18,465 research outputs found
Spatially resolving the thermally inhomogeneous outer atmosphere of the red giant Arcturus in the 2.3 micron CO lines
The outer atmosphere of K giants shows thermally inhomogeneous structures
consisting of the hot chromospheric gas and the cool molecular gas. We present
spectro-interferometric observations of the multicomponent outer atmosphere of
the well-studied K1.5 giant Arcturus (alpha Boo) in the CO first overtone lines
near 2.3 micron. We observed Arcturus with the AMBER instrument at the Very
Large Telescope Interferometer (VLTI) at 2.28--2.31 micron with a spectral
resolution of 12000 and at projected baselines of 7.3, 14.6, and 21.8 m. The
high spectral resolution of the VLTI/AMBER instrument allowed us to spatially
resolve Arcturus in the individual CO lines. Comparison of the observed
interferometric data with the MARCS photospheric model shows that the star
appears to be significantly larger than predicted by the model. It indicates
the presence of an extended component that is not accounted for by the current
photospheric models for this well-studied star. We found out that the observed
AMBER data can be explained by a model with two additional CO layers above the
photosphere. The inner CO layer is located just above the photosphere, at 1.04
+/- 0.02 stellar radii, with a temperature of 1600 +/- 400 K and a CO column
density of 10^{20 +/- 0.3} cm^-2. On the other hand, the outer CO layer is
found to be as extended as to 2.6 +/- 0.2 stellar radii with a temperature of
1800 +/- 100 K and a CO column density of 10^{19 +/- 0.15} cm^-2. The
properties of the inner CO layer are in broad agreement with those previously
inferred from the spatially unresolved spectroscopic analyses. However, our
AMBER observations have revealed that the quasi-static cool molecular component
extends out to 2--3 stellar radii, within which region the chromospheric wind
steeply accelerates.Comment: 10 pages, 9 figures, accepted for publication in Astronomy and
Astrophysic
Laser-boosted lightcraft technology demonstrator
The detailed description and performance analysis of a 1.4 meter diameter Lightcraft Technology Demonstator (LTD) is presented. The launch system employs a 100 MW-class ground-based laser to transmit power directly to an advanced combined-cycle engine that propels the 120 kg LTD to orbit - with a mass ratio of two. The single-stage-to-orbit (SSTO) LTD machine then becomes an autonomous sensor satellite that can deliver precise, high quality information typical of today's large orbital platforms. The dominant motivation behind this study is to provide an example of how laser propulsion and its low launch costs can induce a comparable order-of-magnitude reduction in sensor satellite packaging costs. The issue is simply one of production technology for future, survivable SSTO aerospace vehicles that intimately share both laser propulsion engine and satellite functional hardware
Ionization toward the high-mass star-forming region NGC 6334 I
Context. Ionization plays a central role in the gas-phase chemistry of
molecular clouds. Since ions are coupled with magnetic fields, which can in
turn counteract gravitational collapse, it is of paramount importance to
measure their abundance in star-forming regions. Aims. We use spectral line
observations of the high-mass star-forming region NGC 6334 I to derive the
abundance of two of the most abundant molecular ions, HCO+ and N2H+, and
consequently, the cosmic ray ionization rate. In addition, the line profiles
provide information about the kinematics of this region. Methods. We present
high-resolution spectral line observations conducted with the HIFI instrument
on board the Herschel Space Observatory of the rotational transitions with Jup
> 5 of the molecular species C17O, C18O, HCO+, H13CO+, and N2H+. Results. The
HCO+ and N2H+ line profiles display a redshifted asymmetry consistent with a
region of expanding gas. We identify two emission components in the spectra,
each with a different excitation, associated with the envelope of NGC 6334 I.
The physical parameters obtained for the envelope are in agreement with
previous models of the radial structure of NGC 6334 I based on submillimeter
continuum observations. Based on our new Herschel/HIFI observations, combined
with the predictions from a chemical model, we derive a cosmic ray ionization
rate that is an order of magnitude higher than the canonical value of 10^(-17)
s-1. Conclusions. We find evidence of an expansion of the envelope surrounding
the hot core of NGC 6334 I, which is mainly driven by thermal pressure from the
hot ionized gas in the region. The ionization rate seems to be dominated by
cosmic rays originating from outside the source, although X-ray emission from
the NGC 6334 I core could contribute to the ionization in the inner part of the
envelope.Comment: This paper contains a total of 10 figures and 3 table
Laser Shock Microformingof Thin Metal Sheets with ns Lasers
Continuous and long-pulse lasers have been used for the forming of metal sheets in macroscopic mechanical applications. However, for the manufacturing of micro-electromechanical systems (MEMS), the use of ns laser pulses provides a suitable parameter matching over an important range of sheet components that, preserving the short interaction time scale required for the predominantly mechanical (shock) induction of deformation residual stresses, allows for the successful processing of components in a medium range of miniaturization without appreciable thermal deformation.. In the present paper, the physics of laser shock microforming and the influence of the different experimental parameters on the net bending angle are presented
Bright Source Subtraction Requirements For Redshifted 21 cm Measurements
The \hi 21 cm transition line is expected to be an important probe into the
cosmic dark ages and epoch of reionization. Foreground source removal is one of
the principal challenges for the detection of this signal. This paper
investigates the extragalactic point source contamination and how accurately
bright sources ( ~Jy) must be removed in order to detect 21 cm
emission with upcoming radio telescopes such as the Murchison Widefield Array
(MWA). We consider the residual contamination in 21 cm maps and power spectra
due to position errors in the sky-model for bright sources, as well as
frequency independent calibration errors. We find that a source position
accuracy of 0.1 arcsec will suffice for detection of the \hi power spectrum.
For calibration errors, 0.05 % accuracy in antenna gain amplitude is required
in order to detect the cosmic signal. Both sources of subtraction error produce
residuals that are localized to small angular scales, \kperp \gtrsim 0.05
Mpc, in the two-dimensional power spectrum.Comment: 12 pages, 19 Figures, submitted to Ap
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