1,524 research outputs found
Density Variations over Subparsec Scales in Diffuse Molecular Gas
We present high-resolution observations of interstellar CN, CH, CH^{+},
\ion{Ca}{1}, and \ion{Ca}{2} absorption lines toward the multiple star systems
HD206267 and HD217035. Substantial variations in CN absorption are observed
among three sight lines of HD206267, which are separated by distances of order
10,000 AU; smaller differences are seen for CH, CH^{+}, and \ion{Ca}{1}. Gas
densities for individual velocity components are inferred from a chemical
model, independent of assumptions about cloud shape. While the component
densities can differ by factors of 5.0 between adjacent sightlines, the
densities are always less than 5000 cm^{-3}. Calculations show that the derived
density contrasts are not sensitive to the temperature or reaction rates used
in the chemical model. A large difference in the CH^{+} profiles (a factor of 2
in column density) is seen in the lower density gas toward HD217035.Comment: 9 pages, 2 figures. Accepted for publication in ApJ
OH+ in Diffuse Molecular Clouds
Near ultraviolet observations of OH+ and OH in diffuse molecular clouds
reveal a preference for different environments. The dominant absorption feature
in OH+ arises from a main component seen in CH+ (that with the highest CH+/CH
column density ratio), while OH follows CN absorption. This distinction
provides new constraints on OH chemistry in these clouds. Since CH+ detections
favor low-density gas with small fractions of molecular hydrogen, this must be
true for OH+ as well, confirming OH+ and H2O+ observations with the Herschel
Space Telescope. Our observed correspondence indicates that the cosmic ray
ionization rate derived from these measurements pertains to mainly atomic gas.
The association of OH absorption with gas rich in CN is attributed to the need
for high enough density and molecular fraction before detectable amounts are
seen. Thus, while OH+ leads to OH production, chemical arguments suggest that
their abundances are controlled by different sets of conditions and that they
coexist with different sets of observed species. Of particular note is that
non-thermal chemistry appears to play a limited role in the synthesis of OH in
diffuse molecular clouds.Comment: 15 pages, 4 figures, to appear in ApJ Letter
Variable Interstellar Absorption toward the Halo Star HD 219188 - Implications for Small-Scale Interstellar Structure
Within the last 10 years, strong, narrow Na I absorption has appeared at
v_sun ~ -38 km/s toward the halo star HD 219188; that absorption has continued
to strengthen, by a factor 2-3, over the past three years. The line of sight
appears to be moving into/through a relatively cold, quiescent intermediate
velocity (IV) cloud, due to the 13 mas/yr proper motion of HD 219188; the
variations in Na I probe length scales of 2-38 AU/yr. UV spectra obtained with
the HST GHRS in 1994-1995 suggest N(H_tot) ~ 4.8 X 10^{17} cm^{-2}, ``halo
cloud'' depletions, n_H ~ 25 cm^{-3}, and n_e ~ 0.85-6.2 cm^{-3} (if T ~ 100 K)
for the portion of the IV cloud sampled at that time. The relatively high
fractional ionization, n_e/n_H >~ 0.034, implies that hydrogen must be
partially ionized. The N(Na I)/N(H_tot) ratio is very high; in this case, the
variations in Na I do not imply large local pressures or densities.Comment: 12 pages; aastex; to appear in ApJ
Monitoring the Variable Interstellar Absorption toward HD 219188 with HST/STIS
We discuss the results of continued spectroscopic monitoring of the variable
intermediate-velocity (IV) absorption at v = -38 km/s toward HD 219188. After
reaching maxima in mid-2000, the column densities of both Na I and Ca II in
that IV component declined by factors >= 2 by the end of 2006. Comparisons
between HST/STIS echelle spectra obtained in 2001, 2003, and 2004 and HST/GHRS
echelle spectra obtained in 1994--1995 indicate the following: (1) The
absorption from the dominant species S II, O I, Si II, and Fe II is roughly
constant in all four sets of spectra -- suggesting that the total N(H) and the
(mild) depletions have not changed significantly over a period of nearly ten
years. (2) The column densities of the trace species C I (both ground and
excited fine-structure states) and of the excited state C II* all increased by
factors of 2--5 between 1995 and 2001 -- implying increases in the hydrogen
density n_H (from about 20 cm^{-3} to about 45 cm^{-3}) and in the electron
density n_e (by a factor >= 3) over that 6-year period. (3) The column
densities of C I and C II* -- and the corresponding inferred n_H and n_e --
then decreased slightly between 2001 and 2004. (4) The changes in C I and C II*
are very similar to those seen for Na I and Ca II. The relatively low total
N(H) and the modest n_H suggest that the -38 km/s cloud toward HD 219188 is not
a very dense knot or filament. Partial ionization of hydrogen appears to be
responsible for the enhanced abundances of Na I, C I, Ca II, and C II*. In this
case, the variations in those species appear to reflect differences in density
and ionization [and not N(H)] over scales of tens of AU.Comment: 33 pages, 6 figures, aastex, accepted to Ap
Interstellar absorptions towards the LMC: Small-scale density variations in Milky Way disc gas
Observations show that the ISM contains sub-structure on scales less than 1
pc, detected in the form of spatial and temporal variations in column densities
or optical depth. Despite the number of detections, the nature and ubiquity of
the small-scale structure in the ISM is not yet fully understood. We use UV
absorption data mainly from FUSE and partly from STIS of six LMC stars
(Sk-67{\deg}111, LH54-425, Sk-67{\deg}107, Sk-67{\deg}106, Sk-67{\deg}104, and
Sk-67{\deg}101), all located within 5 arcmin of each other, and analyse the
physical properties of the Galactic disc gas in front of the LMC on sub-pc
scales. We analyse absorption lines of a number of ions within the UV spectral
range. Most importantly, interstellar molecular hydrogen, neutral oxygen, and
fine-structure levels of neutral carbon have been used in order to study
changes in the density and the physical properties of the Galactic disc gas
over small angular scales. While most species do not show any significant
variation in their column densities, we find an enhancement of almost 2 dex for
H_2 from Sk-67{\deg}111 to Sk-67{\deg}101, accompanied by only a small
variation in the OI column density. Based on the formation-dissociation
equilibrium, we trace these variations to the actual density variations in the
molecular gas. On the smallest spatial scale of < 0.08 pc, between
Sk-67{\deg}107 and LH54-425, we find a gas density variation of a factor of
1.8. The line of sight towards LH54-425 does not follow the relatively smooth
change seen from Sk-67{\deg}101 to Sk-67{\deg}111, suggesting that
sub-structure might exist on a smaller spatial scale than the linear extent of
our sight-lines. Our observations suggest that the detected H_2 in these six
lines of sight is not necessarily physically connected, but that we are
sampling molecular cloudlets with pathlengths < 0.1-1.8 pc and possibly
different densities.Comment: 17 pages, 8 figures. Accepted for publication in A&
Observation of interstellar lithium in the low-metallicity Small Magellanic Cloud
The primordial abundances of light elements produced in the standard theory
of Big Bang nucleosynthesis (BBN) depend only on the cosmic ratio of baryons to
photons, a quantity inferred from observations of the microwave background. The
predicted primordial 7Li abundance is four times that measured in the
atmospheres of Galactic halo stars. This discrepancy could be caused by
modification of surface lithium abundances during the stars' lifetimes or by
physics beyond the Standard Model that affects early nucleosynthesis. The
lithium abundance of low-metallicity gas provides an alternative constraint on
the primordial abundance and cosmic evolution of lithium that is not
susceptible to the in situ modifications that may affect stellar atmospheres.
Here we report observations of interstellar 7Li in the low-metallicity gas of
the Small Magellanic Cloud, a nearby galaxy with a quarter the Sun's
metallicity. The present-day 7Li abundance of the Small Magellanic Cloud is
nearly equal to the BBN predictions, severely constraining the amount of
possible subsequent enrichment of the gas by stellar and cosmic-ray
nucleosynthesis. Our measurements can be reconciled with standard BBN with an
extremely fine-tuned depletion of stellar Li with metallicity. They are also
consistent with non-standard BBN.Comment: Published in Nature. Includes main text and Supplementary
Information. Replaced with final title and abstrac
Field evaluation of application variables and plant density for bell pepper pest management
Bell peppers are a valuable vegetable crop in the U.S. Management of pepper insects and diseases relies on chemical control options. Because there are relatively few chemical options available for pepper pest management, it is critical to make efficacious application of pesticides. Foliar spray coverage and spray retention in the middle and bottom of bell pepper canopies were compared using fluorescent dyes and food coloring, respectively. Several delivery systems were evaluated, including air-induction and twin-fan nozzles, air-assisted delivery with conventional hydraulic nozzles, and pneumatic atomization nozzles producing electrically charged sprays. Plant spacing between and within rows was evaluated in the second year of the application trial. Travel speeds of 6.4 and 12.9 km h^-1 were also evaluated. Faster travel speeds did not significantly affect spray retention in twin-row canopies. The electrostatic sprayer produced the greatest differences in deposits between the middle and bottom of the canopy. Although there was no more than a 25 cm difference between leaves sampled from the middle and bottom canopy locations, spray retention on foliage in the bottom canopy locations had significantly lower retention than the middle canopy for almost all sprayer types. Despite differences in atomization characteristics, the performance of the twin-fan nozzle and the air-induction nozzle treatments were similar. Air-assisted delivery provided no advantage in the amount of spray retained on the foliage, but it produced more desirable spray quality on foliage and resulted in more spray retained on whole fruit
Key signal contributions in photothermal deflection spectroscopy
We report on key signal contributions in photothermal deflection spectroscopy
(PDS) of semiconductors at photon energies below the bandgap energy and show
how to extract the actual absorption properties from the measurement data. To
this end, we establish a rigorous computation scheme for the deflection signal
including semi-analytic raytracing to analyze the underlying physical effects.
The computation takes into account linear and nonlinear absorption processes
affecting the refractive index and thus leading to a deflection of the probe
beam. We find that beside the linear mirage effect, nonlinear absorption
mechanisms make a substantial contribution to the signal for strongly focussed
pump beams and sample materials with high two-photon absorption coefficients.
For example, the measured quadratic absorption contribution exceeds 5% at a
pump beam intensity of about in Si and at
in GaAs. In addition, our method also
includes thermal expansion effects as well as spatial gradients of the
attenuation properties. We demonstrate that these effects result in an
additional deflection contribution which substantially depends on the distance
of the photodetector from the readout point. This distance dependent
contribution enhances the surface related PDS signal up to two orders of
magnitude and may be misinterpreted as surface absorption if not corrected in
the analysis of the measurement data. We verify these findings by PDS
measurements on crystalline silicon at a wavelength of 1550 nm and provide
guidelines how to extract the actual attenuation coefficient from the PDS
signal.Comment: 10 pages, 16 figures, submitted to Journal of Applied Physiv
Revisiting the Chlorine Abundance in Diffuse Interstellar Clouds from Measurements with the Copernicus Satellite
We reanalyzed interstellar Cl I and Cl II spectra acquired with the
Copernicus satellite. The directions for this study come from those of Crenny &
Federman and sample the transition from atomic to molecular rich clouds where
the unique chemistry leading to molecules containing chlorine is initiated. Our
profile syntheses relied on up-to-date laboratory oscillator strengths and
component structures derived from published high-resolution measurements of K I
absorption that were supplemented with Ca II and Na I D results. We obtain
self-consistent results for the Cl I lines at 1088, 1097, and 1347 A from which
precise column densities are derived. The improved set of results reveals
clearer correspondences with H2 and total hydrogen column densities. These
linear relationships arise from rapid conversion of Cl^+ to Cl^0 in regions
where H2 is present.Comment: 17 pp, 2 tables, and 3 figures, to appear in The Astrophysical
Journa
- …