1,092 research outputs found
The interstellar D1 line at high resolution
Observations at a resolving power or a velocity resolution are reported of the interstellar D(sub 1) line of Na I in the spectra of gamma Cas, delta Ori, epsilon Ori, pi Sco, delta Cyg, and alpha Cyg. An echelle grating was used in a double-pass configuration with a CCD detector in the coude spectrograph of the 2.7 m reflector at McDonald Observatory. At least 42 kinematically distinct clouds are detected along the light paths to the five more distant stars, in addition to a single cloud seen toward delta Cyg. The absorption lines arising in 13 of the clouds are sufficiently narrow and unblended to reveal clearly resolved hyperfine structure components split by 1.05 km/s. An additional 13 clouds apparently show comparably narrow, but more strongly blended, lines. For each individual cloud, upper limits T(sub max) and (v sub t)(sub max) on the temperature and the turbulent velocity, respectively, are derived by fitting the observed lines with theoretical absorption profiles
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
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
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