The Initial Mass Function: Now and Then

We examine whether existing data in clusters, both old and young, and in the field of the Galactic disk and halo is consistent with a universal slope for the initial mass function (IMF). The most reasonable statement that can be made at the current time is that there is no strong evidence to support a claim of any real variations in this slope. If the IMF slope is universal then this in itself is remarkable implying that variations in metallicity, gas density or other environmental factors in the star formation process play no part in determining the slope of the mass function.Comment: 10 pages, postscript. To appear in "Proceedings 7th Annual Astrophysics Conference in Maryland-STAR FORMATION NEAR AND FAR

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.Comment: 29 pages, 21 Postscript figures, uses aaspp4.sty, to be published in the October 1997 A

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.Comment: 22 pages, ~1Mb postscript file http://www.astro.ubc.ca/~holland/bib.html/ ftp://nukta.astro.ubc.ca/pub/holland/G185_preprint.ps.

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.Comment: 31 pages AASTeX v4.0, 10 Figures avaliable at ftp://ftp.astro.ubc.ca/holland/M31_hal

The White Dwarf Cooling Age of M67

A deep imaging survey covering the entire 23\arcmin diameter of the old open cluster M67 to $V = 25$ has been carried out using the mosaic imager (UHCam) on the Canada-France-Hawaii Telescope. The cluster color-magnitude diagram (CMD) can be traced from stars on its giant branch at $M_{V} = +1$ down through main sequence stars at least as faint as $M_{V} = 13.5$. Stars this low in luminosity have masses below $0.15 M_{\odot}$. A modest white dwarf (WD) cooling sequence is also observed commencing slightly fainter than $M_V = 10$ and, after correction for background galaxy and stellar field contamination, terminating near $M_V = 14.6$. The observed WDs follow quite closely a theoretical cooling sequence for $0.7 M_{\odot}$ pure carbon core WDs with hydrogen-rich atmospheres (DA WDs). The cooling time to an $M_V$ of 14.6 for such WDs is 4.3 Gyr which we take as the WD cooling age of the cluster. A fit of a set of isochrones to the cluster CMD indicates a turnoff age of 4.0 Gyr. The excellent agreement between these results suggests that ages derived from white dwarf cooling should be considered as reliable as those from other dating techniques. The WDs currently contribute about 9% of the total cluster mass but the number seen appears to be somewhat low when compared with the number of giants observed in the cluster.Comment: 15 pages plus 3 diagrams, minor corrections, Accepted for publication in the Astrophysical Journal Letters, to be published September 10, 199

Isochrones and Luminosity Functions for Old White Dwarfs

Using a new grid of models of cooling white dwarfs, we calculate isochrones and luminosity functions in the Johnson-Kron/Cousins and HST filter sets for systems containing old white dwarfs. These new models incorporate a non-grey atmosphere which is necessary to properly describe the effects of molecular opacity at the cool temperatures of old white dwarfs. The various functions calculated and extensively tabulated and plotted are meant to be as utilitarian as possible for observers so all results are listed in quantities that observers will obtain. The tables and plots developed should eventually prove critical in interpreting the results of HST's Advanced Camera observations of the oldest white dwarfs in nearby globular clusters, in understanding the results of searches for old white dwarfs in the Galactic halo, and in determining ages for star clusters of all ages using white dwarfs. As a practical application we demonstrate the use of these results by deriving the white dwarf cooling age of the old Galactic cluster M67.Comment: 7 pages, 8 tables, accepted for publication in the Astrophysical Journa