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1.
The Wolf-Rayet stars represent an advanced stage of evolution of the most massive stars. Their next immediate stadium will be supernova explosion. The most striking property of this very rare but exceptionally hot and bright objects is their extreme mass loss, of the order of 10-5 solar mass per year. In turn of evolution before and during the Wolf-Rayet phase such stars eject a lot of matter (∼ 10 M⊙) with velocity up to 3000 km/s that surrounds the min the form of gas and dust. In the case of binary Wolf-Rayet star such expanding envelope may interact with a companion (usually hot OB star) wind forming a tail extended for ∼ 100 AU. This spectacular phenomenon as well as some other connected with Wolf-Rayet stars that can be studied with high spatial resolution instruments (both astrometric and imaging) are reviewed. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

2.
The stars that will be detectable in the Magellanic Clouds by the DENIS and 2MASS near infrared surveys are enumerated. All thermally-pulsing AGB stars will be observable in I, J, H and K, along with the top two magnitudes of both the early-AGB and the first giant branch. All carbon stars will be visible, and normal (N type) C stars will be easily distinguished by their large J-K colours. However, it will not be possible to separate faint, warm C stars from K and M stars using the photometry alone. Photometry of AGB stars in clusters will allow an accurate evaluation of the AGB tip luminosities as a function of initial mass. Random phase K magnitudes of LPVs and Cepheids should provide a better measure of the LMC tilt and distortions in the SMC. The K survey should turn up 100 to 150 objects undergoing superwind mass loss, these objects being OH/IR stars and the dust-enshrouded C star equivalents of OH/IR stars. It is shown that crowding should not be a problem even in the LMC bar.  相似文献   

3.
Preliminary results are presented from two ongoing complementary surveys intended to investigate the nature and characteristics of the optically invisible secondaries in post-common envelope subdwarf B (sdB) binary stars. We obtain precise radial velocities to derive periods and minimum companion masses for bright field sdB stars. These data are combined with light curves to search for eclipses, reflection effects, or ellipsoidal variations. We emphasize the importance of using complete unbiased samples, without which it will not be possible to understand the details of the multiple processes that produce these stars. It remains true that all known secondary companions in short-period sdB binaries are nearly invisible, thus they must be either low mass main sequence (MS) stars or compact objects, e.g., white dwarfs. In our small, nearly-complete sample, white dwarf secondaries outnumber MS secondaries by about a factor of five. Known MS masses in short-period sdB binaries are all surprisingly low, indicating a possible bimodal mass distribution for all MS secondaries in sdB binaries.  相似文献   

4.
I present in this review the recent results obtained with ISO concerning gas phase molecules in evolved stars. The near, medium and far-infrared spectrum of C-rich and O-rich stars can provide a very important information on the molecular content of these objects. Detailed models for the envelopes of AGB stars are also presented. I also report the detection of triatomic carbon in the far-infrared and I analyze the contribution of the ro-vibrational lines of the low energy bending modes of polyatomic molecules in the far-infrared spectrum of C-rich evolved stars. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

5.
TheUBV photometry of 105 bright stars and theU magnitudes of 36 bright stars in M33 taken by the 2 m RCC telescope of the Rozhen Observatory at the Bulgarian Academy of Sciences were used to obtain colour-magnitude and colour-colour diagrams. A criterion forV, B-V, andU-B is suggested which facilitates the separation of the member stars from the foreground stars. The vast majority of the bright blue stars with strong negativeU-B are M33 members. The existence of objects with an unusual combination of colours (B-V>0.5 and strong negativeU-B) has been confirmed.  相似文献   

6.
We present quantitative analyses of 7 galactic O stars of spectral types from O3 to O6. By means of classical hydrostatic and planeparallel models we find the so-called He and Mass Discrepancies for a number of stars of our sample. We also prove the need of line-blocking for the correct reproduction of these spectra. For stars with effective temperature around 50 000 K and above, there is no possibility of reproducing their spectra with these kind of models. Further analyses with unified models, including sphericity and mass loss, allow us to reproduce even the earliest spectra. They also show that the Mass Discrepancy is not only due to the neglect of sphericity and mass loss in the planeparallel analysis, and give us the mass loss rate of these stars. We find that HD15 570, with a present mass of 100 M⊙, can be the most massive star known in the solar neighbourhood. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

7.
We collected 55 galactic extreme carbon stars from the published literature in this paper. Observational data from IRAS, 2MASS and ISO were analyzed. The results show that the infrared properties of extreme carbon stars are quite different to those for ordinary visual carbon stars. It is shown from IRAS and 2MASS photometric data that extreme carbon stars have much redder infrared colors not only in the far infrared, but also in the near infrared, hence they have much thicker ciucumstellar envelopes and mass loss. It is also indicated from IRAS Low-Resolution Spectra (LRS) and ISO Short Wavelength Spectra (SWS) that they have much redder infrared spectra from 2 μm to 45 μm. The above results are believed to be the signature of undergoing the last stages of AGB evolution for extreme carbon stars.  相似文献   

8.
Hot subdwarf stars (hot sds) are blue subluminous objects. Only a few determinations areavailable to date regarding the study of such aspects as rotation,microturbulent velocities or the magnetic nature of these objects. Over26 sdBs are known to date to be multiperiodic rapid oscillators. Thisproject presents preliminary results of new observations and models of asample of pulsating and non-pulsating hot sds, includingconsiderations on mass loss and eventual magnetic properties.  相似文献   

9.
Variability studies are an important tool to investigate key properties of stars and brown dwarfs. From photometric monitoring we are able to obtain information about rotation and magnetic activity, which are expected to change in the mass range below 0.3 solar masses, since these fully convective objects cannot host a solar‐type dynamo. On the other hand, spectroscopic variability information can be used to obtain a detailed view on the accretion process in very young objects. In this paper, we report about our observational efforts to analyse the variability and rotational evolution of young brown dwarfs and very low‐mass stars. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

10.
Gasdynamic features of detached shells around carbon stars with variable mass loss rate are investigated in detail numerically. It is shown that a shell is unstable and also, 2D perturbations are less developed that 3D ones. The structure of perturbed flows corresponding to different evolution scenarios is compared. The results obtained seem to be promising for interpretation of observations, in particular, the recently obtained detailed data of TT Cyg. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

11.
This is a study of the compact nebulae cn1 and cn2 situated in the extended, bright nebula S235. 12CO observations reveal the presence of blue and red outflows (i.e., a bipolar outflow) from the molecular cloud in which these nebulae are embedded. cn1 and cn2 are shown to be coupled to IR clusters of young stars, some of which have dust disks or envelopes (these are so-called young stellar objects, YSOs), with the YSOs grouped around the center of the clusters. cn1 is coupled to the infrared point source IRAS05377 + 3548, whose IR colors are close to those of T Tau stars. A chain of objects emerging from S235 (which clearly implies they are coupled in terms of evolution) is studied. These are the compact nebulae S235 A, B, and C, and the Herbig-Haro objects GGD5 and GGD6. A group of IR stars associated with GGD6 is also studied.  相似文献   

12.
We present moderate resolution CCD spectra and R photometry for seven KP2001 stars. We revised the spectral classification of the stars in the range λλ4000−8700? . Our photometric data confirmed the behavior of the light curves downloaded from the NSVS (Northern Sky Variability Survey) database. For KP2001-32, presented as a Mira-type variable in NSVS, we estimated absolute bolometric M b and K-band M K magnitudes as well as the distance using the period-luminosity relations. We also estimated the mass loss rate using the calibration relations between mass loss rate and K - [12] index. From the position in infrared color-color diagrams, we confirmed the photometric classification of KP2001-221 as a semiregular variable, based on the light curve of the NSVS database. For the N-type carbon star KP2001-77 we estimated distance and absolute magnitude M K using different calibration methods. For the remaining four objects we derived the absolute magnitudes and distances using our CCD spectra and published JHK S magnitudes. We discuss the nature of these objects on the basis of the obtained results.  相似文献   

13.
Via a study of the evolutionary tracks of 3∼10 M stars on the Hertzsprung-Russell diagram, the variations of the energy, density, temperature at the peak of helium-shell burning, ratio of surface luminosity of helium shell to stellar surface luminosity as well as the stellar radius are analyzed. Then the demarcation point of medium-mass stars in the evolution from early AGB stars to thermally pulsing AGB stars on the HR diagram is determined, and for 119 carbon stars our analysis agrees rather well with observation. At the same time the following is suggested. After arriving at this demarcation point in stellar evolution, in the formula of the loss of stellar wind material it is probably needed to introduce a quantity which is not concerned with the surface luminosity, but it dominates the formation of super stellar wind. On this basis and via the analysis of the structure and evolution of 5 M stars as well as the rate of mass loss of stellar wind, it is found that the effect of turbulent pressure on the mass loss of stellar wind in the stage of thermally pulsing AGB stars is rather great, hence the turbulent pressure of thermally pulsing AGB stars cannot be overlooked. Furthermore, the physical factors which possibly affect the matter loss of the stellar winds of thermally pulsing AGB stars are suggested.  相似文献   

14.
We have analyzed a sample of 74 magnetically active stars toward the Taurus-Auriga starforming region. Based on accurate data on their basic physical parameters obtained from original photometric observations and published data on their proper motions, X-ray luminosities, and equivalent widths of the Hα and Li lines, we have refined the evolutionary status of these objects. We show that 50 objects are young stars with ages of 1–40 Myr and belong to the Taurus-Auriga star-forming region. Other 20 objects have a controversial evolutionary status and can belong to both Taurus-Auriga starforming region and the Gould Belt. The remaining four objects with ages of 70–100 Myr belong to the zero-age main sequence. We have analyzed the relationship between the rotation period, mass, and age for 50 magnetically active stars. The change in the angular momentum of the sample stars within the first 40 Myr of their evolution has been investigated. An active star-protoplanetary disk interaction is shown to occur on a time scale from 0.7 to 10Myr.  相似文献   

15.
We present results of an analysis of a UV image in the direction of Ophiuchus, obtained with the FAUST instrument. The image contains 228 UV sources. Most of these are identified as normal early-type stars through correlations with catalogued objects. For the first time in this project we identify UV sources as such stars by selecting suitable candidates in crowded fields as the bluest objects in colour–colour diagrams using observations from the Wise Observatory. These candidates are then studied using low-resolution spectroscopy, which allows the determination of spectral types to an accuracy of about one-half class, for 60 stars.
Synthetic photometry of spectral data is performed in order to predict the expected UV emission, on the basis of the photometric information. These results are used along with the Hipparcos /Tycho information, to search for subluminous stars. The comparison of the predicted emission with the FAUST measured magnitudes allows us to select 12 stars as highly probable evolved hot stars. High signal-to-noise spectra are obtained for nine of these stars, and Balmer line profiles are compared with the prediction of atmosphere models and with the spectrum of real stellar atmospheres. Among the nine candidates, six are classified as previously unrecognized sdB stars, and two as white dwarfs. Our result indicates that indeed more bright subluminous stars are still unrecognized in the existing samples.  相似文献   

16.
We have secured FUSE observations for a sample of hot, hydrogen-rich subdwarf B stars. The main objective of this project is to map in a more detailed way than has been done in the past pattern of heavy element abundances in sdB stars in order to understand the competing roles of gravitational settling, radiative element support, and mass loss in their photospheres. We present abundances analyses for a subsample of four objects and compare these results with predictions of the parameter-free version of the radiative levitation theory.  相似文献   

17.
Experiments are performed to determine the coherence properties of wave fields, produced by the broadband stellar sources on the earth surface, from the study of spectral changes produced on interference in the Young's double slit experiment. The spectral degree of coherence obtained experimentally in this study for four bright stars in the wavelength range from 325 nm to 660 nm is in close agreement with the value that is expected theoretically from the known angular diameter of the stars. It is shown that the spectral degree of coherence obtained experimentally by this spectral interferometric technique could be used to determine the angular diameter of stars. This revised version was published online in July 2006 with corrections to the Cover Date. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

18.
A summary of our ongoing high and medium resolution optical observational program, devoted to the study of chromospheric activity of pre-main sequence stars, is reported here. The study is centered in bona-fid weak-lined T Tauri stars (WTTS) in Taurus with known rotational period, and in WTTS recently discovered by the ROSAT All-Sky Survey. It is our purpose to quantify the phenomenology of the chromospheric activity of each star determining stellar surface fluxes in the more important chromospheric activity indicators (Ca II H & K, Hβ, Hα, Ca II IRT) as well as to obtain the Li I abundance, a better determination of the stellar parameters, spectral type, and possible binarity. By using these chromospheric excess emissions we will study the flux-flux and rotation-activity relationships in order to get insights into the mechanisms which drive solar-like stellar activity in this kind of low mass pre-main sequence stars. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

19.
The oldest open clusters in our Galaxy set the lower limit to the age of the Galactic Disk (9–10 Gyr). Although they appear to be very rich now, it is clear that their primordial populations were much larger. Often considered as transitional objects, these populous open clusters show structural differences with respect to globular clusters so their dynamics and characteristic evolutionary time scales can also be different. On the other hand, their large membership lead to different dynamical evolution as compared with average open clusters. In this paper, the differential features of the evolution of rich open clusters are studied using N-body simulations, including several of the largest (104 stars) published direct collisional N-body calculations so far, which were performed on a CRAY YMP. The disruption rate of rich open clusters is analysed in detail and the effect of the initial spatial distribution of the stars in the cluster on its dynamics is studied. The results show that cluster life-time depends on this initial distribution, decreasing when it is more concentrated. The effect of stellar evolution on the dynamical evolution of rich clusters is an important subject that also has been considered here. We demonstrate that the cluster's life-expectancy against evaporation increases because of mass loss by evolving high-mass stars. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

20.
We present the first results of a survey carried out on a sample of optically bright post-AGB candidates selected from the IRAS Point Source Catalogue, many of them previously unidentified in the literature. The selection criteria are based on their characteristic far infrared colours. Low resolution spectroscopy of more than 150 sources has been obtained during 6 years. Our preliminary analysis confirms that most of the candidates observed are true post-AGB stars, some of them still strongly reddened. Some young planetary nebulae, a few young stellar objects and active galactic nuclei are also present in the sample. The distribution of post-AGB stars with different spectral types in the IRAS two-colour diagram and its evolutionary implications are briefly discussed. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

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