No abstract

We present a Mass-Luminosity Relation (MLR) for red dwarfs spanning a range of masses from 0.62 M to the end of the stellar main sequence at 0.08 M . The relation is based on 47 stars for which dynamical masses have been determined, primarily using astrometric data from Fine Guidance Sensors (FGS) 3 and 1r, white-light interferometers on the Hubble Space Telescope (HST), and radial velocity data from McDonald Observatory. For our HST/FGS sample of 15 binaries, component mass errors range from 0.4% to 4.0% with a median error of 1.8%. With these and masses from other sources, we construct a V -band MLR for the lower main sequence with 47 stars, and a K-band MLR with 45 stars with fit residuals half of those of the V -band.We use GJ 831 AB as an example, obtaining an absolute trigonometric parallax, π abs = 125.3 ± 0.3 milliseconds of arc, with orbital elements yielding M A = 0.270 ± 0.004M and M B = 0.145 ± 0.002M . The mass precision rivals that derived for eclipsing binaries.

Context. The universality of the Cepheid period-luminosity (PL) relations has been under discussion since metallicity effects were assumed to play a role in the value of the intercept and, more recently, of the slope of these relations. Aims. The goal of the present study is to calibrate the Galactic PL relations in various photometric bands (from B to K) and to compare the results to the well-established PL relations in the LMC. Methods. We use a set of 59 calibrating stars, the distances of which are measured using five different distance indicators: Hubble Space Telescope and revised Hipparcos parallaxes, infrared surface brightness and interferometric Baade-Wesselink parallaxes, and classical Zero-Age-Main-Sequence-fitting parallaxes for Cepheids belonging to open clusters or OB stars associations. A detailed discussion of absorption corrections and projection factor to be used is given. Results. We find no significant difference in the slopes of the PL relations between LMC and our Galaxy. Conclusions. We conclude that the Cepheid PL relations have universal slopes in all photometric bands, not depending on the galaxy under study (at least for LMC and Milky Way). The possible zero-point variation with metal content is not discussed in the present work, but an upper limit of 18.50 for the LMC distance modulus can be deduced from our data.

We report the detection of the lowest mass extra-solar planet yet found around a Sun-like star -a planet with an M sin i of only 14.21 ± 2.91 Earth masses in an extremely short period orbit (P=2.808 days) around ρ 1 Cancri , a planetary system which already has three known planets. Velocities taken from late 2003-2004 at McDonald Observatory with the Hobby-Eberly Telescope (HET) revealed this inner planet at 0.04 AU. We estimate an inclination of the outer planet ρ 1 Cancri d, based upon Hubble Space Telescope Fine Guidance Sensor (FGS) measurements which suggests an inner planet of only 17.7 ± 5.57 Earth masses, if coplanarity is assumed for the system. Subject headings: (stars:) planetary systems -stars:individual (ρ 1 Cancriastrometry

We present new absolute trigonometric parallaxes and relative proper motions for nine Galactic Cepheid variable stars: ℓ Car, ζ Gem, β Dor, W Sgr, X Sgr, Y Sgr, FF Aql, T Vul, and RT Aur. We obtain these results with astrometric data from Fine Guidance Sensor 1r, a white-light interferometer on Hubble Space Telescope. We find absolute parallaxes in milliseconds of arc: ℓ Car, 2.01 ± 0.20 ; ζ Gem, 2.78 ± 0.18 ; β Dor, 3.14 ± 0.16 ; W Sgr, 2.28 ± 0.20 ; X Sgr, 3.00 ± 0.18 ; Y Sgr, 2.13 ± 0.29 ; FF Aql, 2.81 ± 0.18 ; T Vul, 1.90 ± 0.23 ; and RT Aur, 2.40 ± 0.19 , an average σ π /π = 8%. Two stars (FF Aql and W Sgr) required the inclusion of binary astrometric perturbations, providing Cepheid mass estimates. With these parallaxes we compute absolute magnitudes in V, I, K, and Wesenheit W V I bandpasses corrected for interstellar extinction and Lutz-Kelker-Hanson bias. Adding our previous absolute magnitude determination for δ Cep, we construct Period-Luminosity relations for ten Galactic Cepheids.We compare our new Period-Luminosity relations with those adopted by several recent investigations, including the Freedman and Sandage H 0 projects. Adopting our Period-Luminosity relationship would tend to increase the Sandage H 0 value, but leave the Freedman H 0 unchanged. Comparing our Galactic Cepheid PLR with those derived from LMC Cepheids, we find the slopes for K and W V I identical in the two galaxies within their respective errors. Our data lead to a W V I distance modulus for the Large Magellanic Cloud, m-M = 18.50±0.03, uncorrected for any metallicity effects. Applying recently derived metalllcity corrections yields a corrected LMC distance modulus of (m-M) 0 =18.40±0.05. Comparing our Period-Luminosity relationship to solar-metallicity Cepheids in NGC 4258 results in a distance modulus, 29.28 ± 0.08, which agrees with that derived from maser studies.