Young Very Low Mass Stars Research Articles

Investigating the rotational evolution of very low-mass stars and brown dwarfs in young clusters using Monte Carlo simulations

Context. Very low-mass (VLM) stars and brown dwarfs (BDs) present a different rotational behaviour from their solar mass counterparts. Aims. We investigate the rotational evolution of young VLM stars and BDs using Monte Carlo simulations under the hypothesis of disk locking and stellar angular momentum conservation. Methods. We built a set of objects with masses ranging from 0.01 M⊙ to 0.4 M⊙ and considered models with single- and double-peaked initial period distributions with and without disk locking. An object is considered to be diskless when its mass accretion rate is below a given threshold. Results. Models with initial single-peaked period distributions reproduce the observations well given that BDs rotate faster than VLM stars. We observe a correlation between rotational period and mass when we relax the disk locking hypothesis, but with a shallower slope compared to some observational results. The angular momentum evolution of diskless stars is flatter than it is for stars with a disk which occurs because the moment of inertia of objects less massive than 0.2 M⊙ remains pratically constant for a time scale that increases with decreasing stellar mass. Conclusions. Comparing our results with the available observational data we see that disk locking is not as important in the low-mass regime and that the rotational behaviour of VLM stars and BDs is different from what is seen in their solar mass counterparts.

Rotational studies in the Orion Nebula Cluster: from solar mass stars to brown dwarfs

Rotational studies at a variety of ages and masses are important for constraining the angular momentum evolution of young stellar objects (YSO). Of particular interest are the very low mass (VLM) stars and brown dwarfs (BDs), because of the significant lack of known rotational periods in that mass range. We provide for the first time information on rotational periods for a large sample of young VLM stars and BDs. This extensive rotational period study in the 1 Myr old Orion Nebula Cluster (ONC) is based on a deep photometric monitoring campaign using the Wide Field Imager (WFI) camera on the ESO/MPG 2.2m telescope on La Silla, Chile. Accurate I-band photometry of 2908 stars was obtained, extending three magnitudes deeper than previous studies in the ONC. We found 487 periodic variables with estimated masses between 0.5 Msun and 0.015 Msun, 124 of which are BD candidates. This is by far the most extensive and complete rotational period data set for young VLM stars and BDs. In addition, 808 objects show non-periodic brightness variations. We study the dependence of the period distribution on mass and variability level and compare this with known higher mass objects in the ONC (Herbst et al. 2002) and with the 2 Myr old cluster NGC 2264 (Lamm et al., 2004). We find that substellar objects rotate on average faster than the VLM stars. Our rotational data also suggest a dependence of the rotational periods on position within the field, which can be explained by a possible age spread in the ONC. In addition, periodic variables with larger peak-to-peak amplitudes rotate on average slower than those with small peak-to-peak amplitude variations, which can possibly be explained by different magnetic field topologies.

A survey for low-mass spectroscopic binary stars in the young clusters around σ Orionis and λ Orionis

We have obtained multi-epoch, high-resolution spectroscopy of 218 candidate low-mass stars and brown dwarfs (BDs) in the young clusters around a Ori and Ori. We find that 196 targets are cluster members based on their radial velocity, the equivalent width of their Na Ι 8200 lines and the spectral type from their TiO band strength. We have identified 11 new binary stars among the cluster members based on their variable radial velocity and an additional binary from the variation in its linewidth and shape. Of these, six are double-lined spectroscopic binaries (SB2) where the components of the binary are of comparable brightness. The others are single-lined binaries (SB 1) in which the companion is faint or the spectra of the stars are blended. There are three narrow-lined SB 1 binaries in our sample for which the companion is more than 2.5 mag fainter than the primary. This suggests that the mass ratio distribution for the spectroscopic binaries in our sample is broad but that there may be a peak in the distribution near q = 1. The sample covers the magnitude range I C = 14-18.9 (mass ≈0.55-0.03 M ⊙ ), but all of the binary stars are brighter than I C = 16.6 (mass ≈30.12 M ⊙ ) and 10 are brighter than I C = 15.5 (mass ≈0.23 M ⊙ ). There is a significant lack of spectroscopic binaries in our sample at faint magnitudes even when we account for the decrease in sensitivity with increasing magnitude. We can reject the hypothesis that the fraction of spectroscopic binaries is a uniform function of I C magnitude with more than 99 per cent confidence. The spectroscopic binary fraction for stars more massive than about 0.1 M ⊙ (I C < 16.9) is f bright = 0.095 +0.012 -0.028 . The 90 per cent confidence upper limit to the spectroscopic binary fraction for very low-mass (VLM) stars (mass <0.1 M ⊙ ) and BDs is f faint < 7.5 per cent. The hypothesis that f bright and /faint are equal can be rejected with 90 per cent confidence. The average detection probability for our survey is 50 per cent or more for binaries with separations up to 0.28 au for stars with I C < 16.9 and 0.033 au for the fainter stars in our sample. We conclude that we have found strong evidence for a change in the fraction of spectroscopic binaries among young VLM stars and BDs when compared to more massive stars in the same star-forming region. This implies a difference in the total binary fraction between VLM stars and BDs compared to more massive stars or a difference in the distribution of semimajor axes, or both.