X-ray flares in Orion low-mass stars

Free access article

Issue		A&A Volume 471, Number 2, August IV 2007


Page(s)		645 - 654
Section		Stellar structure and evolution
DOI		http://dx.doi.org/10.1051/0004-6361:20077195

A&A 471, 645-654 (2007)
DOI: 10.1051/0004-6361:20077195

X-ray flares in Orion low-mass stars

M. Caramazza^{1, 2}, E. Flaccomio², G. Micela², F. Reale^{1, 2}, S. J. Wolk³, and E. D. Feigelson⁴

¹ Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, Via Archirafi 36, 90123 Palermo, Italy emailmcarama@astropa.unipa.it
² INAF Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA

(Received 30 January 2007 / Accepted 14 June 2007)

Abstract
Context.X-ray flares are common phenomena in pre-main sequence stars. Their analysis gives insights into the physics at work in young stellar coronae. The Orion Nebula Cluster offers a unique opportunity to study large samples of young low mass stars. This work is part of the Chandra Orion Ultradeep project (COUP), an ~10 day long X-ray observation of the Orion Nebula Cluster (ONC).
Aims.Our main goal is to statistically characterize the flare-like variability of 165 low mass (0.1-0.3 $M_{\odot}$ ) ONC members in order to test and constrain the physical scenario in which flares explain all the observed emission.
Methods.We adopt a maximum likelihood piece-wise representation of the observed X-ray light curves and detect flares by taking into account both the amplitude and time derivative of the count-rate. We then derive the frequency and energy distribution of the flares.
Results.The high energy tail of the energy distribution of flares is well described by a power-law with index ~2.2. We test the hypothesis that light curves are built entirely by overlapping flares with a single power law energy distribution. We constrain the parameters of this simple model for every single light curve. The analysis of synthetic light curves obtained from the model indicates a good agreement with the observed data.

Comparing low mass stars with stars in the mass interval (0.9-1.2 $M_{\odot}$ ), we establish that, at ~1 Myr, low mass and solar mass stars of similar X-ray luminosity have very similar flare frequencies.
Conclusions.Our observational results are consistent with the following model/scenario: the light curves are entirely built by overlapping flares with a power-law intensity distribution; the intense flares are individually detected, while the weak ones merge and form a pseudo-quiescent level, which we indicate as the characteristic level.

Key words: stars: activity -- stars: coronae -- stars: flare -- stars: pre-main sequence -- stars: late-type -- X-ray: stars

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.