Carbon Monoxide Isotopologues Research Articles

Collisional excitation of isotopologues of carbon monoxide by molecular hydrogen

ABSTRACTRate coefficients for transitions between the 14 lowest rotational levels, having energies less than 500 K, of the most abundant isotopologue of carbon monoxide,12C16O, caused by collisions with para-H2 and ortho-H2 are reported for temperatures between 5 and 400 K. These data were computed through time-independent close-coupled scattering calculations employing the high-quality potential energy surface (PES) recently calculated by Faure et al. Rate coefficients for collision-induced transitions are required for radiative transfer modelling of molecules present in interstellar clouds under non-equilibrium conditions. In addition, rate coefficients were computed for other carbon monoxide isotopologues, 13C16O, 12C17O, and 12C18O. The PESs for these systems were obtained by transforming the PES for 12C16O–H2 PES. The rotational transitions of 12C17O isotopologue often display resolved hyperfine structure in astronomical spectra. Rate coefficients for collision-induced transitions between the rotational–hyperfine levels of this isotopologue have been computed by the recoupling technique. A simple radiative transfer calculation was performed to illustrate the utility of using such an approach to relate observed intensities in astronomical spectra with molecular abundances.

Reinvestigation of the Rydberg W1Π(ν = 1) level of 12C16O, 13C16O, and 12C18O through rotationally dependent photodissociation branching ratio measurements

A recent high resolution photoabsorption study revealed that the Rydberg W1Π(ν = 1) level of carbon monoxide (CO) is perturbed by the valence E″1Π(ν = 0) level, and the predissociation linewidth shows drastic variation at the crossing point due to the interference effect [Heays et al., J. Chem. Phys. 141(14), 144311 (2014)]. Here, we reinvestigate the Rydberg W1Π(ν = 1) level for the three CO isotopologues, 12C16O, 13C16O, and 12C18O, by measuring the rotationally dependent photodissociation branching ratios. The C+ ion photofragment spectra obtained here reproduce the recent high resolution photoabsorption spectra very well, including the presence of the valence E″1Π(ν = 0) level. The photodissociation branching ratios into the spin-forbidden channel C(1D) + O(3P) show sudden increases at the crossing point between the W1Π(ν = 1) and E″1Π(ν = 0) levels, which is in perfect accordance with the drastic variation of the linewidth observed in the recent spectroscopic study. Further analysis reveals that the partial predissociation rate into the lowest channel C(3P) + O(3P) shows a much more prominent decrease at the crossing point, which is caused by the interference effect between the W1Π(ν = 1) and E″1Π(ν = 0) levels, than that into the spin-forbidden channel C(1D) + O(3P), and this is the reason of the sudden increase as observed in the photodissociation branching ratio measurements. We hope that the current experimental investigation will stimulate further theoretical studies, which could thoroughly address all the experimental observations in a quantitative way.

The G332 molecular cloud ring: I. Morphology and physical characteristics

We present a morphological and physical analysis of a Giant Molecular Cloud (GMC) using the carbon monoxide isotopologues ($^{12}$CO, $^{13}$CO, C$^{18}$O $^{3}P_{2}\rightarrow$ $^{3}P_{1}$) survey of the Galactic Plane (Mopra CO Southern Galactic Plane Survey), supplemented with neutral carbon maps from the HEAT telescope in Antarctica. The giant molecular cloud structure (hereinafter the ring) covers the sky region $332^\circ$ < $\ell$ < $333^\circ$ and $\mathit{b}$ = $\pm 0.5^\circ$ (hereinafter the G332 region). The mass of the ring and its distance are determined to be respectively ~2$\times10^{5}\mathrm{M_{\odot}}$ and ~3.7 kpc from Sun. The dark molecular gas fraction, estimated from the $^{13}$CO and [CI] lines, is $\sim17\%$ for a CO T$_{\mathrm{ex}}$ between [10,20 K]. Comparing the [CI] integrated intensity and N(H$_{2}$) traced by $^{13}$CO and $^{12}$CO, we define an X$\mathrm{_{CI}^{809}}$ factor, analogous to the usual X$_{\mathrm{co}}$, through the [CI] line. X$\mathrm{_{CI}^{809}}$ ranges between [1.8,2.0]$\times10^{21}\mathrm{cm}^{-2}\mathrm{K}^{-1}\mathrm{km}^{-1}\mathrm{s}$. We examined local variation in X$_{\mathrm{co}}$ and T$_{\mathrm{ex}}$ across the cloud, and find in regions where the star formation activity is not in an advanced state, an increase in the mean and dispersion of the X$_{\mathrm{co}}$ factor as the excitation temperature decreases. We present a catalogue of C$^{18}$O clumps within the cloud. The star formation (SF) activity ongoing in the cloud shows a correlation with T$_{\mathrm{ex}}$, [CI] and CO emissions, and anti-correlation with X$_{\mathrm{co}}$, suggesting a North-South spatial gradient in the SF activity. We propose a method to disentangle dust emission across the Galaxy, using HI and $^{13}$CO data. We describe Virtual Reality (VR) and Augmented Reality (AR) data visualisation techniques for the analysis of radio astronomy data.

Study of the filamentary infrared dark cloud G192.76+00.10 in the S254–S258 OB complex

We present results of a high resolution study of the filamentary infrared dark cloud G192.76+00.10 in the S254–S258 OB complex in several molecular species tracing different physical conditions. These include three isotopologues of carbon monoxide (CO), ammonia (NH3) and carbon monosulfide (CS). The aim of this work is to study the general structure and kinematics of the filamentary cloud, and its fragmentation and physical parameters. The gas temperature is derived from the NH3 (J, K) = (1, 1), (2, 2) and 12CO(2–1) lines, and the 13CO(1–0), 13CO(2–1) emission is used to investigate the overall gas distribution and kinematics. Several dense clumps are identified from the CS(2–1) data. Values of the gas temperature lie in the range 10 – 35K, and column density N(H2) reaches the value 5.1 × 1022 cm−2. The width of the filament is of order 1 pc. The masses of the dense clumps range from ∼ 30 M⊙ to ∼ 160 M⊙. They appear to be gravitationally unstable. Themolecular emission shows a gas dynamical coherence along the filament. The velocity pattern may indicate longitudinal collapse.

Disk Masses around Solar-mass Stars are Underestimated by CO Observations

Abstract Gas in protostellar disks provides the raw material for giant planet formation and controls the dynamics of the planetesimal-building dust grains. Accurate gas mass measurements help map the observed properties of planet-forming disks onto the formation environments of known exoplanets. Rare isotopologues of carbon monoxide (CO) have been used as gas mass tracers for disks in the Lupus star-forming region, with an assumed interstellar CO/H2 abundance ratio. Unfortunately, observations of T-Tauri disks show that CO abundance is not interstellar, a finding reproduced by models that show CO abundance decreasing both with distance from the star and as a function of time. Here, we present radiative transfer simulations that assess the accuracy of CO-based disk mass measurements. We find that the combination of CO chemical depletion in the outer disk and optically thick emission from the inner disk leads observers to underestimate gas mass by more than an order of magnitude if they use the standard assumptions of interstellar CO/H2 ratio and optically thin emission. Furthermore, CO abundance changes on million-year timescales, introducing an age/mass degeneracy into observations. To reach a factor of a few accuracy for CO-based disk mass measurements, we suggest that observers and modelers adopt the following strategies: (1) select low-J transitions; (2) observe multiple CO isotopologues and use either intensity ratios or normalized line profiles to diagnose CO chemical depletion; and (3) use spatially resolved observations to measure the CO-abundance distribution.

High-resolution oscillator strength measurements of the v' = 0,1 bands of the B −X, C −X, and E −X systems in five isotopologues of carbon monoxide

The photochemistry of CO and its isotoplogues affects the structure and evolution of many astronomical environments, including interstellar clouds, circumstellar disks around newly formed stars, and the envelopes surrounding highly evolved stars. When in the presence of a strong ultraviolet field, the primary destruction mechanism for interstellar and circumstellar CO is photodissociation, which is entirely governed by discrete line absorption into predissociating levels in the wavelength range 91.2 to 111.8 nm. Because the CO spectrum consists primarily of resolved line features, self-shielding effects in high-column density environments can lead to strong isotopic fractionation signatures in both CO and elemental oxygen and carbon; for example, CO self-shielding in the solar nebula has been invoked to explain the unusual oxygen isotope ratios observed in the earliest solar system condensates, viz. calcium-aluminum inclusions (CAIs) in primitive meteorites.

Observation of a new electronic state of CO perturbing W ¹Π(v=1).

We observe photoabsorption of the W(1) ← X(0) band in five carbon monoxide isotopologues with a vacuum-ultraviolet Fourier-transform spectrometer and a synchrotron radiation source. We deduce transition energies, integrated cross sections, and natural linewidths of the observed rotational transitions and find a perturbation affecting these. Following a deperturbation analysis of all five isotopologues, the perturbing state is assigned to the v = 0 level of a previously unobserved (1)Π state predicted by ab initio calculations to occur with the correct symmetry and equilibrium internuclear distance. We label this new state E″ (1)Π. Both of the interacting levels W(1) and E″(0) are predissociated, leading to dramatic interference effects in their corresponding linewidths.

Survival of molecular gas in cavities of transition disks

(Abridged) Planet formation is closely related to the structure and dispersal of protoplanetary disks. A certain class of disks, called transition disks, exhibit cavities in dust images at scales of up to a few 10s of AU. The formation mechanism of the cavities is still unclear. The gas content of such cavities can be spatially resolved for the first time using the Atacama Large Millimeter/submillimeter Array (ALMA). We have developed a new series of models to simulate the physical conditions and chemical abundances of the gas in cavities to address the question whether the gas is primarily atomic or molecular inside the dust free cavities exposed to intense UV radiation. Molecular/atomic line emission by carbon monoxide (CO), its isotopologues (13CO, C18O, C17O, and 13C18O) and related species ([CI], [CII], and [OI]) is predicted for comparison with ALMA and the Herschel Space Observatory. The gas can remain in molecular form down to very low amounts of gas in the cavity. Shielding of the stellar radiation by a dusty inner disk (pre-transition disk) allows CO to survive down to lower gas masses in the cavity. The main parameter for the CO emission from cavity is the gas mass. Other parameters such as the outer disk mass, bolometric luminosity, shape of the stellar spectrum or PAH abundance are less important. Since the CO pure rotational lines readily become optically thick, the CO isotopologues need to be observed in order to quantitatively determine the amount of gas in the cavity. A wide range of gas masses in the cavity of transition disks (~4 orders of magnitude) can be probed using combined observations of CO isotopologue lines with ALMA. Measuring the gas mass in the cavity will ultimately help to distinguish between different cavity formation theories.

High-Resolution Rovibrational Spectroscopy of Carbon Monoxide Isotopologues Isolated in Solid Parahydrogen

We report high-resolution infrared absorption spectra of six different CO isotopologues isolated in cryogenic parahydrogen (pH2) solids. These data provide a stringent test for theories of nearly free molecular rotors in crystalline solids, such as crystal field theory, rotation-translation coupling theory, and the pseudorotating cage model. A gas-phase molecule rotates about its center-of-mass (C.M.); a trapped molecule instead rotates about its "center of interaction" (C.I.) with the trapping cage, which may differ from the C.M. for heteronuclear diatomics like CO. Isotopic manipulation of CO allows the systematic variation of the C.M. relative to the C.I. We report remarkably good straight line correlation plots between the observed matrix effects and C.M. locations. Extrapolation of these lines to the limit of vanishing matrix effects yields an "experimental prediction" of the C.I. in excellent (fortuitous?) agreement with the C.I. calculated using a linear pH2-CO-pH2 toy model.