High-mass-loss AGB Stars in the South Galactic Cap

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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An ALMA view of SO and SO2 around oxygen-rich AGB stars

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa693 ◽

2020 ◽

Vol 494 (1) ◽

pp. 1323-1347 ◽

Cited By ~ 3

Author(s):

T Danilovich ◽

A M S Richards ◽

L Decin ◽

M Van de Sande ◽

C A Gottlieb

Keyword(s):

Mass Loss ◽

Loss Rate ◽

Mass Loss Rate ◽

Energy Levels ◽

High Mass ◽

High Angular Resolution ◽

Circumstellar Envelope ◽

Agb Stars ◽

Abundance Distribution ◽

High Mass Loss

ABSTRACT We present and analyse SO and SO2, recently observed with high angular resolution and sensitivity in a spectral line survey with ALMA, for two oxygen-rich AGB stars: the low mass-loss rate R Dor and high mass-loss rate IK Tau. We analyse 8 lines of SO detected towards both stars, 78 lines of SO2 detected towards R Dor, and 52 lines of SO2 detected towards IK Tau. We detect several lines of 34SO, 33SO, and 34SO2 towards both stars, and tentatively S18O towards R Dor, and hence derive isotopic ratios for these species. The spatially resolved observations show us that the two sulphur oxides are co-located towards R Dor and trace out the same wind structures in the circumstellar envelope. Much of the emission is well reproduced with a Gaussian abundance distribution spatially centred on the star. Emission from the higher energy levels of SO and SO2 towards R Dor provides evidence in support of a rotating inner region of gas identified in earlier work. The new observations allow us to refine the abundance distribution of SO in IK Tau derived from prior observations with single antennas, and confirm that the distribution is shell like with the peak in the fractional abundance not centred on the star. The confirmation of different types of SO abundance distributions will help fine-tune chemical models and allows for an additional method to discriminate between low and high mass-loss rates for oxygen-rich AGB stars.

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Millimetre Observations of Evolved Stars

Publications of the Astronomical Society of Australia ◽

10.1017/s1323358000020786 ◽

1996 ◽

Vol 13 (2) ◽

pp. 185-186

Author(s):

Jessica M. Chapman

Keyword(s):

Mass Loss ◽

Stellar Atmosphere ◽

Asymptotic Giant Branch ◽

High Mass ◽

Circumstellar Envelope ◽

Agb Stars ◽

Circumstellar Envelopes ◽

Evolved Stars ◽

High Mass Loss ◽

Transfer Momentum

Radio emission at centimetre and millimetre wavelengths provides a powerful tool for studying the circumstellar envelopes of evolved stars. These include stars on the asymptotic giant branch (AGB), post-AGB stars and a small number of massive M-type supergiant stars. The AGB stars and M-type supergiants are characterised by extremely high mass-loss rates. The mass loss in such an evolved star is driven by radiation pressure acting on grains which form in the outer stellar atmosphere. The grains are accelerated outwards and transfer momentum to the gas through grain–gas collisions. The outflowing dust and gas thus form an expanding circumstellar envelope through which matter flows from the star to the interstellar medium, at a typical velocity of 15 km s−1. For a recent review of circumstellar mass loss see Chapman, Habing & Killeen (1995).

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AGB Stars with High Mass Loss Rates in the Bulge of our Galaxy (Abstract)

Light on Dark Matter - Astrophysics and Space Science Library ◽

10.1007/978-94-009-4672-9_23 ◽

1986 ◽

pp. 107-108

Author(s):

W. E. C. J. van der Veen

Keyword(s):

Mass Loss ◽

High Mass ◽

Agb Stars ◽

High Mass Loss ◽

Loss Rates

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Binary post-AGB stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900131146 ◽

1997 ◽

Vol 180 ◽

pp. 313-318 ◽

Cited By ~ 1

Author(s):

L.B.F.M. Waters ◽

C. Waelkens ◽

H. Van Winckel

Keyword(s):

Mass Loss ◽

Central Star ◽

Asymptotic Giant Branch ◽

High Mass ◽

Agb Stars ◽

Chemical Abundances ◽

Intermediate Mass Stars ◽

High Mass Loss ◽

Complicating Factors ◽

Circumstellar Environment

Low and intermediate mass stars leave the Asymptotic Giant Branch (AGB) when the mass in their H-rich envelope is less than about 0.01 M⊙, and the high mass loss drops several orders of magnitude. The central star rapidly evolves to the left part of the HR diagram along a track of constant luminosity (e.g. Schönberner 1983). In principle the evolution of the central star to higher Teff and the expansion and cooling of the AGB remnant are easy to calculate. In practice several complicating factors arise which make it much more difficult to predict the morphology and properties of post-AGB stars, such as binarity, post-AGB mass loss and aspherical AGB mass loss. Binarity of post-AGB stars affects the morphology of the circumstellar environment, and it affects evolutionary timescales and surface chemical abundances of the components in the system. This review discusses some properties of binary post-AGB stars.

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Formation of crystalline silicate around oxygen–rich AGB stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900203124 ◽

1999 ◽

Vol 191 ◽

pp. 239-244 ◽

Cited By ~ 3

Author(s):

Takashi Kozasa ◽

Hisato Sogawa

Keyword(s):

Mass Loss ◽

Loss Rate ◽

Mass Loss Rate ◽

Emission Feature ◽

High Mass ◽

Agb Stars ◽

Dust Grains ◽

High Mass Loss ◽

Calculation Results ◽

History Of

Crystallization of silicate has been investigated within the framework of dust formation in steady state gas outflows around oxygen–rich AGB stars, where silicates are locked not only into homogeneous silicate grains but also into the mantles of heterogeneous grains. Based on the thermal history of dust grains after their formation, the crystallization calculation results in no crystalline silicate for the mass loss rate Ṁ ≤ 2 × 10−5M⊙ yr−1. Only silicate in the mantles of heterogeneous grains can be crystallized for Ṁ ≥ 3 × 10−5M⊙ yr−1, while homogeneous silicate grains remain amorphous. The mass fraction of crystalline silicate increases with increasing Ṁ. The radiation transfer calculations confirm the appearance of an emission feature around 33.5 μm, taking olivine as a representative of crystalline silicates. On the other hand, the 10μm feature appears in absorption, being dominated by homogeneous silicate grains. These trends are consistent with the observations. Thus the crystalline silicate is a diagnostics of high mass loss rate at the late stage of AGB stellar evolution, reflecting the formation process of dust grains.

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Improving the parallaxes of OH bearing Miras

Proceedings of the International Astronomical Union ◽

10.1017/s1743921307013336 ◽

2007 ◽

Vol 3 (S242) ◽

pp. 342-343

Author(s):

W. H. T. Vlemmings ◽

H. J. van Langevelde

Keyword(s):

Physical Properties ◽

Mass Loss ◽

Solar Minimum ◽

Asymptotic Giant Branch ◽

High Mass ◽

Agb Stars ◽

Long Baseline ◽

Mira Variables ◽

High Mass Loss ◽

Very Long Baseline Array

AbstractWe have carried out observations with the Very Long Baseline Array (VLBA) to measure the parallaxes of Mira variables and are able to improve the distance estimates significantly for a fraction of our sample (U Her, S CrB and RR Aql). This is predominantly because we have enhanced our technique by making use of nearby, in-beam calibrators. Additionally, the observing conditions have improved during the current solar minimum. The distances of these stars are of fundamental importance for studying the physical properties of Asymptotic Giant Branch (AGB) stars with high mass loss.

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Variable Winds in Early-B Hypergiants

International Astronomical Union Colloquium ◽

10.1017/s0252921100072031 ◽

1999 ◽

Vol 169 ◽

pp. 222-229

Author(s):

Bernhard Wolf ◽

Thomas Rivinius

Keyword(s):

Mass Loss ◽

Terminal Velocity ◽

Spectral Lines ◽

High Mass ◽

High Dispersion ◽

Echelle Spectrograph ◽

Variability Pattern ◽

Evolutionary Phase ◽

High Mass Loss ◽

Wind Velocities

AbstractEarly-B hypergiants belong to the most luminous stars in the Universe. They are characterized by high mass-loss rates (Ṁ ≈ 10−5Mʘyr−1) and low terminal wind velocities (v∞ʘ400 kms−1) implying very dense winds. They represent a short-lived evolutionary phase and are of particular interest for evolutionary theories of massive stars with mass loss. Due to their high luminosity they play a key role in connection with the “wind momentum - luminosity relation”. Among the main interesting characteristics of early-B hypergiants are the various kinds of photometric and spectroscopic variations. In several recent campaigns our group has performed extensive high dispersion spectroscopy of galactic early-B hypergiants with our fiber-fed echelle spectrograph FLASH/HEROS at the ESO-50 cm telescope. The main outcome was that their dense winds behave hydrodynamically differently to the less luminous supergiants of comparable spectral type. Outwardly accelerated propagating discrete absorption components of the P Cyg-type lines are the typical features rather than rotationally modulated line profile variations. These discrete absorptions could be traced in different spectral lines from photospheric velocities up to 75% of the terminal velocity. The stellar absorption lines show a pulsation-like radial velocity variability pattern lasting up to two weeks as the typical time scale. The radius variations connected with this pulsation-like motions are correlated with the emission height of the P Cyg-type profiles.

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Observational connections between LBV’s and other stars, with emphasis on Wolf-Rayet stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100004498 ◽

1989 ◽

Vol 113 ◽

pp. 229-240

Author(s):

A. F. J. Moffat ◽

L. Drissen ◽

C. Robert

Keyword(s):

Mass Loss ◽

Massive Stars ◽

Underlying Mechanism ◽

High Mass ◽

Strong Line ◽

Essential Step ◽

High Mass Loss ◽

Loss Rates ◽

R Domain

Abstract.We suggest that the LBV mechanism is an essential step to “force” massive stars (M(ZAMS) ≥ 40M⊙) to finally enter the Wolf-Rayet (W-R) domain in the Hertzsprung-Russel diagram (HRD). Just as massive supergiants showincreasingvariability as theyapproachthe Humphreys-Davidson (H-D)instability limit (horizontally in the HRD diagram), so the W-R stars showdecreasingvariability as theyrecede fromthe H-D limit (at first horizontally into the WNL domain, then, with their high mass loss rates, plunging irreversably downwards as ever hotter, smaller and fainter, strong-line W-R stars). Among the W-R stars, the luminous WNL subtypes (especially WN8) are the most variable, probably as a consequence of blob ejection in the wind. The underlying mechanism which triggers this ejection is possibly related to wind instabilities and may thus be quite different from the source of variability in luminous supergiants or LBV’s in quiescence, where photospheric effects dominate.

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Luminous Blue Variables, cool hypergiants and some impostors in the H-R diagram

Symposium - International Astronomical Union ◽

10.1017/s0074180900211625 ◽

2003 ◽

Vol 212 ◽

pp. 38-46

Author(s):

Roberta M. Humphreys

Keyword(s):

Magnetic Fields ◽

Mass Loss ◽

Surface Activity ◽

Massive Star ◽

Massive Stars ◽

High Mass ◽

Luminous Blue Variables ◽

Star Evolution ◽

High Mass Loss ◽

Massive Star Evolution

Current observations of the S Dor/LBVs and candidates and the implications for their important role in massive star evolution are reviewed. Recent observations of the cool hypergiants are altering our ideas about their evolutionary state, their atmospheres and winds, and the possible mechanisms for their asymmetric high mass loss episodes which may involve surface activity and magnetic fields. Recent results for IRC+10420, ρ Cas and VY CMa are highlighted. S Dor/LBVs in eruption, and the cool hypergiants in their high mass loss phases with their optically thick winds are not what their apparent spectra and temperatures imply; they are then ‘impostors’ on the H-R diagram. The importance of the very most massive stars, like η Carinae and the ‘supernovae impostors’ are also discussed.

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