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1.
The combination of a time-dependent spherically symmetric hydrodynamic model of stellar atmosphere pulsation and a radiation transport code, which incorporates maser saturation theory, enabled us to synthesise maps and spectra of H2O maser emission from the circumstellar envelopes of long period variable stars. The synthetic maps and spectra compare favourably with observed 22, 321 and 325 GHz H2O maser emission. As is observed in H2O maser regions the peak emission occurs between 3–8 stellar radii from the star. The calculated H2O maser regions are in conditions of nH2 = 106 − 108 cm−3, assuming a fractional abundance of 10−4; kinetic temperatures of 550–3000 K; dust ensemble temperatures of 500–1200 K and an accelerating velocity field. The IR radiation field is explicitly included in the radiation transport model, incorporating the latest absorption efficiency data for silicates from Draine. We reproduce the features seen in high angular resolution MERLIN spectral line datacubes. This shows that a mass outflow model which extends the photosphere using pulsations and incorporates radiation pressure on silicate based dust particles can produce the observed data on small (10-mas) angular scales. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

2.
Recent studies of the star formation region BBW 36 and associated molecular clouds are presented. The 12CO (1-0) observations, carried out with the 15-m SEST (Swedish-ESO) telescope (Cerro La Silla, Chile), revealed the existence of cloud a, connected with BBW 36 and of cloud b, having elongation in SE-NW direction. A red-shifted molecular outflow with velocity ∼+5 km/s (with respect to cloud a), having a direction parallel to the line of sight, was also observed. VLA observations showed the presence of a source VLA 2 at 3.6 cm with an elongation in the N-S direction. It is suggested that the VLA 2 source coincides with a dust disc (surrounding the object BBW 36). The star 3, which is one of the YSOs in the star-forming region BBW 36 and is connected with a bright comma-like nebula, can be the source of the molecular outflow. The star 3 has very high IR colors and is associated with an IRAS point source IRAS 07280-1829, which has IR colors, typical for an IRAS point source, connected with a water maser. On the 2MASS K image of BBW 36 we can see the existence of a bright nebula; a group of stars is embedded in that nebula, and among these stars there are stars with dust discs (or envelopes). On the 2MASS K image several spiral jets are also present, some of them with a condensation at the end. Published in Astrofizika, Vol. 51, No. 3, pp. 469–477 (August 2008).  相似文献   

3.
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   

4.
Simultaneous MERLIN observations of the OH 1665- and 1667-MHz maser lines in the circumstellar envelope of the semiregular star W Hya have been taken in all Stokes parameters. The 1665-MHz emission comes from two elongated clusters located 80 au from the star. The 1667-MHz emission arises in an incomplete shell of radius 130 au, with the blueshifted features located in the northern part of the envelope and the redshifted components clustered south of the centre. The circularly polarized maser components exhibit spatial separation along the north–south direction. The linearly polarized components were found from the near side of the envelope. Their polarization position angles indicate that the projected axis of the magnetic field at PA ≃ −20° is consistent with spatial segregation of circular polarization. The intensity of the magnetic field, estimated from a tentative measurement of Zeeman splitting, is about 0.6 mG at the location of the 1667-MHz emission, with the field pointing away from the observer. A small change of position angles of linear polarization observed in both maser lines is interpreted as a weak Faraday effect in the maser regions with an electron density of about 2 cm−3. The overall polarization structure of the envelope suggests an ellipsoidal or weak bipolar geometry. In such a configuration, the circumstellar magnetic field may exert a non-negligible influence on mass loss. The velocity field in the circumstellar envelope recovered from observations of SiO, H2O, OH and CO lines at five radial distances reveals a logarithmic velocity gradient of 0.25 and 0.21 in the 1665- and 1667-MHz maser regions respectively. The acceleration within tens of stellar radii cannot be explained by the classical model of radiation pressure on dust.  相似文献   

5.
In an imaging polarimetry survey of candidate post-AGB stars, scattering envelopes are detected around 20 objects. These envelopes represent the final mass-loss phases at the end of the AGB. In all cases, evidence for axisymmetry in the dust density is seen, suggesting that the presence of an axisymmetric outflow may be a ubiquitous phenomenon of the AGB to post-AGB transition. We use the polarized flux images to classify the objects into detached shell, bipolar and unresolved targets. Modelling based on a simple axisymmetric shell geometry supports the contention that post-AGB objects fall into one of two classes that differ in the amount of dust in the circumstellar environment: the detached shells correspond to stars with an optically thin expanding circumstellar envelope (CSE) whereas the bipolar and unresolved targets have optically thick dust structures, probably in the form of discs, which remain bound to the star, rather than partaking in the expansion of the AGB CSE. It is suggested that this bifurcation in morphology is rooted in the presence or absence of a binary companion, which determines whether or not a disc forms. Because the detached shell objects also appear axisymmetric, an additional mechanism for generating the axisymmetry, such as a magnetically shaped outflow, is needed if they do indeed have single star progenitors.  相似文献   

6.
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.
An upper limit for the mass loss rate of rapidly rotating main-sequence O9-B4 stars has been determined. Themaximum mass loss rate of a rotating star is determined by the ability of radiation pressure in lines to remove matter from the gravitational potential well of the star. The maximum mass loss rate in the case of extremely rapid stellar rotation is a factor of 3–7 higher than that in the case of a nonrotating star. A simple formula for determining the ratio of the maximum mass loss rate of a rotating star to the maximum mass loss rate of a nonrotating star with the same mass, luminosity, and volume is suggested.  相似文献   

8.
In this paper, we calculate the mass and velocity distribution of the envelope of Mira stars, for an axisymmetric distribution of the envelope mass. We also calculate the inversion regions of the four 18 cm lines of OH. We assume that the radiation pressure comes from the central star and the thermal emission of the dust itself. We consider pumping mechanisms due to emission from the central star, collision, emission from the dust itself and cosmic microwave background. The model reproduces some of the general features seen in the real sources.  相似文献   

9.
Maser emission from the circumstellar envelopes of four late-type red supergiants has been mapped with milli-arcsecond resolution using MERLIN1. The wind is driven by radiation pressure on dust and the structure and kinematics of the masing regions reflect the dust properties. The unbeamed radius of water maser blobs, ∼ 1012 m, has been measured for the first time. The velocity gradient is used to derive the dust absorption coefficient which increases with radius from ≤ 0.1 to ≤ 1.0 m2 kg−1. Comparison with laboratory studies suggests that small crystalline grains are formed near the star and are annealed into astronomical silicates at larger distances. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

10.
Phase changing variations between Be and shell stars are considered from the viewpoint of the formation of shell absorption lines in the envelopes of these stars. Typical shell stars are characterized by large optical depths of the envelopes in the H line (H) in a range of 2000 to 5000, whereas the envelopes of typical Be stars are optically thinner with the values of (H)100. This infers that the envelopes of Be stars should be fully expanded as compared to those of shell stars, so as to reduce the optical thickness. Spectral formation in shell stars shows that their envelopes are well condensed near the equatorial planes forming disks or rings. In this paper a simple model of transformation from a disk envelope of shell star to a spherical envelope of Be star is considered to show the relative volume and volume emission measures of the envelopes in both phases. The phase change variations observed in Pleione and in other Be and shell stars are discussed based on this simple consideration. Some implications of the present model in the linear polarization, IR-excess, UV spectra and the radiation field of the envelope are also discussed briefly.  相似文献   

11.
The evolved star HD 179821 continues to be the subject of much debate as to whether it is a nearby     post-asymptotic giant branch (post-AGB) star or a distant     high initial mass     post-red supergiant. We have mapped the OH maser emission around HD 179821 in the 1612- and 1667-MHz lines with the MERLIN interferometer array at a resolution of 0.4 arcsec and 0.35 km s−1. The OH emission lies in a thick shell with inner and outer radii of 1.3 and         and expansion velocity of 30 km s−1. Although we find some evidence for acceleration and for deviations from spherical symmetry, the bulk of the maser emission is consistent with a constant-velocity spherical shell. The extent of the shell agrees with H2O and OH dissociation models and supports a distance estimate of 6 kpc. However, the shell is incomplete and appears to have been disrupted by more recent collimated outflow activity within the last 1500 yr. We suggest that this activity is also responsible for the active envelope chemistry (in particular the presence of HCO+) and for the apparent offset of the star from the centre of the shell. The luminous yellow hypergiant star IRC +10420 also shows signs of recent outflows, and HD 179821 may be at a similar, perhaps slightly earlier, phase of evolution. We suggest that the SiO thermal emission arises from the same detached envelope as the OH maser emission as in IRC +10420. If so then this would strengthen the connection between these two stars and probably rule out a post-AGB status for HD 179821.  相似文献   

12.
We have performed millimeter- and submilli- meter-wave survey observations using the Nobeyama millimeter array (NMA) and the Atacama Submillimeter Telescope Experiment (ASTE) in one of the nearest intermediate-mass (IM) star-forming regions: Orion Molecular Cloud-2/3 (OMC-2/3). Using the high-resolution capabilities offered by the NMA (∼several arcsec), we observed dust continuum and H13CO+(1–0) emission in 12 pre- and proto-stellar candidates identified previously in single-dish millimeter observations. We unveiled the evolutionary changes with variations of the morphology and velocity structure of the dense envelopes traced by the H13CO+(1–0) emission. Furthermore, using the high-sensitivity capabilities offered by the ASTE, we searched for large-scale molecular outflows associated with these pre- and proto-stellar candidates observed with the NMA. As a result of the CO(3–2) observations, we detected six molecular outflows associated with the dense gas envelopes traced by H13CO+(1–0) and 3.3 mm continuum emission. The estimated CO outflow momentum increases with the evolutionary sequence from early to late type of the protostellar cores. We also found that the 24 μm flux increases as the dense gas evolutionary sequence. We propose that the enhancement of the 24 μm flux is caused by the growth of the cavity (i.e. the CO outflow destroys the envelope) as the evolutionary sequence. Our results show that the dissipation of the dense gas envelope plays an essential role in the evolution of the IM protostars. The extremely high-sensitivity and high-angular resolution offered by ALMA will reveal unprecedented details of the inner ∼50 AU of these protostars, which will provide us a break through in the classic scenario of IM star/disk formation.  相似文献   

13.
We analyse the differences in infrared circumstellar dust emission between oxygen-rich Mira and non-Mira stars, and find that they are statistically significant. In particular, we find that these stars segregate in the K–[12] versus [12]–[25] colour–colour diagram, and have distinct properties of the IRAS LRS spectra, including the peak position of the silicate emission feature. We show that the infrared emission from the majority of non-Mira stars cannot be explained within the context of standard steady-state outflow models.
The models can be altered to fit the data for non-Mira stars by postulating non-standard optical properties for silicate grains, or by assuming that the dust temperature at the inner envelope radius is significantly lower (300–400 K) than typical silicate grain condensation temperatures (800–1000 K) . We argue that the latter is more probable and provide detailed model fits to the IRAS LRS spectra for 342 stars. These fits imply that two-thirds of non-Mira stars and one-third of Mira stars do not have hot dust (>500 K) in their envelopes.
The absence of hot dust can be interpreted as a recent (∼100 yr) decrease in the mass-loss rate. The distribution of best-fitting model parameters agrees with this interpretation and strongly suggests that the mass loss resumes on similar time-scales. Such a possibility appears to be supported by a number of spatially resolved observations (e.g. recent Hubble Space Telescope images of the multiple shells in the Egg Nebula) and is consistent with new dynamical models for mass loss on the asymptotic giant branch.  相似文献   

14.
We report near-infrared molecular hydrogen and Brackett γ observations towards the massive star formation site G323.74−0.26. The region contains an H  ii region, ∼30 arcsec across, and two Class II methanol maser sites, which are separate from the H  ii region. We show that the spectral type of the star powering the H  ii region is B0. We also show that at least one of the maser sites is powered by an infrared source that appears to be at least as luminous as the star responsible for the H  ii region. However, neither of the two stars associated with the methanol maser sites shows any signs of radio continuum emission above 0.2 mJy. For at least one of these maser sites, this shows a real deficiency in the radio continuum flux, which we suggest is an indication that the star is in an early stage of development, before its H  ii region becomes visible, or it is a multiple intermediate mass star system. A shocked molecular hydrogen outflow is seen extending from one of the maser sites towards the west and possibly in a fan shape, suggesting that the stars associated with the maser sites are indeed at a very early stage of evolution.  相似文献   

15.
Some features of the envelopes of WR stars have not yet been explained in terms of the standard stellar wind. One of these features is the presence of gas condensations (clumps) in the envelopes. In this paper the possible reasons for clump formation are examined along with their role in the structure of envelopes. Clumps can be formed in the transitional zone between a star and its envelope because of instabilities in the gas. This zone lies much deeper than the "optical radus" of the star, so it cannot be observed. A clump expands as it moves under the action of radiation pressure from the star and its density decreases at the same time. The clump mixes with the surrounding gas if its mass is low. Large clumps can reach the visible region of the envelope.  相似文献   

16.
A star formation region connected with SNO 41 is investigated. The observations of this region were carried out in the 12CO (1-0) line and in the 1.2-mm (with SIMBA) with the 15-m SEST mm telescope (Cerro La Silla, Chile). A blue shifted outflow is revealed from the 12CO(1-0) observations, while a bipolar outflow is apparent from the 1.2-mm SIMBA image. In CO it seems that a very faint dust envelope around SNO 41 probably exists, which is expanding with a velocity of ∼10.5 km/s. The distance to SNO 41 is estimated as ∼1500 pc. There are outflows also present in 2MASS images. A spiral jet has a condensation (resembling a HH object) at the end. Another jet has a discontinuity and a bow-shock-like structure on it. In 2MASS images there are also spots resembling HH objects. In this region there is also a rather luminous point source (IRAS 08546-4254), which has IR colors typical for an YSO connected with a water maser. The detection of a strong CS (2-1) line emission toward IRAS 08546-4254, with the same velocity as the CO line, shows the existence of a high density core of molecular gas associated to this source. A methanol maser is also associated with that IRAS source. The existence of CS line emission and a methanol maser (at 6.669 Ghz) is an indication of the presence of a very young massive star. It is not excluded that this IRAS source is the center of outflows mentioned above, because this source coincides with the center of the 1.2-mm SIMBA image and also with the place of origin of the jet with bow-shock-like structure. Published in Astrofizika, Vol. 50, No. 1, pp. 5–15 (February 2007).  相似文献   

17.
We propose that at least two stars on or near the AGB have long-lived orbiting disks: HD 44179, the central star in the Red Rectangle, and BM Gem, a carbon-rich star with an oxygen-rich circumstellar envelope. The CO emission from both of these disks has a spike with a width near ∼2 km s−1, indicating disk radii of ∼1016 cm. The dust in such disks is therefore quite cold (near T ∼ 50 K for the Red Rectangle) and may emit primarily at submillimeter wavelengths. The disks around stars where there is also substantial mass loss may not be easily observable; there could be many as yet undiscovered disks around AGB stars This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

18.
Using the numerical code (`Scenario Machine') we study of number and physical properties of binary Be stars. Evolutionary tracks leading to a formation of the observational binary systems are presented. We conclude that synchronization must be taken into account when calculating binary Be star evolution and calculate the minimal orbital period for Be/evolved companion binary. The obtained distributions over orbital parameters are in good agreement with the observational lack of short-period Be/X-ray binaries. According to our calculations 70% of all Be stars must have a white dwarf. The white dwarfs in these systems should be hot enough with the surface temperature distribution peaking at 10000–20000 K. Their detection is possible during the period of the lack of Be star envelope by the detection of white dwarf extremely UV and soft X-ray emission. This method of registration appears to be particularly promising for `single' early-type Be stars because in these systems the white dwarfs must have a very high surface temperature. However, the loss of the Be disc-like envelope does not often occur and it is a rather rare event for many Be stars. The best possibility of white dwarf detection is given by the study of helium spectral lines found in emission from several Be stars. The ultraviolet continuum energy of these Be stars is found to be not enough to produce the observed helium emission. Besides, we also discuss the orbital properties of binary Be star systems with other evolved companions such as helium stars and neutron stars and give a possible explanation for the lack of Be/black hole binaries. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

19.
From the LRS spectra of almost 2000 IRAS sources showing the 1612 MHz OH emission we found 9 candidates for OH/IR stars with SiC features in their envelopes. Further study indicates that 6 of these are definitely or very probably carbon star with the 11.3 μm SiC emission feature in addition of being OH/IR stars showing the 1612 MHz OH maser emission.  相似文献   

20.
渐近巨星分支恒星 (AGB星 )是一种晚期演化恒星 ,它是恒星作为以核反应释能为发光能源的天体的最后演化阶段。AGB星阶段的恒星具有许多有趣的性质 ,如很大的质量损失率 (因此形成很厚的拱星尘埃气体包层 ) ,光变 ,热脉动 (或He闪耀 ) ,强的红外超量发射 ,分子脉泽发射等 ,弄清AGB星的演化规律是研究恒星演化理论的重要任务。目前人们所知道的AGB星的演化图景是 ,恒星经过漫长的主序演化之后 ,将经过红巨星 (RGB)阶段 ,然后才进入AGB阶段 ,在其演化过程中AGB星的光度和质量损失率要逐渐增大 ,它的光变周期也逐渐变长 ,在其中心星经历了一系列的由He核反应不稳定性引起的热脉动之后 ,它的质量损失很快停止 ,恒星开始向行星状星云 (PN)演化 ,最后行星状星云将会变成一个白矮星 ,这将是许多初始质量不很大的恒星的最终结局。OH/IR星阶段是AGB星演化的一个阶段 ,OH/IR星是那些质量稍大的恒星在AGB阶段后期演化而成的天体。现阶段人们对OH/IR星的具体演化过程还知道得很少。我们利用了球对称包层中的尘埃辐射转移模型来研究OH/IR星的演化性质 ,并且收集了尽量多的具有可靠距离的OH/IR星来研究他们的光度和质量损失率的演化性质。在本文的研究工作中 ,我们主要讨论了OH/IR星在远红外双色图中的分布规律 ,还发现  相似文献   

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