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

Abstract

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 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 Hα emission peak. Using published measurements of infrared excess (Δ(I C − K)), as disk tracer and equivalent width of the nead-infrared Ca ii line λ8542, mid-infrared difference [3.6]-[8.0] μm derived by Spitzer data, and 10% Hα 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 . 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 Δ(I C − K) has been found. While a spread in the rotation rates is seen for stars with no circumstellar disk (Δ(I C − K) < 0.3), stars with a circumstellar disk (Δ(I C − K) > 0.3) show an abrupt drop in their rotation rates by a factor of ∼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.

Physical parameters and long-term photometric variability of V1481 Ori, an SB2 member of Orion nebula Cluster with an accreting component

“…Recently, V1481 Ori was discovered to be a SB2 system by Tobin et al (2009). Biazzo et al (2009) (Furesz et al 2008). The possible presence of a disc is indicated by near infrared (NIR) excess in both 2MASS and IRAC/Spitzer photometry (Werner et al 2004).…”

“…Spectra were collected within the FLAMES (Fibre Large Array Multi Element Spectrograph) GTO with the multi-object GIRAFFE spectrograph in MEDUSA mode 1 . One of these spectra was analyzed by Biazzo et al (2009) with the aim to study the disc-locking scenario for low-mass stars in the Orion Nebula Cluster. Observations were carried out in [2005][2006] from October 15th to January 20th (∼8.4 h on 11 nights).…”

“…A total of eleven spectra of V1481 Ori were obtained and reduced using the GIRAFFE Base-Line Data Reduction Software (girBLDRS; Blecha et al 2000). Sky subtraction was applied following the prescriptions reported in Biazzo et al (2009).…”

“…We measured radial velocities (RVs) and their uncertainties following the procedure described in Biazzo et al (2009), which is based on the analysis of the CCFs between stellar spectra and numerical masks (see that paper for details). In nine spectra, far from the conjunctions, we could resolve both binary components and get their respective RVs from two separate Gaussian fits.…”

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

“…While many of them found a rotation-disk correlation for solar and low mass stars (e.g. H2002; Lamm et al 2005;Rebull et al 2006;Cieza & Baliber 2007;Biazzo 2009), others did not (e.g. Rebull 2001;Makidon 2004;Nguyen et al 2009).…”