S Doradus variables in the Galaxy and the Magellanic Clouds

Abstract

Abstract. The goal in writing this paper is five fold: (1) to summarize the scientific achievements in the 20th century on S Dor variables (or LBVs); (2) to present an inventory of these variables in the Galaxy and the Magellanic Clouds with a description of their physical state and instability properties; (3) to emphasize the photometric achievements of the various types of instabilities. Generally this seems to be a neglected item resulting in a number of misunderstandings continuously wandering through literature; (4) to investigate the structure of the S Dor-area on the HR-diagram; (5) to estimate the total numbers of S Dor variables in the three stellar systems. The position of the strong active S Dor variables in minimum brightness obey the following linear relation on the HR-diagram:log L/L = 1.37 log T eff − 0.03. The relatively small dispersion of less active and supposed exand dormant S Dor variables with respect to this relation is twice as large at the blue side than at the red side. This might be caused by evolution to the WR stage and/or to high rotation. S Dor variables can be subject to five types of instabilities: the very rare genuine eruptive episodes (the "SD-eruptions"), two different brightening phases caused by slow pulsations (the "SD-phases"): one on a time scale of years, the other on a time scale of decades at a more or less constant luminosity and two types of microvariations: one on a time scale of weeks, the other on a time scale of about 100 d. So far, no periodicities of light curve characteristics of any of these instabilities have ever been found. The durations of active and non-active stages are estimated for about half of the sample based on scattered magnitude estimations such as from historical records, and on modern monitoring campaigns. It would be a misunderstanding to believe that all S Dor variables should be always spectacular. It is estimated that most of them will not be spectacular at all for at least 70% of their lifetime as an S Dor variable.

HR Carinae: New spectroscopic data and physical parameters

“…10, we place HR Car in the H-R diagram, showing also the evolutionary tracks of Schaller et al (1992). Our result indicates that HR Car is located near the 40 M track and the line that indicates the total range for the oscillations proposed by van Genderen (2001). The helium abundance that we have found is typical for the He-burning phase.…”

Cyclicities in the light variations of S Doradus stars

“…With this quasi-period, P Cyg fits very well the grid of P = constant lines for α Cyg variations in the H-R diagram (van Genderen & Sterken 1996); 2. The long-term presence of the 17. d 3 oscillations on top of the 100d-type microvariations (∼0.…”

“…Normally, cycle length variations of this type of microvariation (called the α Cyg-type by van Genderen et al 1997a) are much smaller, resulting in at least a marginally significant peak in the amplitude spectrum. The weak-active S Dor variable R 85 in the LMC is to some degree similar to P Cyg considering its mixture of oscillations of different duration , while the normal α Cyg variable (thus no S Dor variable) WRA 977 also shows a stable quasiperiod like P Cyg, over a time interval of 17 years (van Genderen & Sterken 1996). The stability of the 17. d 3 oscillation of P Cyg is reflected by the linear ephemeris for the epochs of its extrema.…”

“…Halfway between the two extrema the one is quickly replaced by the other, probably within a few months (e.g. van Genderen et al 1990van Genderen et al , 1997b. Both can only be seen simultaneously during this short transition stage.…”

Two bi-stability jumps in theoretical wind models for massive stars and the implications for luminous blue variable supernovae

“…In this paper, we investigate whether the second bi-stability jump exists, in conjunction with studying the behaviour of the first bi-stability jump near ∼21 000 K. Whilst the second jump could provide new insights into the properties of late B and A supergiants, for LBVs both bi-stability jumps are relevant, as their T eff changes over a range between ∼ 8 000 and 30 000 K (e.g. van Genderen 2001;Vink 2012). Therefore, if we understand the temperature behaviour ofṀ, we might be able to explain some of the mass loss variations during the different LBV phases (Vink & de Koter 2002).…”