Despite cool effective temperatures and largely neutral photospheres, stars at the bottom of the main- sequence have strong magnetic dynamos. Non-radiative heating processes transfer energy from the magnetic field into
the stellar atmosphere, powering both steady-state emission from X-rays to radio, as well as energetic flares that contribute to the transient population. Although this magnetic heating (or activity) has been observed for decades, the exact mechanisms that control magnetic activity in M dwarfs are still not well understood. In this talk, I will present the results from two of the projects
I have been involved in to understanding the magnetic activity at the bottom of the Main Sequence.
In the first part of my talk, I will present the the detection and an
investigation of
high-energy emission in a transiting super-Earth host system,
GJ 1214, based on an XMM-Newton observations. We derive
an X-ray luminosity of 25.86 erg/s and a corresponding activity level of
log(LX/Lbol )∼ -5.3. Stellar activity can produce large amounts of high-energy
radiation, which is absorbed by the planetary atmosphere leading to
irradiation-driven mass-loss. Assuming that GJ 1214 b formed as a rocky planet,
we estimate that it evaporates at a rate of 1E10 g/s and has lost a
total of ~2-5.6 Earth masses.
In the second part of my talk, I will present our ongoing work on a
systematic and
detailed study of coronal X-ray emission from the bright young pre-main-sequence
RS CVn system–HD 155555, using Chandra HETG/MEG gratings. The RS CVn binaries
are a class of tidally locked, rapidly rotating close binaries
typically composed
of a chromospherically active G- or K-type star with a late-type subgiant or
main-sequence companion. HD 155555 system is a triple star system composed of
a G5 IV, K0 IV and M3V star. The G (HD 155555 A) and K stars (HD 155555 B)
form a short-period binary system with an orbital period of 1.68 days,
that has an M dwarf companion HD 155555 C 33'' away. Both HD 155555 AB and
HD 155555 C are strong X-ray sources with log LX of 30.54 and 29.30 ergs/s,
respectively. We have analysed the dispersed spectra of both HD 155555 AB and
HD 155555 C using discrete temperature APEC models, and have constructed the
emission measure distribution (EMD) using spectral line analysis
(using the ATOMDB 2.0).
We have also obtained the elemental abundances and discuss whether or not
they exhibit the Inverse FIP effect typically seen in active stars.
From an analysis of the He-like triplets of Si XIII, Mg XII and Ne IX,
we have derived coronal electron densities of ~1E10 cm−3. Finally, we compare
our results for HD 155555 with those of other RS CVn systems, such as AR Lac,
a much older binary system, and with TW Hya, which is a somewhat younger single
pre-main sequence star.