FLAMES spectroscopy of low-mass stars in the young clusters σ Ori and λ Ori

Abstract

Aims. We performed a detailed membership selection and studied the accretion properties of low-mass stars in the two apparently very similar young (1-10 Myr) clusters σ Ori and λ Ori. Methods. We observed 98 and 49 low-mass (0.2-1.0 M ) stars in σ Ori and λ Ori respectively, using the multi-object optical spectrograph FLAMES at the VLT, with the high-resolution (R ∼ 17 000) HR15N grating (6470-6790 Å). We used radial velocities, Li and Hα to establish cluster membership and Hα and other optical emission lines to analyze the accretion properties of members. Results. We identified 65 and 45 members of the σ Ori and λ Ori clusters, respectively, and discovered 16 new candidate binary systems. We also measured rotational broadening for 20 stars and estimated the mass accretion rates in 25 stars of the σ Ori cluster, finding values between 10 −11 and 10 −7.7 M yr −1 and in 4 stars of the λ Ori cluster, finding values between 10 −11 and 10 −10.1 M yr −1 . Comparing our results with the infrared photometry obtained by the Spitzer satellite, we find that the fraction of stars with disks and the fraction of active disks is larger in the σ Ori cluster (52 ± 9% and 78 ± 16%) than in λ Ori (28 ± 8% and 40 ± 20%). Conclusions. The different disk and accretion properties of the two clusters could be due either to the effect of the high-mass stars and the supernova explosion in the λ Ori cluster or to different ages of the cluster populations. Further observations are required to draw a definitive conclusion.

Stars and brown dwarfs in theσ Orionis cluster

“…of T Tauri stars in σ Orionis ranges between 500 and 600 mÅ, as expected for ∼3 Ma-old objects (Zapatero Osorio et al 2002;Kenyon et al 2005;Maxted et al 2008;Sacco et al 2008 -but there might be a spread in lithium abundance). Three of the proper-motion contaminants in this work have high signal-to-noise ratio spectroscopy around Hα and Li i, taken by Sacco et al (2008) with FLAMES at the VLT.…”

“…36/Mayrit 1493050). Interestingly, 17 of the 37 proper-motion interlopers (46%) were tabulated by Scholz & Eislöffel (2004), Sherry et al (2004), or Hernández et al (2007 as σ Orionis member candidates, although the membership of a few of them were subsequently discarded by Kenyon et al (2005, 2008c, or Sacco et al (2008). The proper-motion interlopers in this work outnumber those in other comparable studies, such as Caballero (2008c) or Lodieu et al (2009).…”

“…Its low proper motion, of only 9 mas a −1 , confirms the membership of Mayrit 1493050 in σ Orionis. Sacco et al (2008). Of the other 22 stars, 15 had not been investigated in the literature and seven had been discarded as cluster members by Kenyon et al (2005) and Caballero ( , 2008c based on spectroscopy and/or proper motion.…”

“…Three of the proper-motion contaminants in this work have high signal-to-noise ratio spectroscopy around Hα and Li i, taken by Sacco et al (2008) with FLAMES at the VLT. They are the stars Nos.…”

“…If the pEW(Li i)s were correct, then the three M dwarfs should be of the age of the Pleiades or younger (τ < ∼ 120 Ma). I instead consider the feature at the Li i wavelength that Sacco et al (2008) found was not lithium in absorption, but a collection of lines of molecular CN ( 12 C 14 N -Grevesse 1968; Mandell et al 2004;Ghezzi et al 2008). The later the spectral type of a star is, the stronger the molecular bands of lines become.…”