Asteroseismology of massive stars in the young open cluster NGC 884: a status report
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
Asteroseismology of massive stars in the young opencluster NGC 884: a status report
S. Saesen , F. Carrier , A. Pigulski , C. Aerts , G. Handler , A. Narwid ,J. N. Fu , C. Zhang , X. J. Jiang , G. Kopacki , J. Vanautgaerden ,M. St¸e´slicki , B. Acke , E. Poretti , K. Uytterhoeven , W. De Meester ,M. D. Reed , Z. Ko laczkowski , G. Michalska , E. Schmidt , R. Østensen ,C. Gielen , K. Yakut , , A. Leitner , B. Kalomeni , S. Prins ,V. Van Helshoecht , W. Zima , R. Huygen , B. Vandenbussche , P. Lenz ,D. Ladjal , E. Puga Antol´ın , T. Verhoelst , J. De Ridder , P. Niarchos ,A. Liakos , D. Lorenz , S. Dehaes , M. Reyniers , G. Davignon , S.-L. Kim ,D. H. Kim , Y.-J. Lee , C.-U. Lee , J.-H. Kwon , E. Broeders ,H. Van Winckel , E. Vanhollebeke , G. Raskin , Y. Blom , J. R. Eggen ,P. Beck , J. Puschnig , L. Schmitzberger , G. A. Gelven , B. Steininger , andR. Drummond Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Belgium Instytut Astronomiczny, Uniwersytet Wroc lawski, Poland Institut f¨ur Astronomie, Universit¨at Wien, Austria Beijing Normal University, China National Astronomical Observatories, Chinese Academy of Sciences,Beijing, China INAF-Osservatorio Astronomico di Brera, Merate, Italy Department of Physics Astronomy and Material Science,Missouri State University, USA Institute of Astronomy, University of Cambridge, UK Department of Astronomy & Space Sciences, University of Ege, Izmir, Turkey Izmir Institute of Technology, Department of Physics, Izmir, Turkey Department of Astrophysics, Astronomy and Mechanics,University of Athens, Greece Korea Astronomy and Space Science Institute, Daejeon, South Korea
Abstract
To improve our comprehension of the β Cephei stars, we set up a photometricmulti-site campaign on the open cluster NGC 884 ( χ Persei). Thirteen tele-scopes joined the 2005-2007 campaign which resulted in almost 78 000 CCD
Asteroseismology of massive stars in the young open cluster NGC 884: a status report frames. We present an up-to-date status of the analysis of these data, in whichseveral interesting oscillating stars are pointed out. We end with the futureprospects.
Session: Observed frequencies in pulsating massive starsIndividual Objects: NGC 884
Introduction
Recent progress in the seismic interpretation of selected β Cephei stars wasremarkable in the sense that standard stellar structure models are unable toexplain the oscillation data for the best-studied stars: HD 129929 (Aerts etal. 2003), ν Eridani (Pamyatnykh et al. 2004, Ausseloos et al. 2004, Dziem-bowski & Pamyatnykh 2008) and 12 Lacertae (Ausseloos 2005, Handler etal. 2006, Dziembowski & Pamyatnykh 2008). Non-rigid internal rotation andcore convective overshoot are needed to fit the measured oscillation frequenciesand the standard models have now been upgraded to include these effects, al-beit in a crude parametrised way. Pamyatnykh et al. (2004) have suggested toinclude in future models radiative diffusion processes as well, in an attempt toresolve the yet unsolved excitation problem encountered for some of the modesdetected in ν Eridani.A next step in asteroseismology of β Cephei stars was undertaken recently,with the study of these stars in clusters. Indeed, with the current CCD cam-eras we are able to obtain simultaneous measurements of thousands of stars.Another big advantage is the cluster membership of the stars: this gives usmuch tighter constraints when modelling their observed and identified oscilla-tion modes.Krzesi´nski & Pigulski (1997, 2000) discovered one candidate and two bonafida β Cephei stars in NGC 884. The variability study on this cluster conductedby Waelkens et al. (1990) showed that at least half of the brighter stars arevariable, while most of them seem to be Be stars.
Observations
To perform ensemble-asteroseismolgy of NGC 884, we needed time-resolvedmulti-colour differential photometry of a selected field of this cluster, for whichwe organised a large-scale multi-site campaign. An international team moni-tored the cluster with 13 telescopes in the northern hemisphere in the filters U,B, V, I. The data were taken between 2005 and 2007, spread over three obser-vational seasons, spanning in total 800 days. 77 900 CCD images and 92 hoursof photo-electric measurements were collected, representing 1290 hours of data . Saesen, F. Carrier, A. Pigulski, et al.
Figure 1: Spectral windows of the Polish data (left panel), the combined Polish andChinese data (middle panel) and all data (right panel).
To transform the CCD frames into interpretable light curves, we extractedthe fluxes of the stars with Daophot (Stetson 1987), in which we combined PSFand aperture photometry. We performed differential photometry in which wecorrect for atmospheric extinction by taking several reference stars distributedover the CCD frame into account. Currently we are detrending the data withSysRem (Tamuz et al. 2005) to remove the linear systematic effects whichare present in a lot of stars. For sites with many data points we obtain anoverall V accuracy of 3-5 mmag over the entire campaign, depending on thetelescope. The error on the frequency is smaller than 0.000 14 d − and thedetection threshold at high frequencies for Polish data is about 0.3 mmag. Fre-quencies in the β Cephei range can be accepted if their amplitudes are largerthan 1 mmag, but this limit will go down once all data are put together.
Detected variable stars
A preliminary analysis on single-site data led to the confirmation of the twoknown β Cephei stars, Oo 2246 and Oo 2299, and to the discovery of numerousnew pulsators of this type, among which several are multi-periodic (Pigulski et
Asteroseismology of massive stars in the young open cluster NGC 884: a status report al. 2007, Saesen et al. 2008). However, spectroscopy has shed additional lighton two of these newly classified β Cephei stars, Oo 2085 and Oo 2566. Theyshow variations on both short ( ∼ hours) and long ( ∼ days) time scales and turnout to have H α emission. As a consequence, Oo 2085 and Oo 2566 are alsocategorised as Be stars. Oo 2444, Oo 2488 and Oo 2572 remain accepted as β Cephei stars.The β Cephei stars show at least a double-mode behaviour, except Oo 2299,which seems to show a single, predominant mode. Fig. 2 shows the peri-odograms in subsequent stages of prewhitening based on dual-site data of twoof these oscillators. We clearly detect two frequencies for each of them (firsttwo panels) and after subtracting these variations from the data, there is stillpower present in the residuals (lower panels). We expect that additional fre-quencies will peak above the frequency acceptance level after detrending andmerging the data of all sites. The case of Oo 2572 also points out that multi-siteobservations are important to pinpoint the correct frequency peak. In Saesenet al. (2008), which is only based on single-site data, an alias frequency wasmistaken for the correct peak.
Figure 2: Periodograms of subsequent stages of prewhitening based on Polish andChinese data of the β Cephei stars Oo 2488 and Oo 2572.
Besides these five established β Cephei stars, we have five more candidates.Three of them show evidence for low frequencies and might turn out to behybrid oscillators. We have detected seven SPB candidates, amongst whichis Oo 2253: three significant frequencies are extracted and shown in the sub-sequent periodograms of Fig. 3. In addition, the observed field of NGC 884contains several eclipsing binaries (see Fig. 4 for two newly discovered cases)and other variable stars. . Saesen, F. Carrier, A. Pigulski, et al. Figure 3: Periodograms of subsequent stages of prewhitening based on Polish andChinese data of the SPB star Oo 2253.Figure 4: Phase plots of two newly discovered eclipsing binaries Oo 2351 and Oo 2433.
Future prospects
First of all, we will conduct a detailed search for variable stars in the clusterNGC 884. For these variable stars we will do a frequency analysis. We willespecially look for B-type pulsators, for which we will perform a mode iden-tification to determine the degree ℓ . The well-known photometric method,which compares the theoretical amplitude ratios with the observed ones at dif-ferent wavelengths, will be used (Dupret et al. 2003). For this purpose we haveobservations in different bands at our disposal. The photometric amplituderatios are going to be combined with radial-velocity amplitudes deduced fromsimultaneous NOT spectra for the three brightest β Cephei stars. This willmake the mode identification more conclusive than only the amplitude ratios(Daszy´nska-Daszkiewicz et al. 2005).
Asteroseismology of massive stars in the young open cluster NGC 884: a status report
In the end we will fit theoretical frequencies to the observed ones and theirmode identification, simultaneously for all pulsating cluster members. Indeed,as the stars in a cluster were born out of the same cloud, we can assume thatthey have the same age and had the same chemical composition at birth. TheLi`ege stellar evolution code cl´es (Scuflaire et al. 2008) and the non-adiabaticoscillation code
MAD (Dupret et al. 2002) will be applied in this process. Onlymodels that fulfill additional criteria, such as the position in the HR diagramderived from photometry, and the abundances of the stars obtained by NOTspectroscopy, will be retained.In any case, the first results are very promising for our future analysis of allcampaign data. In-depth evaluation of stellar evolution models seems thereforewithin reach, now that the technique of asteroseismology has been extensivelytested on single field stars.
Acknowledgments.
S. Saesen is an Aspirant Fellow and F. Carrier is a Post-doctoral Fellow of the Fund for Scientific Research, Flanders (FWO). K. Uytterhoevenacknowledges financial support from a
European Community Marie Curie Intra-European Fellowship , contract number MEIF-CT-2006-024476.