60,751 research outputs found
The Chandra X-ray view of the power sources in Cepheus A
The central part of the massive star-forming region Cepheus A contains
several radio sources which indicate multiple outflow phenomena, yet the
driving sources of the individual outflows have not been identified. We present
a high-resolution Chandra observation of this region that shows the presence of
bright X-ray sources, consistent with active pre-main sequence stars, while the
strong absorption hampers the detection of less luminous objects. A new source
has been discovered located on the line connecting H_2 emission regions at the
eastern and western parts of Cepheus A. This source could be the driving source
of HH 168. We present a scenario relating the observed X-ray and radio
emission.Comment: 7 pages, 6 figures, accepted for publication in A&
Brain homeostasis : VEGF receptor 1 and 2 ; two unequal brothers in mind
Vascular endothelial growth factors (VEGFs), initially thought to act specifically on the vascular system, exert trophic effects on neural cells during development and adulthood. Therefore, the VEGF system serves as a promising therapeutic target for brain pathologies, but its simultaneous action on vascular cells paves the way for harmful side effects. To circumvent these deleterious effects, many studies have aimed to clarify whether VEGFs directly affect neural cells or if the effects are mediated secondarily via other cell types, like vascular cells. A great number of reports have shown the expression and function of VEGF receptors (VEGFRs), mainly VEGFR-1 and -2, in neural cells, where VEGFR-2 has been described as the major mediator of VEGF-A signals. This review aims to summarize and compare the divergent roles of VEGFR-1 and -2 during CNS development and homeostasis
The evolution of the X-ray emission of HH 2 - Investigating heating and cooling processes
Young stellar objects often drive powerful bipolar outflows which evolve on
time scales of a few years. An increasing number of these outflows has been
detected in X-rays implying the existence of million degree plasma almost
co-spatial with the lower temperature gas observed in the optical and near-IR.
The details of the heating and cooling processes of the X-ray emitting part of
these so-called Herbig-Haro objects are still ambiguous, e.g., whether the
cooling is dominated by expansion, radiation or thermal conduction.
We present a second epoch Chandra observation of the first X-ray detected
Herbig-Haro object (HH 2) and derive the proper-motion of the X-ray emitting
plasma and its cooling history. We argue that the most likely explanation for
the constancy of the X-ray luminosity, the alignment with the optical emission
and the proper-motion is that the cooling is dominated by radiative losses
leading to cooling times exceeding a decade. We explain that a strong shock
caused by fast material ramming into slower gas in front of it about ten years
ago can explain the X-ray emission while being compatible with the available
multi-wavelength data of HH 2.Comment: 5 pages with 4 figures; accepted for publication by Astronomy and
Astrophysic
Superluminal Caustics of Close, Rapidly-Rotating Binary Microlenses
The two outer triangular caustics (regions of infinite magnification) of a
close binary microlens move much faster than the components of the binary
themselves, and can even exceed the speed of light. When , where
is the caustic speed, the usual formalism for calculating the lens
magnification breaks down. We develop a new formalism that makes use of the
gravitational analog of the Li\'enard-Wiechert potential. We find that as the
binary speeds up, the caustics undergo several related changes: First, their
position in space drifts. Second, they rotate about their own axes so that they
no longer have a cusp facing the binary center of mass. Third, they grow larger
and dramatically so for . Fourth, they grow weaker roughly in
proportion to their increasing size. Superluminal caustic-crossing events are
probably not uncommon, but they are difficult to observe.Comment: 12 pages, 7 ps figures, submitted to Ap
The evolution of the jet from Herbig Ae star HD 163296 from 1999 to 2011
Young A and B stars, the so-called Herbig Ae/Be stars (HAeBe), are surrounded
by an active accretion disk and drive outflows. We study the jet HH 409, which
is launched from the HAeBe star HD 163296, using new and archival observations
from Chandra and HST/STIS. In X-rays we can show that the central source is not
significantly extended. The approaching jet, but not the counter-jet, is
detected in Ly alpha. In addition, there is red-shifted Ly alpha emission
extended in the same direction as the jet, that is also absent in the
counter-jet. We can rule out an accretion or disk-wind origin for this feature.
In the optical we find the knots B and B2 in the counter-jet. Knot B has been
observed previously, so we can derive its proper motion of 0.37+-0.01
arcsec/yr. Its electron density is 3000/cm^3, thus the cooling time scale is a
few months only, so the knot needs to be reheated continuously. The shock speed
derived from models of H alpha and forbidden emission lines (FELs) decreased
from 50 km/s in 1999 to 30 km/s in 2011 because the shock front loses energy as
it travels along the jet. Knot B2 is observed at a similar position in 2011 as
knot B was in 1999, but shows a lower ionization fraction and higher mass loss
rate, proving variations in the jet launching conditions.Comment: 14 pages, 8 figures, accepted by A&
Energy-limited escape revised
Gas planets in close proximity to their host stars experience
photoevaporative mass loss. The energy-limited escape concept is generally used
to derive estimates for the planetary mass-loss rates. Our photoionization
hydrodynamics simulations of the thermospheres of hot gas planets show that the
energy-limited escape concept is valid only for planets with a gravitational
potential lower than ergg because in these planets the radiative energy input is
efficiently used to drive the planetary wind. Massive and compact planets with
ergg
exhibit more tightly bound atmospheres in which the complete radiative energy
input is re-emitted through hydrogen Ly and free-free emission. These
planets therefore host hydrodynamically stable thermospheres. Between these two
extremes the strength of the planetary winds rapidly declines as a result of a
decreasing heating efficiency. Small planets undergo enhanced evaporation
because they host expanded atmospheres that expose a larger surface to the
stellar irradiation. We present scaling laws for the heating efficiency and the
expansion radius that depend on the gravitational potential and irradiation
level of the planet. The resulting revised energy-limited escape concept can be
used to derive estimates for the mass-loss rates of super-Earth-sized planets
as well as massive hot Jupiters with hydrogen-dominated atmospheres.Comment: 5 pages, 5 figures, accepted for publication in A&
High-energy irradiation and mass loss rates of hot Jupiters in the solar neighborhood
Giant gas planets in close proximity to their host stars experience strong
irradiation. In extreme cases photoevaporation causes a transonic, planetary
wind and the persistent mass loss can possibly affect the planetary evolution.
We have identified nine hot Jupiter systems in the vicinity of the Sun, in
which expanded planetary atmospheres should be detectable through Lyman alpha
transit spectroscopy according to predictions. We use X-ray observations with
Chandra and XMM-Newton of seven of these targets to derive the high-energy
irradiation level of the planetary atmospheres and the resulting mass loss
rates. We further derive improved Lyman alpha luminosity estimates for the host
stars including interstellar absorption. According to our estimates WASP-80 b,
WASP-77 b, and WASP-43 b experience the strongest mass loss rates, exceeding
the mass loss rate of HD 209458 b, where an expanded atmosphere has been
confirmed. Furthermore, seven out of nine targets might be amenable to Lyman
alpha transit spectroscopy. Finally, we check the possibility of angular
momentum transfer from the hot Jupiters to the host stars in the three binary
systems among our sample, but find only weak indications for increased stellar
rotation periods of WASP-77 and HAT-P-20.Comment: 11 pages, 5 figures, accepted for publication in A&
Noise Pollution? What\u27s the solution?: Understanding Traffic Noise Pollution in Gettysburg, Pennsylvania
Noise pollution can be damaging to a community by impacting its atmosphere as well as the health of its residents, local quality of life, and local economy. Our study sought to gain insight into the level of noise pollution in Gettysburg, both in-town and on the Gettysburg College campus, and how noise pollution may be affecting the local residents and students. We selected 9 sampling sites, 6 in-town and 3 on the college campus, and measured the noise pollution in dBA with portable sound meters three days a week and three times a day over a three week period. Our data showed no major trends in terms of time of day or day of the week, but there were clear differences in noise pollution levels between the different sites in that town sites were generally louder than on campus sites. Noise pollution in town was often louder than 70 dBA, the noise threshold that indicates possible hearing damage overtime set by the World Health Organization. Gettysburg has recently enacted a noise ordinance; our study suggests the Borough possibly could do more to mitigate traffic noise by repairing roads and manipulating road design
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