Cepheid Masses: FUSE Observations of S Mus

S Mus is the Cepheid with the hottest known companion. The large ultraviolet flux means that it is the only Cepheid companion for which the velocity amplitude could be measured with the echelle mode of the HST GHRS. Unfortunately, the high temperature is difficult to constrain at wavelengths longer than 1200 \AA because of the degeneracy between temperature and reddening. We have obtained a FUSE spectrum in order to improve the determination of the temperature of the companion. Two regions which are temperature sensitive near 16,000 K but relatively unaffected by H$_2$ absorption (940 \AA, and the Ly $\beta$ wings) have been identified. By comparing FUSE spectra of S Mus B with spectra of standard stars, we have determined a temperature of 17,000 $\pm$ 500 K. The resultant Cepheid mass is 6.0 $\pm$ 0.4 M$_\odot$. This mass is consistent with main sequence evolutionary tracks with a moderate amount of convective overshoot.Comment: accepted to Ap

Remnants of Sagittarius Dwarf Spheroidal Galaxy around the young globular cluster Palomar 12

Photometry of a large field around the young globular cluster Palomar 12 has revealed the main-sequence of a low surface-brightness stellar system. This main-sequence is indicative of a stellar population that varies significantly in metallicity and/or age, but in the mean is more metal poor than Pal 12. Under different assumptions for the properties of this population, we find distances from the Sun in the range 17-24 kpc, which encompasses the distance to Pal 12, $19.0\pm0.9$ kpc. The stellar system is also detected in a field 2\arcdeg North of Pal 12, which indicates it has a minimum diameter of $\sim0.9$ kpc. The orbit of Pal 12 (Dinescu et al. 2000), the color-magnitude diagram of the stellar system, their positions on the sky, and their distances suggest that they are debris from the tidal disruption of the Sgr dSph galaxy. We discuss briefly the implications for the evolution of Sgr and the Galactic halo.Comment: 16 pages, 2 figures, accepted for ApJ Letters. Some importante changes after revision, including a new figur

Clues on the evolution of the Carina dwarf spheroidal galaxy from the color distribution of its red giant stars

The thin red giant branch (RGB) of the Carina dwarf spheroidal galaxy appears at first sight quite puzzling and seemingly in contrast with the presence of several distinct bursts of star formation. In this Letter, we provide a measurement of the color spread of red giant stars in Carina based on new BVI wide-field observations, and model the width of the RGB by means of synthetic color-magnitude diagrams. The measured color spread, Sigma{V-I}=0.021 +/- 0.005, is quite naturally accounted for by the star-formation history of the galaxy. The thin RGB appears to be essentially related to the limited age range of its dominant stellar populations, with no need for a metallicity dispersion at a given age. This result is relatively robust with respect to changes in the assumed age-metallicity relation, as long as the mean metallicity over the galaxy lifetime matches the observed value ([Fe/H] = -1.91 +/- 0.12 after correction for the age effects). This analysis of photometric data also sets some constraints on the chemical evolution of Carina by indicating that the chemical abundance of the interstellar medium in Carina remained low throughout each episode of star formation even though these episodes occurred over many Gyr.Comment: 4 pages, 3 figures, accepted for publication in the Astrophysical Journal Letter

Galactic Globular Cluster Relative Ages

Based on a new large, homogeneous photometric database of 35 Galactic globular clusters (GGCs), a set of distance and reddening independent relative age indicators has been measured. The observed D(V-I)_2.5 and D(V)(HB-TO) vs. metallicity relations have been compared with the relations predicted by two recent updated libraries of isochrones. Using these models and two independent methods, we have found that self-consistent relative ages can be estimated for our GGC sample. Based on the relative age vs. metallicity distribution, we conclude that: (a) there is no evidence of an age spread for clusters with [Fe/H]<-1.2, all the clusters of our sample in this range being old and coeval; (b) for the intermediate metallicity group (-1.2<=[Fe/H]<-0.9) there is a clear evidence of age dispersion, with clusters up to ~25% younger than the older members; and (c) the clusters within the metal rich group ([Fe/H]>=-0.9) seem to be coeval within the uncertainties (except Pal12), but younger (~17%) than the bulk of the Galactic globulars. The latter result is totally model dependent. From the distribution of the GGC ages with the Galactocentric distance, we can present a possible scenario for the Milky Way formation: The GC formation process started at the same zero age throughout the halo, at least out to ~20 kpc from the Galactic center. According to the present stellar evolution models, the metal-rich globulars are formed at a later time (~ 17% lower age). And finally, significantly younger halo GGCs are found at any R(GC)>8 kpc. For these, a possible scenario associated with mergers of dwarf galaxies to the Milky Way is suggested.Comment: 47 pages, 9 figures. To be published in the Astronomical Journal, November issu

Eccentric Ellipsoidal Red Giant Binaries in the LMC: Complete Orbital Solutions and Comments on Interaction at Periastron

Modelling ellipsoidal variables with known distances can lead to exact determination of the masses of both components, even in the absence of eclipses. We present such modelling using light and radial velocity curves of ellipsoidal red giant binaries in the LMC, where they are also known as sequence E stars. Stars were selected as likely eccentric systems on the basis of light curve shape alone. We have confirmed their eccentric nature and obtained system parameters using the Wilson-Devinney code. Most stars in our sample exhibit unequal light maxima as well as minima, a phenomenon not observed in sequence E variables with circular orbits. We find evidence that the shape of the red giant changes throughout the orbit due to the high eccentricity and the varying influence of the companion. Brief intervals of pulsation are apparent in two of the red giants. We determine pulsation modes and comment on their placement in the period-luminosity plane. Defining the parameters of these systems paves the way for modelling to determine by what mechanism eccentricity is maintained in evolved binaries.Comment: 14 pages, 11 figures. Accepted to MNRAS 2012 January

Tracing out the Northern Tidal Stream of the Sagittarius Dwarf Spheoridal Galaxy

The main aim of this paper is to report two new detections of tidal debris in the northern stream of the Sagittarius dwarf galaxy located at 45 arcdeg and 55 arcdeg from the center of galaxy. Our observational approach is based on deep color-magnitude diagrams, that provides accurate distances, surface brightness and the properties of stellar population of the studied region of this tidal stream. The derived distances for these tidal debris wraps are 45 kpc and 54 kpc respectively.We also confirm these detections with numerical simulations of the Sagittarius dwarf plus the Milky Way. The model reproduces the present position and velocity of the Sagittarius main body and presents a long tidal stream formed by tidal interaction with the Milky Way potential. This model is also in good agreement with the available observations of the Sagittarius tidal stream. We also present a method for estimating the shape of the Milky Way halo potential using numerical simulations. From our simulations we obtain an oblateness of the Milky Way dark halo potential of 0.85, using the current database of distances and radial velocities of the Sagittarius tidal stream. The color-magnitude diagram of the apocenter of Sagittarius shows that this region of the stream shares the complex star formation history observed in the main body of the galaxy. We present the first evidence for a gradient in the stellar population along the stream, possibly correlated with its different pericenter passages. (abridged)Comment: 43 pages (including 15 figures; for high resolution color figures, please contact [email protected]). Submitted to Ap

HST observations of the cometary blue compact dwarf galaxy UGC 4483: a relatively young galaxy?

We present V and I photometry of the resolved stars in the cometary blue compact dwarf galaxy UGC 4483 using Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) images. The resulting I vs. (V-I) color-magnitude diagram (CMD) reaches limiting magnitudes V = 27.5 mag and I = 26.5 mag for photometric errors less than 0.2 mag. It reveals not only a young stellar population of blue main-sequence stars and blue and red supergiants, but also an older evolved population of red giant and asymptotic giant branch stars. The measured magnitude I = 23.65 +/- 0.10 mag of the red giant branch tip results in a distance modulus (m-M) = 27.63 +/- 0.12, corresponding to a distance of 3.4 +/- 0.2 Mpc. The youngest stars are associated with the bright H II region at the northern tip of the galaxy. The population of older stars is found throughout the low-surface-brightness body of the galaxy and is considerably more spread out than the young stellar population, suggesting stellar diffusion. The most striking characteristics of the CMD of UGC 4483 are the very blue colors of the red giant stars and the high luminosity of the asymptotic giant branch stars. Both of these characteristics are consistent with either: 1) a very low metallicity ([Fe/H] = -2.4 like the most metal-deficient globular clusters) and an old age of 10 Gyr, or 2) a higher metallicity ([Fe/H] = -1.4 as derived from the ionized gas emission lines) and a relatively young age of the oldest stellar population in UGC 4483, not exceeding ~ 2 Gyr. Thus our data do not exclude the possibility that UGC 4483 is a relatively young galaxy having formed its first stars only ~ 2 Gyr ago.Comment: 37 pages, 15 PS figures, to appear in Ap

A Possible Massive Asteroid Belt Around zeta Lep

We have used the Keck I telescope to image at 11.7 microns and 17.9 microns the dust emission around zeta Lep, a main sequence A-type star at 21.5 pc from the Sun with an infrared excess. The excess is at most marginally resolved at 17.9 microns. The dust distance from the star is probably less than or equal to 6 AU, although some dust may extend to 9 AU. The mass of observed dust is \~10^22 g. Since the lifetime of dust particles is about 10,000 years because of the Poytning-Robertson effect, we robustly estimate at least 4 10^26 g must reside in parent bodies which may be asteroids if the system is in a steady state and has an age of ~300 Myr. This mass is approximately 200 times that contained within the main asteroid belt in our solar system.Comment: 12 pages, 3 figures, ApJL in pres

The Discovery of a Companion to the Lowest Mass White Dwarf

We report the detection of a radial velocity companion to SDSS J091709.55+463821.8, the lowest mass white dwarf currently known with M~0.17Msun. The radial velocity of the white dwarf shows variations with a semi-amplitude of 148.8 km/s and a period of 7.5936 hours, which implies a companion mass of M > 0.28Msun. The lack of evidence of a companion in the optical photometry forces any main-sequence companion to be smaller than 0.1Msun, hence a low mass main sequence star companion is ruled out for this system. The companion is most likely another white dwarf, and we present tentative evidence for an evolutionary scenario which could have produced it. However, a neutron star companion cannot be ruled out and follow-up radio observations are required to search for a pulsar companion.Comment: ApJ, in press. See the Press Release at http://www.cfa.harvard.edu/press/2007/pr200708.htm

Lupus-TR-3b: A Low-Mass Transiting Hot Jupiter in the Galactic Plane?

We present a strong case for a transiting Hot Jupiter planet identified during a single-field transit survey towards the Lupus Galactic plane. The object, Lupus-TR-3b, transits a V=17.4 K1V host star every 3.91405d. Spectroscopy and stellar colors indicate a host star with effective temperature 5000 +/- 150K, with a stellar mass and radius of 0.87 +/- 0.04M_sun and 0.82 +/- 0.05R_sun, respectively. Limb-darkened transit fitting yields a companion radius of 0.89 +/- 0.07R_J and an orbital inclination of 88.3 +1.3/-0.8 deg. Magellan 6.5m MIKE radial velocity measurements reveal a 2.4 sigma K=114 +/- 25m/s sinusoidal variation in phase with the transit ephemeris. The resulting mass is 0.81 +/- 0.18M_J and density 1.4 +/- 0.4g/cm^3. Y-band PANIC image deconvolution reveal a V>=21 red neighbor 0.4'' away which, although highly unlikely, we cannot conclusively rule out as a blended binary with current data. However, blend simulations show that only the most unusual binary system can reproduce our observations. This object is very likely a planet, detected from a highly efficient observational strategy. Lupus-TR-3b constitutes the faintest ground-based detection to date, and one of the lowest mass Hot Jupiters known.Comment: 4 pages, 4 figures, accepted for publication in ApJ