James M. Nemec

The Star Formation History of the Carina Dwarf Galaxy

We have analyzed deep B and V photometry of the Carina dwarf spheroidal galaxy reaching below the old main-sequence turnoff to V ~ 25. Using simulated color-magnitude diagrams to model a range of star formation scenarios, we have extracted a detailed, global star formation history. Carina experienced three significant episodes of star formation at ~15, 7, and 3 Gyr. Contrary to the generic picture of galaxy evolution, however, the bulk of star formation, at least 50%, occurred during the episode 7 Gyr ago, which may have lasted as long as 2 Gyr. For unknown reasons, Carina formed only 10%–20% of its stars at an ancient epoch and then remained quiescent for more than 4 Gyr. The remainder (~30%) formed relatively recently, only 3 Gyr ago. Interest in the local population of dwarf galaxies has increased lately because of their potential importance in the understanding of faint galaxy counts. We surmise that objects like Carina, which exhibits the most extreme episodic behavior of any of the dwarf spheroidal companions to the Galaxy, are capable of contributing to the observed excess of blue galaxies at B ~ 24 only if the star formation occurred instantaneously.

Blue stragglers as remnants of stellar mergers - The discovery of short-period eclipsing binaries in the globular cluster NGC 5466

Results are reported from a search for short-period variables among blue stragglers in the central region of NGC 5466, based on analysis of 248 B and V CCD images obtained with the U.S. Naval Observatory 1-m, Palomar Observatory 1.5-m, and Steward Observatory 2.3-m telescopes during 1987-1989. The data are presented in extensive tables and graphs and characterized in detail. Nine variable blue stragglers are identified, of which three are eclipsing binaries with periods 0.298-0.511 d (two contact binaries of W UMa type and one detached or semidetached binary) and six are pulsating SX Phe stars. Theoretical models indicate that all of the noneclipsing blue stragglers could be merged close binaries, although other formation mechanisms cannot be completely ruled out. 111 refs.

Variable stars in the Ursa Minor dwarf galaxy

De nouvelles donnees photometriques (CCD et photographiques) de la galaxie naine UMi permettent letude detaillee denviron 95 etoiles variables appartenant a cette galaxie. Pour 88 dentre elles les periodes de pulsation sont determinees avec precision. La periode moyenne des etoiles de type RR Lyrae est calculee et discutee. La distance obtenue est de 70±9 pc. Un diagramme periode-luminosite obtenu par letude des Cepheides anormales permet une nouvelle et meilleure estimation de leur masse

Dwarf Cepheids in the Carina Dwarf Spheroidal Galaxy

We have discovered 20 dwarf Cepheids (DCs) in the Carina dwarf spheroidal galaxy from an analysis of individual CCD images obtained for a deep photometric study of the system. These short-period pulsating variable stars are by far the most distant (~100 kpc) and faintest (V ~ 23.0) DCs known. The Carina DCs obey a well-defined period-luminosity relation, allowing us to readily distinguish between overtone and fundamental pulsators in nearly every case. Unlike RR Lyrae stars, the pulsation mode turns out to be uncorrelated with light-curve shape, and the overtone pulsators do not tend toward shorter periods compared with the fundamental pulsators. Using the period-luminosity relations from Nemec, Nemec, & Lutz and McNamara, we derive (m - M)0 = 20.06 ± 0.12, for E(B - V) = 0.025 and [Fe/H] = -2.0, in good agreement with recent, independent estimates of the distance/reddening of Carina. The error reflects the uncertainties in the DC distance scale, and in the metallicity and reddening of Carina. The frequency of DCs among upper–main-sequence stars in Carina is approximately 3%. The ratio of dwarf Cepheids to RR Lyrae stars in Carina is 0.13 ± 0.10, though this result is highly sensitive to the star formation history of Carina and the evolution of the horizontal branch. We discuss how DCs may be useful to search effectively for substructure in the Galactic halo out to Galactocentric distances of 100 kpc.

Deep CCD photometry and variable stars in the metal-rich globular cluster M71

Two sets of observations of the metal-rich globular cluster M71 are presented. The first uses deep CCD exposures to derive a fiducial in V, B−V and compares this with a cluster of similar metallicity, 47 Tuc. Comparison with oxygen enhanced theoretical isochrones leads to age estimates for M71 of from ∼14 ± 2 Gyr to ∼16 ± 2 Gyr. Within the limits imposed by the quality of the data, no significant age difference between the two clusters is found. The second set of 73 frames is used to search an overlapping 65″ × 104″ area of the cluster for variable stars. Light curves and phase diagrams (where appropriate) are presented for all four of the variables discovered. One variable blue straggler (or SX Phe star) has been identified with a period of 0.050 d

New field RR Lyrae variables in the outer parts of the LMC and the properties of the LMC halo

Four new type-ab RR Lyrae variables that belong to the field of the LMC were found in the direction of NGC 1466; a possible field variable was found near the Reticulum system. A King model that fits the surface densities of the known LMC field RR Lyrae stars of type ab has a limiting radius of 15 kpc. An exponential disk model that fits these densities has a scale length of 2.6 kpc. These models contain ~104^ type ab RR Lyrae stars and have a central surface density of ~200 of these stars per square degree. The LMC field halo of Population II stars has Mv_~ 15.2 and a mass of ~1.6 X 108Msun_. This halo therefore makes up about 2% of the mass of the LMC, a similar percentage to that found for our Galactic halo. Thus, the efficiencies of the earliest star-formation processes may have been comparable in the two systems. The LMC field RR Lyraes have a period-frequency distribution similar to those found in the outer halo of our Galaxy. The ratio (p) of the surface density of these RR Lyraes to that of the older long-period variables (OLPV) (found by Hughes et al. 1991) is ~13 at some 3 to 4 kpc from the LMC center. This measure of the relative strengths of the halo to an old-disk population is within a factor of two of that which would be seen in our Galaxy at the solar distance from its center. The central visual surface brightness of the LMC halo is ~25.5 mag per square arc sec. Its average visual surface brightness (over the area within the effective radius ( ~4 kpc)) is ~26.4 mag per square arc sec. It would be difficult to detect a halo of this faintness in an external galaxy unless the halo could be resolved into individual stars.

Deep CCD photometry of the Sextans dwarf spheroidal galaxy

BV CCD photometry of Sextans obtained with the prime-focus camera of the 4-m telescope at CTIO in March-April 1990 is reported. The data are presented in extensive tables and graphs and analyzed in detail. Features discussed include a main-sequence turnoff at V = about 23.3, a mainly red horizontal branch at V = 20.35 + or - 0.2, a well populated instability strip (implying up to 300 RR Lyrs and Cepheids in the galaxy), Fe/H = -1.5 + or - 0.2, (m-M)0 = 19.7 + or - 0.3 for E(B-V) = 0.02 + or - 0.02, no significant population of stars younger than 12 Gyr, central V surface brightness 17.2 + or - 0.5 mag/sq arcmin, integrated apparent V magnitude 10.3 + or - 0.7 mag, and overabundance 0.4-0.7 dex (compared to other Galactic dwarf spheroidals). 66 refs.

Color-magnitude diagram for the Andromeda II dwarf galaxy

A color-magnitude diagram for the dwarf galaxy Andromeda II (And II) is presented, based on CCD photometry obtained using the Palomar 5 m telescope and Thuan-Gunn g, r filters. The mean metallicity, the metallicity spread, and the distance to And II are estimated. The mean metallicity of the galaxy is found to be =−1.59 −0.12 +0.44 , derived by superimposing globular cluster giant branches. This metallicity is consistent with the accepted M V vs [Fe/H] relationship for dwarf spheroidals. The spread in metallicity seems to be large, σ∼0.43, but we cannot determine the exact value. Andromeda II is estimated to be at a distance of 583 −103 +124 Kpc, ∼120 Kpc closer than M31

Radial velocities of stars in M12 (NGC 6218) and M56 (NGC 6779) and UV-bright stars in globular clusters

Radial velocities of 46 stars in the fields of the globular clusters M 12 (C 1644-018) and M56 (C 1914+300) show that 23 of the stars are cluster members. When these new measurements are combined with all previous ones, the improved cluster velocities are -41.3 ± 1.5 and -134.0 ± 2.7 km s-1, respectively. Six of the observed stars are cluster UV-bright stars. A color-magnitude diagram is constructed for all known UV-bright stars in 29 globular clusters. This composite diagram suggests that UV-bright stars are present in sufficient numbers that they will contribute appreciably to the integrated ultraviolet light of old, metal-poor stellar populations. Although our figure is an improvement over earlier diagrams, it is still seriously incomplete due to selection effects.

Mixture models for studying stellar populations. II: Multivariate finite mixture models

Multivariate finite mixture models provide a framework for analyzing and interpreting observational data that arise from the overlapping spatial, kinematical, chemical, and age distributions of stellar populations. In this paper the basic properties of multivariate finite mixture distributions are summarized. Particular emphasis is placed on the interpretation of observed overall gradients, and on the distinction between models in which a gradient exists in at least one component and those in which there are no gradients within individual stellar population components. Central to this discussion are posterior mixing proportions, which describe how the mixing proportions for an observed sample of stars vary as functions of metal abundance, age, etc., and conditional mixture distributions, which describe the relationships that exist among the variables