861 research outputs found
Heating and cooling in stellar coronae: coronal rain on a young Sun
Recent observations of rapidly-rotating cool dwarfs have revealed H
line asymmetries indicative of clumps of cool, dense plasma in the stars'
coronae. These clumps may be either long-lived (persisting for more than one
stellar rotation) or dynamic. The fastest dynamic features show velocities
greater than the escape speed, suggesting that they may be centrifugally
ejected from the star, contributing to the stellar angular momentum loss. Many
however show lower velocities, similar to coronal rain observed on the Sun. We
present 2.5D magnetohydrodynamic simulations of the formation and dynamics of
these condensations in a rapidly rotating ()
young Sun. Formation is triggered by excess surface heating. This pushes the
system out of thermal equilibrium and triggers a thermal instability. The
resulting condensations fall back towards the surface. They exhibit
quasi-periodic behaviour, with periods longer than typical periods for solar
coronal rain. We find line-of-sight velocities for these clumps in the range
(blue shifted) to $250 \ \mathrm{km} \
\mathrm{s}^{-1}\alpha3.6\times
10^{14}\ \mathrm{g}\simeq~3\%$ of the coronal mass is cool clumps. We conclude that coronal rain
may be common in solar like stars, but may appear on much larger scales in
rapid rotators.Comment: 11 pages, 5 figure
Heating and cooling in stellar coronae: coronal rain on a young Sun
Funding: SD-Y and MJ acknowledge support from STFC consolidated grant number ST/R000824/1. This work was performed using the DiRAC Data Intensive service at Leicester, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility (www.dirac.ac.uk). The equipment was funded by BEIS capital funding via STFC capital grants ST/K000373/1 and ST/R002363/1 and STFC DiRAC Operations grant ST/R001014/1. DiRAC is part of the National e-Infrastructure. CDJ acknowledges support from the NASA GSFC Internal Scientist Funding Model (competitive work package) programme.Recent observations of rapidly rotating cool dwarfs have revealed Hâα line asymmetries indicative of clumps of cool, dense plasma in the starsâ coronae. These clumps may be either long-lived (persisting for more than one stellar rotation) or dynamic. The fastest dynamic features show velocities greater than the escape speed, suggesting that they may be centrifugally ejected from the star, contributing to the stellar angular momentum loss. Many, however, show lower velocities, similar to coronal rain observed on the Sun. We present 2.5D magnetohydrodynamic simulations of the formation and dynamics of these condensations in a rapidly rotating (Prot = 1âd) young Sun. Formation is triggered by excess surface heating. This pushes the system out of thermal equilibrium and triggers a thermal instability. The resulting condensations fall back towards the surface. They exhibit quasi-periodic behaviour, with periods longer than typical periods for solar coronal rain. We find line-of-sight velocities for these clumps in the range of 50âkmâsâ1 (blueshifted) to 250âkmâsâ1 (redshifted). These are typical of those inferred from stellar Hâα line asymmetries, but the inferred clump masses of 3.6 Ă 1014âg are significantly smaller. We find that a maximum of of the coronal mass is cool clumps. We conclude that coronal rain may be common in solar-like stars, but may appear on much larger scales in rapid rotators.Publisher PDFPeer reviewe
THE PENCIL-LESS ARCHITECT\u27S OFFICE: A 66DEVIANT CASE STUDY OF THE DYNAMICS OF STRATEGIC CHANGE AND INFORMATION TECHNOLOGY
The dominant view in the information technology (IT) strategy literature implicitly or explicitly incorporates a normative model of dynamic alignment in which business strategy is seen as the primary driver of strategic adaptation. This paper describes and analyzes a case study of the strategic application of IT where success emerged via a different process. As well as providing evidence of a path to strategic fit that is rarely discussed in the literature, the case points to mastery and the management of risk as critical factors in the process of IT-based strategic change
Chandra X-ray Sources in the Collapsed-Core Globular Cluster M30 (NGC 7099)
We report the detection of six discrete, low-luminosity (Lx < 10^33 erg/s)
X-ray sources, located within 12 arcsec of the center of the collapsed-core
globular cluster M30 (NGC 7099), and a total of 13 sources within the half-mass
radius, from a 50 ksec Chandra ACIS-S exposure. Three sources lie within the
very small upper limit of 1.9 arcsec on the core radius. The brightest of the
three core sources has a luminosity of Lx (0.5-6 keV) = 6x10^32 erg/s and a
blackbody-like soft X-ray spectrum, which are both consistent with it being a
quiescent low-mass X-ray binary (qLMXB). We have identified optical
counterparts to four of the six central sources and a number of the outlying
sources, using deep Hubble Space Telescope and ground-based imaging. While the
two proposed counterparts that lie within the core may represent chance
superpositions, the two identified central sources that lie outside of the core
have X-ray and optical properties consistent with being CVs. Two additional
sources outside of the core have possible active binary counterparts. We
discuss the X-ray source population of M30 in light of its collapsed-core
status.Comment: 18 pages, 13 figures (8 color), resubmitted to ApJ after
incorporating referee comment
ROSAT HRI X-ray Observations of the Open Globular Cluster NGC 288
A ROSAT HRI X-ray image was obtained of the open globular cluster NGC 288,
which is located near the South Galactic Pole. This is the first deep X-ray
image of this system. We detect a Low Luminosity Globular Cluster X-ray source
(LLGCX) RXJ005245.0-263449 with an X-ray luminosity of (5.5+-1.4)x10^32 ergs/s
(0.1-2.0 keV), which is located very close to the cluster center. There is
evidence for X-ray variability on a time scale of <~ 1 day. The presence of
this LLGCX in such an open cluster suggests that dense stellar systems with
high interaction rates are not needed to form LLGCXs. We also searched for
diffuse X-ray emission from NGC 288. Upper limits on the X-ray luminosities are
L_X^h < 9.5x10^32 ergs/s (0.52-2.02 keV) and L_X^s < 9.3x10^32 ergs/s
(0.11-0.41 keV). These imply upper limits to the diffuse X-ray to optical light
ratios in NGC 288 which are lower than the values observed for X-ray faint
early-type galaxies. This indicates that the soft X-ray emission in these
galaxies is due either to a component which is not present in globular clusters
(e.g., interstellar gas, or a stellar component which is not found in low
metallicity Population II systems), or to a relatively small number of bright
Low Mass X-ray Binaries (LMXBs).Comment: The Astrophysical Journal in press. Minor revisions to improve
presentation. 6 pages with 3 embedded Postscript figures in emulateapj.st
Multi-Stranded Coronal Loops: Quantifying Strand Number and Heating Frequency from Simulated Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) Observations
Coronal loops form the basic building blocks of the magnetically closed solar corona yet much is still to be determined concerning their possible fine-scale structuring and the rate of heat deposition within them. Using an improved multi-stranded loop model to better approximate the numerically challenging transition region, this paper examines synthetic NASA Solar Dynamics Observatory's (SDO) Atmospheric Imaging Assembly (AIA) emission simulated in response to a series of prescribed spatially and temporally random, impulsive and localised heating events across numerous sub-loop elements with a strong weighting towards the base of the structure; the nanoflare heating scenario. The total number of strands and nanoflare repetition times are varied systematically in such a way that the total energy content remains approximately constant across all the cases analysed. Repeated time lag detection during an emission time series provides a good approximation for the nanoflare repetition time for low-frequency heating. Furthermore, using a combination of AIA 171/193 and 193/211 channel ratios in combination with spectroscopic determination of the standard deviation of the loop apex temperature over several hours alongside simulations from the outlined multi-stranded loop model, it is demonstrated that both the imposed heating rate and number of strands can be realised
Mapping the Galactic Halo with blue horizontal branch stars from the 2dF quasar redshift survey
We use 666 blue horizontal branch (BHB) stars from the 2Qz redshift survey to
map the Galactic halo in four dimensions (position, distance and velocity). We
find that the halo extends to at least 100 kpc in Galactocentric distance, and
obeys a single power-law density profile of index ~-2.5 in two different
directions separated by 150 degrees on the sky. This suggests that the halo is
spherical. Our map shows no large kinematically coherent structures (streams,
clouds or plumes) and appears homogeneous. However, we find that at least 20%
of the stars in the halo reside in substructures and that these substructures
are dynamically young. The velocity dispersion profile of the halo appears to
increase towards large radii while the stellar velocity distribution is non
Gaussian beyond 60 kpc. We argue that the outer halo consists of a multitude of
low luminosity overlapping tidal streams from recently accreted objects.Comment: Accepted for publication in the Astrophysical Journal Requires
emulateapj to proces
Hilbert Spaces from Path Integrals
It is shown that a Hilbert space can be constructed for a quantum system
starting from a framework in which histories are fundamental. The Decoherence
Functional provides the inner product on this "History Hilbert space". It is
also shown that the History Hilbert space is the standard Hilbert space in the
case of non-relativistic quantum mechanics.Comment: 22 pages. Minor updates to match published versio
Sub-luminous gamma-Ray pulsars
Most pulsars observed by the Fermi LAT have gamma-ray luminosities scaling
with spindown power Edot as L_gamma (Edot x 10^33 erg/s)^{1/2}. However, there
exist one detection and several upper limits an order of magnitude or more
fainter than this trend. We describe these `sub-luminous' gamma-ray pulsars,
and discuss the case for this being an orientation effect. Of the 12 known
young radio pulsars with Edot>10^34 erg/s and d<2kpc several are substantially
sub-luminous. The limited available geometrical constraints favor aligned
geometries for these pulsars, although no one case for alignment is compelling.
In this scenario GeV emission detected from such sub-luminous pulsars can be
due to a lower altitude, lower-power accelerator gap.Comment: 9 pages, 4 figures; accepted to the Astrophysical Journa
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