Photometric campaign on massive stars in the open cluster NGC 5617
F. Carrier, S. Saesen, M. Cherix, G. Bourban, G. Burki, J. Debosscher, D. Debruyne, P. Gruyters, L.M. Sarro, M. Spano, L. Weber
aa r X i v : . [ a s t r o - ph . S R ] J a n Comm. in AsteroseismologyContribution to the Proceedings of the 38 th LIAC / HELAS-ESTA / BAG, 2008
Photometric campaign on massive stars in the open cluster NGC 5617
F. Carrier , S. Saesen , M. Cherix , G. Bourban , G. Burki , J. Debosscher , D. Debruyne ,P. Gruyters , L. M. Sarro , M. Spano , and L. Weber Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Belgium Observatoire de Gen`eve, Universit´e de Gen`eve, Switzerland Dpt. De Inteligencia Artificial, UNED, Madrid, Spain
Abstract
A campaign on the open cluster NGC 5617 was organized in order to characterize the pulsa-tions and to better understand the internal structure of its stars. The variability of the clustermembers was never studied before. We present the observations taken and an up-to-dateanalysis of the obtained time series, especially of several SPB candidates we discovered.
Session: PosterIndividual Objects: NGC 5617
Introduction
Since 2003, we are monitoring a set of selected southern open clusters with the aim ofdetecting and characterizing their variable stars. This program is conducted at Euler, the 1.2-m Swiss telescope at La Silla Observatory, Chile, with the C2 CCD camera. The large timespan of the observations, typically a few years, makes it possible to study long-period variables,while repeated observations during a given run allow us to detect short-term variations aswell.The majority of the clusters is still in the process of being observed, and the obtaineddata are being reduced and analyzed. In these proceedings we present the first results of thissurvey by showing light curves of massive stars, like SPBs, but also δ Scuti and γ Dor stars,as well as eclipsing or ellipsoidal binaries.
Observations and reductions
Over a period of 5.5 years, we took 4400 measurements in the V-band and 750 in each ofthe Geneva U- and B-bands. The fluxes were extracted using a personal revised version of daophot (Stetson 1987) using combined PSF and aperture photometry. Then the effects ofairmass, atmospheric extinction, . . . , were corrected using multi-differential photometry. Thedata presented here are the V-band observations and are still not detrended. Their accuracycurrently reaches 2.5 mmag for the brightest non-variable stars, over a period of more than5 years. We plan to use the software SysRem (Tamuz et al. 2005) to correct the remainingsystematic effects.
Photometric campaign on massive stars in the open cluster NGC 5617
Variable stars in NGC 5617
We found a total of 218 stars displaying variability in the V-band with periods shorter than50 days. The stars with longer periods have to be taken with caution (possible instrumentaldrifts) and will be analyzed later. Our automated classification software (Debosscher etal. 2007) found amongst others about 35 SPB, 30 δ Scuti and 20 γ Dor candidates, and40 eclipsing and 15 ellipsoidal binaries, but the method does not take spectral informationinto account (known thanks to the membership to the cluster). The variability of these starsis thus doubtless but the classification has to be verified. Hereafter we present some of thecandidate pulsators.Two SPB candidatesIn the top panels of Fig. 1, we show the amplitude spectra in different stages of prewhiteningof two SPB star candidates. For one of the candidates, seven significant frequencies, withvalues between 0.495 d − and 0.784 d − , were detected. Peaks near 0.58 d − could belong toa quintuplet ℓ = 2 . The amplitudes range from 16.3 to 4.0 mmag. For the other candidateSPB pulsator, three modes were found: f =0.677 d − , f =0.574 d − , f =0.768 d − . Theamplitudes are 6.3, 3.6 and 2.7 mmag respectively.A δ Scuti and γ Dor candidateIn the middle panels of Fig. 1, we plot the amplitude spectra in different stages of prewhiteningof a δ Scuti and a γ Dor candidate. We determined seven significant frequencies in this δ Scuticandidate, all between 17.995 and 35.228 d − . The amplitudes vary from 3.2 to 0.8 mmag.For the γ Dor candidate we could identify three frequencies: f =1.746 d − , f =1.912 d − , f =1.710 d − . Their amplitudes measure 14.0, 5.1 and 3.3 mmag.Some eclipsing binariesSeveral eclipsing binaries were identified in NGC 5617. In the lower panels of Fig. 1, we showthe phase plots for two of them: one with a short period of P=0.296d and another one witha longer period of P=2.178d. Future work
The first variability results in NGC 5617 encourage a more in depth analysis of the obser-vations. We will search in detail for oscillating stars and try to identify their main modesby means of the multi-colour photometry. With the absolute photometry we have at ourdisposal, we will select some interesting oscillating cluster members and finally attempt tomodel them.
Acknowledgments.
F. Carrier is a Postdoctoral Fellow and S. Saesen is an Aspirant Fellow ofthe Fund for Scientific Research, Flanders (FWO). Part of this work was supported financiallyby the Swiss National Science Foundation.
References
Debosscher, J., Sarro, L. M., Aerts, C. et al. 2007, A&A, 475,1159Stetson, P. B. 1987, PASP, 99, 191Tamuz, O., Mazeh, T., & Zucker, S. 2005, MNRAS, 356, 1466 . Carrier, S. Saesen, M.Cherix, et al.3