X-Shooter study of accretion inρ-Ophiucus: very low-mass stars and brown dwarfs

Abstract

We present new VLT/X-Shooter optical and near-infrared spectra of a sample of 17 candidate young low-mass stars and brown dwarfs located in the ρ-Ophiucus cluster. We derived the spectral type and extinction for all the targets, and then we determined their physical parameters. All the objects but one have M 0.6 M , and eight have mass below or close to the hydrogen-burning limit. Using the intensity of various permitted emission lines present in their spectra, we determined the accretion luminosity and mass accretion rates (Ṁ acc ) for all the objects. When compared with previous works targeting the same sample, we find that, in general, these objects are not as strongly accreting as previously reported, and we suggest that the reason is our more accurate estimate of the photospheric parameters. We also compare our findings with recent works in other slightly older star-forming regions, such as Lupus, to investigate possible differences in the accretion properties, but we find that the accretion properties for our targets have the same dependence on the stellar and substellar parameters as in the other regions. This leads us to conclude that we do not find evidence for a different dependence ofṀ acc with M when comparing low-mass stars and brown dwarfs. Moreover, we find a similar small ( 1 dex) scatter in theṀ acc −M relation as in some of our recent works in other star-forming regions, and no significant differences inṀ acc due to different ages or properties of the regions. The latter result suffers, however, from low statistics and sample selection biases in the current studies. The small scatter in theṀ acc −M correlation confirms that mass accretion rate measurements in the literature based on uncertain photospheric parameters and single accretion indicators, such as the Hα width, can lead to a scatter that is unphysically large. Our studies show that only broadband spectroscopic surveys coupled with a detailed analysis of the photospheric and accretion properties allows us to properly study the evolution of disk accretion rates in star-forming regions.Key words. stars: pre-main sequence -stars: formation -brown dwarfs -protoplanetary disks -accretion, accretion disksopen clusters and associations: individual: rho-Ophiucus IntroductionThe evolution of protoplanetary disks surrounding forming stars has been a major subject of study in recent years. The interest in this topic is driven by the fact that disks are the birthplace of planets. A clear understanding of the physical mechanisms driving the formation, evolution, and dispersal of disks is thus needed to constrain planet formation theories, and to explain the observed properties of our own solar system and of the wealth of exoplanetary systems discovered so far.During their evolution, the central young stellar object (YSO) and the surrounding disk interact through multiple processes, such as photoevaporation, stellar winds, and accretion of matter. The latter is a result of viscous processes happening in the disk, which drive its secular evolu...

X-shooter study of accretion in Chamaeleon I

“…The complete U-band photometric surveys in σ-Orionis (Rigliaco et al 2011a) and in the Orion Nebula Cluster (Manara et al 2012) and the spectroscopic survey of L1641 by Fang et al (2013) came to the similar conclusion of a faster evolutionary timescale of accretion for lower-mass stars than for solar-mass stars. This is a possible explanation for the difference with the results obtained by Manara et al (2015) with a similar procedure on a sample of very low-mass stars and brown dwarfs in the younger ρ-Ophiuchus star-forming region. Manara et al (2015) found that the observedṀ acc -M relation for the objects in ρ-Ophiuchus follows the sameṀ acc ∝M ∼1.8 relation as was found for objects of higher stellar masses derived with an incomplete Article number, page 9 of 28 sample in the Lupus star-forming region by Alcalá et al (2014).…”

“…This is a possible explanation for the difference with the results obtained by Manara et al (2015) with a similar procedure on a sample of very low-mass stars and brown dwarfs in the younger ρ-Ophiuchus star-forming region. Manara et al (2015) found that the observedṀ acc -M relation for the objects in ρ-Ophiuchus follows the sameṀ acc ∝M ∼1.8 relation as was found for objects of higher stellar masses derived with an incomplete Article number, page 9 of 28 sample in the Lupus star-forming region by Alcalá et al (2014). Thus, Manara et al (2015) argued that theṀ acc -M relation is the same from brown dwarfs up to solar-mass stars.…”

An Upper Limit on the Mass of the Circumplanetary Disk for DH Tau b*

“…Assuming these high mass accretion rates are indicative of large disk masses would seem to support formation via turbulent fragmentation. In addition, it seems that most PMCs located in young (<10 Myr), nearby star-forming regions are accreting as has been seen for field brown dwarfs (e.g., Manara et al 2015). Evidence of circumplanetary disks from accretion signatures alone rejects core accretion and subsequent scattering as a possible formation pathway, because it would cause a disk to dissipate.…”

X-Shooter study of accretion in Chamaeleon I

“…In particular, the distribution of M in the two samples is different: more than 70% of the Lupus objects have M 0.5 M , while 60% of the Chamaeleon I targets have M > 0.5 M . It is possible that theṀ acc -M relation is tighter at lower M , as recent results in low-mass stars and brown dwarfs (BDs) in ρ-Ophiuchus suggest (Manara et al 2015), and this would explain why there is a smaller spread oḟ M acc in the Lupus sample. This means that it might be more plausible to have YSOs accreting with L acc ∼ L at higher masses, which would explain why more strongly accreting YSOs are present in the Chamaeleon I sample.…”