Bibcode
DOI
Wargelin, B. J.; Kashyap, V. L.; Drake, J. J.; García-Alvarez, D.; Ratzlaff, P. W.
Bibliographical reference
The Astrophysical Journal, Volume 676, Issue 1, pp. 610-627.
Advertised on:
3
2008
Journal
Citations
26
Refereed citations
23
Description
We present results from two Chandra imaging observations of Ross 154, a
nearby flaring M dwarf star. During a 61 ks ACIS-S exposure, a very
large flare occurred (the equivalent of a solar X3400 event, with
LX=1.8×1030 ergs s-1) in which
the count rate increased by a factor of over 100. The early phase of the
flare shows evidence for the Neupert effect, followed by a further rise
and then a two-component exponential decay. A large flare was also
observed at the end of a later 48 ks HRC-I observation. Emission from
the nonflaring phases of both observations was analyzed for evidence of
low-level flaring. From these temporal studies we find that microflaring
probably accounts for most of the ``quiescent'' emission and that,
unlike for the Sun and the handful of other stars that have been
studied, the distribution of flare intensities does not appear to follow
a power law with a single index. Analysis of the ACIS spectra, which was
complicated by exclusion of the heavily piled-up source core, suggests
that the quiescent Ne/O abundance ratio is enhanced by a factor of ~2.5
compared to the commonly adopted solar abundance ratio and that the Ne/O
ratio and overall coronal metallicity during the flare appear to be
enhanced relative to quiescent abundances. Based on the temperatures and
emission measures derived from the spectral fits, we estimate the length
scales and plasma densities in the flaring volume and also track the
evolution of the flare in color-intensity space. Lastly, we searched for
a stellar wind charge exchange X-ray halo around the star but without
success; because of the relationship between mass-loss rate and the halo
surface brightness, not even an upper limit on the stellar mass-loss
rate can be determined.