Spectroscopy of very low-mass stars and brown dwarfs in the Lambda Orionis star-forming region

Abstract

Context. Most observational studies conducted so far point toward brown dwarfs sharing a similar formation mechanism as the one that is accepted for low-mass stars. However, larger databases and more systematic studies are needed before strong conclusions can be reached. Aims. In this second paper of a series devoted to studying the spectroscopic properties of the Lambda Orionis star-forming region members, we study accretion, activity and rotation for a wide set of spectroscopically confirmed members of the central star cluster Collinder 69 to assess analogies and/or differences between the brown-dwarf and stellar populations of this cluster. Moreover, we present comparisons with other star-forming regions of similar and different ages to address environmental effects on our conclusions. Methods. We studied prominent photospheric lines to derive rotational velocities and emission lines to distinguish between accretion processes and chromospheric activity. In addition, we include information about disk presence and X-ray emission. Results. We report very strong differences in the disk fractions of low-mass stars and brown dwarfs (∼58%) when compared to higher mass stars (26 +4 −3 %), with 0.6 M being the critical mass we find for this dichotomy. As a byproduct, we address the implications of the spatial distribution of disk and diskless members in the formation scenario of the cluster itself. We used the Hα emission to distinguish among accreting and non-accreting sources, finding that 38 +8 −7 % of sources harboring disks undergo active accretion and that his percentage stays similar in the substellar regime. For these sources we have estimated accretion rates. Finally, regarding rotational velocities, we find a high dispersion in v sin(i) that is even higher among the diskless population.

Orion revisited

“…Collinder 69 presents several advantages for the purpose of our study. It is well studied and an extensive list of more than 200 spectroscopically confirmed members is available (Bayo et al 2012). Photometry in almost all the bands presented in the current study has been obtained for its members, making the comparison possible over a broad domain of colors and luminosities.…”

TheHerschel/PACS view of disks around low-mass stars in Chamaleon-I

“…Numerical simulations of planetary population synthesis, such as the ones led by Alibert et al (2011) and Mordasini et al (2012) have shown that the expected planet population should depend on the stellar masses and disk properties. There has been a general effort to better characterize the relationship between the stellar masses and disk properties, which can additionally be related to the formation mechanism of very low-mass stars (Padoan & Nordlund 2002;Reipurth & Clarke 2001;Bayo et al 2011Bayo et al , 2012. It is therefore of prime importance for better constraining the disk characteristics, as well as dust and gas properties, around stars over a wide range of masses (e.g., Pascucci et al 2009).…”

Clusters: Age Scales for Stellar Physics

“…Duerr et al (1982) identified three associations within a very large bubble seen in Hα, later on photometric and spectroscopically characterized by Dolan & Mathieu (1999. Deeper photometry and spectroscopy well inside the substellar domain have been published by Barrado y Navascués et al (2004a, 2007a and Bayo et al (2011Bayo et al ( , 2012.…”

“…Again, just to provide some references: NGC2516 and M35, almost Pleiades twins (Jeffries et al 1998;Barrado y Navascués et al 2001a), IC2602 and IC2391 (Barrado y Navascués et al 1999aRandich 2001;Randich et al 2001;Barrado y Navascués et al 2004b), NGC2547 , IC4665 ), and Collinder 69 (Dolan & Mathieu 1999, Bayo et al 2012). In the very near future, the large scale spectroscopic survey Gaia-ESO (Gilmore et al 2012;Randich et al 2013) will provide an extended database.…”