bluuebunny/crossref_metadata_2025 huggingface dataset:
Asteroseismology: The Impact of Solar Space Observations
Authors: Hudson H.S.
Published: July 2016

Observations from space relevant to solar global properties (oscillations, magnetic activity, etc.) are helpful both scientifically and technically in preparing for stellar observations. This paper summarizes the results from the main previous experiments (ACRIM, SOUP, and IPHIR), and also give an initial technical report from the SXT instrument on board Yohkoh, launched in August 1991. The solar observations to date demonstrate the existence of several mechanisms for low–level variability: spots, faculae, the photospheric network, granulation, and p–mode oscillations. The observations of oscillations have been particularly helpful in setting limits on solar interior rotation. In addition to the solar processes, stars of other types may have different mechanisms of variability. These may include the analogues of coronal holes or solar flares, modes of oscillation not detected in the Sun, collisions with small bodies, duplicity, and probably mechanisms not invented yet but related in interesting ways to stellar convection and magnetism.
Two-Dimensional Helioseismic Inversions
Authors: Schou J. , Christensen-Dalsgaard J. , Thompson M.J.
Published: July 2016

We present results of investigations of different inversion methods for inferring from helioseismic observations the rotation rate in the solar interior as a function of radius and latitude, using a mode set similar to that which is expected from the GONG network.

The rotation of a star perturbs the frequency wnlm of a normal mode by

where n, l and m are the mode ‘quantum’ numbers, θ the colatitude, Ω the rotation rate and Knlm a known function. Until now the observations have not been in terms of individual Δwnlm ’s; rather, coefficients ai(n,l) of a parametrization of the m-dependence of Δwnlm have been given. By representing the latitudinal dependence of the rotation rate in terms of an expansion in cos θ, an inversion of the ai, can be carried out as a series of 1-dimensional inversions. Such inversions have been dubbed “1.5-dimensional”. Our present implementation of a 1.5D procedure follows Schou et al. (1992), and uses a1, a3 and a5.
Generation of Strong Toroidal Magnetic Field Near the Bottom of the Solar Convective Zone
Authors: Krivodubskij V.N.
Published: July 2016

The generation mechanism of the toroidal magnetic field by the angular velocity radial gradient acting on the relict poloidal magnetic field on the boundary between the con-vective and radiative zones is proposed. The magnetic induction magnitude of the toroidal field reaches about 2×10σG, the limiting effect of the magnetic buoyancy being taking into account. This value conforms to the estimation of toroidal field obtained from helioseismological data.
Is Superficial Knowledge about Stellar Surfaces Sufficient?
Authors: Gustafsson Bengt
Published: July 2016

The significance of a detailed knowledge of the atmosphere of a star for a proper understanding of its interior is illustrated and discussed. It is concluded that significant progress in the understanding of stellar atmospheres is needed in order to match methods, discoveries and needs in the study of stellar internal structure and evolution.
Temperature, Abundance and Magnetic Mapping of Stellar Atmospheres
Authors: Donati J.-F.
Published: July 2016

In this paper, I review recent progress in the domain of indirect imaging of stellar atmospheres. Since all stars other than the Sun are too far away for us to disclose any feature on their surface directly, one has to use an indirect tool to image their atmosphere. This method (Doppler Imaging) takes advantage of a star’s rotation to identify and characterise inhomogeneities on its surface. This is done by studyingbothhow inhomogeneities (carried in and out of the observer’s view by rotation) modulate photometric and spectroscopic data withtime, and how surface features distort theshapeof line profiles at a given rotational phase. In the last decade, Doppler Imaging has first been applied to abundance mapping of magnetic Ap stars, then extended to temperature and magnetic mapping of cool active stars. Allowing a solar-like way of studying the surface of stars, this technique provides a wealth of information about stellar atmospheres and, in particular, about the physical processes which dictate their dynamics and energy balance.
[Fe/H] and lithium Abundance for Detached Eclipsing Binaries
Authors: Clausen J.V.
Published: July 2016

We are now seeing a new generation of stellar models based on the recent opacity tables by Rogers and Iglesias (1992). Various changes in other important physical ingredients like nuclear reaction rates, convection parameters, mass loss rates, helium/metal ratios, relative abundances of heavy elements etc. are also included. Besides the Sun the most obvious objects for confrontation of the new grids with observations are binary components, pulsating stars, and well-studied open and globular clusters. Among these candidates the detached, double-lined eclipsing binaries play a unique role as our only source of absolute dimensions which 1) can be considered valid for single stars and 2) are accurate enough for serious tests of the input physics of modern stellar models.
On the Nitrogen Abundance - Age Relation For Main Sequence Early B-Stars
Authors: Lyubimkov Leonid S.
Published: July 2016

The results of the He, N and C abundance determinations for MS early B-stars, summarized in the review [1], lead to the inference that there are evolutionary variations in atmospheric contents of these elements. The conclusion was made that CNO-cycle products emerge on a surface of B-stars during their MS phase yet and such appearance can be considered as a probable manifestation of some mixing process between outer layers and stellar interiors. In particular the N enrichment with the age was found, which reachs about 1.0 dex for most massive B-stars (see [1,2]). However Gies and Lambert [3] (further GL) investigated recently the C, N and O abundances in a sample of early B-stars and did not confirm such great N enrichment. Since the problem of early mixing is very important for the understanding of first evolutionary stages of massive stars, the GL data need further and thorough analysis.
Atmospheric Structure of the Pulsating Ap [CP2] Star HR 3831 from Rapid Multicolour Photometry
Authors: Matthews Jaymie M. , Wehlau William H. , Rice John , Walker Gordon A. H.
Published: July 2016

Rapidly oscillating Ap (roAp) stars are cool magnetic CP2 stars which vary with periods of a few minutes and amplitudes less than 0.01 mag and 1 km/s in light and velocity. Analysis of their p-mode eigenfrequency patterns and splittings gives information about evolutionary state, rotation rate, magnetic field geometry and internal field strength (see Kurtz 1990; Matthews 1991).
We present here an example of how roAp pulsations can be used to obtain an estimate of the temperature structure of an Ap atmosphere.

The pulsation amplitudes of roAp stars decline more rapidly with increasing wavelength than other known pulsators. Matthews et al. (1990) explained this by the wavelength dependence of limb darkening and its weighting effect on the integrated amplitude of an (l, m) = (1,0) mode.
Particle Transport Processes
Authors: Michaud Georges , Proffitt C.R.
Published: July 2016

The effect of gravitational settling and radiation driven diffusion on the evolution of stars near the main sequence is reviewed. New simplified formulae for calculating diffusion are proposed that improve on previous such formulae. The reliability of available diffusion coefficients is discussed and areas where further work is needed are identified. Newly available opacity calculations are used to estimate the effects of radiative acceleration on Fe.

The size of the modifications to the evolution are shown to be modest: a reduction of order 10% on the evolutionary age of globular clusters. There are indications from the Li abundance in the high Teff halo stars that some turbulence is present below the convection zone. Models calibrated using solar properties reproduce the Teff at which the Li gap is observed in F stars as well as its depth without any arbitrary parameter. Similarly the presence of AmFm stars is explained over the Teff range where they are observed.
Line Broadening Data: Stark Broadening of Ca II Sc III And Ti IV Lines
Authors: Dimitrijević Milan S. , Sahal-Bréchot Sylvie
Published: July 2016

Electron– and proton–impact line widths and shifts for important Ca II, Sc III and Ti IV lines, have been calculated using the semiclassical–perturbation formalism. The obtained results were used to investigate the behaviour of Stark broadening parameters within the K I isoelectronic sequence.
Mixing by Internal Waves
Authors: Montalbán Josefina , Schatzman Evry
Published: July 2016

That mixing take place in the radiative zone of many stars, is an event that cannot be forgotten when we try to explain observational results as the lithium abundance in the atmosphere of different stars, its dependence on spectral type, age or rotation velocity... During the last years many processes have been proposed as being responsible of this mixing: overshooting, turbulence induced by rotational instabilities, internal waves, etc... We will consider, following the results obtained by Press (1981), the role of this last mechanism in the transport of lithium to the burning level, not as generators of turbulence (in Press, 1981, and García-López and Spruit, 1991, it is shown that turbulence induced by internal waves decays very quickly inside the radiative zone), but as generators of a diffusive process due to non linear dissipative effects.
Integral Constraints on Convective Overshooting Two-Dimensional Numerical Studies
Authors: Roxburgh Ian W. , Simmons John
Published: July 2016

The integral constraint on convective overshooting (Roxburgh 1976, 1978, 1989) is re-derived for for statistically stationary two-dimensional convection between stress-free horizontal boundaries maintained at constant temperatures, the vertical boundaries being periodic. We report on numerical simulations of such two-dimensional compressible convection in a fluid where the central regions are convectively unstable and the surrounding layers are stable. The results support the conclusion by Roxburgh (1989) that the contribution from the radiative flux is well approximated by the flux carried by the mean temperature gradient, but demonstrate that viscous dissipation is important, reducing the extent of overshooting.
The Depth of the Solar Convection Zone Inferred from Hydrodynamical Models of the Surface Layers
Authors: Steffen M.
Published: July 2016

We have obtained detailed 2-dimensional models of the surface layers of solar-type stars from extensive numerical simulations solving the time-dependent, non-linear equations of hydrodynamics for a stratified compressible fluid. The calculations take into account a realistic equation of state (including ionization of H and He as well as formation of H2-molecules) and use an elaborate scheme to describe multi-dimensional, non-local, frequency-dependent radiative transfer. The hydrodynamical models include the photosphere as well as part of the convective subphotospheric layers, with an open lower boundary located about one pressure scale height (~ 320 km) below TRoss = 1, allowing a free flow of gas out of and into the model. A fixed specific entropy, s*, is (asymptotically) assigned to the gas entering the simulation volume from below, which uniquely determines the effective temperature of the hydrodynamical model. For details about the physical assumptions, numerical method and characteristics of the resulting convective flow at the solar surface see Steffen (1991); Steffen and Freytag (1991).
Stellar Distances and Hipparcos
Authors: Gómez A.E.
Published: July 2016

Hipparcos, the astrometric satellite of the European Space Agency, launched in August 1989, will provide absolute trigonometric parallaxes for about 100 000 stars brighter than the limiting V-magnitude 12.5. In spite of the difficulties caused by a faulty orbit, the mission is successfully going on. A lifetime of more than 4 years is expected which will allow to reach the original objectives of the mission: 2 milli-arcsec for stars brighter than V-magnitude 9 and 4 to 5 milli-arcsec for fainter stars. The observing programme is based upon a single, uniquely-defined Input Catalogue. Field and open cluster stars of almost all effective temperatures, chemical compositions and gravities are being observed. Most types of binaries and variable stars are well represented, but Hipparcos is discovering new ones. Significant parallaxes (relatives errors smaller than 20%) will be obtained for stars closer than 100 pc (about 36 000 stars). Reliable stellar structure models should fit all the possible observational constraints: the main contribution of Hipparcos parallax data will be the huge improvement in the individual luminosities. The determination of stellar masses from the study of visual binaries will also be improved. The preliminary results of the first-year data reduction are an encouraging indication of the expected quality of the final results. On the basis of a mission duration of 4 years, the final results are foreseen for 1995.
Accurate Masses and Radii as Test Data for Stellar Models: Status and Progress on the Copenhagen Project
Authors: Andersen J. , Clausen J.V. , Helt B.E. , Johansen K.T. , Nordström B. , García M. , Giménez A. , Stefanik R.P. , Vaz L.P.R.
Published: July 2016

The most important ingredient of a stellar model is its initial mass. Hence, real stars with known masses are important test objects for stellar models. Initial chemical composition is the other important starting parameter and should therefore also be known (Clausen, 1992). Finally, the most direct indicator of the evolution of a star is its radius. Together, these parameters determine the effective temperature and luminosity of the model, to be tested against a real star.
Determination of Physical Parameters of Binary Systems: The Problem of Spurious Solutions
Authors: Barone F. , Di Fiore L. , Mancuso S. , Milano L. , Russo G.
Published: July 2016

In this paper we apply the Wilson-Price procedure to the solution of the binary system AO Cam using simultaneously all the available information and a statistical criterion to judge about the quality of the solutions found.
Calibration of The Main Sequence Stars: Alpha Centauri
Authors: Neuforge C.
Published: July 2016

New low temperature opacities have been computed and applied together with the new OPAL opacities to recalibrate the α Centauri system. Our solution is in favour of a Z value of about twice Z⊙.
New Evolutionary Tracks of Pre-Main-Sequence Stars
Authors: Palla Francesco , Stahler Steven W. , Parigi Giacomo
Published: July 2016

We present the results of new evolutionary calculations of the pre—main-sequence phase of stars in the mass range 1 to 5 Mʙ. The novelty of our approach is that, unlike in the standard theory of PMS evolution, the initial conditions of the gravitational contraction phase are not arbitrarily set, but derive from protostellar theory.
Radial Pulsation in Variable Stars with Mass Loss
Authors: Pijpers Frank P.
Published: July 2016

It is possible to show that cool giants with very large photospheric scale heights do not perfectly reflect pulsational waves at the photosphere. This means that for these stars the classical formulation of pulsation in which the outer boundary for the resonance cavity is assumed to be perfectly reflecting is not valid. This can have significant consequences for the eigenfrequencies of the pulsation of Long Period Variables such as Mira type variables as well as for the stability of their pulsation.
Observational Constraints on Dynamos in Cool Stars
Authors: Schrijver C.J.
Published: July 2016

Non-radiatively heated outer atmospheres are common among stars on the cool side of the HR diagram. These atmospheres resemble that of the Sun in which the bulk of the heating is associated with magnetic fields. Consequently it is assumed that other cool stars also generate and maintain an internal magnetic field through a dynamo action. This field gives rise to a wide spectrum of phenomena in the stellar atmosphere. The dynamo process that sustains the field thrives on the interaction of rotation and turbulent convection. In this review I formulate a set of propositions outlining our present knowledge of this interaction as distilled from stellar observations. In doing so, I outline the effects of stellar evolution on rotation rate, discuss some of the proposed “dividing lines” in the HR-diagram, and comment on the possible role of the convective turnover time in dynamo efficiency and on dynamo activity for extremely slowly and rapidly rotating stars.
The Musicos Project: Multi-Site Continuous Spectroscopy
Authors: Catala Claude , Baudrand Jacques , Böhm Torsten , Foing Bernard H.
Published: July 2016

Many scientific programs, most of them linked to stellar physics (such as asteroseismology, stellar rotational modulation, surface structures, Doppler imaging, Zeeman-Doppler imaging, variable stellar winds) require a continuous spectroscopic coverage during several days.

MUSICOS (for MUlti-SIte COntinuous Spectroscopy) is an international project for setting up a network of high resolution spectrometers coupled to telescopes of the 2m class, well distributed around the world, and partly dedicated to continuous spectroscopy.

The strategy to reach this objective was defined during two workshops organized at Paris-Meudon Observatory in 1988 and 1990, and consists of three steps: 1) organize multi-site spectroscopie campaigns using resident instruments on various telescopes around the world and transportable fiber-fed spectrographs where adequate spectroscopie equipment is not available; 2) design and develop a cross-dispersed echelle spectrograph, well suited for the scientific programs that require multi-site observations; 3) propose this MUSICOS spectrograph for duplication at several collaborating sites.
Wavelet Analysis of Some Pulsating Stars
Authors: Szatmáry Károly , Gál János
Published: July 2016

The detection of a variable period is very important as it gives information on the evolutionary state or on the binary nature of a pulsating star. If a pulsator is moving in a binary system, its light curve is frequency-modulated by the orbital period (light-time effect). Mode switching or chaos also may be the cause of the changes in the amplitude and period of the light variations.

The determination of period variability is difficult. Usually the conventional Fourier-spectrum cannot give any information about a possible period variation. The 0-C diagram is useful to detect a variable period, but it does not give the amplitude and phase values.
Spectroscopic Observations of The Be Star PHI-PER
Authors: Singh Mahendra , Mon Masahiro , Kogure Tomokazu , Suzuki Masakazu
Published: July 2016

Based on a series of high dispersion Coude spectra taken with 188-cm telescope of Okayama Astrophysical Observatory over an extensive period of time (nearly 20 years), spectroscopic behaviours were measured and discussed. Synchronous variations of V/R (intensity ratio of violet to red emission peaksjvariability with the radial velocity curves (RV) for Balmer and Hel 3888 shell absorption lines were confirmed. Using the peak separations of the Balmer emission lines, envelope extension was also estimated. Main difficulty was found in explaining the envelope extension of the primary star of the system which overflows the Roche-lobe limit when we consider the binary nature of the system.
The Owens Valley Millimeter Array
Authors: Scoville N. , Carlstrom J. , Padin S. , Sargent A. , Scott S. , Woody D.
Published: July 2016

The present status and projected two year developments for the Owens Valley Millimeter Array are described and a brief summary of the astronomical research is presented.
The VLA and VLBA at Millimeter Wavelengths
Authors: Fomalont Edward B.
Published: July 2016

Although the Very Large Array (VLA) and the Very Large Baseline Array (VLBA) were originally intended as centimeter wavelength instruments, the exciting results from high resolution millimeter radio astronomy over the last ten years have generated interest in pushing these arrays into the millimeter region. This report will describe two aspects of recent development at NRAO: the new capability of the VLA at 7mm wavelength which will be operational in 1994, and the completion of the VLBA with its anticipated use at 7mm and 3.6mm.
Distribution of HC5N in IRC+10216
Authors: Takano Shuro , Saito Shuji
Published: July 2016

The distribution of HC5N was observed in the envelope of the carbon star IRC+10216 with the Nobeyama Millimeter Array. A shell like distribution with its diameter of about 29″ was found. The diameter is larger than that of HC3N.
Lunar Occultations of the SiO Masers in RLeo
Authors: Cernicharo J. , Brunswig W. , Paubert G. , Liechti S.
Published: July 2016

VLBI observations show that the SiO maser emitting regions in oxygen-rich stars are very clumpy and that these clumps extend over a few stellar radii (Mclntosh et al. 1987; Colomer et al. 1992). These observations indicate that the ideal instrument for the study of the SiO masers is an interferometer with baselines covering between a few and several hundreds/thousands km. Such an instrument is so far unavailable.

A classical way to get high angular resolution and a full beam synthesis with a single telescope of moderate size is through lunar occultations. This observing technique provides the angular resolution of a single linear antenna several kilometers long. However, at millimeter wavelengths the Fresnel fringes produced by the Moon limb as the source under study is occulted have never been observed. We present here the observation with the 30-m IRAM radio telescope of the v=1 J=2-1 line of SiO during an occultation (and reappearance) of R Leo by the Moon.
CO (J=2-1) Observations of the Molecular Cloud Complex in the Galactic Center
Authors: Oka Tomoharu , Hasegawa Tetsuo , Hayashi Masahiko , Handa Toshihiro , Sakamoto Sei'ichi
Published: July 2016

We report a large scale mapping observation of the Galactic center region in the CO (J=2-1) line using the Tokyo-NRO 60cm survey telescope. Distribution of the CO (J=2-1) emission in the I-V plane suggests that molecular clouds forms a huge complex (Nuclear Molecular cloud Complex, NMC). Tracers of star formation activities in the last 106-108 years show that star formation has occured in a ring ~ 100 pc in radius. Relative to this Star Forming Ring, the molecular gas is distributed mainly on the positive longitude side. This may indicate that much of the gas in NMC is in transient orbit to fall into the star forming ring or to the nucleus in the near future.
Evidence for Protostellar Wind-Cloud Interactions
Authors: Rudolph Alexander L.
Published: July 2016

Recent molecular line observations of HH flows (HH objects and optical jets) have revealed abundant evidence of interactions between protostellar winds and the surrounding molecular clouds. HCO+ observations taken with the BIMA Array have traced out the distribution of both stationary and high-velocity gas near the protostellar objects HH 7-11, L1551, and HH 34.
CS Lines and Dust Continuum Observations of the OMC2 Infrared Cluster
Authors: Umemoto T. , Ohashi N. , Murata Y. , Tatematsu K. , Suzuki M.
Published: July 2016

It is known that stars in GMCs are often born as clusters. Recently, near infrared imaging has enabled us to study the young stars within molecular clouds (e.g., Lada & Lada 1991). Orion Molecular Cloud 2 (OMC2) is located 12' north of the Trapezium cluster in the Orion A cloud, and contains a cluster of about 20 near-IR sources and several FIR sources distributed within a diameter of 0.2 pc (Rayner et al... 1989; Johnson et al. 1990; Mezger, Wink, & Zylka 1990). By large scale mapping observations using the NRO 45 m telescope, this infrared cluster is found to be associated with a dense molecular core (Tatematsu et al. 1993, Umemoto et al. 1993). The region was observed using the Nobeyama Millimeter Array (NMA) to elucidate the structure and cluster formation process within a core.
Millimeter-Wave Continuum Around NGC 7538-IRS1, IRS2, and IRS3
Authors: Akabane Kenji , Tsunekawa Shozo , Inoue Makoto , Kawabe Ryohei , Ohashi Nagayoshi , Kameya Osamu , Ishiguro Masato , Sofue Yoshiaki
Published: July 2016

Millimeter-wave continuum sources in NGC 7538 region were observed with the NRO 45-m telescope and Nobeyama Millimeter Array. NRO 45-m telescope observations showed that the compact region which includes IRS1, IRS2, and IRS3 has a strong millimeter-wave intensity excess, cf. figures 1, and 2.
Warm and Cold Gas in Galactic Nuclei: The Near-IR/Millimeter Connection in NGC 253
Authors: Van Der Werf Paul P. , Cameron M. , Genzel R. , Blietz M. , Krabbe A. , Forbes D. A. , Ward M. J.
Published: July 2016

We discuss the use of seeing-limited near-IR spectroscopic imaging combined with high resolution millimeter and submillimeter wave observations, as a diagnostic in the study of the nuclear interstellar medium in starburst galaxies and active galactic nuclei. As an example, recent near-IR spectroscopic imaging of the starburst galaxy NGC 253 is analyzed. It is shown that the central ~ 100 pc of NGC 253 contains a number of giant star forming complexes, the stellar content of which is at least as large as that of the 30 Dor region in the LMC. We suggest the use of the [FeII]/Brγ ratio as an approximate age indicator for such complexes. The warm component of the nuclear molecular medium in NGC 253 detected in submillimeter CO spectra and in near-IR rovibrational lines of H2 is probably heated by stellar UV radiation or slow shocks in star forming regions, rather than by supernova remnant shocks. There are indications that molecular material is being removed from the nuclear region by the “superwind” observed in optical emission lines.
Self-Gravitating Gas Dynamics in a Nuclear Region of a Weak Barred Galaxy
Authors: Wada K. , Habe A.
Published: July 2016

Starburst regions are frequently located in galactic central regions and CO observations indicate that these regions contain a large amount of molecular gas (e.g. Lo et al. 1987). However, the triggering mechanism for starbursts and the mechanism of the high mass supply rate of gas into a galactic center are still unclear.

It is suggested that tidal encounters of galaxies remove angular momentum of gas and trigger rapid gas accretion and starburst. Noguchi(1988) has shown by computer simulations that galaxy-galaxy interactions induce a stellar bar, and gas loses its angular momentum and accumulates to a galactic center. In his numerical simulation, non-axisymmetric potential of a stellar bar plays an important role in the accretion of gas. However, it is not obvious whether or not gas accretes into a nuclear region within a few hundred pc only by the effects of stellar bar.

Wada and Habe (1992) investigated the dynamics of self-gravitating gas in a barred potential by 2-D numerical simulation, and show that even if the bar is weak, for the initial gas mass ratio to stellar mass grater than 10%, a central elongated gas ring formed at near ILRs becomes unstable and collapses. As a result, a large amount of gas can be supplied to galactic center. They conclude that both the effect of the self-gravity of the gas and the existence of ILRs are necessary to the rapid gas fueling in a weak barred potential.
1.7” Resolution CO(1-0) Observations of ARP220: Nuclear Gas Ring of Merger Remnant
Authors: Okumura S. K. , Kawabe R. , Ishiguro M. , Ishizuki S.
Published: July 2016

We made aperture synthesis CO(l-O) observations of the central region of Arp220 with the Nobeyama Millimeter Array. Central CO emission was resolved with a size of 975 kpc. It shows a ring-like structure (ɪ ~ 500 pc) with a large velocity gradient, 393 km · s−1 · kpc−1, from southwest to northeast direction. The ring-like emission is located around double radio compact sources. No emission peak was found in the center of the double sources within the velocity range 5100 km s−1to 5800 km s−1. These results suggest that an inclined massive gas ring has been or is being formed in the central 1 kpc of Arp220. Most of the molecular gas in Arp220 is concentrated on this nuclear ring. The radio compact sources are probably located at the inner egde of the ring.
An Abundance Analysis of the Silicon CP Star HD 43819
Authors: Lopez-Garcia Zulema , Adelman Saul J.
Published: July 2016

A fine analysis performed using four 4.3 Å/mm IIaO Mt. Wilson Observatory spectrograms shows that HD 43819 has abundance anomalies similar to those of other Silicon stars, namely it is helium, carbon, magnesium, and sulfur poor while being silicon, titanium, chromium, manganese, iron, strontium, zirconium, and rare earth rich.
Hβ Profiles of 25 CP Stars
Authors: Kroll Reinhold
Published: July 2016

I present the Hβ profiles of 25 CP2 and CP4 stars, together with their equivalent widths. Surface gravities are estimated for stars with known effective temperature from the equivalent width of Hβ.
MK Process and the Am Stars
Authors: Sreedhar Rao S. , Abhyankar K. D.
Published: July 2016

The MK morphology of the spectra in the region λλ 3850-4400 Å of 28 Am stars has brought to light the tendency of these stars to form into groups in contrast to earlier findings by various authors (S. Sreedhar Rao & K. D. Abhyankar, 1991). The potential of such a study is seen in the revelation of the inherent flexibility of the stars in this sample, apparently leading to a selective binning of the species with distinctly similar signatures, viz., Ca ll K and Sr ll 4077 features, the blue and violet regions and the Am-shell features. These are found to be the essential discriminants on the basis of which a multi-dimensional classification of Am spectra in the domain of the MK process is attempted.
Magnetic Field Diagnosis Through Spectropolarimetry)
Authors: Mathys Gautier
Published: July 2016

An original approach, the moment technique, is applied to analyze the shapes of spectral lines of Ap stars recorded in both circular polarizations. The longitudinal magnetic field, the asymmetry of the longitudinal magnetic field, and the quadratic field of the studied stars are derived. From the consideration of these quantities and of their variations through the stellar rotation cycle, constraints on the spatially unresolved structure of the magnetic fields are obtained.
Do Bp Stars Have “Flares”?
Authors: Vető Balázs
Published: July 2016

UBV(RI)c photoelectric observations of four variable magnetic Bp stars (HD 21699, HD 177003, HD 184927 and HD 217833) were carried out between 1989 and 1992. Random fluctuation of 0m.1 range was detected in the U colour band in the case of HD 177003 and HD 184927. The U light curve of HD 21699 shows 0m.02 peaks superposed on the periodic light variation which definitely exceeds the typical error of the observations. It is not likely that atmospheric extinction or observational error would cause the phenomena described. On the other hand there are no physical processes known which lead to such changes of the U band continuum radiation in Bp stars.
Rotational Velocities for the Brighter A-Type Stars
Authors: Abt Helmut A. , Morrell Nidia I.
Published: July 2016

In 1968 van den Heuvel showed that the rotational velocities at many A types have peaks below V sin i = 45 km s−1 and above 90 km s−1. We explored this bimodal behavior by obtaining new rotational velocities of 1761 A-type stars with coudé CCD spectra and, to date, new MK classifications for 956 of those. We find that the slow rotators consist of Ap, Am, and short-period binaries while the rapid rotators are normal and λ Bootis stars. The overlap is only 8–10% and can probably be attributed to undetected abnormal stars and binaries.

The λ Bootis stars discovered purely by their having weak λ4481 Mg II constitute about 18% of the early A-type stars but 4% at the late ones. Statistically all such λ Bootis stars have V > 100 km s−1 but they constitute only a small fraction of those stars.

More than half of the stars classified as A2 IV (and some at Al IV and A3 IV) seem to be a hitherto unrecognized class of slowly-rotating abnormal-abundance stars of luminosities like those of class V stars.
Chemical Separation vs Rotation in a and F-Stars
Authors: Charbonneau Paul
Published: July 2016

The role played by rotationally-induced mixing in the diffusion-based models for non-magnetic chemically peculiar stars is investigated. This paper focuses on one specific rotationally controlled mixing mechanism, namely thermally-driven meridional circulation. Its effects on the time evolution of chemical abundances are illustrated by means of three specific examples. The first two concern the diffusion model for FmAm stars, where it is shown that while circulation has a determining influence on the settling of Helium, it has no significant effect on the diffusion of heavier metals once the He superficial convection zone has disappeared. The third example is concerned with the diffusion/mass loss model for λBootis stars. It is shown that the inclusion of circulation prevents the appearance of generalized underabundances at any epoch of the evolution, indicating that the diffusion/mass loss model for these objects must be abandoned.
Models for Vega
Authors: Castelli Fiorella , Kurucz Robert L.
Published: July 2016

By using the ATLAS9 code which requires the new opacity data from Kurucz (1992), we derived a new atmospheric model for Vega with parameters Teff=9500 K, log g=3.95, [M/H]=−0.5, ξ=2 kms s−1. The model was fixed by comparing observed and computed energy distributions and Balmer profiles.
The Magnetospheres of the Helium Peculiar Stars of the Upper Main Sequence
Authors: Shore Steven N.
Published: July 2016

A brief review is presented of the evidence for magnetospheres surrounding the upper main sequence helium peculiar stars. Some of the underlying physical processes are discussed. The helium strong stars all appear to possess comparatively dense magnetospheres. The phenomenon is also present amongsomehelium weak stars, but is quite rare.
Radial Velocity and Magnetic Variations of a Draconis
Authors: Lehmann Holger , Scholz Gerhard
Published: July 2016

Based on photographic Zeeman spectra a magnetic field has been detected for the giant star α Dra. One of the two periods derived for the magnetic changes can be interpreted as the half rotational period corresponding to a slight variation of the radial velocity of 3.57 days. The application of an eguatorially symmetric rotator model with a magnetic field dominated by the quadrupole moment leads to a good agreement with the observed magnetic variation. The orbital elements of α Dra were newly derived.
Dominating Molecules in the Photospheres of Cool Stars
Authors: Jørgensen Uffe Gråe
Published: July 2016

Molecular data bases (including very different numbers of molecular lines, and based on computations spanning a wide range in quality) have been used in the construction of models of the stellar atmospheric structure, and effects of various molecules on the stellar structure and evolution have been described in the literature. A systematic study showing which molecules are of importance in which types of stars is, nevertheless, still lacking, and the line lists used in various model computations necessarily reflect what was at hand at the time of computation rather than a systematic effort to solve the molecular opacity problem. A pioneering investigation was presented in two extensive papers by Tsuji (1964, 1973) where the foundation of the reaction scheme used in most models today was laid down. Later Tsuji (1986) has given a review of the existing work on molecules known in stars, and Johnson (1986) and Gustafsson (1989) have touched upon the same subject in reviews covering a broader range of stellar aspects. Jørgensen (1992) has recently summarized the existing molecular data (and discussed the methods used) available for model atmosphere construction.
Infrared Spectroscopy and Molecules in Circumstellar Envelopes
Authors: Hinkle Kenneth H.
Published: July 2016

During the last two decades there has been a tremendous renewal of interest in the circumstellar environment of evolved stars. This renewal has been mainly driven by sensitive microwave observing techniques. Microwave spectroscopy has resulted in a number of fundamental discoveries in circumstellar shells, including the discovery of maser lines and complex circumstellar chemistry. Historically, interest in circumstellar shells originates from a desire to understand the mass-loss process. It has been known for at least 50 years that circumstellar gas is the origin of interstellar gas and dust. As such the circumstellar shell becomes the pathway through which elements created in the interior of the central star enrich galactic abundances. High resolution spatial and spectral techniques and an ever improving knowledge of stellar evolution has resulted in an increased but still incomplete understanding of the mass-loss process.
Molecular Abundances in the Envelopes Around Evolved Stars
Authors: Olofsson Hans
Published: July 2016

Red giant stars on the asymptotic giant branch (AGB), AGB-stars, lose copious amounts of matter in a slow stellar wind (Olofsson 1993). Mass loss rates in excess of 10-4M⊙yr-1have been measured. The primary observational consequence of this mass loss is the formation of an expanding envelope of gas and dust, a circumstellar envelope (CSE), that surrounds the star. This is a truly extended atmosphere that continues thousands of stellar radii away from the star. At the highest mass loss rates (which probably occur at the end of the AGB evolution) the CSE becomes so opaque that the photosphere is hidden and essentially all information about the object stems from the circumstellar emission. At some point on the AGB a star may change from being O-rich (i.e., the abundance of O is higher than that of C) to becoming C-rich (i.e., a carbon star where the abundance of C is higher than that of O) as a result of nuclear-processed material being dredged up to the surface. The chemical composition of the CSE will follow that of the central star, although with some time delay so that there may be some rare cases of O-rich CSEs around carbon stars. The mass loss decreases and changes its nature as the star leaves the AGB and starts its post-AGB evolution. Eventually the star becomes hot enough to ionize the inner part of the AGB-CSE and a planetary nebula (PN) is formed. The ultimate fate of the star is a long life as a slowly cooling white dwarf. The CSE will gradually disperse and its metal-enriched matter will mix with the interstellar medium, and thereby it contributes to the chemical evolution of a galaxy. The intense mass loss makes it possible for stars as massive as 8 M⊙, i.e., the bulk of all stars in a galaxy, to follow this evolutionary sequence. Similar CSEs are also found around supergiants.
Improvements in the Computation of Grain Opacity
Authors: Alexander David R. , Ferguson Jason W.
Published: July 2016

Atoms and ions dominate the opacity of most astrophysical plasmas at temperatures above about 5000 K. In the range between 5000 K and about 1500 K, molecular processes are important contributors to the opacity. Depending upon the composition and pressure, grains may begin to condense at temperatures below about 1500 K. For the conditions of interest here, the grains which form are generally small (radius < 0.25µm) compared to the wavelength of light near the peak of the Planck curve. Because their absorption and scattering cross section is so large compared to atoms or molecules, grains dominate the mean opacity whenever they are present and must, therefore, be taken into account whenever such temperatures are encountered. Our calculations indicate that failure to do so can lead to errors as large as five orders of magnitude in the mean opacity.
Continuous Molecular Opacities and Photodissociation
Authors: van Dishoeck Ewine F.
Published: July 2016

It is well known that molecules have much more complex electronic spectra than atoms due to their rotational and vibrational motions. Most molecular spectra observed in the laboratory and included in model atmospheres involve transitions between two bound electronic states. However, in contrast to atoms, molecules can also have bound-free transitions into states that are repulsive in at least one nuclear coordinate, even below the first ionization potential. These transitions not only lead to dissociation of the molecule, but also provide a source of continuous opacity which is usually not taken into account in model atmospheres. Moreover, photodissociation plays an important role in the non-LTE chemistry of stellar atmospheres and circumstellar envelopes (see Olofsson this volume). In this chapter, a brief overview of our current knowledge of cross sections for continuous absorption and photodissociation is given.
Experimental Measurement of Weak Band Intensities
Authors: Lehmann Kevin K. , Romanini Daniele
Published: July 2016

In modeling the opacity of astronomical objects, such as stellar atmospheres, the most important transitions are those with absorbance =αL∼ 1, whereαis the absorption coefficient of the atmosphere and L is the pathlength that light travels in leaving the atmosphere. Transitions with much smaller absorbance are not important, while those with much larger absorbance are effectively opaque. The relevant absorption lengthsLare typically orders of magnitude larger than can be realized in the laboratory, and thus obtaining the needed laboratory data is a significant experimental challenge. This volume contains several examples of the power of modernab initiomethods to predict spectra of simple triatomic molecules, such as H2O, but it is still necessary to at least calibrate the predictions against laboratory bands of similar oscillator strength. The theoretical treatment of molecules with four or more atoms lags far behind the theoretical treatment of triatomics and present experimental capabilities.
First Results for CCD Observations with the 70 cm Meniscus Telescope
Authors: Kurtanidze Omar M. , Kvernadze Teimuraz M.
Published: July 2016

From the middle of 1993, a ST-6 CCD Camera (375 × 242 array with 23 × 27 microns pixel size) was mounted at the Newtonian focus (f/3) of the 70 cm meniscus telescope of the Abastumani Observatory. The photometer is now equipped with various sets of narrow-band interference, intermediate and wide band standard niters. Some pilot observations were carried out in direct imaging mode and in combination with an 8° objective prism (166 Å mm−1 at Hγ). Spectrophotometric standard stars with absolute calibrations were observed in the 8° mode to determine the CCD spectral response. Some other observations were performed with a prism-interference filter combination to estimate the observational mode efficiency.
Fibre Optic Light Guide for the Telescope: Some Experiments
Authors: Handojo Andrianto , Fachrizal Noor
Published: July 2016

Optical fibres are used to convey light from star images to the entrance slit of a spectrograph. Using an arrangement, the position of the fibre ends are monitored. The spectral transmission factor of the system and modification in the beam divergence have been measured.
Detecting Extra-solar Planetary Transits with a Schmidt
Authors: Parry Ian R. , Watson Fred G. , Carrasco B. Esperanza
Published: July 2016

We describe an instrumental configuration for detecting large gas-giant planets orbiting main sequence stars via the small drop in stellar brightness that occurs when the planet transits the stellar disk. Our proposed scheme involves the long-term monitoring of the light-curves of tens of thousands of stars using a Schmidt telescope coupled to a CCD via a bundle of 10,000 optical fibres. Using an existing theoretical model of planetary system formation we calculate a detection rate of 14 transits per year for our proposed system.
The Analysis of Digitised Objective Prism Spectra
Authors: Stock J.
Published: July 2016

A method for a fully automatic reduction of digitised objective prism spectra is developed. It leads to positions and radial velocities, magnitudes and intensities of spectral lines. The latter then can be used for classification purposes. The system is tested with plates taken with a specific telescope-prism-filter combination.
Schmidt-Plate Astrometry – Dinosaurs in Action
Authors: Reid Neill
Published: July 2016

Photographic plates are well established as stable detectors, well suited for positional astronomy. The completion of the current second epoch 48 inch Schmidt surveys in both hemispheres will provide material spanning baselines of 20–45 years, allowing proper motions to be determined to an accuracy of 4–7 mas yr−1. This paper discusses some of the Galactic structure problems that these data, combined with accurate photometry, can be used to address.
Large Scale Galactic Structure
Authors: Gilmore Gerard , Ibata Rodrigo
Published: July 2016

Modern models of Galaxy formation make fairly specific predictions which are amenable to detailed tests with galactic kinematic and chemical abundance data. For example, popular Cold Dark Matter models ‘predict’ growth of the Galaxy about a central core, which should contain the oldest stars. Later accretion of material forms the outer halo and the disks, while continuing accretion will continue to affect the kinematic structure of both the outer halo and the thin disk. This picture, which contains aspects of both the monolithic (‘ELS’) and the multifragment (‘Searle-Zinn’) pictures often discussed in chemical evolution models, makes some specific predictions which can be tested. The essential feature of these predictions is that they are believable only for the largest scale effects. Large scale properties of the Galaxy must be measured to test them. It is these studies which need large angular scale data. One specific example of current interest is the ‘prediction’ that mergers of small satellites are an essential feature of galactic evolution. This leads one to look for kinematic and spatial structures, and ‘moving groups’, as a primary test of such models.
Star Counts and Spectral Classification in Star Formation Regions of the Magellanic Clouds
Authors: Kontizas M. , Morgan D. H. , Bellas-Vellidis Y. , Kontizas E.
Published: July 2016

Large complexes of young objects are common features in galaxies and are called superassociations, complexes and supercomplexes according to their size. They contain HII regions, early-type stars, associations and young clusters. Such large structures are expected to trace the star formation regions within galaxies. Star counts and spectral classification over extended areas, using direct and objective prism Schmidt telescope plates are good methods of determining the boundaries of these complexes and supercomplexes where the spatial density of early-type stars is enhanced. A project of tracing the supercomplexes in the Magellanic Clouds has started using plates taken with the UK 1.2 m Schmidt Telescope.
A Survey for Low Surface Brightness Galaxies using Tech Pan Films
Authors: Schwartzenberg Jean Marc , Phillipps Steven , Parker Quentin A.
Published: July 2016

We report on a new survey for low surface brightness galaxies using Kodak Tech Pan 4415 film on the UKST, and demonstrate the advantages of this film over conventional IIIa plates when searching for faint, extended objects. Recent experiments with films of the Virgo cluster have shown that the noise per pixel in deep exposures is only 0.7 % of sky, enabling known objects to be traced out to an isophote around 27 Rµ. In addition, a preliminary survey has revealed several new galaxies with central surface brightnesses around 24 Rµ. With the digital coaddition of several films, we expect to be able to measure objects out to the 28 Rµ isophote and detect galaxies with central surface brightnesses as faint as ∼ 25 Rµ, i.e. below 3 % of sky.