29,915 research outputs found
High purity silica reflective heat shield development
Measurements were made of reflectance in the vacuum ultraviolet down to 0.15 micron. Scattering coefficients (S) and absorption coefficients (K) were also measured. These coefficients express the optical properties and are used directly in a thermodynamic analysis for sizing a heat shield. The effect of the thin silica melt layer formed during entry was also studied from the standpoint of trapped radiant energy
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&
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
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&
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