Lebzelter, Thomas

Sahay, A

Wood, Peter R

Information Bulletin on Variable Stars

English

The Australian National University

COMMISSIONS 27 AND 42 OF THE IAUINFORMATION BULLETIN ON VARIABLE STARSNumber 6107Konkoly ObservatoryBudapest19 May 2014HU ISSN 0374 – 0676LONG PERIOD VARIABLES IN STELLAR CLUSTERS: IC4651SAHAY, ANAND1; LEBZELTER, THOMAS2; WOOD, PETER31 Indian School of Mines, Dhanbad; e-mail: anandsahay@ismu.ac.in2 University of Vienna, Department of Astropysics, Türkenschanzstrasse 17, A1180 Vienna, Austria3 Australian National University, Research School of Astronomy and AstrophysicsIntroductionThe research on long period variables (LPVs) experienced a significant upturn in the pasttwo decades when large photometric surveys led to a huge number of high quality LPVlight curves. Major steps forward in our understanding were possible by the observationof stellar samples with known distance, in particular those from the Magellanic Clouds(e.g. Wood 2000, Lebzelter et al. 2002, Fraser et al. 2005, Soszyński et al. 2007). It wasfound that the pulsation of any LPV follows one of several parallel period-luminosity (PL)relations. To better understand this variability behavior, two of us (TL, PW) initiateda monitoring program to detect and characterize LPVs in stellar clusters. For a detaileddescription of this program we refer to Lebzelter & Wood (2011) and references therein.Observations and data reduction are briefly summarized below.Within this program we monitored the open cluster IC4651. Several photometric andspectroscopic studies exist for this cluster in the literature, among the most recent onesbeing Anthony-Twarog et al. (2000), Meibom et al. (2002), Pasquini et al. (2004), andMikolaitis et al. (2011). IC4651 is an intermediate age open cluster (1.7Gyr) at a distanceof about 1.01±0.05 kpc and a reddening value of E(B−V )=0.1 mag (Meibom et al. 2002;Pasquini et al. 2004). Various spectroscopic studies on cluster dwarfs suggest a slightlysuper-solar metallicity (e.g. Carretta et al. 2004, Pace et al. 2008). A depletion of carbonand an overabundance of nitrogen relative to the sun have been reported by Mikolaitis etal. (2011).Observation and data reductionMonitoring of IC4651 started in May 2002 at the 50 inch telescope at Mount Stromlo.The telescope was equipped with a two channel camera used earlier for the MACHOexperiment (Alcock et al. 1992) and it obtained red and blue images simultaneously. Thewavelength centre of the blue channel was similar to that of Johnson V , while the centreof the red channel was close to that of Johnson R. Observations were obtained once totwice a week. Our monitoring came to an abrupt stop after a few months when MountStromlo observatory was destroyed by a bush fire. Altogether we collected 23 frames over130 days, but only 17 frames were usable for our analysis. All observations were done inqueue observing mode.2Since in the blue band the light amplitude of long period variables is typically largerthan in the red (e.g. Fox & Wood 1982), the blue range received a higher weight fordetecting and characterizing variables. The presence of bad pixels did not allow to searchfor variability in all the stars in our field of view.The image subtraction code ISIS 2.1 (Alard 2000) was used for the detection of variablestars and extraction of light curve data. Stellar fluxes of the candidate stars on the refer-ence frame were measured using standard IRAF routines. To determine the photometriczero point, the known V mag of several standard stars from the Guide Star Catalogue(Lasker et al. 2008) were compared with our calculated values. No correction has beenmade for the difference between Johnson V and MACHO Blue.Among the 150 variable star candidates detected in the observed field, the long periodvariables were selected based on the brightness (on the upper giant branch), timescale ofthe variation (more than 30 days), and a total light amplitude in V of at least 0.05 mag.The period search was done using Period98 (Sperl 1998), a code which computes a discreteFourier transformation in combination with a least-squares fitting of multiple frequencieson the data. We considered a maximum of two periods for each star. We note that for thetypical periods of tens of days found for the variables, the light curves are well sampledso that aliasing should not be a significant problem. Due to the semi-regular nature ofLPVs, any periodicity found represents only a snapshot of a possibly more complex lightcurve, and some non-detected variability of much longer time scales (more than 250 days)may exist as well.ResultsOne likely LPV had been detected in IC4651 in the course of the ASAS survey (Pojmanski& Maciejewski 2004). We could confirm that detection and found three new long periodvariables in the cluster. All four stars are listed in Table 1 with their coordinates, Vand DENIS I magnitudes, periods, amplitudes, and 2MASS magnitudes. We used V1 asidentifier for the known variable while other variables were named using the prefix ’SLW’.In Figure 1, the light variations of the four LPVs are plotted against time startingwith the first date of our time series. Also shown are synthetic light curves using theperiods given in Table 1. From previous work on similar data sets we estimate a typicalphotometric error around 0.02mag. The weakest point in the light curve of SLW3 mayhave a higher photometric uncertainty. As one can see, all the variables have a periodexceeding 60-70 days. At least one complete variability cycle has been covered for eachstar. We cannot say anything about possible long secondary periods in these stars sincethey are expected to exceed the time span of our monitoring by a factor of about six. Theperiod we find for V1 is in good agreement with the literature value of 93.8 d.Along with the variable stars for which we can estimate a period, we have listed 3stars in Table 2 that showed variability but without a clear periodicity. This may be dueto a truly irregular nature of their light change, but the length of our time series is notsufficient to clearly classify them as long period variables.Concerning the membership in IC4651 of the stars in Tables 1 and 2, we can rely on afew studies found in the literature and the stars’ locations in the cluster’s colour-magnitudediagram (CMD, Fig. 2). Unfortunately, there is very little information available for V1.Based on the CMD, its membership is rather unlikely, since the star is clearly offset tothe red from the cluster’s giant branch. Much better data exist for the newly discoveredvariables. SLW1, SLW2, SLW3, SLW5, and SLW6 all show radial velocities very close tothe mean cluster velocity (Meibom et al. 2002, Kordopatis et al. 2013). Including also3Table 1: Long period variables in the field of IC4651Name RA(2000) Dec(2000) Vmean IaJ H K Period ∆V[mag] [mag] [mag] [mag] [mag] [d] [mag]V1 17 25 09.35 −49 52 03.6 12.89 9.413 7.007 6.051 5.646 97 0.75SLW1 17 25 08.95 −49 53 57.1 9.28 8.641 5.747 4.948 4.660 114 0.06SLW2 17 24 49.48 −49 57 26.7 11.01 9.809 8.901 8.350 8.209 73 0.4SLW3 17 24 33.40 −49 54 56.0 10.54 9.109 8.137 7.528 7.341 85 0.27aI-band data taken from DENIS database.Table 2: Long period variable candidates of IC4651 with unknown periodsName RA(2000) Dec(2000) Vmean IaJ H K[mag] [mag] [mag] [mag] [mag]SLW4 17 25 14.50 −50 02 49.2 11.51 10.203 9.408 8.881 8.740SLW5 17 24 54.16 −49 53 07.5 10.53 9.182 8.081 7.469 7.264SLW6 17 24 46.71 −49 54 07 11.04 9.728 8.912 8.391 8.247aI-band data taken from DENIS database.proper motion information, Kharchenko et al. (2004) give a high membership probabilityto these five stars. However, membership probabilities from proper motion data do notgive a homogeneous result (e.g. Dias et al. 2006). SLW4 is probably a field star accordingto its radial velocity.While the radial velocity data suggest a membership of most of the newly detectedvariables, the situation is not as clear when combining variability information and locationin the cluster’s CMD. Using an age of 1.7Gyr, a star on the upper giant branch wouldhave had a main sequence mass of about 1.7M⊙ (Bertelli et al. 2008). The tip of the RGBof IC4651 would then be near K=4.0. SLW1 is therefore located about half a magnitudebelow the RGB tip, SLW2-6 are 3-4 magnitudes below the RGB tip. Thus these variablescould be either RGB or early-AGB stars of the cluster. Two stars, SLW2 and 3, havephotometric amplitudes of a few tenths of a magnitude and periods of ≈80 days. However,such an amplitude and period, while typical of an AGB star, is not expected for an objectthat far below the RGB tip according to results from the Magellanic Clouds (cf. Kiss& Bedding 2003). Their variability behaviour suggests that these stars are more likelybackground objects. However, considering the cluster’s location this would place them≈700 pc above the Galactic plane, which would be quite unlikely. Further studies of thesetwo stars are needed to clarify their nature. SLW1 has a photometric amplitude of lessthan 0.1mag, so we think it is an RGB or AGB star belonging to the cluster.The number of known long period variables with independently defined masses is stillvery small, in particular for masses above 1M⊙. A systematic search of LPVs in openclusters is an important source for increasing this number. In this paper we presentedseveral candidates that are possibly members of the intermediate-age cluster IC4651.Further studies are needed to confirm and characterize their nature.Acknowledgements: The work of TL has been supported by the FWF under projectnumber P23737-N16. This research has made use of the VizieR catalogue access tool,CDS, Strasbourg, France.References:40 50 10013.213.012.812.612.412.212.011.811.60 50 1009.329.309.289.269.249.220 50 10011.211.111.010.910.810.710.610.50 50 10010.810.710.610.510.410.310.210.1V1V[mag]Time[d]SLW1V[mag]Time[d]SLW2V[mag]Time[d]SLW3V[mag]Time[d]Figure 1. Light curves of the four variables detected in IC4651. Fits to the brightness change arebased on the periods given in Table 1. The time axis starts at JD 2452410, the start of our monitoring.0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6141210864SLW4SLW6 SLW2SLW3V1SLW5SLW1  K [mag]J-K [mag]Figure 2. 2MASS colour-magnitude diagram for sources 10 arcminutes around the centre of IC4651.The LPVs from Tables 1 and 2 are marked. The position of the giant branch in this intermediate agecluster is consistent with the positions of variables SLW 1-6 (see the collection of intermediate agegiants in Fig. 2 of Bessel et al. (1983), adjusted for a distance modulus of 10.0).5Alard, C. 2000, A&AS, 144, 363.Alcock, C., Axelrod, T.S., Bennet, D.P., et al. 1992, ASPC, 34, 193.Anthony-Twarog B. J., Twarog B. A., 2000, AJ, 119, 2282.Bertelli, G., Girardi, L., Marigo, P., Nasi, E., 2008, A&A, 484, 815.Bessell, M.S., Wood, P.R., Lloyd Evans, T., 1983, MNRAS, 202, 59.Carretta E., Bragaglia A., Gratton R., Tosi M., 2004, A&A, 422, 951.Dias, W.S., Assafin, M., Florio, V., Alessi, B.S., Libero, V., 2006, A&A, 446, 949.Fox, M.W., Wood, P.R. 1982, ApJ, 259, 198.Fraser, O.J., Hawley, S.L., Cook, K.H., Keller, S.C., 2005, AJ, 129, 768.Kharchenko, N.V., Piskunov, A.E., Röser, S., Schilbach, E., Scholz, R.D., 2004, AN, 325,740.Kiss, L.L., Bedding, T.R., 2003, MNRAS, 343, L79.Kordopatis, G., Gilmore, G., Steinmetz, M., et al., 2013, AJ, 146, 134.Lasker, B.M., Lattanzi, M.G., McLean, B.J., et al., 2008, AJ, 136, 735.Lebzelter, T., Schultheis, M., Melchior, A.L., 2002, A&A, 393, 573.Lebzelter, T., Wood, P.R. 2011, A&A, 529, A137.Meibom S., Andersen J., Nordström B., 2002, A&A, 386, 187.Mikolaitis, S., Tautvaisiene, G., Gratton, R., Bragaglia, A., Carretta, E., 2011, MNRAS,413, 2199.Pace G., Pasquini L., Francois P., 2008, A&A, 489, 403.Pasquini L., Randich S., Zoccali M., Hill V., Charbonnel C., Nordström B., 2004, A&A,424, 951.Pojmanski G., Maciejewski, G., 2004, AcA, 54, 153.Soszyński I., et al., 2007, AcA, 57, 201.Sperl, M., 1998, Comm. in Asteroseismology, 111, 1.Wood, P.R., 2000, PASA, 17, 18.

Long period variables in stellar clusters: IC4651

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Long period variables in stellar clusters: IC4651

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