variable stars in the field of ngc 6882/6885: the case of v381 vulpeculae and v382 vulpeculae

Variable Stars in the Field of NGC 6882/6885: The Case of V381 Vulpeculae and V382VulpeculaeAuthor(s): Eric G. Hintz and Michael B. RoseSource: Publications of the Astronomical Society of the Pacific, Vol. 117, No. 835 (September2005), pp. 955-966Published by: The University of Chicago Press on behalf of the Astronomical Society of the PacificStable URL: http://www.jstor.org/stable/10.1086/432147 .

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955

Publications of the Astronomical Society of the Pacific, 117:955–966, 2005 September� 2005. The Astronomical Society of the Pacific. All rights reserved. Printed in U.S.A.

Variable Stars in the Field of NGC 6882/6885: The Case of V381 Vulpeculaeand V382 Vulpeculae

Eric G. Hintz1 and Michael B. Rose1

Department of Physics and Astronomy, Brigham Young University, N283 ESC, Provo, UT 84602; [email protected], [email protected]

Received 2005 April 28; accepted 2005 May 24; published 2005 July 22

ABSTRACT. We present photometric and spectroscopic results for two reportedd Scuti stars in the field ofNGC 6882/6885. We find that V381 Vul has a period of 0.1185 days and is ad Scuti variable, as previouslyreported. The spectra of V382 Vul shows it to be a B3 star and therefore not ad Scuti. All evidence points toV382 Vul being ab Cephei star with a period of 0.1808 days. Additionally, we report five new variables andeight suspected variable stars. Of the five new variables, two are pulsators and three are eclipsing binary systems.In our search for new variable stars, we use a “robust median statistic” that is proven to be better at finding low-amplitude variables than the traditional error curve approach.

1. INTRODUCTION AND MOTIVATION

During the last 3 years a set of spectral data in the regionof Hb has been collected at the Dominion Astrophysical Ob-servatory in Victoria, Canada. This is a full survey of all north-ernd Scuti stars brighter than 13th magnitude as selected fromRodriguez et al. (2000). During one observing run, two reportedd Scuti stars, V381 Vulpeculae and V382 Vulpeculae, in theopen cluster NGC 6882/6885 were observed. These two starswere both reported as potentiald Scuti variables by Pen˜a et al.(1990). Surprisingly, the spectra for one of these stars, V382Vul, had an emission feature centered in the Hb absorptionline. This would be a very unusual result for ad Scuti variable.

With this result still fresh on the screen, a search was madeof the literature to check for previous reports on V382 Vul. InHenize (1976) the star was reported as an emission-line star,but no other information was given. It is also listed in a catalogof Ha emission stars compiled by Kohoutek & Wehmeyer(1999). A search was then made to confirm the identificationof the star. In Hoag et al. (1961) they presentedUBV photom-etry for stars in the region of NGC 6882/6885 and providedthe generally accepted numbering system. A careful search wasmade of the Hoag et al. (1961) maps for NGC 6882/6885 toconfirm the Pen˜a et al. (1990) identification. We are confidentthat the star identified as a potentiald Scuti variable is the samestar reported as an emission-line star.

The open clusters NGC 6882 and NGC 6885 were originallydescribed by Trumpler (1930) as potentially a single cluster,with NGC 6882 being a condensation inside of NGC 6885.Johnson (1961) did not believe NGC 6885 to be an actual

1 Guest Investigator, Dominion Astrophysical Observatory, Herzberg Insti-tute of Astrophysics, National Research Council of Canada. Observations madewith the 1.8 m Plaskett Telescope.

cluster. However, the work of Svolopoulos (1961) seemed toindicate that NGC 6885 should be considered a cluster. Therecent WIYN open cluster survey (Platais et al. 2003) againbrought up the question of just what constitutes a cluster inthis region. Therefore, for simplicity we refer to the cluster asNGC 6882/6885 throughout this paper.

A number of important physical characteristics for NGC 6882/6885 have been reported in the literature. From high-dispersionspectroscopy of two stars in the region of NGC 6882/6885, Luck(1994) determined . From Washington system[Fe/H] p �0.02photometry, Geisler et al. (1991) found . Fi-[Fe/H] p �0.15nally, Strobel (1991) reported and a distance[M/H] p �0.20of 581 pc. These values all point to NGC 6882/6885 being justslightly metal poor. The distance to NGC 6882/6885 in generalhas been reported as 600 pc (Johnson 1961; Svolopoulos 1961;Becker & Fenkart 1971), with one value a little closer, at 525 pc(Hoag & Applequist 1965). However, from four stars examinedby theHipparcos satellite, Robichon et al. (1999) got a parallaxof mas, which corresponds to a distance of2.52� 0.36

pc. Platais et al. (2003) concluded that NGC 6882397� 50is considerably reddened [ ] and at a muchE(B � V ) p 0.6greater distance (over 1 kpc) than in previous studies. Theyalso believed that NGC 6885 is incorrectly classified as a clusterand might be part of an OB association. Although we do notdirectly address the question of what constitutes a cluster, wecan place the two stars of interest in proper context.

Two unusual possibilities provided the motivation for thisproject. The existence of an emission-lined Scuti would beinteresting, and the contradictions presented by NGC 6882/6885 make the discovery of variables in the region useful.Therefore, follow-up photometric observations of NGC 6882/6885 were taken. The images cover both V381 Vul and V382Vul, along with many other stars in the region. Below, we take



956 HINTZ & ROSE

2005 PASP,117:955–966

Fig. 1.—Portion of field near NGC 6882/6885 covered by CCD frames. The field of view shown is 30� # 30� in order to cover all telescope positions. Thenumbers in the figure match those from Table 3.

a detailed look at both suspectedd Scuti variables and at pre-viously unreported variables in the field.

2. OBSERVATIONS

Spectroscopic data were secured in 2003 September at theDominion Astrophysical Observatory on the 1.8 m PlaskettTelescope. Observations were made with the Cassegrain spec-trograph, using the 21121B grating. This grating is blazed at4100 A and yields 15 A˚ mm�1. Using the SITe-2 CCD with15 mm pixels gives 0.23 A˚ pixel�1. The grating was set to givea central wavelength of 4773 A˚ , with coverage from 4570 to4970 A.

Photometric observations were secured with the 0.4 m DavidDerrick Telescope (DDT) of the Orson Pratt Observatory on

the Brigham Young University campus. The DDT wasequipped with an Apogee Ap47p CCD camera mounted at theNewtonian focus. This provided a plate scale of 1�.32 pixel�1

and a field of view 22�.5 on a side. This field of view allowedV381 Vul and V382 Vul to be observed simultaneously. How-ever, the location of V381 Vul permitted us to observe onlythe northwest corner of the cluster. The field of view is shownin Figure 1, along with our numbering system.

An Optec filter slide was used with a set ofVRI filters anda set ofBVR filters, both modeled after Bessell (1990). Totaltime coverage is 15 hr of data over five nights. In addition,two of the nights were calibrated against standard stars selectedfrom Landolt (1992). All frames were reduced using standardIRAF methods.



V381 VUL AND V382 VUL 957

2005 PASP,117:955–966

Fig. 2.—Comparison of V381 Vul spectra with threed Scuti variables ofknown metal content. These three are SS Psc, RV Ari, and BP Peg, whichhave published [Fe/H] values of�0.10, 0.00, and�0.02, respectively.

Fig. 3.—Continuum-corrected spectra of V382 Vul in the region of Hb.

3. Hb SPECTRAL REGION OF V381 VUL ANDV382 VUL

In Pena et al. (1990) they classified V381 Vul as an F5 star,based entirely on its ( ) color. Published spectral observationsB � Vhave given a spectral type of A7 (Cannon & Mayall 1949). Theseare dramatically different classifications. Our new spectra forV381Vul are shown as the bottom spectra in Figure 2. Using the onlinedigital spectra detailed by Bagnulo et al. (2003), and comparingthe depths of a number of features, we find that we agree withCannon & Mayall (1949) and classify V381 Vul as an A7 IV.The discrepancy between our value and Pen˜a et al. (1990)clearly indicates that V381 Vul has a significant amount ofreddening, which we discuss further in the next section.

A number of additional stars in the A to F spectral rangethat have established values for the Hb index (Rodriguez et al.2000) were also observed on the same night as V381 Vul andV382 Vul. Spectrophotometric indices were calculated usingthe SBANDS package in IRAF and were zero-point–corrected

using the established values. This gave a value for V381 Vulof , which is in good agreement with theHb p 2.77� 0.02spectral type determined earlier. A comparison of V381 Vulto three otherd Scuti stars (BP Peg, SS Psc, and RV Ari) isshown in Figure 2. These three stars have established valuesfor [Fe/H] of �0.02,�0.10, and 0.00, respectively (McNamara1997). The metal content of V381 Vul appears to be similarto or slightly more metal-poor than SS Psc. This estimate forthe metal content is consistent with the value given by Geisleret al. (1991).

The case of V382 Vul is much more complex. Once againPena et al. (1990) gave a spectral type of F6, based entirelyon the ( ) color. The spectral type was given as B8 byB � VCannon & Mayall (1949). Our current spectra of V382 Vulis shown in Figure 3. Because of the emission line in thecenter of Hb, we do not use it for classification. We turn totwo He i lines to determine the spectral type of V382 Vul.The first is the deep line at 4721.929 A˚ , and the second isthe doublet of 4713.143 and 4713.373 A˚ . From a comparisonof these lines with digital spectra from Bagnulo et al. (2003),we determine an earlier spectral type of B3 V for V382 Vul.However, there are additional lines that cannot be explainedby a B3 V. The source of these additional lines is unknown.The combination of these spectral lines can be seen in thechanging shape of the spectral features near the 4713 A˚ He ilines, as shown in Figure 4.

The final information gathered from the spectra of V381 Vuland V382 Vul are the radial velocities for each star. The radialvelocities were determined using the RVIDLINES packagefrom IRAF. We obtain an average velocity from three obser-vations of km s�1 for V381 Vul. The radial velocity31.2� 1.8solutions for V382 Vul are again more complex and are dividedinto the emission line and the absorption lines. For the Hb

emission line, we find a velocity of 14.0 km s�1. From theabsorption lines, we give a best estimate for the velocity of



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2005 PASP,117:955–966

Fig. 4.—Three spectra of V382 Vul centered on 4713 A˚ . The shape of thisline is clearly changing due to the presence of a companion star. Frames areapproximately 15 minutes apart in time.

TABLE 1Radial Velocity Measurements for V381 Vul and V382 Vul

Star HJD 2,450,000.0�RV Absorption

(km s�1)RV Emission

(km s�1)

V381 Vul . . . . . . 2,897.7470 31.4� 1.4 …2,897.7566 31.2� 1.6 …2,897.7664 30.9� 1.8 …

V382 Vul . . . . . . 2,897.7812 1.1� 2.9 13.52,897.7933 �4.0 � 2.5 14.62,897.8058 �10.8 � 2.3 14.0

� km s�1. The complex changing of the lines makes4.6� 2.5this very difficult. However, our value is within the stated valueand error of km s�1 given by Geisler (1988). The1.9� 11.2radial velocities for individual observations are gathered inTable 1.

4. VRI PHOTOMETRY

To aid in our discussion of the photometric measures in theregion of NGC 6882/6885, we collect previously publishedvalues for spectral types, radial velocities, andUBV photometryin Table 2. The first stage in the reduction of the new obser-vations was the calibration of 95 stars in the field, using theLandolt (1992) standards. In the final reduction, star 76 wasfound to be saturated and therefore was removed from furtherconsideration. Between 40 and 50 observations of each starwere secured. The average values forV, ( ), and ( )V � R R � Iare reported in Table 3, along with estimates for the error ofthe mean. We compared ourV magnitudes with those foundin Hoag et al. (1961) and found them to deviate only by asmall zero-point shift of 0.01 in theV filter.

As mentioned in the previous section, there is significantreddening in this region. In Figure 5 we plot the color(V � I)of each star for which we have a published spectral type, asgiven in Table 2, along with standard colors drawn(V � I)from Fitzgerald (1970) and Ducati et al. (2001). From this wefind two distinct groups of stars. The upper left group consistsof V382 Vul and star 22. These two stars give an average

. Using the formulae found in Taylor (1986),E(V � I) p 0.78

we convert this to , which is in excellentE(B � V ) p 0.61agreement with Platais et al. (2003). Correcting for the red-dening and estimating the absolute magnitude from the spectraltypes, we estimate that these two stars are at approximately1200 pc. This places V382 Vul in what Platais et al. (2003)consider NGC 6882. The second group of stars is made up ofV381 Vul and stars 2, 4, 5, 25, 69, and 72. These stars have

or . This gives an averageE(V � I) p 0.22 E(B � V ) p 0.17distance for this group of 540 pc. This group of stars does notbelong to either the cluster favored by Platais et al. (2003) orthe closer group reported in Robichon et al. (1999). In fact,this group seems to be located at the original distance givenfor the combined cluster NGC 6882/6885 (Johnson 1961; Svo-lopoulos 1961; Becker & Fenkart 1971).

The data from Table 3 are used to generate the color-mag-nitude diagram shown in Figure 6 by the open circles. Datafrom five stars observed by theHipparcos project are plottedas filled circles. The two filled circles that overlap with twoopen circles are the two stars in common between the two sets.The agreement is within the errors. Four of theHipparcos starshave good parallax measurements (Robichon et al. 1999), withan average value of mas. This gives a distance of2.52� 0.36

pc. Using theY2 isochrones reported in Yi et al.397� 50(2003), we generated two main sequences, which are alsoshown in Figure 6. The first is for a distance of 400 pc, withno reddening, and is represented by the solid line. The secondis for a distance of 1200 pc, , and the ageE(B � V ) p 0.61estimated by Platais et al. (2003), and is shown by the dashedline that is lower and to the right in Figure 6. The reddeningand distance of the more distant cluster place its main sequenceon the same line as the unreddened nearby stars, making in-terpretation difficult.

5. VARIABLE STARS IN THE FIELD

In addition to V381 Vul and V382 Vul, we examined 92other stars in the field to look for variability. The differentialphotometric reduction techniques are detailed in Hintz et al.(1997). There are three previously known variable stars in thefield of NGC 6882/6885. SU Vul, identified as star 23, is listedas a red irregular variable in the General Catalog of VariableStars (GCVS). V381 Vul and V382 Vul, stars 1 and 28, re-spectively, are both listed as potentiald Scuti stars, as men-




2005 PASP,117:955–966

TABLE 2Published Information for Stars in the NGC 6882/6885Field

Star Number Hoag Number Spectral Typea Memberd dVreV e(B � V) e(U � B)

1 . . . . . . . . . . . 19 A7 … 10.23 0.44 0.252 . . . . . . . . . . . 30 A2 … 10.60 0.21 0.144 . . . . . . . . . . . 27 A5 … 10.53 0.35 0.075 . . . . . . . . . . . 20 A0 … 10.33 0.16 0.119 . . . . . . . . . . . 37 K5 … 10.91 0.93 0.5910 . . . . . . . . . . 31 … M 1.9 10.72 1.38 1.3111 . . . . . . . . . . 12 F2 IIc … 9.57 0.41 0.0612 . . . . . . . . . . 10 A9 IIIc … 9.42 0.36 0.1817 . . . . . . . . . . 47 … … 11.62 0.48 0.1018 . . . . . . . . . . 40 … … 11.27 0.40 0.0119 . . . . . . . . . . 15 K2 IIIb M �3.5 9.87 1.84 1.9021 . . . . . . . . . . 45 … … 11.52 0.22 0.1222 . . . . . . . . . . 55 A0c … 12.48 0.56 0.0624 . . . . . . . . . . 41 … M �3.1 11.33 0.46 0.0325 . . . . . . . . . . 28 A0 … 10.56 0.12 0.0626 . . . . . . . . . . 54 … … 12.31 0.49 0.0628 . . . . . . . . . . 25 B8 M 1.9 10.49 0.42 �0.4731 . . . . . . . . . . 52 … M �0.4 11.83 0.56 0.1433 . . . . . . . . . . 58 … … 12.73 0.69 0.5034 . . . . . . . . . . 62 … … 13.03 0.55 �0.3135 . . . . . . . . . . 57 … … 12.63 0.51 0.1838 . . . . . . . . . . 121 G0c F �24.3 … … …39 . . . . . . . . . . 43 … M �0.6 11.47 0.44 �0.0840 . . . . . . . . . . 56 … … 12.57 0.71 0.1943 . . . . . . . . . . 64 … … 13.09 0.79 0.5160 . . . . . . . . . . 66 … … 13.51 0.66 �0.0269 . . . . . . . . . . 21 A1 Vb … 10.35 0.19 0.1072 . . . . . . . . . . 18 A2 … 10.18 0.15 �0.1475 . . . . . . . . . . 9 A7 IIIc … 9.26 0.28 0.0777 . . . . . . . . . . 8 F6 Vc M �7.5 9.18 0.45 �0.03

a Source is Cannon & Mayall (1949) unless otherwise indicated.b Zug (1933).c Svolopoulos (1961).d Geisler (1988).e Hoag et al. (1961).

tioned earlier. For each night, the ensemble was slightly dif-ferent, due to centering differences, but in general each finalensemble contained 15–20 stars.

The differential magnitudes were then converted to apparentmagnitudes, using the values discussed in the previous section.All magnitudes for each star were used to calculate a meanvalue and a standard deviation. In Figure 7 we show the plotof error per observation versus magnitude for all 94 stars. Al-though a few stars appear as potential variables on this graph,we find that the three known variables are not part of this group.Therefore, we use the robust median statistic (RoMS) detailedin Enoch et al. (2003). This statistic is greater than 1 for var-iables, and Enoch et al. (2003) estimate that stars with a robuststatistic of 0.8 have about a 50% chance of being variable. Inorder to get an appropriate value for the error per observationsused in the RoMS, we fit a line to the lower edge of the standarderror curve. This method overestimates the RoMS, yieldingmore potential variables, but reduces the chance of missing avariable.

From the RoMS, we found 28 stars with values greater than0.8 in at least one of the three filters. All three of the previouslyknown variables are included in this group. At this point, eachof the 28 potential variables was examined visually and withPeriod98. Of the 28 stars, we confirm 5 new variable stars anddetermine new periods for V381 Vul and V382 Vul. An ad-ditional eight stars show varying degrees of variability, withSU Vul being included in this group. However, the time cov-erage was insufficient in all cases to classify the type of var-iation. Below, we examine these 15 stars in more detail. Theresults are summarized in Table 4, and the positions in the HRdiagram are shown in Figure 8.

5.1. Established and New Variable Stars

5.1.1. Star 1—V381 Vul

V381 Vul is one of two variables reported by Pen˜a et al.(1990) as newd Scuti variables. Above, we find that V381 Vulis an A7 IV at a distance of approximately 540 pc. Figure 9



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2005 PASP,117:955–966

TABLE 3Photometric Measure of Stars in the NGC 6882/6885Field

Star Number Hoag Number V �j / NV (V � R) �j / N(V�R) (R � I) �j / N(R�I)

1 . . . . . . . . . . . 19 10.257 0.001 0.237 0.001 0.270 0.0012 . . . . . . . . . . . 30 10.605 0.001 0.092 0.001 0.130 0.0023 . . . . . . . . . . . … 11.690 0.001 0.370 0.002 0.381 0.0024 . . . . . . . . . . . 27 10.554 0.001 0.191 0.001 0.215 0.0015 . . . . . . . . . . . 20 10.353 0.001 0.043 0.001 0.098 0.0016 . . . . . . . . . . . … 12.004 0.002 0.343 0.003 0.346 0.0037 . . . . . . . . . . . … 12.871 0.003 0.385 0.006 0.400 0.0058 . . . . . . . . . . . … 11.724 0.001 0.263 0.002 0.304 0.0029 . . . . . . . . . . . 37 10.925 0.001 0.506 0.002 0.464 0.00210 . . . . . . . . . . 31 10.729 0.001 0.747 0.001 0.657 0.00111 . . . . . . . . . . 12 9.569 0.001 0.218 0.002 0.258 0.00212 . . . . . . . . . . 10 9.425 0.001 0.181 0.002 0.228 0.00213 . . . . . . . . . . … 11.484 0.001 0.293 0.002 0.334 0.00214 . . . . . . . . . . … 12.097 0.002 0.319 0.002 0.352 0.00315 . . . . . . . . . . … 12.907 0.004 0.953 0.005 0.891 0.00316 . . . . . . . . . . … 11.825 0.002 0.281 0.002 0.351 0.00217 . . . . . . . . . . 47 11.600 0.001 0.323 0.002 0.409 0.00118 . . . . . . . . . . 40 11.287 0.001 0.222 0.002 0.251 0.00219 . . . . . . . . . . 15 9.830 0.001 1.055 0.003 0.955 0.00220 . . . . . . . . . . … 12.909 0.004 0.868 0.004 0.802 0.00321 . . . . . . . . . . 45 11.553 0.001 0.088 0.002 0.137 0.00222 . . . . . . . . . . 55 12.469 0.003 0.322 0.004 0.411 0.00523 . . . . . . . . . . 90 10.117 0.001 0.989 0.001 0.874 0.00124 . . . . . . . . . . 41 11.306 0.001 0.250 0.001 0.313 0.00225 . . . . . . . . . . 28 10.574 0.001 0.047 0.001 0.110 0.00126 . . . . . . . . . . 54 12.342 0.002 0.250 0.003 0.319 0.00427 . . . . . . . . . . … 13.721 0.009 0.636 0.011 0.702 0.00928 . . . . . . . . . . 25 10.558 0.001 0.243 0.001 0.298 0.00129 . . . . . . . . . . 89 13.384 0.006 1.255 0.009 1.136 0.00330 . . . . . . . . . . … 12.867 0.003 0.337 0.005 0.406 0.00531 . . . . . . . . . . 52 11.816 0.002 0.334 0.002 0.338 0.00232 . . . . . . . . . . … 12.710 0.002 0.315 0.005 0.332 0.00533 . . . . . . . . . . 58 12.639 0.003 0.438 0.004 0.423 0.00334 . . . . . . . . . . 62 12.979 0.003 0.283 0.005 0.357 0.00535 . . . . . . . . . . 57 12.602 0.002 0.333 0.004 0.355 0.00436 . . . . . . . . . . 143 13.264 0.004 0.269 0.006 0.360 0.00837 . . . . . . . . . . 169 13.411 0.005 0.401 0.006 0.413 0.00638 . . . . . . . . . . 121 9.740 0.001 0.587 0.001 0.536 0.00139 . . . . . . . . . . 43 11.476 0.001 0.263 0.002 0.318 0.00240 . . . . . . . . . . 56 12.566 0.003 0.405 0.004 0.405 0.00341 . . . . . . . . . . … 10.821 0.001 0.263 0.001 0.318 0.00142 . . . . . . . . . . … 13.850 0.008 0.880 0.010 0.777 0.00643 . . . . . . . . . . 64 13.026 0.004 0.457 0.005 0.533 0.00544 . . . . . . . . . . … 13.378 0.006 0.600 0.008 0.601 0.00645 . . . . . . . . . . … 13.049 0.004 0.428 0.006 0.443 0.00646 . . . . . . . . . . … 13.717 0.009 0.465 0.010 0.497 0.01047 . . . . . . . . . . 126 11.852 0.002 0.305 0.002 0.316 0.00248 . . . . . . . . . . 127 10.833 0.001 0.262 0.001 0.305 0.00149 . . . . . . . . . . … 12.086 0.002 0.431 0.004 0.424 0.00450 . . . . . . . . . . … 13.654 0.007 0.444 0.010 0.430 0.00851 . . . . . . . . . . … 13.902 0.008 0.401 0.009 0.368 0.00952 . . . . . . . . . . … 12.677 0.003 0.391 0.005 0.401 0.00553 . . . . . . . . . . … 14.461 0.019 1.823 0.019 1.993 0.00554 . . . . . . . . . . … 13.930 0.009 1.190 0.011 1.087 0.00555 . . . . . . . . . . … 13.596 0.008 0.932 0.009 0.893 0.00556 . . . . . . . . . . … 14.318 0.013 1.235 0.014 1.126 0.00757 . . . . . . . . . . … 13.115 0.005 0.391 0.006 0.470 0.00758 . . . . . . . . . . … 13.583 0.007 0.429 0.009 0.486 0.01259 . . . . . . . . . . 134 11.271 0.001 0.219 0.002 0.258 0.00260 . . . . . . . . . . 66 13.446 0.005 0.386 0.008 0.408 0.009




2005 PASP,117:955–966

TABLE 3 (Continued)

Star Number Hoag Number V �j / NV (V � R) �j / N(V�R) (R � I) �j / N(R�I)

61 . . . . . . . . . . … 12.710 0.003 0.369 0.004 0.363 0.00362 . . . . . . . . . . … 13.087 0.004 0.379 0.005 0.374 0.00663 . . . . . . . . . . … 13.316 0.005 0.277 0.006 0.378 0.00764 . . . . . . . . . . … 13.347 0.005 0.399 0.006 0.457 0.00665 . . . . . . . . . . … 12.874 0.005 0.467 0.004 0.423 0.00466 . . . . . . . . . . … 12.099 0.002 0.399 0.003 0.392 0.00267 . . . . . . . . . . … 11.671 0.002 0.283 0.002 0.322 0.00268 . . . . . . . . . . … 11.422 0.003 0.305 0.009 0.387 0.00869 . . . . . . . . . . 21 10.359 0.001 0.052 0.001 0.139 0.00170 . . . . . . . . . . … 12.532 0.003 0.388 0.004 0.394 0.00471 . . . . . . . . . . … 11.972 0.001 0.231 0.002 0.289 0.00272 . . . . . . . . . . 18 10.229 0.001 0.051 0.001 0.108 0.00173 . . . . . . . . . . … 14.189 0.010 1.810 0.010 2.165 0.00374 . . . . . . . . . . … 13.306 0.006 0.958 0.006 0.866 0.00375 . . . . . . . . . . 9 9.273 0.001 0.144 0.001 0.185 0.00177 . . . . . . . . . . 8 9.191 0.001 0.277 0.001 0.301 0.00178 . . . . . . . . . . … 12.233 0.005 0.249 0.006 0.318 0.00479 . . . . . . . . . . … 13.024 0.003 0.319 0.007 0.433 0.00580 . . . . . . . . . . … 13.820 0.008 0.496 0.010 0.527 0.00881 . . . . . . . . . . … 13.194 0.004 0.458 0.006 0.592 0.00682 . . . . . . . . . . … 13.858 0.008 0.636 0.011 0.566 0.00783 . . . . . . . . . . … 13.322 0.003 0.484 0.007 0.471 0.00584 . . . . . . . . . . … 11.759 0.001 0.475 0.002 0.453 0.00285 . . . . . . . . . . … 13.155 0.004 0.488 0.006 0.492 0.00586 . . . . . . . . . . … 13.178 0.006 0.485 0.011 0.498 0.00587 . . . . . . . . . . … 13.670 0.007 0.367 0.012 0.400 0.00788 . . . . . . . . . . … 13.316 0.006 1.258 0.007 1.129 0.00489 . . . . . . . . . . … 12.229 0.002 1.287 0.002 1.170 0.00190 . . . . . . . . . . … 12.286 0.002 0.397 0.003 0.388 0.00391 . . . . . . . . . . … 12.356 0.002 0.549 0.004 0.496 0.00392 . . . . . . . . . . … 12.481 0.003 0.356 0.005 0.390 0.00393 . . . . . . . . . . … 13.118 0.003 0.455 0.006 0.482 0.00494 . . . . . . . . . . … 13.954 0.009 1.046 0.012 0.945 0.00595 . . . . . . . . . . … 13.358 0.005 0.401 0.008 0.412 0.006

shows the five individual nights of data. We find a period forV381 Vul of 0.1185 days. The amplitudes for theV, R, andIfilters are 0.010, 0.006, and 0.004, respectively. From verylimited data, the amplitude in theB filters is found to be ap-proximately 0.02. These values for the amplitude and periodare significantly different from those of Pen˜a et al. (1990), whofound a period of 0.056 days and an amplitude of 0.034 in theV filter. In Figure 10 we show the phased data in theV andRfilters. Given the spectral type, period, and amplitude mea-surements, we continue to classify this star as ad Scuti.

5.1.2. Star 8—New Variable Star

Star 8 is located near star 1 in the northwest corner of thecluster. Our red data is quite noisy, so the analysis concentratedon theV and I data. The five nights ofV data are shown inFigure 9. We determine a period of 0.2247 days and an am-plitude of 0.015 in both filters. The ( ) color for this starV � Iis 0.567 and is right on the main sequence shown in Figure 8.With the reddening values discussed earlier, this would cor-respond roughly to a G dwarf. From the consistency of the

amplitude and a G dwarf spectral type, we conclude that thisis likely a W UMa system with a period of 0.4494 days.

5.1.3. Star 13—New d Scuti Variable

In Figure 9 the five nights ofV data are shown for star 13.We find this star to have a period of 0.0776 days and amplitudesof 0.031, 0.015, 0.010, and 0.010 in theB, V, R, andI filters,respectively. Using the ( ) color, the data in Figure 5, andV � Ian assumption that the star is in the group at about 540 pc, wefind a spectral type of F3 for this star. The magnitude differencebetween star 1 and star 13 is consistent with the spectral typedifference. From the period, amplitudes, and color, we classifythis star as ad Scuti. Figure 11 shows the phasedV light curveof star 13.

5.1.4. Star 28—V382 Vul

This is the second star reported as ad Scuti by Pen˜a et al.(1990). However, we show above that this star has a spectraltype of B3 V, plus additional unidentified lines, which would



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Fig. 5.—Plot of color vs. spectral type. The solid line was generated(V � I)from data drawn from Fitzgerald (1970) and Ducati et al. (2001). The filledcircles indicate stars from this survey with known spectral types (0 is B0, 10is A0, etc.).

Fig. 6.—Color-Magnitude diagram for region of NGC 6882/6885 coveredby this survey, along with stars from theHipparcos catalog. The open circlesrepresent data from the current survey, and filled circles show stars taken fromthe Hipparcos catalog. Isochrone 1 is for an unreddened clusters at 400 pc.Isochrone 2 is for a heavily reddened cluster at 1200 pc.

Fig. 7.—Plot of error per observation vs. magnitude for all 94 stars examinedin this survey.

clearly preclude the star from being ad Scuti. The spectralobservations also show a large variation in radial velocity forthe spectral features. However, the radial velocity of the emis-sion line is roughly constant at 14 km s�1. This indicates thatthe star is inside a shell that is the source of the emission line.

From the photometry, we find a short-period variation of0.1808 days. The five nights of data are again shown in Fig-ure 9. This new period is far longer than the period reportedby Pena et al. (1990), and we find a nearly constant amplitudeof 0.011 in theV, R, and I filters. From the spectral type ofB3 V, the period of 0.1808 days, and the radial velocity var-iation, we classify this star as ab Cephei. The constant am-plitude is a concern for a pulsating star. However, Sareyan etal. (1997) show fairly constant amplitudes in redder filtersfor anotherb Cephei star, 16 Lacertae.

5.1.5. Star 47—New Eclipsing Binary System

Although we clearly show this star to be variable star, wehave no way to determine the period. In all our nights of data,we caught only one eclipse, as shown in Figure 12. In thisfigure, we show the variation of the star in theB, V, and Rfilters. The depth of the eclipse is 0.22, 0.13, and 0.05 in theB, V, andR filters, respectively. Since star 47 is a redder star,with , we conclude that the companion is bluer(V � I) p 0.621and likely a white dwarf.

5.1.6. Star 72—New Variable

Star 72 is one of the stars farthest from the traditional centerof NGC 6882/6885 that we examined. As given in Table 2, it

has a published spectral type of A2 and appears to be in thegroup at 540 pc. The ( ) of 0.158 calculated from data inV � ITable 3 is consistent with an A2 spectral type. Only four nightsof data were obtained on this star, since it was close to theedge of our field. The four nights are shown in Figure 9. Wefind a period of 0.2220 days, with a constant amplitude acrossthe V, R, andI filters of 0.008. With a spectral type of A2, itis unlikely that this is a W UMa system (Csizmadia & Klagyivik2004). However, we believe that this is an eclipsing systemwith a period of 0.4440 days.

5.1.7. Star 78—New Variable

This star is located in the extreme southeast corner of ourfield of view and for some nights was off the frame. Therefore,we only have three nights of data, and on one of these nightsit appears on only half the frames. The three nights ofV data




2005 PASP,117:955–966

TABLE 4Potential Variable Stars

Star GCVS Name RoMS V Variable?Period(days) Amplitude inV Comments

1 . . . . . . . V381 Vul 1.022 10.257 Yes 0.1185 0.010 Much different than published values. Likely ad Scuti.8 . . . . . . . … 0.993 11.724 Yes 0.4494 0.015 Amplitude inV and I match. No variation seen inR.9 . . . . . . . … 0.801 10.925 No … … Alittle bump in R only.13 . . . . . . … 0.912 11.484 Yes 0.0776 0.015 A new variable. Likely ad Scuti.15 . . . . . . … 0.847 12.907 No … … …16 . . . . . . … 0.848 11.825 No … … …19 . . . . . . … 0.710 9.830 No … … A variationseen inI was due to saturation.23 . . . . . . SU Vul 0.857 10.117 Susp. … … Hard to say anything about this star.25 . . . . . . … 0.844 10.574 Susp. … … Long-period variations, which are hard to pin down.28 . . . . . . V382 Vul 1.020 10.558 Yes 0.3616 0.011 Could be eclipsing system.29 . . . . . . … 0.811 13.384 Susp. 0.1662 0.010 Has statistic of 1.2 inI filter. Period and amplitude forI.34 . . . . . . … 0.834 12.979 No … … …38 . . . . . . … 0.851 9.740 Susp. … … Insufficient coverage, but there are patterns in the data.39 . . . . . . … 0.844 11.476 No … … …41 . . . . . . … 0.869 10.821 Susp. 0.5647 0.009 Statistic higher forR and I. Possible W UMa system.47 . . . . . . … 1.224 11.852 Yes … … Single eclipseseen; indeterminite period.48 . . . . . . … 0.896 10.833 No … … …49 . . . . . . … 0.745 12.086 No … … One nightcaused high statistic. Statistic adjusted.55 . . . . . . … 0.902 13.596 No … … …59 . . . . . . … 0.822 11.271 No … … …67 . . . . . . … 0.880 11.671 No … … …68 . . . . . . … 3.346 11.422 No … … In crowded region and influenced by other stars.72 . . . . . . … 0.903 10.229 Yes 0.4440 0.008 All amplitudes similar; perhaps a W UMa system.78 . . . . . . … 1.067 12.233 Yes 0.0795 0.056 Pulsator near edge of field; only 2.5 nights of data.84 . . . . . . … 1.180 11.759 Susp. … … Variation only found inV filter.88 . . . . . . … 0.887 13.316 No … … Faint star with noisy data.91 . . . . . . … 0.885 12.356 Susp. … … Potential long period.92 . . . . . . … 0.986 12.497 Susp. … … Variation seen only inV filter.

Fig. 8.—HR diagram of the stars from this survey. Stable stars are repre-sented as open circles, variable stars as filled circles, and suspected variablesas filled triangles.

are shown in Figure 9. We find a period of 0.0795 days forstar 78, with amplitudes of 0.056, 0.036, and 0.032 in theV,R, and I filters. Based on the ( ) color and the data inV � IFigure 5, we determine a spectral type of B5 if star 78 is inthe more distant cluster, or F0 if it is in the group at 540 pc.In neither case does the magnitude seem to fit with the deter-

mined spectral type. The period and amplitudes indicate thatthis is ad Scuti variable, but more information is needed toconfirm this classification. The phased light curve for star 78is shown in Figure 13.

5.2. Suspected Variable Stars

In addition to the seven variable stars detailed above, wefind eight stars that show some indication of variation. Fivenights of V data are shown for seven of these stars in Fig-ure 14. Star 29 is not included in the list, since it only showsvariations in theI filter.

5.2.1. Star 23—SU Vul

Star 23 is listed as a variable star in the GCVS. It is listedas a long-period red variable. Our data is insufficient to find aperiod for this star, but does show a long-period variation. Ifthis star had not been listed as a variable, it would have beeneasy to overlook it as a potential variable star.

5.2.2. Star 29

This is an interesting suspect. It is the faintest star for whichwe suspect variability at 13.384 in theV filter. The exposures inthe V filter were not long enough to get good results, and theRoMS was only 0.811 in theV filter. However, in theI filter the



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Fig. 9.—Five nights ofV data for stars 1, 8, 13, 28, 72, and 78.

Fig. 10.—Phased light curves for star 1, V381 Vul, in theV andR filters.Fig. 11.—Phased light curve for the newd Scuti variable star 13 in theB

andV filters.

statistic was 1.2 where the magnitude is 10.993. Therefore, theonly analysis we can perform is in theI filter. From this datawe find a period of 0.1662 days with an amplitude of 0.010.This star has an extremely red color of . More(V � I) p 2.39observations are needed to determine the type of variation forthis star.

5.2.3. Star 41

For star 41 we find a period of 0.2824 days, with a constantamplitude in theVRI filters of 0.009. The ( ) color for thisV � Istar is 0.581, very similar to that found for star 8. The positionin Figure 8 place this star as a G dwarf. The period, amplitude,and position in the HR diagram make it likely that this is a W




2005 PASP,117:955–966

Fig. 12.—One night of data for star 47, showing theB, V, andR data.Fig. 13.—Phased light curve for star 78 in theV filter.

Fig. 14.—Five nights ofV data plotted for the suspected variables. This includes stars 23, 25, 38, 41, 84, 91, and 92.

UMa system with a period of 0.5647 days. Although we areable to get a reasonable solution for this star, we still feel itshould be on the list of suspected variables and not on the listof variables.

5.2.4. Stars 25, 38, 84, 91, and 92

The data for these stars is shown in Figure 14, but there islittle else that can be reported for these stars. No definitive

period was found for any star of this group, and only the trendspresented in the figure give an indication of variability.

6. CONCLUSION

We examine the twod Scuti variables reported by Pen˜a et al.(1990). We find that V381 Vul is indeed ad Scuti; however, wefind a longer period of 0.1185 days and an amplitude 0.010.



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This star is located in a group of stars at approximately 540 pcand is an A7 IV. For V382 Vul we find that the classificationas ad Scuti is in error. V382 Vul has a spectral type of B3 V,plus a late-type companion, and is located in a group at 1200 pc.We classify V382 Vul as ab Cephei star with a period of0.1808 days and an amplitude of 0.011.

Using as a cutoff value, we find 28 candidateRoMSp 0.8variable stars in the field of NGC 6882/6885. Seven stars arefound with . Of these, we find one star with a singleRoMS1 1.0night of bad data and one star with nearby neighbors that causelarge errors. The remaining five stars with are allRoMS1 1.0found to be variable. For we have six stars,0.9 ! RoMS! 1.0with three confirmed as variables, one suspect, and two non-variables, or about 66% variable. For the group with 0.8!

, we examine 15 stars, with six suspects and nineRoMS! 0.9nonvariables. All stars with were found to be non-RoMS! 0.8variable. These statistics are consistent with the predictions ofEnoch et al. (2003). We find the RoMS technique to be superiorover an examination of an error curve in finding low-amplitudevariable stars.

For the region covered by our data, we find stars located attwo distinct distances. There is a group that includes V381 Vul

and that is at the traditional distance of about 600 pc for NGC6882/6885. All but one of these stars is in the northern part ofour field of view. We also find stars, including V382 Vul, ata distance greater than 1 kpc, as favored by Platais et al. (2003).In the end, a more detailed approach is needed in order toseparate the stars in this region into their proper clusters anddefine NGC 6882 and NGC 6885 appropriately.

We acknowledge the Brigham Young University Departmentof Physics and Astronomy for their continued support of ourresearch efforts. In particular, we wish to acknowledge an En-vironment for Mentoring Grant from BYU’s ORCA office. Wealso acknowledge a grant from the Dunham Foundation and aAAS small research grant, which have been used to help equipthe BYU campus observatory. We also acknowledge RonaldHintz for his help acquiring the spectral data.

We acknowledge the use of the 1.8 m Plaskett Telescope atthe Dominion Astrophysical Observatory, Herzberg Instituteof Astrophysics, National Research Council of Canada. We alsoacknowledge the use of the online Library of High ResolutionSpectra of Stars across the HR Diagram of the UVES ParanalObservatory Project (ESO DDT Program ID 266.D-5655).

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variable stars in the field of ngc 6882/6885: the case of v381 vulpeculae and v382 vulpeculae

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