158 research outputs found
Dust and the spectral energy distribution of the OH/IR star OH 127.8+0.0: Evidence for circumstellar metallic iron
We present a fit to the spectral energy distribution of OH 127.8+0.0, a
typical asymptotic giant branch star with an optically thick circumstellar dust
shell. The fit to the dust spectrum is achieved using non-spherical grains
consisting of metallic iron, amorphous and crystalline silicates and water ice.
Previous similar attempts have not resulted in a satisfactory fit to the
observed spectral energy distributions, mainly because of an apparent lack of
opacity in the 3--8 micron region of the spectrum. Non-spherical metallic iron
grains provide an identification for the missing source of opacity in the
near-infrared. Using the derived dust composition, we have calculated spectra
for a range of mass-loss rates in order to perform a consistency check by
comparison with other evolved stars. The L-[12 micron] colours of these models
correctly predict the mass-loss rate of a sample of AGB stars, strengthening
our conclusion that the metallic iron grains dominate the near-infrared flux.
We discuss a formation mechanism for non-spherical metallic iron grains.Comment: 10 pages, 6 figures, accepted for publication by A&
HI in circumstellar environments
We present new results of a spectroscopic survey of circumstellar HI in the
direction of evolved stars made with the Nancay Radiotelescope. The HI line at
21 cm has been detected in the circumstellar shells of a variety of evolved
stars: AGB stars, oxygen-rich and carbon-rich, Semi-Regular and Miras, and
Planetary Nebulae. The emissions are generally spatially resolved, i.e. larger
than 4', indicating shell sizes of the order of 1 pc which opens the
possibility to trace the history of mass loss over the past ~ 10^4-10^5 years.
The line-profiles are sometimes composite. The individual components have
generally a quasi-Gaussian shape; in particular they seldom show the
double-horn profile that would be expected from the spatially resolved
optically thin emission of a uniformly expanding shell. This probably implies
that the expansion velocity decreases outwards in the external shells (0.1-1
pc) of these evolved stars. The HI line-profiles do not necessarily match those
of the CO rotational lines. Furthermore, the centroid velocities do not always
agree with those measured in the CO lines and/or the stellar radial velocities.
The HI emissions may also be shifted in position with respect to the central
stars. Without excluding the possibility of asymmetric mass ejection, we
suggest that these two effects could also be related to a non-isotropic
interaction with the local interstellar medium. HI was detected in emission
towards several sources (rho Per, alpha Her, delta^2 Lyr, U CMi) that otherwise
have not been detected in any radio lines. Conversely it was not detected in
the two oxygen-rich stars with substantial mass-loss rate, NML Tau and WX Psc,
possibly because these sources are young with hydrogen in molecular form,
and/or because the temperature of the circumstellar HI gas is very low (< 5 K).Comment: Accepted for publication in The Astronomical Journa
The variable mass loss of the AGB star WX Psc as traced by the CO J=1-0 through 7-6 lines and the dust emission
Low and intermediate mass stars lose a significant fraction of their mass
through a dust-driven wind during the Asymptotic Giant Branch (AGB) phase.
Recent studies show that winds from late-type stars are far from being smooth.
Mass-loss variations occur on different time scales, from years to tens of
thousands of years. The variations appear to be particularly prominent towards
the end of the AGB evolution. The occurrence, amplitude and time scale of these
variations are still not well understood.
The goal of our study is to gain insight into the structure of the
circumstellar envelope (CSE) of WX Psc and map the possible variability of the
late-AGB mass-loss phenomenon.
We have performed an in-depth analysis of the extreme infrared AGB star WX
Psc by modeling (1) the CO J=1-0 through 7-6 rotational line profiles and the
full spectral energy distribution (SED) ranging from 0.7 to 1300 micron. We
hence are able to trace a geometrically extended region of the CSE.
Both mass-loss diagnostics bear evidence of the occurrence of mass-loss
modulations during the last ~2000 yr. In particular, WX Psc went through a high
mass-loss phase (Mdot~5e-5 Msun/yr) some 800 yr ago. This phase lasted about
600 yr and was followed by a long period of low mass loss (Mdot~5e-8 Msun/yr).
The present day mass-loss rate is estimated to be ~6e-6 Msun/yr.
The AGB star WX Psc has undergone strong mass-loss rate variability on a time
scale of several hundred years during the last few thousand years. These
variations are traced in the strength and profile of the CO rotational lines
and in the SED. We have consistently simulated the behaviour of both tracers
using radiative transfer codes that allow for non-constant mass-loss rates.Comment: 12 pages, accepted for publication in A&
Dusty Infrared Galaxies: Sources of the Cosmic Infrared Background
The discovery of the Cosmic Infrared Background (CIB) in 1996, together with
recent cosmological surveys from the mid-infrared to the millimeter have
revolutionized our view of star formation at high redshifts. It has become
clear, in the last decade, that a population of galaxies that radiate most of
their power in the far-infrared (the so-called ``infrared galaxies'')
contributes an important part of the whole galaxy build-up in the Universe.
Since 1996, detailed (and often painful) investigations of the high-redshift
infrared galaxies have resulted in the spectacular progress covered in this
review. We outline the nature of the sources of the CIB including their
star-formation rate, stellar and total mass, morphology, metallicity and
clustering properties. We discuss their contribution to the stellar content of
the Universe and their origin in the framework of the hierarchical growth of
structures. We finally discuss open questions for a scenario of their evolution
up to the present-day galaxies.Comment: To appear in Annual Reviews of Astronomy and Astrophysics, 2005, vol
43. 31 pages, 12 color figure
An HST Snapshot Survey of Proto-Planetary Nebulae Candidates: Two Types of Axisymmetric Reflection Nebulosities
We report the results from an optical imaging survey of proto-planetary
nebula candidates using the HST. We exploited the high resolving power and wide
dynamic range of HST and detected nebulosities in 21 of 27 sources. All
detected reflection nebulosities show elongation, and the nebula morphology
bifurcates depending on the degree of the central star obscuration. The
Star-Obvious Low-level-Elongated (SOLE) nebulae show a bright central star
embedded in a faint, extended nebulosity, whereas the DUst-Prominent
Longitudinally-EXtended (DUPLEX) nebulae have remarkable bipolar structure with
a completely or partially obscured central star. The intrinsic axisymmetry of
these proto-planetary nebula reflection nebulosities demonstrates that the
axisymmetry frequently found in planetary nebulae predates the proto-planetary
nebula phase, confirming previous independent results. We suggest that
axisymmetry in proto-planetary nebulae is created by an equatorially enhanced
superwind at the end of the asymptotic giant branch phase. We discuss that the
apparent morphological dichotomy is caused by a difference in the optical
thickness of the circumstellar dust/gas shell with a differing equator-to-pole
density contrast. Moreover, we show that SOLE and DUPLEX nebulae are physically
distinct types of proto-planetary nebulae, with a suggestion that higher mass
progenitor AGB stars are more likely to become DUPLEX proto-planetary nebulae.Comment: 27 pages (w/ aaspp4.sty), 6 e/ps figures, 4 tables (w/ apjpt4.sty).
Data images are available via ADIL
(http://imagelib.ncsa.uiuc.edu/document/99.TU.01) To be published in Ap
Observation of periodic variable stars towards the galactic spiral arms by EROS II
We present the results of a massive variability search based on a photometric
survey of a six square degree region along the Galactic plane at (, ) and (, ). This
survey was performed in the framework of the EROS II (Exp\'erience de Recherche
d'Objets Sombres) microlensing program. The variable stars were found among
1,913,576 stars that were monitored between April and June 1998 in two
passbands, with an average of 60 measurements. A new period-search technique is
proposed which makes use of a statistical variable that characterizes the
overall regularity of the flux versus phase diagram. This method is well suited
when the photometric data are unevenly distributed in time, as is our case.
1,362 objects whose luminosity varies were selected. Among them we identified 9
Cepheids, 19 RR Lyrae, 34 Miras, 176 eclipsing binaries and 266 Semi-Regular
stars. Most of them are newly identified objects. The cross-identification with
known catalogues has been performed. The mean distance of the RR Lyrae is
estimated to be kpc undergoing an average absorption of
magnitudes. This distance is in good agreement with the one
of disc stars which contribute to the microlensing source star population.Our
catalogue and light curves are available electronically from the CDS,
Strasbourg and from our Web site http://eros.in2p3.fr.Comment: 15 pages, 11 figures, accepted in A&A (april 2002
Effects of Metallicity on the Chemical Composition of Carbon Stars
We present \emph{Spitzer} IRS data on 19 asymptotic giant branch (AGB) stars
in the Large Magellanic Cloud, complementing existing published data sets of
carbon stars in both Magellanic Clouds and the Milky Way, to investigate the
effects of metallicity on dust and molecular spectral features arising from the
circumstellar envelope. We find that the CH P and R branches at 7.5
micron are affected by dust dilution at higher mass-loss rates -- albeit to a
lesser extent for sources in the Magellanic Clouds, compared to the Milky Way
-- while the narrow 13.7 micron CH Q branch only shows the effect of
dust dilution at low mass-loss rates. A strong metallicity dependence is not
observed for the Q branch. Independent of metallicity, we also provide an
explanation for the observed shifts in the central wavelength of the SiC
emission feature, as we show that these are largely caused by molecular band
absorption on either side of the dust emission feature, dominating over shifts
in the central wavelength caused by self-absorption. We have devised a method
to study the dust condensation history in carbon-rich AGB stars in different
metallicity environments, by measuring the strength of the 11.3 \um SiC and 30
\um MgS features with respect to the continuum, as a function of mass-loss
rate. With this method, it is possible to distinguish in what order SiC and
graphite condense, which is believed to be sensitive to the metallicity, prior
to the eventual deposit of the MgS layer.Comment: 18 pages, 13 figures, Accepted to Ap
Bispectrum speckle interferometry of IRC+10216: the dynamic evolution of the innermost circumstellar environment from 1995 to 2001
We present new near-infrared (JHK) bispectrum speckle-interferometry
monitoring of the carbon star IRC+10216 obtained between 1999 and 2001 with the
SAO 6m telescope. The J-, H-, and K-band resolutions are 50mas, 56mas, and
73mas, resp. The total sequence of K-band observations covers now 8 epochs from
1995 to 2001 and shows the dynamic evolution of the inner dust shell. The
present observations show that the appearance of the dust shell has
considerably changed compared to the epochs of 1995 to 1998. Four main
components within a 0.2" radius can be identified in the K-band images. The
apparent separation of the two initially brightest components A and B increased
from ~191mas in 1995 to ~351mas in 2001. Simultaneously, component B has been
fading and almost disappeared in 2000 whereas the initially faint components C
and D became brighter (relative to peak intensity). These changes can be
related to changes of the optical depth caused, e.g., by mass-loss variations
or new dust condensation in the wind. Our 2D radiative transfer model suggests
that the observed relative motion of A and B is not consistent with the known
terminal wind velocity of 15 km/s. The apparent motion with a deprojected
velocity of 19 km/s on average and of recently 27 km/s appears to be caused by
adisplacement of the dust density peak due to dust evaporation in the optically
thicker and hotter environment. Our monitoring, covering more than 3 pulsation
periods, shows that the structural variations are not related to the stellar
pulsation cycle in a simple way. This is consistent with the predictions of
hydrodynamical models that enhanced dust formation takes place on a timescale
of several pulsation periods. The timescale of the fading of component B can
well be explained by the formation of new dust in the circumstellar envelope.Comment: 11 pages including 6 PostScript figures; also available from
http://www.mpifr-bonn.mpg.de/div/ir-interferometry/publications.html;
Astronomy & Astrophysics, in pres
Removing the Microlensing Blending-Parallax Degeneracy Using Source Variability
Microlensing event MACHO 97-SMC-1 is one of the rare microlensing events for
which the source is a variable star, simply because most variable stars are
systematically eliminated from microlensing studies. Using observational data
for this event, we show that the intrinsic variability of a microlensed star is
a powerful tool to constrain the nature of the lens by breaking the degeneracy
between the microlens parallax and the blended light. We also present a
statistical test for discriminating the location of the lens based on the
\chi^2 contours of the vector \Lambda, the inverse of the projected velocity.
We find that while SMC self lensing is somewhat favored over halo lensing,
neither location can be ruled out with good confidence.Comment: 15 text pages + 2 tables + 7 figures. Published in the Astrophysical
Journa
Multiple Molecular Winds in Evolved Stars I. A Survey of CO(2-1) and CO(3-2) Emission from 45 Nearby AGB Stars
This paper describes observations of a new phenomenon in evolved mass-losing
AGB stars: the presence of two winds with different expansion velocities.
CO(2-1) and CO(3-2) line emission was observed for 45 AGB stars at high
velocity resolution and double winds found in 20% of the sample. Highly
asymmetric lines were found in six other stars. The data tentatively suggest
that double winds occur when the star undergoes a change (pulsational mode,
chemical composition) and that the very narrow components represent the onset
of a new phase of mass loss.Comment: 53 pages, plain TeX, 16 figures in file figs.p
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