multiwavelength observations of cirrus clouds in the north celestial loop: physical parameters of...

Mon. Not. R. Astron. Soc. 416, 1250–1266 (2011) doi:10.1111/j.1365-2966.2011.19121.x

Multiwavelength observations of cirrus clouds in the North CelestialLoop: physical parameters of molecular sites

L. Barriault,1� G. Joncas1� and R. Plume2�1Departement de physique, de genie physique et d’optique and Centre de recherche en astrophysique du Quebec (CRAQ), Universite Laval Quebec,QC G1V 0A6, Canada2Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada

Accepted 2011 May 23. Received 2011 May 11; in original form 2010 May 19

ABSTRACTIn this third paper of the series, we continue to investigate the transition between atomic andmolecular gas in two potential sites of molecule formation at high Galactic latitude, which wecall the Spider and Ursa Major. Using Five College Radio Astronomical Observatory 12CO(J = 1–0) observations and four clump identification algorithms, the clump properties arefirst determined in both regions, showing similar properties compared to the clumps found intranslucent clouds. New higher resolution 12CO (J = 1–0) and 13CO (J = 1–0) observationsfrom the Institut de Radioastronomie Millimetrique and new 12CO (J = 2–1) observationsfrom the James Clerk Maxwell Telescope (resolution ≈20 arcsec) are also presented in fivefields for the Spider (560 spectra) and in two fields for Ursa Major (288 spectra). Using a largevelocity gradient model, physical parameters (density and column density) are determined ineach field. The densities are smaller at the location of the infrared excess peaks (<200 cm−3)than at the locations of the 12CO peaks (≈103 cm−3), confirming that a small density couldexplain the absence of coincidence between the infrared excess peak and the 12CO peak.Self-shielding is probably efficient given the computed 12CO column densities (≈1015 cm−2).

Key words: ISM: clouds – ISM: molecules – radio lines: ISM.

1 IN T RO D U C T I O N

Infrared (IR) cirrus clouds at high Galactic latitudes were first dis-covered by Low et al. (1984). Following this discovery, Boulanger& Perault (1988) showed the existence of a linear correlation be-tween the far-IR intensity and the H I column density in thoseclouds. By examining the far-IR–H I ratio and looking for an ex-cess over that expected from an atomic medium, potential sites ofmolecule formation were discovered (Joncas, Boulanger & Dewd-ney 1992; Reach, Koo & Heiles 1994; Reach, Wall & Odegard 1998;Lockman & Condon 2005; Barriault et al. 2010a). The usual ab-sence of stars in the cirrus clouds with high molecule abundancesallows the study of the transition between the atomic cloud and themolecular cloud.

In diffuse clouds, the rate of H2 formation is determined by twocompetitive processes: the formation of H2 on grain surfaces and thedestruction of H2 by photons (Hollenbach, Werner & Salpeter 1971).Glassgold & Langer (1974) proposed the first one-dimensionalmodel of a diffuse cloud using pressure, thermal, electrical andchemical balance equations. A more complete treatment of the

�E-mail: [email protected] (LB); [email protected] (GJ);[email protected] (RP)

chemistry was done in the model of Black & Dalgarno (1977),whereas a larger chemical network was used in the improved modelof van Dishoeck & Black (1986). Other models were also used to re-produce the physical parameters in diffuse clouds (Snow & McCall2006, and references therein) taking into account processes such asturbulence (Falgarone & Puget 1995; Falgarone, Pineau des Forets& Roueff 1995) and shocks (Flower & Pineau des Forets 1998) toreproduce the abundances of molecules (CH +, HCO +) that needenergy sources (endothermic reaction) larger than the average en-ergy in diffuse clouds. The Meudon photodissociation region (PDR)model (Viala 1986) has also been applied to a diffuse cloud (Le Petit,Roueff & Herbst 2004; Le Petit et al. 2006).

While the development of models has improved our knowledgeof molecule formation, this process is not completely understood.Contrary to the H I observations which are available for large re-gions at high Galactic latitudes (e.g. Hartmann & Burton 1997),the observation of H2 is only possible in absorption for some linesof sight towards stars or active galactic nuclei (Savage et al. 1977;Gillmon & Shull 2006). A surrogate molecule like CO is thereforeused to trace H2 (Heithausen et al. 1993). However, the 12CO peakis not always coincident with the IR excess peak (Barriault et al.2010a). Barriault et al. (2010b) show that OH, a precursor moleculenecessary to CO formation in diffuse regions (Black & Dalgarno1977), could be a better tracer of H2 than CO. The IR excess peak is

C© 2011 The AuthorsMonthly Notices of the Royal Astronomical Society C© 2011 RAS

Physical parameters of molecular sites 1251

coincident with an OH intensity peak, but not with a 12CO intensitypeak, and the OH emission covers an area three times larger thanthat delineated in 12CO. In some regions, the H I column densityis therefore above the threshold for OH and H2, but not for CO.This cloud layering, first noted by Andersson, Wannier & Morris(1991), is in agreement with the results obtained using PDR models(Hollenbach & Tielens 1999, and references therein) which predictthat the C +/C/CO transition occurs deeper in the cloud than theH/H2 transition. However, we do not know if the absence of coinci-dence between the 12CO peak and the IR excess peak is caused bydensities too low for the CO excitation or insufficient self-shielding.A better understanding of the properties and the structure of highGalactic latitude clouds will help elucidate this situation.

In the first and the second paper of the series (Barriault et al.2010a,b), the transition between the atomic and molecular gas wasstudied in two high Galactic latitude clouds which we called the Spi-der and Ursa Major. These fields are located in a prominent H I loop,called the North Celestial Loop (NCL; Meyerdierks, Heithausen &Reif 1991). The first region, the Spider, has a fragmented structure,whereas the morphology of the second region, Ursa Major, is closerto a Galactic plane molecular cloud structure. These studies confirmthe importance of dynamical behaviours in molecule formation andshow that CO might not be the best tracer of H2 in diffuse regions. Inthis third paper of the series, the properties of the molecular gas willbe analysed to understand the conditions in which CO is formed.

First, we will determine the properties of the 12CO clumps in theSpider and in Ursa Major using four clump identification algorithmson the lower resolution (45 arcsec) 12CO (J = 1–0) observationsfrom Five College Radio Astronomical Observatory (FCRAO) (al-ready published in Barriault et al. 2010a). These clump propertieswill be compared with clump properties found in the denser highGalactic latitude translucent clouds.

Secondly, we will present new 12CO (J = 1–0) and 13CO (J = 1–0) observations from the Institut de Radioastronomie Millimetrique(IRAM) and new 12CO (J = 2–1) observations from the JamesClerk Maxwell Telescope (JCMT) with a resolution of ≈20 arcsecfor five fields in the Spider and for two fields in Ursa Major. Thephysical parameters [n(H2), N(CO)] of each field will be determinedusing these observations and a large velocity gradient (LVG) model(Goldreich & Kwan 1974; Scoville & Solomon 1974; Goldsmith,Young & Langer 1983).

The paper is structured as follows. In Section 2 the observationsand the data processing techniques are outlined. In Section 3 theresults are described for each object in turn. In Section 4, the resultsare discussed. We conclude in Section 5.

2 O BSERVATIONS AND DATA PROCESSING

The 12CO (J = 1–0) and 13CO (J = 1–0) observations from FCRAOare described in Barriault et al. (2010a). The angular resolutionis 45 arcsec pixel−1 with 22.5 arcsec pixel−1 for the Spider and20 arcsec pixel−1 for Ursa Major. The initial channel spacing was0.063 km s−1 for the 12CO observations, with a velocity resolutionof 0.077 km s−1, and these data were binned to 0.254 km s−1. Therms noise (Tmb) of the binned data is 0.11 K in 12CO and 0.04 K in13CO. For the clump analysis in Sections 3.1.2 and 3.1.3, the 12COdata were binned to 0.126 km s−1. In the Spider, 13CO emission isdetected only in three well-defined local maxima, which we labelP1, P2 and P3 (see Fig. 1). In the Spider and Ursa Major, new IRAMand JCMT observations were obtained for seven fields with a betterresolution and will be described in the following paragraphs. Thelocations of the new fields were chosen to determine the physical

Figure 1. Location of the five fields observed in 12CO (J = 1–0), 12CO(J = 2–1) and 13CO (J = 1–0). The map shows the FCRAO 12CO integratedintensity. The external limits of the FCRAO map are shown. The fields F1–F5 are shown as black-filled squares. The three regions with 13CO detectionsat the FCRAO are identified (P1–P3, see Section 3.1.1).

conditions in different regions of the Spider and Ursa Major, andmost of the fields are therefore not coincident with the CO clumpsobserved in the FCRAO 12CO integrated intensity map.

The 12CO (J = 1–0), 13CO (J = 1–0), 12CO (J = 2–1) and 13CO(J = 2–1) data were obtained simultaneously at the IRAM 30-mtelescope (Baars et al. 1987) in 2008 May in frequency switchingmode. The IRAM (J = 2–1) observations will not be used in thispaper since the signal-to-noise ratio (S/N) of these observations islower than the S/N of the JCMT (J = 2–1) observations describedin the next paragraph. A total of 30.5 h was spent on the sky. Thedata obtained at the telescope are calibrated to obtain T∗

A (Kutner &Ulich 1981). The calibration at the IRAM 30-m telescope is betterthan 10 per cent for observations at 115 GHz (Kaminski 2008). Themain beam temperatures (Tmb) were obtained using forward andmain beam efficiencies (Feff = 0.97 and Beff = 0.72 at 115 GHz).Tmb is defined by Tmb = (Feff /Beff )T∗

A (e.g. Lisenfeld et al. 2008).The spatial resolution is 20 arcsec and the data were mapped on a10 arcsec grid. To increase the S/N, the data were smoothed witha Gaussian to an effective resolution of 28 arcsec. The channelspacings are 0.203 km s−1 for the 12CO data and 0.213 km s−1 forthe 13CO. All radial velocities are expressed with respect to the localstandard of rest (LSR). The noise σ (Tmb) varies between 0.06 and0.09 K for the 12CO data and varies between 0.04 and 0.06 K for the13CO data. The GILDAS software (Pety 2005) was used to reduce thedata.

The 12CO (J = 2–1) observations were obtained at the JCMT(Prestage 1996), Mauna Kea, Hawaii, using Auto-Correlation Spec-tral Imaging System (ACSIS), in flexible observing mode in 2007,2008 and 2009. We observed in position switching mode and theoff positions were chosen where no CO was detected (IR local min-ima), but close enough to the source to limit the overhead. A totalof 152.65 h was spent on the sky. The data obtained at the telescopeare calibrated to obtain T∗

A. As the data of several nights over a pe-riod of three years were added together, we must take into accountthe variation of the atmospheric conditions. Each night, a calibra-tion spectra was obtained towards one of the following sources:IRC-10216, CRL-2688, CRL-618, OH-231.8 and N2071IR. Thesecalibration spectra were compared with representative spectra and

C© 2011 The Authors, MNRAS 416, 1250–1266Monthly Notices of the Royal Astronomical Society C© 2011 RAS

1252 L. Barriault, G. Joncas and R. Plume

Table 1. Parameters and coordinates of the fields ob-served in the Spider.

Field Name α (2000) δ (2000)(h m s) (◦ ′ ′′)

F1 12CO peak 1 10 35 24.00 73 40 10.7F2 12CO peak 2 10 33 18.00 73 20 45.5F3 Middle 10 36 2.40 73 33 23.9F4 IR excess peak 10 38 19.75 73 28 44.3F5 IR peak 10 41 0.00 73 22 30.0

variations below 10 per cent were found, which is smaller than theuncertainties on the data (10–15 per cent). To determine the mainbeam temperatures, we divided the calibrated T∗

A by the main beamefficiency (0.72). The spatial resolution is 21 arcsec and the datawere mapped on a 10 arcsec grid. To increase the S/N, the data weresmoothed with a Gaussian to an effective resolution of 28 arcsec.The final channel spacing is 0.2 km s−1. The noise σ (Tmb) variesbetween 0.05 and 0.12 K. The STARLINK software (Penny et al. 1993)and the Interactive Data Language (IDL) (Landsman 1995) wereused to reduce the data.

To compare the IRAM data with the JCMT data, all data wereregridded on the same grid using algorithms of the IDL packageSLICE (Miville-Deschenes et al. 2000). The comparison of a 12CO(J = 2–1) spectrum from the JCMT with a 12CO (J = 2–1) spectrumfrom IRAM smoothed and binned to the same pixel size indicatesgood agreement between the calibration of both telescopes. Fig. 1shows the location of the fields observed in the Spider and Table 1provides the central coordinates of each field. Field F1 is locatednear the centre of 12CO peak 1, whereas field F2 is located onthe edge of 12CO peak 2. Field F3 (middle) is located in a regionbetween the 12CO peak and the IR excess peak. Fields F4 and F5are located on the IR excess peak and on the IR peak, respectively.The dimensions of the fields are 2 × 2 arcmin2 (144 points), exceptfor fields F4 and F5 (80 × 80 arcsec2, 64 points).

Fig. 2 shows the location of the fields observed in the Ursa Majorcloud and Table 2 provides the central coordinates of each field.One field (F6) is located on the IR excess peak and another (F7)is located on the 12CO peak. The dimensions of the fields are 2 ×2 arcmin2. The 12CO IRAM line in fields F6 and F7 is contaminatedby the 12CO mesospheric line (Bevilacqua et al. 1989; Forkman et al.2003). The 12CO mesospheric line was then subtracted, assumingthat it is constant everywhere on the field. No contamination isobserved in the Spider.

3 R ESULTS AND INTERPRETATIONS

3.1 The Spider

3.1.1 FCRAO observations in the Spider

The lower resolution FCRAO 12CO (J = 1–0) and 13CO (J = 1–0) data (presented in Barriault et al. 2010a) will first be used tounderstand the kinematical behaviour of the Spider molecular gas.The FCRAO 12CO and 13CO profiles were fitted with at most twoGaussian components. The 12CO velocity range is 0–10 km s−1 andthe velocity distribution is skewed towards larger velocities com-pared to the mean velocity. The southern clumps move with red-shifted velocities (>5 km s−1), while the northern clumps (aboveP3 in Fig. 1) move with blueshifted velocities (<3 km s−1) withrespect to the mean velocity. The mean 12CO velocity for all the

Figure 2. Location of the two fields observed in 12CO (J = 1–0), 12CO(J = 2–1) and 13CO (J = 1–0) in the Ursa Major cloud (white squares). Themap shows the FCRAO 12CO integrated intensity.

Table 2. Parameters and coordinates of the fieldsobserved in the Ursa Major cirrus.

Field Name α (2000) δ (2000)(h m s) (◦ ′ ′′)

F6 IR excess peak 9 51 24.00 70 32 49.0F7 12CO peak 9 51 16.60 70 46 30.0

field (≈15 clumps) is 3.85 ± 0.02 km s−1, while the mean 13COvelocity is 4.21 ± 0.07 km s−1 for three clumps where 13CO isdetected. The difference is explained by the fragmented nature ofthe cloud, where little physical contact exists between the clumps.Table 3 provides a summary of the statistical characteristics of 12COand 13CO radial velocity, linewidth and integrated intensity distri-butions for the subregions P1, P2 and P3 (see Fig. 1). In Table 3, theuncertainties are the standard deviation of the mean velocity and ofthe mean linewidth for each region. We are confident in using the12CO velocities as a reliable measure of the kinematics of the cloudeven if the line is optically thick, since the difference with 13CO issmall. The 12CO linewidth is always at least 10 per cent larger thanthe 13CO linewidth, as expected from an optically thick line. The∫

Tmb(12CO)dv/∫

Tmb(13CO)dv ratio varies a lot, indicating differ-ential excitation, opacity and abundance between 12CO and 13CO.Around 7 per cent of the 12CO profiles have two velocity com-ponents in the Spider, with a velocity difference varying between1.33 and 4.33 km s−1. Those two-component profiles are relatedto two spatially superposed, but kinematically distinct, molecularfeatures.

3.1.2 Comparison of four clump algorithms

To better understand the Spider, we will now compute the prop-erties of the several clumps found in this region using the 12CO



Table 3. Comparison of the LSR radial velocity, linewidth and integrated intensity distribu-tions where 13CO is detected in the Spider.

Parameter P1 P2 P3

Coordinates 10h34m3.s33 10h35m25.s47 10h31m46.s9073◦24′12.′′8 73◦40′40.′′1 73◦47′43.′′1

12CO mean radial velocity (km s−1) 4.69 ± 0.01 4.23 ± 0.01 3.14 ± 0.0112CO mean FWHM (km s−1) 0.805 ± 0.007 0.600 ± 0.002 0.79 ± 0.0113CO mean radial velocity (km s−1) 4.63 ± 0.01 4.22 ± 0.01 3.15 ± 0.0213CO mean FWHM (km s−1) 0.57 ± 0.02 0.52 ± 0.02 0.64 ± 0.05Mean

∫Tmb(12CO)dv (K km s−1) 2.7 ± 0.1 2.00 ± 0.06 1.6 ± 0.2

Mean∫

Tmb(13CO)dv (K km s−1) 0.21 ± 0.01 0.10 ± 0.01 0.048 ± 0.00312CO S/N 13.9–72.0 29.1–55.5 14.4–32.913CO S/N 3.4–32.7 3.4–7.4 3.0–5.7Mean

∫Tmb(12CO)dv/

∫Tmb(13CO)dv 12.8 19.1 34.1

Number of points 94 32 30

FCRAO data. Identification of clumps by eye imposes a subjectiveanalysis (Blitz & Stark 1986). Therefore, the application CUPID ofthe STARLINK project (Berry et al. 2007) is used to find clumps inthe Spider. Four clump identification algorithms are used: GAUSS-CLUMPS (Stutzki & Guesten 1990), CLUMPFIND (Williams, de Geus& Blitz 1994), REINHOLD and FELLWALKER (Berry et al. 2007). Inthe GAUSSCLUMPS algorithm, the local maximum is found and aGaussian clump is fitted. 10 parameters (clump position and ve-locity, full width at half-maximum (FWHM) values along eachaxes, intensity, orientation and velocity gradient in both direction)are varied to minimize the χ 2. Then the clump is subtracted fromthe data cube. Newly found clumps are subtracted in turn. As theS/N decreases, the overlapping clumps are not well separated by thealgorithm. Artificial clumps can also be found in the low-mass endof the distribution (Kramer et al. 1998). According to Chi & Park(2006), for sources that are not very bright, GAUSSCLUMPS is moreeffective.

In the CLUMPFIND algorithm (friends-of-friends algorithm), the in-tegrated intensity contours of the data are computed. The differencebetween the contour levels and the minimum contour level is a func-tion of the noise. At each contour level, the contiguous pixels thatare above the contour are marked as a new clump.

In the REINHOLD algorithm, the edges of the clumps are identifiedby using a set of one-dimensional profiles across the map. Eachprofile is a plot of Tmb with respect to the position in pixels. Thehighest data value in the profile is found. From this peak, the edgeis identified when a pixel already included in another clump is met,when two adjacent pixels are below the background level, when thegradient of the profile over three adjacent pixels is smaller than aspecified value or when the end of the profile is met. The pixelsbetween the edges are associated to the clumps. This is applied toprofiles in other directions and only the clumps found in each direc-tion are kept, producing a three-dimensional set of shells, outliningthe clumps. As the results are often affected by noise, the structuresare cleaned using cellular automata. This algorithm was developedat the Joint Astronomy Centre (JAC) in Hilo, Hawaii.

In the FELLWALKER algorithm (Berry et al. 2007), each data pixelabove the background level is considered as a start for a walkaround the image. At each step, the peak is searched by selectingthe neighbour located in the direction with the steepest gradient.This continues until a local maximum is found.

An S/N of 7 was chosen in order to limit false detections. De-pending on the input parameters, the number of detected clumpsvaries. Using a 45 arcsec beam, only clumps with FWHM largerthan the beam and with a spectral FHWM larger than the velocity

resolution were of course chosen to be real. No clumps were foundusing GAUSSCLUMPS, while more than 10 clumps were found usingthe other algorithms: 15 using CLUMPFIND, 10 using REINHOLD and17 using FELLWALKER. None of the algorithms was able to find threeclumps located to the south-east in Fig. 1 and visible by eye on thechannel maps (see fig. 5 in Barriault et al. 2010a). Some clumpswere identified as two distinct clumps with the FELLWALKER algo-rithm but as single clumps with the CLUMPFIND algorithm. The resultgiven by the FELLWALKER algorithm seems trustworthy as we see thetwo clumps in the channel maps (see fig. 5 in Barriault et al. 2010a)and with an S/N of 7, whereas the CLUMPFIND algorithm was notable to separate the clumps. The REINHOLD algorithm was not ableto find all the clumps found by the other two algorithms. The fittedparameters obtained using the algorithms are the peak value, thespatial and spectral positions as well as the dimensions (rms devia-tion of each pixel with respect to the centroid, where each pixel isweighted by the corresponding pixel data value) of each clump. Thepeak value and the spectral and spatial dimensions are corrected totake account of the smoothing introduced by the instrumental beam.In the following sections, all dimensions are deconvolved from thebeam.

3.1.3 Clump characteristics in the Spider

H2 masses are determined using the clump parameters found in theprevious section and the two methods described below. First, thevirial mass (Mvir) was calculated from (Casasola et al. 2007)

Mvir[M�] = KD�v22.3543, (1)

where D is the diameter of a sphere with radius R =√(�x1 × �x2)/4 in pc and �v is the velocity dispersion (σ =

FWHM/2.354) of the clump in km s−1. �x1 and �x2 are the spa-tial dimensions of the clump. According to Casasola et al. (2007),K is equal to 95 if we assume a sphere with a power-law densitydistribution.

Secondly, the clump mass (Msum) was computed assuming a dis-tance to the cloud of 100 pc (de Vries, Thaddeus & Heithausen1987) and a constant ratio between the H2 column density and the12CO integrated intensity (X factor):

Msum[M�] = m(H2)AW (CO)X, (2)

where m(H2) is the mass of a hydrogen molecule in M�, A isthe area of the clump on the plane of the sky and W(CO) is the12CO integrated intensity. While de Vries et al. (1987) found X =N(H2)/W(CO) = (0.5 ± 0.3) × 1020 cm−2 (K km s−1)−1 in Ursa



Table 4. Dimensions of the clumps found by all the algorithms in the Spider. All the dimensions aredispersions deconvolved from the beam.

CLUMPFIND REINHOLD FELLWALKER

l b v �x1 �x2 �x1 �x2 �x1 �x2

(◦) (◦) (km s−1) (arcsec) (arcsec) (arcsec) (arcsec) (arcsec) (arcsec)

1 135.13 40.31 4.8 44.00 41.38 43.21 41.04 43.88 41.252 134.79 40.16 4.3 32.12 60.55 30.80 53.19 32.60 61.863 134.95 39.90 3.2 26.97 62.15 23.32 52.85 28.45 64.034 134.81 39.80 2.6 35.83 35.84 28.44 23.83 35.32 35.485 135.01 40.47 3.8 28.84 32.23 27.39 29.98 28.17 31.426 134.77 40.23 4.2 55.72 33.94 50.03 23.78 56.22 33.667 135.05 40.11 3.5 81.01 65.26 12.10 28.23 19.39 34.008 134.67 40.64 4.7 28.56 33.22 26.62 31.62 28.08 32.679 134.95 39.97 3.2–3.3 15.99 33.78 9.75 20.52 16.07 33.87

Table 5. Range of other properties of the Spider clumps found by all the three algorithms.

�v Tmbpeak Mean N(H2) Max N(H2) Mvir Msum

(km s−1) (K) (× 1020 cm−2) (× 1020 cm−2) (M�) (M�)

1 0.24–0.25 7.22–7.26 0.85–0.88 2.21 1.51–1.55 0.022–0.0232 0.15 5.51–5.73 0.44–0.48 1.14 0.54–0.62 0.011–0.0123 0.25–0.26 3.31–3.62 0.39–0.46 0.93–0.96 1.32–1.80 0.007–0.0094 0.15–0.16 3.80–4.70 0.29–0.43 0.80–0.83 0.37–0.57 0.004–0.0065 0.15–0.17 3.62–3.87 0.35–0.36 0.72 0.40–0.51 0.004–0.0056 0.16–0.18 2.57–3.04 0.24–0.32 0.54–0.63 0.55–0.86 0.004–0.0087 0.09–0.13 2.26–5.37 0.19–0.22 0.45–0.50 0.09–0.79 0.002–0.0048 0.17–0.18 2.61–2.73 0.30–0.31 0.64 0.48–0.57 0.003–0.0049 0.17–0.26 2.47–4.55 0.26–0.31 0.55–0.75 0.24–0.98 0.001–0.003

Major, Reach et al. (1998) measured X = N(H2)/W(CO) = (1.3 ±0.2) × 1020 cm−2 (K km s−1)−1 for high-latitude molecular cloudstaking into account the lower dust temperature in molecular gas, avalue that they found to be consistent with γ -ray determination forthe same region. However, there is a large uncertainty on these dusttemperatures. As uniform dust properties over the whole NCL wereassumed in Barriault et al. (2010a), we will adopt X = (0.5 ± 0.3) ×1020 cm−2 (K km s−1)−1 in this paper to compute mean N(H2) andmasses. There is a large uncertainty on the X factor as it appearsto vary across the Galaxy. Unfortunately, the number of studies onthe X factor is smaller at high Galactic latitudes than in the Galacticplane. The masses found in this study are therefore approximate.While we found mean masses around ≈5 × 10−3 M� using thismethod, the masses would be around ≈1.5 × 10−2 M� using an Xfactor three times larger.

The uncertainty on the mass is the sum of many factors. Whilethe virial mass does not depend on the value of the X factor, thereare common uncertainties on the distance (factor of 2) and on the Xfactor (at least a factor of 3) for the determination of Msum by eachalgorithm. There are also specific uncertainties for each algorithmbut using four algorithms helps constrain the mean properties of theclumps. Bias introduced by clump identification programs were alsodiscussed by Schneider & Brooks (2004), but contrary to our study,in their analysis the GAUSSCLUMPS algorithm found more clumps(≈2000) than the CLUMPFIND algorithm (≈200) for a giant molecularcloud.

In order to provide robust results, Table 4 provides the dimen-sions of only the nine clumps found at the same location and thesame velocity by the three algorithms. �x1 and �x2 are the spatialdimensions (rms deviation of each pixel centre from the clump cen-troid, where each pixel is weighted by the corresponding pixel data

value) of the clump in arcmin. For the nine clumps, Table 5 providesthe range of velocity dispersion, peak intensity, mean and maximumH2 column density, and mass found using the three clump-findingalgorithms. The clump-finding algorithms provide very differentsolutions when the clumps are less massive than ≈0.004 M�. Thedifferent solutions are probably caused by the way the different al-gorithms respond to small clumps with many pixels close to the S/Nratio limit given by the user. Simulations would be needed to betterunderstand the inner workings of CUPID at low antenna temperaturelevels. To our knowledge no work has been published on the sub-ject. We thus chose not to analyse the clumps detected below thatthreshold. However, even if their existence is questionable, thesevery low-mass clumps are usually associated with structures in thechannel maps.

The mean properties of the clumps found in the Spider are shownin Table 6. The standard deviation of the distribution is written inparentheses. Using FCRAO 12CO data, the total H2 mass of theSpider is 0.30 M� (Barriault et al. 2010a). The H2 mass (Msum)within the clumps is 0.085 M� using CLUMPFIND (28 per cent ofthe total mass of the Spider), 0.058 M� using REINHOLD (19 percent) and 0.081 M� using FELLWALKER (27 per cent). The clumpmasses are between 10−2 and 10−3 M�. The clumps are asymmetric(�x1/�x2 ≈ 0.6–0.7), explaining the failure of GAUSSCLUMPS. Mvir

is always larger than Msum. Mvir is an upper limit, while Msum isunderestimated since the combination of τ < 1 and subthermalexcitation gives low Tmb. A filling factor smaller than 1 can alsoexplain the mass difference. Such large differences between thetwo masses are not unusual, since differences by factors of 100–1000 were found by Kramer et al. (1998) in the high Galacticlatitude dark cloud L1457 and in the high Galactic latitude cloudMCLD 126.6+24.5. The large difference between the two mass



Table 6. Mean parameters of the clumps found using the three algorithms in the Spider.

Algorithm Number Mvir Msum �x1/ �x2

of clumps Mean Range Mean Range(M�) (M�) (M�) (M�)

CLUMPFIND 14 0.87 0.02–3.36 0.005 0.001–0.02 0.70(0.87)a (0.006) (0.20)

REINHOLD 10 0.55 0.05–1.51 0.006 0.001–0.02 0.67(0.49) (0.006) (0.21)

FELLWALKER 17 0.68 0.02–2.15 0.005 0.001–0.02 0.68(0.66) (0.006) (0.20)

aThe standard deviation of the distribution is written in parentheses.

calculations indicates that the clumps are probably gravitationallyunbound objects (see Section 4.2).

While this analysis shows that the number of clumps stronglydepends on the input parameters, such as the minimal S/N, theproperties of the most massive clumps are similar using differentalgorithms, indicating that they are probably real. An absence ofagreement between the algorithms occurs for clumps with massesaround or smaller than 0.004 M� and prevents the analysis of theirindividual properties.

3.1.4 JCMT and IRAM observations in the Spider

Fig. 3 shows the IRAM 12CO and 13CO (J = 1–0) and the JCMT12CO (J = 2–1) profiles at the central location of the five fields ob-served in the Spider. 12CO (J = 1–0) emission above 3σ is detectedat each location, except in F4, whereas 13CO(J = 1–0) emission isstrong (>8σ ) at the location of the 12CO peak 1 (F1). At the otherlocations, there are possible detections of 13CO (<3σ ). The shapesof the 12CO (J = 2–1) and the 12CO (J = 1–0) profiles are verysimilar. For the 12CO (J = 2–1) line, the S/N is smaller than 3 onlyat the location of the IR excess peak (F4). 12CO and 13CO profilesat location F3 show a double component, explained by the presenceof two clumps on the line of sight (see Section 3.1.1). The weakCO emission at the location of the IR excess peak (F4) confirms theabsence of coincidence between the CO and IR excess peaks. Thepoor S/N of the 13CO (J = 1–0) observations will be the limitingfactor of the following analysis.

The integrated intensities were computed by summing the mainbeam temperatures between the appropriate velocities for each fieldand multiplying by the channel spacing. This also provides upperlimits for regions with no 3σ detections. Fig. 4 shows the integratedintensity maps for each line and at each location in the Spider. Atlocation F1, the morphology of the three lines integrated intensitymaps is similar. The strongest emission is located between the centreand the north-west of the field. The 13CO maps for regions F2–F5are essentially noise maps. At location F2, the strongest emission isto the south, while only weak emission is detected to the north-east.At location F3, the strongest emission is located on a line goingfrom the south-west to the north of the field. At location F4, thestrongest emission is to the south-east. At location F5, the strongestemission is to the west and to the south-east. The CO emissionseems to surround the IR peak, but this cannot be determined withcertainty since the position of the IR peak was determined using4.3 arcmin resolution IRAS data, while the F5 field is only 2 arcminwide.

The 12CO (J = 1–0), 13CO (J = 1–0) and 12CO (J = 2–1) pro-files were fitted with at most two Gaussian components. To fit theweaker 13CO (J = 1–0) line, the 12CO (J = 1–0) velocity and

linewidth were given as a parameter guess to the Gaussian fittingprogram. Tables 7–9 provide the mean integrated intensity, veloc-ity and FWHM obtained using Gaussian fitting in each field of theSpider for the IRAM 12CO (J = 1–0) and 13CO (J = 1–0) lines andfor the JCMT 12CO (J = 2–1) line, respectively. The tables alsoprovide the mean rms noise, the S/N range and the percentage ofspectra with an S/N higher than 3. The uncertainties on the meanintegrated intensity, velocity and FWHM are the statistical uncer-tainties calculated from the standard deviation of the distributions.In these tables, F3–1 and F3–2 refer to the redshifted componentand the blueshifted component, respectively, of the field F3. Onlyspectra with an S/N above 3σ are used for the computation of thestatistical parameters, explaining the absence of 13CO mean param-eters in fields F3–F5. Surprisingly, 13CO emission is not detectedat the IR peak (F5), whereas the mean 12CO (J = 1–0) integratedintensity is three times larger at this location than at the 12CO peak2 (F2). This infers variations of the

∫Tmb(12CO)dv/

∫Tmb(13CO)dv

ratio, with the larger ratio occurring in the more diffuse regions. The12CO (J = 1–0) and 12CO (J = 2–1) mean velocities are similarand the small differences can be explained by the fewer numberof 12CO (J = 2–1) detections above 3σ , especially in the field F4.The profiles are usually broader (>0.9 km s−1) to the south-east (F4and F5) and they are narrower to the north-west (≈0.6 km s−1). The12CO (J = 2–1) profiles are usually slightly narrower than the 12CO(J = 1–0) profiles except at location F4 where the broader Gaussianfits are explained by the smaller S/N.

As shown in Fig. 4, the distinction between a noise peak and the13CO line is not obvious, except in field F1 where 13CO emissionis strong. The features fitted in the fields could therefore be noisepeaks. To better determine the parameters for the 13CO line, the13CO data were smoothed with a Gaussian (FWHM = 40 arcsec)and binned to 20 arcsec to increase the S/N. The Gaussian fittingresults of these binned data are shown in Table 8. 13CO is detectedin the binned data in 75 per cent of the profiles of field F1 and in33 per cent of the profiles of field F2. There is also a 3σ detectionin two profiles in field F3, but an inspection of these profiles showsthat these detections are not real. The mean integrated intensity issmaller for the binned data than for the full resolution data for fieldF1 since 13CO is detected on an area almost two times larger whenusing the binned data. More weak spectra are therefore contributingto the mean.

3.1.5 Determination of the physical parameters using an LVGmodel in the Spider

Table 10 provides the mean 12CO (J = 2–1)/12CO (J = 1–0) inte-grated intensity ratio for all regions. The uncertainties on the meanratio and on the standard deviation are the statistical uncertainties



Figure 3. The columns show the IRAM 12CO (J = 1–0), 13CO (J = 1–0) and JCMT 12CO (J = 2–1) profiles in the Spider at the location F1–F5.

calculated from the standard deviation. The largest ratio, observedat the location of the IR excess peak (F4), is not real, since theGaussian fits are poorer due to the lower S/N of the JCMT profilesat this location (see Fig. 3). The peak temperature ratio is 0.46 ±0.05 at this location, probably closer to the real value. The smallestratio is observed at location F5.

For low 12CO (J = 2–1)/12CO (J = 1–0) ratios (≈0.4) andfor small column densities, local thermodynamic equilibrium so-lutions do not exist (Reach et al. 1994). To determine the phys-

ical conditions, an LVG model must be used (Goldreich &Kwan 1974; Scoville & Solomon 1974; Goldsmith et al. 1983).This model assumes that the velocity gradient multiplied by thesize of the region is larger than the local velocity dispersion(Leung & Brown 1977). Then, using an escape probability for-malism, the model solves the coupled equations of detailed bal-ance and radiative transfer to find the best density/column den-sity/kinetic temperature combination that matches the observed lineintensities.



Figure 4. The columns show the IRAM 12CO (J = 1–0), 13CO (J = 1–0) and JCMT 12CO (J = 2–1) integrated intensity maps in the Spider at the locationsF1–F5. The contours show the regions in which the data are above the 3σ level.

The parameters of each line were obtained by the Gaussian fitson the binned and smoothed data (see Section 3.1.4) for the spectrawith an S/N higher than 3. For the 12CO (J = 2–1) or the 13CO (J =1–0) spectra with an S/N lower than 3, the 12CO (J = 1–0) velocityand FWHM and the spectral noise were the parameters given tothe model. For these locations, only upper limits on the columndensities and on the densities will be obtained.

To find the physical conditions in the gas, we created a cube of20 × 20 × 8 LVG models in density/column density/temperaturespace, respectively. The densities ranged from 100 to 105 cm−3, col-umn densities from 5 × 1013 to 5 × 1017 cm−2, and temperatures

from 6 to 20 K. We then used a χ 2 minimization routine to findthe best n(H2)–N(12CO)–Tk combination that fits the observed lineintensities. The first step in this process was to fix the kinetic tem-perature for each source by finding the temperature which producedthe lowest reduced χ 2 value averaged across the entire map.

An isotopic abundance ratio (12CO/13CO) of 55 is usually as-sumed in the models since it is the average value across the Galacticdisc (Garay et al. 2010). Using this ratio the model is not able to finda suitable solution in field F1 since densities are smaller (102 cm−3)where the CO column densities are larger. Those results show thatthe isotopic ratio could be smaller than the canonical value. The



Table 7. Mean integrated intensity, velocity and FWHM of the IRAM 12CO (J = 1–0) observationsin the Spider.

Field Mean integrated Mean Mean Noise S/N Percentageintensity velocity FWHM of spectra

(K km s−1) (km s−1) (km s−1) (K) (>3σ )

F1 1.82 ± 0.08 4.256 ± 0.006 0.642 ± 0.06 0.08 3.6–103.2 95F2 0.51 ± 0.03 4.65 ± 0.01 0.66 ± 0.01 0.07 3.1–31.9 72F3–1 0.34 ± 0.02 4.94 ± 0.02 0.64 ± 0.02 0.06 3.3–21.3 78F3–2 0.45 ± 0.01 3.60 ± 0.01 0.62 ± 0.01 0.06 3.1–26.6 89F4 0.33 ± 0.03 7.07 ± 0.03 0.94 ± 0.06 0.06 3.0–10.9 55F5 1.48 ± 0.06 4.713 ± 0.008 0.92 ± 0.01 0.09 6.0–28.2 100

Table 8. Mean integrated intensity, velocity and FWHM of the IRAM 13CO (J = 1–0)observations in the Spider.


(K km s−1) (km s−1) (km s−1) (K) (>3σ )

Full resolution data

F1 0.167 ± 0.008 4.33 ± 0.01 0.67 ± 0.03 0.04 3.0–12.6 40F2 0.07 ± 0.02 4.62 ± 0.08 0.6 ± 0.2 0.04 3.1–3.5 2.8

Binned data

F1 0.119 ± 0.008 4.25 ± 0.02 0.70 ± 0.01 0.03 3.5–18.7 75F2 0.068 ± 0.003 4.60 ± 0.05 0.74 ± 0.02 0.03 3.1–4.5 33F3–1 – – – 0.02 – –F3–2 – – – 0.02 – –F4 – – – 0.01 – –F5 – – – 0.02 – –

Table 9. Mean integrated intensity, velocity and FWHM of the JCMT 12CO (J = 2–1) observationsin the Spider.


(K km s−1) (km s−1) (km s−1) (K) (>3σ )

F1 1.01 ± 0.05 4.286 ± 0.006 0.590 ± 0.007 0.09 3.0–44.3 84F2 0.24 ± 0.01 4.77 ± 0.04 0.58 ± 0.02 0.07 3.0–11.5 51F3–1 0.15± 0.01 5.04 ± 0.03 0.49 ± 0.03 0.07 3.0–8.5 34F3–2 0.220± 0.008 3.56 ± 0.02 0.58 ± 0.02 0.07 3.0–12.2 65F4 0.247 ± 0.007 6.67 ± 0.02 1.39 ± 0.02 0.05 3.0–3.4 7.8F5 0.44 ± 0.02 4.756 ± 0.009 0.87 ± 0.02 0.05 3.4–16.6 95

Table 10. Mean 12CO (J = 2–1)/12CO(J = 1–0) integrated intensity ratio in theSpider.

Field Mean Standardratio deviation

F1 0.483 ± 0.007 0.082 ± 0.005F2 0.42 ± 0.01 0.090 ± 0.008F3–1 0.37 ± 0.01 0.09 ± 0.01F3–2 0.43 ± 0.01 0.1112 ± 0.008F4 0.57 ± 0.05 0.13 ± 0.05F5 0.282 ± 0.006 0.046 ± 0.004

model was therefore run using a lower isotopic ratio (30) as well asthe canonical isotopic ratio. Table 11 provides the reduced χ 2 forthe fits. While improvements in χ 2 using a ratio of 30 were signif-icant in F1 and marginal in F2–F4, we used 30 for consistency. In

Table 11. Temperature and isotopic ratio in the Spider.

Field Tk Average reduced χ2

(K) 12CO/13CO = 55 12CO/13CO = 30

F1 12 8.54 0.94F2 18 2.55 2.24F3–1 12 1.00 0.76F3–2 12 0.29 0.27F4 14 0.57 0.33F5 12 0.51 1.91

F5, however, a significantly better fit was found using a ratio of 55,and so we used that value.

Table 11 also provides the kinetic temperature for the best so-lution for each field. As the χ 2 values do not change a lot withtemperature in fields F3 and F5, a temperature of 12 K was chosenfor consistency with field F1.



Table 12. Mean 12CO column densities and mean H2 densities found using an LVG model in the Spider.

Field All the field (upper limits) Only the locations with 3σ detectionsMean Mean Mean Mean Mean Mean Mean Mean Maximum

log[

N(12CO)cm−2

]errora log

[n(H2)cm−3

]error log

[N(12CO)

cm−2

]error log

[n(H2)cm−3

]error log

[n(H2)cm−3

]

F1 15.24 ± 0.06b 0.026 3.23 ± 0.03 0.08 15.36 ± 0.07 0.01 3.250 ± 0.009 0.05 3.39 ± 0.07F2 14.7 ± 0.1 0.07 2.66 ± 0.07 0.18 14.72 ± 0.04 0.04 2.92 ± 0.01 0.12 3.05 ± 0.12F3–1 14.54 ± 0.06 0.13 2.90 ± 0.09 0.38 – – – – 3.65 ± 0.34F3–1 14.68 ± 0.04 0.14 2.95 ± 0.07 0.30 – – – – 3.34 ± 0.11F4 15.1 ± 0.1 0.24 2.1 ± 0.1 0.25 – – – – 2.43 ± 0.27F5 15.64 ± 0.07 0.23 2.41 ± 0.05 0.28 – – – – 2.70 ± 0.16

aThe mean error is the error of the LVG model computations.bThe uncertainty of the mean value is the standard deviation of the mean.

Once the temperature and the isotopic ratio were fixed for eachregion, we then fitted each position in the region to obtain maps ofdensity and column density. As expected, the 12CO column densitymaps and the H2 density maps match quite well the 12CO integratedintensity maps shown in Fig. 4. However, only a few points in themaps have signal above 3σ for all the lines and the LVG fits arenot believable over much of the map. We will therefore look at themean values of the >3σ points for the following analysis. Table 12provides the computed mean 12CO column densities and mean H2

densities in each field taking into account all the positions (upperlimits) and only the positions where the three lines have an S/Nhigher than 3. The maximum column density of the field is alsogiven. In Table 12, the uncertainty of the mean value is the standarddeviation of the mean, while the mean error is the error of the LVGmodel computations. In each field, the mean error of the LVG modelis larger than the statistical uncertainty, indicating that a study of thevariations of densities and 12CO column densities at small scales willbe impossible. Given the error on the results, the column densitiesand the densities are relatively constant across the core in each field.The mean densities vary between ≈120 cm−3 at the location of theIR excess peak (F4) and ≈1700 cm−3 at the location of the 12COpeak 1 (F1). In fields F1 and F2, where the results are reliable, thelargest mean density and the largest mean 12CO column density areboth in field F1, suggesting a correlation between both quantities.The largest density (>3σ ) is 2455 +429

−366 cm−3 in field F1. In fieldF3, the upper limits for the mean density and the mean columndensities are similar for both components. In field F4, a large 12COcolumn density is computed, while the 12CO integrated intensity issmall, but we remind the reader that we only have upper limits. Infield F5, as the integrated intensity, the mean 12CO column densityis large, but the mean density is clearly smaller than in the otherfields.

3.2 Ursa Major

3.2.1 FCRAO observations in Ursa Major

The FCRAO 12CO and 13CO profiles were fitted with one Gaussiancomponent to determine the kinematical behaviour of Ursa Major(Barriault et al. 2010a). The 12CO velocity range is 5–9 km s−1, andthe distribution is skewed towards smaller velocities. This asymme-try of the 12CO velocity distribution is explained by the observedvelocity gradient in one part of the region (see Barriault et al. 2010a).The mean 12CO velocity is similar to the mean 13CO velocity eventhough 13CO is detected on a 6.5 times smaller area. This indicatesthat the velocity is not varying a lot and that the opacity effects arelimited. Table 13 provides a summary of the statistical characteris-

Table 13. Comparison of the LSR radial velocity,linewidth and integrated intensity distributions where13CO is detected in the Ursa Major cirrus.

Parameter Ursa Major

12CO mean radial velocity (km s−1) 7.49 ± 0.0212CO mean FWHM (km s−1) 1.318 ± 0.00913CO mean radial velocity (km s−1) 7.51 ± 0.0213CO mean FWHM (km s−1) 0.94 ± 0.01Mean

∫Tmb(12CO)dv (K km s−1) 3.61 ± 0.05

Mean∫

Tmb(12CO)dv (K km s−1) 0.311 ± 0.00912CO S/N 3.4–63.713CO S/N 3.0–23.8Mean

∫Tmb(12CO)dv/

∫Tmb(13CO)dv 11.6

Number of points 346

tics of the 12CO and 13CO radial velocity, linewidth and integratedintensity distributions in the central region of the Ursa Major cloudwhere 13CO is detected. The computation of the uncertainties wasdone using the same method as in Table 3.

3.2.2 Clump characteristics in Ursa Major

The four algorithms of the application CUPID were also used onthe lower resolution FCRAO 12CO observations of Ursa Major todetermine the properties of the individual clumps. As in the Spider,a minimum S/N of 7 was chosen. Six clumps were found usingGAUSSCLUMPS, 11 using CLUMPFIND, eight using REINHOLD and nineusing FELLWALKER. The masses of the clumps were computed usingthe two methods described in Section 3.1.3.

Table 14 provides the dimensions of the clumps found at thesame location and velocity by the four algorithms. �x1 and �x2 arethe spatial dimensions (rms deviation of each pixel centre from theclump centroid, where each pixel is weighted by the correspondingpixel data value) of the clump in arcmin. Table 15 provides therange of velocity dispersion, peak intensity, mean and maximum H2

column density, and mass found using the four algorithms. Thereis clearly no agreement between the properties. We are thereforeable to determine only a possible range of values. Such differencesbetween the results given by the algorithms illustrate the difficultiesinherent to this type of analysis.

The mean properties of the clumps found in Ursa Major are shownin Table 16. The standard deviation of the distribution is written inparentheses. The mean mass of the clumps found by each algo-rithm is similar. Contrary to the other algorithms, the GAUSSCLUMPS

algorithm is unable to find the small clumps (≈0.001 M�) and theclumps located to the south near the IR excess peak (see Fig. 2). The



Table 14. Dimensions of the clumps found by all the algorithms in the Ursa Major cirrus. All the dimensions are dispersionsdeconvolved from the beam.

GAUSSCLUMPS CLUMPFIND REINHOLD FELLWALKER

l b v �x1 �x2 �x1 �x2 �x1 �x2 �x1 �x2

(◦) (◦) (km s−1) (arcsec) (arcsec) (arcsec) (arcsec) (arcsec) (arcsec) (arcsec) (arcsec)

1 140.5 39.6 7.7–7.8 60.60 44.64 75.96 96.80 35.00 36.86 79.13 104.632 140.4 39.5 7.3 39.08 46.42 36.11 42.76 28.25 30.89 35.78 62.373 140.5 39.6 7.2–7.3 47.31 66.69 48.10 63.46 17.16 70.36 36.73 66.74

Table 15. Range of other properties of the Ursa Major clumps found by all the algorithms.

�v Tmbpeak Mean N(H2) Max N(H2) Mvir Msum

(km s−1) (K) (× 1020 cm−2) (× 1020 cm−2) (M�) (M�)

1 0.36–0.41 5.50–6.79 0.90–1.56 3.12–3.17 2.87–8.42 0.017–0.0682 0.35–0.42 5.16–6.30 0.90–1.02 2.45 2.14–4.45 0.011–0.0203 0.38–0.47 3.55–5.29 0.71–0.87 1.85–2.00 3.07–7.57 0.012–0.023

Table 16. Mean parameters of the clumps found using four algorithms in the Ursa Majorcirrus.

Algorithm Number Mvir Msum �x1/ �x2

of clumps Mean Range Mean Range(M�) (M�) (M�) (M�)

GAUSSCLUMPS 6 5.08 3.22–7.57 0.01 0.005–0.02 0.86(1.44)a (0.005) (0.12)

CLUMPFIND 11 2.81 0.05–7.51 0.01 0.001–0.07 0.63(2.45) (0.02) (0.22)

REINHOLD 8 1.95 0.72–3.59 0.009 0.003–0.02 0.52(1.12) (0.007) (0.29)

FELLWALKER 9 3.59 0.19–8.42 0.02 0.001–0.07 0.66(2.89) (0.02) (0.09)

aThe standard deviation of the distribution is written in parentheses.

CLUMPFIND algorithm and the FELLWALKER algorithm find the small-est clumps (≈0.001 M�) and the biggest clumps (≈0.07 M�).Using FCRAO 12CO data, the total H2 mass of the Ursa Majorcloud is 0.32 M� (Barriault et al. 2010a). The H2 mass within theclumps is 0.072 M� using GAUSSCLUMPS (23 per cent of the totalmass of the Ursa Major cloud), 0.075 M� using REINHOLD (23 percent), 0.151 M� using FELLWALKER (47 per cent) and 0.15 M� usingCLUMPFIND (47 per cent).

The mean spatial dimensions are almost equal for the clumpsfound using GAUSSCLUMPS (≈46 arcsec), indicating circular clumps.The clumps found using the other algorithms are not circular, �x1

being smaller than �x2. They are elongated and irregular. Whereas,with the CLUMPFIND and FELLWALKER algorithms, the clumps are con-tiguous, the clumps found with the REINHOLD algorithm are well sep-arated. The mean �v is similar for all the algorithms (≈0.4 km s−1).Some clumps found with CLUMPFIND are too big and contain morethan one clump when looking at the channel maps (see fig. 14in Barriault et al. 2010a). This is explained by the S/N thresholdused (7) which is also used as the gap between the contour levels.When this value is decreased, the biggest clumps are fractionated inmore clumps. The same behaviour is observed with the FELLWALKER

algorithm.

3.2.3 JCMT and IRAM observations in Ursa Major

Fig. 5 shows the IRAM 12CO (J = 1–0), 13CO (J = 1–0)and the JCMT 12CO (J = 2–1) profiles at the central location

of the IR excess peak (F6) and the 12CO peak (F7) fields inthe Ursa Major cloud. At the location of the IR excess peak,13CO is not detected, 12CO (J = 2–1) is very weak (<3σ )and 12CO (J = 1–0) is strong (>12σ ). At the location of the12CO peak, strong detections above 3σ are observed for eachline.

The integrated intensities were obtained by summing the mainbeam temperature over the appropriate velocities and multiply-ing by the channel spacing at both locations. Fig. 6 shows theintegrated intensity maps for each line and at each location inUrsa Major. At location F6, the strongest emission is observedto the north-east. At location F7, three high intensity peaks arevisible. One peak is located to the north, another peak is lo-cated to the centre of the field with an offset to the west com-pared to the first peak, and the last peak is located to thesouth.

Tables 17–19 provide the mean integrated intensity, velocity andFWHM obtained using Gaussian fitting in each field of the UrsaMajor cloud for the IRAM 12CO (J = 1–0) and 13CO (J = 1–0) lines and for the JCMT 12CO (J = 2–1) line, respectively.The tables also provide the mean rms noise, the S/N range andthe percentage of spectra with an S/N higher than 3. Only spec-tra with an S/N above 3 are used for the computation of thestatistical parameters. The mean integrated intensity is 10 timeslarger at the 12CO peak than at the IR excess peak. The profilesare 1.2 times broader at the location of the 12CO peak. Whileno 13CO is detected at the location of the IR excess for the full



Figure 5. The columns show the IRAM 12CO (J = 1–0), 13CO (J = 1–0) and JCMT 12CO (J = 2–1) profiles in the Ursa Major cloud at the central locationof the IR excess peak (F6) and at the central location of the 12CO peak (F7).

Figure 6. The columns show the IRAM 12CO (J = 1–0), 13CO (J = 1–0) and JCMT 12CO (J = 2–1) integrated intensity maps in Ursa Major at locations F6and F7. The contours show the regions in which the data are above the 3σ level.

Table 17. Mean integrated intensity, velocity and FWHM of the IRAM 12CO (J = 1–0) observa-tions in the Ursa Major cloud.


(K km s−1) (km s−1) (km s−1) (K) (>3σ )

F6 0.63 ± 0.03 7.94 ± 0.01 0.94 ± 0.01 0.06 3.1–37.7 91F7 6.3 ± 0.2 7.779 ± 0.007 1.15 ± 0.01 0.09 8.0–116.3 100

resolution data, 13CO is detected in four profiles after the smooth-ing of the data with a Gaussian (FWHM = 40 arcsec) as shown inTable 18.

Table 20 provides the mean 12CO (J = 2–1)/12CO (J = 1–0)integrated intensity ratio in the two fields. The ratio is 1.4 timessmaller at the location of the IR excess peak than at the location ofthe 12CO peak.

3.2.4 Determination of the physical parameters using an LVGmodel in Ursa Major

Using the method described in Section 3.1.5, an LVG model wasalso used in Ursa Major to determine the physical parameters. Asfor the Spider, model grids for temperatures from 6 to 20 K (in stepsof 2 K) have been produced. However, it was necessary to run the



Table 18. Mean integrated intensity, velocity and FWHM of the IRAM 13CO (J = 1–0)observations in the Ursa Major cloud.


(K km s−1) (km s−1) (km s−1) (K) (>3σ )

Full resolution data

F7 0.72 ± 0.02 7.78 ± 0.01 1.02 ± 0.06 0.06 3.0–35.6 96

Binned data

F6 0.051 ± 0.007 8.12 ± 0.04 0.90 ± 0.07 0.02 3.1–3.4 11F7 0.76 ± 0.04 7.79 ± 0.02 1.17 ± 0.02 0.04 4.3–32.8 100

Table 19. Mean integrated intensity, velocity and FWHM of the JCMT 12CO (J = 2–1) obser-vations in the Ursa Major cloud.


(K km s−1) (km s−1) (km s−1) (K) (>3σ )

F6 0.29 ± 0.02 7.94 ± 0.03 0.80 ± 0.04 0.07 3.0–9.5 44F7 3.04 ± 0.09 7.778 ± 0.008 1.13 ± 0.01 0.12 3.2–48.0 99

Table 20. Mean 12CO (J = 2–1) /12CO(J = 1–0) integrated intensity ratio in theUrsa Major cloud.

Field Mean Standardratio deviation

F6 0.36 ± 0.02 0.14 ± 0.01F7 0.474 ± 0.003 0.041 ± 0.002

model with five isotopic ratios in field F6 (20, 30, 40, 45 and 55) andwith three isotopic ratios in field F7 (20, 30 and 55) before findingthe isotopic ratio that gives the smallest χ 2. Table 21 provides thetemperature and the reduced χ 2 for the best solution and for the fitsusing a standard isotopic ratio of 55 in each field. While a smallerisotopic ratio is clearly needed in field F7, the results do not changeappreciably with the isotopic ratio in field F6.

Table 22 provides the computed mean 12CO column densities andmean H2 densities in each field taking into account all the positions(upper limits) and only the positions where the three lines havean S/N higher than 3. The columns are the same as in Table 12.While the 12CO column density is smaller in field F6 than in fieldF7, the mean densities are similar. The maximum density is smaller(891 +311

−231 cm−3) in field F6. The largest density is 1202 +277−225 cm−3 in

field F7. Contrary to all the other fields, the statistical uncertaintyand the mean error of the LVG model are similar in field F7, indi-cating that the spatial variations of the 12CO column densities anddensities may be significant.

Fig. 7 shows the 12CO column density map and the H2 densitymap computed in field F7 of Ursa Major, where there are 3σ detec-

tions everywhere. There are slightly higher densities at the centreof the field, but lower column densities.

4 D ISCUSSION

4.1 Kinematical behaviour

Barriault et al. (2010a) show that the kinematical behaviour of theSpider and of the Ursa Major cirrus is mostly explained by small-scale motions rather than by a large-scale scenario involving theNCL. Whereas 12CO velocities and H I velocities coincide in theSpider, a difference of +1.7 km s−1 is observed in Ursa Major, sug-gesting that molecular and atomic content are well mixed in theSpider, which is probably younger. Moreover, 12CO usually seemsto appear where two H I components merge into one componentor where large H I velocity shear (>20 km s−1 pc−1) is observed, inagreement with models predicting CO formation in such regions(Godard, Falgarone & Pineau Des Forets 2009). While no clump isobserved at the location of the IR excess peak, a large H I velocityshear is observed at this location, possibly indicating the forma-tion of a new clump. The observations of this paper also support ascenario where small-scale motions are the main effect that influ-ences the gas kinematics. The mean 12CO linewidths (FWHM) arebetween 0.6 and 1.0 km s−1, narrower than in the high Galactic lati-tude translucent cloud MBM 7 (around 2 km s−1; Minh et al. 1996)or in the Chamaeleon dark clouds (0.9–3.5 km s−1; Boulanger et al.1998). As the thermal broadening is only ≈0.07 km s−1 for CO atTk = 10–15 K, the linewidth is probably mostly due to turbulentmotions (Minh et al. 1996).

Table 21. Temperature and isotopic ratio in Ursa Major.

Field Tk Best fit Fit 2(K) 12CO/13CO Average reduced χ2 12CO/13CO Average reduced χ2

F6 14 45 1.00 55 1.01F7 16 20 3.85 55 77.87



Table 22. Mean 12CO column densities and mean H2 densities found using an LVG model in the Ursa Major cirrus.

Field All the field (upper limits) Only the locations with 3σ detectionsMean Mean Mean Mean Mean Mean Mean Mean Maximum

log[

N(12CO)cm−2

]errora log

[n(H2)cm−3

]error log

[N(12CO)

cm−2

]error log

[n(H2)cm−3

]error log

[n(H2)cm−3

]

F6 15.11 ± 0.05b 0.18 2.38 ± 0.06 0.23 15.4 ± 0.3 0.13 2.3 ± 0.3 0.17 2.95 ± 0.13F7 16.47 ± 0.08 0.08 2.41 ± 0.06 0.08 16.47 ± 0.08 0.08 2.41 ± 0.06 0.08 3.08 ± 0.09

aThe mean error is the error of the LVG model computations.bThe uncertainty of the mean value is the standard deviation of the mean.

Figure 7. Maps obtained using an LVG model at location F7 in Ursa Major. (a) 12CO column density map. (b) H2 density map.

4.2 Clump characteristics

Based on our analysis of the clumps in the Spider and in Ursa Ma-jor, we were able to determine the properties of the clumps foundby each algorithm. The dimensions (spatial dispersion deconvolvedfrom the beam) of the most massive clumps are ≈30–40 arcsec(≈0.015–0.02 pc) in the Spider and ≈20–80 arcsec (≈0.01–0.04 pc)in Ursa Major and most of the clumps are slightly elongated(�x1/�x2 ≈ 0.5–0.9, see Tables 4, 6, 14 and 16). The velocitydispersions of the clumps are ≈0.15–0.26 km s−1 in the Spider and≈0.35–0.40 km s−1 in Ursa Major. The masses of the clumps arebetween 0.001 and 0.05 M� and all the clumps are substellar (M <

0.08 M�) and gravitationally unbound objects. Approximately one-third of the mass of the cloud is located in the clumps in the Spideraccording to the CLUMPFIND and FELLWALKER algorithm compared tohalf of the mass in Ursa Major, which is an older region.

The clumps found in this study (see Tables 4 and 14) are differentfrom the clumps found in the densest star bearing translucent clouds(McGehee 2008), but they look like those found in other translu-cent clouds. Hily-Blant & Falgarone (2007) applied GAUSSCLUMPS toIRAM 30-m telescope 12CO and 13CO (J = 1–0) data of the MCLD123.5+24.9 molecular cloud in the Polaris Flare (Heithausen &Thaddeus 1990). The angular resolution of these data is 20 arcsec,the spectral resolution is 0.055 km s−1 and the rms noise is 0.30and 0.20 K for the 12CO and the 13CO lines, respectively. As theywere characterizing the properties of the 12CO velocity structuresassociated with the wing of the 12CO profiles, they kept only the12CO clumps moving with such velocities for the analysis, but theyshow the clumps found at all velocities in 13CO.

The most probable spatial dispersions of the structures found byHily-Blant & Falgarone (2007) are 0.012 pc (FWHM = 0.03 pc) in12CO and 0.006 pc (FWHM = 0.015 pc) in 13CO; we found sim-ilar dimensions for our clumps (see Tables 4 and 14). The most

probable velocity dispersions found by Hily-Blant & Falgarone(2007) are 0.15 km s−1 (FWHM = 0.35 km s−1) in 12CO and0.08 km s−1 (FWHM = 0.20 km s−1) in 13CO, similar to the ve-locity dispersions we found for the first five Spider clumps (seeTable 4), but smaller than the velocity dispersions of the three UrsaMajor clumps of Table 14. The clumps found in MCLD 123.5+24.9are roughly two times more elongated than those in our regions(�x1/�x2 ≈ 0.4 compared to ≈0.6–0.7 in our regions). Hily-Blant& Falgarone (2007) also found that the elongated structures have apreferred orientation, a behaviour which is not obvious in our data.

We decided to compare our results with the clumps found innearby and more massive clouds MBM 27–30 (l = 140◦, b = 38◦;Pound & Goodman 1997). These clouds are part of the Ursa Majorcomplex to the north of the Ursa Major cirrus observed in this paperand are oriented towards the centre of the NCL. We applied thefour clump identification algorithms, described in Section 3.1.2, toAT&T Bell Laboratories 7-m 12CO (J = 1–0) data (resolution of100 arcsec) obtained in this region by Pound & Goodman (1997).An S/N threshold of 7 was of course chosen. As in Ursa Major, thereis no agreement between the properties of the clumps found by dif-ferent algorithms. Using GAUSSCLUMPS, the spatial dispersions of theclumps are between ≈100 and 400 arcsec, seven times larger thanthe largest clump found in Ursa Major, while the velocity disper-sions are between ≈0.4 and 0.6 km s−1, compared to ≈0.4 km s−1

in Ursa Major. Even if the cloud is 50 times more massive than eachof our regions, the number of clumps is similar and the masses ofthe clumps are varying between 1 and 10−2 M�.

Kramer et al. (1998) analysed seven molecular clouds with dif-ferent properties. While they surveyed much larger areas and foundbetween 100 and 1000 clumps, a comparison of their clumpsthat have M < 0.08 M� (located in the quiescent high-latitudeclouds MCLD 126.6+24.5 and L1457) with ours reveals the pres-ence of gravitationally unbound clumps in high-latitude clouds.



High-density regions are common at high Galactic latitude.Sakamoto (2002) found a lot of sub-Jeans length structures in alow-density high Galactic latitude molecular cloud, indicating thatother processes than self-gravity are probably responsible for theirformation. Minh et al. (1996) suppose that shocks could be involvedin the formation of the high Galactic latitude clumps and filamentsin MBM 7, whereas Shore et al. (2003) propose that an externalshear flow is responsible for the structure of MBM 40, where noshock compression is found. In the Spider and in Ursa Major, COformation actually occurs where H I velocity shears are observed(20 km s−1 pc−1 in the Spider and 50 km s−1 pc−1 in Ursa Major;Barriault et al. 2010a).

4.3 Physical parameters

The temperatures determined using the LVG model are between 12and 18 K in the Spider and between 14 and 16 K in Ursa Major. vanDishoeck et al. (1991) found that high Galactic latitude translucentclouds have low gas volume densities [n = n(H) + n(H2) = 200–5000 cm−3] and high kinetic temperatures (Tk > 20 K). On the otherhand, Ingalls et al. (2000) found that the most probable solution forhigh Galactic latitude translucent clouds is a high-density and low-temperature solution (n = 104.5 ±0.5 cm−3 and Tk = 8 K). While thetemperatures determined in this study are in agreement with therange of temperatures computed for high Galactic latitude clouds,the densities are clearly smaller than those found by Ingalls et al.(2000).

In principle, the LVG analysis and the clump analysis providesimilar physical information from a different perspective. However,the comparison between the results given by the LVG model andthe results obtained by the clump identification algorithms is onlypossible in two fields which are coincident with a clump. In theSpider, field F1 is coincident with clump 2 of Table 4. Assuming thatthe clump has the same depth as its dimensions on the plane of thesky, we find a maximum density [n(H2)] of ≈1800 cm−3, while themaximum density according to the LVG model is 2455 +429

−366 cm−3.In Ursa Major, field F7 is coincident with clump 1 of Table 14. Wefind a mean density of 700–1250 cm−3 and a maximum density of≈2500 cm−3 for the clump, while the maximum density accordingto the LVG model is 1202 +277

−225 cm−3.According to the LVG model, the isotopic ratio varies between

30 and 55 in the Spider and between 20 and 45 in Ursa Major, oftenlower than the canonical value of 55 (Garay et al. 2010). The isotopicratio is 30 in fields F1 and F2 in the Spider and 20 in field F7 in UrsaMajor, which are the fields where at least some positions have an S/Nhigher than 3 for the three lines. It should be stated that LVG modelsmay not be the best ones to use here. The objects discussed are notquiescent well-developed giant molecular clouds. Thus, there maybe systematical errors involved that remain undefined since onlyone type of model was used. However, in context of the modellingdone here, varying the isotopic ratio is the best approach to improvethe fit considering that the objects are still evolving and prone to theaction of the interstellar radiation field (ISRF). Deriving quantitativestatements on the physical conditions from the inferred isotopic ratiovariations would be hazardous except maybe for fields F1, F5 andF7. In Barriault et al. (2010b) the effect of the ISRF was notedon the OH behaviour. Similarly here field F1 faces the Galacticplane, while field F5 is the furthest from the plane and may bein the shadow of F1, F3 and F4. The ISRF may be affecting theisotopic ratio in F1 (note that F5 has the largest column density).The Ursa cloud is approximately parallel to the Galactic plane. Thelower ratio for F7 is hard to explain using the radiation field (see

equation 3 below). Although its χ 2 improves from 77.87 to 3.85(Table 21), it remains fairly large. Field F7 has the worst model fitof all.

The N(12CO)/N(13CO) ratio is quite variable in translucent cloudssince (i) the photodissociation rate changes with temperature andDoppler widths and (ii) the amount of shielding changes with col-umn density (Visser, van Dishoeck & Black 2009). Enhanced orreduced N(12CO)/N(13CO) ratios by a factor of 2 compared to theelemental isotopic ratio are often observed in translucent clouds athigh Galactic latitudes or in the Galactic plane (Burgh, France &McCandliss 2007; Sheffer et al. 2007; Sonnentrucker et al. 2007).Two competitive processes influence the isotopic ratio: the pho-todissociation of 13CO and the carbon-ion exchange reaction (Stark1995). The first process has an impact since 13CO is more easilydestroyed than 12CO (self-shielding occurring deeper in the cloud,Bally & Langer 1982). The second process is the following reac-tion which cancels the effect of the photodissociation in translucentclouds:

13C+ + 12CO ↔ 13CO + 12C

+ + �E, (3)

where �E/k = 36 K. According to chemical models, the carbon-ionexchange reaction is more efficient than the selective photodissoci-ation of the CO isotopes (Visser et al. 2009) in the clouds with alow temperature, explaining the lower ratios. In the clouds with ahigher temperature (40 K), photodissociation is probably dominant.

The∫

Tmb(12CO)dv/∫

Tmb(13CO)dv ratio computed from the ob-served lines is often smaller than the isotopic ratio since 12COis usually optically thick. While the relation between the excita-tion temperature and the kinetic temperature depends on the colli-sion rate, the difference between both temperatures suggest that theSpider and Ursa Major clouds are not optically thick (τ ≈ 1). A∫

Tmb(12CO)dv/∫

Tmb(13CO)dv ratio of about 10 is found at the lo-cation of the 12CO peaks (F1, F2 and F7), where 13CO was detected.The absence of IRAM 13CO detection in the other fields supportsa ratio larger than 20 in the diffuse regions. In each field, the ratiois effectively smaller than the isotopic ratio adopted to obtain thelowest χ 2 using the LVG model (see Tables 11 and 21).

The∫

Tmb(12CO)dv/∫

Tmb(13CO)dv ratio was also computedusing the lower resolution FCRAO observations in otherand/or larger fields. As shown in Tables 3 and 13, the∫

Tmb(12CO)dv/∫

Tmb(13CO)dv ratio is not constant over the clouds,suggesting changes in the optical depth. According to Stark (1995),the

∫Tmb(12CO)dv/

∫Tmb(13CO)dv ratio varies between 3 and 60

for different diffuse cirrus clouds. The smallest ratio is typical ofgiant molecular clouds, whereas the largest ratio is typical of diffuseclouds. This is in agreement with our results as the ratio is smaller(≈10) for the densest regions (P1 in the Spider and Ursa Major) andis larger (≈30) for the more diffuse region P3 in the Spider whereonly weak 13CO emission is found.

In the Spider and in the Ursa Major cirrus, the 12CO (J = 2–1)/12CO (J = 1–0) integrated intensity ratio is around 0.4 except atthe location of the IR peak in the Spider (F5) and at the locationof the IR excess peak in Ursa Major (F6), where it is smaller (0.3),and at the location of the 12CO peak 1 in the Spider (F1) andat the location of the 12CO peak in Ursa Major (F7), where it islarger (0.48). Reach et al. (1994) also found a low ratio (≈0.4) incirrus clouds, indicating that the excitation of the 12CO rotationallevels is subthermal. Variations in the optical depth and/or excitationtemperature can explain the difference of the intensity ratio since alarger ratio is found at the location of the 12CO peak which is denseraccording to the smaller 12CO (J = 1–0)/13CO (J = 1–0) ratio. TheLVG model confirms that these regions are denser compared to



the other fields. A larger ratio (≈0.6) is usually observed in giantmolecular clouds (Sakamoto et al. 1994) where the ratio is oftenclose to unity (≈0.9) at the centre and decreases at the periphery(≈0.5, Hasegawa 1997).

In the Spider, while densities of about 103 cm−3 are computedwith the LVG model at the location of the 12CO peak 1 (F1) and the12CO peak 2 (F2), the upper limit on the mean density at the locationof the IR excess peak is 120 cm−3, confirming our hypothesis that theabsence of coincidence between the 12CO peaks and the IR excesspeak may be due to a density too low for CO excitation. In UrsaMajor, the density is also smaller at the location of the IR excesspeak, where the mean density is 200 cm−3, but the maximum densityin this field is around 800 cm−3, where CO was actually observed.As the densities are below the critical density of CO (≈103 cm−3,Snow & McCall 2006), the excitation of the line is expected to beproportional to the density, and weaker CO emission at the locationof the IR excess peak could therefore be explained in part by thesmall densities. It is often assumed that IR peak emission locationcoincides with peak column density. This is not always the case. Theintensity of the exciting radiation field also plays a role. Analysisand modelling of Herschel and Spitzer data by Compiegne et al.(2010) have shown that the very small grains (VSG) peak emissiondoes not coincide with the column density peak, whereas the largegrain emission does. The IRAS 100-µm emission used in this studyoriginates from both VSG and large grains emission in proportionsthat depend on the grain distribution. Since Miville-Deschenes et al.(2002) have shown that the grain characteristics change across theUrsa Major cloud, the lack of coincidence between the CO densitypeak and the IR peak is not unexpected.

On the other hand, insufficient self-shielding is probably not themain process explaining the absence of coincidence. For N(12CO) >

1015 cm−2, the photodissociation rate is decreased by a factor of 2due to self-shielding (Lee et al. 1996). The 12CO column densitiescomputed with the LVG model are larger than 1015 cm−2 in theSpider and in Ursa Major, except in fields F2 and F3, indicating thatself-shielding contributes to decreasing the photodissociation rate.Shielding by dust has probably an effect as well since AV > 0.1(Lee et al. 1996) in our regions. Moreover, CO is expected in theSpider and Ursa Major even though ultraviolet shielding is not asefficient as in giant molecular clouds (Barriault et al. 2010a), sincehot chemistry processes may occur in the regions where turbulencedissipates (Falgarone & Puget 1995). While CO formation in diffuseclouds usually results from the low-temperature reaction of C +

with OH (van Dishoeck & Black 1988), the high temperatures dueto turbulence dissipation allow non-equilibrium CH + formationwhich can increase CO formation (Zsargo & Federman 2003).

Whereas our computations suggest that the absence of coinci-dence is probably the result of weak CO excitation at the locationof the IR excess peak, cloud dynamics may also explain this dis-crepancy (Barriault et al. 2010a). As the Spider and Ursa Majorclouds are turbulent (Barriault et al. 2010a), the H2 abundance isexpected to be out of equilibrium since the H2 formation and pho-todissociation time-scales (>107 yr) are larger than the dynamicaltime-scale of the turbulence (≈1 Myr) (Glover & Mac Low 2007).H2 molecules are therefore expected at the location of the IR ex-cess peak even though this field is not as dense as the other fields(≈102 cm−2).

The 12CO column densities computed in Ursa Major are 13 timeslarger than those in the Spider, but the densities are two timessmaller suggesting that Ursa Major is more evolved (Barriault et al.2010a). Moreover, CO depletion on to dust grains could explain theabsence of correlation between column densities and densities in

field F7 of Ursa Major, possibly indicating a more evolved core thanin the Spider. The time-scale for the depletion of a gaseous speciesis ≈109/n(H2) yr (Burke & Hollenbach 1983; Caselli et al. 1999).Assuming n(H2) ≈ 103 cm−3 in field F7, the time-scale is ≈106 yr,similar to the dynamical time-scale of diffuse clouds.

5 C O N C L U S I O N

In this paper, the physical parameters of molecular sites were de-termined in two high Galactic latitude cirrus clouds, the Spider andUrsa Major. Four algorithms have been used to determine the clumpproperties in both regions using the lower resolution 12CO (J =1–0) FCRAO data. Only the clumps with spatial FWHMs largerthan the beam, with a spectral FWHM larger than the velocity reso-lution and with a mass larger than 0.001 M�, were chosen to be real.The clumps have masses between 0.001 and 0.05 M�, comparedto 10−2 and 1 M� in the more massive and nearby MBM 27–30high Galactic latitude molecular clouds. The clump dimensions(≈0.015–0.04 pc) and the velocity dispersion (0.20–0.40 km s−1)are similar to the properties of the clumps found in translucentclouds with no star formation. The clumps are elongated. The low-mass clumps are often barely resolved and the four algorithms donot give the same results, except for the most massive clumps in theSpider. As the input parameters have a strong impact on the numberof clumps and on the properties of the clumps, those algorithmsshould be used with caution in diffuse clouds.

IRAM 12CO (J = 1–0) and 13CO (J = 1–0) and JCMT 12CO(J = 2–1) observations have been analysed at the location of twopotential sites of molecule formation. The absence of 13CO detectionin the fields which do not coincide with CO peaks and the largeerror of the LVG model results prevent a small-scale analysis ofthe physical parameters. Using the upper limits given by the LVGmodel, we are nevertheless able to draw some conclusions. The12CO (J = 1–0)/13CO (J = 1–0) ratio is smaller at the locationof the 12CO peak, whereas the 12CO (J = 2–1)/12CO (J = 1–0)is larger at the same location, indicating a denser area. The LVGmodel confirms that these regions are denser than the other fields.As the 12CO column densities are larger than 1015 cm−2 in almostall the fields, we expect that insufficient self-shielding is not thedominant process explaining the absence of coincidence betweenthe IR excess peak and the 12CO emission peak. We rather suggestthat the small densities (≈200 cm−3) computed at the location of theIR excess peaks cause a low CO excitation temperature explainingthe weaker CO emission and the discrepancy between the 12COpeak and the IR excess peak. The mild depletion observed at thelocation of the 12CO peak in Ursa Major confirms that this regioncould be more evolved than the Spider.

AC K N OW L E D G M E N T S

This research was supported by the Natural Sciences and Engineer-ing Research Council of Canada and by the Fonds Quebecois dela Recherche sur la Nature et les Technologies. LB acknowledgesa Doctoral research scholarships from the Fonds Quebecois de laRecherche sur la Nature et les Technologies. We are grateful toMarc-Antoine Miville-Deschenes for lending his Gaussian fittingprogram. We thank Brenda Matthews, Gerald Schieven and MingZhu for their assistance with the JCMT observations and the datareduction. We thank all the JAC/JCMT staff. LB acknowledges atravel grant from the Natural Science and Engineering ResearchCouncil of Canada (NSERC) for the observations at the JCMT. The



JCMT is supported by the United Kingdom’s Science and Tech-nology Facilities Council (STFC), the National Research CouncilCanada (NRC) and the Netherlands Organization for Scientific Re-search (NWA). We thank Marc W. Pound for his 12CO data cubeof another part of Ursa Major. The FCRAO was supported by theNational Science Foundation (NSF AST-0838222). We thank theIRAM staff for their help with the observations. IRAM is supportedby INSU/CNRS (France), MPG (Germany) and IGN (Spain).

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This paper has been typeset from a TEX/LATEX file prepared by the author.


multiwavelength observations of cirrus clouds in the north celestial loop: physical parameters of...

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