arXiv subjects

Michael Bolte

Publications and source records attributed to Michael Bolte.

At least 55 records · Page 3Linked to original sources

arXiv2004

The Luminosity Function and Color-Magnitude Diagram of the Globular Cluster M12

In this paper we present the V and I luminosity functions and color-magnitude diagrams derived from wide-field (23 arcmin by 23 arcmin) BVI photometry of the intermediate metallicity ([Fe/H]=-1.3) Galactic globular cluster M12. Using observed values (and ranges of values) for the cluster metallicity, reddening, distance modulus, and age we compare these data to recent alpha-enhanced stellar evolution models for low mass metal-poor stars. We describe several methods of making comparisons between theoretical and observed luminosity functions in order to isolate the evolutionary timescale information the luminosity functions contain. We find no significant evidence of excesses of stars on the red giant branch, although the morphology of the subgiant branch in the observed luminosity function does not match theoretical predictions in a satisfactory way. Current uncertainties in Teff-color transformations (and possibly also in other physics inputs to the models) make more detailed conclusions about the subgiant branch morphology impossible. Given the recent constraints on cluster ages from the WMAP experiment (Spergel et al. 2003), we find that good fitting models that do not include He diffusion (both color-magnitude diagrams and luminosity functions) are too old (by approximately 1-2 Gyr) to adequately represent the cluster luminosity function. The inclusion of helium diffusion in the models provides an age reduction (compared to non-diffusive models) that is consistent with the age of the universe being 13.7+/-0.2 Gyr (Bennett et al. 2003).

astro-ph↗

arXiv2004

The s-process in metal-poor stars: Abundances for 22 neutron-capture elements in CS 31062-050

The CH star CS 31062-050 ([Fe/H]=-2.42) is one of the most useful stars yet discovered for evaluating the s-process in metal-poor stars. It is very abundant in heavy elements (e.g., [La/Fe]=2.2), and its relatively cool temperature and low gravity mean that there are many lines of interesting elements present in the spectrum. We measured the abundances of 22 elements with Z>29, including the rarely measured Lu and Pd. We derive an upper limit on the Th abundance as well. The abundances in CS 31062-050 show a similar pattern to many other metal-poor CH stars: high [Pb/Fe] and [Pb/La] ratios, low [Y/La] ratios and high [Eu/La] values compared to the solar system s-process. However, the Th limit, with additional assumptions, is not consistent with the idea that the excess Eu in CS 31062-050 is contributed by the r-process. In addition, the observed [Eu/Tb] cannot be explained by any ratio of solar-system s-process and r-process abundances. We therefore argue that the abundance pattern in CS 31062-050 is most likely the result of the s-process, and we discuss possible modifications that could explain the non-solar-system pattern observed.

astro-ph↗

arXiv2003

The C IV Mass Density of the Universe at Redshift 5

In order to search for metals in the Lyman alpha forest at redshifts z > 4, we have obtained spectra of high S/N and resolution of three QSOs at z > 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 IV) = 12.50 - 13.98 over a total redshift path Delta X = 3.29. In the redshift interval z = 4.5-5.0, where our statistics are most reliable, we deduce a comoving mass density of C IV ions Omega(C IV) = (4.3 +/- 2.5) x 10(-8) (90% confidence limits) for absorption systems with log N(C IV) > 13.0 (for an Einstein-de Sitter cosmology with h = 0.65). This value of Omega(C IV) is entirely consistent with those measured at z < 4; we confirm the earlier finding by Songaila (2001) that neither the column density distribution of C IV absorbers nor its integral show significant redshift evolution over a period of time which 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 redshifts much greater than 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 Lyman alpha forest of lower density than those probed up to now.

astro-ph↗

arXiv2002

The r-process in the early Galaxy

We report Sr, Pd and Ag abundances for a sample of metal-poor field giants and analyze a larger sample of Y, Zr, and Ba abundances. The [Y/Zr] and [Pd/Ag] abundance ratios are similar to those measured for the r-process-rich stars CS 22892-052 and CS 31082-001. The [Pd/Ag] ratio is larger than predicted from the solar-system r-process abundances. The constant[Y/Zr] and [Sr/Y] values in the field stars places strong limits on the contributions of the weak s-process and the main s-process to the light neutron-capture elements. Stars in the globular cluster M 15 possess lower [Y/Zr] values than the field stars. There is a large dispersion in [Y/Ba]. Because the r-process is responsible for the production of the heavy elements in the early Galaxy, these dispersions require varying light-to-heavy ratios in r-process yields.

astro-ph↗

arXiv2002

The white dwarf cooling sequence in the old open cluster NGC 188

We develop the white dwarf luminosity function (LF) of the old open cluster NGC 188 in order to determine a lower limit to the age of the cluster by using the faint end of the cooling sequence. To produce an extensive sequence of the cooling white dwarfs we imaged four contiguous HST-WFPC2 fields in the center of the cluster in the F555W and F814W filters. After imposing selection criteria on the detected objects we found a white dwarf cooling sequence (down to V ~26.5) including 28 candidate white dwarfs in the cluster. The exposures are not deep enough to reach the end of this sequence, but the results of our analysis allow us to establish a lower limit to the age of the cluster independently of the isochrone fit to the cluster turnoff. The most ancient white dwarfs found are ~4 Gyr old, an age that is set solely by the photometric limit of our data. Classical methods provide an estimate of \~7 Gyr (Sarajedini et al., 1999).

astro-ph↗

arXiv2002

Determination of the Dark Matter Profile of Abell 2199 from Integrated Starlight

We have obtained deep, long-slit spectroscopy along the major axis of NGC 6166, the cD galaxy in the cluster Abell 2199, in order to measure the kinematics of intracluster starlight at large radii. The velocity dispersion rises beyond a few kpc, and steadily rises to 660 km/s at a radius of 60 kpc, nearly reaching the velocity dispersion of the cluster (775+-50 km/s). These data suggest that the stars in the halo of the cD trace the potential of the cluster and that the kinematics of these intracluster stars can be used to constrain the mass profile of the cluster. In addition, we find evidence for systematic rotation in the intracluster stars beyond 20 kpc. The surface brightness and velocity dispersion profiles can be fit using a single component mass model only by making unphysical assumptions about the level of anisotropy for both the stars in the cD galaxy and for the kinematics of the galaxies in the cluster. Two-component mass models for the cD and its halo are explored using the kinematics of known cluster members as an additional constraint beyond the extent of the stellar kinematics. Assuming isotropy, the observed kinematics can be reproduced only if the halo has a soft core, i.e., Alpha < 1 (Alpha=1 is excluded due to low implied stellar mass-to-light ratios). This result is inconsistent with the predictions of current N-body simulations with CDM. Assuming that our best-fit mass models for the Abell 2199 were placed at cosmological redshifts between 0.2 <= z <= 0.5, the derived tangential critical radii range from 5'' to 40'' consistent with the strong lensing observed in intermediate redshift clusters. We also present Monte Carlo simulations that test the reliability of velocity dispersions when the S/N is low.

astro-ph↗

arXiv2001

Th Ages for Metal-Poor Stars

With a sample of 22 metal-poor stars, we demonstrate that the heavy element abundance pattern (Z > 55) 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 production ratio of Th/Eu of 0.496. If the Universe is 15 Gyr old, then the initial Th/Eu value should be 0.590, in agreement with some theoretical models of the r-process.

astro-ph↗

arXiv2001

The z=0 Galaxy Luminosity Function: I. Techniques for Identification of Dwarf Galaxies at ~10 Mpc

We present a program to study the galaxy luminosity function (GLF) of the Leo I and Coma I Groups at ~10 Mpc. We have surveyed over seven square degrees in Leo I and ~11 square degrees in Coma I. In this paper, we detail the method we have developed and implemented for identifying on morphological grounds low-surface-brightness, M_R < -10 dwarf galaxies at a distance of 10 Mpc. We also describe extensive Montecarlo simulations of artificial galaxies which we use to tune our detection algorithms and evaluate our detection efficiency and parameter recovery as a function of mu_R(0) and R_T. We find for a sub-set of our Leo I data that at the 90% completeness level we can detect dwarfs comparable to Antlia and Sculptor. Finally, we describe preliminary follow-up observations which confirm we are detecting dwarf spheroidals in Leo I at 10 Mpc.

astro-ph↗

arXiv2000

The Dwarf Galaxy Population of the Dorado group down to Mv=-11

We present V and I CCD photometry of suspected low-surface brightness dwarf galaxies detected in a survey covering ~2.4 deg^2 around the central region of the Dorado group of galaxies. The low-surface brightness galaxies were chosen based on their sizes and magnitudes at the limiting isophote of 26.0Vμ. The selected galaxies have magnitudes brighter than V=20 (Mv=-11 for an assumed distance to the group of 17.2 Mpc), with central surface brightnesses \mu0>22.5 V mag/arcsec^2, scale lengths h>2'', and diameters > 14'' at the limiting isophote. Using these criteria, we identified 69 dwarf galaxy candidates. Four of them are large very low-surface brightness galaxies that were detected on a smoothed image, after masking high surface brightness objects. Monte Carlo simulations performed to estimate completeness, photometric uncertainties and to evaluate our ability to detect extended low-surface brightness galaxies show that the completeness fraction is, on average, > 80% for dwarf galaxies with $-17 14''). The V-I colors of the dwarf candidates vary from -0.3 to 2.3 with a peak on V-I=0.98, suggesting a range of different stellar populations in these galaxies. The projected surface density of the dwarf galaxies shows a concentration towards the group center similar in extent to that found around five X-ray groups and the elliptical galaxy NGC1132 studied by Mulchaey and Zabludoff (1999), suggesting that the dwarf galaxies in Dorado are probably physically associated with the overall potential well of the group.

astro-ph↗

arXiv1998

Wide-Field CCD Photometry of the Globular Cluster M30

We present new VI photometry for the halo globular cluster M30 (NGC 7099), and compute luminosity functions (LFs) in both bands for samples of about 15,000 hydrogen-burning stars from near the tip of the red giant branch (RGB) to over four magnitudes below the main-sequence (MS) turnoff. We confirm previously observed features of the LF that are at odds with canonical theoretical predictions: an excess of stars on subgiant branch (SGB) approximately 0.4 mag above the turnoff and an excess number of RGB stars relative to MS stars. Based on subdwarfs with Hipparcos-measured parallaxes, we compute apparent distance moduli of (m-M)_{V} = 14.87+/-0.12 and 14.65+/-0.12 for reddenings of E(V-I)=0.06 and 0.02 respectively. The implied luminosity for the horizontal branch (HB) at these distances is M_V(HB)=0.11 and 0.37 mag. The two helium indicators we have been able to measure (R and Delta) both indicate that M30's helium content is high relative to other clusters of similar metallicity. M30 has a larger value for the parameter Delta V_{TO}^{HB} than any of the other similarly metal-poor clusters for which this quantity can be reliably measured. This suggests that M30 has either a larger age or higher helium content than all of the other metal-poor clusters examined. The color-difference method for measuring relative ages indicates that M30 is coeval with the metal-poor clusters M68 and M92.

astro-ph↗

arXiv1998

Ages for Globular Clusters in the Outer Galactic Halo: The Second Parameter Clusters Palomar~3, Palomar~4 and Eridanus

We have used the WFPC2 camera on the Hubble Space Telescope to obtain photometry of the outer-halo globular clusters Palomar 3, Palomar 4, and Eridanus. These three are classic examples of the "second parameter" anomaly because of their red horizontal-branch morphologies in combination with their low-to-intermediate metallicities. Our color-magnitude diagrams in (V, V-I) reach $V_{lim} \simeq 27.0$, clearly delineating the subgiant and turnoff regions and about three magnitudes of the unevolved main sequences. The slopes and dereddened colors of the giant branches are consistent with published [Fe/H] estimates that rank the clusters (Pal 3, Eridanus, Pal 4) in order of increasing metallicity, with all three falling near or between the abundance values of the classic nearby halo clusters M3 and M5. Differential fits of their color-magnitude diagrams are made to each other and to M3 and M5 for relative age determinations. We find that the three outer-halo cluster CMDs differ from the nearby clusters in a way that is consistent with their being younger by $\sim 1.5 - 2 $Gyr, if we have correctly estimated the clusters' chemical-abundance ratios. Conversely, the inferred age difference could be smaller ($\ltsim 1 $Gyr) if either [Fe/H] or [$α$/Fe] for the outer-halo clusters is significantly lower than we have assumed. Possible age spreads of order 1$ $Gyr among both the nearby and outer-halo clusters may also be present.

astro-ph↗

arXiv1998

HST Photometry of the Globular Cluster M4

This paper presents a detailed description of the acquisition and processing of a large body of imaging data for three fields in the globular cluster M4 taken with the Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope. Analysis with the ALLFRAME package yielded the deepest photometry yet obtained for this cluster. The resulting data-set for 4708 stars (positions and calibrated photometry in V, I, and, in two fields, U) spanning approximately six cluster core radii is available on the AAS CD-ROM (or email a request to RAI). The scientific analysis is deferred to three companion papers, which investigate the significant white dwarf population discovered and the main sequence population.

astro-ph↗

arXiv1997

White Dwarfs in Globular Clusters: HST 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 sizeable WD populations were found in all three. The location of these WDs in the color-magnitude diagram, their mean mass of 0.51($ \pm 0.03$)M$_{\odot}$, 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 found to be at most a few percent of all the main-sequence stars. The most ancient WDs found are about 9 Gyr old, a level which 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.

astro-ph↗

arXiv1996

Composition Mixing during Blue Straggler Formation and Evolution

We use smoothed-particle hydrodynamics to examine differences between direct collisions of single stars and binary star mergers in their roles as possible blue straggler star formation mechanisms. We find in all cases that core helium in the progenitor stars is largely retained in the core of the remnant, almost independent of the type of interaction or the central concentration of the progenitor stars. We have also modelled the subsequent evolution of the hydrostatic remnants, including mass loss and energy input from the hydrodynamical interaction. The combination of the hydrodynamical and hydrostatic models enables us to predict that little mixing will occur during the merger of two globular cluster stars of equal mass. In contrast to the results of Proctor Sills, Bailyn, & Demarque (1995), we find that neither completely mixed nor unmixed models can match the absolute colors of observed blue stragglers in NGC 6397 at all luminosity levels. We also find that the color distribution is probably the crucial test for explanations of BSS formation - if stellar collisions or mergers are the correct mechanisms, a large fraction of the lifetime of the straggler must be spent away from the main sequence. This constraint appears to rule out the possibility of completely mixed models. For NGC 6397, unmixed models predict blue straggler lifetimes ranging from about 0.1 to 4 Gyr, while completely mixed models predict a range from about 0.6 to 4 Gyr.

astro-ph↗

arXiv1996

CCD Photometry of the Globular Cluster M5. I. The Color-Magnitude Diagram and Luminosity Functions

We present new BVI photometry for the halo globular cluster M5, and examine the B- and I-band luminosity functions (LFs), based on over 20,000 stars. 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. Inclusion of alpha-element enhancements improves the agreement between the observed and predicted positions of the ``red-giant bump''. Using the ΔV_{TO}^{HB} method, we conclude that the clusters M5, Palomar 5, M4, NGC 288, NGC 362, NGC 1261, NGC 1851 and NGC 2808 are the same age at the level of about 1.5 Gyr, with the possible exception of NGC 288 (older by 3.5\pm 1.5 if the reddest NGC 288 HB stars are on the zero-age horizontal branch). Even with NGC 288 set aside, the large range in HB morphology in the remaining clusters appears to eliminate age as the sole second parameter determining HB morphology in the case of constant mass loss between RGB and HB. We are unable to chose between the two competing values for M5's (absolute) metallicity: [Fe/H] = -1.40 (Zinn & West 1984) and -1.17 (Sneden et al. 1992). This level of discrepancy has a signifcant effect on the derivation of the distance modulus and absolute age of M5. From theoretical isochrones and luminosity functions, we find an absolute age for M5 of 13.5 \pm 1 Gyr (internal error, assuming perfect models and no [M/H] error) for the Zinn & West abundance scale and 11 \pm 1 Gyr for the higher abundance value.

astro-ph↗

arXiv1996

The Relative Ages of Galactic Globular Clusters

We discuss the present state of knowledge and thought concerning the spread in age found among Galactic globular clusters, with some discussion of the implications for what happened during the earliest stages of the formation of the Milky Way Galaxy. Differential observational techniques to derive the relative age differences among clusters of similar metallicity are discussed in detail. We conclude that as of the current date (mid-April 1996) the state of the field is still somewhat muddled. However, we believe that there is now a substantial body of evidence --- including a particularly revealing intercomparison of the color-magnitude diagrams of \ngc{1851}, \ngc{288}, and \ngc{362} presented here --- indicating that age is {\it not\/} the dominant second parameter determining the shape of globular clusters' horizontal branches. If our assertion is correct, then apart from a handful of anomalous clusters that may well have been captured from a satellite dwarf galaxy, there is no strong evidence either for a significant spread in age among clusters of a given metal abundance or for a systematic variation of mean age with Galactocentric distance. On the question of whether there is a significant age difference between metal-poor and metal-rich clusters, we feel compelled to fall back on the Scottish verdict: ``Unproven.'' Data now being collected by numerous groups in various subdisciplines may resolve the remaining controversy within a few years.

astro-ph↗

arXiv1996

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.

astro-ph↗

arXiv1995

Hubble Space Telescope Observations of White Dwarfs in the Globular Cluster M4

With the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope, we have discovered in M4 (NGC 6121, C 1620-264) the first extensive sequence of cooling white dwarfs seen in a globular cluster. Adopting a distance modulus of (m-M)_V = 12.65 and a reddening of E(B-V) = 0.37, we show that the sequence, which extends over 9 < M_U < 13, is comprised of white dwarfs of mass \sim 0.5 M_{\odot}. The total mass loss from the present turnoff to the white dwarf sequence is 0.31 M_{\odot} and the intrinsic dispersion in the mean mass appears to be < 0.05 M_{\odot}. Both the location of the white dwarf cooling sequence in the cluster color-magnitude diagram and the cumulative luminosity function attest to the basic correctness and completeness of the physics in theoretical models for the upper three magnitudes of the observed white dwarf cooling sequence. To test the theory in globular clusters at cooling ages beyond \sim 3 \times 10^8 years will require deeper and more complete data.

astro-ph↗