Evidence of early disk-locking among low-mass members of the Orion Nebula Cluster

Context. We present new high-resolution spectroscopic observations for 91 pre-main sequence stars in the Orion Nebular Cluster (ONC) with masses in the range 0.10-0.25 M(circle dot) carried out with the multi-fiber spectrograph flames attached to the UT2 at the Paranal Observatory. Aims. Our aim is to better understand the disk-locking scenario in very low-mass stars. Methods. We have derived radial velocities, projected rotational velocities, and full width at 10% of the Ha emission peak. Using published measurements of infrared excess (Delta(I(C) -K)), as disk tracer and equivalent width of the nead-infrared Ca II line lambda 8542, mid-infrared difference [3.6]-[8.0] mu m derived by Spitzer data, and 10% Ha width as diagnostic of the level of accretion, we looked for any correlation between projected angular rotational velocity divided by the radius (v sin i/R) and presence of disk and accretion. Results. For 4 low-mass stars, the cross-correlation function is clearly double-lined, indicating that the stars are SB2 systems. The distribution of rotation periods derived from our v sin i measurements is unimodal with a peak of a few days, in agreement with previous results for M < 0.25 M(circle dot). The photometric periods were combined with our v sin i to derive the equatorial velocity and the distribution of rotational axes. Our < sin i > is lower than the one expected for a random distribution, as previously found. We find no evidence of a population of fast rotators close to the break-up velocity. A clear correlation between v sin i/R and Delta(I(C) -K) has been found. While a spread in the rotation rates is seen for stars with no circumstellar disk (Delta(I(C) -K) < 0.3), stars with a circumstellar disk (Delta(I(C) -K) > 0.3) show an abrupt drop in their rotation rates by a factor of similar to 5. On the other hand, only a partial correlation between v sin i and accretion is observed when other indicators are used. The X-ray coronal activity level (log L(X)/L(bol)) shows no dependence on v sin i/R, suggesting that all stars are in a saturated regime limit. The critical velocity is probably below our v sin i detection limit of 9 km s(-1). Conclusions. The ONC low-mass stars in our sample, close to the hydrogen burning limit, at present do not seem to be locked, but the clear correlation we find between rotation and infrared color excess suggests that they were locked once. In addition, the percentage of accretors seems to scale inversely to the stellar mass.