Related papers: The sdA problem I: Physical Properties
We present a detailed spectroscopic and photometric analysis of DA and DB white dwarfs drawn from the Sloan Digital Sky Survey with trigonometric parallax measurements available from the Gaia mission. The temperature and mass scales…
We present a study of a large, statistically complete sample of star-forming dwarf galaxies using mid-infrared observations from the {\it Spitzer Space Telescope}. The relationships between metallicity, star formation rate (SFR) and…
We review at length the longstanding problem in the spectroscopic analysis of cool hydrogen-line (DA) white dwarfs (Teff < 13,000 K) where gravities are significantly higher than those found in hotter DA stars. The first solution that has…
The entire evolution of the Milky Way, including its mass-assembly and star-formation history, is imprinted onto the chemo-dynamical distribution function of its member stars, f(x, v, [X/H]), in the multi-dimensional phase space spanned by…
We report on the chemical abundances derived from high-dispersion spectra of 14 red giant stars in the Sagittarius dwarf spheroidal (Sgr dSph) galaxy. The stars span a wide range of metallicities, -1.6 < [Fe/H] < -0.1 dex, and exhibit very…
The cold dark matter (CDM) model has two unsolved issues: simulations overpredict the satellite abundance around the Milky Way (MW) and it disagrees with observations of the central densities of dwarf galaxies which prefer constant density…
For a rotating galaxy, the inner circular-velocity gradient d_{R}V(0) provides a direct estimate of the central dynamical mass density, including gas, stars, and dark matter. We consider 60 low-mass galaxies with high-quality HI and/or…
We present three dimensional spectroscopy of eleven E+A galaxies, selected for their strong H-delta absorption but weak (or non-existent) [OII]3727 and H-alpha emission. This selection suggests that a recent burst of star-formation was…
White dwarf stars are subject to various element transport mechanisms that can cause their surface composition to change radically as they cool, a phenomenon known as spectral evolution. In this paper, we undertake a comprehensive…
The present work is designed to explore the evolution of helium-core white dwarf (HeWD) stars for the case of metallicities much lower than the solar one (Z=0.001 and Z=0.0002). Evolution is followed in a self-consistent way with the…
Dark matter halos with self-interacting dark matter (SIDM) experience a unique evolutionary phenomenon, in that their central regions eventually collapse to high density through the runaway gravothermal process after initially forming a…
Young alpha-rich (YAR) stars have been detected in the past as outliers to the local age $\rm-$ [$\alpha$/Fe] relation. These objects are enhanced in $\alpha$-elements but apparently younger than typical thick disc stars. We study the…
We study dwarf galaxies in the Centaurus A group to investigate their metallicity and possible environmental effects. The Centaurus A group (at ~4 Mpc from the Milky Way) contains about 50 known dwarf companions of different morphologies…
We study the photometric and spectral properties of 39320 early-type galaxies within the SDSS, as a function of both local environment and redshift. The distance to the nearest cluster of galaxies and the distance to the 10th nearest…
We determined the ages, the kinematic parameters and Galactic orbital parameters of six metal-poor (-2.4<[Fe/H]<-1 dex) F-type high proper motion (HPM) stars to investigate their HPM nature and origin. For the kinematical procedure, the…
We report the identification of 26 cool DZ white dwarfs that lie across and below the main sequence in the Sloan Digital Sky Survey (SDSS) u-g vs. g-r two-color diagram; 21 of these stars are new discoveries. The sample was identified by…
Hot subluminous stars (sdO/B) are evolved low mass stars originating from red giants that lost their envelope almost entirely. The multitude of observed phenomena imply that several pathways may form hot subdwarfs, most involving close…
We predict the metallicity distribution of stars and the age-metallicity relation for 6 Dwarf Spheroidal (dSph) galaxies of the Local Group by means of a chemical evolution model which is able to reproduce several observed abundance ratios…
We present infrared (IR) spectral energy distributions (SEDs) of individual star-forming regions in four extremely metal poor (EMP) galaxies with metallicity Z around Zsun/10 as observed by the Herschel Space Observatory. With the good…
We present deep $BVI$ observations of the dwarf irregular galaxy UKS1927-177 in Sagittarius. Statistically cleaned $V$, $(B-I)$ CMDs clearly display the key evolutionary features in this galaxy. Previously detected C stars are located in…