arXiv subjects

S. Thomas Megeath

Publications and source records attributed to S. Thomas Megeath.

79 records · Page 5Linked to original sources

arXiv2007

A map of the day-night contrast of the extrasolar planet HD 189733b

"Hot Jupiter" extrasolar planets are expected to be tidally locked because they are close (<0.05 astronomical units, where 1 AU is the average Sun-Earth distance) to their parent stars, resulting in permanent daysides and nightsides. By observing systems where the planet and star periodically eclipse each other, several groups have been able to estimate the temperatures of the daysides of these planets. A key question is whether the atmosphere is able to transport the energy incident upon the dayside to the nightside, which will determine the temperature at different points on the planet's surface. Here we report observations of HD 189733, the closest of these eclipsing planetary systems, over half an orbital period, from which we can construct a 'map' of the distribution of temperatures. We detected the increase in brightness as the dayside of the planet rotated into view. We estimate a minimum brightness temperature of 973 +/- 33 K and a maximum brightness temperature of 1212 +/- 11 K at a wavelength of 8 microns, indicating that energy from the irradiated dayside is efficiently redistributed throughout the atmosphere, in contrast to a recent claim for another hot Jupiter. Our data indicate that the peak hemisphere-integrated brightness occurs 16$\pm$6 degrees before opposition, corresponding to a hot spot shifted east of the substellar point. The secondary eclipse (when the planet moves behind the star) occurs 120 +/- 24 s later than predicted, which may indicate a slightly eccentric orbit.

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arXiv2006

Spitzer/IRAC Photometry of M, L, and T Dwarfs

We present the results of a program to acquire photometry for eighty-six late-M, L, and T dwarfs using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We examine the behavior of these cool dwarfs in various color-color and color-magnitude diagrams composed of near-IR and IRAC data. The T dwarfs exhibit the most distinctive positions in these diagrams. In M_5.8 versus [5.8]-[8.0], the IRAC data for T dwarfs are not monotonic in either magnitude or color, giving the clearest indication yet that the T dwarfs are not a one parameter family in Teff. Because metallicity does not vary enough in the solar neighborhood to act as the second parameter, the most likely candidate then is gravity, which in turn translates to mass. Among objects with similar spectral type, the range of mass suggested by our sample is about a factor of five (~70 M_Jup to ~15 M_Jup), with the less massive objects making up the younger members of the sample. We also find the IRAC 4.5 micron fluxes to be lower than expected, from which we infer a stronger CO fundamental band at ~4.67 microns. This suggests that equilibrium CH_4/CO chemistry underestimates the abundance of CO in T dwarf atmospheres, confirming earlier results based on M-band observations from the ground. In combining IRAC photometry with near-IR JHK photometry and parallax data, we find the combination of Ks, IRAC 3.6 micron, and 4.5 micron bands to provide the best color-color discrimination for a wide range of M, L, and T dwarfs. Also noteworthy is the M_Ks versus Ks-[4.5] relation, which shows a smooth progression over spectral type and splits the M, L, and T types cleanly.

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arXiv2006

24-micron Detections of Disks around Very Low-mass Stars and Brown Dwarfs in IC 348: Grain Growth/Settling and Inner Holes?

We present observations of six late-type members of the young cluster IC 348 detected at 24 microns with the Multiband Imaging Photometer for Spitzer(MIPS). At least four of the objects are probably substellar. Combining these data with ground-based optical and near-infrared photometry and complementary observations with the Infrared Array Camera (IRAC), we have modeled the spectral energy distributions using detailed models of irradiated accretion disks. We are able to fit the observations with models using a range of maximum grain sizes from ISM-type dust to grains as large as 1 millimeter. Two objects show a lack of excess emission at wavelengths shortward of 5.8-8 microns but significant excess at longer wavelengths, indicative of large optically thin or evacuated inner holes. Our models indicate a inner hole of radius ~ 0.5-0.9 AU for the brown dwarf L316; this is the first brown dwarf with evidence for an AU-scale inner disk hole. We examine several possible mechanisms for the inner disk clearing in this case, including photoevaporation and planet formation.

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arXiv2005

Identifying Primordial Substructure in NGC 2264

We present new Spitzer Space Telescope observations of the young cluster NGC2264. Observations at 24 micron with the Multiband Imaging Photometer has enabled us to identify the most highly embedded and youngest objects in NGC2264. This letter reports on one particular region of NGC2264 where bright 24 micron sources are spatially configured in curious linear structures with quasi-uniform separations. The majority of these sources (~60% are found to be protostellar in nature with Class I spectral energy distributions. Comparison of their spatial distribution with sub-millimeter data from Wolf-Chase (2003) and millimeter data from Peretto et al. (2005) shows a close correlation between the dust filaments and the linear spatial configurations of the protostars, indicating that star formation is occurring primarily within dense dusty filaments. Finally, the quasi-uniform separations of the protostars are found to be comparable in magnitude to the expected Jeans length suggesting thermal fragmentation of the dense filamentary material.

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arXiv2005

Detection of Thermal Emission from an Extrasolar Planet

We present Spitzer Space Telescope infrared photometric time series of the transiting extrasolar planet system TrES-1. The data span a predicted time of secondary eclipse, corresponding to the passage of the planet behind the star. In both bands of our observations, we detect a flux decrement with a timing, amplitude, and duration as predicted by published parameters of the system. This signal represents the first direct detection of (i.e. the observation of photons emitted by) a planet orbiting another star. The observed eclipse depths (in units of relative flux) are 0.00066 +/- 0.00013 at 4.5um and 0.00225 +/- 0.00036 at 8.0um. These estimates provide the first observational constraints on models of the thermal emission of hot Jupiters. Assuming that the planet emits as a blackbody, we estimate an effective temperature of T_p=1060 +/- 50 K. Under the additional assumptions that the planet is in thermal equilibrium with the radiation from the star and emits isotropically, we find a Bond albedo of A = 0.31 +/- 0.14. This would imply that the planet absorbs the majority of stellar radiation incident upon it, a conclusion of significant impact to atmospheric models of these objects. We compare our data to a previously-published model of the planetary thermal emission, which predicts prominent spectral features in our observational bands due to water and carbon monoxide. Based on the time of secondary eclipse, we present an upper limit on the orbital eccentricity that is sufficiently small that we conclude that tidal dissipation is unlikely to provide a significant source of energy interior to the planet.(abridged)

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arXiv2004

IRAC Colors of Young Stellar Objects

We compare the infrared colors predicted by theoretical models of protostellar envelopes and protoplanetary disks with initial observations of young stellar objects made with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope (Werner et al. 2004, Fazio et al. 2004). Disk and envelope models characterized by infall and/or accretion rates found in previous studies can quantitatively account for the range of IRAC colors found in four young embedded clusters: S 140, S 171, NGC 7129, and Cep C. The IRAC color-color diagram ([3.6]-[4.5] vs. [5.8]-[8.0]) can be used to help to distinguish between young stars with only disk emission and protostars with circumstellar envelopes.

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arXiv2003

A Catalog of Young Stellar Groups and Clusters Within 1kpc of the Sun

We present a catalog of near-infrared surveys of young (<= a few 10^6 yr) stellar groups and clusters within 1 kpc from the Sun, based on an extensive search of the literature from the past ten years. We find 143 surveys from 69 published articles, covering 73 different regions. The number distribution of stars in a region has a median of 28 and a mean of 100. About 80% of the stars are in clusters with at least 100 members. By a rough classification of the groups and clusters based on the number of their associated stars, we show that most of the stars form in large clusters. The spatial distribution of cataloged regions in the Galactic plane shows a relative lack of observed stellar groups and clusters in the range 270 deg < l < 60 deg of Galactic longitude, reflecting our location between the Local and Sagittarius arms. This compilation is intended as a useful resource for future studies of nearby young regions of multiple star formation.

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