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Photopolarimetric Characteristics of Brown Dwarfs. I. Uniform Cloud Decks

Sanghavi, Suniti and Shporer, Avi (2018) Photopolarimetric Characteristics of Brown Dwarfs. I. Uniform Cloud Decks. Astrophysical Journal, 866 (1). Art. No. 28. ISSN 1538-4357.

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This work is a theoretical exploration facilitating the interpretation of polarimetric observations in terms of cloudiness, rotational velocities, and effective temperatures of brown dwarfs (BDs). An envelope of scatterers like free electrons, atoms/molecules, or haze/clouds affects the Stokes vector of the radiation emitted by oblate bodies. Due to high rotation rates, BDs can be considerably oblate. We present a conics-based radiative transfer scheme for computing the disk-resolved and disk-integrated polarized emission of an oblate BD or extrasolar giant planet bearing homogeneous or patchy clouds. Assuming a uniform gray atmosphere, we theoretically examine the sensitivity of photopolarimetry to the atmosphere's scattering properties, like cloud optical thickness and grain size, concurrently with BD properties, like oblateness, inclination, and effective temperature, which are all treated as free parameters. Additionally, we examine the potential effects of gravitational darkening (GD), revealing that it could significantly amplify disk-integrated polarization. GD imparts a nonlinear inverse temperature dependence to the resulting polarization. Photopolarimetric observations are sensitive to oblateness and inclination. The degree of polarization increases in response to both, making it potentially useful for assessing the spatial orientation of the BD. Under our model assumptions, increasing droplet size in optically thick clouds causes a blueward shift in the near-infrared colors of BDs, which is interesting in light of the observed J – K brightening in the L/T transition. For large cloud grains, polarization decreases sharply, while the transmitted intensity shows a steady increase. BD polarization is thus a potential indicator not only of the presence of clouds but also provides information on cloud grain size.

Item Type:Article
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URLURL TypeDescription Paper
Sanghavi, Suniti0000-0003-0754-9154
Shporer, Avi0000-0002-1836-3120
Alternate Title:Photo-polarimetric characteristics of brown dwarfs, Part I: uniform cloud decks
Additional Information:© 2018 The American Astronomical Society. Received 2017 March 27; revised 2018 August 16; accepted 2018 August 27; published 2018 October 8. Many thanks are due to three reviewers whose comments greatly improved the quality and expanded the scope of our work. We thank Prof. Heather Knutson for valuable discussions. S.S. thanks Prof. Yuk L. Yung and Prof. John Grotzinger of Caltech for their kind support. This work was partly funded by the ESI Project 01STCR, Task R.18.183.022 at JPL. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.
Funding AgencyGrant Number
Subject Keywords:brown dwarfs – methods: analytical – polarization – radiative transfer – scattering
Issue or Number:1
Record Number:CaltechAUTHORS:20170518-102709338
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Official Citation:Suniti Sanghavi and Avi Shporer 2018 ApJ 866 28
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:77566
Deposited By: Joy Painter
Deposited On:18 May 2017 17:35
Last Modified:03 Oct 2019 17:59

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