Volume 539, March 2012
|Number of page(s)||14|
|Section||Galactic structure, stellar clusters and populations|
|Published online||24 February 2012|
Star formation in the outer Galaxy: coronal properties of NGC 1893⋆
1 INAF Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
2 European Space Agency, 8-10 rue Mario Nikis, 75015 Paris, France
3 Spitzer Science Center, Caltech 314-6, Pasadena, CA 91125, USA
4 INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
5 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
Received: 13 May 2011
Accepted: 2 December 2011
Context. The outer Galaxy, where the environmental conditions are different from the solar neighborhood, is a laboratory in which it is possible to investigate the dependence of star formation process on the environmental parameters.
Aims. We investigate the X-ray properties of NGC 1893, a young cluster (~1–2 Myr) in the outer part of the Galaxy (galactic radius ≥11 kpc) where we expect differences in the disk evolution and in the mass distribution of the stars, to explore the X-ray emission of its members and compare it with that of young stars in star-forming regions near to the Sun.
Methods. We analyze 5 deep Chandra ACIS-I observations with a total exposure time of 450 ks. Source events of the 1021 X-ray sources have been extracted with the IDL-based routine ACIS-Extract. Using spectral fitting and quantile analysis of X-ray spectra, we derive X-ray luminosities and compare the respective properties of Class II and Class III members. We also evaluate the variability of sources using the Kolmogorov-Smirnov test and identify flares in the lightcurves.
Results. The X-ray luminosity of NGC 1893 X-ray members is in the range 1029.5−1031.5 erg s-1. Diskless stars are brighter in X-rays than disk-bearing stars, given the same bolometric luminosity. We found that 34% of the 1021 lightcurves appear variable and that they show 0.16 flare per source, on average. Comparing our results with those relative to the Orion Nebula Cluster, we find that, by accounting for observational biases, the X-ray properties of NGC 1893 and the Orion ones are very similar.
Conclusions. The X-ray properties in NGC 1893 are not affected by the environment and the stellar population in the outer Galaxy may have the same coronal properties of nearby star-forming regions. The X-ray luminosity properties and the X-ray luminosity function appear to be universal and can therefore be used for estimating distances and for determining stellar properties.
Key words: stars: pre-main sequence / stars: luminosity function, mass function / X-rays: stars / stars: coronae / stars: flare / open clusters and associations: individual: NGC 1893
Full Tables 1 and 3 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/539/A74
© ESO, 2012
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