arXiv subjects

Robert Zinn

Publications and source records attributed to Robert Zinn.

24 records · Page 2Linked to original sources

arXiv2003

The Chemical Enrichment History of the Fornax Dwarf Spheroidal Galaxy from the Infrared Calcium Triplet

Near infrared spectra were obtained for 117 red giants in the Fornax dwarf spheroidal galaxy with the FORS1 spectrograph on the VLT, in order to study the metallicity distribution of the stars and to lift the age-metallicity degeneracy of the red giant branch (RGB) in the color-magnitude diagram (CMD). Metallicities are derived from the equivalent widths of the infrared Calcium triplet lines at 8498, 8542, and 8662 A, calibrated with data from globular clusters, the open cluster M67 and the LMC. For a substantial portion of the sample, the strength of the Calcium triplet is unexpectedly high, clearly indicating that the main stellar population of Fornax is significantly more metal-rich than could be inferred from the position of its RGB in the CMD. We show that the relative narrowness of the RGB in Fornax is caused by the superposition of stars of very different ages and metallicities. The metallicity distribution in Fornax is centered at [Fe/H]= -0.9, with a metal-poor tail extending to [Fe/H] ~= -2. While the distribution to higher metallicities is less well determined by our observations, the comparison with LMC data indicates that it extends to [Fe/H] ~ -0.4. By comparing the metallicities of the stars with their positions in the CMD, we have derived the complex age-metallicity relation of Fornax. In the first few Gyr, the metal abundance rose to [Fe/H] ~ -1.0 dex. The enrichment accelerated in the past ~ 1-4 Gyr to reach [Fe/H] ~ -0.4 dex. More than half the sample is constituted of star younger than ~ 4 Gyr, thus indicating sustained recent star formation in Fornax. These results indicate that the capacity of dwarf spheroidal galaxies to retain the heavy elements that they produce is larger than expected. (Abridged)

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arXiv2003

The Selection of RR Lyrae Stars Using Single-epoch Data

We utilize a complete sample of RR Lyrae stars discovered by the QUEST survey using light curves to design selection criteria based on SDSS colors. Thanks to the sensitivity of the u-g color to surface gravity and of g-r color to effective temperature, and to the small photometric errors (~0.02 mag) delivered by SDSS, RR Lyrae stars can be efficiently and robustly recognized even with single-epoch data. In a 100% complete color-selected sample, the selection efficiency (the fraction of RR Lyrae stars in the candidate sample) is 6%, and, by adjusting color cuts, it can be increased to 10% with a completeness of 80%, and to 60% with 28% completeness. Such color selection produces samples that are sufficiently clean for statistical studies of the Milky Way's halo substructure, and we utilize it to select 3,643 candidate RR Lyrae stars from SDSS Data Release 1. We demonstrate that this sample recovers known clumps of RR Lyrae stars associated with the Sgr dwarf tidal tail, and Pal 5 globular cluster, and use it to constrain the halo substructure away from the Sgr dwarf tidal tail. These results suggest that it will be possible to study the halo substructure out to ~70 kpc from the Galactic Center in the entire area imaged by the SDSS, and not only in the multiply observed regions.

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arXiv2003

Halo Substructure in the QUEST RR Lyrae Survey

A survey of 380 sq. deg. of the sky with the 1m Schmidt telescope at the Observatorio Nacional de Llano del Halo and the QUEST camera has found 498 RR Lyrae variables lying from 4 to 60 kpc from the Sun. We describe the halo substructure revealed by these data and the results of measuring some of the stars' radial velocities and metal abundances.

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arXiv2003

Velocities of RR Lyrae Stars in the Sagittarius Tidal Stream

We have measured radial velocities and metallicities of 16 RR Lyrae stars, from the QUEST survey, in the Sagittarius tidal stream at 50 kpc from the galactic center. The distribution of velocities is quite narrow (std dev=25 km/s) indicating that the structure is coherent also in velocity space. The mean heliocentric velocity in this part of the stream is 32 km/s. The mean metallicity of the RR Lyrae stars is [Fe/H]=-1.7. Both results are consistent with previous studies of red giant stars in this part of the stream. The velocities also agree with a theoretical model of the disruption of the Sagittarius galaxy.

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arXiv2002

RR Lyrae Stars in the Halo: Tracers of Streams of Debris of Disrupted Galaxies

We discuss the first part of a survey for RR Lyrae variables in the galactic halo that is being made with the 1m Schmidt telescope at the Venezuelan National Observatory. So far the survey has discovered 497 variables in 380 sq. degrees, lying from 4 to 60 kpc from the Sun. It has detected three statistically significant clumps of variables, which shows that outer halo does not have smooth density contours. One clump is located at 50 kpc from the galactic center, and it is probably tidal debris from the Sagittarius dwarf spheroidal galaxy. A second structure, at 17 kpc from the galactic center, appears to be due to the tidal disruption of the globular cluster Pal 5. The third group, at 19 kpc, is not related with any known globular cluster or dwarf galaxy. The little that is known about its properties is consistent with it being debris from a disrupted galaxy.

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arXiv1999

The Metallicity Dependence of RR Lyrae Absolute Magnitudes from Synthetic Horizontal-Branch Models

A grid of synthetic horizontal-branch (SHB) models based on HB evolutionary tracks with improved physics has been constructed to reconsider the theoretical calibration of the dependence of M_v(RR) on metallicity in globular clusters, and the slope of the mean -[Fe/H] relation. The SHB models confirm Lee's earlier finding (Lee 1991) that the slope of the -[Fe/H] relation is itself a function of the metallicity range considered, and that in addition, for a given [Fe/H], RR Lyrae luminosities depend on HB morphology. This is due to the fact that HB stars pass through the RR Lyrae instability strip at different evolutionary stages, depending on their original position on the HB. At [Fe/H]=-1.9, and for HB type 0, the models yield M_v(RR)=0.47 \pm 0.10. The mean slope for the zero-age HB models is 0.204. Since there is no simple universal relation between M_v(RR) and metallicity that is applicable to all globular clusters, the HB morphology of each individual cluster must be taken into account, in addition to [Fe/H], in deriving the appropriate M_v(RR). Taking HB morphology into account, we find that the slope of the mean -[Fe/H] relation varies between 0.36 for the clusters with galactocentric distances R_gc less than 6 kpc and 0.22 for clusters with 6<R_gc<20 kpc. Implications for interpreting observations of field RR Lyrae variables and for absolute globular cluster ages and galactic chronology are briefly discussed.

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