699 research outputs found
Gas gain on single wire chambers filled with pure isobutane at low pressure
The gas gain of single-wire chambers filled with isobutane, with cell
cross-section 12x12 mm and wire diameters of 15, 25, 50 and 100 m, has
been measured at pressures ranging 12-92 Torr. Contrary to the experience at
atmospheric pressure, at very low pressures the gas gain on thick wires is
higher than that on thin wires at the same applied high voltage as was recently
shown. Bigger wire diameters should be used in wire chambers operating at very
low pressure if multiple scattering on wires is not an issue.Comment: 9 pages, 6 figure
The Abundance Of Boron In Diffuse Interstellar Clouds
We present a comprehensive survey of boron abundances in diffuse interstellar clouds from observations made with the Space Telescope Imaging Spectrograph (STIS) of the Hubble Space Telescope. Our sample of 56 Galactic sight lines is the result of a complete search of archival STIS data for the B II lambda 1362 resonance line, with each detection confirmed by the presence of absorption from O I lambda 1355, Cu II lambda 1358, and Ga II lambda 1414 (when available) at the same velocity. Five previous measurements of interstellar B II from Goddard High Resolution Spectrograph observations are incorporated in our analysis, yielding a combined sample that more than quadruples the number of sight lines with significant boron detections. Our survey also constitutes the first extensive analysis of interstellar gallium from STIS spectra and expands on previously published results for oxygen and copper. The observations probe both high-and low-density diffuse environments, allowing the density-dependent effects of interstellar depletion to be clearly identified in the gas-phase abundance data for each element. In the case of boron, the increase in relative depletion with line-of-sight density amounts to an abundance difference of 0.8 dex between the warm and cold phases of the diffuse interstellar medium. The abundance of boron in warm, low-density gas is found to be B/H = (2.4 +/- 0.6) x 10(-10), which represents a depletion of 60% relative to the meteoritic boron abundance. Beyond the effects of depletion, our survey reveals sight lines with enhanced boron abundances that potentially trace the recent production of B-11, resulting from spallation reactions involving either cosmic rays or neutrinos. Future observations will help to disentangle the relative contributions from the two spallation channels for B-11 synthesis.Robert A. Welch Foundation F-634Space Telescope Science Institute HST-AR-11247.01-AAssociation of Universities for Research in Astronomy, Inc., under NASA NAS5-26555Astronom
High-Resolution Measurements of Intersystem Bands of Carbon Monoxide toward X Persei
In an echelle spectrum of X Per acquired with the Space Telescope Imaging
Spectrograph we have identified individual rotational lines of 11
triplet-singlet (intersystem) absorption bands of ^12CO. Four bands provide
first detections for interstellar clouds. From a comparison with the zeta Oph
sight line we find that X Per is obscured by a higher 12CO column density of
1.4 x 10^16 cm-2. Together with the high spectral resolution of 1.3 km s-1,
this allows (i) an improved measurement of previously published f-values for
seven bands, and (ii) an extraction of the first astrophysical oscillator
strengths for d-X (8-0), (9-0), and (10-0), as well as for e-X (12-0). The
^13CO d-X (12-0) band, previously suspected to exist toward zeta Oph, is now
readily resolved and modeled. Our derived intersystem f-values for ^12CO
include a few mild (leq 34%) disagreements with recent predictions from a
perturbation analysis calculated for the interstellar excitation temperature.
Overall, the comparison confirms the superiority of employing multiple singlet
levels in the calculations of mixing coefficients over previous single-level
predictions.Comment: 11 pages (incl. 1 figure). Accepted by ApJ Letter
Boron Abundances in Diffuse Interstellar Clouds
We present a comprehensive survey of B abundances in diffuse interstellar
clouds from HST/STIS observations along 56 Galactic sight lines. Our sample is
the result of a complete search of archival STIS data for the B II resonance
line at 1362 angstroms, with each detection confirmed by the presence of
absorption from other dominant ions at the same velocity. The data probe a
range of astrophysical environments including both high-density regions of
massive star formation as well as low-density paths through the Galactic halo,
allowing us to clearly define the trend of B depletion onto interstellar grains
as a function of gas density. Many extended sight lines exhibit complex
absorption profiles that trace both local gas and gas associated with either
the Sagittarius-Carina or Perseus spiral arm. Our analysis indicates a higher
B/O ratio in the inner Sagittarius-Carina spiral arm than in the vicinity of
the Sun, which may suggest that B production in the current epoch is dominated
by a secondary process. The average gas-phase B abundance in the warm diffuse
ISM is consistent with the abundances determined for a variety of Galactic disk
stars, but is depleted by 60 percent relative to the solar system value. Our
survey also reveals sight lines with enhanced B abundances that potentially
trace recent production of B-11 either by cosmic-ray or neutrino-induced
spallation. Such sight lines will be key to discerning the relative importance
of the two production routes for B-11 synthesis.Comment: To be published in the proceedings of the IAU Symposium 268, Light
Elements in the Universe, C. Charbonnel, M. Tosi, F. Primas & C. Chiappini,
ed
Recommended from our members
Push-pull experiments to evaluate in Situ arsenic remediation in the Ogallala aquifer
Arsenic concentrations exceed the U.S. Environmental Protection Agency maximum contaminant level (MCL) for drinking water (10 μg/L) in 220 Public Water Systems in Texas. Conventional treatments for arsenic removal are ex-situ treatments, which are costly and produce high concentration arsenic-contaminated sludge which must be disposed of properly.
A pilot study to assess an in-situ arsenic remediation treatment was conducted in the city of Andrews wellfield, Andrews County, Texas. This procedure involved adding dissolved iron to the aquifer through water wells. Under oxidizing conditions, the dissolved iron precipitates as iron oxides near the well. Water pumped from the well following iron precipitation flows through the iron-enriched zone and arsenic is adsorbed onto the iron oxides.
Two experiments were conducted in which 24 g and 72 g of Fe were injected into an experimental well. Prior to the experiments, arsenic concentration in the well water was 43 μg/L. Following the first experiment, arsenic concentration in produced water stabilized at 25 μg/L, and after the second experiment arsenic values stabilized at 20 μg/L, representing an approximate 50% reduction in produced arsenic under the test conditions.
These preliminary results suggest that there is potential for treating groundwater arsenic contamination in small public water systems using in situ treatment based on addition of iron to the wells. The reconnaissance tests showed a 50% reduction in arsenic concentration; however, additional testing is required to determine if arsenic concentrations can be reduced to less than the MCL of 10 mg/L and the frequency and volume (mass) of treatment injections required to maintain these low arsenic concentrations needs to be determined for full demand production rates.Bureau of Economic Geolog
Interstellar CN and CH+ in Diffuse Molecular Clouds: 12C/13C Ratios and CN Excitation
We present very high signal-to-noise ratio absorption-line observations of CN
and CH+ along 13 lines of sight through diffuse molecular clouds. The data are
examined to extract precise isotopologic ratios of 12CN/13CN and 12CH+/13CH+ in
order to assess predictions of diffuse cloud chemistry. Our results on
12CH+/13CH+ confirm that this ratio does not deviate from the ambient 12C/13C
ratio in local interstellar clouds, as expected if the formation of CH+
involves nonthermal processes. We find that 12CN/13CN, however, can be
significantly fractionated away from the ambient value. The dispersion in our
sample of 12CN/13CN ratios is similar to that found in recent surveys of
12CO/13CO. For sight lines where both ratios have been determined, the
12CN/13CN ratios are generally fractionated in the opposite sense compared to
12CO/13CO. Chemical fractionation in CO results from competition between
selective photodissociation and isotopic charge exchange. An inverse
relationship between 12CN/13CN and 12CO/13CO follows from the coexistence of CN
and CO in diffuse cloud cores. However, an isotopic charge exchange reaction
with CN may mitigate the enhancements in 12CN/13CN for lines of sight with low
12CO/13CO ratios. For two sight lines with high values of 12CO/13CO, our
results indicate that about 50 percent of the carbon is locked up in CO, which
is consistent with the notion that these sight lines probe molecular cloud
envelopes where the transition from C+ to CO is expected to occur. An analysis
of CN rotational excitation yields a weighted mean value for T_01(12CN) of
2.754 +/- 0.002 K, which implies an excess over the temperature of the cosmic
microwave background of only 29 +/- 3 mK. This modest excess eliminates the
need for a local excitation mechanism beyond electron and neutral collisions.
The rotational excitation temperatures in 13CN show no excess over the
temperature of the CMB.Comment: 27 pages, 21 figures, emulateapj style, accepted for publication in
Ap
FUSE Measurements of Interstellar Fluorine
The source of fluorine is not well understood, although core-collapse
supernovae, Wolf-Rayet stars, and asymptotic giant branch stars have been
suggested. A search for evidence of the nu process during Type II supernovae is
presented. Absorption from interstellar F I is seen in spectra of HD 208440 and
HD 209339A acquired with the Far Ultraviolet Spectroscopic Explorer. In order
to extract the column density for F I from the line at 954 A, absorption from
H2 has to be modeled and then removed. Our analysis indicates that for H2
column densities less than about 3 x 10^20 cm^-2, the amount of F I can be
determined from lambda 954. For these two sight lines, there is no clear
indication for enhanced F abundances resulting from the nu process in a region
shaped by past supernovae.Comment: 17 pages, 4 figures, accepted for publication in Ap
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