Bibcode
Lodieu, N.; Zapatero Osorio, M. R.; Rebolo, R.; Martín, E. L.; Hambly, N. C.
Bibliographical reference
Astronomy and Astrophysics, Volume 505, Issue 3, 2009, pp.1115-1127
Advertised on:
10
2009
Journal
Citations
59
Refereed citations
45
Description
Context: The knowledge of the initial mass function (IMF) in open
clusters constitutes one way of constraining the formation of low-mass
stars and brown dwarfs, along with the frequency of multiple systems and
the properties of disks. Aims: The aim of the project is to
determine the shape of the mass function in the low-mass and substellar
regimes in the σ Orionis cluster (~3 Myr, ~352 pc, solar
metallicity) as accurately as possible and compare it with the results
in other clusters. Methods: We have analysed the near-infrared
photometric data from the fourth data release (DR4) of the UKIRT
Infrared Deep Sky Suvey (UKIDSS) Galactic clusters survey (GCS) to
derive the cluster luminosity and mass functions, evaluate the extent of
the cluster, and study the distribution and variability of low-mass
stars and brown dwarfs down to the deuterium-burning limit. Results: We have recovered most of the previously published members
and found a total of 287 candidate members within the central 30 arcmin
in the 0.5-0.009 M&sun; mass range, including new objects not
previously reported in the literature. This new catalogue represents a
homogeneous dataset of brown dwarf member candidates over the central 30
arcmin of the cluster. The expected photometric contamination by field
objects with similar magnitudes and colours to σ Orionis members
is ~15%. We present evidence of variability at the 99.5% confidence
level over ~yearly timescales in 10 member candidates that exhibit signs
of youth and the presence of disks. The level of variability is low
(≤0.3 mag) and does not impact the derivation of the cluster
luminosity and mass functions. Furthermore, we find a possible dearth of
brown dwarfs within the central five arcmin of the cluster, which is not
caused by a lower level of photometric sensitivity around the massive,
O-type multiple star σ Ori in the GCS survey. Using
state-of-the-art theoretical models, we derived the luminosity and mass
functions within the central 30 arcmin from the cluster centre, with
completeness down to J = 19 mag, corresponding to masses ranging from
0.5 M&sun; down to the deuterium-burning mass boundary
(~0.013 M&sun;). Conclusions: The mass function of
σ Orionis in this mass interval shows a power law index α =
0.5±0.2 (when expressed as dN/dM ∝ M-α),
which agrees with the one derived for the 3-5 Myr cluster Upper Sco
(based on similar data obtained with the GCS) in the same mass range.
This work is based in part on data obtained as part of the UKIRT
Infrared Deep Sky Survey. The United Kingdom Infrared Telescope is
operated by the Joint Astronomy Centre on behalf of the Science and
Technology Facilities Council of the UK.
Appendices are only available at http://www.aanda.org
Related projects
Very Low Mass Stars, Brown Dwarfs and Planets
Our goal is to study the processes that lead to the formation of low mass stars, brown dwarfs and planets and to characterize the physical properties of these objects in various evolutionary stages. Low mass stars and brown dwarfs are likely the most numerous type of objects in our Galaxy but due to their low intrinsic luminosity they are not so
Rafael
Rebolo López