arXiv subjects

M. S. Marley

Publications and source records attributed to M. S. Marley.

At least 55 records · Page 3Linked to original sources

arXiv2008

CLOUDS search for variability in brown dwarf atmospheres

Context: L-type ultra-cool dwarfs and brown dwarfs have cloudy atmospheres that could host weather-like phenomena. The detection of photometric or spectral variability would provide insight into unresolved atmospheric heterogeneities, such as holes in a global cloud deck. Aims: It has been proposed that growth of heterogeneities in the global cloud deck may account for the L- to T-type transition as brown dwarf photospheres evolve from cloudy to clear conditions. Such a mechanism is compatible with variability. We searched for variability in the spectra of five L6 to T6 brown dwarfs in order to test this hypothesis. Methods: We obtained spectroscopic time series using VLT/ISAAC, over 0.99-1.13um, and IRTF/SpeX for two of our targets, in J, H and K bands. We search for statistically variable lines and correlation between those. Results: High spectral-frequency variations are seen in some objects, but these detections are marginal and need to be confirmed. We find no evidence for large amplitude variations in spectral morphology and we place firm upper limits of 2 to 3% on broad-band variability, on the time scale of a few hours. The T2 transition brown dwarf SDSS J1254-0122 shows numerous variable features, but a secure variability diagnosis would require further observations. Conclusions: Assuming that any variability arises from the rotation of patterns of large-scale clear and cloudy regions across the surface, we find that the typical physical scale of cloud cover disruption should be smaller than 5-8% of the disk area for four of our targets. The possible variations seen in SDSS J1254-0122 are not strong enough to allow us to confirm the cloud breaking hypothesis.

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arXiv2007

Physical and Spectral Characteristics of the T8 and Later-Type Dwarfs

We use new and published near-IR spectra, with synthetic spectra, to derive physical properties of three of the latest-type T dwarfs. A new R~1700 spectrum of the T7.5 dwarf HD 3651B, with existing data, allows a detailed comparison to the well-studied and very similar dwarf, Gl 570D. We find that HD 3651B has both higher gravity and metallicity than Gl 570D, with Teff=820-830K, log g= 5.4-5.5, [m/H]= +0.2 and Kzz=10^4cm^2/s. Its age is 8-12 Gyr and its implied mass is 60-70 M_Jup. We perform a similar analyis of the T8 and T7.5 dwarfs 2MASS J09393548-2448279 and 2MASS J11145133-2618235 using published data, comparing them to the well-studied T8, 2MASS J04151954-0935066. We find that the two dwarfs have the same Teff as the reference dwarf, and similar gravities, but lower metallicities. The parameters are Teff=725-775K and [m/H]= -0.3; log g=5.3-5.45 for 2MASS J09393548-2448279 and log g=5.0-5.3 for 2MASS J11145133- 261823. The age and mass are ~10Gyr and 60M_Jup for 2MASS J09393548-2448279, and ~5 Gyr and 40M_Jup for 2MASS J11145133-261823. A serious limitation is the incompleteness of the line lists of CH4 and NH3 at lambda <1.7um. Spectra of Saturn and Jupiter, and of laboratory CH4 and NH3 gas, suggest that NH3 features in the Y- and J-bands may be useful as indicators of the next cooler spectral type, and not features in the H- and K-bands as previously thought. However large uncertainties remain, as the abundance of NH3 is likely to be significantly below the chemical equilibrium value, and inclusion of laboratory NH3 opacities predicts band shapes that are discrepant with existing data. It is possible that the T spectral class will have to be extended to low temperatures around 400K, when water clouds condense in the atmosphere [abridged].

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arXiv2006

Physical parameters of two very cool T dwarfs

We present new infrared spectra of the T8 brown dwarf 2MASS J04151954-0935066: 2.9-4.1 micron spectra obtained with the Infrared Camera and Spectrograph on the Subaru Telescope, and 5.2-14.5 micron spectra obtained with the Infrared Spectrograph on the Spitzer Space Telescope. We use these data and models to determine an accurate bolometric luminosity of log L_bol/L_sun=-5.67 and to constrain the effective temperature, gravity, mass and age to 725-775K, log g=5.00-5.37, M=33-58 M_Jupiter and age=3-10Gyr. We perform the same analysis using published 0.6-15 micron spectra for the T7.5 dwarf 2MASS J12171110-0311131, for which we find a metal-rich composition ([Fe/H]~0.3) and log L_bol/L_sun=-5.31, T_eff=850-950K, log g=4.80-5.42, M=25-66M_Jupiter and age=1-10Gyr. These luminosities and effective temperatures straddle those determined with the same method and models for Gl 570D by Saumon et al. (2006) and make 2MASS J04151954-0935066 the coolest and least luminous T dwarf with well-determined properties. We find that synthetic spectra generated by the models reproduce the observed red through mid-infrared spectra of 2MASS J04151954-0935066 and 2MASS J12171110-0311131 very well, except for known discrepancies which are most likely due to the incomplete CH4 opacities. Both objects show evidence of departures from strict chemical equilibrium and we discuss this result in the context of other late T dwarfs where disequilibrium phenomena have been observed.

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arXiv2006

3.6-7.9 um Photometry of L and T Dwarfs and the Prevalence of Vertical Mixing in their Atmospheres

We present new L' (3.75 um) photometry of six L and T dwarfs, and M' (4.70 um) photometry of ten L and T dwarfs, observed at Gemini Observatory, and new 3.55, 4.49, 5.73 and 7.87 um photometry of nine L and T dwarfs, obtained with the Spitzer Space Telescope. The sample includes unusually blue and red dwarfs from our near-infrared studies. The data are combined with published L', M' and Spitzer photometry of L and T dwarfs, and trends of colors with spectral type and other colors are examined. Model atmospheres by Marley and Saumon are used to generate synthetic colors for ranges of effective temperature, gravity, grain sedimentation efficiency, metallicity and vertical mixing efficiency. We explore how these parameters affect the mid-infrared colors of L and T dwarfs and find that the data are modelled satisfactorily only if substantial vertical mixing occurs in both L- and T-dwarf atmospheres. The location and range of the L and T dwarf sequences in IRAC color-color and color - magnitude diagrams is also only reproduced if this mixing occurs, with a range of efficiency described by K_zz ~ 10^2-10^6 cm^2/s. The colors of the unusually red dwarfs are best reproduced by non-equilibrium models with low sedimentation efficiency, i.e. thick cloud decks, and those of the unusually blue dwarfs by non-equilibrium models with high sedimentation efficiency, i.e. thin cloud decks. The K-L' and Spitzer [3.55]-[4.49] colors can be used as indicators of effective temperature for L and T dwarfs, but care must be taken to include gravity and metallicity effects for late-T dwarfs and vertical mixing for both late-L and T dwarfs.

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arXiv2006

On the Luminosity of Young Jupiters

Traditional thermal evolution models of giant planets employ arbitrary initial conditions selected more for computational expediency than physical accuracy. Since the initial conditions are eventually forgotten by the evolving planet, this approach is valid for mature planets, if not young ones. To explore the evolution at young ages of jovian mass planets we have employed model planets created by one implementation of the core accretion mechanism as initial conditions for evolutionary calculations. We find that young jovian planets are smaller, cooler, and several to 100 times less luminous than predicted by earlier models. Furthermore the time interval during which the young jupiters are fainter than expected depends on the mass of planet. Jupiter mass planets (1 M_J) align with the conventional model luminosity in as little at 20 million years, but 10 M_J planets can take up to 1 billion years to match commonly cited luminosities, given our implementation of the core accretion mechanism. If our assumptions, especially including our treatment of the accretion shock, are correct, then young jovian planets are substantially fainter at young ages than currently believed. These results have important consequences both for detection strategies and for assigning masses to young jovian planets based on observed luminosities.

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arXiv2006

The Influence of Atmospheric Dynamics on the Infrared Spectra and Light Curves of Hot Jupiters

We explore the infrared spectrum of a three-dimensional dynamical model of planet HD209458b as a function of orbital phase. The dynamical model predicts day-side atmospheric pressure-temperature profiles that are much more isothermal at pressures less than 1 bar than one-dimensional radiative-convective models have found. The resulting day-side thermal spectra are very similar to a blackbody, and only weak water absorption features are seen at short wavelengths. The dayside emission is consequently in significantly better agreement with ground-based and space-based secondary eclipse data than any previous models, which predict strong flux peaks and deep absorption features. At other orbital phases, absorption due to carbon monoxide and methane is also predicted. We compute the spectra under two treatments of atmospheric chemistry: one uses the predictions of equilibrium chemistry, and the other uses non-equilibrium chemistry, which ties the timescales of methane and carbon monoxide chemistry to dynamical timescales. As a function of orbital phase, we predict planet-to-star flux ratios for standard infrared bands and all Spitzer Space Telescope bands. In Spitzer bands, we predict 2-fold to 15-fold variation in planetary flux as a function of orbital phase with equilibrium chemistry, and 2-fold to 4-fold variation with non-equilibrium chemistry. Variation is generally more pronounced in bands from 3-10 $μ$m than at longer wavelengths. The orbital phase of maximum thermal emission in infrared bands is 15--45 orbital degrees before the time of secondary eclipse. Changes in flux as a function of orbital phase for HD209458b should be observable with Spitzer, given the previously acheived observational error bars.

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arXiv2006

A Sensitive Search for Variability in Late L Dwarfs: The Quest for Weather

We have conducted a photometric monitoring program of 3 field late-L brown dwarfs looking for evidence of non-axisymmetric structure or temporal variability in their photospheres. The observations were performed using Spitzer/IRAC 4.5 and 8 micron bandpasses and were designed to cover at least one rotational period of each object. One-sigma RMS (root mean squared) uncertainties of less than 3 mmag at 4.5 micron and around 9 mmag at 8 micron were achieved. Two out of the three objects studied exhibit some modulation in their light curves at 4.5 micron - but not 8 micron - with periods of 7.4 hr and 4.6 hr and peak-to-peak amplitudes of 10 mmag and 8 mmag. Although the lack of detectable 8 micron variation suggests an instrumental origin for the detected variations, the data may nevertheless still be consistent with intrinsic variability since the shorter wavelength IRAC bandpasses probe more deeply into late L dwarf atmospheres than the longer wavelengths. A cloud feature occupying a small percentage (1-2 %) of the visible hemisphere could account for the observed amplitude of variation. If, instead, the variability is indeed instrumental in origin, then our non-variable L dwarfs could be either completely covered with clouds or objects whose clouds are smaller and uniformly distributed. Such scenarios would lead to very small photometric variations. Followup IRAC photometry at 3.6 and 5.8 micron bandpasses should distinguish between the two cases. In any event, the present observations provide the most sensitive search to date for structure in the photospheres of late-L dwarfs at mid-IR wavelengths, and our photometry provides stringent upper limits to the extent to which the photospheres of these transition L dwarfs are structured.

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arXiv2006

A Spitzer Infrared Spectrograph (IRS) Spectral Sequence of M, L, and T Dwarfs

We present a low-resolution (R = 90), 5.5-38 micron spectral sequence of a sample of M, L, and T dwarfs obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope. The spectra exhibit prominent absorption bands of H_2O at 6.27 microns, CH_4 at 7.65 microns, and NH_3 at 10.5 microns and are relatively featureless at lambda > 15 microns. Three spectral indices that measure the strengths of these bands are presented; H_2O absorption features are present throughout the MLT sequence while the CH_4 and NH_3 bands first appear at roughly the L/T transition. Although the spectra are, in general, qualitatively well matched by synthetic spectra that include the formation of spatially homogeneous silicate and iron condensate clouds, the spectra of the mid-type L dwarfs show an unexpected flattening from roughly 9 to 11 microns. We hypothesize that this may be a result of a population of small silicate grains that are not predicted in the cloud models. The spectrum of the peculiar T6 dwarf 2MASS J0937+2931 is suppressed from 5.5-7.5 microns relative to typical T6 dwarfs and may be a consequence of its mildly metal-poor/high surface gravity atmosphere. Finally, we compute bolometric luminosities of a subsample of the M, L, and T dwarfs by combining the IRS spectra with previously published 0.6-4.1 micron spectra and find good agreement with the values of Golimowski et al. who use L'- and M'-band photometry and to account for the flux emitted at lambda > 2.5 microns.

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arXiv2006

Ammonia as a tracer of chemical equilibrium in the T7.5 dwarf Gliese 570D

We present the first analysis of an optical to mid-infrared spectrum of the T7.5 dwarf Gliese 570D with model atmospheres, synthetic spectra, and brown dwarf evolution sequences. We obtain precise values for the basic parameters of Gl 570D: Teff=800 - 820K, log g (cm/s^2)=5.09 - 5.23, and log L/Lsun= -5.525 to -5.551. The Spitzer IRS spectrum shows prominent features of ammonia (NH3) that can only be fitted by reducing the abundance of NH3 by about one order of magnitude from the value obtained with chemical equilibrium models. We model departures from chemical equilibrium in the atmosphere of Gl 570D by considering the kinetics of nitrogen and carbon chemistry in the presence of vertical mixing. The resulting model spectrum reproduces the data very well.

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arXiv2006

Atmosphere, Interior, and Evolution of the Metal-Rich Transiting Planet HD 149026b

We investigate the atmosphere and interior of the new transiting planet HD 149026b, which appears to be very rich in heavy elements. We first compute model atmospheres at metallicities ranging from solar to ten times solar, and show how for cases with high metallicity or inefficient redistribution of energy from the day side, the planet may develop a hot stratosphere due to absorption of stellar flux by TiO and VO. The spectra predicted by these models are very different than cooler atmosphere models without stratospheres. The spectral effects are potentially detectable with the Spitzer Space Telescope. In addition the models with hot stratospheres lead to a large limb brightening, rather than darkening. We compare the atmosphere of HD 149026b to other well-known transiting planets, including the recently discovered HD 189733b, which we show have planet-to-star flux ratios twice that of HD 209458 and TrES-1. The methane abundance in the atmosphere of HD 189733b is a sensitive indicator of atmospheric temperature and metallicity and can be constrained with Spitzer IRAC observations. We then turn to interior studies of HD 149026b and use a grid of self-consistent model atmospheres and high-pressure equations of state for all components to compute thermal evolution models of the planet. We estimate that the mass of heavy elements within the planet is in the range of 60 to 93 M_earth. Finally, we discuss trends in the radii of transiting planets with metallicity in light of this new member of the class.

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arXiv2005

Young Jupiters are Faint: New Models of the Early Evolution of Giant Planets

Here we show preliminary calculations of the cooling and contraction of a 2 MJ planet. These calculations, which are being extended to 1-10 MJ, differ from other published "cooling tracks" in that they include a core accretion-gas capture formation scenario, the leading theory for the formation of gas giant planets. We find that the initial post-accretionary intrinsic luminosity of the planet is ~3 times less than previously published models which use arbitrary initial conditions. These differences last a few tens of millions of years. Young giant planets are intrinsically fainter than has been previously appreciated. We also discuss how uncertainties in atmospheric chemistry and the duration of the formation time of giant planets lead to challenges in deriving planetary physical properties from comparison with tabulated model values.

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arXiv2005

Comparative Planetary Atmospheres: Models of TrES-1 and HD209458b

We present new self-consistent atmosphere models for transiting planets TrES-1 and HD209458b. The planets were recently observed with the Spitzer Space Telescope in bands centered on 4.5 and 8.0 $μ$m, for TrES-1, and 24 $μ$m, for HD209458b. We find that standard solar metallicity models fit the observations for HD209458b. For TrES-1, which has an T_eff ~300 K cooler, we find that models with a metallicity 3-5 times enhanced over solar abundances can match the 1$σ$ error bar at 4.5 $μ$m and 2$σ$ at 8.0$μ$m. Models with solar abundances that included energy deposition into the stratosphere give fluxes that fall within the 2$σ$ error bars in both bands. The best-fit models for both planets assume that reradiation of absorbed stellar flux occurs over the entire planet. For all models of both planets we predict planet/star flux ratios in other Spitzer bandpasses.

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arXiv2004

Near-Infrared Photometry and Spectroscopy of L and T Dwarfs: the Effects of Clouds, Gravity, and Effective Temperature

We present new JHKL'M' photometry on the MKO system for a large sample of L and T dwarfs identified from SDSS and 2MASS and classified according to the scheme of Geballe et al. (2002). We have compiled a sample of 105 L and T dwarfs that are uniformly classified and measured on a single photometric system. The scattered JHK spectral indices and colors of L dwarfs are likely caused by variations in the altitudes, distributions, and thicknesses of condensate clouds. Scatter in the H-K colors of late T dwarfs probably reflects the sensitivity of the K-band flux to pressure induced H_2 opacity, which itself is sensitive to surface gravity. The M' luminosities of late-T dwarfs are 1.5-2.5 times fainter than predicted under conditions of chemical equilibrium. We have computed L_bol and T_eff for 42 L and T dwarfs whose trigonometric parallaxes have been measured. We find that T_eff ~ 1450 K for types L7-T4, which supports recent models that attribute the changing JHK-band luminosities and spectral features across the L-T transition to rapid changes in the condensate clouds over a narrow range of T_eff. We compute T_eff = 600-750 K for 2MASS 0415-0935 (T9), which supplants Gl 570D as the coolest known brown dwarf.

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arXiv2004

Spectral Classification Beyond M

Significant populations of field L and T dwarfs are now known, and we anticipate the discovery of even cooler dwarfs by Spitzer and ground-based infrared surveys. However, as the number of known L and T dwarfs increases so does the range in their observational properties, and difficulties have arisen in interpreting the observations. Although modellers have made significant advances, the complexity of the very low temperature, high pressure, photospheres means that problems remain such as the treatment of grain condensation as well as incomplete and non-equilibrium molecular chemistry. Also, there are several parameters which control the observed spectral energy distribution - effective temperature, grain sedimentation efficiency, metallicity and gravity - and their effects are not well understood. In this paper, based on a splinter session, we discuss classification schemes for L and T dwarfs, their dependency on wavelength, and the effects of the parameters T_eff, f_sed, [m/H] and log g on optical and infrared spectra. We will discuss the various hypotheses that have been presented for the transition from the dusty L types to the clear atmosphere T types. We conclude with a brief discussion of the spectral class beyond T. Authors of each Section are identified by their initials.

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arXiv2004

The L to T Dwarf Transition

While the precise mechanism responsible for the L to T dwarf transition remains unclear, it is clearly caused by changing cloud characteristics. Here we briefly review data relevant to understanding the nature of the transition and argue that changing atmospheric dynamics produce the transition by opening holes through the global iron and silicate cloud decks. Other possibilities, such as a sudden vertical collapse in these cloud decks are also considered. Any acceptable model of the L to T transition must ultimately connect changing cloud properties to the underlying atmospheric dynamics.

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arXiv2004

Spitzer Infrared Spectrograph Observations of M, L, and T Dwarfs

We present the first mid-infrared spectra of brown dwarfs, together with observations of a low-mass star. Our targets are the M3.5 dwarf GJ 1001A, the L8 dwarf DENIS-P J0255-4700, and the T1/T6 binary system epsilon Indi Ba/Bb. As expected, the mid-infrared spectral morphology of these objects changes rapidly with spectral class due to the changes in atmospheric chemistry resulting from their differing effective temperatures and atmospheric structures. By taking advantage of the unprecedented sensitivity of the Infrared Spectrograph on the Spitzer Space Telescope we have detected the 7.8 micron methane and 10 micron ammonia bands for the first time in brown dwarf spectra.

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arXiv2004

L' and M' Photometry of Ultracool Dwarfs

We have compiled L' and M' photometry of 63 single and binary M, L,and T dwarfs obtained at UKIRT using the MKO filter set. This compilation includes new L' data for 8 L dwarfs and 13 T dwarfs and new M' data for 7 L dwarfs, 5 T dwarfs, and Gl 229A. We compute L_bol, BC_K, and T_eff for 42 dwarfs whose flux-calibrated JHK spectra, L' photometry, and trigonometric parallaxes are available, and we estimate these quantities for 9 other dwarfs whose parallaxes and flux-calibrated spectra have been obtained. BC_K is a well-behaved function of near-IR spectral type with a dispersion of ~0.1 mag for types M6-T5; it is significantly more scattered for types T5-T9. T_eff declines steeply and monotonically for types M6-L7 and T4-T9, but is nearly constant at ~1450 K for types L7-T4 with assumed ages of ~3 Gyr. This constant T_eff is evidenced by nearly unchanging values of L'-M' between types L6 and T3. We compare the observed K, L', and M' luminosities of L and T dwarfs in our sample with those predicted by model atmospheres with varying surface gravities and sedimentation efficiencies. The models indicate that the L3-T4.5 dwarfs generally have higher gravities (log g = 5.0-5.5) than the T6-T9 dwarfs (log g = 4.5-5.0). The predicted M' luminosities of T dwarfs are 1.5-2.5 times larger than those derived empirically for the T dwarfs in our sample. This discrepancy is attributed to absorption at 4.5-4.9 um by CO, which is not expected under the condition of chemical equilibrium assumed in the models. We determine that the L3 dwarf Kelu-1 and the T0 dwarf SDSS J0423-0414 are probable binary systems. We compute log(L_bol/L_sun) = -5.73 +/- 0.05 and T_eff = 600-750 K for the T9 dwarf 2MASS J0415-0935, which supplants Gl 570D as the least luminous and coolest brown dwarf presently known.

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arXiv2004

Near-Infrared Photometry and Spectroscopy of L and T Dwarfs: the Effects of Temperature, Clouds, and Gravity

We present JHK photometry and spectroscopy for a sample of newly identified L and T dwarfs from the Sloan Digital Sky Survey (SDSS) and known dwarfs from the SDSS and the Two Micron All Sky Survey (2MASS). Spectral classification has been carried out using four infrared indices; we identify 9 new L8-9.5 dwarfs and 14 new T dwarfs from SDSS, including the latest SDSS dwarf yet found, the T7 SDSS J1758+4633. We classify 2MASS J0415-0935 as T9, the latest and coolest dwarf found to date. We combine these results with previously published data to produce a sample of 59 L dwarfs and 42 T dwarfs. We compare the near-IR colors and absolute magnitudes of dwarfs near the L-T transition with predictions made by models of the distribution and evolution of photospheric condensates. There is scatter in the spectral indices for L dwarfs, suggesting that these indices are probing different levels of the atmosphere. The near-IR colors of the L dwarfs also scatter, likely due to variations in the altitudes, spatial distributions and thicknesses of the condensate clouds. We identify a group of late L dwarfs that are blue for their spectral type and that have enhanced FeH, H2O and KI absorption, possibly due to an unusually small amount of condensates. The scatter seen in the H-K color for late T dwarfs can be reproduced by models with a range in surface gravity. The variation is probably due to the effect on the K-band flux of pressure-induced H2 opacity. The correlation of H-K color with gravity is supported by the strengths of the J-band KI doublet. Gravity is closely related to mass for field T dwarfs and the gravities implied by the H-K colors indicate that the T dwarfs in our sample have masses in the range 15-75 M_Jupiter, except for SDSS J1110+0116 which possibly has a mass of 10-15 M_Jupiter.

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