Mario L. Mateo

The Carina dwarf spheroidal galaxy - How dark is it?

Precise radial velocities obtained with a photon-counting echelle spectrograph for a sample of 17 red giants in the Carina dwarf spheroidal galaxy are presented. The calculation of the systemic velocity and central velocity dispersion of Carina is described, the existing data constraining the structural parameters of Carina are reviewed, and an estimate of the central surface brightness of the galaxy is derived. These data are used to estimate the central mass density of Carina, as well as central and global mass-to-light ratios. It is concluded that the inferred mass densities and mass-density limits for all acceptable models imply the presence of a significant DM component in Carina. DM properties of all well-studied dSph systems are summarized and compared.

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.

Spectroscopy of giants in the Sextans dwarf spheroidal galaxy

We have obtained spectra in the region of the Ca II infrared triplet at 8500 A for 80 stars in the direction of the Sextans dwarf spheroidal galaxy. We have found 43 radial velocity members of the Sextans galaxy and 36 field stars. The mean velocity of the Sextans galaxy is 227.9±1.8 km s −1 , in agreement with the previously determined value of 230±6 km s −1 from Da Costa et al. (1991). We find that the intrinsic velocity dispersion of Sextans is 6.2±0.9 km s −1 based on 33 stars

A kinematic study of the Fornax dwarf spheroidal galaxy

Precise radial velocities of 44 stars and four globular clusters located in two fields of the Fornax dwarf spheroidal galaxy are obtained on the basis of photon-counting echelle spectroscopy with a resolution of approximately 14 km/s. BV CCD photometry of the giant branch of Fornax in both fields are presented as well. A variety of kinematic and photometric criteria are used to identify 10-12 probable nonmembers in the present sample of spectroscopically observed stars. Based on the most probable members, the mean heliocentric systemic velocity of Fornax is 53.0 + or - 1.8 km/s, with no evidence of any significant rotation about the minor axis. The intrinsic velocity dispersion of the stars in Fornaxs central field is 9.9 + or - 1.7 km/s, while for the outer field the velocity dispersion is 1.20 + or - 2.8 km/s. The true central velocity dispersion is not more than 1.6 km/s larger than the observed central dispersions for a number of reasonable models.

Cepheids in Magellanic Cloud star clusters - Fundamental and overtone pulsators in NGC 2157

CCD survey data are employed to examine Cepheids in young Magellanic Cloud star clusters. The properties of three Cepheids observed in NGC 2157 are described. It is detected that the two short-period (3 days) Cepheids have photometric properties that correspond to overtone pulsators and the long-period (7.7 days) Cepheid pulses in the fundamental mode. The pulsational masses for the three Cepheids are calculated to be about 5 solar masses. This mass value does not correlate with the average pulsational mass for Cepheids of 3.0 + or - 0.4 solar masses. The potential cause of this deviation in evolutionary/pulsational mass is investigated. 23 refs.

The Stellar Populations of the Magellanic Clouds

In many ways, studies of the stellar populations in the Magellanic Clouds are more straightforward than in our own Galaxy because of our external vantage point of the Clouds. This allows us to study the global properties of their stellar populations, as well as the individual stars that comprise them. The resulting picture describing the stellar content of the Clouds is obviously complex, defying any attempt to present a complete description in a short review such as this. Thus, my goal is to provide a very broad overview of the global properties of the Magellanic Clouds, along with some slightly more detailed discussions on a few selected topics addressed by recent studies. Recent reviews of the global properties and stellar content of the Magellanic Clouds based on observations ranging from radio to x-ray wavelengths can be found in the Proceedings of IAU Symposium 148 (Haynes and Milne 1991) and in Westerlund (1990). More specialized reviews have been written recently by van den Bergh (1991), Freeman (1989), and Feast (1989), and by numerous authors in the proceedings of a recent European Colloquium on the Magellanic Clouds (de Boer, et al. 1989).

The Initial Mass Functions of Magellanic Cloud star clusters

Luminosity and Initial Mass Functions (IMF) are determined for a sample of six Magellanic Cloud (MC) star clusters. The program objects span a large range in age (10 Myr to 2.5 Gyr), and include the possibly very metal-poor cluster NGC 330 in the SMC. The procedures involved in deriving the luminosity and mass functions are very briefly described. They key result is that the IMFs of all six clusters are consistent with a single IMF slope ofx=2.5, where the Salpeter value is 1.35. This result does not depend on the passband or on the detailed stellar evolution models that are used. A comparison is described between the present results and the few previous and ongoing studies of mass functions of MC clusters. Taken at face value, this comparison suggests that the IMF slopes of MC clusters vary over a wide range, and no apparent correlation with this slope and any global cluster properties are evident. A further comparison with results for galactic clusters (both open and globular) does not clarify the problem. It may be that the slope of the IMF on the scale of even rather massive star clusters is determined either randomly, or by some mechanism we have yet to identify and which does not correlate with obvious cluster properties.

The Stellar Population and Internal Kinematics of the Sextans Dwarf Spheroidal Galaxy

The Sextans dwarf Spheroidal (dSph) galaxy was discovered recently by Irwin, et al. (1990, M. N. R. A. S., 244, 16p). We report results concerning a number of the global properties of the stellar population of this system. Based on deep CCD photometry obtained with the prime-focus CCD camera on the CTIO 4m telescope, we find that the galaxy is dominated by an old stellar population, similar to that observed in the Draco, Sculptor, and Ursa Minor dwarfs. Some blue stragglers are also present; if these are associated with an intermediate age component, they indicate that only a tiny fraction of the stellar population of Sextans is younger than about 10 Gyr. Based on the apparent magnitude of the predominately red horizontal branch, we conclude that the true distance modulus of Sextans is 19.7 ± 0.3, and the total luminosity of the galaxy is 5.2 × 105 L ⊙. A complete description of these results is given in Mateo, et al. (1991, A. J., 101, 892).

Integrated photometric properties of magellanic cloud star clusters

Integrated CCD BVR photometric measurements of Magellanic Cloud (MC) star clusters are described and summarized. The integrated colour-magnitude diagrams and colour-age relations for these systems imply that (a) the colour distribution of MC clusters is distinctly bi-modal (although this bi-modality is not simply explained as a distinction between young and old clusters), and (b) the MCs are probably not forming as many (if any) extremely massive clusters at the present as they did early in their lifetimes (i.e., ≳10 Gyr ago). Structurally, the MC clusters are seen to be intermediate between galactic globular and open clusters and their radial surface brightness profiles can generally be adequately fitted to single-mass King (1966) models (some exceptions do occur, however). Although the distribution of the derived structural parameters of young and old MC clusters are similar, they differ significantly from the distribution of structural parameters observed in intermediate-age clusters, suggesting that these structural properties do not evolve monotonically with time.