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.
- Publication:
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Astronomy and Astrophysics
- Pub Date:
- August 2013
- DOI:
- 10.1051/0004-6361/201321851
- arXiv:
- arXiv:1306.6314
- Bibcode:
- 2013A&A...556A.125M
- Keywords:
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- polarization;
- brown dwarfs;
- stars: atmospheres;
- stars: late-type;
- stars: low-mass;
- stars: general;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 12 pages, 7 figures, recommended for publication in Astronomy and Astrophysics