We present some preliminary results from our program of intensive near-infrared photometric monitoring ofa 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 variation greater than 1 mag in the J band. Other sources, such as LBV 1806-20 showed detectable variability with amplitudes of few tenths of a magnitude with a time-scale of about 60 days

BERNABEI, Stefano

BUEMI, CARLA SIMONA

CUTISPOTO, Giuseppe

DISTEFANO, Elisa Maria Carmela

LETO, PAOLO

MESSINA, Sergio

SCHILLIRO', Francesco

TRIGILIO, CORRADO

UMANA, Grazia Maria Gloria

OA@INAF - Istituto Nazionale di Astrofisica

2011Publication Year2023-01-20T15:50:22ZAcceptance in OA@INAFPhotometric monitoring of Luminous Blue VariablesTitleBUEMI, CARLA SIMONA; DISTEFANO, Elisa Maria Carmela; LETO, PAOLO; SCHILLIRO', Francesco; TRIGILIO, CORRADO; et al.Authorshttp://hdl.handle.net/20.500.12386/32963HandleBULLETIN DE LA SOCIÉTÉ ROYALE DES SCIENCES DE LIÈGEJournal80NumberPhotometric monitoring of Luminous Blue VariablesCarla Buemi1, Elisa Distefano1,2, Paolo Leto1, Francesco Schillirò3, Corrado Trigilio1,Grazia Umana1, Stefano Bernabei4, Giuseppe Cutispoto1, and Sergio Messina11 INAF-Osservatorio Astrofisico di Catania, Catania Italy2Dipartimento di Fisica e Astronomia, Universitá di Catania, Catania Italy3 INAF- Istituto di Radioastronomia, Noto, Italy4 Osservatorio astronomico, Bologna,ItalyAbstract: 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 a time-scale of about 60 days.1 IntroductionThe 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 known 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 objects playin the stellar evolution of massive stars (van Genderen 2001, Kotak & Vink 2006). A detailed anal-ysis of the time scales of variability of LBV stars can provide useful insights in the understanding ofthe evolution of such objects and in the knowledge of the physical mechanisms that trigger the greatgiant eruption. Some objects, like η Car, P Cyg and AG Car, have been extensively monitored, butoften fragmentary observations exist for others. We are thus conducting a long term multiwavelengthphotometric monitoring of a sample of confirmed and candidate LBVs. We present the most inter-esting results of the first two years of observations, performed by using the REMIR infrared imagingcamera available at the REM (Rapid Eye Mount) telescope.2 Observations and resultsHere, 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 April 2008 up to now with a time sampling of one measure per week, aspart of a multiwavelength (optical and infrared) monitoring campaign of 25 LBVs and cLBVs, listed“The multi-wavelength view of hot, massive stars”; 39th Liège Int. Astroph. Coll., 12-16 July 2010Bulletin de la Société Royale des Sciences de Liège, Vol. 80, 2011, p. 351 - 355Table 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 −49:58:24.6 4.44 V4375 Sgr 17:48:14.0 −28:00:53.1 4.82HR Car 10:22:53.8 −59:37:28.4 4.56 LBV 1806−20 18:08:40.3 −20:24:41.1 13.66GSC08958 10:53:59.6 −60:26:44.3 7.32 V4029 Sgr 18:21:14.9 −16:22:31.8 4.60AG Car 10:56:11.6 −60:27:12.8 5.42 V4030 Sgr 18:21:19.5 −16:22:26.1 5.14V432 Car 11:08:40.1 −60:42:51.7 7.96 V481 Sct 18:33:55.3 −06:58:38.7 8.36Sher 25 11:15:07.8 −61:15:17.6 8.60 GAL025 18:37:05.2 −06:29:38.0 15.80W243 16:47:07.5 −45:52:29.2 6.41 V452 Sct 18:39:26.1 −13:50:47.1 7.86ζ Sco 16:53:59.7 −42:21:43.3 3.59 GAL026 18:39:32.2 −05:44:20.5 8.00V1104 Sco 17:06:53.9 −42:36:39.7 6.71 V1672 Aql 19:00:10.9 +03:45:47.1 12.16Wray17−96 17:41:35.4 −30:06:38.8 6.71 V1429 Aql 19:21:34.0 +14:52:56.9 6.09V905 Sco 17:41:59.0 −33:30:13.7 3.55 W 51 19:23:42.3 +14:30:33.0 14.32V4650 Sgr 17:46:18.0 −28:49:03.5 12.31 V1302 Aql 19 26 48.3 +11 21 16.7 5.47WR102ka 17:46:18.1 −29:01:36.6 12.98in Table 1, dedicated to investigate their photometric variability. As the monitoring program is stillongoing, we present just the most interesting, although partial, results from the infrared light curves.Since our goal was to investigate variability of these objects, we carried out differential photometry ofthe candidates with respect to non-variable comparison stars in each target field, using the ensemblephotometry technique (Gilliland & Brown 1988, Everett & Howell 2001) implemented in the softwareARCO (Automatic Reduction of CCD Observation, developed by Distefano et al. 2007). This softwareallows to select the stars for the ensemble among those in common to all frames, iteratively rejectingstars that were found to have either a systematic variation in the instrumental magnitude or largeerrors. The NIR magnitudes were calibrated against magnitude in the 2MASS catalog.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 variations 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.2.1 V481 SctStrong variation in amplitude has been observed in the light curves of this source (Fig. 1). The IRvariability is large (∆J≈1.0) and the rise seems to be faster between the first two runs, although thegap in the coverage does not allow us to follow the entire trend. The star further brightens up to atotal magnitude variation of ∆J≈1.2 on a time scale of about two years. V481 Sct experienced asimilar photometric variation between 2003-2004, as reported by Clark et al. (2005), suggesting asort of cyclic photometric behavior. Smaller (∆J≈0.3) and shorter (time scale of about 60 days) fluxvariations seems to be superimposed to the long term brightening. The infrared colors do not show asystematic trend but exhibit a quite irregular change, making it hard to derive the mechanism of thevariability on the basis of the near-IR photometry alone. We note that the mean J-K and J-H seem todecrease during the first stellar brightening, suggesting a stellar blueing. However, the trend changesduring the third run, suggesting that more than one process is acting.352Bulletin de la Société Royale des Sciences de Liège, Vol. 80, 2011, p. 351 - 355Figure 1: Light and color curves for V481 Sct. The solid lines represent the smoothed brightness.2.2 Wray 17-96As 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≈0.5) variabilitycan be observed also on shorter time scales of about 60 days. As shown in the Figure 3, the infraredcolor became bluer as the flux rises, from the beginning to the end of our observations, indicatingan increase of the stellar temperature. On the contrary, the short time variability is characterized byredder colors when the star becomes brighter. This behavior can be explained in terms of increasingmass loss or/and increased contribution from hot dust.2.3 LBV 1806-20As 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 changes could be present.3 SummaryWe are monitoring a group of LBV and cLBV to characterize the photometric variability changes asa star goes through these final stages of evolution. The aim of this monitoring program is to collect anas complete and homogenous as possible photometric dataset which helps to accurately estimate the353Bulletin de la Société Royale des Sciences de Liège, Vol. 80, 2011, p. 351 - 355Figure 2: Light and color curves for Wray 17-96. The solid lines represent the smoothed brightness.parameters of the photometric variability, as needed for a complete understanding of the properties ofstars 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≈0.2), short time(≈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.AcknowledgementsBased on observations made with the REM Telescope, INAF Chile. We wish to thank the REM teamfor technical support, and in particular Dino Fugazza, for their help in setting-up the observations.We acknowledge partial financial support from PRIN-INAF 2007 and the ASI contract I/038/08/0“HI-GAL.354Bulletin de la Société Royale des Sciences de Liège, Vol. 80, 2011, p. 351 - 355Figure 3: Light and color curves for LBV 1806-20. The solid lines represent the smoothed brightness.ReferencesClark J.S., Larionov V. M., & Arkharov A. 2005, A&A, 435, 239.Distefano E., Messina S., Cutispoto G., et al. 2007, EAS Publication Series, 25, 165Everett, M. E., Howell, S. B. 2001, PASP, 113,1428.Gilliland, R. L., Brown, T. M. 1988, PASP, 100, 754.Humphreys R., Davidson K. 1994, PASP, 106, 1025.Kotak, R., & Vink, J.S. 2006, A&A, 460, L5Langer, N., Hamann, W.R., Lennon, M., Najarro, F., Pauldrach, A.W.A., Puls, J. 1994, A&A, 290, 819.van Genderen, A.M 2001, A&A, 366, 508.355Bulletin de la Société Royale des Sciences de Liège, Vol. 80, 2011, p. 351 - 355

Photometric monitoring of Luminous Blue Variables

https://openaccess.inaf.it/bitstream/20.500.12386/32963/1/document.pdf

Photometric monitoring of Luminous Blue Variables

Abstract

Similar works

Full text

Available Versions

OA@INAF - Istituto Nazionale di Astrofisica