85 research outputs found
A 4% Geometric Distance to the Galaxy NGC4258 from Orbital Motions in a Nuclear Gas Disk
The water maser in the mildly active nucleus in the nearby galaxy NGC4258
traces a thin, nearly edge-on, subparsec-scale Keplerian disk. Using the
technique of very long baseline interferometry, we have detected the proper
motions of these masers as they sweep in front of the central black hole at an
orbital velocity of about 1100 km/s. The average maser proper motion of 31.5
microarcseconds per year is used in conjunction with the observed acceleration
of the masers to derive a purely geometric distance to the galaxy of 7.2 +- 0.3
Mpc. This is the most precise extragalactic distance measured to date, and,
being independent of all other distance indicators, is likely to play an
important role in calibrating the extragalactic distance scale.Comment: 11 pages, 3 figures. Accepted for publication in Natur
Coexisting conical bipolar and equatorial outflows from a high-mass protostar
The BN/KL region in the Orion molecular cloud is an archetype in the study of
the formation of stars much more massive than the Sun. This region contains
luminous young stars and protostars, but it is difficult to study because of
overlying dust and gas. Our basic expectations are shaped to some extent by the
present theoretical picture of star formation, the cornerstone of which is that
protostars acrete gas from rotating equatorial disks, and shed angular momentum
by ejecting gas in bipolar outflows. The main source of the outflow in the
BN/KL region may be an object known as radio source I, which is commonly
believed to be surrounded by a rotating disk of molecular material. Here we
report high-resolution observations of silicon monoxide (SiO) and water maser
emission from the gas surrounding source I; we show that within 60 AU (about
the size of the Solar System), the region is dominated by a conical bipolar
outflow, rather than the expected disk. A slower outflow, close to the
equatorial plane of the protostellar system, extends to radii of 1,000 AU.Comment: 10 pages, 2 figures. Accepted by Nature. To appear December 199
Theory of disk accretion onto supermassive black holes
Accretion onto supermassive black holes produces both the dramatic phenomena
associated with active galactic nuclei and the underwhelming displays seen in
the Galactic Center and most other nearby galaxies. I review selected aspects
of the current theoretical understanding of black hole accretion, emphasizing
the role of magnetohydrodynamic turbulence and gravitational instabilities in
driving the actual accretion and the importance of the efficacy of cooling in
determining the structure and observational appearance of the accretion flow.
Ongoing investigations into the dynamics of the plunging region, the origin of
variability in the accretion process, and the evolution of warped, twisted, or
eccentric disks are summarized.Comment: Mostly introductory review, to appear in "Supermassive black holes in
the distant Universe", ed. A.J. Barger, Kluwer Academic Publishers, in pres
Investigation of the obscuring circumnuclear torus in the active galaxy Mrk231
Here we report on observations of powerful hydroxyl (OH) line emissions that
trace the obscuring material within the circumnuclear environment of the galaxy
Markarian 231. The hydroxyl (mega)-maser emission shows the characteristics of
a rotating, dusty, molecular torus (or thick disk) located between 30 and 100
pc from the central engine. We now have a clear view of the physical
conditions, the kinematics and the spatial structure of this material on
intermediate size scales, confirming the main tenets of unification models.Comment: 10 pages, including 3 Figures, published in Nature Vol 421 2003; the
published pdf--file and higher quality images are available at
http://www.astro.rug.nl/~hrkloeck/np/pubmrk231.htm
The Hubble Constant
I review the current state of determinations of the Hubble constant, which
gives the length scale of the Universe by relating the expansion velocity of
objects to their distance. There are two broad categories of measurements. The
first uses individual astrophysical objects which have some property that
allows their intrinsic luminosity or size to be determined, or allows the
determination of their distance by geometric means. The second category
comprises the use of all-sky cosmic microwave background, or correlations
between large samples of galaxies, to determine information about the geometry
of the Universe and hence the Hubble constant, typically in a combination with
other cosmological parameters. Many, but not all, object-based measurements
give values of around 72-74km/s/Mpc , with typical errors of 2-3km/s/Mpc.
This is in mild discrepancy with CMB-based measurements, in particular those
from the Planck satellite, which give values of 67-68km/s/Mpc and typical
errors of 1-2km/s/Mpc. The size of the remaining systematics indicate that
accuracy rather than precision is the remaining problem in a good determination
of the Hubble constant. Whether a discrepancy exists, and whether new physics
is needed to resolve it, depends on details of the systematics of the
object-based methods, and also on the assumptions about other cosmological
parameters and which datasets are combined in the case of the all-sky methods.Comment: Extensively revised and updated since the 2007 version: accepted by
Living Reviews in Relativity as a major (2014) update of LRR 10, 4, 200
The Murchison Widefield Array Transients Survey (MWATS). A search for low-frequency variability in a bright Southern hemisphere sample
We report on a search for low-frequency radio variability in 944 bright (>4 Jy at 154 MHz) unresolved, extragalactic radio sources monitored monthly for several years with the Murchison Widefield Array. In the majority of sources, we find very low levels of variability with typical modulation indices 2.8 yr) with time-averaged modulation indices M¯¯¯¯¯=3.1−7.1M¯=3.1−7.1 per cent. With 7/15 of these variable sources having peaked spectral energy distributions, and only 5.7 per cent of the overall sample having peaked spectra, we find an increase in the prevalence of variability in this spectral class. We conclude that the variability seen in this survey is most probably a consequence of refractive interstellar scintillation and that these objects must have the majority of their flux density contained within angular diameters less than 50 milliarcsec (which we support with multiwavelength data). At 154 MHz, we demonstrate that interstellar scintillation time-scales become long (∼decades) and have low modulation indices, while synchrotron-driven variability can only produce dynamic changes on time-scales of hundreds of years, with flux density changes less than one milli-jansky (without relativistic boosting). From this work, we infer that the low-frequency extragalactic southern sky, as seen by SKA-Low, will be non-variable on time-scales shorter than 1 yr
Does switching from oral extended-release methylphenidate to the methylphenidate transdermal system affect health-related quality-of-life and medication satisfaction for children with attention-deficit/hyperactivity disorder?
Background:
To evaluate health-related quality of life (HRQL) and medication satisfaction after switching from a stable dose of oral extended-release methylphenidate (ER-MPH) to methylphenidate transdermal system (MTS) via a dose-transition schedule in children with attention-deficit/hyperactivity disorder (ADHD).
Methods:
In a 4-week, multisite, open-label study, 171 children (164 in the intent-to-treat [ITT] population) aged 6-12 years diagnosed with ADHD abruptly switched from a stable dose of oral ER-MPH to MTS nominal dosages of 10, 15, 20, and 30 mg using a predefined dose-transition schedule. Subjects remained on the scheduled dose for the first week, after which the dose was then titrated to an optimal effect. The ADHD Impact Module-Children (AIM-C), a disease-specific validated HRQL survey instrument measuring child and family impact, was used to assess the impact of ADHD symptoms on the lives of children and their families at baseline and study endpoint. Satisfaction with MTS use was assessed via a Medication Satisfaction Survey (MSS) at study endpoint. Both the AIM-C and MSS were completed by a caregiver (parent/legally authorized representative). Tolerability was monitored by spontaneous adverse event (AE) reporting.
Results:
AIM-C child and family HRQL mean scores were above the median possible score at baseline and were further improved at endpoint across all MTS doses. Similar improvements were noted for behavior, missed doses, worry, and economic impact AIM-C item scores. Overall, 93.8% of caregivers indicated a high level of satisfaction with their child's use of the study medication. The majority of treatment-emergent AEs (> 98%) were mild to moderate in intensity, and the most commonly reported AEs included headache, decreased appetite, insomnia, and abdominal pain. Seven subjects discontinued the study due to intolerable AEs (n = 3) and application site reactions (n = 4).
Conclusion:
This study demonstrates that MTS, when carefully titrated to optimal dose, may further improve child and family HRQL, as well as behavioral, medication worry, and economic impact item scores, as measured by the AIM-C in subjects switching to MTS from a stable dose of routinely prescribed oral ER-MPH after a short treatment period. Furthermore, following the abrupt conversion from oral ER-MPH to MTS, the majority of caregivers reported being highly satisfied with MTS as a treatment option for their children with ADHD.
Trial Registration:
NCT0015198
Radio & Optical Interferometry: Basic Observing Techniques and Data Analysis
Astronomers usually need the highest angular resolution possible, but the
blurring effect of diffraction imposes a fundamental limit on the image quality
from any single telescope. Interferometry allows light collected at
widely-separated telescopes to be combined in order to synthesize an aperture
much larger than an individual telescope thereby improving angular resolution
by orders of magnitude. Radio and millimeter wave astronomers depend on
interferometry to achieve image quality on par with conventional visible and
infrared telescopes. Interferometers at visible and infrared wavelengths extend
angular resolution below the milli-arcsecond level to open up unique research
areas in imaging stellar surfaces and circumstellar environments.
In this chapter the basic principles of interferometry are reviewed with an
emphasis on the common features for radio and optical observing. While many
techniques are common to interferometers of all wavelengths, crucial
differences are identified that will help new practitioners avoid unnecessary
confusion and common pitfalls. Concepts essential for writing observing
proposals and for planning observations are described, depending on the science
wavelength, angular resolution, and field of view required. Atmospheric and
ionospheric turbulence degrades the longest-baseline observations by
significantly reducing the stability of interference fringes. Such
instabilities represent a persistent challenge, and the basic techniques of
phase-referencing and phase closure have been developed to deal with them.
Synthesis imaging with large observing datasets has become a routine and
straightforward process at radio observatories, but remains challenging for
optical facilities. In this context the commonly-used image reconstruction
algorithms CLEAN and MEM are presented. Lastly, a concise overview of current
facilities is included as an appendix.Comment: 45 pages, 14 Figures; an abridged version of a chapter to appear in
Volume 2 of Planets, Stars and Stellar Systems, to be published in 2011 by
Springe
Design and characterization of the Large-aperture Experiment to Detect the Dark Age (LEDA) radiometer systems
The Large-Aperture Experiment to Detect the Dark Age (LEDA) was designed to
detect the predicted O(100)mK sky-averaged absorption of the Cosmic Microwave
Background by Hydrogen in the neutral pre- and intergalactic medium just after
the cosmological Dark Age. The spectral signature would be associated with
emergence of a diffuse Ly background from starlight during 'Cosmic
Dawn'. Recently, Bowman et al. (2018) have reported detection of this predicted
absorption feature, with an unexpectedly large amplitude of 530 mK, centered at
78 MHz. Verification of this result by an independent experiment, such as LEDA,
is pressing. In this paper, we detail design and characterization of the LEDA
radiometer systems, and a first-generation pipeline that instantiates a signal
path model. Sited at the Owens Valley Radio Observatory Long Wavelength Array,
LEDA systems include the station correlator, five well-separated redundant dual
polarization radiometers and backend electronics. The radiometers deliver a
30-85MHz band (16<z<34) and operate as part of the larger interferometric
array, for purposes ultimately of in situ calibration. Here, we report on the
LEDA system design, calibration approach, and progress in characterization as
of January 2016. The LEDA systems are currently being modified to improve
performance near 78 MHz in order to verify the purported absorption feature
Networks or structures? : organizing cultural routes around heritage values : case studies from Poland
The most common way of managing cultural heritage recently takes form of cultural routes as they seem to offer a new model of participation in culture to their recipients; they are often a peculiar anchor point for inhabitants to let them understand their identity and form the future; they offer actual tours to enter into interaction with culture and history, to build together that creation of the heritage, which so is becoming not only a touristic product, but, first of all, the space for cultural, social and civic activity. Yet, so far, according to what we know, the research problem concerning the method of cultural route organization (points on the route) into solid structures or more of the networked nature, has not been deliberated. A question arises, what values are brought by routes and how to organize routes to be the carriers of the values important for communities, where routes are functioning. And, as a consequence, if, from the point of view of the values of local communities, organizing solid route structures or organizing more widely-spaced, network-based routes would bring effects and what those effects would be. Thus, the posed question is of course scientifically imprecise because a network is a type of structure but presents a given direction for the development of cultural route structures. Our objective here is to present a certain solidity and rigidity of structure with dynamic and smooth understanding of the network. The research presented in the article is based on 3 case studies. We have selected for this purpose the three largest cultural routes in Poland, organized to various degrees. The outcome of the research was referred also to other cultural route organization research
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