A Constraint on brown dwarf formation via ejection: radial variation of the stellar and substellar mass function of the young open cluster IC2391

Abstract

We present the stellar and substellar mass function of the open cluster IC2391, plus its radial dependence, and use this to put constraints on the formation mechanism of brown dwarfs. Our multiband optical and infrared photometric survey with spectroscopic follow-up covers 11 square degrees, making it the largest survey of this cluster to date. We observe a radial variation in the mass function over the range 0.072 to 0.3Msol, but no significant variation in the mass function below the substellar boundary at the three cluster radius intervals analyzed. This lack of radial variation for low masses is what we would expect with the ejection scenario for brown dwarf formation, although considering that IC2391 has an age about three times older than its crossing time, we expect that brown dwarfs with a velocity greater than the escape velocity have already escaped the cluster. Alternatively, the variation in the mass function of the stellar objects could be an indication that they have undergone mass segregation via dynamical evolution. We also observe a significant variation across the cluster in the colour of the (background) field star locus in colour-magnitude diagrams and conclude that this is due to variable background extinction in the Galactic plane. From our preliminary spectroscopic follow-up to confirm brown dwarf status and cluster membership, we find that all candidates are M dwarfs (in either the field or the cluster), demonstrating the efficiency of our photometric selection method in avoiding contaminants (e.g. red giants). About half of our photometric candidates for which we have spectra are spectroscopically-confirmed as cluster members; two are new spectroscopically-confirmed brown dwarf members of IC2391.

57 pages, 20 figures, accepted for publication in ApJ

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• The low-mass Initial Mass Function in the Orion Nebula cluster based on HST/NICMOS III imaging
• Brown dwarfs and very low mass stars in the Praesepe open cluster: a dynamically unevolved mass function?
• The Prototypical Young L/T-Transition Dwarf HD 203030B Likely Has Planetary Mass
• The kinematical and space structures of IC 2391 open cluster and moving group with Gaia-DR2
• The substellar mass function in the central region of the open cluster Praesepe from deep LBT observations