Michael Bolte

Detailed Abundances for 28 Metal-poor Stars: Stellar Relics in the Milky Way

We present the results of an abundance analysis for a sample of stars with � 4 < ½Fe/H� < � 2. The data were obtained with the HIRES spectrograph at Keck Observatory. The set includes 28 stars, with effective temperature ranging from 4800 to 6600 K. For 13 stars with ½Fe/H� < � 2:6, including nine with ½Fe/H� < � 3:0 and one with ½Fe/H �¼ � 4:0, these are the first reported detailed abundances. For the most metal-poor star in our sample, CS 30336� 049, we measure an abundance pattern that is very similar to stars in the range ½Fe/H ��� 3:5, including a normal C þ N abundance. We also find that it has very low but measurable Sr and Ba, indicating some neutroncapture activity even at this low of a metallicity. We explore this issue further by examining other very neutron captureYdeficient stars and find that, at the lowest levels, [Ba/Sr] exhibits the ratio of the main r-process. We also report on a new r-processYenhanced star, CS 31078� 018. This star has ½Fe/H �¼ � 2:85, ½Eu/Fe �¼ 1:23, and ½Ba/Eu �¼ � 0:51. CS 31078� 018 exhibits an ‘‘actinide boost,’’ i.e., much higher [Th/Eu] than expected and at a similar level to CS 31082� 001. Our spectra allow us to further constrain the abundance scatter at low metallicities, which we then use to fit to the zero-metallicity Type II supernova yields of Heger & Woosley (2008). We find that

Photometry and Spectroscopy of GRB 030329 and Its Associated Supernova 2003dh: The First Two Months

We present extensive optical and infrared photometry of the afterglow of gamma-ray burst (GRB) 030329 and its associated supernova (SN) 2003dh over the first two months after detection (2003 March 30-May 29 UT). Optical spectroscopy from a variety of telescopes is shown and, when combined with the photometry, allows an unambiguous separation between the afterglow and SN contributions. The optical afterglow of the GRB is initially a power-law continuum but shows significant color variations during the first week that are unrelated to the presence of an SN. The early afterglow light curve also shows deviations from the typical power-law decay. An SN spectrum is first detectable ~7 days after the burst and dominates the light after ~11 days. The spectral evolution and the light curve are shown to closely resemble those of SN 1998bw, a peculiar Type Ic SN associated with GRB 980425, and the time of the SN explosion is close to the observed time of the GRB. It is now clear that at least some GRBs arise from core-collapse SNe.

Fossil signatures of ancient accretion events in the halo

The role that minor mergers have played in the formation and structure of the Milky Way is still an open question, about which there is much debate. We use numerical simulations to explore the evolution of debris from a tidally disrupted satellite, with the aim of developing a method that can be used to identify and quantify signatures of accretion in a survey of halo stars. For a Milky Way with a spherical halo, we find that debris from minor mergers can remain aligned along great circles throughout the lifetime of the Galaxy. We exploit this result to develop the method of Great Circle Cell Counts (GC3), which we test by applying it to artificially constructed halo distributions. Our results suggest that if as few as 1\% of the stars in a halo survey are accreted from the disruption of a single subsystem smaller than the Large Magellanic Cloud, GC3 can recover the great circle associated with this debris. The dispersion in GC3 can also be used to detect the presence of structure characteristic of accretion in distributions containing a much smaller percentage of material accreted from any single satellite.

PROBING THE LOWER MASS LIMIT FOR SUPERNOVA PROGENITORS AND THE HIGH-MASS END OF THE INITIAL-FINAL MASS RELATION FROM WHITE DWARFS IN THE OPEN CLUSTER M35 (NGC 2168)*

We present a photometric and spectroscopic study of the white dwarf (WD) population of the populous, intermediate-age open cluster M35 (NGC 2168); this study expands upon our previous study of the WDs in this cluster. We spectroscopically confirm 14 WDs in the field of the cluster: 12 DAs, 1 hot DQ, and 1 DB star. For each DA, we determine the WD mass and cooling age, from which we derive each stars progenitor mass. These data are then added to the empirical initial-final mass relation (IFMR), where the M35 WDs contribute significantly to the high-mass end of the relation. The resulting points are consistent with previously published linear fits to the IFMR, modulo moderate systematics introduced by the uncertainty in the star cluster age. Based on this cluster alone, the observational lower limit on the maximum mass of WD progenitors is found to be ~5.1 M ? ? 5.2 M ? at the 95% confidence level; including data from other young open clusters raises this limit to as high as 7.1 M ?, depending on the cluster membership of three massive WDs and the core composition of the most massive WDs. We find that the apparent distance modulus and extinction derived solely from the cluster WDs ((m ? M) V = 10.45 ? 0.08 and E(B-V) = 0.185 ? 0.010, respectively) is fully consistent with that derived from main-sequence fitting techniques. Four M35 WDs may be massive enough to have oxygen-neon cores; the assumed core composition does not significantly affect the empirical IFMR. Finally, the two non-DA WDs in M35 are photometrically consistent with cluster membership; further analysis is required to determine their memberships.

White Dwarfs in Globular Clusters: Hubble Space Telescope Observations of M4*

Using WFPC2 on the Hubble Space Telescope, we have isolated a sample of 258 white dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radial distances from the cluster center were observed, and sizable WD populations were found in all three. The location of these WDs in the color-magnitude diagram, their mean mass of 0.51(±0.03) M☉, and their luminosity function confirm basic tenets of stellar evolution theory and support the results from current WD cooling theory. The WDs are used to extend the cluster main-sequence mass function upward to stars that have already completed their nuclear evolution. The WD/red dwarf binary frequency in M4 is investigated and is found to be at most a few percent of all the main-sequence stars. The most ancient WDs found are ~9 Gyr old, a level that is set solely by the photometric limits of our data. Even though this is less than the age of M4, we discuss how these cooling WDs can eventually be used to check the turnoff ages of globular clusters and hence constrain the age of the universe.

Measuring age differences among globular clusters having similar metallicities - A new method and first results

A color-difference technique for estimating the relative ages of globular clusters with similar chemical compositions on the basis of their CM diagrams is described and demonstrated. The theoretical basis and implementation of the procedure are explained, and results for groups of globular clusters with m/H = about -2, -1.6, and -1.3, and for two special cases (Palomar 12 and NGC 5139) are presented in extensive tables and graphs and discussed in detail. It is found that the more metal-deficient globular clusters are nearly coeval (differences less than 0.5 Gyr), whereas the most metal-rich globular clusters exhibit significant age differences (about 2 Gyr). This result is shown to contradict Galactic evolution models postulating halo collapse in less than a few times 100 Myr. 77 refs.

The c IV mass density of the universe at redshift 5

In order to search for metals in the Lyα forest at redshifts zabs > 4, we have obtained spectra of high signal-to-noise ratio and moderately high resolution of three QSOs at zem > 5.4 discovered by the Sloan Digital Sky Survey. These data allow us to probe to metal enrichment of the intergalactic medium at early times with higher sensitivity than previous studies. We find 16 C IV absorption systems with column densities log N(C ) = 12.50-13.98 over a total redshift path ΔX = 3.29. In the redshift interval z = 4.5-5.0, where our statistics are most reliable, we deduce a comoving mass density of C3+ ions Ω = (4.3 ± 2.5) × 10-8 (90% confidence limits) for absorption systems with log N(C ) ≥ 13.0 (for an Einstein-de Sitter cosmology with h = 0.65). This value of Ω is entirely consistent with those measured at z < 4; we confirm the earlier finding by Songaila that neither the column density distribution of C IV absorbers nor its integral show significant redshift evolution over a period of time that stretches from ~1.25 to ~4.5 Gyr after the big bang. This somewhat surprising conclusion may be an indication that the intergalactic medium was enriched in metals at z 5, perhaps by the sources responsible for its reionization. Alternatively, the C IV systems we see may be associated with outflows from massive star-forming galaxies at later times, while the truly intergalactic metals may reside in regions of the Lyα forest of lower density than those probed up to now.

Ccd photometry of the globular cluster m5. I. the color-magnitude diagram and luminosity functions

We present new BV I photometry for the halo globular cluster M5 (NGC 5904 = C1516+022), and examine the B- and I-band luminosity functions (LFs), based on over 20,000 stars — one of the largest samples ever gathered for a cluster luminosity function. Extensive artificial star tests have been conducted to quantify incompleteness as a function of magnitude and cluster radius. We do not see evidence in the LF of a “subgiant excess” or of a discrepancy in the relative numbers of stars on the red-giant branch and main sequence, both of which have been claimed in more metal-poor clusters. Enhancements of �-element have been taken into account in our analysis. This improves the agreement between the observed and predicted positions of the “red-giant bump”. Depending on the average �-element enhancement among globular clusters and the distance calibration, the observed discrepancy between the theoretical and observed position for a large number of clusters (Fusi Pecci et al. 1990) can be almost completely removed. The helium abundance of M5, as determined by the population ratio R, is found to be Y = 0.19 ± 0.02. However, there is no other indication that the helium abundance is different from other clusters of similar metallicity, and values calculated for other helium indicators are consistent with Y ≈ 0.23. The relative ages of M5, Palomar 5, M4, NGC 288, NGC 362, NGC 1261, NGC 1851 and � , , !

Th Ages for Metal-poor Stars

With a sample of 22 metal-poor stars, we demonstrate that the heavy-element abundance pattern (Z ≥ 56) is the same as the r-process contributions to the solar nebula. This bolsters the results of previous studies that there is a universal r-process production pattern. We use the abundance of thorium in five metal-poor stars, along with an estimate of the initial Th abundance based on the abundances of stable r-process elements, to measure their ages. We have four field red giants with errors of 4.2 Gyr in their ages and one M92 giant with an error of 5.6 Gyr, based on considering the sources of observational error only. We obtain an average age of 11.4 Gyr, which depends critically on the assumption of an initial Th/Eu production ratio of 0.496. If the universe is 15 Gyr old, then the (Th/Eu)0 should be 0.590, in agreement with some theoretical models of the r-process.

Hubble Space Telescope Observations of the Oldest Star Clusters in the Large Magellanic Cloud

We present V, V-I color-magnitude diagrams (CMDs) for three old star clusters in the Large Magellanic Cloud (LMC): NGC 1466, NGC 2257, and Hodge 11. Our data extend ~3 mag below the main-sequence turnoff, allowing us to determine accurate relative ages and the blue straggler frequencies. Based on a differential comparison of the CMDs, any age difference between the three LMC clusters is less than 1.5 Gyr. Comparing their CMDs to those of M92 and M3, the LMC clusters, unless their published metallicities are significantly in error, are the same age as the old Galactic globulars. The similar ages to Galactic globulars are shown to be consistent with hierarchial clustering models of galaxy formation. The blue straggler frequencies are also similar to those of Galactic globular clusters. We derive a true distance modulus to the LMC of (m - M)0 = 18.46 ± 0.09 [assuming (m - M)0 = 14.61 for M92] using these three LMC clusters.