3. Variable stars

In this paper we present results for 42 variables identified in the five observed fields. All of them are new discoveries and were assigned names OGLEGC212-216, OGLEGC218-223 and OGLEGC225-255. Names OGLEGC217 and OGLEGC224 were given to previously known variables V9 and V3 (e.g. Hogg 1973). Photometry obtained for these two stars was poor because their images were badly overexposed on most of analyzed frames. Therefore, we decided to drop OGLEGC217=V9 and OGLEGC224=V3 from our list of variables. Our survey did not cover the central part of the cluster which was invesigated recently by Edmonds et al. (1996).

The rectangular and equatorial coordinates of the 42 newly identified variables are listed in Table 4 (click here). The rectangular coordinates correspond to positions of variables on the V-band "template" images. These images allow easy identification of all objects listed in Table 4 (click here). The name of the field in which a given variable can be identified is given in the 6th column. All frames collected by the OGLE team were deposited at the NASA NSS Data Center. Frames mr5228, mr5227, mr7890, mr14597 and mr14595 were used as templates for fields 104A, 104B, 104C, 104D and 104E, respectively. The transformation from rectangular to equatorial coordinates was derived from positions of stars which could be matched with objects from the astrometric list kindly provided by Kyle Cudworth. The number of "transformation stars" identified in a given field ranged from 55 to 100. The adopted frame solutions reproduce equatorial coordinates of these stars with residuals rarely exceeding 0.5 arcsec. According to Cudworth the absolute accuracy of equatorial coordinates for stars from his table is not worse than .

 

 

Name X Y RA(1950) Dec(1950) Field h:m:s

OGLEGC212 220.9 254.7 0:18:41.83 -72:28:44.7 A

OGLEGC213 226.8 1488.2 0:18:33.61 -72:19:49.3 A

OGLEGC214 993.9 1581.3 0:19:45.95 -72:18:42.8 A

OGLEGC215 1338.7 541.9 0:20:26.80 -72:26:01.4 A

OGLEGC216 1364.1 1905.1 0:20:18.63 -72:16:09.3 A

OGLEGC218 1706.3 1622.4 0:20:53.31 -72:17:59.4 A

OGLEGC219 344.5 244.3 0:22:54.32 -72:27:00.8 B

OGLEGC220 206.5 1776.8 0:22:30.21 -72:16:00.3 B

OGLEGC221 390.8 1673.4 0:22:48.44 -72:16:39.2 B

OGLEGC222 643.2 280.3 0:23:22.66 -72:26:35.0 B

OGLEGC223 768.9 816.9 0:23:30.69 -72:22:37.9 B

OGLEGC225 709.6 1398.1 0:23:20.72 -72:18:27.9 B

OGLEGC226 931.0 1853.5 0:23:38.38 -72:15:02.5 B

OGLEGC227 1014.1 90.7 0:23:59.63 -72:27:44.3 B

OGLEGC228 1041.4 657.1 0:23:57.90 -72:23:37.6 B

OGLEGC229 1023.4 1208.5 0:23:52.00 -72:19:39.2 B

OGLEGC230 1131.5 1315.2 0:24:01.46 -72:18:49.1 B

OGLEGC231 1070.9 1853.6 0:23:51.64 -72:14:57.5 B

OGLEGC232 1512.2 176.1 0:24:46.64 -72:26:49.3 B

OGLEGC233 1781.4 591.1 0:25:09.02 -72:23:39.6 B

OGLEGC234 163.2 618.6 0:20:29.83 -72:13:58.7 C

OGLEGC235 223.1 1394.6 0:20:30.00 -72:08:20.0 C

OGLEGC236 800.0 337.9 0:21:32.23 -72:15:38.6 C

OGLEGC237 1403.4 1232.5 0:22:22.42 -72:08:48.9 C

OGLEGC238 1800.1 661.7 0:23:04.39 -72:12:41.6 C

OGLEGC239 1359.7 528.5 0:22:23.80 -72:13:55.8 C

OGLEGC240 1552.8 1853.8 0:22:31.56 -72:04:13.9 C

OGLEGC241 1649.5 992.6 0:22:47.51 -72:10:23.8 C

OGLEGC242 130.6 969.0 0:18:49.43 -72:32:03.6 D

OGLEGC243 328.9 931.3 0:19:08.78 -72:32:14.1 D

OGLEGC244 1467.4 389.3 0:21:02.32 -72:35:33.2 D

OGLEGC245 1227.3 1183.7 0:20:33.52 -72:29:56.4 D

OGLEGC246 1074.1 1119.8 0:20:19.25 -72:30:29.1 D

OGLEGC247 1559.2 285.6 0:21:11.95 -72:36:15.1 D

OGLEGC248 1604.9 520.4 0:21:14.64 -72:34:31.7 D

OGLEGC249 698.3 12.0 0:22:46.26 -72:38:49.3 E

OGLEGC250 930.6 1371.6 0:22:59.38 -72:28:51.8 E

OGLEGC251 542.1 1778.9 0:22:19.39 -72:26:07.2 E

OGLEGC252 1171.0 909.6 0:23:25.73 -72:32:04.6 E

OGLEGC253 1863.2 873.3 0:24:32.62 -72:31:57.2 E

OGLEGC254 1629.1 1072.4 0:24:08.63 -72:30:38.8 E

OGLEGC255 1540.6 1294.4 0:23:58.50 -72:29:05.4 E

Table 4: Rectangular and equatorial coordinates for variables identified in the field of 47 Tuc. The X and Y coordinates give positions of the variables on the template images (see text for details)

   

Name P V-I V A_V

OGLEGC day mean

212 0.6946 0.63 19.5 0.8

213 0.6329 0.56 19.8 0.4

216 0.3617 0.47 19.9 0.4

223 0.2971 0.33 17.6 0.45

226 0.6474 ? 19.4 0.45

232 0.3635 0.53 19.5 0.5

234 0.6159 0.79 19.55 0.6

235 0.5317 0.42 19.8 0.6

236 0.5083 0.77 19.8 0.3

243 0.6255 0.56 19.8 0.55

246 0.5719 0.80 19.65 0.8

247 0.5115 0.51 19.9 0.65

255 0.5251 0.50 19.8 1.0

Table 5: Light curve parameters for RR Lyr stars from the field of 47 Tuc. A_V is the full range of variability

  

Name Type Period V-I

OGLEGC days

214 Ecl 0.2737 0.82 17.96 18.34

215 8.666 1.14 16.56 16.68

218 ? 1.69 15.80 16.17

219 K 36.05 1.08 15.28 15.46

220 K 10.69 1.03 16.265 16.34

221 Ecl 0.3135 0.79 17.78 18.22

222 K 18.93 0.95 16.62 16.80

225 Ecl 0.2346 1.04 19.47 20.0

227 Ecl 0.3788 0.52 16.49 16.77

228 Ecl 1.1504 0.34 15.90 16.30

229 K 8.378 1.06 14.92 15.05

230 4.814 1.23 17.51 17.71

231 K 6.498 0.93 14.225 14.325

233 28.69 1.45 16.55 16.72

237 K 18.80 0.85 16.87 16.95

238 Ecl 0.2506 0.77 18.46 18.80

239 ? 1.53 16.58 16.67

240 Ecl 4.3158 0.00 19.93 20.65

241 ? 1.67 16.72 16.83

242 ? 2.48 16.55 17.42

244 Ecl 0.3837 0.51 16.16 16.38

245 Ecl 0.2789 0.69 15.49 15.87

248 1.9967? 1.26 17.55: ?

249 Ecl 0.3226 0.64 17.33 17.66

250 Ecl 0.3514 0.43 16.34 16.56

251 3.4629 1.12 16.56 16.87

252 ? 2.90 17.04 16.68

253 Ecl 0.4462 0.57 16.77 17.12

254 ? 1.81 16.47 16.62

Table 6: Light-curve parameters for eclipsing binaries and red variables identified in the field of 47 Tuc. Certain eclipsing systems and likely K giants belonging to the cluster are marked in the second column

Our sample of variables includes 13 RR Lyr stars. Table 5 (click here) lists basic characteristics of the light curves of these stars. The mean V magnitudes were calculated by numerically integrating the phased light curves after converting them into an intensity scale. Photometric data for the remaining variables are given in Table 6 (click here). The V-I colors listed in Tables 5 (click here) and 6 (click here) were measured at random phases. For each of fields we used a single exposure in the I band bracketed by two exposures in the V band. To determine the periods of identified variables we used an aov statistic (Schwarzenberg-Czerny 1989, 1991). This statistic allows - in particular - reliable determination of periods for variables with non-sinusoidal light curves (eg. eclipsing binaries). Phased light curves of RR Lyr stars are shown in Figs. 2 (click here) and 3 (click here) while Fig. 4 (click here) presents phased light curves for the remaining variables with determined periods. Time domain light curves for these variables for which we were unable to determine periods are shown in Fig. 5 (click here).

  
Figure 2: Phased V light curves for RR Lyr stars from the SMC. Inserted labels give the names of variables

  
Figure 3: Phased V light curve for the halo RR Lyr star OGLEGC223

  
Figure 4: Phased V light curves for the variables listed in Table 6 (click here). Inserted labels give the names of variables and their periods in days

 
Figure 4: continued

  
Figure 5: Time domain light curves for variables with unknown periods. Light curves for 1993 and 1994 seasons are shown for OGLEGC218

  
Figure 6: A schematic CMD for 47 Tuc with the positions of the variables from fields A-E marked. The triangles represent certain eclipsing binaries, the asterisks RR Lyr stars and the open circles the remaining variables. Positions of stars from Table 6 (click here) are labeled

Figure 6 (click here) shows the location of all variables with known colors on the cluster color-magnitude diagram (CMD). For the RR Lyr stars marked positions correspond to the intensity-averaged magnitudes. For the remaining variables we marked positions corresponding to the magnitude at maximum light. All but one RR Lyr stars are grouped around indicating that they belong to the SMC. RR Lyr variable OGLEGC223 is a background object in the galactic halo.

There are 12 certain eclipsing binaries in our sample of variables. This group of stars is dominated by contact binaries with EW-type light curves and periods shorter than 0.4 day. The only 3 stars whose light curves indicate a detached or semi-detached configuration are OGLEGC228, OGLEGC240 and OGLEGC253. OGLEGC240 is a detached binary with an EA-type light curve. The light curve of this variable is relatively noisy due to the faintness of the object. None the less examination of the individual frames leaves no doubts about the reality of the observed changes. The blue color and apparent magnitude of OGLEGC240 indicates that it is an A spectral type binary in the SMC.
OGLEGC228 shows a light curve typical of semi-detached binaries. This star is located among candidate blue-stragglers on the cluster CMD. OGLEGC253 is also a potential blue straggler. Its light curve shows two minima of very different depth but we cannot exclude possibility that the components of this binary are in geometrical contact. Several systems with light curves similar to the light curve of OGLEGC253 were analyzed during last decade (e.g. Hilditch et al. 1989). Although most of detected binaries are candidate blue stragglers, there are four contact systems located slightly to the red of the cluster main sequence. These four binaries are potential main sequence systems belonging to 47 Tuc. We shall return below to the question about membership of identified contact binaries.

Variables which could not be classified as either RR Lyr stars or eclipsing binaries are generally red stars with periods ranging from 2 days to several weeks. Six red variables which are located on or near the subgiant branch of 47 Tuc can be considered candidates for cluster members. Recently Edmonds & Gilliland (1996) reported discovery of low amplitude variability among a large fraction of K giants in 47 Tuc. Using the data collected with the HST they estimated that most of variable giants have periods between 2 and 4 days and V amplitudes in the range 5-25 mmag. Edmonds & Gilliliand (1996) argue that the observed variability of K giants from 47 Tuc is caused by low-overtone pulsations. The variable K giants from our sample have periods ranging from 2 to 36 days and show full amplitudes in the V band ranging from 0.08 to 0.18 mag. Based on the quality of our data we estimate conservatively that we should be able to detect any periodic variables among cluster giants with periods up to 2 weeks and full amplitudes exceeding 0.05 mag. We note that six candidates for variable K giants identified by us can easily be studied spectroscopically. Such observations would answer the question about the mechanism of observed photometric variability. Observed light variations are sufficiently large to imply detectable changes of if the variability is indeed due to pulsations.

Variables with V-I>1.1 and V>15.5 are likely to be evolved stars on the AGB in the SMC. We note that SMC stars can be easily distinguished from 47 Tuc members based on their radial velocities (heliocentric radial velocities of SMC and 47 Tuc are +175 km/s and -18.7 km/s, respectively).

We consider some of our period determinations as preliminary. Particularly, for OGLEGC229 we adopted P=8.38 d because the light curve seems to show two distinct minima. However, we cannot exclude the possibility that the correct period is in fact half this value. Also the period of OGLEGC240 can be half the adopted value of P=4.32 d. For P=2.16 d our light curve of OGLEGC240 would show just one detectable eclipse.

3.1. Cluster membership of the contact binaries

The 47 Tuc cluster is located at a hight galactic latitude of b=-45 deg. However, we cannot assume that all eclipsing binaries listed in Table 6 (click here) are cluster members. In particular, faint contact binaries with V>16 are known to occur at high galactic latitudes (e.g. Saha 1984). We have applied the absolute brightness calibration established by Rucinski (1995) to calculate M_V for the newly discovered contact binaries. Rucinski's calibration gives M_V as a function of period, unreddened color (V-I)₀ and metallicity:

We adopted for all systems and E(V-I)=0.05 (Harris 1996). Figure 7 (click here) shows the period versus an apparent distance modulus diagram for contact binaries identified in fields 104A-E. An apparent distance modulus was calculated for each system as a difference between its magnitude and . An apparent distance modulus for 47 Tuc is estimated at (m-M) _V=13.21 (Harris 1996). The only system with significantly deviating value of (m-M) _V is OGLEGC245. This binary is most probably a foreground variable. The remaining 8 systems plotted in Fig. 7 (click here) are likely members of the cluster.

  
Figure 7: Period vs. apparent distance modulus diagram for contact binaries from the field of 47 Tuc. A horizontal line at (m-M)_V=13.21 corresponds to the distance modulus of the cluster. Error bars correspond to the formal uncertainty in the absolute magnitudes derived using Rucinski's (1995) calibration

3.2. Completeness of the survey for contact binaries

Our survey resulted in the identification of 8 contact binaries which are likely members of the cluster and 2 detached/semidetached binaries which are possible blue stragglers belonging to the cluster. Only 4 contact systems were identified below the cluster turnoff. These numbers are surprisingly small considering that we analyzed the light curves of 76119 stars with average magnitudes V<19.5, mostly main sequence stars belonging to the cluster. For the clusters members the limiting magnitude V=19.5 corresponds to M _V=6.1. We adopted here (m-M) _V=13.4 for the apparent distance modulus of 47 Tuc (Hesser et al. 1987). The quality and quantity of photometry was sufficient to allow the detection of potential eclipsing binaries with periods shorter than 1 day and exhibiting eclipses deeper than about 0.3 mag (see Tables 1 (click here) and 3 (click here)).

A hint that our survey is quite complete with respect to faint short period variables comes from the fact that we detected 12 RR Lyr stars from the SMC. Graham (1975) searched for variables a field covering an area . His field was centered north of 47 Tuc and included a small part of the cluster. Graham identified 76 RR Lyr stars, with surface density of 0.016 variables per arcmin². The effective area covered by our survey was 935 arcmin² yelding surface density of RR Lyr stars of about 0.013 variables per arcmin². Apparently we did not miss in our survey too many RR Lyr stars from the SMC.

The relative frequency of occurrence of detectable contact binaries in our sample is . This frequency is more than an order of magnitude lower than the binary frequency observed for fields containing galactic open clusters (Kaluzny & Rucinski 1993; Mazur et al. 1995) and for fields located near the galactic center which were monitored by OGLE (Rucinski 1997). Recent surveys of globular clusters M 71 (Yan & Mateo 1994) and M 5 (Yan & Reed 1996) gave and , respectively.

To get a quantitative estimate of the completeness of our sample we performed tests with artificial variables for fields 104B and 104E. Results of test for field 104B should apply also to the fields 104A and 104C because all three fields contain similar numbers of measurable stars and were observed with comparable frequency. Similarly, results for field 104E should apply to field 104D. For both fields we selected 5 samples of objects from sets of stars whose light curves were examined for variability. The brightest sample included stars with 16.0<V<17.0 and the faintest sample included stars with 19.0<V<19.5. a total of 100 stars were selected at random from each sample. The observed light curves of these stars were then interlaced with the synthetic light curves of model contact binaries. The synthetic light curves were generated using a simple prescription given by Rucinski (1993). Two separate cases were considered. Case I - a contact binary with the inclination and the mass ratio q=0.10. Case II - a contact binary with the inclination and the mass ratio q=0.30. In both cases the so called "fill-out-parameter" was set to f=0.5. The light curves corresponding to Case-I and Case-II show depths of primary eclipses equal to 0.15 and 0.32 mag, respectively. For each of the artificially generated light curves a period was drawn in a random way from the range 0.2-0.45 d. Also the phase for the first point of the given light curve was randomly selected. The simulated light curves were then analysed in the manner as the observed light curves. Specifically, we applied a procedure based on the test. The number of artificial variables which were "recovered" for Cases I-II and 5 magnitude ranges is given in Table 7 (click here). It may be concluded that the completeness of our sample of contact binaries is better than 88% for systems with V<19.5 and depth of eclipses higher than 0.32 mag. For systems with full amplitudes as small as 0.15 mag the completeness is higher than 73% for V<19.0.

   

Range Field 104B Field 104B Field 104E Field 104E

of V Case-I Case-II Case-I Case-II

16.0-17.0 89 90 99 99

17.0-18.0 88 94 90 94

18.0-18.5 81 96 89 94

18.5-19.0 74 89 73 92

19.0-19.5 52 88 35 88

Table 7: Results of a test with artificial variables. Columns 2-5 give numbers of recovered variables. See text for details

It has been noted by Kaluzny et al. (1997c) that the frequency of occurrence of contact binaries in 47 Tuc is very low in comparison with open clusters and with several globular clusters which have been recently surveyed for eclipsing binaries by various groups. However, results presented here are based on a larger sample of stars than the sample analyzed by Kaluzny et al. (1997c). A more extended discussion of this topic is given in Kaluzny et al. (1997c). It is appropriate to note at this point that the low frequency of occurrence of contact binaries among 47 Tuc stars was first suggested by Shara et al. (1988).