bluuebunny/crossref_metadata_2025 huggingface dataset:
Search Methods for Globular Cluster Candidates in M31
Authors: C. Seguel Juan , Geisler Doug , Gyoon Lee Myung , Chul Kim Sang , Sarajedini Ata , E. Harris William
Published: June 2019

Searching for globular cluster candidates in nearby galaxies such as M31 is the first step to study the characteristics of extragalactic globular cluster systems. Previous searches for M31 globular clusters were mostly based on visual inspection of photograpic plates. We have selected globular cluster candidates from a wide-field Washington CCD survey of M31, using various methods: color-magnitude diagrams, color-color diagrams, point spread function subtraction and visual inspection of the objects. The efficiency and accuracy of these methods for finding globular clusters are presented.
Near-Infrared Imaging of the LMC Nebula N11B
Authors: Barbá Rodolfo H. , Rubio Monica
Published: June 2019

We present new near-infrared observations of the N11B nebula in the LMC. The presence of IR sources with intrinsic IR excess, a methanol maser and a very prominent PDR regions suggest strong star forming activity in N11B.
Ages and Metallicities of Globular Clusters in NGC 4472
Authors: Beasley M.A. , Sharples R.M. , Bridges T.J. , Hanes D.A. , Zepf S.E. , Ashman K.M. , Geisler D.
Published: June 2019

We have derived ages and metallicities from co-added spectra of 131 globular clusters (GCs) associated with the giant Virgo elliptical NGC 4472. ¿From their metal-line indices, we find that our sample of GCs span a metallicity range of approximately −1.6 ≤ [Fe/H] ≤ 0 dex. The SSP models of Worthey (1994; W94) predict that the metal-poor population of GCs has an age of 14.5 ± 4 Gyr and that the metal-rich population is 13.8 ± 6 Gyr old, whilst the models of Kurth, Fritze-v. Alvensleben & Fricke (1999; KFF99) predict ages of 6.0 ± 2 and 8.0 ± 5 respectively. We conclude that, within the uncertainties, the GCs are old and coeval, and that the bimodality seen in the broadband colours primarily reflects metallicity and not age differences.
Dynamics of Globular Cluster Systems in Elliptical Galaxies
Authors: J. Romanowsky Aaron
Published: June 2019

One of the most promising avenues for exploring the dynamics of the outer parts of elliptical galaxies involves using bright discrete objects as kinematical tracers: globular clusters and planetary nebulae. As large data sets are becoming available, rigorous dynamical analyses are needed to interpret them. To this end, we have developed an extension of orbit modeling methods, which we apply to the dynamics of the giant ellipticals M87 and M49. We find that a sample of ∼ 200 globular clusters velocities is effective at demonstrating the presence of a dark halo and providing hints about the detailed distribution of the dark matter.
Resolved RSG and Bright AGB Stars in NGC 1313
Authors: Pompei Emanuela , Brillant S. , Marconi G. , Vanzi L.
Published: June 2019

Red supergiants and bright AGB stars have been detected in NGC 1313, a galaxy outside of the Local Group, using NIR data obtained at ESO-NTT in La Silla Observatory. The derived color-magnitude diagram (CMD) compared with synthetic CMD strongly support the existence of an intermediate age stellar population.
Old and New Tools for Understanding the Evolution of Stars in Clusters
Authors: D'Antona Francesca
Published: June 2019

The uncertainties in the modelling of some important inputs of stellar evolution must be taken into account for a correct interpretation, both of the HR diagrams of individual stars, and of the integrated colors, of stellar clusters. After a short discussion of Globular Cluster ages, we focus on the problem of convection and discuss the parametrization of convection efficiency and overshooting.

Convective efficiency affects the reliability of the giants colors, especially for high metallicity: age - metallicity relations found from integrated colors of clusters must be regarded with caution.

Non-instantaneous mixing, both in the formal convective region and in the overshooting region must be taken into account: this is necessary to compute nucleosynthesis in ‘hot bottom burning’ envelopes, but it may also affect the color distribution in clump (core helium burning) stars, as shown here for the case of the LMC cluster NGC 1866.
On the Problem of the Mechanism of the Origin of Stars in Stellar Associations
Authors: Ambartsumian V. A.
Published: June 2019

Almost ten years have passed since the idea of stellar associations as nonstable stellar systems was formulated. The complex of observational data obtained during this time indicates that stars contained in the associations are young objects of some million years of age. We would like to stress here that this concerns both the O and T associations. It is also known that those O associations which could be sufficiently investigated in this respect, contain, as a rule, T Tauri type stars and are consequently T associations as well. There are, on the other hand, T associations which do not contain hot giants. But apparently the mechanisms of stellar formations must be similar in O and T associations. This means that any theory of stellar origin for a given type of association must permit variations, which will provide an explanation of the origin of stars in associations of other type.

Two hypotheses on the origin of stellar associations have been thus far discussed. One of them, suggested by the author at the initial stage of the idea about associations, supposes that each association has originated as a result of an expansion from a body or a system, the volume of which was initially very small. The dimensions of the latter was in any case less than one parsec. According to this point of view, these initial bodies (protostars) have either not been observed up to the present, or have not yet been identified with any known object. This point of view does not give any indication about a concrete mechanism of stellar origin, postponing its explanation to the time, when the earliest stages of the expansion of the association may be studied in detail.
Aerodynamic Dissipation
Authors: Petschek H. E.
Published: June 2019

Analyses of aerodynamic dissipation in ordinary un-ionized gases are all based upon the Navier-Stokes equations. These equations relate the rate of dissipation to the local gradients in velocity and temperature through the viscosity and heat conduction coefficients. Although it is true that in many flow situations the magnitude of the total dissipation in the gas does not depend on the magnitude of the viscosity coefficient, this coefficient does determine the minimum scale of variations observed in the gas and the form of the Navier-Stokes equations determines the type of phenomena which are observed on a small scale. In order to discuss dissipation in an ionized gas in the presence of a magnetic field, it is therefore necessary to re-examine the derivation of the basic flow equations. This paper attempts to do this for a case of a completely ionized gas and demonstrates that the basic microscopic dissipation mechanism is appreciably different. For example, it is shown that the minimum length in which the properties of the flow field can change noticeably is appreciably less than one mean free path.
On the Condensation of Interstellar Material
Authors: Schatzman E.
Published: June 2019

One seeks the condition of spontaneous collapse of a layer of interstellar matter by the method of marginal stability. The radiation field is included in this treatment of the gas-dynamic properties of the interstellar material. The optical model used is that of Ambartsumian's, which neglects interlocking.
On the Gravitational Instability of a Medium in Nonuniform Rotation
Authors: Bel N. , Schatzman E.
Published: June 2019

We examine here the problem of the gravitational instability of a medium in nonuniform rotation. We seek therefore to resolve a problem differing somewhat from that examined by Chandrasekhar and Fricke. We imagine a gaseous medium in rotation in a potential field, ɸ, and consider only the plane problem. In the unperturbed motion, the particles describe circular orbits in uniform motion. We examine the stability against perturbations.
Magnetic Field of a Contracting Protostar
Authors: Mestel L.
Published: June 2019

In a recent paper, Mestel and Spitzer discussed the problem of star formation from cool matter in the presence of a magnetic field of energy density comparable with the thermal energy density. If the field were frozen into the gas, the magnetic pressure would put a lower limit of order 103M⊙ to the mass that could be gravitationally bound, and this limit is unaltered by contraction of the cloud. However, in a lightly ionized gas, the field moves not with the gas as a whole, but with the plasma, and the motion of the plasma through the cloud is determined by a balance between mgnetic force and friction between neutral gas and plasma. A cloud containing sufficient dust can extinguish the galactic ionizing radiation; the plasma density decays quickly enough for the plasma and neutral gas to become uncoupled during the time of gravitational contraction. In this way high densities are built up without correspondingly high magnetic energy, and the cloud can break up into stars.
On the Turbulence in the Protoplanetary Cloud
Authors: Safronov V. S.
Published: June 2019

The problem of turbulence in the protoplanetary cloud is of importance for planetary cosmogony. Chaotic macroscopic motions probably existed in the cloud during its formation. Further evolution of the cloud depended to a great extent upon whether these original motions damped in a short time, or turbulence supported by some source of energy existed during planet formation. According to Kuiper and Fessenkov's hypotheses, massive protoplanets formed as a result of gravitational instability and turned into planets after the dissipation of light elements. Large-scale turbulent motions with mean velocities exceeding the thermal velocities of atoms and molecules would prevent, however, gravitational instability in the cloud, even if its mass was of the order of the mass of the sun. According to Edgeworth and to Gurevitch and Lebedinsky the planets grew gradually from small condensations formed in a flattened dust disk with a mass equal to that of the present planetary system. But even small scale turbulent motions would prevent extreme flattening of the disk necessary in this case for gravitational instability. The problem of turbulence is also connected with the problem of present distribution of angular momentum between the sun and planets, as large-scale turbulence produces redistribution of matter and of angular momentum in the cloud.
Kinematics of the Filaments in the Crab Nebula
Authors: Münch G.
Published: June 2019

The Crab Nebula, remnant of the supernova of 1054, may be observed today in the recombination spectrum of the filamentary component, or in the continuous spectrum of the central amorphous mass. The recent optical discovery of a high degree of polarization in the amorphous component has led to a striking confirmation and general acceptance of the hypothesis first advanced by Shklovsky, which considers the continuous spectrum as emitted by relativistic electrons moving in a magnetic field.
Confrontation of Lemaître Models and the Cosmological Constant with Observations
Authors: Petrosian Vahé
Published: June 2019

The history of the cosmological constant and the Lemaître models is reviewed briefly. Using recent cosmological observations, it is found that the cosmological constant if non-zero must be in absolute value less than 2 × 10-56 cm-2. The predictions of the Lemaître models are compared with modern observations. It is shown that Lemaître models without evolution fail to reproduce the observed radio source counts. The existence of quasars with large redshift (z > 2.5) is shown to be strong evidence against the Lemaître models.
Recent Progress in Linear and Non-Linear Calculations of Radial Stellar Pulsation
Authors: Cox Arthur N.
Published: June 2019

This review consists of a discussion about the agreement of non-linear calculations at very low amplitude with linear theory results, the recent computational advances in progress, the current problems especially those relating to observations, a review of current computations, and finally the presentation of some recent extensive non-linear calculations.
The Abundances and Gravities of the δ Scuti, AI Velorum and RR Lyrae Stars
Authors: Bessell M. S.
Published: June 2019

A comparative investigation of the temperatures, gravities and abundances of the AI Velorum stars and some selected δ Scuti stars and RR Lyrae stars has been carried out at Mount Stromlo during the past few years. Eleven AI Velorum stars, 6 δ Scuti and 6 RR Lyrae stars together with 6 established standard stars have been observed. The eleven AI Velorum stars comprise almost all those accessible from the Southern Hemisphere. The RR Lyrae stars were selected as examples of the weakest and strongest line stars of type a and c, and the 6 δ Scuti stars observed also represent the strongest and weakest line stars of the group.
On the Variability of Supergiants of Types B–G
Authors: Maeder A. , Rufener F.
Published: June 2019

The variability of the B–G supergiants is studied here with a set of 500 observations made for 76 stars. In addition to the discussion on the variability in colour indices (Maeder and Rufener, 1972), we also include the analysis of the apparent magnitudes. The observations, made randomly in time over several years, are selected from the new Catalogue (Rufener, 1974) of stars measured in the Geneva Observatory system. Some results on the luminosity dependence of the variations are presented in Table I.
Red Variables of the Old Disc and Halo Populations
Authors: Feast M. W.
Published: June 2019

This paper discusses the following topics: (1) Mira Variables in Globular Clusters; (2) infrared observations of Globular Cluster Variables; (3) carbon Stars in Magellanic Cloud Clusters; (4) the Se Variables and the superlithium rich Variables.
On Some Peculiar Features in the Sequences of the Old Open Star Clusters
Authors: Maeder A.
Published: June 2019

In spite of the rather good agreement between the theory of stellar evolution and the observations, there exist some difficulties when one compares closely the sequences of open star clusters and the theoretical isochrones. Several, if not all, of the old open star clusters seem to be concerned, especially those which are accurately measured, namely Praesepe, NGC 2360, 752, 3680 and M67. The problem concerns the gap occuring in the HR diagram at the end of the phase of hydrogen burning in the core; it corresponds to the phase of hydrogen exhaustion (or of overall contraction). The sequence of M67 has been studied by Racine (1971) and Torres-Peimbert (1971). The well apparent gap is located farther from the zero-age main sequence than indicated by the models and the hook towards a larger Teff predicted during this phase is not observed. Differences in chemical composition may not be held responsible for these anomalies. From Torres-Peimbert's models, it may be assumed that neither solar type, nor super metal rich composition are able to reduce the discrepancies. As a further illustration, let us mention the case of NGC 752. In Table I, the main features related to the gap are examined: the disagreement, like in M67, essentially concern features 1 and 2. The observations are based on a recent study of Grenon and Mermillod (1973) and on Bell's data (1972). Bell has also mentioned the existence of discrepancies. As in M67, the gap is too far from the zero-age main sequence and does not present any sudden turning towards a larger Teff.
Non-Radial Oscillations
Authors: Ledoux P.
Published: June 2019

The problem of the adiabatic non-radial oscillations of spherical stars is reviewed and results recalled for a variety of models. The anomalous behaviour of the eigenfunctions for highly condensed models is related to the apparent mobile singularities depending on the eigenvalues. Tables of Q-values are provided to facilitate possible applications to variable stars.

In the case of the gravity modes, the existence of multiple spectra some stable (g+ modes) some unstable (g− modes) if superadiabatic and subadiabatic regions alternate is discussed and the interest of further investigations underlined.

As far as vibrational stability (effects of the non adiabatic terms) is concerned, a general expression is given for the ‘damping coefficient’. The attention is drawn to the possibility for g+ modes of becoming vibrationally unstable under the effect of various factors and in various models, including the Sun where this was advocated as a possibility of relieving the neutrinos difficulty.

Finally the present status of the most obvious candidates among variable stars for non-radial oscillations, the β Canis Majoris stars and the rapid blue variables (white dwarfs) is briefly reviewed.
The Sun as a Pulsating Rotating Star
Authors: Schatten Kenneth H.
Published: June 2019

Physical arguments are provided which suggest the following:

(a) The Sun rotates rapidly internally, with a period near one day. The arguments are based upon a low ‘effective’ plasma dynamic viscosity associated with a negative ‘effective’ magnetic density, ϱM = −B2/4πν2. This low (near zero) viscosity allows several calculations of the angular velocity of the solar core to be made. A fluid dynamical argument based upon the inviscous Navier-Stokes relation shows that for objects seated in a non-expanding magnetohydrodynamic fluid, the usual Kepler law should be replaced by T2 ∝ r4.

(b) The rapid rotation suggests that the solar magnetic field is deeply buried and, furthermore, that the Sun violates the Ferraro theorem. This violation results from a radial electric field necessary to support the solar plasma. This radial field requires the Sun to be charged with a charge q = + 2 × 1011 esu. Thus the Sun's differential rotation may be viewed as arising from B']θ=0 in the rest frame of the fluid where Er ≠ 0 or alternatively as a polar spin down by the solar wind flow. Thús in the fluid frame, the solar activity cycle may be viewed as an Alfvén wave, with T= Ω/vA = 20 yr. The velocity of material along the field is such that (G−1 cm−2 s−1), governed by the collapse of mass in the solar core; (4H + 4e → 1He + + 2e) results in a reduction of gas pressure unless ~7 × 1014 gm s−1 of material continually collapse so as to conserve the particle number in the solar interior.

(c) This requires the Sun to emit gravity waves with an energy comparable to its luminosity. The virial of the nuclear reactions in the core governs whether the energy goes into gravity waves or heat. A variable positron-electron core allows this.
Data for Probing the Sun
Authors: Elsworth Yvonne
Published: June 2019

The observations of solar oscillations provide an unrivalled, precise way of probing the solar interior. In this paper, I consider the observations and their interpretation in terms of the physics of the Sun. The oscillations that we are concerned with here are the so-called p modes, i.e. oscillations for which pressure is the restoring force. The modes for which gravity is the restoring force have yet to be unambiguously detected on the Sun. The observations are made either as Doppler velocity or as intensity and are, in general, very small effects. To get an impression of the precision required, consider that in integrated velocity the total signal is ~ 1 m s−1 with the strongest individual modes being about 15-20 cm s−1. The weakest, detected modes are of order a few mm s−1. When this signal is measured as a Doppler shift, v/c is a few parts in 1011. The observations are made by a variety of instruments on Earth or in Space which can be simply divided into those which observe the Sun as a star and those which image the solar surface into many pixels Although there are many different observers using many different techniques, in all cases one is analysing light emitted from a region relatively high in the atmosphere of the Sun. When one considers how these measurements can be interpreted in terms of the solar oscillations, two issues arise:

1.

Roughly where in the solar atmosphere are the lines formed?

2.

How different are the heights of formation for different lines?
LOI/SOHO Constraints on Oblique Rotation of the Solar Core
Authors: Gizon L. , Appourchaux T. , Gough D.O.
Published: June 2019

The axis of rotation of the Sun's surface is inclined from the normal to the ecliptic by 7°.25. Is that true also of the rotation of the rest of the Sun? Knowledge of the direction of the angular momentum is pertinent to studies of the formation of the solar system. Moreover, Bai and Sturrock (1993) have recently interpreted temporal variations in the spatial distribution of solar flares as the outcome of the interaction of the Sun's envelope with an obliquely rotating core. We report here an attempt to determine the principal seismic axes of oscillation of the dipole and quadrupole p modes from LOI data obtained as a component of the VIRGO investigation on the spacecraft SOHO. We find that formally their most likely orientation is somewhat closer to being normal to the ecliptic than is the axis of the surface rotation. However, the uncertainty in the determination well encompasses the possibility of them being parallel to the surface rotation axis, yet it does not reject (at a level marginally greater than one standard deviation) the possibility that the Sun's angular momentum is parallel to that of the rest of the solar system.
Autoregressive Modeling of Golf Velocity Time Series
Authors: Ulrich R. , Varadi F. , Bertello L. ,   The GOLF Team
Published: June 2019

The frequencies of solar p-modes are usually computed by fitting Lorentzian profiles to the observed signals’ Fourier spectra. We offer a new technique to estimate mode parameters: direct modeling by autoregressive stochastic processes, in conjunction with Singular Spectrum Analysis.
The Convection Zone and Oscillations
Authors: Baturin V.A. , Mironova I.V. , Ayukov S.V.
Published: June 2019

We study the influence of the structure and physics of the uppermost superadiabatic layers on the oscillation spectrum of p-modes. These modes are trapped inside the Sun by reflecting at the upper layers of the Sun, so these layers are important for the oscillation spectrum. Indeed, the main difference between the computed and observed frequencies seems to come from these layers (Christensen-Dalsgaard, 1985; Christensen-Dalsgaard and Thompson, 1997).
Observations of the Latitudinal Variation of the Solar Radiance of Non Active Regions of the Sun
Authors: Domingo V. , Sanchez L. , Appourchaux T.
Published: June 2019

Kuhn et al. (1988) have found that there are variations in the photospheric temperature with the solar cycle that depend on solar latitude. This would be an independent mechanism, other than the effects of sunspots and faculae, contributing to the change in solar irradiance (Kuhn 1991). It is important to pursue this investigation as such variations would be related to the transport of energy through the convection zone, and thus give a good indication of its structure and evolution.
Inversion Methods
Authors: Thompson M.J.
Published: June 2019

I want to start by addressing the question, ‘What is inversion?’ My answer would be that inversion is the process of going from data to making inferences about the object under study. In the case of helioseismology, the data at the present time are principally the mode frequencies, and the object under study is the solar interior.
Helioseismic Constraints on the Solar Structure: Evidences of Element Segregation and Mild Mixing Below the Convection Zone
Authors: Vauclair S.
Published: June 2019

The study of the internal structure of the Sun entered a new age with helioseismology. Several ground based networks, as well as the space mission SoHO, continuously observe the solar oscillations. In particular GONG, the “Global Oscillation Network Project”, gathers six sites around the world with six identical Doppler instruments. These instruments observe the phase shift of the Ni 676.8nm line with 3 images every minute, 1.8 sites observing simultaneously. Millions of solar p-modes have been detected. The inversion of the measured frequencies yields accurate and detailed information about the sound velocity in the Sun's interior, which in turn leads to constraints on the equation of state, opacities, chemical composition. Precise informations on the differential rotation inside the Sun have also been obtained. (See the special Science issue on GONG, vol 272, 31 May 1996).
Solar Rotation and Large-Scale Flows Measured by Time-Distance Helioseismology from Mdi
Authors: Giles P.M. , Duvall T.L.
Published: June 2019

The technique of time-distance helioseismology provides a new tool for examining large-scale flows beneath the sun's surface. We have used this technique to study the meridional flow and have found that the poleward flow observed at the surface appears to persist to a depth of at least 26,000 km. We have also examined the differential rotation in these uppermost layers, and find a slight asymmetry between the northern and southern hemispheres.
Spherical and Aspherical Structure of the Sun: First Year of Soho/Mdi Observations
Authors: Kosovichev A.G. , Nigam R. , Scherrer P.H. , Schou J. , Christensen-Dalsgaard J. , Dziembowski W.A. , Goode P.H. , Gough D.O. , Reiter J. , Rhodes E.J.
Published: June 2019

We report the initial results of one year of continuous observations of the Sun‘s internal structure from the Michelson Doppler Imager (MDI) on board SOHO. The results have been obtained by inverting frequencies of p and f modes determined with two different methods of averaging over split multiplets. Small systematic differences between the two frequency sets depend primarily on mode frequencies, and, thus, did not significantly affect the inversions. A preliminary study of the systematic effects resulting from asymmetry of oscillation power peaks has also shown no significant influence on the inversion results. The inferred sound-speed profile is in general agreement with the previous data from MDI and ground-based networks. In the energy-generating core, the resolution is substantially improved, and the inversion results indicate a sharp negative perturbation of the sound speed in the core, tending to a positive value near the center. High-precision measurements of the f-mode frequencies have been used to determine the seismic radius of the Sun. The global asphericity estimated from frequency variation across the split mode multiplets has been found to be small, and is consistent with the asphericity during the previous activity minimum. Variations of the solar frequencies during the first year of MDI observations have also been detected.
Loi and Gong Low-Degree Rotational Splittings
Authors: Appourchaux T. , Rabello-Soares M.C. , Gizon L.
Published: June 2019

Two different data sets have been used to derive low-degree rotational splittings. One data set comes from the Luminosity Oscillations Imager of VIRGO on board SOHO; the observation starts on 27 March 96 and ends on 26 March 97, and are made of intensity time series of 12 pixels (Appourchaux et al, 1997, Sol. Phys., 170, 27). The other data set was kindly made available by the GONG project; the observation starts on 26 August 1995 and ends on 21 August 1996, and are made of complex Fourier spectra of velocity time series for l = 0 − 9. For the GONG data, the contamination of l = 1 from the spatial aliases of l = 6 and l = 9 required some cleaning. To achieve this, we applied the inverse of the leakage matrix of l = 1, 6 and 9 to the original Fourier spectra of the same degrees; cleaning of all 3 degrees was achieved simultaneously (Appourchaux and Gizon, 1997, these proceedings).
Low-Degree P-Mode Solar Cycle Trends from 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).

In order to investigate the solar-cycle dependence of the low-degree p-mode frequencies, we have analysed eighteen 4-month frequency spectra generated from BiSON Doppler velocity residuals collected between 1991 January 01 and 1997 January 05. These data cover the falling phase of solar activity cycle 22. up to the cycle 22/23 boundary. Fig. 1(a) shows frequency shifts, averaged over two orders in n. up to ~ 3900 μHz - as derived from the analysis of the 4-month spectra - normalised to unit change in the 10.7-cm radio flux. The dashed line lying above the BiSON data is a fit to the BBSO 1989-minus-1986 frequency shifts, for 4 ≤ ℓ ≤ 140 (Libbrecht k Woodard. 1991). The dotted line passing through the data corresponds to the bestscaled fit of the BBSO data to the BiSON data - the best-fit requires the BBSO data to be scaled by 0.71 ± 0.03. This is reasonably consistent - as expected - with the mean, overall ratio of the inverse mode masses of those data used in the BiSON and BBSO analyses (≈ 0.67). Fig. 1(b) shows the BiSON activity-normalised frequency shifts, plotted as a function of inverse mode mass - here, as anticipated, there is a clear correlation between the variables.
Exploratory Simulation of Solar Granules: How Sharp is the Convection/Radiation Transition?
Authors: Chan Kwing L. , Kim Y.C.
Published: June 2019

Currently, the most successful direct simulation of the solar granules (and the convection/radiation transition layer) is the three-dimensional (3D) model computed by Stein and Nordlund (1989). So far, there is no other similar 3D models available for comparison [however, see Ludwig et al. (1997) for a recent 2D calculation]. We are developing an alternative numerical approach to simulate the 3D radiation hydrodynamics of this layer. In this approach, the Eddington approximation is used to handle the radiation rather than solving the radiative transfer equations along rays, and the ADISM method (Chan and Wolff 1982) which solves the Navier Stokes equations in conservative forms is used to speed up the thermal relaxation of the fluid layer. We are in the process of testing the numerical accuracy of the codes. This paper summarizes the results of a test that illustrate the effects of vertical space resolution on the mean profiles of some important quantities.
Acoustic Imaging and Subsurface Absorption Structure of Sunspots
Authors: Chang H.-K. , Chou D.-Y. , Labonte B.
Published: June 2019

Recently, Chang et al. (1997, hereafter Paper I) developed a new method, based on time-distance curves (Duvall et al. 1993), to construct a three-dimensional acoustic intensity image of the solar interior. Here we report results from another data set.

Solar p-mode waves, which are continually generated and dissipated stochastically by the turbulent convection, are scattered and absorbed by local inhomogeneities. If we appropriately add p-mode waves observed at the surface, based on the knowledge of how the waves propagate (time-distance curves), such that acoustic signals emanating from a particular point at a particular time are collected in phase, we can reconstruct the p-mode amplitude at the target point at the target time. The scheme of adding signals from a target point in phase plays the same role as a lens in optics. Thus we call it the “computational acoustic lens” (Paper I). This imaging technique can be used to construct acoustic amplitudes at any point in the solar interior based on the time-distance curve between that depth and the surface, which can be computed from a standard solar model based on the ray theory. The time series of constructed a plitudes provides information of both the intensity and the phase of the wavetrain. The intensity can be used to study p-mode absorption regions, and the phase can be used to probe other properties of local inhomogeneities, such as flow and change in wave speed.
Theoretical Aspects of Asteroseismology
Authors: Christensen-Dalsgaard J.
Published: June 2019

Asteroseismology depends on the ability to determine frequencies for identified modes of stellar oscillation, and to use these frequencies to probe stellar interiors. Thus crucial aspects are the dependence of the frequencies on stellar properties, and the likelihood that modes are excited to observable amplitudes. I provide a brief overview of these issues and some remarks about how the frequencies may be analyzed, as a background for the more detailed presentations elsewhere in the proceedings.
roAp Stars through Theorists' Eyes - Excitation Mechanism
Authors: Gautschy Alfred , Saio Hideyuki
Published: June 2019

This exposition focuses on the still elusive excitation mechanism of the rapid oscillations in some Ap stars. We visit the domain of overstable magnetic convection which was conjectured as the origin of the observed high radial-order p modes. Mostly we dwell, however, on the classical k-mechanism. In roAp stars, the zone of partial hydrogen ionization is driving rather effectively. Furthermore, we argue that an atmospheric temperature inversion supports the κ-mechanism driving by preventing oscillation energy from leaking into the atmosphere.
A Search for Solar-Like Oscillations In α Cen A
Authors: Bedding T.R. , Kjeldsen H. , Frandsen S. , Dall T.H.
Published: June 2019

We have been using a new method to search for solar-like oscillations that involves measuring temperature changes via their effect on the equivalent widths of the Balmer hydrogen lines. We observed α Cen A over six nights in 1995 with the 3.9-metre Anglo-Australian Telescope and the European Southern Observatory's 3.6-metre telescope in Chile. Oscillations were not detected, with an upper limit only slightly higher than the expected signal.
Slowly Pulsating B Stars: New Insights from Hipparcos
Authors: Aerts C. , De Cat P. , Waelkens C.
Published: June 2019

The photometric experiment on board of Hipparcos has discovered, among other types of variables, a large amount of new Slowly Pulsating B Stars. We have selected the fourteen brightest stars of this sample, together with five previously known Slowly Pulsating B Stars, for long-term spectroscopic and photometric monitoring. The selected stars have spectral types ranging from B 2 up to B 9 and are thus nicely spread across the instability strip. We here present the results of a preliminary analysis of our data and point out that our sample is unique in the sense that it allows us to perform seismology of massive early-type stars.
Bisector Velocities of Hα in the roAp Star α Cir: Probing the Principal Pulsation Mode
Authors: Baldry I. K. , Bedding T. R. , Viskum M. , Kjeldsen H. , Frandsen S.
Published: June 2019

α Circini is the brightest of the known rapidly oscillating Ap (roAp) stars. Previous observations of this star in photometry (Kurtz et al. 1994) have shown that it has one dominant pulsation mode, which is a pure oblique dipole mode (ℓ = 1) with a frequency of 2442 μHz (P = 6.825 min). Kurtz et al. (1994) measured the amplitude of the principal mode to be 2.55 mmag (Strömgren v).
B Star Pulsation - Theory and Seismological Prospects
Authors: Dziembowski W.A.
Published: June 2019

Progress in understanding B-type pulsators has been reviewed at several recent meetings (e.g. Dziembowski, 1993, 1995; Moskalik, 1995). Not much happened afterwards. Owing to further improvement in the stellar opacity calculations (Iglesias and Rogers 1996), the theoretical pulsation-instability domain in the upper Main Sequence is now more precisely determined. In the next section I will compare the predicted domain with the positions of various B-type pulsators in the H-R diagram.
Spectral Line-Profile Variability as a Probe for ℓ and m
Authors: Schrijvers C. , Telting J.H.
Published: June 2019

We investigate the observable spectroscopic characteristics of non-radial pulsation for stars with rotation rates large enough to resolve the stellar surface by Doppler imaging. We show that the intensity variations in time series of theoretical spectra, at each position in the line profile, cannot be described by a single sinusoid: at least one harmonic sinusoid needs to be included to describe the data. Across the line profile the relative amplitudes and phases of both these sinusoids vary independently.
Oscillations of HB Red Variable Stars
Authors: Xiong D.R. , Cheng Q. L. , Deng L.
Published: June 2019

Using a nonlocal time-dependent theory of convection, we have calculated the linear non-adiabatic oscillations of the Horizontal Branch (HB) stars, with both the dynamic and thermodynamic coupling between convection and oscillations been carefully treated. Turbulent pressure and turbulent viscosity have been included consistently in our equations of non-adiabatic pulsation. When the coupling between convection and oscillations is ignored, for all models with Te ≤ 7350K, the fundamental through the second overtone are pulsationally unstable; while for Te ≤ 6200K all the models are unstable up to (at least) the 9th overtone. When the coupling between convection and oscillations is included, the RR Lyrae instability strip is very well predicted. Within the strip most models are pulsationally unstable only for the fundamental and the first few overtones. Turbulent viscosity is an important damping mechanism. Being exclusively distinct from the luminous red variables (long period variables), the HB stars to the right of the RR Lyrae strip are pulsationally stable for the fundamental and low-order overtones, but become unstable for some of the high-order overtones. This may provide a valuable clue for the short period, low amplitude red variables found outside the red edge of the RR Lyrae strip on the H-R diagram of globular clusters. Moreover, we present a new radiation modulated excitation mechanism functioning in radiation flux gradient regions. The effects of nonlocal convection and the dynamic coupling between convection and oscillations are discussed. The spatial oscillations of the thermal variables in the pulsational calculations have been effectively suppressed.
The Influence of A Magnetic Field on Radiative Damping of Magnetoatmospheric Oscillations
Authors: Banerjee Dipankar , Hasan S.S. , Christensen-Dalsgaard J.
Published: June 2019

We investigate the influence of a magnetic field on the radiative damping of magnetoatmospheric waves, extending our previous work on the adiabatic modes of an isothermal stratified atmosphere with a uniform vertical magnetic field. Banerjee, Hasan & Christensen-Dalsgaard (1996, 1997) generalized this work to include radiative effects using Newton's law of cooling for a weak magnetic field. The present study examines the variation of the mode damping rate with increasing magnetic field strength. We find that a moderate field suppresses radiative damping.
A Spatial Analysis of Local Sources of Oscillation
Authors: Yan Y. , Deubner F.-L. , Kleineisel R.
Published: June 2019

Recent observational studies of the spatial and temporal relations between impulsive and periodic perturbations (Deubner and Kleineisel, 1997; Espagnet et al., 1996; see also Goode et al., these proceedings) suggest that oscillations in the solar atmosphere are excited locally and stochastically by turbulent convection. Proper characterization of the dynamical processes involved requires careful analysis of the observations in the spatial as well as in the temporal domain. Wavelet transforms (Combes et al., 1984) allow the characterization of waveforms together with the temporal delays that occur between the observed fluctuations. We have employed Morlet wavelets (Daubechies, 1992) in the present analysis. In addition to the phase relation between convective eddies and the atmospheric waves we study in particular the 2-D spatial evolution of the perturbations, to better distinguish different modes of oscillation. The observational material used is the same as in Deubner and Kleineisel (1997).
Synthesis of Solar-Stellar Seismology — Meeting Summary
Authors: Chitre S. M.
Published: June 2019

The Sun has generally been described as the Rosetta Stone of astronomy. This label is especially apt because our star is sufficiently close to Earth for a detailed scrutiny of its surface, a luxury that is not readily available for distant stars. It is our fond hope that the interior of the Sun and stars can be used as an astrophysical laboratory for testing theories of atomic and nuclear physics, high-temperature plasma physics, magnetohydrodynamics and neutrino physics.
The Structure of Cold Molecular Cloud Cores
Authors: Ward-Thompson D. , Nutter D. J. , Kirk J. M. , André P.
Published: June 2019

A brief summary is presented of our current knowledge of the structure of cold molecular cloud cores that do not contain protostars, sometimes known as starless cores. The most centrally condensed starless cores are known as pre-stellar cores. These cores probably represent observationally the initial conditions for protostellar collapse that must be input into all models of star formation. The current debate over the nature of core density profiles is summarised. A cautionary note is sounded over the use of such profiles to ascertain the equilibrium status of cores. The magnetic field structure of pre-stellar cores is also briefly discussed.
High-Resolution Mid-Infrared Observations of High Mass Protostellar Objects
Authors: De Buizer James M.
Published: June 2019

This article presents a review of all high-resolution ground-based mid-infrared observations of high mass protostellar objects (HMPOs) to date. An introduction to the characteristics of HMPOs and their relationship to the hot molecular core phase of massive star formation is outlined. I explain why observations in the mid-infrared of these types of sources are important, and in particular why high-resolution observations are needed. A summary is given of the earliest mid-infrared detections of HMPOs, followed by a detailed account of the most recent observations.
Infrared Studies of Protostellar Binaries
Authors: Greene Tom
Published: June 2019

Although much progress has been made in studying the multiplicity properties of pre-main-sequence T Tauri stars over the past decade, younger and more embedded protostellar systems are just now being surveyed for multiplicity. I examine several recent infrared imaging and spectroscopic studies which reveal the multiplicity properties of embedded protostars for the first time. The overall binary / multiplicity fraction of protostars is statistically indistinguishable from that of pre-main-sequence stars when measured over similar separation distances. Some individual components of protostellar binaries appear to be coeval from their spectral energy distributions or spectra. However, there are some systems which appear to consist of both a protostar and a pre-main-sequence star, and the coevality of these systems is very uncertain. There is much yet to be learned about the multiplicity properties of protostars, and future high resolution near-IR imaging surveys will likely make much headway.
X-Rays From Class 0/I Protostars
Authors: Tsuboi Yohko
Published: June 2019

In the last decade, our interest moved significantly to the youngest stellar phase, “protostars”. Evidence for infalls and outflows are implying violent star-forming activities close vicinity of the protostar itself. However, due to the very high extinction in optical and near-infrared band or even in soft X-ray band, we had no tool to access the star itself. In such situation, Chandra X-ray Observatory, with the high transparency in the hard X-ray band and the superior spatial resolution, are beginning to reveal X-ray emitting phenomena deep inside the protostellar cores. In this paper, the high energy activities in Class I and 0 sources detected with Chandra is reviewed.
Early Results from the SMA
Authors: Zhang Qizhou
Published: June 2019

The Submillimeter Array (SMA), a collaborative project of the Smithsonian Astrophysical Observatory (SAO) and the Academia Sinica Institute of Astronomy and Astrophysics of Taiwan (ASIAA), is approaching the end of the construction phase. In this paper, I review its current capability and describe the scientific programs carried out in recent months. I will also present some preliminary results obtained with the SMA in the area of star formation.
Studying Star Formation with the Keck Interferometer
Authors: Akeson Rachel
Published: June 2019

The Keck Interferometer utilizes the two 10-meter Keck telescopes as a direct detection interferometer in the infrared. The 85-meter baseline produces a fringe spacing of 5 milliarcseconds at 2 microns, or 0.7 AU for sources in the Taurus star forming region. The first observations with the Keck Interferometer of T Tauri and Herbig Ae/Be sources are presented and the implications for circumstellar disk models are discussed.
From Infall to Rotation around Young Stars: The Origin of Protoplanetary Disks
Authors: Hogerheijde Michiel R.
Published: June 2019

The origin of disks surrounding young stars has direct implications for our understanding of the formation of planetary systems. In the interstellar clouds from which star form, angular momentum is regulated by magnetic fields, preventing the spin up of contracting cores. When ∼ 0.03 pc-sized dense cores decouple from the magnetic field and collapse dynamically, ∼ 10−3 km s−1 pc of specific angular momentum is locked into the system. A viscous accretion disk is one of two possible mechanisms available for the necessary redistribution of angular momentum; the other one is the formation of a multiple stellar system. Recent observational results involving high-angular resolution observations are reviewed: the presence of disks deep inside collapsing envelopes; an accretion shock surrounding a disk; the velocity field in collapsing and slowly rotating envelopes; a possible transitional object, characterized as a large, contracting disk; and the velocity field in disks around T Tauri stars. Observational facilities becoming available over the next several years promise to offer significant progress in the study of the origin of protoplanetary disks.
Early Disk Evolution
Authors: Wardle Mark
Published: June 2019

A variety of processes play a role in the evolution of protostellar disks. Here I focus on the uncertain issue of magnetic field-disk coupling and its implications for magnetically-driven turbulence and disk-driven winds. At present it is clear that the magnetic field plays a crucial role in disk evolution, but detailed conclusions cannot be drawn because the complicated interplay between dynamics and the evolution of the grain population remains to be explored.
A Search for Disks around Massive Young Stellar Objects
Authors: Gibb A. G. , Hoare M. G. , Mundy L. G. , Wyrowski F.
Published: June 2019

We present subarcsecond observations at 2.7 and 1.4 mm of a sample of massive young stellar objects made with the BIMA millimetre array. For most sources the continuum emission on the smallest scales at 2.7 mm is dominated by free-free emission from the stellar wind or jet. Strong emission at 1.4 mm shows the presence of significant dust associated with Cep A and GL 490 but our resolution is not sufficient to resolve any structure. The 2.7-mm emission from GL 490 is resolved but it is not clear whether we are seeing a single circumstellar disk or a secondary companion, although near-infrared data support the disk hypothesis. Estimates of the dust mass yield values of ∼1–4 M⊙ within radii of 150 to 1000 AU.
Galactic rotation and velocity fields
Authors: Shuter W.L.H. , Gill A.
Published: June 2019

Using unpublished 21-cm data from Jackson and Kerr of the entire Galaxy at b = 0°, we have determined the terminal velocities, vm, to be (Gill & Shuter, 1983)