Heavy Mass Loss Research Articles

Dust driven winds

The status of dust driven winds, constituting an important subclass of essentially radiation generated winds, is surveyed. Dust driven winds are conceived as a long lasting phenomenon of heavy mass loss concerning those luminous cool giants and supergiants, where dust condensation in the expanding flow determines both thestellar mass loss rate and thesubsonic-supersonic transition of the velocity field. Our contribution aims at a self-consistent description of the dynamical shell structure with particular emphasis to the theoretical aspects of this important phenomenon. Thus, not only the complex coupling of the various ingredients (hydrodynamics, chemistry, radiative transfer, dust nucleation, and growth) is outlined in detail, but also general arguments regarding the overall structure of such winds and the expected position of their central objects in the Hertzsprung-Russel diagram are conducted. A selected typical self-consistent model for a stationary C-star shell demonstrates the characteristic wind structure and gives insight into the close nonlinear interplay between dust formation and wind generation. During the late evolutionary stages of a star along the AGB dust driven mass loss provides a natural self-accelerating mechanism which easily can produce very high mass loss rates, an effect which possibly might play an important role for the Tip-AGB objects and the AGB-PN-transition.

ROSAT observations of recent supernovae

Results from ROSAT observations of recent supernova events are discussed in the light of a model, where the soft X-rays originate in the shock interaction of the expanding supernova ejecta with the circumstellar material, left from heavy mass loss of the progenitor object. Special attention is given to the observations of SN1993J in M81, where lightcurves and spectral measurements allow a more detailed analysis and comparison to numerical model calculations.

Wolf Rayet stars in galaxies and the role of binaries and hydrodynamics

WR stars obey M-L-·-Teff-R-chemistry relations, which are in general model independent (with the exception of chemistry). The processes of WR formation (stellar winds, hydrodynamical mixing, binary mass transfer etc.) influence, however, the domain of parameters occupied by WR stars. We specifically examine the distribution of luminosities and H-contents of WN stars, which both support heavy mass loss rates, or possibly mixing, in the main sequence and WNL phases. Detailed studies of the number ratios WR/O, WC/WN, WC/WR, etc. are made for galaxies at various metallicities Z which exhibit extremely different WR populations. Good agreement of models and observations is found. It is also shown that the data are better explained if a certain fraction Φ (less than 10%) of the O-stars become WR stars, preferentially of type WNE as a result of Roche lobe overflow (RLOF) in binaries. This result necessarily implies that the fraction of WR stars, owing their existence to RLOF, is variable with Z, being nearly 100% at low Z and much smaller at high Z. We also identify several hydrodynamical developments physically required in stellar models. Among them, we collect the available observational and theoretical arguments supporting an important role of mixing in massive star evolution.

Excitation of the inner pc of the Galactic Center: Results from near-infrared spectroscopy

We present near-infrared spectroscopy and line imaging of the central 0.5 pc of the Galaxy. The “mini-cavity” region south-west of SgrA∗ is a source of bright 2.217 μm emission now identified as due to [Fe III]. We infer that iron is released from grains in shocks triggered by fast wind blowing into the Galactic Center gas streamers. After cooling the shocked streamer gas is maintained at about 7000 K by the central UV field. Morphology and dynamics can be explained either by models invoking HeI emission line stars as a local wind source or by models in which a jet from SgrA∗ acts as an external wind source. Velocity-resolved spectroscopy of the brightest HeI emission line star in the Galactic Center has been used to constrain atmospheric models. The star is found to be similar to evolved massive stars of type Ofpe/WN9, undergoing heavy mass loss. Helium is overabundant in the atmosphere. Extrapolating from this star it is likely that the Galactic Center HeI stars contribute a large fraction of the total luminosity and a significant fraction of the Lyman continuum luminosity of the central parsec. Hotter stars are needed to provide the HeI ionizing flux.

Heavy mass loss from the symbiotic star AS 304*

IUE, optical and infrared observations of the previously poorly studied symbiotic star AS 304 are discussed. The IUE low-resolution spectra are dominated by very strong P Cygni profiles of the resonant lines of N v, Si IV and C IV, indicating that the hot component of AS 304 is losing mass at a high rate via a fast wind. These P Cygni profiles make AS 304 almost unique among the symbiotic stars so far observed by the IUE. The hot component of AS 304 resembles the mass-losing nuclei of planetary nebulae

Circumstellar dust emission in five Large Magellanic Cloud supergiants

Spectra at 8-1 3 μm are presented of five Large Magellanic Cloud supergiant stars with circumstellar dust emission. The brightest two objects were discovered in the IRAS survey and have thick dust shells with deep silicate absorption features; they are probably late-type M supergiants undergoing very heavy mass loss. Comparison between the current data and previous mid-infrared photometry suggests that the apparent depth of the silicate feature in the OH/IR star IRAS 04553-6825 has varied over a 5-yr interval. The peculiar B[e] hypergiant R 126 displays a strong circumstellar dust shell, but with a very muted silicate emission feature

Early maternal investment and growth in reindeer

I studied whether females in a herd of semidomesticated reindeer (Rangifer tarandus) in Finland invest more heavily in male than in female offspring and whether the mother's body size and condition have different effects on growth rates of male and female offspring. The results did not provide evidence of different preweaning investment in male and female offspring. Proportional mass loss of breeding females during calving was greater when the calf was male than when it was female, but change in mass between consecutive autumns did not depend on the sex of the calf. In general, the growth of male and female calves depended in approximately similar fashion on maternal characteristics, but amongst the male calves only, the growth rate from birth until the following November depended on the mother's change in mass the previous winter. Mass loss of parturient females was inversely related to mass loss during pregnancy. Females that experienced heavy mass loss from November until postcalving gained the most mass before the following November. Winter food supplementation probably led to large body masses and a low mortality rate of calves and might have masked differences in reproductive costs associated with producing male and female calves.

An extended disc around SS 433

Observational implications of heavy mass loss by the SS 433 system are considered. The gas flowing out through the outer Lagrangian point forms a disk-like envelope (extended disc) which is illuminated by the precessing accretion disc of SS 433. The extended disc has the shape of a double cone, whose surface brightness at about 1 arcsec scale is sufficient for the disc to be detected by the Hubble Space Telescope and probably by a ground-based telescope. At a distance r <0.15 arcsec from SS 433, one might expect to find a spiral-like structure in the extended disc, which could be variable in the course of precession. Some evidence for outflowing gas streams near the system is discussed.

A magnetospheric heating model for the evaporation of the companion to PSR 1957 + 20

An orbital period derivative for the PSR 1957+20 system has recently been reported, which indicates orbital contraction over a time-scale of 10 15 s. The angular momentum loss of the light companion is probably indicative of heavy mass loss, too high to be explained by previous evaporation scenarios. In addition, an eclipse was reported in the time-integrated component of the pulsar emission at 1400 MHz at a phase of 0.23 to 0.24, slightly before the middle of the eclipse at lower frequencies, implying that the wind is shot out asymmetrically from the companion star's surface. It is suggested that the companion has a magnetosphere which allows for extremely efficient and possibly uneven heating of the companion's surface, and partial collimation of the outflow

A cluster of He I emission-line stars in the Galactic center

We report 1″.9/300 km s −1 resolution imaging spectroscopy of He I n=2 1 P→n=2 1 S 2.06 μm line emission in the Galactic center. There is a group of about a dozen compact, broad-line He I emission-line stars most of which are positionally coincident with compact 2 μm continuum sources. IRS 16 and its broad-line region is probably a central concentration of this 1 pc diameter He I star cluster. The He I stars may be blue supergiants with heavy mass loss. The high He I/H I (Brα, Brγ) ratios suggest an overabundance of helium in their atmospheres indicative of a post-main-sequence phase.

OH–IR sources as precursors to protoplanetary nebulae

IT has long been suspected1 that OH–IR sources, highly evolved red giant stars that have built up massive, cool gaseous envelopes through heavy mass loss, are precursors to planetary nebulae. The two kinds of object share a similar galactic distribution, and their circumstellar envelopes have comparable masses and expansion velocities2,3. Recently, several hybrid objects have been found with the far infrared and OH maser emission characteristic of the OH–IR sources, but also with radio continuum emission from a central H II region4–6. There is strong circumstantial evidence that these objects, of which the prototype is Vy2–2 (refs 4,7), seem to be in a transitional state, but their precise evolutionary status remains unclear. Here we present radio-interferometer maps, obtained with the MERLIN network, of OH maser emission at 1,612 MHz from Vy27#150;2 and OH0.9 + 1.3, another of the hybrid sources. Our maps reveal shell structure in the masers, which we take to be conclusive evidence of an evolutionary link between OH–IR sources and planetary nebulae.

On the evolutionary status and instability mechanism of the Luminous Blue Variables (LBV)

AbstractVarious evolutionary sequences leading to LBV are examined. The sequence O-Of-LBV-WR-SN is well supported by the models; some LBV with relatively lower luminosity may turn into OH/IR sources. The overall duration of the LBV phase depends mainly on the average mass loss rate; for &amp;lt;Ṁ&amp;gt; = 10−3M⊙y−1, it lasts about 104y.Very massive stars undergo, when they reach logTeff= 3.9, strong departure from hydrostatic equilibrium due to supra-Eddington luminosities at some depth in the outer layers. This results in heavy mass loss, as the growth rate of the instability is very fast. We suggest that the amount of mass ejected in a shell episode is mainly determined by the mass of such a layer that its thermal adjustment timescale is within an order of magnitude of the stellar dynamical timescale. Simulations of B-light curves due to shell ejections by LBV are performed and some sensitive properties are identified.

Abundances in Hot Evolved Stars

In the upper left corner of the HR-diagram various stars have been found that are clearly not members of the hydrogen main sequence. The majority of them lie to the left and below the main sequence, indicating that they are highly evolved stars close to the extinction of their thermonuclear power source and hence to rapid cooling towards the white dwarf domain. The implication is that a “short” time ago they were red giants and as such experienced phenomena like mixing, dredge-up and heavy mass loss. Therefore, one expects the hot evolved stars to display the consequences of the afore mentioned processes that are up to now not satisfactorily understood.

The infrared size of IRc2/KL and its structure on an arcsecond scale

Small-aperture diffraction-limited scans of IRc2/KL at 7.8 and 12.5 microns are discussed. These scans, which were made at several position angles, spatially resolve the region within a 250 AU radius of this source, which is thought to be a newly formed luminous star undergoing heavy mass loss. IRc2 is found to have a pronounced elongation. The shape and orientation of this extended structure is similar to that seen on a larger scale in low-excitation molecular transitions, and is very similar to the distribution of H2O 'shell' masers. The size of the object may be reconciled with its dereddened blackbody temperature and total luminosity of 0.00001 solar luminosities (which is appropriate to OMC1) by the presence of small-scale structure. The observations thus support the idea that IRc2 is the primary luminosity source for OMC1. New astrometric measurements indicate that the 12.5 microns peak of IRc2 is coincident with the positions of the two bright SiO masers to within 0.3 arcsec.

Convective overshooting in the evolution of very massive stars

Possible convective overshooting in stars of 30-120 solar masses are considered, including a merger between the convective core and the intermediate zone, and penetration by the outer convection zone into the hydrogen-shell region when the star is a supergiant. Convective mixing between the core and inner envelopes is found to lead to a brief renewal of hydrogen burning in the core, and a moderate widening of the main sequence bond in the H-R diagram. Deep penetration by the outer convection zone is found to force the star out of the red supergiant configuration and into a configuration near the main sequence. This would account for the apparent spread of the uppermost part of the main sequence and the concentration of luminous supergiants towards earlier spectral types. In addition, heavy mass loss need not be assumed to achieve the points of agreement, and are tentatively considered unimportant from an evolutionary point of view.

Stellar evolution at high mass including the effect of a stellar wind

view Abstract Citations (100) References (52) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stellar evolution at high mass including the effect of a stellar wind. Stothers, R. ; Chin, C. -W. Abstract The effect of a stellar wind on the evolution of stars in the mass range from 15 to 120 solar masses is investigated. All the stellar models are constructed with the use of Cox-Stewart opacities. Four possible cases of mass loss are considered: (1) no mass loss at all; (2) substantial mass loss from stars in all stages of evolution; (3) heavy mass loss from red supergiants only; and (4) sudden and very heavy mass loss from luminous yellow supergiants. The assumption of mass loss during the main-sequence phase of evolution is found to lead to a lowering of the luminosity and, unless the mass loss is extremely heavy, of the effective temperature as well. A comparison of the adopted mass-loss rates with observed rates suggests that stellar winds are probably not an important factor in the evolution of main-sequence stars and supergiants unless the initial masses are greater than about 30 solar masses. Publication: The Astrophysical Journal Pub Date: October 1979 DOI: 10.1086/157388 Bibcode: 1979ApJ...233..267S Keywords: Early Stars; Main Sequence Stars; Stellar Evolution; Stellar Mass Ejection; Stellar Winds; Abundance; Helium; Hydrogen; Schwarzschild Metric; Stellar Models; Stellar Structure; Supergiant Stars; Astrophysics; Mass Loss:Stellar Models; Massive Stars:Evolution; Stellar Evolution:Stellar Winds full text sources ADS |

Stellar winds and the evolution of luminous stars

The effect of a stellar wind on the evolution of stars in the mass range 7-60 solar masses has been investigated for stellar models in which Carson's opacities have been employed. Several cases of mass loss have been considered. It is found that the assumption of heavy mass loss from both blue and red supergiants can account well for the relevant observations of OBN stars, WN stars, and very luminous supergiants of all spectral types. But no amount of mass loss can account adequately for the properties of the B supergiants of lowest luminosity. A critical comparison is made between the present results and some earlier results based on the adoption of Cox-Stewart opacities.