With VODataService 1.2, service providers in the Virtual Observatory (VO) have a reasonably straightforward way to declare where in space, time, and spectrum the data within a resource (i.e., service or data collection) lie. Here, we discuss the the
mechanism and design choices, current limitations (e.g., regarding non-electromagnetic or solar system resources) as well as ways to overcome them. We also show how users and clients can already run queries against resource coverage using a scheme that is expected to become part of RegTAP 1.2 (or a separate standard). We conclude with an ardent plea to all resource creators to provide STC metadata -- only wide adoption will make this facility useful.
This document describes a recommended syntax for writing the string representation of unit labels (VOUnits). In addition, it describes a set of recognised and deprecated units, which is as far as possible consistent with other relevant standards (BIP
M, ISO/IEC and the IAU). The intention is that units written to conform to this specification will likely also be parsable by other well-known parsers. To this end, we include machine-readable grammars for other units syntaxes.
The Virtual Observatory Registry is a distributed directory of information systems and other resources relevant to astronomy. To make it useful, facilities to query that directory must be provided to humans and machines alike. This article reviews th
e development and status of such facilities, also considering the lessons learnt from about a decade of experience with Registry interfaces. After a brief outline of the history of the standards development, it describes the use of Registry interfaces in some popular clients as well as dedicated UIs for interrogating the Registry. It continues with a thorough discussion of the design of the two most recent Registry interface standards, RegTAP on the one hand and a full-text-based interface on the other hand. The article finally lays out some of the less obvious conventions that emerged in the interaction between providers of registry records and Registry users as well as remaining challenges and current developments.
The Data Center Helper Suite DaCHS is an integrated publication package for building Virtual Observatory (VO) and Web services, supporting the entire workflow from ingestion to data mapping to service definition. It implements all major data discover
y, data access, and registry protocols defined by the VO. DaCHS in this sense works as glue between data produced by the data providers and the standard protocols and formats defined by the VO. This paper discusses central elements of the design of the package and gives two case studies of how VO protocols are implemented using DaCHS concepts.
In the Virtual Observatory (VO), the Registry provides the mechanism with which users and applications discover and select resources -- typically, data and services -- that are relevant for a particular scientific problem. Even though the VO adopted
technologies in particular from the bibliographic community where available, building the Registry system involved a major standardisation effort, involving about a dozen interdependent standard texts. This paper discusses the server-side aspects of the standards and their application, as regards the functional components (registries), the resource records in both format and content, the exchange of resource records between registries (harvesting), as well as the creation and management of the identifiers used in the system based on the notion of authorities. Registry record authors, registry operators or even advanced users thus receive a big picture serving as a guideline through the body of relevant standard texts. To complete this picture, we also mention common usage patterns and open issues as appropriate.
We describe a system used by the NASA Astrophysics Data System to identify bibliographic references obtained from scanned article pages by OCR methods with records in a bibliographic database. We analyze the process generating the noisy references an
d conclude that the three-step procedure of correcting the OCR results, parsing the corrected string and matching it against the database provides unsatisfactory results. Instead, we propose a method that allows a controlled merging of correction, parsing and matching, inspired by dependency grammars. We also report on the effectiveness of various heuristics that we have employed to improve recall.
We examine the modes admitted by the Mestel disk, a disk with a globally flat rotation curve. In contrast to previous analyses of this problem by Zang (cite{1976PhDT........26Z}) and Evans & Read (cite{1998MNRAS.300...83E}, cite{1998MNRAS.300..106E})
, we approximate the orbits to obtain almost closed expressions for the kernel of the integral equation governing the behaviour of the modes. Otherwise we, like them, follow Kalnajs programme to simultaneously solve the Boltzmann and Poisson equations. We investigate the modes admitted by both the self-consistent and a cut-out Mestel disk, the difference being that in the latter, a part of the matter in the disk is immobilised. This breaks the self-similarity and produces a pronouncedly different picture, both technically and in terms of the disk properties. The self-consistent disk is governed by a Cauchy integral equation, the cut-out disk by an integral equation that can be treated as a Fredholm equation of the second kind. In general, our approximation reproduces the results of the previous works remarkably well, yielding quantities mostly within 5% of the values reported by Zang and Evans & Read and thus also the basic result that in a ``standard cut-out disk, only one-armed modes are unstable at the limit of axisymmetric stability. In the self-consistent disk, relatively compact expressions for the kernel allow an intuitive understanding of most of the properties of neutral (non-rotating, non-growing) modes there. We finally show that self-consistent Mestel disks do not admit growing or rotating modes in this sort of stellar-dynamical analysis.
We examine the development of a transient spiral arm in a disk galaxy made up of both gas and stars. To this end we have performed numerical simulations in a shearing sheet (basically a rectangular patch of a disc) that contains gas in the form of cl
ouds behaving like Brahics (1977) sticky particles, and stars that appear as a background continuum providing the perturbation forces. These are computed from the theory of swing amplification, using Fuchs (1991) work. We describe the evolution of our model under a single and under recurring swing amplification events, discerning three phases. Furthermore, we give an interpretation of this evolution in terms of a variation of the epicyclic frequency with the distance to the wave crest. We also assess the importance of self gravity in the gas for our results.
