Circumstellar Envelopes Of AGB Stars Research Articles

AGB Stars and Their Circumstellar Envelopes. I. the VULCAN Code

The interplay between AGB interiors and their outermost layers, where molecules and dust form, is a problem of high complexity. As a consequence, physical processes like mass loss, which depend on the chemistry of the circumstellar envelope, are often oversimplified. The best candidates to drive mass-loss in AGB stars are dust grains, which trap the outgoing radiation and drag the surrounding gas. Grains build up, however, is far from being completely understood. Our aim is to model both the physics and the chemistry of the cool expanding layers around AGB stars in order to characterize the on-going chemistry, from atoms to dust grains. This has been our rationale to develop ab initio VULCAN, a FORTRAN hydro code able to follow the propagation of shocks in the circumstellar envelopes of AGB stars. The version presented in this paper adopts a perfect gas law and a very simplified treatment of the radiative transfer effects and dust nucleation. In this paper, we present preliminary results obtained with our code.

Early Results from SOFIA/EXES High Resolution Spectroscopic Observations of Circumstellar Envelopes of AGB Stars

Early Results from SOFIA/EXES High Resolution Spectroscopic Observations of Circumstellar Envelopes of AGB Stars

Chemical equilibrium in AGB atmospheres: successes, failures, and prospects for small molecules, clusters, and condensates

Chemical equilibrium has proven extremely useful for predicting the chemical composition of AGB atmospheres. Here we use a recently developed code and an updated thermochemical database that includes gaseous and condensed species involving 34 elements to compute the chemical equilibrium composition of AGB atmospheres of M-, S-, and C-type stars. We include for the first time Ti x C y clusters, with x = 1-4 and y = 1-4, and selected larger clusters ranging up to Ti13C22, for which thermochemical data are obtained from quantum-chemical calculations. Our main aims are to systematically survey the main reservoirs of each element in AGB atmospheres, review the successes and failures of chemical equilibrium by comparing it with the latest observational data, identify potentially detectable molecules that have not yet been observed, and diagnose the most likely gas-phase precursors of dust and determine which clusters might act as building blocks of dust grains. We find that in general, chemical equilibrium reproduces the observed abundances of parent molecules in circumstellar envelopes of AGB stars well. There are, however, severe discrepancies of several orders of magnitude for some parent molecules that are observed to be anomalously overabundant with respect to the predictions of chemical equilibrium. These are HCN, CS, NH3, and SO2 in M-type stars, H2O and NH3 in S-type stars, and the hydrides H2O, NH3, SiH4, and PH3 in C-type stars. Several molecules have not yet been observed in AGB atmospheres but are predicted with non-negligible abundances and are good candidates for detection with observatories such as ALMA. The most interesting ones are SiC5, SiNH, SiCl, PS, HBO, and the metal-containing molecules MgS, CaS, CaOH, CaCl, CaF, ScO, ZrO, VO, FeS, CoH, and NiS. In agreement with previous studies, the first condensates predicted to appear in C-rich atmospheres are found to be carbon, TiC, and SiC, while Al2O3 is the first major condensate expected in O-rich outflows. According to our chemical equilibrium calculations, the gas-phase precursors of carbon dust are probably acetylene, atomic carbon, and/or C3, while for silicon carbide dust, the most likely precursors are the molecules SiC2 and Si2C. In the case of titanium carbide dust, atomic Ti is the major reservoir of this element in the inner regions of AGB atmospheres, and therefore it is probably the main supplier of titanium during the formation of TiC dust. However, chemical equilibrium predicts that large titanium-carbon clusters such as Ti8C12 and Ti13C22 become the major reservoirs of titanium at the expense of atomic Ti in the region where condensation of TiC is expected to occur. This suggests that the assembly of large Ti x C y clusters might be related to the formation of the first condensation nuclei of TiC. In the case of Al2O3 dust, chemical equilibrium indicates that atomic Al and the carriers of Al-O bonds AlOH, AlO, and Al2O are the most likely gas-phase precursors.

The Role of Internal Photons on the Chemistry of the Circumstellar Envelopes of AGB Stars

Abstract Recent high spatial resolution observations of gas and dust in the circumstellar envelopes (CSEs) of AGB stars indicate morphologies much more complex than the smooth density distributions generated by spherically symmetric, constant mass-loss rates. In particular, the observation of spiral arcs and disks indicates the likely presence of a binary companion that in some cases gives rise to the UV photons detected by Galaxy Evolution Explorer. In this paper, we extend our recent model of the chemistry in a clumpy, porous CSE around an AGB star to include the influence of stellar blackbody photons on the CSE chemistry. Our results indicate that internal photons, in a clumpy, porous CSE, can alter chemistry within a few stellar radii and, for some molecules, alter abundances out to several hundred stellar radii. They further suggest that harder radiation from companion stars or accretion disks will have a substantial impact on chemistry in the dust formation zones and inner CSEs of AGB stars.

Zooming into the complex dusty envelopes of C-rich AGB stars

AbstractRecent advances in high-angular resolution instruments (VLT and VLTI, ALMA) have enabled us to delve deep into the circumstellar envelopes of AGB stars from the optical to the sub-mm wavelengths, thus allowing us to study in detail the gas and dust formation zones (e.g., their geometry, chemistry and kinematics). This work focuses on four (4) C-rich AGB stars observed with a high-angular resolution technique in the near-infrared: a multi-wavelength tomographic study of the dusty layers of the circumstellar envelopes of these C-rich stars, i.e. the variations in the morphology and temperature distribution.

Chemistry in AGB stars: successes and challenges

Emission and absorption line observations of molecules in late-type stars are a vital component in our understanding of stellar evolution, dust formation and mass loss in these objects. The molecular composition of the gas in the circumstellar envelopes of AGB stars reflects chemical processes in gas whose properties are strong functions of radius with density and temperature varying by more than ten and two orders of magnitude, respectively. In addition, the interstellar UV field plays a critical role in determining not only molecular abundances but also their radial distributions. In this article, I shall briefly review some recent successful approaches to describing chemistry in both the inner and outer envelopes and outline areas of challenge for the future.

The abundance of HCN in circumstellar envelopes of AGB stars of different chemical type

A multi-transition survey of HCN (sub-) millimeter line emission from a large sample of AGB stars of different chemical type is presented. The data are analysed and circumstellar HCN abundances are estimated. The sample stars span a large range of properties such as mass-loss rate and photospheric C/O-ratio. The analysis of the new data allows for more accurate estimates of the circumstellar HCN abundances and puts new constraints on chemical models. In order to constrain the circumstellar HCN abundance distribution a detailed non-LTE excitation analysis, based on the Monte Carlo method, is performed. Effects of line overlaps and radiative excitation from dust grains are included. The median values for the derived abundances of HCN (with respect to H2) are 3x10-5, 7x10-7 and 10-7 for carbon stars (25 stars), S-type AGB stars (19 stars) and M-type AGB stars (25 stars), respectively. The estimated sizes of the HCN envelopes are similar to those obtained in the case of SiO for the same sample of sources and agree well with previous results from interferometric observations, when these are available. We find that there is a clear dependence of the derived circumstellar HCN abundance on the C/O-ratio of the star, in that carbon stars have about two orders of magnitude higher abundances than M-type AGB stars, on average. The derived HCN abundances of the S-type AGB stars have a larger spread and typically fall in between those of the two other types, however, slightly closer to the values for the M-type AGB stars. For the M-type stars, the estimated abundances are much higher than what would be expected if HCN is formed in thermal equilibrium. However, the results are also in contrast to predictions from recent non-LTE chemical models, where very little difference is expected in the HCN abundances between the various types of AGB stars.

CO observations of water-maser post-asymptotic giant branch stars and detection of a high-velocity outflow in IRAS 15452–5459

Many aspects of the evolutionary phase in which Asymptotic Giant Branch stars (AGB stars) are in transition to become Planetary Nebulae (PNe) are still poorly understood. An important question is how the circumstellar envelopes of AGB stars switch from spherical symmetry to the axially symmetric structures frequently observed in PNe. In many cases there is clear evidence that the shaping of the circumstellar envelopes of PNe is linked to the formation of jets/collimated winds and their interaction with the remnant AGB envelope. Because of the short evolutionary time, objects in this phase are rare, but their identification provides valuable probes for testing evolutionary models. We have observed (sub)millimeter CO rotational transitions with the APEX telescope in a small sample of stars hosting high-velocity OH and water masers. These targets are supposed to have recently left the AGB, as indicated by the presence of winds traced by masers, with velocities larger than observed during that phase. We have carried out observations of several CO lines, ranging from J=2-1 up to J=7-6. In IRAS 15452-5459 we detect a fast molecular outflow in the central region of the nebula and estimate a mass-loss rate between 1.2x10^{-4} Msun yr^{-1} (assuming optically thin emission) and 4.9x10^{-4} Msun yr^{-1} (optically thick emission). We model the SED of this target taking advantage of our continuum measurement at 345 GHz to constrain the emission at long wavelengths. For a distance of 2.5 kpc, we estimate a luminosity of 8000 Lsun and a dust mass of 0.01 Msun. Through the flux in the [CII] line (158 um), we calculate a total mass of about 12 Msun for the circumstellar envelope, but the line is likely affected by interstellar contamination.

The physics and chemistry of circumstellar envelopes of S-stars on the AGB

AbstractPresented here are the preliminary results of a long-term study of S-stars on the AGB. S-stars are important as possible transition objects between oxygen-rich M-stars and carbon stars. The aim of the study is to compare results from our newly gathered observational database for the S-stars with those already obtained for the M- and carbon stars. We can thus follow the changes as the stars evolve along the AGB and more firmly establish the suggested M-MS-S-SC-C evolutionary sequence. It will also allow us to determine the relative importance of processes such as non-equilibrium chemistry, grain formation, and photodissociation in regulating the chemistry in circumstellar envelopes of AGB stars.

SiO in C-rich circumstellar envelopes of AGB stars: effects of non-LTE chemistry and grain adsorption

Aims.New SiO multi-transition millimetre line observations of a sample of carbon stars, including J = observations with the APEX telescope, are used to probe the role of non-equilibrium chemistry and the influence of grains in circumstellar envelopes of carbon stars.

Molecular Abundances in AGB Circumstellar Envelopes

In this review the present status of molecules in circumstellar envelopes of AGB stars is presented. Emphasis is put on the determination of abundances, and estimates of their uncertainties, from an observational point of view. Despite an impressive number of circumstellar species detected, about 60, there remains much work before general conclusions can be drawn. In particular, sophisticated radiative transfer modelling of circumstellar line emission must be done. This requires a detailed knowledge of the stellar and circumstellar properties, as well as basic molecular physics/chemistry data.

AGB maser stars as tracers of stellar populations

AbstractMasers are known to occur in circumstellar envelopes of AGB stars that are long-period variables and have entered the TP-AGB phase. The maser radiation is intrinsically weak and detection limits prevent us from seeing maser stars much beyond the Galactic Center. Very deep searches have led to the detection of a handful of maser stars in the Large Magellanic Cloud. Some 1000 stellar masers are known. They trace a population of stars with main sequence masses between 1 and 6 M⊙ and ages between 2 and 8 Gyr, the same population that is traced by Cepheids. Their distribution in our Galaxy is the same as that of the red giants, and therefore the stars trace the galactic bar. An outstanding feature is a small disk of radius 150 pc around the galactic center. The disk is in rapid, solid body rotation and leads to a good measurement of the mass density within that distance for Sgr A.

Mid Infrared Colors of Early Type Galaxies

We compare the mid infrared colors of early type galaxies with isochrones that suitably account for the effects of dust in the circumstellar envelopes of AGB stars. We show that these stars affect significantly the integrated colors in the ISO bands. The excess at 10μm observed in some early type galaxies combined with the optical spectrum, is compatible with a 100% old metal rich population. Limits on the additional young components are: age 0.2 Gyr, mass=0.5% and E(B-V)≥0.5 or age 3 Gyr, mass 10% and E(B-V)≥0.2.

ISO's View of the Molecular Content of Evolved Stars

The recent results of the ISO satellite in the field of molecular spectrocopy of AGB stars are reviewed. For the fist time, the two spectrometers onboard ISO have provided the opportunity to observe the pure rotational lines of several molecules in the far infrared and the ro-vibrational bands of the most abundant molecular species in the near and mid-infrared. These data allow to carry out a systematic study of the circumstellar envelopes of AGB stars and Planetary Nebulae. I analyze in this paper the role of resonant scatterring in the pumping of the ro-vibrational molecular levels in CSEs.

Model Atmospheres of AGB Stars: Dynamics, Molecules and Dust

The atmospheres and circumstellar envelopes of AGB stars are characterized by complex physical phenomena like shock waves caused by stellar pulsation or formation of molecules and dust which often lead to a heavy mass loss and have a strong influence on IR properties as observed by ISO. To allow a physical interpretation of various observations we have constructed improved dynamical model atmospheres of long-period variables. In this contribution we mainly investigate the dependence of the atmospheric structure and its variability on stellar pulsation, molecular opacities and time-dependent dust formation. IR spectra resulting from our models are discussed in detail by Loidl et al. (1997b) and compared to ISO-SWS spectra obtained by Hron et al. (1997).

ON THE EXPANSION VELOCITIES OF PLANETARY NEBULAE AND THE CIRCUMSTELLAR ENVELOPES OF AGB STARS

ABSTRACT The distribution of expansion velocities of the gas of planetary nebulae are compared to the distribution of the circumstellar envelopes of AGB stars. We have examined the relationship between ionized and neutral expansion velocities for the nebulae with detected circumnebular neutral shells. It is found that compact planetary nebulae with ionized radii less than about 0.05 pc show a distribution of expansion velocities that is similar to AGB envelopes, while large planetary nebulae with ionized radii greater than about 0.30 pc show a significantly different expansion velocity distribution than AGB envelopes. Among the large planetary nebula subset, only those nebulae of Greig's class C have an expansion velocity distribution significantly different than AGB stars, while large Grieg's class B nebulae have the same expansion velocity distribution as AGB stars. The average velocity difference between AGB stars and large Grieg's class C nebulae is well predicted by the interacting stellar winds model of planetary nebula evolution. However, the acceleration of ionized gas predicted by the interaction of stellar winds is not evident in the observed expansion velocity distribution of large Grieg's class B nebulae.

Interferometric observations of non-maser SiO emission from circumstellar envelopes of AGB stars - Acceleration regions and SiO depletion

High- and medium-resolution images of SiO J = 2-1(V = 0) from the circumstellar envelopes (CSEs) of three oxygen-rich stars, Chi Cyg, RX Boo, and IK Tau, were obtained. The SIO images were found to be roughly circular, implying that the CSEs are spherically symmetric on angular-size scales of about 3-9 arcsec. The observed angular half-maximum intensity source radius is nearly independent of the LSR velocity for all three CSEs. Chi Cyg and RX Boo are argued to be less than 450 pc distant, and have mass-loss rates larger than about 10 exp -6 solar mass/yr. In Chi Cyg and RX Boo, the line profiles at the peak of the brightness distribution are rounded, typical of optically-thick emission from a spherical envelope expanding with a constant velocity. In the IK Tau line profiles, an additional narrower central component is present, probably a result of emission from an inner circumstellar shell with a significantly smaller expansion velocity than the extended envelope.