Ring nebulae associated with of stars: Statistics,classification, origin

The ring nebulae associated with galactic Of stars is considered on the grounds of the list of Of nebulae proposed by lozinskaya and Lomovsky (1982). Taking into account the selection effects, about 80% of Of stars are shown to be associated withHii regions and about 30–50% of these regions have shell structures. Four types of nebulae associated with Of stars are resolved: amorphousHii regions, ring-likeHii regions, wind-blown bubbles, and stellar ejectas. These types appear to be identical to the morphology of nebulae around WR stars proposed by Chu (1981). Observational data are presented and the nature of a number of Of ring nebulae of different types is discussed.     

Neutral hydrogen in the vicinity of galactic radio sources

In the preceding paper by Vengeret al. (1984) the results of observations of neutral gas in the vicinity of some galacticHii regions were considered; and it was demonstrated that 17 of the regions observed are surrounded by expandingHi envelopes. This paper describes a model of interaction between theHii regions and the surrounding interstellar medium constructed on the basis of the said results. It is assumed that the main dynamic factor in the envelope formation mechanism is the total stellar wind from the stars which ionze theHii regions. The employment of the observations of the line at =21 cm, of the radio continuum and IR dust continuum made it possible to determine the differential mass spectra of the stars exciting theHii zones and calculate some characteristics of the stellar population. The mass spectrum index of the objects considered turned out to be much higher than that for the mass spectra of background stars and scattered clusters of stars.     

Nebular complex in the region of Cas OB2 association; ring nebula Sh157

This paper presents the results of monochromatic [Oiii], [Nii], and [Sii] observations of ring nebula Sh157 around the star (WR + B0III) HD 219460 belonging to the Ba 3 cluster. A stratification of radiation typical for photoionization excitation has been revealed. The observations suggest that the Sh157 ring structure may arise as a result of the HD 219460 stellar wind blowing the surroundingHii region, and the bubble age is found to bet(2–5)×10⁵ yr. Three outer envelopes have been distinguished: a weaker extended emission shell apparently blown out by the wind from B-stars of the Ba 3 cluster, and two dust shells which are likely to be associated with NGC 7510 and Cas OB2. The paper is also concerned with the discussion of young star aggregates Ba 3, NGC 7510, Cas OB1, OB2, OB4, OB5, OB7, and Cep OBI and the associatedHii regions, shells and supershells of gas and dust, molecular clouds, and supernova remnants which may be probable members of a single giant stellar complex where the star formation process is in progress.     

The atomic physics underlying the spectroscopic analysis of massive stars and supernovae

We have developed a radiative transfer code, cmfgen, which allows us to model the spectra of massive stars and supernovae. Using cmfgen we can derive fundamental parameters such as effective temperatures and surface gravities, derive abundances, and place constraints on stellar wind properties. The last of these is important since all massive stars are losing mass via a stellar wind that is driven from the star by radiation pressure, and this mass loss can substantially influence the spectral appearance and evolution of the star. Recently we have extended cmfgen to allow us to undertake time-dependent radiative transfer calculations of supernovae. Such calculations will be used to place constraints on the supernova progenitor, to place constraints on the supernova explosion and nucleosynthesis, and to derive distances using a physical approach called the “Expanding Photosphere Method”. We describe the assumptions underlying the code and the atomic processes involved. A crucial ingredient in the code is the atomic data. For the modeling we require accurate transition wavelengths, oscillator strengths, photoionization cross-sections, collision strengths, autoionization rates, and charge exchange rates for virtually all species up to, and including, cobalt. Presently, the available atomic data varies substantially in both quantity and quality.     

大质量双星系统的非守恒演化

由于大质量双星系统有强大的星风物质损失,因而在研究其结构和演化时必须考虑星风物质损失,动量损失,物质交换以及由以上原因引起的轨道参量的变化,此外,天文观测又证实,一些大质量双星系统中存在星风冲击波,有Ｘ射线辐射以及有致密天体（白矮星,中子星）的存在,因此在研究大质量双星的演化时,又会遇到在星风冲击波理论及其对演化的影响,双星系统何时会演化成为公共外壳的系统,以及双星系统中如果发生超新星爆发,是否会     

Global photometric observations and physical properties ofHii regions in the small magellanic cloud

Integrated photoelectric measurements of the equivalent widthW _H, the [Oiii]/H ratio and the H emission line flux were obtained for 30Hii regions in the SMC. Physical properties of theHii regions and their ionizing stellar associations were derived. Some aspects of the recent star formation in the SMC and the evolution ofHii regions are discussed.     

Metal enrichment history of the proto-galactic interstellar medium

To investigate the metal enrichment history of the primordial interstellar medium (ISM), we have studied the long-term evolution of supernova remnants (SNRs) and how SNRs distribute the heavy metals into the ISM when they explode. With the assumed IMF for massive stars, we have computed the multiple supernova explosions and evolution in an inhomogeneous ISM. We compare the predicted metallicity distribution of metal deficient halo stars with the observed one. This revised version was published online in September 2006 with corrections to the Cover Date.     

The colours of the interstellar medium

On the basis ofI-I plots, we find that the ISM radiates preferentially at two pairs of far-infrared frequencies which correspond to (scattered) black-body temperatures of (23 ± 1, 187 ± 5) K and (39 ± 1, 104 ± 5) K. The first pair is emitted by the cold matrix, the second pair byHii regions and supernova shells.     

Massive stars

We describe the present state of massive star research seen from the viewpoint of stellar evolution, with special emphasis on close binaries. Statistics of massive close binaries are reasonably complete for the Solar neighbourhood. We defend the thesis that within our knowledge, many scientific results where the effects of binaries are not included, have an academic value, but may be far from reality. In chapter I, we summarize general observations of massive stars where we focus on the HR diagram, stellar wind mass loss rates, the stellar surface chemistry, rotation, circumstellar environments, supernovae. Close binaries can not be studied separately from single stars and vice versa. First, the evolution of single stars is discussed (chapter I). We refer to new calculations with updated stellar wind mass loss rate formalisms and conclusions are proposed resulting from a comparison with representative observations. Massive binaries are considered in chapter II. Basic processes are briefly described, i.e. the Roche lobe overflow and mass transfer, the common envelope process, the spiral-in process in binaries with extreme mass ratio, the effects of mass accretion and the merging process, the implications of the (asymmetric) supernova explosion of one of the components on the orbital parameters of the binary. Evolutionary computations of interacting close binaries are discussed and general conclusions are drawn. The enormous amount of observational data of massive binaries is summarized. We separately consider the non-evolved and evolved systems. The latter class includes the semi-detached and contact binaries, the WR binaries, the X-ray binaries, the runaways, the single and binary pulsars. A general comparison between theoretical evolution and observations is combined with a discussion of specially interesting binaries: the evolved binaries HD 163181, HD 12323, HD 14633, HD 193516, HD 25638, HD 209481, Per and Sgr; the WR+OB binary V444 Cyg; the high mass X-ray binaries Vela X-1, Wray 977, Cyg X-1; the low mass X-ray binaries Her X-1 and those with a black hole candidate; the runaway Pup, the WR+compact companion candidates Cyg X-3, HD 50896 and HD 197406. We finally propose an overall evolutionary model of massive close binaries as a function of primary mass, mass ratio and orbital period. Chapter III deals with massive star population synthesis with a realistic population of binaries. We discuss the massive close binary frequency, mass ratio and period distribution, the observations that allow to constrain possible asymmetries during the supernova explosion of a massive star. We focuss on the comparison between observed star numbers (as a function of metallicity) and theoretically predicted numbers of stellar populations in regions of continuous star formation and in starburst regions. Special attention is given to the O-type star/WR star/red supergiant star population, the pulsar and binary pulsar population, the supernova rates. Received 17 July 1998     

The chemo-dynamical evolution of a disk galaxy

I present a model for the formation and evolution of a massive disk galaxy, within a growing dark halo whose mass evolves according to cosmological simulations of structure formation. The galactic evolution is simulated with a new three-dimensional chemo-dynamical code, including dark matter, stars and a multi-phase ISM. We follow the evolution from redshift z= 4.85 until the present epoch. The energy release by massive stars and supernovae prevents a rapid collapse of the baryonic matter and delays the maximum star formation until redshift z ≈ 1. The galaxy forms radially from inside-out and vertically from top-to-bottom. Correspondingly, the inner halo is the oldest component, followed by the outer halo, the bar/bulge, the thick and the thin disk. The bulge in the model consists of at least two stellar subpopulations, an early collapse population and a population that formed later in the bar. This revised version was published online in August 2006 with corrections to the Cover Date.     

Charge exchange in massive star‐forming regions

As a result of feedback from massive stars, via their intense winds and/or supernova explosions, massive star‐forming regions are entirely filled with hot, X‐ray emitting plasmas, which escape into the ambient ISM. As shown recently by Townsley et al. for several “extreme” cases (Carina, M17, NGC 3576, NGC 3603, 30 Dor), by way of large Chandra ACIS mosaics, extra, non‐thermal emission lines are present on top of the standard lines emitted by hot plasmas. Some of them are very close to lines characteristic of charge‐exchange reactions between the hot plasma and the cold surrounding material, suggesting that this mechanism operates on large spatial scales (several 10 pc) in star‐forming regions in general. The connection with starburst galaxies is briefly mentioned, and it is pointed out that supernovae interacting with molecular clouds may also provide a good environment to look for charge exchange processes (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preliminary results of the multisite time-series campaign on the sdB pulsator PG 1325+101

I present a model for the formation and evolution of a massive disk galaxy, within a growing dark halo whose mass evolves according to cosmological simulations of structure formation. The galactic evolution is simulated with a new 3D chemo-dynamical code, including dark matter, stars and a multi-phase ISM. We follow the evolution from redshift z = 4.85 until the present epoch. The energy release by massive stars and supernovae prevents a rapid collapse of the baryonic matter and delays the maximum star formation until redshift z ≈ 1. The galaxy forms radially from inside-out and vertically from top-to-bottom. The feedback of stars leads to turbulent motions and large-scale flows in the ISM. As one result the galactic disk is significantly enriched by chemical elements synthesized in bulge stars.     

The Impact of Massive Stars on their Surrounding Interstellar Medium

In this paper, I present a brief overview of the observational evidence for the interaction between the winds of hot stars and their surrounding interstellar medium (ISM). As the star evolves, it will impinge on the gas in different ways depending on the nature of its wind and on the physical state of the interstellar gas. The three main evolutionary phases of massive stars, the main sequence O phase, the intermediate red supergiant or luminous blue variable phase and finally the Wolf-Rayet phase are discussed. The study of the ISM in the vicinity of hot stars is shown to be a powerful tool to study the history of the gas and gain more insight into the evolution of the star. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stellar dynamics in young clusters: the formation of massive runaways and very massive runaway mergers

In the present paper we combine an N-body code that simulates the dynamics of young dense stellar systems with a massive star evolution handler that accounts in a realistic way for the effects of stellar wind mass loss. We discuss two topics.

1. The formation and the evolution of very massive stars (with masses >120 M_⊙) is followed in detail. These very massive stars are formed in the cluster core as a consequence of the successive (physical) collisions of the 10–20 most massive stars in the cluster (this process is known as ‘runaway merging’). The further evolution is governed by stellar wind mass loss during core hydrogen and core helium burning (the WR phase of very massive stars). Our simulations reveal that, as a consequence of runaway merging in clusters with solar and supersolar values, massive black holes can be formed, but with a maximum mass ≈70 M_⊙. In low-metallicity clusters, however, it cannot be excluded that the runaway-merging process is responsible for pair-instability supernovae or for the formation of intermediate-mass black holes with a mass of several 100 M_⊙.
2. Massive runaways can be formed via the supernova explosion of one of the components in a binary system (the Blaauw scenario), or via dynamical interaction of a single star and a binary or between two binaries in a star cluster. We explore the possibility that the most massive runaways (e.g. ζ Pup, λ Cep, BD+43°3654) are the product of the collision and merger of two or three massive stars.

     

The ISM in nearby galaxies

The SKA will revolutionise the study of the principles underlying star formation (SF), resolving interstellar cloud complexes which are the birthplaces of stars and answering such questions as which are the sufficient and necessary conditions for SF to commence. Also, massive SF is intimately related to stellar death. The SKA will be able to study the structure of the ISM at 100 pc resolution out to distances of up to 20 Mpc and will quantify the impact the demise of massive stars has on their environment. Importantly, the SKA will probe the transition region between ISM and IGM, linking star formation and stellar death in the disks of galaxies to faint HI structures further afield, such as “anomalous gas” and (Compact) High Velocity Clouds. Lastly, the superb sensitivity of the SKA will result in some hundred background sources per square degree against which HI absorption lines can be searched for, probing not only the relative importance of the different phases of the gas in galaxies but also the low density gas in the outskirts and between galaxies.     

Mass pick-up in astronomical flows

Many astrophysical sources are clumpy. The flows in the diffuse media in these sources are affected by the mass picked up through the ablation of the embedded clumps. The diffuse media-clump interfaces can be observable sources themselves. Flows in clumpy stellar wind blown bubbles, supernova remnants, quasar winds, cooling gas accreting on to galaxies, and in pre-galactic bubbles blown by energy input from young galaxies, are discussed. Pick-up has important chemical as well as dynamical effects in star forming regions. The natures of the interfaces are considered.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.     

Feedback and metal enrichment in cosmological SPH simulations – II. A multiphase model with supernova energy feedback

We have developed a new scheme to treat a multiphase interstellar medium in smoothed particle hydrodynamics simulations of galaxy formation. This scheme can represent a co-spatial mixture of cold and hot ISM components, and is formulated without scale-dependent parameters. It is thus particularly suited to studies of cosmological structure formation where galaxies with a wide range of masses form simultaneously. We also present new algorithms for energy and heavy element injection by supernovae, and show that together these schemes can reproduce several important observed effects in galaxy evolution. Both in collapsing systems and in quiescent galaxies our codes can reproduce the Kennicutt relation between the surface densities of gas and of star formation. Strongly metal-enhanced winds are generated in both cases with ratios of mass-loss to star formation which are similar to those observed. This leads to a self-regulated cycle for star formation activity. The overall impact of feedback depends on galaxy mass. Star formation is suppressed at most by a factor of a few in massive galaxies, but in low-mass systems the effects can be much larger, giving star formation an episodic, bursty character. The larger the energy fraction assumed available in feedback, the more massive the outflows and the lower the final stellar masses. Winds from forming discs are collimated perpendicular to the disc plane, reach velocities up to  ∼1000 km s⁻¹  , and efficiently transport metals out of the galaxies. The asymptotically unbound baryon fraction drops from >95 per cent to ∼30 per cent from the least to the most massive of our idealized galaxies, but the fraction of all metals ejected with this component exceeds 60 per cent regardless of mass. Such winds could plausibly enrich the intergalactic medium to observed levels.     

Effect of the rotation and tidal dissipation history of stars on the evolution of close-in planets

Since 20 years, a large population of close-in planets orbiting various classes of low-mass stars (from M-type to A-type stars) has been discovered. In such systems, the dissipation of the kinetic energy of tidal flows in the host star may modify its rotational evolution and shape the orbital architecture of the surrounding planetary system. In this context, recent observational and theoretical works demonstrated that the amplitude of this dissipation can vary over several orders of magnitude as a function of stellar mass, age and rotation. In addition, stellar spin-up occurring during the Pre-Main-Sequence (PMS) phase because of the contraction of stars and their spin-down because of the torque applied by magnetized stellar winds strongly impact angular momentum exchanges within star–planet systems. Therefore, it is now necessary to take into account the structural and rotational evolution of stars when studying the orbital evolution of close-in planets. At the same time, the presence of planets may modify the rotational dynamics of the host stars and as a consequence their evolution, magnetic activity and mixing. In this work, we present the first study of the dynamics of close-in planets of various masses orbiting low-mass stars (from \(0.6~M_\odot \) to \(1.2~M_\odot \)) where we compute the simultaneous evolution of the star’s structure, rotation and tidal dissipation in its external convective envelope. We demonstrate that tidal friction due to the stellar dynamical tide, i.e. tidal inertial waves excited in the convection zone, can be larger by several orders of magnitude than the one of the equilibrium tide currently used in Celestial Mechanics, especially during the PMS phase. Moreover, because of this stronger tidal friction in the star, the orbital migration of the planet is now more pronounced and depends more on the stellar mass, rotation and age. This would very weakly affect the planets in the habitable zone because they are located at orbital distances such that stellar tide-induced migration happens on very long timescales. We also demonstrate that the rotational evolution of host stars is only weakly affected by the presence of planets except for massive companions.     

Supernova SN 1961v: An explosion of a very massive star

An investigation of the outburst of the unique supernova SN 1961v in the galaxy NGC 1058 is carried out. An analysis of hydrodynamic models of supernova outbursts and a comparison with a considerable body of observational data on SN 1961v clearly show that the SN 1961v phenomenon is an explosion of a very massive star (VMS) of a mass of 2000M and a radius of about 100R which results in expelling the envelope with a kinetic energy of 1.8×10⁵² erg. The light curve of SN 1961v (Figures 1, 7a, and 7b) furnishes direct evidence for the heterogeneity of the presupernova interior. The chemical composition profile produced during the evolution of the VMS and in the final explosion must have a number of the essential features (Figure 11). In particular, hydrogen has to be underabundant relative to the solar content and distributed in a specific manner throughout the star. At the late stages from February, 1963 to February, 1967, the light curve of SN 1961v (Figure 1) may be accounted for by the interaction of the expelled envelope with the stellar wind of the presupernova. The latest observations of SN 1961v in 1968 and 1970 are virtually those of a giantHii region created by the VMS before the explosion. Two astrophysical phenomena-the peculiar outburst of SN 1961v and the most luminous object R136a in the Large Magellacnic Cloud (LMC) which reveals a striking similarity with the presupernova-are evidence for the existence of VMSs. The evolution of VMSs similar to the object R136a may be terminated by explosions like the outburst of SN 1961v. Such explosions give rise to the formation of energetic supernova remnants whose examples may be the Cygnus superbubble and the supergiant shells in the LMC. A comparison of the internal structure of the presupernova with the available evolutionary calculations allows one to conclude that the influence of mass loss on the evolution of VMSs is negligible.     

Stellar associations and complexes in M33

About 460 OB associations were selected by a comparison of theUBV plates. TheUBV photographic photometry of 1944 blue stars in the associations was made. The new associations appear like cores within Humphreys and Sandage's associations. Their star content, size distribution, and mean size 80 pc confirm their identity with the OB associations in the Galaxy and in the Magellanic clouds. The boundaries of the associations are delineated mainly by the density of the resolved stars on theU plates. It is impossible to divide them into smaller areas. The genuine OB associations form groups of two or more members with a length scale of 250 pc. Their boundaries were delineated independently, but they coincide with the OB associations of Humphreys and Sandage (1980). These groups represent real concentration of blue massive stars with a large age dispersion. The star complexes unify a group of associations,Hii regions, andHi peak distribution. Their mean size is 570 pc. The extensiveHi clouds with a mean size of 1.2 kpc contain two or more star complexes. The questions related to star formation are briefly discussed.