bluuebunny/crossref_metadata_2025_split huggingface dataset:
Cluster Mass and Metallicity Distributions: Reconstructing the Events During Halo Formation
Authors: Harris William E. 
Published: June 2019

Globular clusters in most large galaxies are a mixture of metal-poor and metal-rich (bimodal), but the halo stars are almost entirely metal-rich. This and other lines of evidence argue that the metal-poor globular clusters formed within widely distributed 108 − 109M⊙ gas clouds (supergiant GMCs) during an early burst in which most of the gas was ejected or unused till later rounds of star formation.New simulations of the growth of pre-galactic potential wells in the early universe now indicate that the initial power-law form of the globular cluster mass distribution (dN/dM ∼ M-1.8) is a miniature replica of the mass distribution of the SGMCs themselves, which grow hierarchically in the CDM potential wells of large protogalaxies.
On the Formation of Interstellar Gas Clouds
Authors: Kaplan S. A. 
Published: June 2019

The characteristic features of interstellar gas clouds—existence of large density fluctuations, their connection with cosmic dust, stretching along the magnetic fields, and so on—may be described by a shock wave theory in interstellar space.The author has developed a theory of stationary shock waves accompanied by losses of energy by means of radiation. Choosing two surfaces on both sides of the front, so that the regions of energy radiation should lie between them, we can write an equation for the mass flow and for the impulse conservation between these surfaces, and two equations which determine the stationary temperature of the gas in the field of interstellar radiation. The solution of this system of equations permits one to determine the general changes of thermodynamic and other parameters for the transition of gas through the shock wave with regions of radiative cooling. If changes of the degree of ionization take place, and a magnetic field is present, some terms should necessarily be added to the corresponding equations.The boundary between the interstellar gas cloud and the intercloud medium must represent the shock wave accompanied by losses of energy by means of radiation, because such ruptures may probably be supposed as the sole explanation of stability of the great density changes (a hundred times and more) often observed in the interstellar space.In this paper we give some results of the theory of shock waves accompanied by radiative losses of energy.
Statistical Study of Globules Projected on Stellar Clouds
Authors: Rojkovsky D. A. 
Published: June 2019

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

The high speed photometric observations of dwarf novae during outbursts that led to the discovery (Warner and Robinson, 1972) of rapid pulsations have been extended to other objects. Pulsations are usually seen near outburst maximum; an exception is VW Hyi which showed oscillations on December 25, 1972, when it was well down the declining part of its light curve.The periods of the oscillations are found to vary; in most cases a rapid change from one discrete mode to another is observed. The dwarf nova stays in one oscillation mode for about an hour, and changes mode over a time of about 10 min. In the case of the VW Hyi observation, the period changed continuously from 28 s to 34 s over a time of 4 h, during which the apparent brightness decreased by 40%.The existence of many closely spaced oscillation modes suggests that the dwarf novae are oscillating in non-radial modes. This is supported by observations of the 71-s oscillations of DQ Her: during eclipse a phase shift is seen indicating an l = 2, m = 2 mode of oscillation. In UX UMa, the 29-s oscillations show an opposite phase shift from that in DQ Her and indicate an l = 2, m = −2 mode.A period-luminosity relationship can be deduced for the oscillations in dwarf novae, and this is in good agreement with recent calculations on quadrupole oscillations of hot white dwarfs made by Osaki and Hansen (1973).Pulsations have so far been detected in the following objects: Z Cam (17 s), SY Cnc (24.6), CN Ori (25), KT Per (27), UX UMa (29), VW Hyi (28–34), CD −42° 14462 (29 and 30), AH Her (31).
Tests of the Binary Hypothesis of Novae Through Nova Nebulae Observations
Authors: Hutchings J. B. 
Published: June 2019

I should like to follow up Dr Schatzman's talk with a few words on models of Nova nebulae. Geometrical models for novae Aql 1921 and Her 1934 have been derived by Mustel and Boyarchuk (1970), for novae Del 1967, Vul 1968 and Ser 1970 by Hutchings (1972), for Aql 1921 by Weaver (1974) and for Del 1967 by Malakpur (1973). Very different techniques and lines of reasoning have been used by all these people, but the models derived all have very strong similarities which should lead us to take them seriously in considering the nature of the nova mechanism.It appears that nova remnants are characterized by two polar cones or blobs of similar but opposite velocity and similar density, moving along the axis of rotation of the system. In addition, one or more rings of matter may exist symmetrically about the equatorial plane, with a recession velocity and density which, in general, are different from the polar matter. The polar blobs appear to be associated with the matter giving rise to the ‘principal’ spectrum in the bright phases of the nova.Three possible main causes have been suggested for the formation of such remnants, all connected with the initial outburst: (1) Weaver has proposed that an initially spherically symmetrical expansion is impeded and deflected by matter circulating in an accretion ring around the degenerate star. (2) Schatzman has suggested that the presence of the ring may favour an initial outburst preferentially in the polar regions. (3) Hutchings has suggested that physical interference of an initially spherical expansion may occur by a secondary component whose size is a large fraction of the separation of the two stars.I feel that these points need to be considered by theoreticians in proposing detailed mechanisms of the nova outburst.
Instabilities of Magnetic Fields in Stars
Authors: Tayler R. J. 
Published: June 2019

It has been shown (Markey and Tayler, 1973; Tayler, 1973; Wright, 1973) that a wide range of simple magnetic field configurations in stars are unstable. Although the ultimate effect of the instabilities is unclear, it seems likely that they would lead to enhanced destruction of magnetic flux, so that magnetic field decay would be much more rapid than previously supposed. Instability is almost certain in a non-rotating star containing either a purely toroidal field or a purely poloidal field, which has closed field lines inside the star. In both cases the instability resembles the well known instabilities of cylindrical and toroidal current channels, modified by the constraint that motion must be almost entirely along surfaces of constant gravitational potential.If both toroidal and poloidal fields are present, the problem is more complicated. In a toroidal plasma with a helical field, the worst instabilities are also helical but it is impossible for a helical disturbance to be parallel to a surface of constant gravitational potential everywhere. As a result, the admixture of toroidal and poloidal fields has a stabilizing influence, but it is not at present clear whether the majority of such configurations are completely stable.The effect of rotation has not yet been studied but it will certainly be important if the rotation period is less than the time taken for an Alfvén wave to cross the region of interest. This is true in most stars unless the internal magnetic field is very much stronger than any observed field.
Solar Irradiance Variations: Theory
Authors: Spruit H.C. 
Published: June 2019

The following is a somewhat condensed version of discussions previously given elsewhere (Spruit, 1991, 1992). Some new developments not discussed there are presented in sections 4 and 5.Since the observed irradiance variations are so clearly associated with manifestations of the solar magnetic field, I focus here on magnetic causes. Much of the physics of irradiance variations, however, is governed by the thermal response of the convective envelope and this response is similar for other possible causes of irradiance variations.
Oscillation of the Magnetic Field in an Active Region
Authors: Horn T. , Staude J. 
Published: June 2019

The Vacuum Tower Telescope (VTT) at Tenerife has been used together with a polarimeter in front of a two-dimensional imaging spectrometer with a Fabry-Perot interferometer. The spectrometer has been described by Bendlin et al. (1992) and Bendlin et al. (1995), the remaining instrumentation, calibration, and data reduction by Horn et al. (1996). Some parameters of the setup were: 384×286 pixel CCD-camera (0.2” per pixel); resolution for the images better than 0.8”; filtergrams in the photospheric spectral line Fe I 6173.4 Å with Δλ = 10.9 mÅ; 45 filtergrams for each scan; 127 scans within 114 min. Δt = 54 s. White-light images from the same field of view were taken strictly simultaneously to all narrow-band pictures to correct for image blurring and motion.The present observations were obtained on July 20, 1994, and were focused on the main spot of the active region NOAA 7757, 30° NE from the disk center.
Extra-Galactic Star Formation Revealed
Authors: Schinnerer Eva 
Published: June 2019

High angular resolution observations of nearby galaxies in the optical using ground-based and space-based telescopes have not only revealed the presence of young stellar clusters, but also allowed to study their properties in various dynamical environments. These studies have shown that young massive clusters (YMCs) have typical masses of a few 1000 M⊙ and sizes of a few parsec irrespective of their site of formation (such as bulges, spiral arms, starburst rings, or mergers). This points toward a universal formation mechanism for these stellar clusters.Observations of the dust and gas content in high redshift galaxies allows one to study the reservoir for star formation in the early universe. These studies reveal extremely high star formation rates of a few 1000 M⊙ yr−1, while the distribution of the molecular gas still seems to be comparable to what is observed in the local universe. The detection of considerable amounts of molecular gas via its CO lines in the highest redshifted QSOs known today (up to z=6.4) indicates that star formation in the early universe has already produced considerable amounts of metals.
Masers: High Resolution Probes of Massive Star Formation
Authors: Ellingsen S P 
Published: June 2019

Astrophysical masers are one of the most readily detected signposts of high-mass star formation. Their presence indicates special conditions, probably indicative of a specific evolutionary phase. Masers also represent the ultimate high-resolution probe of star formation with the potential to reveal information on the kinematics and physical conditions within the region at milli-arcsecond resolution. To date this potential has largely remained unfulfilled, however, recent advances suggest that this will soon change.The key to unlocking the potential of masers lies in identifying where they fit within the star formation jigsaw puzzle. I will review recent high resolution observations of OH, water and methanol maser transitions and what they reveal. I also briefly discuss how multi-transition observations of OH and methanol masers are being used to constrain maser pumping models and through this estimate the physical conditions in the masing region.
Distribution and motions of CO in M51
Authors: Rydbeck G. , Hjalmarson å. , Rydbeck O. 
Published: June 2019

We present results (partly preliminary) of an extensive map (73 positions) of CO (J=1–0) emission in M51 (Fig. 1). The spectra were obtained with the Onsala 20-m antenna (beam size 33″), equipped with a cooled mixer and a 512×1 MHz multichannel receiver. The data are not yet fully analyzed but our preliminary results are as follows:1. A small central minimum in the CO emission is apparent (Fig. 2). The average radial CO distribution shows a maximum at ~ 15″ (corresponding to 0.7 kpc, for an assumed distance of 9.6 Mpc).2. The above minimum results from what seems to be an oval ridge structure in the emission intensity around the center, with an extent of about 30″ by 40″ (Fig. 3). This could be the markings of an elongated hole or the beginning of the spiral arms. (The ridges are fairly well correlated with the innermost parts of the spiral arms as delineated by continuum observations by Segalowitz, 1976.)3. It seems that a central oval velocity pattern is needed to explain a dip at 450 km s-1 in the central spectrum. (The dip is present in all and independent parts of our data.)4. The outer parts of the galaxy have only been covered in strips (Fig. 1). Clearly there is structure in these strips, i.e. not only a radial decrease. The “on-arm” spectrum (88″, 0″) has for example a greater integrated and peak intensity than the “interarm” spectrum (77″, 0″). Such arm-interarm contrast is not always clear, though. Observations covering a larger section of an arm-interarm region, where contrast has been seen, are being planned for the next observing season.5. We also note that the observed apparent velocity differences between CO and ionized gas, reported by Rydbeck et al. (1983), persist. A fully convincing argument on this point (taking into account the finite beam etc.) requires some further model work, however.
Loop I (the north Polar Spur) region - a quasi radio halo
Authors: Milogradov-Tuin Jelena 
Published: June 2019

The distribution of total spectral indices between 38 and 408 MHz with a resolution of for δ>-25° has the following main properties:1. Relatively small variations of spectral indices over the sky.2. High indices in the central region at high galactic latitudes.3. Moderately low spectral indices in the anticentre region.4. Lower indices in the low-brightness regions (cold holes).5. The lowest indices in regions containing large amounts of HII.
Two Aspects of Using Hipparcos Data for Studying Multiperiodic Stellar Pulsations
Authors: Jerzykiewicz M. 
Published: June 2019

AbstractTwo aspects of using Hipparcos data for studying multiperiodic stellar pulsations involve (1) deriving frequencies of multiperiodic pulsators from Hipparcos Epoch Photometry, and (2) using Hipparcos parallaxes together with other data, such as the frequencies, frequency ratios and multicolor photometry, to identify pulsation modes. Details are examined by looking at the δ Scuti star DK Virginis. From Hipparcos Epoch Photometry, two frequencies are derived. These frequencies are then verified by showing that they also fit all available ground-based observations of the star. In addition, Hipparcos parallax and a photometric effective temperature are used to compare the position of DK Vir in the HR diagram with evolutionary tracks. The star turns out to be at the end of core-hydrogen-burning stage of its evolution or slightly beyond, with a mass of 2.2 M⊙, or somewhat smaller. The observed frequencies indicate that the two detected modes are p3 and p2, while the relatively large photometric amplitudes imply ℓ ≲ 2.
A Search of Variable Stars in the Tycho Observations
Authors: Piquard Sandrine , Halbwachs Jean-Louis , Fabricius Claus , Geckeler Ralf , Wicenec Andreas 
Published: June 2019

AbstractThe selection of variable stars in the Tycho catalogue was re-considered. A method based on a Kolmogorov–Smirnov test was elaborated, giving 729 candidate variable stars. This number could increase to about 1000 in the future.
RV Tauri Stars and Type II Cepheids in the Magellanic Clouds - Results from the MACHO Database
Authors: Pollard Karen R. , Alcock C. , Allsman R. A. , Alves D. , Axelrod T. S. , Becker A. C. , Bennett D. P. , Cook K. H. , Freeman K. C. , Griest K. , Lehner M. J. , Marshall S. L. , Peterson B. A. , Pratt M. R. , Quinn P. J. , Sutherland W. , Tomaney A. , Welch D. L. ,   The MACHO Collaboration
Published: June 2019

AbstractA review of the properties of Type II Cepheids and RV Tauri stars in the Magellanic Clouds is presented. In the behaviour of their light and colour curves, the RV Tauri stars appear to be a direct extension of the Type II Cepheids to longer periods. A single P – L – C relationship describes both the Type II Cepheids and RV Tauri stars in the LMC. The derived high intrinsic magnitudes for the RV Tauri variables supports the proposition that these objects are luminous stars evolving off the AGB. Preliminary analysis of the long time-series MACHO photometry indicates one star (MACHO*05:37:45.0–69:54:16) has an obvious ‘period-quadrupled’ periodicity, which is supporting evidence for a period-doubling bifurcation transition to chaotic pulsations.
Polarization Observations of RV Tauri Stars at the Dodaira Station of the National Astronomical Observatory in Japan
Authors: Yoshioka K. , Saijo K. , Sato H. 
Published: June 2019

AbstractWe made multicolor linear polarimetric observations of RV Tauri stars using the 91-cm reflector at the Dodaira Station of the National Astronomical Observatory in Japan. We report the main results for 17 RV Tauri stars obtained until now.
UBVRIJHK Photometry of Short-Period Red Variables Identified by the Hipparcos Survey
Authors: Laney C. D. , Koen C. 
Published: June 2019

AbstractThe Hipparcos survey team discovered thousands of new variables, including M giants with periods as short as a day, which appear to have maintained rough phase coherence over a period of more than three years. New optical and infrared photometry has been obtained as part of an effort to understand the nature of these anomalous stars.
Cepheid Diameters from Optical Interferometry: The NPOI Survey
Authors: Nordgren Tyler E. , Germain M. E. , Sudol J. J. , Hindsley R. B. , Armstrong J. T. , Hajian Arsen R. 
Published: June 2019

AbstractWe present linear radii for four Cepheid variable stars, spanning a pulsation period range from 3 to 11 d, measured with the Navy Prototype Optical Interferometer (NPOI). We compare these radii to those found using traditional indirect methods, and to various period–radius relations found in the literature.
RR Lyrae Stars in Globular Clusters
Authors: Walker Alistair R. 
Published: June 2019

AbstractOf the several types of variable stars that occur in globular clusters, RR Lyrae stars have deservedly received the most attention. These stars are ideally suited for comparisons between evolution and pulsation theory, they play a prominent role in distance determinations, and are valuable tracers of the oldest stellar populations. After a brief discussion of technical aspects, we review some of the recent observational work.
Light Curves and Metal Abundances of RR Lyrae Variables in the Bar of the Large Magellanic Cloud
Authors: Clementini G. , Bragaglia A. , Fabrizio L. Di , Carretta E. , Gratton R. G. 
Published: June 2019

AbstractThe Large Magellanic Cloud (LMC) is widely considered a corner-stone of the astronomical distance scale. However, a difference of 0.2−0.3 mag exists in its distance as predicted by the short and long distance scales. Distances to the LMC from Population II objects are founded on the RR Lyrae variables. We have undertaken an observational campaign devoted to the definition of the average apparent luminosity, and to the study of the mass–metallicity relation for RR Lyrae stars in the bar of the LMC. These are compared with analogous quantities for cluster RR Lyrae stars. The purpose is to see whether an intrinsic difference in luminosity, possibly due to a difference in mass, might exist between field and cluster RR Lyrae stars, which could be responsible for the well-known dichotomy between short and long distance scales. Preliminary results are presented on the V and B − V light curves, the average apparent visual magnitude, and the pulsational properties of 102 RR Lyrae stars in the bar of the LMC, observed at ESO in January 1999. The photometric data are accurately tied to the Johnson photometric system. Comparison is presented with the photometry of RR Lyrae stars in the bar of the LMC obtained by the MACHO collaboration (Alcock et al. 1996). Our sample includes 9 double-mode RR Lyrae stars selected from Alcock et al. (1997) for which an estimate of the metal abundance from the ΔS method is presented.
Revised Baade–Wesselink Analysis of RR Lyrae Stars
Authors: Cacciari Carla , Clementini1 Gisella , Castelli Fiorella , Melandri Fabrizio 
Published: June 2019

AbstractWe have applied the Baade–Wesselink method to two field RR Lyrae stars, i.e. SW And and RR Cet, and derived their distances and physical parameters. With respect to previous B–W analyses we have applied the following improvements: i) use of all sets of available data, after proper comparison for homogeneity and compatibility; ii) use of the most recent and accurate model atmospheres, with turbulent velocity Vturb = 4 km s−1 and the no-overshooting approximation, and comparison with other treatments of convection; iii) use of the instantaneous gravity along the pulsation cycle rather than the mean value; iv) comparison with modified radial velocity curves according to various assumptions on radial velocity gradients in the atmosphere; and v) careful reanalysis of the temperature scale. The main aim of this study is to evaluate the effect of the above items on the B–W results and verify whether any (or a combination) of them can possibly account for the discrepancy of the absolute magnitude zero-point with respect to other independent determinations.
New Pulsational Constraints to the Distance of Globular Clusters and the MV(RR)–[Fe/H] Relation
Authors: Ripepi Vincenzo , Caputo Filippina , Castellani Vittorio , Marconi Marcella 
Published: June 2019

AbstractWe applied the pulsational method (Caputo 1997) to derive the distances to a sample of galactic globular clusters with well-observed RR Lyrae populations. To apply the method we calculated a set of pulsational theoretical boundaries of the instability strip for the range of masses and chemical compositions spanned by the analysed clusters. In this way we were able to fix simultaneously the apparent distance modulus and the absolute visual magnitude of the RR Lyrae population of each cluster in the sample. As a result we derived the following relations:
Origin of Irregularities in RR Lyrae Pulsations
Authors: Chadid Merieme 
Published: June 2019

AbstractFor the first time important irregularities in atmospheric pulsations of RR Lyrae itself are clearly detected. They occur from one pulsation cycle to another. Their origins cannot be directly explained by the Blazhko effect. We suggest that these irregularities are mostly provoked by strong shock waves propagating within the atmosphere.