15,451 research outputs found
Asteroseismic Signatures of Stellar Magnetic Activity Cycles
Observations of stellar activity cycles provide an opportunity to study
magnetic dynamos under many different physical conditions. Space-based
asteroseismology missions will soon yield useful constraints on the interior
conditions that nurture such magnetic cycles, and will be sensitive enough to
detect shifts in the oscillation frequencies due to the magnetic variations. We
derive a method for predicting these shifts from changes in the Mg II activity
index by scaling from solar data. We demonstrate this technique on the
solar-type subgiant beta Hyi, using archival International Ultraviolet Explorer
spectra and two epochs of ground-based asteroseismic observations. We find
qualitative evidence of the expected frequency shifts and predict the optimal
timing for future asteroseismic observations of this star.Comment: 5 pages including 3 figures and 1 table, MNRAS Letters accepte
The Abundance of Interstellar Fluorine and Its Implications
We report results from a survey of neutral fluorine (F I) in the interstellar
medium. Data from the Far Ultraviolet Spectroscopic Explorer (FUSE) were used
to analyze 26 lines of sight lying both in the galactic disk and halo,
including lines to Wolf-Rayet stars and through known supernova remnants. The
equivalent widths of fluorine resonance lines at 951.871 A and 954.827 A were
measured or assigned upper limits and combined with a nitrogen curve of growth
to obtain F I column densities. These column densities were then used to
calculate fluorine depletions. Comparisons are made to the previous study of F
I by Federman et al. (2005) and implications for F I formation and depletion
are discussed.Comment: 32 pages, 10 figures, Accepted to Ap
A method for predicting the stability characteristics of an elastic airplane. Volume 4: FLEXSTAB 1.02.00 demonstration case and results
Circumstellar grain extinction properties of recently discovered post AGB stars
The circumstellar grains of two hot evolved post asymptotic giant branch (post AGB) stars, HD 89353 and HD 213985 were examined. From ultraviolet spectra, energy balance of the flux, and Kurucz models, the extinction around 2175 A was derived. With visual spectra, an attempt was made to detect 6614 A diffuse band absorption arising from the circumstellar grains so that we could examine the relationship of these features to the infrared features. For both stars, we did not detect any diffuse band absorption at 6614 A, implying the carrier of this diffuse band is not the carrier of the unidentified infrared features not of the 2175 A bump. The linear ultraviolet extinction of the carbon-rich star HD 89353 was determined to continue across the 2175 A region with no sign of the bump; for HD 213985 it was found to be the reverse: a strong, wide bump in the mid-ultraviolet. The 213985 bump was found to be positioned at 2340 A, longward of its usual position in the interstellar medium. Since HD 213985 was determined to have excess carbon, the bump probably arises from a carbonaceous grain. Thus, in view of the ultraviolet and infrared properties of the two post AGB stars, ubiquitous interstellar infrared emission features do not seem to be associated with the 2175 A bump. Instead, the infrared features seem related to the linear ultraviolet extinction component: hydrocarbon grains of radius less than 300 A are present with the linear HD 89353 extinction; amorphous anhydrous carbonaceous grains of radius less than 50 A might cause the shifted ultraviolet extinction bump of HD 213985
Search for Spin-Dependent Short-Range Force Using Optically Polarized He Gas
We propose a new method to detect short-range \textit{P-} and \textit{T-}
violating interactions between nucleons, based on measuring the precession
frequency shift of polarized He nuclei in the presence of an unpolarized
mass. To maximize the sensitivity, a high-pressure He cell with thin glass
windows (250 ) is used to minimize the distance between the mass and
He. The magnetic field fluctuation is suppressed by using the He gas in
a different region of the cell as a magnetometer. Systematic uncertainties from
the magnetic properties of the mass are suppressed by flipping both the
magnetic field and spin directions. Without any magnetic shielding, our result
has already reached the sensitivity of the current best limit. With improvement
in uniformity and stability of the field, we can further improve the
sensitivity by two orders of magnitude over the force range from
m
Precision determination of absolute neutron flux
A technique for establishing the total neutron rate of a highly-collimated
monochromatic cold neutron beam was demonstrated using a method of an
alpha-gamma counter. The method involves only the counting of measured rates
and is independent of neutron cross sections, decay chain branching ratios, and
neutron beam energy. For the measurement, a target of 10B-enriched boron
carbide totally absorbed the neutrons in a monochromatic beam, and the rate of
absorbed neutrons was determined by counting 478keV gamma rays from neutron
capture on 10B with calibrated high-purity germanium detectors. A second
measurement based on Bragg diffraction from a perfect silicon crystal was
performed to determine the mean de Broglie wavelength of the beam to a
precision of 0.024 %. With these measurements, the detection efficiency of a
neutron monitor based on neutron absorption on 6Li was determined to an overall
uncertainty of 0.058 %. We discuss the principle of the alpha-gamma method and
present details of how the measurement was performed including the systematic
effects. We also describe how this method may be used for applications in
neutron dosimetry and metrology, fundamental neutron physics, and neutron cross
section measurements.Comment: 44 page
Formation of buckminsterfullerene (C60) in interstellar space
Buckminsterfullerene (C60) was recently confirmed to be the largest molecule
identified in space. However, it remains unclear how, and where this molecule
is formed. It is generally believed that C60 is formed from the build up of
small carbonaceous compounds, in the hot and dense envelopes of evolved stars.
Analyzing infrared observations, obtained by Spitzer and Herschel, we found
that C60 is efficiently formed in the tenuous and cold environment of an
interstellar cloud illuminated by strong ultraviolet (UV) radiation fields.
This implies that another formation pathway, efficient at low densities, must
exist. Based on recent laboratory and theoretical studies, we argue that
Polycyclic Aromatic Hydrocarbons are converted into graphene, and subsequently
C60, under UV irradiation from massive stars. This shows that alternative -
top-down - routes are key to understanding the organic inventory in space.Comment: 22 pages, 4 figures, 3 sup. figures, 1 sup. vide
Intra-Arterial Infusion with Methotrexate in the Rat
The superiority of intra-arterial infusion with methotrexate (MTX) over its systemic use in the treatment of head and neck tumours is still being questioned. A model in the rat, suitable for intra-arterial administration of MTX could be constructed. In this model 3 schedules have been investigated: (1) 7 days continuous intra-arterial infusion with MTX; (2) the same schedule combined with leucovorin (CF) 6-hourly intraperitoneally (i.p.) after Sullivan et al. (1959); (3) intermittent administration of MTX 2 × 24 h intra-arterial infusion on Day 1 and 4, while on Day 2, 3, 5, 6 and 7 the catheter is kept open by the continuous intra-arterial infusion of saline. For all the three schedules intra-arterial MTX proved to be superior to its systemic use
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