arXiv subjects

M. Bolte

Publications and source records attributed to M. Bolte.

22 records · Page 2Linked to original sources

arXiv1997

Galactic Globular Cluster Metallicity Scale from the Ca II Triplet. I. Catalog

We have obtained 2640 CCD spectra with resolution ~4 Angstrom in the region 7250-9000 Angstroms for 976 stars lying near the red giant branches in color-magnitude diagrams of 52 Galactic globular clusters. Radial velocities of ~16 km/second accuracy per star determined from the spectra are combined with other criteria to assess quantitative membership probabilities. Measurements of the equivalent widths of the infrared calcium triplet lines yield a relative metal-abundance ranking with a precision that compares favorably to other techniques. Regressions between our system and those of others are derived. Our reduction procedures are discussed in detail, and the resultant catalog of derived velocities and equivalent widths is presented. The metal abundances derived from these data will be the subject of a future paper.

astro-ph↗

arXiv1997

NGC 2419, M92, and the Age Gradient in the Galactic Halo

The WFPC2 camera on HST has been used to obtain deep main sequence photometry of the low-metallicity ([Fe/H]=-2.14), outer-halo globular cluster NGC 2419. A differential fit of the NGC 2419 CMD to that of the similarly metal-poor \ standard cluster M92 shows that they have virtually identical principal sequences and thus the same age to well within 1 Gyr. Since other low-metallicity clusters throughout the Milky Way halo have this same age to within the 1-Gyr precision of the differential age technique, we conclude that the earliest star (or globular cluster) formation began at essentially the same time everywhere in the Galactic halo throughout a region now almost 200 kpc in diameter. Thus for the metal-poorest clusters in the halo there is no detectable age gradient with Galactocentric distance. To estimate the absolute age of NGC 2419 and M92, we fit newly computed isochrones transformed through model-atmosphere calculations to the (M_V,V-I) plane, with assumed distance scales that represent the range currently debated in the literature. Unconstrained isochrone fits give M_V(RR) = 0.55 \pm 0.06 and a resulting age of 14 to 15 Gyr. Incorporating the full effects of helium diffusion would further reduce this estimate by about 1 Gyr. A distance scale as bright as M_V(RR) = 0.15 for [Fe/H] = -2, as has recently been reported, would leave several serious problems which have no obvious solution in the context of current stellar models.

astro-ph↗

arXiv1997

Formation of the Milky Way

We review observational evidence bearing on the formation of a prototypical large spiral galaxy, the Milky Way. New ground- and space-based studies of globular star clusters and dwarf spheroidal galaxies provide a wealth of information to constrain theories of galaxy formation. It appears likely that the Milky Way formed by a combination of rapid, dissipative collapse and mergers, but the relative contributions of these two mechanisms remain controversial. New evidence, however, indicates that initial star and star cluster formation occurred simultaneously over a volume that presently extends to twice the distance of the Magellanic Clouds.

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arXiv1994

Anticorrelated CN and CH Variations on the 47 Tucanae Main-Sequence Turnoff

Observations of CN and CH band strengths among a random sample of main-sequence turn-off stars (+3.9 < MV < +4.6) in the globular cluster 47 Tuc (NGC 104, C0021-723) were made with the CTIO Argus fiber spectrograph for the purpose of determining the ratio of CN-strong to CN-weak stars and investigating the behavior of CH relative to CN. Of the 20 turn-off stars, 12 were found to be CN-strong while 8 appear to be CN-weak. This ratio of CN-strong to CN-weak stars is similar to the ratios found among the more luminous 47 Tuc stars and implies little change in the overall distribution of CN with evolutionary state, although the present sample size is small. A general anticorrelation between CN and CH is also observed in that the CN-weak main-sequence stars all (with one possible exception) exhibit strong CH bands - a trend similar to that found among the brighter stars. That these variations occur among such relatively un-evolved stars and that the overall CN distribution appears to be independent of evolutionary state presents serious challenges to internal or mixing theories of their origin. We therefore suggest that at least some component of the C and N abundance inhomogeneities observed among the brighter (more evolved) stars of this cluster appears to have been established prior to the commencement of evolution up the red giant branch.

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