A deep survey of brown dwarfs in Orion with Gemini

Abstract

We report the results of a deep near infrared (JHK) survey of the outer parts of the Trapezium Cluster with Gemini South/Flamingos. 396 sources were detected in a 26 arcmin 2 area, including 138 brown dwarf candidates, defined as M< 0.075M ⊙ for an assumed age of 1 Myr. Only 33 of the brown dwarf candidates are planetary mass candidates (PMCs) with estimated masses in the range 0.003 <M< 0.012M ⊙ . In an extinction limited sample (A V < 5) complete to approximately 0.005M ⊙ (5 M Jup ) the mass function appears to drop by a factor of 2 at the deuterium burning threshold, i.e. at planetary masses. After allowing for background contamination it is likely that planetary mass objects at 3-13 M Jup number < 10% of the cluster population, with an upper limit of 13%. Analysis of the spatial distribution of stars and brown dwarf candidates suggests that brown dwarfs and very low mass stars (M< 0.1M ⊙ ) are less likely than more massive stars to have wide (> 150 AU) binary companions. This result has modest statistical significance (96%) in our data but is supported at 93% confidence by analysis of an completely independent sample taken from the Subaru data of Kaifu et al.(2000). There is a statistically very significant excess of both stars and brown dwarfs with small separations from each other (< 6 arcsec or 2600 AU). This appears to be due to the presence of small N subgroups, which are likely to be dynamically unstable in the long term. Hence these results are consistent with the 'ejected stellar embryo' hypothesis for brown dwarf formation (Reipurth & Clarke 2001). We also report the discovery of two new bipolar nebulae, which are interpreted as Class I protostars.

Young, Low‐Mass Brown Dwarfs with Mid‐Infrared Excesses

“…To date, searches for planetary-mass brown dwarfs have focused on dense young stellar clusters with large extinctions in order to eliminate contamination by background objects. Several surveys have reported sources with masses possibly below 12 M J in Orion (Lucas et al 2005;Zapatero Osorio et al 2002;Lucas et al 2001), but the intrinsic faintness of these objects, combined with the large distances to the sources, makes it difficult to confirm spectroscopically the low gravity and hence young age and low mass of the objects. S Ori 70, originally thought to be a young, 3 M J object in the σ Orionis cluster (Zapatero Osorio et al 2002), may actually be an older, more massive field brown dwarf (Burgasser et al 2004).…”

A Universal Stellar Initial Mass Function? A Critical Look at Variations

“…These techniques have been applied extensively to the Orion Nebula Cluster, a valuable young cluster for star formation studies due to its proximity and richness (see review by Muench et al 2008). Numerous studies of the IMF of the Orion Nebular Cluster have been made over the past decade (Hillenbrand 1997, Luhman et al 2000, Muench et al 2000, Slesnick et al 2004, Lucas et al 2005, Weights et al 2009). These independent IMF determinations are strikingly consistent, deriving results that strongly resemble a Kroupa/Chabriertype IMF: a Salpeter-like slope in the super-solar regime that breaks below 1 M ⊙ to a broad peak (in logarithmic space) at 0.2-0.3 M ⊙ .…”

Towards precise ages and masses of Free Floating Planetary Mass Brown Dwarfs

“…The three ONC PMOs are 183−729 (18), 152−717 (27) and 137−532 (172) (Lucas & Roche 2000;Lucas, Roche, & Tamura 2005, hereafter LRT05) (the numbers in brackets are the source catalogue numbers quoted in LRT05). They were selected because each has a bright star IMAG=15−16 and RMAG=17−18 within ∼25 that could potentially be used as a tip/tilt guide star for laser-guided adaptive optics (AO) and they are bright enough for medium resolution ground based spectroscopy.…”