CASCA 2010: Stars 3 session

Variable stars in the Hyades Cluster (Jayme Derrah)

To establish distance scales you need to measure variable stars at known distances; the Hyades cluster is the closest open cluster to the Earth. Unfortunately it's just out of range for accurate parallax measurements, so distances are measured using the "convergent point method". Variable stars of interest re eclipsing binaries and pulsational variables (though few new examples of the latter are expected). The project uses the Baker-Nunn patrol camera, which has a large enough field of view to include the whole cluster and reaches a limiting magnitude of 19.5 in two minutes; on the other hand there's only one filter, a light pollution blocker. Follow-up of discovered variables will use B and V.

Type 1A Supernova Progenitor Diversity (Ashley J. Ruiter)

Sub-Chandrasekhar-mass WDs should be looked at as SN1A progenitors. SN1A light curves are driven by the amount of nickel produced, so we can use them as standard candles almost without any idea of what the progenitors look like. Theories are CO WD mergers and CO WD collapse triggered by companion Roche lobe overflow. Simulations predict that the former ought to produce NSes instead, while models (including this work) have difficulty producing enough of the latter. In particular, she built a population synthesis code and found an order of magnitude too few overflow models. The idea is that perhaps allowing sub-Chandrasekhar-mass collapse can help. More detailed models, in particular using helium accretion, allow collapse to occur with 0.8-0.9 Msun: you get detonation in a shell of accreted helium which triggers the explosion. She built simulated light curves based on such explosions and found reasonable agreement with observed light curves [but how hard is this, if nickel mass is all you see?]. From a population synthesis point of view, though, they do provide enough. It's also theoretically nice because it explains the observed variety in light curves: it's a function of mass at collapse.

Questions: Where do these helium-rich companions come from? Many are He white dwarfs, which arise naturally in such binaries (lost their hydrogen through Roche lobe overflow). What's that blue line on the graph? It turns out if you start with 0.9 Msun WDs the merger scenario looks a little more plausible. What assumptions go into the pop. synth? Standard IMF, etc. Where does the mass go during binary evolution? Eddington-limited transfer, so probably outflows.

Tau Sco: The Discovery of the Clones (Veronique Petit)

Part of the MiMeS (Magnetic Field in Massive Star) collaboration. There ~35 known magnetic OB stars (i.e. B directly detected, which is hard), but it has long been suspected that B is ubiquitous in massive stars. ("Magnetic fields are to astrophysics as sex is to psychology." - someone) These stars are short-lived but important, and the key complicating parameters are rotation and mass loss; we expect B to substantially affect the wind (as it does in the Sun). Tau Sco, a B 0.5 V star, is the focus of this talk. We can measure B by the Zeeman effect, i.e. circular polarization across a spectral line. If you sample the stellar rotation, you can actually produce B maps. Unlike all other massive magnetic stars which have roughly dipolar fields, tau Sco's field is complex and multipolar. Studies of the wind using UV line profiles show strange inconsistencies between different line traces; there seems to be some correlation between the UV variability as a function of stellar rotation and the B. It's natural to assume that the wind anomalies arise from the complexity of B, but it was hard to test until their recent discovery of two "clones" that have similar wind anomalies and slow rotation. ESPADON shows that these stars are magnetic; sadly it has not yet been possible to sample the stellar rotation densely enough to map the magnetic field. Preliminarily, though, a dipole model suggests that surface B is roughly equal to that of tau Sco. 

Questions: Have people measured this supposed correlation between B and the wind? Yes, but it doesn't really predict tau Sco. How strong are these Bs? ~500 G. Does B of tau Sco vary with time, particularly in flares? We don't know, they had to assume it depended only on rotation. Could you use the Bayesian inference to estimate non-dipolar fields? There's too much uncertainty when you don't know about the stellar rotation.

Tracing Wolf-Rayet wind structures (Alexandre David-Uraz)

This work focuses on WR113. WR stars have very high mass loss rates, high enough that radiation pressure alone can't explain it; we need to understand the physics of clumping. WR113 = CV Ser is a binary system in which the companion can often be seen through the WR wind. It's a double-line binary, so they can in principle get a decent picture of the orbit, but so far the spectrum has been too messy. Once this is accomplished, they should be able to separate the two spectra quite well - average everything in the WR frame, then subtract this and switch to the companion's frame, then back and forth. In any case, the "eclipse" is of depth ~0.6 mag - well, in Lamontagne et al. from the 90s; in a 1963 paper the eclipse seems variable (0.1 mag sometimes and 0.6 mag), and in 1970 they didn't see an eclipse at all. MOST sees a shallow eclipse but variable - in fact, two successive eclipses differ very substantially. The real focus of the research is random variations due to clumping, and indeed they see both photometric and spectroscopic evidence for clumping. There's also the issue of a colliding-wind shock, which shows up in the lines (in particular there's streaming along the shock); with luck this should help establish system geometry. The next step is Fourier analysis for pulsations in star or wind, and wavelet analysis for random clumping. The goal is to link these to spectroscopic data and constrain the clumping.  

Questions: This is a complicated system; is there evidence for an accretion disk? Unclear, but the colliding winds suggest not. Is this the only eclipsing WR star? No, there are others. 

Limb-Darkening and Stellar Atmospheres (Hilding Neilson)

Limb darkening is particularly interesting these days as observations become more constraining. They affect planet parameters inferred from transits (and fitting can constrain limb darkening). Optical interferometry can directly measure it, and microlensing can help as well. Limb darkening tells you something about conditions in stellar atmospheres; Schwarzschild used solar eclipse observations to show that the solar atmosphere is in radiative rather than adiabatic equilibrium. Traditionally, though, people just use empirical limb darkening laws that are pretty crummy; in fact popular parameterizations have fixed points that are more or less independent of the parameters, and fit observations rather badly. The fixed point arises because the models the empirical laws are based on all make the Eddington approximation. Spherical symmetry rather than plane-parallel atmospheres helps spread the fixed point out. Detailed modelling suggests that the true value at the fixed point probes atmosphere physics. Observations suggest that you can make these inference even with the wrong models.

No comments: