1,894 research outputs found
Ionic charge distributions of energetic particles from solar flares
The effects which solar flare X-rays have on the charge states of solar cosmic rays is determined quantitatively. Rather than to characterize the charge distribution by temperature alone, it is proposed that the X-ray flux at the acceleration site also is used. The effects of flare X-rays are modeled mathematically
A search for outflows from X-ray bright points in coronal holes
Properties of X-ray bright points using two of the instruments on Solar Maximum Mission were investigated. The mass outflows from magnetic regions were modeled using a two dimensional MHD code. It was concluded that mass can be detected from X-ray bright points provided that the magnetic topology is favorable
Phase-dependent X-ray observations of the beta Lyrae system: No eclipse in the soft band
We report on observations of the eclipsing and interacting binary beta Lyrae
from the Suzaku X-ray telescope. This system involves an early B star embedded
in an optically and geometrically thick disk that is siphoning atmospheric
gases from a less massive late B II companion. Motivated by an unpublished
X-ray spectrum from the Einstein X-ray telescope suggesting unusually hard
emission, we obtained time with Suzaku for pointings at three different phases
within a single orbit. From the XIS detectors, the softer X-ray emission
appears typical of an early-type star. What is surprising is the remarkably
unchanging character of this emission, both in luminosity and in spectral
shape, despite the highly asymmetric geometry of the system. We see no eclipse
effect below 10 keV. The constancy of the soft emission is plausibly related to
the wind of the embedded B star and Thomson scattering of X-rays in the system,
although it might be due to extended shock structures arising near the
accretion disk as a result of the unusually high mass-transfer rate. There is
some evidence from the PIN instrument for hard emission in the 10-60 keV range.
Follow-up observations with the RXTE satellite will confirm this preliminary
detection.Comment: to appear in A&A Letter
X-ray Emission from Magnetically Torqued Disks of Oe/Be Stars
We focus attention on the Oe/Be stars to test the concept that the disks of
these stars form by magnetic channeling of wind material toward the equator.
Calculations are made of the X-rays expected from the Magnetically Torqued Disk
(MTD) model for Be stars discussed by Cassinelli et al. (2002), by Maheswaran
(2003), and by Brown et al. (2004). The dominant parameters in the model are
the value of the velocity law, the rotation rate of the star, ,
and the ratio of the magnetic field energy density to the disk gravitational
energy density, .
The model predictions are compared with the observations obtained for
an O9.5 star Oph from \Berghofer et al. (1996) and for 7 Be stars from
Cohen et al. (1997). Extra considerations are also given here to the well
studied Oe star Oph for which we have observations of the
X-ray line profiles of the triad of He-like lines from the ion Mg XI.Comment: 28 pages with 6 figures. Accepted for publication in Ap
New Challenges For Wind Shock Models: The Chandra Spectrum Of The Hot Star Delta Orionis
The Chandra spectrum of delta Ori A shows emission lines from hydrogen- and helium-like states of Si, Mg, Ne, and O, along with N VII Lyalpha and lines from ions in the range Fe XVII-Fe XXI In contrast to the broad lines seen in zeta Pup and zeta Ori (850 +/- 40 and 1000 +/- 240 km s(-1) half-width at half-maximum [HWHM], respectively), these lines are broadened to only 430 +/- 60 km s(-1) HWHM. This is much lower than the measured wind terminal velocity of 2000 km s(-1). The forbidden, intercombination, and resonance (fir) lines from He-like ions indicate that the majority of the X-ray line emission does not originate at the base of the wind, in agreement with the standard wind shock models for these objects. However, in that model the X-ray emission is distributed throughout an expanding, X-ray-absorbing wind, and it is therefore surprising that the emission lines appear relatively narrow, unshifted, and symmetric. We compare the observed line profiles to recent detailed models for X-ray line pro le generation in hot stars, but none of them offers a fully satisfactory explanation for the observed line profiles
Collisionless Damping of Fast MHD Waves in Magneto-rotational Winds
We propose collisionless damping of fast MHD waves as an important mechanism
for the heating and acceleration of winds from rotating stars. Stellar rotation
causes magnetic field lines anchored at the surface to form a spiral pattern
and magneto-rotational winds can be driven. If the structure is a magnetically
dominated, fast MHD waves generated at the surface can propagate almost
radially outward and cross the field lines. The propagating waves undergo
collisionless damping owing to interactions with particles surfing on magnetic
mirrors that are formed by the waves themselves. The damping is especially
effective where the angle between the wave propagation and the field lines
becomes moderately large ( to ). The angle tends naturally
to increase into this range because the field in magneto-rotational winds
develops an increasingly large azimuthal component. The dissipation of the wave
energy produces heating and acceleration of the outflow. We show using
specified wind structures that this damping process can be important in both
solar-type stars and massive stars that have moderately large rotation rates.
This mechanism can play a role in coronae of young solar-type stars which are
rapidly rotating and show X-ray luminosities much larger than the sun. The
mechanism could also be important for producing the extended X-ray emitting
regions inferred to exist in massive stars of spectral type middle B and later.Comment: 12 pages, including 7 figures, accepted for publication in Ap
X-ray emission from the double-binary OB-star system QZ Car (HD 93206)
X-ray observations of the double-binary OB-star system QZ Car (HD 93206)
obtained with the Chandra X-ray Observatory over a period of roughly 2 years
are presented. The orbit of systems A (O9.7 I+b2 v, PA = 21 d) and B (O8 III+o9
v, PB = 6 d) are reasonably well sampled by the observations, allowing the
origin of the X-ray emission to be examined in detail. The X-ray spectra can be
well fitted by an attenuated three temperature thermal plasma model,
characterised by cool, moderate, and hot plasma components at kT ~ 0.2, 0.7,
and 2 keV, respectively, and a circumstellar absorption of ~ 0.2 x 10^22 cm-2.
Although the hot plasma component could be indicating the presence of wind-wind
collision shocks in the system, the model fluxes calculated from spectral fits,
with an average value of ~ 7 x 10^-13 erg s-1 cm-2, do not show a clear
correlation with the orbits of the two constituent binaries. A semi-analytical
model of QZ Car reveals that a stable momentum balance may not be established
in either system A or B. Yet, despite this, system B is expected to produce an
observed X-ray flux well in excess of the observations. If one considers the
wind of the O8 III star to be disrupted by mass transfer the model and
observations are in far better agreement, which lends support to the previous
suggestion of mass-transfer in the O8 III + o9 v binary. We conclude that the
X-ray emission from QZ Car can be reasonably well accounted for by a
combination of contributions mainly from the single stars and the mutual
wind-wind collision between systems A and B.Comment: 11 pages, 7 figures. Accepted for the ApJS Special Issue on the
Chandra Carina Complex Project (CCCP), scheduled for publication in May 2011.
All 16 CCCP Special Issue papers are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at
leas
An Extensive Collection of Stellar Wind X-ray Source Region Emission Line Parameters,Temperatures, Velocities, and Their Radial Distributions as Obtained from Chandra Observations of 17 OB Stars
Chandra high energy resolution observations have now been obtained from
numerous non-peculiar O and early B stars. The observed X-ray emission line
properties differ from pre-launch predictions, and the interpretations are
still problematic. We present a straightforward analysis of a broad collection
of OB stellar line profile data to search for morphological trends. X-ray line
emission parameters and the spatial distributions of derived quantities are
examined with respect to luminosity class. The X-ray source locations and their
corresponding temperatures are extracted by using the He-like f/i line ratios
and the H-like to He-like line ratios respectively. Our luminosity class study
reveals line widths increasing with luminosity. Although the majority of the OB
emission lines are found to be symmetric, with little central line
displacement, there is evidence for small, but finite, blue-ward line-shifts
that also increase with luminosity. The spatial X-ray temperature distributions
indicate that the highest temperatures occur near the star and steadily
decrease outward. This trend is most pronounced in the OB supergiants. For the
lower density wind stars, both high and low X-ray source temperatures exist
near the star. However, we find no evidence of any high temperature X-ray
emission in the outer wind regions for any OB star. Since the temperature
distributions are counter to basic shock model predictions, we call this the
"near-star high-ion problem" for OB stars. By invoking the traditional OB
stellar mass loss rates, we find a good correlation between the fir-inferred
radii and their associated X-ray continuum optical depth unity radii. We
conclude by presenting some possible explanations to the X-ray source problems
that have been revealed by this study.Comment: Published in 2007, ApJ, 668, 456. An Erratum scheduled for
publication in 2008, ApJ, 680, is included as an Appendix. The Erratum
corrects some tabulated data in 5 tables and 2 figure
- …