A Chandra X-ray detection of the L dwarf binary Kelu-1. Simultaneous Chandra and Very Large Array observations
Context: Magnetic activity in ultracool dwarfs, as measured in X-rays and Hα, 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 LX = 2.9-1.3+1.8 × 1025 erg s-1, while it remained undetected in the radio down to a 3 σ limit of LR ≤ 1.4 × 1013 erg s-1 Hz-1. We argue that, whereas the X-ray and Hα 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.
Astronomy and Astrophysics
- Pub Date:
- September 2007
- radio continuum: stars;
- stars: activity;
- stars: coronae;
- stars: individual: Kelu-1;
- stars: low-mass;
- brown dwarfs;
- X-rays: stars;
- Accepted for publication as a Letter in Astronomy &