Issue |
A&A
Volume 521, October 2010
|
|
---|---|---|
Article Number | A45 | |
Number of page(s) | 23 | |
Section | Catalogs and data | |
DOI | https://doi.org/10.1051/0004-6361/201014861 | |
Published online | 19 October 2010 |
HRC-I/Chandra X-ray observations towards σ Orionis
1
Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir km 4,
28850 Torrejón de Ardoz, Madrid, Spain e-mail: caballero@cab.inta-csic.es
2
Departamento de Astrofísica y Ciencias de la Atmósfera,
Facultad de Física, Universidad Complutense de Madrid, 28040
Madrid, Spain
3
Centro Universitario Regional Zona Atlántica, Universidad Nacional del
Comahue, Monseñor Esandi y Ayacucho, 8500 Viedma, Río Negro, Argentina
Received:
25
April
2010
Accepted:
11
June
2010
Aims. We investigated the X-ray emission from young stars and brown dwarfs in the σ Orionis cluster (τ ~ 3 Ma, d ~ 385 pc) and its relation to mass, the presence of circumstellar discs, and separation to the cluster centre by taking advantage of the superb spatial resolution of the Chandra X-ray Observatory.
Methods. We used public HRC-I/Chandra data from a 97.6 ks pointing towards the cluster centre and complemented them with X-ray data from IPC/Einstein, HRI/ROSAT, EPIC/XMM-Newton, and ACIS-S/Chandra together with optical and infrared photometry and spectroscopy from the literature and public catalogues. On our HRC-I/Chandra data, we measured count rates, estimated X-ray fluxes, and searched for short-term variability. We also looked for long-term variability by comparing with previous X-ray observations.
Results. Among the 107 detected X-ray sources, there were 70 cluster stars with known
signposts of youth, two young brown dwarfs, 12 cluster member candidates, four
field dwarfs, and two galaxies with optical-infrared counterpart.
The remaining sources were extragalactic.
Based on a robust Poisson-χ2 analysis, nine cluster stars displayed flares
or rotational modulation during the HRC-I observations, while eight other
stars and one brown dwarf showed X-ray flux variations between the HRC-I and
IPC, HRI, and EPIC epochs.
We constructed a cluster X-ray luminosity function from O9.5 (about
18 ) to M6.5 (about 0.06
).
We found: (i) that early-type stars in multiple systems or with
spectroscopic peculiarities tend to display X-ray emission; (ii) that the two
detected brown dwarfs and the least-massive star are among the σ Orionis
objects with the highest LX/LJ ratios; and (iii) that a large fraction of
known classical T Tauri stars in the cluster are absent in this and other X-ray
surveys.
Finally, from a spatial distribution analysis, we quantified the impact of
sensitivity degradation towards the HRC-I borders on the detection of faint
X-ray sources and concluded that dozens X-ray σ Orionis stars and brown
dwarfs still need to be detected.
Key words: brown dwarfs / stars: early-type / stars: flare / stars: variables: T Tauri, Herbig Ae/Be / X-rays: stars / open clusters and associations: individual: σ Orionis
© ESO, 2010
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