Linear polarization of rapidly rotating ultracool dwarfs

Abstract

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.

Simultaneous optical and near-infrared linear spectropolarimetry of the earthshine

“…• 3) at the LIRIS Z, J, H, and K wavelengths (which agrees with the assumption made by Miles-Páez et al 2013), while the angle correction is wavelength-dependent for ALFOSC: from −2.…”

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

“…following the equations given in Miles-Páez et al (2013) and Wardle & Kronberg (1974). We caution that the expression for the Θ uncertainty is valid for P/σ P ≥ 3, where σ P is the error in linear polarimetry.…”

“…A.4), as opposed to the anti-phased g and i light curves discussed by Littlefair et al (2008). The previous J-band (1.2 μm) linear polarimetry measurement by our group (Miles-Páez et al 2013) is also included. It was taken 3.7 months prior to the ALFOSC observations.…”

“…Two phases are presented for clarity. The green triangle stands for a J band measurement taken from Miles-Páez et al (2013) 0.18 ± 0.01 or about 21 min. Wolszczan & Route (2014) reported that the radio emission bursts of TVLM 513−46 are detected 0.41 ± 0.02 of the period earlier than the peak of the intensity curve, which indicates the dipolar nature of the magnetic field and the inclination of the magnetic field with respect to the rotation axis (Berger et al 2009).…”

Polarized scattered light from self-luminous exoplanets

“…The trend was confirmed by Zapatero Osorio et al (2005) who measured polarization degrees in the range of 0.2-2.5% caused by possible cloud inhomogeneities, rotationally-induced oblateness, or in some cases the presence of a dusty disk or envelope. Near-infrared polarimetry by Miles-Páez et al (2013) showed that the fast rotating dwarfs from their sample (M7 through T2 spectral types) had on average a larger polarization degree than the moderately rotating dwarfs (see also Goldman et al 2009;Tata et al 2009;Zapatero Osorio et al 2011;Miles-Páez et al 2017). Polarimetric observations of self-luminous gas giants and brown dwarf companions are technically more demanding than observations of field dwarfs because of the companion-to-star flux contrast and the small angular separations involved.…”