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

¹ Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38205 La Laguna, Spain
e-mail: pamp@iac.es; epalle@iac.es
² Dpt. de Astrofísica, Univ. de La Laguna, Avda. Astrofísico Francisco Sánchez s/n, 38206 La Laguna, Tenerife, Spain
³ Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
e-mail: mosorio@cab.inta-csic.es

Received: 17 July 2014
Accepted: 24 July 2015

Abstract

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.