bluuebunny/crossref_metadata_2025 huggingface dataset:
OH/IR Stars Near the Galactic Centre
Authors: Habing H.J. , Olnon F.M. , Winnberg A. , Matthews H.E. , Baud B.
Published: June 2019

We have detected 34 OH/IR stars within 1 degree of the galactic centre by their OH emission line at 1612 MHz (18 cm) using the Effelsberg 100 m telescope and the Very Large Array. The spatial distribution and the distribution of the radial velocities show that practically all stars are within 150 pc from the Galactic centre, and that the number of foreground objects is very small. The projected distribution of the stars is similar to that of the surface brightness at 2.4 μm. Since the 2.4 μm radiation is supposed to be due to red giants, the OH/IR stars are probably members of the same population. The stars have considerable random velocities (velocity dispersion in one coordinate of 150 ± 50 km s−1), but show general Galactic rotation. The high velocity dispersion is remarkable for objects of this population.
Star Formation: From OB Associations to Protostars
Authors: Lada Charles J.
Published: June 2019

The study of star formation is a relatively young discipline of the field of astronomy. Up until the mid point of the twentieth century only a most rudimentary understanding of the subject was possible. This is because prior to that time there did not exist any substantive body of empirical data which could be used to critically test even the most basic hypotheses concerning stellar origins. However, as a result of impressive advances in observational technology and in our understanding of stellar evolution during the last forty years, the subject of star formation has developed into one of the most important branches of modern astrophysical research. A large body of observational data and a considerable literature pertaining to this subject now exist and a significant fraction of the international astronomical community devotes their efforts towards trying to comprehend the origins of stars and planets. Yet, despite these efforts we have yet to observationally identify, with any certainty, a single object in the process of stellar birth! Moreover, we have not yet produced a viable theory of star formation, one capable of being tested and refined by critical experiment. In many ways, stellar birth is as much a mystery today as it was forty years ago. However, there can be little doubt that during the last two decades truly revolutionary progress has been made in the quest to understand the star formation process in our galaxy. This apparent paradox in the state of our knowledge concerning stellar origins is resolved with the realization that the history of the study of star formation has been a history of the study of progressively earlier and earlier stages of stellar evolution. Indeed, it is in precisely this area of endeavor that we have learned so much.
IRAS and Star Formation in Dark Clouds
Authors: Emerson J. P.
Published: June 2019

As a first step in systematically studying star formation in dark clouds we report a search for IRAS Point Source Catalog detections lieing within the boundaries of Southern Dark Clouds in the catalog of Hartley et al. (1986). To aid in further classifying the 1099 objects by their infrared colours the colours of the whole IRAS Point Source Catalog are discussed and plotted, and the regions occupied by various types of objects tabulated. The presence of Cirrus makes it difficult to confidently identify protostellar like objects from IRAS data alone. Nevertheless 247 sources have colours characteristic of objects deeply embedded in the dark clouds and are probably at least young stars of low mass. These sources appear to be located at random positions within the dark cloud volumes and there is no evidence to suggest that formation of low mass stars in this dark cloud sample is externally triggered.
The Most Luminous Star Formation Regions in the Galaxy
Authors: Jaffe D. T. , Genzel R. , Harper D. A. , Harris A. I. , Ho P.T.P.
Published: June 2019

We present new far-IR and submillimeter broad-band and spectroscopic results on the dense and very luminous cores of massive star formation regions. The best-studied region, W51, contains one core around the source IRS2 and another around W51 MAIN. Our earlier submillimeter continuum mapping has shown that these two cores are very massive (2-4 × 104 M⊙) and have average densities of nH2 ∼ 105 over their inner parsec. New far-IR maps show that both cores are very luminous (L(MAIN) ∼2 × 106 L⊙; L(IRS2) ∼4x106 L⊙). Observations of the (1,1) and (2,1) transitions of NH3, indicate high kinetic temperatures (200-400 K) for the quiescent gas in the inner several arc seconds (0.1 pc) of both cores. Spectroscopy of the 370 μm J = 7 → 6 and 163 μm J = 16 → 15 transitions of CO toward the cores allows us to characterize the hot high velocity material seen previously on the H2O maser transitions and not readily visible in the low J transitions of CO. The high velocity flow in IRS2 is ∼ 60 times more massive than the very similar outflow in the ∼ 30 times less luminous Orion/KL core. The mass loss rate is ∼ 30 times greater than in Orion. Additional observations of W49 allow us to draw a few general conclusions about the most luminous star formation regions in our galaxy: (1) The luminous cores are 102-103 more massive than the Orion core with the same density. (2) Outflows and warm regions in these cores have physical conditions similar to those in their less luminous counterparts but far more mass is involved in the flows.
The Shocked Molecular Gas in the Orion Bright Bar
Authors: Omodaka Toshihiro , Hayashi Masahiko , Hasegawa Tetsuo , Gatley Ian
Published: June 2019

The Orion bright bar, a bar shaped optical feature, is seen about 2 arcmin southeast of the Trapezium stars in the Orion nebula. Since the bright bar is probably an ionization front seen edge on, this region has an ideal configuration to investigate a shocked region generated by an expanding H II region.
The Structure of the W49A Molecular Cloud Complex: Burst of Star Formation in the 105 M⊙ Core
Authors: Miyawaki Ryosuke , Hayashi Masahiko , Hasegawa Tetsuo
Published: June 2019

We have observed the CS (J = 1-0), C34S (J = 1-0) and H51α emission toward the W49A molecular cloud complex in an area of 3'x 2′ (α x δ) with an angular resolution of 33″. The CS emitting region is 100″ x 80″ or 6.7 pc x 5.4 pc (α x δ) at the half maximum level. Although the CO emission is self-absorbed due to the foreground cold gas, the CS optical depth of the foreground gas is found to be small. Therefore, the two CS peaks at VLSR = 4 km s−1 and 12 km s−1 imply the presence of two dense molecular clouds toward W49A. The brighter 12 km s−1 cloud peaks 35″ southeast of W49A IRS, the infrared and H2O/OH maser sources associated with the compact H II region, while the 4 km s−1 cloud has a peak at W49A IRS. The hydrogen column density through the c34S emitting region is (0.3-1.7) x 1024 cm−2. The estimated core mass of the W49A molecular cloud is (0.5-2.5) x 104 M⊙. This mass is closely packed in a small region of 3.4 pc in diameter, and is about an order of magnitude larger than the virial mass of the system. The massive core will collapse within 10 years unless there is some special supporting mechanism. There was a sudden increase in the star formation rate 104– 105 years ago, suggesting a triggered burst of star formation in the core of W49A. The collision of two velocity clouds might have triggered the formation of this massive core and the burst of star formation.
The Populations of H2 Vibrational States in Intense UV Fields Near O-B Stars
Authors: Dewdney P. E. , Roger R. S. , Robert N.
Published: June 2019

In most places where molecular hydrogen exists in the interstellar medium, it will be found in the ground vibrational and ground electronic state. This will not be so, however, near 0 or early B stars where, in the region just beyond the ionization boundary, populations will be determined by UV fields up to 105 times more intense than the mean interstellar value (4 × 10−16 ergs cm−3 s−1 = 1 Habing unit). The H2 absorbs Lyman-Werner band photons longwards of λ91 nm and subsequent decays to the ground electronic state may lead to dissociation (vibrational continuum) or to one of 14 vibrationally excited states. Molecules in these states have lifetimes of order 1010 s and, in the intense fields, will be exposed to further Lyman-Werner excitation. The probability of dissociation is therefore greatly enhanced by this ‘multiple excitation’, since the number of lines available to vibrationally excited H2 is many times that available to ground-state H2 (Shull, 1978).
Protostellar Candidates in W75N
Authors: Mountain C. M. , Gatley I. , Hartquist T. W. , Leggett S. K. , Selby M. J.
Published: June 2019

W75N is a highly obscured region containing far-infrared sources, masers and molecular clouds. New results are presented showing highly reddened point sources near the maser source W75N(OH). One of these sources appears to be illuminating a newly discovered reflection nebula exhibiting large scale fluorescent molecular hydrogen.
The Dust Cavity Around the Exciting Star of Small H II Regions
Authors: Yoshida S. , Mizuno S. , Nakano M. , Kogure T. , Sasaki T. , Wiramihardja S. D.
Published: June 2019

In a series of observations (Mizuno et al. 1981; Mizuno 1982; Nakano et al. 1983), we have carried out the surface photometry of small H II regions which were selected from Sharpless catalogue (1959) by the following properties: (1) small angular size (≦ 10 arcmin), (2) round and simple appearance, and (3) a single BO V star as the exciting star. Properties (1) and (2) are for the convenience of image processing, and property (3) is to avoid the contamination of [O III] emission in the V-band so as to get pure continuum intensity.
Bipolar Outflows and Stellar Jets
Authors: Snell Ronald L.
Published: June 2019

A wealth of data is now available on the energetic mass outflows that are associated with young stellar objects. This phenomenon is thought to occur at a very early stage in the evolution of stars of almost all masses. The discovery of this energetic event was first made through observations of the rapidly expanding molecular gas that surrounds many of these young stellar objects. A review of the physical properties, including the energetics and morphology, of the expanding molecular gas is presented in this paper. In addition, the role these energetic winds play in affecting the dynamics of the parental molecular clouds is also discussed. Finally, the results of detailed studies of the structure and kinematics of the high velocity molecular gas are reviewed and the evidence for existance of wind-swept cavities and molecular shells within the clouds are presented.
The Star Forming Region in Bok Globule 210-6a
Authors: Kuiper T.B.H. , Peters W.L. , Gardner F. F. , Whiteoak J. B.
Published: June 2019

The NASA 64-m antenna at Tidbinbilla and the CSIRO 64-m antenna at Parkes have been used to observe the Bok Globule 210-6a (“Valentine's Night”) in the (1,1), (2,2), and (3,3) transitions of ammonia. The beam sizes of the two telescopes were 55 arcsec and 81 arcsec, respectively.
Formation of OB Stars by Radiatively-Driven Implosion
Authors: Klein R.I. , McKee C.F. , Sandford M.T. , Whitaker R. , Ho P.T.P.
Published: June 2019

A theory for the sequential formation of OB stars in dense molecular clouds was first quantitatively investigated by Elmegreen and Lada (1976). The model was one-dimensional in nature and assumed shock propagation into homogeneous clouds. This picture has provided a successful explanation of the morphology of the Ori OB1 association.
A Model for Self-Regulating Star Formation
Authors: Wiklind Tommy
Published: June 2019

To model the nearly constant star formation rate, SFR, observed in most late-type galaxies (Gallagher et al. 1984), as well as the starbursting behaviour of some interacting galaxies (Joseph et al. 1984) we have developed a phenomenological model of the regulatory coupling between the density of the star forming part of the interstellar medium, i.e. molecular gas, and the rate of formation of massive stars.
The Underlying Population in Young Stellar Systems
Authors: Tarrab I. , Bruzual G.
Published: June 2019

The problem of stellar population present in a young stellar system is studied from the point of view of the evolutionary synthesis. The dependence of the observable properties on: a) stellar mass loss, b) initial mass function, c) star formation rate, and d) age of stellar population is explored. Properties, such as colors and number of lyman continuum photons, that vary rapidly in time are derived and compared with existing data. The range from the UV to the IR is covered in our predictions. We find good agreement between the observations and our predictions for reasonable values of our free parameters.
Far-Infrared Emission from Clumpy Irregular Galaxies
Authors: Klein U. , Heidmann J. , Wielebinski R. , Wunderlich E.
Published: June 2019

The four clumpy irregular galaxies Mkr 8, 296,297 and 325 have been observed by IRAS. All galaxies have been detected in at least two of the four detector bands. The ratios of the 100 to 60-m flux densities are comparable to those of HII regions or violently star forming galaxies. The average star formation rate in clumpy irregular galaxies is of the order of a few solar masses per year (based on their average far-infrared luminosity and a Hubble constant of 75 km s−1 Mpc−1.
Report on the Progress in Stellar Evolution to 1950
Authors: Gingerich Owen
Published: June 2019

At a joint meeting of the American Astronomical Society and the American Physical Society held in June of 1940, the University of Michigan astronomer Dean McLaughlin (1940) gave a review of the current understanding of stellar evolution. At the end he somewhat facetiously remarked that, “For several years I have told students that I knew all about stellar evolution in 1923, less is 1925, and nothing at all since 1930.” I would like to suggest that those dates were not chosen randomly, and in the first part of my own survey of stellar evolution to 1950, I would like to explain the significance of those dates.
Walter Baade's Discovery of the Two Stellar Populations
Authors: Osterbrock Donald E.
Published: June 2019

Walter Baade was born one hundred and one years ago; he died in 1960, a third of a century ago. In my opinion he was the second most important observational astronomer of this century, after Edwin Hubble, who changed our place in the universe and our perception of it. I will give some of the evidence in this paper.
Population and Evolution of Pulsating A-F Stars
Authors: Shiyang Jiang , Yanying Liu
Published: June 2019

Pulsating A-F variables include all the stellar types listed in teh Table, as well as the pulsating white dwarfs. Stars near the zero-age-main-sequence have faster rotation velocity, which slows as expected with age (Villata 1992) and a smaller amplitude of light variation, so we suggest that rotation velocity be considered in Population classifications. Also, in the Galaxy, the galactic rotation constant A is related to stellar age T by: A(kms−1kpc−1) = (−2.4±0.8)T(109yr) + (32±2) (Kharchenko 1992). The linear rotation velocity is also a function of the Z coordinate of the object inside the Galaxy: the mean Z-gradient is −10kms−1kpc−1 (Malakhova & Petrovskaya, 1992). Thus the population is strongly correlated with the rotation velocity and the evolutionary age.
New Algorithms to Calibrate the Strömgren Photometric System in Metal-to-Hydrogen Ratio
Authors: Malyuto V.
Published: June 2019

The previous calibration of the Strömgren photometric indices in terms of spectroscopic [Fe/H] values was performed by Schuster and Nissen (1989) with the application to their sample of calibration stars a mathematical package handling the regression of [Fe/H] values against the (b – y), m1, and c1 indices, and their combinations. To provide a similar regression we applied a quite another independent statistical method (notably that of of stepwise linear regression) to the same sample and also to other available calibration samples of F stars. We conclude that in our case the quality of calibration was slightly better and the analytical form of calibration equation was much simpler.
On role of Triple Systems in the Dynamics of the Galaxy: Stars with High Velocities
Authors: Anosova J. , Colin J. , Kiseleva L.
Published: June 2019

There are a number of stars with very high pecular velocities in the Galactic halo within the region of about 1.5 kpc from the Galactic plane. At least some of these stars could be the result of an escape process in multiple stellar systems located in the Galactic disc.
Pal 1: A Globular Cluster Anyway?
Authors: Borissova J. , Spassova N.
Published: June 2019

CCD photometry of the halo cluster Pall in Thuan-Gunn system is presented. The color - magnitude diagrams (Fig. 1) show a well defined red horizontal branch, lower giant branch and main-sequence down to about two magnitudes below the main-sequence turnoff. The giant branch is absent and the brightest stars are horizontal-branch stars. The horizontal branch is composed of red stars only. The age of the cluster, determined by comparison with the isochrones of Bell and Vanden Berg (1987) is consistent with an age between 12 – 14 Gyr. and a metallicity [Fe/H] = −0.79. The observed luminosity function shows a deficiency of stars from about 0.5 mag below turnoff up to g = 21 mag. A distance modulus of (m – M)g0 = 15.35 ± 0.1 magnitude has been derived. An estimate of the cluster structural parameters gives rc = 1.7 pc and c = 1.46. A mass estimate of 1.6 × 103M⊙ and mass-to-light ratio 1.77 has been obtained, using King's (1966) method.
CCD Photometry in the Globular Cluster M4
Authors: Griffiths W.K. , Kanatas I.N. , Dickens R.J. , Penny A.J.
Published: June 2019

A V, B- V composite colour-magnitude diagram, based upon CCD photometry from V~ 12 on the red giant branch to V~ 25 on the main-sequence has been derived for the globular cluster M4. A distance to the cluster of (m – M)v = 12.84 ± 0.19 is determined and the best match to theoretical isochrones is for the case [Fe/H]=−1.27 and an age of 16±1 Gyr. A differential age comparison with NGC 362 shows that M4 is approximately 1.7 Gyr older.
Emission Lines in Star Forming Galaxies
Authors: Gustavo Bruzual A. , Gladis Magris C.
Published: June 2019

Magris et al. (1993, Proceedings of the ESO/OHP Workshop on Dwarf Galaxies, ed. G. Meylan) have combined the spectral evolution population synthesis code of Bruzual & Charlot (1993, ApJ 405, 538) with the multipurpose photoionization-shock code MAPPINGS (Binette et al. 1993, AJ 105, 797) in order to calculate self-consistently the evolution of the nebular emission corresponding to the ionizing spectra produced by the synthetic population. In this poster we compare the redshift dependence of the EW[OII]3727 predicted by the Magris et al. models with the observations available (Colles et al. 1990, MNRAS 244, 408; Kennicutt 1992, ApJS 79, 255; Le Fevre et al. 1994, ApJ 423, L89). Models in which stars form following the Salpeter IMF according to the SFR Ψ(t) = 1M⊙τ−1exp(–t/τ), for τ = 1, 2, 3, 5, 7, and ∞ Gyr have been considered. We assume Ho = 50, Ω = 0, tg = 16 Gyr. About 2/3 of the galaxies in these samples have EW[OII]3727 from 2 to 5 times larger than the ones predicted by our models for steadily decreasing or even constant (τ = ∞) SFR. To cover the range of observed values of EW[OII]3727 we assume that instantaneous bursts (Salpeter IMF) of star formation take place in these galaxies, superimposed on the steady SFR used in the models. The mass added by the bursts as newly formed stars per unit mass of stars in the underlying galaxy is in the range 1 – 4 × 10−4. Depending on the rest-frame wavelength, the color of the galaxies may remain unaffected or become considerably bluer as a consequence of the bursting activity.
Dark Matter in the Polar Ring Spiral NGC 660
Authors: Van Driel Wim
Published: June 2019

Polar ring galaxies, where rotation velocities can be measured in two orthogonal planes, are generally considered to be ideal objects for studies of the three-dimensional distribution of dark matter in galaxies.
Near-IR Imaging of Elliptical Galaxies
Authors: Pahre M.A. , Djorgovski S. , Matthews K. , Shupe D. , De Carvalho R. , Mould J.R.
Published: June 2019

We have imaged more than thirty early-type galaxies in the K-band to investigate their stellar populations. Our surface brightness fluctuations (SBF) measurements for the nearest 14 galaxies produce a mean fluctuations magnitude in the K-band of The scatter in the Virgo cluster is small at 0.18 mag, which implies that infrared SBF is potentially a good distance indicator (Pahre & Mould 1994). Inspection of the simple stellar population tracks of Worthey (1994) suggests that a plot of the fluctuation color against broadband color (V – I) might be useful in discriminating between age and metallicity effects in elliptical galaxies. We have measured (r – K) color gradients for the entire sample utilizing three methods, two of them independent of sky-subtraction errors, as found in Sparks & Jørgensen (1993). Our color gradients are consistent with a mean metallicity gradient of 0.14 mag dex−1, which is somewhat smaller than that implied by optical color and line gradients alone, suggesting that age gradients may also be important. Finally, we have constructed an infrared Fundamental Plane (FP) which is consistent with its optical counterpart; continued work will determine if there is a significant change in the tilt of the FP between the optical and infrared.
Reference frames in astrometry
Authors: de Vegt Chr. , Johnston K. J.
Published: June 2019

Present and near future advances in astrometry at radio and optical wavelengths will allow at least an order of magnitude increase in the precision with which fundamental reference frames are defined compared with those available ten years ago. A brief review is given of the present status of fundamental celestial reference frames with a view towards defining the problems encountered in establishing a fundamental reference frame and methods of linking these reference frames. An estimate is made of the progress to be made in reference frames over the next ten years.
Reference frames: determinations and connections
Authors: Dickey J. O. , Fanselow J. L. , Melbourne W. G. , Newhall X. X. , Standish E. M. , Williams J. G.
Published: June 2019

The modern space techniques have achieved such an unprecedented accuracy and precision in position measurements that there is required a rethinking and reformulation of reference frames, both in their concepts and in their realizations. Various reference frames are a natural consequence of the different observational techniques and data types. Not only are the determination and the necessity of these various frames stressed, but also their evolving nature is recognized. The current status of frame ties is highlighted, and the need for improved ties is motivated. We conclude with our concerns and recommendations.
The mobile laser-ranging system as a comparative tool for monitoring the terrestrial reference frame
Authors: Wilson Peter
Published: June 2019

The proposed earth rotation service and the related efforts to monitor the terrestral reference frame will employ instrumental and computational techniques of widely different kinds. It has already been demonstrated that the results produced by these techniques are sensitive to systematic errors occurring at the level of a few millimeters. As a result it will be extremely important to verify the performance of the different systems by making comparative observations at the same sites. Only mobile laser-ranging systems are currently in a position to perform this kind of service. To demonstrate the status of laser ranging during co-location, this paper presents a proposal for a controlled collocation experiment involving U.S. and European stations of the IRIS network.
High frequency variations in Earth rotation
Authors: Carter W. E. , Robertson D. S. , Fallon F. W.
Published: June 2019

The combined POLARIS-IRIS Earth orientation time series now span nearly a full cycle of the Chandler-annual beat period, beginning in late 1980. Since April 1985 there is also a nearly continuous coverage of UT1 at daily intervals. We have fit a simple model, consisting of circular 14-month and annual components and a linear drift to the polar motion series, then computed the “along-track” and “cross-track” residuals. Both sets of residuals display structure with amplitudes of tens of milliseconds of arc on time scales of months, but Fourier analysis reveals no significant peaks at shorter periods, including the 40-60 day period found in the UT1 time series.

During September, 1986, we introduced a new “quick-look” UT1 time series. The values are typically available within 7 days. The accuracy, which depends strongly on the accuracy of the X and Y pole coordinates used in the computations, ranged from 0.3 to 0.7 milliseconds during the first two weeks, but improved to about 0.1 milliseconds during the latter two weeks of the month. We plan to continue the quick-look UT1 series as a standard product of the IRIS Earth orientation monitoring service.
Radiointerferometric polar motion determination using a very long north-south baseline
Authors: Nothnagel A , Nicolson G. D. , Schuh H , Campbell J , Cloppenburg H , Kilger R
Published: June 2019

The first high accuracy VLBI measurements with the Hartebeesthoek Radio Astronomy Observatory (HartRAO) at the southern end of the African tectonic plate were made possible at the beginning of 1986 through the loan of a MARK III DAT to HartRAO by the US National Geodetic Survey. Six twenty-four hour experiments spread over thirty-three days were used to precisely determine the HartRAO station position and to measure baseline lengths to Europe and North America. Interleaved between these multi-station experiments, a single baseline from Wettzell to HartRAO was used for two hours on a daily basis in order to measure pole positions. The formal errors of the x and y pole component determinations for each day are about ±2 mas and ±1 mas respectively, but an offset of about 6 mas from the IRIS values remains to be investigated.
Earth rotation from radio interferometric tracking of GPS satellites
Authors: Abbot R. I. , King R. W. , Bock Y. , Counselman C. C.
Published: June 2019

Radio-interferometric tracking of the Global Positioning System (GPS) satellites offers a new technique for regular monitoring of variations in the earth's rotation. The observations are sensitive to pole position and length-of-day, at a level of precision which may make this technique competitive with satellite and lunar laser ranging and very long baseline interferometry (VLBI). The present limitations are the number of satellites and tracking stations available and inadequate modeling of non-gravitational forces on the satellites. The potential advantages are rapid turn-around and minimal incremental cost. We have performed a preliminary analysis using six days of observations from a four-station network. Comparison of earth rotation values from our GPS analysis with values obtained by VLBI and laser ranging reveals differences after five days of 0.9 ms in UT1, 0.04″ in x and 0.07″ in y. These differences reflect errors in the GPS determinations due primarily to inadequate modeling of non-gravitational forces.
The combination of polar motion observations
Authors: Wilson Clark R. , Vicente R. O.
Published: June 2019

Polar motion data for the period 1981–1985 are used to obtain a combined solution from Doppler, Satellite Laser Ranging, and Astrometric observations. The combined solution is a weighted average of the three series, with weights determined from reported errors which are scaled so that they agree with errors estimated from differences among the various series. The combined solution is effective in removing spurious deviations in the pole path which appear in a single series. However, we also show that estimated errors can be unreliable when derived from short time series, when one series is much less noisy than the others. Thus, a combined solution where weights depend upon estimated errors can yield poor results, and we demonstrate this effect by comparing a combined solution for 1984–85 with the independent IRIS series.
Kalman filtering of Earth orientation changes
Authors: Morabito David D. , Eubanks T. Marshall , Steppe J. Alan
Published: June 2019

The orientation of the earth in space changes unpredictably in a rapid and irregular manner, in addition to the uniform rotation of the earth. Observations of extra-terrestrial objects from the surface of the earth are affected by these variations, and knowledge of these changes is required for a variety of geodetic and astrometric purposes as well as being of interest in its own right. The orientation of the earth (specified by a three dimensional rotation vector) is measured by a variety of techniques; combination of these data sets is complicated by irregular changes in the spacing and accuracy of the various time series, and also by the existence of lower dimensional measurements of different linear combinations of the rotation vector components. A Kalman filter has been developed at JPL to smooth and predict earth orientation changes for application to spacecraft navigation by the NASA Deep Space Network. The filter, which provides estimates of the earth orientation changes (and of the excitation of these changes) based on whatever measurements are available, has been used for a number of research applications, both in the reduction of geodetic and astrometric data, and in research into the geophysical causes of earth orientation changes. The JPL Kalman filter uses stochastic models to account statistically for otherwise unpredictable changes in earth orientation; these models make it possible to provide reasonable estimates of the error in the smoothed time series, and to automatically vary the amount of smoothing according to the accuracy and density of the data. The derivation of the stochastic models used by the filter, the implementation of the models into the filter, a statistical description of what the filter does, and the results of filtering specific data sets will be discussed.
Bound on the amplitude of the Earth's free core–nutation
Authors: Herring T. A. , Gwinn C. R. , Buffett B. A. , Shapiro I. I.
Published: June 2019

We analyzed six years of very–long–baseline interferometry (VLBI) data and determined corrections to the coefficients of the seven terms with the largest amplitudes in the IAU 1980 nutation series. Our analysis yields results consistent with earlier analyses of smaller sets of VLBI data, within the uncertainties of the latter. Here, we restrict discussion to the freely excited core–nutation or “free core–nutation” (FCN). Our analysis yields an estimate of 0.33 ± 0.12 mas for an assumed constant amplitude of the FCN, which allows us to place an upper bound on it of 0.6 mas (99.5% confidence limit). We also studied possible temporal variations of the complex amplitude of the FCN by modeling it as a stochastic process with a white noise excitation. We detected no statistically significant variations of this amplitude for the six–year interval spanned by the VLBI data. However, in the neighborhood of one cycle per day, the power spectral density of the atmospheric surface loading is estimated from global weather data to be 0.24 (g cm−2)2 day, about five times larger than the largest such power spectral density that would be consistent with the upper bound on the amplitude of the FCN placed by the VLBI data. Thus, we conclude that this estimate is too high and that, if the FCN were excited by surface loads with frequencies near one cycle per day, then the power spectral density of these loads must be <0.06 (g cm−2)2 day (99.9% confidence limit).
Direct estimation of nutation coefficients from VLBI data
Authors: Himwich W. E. , Harder E. J.
Published: June 2019

Previous analyses of nutation angle errors from Mark III VLBI data have been made by estimating offset corrections to the instantaneous nutation angles from single-day observing sessions. Adjustments to the coefficients of the nutation model were then estimated from the time series of offset corrections. In this paper we estimate the corrections to the nutation series coefficients directly from the multi-year ensemble of data. The resulting corrections have smaller uncertainties than corrections estimated from the daily offsets because the direct solution has many fewer degrees of freedom. We have forced the corrections to occur at the periods of the dominant terms of the IAU 1980 nutation series instead of allowing the pole orientation to vary freely on each day. Results from daily offset adjustment and direct nutation series adjustment from nearly seven years of data agree at the level of the combined one-sigma uncertainties, 0.1 milliarcseconds. The amplitude of the free core nutation was also estimated from the daily offset data; no significant amplitude was observed.
Snow-load excitation of the Earth's annual wobble
Authors: Chao B. Fong , O'Connor William P. , Chang Alfred T. C.
Published: June 2019

A global, monthly snow depth data set has been generated from weather satellite (Nimbus 7) observations using passive microwave remote-sensing techniques. In this paper we analyzed five years of data, 1980–1984, to compute the snow-load excitation of the annual wobble of the Earth's rotation axis. A uniform sea-level decrease has been assumed in order to conserve water mass. The result shows dominant seasonal cycles. The prograde component of the annual excitation is Ψ+ = (5.0 milliarcsec, −110*) and the retrograde component Ψ− = (5.0 milliarcsec, −31*). These computed values are compared with previous groundwater estimates, as well as the inferred values from ILS and LAGEOS polar motion measurements. The importance of accurate data is stressed and future plans proposed.
Detached Main-Sequence Binaries: A Laboratory for the Study of Stellar Structure
Authors: Gimenez A.
Published: June 2019

Our knowledge of the evolutionary processes taking place in interacting binary stars is based primarily in our understanding of the stellar structure and evolution of single stars. The determination of absolute dimensions in close doble-lined eclipsing binaries, with well-detached components, is the best empirical approach available into stellar structure.

Important physical parameters and processes can be studied in some detail within the framework of detached main-sequence binaries, e.g., initial chemical composition, the mixing-length ratio, the relevance of convective overshooting, interactions between matter and radiation in the stellar interior, the concentration of mass towards the center of the stars or the response of stellar envelopes to tidal influences by an external gravitational potential.

A combination of observational results from photometry and spectroscopy are used to obtain relevant stellar parameters: absolute dimensions, ages, chemical composition, apsidal motion rate, degree of synchronization and circularization, etc., and a comparison with theoretical models allow us to derive some information about the above mentioned physical mechanisms.
W UMa Binaries
Authors: Vilhu O.
Published: June 2019

The origin and evolution of contact (W UMa) binaries are discussed with the help of the braking law of eq.(1). Best observationally justified parameter-values in (1) for W UMa, CV and LMXB binaries are: α ≈ 1.5, γ = 2 and f computed from eq.(2). A semi-empirical relation (eq.(5)) between the scaled magnetic torque and the fractional X-ray luminosity is proposed. First attempts of simultaneous photometric and spectroscopic surface imaging of contact binaries are described.
X-Ray Binaries and Related Systems
Authors: Trimble Virginia
Published: June 2019

Neutron stars and a few black holes in binaries reveal their presence by emitting X-rays when they accrete gas from their companions via a wind or disk. Related objects include SS 433, Geminga, gamma ray bursters, TeV/PeV sources, and the source in CTB 108. Systems with secondaries 8 Mo are the natural descendents of main sequence OB binaries. Those with secondaries ≤ 1 Mo arguably form some other way. These systems display a wealth of structure in both wavelength and time domains, much of which is reasonably well understood. Among the things we would like to know more about are the masses and rotation periods of the neutron stars in the two main kinds of systems.
Wolf-Rayet Stars
Authors: Cherepashchuk A. M.
Published: June 2019

Many new results have been obtained up to now in the problem of Wolf-Rayet (WR) stars. Most of these results are published now in the Proceedings of IAU Symposium N 143 (Van der Hucht and Hidayat, 1991), held in Baly in June 1990. Here we report some new additional results concerning mainly WR stars in binary systems.
Binary S and MS Stars
Authors: Johnson Hollis R.
Published: June 2019

We assemble and discuss evidence for binarity in S and MS stars - stars that are enriched in s-process elements. A popular view is that Tc-deficient S and MS stars are not thermally pulsing AGB stars but are mass-transfer binaries. We describe several methods used to test this hypothesis through a search for the putative white-dwarf companion: (1) periodic radial velocity variations, (2) direct observation of the hot companions with IUE, (3) evidence of a hot gas cloud (hotter than a chromosphere) in the system, and (4) evidence of circumstellar dust as revealed through infrared radiation. Results of these methods are compared. All evidence supports the idea that Tc-deficient S and MS stars are mass-transfer binaries, and a large fraction appear to be interactive.
First Results of Continuous IUE Observations of Algol
Authors: González-Riestra R. , Gimènez A. , Guinan E. F.
Published: June 2019

We present the first results of a continuous monitoring of Algol with IUE along 1.5 orbital period (4 1/3 days). A total of 32 high resolution spectra were obtained. Algol represents the final slow stage of mass transfer. The spectra show the presence of high ionization lines (NV, SiIV, CIV) due to the heating of the gas that impacts the accreting star. The depth of the primary eclipse is larger at shorter wavelengths and the shape of the continuum out of the eclipse is compatible with a B star, ruling out the presence of an extra light source at these wavelengths.
A New Catalogue of Semidetached Algol-Type Binaries with Well Determined Absolute Dimensions
Authors: García J. M. , Giménez A.
Published: June 2019

Data on a selected sample of semidetached Algol-type eclipsing binaries with well-known absolute dimensions have been collected. The strict criteria of selection adopted are given together with the provisional form of the catalogue. This includes besides the absolute dimensions other relevant parameters that may be of interest.
HR 2142: A Semi-Detached Interacting Binary?
Authors: Coté J. , Pols O. R. , Waters L. B. F. M.
Published: June 2019

The energy distribution of HR 2142 is studied in order to test the hypothesis that this star is an interacting binary with a cool Roche–lobe filling companion. We find that, for any reasonable choice of Teff, the companion should have been detected in the red part of the spectrum. We propose instead that the Be star is the outcome of a case B mass-transfer, which has pun it up. It is now surrounded by a mass-loss disc, rather than an accretion disc and is accompanied by either a helium star (resembling the Φ Per system) or by a white dwarf. If the binary was born spinning rapidly, alternatively the companion may be a solar-type, unevolved main sequence star. In case of a Helium star companion, there may be Helium lines visible (like in Φ Per), or the helium star may be detectable in the XUV, e.g. by the ROSAT XUV instrument. A white dwarf companion may be accreting material from the disc around the Be star and may show low luminosity X-ray emission. This emission may have been seen in the ROSAT PSPC X-Ray Survey.
Colliding Stellar Winds in Close Binary Systems
Authors: Shore Steven N. , Corcoran Michael J.
Published: June 2019

We describe some of the observational phenomena associated with colliding stellar winds, focussing especially on close WR+O binaries. In particular, we report some new ultraviolet observations of CQ Cep and V444 Cyg. We also discuss some of the general theory of such wind collisions.
The Masive Binary SV Centauri
Authors: Linnell Albert P.
Published: June 2019

Current light synthesis models of SV Centauri require a highly overcontact system with an 8000K temperature discontinuity between components. Current evolutionary models support the overcontact status, and describe the system as in the final stage of rapid mass transfer prior to mass reversal, calculated to occur within 2500 years. Light synthesis simulation provides a poor fit to the observations, and requires an ad hoc assumption to fit the substantial O'Connell effect.

IUE spectra have been reported to indicate the presence of a 200 000K source of small dimensions in the system. New spectrum synthesis results demonstrate there is no hot spot, but there is extra radiation longwards of 200nm that is unaccounted for by the overcontact model.

Postulated presence of an accretion disk around the mass gainer prospectively overcomes the observational anomalies. Formation of the accretion disk apparently violates the Lubow-Shu criterion. A reasonable scenario nonetheless supports accretion disk formation. Conflict with the theoretical models can be resolved on the postulate that the spherical accretion assumed in those models render them inapplicable to this system.
An Ultraviolet Study of the Near-Contact Binaries
Authors: Shaw J. S.
Published: June 2019

Five members of the class of near-contact binaries have been investigated in the ultraviolet using the IUE. The spectra have been analyzed to find masses of two of the systems and to indicate the possibility of mass transfer.
The Spotted Contact Binary SS Arietis — Spectroscopy and Infrared Photometry
Authors: Rainger P.P. , Bell S.A. , Hilditch R.W.
Published: June 2019

The first infrared photometry for the W-UMa system SS Ari is presented. An analysis based on medium resolution spectroscopy presented here shows that SS Ari is a W-type system with a mass ratio of 0.33. It seems certain that the asymmetry in the published light curves and those obtained for this study can be explained by the effect of spots on one or possibly both components of the system. The precise location, size and temperature of these spots require the use of Doppler Imaging techniques in conjunction with high quality multi-band photometry.
White Light Photometry of the Cataclysmic Variable BZ Camelopardalis
Authors: Pajdosz Gabriel , Zoła Stanisław
Published: June 2019

We present high-speed photoelectric observations of a 13th-mag cataclysmic binary BZ Cam (=0623+71), gathered in 1989 and 1990. The data were analyzed by means of the Fourier technique. The most prominent feature detected in the power spectrum is a 3h36m period which we ascribe as being a photometric orbital modulation. This period is significantly different from the periodicity previously known from spectroscopic observations. The power spectrum shows also quasi-periodic oscillations of shorter time-scales but any conclusions about their stability can not be drawn yet.
Behavior of Matter Near the Outer Region of the Accretion Disk in Her X-1
Authors: Bochkarev Nikolai G. , Karitskaya Eugenia A.
Published: June 2019

Detailed analysis of numerous observational data for Her X-1 / HZ Her shows that Her X-1 has a rare type of accretion regime: impulses of mass transfer every 0.81 d. Semi-empirical analysis show complicated 3 D type of gas flow near the outer disk rim of Her X-1: interaction of gas stream with disk forms blobs, which are disintegrated into small drops of matter (“sprays”) by an interaction with gaseous corona around the outer disk rim. Fall of the sprays on different parts of the disk decreases time of accreting mass flow along disk radius. Evaporation of sprays is a source of mass supporting the disk corona. The corona and disintegrated blobs (sprays) have numerous observational manifestations in optical and X-ray ranges. Evaporation of the corona from the Roche lobe could be an important source of orbital angular momentum loss and, consequently, a source of orbital period variation.
The Blue Straggler F190: A Case for Mass Transfer
Authors: Milone Alejandra A. E. , Latham David W.
Published: June 2019

The blue straggler F190 is a member of the old open cluster M67. We argue that F190 may still be in the final stages of the mass transfer that has made it into a blue straggler. We use the turnoff mass of the cluster and the mass function derived from the spectroscopic orbit for F190 to constrain the masses of each member of the binary, both before mass transfer and now. We find that the mass transfer must have been nearly 100 percent efficient.
Massive Close Binaries in the Magellanic Clouds
Authors: Niemela Virpi S. , Bassino Lilia P.
Published: June 2019

We present spectroscopic orbits for 4 eclipsing binary systems in the Magellanic Clouds. The minimum masses that we find for the binary components in the LMC are: 34 + 17 M⊙ for HV 2241 and 26 + 15 M⊙ for HV 2543, and for the ones in the SMC: 21 + 25 M⊙ for AzV 73 and 17 + 19 M⊙ for HV 1620.
Close-Binary Nuclei of Planetary Nebulae
Authors: Bond Howard E. , Ciardullo Robin , Meakes Michael G.
Published: June 2019

Close-binary planetary-nebula nuclei (PNNs) provide direct evidence for occurrence of a common-envelope phase in binary-star evolution. Their descendants are V471 Tauri-type detached binaries, cataclysmic binaries, and possibly Type I supernovae. Thirteen close-binary PNNs are now known from periodic photometric or radial-velocity variations, or from composite optical/UV spectra. At least 10% of PNNs are close binaries, a fraction more than sufficient to account for the formation of all of the cataclysmic variables in the solar neighborhood. The Abell 35-type binary PNNs, a class with three known members, contain rapidly rotating, chromospherically active late-type primary stars along with extremely hot companions detected with the IUE satellite.
Properties of Faculae from Observations Near the Opacity Minimum
Authors: Foukal P , Moran T
Published: June 2019

Imaging of active regions in continuum around 1.6 μm shows that many facular regions are less bright than the photosphere when observed nearer to disk center than μ = cos θ ~ 0.75. The contrast of these dark faculae increases with magnetic flux above a threshold of approximately 2 × 1018 Mx. This explains why not all faculae are dark at 1.6 μm, since the magnetic flux density in many regions of bright Ca K plage emission falls below this threshold. After correction for blurring, the typical contrast value is about 4-5%, so the brightness temperature deficit is about 130 K. Faculae are brighter than the photosphere at 1.63 μm nearer to the limb than μ ~ 0.5. The negative contrast of dark faculae may arise from cooling of the surrounding photosphere, or from increased visibility of cool layers of the facular flux tube itself. Quantitative comparison of these IR data with MHD models awaits calculation of flux tube contrasts at realistic angular resolution.
Simultaneous IR and Visible Light Measurements of the Solar Granulation
Authors: Keil S. , Kuhn J. , Lin H. , Reardon K.
Published: June 2019

Movies of the solar granulation were made simultaneously at 5575 Å and 1.64 μm using the Vacuum Tower Telescope at NSO/SP. A 128 × 128 HgCdTe array was used in the infrared and an RCA 504 CCD in the visible. From the movies, we determine and compare statistical properties of the granulation and seeing conditions.