3,368 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
Pulsational instability of yellow hypergiants
Instability of population I (X=0.7, Y=0.02) massive stars against radial
oscillations during the post-main sequence gravitational contraction of the
helium core is investigated. Initial stellar masses are in the range from
65M_\odot to 90M_\odot. In hydrodynamic computations of self-exciting stellar
oscillations we assumed that energy transfer in the envelope of the pulsating
star is due to radiative heat conduction and convection. The convective heat
transfer was treated in the framework of the theory of time-dependent turbulent
convection. During evolutionary expansion of outer layers after hydrogen
exhaustion in the stellar core the star is shown to be unstable against radial
oscillations while its effective temperature is Teff > 6700K for
Mzams=65M_\odot and Teff > 7200K for mzams=90M_\odot. Pulsational instability
is due to the \kappa-mechanism in helium ionization zones and at lower
effective temperature oscillations decay because of significantly increasing
convection. The upper limit of the period of radial pulsations on this stage of
evolution does not exceed 200 day. Radial oscillations of the hypergiant resume
during evolutionary contraction of outer layers when the effective temperature
is Teff > 7300K for Mzams=65M_\odot and Teff > 7600K for Mzams=90M_\odot.
Initially radial oscillations are due to instability of the first overtone and
transition to fundamental mode pulsations takes place at higher effective
temperatures (Teff > 7700K for Mzams=65M_\odot and Teff > 8200K for
Mzams=90M_\odot). The upper limit of the period of radial oscillations of
evolving blueward yellow hypergiants does not exceed 130 day. Thus, yellow
hypergiants are stable against radial stellar pulsations during the major part
of their evolutionary stage.Comment: 20 pages, 7 gigures. Accepted for publication in Astronomy Letter
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
Pulsar Magnetospheric Emission Mapping: Images and Implications of Polar-Cap Weather
The beautiful sequences of ``drifting'' subpulses observed in some radio
pulsars have been regarded as among the most salient and potentially
instructive characteristics of their emission, not least because they have
appeared to represent a system of subbeams in motion within the emission zone
of the star. Numerous studies of these ``drift'' sequences have been published,
and a model of their generation and motion articulated long ago by Ruderman &
Sutherland (1975); but efforts thus far have failed to establish an
illuminating connection between the drift phemomenon and the actual sites of
radio emission. Through a detailed analysis of a nearly coherent sequence of
``drifting'' pulses from pulsar B0943+10, we have in fact identified a system
of subbeams circulating around the magnetic axis of the star. A mapping
technique, involving a ``cartographic'' transform and its inverse, permits us
to study the character of the polar-cap emission ``map'' and then to confirm
that it, in turn, represents the observed pulse sequence. On this basis, we
have been able to trace the physical origin of the ``drifting-subpulse''
emission to a stably rotating and remarkably organized configuration of
emission columns, in turn traceable possibly to the magnetic polar-cap ``gap''
region envisioned by some theories.Comment: latex with five eps figure
Depth concentrations of deuterium ions implanted into some pure metals and alloys
Pure metals (Cu, Ti, Zr, V, Pd) and diluted Pd-alloys (Pd-Ag, Pd-Pt, Pd-Ru,
Pd-Rh) were implanted by 25 keV deuterium ions at fluences in the range
(1.2{\div}2.3)x1022 D+/m2. The post-treatment depth distributions of deuterium
ions were measured 10 days and three months after the implantation using
Elastic Recoil Detection Analysis (ERDA) and Rutherford Backscattering (RBS).
Comparison of the obtained results allowed to make conclusions about relative
stability of deuterium and hydrogen gases in pure metals and diluted Pd alloys.
Very high diffusion rates of implanted deuterium ions from V and Pd pure metals
and Pd alloys were observed. Small-angle X-ray scattering revealed formation of
nanosized defects in implanted corundum and titanium.Comment: 12 pages, 9 figure
Effective Lagrangian approach to nuclear mu-e conversion and the role of vector mesons
We study nuclear mu-e conversion in the general framework of an effective
Lagrangian approach without referring to any specific realization of the
physics beyond the standard model (SM) responsible for lepton flavor violation
(LFV). We examine the impact of a specific hadronization prescription on the
analysis of new physics in nuclear mu-e conversion and stress the importance of
vector meson exchange between lepton and nucleon currents. A new issue of this
mechanism is the presence of the strange quark vector current contribution
induced by the phi meson. This allows us to extract new limits on the LFV
lepton-quark effective couplings from the existing experimental data.Comment: 19 pages, 3 figures, to be published in Phys Rev
Next-to-leading order QCD corrections to Higgs boson production in association with a photon via weak-boson fusion at the LHC
Higgs boson production in association with a hard central photon and two
forward tagging jets is expected to provide valuable information on Higgs boson
couplings in a range where it is difficult to disentangle weak-boson fusion
processes from large QCD backgrounds. We present next-to-leading order QCD
corrections to Higgs production in association with a photon via weak-boson
fusion at a hadron collider in the form of a flexible parton-level Monte Carlo
program. The QCD corrections to integrated cross sections are found to be small
for experimentally relevant selection cuts, while the shape of kinematic
distributions can be distorted by up to 20% in some regions of phase space.
Residual scale uncertainties at next-to-leading order are at the few-percent
level.Comment: 17 pages, 7 figures, 1 tabl
Impact of diabetes duration and cardiovascular risk factors on mortality in type 2 diabetes: the Hoorn Study
Background. Several studies have reported differences in the mortality risk between diabetic subjects detected by screening and known diabetic patients. We studied mortality in relation to diabetes duration, and the contribution of other cardiovascular risk factors to the elevated risk. Materials and methods. Participants were type 2 diabetic subjects (n = 174) of a population-based cohort study. Of these, 95 were diagnosed by screening. Known diabetic subjects were grouped into two categories of diabetes duration, with a median duration of 2.4 and 11.2 years, respectively. We assessed the contribution of classical cardiovascular risk factors (dyslipidaemia, hypertension, and prior myocardial infarction), and of new cardiovascular risk factors (microalbuminuria, von Willebrand factor, sVCAM-1 and C-reactive protein) to the mortality risk during nearly 10 years of follow up. Cox's proportional hazards model was used to study the association of diabetes duration and mortality. Results. The age- and sex-adjusted relative risks of mortality were 2.06 (95% C.I. 1.04-4.10) and 3.19 (1.64-6.20) for the patients with short- and long-term diabetes compared with the screening-detected diabetic subjects, respectively. Adjustment for cardiovascular risk factors resulted in a reduction of mortality risk in both groups: 1.13 (0.51-2.50) and 2.39 (1.18-4.83), respectively. Mortality risk significantly increased with increasing diabetes duration, even after multiple adjustment (P-value for trend ranged from < 0.001-0.018). Conclusions. Mortality risk increased with increasing diabetes duration. In subjects with short diabetes duration the mortality risk could largely be attributed to other risk factors. In subjects with a longer diabetes duration, however, the elevated mortality risk was independent of these cardiovascular risk factors
Measuring the Magnetic Field on the Classical T Tauri Star TW Hydrae
We present infrared (IR) and optical echelle spectra of the Classical T Tauri
star TW Hydrae. Using the optical data, we perform detailed spectrum synthesis
to fit atomic and molecular absorption lines and determine key stellar
parameters: Teff = 4126 \pm 24 K, log g = 4.84 \pm 0.16, [M/H] = -0.10 \pm
0.12, vsini = 5.8 \pm 0.6 km/s. The IR spectrum is used to look for Zeeman
broadening of photospheric absorption lines. We fit four Zeeman sensitive Ti I
lines near 2.2 microns and find the average value of the magnetic field over
the entire surface is 2.61 \pm 0.23 kG. In addition, several nearby
magnetically insensitive CO lines show no excess broadening above that produced
by stellar rotation and instrumental broadening, reinforcing the magnetic
interpretation for the width of the Ti I lines. We carry out extensive tests to
quantify systematic errors in our analysis technique which may result from
inaccurate knowledge of the effective temperature or gravity, finding that
reasonable errors in these quantities produce a 10% uncertainty in the mean
field measurement.Comment: The tar file includes one Tex file and four .eps figures. The paper
is accepted and tentatively scheduled for the ApJ 1 December 2005, v634, 2
issue. ApJ manuscript submission # 6310
On the role of continuum-driven eruptions in the evolution of very massive stars and Population III stars
We suggest that the mass lost during the evolution of very massive stars may
be dominated by optically thick, continuum-driven outbursts or explosions,
instead of by steady line-driven winds. In order for a massive star to become a
WR star, it must shed its H envelope, but new estimates of the effects of
clumping in winds indicate that line driving is vastly insufficient. We discuss
massive stars above roughly 40-50 Msun, for which the best alternative is mass
loss during brief eruptions of luminous blue variables (LBVs). Our clearest
example of this phenomenon is the 19th century outburst of eta Car, when the
star shed 12-20 Msun or more in less than a decade. Other examples are
circumstellar nebulae of LBVs, extragalactic eta Car analogs (``supernova
impostors''), and massive shells around SNe and GRBs. We do not yet fully
understand what triggers LBV outbursts, but they occur nonetheless, and present
a fundamental mystery in stellar astrophysics. Since line opacity from metals
becomes too saturated, the extreme mass loss probably arises from a
continuum-driven wind or a hydrodynamic explosion, both of which are
insensitive to metallicity. As such, eruptive mass loss could have played a
pivotal role in the evolution and fate of massive metal-poor stars in the early
universe. If they occur in these Population III stars, such eruptions would
profoundly affect the chemical yield and types of remnants from early SNe and
hypernovae.Comment: 4 pages, 1 figure, accepted by ApJ Letter
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