Rotation-disk connection for very low mass and substellar objects in the Orion Nebula Cluster^*

¹ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany e-mail: vicrodriguez@mpia.de
² International Max Planck Research School for Astronomy & Cosmic Physics at the University of Heidelberg, Germany
³ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁴ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany

Received: 17 October 2009
Accepted: 13 January 2010

Abstract

Aims. Angular momentum (J) loss requires magnetic interaction between the forming star and both the circumstellar disk and the magnetically driven outflows. In order to test these predictions many authors have investigated a rotation-disk connection in pre-main sequence objects with masses larger than about 0.4 . For brown dwarfs (BDs) this connection was not investigated as yet because there are very few samples available. We aim to extend this investigation well down into the substellar regime for our large sample of ≈80 BDs in the Orion Nebula Cluster, for which we have recently measured rotational periods.

Methods. In order to investigate a rotation-disk correlation, we derived near-infrared (NIR) excesses for a sample of 732 periodic variables in the Orion Nebula Cluster with masses ranging between ≈1.5–0.02  and whose IJHK colors are available. Circumstellar NIR excesses were derived from the Δ[ I–K] index. We performed our analysis in three mass bins (>0.4 , 0.4–0.075 , and <0.075).

Results. We found a rotation-disk correlation in the high and intermediate mass regime, in which objects with NIR excess tend to rotate slower than objects without NIR excess. Interestingly, we found no correlation in the substellar regime. A tight correlation between the peak-to-peak (ptp) amplitude of the rotational modulation and the NIR excess was found however for all objects with available ptp values (<0.4 ). We discuss possible scenarios which may explain the lack of rotation-disk connection in the substellar mass regime. One possible reason could be the strong dependence of the mass accretion rate on stellar mass in the investigated mass range ( ), which is expected to result in a corresponding mass dependence of /J.