bluuebunny/crossref_metadata_2025 huggingface dataset:
A High-Resolution View of the Solar Chromosphere and Corona
Authors: Brueckner G. E.
Published: March 2016

Skylab has demonstrated the dominant role of magnetic fields in the solar atmosphere. The solar wind is not a necessary consequence of the pressure imbalance between the hot corona and the interplanetary medium. High speed solar windstreams are originating in coronal holes where coronal density and temperature are less than in the quiet sun. Older models of the solar wind invoke heat conduction from the hot corona as the prime energy source for the solar wind. However in coronal holes the energy supplied by heat conduction is less than in the ordinary sun while the high speed windstreams require an amount of energy which cannot be supplied by conduction alone.
X-Ray Properties of Galactic Nuclei
Authors: Weedman Daniel W.
Published: March 2016

Preparing this review was my just punishment for stating only two years ago - in another review (Weedman 1977) - that Seyfert galaxies are not strong X-ray sources. I said that because, as recently as three years ago, NGC 4151 was the only Seyfert galaxy known as an X-ray source. Now we have 36 Seyfert 1 galaxies, along with 12 other galaxies with strong emission-line nuclei, that are X-ray sources. And this is all without even having HEAO-2 data at our disposal yet. The study of active galactic nuclei with X-ray astronomy is progressing so rapidly that a reviewer feels almost hopeless. The best I can do is summarize what is known as of the summer of 1979 and give a simple overview of how X-ray and other properties relate.

Some excellent reviews of the X-ray properties of Seyfert and other emission-line galaxies already exist. I especially recommend that by Andrew Wilson (1979). He provides very complete references as of a year ago, but X-ray astronomy is progressing so rapidly that he then had only somewhat more than half the active nuclei now in Tables 1 and 2. It was the group working with the Ariel V SSI that made the initial comprehensive X-ray studies of Seyfert galaxies (Ward et al. 1977, Elvis et al. 1978). The UHURU results for Seyfert galaxies followed soon after and are summarized by Tananbaum et al. (1978); the HEAO-A-2 survey results are now in press (Marshall et al. 1979) I have tried to incorporate these and other recent results in Tables 1 and 2.
Recent X-Ray Observations of Seyferts and Emission-Line Galaxies
Authors: Griffiths R. E.
Published: March 2016

It has been just a few years since Type 1 Seyferts were established as a class of X-ray sources with luminosities in the range 1042 - 1045 ergs s-1 by Elvis et al. (1978) using data from the sky survey instrument on Ariel V, and by Tananbaum et al. (1978) using data from UHURU.

The average error-box sizes for X-ray sources identified with Type 1 Seyferts in the 2A catalog (Cooke et al. 1978) is ˜ 0.4 sq. degrees, and ˜ 1.0 sq. degrees for those in the 4U catalog (Forman et al. 1978). Improvement in these positions has been made over the past two years by the modulation collimators on board the satellites SAS-3 and HEAO-1. In particular, the HEAO-1 scanning modulation collimator has been used to position a total of 20 X-ray sources, confirming the identification in each case, with the possible exception of Mkn 279 (Dower et al. 1979, Griffiths et al. 1979a). Of the 37 X-ray sources which were discovered prior to the launch of the Einstein Observatory and which have been associated with Type 1 Seyferts, 21 have been positioned to ˜ 1 arc minute, representing an improvement by factors of ˜ 20 to 100 over the previous 2A and 4U error box sizes. Some examples of the error boxes and identifications confirmed with the HEAO-1 scanning modulation collimator are shown in figs. 1 and 2. In fig. 1 both NGC 7213 (Philips 1979) and MCG - 2 - 58 - 22 (Ward et al. 1978) were discovered to be Seyferts by optical spectroscopy of candidate objects in the error regions of the corresponding X-ray sources. NGC 7213 is a Seyfert nucleus in a galaxy of Type SO (Philips 1979). In fig. 2, NGC 931 was likewise discovered to be a Seyfert as a result of its X-ray emission (Ward and Wilson 1978).
The Active Region in Galactic Nuclei: An Outline
Authors: Pacini F.
Published: March 2016

The activity of galactic nuclei entails a continuous release of energy in the form of hot gas, violent motions, relativistic particles and ordered magnetic fields. The spectrum radiated spans most of the electromagnetic band, from radiowaves to γ-rays, and the power can reach or exceed 10441 - 1046 ergs sec-1. Various extensive surveys dealing with this subject have been published recently (see, e.g., Hazard and Mitton 1979; Physica Scripta 1978; Setti 1976; Wolfe 1979): we refer the reader to these publications for details. In the following we shall just outline the main ideas which are being discussed to explain the origin of these phenomena which pose one of the most important problems of modern astronomy.
Energy Transport in Radio Sources
Authors: Scheuer P. A. G.
Published: March 2016

Dr. Pacini has dealt with the innermost parsec; my job is to comment on the next 999,999. I have four comments.

1. A reminder of the importance of using the bulk kinetic energy of whatever beam or jet there is - forms of energy that exert pressure will get lost on the way out because of adiabatic expansion (Longair et al. 1973).

2. What is the asymmetry of jets trying to tell us?
Optical Observations of Radio Jets
Authors: van Breugel Wil , Miley George , Butcher Harvey R.
Published: March 2016

Over the past several years a considerable body of evidence has accumulated, suggesting that extended radio sources are powered quasi-continuously from the nuclei of their parent galaxies. This view is supported by the recent discovery that several radio galaxies have narrow radio jets which connect their active nuclei with the large radio lobes and which often extend for several tens of kiloparsecs. Because of their presumed association with the energy transport outward from the active nuclei, radio jets are at present being intensively studied with high-resolution radio techniques.

The closest galaxy known to have a radio jet is the giant elliptical M87 (e.g., Wilkinson 1974), and in this case there is a well-known optical counterpart (e.g., Curtis 1918; de Vaucouleurs, Angione and Fraser 1968), This optical jet is highly polarized (Baade 1956), implying that at least part of the emission is non-thermal. This and the good agreement between the optical and radio structure suggests that these features are closely related.
Implications of Results from The Einstein Observatory for The X-Ray Background
Authors: Gorenstein Paul , Schwartz Daniel A.
Published: March 2016

The origin of the diffuse background has been an important question in X-ray astronomy starting from the earliest measurements (Giacconi, et al. 1962). When it was recognized that the X-ray background above 2 keV was isotropic and hence extragalactic (c.f. Schwartz 1970, 1979), it became evident that understanding its origin would have cosmological implications (c.f. Hoyle 1963, Rees 1973). Two general explanations representing opposite points of view have received the most attention. One is that the background is composed of faint unresolved objects which are of the same classes as, but perhaps in earlier phases than, the objects which can be detected directly and identified. If this explanation is correct, then the luminosities and/or proper densities of the objects must be larger at earlier times in the universe, and the magnitude and graininess of the background can be used to place important constraints upon the evolution of the objects. The other is that the background is truly diffuse. The most probable explanation of this type is a hot plasma that pervades the intergalactic medium. If it accounted for only 10% of the background, the mass that could be attributed to such a plasma would represent more matter in the universe than has been detected so far by all other means. Such a large mass would be important dynamically, and in determining the deceleration parameter, although it would not “close” the universe. Furthermore, the density of such a plasma would place constraints upon determining the epoch of galaxy formation.
The European Astrometry Satellite, Hipparcos
Authors: Høg Erik
Published: March 2016

A description of the instrument is given. It is expected to obtain positions, annual proper motions and parallaxes for 100 000 stars in a 2.5 years mission. The accuracy (s.e.) will be for stars of mB < 11 and about at mB=14. An extension of the mission to 3.5 years would improve the annual proper motion to for stars brighter than mB = 9.
The Impact of Star Parallaxes and Very Accurate Proper Motions on Galactic Structure and Dynamics Studies
Authors: Lindblad Per Olof
Published: March 2016

Assuming that we have the absolute parallaxes and homogeneous set of proper motions for a large number of stars as described by the previous speakers - how should we use them in studies of galactic structure and dynamics?

Concerning the parallaxes I will be very brief - the subject has already been discussed by Dr. Pagel. Clearly, the main importance of the absolute parallaxes obtained is in the calibration of photometric or spectrophotometric absolute magnitude scales. For a sufficient number of stars observed in the Hyades cluster at a distance of 50 pc and in the Scorpius-Centaurus association the systematic error, which lies at the bottom of all subsequent calibration of distance indicators, may be reduced to the same order as the accuracy of photometric measurements.
Stellar Atmospheres and Chemical Compositions of Galaxies
Authors: Aller Lawrence H.
Published: March 2016

The chemical compositions of external galaxies are usually found from their HII regions or composite absorption line spectra of their nuclei. In the Magellanic Clouds, however, individual stars are observable, but objects heretofore studied necessarily have been luminous la supergiants which border on the brink of instability. The image photon counting system on the Anglo-Australian Telescope makes it possible to observe fainter, more stable Ib supergiants. Energy distributions and Balmer line profiles for these stars can be fitted with theoretical predictions by Kurucz. A joint effort by J.E. Ross and B.J. O’Mara of the University of Queensland, Bruce Peterson of the Anglo-Australian Observatory, and a group at the University of California, Los Angeles to analyze three lb supergiants suggests that metals of the iron group are depleted by a factor of 2-3.5 with respect to the normal chemical composition of our own galaxy.
Rotation in Close Binary Stars
Authors: Plavec Mirek J.
Published: March 2016

In binary stars of short period, axial rotation of the components tends to be synchronized with the orbital revolution. Rotation of B. and A stars is therefore slowed down, while for the later-type stars, it is accelerated. This latter fact probably contributes to the phenomenon of the RS CVn stars. Notable deviations from synchronism among short-period systems are probably related to mass transfer between the components.

Binary stars play a double role in astrophysics. On the one hand, they reveal properties and structures that would otherwise be almost unobservable, and therefore help considerably to understand the character and evolution of single stars. On the other hand, they create objects and phenomena that otherwise would not exist at all, thereby actually creating more problems than they help to solve. I think that the first aspect makes the binary stars extremely useful; the other aspect makes them extremely exciting and interesting. This dual role also appears in connection with rotation. The “revealing” property is best documented by the fact that the phenomenon of stellar rotation was discovered in binary stars, through the “rotation effect” distorting the radial velocity curve of an eclipsed star. A more recent example is the probable identification of a star considerably flattened and distorted by differential rotation, in agreement with the theoretical models advanced by Bodenheimer and Ostriker (1970). The star is the secondary component of BM Orionis, which is the faintest of the four stars forming the famous Orion Trapezium. BM Orionis was once suspected of harboring a black hole, since during the apparently total eclipse of the primary B3 star, is spectrum remained visible and no other spectrum could be detected (Doremus, 1970). A higher dispersion of our Lick spectra permitted us (Popper and Plavec, 1976) to detect weak lines of the secondary star, which is most likely a late A giant still contracting to the main sequence. The flat bottom of the light curve at the primary eclipse is best explained by a model first suggested by Hall (1971), involving a disk-shaped, differentially rotating star. It would be difficult to recognize the shape of such a star if it were not member of an eclipsing system.
Polarimetry of Emission Lines as a Probe of Stellar Winds
Authors: McLean Ian S.
Published: March 2016

The motivation for obtaining optical polarization data of close binary systems and/or stars exhibiting stellar activity (i.e., flares, winds, chromospheres, spots, etc.) is that polarization is not only dependent on the physical condition of the stellar environment but is actually rather sensitive to geometric factors and can act as an excellent diagnostic of the latter.

High resolution, multichannel spectropolarimetry of certain emission-line stars was demonstrated to be a powerful new technique in studying the physics and kinematics of stellar envelopes. Two sources of polarization were discussed viz., (l) fluorescent (resonant scattered) emission lines similar to those found in the solar chromosphere and corona, which can be highly polarized and sensitive to magnetic fields, and (2) Doppler broadened emission lines formed in a rotating/expanding stellar wind. The Doppler broadening effectively provides spatial resolution, so that as one scans a line polarimetrically, scattering from different regions of the stellar wind is observed according to its kinematical motion.
The Distance of the Virgo Cluster
Authors: Hanes David A.
Published: March 2016

Until the discovery (Corey & Wilkinson, 1976) of the anisotropy of the cosmic microwave background, the Virgo cluster represented something like a Rosetta Stone for many observational cosmologists: in the absence of a significant peculiar velocity component for the Local Group in the direction of the Virgo cluster, its distance, accurately measured, might reveal the global expansion rate and the Hubble age. Although this simple picture has changed, the distance of the Virgo cluster remains important, partly for a sharper understanding of the properties of rich clusters and the galaxies they contain, but more importantly (for my purposes here) as an interesting distance over which we may test various constructions of the extragalactic distance scale.
The Tully-Fisher Relation and its Application to the Distance Scale
Authors: Aaronson M.
Published: March 2016

The Tully-Fisher relation applied in the infrared appears to be the best global distance indicator presently available for determining the expansion rate and deviations from uniform Hubble flow. In this article recent results obtained using the IR/H I method are reviewed. A Virgo-directed Local Group velocity of about 300 km s–1 is indicated (implying a local value for the deceleration parameter qo ~ 0.05 – 0.1) along with a “best guess” value for the Hubble Constant of 85 km s-1 Mpc-1.
Zodiacal Light: A Probe of the Properties and Evolution of Interplanetary Dust
Authors: Schuerman† D. W. , Weinberg J. L.
Published: March 2016

A brief discussion is presented on the use of zodiacal light observations to derive information on the optical and physical properties of interplanetary dust and its spatial distribution.
Optical, Infrared and Ultraviolet Observations of Active Nuclei
Authors: Longair M.S.
Published: March 2016

I have been invited to present a brief review of optical, infrared and ultraviolet observations of active nuclei. I will only describe a small sub-set of these observations, in particular those which I believe are most likely to be important in elucidating the properties of the central energy source to which we attribute the origin of all high energy phenomena. I will therefore restrict my remarks to certain aspects of BL Lac objects, optically violently variable quasars, quasars, Seyfert I galaxies and broad line radio galaxies. If we could understand these systems, we should be able to account for most other types of active nuclei, the Seyfert II galaxies, narrow line radio galaxies, all other types of Markarian galaxy, compact blue galaxy, etc. In my view, the recent observations of NGC 4151 provide us with a benchmark against which we should measure all other observations of active nuclei.
Uv Galaxies
Authors: Ye Khachikian E.
Published: March 2016

The systematical statement of the concept of the activity of the nuclei of galaxies was first made public by Ambartsumian about 25 years ago (Ambartsumian 1958, 1962, 1965). Since 1965 he has been advancing and developing the idea of the basic role of the nuclei of galaxies in their life and evolution.

According to Ambartsumian the “activity” of the nuclei of galaxies manifests itself mainly in the following forms:

1.

Outflow of ordinary gas matter (in form of jets or clouds) from the nuclear region at the velocity of up to hundreds of kilometres per second.

2.

Continuous emission of the flux of relativistic particles or other agents, producing high energy particles, as a result of which a radio halo may form around the nucleus.

3.

Eruptive ejections of gas matter (M82 type).

4.

Eruptive ejections of concentrations of relativistic plasma (NGC 4486, 5128, etc.)

I should like to emphasize on the form of activity which says:

5.

Ejection of compact blue condensations with an absolute magnitude of the order of luminosity of dward galaxies (NGC 3561, IC 1182). Here the division of the nucleus into two or more comparable componentsis also presumed; initiating the formation of multiple galaxies.
Radio Observations of Giant Extragalactic HII Regions
Authors: Heeschen D. S.
Published: March 2016

Giant extragalactic HII regions are found in the disks of nearby spiral and irregular galaxies, in the nuclear regions of spiral and elliptical galaxies, and in a variety of peculiar and interacting systems. At radio wavelengths they may emit thermal continuum radiation from the ionized gas and/or nonthermal synchrotron radiation if high energy electrons and magnetic fields are present. In some instances line radiation from associated molecular and neutral hydrogen clouds may also be detected. Table 1 illustrates the sorts of objects in which radio HII regions are observed and indicates the range of radio parameters found. Columns 1 and 2 give the galaxy and specific HII regions within the galaxy. Column 3 is the adopted distance of the galaxy. Column 4 indicates whether the emission is thermal (T) or nonthermal (N-T); column 5 is the 20 cm luminosity, and column 6 the linear size of the radio emitting region. For thermal sources the electron density, derived from the luminosity and size, is listed in column 7. The final column gives references. Note that almost all of the observational information obtainable from radio continuum observations is contained in columns 4, 5, and 6. Very occasionally there may be additional data concerning polarization or variability.
Giant H II Complexes Outside Our Galaxy at Optical Wavelengths
Authors: Rosa Michael
Published: March 2016

By definition giant H II complexes are luminous sources in the spectra of thermal photoionized interstellar gas. Therefore they are easily detected out to large distances. As a consequence the list of objects is vast and it is impossible to cover in detail all the results brought together by an increasing number of investigators. For more complete bibliography the reader is referred to the reference lists in the papers quoted. Let me line out here instead a number of aspects which have received considerable attention or which are still subject to major improvements.
Mass Outflow in Molecular Clouds: A New Phase in the Evolution of Newly-Formed Stars?
Authors: Genzel R. , Downes D.
Published: March 2016

Molecular gas with velocity dispersions exceeding 100 km s-1 toward dense, interstellar clouds was discovered by microwave observers about a decade ago. It has now been established that this high velocity gas is a result of violent mass outflows (winds) originating from T-Tau stars, compact infrared sources or ultra-compact HII regions at the cores of the clouds. The mass outflow phenomena occur in regions of star formation covering five orders of magnitude in luminosity, and are of long duration . The observations suggest that violent mass loss is a new, important phase in the pre-main sequence evolution of newly-formed stars more massive than a few solar masses. The impact of the flows on the energy balance and dynamical stability of molecular clouds may be substantial. This review is mainly a summary of the observational facts, and gives a description of the physical conditions in an outflow zone. The impact of the flows on molecular clouds is discussed. Recent reviews of the outflow phenomena can also be found in (1, 2, 3, ).
Large-Scale Radio Jets
Authors: Laing R.A.
Published: March 2016

The purpose of this review is to outline the systematic properties of radio jets on kpc scales, as derived from the basic observations of surface brightness and linear polarization and to emphasize the uncertainties in the determination of their physical parameters. These results come primarily from observations of about 100 jets with the VLA: a fuller account is given by Bridle (1982) and the proceedings of IAU Symposium 97 contain many illustrations and references, which must be omitted here.

I take a “jet” to be a feature in the radio brightness distribution which is at least four times as long as it is wide, which can be clearly separated (spatially or by brightness contrast) from the rest of the source and points away from a radio core. Wilson (1982) has considered jets in spiral galaxies and I shall discuss only the more luminous jets found in elliptical radio galaxies and quasars.
Women in Astronomy - Italy
Authors: Padrielli L.
Published: March 2016

Let me start with a short historical excursion, taking the Bologna University as an example. The Bologna University was founded in 1088, but only at the beginning of 1700, when a deep transformation in the tradition and female behaviour model occurred, women started to approach the academic life mostly in humanities. There were also examples of scientist women, often
without a real academic title working side by side with men (generally fathers or husbands).

During the 19th century the female presence in the italian universities slowly increased, becoming a reality at the beginning of the 20th century. In the time interval from 1884 to 1900, 224 degrees were assigned to women in Italy (less than 10% of the total): 68.9% in Literature and Philosophy,7.8% in Mathematics, 11.7% in Natural Science, 9.3% in Medicine, and 2.3% in Law. Women were mostly involved in fields related to educational activities, however six out of 224 got a chair at the Universities, five of which in scientific fields.
Status of Women Astronomers in China
Authors: Shuhua Ye
Published: March 2016

The percentages of women working in Astronomy including research and technical positions are as follows:

If we look at the percentages of female students:

It is easy to see, that since fewer women have the chance of higher education, it is not a surprise that fewer women can reach to the higher level in astronomy.

Social discrimination and prejudice have been the obstacles for women to get higher education and choose a career in astronomy. For example:

—

1. Home influence. A girl from her early childhood is educated to play with dolls and help in the kitchen.

—

2. Marriage. In the view of public, a man should not choose a wife with higher education nor in a higher position as compared to him. And thus, well educated women have less chance to get married.
Are the Opportunities in Astronomy and the Working Conditions Different for Women and Men?
Authors: Burbidge E. Margaret
Published: March 2016

As the oldest speaker at this Joint Discussion, my lifetime in astronomy spans a larger number of decades than the lives of most of us here, and consequently I have seen great changes in the opportunities, working conditions, status, and rewards for their work that are now available to women, in comparison with the situation as it was 50 years ago. Nevertheless, one has to admit that the answers to two of the questions we have been asked to address have to be a qualified yes: there is still a problem, and there is need for improvement.

The advances that we have seen, especially in the last 20 years, have been recognized throughout the worldwide community as being of immense benefit to astronomy. It is also recognized by the general public that the situation for women in astronomy is much different now from what it was in my early years.
Opportunities and Working Conditions
Authors: Griffin Elizabeth
Published: March 2016

It is impossible to pose, explain and solve in a mere 15 minutes a complex puzzle like this that has been with us for centuries. There are so many different facets; it is hard to focus on a problem that has no nucleus and to summarize a situation that has neither beginning nor end, and some of you are therefore bound to disagree with at least some of what I say.

I singled out the three questions, “Is there a problem?”, “Is there need for improvement?” and “What are the possible ways to improve the situation?”, because each seems to beg the next and all three can therefore be tackled together. The official answer to the first question is “No, there is no discrimination, and no Problem”, because the large majority of institutions declare themselves Equal Opportunity Employers, and statistics show that women are usually appointed to positions in at least the same proportion, relative to their malepeers, as they apply. The nagging worry is why they do not feel able to apply in the sort of numbers that reflect their population. This is more than a social problem; anyone who counts the cost of educating his or her daughter to the age of 25 will know that it has an economic side too. And the reality of the Problem? I attended the 1992 Baltimore meeting on “Women in Astronomy”, and for the first of the two days the 175 or so women in the 200-strong audience simply let off steam, flung mud, aired grievances, and demonstrated that a Problem was very much in evidence; everyone had one, in some disguise - except, possibly, those who occupied the seats on the Panel the next day and answered the political questions. But put 175 men together in a free-for-all, and they would probably discuss .... maybe how best to spend the next $M100 on space astronomy.
Two-tier system of astronomical constants
Authors: Lieske J.H.
Published: March 2016

Conventional astronomical standards are applied to a wide range of applications, many of which do not require state-of-the-art precision. Stability of the standards is of great importance, although there is a trend by some in the astronomical community to promote frequent revision of the values of astronomical constants. It is possible to employ stable standards and to differentially correct them by use of the IERS formulation when very precise observations require such action.

The IERS formulation for the transformation from a fixed frame to an of-date frame involves precession and nutation along with two parameters δψ and δ which can represent modifications to the standard precession and nutation formulation. The transformation from the mean-of-date frame to the true-of-date frame in the IERS formulation using rotation matrices is rtrue = NrMean where

N = R1(−εm − Δε − δε) R3 (−ΔΨ − δΨ) R1(ε)m

where εm is the mean obliquity, ΔΨ and Δε are the nutations in longitude and obliquity, and where δψ and δε are IERS parameters which allow for changes to the IAU system. They are tabulated in the IERS reports and are generally thought of as observationally-determined values, but the IERS formulation can be used to introduce corrections of an arbitrary nature.
Observations of Luni-Solar Precession
Authors: McCarthy D.D. , Luzum B.J.
Published: March 2016

The observations of dψ and dε used in this analysis were taken from the combination solution of the IERS Sub-Bureau for Rapid Service and Precition (McCarthy and Luzum 1991a). Besides corrections to the coefficients determined from VLBI, additional estimates can be derived from lunar laser ranging (LLR) observations (Williams et al. 1991; Whipple 1993). Table ?? shows a comparison of the estimates of the change in longitude and obliquity derived in this analysis with corresponding terms from analyses by other authors using different observations. The theoretical value of obliquity by Williams (1994) is also included for comparison. There are significant unexplained discrepancies among the rate estimates. Difference in the methods of the analyses such as the procesure for the estimation of the nutation coefficients and correlations within the solutions probably account for the greater part of these discrepancies. In considering the adoption of changes in the IAU model for nutation, it is important to recall that changes must also be made in the precession constant. Introduction of changes in nutation without corresponding changes in precession will not improve the agreement between observations and theory.
Astronomical Units and Constants in a Relativistic Framework
Authors: Capitaine N. , Guinot B.
Published: March 2016

In 1991, IAU Resolution A4 introduced General Relativity as the theoretical background for defining celestial space-time reference sytems. It is now essential that units and constants used in dynamical astronomy be defined in the same framework, at least in a manner which is compatible with the minimum degree of approximation of the metrics given in Resolution A4.

This resolution states that astronomical constants and quantities should be expressed in SI units, but does not consider the use of astronomical units. We should first evaluate the usefulness of maintaining the system of astronomical units. If this system is kept, it must be defined in the spirit of Resolution A4. According to Huang T.-Y., Han C.-H., Yi Z.-H., Xu B.-X. (What is the astronomical unit of length?, to be published in Asttron. Astrophys.), the astronomical units for time and length are units for proper quantities and are therefore proper quantities. We fully concur with this point of view. Astronomical units are used to establish the system of graduation of coordinates which appear in ephemerides: the graduation units are not, properly speaking astronomical units. Astronomical constants, expressed in SI or astronomical units, are also proper quantities.
Expressions for Precession Formulae
Authors: Bretagnon P.
Published: March 2016

In 1976, the IAU recommended for the value of the precession constant 50".290 966 per Julian year and, since 1984, most of the ephemerides have used the formulas of Lieskeet al.(1977). In this paper, the motion of the ecliptic was based on Newcomb’s theory. The ecliptic was defined with an accuracy of 0".3 over 1000 years and of 6 mas over one century around J2000. For several years, new determinations of the precession constant have showed that this value is too high by about 3 mas per year. For instance, Williamset al.(1991) give 50".2882 per year. From this value and from the mean elements defining the ecliptic given by Simonet al(1994), we have built new expressions for precession formulae. The motion of the mean ecliptic is described through variablesqandp:wheretis the time reckoned in thousands Julian years from J2000. This solution has an accuracy of 1 mas over several thousands years and 0.1 mas over one century around J2000. The variations of the equator are obtained by integration of the differential equations of the precession in longitude and of the obliquity in Kinoshita (1977) and Kinoshita and Souchay (1990). The dynamical ellipticity is determined in order to obtain (dP/dt) = 50 288".2 att= 0 asHd= 0.003 273 7752. From this value forHd, we can integrate the differential equations forPand ε:
Modulation of the Cosmic Radiation and its Manifestation in Cosmogenic Isotopes
Authors: Beer J.
Published: March 2016

An issue of increasing importance in solar physics and climatology is solar variability (Sofia and Fox, 1994). It seems that most solar type stars show some magnetic variability at the surface (Baliunas and Jastrow, 1990). To understand the underlying processes and to model them is a big challengein solar physics.

When satellites began to observe the sun outside the disturbing atmosphere it soon became clear that the solar irradiance is also variable and seems to be related to the 11- year Schwabe cycle (Willson, et al., 1986), (Foukal, this volume). This result is of great importance because the sun is the engine which drives the climate system on earth. Although the variations are small (0.1-0.2%) there is clear evidence from observational data of solar type stars (Baliunas and Jastrow, 1990) and from theoretical considerations as well that much larger fluctuations over longer time scales potentially could occur.

This raises the question what role the sun plays in today’s climate change and possibly in the near future. There are basically two approaches to address this question. 1. The solar irradiance and the climate have to be monitored continuously with high precision in order to detect changes and to understand how they are caused. 2. Since there are reasons to believe that the sun also exhibits long-term changes which cannot be detected during short periods of direct observations, one has to investigate the past by looking for connections between climate and solar variability.

To reconstruct past climate changes is comparatively simple. There is a large amount of climatic observations available for the last 2-3 centuries. The climatic conditions of earlier times can be reconstructed quite reliably based on isotopic ratios, pollen assemblages, and many other parameters measured in natural archives.
p-mode frequency corrections due to convection
Authors: Stix M.
Published: March 2016

Convection consists of rising hot and descending cool parcels of gas. In order to assess the effect of such an in homogeneity on acoustic waves a simple model has been proposed by Zhugzhda and Stix (1994). The model consists of a sequence of alternating vertical layers with temperatures T1 and T2 and upward and downward velocities V1 and V2. At the interfaces between the layers the horizontal component of the velocity and the pressure are continuous. This model allows to determine the phase velocity of a vertically propagating acoustic wave. The main result is that, with increasing frequency, this phase velocity approaches the sound speed of the cooler layers. The reason of such a behavior is that the horizontal structure of the wave is oscillatory in the cool layers, but exponential (evanescent) in the hot layers, so that there is a certain amount of wave trapping in the cool layers. An analogous effect of trapping in a horizontal layer has been described by Kahn (1961) for the temperature minimum of the solar atmosphere.
Asteroseismology of white dwarf stars
Authors: Nather R.E.
Published: March 2016

Perhaps the hardest scientific problems to solve are those most scientists believe have already been solved, but which have not. Any model we make of an astronomical process is doomed to be incomplete at some level; the “broad-brush” picture of stellar structure and evolution is often accepted as a solved problem, but in fact many discrepancies exist between our models and the observations.

For example, the “standard solar model” fails to predict the observed neutrino flux from the Sun, and its insistence that the Sun’s luminosity increased by 30% over geological time is not in accord with the evidence. White dwarf stars were thought to be stable against pulsation because the models did not pulsate, and it took a long time to improve them to the point where they did, and could then begin to match our observations of pulsating white dwarfs. For several years their outer hydrogen layers were thought to be extremely thin because models with thicker layers did not pulsate; once the zoning problem was found and solved in the models they could begin to match observations better. These past difficulties illustrate a common danger: it is very easy to confuse our models with the observed reality they try to mimic, and to assume they match better than they do. In what follows I will try to keep the observations and our models of them separate.
Recent Advances in Convection Theory and Modelling
Authors: Sofia S.
Published: March 2016

This Joint Discussion (Number 13), took place on August 22, 1994 at The Hague, in connection with the XXII General Assembly of the IAU. At the one-day long meeting, there were presentations by 15 invited speakers and 15 posters.

The Joint Discussions had been organized in response to the considerable progress made in this field of research during the previous decade. Although it had long been known that the prevailing mixing length theory (MLT), used extensively and very successfully in Astrophysics for several decades had become needlessly limited, until recently it was impractical to contemplate more realistic approaches. The situation has changed recently as a consequence of advances in numerical techniques and computational capabilities, and thus JD 13 was organized to discuss the advances, and perhaps to understand the strengths and weaknesses of each approach.

There were two presentations which addressed the main issues in convection theory (E. Schatzman), and the astrophysical implications (P. Demarque). Several talks covered current numerical codes, which included deep convection in a rotating reference frame (K. Chan), convection in the presence of magnetic fields (P. Fox), and shallower solar convection simulations on a wide range of spatial scales (A. Nordlund). Although these approaches have enriched (and are continuing to enrich) our understanding of the physics of convective fluids, they are much too detailed (both in space and in time) to be integrated in the study of stellar evolution. To overcome this shortcoming, S. Sofia described a technique developed together with Lydon and Fox to use relationships between dynamical and thermodynamic properties of convective flows derived in numerical models to be applied in stellar structure and evolution codes by performing small modifications of the standard MLT formalism. The advantage of this technique is that it does not contain a mixing length or any other arbitrary parameter, and it was used successfully in modeling the evolution of the Sun and other solar analogues. V. Canuto also presented a formulation of convection both amenable to be used in stellar evolution studies, and not requiring an arbitrary mixing length-like parameter. His formulation uses the Reynolds stress method, which has the advantage of modeling the full eddy spectrum of the turbulence, rather than the narrow wave number range for energy containing eddies assumed in the MLT. Additionally, this technique can address the problems of non-locality and overshoot. M. Stix also addressed non-locality and overshoot by presenting results of a non-local mixing length model of the Sun derived from the Shaviv and Salpeter model.
Lithium Abundances of Solar-Type Stars in Open Clusters
Authors: Soderblom D.R.
Published: March 2016

This paper discusses the lithium abundances of late-F through mid-F dwarfs in the nearby open clusters. That means that I am considering young disk stars, of course, with metallicities similar to the Sun’s. I will not discuss the chasm seen among mid-F dwarfs. Li is not easily studied because only one spectroscopic feature is available and that feature is often weak. Thus high spectroscopic resolution and good signal to noise are needed. But Li is still the most easily studied of the light-element pentateuch (D, He, Li, Be, B).

The few clusters now accessible to large telescopes at high spectroscopic resolution include the Hyades, Pleiades, a Persei, as well as smaller samples in a few others (such as Coma, M67, NGC 752, the Ursa Major Group). The samples within clusters are often substantial, and allow one to examine differences in Li between stars of the same mass, age, and composition. However the total number of clusters observed is still small, so one must assume that each cluster is fully representative for its age and composition.

Much recent work on Li in stars has been for Population II objects because of the importance of Li for cosmology. We study Li in Pop I stars for three reasons: First, to understand how the Galaxy has come to be enriched in Li since the Pop II stars formed. Second, to understand the stellar physics behind Li depletion in Pop I stars as a means to understanding what happens in the outer layers of such a star over time. Third, to apply that understanding of the stellar physics to Pop II stars so we can correctly infer the Big Bang Li abundance.
Lithium in Giants
Authors: Balachandran Suchitra
Published: March 2016

In the standard stellar model, lithium burning does not occur during the first-ascent giant branch phase. The surface lithium abundance in a first-ascent or clump giant is thus the product of the lithium abundance of its main sequence progenitor and subsequent dilution by the deepening convective envelope. Iben’s (1967a, b) early calculations predicted that the dilution factor would vary between a factor of about 28 in a 1 M⊙ star to a factor of 60 in a 5 M⊙ star. More recent calculations with improved physics show remarkably little change in these predicted dilution factors (Pinson-neault, pvt. comm.). Since Li only survives in the outer 2-3 % by mass of a main sequence star, the maximum redgiant ∼ abundance is predicted to be ∼ log e(Li)=1.5 to within a factor of two. Even lower abundances will result if surface Li is destroyed during the main sequence phase. Lithium depletion on the main sequence has been well documented in stars of spectral type mid-F and later and have been explained by a variety of non-standard mechanisms (e.g., rotation, diffusion and turbulence). Mixing induced by such mechanisms is not expected in high-mass stars (M⊙ > 1.5) which have insignificant surface convection. Yet recent observations reveal that non-standard processes may be at work in these stars as well.

Lithium abundances far below the predicted maximum value are seen in high-mass cluster giants, and in some clusters large spreads in Li are observed in giants of roughly the same mass (Gilroy 1989). If, in concordance with the standard-model prediction, one assumes that Li burning on the giant branch is unlikely, then both the destruction and the scatter of Li must be accounted for on the main sequence. Since the surface convection zones are insignificant, other forms of Li-loss (e.g. mass loss, meridional circulation) may have to be resorted to. Unfortunately the main sequence progenitors of most of these giants are too hot to have a measurable Li I feature, but observations of field late-A stars by Burkhart and Coupry (1991) reveal some evidence for surface Li-loss. Follow-up observations of Li inclusters, and Be and B abundance determinations would be useful. Lithium burning on the giant branch should be investigated theoretically.
Chemical Abundances from Planetary Nebulae in the Magellanic Clouds
Authors: Barlow M.J.
Published: March 2016

Heavy element abundances, in particular those of oxygen, obtained from recent spectroscopic surveys of Magellanic Cloud planetary nebulae (PN), are reviewed and compared with those derived for H regions and objects in our own galaxy. These abundances have been based on collisionally excited lines and are very sensitive to the adopted electron temperature. There is increasing evidence that temperature or density fluctuations within nebulae lead to the electron temperatures being overestimated, with the corollary that the heavy element abundances have been underestimated.
Challenges of Using Planetary Nebulae for Extragalactic Distances
Authors: Bottinelli L. , Gouguenheim L. , Paturel G. , Teerikorpi P.
Published: March 2016

The planetary nebulae (hereafter PN) distance criterion (Ciardullo et al. 1989a) relies on the shape adopted for the luminosity function (hereafter LF) and its adequate fit to the observed data. Crucial hypothesis concerning the LF are its universal shape with an universal value of the absolute magnitude cut-off and concerning the sample, its completeness and the selection of the rejected data.

Because the completeness is achieved only in the bright part of the LF, the brighest PN play a crucial role in the distance determination and it is well known that the brightest objects (with intrinsic scatter) in a giant galaxy are brighter than in a dwarf galaxy. Thus more luminous galaxies are given smaller distances. Because giant galaxies are rare they are not seen at small distances; on the contrary, they are typically observed at large distances. This effect induces a progressive compression of the distance scale.
Observations of Gas in Interacting Galaxies
Authors: Combes F.
Published: March 2016

This is a brief review of the recent progress made on this domain, earlier work is nicely summarised either in the review of Barnes & Hernquist (1992a), or in Proceedings of Rencontres de Moriond in spring 1992 (e.g. Thuan et al 1993), or of Lexington Conference in spring 1993 (e.g. Shlosman 1994).
The Circumnuclear Region in NGC 4321
Authors: Knapen J.H. , Beckman J.E. , Shlosman I. , Heller C.H. , Peletier R.F. , De Jong R.S.
Published: March 2016

We present results from a recent study of the inner 1 kpc zone of the weakly barred spiral galaxy NGC 4321 (M100), which reveals a circumnuclear starburst, organized in a pair of tightly-wound spiral arms. Combining high-resolution imaging in the optical and near-IR with a detailed dynamical model, we show that the morphology of this region can be explained by a resonance interaction between the disk and the stellar bar and its effects on the gas.
The OPAL opacity Code
Authors: Rogers F.J. , Iglesias C.A.
Published: March 2016

The OPAL opacity effort was undertaken in 1985 in response to speculation that the existing opacity data were not of sufficient accuracy to model observed stellar properties. We have taken the view that a completely new code with improved equation of state, atomic physics and line broadening was required. It was apparent that the most computationally intensive part of the calculations would be to obtain the vast amount of atomic data needed for the bound-bound and bound-free absorption cross-sections. To meet this challenge we developed a parametric potential method, that was fast enough to allow on-line calculations, while achieving accuracy comparable to that of the single-configuration Dirac-Fock method. This on-line capability was also chosen to allow flexibility to study the effect of various coupling and data averaging methods. It also makes it easy to study the effect of adding more elements.

The OPAL equation of state is based on the many-body statistical mechanics of partially ionized plasmas in the grand canonical ensemble. This is the so-called “physical picture” method. In this approach one works directly with the electrons and nuclei present in the plasma. The effect of the plasma environment on the internal states is obtained directly from the statistical mechanical analysis. A convergent partition function is a natural consequence of this approach.

OPAL includes degeneracy and plasma collective effects in the free-free absorption using a screened form of the parametric potentials. Similar corrections to the Thomson scattering are obtained from the method of Boercker (1987). The spectral line broadening for one, two and three electrons ions are obtained from a suite of codes provided by R.W. Lee (1988) that include linear Stark theory. For all other transitions we use Voigt profiles where the Gaussian width is due to Doppler broadening and the Lorentz width is due to the natural width plus fits to the electron impact collision width (Dimitrievic and Konjevic 1980).