Ruth C. Peterson

The Chemical Composition Contrast between M3 and M13 Revisited: New Abundances for 28 Giant Stars in M3

We report new chemical abundances of 23 bright red giant members of the globular cluster M3, based on high-resolution (R ~ 45,000) spectra obtained with the Keck I telescope. The observations, which involve the use of multislits in the HIRES Keck I spectrograph, are described in detail. Combining these data with a previously reported small sample of M3 giants obtained with the Lick 3 m telescope, we compare metallicities and [X/Fe] ratios for 28 M3 giants with a 35-star sample in the similar-metallicity cluster M13, and with Galactic halo field stars having [Fe/H] < -1. For elements having atomic number A ? A(Si), we derive little difference in [X/Fe] ratios in the M3, M13, or halo field samples. All three groups exhibit C depletion with advancing evolutionary state beginning at the level of the red giant branch bump, but the overall depletion of about 0.7?0.9 dex seen in the clusters is larger than that associated with the field stars. The behaviors of O, Na, Mg, and Al are distinctively different among the three stellar samples. Field halo giants and subdwarfs have a positive correlation of Na with Mg, as predicted from explosive or hydrostatic carbon burning in Type II supernova sites. Both M3 and M13 show evidence of high-temperature proton-capture synthesis from the ON, NeNa, and MgAl cycles, while there is no evidence for such synthesis among halo field stars. But the degree of such extreme proton-capture synthesis in M3 is smaller than it is in M13: the M3 giants exhibit only modest deficiencies of O and corresponding enhancements of Na, less extreme overabundances of Al, fewer stars with low Mg and correspondingly high Na, and no indication that O depletions are a function of advancing evolutionary state, as has been claimed for M13. We have also considered NGC 6752, for which Mg isotopic abundances have been reported by Yong et al. Giants in NGC 6752 and M13 satisfy the same anticorrelation of O abundances with the ratio (25Mg + 26Mg)/24Mg, which measures the relative contribution of rare to abundant isotopes of Mg. This points to a scenario in which these abundance ratios arose in the ejected material of 3?6 M? cluster stars, material that was then used to form the atmospheres of the presently evolving low-mass cluster stars. It also suggests that the low oxygen abundance seen among the most evolved M13 giants arose in hot bottom O-to-N processing in these same intermediate-mass cluster stars. Thus, mixing is required by the dependence of some abundance ratios on luminosity, but an earlier nucleosynthesis process in a hotter environment than giants or main-sequence stars is required by the variations previously seen in stars near the main sequence. The nature and the site of the earlier process is constrained but not pinpointed by the observed Mg isotopic ratio.

Velocities of stars in remote Galactic satellites and the mass of the Galaxy

Observations of the two most distant known dwarf spheroidal Galactic satellites, Leo I and II, and of two remote globular clusters, Eridanus and Pal 14, are presented. It is demonstrated that etalon spectra can be used for accurate wavelength calibration across the entire observed spectral range. Measured velocities of three stars in Eridanus, two stars in Pal 14, six stars in Leo I, and five stars in Leo II are used to derive that the heliocentric systemic velocities of these systems are - 21 + or - 4, 72 + or - 4, 285 + or - 3, and 70 + or - 4 km/s, respectively. The value of the velocity for Leo I is in significant disagreement with previously published values. Timing arguments are used to estimate that the mass of the Galaxy is at least 13 x 10 to the 11th solar. This value is valid only if Leo I is gravitationally bound to the Galaxy, and arguments supporting this assumption are presented. 70 refs.

The nonsolar abundance ratios of Arcturus deduced from spectrum synthesis

Using opacity distribution functions based on a newly expanded atomic and molecular line list, we have calculated a model atmosphere for Arcturus that reproduces the observed flux distribution. Individual line parameters in the list were adjusted to match the solar spectrum in a preliminary way, in the regions 5000-5500 A, 6000-6500 A, and 7500-9000 A. The Arcturus model spectrum calculated with these adjustments reproduces well the profiles of all lines in the observed spectrum of the Griffin atlas for which the solar gf-values are well determined. The Arcturus model has an iron abundance [Fe/H]=−0.5±0.1, a temperature T eff =4300±30 K, gravity log g=1.5±0.15, and an overabundance of the light metals

Addendum: Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15. II. Kinematic Analysis and Dynamical Modeling [Astron. J. 124, 3270 (2002)]

It has been reported that there is an error in the figure in Dull et al. (1997, D97) that shows the radial M/L profile in Fokker-Planck models of M15. We discuss how this modifies the interpretation of our kinematical data. These imply the existence of a dark and compact mass component near the center of M15, either a single black hole (BH) or a collection of dark remnants that have sunk to the cluster center due to mass segregation. We previously showed that the latter interpretation is in conflict with the D97 M/L profile, which supported the BH interpretation. We repeat our analysis here with the corrected D97 profile. Models without a BH are now found to be statistically acceptable (within 1-sigma), although inclusion of a BH still provides a marginally better fit. It does not necessarily follow that dark remnants are now the preferred interpretation of the data. The D97 models, as well as N-body models by Baumgardt et al. (2002), assume that all neutron stars are retained during cluster evolution. This conflicts with predictions of the neutron-star retention rate (typically below 10 per cent) based on pulsar kick velocities. The presence of a single BH therefore continues to be a viable interpretation of the data. The best fit BH mass with the corrected D97 M/L profile is M_BH = 1.7^{+2.7}_{-1.7} x 10^3 solar masses, and with a constant M/L it is M_BH = 3.2^{+2.2}_{-2.2} x 10^3 solar masses. A model that includes both neutron star escape and mass segregation would probably yield a value between these numbers. This agrees with the correlation between velocity dispersion and BH mass inferred for galaxies. However, with the presently available models and data it is neither uniquely implied nor ruled out that M15 has an intermediate-mass BH.

Stellar velocity dispersions of dwarf elliptical galaxies

We present new measurements of the central stellar velocity dispersions (σ) of dwarf elliptical galaxies (dEs), most of them of the nucleated variety. The data consist of photon counting spectra taken at the MMT with spectral resolution of 12 and 32 km s −1 and CCD data from CTIO and KPNO at 8 and 18 km s −1 (FWHM). The measured dispersions range from 14 to 39 km −1 , and are lower than previous measurements done at more coarse resolution by about 50%. M/L ratios are derived for the galaxies. These are not unusually high (1-7, in solar units), but the change in M/L with luminosity for all dE galaxies confirms a prediction made by Dekel and Silk [ApJ, 303, 39 (1986)] (M/L∼L −0.40±0.06 ) as does the observed relation between L and velocity dispersion (L∼σ 5.6±0.9 )

Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15. I. STIS Spectroscopy and WFPC2 Photometry*

In this series of two papers, we describe a project with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) to measure the line-of-sight velocities of stars in the central few arcseconds of the dense globular cluster M15. The main goal of this project is to search for the possible presence of an intermediate-mass central black hole. This first paper focuses on the observations and reduction of the data. We scanned the central region of M15 spectroscopically by consecutively placing the 01 HST/STIS slit at 18 adjacent positions. The spectral pixel size exceeds the velocity dispersion of M15. This puts the project at the limit of what is feasible with STIS, and exceedingly careful and complicated data reduction and analysis were required. We applied corrections for the following effects: drifts in the STIS wavelength scale during an HST orbit; the orbital velocity component of HST along the line of sight to the cluster, and its variations during the HST orbit; and the apparent wavelength shift that is perceived for a star that is not centered in the slit. The latter correction is particularly complicated and requires many pieces of information: (1) the positions and magnitudes of all the stars near the center of M15; (2) accurate positionings of the STIS slits during the observations; (3) and the HST/STIS point-spread function (PSF) and line-spread function (LSF). To address the first issue we created a stellar catalog of M15 from the existing HST/WFPC2 data discussed previously by Guhathakurta et al., but with an improved astrometric and photometric calibration. The catalog is distributed electronically as part of this paper. It contains 31,983 stars with their positions and U, B, and V magnitudes. To address the second issue, we model the observed intensity profiles along the STIS slits to determine the slit positionings to 0007 accuracy in each coordinate. To address the third issue, we obtained observations of a bright field star to which we fitted multi-Gaussian PSF and LSF models. Upon reduction of the M15 spectroscopy we ultimately obtain 19,200 one-dimensional STIS spectra, each for a different aperture position in M15, with a velocity scale accurate to better than 2.5 km s-1. We develop an algorithm that co-adds the spectra for individual apertures and use it to extract spectra of individual stars with minimum blending and maximum signal-to-noise ratio. In Paper II we use these spectra to extract reliable line-of-sight velocities for 64 stars, half of which reside within R = 24 from the cluster center. These velocities constrain the central structure, dynamics, and mass distribution of the cluster.

Heavy-Element Abundances of the Old Open Cluster NGC 6791*

The old open cluster NGC 6791 is believed to be more metal-rich than any other, yet several hot blue horizontal-branch (BHB) stars are probable members. We have performed an abundance analysis of the coolest BHB candidate, 2-17, whose proper motion and radial velocity both support cluster membership. Its luminosity and its low rotational velocity, vsin

An Extremely Lithium-rich Bright Red Giant in the Globular Cluster M3

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Relative abundance determinations in extremely metal poor giants. II. Transition probabilities and the abundance determinations

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Radial velocities of stars in the globular cluster M4 and the cluster distance

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