Transcript DoAr21_AAS2005 - Astronomy at Swarthmore College

High-Resolution X-ray Spectroscopy of the Accreting Weak-Line T Tauri Star DoAr 21
Victoria Swisher, Eric L. N. Jensen, David H. Cohen (Swarthmore College), and Marc Gagné (West Chester University)
X-rays from Pre-Main-Sequence (PMS) stars
New Chandra grating (HETGS) spectroscopy
PMS stars have stronger and harder x-ray emission than cool mainsequence stars. They also exhibit significant x-ray time variability. Is PMS
x-ray activity similar in origin to solar-type x-ray activity (alpha-omega
dynamo driven)? If so, why is it stronger? Is it connected to accretion?
94 ks observation in May 2003. The spectrum (at right)
is very hard – dominated by bremsstrahlung continuum,
but emission lines from some highly ionized species are
also seen (up to helium-like iron, Fe XXV).
DoAr 21 (V2246 Oph) in the ρ
Oph star forming cloud, is a
Weak-lined T Tauri Star (WTTS),
having shown H-alpha in emission in
the 1950s but without H-alpha
emission reported subsequently and
without a strong IR excess. It is only
about 1 Myr old (left) and has
already been identified, via lowresolution x-ray spectroscopy, to
have strong, hard, and variable x-ray
emission (Imanishi et al. 2002).
One of the few PMS stars that is x-ray bright enough to produce a highquality grating spectrum, DoAr 21 is perhaps the youngest PMS star
observed with the Chandra gratings. In terms of its disk properties, it
is an intermediate case between the CTTS TW Hya, which has x-ray
properties that have been interpreted in terms of accretion (Kastner et al.
2001) and HD 98800, which is a naked T Tauri star that has x-ray properties
similar to main-sequence stars (Kastner et al. 2003).
The SED (above) shows a modest
IR excess with a PAH feature
(inset) near 11 microns. Recently,
evidence has been presented for
circumstellar molecular hydrogen
emission (Bary et al. 2003).
New high-resolution spectroscopy: We
obtained H-alpha data showing broad
emission wings and night-to-night
variability. We also measured v sin i ~ 80
km/s.
There seem to be several indicators of
circumstellar material including at
least modest accretion.
MEG
Global spectral modeling produces a good fit with
a two-temperature thermal plasma (APEC): kT =
1.5 keV, 5.6 keV; abundances ~ 0.3 solar. ISM
absorption consistent with extinction.
HEG
High-resolution x-ray diagnostics
Chandra grating spectra provide several
diagnostics of the physical properties of hot
plasma that might be useful for discriminating
among theories of PMS x-ray production.
The strongest line in
the spectrum – Si
Lyman-alpha (left) –
shows modest, but
statistically
significant
broadening (FWHM
= 460 +/- 120
km/s). Widths for
the strongest lines
are summarized at
left.
Forbidden-to-intercombination line
intensity ratios in He-like ions are
diagnostics of density. Kastner et al.
(2002) find small f/i ratios in the
CTTS TW Hya for O and Ne, indicating
high densities (~ 1013 cm-3). The
naked T Tauri star HD 98800 shows
large f/i ratios, indicating low
densities. Our Chandra observations
of DoAr 21 show intermediate f/i
ratios for Si (right) and S. The
implied densities are more in-line
with the high values seen in TW Hya,
but our results have large
uncertainties.
A large x-ray flare (below) was seen
in our Chandra observation. The x-ray
flux increased by a factor of three and
the emission hardened during the flare.
Spectral modeling showed that the hard
component nearly doubled in
temperature (from kT = 2.8 keV to kT =
5.8 keV).
R I
F
Copies of this poster are available at astro.swarthmore.edu/~cohen/projects/doar21/