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

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