White Dwarfs & Clusters Schedule

CASCA 2013 Schedule for White Dwarfs & Clusters

2013-05-29T14:00 White Dwarfs & Clusters
Location: Hebb Theatre Chair: Harvey Richer
1400	Goldsbury, Ryan *	Constraining the High Temperature End of White Dwarf Cooling Models
	We present an empirical determination of the white dwarf cooling sequence in the globular cluster 47 Tucanae. Using spectral models, we determine temperatures for 887 objects from Wide Field Camera 3 data, as well as 292 objects from data taken with the Advanced Camera for Surveys. We make the assumption that the rate of white dwarf formation in the cluster is constant. Stellar evolution models are then used to determine the rate at which objects are leaving the main sequence, which must be the same as the rate at which objects are arriving on the white dwarf sequence in our field. The result is an empirically derived relation between temperature (Teff ) and time (t) on the white dwarf cooling sequence. Comparing this result to theoretical cooling models, we find general agreement with the expected slopes between 20,000K and 30,000K and between 6,000K and 20,000K, but the transition to the Mestel cooling rate of Teff ∝ t−0.4 is found to occur at hotter temperatures, and more abruptly than is predicted by any of these models.
1415	Prodan, Snezana *	Three body dynamics in extreme stellar enviroments
	The majority of stars are believed to be in binaries and even higher multiplicity systems, both in the field and in the dense stellar environments of globular clusters and galactic nuclei. Dense environment of globular clusters is implies dynamical formation channels where 30% − 40% of all formed triples are affected by interactions between the inner and outer binary. A combination of triple dynamics with tidal effects may lead to or significantly alter formation of interacting binaries and their products. Similarly, in the vicinity (within ∼ 1 pc) of the Galactic massive black hole (MBH) where the gravitational potential is dominated by its mass, binaries are on orbits that are bound to the MBH and can form a hierarchical triple system with the MBH in which the binary orbit around the Galactic center is the outer orbit of the triple. If the orbit of a binary is highly inclined with respect to its orbit around the third body (either another star or in case of Galactic centre a MBH), strong oscillations of the inner orbit eccentricity are induced on the secular timescale which is often shorter then the timescale over which gravitational interactions with background stars would significantly affect both the internal and external orbit of the binary. The induced high eccentricities could lead to rapid tidal energy dissipation which will very rapidly “shrink” the inner binary orbit. A combination of perturbations due to the third body and tidal effects in the binary could therefore provide an effective channel for formation of close/contact binaries and their products, including stellar mergers, X-ray binaries, supernovae and γ- ray bursts.
1430	Kirk, Helen	Mass Segregation in Simulated Small-Cluster Formation
	Most stars form within clustered environments, and yet this formation regime is relatively poorly understood. Although large clusters tend to be distant, there are a number of small young clusters which are quite nearby (several hundred parsecs) which offer a window into clustered star formation. Observations of nearby young stellar groupings with only tens of members have revealed that the most massive member of the system is already located near the cluster centre. This is reminiscent of the mass segregation observed in older, larger clusters, and suggests that star clusters may be born mass segregated (Kirk & Myers 2011, 2012). Numerical simulations provide an avenue to gain additional insight into the processes occurring. I show results from the analysis of a suite of numerical simulations which span a variety of initial conditions (turbulent Mach numbers, driving scale, and radiative transfer effects). These simulations all form small clustered systems which match the scale of the observations. We track the formation and evolution of these small clusters, using the same analysis tools that were applied to the observational data, and find that the most massive cluster members tend to form relatively early, with the rest of the cluster forming later around them. The simulations lend support to the idea that cluster mass segregation is at least primordial rather than being solely attributed to dynamics.
1445	Samra, Raminder *	3D Velocity Tomography in the core of Messier 71
	Using the Gemini North Telescope at Mauna Kea we have obtained astrometric and spectroscopic data for stars in the core of the galactic globular cluster Messier 71 (NGC 6838). This data has allowed us to for the first time ever to obtain three dimensional velocity profiles for stars in the vicinity of centre of a globular cluster. Using the Near Infrared Imager with Adaptive Optics and a 3.8 year baseline for our astrometric study we have resolved the internal proper motion dispersion. The proper motion dispersion is found to be 179+/-17 micro-arcsec/year, we have put a strict limit to the size of any central Intermediate Mass Black Hole at ~150 solar masses at 90% confidence, additionally we find no evidence of core anisotropy. Using our GMOS Integrated Field Unit spectroscopic data we have obtained a radial velocity dispersion of 3.54+/-0.64 km/s. Combining our proper motion and radial velocity dispersions we find the geometric distance to the cluster to be 4.1+/-1.2 kpc. We then compare our geometric distance to a distance found from fitting stellar evolution models. We have developed a new technique for fitting models, using this technique we find the stellar evolution model distance to be 3.9+/-0.2 kpc.
1500	Kalirai, Jason	Exploring the Stellar Graveyard of the Milky Way
	98% of all stars will end their lives as white dwarfs. In old stellar populations, such as globular clusters and stellar halos, the bulk of the progenitor stellar mass function above the present day turnoff is therefore now populated on the white dwarf cooling sequence. These remnants have remarkable properties and can be studied in exquisite detail to reveal their temperatures, gravities, and masses. In this talk, I will describe unprecedented HST imaging and Keck spectroscopic observations of these stars in old stellar populations. This work has led to the first global constraints on the mapping between initial stellar mass and final mass, and therefore has broad applications for understanding stellar evolution theory, mass loss, and chemical enrichment of the interstellar medium. Additionally, through a new technique, I will describe how we can invert the process of stellar evolution to establish a relation between the remnant mass in an old stellar population and the parent age. By applying this technique to nearby Milky Way halo stars, we measure the age of the inner halo of the Milky Way to be 11.4 +/- 0.7 Gyr.