Edmund P. Nelan

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.

A Mass for the Extrasolar Planet Gliese 876b Determined from Hubble Space Telescope Fine Guidance Sensor 3 Astrometry and High-Precision Radial Velocities*

We report the first astrometrically determined mass of an extrasolar planet, a companion previously detected by Doppler spectroscopy. Radial velocities first provided an ephemeris with which to schedule a significant fraction of the Hubble Space Telescope(HST) observations near companion peri- and apastron. The astrometry residuals at these orbital phases exhibit a systematic deviation consistent with a perturbation due to a planetary mass companion. Combining HST astrometry with radial velocities, we solve for the proper motion, parallax, perturbation size, inclination, and position angle of the line of nodes, while constraining period, velocity amplitude, longitude of periastron, and eccentricity to values determined from radial velocities. We find a perturbation semimajor axis and inclination, mas, , and Gl 876 absolute parallax, a p 0.25 0.06 i p 84 6 p p abs mas. Assuming that the mass of the primary star is , we find the mass of the planet, 214.6 0.2 M p 0.32 M ∗ , Gl 876b, . M p 1.89 0.34 M b Jup

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.

THE SOLAR NEIGHBORHOOD. XXXIII. PARALLAX RESULTS FROM THE CTIOPI 0.9 m PROGRAM: TRIGONOMETRIC PARALLAXES OF NEARBY LOW-MASS ACTIVE AND YOUNG SYSTEMS

We present basic observational data and association membership analysis for 45 young and active low-mass stellar systems from the ongoing RECONS photometry and astrometry program at the Cerro Tololo Inter-American Observatory. Most of these systems have saturated X-ray emission (log(Lx/Lbol) > -3.5) based on X-ray fluxes from the ROSAT All-Sky Survey, and many are significantly more luminous than main-sequence stars of comparable color. We present parallaxes and proper motions, Johnson-Kron-Cousins VRI photometry, and multiplicity observations from the CTIOPI program on the CTIO 0.9m telescope. To this we add low-resolution optical spectroscopy and line measurements from the CTIO 1.5m telescope, and interferometric binary measurements from the Hubble Space Telescope Fine Guidance Sensors. We also incorporate data from published sources: JHKs photometry from the 2MASS point source catalog; X-ray data from the ROSAT All-Sky Survey; and radial velocities from literature sources. Within the sample of 45 systems, we identify 21 candidate low-mass pre-main-sequence members of nearby associations, including members of beta Pictoris, TW Hydrae, Argus, AB Doradus, two ambiguous 30 Myr old systems, and one object that may be a member of the Ursa Major moving group. Of the 21 candidate young systems, 14 are newly identified as a result of this work, and six of those are within 25 parsecs of the Sun.

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.

Stars in the Hubble Ultra Deep Field

We identified 46 unresolved source candidates in the Hubble Ultra Deep Field (HUDF) down to i775 ¼ 29:5. UnresolvedobjectswereidentifiedusingaparameterS,whichmeasuresthedeviationfromthecurveofgrowthofa point source. Extensive testing of this parameter was carried out, including the effects of decreasing signal-to-noise ratio and of the apparent motions of stars, which demonstrated that stars brighter thani775 ¼ 27:0 could be robustly identified. Low-resolution grism spectra of the 28 objects brighter than i775 ¼ 27:0 identify 18 M and later stellar type dwarfs, two candidate L dwarfs, two QSOs, and four white dwarfs. Using the observed population of dwarfs with spectral type M4 or later, we derive a Galactic disk scale height of 400 � 100 pc for M and L stars. The local white dwarf density is computed to be as high as (1:1 � 0:3) ;10 � 2 stars pc � 3 . On the basis of observations taken 73 days apart, we determined that no object in the field has a proper motion larger than 0B027 yr � 1 (3 � detection limit). No high-velocity white dwarfs were identified inthe HUDF, and all four candidates appear more likely to be part of the Galactic thick disk. The lack of detected halo white dwarfs implies that if the dark matter halo is 12 Gyr old, white dwarfs account for less than 10% of the dark matter halo mass. Subject headingg Galaxy: disk — Galaxy: stellar content — Galaxy: structure — stars: late-type — white dwarfs Online material: color figures

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.

ASTEROSEISMOLOGY OF THE TRANSITING EXOPLANET HOST HD 17156 WITH HUBBLE SPACE TELESCOPE FINE GUIDANCE SENSOR

Observations conducted with the Fine Guidance Sensor on Hubble Space Telescope (HST) providing high cadence and precision time-series photometry were obtained over 10 consecutive days in December 2008 on the host star of the transiting exoplanet HD 17156b. During this time 10^12 photons (corrected for detector deadtime) were collected in which a noise level of 163 parts per million per 30 second sum resulted, thus providing excellent sensitivity to detection of the analog of the solar 5-minute p-mode oscillations. For HD 17156 robust detection of p-modes supports determination of the stellar mean density of 0.5301 +/- 0.0044 g/cm^3 from a detailed fit to the observed frequencies of modes of degree l = 0, 1, and 2. This is the first star for which direct determination of the mean stellar density has been possible using both asteroseismology and detailed analysis of a transiting planet light curve. Using the density constraint from asteroseismology, and stellar evolution modeling results in M_star = 1.285 +/- 0.026 solar, R_star = 1.507 +/- 0.012 solar, and a stellar age of 3.2 +/- 0.3 Gyr.Observations conducted with the Fine Guidance Sensor on the Hubble Space Telescope (HST) providing high cadence and precision time-series photometry were obtained over 10 consecutive days in 2008 December on the host star of the transiting exoplanet HD?17156b. During this time, 1.0 ? 1012 photons (corrected for detector dead time) were collected in which a noise level of 163 parts per million per 30?s sum resulted, thus providing excellent sensitivity to the detection of the analog of the solar 5-minute p-mode oscillations. For HD?17156, robust detection of p modes supports the determination of the stellar mean density of ?* = 0.5301 ? 0.0044?g?cm?3 from a detailed fit to the observed frequencies of modes of degree l = 0, 1, and 2. This is the first star for which the direct determination of ?* has been possible using both asteroseismology and detailed analysis of a transiting planet light curve. Using the density constraint from asteroseismology, and stellar evolution modeling results in M * = 1.285 ? 0.026 M ?, R * = 1.507 ? 0.012 R ?, and a stellar age of 3.2 ? 0.3?Gyr.

Photometry of Proxima Centauri and Barnard's Star Using HST 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.