16,162 research outputs found
The planetary nebula IC 5148 and its ionized halo
Many round or nearly roundish Planetary Nebulae (PNe) show multiple shells
and halo structures during their evolutionary stage near the maximum
temperature of their central star. Controversial debate is currently ongoing if
these structures are recombination halos, as suggested by hydrodynamic
modelling efforts, or ionized material. Recently we discovered a halo with even
somewhat unusual structures around the sparsely studied PN IC~5148 and present
for the first time spectroscopy going out to the halo of such a PN.} resolution
spectroscopy is used to derive dust chemistry and mineralogy. We investigate
the spatial distribution of material and its ionization state from the center
of the nebula up to the very outskirts of the halo. We obtained long-slit low
resolution spectroscopy (FORS2@VLT) of the nebula in two position angles, which
we used to investigate the nebular structure and its halo in the optical range
from 450 to 880\,nm. In addition we used medium resolution spectra taken with
X-SHOOTER@VLT ranging from 320 nm to 2.4 mu to derive atmospheric parameters
for the central star. We obtained the distance and position in the Galaxy from
various methods combined with GAIA DR2 data. We also applied Cloudy models to
the nebula in order to derive physical parameters of the various regions. We
obtained spatially resolved structures and detailed descriptions of the
outrunning shock front and a set of unusual halo structures denoted to further
shock. The halo structures appears clearly as hot ionized material. Furthermore
we derived a reliable photometric value for the central star at a GAIA distance
of D=1.3kpc. Considering the large distance \,kpc from the galactic
plane together to its non-circular motion in the galaxy and, a metallicity only
slightly below that of typical disk PNe, most likely IC 5148 originates from a
thick disk population star.Comment: 12 pages, 17 figures, accepted for publication in Astronomy &
Astrophysic
Absence of structural correlations of magnetic defects in heavy fermion LiV2O4
Magnetic defects have pronounced effects on the magnetic properties of the
face-centered cubic compound LiV2O4. The magnetic defects arise from crystal
defects present within the normal spinel structure. High-energy x-ray
diffraction studies were performed on LiV2O4 single crystals to search for
superstructure peaks or any other evidence of periodicity in the arrangement of
the crystal defects present in the lattice. Entire reciprocal lattice planes
are mapped out with help of synchrotron radiation. No noticeable differences in
the x-ray diffraction data between a crystal with high magnetic defect
concentration and a crystal with low magnetic defect concentration have been
found. This indicates the absence of any long-range periodicity or short-range
correlations in the arrangements of the crystal/magnetic defects.Comment: 6 pages, 4 figure
Organic Haze as a Biosignature in Anoxic Earth-like Atmospheres
Early Earth may have hosted a biologically-mediated global organic haze
during the Archean eon (3.8-2.5 billion years ago). This haze would have
significantly impacted multiple aspects of our planet, including its potential
for habitability and its spectral appearance. Here, we model worlds with
Archean-like levels of carbon dioxide orbiting the ancient sun and an M4V dwarf
(GJ 876) and show that organic haze formation requires methane fluxes
consistent with estimated Earth-like biological production rates. On planets
with high fluxes of biogenic organic sulfur gases (CS2, OCS, CH3SH, and
CH3SCH3), photochemistry involving these gases can drive haze formation at
lower CH4/CO2 ratios than methane photochemistry alone. For a planet orbiting
the sun, at 30x the modern organic sulfur gas flux, haze forms at a CH4/CO2
ratio 20% lower than at 1x the modern organic sulfur flux. For a planet
orbiting the M4V star, the impact of organic sulfur gases is more pronounced:
at 1x the modern Earth organic sulfur flux, a substantial haze forms at CH4/CO2
~ 0.2, but at 30x the organic sulfur flux, the CH4/CO2 ratio needed to form
haze decreases by a full order of magnitude. Detection of haze at an
anomalously low CH4/CO2 ratio could suggest the influence of these biogenic
sulfur gases, and therefore imply biological activity on an exoplanet. When
these organic sulfur gases are not readily detectable in the spectrum of an
Earth-like exoplanet, the thick organic haze they can help produce creates a
very strong absorption feature at UV-blue wavelengths detectable in reflected
light at a spectral resolution as low as 10. In direct imaging, constraining
CH4 and CO2 concentrations will require higher spectral resolution, and R > 170
is needed to accurately resolve the structure of the CO2 feature at 1.57
{\mu}m, likely, the most accessible CO2 feature on an Archean-like exoplanet.Comment: accepted for publication in Astrobiolog
Possible Implications of Asymmetric Fermionic Dark Matter for Neutron Stars
We consider the implications of fermionic asymmetric dark matter for a "mixed
neutron star" composed of ordinary baryons and dark fermions. We find examples,
where for a certain range of dark fermion mass -- when it is less than that of
ordinary baryons -- such systems can reach higher masses than the maximal
values allowed for ordinary ("pure") neutron stars. This is shown both within a
simplified, heuristic Newtonian analytic framework with non-interacting
particles and via a general relativistic numerical calculation, under certain
assumptions for the dark matter equation of state. Our work applies to various
dark fermion models such as mirror matter models and to other models where the
dark fermions have self interactions.Comment: 20 pages, 6 figure
Improved Astrometry and Photometry for the Luyten Catalog. I. Bright Stars
We outline the construction of an updated version of the New Luyten
Two-Tenths (NLTT) catalog of high proper motion stars, which will contain
improved astrometry and photometry for the vast majority of the ~59,000 stars
in NLTT. The bright end is constructed by matching NLTT stars to Hipparcos,
Tycho-2, and Starnet; the faint end by matching to USNO-A and 2MASS. In this
first paper, we detail the bright-end matching procedure. We show that for the
majority of stars in his catalog, Luyten measured positions accurate to 1" even
though he recorded his results much more coarsely. However, there is a long
tail of position errors, with one error as large as 11 deg. Proper-motion
errors for the stars with small position errors are 24 mas/yr (1 sigma) but
deteriorate to 34 mas/yr for stars with inferior positions. NLTT is virtually
100% complete for V15 deg, but completeness in this magnitude
range falls to about 75% at the Galactic plane. Incompleteness near the plane
is not uniform, but is rather concentrated in the interval -80<l<20, where the
Milky Way is brightest.Comment: Submitted to ApJ, 28 pages including 7 figure
Current-induced spin polarization in InGaAs and GaAs epilayers with varying doping densities
The current-induced spin polarization and momentum-dependent spin-orbit field
were measured in InGaAs epilayers with varying indium
concentrations and silicon doping densities. Samples with higher indium
concentrations and carrier concentrations and lower mobilities were found to
have larger electrical spin generation efficiencies. Furthermore,
current-induced spin polarization was detected in GaAs epilayers despite the
absence of measurable spin-orbit fields, indicating that the extrinsic
contributions to the spin polarization mechanism must be considered.
Theoretical calculations based on a model that includes extrinsic contributions
to the spin dephasing and the spin Hall effect, in addition to the intrinsic
Rashba and Dresselhaus spin-orbit coupling, are found to qualitatively agree
with the experimental results.Comment: 16 pages, 8 figure
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