Laurence W. Fredrick

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.

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.

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.

Interferometric Astrometry with Hubble Space Telescope Fine Guidance Sensor 3: The Parallax of the Cataclysmic Variable TV Columbae

TV Col is a 13th magnitude intermediate polar cataclysmic variable with multiple periods found in the light curves. Past estimates predicted a distance of 400 pc to greater than 500 pc. Recently completed Hubble Space Telescope fine guidance sensor interferometric observations allow us to determine the first trigonometric parallax to TV Col. This determination puts the distance of TV Col at 368 pc. CD-32 2376, a 10th magnitude Tycho Catalog star, is a reference star in the TV Col frame. We find a distance of 127.7 ± 1 pc.

Astrometry with Hubble Space Telescope Fine Guidance Sensor 3: The Parallax of the Cataclysmic Variable RW Triangulum

RW Triangulum (RW Tri) is a 13th magnitude nova-like cataclysmic variable star with an orbital period of 0.2319 days (5.56 hr). Infrared observations of RW Tri indicate that its secondary is most likely a late-K dwarf (Dhillon). Past analyses predicted a distance of 270 pc, derived from a blackbody fit to the spectrum of the central part of the disk (Rutten, van Paradijs, & Tinbergen). Recently completed Hubble Space Telescope Fine Guidance Sensor interferometric observations allow us to determine the first trigonometric parallax to RW Tri. This determination puts the distance of RW Tri at 341, one of the most distant objects with a direct parallax measurement. We compare our result with methods previously employed to estimate distances to cataclysmic variables.

Precise masses for wolf 1062 AB from hubble space telescope interferometric astrometry and Mcdonald observatory radial velocities

We present an analysis of astrometric data from Fine Guidance Sensor 3 (FGS 3), a white-light interferometer on HST , and of radial velocity data from two ground-based campaigns. We model the astrometric and radial velocity measurements simultaneously to obtain parallax, proper motion, and component masses for Wolf 1062 (Gl 748; M3.5 V). To derive the mass fraction, we relate FGS 3 fringe scanning observations of the science target to a reference frame provided by fringe tracking observations of a surrounding star —eld. We obtain an absolute parallax mas) yielding (n abs \ 98.0 ^ 0.4 M A \ 0.379 and high-quality component masses with errors of only 1.5%. ^ 0.005 M _ M B \ 0.192 ^ 0.003 M _ ,

Astrometry with Hubble Space Telescope Fine Guidance Sensor number 3: Position-mode stability and precision

We report results from a test exploring the long- and short-term astrometric stability of Hubble Space Telescope Fine Guidance Sensor #3. A test field was observed 40 times over 522 days to determine the precision and accuracy of FGS astrometry and to measure the character and magnitude of possible secular scale changes. We examine the astrometric data and the associated guide star data to determine random errors. These data are also explored to find sources of systematic error. After correcting for some systematic effects we obtain a precision of 0.002 arcsec (2 mas) per observation (RSS of x and y). This is relative astrometry within a central 2.5 arc minute FGS field of view of any orientation. We find that the scale varies over time and confirm the sense of the trend with independent data. From the 40 observation sets we produce a catalog of an astrometry test field containing 8 stars whose relative positions are well known to an average 0.7 and 0.9 mas in x and y. One reference star has a relative parallax of 3.1 ± 0.5 mas. Finally, we report that 11 observation sets acquired over 387 days produce parallaxes and relative positions with 1 mas precision.

Astrometry with The Hubble Space Telescope: A Parallax of the Central Star of the Planetary Nebula NGC 6853

We present an absolute parallax and relative proper motion for the central star of the planetary nebula NGC 6853 (the Dumbbell). We obtain these with astrometric data from the Fine Guidance Sensor 3, a white-light interferometer on the Hubble Space Telescope. Spectral classifications and VRIJHKT2M and DDO51 photometry of the stars making up the astrometric reference frame provide spectrophotometric estimates of their absolute parallaxes. Introducing these into our model as observations with error, we find πabs = 2.10 ± 0.48 mas for the DAO central star of NGC 6853. A weighted average with a previous ground-based USNO determination yields πabs = 2.40 ± 0.32. We assume that the extinction suffered by the reference stars nearest (in angular separation and distance) to the central star is the same as for the central star. Correcting for color differences, we find AV = 0.30 ± 0.06 for the central star, hence, an absolute magnitude MV = 5.48. A recent determination of the central star effective temperature aided in estimating the central star radius, R* = 0.055 ± 0.02 R⊙, a star that may be descending to the white dwarf cooling track.

Interferometric Astrometry of the Detached White Dwarf-M Dwarf Binary Feige 24 Using HST Fine Guidance Sensor 3: White Dwarf Radius and Component Mass Estimates*

With Hubble Space Telescope Fine Guidance Sensor 3 we have determined a parallax for the white dwarf–M dwarf interacting binary, Feige 24. The white dwarf (DA) component has an effective temperature Teff ~ 56,000 K. A weighted average with past parallax determinations (πabs = 14.6 ± 0.4 mas) narrows the range of possible radius values, compared with past estimates. We obtain RDA = 0.0185 ± 0.0008 R⊙ with uncertainty in the temperature and bolometric correction the dominant contributors to the error. Fine Guidance Sensor 3 photometry provides a light curve entirely consistent with reflection effects. A recently refined model mass-luminosity relation for low-mass stars provides a mass estimate for the M dwarf companion, MdM = 0.37 ± 0.20 M⊙, where the mass range is due to metallicity and age uncertainties. Radial velocities from Vennes and Thorstensen provide a mass ratio from which we obtain MDA = 0.49 M⊙. Independently, our radius and recent log g determinations yield 0.44 M⊙ < MDA < 0.47 M⊙. In each case, the minimum DA mass is that derived by Vennes & Thorstensen from their radial velocities and Keplerian circular orbits with i ≤ 90°. Locating Feige 24 on an (M, R)-plane suggests a carbon core. Our radius and these mass estimates yield a value of γgrav inconsistent with that derived by Vennes & Thorstensen. We speculate on the nature of a third component whose existence would resolve the discrepancy.