Stars and binaries

CASCA 2013 Schedule for Stars and Binaries

Stars and Binaries
Location: Hennings 202 Chair: Kim Venn
1600 Wade, Gregg The BinaMIcS Large Programs
  Magnetic fields are a crucial ingredient in a star’s evolution, influencing its formation, the structure of its atmosphere and interior, as well as controlling the interaction with its environment. For binary stars magnetism is even more significant, as magnetic fields in binary systems will be strongly affected by, and may also strongly affect, the transfer of energy, mass and angular momentum between the components in these important stellar systems. However, the interplay between stellar magnetic fields and binarity has yet to be investigated in any real detail, from either an observational or theoretical point-of-view. The BinaMIcS project represents innovative large programmes at the CFHT and TBL telescopes to study the complex phenomenon of stellar magnetism under the influence of the unique physical processes and interactions occurring in close binary systems. In this talk I will present the main scientific objectives of the project, as well as the observational and theoretical strategies that have been chosen to reach them.
1615 Antonini, Fabio The origin of the S-star claster at the Galactic Center
  This talk focuses on the origin of the S-stars, the young stars near the supermassive black hole (SMBH) at the Galactic center. We apply recent insights about how relativity interacts with Newtonian (star-star) perturbations near Schwarzschild and Kerr SBHs to the S-stars and put constraints on competing models for their origin and stellar cusp parameters. For the first time we show that the orbital distribution of the S-stars predicted by formation scenarios which invoke the tidal disruption of binary stars by the SBH are consistent with observations even when relativistic effects are considered. I show that the S-star orbits can only be reproduced assuming dynamically relaxed models (i.e., steep density cusps) for the Galactic center. I conclude by discussing the importance of resonant relaxation in terms of tidal disruption events in galactic nuclei.
1630 A. Nandez, Jose Luis What V1309 Sco can teach us
  V1309 Sco is an unusual transient event observed in 2008. The progenitor of this object was determined to be a contact binary that ended up merging and producing the observed mysterious red outburst. This outburst can be explained by having some matter ejected during the merger. Using SPH, we modeled the merger of a likely progenitor binary and studied how initial orbital period, synchronization, Roche lobe overflow and secondary structure (degenerate or main-sequence star) could affect the outcome. We find that for a large range of initial condition the total mass ejecta is within a range between $0.04-0.08\ M_\odot$ and this ejecta proceeds in at least two well distinguished episodes; this makes our initial conditions very robust for matching both morphology and the energy of the observed outburst. In order to explain the pre-merger orbital period decay, we can analyze how it depends on the nature of the low-mass companion. We find that we better match the observations while considering the scenario where the companion prior the merger was a stripped red giant core, and hence, during V1309 Sco outburst, the system evolved through its second episode of the mass transfer.
1645 David-Uraz, Alexandre * Searching for the origin of the wind variability of OB stars
  OB stars are known to exhibit various types of spectral variability, especially in their ultraviolet resonance lines. Discrete absorption components (DAC), localized blueward-migrating UV absorption features, seem to be ubiquitous amongst these stars. These systematic cyclical features have been associated to large-scale azimuthal structures extending from the base of the wind to its outer regions: corotating interaction regions (CIR). Historically, there have been two main competing hypotheses as to what physical processes may perturb the star/s surface and locally drive a faster outflow, ultimately generating CIRs, namely magnetic fields and non-radial pulsations (NRP). As part of a systematic study of the origin of the cyclical wind variability in OB stars, here we evaluate the possible relation between large-scale, dipolar magnetic fields and the CIR phenomenon. We report the results of our search for weak magnetic fields in a sample of 14 stars exhibiting well-documented DAC behaviour. Using high-resolution spectropolarimetric data, we find no evidence of magnetic fields capable of significant chanelling of the stellar winds of any of these stars (i.e. wind confinement parameter \eta_{*} above unity). It thus appears that dipolar fields are not likely to be responsible for these structures in massive star winds, meaning that some other, as of yet unknown, mechanism must come into play.
1700 Hoffman, Kelsey The Effect of Micro-lensing in Eclipsing Binary-star Systems
  Using photometric observations from the Kepler Space Telescope of eclipsing binary star systems where one component is a white dwarf we have investigated the strength of the micro lensing effect. We have examined the stellar binary KOI-81 from the Kepler mission. KOI-81 is composed of a white-dwarf and a A-type main-sequence star in a 24 day circular orbit and have found that micro-lensing is detectable. We use our lightcurve models to measure the strength of the micro-lensing signal and refine the radius of the eclipsing white dwarf.
1715 Hejazi, Neda *

"New Photometric Calibrations of Metallicity and Temperature for M dwarfs"

  M stars( M dwarfs) are among the least luminous stars in the Milky Way, but they are by far the most numerous stars in the Galaxy, making up a considerable part of its baryonic content; about 70% of all stars by number and about 40% of the total stellar mass. The lifetimes of these small stars are much longer than the current age of the universe and they can therefore be used as excellent tracers of chemical, kinematical and dynamical evolution of our Galaxy. Evidently, the determination of fundamental parameters of M stars such as temperature and metallicity can provide us with tremendous information about the Galaxy. However, the determination of these fundamental properties for M dwarfs is a very difficult job. Due to their cool atmospheres, there exist many molecules, causing their spectra so more complicated than those of hotter stars (For example, G or F stars). As a result, the determination of metallicity and temperature of these cool stars through the spectral analysis is so challenging. On the other hand, the location of a M dwarf in a color-color diagram , in general, correlates to its temperature and metallicity. This has led to develop a technique in which synthetic, broad-band photometry is generated and sythetic color-color diagrams , thus , are calibrated over a specific range of surface temperature and metallicity. The temperature and metallicity of a particular star, therefore, can be achieved by comparing the observed and synthetic colors. In this work, by using the best available model stellar atmospheres and taking advantage of the highly accurate CFHTLS/WIRCAM photometry, the metallicity and effective temperature of a sample of M dwarfs are obtained.