17 research outputs found
The Absolute Abundance of Iron in the Solar Corona
We present a measurement of the abundance of Fe relative to H in the solar
corona using a technique which differs from previous spectroscopic and solar
wind measurements. Our method combines EUV line data from the CDS spectrometer
on SOHO with thermal bremsstrahlung radio data from the VLA. The coronal Fe
abundance is derived by equating the thermal bremsstrahlung radio emission
calculated from the EUV Fe line data to that observed with the VLA, treating
the Fe/H abundance as the sole unknown. We apply this technique to a compact
cool active region and find Fe/H
= 1.56 x 10^{-4}, or about 4 times its value in the solar photosphere.
Uncertainties in the CDS radiometric calibration, the VLA intensity
measurements, the atomic parameters, and the assumptions made in the spectral
analysis yield net uncertainties of order 20%. This result implies that low
first ionization potential elements such as Fe are enhanced in the solar corona
relative to photospheric values.Comment: Astrophysical Journal Letters, in pres
Coronal Emission Measures and Abundances for Moderately Active K Dwarfs Observed by Chandra
We have used Chandra to resolve the nearby 70 Oph (K0 V+K5 V) and 36 Oph (K1
V+K1 V) binary systems for the first time in X-rays. The LETG/HRC-S spectra of
all four of these stars are presented and compared with an archival LETG
spectrum of another moderately active K dwarf, Epsilon Eri. Coronal densities
are estimated from O VII line ratios and emission measure distributions are
computed for all five of these stars. We see no substantial differences in
coronal density or temperature among these stars, which is not surprising
considering that they are all early K dwarfs with similar activity levels.
However, we do see significant differences in coronal abundance patterns.
Coronal abundance anomalies are generally associated with the first ionization
potential (FIP) of the elements. On the Sun, low-FIP elements are enhanced in
the corona relative to high-FIP elements, the so-called "FIP effect." Different
levels of FIP effect are seen for our stellar sample, ranging from 70 Oph A,
which shows a prominent solar-like FIP effect, to 70 Oph B, which has no FIP
bias at all or possibly even a weak inverse FIP effect. The strong abundance
difference exhibited by the two 70 Oph stars is unexpected considering how
similar these stars are in all other respects (spectral type, age, rotation
period, X-ray flux). It will be difficult for any theoretical explanation for
the FIP effect to explain how two stars so similar in all other respects can
have coronae with different degrees of FIP bias. Finally, for the stars in our
sample exhibiting a FIP effect, a curious difference from the solar version of
the phenomenon is that the data seem to be more consistent with the high-FIP
elements being depleted in the corona rather than a with a low-FIP enhancementComment: 35 pages, 8 figures, AASTEX v5.0 plus EPSF extensions in mkfig.sty;
accepted by Ap
Argon Abundances in the Solar Neighborhood: Non-LTE Analysis of Orion Association B-type Stars
Argon abundances have been derived for a sample of B main-sequence stars in
the Orion association. The abundance calculations are based on NLTE metal
line-blanketed model atmospheres calculated with the NLTE code TLUSTY and an
updated and complete argon model atom. We derive an average argon abundance for
this young population of A(Ar) = 6.66 +- 0.06. While our result is in excellent
agreement with a recent analysis of the Orion nebula, it is significantly
higher than the currently recommended solar value which is based on abundance
measurements in the solar corona. Moreover, the derived argon abundances in the
Orion B stars agree very well with a measurement from a solar impulsive flare
during which unmodified solar photospheric material was brought to flare
conditions. We therefore argue that the argon abundances obtained independently
for both the Orion B stars and the Orion nebula are representative of the disk
abundance value in the solar neighborhood. The lower coronal abundance may
reflect a depletion related to the FIP effect. We propose a new reference value
for the abundance of argon in the solar neighborhood, A(Ar) = 6.63 +- 0.10,
corresponding to Ar/O = 0.009.Comment: to appear in Astrophysical Journal, 24 pages, 3 figures; minor
corrections after referee's repor
Preferences for Domestic Water Services in the Middle Olifants Sub-Basin of South Africa
Using household survey data, this study investigates preferences for domestic water services in the Middle Olifants sub-basin of South Africa. Water is a relatively scarce resource in South Africa that is distributed unevenly both geographically and seasonally as well as sociopolitically. For a water management addressing the policy objectives of efficiency in use, equity in access and benefits and long-term sustainability, economic valuation of the different water uses is required. In order to detect households' preferences, a choice experiment of the various water services was conducted. Results suggested the presence of preference heterogeneity and therefore, a latent class model was applied, dividing households into homogeneous groups according to their preferences. Four distinct groups of households could be identified which differ significantly in terms of their socio-economic characteristics, their attitudes toward pricing of water and their satisfaction with current water service levels. Willingness to pay (WTP) estimates of different water service characteristics in all groups indicate that households are willing to pay higher prices for a better and more reliable water services provision. But the amount households are willing to pay differs among the groups. This information is helpful for policy-makers to enable the design of water services in the Middle Olifants according to preferences of local households. Besides, WTP estimation can provide a basis for setting water tariffs
§Royal Society for the Protection of Birds, The Lodge, Sandy,
Abstract. This work was undertaken to devise a technique to measure the height of crops in farmland � elds through remote sensing. Crop height is a useful spatial variable which, when measured by ground-based manual survey, has proven to be an important predictor of bird species population. An airborne scanning laser system capable of measuring topography to a height accuracy of better than 10 cm was used to acquire height data over a region of farmland near Oxford, UK. A scanning laser was pulsed from an aircraft at the ground, measuring the time between transmission and receipt of the last signi � cant return signal. DiŒerential Geographical Positioning System (GPS) and onboard attitude sensors were combined with these delay times to construct a set of spot heights through the region. Crop height was also measured from the ground. Pulses were returned from mainly within the crop, rather than predominantly the canopy or ground, so an algorithm to measure the variation of the returned height, after detrending the heights for topography, was developed. A simple relationship was found between the mean crop height and the standard deviation of detrended return heights within a � eld. This relationship could be used to derive crop height from Light Detection and Ranging (LiDAR) data with an accuracy better than 10 cm. 1