We present new, high resolution, infrared spectra of the T dwarf Gliese 229B in the J, H, and K bandpasses. We analyze each of these as well as previously published spectra to determine its metallicity and the abundances of NH3 and CO in terms of the surface gravity of Gl 229B, which remains poorly constrained. The metallicity increases with increasing gravity and is below the solar value unless Gl 229B is a high-gravity brown dwarf with log g(cgs) ~ 5.5. The NH3 abundance is determined from both the H and the K band spectra which probe two different levels in the atmosphere. We find that the abundance from the K band data is well below that expected from chemical equilibrium, which we interpret as strong evidence for dynamical transport of NH3 in the atmosphere. This is consistent with the previous detection of CO and provides additional constraints on the dynamics of the atmosphere of this T dwarf.Comment: 22 pages and 19 figures. Accepted for publication in the Astrophysical Journa
It has been becoming clear that the atmospheres of the young, self‐luminous extrasolar giant planets imaged to date are dusty. Planets with dusty atmospheres may exhibit detectable amounts of linear polarization in the near‐infrared, as has been observed from some field L dwarfs. The asymmetry required in the thermal radiation field to produce polarization may arise either from the rotation‐induced oblateness or from surface inhomogeneities, such as partial cloudiness. While it is not possible at present to predict the extent to which atmospheric dynamics on a given planet may produce surface inhomogeneities substantial enough to produce net non‐zero disc‐integrated polarization, the contribution of rotation‐induced oblateness can be estimated. Using a self‐consistent, spatially homogeneous atmospheric model and a multiple scattering polarization formalism for this class of exoplanets, we show that polarization of the order of 1 per cent may arise due to the rotation‐induced oblateness of the planets. The degree of polarization for cloudy planets should peak at the same wavelengths at which the planets are brightest in the near‐infrared. The observed polarization may be even higher if surface inhomogeneities exist and play a significant role. Polarized radiation from self‐luminous gas giant exoplanets, if detected, provides an additional tool to characterize these young planets and a new method to constrain their surface gravity and masses.
Light scattering by atmospheric dust particles is responsible for the polarization observed in some L dwarfs. Whether this polarization arises from an inhomogeneous distribution of dust across the disk or an oblate shape induced by rotation remains unclear. Here we argue that the latter case is plausible and, for many L dwarfs, the more likely one. Furthermore evolutionary models of mature field L dwarfs predict surface gravities ranging from about 200 to 2500 m s −2 (corresponding to masses of ∼ 15 to 70 M Jupiter ). Yet comparison of observed spectra to available synthetic spectra often does not permit more precise determination of the surface gravity of individual field L dwarfs, leading to important uncertainties in their properties. Since rotationally-induced non-sphericity, which gives rise to non-zero disk-integrated polarization, is more pronounced at lower gravities, polarization is a promising low gravity indicator. Here we combine a rigorous multiple scattering analysis with a self-consistent cloudy atmospheric model and observationally inferred rotational velocities and find that the observed optical polarization can be explained if the surface gravity of the polarized objects is about 300 m s −2 or less, potentially providing a new method for constraining L dwarf masses.
While there have been multiple observational programs aimed at detecting linear polarization of optical radiation emitted by ultracool dwarfs, there has been comparatively less rigorous theoretical analysis of the problem. The general expectation has been that the atmospheres of those substellar-mass objects with condensate clouds would give rise to linear polarization due to scattering. Because of rotation-induced non-sphericity, there is expected to be incomplete cancellation of disk-integrated net polarization and thus a finite polarization. For cloudless objects, however, only molecular Rayleigh scattering will contribute to any net polarization and this limit has not been well studied. Hence in this paper we present a detailed multiple scattering analysis of the polarization expected from those T-dwarfs whose spectra show absence of condensates. For this, we develop and solve the full radiative transfer equations for linearly polarized radiation. Only atomic and molecular Rayleigh scattering are considered to be the source of polarization. We compute the local polarization at different angular directions in a plane-parallel atmospheres calculated for the range of effective temperatures of T dwarfs and then average over the whole surface of the object. The effects of gravity and limb darkening as well as rotation induced non-sphericity are included. It is found that the amount of polarization decreases with the increase in effective temperature. It is also found that significant polarization at any local point in the atmosphere arises only in the optical (B-band). However, the disk integrated polarization-even in the B-band-is negligible. Hence we conclude that, unlike the case for cloudy L dwarfs, polarization of cloudless T-dwarfs by atomic and molecular scattering may not be detectable. In the future we will extend this work to cloudy L-and T-dwarf atmospheres.
Received; accepted 1 sujan@iiap.ernet.in -2 - ABSTRACTTheoretical analysis and observational evidences indicate that a brown dwarf with effective temperature greater than 1300 K would have dust cloud in its atmosphere. In this letter, we show that dust scattering should yield polarized continuum radiation from the relatively warm brown dwarfs and the polarized flux profile could be a potential diagnosis tool for the optical and the physical properties of dust grains. The degree of polarization due to multiple scattering will be more in the optical region if the particle size is small while significant polarization should be detected in the infra-red region if the particle size is large. It is pointed out that the departure from sphericity in the shape of the object due to rapid rotation and due to tidal effect by the companion in a binary system ensures the disc integrated polarization to be non-zero.
Brown dwarfs exhibit patchy or spatially varying banded cloud structures that are inferred through photometric and spectroscopic variability modeling techniques. However, these methods are insensitive to rotationally invariant structures, such as the bands seen in Jupiter. Here, we present H-band Very Large Telescope/NaCo linear polarization measurements of the nearby Luhman 16 L/T transition binary, which suggest that Luhman 16A exhibits constant longitudinal cloud bands. The instrument was operated in pupil tracking mode, allowing us to unambiguously distinguish between a small astrophysical polarization and the ∼2% instrumental linear polarization. We measure the degree and angle of linear polarization of Luhman 16A and B to be p A = 0.031% ± 0.004% and ψ A = −32° ± 4°, and p B = 0.010% ± 0.004% and , respectively. Using known physical parameters of the system, we demonstrate that an oblate homogeneous atmosphere cannot account for the polarization measured in Luhman 16A, but could be responsible for that of the B component. Through a nonexhaustive search of banded cloud morphologies, we demonstrate a two-banded scenario that can achieve a degree of linear polarization of p = 0.03% and conclude that the measured polarization of the A component must be predominantly due to cloud banding. For Luhman 16B, either oblateness or cloud banding could be the dominant source of the measured polarization. The misaligned polarization angles of the two binary components tentatively suggest spin–orbit misalignment. These measurements provide new evidence for the prevalence of cloud banding in brown dwarfs while at the same time demonstrating a new method—complementary to photometric and spectroscopic variability methods—for characterizing the cloud morphologies of substellar objects without signs of variability.
We present new grids of transmission spectra for hot-Jupiters by solving the multiple scattering radiative transfer equations with non-zero scattering albedo instead of using the Beer-Bouguer-Lambert law for the change in the transmitted stellar intensity. The diffused reflection and transmission due to scattering increases the transmitted stellar flux resulting into a decrease in the transmission depth. Thus we demonstrate that scattering plays a double role in determining the optical transmission spectra -increasing the total optical depth of the medium and adding the diffused radiation due to scattering to the transmitted stellar radiation. The resulting effects yield into an increase in the transmitted flux and hence reduction in the transmission depth. For a cloudless planetary atmosphere, Rayleigh scattering albedo alters the transmission depth up to about 0.6 micron but the change in the transmission depth due to forward scattering by cloud or haze is significant throughout the optical and near-infrared regions. However, at wavelength longer than about 1.2 µm, the scattering albedo becomes negligible and hence the transmission spectra match with that calculated without solving the radiative transfer equations. We compare our model spectra with existing theoretical models and find significant difference at wavelength shorter than one micron. We also compare our models with observational data for a few hot-Jupiters which may help constructing better retrieval models in future.
We report the results of the high precision photometric follow-up observations of five transiting hot jupiters -WASP-33b, WASP-50b, WASP-12b, HATS-18b and HAT-P-36b. The observations are made from the 2m Himalayan Chandra Telescope at Indian Astronomical Observatory, Hanle and the 1.3m J. C. Bhattacharyya Telescope at Vainu Bappu Observatory, Kavalur. This exercise is a part of the capability testing of the two telescopes and their back-end instruments. Leveraging the large aperture of both the telescopes used, the images taken during several nights were used to produce the transit light curves with high photometric S/N (> 200) by performing differential photometry. In order to reduce the fluctuations in the transit light curves due to various sources such as stellar activity, varying sky transparency etc. we preprocessed them using wavelet denoising and applied Gaussian process correlated noise modeling technique while modeling the transit light curves. To demonstrate the efficiency of the wavelet denoising process we have also included the results without the denoising process. A state-of-the-art algorithm used for modeling the transit light curves provided the physical parameters of the planets with more precise values than reported earlier.
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