K Giants in Baade's Window. II. The Abundance Distribution

This is the second in a series of papers in which we analyze spectra of over 400 K and M giants in Baade's Window, including most of the stars with proper motions measured by Spaenhauer et al. [AJ, 103, 297 (1992)]. In our first paper, we measured line--strength indices of Fe, Mg, CN and H$\beta$ and calibrated them on the system of Faber et al. [ApJS, 57, 711 (1985)]. Here, we use the $\langle{\rm Fe}\rangle$ index to derive an abundance distribution in [Fe/H] for 322 stars with effective temperatures between 3900 K and 5160 K. Our derived values of [Fe/H] agree well with those measured from high--resolution echelle spectra (e.g., McWilliam \& Rich [ApJS, 91, 749 (1994)]) for the small number of stars in common. We find a mean abundance $\langle{\rm [Fe/H]}\rangle = -0.11 \pm 0.04$ for our sample of Baade's Window K giants. More than half the sample lie in the range $-0.4 <$ \feh\ $<+0.3$. We estimate line--of--sight distances for individual stars in our sample and confirm that, in Baade's Window, most K giants with $V < 15.5$ are foreground disk stars, but the great majority (more than 80\%) with $V > 16$ belong to the bulge. We also compare the metallicities derived from the CN and Mg$_2$ indices to those from iron. Most of the metal--rich stars in our sample appear to be CN--weak, in contrast to the situation in metal--rich globular clusters and elliptical galaxies. The metal--poor half of our sample ([Fe/H] $< 0$) shows evidence for a mild Mg overenhancement ([Mg/Fe] $\sim +0.2$); but this is not seen in the more metal--rich stars ([Fe/H] $\geq$ 0). The K giants in Baade's Window therefore share some, but not all, of the characteristics of stars in elliptical galaxies as inferred from their integrated light.Comment: Accepted for publication in the Astronomical Journal, tentatively scheduled for July, 1996. LaTex source which generates 40 pages of text (no figures or tables). Complete (text + 15 figs + 5 tables) preprint in gzip/tar format is also available at ftp://bessel.mps.ohio-state.edu/pub/terndrup/kg2.tar.gz (227 kbyte

Ages, Distances, and the Initial Mass Functions of Stellar Clusters

We provide a review of the current status of several topics on the ages, distances, and mass functions of open clusters, with a particular emphasis on illuminating the areas of uncertainty. Hipparcos has obtained parallaxes for nearby open clusters that have expected accuracies much better than has been previously achievable. By using the lithium depletion boundary method and isochrone fitting based on much improved new theoretical evolutionary models for low mass stars, it is arguable that we will soon have have much better age scales for clusters and star-forming regions. With improved optical and near-IR cameras, we are just now beginning to extend the mass function of open clusters like the Pleiades into the regime below the hydrogen burning mass limit. Meanwhile, observations in star-forming regions are in principle capable of identifying objects down to of order 10 Jupiter masses.Comment: 13 pages, including 3 embedded figures (4 EPS files). To appear in "11th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun," ed. R. J. Garcia Lopez, R. Rebolo, and M. R. Zapatero Osori

GRB 060218/SN 2006aj: A Gamma-Ray Burst and Prompt Supernova at z=0.0335

We report the imaging and spectroscopic localization of GRB 060218 to a low-metallicity dwarf starburst galaxy at z = 0.03345 +/- 0.00006. In addition to making it the second nearest gamma-ray burst known, optical spectroscopy reveals the earliest detection of weak, supernova-like Si II near 5720 Angstroms (0.1c), starting 1.95 days after the burst trigger. UBVRI photometry obtained between 1 and 26 days post-burst confirms the early rise of supernova light, and suggests a short time delay between the gamma-ray burst and the onset of SN 2006aj if the early appearance of a soft component in the X-ray spectrum is understood as a ``shock breakout''. Together, these results verify the long-hypothesized origin of soft gamma-ray bursts in the deaths of massive stars.Comment: 5 pages, 2 figure

Resolving the pulsations of subdwarf B stars: HS 0039+4302, HS 0444+0458, and an examination of the group properties of resolved pulsators

We continue our program of single-site observations of pulsating subdwarf B (sdB) stars and present the results of extensive time series photometry of HS 0039+4302 and HS 0444+0458. Both were observed at MDM Observatory during the fall of 2005. We extend the number of known frequencies for HS 0039+4302 from 4 to 14 and discover one additional frequency for HS 0444+0458, bringing the total to three. We perform standard tests to search for multiplet structure, measure amplitude variations, and examine the frequency density to constrain the mode degree $\ell$. Including the two stars in this paper, 23 pulsating sdB stars have received follow-up observations designed to decipher their pulsation spectra. It is worth an examination of what has been detected. We compare and contrast the frequency content in terms of richness and range and the amplitudes with regards to variability and diversity. We use this information to examine observational correlations with the proposed $\kappa$ pulsation mechanism as well as alternative theories.Comment: 32 pages, 18 figures, 7 tables. Accepted for publication in MNRA

GRB 021004: A Possible Shell Nebula around a Wolf-Rayet Star Gamma-Ray Burst Progenitor

The rapid localization of GRB 021004 by the HETE-2 satellite allowed nearly continuous monitoring of its early optical afterglow decay, as well as high-quality optical spectra that determined a redshift of z=2.328 for its host, an active starburst galaxy with strong Lyman-alpha emission and several absorption lines. Spectral observations show multiple absorbers blueshifted by up to 3,155 km/s relative to the host galaxy Lyman-alpha emission.We argue that these correspond to a fragmented shell nebula, gradually enriched by a Wolf-Rayet wind over the lifetime of a massive progenitor bubble. In this scenario, the absorbers can be explained by circumstellar material that have been radiatively accelerated by the GRB emission. Dynamical and photoionization models are used to provide constraints on the radiative acceleration from the early afterglow.Comment: 5 pages, 3 figures, to appear in the proceedings of the 2003 GRB Conferenc

K giants in Baade's window; 1, velocity and line-strength measurements

This is the first in a series of papers in which we analyze medium--resolution spectra of over 400 K and M giants in Baade's Window. Our sample was selected from the proper motion study of Spaenhauer et al. [AJ, 103, 297 (1992)]. We have measured radial velocities for most of the sample, as well as line--strength indices on the system of Faber et al. [ApJS, 57, 711 (1985)]. We analyze the random and systematic errors in velocities and line strengths, and show that the bright (V < 16.0) stars in our sample are predominantly foreground disk stars along the line--of--sight toward Baade's Window. We find that most of the bulge K giants have stronger Mg absorption at a given color than do stars in the solar neighborhood. If the K giants in our sample are moderately old, we suggest that on average they may have [Mg/Fe] ~ +0.3, consistent with the results of recent high--resolution spectroscopy in Baade's Window

Abundance Patterns in Stars in the Bulge and Galactic Center

We discuss oxygen and iron abundance patterns in K and M red-giant members of the Galactic bulge and in the young and massive M-type stars inhabiting the very center of the Milky Way. The abundance results from the different bulge studies in the literature, both in the optical and the infrared, indicate that the [O/Fe]-[Fe/H] relation in the bulge does not follow the disk relation, with [O/Fe] values falling above those of the disk. Based on these elevated values of [O/Fe] extending to large Fe abundances, it is suggested that the bulge underwent a rapid chemical enrichment with perhaps a top-heavy initial mass function. The Galactic Center stars reveal a nearly uniform and slightly elevated (relative to solar) iron abundance for a studied sample which is composed of 10 red giants and supergiants. Perhaps of more significance is the fact that the young Galactic Center M-type stars show abundance patterns that are reminiscent of those observed for the bulge population and contain enhanced abundance ratios of alpha-elements relative to either the Sun or Milky Way disk at near-solar metallicities.Comment: requires iaus.cls; to appear in Formation and Evolution of Galaxy Bulges, Proceedings IAU Symposium No. 245, 2007, M. Bureau et al. eds., in pres

Implications of New JHK Photometry and a Deep Infrared Luminosity Function for the Galactic Bulge

We present deep near-IR photometry for Galactic bulge stars in Baade's Window, $(l,b) = (1.0\deg, -3.9\deg),$ and another minor axis field at $(l,b) = (0^\circ,-6^\circ)$. We combine our data with previously published photometry and construct a luminosity function over the range $5.5 \leq K_0 \leq 16.5$, deeper than any previously published. The slope of this luminosity function and the magnitude of the tip of the first ascent giant branch are consistent with theoretical values derived from isochrones with appropriate age and metallicity. We use the relationship between [Fe/H] and the giant branch slope derived from near-IR observations of metal rich globular clusters by Kuchinski {\it et al.} [AJ, 109, 1131 (1995)] to calculate the mean metallicity for several bulge fields along the minor axis. For Baade's Window we derive $\langle {\rm[Fe/H]}\rangle = -0.28 \pm 0.16$, consistent with the recent estimate of McWilliam \& Rich [ApJS, 91, 749 (1994)], but somewhat lower than previous estimates based on CO and TiO absorption bands and the $JHK$ colors of M giants by Frogel {\it et al.} [ApJ, 353, 494 (1990)]. Between $b = -3\deg$ and $-12\deg$ we find a gradient in $\langle {\rm [Fe/H]}\rangle$ of $-0.06 \pm 0.03$ dex/degree or $-0.43 \pm 0.21$ dex/kpc for $R_0 = 8$ kpc, consistent with other independent derivations. We derive a helium abundance for Baade's Window with the $R$ and $R^\prime$ methods and find that $Y = 0.27 \pm 0.03$ implying $\Delta Y / \Delta Z = 3.3 \pm 1.3$. Next, we find that the bolometric corrections for bulge K giants ($V - K \leq 2$) are in excellent agreement with empirical derivations based on observations of globular cluster and local field stars. However, for the redder M giants weComment: Accepted by the Astronomical Journal. 43 pages, uuencoded compressed PostScript, no figures or tables. A complete (text, figs and tables) preprint is also available at ftp://bessel.mps.ohio-state.edu/pub/terndrup/bwphot.tar.Z (compressed tar file with PostScript

The Color-Period Diagram and Stellar Rotational Evolution - New Rotation Period Measurements in the Open Cluster M34

We present results from a 5-month photometric survey for stellar rotation periods combined with a 4-year radial-velocity survey for membership and binarity in the 220Myr open cluster M34. We report surface rotation periods for 120 stars, 83 of which are late-type cluster members. A comparison to previous work serves to illustrate the importance of high cadence long baseline photometric observations and membership information. The new M34 periods are less biased against slow rotation and cleaned for non-members. The rotation periods of the cluster members span more than an order of magnitude from 0.5 day up to 11.5 days, and trace two distinct rotational sequences - fast (C) and moderate-to-slow (I) - in the color-period diagram. The sequences represent two different states in the rotational evolution of the late-type cluster members. We use the color-period diagrams for M34 and for younger and older clusters to estimate the timescale for the transition from the C to the I sequence and find ~<150Myr, ~150-300Myr, and ~300-600Myr for G, early-mid K, and late K dwarfs, respectively. The small number of stars in the gap between C and I suggest a quick transition. We estimate a lower limit on the maximum spin-down rate (dP/dt) during this transition to be ~0.06 days/Myr and ~0.08 days/Myr for early and late K dwarfs, respectively. We compare the I sequence rotation periods in M34 and the Hyades for G and K dwarfs and find that K dwarfs spin down slower than the Skumanich rate. We determine a gyrochronology age of 240Myr for M34. We measure the effect of cluster age uncertainties on the gyrochronology age for M34 and find the resulting error to be consistent with the error estimate for the technique. We use the M34 I sequence to redetermine the coefficients in the expression for rotational dependence on color used in gyrochronology (abridged).Comment: 47 pages (12pt, preprint), 14 figures, 2 tables, Accepted for publication in ApJ, format of RA coordinates in Table 2 corrected in latest versio

Chemical Abundances in the Galactic Center

We present chemical abundances in a sample of luminous cool stars located within 50 pc of the Galactic Center. Abundances of C, N, O, Ca and Fe are derived using high-resolution infrared spectra in the H- and K- bands. We report solar iron abundance, enhanced alpha element abundances, and CN-cycle mixed material in the atmospheres of these evolved stars