arXiv subjects

Travis S. Metcalfe

Publications and source records attributed to Travis S. Metcalfe.

At least 73 records · Page 4Linked to original sources

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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arXiv2013

Kepler-68: Three Planets, One With a Density Between That of Earth and Ice Giants

NASA's Kepler Mission has revealed two transiting planets orbiting Kepler-68. Follow-up Doppler measurements have established the mass of the innermost planet and revealed a third jovian-mass planet orbiting beyond the two transiting planets. Kepler-68b, in a 5.4 day orbit has mass 8.3 +/- 2.3 Earth, radius 2.31 +/- 0.07 Earth radii, and a density of 3.32 +/- 0.92 (cgs), giving Kepler-68b a density intermediate between that of the ice giants and Earth. Kepler-68c is Earth-sized with a radius of 0.953 Earth and transits on a 9.6 day orbit; validation of Kepler-68c posed unique challenges. Kepler-68d has an orbital period of 580 +/- 15 days and minimum mass of Msin(i) = 0.947 Jupiter. Power spectra of the Kepler photometry at 1-minute cadence exhibit a rich and strong set of asteroseismic pulsation modes enabling detailed analysis of the stellar interior. Spectroscopy of the star coupled with asteroseismic modeling of the multiple pulsation modes yield precise measurements of stellar properties, notably Teff = 5793 +/- 74 K, M = 1.079 +/- 0.051 Msun, R = 1.243 +/- 0.019 Rsun, and density 0.7903 +/- 0.0054 (cgs), all measured with fractional uncertainties of only a few percent. Models of Kepler-68b suggest it is likely composed of rock and water, or has a H and He envelope to yield its density of about 3 (cgs).

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arXiv2012

Calibrating Convective properties of Solar-like Stars in the Kepler Field of View

Stellar models generally use simple parametrizations to treat convection. The most widely used parametrization is the so-called "Mixing Length Theory" where the convective eddy sizes are described using a single number, α, the mixing-length parameter. This is a free parameter, and the general practice is to calibrate αusing the known properties of the Sun and apply that to all stars. Using data from NASA's Kepler mission we show that using the solar-calibrated αis not always appropriate, and that in many cases it would lead to estimates of initial helium abundances that are lower than the primordial helium abundance. Kepler data allow us to calibrate αfor many other stars and we show that for the sample of stars we have studied, the mixing-length parameter is generally lower than the solar value. We studied the correlation between αand stellar properties, and we find that αincreases with metallicity. We therefore conclude that results obtained by fitting stellar models or by using population-synthesis models constructed with solar values of αare likely to have large systematic errors. Our results also confirm theoretical expectations that the mixing-length parameter should vary with stellar properties.

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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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arXiv2011

Kepler-21b: A 1.6REarth Planet Transiting the Bright Oscillating F Subgiant Star HD 179070

We present Kepler observations of the bright (V=8.3), oscillating star HD 179070. The observations show transit-like events which reveal that the star is orbited every 2.8 days by a small, 1.6 R_Earth object. Seismic studies of HD 179070 using short cadence Kepler observations show that HD 179070 has a frequencypower spectrum consistent with solar-like oscillations that are acoustic p-modes. Asteroseismic analysis provides robust values for the mass and radius of HD 179070, 1.34{\pm}0.06 M{\circ} and 1.86{\pm}0.04 R{\circ} respectively, as well as yielding an age of 2.84{\pm}0.34 Gyr for this F5 subgiant. Together with ground-based follow-up observations, analysis of the Kepler light curves and image data, and blend scenario models, we conservatively show at the >99.7% confidence level (3σ) that the transit event is caused by a 1.64{\pm}0.04 R_Earth exoplanet in a 2.785755{\pm}0.000032 day orbit. The exoplanet is only 0.04 AU away from the star and our spectroscopic observations provide an upper limit to its mass of ~10 M_Earth (2-σ). HD 179070 is the brightest exoplanet host star yet discovered by Kepler.

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arXiv2011

Kepler-22b: A 2.4 Earth-radius Planet in the Habitable Zone of a Sun-like Star

A search of the time-series photometry from NASA's Kepler spacecraft reveals a transiting planet candidate orbiting the 11th magnitude G5 dwarf KIC 10593626 with a period of 290 days. The characteristics of the host star are well constrained by high-resolution spectroscopy combined with an asteroseismic analysis of the Kepler photometry, leading to an estimated mass and radius of 0.970 +/- 0.060 MSun and 0.979 +/- 0.020 RSun. The depth of 492 +/- 10ppm for the three observed transits yields a radius of 2.38 +/- 0.13 REarth for the planet. The system passes a battery of tests for false positives, including reconnaissance spectroscopy, high-resolution imaging, and centroid motion. A full BLENDER analysis provides further validation of the planet interpretation by showing that contamination of the target by an eclipsing system would rarely mimic the observed shape of the transits. The final validation of the planet is provided by 16 radial velocities obtained with HIRES on Keck 1 over a one year span. Although the velocities do not lead to a reliable orbit and mass determination, they are able to constrain the mass to a 3σ upper limit of 124 MEarth, safely in the regime of planetary masses, thus earning the designation Kepler-22b. The radiative equilibrium temperature is 262K for a planet in Kepler-22b's orbit. Although there is no evidence that Kepler-22b is a rocky planet, it is the first confirmed planet with a measured radius to orbit in the Habitable Zone of any star other than the Sun.

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arXiv2011

Asteroseismic constraints on diffusion in WD envelopes

The asteroseismic analysis of white dwarfs allows us to peer below their photospheres and determine their internal structure. At ~ 28,000 K EC20058-5234 is the hottest known pulsating helium atmosphere white dwarf. As such, it constitutes an important link in the evolution of white dwarfs down the cooling track. It is also astrophysically interesting because it is at a temperature where white dwarfs are expected to cool mainly through the emission of plasmon neutrinos. In the present work, we perform an asteroseismic analysis of EC20058-5234 and place the results in the context of stellar evolution and time dependent diffusion calculations. We use a parallel genetic algorithm complemented with targeted grid searches to find the models that fit the observed periods best. Comparing our results with similar modeling of EC20058-5234's cooler cousin CBS114, we find a helium envelope thickness consistent with time dependent diffusion calculations and obtain a precise mode identification for EC20058-5234.

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arXiv2010

Unveiling stellar magnetic activity using CoRoT seismic observations

It is well known that in the Sun, the frequencies and amplitudes of acoustic modes vary throughout the solar cycle. Indeed, while the magnetic activity goes towards its maximum, the frequencies of the modes increase and their amplitudes decrease. We have analyzed data from the CoRoT mission on a few stars that exhibit solar-like oscillations. The study of HD49933 (observed during 60 days and 137 days spanning a total of 400 days) showed a modulation of the maximum amplitude per radial mode and the frequency shifts of the modes, showing magnetic activity in this rapidly rotating star. Moreover, both properties vary in an anticorrelated way and the data allowed us to establish a lower limit for the activity-cycle period of ~120 days. Measurements in Ca H and K lines confirmed that this star is in the "active stars" category. We will also discuss the results obtained for other targets such as HD181420 and HD49835 for which we have investigated a similar behavior.

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arXiv2010

CoRoT reveals a magnetic activity cycle in a Sun-like star

The 11-year activity cycle of the Sun is a consequence of a dynamo process occurring beneath its surface. We analyzed photometric data obtained by the CoRoT space mission, showing solar-like oscillations in the star HD49933, for signatures of stellar magnetic activity. Asteroseismic measurements of global changes in the oscillation frequencies and mode amplitudes reveal a modulation of at least 120 days, with the minimum frequency shift corresponding to maximum amplitude as in the Sun. These observations are evidence of a stellar magnetic activity cycle taking place beneath the surface of HD49933 and provide constraints for stellar dynamo models under conditions different from those of the Sun.

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arXiv2008

Asteroseismology and the Solar-Stellar Connection

In March 2009, NASA will launch the Kepler satellite -- a mission designed to discover habitable Earth-like planets around distant Sun-like stars. The method that Kepler will use to detect distant worlds will only reveal the size of the planet relative to the size of the host star, so part of the mission is devoted to characterizing other suns using asteroseismology. In this proceedings, I give a broad overview of the Kepler mission and the data that it will produce, with a special emphasis on how it could improve our understanding of solar and stellar dynamos. I conclude with an update on the development of a stellar modeling pipeline for interpreting asteroseismic observations.

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arXiv2008

A multi-site campaign to measure solar-like oscillations in Procyon. I. Observations, Data Reduction and Slow Variations

We have carried out a multi-site campaign to measure oscillations in the F5 star Procyon A. We obtained high-precision velocity observations over more than three weeks with eleven telescopes, with almost continuous coverage for the central ten days. This represents the most extensive campaign so far organized on any solar-type oscillator. We describe in detail the methods we used for processing and combining the data. These involved calculating weights for the velocity time series from the measurement uncertainties and adjusting them in order to minimize the noise level of the combined data. The time series of velocities for Procyon shows the clear signature of oscillations, with a plateau of excess power that is centred at 0.9 mHz and is broader than has been seen for other stars. The mean amplitude of the radial modes is 38.1 +/- 1.3 cm/s (2.0 times solar), which is consistent with previous detections from the ground and by the WIRE spacecraft, and also with the upper limit set by the MOST spacecraft. The variation of the amplitude during the observing campaign allows us to estimate the mode lifetime to be 1.5 d (+1.9/-0.8 d). We also find a slow variation in the radial velocity of Procyon, with good agreement between different telescopes. These variations are remarkably similar to those seen in the Sun, and we interpret them as being due to rotational modulation from active regions on the stellar surface. The variations appear to have a period of about 10 days, which presumably equals the stellar rotation period or, perhaps, half of it. The amount of power in these slow variations indicates that the fractional area of Procyon covered by active regions is slightly higher than for the Sun.

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arXiv2007

The Production Rate and Employment of Ph.D. Astronomers

In an effort to encourage self-regulation of the astronomy job market, I examine the supply of, and demand for, astronomers over time. On the supply side, I document the production rate of Ph.D. astronomers from 1970 to 2006 using the UMI Dissertation Abstracts database, along with data from other independent sources. I compare the long-term trends in Ph.D. production with federal astronomy research funding over the same time period, and I demonstrate that additional funding is correlated with higher subsequent Ph.D. production. On the demand side, I monitor the changing patterns of employment using statistics about the number and types of jobs advertised in the AAS Job Register from 1984 to 2006. Finally, I assess the sustainability of the job market by normalizing this demand by the annual Ph.D. production. The most recent data suggest that there are now annual advertisements for about one postdoctoral job, half a faculty job, and half a research/support position for every new domestic Ph.D. recipient in astronomy and astrophysics. The average new astronomer might expect to hold up to 3 jobs before finding a steady position.

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arXiv2006

The Future of Computational Asteroseismology

The history of stellar seismology suggests that observation and theory often take turns advancing our understanding. The recent tripling of the sample of pulsating white dwarfs generated by the Sloan Digital Sky Survey represents a giant leap on the observational side. The time is ripe for a comparable advance on the theoretical side. There are basically two ways we can improve our theoretical understanding of pulsating stars: we can improve the fundamental ingredients of the models, or we can explore the existing models in greater computational detail. For pulsating white dwarfs, much progress has recently been made on both fronts: models now exist that connect the interior structure to its complete evolutionary history, while a method of using parallel computers for global exploration of relatively simple models has also been developed. Future advances in theoretical white dwarf asteroseismology will emerge by combining these two approaches, yielding unprecedented insight into the physics of diffusion, nuclear burning, and mixing.

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arXiv2006

The Citation Impact of Digital Preprint Archives for Solar Physics Papers

Papers that are posted to a digital preprint archive are typically cited twice as often as papers that are not posted. This has been demonstrated for papers published in a wide variety of journals, and in many different subfields of astronomy. Most astronomers now use the arXiv.org server (astro-ph) to distribute preprints, but the solar physics community has an independent archive hosted at Montana State University. For several samples of solar physics papers published in 2003, I quantify the boost in citation rates for preprints posted to each of these servers. I show that papers on the MSU archive typically have citation rates 1.7 times higher than the average of similar papers that are not posted as preprints, while those posted to astro-ph get 2.6 times the average. A comparable boost is found for papers published in conference proceedings, suggesting that the higher citation rates are not the result of self-selection of above-average papers.

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arXiv2005

An Asteroseismic Test of Diffusion Theory

The helium-atmosphere (DB) white dwarfs are commonly thought to be the descendants of the hotter PG 1159 stars, which initially have uniform He/C/O atmospheres. In this evolutionary scenario, diffusion builds a pure He surface layer which gradually thickens as the star cools. In the temperature range of the pulsating DB white dwarfs (T_eff ~ 25,000 K) this transformation is still taking place, allowing asteroseismic tests of the theory. Objective global fitting of our updated double-layered envelope models to recent observations of the pulsating DB star CBS 114, and to existing observations of the slightly cooler star GD 358, lead to determinations of the envelope masses and pure He surface layers that qualitatively agree with the expectations of diffusion theory. These results provide new asteroseismic evidence supporting one of the central assumptions of spectral evolution theory, linking the DB white dwarfs to PG 1159 stars.

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arXiv2005

A deeper understanding of white dwarf interiors

A detailed record of the physical processes that operate during post-main-sequence evolution is contained in the internal chemical structure of white dwarfs. Global pulsations allow us to probe the stellar interior through asteroseismology, revealing the signatures of prior nuclear burning, mixing, and diffusion in these stars. I review the rapid evolution of structural models for helium-atmosphere variable (DBV) white dwarfs over the past five years, and I present a new series of model-fits using recent observations to illustrate the relative importance of various interior structures. By incorporating physically motivated C/O profiles into double-layered envelope models for the first time, I finally identify an optimal asteroseismic model that agrees with both diffusion theory and the expected nuclear burning history of the progenitor. I discuss the implications of this fundamental result, and I evaluate the prospects for continued progress in the future.

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arXiv2005

Cool White Dwarfs in the Sloan Digital Sky Survey

A reduced proper motion diagram utilizing Sloan Digital Sky Survey (SDSS) photometry and astrometry and USNO-B plate astrometry is used to separate cool white dwarf candidates from metal-weak, high-velocity main sequence Population II stars (subdwarfs) in the SDSS Data Release 2 imaging area. Follow-up spectroscopy using the Hobby-Eberly Telescope, the MMT, and the McDonald 2.7m Telescope is used to demonstrate that the white dwarf and subdwarf loci separate cleanly in the reduced proper motion diagram, and that the contamination by subdwarfs is small near the cool white dwarf locus. This enables large statistically complete samples of white dwarfs, particularly the poorly understood cool white dwarfs, to be created from the SDSS imaging survey, with important implications for white dwarf luminosity function studies. SDSS photometry for our sample of cool white dwarfs is compared to current white dwarf models.

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arXiv2005

The Rise and Citation Impact of astro-ph in Major Journals

The rise in the use of the arXiv preprint server (astro-ph) over the past decade has led to a major shift in the way astronomical research is disseminated. Schwarz & Kennicutt (2004) recently found that Astrophysical Journal papers posted to astro-ph are cited roughly twice as often as papers that are not posted, suggesting that the preprint server has become the primary resource for many astronomers to keep up with the literature. I describe a simple method to determine the adoption rate and citation impact of astro-ph over time for any journal using NASA's Astrophysics Data System (ADS). I use the ADS to document the rise in the adoption of astro-ph for three major astronomy journals, and to conduct a broad survey of the citation impact of astro-ph in 13 different journals. I find that the factor of two boost in citations for astro-ph papers is a common feature across most of the major astronomy journals.

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