Rotation/saturation/dynamos -- Poster Presentation
What are the Drivers of X-ray Production in Pre-Main-Sequence
Stars
Keivan Stassun, Vanderbilt University
We present an analysis of Chandra observations of the Orion
Nebula Cluster (ONC) to study the X-ray properties of a large sample of
pre-main-sequence (PMS) stars with optically determined rotation periods.
Our goal is to elucidate the origins of X-rays in PMS stars by seeking
out connections between the X-rays and the mechanisms most likely
driving their production---rotation and accretion. We find that these
stars have L$_x$/L$_{bol}$ near, but below, the ``saturation'' value of
10$^{-3}$, and that X-ray luminosity is significantly correlated with
stellar rotation, in the sense of decreasing L$_x$/L$_{bol}$ with more
rapid rotation. These findings suggest that stars with optical rotation
periods are in the ``super-saturated'' regime of the rotation-activity
relationship, consistent with their Rossby numbers. However, we also
find that stars with optical rotation periods are significantly biased
to high L$_x$. This is not the result of magnitude bias in the optical
rotation-period sample, but rather of the diminishingly small amplitude
of optical variations in stars with low L$_x$. Evidently, there exists
in the ONC a population of stars whose rotation periods are unknown and
that possess lower average X-ray luminosities than those of stars with
known rotation periods. These stars may sample the linear regime of the
rotation-activity relationship. Accretion also manifests itself in
X-rays, though in a somewhat counterintuitive fashion: while stars with
spectroscopic signatures of accretion show harder X-ray spectra than
nonaccretors, they show lower X-ray luminosities and no enhancement of
X-ray variability. We interpret these findings in terms of a common
origin for the X-ray emission observed from both accreting and nonaccreting
stars, with the X-rays from accreting stars simply being attenuated by
magnetospheric accretion columns. We also present results from a simultaneous
optical/X-ray monitoring study of variability in the ONC, where we find
very little evidence for correlated variability, indicating that X-ray
events are not temporally related to accretion events. These findings
suggest that X-rays from PMS stars have their origins primarily in
chromospheres/coronae, not accretion.