A Chandra X-ray detection of the L dwarf binary Kelu-1

Free access article

Issue		A&A Volume 471, Number 3, September I 2007


Page(s)		L63 - L66
Section		Letters
DOI		http://dx.doi.org/10.1051/0004-6361:20078093

A&A 471, L63-L66 (2007)
DOI: 10.1051/0004-6361:20078093

Letter

Simultaneous Chandra and Very Large Array observations

M. Audard^{1, 2}, R. A. Osten³, A. Brown⁴, K. R. Briggs⁵, M. Güdel⁵, E. Hodges-Kluck^{3, 4}, and J. E. Gizis⁶

¹ ISDC, Ch. d'Ecogia 16, 1290 Versoix, Switzerland
e-mail: Marc.Audard@obs.unige.ch
² Observatoire de Genève, University of Geneva, Ch. des Maillettes 51, 1290 Sauverny, Switzerland
³ Astronomy Department, University of Maryland, College Park, MD, USA
⁴ Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO 80309-0389, USA
⁵ Paul Scherrer Institut, Villigen & Würenlingen, 5232 Villigen PSI, Switzerland
⁶ Department of Physics and Astronomy, 223 Sharp Lab, University of Delaware, Newark, DE 19716, USA

(Received 15 June 2007 / Accepted 6 July 2007)

Abstract
Context.Magnetic activity in ultracool dwarfs, as measured in X-rays and H $\alpha$ , shows a steep decline after spectral type M7-M8. So far, no L dwarf has been detected in X-rays. In contrast, L dwarfs may have higher radio activity than M dwarfs.
Aims.We observe L and T dwarfs simultaneously in X-rays and radio to determine their level of magnetic activity in the context of the general decline of magnetic activity with cooler effective temperatures.
Methods.The field L dwarf binary Kelu-1 was observed simultaneously with Chandra and the Very Large Array.
Results.Kelu-1AB was detected in X-rays with $L_{\rm X} = 2.9_{-1.3}^{+1.8}$ $\times$ 10²⁵ erg s^-1, while it remained undetected in the radio down to a $3 \sigma$ limit of $L_{\rm R} \leq 1.4$ $\times$ 10¹³ erg s^-1 Hz^-1. We argue that, whereas the X-ray and H $\alpha$ emissions decline in ultracool dwarfs with decreasing effective temperature, the radio luminosity stays (more or less) constant across M and early-L dwarfs. The radio surface flux or the luminosity may better trace magnetic activity in ultracool dwarfs than the ratio of the luminosity to the bolometric luminosity.
Conclusions.Deeper radio observations (and at short frequencies) are required to determine if and when the cut-off in radio activity occurs in L and T dwarfs, and what kind of emission mechanism takes place in ultracool dwarfs.

Key words: radio continuum: stars -- stars: activity -- stars: coronae -- stars: individual: Kelu-1 -- stars: low-mass, brown dwarfs -- X-rays: stars

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