bluuebunny/crossref_metadata_2025 huggingface dataset:
Hierarchical fragmented structure of molecular clouds produced by supersonic turbulence
Authors: Kolesnik Igor G. , Ohul'chansky Yaroslav Yu.
Published: June 2019

For GMC's the two prominent properties are typical. They consist of dense molecular gas clumps concentrating to the GMC centre and filling only a few percent of a total volume. And these clumps participate in chaotic motions with velocities vt exceeding as a rule the sound velocity co at the temperature of molecular gas. This phenomenon is considered as a supersonic molecular cloud's turbulence. The compressibility of turbulent matter becomes very important with such velocities. Thus in application to GMC it is necessary to develop the theory of turbulence and fragmentation under transsonic and supersonic random motions. The hydrodynamic flow velocity field can be divided into the potential and vortical components. When transsonic or supersonic motions prevail the potential component is become more important that stimulates the shock wave's stochastic field development. Ohul'chansky (1988, Kinematics and Physics of Celestial Bodies 4,3) has described this process on the base of Burgers' equation treatment. In this paper we apply this approach for conditions of GMCs that permit the supersonic turbulence' spectrum evolution, the large density fluctuations development, and clumps formation to consider.
The bipolar-flow phenomenon
Authors: Kundt Wolfgang
Published: June 2019

Very young stars, known as ‘pre-T-Tauri stars’ (PTTS) or ‘young stellar objects’ (YSO), tend to be surrounded by elongated outflow regions involving ‘Herbig-Haro objects’ (HH). Such ‘bipolar flows’ (BF) are reminiscent of the extragalactic radio sources for which a consensus has formed in 1986 that their jets consist of extremely relativistic pair plasma, of typical Lorentz factor 104±1, generated by an ‘active galactic nucleus’ (AGN); [refs 6,7]. Here I collect new circumstantial evidence for the relativistic nature of BFs.
Large scale interaction of the outflow and quiescent gas in Orion
Authors: Martin-Pintado J. , Rodriguez-Franco A. , Bachiller R.
Published: June 2019

The IRAM 30-m radiotelescope have been used to obtain, with high angular resolution, the spatial distribution and the physical conditions of the quiescent gas in Orion A, and to search for high velocity molecular gas far away from the well known molecular outflow around IRc2. To study the quiescent gas we mapped a region of 200″×300″ around IRc2 in the J=12-11 and J=16-15 lines of HC3N with angular resolutions of 22″ and 17″ respectively. The left panel of Fig. 1 shows the spatial distribution of the high density quiescent gas around IRc2 for different radial velocities. Beside the already known molecular ridge north of IRc2 (see e. g. Bartla et al. 1983), we find four very thin (nearly unresolved) and long filaments, like “fingers”, stretching from IRc2 to the north and west. The deconvolved size of the longest fingers is ≈180″×15″. From a multi-transition analysis of the HC3N emission we derive H2 densities of 1−8 105 cm−3, kinetic temperatures larger than 40 K and masses of ≈10 Mo. Our high sensitivity observations of the J=2-1 line of CO at selected positions (see right panel ib Fig. 1) show widespread molecular gas with high velocities wings over the region where the molecular fingers and the HH objects are observed (see Fig.1). The high velocity emission occurs over a range of ±40 kms−1. This high velocity gas is more extended (up to 150″ from IRc2) than the very compact (40″) and well studied molecular outflow around IRc2 (see e.g. Wilson et al. 1986). The terminal velocities of the CO wings decrease from 100 km s−1 (corresponding to the very fast molecular flow) to the typical terminal velocities of the extended high velocity gas when the distance to IRc2 changes from 40″ to 60″. The origin of the large scale high velocity gas is unknown, but it is very likely the link between the very compact (40″) and fast (±100 km s−1) molecular outflow around IRc2 and the ionized high velocity gas and the HH objects (Martín-Pintado et al. 1990). The mass, momentum and energy of the extended high velocity gas are crudely estimated to be ≈1 Mo, ≈20 Mo km s−1 and ≈2 1045 erg respectively (i.e. a factor of ≈10 smaller than those of the fast molecular outflow). The location, at the edges of the molecular fingers, and the proper motions of the HH objects (see Fig. 1) suggest the stellar wind is interacting with the molecular fingers. If this interpretation is correct, the influence of the molecular outflow in Orion on the surrounding molecular clouds must be revised.
A hydrodynamical study of fragmenting gas clouds
Authors: Monaghan Joe , Lattanzio John
Published: June 2019

We present 3-D hydrodynamical calculations of collapsing rotating gas clouds, with molecular cooling. We find that cooling significantly inreases the number of fragments.
A model of the distribution of ionised carbon in M17SW
Authors: Pinnock Simon , Monteiro Tania S.
Published: June 2019

The observed distribution of ionised carbon with depth into the edge-on photodissociation region (PDR) in M17SW is not consistent with results from chemical models which assume a constant or smoothly varying density. These observations have been interpreted to indicate that the PDR actually consists of many clumps embedded in a low density interclump medium (1). A chemical model is described which is designed to simulate such a clumpy PDR.
15NH3 millimasers toward NGC 7538—IRS 1
Authors: Schilke Peter , Walmsley Malcolm
Published: June 2019

Ammonia towards the UCHII—region NGC 7538—IRS 1 shows an extremely hot component observed in absorption in several transitions and a so far unexplained (9,6) maser (see Fig. 1). This source is the only known source in the Galaxy where the 141 GHz line (1450 K above ground) of vibrationally excited Ammonia is seen in absorption (Schilke et al. 1990). An additional unique feature is the 15NH3(3,3) maser observed by Mauersberger et al. (1986) in Effelsberg and by Johnston et al. (1989) with the VLA. There are theoretical models to explain such a maser (see e.g. Flower, Offer and Schilke 1990) in low density regions where ortho-H2 is underabundant. In this study, our aim was to investigate 15NH3 in more detail. We succeeded in observing the (1,1) and (2,2) lines of 15NH3 in absorption and, surprisingly, the (4,3) and (4,4) lines in emission (see Fig. 1).
A very straight and collimated outflow in the core of OMC-1
Authors: Schmid-Burgk J. , Güsten R. , Mauersberger R. , Schulz A. , Wilson T. L.
Published: June 2019

We have recently discovered a large-scale (200″) outflow system in the core of OMC-1 (fig. 1), centered about 100″ South of IRc2 and extending over some 120″ (red lobe) resp. 60″ (blue) along a position angle of —31° (Schmid-Burgk et al. 1990). The blue lobe which might actually protrude into the HII region M42 is poorly defined in CO 2-1, but the red lobe reveals a number of remarkable properties which we summarize here:

The outflow is very straight and smooth. Over the full length of 120″, the center of any cross scan deviates by not more than about 1″ from a straight line. This line passes to within 2″ the peak of the submm source FIR4 of OMC-1 (Mezger, Wink and Zylka 1990) and the mm continuum peak CS3 (Mundy et al. 1986); it also cuts across the red and blue SiO-outflow lobes recently discovered some 5-10″ to either side of FIR4 (Ziurys, Wilson and Mauersberger 1990). It thus seems that the “base” of our large-scale CO jet can be seen as well.
Density and kinematics of the W49A cloud core
Authors: Serabyn E. , Güsten R.
Published: June 2019

The dense core of the W49A molecular cloud (Miyawaki et al. 1986, Schloerb et al. 1987) has been mapped in 5 different transitions of CS and C34S, in order to determine its density structure. The three lower frequency transitions (CS J=3-2, CS J=5-4 and C34S J=5-4) were observed with the IRAM 30m telescope, and the two highest frequency transitions (CS J=7-6 and C34S J=7-6) with the Caltech Submillimeter Observatory. The beamsizes were in the range 12″ to 20″. As a calibration check, the CS J=7-6 line was observed with both telescopes, and was found to give a consistent temperature scale. The spectra at the peak of the emission are shown in Fig. 1.
Radiative transfer of CO through clumpy molecular clouds with external UV heating
Authors: Tauber Jan A. , Goldsmith Paul F.
Published: June 2019

We have developed a model which simulates the radiative transfer of molecular line emission through clumpy molecular clouds. The dynamical structure of the model cloud is based on the work of Kwan and Sanders (1986). The model incorporates the existence of an intense source of UV photons at the surface of the cloud. The UV source heats the clumps and creates kinetic temperature and CO abundance gradients within them. The amount of heating depends on the intensity of the UV field, which decreases from the surface to the core of the cloud due to attenuation by dust. We treat in detail the photochemistry and self-shielding properties of CO as a function of UV intensity and gas density in order to obtain the CO line intensities emerging from each clump. The line intensity emerging from the cloud is obtained by integrating the emission from all clumps along the line of sight, weighted by an area covering factor, and attenuated by the opacity of intervening clumps. The effects of the heating are significantly noticeable on the line intensities of CO transitions arising from levels with J between ∼ 3 and ∼ 7. We apply our model to the case of the Orion A molecular cloud, and in particular to observations of the J=3 →2 12CO and 13CO lines. The model is in general agreement with the observed enhanced intensity of the 12CO J=3 →2 transition relative to the J=1 →0 transition throughout the central ∼ 10′ region of Orion. It also produces centrally peaked spectral lines whose intensity is maximum in a shell-like distribution centered on the Trapezium HII region, as is observed.
The W75 — Cygnus - X IRAS loop:- OB - ‘Bubble’ or SNR?
Authors: Ward-Thompson D. , Robson E. I.
Published: June 2019

IRAS Calibrated Raw Detector Data (CRDD) are presented of a part of the Cygnus-X region, incorporating W75, DR21 and W75N, and a previously unknown loop of dust emission is observed. This loop is interpreted as a spherical shell-like shock front, and two alternative explanations for its origin are explored — a wind-blown bubble around an OB association, and an old supernova remnant. The arguments for each are outlined, and it is deduced that there are insufficient OB stars old enough to have formed the loop by combined stellar wind action, although a SNR appears consistent with the data.
A binary star formation mechanism through the fragmentation of prolate dense cores rotating end over end
Authors: Zinnecker Hans
Published: June 2019

I propose and briefly elaborate on a major new mechanism for the formation of wide, low-mass binary stars: the fragmentation of a collapsing, initially elongated dense molecular core rotating end over end. This initial structure will develop into two independent gravitationally bound stellar condensations orbiting each other in a rather eccentric orbit.
The interstellar medium around Wolf-Rayet stars: clues to evolution
Authors: Nichols Joy S.
Published: June 2019

Multi-wavelength investigations of the interstellar environment around WR stars usingIUE, IRAS, and optical data have led to the discovery of extended shells of gas and dust 50-100 pc in diameter in the lines of sight to the WR stars HD 50896, HD 96548, and HD 192163. These three stars share several common characteristics: (1) associated N- and He-enriched ring nebula, (2) spectral class WN5-8, and (3) spectral and photometric variability. Surveys ofIUEspectra andIRASimages reveal a few additional candidates for such extended shells. Although positional coincidences cannot be excluded, possible origins of these extended shells include interstellar bubbles formed by stellar winds, non-conservative mass loss from binary systems, and supernova remnants. Current data suggest these three stars may be in a post X-ray binary stage, representing the second WR phase of massive binary star evolution. The discovery of extended shells of gas and dust possibly related to WR stars is important because the search for stars in the second WR phase, after the supernova of the primary, has been quite elusive. However, the observational evidence and available theoretical models for post-supernova massive binary star evolution and subsequent non-conservative mass transfer are not yet sufficient to firmly identify the origins of these extended shells.
Wavelet analysis of variable Wolf-Rayet emission lines
Authors: Lépine Sébastien
Published: June 2019

A new powerful mathematical tool for the analysis of structures on spectral
lines is presented: wavelet analysis. Applied to variable Wolf-Rayet emission
lines, it allows one to extract efficiently and in an objective way, individual
sub-structures and compare some of their physical properties: integrated flux
f, velocity dispersion σv, projected velocity v and acceleration a.
Optically thick winds in novae
Authors: Shaviv Giora
Published: June 2019

We discuss the theory of optically thick winds from Novae, how they are treated numerically, and the effect of the mass loss on the long term evolution of the WD.
Properties of Wolf-Rayet binaries: the key to understanding Wolf-Rayet stars
Authors: Moffat Anthony F.J.
Published: June 2019

It is argued that, with few exceptions, WR stars in binaries are essentially indistinguishable from single WR stars of the same subtype. In particular, mass transfer from one star to the other can be neglected, being completely dominated by the winds and their interaction. Empirical masses and mass-loss rates derived for WR stars in binaries are then examined as typifying WR stars in general; they are used to derive a plausible scenario for the evolution of WR stars.
X-ray variability in V444 Cygni: evidence for colliding winds
Authors: Corcoran M.F. , Stevens I.R. , Pollock A.M.T. , Swank J.H. , Shore S.N. , Rawley G.L.
Published: June 2019

Phase-resolved observations of the soft X-ray flux from the WN5+O6 binary V444 Cyg (WR139) have been obtained by ROSAT. The objective of these observations were: (1) to compare our results with previous observations made with the IPC (Moffat et at. 1982); (2) to explore the phase dependence of the X-ray variations; and (3) to look for evidence of the generation of X-rays from colliding winds in the system. ROSAT observed V444 Cygni with the PSPC five separate times, for a total integration time of about 20 ksec. Figure 1 shows the phase-binned light-curve along with archival IPC data. The observed X-ray variability can be modeled as a wind eclipse of an X-ray source by the WR wind. If most of the X-rays from the system are produced in a shock-heated region of wind interaction between the two stars, then the shocked gas has a large physical extent (r ≈ 140RW R) if the observed variability is produced solely by the eclipse of the region by the WR wind. However, since the O star makes a significant contribution to the total X-ray flux from the system, the actual size of the shocked interaction region is probably much smaller.
Type Ia Supernovae and the Value of the Hubble Constant
Authors: Gibson Brad K. , Brook Chris B.
Published: June 2019

The methodology involved in deriving the Hubble Constant via the calibration of the corrected peak luminosities of Type Ia supernovae (SNe) is reviewed. We first present a re-analysis of the Calán-Tololo (C-T) and Center for Astrophysics (CfA) Type Ia SN surveys. Bivariate linear least squares and quadratic boot-strapped fits in peak apparent magnitude and light curve shape are employed to correct this heterogeneous sample of peak apparent magnitudes, resulting in an homogeneous (and excellent) secondary distance indicator: the so-called corrected peak luminosity. We next provide an empirical calibration for this corrected luminosity, using Cepheid-based distances for seven nearby spiral galaxies host to Type Ia SNe. Included in this sample is the spectroscopically peculiar SN 1991T (in NGC 4527), whose corrected peak luminosity is shown to be indistinguishable from that of so-called “normal” SNe. A robust value of the Hubble Constant is derived and shown to be H0=73±2(r)±7(s)km s-1 Mpc-1.
The Rise Time of the Normal Type Ia SN 1990N
Authors: Mazzali Paolo A. , Schmidt Brian P.
Published: June 2019

The earliest spectrum available of a well-observed, local type Ia SN, the UV-optical spectrum of SN 1990N obtained 14 days beforeBmaximum, was modelled with a Monte Carlo code in order to determine its epoch and consequently the rise time from explosion toBmax, which is important for fitting template light curves. If a standard density distribution and composition for the SN ejecta are used, and the Cepheid distance modulus to the host galaxy, NGC4639 in the Virgo cluster, μ = 32.0, is adopted, the spectrum is best reproduced for an epocht= 5.5 ± 1 days, implying for the SN a rise time of 19.5 ± 1 days. For a shorter μ,tis smaller, but for a T-F distance μ = 31.4 the solution is not consistent.
Realistic Galaxy Models for Lensing Statistics
Authors: Rusin David. , Tegmark Max.
Published: June 2019

The gravitational lensing rate in a well-defined sample of sources can place strong bounds on the cosmological constant, but only if the lensing optical depth is robustly calculated. Significant progress is likely to be achieved by employing more realistic models to describe the population of lens galaxies. Here we investigate the role of elliptical deflectors in lensing statistics, and their effect on cosmological constraints derived from the JVAS survey. We also evaluate the prospects for constraining the cosmological constant using the much larger CLASS data set.
Weak lensing study of low mass groups: implications for Ωm
Authors: Hoekstra H. , Franx M. , Kuijken K. , Carlberg R. G. , C. Yee H. K.
Published: June 2019

We report on the first measurement of the average mass and mass-to-light ratio of galaxy groups by analysing the weak lensing signal induced by these systems. The Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2) allows the identification of a large number of groups at intermediate redshifts. For our analysis we use a sample of 50 groups which are selected on the basis of a careful dynamical analysis of group candidates. We detect a signal at the 99% confidence limit. The best fit singular isothermal sphere model yields an Einstein radius rE = 0″.72 ± 0″.29. This corresponds to a velocity dispersion of 〈σ2〉1/2 = 274±+48-59 km/s, which is in good agreement with the dynamical estimate. Under the assumption that the light traces the mass, we find an average mass-to-light ratio of 191 ± 83 h in the restframe B band. Unlike dynamical estimates, this result is insensitive to problems associated with determining group membership. We use the observed mass-to-light ratio to estimate the matter density of the universe, for which we find Ωm = 0.19 ± 0.10 (ΩΛ = 0), in good agreement with other recent estimates. For a closed universe (Ωm + ΩΛ = 1), we obtain Ωm = 0.13 ± 0.07.
Concluding Perspective
Authors: Rees Martin J.
Published: June 2019

This meeting has covered a broad agenda. This contribution to the concluding session offers comments on some current controversies, and on the prospects for cosmology in the coming decade.
Wrinkles in the Galaxy
Authors: López-Corredoira Martín
Published: June 2019

New calculations of the Galactic contribution to microwave background radiation anisotropies are carried out and a probable Galactic predominance over cosmological signals at large scales is explored for these frequencies. When we take into account a frequency-dependent contrast of molecular clouds with respect to the Galactic background of the diffuse interstellar medium, the anisotropic amplitude produced by Galactic dust is of the same order as that of the data from the observations. The frequency independence of anisotropies in the microwave range is not necessarily an argument against the Galactic predominance if we take into account an aditional rotational dust emission, for instance. This provides a basis for questioning the validity of considering negligible the Galactic contribution of the microwave background radiation anisotropies. Moreover, the size of the clouds is nearly coincident with that of the structures observed in the microwaves.
The WEB Structures at Small Scale Related to Λ0 Problem
Authors: Doru Suran Marian , Antonia Popescu Nedelia
Published: June 2019

In the present paper, using a variety of N-body and gas codes, we study the large and small scale structures. Furthermore, we identify coherent objects at various threshold levels and calculate their planarity and filamentarity. The WEB theory is used to constrain the cosmological parameters of inflation-based Λ0 CDM models.
Observation of the Sunyaev-Zel'dovich Effect toward CL0016+16 at 43 G Hz
Authors: Tsuboi Masato. , Ohno Takeshi. , Tanabe Yaichiro. , Kasuga Takashi.
Published: June 2019

We performed a mapping observation of the Sunyaev-Zel'dovich effect toward a galaxy cluster, CL0016+16, at 43 GHz using a newly developed 6-beam receiver installed in the Nobeyama 45-m telescope. The temperature decrement of the CMB was measured to be δT*A = −0.49 ± 0.06 mK at the center of the cluster. Fitting the isothermal, spherical-symmetrical β -model to the observed temperature decrement distribution, the S-Z effect at the center is inferred to be δTRJ(0) = −1.0 ± 0.1 mK at the Rayleigh-Jeans limit. This suggests that the Hubble constant is H0{q0 = 0.5} = 67+16−11 km s−1 Mpc−1.
Methanol masers: Evolutionary and kinematic tracers of massive star formation
Authors: Minier Vincent , Booth Roy , Conway John , Pestalozzi Michele
Published: June 2019

We summarise our recent VLBI observations of a large sample of methanol maser sources associated with high-mass star-forming regions.
Masers in G34.3+0.2: What more can 6.7-GHz methanol masers tell us?
Authors: Yi Jiyune , Phillips Chris J. , Booth Roy S.
Published: June 2019

We present results of 6.7 GHz methanol maser observations using the European VLBI Network (EVN) toward the G34.3+0.2 complex, which contains a prototypical cometary HII region and a dust enshrouded, probably emerging massive star, G34.24+0.13. We have investigated the methanol maser distribution in both objects and compared it with the OH and water masers in the main HII region. The masers in G34.24+0.13 lie on top of the 1.3 mm continuum emission showing that methanol masers are present, even at this early evolutionary stage.
SiO masers in red giants
Authors: Humphreys E. M. L.
Published: June 2019

In recent years, it has become possible to trace rapid structural changes occurring in the extended atmospheres of red giant stars through VLBI monitoring of SiO masers. The observations reveal a complex picture. Firstly, material is predominantly outflowing, but also infalls towards the star, in a pulsating inner envelope periodically disrupted by shocks. Secondly, circular polarization measurements may indicate that the inner circumstellar envelope is permeated by a magnetic field strong enough to be of dynamical importance. Finally, towards a few stars, observations also show evidence for rotation of the SiO maser region. Current SiO maser models can reproduce many of the key features of circumstellar SiO maser emission, and have successfully predicted e.g. the infall of SiO masing gas and the observed spatial separation ofv= 1 andv= 2 43 GHz masers. However, both stellar hydrodynamical and maser codes need to increase in complexity in order to model fully these regions. In this review I will discuss the current developments in circumstellar SiO maser observations and models.
Structure of SiO masers in Orion-KL
Authors: Doeleman Sheperd , Lonsdale Colin , Kondratko Paul , Raas Whitney , Read Predmore C.
Published: June 2019

The Orion-KL nebula is the closest (450pc) site of high-mass star formation and exhibits powerful outflows associated with protostars. It is also one of only three known star forming regions to exhibit SiO maser emission. Emission in three SiO maser transitions (v=1 J=1 → 0, v=1 J=2 → 1, and v=2 J=1 → 0) imaged by VLBI exhibits an “X” morphology suggesting that the Orion masers form along the outlines of two opposing conical outflows to the NW and SE. At the center of this “X”, VLA observations find emission from an HII region presumably associated with a young star whose wind drives the outflow. The SiO masers probably form along the interface between the stellar wind and surrounding parent cloud. We find that SiO maser emission from different transitions preferentially occurs at different radii from the central star implying that the masers are tracers for physical conditions in the wind-cloud interaction region. On the smallest scales, some individual maser features in each transition overlap both spatially and in velocity providing strong evidence that more than one transition can mase within the same volume of gas.
A 3mm SiO maser survey in the galactic bulge region
Authors: Lüdke Everton , Migenes Victor , Balasubramanyam Ramesh , Cassol Nadiane C.
Published: June 2019

We have made a blind survey of SiO masers, of the J=2-1 v = 1 transition, toward 106 bright OH/IR stars which have been previously detected in the OH main line at 1662 MHz with the ATCA in the galactic bulge by Sevenster et al (2000) and in the galactic disk at J=1-0 (v = 1 − 2) line by Miyazaki (2001). A comparison with published data shows that new SiO masers are detected for OH/IR stars in the galactic center, suggesting that those stars are undergoing strong mass loss with dense circumstellar envelopes with apparent angular sizes far smaller than the OH maser shell as obtained by interferometric observations. At the moment, our detection rate is about 23% and it may be increased when the project is completed.
Infrared Imaging of the Arches Cluster - Adaptive Optics in the Densest Region of the Milky Way
Authors: Stolte Andrea , K. Grebel Eva , Brandner Wolfgang , F. Figer Donald
Published: June 2019

The Arches cluster - located only 11′ from the Galactic Center (GC) - is one of the densest and richest young star clusters in the Milky Way. With an age of only about 2 Myr, it is ideally suited to study massive cluster formation in an extreme environment. We find an IMF slope of Γ = −0.77 from 5 to 100 M⊙, in good agreement with the results from HST/NICMOS from Figer et al. (1999). The limiting factor in the dense cluster center is crowding. With the new AO systems, high resolution analysis of the dense cluster region combined with very deep infrared photometry is available. We have analysed deep H and K′ images of the cluster center obtained with the GEMINI/Hokupa'a adaptive optics system. Colour-magnitude diagrams and the IMF are constructed from these data. A comparison with isochrones yields the mass function.
Washington Photometry of New Globular Clusters in M31
Authors: Chul Kim Sang , Gyoon Lee Myung , Geisler Doug , Seguel Juan , Sarajedini Ata , E. Harris William
Published: June 2019

We present a progress report on Washington photometry of several hundred new globular cluster (GC) candidates in M31 which were recently found from our new CCD survey of GCs. The Washington CMT1 filters that we used are very efficient to survey extragalactic GCs and to estimate the metallicity of GCs. Preliminary color-magnitude diagrams and color-color diagrams of the new GC candidates and known GCs in M31 are obtained.
3-Gyr-old Globular Clusters in the Merger Remnant NGC 1316: Implications for the Fate of Globular Clusters Formed During Gas-rich Galaxy Mergers
Authors: Goudfrooij Paul
Published: June 2019

The giant early-type merger remnant galaxy NGC 1316 is an ideal probe for studying the long-term effects of a major merger on its globular cluster (GC) system, given its spectroscopically derived merger age of ∼3 Gyr which we reported in a recent paper. Here we report several pieces of photometric evidence showing that the second-generation GCs in NGC 1316 are at an evolutionary phase in between that of luminous GCs found in younger merger remnants such as (e.g.) NGC 7252 and that of ‘red’ GCs found in ‘normal, old’ ellipticals. The observation that massive, second-generation GCs formed during major mergers can survive for at least 3 Gyr provides strong evidence that these clusters can have ‘normal’ mass functions including low-mass stars, and hence that they can survive to reach ‘old age’ similar to those of ‘normal’ ellipticals.
Deep Wide-Field Photometry of the NGC 5128 Globular Cluster System: A Progress Report
Authors: Harris G. L. H. , Geisler D. , Harris W. E. , Hesser J. E.
Published: June 2019

We have obtained CMR photometry for a roughly 1° square region centered on NGC 5128. Preliminary results indicate that the limiting magnitude of the images is ≳ 1 magnitude fainter than the peak of the globular cluster luminosity function (GCLF) at R ⋍ 21.
Evolution of Globular Clusters Formed in Mergers
Authors: Schweizer François
Published: June 2019

Globular clusters formed in galactic mergers (e.g., The Antennae) can now be studied at different stages of their evolution. In young merger remnants (e.g., NGC 7252) these “second-generation” globulars appear by the hundreds as young halo clusters of roughly solar metallicity. While at first bluer and much more luminous than old metal-poor globulars, they become redder after 1–1.5 Gyr and can then be observed as still overluminous red clusters of intermediate age in perturbed-looking E and S0 galaxies (e.g., NGC 1316, 1700, 3610). There is evidence from the color distributions, projected radial distributions, and perhaps also luminosity functions that these clusters eventually assume the properties of red metal-rich globulars observed in many giant ellipticals. Studies of globular clusters in ongoing mergers and young remnants suggest that second-generation globulars form from giant molecular clouds shocked by the rapid pressure increase in the merger-induced starburst. This pressure-induced formation lends credence to Cen's (2001) argument that the general pressure increase during cosmological reionization at z ≈ 7–15 triggered the near-simultaneous formation of the universal population of first-generation metal-poor globulars observed in galaxies of all types.
The Hβ Index As a Chronometer of Extragalactic Globular Cluster Systems
Authors: Lee Hyun-chul , Yoon Suk-Jin , Lee Young-Wook
Published: June 2019

For the first time, we have taken into account the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity in our population synthesis models and they result that the integrated Hβ index is significantly affected by the presence of blue HB stars. As a matter of fact, due to the systematic HB morphology variation, it is found that Hβ does not monotonically decrease as metallicity increases at given ages, but shows a kind of wavy feature. According to our models, a systematic difference between the globular cluster system in the Milky Way Galaxy and that in NGC 1399 in the Hβ vs. Mg2 plane is explained if globular cluster systems in giant elliptical galaxies are a couple of billion years older, in the mean, than the Galactic counterpart.
On the Nature of the Emission of the Crab Nebula
Authors: Shklovsky I. S.
Published: June 2019

The emission of the Crab Nebula is perhaps connected with the internal motions of the gas. As shown by the observations, the power of the emission sources differs by some hundred times in different points of the amorphous mass (“wisps” and other details). Oort and Walraven believe the “wisps” to be condensations of relativistic electrons ejected from the central star. However, there are some other variable details elsewhere in the nebula (as described by Lampland, and Oort and Walraven).

Oort and Walraven's hypothesis met some important difficulties: (a) the direction of the magnetic lines in this region of the nebula is perpendicular to the wisp's velocity; (b) the dimension of the wisp is about three light months, while it appears during a month and faster, so that the relativistic electrons moving along the lines of force have no time to spread along the wisp; and (c) if the strength is not changing, the energy of the relativistic electrons in the wisp must be some hundred times greater than the density of the magnetic energy, consequently the field strength must grow in the wisp to keep the electrons in the volume.
Cygnus Loop and Some Related Nebulosities
Authors: Minkowski R.
Published: June 2019

The suggestion by Oort that the Cygnus loop, an object of a diameter of about 3° whose brightest parts are NGC 6960 and NGC 6992, is an expanding supernova shell slowed down by interaction with the interstellar medium has led to an increased interest in objects of this type. As better information on the Cygnus loop becomes available, it seems to become increasingly probable that Oort's suggestion provides an acceptable interpretation.
CCD Astrometry
Authors: Stone R.C. , Dahn C.C.
Published: June 2019

CCD transit telescopes are now determining star positions at faint magnitudes (i.e., down to V ∼ 17 mag) and with accuracies as good as ∼ ±150 mas. CCD parallaxes with errors ∼ ±1 mas are now being determined routinely in time periods less than 3 years.
The Extragalactic Optical-Radio Reference Frame
Authors: de Vegt C. , Johnston K.J.
Published: June 2019

The new extragalactic reference frame will be based primarily on the positions of a globally selected number of about 500 compact radio sources, which almost all display optical counterparts, mainly quasars or BL Lac objects. Precise radio positions of these objects on the mas level have been determined by VLBI techniques and will be monitored and updated on a regular basis. In parallel an extensive optical observing program is underway to determine precise optical positions of these objects. The present optical reference frame is based on the FK5 fundamental catalog, containing 4652 brighter stars. The transfer of the VLBI radio frame to the optical domain will be achieved now by the Hipparcos stellar net, containing about 120,000 stars with comparable accuracy. The paper will address details and consequences of the implemenation of the new system for astrometry and general astronomical applications.
Fine Structure in the Main Sequence: Primordial Helium and ΔY/ΔZ
Authors: Pagel B.E.J.
Published: June 2019

The primordial helium abundance YP is important for cosmology and the ratio δY/δZ constrains models of stellar evolution. While the most accurate estimates of both quantities now come from emission lines in HII regions, significant information comes from effects of helium content on stellar structure including in particular the location of the main sequence as a function of metallicity and age. HIPPARCOS parallaxes with 1 or 2 mas accuracy will naturally lead to great advances in this type of study for stars with metallicities down to about 0.1 solar, but sub-mas accuracy will be needed in order to extend it to stars of still lower metallicity.
A Galactic Population Census in Support of Astrometric Measurements
Authors: Perryman M.A.C.
Published: June 2019

A galactic counterpart of the Sloan Digital Sky Survey, consisting of two dedicated 2–3-m class telescopes (one in each hemisphere), could provide precisely those stellar data necessary to complement the information furnished by a future astrometric space mission: in particular, an automated acquisition and analysis system could provide spectral types, metallicities, and radial velocities of several tens of millions of stars down to about 15–16 mag. Such a dedicated spectroscopic stellar survey would provide considerably more information about the details of the stellar distribution within our Galaxy than is known at present. It is suggested that a galactic version of the Sloan Digital Sky Survey, utilising the GSC-II as an observing list, and capitalising on recent developments in multi-fibre spectroscopic survey capabilities, would be a timely initiative.
Better Parallaxes and the Cosmic Distance Scale
Authors: Van Den Bergh Sidney
Published: June 2019

It is generally agreed that the Hubble parameter lies in the range 50 < Ho(km s−1Mpc−1) < 100. Recent observations, which are discussed in the present paper, favor Ho≳ 75 km s−1Mpc−1. Hubble Space Telescope observations of Cepheids in Virgo cluster galaxies will probably reduce the uncertainty in Hoto ∼ 20%. It should be possible to lower this remaining uncertainty in the Hubble parameter by strengthening the calibration of the Cepheid period-luminosity relation, the MV(RR) versus [Fe/H] relation of RR Lyrae stars, and the luminosity calibration of the subdwarf main sequence, by using parallaxes obtained with Hipparcos.
Secular Motions of Extragalactic Radio-Sources and the Stability of the Radio Reference Frame
Authors: Eubanks T.M. , Matsakis D.N. , Josties F.J. , Archinal B.A. , Kingham K.A. , Martin J.O. , McCarthy D.D. , Klioner S.A. , Herring T.A.
Published: June 2019

The best current approximation to an inertial reference frame is provided by Very Long Baseline Interferometry (VLBI) observations of extragalactic radio sources with red shifts (z) up to 3.8. The stability of the resulting reference frame directly depends on the amount of any secular changes in the observed source positions.

Two types of potentially observable secular motions should be present in extragalactic source positions. Gravitational accelerations of the solar system will cause secular motions through aberration, amounting to, e.g., about 4 microarcsec (μas) year–1 due to the mass of the galaxy. Extragalactic mass concentrations will cause gravitational deflections in the apparent positions of more distant radio sources, and these will change with time as the mass concentrations evolve. This effect could easily cause secular motions of order 1 μas year–1 in some, or even most, radio sources with z ≥ 1.

The present astrometric VLBI data set contains about one million observations over a 15 year period, with current source proper motion formal errors being as small as 2.5 μas year–1. Proper motion estimates from these data reveal many sources with statistically significant proper motion estimates of order 30 μas year–1, about an order of magnitude larger than expected. Work continues to determine if the observed motions are due to systematic errors or reflect true secular changes in source positions. The results from a continued proper motion analysis of the complete astrometric VLBI data set will be presented.
Gravitational Lensing by Stars and Machos and the Orbital Motion of the Earth
Authors: Hosokawa M. , Ohnishi K. , Fukushima T. , Takeuti M.
Published: June 2019

We showed that it is feasible to measure the mass of a single star by observing the variation of gravitational deflection caused by the orbital motion of the Earth. When the distance of a star is less than 60 pc and some appropriate sources are within 1 arcsec. in its background, not only the distance but also the mass of the star may be determined by measuring the deflection with an accuracy of 10 μ arcsec. In the case of photometric microlensing by a MACHO, the observation of astrometric gravitational deflection is also useful. By measuring the separation between the primary image and the secondary image, the ratio of mass to distance of the MACHO will be obtained. Further, the orbital motion of the Earth modifying the light curve of the source is discussed.
The Observations of Gliese 623 and Some Other Objects with Suspected Unseen Components
Authors: Shakht N.A.
Published: June 2019

The star Gliese 623 (RA = 16h 22.6m; Deal = +48°28′ [1950]), π = 0.″138; mv = 10.3; Sp dM3, (Woolley et al., 1970)) is observed at Pulkovo by means of the 26-inch Refractor. The series of 95 plates (about 600 individual positions) obtained during 1979–1994 has been studied. This set has shown perturbations in the motion of this star under the influence of a dark component which has been investigated earlier by different methods (see, for instance, Marcy & Moore, 1989). The following values of relative proper motion and parallax with their mean errors and preliminary elements of the photocentric orbit have been adopted on the base of our observations: μx = +1.″1483 ± 0.″0010; μy = −0.″4462 ± 0.″0009; πx = 0.″126 ± 0.″019; πy = 0.″136 ± 0.″017; P = 3.76 years; e = 0.50; To = 1984.28. With the use of modified Thiele – Innes constants the following elements of the orbit have been determined: α = 0.″052 ± 0.″006 (m.e.) and i = 141°; ω = 265°; ω = 126°. We may suppose that the lower limit of the mass of the component is about 0.09 masses of the Sun. Our study has shown small periodic deviations in the distances between the components of δ Gem and ADS 11632, which may be evoked by substellar and stellar companions of low masses (Shakht, 1988).
CCD Photometry of Hipparcos Stellar Systems
Authors: Oblak E. , Chareton M.
Published: June 2019

With the introduction of CCD detectors, it now appears feasible to obtain accurate photometric data for each of the components of close visual double stars with an angular separations between 1 and 12″.
Some Physical Parameters and UVBY β Photometry of Trapezium Type Multiple Systems
Authors: Salukvadze G.N. , Javakhishvili G. Sh.
Published: June 2019

The presented paper deals with the results of electrophotometric observations of 59 components of 19 trapezia in Strömgren and Crawford six-colour photometric system. The multiple systems, selected from the Abastumani Catalogue of Trapezia (Salukvadze, 1978), are: ABAO 2, 8, 34, 48, 51, 62, 75, 94, 245, 312, 313, 316, 324, 348, 356, 363, 387, 396.

Observations were made on the 125-cm mirror telescope with the use of a one-channel photometer, based on photon counting, with diaphragms 10″ and 20″. Reduction was done on the Observatory computer with a procedure described by Salukvadze and Javakhishvili (1989).

We calculated the indices [m1],[c1] and [u-b] as in (Strömgren 1967, Philip and Egret 1980). The unreddened indices (b-y), m1 and c1 were calculated by the formulae of Crawford (1975).

Semi-empirical calibrations for effective temperature, bolometric correction and mass for early-type stars, using Strömgren photometric indices c0 and beta, are given by Balona (1984). In order to determine absolute magnitudes we used the calibration from Balona and Shobbrook (1984).
The Hubble Constant and the Deceleration Parameter
Authors: Tammann G. A.
Published: June 2019

A preliminary report is given of recent work with A. Sandage on the Hubble constant. Through a chain of distance indicators in Sc and Ir galaxies (cepheids, brightest stars, H iiregions, and luminosity classes) the distance scale is carried beyond any possible local anisotropy of the velocity field. Special care is taken to allow for the dependence of the intrinsic properties of the distance indicators on the size of the parent galaxy, and for the effect of the Malmquist correction.H0is found to be 55 ± 7 km s-1Mpc-1; within the errors no systematic changes with distance were found.

Aformalvalue of the deceleration constantq0= 1 ± 1 was recently derived by Sandage (1972a) and Sandage and Hardy (1973). The most important correction to this value is probably the luminosity evolution of galaxies, which tends to pushq0below 0.5. The ensuing evidence for anopen universeis also favored by independent arguments.
Superclustering of Galaxies
Authors: Abell G. O.
Published: June 2019

Evidence for superclustering of galaxies includes the local supercluster, the large deviation of the observed surface distribution of clusters of galaxies from a random one, the cell size dependence of the index of clumpiness, the results of covariance function tests for correlations between different cluster centers and between clusters and individual galaxies, and a power-spectrum analysis. All tests give results that indicate or are compatible with the existence of inhomogeneities in the distribution of matter in space with linear diameters of 50 to 100 Mpc. If these are dynamically stable superclusters, they probably have internal velocity dispersions in the range 1000 to 3000 km s-1. The possible effects of superclustering on dynamical studies of individual clusters are discussed briefly.
The Anisotropy of the Universe at Large Times
Authors: Hawking S. W.
Published: June 2019

The most important cosmological observation in the last forty years has undoubtedly been the discovery of the microwave background. As well as confirming the existence of a hot early phase of the Universe, by its spectrum, its remarkable isotropy indicates that the Universe must be very nearly spherically symmetric about us. Because of the revolution of thought brought about by Copernicus, we are no longer vain enough to believe that we occupy any special position in the Universe. We must assume, therefore, that the radiation would appear similarly isotropic in any other place. One can show that the microwave radiation can be exactly isotropic at every point only if the Universe is exactly spatially homogeneous and isotropic, that is to say, it is described by one of the Friedmann models. (Ehlers et al., 1968). Of course, the Universe is neither homogeneous nor isotropic locally. This must mean that the background radiation is not exactly isotropic, but only isotropic to within the very good limits set by the observations (about 0.1%). One would like to know, however, what limits the observations place on the large-scale anisotropies and inhomogeneities of the Universe. One would also like to know why it is that the Universe is so nearly, but not exactly, isotropic.
Interaction of Gravitational Radiation with a Uniformly Magnetized Sphere
Authors: DE SABBATA V. , Fortini P. , Gualdi C. , Baroni L. Fortini
Published: June 2019

Maxwell equations in the field of a gravitational wave are linearized by means of the weak field approximation. Then the equations are solved in the case of a uniformly magnetized sphere and the dipole electromagnetic radiation power is calculated. These results are applied to compute the electromagnetic radiation emitted by magnetic neutron stars and by the Earth when hit by gravitational radiation.
On the Distance to Cygnus X-1 (HDE 226868)
Authors: Bregman Jesse , Butler Dennis , Kemper Edward , Koski Alan , Kraft R. P. , Stone R. P. S.
Published: June 2019

From U, B, V photometry of 104 stars in a field of radius ~ 30′ centered on the X-ray binary star Cyg X-1 (HDE 226868), we have studied the color excess E(B–V) as a function of distance. Spectral types were observed de novo for 42 of these stars. We conclude that HDE 226868 cannot be nearer than 1 kpc, and is probably at a distance of 2.5 kpc or more. The primary component is therefore a luminous OB star of mass ~30 M⊙, and the X-ray component has a minimum mass ~6 M⊙.
Non-Pulsating Stars and the Population I and II Instability Strips
Authors: Cox Arthur N. , Tabor James E. , King David S.
Published: June 2019

With specially computed detailed tables of equations of state and opacities, the instability strips for δ Scuti stars and Cepheids of population I and RR Lyrae and W Virginis stars of population II have been compared using the linear pulsation theory. Uncertainties in the observed strip locations and sometimes the mode of the observed pulsations do not allow high accuracy in fixing helium contents or the variables masses. Nevertheless, if masses close to those given by evolutionary theory are used, the helium content in population II objects is likely less than Y = 0.25. A helium content of close to zero would put the theoretical blue edge of the instability strip to the red of the observed red edge, and have all the hotter stars which are in the strip as non-pulsating. For population I, Y can be more than 0.3, (more than 0.4 if half evolutionary masses are used), but if a given star has Y less than about 0.2 (full mass) or 0.25 (half mass), it can appear in the dwarf and classical Cepheid strips as non-pulsating.