ACTA ASTRONOMICA Vol. 47 (1997) pp. 211±223
A CCD Search for Variable Stars in Young Open Cluster 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. Star 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 6 5 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