Chris Laws

Parent Stars of Extrasolar Planets. VI. Abundance Analyses of 20 New Systems

The results of new spectroscopic analyses of 20 recently reported extrasolar planet parent stars are presented. The companion of one of these stars, HD 10697, has recently been shown to have a mass in the brown dwarf regime; we find [Fe/H] = +0.16 for it. For the remaining sample, we derive [Fe/H] estimates ranging from -0.41 to +0.37, with an average value of +0.18 ± 0.19. If we add the 13 stars included in the previous papers of this series and six other stars with companions below the 11 MJ limit from the recent studies of Santos et al., we derive [Fe/H] = +0.17 ± 0.20. Among the youngest stars with planets with F or G0 spectral types, [Fe/H] is systematically larger than young field stars of the same Galactocentric distance by 0.15 to 0.20 dex. This confirms the recent finding of Laughlin that the most massive stars with planets are systematically more metal-rich than field stars of the same mass. We interpret these trends as supporting a scenario in which these stars accreted high-Z material after their convective envelopes shrunk to near their present masses. Correcting these young star metallicities by 0.15 dex still does not fully account for the difference in mean metallicity between the field stars and the full parent stars sample. The stars with planets appear to have smaller [Na/Fe], [Mg/Fe], and [Al/Fe] values than field dwarfs of the same [Fe/H]. They do not appear to have significantly different values of [O/Fe], [Si/Fe], [Ca/Fe], or [Ti/Fe], though. The claim made in Paper V that stars with planets have low [C/Fe] is found to be spurious, due to unrecognized systematic differences among published studies. When corrected for these differences, they instead display slightly enhanced [C/Fe] (but not significantly so). If these abundance anomalies are due to the accretion of high-Z matter, it must have a composition different from that of the Earth.

Parent Stars of Extrasolar Planets. VII. New Abundance Analyses of 30 Systems

The results of new spectroscopic analyses of 30 stars with giant planet and/or brown dwarf companions are presented. Values for Teff and [Fe/H] are used in conjunction with Hipparcos data and Padua isochrones to derive masses, ages, and theoretical surface gravities. These new data are combined with spectroscopic and photometric metallicity estimates of other stars harboring planets and published samples of F, G, and K dwarfs to compare several subsets of planet bearing stars with similarly well-constrained control groups. The distribution of [Fe/H] values continues the trend uncovered in previous studies in that stars hosting planetary companions have a higher mean value than otherwise similar nearby stars. We also investigate the relationship between stellar mass and the presence of giant planets, and we find statistically marginal but suggestive evidence of a decrease in the incidence of radial velocity companions orbiting relatively less massive stars. If confirmed with larger samples, this would represent a critical constraint to both planetary formation models, as well as to estimates of the distribution of planetary systems in our Galaxy.

Gravitational Microlensing Events Due to Stellar-Mass Black Holes

We present an analysis of the longest timescale microlensing events discovered by the MACHO Collaboration during a 7 year survey of the Galactic bulge. We find six events that exhibit very strong microlensing parallax signals due, in part, to accurate photometric data from the GMAN and MPS collaborations. The microlensing parallax fit parameters are used in a likelihood analysis, which is able to estimate the distances and masses of the lens objects based on a standard model of the Galactic velocity distribution. This analysis indicates that the most likely masses of five of the six lenses are greater than 1 M☉, which suggests that a substantial fraction of the Galactic lenses may be massive stellar remnants. This could explain the observed excess of long-timescale microlensing events. The lenses for events MACHO-96-BLG-5 and MACHO-98-BLG-6 are the most massive, with mass estimates of M/M☉ = 6 and M/M☉ = 6, respectively. The observed upper limits on the absolute brightness of main-sequence stars for these lenses are less than 1 L☉, so both lenses are black hole candidates. The black hole interpretation is also favored by a likelihood analysis with a Bayesian prior using a conventional model for the lens mass function. We consider the possibility that the source stars for some of these six events may lie in the foreground Galactic disk or in the Sagittarius (Sgr) dwarf galaxy behind the bulge, but we find that bulge sources are likely to dominate our microlensing parallax event sample. Future Hubble Space Telescope observations of these events can either confirm the black hole lens hypothesis or detect the lens stars and provide a direct measurement of their masses. Future observations of similar events by the Space Interferometry Mission or the Keck or VLT interferometers, as explained by Delplancke, Gorski, & Richichi, will allow direct measurements of the lens masses for stellar remnant lenses as well.

A search for 6Li in stars with planets

Using very high-resolution (R ∼ 125 000) and high-quality (signal-to-noise ratio ≥ 350) spectra, we have searched for 6 Li in stars hosting extrasolar planets. From detailed profile-fitting of the Li I resonance line at 6707.7 A, we find no significant amount of 6 Li relative to 7 Li for any of eight planet-bearing stars ( 6 Lι/ 7 Li ≤ 0.0-4.03) with strong Li I lines. In particular, we do not confirm the presence of 6 Li with 6 Lι/ 7 Li = 0.13 reported by Israelian et al. for HD 82943, a star with two known planets. Several of the eight stars plus HD 219542 A, the planetless primary of a binary, have been identified in the literature as possible recipients of accreted terrestrial material. For all of the planet-hosting stars and an additional five planetless stars, we find no 6 Li.

Parent Stars of Extrasolar Planets. VIII. Chemical Abundances for 18 Elements in 31 Stars

We present the results of detailed spectroscopic abundance analyses for 18 elements in 31 nearby stars with planets (SWPs). The resulting abundances are combined with other similar studies of nearby SWPs and compared to a sample of nearby stars without detected planets. We find some evidence for abundance differences between these two samples for Al, Si and Ti. Some of our results are in conflict with a recent study of SWPs in the SPOCS data base. We encourage continued study of the abundance patterns of SWPs to resolve these discrepancies.

MARVELS-1b: A Short-period, Brown Dwarf Desert Candidate from the SDSS-III Marvels Planet Search

We present a new short-period brown dwarf (BD) candidate around the star TYC 1240-00945-1. This candidate was discovered in the first year of the Multi-object APO Radial Velocity Exoplanets Large-area Survey (MARVELS), which is part of the Sloan Digital Sky Survey (SDSS) III, and we designate the BD as MARVELS-1b. MARVELS uses the technique of dispersed fixed-delay interferometery to simultaneously obtain radial velocity (RV) measurements for 60 objects per field using a single, custom-built instrument that is fiber fed from the SDSS 2.5 m telescope. From our 20 RV measurements spread over a ~370 day time baseline, we derive a Keplerian orbital fit with semi-amplitude K = 2.533 ± 0.025 km s^(–1), period P = 5.8953 ± 0.0004 days, and eccentricity consistent with circular. Independent follow-up RV data confirm the orbit. Adopting a mass of 1.37 ± 0.11 M_☉ for the slightly evolved F9 host star, we infer that the companion has a minimum mass of 28.0 ± 1.5 M_(Jup), a semimajor axis 0.071 ± 0.002 AU assuming an edge-on orbit, and is probably tidally synchronized. We find no evidence for coherent intrinsic variability of the host star at the period of the companion at levels greater than a few millimagnitudes. The companion has an a priori transit probability of ~14%. Although we find no evidence for transits, we cannot definitively rule them out for companion radii ≲ R_(Jup).

Two-Micron All-Sky Survey J01542930+0053266: a new eclipsing M dwarf binary system

We report on 2MASS J01542930+0053266, a faint eclipsing system composed of two M dwarfs. The variability of this system was originally discovered during a pilot study of the 2MASS Calibration Point Source Working Database. Additional photometry from the Sloan Digital Sky Survey yields an 8–passband lightcurve, from which we derive an orbital period of 2.6390157±0.0000016 days. Spectroscopic followup confirms our photometric classification of the system, which is likely composed of M0 and M1 dwarfs. Radial velocity measurements allow us to derive the masses (M1 = 0.66 ± 0.03M⊙; M2 = 0.62±0.03M⊙) and radii (R1 = 0.64±0.08R⊙; R2 = 0.61±0.09R⊙) of the components, which are consistent with empirical mass–radius relationships for low– mass stars in binary systems. We perform Monte Carlo simulations of the lightcurves which allow us to uncover complicated degeneracies between the system parameters. Both stars show evidence of Hα emission, something not common in early–type M dwarfs. This suggests that binarity may influence the magnetic activity properties of low-mass stars; activity in the binary may persist long after the dynamos in their isolated counterparts have decayed, yielding a new potential foreground of flaring activity for next generation variability surveys.

A Reevaluation of the Super-LITHIUM-rich Star in NGC 6633

We present a new abundance analysis of the super-Li-rich star J37 and a comparison star in NGC 6633. We confirm the result of Deliyannis et al. that J37 has a Li abundance well above the meteoritic value, and we also confirm that Al, S, Si, Ca, Fe, and Ni are supersolar and that C is subsolar. We additionally find that Na, Sc, Ti are supersolar, while O is subsolar. The abundance pattern of metals in J37 is generally consistent with the bulk composition of the Earth. We propose that accretion of circumstellar matter is the best explanation for these abundance anomalies, although we cannot rule out a secondary contribution from diffusion.