A search for delta Scuti stars in northern open clusters

Transcription

1 Astron. Astrophys. 328, (1997) ASTRONOMY AND ASTROPHYSICS A search for delta Scuti stars in northern open clusters I. CCD photometry of NGC 7245, NGC 7062, NGC 7226 and NGC 7654 M. Viskum 1, M.M. Hernández 2, J.A. Belmonte 2, and S. Frandsen 1 1 Institute of Physics and Astronomy, University of Aarhus, DK-8000 Århus C, Denmark 2 Instituto de Astrofísica de Canarias, E La Laguna, Tenerife, Spain Received 7 April 1997 / Accepted 21 July 1997 Abstract. In an effort to test stellar structure and evolution models, analysis of oscillations in A and F stars in open clusters is carried out by the STACC network (Frandsen 1992). In this paper we describe our effort to locate a suitable open cluster in the northern hemisphere for a future multi-site campaign. We present BV Johnson and time series CCD photometry of four poorly studied open clusters: NGC 7245, NGC 7062, NGC 7226 and NGC New improved colour-magnitude diagrams for these four northern hemisphere clusters, together with the results from the search for variable stars in the clusters are presented. The cluster reddening, distance and age are estimated from isochrone fitting. In the four clusters, we discovered a total of two δ Scuti stars, one eclipsing binary, one variable of unknown type and evidence for 3 potential variables all situated within the δ Scuti instability strip. We find that the fraction of δ Scuti stars in these four open clusters is much lower than among field stars and in other open clusters suggesting that some additional parameters control the pulsation, parameters that we do not understand at this moment. Key words: open clusters HR diagrams stars: oscillations stars: δ Scuti stars: evolution 1. Introduction δ Scuti stars are becoming an important tool in the test of stellar structure and evolution theory (see e.g. Breger 1995; Goupil et al. 1993; Guzik & Bradley 1995; Pérez Hernández et al. 1995; Hernández et al. 1996). This is mainly because, in contrast to the classical variables like the Cepheids and RR Lyrae stars, δ Scuti stars are believed to be normal A and F main-sequence stars and many are multi-periodic pulsators. δ Scuti stars are situated on, or just above, the zero age main-sequence (ZAMS) within the classical instability strip. These stars have masses of Send offprint requests to: M. Viskum M and have convective cores. They are therefore important tools in obtaining information about properties of such cores, and related phenomena such as convective overshooting. Seismic studies of stars is in principle a simple task: One has to compare observed oscillations with theoretical oscillations in a model star. A stellar oscillation can be described in two steps: The measurement of the eigenfrequency of the oscillation and a classification of the oscillation in terms of quantum numbers: n, l and m. One must perform both steps for asteroseismology, but so far there are few convincing mode identifications available. This is mainly because δ Scuti stars have a dense theoretical eigenfrequency spectrum that is further complicated by rotation. Since only a small fraction of the possible eigenmodes oscillate with high enough amplitudes to be detected, we have so far only been able to detect frequencies but not been able to use those for asteroseismology. To overcome this, long multi-site campaigns are necessary. Acknowledging this, several networks have been founded to overcome the daily alias problem and increase the frequency resolution. STACC (Small Telescope Array with CCD Cameras) is such a network and is organized from IFA, University of Aarhus, Denmark (Frandsen 1992). STACC is focussed on open clusters. By observing open clusters with a CCD, we are able to observe all the stars at the same time, saving not only observing time, but also having the advantage of observing the stars under the same weather conditions, which enable us to perform high precision differential photometry. Another important advantage is that in open clusters, the same distance, age, initial chemical abundance and interstellar reddening can be assumed for all the stars, giving strong constraints on these parameters and on the mode identification of variable stars. In June 1993 STACC had its first campaign, where the target was the southern open cluster NGC The success of this campaign is described in detail by Frandsen et al. (1996). Seven δ Scuti stars were observed in NGC 6134, which makes this cluster an important target for testing stellar structure and evolution models. Because many observatories are situated north of the Equator, we have started a search for good targets in the northern

2 M. Viskum et al.: A search for δ Scuti stars in northern open clusters. I 159 hemisphere, i.e. a cluster containing several δ Scuti stars, like NGC The observations presented here are the first attempt to locate such a cluster and have been carried out with a single telescope. Detection of a suitable cluster will be followed up by an extensive STACC campaign. A suitable open cluster for δ Scuti star seismology observed with CCD s is a cluster with an age between Gyr and a distance between 1 2 Kpc. Younger clusters have only main sequence δ Scuti stars with very low amplitudes and therefore hard to locate and hard to obtain good precision for. Older clusters have turnoff below the instability strip. The distance limits assure that the angular size of the cluster on the sky fits into the field of view of the telescope/ccd system typically available and the field do not get too crowded. For a more detailed description of our definition of a suitable open cluster see Frandsen & Viskum (1995). During an observing run from Observatorio del Teide (Tenerife, Spain) in 1994, we obtained BV Johnson and time series CCD photometry of four open clusters. Below we present our results of these observations. For each cluster a CCD image of the field and a colour-magnitude diagram are presented together with a discussion of detected variables. 2. Observations The observations were carried out during August 1994 with the 80 cm telescope (IAC80) at Observatorio del Teide (Tenerife, Spain). A CCD camera with a 1024 x 1024 pixel Thomson chip was used. The image scale was per pixel, which gives a field of view of 7. 9x7. 9. Table 1 gives an overview of the observations. The first four nights were characterized by very unstable weather with dust and clouds. The seeing was in general poor ranging from 4 in the beginning of the observing run down to about 2 the last two nights. Further there was a problem with focusing the telescope therefore the star images shown here of the four clusters are somewhat fuzzy. 3 nights in the middle of the run were allocated to observations of the pulsating White Dwarf RXJ , as a part of a WET campaign. One night of time series measurements was obtained for NGC 7062, NGC 7226 and NGC In the case of NGC 7245, we obtained 3 nights. No filter was used in the time series observations. In addition to the time series observations, we carried out BV-photometry for all of the clusters during one night using standard V and B filters. 3. Data reductions 3.1. Photometric reductions The basic CCD reductions, such as bias subtraction and calibration for pixel sensitivity variations (flat-fielding), have been done using the IRAF CCD image reduction package CCDRED. After each observing night, twilight flat-field images were taken together with some bias frames. In the flat-fielding procedure, we used a nightly mean flat-field. The linearity of the CCD camera has not been measured. We have discovered several bad pixel Table 1. Log of the observations Date UT Object α δ N obs 6/8 00:28-04:32 NGC /8 23:08-04:55 NGC /8 22:53-05:33 NGC /8 22:31-05:35 NGC /8 23:00-06:05 BV 18 14/8 22:29-05:30 NGC /8 22:21-05:28 NGC columns on the CCD detector. We have not tried to correct for these bad pixels. Instead we simply do not use the photometry obtained from stars falling on or near these pixels. After the basic CCD reductions, we found the relative brightness of each star on the frame by using a CCD photometry package: MOMF (Kjeldsen & Frandsen 1992). MOMF has been developed especially to handle time series of CCD frames. The photometry in MOMF is a combined aperture/psf-fitting procedure, which gives low noise in moderately crowded fields Transformation to the standard system Because the weather during the whole observing run turned out to be non-photometric, it was impossible to use standard stars for the photometric calibration. Instead the instrumental B i and V i magnitudes have been transformed into standard Johnson B and V magnitudes using fitting coefficients (zero point and colour term) derived by comparing our observations with already published data. The transformations used are of the form: V = V i + a 0 + a 1 (B V ) and B = B i + c 0 + c 1 (B V ), where V, B and (B V ) refers to the magnitudes in the standard system while V i and B i are our instrumental magnitudes. Only for NGC 7245 we were able to compare our photometry with earlier CCD photometry. Due to the better intrinsic precision of CCD photometry we have determined the fitting coefficients a 0, a 1, c 0 and c 1 for NGC 7245 that permit us to transform our measurements to the standard system. These coefficients are then used for all the other clusters. A colour-magnitude diagram for each cluster is presented including the borders of the instability strip. These are taken from Breger (1979) transformed to the UBV system Frequency analysis In the search for variable stars, we start out by looking at a pulsation parameter d (Kjeldsen & Frandsen 1992), which is defined as the ratio between the total rms scatter of a given time series (σ rms ) and the internal scatter (σ int ): d = σ rms, (1) σ int where σ int is defined as: σ 2 int = N 1 1 (m i m i+1 ) 2, (2) 2 (N 1) i=1

3 160 M. Viskum et al.: A search for δ Scuti stars in northern open clusters. I where N is the number of data points in the time series, and m i is the magnitude of the star at the data point i. The internal scatter is a measurement of the point to point variation in the time series. If the timescale of a star s variability is much longer than the time between successively exposures, then σ int will be an estimate of photon- and scintillation noise and seeing effects, while the rms scatter will also contain instrumental drifts problems, slowly varying atmospheric effects and the variability of the star. Stars with a high d value are potential variables and for such stars we do a very detailed analysis of the time series. The d parameter is useful in the first selection of potential variables in a cluster containing several hundred stars. For all the stars within the instability strip we do a detailed analysis examining the light curve for any odd behaviour and the corresponding power spectrum is searched for significant oscillation peaks. The power spectrum is calculated by a least-squares sinewave fit to the time series using weighted data points (cf. Frandsen et al. 1995). We calculate the weight (w j =1/σj 2 ) for each data point from the deviation of that point from a local mean in the data: σ 2 j = 1 4 j+ i=j (m i m i 1 ) 2 + j+ i=j (m i m i+1 ) 2 (3) Here is a local scale (typical 10-20) and m i is the data point number i. This method assigns low weight to points that strongly deviate from the local mean. Using these weights was found to significantly reduce the noise level in the power spectrum. The power spectrum is normally calculated between 0 1 mhz, with an over-resolution factor of about 10. For a typical night of observations, we have around 6 hours of data, which gives a formal resolution of 46 µhz. The short time span of observations for each cluster does not allow any detailed frequency analysis or mode identification, but it does allow us to establish the timescale of variability Ages, distances and reddening The knowledge of the reddening, distances and ages for these clusters is limited. The reddening and the distance were determined by fitting the ZAMS given by Vandenberg (1985) to the shape of the cluster main-sequence. The cluster ages were then obtained by isochrone fitting to the cluster main-sequence using the convective overshooting and mass-loss models of Maeder & Meynet (1989) and Schaller et al. (1992). The program to compute isochrones from the evolutionary models was kindly provided by A. Maeder and G. Meynet. We have used standard evolutionary models with moderate convective overshooting (d over /H p =0.2) and standard mass-loss rate and, lacking of any knowledge of the cluster metallicity, we have only used isochrones with a solar (Z = 0.020) metal content. As discussed by Vandenberg & Poll (1989) the surface temperatures of the stellar models of Vandenberg (1985) are slightly too high due to problems with the model atmospheres used to provide the boundary conditions. In order to match the Maeder & Meynet isochrones, we have therefore shifted the ZAMS of Vandenberg Fig. 1. CCD frame of the cluster NGC The scale is per pixel. Also indicated are the four detected variables (1985) 0.06 mag. in (B V ). The errors given below on the determined ages and reddening are estimates based on a visual judgement of the fitting procedure. 4. Results 4.1. NGC 7245 Three nights of data were obtained for NGC 7245 giving a total of 21.5 hours of time series observations. NGC 7245 is fairly distant and not much is known about it. In the literature different estimates are given for the distance and the reddening. Yilmaz (1970) presented RGU photographic photometry and a colourmagnitude diagram, obtaining a distance of 1925 pc and E(G R) =0.68 corresponding to a value of E(B V )=0.60. Lang (1992) gives the value E(B V )=0.49 and a distance of 1900 pc for NGC Below, we show the selected CCD field (Fig. 1) and a colourmagnitude diagram (Fig. 2) of NGC The V and B V magnitudes are standard magnitudes calibrated using 13 stars in common between our data and the CCD photometry published by Petry & DeGioia-Eastwood (1992). We have procured the data by Petry & DeGioia-Eastwood (1992) from the database given by Mermilliod (1994). The standard deviation of the difference between the fitted magnitudes and the Petry & DeGioia- Eastwood magnitudes in the two filters are σ V =0.012 mag and σ B =0.037 mag. There is no indication of any higherorder colour terms. We have obtained photometry for 650 stars within the field of view. Some of these stars are field stars rather than members of the cluster which explains some of the scatter of the main-sequence band seen in Fig. 2. Also indicated are the approximate borders of the instability strip given by Breger (1979), and indicated by separated symbols are the detected variables discussed in the next paragraph. The main-sequence extends 6 magnitudes below the turn off point, located at V 13,

4 M. Viskum et al.: A search for δ Scuti stars in northern open clusters. I 161 NGC 7245 Fig. 2. Colour-magnitude diagram for NGC From the isochrone fitting we found E(B V ) = 0.40, a distance of 2800 pc and a log(age) = 8.5. Superimposed are the three isochrones with log(age) = 8.4, 8.5 and 8.6 all with solar metal content. The long-dashed line is the ZAMS given by Vandenberg (1985). The short-dashed lines are the borders of the instability strip given by Breger (1979). Also indicated in the diagram are the four detected variables: #493 is represented by a diamond; #456 by a square; #417 by a big black dot and #469 by a triangle Table 2. Detected variable stars in NGC 7245 ID V B-V Frequency Amplitude Type (µhz) (mmag) ? δ Scuti EB δ Scuti (B V ) 0.40, and the morphology resembles that given by Yilmaz (1970). There are a few red giant stars which can be used in the isochrone fitting. From the isochrone fitting the best fit is obtained with a reddening of E(B V ) = 0.40 ± 0.02, a true distance of 2800 pc±200 pc and an age of 320 Myr (log(age) = 8.5). Superimposed in Fig. 2 is also the log(age) =8.4 and 8.6 isochrones for comparison. We find a somewhat smaller reddening than Lang (1992), but we need a much larger distance to fit the data. Because our data covers a wide magnitude interval we believe that our distance and reddening estimates are more precise than earlier estimates. Furthermore, the two δ Scuti stars we find are within the instability strip, in agreement with the reddening and distance. Fig. 3. Light curve of star #456 in NGC This is a nice example of a multi-periodic δ Scuti star. More modes, than the two given in Table 2, may be present, but to establish this, further observations are needed with higher frequency resolution. The two modes detected have periods of 2.8 and 1.9 hours, respectively Variable stars in NGC 7245 Three nights of data of NGC 7245 and a formal resolution of 2.4 µhz give us a reasonable chance to detect even single modes. The mean error per data point for the nights Aug. 14 and Aug. 15 for the brightest stars is mag and for the brightest 200 stars it is mag. The scatter for the first night on NGC 7245 is a bit higher due to poor weather conditions. We have found evidence for two δ Scuti stars. Star #456 has a main peak at 99 µhz with an amplitude of 7.9 mmag. This was the first variable star to be detected because it has a d value of 2.3. In Fig. 3 we show the light curve of #456 and in Fig. 4 the corresponding power spectrum. In the colour-magnitude diagram #456 is found inside the instability strip. We therefore propose that this star is a main-sequence δ Scuti star. After prewhitening with the frequency at 99.2 µhz there is an indication of another peak at µhz with an amplitude of 4.1 mmag. Due to the complicated spectral window of these observations, we regard this peak as very tentative, but it is included in the list of detected variables (Table 2) for completeness. Another star (#493) which lies next to star #456 in the colour-magnitude diagram is also found to be variable. The pulsation parameter for this star is d =1.52 and we find that it has a main oscillation frequency at µhz. In Fig. 5 the power spectrum of #493 in the range from 0 1 mhz is given. The time series has been de-correlated with external parameters (seeing, x- and y-offsets on the CCD). A detailed discussion of our de-correlation procedure can be found in Frandsen et al. (1996). Besides the main peak, two bumps can be seen around 50 µhz and 315 µhz. After pre-whitening with the principal frequency the highest peak in the pre-whitened spectrum is at 315 µhz with an amplitude of 2.3 mmag. The mean noise level in the pre-whitened amplitude spectrum in the frequency range

5 162 M. Viskum et al.: A search for δ Scuti stars in northern open clusters. I Fig. 4. Power spectrum of star #456 in NGC 7245 for all three nights of data. The inset shows the power spectrum of the window function Fig. 6. Light curve of star #469 in NGC 7245, which we believe is an eclipsing binary Fig. 5. Power spectrum of star #493 in NGC 7245 for all three nights of data. The time series has been de-correlated with external parameters. The star has an oscillation frequency at µhz. See the text for more details Fig. 7. Phase plot of star #417 in NGC The position in the colour-magnitude diagram indicates that, if it is a cluster member, #417 is neither a δ Scuti star, nor an eclipsing binary. The star is most probably a background δ Scuti star of interest is 0.93 mmag. This gives a S/N =4.2 for the main peak and a S/N=2.5 for the peak at 315 µhz. We therefore conclude that the only significant oscillation mode in star #493 is the one at µhz. In Fig. 6 we show the light curve of a poorly sampled eclipsing binary. In the colour-magnitude diagram the star #469 is situated above the ZAMS, as expected for a normal eclipsing type binary. Further observations are needed to extract the period of this binary system. A phase plot of a fourth star, #417, is shown in Fig. 7. The light curve is well described by a single sinusoidal with a period of days and an amplitude of 0.01 mag. The nature of this light variation is not clear. From the shape of the light variation, this star could be an eclipsing binary of the W UMA type, but the position of #417 to the left of the main-sequence indicates rather that the star does not belong to the cluster but is a background reddened δ Scuti star. Radial velocity measurements or uvby-β photometry of this star could answer this question. None of the more evolved stars, nor any of the other stars within the instability strip, seem to oscillate with detectable amplitudes. From analysing the power spectra we find that the rest of the stars are stable at a detection level of 2-3 mmag. NGC 7245 has 69 stars within the instability strip. If we assume that most of these stars are cluster members then only a fraction of less than 2% are variables at this detection level. This incidence of variable stars in NGC 7245 is much lower than among field stars, where the incidence of variability within the instability strip is 30% with amplitudes of 0.01 mag. or more as discussed by Breger (1979) and Wolff (1983). Also, it is much less frequent than in other open clusters, where the incidence of variability is similar to field stars: In the Hyades cluster the incidence of variability is 37% (Horan 1979) at a 0.01 mag. detection level, and in the Praesepe cluster about 40% are variables with amplitudes above 2 mmag. Frandsen et al. (1996) found that 20% of the stars in NGC 6134 within the instability strip were variables at a detection level of 1-2 mmag.

6 M. Viskum et al.: A search for δ Scuti stars in northern open clusters. I 163 NGC 7062 Fig. 8. CCD frame of the cluster NGC The position of the detected variable is given by its number. The stars are somewhat fuzzy due to problems with focusing the telescope and poor seeing 4.2. NGC 7062 NGC 7062 is a relatively well studied cluster. UBV photometric data have been given both by Hoag et al. (1961) and Hassan (1973). Strömgren photoelectric photometry has been carried out by Peniche et al. (1990), while Peniche et al. (1987) have performed a photometric study of short periodic variables in NGC The fundamental parameters for NGC 7062 have been discussed by several authors: Hagen (1970) gives a reddening of 0.25 mag. while Becker & Fenkart (1971) have derived the value 0.45 mag from ZAMS fitting. Hassan (1973) found a distance of 1786 pc, E(B V )=0.48 and an age of 7.5x10 8 yr for NGC Peniche et al. (1990) determined E(B V )=0.456, a distance modulus of and the age to be 7.0 x 10 8 yr. In Fig. 8 the CCD field of NGC 7062 is shown. Photometry for 150 stars in the field of view have been measured. A colourmagnitude diagram of these, can be seen in Fig. 9. We have compared our photometry with that of Hassan (1973) from 20 stars in common between the two data sets. The rms scatter of the residuals is σ V = mag and σ B = mag. The higher scatter is reflecting the high intrinsic uncertainty in Hassan s photographic data. There is a considerable scatter in the main-sequence for NGC 7062, which could be due to unresolved binaries and field stars. There is a clear turn-off at V 13, (B V ) 0.6, which can be used in the cluster age determination. From the ZAMS and isochrone fitting we find that a value of E(B V )=0.47±0.03, a distance of 1800±400 pc and an age of 500 Myr (log(age) =8.7) fit the data for NGC 7062 best. This age and reddening are in good agreement with earlier results. Our distance to NGC 7062 agrees well with Hassan (1973) but is somewhat lower than the distance of 2726 pc found by Peniche Fig. 9. Colour-magnitude diagram of NGC From the isochrone fitting we find E(B V )=0.47, a distance of 1800 pc and the age of NGC 7062 to be 500 Myr (log(age) =8.7). Superimposed are the three isochrones with log(age) = 8.6, 8.7 and 8.8 all calculated for solar metal content. The long-dashed line is the ZAMS given by Vandenberg (1985). The short-dashed lines are the borders of the instability strip given by Breger (1979). Also indicated is the detected variable star #180 represented as a square et al. (1990). Peniche et al. (1990) determined the distance to NGC 7062 from the distance modulus of only 20 stars where a normal distribution was fitted, resulting in a mean distance modulus of mag with a standard deviation of 1.14 mag. So our result is within the uncertainty given by Peniche et al. (1990) and because our results are based on a much larger data sample we believe that our distance estimate is more precise Variable stars in NGC 7062 The data obtained for NGC 7062 have a fairly high noise level due to bad weather and only 55 data points were obtained. Therefore, the present observations of this cluster can only indicate potential variables. The stars with best photometry have rms scatter of mmag but it is in general higher. Only one star has been detected with visible periodic variation of the light curve. Star #180 is a V =14.01, (B V )=0.81 magnitude star situated in the middle of the instability strip with a d =1.46 and a photometric variation of the order 0.01 mag., so it is a potential δ Scuti variable. The light curve is shown in Fig. 10. Peniche et al. (1990) discussed 3 potential variables in NGC Their ID80, ID13 and ID17 had dispersions in the V magnitude of order 2 3%. We have identified ID17 and ID13 as our #292 and #217, respectively. ID80 is not in our field. The stars #292 and #217 have rms scatter of mag and mag, respectively, and show no obvious sign of variability. We

7 164 M. Viskum et al.: A search for δ Scuti stars in northern open clusters. I Fig. 10. Light curve for star #180 in NGC 7062 NGC 7226 Fig. 12. Colour-magnitude diagram of NGC From the isochrone fitting we find E(B V )=0.47, a distance of 2500 pc and the age of NGC 7226 to be 500 Myr (log(age) =8.7). Superimposed are the three isochrones with log(age) = 8.6, 8.7 and 8.8 all calculated for solar metal content. The long-dashed line is the ZAMS given by Vandenberg (1985). The short-dashed lines are the borders of the instability strip given by Breger (1979) Fig. 11. CCD frame of the cluster NGC 7226 therefore conclude, that these two stars are stable, at least, at the mmag level NGC 7226 NGC 7226 is a poorly studied cluster. The latest available published photometry for this cluster is that of Yilmaz (1970) who had obtained photographic RGU photometry of 160 stars and found a distance of 2240 pc. He also calculated the colour excess E(G R) =0.68 which corresponds to an E(B V )=0.49 using a transformation equation given by Steinlin (1968). In Fig. 11 a CCD frame of the field is shown. We have compared our photometry with photometry obtained by Yilmaz (1970) by using 25 stars in common between the two data sets. The standard deviation of the residuals for V and B magnitudes are σ V =0.143 mag. and σ B =0.134 mag. respectively. These residuals are much higher than expected, which we believe is due to the high intrinsic scatter in Yilmaz s photographic data and due to the transformation from the RGU system to the BV R system, where we have used the transformation equations given by Steinlin (1968). The colour-magnitude diagram of NGC 7226 is shown in Fig. 12. We have obtained photometry for 500 stars within the field of view. The cluster main-sequence extends almost 6 mag. below the turn-off point, located at V 13.5, (B V ) 0.50, and is contaminated by many field stars. We find the reddening to E(B V )=0.47 ± 0.03 and a distance of 2500 ± 400 pc. The uncertainties in these two numbers are quite high due to the larger scatter around the cluster main-sequence. The best fit is obtained by an 5x10 8 yr isochrone (log(age) =8.7), but the log(age) = 8.6 and log(age) = 8.8 isochrones also fit the data quite well Variable stars in NGC 7226 We have examined the light curves of all stars down to V 16 and all 68 stars within the instability strip and no variables were found. We therefore conclude that all the stars within the instability strip for this cluster are stable at a 2-3 mmag. level NGC 7654 (M 52) NGC 7654 is a cluster containing many stars and having a fairly dense core with some very bright B-stars. The bright B-stars constitute a problem, because these stars saturate the CCD when we try to detect the fainter stars. As evident from the CCD image (Fig. 13), this saturation affects the photometry of stars close to the B-stars. The cluster appears in a region of strong interstellar absorption. Danford & Thomas (1981) have done uvby-β photometry on NGC 7654 and found that it has nonuniform

8 M. Viskum et al.: A search for δ Scuti stars in northern open clusters. I 165 NGC 7654 Fig. 13. CCD frame of the cluster NGC Also shown are the positions of the two detected variables. See the text for further details reddening across the cluster with an colour excess of up to 0.11 for stars in the northern half of the cluster compared to stars of the southern half. They found the mean reddening to be E(B V )=0.57, a distance of 1470 pc and an age of 65 Myr. Harris (1976) estimated the age of NGC 7654 to be 25 Myr. We present here the first CCD photometry done on this cluster. We have obtained precise photometry for 360 stars in the field of view (Fig. 13). A colour-magnitude diagram of the cluster is shown in Fig. 14. The best fit to the cluster main-sequence gives a value of the colour excess E(B V )=0.58 ± 0.02 and a distance of 1400 ± 200 pc. This is consistent with the values found by Danford & Thomas (1981). The best fit to the turn-off region is a 158 Myr isochrone (log(age) =8.2). The age of the cluster found by us is much higher than given by other authors. To investigate this discrepancy the photometry of bright stars given by Bouigue (1959) and Hoag et al. (1961) are also shown in Fig. 14. We have procured the data by Bouigue (1959) and Hoag et al. (1961) from the database given by Mermilliod (1994). The stars measured by Bouigue (1959) are bright stars from a larger field than ours on the sky, but they are all classified as cluster members in the database by Mermilliod (1994). One clearly sees that some of the bright stars lie above the plotted isochrones. We have tried other isochrones with much younger ages, but none of them can fit the bright stars together with the rest of the cluster main-sequence. Further, our isochrones clearly follow a turn-off of stars. Some of the bright stars could be blue stragglers, but most of them are too evolved. So our conclusion is that we see an older cluster behind some younger stars, perhaps also constituting a small cluster. This interpretation should of cause be confirmed by further observations. Fig. 14. Colour-magnitude diagram for NGC The filled circles are our data, the small diamonds are data from Bouigue (1959) while the crosses are data given by Hoag et al. (1961). See the text for further details about the data points. Also two potential variables are indicated: Star # 501 as a square and star #215 as a big diamond. We find NGC 7654 to have a E(B V )=0.58 and a distance of 1400 pc. The best fit is obtained with an isochrone of log(age) =8.2. Superimposed are three isochrones with log(age) = 8.1, 8.2 and 8.3 respectively. The long-dashed line is the ZAMS given by Vandenberg (1985). The short-dashed lines are the borders of the instability strip given by Breger (1979) Variable stars in NGC 7654 We have found two potential variables as shown in Fig. 15a and b: Star #215 has a position in the colour-magnitude diagram at the main-sequence within the instability strip and has V = 14.95, (B V )=0.84, d =5.1 and σ int = mag. The other potential variable (#501) has V =14.37, (B V )=0.72, d =2.1, σ int = and is also a main-sequence star near the hot border of the instability strip. Thus both stars could be δ Scuti variables. 5. Summary of results New improved CCD photometry have been obtained for the four intermediate age open clusters: NGC 7245, NGC 7062, NGC 7226, NGC 7654 and calibrated colour-magnitude diagrams are presented. These observations were intended as a search for open cluster containing variable stars, especially δ Scuti stars. We have found one good target for a future campaign, namely NGC In NGC 7245 we have detected two δ Scuti variables, one eclipsing binary and one other variable of unknown type. In NGC 7062 only one potential variable was found. The time series measurements of NGC 7062 were too noisy to look for low amplitude variables, but we can conclude that NGC 7062 does not contain variable stars with amplitudes

9 166 M. Viskum et al.: A search for δ Scuti stars in northern open clusters. I References Fig. 15a and b. Light curves for two potential variables in NGC Both stars are situated within the instability strip near the main-sequence and could therefore be δ Scuti stars Table 3. Resulting basic parameters for the four clusters Object E(B V ) distance age (pc) (Myr) NGC NGC NGC NGC higher than about 0.01 mag. NGC 7226 was found not to contain variables with amplitudes above the detection limit ( mag). NGC 7654 was found to have two potential variables, both good δ Scuti star candidates. This incidence of variability in these four clusters is much smaller than usual seen among field stars and in other extensive studied open clusters such as the Praesepe cluster, the Hyades cluster and NGC In Table 3 we give a summary of the basic parameters for NGC 7062, NGC 7226, NGC 7245 and NGC 7654 obtained in this study. Acknowledgements. We would like to thank the staff at the IAC80 Telescope for their assistance during the observations. We are grateful to H. Kjeldsen for his support and advice throughout this work, and to T. Bedding for his useful comments on the manuscript. A. Maeder and P. Meynet are thanked for providing us with their evolutionary code and the program used in the cluster age determinations. M. Viskum acknowledges financial support from the Carlsberg foundation and from the Instituto de Astrofísica de Canarias, on behalf of the ANTENA Network, of the E.U. This research has made use of the Simbad database, operated at CDS, Strasbourg, France. Becker, W., Fenkart, R. 1971, A&AS, 4, 241 Bouigue, R. 1959, Ann. Obs. Toulouse 27, 47 Breger, M. 1979, PASP, 91, 5 Breger, M. 1995, in: Astrophysical Applications of Stellar Pulsation, R.S. Stobie & P.A. Whitelock (eds.), ASP Conference Series, Vol. 83, p. 70 Danford, S.C., Thomas, J. 1981, PASP, 93, 447 Frandsen, S. 1992, Delta Scuti Star Newsletter, M. Breger (ed.), Wien, Austria, 5, 12 Frandsen, S., Balona, L.A., Viskum, M., Koen, C., Kjeldsen, H. 1996, A&A, 308, 132 Frandsen, S., Jones, A., Kjeldsen, H., Viskum, M., Hjorth, J., Andersen, N.H., Thomsen, B. 1995, A&A, 301, 123 Frandsen, S., Viskum, M. 1995, in: Astrophysical Applications of Stellar Pulsation, R.S. Stobie & P.A. Whitelock (eds.), ASP Conference Series, Vol. 83, p. 327 Guzik, J.A., Bradley, P.A. 1995, Baltic Astronomy, vol. 4., 442 Goupil, M.J., Michel, E., Lebreton, Y., Baglin, A. 1993, A&A, 268, 546 Hagen, G.L. 1970, An Atlas of Open Cluster Colour-Magnitude diagrams, Publ. David Dunlap Obs., 4, 1-62 Harris, G.L.H. 1976, ApJS, 30, 451 Hassan, S.M. 1973, A&AS, 9, 261 Hernández, M.M., Pérez Hernández, F., Michel, E., Belmonte, J.A., Goupil, M.J., Lebreton, Y. 1996, in: Sounding solar and stellar interiors, IAU Symposium 181, (in press) Hoag, A.A., Johnson, H.L., Iriarte, B., Mitchell, R.I., Hallam, K.L., Sharpless, S. 1961, in: Publications of the United States Naval Observatory, Second Series, Vol. XVII, Part VII Horan, S. 1979, AJ, 84, 1770 Kjeldsen, H., Frandsen, S. 1992, PASP, 104, 413 Lang, K.R. 1992, Astrophysical Data: Planets and Stars, Springer- Verlag Maeder, A., Meynet, G. 1989, A&A 210, 155 Mermilliod, J.-C. 1994, Open Cluster Data Base, Version 2.0, Technical report, Institut d Astronomie de L Université de Lausanne Peniche, R., Peña, J.H. 1987, Rev. Mex. Astron. Astrofis., 14, 420 Peniche, R., Peña, J.H., Díaz Martínez, S.H., Gómez, T. 1990, Rev. Mex. Astron. Astrofis., 20, 127 Pérez Hernández, F., Claret, A., Belmonte, J.A. 1995, A&A, 295, 113 Petry, C.E., DeGioia-Eastwood, K. 1992, BAAS, 24, 1235 Schaller, G., Schaerer, D., Meynet, G., Maeder, A. 1992, A&AS 96, 269 Steinlin, U. 1968, Zeitschrift für Astrophysik 69, 276 Vandenberg, D.A. 1985, ApJS, 58, 711 Vandenberg, D.A., Poll, H.E. 1989, AJ, 98, 1451 Wolff, S.C. 1983, The A-Stars: Problems and Perspectives, NASA SP- 463, p. 93 Yilmaz, F. 1970, A&A, 8, 213 This article was processed by the author using Springer-Verlag LaT E X A&A style file L-AA version 3.