Abstract
Angular momentum 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 0.4Msun. For brown dwarfs 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 BDs in the Orion Nebula Cluster, for which we have recently measured rotational periods. 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 Msun and whose IJHK colors are available. Circumstellar NIR excesses were derived from the Delta[I-K] index. We performed our analysis in three mass bins.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. 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.
Highlights
At present observations and theory together give a comprehensive picture of star formation
We aim to extend this investigation well down into the substellar regime for our large sample of ≈80 brown dwarfs (BDs) in the Orion Nebula Cluster, for which we have recently measured rotational periods
One possible reason could be the strong dependence of the mass accretion rate Mon stellar mass in the investigated mass range (M ∝ Ms2ta−r2.8), which is expected to result in a corresponding mass dependence of J/J
Summary
At present observations and theory together give a comprehensive picture of star formation. NIR observations are less efficient in detecting disks than observations at longer wavelengths, the relatively high disk fraction of ≈50% found by Lada (2000), Muench et al (2001) among large samples of low mass stars and brown dwarfs by means of JHK excesses provides valid support that NIR excess is a suitable disk indicator for low mass and substellar objects. In order to constrain a possible rotation-disk connection, in particular for the so far very poorly investigated very low mass and substellar regime, we analyze here a sample of 732 periodic variables from Paper I and Herbst et al (2002) which have available JHK colors to be used as disk indicators.
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