Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
T. Preibisch and H. Zinnecker:
X-ray properties of the young stellar and
substellar objects in the IC 348 cluster - the CHANDRA View
Astronomical Journal 123, 1613-1628 (2002)
Abstract.
We explore the X-ray properties of the young stellar and substellar
objects in the open cluster IC 348 as seen in our deep CHANDRA X-Ray
Observatory
ACIS image. First, we give identifications of all X-ray sources and
determine upper limits for the X-ray luminosities of the undetected
cluster members. Then,
we analyse the X-ray spectra of the young stellar objects, deriving
plasma temperatures between ~0.7 keV and ~3 keV for the T Tauri stars
in IC 348, and
higher temperatures between ~3 keV and ~7 keV for flaring sources and
two embedded young stellar objects. We find several large X-ray flares,
in some of
which a clear hardening of the X-ray spectra during the flare peak is
seen. Next, we use the exceptional optical, infrared, and X-ray data
set of this cluster to study various correlations and their
implications, and to discuss new
answers to some long-standing questions related to X-ray emission from
young (sub-) stellar objects. The X-ray luminosities of the young
low-mass stars
are strongly correlated to the stellar bolometric luminosities (L_X ~
10^-4 x L_bol). Also, a good correlation between X-ray luminosity and
stellar mass is
found (L_X propto M^2). For the weak line T Tauri stars, we find a
tight correlation between X-ray activity and chromospheric activity
(L_X propto
L_Halpha^0.8), supporting the hypothesis that the chromosphere is
heated by X-rays from the overlying corona. The observed X-ray
properties of the brown dwarfs (and brown dwarf candidates) are very
similar to those of late-type stars; we explain this behaviour as
the consequence of the fact that very young substellar objects are
still warm enough to maintain partially ionized atmospheres which are
capable of sustaining
electrical currents, while in the cooler neutral atmospheres of L and T
dwarfs such currents are shut off (hence no X-ray emission). Finally,
we explore the difference between the X-ray luminosity functions of
classical and weak-line T Tauri stars. We find that the classical T
Tauri stars in
IC 348 seem to be on average less X-ray luminous than the weak-line T
Tauri stars. However, we suggest that this apparent difference is
caused by a
selection effect: there is a strong detection bias against those
weak-line T Tauri stars which are optically faint and hence X-ray
faint; the population of
classical T Tauri stars, on the other hand, is essentially completely
known due to its very prominent Halpha emission. This conclusion is
corroborated by
another new result: When using a photometrically selected, magnitude
limited, complete sample of T Tauri stars and taking the K-L infrared
excess as a tracer
of circumstellar material, we find no evidence in IC 348 for a
difference in X-ray properties of young stars with and without
circumstellar matter; i.e. classical
and ``naked'' T Tauri stars.
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