6,752 research outputs found
UV Surface Environment of Earth-like Planets Orbiting FGKM Stars Through Geological Evolution
The UV environment of a host star affects the photochemistry in the
atmosphere, and ultimately the surface UV environment for terrestrial planets
and therefore the conditions for the origin and evolution of life. We model the
surface UV radiation environment for Earth-sized planets orbiting FGKM stars at
the 1AU equivalent distance for Earth through its geological evolution. We
explore four different types of atmospheres corresponding to an early Earth
atmosphere at 3.9 Gyr ago and three atmospheres covering the rise of oxygen to
present day levels at 2.0 Gyr ago, 0.8 Gyr ago and modern Earth (Following
Kaltenegger et al. 2007). In addition to calculating the UV flux on the surface
of the planet, we model the biologically effective irradiance, using DNA damage
as a proxy for biological damage. We find that a pre-biotic Earth (3.9 Gyr ago)
orbiting an F0V star receives 6 times the biologically effective radiation as
around the early Sun and 3520 times the modern Earth-Sun levels. A pre-biotic
Earth orbiting GJ 581 (M3.5V) receives 300 times less biologically effective
radiation, about 2 times modern Earth-Sun levels. The UV fluxes calculated here
provide a grid of model UV environments during the evolution of an Earth-like
planet orbiting a range of stars. These models can be used as inputs into
photo-biological experiments and for pre-biotic chemistry and early life
evolution experiments.Comment: 10 pages, 5 figure
Effect of UV Radiation on the Spectral Fingerprints of Earth-like Planets Orbiting M dwarfs
We model the atmospheres and spectra of Earth-like planets orbiting the
entire grid of M dwarfs for active and inactive stellar models with =
2300K to = 3800K and for six observed MUSCLES M dwarfs with UV
radiation data. We set the Earth-like planets at the 1AU equivalent distance
and show spectra from the VIS to IR (0.4m - 20m) to compare
detectability of features in different wavelength ranges with JWST and other
future ground- and spaced-based missions to characterize exo-Earths. We focus
on the effect of UV activity levels on detectable atmospheric features that
indicate habitability on Earth, namely: HO, O, CH, NO and
CHCl.
To observe signatures of life - O/O in combination with reducing
species like CH, we find that early and active M dwarfs are the best
targets of the M star grid for future telescopes. The O spectral feature at
0.76m is increasingly difficult to detect in reflected light of later M
dwarfs due to low stellar flux in that wavelength region. NO, another
biosignature detectable in the IR, builds up to observable concentrations in
our planetary models around M dwarfs with low UV flux. CHCl could become
detectable, depending on the depth of the overlapping NO feature.
We present a spectral database of Earth-like planets around cool stars for
directly imaged planets as a framework for interpreting future lightcurves,
direct imaging, and secondary eclipse measurements of the atmospheres of
terrestrial planets in the HZ to design and assess future telescope
capabilities.Comment: in press, ApJ (submitted August 18, 2014), 16 pages, 12 figure
High-pressure study of substrate material ScAlMgO4
We report on the structural properties of ScAlMgO4 studied under
quasi-hydrostatic pressure using synchrotron high-pressure x-ray diffraction up
to 40 GPa. We also report on single-crystal studies of ScAlMgO4 performed at
300 K and 100 K. We found that the low-pressure phase remains stable up to 24
GPa. At 28 GPa, we detected a reversible phase transformation. The
high-pressure phase is assigned to a monoclinic distortion of the low-pressure
phase. No additional phase transition is observed up to 40 GPa. In addition,
the equation of state, compressibility tensor, and thermal expansion
coefficients of ScAlMgO4 are determined. The bulk modulus of ScAlMgO4 is found
to be 143(8) GPa, with a strong compressibility anisotropy. For the trigonal
low-pressure phase, the compressibility along the c-axis is twice than
perpendicular one. A perfect lattice match with ZnO is retained under pressure
in the pressure range of stability of wurtzite ZnO.Comment: 22 pages, 5 figures, 4 tables, 24 reference
FUSE Observations of Nebular O VI Emission from NGC 6543
NGC 6543 is one of the few planetary nebulae (PNe) whose X-ray emission has
been shown to be extended and originate from hot interior gas. Using FUSE
observations we have now detected nebular O VI emission from NGC 6543. Its
central star, with an effective temperature of ~50,000 K, is too cool to
photoionize O V, so the O VI ions must have been produced by thermal collisions
at the interface between the hot interior gas and the cool nebular shell. We
modeled the O VI emission incorporating thermal conduction, but find that
simplistic assumptions for the AGB and fast wind mass loss rates overproduce
X-ray emission and O VI emission. We have therefore adopted the pressure of the
interior hot gas for the interface layer and find that expected O VI emission
to be comparable to the observations.Comment: 4 pages, 4 figures, 1 table, using emulateapj.cls style. Accepted for
publication in ApJ Letter
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