989 research outputs found
On the origin of continuum and line emission in CTTSs
We calculated profiles of CIV 1550, Si IV 1400, NV 1240 and OVI 1035 doublet
lines using results of 3D MHD simulations of disc accretion onto young stars
with dipole magnetic field. It appeared that our calculations can not reproduce
profiles of these lines observed (HST/GHRS-STIS and FUSE) in CTTSs's spectra.
We also found that the theory predicts much larger C IV 1550 line flux than
observed (up to two orders of magnitude in some cases) and argue that the main
portion of accretion energy in CTTSs is liberated outside accretion shock. We
conclude that the reason of disagreement between the theory and observation is
strongly non-dipole character of CTTS's magnetic field near its surface.Comment: 9 pages, 5 figure, 1 table, to appear in the proceedings of IAU
Symposium 243 "Star-Disk Interaction in Young Stars" (Grenoble, France,
May/2007
2MASS 1315-2649: A High Proper Motion L Dwarf with Strong H-alpha Emission
In Hall (2002) I reported that 2MASSI J1315309-264951 is an L dwarf with
strong H-alpha emission. Two spectroscopic epochs appeared to show that the
H-alpha was variable, decreasing from 121 Angstroms to 25 Angstroms EW, which I
interpreted as a flare during the first observation. Gizis (2002) independently
discovered this object, and his intermediate spectroscopic epoch shows H-alpha
with 97 Angstroms EW. A new fourth epoch of spectroscopy again shows a very
large H-alpha EW (124 Angstroms), confirming this object to be a persistent,
strong H-alpha emitter. Whether the H-alpha is steady (like 2MASS 1237+6526) or
from continuous strong flaring (like PC0025+0447) remains unclear. Imaging
confirms that 2MASS 1315-2649 has a high proper motion (0.71"/year),
corresponding to a transverse velocity of ~76 km/s at its distance of ~23 pc.
Thus 2MASS 1315-2649 is consistent with being >~2 Gyr old and therefore
relatively massive. If that is so, the correlation of H-alph activity with mass
found by Gizis et al. (2000) would seem to support the continuous strong
flaring scenario, though it does not rule out a brown dwarf binary accretion
scenario.Comment: 2 pages, ApJL accepte
Chandra Reveals Variable Multi-Component X-ray Emission from FU Orionis
FU Orionis is the prototype of a class of eruptive young stars (``FUors'')
characterized by strong optical outbursts. We recently completed an exploratory
survey of FUors using XMM-Newton to determine their X-ray properties, about
which little was previously known. The prototype FU Ori and V1735 Cyg were
detected. The X-ray spectrum of FU Ori was found to be unusual, consisting of a
cool moderately-absorbed component plus a hotter component viewed through an
absorption column density that is an order of magnitude higher. We present here
a sensitive (99 ks) follow-up X-ray observation of FU Ori obtained at higher
angular resolution with Chandra ACIS-S. The unusual multi-component spectrum is
confirmed. The hot component is centered on FU Ori and dominates the emission
above 2 keV. It is variable (a signature of magnetic activity) and is probably
coronal emission originating close to FU Ori's surface viewed through cool gas
in FU Ori's strong wind or accretion stream. In contrast, the X-ray centroid of
the soft emission below 2 keV is offset 0.20 arcsec to the southeast of FU Ori,
toward the near-IR companion (FU Ori S). This offset amounts to slightly less
than half the separation between the two stars. The most likely explanation for
the offset is that the companion contributes significantly to the softer X-ray
emission below 2 keV (and weakly above 2 keV). The superimposed X-ray
contributions from FU Ori and the companion resolve the paradox posed by
XMM-Newton of an apparently single X-ray source viewed through two different
absorption columns.Comment: 21 pages, 3 tables, 6 figure
Interpretation of the Veiling of the Photospheric Spectrum for T Tauri Stars in Terms of an Accretion Model
The problem on heating the atmospheres of T Tauri stars by radiation from an
accretion shock has been solved. The structure and radiation spectrum of the
emerging so-called hot spot have been calculated in the LTE approximation. The
emission not only in continuum but also in lines has been taken into account
for the first time when calculating the spot spectrum. Comparison with
observations has shown that the strongest of these lines manifest themselves as
narrow components of helium and metal emission lines, while the weaker ones
decrease significantly the depth of photospheric absorption lines, although
until now, this effect has been thought to be due to the emission continuum
alone. The veiling by lines changes the depth of different photospheric lines
to a very different degree even within a narrow spectral range. Therefore, the
nonmonotonic wavelength dependence of the degree of veiling r found for some
CTTS does not suggest a nontrivial spectral energy distribution of the veiling
continuum. In general, it makes sense to specify the degree of veiling r only
by providing the set of photospheric lines from which this quantity was
determined. We show that taking into account the contribution of lines to the
veiling of the photospheric spectrum can cause the existing estimates of the
accretion rate onto T Tauri stars to decrease by several times, with this being
also true for stars with a comparatively weakly veiled spectrum. Neglecting the
contribution of lines to the veiling can also lead to appreciable errors in
determining the effective temperature, interstellar extinction, radial
velocity, and vsin(i)
Accelerated X-ray Structure Elucidation of a 36 kDa Muramidase/Transglycosylase Using wARP
The X-ray structure of the 36kDa soluble lytic transglycosylase from Escherichia coli has been determined starting with the multiple isomorphous replacement method with inclusion of anomalous scattering at 2.7 Å resolution. Subsequently, before any model building was carried out, phases were extended to 1.7 Å, resolution with the weighted automated refinement procedure wARP, which gave a dramatic improvement in the phases. The electron-density maps from wARP were of outstanding quality for both the main chain and the side chains of the protein, which allowed the time spent on the tracing, interpretation and building of the X-ray structure to be substantially shortened. The structure of the soluble lyric transglycosylase was refined at 1.7 Å, resolution with X-PLOR to a final crystallographic R factor of 18.9%. Analysis of the wARP procedure revealed that the use of the maximum-likelihood refinement in wARP gave much better phases than least-squares refinement, provided that the ratio of reflections to protein atom parameters was approximately 1.8 or higher. Furthermore, setting aside 5% of the data for an Rfree test set had a negative effect on the phase improvement. The mean WwARP, a weight determined at the end of the wARP procedure and based on the variance of structure factors from six individually refined wARP models, proved to be a better indicator than the Rfree factor to judge different phase improvement protocols. The elongated Slt35 structure has three domains named the alpha, beta and core domains. The alpha domain contains mainly α-helices, while the beta domain consists of a five-stranded antiparallel β-sheet flanked by a short α-helix. Sandwiched between the alpha and beta domains is the core domain, which bears some resemblance to the fold of the catalytic domain of the previously elucidated 70 kDa soluble lytic transglycosylase from E. coli. The putative active site is at the bottom of a large deep groove in the core domain.
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