Photometric monitoring of Luminous Blue Variables
Carla Buemi, Elisa Distefano, Paolo Leto, Francesco Schilliro', Corrado Trigilio, Grazia Umana, Stefano Bernabei, Giuseppe Cutispoto, Sergio Messina
PPhotometric monitoring of Luminous Blue Variables
Carla Buemi , Elisa Distefano , , Paolo Leto , Francesco Schillir `o , Corrado Trigilio ,Grazia Umana , Stefano Bernabei , Giuseppe Cutispoto , and Sergio Messina INAF-Osservatorio Astrofisico di Catania, Catania Italy Dipartimento di Fisica e Astronomia, Universit´a di Catania, Catania Italy INAF- Istituto di Radioastronomia, Noto, Italy Osservatorio astronomico, Bologna,Italy
Abstract:
We present some preliminary results from our program of intensive near-infrared photometric moni-toring of a sample of confirmed and candidate Luminous Blue Variables (LBVs) conducted from 2008 to 2010.Clear long-term variability has been observed for Wray 17-96 and V481 Sct, with overall brightness variationgreater than 1 mag in the J band. Other sources, such as LBV 1806-20 showed detectable variability withamplitudes of few tenths of a magnitude with time-scale of about 60 days.
The class of Luminous Blue Variables (LBVs) consists of luminous and massive stars, that are be-lieved to go through a short but violent transition phase of evolution from the main sequence towardsthe Wolf-Rayet stage (Humphreys & Davidson, 1994; Langer et al., 1994). LBVs are well know toshow a combination of spectral and photometric variability (S-Doradus type variability and/or η -Cartype eruptions), whose origin is not yet well understood, despite the crucial role that such objectsplay in the stellar evolution of massive stars (van Genderen, 2001; Kotak & Vink, 2006). A detailedanalysis of the time scale variability of LBV stars can provide useful insights in the understanding theevolution of such objects and in the knowledge of the physical mechanisms that trigger the great gianteruption. Some objects, like η Car, P Cyg and AG Car, has been extensively monitored, but oftenfragmentary observations exists for others. We are thus conducting a long term multiwavelength pho-tometric monitoring of a sample of confirmed and candidate LBVs. We present the most interestingresults of the first two years of observations, performed by using the REMIR infrared imaging cameraavailable at the REM (Rapid Eye Mount) telescope.
Here, we present observations taken with the 60 cm robotic Rapid Eye Mount (REM) telescope,located at the European Southern Observatory (ESO) (La Silla, Chile). Data were acquired in the V,R, I, J, H and K bands since 2008 April up to now with a time sampling of one measure per week,as a part of a multiwavelength (optical and infrared) monitoring campaign of 25 LBVs and cLBVs, “The multi-wavelength view of hot, massive stars”; 39 th Li`ege Int. Astroph. Coll., 12-16 July 2010 a r X i v : . [ a s t r o - ph . S R ] N ov able 1: List of targets. The J magnitude are from the 2MASS catalog. Name α (2000) δ (2000) J mag Name α (2000) δ (2000) J magPV Vel 09:15:54.8 − V4375 Sgr 17:48:14.0 − HR Car 10:22:53.8 − −
20 18:08:40.3 − − − − − V432 Car 11:08:40.1 − − − − W243 16:47:07.5 − − ζ Sco 16:53:59.7 − − − + −
96 17:41:35.4 − + − + V4650 Sgr 17:46:18.0 − +
11 21 16.7 5.47WR102ka 17:46:18.1 − listed in Table 1,dedicated to investigate their photometric variability. As the monitoring programis still ongoing, we present just the most interesting, although partial, results from the infrared lightcurves. Since our goal was to investigate variability of these objects, we carried out differentialphotometry of the candidates with respect to non-variable comparison stars in each target field, usingthe ensemble photometry technique (Gilliland & Brown 1988; Everett & Howell 2001) implementedin the software ARCO (Automatic Reduction of CCD Observation, developed by Distefano et al.(2007). This software allows to select the stars for the ensemble among those in common to all frames,iteratively rejecting stars that were found to have either a systematic variation in the instrumentalmagnitude or large errors. The NIR magnitudes were calibrated against magnitude in the 2MASScatalog.The accuracy and time resolution of the monitoring allow us to detect even small amplitude vari-ability. Photometric errors, as derived following the standard method described by Everett & Howell(2001), are about 0.01 mag on most of the individual measurements; thus the photometric data qualityis good enough to distinguish photometric variation at close to 0.05 mag level.Flux curves and observed color variations are presented for the clearest cases of both long and/orshort term detected variability.
Strong variation in amplitude has been observed in the light curves of this source (Fig. 1). The IRvariability is large ( ∆ J ≈ ∆ J ≈ ∆ J ≈ As for V481 Sct, the brightness of Wray 17-96 varies significantly over the entire interval of ourobservations (Fig. 2). The total variation amplitudes tend to decrease with wavelength, spanningfrom 1 mag in the K band, to 1.45 mag in the J band, but clear small amplitude ( ∆ K ≈ As shown in Fig. 3, the observed light curves of this source show a long term rising trend, with totalbrightness variations having an amplitude of about 0.4 mag in all the bands from the beginning tothe end of the observations. Short term variations with amplitude no larger than 0.2-0.25 mag are ob-served too. During the long term variation, the colors remain practically constant, with a weak blueingassociated to the maxima of the short term flux oscillations, suggesting that both stellar temperatureand radial changing could be present.
We are monitoring a select group of LBV and cLBV to characterize the photometric variabilitychanges as a star goes through these final stages of evolution. The aim of this monitoring program isto collect an as complete and homogenous as possible photometric dataset which helps to accuratelyigure 2: Light and color curves for Wray 17-96. The solid lines represent the smoothed brightness.estimate the parameters of the photometric variability, as needed for a complete understanding of theproperties of stars in this unstable regime.We observed two time scales of variation in our dataset: long term light variation covering theentire interval of our observations (about 2 years) and a small amplitude ( ∆ mag ≈ ≈
60 days) oscillations. We found that about 40% of the sources in our sample are characterizedby one or both such kinds of variations. Such preliminary results, support the need of a regular andhomogeneous monitoring of such objects. A long term homogeneous monitoring of the objects in oursample could help to better define the distribution of the light curve parameters and to set limits onpossible mechanisms generating such photometric behavior.The complete results of our ongoing studies will be published in Buemi et al. (2011, in prepara-tion), where the observed light curves from the entire sample will be discussed.
Acknowledgements
Based on observations made with the REM Telescope, INAF Chile.We wish to thank the REM team for technical support, and in particular Dino Fugazza, for their helpin setting-up the observations.We acknowledge partial financial support from PRIN-INAF 2007 and the ASI contract I/038/08/0“HI-GAL.igure 3: Light and color curves for LBV 1806-20. The solid lines represent the smoothed brightness.