scholarly journals Mass-losing AGB Stars in the LMC

2000 ◽  
Vol 177 ◽  
pp. 145-151
Author(s):  
Jacco Th. Van Loon ◽  
Albert A. Zijlstra ◽  
Patricia A. Whitelock ◽  
Cecile Loup ◽  
L.B.F.M. Waters
Keyword(s):  
Mass Loss ◽  
Optical Depth ◽  
Agb Stars ◽  
Circumstellar Envelopes ◽  
Carbon Stars ◽  
Infrared Study ◽  
Tentative Evidence ◽  
Loss Rates

We show the results of an infrared study of a sample of heavily obscured AGB stars in the LMC. Both carbon-rich and oxygen-rich mass-losing AGB stars can be found at both high and low luminosities, but the percentage of carbon stars decreases with increasing luminosity. The optical depth of the circumstellar envelopes also decreases with increasing luminosity, while the mass-loss rates are (nearly) constant with luminosity. We also show tentative evidence for having found the first post-AGB stars in the LMC.

scholarly journals Carbon star wind models at solar and sub-solar metallicities: a comparative study

2019 ◽  
Vol 623 ◽  
pp. A119 ◽  
Author(s):  
S. Bladh ◽  
K. Eriksson ◽  
P. Marigo ◽  
S. Liljegren ◽  
B. Aringer
Keyword(s):  
Mass Loss ◽  
Agb Stars ◽  
Carbon Stars ◽  
Dust Production ◽  
Grain Sizes ◽  
Solar Metallicity ◽  
Effective Temperatures ◽  
Carbon Excess ◽  
Thermal Pulses ◽  
Loss Rates

Context. The heavy mass loss observed in evolved stars on the asymptotic giant branch (AGB) is usually attributed to dust-driven winds, but it is still an open question how much AGB stars contribute to the dust production in the interstellar medium, especially at lower metallicities. In the case of C-type AGB stars, where the wind is thought to be driven by radiation pressure on amorphous carbon grains, there should be significant dust production even in metal-poor environments. Carbon stars can manufacture the building blocks needed to form the wind-driving dust species themselves, irrespective of the chemical composition they have, by dredging up carbon from the stellar interior during thermal pulses. Aims. We investigate how the mass loss in carbon stars is affected by a low-metallicity environment, similar to the Large and Small Magellanic Clouds (LMC and SMC). Methods. The atmospheres and winds of C-type AGB stars are modeled with the 1D spherically symmetric radiation-hydrodynamical code Dynamic Atmosphere and Radiation-driven Wind models based on Implicit Numerics (DARWIN). The models include a time-dependent description for nucleation, growth, and evaporation of amorphous carbon grains directly out of the gas phase. To explore the metallicity-dependence of mass loss we calculate model grids at three different chemical abundances (solar, LMC, and SMC). Since carbon may be dredged up during the thermal pulses as AGB stars evolve, we keep the carbon abundance as a free parameter. The models in these three different grids all have a current mass of one solar mass; effective temperatures of 2600, 2800, 3000, or 3200 K; and stellar luminosities equal to logL*∕L⊙ = 3.70, 3.85, or 4.00. Results. The DARWIN models show that mass loss in carbon stars is facilitated by high luminosities, low effective temperatures, and a high carbon excess (C–O) at both solar and subsolar metallicities. Similar combinations of effective temperature, luminosity, and carbon excess produce outflows at both solar and subsolar metallicities. There are no large systematic differences in the mass-loss rates and wind velocities produced by these wind models with respect to metallicity, nor any systematic difference concerning the distribution of grain sizes or how much carbon is condensed into dust. DARWIN models at subsolar metallicity have approximately 15% lower mass-loss rates compared to DARWIN models at solar metallicity with the same stellar parameters and carbon excess. For both solar and subsolar environments typical grain sizes range between 0.1 and 0.5 μm, the degree of condensed carbon varies between 5 and 40%, and the gas-to-dust ratios between 500 and 10 000. Conclusions. C-type AGB stars can contribute to the dust production at subsolar metallicities (down to at least [Fe∕H] = −1) as long as they dredge up sufficient amounts of carbon from the stellar interior. Furthermore, stellar evolution models can use the mass-loss rates calculated from DARWIN models at solar metallicity when modeling the AGB phase at subsolar metallicities if carbon excess is used as the critical abundance parameter instead of the C/O ratio.

scholarly journals The Evolution of AGB Stars

1992 ◽  
Vol 9 ◽  
pp. 617-619 ◽  
Author(s):  
P.R. Wood ◽  
E. Vassiliadis
Keyword(s):  
Mass Loss ◽  
Radiation Pressure ◽  
Stellar Evolution ◽  
Large Mass ◽  
Dual Process ◽  
Radial Pulsation ◽  
Agb Stars ◽  
Carbon Stars ◽  
Ir Stars ◽  
Loss Rates

Computations of AGB stellar evolution which include the effects of mass loss are still relatively rare. However, in order to relate numbers of Mira variables, OH/IR stars and carbon stars to associated stellar populations, it is necessary to understand evolutionary timescales on the AGB.The dominant factors controlling very late AGB evolution are shell flashes and mass loss, and some quantitative estimate of the latter is needed for stellar evolution calculations. The favoured mechanism for the production of the large mass loss rates observed in late AGB stars such as OH/IR stars and dust-enshrouded carbon stars, which have mass loss rates up to a few times 10−5 M⊙ yr−1 (see van der Veen and Rugers 1989 for a compilation), is a dual process involving the lévitation of matter above the photosphere by large-amplitude radial pulsation followed by the formation of grains on which radiation pressure acts to drive the circumstellar material away from the star (Castor 1981; Holzer and MacGregor 1985; Hearn 1990). The studies by Wood (1979) and Bowen (1988) show that, by themselves, neither pulsation nor radiation pressure acting on grains can produce the very large mass loss rates from AGB stars.

scholarly journals The abundance of S- and Si-bearing molecules in O-rich circumstellar envelopes of AGB stars

2020 ◽  
Vol 641 ◽  
pp. A57
Author(s):  
S. Massalkhi ◽  
M. Agúndez ◽  
J. Cernicharo ◽  
L. Velilla-Prieto
Keyword(s):  
Mass Loss ◽  
Mass Loss Rate ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Circumstellar Envelopes ◽  
Fractional Abundance ◽  
Wide Range ◽  
Order Of Magnitude ◽  
Large Velocity Gradient ◽  
Loss Rates

Aims. We aim to determine the abundances of SiO, CS, SiS, SO, and SO2 in a large sample of oxygen-rich asymptotic giant branch (AGB) envelopes covering a wide range of mass loss rates to investigate the potential role that these molecules could play in the formation of dust in these environments. Methods. We surveyed a sample of 30 oxygen-rich AGB stars in the λ 2 mm band using the IRAM 30m telescope. We performed excitation and radiative transfer calculations based on the large velocity gradient method to model the observed lines of the molecules and to derive their fractional abundances in the observed envelopes. Results. We detected SiO in all 30 targeted envelopes, as well as CS, SiS, SO, and SO2 in 18, 13, 26, and 19 sources, respectively. Remarkably, SiS is not detected in any envelope with a mass loss rate below 10−6 M⊙ yr−1, whereas it is detected in all envelopes with mass loss rates above that threshold. From a comparison with a previous, similar study on C-rich sources, it becomes evident that the fractional abundances of CS and SiS show a marked differentiation between C-rich and O-rich sources, being two orders of magnitude and one order of magnitude more abundant in C-rich sources, respectively, while the fractional abundance of SiO turns out to be insensitive to the C/O ratio. The abundance of SiO in O-rich envelopes behaves similarly to C-rich sources, that is, the denser the envelope the lower its abundance. A similar trend, albeit less clear than for SiO, is observed for SO in O-rich sources. Conclusions. The marked dependence of CS and SiS abundances on the C/O ratio indicates that these two molecules form more efficiently in C- than O-rich envelopes. The decline in the abundance of SiO with increasing envelope density and the tentative one for SO indicate that SiO and possibly SO act as gas-phase precursors of dust in circumstellar envelopes around O-rich AGB stars.

scholarly journals CO in OH/IR-Stars - on Excitation and Mass Loss

1989 ◽  
Vol 106 ◽  
pp. 368-368
Author(s):  
A. Heske ◽  
H.J. Habing ◽  
W.E.C.J. van der Veen ◽  
A. Omont ◽  
T. Forveille
Keyword(s):  
Mass Loss ◽  
Agb Stars ◽  
Line Formation ◽  
Circumstellar Envelopes ◽  
Simple Method ◽  
Long Period ◽  
Ir Stars ◽  
Low Mass ◽  
The Impact ◽  
Loss Rates

Observations of CO in long period variables have been widely used to determine mass loss rates by applying models for CO line formation (e.g. Knapp and Morris, 1985) which use a simple method to take the impact from infrared radiation into account. Recent 00(2-1) and (1-0) observations of some more evolved OH/IR stars yielded much too low mass loss rates using these simple models, thus indicating that they cannot be extrapolated to far evolved AGB stars with optically thick circumstellar envelopes.

Unravelling the sulphur chemistry of AGB stars

2018 ◽  
Vol 14 (S343) ◽  
pp. 379-380
Author(s):  
Taïssa Danilovich
Keyword(s):  
Mass Loss ◽  
Radial Distribution ◽  
Loss Rate ◽  
Mass Loss Rate ◽  
Expansion Velocity ◽  
Agb Stars ◽  
Circumstellar Envelopes ◽  
Carbon Stars

AbstractThere are clear differences in what sulphur molecules form in AGB circumstellar envelopes (CSEs) across chemical types. CS forms more readily in the CSEs of carbon stars, while SO and SO2 have only been detected towards oxygen-rich stars. However, we have also discovered differences in sulphur chemistry based on the density of the CSE, as traced by mass-loss rate divided by expansion velocity. For example, the radial distribution of SO is drastically different between AGB stars with lower and higher density CSEs. H2S can be found in high abundances towards higher density oxygen-rich stars, whereas SiS accounts for a significant portion of the circumstellar sulphur for higher density carbon stars.

scholarly journals Sulphur-bearing molecules in AGB stars

2018 ◽  
Vol 617 ◽  
pp. A132 ◽  
Author(s):  
T. Danilovich ◽  
S. Ramstedt ◽  
D. Gobrecht ◽  
L. Decin ◽  
E. De Beck ◽  
...  
Keyword(s):  
Chemical Composition ◽  
Mass Loss ◽  
Loss Rate ◽  
Mass Loss Rate ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Circumstellar Envelopes ◽  
Carbon Stars ◽  
Order Of Magnitude ◽  
Transfer Modelling

Context. Sulphur has long been known to form different molecules depending on the chemical composition of its environment. More recently, the sulphur-bearing molecules SO and H2S have been shown to behave differently in oxygen-rich asymptotic giant branch (AGB) circumstellar envelopes of different densities. Aims. By surveying a diverse sample of AGB stars for CS and SiS emission, we aim to determine in which environments these sulphur-bearing molecules most readily occur. We include sources with a range of mass-loss rates and carbon-rich, oxygen-rich, and mixed S-type chemistries. Where these molecules are detected, we aim to determine their CS and SiS abundances. Methods. We surveyed 20 AGB stars of different chemical types using the APEX telescope, and combined this with an IRAM 30 m and APEX survey of CS and SiS emission towards over 30 S-type stars. For those stars with detections, we performed radiative transfer modelling to determine abundances and abundance distributions. Results. We detect CS towards all the surveyed carbon stars, some S-type stars, and the highest mass-loss rate oxygen-rich stars, (Ṁ ≥ 5 × 10−6 M⊙ yr−1). SiS is detected towards the highest mass-loss rate sources of all chemical types (Ṁ ≥ 8 × 10−7 M⊙ yr−1). We find CS peak fractional abundances ranging from ~4 × 10−7 to ~2 × 10−5 for the carbon stars, from ~3 × 10−8 to ~1 × 10−7 for the oxygen-rich stars, and from ~1 × 10−7 to ~8 × 10−6 for the S-type stars. We find SiS peak fractional abundances ranging from ~9 × 10−6 to ~2 × 10−5 for the carbon stars, from ~5 × 10−7 to ~2 × 10−6 for the oxygen-rich stars, and from ~2 × 10−7 to ~2 × 10−6 for the S-type stars. Conclusions. Overall, we find that wind density plays an important role in determining the chemical composition of AGB circumstellar envelopes. It is seen that for oxygen-rich AGB stars both CS and SiS are detected only in the highest density circumstellar envelopes and their abundances are generally lower than for carbon-rich AGB stars by around an order of magnitude. For carbon-rich and S-type stars SiS was also only detected in the highest density circumstellar envelopes, while CS was detected consistently in all surveyed carbon stars and sporadically among the S-type stars.

scholarly journals Carbon Stars as Planetary Nebula Progenitors

1989 ◽  
Vol 131 ◽  
pp. 381-390
Author(s):  
G. R. Knapp
Keyword(s):  
Mass Loss ◽  
Planetary Nebula ◽  
Total Mass ◽  
Carbon Star ◽  
Substantial Contribution ◽  
Agb Stars ◽  
Carbon Stars ◽  
Molecular Line ◽  
Wide Range ◽  
Loss Rates

Molecular line observations show that some planetary nebulae are still only partially ionized and are surrounded by the remains of the mass loss envelope shed by the preceding AGB star. The mass loss rates and outflow velocities of these envelopes are similar to those of the cool winds from luminous AGB stars. Both the kinematics of carbon stars and observations of the molecular envelopes around young planetaries show that the carbon star progenitors have a wide range of ages and of mass loss rates. There is increasing evidence that a significant fraction of AGB stars are carbon stars and that these provide a substantial contribution to the total mass returned to the interstellar medium.

scholarly journals Two-dimensional modeling of density and thermal structure of dense circumstellar outflowing disks

2018 ◽  
Vol 613 ◽  
pp. A75 ◽  
Author(s):  
P. Kurfürst ◽  
A. Feldmeier ◽  
J. Krtička
Keyword(s):  
Mass Loss ◽  
Optical Depth ◽  
Massive Stars ◽  
Thermal Structure ◽  
Time Dependent ◽  
Navier Stokes ◽  
Viscous Heating ◽  
Two Dimensional ◽  
Dense Region ◽  
Loss Rates

Context. Evolution of massive stars is affected by a significant loss of mass either via (nearly) spherically symmetric stellar winds or by aspherical mass-loss mechanisms, namely the outflowing equatorial disks. However, the scenario that leads to the formation of a disk or rings of gas and dust around massive stars is still under debate. It is also unclear how various forming physical mechanisms of the circumstellar environment affect its shape and density, as well as its kinematic and thermal structure. Aims. We study the hydrodynamic and thermal structure of optically thick, dense parts of outflowing circumstellar disks that may be formed around various types of critically rotating massive stars, for example, Be stars, B[e] supergiant (sgB[e]) stars or Pop III stars. We calculate self-consistent time-dependent models of temperature and density structure in the disk’s inner dense region that is strongly affected by irradiation from a rotationally oblate central star and by viscous heating. Methods. Using the method of short characteristics, we specify the optical depth of the disk along the line-of-sight from stellar poles. Within the optically thick dense region with an optical depth of τ > 2∕3 we calculate the vertical disk thermal structure using the diffusion approximation while for the optically thin outer layers we assume a local thermodynamic equilibrium with the impinging stellar irradiation. For time-dependent hydrodynamic modeling, we use two of our own types of hydrodynamic codes: two-dimensional operator-split numerical code based on an explicit Eulerian finite volume scheme on a staggered grid, and unsplit code based on the Roe’s method, both including full second-order Navier-Stokes shear viscosity. Results. Our models show the geometric distribution and contribution of viscous heating that begins to dominate in the central part of the disk for mass-loss rates higher than Ṁ ≳ 10−10 M⊙ yr−1. In the models of dense viscous disks with Ṁ > 10−8 M⊙ yr−1, the viscosity increases the central temperature up to several tens of thousands of Kelvins, however the temperature rapidly drops with radius and with distance from the disk midplane. The high mass-loss rates and high viscosity lead to instabilities with significant waves or bumps in density and temperature in the very inner disk region. Conclusions. The two-dimensional radial-vertical models of dense outflowing disks including the full Navier-Stokes viscosity terms show very high temperatures that are however limited to only the central disk cores inside the optically thick area, while near the edge of the optically thick region the temperature may be low enough for the existence of neutral hydrogen, for example.

scholarly journals The Production of Low Mass Carbon Stars; Carbon-rich Dredge Up or Oxygen-rich Mass Loss?

1989 ◽  
Vol 106 ◽  
pp. 229-231
Author(s):  
R.E. Stencel ◽  
J.E. Pesce ◽  
K.M. MacGregor
Keyword(s):  
Mass Loss ◽  
Selective Removal ◽  
Secondary Effect ◽  
Agb Stars ◽  
Radiative Force ◽  
Conventional Theory ◽  
Carbon Stars ◽  
Solar Mass ◽  
Low Mass ◽  
Momentum Coupling

AbstractConventional theory explains the origin of carbon stars as due to dredge up of carbon enriched material from the stellar core during helium flash events late in the life of solar mass AGB stars (e.g. Boothroyd and Sackmann 1988). This relatively efficient process however, seems to produce a larger C/O ratio than observed (Lambert et al. 1987). A secondary effect which could contribute to the appearance of carbon stars, is the selective removal of oxygen from the atmosphere by radiative force expulsion of oxygen rich dust grains (e.g. silicates like [Mg, Fe2SiO4]). We present calculations for this scenario which evaluate the degree of momentum coupling between the grains and gas under the thermodynamical conditions of AGB star atmospheres.

scholarly journals ISOCAM and DENIS Survey of 0.5 square degrees in the Bar of the LMC. Detection of the whole TP-AGB Star Population

1999 ◽  
Vol 191 ◽  
pp. 561-566
Author(s):  
C. Loup ◽  
E. Josselin ◽  
M.-R. Cioni ◽  
H.J. Habing ◽  
J.A.D.L. Blommaert ◽  
...  
Keyword(s):  
Mass Loss ◽  
High Mass ◽  
Evolutionary Models ◽  
Agb Stars ◽  
Complete Sample ◽  
High Mass Loss ◽  
Low Mass ◽  
The One ◽  
Good Agreement ◽  
Loss Rates

We surveyed 0.5 square degrees in the Bar of the LMC with ISOCAM at 4.5 and 12 μm, and with DENIS in the I, J, and Ks bands. Our goal was to build a complete sample of Thermally-Pulsing AGB stars. Here we present the first analysis of 0.14 square degrees. In total we find about 300 TP-AGB stars. Among these TP-AGB stars, 9% are obscured AGB stars (high mass-loss rates); 9 of them were detected by IRAS, and only 1 was previously identified. Their luminosities range from 2 500 to 14 000 L⊙, with a distribution very similar to the one of optical TP-AGB stars (i.e. those with low mass-loss rates). Such a luminosity distribution, as well as the percentage of obscured stars among TP-AGB stars, is in very good agreement with the evolutionary models of Vassiliadis & Wood (1993) if most of the TP-AGB stars that we find have initial masses smaller than 1.5 to 2 M⊙.

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