arXiv subjects

Stephen Holland

Publications and source records attributed to Stephen Holland.

24 records · Page 2Linked to original sources

arXiv1998

Stellar Populations Beyond the Local Group with the NGST

We present simulated J- and K-band observations of stars in the Virgo and Coma clusters of galaxies using the proposed Next Generation Space Telescope with a Near-Infrared Camera, and discuss some of the science results that might be obtained. The proposed telescope will be able to resolve the brightest ~3 magnitudes of the red giant branch in the halos of galaxies in the Virgo Cluster and may be able to resolve stars at the tip of the red giant branch in the Coma Cluster. The simulations show that the background light is more important than the size of the aperture in determining the limiting magnitude of the observations. Therefore, we recomment that the Next Generation Space Telescope be placed in a 1 x 3 a.u. orbit to minimize background light.

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arXiv1998

The Distance to the M31 Globular Cluster System

The distance to the centroid of the M31 globular cluster system is determined by fitting theoretical isochrones to the observed red-giant branches of fourteen globular clusters in M31. The mean true distance modulus of the M31 globular clusters is found to be 24.47 +/- 0.07 mag. This is consistent with distance modulii for M31 that have been obtained using other distance indicators.

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arXiv1997

Deep HST V- and I-Band Observations of Two Globular Clusters in the Halo of M31

We present deep (V ~= 27) V- and I-band stellar photometry of G302 and G312, two globular star clusters in the halo of M31. These data were obtained using the Hubble Space Telescope's Wide Field/Planetary Camera 2. We find iron abundances of [Fe/H] = -1.85 +/- 0.12 for G302 and [Fe/H] = -0.56 +/- 0.03 for G312, consistent with spectroscopic measurements. The color-magnitude diagrams for each cluster show no evidence for an intermediate-aged population of stars, or a second parameter effect in the morphology of the horizontal branch. G302 shows no evidence for a color gradient but the inner regions of G312 are bluer than the outer regions. G312 shows no evidence of ellipticity or an extended halo of unbound stars. G302 has a projected ellipticity of 0.195 +/- 0.012 with the projected major axis oriented towards the center of M31. G302 also shows evidence of an extended asymmetric stellar halo extending to at least twice the fitted Michie-King tidal radius. The amount of mass beyond the tidal radius of G302 is consistent with the stellar escape rates which have been predicted by N-body simulations of globular clusters in the Galactic tidal field.

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arXiv1996

Deep HST V- and I-Band Observations of the Halo of M31: Evidence for Multiple Stellar Populations

We present deep V & I WFPC2 photometry in two fields in the M31 halo 32' and 50' from the center of M31 along the SE minor axis at the locations of the M31 GCs G302 and G312 respectively. The M31 halo LFs are not consistent with a single high-metallicity population but are consistent with a mix of 50% to 75% metal-rich stars and 25% to 50% metal-poor stars. This agrees with the RGB morphology, the HB luminosity, and the RR Lyrae stars in the M31 halo. The RGB morphology indicates a metallicity spread of -2 < [m/H] < -0.2 with most stars having [m/H] = -0.6, making the M31 halo significantly more metal- rich than either the Galactic halo or the M31 GC system. The HB is dominated by a red clump similar to the 47 Tuc HB but a small number of blue HB stars are visible, supporting the conclusion that there is a metal-poor component to the M31 halo. The number of HB stars is smaller than would be expected from the observed metallicity distribution but it is not clear if this is due to the photometric limits of our data or a 2nd parameter effect. We find a He abundance of Y >~ 0.20 to 0.27, comparable with the Galaxy. LFs show weak evidence that the R = 50' field contains a higher fraction of metal-poor stars than the R = 32' field but the metallicity distributions of the RGB stars strongly suggest that both fields have the same mix of stellar populations.

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arXiv1996

Age Gradient and the Second Parameter Problem in the Galactic Halo

We establish a framework for determing absolute ages of Galactic globular clusters and then use these ages to investigate the age-metallicity and age-Galactocentric distance relations for the 36 clusters with the most reliable age data. The clusters span Galactocentric distances from 4 through 100 kpc and cover a metallicity range from $[Fe/H] = -0.6$ to $-2.3$. Adopting currently plausible choices for the relation between cluster metallicity and horizontal-branch luminosity, and alpha-enhancement ratios, we find that the majority of the globular clusters form an age distribution with a dispersion $σ(t)$ about $10^9$ years, and a total age spread smaller than 4 Gyr. Clusters in the lowest metallicity group ($[Fe/H] < -1.8$) appear to be the same age to well within 1 Gyr at all locations in the Milky Way halo, suggesting that star formation began throughout the halo nearly simultaneously in its earliest stages. We find no statistically significant correlation between mean cluster age and Galactocentric distance (no age gradient) from 4 to 100 kpc. The correlation between cluster age and horizontal-branch type suggests that causes in addition to metallicity and age are required to understand the distribution of stars along the horizontal branches in globular cluster color-magnitude diagrams.

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arXiv1995

The Double Cluster G185 in M31

We have identified a small globular cluster in M31 located approximately 4 arcseconds northwest of the M31 globular cluster G185. While several multiple globular clusters have been observed in the Magellanic Clouds none have been found in the Galaxy or in M31. We estimate the probability of such a chance line-of-sight alignment occuring near the nucleus of M31 to be 0.09 +/- 0.03 and find no obvious indication of any tidal deformation in either cluster, as would be expected if the clusters were interacting. Two-dimensional modelling suggests G185 has a King (1966) [AJ, 71, 64] concentration of c = 1.11 +/- 0.08 while the companion has c = 0.67 +/- 0.17 and is physically smaller than G185. Both objects have integrated dereddened colors similar to those of Galactic globular clusters.

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