X-ray variability of PMS stars - Toward an explanation of the different X-ray properties of CTTS and WTTS. (arXiv:1210.6770v1 [astro-ph.SR])

X-ray variability of PMS stars - Toward an explanation of the different X-ray properties of CTTS and WTTS. (arXiv:1210.6770v1 [astro-ph.SR]):
The intense X-ray emission from coronae and accretion shocks in young PMS
stars is likely to play an important role in the evolution and dispersal of
circumstellar disks. Several aspects of the physics of this X-ray emission
remain mysterious, e.g., whether and how much accretion affects coronal
emission.

We studied the X-ray variability of ~1 Myr old low-mass PMS stars as a
function of timescale, stellar rotation, and stellar characteristics, in order
to gain insights on the working of PMS coronae, their X-ray emission, and the
circumstellar environment in which they are immersed.

We have exploited the ~850 ksec long Chandra observation of the Orion Nebula
Cluster obtained by the COUP collaboration in Jan. 2003, and statistically
analyzed the X-ray lightcurves of low-mass stars in several subsamples. In
particular, we characterized the different X-ray behavior of stars with and
without circumstellar accretion disks.

Accreting stars (Classical T Tauri Stars, CTTSs) are found to be more
variable than non accreting ones (Weak-lined T Tauri Stars, WTTSs) at all
timescales and in all the X-ray energy bands considered. Variability is seen to
increase with time-scale up to $\sim$10 days, i.e. the longest probed.
Signatures of rotational modulation are observed for both CTTSs and WTTSs, and
most clearly for CTTSs in the soft X-ray band. Lower mass stars are more
variable than higher mass ones.

We propose that the difference in variability between CTTSs and WTTSs may be
explained assuming that the X-ray emission of CTTS is affected by time-variable
absorption due circumstellar structures, such as warps in the inner disk and/or
accretion streams. This suggestion is appealing because, in the hypothesis that
the coronae of CTTSs and WTTSs are similar, it may also explain why CTTSs have
lower and more scattered X-ray emission levels with respect to WTTSs.