bluuebunny/crossref_metadata_2025_split huggingface dataset:
Observational and Theoretical Modeling of Nutation
Authors: Mathews P.M. , Herring T.A. 
Published: June 2021

The IERS nutation series was developed as a nutation series which would match observations and could be used with geodetic systems which have sensitivity to nutations but are not able to determine accurately corrections to the IAU-1980 nutation series. After experimentation with different methods for parameterizing the nutation estimation problem, the best fit to the VLBI observed nutation angle data was found by estimating selected coefficients in the Mathewset al.(1991a) nutation response function (see equation (1) below), along with other parameters to be discussed below. The estimation of the response function coefficients was also found to account for mantle anelasticity and the loading part of the ocean tide correction and therefore no corrections were applied for these effects. Their contributions should be absorbed into the estimated response coefficients. These initial analyses also showed: setting the inner-core response to zero yielded better fits to the nutation angle data than the Mathewset al.(1991b) theoretical response and resonance frequency; the amplitude of the RFCN free mode appeared to be time dependent; and the prograde annual nutation was significantly affected by the SI thermal tide which is not proportional to the S1gravitational potential.The data set used to estimate the IERS conventions series was the GSFC nutation angle data set with 2040 pairs of adjustments to the IAU 1980 nutations in obliquity and longitude spanning the interval between 1979 and 1995. Here we also compare the IERS conventions series with a US Naval Observatory analysis of VLBI data between 1979 and August 1997. This data set consists of 2152 pairs of nutation angle estimates and extends by 1.8 years the interval covered by the GSFC data set. It also contains less pre-1984 data than the GSFC data set which accounts for the smaller increase in total number of observations than would be expected for the additional time interval covered.
An Update on The Problem of Small Comets
Authors: Brandt J.C. , A’Hearn M.F. 
Published: June 2021

The evidence for a major population of small comets (SCs) is summarized in two steps. First, we briefly summarize our previous work. Second, we describe recent work that continues to find SCs. When new capabilities become available that can detect SCs, we expect to find them and this is occurring. At present, their detection is a haphazard process and we advocate a dedicated, optimized search program.We define SCs as those icy bodies (i.e., sublimating bodies) with radius less than 1000 meters. Often the existence of SCs is inferred from effects ascribed to them. These “effects” include: lunar cratering; cratering on Ganymede; excess interplanetary hydrogen; delivery of volatile materials to the terrestrial planets; and a source of fragile bodies entering the Earth’s atmosphere. While some of these indirect methods support the existence of SCs, direct observations are clearly preferred.
The Progress of The Reference Frame
Authors: Morrison L.V. 
Published: June 2021

F.W. Argelander proposed to a meeting of the Astronomischen Gessellschaft in 1867 that an accurate catalogue of all stars down to 9th magnitude should be compiled. The sky was divided into zones of declination and observations were undertaken with transit circles at a number of observatories in an international collaboration. The importance of fitting these zonal observations to one global reference frame was realized, and for this purpose A. Auwers produced a Fundamental Catalog (FC) for the Northern and Southern hemispheres in 1879 and 1883, respectively. The accuracy of the positions was about half of an arcsecond. This was the first international celestial reference frame.This reference frame was improved over the next hundred years by continued international cooperation which was later coordinated through the IAU. These efforts culminated in the FK5 which was issued in 1988. The progress in accuracy from the FC to the FK5 is shown in Figure 1. Meanwhile, radio astronomy through the technique of Very Long Baseline Interferometry (VLBI) began to produce positions of extragalactic sources with an accuracy of a milliarcsecond (mas), which is almost two orders of magnitude better than that of the optical positions of the stars in the FK5.
Formation of The International Celestial Reference Frame
Authors: Ma C. , Arias E.F. , Eubanks T.M. , Fey A.L. , Gontier A.-M. , Jacobs C.S. , Sovers O.J. 
Published: June 2021

The goal of the work described here is to create the definitive catalogue for the new International Celestial Reference Frame (ICRF) using the best data and methods available at the time the work was done. This work is the joint cooperative effort of a subgroup of the IAU Working Group on Reference Frames which was formed expressly for this purpose in February 1995. The authors of this report constituted the subgroup. A fuller account of this report can be found in the introduction to the ICRF catalog (IERS 1997).The ICRF of 608 sources presented here is based on essentially all the VLBI observations accu-mulated over about 15 years in several worldwide programs. Dual frequency Mark III data have both geodetic and astrometric applications. Most of the data (95% of nearly 2 million observations) were acquired primarily for geodetic purposes. The major geodetic programs include: NASA’s Crustal Dynamics Project/Space Geodesy Program and USNO’s NAVEX sessions for the terrestrial reference frame, as well as IRIS, NAVNET and NEOS sessions for monitoring Earth rotation. The geodetic programs have used the brightest radio sources, gradually concentrating on the most com-pact as sensitivity improved. These geodetic sources were also the foundation of astrometric work because of the large number of observations for the ~150 most commonly used. The astrometric programs which densify the sky include the Radio-Optical Reference Frame sessions done by US Naval Research Laboratory (NRL) and USNO and the space navigation efforts of Jet Propulsion Laboratory (JPL).
Link of the Hipparcos Catalogue to the ICRS
Authors: Kovalevsky J. 
Published: June 2021

AbstractIn compliance with the 1991 IAU resolutions on reference frames, the merged Hipparcos Catalogue had to be aligned in such a way that it represents at optical wavelengths the International Celestial Reference System (ICRS). This was realized as a combination of particular solutions obtained from observations of radio stars by radio interferometry, observation of quasars relative to Hipparcos stars, photographic plates including galaxies and Hipparcos stars, star catalogues referred to extragalactic objects, HST observations, and comparison of Earth orientation parameters obtained by VLBI and ground-based optical observations reduced with Hipparcos positions and proper motions. The individual results generally agree within 10 mas in orientation at epoch and 1 mas/yr in spin of the system. Two methods were used to combine the results and yielded uncertainties of 0.6 mas in orientation and 0.25 mas/yr in spin.
Linking the FK5 to the ICRF
Authors: Mignard F. , Froeschile M. 
Published: June 2021

AbstractThe Hipparcos optical reference frame is compared to the basic FK5 in order to determine the orientation at T0 = 1991.25 and the global spin between the two frames. The components of the spin are significant and suggest a correction the IAU76 value of the precession constant and to a possible non-precessional motion of the equinox of the FK5. The regional errors are analysed with harmonic functions and found to be as large as 150 mas in position and 3 mas/yr in proper motion.
Joint Discussion 8: Stellar Evolution in Real Time, Part I Explicit Evidences for Stellar Evolution
Authors: Guinan E.F. , Koch R.H. 
Published: June 2021

JD8 assembled for half-day sessions on August 22 & 23, 1997 in a satisfactory meeting hall. Between 350 and 400 delegates - about 50% larger than anticipated - attended most of the sessions.The theme of the meeting developed from initial conversations between R. H. Koch and Y. Kondo and was extended by E. F. Guinan. Opinions from the officers of Commissions 27 and 42 were favorable to seeking IAU approval and eventually the co-sponsoring Commissions came on board.In essence, all the organizing workers recognized that there did not exist a broad recognition that we really do know specific stars whose evolution we have actually perceived and that there are also statistical interpretations (not the conventional isochrones in an HRD or CMD) for ensembles of stars which lead to the same conclusion.The presentation compiled here is really a distillation of short offerings from the JD participants provided against a deadline which was regrettably close at hand. A full presentation of the meeting Proceedings will appear in The New Astronomy series of Elsevier. E. R. Jewell prepared the final files.
Watching Stars Evolve: Rapid Evolutionary Changes of the Post-AGB Star - FG SGE
Authors: Guinan E.F. , Efimov Yu. S. , Margheim S.J. 
Published: June 2021

The variable post-AGB star FG Sge has provided an unique opportunity to study phenomenon which may be characteristic of the post-PN stage of stellar evolution. FG Sge is the central star in the 36-arcsec PN He 1-5 that over the last 50 yrs has been undergoing rapid evolutionary changes. Most notable are changes in temperature, luminosity, chemical composition and pulsation period. Starting in mid-1992, the star has undergone several large dimming events from which it has yet to recover. These events closely resemble the dimming events commonly observed in R CrB stars which result from dust formation in ejected envelopes. From all the evidence, it now appears that FG Sge has evolved into a R CrB star.In this paper we first summarize the recent photometric history of the FG Sge from UBVRI photometry obtained during 1990-1997. We then combine these data with earlier photometry to define the star’s light bevavior over the last 100 yrs. The photometry is also combined with available spectroscopic polarimetric, and IR observations. Particlar attention is given to comparing FG Sge’s post-dimming behavior with contemporaneous observations of R CrB itself, also done on the same photometric system.
Mode and Period Changes in Pulsating Stars near the main Sequence: δ Scuti Stars
Authors: Breger M. 
Published: June 2021

Reported mode changes in δ Scuti stars cna usually be explained by insufficient data to obtain multiperiodic solutions and by the effect of time-variable amplitudes. The extensive data on 4 CVn obtained by the Delta Scuti network are used to illustrate that the large changes in the appearance of the power spectra in the years from 1966 to 1996 can be ascribed to amplitude variability.The period changes of δ Scuti stars have been collected or redetermined from the available observations and are compared with values computed from evolutionary models. For the radial pulsators of Pop.I, the observations indicate (l/P)dP/dt values around 10−7 year−1 with equal distribution between period increases and decreases. The evolutionary models, on the other hand, predict that the vast majority should show increasing periods a factor of about 10 smaller than observed. Arguments are given why the rate of evolution for these relatively unevolved stars cannot yet be deduced from the observed period changes.
JD 10: Low-Luminosity Stars: Scientific Organizing Committee
Authors: Axelrod T. , Binney J.J. , Burrows A.S. , da Costa G.S. , Grenon M. , Nakano T. , Perryman M.A.C. , Richer H.B. , Seilwood J.A. 
Published: June 2021

Most of the mass near the Sun takes the form of stars less luminous than the Sun. A central problem of Galactic astronomy is the determination of how much mass these stars contain.The only thoroughly reliable way of detecting mass is through its gravitational field. Classically this has been done by studying the Galaxy’s rotation curve and the Oort limit, both of which remain active areas of research. In the last few years an exciting new way of probing the Galaxy’s gravitational field has been opened up by large surveys for microlensing events. These surveys are yielding important information about the numbers of low-mass objects both in the disk and above it. Another truly dynamical probe for low-luminosity objects is provided by studies of clusters. JD 10 reviewed what we have learned about the density of low-luminosity stars from each of these approaches.An important orthogonal approach to determining the density of low-luminosity stars involves seperately determining the luminosity function and the mass-luminosity relation for stars of a given spectral class (e.g. main-sequence stars) and then combining them to obtain the mass function. The development of infrared detectors and the refurbishment of the The Hubble Space Telescope have recently yielded important advances in each of these areas. A major difficulty with this line of research is the fact that, in the neighbourhood of M = 0.08 Mʘ, the main-sequence mass-luminosity relation is (i) steep and non-linear, and (ii) dependent not only on the masses of the faintest stars, but also on their ages, metallicities, rotation rates, binarity etc. Moreover, cool low-luminosity stars have extremely complex spectra, which are difficult to simulate with radiative transfer calculations. Similar difficulties are encountered in the determination of the mass-luminosity relation for white dwarfs. JD 10 reviewed the current status of these very difficult problems.
White Dwarf Cooling Curves and Searches For White Dwarfs
Authors: Wood Matt A. 
Published: June 2021

White dwarf (WD) evolution is essentially a cooling problem in two parts. Degenerate electrons in the core provide the support while non-degenerate C and O ions provide the thermal reservoir (Koester & Chanmugam 1990, Kawaler 1996). The rate of energy loss through the non-degenerate envelope of H and He—segregated by gravitational settling—is meager enough that the vast majority of WDs have not had time to cool to undetectability in the ~10 Gyr age of the Galactic disk. Because WDs are a remarkably homogeneous class with a mass function sharply peaked about 0.6Mʘ, and because the cooling physics is relatively straightforward to calculate, the observed deficit of WDs below Mv = 16.5 [log(L/Lʘ) ≈ -4.5, Teff ʘ 4000 K; see Liebert et al. 1988 (LDM), Oswalt et al. 1996 (OSWH), and Leggett et al. 1997] provides constraints on the age and star-formation history of the Galactic disk (Winget et al. 1987 (WEA97), Wood 1992 (W92), Yuan 1992, Hernanz et al. 1994).The maximum fractional surface layer masses that can remain following post-AGB evolution are log(qH) ~ —4 and log(qHe) ~ —2 (for a 0.6 Mʘ remnant—the values are inversely correlated with remnant mass), although we know from WD asteroseismology that surface H layer masses range from this value to smaller than logqH ~ —6 (e.g., Fontaine & Wesemael 1997). Some 75% ofWDs in the McCook & Sion (1987) catalog reveal pure-H photospheres (spectral type DA). The remainder are broadly classified as non-DAs, with most showing show pure-He spectra (DB) or continuous spectra (DC), and with ahandful that show mixed compositions (DAB), or C (DQ) or metal lines (DZ). See Bergeron et al. 1997 for a thorough discusion of the chemical evolutionof cool WDs.
Wide Binaries: Probes of The Galaxy’s Dark Matter Content
Authors: Oswalt Terry D. , Smith J.Allyn , Wood Matt A. 
Published: June 2021

Photometric parallaxes derived from B, V, R, I photometry of wide binaries with white dwarf components were used to set a firm lower limit to the age of the Galactic disk (see Oswalt et al. 1996 and the article by Wood elsewhere in these proceedings). Here we show the distribution of projected semi-major axes derived from new B, V, R, I data for over 200 wide pairs.Like clusters, wide binaries are gradually disrupted, primarily by the gentle but cumulative effects of encounters with the denser parts of giant molecular clouds (see Wasserman & Wienberg 1991 for a review). A sharp cut-off in semi-major axes > 0.1 pc was one of the predictions of competing models. Wood & Oswalt (1992) showed that additional orbital expansion occurs in wide pairs which have experienced isotropic post-MS mass loss. Can we see the signatures of these events in the wide binary sample?
The Large Sky Area Multi-Object Fibre Spectroscopy Telescope (Lamost)
Authors: Chu Yaoquan 
Published: June 2021

AbstractIn China, we will start to build a new spectroscopic survey telescope. It will be a meridian reflecting Schmidt telescope. The optical system is horizontal. The primary mirror is spherical and segmented. It faces towards the north. In its spherical center there is a plane mirror which is segmented too. The different shape of reflecting Schmidt plate is realized by active optics. The clear aperture is 4 m and f-ratio is 5. The angular field of view is 5 square degree, 4000 objects can be observed simultaneously by using optical fibre. The science goal is to make a 20000 square degrees spectroscopic survey, including ten million galaxies and stars, many variable objects and the identification of numerous objects found by radio, x-ray, IR and other survey in these sky areas.
The Tycho Catalogue: Astrometric And Photometric Results
Authors: Høg E. 
Published: June 2021

AbstractThe Tycho Catalogue contains basic astrometric and photometric data for a million stars on the sky, complete to about V = 10.5 mag. The accuracy at the catalogue epoch (J1991.25) of positions and two-colour magnitudes is generally an order of magnitude better than has hitherto been available for the majority of these stars. Results from the production and verification of the catalogue is presented. The catalogue and associated epoch photometry annex are available in machine-readable form as part of the final Hipparcos mission products.
Distance and Age of M92 From Hipparcos Subdwarfs
Authors: Pont F. , Mayor M. , Turon C. , Vandenberg D.A. 
Published: June 2021

The parallax data gathered by Hipparcos for field subdwarfs allow a muchmore precise determination of globular cluster distances by direct sequencefitting than was previously possible. We determined the distance and age ofthe old, representative globular cluster M92 from a set of more than 500 subdwarf candidates with Hipparcos parallaxes. Precise [Fe/H] values were derived using the equivalent width of the CORAVEL cross-correlation function.Our best estimate of the distance of M92 is (m — M)v = 14.67 ± 0.08 (including binaries with a statistical correction) or (m — M)v = 14.74 ± 0.08 (classic treatment, i.e. excluding binaries). The agreement of the cluster sequence with the position of extreme metal-poor field subdwarfs is remarkable [figure, left]. The distance found is slightly higher than previously thought. The corresponding ages, derived by comparing the luminosity of the turnoff and subgiant-branchstars with up-to-date evolution models [figure, right], are 14±1.2 Gyr or 13±1.2 Gyr respectively, implying a minimum age of 13 Gyr for the Universe.Other authors have claimed that larger distances, and smaller ages, resulted from Hipparcos subdwarf parallaxes. Although there are some differences in the data sets used by each author, the main difference resides in the treatment of systematic and selection biases. We have examined at some length the biases affecting the determination of the mean luminosity of a set of subdwarfs selected in the usual way. By means of Monte Carlo simulations, we show that selection biases act in a direction opposite to the classic Lutz-Kelker bias affecting parallax data, and that they can be dominant. The biases introduced are of the order of 0.1 mag.It is pointed out that, now that the whole Hipparcos catalogue is available, we shall be able to greatly reduce the systematic biases by refining the fitting procedure. Firstly, by imposing no [Fe/H] limits and fitting the subdwarf locus for all [Fe/H] values at the same time. Secondly, by imposing no parallax error limit and fitting the data in parallax space instead of magnitude space (comparing observed parallaxes directly with parallaxes predicted from the models).
The Distance, Absolute Magnitude and Space Motion of Alpha Orionis
Authors: Wing R.F. , Guinan E.F. 
Published: June 2021

Alpha Orionis (Betelgeuse) is the brightest, and presumably the nearest, of the M-type supergiants. It is also one of the least reddened. These features make it important for the calibration of the absolute magnitudes and intrinsic colors of M supergiants. However, its distance has been difficult
to establish, and there has been considerable debate concerning its reddening.The Hipparcos parallax for α Ori is 7.63 ± 1.64 mas, corresponding to a distance of 131 ± 30 pc and a true distance modulus (m — M)o of 5.59 ± 0.48 mag. If we adopt <V> = 0.50 and E(B - V) = 0.05, the mean absolute visual magnitude becomes <Mv> = -5.24, a result consistent with a luminosity classification of lab.The CN strength of α Ori measured in the near infrared also indicates luminosity class Iab. Furthermore, the star’s absolute magnitude at 1.04 μm, for the Hipparcos distance, is -8.34, in agreement with the M2 Iab supergiants in h &. χ Per for a distance modulus of (m — M)o = 12.0.From the Hipparcos proper-motion components and distance and a published radial velocity, we derive a space motion of 27.8 km s-1 with components U = +19.7, V = -7.3, and W = -1.5 km s-1 with respect to the Sun. This motion is not very different from the local standard of rest. It seems unlikely that μ Ori, at a distance only one-third that of the Trapezium, is physically associated with the Orion Complex.At 131 pc, the observed optical angular diameter of μ Ori, 0.055 arcsec, translates to a linear diameter of 7.2 AU. Centered on the Sun, itsphotosphere would be in the asteroid belt. Its angular diameter measured bythe Hubble Space Telescope in the UV (corresponding to the size of the chromosphere) is a factor of 2 larger. Surface features detected interferometrically are believed to be ~ 10% of the diameter in extent, corresponding to 0.7 AU or 108 km.
Kinematics of Disk Stars in the Solar Neighbourhood
Authors: Gómez A.E. , Grenier S. , Udry S. , Haywood M. , Sabas V. , Meillon L. , Royer F. , Lebreton Y. 
Published: June 2021

Using Hipparcos parallaxes and proper motions together with radial velocity data and individual ages estimated from isochones, the velocity ellipsoid has been determined as a function of age. On the basis of the available kinematic data two different samples were considered: a first one (7789 stars) for which only tangential velocities were calculated and a second one containing 3104 stars with available U, V and W velocity components and total velocities ≤ 65 km.s-1.The main conclusions are: -Mixing is not complete at about 0.8-1 Gyr. -The shape of the velocity ellipsoid changes with time getting rounder from σu/σv/σ-w = 1/0.63/0.42 ± 0.04 at about 1 Gyr to1/0.7/0.62 ±0.04 at 4-5 Gyr. -The age-velocity-dispersion relation (from the sample with kinematical selection) rises to a maximum, thereafter remaining roughly constant; there is no dynamically significant evolution of the disk after about 4-5 Gyr. -Among the stars with solar metallicities and log(age) > 9.8 two groups are identified: one has typical thin disk characteristics, the other is older than 10 Gyr and lags the LSR at about 40 km.s-1 . -The variation of the tangential velocity with age(without selection on the tangential velocity) shows a discontinuity at about 10 Gyr, which may be attributed to stars typically of the thick disk populations for ages > 10 Gyr.