Showing papers in "Publications of the Astronomical Society of the Pacific in 1998"
TL;DR: In this article, the authors describe version 90 (C90) of the code, paying particular attention to changes in the atomic database and numerical methods that have affected predictions since the last publicly available version, C84.
Abstract: CLOUDY is a large‐scale spectral synthesis code designed to simulate fully physical conditions within an astronomical plasma and then predict the emitted spectrum. Here we describe version 90 (C90) of the code, paying particular attention to changes in the atomic database and numerical methods that have affected predictions since the last publicly available version, C84. The computational methods and uncertainties are outlined together with the direction future development will take. The code is freely available and is widely used in the analysis and interpretation of emission‐line spectra. Web access to the Fortran source for CLOUDY, its documentation Hazy, and an independent electronic form of the atomic database is also described.
2,571 citations
TL;DR: A stellar spectral flux library of wide spectral coverage and an example of its application are presented in this paper, which consists of 131 flux-calibrated spectra, encompassing all normal spectral types and luminosity classes at solar abundance, and metal-weak and metalrich F-K dwarf and G-K giant components.
Abstract: A stellar spectral flux library of wide spectral coverage and an example of its application are presented. The new library consists of 131 flux-calibrated spectra, encompassing all normal spectral types and luminosity classes at solar abundance, and metal-weak and metal-rich F-K dwarf and G-K giant components. Each library spectrum was formed by combining data from several sources overlapping in wavelength coverage. The SIMBAD database, measured colors, and line strengths were used to check that each input component has closely similar stellar type. The library has complete spectral coverage from 1150 to 10620 Afor all components and to 25000 Afor about half of them, mainly later types of solar abundance. Missing spectral coverage in the infrared currently consists of a smooth energy distribution formed from standard colors for the relevant types. The library is designed to permit inclusion of additional digital spectra, particularly of non-solar abundance stars in the infrared, as they become available. The library spectra are each given as Fl versus l, from 1150 to 25000 Ain steps of 5 A ˚. A program to combine the library spectra in the ratios appropriate to a selected isochrone is described and an example of a spectral component signature of a composite population of solar age and metallicity is illustrated. The library spectra and associated tables are available as text files by remote electronic access.
1,424 citations
TL;DR: In this article, the radio to X-ray properties of GPS and CSS sources are reviewed and the current hypotheses for their origin and their use to constrain the evolution of powerful radio galaxies.
Abstract: I review the radio to X-ray properties of gigahertz peaked-spectrum (GPS) and compact steep- spectrum (CSS) sources, the current hypotheses for their origin, and their use to constrain the evolution of powerful radio galaxies. The GPS and CSS sources are compact, powerful radio sources with well-defined peaks in their radio spectra (near 1 GHz in the GPS and near 100 MHz in the CSS). The GPS sources are entirely contained within the extent of the narrow-line region (&1 kpc), while the CSS sources are contained entirely within the host galaxy (&15 kpc). The peaks in the spectra are probably due to synchrotron self-absorption, though free-free absorption through an inhomogeneous screen may also play a role. The turnover frequency varies with linear size l as , suggesting a simple physical relationship between these parameters. The radio 20.65 n / l m morphologies are strikingly like those of the large-scale classical doubles, though some sources can have very distorted morphologies suggestive of interactions. Radio polarization tends to be low, and in some cases the Faraday rotation measures can be extremely large. The IR properties are consistent with stellar populations and active galactic nucleus (AGN) bolometric luminosity similar to that of the 3CR classical doubles. The optical host galaxy properties (absolute magnitude, Hubble diagram, evidence for interaction) are consistent with those of the 3CR classical doubles. CSS sources at all redshifts exhibit high surface brightness optical light (most likely emission-line gas) that is aligned with the radio axis. The optical emission-line properties suggest (1) interaction of the radio source with the emission-line gas and (2) the presence of dust toward the emission-line regions. X- ray observations of high-redshift GPS quasars and a couple of GPS galaxies suggest the presence of significant columns of gas toward the nuclei. Searches for cold gas in the host galaxies have revealed large amounts of molecular gas and smaller amounts of atomic gas in several sources, though probably not enough to confine the radio sources. The main competing models for the GPS and CSS sources are that (1) they are frustrated by interaction with dense gas in their environments and (2) they are young and evolving radio sources that will become large-scale sources. Combining the bright GPS and CSS samples with the 3CR results in a sample spanning a range in source size of 10 5 that can be used to study source evolution. The number density versus linear size relation is consistent with a picture in which the sources expand with constant velocity and the radio power drops with linear size l according to . This strong evolution suggests that at least some of the 20.5 P / l GPS and CSS sources evolve to become lower luminosity FR 1 radio sources. The GPS and CSS sources are important probes of their host galaxies and will provide critical clues to the origin and evolution of powerful radio sources.
1,015 citations
TL;DR: In this article, the authors developed a new methodology for building spectral templates for galaxy redshifts, using the new templates for the FAST spectrograph as an example, and showed that a two-dimensional spectral classification of galaxy spectra can be developed using their emission- and absorption-line templates as physically orthogonal basis vectors.
Abstract: RVSAO is a set of programs to obtain redshifts and radial velocities from digital spectra. RVSAO operates in the IRAF environment. The heart of the system is xcsao, which implements the cross-correlation method and is a direct descendant of the system built by Tonry & Davis. emsao uses intelligent heuristics to search for emission lines in spectra, and then fits them to obtain a redshift. sumspec shifts and sums spectra to build templates for cross-correlation. linespec builds synthetic spectra given a list of spectral lines. bcvcorr corrects velocities for the motion of the Earth. We discuss in detail the parameters necessary to run xcsao and emsao properly. We discuss the reliability and error associated with xcsao-derived redshifts. We develop an internal error estimator, and we show how large, stable surveys can be used to develop more accurate error estimators. We also develop a new methodology for building spectral templates for galaxy redshifts, using the new templates for the FAST spectrograph as an example. We show how to obtain correlation velocities using emission-line templates. Emission-line correlations are substantially more efficient than the previous standard technique, automated emission-line fitting. Using this machinery, the blunder rate for redshift measurements can be kept near zero; the automation rate for FAST spectra is ∼95%. We use emsao to measure the instrumental zero-point offset and instrumental stability of the Z-Machine and FAST spectrographs. We compare the use of RVSAO with new methods, which use singular value decomposition and x 2 fitting techniques, and conclude that the methods we use are either equal or superior. We show that a two-dimensional spectral classification of galaxy spectra can be developed using our emission- and absorption-line templates as physically orthogonal basis vectors.
605 citations
TL;DR: In this paper, a model-independent method of assessing the uncertainties in cross-correlation lags determined from the light curves of active galactic nuclei (AGNs) was proposed.
Abstract: We describe a model-independent method of assessing the uncertainties in cross-correlation lags determined from the light curves of active galactic nuclei (AGNs) and use this method to investigate the reality of lags between UV and optical continuum variations in well-studied AGNs. Our results confirm the existence of such lags in NGC 7469. We find that the continuum variations at 1825, 4845, and 6962 A follow those at 1315 A by , and days, respectively, based on the centroids of the cross-correlation functions; the error intervals quoted correspond to 68% confidence levels, and each of these lags is greater than zero at no less than 97% confidence. We do not find statistically significant interband continuum lags in NGC 5548, NGC 3783, or Fairall 9. Wavelength-dependent continuum lags may be marginally detected in the case of NGC 4151. However, on the basis of theoretical considerations, wavelength-dependent continuum lags in sources other than NGC 7469 are not expected to have been detectable in previous experiments. We also confirm the existence of a statistically significant lag between X-ray and UV continuum variations in the blazar PKS 2155–304.
543 citations
TL;DR: The Space Telescope Imaging Spectrograph (STIS) instrument was installed on the Hubble Space Telescope (HST) during the second servicing mission, in 1997 February, and four bands cover the wavelength range of 115-1000 nm, with spectral resolving powers between 26 and 200,000 as discussed by the authors.
Abstract: The Space Telescope Imaging Spectrograph (STIS) instrument was installed on the Hubble Space Telescope (HST) during the second servicing mission, in 1997 February. Four bands cover the wavelength range of 115–1000 nm, with spectral resolving powers between 26 and 200,000. Camera modes are used for target acquisition and deep imaging. Correction for HST's spherical aberration and astigmatism is included. The 115–170 nm range is covered by a CsI MAMA (Multianode Microchannel Array) detector and the 165–310 nm range by a Cs2Te MAMA, each with a format of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc}
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402 citations
TL;DR: In this article, it was shown that most of the heavy isotope anomalies attributable to novae are most likely produced by the approximately 25%-33% of novae that occur in systems containing massive (M, O) oxygen-neon-magnesium (ONeMg) WDs.
Abstract: Classical novae, explosions that result from thermonuclear runaways (TNRs) on the surfaces of white dwarfs (WDs) accreting hydrogen-rich matter in close binary systems, are sporadically injecting material processed by explosive hydrogen-burning nucleosynthesis into the interstellar medium (ISM). Although novae probably have processed less than »0.3% of the interstellar matter in the Galaxy, both theoretical and observational evidence suggests that they may be important sources of the nuclides 7 Li, 15 N, and 17 O, as well as the radioactive isotopes 22 Na and 26 Al. The latter nuclides are astrophysically important in that they may have been involved in the production of the 22 Ne (Ne-E) and 26 Mg enrichments identified in meteoritic inclusions, the composition of which is thought to be representative of the chemical and mineral contents of the primitive solar nebula. These inclusions may be partially composed of dust condensed in nova outbursts. We review theoretical expectations for the yields of various isotopes in nova outbursts and conclude that any of the heavy isotope anomalies attributable to novae are most likely produced by the approximately 25%-33% of novae that occur in systems containing massive ( M,) oxygen-neon-magnesium (ONeMg) WDs. We attempt to place quantitative constraints M 1 1.2 ⁄ on the degree to which classical novae participate in the production of chemical anomalies, both in the primitive solar system and on a Galactic scale. Diffuse Galactic g-ray fluxes provide particularly important clues to and constraints on the 22 Na and 26 Al yields from novae. Ultraviolet (UV), optical, and infrared (IR) emission-line spectra of classical novae reveal the abundances of some of the gas-phase elements present in the ejecta; recent results are reviewed. We describe how IR observations of novae reveal dust formation and gas-phase line emission and how they distinguish the temporal development of nova explosions on carbon-oxygen (CO) WDs (CO novae) from those on ONeMg WDs (ONeMg or "neon" novae). Recent studies show that the ejecta in some novae can be strongly cooled by near- and mid-IR forbidden-line radiation from highly ionized ("coronal") atomic states. We compare the abundances deduced from recent UV, optical, and IR observations with theoretical predictions, and we suggest that future studies of IR coronal emission lines may provide additional key information. Novae produce only about 0.1% of the Galactic "stardust" (dust condensed in stellar outflows), but IR observations show that it may be some of the more interesting dust. Novae appear capable of producing astrophysical dust of virtually every known chemical and mineral composition. We summarize recent IR observations of the dust production scenario in novae and argue that neon novae may lead to the formation of dust grains that carry the Ne-E and 26 Mg anomalies.
382 citations
TL;DR: FAST as discussed by the authors is a high-throughput optical spectrograph that has been in operation at the Cassegrain focus of the 15m Tillinghast reflector since 1994.
Abstract: We describe a high‐throughput optical spectrograph that has been in operation at the Cassegrain focus of the 15‐m Tillinghast reflector since 1994 January FAST has a 3 \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc}
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347 citations
280 citations
TL;DR: In this paper, the authors studied the evolution of hydrogen-rich CVs near minimum orbital period at ~78 minutes and found that the total space density of these CVs is ~10−5 pc−3, with short-period systems constituting ~75% of the total.
Abstract: We study the evolution of hydrogen-rich cataclysmic variables (CVs) near minimum orbital period at ~78 minutes. As has been known for many years, these are among the most intrinsically common CVs, but they hide fairly well because of their faintness and low incidence of eruptions. We discuss their number and observational signatures, paying special attention to those that may have passed minimum orbital period—the "period bouncers." The status of binaries near minimum period is best determined by the mass ratio, and this is best constrained by measuring the accretion disk precession frequency, because that frequency is readily accessible to observation and proportional to the secondary star's mass. This method reveals four stars that are good candidates to have survived period bounce; two appear to have secondaries as puny as 0.02 M⊙. But each star can have bounced only recently if at all. There is still no strong evidence of any long era of evolution in a state of increasing period. This conflicts sharply with discussions of observational data that have identified dozens of known CVs with this state. The total space density of cataclysmic variables is ~10–5 pc–3, with short-period systems constituting ~75% of the total. Both estimates are far less than predicted by simple theories of evolution. It is probably necessary to have some means of destroying CVs before they reach the predicted very high space densities. This can be done by invoking an angular momentum loss mechanism that does not quickly subside as the mass ratio becomes very low.
271 citations
TL;DR: In this article, extensive results from the commissioning phase of PUEO, the adaptive optics instrument adaptor for the Canada-France-Hawaii Telescope (CFHT), are presented and discussed.
Abstract: Extensive results from the commissioning phase of PUEO, the adaptive optics instrument adaptor for the Canada‐France‐Hawaii Telescope (CFHT), are presented and discussed. Analyses of more than 750 images recorded with a CCD and a near‐IR camera on 16 nights in wavelengths from B to H are used to derive the properties of the compensated wavefront and images in a variety of conditions. The performance characteristics of the system are analyzed and presented in several ways, in terms of delivered Strehl ratios, full width half‐maxima (FWHM), and quantities describing the improvements of both. A qualitative description is given of how the properties of the corrected images result from the structure function of the compensated phase. Under median seeing conditions, PUEO delivers essentially diffraction‐limited images at H and K and images with FWHM ∼ 0 \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepac...
TL;DR: In this article, the Johnson B, Kron-Cousins R photometric system for approximately 100,000 stars in a 1/3 deg2 field centered on the dwarf spheroidal galaxy in Fornax was presented.
Abstract: We present accurate photometry on the Johnson B, Kron-Cousins R photometric system for approximately 100,000 stars in a 1/3 deg2 field centered on the dwarf spheroidal galaxy in Fornax. We identify numerous probable short-period variable stars, blue stars that appear to be the main sequence of a small population with an age of order 108 yr, and two distinct types of luminous red star: an extended sequence of primarily carbon stars and a clump of mostly M giants slightly more luminous than the giant-branch tip. The spatial distribution of each of these subpopulations within the Fornax dwarf galaxy is considered.
TL;DR: In this article, the abundance patterns in our own Galaxy and in external galaxies requires the presence of at least three sources of chemical enrichment whose relative contributions can vary from environment to environment, i.e., Type Ia supernovae, Type II SNe, and N-rich mass loss from intermediate-mass stars.
Abstract: Model and observed spectral feature indices indicate that, in galaxies of all types with velocity dispersions larger than about 225 km s–1, [Mg/Fe] progressively drifts greater than zero, until it reaches about 0.3 in the largest ellipticals. For elliptical galaxies that have absorption-line data for more elements, the abundances of Na and N elevate in a similar fashion, relative to both Ca and Fe ([Ca/Fe] ≈ 0; [Na, N/Fe] progressively > 0). Titanium may also share this apparently generic light-element behavior. The abundance pattern in elliptical galaxies matches neither the disk, the halo, nor the bulge of our own Galaxy, although the bulge appears to come the closest. N is depleted in the Galactic bulge but elevated in M31, M31's metal-rich globular clusters, and large elliptical galaxies. If all measured elements are considered, the abundance patterns in our own Galaxy and in external galaxies requires the presence of at least three sources of chemical enrichment whose relative contributions can vary from environment to environment. These three sources of enrichment may correspond to Type Ia supernovae (SNe), Type II SNe, and N-rich mass loss from intermediate-mass stars, but the behavior of O, Ca, Si, Sc, V, and Ti in the Galactic bulge, disk, and halo seems to require at least one additional supernova flavor.Abundance ratio effects represent a barrier to the estimation of mean ages from integrated light, a barrier that is greater than that of isochrone error. Isochrone grids allowing for the variation of individual elements are needed, but relative changes in isochrone temperatures computed as a function of abundance pattern need to be accurate to roughly 7 K if 5% age estimates are desired.
TL;DR: In this article, a sequence of power spectra measured for smaller apertures is extrapolated up to very large (8 m) telescopes, with a gradual transmission falloff near the edges.
Abstract: Stellar intensity scintillation in the optical was extensively studied at the astronomical observatory on La Palma (Canary Islands). Atmospheric turbulence causes "flying shadows" on the ground, and intensity fluctuations occur both because this pattern is carried by winds and is intrinsically changing. Temporal statistics and time changes were treated in Paper I, and the dependence on optical wavelength in Paper II. This paper discusses the structure of these flying shadows and analyzes the scintillation signals recorded in telescopes of different size and with different (secondary-mirror) obscurations. Using scintillation theory, a sequence of power spectra measured for smaller apertures is extrapolated up to very large (8 m) telescopes. Apodized apertures (with a gradual transmission falloff near the edges) are experimentally tested and modeled for suppressing the most rapid scintillation components. Double apertures determine the speed and direction of the flying shadows. Challenging photometry tasks (e.g., stellar microvariability) require methods for decreasing the scintillation "noise." The true source intensity I(l) may be segregated from the scintillation component in DI(t,l,x,y) postdetection computation, using physical modeling of the temporal, chromatic, and spatial properties of scintillation, rather than treating it as mere "noise." Such a scheme ideally requires multicolor high-speed (&10 ms) photometry on the flying shadows over the spatially resolved ( &10 cm) telescope entrance pupil. Adaptive correction in real time of the two-dimensional intensity excursions across the telescope pupil also appears feasible, but would probably not offer photometric precision. However, such "second-order" adaptive optics, correcting not only the wavefront phase but also scintillation effects, is required for other critical tasks such as the direct imaging of extrasolar planets with large ground-based telescopes.
TL;DR: In this paper, the authors describe an original technique that involves moving a virtual plane of analysis in the atmosphere, using this technique, a simple optical combination allows us to explore the atmosphere and wipe out a particular layer while distant layers located beneath or above are reinforced.
Abstract: During the last two decades, much effort has been put into the remote sensing of atmospheric turbulent layers, using optical devices. The main scientific interest concerns optical path fluctuations relevant to communication and astronomy and the widening of our understanding of atmospheric physics. Here we describe an original technique that involves moving a virtual plane of analysis in the atmosphere. Using this technique, a simple optical combination allows us to explore the atmosphere, wiping out a particular layer while distant layers located beneath or above are reinforced. When combined with the SCIDAR technique, this phenomenon leads to the "generalized SCIDAR" concept, allowing turbulence profiles to be remotely assessed throughout the whole atmosphere, including the boundary layer. The potential of this new concept is shown in a laboratory experiment using a vein to simulate an atmospheric turbulent layer.
TL;DR: In this paper, the authors present corrections that account for the bias of signal-to-noise ratio and the (true) slope of the faint-source number-flux relation.
Abstract: Flux estimates for faint sources or transients are systematically biased high because there are far more truly faint sources than bright Corrections that account for this effect are presented as a function of signal- to-noise ratio and the (true) slope of the faint-source number-flux relation The corrections depend on the source being originally identified in the image in which it is being photometered If a source has been identified in other data, the corrections are different; a prescription for calculating the corrections is presented Implications of these corrections for analyses of surveys are discussed; the most important is that sources identified at signal-to-noise ratios of 4 or less are practically useless
TL;DR: The IAU Colloquium 170, on Precise Stellar Radial Velocities, was originally proposed by IAU Commission 30, and the Scientific Organizing Committee was chaired by John Hearnshaw, president of the commission, while the Local Organising Committee was led by Colin Scarfe, the past president as mentioned in this paper.
Abstract: IAU Colloquium 170, on Precise Stellar Radial Velocities, was originally proposed by IAU Commission 30, and the Scientific Organizing Committee was chaired by John Hearnshaw, president of the commission, while the Local Organizing Committee was chaired by Colin Scarfe, the past president. The meeting took place at Dunsmuir Lodge, the conference center of the University of Victoria, the host institution. Most of the participants were accommodated in the Lodge, an arrangement that facilitated informal discussion away from sessions and permitted social events to be easily organized. About 70 astronomers from 20 countries participated in the meeting, and there was a wide range of papers covering many topics, including instrumentation, detection of extrasolar planets, asteroseismology, line broadening and asymmetries in late-type stellar spectra, binary stars, pulsating stars, rotating and spotted stars, standard stars, etc. Some aspects of the meeting of particular interest included the following:
TL;DR: In this article, the authors collect measurements of the Galactic γ-ray sky in spectral lines attributed to the decay of radioactive 7Be, 22Na, 26Al, 44Ti, 56Ni, 57Ni, and 60Fe.
Abstract: Our modern laboratory of nuclear physics has expanded to encompass parts of the universe, or at least our Galaxy. Gamma rays emitted by the decays of radioactive nuclei testify to the production of isotopes through nuclear processes in astrophysical events. We collect measurements of the Galactic γ-ray sky in spectral lines attributed to the decay of radioactive 7Be, 22Na, 26Al, 44Ti, 56Ni, 57Ni, and 60Fe. We organize and collate these measurements with models for the production sites in novae, supernovae, stellar interiors, and interstellar cosmic-ray interactions. We discuss the physical processes and the spatial distribution of these production sites, along with models of the chemical evolution of the Galaxy. Highlights of measurements made in the last decade include detailed images of the Galaxy in 26Al radioactivity and detection of 56Co and 57Co from SN 1987A, 44Ti from Cas A, and possibly 56Ni from SN 1991T. The 26Al mapping of recent Galactic nucleosynthesis may be considered as a new view on the entire ensemble of massive stars in the Galaxy. The local Cygnus region shows prominent radioactive emission from well-known stellar clusters, but the absence of γ-rays from the closest Wolf-Rayet star, WR 11, in the Vela region is puzzling. SN 1987A studies in γ-rays measure the radioactive powering of the supernova light curve directly, which will be particularly important for the dim late phase powered by 44Ti. The 57Ni/56Ni isotopic ratio determinations from γ-rays provide additional guidance for understanding SN 1987A's complex light curve and now appear to be uniformly settling to about twice the solar ratio. Cas A 44Ti production as measured through γ-rays presents the interesting puzzle of hiding the expected, coproduced, and large 56Ni radioactivity. Core-collapse supernova models need to parameterize the inner boundary conditions of the supernova in one way or another, and now enjoy another measurement of the ejecta that is definitely originating from very close to the difficult regime of the mass cut between ejecta and compact remnant. Other relevant measurements of cosmic element abundances, such as observations of atomic lines from the outer shells of the production sites or meteoritic analysis of interstellar grains, complement the rather direct measurements of penetrating γ-rays, thus enhancing the observational constraints of nuclear astrophysics models.
TL;DR: In this article, photometry and spectroscopy of the dwarf nova EG Cancri in its 1996/1997 episode of eruptions were reported, which established the star as a new member of the SU UMa class of dwarf novae.
Abstract: We report photometry and spectroscopy of the dwarf nova EG Cancri in its 1996/1997 episode of eruptions. The main eruption was clearly a superoutburst featuring common superhumps with a period of 0.06036(2) days, establishing the star as a new member of the SU UMa class of dwarf novae. At the end of the main eruption, the superhumps were replaced by another wave of slightly longer period (0.06045 days), which endured for at least another 90 days. The properties of the latter wave suggest identification as a "late superhump." The recurrence time for superoutbursts is long, probably in the range 7-20 yr. After the 15 day superoutburst, the star displayed six remarkable surges in light, with an average interval of 7 days; these "echo outbursts" strongly resemble ordinary dwarf nova eruptions. This suggests that a high accretion rate persisted in the disk for ~40 days after the main superoutburst. By a year after outburst, the star faded to V = 18.7, with most of the continuum light from a white dwarf of T ~ 15,000 K. We estimate a distance of 320 pc and a Mv = +11.5 for accretion light. The extreme faintness, the rarity of eruptions, the enormous duration of superhumps, and the quiescent spectrum together establish membership in the WZ Sge subclass, the most sluggish of dwarf novae. Spectroscopy at quiescence and a transient wave in early outburst establish an orbital period of 0.05997(9) days. This suggests a secondary star with M ~ 0.02 M⊙, yet R ~ 0.08 R⊙. A binary can reach such a state after the secondary is forced to lose mass on a timescale shorter than its Kelvin-Helmholtz timescale.
TL;DR: The IAU Symposium 192 was devoted to the fundamental question of Local Group membership, with review papers by both E. K. Jerjen et al. and T. E. Armandroff touching on this subject as mentioned in this paper.
Abstract: IAU Symposium 192 was both broader and narrower than its title would suggest. As will be discussed below, this meeting covered many aspects of the Local Group other than stellar content. On the other hand, discussion of the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) was cut back, since a previous IAU Symposium (190) on the Magellanic Clouds had been held only 2 months earlier in Victoria, Canada. Local Group membership.—A substantial portion of the conference was devoted to the fundamental question of Local Group (LG) membership, with review papers by both E. K. Grebel and T. E. Armandroff touching on this subject. In the last year the number of known companions to M31 has doubled, thanks to work by Armandroff, I. D. Karachentsev, and collaborators. The Armandroff search is based on a matched filter applied to the Digital Sky Survey and has produced many more candidates that are still being investigated. However, all the new LG objects to date are brighter than , M 10 v suggesting that many other fainter galaxies are lurking, waiting to be discovered. (Contrast this work on M31 companions with a search by M. Irwin and collaborators, who have turned up only one new object after searching two-thirds of the sky.) On several occasions it was remarked that NGC 55 (which is usually considered to be a member of the nearby Sculptor Group) lies on the zero velocity surface of the Local Group and may actually be considered a member of our Group. (A recent preprint by H. Jerjen et al. [astro-ph/9809046] suggests that it may be more appropriate to consider the Local and Sculptor Groups as a single supergalactic structure.) A related question is that of the luminosity function of the Local Group. If the inventory of LG galaxies is not complete below 10, then one cannot rule out the possibility of a steep upturn in the LG luminosity function (as observed in some other samples of galaxy populations). Morphology.—Sidney Van den Bergh highlighted the absence of large dwarf spheroidals (such as those in the Virgo cluster or the one known in the M81 Group). K. Freeman pointed out the difference in the structure of bulges in earlytype systems (e.g., M31, the Sombrero galaxy, etc.) that possess
TL;DR: An automated approach for determining the noise associated with astronomical images is described, based on a multiresolution transform of the image, the atrouswavelet transform, which can be used for high-quality image filtering or compression.
Abstract: We describe an automated approach for determining the noise associated with astronomical images. Detector noise is ever present and must be determined for high-quality image filtering or compression. We also show that the method can be used for very high quality cosmic-ray hit removal. Our method is based on a multiresolution transform of the image, the atrouswavelet transform. We present a range of examples and applications to illustrate the effectiveness of this approach.
TL;DR: The workshop "1051 Ergs: The Evolution of Shell Supernova Remnants" as discussed by the authors was organized around chronological sessions for "very young," "young", "mature", and "old" supernova remnants, with implicit recognition that these labels are often difficult to apply.
Abstract: This paper reports on the workshop "1051 Ergs: The Evolution of Shell Supernova Remnants," hosted by the University of Minnesota, 1997 March 23-26. The workshop was designed to address fundamental dynamical issues associated with the evolution of shell supernova remnants and to understand better the relationships between supernova remnants and their environments. Although the title points only to classical, shell SNR structures, the workshop also considered dynamical issues involving X-ray-filled composite remnants and pulsar-driven shells, such as that in the Crab Nebula. Approximately 75 observers, theorists, and numerical simulators with wide-ranging interests attended the workshop. An even larger community helped through extensive on-line debates prior to the meeting to focus issues and galvanize discussion. In order to deflect thinking away from traditional patterns, the workshop was organized around chronological sessions for "very young," "young," "mature," and "old" remnants, with the implicit recognition that these labels are often difficult to apply. Special sessions were devoted to related issues in plerions and "thermal X-ray composites." Controversy and debate were encouraged. Each session also addressed some underlying, general physical themes: How are supernova remnant (SNR) dynamics and structures modified by the character of the circumstellar medium (CSM) and the interstellar medium (ISM), and vice versa? How are magnetic fields generated in SNRs and how do magnetic fields influence SNRs? Where and how are cosmic rays (electrons and ions) produced in SNRs, and how does their presence influence or reveal SNR dynamics? How does SNR blast energy partition into various components over time, and what controls conversion between components? In lieu of a proceedings volume, we present here a synopsis of the workshop in the form of brief summaries of the workshop sessions. The sharpest impressions from the workshop were the crucial and underappreciated roles that environments have on SNR appearance and dynamics and the critical need for broad-based studies to understand these beautiful but enigmatic objects.
TL;DR: In this article, a catalog of 38 young solar analogs within 25 pc, stars which are uniquely well-suited for observations of thier circumstellar environments to improve our understanding of conditions within the Solar System during the Hadean/early Archean eons (prior to 3.8 Ga).
Abstract: We present a catalog of 38 young solar analogs within 25 pc, stars which are uniquely well-suited for observations of thier circumstellar environments to improve our understanding of conditions within the Solar System during the Hadean/early Archean eons (prior to 3.8 Ga).
TL;DR: In this paper, the star's soft X-ray orbital light curve shows a strong but transient energy-dependent dip around orbital phase 0.7, which is probably due to absorption by an extended gas cloud in which the mass transfer stream strikes the disk.
Abstract: We report new observations of the dwarf nova WZ Sagittae, showing the return of the 27.87 s oscillation in 1995/1996 high-speed optical light curves. As was true in 1978, the main peak of the power spectrum generally occurred at 27.87 or 28.96 s, with the signals sometimes simultaneously present. Weak transient signals were also seen at 28.20 and 29.69 s. An ASCA X-ray observation showed a significant peak at 27.86±0.01 s in the 2-6 keV band, which appeared to establish the star's membership as a "DQ Herculis star," in which accretion onto the white dwarf is channeled by a strong magnetic field. The X-ray period is most likely the true white dwarf spin period. However, we certainly do not understand the distribution of power among these various signals of slightly different period. This puzzling multiperiodic structure remains an impediment to full acceptance of WZ Sge as a DQ Her star. The star's soft X-ray orbital light curve shows a strong but transient energy-dependent dip around orbital phase 0.7, which is probably due to absorption by an extended gas cloud in which the mass-transfer stream strikes the disk.
TL;DR: Doppler measurements from the Keck/HIRES spectrometer of the G3 V star HD 187123 reveal Keplerian variations with a period of 3097 days and a semiamplitude of 72 m s−1.
Abstract: Doppler measurements from the Keck/HIRES spectrometer of the G3 V star HD 187123 reveal Keplerian variations with a period of 3097 days and a semiamplitude of 72 m s–1 An orbital fit yields a companion mass of M = 052 MJUP/ sin i}, a semimajor axis of a = 0042 AU, and an eccentricity of e = 003 (consistent with zero) HD 187123 appears similar to the Sun in mass, age, chromosphere, and rotation rate Although unlikely, non-Keplerian explanations for the Doppler variations, such as spots and pulsation, cannot be ruled out and require future photometry and spectroscopy
TL;DR: In this paper, the authors investigated charge transfer effects in photometry with Wide Field Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope and compared WFPC2 observations with ground-based photometry for the Galactic globular clusters ω Centauri and NGC 2419.
Abstract: Charge-transfer effects in photometry with Wide Field Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope are investigated by a comparison of WFPC2 observations with ground-based photometry for the Galactic globular clusters ω Centauri and NGC 2419. Simple numerical formulae describing the fraction of lost signal as functions of position on the detector, stellar brightness, and the diffuse sky brightness recorded in an image are presented, and the resulting corrections are compared with those previously derived by Whitmore & Heyer. Significant lost-charge effects are seen that are proportional to both the Y-coordinate (i.e., the number of shifts along the parallel register during readout) and the X-coordinate (number of shifts along the serial register). A "typical" star image (one containing ~104 photoelectrons) near the center of a "typical" intermediate-length exposure (one with a diffuse sky brightness of ~10 e– pixel–1, obtained at a camera temperature of –88° C) loses approximately 2% of its electrons to charge traps during readout; a star in the corner of the image most remote from the readout electronics loses twice that. The percentage of charge lost decreases as the star brightness or the diffuse sky brightness increases. Charge losses during the brief period when WFPC2 was operated at a temperature of –76° C were approximately 85% greater, but apart from that, no significant change in the charge-transfer losses with time during the first 3.5 years of WFPC2's mission is evident, except possibly a weak effect for the very faintest star images. These results are quite similar to those of Whitmore & Heyer, which were based on a much smaller data set, but there are some differences in detail. Even with the present set of corrections, additional sources of calibration uncertainty that I am unable identify or characterize with the available data probably limit the external accuracy of photometry from WFPC2 to of order 1%-2%.
TL;DR: In this article, the authors discuss the nature of V Sagittae and T Pyxidis, two enigmatic blue variable stars commonly classed among the cataclysmic variables, and show that they provide a good match for the newly discovered class of supersoft" X-ray binaries, probably powered by quasi-steady nuclear burning of accreted gas on a white dwarf.
Abstract: We discuss the nature of V Sagittae and T Pyxidis, two enigmatic blue variable stars commonly classed among the cataclysmic variables. These stars have bolometric luminosities in the range 36 (1-50) # 10 ergs s 1 , far exceeding that of any accretion-powered cataclysmic variable. They also show extremely blue colors ( and after dereddening) and orbital light curves that are quite similar and yet are B V 0.3 U B 1.3 not seen in any normal cataclysmic variable. But in all these respects, as well as in the rich and highly excited emission-line spectrum, the stars provide a good match for the newly discovered class of "supersoft" X-ray binaries, probably powered by quasi-steady nuclear burning of accreted gas on a white dwarf. Both stars show photometric waves at the orbital period. V Sge also shows a deep minimum, a true eclipse of the accretion disk arising from fairly high binary inclination. T Pyx is nearly face-on (probably ), which results in i ∼ 10 -20 narrow lines and a low amplitude for the orbital signal. T Pyx shows a very stable photometric wave at P days, but interpretation of this is hampered by another transient signal at 0.1098 days. This might be 0.076227 construed as evidence for a magnetic white dwarf.
TL;DR: Techniques are introduced that simultaneously minimize these artifacts and maximize the signal-to-noise ratio of the digitized data and tables of the optimum digitizer thresholds for both uniform and nonuniform input threshold digitizers are presented.
Abstract: The process of digitizing a stochastic signal introduces systematic distortions into the resulting digitized data. Further processing of these data may result in the appearance of unwanted artifacts, especially when the input signal was generated by a nonstationary stochastic process. In this paper the magnitude of these distortions are calculated analytically and the results are applied to a specific example found in pulsar signal processing. A pulsar signal is an excellent example of a nonstationary stochastic process. When analyzing pulsar data, the effects of interstellar medium (ISM) dispersion must be removed by digitally filtering the received signal. The distortions introduced through the digitization process cause unwanted artifacts to appear in the final "dedispersed" signal. These artifacts are demonstrated using actual 2-bit (4-level) digitized data of the pulsar PSR B083345 (Vela). Techniques are introduced that simultaneously minimize these artifacts and maximize the signal-to-noise ratio of the digitized data. The distortion analysis and artifact removal techniques described in this paper hold for an arbitrary number of input digitization thresholds (i.e., number of bits). Also presented are tables of the optimum digitizer thresholds for both uniform and nonuniform input threshold digitizers.
TL;DR: In this article, a list of photometric VRI comparison sequences in the fields of 16 blazars is presented, where comparison stars were calibrated for the first time during the present work.
Abstract: We present a list of photometric VRI comparison sequences in the fields of 16 blazars. For six of these objects, comparison stars were calibrated for the first time during the present work. For most of the other 10 blazars, we improved the old sequences by either calibrating stars in the R and I bands or adding new comparison stars. Finding charts for 15 of these sequences are also reported.
TL;DR: In this paper, the authors used a genetic algorithm to find an objective deconvolution of the observations, applied to the polar V347 Pav, gave the most unbiased picture of the accretion regions in a polar (Stephen Potter et al., 1998).
Abstract: The “Annapolis Workshop on Magnetic Cataclysmic Variables,” genially known as MCV2 (to acknowledge that the first such specialized workshop occurred in Cape Town 3 years earlier), was held 1998 July 13–17 on the campus of St. John’s College, Annapolis, Maryland. The stimulation for this series arises from the prominence of magnetic cataclysmic variables (CVs) in the EUV and X-ray sky: for the strongly magnetic systems (polars), most of the accretion luminosity is emitted in UV and soft X-rays; for the less magnetic CVs (intermediate polars, in which the white dwarf primary’s rotation is not synchronized with binary revolution), there is a strong harder X-ray component. Therefore, these objects are being studied by the contemporary satellites HST, EUVE, ROSAT, BeppoSAX, Ginga, RXTE, and ASCA and will be prime candidates for next-generation instruments, which include AXAF, XMM, Astro-E, SpectrumXG, and Constellation-X. Despite these comments, the reality is that high time resolution, ground-based optical spectroscopy with large telescopes continues to be the principal source of structural information about polars. In particular, as shown by Axel Schwope, Doppler tomography gives quantitative results for the interstar stream ballistic trajectory and the path of the gas after it has coupled onto the magnetic field lines of the primary. The results for HU Aqr, which, like probably all polars, has high and low states of mass transfer, show that the stream is threaded earlier when the system is in the low state. The same star has been studied with independent mapping techniques by Sonja Vrielmann and Kate HarropAllin et al. The resulting details of the line and continuum brightness distributions and the velocity structure will provide the stimulus for physical models of the entire interstar plasma stream. A new technique, using observed Stokes parameters and a genetic algorithm to find an objective deconvolution of the observations, applied to the polar V347 Pav, gives the most unbiased picture of the accretion regions in a polar (Stephen Potter et al.). Spin-period tomograms, constructed by Coel Hellier from time-resolved emission-line profiles in intermediate polars, convincingly confirm the existence of accretion curtains. In AO Psc only one curtain is visible, extending over ∼120 of the inner edge of the truncated accretion disk, but in PQ Gem