arXiv subjects

J. A. Holtzman

Publications and source records attributed to J. A. Holtzman.

25 records · Page 2Linked to original sources

arXiv2005

Hi-Res Spectroscopy of a Volume-Limited Hipparcos Sample within 100 parsec: Austin Bash Symposium Conference Proceeding

Accurate parallaxes from the Hipparcos catalog have enabled detailed studies of stellar properties. Subgiants are of particular interest because they lie in an area where isochrones are well separated, enabling dependable age determination. We have initiated a project to obtain hi-res spectra of Hipparcos subgiants in a volume-limited sample within 100 pc. We obtain stellar properties and abundances via fully automatic analysis. We will use our results to constrain star formation history and chemical evolution in the solar neighborhood. We present initial results for our observed sample and discuss future development for this project.

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arXiv2005

Hi-Res Spectroscopy of a Volume-Limited Hipparcos Sample within 100 parsec

We obtain high-resolution echelle spectra of evolved Hipparcos stars within 100 parsec for detailed abundance analysis. We are particularly interested in subgiants because their ages can be derived accurately. Here, we present preliminary results for 10 subgiants and discuss the future work for this project. Our main scientific interest is to incorporate our results to constrain the star formation history in the solar neighborhood.

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arXiv2002

Time-resolved, multi-color photometry and spectroscopy of Virgo 4 (OU Vir): a high orbital inclination, short orbital period dwarf nova

We present multi-color photometry and time resolved spectroscopy of OU Vir. The analysis of the quiescent light curve shows that OU Vir is characterized by i) strong cycle-to-cycle brightness variations, and ii) hot spot modulated light curve with grazing eclipse of the impact region. Colors are derived both in- and out- of eclipse. The time-resolved spectroscopy allows us to produce the radial velocity curve from the H$α$ accretion disk emission line which possibly reveals only weak evidence for hot spot line emission. The hot spot is believed to be a turbulent optically thick region, producing mostly continuum emission.

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arXiv2001

Keck Spectroscopy and HST Imaging of GRB 000926: Probing a Host Galaxy at z = 2. 038

We present early-time Keck spectroscopic observations and late-time Hubble Space Telescope (HST) imaging of GRB 000926. The HST images show a small offset between the optical transient and the compact host galaxy. Combined with the large equivalent widths measured for metallic absorption lines by the Keck Echellette Spectrograph and Imager (ESI) and the Low-Resolution Imaging Spectrometer (LRIS), this indicates that the GRB exploded near the center of its host. The ESI spectroscopy reveals two absorption systems centered at z = 2.0379 \pm 0.0008 with a velocity separation of 168 km s^{-1}, which we interpret as being due to individual clouds in the host galaxy. The ratios of chromium to zinc equivalent widths indicates the host is depleted in dust relative to local values to a similar degree as damped Lyman alpha systems at the same redshift. Further, the two clouds appear to have similar relative metal abundance and dust to gas ratio. If one cloud is associated with the GRB site, this implies the explosion did not significantly alter the surrounding environment.

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arXiv2001

Multi-Color Observations of the GRB000926 Afterglow

We present multi-color light-curves of the optical afterglow of GRB 000926. Beginning ~1.5 days after the burst, the light-curves of this GRB steepen measurably. The existence of such achromatic breaks are usually taken to be an important observational signature that the ejecta are not expanding isotropically, but rather have a collimated jet-like geometry. If we interpret the data in this context, we derive an opening angle of 5 degrees, which reduces the energy release compared to an isotropic model by a factor of ~275, to 1.7 x 10^51 erg. To fit the data with a simple jet model requires extinction along the line of sight. The derived A_V is in the range 0.11 - 0.82 mag, depending on the adopted extinction law and whether the electrons giving rise to the optical emission are undergoing synchrotron cooling or not. Since this is in excess of the expected extinction from our Galaxy, we attribute this to the GRB host. We note that this extinction is typical of a galactic disk, and therefore the event likely took place in the disk of its host.

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arXiv1999

Observations and Implications of the Star Formation History of the LMC

We present derivations of star formation histories based on color-magnitude diagrams of three fields in the LMC from HST/WFPC2 observations. A significant component of stars older than 4 Gyr is required to match the observed color-magnitude diagrams. Models with a dispersion-free age-metallicity relation are unable to reproduce the width of the observed main sequence; models with a range of metallicity at a given age provide a much better fit. Such models allow us to construct complete ``population boxes'' for the LMC based entirely on color-magnitude diagrams; remarkably, these qualitatively reproduce the age-metallicity relation observed in LMC clusters. We discuss some of the uncertainties in deriving star formation histories. We find, independently of the models, that the LMC bar field has a larger relative component of older stars than the outer fields. The main implications suggested by this study are: 1) the star formation history of field stars appears to differ from the age distribution of clusters, 2) there is no obvious evidence for bursty star formation, but our ability to measure bursts shorter in duration than $\sim$ 25% of any given age is limited by the statistics of the observed number of stars, 3) there may be some correlation of the star formation rate with the last close passage of the LMC/SMC/Milky Way, but there is no dramatic effect, and 4) the derived star formation history is probably consistent with observed abundances, based on recent chemical evolution models.

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arXiv1996

Hubble Space Telescope Observations of Globular Clusters in M31 II: Structural Parameters

We analyze post-refurbishment Hubble Space Telescope images of four globular clusters in M31. The ability to resolve stars to below the horizontal branch permits us to use star counts to extend the surface brightness profiles determined using aperture photometry to almost 5 orders of magnitude below the central surface density. Three of the resulting cluster profiles are reasonably well-fit using single-mass King models, with core and tidal radii typical of those seen in Galactic globular clusters. We confirm an earlier report of the discovery of a cluster which has apparently undergone core collapse. Three of the four clusters show departures in their outskirts from King model behavior which, based on recent results for Galactic globulars, may indicate the presence of tidal tails.

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