Triggered star formation and evolution of T-Tauri stars in and around bright-rimmed clouds

The aim of this paper is to quantitatively testify the ' small-scale sequential star formation ' hypothesis in and around bright-rimmed clouds (BRCs). As a continuation of the recent attempt by Ogura et al., we have carried out   BVI_c   photometry of four more BRC aggregates along with deeper re-observations of two previously observed BRCs. Again, quantitative age gradients are found in almost all the BRCs studied in the present work. Archival Spitzer /Infrared Array Camera data also support this result. The global distribution of near-infrared excess stars in each H  ii region studied here clearly shows evidence that a series of radiation-driven implosion processes proceeded in the past from near the central O star(s) towards the peripheries of the H  ii region. We found that in general weak-line T-Tauri stars (WTTSs) are somewhat older than classical T-Tauri stars (CTTSs). Also the fraction of CTTSs among the T-Tauri stars (TTSs) associated with the BRCs is found to decrease with age. These facts are in accordance with the recent conclusion by Bertout, Siess & Cabrit that CTTSs evolve into WTTSs. It seems that in general the equivalent width of Hα emission in TTSs associated with the BRCs decreases with age. The mass function (MF) of the aggregates associated with the BRCs of the morphological type 'A' seems to follow that found in young open clusters, whereas 'B/C'-type BRCs show significantly steeper MF.     

Hydrated sulphuric acid in dense molecular clouds

We consider sulphur depletion in dense molecular clouds, and suggest hydrated sulphuric acid, H₂SO₄ ·  n H₂O, as a component of interstellar dust in icy mantles. We discuss the formation of hydrated sulphuric acid in collapsing clouds and its instability in heated regions in terms of the existing hot core models and observations. We also show that some features of the infrared spectrum of hydrated sulphuric acid have correspondence in the observed spectra of young stellar objects.     

The locations of newly formed stars in molecular clouds

Observations of very massive stars (M10M ) are suggestive of a star formation process which requires an external trigger. However, observations pertaining to the formation of stars of lower mass (M9M ) require no such triggering mechanism and are consistent with the idea that such stars form as a natural consequence of the evolution, gravitational collapse and fragmentation of a proto-stellar molecular cloud.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.     

Submillimetre co rotational lines from dense cores in molecular clouds

A simple, spherically-symmetric, centrally-condensed model is constructed for a dense core in a molecular cloud. Optical depths and peak brightness temperatures are calculated for the 10 lowest rotational transitions of carbon monoxide. The cloud, using parameters given by observation for dark condensations in molecular clouds, turns out to be optically thin in these transitions, which allows the maximum density and density distribution to be estimated.     

Models of dense stars

The aim of this paper is to present the results of a construction of five models of composite stellar configurations, consisting of an energy-generating convective core surrounded by source-free envelope in radiative equilibrium, and of sufficient density for the coefficient of absorption to vary approximately as inverse square of the local temperature. The principal characteristic of such models proved to be their very high degree of central condensation; their central densities being 10³–10⁴ times as large as the mean density of the composite configuration. The relevance of such models to the internal structure of subdwarf components of close binary systems with periods less than a day is briefly pointed out.Investigation supported in part by Contract N5 ori-07843 with the Office of Naval Research of the U.S. Navy Department.     

GEMINI near-infrared spectroscopic observations of young massive stars embedded in molecular clouds

K -band spectra of young stellar candidates in four Southern hemisphere clusters have been obtained with the Gemini Near-Infrared Spectrograph in Gemini South. The clusters are associated with IRAS sources that have colours characteristic of ultracompact H  ii regions. Spectral types were obtained by comparison of the observed spectra with those of a near-infrared (NIR) library; the results include the spectral classification of nine massive stars and seven objects confirmed as background late-type stars. Two of the studied sources have K -band spectra compatible with those characteristic of very hot stars, as inferred from the presence of C  iv , N  iii and N  v emission lines at 2.078, 2.116 and 2.100 μm, respectively. One of them, I16177_IRS1, has a K -band spectrum similar to that of Cyg OB2 7, an O3If* supergiant star. The nebular K -band spectrum of the associated Ultra-Compact (UC) H  ii region shows the s-process [Kr  iii ] and [Se  iv ] high excitation emission lines, previously identified only in planetary nebula. One young stellar object was found in each cluster, associated with either the main IRAS source or a nearby resolved Midecourse Space eXperiment ( MSX ) component, confirming the results obtained from previous NIR photometric surveys. The distances to the stars were derived from their spectral types and previously determined JHK magnitudes; they agree well with the values obtained from the kinematic method, except in the case of IRAS  15408−5356  , for which the spectroscopic distance is about a factor of 2 smaller than the kinematic value.     

Silicon chemistry in dense clouds

The equilibrium chemistry of silicon in dense interstellar clouds is discussed in terms of both gas phase and grain surface reactions. Unless the metal depletion is very large, the gas phase scheme tends to over-produce SiO and/or SiS when compared to the observations of Sgr B2. The scheme also predicts SiC to be an abundant form of silicon. There is a great need for relevant laboratory data on the reactions used here—of the 35 rate coefficients adopted in the scheme, only three have been measured in the laboratory. Reactions between positively charged gas phase ions and small grains can lead to the formation of SiO and SiS. This type of reaction seems to offer a simple explanation for the observed differences between sulphur and silicon chemistry in dense clouds.     

On the possible identification of ‘solid-state’ lines in T-Tauri stars

The possibility that the unidentified absorption lines observed in the spectrum of YY Orionis star S CrA can be attributed to the absorption features of irradiated amethyst is discussed.     

Interstellar clouds and the formation of stars

Part I gives a survey of the drastic revision of cosmic plasma physics which is precipitated by the exploration of the magnetosphere throughin situ measurements. The pseudo-plasma formalism, which until now has almost completely dominated theoretical astrophysics, must be replaced by an experimentally based approach involving the introduction of a number of neglected plasma phenomena, such as electric double layers, critical velocity, and pinch effect. The general belief that star light is the main ionizer is shown to be doubtful; hydromagnetic conversion of gravitational and kinetic energy may often be much more important.In Part II the revised plasma physics is applied to dark clouds and star formation. Magnetic fields do not necessarily counteract the contraction of a cloud; they may just as well pinch the cloud. Magnetic compression may be the main mechanism for forming interstellar clouds and keeping them together.Part III treats the formation of stars in a dusty cosmic plasma cloud. Star formation is due to an instability, but it is very unlikely that it has anything to do with the Jeans instability. A reasonable mechanism is that the sedimentation of dust (including solid bodies of different size) is triggering off a gravitationally assisted accretion. A stellesimal accretion analogous to the planetesimal accretion leads to the formation of a star surrounded by a very low density hollow in the cloud. Matter falling in from the cloud towards the star is the raw material for the formation of planets and satellites.The study of the evolution of a dark cloud leads to a scenario of planet formation which is reconcilable with the results obtained from studies based on solar system data. This means that the new approach to cosmical plasma physics discussed in Part I logically leads to a consistent picture of the evolution of dark clouds and the formation of solar systems.     

Turbulent molecular clouds

Stars form within molecular clouds but our understanding of this fundamental process remains hampered by the complexity of the physics that drives their evolution. We review our observational and theoretical knowledge of molecular clouds trying to confront the two approaches wherever possible. After a broad presentation of the cold interstellar medium and molecular clouds, we emphasize the dynamical processes with special focus to turbulence and its impact on cloud evolution. We then review our knowledge of the velocity, density and magnetic fields. We end by openings towards new chemistry models and the links between molecular cloud structure and star-formation rates.     

Thermal coagulation of charged grains in dense clouds

A treatment is given of the thermal coagulation of interstellar grains, taking their charge fully into account. The original calculations showed this charge to be negative unless photoionization dominated when it became positive. More recently, the effects of discreteness in the charge and of grain polarization have been considered and all these effects are included in the present work. The general conclusion is that the effect of charge is to decrease grain coagulation although a very small increase can occur for certain situations in the presence of photoionization.     

The formation of molecular clouds

In a recent paper, Elmegreen has made a cogent case, from an observational point of view, that the lifetimes of molecular clouds are comparable to their dynamical time-scales. If so, this has important implications for the mechanisms by which molecular clouds form. In particular, we consider the hypothesis that molecular clouds may form not by in situ cooling of atomic gas, but rather by the agglomeration of the dense phase of the interstellar medium, much, if not most, of which is already in molecular form.     

Physical conditions in the convective envelope of stars

Summary The mechanical flux originating in the convective envelope of stars is shown to depend critically by the treatment of convection. In particular, in the framework of the mixing-length theory, in passing from a mixing lengthl=H _P to a mixing lengthl=H the presence of mechanical fluxes shifts from being a marginal phenomenon to a dominant one.Possible implications concerning atmospheric microturbulence in Main Sequence stars, as so asA _p andA _m stars, several types of variables and mass loss are briefly discussed.

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Sommario Si mostra come il flusso cinetico originato negli inviluppi convettivi di una stella dipenda criticamente dalle assunzioni fatte nel trattamento della convezione superadiabatica. In particolare, sulla base della teoria della lunghezza di rimescolamento, a seconda che si assumal=H _P o l=H i flussi cinetici passano dall'essere un fenomeno marginale a contributi determinanti.E' discussa una serie di possibili implicazioni riguardanti la microturbolenza atmosferica in stelle di sequenza principale, le stelleA _p eA _m, diversi tipi di variabili e la perdita di massa in fase di gigante.

     

Neutron stars and the dense matter equation of state

Neutron stars provide a unique laboratory with which to study cold, dense matter. The observational quantities of primary astrophysics interest are the maximum mass and the typical radius of a neutron star. These quantities are related to the relative stiffness of neutron-rich matter at supernuclear densities and the density dependence of the nuclear symmetry energy near the nuclear saturation density. The measurements of these nuclear properties via nuclear systematics and structure, heavy-ion collisions and parity-violating electron scattering from neutron-rich nuclei, are discussed. Several new observations, including mass measurements of binary pulsars and a confirmed distance determination for a nearby cooling neutron star, will be summarized. Additionally addressed will be observations of thermal emissions from cooling neutron stars in globular clusters and thermonuclear explosions from accreting stars. It will be demonstrated how this astrophysical data is shedding light on the pressure-density relation of extremely dense matter.     

Gamma rays from molecular clouds

It is believed that the observed diffuse gamma-ray emission from the galactic plane is the result of interactions between cosmic rays and the interstellar gas. Such emission can be amplified if cosmic rays penetrate into dense molecular clouds. The propagation of cosmic rays inside a molecular cloud has been studied assuming an arbitrary energy and space dependent diffusion coefficient. If the diffusion coefficient inside the cloud is significantly smaller compared to the average one derived for the galactic disk, the observed gamma-ray spectrum appears harder than the cosmic ray spectrum, mainly due to the slower penetration of the low energy particles towards the core of the cloud. This may produce a great variety of gamma-ray spectra.     

Small-scale clumping in molecular clouds

Summary This review consists of five sections. In the introduction, we briefly review the development of the study of molecular clouds. In the second section, we review the theories of molecular cloud structure and compare these predictions with the statistical properties of the clouds. In Sect. 3 we give an overview of current approaches to determinations of mass and local density in clouds. In the fourth part, we discuss the observations of a selected sample of individual sources. The emphasis here is on high resolution studies of regions of star formation. The final section contains a discussion of instrumental limitations and mentions some future developments.