424 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&
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&
The Binarity of Eta Carinae and its Similarity to Related Astrophysical Objects
I examine some aspects of the interaction between the massive star Eta
Carinae and its companion, in particular during the eclipse-like event, known
as the spectroscopic event or the shell event. The spectroscopic event is
thought to occur when near periastron passages the stellar companion induces
much higher mass loss rate from the primary star, and/or enters into a much
denser environment around the primary star. I find that enhanced mass loss rate
during periastron passages, if it occurs, might explain the high eccentricity
of the system. However, there is not yet a good model to explain the presumed
enhanced mass loss rate during periastron passages. In the region where the
winds from the two stars collide, a dense slow flow is formed, such that large
dust grains may be formed. Unlike the case during the 19th century Great
Eruption, the companion does not accrete mass during most of its orbital
motion. However, near periastron passages short accretion episodes may occur,
which may lead to pulsed ejection of two jets by the companion. The companion
may ionize a non-negligible region in its surrounding, resembling the situation
in symbiotic systems. I discuss the relation of some of these processes to
other astrophysical objects, by that incorporating Eta Car to a large class of
astrophysical bipolar nebulae.Comment: Updated version. ApJ, in pres
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
Herschel/PACS observations of the 69 band of crystalline olivine around evolved stars
We present 48 Herschel/PACS spectra of evolved stars in the wavelength range
of 67-72 m. This wavelength range covers the 69 m band of crystalline
olivine (). The width and
wavelength position of this band are sensitive to the temperature and
composition of the crystalline olivine. Our sample covers a wide range of
objects: from high mass-loss rate AGB stars (OH/IR stars,
M/yr), through post-AGB stars with and without circumbinary disks, to
planetary nebulae and even a few massive evolved stars. The goal of this study
is to exploit the spectral properties of the 69 m band to determine the
composition and temperature of the crystalline olivine. Since the objects cover
a range of evolutionary phases, we study the physical and chemical properties
in this range of physical environments. We fit the 69 m band and use its
width and position to probe the composition and temperature of the crystalline
olivine. For 27 sources in the sample, we detected the 69 m band of
crystalline olivine (). The
69 m band shows that all the sources produce pure forsterite grains
containing no iron in their lattice structure. The temperature of the
crystalline olivine as indicated by the 69 m band, shows that on average
the temperature of the crystalline olivine is highest in the group of OH/IR
stars and the post-AGB stars with confirmed Keplerian disks. The temperature is
lower for the other post-AGB stars and lowest for the planetary nebulae. A
couple of the detected 69 m bands are broader than those of pure
magnesium-rich crystalline olivine, which we show can be due to a temperature
gradient in the circumstellar environment of these stars. continued...Comment: Accepted for publication in A&
Constraints on the H2O formation mechanism in the wind of carbon-rich AGB stars
Context. The recent detection of warm HO vapor emission from the outflows
of carbon-rich asymptotic giant branch (AGB) stars challenges the current
understanding of circumstellar chemistry. Two mechanisms have been invoked to
explain warm HO vapor formation. In the first, periodic shocks passing
through the medium immediately above the stellar surface lead to HO
formation. In the second, penetration of ultraviolet interstellar radiation
through a clumpy circumstellar medium leads to the formation of HO
molecules in the intermediate wind.
Aims. We aim to determine the properties of HO emission for a sample of
18 carbon-rich AGB stars and subsequently constrain which of the above
mechanisms provides the most likely warm HO formation pathway.
Methods, Results, and Conclusions. See paper
Resolving the compact dusty discs around binary post-AGB stars using N-band interferometry
We present the first mid-IR long baseline interferometric observations of the
circumstellar matter around binary post-AGB stars. Two objects, SX Cen and HD
52961, were observed using the VLTI/MIDI instrument during Science
Demonstration Time. Both objects are known binaries for which a stable
circumbinary disc is proposed to explain the SED characteristics. This is
corroborated by our N-band spectrum showing a crystallinity fraction of more
than 50 % for both objects, pointing to a stable environment where dust
processing can occur. Surprisingly, the dust surrounding SX Cen is not resolved
in the interferometric observations providing an upper limit of 11 mas (or 18
AU at the distance of this object) on the diameter of the dust emission. This
confirms the very compact nature of its circumstellar environment. The dust
emission around HD 52961 originates from a very small but resolved region,
estimated to be ~ 35 mas at 8 micron and ~ 55 mas at 13 micron. These results
confirm the disc interpretation of the SED of both stars. In HD 52961, the dust
is not homogeneous in its chemical composition: the crystallinity is clearly
concentrated in the hotter inner region. Whether this is a result of the
formation process of the disc, or due to annealing during the long storage time
in the disc is not clear.Comment: 12 pages, 10 figures, accepted for publication in A &
Reconstructing Regional Trajectories: the Provenance and Distribution of Archaic to Hellenistic Ceramics in Central Pisidia (South-west Turkey)
Material Culture Studie
Catalog of Galactic Beta Cephei Stars
We present an extensive and up-to-date catalog of Galactic Beta Cephei stars.
This catalog is intended to give a comprehensive overview of observational
characteristics of all known Beta Cephei stars. 93 stars could be confirmed to
be Beta Cephei stars. For some stars we re-analyzed published data or conducted
our own analyses. 61 stars were rejected from the final Beta Cephei list, and
77 stars are suspected to be Beta Cephei stars. A list of critically selected
pulsation frequencies for confirmed Beta Cephei stars is also presented. We
analyze the Beta Cephei stars as a group, such as the distributions of their
spectral types, projected rotational velocities, radial velocities, pulsation
periods, and Galactic coordinates. We confirm that the majority of these stars
are multiperiodic pulsators. We show that, besides two exceptions, the Beta
Cephei stars with high pulsation amplitudes are slow rotators. We construct a
theoretical HR diagram that suggests that almost all 93 Beta Cephei stars are
MS objects. We discuss the observational boundaries of Beta Cephei pulsation
and their physical parameters. We corroborate that the excited pulsation modes
are near to the radial fundamental mode in frequency and we show that the mass
distribution of the stars peaks at 12 solar masses. We point out that the
theoretical instability strip of the Beta Cephei stars is filled neither at the
cool nor at the hot end and attempt to explain this observation
The enigmatic nature of the circumstellar envelope and bow shock surrounding Betelgeuse as revealed by Herschel. I. Evidence of clumps, multiple arcs, and a linear bar-like structure
Context. The interaction between stellar winds and the interstellar medium
(ISM) can create complex bow shocks. The photometers on board the Herschel
Space Observatory are ideally suited to studying the morphologies of these bow
shocks. Aims. We aim to study the circumstellar environment and wind-ISM
interaction of the nearest red supergiant, Betelgeuse. Methods. Herschel PACS
images at 70, 100, and 160 micron and SPIRE images at 250, 350, and 500 micron
were obtained by scanning the region around Betelgeuse. These data were
complemented with ultraviolet GALEX data, near-infrared WISE data, and radio 21
cm GALFA-HI data. The observational properties of the bow shock structure were
deduced from the data and compared with hydrodynamical simulations. Results.
The infrared Herschel images of the environment around Betelgeuse are
spectacular, showing the occurrence of multiple arcs at 6-7 arcmin from the
central target and the presence of a linear bar at 9 arcmin. Remarkably, no
large-scale instabilities are seen in the outer arcs and linear bar. The dust
temperature in the outer arcs varies between 40 and 140 K, with the linear bar
having the same colour temperature as the arcs. The inner envelope shows clear
evidence of a non-homogeneous clumpy structure (beyond 15 arcsec), probably
related to the giant convection cells of the outer atmosphere. The
non-homogeneous distribution of the material even persists until the collision
with the ISM. A strong variation in brightness of the inner clumps at a radius
of 2 arcmin suggests a drastic change in mean gas and dust density some 32 000
yr ago. Using hydrodynamical simulations, we try to explain the observed
morphology of the bow shock around Betelgeuse. Conclusions: [abbreviated]Comment: 26 page
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