73 research outputs found
Oscillator Strengths for B-X, C-X, and E-X Transitions in Carbon Monoxide
Band oscillator strengths for electronic transitions in CO were obtained at
the Synchrotron Radiation Center of the University of Wisconsin-Madison. Our
focus was on transitions that are observed in interstellar spectra with the Far
Ultraviolet Spectroscopic Explorer; these transitions are also important in
studies of selective isotope photodissociation where fractionation among
isotopomers can occur. Absorption from the ground state (X ^1Sigma^+ v'' = 0)
to A ^1Pi (v'= 5), B ^1Sigma^+ (v' = 0, 1), C ^1Sigma^+ (v' = 0, 1), and E ^1Pi
(v' = 0) was measured. Fits to the A - X (5, 0) band, whose oscillator strength
is well known, yielded the necessary column density and excitation temperature.
These parameters were used in a least-squares fit of the observed profiles for
the transitions of interest to extract their band oscillator strengths. Our
oscillator strengths are in excellent agreement with results from recent
experiments using a variety of techniques. This agreement provides the basis
for a self-consistent set of f-values at far ultraviolet wavelengths for
studies of interstellar (and stellar) CO.Comment: 22 pages, 3 figures, ApJS (in press
Reanalysis of Copernicus Measurements on Interstellar Carbon Monoxide
We used archival data acquired with the Copernicus satellite to reexamine CO
column densities because self-consistent oscillator strengths are now
available. Our focus is on lines of sight containing modest amounts of
molecular species. Our resulting column densities are small enough that
self-shielding from photodissociation is not occurring in the clouds probed by
the observations. While our sample shows that the column densities of CO and H2
are related, no correspondence with the CH column density is evident. The case
for the CH+ column density is less clear. Recent chemical models for these
sight lines suggest that CH is mainly a by-product of CH+ synthesis in low
density gas. The models are most successful in reproducing the amounts of CO in
the densest sight lines. Thus, much of the CO absorption must arise from denser
clumps along the line of sight to account for the trend with H2.Comment: 19 pages, 6 figures. Accepted for publication in Ap
Oscillator Strengths and Predissociation Rates for Rydberg Transitions in 12C16O, 13C16O, and 13C18O Involving the E 1Pi, B 1Sigma+, and W 1Pi States
One of the processes controlling the interstellar CO abundance and the ratio
of its isotopologues is photodissociation. Accurate oscillator strengths and
predissociation rates for Rydberg transitions are needed for modeling this
process. We present results on absorption from the E ^1Pi-X ^1Sigma^+ (1-0) and
B ^1Sigma^+-X ^1Sigma^+ (6-0) bands at 1051 and 1002 \AA, respectively, and the
vibrational progression W ^1Pi-X ^1Sigma^+ (v'-0) bands with v' = 0 to 3 at
972, 956, 941, and 925 \AA, respectively. The corresponding spectra were
acquired at the high resolution (R ~ 30,000) SU5 beam line at the Super ACO
Synchrotron in Orsay, France. Spectra were obtained for the ^12C^16O, ^13C^16O,
and ^13C^18O isotopologues. These represent the most complete set of
measurements available. Comparison is made with earlier results, both empirical
and theoretical. While earlier determinations of oscillator strengths based on
absorption from synchrotron radiation tend to be somewhat smaller than ours,
the suite of measurements from a variety of techniques agree for the most part
considering the mutual uncertainties. For the bands studied here, their
relative weakness, or their significant line widths arising from
predissociation, minimizes potential problems from large optical depths at line
center in absorption measurements. Predissociating line widths could generally
be extracted from the spectra thanks to the profile simulations used in the
analysis. In many cases, these simulations allowed us to consider e and f
parity levels separately and to determine the dependence of the width on
rotational quantum number, J. Our results are consistent with earlier
determinations, especially the widths inferred from laser experiments
Oscillator strengths for transitions to Rydberg levels in , and between 967 and 972 A
Absorption oscillator strengths have been determined from high-resolution
spectra in the 967-972 \AA region of three CO isotopomers for transitions to
the Rydberg levels 4{\it p}(0), 3{\it d}(1) and 4{\it p}(0),
as well as to the mixed {\it E(6)} level recently characterized by Eidelsberg
et al. (2004). Synchrotron radiation from the Super-ACO electron storage ring
at Orsay (LURE) was used as a light source. Oscillator strengths were extracted
from the recorded spectra by least-squares fitting of the experimental profiles
with synthetic spectra taking into account the homogeneous and heterogeneous
interactions of the four levels. Column densities were derived from fits to the
3{\it p}(0) absorption band whose oscillator strength is well established.
These are the first reported measurements for CO. For
CO, our results are consistent with the larger values obtained in
the most recent laboratory and astronomical studies.Comment: 9 pages 7 figures 3 tables. Accepted in A&A, date of acceptance
11/05/200
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
Far-ultraviolet Spectroscopy of Venus and Mars at 4 A Resolution with the Hopkins Ultraviolet Telescope on Astro-2
Far-ultraviolet spectra of Venus and Mars in the range 820-1840 A at 4 A
resolution were obtained on 13 and 12 March 1995, respectively, by the Hopkins
Ultraviolet Telescope (HUT), which was part of the Astro-2 observatory on the
Space Shuttle Endeavour. Longward of 1250 A, the spectra of both planets are
dominated by emission of the CO Fourth Positive band system and strong OI and
CI multiplets. In addition, CO Hopfield-Birge bands, B - X (0,0) at 1151 A and
C - X (0,0) at 1088 A, are detected for the first time, and there is a weak
indication of the E - X (0,0) band at 1076 A in the spectrum of Venus. The B -
X band is blended with emission from OI 1152. Modeling the relative intensities
of these bands suggests that resonance fluorescence of CO is the dominant
source of the emission, as it is for the Fourth Positive system. Shortward of
Lyman-alpha, other emission features detected include OII 834, OI lambda 989,
HI Lyman-beta, and NI 1134 and 1200. For Venus, the derived disk brightnesses
of the OI, OII, and HI features are about one-half of those reported by Hord et
al. (1991) from Galileo EUV measurements made in February 1990. This result is
consistent with the expected variation from solar maximum to solar minimum. The
ArI 1048, 1066 doublet is detected only in the spectrum of Mars and the derived
mixing ratio of Ar is of the order of 2%, consistent with previous
determinations.Comment: 8 pages, 5 figures, accepted for publication in ApJ, July 20, 200
Further Evidence for Chemical Fractionation from Ultraviolet Observations of Carbon Monoxide
Ultraviolet absorption from interstellar 12CO and 13CO was detected toward
rho Oph A and chi Oph. The measurements were obtained at medium resolution with
the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Column
density ratios, N(12CO)/N(13CO), of 125 \pm 23 and 117 \pm 35 were derived for
the sight lines toward rho Oph A and chi Oph, respectively. A value of 1100 \pm
600 for the ratio N(12C16O)/N(12C18O) toward rho Oph A was also obtained.
Absorption from vibrationally excited H_2 (v" = 3) was clearly seen toward this
star as well.
The ratios are larger than the isotopic ratios for carbon and oxygen
appropriate for ambient interstellar material. Since for both carbon and oxygen
the more abundant isotopomer is enhanced, selective isotopic photodissociation
plays the key role in the fractionation process for these directions. The
enhancement arises because the more abundant isotopomer has lines that are more
optically thick, resulting in more self shielding from dissociating radiation.
A simple argument involving the amount of self shielding [from N(12CO)] and the
strength of the ultraviolet radiation field premeating the gas (from the amount
of vibrationally excited H_2) shows that selective isotopic photodissociation
controls the fractionation seen in these two sight lines, as well as the sight
line to zeta Oph.Comment: 40 pages, 8 figures, to appear in 10 July 2003 issue of Ap
The Fourth Positive System of Carbon Monoxide in the Hubble Space Telescope Spectra of Comets
The rich structure of the Fourth Positive System (A-X) of carbon monoxide
accounts for many of the spectral features seen in long slit HST-STIS
observations of comets 153P/Ikeya-Zhang, C/2001 Q4 (NEAT), and C/2000 WM1
(LINEAR), as well as in the HST-GHRS spectrum of comet C/1996 B2 Hyakutake. A
detailed CO fluorescence model is developed to derive the CO abundances in
these comets by simultaneously fitting all of the observed A-X bands. The model
includes the latest values for the oscillator strengths and state parameters,
and accounts for optical depth effects due to line overlap and self-absorption.
The model fits yield radial profiles of CO column density that are consistent
with a predominantly native source for all the comets observed by STIS. The
derived CO abundances relative to water in these comets span a wide range, from
0.44% for C/2000 WM1 (LINEAR), 7.2% for 153P/Ikeya-Zhang, 8.8% for C/2001 Q4
(NEAT) to 20.9% for C/1996 B2 (Hyakutake). The subtraction of the CO spectral
features using this model leads to the first identification of a molecular
hydrogen line pumped by solar HI Lyman-beta longward of 1200A in the spectrum
of comet 153P/Ikeya-Zhang. (Abridged)Comment: 12 pages, 11 figures, ApJ accepte
Hubble Space Telescope Survey of Interstellar ^12CO/^13CO in the Solar Neighborhood
We examine 20 diffuse and translucent Galactic sight lines and extract the
column densities of the ^12CO and ^13CO isotopologues from their ultraviolet
A--X absorption bands detected in archival Space Telescope Imaging Spectrograph
data with lambda/Deltalambda geq 46,000. Five more targets with Goddard
High-Resolution Spectrograph data are added to the sample that more than
doubles the number of sight lines with published Hubble Space Telescope
observations of ^13CO. Most sight lines have 12-to-13 isotopic ratios that are
not significantly different from the local value of 70 for ^12C/^13C, which is
based on mm-wave observations of rotational lines in emission from CO and H_2CO
inside dense molecular clouds, as well as on results from optical measurements
of CH^+. Five of the 25 sight lines are found to be fractionated toward lower
12-to-13 values, while three sight lines in the sample are fractionated toward
higher ratios, signaling the predominance of either isotopic charge exchange or
selective photodissociation, respectively. There are no obvious trends of the
^12CO-to-^13CO ratio with physical conditions such as gas temperature or
density, yet ^12CO/^13CO does vary in a complicated manner with the column
density of either CO isotopologue, owing to varying levels of competition
between isotopic charge exchange and selective photodissociation in the
fractionation of CO. Finally, rotational temperatures of H_2 show that all
sight lines with detected amounts of ^13CO pass through gas that is on average
colder by 20 K than the gas without ^13CO. This colder gas is also sampled by
CN and C_2 molecules, the latter indicating gas kinetic temperatures of only 28
K, enough to facilitate an efficient charge exchange reaction that lowers the
value of ^12CO/^13CO.Comment: 1-column emulateapj, 23 pages, 9 figure
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