Skyler H. Grammer

LIGHT CURVE TEMPLATES AND GALACTIC DISTRIBUTION OF RR LYRAE STARS FROM SLOAN DIGITAL SKY SURVEY STRIPE 82

We present an improved analysis of halo substructure traced by RR Lyrae stars in the Sloan Digital Sky Survey (SDSS) stripe 82 region. With the addition of SDSS-II data, a revised selection method based on new ugriz light curve templates results in a sample of 483 RR Lyrae stars that is essentially free of contamination. The main result from our first study persists: the spatial distribution of halo stars at galactocentric distances 5-100 kpc is highly inhomogeneous. At least 20% of halo stars within 30 kpc from the Galactic center can be statistically associated with substructure. We present strong direct evidence, based on both RR Lyrae stars and main-sequence stars, that the halo stellar number density profile significantly steepens beyond a Galactocentric distance of ~30 kpc, and a larger fraction of the stars are associated with substructure. By using a novel method that simultaneously combines data for RR Lyrae and main-sequence stars, and using photometric metallicity estimates for main-sequence stars derived from deep co-added u-band data, we measure the metallicity of the Sagittarius dSph tidal stream (trailing arm) toward R.A. ~2h-3h and decl. ~ 0? to be 0.3 dex higher ([Fe/H] = ?1.2) than that of surrounding halo field stars. Together with a similar result for another major halo substructure, the Monoceros stream, these results support theoretical predictions that an early forming, smooth inner halo, is metal-poor compared to high surface brightness material that have been accreted onto a later-forming outer halo. The mean metallicity of stars in the outer halo that are not associated with detectable clumps may still be more metal-poor than the bulk of inner-halo stars, as has been argued from other data sets.

The Unusual Temporal and Spectral Evolution of SN2011ht. II. Peculiar Type IIn or Impostor

SN2011ht has been described both as a true supernova (SN) and as an impostor. In this paper, we conclude that it does not match some basic expectations for a core-collapse event. We discuss SN2011hts spectral evolution from a hot dense wind to a cool dense wind, followed by the post-plateau appearance of a faster low density wind during a rapid decline in luminosity. We identify a slow dense wind expanding at only 500-600?km?s?1, present throughout the eruption. A faster wind speed V ~ 900?km?s?1 occurred in a second phase of the outburst. There is no direct or significant evidence for any flow speed above 1000?km?s?1; the broad asymmetric wings of Balmer emission lines in the hot wind phase were due to Thomson scattering, not bulk motion. We estimate a mass-loss rate of order 0.05 M ??yr?1 during the hot dense wind phase of the event. The same calculations present difficulties for a hypothetical unseen SN blast wave. There is no evidence that the kinetic energy greatly exceeded the luminous energy, roughly 3 ? 1049?erg; so the radiative plus kinetic energy was small compared to a typical SN. We suggest that SN2011ht may have been a giant eruption driven by super-Eddington radiation pressure, perhaps beginning a few months before the discovery. A strongly non-spherical SN might also account for the data at the cost of more free parameters.

LUMINOUS AND VARIABLE STARS IN M31 AND M33. I. THE WARM HYPERGIANTS AND POST-RED SUPERGIANT EVOLUTION*

The progenitors of Type IIP supernovae (SNe) have an apparent upper limit to their initial masses of about 20 M ☉, suggesting that the most massive red supergiants evolve to warmer temperatures before their terminal explosion. But very few post-red supergiants are known. We have identified a small group of luminous stars in M31 and M33 that are candidates for post-red supergiant evolution. These stars have A-F-type supergiant absorption line spectra and strong hydrogen emission. Their spectra are also distinguished by the Ca II triplet and [Ca II] doublet in emission formed in a low-density circumstellar environment. They all have significant near- and mid-infrared excess radiation due to free-free emission and thermal emission from dust. We estimate the amount of mass they have shed and discuss their wind parameters and mass loss rates, which range from a few × 10–6 to 10–4 M ☉ yr–1. On an H-R diagram, these stars will overlap the region of the luminous blue variables (LBVs) at maximum light; however, the warm hypergiants are not LBVs. Their non-spherical winds are not optically thick, and they have not exhibited any significant variability. We suggest, however, that the warm hypergiants may be the progenitors of the less luminous LBVs such as R71 and even SN1987A.

Mapping the Asymmetric Thick Disk. III. The Kinematics and Interaction with the Galactic Bar

In the first two papers of this series, Larsen et?al. describe our faint CCD survey in the inner Galaxy and map the overdensity of thick disk stars in Quadrant 1 (Q1) to 5?kpc or more along the line of sight. The regions showing the strongest excess are above the density contours of the bar in the Galactic disk. In this third paper on the asymmetric thick disk, we report on radial velocities and derived metallicity parameters for over 4000 stars in Q1, above and below the plane, and in Quadrant 4 (Q4) above the plane. We confirm the corresponding kinematic asymmetry first reported by Parker et?al., extended to greater distances and with more spatial coverage. The thick disk stars in Q1 have a rotational lag of 60-70?km?s?1 relative to circular rotation, and the metal-weak thick disk stars have an even greater lag of 100?km?s?1. Both lag their corresponding populations in Q4 by 30?km?s?1. Interestingly, the disk stars in Q1 also appear to participate in the rotational lag by about 30?km?s?1. The enhanced rotational lag for the thick disk in Q1 extends to 4?kpc or more from the Sun. At 3-4?kpc, our sight lines extend above the density contours on the near side of the bar, and as our lines of sight pass directly over the bar the rotational lag appears to decrease. This is consistent with a gravitational wake induced by the rotating bar in the disk which would trap and pile up stars behind it. We conclude that a dynamical interaction with the stellar bar is the most probable explanation for the observed kinematic and spatial asymmetries.