27 research outputs found
Dust grain properties in atmospheres of AGB stars
We present self-consistent dynamical models for dust driven winds of
carbon-rich AGB stars. The models are based on the coupled system of
frequency-dependent radiation hydrodynamics and time-dependent dust formation.
We investigate in detail how the wind properties of the models are influenced
by the micro-physical properties of the dust grains that enter as parameters.
The models are now at a level where it is necessary to be quantitatively
consistent when choosing the dust properties that enters as input into the
models. At our current level of sophistication the choice of dust parameters is
significant for the derived outflow velocity, the degree of condensation and
the estimated mass loss rates of the models. In the transition between models
with and without mass-loss the choice ofmicro-physical parameters turns out to
be very significant for whether a particular set of stellar parameters will
give rise to a dust-driven mass loss or not.Comment: 10 pages, 3 figures. To appear in: Modelling of Stellar Atmospheres,
N.E. Piskunov, W.W. Weiss, D.F. Gray (eds.), IAU Symposium Vol. xxx.
Proceedings for the IAU Symposium 210, Uppsala, June 200
Dynamic model atmospheres of cool giants
Cool giant stars are highly dynamical objects, and complex micro-physical
processes play an important role in their extended atmospheres and winds. The
interpretation of observations, and in particular of high-resolution IR
spectra, requires realistic self-consistent model atmospheres. Current
dynamical models include rather detailed micro-physics, and the resulting
synthetic spectra compare reasonably well with observations. A transition from
qualitative to quantitative modelling is taking place at present. We give an
overview of existing dynamical model atmospheres for AGB stars, discussing
recent advances and current trends in modelling, including 3D 'star-in-a-box'
models. When comparing synthetic spectra and other observable properties
resulting from dynamical models with observations we focus on the near- and
mid-IR wavelength range.Comment: 12 pages, 3 figures, to be published in Proc. of ESO Workshop on High
Resolution Infrared Spectroscopy in Astronomy, eds. Kaeufl H.U., Siebenmorgen
R., Moorwood A., ESO Astrophysics Symposia, Springe
Intense Mass Loss from C-rich AGB Stars at low Metallicity?
We argue that the energy injection of pulsations may be of greater importance
to the mass-loss rate of AGB stars than metallicity, and that the mass-loss
trend with metallicity is not as simple as sometimes assumed. Using our
detailed radiation hydrodynamical models that include dust formation, we
illustrate the effects of pulsation energy on wind properties. We find that the
mass-loss rate scales with the kinetic energy input by pulsations as long as a
dust-saturated wind does not occur, and all other stellar parameters are kept
constant. This includes the absolute abundance of condensible carbon (not bound
in CO), which is more relevant than keeping the C/O-ratio constant when
comparing stars of different metallicity. The pressure and temperature
gradients in the atmospheres of stars, become steeper and flatter,
respectively, when the metallicity is reduced, while the radius where the
atmosphere becomes opaque is typically associated with a higher gas pressure.
This effect can be compensated for by adjusting the velocity amplitude of the
variable inner boundary (piston), which is used to simulate the effects of
pulsation, to obtain models with comparable kinetic-energy input. Hence, it is
more relevant to compare models with similar energy-injections than of similar
velocity amplitude. Since there is no evidence for weaker pulsations in
low-metallicity AGB stars, we conclude that it is unlikely that low-metallicity
C-stars have a lower mass-loss rate, than their more metal-rich counterparts
with similar stellar parameters, as long as they have a comparable amount of
condensible carbon.Comment: 4 pages, 3 figures. Accepted for publication in A&A. Updated after
language editing. Additional typos fixe
Atmospheric dynamics in carbon-rich Miras. I. Model atmospheres and synthetic line profiles
Atmospheres of evolved AGB stars are heavily affected by pulsation, dust
formation and mass loss, and they can become very extended. Time series of
observed high-resolution spectra proved to be a useful tool to study
atmospheric dynamics throughout the outer layers of these pulsating red giants.
Originating at various depths, different molecular spectral lines observed in
the near-infrared can be used to probe gas velocities there for different
phases during the lightcycle. Dynamic model atmospheres are needed to represent
the complicated structures of Mira variables properly. An important aspect
which should be reproduced by the models is the variation of line profiles due
to the influence of gas velocities. Based on a dynamic model, synthetic spectra
(containing CO and CN lines) were calculated, using an LTE radiative transfer
code that includes velocity effects. It is shown that profiles of lines that
sample different depths qualitatively reproduce the behaviour expected from
observations.Comment: accepted by A&A, 12 pages, 9 figure
Period and chemical evolution of SC stars
The SC and CS stars are thermal-pulsing AGB stars with C/O ratio close to
unity. Within this small group, the Mira variable BH Cru recently evolved from
spectral type SC (showing ZrO bands) to CS (showing weak C2). Wavelet analysis
shows that the spectral evolution was accompanied by a dramatic period
increase, from 420 to 540 days, indicating an expanding radius. The pulsation
amplitude also increased. Old photographic plates are used to establish that
the period before 1940 was around 490 days. Chemical models indicate that the
spectral changes were caused by a decrease in stellar temperature, related to
the increasing radius. There is no evidence for a change in C/O ratio. The
evolution in BH Cru is unlikely to be related to an on-going thermal pulse.
Periods of the other SC and CS stars, including nine new periods, are
determined. A second SC star, LX Cyg, also shows evidence for a large increase
in period, and one further star shows a period inconsistent with a previous
determination. Mira periods may be intrinsically unstable for C/O ~ 1; possibly
because of a feedback between the molecular opacities, pulsation amplitude, and
period. LRS spectra of 6 SC stars suggest a feature at wavelength > 15 micron,
which resembles one recently attributed to the iron-sulfide troilite. Chemical
models predict a large abundance of FeS in SC stars, in agreement with the
proposed association.Comment: 14 pages, 20 figures. MNRAS, 2004, accepted for publication. Janet
Mattei, one of the authors, died on 22 March, 2004. This paper is dedicated
to her memor
Dust formation in winds of long-period variables. V. The influence of micro-physical dust properties in carbon stars
We present self-consistent dynamical models for dust-driven winds of
carbon-rich AGB stars. The models are based on the coupled system of
frequency-dependent radiation hydrodynamics and time-dependent dust formation.
We investigate in detail how the wind properties of the models are influenced
by the micro-physical properties of the dust grains that are required by the
description of grain formation. The choice of dust parameters is significant
for the derived outflow velocity, the degree of condensation and the resulting
mass loss rates of the models. In the transition region between models with and
without mass loss the choice ofmicro-physical parameters turns out to be very
significant for whether a particular set of stellar parameters will give rise
to a dust-driven mass loss or not. We also calculate near-infrared colors to
test how the dust parameters influence the observable properties of the models,
however, at this point we do not attempt to fit particular stars.Comment: 13 pages, 8 figures, A&A in pres
Dust and molecular shells in asymptotic giant branch stars - Mid-infrared interferometric observations of R Aql, R Aqr, R Hya, W Hya and V Hya
Mid-IR (8 - 13 micron) interferometric data of four oxygen-rich AGB stars (R
Aql, R Aqr, R Hya, and W Hya) and one carbon-rich AGB star (V Hya) were
obtained with MIDI/VLTI between April 2007 and September 2009. The spectrally
dispersed visibility data are analyzed by fitting a circular fully
limb-darkened disk (FDD). Results. The FDD diameter as function of wavelength
is similar for all oxygen-rich stars. The apparent size is almost constant
between 8 and 10 micron and gradually increases at wavelengths longer than 10
micron. The apparent FDD diameter in the carbon-rich star V Hya essentially
decreases from 8 to 12 micron. The FDD diameters are about 2.2 times larger
than the photospheric diameters estimated from K-band observations found in the
literature. The silicate dust shells of R Aql, R Hya and W Hya are located
fairly far away from the star, while the silicate dust shell of R Aqr and the
amorphous carbon (AMC) and SiC dust shell of V Hya are found to be closer to
the star at around 8 photospheric radii. Phase-to-phase variations of the
diameters of the oxygen-rich stars could be measured and are on the order of
15% but with large uncertainties. From a comparison of the diameter trend with
the trends in RR Sco and S Ori it can be concluded that in oxygen-rich stars
the overall larger diameter originates from a warm molecular layer of H2O, and
the gradual increase longward of 10 micron can be most likely attributed to the
contribution of a close Al2O3 dust shell. The chromatic trend of the Gaussian
FWHM in V Hya can be explained with the presence of AMC and SiC dust. The
observations suggest that the formation of amorphous Al2O3 in oxygen- rich
stars occurs mainly around or after visual minimum. However, no firm
conclusions can be drawn concerning the mass-loss mechanism.Comment: 32 pages (including 7 pages appendix), 10 figure
Production of dust by massive stars at high redshift
The large amounts of dust detected in sub-millimeter galaxies and quasars at
high redshift pose a challenge to galaxy formation models and theories of
cosmic dust formation. At z > 6 only stars of relatively high mass (> 3 Msun)
are sufficiently short-lived to be potential stellar sources of dust. This
review is devoted to identifying and quantifying the most important stellar
channels of rapid dust formation. We ascertain the dust production efficiency
of stars in the mass range 3-40 Msun using both observed and theoretical dust
yields of evolved massive stars and supernovae (SNe) and provide analytical
expressions for the dust production efficiencies in various scenarios. We also
address the strong sensitivity of the total dust productivity to the initial
mass function. From simple considerations, we find that, in the early Universe,
high-mass (> 3 Msun) asymptotic giant branch stars can only be dominant dust
producers if SNe generate <~ 3 x 10^-3 Msun of dust whereas SNe prevail if they
are more efficient. We address the challenges in inferring dust masses and
star-formation rates from observations of high-redshift galaxies. We conclude
that significant SN dust production at high redshift is likely required to
reproduce current dust mass estimates, possibly coupled with rapid dust grain
growth in the interstellar medium.Comment: 72 pages, 9 figures, 5 tables; to be published in The Astronomy and
Astrophysics Revie