bluuebunny/crossref_metadata_2025 huggingface dataset:
A comparative study of star formation efficiencies in nearby molecular cloud complexes
Authors: Fukui Yasuo , Mizuno Akira
Published: June 2019

We present new observational data of 13CO J=1−0 emission in nearby molecular cloud complexes in Taurus, Ophiuchus, and Orion, and compare star formation efficiencies in the three regions. The 13CO data have been obtained with the 4m millimeter telescope at Nagoya equipped with a SIS receiver having receiver temperature of 23 K in double side band. The molecular distribution is often characterized by highly filamentary, continuous distribution, rather than well defined clumps. Active star formation is taking place almost exclusively in the densest regions in the molecular distributions. The filamentary distributions often show velocity gradients perpendicular to their elongation, suggesting that they are spinning around their major axes. Roles of photoionization and the spinning motion on formation of stars and clouds are discussed as a step toward understanding the mechanism regulating star formation efficiencies.
Star formation in three nearby cloud complexes
Authors: Evans Neal J. , Lada Elizabeth A.
Published: June 2019

We consider the distribution and nature of the gas and star formation in three nearby molecular cloud complexes: Taurus-Auriga, Ophiuchus, and Orion. Large-scale studies show that quite different distributions of dense gas and star formation exist in these regions, but that the global efficiency of star formation, so far, is about 1% in all of these complexes, very similar to the final efficiencies deduced for open clusters and associations. Furthermore, evidence for differences in the initial mass function among these regions is weak; all three regions may be compatible with the initial mass function for open clusters and field stars. Finally, we consider specific examples of star formation in each complex; the results indicate that the ideas of Adams, Lada, and Shu (1987) are reasonably consistent with the data on L1551 IRS 5, in the Taurus cloud, but that modifications are likely to be needed for IRAS 16293-2422 in Ophiuchus and for NGC2071 in Orion.
The effects of ionizing radiation on star formation in molecular clouds
Authors: Bertoldi F. , McKee C.F. , Klein R.I.
Published: June 2019

The gravitational stability of molecular cloud clumps before and after the onset of massive star formation is discussed. We suggest that the most massive clumps are magnetically supercritical but gravitationally stabilized by the hydromagnetic turbulence caused by FUV photoionization-regulated low-mass star formation in their interiors. The ionizing radiation of an O star can trigger star formation in initially sub- and supercritical clumps.
Do random velocity fields inhibit or induce gravitational collapse and fragmentation?
Authors: Chantry Philippe , Grappin Roland , Léorat Jacques
Published: June 2019

Recent numerical works on self-gravitating turbulence (Bonazzola et al., 1987; Léorat, Passot, Pouquet 1990) tend to show that turbulent velocity fields could act as a “turbulent” pressure, i.e. inhibit the collapse of self-gravitating clouds. This effect is efficient when the caracteristic scale of the turbulent flow is small compared to the Jeans's length. When these two scales are comparable, we show (Chantry, Grappin, Léorat, 1990) that the turbulence do no more inhibit the collapse, but seems to trigger it.
MHD instabilities and fragmentation of molecular clouds
Authors: Dudorov A.E.
Published: June 2019

The properties of the interstellar magnetic clouds, their hierarchy, turbulence and their origin in a processes of the rising of MHD-instabilities are discussed.
Effects of the UV radiation on the surrounding gas and dust
Authors: Fuente A. , Martin-Pintado J. , Cernicharo J. , Brouillet N. , Duvert G.
Published: June 2019

Two molecular clouds associated with well known reflection nebulosities have been studied in order to stablish the effects of the UV radiation on the surrounding gas and dust. Observations of the J=2 →1 and J=1 →0 transitions of CO and 13CO have been carried out with the telescopes POM 1 and POM 2 towards the clouds associated with LKHα 198 and HD200775. The IRAS maps at 12, 25, 60 and 100 μm towards these globules have been also examined.
Interpretation of low j 12CO and 13CO observations of Orion A by means of an onion shell radiative transfer model
Authors: Gierens Klaus M.
Published: June 2019

12CO and 13CO J = 1 → 0 and J = 2 → 1 observations of a 1 deg × 2 deg region centered on the BN/KL nebula in Orion A showed almost everywhere surprising intensity ratios. According to the standard interpretation of CO lines the 13CO TA(2 → 1)/TA(1 → 0) ratio meant optically thick and thermalized 13CO emission whereas at the same positions the 12CO/13CO intensity ratios indicated optically thin 13CO (Castets et al. 1989). Castets et al. (1990) suggested that temperature gradients in the observed clumps caused by external UV heating could be responsible for these results.
Global properties of star formation in Taurus
Authors: de Castro Ana I. Gomez , Pudritz Ralph E.
Published: June 2019

A careful analysis of the data available in the literature has been carried out to determine the global properties of star formation in Taurus. Three main trends arise in the data:

(1)

the magnetic field in Taurus is distorted in the direction of the proper motions of the T Tauri Stars (TIS) (see below).

(2)

the groups with different proper motions seem to have different stellar contents. An analysis of the stellar population based on the equivalent width of Hα, W(Hα), shows that groups I and III (Jones and Herbig, 1979) have approximately the same population (TTS show either large or small W(Hα)). However in group II (central part of the cloud) there are also a large number of stars with intermediate W(Hα) (Gomez de Castro and Pudritz, 1990).

(3)

the mass function of the NH3 cores (thought to be "protostellar cores") is dN/dM< M∝ with α=−1.5, in contrast with the IMF of the TTS given by Cohen and Kuhi (1979), α=−2.5.

Only point (1) will be outlined in this contribution.
Polarization of background starlight and the structure of the interstellar magnetic field
Authors: Goodman Alyssa A. , Myers Philip C. , Bastien Pierre
Published: June 2019

We discuss the use of polarization maps of background starlight in studying the structure of the interstellar magnetic field. We make the assumption that the polarization observed is due to magnetically aligned dust grains associated with interstellar clouds along the line of sight, and that the position angle (ΘE) of polarization observed gives the direction (modulo 180°) of , the plane-of-the-sky projection of the (line-of-sight-averaged) magnetic field.

There are two basic points in this paper. (1.) Out of context, the projected orientation of an elongated dark cloud may appear special (e.g. roughly "parallel" or “perpendicular”) in relation to the local (plane-of-the-sky) field direction given by polarization observations, but, when the view is expanded to include an entire complex of dark clouds, the shape and orientation of clouds within a complex often appears unrelated to the field structure. (2.) the dispersion in the postion angle of polarization observed in a region of the sky contains information about the ratio of the strength of the uniform (straight) component of the local magnetic field as compared to the dispersion (nonuniform component) in the field. Furthermore, in a region where Zeeman measurements covering the same region as polarization observations have been made, the uniform-to-nonuniform ratio deduced for the field from polarization data, can be combined with information about the line-of-sight field and an estimate of the correlation length of the field, to describe the magnetic field in three dimensions. We discuss the results of such an analysis for the dark cloud Lynds 204 (L204).
Megamasers as probes of galaxy mass spectrum evolution
Authors: Khersonskii V.K. , Voshchinnikov N.V.
Published: June 2019

OH megamasers having very high luminosities in the spectral line can be effectively used for the probing of the evolutionary properties of the galaxies in the earliest cosmological epochs. The frequency shift of the emission line uniqually determines the redshift z, which tells about the epoch of emission. One of the important cosmological problems is the investigation of the galaxy mass spectrum in the expanding Universe. There is the empirical relation between the OH and far-infrared luminosities of galaxies. Therefore, if in the earliest cosmological epochs, there were galaxies with sufficient powerful infrared excesses and containing molecular material, they can be detected using the observations of their OH maser emission. The interacting and merging galaxies can be considered as the best candidates for such objects.
The high latitude cloud Lynds 1642 is not breaking up
Authors: Liljeström T.
Published: June 2019

CO, HCO+ and NH3 observations have been carried out towards the high latitude cloud L1642 using the 1.2-m GISS, the 11-m NRAO, the 14-m Metsähovi and the 100-m Effelsberg radio telescopes. The velocity field of the CO gas indicates a core-halo structure. The core component has a constant radial velocity, whereas the halo gas is slightly (> 1 km/s) redshifted, as compared to the radial velocity of the core, and shows velocity gradients towards the cloud edges in a similar way as the HI gas associated with L1642. Within the border of the 13CO emission the mass of the cloud is estimated to be some 76M⊙. In contradiction to Magnani et al.(1985), who claimed that L1642 belongs to a population of very young high latitude clouds which are breaking up, this study supports the view that L1642 is in virial equilibrium and significantly older than 106 yr. The virial equilibrium of L1642 enables a distance determination of ˜ 190 pc to the cloud core.
A large scale survey of dense cores and molecular outflows in Ophiuchus
Authors: Mizuno Akira , Nozawa Satonori , Iwata Takahiro , Fukui Yasuo
Published: June 2019

We have been surveying dense molecular cores in Ophiuchus region including ρ Oph, L234, and L43 with the 4m radio telescope at Nagoya University since 1985. We have already mapped ∼18° × 12° area with 2′ or 4′ grid spacing in 13CO (J=1-0) spectra. We have identified ∼50 dense cores (we call ”13CO cores”). Typical mass, density, and size of the 13CO cores are ∼20 M⊙, ∼3 × 103 cm−3, and ∼0.3 pc, respectively (Nozawa et al. 1990). We also surveyed molecular outflows in 12CO (J=1-0) spectra toward 13 IRAS point sources associated with 13CO cores in Ophiuchus. As a result of the survey, we have found 5 molecular outflows in the filamentary dark clouds and 5 regions exhibiting high velocity wings in the ρ Oph main body.
Comparison of turbulence in HII regions and molecular clouds
Authors: O'Dell C. R.
Published: June 2019

Both the HII Regions and the Molecular Clouds show broadening of their emission lines beyond that expected from thermal motion and this is ascribed to turbulence. Turbulence in molecular clouds generally agrees with a model where the velocity of motion is determined by the Alfv én velocity.

Turbulence in Galactic HII Regions and Giant Extragalactic HII Regions can also be studied by the width of the emission lines. The magnitude of the turbulent velocities in these regions are characteristically about 10 km/s. There is a general increase in turbulent velocity with the size of the HII Region, and this relation is close to but different from the one third power dependence expected from the most naive application of Kolmogorov theory. When a detailed study is conducted of each Galactic HII Region by means of the structure function, one finds that there is not agreement with Kolmogorov theory.

The Size-Turbulent versus Velocity relation for Galactic HII Regions differs slightly from the better defined velocity relation for Giant Extragalactic HII Regions. This difference is probably due to the fact that the larger extragalactic objects are probably complexes of multiple individual HII Regions. There is no evidence that broadening of extragalactic HII Regions is due to motion about a common center of mass.
Fragmentary structure in the L1551 molecular outflow
Authors: Parker Nicholas D. , White Glenn J.
Published: June 2019

We have obtained a high resolution map of the 12CO J=3—2 emission in the vicinity of HH102 in the L1551 molecular outflow. The data reveal the presence of several bright clumps within the thin shell of low-velocity blue-shifted outflowing gas. There is evidence for further fragmentation in the clumps, as signified by the high derived excitation temperatures and low beam-filling factors. The region of peculiarly energetic activity associated with HH102 coincides with the projected location of impact of a radio jet from IRS5 with the dense ambient gas surrounding the outflow cavity.
Observations of variability of H2O Maser sources associated with star formation regions
Authors: Pashchenko M. I. , Lekht E. E. , Berulis I. I. , Sorochenko R. L.
Published: June 2019

An outstanding feature of the water maser emission from star formation regions is the strong time variability, which was recognised soon after the discovery of interstellar H2O masers. It is likely that the observed considerable variations of the H2O line profiles as well as of the flux density reflect the disturbancies in the protostellar nebule probably associated with the outflow from the newly formed stars.
Synthetic linemaps for hierarchial clouds
Authors: Stenholm L. G.
Published: June 2019

Molecular clouds are characterized by large density contrasts and pronounced internal motions. Some information on their internal physics has been acquired through relatively simple data analysis and through theoretical modelling, mainly hydrodynamical calculations. More detailed radiative transfer models have been limited to spherical cloud models.

A related but less recognized problem is the classification of molecular clouds, or otherwise stated, can we compress the information in the observed two-dimensional maps for various line parameters in such a way that the physical differences are retained, while the random component has been removed? This is a more fundamental formulation of the special problem of comparing the appearance of two molecular clouds, or of the problems of quantifying the velocity correlation within the clouds.
Cloud-Cloud collisions and fragmentation
Authors: Whitworth Anthony , Pongracic Helen
Published: June 2019

Supersonic head-on collisions between quiescent clouds produce flattened sheets of shocked gas. We derive the condition which the cooling law must satisfy if this sheet is to fragment into protostellar condensations (i.e. gravitationally unstable lumps). If this condition is not satisfied, colliding clouds are likely to be disrupted and dispersed. We show that under the conditions obtaining in GMCs, most cloud-cloud collisions probably do not result in fragmentation.
The Planck Mission
Authors: Tauber J. A.
Published: June 2019

We present an overview of the European Space Agency's Planck mission, its scientific objectives and the key elements of its technical design. The current programmatic status of Planck within ESA's Scientific Programme, implementation plans, and near-term milestones are also addressed.
Constraints in the Δ0-Ω0 plane from gravitational lensing
Authors: Helbig Phillip.
Published: June 2019

I review simultaneous constraints on the cosmological parameters Δ0 and Ω0 from gravitational lensing. The emphasis is on systematic extragalactic surveys for strong gravitational lenses, mainly the largest and best-defined such survey, JVAS/CLASS.
Gravitational Lensing as Probe of Large Scale Structure of the Universe
Authors: Bhatia V. B. , Jain D. , Mahajan S. , Panchapakesan N.
Published: June 2019

We apply gravitational lensing statistics to: (1) place a limit on the cosmological constant (ΩΛ); (2) place a limit on the average red-shift (< z >) of gamma-ray bursters (GRBs); (3) investigate models of galaxy evolution to see how compatible these models are with lensing statistics. We also point out the sources of uncertainty in lensing statistics, leading to uncertainty in the results.
The X-ray Fundamental Plane of Clusters of Galaxies
Authors: Fujita Yutaka.
Published: June 2019

We analyze the relations among the central gas density, core radius, and temperature of X-ray clusters by plotting the observational data in the three-dimensional (log ρgas,0, log rc, and log Tgas) space and find that the data lie on a ‘fundamental plane’. We discuss the implications of the plane.
To Solve Dark Matter Problems Without Dark Matter
Authors: Xu Chongming. , Wu Xuejun.
Published: June 2019

We consider solving the dark matter problem via a modified gravitational Lagrangian.
Multi-epoch water maser survey towards low-mass YSOs
Authors: Furuya Ray S. , Kitamura Yoshimi , Wootten Alwyn , Claussen Mark J. , Kawabe Ryohei
Published: June 2019

We present the results from a series of multi-epoch 22 GHz H2O maser surveys with the Nobeyama 45 m telescope and the VLA towards low-mass young stellar objects, including all the class 0 sources in the northern sky. Our Nobeyama 45 m survey is the deepest survey - down to an isotopic H2O luminosity of ∼ 10−13L⊙ - performed so far. From this survey, we obtained the following results. (1) Class 0 sources show high H2O maser activity: our derived detection rates are ∼ 38% for class 0, but only ∼ 4% for class I sources. (2) Activity of the H2O masers is more likely related to 100 AU scale ionized jets than to large scale molecular outflows.
The OH/IR star population at the centre of the Galaxy
Authors: Ortiz Roberto , Omont Alain , Schultheis Mathias , Copet Eric , Habing Harm J. , Messineo Maria , Ganesh Shashikiran , Blommaert Joris A. D. L.
Published: June 2019

In the last years various OH surveys at 1612 MHz have been carried out in the galactic bulge region with high spatial resolution (Lindqvist et al. 1992; Sevenster et al. 1997; Sjouwerman et al. 1998) which have resulted in the detection of over five hundred double-peaked objects commonly associated with oxygen-rich AGB stars. More recently, the ISOGAL survey, which is part of the ISO mission, scanned several regions along the galactic plane, including the inner bulge, at 6.8 and 15 micron wavelengths. In this work we report an analysis of 104 OH/IR stars in the bulge, based on near and mid-infrared observations. Mass-loss rates, luminosities and masses are determined for a sub-sample of stars, for which near-infrared data are available in the literature.
Observational studies of maser polarization
Authors: Kemball Athol
Published: June 2019

This paper reviews observational studies of maser polarization, with particular reference to observational results reported in the intervening eight years since the last maser conference (Clegg & Nedoluha 1993). The scientific role of maser polarization observations is discussed and the theoretical interpretation of such data is considered. Recent observational results are presented for several common maser transitions, including OH, H2O, and SiO.
Metal-Poor Globular Clusters of the Milky Way and Environs
Authors: E. Hesser James , Fulbright Jon P.
Published: June 2019

We review properties of the most metal-deficient globular clusters and halo stars in the local universe as of March 2001, with goals of examining present evidence for the duration of the initial burst of massive cluster formation in the Milky Way, determining when that burst occurred, and elucidating what appears to be limiting our present understanding. Such issues bear upon many topics to arise later in the Symposium.
The Globular Cluster Terzan 7 in the Sagittarius Dwarf Spheroidal Galaxy
Authors: Held Enrico V. , Rosenberg Alfred , Saviane Ivo , Momany Yazan
Published: June 2019

We present deep V, I photometry of the globular cluster Terzan 7, a probable member of the globular cluster system of the Sagittarius dwarf spheroidal galaxy. The metallicity, estimated from a new method based on analytic RGB fits, agrees with previous estimates based on color-magnitude diagrams ([Fe/H]= −0.9 ± 0.1 dex). This result confirms a discrepancy between photometric and spectroscopic determinations. Using both the horizontal and vertical methods to estimate relative ages, we confirm that the age of Terzan 7 is about 70% that of 47 Tuc. A rich population of blue stragglers is found, strongly concentrated toward the center of the cluster.
Metal-rich and Metal-poor Globular Clusters in Ellipticals: Did we Learn Anything? or Constraints on Galaxy Formation and Evolution from Globular Cluster Sub-populations
Authors: Kissler-Patig Markus
Published: June 2019

A brief review on globular cluster sub-populations in galaxies, and their constraints on galaxy formation and evolution is given. The metal-poor and metal-rich sub-populations are put in a historical context, and their properties, as known to date, are summarized. We review why the study of these sub-populations is extremely useful for the study of galaxy formation and evolution, but highlight a few caveats with the current interpretations. We re-visit the current globular cluster system formation scenarios and show how they boil down to a single scenario for the metal-poor clusters (namely the formation in “universal”, small fragments at high z) and that a hierarchical formation seems favored for the metal-rich clusters.
The Mass Function of Young Star Clusters in our Galaxy and Nearby Galaxies: Is It Universal?
Authors: Sagar Ram
Published: June 2019

Mass functions (MFs) derived from photometric observations of young star clusters of our Galaxy, the Magellanic Clouds (MCs), M31 and M33 have been used to investigate the question of universality of the initial mass function and presence of mass segregation in these systems. Observational determination of the MF slope of young star clusters have an inherent uncertainty of at least ∼ 1.0 dex in the Milky Way and of ∼ 0.4 dex in the MCs. There is no obvious dependence of the MF slope on either galactocentric distance or age of the young star clusters or on the spatial concentration of the stars formed or on the galactic characteristics including metallicity. Effects of mass segregation have been observed in a good number of young stellar groups of our Galaxy and MCs. As their ages are much smaller than their dynamical evolution times, star formation processes seem to be responsible for the observed mass segregation in them.
Analysis of Colour-Magnitude Diagrams of Rich LMC Clusters: NGC 1831
Authors: Santiago Basílio X. , Kerber Leandro , Castro Rodrigo , Wilkinson Mark , de Grijs Richard , Johnson Rachel , Gilmore Gerry , Beaulieu Sylvie
Published: June 2019

We present a preliminary analysis of a deep colour-magnitude diagram of NGC 1831, a rich cluster in the LMC. The data were obtained with HST/WFPC2 in the F555W and F814W filters, reaching m555 ⋍ 25 mag. Efficient use of the two-dimensional distribution of data was made by comparing the observed colour-magnitude diagram with model ones. We discuss the process used in accounting for field star contamination and sampling incompleteness in the data. We also describe the algorithm used in building artificial diagrams from model isochrones and the statistical method used in comparing model and data colour-magnitude diagrams. From these comparisons we infer a metallicity Z = 0.008 and a low value, fbin ⋍ 0.25, for the global binary fraction of NGC 1831. We also rule out ages τ ≲ 500 Myr and mass function slope values α ≳ 2.
Dynamical Evolution of Rich Star Clusters in the LMC
Authors: Wilkinson Mark I. , Gilmore Gerard F. , Hurley Jarrod
Published: June 2019

We combine the results of a large HST study of rich LMC clusters with state of the art N-body simulations. We summarise recent highlights from our project: the estimation of age spreads in young LMC clusters, the measurement of the binary fraction of NGC1818, and the determination of deep cluster luminosity functions (LF). We also present preliminary results from an investigation of the core radius versus age relation evident among LMC clusters.
47 Tucanae: The Spectroscopic versus CMD Age Discrepancy
Authors: Vazdekis Alexandre , Salaris Maurizio , Arimoto Nobuo , A. Rose James
Published: June 2019

We investigate the origin of the discrepancy between the age estimates based on the Color-Magnitude-Diagram and the integrated spectra of 47 Tuc. Using a newly defined Hγ age indicator, with unprecedented power to disentangle the degeneracy between age and metallicity, we confirm the very large spectroscopic age derived for this cluster. We explore a number of key parameters with potential to decrease the Balmer lines and show the necessity of turnoff stars with lower temperatures. We find that alpha-enhanced isochrones with atomic diffusion leads to a younger spectroscopic age in better agreement with the CMD age.
Testing Intermediate-age Stellar Evolution with Magellanic Cloud Clusters
Authors: Gallart Carme , Zoccali Manuela , Bertelli Giampaolo , Chiosi Cesare , Demarque Pierre , Girardi Leo , Yi Sukyoung
Published: June 2019

We present new deep VLT color-magnitude diagrams of three intermediate-age populous star clusters in the LMC. These data will be used independently by the Padova and Yale stellar evolution Groups to address issues relevant to the evolution of intermediate-mass stars: mainly the extent of the core convective overshoot, as well as other poorly determined parameters, such as mass loss during stellar evolution, internal rotation, and He content.
Age Dating of Globular Clusters Using UBV(RI) Main-sequence Photometry in the Two-color Diagram: Age of NGC 6397
Authors: Kravtsov Valery V.
Published: June 2019

I propose and apply a method for deriving ages of the metal-poor globular clusters (GCs) in a distance-independent way, which is based on an age indicator related to the main sequence in the two-color diagrams using the U-B index. The age of the metal-poor GC NGC 6397 has been estimated, using Yale isochrones of Demarque et al. (1996), to be close to 16 Gyr provided the cluster metallicity is near [Fe/H] = −2.0.
Metallicity Gradients in Globular Cluster Systems: the Trace of a Self-Enrichment Process?
Authors: Parmentier G. , Magain P. , Noels A. , Thoul A. , Jehin E.
Published: June 2019

We have developed a model of globular cluster self-enrichment, based on the ability of the globular cluster gaseous progenitors to retain the ejecta of a first generation of Type II Supernovae. The key point is that this ability depends on the pressure exerted on the progenitor cloud by the surrounding protogalactic medium and therefore on the location of the cloud in the protogalaxy. The model is able to explain the galactic halo metallicities and the metallicity gradient of the Old Halo which is thought to be the genuine galactic globular cluster system. The possibility that metallicity gradients are a common property of extragalactic globular cluster systems is also presented.
Twenty-one-cm Studies of Some Interstellar Clouds
Authors: Davies R. D.
Published: June 2019

Cygnus-x is an extended radio source centered at δ=40° and R.A. = 20h25m (l=48°, b=0°). Its emission in the continuous spectrum has been studied by Davies using high resolution surveys at the following frequencies: 92, 158, and 1425 Mcps, 242, 400, 600, and 900 Mcps. The background emission at the position of the source was removed by a smooth interpolation in galactic longitude between the two sides of the source. The resultant flux density did not change significantly with frequency and had the value 4.5 X 10−23 wm2 (cps)−1. Moreover, the brightness temperature Ts of the center of the source varied inversely as the square of the frequency v; Ts·v2 had the value 3.5X1019 °K sec−2.
Star Chains and Dark Filaments in Galactic Nebulae
Authors: Fessenkov V. G.
Published: June 2019

When investigating sufficiently dense star fields, one often finds some short chains of stars oriented with a certain regularity and in a few cases connected with dark filaments of the galactic nebulae. It is necessary to be very cautious in the interpretation of this phenomenon, because even completely accidental clustering of some objects can sometimes lead to interesting deviations from homogeneity.
Examples of Gas Motion and Certain Hypotheses on the Mechanism of Stellar Outbursts
Authors: Sedov L. I.
Published: June 2019

In connection with the problem of the explanation of stellar outbursts exact solutions for gas flows with spherical symmetry can be given in the following three cases.

1. Propagation of detonation waves from the interior to the surface of a star, accompanied by an output of nuclear energy on the wave front. Effects of the increase of detonation velocity depending upon the law of density decrease from the center to the outer layer (failure of Chapman-Jourguet's rule) are investigated. As a result of a sufficiently rapid density decrease one obtains a complete dispersion of the detonation products with the formation of a vacuum near the center. Similar solutions are obtained for the spherical problem of the propagation of a rarefaction jump, accompanied by an energy output (a jump of flame front type) through a gas at rest.

2. Perturbed gas motion due to an explosion caused by a sudden large output of energy inside a star. The energy is transferred to the surface together with the shock wave. Exact automodel solutions of the equations for adiabatic time-dependent gas motion, accompanied by the formation of a vacuum when γ=Cp/Cv = 4/3 and without it, are given, gravitation being taken into account. Some solutions for larger values of y are studied.

3. Examples of dynamically unstable equilibrium states disturbed by an explosion followed by the development of a shock wave, propagating through a gas at rest with density gradient. A motion without energy output develops. The energy of the disturbed motion at any time is equal to the initial energy in the equilibrium state.
Some Exact Solutions of the Equations of Magnetohydrodynamics when Both Self-Attraction and Magnetic Fields Are Present
Authors: McVittie G. C.
Published: June 2019

This paper describes recent work on magnetohydrodynamics by M. H. Rogers and myself. It has seemed to us that the large-scale dynamics of cosmical gas clouds must take into account the fact that a gas is a compressible material, so that the hypothesis of a constant density must be avoided. Secondly, there must be present a pressure-gradient force. Thirdly, self-gravitation cannot be omitted. Fourthly, the motion should be adiabatic, in the first instance at least. To these conditions we have now also added the effects of the presence of magnetic fields, but have assumed that the conductivity of the material is infinite and that it is of constant permeability. We have neglected the effects of turbulence, of viscosity, and of heat conduction.
Shock Waves in Magnetogasodynamic Turbulence
Authors: Kaplan S. A.
Published: June 2019

The spectral theory of isotropic magnetogasodynamic turbulence proposed by the author some years ago gives the following expression for the spectral functions of kinetic and magnetic energies.
The Spectrum of Density Perturbations in an Expanding Universe
Authors: Silk Joseph
Published: June 2019

Perhaps the most challenging problem confronting a cosmologist is to reconcile the observed large-scale structure of the Universe with the Friedmann-Lemaître cosmological models that have gained such widespread acceptance in recent years (cf. however the alternative viewpoint, as exemplified in this Symposium by Arp and others). In this review, I shall look anew at the spectrum of density inhomogeneities that survive decoupling of matter and radiation at z ~ 1000 and provide the primordial fluctuations that can eventually generate galaxies. A closely related matter, that of the associated fluctuations in the background radiation, is discussed elsewhere in this volume by Doroshkevich, Sunyaev and Zel'dovich.
The Theoretical Situation
Authors: Iben Icko
Published: June 2019

The domain of this symposium is so wide – essentially anything at all having to do with stars – that it is impossible to do more than cursory justice to even a small portion of the matters to be discussed. My contribution will therefore be limited primarily to a discussion of the status of theoretical work bearing on the behavior of stars that evolve through the classical instability strip that extends from the region of Cepheids to the domain of RR Lyrae stars. Discussion of other extremely important variable stars such as cataclysmic variables, Mira and irregular variables, flare stars, β-Canis Majoris stars, and δ-Scuti and small amplitude variables will here be mentioned only in passing; presumably, most of these stars will be discussed at length by other speakers at this symposium. Further little attention will be paid to the thermal instability that is initiated in the helium-burning region of a double-shell-source star and to current thinking about the progenitors of type I and type II supernovae; presumably, these topics will be discussed in Warsaw in the symposium on advanced stages of evolution.
The Temperature Scale for Classical Cepheids
Authors: Parsons Sidney B.
Published: June 2019

In the study of Cepheid pulsation and evolution, it is very important to be able to obtain effective temperatures from some easily observable spectral characteristic. A review is given of the several discrepant relations between Teff and intrinsicB—V index given for classical Cepheids and/or yellow supergiants by Rodgers (1970) and Parsons (1971) (the same as Kraft, 1961; Böhm-Vitense, 1972; Schmidt, 1972, and van Paradijs, 1973). All temperatures agree well at (B – V)0 = 0.4 but diverge toward redder colors; at (B – V)0 = 0.9 the values range from 4900° (Böhm-Vitense, for class Ia-Ib) to 5770° (Schmidt, for class I stars). In each case the analysis leading to the assignment of Teff to a given color index is not completely satisfactory, so it is difficult to make value judgements. Comparison of relations between Teff and spectral class shows similar discrepancies.
A Search for β Cephei Pulsation in Extreme Composition Models
Authors: Cox Arthur N. , Tabor James E.
Published: June 2019

Following Davey (1973), we have considered the linear radial pulsational stability of stellar models in the region of the HR diagram populated by β Cephei stars. Instead of one composition, however, we have considered many compositions, some of them rather extreme. No consideration of composition changes for temperatures above a few-million degrees was given, and therefore the stellar structure and its pulsational stability may not be evaluated very accurately. Modulation of nuclear reactions was also neglected. In no case was radial pulsational instability found.
Non-Linear Instability of Stars With M > 100 M⊙
Authors: Papaloizou J. C. B. , Tayler R. J.
Published: June 2019

Massive main sequence stars are vibrationally unstable to small perturbations excited in the region of nuclear energy generation. This instability may, however, be limited at finite amplitude. Only the fundamental radial mode is unstable with an e-folding time of 105 to 106 pulsation periods; in contrast the overtones have damping times short compared with the growth time of the fundamental. Non-linear effects couple the linear modes. At very low amplitude the energy transfer time between modes is long compared with both growth and damping times and, as the overtones decay, a pure fundamental should result. However, as the amplitude increases, the transfer time becomes less than the growth and damping times. Energy can then be transferred from the fundamental to the overtones and damping can lead to a limitation of amplitude.
Variations in the Line Spectrum of A-Type Supergiants
Authors: Buscombe William
Published: June 2019

Attention is drawn to a group of stars of luminosity class Ia, in which variation of radial velocity has long been known in semi-regular cycles of 5 to 30 days. The interval of approximate repetition appears to be positively correlated with the luminosity of the star.

Recent spectrophotometry of the absorption lines, especially by Rosendhal, Aydin, Groth, and Buscombe shows a considerable variation in the strength of Balmer lines (with emission components detected at H∝), as well as those of metallic ions. The microturbulence parameter derived from curves of growth for Fe ii varies by a factor of two or more during some cycles.
On the Binary Nature of the Slow Nova, RR Telescopii
Authors: Webster B. L.
Published: June 2019

Those novae known to be binaries generally have orbital periods of the order of hours, exceptions being the atypical recurrent novae T CrB and RS Oph, which have giant companions and probably much longer periods. Since the orbital period in a semi-detached system relates to the mechanism of current mass exchange and also to the extent to which the primary evolved before mass exchange took place at an earlier stage, it is of interest to see if any classical novae are in more widely separated systems.

This communication concerns the star RR Telescopii, which has all the characteristics of a slow nova – a range in amplitude greater than seven magnitudes, a spectral type at maximum of F, and a decline through a nebular stage of increasing ionization level (e.g. Thackeray, 1955). RR Tel was seen as a variable before outburst, but little is known about this variable apart from its period of 387 days, although doubts have been expressed about its being a red variable (Payne-Gaposchkin, 1957). Dr Thackeray has Radcliffe spectrograms of RR Tel from soon after maximum to the present. On some of the more recent of these, bands of TiO have become visible, presumably as the hot star has faded, and Dr Thackeray and I interpret these as meaning that the original variable is still there and is an M giant. Thus RR Tel is in a binary system containing a giant M star and a hot companion and such a system cannot have an orbital period of hours like the classical novae mentioned above.

In 1972, Dr Glass and I examined RR Tel in the infrared between 1.2 and 20 µ. The energy distribution does not resemble the cool star that might be expected, but is exactly like that of free-free radiation over the whole wavelength range. The puzzle is that the infrared is two orders of magnitude stronger than we would predict from the optical spectrum for free-free radiation.
Mass-Ratio Determination in Contact Binaries
Authors: Hutchings J. B.
Published: June 2019

In the wake of recent theoretical work on contact systems (e.g. Whelan, 1972; Biermann and Thomas, 1972; Lucy, 1968), it is of importance to determine fundamental data from observations. This has been done recently by several groups in analysing light curves (Mochnacki and Doughty, 1972; Hutchings and Hill, 1973; Wilson and Devinney, 1973), and it is found that shapes, temperature differences and distributions, and mass-ratios, can be determined in many cases. However, where spectroscopic data are also available, the mass-ratios are not always in agreement. Using the photometric models, it is possible (Hutchings, 1973) to calculate the distortion of line profiles resulting (primarily) from the non-uniform brightness over the component stars in these systems. This distortion leads to the characteristically observed ‘square’ velocity curves for the systems (e.g. Binnendijk, 1967). Correction for the effect in most cases (a) resolves the mass-ratio discrepancy and (b) leads to better estimates for the masses. The faintness of most contact systems makes detailed spectroscopy difficult, but there appears to be a need for further work in the directions outlined here to improve the fundamental data available on them. These results should also be borne in mind in inspecting previous work on contact binaries.
Touching on the Effects of an Imperfect Window Function
Authors: Chaplin W.J. , Elsworth Y. , Isaak G.R. , McLeod C.P. , Miller B.A. , New R.
Published: June 2019

BiSON currently consists of four fully- and two semi-automated observatories, dedicated to the round-the-clock collection of low-degree (low-l) helioseismological data. A complete historical record of the network can be found in Chaplin et al. (1996). The annual duty cycles achieved by BiSON have improved steadily since our sixth site was commissioned in 1992. Over the period 1992 – 1996 inclusive, we have recorded annual duty cycles of: 54. 68. 78. 74 and 78 per cent respectively.
Vamos: Velocity and Magnetic Observations of the Sun: Calibration and Data Analysis
Authors: Oliviero M. , Severino G. , Straus Th. , Cacciani A. , Dolci M. , Moretti P.F.
Published: June 2019

This poster illustrates the calibration procedure and the analysis pipeline developed for the helioseismology data acquired with the VAMOS (Velocity And Magnetic Observations of the Sun) instrument. The VAMOS, based on the MOF (Magneto-Optical Filter) technology, and its operation are discussed in detail elsewhere (Cacciani et al., 1997 and Moretti et al., 1997).
What Do Solar F-Mode Frequencies Tell US?
Authors: Antia H.M. , Chitre S.M.
Published: June 2019

The fundamental mode (f-mode) which is essentially a surface mode can be expected to provide a diagnostic of flows and magnetic fields etc. present in the surface regions (Ghosh, Chitre & Antia 1995; Rosenthal & Christensen-Dalsgaard 1995). It turns out that the observed f-mode frequencies can also provide an accurate measure of solar radius (Antia 1997; Schou et al. 1997).
A Search for ℓ = 2 Asymmetries in Bison Data
Authors: Chaplin W.J. , Elsworth Y. , Isaak G.R. , McLeod C.P. , Miller B.A. , New R.
Published: June 2019

An in-depth discussion of the analysis presented here can be found in an up-coming paper (Chaplin et al., 1997).

The presence of a magnetic field will raise the degeneracy in ℓ of the resonant p-mode oscillations, via perturbations resulting from the Lorentz force. These degeneracyraising effects will give rise to asymmetric mode-multiplet structures. Both Gough & Thompson (1990). and Dziembowski & Goode (1997) have addressed the implications and potential complications that might result from such phenomena. Here, in an attempt to reveal the presence of an asymmetric frequency structure in the low-degree ℓ = 2 modes, i.e. to measure the asymmetries
Excitation of Oscillations - An Update of Bison Data
Authors: Chaplin W. J. , Elsworth Y. , Isaak G. R. , McLeod C. P. , Miller B. A. , New R.
Published: June 2019

BiSON is a 6-station, world-wide network of instruments which observe low-degree solar oscillations.

Theory (Goldreich & Keeley, 1977) predicts that the probability distribution of the mode powers is expected to follow negative exponential statistics under the condition that the interval of time for which the modes are averaged is less than the lifetime of the mode. Simulations (Chaplin et al, 1997) confirm this. The first publication of such an analysis for real data (Elsworth et al., 1995) showed that the oscillations largely follow the predictions with a small excess of the very largest power in the modes. In this paper we extend the previous observations of BiSON data to 80 months of data taken during the declining phase of the solar cycle. The data span the period January 1990 to August 1996. The data have been analysed using Fourier transforms each about 1/2 day in length. The minimum data fill accepted in any one period was 0.7 (Elsworth et al, 1995). Below this fill the data were discarded. The resolution of such short transforms is insufficient to distinguish between the components of the even and odd mode pairs. The mode powers are calculated for mode pairs ℓ = 0 & 2 and ℓ=1&3.

Comparison with theoretical predictions show that, as seen previously, there is still an excess of very high excitations which can be seen throughout the 80 months.
The Acoustic Cut-Off Frequency of roAp Stars
Authors: Audard N. , Kupka F. , Weiss W. W. , Morel P. , Provost J.
Published: June 2019

For 5 out of 28 known rapidly oscillating magnetic chemically peculiar (roAp) stars, the largest observed frequency seems to exceed the theoretical acoustic cutoff frequency, which is determined by the outermost stellar regions. We show that a better modelling of the atmosphere reconciles the theory with the observations for at least the roAp star α Cir.
Asteroseismology of δ Scuti Stars: Observations
Authors: Breger Michel
Published: June 2019

We review recent observational developments which provide important asteroseismological tools. Extensive multisite campaigns of individual δ Scuti stars show that 24 or more pulsation modes with ℓ = 0 to 2 can be detected photometrically. Spectroscopically, also about 30 modes have been detected, and these can be identified with modes of ℓ values up to 20. Since each technique favors the detection of specific types of modes, hundreds or thousands of modes must be excited in δ Scuti stars.

We examine the quantities which can be matched between observations and theoretical models specifically computed for each star. Recent progress in tbe mode identification of multiple pulsation modes is illustrated by presenting an application of the phase shift method for the two stars, FG Vir and 4 CVn, recently measured by the Delta Scuti Network.
EC14026 Stars: Theoretical Considerations
Authors: Fontaine G. , Charpinet S. , Brassard P. , Chayer P. , Rogers F.J. , Iglesias C.A. , Dorman B.
Published: June 2019

The recent discovery of pulsating subdwarf B (sdB) stars at the South African Astronomical Observatory (Kilkenny et al. 1997; Koen et al. 1997; Stobie et al. 1997; O'Donoghue et al. 1997) has opened a brand new avenue in the field of asteroseismology. The study of these pulsators (dubbed EC14026 stars, after the prototype) offers the exciting possibility of exploiting the full power of asteroseismology to investigate the sdB phase of stellar evolution, one of the last frontiers in our general understanding of the history of stars.
A High Speed Photometric Survey of Normal and Peculiar A-Type Stars
Authors: Kurtanidze O.M. , Nikolashvili M.G.
Published: June 2019

The programme of long-term high-speed photometric survey about two hundred Normal and Peculiar A-type stars with 125cm RC telescope equipped by Two-Star Photometer is presented.
Pulsation of PMS 1.8 M⊙ Stars: A Theoretical Investigation
Authors: Suran Doru Marian
Published: June 2019

Herbig Ae stars are pre-main sequence of masses ~2M⊙. As they trace their course toward the ZAMS, they cross the classical instability strip. At this stage, their evolution has considerably slowed down. Their structure differs from that of stars evolving after the ZAMS (δ Scuti stars) in the very deep layers where nuclear burning has only recently started. But the outer layers for stars before and after the ZAMS are expected to be similar. As these layers drive the pulsation of δ Scuti stars, it is reasonable to expect that PMS stars in the instability strip also are destabilized by the κ mechanism with a similar range of unstable modes. Indeed, recently a Herbig Ae star has been reported as a variable (Kurtz and Marang 1995).
Oscillations of Long-Period Variables
Authors: Xiong D.R. , Deng L. , Cheng Q. L.
Published: June 2019

We have performed a linear pulsational stability survey of long period variable models. The dynamic and thermodynamic couplings between convection and oscillations are treated by using a statistical theory of nonlocal and time-dependent convection. The results show that the fundamental and all the low overtones are always pulsationally unstable for the low-temperature models when the coupling between convection and oscillations is ignored. When the coupling is considered, there is indeed a “Mira” pulsationally instability region outside of the Cepheid instability strip on the H-R diagram. The coolest models near the Hayashi track are pulsationally stable. Towards high temperature the fundamental mode first becomes unstable, and then the first overtone. Some of the 2nd −4th overtones may become unstable for the hotter models. All the modes higher than 4th (n > 4) are pulsationally stable. The position and the width of such an instability region on the H-R diagram critically depends on the mass, luminosity and metal abundance of the star.