931 research outputs found
Herschel imaging and spectroscopy of the nebula around the luminous blue variable star WRAY 15-751
We have obtained far-infrared Herschel PACS imaging and spectroscopic
observations of the nebular environment of the luminous blue variable WRAY
15-751. These images clearly show that the main, dusty nebula is a shell of
radius 0.5 pc and width 0.35 pc extending outside the H-alpha nebula. They also
reveal a second, bigger and fainter dust nebula, observed for the first time.
Both nebulae lie in an empty cavity, likely the remnant of the O-star wind
bubble formed when the star was on the main sequence. The kinematic ages of the
nebulae are about 20000 and 80000 years and each nebula contains about 0.05
Msun of dust. Modeling of the inner nebula indicates a Fe-rich dust. The
far-infrared spectrum of the main nebula revealed forbidden emission lines
coming from ionized and neutral gas. Our study shows that the main nebula
consists of a shell of ionized gas surrounded by a thin photodissociation
region illuminated by an "average" early-B star. The derived abundance ratios
N/O=1.0+/-0.4 and C/O=0.4+/-0.2 indicate a mild N/O enrichment. We estimate
that the inner shell contains 1.7+/-0.6 Msun of gas. Assuming a similar
dust-to-gas ratio for the outer nebula, the total mass ejected by WRAY 15-751
amounts to 4+/-2 Msun. The measured abundances, masses and kinematic ages of
the nebulae were used to constrain the evolution of the star and the epoch at
which the nebulae were ejected. Our results point to an ejection of the nebulae
during the RSG evolutionary phase of an ~ 40 Msun star. The presence of
multiple shells around the star suggests that the mass-loss was not a
continuous ejection but rather a series of episodes of extreme mass-loss. Our
measurements are compatible with the recent evolutionary tracks computed for an
40 Msun star with little rotation. They support the O-BSG-RSG-YSG-LBV filiation
and the idea that high-luminosity and low-luminosity LBVs follow different
evolutionary paths.Comment: 19 pages, 13 figures, accepted for publication in A&
Evolution and Nucleosynthesis of Zero Metal Intermediate Mass Stars
New stellar models with mass ranging between 4 and 8 Mo, Z=0 and Y=0.23 are
presented. The models have been evolved from the pre Main Sequence up to the
Asymptotic Giant Branch (AGB). At variance with previous claims, we find that
these updated stellar models do experience thermal pulses in the AGB phase. In
particular we show that: a) in models with mass larger than 6 Mo, the second
dredge up is able to raise the CNO abundance in the envelope enough to allow a
"normal" AGB evolution, in the sense that the thermal pulses and the third
dredge up settle on; b) in models of lower mass, the efficiency of the CNO
cycle in the H-burning shell is controlled by the carbon produced locally via
the 3alpha reactions. Nevertheless the He-burning shell becomes thermally
unstable after the early AGB. The expansion of the overlying layers induced by
these weak He-shell flashes is not sufficient by itself to allow a deep
penetration of the convective envelope. However, immediately after that, the
maximum luminosity of the He flash is attained and a convective shell
systematically forms at the base of the H-rich envelope. The innermost part of
this convective shell probably overlaps the underlying C-rich region left by
the inter-shell convection during the thermal pulse, so that fresh carbon is
dredged up in a "hot" H-rich environment and a H flash occurs. This flash
favours the expansion of the outermost layers already started by the weak
thermal pulse and a deeper penetration of the convective envelope takes place.
Then, the carbon abundance in the envelope rises to a level high enough that
the further evolution of these models closely resembles that of more metal rich
AGB stars. These stars provide an important source of primary carbon and
nitrogen.Comment: 28 pages, 5 tables and 17 figures. Accepted for publication in Ap
Terminal velocities of luminous, early-type SMC stars
Ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS) are
used to determine terminal velocities for 11 O and B-type giants and
supergiants in the Small Magellanic Cloud (SMC) from the Si IV and C IV
resonance lines. Using archival data from observations with the Goddard
High-Resolution Spectrograph and the International Ultraviolet Explorer
telescope, terminal velocities are obtained for a further five B-type
supergiants. We discuss the metallicity dependence of stellar terminal
velocities, finding no evidence for a significant scaling between Galactic and
SMC metallicities for Teff < 30,000 K, consistent with the predictions of
radiation driven wind theory for supergiant stars. A comparison of the
ratio between the SMC and Galactic samples, while
consistent with the above statement, emphasizes that the uncertainties in the
distances to galactic O-stars are a serious obstacle to a detailed comparison
with theory. For the SMC sample there is considerable scatter in this ratio at
a given effective temperature, perhaps indicative of uncertainties in stellar
masses.Comment: 28 pages, 8 figures, accepted by ApJ; minor revisions prior to
acceptanc
The Herschel view of the nebula around the luminous blue variable star AG Carinae
Far-infrared Herschel PACS imaging and spectroscopic observations of the
nebula around the luminous blue variable (LBV) star AG Car have been obtained
along with optical imaging in the Halpha+[NII] filter. In the infrared light,
the nebula appears as a clumpy ring shell that extends up to 1.2 pc with an
inner radius of 0.4 pc. It coincides with the Halpha nebula, but extends
further out. Dust modeling of the nebula was performed and indicates the
presence of large grains. The dust mass is estimated to be ~ 0.2 Msun. The
infrared spectrum of the nebula consists of forbidden emission lines over a
dust continuum. Apart from ionized gas, these lines also indicate the existence
of neutral gas in a photodissociation region that surrounds the ionized region.
The abundance ratios point towards enrichment by processed material. The total
mass of the nebula ejected from the central star amounts to ~ 15 Msun, assuming
a dust-to-gas ratio typical of LBVs. The abundances and the mass-loss rate were
used to constrain the evolutionary path of the central star and the epoch at
which the nebula was ejected, with the help of available evolutionary models.
This suggests an ejection during a cool LBV phase for a star of ~ 55 Msun with
little rotation.Comment: accepted for publication in A&
Molecular form factors in X-ray crystallography
The calculation of molecular form factors from ab initio molecular electronic wavefunctions is discussed, and a scheme for application to X-ray diffraction structure analysis is given. The method is used to calculate the form factor of the NH+4 molecular ion from three accurate molecular wavefunctions, as well as for the corresponding isolated-atoms arrangement. The merits of the different form factors obtained are tested on experimental single-crystal X-ray diffraction data for NH4F. It appears that the quality of the form factor is determined by the number of centres in the wavefunction basis rather than by the wavefunction energy
An independent distance estimate to CW Leo
CW Leo has been observed six times between October 2009 and June 2012 with
the SPIRE instrument on board the Herschel satellite. Variability has been
detected in the flux emitted by the central star with a period of 639 \pm 4
days, in good agreement with determinations in the literature. Variability is
also detected in the bow shock around CW Leo that had previously been detected
in the ultraviolet and Herschel PACS/SPIRE data. Although difficult to prove
directly, our working hypothesis is that this variability is directly related
to that of the central star. In this case, fitting a sine curve with the period
fixed to 639 days results in a time-lag in the variability between bow shock
and the central star of 402 \pm 37 days. The orientation of the bow shock
relative to the plane of the sky is unknown (but see below). For an inclination
angle of zero degrees, the observed time-lag translates into a distance to CW
Leo of 130 \pm 13 pc, and for non-zero inclination angles the distance is
smaller. Fitting the shape of the bow shock with an analytical model (Wilkin
1996), the effect of the inclination angle on the distance may be estimated.
Making the additional assumption that the relative peculiar velocity between
the interstellar medium (ISM) and CW Leo is determined entirely by the star
space velocity with respect to the local standard of rest (i.e. a stationary
ISM), the inclination angle is found to be (-33.3 \pm 0.8) degrees based on the
observed proper motion and radial velocity. Using the Wilkin model, our current
best estimate of the distance to CW Leo is 123 \pm 14 pc. For a distance of 123
pc, we derive a mean luminosity of 7790 \pm 150 Lsol (internal error).Comment: Accepted A&A Letter
Large-scale environments of binary AGB stars probed by Herschel. II: Two companions interacting with the wind of pi1 Gruis
Context. The Mass loss of Evolved StarS (MESS) sample observed with PACS on
board the Herschel Space Observatory revealed that several asymptotic giant
branch (AGB) stars are surrounded by an asymmetric circumstellar envelope (CSE)
whose morphology is most likely caused by the interaction with a stellar
companion. The evolution of AGB stars in binary systems plays a crucial role in
understanding the formation of asymmetries in planetary nebul{\ae} (PNe), but
at present, only a handful of cases are known where the interaction of a
companion with the stellar AGB wind is observed.
Aims. We probe the environment of the very evolved AGB star Gruis on
large and small scales to identify the triggers of the observed asymmetries.
Methods. Observations made with Herschel/PACS at 70 m and 160 m
picture the large-scale environment of Gru. The close surroundings of
the star are probed by interferometric observations from the VLTI/AMBER
archive. An analysis of the proper motion data of Hipparcos and Tycho-2
together with the Hipparcos Intermediate Astrometric Data help identify the
possible cause for the observed asymmetry.
Results. The Herschel/PACS images of Gru show an elliptical CSE whose
properties agree with those derived from a CO map published in the literature.
In addition, an arc east of the star is visible at a distance of
from the primary. This arc is most likely part of an
Archimedean spiral caused by an already known G0V companion that is orbiting
the primary at a projected distance of 460 au with a period of more than 6200
yr. However, the presence of the elliptical CSE, proper motion variations, and
geometric modelling of the VLTI/AMBER observations point towards a third
component in the system, with an orbital period shorter than 10 yr, orbiting
much closer to the primary than the G0V star.Comment: 13 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
MSX, 2MASS, and the LMC: A Combined Near and Mid Infrared View
The Large Magellanic Cloud (LMC) has been observed by both the Midcourse
Space Experiment (MSX) in the mid-infrared and the Two Micron All Sky Survey
(2MASS) in the near-infrared. We have performed a cross-correlation of the 1806
MSX catalog sources and nearly 1.4 million 2MASS catalogued point and extended
sources and find 1664 matches. Using the available color information, we
identify a number of stellar populations and nebulae, including main sequence
stars, giant stars, red supergiants, carbon- and oxygen-rich asymptotic giant
branch (AGB) stars, planetary nebulae, H II regions, and other dusty objects
likely associated with early-type stars. 731 of these sources have no previous
identification. We compile a listing of all objects, which includes photometry
and astrometry. The 8.3 micron MSX sensitivity is the limiting factor for
object detection: only the brighter red objects, specifically the red
supergiants, AGB stars, planetary nebulae and HII regions, are detected in the
LMC. The remaining objects are likely in the Galactic foreground. The spatial
distribution of the infrared LMC sources may contribute to understanding
stellar formation and evolution and the overall galactic evolution. We
demonstrate that a combined mid- and near-infrared photometric baseline
provides a powerful means of identifying new objects in the LMC for future
ground-based and space-based follow-up observations.Comment: 23 pages, 10 figures, to appear in the AJ (2001 Oct issue). N.B:
Tables 2 & 3 corrected and available as html file
The detached dust shells of AQ And, U Ant, and TT Cyg
Detached circumstellar dust shells are detected around three carbon variables
using Herschel-PACS. Two of them are already known on the basis of their
thermal CO emission and two are visible as extensions in IRAS imaging data. By
model fits to the new data sets, physical sizes, expansion timescales, dust
temperatures, and more are deduced. A comparison with existing molecular CO
material shows a high degree of correlation for TT Cyg and U Ant but a few
distinct differences with other observables are also found.Comment: Letter accepted for publication on the A&A Herschel Special Issu
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