G. Fritz Benedict

HUBBLE SPACE TELESCOPE FINE GUIDANCE SENSOR PARALLAXES OF GALACTIC CEPHEID VARIABLE STARS: PERIOD-LUMINOSITY RELATIONS ∗

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 the Hubble Space Telescope. We find absolute parallaxes in milliarcseconds: l Car, 2.01 ± 0.20 mas; ζ Gem, 2.78 ± 0.18 mas; β Dor, 3.14 ± 0.16 mas; W Sgr, 2.28 ± 0.20 mas; X Sgr, 3.00 ± 0.18 mas; Y Sgr, 2.13 ± 0.29 mas; FF Aql, 2.81 ± 0.18 mas; T Vul, 1.90 ± 0.23 mas; and RT Aur, 2.40 ± 0.19 mas; average σπ/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 WVI bandpasses, corrected for interstellar extinction and Lutz-Kelker-Hanson bias. Adding our previous absolute magnitude determination for δ Cep, we construct period-luminosity relations (PLRs) for 10 Galactic Cepheids. We compare our new PLRs with those adopted by several recent investigations, including the Freedman and Sandage H0 projects. Adopting our PLR would tend to increase the Sandage H0 value, but leave the Freedman H0 unchanged. Comparing our Galactic Cepheid PLR with those derived from LMC Cepheids, we find the slopes for K and WVI to be identical in the two galaxies within their respective errors. Our data lead to a WVI distance modulus for the LMC m - M = 18.50 ± 0.03, uncorrected for any metallicity effects. Applying recently derived metallicity corrections yields a corrected LMC distance modulus of (m - M)0 = 18.40 ± 0.05. Comparing our PLR to solar-metallicity Cepheids in NGC 4258 results in a distance modulus 29.28 ± 0.08 that agrees with one derived from maser studies.

Detection of a Neptune-Mass Planet in the ρ1 Cancri System Using the Hobby-Eberly Telescope

We report the detection of the lowest mass extrasolar planet yet found around a Sun-like star—a planet with an M sin i of only 14.21 ± 2.91 M⊕ in an extremely short period orbit (P = 2.808 days) around ρ1 Cancri, a planetary system that already has three known planets. Velocities taken from late 2003-2004 at McDonald Observatory with the Hobby-Eberly Telescope revealed this inner planet at 0.04 AU. We estimate an inclination of the outer planet ρ1 Cancri d, based on Hubble Space Telescope Fine Guidance Sensor measurements that suggest an inner planet of only 17.7 ± 5.57 M⊕, if coplanarity is assumed for the system.

The Optical Mass-Luminosity Relation at the End of the Main Sequence (0.08-0.20 M☉)*

The empirical mass-luminosity relation at M is presented for stars with masses 0.08-0.20 M☉ based upon new observations made with Fine Guidance Sensor 3 on the Hubble Space Telescope. The targets are nearby, red dwarf multiple systems in which the magnitude differences are typically measured to ±0.1 mag or better. The M values are generated using the best available parallaxes and are also accurate to ±0.1 mag, because the errors in the magnitude differences are the dominant error source. In several cases this is the first time the observed sub-arcsecond multiples have been resolved at optical wavelengths. The mass-luminosity relation defined by these data reaches to M=18.5 and provides a powerful empirical test for discriminating the lowest mass stars from high-mass brown dwarfs at wavelengths shorter than 1 μm.

CONFIRMATION OF ERRORS IN HIPPARCOS PARALLAXES FROM HUBBLE SPACE TELESCOPE FINE GUIDANCE SENSOR ASTROMETRY OF THE PLEIADES

We present absolute trigonometric parallaxes and relative proper motions for three members of the Pleiades, obtained with the Hubble Space Telescopes Fine Guidance Sensor 1r, a white-light interferometer. We estimate spectral types and luminosity classes of the stars comprising the astrometric reference frame from R ? 2000 spectra, VJHK photometry, and reduced proper motions. From these we derive estimates of absolute parallaxes and introduce them into our model as observations with error. We constrain the three cluster members to have a 1 ? dispersion in distance less than 6.4 pc and find an average ?abs = 7.43 ? 0.17 ? 0.20 mas, where the second error is systematic due to member placement within the cluster. This parallax corresponds to a distance of 134.6 ? 3.1 pc or a distance modulus of m - M = 5.65 ? 0.05 for these three Pleiades stars, presuming a central location. This result agrees with three other independent determinations of the Pleiades distance. Presuming that the cluster depth systematic error can be significantly reduced because of the random placement of these many members within the cluster, these four independent measures yield a best-estimate Pleiades distance of ?abs = 7.49 ? 0.07 mas, corresponding to a distance of 133.5 ? 1.2 pc or a distance modulus of m - M = 5.63 ? 0.02. This resolves the dispute between the main-sequence fitting and the Hipparcos distance moduli in favor of main-sequence fitting.

The Extrasolar Planet ϵ Eridani b: Orbit and Mass*

Hubble Space Telescope (HST) observations of the nearby (3.22 pc) K2 V star Eridani have been combined with ground-based astrometric and radial velocity data to determine the mass of its known companion. We model the astrometric and radial velocity measurements simultaneously to obtain the parallax, proper motion, perturbation period, perturbation inclination, and perturbation size. Because of the long period of the companion, Eri b, we extend our astrometric coverage to a total of 14.94 yr (including the 3 yr span of the HST data) by including lower precision ground-based astrometry from the Allegheny Multichannel Astrometric Photometer. Radial velocities now span 1980.8-2006.3. We obtain a perturbation period, P = 6.85 ± 0.03 yr, semimajor axis α = 1.88 ± 0.20 mas, and inclination i = 301 ± 38. This inclination is consistent with a previously measured dust disk inclination, suggesting coplanarity. Assuming a primary mass M* = 0.83 M⊙, we obtain a companion mass M = 1.55MJ ± 0.24MJ. Given the relatively young age of Eri (~800 Myr), this accurate exoplanet mass and orbit can usefully inform future direct-imaging attempts. We predict the next periastron at 2007.3 with a total separation ρ = 03 at position angle P.A. = -27°. Orbit orientation and geometry dictate that Eri b will appear brightest in reflected light very nearly at periastron. Radial velocities spanning over 25 yr indicate an acceleration consistent with a Jupiter-mass object with a period in excess of 50 yr, possibly responsible for one feature of the dust morphology, the inner cavity.

Astrometry with the Hubble Space Telescope: A Parallax of the Fundamental Distance Calibrator RR Lyrae*

We present an absolute parallax and relative proper motion for the fundamental distance scale calibrator � Cep. We obtain these with astrometric data from FGS 3, a white-light interferometer on the Hubble Space Telescope (HST). Utilizing spectrophotometric estimates of the absolute parallaxes of our astrometric reference stars and constrainingCep and reference star HD 213307 to belong to the same association (Cep OB6), we findabs = 3.66 � 0.15 mas. The larger than typical astrometric residuals for the nearby astrome- tric reference star HD 213307 are found to satisfy Keplerian motion with P = 1.07 � 0.02 yr, a perturbation and period that could be due to an F0 V companion � 7 mas distant from and � 4 mag fainter than the pri- mary. Spectral classifications and VRIJHKT2M and DDO51 photometry of the astrometric reference frame surroundingCep indicate that field extinction is high and variable along this line of sight. However the extinction suffered by the reference star nearest (in angular separation and distance) toCep, HD 213307, is lower and nearly the same as forCep. Correcting for color differences, we find hAVi = 0.23 � 0.03 for � Cep and hence an absolute magnitude MV = � 3.47 � 0.10. Adopting an average V magnitude, hVi = 15.03 � 0.03, for Cepheids with log P = 0.73 in the large Magellanic Cloud (LMC) from Udalski et al., we find a V-band distance modulus for the LMC, mM = 18.50 � 0.13, or 18.58 � 0.15, where the lat- ter value results from a highly uncertain metallicity correction. These agree with our previous RR Lyr HST parallax-based determination of the distance modulus of the LMC.

Interferometric Astrometry of Proxima Centauri and Barnard's Star Using HUBBLE SPACE TELESCOPE Fine Guidance Sensor 3: Detection Limits for Substellar Companions

We report on a substellar-companion search utilizing interferometric fringe-tracking astrometry acquired with Fine Guidance Sensor 3 on the Hubble Space Telescope. Our targets were Proxima Centauri and Barnards star. We obtain absolute parallax values of πabs = 07687 ± 00003 for Proxima Cen and πabs = 05454 ± 00003 for Barnards star. Once low-amplitude instrumental systematic errors are identified and removed, our companion detection sensitivity is less than or equal to one Jupiter mass for periods longer than 60 days for Proxima Cen. Between the astrometry and the recent radial velocity results of Kurster et al., we exclude all companions with M > 0.8MJup for the range of periods 1 day < P < 1000 days. For Barnards star, our companion detection sensitivity is less than or equal to one Jupiter mass for periods longer than 150 days. Our null results for Barnards star are consistent with those reported by Gatewood in 1995.

HD 98800: A unique stellar system of post-T tauri stars

HD 98800 is a system of four stars, and it has a large infrared excess that is thought to be due to a dust disk within the system. In this paper we present new astrometric observations made with Hipparcos, as well as photometry from Hubble Space Telescope WFPC2 images. Combining these observations and reanalyzing previous work allow us to estimate the age and masses of the stars in the system. Uncertainty in these ages and masses results from uncertainty in the temperatures of the stars and any reddening they may have. We find that HD 98800 is most probably about 10 Myr old, although it may be as young as 5 Myr or as old as 20 Myr old. The stars in HD 98800 appear to have metallicities that are about solar. An age of 10 Myr means that HD 98800 is a member of the post T Tauri class of objects, and we argue that the stars in HD 98800 can help us understand why post T Tauris have been so elusive, HD 98800 may have formed in the Centaurus star-forming region, but it is extraordinary in being so young and yet so far from where it was born.

Accurate M DWARF metallicities from spectral synthesis : A critical test of model atmospheres

We describe a method for accurately determining M dwarf metallicities with spectral synthesis based on abundance analyses of visual binary stars. We obtained high-resolution, high-signal-to-noise ratio spectra of each component of five visual binary pairs at McDonald Observatory. The spectral types of the components range from F7 to K3 V for the primaries and from M0.5 to M3.5 V for the secondaries. We have determined the metallicities of the primaries differentially with respect to the Sun by fitting synthetic spectra to Fe I line profiles in the observed spectra. In the course of our analysis of the M dwarf secondaries, we have made significant improvements to the PHOENIX cool-star model atmospheres and the spectrum analysis code MOOG. Our analysis yields an rms deviation of 0.11 dex in metallicity values between the binary pairs. We estimate the uncertainties in the derived stellar parameters for the M dwarfs to be 48 K, 0.10 dex, 0.12 dex, 0.15 km s-1, and 0.20 km s-1 for Teff, log g, [M/H], ξ, and η, respectively. Accurate stellar evolutionary models are needed to progress further in the analysis of cool-star spectra; the new model atmospheres warrant recalculation of the evolutionary models.

Photometry of Proxima Centauri and Barnard's Star Using Hubble Space Telescope Fine Guidance Sensor 3: A Search for Periodic Variations

We have observed Proxima Centauri and Barnards star with the Hubble Space Telescope Fine Guidance Sensor 3. Proxima Cen exhibits small-amplitude, periodic photometric variations. Once several sources of systematic photometric error are corrected, we obtain 2 mmag internal photometric precision. We identify two distinct behavior modes over the past 4 years: higher amplitude, longer period and smaller amplitude, shorter period. Within the errors, one period (P ~ 83 days) is twice the other. Barnards star shows very weak evidence for periodicity on a timescale of approximately 130 days. If we interpret these periodic phenomena as rotational modulation of starspots, we identify three discrete spots on Proxima Cen and possibly one spot on Barnards star. We find that the disturbances change significantly on timescales as short as one rotation period.