Low Mass Stars and Brown Dwarfs around Sigma Orionis

We present optical spectroscopy of 71 photometric candidate low-mass members of the cluster associated with Sigma Orionis. Thirty-five of these are found to pass the lithium test and hence are confirmed as true cluster members, covering a mass range of <0.055-0.3M_{sun}, assuming a mean cluster age of <5 Myr. We find evidence for an age spread on the (I, I-J) colour magnitude diagram, members appearing to lie in the range 1-7 Myr. There are, however, a significant fraction of candidates that are non-members, including some previously identified as members based on photometry alone. We see some evidence that the ratio of spectroscopically confirmed members to photometric candidates decreases with brightness and mass. This highlights the importance of spectroscopy in determining the true initial mass-function.Comment: To appear in the 12th Cambridge Workshop on Cool Stars Stellar Systems and the Su

The low-mass Initial Mass Function in the young cluster NGC 6611

NGC 6611 is the massive young cluster (2-3 Myr) that ionises the Eagle Nebula. We present very deep photometric observations of the central region of NGC 6611 obtained with the Hubble Space Telescope and the following filters: ACS/WFC F775W and F850LP and NIC2 F110W and F160W, loosely equivalent to ground-based IZJH filters. This survey reaches down to I ~ 26 mag. We construct the Initial Mass Function (IMF) from ~ 1.5 Msun well into the brown dwarf regime (down to ~ 0.02 Msun). We have detected 30-35 brown dwarf candidates in this sample. The low-mass IMF is combined with a higher-mass IMF constructed from the groundbased catalogue from Oliveira et al. (2005). We compare the final IMF with those of well studied star forming regions: we find that the IMF of NGC 6611 more closely resembles that of the low-mass star forming region in Taurus than that of the more massive Orion Nebula Cluster (ONC). We conclude that there seems to be no severe environmental effect in the IMF due to the proximity of the massive stars in NGC 6611.Comment: accepted for publication in MNRAS (main journal); 18 pages, 12 figures and 3 table

Star Formation in the Eagle Nebula and NGC 6611

We present IZJHKL' photometry of the core of the cluster NGC 6611 in the Eagle Nebula. This photometry is used to constrain the Initial Mass Function (IMF) and the circumstellar disk frequency of the young stellar objects. Optical spectroscopy of 258 objects is used to confirm membership and constrain contamination as well as individual reddening estimates. Our overall aim is to assess the influence of the ionizing radiation from the massive stars on the formation and evolution of young low-mass stars and their disks. The disk frequency determined from the JHKL' colour-colour diagram suggests that the ionizing radiation from the massive stars has little effect on disk evolution (Oliveira et al. 2005). The cluster IMF seems indistinguishable from those of quieter environments; however towards lower masses the tell-tale signs of an environmental influence are expected to become more noticeable, a question we are currently addressing with our recently acquired ultra-deep (ACS and NICMOS) HST images.Comment: in "Triggered Star Formation in a Turbulent ISM", IAU symposium, poster contribution; a full version of the poster can be found at http://www.astro.keele.ac.uk/~jacco/papers/IAUS237_Eagle_2006.pd

Cool stars in NGC 2547 and pre main sequence lithium depletion

We present the results of a spectroscopic survey of X-ray selected, low-mass candidate members of the young open cluster NGC 2547. Using a combination of photometry, spectroscopic indices and radial velocities we refine our candidate list and then use our spectroscopy to study the progression of lithium depletion in low-mass pre main sequence stars. We derive lithium abundances or upper limits for all our candidate members, which have effective temperatures in the range 5000>Teff>3200K, and compare these with predictions for lithium burning and depletion provided by a number of models and also with the lithium depletion seen in younger and older stars. We find that some models can reproduce the lithium abundance pattern of NGC 2547 if the cluster has an age of ~20-35Myr, which is also indicated by fits to low-mass isochrones in the Hertzsprung-Russell diagram. But the lack of significant further lithium depletion between NGC 2547 and older clusters argues for an age of at least 50Myr, more in keeping with the lack of lithium observed in even fainter NGC 2547 candidates. We show that reconciliation of these age estimates may require additions to the physics incorporated in current generations of pre main sequence models.Comment: Accepted for publication in MNRAS (better version of Fig.1 available at http://www.astro.keele.ac.uk/~rdj/

The discovery of a low mass, pre-main-sequence stellar association around gamma Velorum

We report the serendipitous discovery of a population of low mass, pre-main sequence stars (PMS) in the direction of the Wolf-Rayet/O-star binary system gamma^{2} Vel and the Vela OB2 association. We argue that gamma^{2} Vel and the low mass stars are truly associated, are approximately coeval and that both are at distances between 360-490 pc, disagreeing at the 2 sigma level with the recent Hipparcos parallax of gamma^{2} Vel, but consistent with older distance estimates. Our results clearly have implications for the physical parameters of the gamma^{2} Vel system, but also offer an exciting opportunity to investigate the influence of high mass stars on the mass function and circumstellar disc lifetimes of their lower mass PMS siblings.Comment: Monthly Notices of the Royal Astronomical Society, Letters - in pres

Ages, Distances, and the Initial Mass Functions of Stellar Clusters

We provide a review of the current status of several topics on the ages, distances, and mass functions of open clusters, with a particular emphasis on illuminating the areas of uncertainty. Hipparcos has obtained parallaxes for nearby open clusters that have expected accuracies much better than has been previously achievable. By using the lithium depletion boundary method and isochrone fitting based on much improved new theoretical evolutionary models for low mass stars, it is arguable that we will soon have have much better age scales for clusters and star-forming regions. With improved optical and near-IR cameras, we are just now beginning to extend the mass function of open clusters like the Pleiades into the regime below the hydrogen burning mass limit. Meanwhile, observations in star-forming regions are in principle capable of identifying objects down to of order 10 Jupiter masses.Comment: 13 pages, including 3 embedded figures (4 EPS files). To appear in "11th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun," ed. R. J. Garcia Lopez, R. Rebolo, and M. R. Zapatero Osori

No evidence for intense, cold accretion onto YSOs from measurements of Li in T-Tauri stars

We have used medium resolution spectra to search for evidence that proto-stellar objects accrete at high rates during their early 'assembly phase'. Models predict that depleted lithium and reduced luminosity in T-Tauri stars are key signatures of 'cold' high-rate accretion occurring early in a star's evolution. We found no evidence in 168 stars in NGC 2264 and the Orion Nebula Cluster for strong lithium depletion through analysis of veiling corrected 6708 angstrom lithium spectral line strengths. This suggests that 'cold' accretion at high rates (M_dot > 5 x 10-4 M_sol yr-1) occurs in the assembly phase of fewer than 0.5 per cent of 0.3 < M < 1.9 M_sol stars. We also find that the dispersion in the strength of the 6708 angstrom lithium line might imply an age spread that is similar in magnitude to the apparent age spread implied by the luminosity dispersion seen in colour magnitude diagrams. Evidence for weak lithium depletion (< 10 per cent in equivalent width) that is correlated with luminosity is also apparent, but we are unable to determine whether age spreads or accretion at rates less than 5 x 10-4 M_sol yr-1 are responsible.Comment: 13 pages, 10 figures; Accepted for publication in Monthly Notices of the Royal Astronomical Society, 2013 June 0