Chin.Astron.Astrophys.(1991)15/4,375-385 0 Pergamon Press plc A translation of Printed in Great Britain Acta Astron.Sin. (1991)32/2,134-144 0275-1062/91$10.00t.00

NEAR INFRARED OBSERVATIONS AND ANALYSES

OF MOLECULAR OURLOWS’

SUN Jin WU Yue-fang Department of Astronomy, Beijing Normal University

MAO Xin-jie LI Shou-shong Department of Geophysics, Beijing University

ABSTRACT We present results of near infrared observation of 21 molecular outflow sources and two non-outflow sources with compact cores. Combined with IRAS and other surface station observations we analyse their spectra and find that the outflow sources have, on average, steeper spectral gradients than the non-outflow sources in the range 2.28-25~. Most of the bipolar outflow sources have gradients greater than 2.0. Using a revised blackbody photosphere model we calculate the contributions to the JHK fluxes by the central young star and the circumstellar envelope. For the sources with known bolometric luminosity we derive the photospheric temperature of the central star and the circumstellar extinction. Results show that most of the young stars associated with molecular outflows are probably T Tauri stars (5000-7000K) or emission line stars (9000-26000K). The circumstellar extinction in JHK is around 10 to 20 magnitudes. These facts show that molecular outflow sources are young objects still embedded deep inside or around the interior of compact cores. Fitting the 3.5-25 p and 60-100~ spectra with a 1-l dust emission model to five source gives a negative power law for the temperature profile of the circumstellar shell with exponents between 0.39 and 0.48, close to the theoretical results for molecular clouds associated with HII regions.

1. INTRODUCTION

Over the past ten years fast outflows of cold molecular gas have been continually observed in regions of star formation. This shows that in the early stage of stellar evolution, the pre-main sequence stage, most stars would undergo a stage of energetic matter ejection [l]. As a result of a wide-ranging search we now know more than 100 sources of molecular outflow [l-3]. However, the data do not yet constitute a complete sample. Using the IRAS data, some authors have proposed criteria for searching for outflows in unidentified IRAS sources [4, 51. Also, many observations have shown that the young objects associated with molecular outflows are 'i Received 1990 December 25 Program supported by the National Natural Science Foundation.

376 SUN Jin et al.

always surrounded by rotating, compact disks. It is expected that radiation from the disk will cause further reddening of the spectra of these young objects. Analysis of the IRAS data tells us that most outflow sources have strong emission in the far infrared and a low colour temperature. Observations in the near infrared will provide uore informtion for the analysis of the entire infrared spectrum such ss the gradient between the near and far infrared, the circumstellar extinction and the temperature structure of the circumstellar envelope. These data would be useful for the study of the early evolution of stars and for the search for more molecular outflow sources.

Over the paat few years we used the 1.26 l infrared telescope of Beijing Observatory Xinlong Observing Station and observed the near infrared emission of a series of young object5 with molecular outflows. For comparison we also observed two young stars with no detected outflows. These, like the previous series, are all objects embedded in or near the compact cores of molecular clouds. In Section 2 we present the results of our JHK photometry of 21 outflow sources and 2 non-outflow sources. Section 3 combines their IRAS data to calculate their spectral gradients and infrared luminosity over l-lOOc(. Section 4 analyses the properties of the circumstellar envelopes through model fitting of the near, middle and far infrared spectra, and concludes with a discussion of the probable properties of the central young stars.

2. ORSRRVATIONS

In January and August of 1988 and 1989 and September of 1990 we carried out near infrared observation in the JHR bands on 24 outflow sources and 2 non-outflow sources with compact nuclei using the 1.26 m infrared telescope of Beijing Observatory Xinlong Observing Station. For some of the fainter sources, repeated measurements were made. Sources with large discrepancies in the results, belonging to some very faint objects, are not reported here.

The filter characteristics of the photometer and the flux density calibration are given in TABLE 1.

TARLR 1 Filter Characteristicm and Flux Density Calibration

Filter xo(pr) AX(P) T I

J 1.26 0.25 60% 1600 Ii 1.68 0.40 71% 1020 K 2.28 0.48 79% 660

xo and ALLX are the effective wavelength snd bandwidth of the filter, T is its transmission and C is the flux density for magnitude zero. The relation between the measured flux density $, and the apparent magnitude I is therefore $, = C. 1O-o.4’. The magnitude measuring accuracy of the telescope is 0.04-0.05mag. For correction for interstellar reddening we use Van Herk’s (1965) formula,

Molecular Outflows 377

.4.- o.I4crcl~l[I-up(--l0tdn~6j)l. (11

where r is the distance in kpc. After obtaining k, the extinction in the J, II, K bands are found according to the interstellar reddening relation of Rieke and Lebofsky [%I. The JHK magnitudes, corrected for atmospheric extinction and interstellar reddening, are given Columns 4, 5 and 6 of TABLE 2. Column 2 is the IRAS name of the source, and Cal. 3 gives the velocity of the molecular outflow, derived from the wing width of CO molecular lines. The last two lines refer to the two non-outflow sources.

TADLII 2 JIIK Photometry ofMolecularGut.flou Source8

V (K41) Name IRAS J <m*r) H (-r)

LKH, too 00097+vl33 I3 II.05 IO.13

WJ-mx 02219+6152 32 *IO.92 l 9.40

AFCL 437 03031+5119 24 10.2s 9.54

APGL 490 03236+S836 65 910.40 1.94

LKH. 101 04269+3110 15 * 7.60 1.61

HL T.u 04297+l901 II 9.68 D.S2

HH 24 05433-0011 1s 12.35 11.52

Man 12 06013-0622 31 '11.63 9.66

-GGD 12-15 06094-0611 2s l 10.9s *Il.69

SZSI-IRS1 06099+1900 26 IO.92 21.43

AFGL 961 06389+04i5 30 10.94 6.96

NGC 2264 06394+0932 28 10.10 9.25

s Bl 19442+2427 10 * 9.51 l 1.46

Sl8ll 19446+2501 I7 *LO.62 l 9.36

s196 202SS+3112 18 l 7.10 1.95

AFGL 2191 20275+4001 42 a.93 1.1)

VI33, Cl‘ 20191+5009 7 * 9.Y7 l 9.07

vats cy; 21311+1000 29 1.93 1.49

St40 22%76-i-6303 42 to.40 7.61

NGC 1536 23116-1-6111 35 * 9.11 9.42

YWC I080 131%!+6034 60 9.01 9.29

cw r*u Olll2+2903 9.10 6.17

DO T.v 043S3+2604 9.66 8.SO

Note: asterisks mark nagnitudes measured for the first time.

K t-s) --

1.n

91.93

7.43

5.39

3.03

1.49

10.04

7.09

*9S6

9.03

6.10

5.19

l 6.91

v.53

6.69

I.54

9.182

6.56

S.9,

6.94

1.60

6.92

'1.12

3. INPRAR&DGUADIENT AND LIJMINGSITY

Combining our n eaurements with the IRAS data and data from other ground-based mtations we obtained the infrared spectra for the measured outflow sourcea and analyse their aain features.

1) Host outflow sources have a steep infrared spectrum. Figs. l-4 give soae such examples. IRAS measurements are marked with plus signs, ours with triangles and the others with squares. The figures show that outflow sources are almost all strong emission sources in the far infrared. Uecently Snell et al. [T] searched for high-velocity GCJ emission features in nolecular clouds associated

378 SUN Jin et al.

with strong far infrared sources (&or > 500 Jyf in the region o = oh-12”, 6 z 0’ and found a detection rate of about 50%. This means that when a source becomes a bright source in tbe far infrared, it is soon accompanied by strong outflow activity.

-a.0

0.0 f

1.0 20 3.0

Mn(u)

Fig.1 The spectrum of AFGL961 Fig.2 The spectrum of HL Tau

Fig.3 The spectrum of S88B

2) In some of the spectra of outflow sources there is an enhancement in the near or middle infrared. See, for example, Fig. 5 (AFGL 490) and Fig.6 (AFGL 2591). The reason for this enhancement is not clear. Figs. 7 and 8 show the spectra of the two non-outflow sources, CW Tau and DC Tau. They show clearly that these sources do not have far infrared excesses. Of course, some optically invisible non-outflow sources embedded deep in compact cores of molecular clouds could also have steeper &ectra [il.

We define formula

the spectral gradient between 2.28~ and 25~ by the

(2)

r Hi Tml

4.0. .

-2.0.

0.0 1.0 2.0 3.

lolh)

Fig.4 The spectrum of S140

Molecular Outflows 379

AFGL 2%

4.0 . w

.

-20

0.0 1.0 2.0

JKAG)

Fig.5 The spectrum of AFGL490 Fig.6 The spectrum of AFGL2591

CW Tau

‘.OT ‘,Or

Fig.7 The spectrum of CW Tau Fig.8 The Spectrum of Do Tau

and calculate S for all the measured sources. Because the number of sources is small, the sample is statistically incomplete and we are unable to derive any clear relations between S and other physical parameters. However, on average, the outflow sources have a steeper gradient than the non-outflow sources. For the 21 outflow sources given here, the mean gradient is 2.4, whereas for the 20 non-outflow sources located in or near the cores of clouds, observed by Myer et al. [al, we find a mean gradient of 1.31. For the 10 bipolar outflows observed here, the mean gradient is 2.51. TABLE 3 lists, for all the measured sources, their distance, the configuration of the outflow and the gradient between 2.28 p and 25 p. We see that, of the 10 bipolar outflow sources, 8 have steep slopes (S> 2.0). This aeans that bipolar outflow sources are redder sources. We expect that the slope will decrease with increasing opening angle of the outflow: the outflow will blow away the ambient satter in tise and reduce the far infrared excess. More and wider observations of the different types of outflow are required to elucidate this point.

380 SUN Jin et al.

TABLE 3 Spectral Slopes and Infrared Luminosities

Slope L.<Ld

1.56

3.93

1.65

I .96

1.53

I.59

2.09

1.53

3.64

3.11

I .36

1.94

1.50

1.14.

1.63

1.35

1.31

1.06

1.51

z.a5

1.56

I .0(3)

1.1(6)

3.0(4).

3.1(3)’

I .1(4)

6.5’

20

5.0(4)

1.0(I)

4.6(3)

1.4(6)

5.5(3)

Name istancc (Kpc)

LKH 19S 0.95’ bipolar

W&IRS5 2.3 bipolar

AFCL 437 bipolar AFCL 490 0.9’ bipolar LKH I01 0.8 ? HL Tau 0.14’ ,

HH 14 0.5 bipolar

Man P.? 0.1) bipolar

COD 12-15 I.0 bipolar

SZSS-IRS1 o.al ? AFCL 961 I .6 bipular NGC 2264 0.76 ITdOpOl~l 5 97 1.7 ? S86B 2.0 ? S 106 1.3 I AFCL 1591 1.2 bipolar

VI331 cy# 0.7 ? V645 Cyg 5.6’ bipolar

s 140 I.0 irmropic

NGC.7536 1.8 isotropic

~wc loao 2.5 isotropic

Note: “a” refers to Bef. [9], “b” to Ref.[lO]; the luminosities in the last column are derived here; other data are from 111.

9.1(l)

7.6(3)

1.6

4.3

L .1(3)

4.9(l)

1.7(3)

I .4(3)

9.4(l)

1.00)

3.0(4)

36’

1.2(S)’

I .4(4)

1.5(s)-

1.7(4j

I .6(3)

1.1(3)

4.6(3)

I .7(4)

1.40)

6.1(3)

l.)(3)

TABLE 3 lists also the total luminosity L* and the infrared luminosity 4s. The latter is obtained by integrating the curve fitted to the measured fluxes between 1.26~ and 100~. For the majority of outflow sources, their infrared luminosity is of about the same order of magnitude as their total luminosity.

4. CIRCUMSTELLAB PBCPEBTIES OF OUTFLOW SCUBCES

The infrared emission of outflow sources should be directly related to the emissions of the central star, the circumstellar envelope of dust and gas and the disk matter surrounding the central star. Here, we shall ignore the effect of the disk emission for the time being. Nor shall we attempt a rigorous solution of the equation of radiative transfer for the envelope. We shall, instead, use a simplified model and by fitting the fluxes in the near, middle and far infrared bands, attempt an understanding of the properties of the circumstellar envelops and the central young star.

1) Model Fit of the Near Infrared Emission--Circumstellar Extinction and Photosphere Temperature.

In this section we shall use a revised blackbody photosphere model similar to that in [8] to fit the JHK fluxes. The model assumes that the near infrared flux is mainly from a photosphere black body

Molecular Outflows 381

of temperature TOP and a circumstellar black body of temperature To. Then, with x denoting the ratio of the equivalent solid angle of the circwstellar black body to the solid angle of the central star, the total flux ZeQ contributed by the two components is

,,p_ hcLe ~{[+--)- I]-‘++P($)- II_‘].

where La is the bolometric luminosity of the source, and D is its distance. For low luminosity sources, typically the T Tauri stars, we take TP = 1200 K and x = 35 for the model parameters 18, 121. For the higher luminosity sources IL* _ 103-10’ hco). we take the equivalent-radius of the-circumsteilar black body,-&, to be some 30-50 times the radius of the central star, RR. This choice of RC is determined by the least. scatter in the model fit and is consistent with the estimated thermal structure of the circusstellar envelope discussed in the next section.

Extinction of the light of the central star by the circumstellar envelope is found from the ratio between ZvQ and the observed flux 8, in the JHK bands according to the formula,

A.- I.O86[~]In(9 (4)

[&/Al is taken from the interstellar reddening relation given in 161. Substituting (3) in (4) gives an explicit expression for Av. In principle, if the model parameters are correctly chosen then we should get the same Av from the fluxes of all three JHK bands. So we iterate on the photosphere black body temperature TOP until we get the least scatter in the values of Av from the three bands.

Using this model we ca also estimate the optical thickness of the circumstellar envelope in the three bands,

?I - h(rvQls.>. (5)

TABLE 4 gives all the results from near infrared model fit for 14 sources with known bolometric lusinosity. Col. 2 gives the extinction, Col. 3, the standard error in the extinction from derivations in the three bands, Cal. 4 is the photospheric temperature TOP of the central young star in the optimal model fit, and Cola. 5-7 are the optical thicknesses at the wavelengths indicated.

The circumstellar extinctions around central young stars associated with molecular outflow (including T Tauri stars and emission line stars) found from our model are close to the estimates by Alonso-Costa, Kwan and other authors (see Bef. [ill, Appendix, Table 2). The uncertainty in our model comes mainly from uncertainties in the estimates for the equivalent black body temperature of the circumstellar envelope and its angular eke. TABLE 4 shows that the great majority of sources have a scatter in the extinction of less than 1.5 l ag, which is better than the results given in [al.

382 SUN Jin et al.

TABLR 4 Circuustellar Rxtinction aud Photosphere Temperature derived from Near-Iafrared Photometry

Namt Av (1R) QA.(IR> T., (mar) Cm4 (lOOOK)

LKH 190 7.9 0.42 17.7 1.93 I .35 0.79

AFGL 490 15.9 0.60 11.0 4.10 2.69 1.57

LKH 101 IS.0 i.15 9.1 3.65 2.68 1.47

HL TlU 9.7 0.36 5.5 2.42 1.61 0.99

cw I-au* 6.5 0.23 3.0 1.62 1.05 1.96

DO Tau* 4.3 0.23 4.2 I.24 0.64 0.42

HH 24 II.4 1.50 6.1 2.89 2.34 1.20

Mea R2 14.6 1.71 24.0 5.98 4.3s 2.43

AFCL 96f 16.2 0.90 10.2 4 .o, 2.79 I.61

AFCL 2591 14.4 1 .I$ IS.0 3.36 2.72 1.39

v 1331 cyg * .4 t.01 6.1 0.60 0.01 0.19

$140 19.8 “.X0 Il.2 1.18 3.111 2.04

NGC 7S38 ‘1.0 ! .30 26.0 2 .Sb 2.03 1.05

MWC lOS0 2.2 1.12 29.0 0.42 0.18 0.3:

-

-

63

73

76

If

3a

76

72

*Non-outflow source.

The above uodel has given estimates for the circuastellar extinction of between 10 and 20 mag, and estimates for the photosphere temperature of between 5000 aud 26000 K. Spectral classification of these young objects wm involved in Refs. [lo) and 1121. Using the relation between spectral type and effective temperature of the photosphere, given in (123, appropriate for the young stars, we reproduce here the data for 7 objects included in TABLE 4:

STAR SPECK

LKH 198 83 HL Tau Kl CW Tau K3 UC Tau KT-MO Mon R2 BI

v1331 cyg PO MWC 1080 BO

PHOTOS. TEMP.

17.9 x lo3 K 4.0 4.7 4.0

23.0 7.0

30.0

Comparing this list with the values of Tea given in TABLE 4 we find that our values, apart from MWC 1080, have uncertainties between 200 and 2000 K. From TABLE 4 we see that the young stare associated with molecular outflows can be roughly divided into two kinds, one kind have Top = 5000-TOOOK, and correspond to the several low luainosity stars; they are T Tauri stars and HH objects, The other kind have Top= 9000-26OOOK, they belong to high-luminosity young stars. According to spectral observations over 0.6-l.Om and near infrared observations of HI, they belong to the category of emission line stars [9, 111. These young objects, deeply eubedded in or near the cores of dark clouds have greater infrared excesses than the eniasion line stars on the sain sequence, therefore aany

Molecular Outflows 383

of them can be assigned to pre-main sequence Herbig AJ& stars; for example, LEB 198, V645 Cyg and MWC 1080. The Herbig a/i% stars pre-uain sequence stars with masses between T Tauri and OB stars. Host of thes are associated with molecular or optical outflows and often have the silicate and silicon carbonate features in the spectrum. The last column of TABLE 4 gives the IBAS-LB8 spectral classification, from which we see that the sources LEB 198, AFGL 490, Mon R2, AFGL 2591, Sl40 and NW 1080 all have the 9.7u silicate feature either in emission or in absorption.

2) Model Fit of the Middle and Far Infrared esission-Dust Temperature Profile of the Circumstellar Envelope.

In this section we fit the 3.5-25 u and 60-160~ infrared spectra of outflow sources to a Plaucb curve plus a 1-l dust emission model for A > 35 JI. The quantity to be uinimised is defined as

where F is the observed flux, B(Td) is the Plan& function for a dust temperature Td, q is the euissivity of the dust, Q is the angular siee of the envelope at Td aud N is the number of points to be fitted. The Planck function has the fors

B,(T,) - Al-‘[erP(hr/4T,) - II-‘/Q. (71

where

A = 2hc%, a = nffi2/o2.

& is the radius of the dust envelope at Td. The sodel paraseters Td and Q are found by sinimising x. With an iteration accuracy of 10-3, the results for five sources with complete spectral data are given in TABLE 5. The last column gives the exponent a if we amuse the tesperature profile follows a power law.

(~,,/‘r,,) = (R.,/R.,)-‘. u > 0 (81

TABLE 5 Thermal Structure of the Dust Envelops for Five Sources

LL;H 198

ArGL I?0

AFGL 961

AFGL tS9,

SIIO

Figs. 9 and 10 gives two exauples (AFGL 490 and LEB 198) of separate fits to the near, siddle and far infrared fluxes. For the

384 SUN Jin et al.

five sources of TABLE 5, model calculations show that the duet temperature varies with the distance fror the central star according to a power law with exponent ranging from -0.39 to -.048. This is in good agreement with Leung’s result for the molecular clouds associated with HI1 regions. Embedded in these clouds are OB stars with a photosphere temperature of 4 x 10’R and a luminosity of lo6 l&. The close agreement probably l esns that, as a star evolves from the pre-main sequence stage to an OB star of the main sequence, the thermal structure of its circumstellar envelope undergoes little change.

4.0

t T,=47SK

R.=S.?x IO”cm

-*.o[ ,* , , ( , , 0.0 1.0 2.”

Wb)

Fig.9 Model fitting of the spectrum of AFGL 490

4.0 t

t T*=a.rK

Ta = ts5.7K R.,“LZX 10%

Fig.10 Model fitting of the spectrum of LKH 198

In sum, a combined analysis of near infrared photometry of molecular outflow sources and the IRAS and others’ data has provided some probable properties of circumstellar extinction around pre-main sequence stars, thernal structure of the dust envelope aud the central young stars. The results, steep infrared spectra, strong circumstellar extinction, high infrared luminosity ratio and the presence of the 9.7 p silicate feature (especially the absorption feature) show again and again that the molecular outflow sources are young objects still deeply embedded inside or in the vicinity of compact cores of dark clouds.

Molecular Outflows 385

AC~NOWL~~E~ We thank Colleague QIAN Zbong-yu of Beijing Astronomical Observatory for support and help with this work.

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