bluuebunny/crossref_metadata_2025_split huggingface dataset:
Monte Carlo Simulations of Radio Pulsars and Their Progenitors
Authors: Dewey Rachel J. , Cordes James M. 
Published: June 2019

The formation of neutron stars in binary systems is often used to explain the nature of specific radio pulsars and characteristics of the pulsar population as a whole. We have investigated the extent to which such scenarios provide a self-consistent description of the pulsar population. Using a computer simulation, we modeled the evolution of the main sequence stellar population and compared the predicted neutron star population to the observed radio pulsar population, focusing our attention on the pulsar velocity distribution and the incidence of binary pulsars. These characteristics relate very directly to the binary nature of pulsar progenitors, and are not strongly dependent on models of pulsar magentic field and luminosity evolution.The need to reproduce both the high velocities typical of pulsars and the low incidence of binary pulsars strongly constrains the formation of pulsars in binary systems. Unless one assumes that virtually all pulsars originate in close binary systems, the observed velocity distribution cannot result from the disruption of binary systems by symmetric supernova explosions; some additional acceleration process (e.g. asymmetric supernova mass ejection or asymmetries in pulsar radiation) must act during or soon after a pulsar's formation. It is possible to reproduce the velocity distribution by assuming that all pulsars are born in binary systems with initial orbital periods less than about 30 years. However, the predicted incidence of binaries is then too large by more than an order of magnitude, unless one also assumes that the process of mass transfer from the primary to the secondary is almost always non-conservative, or that the minimum mass necessary for a stripped helium core to explode as a supernova is larger (over 4 M⊙) than currently believed. Further analyses of the radio pulsar population, the X-ray binary population and the abundances of elements ejected in supernovae should help determine which of these alternatives is most reasonble. Additional studies of the main sequence stellar population, accounting more accurately for evolutionary and observational selection effects, will reduce the uncertainties in modeling the formation of the neutron star population.It has also been suggested that the observed correlation between pulsar velocities and magnetic moments (see Cordes, these Proceedings) is induced by the differing evolutionary paths by which stars in binary systems form radio pulsars. Our simulation does not reproduce this correlation, and we do not find any paths likely to produce low velocity, low magnetic field neutron stars not in binary systems.We are submitting a full description of our model and results to The Astrophysical Journal.
Constraints to Possible Progenitor Systems of PSR 1831-00
Authors: Pylyser E.H.P. 
Published: June 2019

Numerical computations were carried out to simulate the evolution of the recently discovered radio pulsar binary PSR 1831-00 with a very low mass function (Porb = 1.81 days, Dewey et al. 1986). These show that the minimum value of the initial mass of the progenitor of the mass-losing secondary is about 1 M⊙. All computed systems with a final period equal to the observed one, and an initial progenitor mass of 1 M⊙ have an age of the order of the Hubble time (∼ 1.3 × 1010years). For a 1.5 M⊙ progenitor of the mass-losing secondary we find that the final (= presently observed) orbital period sets strong constraints to the possible initial system configuration: the binary period should have been such that the 1.5 M⊙ star was just at the onset of the over-all contraction phase (Porb = 0.74 days, XC = 0.05) when it began to fill its Roche-lobe, and the initial mass or the compact mass-accreting star must have been < 0.85 M⊙. (This star was then presumably a white dwarf, which later collapsed due to the accretion). For example, the final period and remnant secondary mass for a system with an initial configuration of 1.5 + 0.8 M⊙ are 1.83 days and 0.20 M⊙, respectively.If one would start out from a more evolved initial state of the 1.5 M⊙ secondary than the end of the overall-contraction phase, the maximum initial mass of the compact object will become drastically smaller and the final remnant mass of the secondary becomes larger than 0.20 M⊙ in all cases.Calculations for a 2.0 M⊙ progenitor of the secondary suggest that if such a progenitor can produce the present system, the initial mass of the accreting primary was at maximum 1.30 M⊙.All the calculations were performed with the inclusion of magnetic braking in so far as the extent of the convective envelope allowed this (see Pylyser & Savonije 1986, in preparation). The effects of the explosion effects due to the accretion induced collapse of the white dwarf were not taken into account.
Geodetic Precession in Binary Pulsars
Authors: Bailes M. 
Published: June 2019

We have calculated the probability of observing geodetic precession in the binary pulsar PSR1913+16 for several different progenitor systems. Such an observation would support the asymmetric kick hypothesis for the origin of pular velocities. The results are shown to be dependent on the assumed progenitor but not to a strong degree. It is concluded that the probability of an observation to date is less than 20 percent in contrast to past predictions. In 15 years we expect this figure to be near 60 percent. We conclude that the null result to date cannot be taken as evidence against the asymmetric kick hypothesis for the origin of pulsar velocities. We develop our model and apply it to binary pulsars in general. We conclude that the likelihood of observing geodetic precession in the near future is low.A complete version of this paper is soon to be submitted to a scientific journal.
An Iterative Method for the Reconstruction of Two-Dimensional Density Distributions
Authors: Bremer M. , Grewing M. 
Published: June 2019

As hydrodynamic models of planetary nebulae advance from 1D to 2D calculations, it becomes desirable to make the same step for reconstruction techniques, which aim at deriving from the 2D intensity distributions of PNe images their 3D structure. A basic step is the determination of a symmetry axis and its orientation in space which can be described by two angles, one measured in the plane of the sky, and one measured with respect to the tangential plane. While the first one can be determined from the images, e.g. by applying the criterion of maximum normalized correlation between the object halves, which we found to yield the best results, the second angle is treated as a free parameter (see below). The steps of the iterative reconstruction algorithm are the following (with image(x, y) = input image):
1.Fill a 3D kartesian density grid(xyz) (random or continuous), observing the constraint ∫ grid(xyz)dz = image(x, y), where z is along the line of sight.2.Transform to cylindrical symmetry system (r, z', ϕ) and read out density (r, z') averaging over ϕ.3.Return density(r, z') into grid(xyz) and normalize the ∫ grid(xyz)dz to match the input image.4.Return to (2.) as long as two subsequent density(r, z') estimations differ by more than a specified limit.
How to Model the Chemical Evolution of Galaxies
Authors: Köppen Joachim 
Published: June 2019

For a first interpretation of the comparison of observational data, the crude “Simple Model” of chemical evolution is quite useful. Since it has well been described in the literature (e.g. Pagel and Patchett 1975, Tinsley 1980), let us here just review the assumptions and whether they are satisfied:
1.The galaxy is a closed system, with no exchange of matter with its surroundings: For the solar neighbourhood this probably is not true (the infamous Gdwarf-“problem”, Pagel 1989b). For the Magellanic Clouds this is most certainly wrong, because of the presence of the Inter-Cloud Region and the Magellanic Stream, and evidence for interaction with each other and the Galaxy as well (cf. e.g. Westerlund 1990).2.It initially consists entirely of gas (without loss of generality of primordial composition): This is good approximation also for models with gas infall, as long as the infall occurs with a time scale shorter than the star formation time scale.3.The metal production of the average stellar generation (the yield y) is constant with time: Initially, it is reasonable to make this assumption. For tables of the oxygen yield see Koppen and Arimoto (1991).4.The metal rich gas ejected by the stars is completely mixed with the ambient gas. To neglect the finite stellar life times (“instantaneous recycling approximation”) is appropriate for elements synthesized in stars whose life time is much shorter than the star formation time scale, such as oxygen, neon, sulphur, and argon.5.The gas is well mixed at all times: We don't know. The dispersion of H II region abundances may give an indication. In the Magellanic Clouds Dufour (1984) finds quite a low value (±0.08 dex for oyxgen).
Comparison and Calculation of Different Atmospheric Delay Models Using Atmospheric Parameters of VLBI Stations of China
Authors: Yang Zhigen 
Published: June 2019

The values of the atmospheric time delay for the “Chao”, “Marini” and “CfA−2.2” mapping function are calculated by using the atmospheric parameters in summer and winter at Shanghai, Kunming and Urumqi station respectively. A comparison among these values shows that the derivations of “Marini” from “Chao” and “CfA” are relatively large. On the other hand, the difference of values between the “Chao” and the “CfA” in the case of ∊ = 40° ∼ 10°, which is the average for the three stations, is from +1 mm to +47 mm for the “wet” part of the delay in summer, while is from −2 mm to −28 mm for the “dry” part in winter. For the case of low elevation angle ɛ ≐ 5°, the difference for the “wet” part can be about 400 mm in summer. Therefore, it is indispensable to make a further comparison between “Chao” and “CfA” mapping function by using the data of VLBI observations, in order to make a better revision to the adopted models of atmospheric delay.The monthly averages of the height of tropopause ht and the tropospheric temperature lapse rate βt for the three stations mentioned above are used to calculate the dry atmospheric delay by the “CfA” mapping function. The results show that the amplitudes of the annual changing of delay dτa, which is caused by ht and βt for the case of ∊ = 20° ∼ 10° at Urumqi station, are about 1 ∼ 5 mm and 2 ∼ 15 mm respectively. Therefore, taking the parameters of ht and βt of the stations into account in “CfA” model, instead of using fixed constants, would be much favourable for the requirements of 1 ps precision of VLBI physical models.
High-Precision Parallaxes and Their Astrophysical Applications
Authors: Lai Wan 
Published: June 2019

As demonstrated by USNO CCD parallax program, a CCD attached to a modest aperature telescope is capable of measuring stellar trigonometric parallax to a precision better than 1.0 mas. consequently, many objects previously regarded as being too distant for ground-based trigonometric distance determinations can now be considered.In this paper we show the High-precision parallaxes will provide a very good opportunity for the refinement of H-R digrammes and the calibration of luminosity criteria, we also refer to the open cluster and to determination of accurate stellar masses from close binanies.
Issues of Time for Reference Systems
Authors: Seidelmann P. K. 
Published: June 2019

At the International Astronomical Union General Assembly in 1991 the terminologies Geocentric Coordinate Time (TCG) and Barycentric Coordinate Time (TCB) were introduced as the time coordinates of four dimensional geocentric and barycentric coordinate systems, respectively. The reasons for these time-like arguments and their applications will be discussed. The relationships between the different time-like arguments will be discussed along with their applications and relationships to the astronomical constants.Since these time-like arguments have secular differences, there are new issues to be resolved. These new issues include the definition of the epoch J2000.0, relationship between mean and apparent sidereal time, the time-like argument for specifying constants, the definition and future use of ephemeris time, delta T values and the transition to, or future use of, TCG, TCB and TDB.
Formal Inversion of the Self–Consistent Problem for Triaxial Galaxies
Authors: Dejonghe Herwig 
Published: June 2019

Triaxial separable potentials V(λ, μ,ν) = fλ + fμ + fν ≥ 0, with

admit three constants of the motion (see de Zeeuw 1985 for more details), for which we can take the following set:
Here vλ, and vν are the velocity components in the (λ, μ,ν) coordinate system, and we recognize in E the binding energy per unit mass.
Cold Collapse as a Way of Making Elliptical Galaxies
Authors: Aguilar L.A. , Merritt D. , Duncan M. 
Published: June 2019

SummaryWe investigate whether dissipationless collapse starting from very cold, non-rotating initial conditions can produce objects resembling real elliptical galaxies. We also study the effect of various initial geometries on the shape of the final object. Collapses that are initially very cold (2T/W < 0.1) are different from warmer collapses, due to the presence of a dynamical instability associated with clumping of nearly-radial orbits (Polyachenko 1981). This instability can produce very elongated bars (1.6 to 2.1 axis ratio) from spherical initial conditions. the instability is also present in models evolved from oblate and triaxial initial conditions. Warm collapses tend to preserve their initial shapes. Cold initial conditions produce objects whose surface density profiles are well fit by a de-Vaucouleurs law; warm collapses, on the other hand, produce a core-halo profile. A large collapse factor seems necessary to produce objects resembling real galaxies; the same collapse factor guarantees the presence of the radial orbit instability. It thus appears that initial flattening is not crucial for producing prolate or nearly prolate galaxies. Oblate galaxies, on the contrary, seem very difficult to form, unless extremely flattened initial conditions are invoked. Preliminary experiments suggest that these results are not changed by realistic amounts of angular momentum.
Abundances in the Galactic Center
Authors: Wannier Peter G. 
Published: June 2019

Abundance measurements in the Galactic Center (GC) probe material with a nucleosynthetic history unique in our Galaxy. The measurements are of two types: probing interstellar and stellar material. Measurements of gas-phase abundances are mostly toward SgrB2 and SgrA. They reflect the current state of nuclear evolution in the GC and include several important isotope abundance ratios. The isotope ratios provide the most accurate information and allow for comparison with results elsewhere in the interstellar medium. The second type of measurement is of abundances in (and around) stars, yielding chemical abundances in the bulge and disc populations, and reflecting the state of nuclear evolution when the stars were born.When combined with our knowledge of evolution in the solar neighborhood, several of the results are consistent with the the greater nuclear “maturity” of the inner Galaxy. However, there are several important exceptions, which point to the fact that we really do not understand nuclear processing in the GC region. Some of the open issues may be resolved observationally, and with new infrared and millimeterwave techniques a clear opportunity exists to improve the observational record. Results should increase our understanding of stellar nucleosynthesis and of the history of star formation in the GC.
Magnetized Structures in Sgr A West
Authors: Aitken D. K. 
Published: June 2019

Spectropolarimetric studies of Sgr A West at 10μm reveal intrinsically polarized emission from the northern arm. The polarization, of amount 7–8 %, is attributed to emission from dust grains which are aligned by a magnetic field directed along the line of the northern arm. Along the eastern arm the polarization is smaller and indicates either a weaker field or one directed closer to the line of sight.Polarized emission has also been observed at 100μm from aligned grains in the ring of molecular gas which apparently encircles Sgr A. The aligning magnetic field lies in the plane of the ring and is probably toroidal. The correspondence between the polarization position angles at 10 and 100μm suggests a model in which the northern arm is a disrupted filament from the molecular ring.While the uncertain magnetic properties of grains prevent a reliable direct estimate of field strength, circumstantial evidence suggests it is larger than elsewhere in the Galaxy and possibly large enough to influence the dynamics of the region.
The UBVRI photometric and polarimetric observations of the T Tau star HDE 283572
Authors: Shakhovskaya N.I. 
Published: June 2019

The preliminary analysis of UBVRI photometric and polarimetric observations of the naked T Tau star HDE 283572 indicates that:(i)there is intrinsic and variable component of polarisation;(ii) the distribution and the area of the active regions on stellar surface are variable on time scale of a year.
Teaching Astrobiology Online
Authors: Maddison S. T. 
Published: June 2019

As part of Swinburne Astronomy Online (SAO), we run an online short course entitled Searching for Extrasolar Planets and Extraterrestrial Life. The main aim of the short course is to act as a “feeder” into our graduate programs and allow students to trial online education while exploring one of the new hot topics of astronomy – astrobiology.I will present a brief overview of how SAO works, followed by an outline of our short course which has been running for four semesters. In particular, I will focus on why astrobiology is a good choice of topics for an online short course, and look at the successes (and failures) of the course in attracting students to both online education and astronomy –and astrobiology in particular.
The ATCA/VLA OH 1612 MHz survey: identification of post-AGB stars
Authors: Chapman Jessica , Sevenster Maartje 
Published: June 2019

The ATCA/VLA Galactic Plane survey detected OH 1612 MHz maser emission from a total of 766 sources. In most cases the detected sources have double-peaked spectral profiles which are characteristic of OH/IR stars. A small number of sources however have irregular spectra and uncertain classifications. We discuss the maser results for two sources with irregular OH spectra, D046 (OH 326.5-0.4) and B292 (OH009.1-0.4). For B292 we detected OH 1720 MHz emission - a transition previously unknown for evolved stars. D046 has exceptionally broad maser profiles and strong non-thermal radio continuum emission. Both are likely to be bipolar post-AGB sources.Post-AGB sources can be identified from their MSX far-infrared properties. For the ATCA/VLA OH-selected sample, most of the likely post-AGB stars do not have unusual OH 1612 MHz spectra.
Relative Astrometry with Imaging Transit Telescopes
Authors: Benedict G. F. , McGraw J. T. , Hess T. R. 
Published: June 2019

A CCD/Transit Instrument (CTI) has produced relative astrometry with standard errors less than 2.6% of a 1.55 arcsecond pixel for stars with V ≤ 17. Additional astrometric studies with existing data are required to better understand the ultimate contribution these devices can make to our science.The CTI is presently dismantled, awaiting a move to a new site. We briefly discuss the potential astrometric scientific returns from the exisiting data set, from a refurbished CTI, and from a similar device emplaced on the Moon.
Stand-Alone Program Package for Improvement of Parameters of Celestial Bodies Motion
Authors: Ivanova T. V. , Novikov F. A. , Pariiskaya E. Yu. 
Published: June 2019

A stand-alone Program Package for Improvement (PPI) of motion parameters of celestial bodies from observations is described. The PPI is oriented for solving conditional equations system by the method of least squares and is implemented for IBM PC in MS-DOS in the Turbo-Pascal programming language v.5.0. There are two versions of the system:
A)using the standard conventional memory 640 Kb andB)using the external memory.
Field Galaxies and Cluster Galaxies: Abundances of Morphological Types and Corresponding Luminosity Functions
Authors: Neyman Jerzy , Scott Elizabeth L. 
Published: June 2019

With reference to theory published earlier, formulas are given for the estimation of (i) abundances of morphological types among field galaxies, (ii) of selection probabilities, and (iii) of ‘space luminosity functions’. Strictly, the theory applies to ‘homogeneous classes’ of galaxies. This term designates a category of galaxies, say C, so finely defined that the probability, say Φ(m | C), that a galaxy of category C will be included in the catalogue depends on its photographic apparent magnitude m and on nothing else. The practical use of the theory is illustrated on data in the HMS Catalogue. It appears that certain combinations of the Hubble morphological types satisfy the definition of a homogeneous class. Such, for example, is the case for combinations of ellipticals E0–E3 and, separately, of spirals Sc, Scp, SBc. However, the combination of these two categories is not a homogeneous class.In order to validate the theory empirically, calculations were performed to predict the abundances of eight combinations of morphological types among cluster galaxies listed in the HMS Catalogue, each combination being treated as a distinct homogeneous class. Additional hypotheses underlying these calculations are: (a) abundances of morphological types, (b) luminosity functions of these types, and (c) selection probabilities for cluster galaxies coincide with those for field galaxies. A comparison with the observations, reaching the value of z = 0.07, is satisfactory. This tends to validate the combination of formulas (i), (ii), (iii) with the additional hypotheses (a), (b) and (c). Incidentally, the result tends to support the steady state cosmology.
Geological Considerations
Authors: Jennings Jesse D. 
Published: June 2019

If the data recovered at the Wendover caves are to be fully evaluated, the student must turn to the geologic and climatic events of the late Pleistocene and the Recent geologic epochs. Some of the cave data may even contribute to the study of the sequence and duration of climatic fluctuations within late Pleistocene and Recent times.The archeological interest in geologic/climatic matters develops for various reasons. First is the matter of chronology. By means of radiocarbon dating we know that man visited Danger Cave about 9000 B.C. It is necessary to correlate these deposits with already recognized geologic and climatic events. Fortunately for this purpose a fairly specific calendar of geological time exists. If the cave deposits could be correlated with natural events this correlation automatically provides, at very least, estimates of the time of occurrence of the natural cave deposits. Another interest the archeologist has in a valid geologic, and hence climatic, correlation lies in the insight he may gain into the environmental circumstances which man faced at any given time during his conquest of the Americas.
Spin Correlation in Binary Systems
Authors: Farbiash N. , Steinitz R. 
Published: June 2019

AbstractWe examine the correlation of projected rotational velocities in binary systems. It is an extension of previous work (Steinitz and Pyper, 1970; Levato, 1974). An enlarged data basis and new tests enable us to conclude that there is indeed correlation between the projected rotational velocities of components of binaries. In fact we suggest that spins are already correlated.