91 research outputs found
Is the Wind of the Galactic Oe Star HD 155806 Magnetically Confined?
Spectropolarimetric observations of HD 155806 - the hottest Galactic Oe star
- were obtained with CFHT/ESPaDOnS to test the hypothesis that disk signatures
in its spectrum are due to magnetic channeling and confinement of its stellar
wind. We did not detect a dipole field of sufficient strength to confine the
wind, and could not confirm previous reports of a magnetic detection. It
appears that stellar magnetism is not responsible for producing the disk of HD
155806.Comment: 2 pages, 1 figure; to appear in the proceedings of IAU Symposium 27
A Revised Geometry for the Magnetic Wind of theta^1 Orionis C
Theta^1 Ori is thought to be a hot analog of Bp variables because its optical
and UV line and X-ray continuum fluxes modulate regularly over the
magnetic/rotational period. A flattened magnetosphere surrounding co-rotates
with these stars, producing a periodic modulation of emission and absorption
components of the UV resonance lines, as well as of optical H and He lines. In
this paper we examine these modulations in detail and point out that the
far-blue and near-red wings of C IV and N V resonance lines exhibit
anticorrelated modulations, causing mild flux elevations at moderate redshifts
at edge-on phase (phi=0.5). However, the lines do not exhibit rest-frame
absorption features, the usual signatures of cool static disks surrounding Bp
stars. We suggest that this behavior can be explained by the existence of two
geometrically distinct wind regions separated by the local magnetic Alfven
radius. Wind streams emerging outside this point are forced outward by
radiative forces and eventually expand outward radially to infinity - this
matter produces the far-blue wing absorptions at phi=0.5. Interior streams
follow closed loops and collide at the magnetic equator with counterstreams.
There they coalesce and fall back to the star along their original field lines
- these are responsible for mild emissions at this same phase. The rapid
circulation of the interior wind component back to the star is responsible for
the absence of static disk features.Comment: 7 figure
The changing UV and X-ray properties of the Of?p star CPD -28 2561
The Of?p star CPD -28 2561 was monitored at high energies with XMM-Newton and
HST. In X-rays, this magnetic oblique rotator displays bright and hard emission
that varies by ~55% with rotational phase. These changes occur in phase with
optical variations, as expected for magnetically confined winds; there are two
maxima and two minima in X-rays during the 73d rotational period of CPD -28
2561. However, contrary to previously studied cases, no significant hardness
variation is detected between minima and maxima, with the exception of the
second minimum which is slightly distinct from the first one. In the UV domain,
broad-band fluxes remain stable while line profiles display large variations.
Stronger absorptions at low velocities are observed when the magnetic equator
is seen edge-on, which can be reproduced by a detailed 3D model. However, a
difference in absorption at high velocities in the CIV and NV lines is also
detected for the two phases where the confined wind is seen nearly pole-on.
This suggests the presence of strong asymmetries about the magnetic equator,
mostly in the free-flowing wind (rather than in the confined dynamical
magnetosphere).Comment: 14 pages, 11 figures, accepted for publication by MNRA
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Bringing high-grade arteriovenous malformations under control: clinical outcomes following multimodality treatment in children.
OBJECTIVE:Brain arteriovenous malformations (AVMs) consist of dysplastic blood vessels with direct arteriovenous shunts that can hemorrhage spontaneously. In children, a higher lifetime hemorrhage risk must be balanced with treatment-related morbidity. The authors describe a collaborative, multimodal strategy resulting in effective and safe treatment of pediatric AVMs. METHODS:A retrospective analysis of a prospectively maintained database was performed in children with treated and nontreated pediatric AVMs at the University of California, San Francisco, from 1998 to 2017. Inclusion criteria were age ≤ 18 years at time of diagnosis and an AVM confirmed by a catheter angiogram. RESULTS:The authors evaluated 189 pediatric patients with AVMs over the study period, including 119 ruptured (63%) and 70 unruptured (37%) AVMs. The mean age at diagnosis was 11.6 ± 4.3 years. With respect to Spetzler-Martin (SM) grade, there were 38 (20.1%) grade I, 40 (21.2%) grade II, 62 (32.8%) grade III, 40 (21.2%) grade IV, and 9 (4.8%) grade V lesions. Six patients were managed conservatively, and 183 patients underwent treatment, including 120 resections, 82 stereotactic radiosurgery (SRS), and 37 endovascular embolizations. Forty-four of 49 (89.8%) high-grade AVMs (SM grade IV or V) were treated. Multiple treatment modalities were used in 29.5% of low-grade and 27.3% of high-grade AVMs. Complete angiographic obliteration was obtained in 73.4% of low-grade lesions (SM grade I-III) and in 45.2% of high-grade lesions. A periprocedural stroke occurred in a single patient (0.5%), and there was 1 treatment-related death. The mean clinical follow-up for the cohort was 4.1 ± 4.6 years, and 96.6% and 84.3% of patients neurologically improved or remained unchanged in the ruptured and unruptured AVM groups following treatment, respectively. There were 16 bleeding events following initiation of AVM treatment (annual rate: 0.02 events per person-year). CONCLUSIONS:Coordinated multidisciplinary evaluation and individualized planning can result in safe and effective treatment of children with AVMs. In particular, it is possible to treat the majority of high-grade AVMs with an acceptable safety profile. Judicious use of multimodality therapy should be limited to appropriately selected patients after thorough team-based discussions to avoid additive morbidity. Future multicenter studies are required to better design predictive models to aid with patient selection for multimodal pediatric care, especially with high-grade AVMs
Measuring mass-loss rates and constraining shock physics using X-ray line profiles of O stars from the Chandra archive
We quantitatively investigate the extent of wind absorption signatures in the X-ray grating spectra of all non-magnetic, effectively single O stars in the Chandra archive via line profile fitting. Under the usual assumption of a spherically symmetric wind with embedded shocks, we confirm previous claims that some objects show little or no wind absorption. However, many other objects do show asymmetric and blueshifted line profiles, indicative of wind absorption. For these stars, we are able to derive wind mass-loss rates from the ensemble of line profiles, and find values lower by an average factor of 3 than those predicted by current theoretical models, and consistent with Hα if clumping factors of fcl ≈ 20 are assumed. The same profile fitting indicates an onset radius of X-rays typically at r ≈ 1.5R*, and terminal velocities for the X-ray emitting wind component that are consistent with that of the bulk wind. We explore the likelihood that the stars in the sample that do not show significant wind absorption signatures in their line profiles have at least some X-ray emission that arises from colliding wind shocks with a close binary companion. The one clear exception is ζ Oph, a weak-wind star that appears to simply have a very low mass-loss rate. We also reanalyse the results from the canonical O supergiant ζ Pup, using a solar-metallicity wind opacity model and find M^˙=1.8×10−6 M_ ⊙yr^−1, consistent with recent multiwavelength determinations
Chandra X-ray spectroscopy of the very early O supergiant HD 93129A: constraints on wind shocks and the mass-loss rate
We present analysis of both the resolved X-ray emission line profiles and the
broadband X-ray spectrum of the O2 If* star HD 93129A, measured with the
Chandra HETGS. This star is among the earliest and most massive stars in the
Galaxy, and provides a test of the embedded wind shock scenario in a very dense
and powerful wind. A major new result is that continuum absorption by the dense
wind is the primary cause of the hardness of the observed X-ray spectrum, while
intrinsically hard emission from colliding wind shocks contributes less than
10% of the X-ray flux. We find results consistent with the predictions of
numerical simulations of the line-driving instability, including line
broadening indicating an onset radius of X-ray emission of several tenths
Rstar. Helium-like forbidden-to-intercombination line ratios are consistent
with this onset radius, and inconsistent with being formed in a wind-collision
interface with the star's closest visual companion at a distance of ~100 AU.
The broadband X-ray spectrum is fit with a dominant emission temperature of
just kT = 0.6 keV along with significant wind absorption. The broadband wind
absorption and the line profiles provide two independent measurements of the
wind mass-loss rate: Mdot = 5.2_{-1.5}^{+1.8} \times 10^{-6} Msun/yr and Mdot =
6.8_{-2.2}^{+2.8} \times 10^{-6} Msun/yr, respectively. This is the first
consistent modeling of the X-ray line profile shapes and broadband X-ray
spectral energy distribution in a massive star, and represents a reduction of a
factor of 3 to 4 compared to the standard H-alpha mass-loss rate that assumes a
smooth wind.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society. 12 pages, 10 figures (incl. 5 color
A FUSE Survey of Interstellar Molecular Hydrogen in the Small and Large Magellanic Clouds
We describe a moderate-resolution FUSE survey of H2 along 70 sight lines to
the Small and Large Magellanic Clouds, using hot stars as background sources.
FUSE spectra of 67% of observed Magellanic Cloud sources (52% of LMC and 92% of
SMC) exhibit absorption lines from the H2 Lyman and Werner bands between 912
and 1120 A. Our survey is sensitive to N(H2) >= 10^14 cm^-2; the highest column
densities are log N(H2) = 19.9 in the LMC and 20.6 in the SMC. We find reduced
H2 abundances in the Magellanic Clouds relative to the Milky Way, with average
molecular fractions = 0.010 (+0.005, -0.002) for the SMC and =
0.012 (+0.006, -0.003) for the LMC, compared with = 0.095 for the
Galactic disk over a similar range of reddening. The dominant uncertainty in
this measurement results from the systematic differences between 21 cm radio
emission and Lya in pencil-beam sight lines as measures of N(HI). These results
imply that the diffuse H2 masses of the LMC and SMC are 8 x 10^6 Msun and 2 x
10^6 Msun, respectively, 2% and 0.5% of the H I masses derived from 21 cm
emission measurements. The LMC and SMC abundance patterns can be reproduced in
ensembles of model clouds with a reduced H2 formation rate coefficient, R ~ 3 x
10^-18 cm^3 s^-1, and incident radiation fields ranging from 10 - 100 times the
Galactic mean value. We find that these high-radiation, low-formation-rate
models can also explain the enhanced N(4)/N(2) and N(5)/N(3) rotational
excitation ratios in the Clouds. We use H2 column densities in low rotational
states (J = 0 and 1) to derive a mean kinetic and/or rotational temperature
= 82 +/- 21 K for clouds with N(H2) >= 10^16 cm^-2, similar to Galactic
gas. We discuss the implications of this work for theories of star formation in
low-metallicity environments. [Abstract abridged]Comment: 30 pages emulateapj, 14 figures (7 color), 7 tables, accepted for
publication in the Astrophysical Journal, figures 11 and 12 compressed at
slight loss of quality, see http://casa.colorado.edu/~tumlinso/h2/ for full
version
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