ACTA ASTRONOMICA Vol. 47 (1997) pp. 211±223

A CCD Search for Variable in Young NGC 663

by G. P i e t r z y nski Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warsaw e-mail [email protected]

Received March 1, 1997

ABSTRACT

The results of search for variable stars in the young open cluster NGC 663 are presented. Based on 32 nights of observations ®ve new variable stars have been discovered. Two of them are Be stars.

Another two are pulsating ± most probably ± RR Lyrae variables. designated temporarily as v 7

is a background Cephei star. The unambiguous periods and more complete light curves for two eclipsing stars discovered earlier were also obtained. Key words: Stars: oscillations ± binaries: eclipsing ± open clusters and associations: individual: NGC 663

1. Introduction

This paper is a contribution to ongoing program of searching for variable stars in young open clusters, which until now covers about a hundred observing nights. Our previous researches concerned two clusters: NGC 654 (Kubiak and Pietrzynski 1995, Pietrzynski 1996a) and IC 4996 (Pietrzynski 1996b, 1996c). Seven new variable stars have been discovered in those clusters. Four detached eclipsing systems are probably cluster members. One W UMa and two probable RR Lyrae stars are backgroundobjects. Our results showthat searching for variable stars in open clusters can be successfully made with small telescope equipped with CCD camera. Preliminary results for NGC 663 were presented in Pietrzynski (1996d). In that paper, hereinafter called Paper I, discovery of two eclipsing systems were reported. Unfortunately because of narrow minima of the variable designated temporarily

as v 1 only a few points were located near the phases 0.0 and 0.5 so the period

derived for that star was somewhat ambiguous. In the case of star marked as v 2 a gap in observations close to the expected secondary minimum makes it impossible to decide which of two periods, 1.03 or 0.515, is correct. The aim of the present 212 A. A. project was to obtain unambiguous periods for variables discovered in Paper I and to search for variable stars in three new ®elds of the cluster. After description of instrumental system (Section 2) and data reduction procedure (Section 3) the variable stars observed in this project are presented in Section 4.

2. Observations

The presented data were collected on 32 nights at the Ostrowik Station of the Warsaw University Observatory using 0.6 m Zeiss re¯ector. The detector was a

Tektronix TK512CB backside illuminated thin CCD chip. The size of the detector 00

was 512  512 pixels with one pixel corresponding to 0. 74. The ®eld of view

 was 65 6 5 arcmin. The gain and readout noise were 9.35 electrons/ADU and 13.7 electrons, respectively. Instrument and data acquisition system are described in details in Udalski and Pych (1992). N

500

400

300 E v1 200 cI v3

100 v2 0 0 100 200 300 400 500

Fig. 1. Map of NGC 663 ± Region I based on an I frame taken with 500 s exposure time. One pixel

00

v v cI corresponds to 0. 74. v 1, 2, 3 and mark the variables and comparison star, respectively.

As can be seen from the map given in Hoag et al. (1961) the area occupied by

NGC 663 is apparently larger than our ®eld of view (14  14 arcmin). The cluster was divided then into four regions. In this way we monitored for variability the

stars from the ®eld of about 12  12 arcmin around cluster center. Maps of that regions are presented in Figs. 1±4. Region I, Region II, Region III and Region IV were monitored on 27, 11, 10 and 7 nights respectively. Some of the data were collected through thin clouds. Vol. 47 213 N

0

100

200 E v5 300 v4

400 cII

500

500 400 300 200 100 0

Fig. 2. Map of NGC 663 ± Region II based on an I frame taken with 500 s exposure time. One pixel

00

v cI I corresponds to 0. 74. v 4 and 5 mark variable stars. points the comparison star. N

0

100

cIII 200 E 300

v6 400

500

500 400 300 200 100 0

Fig. 3. Map of NGC 663 ± Region III based on an I frame taken with 500 s exposure time. One

00 cI I I pixel corresponds to 0. 74. v 6 points variable star. marks the comparison star. 214 A. A. N

0

100 cIV

200 E 300

400 v7

500

500 400 300 200 100 0

Fig. 4. Map of NGC 663 ± Region IV based on an I frame taken with 500 s exposure time. One

00 cI V pixel corresponds to 0. 74. v 7 and mark the variable and comparison star respectively.

The frames were taken through the Cousins I ®lter. The autoguider was used during the observations. The tracking of the telescope was good enough to make possible the long exposures, up to 500 s. Due to the young age of the cluster some bright B-type stars are present in the ®eld of view. In order to obtain good photometry for these stars short exposures, ranging from 30 s to 120 s depending on the weather conditions and presence of bright stars, were also made. On one

night some V -®lter frames with exposure time 240 s were also taken in a purpose to transform instrumental brightness into standard system. Stars with photometry obtained by van den Berg and de Roux (1978) were used as secondary standards. Because of non photometric weather condition and low precision of photographic photometry of the standards the achieved accuracy is very low (about 0.06 mag).

3. Data Reduction

Regions of the sky relatively free from stars were imaged during twilight to construct ¯at-®eld images. Usually 20 such frames each about 15000 ADU above bias were obtained with the telescope position randomly moved several tens of arcseconds between exposures. The raw frames have been de-biased, dark signal subtractioned and ¯at-®elded in a standard way using IRAF 1 package. The pro®le

1 IRAF is distributed by National Optical Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with National Science Foundation. Vol. 47 215 photometry for all stars was derived with DAOphot II software. The stellar point spread function was evaluated from a sum of ®ve isolated stars. The formal error

returned by DAOphot for stars with brightness about 16 mag in V was about 0.04 mag. The procedure of constructing database is the same as in Pietrzynski (1996a). The best frame from each region was used to make reference list. After careful inspection of the frames all spurious stars were removed. The resulting reference

lists consist of 283 (Reg. I), 173 (Reg. II), 286 (Reg. III) and 202 (Reg. IV) stars.

cI I cI I I Several stars were tried as comparison stars. Finally stars marked as cI , ,

and cI V were used as comparison stars in this project. Description of these stars is given in Table 1.

Table1

Comparison stars used in this project. Spectral types are from Tapia et al. (1991)

V

Star [mag] I [mag] Spectral type Region

cI 11.4 0.003 B2 V Region I

cI I 11.0 0.003 B2 I Region II

cI I I 14.7 0.006 ± Region III

cI V ± 0.005 ± Region IV

Thesearchforvariablestarshavebeendoneinthesamemanner as in Pietrzynski (1996b). First all light curves were visually examined. Then for suspected vari- ables the power spectra based on the CLEAN algorithm (Roberts,LeharsandDreher 1987) and/or AoV periodograms (Schwarzenberg-Czerny 1989) were calculated. This procedure allows us to ®nd ®ve new variable stars described below.

4. Variable Stars

4.1. Variable Star v1

= Based on the data presented in Paper I only a crude period P 0 6 d could be

derived for v 1. Observed minima are narrow so only a few points were located near the phase 0.0 and 0.5. In the present observations much more data have

been collected. On two nights both primary and secondary minima were observed

= during each night. In that case an unambiguousperiod P 0 461 d was evaluated.

Light curve of variable v 1 phased with that period is presented in Fig. 5. Shape of that light curve is characteristic for EA eclipsing systems. The observed times of 216 A. A. minimum light are given in Table 2. From the data the following linear ephemeris

is derived:

= +  E Min I J D Hel 2450368 382 0 461

The brightness in V ®lter at maximum light is 14.6 mag. This relatively bright detached system with two distinct minima of comparable depth and relatively short period is very promising candidate for spectroscopic study.

2.6

2.7

2.8 I [mag] ∆

2.9

v1 P = 0.461 d

3 -.4 -.2 0 .2 .4 .6 .8 1 1.2 1.4

phase

I v cI Fig. 5. Light curve of the variable v 1. stands for the difference 1 .

Table2

Times of minimum light for v 1

O C J.D.Hel E

+ 2450000

171.45 363 0 0.01

174.44 355 5 0 01

211.32 275 5 0.01

211.57 275 0 0 03 338.38 0.0 0 01

349.45 24.0 0.01

358.44 43.5 0 01 365.59 59.0 0 01 Vol. 47 217

4.2. Variable Star v 2 In the light curve presented in Paper I there was an unfortunate gap in observa- tion at the expected secondary minimum so the period given there was somewhat ambiguous. Fig. 6 shows light curve of this star obtained in the present project. Both kinds of minima are clearly visible. The depths of primary and secondary minima are 0.55 mag and 0.15 mag respectively. The observed times of minima

are given in Table 3. The linear ephemeris derived from the data is:

= +  E Min I J D Hel 2450171 319 1 025

Shape of the light curve is that of EB stars. Unfortunately we have no informa-

tion about its brightness in V ®lter. This star is also a good object for spectroscopic study.

Table3

Times of minimum light for v 2

O C J.D.Hel E + 2450000 171.32 0.0 0.001

174.40 3.0 0.005 211.29 39.0 0 008 338.41 163.0 0.001

358.40 182.5 0.002 393.25 216.5 0 002

3.6

3.8

4 I [mag] ∆

4.2

v2 P = 1.025 d 4.4

-.4 -.2 0 .2 .4 .6 .8 1 1.2 1.4

phase

I v cI Fig. 6. Light curve of the variable v 2. stands for the difference 2 .

218 A. A.

 + 4.3. Variable Star v 3 (BD 60 341) This star was observed in the past by many authors. According to Svolopoulos

(1962) the spectral type of v 3 isB9Vne. Liu et al. (1991) measured radial velocity

of 27 3 km/s. In Coyne, Wisniewski and Otten (1978) this star has number 619 H

and was discoveredto show emission in . Sanduleak(1979, 1990) also reported strong emission in spectrum of this star. He failed however to ®nd variations in the

emission strength. The V magnitude obtained in this paper at the minimum light is 10.6 mag. It is in good agreement with previous studies. Only the value given in Tapia et al. , 10.74 mag, differs by about 0.1 mag from the present value.

Fig. 7 presents all observations collected for v 3 in this project. From this

light curve we can see that v 3 brightened by at least 0.13 mag during ®rst 60 days of observations. After 280 days similar feature appeared on the light curve. The present data are too scarce to make any ®nal conclusions about period of this

star. All the data were obtained through I ®lter so we have no informations about V brightening in V . We can only speculate that differences between magnitudes published in the past by different authors may be explained by this phenomenon.

-1.1

-1.05

-1 I [mag] ∆

-.95

-.9

-.85

100 150 200 250 300 350 400 450 500

JDHel + 2450000.0

I v cI I Fig. 7. Observations of v 3. stands for the difference 3 .

4.4. Variable Star v 4 H

Tapia et al. (1980) derived from ubvy and photometry the spectral type B1 I for this star. They also pointed out that this is a ®eld star. The data obtained in this paper show that this star pulsate with an amplitude of about 0.03 mag and Vol. 47 219 a period of about 0.197 d. Fig. 8. presents light curve of this variable. Observed times of maximum light are given in Table 4. This data lead to the following linear

ephemeris :

= +  E

Max I J D Hel 2450410 622 0 197

= From approximate transformation we get V 11 1 mag at the phase 0.25.

Taking into account the crude spectral type of v 4 we conclude that this is probably

a Cephei variable with relatively large amplitude. This very interesting star deserves further photometric and spectroscopic studies.

.04

.06

.08 I [mag] ∆ .1

.12

v4 P = 0.197 d

.14 -.4 -.2 0 .2 .4 .6 .8 1 1.2 1.4

phase

I v cI I Fig. 8. Observations of v 4. stands for the difference 4 .

Table4

Times of maximum light for v 4

O C J.D.Hel E

+2450000

410.44 1 0 0 02

410.64 0.0 0.02

434.21 120.0 0 04 434.41 121.0 0 03 437.42 136.0 0.02 437.60 137.0 0.01 476.40 334.0 0.01

476.60 335.0 0.01 477.57 340.0 0 01 220 A. A.

4.5. Variable Star v 5 This star has number 620 in Coyne, Wisniewski, and Otten (1978) and is

designated there as a . The three V magnitudes obtained by these authors differ by more than 1 mag. The emission line of this star was con®rmed by Sanduleak(1979, 1990). He also discoveredvariations in its emission line strength.

The brightness in V ®lter at the minimum light obtained in this paper is 12.1 mag.

The spectral type B8 is given for v 5 in Zugg (1933). All observations collected in this project are displayed in Fig. 9. Our data con®rm photometric variability of this star but are to scarce to say something about its periodicity and type.

.6

.65

.7 I [mag] ∆ .75

.8

.85

400 410 420 430 440 450 460 470 480 490

JDHel - 2450000.0

I v cI I Fig. 9. Light curve of v 5. is a difference 5 .

4.6. Variable Star v 6

Fig. 10 presents the light curve of variable designated temporarily as v 6 phased with period of 0.95 d. The shape of the light curve is typical for RR Lyrae variable. Times of maximum light for that star are given in Table 5. This data lead to the

following linear ephemeris:

= +  E Max I J D Hel 2450450 25 0 94

Large scatter of observations at maximum light may be due to the change of

amplitude caused by BøazkoÇ effect. The V magnitude at maximum light is about 10.7 mag. Vol. 47 221

-3

-2.9 v6 P = 0.94 d

-2.8 I [mag] ∆

-2.7

-2.6 -.4 -.2 0 .2 .4 .6 .8 1 1.2 1.4

phase

I v cI I I Fig. 10. Light curve of v 6. is a difference 6 .

Table5

Times of maximum light for v 6

O C J.D.Hel E

+ 2450000

444.65 6 0 0.01

448.32 2 0 0 06

449.38 1 0 0.07 450.23 0.0 0 02

4.7. Variable Star v 7

This is another RR Lyrae type variable discovered in the ®eld of NGC 663.

Light curve of v 7 phased with the period of 0.614 d is displayed in Fig. 11. There

is no data in V ®lter for this star. Times of maximum light are given in Table 6.

Table6

Times of maximum light for v 7

O C J.D.Hel E + 2450000 171.32 0.0 0.001

174.40 3.0 0.005 211.29 39.0 0 008 338.41 163.0 0.001

358.40 182.5 0.002 393.25 216.5 0 002 222 A. A.

The calculated linear ephemeris is:

= +  E Max I J D Hel 2450171 319 0 614

4.2

v7 P = 0.614 d 4.4

4.6

4.8 I [mag] ∆

5

5.2

5.4 -.4 -.2 0 .2 .4 .6 .8 1 1.2 1.4

phase

I v cI V Fig. 11. Light curve of v 7. is a difference 7 .

5. Remarks

Photometric variability of Be stars is clearly established (Feinstein 1968). NGC 663 has the largest known number of Be stars among open clusters. This provides a unique opportunity to study such a large and uniform group of Be stars.

Table7 Variable stars discovered in the ®eld of NGC 663

Variable Type Period [d] Max. amplitude [mag] Brightness [mag] Spectral type

I = V =

v 1 EA 0.461 0 12 14 6 ±

I =

v 2 EB 1.025 0 55 ± ±

I = V =

v 3 Be ? 0 1 10 6 B9 V ne

I = V =

v 4 Cephei 0.197 0 03 11 1 B1 I

I = V =

v 5 Be ? 0 2 12 1 B8

I = V =

v 6 RR Lyrae 0.94 0 2 10 7 ±

I = v 7 RR Lyrae 0.614 0 6 ± ±

In this project 21 Be stars have been monitored. Two of them show clear changes

in brightness by more than 0.1 mag in I ®lter. Beside the two Be stars ®ve other Vol. 47 223 variable stars have been found. Two of them are eclipsing systems with relatively short periods. The light curves of two remaining stars are typical for pulsating

stars. Most probably they are ®eld RR Lyrae variables. Based on the spectral type and shape of the light curve v 4 seems to be a Cephei variable. Description of all variable stars discovered in NGC 663 is given in Table 7.

Acknowledgements. I would like to thank Professor Marcin Kubiak for his help in this project. Thanksare also due to ProfessorJ.-C. Mermilliod for obtaining his Open Cluster DataBase. The partial support by the grant 2.PO3D.030.09 is acknowledged.

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