bluuebunny/crossref_metadata_2025 huggingface dataset:
The TRUMP Astrophysics Project: resources for physics teaching
Authors: Swinbank E.
Published: June 2019

Comets and quasars, black holes and the big bang, pulsars and planets all feature in the media and excite people to find out more - astronomy might be described as the popular face of modern science. In the UK, recent changes in Advanced Level (A-level) physics courses mean that many students have the option of studying astrophysics to a depth beyond the merely descriptive. This option is proving popular with teachers and students, but presents particular challenges shared by few other areas of A-level physics courses.
About the Spectral Classification of Be Stars
Authors: Feinstein A. , Tignanelli H.
Published: June 2019

The problem of describing the Be stars is especially interesting to investigate. A large collection of these objects are organized into information with preserving structures, such as classifications. In this case the difficulties consist of: 1) determining which are the main properties in the classification, and 2) deciding which one of the possible classifications should be used. For the first part there are a MK spectral system and a scheme of classifications presented by Jaschek et al. (1980), among others. The second part has been worked out in many ways. Here we describe, following the Information Theory (IT), an analytical method to compare both classifications of Be stars.
Simultaneous Spectroscopy and Polarimetry of Be Stars
Authors: Gies D. R. , McDavid David
Published: June 2019

Evidence is now accumulating that many Be stars display photospheric line profile variations on timescales of days or less that are probably caused by nonradial pulsations (Baade 1984; Penrod 1986). In some circumstances these pulsations can promote mass loss into the circumstellar envelope, and consequently the conditions in the inner part of the envelope may vary on similar timescales. Changes in the envelope could produce variations in the polarization and emission line profiles, and observers have reported rapid variability in both. We describe here an initial attempt to search for simultaneous variations in continuum polarization, Hα emission, and the He I λ6678 photospheric absorption line in order to investigate correlated changes on short timescales.
A Transient Shock-Wave in the Shell of the Be Star HD 184279
Authors: Ballereau D. , Chauville J.
Published: June 2019

HD 184279 (V1294 Aql) has presented in the past an emission spectrum on the first terms of the Balmer series, and shell absorptions on the hydrogen and neutral helium lines. Their variations have been reported by Merrill (1952), Merrill and Lowen (1953) and Svolopoulos (1975). Ballereau and Hubert-Delplace (1982) evidenced long-term V/R variations with an amplitude of ∼100 km s-1. Short-term photometric variations are irregular (Tempesti and Patriarca, 1976), while long-term variations are correlated with radial velocity (RV) of shell lines (Horn et al., 1982). Ballereau and Chauville (in preparation) extended the spectroscopic observations until 1984 and confirmed the pseudo-periodic variations, the last half-period ranging over 3.4 years (April 1, 1980-August 1, 1983).
Optical Emission-Line Spectra of Be Stars (Review Paper)
Authors: Dachs Joachim
Published: June 2019

Recent observational work on spectra obtained at optical and near-infrared wavelengths is reviewed for “ordinary” (non-supergiant non-peculiar) Be and Be-shell stars, with particular emphasis on comparison between high-resolution spectral measurements and current Be star models. Emission-line profiles are interpreted in terms of geometry, dimensions and dynamics of a dense (N ≃ 1011 cm-3), cool (T ≃ 104 k) line-emitting region in the circumstellar envelope, separated from the thin, hot expanding stellar wind region. Present observations support a rotating-disk model for the cool, line-emitting region. Indications pointing towards structuring of cool circumstellar envelope regions are reported. Significant spectral variations are detected both on short (less than one day) and long time scales (months to years). Typical cycle times for large-amplitude V/R variations and intervals between shell episodes depend on spectral type, increasing with decreasing effective temperature of the central stars and ranging from a few years for early-Be type stars to a few decades for late Be stars.
Observational Constraints on Cool Disk Models of Be Stars
Authors: Hubert A.M. , Floquet M. , Chauville J. , Chambon M.Th.
Published: June 2019

At first we examine the main characteristics of 3 well-known shell stars (EW Lac, ζ Tau, 48 Lib) before and during their variable shell phase (long time scale V/R and RV cycles). For that we have investigated a) recent studies of our Meudon working group, based on long sequences of spectroscopic observations (Hubert et al. 1983, 1986a; Mon 1984) b) previous data found in the literature c) additional information by a reexamination of line profiles obtained from our material. For ζ Tau and 48 Lib the selected cycles have been chosen among the most representative ones in the history of their respective shell episode. For EW Lac, which entered an active phase in 1977, we have studied the only recently observed cycle of shell feature variations. A summary of the 3 V/R shell stars characteristics during 1) the pre variable shell phase 2) the variable shell phase is reported elsewhere (Hubert et al. 1986b); we only give here general trends of it, Table 1.
An Optical Study of Southern Be Stars
Authors: Giovannelli F. , Rossi C. , Vittone A. A.
Published: June 2019

Within the framework of a coordinated campaign of observations in optical and IR of a sample of 21 southern Be stars, we report some results from spectra taken at ESO La Silla Observatory with 1.5m telescope equipped with IDS and the Boller & Chivens grating spectrograph used at 220 A/mm reciprocal dispersion. The spectra, taken in March 1985, range from about 4250 to 7000 A. The equivalent widths of the Balmer lines, either in emission or in absorption, have been computed. Fig. 1 shows the relationship between the logarithm of the absolute value of the Hα, Hγ equivalent widths ratio and the logarithm of the luminosity of the stars in solar units.
FeII And FeIII Lines As a Diagnostic of the Physical Conditions in the Atmospheres of Be Stars
Authors: Baratta G.B. , Friedjung M. , Muratorio G. , Viotti R.
Published: June 2019

Fell lines are frequently observed in the optical spectra of early type emission line stars. Space observations of their ultraviolet spectra led to the discovery of a considerable line absorption due to ionized metal lines formed in the stellar envelopes. We describe some examples and show that spectral synthesis is required especially for the analysis of the low resolution spectra, and may provide information about the structure of the envelopes and the mass loss rates. We also discuss the Felll lines in the UV spectra of a Herbig Be star and of β Lyrae.
The Spheroidal/Ellipsoidal, Variable Mass-Loss, Decelerated Be Star Model (Review Paper)
Authors: Doazan V.
Published: June 2019

The proposed model is empirical; it is based on analysis of the available data on Be stars obtained in all the observable spectral regions, and it is required to be thermodynamically self-consistent. Rather than trying to answer the question: “What is the origin of the Be-phenomenon?” We ask: First. “What phenomena characterize empirically Be stars?” Second, “What thermodynamic characteristics are implied by the existence of such phenomena?” Third. “What inferences may be made an the atmospheric structure of a Be star from these empirical and thermodynamical characteristics?” The observed phenomena, their thermodynamical implications and the resulting model have each two aspects. On the one hand, the observational evidence for a nonradiatively heated, expanding chromosphere-corona implies the existence of both a nonradiative energy flux and a mass outflow from the photosphere. This first aspect is common to both Be and normal B stars, at least for the earliest subtypes.
RR Lyraes in the Magellanic Clouds
Authors: Walker Alistair R.
Published: June 2019

Recent work on the Magellanic Cloud RR Lyrae stars is reviewed. The absolute magnitudes of LMC RR Lyraes, when calibrated from a distance modulus of 18.5 mag, disagrees with the Galactic calibration. The revised distance scale makes distances greater, and ages younger, within our galaxy. Field star studies show that the “halo” population of the LMC is very similar to that of our own Galaxy outside of the solar circle. This result supports a Searle and Zinn model of galaxy formation.
I-band Cepheid Distance to WLM
Authors: Lee Myung Gyoon , Freedman Wendy L. , Madore Barry F.
Published: June 2019

WLM (DD0221) is a highly resolved dwarf irregular galaxy in the Local Group. Fifteen Cepheids in this galaxy have been studied by Sandage & Carlson (1985, AJ, 90, 1464) using photographic photometry. We have obtained BVRI CCD photometry of stars in the central area (2.2' x 3.5') of WLM. Figure 1 shows an I–(V – I) diagram of ∼ 2,600 measured stars including five known Cepheids. The distance to WLM has been estimated using the random-phase I band photometr y of five Cepheids (V7(I=20.52), V29(I=20.77), V48(I=20.43), V66(I=21.19), V67(I=21.14)) combined with their periods given by Sandage & Carlson. We have obtained Δ(m - M)II(WLM – LMC) = 6.25 ± 0.11 and (m - M)0 = 24.92 ± 0.21, adopting (m - M)o(LMC) = 18.5 and AI(WLM) = 0.04.
White Dwarf and Pre-White Dwarf Oscillations
Authors: Cox Arthur N.
Published: June 2019

Compact stars that result from extreme mass loss on the asymptotic giant branch and planetary nebula formation are observed to pulsate in a very large surface effective temperature range as they cool to become the classical white dwarfs. The hottest and most luminous of these display periods in excess of 1000 seconds because they are large, but when the stars arrive on the cooling line on the Hertzsprung-Russell diagram, their periods become generally less than 1000 seconds. Then the stars have masses near 0.6 M⊙ and radii near 109 cm. Their luminosity depends then almost entirely on the surface effective temperature as the entire star with its legacy of complicated internal luminosity peaks cools to the classical simple electron degenerate structure. Very thin surface layers of hydrogen and helium cover the bulk of the carbon- and oxygen-rich mass that results from hydrogen and helium burning in earlier intermediate mass stellar evolution. The cause of the nonradial pulsations of low angular degree, but rather high radial order, for the most luminous of these stars is the cyclical ionization of carbon and oxygen in layers not too deep that their effectiveness is limited by a long luminosity time scale. Thus the surface hydrogen and helium must be thin, probably thinner than the current period spacings interpretation for PG 1159-035 suggests. For the classical DBV and DAV pulsators, it appears that neither the hydrogen ionization K and γ effects or convection blocking at the bottom of a hydrogen convection zone can destabilize the observed pulsations when the overriding short time scale effects of time-dependent convection are included. It appears, however, that a thin CO convection shell can produce pulsations by its time-dependent effects, but again only very thin H and He surface layers are allowed. This new pulsation mechanism can alleviate the serious problem that DAV variables are observed hotter than the hydrogen K and γ effects and convection blocking can predict. The appearance of non-pulsators in the DAV and DBV instability strips can be explained by a too-thick hydrogen and helium surface layer that interferes with (poisons) the CO ionization convection zone. Finally time-dependent convection predicts that only a few of the many possible modes exist due to their internal amplitude structure that can result in both strong driving and strong damping. Thus actually observed pulsating modes can assist in mapping individual internal white dwarf composition structures, not only by their periods but also the fact that they pulsate.
Nonradial Pulsation amongδScuti Stars
Authors: Breger Michel
Published: June 2019

The review examines recent observational and theoretical results on the nonradial pulsation ofδScuti stars. The dominant pulsation modes are rotationally split p modes with= 1 and 2, and radial orders 1 to 4. Line-profile analyses also reveal the existence of additional high-degree modes. Nonradial pulsation is usually accompanied by slow amplitude and period variability. The small-amplitude single-mode variables are shown to be typical nonradial pulsators.

The interesting cases of 1 Mon andθ2Tau are examined in more detail. The present status of asteroseismolgy of theδScuti stars is reviewed.

The periods of the four evolvedδScuti variables studied are found to be decreasing, although stellar evolution predicts period increases due to larger radii. New work on the multiple periods of AI Vel shows that, apart from stellar evolution, an additional mechanism must exist to cause temporary period changes. It is argued that for a larger sample of stars the average period change must nevertheless reflect evolutionary changes.
Frequency analysis of multiperiodic δ Scuti stars
Authors: Mantegazza L. , Poretti E. , Zerbi F.M.
Published: June 2019

We report some of the recent results of our studies on δ Scuti star pulsation which are based on observations made at Merate and La Silla Observatories, sometimes in two site campaigns. Our recent experiences on X Caeli and 44 Tauri have shown that quite complicate light curves can be resolved even with observations obtained from one site only, if compact and accurate datasets are collected on sufficiently long time baselines (Mantegazza and Poretti, 1992; Poretti et al., 1992). Here we report the preliminary results of the light curve analysis of three more stars surveyed for more than 120 hours each: BI CMi, HD 18878 and HD 224639.

BI CMi has been observed in a two-site campaign in January-February 1991, the observations being collected during 17 nights. The frequency analysis of the light curve allowed us to identify unambiguously 4 pulsation modes at 8.247, 8.863, 8.514 and 7.424 c/d with semi-amplitudes of 22, 19, 5 and 5 mmag respectively.
A Striking Similarity Between the Sun, Binary Systems and RR Lyrae Stars
Authors: Kotov V. A.
Published: June 2019

The periodicity of P0=160.0101 (±1) min discovered in the Sun's global oscillations, then in rapid variability of active galactic nuclei (AGNs) (Kotov & Lyuty 1990), might have a cosmological origin. Analysing the distribution of periods of ∼4000 close binaries we found that the most commensurate (with an odd-even parity taken into account) period of galactic binaries is equal to 160.0(±1) min (∼ 4σ). Accordingly, it is argued that the famous 2-3 hr gap of cataclysmic binaries might be closely associated with the “universal” P0- oscillation. A similar analysis applied to the ∼1200 RR Lyrae variables in galactic globular clusters (GCs) and 72 RR Lyraes in the LMC GCs (Graham 1985; Nemec et al. 1985) showed that for all those stars the P0- period appears to be the most “resonant” one (Fig. 1). This strongly supports the cosmological origin of the P0- oscillation. Since the P0- resonance for RR Lyraes appears to be most pronounced when the “odd-even” parity is taken into account, it is hypothesized that variability in the majority of GC RR Lyraes should be related to their binary nature and may be relevant to explanation of Blazhko effect.
Focal Points in Contemporary β Cephei Star Research
Authors: Sterken C.
Published: June 2019

We review the latest results on searches for new β Cephei stars, and discuss the most recent findings from high-resolution spectrographic observations. The implications of analyses of archival data and new observations in the light of period changes, binarity, and aspects of the evolutionary state of β Cephei stars are evaluated.
Time-Series Spectroscopy of ζ Ophiuchi
Authors: Reid A.H.N. , Bolton C.T. , Crowe R.A. , Fieldus M.S. , Fullerton A.W. , Gies D.R. , Howarth I.D. , McDavid D. , Prinja R.K. , Smith K.C.
Published: June 2019

We have undertaken a multi-site, multi-wavelength observing campaign on the archetypal O stars ζ Puppis (O4 I(n)f) and ζ Ophiuchi (O9.5 V). Both stars are well known for the strength of their line profile variations (lpv's), and represent extremes of O spectral type and luminosity class. UV time-series spectroscopy of ζ Pup and ζ Oph is described by Prinja et al. (Ap.J. 1992, 390, 266), and Howarth et al. (Ap.J. 1992, submitted) respectively. The optical spectroscopic results of ζ Oph are reported by Reid et al. (1992, ApJ submitted), of which some of the principal results are given here.

During late April, and early May, 1989, we obtained high-resolution, high signal-to-noise optical spectra of the late O-type, rapid rotator ζ Oph. Time-series analysis, using the CLEAN algorithm, has shown that the characteristic lpv seen in HeI λ447lÅ, Si III λλ4552, 4567, 4575Å, and MgII λ448lÅ can be satisfactorily represented as a set of 4 sinusoids. No substantial variation is observed in HeII λ4541, or NIII λ4517Å. We attribute this behaviour to a combination of equatorial gravity-darkening and a latitudinally-confined origin for the lpv.

The phase changes over the line profiles indicate repetitive patterns of axial symmetry, rotating prograde in the co-rotating frame of the star. The periods are 3.339 hours (-m = 4), 2.435 hours (-m = 5 or -m = 6), 1.859 hours (-m = 9 ± 1), and either 1.366 hours or 1.292 hours (-m = 11 ± 1); -m represents the spatial frequency around the stellar equator. The first three periods confirm those found at earlier epochs, and we conclude that some lpv characteristics are reproduced over at least a 2-year interval.

Since no commensurate superperiod (|m|P) exists, and since the super-periods are less than our estimated minimum rotation period for ζ Oph (> 18 hours), we reject a rotational modulation origin for the lpv and conclude that the star is undergoing multi-mode, sectorial, non-radial pulsations.
Dust Induced Dynamics of Circumstellar Shells around Long—Period Variables
Authors: Gauger A. , Fleischer A.J. , Sedlmayr E.
Published: June 2019

The presence of dust essentially affects the optical appearance and the dynamics of circumstellar shells around LPVs, while in turn the formation of solid particles as well as their properties critically depend on the actual local conditions. Thus, a reliable modelling of the structure and dynamics of such shells requires a detailed treatment of the dust complex.

In our hydrocode, which is described in more detail in Fleischer et al. (1992), the time evolution of the dust component is treated consistently by means of a moment method (Gauger et al. 1990). For the solution of the hydrodynamical and thermodynamical problem we essentially follow the approach of Bowen (1988), except for the shocks, which so far are assumed to be isothermal.
A spectroscopic study of RCB stars in the Galaxy and the LMC
Authors: Pollard Karen , Cottrell P.L. , Lawson W.A.
Published: June 2019

High resolution échelle spectra have been obtained of two Large Magellanic Cloud (LMC) and one galactic R Coronae Borealis (RCB) star with the Anglo-Australian Telescope. An analysis of these data using He- and C-rich models and the model atmosphere code WIDTH6 of Kurucz indicates that the galactic RCB star SU Tau and the two LMC stars, W Men and HV12842, have similar atmospheric parameters to the warmer galactic RCB and hydrogen-deficient Carbon (HdC) stars e.g., R CrB and XX Cam. Specifically, the new stars have Teff ∼7000 K, log g = 0.5-1.0, microturbulent velocities between 6 and 8 km.s-1 and C/He ratios from 0.004 to 0.006.

An abundance analysis has been performed on these stars for a wide range of species. Special emphasis has been placed on particular elements H, He, Li, C, N, O, & Fe (see figure 1), which are key indicators of the extent and relative importance of nuclear processes such as the CNO cycle and triple-alpha process, and also of the physical processes such as convective mixing and mass loss. Specific values for the abundances of [H/He], [Li/Fe] and [Fe/total] (expressed relative to the total abundance of all species) in SU Tau and HV12842 are -5.4 & -6.4, +2.5 & +2.4, -0.4 & -0.8.
Oosterhoff I and II RR Lyrae Variable Masses
Authors: Cox Arthur N.
Published: June 2019

The anomalously low theoretical masses for double-mode RR Lyrae variable stars obtained from accurately observed period ratios recently have been increased by using stellar models constructed with the new Livermore OPAL opacities. These new models appear to the lowest order radial modes to be less concentrated in density, and they thus produce longer periods. Since the fundamental mode is increased in period more than the first overtone, this period ratio from the model is decreased. Cox (1991) showed that now these pulsation-based masses agree with evolution masses, and all RR Lyrae variable stars show little mass loss from the red giant tip in earlier evolution. This investigation has been validated by two additional papers that use more directly the OPAL opacities rather than simple adjustments to the Stellingwerf (1975ab) fit. These new papers (Kovacs, Buchler, and Marom, 1991, and Kovacs, Buchler, Marom, Iglesias, and Rogers, 1992) also display composition effects for Population II mixtures.

My new study uses directly the latest OPAL intermediate coupling opacities for the Oosterhoff I and II (Z=0.0003 and 0.0001, respectively) compositions in frozen-in convection (mixing length/pressure scale height ratio=1.25) models to see if there is any difference between the masses of variable stars in these two globular cluster classes. Derived masses are just over 0.65 and 0.75 M⊙, respectively. The Oosterhoff I mass could be as large as the Oosterhoff II mass only if the iron abundance in the Z mixture is increased by more than 30% over that for the solar mixture.

The Sandage effect of higher periods for variables in Oosterhoff II clusters at lower Z than for the Oosterhoff I clusters might be explained with my new masses by having the Oosterhoff II variables born much to the blue of the instability strip and evolving at increasing luminosity to the red. For Oosterhoff I variables, they can be born at their lower mass and higher Z inside the instability strip at lower luminosity, evolving both to the red and blue with little evolutionary luminosity increase before they become red nonvariables.
A Full-Amplitude Nonlinear Model for RR Lyr: Pulsations, Shock Waves and Hα Peculiarities
Authors: Fokin Andrew
Published: June 2019

The shock phenomena and Hα formation in the atmosphere of RR Lyrae are investigated by means of numerical simulations. The full-amplitude hydrodynamical model is generated, adopting M = 0.578M⊙, L = 62L⊙, Teff = 7175K and Population II composition. The Hα profiles are obtained by solution of the non-LTE line transfer problem for the multilevel hydrogen atom. In the course of pulsations an extended (about 16 static scales) low-density atmosphere is produced with nearly exponential density decrease. Two shocks, propagating outwards, are successively generated during one period. The shock amplitude reaches 140 km/s. No mass loss has been found. These results confirm the earlier results of S. Hill (1972). The main shock develops very high, at mass depths near 0.001 g per area, whereas computations for W Vir yield 100. An analysis of the Fe I and Ti II lines shows that this difference is sufficient to explain the lack of metallic line-doubling in RR Lyrae stars. The bump on the light curve is due to a weak shock, propagating below the photosphere, which is generated with the early shock at the stage of expansion at the H-recombination front. The central intensities, Dopplerian shifts, amplitudes and phases of splitting of the computed Hα profiles agree well with the observed values (Gillet & Crowe 1988). The lack of strong emission also fits the observations, resulting from strong scattering processes near the shock. The predicted Lyman lines show strong emission, reaching maximum at the maximum light.
The Red Edge of the Cepheid Instability Strip
Authors: Li Yan
Published: June 2019

Recent results of investigations of the nonequilibrium effects between gas and radiation on the red edge of the Cepheid instability strip are presented.
Mass-Loss During the RR Lyrae Phase of the HB: Mass Dispersion on the HB and RR Lyrae Period Changes
Authors: Koopmann Rebecca A. , Lee Young-Wook , Demarque Pierre , Howard Jamie M.
Published: June 2019

Horizontal branch (HB) models were evolved using the Yale stellar evolution code, YREC, to test the possibility that mass loss during the RR Lyrae phase is able to produce the observed color (mass) dispersion on the HB (Willson and Bowen 1984) and the anomalous period changes in RR Lyrae stars (Laskarides 1974). Models of total mass 0.64, 0.66, 0.68, 0.70, and 0.72 M⊙ (YMS = 0.23, Z = 0.001) were evolved with constant mass loss rates of 0, 10-10, and 10-9 M⊙ yr-1. Mass loss was assumed to occur only in the RR Lyrae phase, and the instability strip was defined by 3.800 < log Teff < 3.875.

HB stars which lose mass evolve further to the blue. Low mass loss rates do not affect the shape of the tracks significantly. Stars, which without mass loss could not become blue HB stars, were able to emerge from the instability strip on the blue side.
The Masses and Luminosities of Globular Cluster RRc Stars
Authors: Simon Norman R. , Clement Christine M.
Published: June 2019

A large number of hydrodynamic pulsation models are converged in the first overtone, and their mean properties compared with observations of RRc stars in six globular clusters. The two observed quantities, period and Fourier parameter ϕ31, lead, via the models, to inferred values for mean mass, luminosity and temperature of the RRc sample in each cluster. We find a narrow range in intracluster RRc luminosity and temperature, but a wider range in mass. At the same time, the intercluster spread is wide in all three parameters. A full discussion of our techniques and results will be given elsewhere (Simon and Clement, in preparation).
CCD Photometry of RR Lyrae Stars in M3
Authors: Cacciari C. , Carretta E. , Ferraro F. , Fusi Pecci F. , Tessicini G. , Nemec J.M. , Walker A.R.
Published: June 2019

New CCD BVI observations of RR Lyrae variables in M3 are presented. Mean magnitudes and colours are derived, as well as their relations with periods and amplitudes, and comparisons are made with previous data (Sandage 1981, 1990). Preliminary results are presented on the temperature distribution of the variables and the period-shift effect with respect to M15 and M68.
CCD Photometry of RR Lyrae Stars in NGC 6388 and M15
Authors: Silbermann N.A. , Smith H.A. , Bolte M.
Published: June 2019

We present preliminary results of a program of CCD photometry of RR Lyrae variable stars in the globular clusters NGC 6388 and M15.
Resonance Effects in Fundamental Mode, First-Overtone, and Double-Mode Cepheids
Authors: Antonello Elio
Published: June 2019

Linear adiabatic periods, period ratios and frequencies of Cepheid models with. P0 less than about 10 days have been computed, taking into account standard and nonstandard mass-luminosity relations and the new (or augmented) opacities. A comparison of the results with the observed properties of Cepheids has yielded the following conclusions:

(a)

A non-standard mass-luminosity relation is needed in order to have good agreement between observed stars and models on the temperature-period (or luminosity) diagram; ‘standard’ models have much lower temperatures and luminosities than observed stars.

(b)

The linear models predict the following resonances: f2/f0 = 2 (well known) at P0 ∼ 10 days, f4/f0 = 3 at P0 ∼ 6.8 days, f4/f1 = 2 at P1 ∼ 3.2 days, and f1 + f0 = f3 at P0 ∼ 6.5 days.

(c)

All these resonances yield effects which have been observed in light curves of Cepheids at the predicted periods: f2/f0 = 2 and f4/f0 = 3 in classical Cepheids; f4/f1 = 2 in first-overtone Cepheids; and f4/f1 = 2 and f1 + f0 = f3 in double-mode Cepheids.

The discussion will include comparison with the results obtained with nonlinear models by Moskalik et al. (1992) for fundamental-mode Cepheids, and by Aikawa (1992) for first-overtone Cepheids.
Variable Stars in the Sculptor Dwarf Galaxy
Authors: Goldsmith C. G.
Published: June 2019

This project was initiated in 1985 by James Nemec (University of Washington) and Nicholas Suntzeff (C.T.I.O.). The goal was to study the system of ∼600 variable stars in the Sculptor dwarf galaxy. In 1987 the author became the recipient of the plate collection, which formed the basis for his Ph.D. dissertation. In this paper preliminary results are presented. Briefly, 612 stars were studied, of which 432 are van Agt (1978) stars and 180 are newly discovered variable stars. A total of 381 stars are confirmed variables. Most of these are RR Lyraes, but many anomalous Cepheids and some candidate eclipsing variables were also found. Several candidate double-mode RR Lyrae stars were also identified. The mean period of the ab-type RR Lyrae stars is 0.60±0.08 day, and the mean period of the c-type stars is 0.35±0.03 day, not unlike the mean periods of other nearby dwarf galaxies.
A new pulsating white dwarf in the Wide Field Camera?
Authors: Wonnacott D.
Published: June 2019

Recent observations in the extreme ultraviolet have shown that the binary companion of the pulsating metallic-lined A8m: star IK Peg is a hot DA white dwarf. The EUV light curve is shown to be subject to too much photon noise to identify any periods, and simulations of the effect of a hotspot on the compact object indicate that it is unlikely that the observed photometric variability is not intrinsic to the A8m: star.
B,V Photometry of the Variable Star V9 in the Globular Cluster 47 Tuc
Authors: Corwin Michael , Carney Bruce
Published: June 2019

We present BV CCD photometry of the variable star V9 in the globular cluster 47 Tuc. V, B, and (B – V) light curves are given. A colour-magnitude diagram based on four V and four B frames is given. V9's location on the diagram is considerably brighter and bluer than the edge of the red horizontal branch. Its radial velocity indicates that V9 is a member of the cluster.
Some Observational Limitations of Extinction Measurements in the Rocket Ultraviolet
Authors: Borgman Jan
Published: June 2019

Even Before any Measurement of stellar fluxes in the ultraviolet had been obtained, predictions were being made on the basis of stellar atmosphere models. At the moment, it seems that such com-putations (for example, see ref. 1) are in reasonable agreement with actual observations from rockets (ref. 2) in the spectral region down to about 2000 Å. At shorter wavelengths it is necessary to take into account the appreciable blanketing by spectral lines which unavoidably is present in the transmission bands of any wide-band photometric system, even in the case of early B stars, as has been shown in reference 3.

It must be anticipated that the reduction of extinction measurements in the UV from spaceborne instruments will follow the same procedure that has been used so far: stars with the same intrinsic energy distribution but with different amounts of reddening material in the line of sight are compared, the difference in energy distribution being attributed to the wavelength dependence of interstellar extinction.
Optical Thickness of the Galactic Absorption Layer and Its Ratio of Total to Selective Absorption
Authors: Neckel Heinz
Published: June 2019

As is Well Known, the surface brightness within the galaxies decreases strongly with increasing distance from the center. Therefore, if the diameters of the galaxies are defined by the isophote of a distinct apparent surface brightness, these diameters become smaller and smaller with increasing galactic absorption. The diameters are defined in reference 1 in this manner (by the isophote 26ṃ15/sec2); therefore, the mean surface brightnesses derived in that reference correspond more and more to the inner, brighter parts of the galaxies as the absorption increases. Consequently, in the derivation of the half optical thickness a of the galactic absorption layer from the variation of the mean surface brightnesses S of 113 spiral galaxies with galactic latitude β, the assumption in reference 1, dS/d(csc β) = a, is not correct but has to be replaced by the more complicated formula
Color and Polarization of Reflexion Nebulae NGC 2068, NGC 7023, and Merope Nebula in Three Spectral Regions
Authors: Elvius Aina , Hall John S.
Published: June 2019

A Survey of the Polarization of Light by the nebulosity within 67 regions of the Pleiades cluster was made in early 1962. The survey was made without color filters. On two nights, however, some preliminary polarization measures were made with color filters; the results obtained in this investigation seemed to be promising enough to justify a more thorough study of polarization in three colors. The polarimeter used in 1962 was not suited for observations in the UV, however, because its polaroid analyzer would have absorbed most of the ultraviolet light.

In early 1963 color measures of nebulosity were made within the Pleiades cluster in the blue and yellow spectral regions. Later that year a few more observations of polarization were made in three colors with a polarimeter which contained a Glan-Thompson analyzer. An entirely new polarimeter with a Glan-Foucault prism was built in 1964.
A Unified Model of Interstellar Extinction and Polarization
Authors: Mayo Greenberg J. , Shah G.
Published: June 2019

The Most Completely Observed Characteristics of Interstellar Grains are their ability to dim and polarize the light of distant stars in our own galaxy. The principal limitation in previous theoretical models of interstellar dust clouds has been the fact that the extinction and polarization have been calculated for different particles, the connection between these types of particles being semiquantitative at best. A knowledge of the electromagnetic scattering properties of the various proposed grains is a fundamental ingredient of any theory of extinction and polarization. All the types of grains which have been suggested, except those of reference 1, scatter electromagnetic radiation by a classical process, i.e., a process which is characterized by a classical application of Maxwell's equations and for which the particles are defined by a size, shape, and index of refraction. In spite of the fact that the methods of application of the classical theory are completely understood, numerical results for the particular range of application which appears to be needed in the interstellar dust problem have been somewhat limited.
Classical Grains and Piatt Particles: Structural Characteristics and Interaction With Galactic Radiation
Authors: McMillan J. A.
Published: June 2019

The Last 20 Years much work has been done, both theoretical and experimental, that has improved the understanding of the nature and characteristics of interstellar grains; most of it, however, has not been primarily involved in this particular field. Unfortunately, the problem is extremelv complex. As recently pointed out (ref. 1), an analysis of the nature and growth of interstellar grains calls for consideration of: (1) nucleation of condensing vapors; (2) mechanisms of crystal growth; (3) chemical reactions at low temperature including surface reactions of free radicals; and (4) radiation chemistry of solids. Simultaneous analysis of all these factors seems to be, at the present time, a hopeless endeavor. With the exception of radiation effects, separate analyses of each one have extensively been made. So far, however, it seems that combined effects cannot be overlooked. Partial aspects of the problem are discussed in this paper. The treatment of radiation is essentially exploratory, and its relative success ought not to blind us to the uncertainty of the data and to the speculative character of some of the assumptions.
High Angular Resolution Binary Star Interferometry
Authors: McAlister Harold A.
Published: June 2019

The benefits and limitations of the application of high angular resolution interferometry to the study of binary stars are considered. The principle advantage of such observations is that they provide data complementary to parameters determined by other techniques and permit the direct determination of masses, angular diameters and distances to systems which would otherwise be inaccessible to such complete analysis. These values then directly lead to linear diameters, absolute magnitudes, absolute emergent fluxes and effective temperatures. The prospects for significantly increasing our knowledge of these fundamental astrophysical parameters are extremely promising. The applications of interferometry to surveys for new binary stars, to star clusters, and to wider visual binaries with the goal of detecting planetary perturbations are also briefly considered.
Effects of Ground Motions on Amplitude Interferometry
Authors: Bender Peter L.
Published: June 2019

Some examples are given of types of ground motion which hs.ve been observed with seismic and geodetic instruments. In general, away from sources of man-made noise, the amplitudes of ground strains and tilts in the microseism band with periods of a few seconds normally are of the order of a few times 10-10. Strain and tilt tides give amplitudes of a few times 10-8, and long term average strain and tilt rates at relatively good sites may be roughly 10-7/yr. Superposed on such relatively long wavelength motions can be larger local pier motions, which are likely to be irregular. However, very high ground stability has been demonstrated at a few sites The limitations of ground motions on long term amplitude interferometer stability are serious if applications such as earth rotation and polar motion measurements are considered, but at shorter periods the environmental problems will be almost completely atmospheric.
The Scope of the Lunar Occulation for Measurement of Stellar Angular Diameters
Authors: Ridgway Stephen T.
Published: June 2019

This review of the lunar occultation technique summarizes results of 94 diameter measurements for 62 stars. The range of angular diameters measured, the coverage of spectral types, and the wavelengths studied are illustrated and discussedo Comparison with other techniques, magnitude limits and possible future developments are also considered.
The X-Ray Spectra of Laser Produced Plasmas with a Concave Mica Crystal Spectrograph and their Time Dependence
Authors: Mazing M. , Shevelko A.P. , Kasjanov Yu.S. , Chevokin W.K.
Published: June 2019

Soft X-ray spectra of aluminum and titanium and some other elements were observed in laser produced plasmas. The hydrogen and helium-like ion resonance lines and their satellites were obtained for one laser pulse by means of a concave mica crystal spectrograph. The experimental wavelengths were compared with theoretical ones. Time dependence of the aluminum spectra was studied using the electron optical camera and scintillator as the X-ray detector. The duration time of the resonance line emission is approximately equal to that of laser pulse for helium-like ions and is noticeably shorter for hydrogen-like ions. In the latter case this time is rather different for each laser pulse.
Extended Atmospheres of Planetary Nuclei
Authors: Böhm K. H. , Cassinelli J.
Published: June 2019

Exploratory calculations on nongray, hydrostatic-equilibrium model envelopes for central stars of planetary nebulae of high temperature and possibly near the instability limit are reported. It is conjectured that these may be related to Wolf-Rayet type nuclei; it appears possible to obtain an OV absorption, and OVI emission, spectrum even in an LTE calculation.
Laser Action in Non-LTE Atmospheres
Authors: Menzel Donald H.
Published: June 2019

The radiative transfer equation is written in microscopic form, and from some simplifications on the ratio of occupation numbers for upper and lower level, a laser action is suggested.
The Central Star of the Planetary Nebula LMC-N66: A Massive Accreting White Dwarf?
Authors: Реñа M. , Hamann W.-R.
Published: June 2019

The central star of the PN LMC-N66 showed an impressive outburst in 1993 - 1994, returning to its initial conditions about 8 years later. Its spectrum resembles that of a WN4.5 star, being the only confirmed central star of planetary nebulae showing such a spectral type. Recent analysis for the central star parameters, performed by Hamann et al. (2003) is presented. They have found that the bolometric luminosity increased by a factor larger than 6, during the outburst. We discuss the possible scenarios which have been proposed to explain the exceptional stellar parameters and the outburst mechanism. The stellar characteristics and the morphology and kinematics of the planetary nebula suggest the presence of binary system (massive star with a less massive companion or, a white dwarf accreting matter in a close-binary system). These cases pose the least severe contradictions with observational constraints.
Chandra Monitoring Observation of the Antennae Galaxies: The X-Ray Source Populations and the Shape of their Luminosity Function
Authors: Zezas A. , Fabbiano G. , Baldi A. , King A. R. , Ponman T. J. , Raymond J. C. , Schweizer F.
Published: June 2019

We present the X-ray luminosity function (XLF) of the Antennae galaxies based on 8 observation performed with Chandra. We detect between 37 and 49 sources in each observation. After combining all observations we detect a total of 120 sources down to a limiting luminosity of ~ 2 x 1037 erg s–1. Comparison between the XLFs of the individual observations shows that they are not affected by source variability. The cumulative XLF of the coadded observations is well represented by a single power-law with a slope of ~ –0.5.
X-Ray Binaries in Early Type Galaxies and the Globular Cluster Connection
Authors: Maccarone T. J. , Kundu A , Zepf S. E. , Puzia T. H.
Published: June 2019

We summarize the key observations made in recent observations of X-ray sources in early-type galaxies. Typically about half of the X-ray binaries in early-type galaxies are in globular clusters, they are preferentially found in metal rich globular clusters, and there is no indication that cluster-age is an important parameter. Theoretical challenges are presented by these results.
The Formation and Evolution of Compact Stars in Binaries
Authors: Taam Ronald E.
Published: June 2019

The stellar evolutionary processes responsible for the formation of compact objects in interacting binary systems and their evolution are described. The common envelope phase plays a crucial role in their formation and angular momentum losses associated with magnetic braking and/or mass loss are important for their evolution. An application of these processes provides the evolutionary link between classes of interacting binary systems.
The Effects of Binaries on Population Studies
Authors: Vanbeveren Dany
Published: June 2019

We give an overview of the work that has been done in Brussels since 1997 on various aspects of population studies and the effects of binaries: stellar populations and supernova rates in regions where star formation is continuous and in starbursts, the expected spectral evolution of starburst regions, the chemical evolution of galaxies.
Accretion Processes in Mira AB
Authors: Karovska M. , Wood B. , Marengo M. , Raymond J. C. , Hack W. , Guinan E.
Published: June 2019

The Mira AB interacting binary provides an ideal laboratory for studying mass loss and accretion processes in systems containing an AGB star and a compact accretor, because its components have been resolved and can be studied individually. We present here results from our long term study of accretion processes in this system. Those include recent HST observations suggesting that the accretion rate onto Mira B is much lower than before, indicating possible disruption of the accretion disk.
High Speed Photometry of AM CVns with the UCT CCD
Authors: Woudt Patrick A. , Warner Brian
Published: June 2019

Following our high speed photometric survey of faint cataclysmic variables, two CVs were recognised to be helium-transferring double degenerates; these are ES Cet (Рorb – 620.21144 s) and 2003aw (Psh= 2041.5 s). We have now observed ES Cet photometrically for over two years in order to follow the evolution of the orbital period. We oiler an alternative; explanation for the nature of V407 Vul (which was classified as an ultra-compact together with RX J0806 and ES Cet); this will take V407 Vul out of the AM CVn classification.
The Mysterious Nature of HS 2331+3905
Authors: Aranjo-Betancor S. , Gänsicke B. T. , Hagen H.-J. , Marsh T. R. , Thorstensen J. , Harlaftis E. T. , Fried R. E. , Engels D.
Published: June 2019

We report the discovery of one unique cataclysmic variable drawn from the Hamburg Quasar Survey, HS 2331 + 3905. Follow-up observations obtained over three years unveiled a very unusual picture. The large amplitude 3.5 h radial velocity variations obtained from our optical spectroscopy is not the orbital period of the system, as one would normally expect. Instead, extensive CCD photometry strongly suggests that HS 2331 + 3005 is a short orbital period cataclysmic variable with Рorb = 81.09 min, containing a cold white dwarf which appears to exhibit ZZ Ceti pulsations.
CH Cygni and Other Symbiotic Stars with Low Luminosity Hot Components
Authors: Leedjärv L. , Tomov T. , Mikołajewski M. , Burmeister M.
Published: June 2019

Some aspects of the recent activity period of the peculiar symbiotic binary CH Cyg, which has ejected both bipolar jets and discrete centrifugal outflows, are discussed.

In symbiotic binaries, the red giant's wind is accreted onto the white dwarf. High luminosity of the hot components (Lhot ~103L⊙, reaching ~104L⊙ in the outbursts) (Mikołajewska 2003; Sokoloski 2003) indicates that the accreted matter undergoes thermonuclear burning. There are about 10 symbiotic stars which have ejected high-velocity, collimated, bipolar jets. Some of them show flickering of the optical light on the time scale of minutes. Both jet ejection and flickering indicate that an accretion disk should be present in those systems. Jet emitting symbiotic stars seem to have lower luminosity of the hot component ( ~1 – 100 L⊙) than in most other symbiotics.
Modeling He-Rich Disks in AM CVn Binaries
Authors: Nagel T. , Dreizler S. , Rauch T. , Werner K.
Published: June 2019

We have developed a new code for the calculation of synthetic spectra and vertical structures of accretion disks in cataclysmic variables and compact X-ray binaries. Here we present results for the CV system AM CVn.

AM CVn stars are a special type of cataclysmic variables, also called helium cataclysmics. They are systems of interacting binary white dwarfs, consisting of a degenerate C-O white dwarf primary and a low mass semi-degenerate secondary. The secondary loses mass, almost, pure helium, to the primary, forming an accretion disk. They have all in common a helium-rich composition, analoguous to the hydrogen-rich cataclysmic variables. They show photometric variabilities on time scales of ~ 1000s, the prototype of the class, AM CVn, e.g. exhibits a variability of ~ 18 min (Nelemans et al. 2001).
Time-Series Photometry of WZ Sge After the 2001 Outburst
Authors: Kitsionas S. , Giannakis O. , Harlaftis E. T. , Skidmore W. , Boffin H. , Gänsicke B. , Papadimitriou С.
Published: June 2019

We present R-band CCD photometry of the eclipsing dwarf-nova WZ Sge from six epochs after its 2001 outburst, using the 1.2m telescope of the NOA in Greece.