22 research outputs found
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
Ultraviolet Survey of CO and H_2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
(Abridged) We carried out a comprehensive far-ultraviolet (UV) survey of
^12CO and H_2 column densities along diffuse molecular Galactic sight lines in
order to explore in detail the relationship between CO and H_2. We measured new
CO abundances from HST spectra, new H_2 abundances from FUSE data, and new CH,
CH^+, and CN abundances from the McDonald and European Southern Observatories.
A plot of log N(CO) versus log N(H_2) shows that two power-law relationships
are needed for a good fit of the entire sample, with a break located at log
N(CO, cm^-2) = 14.1 and log N(H_2) = 20.4, corresponding to a change in
production route for CO in higher-density gas. Similar logarithmic plots among
all five diatomic molecules allow us to probe their relationships, revealing
additional examples of dual slopes in the cases of CO versus CH (break at log N
= 14.1, 13.0), CH^+ versus H_2 (13.1, 20.3), and CH^+ versus CO (13.2, 14.1).
These breaks are all in excellent agreement with each other, confirming the
break in the CO versus H_2 relationship, as well as the one-to-one
correspondence between CH and H_2 abundances. Our new sight lines were selected
according to detectable amounts of CO in their spectra and they provide
information on both lower-density (< 100 cm^-3) and higher-density diffuse
clouds. The CO versus H_2 correlation and its intrinsic width are shown to be
empirically related to the changing total gas density among the sight lines of
the sample. We employ both analytical and numerical chemical schemes in order
to derive details of the molecular environments. In the low-density gas, where
equilibrium-chemistry studies have failed to reproduce the abundance of CH^+,
our numerical analysis shows that nonequilibrium chemistry must be employed for
correctly predicting the abundances of both CH^+ and CO.Comment: 40 pages in emulateapj style, to appear in the Astrophysical Journa
Current assessment of the Red Rectangle band problem
In this paper we discuss our insights into several key problems in the
identification of the Red Rectangle Bands (RRBs). We have combined three
independent sets of observations in order to try to define the constraints
guiding the bands. We provide a summary of the general behavior of the bands
and review the evidence for a molecular origin of the bands. The extent,
composition, and possible absorption effects of the bands are discussed.
Comparison spectra of the strongest band obtained at three different spectral
resolutions suggests that an intrinsic line width of individual rotational
lines can be deduced. Spectroscopic models of several relatively simple
molecules were examined in order to investigate where the current data are
weak. Suggestions are made for future studies to enhance our understanding of
these enigmatic bands