a search for delta scuti stars in northern open...

Astron. Astrophys. 328, 158–166 (1997) ASTRONOMYAND

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. Viskum1, M.M. Hernandez2, J.A. Belmonte2, and S. Frandsen1

1 Institute of Physics and Astronomy, University of Aarhus, DK-8000 Arhus C, Denmark2 Instituto de Astrofısica de Canarias, E-38200 La Laguna, Tenerife, Spain

Received 7 April 1997 / Accepted 21 July 1997

Abstract. In an effort to test stellar structure and evolutionmodels, analysis of oscillations in A and F stars in open clus-ters is carried out by the STACC network (Frandsen 1992).In this paper we describe our effort to locate a suitable opencluster in the northern hemisphere for a future multi-site cam-paign. We present BV Johnson and time series CCD photometryof four poorly studied open clusters: NGC 7245, NGC 7062,NGC 7226 and NGC 7654. New improved colour-magnitudediagrams for these four northern hemisphere clusters, togetherwith the results from the search for variable stars in the clus-ters are presented. The cluster reddening, distance and age areestimated from isochrone fitting.

In the four clusters, we discovered a total of two δ Scutistars, one eclipsing binary, one variable of unknown type andevidence for 3 potential variables all situated within the δ Scutiinstability strip. We find that the fraction of δ Scuti stars in thesefour open clusters is much lower than among field stars and inother open clusters suggesting that some additional parameterscontrol the pulsation, parameters that we do not understand atthis 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 stellarstructure and evolution theory (see e.g. Breger 1995; Goupil etal. 1993; Guzik & Bradley 1995; Perez Hernandez et al. 1995;Hernandez et al. 1996). This is mainly because, in contrast to theclassical variables like the Cepheids and RR Lyrae stars, δ Scutistars are believed to be normal A and F main-sequence stars andmany are multi-periodic pulsators. δ Scuti stars are situated on,or just above, the zero age main-sequence (ZAMS) within theclassical instability strip. These stars have masses of 1.5− 2.5

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M� and have convective cores. They are therefore importanttools 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 hasto compare observed oscillations with theoretical oscillations ina model star. A stellar oscillation can be described in two steps:The measurement of the eigenfrequency of the oscillation and aclassification of the oscillation in terms of quantum numbers: n,l and m. One must perform both steps for asteroseismology, butso far there are few convincing mode identifications available.This is mainly because δ Scuti stars have a dense theoreticaleigenfrequency spectrum that is further complicated by rotation.Since only a small fraction of the possible eigenmodes oscillatewith high enough amplitudes to be detected, we have so far onlybeen able to detect frequencies but not been able to use those forasteroseismology. To overcome this, long multi-site campaignsare necessary. Acknowledging this, several networks have beenfounded to overcome the daily alias problem and increase thefrequency resolution.

STACC (Small Telescope Array with CCD Cameras) is sucha network and is organized from IFA, University of Aarhus,Denmark (Frandsen 1992). STACC is focussed on open clus-ters. By observing open clusters with a CCD, we are able toobserve all the stars at the same time, saving not only observingtime, but also having the advantage of observing the stars underthe same weather conditions, which enable us to perform highprecision differential photometry. Another important advantageis that in open clusters, the same distance, age, initial chemicalabundance and interstellar reddening can be assumed for all thestars, giving strong constraints on these parameters and on themode identification of variable stars.

In June 1993 STACC had its first campaign, where the tar-get was the southern open cluster NGC 6134. The success ofthis campaign is described in detail by Frandsen et al. (1996).Seven δ Scuti stars were observed in NGC 6134, which makesthis cluster an important target for testing stellar structure andevolution models.

Because many observatories are situated north of the Equa-tor, we have started a search for good targets in the northern

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, likeNGC 6134. The observations presented here are the first attemptto locate such a cluster and have been carried out with a singletelescope. Detection of a suitable cluster will be followed up byan extensive STACC campaign.

A suitable open cluster for δ Scuti star seismology observedwith CCD’s is a cluster with an age between ∼ 0.3 − 1.0 Gyrand a distance between ∼ 1 − 2 Kpc. Younger clusters haveonly main sequence δ Scuti stars with very low amplitudes andtherefore hard to locate and hard to obtain good precision for.Older clusters have turnoff below the instability strip. The dis-tance limits assure that the angular size of the cluster on the skyfits into the field of view of the telescope/CCD system typicallyavailable and the field do not get too crowded. For a more de-tailed description of our definition of a suitable open cluster seeFrandsen & Viskum (1995).

During an observing run from Observatorio del Teide(Tenerife, Spain) in 1994, we obtained BV Johnson and timeseries CCD photometry of four open clusters. Below we presentour results of these observations. For each cluster a CCD im-age of the field and a colour-magnitude diagram are presentedtogether with a discussion of detected variables.

2. Observations

The observations were carried out during August 1994 with the80 cm telescope (IAC80) at Observatorio del Teide (Tenerife,Spain). A CCD camera with a 1024 x 1024 pixel Thomson chipwas used. The image scale was 0.′′46 per pixel, which gives afield of view of 7.′9 x 7.′9. Table 1 gives an overview of theobservations. The first four nights were characterized by veryunstable weather with dust and clouds. The seeing was in gen-eral poor ranging from 4′′ in the beginning of the observingrun down to about 2′′ the last two nights. Further there was aproblem with focusing the telescope therefore the star imagesshown here of the four clusters are somewhat fuzzy. 3 nightsin the middle of the run were allocated to observations of thepulsating White Dwarf RXJ 2117.1+3412, as a part of a WETcampaign.

One night of time series measurements was obtainedfor NGC 7062, NGC 7226 and NGC 7654. In the case ofNGC 7245, we obtained 3 nights. No filter was used in the timeseries observations. In addition to the time series observations,we carried out BV-photometry for all of the clusters during onenight using standard V and B filters.

3. Data reductions

3.1. Photometric reductions

The basic CCD reductions, such as bias subtraction and cali-bration for pixel sensitivity variations (flat-fielding), have beendone using the IRAF CCD image reduction package CCDRED.After each observing night, twilight flat-field images were takentogether with some bias frames. In the flat-fielding procedure,we used a nightly mean flat-field. The linearity of the CCD cam-era has not been measured. We have discovered several bad pixel

Table 1. Log of the observations

Date UT Object α1994.6 δ1994.6 Nobs

6/8 00:28-04:32 NGC 7062 21 23.5 +46 21 557/8 23:08-04:55 NGC 7226 22 10.3 +55 22 818/8 22:53-05:33 NGC 7245 22 14.9 +54 16 90

12/8 22:31-05:35 NGC 7654 23 24.5 +61 34 10413/8 23:00-06:05 BV — — 1814/8 22:29-05:30 NGC 7245 22 14.9 +54 16 11015/8 22:21-05:28 NGC 7245 22 14.9 +54 16 116

columns on the CCD detector. We have not tried to correct forthese bad pixels. Instead we simply do not use the photometryobtained from stars falling on or near these pixels.

After the basic CCD reductions, we found the relative bright-ness of each star on the frame by using a CCD photometry pack-age: MOMF (Kjeldsen & Frandsen 1992). MOMF has been de-veloped especially to handle time series of CCD frames. Thephotometry in MOMF is a combined aperture/PSF-fitting pro-cedure, which gives low noise in moderately crowded fields.

3.2. Transformation to the standard system

Because the weather during the whole observing run turned outto be non-photometric, it was impossible to use standard starsfor the photometric calibration. Instead the instrumentalBi andVi magnitudes have been transformed into standard JohnsonB and V magnitudes using fitting coefficients (zero point andcolour term) derived by comparing our observations with al-ready published data. The transformations used are of the form:V = Vi + a0 + a1 · (B − V ) and B = Bi + c0 + c1 · (B − V ),whereV ,B and (B−V ) refers to the magnitudes in the standardsystem while Vi and Bi are our instrumental magnitudes. Onlyfor NGC 7245 we were able to compare our photometry withearlier CCD photometry. Due to the better intrinsic precisionof CCD photometry we have determined the fitting coefficientsa0, a1, c0 and c1 for NGC 7245 that permit us to transform ourmeasurements to the standard system. These coefficients arethen used for all the other clusters.

A colour-magnitude diagram for each cluster is presentedincluding the borders of the instability strip. These are takenfrom Breger (1979) transformed to the UBV system.

3.3. Frequency analysis

In the search for variable stars, we start out by looking at a“pulsation parameter” d (Kjeldsen & Frandsen 1992), which isdefined as the ratio between the total rms scatter of a given timeseries (σrms) and the internal scatter (σint):

d =σrms

σint, (1)

where σint is defined as:

σ2int =

12 · (N − 1)

N−1∑i=1

(mi −mi+1)2 , (2)

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, andmi is the magnitude of the star at the data point i. The internalscatter is a measurement of the point to point variation in thetime series. If the timescale of a star’s variability is much longerthan the time between successively exposures, then σint will bean estimate of photon- and scintillation noise and seeing effects,while the rms scatter will also contain instrumental drifts prob-lems, slowly varying atmospheric effects and the variability ofthe star. Stars with a high d value are potential variables and forsuch stars we do a very detailed analysis of the time series. Thed parameter is useful in the first selection of potential variablesin a cluster containing several hundred stars. For all the starswithin the instability strip we do a detailed analysis examiningthe light curve for any odd behaviour and the correspondingpower spectrum is searched for significant oscillation peaks.

The power spectrum is calculated by a least-squares sine-wave fit to the time series using weighted data points (cf. Frand-sen et al. 1995). We calculate the weight (wj = 1/σ2

j ) for eachdata point from the deviation of that point from a local mean inthe data:

σ2j =

14∆

j+∆∑i=j−∆

(mi −mi−1)2 +j+∆∑

i=j−∆

(mi −mi+1)2

(3)

Here ∆ is a local scale (typical 10-20) and mi is the data pointnumber i. This method assigns low weight to points that stronglydeviate from the local mean. Using these weights was found tosignificantly reduce the noise level in the power spectrum.

The power spectrum is normally calculated between 0–1mHz, with an over-resolution factor of about 10. For a typicalnight of observations, we have around 6 hours of data, whichgives a formal resolution of 46 µHz . The short time span ofobservations for each cluster does not allow any detailed fre-quency analysis or mode identification, but it does allow us toestablish the timescale of variability.

3.4. Ages, distances and reddening

The knowledge of the reddening, distances and ages for theseclusters is limited. The reddening and the distance were deter-mined by fitting the ZAMS given by Vandenberg (1985) to theshape of the cluster main-sequence. The cluster ages were thenobtained by isochrone fitting to the cluster main-sequence usingthe convective overshooting and mass-loss models of Maeder& Meynet (1989) and Schaller et al. (1992). The program tocompute isochrones from the evolutionary models was kindlyprovided by A. Maeder and G. Meynet. We have used stan-dard evolutionary models with moderate convective overshoot-ing (dover/Hp = 0.2) and standard mass-loss rate and, lackingof any knowledge of the cluster metallicity, we have only usedisochrones with a solar (Z = 0.020) metal content. As discussedby Vandenberg & Poll (1989) the surface temperatures of thestellar models of Vandenberg (1985) are slightly too high dueto problems with the model atmospheres used to provide theboundary conditions. In order to match the Maeder & Meynetisochrones, we have therefore shifted the ZAMS of Vandenberg

Fig. 1. CCD frame of the cluster NGC 7245. The scale is 0.”46 perpixel. Also indicated are the four detected variables

(1985) 0.06 mag. in (B − V ). The errors given below on thedetermined ages and reddening are estimates based on a visualjudgement of the fitting procedure.

4. Results

4.1. NGC 7245

Three nights of data were obtained for NGC 7245 giving a totalof 21.5 hours of time series observations. NGC 7245 is fairlydistant and not much is known about it. In the literature differentestimates are given for the distance and the reddening. Yilmaz(1970) presented RGU photographic photometry and a colour-magnitude diagram, obtaining a distance of 1925 pc andE(G−R) = 0.68 corresponding to a value of E(B − V ) = 0.60. Lang(1992) gives the valueE(B−V ) = 0.49 and a distance of 1900pc for NGC 7245.

Below, we show the selected CCD field (Fig. 1) and a colour-magnitude diagram (Fig. 2) of NGC 7245. The V and B − Vmagnitudes are standard magnitudes calibrated using 13 stars incommon between our data and the CCD photometry publishedby Petry & DeGioia-Eastwood (1992). We have procured thedata by Petry & DeGioia-Eastwood (1992) from the databasegiven by Mermilliod (1994). The standard deviation of the dif-ference between the fitted magnitudes and the Petry & DeGioia-Eastwood magnitudes in the two filters are σV = 0.012 magand σB = 0.037 mag. There is no indication of any higher-order colour terms. We have obtained photometry for 650 starswithin the field of view. Some of these stars are field stars ratherthan members of the cluster which explains some of the scatterof the main-sequence band seen in Fig. 2. Also indicated arethe approximate borders of the instability strip given by Breger(1979), and indicated by separated symbols are the detected vari-ables discussed in the next paragraph. The main-sequence ex-tends 6 magnitudes below the turn off point, located at V ' 13,


NGC 7245

Fig. 2. Colour-magnitude diagram for NGC 7245. From the isochronefitting we found E(B − V ) = 0.40, a distance of 2800 pc anda log(age) = 8.5. Superimposed are the three isochrones withlog(age) = 8.4, 8.5 and 8.6 all with solar metal content. Thelong-dashed line is the ZAMS given by Vandenberg (1985). Theshort-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 bigblack dot and #469 by a triangle

Table 2. Detected variable stars in NGC 7245

ID V B-V Frequency Amplitude Type(µHz) (mmag)

417 17.34 0.77 51 98 ?

456 14.70 0.55 99.2 7.9 δ Scuti143.7 4.1

469 15.55 0.90 EB

493 14.89 0.55 146.0 3.9 δ Scuti

(B − V ) ' 0.40, and the morphology resembles that given byYilmaz (1970). There are a few red giant stars which can beused in the isochrone fitting.

From the isochrone fitting the best fit is obtained with areddening of E(B − V ) = 0.40 ± 0.02, a true distance of2800 pc±200 pc and an age of 320 Myr (log(age) = 8.5). Super-imposed in Fig. 2 is also the log(age) = 8.4 and 8.6 isochronesfor comparison.

We find a somewhat smaller reddening than Lang (1992),but we need a much larger distance to fit the data. Because ourdata covers a wide magnitude interval we believe that our dis-tance and reddening estimates are more precise than earlier esti-mates. Furthermore, the two δ Scuti stars we find are within theinstability strip, in agreement with the reddening and distance.

Fig. 3. Light curve of star #456 in NGC 7245. This is a nice exampleof a multi-periodic δ Scuti star. More modes, than the two given inTable 2, may be present, but to establish this, further observations areneeded with higher frequency resolution. The two modes detected haveperiods of 2.8 and 1.9 hours, respectively

4.1.1. Variable stars in NGC 7245

Three nights of data of NGC 7245 and a formal resolutionof 2.4 µHz give us a reasonable chance to detect even singlemodes. The mean error per data point for the nights Aug. 14and Aug. 15 for the brightest stars is 0.002 mag and for thebrightest 200 stars it is 0.007 mag. The scatter for the first nighton NGC 7245 is a bit higher due to poor weather conditions.We have found evidence for two δ Scuti stars. Star #456 hasa main peak at 99 µHz with an amplitude of 7.9 mmag. Thiswas the first variable star to be detected because it has a d valueof 2.3. In Fig. 3 we show the light curve of #456 and in Fig. 4the corresponding power spectrum. In the colour-magnitude di-agram #456 is found inside the instability strip. We thereforepropose that this star is a main-sequence δ Scuti star. After pre-whitening with the frequency at 99.2 µHz there is an indicationof another peak at 143.7 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 thelist of detected variables (Table 2) for completeness.

Another star (#493) which lies next to star #456 in thecolour-magnitude diagram is also found to be variable. Thepulsation parameter for this star is d = 1.52 and we find thatit has a main oscillation frequency at 146.0 µHz . In Fig. 5 thepower 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 discussionof our de-correlation procedure can be found in Frandsen et al.(1996). Besides the main peak, two bumps can be seen around50 µHz and 315 µHz . After pre-whitening with the principalfrequency the highest peak in the pre-whitened spectrum is at315 µHz with an amplitude of 2.3 mmag. The mean noise levelin the pre-whitened amplitude spectrum in the frequency range


Fig. 4. Power spectrum of star #456 in NGC 7245 for all three nightsof data. The inset shows the power spectrum of the window function

Fig. 5. Power spectrum of star #493 in NGC 7245 for all three nights ofdata. The time series has been de-correlated with external parameters.The star has an oscillation frequency at 146.0 µHz . See the text formore details

of interest is 0.93 mmag. This gives a S/N = 4.2 for the mainpeak and a S/N=2.5 for the peak at 315 µHz . We therefore con-clude that the only significant oscillation mode in star #493 isthe one at 146.0 µHz .

In Fig. 6 we show the light curve of a poorly sampled eclips-ing binary. In the colour-magnitude diagram the star #469 is sit-uated above the ZAMS, as expected for a normal eclipsing typebinary. Further observations are needed to extract the period ofthis binary system.

A phase plot of a fourth star, #417, is shown in Fig. 7. Thelight curve is well described by a single sinusoidal with a periodof 0.2271 days and an amplitude of 0.01 mag. The nature of thislight variation is not clear. From the shape of the light variation,this star could be an eclipsing binary of the W UMA type, but theposition of #417 to the left of the main-sequence indicates ratherthat the star does not belong to the cluster but is a backgroundreddened δ Scuti star. Radial velocity measurements or uvby-βphotometry of this star could answer this question.

Fig. 6. Light curve of star #469 in NGC 7245, which we believe is aneclipsing binary

Fig. 7. Phase plot of star #417 in NGC 7245. The position in thecolour-magnitude diagram indicates that, if it is a cluster member, #417is neither a δ Scuti star, nor an eclipsing binary. The star is most prob-ably a background δ Scuti star

None of the more evolved stars, nor any of the other starswithin the instability strip, seem to oscillate with detectableamplitudes. From analysing the power spectra we find that therest of the stars are stable at a detection level of 2-3 mmag.NGC 7245 has 69 stars within the instability strip. If we as-sume that most of these stars are cluster members then onlya fraction of less than 2% are variables at this detection level.This incidence of variable stars in NGC 7245 is much lowerthan among field stars, where the incidence of variability withinthe instability strip is ∼30% with amplitudes of 0.01 mag. ormore as discussed by Breger (1979) and Wolff (1983). Also,it is much less frequent than in other open clusters, where theincidence of variability is similar to field stars: In the Hyadescluster the incidence of variability is ∼ 37% (Horan 1979) ata 0.01 mag. detection level, and in the Praesepe cluster about40% are variables with amplitudes above 2 mmag. Frandsen etal. (1996) found that∼20% of the stars in NGC 6134 within theinstability strip were variables at a detection level of 1-2 mmag.


Fig. 8. CCD frame of the cluster NGC 7062. The position of the de-tected variable is given by its number. The stars are somewhat fuzzydue to problems with focusing the telescope and poor seeing

4.2. NGC 7062

NGC 7062 is a relatively well studied cluster. UBV photomet-ric data have been given both by Hoag et al. (1961) and Hassan(1973). Stromgren photoelectric photometry has been carriedout by Peniche et al. (1990), while Peniche et al. (1987) haveperformed a photometric study of short periodic variables inNGC 7062. The fundamental parameters for NGC 7062 havebeen discussed by several authors: Hagen (1970) gives a red-dening of 0.25 mag. while Becker & Fenkart (1971) have de-rived the value 0.45 mag from ZAMS fitting. Hassan (1973)found a distance of 1786 pc, E(B − V ) = 0.48 and an age of7.5 x 108 yr for NGC 7062. Peniche et al. (1990) determinedE(B − V ) = 0.456, a distance modulus of 12.18 and the age tobe 7.0 x 108 yr.

In Fig. 8 the CCD field of NGC 7062 is shown. Photometryfor 150 stars in the field of view have been measured. A colour-magnitude diagram of these, can be seen in Fig. 9. We havecompared our photometry with that of Hassan (1973) from 20stars in common between the two data sets. The rms scatterof the residuals is σV = 0.031 mag and σB = 0.054 mag.The higher scatter is reflecting the high intrinsic uncertaintyin Hassan’s photographic data. There is a considerable scatterin the main-sequence for NGC 7062, which could be due tounresolved binaries and field stars. There is a clear turn-off atV ' 13, (B − V ) ' 0.6, which can be used in the cluster agedetermination.

From the ZAMS and isochrone fitting we find that a value ofE(B−V ) = 0.47±0.03, a distance of 1800±400 pc and an ageof 500 Myr (log(age) = 8.7) fit the data for NGC 7062 best. Thisage and reddening are in good agreement with earlier results.Our distance to NGC 7062 agrees well with Hassan (1973) but issomewhat lower than the distance of 2726 pc found by Peniche

NGC 7062

Fig. 9. Colour-magnitude diagram of NGC 7062. From the isochronefitting we find E(B − V ) = 0.47, a distance of 1800 pc and the ageof NGC 7062 to be 500 Myr (log(age) = 8.7). Superimposed are thethree isochrones with log(age) = 8.6, 8.7 and 8.8 all calculated for solarmetal content. The long-dashed line is the ZAMS given by Vandenberg(1985). The short-dashed lines are the borders of the instability stripgiven 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 toNGC 7062 from the distance modulus of only 20 stars wherea normal distribution was fitted, resulting in a mean distancemodulus of 12.18 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 datasample we believe that our distance estimate is more precise.


The data obtained for NGC 7062 have a fairly high noise leveldue to bad weather and only 55 data points were obtained. There-fore, the present observations of this cluster can only indicatepotential variables. The stars with best photometry have rmsscatter of 0.002 mmag but it is in general higher. Only one starhas been detected with visible periodic variation of the lightcurve. Star #180 is a V = 14.01, (B − V ) = 0.81 magnitudestar situated in the middle of the instability strip with a d = 1.46and a photometric variation of the order 0.01 mag., so it is apotential δ Scuti variable. The light curve is shown in Fig. 10.

Peniche et al. (1990) discussed 3 potential variables inNGC 7062. Their ID80, ID13 and ID17 had dispersions in theV magnitude of order 2–3%. We have identified ID17 and ID13as our #292 and #217, respectively. ID80 is not in our field. Thestars #292 and #217 have rms scatter of 0.0043 mag and 0.0040mag, respectively, and show no obvious sign of variability. We


Fig. 10. Light curve for star #180 in NGC 7062

Fig. 11. CCD frame of the cluster NGC 7226

therefore conclude, that these two stars are stable, at least, at themmag level.

4.3. NGC 7226

NGC 7226 is a poorly studied cluster. The latest available pub-lished photometry for this cluster is that of Yilmaz (1970) whohad obtained photographic RGU photometry of 160 stars andfound a distance of 2240 pc. He also calculated the colour excessE(G − R) = 0.68 which corresponds to an E(B − V ) = 0.49using a transformation equation given by Steinlin (1968). InFig. 11 a CCD frame of the field is shown.

We have compared our photometry with photometry ob-tained by Yilmaz (1970) by using 25 stars in common betweenthe two data sets. The standard deviation of the residuals for Vand B magnitudes are σV = 0.143 mag. and σB = 0.134 mag.respectively. These residuals are much higher than expected,which we believe is due to the high intrinsic scatter in Yilmaz’sphotographic data and due to the transformation from theRGUsystem to the BV R system, where we have used the transfor-mation equations given by Steinlin (1968).

NGC 7226

Fig. 12. Colour-magnitude diagram of NGC 7226. From the isochronefitting we find E(B − V ) = 0.47, a distance of 2500 pc and the ageof NGC 7226 to be 500 Myr (log(age) = 8.7). Superimposed are thethree isochrones with log(age) = 8.6, 8.7 and 8.8 all calculated for solarmetal content. The long-dashed line is the ZAMS given by Vandenberg(1985). The short-dashed lines are the borders of the instability stripgiven by Breger (1979)

The colour-magnitude diagram of NGC 7226 is shown inFig. 12. We have obtained photometry for 500 stars within thefield 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 reddeningto 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 thelarger scatter around the cluster main-sequence. The best fit isobtained by an 5 x 108 yr isochrone (log(age) = 8.7), but thelog(age) = 8.6 and log(age) = 8.8 isochrones also fit the dataquite well.


We have examined the light curves of all stars down to V '16 and all 68 stars within the instability strip and no variableswere found. We therefore conclude that all the stars within theinstability strip for this cluster are stable at a 2-3 mmag. level.

4.4. NGC 7654 (M 52)

NGC 7654 is a cluster containing many stars and having a fairlydense core with some very bright B-stars. The bright B-starsconstitute a problem, because these stars saturate the CCD whenwe try to detect the fainter stars. As evident from the CCD image(Fig. 13), this saturation affects the photometry of stars close tothe B-stars. The cluster appears in a region of strong interstel-lar absorption. Danford & Thomas (1981) have done uvby-βphotometry on NGC 7654 and found that it has nonuniform


Fig. 13. CCD frame of the cluster NGC 7654. Also shown are thepositions of the two detected variables. See the text for further details

reddening across the cluster with an colour excess of up to0.11 for stars in the northern half of the cluster compared tostars of the southern half. They found the mean reddening to beE(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. Wepresent here the first CCD photometry done on this cluster.

We have obtained precise photometry for 360 stars in thefield of view (Fig. 13). A colour-magnitude diagram of the clus-ter is shown in Fig. 14. The best fit to the cluster main-sequencegives a value of the colour excess E(B−V ) = 0.58± 0.02 anda distance of 1400 ± 200 pc. This is consistent with the valuesfound by Danford & Thomas (1981). The best fit to the turn-offregion is a 158 Myr isochrone (log(age) = 8.2).

The age of the cluster found by us is much higher than givenby other authors. To investigate this discrepancy the photome-try of bright stars given by Bouigue (1959) and Hoag et al.(1961) are also shown in Fig. 14. We have procured the data byBouigue (1959) and Hoag et al. (1961) from the database givenby Mermilliod (1994). The stars measured by Bouigue (1959)are bright stars from a larger field than ours on the sky, but theyare all classified as cluster members in the database by Mer-milliod (1994). One clearly sees that some of the bright starslie above the plotted isochrones. We have tried other isochroneswith much younger ages, but none of them can fit the brightstars together with the rest of the cluster main-sequence. Fur-ther, our isochrones clearly follow a turn-off of stars. Some ofthe bright stars could be blue stragglers, but most of them aretoo evolved. So our conclusion is that we see an older clusterbehind some younger stars, perhaps also constituting a smallcluster. This interpretation should of cause be confirmed by fur-ther observations.

NGC 7654

Fig. 14. Colour-magnitude diagram for NGC 7654. The filled circlesare our data, the small diamonds are data from Bouigue (1959) whilethe crosses are data given by Hoag et al. (1961). See the text for furtherdetails about the data points. Also two potential variables are indi-cated: Star # 501 as a square and star #215 as a big diamond. We findNGC 7654 to have a E(B−V ) = 0.58 and a distance of 1400 pc. Thebest fit is obtained with an isochrone of log(age) = 8.2. Superimposedare three isochrones with log(age) = 8.1, 8.2 and 8.3 respectively.The long-dashed line is the ZAMS given by Vandenberg (1985). Theshort-dashed lines are the borders of the instability strip given by Breger(1979)


We have found two potential variables as shown in Fig. 15a andb: Star #215 has a position in the colour-magnitude diagramat the main-sequence within the instability strip and has V =14.95, (B − V ) = 0.84, d = 5.1 and σint = 0.0029 mag. Theother potential variable (#501) has V = 14.37, (B−V ) = 0.72,d = 2.1, σint = 0.0031 and is also a main-sequence star nearthe hot border of the instability strip. Thus both stars could beδ Scuti variables.

5. Summary of results

New improved CCD photometry have been obtained for thefour intermediate age open clusters: NGC 7245, NGC 7062,NGC 7226, NGC 7654 and calibrated colour-magnitude dia-grams are presented. These observations were intended as asearch for open cluster containing variable stars, especially δScuti stars. We have found one good target for a future cam-paign, namely NGC 7245. In NGC 7245 we have detected twoδ Scuti variables, one eclipsing binary and one other variableof unknown type. In NGC 7062 only one potential variable wasfound. The time series measurements of NGC 7062 were toonoisy to look for low amplitude variables, but we can concludethat NGC 7062 does not contain variable stars with amplitudes


Fig. 15a and b. Light curves for two potential variables inNGC 7654. Both stars are situated within the instability strip near themain-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 7062 0.47 1800 500NGC 7226 0.47 2500 500NGC 7245 0.40 2800 320NGC 7654 0.58 1400 160

higher than about 0.01 mag. NGC 7226 was found not to containvariables with amplitudes above the detection limit (∼ 0.005mag). NGC 7654 was found to have two potential variables,both good δ Scuti star candidates. This incidence of variabilityin these four clusters is much smaller than usual seen amongfield stars and in other extensive studied open clusters such asthe Praesepe cluster, the Hyades cluster and NGC 6134. In Ta-ble 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 Tele-scope for their assistance during the observations. We are grateful toH. Kjeldsen for his support and advice throughout this work, and to T.Bedding for his useful comments on the manuscript. A. Maeder andP. Meynet are thanked for providing us with their evolutionary codeand the program used in the cluster age determinations. M. Viskum ac-knowledges financial support from the Carlsberg foundation and fromthe Instituto de Astrofısica de Canarias, on behalf of the ANTENA Net-work, of the E.U. This research has made use of the Simbad database,operated at CDS, Strasbourg, France.

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