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 greater than or similar to 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 similar to 0.39% with a confidence interval of >= 3 sigma. 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.