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تسجيل مستخدم جديدThe substellar population of Sigma Orionis: A deep wide survey
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We present a deep I,Z photometric survey covering a total area of 1.12 deg^{2} of the Sigma Orionis cluster (Icompl=22 and Zcompl=21.5mag). From I, I-Z color-magnitude diagrams we have selected 153 candidates that fit the previously known sequence of the cluster. Using J-band photometry, we find that 124 of the 151 candidates follow the previously known infrared photometric sequence of the cluster and are probably members. We have studied the spatial distribution of these candidates and found that there are objects located at distances greater than 30 arcmin to the north and west of Sigma Orionis that probably belong to different populations of the Orions Belt. For the 102 bona fide Sigma Orionis cluster member candidates, we find that the radial surface density can be represented by a decreasing exponential function (sigma = sigma_0 e^{-r/r_0}) with a central density of sigma_0=0.23+/-0.03 object/arcmin^{2} and a characteristic radius of r_0=9.5+/-0.7 arcmin. From a statistical comparison with Monte Carlo simulations, we conclude that the spatial distribution of the cluster member candidates is compatible with a Poissonian distribution and, hence, they are not mainly forming aggregations or sub-clustering. Using near-infrared JHK-band data from 2MASS and UKIDSS and mid-infrared data from IRAC/Spitzer, we find that 5-9 % of the brown dwarf candidates in the Sigma Orionis cluster have K-band excesses and 31+/-7 % of them show mid-infrared excesses at wavelengths longer than 5.8 microns, which are probably related to the presence of disks. We have also calculated the initial mass spectrum (dN/dm) of Sigma Orionis from very low mass stars (0.10 Msol) to the deuterium-burning mass limit (0.012-0.013 Msol). This is a rising function toward lower masses and can be represented by a power-law distribution (dN/dm = m^{-alpha}) with an exponent alpha of 0.7+/-0.3 for an age of 3 Myr.
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VLT/FORS spectroscopy and 2MASS near-infrared photometry, together with previously known data, have been used to establish the membership and the properties of a sample of low-mass candidate members of the sigma Orionis cluster with masses spanning f
rom 1 Msun down to about 0.013 Msun (i.e., deuterium-burning mass limit). We have observed K-band infrared excess and remarkably intense H(alpha) emission in various cluster members, which, in addition to the previously detected forbidden emision lines and the presence of LiI in absorption at 6708 A, have allowed us to tentatively classify sigma Orionis members as classical or weak-line TTauri stars and substellar analogs. Variability of the H(alpha) line has been investigated and detected in some objects. Based on the K-band infrared excesses and the intensity of H(alpha) emission, we estimate that the minimum disk frequency of the sigma Orionis low-mass population is in the range 5-12%.
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th angular separations between 0.4 and 4.0 arcsec. We search for companions to 331 catalogued cluster stellar members and candidates in public K-band UKIDSS images outside the innermost 1 arcmin, which is affected by the glare of the bright, eponymous sigma Ori multiple system, and investigate their cluster membership with colour-magnitude diagrams and previous knowledge of youth features. Of the 18 identified pairs, ten have very low individual probabilities of chance alignment (< 1 %) and are considered here as physical pairs. Four of them are new, while the other six had been discovered previously, but never investigated homogeneously and in detail. Projected physical separations and magnitude differences of the ten probably bound pairs range from 180 to 1220 au, and from 0.0 to 3.4 mag in K, respectively. Besides, we identify two cluster stars with elongated point spread functions. We determine the minimum frequency of wide multiplicity in the interval of projected physical separations s = 160-1600 au in sigma Orionis at 3.0^{+1.2}_{-1.1} %. We discover a new Lindroos system, find that massive and X-ray stars tend to be in pairs or trios, conclude that multiplicity truncates circumstellar discs and enhances X-ray emission, and ascribe a reported lithium depletion in a young star to unresolved binarity in spectra of moderate resolution. When accounting for all know multiples, including spectroscopic binaries, the minimum frequency of multiplicity increases to about 10 %, which implies that of the order of 80-100 unknown multiple systems still await discovery in sigma Orionis.
By collecting optical and infrared photometry and low resolution spectroscopy, we have identified a large number of low mass stars and brown dwarf candidates belonging to the young cluster (~5 Myr) associated with the binary star lambda Orionis. The
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