The σ Orionis Cluster (original) (raw)

Stars and brown dwarfs in the σ Orionis cluster

Astronomy and Astrophysics, 2010

Aims. I attempt to fully understand the origin of the stellar and substellar populations in the young σ Orionis open cluster, which is a benchmark for star-forming studies. Because of the very low proper motion of the cluster, late-type dwarfs with appreciable proper motion in the foreground of σ Orionis can be easily discarded as targets from expensive spectroscopic follow-up studies. Methods. I use the Aladin sky atlas, USNO-B1, public astrometric catalogues, and photographic plate digitisations to identify stars with proper motions that are inconsistent with cluster membership in a circular area of radius 30 arcmin centred on the early-type multiple system σ Ori. Primarily because of the long time baseline of more than half a century, the errors in the measured proper motions are lower than 2 mas a −1 . Results. Of the 42 stars selected for astrometric follow-up, 37 of them are proper-motion cluster interlopers. Some USNO-B1 measurements were affected by partially resolved (visual) multiplicity and target faintness. Because of their late spectral types and, hence, red colours, 24 contaminants had been considered at some point as σ Orionis members. I discuss how contamination may have affected previous work (especially related to disc frequencies) and the curious presence of lithium absorption in three M-dwarf proper motion contaminants. Finally, I classify the bright star HD 294297 as a late-F field dwarf unrelated to the cluster based on a new proper motion measurement. Conclusions. Although proper motions cannot be used to confirm membership in σ Orionis, they can be instead used to discard a number of cluster member candidates without spectroscopy.

The Substellar Population of Σ Orionis: A Deep Wide Survey

The Astrophysical Journal, 2011

We present a deep I,Z photometric survey covering a total area of 1.12 deg 2 of the σ Orionis cluster and reaching completeness magnitudes of I=22 and Z=21.5 mag. From I, I − Z color-magnitude diagrams we have selected 153 candidates that fit the previously known sequence of the cluster. They have magnitudes in the range I=16-23 mag, which corresponds to a mass interval from 0.1 down to 0.008 M ⊙ at the most probable age of σ Orionis (2-4 Myr). Using J-band photometry, we find that 124 of the 151 candidates within the completeness of the optical survey (82 %) follow the previously known infrared photometric sequence of the cluster and are probably members. We have studied the spatial distribution of the very low mass stars and brown dwarf population of the cluster and found that there are objects located at distances greater than 30 arcmin to the north and west of σ Orionis that probably belong to different populations of the Orion's Belt. For the 102 bona fide σ Orionis cluster member candidates, we find that the radial surface density can be represented by a decreasing exponential function (σ = σ 0 e −r/r0) with a central density of σ 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 objects located at the same distance from the center of the cluster is compatible with a Poissonian distribution and, hence, that very low mass stars and brown dwarfs 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 about 5-9 % of the brown dwarf candidates in the σ Orionis cluster have K-band excesses and 31±7 % of them show mid-infrared excesses at wavelengths longer than 5.8 µ m. These are probably related to the presence of disks, most of which are "transition disks". We have also calculated the initial mass spectrum (dN/dm) of σ Orionis from very low mass stars (∼ 0.10 M ⊙) to the deuterium-burning mass limit (0.012-0.013 M ⊙), i.e., complete in the entire brown dwarf regime. This mass spectrum is a rising function toward lower masses and can be represented by a power-law distribution (dN/dm ∝ m −α) with an exponent α of 0.7± 0.3 for an age of 3 Myr.

A near-infrared/optical/X-ray survey in the centre of sigma Orionis

Astron Nachr, 2007

Because of the intense brightness of the OB-type multiple star system σ Ori, the low-mass stellar and substellar populations close to the centre of the very young σ Orionis cluster is poorly know. I present an IJHK_s survey in the cluster centre, able to detect from the massive early-type stars down to cluster members below the deuterium burning mass limit. The near-infrared and optical data have been complemented with X-ray imaging. Ten objects have been found for the first time to display high-energy emission. Previously known stars with clear spectroscopic youth indicators and/or X-ray emission define a clear sequence in the I vs. I-K_s diagram. I have found six new candidate cluster members that follow this sequence. One of them, in the magnitude interval of the brown dwarfs in the cluster, displays X-ray emission and a very red J-K_s colour, indicative of a disc. Other three low-mass stars have excesses in the K_s band as well. The frequency of X-ray emitters in the area is 80±20 %. The spatial density of stars is very high, of up to 1.6±0.1 arcmin-2. There is no indication of lower abundance of substellar objects in the cluster centre. Finally, I also report two cluster stars with X-ray emission located at only 8000-11000 AU to σ Ori AB, two sources with peculiar colours and an object with X-ray emission and near-infrared magnitudes similar to those of previously-known substellar objects in the cluster.

Search and characterization of T-type planetary mass candidates in the σ Orionis cluster

Proceedings of the International Astronomical Union, 2010

Context. The proper characterization of the least massive population of the young σ Orionis star cluster is required to understand the form of the cluster mass function and its impact on our comprehension of the substellar formation processes. S Ori 70 (T5.5 ± 1) and 73, two T-type cluster member candidates, would have likely masses between 3 and 7 M Jup if their age is 3 Myr. S Ori 73 awaits confirmation of its methane atmosphere. Aims. We aim to: i) confirm the presence of methane absorption in S Ori 73 through methane imaging; ii) study S Ori 70 and 73 cluster membership via photometric colors and accurate proper motion analysis; iii) perform a new search to identify additional T-type σ Orionis member candidates. Methods. We obtained HAWK-I (VLT) J, H, and CH 4off photometry of an area of 119.15 arcmin 2 in σ Orionis down to J comp = 21.7 and H comp = 21 mag. S Ori 70 and 73 are contained in the explored area. Near-infrared data were complemented with optical photometry using images acquired with OSIRIS (GTC) and VISTA as part of the VISTA Orion survey. Color-magnitude and color-color diagrams were constructed to characterize S Ori 70 and 73 photometrically, and to identify new objects with methane absorption and masses below 7 M Jup . We derived proper motions by comparison of the new HAWK-I and VISTA images with published near-infrared data taken 3.4 -7.9 yr ago. Results. S Ori 73 has a red H − CH 4off color indicating methane absorption in the H-band and a spectral type of T4 ± 1. S Ori 70 displays a redder methane color than S Ori 73 in agreement with its latter spectral classification. Our proper motion measurements (µ α cos δ = 26.7 ± 6.1, µ δ = 21.3 ± 6.1 mas yr −1 for S Ori 70, and µ α cos δ = 46.7 ± 4.9, µ δ = −6.3 ± 4.7 mas yr −1 for S Ori 73) are larger than the motion of σ Orionis, rendering S Ori 70 and 73 cluster membership uncertain. From our survey, we identified one new photometric candidate with J = 21.69 ± 0.12 mag and methane color consistent with spectral type ≥ T8. Conclusions. S Ori 73 has colors similar to those of T3-T5 field dwarfs, which in addition to its high proper motion suggests that it is probably a field dwarf located at 170-200 pc. The origin of S Ori 70 remains unclear: it can be a field, foreground mid-to late-T free-floating dwarf with peculiar colors, or an orphan planet ejected through strong dynamical interactions from σ Orionis or from a nearby star-forming region in Orion.

Theσ Orionis substellar population

Astronomy & Astrophysics, 2003

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 σ Orionis cluster with masses spanning from 1 M down to about 0.013 M (i.e., deuterium-burning mass limit). We have observed K-band infrared excess and remarkably intense Hα emission in various cluster members, which, in addition to the previously detected forbidden emision lines and the presence of Li  in absorption at 6708 Å, have allowed us to tentatively classify σ Orionis members as classical or weak-line T Tauri stars and substellar analogs. Variability of the Hα line has been investigated and detected in some objects. Based on the K-band infrared excesses and the intensity of Hα emission, we estimate that the minimum disk frequency of the σ Orionis low-mass population is in the range 5-12%.

Search and characterization of T-type planetary mass candidates in the sigma Orionis cluster

Astronomy & Astrophysics, 2011

A near-infrared/optical/X-ray survey in the centre ofσ Orionis

Astronomische Nachrichten, 2007

Key words open clusters and associations: individual: (σ Orionis) -stars: pre-main sequence -stars: low mass, brown dwarfs -planetary systems -X-rays: stars Because of the intense brightness of the OB-type multiple star system σ Ori, the low-mass stellar and substellar populations close to the centre of the very young σ Orionis cluster is poorly know. I present an IJHKs survey in the cluster centre, able to detect from the massive early-type stars down to cluster members below the deuterium burning mass limit. The near-infrared and optical data have been complemented with X-ray imaging. Ten objects have been found for the first time to display high-energy emission. Previously known stars with clear spectroscopic youth indicators and/or X-ray emission define a clear sequence in the I vs. I − Ks diagram. I have found six new candidate cluster members that follow this sequence. One of them, in the magnitude interval of the brown dwarfs in the cluster, displays X-ray emission and a very red J − Ks colour, indicative of a disc. Other three low-mass stars have excesses in the Ks band as well. The frequency of X-ray emitters in the area is 80±20 %. The spatial density of stars is very high, of up to 1.6±0.1 arcmin −2 . There is no indication of lower abundance of substellar objects in the cluster centre. Finally, I also report two cluster stars with X-ray emission located at only 8000-11000 AU to σ Ori AB, two sources with peculiar colours and an object with X-ray emission and near-infrared magnitudes similar to those of previously-known substellar objects in the cluster.

Young Stars and Their Circumstellar Disks in the σ Orionis Cluster

Open Issues in Local Star Formation, 2003

The σ Orionis cluster is a young association evolving under the disruptive influence of its massive O-star namesake. We are analysing this cluster as part of a program to characterise the influence of O-stars on the early stages of stellar evolution. At an age of approximately 4 Myr, this cluster is at a crucial stage in terms of disk evolution and therefore it is a key case to better constrain disk dissipation timescales. We have obtained RI photometry and optical spectroscopy of the σ Ori cluster; we have analysed the Li i and Na i features to establish cluster membership. We have thus gathered a unique sample of spectroscopically confirmed low-mass cluster members and brown dwarfs. Disk frequencies from K-band excesses from 2MASS suggest that less than 7% of the very low-mass σ Ori members have disks (Oliveira et al. 2002), in stark contrast with even younger clusters (e.g. Trapezium). However, near-infrared disk frequencies have to be taken with caution. We are currently undertaking an L-band (imaging) and mid-infrared (imaging and spectroscopy) program to identify and probe the properties of circumstellar disks around young stars in this cluster. Preliminary results indicate that at least 30% of cluster members have circumstellar disks.

Photometric Identification of the Low-Mass Population of Orion OB1b. I. The σ Orionis Cluster

The Astronomical Journal, 2004

We report an optical photometric survey of 0.89 deg 2 of the Orion OB1b association centered on σ Ori. This region includes most of the σ Ori cluster, the highest density region within Orion OB1b. We have developed a statistical procedure to identify the young, low-mass, pre-main sequence population of the association. We estimate that the cluster has ∼160 members in the mass range (0.2≤M≤1.0 M ⊙ ). The cluster has a radius of ∼3-5 pc and an estimated age of 2.5±0.3 Myrs. We estimate that the total mass of the cluster is 225±30 M ⊙ . This mass is similar to the estimated mass of the ∼5×10 5 year old cluster NGC 2024. NGC 2024 and σ Ori appear to be a well matched pair of clusters, except for the ∼2 Myr difference in their ages.

The σ Orionis Cluster (original) (raw)

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