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1.
In this work, a sample of luminous M-type giants in the Baade's Windows towards the inner Galactic bulge is investigated in the near-infrared. The ISOGAL survey at 7 and 15 μm has given information concerning the mass-loss rates of these stars and their variability characteristics have been extracted from the MACHO data base. Most are known to be semi-regular variables (SRVs). Here we discuss how their IJHK S -region colours depend on period and on the presence or absence of mass loss, using results mainly taken from the DENIS and 2MASS surveys.
In order to compare their colours with solar neighbourhood stars, photometric colours on the DENIS, 2MASS and ESO photometric systems have been synthesized for objects in the spectrophotometric atlas of Lançon & Wood. In addition, they have been used to predict the differences in colour indices when stars with strong molecular bands are observed using different photometric systems.
The SRVs are found to inhabit the upper end of the , KS colour–magnitude diagram, lying just below the Miras. High mass-loss rates are associated with high luminosity. The near-infrared colours of the SRVs increase in a general way with period and are the reddest for stars with significant mass loss. The average colours of Mira variables, whose periods start at around 200 d in the bulge, are bluer than those of the semi-regulars at this period, particularly in , thanks to the association of deep water vapour bands with large amplitude. 相似文献
In order to compare their colours with solar neighbourhood stars, photometric colours on the DENIS, 2MASS and ESO photometric systems have been synthesized for objects in the spectrophotometric atlas of Lançon & Wood. In addition, they have been used to predict the differences in colour indices when stars with strong molecular bands are observed using different photometric systems.
The SRVs are found to inhabit the upper end of the , K
2.
W.-R. Hamann 《Astrophysics and Space Science》2010,329(1-2):151-158
Stellar winds appear as a persistent feature of hot stars, irrespective of their wide range of different luminosities, masses, and chemical composition. Among the massive stars, the Wolf–Rayet types show considerably stronger mass loss than the O stars. Among hot low-mass stars, stellar winds are seen at central stars of planetary nebulae, where again the hydrogen-deficient stars show much stronger winds than those central stars with “normal” composition. We also studied mass-loss from a few extreme helium stars and sdOs. Their mass-loss rate roughly follows the same proportionality with luminosity to the power 1.5 as the massive O stars. This relation roughly marks a lower limit for the mass loss from hot stars of all kinds, and provides evidence that radiation pressure on spectral lines is the basic mechanism at work. For certain classes of stars the mass-loss rates lie significantly above this relation, for reasons that are not yet fully understood. Mass loss from low-mass stars may affect their evolution, by reducing the envelope mass, and can easily prevent diffusion from establishing atmospheric abundance patterns. In close binary systems, their winds can feed the accretion onto a companion. 相似文献
3.
We present a brief overview of the theory of stellar winds with a strong emphasis on the radiation-driven outflows from massive
stars. The resulting implications for the evolution and fate of massive stars are also discussed. Furthermore, we relate the
effects of mass loss to the angular momentum evolution, which is particularly relevant for the production of long and soft
gamma-ray bursts. Mass-loss rates are not only a function of the metallicity, but are also found to depend on temperature,
particularly in the region of the bi-stability jump at 21 000 Kelvin. We highlight the role of the bi-stability jump for Luminous
Blue Variable (LBV) stars, and discuss suggestions that LBVs might be direct progenitors of supernovae. We emphasize that
radiation-driven wind studies rely heavily on the input opacity data and linelists, and that these are thus of fundamental
importance to both the mass-loss predictions themselves, as well as to our overall understanding of the lives and deaths of
massive stars. 相似文献
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6.
Hans Olofsson 《Astrophysics and Space Science》2008,313(1-3):201-207
Intense mass loss occurs for low- and intermediate-mass stars on the asymptotic giant branch (AGB), and for the higher mass
(≳8 M⊙) stars during their red supergiant evolution. These winds affect the evolution of the stars profoundly, creates circumstellar
envelopes of gas and dust, as well as enrich the interstellar medium with heavy elements and grain particles. The mass loss
characteristics are well-studied, but the basic processes are still not understood in detail, and the mass-loss rate of an
individual star cannot be derived from first principles. These objects also provide us with fascinating systems, in which
intricate interplays between various physical and chemical processes take place, and their relative simplicity in terms of
geometry, density distribution, and kinematics makes them excellent astrophysical laboratories. The review concentrates on
the aspects of AGB stars and their mass loss which are of particular interest in connection with ALMA. 相似文献
7.
8.
Ju. L. Frantsman 《Astrophysics and Space Science》1988,145(2):287-292
The observed maximum luminosities of AGB stars in the Magellanic Cloud clusters were compared with luminosities obtained theoretically in order to estimate the ages of the clusters. Several rates of mass loss by AGB stars are considered. The ages for 10 clusters in the SMC and 25 in the LMC are presented in the tables with the compilations of ages from the literature quoted. The discrepancies between ages derived from the AGB peak luminosities and from the Main-Sequence turn off and maximum luminosity are explained by allowing for intensive mass-loss during the AGB evolutionary phase. The method is useful only if the brightest star is on the thermally pulse AGB phase but not in the early-AGB phase. 相似文献
9.
Jorick S. Vink 《Astrophysics and Space Science》2004,291(3-4):239-245
We present the results of Monte Carlo mass-loss computations for hot low-mass stars, specifically for subdwarf B (sdB) stars. It is shown that the mass-loss rates on the Horizontal Branch (HB) computed from radiative line-driven wind models are not high enough to create sdB stars. We argue, however, that mass loss plays a role in the chemical abundance patterns observed both in field sdB stars, as well as in cluster HB stars. The derived mass loss recipe for these (extremely) hot HB stars may also be applied to other groups of hot low-mass stars, such as post-HB (AGB-manqué, UV-bright) stars, over a range in effective temperatures between ?10 000 and 50 000 K. Finally, we present preliminary spectral synthesis on the more luminous sdB stars for which emission cores in Hα have been detected (Heber, U., et al.: 2003, in:Stellar Atmosphere Modeling, ASP Conference Proceedings, p. 251). We find that these line profiles can indeed be interpreted as the presence of a stellar wind with mass loss on the order of 10?11?M ⊙ yr ?1. 相似文献
10.
《New Astronomy Reviews》1999,43(6-7):437-440
After a brief review of our current understanding of Miras and their evolutionary status, three aspects of “real-time” evolution in these and related stars are examined: (i) Chemical changes (O-rich to C-rich) due to third dredge-up. (ii) Period changes due to the effects of the helium-shell flash. (iii) The existence of “fossil” dust and gas shells. Studies of resolved gas and dust shells are highlighted as particularly interesting. They will enable us to examine the mass-loss histories of many late-type stars over the last ten thousand years or so. Such observations have only recently become technically feasible and they are expected to provide important new insights into the late stages of stellar evolution. 相似文献
11.
The R Coronae Borealis (RCB) stars represent a rare, or short-lived stage of low and intermediate mass stellar evolution. They are important in the context of mass-loss anddust formation on the Asymptotic Giant Branch.These stars are defined by their large declines in brightness at irregular intervals caused by dust formation.There are two major evolutionary models for the origin of RCB stars, the Double Degenerate and the Final Helium Shell Flash.In the final flash model, RCB stars lie at the centers of old Planetary Nebulae (PN). Here I discuss possible relationships with the [WC] stars. 相似文献
12.
G. R. Knapp 《Astrophysics and Space Science》1995,224(1-2):333-333
Copious mass loss on the Asymptotic Giant Branch dominates the late stages of stellar evolution. Maps of extended circumstellar envelopes provide a history of mass loss and trace out anisotropic mass loss. This review concentrates on observations of millimeter wavelength molecular line emission, on high resolution maps of maser emission and on observations of submillimeter, millimeter and radio wavelength continuum emission. Radio continuum observations show that AGB stars are larger at radio than at optical wavelengths. The extended chromospheres indicated by these observations extend to distances from the star large enough for dust to form, thereby initiating mass loss. Molecular line maps have found time-variable mass loss for some stars, including detached shells indicating interrupted mass loss and evidence for a rapid increase in the mass loss rate at the end of the AGB phase. Maps of circumstellar envelopes show evidence of flattening, bipolar outflow and angular variations in both the mass loss rate and the outflow velocity. As stars evolve away from the AGB and planetary nebula formation begins, these structures become more pronounced, and fast bipolar molecular winds are observed. The time scales derived from the dynamical times of these winds and from the expansion rates of the central planetary nebulae are very rapid in some cases, about 100 years, in agreement with the predictions of stellar evolution theory. 相似文献
13.
Noam Soker 《Monthly notices of the Royal Astronomical Society》2001,324(3):699-704
I examine the implications of the recently found extrasolar planets on the planet-induced axisymmetric mass-loss model for the formation of elliptical planetary nebulae (PNe). This model attributes the low departure from spherical mass-loss of upper asymptotic giant branch (AGB) stars to envelope rotation which results from deposition of orbital angular momentum of the planets. Since about half of all PNe are elliptical, i.e., have low equatorial to polar density contrast, it was predicted that about 50 per cent of all Sun-like stars have Jupiter-like planets around them, i.e., a mass about equal to that of Jupiter, M J , or more massive. In the light of the new findings that only 5 per cent of Sun-like stars have such planets, and a newly proposed mechanism for axisymmetric mass-loss, the cool magnetic spots model, I revise this prediction. I predict that indeed ∼50 per cent of PN progenitors do have close planets around them, but the planets can have much lower masses, as low as ∼0.01 M J , in order to spin-up the envelopes of AGB stars efficiently. To support this claim, I follow the angular momentum evolution of single stars with main-sequence mass in the range of 1.3–2.4 M⊙ , as they evolve to the post-AGB phase. I find that single stars rotate much too slowly to possess any significant non-spherical mass-loss as they reach the upper AGB. It seems, therefore, that planets, in some cases even Earth-like planets, are sufficient to spin-up the envelope of these AGB stars for them to form elliptical PNe. The prediction that on average several such planets orbit each star, as in the Solar system, still holds. 相似文献
14.
中等质量恒星在赫罗图中由E-AGB星进入TP-AGB星的分界点 总被引：1，自引：0，他引：1
通过对3～10 M_☉恒星在赫罗图上演化轨迹的研究,分析恒星内部氦壳层燃烧峰值处能量、密度、温度、氦壳层表面光度与恒星表面光度比及恒星半径的变化,给出了中等质量恒星由早期AGB星演化至热脉冲AGB星阶段在赫罗图上的分界点,与119颗碳星的观测结果吻合得相当好.同时提出:在恒星演化至该分界点之后,其星风物质损失公式可能需要引入一个与表面光度无关的量以主导超星风的形成.在此基础上,通过对考虑湍流压效应下5 M_☉恒星的结构和演化及星风物质损失率的分析,发现湍流压在热脉冲AGB星阶段对星风物质损失影响较大,从而使得热脉冲AGB星的湍流压不可忽略,进而提出了影响热脉冲AGB星星风物质损失的可能的物理因素. 相似文献
15.
D. Valls-Gabaud 《Astrophysics and Space Science》1988,142(1-2):289-304
Summary The present status of the observational evidence about mass loss in visual binary and multiple systems is reviewed. Particular
emphasis is placed on the importance of mass-loss rates measures in these stars, as a systematic study would provide useful
constraints in two fields. First, as mass-outflows processes are not modified by the interaction of the two stars, and distances,
masses, and evolutionary stages are usually fairly good known, an absolute stellar scale for
can more easily and accurately be derived for these systems. Second,
rates are essential to study the detailed orbital evolution of wide pairs, providing clues to their origin. Detailed numerical
simulations including mass loss have been carried out. It is suggested that mass loss may explain in part the recent statistics
on separation of wide visual binaries as a function of spectral type and age. 相似文献
16.
We examine the envelope properties of asymptotic giant branch (AGB) stars as they evolve on the upper AGB and during the early post-AGB phase. Because of the high mass-loss rate, the envelope mass decreases by more than an order of magnitude. This makes the density profile below the photosphere much shallower, and the entropy profile much steeper. We discuss the possible role of these changes in the profiles in the onset of the high mass-loss rate (superwind) and the large deviation from spherical mass loss at the termination of the AGB. We concentrate on the idea that the shallower density profile and steeper entropy profile allow the formation of cool magnetic spots, above which dust forms much more easily. 相似文献
17.
Hai-juan Zhang Jian-jun Zhou Guo-liang Dong Jarken Esimbek Ji-mang Mu 《Astrophysics and Space Science》2010,330(1):23-36
We collected infrared and radio data on 110 OH/IR stars, 65 S stars and 184 C-rich stars from the literature. We analyzed
their spectral energy distributions, bolometric magnitudes, infrared colors and mass-loss rates. Our study confirms that OH/IR
stars and C-rich stars reach similarly high mass-loss rates at similar luminosities, supporting the idea that mass-loss rates
are determined by internal stellar activities such as pulsation rather than the properties of their envelopes. The mass-loss
rates of OH/IR stars and C-rich stars are strongly correlated with the color indices K-[A], K-[C], K-[D] and K-[E], and the
two populations can be described with one formula. Our study also reveals that only those stars with color indices K-[A]<4
or K-[C]<5 have SEDs that peak in the near-infrared waveband. 相似文献
18.
Near-infrared photometry of 35 SiO maser stars (v = 1, J = 1 - 0) of various types is presented in this paper. Combining the JHK flux densities from IRAS, the distributions of the stars on two color-color diagrams are obtained. The spectral slopes, equivalent black-body temperatures, maser luminosities and mass-loss rates of the central stars are calculated from the infrared or radio data. These parameters are then analysed together with the integrated SiO fluxes. The results show that SiO masers are common in AGB stages. The integrated SiO maser flux is only weakly correlated with the mass-loss rate, and increase slowly along the sequence from Mira stars to OH/IR stars. It is more closely correlated with the near-infrared colors and the equivalent black-body temperatures. Also, its upper limit is correlated with 12μm and 25μm fluxes. We also briefly discusses the general function and specific role of SiO maser in the evolution of AGB stars. 相似文献
19.
Ian R. Stevens 《Monthly notices of the Royal Astronomical Society》2005,356(3):1053-1063
We present a new analysis of the expected magnetospheric radio emission from extrasolar giant planets (EGPs) for a distance limited sample of the nearest known extrasolar planets. Using recent results on the correlation between stellar X-ray flux and mass-loss rates from nearby stars, we estimate the expected mass-loss rates of the host stars of extrasolar planets that lie within 20 pc of the Earth. We find that some of the host stars have mass-loss rates that are more than 100 times that of the Sun and, given the expected dependence of the planetary magnetospheric radio flux on stellar wind properties, this has a very substantial effect. Using these results and extrapolations of the likely magnetic properties of the extrasolar planets, we infer their likely radio properties.
We compile a list of the most promising radio targets and conclude that the planets orbiting Tau Bootes, Gliese 86, Upsilon Andromeda and HD 1237 (as well as HD 179949) are the most promising candidates, with expected flux levels that should be detectable in the near future with upcoming telescope arrays. The expected emission peak from these candidate radio emitting planets is typically ∼40–50 MHz. We also discuss a range of observational considerations for detecting EGPs. 相似文献
We compile a list of the most promising radio targets and conclude that the planets orbiting Tau Bootes, Gliese 86, Upsilon Andromeda and HD 1237 (as well as HD 179949) are the most promising candidates, with expected flux levels that should be detectable in the near future with upcoming telescope arrays. The expected emission peak from these candidate radio emitting planets is typically ∼40–50 MHz. We also discuss a range of observational considerations for detecting EGPs. 相似文献
20.
Knut Jørgen Røed Ødegaard 《Astrophysics and Space Science》1996,238(1):107-111
New computations of massive stars follow the evolution up to advanced stages and include: -A large and flexible nuclear network consisting of 174 nuclear species that are linked by 1742 nuclear reactions. -Semiconvection, overshooting and mass loss. -Modern rates for both strong and weak interaction processes as well as the latest rates for the neutrino processes. -Improved grid distribution and a large number of grid points. The nuclear network and the diffusion equation are solved for each time step during the whole evolution. In this way the accuracy of nuclear yields and chemical abundances are mainly limited by uncertainties in the diffusion coefficient found from the convection theories. Several instability mechanisms may affect the mass loss rates of massive stars and thereby the structure and abundances of WR stars. Due to heavy mass loss at the LBV and WR stages, the masses at the pre-SN stage may be less than 5M ⊙. Yields and abundances throughout the stars are discussed together with the amount of all elements expelled. 相似文献