bluuebunny/crossref_metadata_2025 huggingface dataset:
The detection of new methanol masers in the 5−1 — 40E Line
Authors: Kalenskii Sergei V. , Slysh Vyacheslav I. , Val'tts Irina E. , Winnberg Anders , Johansson Lars E. B.
Published: June 2019

Fifty-one objects in the 5−1 − 40E methanol line at 84.5 GHz was detected during a survey of Class I maser sources. Narrow maser features were found in 17 of these. Broad quasi-thermal lines were detected towards other sources. One of the objects with narrow features, the young bipolar outflow L 1157 was also observed in the 80 − 71A+ line at 95.2 GHz; a narrow line was detected at this frequency. Analysis showed that the broad lines are usually inverted. The quasi-thermal profiles imply that the line opacities are not larger than several units. These results confirm the plausibility of models in which compact Class I masers appear in extended sources as a result of an appropriate velocity field.

Measurements of linear polarization at 84.5 GHz in 13 sources were made. No polarization was found except a tentative detection of a weak polarization in M 8E.
Six years of astrometric monitoring of the OH maser in U Herculis
Authors: Vlemmings Wouter , van Langevelde Huib Jan , Diamond Phil
Published: June 2019

Using VLBI phase referencing, we have been registering the position of the most blue-shifted OH 1667 MHz maser spot with respect to nearby extragalactic reference sources for almost seven years. The data for these 10 epochs allow us to determine the proper motion and parallax of U Her. It also yields the opportunity to compare the maser position to the position of the star, as measured by the Hipparcos satellite. Results support the theory that the compact bright spot is the amplified stellar image.
Topics in basic maser theory
Authors: Elitzur Moshe
Published: June 2019

This review covers some of the developments in basic theory of astronomical masers over the past ten years. Topics included are the effects of three dimensional geometry and polarization, with special emphasis on the differences between maser and non-maser radiation.
Theory and extragalactic masers
Authors: Watson William D.
Published: June 2019

Theoretical research on extragalactic masers (and hence this review) is almost entirely directed toward 22 GHz water masers in circumnuclear disks with the focus being the near ideal masing disk at the nucleus of the galaxy NGC4258. The discussion here is organized around (1) the excitation and conditions for the masers, (2) the spatial and spectral appearance of the maser emission, and (3) the mass accretion rate and structure of the disk. In addition, a summary is given of the basic physics (spectral linebreadths and maser polarization) that underlies certain interpretations of these and other astrophysical masers.
Chaotic temporal variations in cosmic masers
Authors: Chian Abraham C.-L. , Rempel Erico L. , Borotto Félix A.
Published: June 2019

Electron cyclotron maser emission is an accepted physical mechanism for generating coherent planetary and stellar radio emissions. Observational data has indicated evidence of nonlinear and chaotic temporal variability in some cosmic masers such as solar microwave spikes. The nonlinear and chaotic characteristics of cosmic masers can be attributed to plasma turbulence, such as Alfvén chaos, embedded in the emission region. We report a chaos theory of Alfvén waves which can account for chaotic acceleration of electrons in the source region of cosmic masers. Two types of Alfvén intermittency are identified: Pomeau-Manneville intermittency and crisis-induced intermittency. Since Alfvén waves may be responsible for accelerating electrons that emit maser radiations, the chaotic dynamics of Alfvén waves may be the origin of chaotic time variations of cosmic masers. Hence, we suggest that Alfvén intermittency may cause intermittent temporal fluctuations which can be observed in cosmic masers.
Near Infrared Photometry of New Galactic Globular Clusters GC 01 and GC 02
Authors: Borissova J. , Ivanov V. D. , Vanzi L.
Published: June 2019

We present a preliminary report on the first deep near infrared photometry of 2MASS GC 01 and 2MASS GC 02 - new Galactic globular cluster candidates, discovered by the 2MASS. The red giant branch slopes yielded [Fe/H]=-0.42 ± 0.15 dex and [Fe/H]=-0.66 ± 0.17 dex, respectively for GC 01 and GC 02. We estimated the reddening towards GC 01 and GC 02: E(B - V) = 5.36 ± 0.20, and E(B - V) = 4.55 ± 0.17. The calculated distance moduli to the clusters are: (m - M)0 = 13.53 ± 0.27 and (m - M)0 = 14.53 ± 0.31 for GC 01 and GC 02. Our best fit for the radial surface brightness profile of GC 02 yields: lg(rc) = 1.40, lg(rh) = 1.54, lg(rt) = 1.31, and c=1.60. CC 01 is less concentrated: lg(rc) = 1.63, lg(rh) = 1.7, lg(rt) = 1.25, and c=1.41.
Second-parameter Globular Clusters in the Milky Way and in M33 as Tracers of Mass Loss from M31 in the Early Epoch?
Authors: V. Kravtsov Valery
Published: June 2019

I suggest that the bulk of the “young”, second-parameter globular clusters (SPGCs) observed in the outer halo of our Galaxy and recently found in the other massive spiral of the Local Group (LG), M33, may have originated due to mass outflow from M31 and subsequent accretion of gas on the Galaxy and M33 in the early epoch.
Surface Photometry of Star Clusters in the Dwarf Elliptical Galaxies NGC 185 and NGC 205
Authors: Chul Kim Sang , Gyoon Lee Myung , Geisler Doug , Sarajedini Ata , E. Armandroff Taft , S. Da Costa Gary
Published: June 2019

We present the surface photometry of star clusters in the nearby dwarf elliptical galaxies NGC 185 and NGC 205, obtained from deep HST WFPC2 F555W (V) and F814W (I) images. We have obtained surface brightness and color profiles of six star clusters in NGC 185, seven star clusters in NGC 205, and one recently discovered non-stellar object in NGC 205. The surface brightness profiles of ten star clusters are fitted well by the King model, and those of four star clusters are fitted well by the power-law. Three out of ten star clusters fitted well with King model show signs of tidal tails.
New Insights from a Study of the Globular Cluster Systems of 60 Early-Type Galaxies
Authors: Kundu Arunav , Whitmore Brad
Published: June 2019

We present the results of our detailed WFPC2-based photometric study of the globular cluster systems (GCS) of over 60 elliptical and SO galaxies. Approximately 50% of the GCSs of ellipticals, and at least 15–20% of S0s reveal bimodality in the color distribution. We trace the variation in GCS properties with Hubble type and discuss the implications on the various models of galaxy (and cluster system) formation. We also provide evidence that the globular cluster luminosity function is an excellent distance indicator with an accuracy comparable to the surface brightness fluctuation method.
Ages and Metallicities of Globular Clusters in NGC 5128
Authors: Held Enrico V. , Federici Luciana , Cacciari Carla , Testa Vincenzo
Published: June 2019

We have obtained multislit spectroscopy of 40 known globular clusters in the nearby peculiar elliptical galaxy NGC 5128 at the ESO NTT telescope. Measurements of metal and hydrogen absorption line indices have been used to estimate the age and metal content of individual clusters. The results have been combined with previous data from multifibre spectroscopy of clusters in the halo of NGC 5128 to study the age and metallicity distribution of the globular cluster system.
Globular Clusters in Dwarf Elliptical Galaxies
Authors: W. Miller Bryan , M. Lotz Jennifer , C. Ferguson Henry , Stiavelli Massimo , C. Whitmore Bradley
Published: June 2019

We present preliminary results on the shape of the globular cluster luminosity function and the colors and inferred metallicities of the clusters in dwarf elliptical galaxies imaged with HST. The luminosity function (LF) of the GC candidates is consistent with a Gaussian-shaped LF similar to that in giant ellipticals. Also, with a mean color of (V - I) = 0.94, most of the GCs appear to be old and metal-poor ([Fe/H] = −1.4) like GCs in the Galaxy and in nearby giant ellipticals. This suggests that the bulk of the clusters were formed more than 10 Gyr ago.
A Search for Globular Clusters in the Surroundings of Dwarf Galaxies in Fornax: Intracluster Globulars?
Authors: P. Bassino Lilia , A. Cellone Sergio , C. Forte Juan
Published: June 2019

We present the results of a search for globular clusters in the surroundings of 15 low surface brightness dwarf galaxies in the Fornax cluster, on CCD images in the C and T1 bands. Globular cluster candidates show a clear bimodal color distribution. Their surface density distribution shows no concentration towards the respective dwarf galaxies but it does show concentration towards the center of the Fornax cluster. We suggest that the potential globular clusters might not be bound to the dwarf galaxies, but might instead belong to the intra-cluster medium.
Globular Cluster Specific Frequencies in Coma
Authors: Marín-Franch A. , Aparicio A.
Published: June 2019

Globular cluster systems (GCSs) have been observed in 17 elliptical galaxies located in Coma. Surface brightness fluctuations (SBF) and a 2.5-m ground-based telescope (INT, La Palma) have been used to determine total populations of Globular clusters (GCs) and specific frequency (SN) has been evaluated for each individual galaxy.
Embedded Clusters in the Low Metallicity Galaxy SBS 0335-052
Authors: Vanzi Leonardo , Hunt L.K. , Thuan T.X.
Published: June 2019

We present new infrared observations of the low-metallicity blue compact galaxy SBS 0335-052. We find that the NIR emission is consistent with star formation occurring in clusters younger than 5 Myr. We detect optically hidden star formation at 4 μm obscured by more than 15 mag. of visual extinction. We quantify the possible contribution from an evolved stellar population and from warm dust. We finally discuss the possible origin of the dust observed.
Kinematic Masses of Super Star Clusters in NGC 1569
Authors: M. Gilbert Andrea , R. Graham James
Published: June 2019

We summarize recent results from our study of the massive young super star clusters (SSCs) in the dwarf galaxy NGC 1569, and present new high-resolution NIRSPEC spectra that permit the measurement of a cluster's intrinsic stellar velocity dispersion. Thus we derive kinematic masses for the two brightest SSCs: 2.3 × 105 Msun for SSC B, and 3.9 × 105 and 4.4 × 105 Msun for the two components of SSC A.
HST Observations of Young Star Clusters in Interacting Galaxies
Authors: D. Borne Kirk , C. Keel William , N. Appleton Philip , Struck Curtis , A. Lucas Ray , B. Schultz Alfred
Published: June 2019

We present early results from the analysis of HST imaging observations for several pairs of interacting galaxies. We include two cases that were specifically chosen to represent a strong early (young) encounter and a weak late (old) encounter. The goals of the project include a determination of the timing, frequency, strength, and characteristics of the young star clusters formed in these two limiting cases of tidal encounters.
Cluster Mass and Metallicity Distributions: Reconstructing the Events During Halo Formation
Authors: Harris William E.
Published: June 2019

Globular clusters in most large galaxies are a mixture of metal-poor and metal-rich (bimodal), but the halo stars are almost entirely metal-rich. This and other lines of evidence argue that the metal-poor globular clusters formed within widely distributed 108 − 109M⊙ gas clouds (supergiant GMCs) during an early burst in which most of the gas was ejected or unused till later rounds of star formation.

New simulations of the growth of pre-galactic potential wells in the early universe now indicate that the initial power-law form of the globular cluster mass distribution (dN/dM ∼ M-1.8) is a miniature replica of the mass distribution of the SGMCs themselves, which grow hierarchically in the CDM potential wells of large protogalaxies.
Stellar Populations in Star Clusters: The Role Played by Stochastic Effects
Authors: Bruzual A. Gustavo
Published: June 2019

In this paper I combine the results of a set of population synthesis models with simple Montecarlo simulations of stochastic effects in the number of stars occupying sparsely populated stellar evolutionary phases, to show that the scatter observed in the magnitudes and colors of LMC and NGC 7252 star clusters can be understood in the framework of current stellar evolution theory, without the need to introduce ad-hoc corrections (e.g. artificially increasing the number of AGB stars).
Formation Scenarios for Globular Clusters and Their Host Galaxies
Authors: Zepf Stephen E.
Published: June 2019

This review focuses on how galaxies and their globular cluster systems form. I first discuss the now fairly convincing evidence that some globular clusters form in galaxy starbursts/mergers. One way these observations are valuable is they place important constraints on the physics of the formation of globular clusters. Moreover, it is natural to associate the typically metal-rich clusters forming in mergers with the substantial metal-rich population of globulars around ellipticals, thereby implying an important role for galaxy mergers in the evolution of elliptical galaxies. I also highlight some new observational efforts aimed at constraining how and when elliptical galaxies and their globular cluster systems formed. These include systematic studies of the number of globular clusters around galaxies as a function of morphological type, studies of the kinematics of globular cluster populations in elliptical galaxies, and a variety of observational programs aimed at constraining the relative ages of globular clusters within galaxies as a function of cluster metallicity. The understanding of the formation of globular cluster systems and their host galaxies has grown dramatically in recent years, and the future looks equally promising.
Dating Star Clusters in the Small Magellanic Cloud
Authors: Ahumada Andrea V. , Clariá Juan J. , Bica Eduardo , Dutra Carlos M.
Published: June 2019

We have mainly estimated ages and foreground interstellar reddening values for 15 SMC concentrated star clusters, from integrated spectra in the range (3600–6800) Å. Most of the sample are young blue objects (6–60 Myr), while L28, NGC 643 and L 114 are found to be intermediate-age clusters (1–6 Gyr). The present data also constitute a spectral library at the metallicity level of SMC clusters.
Large-Scale Structure and Direction of Rotation in Galaxies
Authors: de Vaucouleurs G.
Published: June 2019

The large-scale distribution of gas and dust in external galaxies seems to be closely correlated with the spiral arms and other main structural patterns. This was shown, in particular, by Baade and Mayall at the 1949 Symposium. A great many structural patterns exist among external galaxies; it is likely that these will prove to be important indicators of the dynamics and possibly of the evolution of cosmic gas clouds in stellar systems. This was pointed out at the 1953 Symposium by Hubble, whose original classification system was described in the report by Blaauw which includes also some indications given by Sandage of the program of revision and illustration of this system.
On the Formation of Interstellar Gas Clouds
Authors: Kaplan S. A.
Published: June 2019

The characteristic features of interstellar gas clouds—existence of large density fluctuations, their connection with cosmic dust, stretching along the magnetic fields, and so on—may be described by a shock wave theory in interstellar space.

The author has developed a theory of stationary shock waves accompanied by losses of energy by means of radiation. Choosing two surfaces on both sides of the front, so that the regions of energy radiation should lie between them, we can write an equation for the mass flow and for the impulse conservation between these surfaces, and two equations which determine the stationary temperature of the gas in the field of interstellar radiation. The solution of this system of equations permits one to determine the general changes of thermodynamic and other parameters for the transition of gas through the shock wave with regions of radiative cooling. If changes of the degree of ionization take place, and a magnetic field is present, some terms should necessarily be added to the corresponding equations.

The boundary between the interstellar gas cloud and the intercloud medium must represent the shock wave accompanied by losses of energy by means of radiation, because such ruptures may probably be supposed as the sole explanation of stability of the great density changes (a hundred times and more) often observed in the interstellar space.

In this paper we give some results of the theory of shock waves accompanied by radiative losses of energy.
Statistical Study of Globules Projected on Stellar Clouds
Authors: Rojkovsky D. A.
Published: June 2019

The discovery of globules by Bok and Reilly leads to various speculation concerning their cosmogonical meaning as protostellar bodies in our galaxy.

It is well known that the most favorable conditions for the discovery and study of globules are reached when they are connected with diffuse galactic nebulae (Dufay). Observations show that in this case globules possess various and generally irregular shapes. It is quite probable that these globules are connected with large dark clouds, genetically linked with bright nebulae. The number of globules in a nebula depends essentially on the peculiarity of its structure. Nevertheless the total area of the sky covered by the nebulae is relatively small and their physical conditions depending on nearby hot stars are peculiar. Consequently it is hardly possible to suppose that the properties of globules present in nebulae are the same as in the other regions of the galaxy. It is important to collect more data concerning the spatial distribution of globules, their density, sizes etc. The stellar clouds of the Milky Way give some possibility for further study in this respect. In the case of a sufficiently dense stellar field the projecting globules can be discovered as some fluctuations in the stellar population as observed on the photograph.
Gross Dynamics of the Interstellar Medium
Authors: Parker E. N.
Published: June 2019

This paper reviews the many, by now familiar, processes that have at various times been attributed to the interstellar motions, in an effort to piece together some over-all picture of the grosser dynamical relations which might be expected to apply to the motions of the material and electromagnetic fields between the stars. We are interested in numerical values for the material densities, velocity fields, magnetic fields, etc., and we should also like to know to what extent the observed properties of the different fields are dependent upon each other.
Magnetohydrodynamic Dissipation
Authors: Biermann L. , Schlüter A.
Published: June 2019

Before entering the subject of magnetohydrodynamic dissipation we comment briefly on the question of thesupplyof kinetic energy to the interstellar gas. This topic has been considered in the 1953 Symposium by Schlüter and myself, and also by Oort. As had been pointed out already by Spitzer in Paris, 1949, the visible HII regions, owing to their excess pressure as compared with the HI regions and the dilute HII regions, must be assumed to expand with a velocity of the order of 10 or 20 km/sec. By this expansion part of the radiation energy of the star is converted into kinetic energy. It was estimated (p. 153 of the proceedings) that a typical HII region around a BO star feeds 1035ergs/sec to the instellar gas, and that the number of these regions is such, that each region has to provide kinetic energy on the average to 1036−1037g of interstellar material. Thus a value of 10−2−10−1erg g−1sec−1was found (which corresponds to 10−26−10−25erg cm−3sec−1, assuming 10−24g/cm3for the mean density of the interstellar material in the disk).
Cross Sections for Photoionization from Valence-Electron States
Authors: Burgess A. , Seaton M. J.
Published: June 2019

In order to study recombination processes in gaseous nebulae, one requires the coefficients for radiative recombination to many excited states of neutral atoms and positive ions. These may be calculated as functions of electron temperature once the corresponding photoionization cross sections are known. For photoionization from valence-electron states a general formula has been obtained using a method which is an extension of that used by Bates and Damgaard for bound-bound transition probabilities.
Internal Kinematics of the Planetary Nebulae
Authors: Wilson O. C.
Published: June 2019

A common form of planetary nebula consists of a high temperature nuclear star surrounded by an ovoid or ellipsoidal shell of gas which has originated in some manner in the nucleus. The radiation from the nebula consists of the ordinary spectral lines and continua of hydrogen and helium plus the so-called forbidden lines of atoms and ions of oxygen, nitrogen, neon, argon, etc. It has become clear that, as in other cosmic bodies, the bulk of the nebula consists of hydrogen and helium, and the remainder of the chemical elements occur only as a slight admixture of impurities. For instance, recent data by Aller and Minkowski lead to relative numbers of atoms of various chemical species in a particular nebula (NGC 7027) as follows (oxygen arbitrarily taken as 104):
Comparison of Gas and Radiation Pressure in the Problems of the Dynamics of Planetary Nebulae
Authors: Zanstra H.
Published: June 2019

We may summarize Wilson's extended study of internal motions in planetary nebulae by stating that he finds the average expansion velocity in the regions whence originate the lines of H, NeIII, OIII to be roughly 20 km/sec, with rare examples ranging as low as O and as high as 50 km/sec. We ask whether radiation pressure and/or gas pressure may give, in an elementary calculation, velocities anywhere near these values.
Bright Rims in Diffuse Nebulae
Authors: Pottasch Stuart R.
Published: June 2019

Bright rims at the edge of dark markings associated with diffuse nebulae have been noticed for more than thirty years. An example is shown in Fig. 1, a photographic negative of the diffuse nebula IC 1396. Figure 1(a) shows the entire nebula, which is excited by an O6 star (circled). Figure 1(b) is an enlargement of one of the bright rims in this nebula. A number of general characteristics can be pointed out. The bright rim is always at the edge of what appears to be dark matter and always is bright on that side of the matter closest to the exciting star. If a line of symmetry is drawn through the dark material it is found in general to pass very near to the exciting star. Sometimes isolated matter similar to Bok's globules are present, and when present also show bright rims. These globules are included in the observational study.
Gas-Dynamical Effects of Star Formation
Authors: Savedoff M. P.
Published: June 2019

At the Second Symposium, various contributors discussed the possibility that star formation might provide the energy to maintain the motion field in the interstellar medium. Although Oort and Spitzer emphasized the importance of the recoil momentum produced by the gradual ionization of the surrounding gas, the important effect appears to be the gas pressure itself when the ionized region is enclosed by the surrounding gas. Indeed, Kahn has shown (for one-dimensional flow into a vacuum) that the recoil momentum is inefficient in producing mass motion; only a limited range in photon fluxes result in acceleration of neutral hydrogen.
Gas Dynamics of Galaxy Collisions
Authors: Savedoff M. P.
Published: June 2019

When two galaxies collide at extremely high velocities, Baade and Spitzer note that negligible stellar effects are anticipated but that the interstellar medium is violently affected. Recently the radio source Cygnus A has been identified with what appears to be two colliding galaxies. Baade and Minkowski describe the optical nebula as two … “late-type systems, judging by low density gradients of the two disks. Spatially they are oriented face to face, they are slightly decentered, and we look upon them at an angle not far from 45°.” “ … the two systems must be in close contact because of the strong signs of tidal distortion which the nuclei show.” The lines are noted as diffuse.
On the Mathematical Structure of Turbulence
Authors: Bass J.
Published: June 2019

One is able to formulate in a very general way the mathematical problem of turbulence in the following form.

One has a system of second-order nonlinear partial differential equations with three space variables and one time variable. One looks for solutions sufficiently complex and nonstationary to represent irregular oscillations.
Large Scale Anisotropy of the Cosmic Microwave Background
Authors: Partridge R. B.
Published: June 2019

It is now generally accepted that the microwave background radiation, discovered in 1965 (Penzias and Wilson, 1965; Dicke et al., 1965), is cosmological in origin. Measurements of the spectrum of the radiation, discussed earlier in this volume by Blair, are consistent with the idea that the radiation is in fact a relic of a hot, dense, initial state of the Universe – the Big Bang. If the radiation is cosmological, measurements of both its spectrum and its angular distribution are capable of providing important – and remarkably precise – cosmological data.
Fine-Scale Anisotropy of the Cosmic Microwave Background Radiation
Authors: Boynton Paul E.
Published: June 2019

In keeping with the title of Symposium No. 63, I would like to confront the assembly with a short review of observational upper limits on the amplitude of intensity fluctuations in the microwave background radiation on angular scales of less than one degree. These fluctuations are presently of interest for at least two important cosmological concerns:
β Canis Majoris Stars in the (β, [c1]) Plane
Authors: Shobbrook R. R.
Published: June 2019

Recent publications have discussed the existence of stable B stars in the β CMa instability strip of the (β, Q) plane (Lesh and Aizenman, 1973) and the (log g, θe) and (Mbol, log Te) planes (Watson, 1972). The new lists of Strömgren uvbyβ photometry published by Crawford et al. (1970, 1971a, 1971b) for most of the B stars in the Bright Star Catalogue suggested that one might determine quite precisely the proportion of stable B stars (that is, stars not known as β CMa variables) in a luminosity/colour plot in the form of the β index against [c1] = c1 −0.2 (b – y).
Dynamical Models of Asymptotic-Giant-Branch Stars
Authors: Wood P. R.
Published: June 2019

The dynamical behaviour of the envelopes of four asymptotic-giant-branch stars of composition (X, Z)=(0.68, 0.02) and luminosity log L/L⊙ = 3.41, 3.60, 3.85 and 4.14 has been studied. At log L/L⊙ = 3.41, the envelope pulsates in the first-overtone mode and exhibits properties similar to those of the Mira variables. The more luminous models were all found to pulsate in the fundamental mode while simultaneously undergoing violent envelope relaxation oscillations. A significant amount of mass loss occurred from the model with luminosity log L/L⊙ = 3.60. It is suggested that a redgiant star undergoing envelope relaxation oscillations would be recognised as a symbiotic star. In the two most luminous models, a distinct outward-moving shell containing almost all the hydrogen-rich material in the star is formed. A suggestion is made that a further increase in luminosity would completely eject this shell to form a planetary nebula.
The Characteristics of the Field RR Lyrae Stars in the Magellanic Clouds
Authors: Graham J. A.
Published: June 2019

The Magellanic Clouds are well known as being very suitable for observing the various stages of stellar evolution. During the last few years, I have been studying the RR Lyrae variable stars in each of the two Clouds. Some first results were reported at IAU Colloquium No. 21 in 1972 (Graham, 1973). Here, I would like to update these results on the basis of more recent data and to comment on some of the characteristics of the field RR Lyrae stars in each system. Periods and light curves are now available for 63 RR Lyrae stars in a 1° x 1.3° field centered on the cluster NGC 1783 in the Large Magellanic Cloud (LMC) and for 62 stars in a 1° x 1.3° field centered on the cluster NGC 121 in the Small Magellanic Cloud (SMC). Both ab and c type variables are represented and, viewed individually, the Cloud RR Lyraes are identical in characteristics to those known in our Galaxy. Studied as groups, however, there are small but significant differences between the RR Lyrae stars in each system. The following four specific features seem to be emerging from the study.
Models for Nuclei of Planetary Nebulae and for Ultraviolet Stars
Authors: Katz J. , Malone R. , Salpeter E. E.
Published: June 2019

A series of stellar models were evolved, all with a total mass of 0.65 M⊙, an initial carbon-oxygen core of mass 0.60 M⊙, an intermediate helium mantle and an outer hydrogen-rich envelope with mass varying from case to case. Although the most hydrogen-rich cases resulted in red giants, cases with ≲ 0.01 M⊙ in the hydrogen envelope evolved at high surface temperature. The early stages of development of these models are similar to observed central stars of planetary nebulae. The later stages (when the nebula should be very thin optically) still have a high luminosity; the relevance to ‘ultraviolet stars’ will be discussed.
Non-Radial Pulsations of Dwarf Novae
Authors: Warner Brian
Published: June 2019

The high speed photometric observations of dwarf novae during outbursts that led to the discovery (Warner and Robinson, 1972) of rapid pulsations have been extended to other objects. Pulsations are usually seen near outburst maximum; an exception is VW Hyi which showed oscillations on December 25, 1972, when it was well down the declining part of its light curve.

The periods of the oscillations are found to vary; in most cases a rapid change from one discrete mode to another is observed. The dwarf nova stays in one oscillation mode for about an hour, and changes mode over a time of about 10 min. In the case of the VW Hyi observation, the period changed continuously from 28 s to 34 s over a time of 4 h, during which the apparent brightness decreased by 40%.

The existence of many closely spaced oscillation modes suggests that the dwarf novae are oscillating in non-radial modes. This is supported by observations of the 71-s oscillations of DQ Her: during eclipse a phase shift is seen indicating an l = 2, m = 2 mode of oscillation. In UX UMa, the 29-s oscillations show an opposite phase shift from that in DQ Her and indicate an l = 2, m = −2 mode.

A period-luminosity relationship can be deduced for the oscillations in dwarf novae, and this is in good agreement with recent calculations on quadrupole oscillations of hot white dwarfs made by Osaki and Hansen (1973).

Pulsations have so far been detected in the following objects: Z Cam (17 s), SY Cnc (24.6), CN Ori (25), KT Per (27), UX UMa (29), VW Hyi (28–34), CD −42° 14462 (29 and 30), AH Her (31).
Tests of the Binary Hypothesis of Novae Through Nova Nebulae Observations
Authors: Hutchings J. B.
Published: June 2019

I should like to follow up Dr Schatzman's talk with a few words on models of Nova nebulae. Geometrical models for novae Aql 1921 and Her 1934 have been derived by Mustel and Boyarchuk (1970), for novae Del 1967, Vul 1968 and Ser 1970 by Hutchings (1972), for Aql 1921 by Weaver (1974) and for Del 1967 by Malakpur (1973). Very different techniques and lines of reasoning have been used by all these people, but the models derived all have very strong similarities which should lead us to take them seriously in considering the nature of the nova mechanism.

It appears that nova remnants are characterized by two polar cones or blobs of similar but opposite velocity and similar density, moving along the axis of rotation of the system. In addition, one or more rings of matter may exist symmetrically about the equatorial plane, with a recession velocity and density which, in general, are different from the polar matter. The polar blobs appear to be associated with the matter giving rise to the ‘principal’ spectrum in the bright phases of the nova.

Three possible main causes have been suggested for the formation of such remnants, all connected with the initial outburst: (1) Weaver has proposed that an initially spherically symmetrical expansion is impeded and deflected by matter circulating in an accretion ring around the degenerate star. (2) Schatzman has suggested that the presence of the ring may favour an initial outburst preferentially in the polar regions. (3) Hutchings has suggested that physical interference of an initially spherical expansion may occur by a secondary component whose size is a large fraction of the separation of the two stars.

I feel that these points need to be considered by theoreticians in proposing detailed mechanisms of the nova outburst.
Instabilities of Magnetic Fields in Stars
Authors: Tayler R. J.
Published: June 2019

It has been shown (Markey and Tayler, 1973; Tayler, 1973; Wright, 1973) that a wide range of simple magnetic field configurations in stars are unstable. Although the ultimate effect of the instabilities is unclear, it seems likely that they would lead to enhanced destruction of magnetic flux, so that magnetic field decay would be much more rapid than previously supposed. Instability is almost certain in a non-rotating star containing either a purely toroidal field or a purely poloidal field, which has closed field lines inside the star. In both cases the instability resembles the well known instabilities of cylindrical and toroidal current channels, modified by the constraint that motion must be almost entirely along surfaces of constant gravitational potential.

If both toroidal and poloidal fields are present, the problem is more complicated. In a toroidal plasma with a helical field, the worst instabilities are also helical but it is impossible for a helical disturbance to be parallel to a surface of constant gravitational potential everywhere. As a result, the admixture of toroidal and poloidal fields has a stabilizing influence, but it is not at present clear whether the majority of such configurations are completely stable.

The effect of rotation has not yet been studied but it will certainly be important if the rotation period is less than the time taken for an Alfvén wave to cross the region of interest. This is true in most stars unless the internal magnetic field is very much stronger than any observed field.
Seismic Solar Models and the Neutrino Problem
Authors: Takata M. , Shibahashi H.
Published: June 2019

We determine the structure of the solar radiative zone with the imposition of the sound speed profile and the depth of the convection zone obtained from helioseismic analysis. We discuss the neutrino fluxes and capture rates using the resultant seismic solar model. We find that the seismic solar model cannot resolve the solar neutrino problem. The hydrogen and helium profiles of the Sun are obtained as a part of the solutions. We find that hydrogen is reduced in the core as expected in the theory of stellar evolution.
Helioseismology and the Solar Neutrino Problem
Authors: Antia H. M. , Chitre S. M.
Published: June 2019

The precisely measured frequencies of solar oscillations provide us with a unique tool to probe the solar interior with sufficient accuracy. These frequencies are principally determined by the dynamical quantities like sound speed, density or the adiabatic index of the solar material and a primary inversion of the observed frequencies yields the sound speed and density profiles inside the Sun (Gough et al. 1996). The equations of thermal equilibrium enable us to determine the temperature and chemical composition profiles, but for this additional prescriptions regarding the input physics (i.e., opacities, equation of state and nuclear energy generation rate) are required (Shibahashi 1993; Antia & Chitre 1995; Shibahashi & Takata 1996; Kosovichev 1996). This information in turn can be used to calculate the neutrino fluxes, and the seismic models can thus be used to explore the possibility of an astrophysical solution to the solar neutrino problem (Roxburgh 1996; Antia & Chitre 1997).
A Comparison Of Low-Degree Solar P-Mode Frequencies from Bison and Loi
Authors: Appourchaux T. , Chaplin W.J. , Elsworth Y. , Isaak G.R. , McLeod C.P. , Miller B.A. , New R.
Published: June 2019

Here, we compare the low-degree solar p-mode frequencies returned from the analysis of two, contemporaneous, independent helioseismological data sets collected during 1996. The first comprises Doppler velocity observations of the 770-nm line of potassium, made in integrated sunlight by the six-station, terrestrial Birmingham Solar-Oscillations Network (BiSON). The second consists of irradiance distribution measurements of the solar disc, made at 500 nm, by the Luminosity Oscillations Imager (LOI), which is part of the VIRGO experiment on the ESA/NASA SOHO satellite. The starting date is 27 March 96. The power spectra corresponds to an 8-month time series. Shorter time series of 4 months were also used to verify the starting dates and the temporal behaviour of the p modes.
Aplication of A New Observational Strategy to the Study of Gravitational Solar Modes
Authors: Mateos I. Martin , Pallé P.L.
Published: June 2019

The aim of the present work is the detection of solar g-modes, making use of their spatial and temporal properties, by means of a new observational strategy. The basic data, gathered at the Observatorio del Teide in 1993, consists on daily solar velocity measurements taken continuous and sequentially at six different and symmetric positions on the solar disk. By correlating the time series obtained from different positions, and considering the geometrical properties of different modes (l, m) on the Sun‘s surface, some of them can selectively be eliminated or enhanced. In particular, the main spectral features present in the resulting power spectra must have precise phase relations if they correspond to global solar g-modes.
Solar Irradiance Variations: Theory
Authors: Spruit H.C.
Published: June 2019

The following is a somewhat condensed version of discussions previously given elsewhere (Spruit, 1991, 1992). Some new developments not discussed there are presented in sections 4 and 5.

Since the observed irradiance variations are so clearly associated with manifestations of the solar magnetic field, I focus here on magnetic causes. Much of the physics of irradiance variations, however, is governed by the thermal response of the convective envelope and this response is similar for other possible causes of irradiance variations.
An Improved Calibration of the Mixing-Length Based on Simulations of Solar-Type Convection
Authors: Ludwig H.-G. , Freytag B. , Steffen M.
Published: June 2019

Based on detailed 2D numerical radiation hydrodynamics (RHD) calculations of time-dependent compressible convection, we have studied the dynamics and thermal structure of the convective surface layers of stars in the range of effective temperatures and gravities between 4500 K ≤ Teff ≤ 7100 K and 2.54 ≤ log g ≤ 4.74. Although our hydrodynamical models describe only the shallow, strongly superadiabatic layers at the top of the convective stellar envelope, they provide information about the value of the entropy s∗ of the deeper, adiabatically stratified regions. E.g. in the solar case the helioseismically measured entropy jump is predicted within 9% of its actual value.
Numerical Simulation of Penetrative Convection: A Parametric Study
Authors: Singh Harinder P. , Roxburgh Ian W. , Chan Kwing L.
Published: June 2019

In continuation of our earlier studies (Singh, Roxburgh & Chan 1997a,b; hereafter SRC97a,b) we perform some further tests to study the general behaviour of penetrative convection and its scaling with rms vertical velocity by varying a number of input parameters like the aspect ratio and the positioning of the interface between the unstable-lower stable layer.
Line Profiles and Rotational Splitting of Individual P-Modes
Authors: Gavryusev V.G. , Gavryuseva E.A.
Published: June 2019

We used GONG time series for ℓ = 0,1,2,3 from June 10, 1995 to January 7, 1997, 578 days in total. The duty cycle varies significantly during this period of time but on average it is sufficiently high, and the remaining gaps are irregular. Because of this we do not see daily side lobes in the spectra obtained.
Observation of Low-Degree Modes from SOHO/MDI Using Optimal Masks
Authors: Toutain T. , Kosovichev A. , Appourchaux T.
Published: June 2019

The overlap of peaks of split multiplets in oscillation power spectra and the leakage of other degree modes could significantly affect the measurements of the rotational frequency splitting. We have developed optimal masks which allow to isolate individual components of the multiplets. The method is applied to the Michelson Doppler Imager Low-l data. The results of the mode frequency measurements are reported.
The Internal Rotation Rate Inferred from Lowl and Gong Data: Using a Forward Method
Authors: Y Li. , Wilson P.R.
Published: June 2019

Recently the LOWL Group has made available 2-year averages of frequency splitting data obtained mainly for low values of the degree ℓ. Charbonneau et al. (1997) find a nearly flat rotational curve in the deep radiative core from these data using a Genetic Forward Modeling method. Subsequently, they test the assumption of latitude independence for r < 0.5 by performing a 1.5-D inversion, and find a rotation rate which is 25% greater at the pole than at the equator for 0.1 < r < 0.2.
New Developments in Asteroseismology
Authors: Kurtz D. W.
Published: June 2019

The day before the start of IAU Symposium 185 Wojtek Dziembowski and I were sitting in the beautiful 300-year-old garden of Shisendo in Kyoto discussing asteroseismology. Wojtek put his fingertips together to make a little temple of his hands in front of him. Then, slowly moving this temple up and down in front of him, while staring off at the green Japanese mosses in the dappled sun and shade of Shisendo‘s garden forest he said to me in his studied way, “Don, I am not sure that there is any asteroseismology yet”. I thought, “Uh-oh. If others believe that is true, then they may not think I have much to say in my review of new developments in asteroseismology at S185.” Yet I do have much to say, so I think I had better define what I mean by asteroseismology.
Asteroseismology in Action: Pulsating Hot White Dwarfs
Authors: Kawaler Steven D.
Published: June 2019

The development of helioseismology as a forefront area of solar and stellar astronomy has been fascinating to watch. Helioseismology takes full advantage of the benefits of lots of photons and a spatially resolved target. These allow measurement of tiny amplitude photospheric velocity and flux variations, and the precise identification of the oscillation modes corresponding to each observed frequency.
Asteroseismology with the Space Mission Corot
Authors: Baglin A. ,   The COROT Team
Published: June 2019

Seismology from the ground has already provided many important results, as discussed for instance in Frandsen (1997) and in this symposium. The limitations of this approach are essentially the day-night alternance and the meteorological instabilities for photometry, the lack of photons and the broadening of spectral lines due to rotation for spectroscopy. Seismology from space is less limited, as photometry at a very high accuracy on long uninterrupted observing runs is possible.
Magnetic Properties of Rapidly Oscillating Ap Stars
Authors: Mathys G. , Hubrig S.
Published: June 2019

Rapidly oscillating Ap stars generally pulsate in multiple modes, characterized by different frequencies. The amplitudes of these modes may furthermore be modulated with the rotation frequency of the star. For the two roAp stars whose magnetic fields have been sufficiently studied, the maximum pulsation amplitude coincides in phase with one of the extrema of the mean longitudinal magnetic field. Two interpretations of this property have been proposed: the oblique pulsator model, according to which the pulsation modes are aligned with the magnetic axis of the star, and the spotted pulsator model, which assumes that the pulsation modes are symmetric about the rotation axis of the star, and that pulsation amplitude modulation is due to the inhomogeneity of the stellar surface (which, itself, is related to the magnetic field geometry). At present, no definite choice between these two models can be made, though the oblique pulsator model is often preferred.
Seismic Detection of Boundaries of Stellar Convective Regions
Authors: Monteiro Mário J.P.F.G. , Christensen-Dalsgaard Jørgen , Thompson Michael J.
Published: June 2019

The edge of a convective region inside a star gives rise to a characteristic periodic signal in the frequencies of its global p-modes (e.g. [1], [4]), such that the frequencies ω are then essentially a smooth function of the mode order n plus a periodic component . Here the amplitude is , with A1 and A2 being values that depend weakly on frequency ω: A1 is always present in general, but A2 will be non-zero only if there is overshoot; is essentially the acoustical depth τ (i.e. the sound travel time) of the edge of the convection zone measured from the surface of the star; and Φ0 is a constant related to the phase of the eigenfunctions. To facilitate the comparison between different stars, we consider the amplitude evaluated at a fiducial frequency by defining . For the Sun, we chose as the reference frequency . If we take this value and scale it for other stars (using just a standard homology scaling for frequencies), we find .
Discovery of Non-Radial Pulsations in the White Dwarf Primary of a Cataclysmic Variable Star
Authors: Warner Brian , Van Zyl Liza
Published: June 2019

Non-radial pulsations in isolated white dwarfs have been known for 25 years and it has been shown that the hydrogen-rich (DA) white dwarfs have a high probability of pulsating if they lie in the instability strip with effective temperature between 11500 and 13200 K - the ZZ Ceti stars (e.g. Kepler and Nelan 1993). Analysing techniques developed for such stars allow derivation of masses, luminosities, rotation periods, hydrogen surface layer masses and other properties (e.g. Kepler and Bradley 1995). A number of binary systems are known in which the primary is a white dwarf; dominant in this class are the cataclysmic variable (CV) stars. Until now no CV primary has been found to have non-radial pulsations.
Phase Relations and Other Diagnostics of Solar Atmospheric Structure and Dynamics
Authors: Deubner Franz - Ludwig
Published: June 2019

Based on spectral observations with high spatial and temporal resolution, we discuss in this brief review the dynamic behaviour of the upper layers of the quiet solar atmosphere. A proper description of this behaviour is fundamental for the understanding (and simulation) of the temperature structure and the heating of the chromosphere, and the interpretation of such conspicuous phenomena like the chromospheric “grains”. The evidence is drawn from waveforms, power spectra, and phase spectra of velocity and brightness fluctuations in numerous Fraunhofer lines formed over an extended radial range of the atmosphere. A comparison with recent predictions of simulation calculations documents the strong need to extend the simulation codes to 3-D, and also to incorporate a realistic chromosphere/coronal transition layer, to better reproduce the observations.
High Frequency Signal in Golf Data
Authors: García R.A. , Pallé P.L. ,   The GOLF Team
Published: June 2019

The analysis of ≈ 460 days of high quality data provided by the GOLF experiment on board SOHO, has unambiguously revealed the presence of signal in the power spectrum in the region 5.8 to 7.5 mHz, well above the p-modes cut-off frequency. In this contribution, the observed structure of these full-disk observations is presented. High frequency peaks (hereafter HFPs) well above the acoustic cutoff frequency of the solar p modes (≈ 5.5 mHz) have been already observed in intermediate and high spatial resolution oscillation data (Duvall et al. 1991). The different interpretations of the observed pattern (Kumar 1994 and references therein; Rast and Gough 1995) agree that it provides direct information on the location and some characteristics of the source of the acoustic oscillations (see also Vorontsov et al. 1997).