John B. Laird

An abundance of small exoplanets around stars with a wide range of metallicities

The abundance of heavy elements (metallicity) in the photospheres of stars similar to the Sun provides a ‘fossil’ record of the chemical composition of the initial protoplanetary disk. Metal-rich stars are much more likely to harbour gas giant planets, supporting the model that planets form by accumulation of dust and ice particles. Recent ground-based surveys suggest that this correlation is weakened for Neptunian-sized planets. However, how the relationship between size and metallicity extends into the regime of terrestrial-sized exoplanets is unknown. Here we report spectroscopic metallicities of the host stars of 226 small exoplanet candidates discovered by NASA’s Kepler mission, including objects that are comparable in size to the terrestrial planets in the Solar System. We find that planets with radii less than four Earth radii form around host stars with a wide range of metallicities (but on average a metallicity close to that of the Sun), whereas large planets preferentially form around stars with higher metallicities. This observation suggests that terrestrial planets may be widespread in the disk of the Galaxy, with no special requirement of enhanced metallicity for their formation.

Improving Stellar and Planetary Parameters of Transiting Planet Systems: The Case of TrES-2

We report on a spectroscopic determination of the atmospheric parameters and chemical abundance of the parent star of the recently discovered transiting planet TrES-2. A detailed LTE analysis of a set of Fe I and Fe II lines from our Keck spectra yields T_(eff) = 5850 ± 50 K, log g = 4.4 ± 0.1, and [Fe/H] = -0.15 ± 0.10. Several independent checks (e.g., additional spectroscopy, line-depth ratios) confirm the reliability of our spectroscopic T_(eff) estimate. The mass and radius of the star, needed to determine the properties of the planet, are traditionally inferred by comparison with stellar evolution models using T_(eff) and some measure of the stellar luminosity, such as the spectroscopic surface gravity. We apply here a new method in which we use instead of log g the normalized separation a/R_* (related to the stellar density), directly measurabele from the light curves of transiting planets with much greater precision. With the a/R_* value from the light-curve analysis of Holman and coworkers and our T_(eff) estimate, we obtain M_* = 0.980 ± 0.062 M_☉ and R_* = 1.000^(+0.036)_(-0.033) R_☉, and an evolutionary age of 5.1^(+2.7)_(-2.3) Gyr, in good agreement with other constraints (Ca II H and K line cores, lithium abundance, and rotation). The new stellar parameters yield improved values for the planetary mass and radius of M_p = 1.198 ± 0.053 M_J and R_p = 1.220^(+0.045)_(-0.042) R_J, confirming that TrES-2 is the most massive among the currently known nearby (d ≲ 300 pc) transiting hot Jupiters. The surface gravity of the planet, log g_p = 3.299 ± 0.016, can be derived independently of the knowledge of the stellar parameters (i.e., directly from observations), and with a very high precision rivaling that of the best known double-lined eclipsing binaries.

The Spectroscopic Orbit of the Planetary Companion Transiting HD 209458

We report a spectroscopic orbit with period P=3.52433+/-0.00027 days for the planetary companion that transits the solar-type star HD 209458. For the metallicity, mass, and radius of the star, we derive [Fe/H&sqbr0;=0.00+/-0.02, M*=1.1+/-0.1 M middle dot in circle, and R*=1.2+/-0.1 R middle dot in circle. This is based on a new analysis of the iron lines in our HIRES template spectrum and also on the absolute magnitude, effective temperature, and color of the star, and it uses isochrones from four different sets of stellar evolution models. Using these values for the stellar parameters, we reanalyze the transit data and derive an orbital inclination of i=86&fdg;1+/-1&fdg;6. For the planet, we derive a mass of Mp=0.69+/-0.05 MJup, a radius of Rp=1.40+/-0.17 RJup, and a density of rho=0.31+/-0.07 g cm-3.

A Survey of Proper-Motion Stars. XVI. Orbital Solutions for 171 Single-lined Spectroscopic Binaries

We report 25,563 radial velocity measurements for 1359 single-lined stars in the Carney-Latham sample of 1464 stars selected for high proper motion. For 171 of these, we present spectroscopic orbital solutions. We find no obvious difference between the binary characteristics in the halo and the disk populations. The observed frequency is the same, and the period distributions are consistent with the hypothesis that the two sets of binaries were drawn from the same parent population. This suggests that metallicity in general, and radiative opacities in particular, have little influence over the fragmentation process that leads to short-period binaries. All the binaries with periods shorter than 10 days have nearly circular orbits, while the binaries with periods longer than 20 days exhibit a wide range of eccentricities and a median value of 0.37. For the metal-poor high-velocity halo binaries in our sample, the transition from circular to eccentric orbits appears to occur at about 20 days, supporting the conclusion that tidal circularization on the main sequence is important for the oldest binaries in the Galaxy.

Spectroscopic Binaries, Velocity Jitter, and Rotation in Field Metal-poor Red Giant and Red Horizontal-Branch Stars

We summarize 2007 radial velocity measurements of 91 metal-poor field red giants. Excluding binary systems with orbital solutions, our coverage averages 13.7 yr per star, with a maximum of 18.0 yr. We report four significant findings. (1) Sixteen stars are found to be spectroscopic binaries, and we present orbital solutions for 14 of them. The spectroscopic binary frequency of the metal-poor red giants, with [Fe/H] ≤ -1.4, for periods less than 6000 days, is 16% ± 4%, which is not significantly different from that of comparable-metallicity field dwarfs, 17% ± 2%. The two CH stars in our program, BD -1°2582 and HD 135148, are both spectroscopic binaries. (2) Velocity jitter is present among about 40% of the giants with MV ≤ -1.4. The two best-observed cases, HD 3008 and BD +22°2411, show pseudoperiodicities of 172 and 186 days, longer than any known long-period variable in metal-poor globular clusters. Photometric variability seen in HD 3008 and three other stars showing velocity jitter hints that starspots are the cause. However, the phasing of the velocity data with the photometry data from Hipparcos is not consistent with a simple starspot model for HD 3008. We argue against orbital motion effects and radial pulsation, so rotational modulation remains the best explanation. The implied rotational velocities for HD 3008 and BD +22°2411, both with MV ≤ -1.4 and R ≈ 50 R⊙, exceed 12 km s-1. (3) Including HD 3008 and BD +22°2411, we have found signs of significant excess line broadening in eight of the 17 red giants with MV ≤ -1.4, which we interpret as rotation. In three cases, BD +30°2034, CD -37°14010, and HD 218732, the rotation is probably induced by tidal locking between axial rotation and the observed orbital motion with a stellar companion. But this cannot explain the other five stars in our sample that display signs of significant rotation. This high frequency of elevated rotational velocities does not appear to be caused by stellar mass transfer or mergers: there are too few main-sequence binaries with short enough periods. We also note that the lack of any noticeable increase in mean rotation at the magnitude level of the red giant branch luminosity function bump argues against the rapid rotations being caused by the transport of internal angular momentum to the surface. Capture of a planetary-mass companion as a red giant expands in radius could explain the high rotational velocities. (4) We also find significant rotation in at least six of the roughly 15 (40%) red horizontal-branch stars in our survey. It is likely that the enhanced rotation seen among a significant fraction of both blue and red horizontal-branch stars arose when these stars were luminous red giants. Rapid rotation alone therefore appears insufficient cause to populate the blue side of the horizontal branch. While the largest projected rotational velocities seen among field blue and red horizontal-branch stars are consistent with their different sizes, neither are consistent with the large values we find for the largest red giants. This suggests that some form of angular momentum loss (and possibly mass loss) has been at work. Also puzzling is the apparent absence of rotation seen in field RR Lyrae variables. Angular momentum transfer and conservation in evolved metal-poor field stars thus pose many interesting questions for the evolution of low-mass stars.

A NEW SPECTROSCOPIC AND PHOTOMETRIC ANALYSIS OF THE TRANSITING PLANET SYSTEMS TrES-3 AND TrES-4

We report new spectroscopic and photometric observations of the parent stars of the recently discovered transiting planets TrES-3 and TrES-4. A detailed abundance analysis based on high-resolution spectra yields [Fe/H] = –0.19 ± 0.08, T_(eff) = 5650 ± 75 K, and log g = 4.4 ± 0.1 for TrES-3, and [Fe/H] = +0.14 ± 0.09, T_(eff) = 6200 ± 75 K, and log g = 4.0 ± 0.1 for TrES-4. The accuracy of the effective temperatures is supported by a number of independent consistency checks. The spectroscopic orbital solution for TrES-3 is improved with our new radial velocity measurements of that system, as are the light-curve parameters for both systems based on newly acquired photometry for TrES-3 and a reanalysis of existing photometry for TrES-4. We have redetermined the stellar parameters taking advantage of the strong constraint provided by the light curves in the form of the normalized separation a/R_* (related to the stellar density) in conjunction with our new temperatures and metallicities. The masses and radii we derive are M_* = 0.928^(+0.028)_(–0.048) M_⊙, R_* = 0.829^(+0.015)_(–0.022) R_⊙, and M_* = 1.404^(+0.066)_(–0.134) M_⊙, R_* = 1.846^(+0.096)_(–0.087) R_⊙ for TrES-3 and TrES-4, respectively. With these revised stellar parameters, we obtain improved values for the planetary masses and radii. We find M_p = 1.910^(+0.075)_(–0.080) M_(Jup), R_p = 1.336^(+0.031)_(–0.036) R_(Jup) for TrES-3, and M_p = 0.925 ± 0.082 M_(Jup), R_p = 1.783^(+0.093)_(–0.086) R_(Jup) for TrES-4. We confirm TrES-4 as the planet with the largest radius among the currently known transiting hot Jupiters.

A survey of proper-motion stars. X. The early evolution of the Galaxy's halo

The recent history of work bearing on the early chemical/dynamical evolution of the Galaxy, and the relation of the halo population to the disk population(s) are discussed. In particular, the mean rotational velocity v(rot), and planar orbital eccentricity (e) are considered as functions of time, inferred from mean metallicity. For m/H less than about -1, signs of dynamicl evolution in mean v(rot) or mean e are not detected. It is argued that (e) for metal-poor stars is about 0.6, not 0.8 as previously believed, and is even lower in the three-dimensional case. It is pointed out that all the data are consistent with the high-velocity, low metallicity halo population having had a chemical and dynamical history almost independent of the Galactic disk. It is proposed that the halo was assembled from mergers of small satellites with the Galaxy along the lines discussed by Searle and Zinn (1978). It is further proposed that the Galactic bulge originated from the gaseous outflow from the Galaxys halo and these fragments. 89 refs.

High-Resolution Spectroscopy of the Transiting Planet Host Star TrES-1

We report on a spectroscopic determination of the stellar parameters and chemical abundances for the parent star of the transiting planet TrES-1. Based on a detailed analysis of iron lines in our Keck and Hobby-Eberly Telescope spectra, we derive K, , and . By measuring the Ca ii T p 5250 75 log g p 4.6 0.2 [Fe/H] p 0.00 0.09 eff activity indicator and by putting useful upper limits on the Li abundance, we constrain the age of TrES-1 to be Gyr. By comparing theoretical stellar evolution models with the observational parameters, we obtain 2.5 1.5 and . Our improved estimates of the stellar parameters are utilized M p 0.89 0.05 MR p 0.83 0.05 R , , in a new analysis of the transit photometry of TrES-1 to derive a mass , a radius M p (0.76 0.05) MR p p J p , and an inclination deg. The improved planetary mass and radius estimates provide the 0.08 0.5 1.04 Ri p 89.5 0.05 J 1.3 grounds for new crucial tests of theoretical models of evolution and evaporation of irradiated extrasolar giant planets. Subject headings: planetary systems: formation — stars: abundances — stars: individual (TrES-1) Online material: color figure

HUNTING FOR THE PROGENITOR OF SN 1006: HIGH-RESOLUTION SPECTROSCOPIC SEARCH WITH THE FLAMES INSTRUMENT*

Type Ia supernovae play a significant role in the evolution of the universe and have a wide range of applications. It is widely believed that these events are the thermonuclear explosions of carbon-oxygen white dwarfs close to the Chandrasekhar mass (1.38 M ☉). However, CO white dwarfs are born with masses much below the Chandrasekhar limit and thus require mass accretion to become Type Ia supernovae. There are two main scenarios for accretion: first, the merger of two white dwarfs and, second, a stable mass accretion from a companion star. According to predictions, this companion star (also referred to as donor star) survives the explosion and thus should be visible in the center of Type Ia remnants. In this paper, we scrutinize the central stars (79 in total) of the SN 1006 remnant to search for the surviving donor star as predicted by this scenario. We find no star consistent with the traditional accretion scenario in SN 1006.

Metal-poor Field Blue Stragglers: More Evidence for Mass Transfer

We report radial velocity studies of five candidate metal-poor field blue stragglers, all known to be deficient in lithium. Four of the five stars are single-lined spectroscopic binaries, with periods ranging from 302 to 840 days, and low orbital eccentricities, in agreement with similar behavior found for other blue straggler candidates by Preston & Sneden and Carney et al. The limited data available for lithium abundances indicate that all blue straggler binaries have depleted lithium, but that constant velocity stars generally have normal lithium abundances. This suggests that the lithium gap for hot metal-poor main-sequence stars may not exist or lies at higher temperatures than found in the Hyades. Our results and those of Preston & Sneden show higher values of vrot sin i for the binary stars than those of comparable temperature constant velocity stars. The orbital periods are too long for tidal effects to be important, implying that spin-up during mass transfer when the orbital separations and periods were smaller is the cause of the enhanced rotation. The mass function distribution is steeper for the blue straggler binary stars than that of lower mass single-lined spectroscopic binaries, indicating a narrower range in secondary masses. We argue that if all secondaries are white dwarfs with the same mass, it is probably around 0.55 M?. The models of Rappaport et al., applied to white dwarf secondaries, suggest that the orbital elements of all metal-poor binary blue stragglers are consistent with stable mass transfer, with the possible exception of G202-65.