Photometric variability of luminous blue variable stars on different time scales

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1 Photometric variability of luminous blue variable stars on different time scales Gantchev G. 1, Nedialkov P. 1, Ivanov V. 2, Ovcharov E. 1, Valcheva A. 1, Minev M. 1 ( 1 Department of Astronomy, Sofia University, Bulgaria 2 ESO, Garching bei München, Germany) g.gantchev@phys.uni-sofia.bg

2 Some Properties of LBVs LBVs are the most luminous stellar objects with luminosities up to 10 6 L Conti, 1984 LBVs are very rare: only <20 confirmed LBVs in our Galaxy and a few tens in other galaxies of the Local Group with poorly determined stellar parameters Irregular and unpredictable brightness variations with time scales from microvariations to rare catastrophic mass loss Represent a short (~40000 yrs.) critical phase in the evolution of very massive stars Variations due to changes in the radius and surface temperature of the stars Prominent emission lines of H, HeI, FeII and [FeII], often with P Cygni profiles Murdin, P.: 2001, Encyclopedia of Astronomy and Astrophysics Minimum brightness: BSG (T ~15-20,000 K) Maximum brightness: tater A-or F-type supergiants(t ~ 8000 K) Constant bolometric magnitude: absorption and re-radiation of the starlight by the ejected matter

3 (a) Short time variations t ~ months with m ~ m magnitudes; (α Cygni variations) (b) Typical LBV variations on a t ~ years with m ~ m radius grow and shrink unknown inside instability r/r~10 times L=const.: visual light UV radiation Sp. Class: A early B (S Doradus variations) (c) Large eruptions t ~ n x 100 yr, m ~ 5 m, M~1M or more (observed in P Cygni & η Carinae, not in S Dor) unknown origin: - outward directed radiation pressure force? - large instability created by minor disturbances? The LBV classification is phenomenological, where one of the following properties has to be present. Either the star shows S-Doradus type variability, or giant eruptions of several solar masses must have occurred...

4 General Evolutionary Scheme for Massive Stars (Humphreys, R. & Davidson, K., 2014)

5 Historical light curves and data for M31 and M33

6 A (very) famous LBV - η Carina Historic light curve of supernova impostor Eta Carinae Visual, photographic, photoelectric and CCD data from Fernández-Lajús et al. (2009)

7 The structure function: log(time-lag) Schematic showing the ideal structure function for a time series plus measurement noise. log(time-lag) Structure function for the quasar QSO J Hughes et al, 1992, ApJ, 396, 469

8 First results of the analysis LBVs in M31 galaxy LBVs in M33, LMC and Milky Way galaxies Structure function for the quasar 3C 120 Hughes et al, 1992 The true ladder-like structure function (?)

9 Observations 2m RCC Carl Zeiss, 50/70cm Shmidt & 60cm Cassegrain telescopes at Rozhen national observatory (Bulgaria) 6m telescope BTA-6 (Russia) Mobile Private Student Astronomical Observatory (MPSAO) Gantchev New Student Astronomical Observatory (SAO) Plana (Sofia, Bulgaria)

10 Target: M33 Telescope: 2.6m, VST, VLT Survey Telescope, Cerro Paranal, Chile. Band r (SDSS) Images: 511 (r), 289 (g), 85 (u), 35 (Halpha) Detector: OmegaCAM, (16k x16k pixels) CCD camera Resolution: 0.2arcsec/px FoV: 1x1 sq. deg. Aperture photometry and astrometry of ~ stellar-like objects performed bycasu (Cambridge Astronomy Survey Unit) Current time span: 2.1 yrs until the end of 2014VST

11 The structure function analysis (Hughes et al. 1992) will be applied in order to study the variability of ~30 known or suspected LBVs in the M33 galaxy (Massey et al. 2007) on different time scales. In some cases like Var C the time resolution of the data allows us to confirm an enhanced weekly variations m ~ 0.3m which is somehow shorter than the previously know typical monthly variations with the same maximum amplitude thought to be caused by non-radial pulsations. Distribution of the stars on the 24 CCD detectors of the OMEGACAM

12 First results for VST M33 data Comparison between two structure functions, one of the LBV Var C and one of a comparison standard star showing close to none variations, especially at the shorter time scales (less than a week). We observe that on the longest time scales Var C has the biggest variations in magnitude (more than 2m) and on the shortest ones, close to the sigma noise, we see only very little variations, when the star is observed for 100sec

13 Structural function analysis for the full data sets of Var C (B and V from Burggraf and r from OMEGACAM) in the three available bands showing the variations on different time scales

14 Future work The candidate star with a constructed structure function IS or ISN T an LBV? Diagnostic on diagram Slope Timescale, period?, change step of structure function and analyse on shorter timescales Structure functions for equivalent line widths of H-alpha, IR bands and other spectral features Parameter space: slope vs timescale Check for the sustainability of the results: quiescence vs eruption phase Could LBVs be used as distance indicators? Period-Luminosity relation between the four studied LBVs in M31 and Cepheids from the DIRECT project survey in the V band. (Ivanov G.R., 1990 In Physics of LBV, 1988 Wagner-Kaiser, R. et al., 2015, MNRAS, Stanek, K. Z , ApJ, A-Y)

15 Concluding remarks We have gathered and combined archived observational data (timespan~100 yrs.) for the most studied LBVs in the Local Group and plan to do so for the rest of them. For the first time, we constructed their structure functions and for each variable we calculated their characteristic, valuable parameters: slope and timescale. Gantchev, 2017 The consistency and sustainability of these parameters has been tested throughout historical photometric data, time coverage and type of variability (quiescence or eruption ) the LBV star was in. Amongst the four most studied LBVs in M31, only one (AF And) shows high stability of it s structure function s slope and time scale. We plan to perform a long and short term observational campaign with all the available instruments to us of all known Благодаря and candidate за LBVs вниманието in M31, M33 and to pursue the same methodology for the other known stars in the Local Group. True micro variations of the LBV at the time scales day-hour-minutes, if any, yet have to be proven We expect, with the different types of behavior in light curves and structure functions, to be able to confirm or reject the membership of the candidate star to the LBV type or not and apply our structure function analysis to other candidates or type of variable stars.

16 Thank you for your attention! To be continued