984 research outputs found
Near-infrared spectroscopy of the very low mass companion to the hot DA white dwarf PG1234+482
We present a near-infrared spectrum of the hot (
55,000 K) DA white dwarf PG 1234+482. We confirm that a very low mass companion
is responsible for the previously recognised infrared photometric excess. We
compare spectra of M and L dwarfs, combined with an appropriate white dwarf
model, to the data to constrain the spectral type of the secondary. We find
that uncertainties in the 2MASS photometry of the white dwarf prevent us
from distinguishing whether the secondary is stellar or substellar, and assign
a spectral type of L01 (M9-L1).Therefore, this is the hottest and youngest
( yr) DA white dwarf with a possible brown dwarf companion.Comment: 5 pages, 2 figures, accepted by MNRA
Comparing global optimization and default settings of stream-based joins
One problem encountered in real-time data integration is the join of a continuous incoming data stream with a disk-based relation. In this paper we investigate a stream-based join algorithm, called mesh join (MESHJOIN), and focus on a critical component in the algorithm, called the disk-buffer. In MESHJOIN the size of disk-buffer varies with a change in total memory budget and tuning is required to get the maximum service rate within limited available memory. Until now there was little data on the position of the optimum value depending on the memory size, and no performance comparison has been carried out between the optimum and reasonable default sizes for the disk-buffer. To avoid tuning, we propose a reasonable default value for the disk-buffer size with a small and acceptable performance loss. The experimental results validate our arguments
An event-based near real-time data integration architecture
Extract-Transform-Load (ETL) tools feed data from operational databases into data warehouses. Traditionally, these ETL tools use batch processing and operate offline at regular time intervals, for example on a nightly or weekly basis. Naturally, users prefer to have up-to-date data to make their decisions, therefore there is a demand for real-time ETL tools. In this paper we investigate an event-based near real-time ETL layer for transferring and transforming data from the operational database to the data warehouse. One of our main concerns in this paper is master data management in the ETL layer. We present the architecture of a novel, general purpose, event-driven, and near real-time ETL layer that uses a Database Queue (DBQ), works on a push technology principle and directly supports content enrichment. We also observe that the system architecture is consistent with the information architecture of a classical Online Transaction Processing (OLTP) application, allowing us to distinguish between different kinds of data to increase the clarity of the design. Keywords: event-based architecture, content enrichment, master data, extract-transform-load, enterprise service bus
A high Eddington-ratio, true Seyfert 2 galaxy candidate: implications for broad-line-region models
A bright, soft X-ray source was detected on 2010 July 14 during an
XMM--Newton slew at a position consistent with the galaxy GSN 069 (z=0.018).
Previous ROSAT observations failed to detect the source and imply that GSN 069
is now >240 times brighter than it was in 1994 in the soft X-ray band. We
report here results from a ~1 yr monitoring with Swift and XMM-Newton, as well
as from optical spectroscopy. GSN 069 is an unabsorbed, ultra-soft source in
X-rays, with no flux detected above ~1 keV. The soft X-rays exhibit significant
variability down to timescales of hundreds of seconds. The UV-to-X-ray spectrum
of GSN 069 is consistent with a pure accretion disc model which implies an
Eddington ratio of ~0.5 and a black hole mass of ~ 1.2 million solar masses. A
new optical spectrum, obtained ~3.5 months after the XMM-Newton slew detection,
is consistent with earlier spectra and lacks any broad line component,
classifying the source as a Seyfert 2 galaxy. The lack of cold X-ray absorption
and the short timescale variability in the soft X-rays rule out a standard
Seyfert 2 interpretation of the X-ray data. We discuss our results within the
framework of two possible scenarios for the broad-line-region (BLR) in AGN,
namely the two-phase model (cold BLR clouds in pressure equilibrium with a
hotter medium), and models in which the BLR is part of an outflow, or
disc-wind. Finally, we point out that GSN 069 may be a member of a population
of super-soft AGN whose SED is completely dominated by accretion disc emission,
as it is the case in some black hole X-ray binary transients during their
outburst evolution. The disc emission for a typical AGN with larger black hole
mass than GSN 069 does not enters the soft X-ray band, so that GSN 069-like
objects would likely be missed by current X-ray surveys, or mis-classified as
Compton-thick candidates. (ABRIDGED)Comment: Accepted for publication in MNRA
PHL 5038: a spatially resolved white dwarf + brown dwarf binary
A near-infrared excess is detected at the white dwarf PHL 5038 in UKIDSS photometry, consistent with the presence of a cool, substellar companion. We have obtained H- and K-grism spectra and images of PHL 5038 using NIRI on Gemini North. The target is spatially and spectrally resolved into two components: an 8000 K DA white dwarf, and a likely L8 brown dwarf companion, separated by 0.94\arcsec. The spectral type of the secondary was determined using standard spectral indices for late L and T dwarfs. The projected orbital separation of the binary is 55 AU, so it becomes only the second known wide WD+dL binary to be found after GD 165AB. This object could potentially be used as a benchmark for testing substellar evolutionary models at intermediate to older ages
Irradiated brown dwarfs
We have observed the post common envelope binary WD0137-349 in the near
infrared , and bands and have determined that the photometry varies
on the system period (116 min). The amplitude of the variability increases with
increasing wavelength, indicating that the brown dwarf in the system is likely
being irradiated by its 16500 K white dwarf companion. The effect of the
(primarily) UV irradiation on the brown dwarf atmosphere is unknown, but it is
possible that stratospheric hazes are formed. It is also possible that the
brown dwarf (an L-T transition object) itself is variable due to patchy cloud
cover. Both these scenarios are discussed, and suggestions for further study
are made.Comment: 5 pages, 2 figures. Proceedings from "Brown dwarfs come of age"
meeting in Fuerteventura 201
Further investigation of white dwarfs in the open clusters NGC2287 and NGC3532
We report the results of a CCD imaging survey, complimented by astrometric
and spectroscopic follow-up studies, that aims to probe the fate of
heavy-weight intermediate mass stars by unearthing new, faint, white dwarf
members of the rich, nearby, intermediate age open clusters NGC3532 and
NGC2287. We identify a total of four white dwarfs with distances, proper
motions and cooling times which can be reconciled with membership of these
populations. We find that WDJ0643-203 in NGC2287, with an estimated mass of
M=1.02-1.16Msun, is potentially the most massive white dwarf so far identified
within an open cluster. Guided by the predictions of modern theoretical models
of the late-stage evolution of heavy-weight intermediate mass stars, we
conclude that there is a distinct possibility it has a core composed of O and
Ne. We also determine that despite the cooling times of the three new white
dwarfs in NGC3532 and the previously known degenerate member NGC3532-10
spanning ~90Myr, they all have remarkably similar masses (M~0.9-1Msun). This is
fully consistent with the results from our prior work on a heterogeneous sample
of ~50 white dwarfs from 12 stellar populations, on the basis of which we
argued that the stellar initial mass-final mass relation is less steep at
Minit>4Msun than in the adjacent lower initial mass regime. This change in the
gradient of the relation could account for the secondary peak observed in the
mass distribution of the field white dwarf population and mitigate the need to
invoke close binary evolution to explain its existence. Spectroscopic
investigation of numerous additional candidate white dwarf members of NGC3532
unearthed by a recent independent study would be useful to confirm (or
otherwise) these conclusions.Comment: 8 Figures, 8 tables. Accepted for publication in MNRA
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