首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Having analysed the accretion-based model of the dust–gas separation, which is regarded to be the most promising for the explanation of the anomalous properties of λ Bootis-type stars, we can conclude that: (i) for any reasonable density profiles of the shell, dust grains appear to be decoupled from the gaseous background within the region where the temperature drops to a value that is less than the condensation temperature of heavy elements such as Mg, Ca, Fe, etc; (ii) most likely, in the shell of λ Bootis-type stars only small dust particles (of less than ≈10−6 cm in size) can be created; (iii) significant alteration of the initial atmospheric chemical composition can take place in the case when the density in the shell changes as ∼ r −2.  相似文献   

2.
1 INTRODUCTIONThe mixing length theory (MLT) for stellar convection originally developed by Vitense(1953, 1958) has been the most popularly used local convection theory in the studies of stellarstructure and evolution. The theory was later modified and revised by many investigators,who suggested some different expressions. In fact, MLT is not a real hydrodynamic theory,rather, it is a simple "ballistic" theory which traces the motion of imaginary convective elements. In reality j stell…  相似文献   

3.
We employ spectra of resolution 20–35000 of seven SC stars, four S stars, two Ba stars and two K–M stars to derive abundances of a variety of elements from Sr to Eu relative to iron. Special attention is paid to Rb and Tc, and to the ratio of the heavy s-process species to the light s-process elements. Abundances are derived in LTE, both by using model atmospheres in which the carbon and oxygen abundances are nearly equal and by using curves of growth. Spectrum synthesis is used for critical lines such as the 5924-Å line of Tc and the 7800-Å line of Rb. For most of the heavy-element stars the enhancement of the s-process elements is about a factor of 10. The ratio of the heavy to light s-process species is not far from solar, except for RR Her for which the same ratio is +0.45 dex. For Tc the blending by other lines is severe. While we have probably detected the 5924-Å line, we can only present abundances in the less-than-or-equal-to category. For Rb, whose abundance is sensitive to the 85Rb/87Rb ratio and hence to the neutron density during s-process production, we find a considerable range of abundances, indicating a neutron density from 106 to ≳108 cm−3 for the SC stars. For the four S stars the range is from 107 to ≳108 cm−3. Recent calculations by Gallino et al. show that neutron densities near 107 cm−3 favour the 13C source for neutrons, while densities greater than 108 cm−3 may be associated with neutrons from the 22Ne source.  相似文献   

4.
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   

5.
We have collected nearly all the available observed data of the elements from Ba to Dy in halo and disk stars in the metallicity range -4.0 <[Fe/H]< 0.5. Based on the observed data of Ba and Eu, we evaluated the least-squares regressions of [Ba/Fe] on [Fe/H], and [Eu/H] on [Ba/H]. Assuming that the heavy elements (heavier than Ba) are produced by a combination of the main components of s- and r-processes in metal-poor stars, and choosing Ba and Eu as respective representative elements of the main s- and the main r-processes, a statistical model for predicting the Galactic chemical evolution of the heavy elements is presented. With this model, we calculate the mean abundance trends of the heavy elements La, Ce, Pr, Nd, Sm, and Dy with the metallicity. We compare our results with the observed data at various metallicities, showing that the predicted trends are in good agreement with the observed trends, at least for the metallicity range [Fe/H]> -2.5. Finally, we discuss our results and deduce some importa  相似文献   

6.
The abundances of long-lived radioactive elements Th and U observed in metal-poor halo stars can be used as chronometers to determine the age of individual stars, and hence set a lower limit on the age of the Galaxy and hence of the universe. This radioactive dating requires the zero-decay productions of Th and U, which involves complicated r-process nucleosynthesis calculations. Several parametric r-process models have been used to calculate the initial abundance ratios of Th/Eu and U/Th, but, due to the sharp sensitivity of these models to nuclear physics inputs, the calculations have relatively large uncertainties which lead to large uncertainties in the age determinations. In order to reduce these uncertainties, we present a simple method to estimate the initial productions of Th and U, which only depends on the solar system abundances and the stellar abundances of stable r-process elements. From our calculations of the initial abundance ratios of Th/Eu and U/Th, we re-estimate the ages of those ver  相似文献   

7.
Using the latest non-local thermodynamic equilibrium (non-LTE) synthetic spectra and stellar model calculations, we have evaluated the potential effect of the presence of heavy elements in the photospheres of hot H-rich DA white dwarfs. In particular, we have examined their influence on the effective temperature and surface gravity perceived from analysis of the Balmer line profiles. It is apparent that both the inclusion of non-LTE effects in the models and significant quantities of heavy elements act independently to lower the value of T eff determined from a particular spectrum. Hence, the true effective temperatures of the heavy element-rich DA white dwarfs, currently estimated to be above 55 000 K, are apparently lower than previously reported from pure-H LTE analyses, by some 4000–7000 K. We do not see any similar influence on measurements of log g . This work concentrates on a group of relatively bright well-studied objects, for which heavy element abundances are known. As a consequence of this, establishment of correct temperatures for all other hot white dwarfs will require a programme of far-UV spectroscopy in order to obtain the essential compositional information. Since only stars with effective temperatures lying notionally in the range from ≈ 55 000 to 70 000 K (52 000–62 000 K when the non-LTE effects and heavy elements are taken into account) have been considered here, important questions remain regarding the magnitude of any similar effects in even hotter white dwarfs and pre-white dwarfs. The resulting implications for the plausibility of the evolutionary link between the main hot DA population and their proposed precursors, the H-rich central stars of planetary nebulae, need to be investigated.  相似文献   

8.
邱红梅  赵刚  仲佳勇 《天文学报》2002,43(3):257-263
在第1篇论文的基础上，确定了样本星的恒星大气参数，得到这些星中9种元素的丰度。讨论了各种元素丰度随[Fe/H]的变化。平均的[Na/Fe]～－0．01dex，接近于太阳丰度。α元素Si和Ca具有几乎相同的丰度模式，而[Ti/Fe]弥散较大，但三者均有随[Fe/H]的减小而增加的趋势。铁峰元素V、Cr、Ni在不同丰度处有较大的弥散，[Cr/Fe]在所有样本星中均表现超丰；而[Mn/Fe]却明显过贫，且随金属丰度的增加而增加。  相似文献   

9.
We introduce a new method to derive the orbital parameters of spectroscopic bi-nary stars by nonlinear least squares of (o-c). Using the measured radial velocity data of the four double lined spectroscopic binary systems, AI Phe, GM Dra, HD 93917 and V502 Oph, we derived both the orbital and combined spectroscopic elements of these systems. Our numerical results are in good agreement with the those obtained using the method of Lehmann-Filhe's.  相似文献   

10.
We monitored 16 X-ray selected young solar-type stars for light variation and found appreciable periodic light variability with amplitudes of a few hundredths of a magni-tude in nine of the objects. Using the method of Phase Dispersion Minimization (PDM) and Fourier analysis (software PERIOD04), the rotation periods of these stars were determined from the photometric data. The rotation periods of all nine stars are shorter than about 3days. It is suggested that, as with the Pleiades cluster, small amplitude light variations are quite common among young solar-type stars with rotation periods around 3 days or less. This gives further evidence for the spin up of solar-type stars predicted by models of angular momentum evolution of pre-main sequence stars.  相似文献   

11.
In our previous search for magnetic fields in Herbig Ae stars, we pointed out that HD 101412 possesses the strongest magnetic field among the Herbig Ae stars and hence is of special interest for follow‐up studies of magnetism among young pre‐main‐sequence stars. We obtained high‐resolution, high signal‐to‐noise UVES and a few lower quality HARPS spectra revealing the presence of resolved magnetically split lines. HD 101412 is the first Herbig Ae star for which the rotational Doppler effect was found to be small in comparison to the magnetic splitting and several spectral lines observed in unpolarized light at high dispersion are resolved into magnetically split components. The measured mean magnetic field modulus varies from 2.5 to 3.5kG, while the mean quadratic field was found to vary in the range of 3.5 to 4.8 kG. To determine the period of variations, we used radial velocity, equivalent width, line width, and line asymmetry measurements of variable spectral lines of several elements, as well as magnetic field measurements. The period determination was done using the Lomb‐Scargle method. The most pronounced variability was detected for spectral lines of He I and the iron peak elements, whereas the spectral lines of CNO elements are only slightly variable. From spectral variations and magnetic field measurements we derived a potential rotation period Prot = 13.86 d, which has to be proven in future studies with a larger number of observations. It is the first time that the presence of element spots is detected on the surface of a Herbig Ae/Be star. Our previous study of Herbig Ae stars revealed a trend towards stronger magnetic fields for younger Herbig Ae stars, confirmed by statistical tests. This is in contrast to a few other (non‐statistical) studies claiming that magnetic Herbig Ae stars are progenitors of the magnetic Ap stars. New developments in MHD theory show that the measured magnetic field strengths are compatible with a current‐driven instability of toroidal fields generated by differential rotation in the stellar interior. This explanation for magnetic intermediate‐mass stars could be an alternative to a frozen‐in fossil field (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

12.
We model the evolution of the abundances of light elements in carbon-enhanced metal-poor (CEMP) stars, under the assumption that such stars are formed by mass transfer in a binary system. We have modelled the accretion of material ejected by an asymptotic giant branch star on to the surface of a companion star. We then examine three different scenarios: one in which the material is mixed only by convective processes, one in which thermohaline mixing is present and a third in which both thermohaline mixing and gravitational settling are taken in to account. The results of these runs are compared to light element abundance measurements in CEMP stars (primarily CEMP- s stars, which are rich in s -processes elements and likely to have formed by mass transfer from an AGB star), focusing on the elements Li, F, Na and Mg. None of the elements is able to provide a conclusive picture of the extent of mixing of accreted material. We confirm that lithium can only be preserved if little mixing takes place. The bulk of the sodium observations suggest that accreted material is effectively mixed but there are also several highly Na and Mg-rich objects that can only be explained if the accreted material is unmixed. We suggest that the available sodium data may hint that extra mixing is taking place on the giant branch, though we caution that the data are sparse.  相似文献   

13.
High-resolution, high signal-to-noise ratio spectra have been obtained for 32 metal-poor stars. The equivalent widths of Li A6708 A were measured and the lithium abundances were derived. The average lithium abundance of 21 stars on the lithium plateau is 2.33±0.02 dex. The Lithium plateau exhibits a marginal trend along metallicity, dA(Li)/d[Fe/H] ?0.12±0.06, and no clear trend with the effective temperature. The trend indicates that the abundance of lithium plateau may not be primordial and that a part of the lithium was produced in Galactic Chemical Evolution (GCE).  相似文献   

14.
We review existing ROSAT detections of single Galactic Wolf–Rayet (WR) stars and develop wind models to interpret the X-ray emission. The ROSAT data, consisting of bandpass detections from the ROSAT All-Sky Survey (RASS) and some pointed observations, exhibit no correlations of the WR X-ray luminosity ( L X) with any star or wind parameters of interest (e.g. bolometric luminosity, mass-loss rate or wind kinetic energy), although the dispersion in the measurements is quite large. The lack of correlation between X-ray luminosity and wind parameters among the WR stars is unlike that of their progenitors, the O stars, which show trends with such parameters. In this paper we seek to (i) test by how much the X-ray properties of the WR stars differ from the O stars and (ii) place limits on the temperature T X and filling factor f X of the X-ray-emitting gas in the WR winds. Adopting empirically derived relationships for T X and f X from O-star winds, the predicted X-ray emission from WR stars is much smaller than observed with ROSAT . Abandoning the T X relation from O stars, we maximize the cooling from a single-temperature hot gas to derive lower limits for the filling factors in WR winds. Although these filling factors are consistently found to be an order of magnitude greater than those for O stars, we find that the data are consistent (albeit the data are noisy) with a trend of in WR stars, as is also the case for O stars.  相似文献   

15.
1 INTRODUCTION In the past years, we were thrilled to the reports of discoveries of many planets around stars.These planetary systems outside the solar system (if exist) provide not only an independenttest of the formation theory of the solar system but also a chance to search for extraterrestriallife in the universe. Many studies have been made to identify the particularities of these stars,among which spectroscopic studies (e.g. Gonzalez et al. 2001; Santos et al. 2001; Zhao etaL. 2001…  相似文献   

16.
Minimal models of cooling neutron stars with accreted envelopes   总被引：1，自引：0，他引：1  
We study the 'minimal' cooling scenario of superfluid neutron stars with nucleon cores, where the direct Urca process is forbidden and enhanced cooling is produced by neutrino emission due to the Cooper pairing of neutrons. Extending our recent previous work, we include the effects of surface accreted envelopes of light elements. We employ the phenomenological density-dependent critical temperatures   T cp(ρ)  and   T cnt(ρ)  of singlet-state proton and triplet-state neutron pairing in a stellar core, as well as the critical temperature   T cns(ρ)  of singlet-state neutron pairing in a stellar crust. We show that the presence of accreted envelopes simplifies the interpretation of observations of thermal radiation from isolated neutron stars in the scenario of our recent previous work and widens the class of models for nucleon superfluidity in neutron star interiors consistent with the observations.  相似文献   

17.
We study spectra of 18 active stars and stellar systems, drawn from lists of known active binaries or newly discovered extreme-ultraviolet and soft X-ray sources. Using spectral synthesis and template modelling, we measure (often for the first time) a variety of physical properties of the stars, including spectral types, luminosities and v  sin  i values. We report direct observation of previously undetected secondary components in three systems, and refine the orbital elements for two of these (HR 7275 and HD 217344).  相似文献   

18.
It is generally acknowledged that the mass-loss of asymptotic giant branch (AGB) stars undergoes variations on different time-scales. We constructed models for the dust envelopes for a sample of AGB stars to assess whether mass-loss variations influence the spectral energy distribution. To constrain the variability, extra observations at millimetre wavelengths (1.2 mm) were acquired. From the analysis of the dust models, two indications for the presence of mass-loss variations can be found, being (1) a dust temperature at the inner boundary of the dust envelope that is far below the dust condensation temperature and (2) an altered density distribution with respect to  ρ( r ) ∝ r −2  resulting from a constant mass-loss rate. For five out of the 18 studied sources a two-component model of the envelope is required, consisting of an inner region with a constant mass-loss rate and an outer region with a less steep density distribution. For one source an outer region with a steeper density distribution was found. Moreover, in a search for time variability in our data set at 1.2 mm, we found that WX Psc shows a large relative time variation of 34 per cent which might partially be caused by variable molecular line emission.  相似文献   

19.
A detailed study was performed for a sample of low-mass pre-main-sequence (PMS) stars, previously identified as weak-line T Tauri stars, which are compared to members of the Tucanae and Horologium Associations. Aiming to verify if there is any pattern of abundances when comparing the young stars at different phases, we selected objects in the range from 1 to 100 Myr, which covers most of PMS evolution. High-resolution optical spectra were acquired at European Southern Observatory and Observatório do Pico dos Dias . The stellar fundamental parameters effective temperature and gravity were calculated by excitation and ionization equilibria of iron absorption lines. Chemical abundances were obtained via equivalent width calculations and spectral synthesis for 44 per cent of the sample, which shows metallicities within 0.5 dex solar. A classification was developed based on equivalent width of Li  i 6708 Å and Hα lines and spectral types of the studied stars. This classification allowed a separation of the sample into categories that correspond to different evolutive stages in the PMS. The position of these stars in the Hertzsprung–Russell diagram was also inspected in order to estimate their ages and masses. Among the studied objects, it was verified that our sample actually contains seven weak-line T Tauri stars, three are Classical T Tauri, 12 are Fe/Ge PMS stars and 21 are post-T Tauri or young main-sequence stars. An estimation of circumstellar luminosity was obtained using a disc model to reproduce the observed spectral energy distribution. Most of the stars show low levels of circumstellar emission, corresponding to less than 30 per cent of the total emission.  相似文献   

20.
We report observations of the He  i λ 5876 (D3) line in the late A- and early F-type stars in the Pleiades and Alpha Persei star clusters used to determine chromospheric activity levels. This represents the first sample of young stars in this temperature range with chromospheric activity measurements. We find the same average activity level in the young early F stars as in Hyades-age stars and field stars. In addition, the young star sample shows the same large star-to-star variation in activity as seen in the older stars. Thus, as a whole, chromospheric activity in this photospheric temperature range remains the same over nearly a factor of 100 in stellar age (50 Myr to 3 Gyr), in striking contrast to the behaviour of later-type stars. In the five late A stars we find three certain detections of D3 and one likely detection. This includes the bluest star yet observed with a chromospheric D3 line, Pleiades star HII 1362 at ( B − V )0=0.22, making it one of the earliest stars with an observed chromosphere. The late A stars have D3 equivalent widths comparable to the weakest early F stars. However, when comparing D3 measurements in the young late A stars with older late A stars, we find evidence for a slight decrease in activity with age based on the large number of non-detections in the older stars. We find an apparently linear relationship between the activity upper limit and B − V over our entire range of B − V . Extrapolated blueward, this relationship predicts that the chromospheric D3 line would disappear for all stars at B − V ≈0.13.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号