Probing Dust in the Atmosphere of Brown Dwarfs through Polarization

Sengupta, Sujan; Krishan, V.

doi:10.1086/324559

Cited by 43 publications

(57 citation statements)

References 25 publications

(21 reference statements)

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“…However, at a given projected rotational velocity, the true rotational velocity would increase by (sin i) −1 , and the flattening would increase: following the Eqs. (1) and (2) of Sengupta & Krishan (2001), for most relevant eccentricities, i.e. for 0.04 < e < 0.9 4 , the difference of the equatorial and polar radii is roughly proportional to (sin i) −2 , and therefore the polarisation degree is nearly independent of sin i.…”

Section: Discussionmentioning

confidence: 89%

“…We can however attempt to compare our results with the models of Sengupta & Kwok (2005) and with those of the atmospheric variability surveys. Sengupta & Krishan (2001) v sin i measurements (see Table 1 and Fig. 2).…”

Section: Discussionmentioning

confidence: 99%

“…However, Sengupta & Krishan (2001) and Sengupta & Kwok (2005) both predict an increasing polarisation degree with wavelength over 0.6-1 μm for a wide range of dust grain size (results are presented for 0.1-1 μm). Sengupta (2003) presents similar conclusions over 0.7-0.9 μm for particle sizes between 0.2 μm and 30 μm.…”

Section: Comparison With Previous Surveysmentioning

confidence: 94%

“…Gravitational pulling by a close companion could also break the symmetry of the brown dwarf surface, as it is expected rotational acceleration does. Sengupta & Krishan (2001) calculated the polarisation of rotationally-flattened ultra-cool dwarfs and predicted that they could have non-zero polarised light. They estimated the polarisation degree due to the rotationally-induced flattening for single scattering and various grain-size distributions, while Sengupta & Kwok (2005) studied the effect of multiple scattering.…”

Section: Introductionmentioning

confidence: 99%

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Polarisation of very-low-mass stars and brown dwarfs

Goldman¹,

Pitann²,

Osorio

et al. 2009

A&A

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Context. Ultra-cool dwarfs of the L spectral type (T eff = 1400-2200 K) are known to have dusty atmospheres. Asymmetries of the dwarf surface may arise from rotationally-induced flattening and dust-cloud coverage, and may result in non-zero linear polarisation through dust scattering. Aims. We aim to study the heterogeneity of ultra-cool dwarfs' atmospheres and the grain-size effects on the polarisation degree in a sample of nine late M, L and early T dwarfs. Methods. We obtain linear polarimetric imaging measurements using FORS1 at the Very Large Telescope, in the Bessel I filter, and for a subset in the Bessel R and the Gunn z filters. Results. We measure a polarisation degree of (0.31 ± 0.06)% for LHS102BC. We fail to detect linear polarisation in the rest of our sample, with upper-limits on the polarisation degree of each object of 0.09% to 0.76% (95% of confidence level), depending on the targets and the bands. For those targets we do not find evidence of large-scale cloud horizontal structure in our data. Together with previous surveys, our results set the fraction of ultra-cool dwarfs with detected linear polarisation to 30 +10 −6 % (1-σ errors). From the whole sample of well-measured objects with errors smaller than 0.1%, the fraction of ultra-cool dwarfs with polarisation degree larger than 0.3% is smaller than 16% (95% confidence level). Conclusions. For three brown dwarfs, our observations indicate polarisation degrees different (at the 3-σ level) than previously reported, giving hints of possible variations. Our results fail to correlate with the current model predictions for ultra-cool dwarf polarisation for a flattening-induced polarisation, or with the variability studies for a polarisation induced by an heterogeneous cloud cover. This stresses the intricacy of each of those tasks, but may arise as well from complex and dynamic atmospheric processes.

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Section: Discussionmentioning

confidence: 89%

Section: Discussionmentioning

confidence: 99%

Section: Comparison With Previous Surveysmentioning

confidence: 94%

Section: Introductionmentioning

confidence: 99%

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Polarisation of very-low-mass stars and brown dwarfs

Goldman¹,

Pitann²,

Osorio

et al. 2009

A&A

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“…This result combined with convection and the presence of dust could give rise to intricated atmospheric dynamics, likely generating periodic and non-periodic photometric variability as seen in some late-M, L, and T dwarfs (Bailer-Jones & Mundt 2001;Martín et al 2001;Koen 2004;Buenzli et al 2012;Khandrika et al 2013). From a theoretical perspective, Sengupta & Krishan (2001), Sengupta (2003), Sengupta & Kwok (2005), Sengupta & Marley (2010), and de Kok et al (2011 predicted that ultracool dwarfs with atmospheric condensates and high v sin i's show measurable linear polarization degrees that are typically < ∼ 1% in the optical and near-infrared. Fast rotation induces photospheres into the form of an oblate ellipsoid, and this lack of symmetry leads to the incomplete cancellation of the polarization from different areas of the dwarfs surfaces.…”

Section: Introductionmentioning

confidence: 99%

Linear polarization of rapidly rotating ultracool dwarfs

Miles-Páez

Osorio

Πάλλη

et al. 2013

A&A

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Aims. We aim to study the near-infrared linear polarization signal of rapidly rotating ultracool dwarfs with spectral types ranging from M7 through T2 and projected rotational velocities of v sin i 30 km s −1 . These dwarfs are believed to have dusty atmospheres and oblate shapes, which is an appropriate scenario to produce measurable linear polarization of the continuum light. Methods. Linear polarimetric images were collected in the J-band for a sample of 18 fast-rotating ultracool dwarfs, of which five were also observed in the Z-band using the Long-slit Intermediate Resolution Infrared Spectrograph (LIRIS) on the Cassegrain focus of the 4.2-m William Herschel Telescope. The measured median uncertainty in the linear polarization degree is ±0.13% for our sample, which allowed us to detect polarization signatures above ∼0.39% with a confidence interval of ≥3σ. Results. About 40 ± 15% of the sample is linearly polarized in the Z-and J-bands. All positive detections have linear polarization degrees ranging from 0.4% to 0.8% in both filters independent of spectral type and spectroscopic rotational velocity. However, simple statistics point at the fastest rotators (v sin i 60 km s −1 ) having a larger fraction of positive detections and a larger averaged linear polarization degree than the moderately rotating dwarfs (v sin i = 30-60 km s −1 ). Our data suggest little linear polarimetric variability on short timescales (i.e., observations separated by a few ten rotation periods), and significant variability on long timescales (i.e., hundred to thousand rotation cycles), supporting the presence of long-term weather in ultracool dwarf atmospheres.

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Spectro-Polarimetry of Self-Luminous Extrasolar Planets

Sengupta

2013

J Astrophys Astron

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Probing Dust in the Atmosphere of Brown Dwarfs through Polarization

Cited by 43 publications

References 25 publications

Polarisation of very-low-mass stars and brown dwarfs

Polarisation of very-low-mass stars and brown dwarfs

Linear polarization of rapidly rotating ultracool dwarfs

Spectro-Polarimetry of Self-Luminous Extrasolar Planets

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