bluuebunny/crossref_metadata_2025 huggingface dataset:
The WO stars and their environment
Authors: Polcaro V.F. , Viotti R. , Rossi C. , Norci L.
Published: June 2019

The WO stars form a subgroup of the WR class characterized by a strong O VI emission at 382 nm (Sanduleak 1971; Barlow & Hummer 1982). In 1989–90 we observed intermediate resolution spectra of the four Sanduleak WO stars and of their surrounding nebulosities using the Bologna-Loiano 1.52m and ESO-La Silla 1.52m and 2.2m telescopes (e.g., Polcaro et al. 1991, 1992, 1994 in preparation). Fig. 1 shows that the spectra are similar but with some striking differences among the four stars. They can be arranged in a sequence in order of increasing ionization: Sand 2, Sand 1, Sand 5, Sand 4 (see Table 1). Sand 4 is a binary system, and the continuum of the O4 companion dominates over that of the WO star. In the other objects, the reddening-corrected continuum can be fitted by a 80 000 K BB, in agreement with the high ionization level of their emission spectra.
Continuum and emission line observations of Wolf-Rayet stars: single and “WR+abs” objects
Authors: Morris Patrick W.
Published: June 2019

The UV — IR properties of the continuous energy distribution of single WR stars in the Galaxy and LMC are reviewed. The observed, dereddened distributions may be approximated as a power law Fλ ∼ λ−α with a non-Rayleigh Jeans range of α directly associated with the emission of excess flux over the full wavelength range and across all spectral subtypes. The observational results are compared to published NLTE models, where we find an important discrepancy in the frequency distribution of α for Galactic WN stars. A dependency of emission line equivalent widths on the continuum flux at line center is discussed; the systematic offset of the absorption line (but otherwise presumed single) objects is examined, and the observed behavior is compared to models and explained with simple theory.
The dynamics of Wolf-Rayet winds
Authors: Owocki Stanley P. , Gayley Kenneth G.
Published: June 2019

We review the dynamics of Wolf-Rayet stellar winds, with emphasis on how multi-line scattering can lead to mass loss rates for which the wind's radial momentum flux Ṁv∞ greatly exceeds the limit for single-scattering of the star's radiative momentum L*/c. The geometrical considerations that allow this to occur are illuminated through connections to multiple scattering in an effectively gray medium. The so-called “WR wind momentum problem” would be better characterized as an “opacity problem” of simply identifying a sufficiently dense ensemble of optically thick lines.
The non-isotropic diffusion approximation in Wolf-Rayet winds
Authors: Gayley Kenneth G. , Owocki Stanley P.
Published: June 2019

We derive the dynamics of a steady-state Wolf-Rayet wind using a non-isotropic diffusion approximation applied to the transfer between strongly overlapping spectral lines. Following the approach of Friend and Castor (1983), the line list is assumed to approximate a statistically parametrized Poisson distribution in frequency, so that photon transport is controlled by an angle-dependent, effectively gray opacity. We find that multiple radiative momentum deposition is greatest when photons undergo a nearly local diffusion, e.g. through scattering between many lines closely spaced in frequency. Our results reiterate the view that the so-called “momentum problem” of WR winds is better characterized as an “opacity problem” of simply identifying enough driving lines. One way of increasing the number of thick lines in WR winds is to transfer opacity from saturated to unsaturated lines, yielding a steeper opacity distribution than that found in OB winds. We also consider the growth of instabilities, and find that WR winds are up to an order of magnitude less unstable than OB winds if both are driven radiatively. However, initially small perturbations can still be multiplied by many orders of magnitude as they advect through WR winds.
Revised Wolf-Rayet binary orbits and new binaries
Authors: Niemela V.S.
Published: June 2019

Revised orbits are presented for the double-lined binary WR+O systems HD 63099 (WR9) and HDE 320102 (WR97). The orbit derived for WR9 from OVI emission and HeII absorptions is considerably different from that previously published based on the CIVΛ4658 emission line velocities. WR97 is found to have a period of 12.6 days and small minimum masses, probably due to a low orbital inclination. Two stars in our Galaxy, WR30a and WR70, and two in the Magellanic Clouds, Brey44 and AB7, show radial velocity variations of emission and absorption lines in opposite sense. Although no periods are yet available, preliminary values of the mass ratios can be estimated from the velocity variations.
A seven year campaign on WR22
Authors: Rauw G. , Vreux J.-M. , Gosset E. , Hutsemekers D. , Magain P. , Manfroid J. , Remy M. , Rochowicz K.
Published: June 2019

New spectroscopic and photometric observations of WR22 are presented. High signal-to-noise digital spectra with high dispersion are used to improve the parameters of the orbit (P = 80.31 ±0.01 d), formerly determined on the basis of photographic spectra only. The intrinsic photometric variability of WR22 and recent observations of the eclipse are also discussed.
Complete stellar models for massive stars
Authors: Schaerer D. , de Koter A. , Schmutz W.
Published: June 2019

We present a first step towards complete stellar models for massive stars that treat the entire star including its wind. Our models are based on state-of-the art non-LTE atmospheric and interior modelling. The models yield the stellar evolution under the constraint of realistic atmospheres as well as line-blanketed continuum fluxes and H and He line profiles. We follow the evolution from OB to LBV, Ofpe/WN, and WR stars. We investigate the effect of line-blanketing during MS evolution. We adress the effect of an the extended atmosphere on the evolution and discuss the first results for WR stars.
The galactic number of Wolf-Rayet stars produced via close binary evolution
Authors: Meurs E.J.A. , van den Heuvel E.P.J.
Published: June 2019

Wolf-Rayet stars may have evolved from massive close binary systems, as a result of the mass transfer processes in such systems. A substantial fraction of all WR stars known is, indeed, found in close binaries, with massive early-type companions. After the outer layers of the initial primary have been removed during Roche Lobe Over-Flow, the remaining stellar core may be a helium-burning helium star, classified observation ally as a WR star. The fraction of such evolved binaries among main-sequence stars can be computed, following recipes from stellar- and galactic evolution and assuming steady, standard-IMF star formation over the last ∼ 109 yr in our Galaxy. Combining these results with the stellar PMF yields the total galactic number of such objects, or alternatively the number expected within, e.g., 3 kpc from the Sun. Comparison with observed numbers shows that the close binary produced WR stars should constitute a significant fraction of all WR stars, the exact fraction being dependent on the lower mass limit for WR star production (and also the duration of WR characteristics in the helium star's lifetime). The mass transfer processes in close binary systems probably allow this limit to be lower than in the case of a single WR star. Another favourable comparison regards the computed and observed numbers of persistent strong, massive X-ray binaries. Finally, second-phase WR stars are likely to comprise 5 — 10 % of the number of normal, first-stage WR binaries.
Dust formation around WC stars
Authors: Williams P. M.
Published: June 2019

The presence of heated circumstellar dust around WC type Wolf-Rayet stars is reviewed in terms of the episodic or persistent condensation of carbon grains in their stellar winds. At present, 19 WC stars are known to form dust persistently and 7 episodically. Infrared spectroscopy suggests that the grains are amorphous rather than graphitic carbon. The properties of the stars which make dust episodically are reviewed in terms of processes in colliding winds in WR binary systems and the possible applicability of these processes to the stars which make dust persistently in examined.
The ROSAT view of the massive eclipsing O-type binary system 29 UW Canis Majoris
Authors: Berghöfer Thomas W. , Schmitt Jürgen H. M. M.
Published: June 2019

We have investigated a recently obtained ROSAT X-ray observation of the massive binary 29 UW Canis Majoris (HR 2781; HD 57060). This short-period binary (4.3934 d) consists of two eclipsing O-type stars and has been observed for about 9 days with the ROSAT PSPC. We discuss the origin of the X-ray emission in the close binary system 29 CMa in the context of the traditional colliding wind scenario as well as the scenario of X-ray emission from shock heated gas in the stellar winds.
An eight year study of the radio emission from the WR binary HD 193793
Authors: Becker Robert H. , White Richard L.
Published: June 2019

Although we now know of quite a few WR-O star binary systems, only HD 193793 is well studied across the entire electromagnetic spectrum. Hence it affords us the best opportunity to test various models for the system against a wealth of observational data. In this paper we present the results of 8 years of monitoring the radio flux density from HD 193793 with the VLA. This database is unique both in terms of its dense coverage of an entire binary cycle and because it extends the radio coverage to 2 cm wavelength, a shorter wavelength than previously available. With this data we are able to simultaneously solve for the time dependent attenuation in the system and the intrinsic radio luminosity. The standard model of spherically symmetric colliding winds faces severe difficulties in explaining the observations. We conclude that the radio data are most readily interpreted in the context of a WR star wind which is confined to a disk. A disk model for the WR wind also provides a natural explanation for the sudden formation of dust just after periastron.
X-ray selected Wolf-Rayet stars: The discovery of Th35-42 and prospects for future X-ray surveys
Authors: Mereghetti S. , Belloni T. , Haberl F. , Voges W.
Published: June 2019

The X-ray source 1E 1024.0-5732, serendipitously discovered with the Einstein Observatory, was previously interpreted as a rapidly spinning neutron star, accreting matter from its massive early-type companion, the emission-line star Th35-42. However, new ROSAT data do not support the presence of a neutron star in this source. A more likely scenario, also indicated by recent optical spectroscopy, involves X-ray emission from the colliding winds of a WR+O binary. Surprisingly, this star remained unnoticed during an extensive optical search for new Wolf-Rayet stars, carried out in this region of sky. Stimulated by the discovery of this first X-ray selected WR star, we have undertaken a search for similar objects in the data from the ROSAT All Sky Survey.
Magnetic annihilation in colliding-wind binary systems
Authors: Jardine M. , Allen H.R. , Pollock A.M.T.
Published: June 2019

We investigate the possibility that a stagnation-point magnetic reconnection model may account for the particle acceleration necessary for the generation of nonthermal radio emission in the Wolf-Rayet binary systems exemplified by WR140.
The Cosmic Background Imager
Authors: Pearson T. J. , Mason B. S. , Padin S. , Readhead A. C. S. , Shepherd M. C. , Sievers J. , Udomprasert P. S. , Cartwright J. K.
Published: June 2019

The Cosmic Background Imager (CBI) is an instrument designed to make images of the cosmic microwave background radiation and to measure its statistical properties on angular scales from about 3 arc minutes to one degree (spherical harmonic scales from l ˜ 4250 down to l ˜ 400). The CBI is a 13-element interferometer mounted on a 6 meter platform operating in ten 1-GHz frequency bands from 26 GHz to 36 GHz. The instantaneous field of view of the instrument is 45 arcmin (FWHM) and its resolution ranges from 3 to 10 arcmin; larger fields can be imaged by mosaicing. At this frequency and resolution, the primary foreground is due to discrete extragalactic sources, which are monitored at the Owens Valley Radio Observatory and subtracted from the CBI visibility measurements.

The instrument has been making observations since late 1999 of both primordial CMB fluctuations and the Sunyaev-Zeldovich effect in clusters of galaxies from its site at an altitude of 5080 meters near San Pedro de Atacama, in northern Chile. Observations will continue until August 2001 or later. We present preliminary results from the first few months of observations.
The MAP Satellite Mission to Map the CMB Anisotropy
Authors: Page Lyman.
Published: June 2019

The Microwave Anisotropy Probe (MAP) satellite is scheduled to launch in mid-2001. MAP's goal is to produce a map of the anisotropy in the cosmic microwave background of unprecedented accuracy and precision. The guiding design principle has been the minimization of systematic effects. The instrument design and mapping strategy work in concert to take advantage of the unique opportunities afforded by deep space. We give an overview of the mission and compare the projected MAP error bars to recent measurements.
Ω0 from Clusters of Galaxies
Authors: Blanchard Alain. , Sadat Rachida. , Bartlett Jim.
Published: June 2019

Clusters constitute a very rich source of information for cosmology. Their present day abundance can be used to found the normalization and the shape of the power spectrum. Clusters can also be used to determine the parameter density of the universe Ω − 0. The evolution of their number density is a powerful cosmological global test of the mean density of the Universe. It is fashionable to claim that the abundance of clusters does not change very much with clusters redshift and therefore favor a low density universe. This is an overstatement and analyses based on the most recent data rather favor a high density universe. The baryon fraction in clusters is an alternative method to derive the mean density of the Universe. Here again, taking into account several biases in the baryon fraction is derived from data, the actual baryon fraction seen in clusters can be reconcilied with a high density universe.
The Local Group Dwarf Spheroidal Galaxies: A Key to Building Blocks in the Universe
Authors: Hirashita Hiroyuki. , Tamura Naoyuki. , Takeuchi Tsutomu T.
Published: June 2019

Recent studies have been revealing the properties of dwarf spheroidal galaxies (dSphs). Their low mass indicates that the dSphs may provide a clue to physical properties of the building blocks in the hierarchical structure formation. We select the Local Group dSphs as a sample. To obtain the information on the star formation history of dSphs, we investigate the relation between their metallicity and virial mass. According to our scenario, the star formation efficiency of the dSphs is low because of strong regulation. This is consistent with their high mass-to-light ratios. We also comment on the environmental effects on the dSphs.
Galaxy Luminosity Function: Applications and Cosmological Implications
Authors: Takeuchi Tsutomu T. , Yoshikawa Kohji. , Ishii Takako T.
Published: June 2019

We studied the statistical methods for the estimation of the luminosity function (LF) of galaxies by Monte Carlo simulations. After examining the performance of these methods, we analyzed the photometric redshift data of the Hubble Deep Field prepared by Fernández-Soto et al. (1999). We also derived luminosity density ρL at B- and I-band. Our B-band estimation is roughly consistent with that of Sawicki, Lin, & Yee (1997), but a few times lower at 2.0 < z < 3.0. The evolution of ρL(I) is found to be less prominent.
Dust, HII and molecules towards OH and H2O masers
Authors: Forster James R.
Published: June 2019

Radio synthesis observations made with the BIMA interferometer at 29, 86 and 216 GHz are presented for twelve galactic fields containing multiple interstellar OH and H2O maser sites. A dusty molecular cloud was found at 20 of the 23 maser sites in the fields studied. The clouds have masses in the range 50 to 800 M⊙ and diameters between 0.1 and 0.5 pc.

The data show that most masers are located near the centers of massive, dusty molecular cores. The cores appear to be centrally condensed and internally excited. These results suggest that most masers found in star-forming regions are associated with a massive young object at the center of a collapsing molecular cloud. The kinematics of the core gas, and association with ultra-compact HII regions, implies that the duration of the maser phase includes collapse, expansion and early formation of an HII region.
The relationship between masers and massive star formation: what can be learned from the infrared?
Authors: De Buizer James M.
Published: June 2019

The infrared represents an alternative wavelength regime in which to study the environments of maser emission, while at the same time complementing the information obtained through radio techniques. The near infrared (1–2 μm) yields information on outflows, shocks, and reflected dust emission, while the thermal infrared (3–30 μm) yields information on the thermal dust distribution around stars. Thus, the infrared regime yields important clues in determining whether masers exist in shocks, outflows, circumstellar accretion disks, or in the dense medium close to protostars.
First detection of vibrationally excited water masers toward embedded young stellar objects
Authors: Liljeström Tarja , Winnberg Anders , Booth Roy
Published: June 2019

We report the first detection of vibrationally excited water maser emission in the circumstellar envelope of two young stellar objects (W51M, Z CMa). The results suggest that the observed 96 GHz water maser emission traces hot gas in an accretional stage of the protostellar evolution.
Methanol maser monitoring at 6.7 and 12.2 GHz
Authors: Gaylard Michael J , Goedhart Sharmila , Dhlamini Dumisani
Published: June 2019

Methanol masers occuring in thirteen star-forming regions at both 6.7 and 12.2 GHz were monitored for a year at one to two week intervals using the 26-m Hartebeesthoek telescope. Correlated variations occurred in the masers in six of the ten sources that provided useful data.
Milliarcsecond structure of methanol masers in L1206 and GL2789
Authors: Voronkov Maxim A. , Slysh Vyacheslav I.
Published: June 2019

We report results of EVN interferometric study of two star-forming regions L1206 and GL2789 in the brightest methanol maser line at 6.7 GHz. Using measured absolute positions both methanol masers were identified with protostars which are sources of bipolar outflows. Both masers consist of several maser spots, with some of them being aligned in a linear structure with velocity gradient probably delineating edge-on circumstellar disks. We estimated the radii of such disks to be 140 AU and 280 AU (or 700 AU for the whole structure treated as a disk) for L1206 and GL2789 respectively. The brightness temperatures of the most intense features in L1206 and GL2789 are at least 1.1 × 1010 K and 1.4 × 109K respectively.
Models of class II methanol masers
Authors: Sobolev Andrej M. , Ostrovskii Andrei B. , Malyshev Alexey V. , Cragg Dinah M. , Godfrey Peter D. , Sutton Edmund C. , Watson William D. , Ellingsen Simon P. , Caswell Jim L.
Published: June 2019

Current models of class II methanol masers are able to describe the brightnesses of the strongest masers and provide a basis for explaining observed line ratios. Determination of the physical parameters in the source requires observational data in many maser transitions. In order to provide observational constraints for models we searched for and detected 7 new methanol masers. This allowed us to constrain the physical parameters of the 3 sources with the greatest number of detected methanol maser lines: W3(OH), NGC6334F, and G345.01 + 1.79. The models accurately account for the fluxes of the bulk of the detected maser lines. Remaining discrepancies most probably reflect the fact that the most prominent components of the different maser lines are formed under different conditions. This is supported by comparison of the line profiles. We outline directions for future studies in the field.
Supernova remnants and OH (1720 MHz) masers
Authors: Green Anne J.
Published: June 2019

Anomalous OH masers were detected towards supernova remnants (SNRs) more than 30 years ago. The satellite line at 1720 MHz was detected in emission, while the main lines (1665 and 1667 MHz) and the other satellite line, at 1612 MHz, occurred only in absorption. Independently, 25 years ago, a theoretical model was proposed whereby 1720 MHz OH masers could be collisionally excited by the passage of a shock through a molecular cloud. For an efficient inversion of the 1720 MHz transition, the gas should have a narrow range of physical properties, namely a kinetic temperature 50 ≤ TK ≤ 125 K, a volume density nH2 ∼ 105 cm−3, and a column density of OH gas 1016–1017 cm−2. However, it was not until 1994, with interferometric observations, that the importance of studies of these masers, formed when an SNR is in direct interaction with a molecular cloud, was realised. This discovery triggered a series of surveys to search all known Galactic SNRs. I will review the outcomes of these surveys and discuss the consequences of using 1720 MHz masers as a diagnostic tool for calculating shocked gas conditions, for magnetic field determinations (from Zeeman splitting measurements) and for measuring the properties of SNRs.
Magnetic fields in supernova remnants from OH (1720 MHz) masers
Authors: Brogan Crystal L. , Claussen Mark J. , Goss William M.
Published: June 2019

Supernovae have a profound effect on the morphology, kinematics, and metallicity of galaxies. The impact of supernova shocks on surrounding molecular clouds is also thought to trigger new generations of star formation. A critical ingredient in such interactions and, indeed, all aspects of supernova remnant (SNR) evolution are magnetic fields. In recent years, OH (1720 MHz) masers have been used as signposts for the interaction of SNRs with molecular gas. In addition to tracing SNR/molecular cloud interactions, the OH (1720 MHz) maser line also provides a unique opportunity to measure the strength of the post-shock magnetic field via Zeeman splitting. Recent results from efforts to both detect the magnetic fields and resolve the maser spot sizes of OH (1720 MHz) masers toward W51C using the VLBA and W44 using MERLIN are presented. These observations have yielded magnetic field detections between 0.5 and 2.5 mG and large maser spot sizes of about 1015 cm.
VLBA observations of the 7mm SiO masers in TX Cam and R Cas
Authors: Yi Jiyune , Booth Roy S. , Conway John E. , Winnberg Anders , Diamond Phillip J.
Published: June 2019

Simultaneous VLBI observations of the SiO masers of the J = 1 → 0 rotational line in the v = 1 and v = 2 vibrational levels toward Mira variables are presented. Because SiO maser lines are formed deep in circumstellar envelopes they serve as a unique tool to study the innermost envelopes of evolved stars. Although the first interpretation of SiO maser emission was made in 1974, observational features are only partially explained by models which have been suggested since then. Positional coincidence of the J = 1 → 0 masers of v = 1 and v = 2 has been argued as a way to distinguish among the maser pumping models, but it requires simultaneous observations of the two lines using high resolution. We have developed the technique for such observations during the last few years and here we report successful results of our simultaneous observations of the SiO v = 1 and v = 2 J = 1 → 0 masers using the Very Long Baseline Array (VLBA). We discuss the pumping mechanism in terms of our observational results.
OH masers in semiregular variables: Insights from long-term monitoring
Authors: Błaszkiewicz Leszek , Szymczak Marian , Etoka Sandra , Le Squeren A.M.
Published: June 2019

Drifts in radial velocities of OH maser features of the three semiregular variable stars R Crt, RT Vir and W Hya have been analyzed. The radial velocities of the extreme blue- and red-shifted features drifted by about 0.2 kms−1 to 1 km s−1 on time-scales of few months to several years. In the case of W Hya a blending effect of spectral features can account for those drifts. In the two other sources the velocity drifts are likely to be intrinsic to the clumps for which the velocities of peak emission vary due to appearance or disappearance of maser emission in the regions where circumstellar matter is still accelerated.
Monitoring of extragalactic water masers with the MPIfR 100-m telescope
Authors: Hagiwara Yoshiaki , Henkel Christian , Sherwood William A.
Published: June 2019

We present single-dish monitoring of the 22 GHz water maser lines from the Seyfert 2 galaxies NGC 3079, M51(NGC 5194), NGC 5793, and the radio galaxy NGC 315 with the Effelsberg 100-m radio telescope. During the monitoring period of 1995 − 2001, the H2O masers flared in M51 and NGC 5793, while maser emission from NGC 315 was not detected in 1996 and 2000. During 2000, we discovered new red-shifted velocity features in NGC 3079 and blue-shifted features in M51. These velocity components are crucial to model the distribution of maser emission in each galaxy.
Star Clusters and Stellar Populations of the M31 Disk as Viewed by HST
Authors: F. Williams Benjamin , W. Hodge Paul
Published: June 2019

We present a new method for finding associations of young stars in M31 using broadband WFPC2 data from the HST data archive. Applying our identification method to 13 WFPC2 fields, covering an area of ∼60 arcmin2, has revealed 80 new candidate young star clusters in these portions of the M31 disk. Most of these clusters are small (≳5 pc) young (∼10–200 Myr) star groups located within large OB associations. We have estimated the reddening values and the ages of each candidate individually by fitting isochrones to the stellar photometry. We provide a catalogue of the candidates including rough approximations of their reddenings and ages.
The RR Lyrae Variables in M54 and the Sgr Dwarf Galaxy
Authors: Cacciari C. , Bellazzini M. , Colucci S.
Published: June 2019

We report on new B, V and I CCD photometry of the globular cluster M54 that was aimed at the study of its variable stars. With respect to the previous most recent work on M54 we have nearly doubled the number of detected variable stars: M54 can now be classified as intermediate in the Oosterhoff groups. The metallicity can be estimated for the cluster and field red giant stellar population, and for the variables.
Globular Clusters around Galaxies in Groups
Authors: Da Rocha Cristiano , de Oliveira Claudia Mendes , Bolte Michael , L. Ziegler Bodo , H. Puzia Thomas
Published: June 2019

We have obtained deep photometry of NGC 1199 (in HCG 22) and NGC 6868 (in the Telescopium group). Both galaxies are the optically brightest galaxies of their groups. Our analysis of B and R images taken with the Keck II and the VLT/ESO telescopes, detected a population of globular clusters around both galaxies, with total specific frequencies SN = 1.7 ± 0.6 for NGC 1199 and SN = 1.3 ± 0.6 for NGC 6868. The color distributions of the globular cluster systems shows bimodal peaks centered at (B - R)0 = 1.13 ± 0.10 and 1.42 ± 0.10 (NGC 1199) and (B - R)0 = 1.12 ± 0.10 and 1.42 ± 0.10 (NGC 6868).
The Young Star Clusters Parameters in Late Type Spiral Galaxies
Authors: Gusev Alexander S.
Published: June 2019

An analysis of parameters of the 566 young star clusters in the 35 late type spiral and irregular galaxies is presented. The disposition, the sizes and the colors of the star clusters are studied, based on the multicolor surface photometry data. The ages of the young star clusters are estimated using evolutionary synthesis method. A problem of taking into account the dust contribution is discussed. Most of the clusters are located in the Inner Lindblad Resonance ring and in other resonance rings. In some ring galaxies the distances between neighbouring star clusters are similar. For some of investigated galaxies, the star clusters belonging to a given galaxy have similar ages. A correlation between age and size of star forming regions exists. The younger objects have smaller average sizes. There are no single star forming regions with sizes > 550 pc. In general, the star forming regions in a given galaxy have similar sizes. A correlation between maximal diameter of star clusters and absolute blue magnitude of parental galaxy exists.
Star Formation in Large N Clusters
Authors: Clarke C.J.
Published: June 2019

We focus here on the gas dynamics of cluster formation and the early stellar dynamical evolution of young clusters. We point out that the condition that a cloud can fragment into a large number of pieces places rather particular constraints on its initial state; we also review the processes that shape the stellar IMF in cluster formation simulations. We show how N-body calculations and observations can be combined to discover the properties of clusters at the point at which they first become stellar dynamical (as opposed to gas dynamical systems). Finally, we touch on the question of how proto-cluster clouds are assembled and reopen the issue of whether dark matter may play a role in globular cluster formation.
Formation and Disruption of Globular Star Clusters
Authors: Fall S. Michael , Zhang Qing
Published: June 2019

In the first part of this article, we review observations of the mass and luminosity functions of young and old star cluster systems. We also review some of the physical processes that may determine the characteristic mass of globular clusters and the form of their mass function. In the second part of this article, we summarize our models for the disruption of clusters and the corresponding evolution of the mass function. Much of our focus here is on understanding why the mass function of globular clusters has no more than a weak dependence on radius within their host galaxies.
NANTEN Observations of the LMC: Giant Molecular Clouds as Formation Sites of Stellar Clusters
Authors: Mizuno Norikazu , Yamaguchi Reiko , Onishi Toshikazu , Mizuno Akira , Fukui Yasuo
Published: June 2019

A CO survey of the LMC has been completed in the J = 1−0 carbon monoxide emission at 2.6 mm wavelength with NANTEN. This survey has revealed the first complete sample of giant molecular clouds (GMCs) in the LMC at a linear resolution of ∼ 40 pc. The GMCs exhibit a good spatial correlation with youngest stellar clusters often associated with the giant H II regions. The ages of clusters associated with GMCs are fairly young, ≲ 10 Myr, demonstrating that cluster formation is ongoing in these GMCs. In addition, compact groups of the young clusters are often found at the peak position of the GMCs, e.g., N159 and N44, while much looser groups are away from the GMCs. This suggests that the clusters are formed in groups and are dispersed as they become old.
The USNO Astrometric Optical Interferometer
Authors: Hutter D.J. , Johnston K.J. , Mozurkewich D.
Published: June 2019

The U.S. Naval Observatory Astrometric Optical Interferometer (AOI) began operation on Anderson Mesa, near Flagstaff, Arizona, in the autumn of 1994. The AOI incorporates four siderostats that are located in a Y-shaped configuration, and features a full-array laser metrology system to monitor baseline motion. The AOI incorporates state-of-the-art delay lines and a real-time fringe-tracking system. The AOI will have a limiting visual magnitude of 10, under typical observing conditions, and will produce star positions accurate to a few milliarcseconds (mas). With a planned operational lifetime of several decades, this instrument will be capable of maintaining the optical reference frame by improving the proper motions of thousands of the brighter HIPPARCOS stars through repeated observations.
Astrometry of Large Sky Surveys
Authors: Van Altena W.F. , Girard T.M. , Platais I.
Published: June 2019

One of the major uses of the Large Sky Surveys such as the Palomar and SERC Schmidt Surveys is the derivation of positions for faint objects discovered in the course of other surveys. The HST Guide Star Catalogue has proved to be of great help in deriving positions for such objects, but the well-known, and unavoidable, systematic errors in the GSC positions limit the accuracy of the final positions. In this Review, we will describe techniques that can be used to successfully transfer a reference frame from the bright FK5 stars to the small confines of a CCD chip containing only faint objects with the goal of maintaining an accuracy of approximately 0.1 arcsecond in the final positions.
Accurate Parallaxes and Stellar Ages Determinations
Authors: Lebreton Y. , Michel E. , Goupil M.J. , Baglin A. , Fernandes J.
Published: June 2019

Uncertainties on stellar ages due to the physical description of the stellar material entering the models and to measurements of global parameters and chemical composition are estimated in the case of A/F stars.

With combined efforts on atmosphere modeling (to improve global parameters), on asteroseismology (to improve the physical description of the stellar interior) and on distance determination at the level of the sub-milliarcsecond (to improve luminosity), an accuracy on age determination of about 10% is foreseeable.
Peculiar Stars of Low Luminosity
Authors: Liebert J. , Dahn C.C.
Published: June 2019

Precise trigonometric parallax measurements orders of magnitude more accurate than a milliarcsecond will contribute greatly to our understanding of peculiar, low luminosity stellar objects of several types. First, the volume of space out to which luminosities may be determined to the accuracy of the best, very-nearby stars will be increased greatly. For the relatively rare field Population II stars, this will lead to the first accurate empirical calibrations of the main sequence at the low mass end, for comparison with globular clusters of various metallicities. Parallaxes at 1 kpc or farther will be adequate to help in the discovery or confirmation of the rare carbon dwarfs - main sequence stars with carbon-rich atmospheres. For cool white dwarfs, luminosities accurate to a few per cent or better will identify unresolved binaries, and objects of unusually high and low mass. For our most numerous solar neighbors, the M dwarfs and especially those near the stellar mass limit, accurate luminosities can help in the determinations of the chemical composition and age distributions.
The New Catalogue of Positions and Proper Motions of 5115 Bright Stars Compiled as the Result of the International “Bright Stars” Program
Authors: Molotaj O.A. , Tel'Nyuk-Adamchuk V.V. , Chernega N.A. , Kanivec' N.D.
Published: June 2019

At Kyiv University Observatory, in accordance with IAU resolution, the Combined Catalogue of Positions and Proper Motions of 5115 Bright Stars in the FK5 system for Epoch and Equinox J2000.0 (BSC, see reference) has been compiled using 20 source catalogues obtained within the framework of ‘Bright Stars’ International Program. The source catalogs are Bordeaux-66, Bucharest-65, Bucharest-68, Belgrade-79, Cape-68, Kharkiv-70n, Kharkiv-70s, Kyiv-66, Kyiv-73, El Leoncito-70, Moscow-76, Mykolaiv-65, Perth-70, Perth-75, Santiago-65, Santiago-67, Strasbourg-65, Tashkent-64, Tokyo-68, Washington-66. The BSC accumulates around 30,000 source catalogue positions observed within 1960-1980. The standard errors of BSC do not exceed 0.1 arcsec and 0.25 arcsec/cy for positions and proper motions, respectively. The systematic differences between BSC and FK5 in positions for the mean BSC epoch 1970 as a rule do not exceed several hundredths of arcsec. A comparison of the BSC with the PPM catalogue indicates that in the latter the accuracy of positions and proper motions of the ordinary bright stars is slightly worse than that of average values given by the authors of the PPM. It is important to note that the comparison of the BSC with PPM for various star subsets shows deviations of both the HPS bright star system and the rest of the PPM bright stars, on the one hand, from the stars of the FK5 Extension contained in the PPM on the other hand. To examine the BSC proper motion system the stellar astronomy constants and correction to precession have been determined using the BSC proper motions. The parameters obtained agree with the standard ones. But the Oort constants are slightly smaller than those of standard.

The BSC catalogue as a whole is acceptable for astrometry usage with respect to both accidental and systematic accuracy.
Placing the Guide Star Catalog on the Hipparcos System
Authors: Bastian U. , Bucciarelli B. , Hayes J. , Lasker B. M. , Morrison J. E. , Röser S. , Smart R. L. , Sturch C. R. , Taff L. G.
Published: June 2019

The HIPPARCOS Catalogue, linked to the system of extra-galactic radio sources, will soon be the reference system in optical astronomy. The Guide Star Catalog (GSC; Lasker et al. AJ 99, 2019ff, 1990), with its 18 million objects, already serves most astronomers for the planning of small field observations, optical identifications, and the reduction of CCD images. The utility of the GSC will be greatly enhanced if it is transformed to the HIPPARCOS system and its remaining systematic errors minimized. To do so we will use the forthcoming TYCHO Reference Catalogue (Röser & Høg, in Workshop on Databases for Galactic Structure, eds. Davis Philip, Hauck & Upgren, pp. 137, 1993). The reductions will use a combination of the mask and sub-plate methods (Taff, AJ 98, 1912, 1989; Taff, Lattanzi & Bucciarelli ApJ 361, 667, 1990), the co-location method (Bucciarelli, Lattanzi & Taff, ApJ Suppl. 84, 91, 1993), the new “fixed number of stars filter” of Röser, Bastian & Kuzmin (in press in the proceedings of IAU 148, 1994), and the infinitely overlapping circles (IOC) version of the latter (Taff, Bucciarelli & Lattanzi, ApJ 358, 359, 1990; Bucciarelli, Taff & Lattanzi, J. Stat. Comp. Sim. 48, 29, 1993). We present in the table below test results using the PPM as the reference catalog (typically only 4 sub-plates per plate; 25 stars for the Square Filter; 16 stars per circle for the IOC method). The first line presents the estimates for σαcosδ and σδ, both in arcsec, relative the to PPM Suppl (Röser, Bastian & Kuzmin, A&A Suppl. 105, 301, 1994); the second line contains them based on plate overlaps (all for the −30° declination zone). When comparing the numbers note that in the first line the GSC and PPM Suppl (0.″12) errors are contributing whereas in the second line the GSC errors are present twice (in the rms sense).
Analysis of Stellar Observations: Systematic Effects, Radio Stars, Proper Motions
Authors: Martin V.A.F. , Leister N.V.
Published: June 2019

An observational stellar program with a prismatic astrolabe was started at “Observatório Abrahão de Moraes” - OAM - Sao Paulo, Brazil, in 1974
Concluding Remarks
Authors: Kovalevsky Jean
Published: June 2019

We are now at the end of a long symposium which lasted five (but not all full) days. The objective was first to review the best present achievements of astrometry and to present projects for a much more advanced astrometry in terms of precision attained versus the number of stars and limiting magnitudes. The word sub-milliarcsecond astrometry was intentionaly vague. It has rightly been often understood as designating a range of precisions of the order of a few tens microarcseconds.
Confrontation of Antimatter Cosmologies with Observational Data
Authors: Steigman Gary
Published: June 2019

The presence of large amounts of antimatter in the Universe would be detectable directly via the cosmic rays and indirectly via the annihilation γ-rays. The observational data is reviewed. There is no evidence whatever indicating the presence of astrophysically interesting amounts of antimatter in the Universe. From the available data we may conclude that the Galaxy is made entirely of ordinary matter and that if any antimatter at all is present in the Universe, it must be very well separated from ordinary matter. Furthermore, we show that the observational constraints on various symmetrical cosmological models strengthen the case against antimatter in the Universe.
A New General Covariant Approach to the General Relativistic Two-Body Problem
Authors: Rosenblum Arnold
Published: June 2019

A new general covariant approach to the general relativistic equations of motion is presented. It is stressed that our present understanding of the development of binaries due to general relativistic effects and of the power emitted by these systems in the form of gravitational radiation is highly unsatisfactory.
On a Possible Influence of Magnetic Fields on the Structure of a Disk Formed During Accretion of Plasma in Binary Systems
Authors: Pustilnik L. A. , Shvartsman V. F.
Published: June 2019

During accretion of plasma in binary systems containing compact objects magnetic fields are most likely to become arranged quickly, grow and fully determine the disk structure. The disk divides into separate dense clots, and a corona appears over the equatorial plane of the system. In the corona magnetic lines of force reconnect and beams of relativistic particles are generated.
Polarization of Galactic Background Light in the Visible Spectral Region
Authors: Leinert Ch.
Published: June 2019

This is a summary of the few observations available to date. Although there remain doubts concerning the reliability of most of the published results, a linear polarization of a few percent, perpendicular to the galactic plane, is indicated. Therefore, scattering of galactic light by interstellar dust appears to be the dominating source of galactic background polarization. There is a severe lack of model calculations, which would be needed for a physical interpretation of the data.
Fluctuations of the Diffuse Galactic Background
Authors: Mattila K.
Published: June 2019

An analysis of fluctuations in the brightness of the Milky Way using the concept that interstellar matter occurs in the form of discrete clouds was first applied by Ambarzumian (1940, 1944). This theory was formulated in a general way and discussed in great detail in a series of papers by Chandrasekhar and Münch (1950a, 1950b, 1951, 1952), by Münch and Chandrasekhar (1952), and by Limber (1953). More recently Peters (1970) presented an analysis of this kind based on extensive photographic observational material. Although the influence of clumpiness of the dust distribution on the mean integrated starlight was thoroughly discussed in these papers, it has not been properly included in most of the photometric models of the Galaxy. Only the models of Caplan and Grec (1979) and Mattila (1980a, 1980b) incorporate these effects.
Blue and Red Brightnesses of the Integrated Starlight Obtained by Star-Counts in 24 Regions
Authors: Tanabe H. , Mori K.
Published: June 2019

Star-counts on the Palomar Sky Survey Atlas were made in 1968-87 in order to estimate the background brightness in zodiacal light and airglow observations. Measured diameters of blue and red images of each star were converted to the magnitude with empirical diameter-magnitude relations. The resultant blue and red brightnesses and colors of the integrated starlight are presented for 24 sky regions, including 4 polar (North Celestial, North Ecliptic, North and South Galactic) and 10 ecliptic regions.
Infrared Radiation from Evolved Stars
Authors: Rose William K.
Published: June 2019

OH/IR stars and carbon stars are examples of evolved stars known to lose mass rapidly. In a recent paper (Rose 1987) we have described calculations of infrared continuum radiation from dust in stellar winds from red giants. We compare calculated radiative models with recent infrared measurements and estimate the total contribution of evolved stars to the mid-infrared background radiation observed from the Milky Way and M31.
Diffuse Galactic Light in the UV and Visible
Authors: Witt A. N.
Published: June 2019

The diffuse galactic light, resulting from the coherent scattering of galactic starlight by dust grains contained in the general interstellar medium, has been the subject of active investigation for nearly 60 years. The separation of the diffuse galactic light from the other sources contributing to the light from the night sky has proven difficult, and different attempts at measuring the intensity and galactic distribution of the diffuse galactic light, both in the visible and the UV, are reviewed here. The interpretation of such measurements in terms of average scattering properties of interstellar grains is subject to additional uncertainties, stemming from the high degree of idealization imposed on the galaxy models used to study the radiative transfer problem. In the visible, the observations are more nearly definitive and the model problems less severe; reasonably consistent scattering properties have therefore been derived for this spectral region. In the UV, the situation is considerably less satisfactory, mainly due to a lack of sufficiently extensive, reliable measurements of the diffuse galactic light intensity at λ < 200 nm. A dedicated space mission may be the required solution. The radiative transfer in the UV presents serious difficulties due to the increasing opacity of the interstellar medium with shorter wavelengths and the resulting growing importance of the local galactic structure.
Observations of the Diffuse Ultraviolet Background from the UVX Experiment On Space Shuttle
Authors: Henry Richard C. , Feldman Paul D. , Murthy Jayant
Published: June 2019

The UVX experiment has resulted in confirmation that the diffuse ultraviolet background is low in intensity everywhere and that it is fairly uniform in intensity, both spatially and spectrally. There is no clear evidence that any significant portion of the moderate galactic latitude diffuse cosmic background originates in galactic plane starlight scattering from interstellar dust.
The Far Ultraviolet Background: Dust Scattering and the Extragalactic Continuum
Authors: Hurwitz Mark , Bowyer Stuart , Martin Christopher
Published: June 2019

NASA's Ultraviolet Experiment (UVX) payload, which flew aboard space shuttle Columbia in January 1986, contained a spectrograph built by the Space Astrophysics Group at the University of California, Berkeley. The wavelength range is 1400–1850 Å with a FWHM resolution of ~15 ± 2 Å. A full description of the instrument can be found in Martin and Bowyer (1984). The instrument was pointed at various regions of the sky for 8 nighttime orbits. Targets spanning a wide range of galactic latitudes and neutral hydrogen column densities were observed.
The Global Distribution of Zodiacal Dust
Authors: Kneißel B.
Published: June 2019

From visible or infrared brightness observations of the zodiacal light, a large variety of models of the three-dimensional structure of the zodiacal dust cloud has been proposed. To assess the reliability of these models, we must first investigate their fit to a selected set of observational data. The fit is best for bulge models, which have an appreciable density over the solar poles. Next, we check the orbital inclination distributions predicted by the various models. A comparison of these distributions with those of minor bodies in the solar system does not support the preference for bulge models, but instead supports polar hole models with a negligible density over the solar pole. These uncertainties of modelling have to be kept in mind when models are used to derive the brightness contribution of zodiacal light, particularly in the infrared, where the data base is still limited.