3,397 research outputs found
The yellow hypergiants HR 8752 and rho Cassiopeiae near the evolutionary border of instability
High-resolution near-ultraviolet spectra of the yellow hypergiants HR 8752
and rho Cassiopeiae indicate high effective temperatures placing both stars
near the T_eff border of the ``yellow evolutionary void''. At present, the
temperature of HR 8752 is higher than ever. For this star we found
Teff=7900+-200 K, whereas rho Cassiopeiae has Teff=7300+-200 K. Both, HR 8752
and rho Cassiopeiae have developed strong stellar winds with Vinf ~ 120 km/s
and Vinf ~ 100 km/s, respectively. For HR 8752 we estimate an upper limit for
the spherically symmetric mass-loss of 6.7X10^{-6}M_solar/yr. Over the past
decades two yellow hypergiants appear to have approached an evolutionary phase,
which has never been observed before. We present the first spectroscopic
evidence of the blueward motion of a cool super/hypergiant on the HR diagram.Comment: 13 pages including 3 figures. Accepted for publication in ApJ Letter
Inclusive Quasi-Elastic Charged-Current Neutrino-Nucleus Reactions
The Quasi-Elastic (QE) contribution of the nuclear inclusive electron
scattering model developed in Nucl. Phys. A627 (1997) 543 is extended to the
study of electroweak Charged Current (CC) induced nuclear reactions, at
intermediate energies of interest for future neutrino oscillation experiments.
The model accounts for, among other nuclear effects, long range nuclear (RPA)
correlations, Final State Interaction (FSI) and Coulomb corrections.
Predictions for the inclusive muon capture in C and the reaction
C near threshold are also given. RPA correlations are
shown to play a crucial role and their inclusion leads to one of the best
existing simultaneous description of both processes, with accuracies of the
order of 10-15% per cent for the muon capture rate and even better for the LSND
measurement.Comment: 31 pages and 14 figures, accepted for publication as a regular
article in Physical Review
On the universal X-ray luminosity function of binary X-ray sources in galaxies
The empirically determined universal power-law shape of X-ray luminosity
function of high mass X-ray binaries in galaxies is explained by fundamental
mass-luminosity and mass-radius relations for massive stars.Comment: 4 pages, plain LaTeX, no figures. Submitted to Astronomy Letter
X-Ray Evidence for Flare Density Variations and Continual Chromospheric Evaporation in Proxima Centauri
Using the XMM-Newton X-ray observatory to monitor the nearest star to the
Sun, Proxima Centauri, we recorded the weakest X-ray flares on a magnetically
active star ever observed. Correlated X-ray and optical variability provide
strong support for coronal energy and mass supply by a nearly continuous
sequence of rapid explosive energy releases. Variable emission line fluxes were
observed in the He-like triplets of OVII and NeIX during a giant flare. They
give direct X-ray evidence for density variations, implying densities between
2x10^{10} - 4x10^{11} cm^{-3} and providing estimates of the mass and the
volume of the line-emitting plasma. We discuss the data in the context of the
chromospheric evaporation scenario.Comment: 10 pages, 2 figures, accepted by The Astrophysical Journal, Letters;
improved calculations of radiative loss of cool plasma (toward end of paper
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
Crossing the `Yellow Void' -- Spatially Resolved Spectroscopy of the Post- Red Supergiant IRC+10420 and Its Circumstellar Ejecta
IRC +10420 is one of the extreme hypergiant stars that define the empirical
upper luminosity boundary in the HR diagram. During their post--RSG evolution,
these massive stars enter a temperature range (6000-9000 K) of increased
dynamical instability, high mass loss, and increasing opacity, a
semi--forbidden region, that de Jager and his collaborators have called the
`yellow void'. We report HST/STIS spatially resolved spectroscopy of IRC +10420
and its reflection nebula with some surprising results. Long slit spectroscopy
of the reflected spectrum allows us to effectively view the star from different
directions. Measurements of the double--peaked Halpha emission profile show a
uniform outflow of gas in a nearly spherical distribution, contrary to previous
models with an equatorial disk or bipolar outflow. Based on the temperature and
mass loss rate estimates that are usually quoted for this object, the wind is
optically thick to the continuum at some and possibly all wavelengths.
Consequently the observed variations in apparent spectral type and inferred
temperature are changes in the wind and do not necessarily mean that the
underlying stellar radius and interior structure are evolving on such a short
timescale. To explain the evidence for simultaneous outflow and infall of
material near the star, we propose a `rain' model in which blobs of gas
condense in regions of lowered opacity outside the dense wind. With the
apparent warming of its wind, the recent appearance of strong emission, and a
decline in the mass loss rate, IRC +10420 may be about to shed its opaque wind,
cross the `yellow void', and emerge as a hotter star.Comment: To appear in the Astronomical Journal, August 200
The composition and nature of the dust shell surrounding the binary AFGL 4106
We present infrared spectroscopy and imaging of AFGL~4106. The 2.4-5 micron
ISO-SWS spectrum reveals the presence of a cool, luminous star (T_eff ~ 3750 K)
in addition to an almost equally luminous F star (T_eff ~ 7250 K). The 5-195
micron SWS and LWS spectra are dominated by strong emission from circumstellar
dust. We find that the dust consists of amorphous silicates, with a minor but
significant contribution from crystalline silicates. The amorphous silicates
consist of Fe-rich olivines. The presence of amorphous pyroxenes cannot be
excluded but if present they contain much less Fe than the amorphous olivines.
Comparison with laboratory data shows that the pure Mg-end members of the
crystalline olivine and pyroxene solid solution series are present. In
addition, we find strong evidence for simple oxides (FeO and Al2O3) as well as
crystalline H2O ice. Several narrow emission features remain unidentified.
Modelling of the dust emission using a dust radiation transfer code shows that
large grains (~1 micron) must be present and that the abundance of the
crystalline silicates is between 7 and 15% of the total dust mass, depending on
the assumed enstatite to forsterite ratio, which is estimated to be between 1
and 3. The amorphous and crystalline dust components in the shell do not have
the same temperature, implying that the different dust species are not
thermally coupled. We find a dust mass of ~3.9 x 10^-2 M_sol expelled over a
period of 4 x 10^3 years for a distance of 3.3 kpc. The F-star in the AFGL~4106
binary is likely a post-red-supergiant in transition to a blue supergiant or WR
phase.Comment: 22 pages (including 12 figures), accepted by Astronomy and
Astrophysic
Recent Outbursts from the Transient X-Ray Pulsar Cep X-4 (GS 2138+56)
We report on X-ray observations of the 66 s period transient X-ray pulsar Cep
X-4 (GS 2138+56) with the Burst and Transient Source Experiment (BATSE) on the
Compton Gamma-Ray Observatory (CGRO) and with the Rossi X-ray Timing Explorer
(RXTE). Two outbursts from Cep X-4 were observed with BATSE in 1993 June-July
and 1997 July. Pulse frequencies of 15.0941 +/- 0.0002 mHz on 1993 June 25 (MJD
49,163) and 15.0882 +/- 0.0002 mHz on 1997 July 12 (MJD 50,641) were each
measured from 2 day spans of BATSE data near each outburst's peak. Cep X-4
showed an average spin down rate of (-4.14 +/- 0.08)*10^(-14) Hz/s between the
1993 and 1997 outbursts. After BATSE could no longer detect Cep X-4, public
observations were performed on 1997 July 18 & 25 with the Proportional Counter
Array (PCA) on RXTE. A pulse frequency of 15.088 +/- 0.004 mHz was measured
from observations on 1997 July 18 (MJD 50,647). Significant aperiodic noise,
with an rms variance of ~18% in the frequency range 0.01-1.0 Hz was observed on
both days. Energy and intensity dependent pulse shape variations were also seen
in these data. Recently published optical observations associate Cep X-4 with a
Be companion star. If all 4 outbursts observed from Cep X-4 are assumed to
occur at the same orbital phase, we find that the orbital period is between 23
days and 147.3 days.Comment: 19 pages (LaTeX) including 9 figures. Accepted for publication in the
Astrophysical Journa
Psychosocial predictors of exclusive breastfeeding duration to six moths postpartum
The World Health Organization recommend that for optimal growth and development all infants worldwide should be exclusively breastfed for the first six months of life. Previous studies have identified that psychosocial factors are important for the maintenance of exclusive breastfeeding. This study aimed to examine the psychosocial factors associated with exclusive breastfeeding duration to six months postpartum. Method: 125 women completed questionnaires at three time points; 32 weeks gestation, two and six months postpartum. Self-efficacy, body attitude, psychological adjustment, attitude, intention, confidence, motivation and importance of exclusive breastfeeding and breastfeeding status were measured. Findings: At 32 weeks gestation a womanâs confidence to achieve exclusive breastfeeding was a predictor of exclusive breastfeeding to six months postpartum. At two months postpartum, psychological adjustment and self-efficacy were predictive of exclusive breastfeeding duration. At six months postpartum, psychological adjustment, self-efficacy, confidence to maintain and feeling fat were predictive of exclusive breastfeeding duration. Conclusion: Self-efficacy, psychological adjustment, body image, motivation and confidence are important for the maintenance of exclusive breastfeeding to six months. This has clinical application for health psychology as antenatal breastfeeding education and postpartum support may be strengthened by strategies that build a womanâs confidence to exclusively breastfeed
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