515 research outputs found
A critical test of empirical mass loss formulae applied to individual giants and supergiants
To test our new, improved Reimers-type mass-loss relation, given by Schroder
& Cuntz in 2005 (ApJL 630, L73), we take a look at the best studied galactic
giants and supergiants - particularly those with spatially resolved
circumstellar shells and winds, obtained directly or by means of a companion
acting as a probing light source. Together with well-known physical parameters,
the selected stars provide the most powerful and critical observational venues
for assessing the validity of parameterized mass-loss relations for cool winds
not driven by molecules or dust.
In this study, star by star, we compare our previously published relation
with the original Reimers relation (1975), the Lamers relation (1981), and the
two relations by de Jager and his group (1988, 1990). The input data,
especially the stellar masses, have been constrained using detailed stellar
evolution models. We find that only the relationship by Schroder & Cuntz
agrees, within the error bars, with the observed mass-loss rates for all giants
and supergiants.Comment: 11 pages, 5 Figs. accepted by Astronomy & Astrophysic
Recommended from our members
Reactive phase change materials for enhanced thermal energy storage
Effective storage and release of low-to-moderate temperature thermal energy (e.g. solar thermal or geothermal) could be transformational for applications such as space heating/cooling, domestic hot water, or off-grid cooking. Good candidates for thermal energy storage in this temperature range include latent heat storage (LHS) systems and thermochemical energy storage (TCES) systems using reversible salt-hydrate dehydration reactions. Here we propose that an energy storage system by use of magnesium nitrate hexahydrate can potentially improve upon independent TCES or LHS systems by utilizing both the thermochemical hydration reaction and the latent heat available through the solid-liquid phase change of one magnesium nitrate hydrate eutectic. This chemistry is investigated through TGA/DSC analysis and shows a total energy density of approximately 1170±94 kJ/kg when dehydrating the material up to 145°C. Reversible latent heat cycling at a eutectic melting temperature of 130°C is shown by the DSC signal and estimated to be on the order of 115±9.2 kJ/kg—a 10% increase over the thermochemical energy storage alone. Although the latent energy release was found to decrease slightly over several cycles, the mass was found to stabilize near an asymptotic value corresponding to the published eutectic composition. These results suggest the concept of reactive phase change materials could be a promising solution to increasing volumetric stored energy density
Structure of the outer layers of cool standard stars
Context: Among late-type red giants, an interesting change occurs in the
structure of the outer atmospheric layers as one moves to later spectral types
in the Hertzsprung-Russell diagram: a chromosphere is always present, but the
coronal emission diminishes and a cool massive wind steps in.
Aims: Where most studies have focussed on short-wavelength observations, this
article explores the influence of the chromosphere and the wind on
long-wavelength photometric measurements.
Methods: The observational spectral energy distributions are compared with
the theoretical predictions of the MARCS atmosphere models for a sample of 9 K-
and M-giants. The discrepancies found are explained using basic models for flux
emission originating from a chromosphere or an ionized wind.
Results: For 7 out of 9 sample stars, a clear flux excess is detected at
(sub)millimeter and/or centimeter wavelengths. The precise start of the excess
depends upon the star under consideration. The flux at wavelengths shorter than
about 1 mm is most likely dominated by an optically thick chromosphere, where
an optically thick ionized wind is the main flux contributor at longer
wavelengths.
Conclusions: Although the optical to mid-infrared spectrum of the studied K-
and M-giants is well represented by a radiative equilibrium atmospheric model,
the presence of a chromosphere and/or ionized stellar wind at higher altitudes
dominates the spectrum in the (sub)millimeter and centimeter wavelength ranges.
The presence of a flux excess also has implications on the role of these stars
as fiducial spectrophotometric calibrators in the (sub)millimeter and
centimeter wavelength range.Comment: 13 pages, 6 figures, 7 pages of online material, submitted to A&
Fe XVII X-ray Line Ratios for Accurate Astrophysical Plasma Diagnostics
New laboratory measurements using an Electron Beam Ion Trap (EBIT) and an
x-ray microcalorimeter are presented for the n=3 to n=2 Fe XVII emission lines
in the 15 {\AA} to 17 {\AA} range, along with new theoretical predictions for a
variety of electron energy distributions. This work improves upon our earlier
work on these lines by providing measurements at more electron impact energies
(seven values from 846 to 1185 eV), performing an in situ determination of the
x-ray window transmission, taking steps to minimize the ion impurity
concentrations, correcting the electron energies for space charge shifts, and
estimating the residual electron energy uncertainties. The results for the
3C/3D and 3s/3C line ratios are generally in agreement with the closest theory
to within 10%, and in agreement with previous measurements from an independent
group to within 20%. Better consistency between the two experimental groups is
obtained at the lowest electron energies by using theory to interpolate, taking
into account the significantly different electron energy distributions.
Evidence for resonance collision effects in the spectra is discussed.
Renormalized values for the absolute cross sections of the 3C and 3D lines are
obtained by combining previously published results, and shown to be in
agreement with the predictions of converged R-matrix theory. This work
establishes consistency between results from independent laboratories and
improves the reliability of these lines for astrophysical diagnostics. Factors
that should be taken into account for accurate diagnostics are discussed,
including electron energy distribution, polarization, absorption/scattering,
and line blends.Comment: 29 pages, including 7 figure
Radiative transition rates and collision strengths for Si II
Aims. This work reports radiative transition rates and electron impact
excitation collision strengths for levels of the 3s23p, 3s3p2, 3s24s, and 3s23d
configurations of Siii. Methods. The radiative data were computed using the
Thomas-Fermi-Dirac-Amaldi central potential, but with the modifications
introduced by Bautista (2008) that account for the effects of electron-electron
interactions. We also introduce new schemes for the optimization of the
variational parameters of the potential. Additional calculations were carried
out with the Relativistic Hartree-Fock and the multiconfiguration Dirac-Fock
methods. Collision strengths in LS-coupling were calculated in the close
coupling approximation with the R-matrix method. Then, fine structure collision
strengths were obtained by means of the intermediate-coupling frame
transformation (ICFT) method which accounts for spin-orbit coupling effects.
Results. We present extensive comparisons between the results of different
approximations and with the most recent calculations and experiment available
in the literature. From these comparisons we derive a recommended set of gf-
values and radiative transition rates with their corresponding estimated
uncertainties. We also study the effects of different approximations in the
representation of the target ion on the electron-impact collision strengths.
Our most accurate set of collision strengths were integrated over a Maxwellian
distribution of electron energies and the resulting effective collision
strengths are given for a wide range of temperatures. Our results present
significant differences from recent calculations with the B-spline
non-orthogonal R-matrix method. We discuss the sources of the differences.Comment: 6 figures, 5 tables within text, 2 electronic table
Observable Signatures of Planet Accretion in Red Giant Stars I: Rapid Rotation and Light Element Replenishment
The orbital angular momentum of a close-orbiting giant planet can be
sufficiently large that, if transferred to the envelope of the host star during
the red giant branch (RGB) evolution, it can spin-up the star's rotation to
unusually large speeds. This spin-up mechanism is one possible explanation for
the rapid rotators detected among the population of generally slow-rotating red
giant stars. These rapid rotators thus comprise a unique stellar sample
suitable for searching for signatures of planet accretion in the form of
unusual stellar abundances due to the dissemination of the accreted planet in
the stellar envelope. In this study, we look for signatures of replenishment in
the Li abundances and (to a lesser extent) 12C/13C, which are both normally
lowered during RGB evolution. Accurate abundances were measured from high
signal-to-noise echelle spectra for samples of both slow and rapid rotator red
giant stars. We find that the rapid rotators are on average enriched in lithium
compared to the slow rotators, but both groups of stars have identical
distributions of 12C/13C within our measurement precision. Both of these
abundance results are consistent with the accretion of planets of only a few
Jupiter masses. We also explore alternative scenarios for understanding the
most Li-rich stars in our sample---particularly Li regeneration during various
stages of stellar evolution. Finally, we find that our stellar samples show
non-standard abundances even at early RGB stages, suggesting that initial
protostellar Li abundances and 12C/13C may be more variable than originally
thought.Comment: Accepted for publication in the Astrophysical Journal. 29 pages in
emulateapj format, including 16 figures and 12 tables. Tables 4 and 8 are
provided in their entirety as plain text ancillary files (and will also be
available in the electronic edition of ApJ
Weak G-band stars on the H-R Diagram: Clues to the origin of Li anomaly
Weak G-band (WGB) stars are a rare class of cool luminous stars that present
a strong depletion in carbon, but also lithium abundance anomalies that have
been little explored in the literature since the first discovery of these
peculiar objects in the early 50's. Here we focus on the Li-rich WGB stars and
report on their evolutionary status. We explore different paths to propose a
tentative explanation for the lithium anomaly. Using archive data, we derive
the fundamental parameters of WGB (Teff, log g, log(L/Lsun)) using Hipparcos
parallaxes and recent temperature scales. From the equivalent widths of Li
resonance line at 6707 {\AA}, we uniformly derive the lithium abundances and
apply when possible NLTE corrections following the procedure described by Lind
et al. (2009). We also compute dedicated stellar evolution models in the mass
range 3.0 to 4.5 Msun, exploring the effects of rotation-induced and
thermohaline mixing. These models are used to locate the WGB stars in the H-R
diagram and to explore the origin of the abundance anomalies. The location of
WGB stars in the H-R diagram shows that these are intermediate mass stars of
masses ranging from 3.0 to 4.5 Msun located at the clump, which implies a
degeneracy of their evolutionary status between subgiant/red giant branch and
core helium burning phases. The atmospheres of a large proportion of WGB stars
(more than 50%) exhibit lithium abundances A(Li) \geq 1.4 dex similar to
Li-rich K giants. The position of WGB stars along with the Li-rich K giants in
the H-R diagram however indicates that both are well separated groups. The
combined and tentatively consistent analysis of the abundance pattern for
lithium, carbon and nitrogen of WGB stars seems to indicate that carbon
underabundance could be decorrelated from the lithium and nitrogen
overabundances.Comment: 13 pages, 3 figures, Accepted for publication in Astronomy and
Astrophysic
Nursing Care for Pregnant Adolescents
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73476/1/0884217503252191.pd
Doppler Shifts and Broadening and the Structure of the X-ray Emission from Algol
In a study of Chandra High Energy Transmission Grating spectra of Algol, we
clearly detect Doppler shifts caused by the orbital motion of Algol B. These
data provide the first definitive proof that the X-ray emission of Algol is
dominated by the secondary, in concordance with expectations that Algol A (B8)
is X-ray dark. The measured Doppler shifts are slightly smaller than expected,
implying an effective orbital radius of about 10 Rsolar, instead of 11.5 Rsolar
for the Algol B center of mass. This could be caused by a small contribution of
X-ray flux from Algol A (10-15%), possibly through accretion. The more likely
explanation is an asymmetric corona biased toward the system center of mass by
the tidal distortion of the surface of Algol B. Analysis of the strongest lines
indicates excess line broadening of ~150 km/s above that expected from thermal
motion and surface rotation. Possible explanations include turbulence, flows or
explosive events, or rotational broadening from a radially extended corona. We
favor the latter scenario and infer that a significant component of the corona
at temperatures <10^7 K has a scale height of order the stellar radius. This is
supported by the shape of the X-ray lightcurve and the shallow dip at secondary
eclipse. We also examine the O VII intercombination and forbidden lines in a
Low Energy Transmission Grating Spectrograph observation and find no change in
their relative line fluxes as the system goes from quadrature to primary
eclipse. Since these lines are strongly affected by UV irradiation from Algol
A, this supports the conjecture that the corona of Algol B at temperatures of
several million K must be significantly extended and/or located toward the
poles to avoid being shadowed from Algol A during primary eclipse.Comment: 36 pages, 10 figure
On metal-deficient barium stars and their link with yellow symbiotic stars
This paper addresses the question of why metal-deficient barium stars are not
yellow symbiotic stars (YSyS). Samples of (suspected) metal-deficient barium
(mdBa) stars and YSyS have been collected from the literature, and their
properties reviewed. It appears in particular that the barium nature of the
suspected mdBa stars needs to be ascertained by detailed abundance analyses.
Abundances are therefore derived for two of them, HD 139409 and HD 148897,
which reveal that HD 148897 should not be considered a barium star. HD 139409
is a mild barium star, with overabundances observed only for elements belonging
to the first s-process peak (Y and Zr). The evidence for binarity among mdBa
stars is then reviewed, using three different methods: (i) radial-velocity
variations (from CORAVEL observations), (ii) Hipparcos astrometric data, and
(iii) a method based on the comparison between the Hipparcos and Tycho-2 proper
motions. A first-time orbit is obtained for HIP 55852, whereas evidence for the
(so far unknown) binary nature of HIP 34795, HIP 76605, HIP 97874 and HIP
107478 is presented. Two stars with no evidence for binarity whatsoever (HIP
58596 and BD +3 2688) are candidates low-metallicity thermally-pulsing
asymptotic giant branch stars, as inferred from their large luminosities. The
reason why mdBa stars are not YSyS is suggested to lie in their different
orbital period distributions: mdBa stars have on average longer orbital periods
than YSyS, and hence their companion accretes matter at a lower rate, for a
given mass loss rate of the giant star. The definite validation of this
explanation should nevertheless await the determination of the orbital periods
for the many mdBa stars still lacking periods, in order to make the comparison
more significant.Comment: Astronomy & Astrophysics, in press; 16 pages, 14 figures; also
available at http://www.astro.ulb.ac.be/Html/ps.html#PR
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