Effects of mass loss on the late stages of stellar evolution

Advances in the infrared and radio observational techniques in the last decade have led to a revolution in our understanding of the late stages of stellar evolution. Intermediate (1–8 M_⊙) mass stars are found to be undergoing rapid mass loss in the form of a stellar wind during the asymptotic-giant-branch after the exhaustion of helium burning in the core. Significant fraction of the original stellar mass can be lost in short time scales of < 10⁶ yr. The ejected mass constitutes the major component of matter returned by stars to the interstellar medium. Since such material has been heavily nuclear processed, they also represent the dominant mechanism of chemical enrichment of the Galaxy. The high rate of mass loss implies that the majority of Population I stars end their evolution as planetary nebulae and white dwarfs rather than superovae and neutron stars.In this review, we summarize recent observational methods in the determination of the mass loss rate and the associated physical parameters of the stellar wind. Since the observed mass loss rate greatly exceeds the nuclear burning rate, we also discuss the theoretical models on how such mass loss affects the asymptotic giant branch evolution. A scenario is presented on how red giants evolve into planetary nebulae, a process which has been very poorly understood until recently. Conjectures on how the current evolutionary “missing link” - the proto-planetary nebulae - could be identified are also considered.     

Les premières étoiles

The oldest stars of the Galaxy are quite different from common stars, like our Sun. Understanding why it is so, requires to open the question in a cosmological perspective. After the Big Bang, and for at least 300 000 years, the Universe was nearly uniform, and had a very simple chemical composition formed during the hot phase of the Big Bang: only hydrogen, helium and traces of other light elements, deuterium, ³ He, and ⁷ Li. This composition is known as “primordial”. At a later time, about one or two billion years after the Big Bang, condensations developped at all scales, the smallest ones being stars. The most massive stars, reaching very high temperatures at their center, transformed their initial composition by thermonuclear reactions, producing all common elements observed in the solar system, carbon, nitrogen, oxygen, etc. These elements were dispersed into the interstellar medium by mass-ejection at the final stage of evolution of these massive stars, and recycled by subsequent generations of stars. The first stars must have been formed with the primordial composition, whereas later generations had an increasing proportion of elements produced by stellar nucleosynthesis. Intensive searches of stars with no, or very little elements produced by stellar nucleosynthesis have been performed during the last 20 years. Actually more than 100 stars were discovered with a very low proportion of such elements, one thousandth of the proportion in the Sun (in which they amount to about 1.7% by mass), or less. But no star was found with less that 1/10,000 of the solar proportion. So no “primordial” star has been observed yet. The reason why is still an open question.     

On the Cosmological Origin of Population III Objects

It is the aim of our letter to embedd the presently well understood scale of stellar mass into a simple cosmological scenario for the origin of Population III objects. The result of our consideration will be that if the first bound objects with typical stellar mass of the order of 30 M_⊙ could evolve as ordinary stars ending up exploding (Type II supernovae) the problem of the pregalactic enrichment of a primordial gas cloud be solved.     

Microquasars

Microquasars are compact objects (stellar-mass black holes and neutron stars) in our Galaxy that mimic, on a smaller scale, many of the phenomena seen in quasars. Their discovery provided new insights into the physics of relativistic jets observed elsewhere in the universe, and the accretion–jet coupling. Furthermore, microquasars are opening new horizons for the understanding of ultraluminous X-ray sources observed in external galaxies, gamma-ray bursts of long duration, and the origin of stellar black holes and neutron stars. Microquasars are one of the best laboratories to probe General Relativity in the limit of the strongest gravitational fields, and, as such, have become an area of topical interest for both high energy physics and astrophysics. To cite this article: I.F. Mirabel, C. R. Physique 8 (2007).     

Maximum stellar iron core mass

An analytical method of estimating the mass of a stellar iron core, just prior to core collapse, is described in this paper. The method employed depends, in part, upon an estimate of the true relativistic mass increase experienced by electrons within a highly compressed iron core, just prior to core collapse, and is significantly different from a more typical Chandrasekhar mass limit approach. This technique produced a maximum stellar iron core mass value of 269 × 10³⁰ kg (1.35 solar masses). This mass value is very near to the typical mass values found for neutron stars in a recent survey of actual neutron star masses. Although slightly lower and higher neutron star masses may also be found, lower mass neutron stars are believed to be formed as a result of enhanced iron core compression due to the weight of non-ferrous matter overlying the iron cores within large stars. And, higher mass neutron stars are likely to be formed as a result of fallback or accretion of additional matter after an initial collapse event involving an iron core having a mass no greater than 2.69 × 10³⁰ kg     

Future solar systems

Widely regarded as pathological variable stars – with erratic photometric and spectroscopic behavior of unknown physical origin – 20 years ago, T Tauri stars (TTSs) turned out in the last decade to be promising laboratories for observing the formation of solar systems such as ours. This is because circumstellar, presumably protoplanetary disks were found to surround a large fraction of them. While evidence for disks was primarily indirect until 1995, recent high resolution imaging confirmed earlier claims that the infrared (IR) and ultraviolet (UV) excesses seen in the spectral energy distribution (SED) of these stars were due to disk emission. The activity displayed by young stellar objects at all wavelengths is due to the interaction between the circumstellar disk and the magnetized star and to non-stationary accretion/ejection phenomena. In this review, we briefly summarize properties of these young solar-type stars and describe their circumstellar disks in some detail, focusing on current optical, infrared and millimeter high angular resolution observations that now allow us to resolve the disks.     

Abundance gradients in the galactic disk

The relationship between abundances and orbital parameters for 235 F- and G-type intermediate- and low- mass stars in the Galaxy is analyzed. We foundthat there are abundance gradients in the thin disk in both radial and verticaldirections (-0.116 dex kpc-1 and -0.309 dex kpc-1 respectively). The gradients appear to be flatter as the Galaxy evolves. No gradient is found in the thick disk based on 18 thick disk stars. These results indicate that the ELS model is mainly suitable for the evolution of the thin disk, while the SZ model is more suitable for the evolution of the thick disk. Additionally, these results indicate that in-fall and out-flow processes play important roles in the chemical evolution of the Galaxy.     

Estimate of stellar masses from their QPO frequencies

Kilohertz quasiperiodic oscillations (kHz QPOs) are observed in binary stellar systems. For such a system, the stellar radius is very close to the marginally stable orbit R _ms as predicted by Einstein’s general relativity. Many models have been proposed to explain the origin of the kHz QPO features in the binaries. Here we start from the work of Li et al (Phys. Rev . Lett. 83, 3776 (1999)) who in 1999, from the unique millisecond X-ray pulsations, suggested SAX J1808.4−3658 to be a strange star, from an accurate determination of its rotation period. It showed kHz QPOs eight years ago and so far it is the only set that has been observed. We suggest that the mass of four compact stars SAX J1808.4−3658, KS 1731−260, SAX J1750.8−2900 and IGR J17191−2821 can be determined from the difference in the observed kHz QPOs of these stars. It is exciting to be able to give an estimate of the mass of the star and three other compact stars in low-mass X-ray binaries using their observed kHz QPOs.     

The LVD experiment at Gran Sasso

Summary The Large-Volume Detector (LVD) in the Gran Sasso underground Laboratory is a multipurpose detector consisting of a large volume of liquid scintillator (at present 562 tons are in data taking) interleaved with limited-streamer tubes. Several physical problems are investigated with LVD, the major being the search for neutrino bursts from gravitational stellar collapses in our Galaxy. In this paper we discuss some results on cosmic neutrinos and cosmic-ray muons obtained with the first of the five towers of LVD (operational since June 1992) and part of the second tower (operational since June 1994). The results of the search for supernovae neutrinos show that LVD is a neutrino observatory able to detect neutrinos of different flavours from gravitational stellar collapses in all our Galaxy, over a wide range of burst durations. Indeed, the carbon-based liquid-scintillator target gives a unique possibility to directly detect neutral- and charged-currents neutrino interactions with a very good signature. This characteristic of LVD allows us to make an indirect estimate of the neutrino rest mass and of neutrino oscillations from supernovae in our Galaxy. No evidence for burst candidates has been found in the data recorded from June 1992 to March 1995, for a total live time of 682 days and a total exposure of 613 tons per year. We present the results of a time coincidence analysis between low-energy signals, eventually due to neutrinos of different flavours, and γ-ray bursts (GRBs) detected by the BATSE experiment. This search covers the period from June 1993 to March 1995, during which 41 GRBs have been selected from the BATSE data. Since no excess of events in LVD has been found, upper limits on the neutrino fluxes are reported for (ν_e, p), and for neutral- and charged-currents neutrino interactions of different flavours with the C-nuclei of the scintillator. The muon intensity as a function of slant depth is presented. These measurements, obtained during a live time period of 11.556 hours, cover a slant depths range from about 3000 to about 20 000 hg/cm² of standard rock and extend over five decades of intensity. An interesting result is that the muon flux is independent of slant depth beyond a depth of about 14 000 hg/cm² of standard rock, and corresponds to near horizontal muons. This is direct evidence that this flux is due to atmospheric neutrinos interacting in the rock surrounding LVD.     

Relativistic models of a class of compact objects

A class of general relativistic solutions in isotropic spherical polar coordinates which describe compact stars in hydrostatic equilibrium are discussed. The stellar models obtained here are characterized by four parameters, namely, ??, k, A and R of geometrical significance related to the inhomogeneity of the matter content of the star. The stellar models obtained using the solutions are physically viable for a wide range of values of the parameters. The physical features of the compact objects taken up here are studied numerically for a number of admissible values of the parameters. Observational stellar mass data are used to construct suitable models of the compact stars.     

Temperature fluctuations of cosmic microwave background induced by gravitational lensing

Adopting the accepted interpretation of the cosmic microwave background (CMBR) as a relic of the early hot universe we show that any angular intensity variations existing in the background at the “last scattering surface” at the redshift of ∼10³ will induce bright fluctuations through gravitational lensing by intervening masses. The resulting temperature variations ΔT are estimated for gravitating masses like distant galaxies (z ∼ 1) and local dark objects (e.g. population III stars) in our own Galaxy. It is found that the value of ΔT/T produced by the above mechanisms further constrains the theories of galaxy formation. The calculation also limits the amount of matter present in the form of population III objects in the galaxy.     

紫外光子在星周天体化学中的作用

介于1到8个太阳质量之间的恒星在演化后期的渐近巨星分支(AGB)阶段,会以气体和尘埃的形式损失大量物质. AGB星提供了星际介质中高达35%的尘埃,其中包含形成太阳系所需要的物质. 此外,AGB星周包层是复杂有机化学所发生的重要场所之一,在其中已探测到的有机分子已超过80多个. 这里展示了AGB星自身或者其近距离的伴星所辐射的紫外光子会显著地改变星周包层的化学特性,尤其是具有团块结构的星周包层. 研究发现,在恒星存在伴星(例如白矮星)的情况下,高通量的紫外光子会破坏富碳AGB星周包层内部的H₂O分子,使其含量低于观测到的水平,并且在星周包层内部产生C⁺等物质,这与以往的星周化学模型的研究结果是不同的.     

The formation of stars

We review the process of star formation, detailing the theories underlying the stability of molecular clouds and their collapse to protostars, and discussing the empirical evidence and models which inform them. We give emphasis to the role that the magnetic field plays in influencing the stability of molecular clouds and hence the star formation rate. The end result of star formation is a mass function which appears constant within our Galaxy. A relative abundance of low mass stars is observed over high mass stars and most of the stars that do form are found to exist as members of a binary system. The origin of binarity is reviewed as is the discovery, formation and observations of some of the lowest mass stars known, the brown dwarfs.     

不同光谱型的低分辨率恒星光谱在不同信噪比条件下视向速度测量精度的分析

恒星的视向速度对于研究银河系的演化结构和动力学有很重要的意义,同时也是寻找变源和特殊天体的一种手段。不同的研究对其测量精度有不一样的要求。使用模板匹配的方法计算不同类型的低分辨率可见光波段恒星光谱的视向速度精度,从而为不同方面的科学研究提供有效可靠的参考。分别选取不同光谱型高信噪比的美国斯隆巡天恒星光谱,并加以噪声来模拟不同信噪比条件下的恒星光谱。通过分别计算这些恒星样本的视向速度,定量分析了各种类型的恒星在不同信噪比条件下能达到的视向速度测量精度。同时,还就白矮星的视向速度测量精度进行了分析。结果显示,对于相同信噪比的早型恒星的视向速度测量精度远没有晚型恒星的测量精度高,尤其是A型星的视向速度测量标准误差是K型星和M型星的5~8倍。分析其原因,主要是由于不同类型恒星的具有不同宽度的谱线所导致的。因此对于具有更宽谱线的白矮星光谱的视向速度测量误差更大,可达50 km·s-1。以上结论将为恒星科学研究提供很好的参考。     

Nearby stars as sources of gamma-ray bursts

Observations of ground-based telescopes and the Hubble space telescope made it possible to identify a part of gamma-ray bursts with far objects (redshift parameter Z ≥ 1).However, it remains unclear what are other bursts and what are their sources. The possibility of identifying other bursts with close sources known as small-mass flare stars is considered. The coordinates of space gamma-ray bursts (GRBs) for 2008–2013 and close stars (within the radius r < 25 pc) were compared by the correlation analysis method. Six coincidences were found with an accuracy of ～0.1°. The probability of accidental coincidence of GRBs with stars is 4 · 10^?8, which undoubtedly proves their stellar origin.     

The Antikythera mechanism and the mechanical universe

Astronomers have two approaches to trying to determine the age of the Universe. They can estimate the ages of individual objects in the Universe and specifically in our Galaxy. These estimates use either the observed properties of stars and theoretical ideas concerning stellar evolution or the abundances of long-lived radioactive isotopes and their decay products. Alternatively they can use cosmological theories and observations to try to determine the age of the entire Universe. Obviously the Universe must be older than its component parts but neither of the above methods is sufficiently reliable that this is true of the deduced ages. As a result, it is from time to time reported that some object in the Universe is older than the Universe itself. In this article we discuss the methods that are currently being used to determine the age and we emphasize the problems in obtaining reliable results. It is not at present possible to provide a definite value for the age of the Universe.     

大质量恒星的形成:理论与观测

文章简要叙述了有关大质量恒星形成的理论以及相关观测证据。目前大质量恒星形成的理论主要有两种，即吸积说和并合说．吸积说认为，大质量星可能与小质量星形成于相似的过程；并合说主张大质量星可能是由小质量年轻星碰撞合并而成．解决这两种理论争论的关键在于在大质量星附近能否观测到吸积盘的存在，最新的观测表明大质量星更有可能是通过吸积增加自身的质量，但最终解决这一问题可能还需要更多的观测证据。文章还提出了一些本领域尚未解决的问题，为感兴趣的研究者提供参考。     

High-energy astrophysics with neutrino telescopes

Neutrino astrophysics offers new perspectives on the Universe investigation: high-energy neutrinos, produced by the most energetic phenomena in our Galaxy and in the Universe, carry complementary (if not exclusive) information about the cosmos with respect to photons. While the small interaction cross section of neutrinos allows them to come from the core of astrophysical objects, it is also a drawback, as their detection requires a large target mass. This is why it is convenient to put huge cosmic neutrino detectors in natural locations, like deep underwater or under-ice sites. In order to supply for such extremely hostile environmental conditions, new frontier technologies are under development. The aim of this work is to review the motivations for high-energy neutrino astrophysics, the present status of experimental results and the technologies used in underwater/ice Cherenkov experiments, with a special focus on the efforts for the construction of a km³-scale detector in the Mediterranean Sea.     

The spatial distribution of old neutron stars in the Galaxy

The spatial distributions of old neutron stars (NSs) with ages 109 to 1010 yr in our Galaxy are investigated by Monte Carlo simulation under two different initial random velocity models.It is found that the scale heights of the distribution increase with the Galactic radial distance.The location of the peak of the NS distribution is closer to the Galactic center than that of their progenitors.The results from our detailed numerical analysis reveal that there is resemblance between the simulated old NS distribution and the structure of the observed HI disk.     

Anisotropic relativistic stellar models

We present a class of exact solutions of Einstein's gravitational field equations describing spherically symmetric and static anisotropic stellar type configurations. The solutions are obtained by assuming a particular form of the anisotropy factor. The energy density and both radial and tangential pressures are finite and positive inside the anisotropic star. Numerical results show that the basic physical parameters (mass and radius) of the model can describe realistic astrophysical objects like neutron stars.