Multi-fibre optical spectroscopy of low-mass stars and brown dwarfs in Upper Scorpius

Abstract

Context. Knowledge of the mass function in open clusters constitutes one way to critically examine the formation mechanisms proposed to explain the existence of low-mass stars and brown dwarfs. Aims. The aim of the project is to determine as accurately as possible the shape of the mass function across the stellar/substellar boundary in the young (5 Myr) and nearby (d = 145 pc) Upper Sco association. Methods. We have obtained multi-fibre intermediate-resolution (R ∼ 1100) optical (∼5750−8800 Å) spectroscopy of 94 photometric and proper motion selected low-mass star and brown dwarf candidates in Upper Sco with the AAOmega spectrograph on the AngloAustralian Telescope. Results. We have estimated the spectral types and measured the equivalent widths of youth (Hα) and gravity (Na I and K I) diagnostic features to confirm the spectroscopic membership of about 95% of the photometric and proper motion candidates extracted from 6.5 square degrees surveyed in Upper Sco by the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey (GCS). We also detect lithium in the spectra with the highest signal-to-noise, consolidating our conclusions about their youth. Furthermore, we derive an estimate of the efficiency of the photometric and proper motion selections used in our earlier studies using spectroscopic data obtained for a large number of stars falling into the instrument's field-of-view. We have estimated the effective temperatures and masses for each new spectroscopic member using the latest evolutionary models available for low-mass stars and brown dwarfs. Combining the current optical spectroscopy presented here with near-infrared spectroscopy obtained for the faintest photometric candidates, we confirm the shape and slope of our earlier photometric mass function. The luminosity function drawn from the spectroscopic sample of 113 USco members peaks at around M6 and is flat at later spectral type. We may detect the presence of the M7/M8 gap in the luminosity function as a result of the dust properties in substellar atmospheres. The mass function may peak at 0.2 M and is quite flat in the substellar regime. We observe a possible excess of cool low-mass brown dwarfs compared to IC 348 and the extrapolation of the field mass functions, supporting the original hypothesis that Upper Sco may possess an excess of brown dwarfs compared to other young regions. Conclusions. This result shows that the selection of photometric candidates based on five band photometry available from the UKIDSS GCS and complemented partially by proper motions can lead to a good representation of the spectroscopic mass function.

The spectral type of CHS 7797 - an intriguing very low mass periodic variable in the Orion Nebula Cluster

“…These low values agree with the PEWs of Na I doublet measured in other young stars (e.g. Lodieu et al 2011) and are lower than the ≈6 Å typical equivalent widths measured in M old field dwarfs (Martin et al 1996).…”

“…1 and also from the values presented in Table 2 the two spectra are similar, but not identical. A direct comparison of the spectra of CHS 7797 at the two observed phases and spectra of three members of the ≈5 Myr-old Upper Scorpius Association from Lodieu et al (2011) is shown in Fig. 2 2 .…”

“…Lodieu et al 2011;Riddick et al 2007b;Luhman 2004). In addition, photometric monitoring programs, mostly aimed at studying variability and rotation evolution, have also played an important role in the detection of many VLM stars and even brown dwarf candidates (e.g.…”

No substellar objects at the center of the Lupus 3 star-forming cloud

“…The comparison between results based on visible and near-infrared spectroscopy may be affected by such biases, given the greater uncertainties in nearinfrared spectral classifications. However, Lodieu et al (2011) have recently used visible spectroscopy to derive a star-to-brown dwarf ratio of only 2.5 for the Upper Scorpius association, adjacent to the Lupus clouds, based on spectroscopy in the visible. The departure with respect to values found in other regions is mainly due to an overabundance of brown dwarfs with masses below ∼0.04 M that includes a possible secondary peak of the mass function around 0.015 M .…”

“…However, much progress has been made in the last decade with the discovery of ever fainter, less massive members now reaching down to a few Jupiter masses in some nearby star-forming regions (e.g. Luhman 2007;Luhman et al 2009;Spezzi et al 2008;Bihain et al 2009;Boudreault & Bailer-Jones 2009;Scholz et al 2009;Haisch et al 2010;Geers et al 2011;Lodieu et al 2011, for some recent surveys). Much of the progress has come from dedicated surveys using wide-field instruments complemented by A&A 531, A33 (2011) Fig.…”