[Paper Review] The substellar mass function in sigma Orionis. II. Optical, near-infrared and IRAC/Spitzer photometry of young cluster brown dwarfs and planetary-mass objects

J. A. Caballero, V. J. S. Béjar+9 more|arXiv (Cornell University)|May 7, 2007

Astrophysics and Star Formation StudiesPhysics and Astronomy99 references82 citations

TL;DR

This study presents deep optical, near-infrared, and Spitzer IRAC photometry of young substellar objects in the σ Orionis cluster, identifying 30 bona fide members down to ~0.006 M☉. It finds a rising substellar mass function with α = 0.6 ± 0.2, indicating no evidence for an opacity mass-limit below 0.006 M☉, supporting a continuous formation mechanism for brown dwarfs and planetary-mass objects via cloud fragmentation.

ABSTRACT

We investigate the mass function in the substellar domain down to a few Jupiter masses in the young sigma Orionis open cluster (3+/-2 Ma, d = 360^+70_-60 pc). We have performed a deep IJ-band search, covering an area of 790 arcmin^2 close to the cluster centre. This survey was complemented with an infrared follow-up in the HKs- and Spitzer 3.6-8.0 mum-bands. Using colour-magnitude diagrams, we have selected 49 candidate cluster members in the magnitude interval 16.1 mag < I < 23.0 mag. Accounting for flux excesses at 8.0 mum and previously known spectral features of youth, 30 objects are bona fide cluster members. Four are first identified from our optical-near infrared data. Eleven have most probable masses below the deuterium burning limit and are classified as planetary-mass object candidates. The slope of the substellar mass spectrum (Delta N / Delta M = a M^-alpha) in the mass interval 0.11 Msol M < 0.006 Msol is alpha = +0.6+/-0.2. Any opacity mass-limit, if these objects form via fragmentation, may lie below 0.006 Msol. The frequency of sigma Orionis brown dwarfs with circumsubstellar discs is 47+/-15 %. The continuity in the mass function and in the frequency of discs suggests that very low-mass stars and substellar objects, even below the deuterium-burning mass limit, may share the same formation mechanism.

Motivation & Objective

• To determine the shape of the substellar mass function in the σ Orionis cluster down to a few Jupiter masses.
• To identify and characterize brown dwarfs and planetary-mass object candidates in a young, nearby open cluster.
• To assess the frequency of circumsubstellar discs around very low-mass objects to probe formation mechanisms.
• To test the existence of an opacity mass-limit for objects formed via gravitational fragmentation in molecular clouds.
• To establish whether planetary-mass objects form via the same process as low-mass stars and brown dwarfs.

Proposed method

• Conducted a deep $IJ$-band survey over 790 arcmin² centered on σ Orionis, reaching limiting magnitudes of $I \sim 24.1$ mag and $J \sim 21.8$ mag.
• Performed infrared follow-up in $H$-, $K_{\rm s}$-bands and Spitzer IRAC 3.6–8.0 μm bands to detect flux excesses indicative of discs.
• Used color-magnitude diagrams ($I$ vs. $I-J$) to select 49 candidate cluster members in the magnitude range 16.1 < $I$ < 23.0 mag.
• Applied spectroscopic features of youth and 8.0 μm flux excesses to confirm cluster membership and identify disc-bearing objects.
• Fitted the substellar mass function as a power law: $\Delta N/\Delta M \propto M^{-\alpha}$, with $\alpha$ derived from the mass interval 0.11–0.006 $M_\odot$.
• Estimated the frequency of brown dwarfs with discs using IRAC 8.0 μm excess and confirmed membership criteria.

Experimental results

Research questions

• RQ1 What is the slope of the substellar mass function in σ Orionis down to ~0.006 $M_\odot$?
• RQ2 Do planetary-mass object candidates in σ Orionis show signs of circumstellar discs, and what is the frequency of disc-bearing brown dwarfs?
• RQ3 Is there evidence for a mass cut-off in the initial mass function due to opacity-limited fragmentation in molecular clouds?
• RQ4 Do the observed properties of very low-mass objects support a continuous formation mechanism from low-mass stars to planetary-mass objects?
• RQ5 How do the photometric and spectroscopic properties of faint substellar objects in σ Orionis compare to known young brown dwarfs and isolated planetary-mass objects?

Key findings

• The substellar mass function in σ Orionis is rising, with a power-law slope of $\alpha = 0.6 \pm 0.2$ in the mass range 0.11–0.006 $M_\odot$.
• Among the 49 candidates, 30 are confirmed as bona fide cluster members based on youth indicators and IRAC 8.0 μm excesses.
• Eleven objects have most probable masses below the deuterium burning limit (0.013 $M_\odot$) and are classified as planetary-mass object candidates.
• The frequency of brown dwarfs with circumsubstellar discs is 47 ± 15%, indicating a high incidence of disc retention in the substellar domain.
• No break or cut-off in the mass function is observed down to 0.006 $M_\odot$, suggesting no evidence for an opacity mass-limit in this cluster.
• Four brown dwarfs and two planetary-mass object candidates are reported for the first time in this study.