Dynamical Masses of Young Stars in Multiple Systems

We present recent measurements of the orbital motion in the young binaries DF Tau and ZZ Tau, and the hierarchical triple Elias 12, that were obtained with the Fine Guidance Sensors on the HST and at the Keck Observatory using adaptive optics. Combining these observations with previous measurements from the literature, we compute preliminary orbital parameters for DF Tau and ZZ Tau. We find that the orbital elements cannot yet be determined precisely because the orbital coverage spans only about 90 degr in position angle. Nonetheless, the range of possible values for the period and semi-major axis already defines a useful estimate for the total mass in DF Tau and ZZ Tau, with values of 0.90{+0.85}{-0.35} M_sun and 0.81{+0.44}{-0.25} M_sun, respectively, at a fiducial distance of 140 pc.Comment: 26 pages, 9 figures, accepted for publication in A

NOAO Fundamental Plane Survey. II. Age and Metallicity along the Red Sequence from Line‐Strength Data

We present spectroscopic line-strength data for 4097 red-sequence galaxies in 93 low-redshift galaxy clusters and use these to investigate variations in average stellar populations as a function of galaxy mass. Our analysis includes an improved treatment of nebular emission contamination, which affects ~10% of the sample galaxies. Using the stellar population models of D. Thomas and collaborators, we simultaneously fit 12 observed line-strength-σ relations in terms of common underlying trends of age, [Z/H] (total metallicity), and [α/Fe] (α-element enhancement). We find that the observed line-strength-σ relations can be explained only if higher mass red-sequence galaxies are, on average, older, more metal-rich, and more α-enhanced than lower mass galaxies. Quantitatively, the scaling relations are age ∝ σ0.59±0.13, Z/H ∝ σ0.53±0.08, and α/Fe ∝ σ0.31±0.06, where the errors reflect the range obtained using different subsets of indices. Our conclusions are not strongly dependent on which Balmer lines are used as age indicators. The derived age-σ relation is such that if the largest (σ ~ 400 km s-1) galaxies formed their stars ~13 Gyr ago, then the mean age of low-mass (σ ~ 50 km s-1) objects is only ~4 Gyr. The data also suggest a large spread in age at the low-mass end of the red sequence, with 68% of the galaxies having ages between 2 and 8 Gyr. We conclude that although the stars in giant red galaxies in clusters formed early, most of the galaxies at the faint end joined the red sequence only at recent epochs. This downsizing trend is in good qualitative agreement with observations of the red sequence at higher redshifts but is not predicted by semianalytic models of galaxy formation

Toxoplasma gondii in the food supply

Toxoplasmosis is caused by infection with the protozoan parasite Toxoplasma gondii. Infections are usually either asymptomatic or develop mild symptoms that are self-limited, but infections in immunosuppressed persons can be severe. Infections in pregnant women can cause serious health problems in the child such as mental retardation and blindness. Infection with T. gondii in immunocompetent adults can lead to impaired eyesight. Toxoplasmosis has ranked very highly in two studies of death and disability attributable to foodborne pathogens. The consumption of raw or undercooked meat containing T. gondii tissue cysts and the consumption of raw vegetables or water contaminated with T. gondii oocysts from cat feces is most frequently associated with human illness. The risk of acquiring a Toxoplasma infection via food varies with cultural and eating habits in different human populations

Precise Estimates of the Physical Parameters for the Exoplanet System HD-17156 Enabled by HST FGS Transit and Asteroseismic Observations

We present observations of three distinct transits of HD 17156b obtained with the Fine Guidance Sensors (FGS) on board the Hubble Space Telescope} (HST). We analyzed both the transit photometry and previously published radial velocities to find the planet-star radius ratio R_p/R_s = 0.07454 +/- 0.00035, inclination i=86.49 +0.24/-0.20 deg, and scaled semi-major axis a/R = 23.19 +0.32/-0.27. This last value translates directly to a mean stellar density determination of 0.522 +0.021/-0.018 g cm^-3. Analysis of asteroseismology observations by the companion paper of Gilliland et al. (2009) provides a consistent but significantly refined measurement of the stellar mean density. We compare stellar isochrones to this density estimate and find M_s = 1.275 +/- 0.018 M_sun and a stellar age of $3.37 +0.20/-0.47 Gyr. Using this estimate of M_s and incorporating the density constraint from asteroseismology, we model both the photometry and published radial velocities to estimate the planet radius R_p= 1.0870 +/- 0.0066 Jupiter radii and the stellar radius R_s = 1.5007 +/- 0.0076 R_sun. The planet radius is larger than that found in previous studies and consistent with theoretical models of a solar-composition gas giant of the same mass and equilibrium temperature. For the three transits, we determine the times of mid-transit to a precision of 6.2 s, 7.6 s, and 6.9 s, and the transit times for HD 17156 do not show any significant departures from a constant period. The joint analysis of transit photometry and asteroseismology presages similar studies that will be enabled by the NASA Kepler Mission.Comment: Accepted for publication to Ap

Heavy‐Element Diffusion in Metal‐poor Stars

Stellar evolution models that include the effect of helium and heavy-element diffusion have been calculated for initial iron abundances of [Fe/H] = -2.3, -2.1, -1.9, and -1.7. These models were calculated for a large variety of masses and three separate mixing lengths, α = 1.50, 1.75, and 2.00 (with α = 1.75 being the solar calibrated mixing length). The change in the surface iron abundance for stars of different masses was determined for the ages of 11, 13, and 15 Gyr. Iron settles out of the surface convection zone on the main sequence ; this iron is dredged back up when the convection zone deepens on the giant branch. In all cases, the surface [Fe/H] abundance in the turnoff stars was at least 0.28 dex lower than the surface [Fe/H] abundance in giant branch stars of the same age. However, Gratton et al. recently found, based on high-dispersion spectra of stars in the globular cluster NGC 6397, that the turnoff and giant branch stars had identical (within a few percent) iron abundances of [Fe/H] = -2.03. These observations prove that heavy-element diffusion must be inhibited in the surface layers of metal- poor stars. When diffusion is inhibited in the outer layers of a stellar model, the predicted temperatures of the models are similar to those of models evolved without diffusion, while the predicted lifetimes are similar to those of stars in which diffusion is not inhibited. Isochrones constructed from the models in which diffusion is inhibited fall halfway between isochrones without diffusion and isochrones with full diffusion. As a result, absolute globular cluster ages based upon the absolute magnitude of the turnoff are 4% larger than ages inferred from full-diffusion isochrones and 4% smaller than ages inferred from non-diffusion isochrones

A High Angular Resolution Survey of Massive Stars in Cygnus OB2: Results from the Hubble Space Telescope Fine Guidance Sensors

We present results of a high angular resolution survey of massive OB stars in the Cygnus OB2 association that we conducted with the Fine Guidance Sensor 1R (FGS1r) on the Hubble Space Telescope. FGS1r is able to resolve binary systems with a magnitude difference delta-V < 4 down to separations as small as 0.01 arcsec. The sample includes 58 of the brighter members of Cyg OB2, one of the closest examples of an environment containing a large number of very young and massive stars. We resolved binary companions for 12 targets and confirmed the triple nature of one other target, and we offer evidence of marginally resolved companions for two additional stars. We confirm the binary nature of 11 of these systems from complementary adaptive optics imaging observations. The overall binary frequency in our study is 22% to 26% corresponding to orbital periods ranging from 20 - 20,000 years. When combined with the known short-period spectroscopic binaries, the results supports the hypothesis that the binary fraction among massive stars is > 60%. One of the new discoveries is a companion to the hypergiant star MT 304 = Cyg OB2-12, and future measurements of orbital motion should provide mass estimates for this very luminous star.Comment: accepted for AJ, 84 pages, 61 figure

Pismis 24-1: The Stellar Upper Mass Limit Preserved

Is there a stellar upper mass limit? Recent statistical work seems to indicate that there is and that it is in the vicinity of 150 solar masses. In this paper we use HST and ground-based data to investigate the brightest members of the cluster Pismis 24, which was previously inferred to have a mass greater than 200 solar masses, in apparent disagreement with that limit. We determine that Pismis 24-1 is composed of at least three objects, the resolved Pismis 24-1SW and the unresolved spectroscopic binary Pismis 24-1NE. The evolutionary zero-age masses of those two objects and that of the nearby Pismis 24-17 are all approximately 100 solar masses, very large but under the stellar upper mass limit