bluuebunny/crossref_metadata_2025_split huggingface dataset:
2D Chemodynamical Simulations of Low-Mass Galaxies
Authors: Hensler G. , Rieschick A. 
Published: June 2021

Because of their low gravitational energy, low-mass galaxies are seriously affected by energetical processes in their interstellar medium, such as supernova explosions, or by gravitational perturbations, e.g., by neighbouring galaxies. This can reasonably explain their variety of morphological types. If the evolutionary timescales of galaxies are predominantly determined by internal processes, the multi-phase character as well as star-gas interactions and phase transitions have to be taken into account. For this purpose we have developed a numerical treatment of the dynamical behaviour of gas and stars, which also accounts for the metal dependence of some processes and which can trace the chemical evolution for different elements. This so-called chemodynamical treatment is described in detail in Theis et al. (1992) and Samland et al. (1997). It considers three stellar components and devides the gas into clouds (CM, with a mass spectrum) and a hot intercloud medium (ICM). Since the element enhancement of the interstellar medium is produced by different processes with different lifetimes of their progenitors, O, Fe, and N are used as tracer elements to represent supernovae type II (SNell), type la (SNela), and planetary nebulae (PNe) contributions. While supernovae form the ICM, PNe only attribute to the CM so that only mixing effects of both gas phases can alter abundance ratios.Due to limited computer capacities the first chemodynamical simulations of dwarf galaxies could be performed only one-dimensionally so far (see e.g., Hensler et al. 1993, 1998). The recently developed two-dimensional chemodynamical code CoDEx (Samland 1994) was first applied to massive disk galaxies and produced models of which a particular one could represent various chemical and structural observations of the Milky Way with striking agreement (Samland et al. 1997).
Fundamental Arguments of the Current Nutation Theory: Dynamical Reference Frame
Authors: Standish E.M. 
Published: June 2021

The latest JPL planetary and lunar ephemerides, DE405, are now referenced to the ICRF with an accuracy of about 1 mas. This has been accomplished mainly by fitting the ephemerides to 18 VLBI observations of the Magellan Spacecraft in orbit around Venus, 1990-1994, and to 2 VLBI observations of the Phobos Spacecraft in its approach to Mars, 1989. The orientation of DE405 is discussed in more detail elsewhere in this volume (Standish, 1997). Since all of the parameters of the inner solar system are now determined to 1 mas or better, one should be able to extract numerically the fundamental arguments of the nutation theories to the level of 1 mas.There are two ways of extracting the ecliptic, for example, from a numerical ephemeris: 1) one computes the node and obliquity of the instantaneous ecliptic at multiple points in time and then fits these with analytic functions, or 2) one fits an analytical planetary theory to the ephemerides and then computes the node and obliquity from the theory’s parameters. This paper relates a short example using method #1 and concludes that method #2 is probably more preferable.
Relativistic Considerations for Precession and Nutation
Authors: Klioner S.A. , Soffel M. 
Published: June 2021

AbstractThe whole scope of problems related with the rotational motion of celestial bodies is briefly discussed. Relativistic modeling of the Earth rotation is considered from a conceptual point of view. Relativistic effects in rotational equations of motion of an extended body in general relativity are discussed. Numerical values of the effects are given.
Astronomical Effects of Current Changes in Fundamental Astrometric References
Authors: Kovalevsky J. , McCarthy D. 
Published: June 2021

AbstractWe present a first description of the changes in the current procedures that will have to be performed in order to take into account the adoption of the ICRS as the new reference system and new nutation-precession series. A number of choices will have to be made: they are presented in this paper.
Impact-Induced Activity of The Asteroid-Comet P/1996N2 Elst-Pizarro: Yes or No?
Authors: Boehnhardt H. , Sekanina Z. , Fiedler A. , Rauer H. , Schulz R. , Tozzi G. 
Published: June 2021

AbstractP/1996 N2 Elst-Pizarro was discovered with a tail of dust in July 1996 and was thus classified as periodic comet. From the orbit point of view it belongs to the outer asteroid belt and is very likely a member of the Themis family, a group of bodies originating from an asteroid collision.The absence of a coma and the presence of the needle-like anti-tail in August 1996 did not exclude the possibility of a sudden emission event which could have been caused by the impact of another minor body on the object. However, with the development of a normal tail between September and November 1996 it became obvious that the dust emission episode continued at least for 2 months after perihelion, a scenario that is typical for gas-driven cometary activity. However, it is still possible that a collision triggered the event.The paper summarizes the observational pro and cons for impact-induced activity in this object, possibly the first ever detected comet-asteroid of the solar system.
Mass Extinctions, Comet Impacts, and The Galaxy
Authors: Rampino Michael R. , Stothers Richard B. 
Published: June 2021

AbstractThe hypothesis relating mass extinctions of life on Earth to impacts of comets whose flux is partly modulated by the dynamics of the Milky Way Galaxy contains a number of postulates that can be tested by geologic evidence and statistical analyses. In an increasing number of cases, geologic evidence for impact (widespread impact debris and/or large impact craters) is found at times of mass extinction events, and the record of dated impact craters has been found to show a significant correlation with mass extinctions. Statistical analyses suggest that mass extinction events exhibit a periodic component of about 26 to 30 Myr, and periodicities of 30± 0.5 Myr and 35 ±2 Myr have been extracted from sets of well-dated impact craters. The evidence is consistent with periodic or quasi-periodic showers of impactors, probably Oort Cloud comets, with an approximately 30-Myr cycle. The best explanation for these proposed quasi-periodic comet showers involves the Sun’s vertical oscillation through the galactic disk, which may have a similar cycle time between crossings of the galactic plane.
Optical Counterpart Of ICRF: Hipparcos
Authors: Lindegren L. , Perryman M.A.C. 
Published: June 2021

The Hipparcos reference frame is defined by the positions and proper motions of ~105 stars given in the Hipparcos Catalogue (ESA, 1997). It is the optical counterpart of the International Celestial Reference Frame defined by the radio positions of extragalactic sources. The linking of the Hipparcos frame to the extragalactic frame is described by Kovalevsky et al. (1997) and Kovalevsky (1997). In the present review we consider only the intrinsic properties of the Hipparcos reference frame.A complete description of the contents of the Hipparcos Catalogue is contained in Volume 1 of ESA (1997). Recent overviews have been given by Perryman et al. (1997) and Perryman (1997). The principal astrometric data provided are the barycentric coordinates (α, δ) of the stars, the trigonometric parallaxes (π) and the proper motion components (μα* = μα cos δ, μδ). These parameters are given in the main catalogue for 117955 entries along with estimated standard errors and correlation coefficients, all data being referred to the common reference epoch To = J1991.25 on the Terrestrial Time (TT) scale. Typical ranges of the astrometric standard errors and some other characteristics of the catalogue are shown in Table 1.
The Tycho Catalogue: Extension of Optical Reference Frame
Authors: HØG E. 
Published: June 2021

AbstractThe Tycho Catalogue contains astrometric and photometric data for the one million brightest stars on the sky. About 900000 stars are high quality astrometric reference stars. Photometry in a blue and visual band is provided.Two extensions of the catalogue are under way. One is the extension to one magnitude fainter stars by a second reduction of the original Tycho observations, resulting in a catalogue with a total of about three million stars. The other extension is the derivation of more accurate proper motions by means of the Astrographic Catalogue positions as first epoch. This latter work will be carried out first for the one million stars, and later for all the three million, resulting in the Tycho Reference Catalogue (TRC).
Linking the Dynamical Reference frame to the ICRF
Authors: Standish E.M. 
Published: June 2021

AbstractThe latest JPL planetary and lunar ephemerides, DE405, are referenced to the International Celestial Reference Frame (ICRF) with an accuracy that approaches 1 mas for the four innermost planets, the sun, and the moon. This has been accomplished mainly by 18 VLBI observations of the Magellan Spacecraft in orbit around Venus. The ephemeris of Jupiter, however, is not well-determined since the various observations are not consistent within each other. The outer four planets continue to rely almost entirely upon optical observations; their ephemeris uncertainties lie in the 100-200 mas range.
Semi-Empirical Determination of the Position of RRAB Stars on The H-R Diagram
Authors: Jurcsik J. , Kovács G. 
Published: June 2021

In a recent series of papers, it was shown that the metallicity and the absolute V, I and K mean magnitudes of RRab stars can be accurately calculated using only their periods and the low order Fourier parameters of the V light curves. The application of these formulae in conjunction with the synthetic colours of the model atmospheres and with the theoretical pulsation periods enables us to determine all the basic physical parameters (i.e. logL, logT, logM and [Fe/H]) of any variable with an accurate light curve. As a result, we might trace evolutionary tracks on the H—R diagram by plotting the stars with the same mass and metallicity. From a present sample of nearly 300 stars, signs of differences in the evolutionary statescan be suspected but not yet proven.The accumulation of large numbers of precise light and colour curves, onthe one hand, will further improve the empirical formulae and therefore theaccuracy of the derived parameters and, on the other hand, will increase our chances of constructing better-populated empirical evolutionary tracks.
R Coronae Borealis Stars: Long-Term Photometric & Spectroscopic Studies
Authors: Cottrell P.L. , Skuljan L. , Kilmartin P.M. , Gilmore C. , Lawson W.A. 
Published: June 2021

For more than a decade we have been able to acquire and analyse a significant amount of photometric data of the highly variable R Coronae Borealis (RCB) stars. This has made been possible by a photometric service observing programme instigated at the Observatory. These photometric data have been combined with less extensive spectroscopic coverage, particularly of the decline phase of these stars. These have been supplemented by observations obtained at Mount Stromlo and Siding Spring Observatories for a radial velocity study. Significantly more spectroscopic observations are now being acquired with the development of a new medium resolution spectrograph at Mount John University Observatory.In this poster we will present recent photometric and spectroscopic results for a number of the RCB stars in our sample. This observational and analysis work can be used to provide further insight into the nature of these stars, their likely progeny and progenitors and the processes that are involved in the formation and evolution of the obscuring dust clouds which cause the decline phase.
Ground-Based Surveys For Low-Luminosity Stars
Authors: Jones Hugh R.A. 
Published: June 2021

Most of what we know about the Luminosity and mass function of low-mass stars has been derived from analysis of ground-based photometric and astrometric Surveys. A major uncertainty in the Interpretation of such surveys lies in the appropriate correction for unresolved binaries and in the use of a reliable colour-magnitude relation. various new surveys are now uncovering large numbers of objects which should rapidly ameliorate these ambiguities.The recent discovery of brown dwarfs and planets has highlighted the importance of low-luminosity stars. once we understand their spectra and colours and have statistically significant samples, it will be important to use them to reliably constrain theories of star formation and chemical evolution as well as to use them as a probe of galactic structure and to reliably ascertain their contribution to baryonic dark matter. In this brief review ishall focus on ground-based field searches. Hambly (1997) reviews the very successful searches that have recently been carried out in nearby clusters.Until the 1970s, most work on the faint-star luminosity function was based on proper-motion searches primarily by dutch researchers, in particular Luyten (e.g. 1938, 1979). The Last 20 years have seen a switch from astrometric to photometric surveys employing plate-scanning machines or CCD cameras. Combined with substantial increases in computing power these technologies have led to the discovery of large numbers of very low-luminosity Stars.
Parallaxes Using Infrared Arrays
Authors: Brockie R.M. , Jones H.R.A. , Wells M. , Longmore A.J. 
Published: June 2021

AbstractWe have recently carried out an infrared parallax programme on the UKIRT conducted with a 2562 array. Our observations were made in the near-infrared K-band which is ideal for observing red stars and brown dwarf candidates and has the advantage that differential colour refraction is negligible. We find a parallax of πabs = 156.0 ±13.3 mas for the archetypal late-type M dwarf vB 10 and πabs =11.1 ± 7.5 mas for PC 0025+0447 which we conclude is too distant to be a clear case of a brown dwarf and is probably a late-type M dwarf close to the hydrogen-burning limit. Recent improvements in telescopes and instrumentation working in the infrared should enable accuracies of ~ 1 mas to be obtained for future infrared parallax programmes.
Completing the Local Sample with Tycho
Authors: Smart R.L. , Pannunzio R. , Lattanzi M.G. , Mclean B. 
Published: June 2021

The Tycho Catalogue contains proper motions and low precision parallaxes with a median precisions of 25 mas, while having high precision B and V magnitudes (median precision of 0.07 and 0.06 respectivly). The number of highparallax stars is quite unexpected, with over 30 previously unreferenced, non-Hipparcos, stars with a parallax >500 mas and over 2000 with a parallax >250 mas. While many of these are probably mistakes, the completeness limit of Tycho and the precise magnitudes provide us with a way to intelligently build a complete picture of the nearby star field. Catalogues of nearby stars provide us with many fundamental parameters in the field of stellar astronomy, but the completness of known stars within say even 8pcs is still underestimated by about 30%, i.e., 50-60 systems (T.J. Henry, CSSS8, ASP Conf Series 64, 1994).In a new program at Torino we are using the Tycho Catalogue to intelligently build a list of possible candidates for a 1Opc volume. Figure 1 shows that Tycho will contain basically all ZAMS to spectral type M3 within 10 pes, M5 to 4pcs and M6 to 2pcs. A comparison of the photometric and Tycho distances provides a list free of giants and systems where binarity has affected the color. This list is then added to the Torino Parallax program where we expect to provide more precise parallaxes.
Hipparcos Stars in The GCVS
Authors: Samus N.N. 
Published: June 2021

Prof. M. Grenon asked the team of the General Catalogue of Variable Stars to attribute GCVS names to new Hipparcos variables in order to present, in the Hipparcos Catalogue, the information on these stars already with final designations. We received lists of 5665 variables. Of them, 3157 qualified to the standard GCVS criteria and will appear in the 74th Namelist of variable stars.The main problems in compiling the Namelist were the following:1.Cases of spurious variability for double and multiple stars and for misidentified stars (wrong colour indices used in reductions). Some wrong identifications come from the SIMBAD data base.2.Lacking identifications with catalogues of spectroscopic binaries. Some stars attributed to slowly pulsating B variables are in fact ellipsoidal variables. HIP 98954 = V1472 Aql was classified as a semiregular star, but it is a rather unusual, probably eclipsing, star (Samus, 1997), a known spectroscopic-binary M giant with a 200d period (Lucke and Mayor, 1982).
The Luminosity Calibration of the HR Diagram
Authors: Gómez A.E. , Luri X. , Mennessier M.O. , Torra J. , Figueras F. , Royer F. 
Published: June 2021

An extensive work on the luminosity calibration of the HR diagram has been performed using the LM method based on the Maximum Likelihood estimation (Luri et al. 1996). The method uses all available information: Hipparcos parallax and proper motion data as well as radial velocities and some measured spectroscopic or photometric parameters related to luminosity. On the other hand, the method takes into account the censorship of the sample and the errors of the data. Finally, it is able to identify and separate in a given sample, groups with different luminosity, kinematical or spatial characteristics providing not only the corresponding luminosity calibration relationship, but also improved individual absolute magnitude estimates.In order to asses how reliable the luminosity calibrations based on the MK spectral classification are, the LM method has been applied to a sample of B to K stars with MK classification selected from the Hipparcos survey. All known binaries, variables and spectroscopically peculiar stars were removed from the sample. The final sample contains 22054 stars. Our results show that the relation between absolute magnitude and luminosity class has a large intrinsic dispersion. As a consequence, the assignation to a star of a mean absolute magnitude given by a MK classification calibration is a rough procedure. It is preferable to use the individual absolute magnitudes estimated using all the available information for each star, like the estimates provided by the LM method (Gómez et al. 1997).
Absolute Proper Motions of Bulge Giants in the Hipparcos System
Authors: Minniti D. , Lattanzi M.G. , Claria J.J. , Massone G. , Casalegno R. 
Published: June 2021

Three windows towards the Galactic bulge, located at (l, b) = (1.6,-2.8), (5.2,-3.5), and (4.2,-4.8), were identified from deep astrometric plates. Deep BVRI CCD photometry of these fields reaching V = 18.5 was obtained at the ESO Dutch telescope. The color-color and color magnitude diagrams of these fields allow us to select different bulge tracers, including blue horizontal branch stars, red clump giants, and RGB tip stars, as well as disk main-sequence stars. A first catalogue of some 2000 red clump giants was constructed, and measurement of their proper motions in under way. We are deriving absolute proper motions in the Hipparcos system using a two step procedure. First, plates from the Cordoba archive of the Astrographic Catalogue (epochs 1903-1913) are used in combination with recent plates from theESO GPO telescope to tie a set of intermediate magnitude field stars in the Hipparcos system of positions and proper motions. Second, deeper intermediate epoch (1958) and final epoch CCD images (epoch 1996.5) are used to determine the absolute proper motions of the fainter bulge stars. The average uncertainty in the absolute proper motion of a single giant, based on a preliminary sample of few tens of stars, is 0”.006 yr-1. This
relatively small error, in combination with the sample size, would allow usto study the shape of the velocity ellipsoid for the different Galactic components present in the inner regions as function of Galactocentric distance, and also to measure an accurate distance to the Galactic center and the
mass of the bulge. In addition, we identified about 20 faint objects which are potential members of the Sgr dwarf, based on their location in the color-magnitude diagrams, and are also measuring their proper motions.Other windows with measured proper motions are located along the Galactic minor axis, including Baade’s window at l, b = (1, −4) (Spaenhauer, A., Jones, B. F., &: Whitford, A. E., 1992, A3, 103, 297), and the Plaut field at l, b = (0, - 8 ) (Mendez, R. et al., 1997, in 4th ESO/CTIO Workshop on “The Galactic Center”, in press). Our proper motions are complementary, probing the kinematics off the Galactic minor axis, where the signature of rotation should be evident. The
fields chosen for this study overlap fields that the MACHO project is following since 1993. Their variable star database would provide with excellent tracers of different populations. The 3-D motions of these different Galactic components would be measured for the first time. These include RR Lyrae tracers of the inner metal-poor halo, Miras, LPVs and delta Scuti stars tracers of the metal-rich bulge, and eclipsing binaries tracers of the Galactic disk (Minniti, D., et al., 1996, in IAP Coll. on “Variable Stars and the Astrophysical Returns of Microlensing Surveys”, ed. R. Ferlet, p. 257; Alcock, C, et al., 1997, astro-ph/9707311).
Populations Among High-Velocity Early-Type Stars
Authors: Royer F. , Gómez A.E. 
Published: June 2021

As far as the standard picture of galactic evolution and structure is concerned, early-type stars belong to disk populations. However, the existence of a population of early-type stars with nearsolar metallicities and main-sequence surface gravities, together with high velocities and/or standing at large distances away from the plane, is a well known anomaly to this picture.In order to provide an insight into the different populations of early-type stars, a sample of high-velocity stars has been gathered from the Hipparcos catalogue. We have selected the stars according to B — V < 0.4,and a tangential velocity larger than 80 km/s. The sample is composed of 101 stars.The high-velocity early-type stars in the solar neighbourhood are a mixture of different stellar populations: blue horizontal halo stars passing near the sun and high-velocity disk stars. We have decomposed into gaussiansthe V and W-distributions of our sample, to isolate the disk from the halo populations. The U-distribution of high-velocity disk stars is clearly asymmetric. The selection bias over the tangential velocities creates a depletion of low U values. Moreover, two thirds of the sample have U < 0 (toward the galactic anticentre, l = 180°). This cannot be explained by the distribution tail of disk stars or stochastic events such as runaway stars, which privilege no direction.
Suggestion to Revise the Hipparcos-Parallax of Cepheids Belonging to Binary Systems
Authors: Szabados L. 
Published: June 2021

It has been suspected (Szabados 1997) that the apparent orbit of previously unresolved binaries involving a Cepheid component falsifies the Hipparcos-parallaxes deduced for these Cepheids. Since the incidence of binaries among Cepheids exceeds 50 per cent, this adverse effect on the parallax determination is a major error source.An obvious remedy is to repeat the determination of the five astrometric parameters for the Cepheids, members in binary systems, taking into accountthe orbital elements known from spectroscopic studies. This will result in more precise parallaxes and cosmic distance scale. Moreover, these Cepheidsare ideal targets for the future microarcsec astrometric missions.This study was partly supported by OTKA-grants T 014852 and T 022946.