Observed Polarization of Brown Dwarfs Suggests Low Surface Gravity

Sengupta, Sujan; Marley, Mark S.

doi:10.1088/2041-8205/722/2/l142

Cited by 49 publications

(73 citation statements)

References 35 publications

(69 reference statements)

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“…Dust is also present in the photospheres of dwarfs cooler than T eff = 2700 K (e.g., Tsuji et al 1996) as a result of the natural chemistry of low-temperature and high-gravity atmospheres that includes the condensation of refractory elements into liquid and solid particles. Based on a scenario of large rotation velocities (that induce oblate shapes) and/or inhomogeneous distributions of dusty clouds, Sengupta & Marley (2010) and Marley & Sengupta (2011) theoretically show that the net linear polarization of dusty dwarfs by single scattering processes can be as high as a few per cent at optical and near-infrared wavelengths. Miles-Páez et al (2013, and references therein) provided observational proof of linear polarization detections in ultracool dwarfs.…”

Section: Discussionmentioning

confidence: 99%

Rotational modulation of the linear polarimetric variability of the cool dwarf TVLM 513−46546

Miles-Páez

Osorio

Πάλλη

2015

A&A

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Aims. We aim to monitor the optical linear polarimetric signal of the magnetized, rapidly rotating M8.5 dwarf TVLM 513−46546. Methods. R-and I-band linear polarimetry images were collected with the Andalucía Faint Object Spectrograph and Camera (ALFOSC) instrument of the 2.56-m Nordic Optical Telescope (NOT) on two consecutive nights, covering about 0.5 and four rotation cycles in the R and I filters, respectively. We also obtained simultaneous intensity curves by means of differential photometry. The typical precision of the data is ±0.46% (R), ±0.35% (I) in the linear polarization degree and ±9 mmag (R), ±1.6 mmag (I) in the differential intensity curves. Results. Strong and variable linear polarization is detected in the R and I filters, with values of maximum polarization (p * = 1.30 ± 0.35 %), that are similar for both bands. The intensity and the polarimetric curves present a sinusoid-like pattern with a periodicity of ∼1.98 h, which we ascribe to structures in the surface of TVLM 513−46546 synchronized with its rotation. We found that the peaks of intensity and polarimetric curves occur with a phase difference of 0.18 ± 0.01 and that the maximum of the linear polarization occurs nearly half a period (0.59 ± 0.03) after the radio pulse. We discuss different scenarios to account for the observed properties of the light curves.

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Section: Discussionmentioning

confidence: 99%

Rotational modulation of the linear polarimetric variability of the cool dwarf TVLM 513−46546

Miles-Páez

Osorio

Πάλλη

2015

A&A

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“…This may be explained by a true moderate rotation velocity (v sin i = 32.8 km s −1 ) or a low inclination angle of the spin axis (i.e., the object is seen near pole-on). To test the linear polarimetric predictions of low gravity (young), cool lowmass dwarfs made by Sengupta & Marley (2010) and , a larger number of observations is demanded.…”

Section: Linear Polarization Versus Spectral Typementioning

confidence: 99%

“…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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“…Several numerical studies have been undertaken to investigate the effect of atmospheric asymmetries on the degree of polarization of self-luminous gas giants and brown dwarfs, showing typical values of linear polarization up to 1-2% in the optical and near-infrared. Sengupta & Marley (2010) argued that the Iband degree of polarization of field L dwarfs can be explained by rotationally-induced oblateness and a uniform cloud layer, suggesting low surface gravity (see also Sengupta & Kwok 2005). The same authors have extended their work to gas giant exoplanets and pointed out the potential of infrared polarization measurements of directly imaged planets for the study of their surface gravity and cloud inhomogeneities (Marley & Sengupta 2011).…”

Section: Introductionmentioning

confidence: 98%

“…Thermal radiation from a planetary atmosphere can also be polarized when it has been scattered. However, the disk-integrated polarization from the thermal photons will be negligible unless scattering occurs in an atmosphere that deviates from spherical symmetry (Sengupta & Marley 2010;Marley & Sengupta 2011;de Kok et al 2011). Therefore, a polarization measurement may provide information on the oblateness of an atmosphere or the presence of horizontal cloud variations (e.g., bands or patches).…”

Section: Introductionmentioning

confidence: 99%

Polarized scattered light from self-luminous exoplanets

Stolker

Min

Stam

et al. 2017

A&A

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Context. Direct imaging has paved the way for atmospheric characterization of young and self-luminous gas giants. Scattering in a horizontally-inhomogeneous atmosphere causes the disk-integrated polarization of the thermal radiation to be linearly polarized, possibly detectable with the newest generation of high-contrast imaging instruments. Aims. We aim to investigate the effect of latitudinal and longitudinal cloud variations, circumplanetary disks, atmospheric oblateness, and cloud particle properties on the integrated degree and direction of polarization in the near-infrared. We want to understand how 3D atmospheric asymmetries affect the polarization signal in order to assess the potential of infrared polarimetry for direct imaging observations of planetary-mass companions. Methods. We have developed a three-dimensional Monte Carlo radiative transfer code (ARTES) for scattered light simulations in (exo)planetary atmospheres. The code is applicable to calculations of reflected light and thermal radiation in a spherical grid with a parameterized distribution of gas, clouds, hazes, and circumplanetary material. A gray atmosphere approximation is used for the thermal structure. Results. The disk-integrated degree of polarization of a horizontally-inhomogeneous atmosphere is maximal when the planet is flattened, the optical thickness of the equatorial clouds is large compared to the polar clouds, and the clouds are located at high altitude. For a flattened planet, the integrated polarization can both increase or decrease with respect to a spherical planet which depends on the horizontal distribution and optical thickness of the clouds. The direction of polarization can be either parallel or perpendicular to the projected direction of the rotation axis when clouds are zonally distributed. Rayleigh scattering by submicronsized cloud particles will maximize the polarimetric signal whereas the integrated degree of polarization is significantly reduced with micron-sized cloud particles as a result of forward scattering. The presence of a cold or hot circumplanetary disk may also produce a detectable degree of polarization ( 1%) even with a uniform cloud layer in the atmosphere.

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Observed Polarization of Brown Dwarfs Suggests Low Surface Gravity

Cited by 49 publications

References 35 publications

Rotational modulation of the linear polarimetric variability of the cool dwarf TVLM 513−46546

Rotational modulation of the linear polarimetric variability of the cool dwarf TVLM 513−46546

Linear polarization of rapidly rotating ultracool dwarfs

Polarized scattered light from self-luminous exoplanets

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