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

We present high-resolution spectroscopic observations for 91 PMS stars in ONC with masses in the range 0.10-0.25Msun carried out with the multi-fiber spectrograph FLAMES@ESO. Our aim is to better understand the disk-locking scenario in very low-mass stars. We have derived radial velocities, vsini, and full width at 10% of the Halpha emission peak. Using published measurements of infrared excess as disk tracer, and equivalent width of the NIR CaII line lambda8542, mid-IR difference [3.6]-[8.0]micron derived by Spitzer data, and 10% Halpha width as diagnostic of the level of accretion, we have looked for any correlation between vsini divided by the radius and presence of disk and accretion. Four low-mass stars are SB2 systems. The distribution of rotation periods derived from our vsini measurements is unimodal with a peak of few days. Our <sini> is lower than the one expected for a random distribution. We find no evidence for a population of fast rotators close to the break-up velocity. A clear correlation between vsini/R and Delta(Ic-K) has been found. While for stars with no circumstellar disk a spread in the rotation rates is seen, stars with a circumstellar disk show an abrupt drop in their rotation rates by a factor of ~5. On the other hand, only a partial correlation between vsini and accretion is observed when other indicators are used. The X-ray coronal activity level shows no dependence on vsini/R suggesting that all stars are in a saturated regime limit. The critical velocity is probably below our vsini detection limit of 9 km/s. The ONC low-mass stars in our sample at present seem to be not locked, but the clear correlation we find between rotation and IR color excess suggests that they were locked once. In addition, the percentage of accretors seems to scale inversely to the stellar mass.