arXiv subjects

David W. Latham

Publications and source records attributed to David W. Latham.

At least 433 records · Page 24Linked to original sources

arXiv2014

The Kepler-10 planetary system revisited by HARPS-N: A hot rocky world and a solid Neptune-mass planet

Kepler-10b was the first rocky planet detected by the Kepler satellite and con- firmed with radial velocity follow-up observations from Keck-HIRES. The mass of the planet was measured with a precision of around 30%, which was insufficient to constrain models of its internal structure and composition in detail. In addition to Kepler-10b, a second planet transiting the same star with a period of 45 days was sta- tistically validated, but the radial velocities were only good enough to set an upper limit of 20 Mearth for the mass of Kepler-10c. To improve the precision on the mass for planet b, the HARPS-N Collaboration decided to observe Kepler-10 intensively with the HARPS-N spectrograph on the Telescopio Nazionale Galileo on La Palma. In to- tal, 148 high-quality radial-velocity measurements were obtained over two observing seasons. These new data allow us to improve the precision of the mass determina- tion for Kepler-10b to 15%. With a mass of 3.33 +/- 0.49 Mearth and an updated radius of 1.47 +0.03 -0.02 Rearth, Kepler-10b has a density of 5.8 +/- 0.8 g cm-3, very close to the value -0.02 predicted by models with the same internal structure and composition as the Earth. We were also able to determine a mass for the 45-day period planet Kepler-10c, with an even better precision of 11%. With a mass of 17.2 +/- 1.9 Mearth and radius of 2.35 +0.09 -0.04 Rearth, -0.04 Kepler-10c has a density of 7.1 +/- 1.0 g cm-3. Kepler-10c appears to be the first strong evidence of a class of more massive solid planets with longer orbital periods.

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arXiv2014

Three regimes of extrasolar planets inferred from host star metallicities

Approximately half of the extrasolar planets (exoplanets) with radii less than four Earth radii are in orbits with short periods. Despite their sheer abundance, the compositions of such planets are largely unknown. The available evidence suggests that they range in composition from small, high-density rocky planets to low-density planets consisting of rocky cores surrounded by thick hydrogen and helium gas envelopes. Understanding the transition from the gaseous planets to Earth-like rocky worlds is important to estimate the number of potentially habitable planets in our Galaxy and provide constraints on planet formation theories. Here we report the abundances of heavy elements (that is, the metallicities) of more than 400 stars hosting 600 exoplanet candidates, and find that the exoplanets can be categorized into three populations defined by statistically distinct (~ 4.5σ) metallicity regions. We interpret these regions as reflecting the formation regimes of terrestrial-like planets (radii less than 1.7 Earth radii), gas dwarf planets with rocky cores and hydrogen-helium envelopes (radii between 1.7 and 3.9 Earth radii) and ice or gas giant planets (radii greater than 3.9 Earth radii). These transitions correspond well with those inferred from dynamical mass estimates, implying that host star metallicity, which is a proxy for the initial solid inventory of the protoplanetary disk, is a key ingredient regulating the structure of planetary systems.

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arXiv2014

Stellar rotational periods in the planet hosting open cluster Praesepe

By using the dense coverage of the extrasolar planet survey project HATNet, we Fourier analyze 381 high-probability members of the nearby open cluster Praesepe (Beehive/M44/NGC 2632). In addition to the detection of 10 variables (of δScuti and other types), we identify 180 rotational variables (including the two known planet hosts). This sample increases the number of known rotational variables in this cluster for spectral classes earlier than M by more than a factor of three. These stars closely follow a color/magnitude -- period relation from early F to late K stars. We approximate this relation by polynomials for an easier reference to the rotational characteristics in different colors. The total (peak-to-peak) amplitudes of the large majority (94%) of these variables span the range of 0.005 to 0.04 mag. The periods cover a range from 2.5 to 15 days. These data strongly confirm that Praesepe and the Hyades have the same gyrochronological ages. Regarding the two planet hosts, Pr0211 (the one with the shorter orbital period) has a rotational period that is ~2 days shorter than the one expected from the main rotational pattern in this cluster. This, together with other examples discussed in the paper, may hint that star-planet interaction via tidal dissipation can be significant in some cases in the rotational evolution of stars hosting Hot Jupiters.

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arXiv2014

KELT-6b: A P~7.9 d Hot Saturn Transiting a Metal-Poor Star with a Long-Period Companion

We report the discovery of KELT-6b, a mildly-inflated Saturn-mass planet transiting a metal-poor host. The initial transit signal was identified in KELT-North survey data, and the planetary nature of the occulter was established using a combination of follow-up photometry, high-resolution imaging, high-resolution spectroscopy, and precise radial velocity measurements. The fiducial model from a global analysis including constraints from isochrones indicates that the V=10.38 host star (BD+31 2447) is a mildly evolved, late-F star with T_eff=6102 \pm 43 K, log(g_*)=4.07_{-0.07}^{+0.04} and [Fe/H]=-0.28 \pm 0.04, with an inferred mass M_*=1.09 \pm 0.04 M_sun and radius R_star=1.58_{-0.09}^{+0.16} R_sun. The planetary companion has mass M_P=0.43 \pm 0.05 M_J, radius R_P=1.19_{-0.08}^{+0.13} R_J, surface gravity log(g_P)=2.86_{-0.08}^{+0.06}, and density rho_P=0.31_{-0.08}^{+0.07} g~cm^{-3}. The planet is on an orbit with semimajor axis a=0.079 \pm 0.001 AU and eccentricity e=0.22_{-0.10}^{+0.12}, which is roughly consistent with circular, and has ephemeris of T_c(BJD_TDB)=2456347.79679 \pm 0.00036 and P=7.845631 \pm 0.000046 d. Equally plausible fits that employ empirical constraints on the host star parameters rather than isochrones yield a larger planet mass and radius by ~4-7%. KELT-6b has surface gravity and incident flux similar to HD209458b, but orbits a host that is more metal poor than HD209458 by ~0.3 dex. Thus, the KELT-6 system offers an opportunity to perform a comparative measurement of two similar planets in similar environments around stars of very different metallicities. The precise radial velocity data also reveal an acceleration indicative of a longer-period third body in the system, although the companion is not detected in Keck adaptive optics images.

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arXiv2014

Masses, Radii, and Orbits of Small Kepler Planets: The Transition from Gaseous to Rocky Planets

We report on the masses, sizes, and orbits of the planets orbiting 22 Kepler stars. There are 49 planet candidates around these stars, including 42 detected through transits and 7 revealed by precise Doppler measurements of the host stars. Based on an analysis of the Kepler brightness measurements, along with high-resolution imaging and spectroscopy, Doppler spectroscopy, and (for 11 stars) asteroseismology, we establish low false-positive probabilities for all of the transiting planets (41 of 42 have a false-positive probability under 1%), and we constrain their sizes and masses. Most of the transiting planets are smaller than 3X the size of Earth. For 16 planets, the Doppler signal was securely detected, providing a direct measurement of the planet's mass. For the other 26 planets we provide either marginal mass measurements or upper limits to their masses and densities; in many cases we can rule out a rocky composition. We identify 6 planets with densities above 5 g/cc, suggesting a mostly rocky interior for them. Indeed, the only planets that are compatible with a purely rocky composition are smaller than ~2 R_earth. Larger planets evidently contain a larger fraction of low-density material (H, He, and H2O).

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arXiv2013

Planetary Candidates Observed by Kepler IV: Planet Sample From Q1-Q8 (22 Months)

We provide updates to the Kepler planet candidate sample based upon nearly two years of high-precision photometry (i.e., Q1-Q8). From an initial list of nearly 13,400 Threshold Crossing Events (TCEs), 480 new host stars are identified from their flux time series as consistent with hosting transiting planets. Potential transit signals are subjected to further analysis using the pixel-level data, which allows background eclipsing binaries to be identified through small image position shifts during transit. We also re-evaluate Kepler Objects of Interest (KOI) 1-1609, which were identified early in the mission, using substantially more data to test for background false positives and to find additional multiple systems. Combining the new and previous KOI samples, we provide updated parameters for 2,738 Kepler planet candidates distributed across 2,017 host stars. From the combined Kepler planet candidates, 472 are new from the Q1-Q8 data examined in this study. The new Kepler planet candidates represent ~40% of the sample with Rp~1 Rearth and represent ~40% of the low equilibrium temperature (Teq<300 K) sample. We review the known biases in the current sample of Kepler planet candidates relevant to evaluating planet population statistics with the current Kepler planet candidate sample.

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arXiv2013

An Earth-sized planet with an Earth-like density

Kepler-78 (KIC 8435766) was identified by Sanchis-Ojeda et al. (2013) as harbouring a transiting planet of 1.16 times the size of the Earth and an orbital period of only 8.5 hours. While the exquisite Kepler photometry was able to determine its radius and period, the mass of the planet (and thus its mean density) remained unconstrained in the absence of precise radial-velocity measurements. Here we present an accurate mass measurement of Kepler-78b using the HARPS-N spectrograph, recently installed on the Telescopio Nazionale Galileo (INAF) at the Roque de los Muchachos Observatory, La Palma, Spain. These new data yield a mass of 1.86 Earth masses. The resulting mean density of the planet is 5.57 grams per cubic centimetre, which is similar to that of the Earth and implies a composition of iron and rock. Kepler-78b, which orbits a Sun-like star called Kepler 78 located in the Cygnus constellation at a distance of about 400 light years from us, is now the smallest exoplanet for which both the mass and radius are known accurately.

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arXiv2013

Stellar Spin-Orbit Misalignment in a Multiplanet System

Stars hosting hot Jupiters are often observed to have high obliquities, whereas stars with multiple co-planar planets have been seen to have low obliquities. This has been interpreted as evidence that hot-Jupiter formation is linked to dynamical disruption, as opposed to planet migration through a protoplanetary disk. We used asteroseismology to measure a large obliquity for Kepler-56, a red giant star hosting two transiting co-planar planets. These observations show that spin-orbit misalignments are not confined to hot-Jupiter systems. Misalignments in a broader class of systems had been predicted as a consequence of torques from wide-orbiting companions, and indeed radial-velocity measurements revealed a third companion in a wide orbit in the Kepler-56 system.

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arXiv2013

A Combined Astrometric and Spectroscopic Study of Metal-Poor Binaries

In this work we present a study of binary systems in a metal-poor sample of solar type stars. The stars analyzed were rejected from two planet search samples because they were found to be binaries. Using available radial velocity and Hipparcos astrometric data, we apply different methods to find, for every binary system, a possible range of solutions for the mass of the companion and its orbital period. In one case we find that the solution depends on the Hipparcos data used: the old and new reductions give different results. Some candidate low-mass companions are found, including some close to the brown dwarf regime.

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arXiv2013

Kepler-63b: A Giant Planet in a Polar Orbit around a Young Sun-like Star

We present the discovery and characterization of a giant planet orbiting the young Sun-like star Kepler-63 (KOI-63, $m_{\rm Kp} = 11.6$, $T_{\rm eff} = 5576$ K, $M_\star = 0.98\, M_\odot$). The planet transits every 9.43 days, with apparent depth variations and brightening anomalies caused by large starspots. The planet's radius is $6.1 \pm 0.2 R_{\earth}$, based on the transit light curve and the estimated stellar parameters. The planet's mass could not be measured with the existing radial-velocity data, due to the high level of stellar activity, but if we assume a circular orbit we can place a rough upper bound of $120 M_{\earth}$ (3$σ$). The host star has a high obliquity ($ψ$ = $104^{\circ}$), based on the Rossiter-McLaughlin effect and an analysis of starspot-crossing events. This result is valuable because almost all previous obliquity measurements are for stars with more massive planets and shorter-period orbits. In addition, the polar orbit of the planet combined with an analysis of spot-crossing events reveals a large and persistent polar starspot. Such spots have previously been inferred using Doppler tomography, and predicted in simulations of magnetic activity of young Sun-like stars.

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arXiv2013

The same frequency of planets inside and outside open clusters of stars

Most stars and their planets form in open clusters. Over 95 per cent of such clusters have stellar densities too low (less than a hundred stars per cubic parsec) to withstand internal and external dynamical stresses and fall apart within a few hundred million years. Older open clusters have survived by virtue of being richer and denser in stars (1,000 to 10,000 per cubic parsec) when they formed. Such clusters represent a stellar environment very different from the birthplace of the Sun and other planet-hosting field stars. So far more than 800 planets have been found around Sun-like stars in the field. The field planets are usually the size of Neptune or smaller. In contrast, only four planets have been found orbiting stars in open clusters, all with masses similar to or greater than that of Jupiter. Here we report observations of the transits of two Sun-like stars by planets smaller than Neptune in the billion-year-old open cluster NGC6811. This demonstrates that small planets can form and survive in a dense cluster environment, and implies that the frequency and properties of planets in open clusters are consistent with those of planets around field stars in the Galaxy.

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arXiv2013

Transits and Occultations of an Earth-Sized Planet in an 8.5-Hour Orbit

We report the discovery of an Earth-sized planet ($1.16\pm 0.19 R_\oplus$) in an 8.5-hour orbit around a late G-type star (KIC 8435766, Kepler-78). The object was identified in a search for short-period planets in the {\it Kepler} database and confirmed to be a transiting planet (as opposed to an eclipsing stellar system) through the absence of ellipsoidal light variations or substantial radial-velocity variations. The unusually short orbital period and the relative brightness of the host star ($m_{\rm Kep}$ = 11.5) enable robust detections of the changing illumination of the visible hemisphere of the planet, as well as the occultations of the planet by the star. We interpret these signals as representing a combination of reflected and reprocessed light, with the highest planet dayside temperature in the range of 2300 K to 3100 K. Follow-up spectroscopy combined with finer sampling photometric observations will further pin down the system parameters and may even yield the mass of the planet.

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arXiv2013

The quadruple pre-main sequence system LkCa3: Implications for stellar evolution models

We report the discovery that the pre-main sequence object LkCa3 in the Taurus-Auriga star-forming region is a hierarchical quadruple system of M stars. It was previously known to be a close (~0.5 arc sec) visual pair, with one component being a moderately eccentric 12.94-day single-lined spectroscopic binary. A re-analysis of archival optical spectra complemented with new near-infrared spectroscopy shows both visual components to be double-lined, the second one having a period of 4.06 days and a circular orbit. In addition to the orbital elements, we determine optical and near-infrared flux ratios, effective temperatures, and projected rotational velocities for all four stars. Using existing photometric monitoring observations of the system that had previously revealed the rotational period of the primary in the longer-period binary, we detect also the rotational signal of the primary in the 4.06-day binary, which is synchronized with the orbital motion. With only the assumption of coevality, a comparison of all of these constraints with current stellar evolution models from the Dartmouth series points to an age of 1.4 Myr and a distance of 133 pc, consistent with previous estimates for the region and suggesting the system is on the near side of the Taurus complex. Similar comparisons of the properties of LkCa3 and of the well-known quadruple pre-main sequence system GG Tau with the widely used models from the Lyon series for a mixing length parameter of alpha_ML = 1.0 strongly favor the Dartmouth models.

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arXiv2013

KELT-3b: A Hot Jupiter Transiting a V=9.8 Late-F Star

We report the discovery of KELT-3b, a moderately inflated transiting hot Jupiter with a mass of 1.477 (-0.067, +0.066) M_J, and radius of 1.345 +/- 0.072 R_J, with an orbital period of 2.7033904 +/- 0.000010 days. The host star, KELT-3, is a V=9.8 late F star with M_* = 1.278 (-0.061, +0.063) M_sun, R_* = 1.472 (-0.067, +0.065) R_sun, T_eff = 6306 (-49, +50) K, log(g) = 4.209 (-0.031, +0.033), and [Fe/H] = 0.044 (-0.082, +0.080), and has a likely proper motion companion. KELT-3b is the third transiting exoplanet discovered by the KELT survey, and is orbiting one of the 20 brightest known transiting planet host stars, making it a promising candidate for detailed characterization studies. Although we infer that KELT-3 is significantly evolved, a preliminary analysis of the stellar and orbital evolution of the system suggests that the planet has likely always received a level of incident flux above the empirically-identified threshold for radius inflation suggested by Demory & Seager (2011).

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arXiv2013

A sub-Mercury-sized exoplanet

Since the discovery of the first exoplanet we have known that other planetary systems can look quite unlike our own. However, until recently we have only been able to probe the upper range of the planet size distribution. The high precision of the Kepler space telescope has allowed us to detect planets that are the size of Earth and somewhat smaller, but no previous planets have been found that are smaller than those we see in our own Solar System. Here we report the discovery of a planet significantly smaller than Mercury. This tiny planet is the innermost of three planets that orbit the Sun-like host star, which we have designated Kepler-37. Owing to its extremely small size, similar to that of Earth's Moon, and highly irradiated surface, Kepler-37b is probably a rocky planet with no atmosphere or water, similar to Mercury.

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arXiv2013

Fundamental Properties of Kepler Planet-Candidate Host Stars using Asteroseismology

We have used asteroseismology to determine fundamental properties for 66 Kepler planet-candidate host stars, with typical uncertainties of 3% and 7% in radius and mass, respectively. The results include new asteroseismic solutions for four host stars with confirmed planets (Kepler-4, Kepler-14, Kepler-23 and Kepler-25) and increase the total number of Kepler host stars with asteroseismic solutions to 77. A comparison with stellar properties in the planet-candidate catalog by Batalha et al. shows that radii for subgiants and giants obtained from spectroscopic follow-up are systematically too low by up to a factor of 1.5, while the properties for unevolved stars are in good agreement. We furthermore apply asteroseismology to confirm that a large majority of cool main-sequence hosts are indeed dwarfs and not misclassified giants. Using the revised stellar properties, we recalculate the radii for 107 planet candidates in our sample, and comment on candidates for which the radii change from a previously giant-planet/brown-dwarf/stellar regime to a sub-Jupiter size, or vice versa. A comparison of stellar densities from asteroseismology with densities derived from transit models in Batalha et al. assuming circular orbits shows significant disagreement for more than half of the sample due to systematics in the modeled impact parameters, or due to planet candidates which may be in eccentric orbits. Finally, we investigate tentative correlations between host-star masses and planet candidate radii, orbital periods, and multiplicity, but caution that these results may be influenced by the small sample size and detection biases.

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arXiv2012

KELT-2Ab: A Hot Jupiter Transiting the Bright (V=8.77) Primary Star of a Binary System

We report the discovery of KELT-2Ab, a hot Jupiter transiting the bright (V=8.77) primary star of the HD 42176 binary system. The host is a slightly evolved late F-star likely in the very short-lived "blue-hook" stage of evolution, with $\teff=6148\pm48{\rm K}$, $\log{g}=4.030_{-0.026}^{+0.015}$ and $\feh=0.034\pm0.78$. The inferred stellar mass is $M_*=1.314_{-0.060}^{+0.063}$\msun\ and the star has a relatively large radius of $R_*=1.836_{-0.046}^{+0.066}$\rsun. The planet is a typical hot Jupiter with period $4.11379\pm0.00001$ days and a mass of $M_P=1.524\pm0.088$\mj\ and radius of $R_P=1.290_{-0.050}^{+0.064}$\rj. This is mildly inflated as compared to models of irradiated giant planets at the $\sim$4 Gyr age of the system. KELT-2A is the third brightest star with a transiting planet identified by ground-based transit surveys, and the ninth brightest star overall with a transiting planet. KELT-2Ab's mass and radius are unique among the subset of planets with $V<9$ host stars, and therefore increases the diversity of bright benchmark systems. We also measure the relative motion of KELT-2A and -2B over a baseline of 38 years, robustly demonstrating for the first time that the stars are bound. This allows us to infer that KELT-2B is an early K-dwarf. We hypothesize that through the eccentric Kozai mechanism KELT-2B may have emplaced KELT-2Ab in its current orbit. This scenario is potentially testable with Rossiter-McLaughlin measurements, which should have an amplitude of $\sim$44 m s$^{-1}$.

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arXiv2012

Kepler-36: A Pair of Planets with Neighboring Orbits and Dissimilar Densities

In the Solar system the planets' compositions vary with orbital distance, with rocky planets in close orbits and lower-density gas giants in wider orbits. The detection of close-in giant planets around other stars was the first clue that this pattern is not universal, and that planets' orbits can change substantially after their formation. Here we report another violation of the orbit-composition pattern: two planets orbiting the same star with orbital distances differing by only 10%, and densities differing by a factor of 8. One planet is likely a rocky `super-Earth', whereas the other is more akin to Neptune. These planets are thirty times more closely spaced--and have a larger density contrast--than any adjacent pair of planets in the Solar system.

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