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1.
We put forward evidence that relates the phenomenon of the chemically peculiar stars of the upper main sequence to Supernovae explosions in young clusters. In the Upper Scorpius region we find that a supernova shell has interacted with dense clouds and that the peculiar stars lie close to or along the edges of interaction. We argue that the stars at or near the cloud faces are capable of acquiring this enriched material which is sufficiently slowed down. The magnetic accretion process of Havnes and Conti provides the mechanism of acquisition. This process with the associated magnetic braking accounts for the build-up in abundance anomalies and the slowing down of rotation with age.  相似文献   

2.
We analyze the temperature dependence of the abundances of the chemical elements Si, Ca, Cr, and Fe in the atmospheres of normal, metallic-line (Am), magnetic peculiar (Ap), and pulsating magnetic peculiar (roAp) stars in the range 6000–15000 K. The Cr and Fe abundances in the atmospheres of Ap stars increase rapidly as the temperature rises from 6000 to 9000–10000 K. Subsequently, the Cr abundance decreases to values that exceed the solar abundance by an order of magnitude, while the Fe abundance remains enhanced by approximately +1.0 dex compared to the solar value. The temperature dependence of the abundances of these elements in the atmospheres of normal and Am stars is similar in shape, but its maximum is several orders of magnitude lower than that observed for Ap stars. In the range 6000–9500 K, the observed temperature dependences for Ap stars are satisfactorily described in terms of element diffusion under the combined action of gravitational settling and radiative acceleration. It may well be that diffusion also takes place in the atmospheres of normal stars, but its efficiency is very low due to the presence of microturbulence. We show that the magnetic field has virtually no effect on the Cr and Fe diffusion in Ap stars in the range of effective temperatures 6000–9500 K. The Ca abundance and its variation in the atmospheres of Ap stars can also be explained in terms of the diffusion model if we assume the existence of a stellar wind with a variable moderate rate of ～(2–4) × 10? 15M⊙ yr?1.  相似文献   

3.
We present stellar evolution calculations for Population III stars for both single- and binary-star evolutions. Our models include 10- and  16.5-M⊙  single stars and a  10-M⊙  model star that undergoes an episode of accretion resulting in a final mass of  16.1 M⊙  . For comparison, we present the evolution of a solar heavy element abundance model. We use the structure from late-stage evolution models to calculate simulated supernova light curves. Light curve comparisons are made between accretion and non-accretion progenitor models, and models for single-star evolution of comparable masses. Where possible, we make comparisons to previous works. Similar investigations have been carried out, but primarily for solar or near-solar heavy metal abundance stars and not including both the evolution and the supernova explosions in one work.  相似文献   

4.
The loss of angular momentum owing to unstable r-modes in hot young neutron stars has been proposed as a mechanism for achieving the spin rates inferred for young pulsars. One factor that could have a significant effect on the action of the r-mode instability is fallback of supernova remnant material. The associated accretion torque could potentially counteract any gravitational-wave-induced spin-down, and accretion heating could affect the viscous damping rates and hence the instability. We discuss the effects of various external agents on the r-mode instability scenario within a simple model of supernova fallback on to a hot young magnetized neutron star. We find that the outcome depends strongly on the strength of the magnetic field of the star. Our model is capable of generating spin rates for young neutron stars that accord well with initial spin rates inferred from pulsar observations. The combined action of r-mode instability and fallback appears to cause the spin rates of neutron stars born with very different spin rates to converge, on a time-scale of approximately 1 year. The results suggest that stars with magnetic fields ≤1013 G could emit a detectable gravitational wave signal for perhaps several years after the supernova event. Stars with higher fields (magnetars) are unlikely to emit a detectable gravitational wave signal via the r-mode instability. The model also suggests that the r-mode instability could be extremely effective in preventing young neutron stars from going dynamically unstable to the bar-mode.  相似文献   

5.
We have derived the abundances of the rare-earth elements (REEs) Ce, Pr, Nd, and Eu in the atmospheres of 26 magnetic peculiar (Ap) stars in the range of effective temperatures 7000–10 000 K from spectra with resolutions R = 48 000 and 80 000 and investigated the dependence of the CePrNdEu anomalies (the difference in the element abundances determined separately from lines of the first and second ionization stages) on the effective temperature. The REE anomaly is shown to decrease with increasing effective temperature virtually to the point of disappearance for all of the investigated elements, except Eu. For the best-studied element Nd the Nd anomaly has also been found to decrease with increasing magnetic field strength for cool stars. For hot stars there is no Nd anomaly in a wide range of magnetic field strengths. Since the presence of anomalies in cool Ap stars is associated with the REE concentration in the upper atmospheric layers, the lower boundary of the REE layer apparently sinks into deeper layers with increasing effective temperature and magnetic field, causing the anomalies to disappear. We have detected an anticorrelation between the abundances of iron-peak elements and rare-earth elements, which serves as additional evidence for different stratification of these elements in the atmospheres of Ap stars.  相似文献   

6.
By estimating the orbital inclinations of non-eclipsing Ap spectroscopic binaries with periods less than 13 days, we can obtain a marked statistical result on the frequency distribution of the inclinations which strongly suggests that the abundance anomalies of Ap components tend to concentrate towards the stellar polar region. This would provide a critical test for models of Ap stars.  相似文献   

7.
We employ the supernova fallback disk model to simulate the spin evolution of isolated young neutron stars(NSs). We consider the submergence of the NS magnetic fields during the supercritical accretion stage and its succeeding reemergence. It is shown that the evolution of the spin periods and the magnetic fields in this model is able to account for the relatively weak magnetic fields of central compact objects and the measured braking indices of young pulsars. For a range of initial parameters, evolutionary links can be established among various kinds of NS sub-populations including magnetars, central compact objects and young pulsars. Thus, the diversity of young NSs could be unified in the framework of the supernova fallback accretion model.  相似文献   

8.
《Astronomische Nachrichten》1976,297(5):217-227
The published data on the abundance determinations for magnetic Ap-stars were considered critically. The traditional methods of curve of growth were shown to be unsufficient for quantitative analysis of the atmospheres of magnetic Ap-stars due to inhomogeneous structure of the surface of these stars. Only qualitative results can be obtained by these methods. Correlations between overabundances of different elements are considered. Very distinct correlations exist between overabundances of Mn and Zr, between Fe and rare earths and others. No correlation could be found between overabundance of elements and Teff. The methods of determination of local abundances and the results obtained — overabundances of some elements in spots and underabundances of them out of spots - are considered. The significanse of these data for the theory of chemical anomalies in magnetic Ap stars are mentioned.  相似文献   

9.
In quiet stellar atmospheres that are stabilized, for example, by the presence of strong magnetic fields, a non-uniform distribution of chemical elements (abundance stratification) is predicted by the diffusion theory. Spectroscopic observations of magnetic Ap stars provide a strong support for the diffusion theory. Stratification predictions in the upper atmosphere, in particular, strongly depend on the magnetic field geometry. Optical lines used in stratification analysis of Ap stars do not probe the upper atmospheric layers and, hence, cannot provide reliable information on abundance gradients there. We show the expected effect of stratification on the profiles of strong lines in the UV region and how these lines may be used for constraining diffusion theory.  相似文献   

10.
The deficiency of spectroscopic binaries among Ap stars compared with normal stars is confirmed. In systems with Ap components of other than Mn-Hg type we have a lack of short periods. For these systems we have established relativ large excentricities of their orbits. Circular orbits are absent. These observational facts must be taken into consideration in order to explain the generation of stellar magnetic fields and Ap characteristics.  相似文献   

11.
The studies of the abundances of superheavy chemical elements in magnetic stars conducted at the Crimean Astrophysical Observatory were facilitated by the launch of the Astron space station in 1983. This spacecraft observed the ultraviolet spectra of such stars. The present brief review is focused on the abundances of superheavy elements (Pt, Au, Hg, Tl, Pb, Bi, Th, and U) in the atmospheres of magnetic Ap and HgMn stars. These results were obtained basing on the visible and ultraviolet spectra of stars. The data accumulated over more than 30 years show that these stars are characterized by significant overabundances (up to 6–7 dex) of such elements. The following important fact is noted: the superheavy element anomalies follow the trend in the anomalies that are characteristic of less heavy elements. Therefore, it may be assumed that all these anomalies (including the significant overabundances of superheavy elements) share a common explanation. Certain unresolved problems are discussed briefly.  相似文献   

12.
Helium, which was first discovered on the sun with the help of spectral analysis, plays, together with hydrogen, a principal role in astrophysics. We consider here two fundamental quantities: primordial helium abundance formed during Big Bang nucleosynthesis and the current initial helium abundances in nearby stars. It is shown that stellar atmospheres are enriched in helium during the main-sequence stage. Observational evidence for helium contamination in close OB-binaries is discussed. Stars with strong abundance anomalies are considered, such as chemically peculiar Ap and Bp helium-deficient stars and some types of objects with helium atmospheres.  相似文献   

13.
The energy distribution curves of eleven Ap stars, three Am stars, four normal A stars and one F0 V magnetic star have been obtained between 478 nm and 680 nm. For four of the Ap stars, two Am stars and all the four normal A stars, the effective temperatures are believed to have been estimated for the first time. For the rest, these estimates are expected to be an improvement over previously available values.It is concluded that the Ap and Am stars are not much different from the normal A stars in so far as their temperatures are concerned.  相似文献   

14.
Neutron stars in X-ray binary systems are fascinating objects that display a wide range of timing and spectral phenomena in the X-rays. Not only parameters of the neutron stars, like magnetic field strength and spin period evolve in their active binary phase, the neutron stars also affect the binary systems and their immediate surroundings in many ways. Here we discuss some aspects of the interactions of the neutron stars with their environments that are revelaed from their X-ray emission. We discuss some recent developments involving the process of accretion onto high magnetic field neutron stars: accretion stream structure and formation, shape of pulse profile and its changes with accretion torque. Various recent studies of reprocessing of X-rays in the accretion disk surface, vertical structures of the accretion disk and wind of companion star are also discussed here. The X-ray pulsars among the binary neutron stars provide excellent handle to make accurate measurement of the orbital parameters and thus also evolution of the binray orbits that take place over time scale of a fraction of a million years to tens of millions of years. The orbital period evolution of X-ray binaries have shown them to be rather complex systems. Orbital evolution of X-ray binaries can also be carried out from timing of the X-ray eclipses and there have been some surprising results in that direction, including orbital period glitches in two X-ray binaries and possible detection of the most massive circum-binary planet around a Low Mass X-ray Binary.  相似文献   

15.
频繁的超新星爆发可用以解释活动星系核与类星体的超高金属丰度。星团中的恒星与吸积盘相互作用导致致密天体中子星和黑洞的形成率很高,这些致密天体与红巨星很容易形成双星系统,而后形成（ＮＳ／ＢＨ,ＮＳ／ＢＨ） 双星由于引力辐射最终导致并合而形成伽玛射线爆发。我们估计了伽玛射线爆的爆发频率,发现与观测相符。  相似文献   

16.
Ap star magnetism is often attributed to fossil magnetic fields which have not changed much since the pre‐main‐sequence epoch of the stars. Stable magnetic field configurations are known which could persist probably for the entire mainsequence life of the star, but they may not show the complexity and diversity exhibited by the Ap stars observed. We suggest that the Ap star magnetism is not a result of stable configurations, but is the result of an instability based on strong toroidal magnetic fields buried in the stars. The highly nonaxisymmetric remainders of the instability are reminiscent of the diversity of fields seen on Ap stars. The strengths of these remnant magnetic fields are actually between a few per cent up to considerable fractions of the internal toroidal field; this means field strengths of the order of kGauss being compatible with what is observed. The magnetic fields emerge at the surface rather quickly; rough estimates deliver time‐scales of the order of a few years. Since rotation stabilizes the instability, normal A stars may still host considerable, invisible toroidal magnetic fields (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

17.
The fast neutron capture process (the r-process) occurs in the neutron-rich circumstance. However its concrete physical environment is not very clear. With recent progress in observations, many extremely metal-poor halo stars have been discovered. They have two characteristics: one is the overabundance of fast neutron elements with the relative abundance consistent with that of the sun; the other is that fast neutron element contents in stars at the same metal abundance have a very large dispersion. This provides a particular way to study the origin of the r-process. Simulation was used to study the galaxy's evolution process and the resulting dispersion of fast neutron nuclide contents in stars. The model of galaxy evolution obtained in this way not only contains spontaneous star formation in the gas region, but also includes the star formation excited by the supernova explosion. It is shown from our results that the supernovae at the low mass end should be the place producing the fast neutron nuclides. In addition, it is also shown that the non-uniformity of the galaxy evolution caused by the supernova explosion is not enough to explain the observed dispersion of fast neutron element contents in halo stars. This problem should be further studied.  相似文献   

18.
The infrared properties of barium stars are studied using published data in the K band and from IRAS . At 12 and 25 μm the emission from barium stars shows no excess over photospheric emission. Thus the claim made by Hakkila that some barium stars show evidence of the presence of warm (∼300  K ) circumstellar material is not supported. The 60-μm properties of barium stars are studied using survival analysis methods, and it is found that very few (3.7 ± 2.6 per cent) barium stars exhibit far-infrared excesses. Furthermore, it is found that the incidence of excess emission at 60 μm is lower in barium stars than for normal G and K giants. This may indicate that the mass-transfer event that is assumed to have taken place in barium stars has removed any cool circumstellar material that may have existed in these systems. Alternatively, it is suggested that the incidence of infrared excesses in normal G and K giants may have been over-estimated as a result of not fully accounting for foreground contamination by interstellar cirrus.  相似文献   

19.
Dynamo action within the cores of Ap stars may offer intriguing possibilities for understanding the persistent magnetic fields observed on the surfaces of these stars. Deep within the cores of Ap stars, the coupling of convection with rotation likely yields magnetic dynamo action, generating strong magnetic fields. However, the surface fields of the magnetic Ap stars are generally thought to be of primordial origin. Recent numerical models suggest that a primordial field in the radiative envelope may possess a highly twisted toroidal shape. We have used detailed 3-D simulations to study the interaction of such a twisted magnetic field in the radiative envelope with the core-dynamo operating in the interior of a 2 solar mass A-type star. The resulting dynamo action is much more vigorous than in the absence of such a fossil field, yielding magnetic field strengths (of order 100 kG) much higher than their equipartition values relative to the convective velocities. We examine the generation of these fields, as well as the growth of large-scale magnetic structure that results from imposing a fossil magnetic field. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

20.
The Hertzsprung-Russell diagram of the Large Magellanic Cloud compiled recently by Fitzpatrick & Garmany (1990) shows that there are a number of supergiant stars immediately redward of the main sequence although theoretical models of massive stars with normal hydrogen abundance predict that the region 4.5 ≤ logT eff ≤ 4.3 should be un-populated (“gap”). Supergiants having surface enrichment of helium acquired for example from a previous phase of accretion from a binary companion, however, evolve in a way so that the evolved models and observed data are consistent — an observation first made by Tuchman & Wheeler (1990). We compare the available optical data on OB supergiants with computed evolutionary tracks of massive stars of metallicity relevant to the LMC with and without helium-enriched envelopes and conclude that a large fraction (≈ 60 per cent) of supergiant stars may occur in binaries. As these less evolved binaries will later evolve into massive X-ray binaries, the observed number and orbital period distribution of the latter can constrain the evolutionary scenarios of the supergiant binaries. The distributions of post main sequence binaries and closely related systems like WR + O stars are bimodal-consisting of close and wide binaries in which the latter type is numerically dominating. When the primary star explodes as a supernova leaving behind a neutron star, the system receives a kick and in some cases can lead to runaway O-stars. We calculate the expected space velocity distribution for these systems. After the second supernova explosion, the binaries in most cases, will be disrupted leading to two runaway neutron stars. In between the two explosions, the first born neutron star’s spin evolution will be affected by accretion of mass from the companion star. We determine the steady-state spin and radio luminosity distributions of single pulsars born from the massive stars under some simple assumptions. Due to their great distance, only the brightest radio pulsars may be detected in a flux-limited survey of the LMC. A small but significant number of observable single radio pulsars arising out of the disrupted massive binaries may appear in the short spin period range. Most pulsars will have a low velocity of ejection and therefore may cluster around the OB associations in the LMC.  相似文献   

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