bluuebunny/crossref_metadata_2025 huggingface dataset:
Outflows from Newborn Multiple Stars
Authors: Reipurth Bo
Published: July 2016

Analysis of a sample of giant Herbig-Haro flow sources shows that 79%–86% are binaries or higher order multiples. This represents the youngest sample of stars studied so far for binarity. A stellar dynamics jet hypothesis is proposed in which the dynamical decay of triple or multiple systems leads to giant outflow activity. Close triple approaches will cause serious perturbations and probably direct collisions among individual circumstellar disks, with a consequent burst of outflow activity, which can produce giant HH bow shocks. As one component is ejected, the two remaining stars and their small truncated disks form a closer bound pair with high eccentricity. Gas streams from a circumbinary disk feed the stars and this as well as other dynamical effects cause the binary orbit to shrink. As the stellar components gradually spiral towards each other, accretion and outflow becomes cyclic, modulated on an orbital time scale. The resulting HH flows can be read as a fossil record of the evolution of orbital motions of the newly formed binary as it shrinks from a typical separation of 100 AU or more to 10 AU or less. After a triple disintegration event, both components (star and close binary) leave their nascent envelope, and while one component becomes visible as a T Tauri star, the other will be obscured for a while by the envelope and will appear as a bright near-infrared object, thus explaining the socalled IRC binaries which are infrequently found in star forming regions.
Disks in Young Binary Systems: Unresolved Millimeter-Wave Observations
Authors: Jensen Eric L. N.
Published: July 2016

Observations at millimeter and submillimeter wavelengths are sensitive to the total mass of circumstellar and circumbinary dust in a multiple system, and in some cases single-dish observations can help constrain the location of disk material in spite of their lack of spatial resolution. Young binary stars show a great diversity of disk properties, with a large part of the variation accounted for by binary separation. Many of the closest binaries (those with separations a less than a few AU) harbor massive circumbinary disks. Binaries with a ≳ 100 AU tend to have massive circumstellar disks. In both cases, the properties of these disks (as deduced from millimeter and infrared fluxes) are indistinguishable from those around single stars. In the intermediate separation range (10 ≲ a ≲ 100), however, while disks do exist in most binaries, they are strongly limited by the presence of stellar companion, with inferred dust masses of order an Earth mass. While comparison of sample properties is secure, calculating masses in individual systems is limited by the uncertainty in dust opacity and surface density distribution laws (as in single stars), with the additional complication of the uncertain disk geometry in the system.
Pre-Main Sequence Binaries with Aligned Disks?
Authors: Wolf Sebastian , Stecklum Bringfried , Henning Thomas
Published: July 2016

We present the results of a study performed with the goal to investigate whether low-mass pre-main sequence binary stars are formed by multiple fragmentation or via stellar capture. If binaries form preferentially by fragmentation, we expect their disks to be co-planar. On the other hand, the capture scenario will lead to a random distribution of disk orientations. We performed near-infrared polarization measurements of 49 young visual binary stars in the K band with SOFI at the NTT. The near-infrared excess radiation of the targets mostly point to the presence of disks. For a major fraction of the sample, evidence for disks is also obvious from other features (outflows, jets, Herbig-Haro objects). We derived the disk orientation from the orientation of the polarization vector of both components of each binary. This statistical study allows to test which hypothesis (co-planarity, random orientation) is consistent with the observed distribution of polarimetric position angles. We find evidence that the disks are preferentially aligned.
Correlations of Spectral Accretion Signatures in Young Binaries
Authors: Prato Lisa , Monin Jean-Louis
Published: July 2016

We review correlations of spectral accretion diagnostics in young binary star systems. Hydrogen emission lines at visual (Hα) and near-infrared (Brγ) wavelengths indicate the presence of active accretion flows onto young stars. We examine the simultaneity of this process in both components of close binaries (separations 100–1500 AU) and find that active accretion is not a random process in these systems: the two stars are typically both active or both inactive. Only 19% of our sample consists of mixed pairs in which just one component displays evidence of accretion, usually the primary. In systems with two active stars, the equivalent width and in particular the line flux of the primary star is generally dominant, indicating stronger accretion activity. We discuss accretion and disk dissipation processes in close binaries and propose that a circumbinary envelope model accounts for the correlated evolution of circumstellar disks around young binaries.
First Results from a STIS Survey of Close Pre-Main Sequence Binaries in Taurus
Authors: Kenyon Scott J. , Hartigan Patrick
Published: July 2016

We describe preliminary results of an HST survey of pre-main sequence binary stars in the Taurus-Auriga cloud. We use STIS medium and low resolution spectra to derive the reddening, spectral type, and veiling for each component of close pairs with angular separations ≲ 1″. This sample yields a frequency of mixed pairs, containing one classical and one weak emission T Tauri star, similar to that of wide pre-main sequence binaries. The components of most pairs are coeval, but the secondaries in four pairs are significantly younger than the primaries for the three sets of evolutionary tracks we considered.
Filament Fragmentation
Authors: Inutsuka Shu-ichiro , Tsuribe Toru
Published: July 2016

The formation and evolution processes of magnetized filamentary molecular clouds are investigated in detail by linear stability analyses and non-linear numerical calculations. A one-dimensionally compressed self-gravitating sheet-like cloud breaks up into filamentary clouds. The directions of the longitudinal axes of the resulting filaments are perpendicular to the directions of magnetic field lines unless the column density of the sheet is very small. These magnetized filaments tend to collapse radially without characteristic density, length, and mass scale for the further fragmentation during the isothermal phase. The characteristic minimum mass for the final fragmentation is obtained by the investigation of thermal processes. The essential points of the above processes are analytically explained in terms of the basic physics. A theory for the expected mass function of dense molecular cloud cores is obtained. The expected mean surface density of companions of dense cores is also discussed.
Measuring the Masses of Young Stars
Authors: Simon Michal
Published: July 2016

This paper presents the masses of young stars measured by the rotation of their circumstellar disks (Simon, Dutrey, and Guilloteau 2000). Their precision is good enough to enable meaningful tests of theoretical models of pre-main sequence evolution. The tests are however limited by the imprecision with which the distances to the stars are known. The astrometric instruments now being developed have the potential to remove this limitation.
A Relative Age Test of Pre-Main Sequence Evolutionary Models Based on the Young Quadruple System GG Tauri
Authors: White Russel J.
Published: July 2016

The components of the young hierarchical quadruple GG Tau, which span a wide range in spectral type (K7 – M7.5), are used to test both evolutionary models and the temperature scale for very young, low mass stars under the assumption of coeval formation. Of the evolutionary models tested which extend into the substellar regime, those of Baraffe et al. yield the most consistent ages when combined with a temperature scale intermediate between that of dwarfs and giants. The Palla & Stahler and Siess et al. models are also capable of yielding a coeval age down to their lowest mass (0.1 M⊙). These latter two models, which extend to much higher masses than the Baraffe et al. models, agree reasonably well with the Baraffe et al. models at 1.0 M⊙ and thus could be combined to construct a set evolutionary models that extends from Substellar to several solar masses.
A Dialogue on Dynamical Pre-Main Sequence Tracks
Authors: Wuchterl Günther
Published: July 2016

Based on the theory of stellar structure and evolution combined with the theory of stellar atmospheres theoretical properties of young stars can be calculated. These calculations of pre-main sequence evolution have been refined over the last decades and do now provide theoretical spectra and colours even for very cool objects like young stars brown dwarfs and planets. Two of their key assumptions must become invalid towards the formation phases: (1) the hydrostatic equilibrium of pressure forces and gravity that assumes stellar matter to be at rest and (2) the non-dependence on the initial thermal structure. The former (1) is violated by accretion- and collapse flows, the latter (2) because a new born young star is observed with the specific thermal structure produced by the cloud collapse. I discuss changes in the theoretical properties of young stars that follow from calculating the pre-main sequence evolution as the consequence of the collapse of Bonnor-Ebert spheres.
Unresolved Pre-Main Sequence Binaries in the HR Diagram
Authors: Hartigan Patrick , Kenyon Scott
Published: July 2016

We consider how positions of unresolved pre-main-sequence binaries in the HR diagram compare with the positions of their corresponding primaries and secondaries. As one might expect from simple considerations, the composites appear younger than their components, and have masses intermediate between those of the two stars. However, the amount of the age difference between the composite and its components ranges from nearly zero to as much as an order of magnitude depending upon the specifics of the system such as the reddening and relative spectral types of the objects. We present four examples that illustrate these issues.
The Effect of Unresolved Binaries on the Low-Mass IMF
Authors: Malkov Oleg , Zinnecker Hans
Published: July 2016

We have carried out a thought experiment in which we generate a random pairing of objects drawn from a pre-assumed single star power-law IMF, which we call the fundamental IMF. We show how the mass function of primary stars, secondary stars (if we could resolve them) and the mass function of the total mass of systems differ from the underlying fundamental IMF. We also compare our results with observations and conclude that the fundamental IMF of subsolar mass stars could be steeper than currently believed. In other words: the low-mass turn-over of the observed (“apparent”) IMF could be spurious, if the binary fraction of field stars is close to 100% (perhaps due to invisible companions).
Comparison between Extrasolar Planets and Low-Mass Secondaries
Authors: Mazeh Tsevi , Zucker Shay
Published: July 2016

This paper compares the statistical features of the sample of discovered extrasolar planets with those of the secondaries in nearby spectroscopic binaries, in order to enable us to distinguish between the two populations. Based on 32 planet candidates discovered until March 2000, we find that their eccentricity and period distribution are surprisingly similar to those of the binary population, while their mass distribution is remarkably different. The mass distributions definitely support the idea of two distinct populations, suggesting the planet candidates are indeed extrasolar planets. The transition between the two populations probably occurs at 10–30 Jupiter masses. We point out a possible negative correlation between the orbital period of the planets and the metallicity of their parent stars, which holds only for periods less than about 100 days. These short-period systems are characterized by circular or almost circular orbits.
Microlensing: A Tool to Probe Distant Binary Populations
Authors: Di Stefano Rosanne
Published: July 2016

Programs designed to observe gravitational microlensing are poised to provide a great deal of information about binary populations far from Earth, including those in the Galactic Bulge, in the Magellanic Clouds, in M31, and perhaps in other external galaxies. Because many millions of stars are monitored, microlensing observations allow us to study binaries in three ways: (1) when they are “involved” in a microlensing event (as either a lens or lensed source), (2) when variability due to binarity produces significant variations in the light curve, and (3) when light from a more distant star is attenuated or refracted by matter associated with the binary system (e.g., in a disk).

Microlensing observations will discover large numbers of binaries and planetary systems in a variety of galactic environments. Thus, comparative statistical studies of binary properties (distributions of mass ratios and orbital separations) are possible.

An intriguing sign that we have already begun to learn about binaries through microlensing observations comes from work indicating that all of the lenses detected to date may in fact be binaries. For observations along the direction of the Magellanic Clouds this would imply that, if the lenses are primarily located in the Halo, then MACHOs tend to be binaries. If, on the other hand, most of the lenses are located in the Magellanic Clouds, microlensing observations are giving us a unique way to explore a distant stellar population of binaries.
Adaptive Optics Imaging of Faint Companions: Current & Future Prospects
Authors: Close Laird M.
Published: July 2016

I briefly describe how diffraction-limited imaging with adaptive optics (AO) can detect low mass companions (young massive brown dwarfs for example). I review how current curvature AO systems can already detect point sources 1 million times fainter at separations of 3 arcsec in median seeing (0.65 arcsec). I show real examples of very faint companion detections made with the University of Hawaii AO system located at CFHT on Mauna Kea around the young (2 Myr) nearby (132 pc) Herbig Ae/Be star MWC480. Moreover, I show that the four faint (H=18–19 mag) companions within 6 arcsec of MWC480 (H=7.0 mag) are unlikely to be physical since they are non-common proper motion objects. I point out that the current 8–10m class AO systems will detect even fainter companions at closer separations with 0.03–0.06 arcsec NIR imaging.
Future Opportunities in Young Binary Star Research with Space Observatories
Authors: Stapelfeldt Karl
Published: July 2016

Ambitious infrared and astrometric space observatories are planned by both NASA and ESA for the first decade of the new millenium. I present a brief overview of their capabilities and suggest some of their likely contributions to research on young binary stars.
Summary and Reflections
Authors: Mathieu Robert D.
Published: July 2016

Let me begin by taking a very broad historical perspective on the study of pre-main-sequence binaries. You have seen reference several times at this meeting to that first paper on T Tauri binaries by Joy & Van Biesbroeck (1944). Following that paper there followed a long period that might be called the Decades of Ignorance. I refer here to a period of time in the study of star formation where we largely ignored the fact that we knew that most stars were binaries. The high frequency of binaries among stars is something that we've known for a very long time, well over a century. Indeed, looking back, it is remarkable that the paper in 1982 by Mel Dyck and his collaborators identifying T Tauri as a binary caused such a stir. This paper is sometimes cited as the birth of modern research into young binaries. But how many of our colleagues in the realm of main-sequence binaries had already told us over and over again that most of these young stars had to be binaries? Of course, the reason this important paper caused much ado was in part because T Tauri is the very prototype star of star-formation study.
The Optical Extragalactic Background Light from Resolved Galaxies
Authors: Pozzetti Lucia , Madau Piero
Published: July 2016

We discuss the ultraviolet to near-IR galaxy counts from the deepest imaging surveys, including the northern and southern Hubble Deep Fields. The logarithmic slope of the galaxy number-magnitude relation is flatter than 0.4 in all seven UBVIJHK optical passbands at faint magnitudes, i.e. the light from resolved galaxies has converged from the UV to the near-IR. Most of the galaxy contribution to the extragalactic background light (BEL) comes from relatively bright, low-redshift objects (50% at VAB ≲ 21 and 90% at VAB ≲ 25.5). We find a lower limit to the surface brightness of the optical EBL of about 15 nW m−2 sr−1, comparable to the intensity of the far-IR background from COBE data. Diffuse light, lost because of surface brightness selection effects, may add substantially to the EBL.
Observations of the Near-Infrared Extragalactic Background Light
Authors: Matsumoto Toshio
Published: July 2016

We searched for the near infrared extragalactic background light (IREBL) in data from the Near Infrared Spectrometer (NIRS) on the Infrared Telescope in Space (IRTS). After subtracting the contribution of faint stars and a modeled zodiacal component, a significant isotropic emission is detected whose in-band flux amounts to ~ 30 nWm−2sr−1. This brightness is consistent with upper limits of COBE/DIRBE, but is significantly brighter than the integrated light of faint galaxies. The star subtraction analyses from DIRBE data show essentially the same results apart from the uncertainty in the model of the zodiacal light. A significant fluctuation of the sky brightness was also detected. A 2-point correlation analysis indicates that the fluctuations have a characteristic spatial structure of 100 ~ 200 arcmin. This could be an indication of the large scale structure at high redshift. Combined with the far infrared and submillimeter EBL, the total energy flux amounts to 50 ~ 80 nWm−2sr−1 which is so bright that unknown energy sources at high redshifts are required.
Study of Galaxy Evolution Using VHE γ-ray Observations with Ground-based Čerenkov Telescopes
Authors: Konopelko A.
Published: July 2016

The spectral distribution of the extragalactic background light (EBL) in the infrared yields important information about the evolution of galaxies. The spectrum of a galaxy in the 0.1–200 μm region is a footprint of the intrinsic starlight at ~ 1μm and its extinction by dust with re-emission at ~ 100 μm. The overall spectral energy distribution of the EBL is then determined by the galaxy luminosity evolution. High-energy γ-rays are absorbed by the EBL photons through γγ → e+e- reactions. Such an effect has been seen recently in the Mkn 501 TeV spectrum measured by the HEGRA (High Energy Gamma Ray Astronomy) collaboration using an advanced system of imaging atmospheric Čerenkov telescopes (IACTs). The intrinsic spectra of AGNs in the 50 GeV-1 TeV energy range may be constrained by the X-ray fluxes measured with satellite instruments aboard missions such as RXTE, XMM, and ASCA. By reducing the energy threshold down to 50 GeV, forthcoming ground-based IACTs systems (CANGAROO IV, H.E.S.S., VERITAS) may be able to study the absorption cutoff in energy spectra of distant AGNs (z< 0.4), to unfold the true galaxy luminosity evolution function.
The Chemical Evolution of the Universe
Authors: Songaila Antoinette , Cowie Lennox L.
Published: July 2016

We still know relatively little about the local metal density outside of galaxies and clusters, but at high redshifts (z ~ 3) the advent of highly efficient high-resolution spectrographs on the new 8–10m telescopes has revolutionized the study of quasar absorption lines. This allows us to track the bulk of the baryons in the Universe, which are in highly observable form at this redshift, and to determine the metal density of the gas that contains most of the baryonic material. One of the more surprising results that has emerged is how uniformly the intergalactic gas at redshifts 2–4 is pervaded by metals, and we discuss current limits on such metal contamination of the low density gas. These measurements will be useful in understanding and calibrating feedback processes in early generations of small galaxies. Future work will track the metal distribution at higher redshift using bright z = 5 SDSS quasars.
Extragalactic Background Light, MACHOs, and the Cosmic Stellar Baryon Budget
Authors: Madau Piero , Haardt Francesco , Pozzetti Lucia
Published: July 2016

The optical/far–IR extragalactic background light (EBL) from both resolved and unresolved extragalactic sources is an indicator of the total luminosity of cosmic structures, as the cumulative emission from young and evolved galactic systems, as well as from active galactic nuclei (AGNs), is recorded in this radiation. This is a brief review of some of the implications of the observed brightness of the night sky for the stellar mass density and average metallicity of the universe today, and of the possible contribution of MACHO progenitors and QSOs to the EBL. Assuming a Salpeter initial mass function with a cutoff below 0.6 M⊙, a lower limit of Ωg+Sh2 > 0.0015 I60 can be derived to the visible (recycled gas + stars) mass density required to generate an EBL at a level of IEBL = 60 I60 nW m−2 sr−1. Our latest, ‘best–guess’ estimate is Ωg+sh2 ≈ 0.0023 I60, which implies a mean metallicity at the present–epoch of yZΩg+s/Ωb ≈ 0.2 Z⊙. If massive dark halos around spiral galaxies are partially composed of faint, old white dwarfs, i.e., if a non–negligible fraction (~ a few percent) of the nucleosynthetic baryons is locked in the remnants (MACHOs) of intermediate–mass stars forming at very high redshifts, then the bright early phases of such halos should contribute significantly to the observed EBL. Assuming a standard black hole accretion model for quasar activity and using recent observations of the quasar population and new synthesis models for the cosmic X-ray background, we estimate a present mass density of QSO remnants of ρBH ≈ 3 x 105 M⊙ Mpc−3 for a 10% efficiency of accreted mass–to–radiation conversion. The quasar contribution to the brightness of the night sky is IQSO ≈ 2 nW m−2 sr−1.
Star Formation and Chemical Evolution of Damped Ly-α Systems
Authors: Ma Jun
Published: July 2016

In this paper, we investigate the star formation and chemical evolution of damped Lyman-α systems (DLAs) based on the disk galaxy formation model developed by H. J. Mo, S. Mao, & S. D. M. White (1998, MNRAS, 295, 319). We propose that the DLAs are the central galaxies of less massive dark haloes present at redshifts z ~ 3, and that they should inhabit haloes of moderately low circular velocity. We adopt the empirical Schmidt law of star formation rates, and a closed box model of chemical evolution in which an approximation known as instantaneous recycling is assumed. In calculating the predicted distribution of metallicities for DLAs in our models, two cases are considered. One is that, using the closed box model, empirical Schmidt law, and star formation epoch, the distribution of metallicity can be directly calculated. The other is that, when the simple gravitational instability of a thin isothermal gas disk first discussed by A. Toomre (1964, ApJ, 139, 1217) is considered, star formation occurs only in the region where the surface density of gas satisfies the critical value — rather than everywhere in the gas disk. We first obtain the region where star formation can occur by assuming that the disk has a flat rotation curve and that the rotational velocity is equal to the circular velocity of the surrounding dark matter halo. We then calculate the metallicity distribution for case one. We assume that star formation in each DLA lasts for a period of 1 Gyr from redshifts z = 3. There is only one output parameter in our model, i.e., the stellar yield, which relates to the epoch of star formation. It is obtained by normalizing the predicted distribution of metallicity to the mean value of 1/13 Z⊙ as presented by M. Pettini, L. J. Smith, D. L. Kind, & R. W. Hunstead (1997, ApJ, 486, 665). The predicted metallicity distribution is consistent with current (rather limited) observational data. A random distribution of galactic disks is taken into account.
The Next Generation Space Telescope
Authors: Stockman H. S. , Mather John C.
Published: July 2016

The Next Generation Space Telescope (NGST) will be an 8 m deployable telescope, radiatively cooled to 30 K and diffraction-limited at 2 μm, operating at the Sun-Earth Lagrangian point L2. It will be built by a partnership of NASA, ESA, and CSA (Canadian Space Agency). The camera sensitivity should be limited by the zodiacal light for wavelengths < 10 μm. The main scientific objectives are the study of the origin and evolution of galaxies, stars, and planets, beginning with the first luminous objects to form from the Big Bang. Other objectives include studies of dark matter, supernovae, the intergalactic medium, gamma ray bursts, star ages, and exobiology. The telescope will be operated like the Hubble Space Telescope (HST) by the Space Telescope Science Institute, with all observing programs openly solicited and selected by peer review.

The NGST scientific requirements originated with the report of the Dressler Committee HST & Beyond. The instruments recommended by the Ad Hoc Science Working Group (ASWG) include 1) a wide field near infrared (NIR) camera with an 8K2 detector array covering 0.6 - 5 μm, 2) a multi-object NIR spectrograph capable of simultaneously observing > 100 objects with a resolution of R (λ/δλ) = 1000, and 3) a combined mid-infrared (MIR) camera and spectrograph from 5-27 μm, with a resolution of R > 1500.
The Planck Mission
Authors: Tauber J. A.
Published: July 2016

We present an overview of the European Space Agency's Planck mission, its scientific objectives, and the key elements of its technical design. The current programmatic status of Planck within ESA's Scientific Programme, implementation plans, and near-term milestones, are also addressed.
Elements in the Cosmos: Origin of the Light Elements
Authors: Reeves Hubert
Published: July 2016

In the first part of this paper, a review is given of the situation of the Big Bang nucleosynthesis of the nuclides D, 3He, 4He and 7Li, taking into account the latest experimental data (number of neutrino species, lifetime of the neutron) and theoretical developments (quark-hadron phase transition). In the second part. I review the process of Galactic Cosmic Ray formation of lithium, beryllium and boron throughout the life of the galaxy, taking advantage of recent measurements of Be and Li in iron deficient stars.
Solar Abundances: The Reference System
Authors: Grevesse N.
Published: July 2016

We summarize the recent critical compilation and analysis of the solar and meteoritic abundances by Anders and Grevesse (1989). We also update a few solar photospheric results and present a detailed discussion of the problem of the solar abundance of iron.
Mixing Due to Angular Momentum Transfer in Evolving Sunlike Stars
Authors: Demarque Pierre
Published: July 2016

The physical mechanisms that control the rotational history of stars like the Sun are reviewed, and the likely role of the associated rotational mixing is described. Results of stellar evolution calculations that include the effects of rotation are also described. The same theory, based on a single calibration for stars of different masses and metallicities, can successfully explain the observed Li and Be depletion in young open clusters and Li in field dwarf stars of the halo population, and the striking difference between metal-rich and metal- poor abundance patterns. There is evidence, both observational and theoretical, that angular momentum remains hidden in stellar interiors into advanced phases of evolution. Internal rotation also offers a natural explanation for the CNO isotopic anomalies observed among red giants. Finally, some of the questions raised by recent progress are considered.
A Brief Review of the s-process of Nucleosynthesis
Authors: Arnould M.
Published: July 2016

A brief overview is presented of the s-process of nucleosynthesis. Special emphasis is put on some of its nuclear physics aspects, and in particular on the uncertainties that still affect the rates of the 13C(α,n)16O and 22Ne(α,n)25Mg neutron producing reactions. We also briefly review the so-called “classical” s-process model, and put it in perspective with recent s-process calculations conducted in the framework of detailed models for massive stars or for thermally pulsing AGB stars.
The Hubble Constant: Present Status
Authors: Fukugita Masataka
Published: July 2016

The current status on the value of the Hubble constant is reviewed with the emphasis given to the origin of the discrepancy among authors. I argue that the situation is not too controversial and straightforward reading of observations indicates a high value (H0 ≃ 75 km s−1 Mpc−1).
Light Propagation in a Clumpy Universe
Authors: Hui Lam , Seljak Uroš
Published: July 2016

The propagation of light in an inhomogeneous universe is a long standing problem. Its resolution requires, first, a realistic description of the geometry of a clumpy universe and, second, solutions to the null geodesic equations given the metric of such a universe. The Friedmann-Robertson-Walker metric has become the standard description of the large scale geometry of the universe. However, the observable universe today is manifestly inhomogeneous. The weakly perturbed Friedmann-Robertson-Walker metric is often used to describe such a universe. But its validity is only guaranteed for a weakly inhomogeneous universe, where, for instance, overdensities are small , which is not true for sufficiently small scales in the universe today. It is well known, however, that the metric perturbations can still be small even if the overdensity is not small, given the right conditions and coordinates. However, spatial gradients of metric perturbations are not necessarily small any more. Here we estimate whether the second-order corrections involving them can affect significantly the expansion of the universe or the light propagation in it.
The Velocity Dispersion And Dispersion Profile Of Abell 963
Authors: Lavery R.J. , Henry J.P.
Published: July 2016

Abell 963 (z = 0.206) is still the best candidate for a true arc-counterarc lens configuration (Lavery & Henry 1988). A simple model explains the positions, lengths and patchy light distribution of the two arcs. Photometry indicates the arcs also have the same B-R color (Lavery & Henry 1988; Ellis et al. 1991). However, spectroscopic observations have not confirmed that these arcs originate from the same background galaxy. Ellis et al. (1991) detected a single emission line in the spectrum of the smaller northern arc, identifying it as [O II] λ3727 at a redshift of 0.77. Neither Lavery (1989) or Ellis et al. (1991) detected this emission line in the larger southern arc.
A Search for Dark Matter in the Halos of Lensing Galaxies using VLBI
Authors: Garrett M.A. , Nair S. , Porcas R.W. , Patnaik A.R.
Published: July 2016

Baryonic Dark Matter (BDM) candidates are segregated into two main mass ranges: (i) sub-solar mass dwarf stars (MACHOS) and (ii) ∼ 104–106M⊙ Very Massive Objects (VMOs). The lower mass range has been the target of the various micro-lensing programs but the first, tentative conclusions (see Stubbs et al. these proceedings) seem to suggest that MACHOs are unlikely to provide the bulk of the dark matter in the galactic halo. Meanwhile the upper mass range (104–106M⊙) remains largely unexplored. However, Wambsganss & Paczynski 1992 (hereafter WP92), have shown that this mass range is perfectly tuned to a straightforward and direct test: gravitational milli-lensing of macro-lensed images (Fig 1).
Binary Micro-Parallax Effects
Authors: Hardy S.J. , Walker M.A.
Published: July 2016

Of the large number of microlensing events detected towards the galactic bulge, there has been at least one clear case of a binary lens system, the OGLE 7 event. If this event had been observed simultaneously from three 1 m class ground-based telescopes during its second caustic crossing, an identification of the lens as a bulge or disk object and a determination of the orientation of the lens velocity on the sky could have been made.
Far-UV Spectral Variability In UM425 & PG1115+080
Authors: Michalitsianos A.G. , Oliversen R.J.
Published: July 2016

Determining the nature of spectral variability in gravitationally-lensed BAL (broad absorption line) quasars is important for time-delay and microlensing studies because the delay-times can be comparable with the time scales of intrinsic QSO ionization changes. Far-UV spectra of the candidate lens UM 425 = Q1120+019A and PG1115+080A (Triple Quasar) revealed the presence of strong O VI λ1033 emission. In both cases, absorption in the blue-wing of the O VI line profile indicated characteristic BAL outflow.
Superluminal Gravitational Microlensing Events In PKS 0537-441 And Prospects For Future Detections
Authors: Surpi G.C. , Romero G.E. , Vucetich H.
Published: July 2016

The BL-Lac object PKS 0537-441 has recently displayed strong intraday variability at 1.42 GHz on time scales of ∼ 104 s. Such kind of variability is hardly consistent with an intrinsic-to-source origin, since apparent brightness temperatures higher than 1021 K are implied for the emitting region in the source.

An alternative explanation for this ultra-rapid radio variability in PKS 0537-441 is developed based on gravitational microlensing of a superluminal component in the blazar caused by compact objects in the halo of an intervening galaxy. The mass of the microlenses is estimated and the blazar is proposed as a good candidate in a systematic search for extragalactic MACHOs.
The Q0957+561 Microlensing
Authors: Thomson David J. , Schild Rudolph E.
Published: July 2016

The TwQSO microlensing shows rich structure with cusp-profiled features on time scales of weeks to months, and with periodic effects on time scales of a few years.
Prospects For The Detection Of Microlensing Time Delays
Authors: Moore Christopher B. , Hewitt Jacqueline N.
Published: July 2016

Since the image separations produced by microlensing are inaccessible to observation, we are left to observe either a change in total amplitude or the time delay between the images. Spillar (1993) has suggested that a time delay might be observed in the autocorrelation of a microlensed signal. The time delays are of order a few microseconds and are easily accessible to a sufficiently wide bandwidth system. We calculate expected observational results using the Green Bank Telescope with a VLBA recording system, and find that brightness temperatures exceeding the Compton limit are required for detection.
Gravitational Microlensing By Random Motion Of Stars: Movie and Analysis of Light Curves
Authors: Wambsganss Joachim , Kundić Tomislav
Published: July 2016

We present a quantitative analysis of the effect of microlensing caused by random motion of individual stars in a galaxy lensing a background quasar. We calculate a large number of magnification patterns for positions of the stars slightly offset from one frame to the next, and thus obtain light curves for fixed quasar and galaxy positions, only due to the change in the relative star positions. These light curves are analyzed to identify microlensing events, which are then classified with respect to height, duration, and slope. These random motion microlensing events are compared with the corresponding ones caused by the bulk motion of the galaxy.

We find that microlensing events produced by random motion of stars are shorter, steeper, and more frequent than bulk motion events, assuming the velocity dispersion of the stars equals the bulk velocity of the galaxy. The reason for this difference is that in the case of random motion, caustics can move with an arbitrarily high velocity, producing very short events, whereas in the comparison case for bulk motion a microlensing event can never be shorter than it takes a fold caustic, which moves with the velocity of the lensing galaxy projected onto the quasar plane, to cross the quasar. An accompanying video illustrates these results. For three different values of the surface mass density κ, it shows time sequences of 1000 magnification patterns for slowly changing lens positions, together with the positions and velocity vectors of the microlensing stars. The full paper including the video can be found in Wambsganss & Kundić (1995). A short version of the video is available as an MPEG movie under anonymous ftp at astro.princeton.edu, in the directory jkw/microlensing/moving_stars.
New optical and MERLIN images of the quadruple gravitational lens B1422+231
Authors: Akujor C. E. , Patnaik A. R. , Smoker J. V. , Garrington S. T.
Published: July 2016

B1422+231, a multiple component quasar at a redshift of 3.62, is believed to be a gravitationally lensed system (Patnaik et al. 1992). It has 4 components with maximum image separation of 1.3 arcsec. The three brighter components, A, B and C have similar polarization properties at 8.4 GHz. The radio spectra between 5 and 8.4 GHz of A, B and C are similar; since D is very weak its spectrum is not accurately determined.
Flux Density Variations Of PKS 1830–211
Authors: Lovell J.E.J. , McCulloch P.M. , King E.A. , Jauncey D.L.
Published: July 2016

We have been observing the strong radio source and Einstein ring gravitational lens PKS 1830–211 as part of a flux density monitoring program at 2.3 and 8.4 GHz using the 26 m antenna at the Mt Pleasant Observatory. As can be seen from Figure 1 this source is variable at both frequencies. The 2.3 GHz flux density decreased from 10 Jy in 1990 November to a minimum of 9 Jy during 1992, and has since steadily increased to its present ∼12 Jy. Over the same interval, the flux density at 8.4 GHz has exhibited greater variability, more than doubling from an initial value of 5 Jy in late 1990 to a peak of ∼11 Jy early in 1992, before decreasing to a level of ∼7.5 Jy. VLA observations by van Ommen et al. (1995) show that the 15 GHz total flux density increased before the 8.4 GHz brightening seen at Mount Pleasant. Although the 15 GHz time series is not well sampled, it is clear from these data that the 15 GHz outburst precedes the 8.4 GHz feature by ∼400 days.
Compact Doubles: Testing The Lensing Hypothesis
Authors: Gopal-Krishna  , Subramanian Kandaswamy
Published: July 2016

If a compact double (CD) is caused by lensing, the orientation and/or flux ratio of its components can substantially change in ≈ 1 year.
A Radio Survey For Gravitational Lenses In The Southern Hemisphere
Authors: Lovell J.E.J. , McCulloch P.M. , Jauncey D.L.
Published: July 2016

We are undertaking an imaging survey with the Australia Telescope Compact Array (ATCA) to find gravitational lens candidates in flat-spectrum Parkes Catalogue radio sources. Flat-spectrum radio sources typically possess a single high brightness temperature nucleus of milliarcsecond size. Such sources, if lensed, will show multiply imaged nuclei with separations that are large compared to their milliarcsecond sizes. Our flat-spectrum sample was selected using the criteria α2.7/5.0 > −0.5 (S(v) ∝ vα), S2.7 > 0.34Jy and δ ≤ −20°, and comprises a total of 461 sources.
A VLBA 15 GHz Small Separation Gravitational Lens Survey
Authors: Patnaik A.R. , Garrett M.A. , Polatidis A. , Bagri D.
Published: July 2016

We have embarked on a 15 GHz VLBA survey of 1000 flat spectrum sources. We present the results from a 24 hour pilot observing run in which 72 sources were mapped. The primary aims of this project are:

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to search for small separation (1-150 mas) gravitational lens systems

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to identify targets for current mm and anticipated Space VLBI programs

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a morphological classification of compact radio sources at relatively high frequency with sub-mas resolution.
Dynamical Constraints on the Formation of Elliptical Galaxies
Authors: de Zeeuw P.T. , Carollo C.M.
Published: July 2016

Recent work on the construction of spherical, axisymmetric and triaxial dynamical models for elliptical galaxies is reviewed briefly, including their role in providing evidence for dark halos and central black holes. The different orbital structures and shapes of low-mass and giant elliptical galaxies provide essential constraints on scenarios of galaxy formation.
Elliptical Galaxies: The Age - Metallicity Dilemma
Authors: Chiosi Cesare
Published: July 2016

In this paper, we briefly report on the study by Bressan et al. (1995, BCT) who examine the properties of elliptical galaxies in the space of the parameters Hβ, [MgFe], (1550-V), and velocity dispersion Σ, try to infer the age of these systems, and cast light on the age-metallicity dilemma.
The Formation of Globular Cluster Systems: how, when, and where?
Authors: Harris William E.
Published: July 2016

Globular clusters, as fossil remnants of the protogalactic era, provide unique traces of the earliest events of galaxy formation. However, new observations – especially from HST – are showing that massive, globular-like star clusters belong not only to the pregalactic era but can form right up to the present day under the right circumstances. Appropriate interpretation may now let us learnsimultaneouslyabout the process of cluster formation as well as the nature of the gaseous fragments from which the galaxies were assembled.
Dwarf and Low Surface Brightness Galaxies
Authors: McGaugh Stacy S.
Published: July 2016

The terms ‘dwarf’ and ‘low surface brightness’ are commonly used to mean a variety of different things, and are sometimes used interchangeably. It is thus necessary to explicitly quantify the definitions which are adopted. Here I shall limit the discussion to field galaxies which are [exponential] disks characterized by a scale size h and central surface brightness μ0.
Dynamical Effects of Mergers
Authors: Barnes J.E.
Published: July 2016

The bridges and tails of interacting galaxies were elegantly explained when Toomre & Toomre (1972) showed that such features arise from tides acting on disk galaxies. Emboldened by this success, Toomre & Toomre proposed that certain twin-tailed fuzzballs without obvious interaction partners were in fact the merged relics of interacting pairs, and that such relics eventually become elliptical galaxies. Observations of twin-tailed systems found evidence for both their tidal origins and their elliptical destinies (e.g. Schweizer 1986), while self-consistent numerical simulations substantiated theoretical predictions of rapid orbital decay and the elliptical-like outcome of violent relaxation during merging (e.g. Barnes 1988, and references therein). But while our basic picture of merging seems solid, the precise role of this process in galaxy evolution is not so clear. Dynamical studies provide some insight into this issue; at present we can distinguish three levels of reliability in the numerical work: solid results, good bets, and hopeful guesses.
Characterizing distant blue galaxies with HST images and Keck spectra
Authors: Illingworth Garth D.
Published: July 2016

Keck spectra and HST images have been used to derive characteristic velocity and length scales for an enigmatic component of the faint, blue, field galaxy population: the compact, narrow emission-line galaxies (CNELGs). These galaxies are very luminous, but have been found to be quite low mass systems (with typical masses ∼ 109 M⊙). Their blue colors and strong emission lines indicate that they are undergoing a major burst of star formation. Following the completion of their current burst they will fade, becoming, in the absence of further major bursts, objects very similar to contemporary spheroidal galaxies. With mean sizes Re ∼ 1.4 kpc and Gaussian velocity profiles with mean σ = 45 km s–1, the length scales and velocity widths of CNELGs are also quite consistent with the measured length scales and velocity widths of current spheroidals.
The Linewidth-Luminosity Relation for Blue Galaxies at a redshift of ∼ 0.25
Authors: Rix Hans-Walter , Colless Matthew , Guhathakurta Puragra
Published: July 2016

This is a progress report on a collaborative effort to probe galaxy evolution by constraining the central mass-to-light ratios of distant field galaxies. Details can be found in a forthcoming paper (Rix, Guhathakurta, Colless & Ing 1996, RGCI). At the moment, the program is focused on the question of how over-luminous the abundant blue field galaxies at z ∼ 0.3 are (see Koo, K. and Kron, R. 1992), compared to galaxies of the same circular velocity and type at z ∼ 0. We have measured the integrated [OII] linewidths for a complete sample of distant blue galaxies to establish a linewidth-luminosity-color relation for this population and to compare this relation to the one for local galaxies. In addition, we explore practical strategies to obtain and analyze data so as to best relate the observable quantities (e.g. linewidths) to the physical quantities of interest (such as vcirc or M/L).
The Ages of Elliptical Galaxies in a Merger Model
Authors: Kauffmann G.
Published: July 2016

There have long been two competing views on the formation history of the ellipticals galaxies we see today. One is that most of the stars in present-day galactic bulges and ellipticals were produced during a relatively short, early phase of intense star formation at high redshift. The second view is that elliptical galaxies are relative latecomers, having been produced as the result of the merging of disk galaxies drawn together by gravity as their surrounding dark matter halos coalesced.
What Quasars Tell Us about Galactic Evolution
Authors: Rees Martin J
Published: July 2016

Quasars offer three types of clue to galactic evolution. (i) The formation of massive black holes in the centres of young galaxies; (ii) Clues to subgalactic structure from quasar absorption spectra; (iii) Implications for redshifts higher than 5. These are briefly summarised in this text.
The Radio Spectrum of SGR A∗
Authors: Beckert T. , Duschl W.J. , Mezger P.G. , Zylka R.
Published: July 2016

Sgr A∗, the enigmatic radio source located at the dynamical center of the Galaxy, is firmly detected in the frequency range of ∼ 1 – few 102 GHz. For ∼ 0.5 – 1 GHz and in the MIR range only significant upper limits of the flux density are known. Between ∼ 1.5 and 600 GHz the time averaged flux density Sv is proportional to v1/3 (v: frequency). For frequencies higher than ∼ 600 GHz as well as for those lower than ∼ 1.5 GHz, Sv drops sharply.
The Total Mass of DDO 154
Authors: Carignan Claude
Published: July 2016

The total mass and total extent of galaxies (including their dark halos) are fundamental parameters that are completely unknown for all galaxies. The best estimates we have for spiral and dwarf irregular galaxies come from detailed mass models using extended HI rotation curves. But, in every galaxy studied so far, such analysis only succeeded to derive lower limits of the total mass and total extent of their dark halo out to the last measured velocity point of the rotation curves which are still flat or even rising, implying that more dark mass is present at larger radii.
Recovering the Intrinsic Metallicity Distribution of Elliptical Galaxies
Authors: Ciotti L. , Stiavelli M. , Braccesi A.
Published: July 2016

We present a simple recipe to derive the metallicity distribution of galaxies as a function of their integrals of the motion. Elliptical galaxies are known to possess metallicity gradients that frequently show variations of a factor of 2 from the center to the effective radius (Peletier, 1989). The observed gradients are the result of projection and orbital mixing. Orbital mixing arises because stars at a given radius may have their apocenters spanning a wide range of radii. Thus, the mean metallicity at any given point inside the galaxy is the result of the metallicity distribution in phase space weighted by the galaxian distribution function. We have proposed (Ciotti, Stiavelli & Braccesi, 1995) a simple inversion procedure allowing one to derive the dependence of metallicity on the integrals of motion for a spherical galaxy. The ideas of this paper can be generalized to oblate two-integrals models following the Hunter & Qian (1993) technique.