The Substellar Mass Function in σ Orionis

By V. Béjar, E. Martín, M. Z. Osorio, R. Rebolo, D. B. Navascues, Coryn A. L. Bailer-Jones, R. Mundt, Isabelle Baraffe, C. Chabrier, F. Allard. 2001

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We combine results from imaging searches for substellar objects in the σ Orionis cluster and follow-up photometric and spectroscopic observations to derive a census of the brown dwarf population in a region of 847 arcmin2. We identify 64 very low mass cluster member candidates in this region. We have available three-color (I, Z, and J) photometry for all of them, spectra for 24 objects, and K photometry for 27% of our sample. These data provide a well-defined sequence in the I versus I-J and I versus I-K color-magnitude diagrams and indicate that the cluster exhibits little reddening despite its young age (~5 Myr). Using state-of-the-art evolutionary models, we derive a mass function from the low-mass stars (0.2 M☉) across the complete brown dwarf domain (0.075 to 0.013 M☉) and into the realm of free-floating planetary-mass objects (≤0.013 M☉). We find that the mass spectrum (dN/dm) ∝ m-α increases toward lower masses, with an exponent α = 0.8 ± 0.4. Our results suggest that planetary-mass isolated objects could be as common as brown dwarfs; both kinds of objects together would be as numerous as stars in the cluster. If the distribution of stellar and substellar masses in σ Orionis is representative of the Galactic disk, older and much lower luminosity free-floating planetary-mass objects with masses down to about 0.005 M☉ should be abundant in the solar vicinity, with a density similar to M-type stars.

Subfields: Astronomy and Astrophysics

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Galactic Stellar and Substellar Initial Mass Function (2003, background): “…limit and below the deuterium-burning minimum mass, and the rising mass spectrum down to this limit in several young clusters (see Figure 5 and Najita et al. 2000, Béjar et al. 2001, Mart́ın et al 2001, Lucas et al. 2001) seem to exclude deuterium burning as a peculiar process in star formation.”
The “Mysterious” Origin of Brown Dwarfs (2002, background): “…Martı́n et al. 1998, 2001; Zapatero Osorio et al. 2000; Luhman et al. 2000; Hillenbrand & Carpenter 2000; Najita et al. 2000; Comerón et al. 2000; Luhman 2000; Moraux et al. 2001; Béjar et al. 2001; Lodieu et al. 2002) and among field stars (e.g., Kirkpatrick et al. 1999, 2000; Chabrier 2002).”
Planet formation and migration (2005, background): “Béjar et al (2001) have pointed out that if the distribution of stellar and substellar masses in σ Orionis is representative of the Galactic disk, older and much lower luminosity free–floating objects with masses down to a few MJ should be abundant in the solar vicinity, with a density similar to…”
The Initial Mass Function in the Taurus Star-forming Region (2002, methodology): “In the Ori cluster, the mass function measured by Béjar et al. (2001) has good number statistics for brown dwarfs down to 0.01 M but does not extend above 0.2 M .”
The Luminosity and Mass Function of the Trapezium Cluster: From B Stars to the Deuterium-burning Limit (2002, result): “…IMF feature has yet to be identified in the substellar regime of the well-studied IC 348 young cluster (see Najita et al. 2000), the recent work of Béjar et al. (2001) suggests a mass function for the young Orionis cluster (Walter et al. 1997) that is slowly falling throughout the brown dwarf…”
Brown dwarfs in the Pleiades cluster: Clues to the substellar mass function , (2002, result): “We used the 120 Myr isochrone from the DUSTY models of Chabrier et al. (2000) to estimate masses.”
A Universal Stellar Initial Mass Function? A critical look at variations in extreme environments (2010, background): “Some nearby regions, in particular Taurus, exhibit stellar IMFs which appear to be inconsistent with those observed in other regions (e.g. Orion, IC 348, Cham I).”
Evolutionary models for low-mass stars and brown dwarfs: uncertainties and limits at very young ages (2001, methodology): “They also offer the best chance to determine the Initial Mass Function (IMF) down to the substellar regime and the minimum mass formed by a collapse process (see Béjar et al. 2001).”
The Formation and Early Evolution of Low-Mass Stars and Brown Dwarfs (2012, background): “As an alternative to applying a temperature scale to spectral types, temperatures (and surface gravities) for young low-mass objects have been estimated by fitting absorption lines in high-resolution spectra with lines produced by model atmospheres (Mohanty, Jayawardhana & Basri 2004; Mohanty et al.”
The formation of free-floating brown dwarves and planetary-mass objects by photo-erosion of prestellar cores (2004, background): “Now they are routinely observed (e.g. McCaughrean et al. 1995; Luhman et al. 1998; Wilking et al. 1999; Luhman & Rieke 1999; Lucas & Roche 2000; Martín et al. 2000; Luhman et al. 2000; Béjar et al. 2001; Martín et al. 2001; Wilking et al. 2002; McCaughrean et al. 2002).”
The Stellar Population of the Chamaeleon I Star-forming Region (2007, background): “…& Carpenter 2000; Muench et al. 2002, 2003; Preibisch et al. 2003; Lucas et al. 2005) and spectroscopy (Hillenbrand 1997; Comerón et al. 2000; Béjar et al. 2001; Luhman et al. 2003b; Briceño et al. 2002; Barrado y Navascués et al. 2004; Luhman 2004d; Slesnick et al. 2004; Levine et…”
The formation of a star cluster: predicting the properties of stars and brown dwarfs (2002, background): “Surveys of young star clusters are beginning to probe masses down to this theoretical minimum mass (Zapatero Osorio et al. 1999; Lucas & Roche 2000; Béjar et al. 2001; Mart́in et al. 2001b; Lucas et al. 2001), with the masses of some objects estimated to be as low as 3 Jupiter-masses (Zapatero…”
A Spitzer Space Telescope Study of Disks in the Young σ Orionis Cluster (2007, methodology): “Adopting an age interval of 2Y7 Myr and the Hipparcos distance of the star Ori (352þ166 85 pc), Béjar et al. (1999, 2001, 2004) presented several lists of low-mass stars, brown dwarfs, and free-floating planets belonging to the Ori cluster, selected from optical and NIR color-magnitude diagrams.”