bluuebunny/crossref_metadata_2025 huggingface dataset:
An Ellipticity-Age Relation for Globular Clusters in the Large Magellanic Cloud
Authors: Fall S. Michael , Frenk Carlos S.
Published: June 2019

We have measured the ellipticities of 52 globular clusters in the LMC and 93 in the Galaxy by eye from polaroid enlargements of the sky surveys (Frenk & Fall 1982). In most cases, the measurements pertain to regions between (1–2)rh where rh is the median radius of a cluster; i.e. the radius containing half of the light in three dimensions. These were compared with determinations based on star counts for 12 members of the LMC sample and 19 members of the Galactic sample. We found no systematic difference between the ellipticities from the two methods and concluded that the eye-measurements are free of any major bias. They are also in reasonable agreement with the measurements by Geyer & Richtler (1981) and Geyer, Hopp & Nelles (1983), who used the Agfa contourfilm technique. The ellipticities measured by Geisler & Hodge (1980) from microdensitometer scans are systematically large in comparison with our results and those of Geyer and his associates. Since the scans cover only a small part of each cluster, a few bright stars can cause spurious elongations in the fitted contours.
A Search for Low-Velocity Neutral Hydrogen in the Magellanic Stream
Authors: Bystrova N. V.
Published: June 2019

The large velocity range of the HI clouds belonging to the Magellanic Stream, as found by Haynes (1979), makes it reasonable to search for some low LSR velocity neutral hydrogen possibly belonging to the Stream. This gas, if any, would be severely blended by the local galactic gas.
Spectroscopy of Red Variables and Other Luminous Red Stars
Authors: Bessell M. S.
Published: June 2019

Long period variable stars represent the most advanced stage of asymptotic giant branch evolution prior to planetary nebula ejection or catastrophic core collapse. In the Magellanic Clouds through study of the LPVs it has been possible to identify for the first time, stars on the AGB with luminosities right up to the AGB limit (Mbol ~ −7.1) providing direct evidence that the more massive AGB stars produce supernovae. Because the stars have well defined periods, knowledge of the temperature and luminosity enables the mass to be derived. This review will highlight spectroscopic observations of the LPVs and discuss what information they provide on nucleosynthesis on the AGB in stars of different mass.
A Search for Red Variable Stars in the LMC
Authors: Paltoglou G. , Wood P. R. , Bessell M. S. , Ratnatunga K.
Published: June 2019

The observational material for this study consists of a series of 19 I plates (IVN+RG715) of the southern half of the LMC taken by the UK Schmidt Telescope over a six year period from 1977–1983. A small region of size 28′x56′ centred on α(1950) = 5 28 50.7, δ(1950) = −69 31 56 was scanned on all plates with a PDS microdensitometer and magnitudes were derived for typically 14000 stars per scan area. All magnitudes were converted to a common system by comparison of magnitudes to a standard plate; this comparison showed the individual rms measurement error to be 0.13 mag. No photoelectric standard sequence exists in the region studied but the instrumental magnitudes have been converted to preliminary IK magnitudes by using the IK magnitudes given by Blanco, McCarthy and Blanco (1980) for some of the red stars in the field.
Are the Supernova Remnants of the LMC in the Adiabatic Phase?
Authors: Fusco-Femiano R. , Preite-Martinez A.
Published: June 2019

A catalog of supernova remnants (SNRs) observed in the soft X-ray band 0.15–4.5 KeV by the imaging instruments on board the Einstein Observatory has been recently published by Mathewson et al. (1983). One of the conclusions in their paper is that SNRs in the LMC appear to evolve to large diameters much faster than predicted by the adiabatic expansion theory (Sedov 1959). This is based on the observed cumulative number-radius (N-R) relation that for the remnants in the LMC turns out to be approximatively linear. As suggested by Mathewson et al. (1983), a linear N-R relation can be obtained assuming a population of SNRs in the free-expansion phase (phase I, Woltjer 1972) expanding in a homogeneous medium. In the following we show that the X-ray surface brightness as a function of linear radius (Σ-R diagram) and the N-R relation can be alternatively explained assuming that the SNRs in the LMC are in the adiabatic phase.
Interstellar Extinction in the Magellanic Clouds
Authors: Nandy K.
Published: June 2019

The extinction properties of interstellar dust in the Large and Small Magellanic Clouds have been systematically investigated, using recent UV observations of early type Cloud members along with complementary visible data. The extinction curves differ systematically from the standard Galactic curve. The latter shows a broad absorption feature centred near 2200Å in virtually all sight lines but this is absent or only weakly present in the SMC; also the SMC extinction in the far UV is the largest known relative to EB-V. Dust in the LMC appears to be intermediate in extinction properties between the SMC and normal Galactic material. However, exceptions from the average extinction curves have been found in both Clouds.

Model computations show that the range of grain sizes and their number distribution law may not be significantly different in the Clouds and the Galaxy; the difference in extinction laws can be accounted for by varying the graphite contribution relative to silicate.
Temperature Diagnostics of the Upper Photosphere
Authors: Shchukina N. G. , Shcherbina T. G. , Rutten R. J.
Published: June 2019

We use NLTE modelling of the alkali resonance lines and of C I and 0 I high-excitation multiplets to test their quality as temperature diagnostics of the upper photosphere, in the context of Avrett's NLTE recovery of the LTE Holweger-Müller model below the temperature minimum and Ayres' bifurcation into hot and cool components above the temperature minimum.
Empirical Photospheric Fluxtube Models from Inversion of Stokes V Data
Authors: Keller C. U.
Published: June 2019

We present results of an inversion procedure that derives the turbulent velocity, the magnetic field strength, and the temperature stratification of the photospheric layers of solar magnetic fluxtubes from 10 FeI and FeII Stokes V line profiles around 5250 Å and from the continuum contrast. The free parameters of two-dimensional magnetohydrostatic fluxtube models are determined by minimizing the difference between observed and calculated Stokes V parameters in an iterative manner. Results of this inversion procedure applied to observations of a plage and a network region at disk center indicate a temperature deficit (at equal geometrical height) of the fluxtubes at the level of continuum formation and a temperature excess at the highest levels of line formation in general agreement with the latest theoretical fluxtube models.
On the 5-Minute Photospheric Oscillation and its Modeling
Authors: Marmolino C. , Severino G.
Published: June 2019

The 5-minute oscillations are standing waves trapped in subphotospheric cavities, whose evanescent part affect the profiles of photospheric lines.
Generation of Magnetic Fields and Electric Currents in the Solar Photosphere
Authors: Henoux J.C. , Somov B.V.
Published: June 2019

Velocities of electrons, ions and neutrals are computed in the three-fluid approximation for an axisymmetrical magnetic field. By prescribing a radial dependence of the velocity of neutrals in agreement with a downflow, the radial dependence of the magnetic field energy density is derived for a given set of values of the magnetic field at the central and external boundaries. Flux-tube cooling by advection of ionization energy is found to be significant. Vortices in the low photosphere could produce significant electric power and DC current intensity along the coronal magnetic lines of forces. The velocities of neutrals, the size and the number of flux-tubes required to power flares in plage regions, are estimated.
Convection and its Stability in the Equatorial Regions of the Convection Zone
Authors: Klyachkin A.V.
Published: June 2019

The problem of the existence, evolution, and stability of spatial structures in convection is of considerable importance to astrophysics as well as to geophysical hydrodynamics. The Boussinesq approximation will be used because the considered motions in stars are sufficiently slow. The system of hydrodynamic equations describing convection in a rotating inhomogeneous medium has the form:
Here Dt is the total time derivative, U the velocity, P, T, and C the deviations of the pressure, temperature, and helium abundance (by mass) from the basic equilibrium values, ρm, νm, χm, and Dm the values averaged over the considered layer of the density, viscosity, thermal and helium diffusivities, βT and βc the averaged coefficients of the thermal and helium expansions, g and Ω the gravitational acceleration and angular velocity, ∇Tb, and ∇Cb the values of the basic equilibrium temperature and helium gradients, and ñTad the adiabatic temperature gradient.
Symmetry Breaking in the Solar Dynamo: Nonlinear Solutions
Authors: Jennings R.L. , Weiss N.O.
Published: June 2019

We examine an idealized αω-dynamo model in which the magnetic fields depend only on latitude and time. The solutions that bifurcate from the field-free state are either symmetric or antisymmetric about the equator (quadrupolar or dipolar respectively). Nonlinear steady and periodic solutions, whether stable or unstable, can be followed numerically as the dynamo number is varied, revealing a rich bifurcation structure with mixed-mode solutions (lacking symmetry about the equator) appearing at secondary bifurcations. These results show how stable asymmetric fields can occur in the sun and illustrate the formation of complicated spatial structure in more active stars.
Observing, Modeling, and Understanding Stellar Granulation
Authors: Dravins Dainis
Published: June 2019

Numerical simulations of the three-dimensional structure and time evolution of stellar surface convection are now possible, at least for solar-type stars. Using the output from such simulations as sets of spatially and temporally varying model atmospheres, synthetic granulation images and spectral line profiles are computed, and compared to observations. Thus obtained disk-integrated data agree with observed lineshapes and bisector patterns in different stars, and also permit stellar rotation to be determined. Such simulations represent the first generation of models that are free from the classical ad hoc parameters of ‘mixing-length’, ‘micro-’ or ‘macro-turbulence’, parameters which in the past have characterized and limited stellar astrophysics.

To infer the surface structure also in more exotic stars, simpler and parametrized models must still be used to interpret observed line asymmetries. Such models suggest that rapidly rising ‘granules’ cover only a small surface fraction on early-type stars, a situation opposite to that in solar-type ones, and one likely to affect magnetic fields and stellar activity. Theoretical challenges for the future include detailed modeling also of early-type, giant, and other non-solar type stars of different rotational velocities; the hydrodynamics of entire stellar envelopes (including the interaction with global oscillations); and the interaction with magnetic fields (including their generation). Greatly increased computing power will be needed for such detailed modeling throughout the Herzsprung-Russell diagram, possibly requiring custom-designed computers.

Signatures of stellar granulation are primarily observed as asymmetries and wavelength shifts in photospheric absorption lines. Observational challenges include achieving sufficient spectral resolution to fully resolve such asymmetries; identifying granulation signatures throughout the HR-diagram (including the blended spectra of cool stars); observing how line asymmetries for a given spectral type depend on stellar rotational velocity, measuring wavelength shifts between groups of different lines in the same star, and between different stars; monitoring lineshift variations during stellar activity cycles; and ultimately high-resolution spectroscopy of spatially resolved granulation structures across stellar disks. The latter will require active optics on future very large telescopes, or the use of long-baseline optical interferometers.
A Model for Stellar Convection and Spectral Line Asymmetries
Authors: Antia H. M.
Published: June 2019

A model for stellar convection zones based on linear convective modes using a nonlocal mixing length theory is developed to study the spectral line asymmetries and the line shifts resulting from convective motions in the stellar photospheric region. The amplitudes of these linear convective modes is estimated by requiring the convective flux due to a linear superposition of such modes to reproduce the convective flux in the mixing length model. To study the spectral line asymmetries the convective mode with the largest amplitude in the photospheric line formation region is chosen to represent the stellar velocity field and the accompanying intensity fluctuations. Synthetic spectral line profiles are obtained by summing locally symmetric profiles over the stellar disk according to the local Doppler velocity and intensity fluctuations. The resulting line bisector shapes and the line shifts are compared with observations for α-Cen B. It is found that while the simple model proposed here can explain either the line shifts or the line bisector shape reasonably well, it fails to explain both these characteristics simultaneously.
Magnetic Fields on Solar–Like Stars: The First Decade
Authors: Saar S. H.
Published: June 2019

I review the progress made over the past decade in the measurement of magnetic fields on solar–like stars. I describe the evolution of magnetic analysis techniques, summarize our current understanding of stellar magnetic properties, and outline some future research directions.
Evidence That Wolf-Rayet Stars are Pre-Mainsequence Objects
Authors: Underhill Anne B.
Published: June 2019

The evidence is reviewed that Population I Wolf-Rayet stars have solar abundances, that they are surrounded by remnant disks formed from their natal clouds, and that their rate of mass loss is moderate. These properties are consistent with Wolf-Rayet stars being young objects recently arrived on the main-sequence rather than the evolved, peeled-down remnants of massive stars.
Analyses of Wolf-Rayet Spectra
Authors: Hamann W.-R.
Published: June 2019

Wolf-Rayet stars represent an important stage in the evolution of massive stars, but are only poorly understood so far. For a better knowledge one might determine their effective temperatures, luminosities and atmospheric compositions. But the emission-line dominated Wolf-Rayet spectra were not accessible to a quantitative analysis for a long time, because “standard” model calculations for static, plane-parallel stellar atmospheres are not adequate to that type of stars.
Optical Continuum Formation in Rotating-Wind Models of WR Stars
Authors: Doherty L. R.
Published: June 2019

Wolf-Rayet stars could be rapid rotators. We must ask if a rotationally flattened wind can lead to an observed continuous spectrum that depends on the angle of inclination. Schmid-Burgk (1982) showed that in the radio region the spectral shape is not changed. Here we consider the optical region, where both thermal emission and electron scattering are important. This study uses a simple, idealized model of a rotating wind in which v(r) is from Friend and Abbott (1986), but depends on polar angle in the manner of Poe et al. (1989). Equatorial densities are five times polar values at the same radial distance. The wind consists only of fully ionized He surrounding a spherical core and has a radial optical depth of 2.0 in the equator in the limit of pure electron opacity, essentially at wavelengths less than 2000A. Radiative transfer in the wind is fully treated by a second-order moment method based on the equation of transfer in general spherical coordinates (Doherty 1989). A constant flux at the core boundary is assumed. This flux corresponds to a T = 4×104K blackbody and the wind temperature is 2×104 . The shape of the emergent spectrum in the region computed (2500—7500A) changes little with inclination and resembles the spectrum of a spherical wind with intermediate density. Thus a rotating wind may masquerade as spherical but, in the present model at least, vary in flux by nearly a factor of two from pole to equator. This can affect the apparent magnitude, wind polarization, and possibly the line spectrum and wind acceleration.
Sporadic Variations in Wolf-Rayet Stars
Authors: Shylaja B.S.
Published: June 2019

Many Wolf-Rayet stars display variations which are observable either spectroscopically or photometrically or both. These are not exclusively associated with binary systems. A study is made of the sporadic events in the context of extended atmospheric structure.
Theoretical Results on WR Instabilities
Authors: Maeder André , Schaller Gérard
Published: June 2019

The instabilities of WR stars are discussed in the framework of both the quasi-adiabatic and non-adiabatic methods. We show that a very careful treatment of stellar opacities is necessary. Both methods show that WR stars, with little or no H left, are vibrationally unstable in the fundamental radial mode due to the ε-mechanism, confirming earlier results by Maeder (1985). Thus, from the point of view of stability, WR stars are fundametally different from O stars. Our new analysis also supports our previous view that WR stars evolve, keeping at the edge of vibrational instability.
The Impact of Polarimetry on Wolf-Rayet Star Models
Authors: Schulte-Ladbeck Regina E.
Published: June 2019

Recent observational efforts have presented new results concerning (linear) polarization variations as a function of time as well as wavelength that have considerable impact on WR star models:

Polarimetry throughout orbital phase has provided inclinations of double-line spectroscopic binaries containing WR stars with O star companions, thus improving our understanding of the masses and evolution of WR binaries (Schulte-Ladbeck 1989, A.J. 97, 1471). New binary-model calculations were presented at the meeting and continuing comparison with observations should prove productive.
Wolf-Rayet Binaries: Observational Aspects
Authors: Cherepashchuk A. M.
Published: June 2019

New data concerning observational aspects of Wolf-Rayet binaries are summarised. WR+O and WR+c binary systems are considered. All the data concerning WR binaries agree well with the modern understanding of WR stars as the helium remnants formed as a result of mass loss by massive O-type stars.
Terminal Velocities for a Large Sample of O Stars, B Supergiants, and Wolf-Rayet Stars
Authors: Prinja R. K. , Barlow M. J. , Howarth I. D.
Published: June 2019

We argue that easily measured, reliable estimates of terminal velocities for early-type stars are provided (1) by the central velocity asymptotically approached by narrow absorption features in unsaturated UV P Cygni profiles, and (2) by the violet limit of zero residual intensity in saturated P Cygni profiles. We use these estimators and high resolution IUE data to determine terminal velocities, v∞, for 181 O stars, 70 early B supergiants, and 35 Wolf-Rayet stars. For OB stars our values are typically 15-20% smaller than the extreme violet edge velocities, vedge, while for WR stars v∞ = 0.76vedge on average. We give new mass-loss rates for WR stars which are thermal radio emitters, taking into account our new terminal velocities and recent revisions to estimates of distances and to the mean nuclear mass per electron. We examine the relationships between v∞, the surface escape velocities, and effective temperatures.
An Analytical Approach to the Problem of Mass Loss from Wolf-Rayet Stars
Authors: Turolla R.
Published: June 2019

We investigate the dependence of the wind parameters on the properties of the parent star by means of an approximate analytical approach, largely based upon dimensional arguments (Bandiera and Turolla, 1990). The main assumption we use are.
Enrichment by Wolf-Rayet Stars and Other Massive Stars in Gas, Dust and Energy
Authors: Cohen Martin
Published: June 2019

I update previous estimates of the separate contributions for radiative energy, integrated total stellar wind mass and dust mass from Wolf-Rayet stars and other massive (OBA) stars. In the context of the intriguing dusty WC9 stars, I: (1) discuss the observability (or otherwise) between 0.4 and 23 μm of the condensation route from hot gas to carbon-rich grains; (2) urge caution in the use of 10 μm infrared spectra of these luminous stars to deduce the importance of silicates as a component of the interstellar medium, and (3) speculate on a possible new method for discovering new members of this relatively rare subtype based on IRAS Low Resolution Spectra. I review the observational evidence for dust condensation around SN 1987A.
Ring Nebulae Around of and WO Stars
Authors: Lozinskaya Tatiana A.
Published: June 2019

To understand the origin of WR-rings it is of interest to consider nebulae associated with the precursors of WR stars and with stars at later stages of evolution. The former are represented by numerous Of stars and the latter by only four WO stars which, according to Barlow and Hummer (1982), are the most extreme WR stars near the end of their lives.
Estimates of Bolometric Corrections for WR Stars and Progenitors of WN Binaries
Authors: de Loore C. , de Greve J.P.
Published: June 2019

Using theoretical models of WR binary evolution, and adopting bolometric corrections for the O-type companions, the B.C. of a sample of WN stars are derived using the luminosity difference and the mass ratio. From the present characteristics of observed WN binaries the initial parameters are estimated assuming conservative case B of mass transfer and stellar wind as mass modifying processes. The influence of nonconservative interaction is discussed.
LMC OB Associations Containing Wolf-Rayet Stars
Authors: Schild H. , Lortet M. C. , Testor G.
Published: June 2019

We observed four OB associations in the Large Magellanic Cloud which contain Wolf-Rayet stars. The associations are located in the neighbourhood of 30 Dor. They contain 15 WR stars of WCE and most WN subtypes. The observations include UBV photometry for several thousand objects and spectroscopy in the range from 3500 to 5000 Å for about 250 stars. Two new extreme Of stars and a new WR star as well as many hitherto unknown O stars were identified. Cluster HR diagrams reveal the age structure of the association stars and yield tentative conclusions about evolutionary connections between Wolf-Rayet stars in the LMC.
The Wolf-Rayet Connection - Luminous Blue Variables and Evolved Supergiants
Authors: Humphreys Roberta M.
Published: June 2019

The physical characteristics and behavior of evolved massive stars in three different mass ranges are reviewed with application to whether they may eventually evolve to the WR stage 1. >40-50 M⊙ as LBV's, 2. ∼30-40 M⊙ as cool hypergiant-OH/IR stars and 3. ∼10-30 M⊙ as red supergiant-OH/IR stars. I emphasize the importance of the relatively short but high mass loss phases as LBV's and as OH/IR stars in determining the fate of massive stars from 10 to 100 M⊙.
Explosions of Helium Stars and Type IB/IC/IIB Supernovae
Authors: Nomoto K.
Published: June 2019

Theoretical models of supernova explosions of helium stars with various masses are reviewed to examine possible connections between Wolf-Rayet stars and Type Ib/Ic/IIb supernovae. Nucleosynthesis, Rayleigh-Taylor instabilities, and light curves are compared with observations. Maximum brightness and the fast decline of the light curves of typical SNe Ib/Ic can be well accounted for by the helium star models if the helium star mass is as low as 3-5 M⊙. These low mass helium stars can form from stars of 12-18 M⊙ after Roche-lobe overflow in close binary systems. Probably progenitors of typical SNe Ib/Ic are not classified as Wolf-Rayet stars.
X-Ray and Gamma-Ray Signatures of Wolf-Rayet Supernova Explosions
Authors: The Lih-Sin , Clayton Donald D. , Burrows Adam
Published: June 2019

It is widely speculated that a Type Ib supernova is the explosion of a Wolf-Rayet star. We calculate the X-ray and gamma-ray signatures of models of that type, assuming all hard photons to have originated with Ni decay chains, in hopes of providing diagnostics of the exposed-core models of massive stars, which constitute one model of the Wolf-Rayet stars, calculated by Ensman and Woosley (1988). These provide the characteristic luminosity peak and light curve of Type Ib supernovae for helium-core masses between 4 and 6 M⊙. We compute gamma-ray line shapes and fluxes and the Comptonized X-ray continuum resulting from the decay of the radioactive 56Co and 57Co isotopes that are synthesized by the explosion of the presupernova star (the suggested Wolf-Rayet or post-Wolf-Rayet star) with a Monte Carlo transport code. The expansion velocity, the total mass of the ejecta, the radial mixing of radioactivity in that ejecta, and the 56Ni yield effect both the strength and the evolution of the hard radiation. With the anticipated launch of Gamma Ray Observatory, we can hope to detect Type Ib supernovae to distances of 3 Mpc and utilize the characteristics of the gamma lines and X-ray spectrum to distinguish between differing Type Ib supernova models and to address their suggested relationship to Wolf-Rayet stars.
From LBV Binary to WR+OB Binary
Authors: Vanbeveren D.
Published: June 2019

The LBV phase is generally identified with the hydrogen shell burning phase of a star with initial mass larger than 40-50 M⊙ (see also Humphreys, this volume) and therefore in binaries with ZAMS component masses larger than 40-50 M⊙ and periods larger than 4-7 days the primary may experience a LBV mass loss phase before it reaches its critical equipotential surface (the Roche lobe). I then define the ‘LBV scenario’ of massive close binaries as follows:

when a binary component (initial mass larger than 40-50 M⊙) reaches the LBV phase prior to its Roche lobe overflow phase (RLOF), a stellar wind mass loss phase sets in at rates comparable to the rates encounterd during a RLOF process and which are large enough in order to prohibit the occurence of a RLOF.

Evolutionary models are computed for close binaries with initial primary masses larger than 50 M⊙ and mass ratios ranging between 0.2 and 1. Special attention is given at the predicted spectral type of the secondary component. In order to determine the mass of the primary at the end of its LBV phase, I have used the following general theorem holding for the most massive stars, i.e.

a hydrogen shell burning mass loser with mass larger than 50 M⊙ in a massive close binary restores thermal equilibrium (and becomes a WR star) when helium starts burning in its core and when its atmospherical hydrogen abundance has dropped to Xatm = 0.2-0.3 (by weight).
The Infrared Spectrum of P Cygni
Authors: Deacon J. R. , Barlow M. J.
Published: June 2019

An energy distribution for P Cyg (B1 Ia+) has been produced using UKIRT photometry and spectra, flux-calibrated IUE high-resolution spectra, Johnson and Mitchell 13-colour photometry and IRAS photometry. Infrared excesses due to free-free emission from the wind have then been derived and modelled.
Observational Evidence for Evolutionary Connections Between Wolf-Rayet Stars and Red Supergiants
Authors: Georgiev Leonid N.
Published: June 2019

The main question in our understanding of the nature of W R stars is: which stars are their progenitors and how do they evolve to reach the WR stage? We have some ideas about progenitor's masses and, by mass luminosity relations, about their brightness when they are 0 stars. But, it is difficult to say anything for their possible Red phase.
Progenitors of Type Ib Supernovae
Authors: Panagia Nino , Laidler Victoria G.
Published: June 2019

We argue that the progenitors of type Ib supernovae are moderately massive stars (M ~ 7 M⊙) in binary systems whereas the hypothesis that they originate from very massive stars (M > 20 M⊙) is not consistent with the observational evidence on SNIb.
Large Number of Wolf-Rayet Stars in Emission-Line Galaxies
Authors: Mas-Hesse Jose Miguel , Kunth Daniel
Published: June 2019

We present synthetic populations of massive stars that constrain the properties of the star formation episodes, predict the occurence, subtype distribution and evolution of WR star populations at solar and SMC metallicities. Instantaneous burst and continuous star formation rates have been considered. Our predictions have been tested on a sample of 17 galaxies. The evolution of the WR stars is qualitatively well reproduced. The IMF slope is flatter on average than in the solar vicinity and is not correlated with the metallicity. The extinction law is in general similar to that of the SMC, independently of metallicity.
On the Detection of WR Stars in Nearby Galaxies
Authors: Lequeux J. , Azzopardi M.
Published: June 2019

The different methods for detecting WR stars in nearby galaxies are reviewed and compared. Each one has its own advantages and drawbacks, and they are complementary rather than competing. Both monochromatic filter imaging and field- or single-object spectroscopy are required for an absolutely safe detection of objects near the limit. The results obtained to-date are shortly reviewed and perspectives are discussed.
Determination of the Interstellar Extinction Toward WN Stars from Observed UBV Color Indices
Authors: Vacca William D. , Schmutz Werner
Published: June 2019

Recent observations of Wolf-Rayet stars by Massey (1984) and Torres-Dodgen and Massey (1988) have yielded high quality, absolutely calibrated spectra of nearly all known Wolf- Rayet objects. These observations also indicate that discontinuities, or “jumps”, are present in the continuum spectra of some Wolf-Rayet stars. Such continuum jumps are predicted by the current theoretical models of Wolf-Rayet atmospheres. In general, these models provide good fits to the observed spectra of Wolf-Rayet stars. The models also indicate that, between jumps, the intrinsic continuum can be closely approximated by a power law in wavelength. In this case, we have the following relation between the intrinsic colors:

where

and Z3645 is the strength of the He II (η = 4) jump at 3645 Å in mags. This relation holds because the central wavelength of the u filter (λu = 3650 Å) is nearly coincident with that of the He II jump. In addition, models of WN stars with helium-dominated atmospheres predict a correlation between D3645 and (6 - v)o:
The Distribution of M Supergiants with Large Infrared Excess and WR Stars in the Galaxy
Authors: Raharto M.
Published: June 2019

From the m12 – m25vs. spectral type diagram (see for example: Habing, 1987, Figure 5), it can be seen that there are no early M giant stars with m12 – m25 > 0.7. On the other hand all early M stars with m12 – m25 > 0.7 are M supergiants. These early type M supergiants have typically infrared excesses at 12 μm in the range of 20 to 170 times higher than their photosperic fluxes.
Note on Absolute Magnitudes in the Red and Infrared of Late M-Type Dwarfs
Authors: Lippincott S. L.
Published: June 2019

The importance of red and infrared magnitudes for late dwarf M-type stars is stressed. The absolute magnitudes for the nearest stars with (R − I) ⩾ 1.20 is re-assessed.
Photo-Electric Measures of Hα and Hβ in the Scorpio-Centaurus Association
Authors: Jones D. H. P. , Carrick D. W.
Published: June 2019

Photo-electric measures of Hα and Hβ have been made in an attempt to confirm the scatter in the Luminosity-Balmer line relation predicted by Hardorp and Strittmatter as an effect of Stellar rotation.
Rotational Effects on the Photometric Determinations of Absolute Magnitudes for A- and F-Type Stars
Authors: Maeder A.
Published: June 2019

The effects of rotation on the determinations of absolute magnitudes by means of the c1 vs (b – y) and d vs (B2 – V1) diagrams are discussed.
Absolute Magnitude Determinations from Hydrogen-Line Photometry
Authors: Crawford D. L.
Published: June 2019

The history and status of Balmer hydrogen-line photometry are briefly reviewed, and problems associated with calibrations in general are commented on.

The calibration work at Kitt Peak is described in some detail for B, A, and F type stars. The data used, the determination of intrinsic colours and colour excesses, photometric classification, and the relation of our indices to other people's are reviewed. With this background, the procedure used in establishing the absolute magnitude — Hβ calibration is given, and the preliminary calibration is presented. Finally, the work remaining to be done before the calibration is final is noted, and a comparison is given to Blaauw's zero-age main sequence calibration.
The Hγ-Absolute Magnitude Calibration
Authors: Crampton D.
Published: June 2019

A new calibration of Hγ equivalent widths is presented. Unlike Petrie's method, the new calibration does not require an estimate of the luminosity class or spectral type of the star. A brief comparison with other calibrations is made.
The Absolute Magnitude and Stellar Population of a Ba II Star
Authors: McClure R. D.
Published: June 2019

A star showing extreme features characteristic of the Ba II stars has been found in the old open cluster NGC 2420. A new colour-magnitude diagram has been constructed for the cluster, and a distance modulus of 11m.7 is obtained by fitting the unevolved main sequence to the zero-age main sequence. The location of the Ba II star is consistent with its being a disc population star of absolute magnitude Mv = −0m.3, mass 1.4 M⊙ and age 2.4 × 109 yr.
Scientific CCD Prospects for 1994 and Beyond
Authors: Jorden Paul R. , Oates A. P.
Published: June 2019

We review the present status of CCD use at major observatories, discuss the various developments that are leading to improved sensors. The choice of devices is quite extensive, but far from stable! A look at progress over the last few years helps to put present and future developments in perspective. Several areas of active CCD development are presented and discussed.
Observational Concerns and Techniques for High-Background Mid-Infrared (5–20 Micron) Array Imaging
Authors: Gezari Daniel Y.
Published: June 2019

Broadband observations at 5–20 micron with large ground-based telescopes are dominated by thermal background radiation from the telescope optics, instrument and sky. Astronomical sources typically contribute less than one percent of the total detected flux. In most cases the direct, unprocessed image of a bright source (before background subtraction) is indistinguishable from an image of blank sky (Fig. 1). This paper gives an overview of the fundamental concepts, conditions and limitations of high-background mid-infrared imaging. The techniques used to acquire and process the image data are summarized, illustrated with examples from our 58 × 62 pixel mid-infrared camera astronomy program.
CCD Photometry: Some Basic Concerns
Authors: Sterken C.
Published: June 2019

We review the status of CCD photometry with emphasis on applications in projects that require considerable amounts of observing time at medium-aperture telescopes and at tasks complementing projects carried out at large telescopes. Associated problems of CCDs are discussed, together with unsuspected difficulties affecting millimagnitude accuracy mainly caused by pixel-size mismatch, cooling, data acquisition, non-linearity of response and improper standardization.
Automated CCD Scanning for Near Earth Asteroids
Authors: Jedicke Robert
Published: June 2019

The Spacewatch group at the University of Arizona's Lunar and Planetary Laboratory was probably the first (1984) to implement CCD-scanning in a major astronomical program. In the past three years, using a Tektronix 2048 × 2048 CCD, the program has discovered ∼ 45% of the new Earth approaching asteroids, measured astrometric positions for over 50,000 main belt asteroids, discovered two of the three known Centaurs, and found evidence for an unheralded population of small (∼ 10-m) objects in the inner solar system. This success is due to the automated Moving Object Detection Program (MODP) which searches successive scans over the same region for objects showing consistent motion. While visual examination of photographic plates may have a higher efficiency, an automated routine for detecting moving objects does not tire and is repeatable. Our recent work quantifies the efficiency of MODP as a function of the asteroid's magnitude, rate of motion, and orbital parameters. Other work suggests that the observational detection of faint, trailed, Fast Moving Objects may be improved by incorporating linear darkfield subtraction and flatfielding during scans.
High Precision Stellar Photometry by CCDs. I
Authors: Penny A. J.
Published: June 2019

This talk is about the limits to the precision of stellar photometry in comparing one star with another in a single CCD frame. This is concerned with bright stars, and concentrates on three problems at the 0.1 percent level of accuracy: how to flatfield; how to deal with varying point-spread-functions that vary across an image; how to deal with the fact that the response inside a pixel is not uniform. The first is the well-known difficulty of getting a uniform illumination across the CCD to use as a flatfield; the use of a rotatable CCD mounting and of drift-scanning is discussed. The second depends on the ability to detect and define small, but significant, changes in the PSF. The third is the fact that the pixels of optical CCDs can have non-uniformities inside them of ten percent, and these when folded with the PSF produce systematic errors significant at the 0.1 percent level; with infra-red arrays these problems can be much worse. The use of software to model these variations and reduce these errors is described.
NICMOS3 Detector for Spectroscopy
Authors: Vanzi L. , Marconi A. , Gennari S.
Published: June 2019

We present some results about the use of an engineering grade NICMOS3 array for medium resolution spectroscopy. All the work described here was performed at the Infrared Laboratory of the Osservatorio Astrofisico di Arcetri in Florence and at the Italian Telescope TIRGO (Citterio 1978). The detector has been mounted on the LonGSp (Longslit Gornergrat Spectrometer), an instrument designed and set into operation in Arcetri and described in Gennari and Vanzi (1994).
A Dual CCD Mosaic Camera System Searching for Massive Compact Halo Objects (Machos)
Authors: Cook Kem H.
Published: June 2019

The Macho Collaboration uses a dedicated 1.27-m telescope (The Great Melbourne Telescope) at Mount Stromlo to make photometric measurements of tens of millions of stars per night searching for the gravitational microlensing signature of MACHOs in the halo and disk of the Milky Way. A prime focus corrector and dichroic beamsplitter provide red (6300–7800 å) and blue (4500–6300 å) foci with one degree fields. A two by two mosaic of 2048 × 2048 pixel CCDs in each focal plane provides simultaneous images of 0.5 square degrees. By August of 1994, more than 20,000, 32 megapixel images will have been collected of fields in the Large Magellanic Cloud (LMC), Small Magellanic Cloud and the bulge of the Milky Way. We have implemented an online analysis system which produces photometric reductions of a night's data (five Gbyte of images) within 24 hours. This system allows us to identify and follow interesting events in real-time. In this search, we have identified more than 60,000 variable stars, and a preliminary analysis of their types and distribution will be presented. Microlensing events toward the LMC and the bulge have been discovered and detection efficiencies are being calculated to constrain the MACHO content of the Milky Way's halo.
CCD Mosaic Development for Large Optical Telescopes
Authors: Luppino G. A. , Metzger M. R. , Miyazaki S.
Published: June 2019

We outline the recent developments in CCD imager technology aimed at producing the very large format (8192 × 8192 pixels and larger) detector mosaics required for existing 4-m class and new 8 to 10-m class telescopes. The key technology areas include buttable array design and buttable element packaging, and optimization schemes for QE and readout time. As an example, we highlight the University of Hawaii effort to develop an 8192 × 8192 15 μm pixel CCD mosaic.
Highlights of IAU Symposium No. 167
Authors: D'Odorico Sandro
Published: June 2019

The talks at this IAU Symposium have illustrated the spectacular development which has taken place in the last decade in the field of array detectors for astronomy. Just a few years ago it was possible to speak of two-D detectors for the UV-red wavelength range only. At this meeting we have witnessed presentations on array characteristics from the extreme UV (Bonanno 1995), through the blue-visual range (D'Odorico 1995, Jorden and Oates 1995, Iwert 1995 and Luppino et al. 1995); the infrared 1 to 5 μm window (McLean 1995, Finger et al. 1995, Gilmore et al. 1995, Fazio 1995, Glass et al. 1995 and Ueno et al. 1995); the 10–20 μm window (Fazio 1995, Gezari 1995) and finally to an array of bolometers to operate at submillimeter wavelengths (Moseley 1995). Field imaging and spectroscopy are now possible across this entire energy spectrum and some of the first exciting astronomical results obtained with these devices have been presented here.
The Rutherford-SAAO CCD Controllers and Their Applications
Authors: Glass I. S. , Carter D. B. , Woodhouse G. F. , Waltham N. A. , Newton G. M.
Published: June 2019

We describe a versatile array controller developed at RAL and SAAO. The original concept was due to Waltham, van Breda and Newton (1990). A Transputer-based microcomputer forms the heart of the device.
The Feasibility of a CCD for an Astrometric Refractor
Authors: Upgren A. R. , Morales Alice , Herrero Jose , Griese J. W. , Vincent J. M. , Lee John T.
Published: June 2019

The 0.5m refracting telescope of the Van Vleck Observatory has been active in the determination of trigonometric parallaxes since its first observations in 1922. Its lenses were ground by C.A.R. Lundin of the Alvan Clark Co. for photographic use. Coma was minimized across the field and vignetting was also kept to a minimum. Partly as a consequence the focal curve is very steep in the blue and green regions of the spectrum, as is shown in Fig. 1. A Wratten No. 12 minus blue filter is used to filter out all wavelengths to the blue of about 5200 å. The region between 5200 å and 6000 å is very flat with the focal plane varying over a range of about one millimeter. Towards the red region it steepens, although not enough to impair images on photographic plates of emulsion types 103a-D and IIIa-F, the two in widespread use in recent years.
Stellar Positions from CCD Images
Authors: Upgren A. R. , Abad C. , Stock J.
Published: June 2019

The CIDA visual refractor of 65-cm aperture and focal length 10.5 m, has been used extensively for position determinations on photographic plates. The combination of Kodak D plates and a yellow filter permit an almost perfect adaptation to the focal curve of the telescope. It appeared of interest to test whether the telescope could be used for astrometric purposes with a CCD detector. As is well known, the spectral sensitivity of these detectors extends well into the infrared where the images formed by the telescope optics will be far out of focus. The blue spectral region where this would also be the case can easily be cut off by a yellow filter. There are no filters which would produce a similarly sharp cut-off towards the red region. On the other hand, given the small field covered by a CCD, the displacement of the red out-of-focus image with respect to the center of the visual image might be negligible. Recently obtained accurate positions in the area of the Perseus Double Cluster made this field suitable for the test of this possibility.
Combined Visual and Near-IR Digital Photometry: The Very Young Cluster Westerlund 2
Authors: Guarnieri M. D. , Gai M. , Massone G. , Lattanzi M. G. , Munari U. , Moneti A.
Published: June 2019

We discuss CCD UBVRI and NICMOS3 JHK observations of the young open cluster Westerlund 2, with emphasis on both reduction techniques and absolute calibrations.
A Dual CCD Mosaic Camera System Searching for Massive Compact Halo Objects (Machos)
Authors: Cook Kem H.
Published: June 2019

The Macho Collaboration uses a dedicated 1.27-m telescope (The Great Melbourne Telescope) at Mount Stromlo to make photometric measurements of tens of millions of stars per night searching for the gravitational microlensing signature of MACHOs in the halo and disk of the Milky Way. A prime focus corrector and dichroic beamsplitter provide red (6300–7800 å) and blue (4500–6300 å) foci with one degree fields. A two by two mosaic of 2048 × 2048 pixel CCDs in each focal plane provides simultaneous images of 0.5 square degrees. By August of 1994, more than 20,000, 32 Megapixel images will have been collected of fields in the Large Magellanic Cloud (LMC), Small Magellanic Cloud and the bulge of the Milky Way. We have implemented an online analysis system which produces photometric reductions of a night's data (five Gbyte of images) within 24 hours. This system allows us to identify and follow interesting events in real-time. In this search, we have identified more than 60,000 variable stars, and a preliminary analysis of their types and distribution will be presented. Microlensing events toward the LMC and the bulge have been discovered and detection efficiencies are being calculated to constrain the MACHO content of the Milky Way's halo.
CCD Photometry of the M 67 Cluster in the Vilnius Photometric System
Authors: Boyle R. , Straižys V. , Vrba F. , Smriglio F. , Dasgupta A. K.
Published: June 2019

The Vilnius photometric system, consisting of seven passbands at 345, 374, 405, 466, 516, 544 and 656 nm makes it possible to determine spectral classes (or temperatures), absolute magnitudes (or surface gravities), metallicities and peculiarity types for stars of all spectral types in the presence of interstellar reddening (Straizys 1977, 1992a, b). This makes the system very useful for the determination of the physical parameters of stars which are too faint to be studied by spectroscopic methods. The system is especially effective when used with CCD detectors which combine a wide field, high sensitivity and high photometric accuracy.
Magnetic Flux Ropes of Venus: A Paradigm for Helical Magnetic Structures in Astrophysical Systems
Authors: Elphic R. C.
Published: June 2019

The magnetic flux ropes of Venus are small scale (ion gyroradius) cylindrically symmetric structures observed in situ by the Pioneer Venus orbiter in the largely magnetic field-free ionosphere of the planet. They are so named because of their helical magnetic structure, which in turn is due to primarily field-aligned currents within the rope. Empirical models can be used to examine the current structure in detail, and these models indicate that flux ropes may be unstable to the helical kink mode. Statistics of rope distribution and orientation also support this instability picture. The results of investigations into the direct measurements of Venus flux ropes may be relevant to certain astrophysical phenomena that must be observed remotely.