The Ursa Major Group (UMaG) is studied as a test case for the authenticity of Stellar Kinematic Groups, using Coravel radial velocities, recent compilations of astrometric data and new spectroscopic observations. Spectroscopic age indicators, particularly indices of the strength of chromospheric emission, are applied to solar-type candidate members of UMaG, and it is shown that stars that meet the spectroscopic criteria also have kinematics that agree better with the space motions of the nucleus of UMaG than does the starting sample as a whole. The primary limitation on the precision of kinematics is now parallaxes instead of radial velocities. These more restrictive kinematic criteria are then applied to other UMaG candidates and a list summarizing membership is presented. UMaG is also examined as a cluster, confirming its traditional age of 0.3 Gyr and a mean [Fe/H] of -0.08 +/- 0.09 for those stars most likely to be bona fide members.
We have obtained uvby data of a sample of 232 stars in Baade's Window. These observations are presented along with the corresponding diagrams. For a description of the uvby photometric system, see e.g. <GCPD/04>
We have computed evolutionary synthesis models covering metallicities from PopIII to solar, a wide range of IMFs, and two star-formation histories (bursts and constant star-formation). The detailed predictions these models, including their time dependence, are given here. The predicted quantities include ionising fluxes, UV continuum properties, recombination line strengths (for Ly{alpha}, HeII and other lines), as well as several other quantities of interest. The models provided here cover a wider range of parameter space than our earlier calculations (Schaerer, 2003A&A...397..527S, Cat. VI/109).
The Vela OB2 association is a group of 10Myr stars exhibiting a complex spatial and kinematic substructure. The all-sky Gaia DR2 catalogue contains proper motions, parallaxes (a proxy for distance) and photometry that allow us to separate the various components of Vela OB2. We characterise the distribution of the Vela OB2 stars on a large spatial scale, and study its internal kinematics and dynamic history. We make use of Gaia DR2 astrometry and published Gaia-ESO Survey data. We apply an unsupervised classification algorithm to determine groups of stars with common proper motions and parallaxes. We find that the association is made up of a number of small groups, with a total current mass over 2330M_{sun}_. The three-dimensional distribution of these young stars trace the edge of the gas and dust structure known as the IRAS Vela Shell across 180 pc and shows clear signs of expansion. We propose a common history for Vela OB2 and the IRAS Vela Shell. The event that caused the expansion of the shell happened before the Vela OB2 stars formed, imprinted the expansion in the gas the stars formed from, and most likely triggered star formation.
We study the red giant populations of two dwarf elliptical (dE) galaxies, AM 1339-445 and AM 1343-452. The galaxies are members of the Centaurus A group (D~3.8Mpc), and are classified as outlying (R~350kpc) satellites of Cen A. The analysis is based on near-IR photometry for individual red giant stars, derived from images obtained with ISAAC on the VLT, and on optical V and I photometry, derived from WFPC2 images retrieved from the HST science archive. The data enable us to investigate the stellar populations of these galaxies in the vicinity of the red giant branch (RGB) tip. In both systems we find stars above the RGB tip, which we interpret as intermediate-age upper-AGB stars. Their luminosities suggest ages of approximately 6.5+/-1 and 4+/-1Gyr as estimates for the epoch of the last episode of significant star formation in these systems. In both cases the number of upper-AGB stars suggests that ~15% of the total stellar population is in the form of intermediate-age stars.
We analyzed the massive star population of the Virgo Cluster galaxy NGC 4535 using archival Hubble Space Telescope Wide Field Planetary Camera 2 images in filters F555W and F814W, equivalent to Johnson V and Kron-Cousins I. We performed high precision point spread function fitting photometry of 24353 sources including 3762 candidate blue supergiants, 841 candidate yellow supergiants and 370 candidate red supergiants. We estimated the ratio of blue to red supergiants as a decreasing function of galactocentric radius. Using Modules for Experiments in Stellar Astrophysics isochrones at solar metallicity, we defined the luminosity function and estimated the star formation history of the galaxy over the last 60Myrs. We conducted a variability search in the V and I filters using three variability indexes: the median absolute deviation, the interquartile range and the inverse von-Neumann ratio. This analysis yielded 120 new variable candidates with absolute magnitudes ranging from M_V_=-4 to -11mag. We used the MESA evolutionary tracks at solar metallicity, to classify the variables based on their absolute magnitude and their position on the color-magnitude diagram. Among the new candidate variable sources are eight candidate variable red supergiants, three candidate variable yellow supergiants and one candidate luminous blue variable, which we suggest for follow-up observations.
We have studied the stellar populations near the nucleus of the nearby spiral galaxy M33 using archival observations of the Hubble Space Telescope Planetary Camera in the F555W and F785LP passbands. The I vs (V-I) color-magnitude diagram reveals a young stellar population with a blue main sequence and red supergiants, and stars in a blue loop that are Cepheid candidates. An old red giant branch is clearly visible, with a break at I=20.7mag which is, within the observational errors, identical to the giant branch termination point of the Mould & Kristian (1986ApJ...305..591M) halo field. However, red giants are seen up to I=19.5mag, and many bright red giants are undetected in the V band. We find evidence for a wide abundance spread of at least 1.5dex from metal-poor (~M15) to metal-rich (>=47Tucanae) in the Population II stars, in sharp contrast to the M33 halo field of Mould & Kristian (1986ApJ...305..591M) which is metal-poor (~M92) with a narrow abundance range. We show that the various stellar populations have different spatial distributions. The Population II stars are more centrally concentrated than the Population I stars. The red giants brighter than I=20.7 (the termination point for a Galactic globular cluster giant branch) are found to be more centrally concentrated than the (presumably) old red giants. The most-centrally concentrated bright Population II giants are probably associated with the many bright red giants that are undetected in the V band. These infrared-bright giants were probably created in an intermediate-age burst of star formation that took place well after the formation of the oldest halo stars but long before the recent burst of star formation seen in nearby OB associations. This stellar population probably corresponds to the bulge identified in the infrared H band by Minniti et al. (1993ApJ...410L..79M).
We present Washington CT_1_T_2_ photometry of a field central to the Bootes I dwarf spheroidal galaxy, which was discovered as a stellar overdensity in the Sloan Digital Sky Survey (DR5). We show that the Washington filters are much more effective than the Sloan filters in separating the metal-poor turn-off stars in the dwarf galaxy from the foreground stars. We detect 165 objects in the field, and statistically determine that just over 40% of the objects are nonmembers. Our statistical analysis mostly agrees with radial velocity measurements of the brighter stars. We find that there is a distinct main-sequence turn-off and subgiant branch, where there is some evidence of a spread in chemical abundance.
