CO Number Density Research Articles

Stochastic star formation in the Milky Way inferred from the unity index of the Kennicutt–Schmidt law

Abstract We performed a correlation analysis between the brightness temperature of the CO line and number density of H ii regions in the longitude–velocity diagram (LVD) of the Milky Way in order to investigate the volumetric star-formation law. We determined the index α of the Kennicut–Schmidt law for the molecular gas defined by $\rho _{\rm SFR}\propto \rho _{\rm H_2}^\alpha$, where ρSFR is the SFR (star-formation rate) density and $\rho _{\rm H_2}$ is the molecular-gas density. We obtained α = 1.053 ± 0.075 and 1.031 ± 0.067 for the CO-line data from the Nobeyama 45 m and Columbia 1.2 m telescope Galactic plane surveys, respectively. This result is consistent with the KS indices currently determined for the molecular gas in the Milky Way as well as in spiral and starburst galaxies. We argue that an index close to 1 is universal in favour of stochastic (spontaneous) star formation, but is inconsistent with the cloud-collision model, which predicts a steeper index of α = 2. We also suggest that the efficiency of star formation in the Galactic Centre is an order of magnitude lower than that in the disc.

A Constraint on the Amount of Hydrogen from the CO Chemistry in Debris Disks

The faint CO gases in debris disks are easily dissolved into C by UV irradiation, while CO can be reformed via reactions with hydrogen. The abundance ratio of C/CO could thus be a probe of the amount of hydrogen in the debris disks. We conduct radiative transfer calculations with chemical reactions for debris disks. For a typical dust-to-gas mass ratio of debris disks, CO formation proceeds without the involvement of H2 because a small amount of dust grains makes H2 formation inefficient. We find that the CO to C number density ratio depends on a combination of n H Z 0.4 χ −1.1, where n H is the hydrogen nucleus number density, Z is the metallicity, and χ is the far-UV flux normalized by the Habing flux. Using an analytic formula for the CO number density, we give constraints on the amount of hydrogen and metallicity for debris disks. CO formation is accelerated by excited H2 when either the dust-to-gas mass ratio is increased or the energy barrier of chemisorption of hydrogen on the dust surface is decreased. This acceleration of CO formation occurs only when the shielding effects of CO are insignificant. In shielded regions, the CO fractions are almost independent of the parameters of dust grains.

Two-dimensional laser-induced fluorescence thermometry in a medium- to high-enthalpy CO2 flow

Ground-based experimental facilities are needed to qualify materials subjected to aerothermodynamic loads during an entry into the Martian atmosphere. In the present work, the focus is on quantifying the temperature field in a medium- to high-enthalpy CO_{2} flow (from 4.5 to {10.5}, hbox {MJ}/hbox {kg}) of a plasma wind tunnel using one- and two-dimensional two-photon absorption laser-induced fluorescence (CO-TALIF) thermometry at a CO number density in the range of 1–2 times 10^{17} cm^{-3}. However, the measurement method developed here can also be used to study satellite propulsion systems. Due to absorption by air, a two-photon excitation scheme of the CO molecule at 115 nm (VUV) was used. Experimental and simulated excitation spectra are compared for temperature determination. For spectrum simulation in the B^1varSigma ^+leftarrow X^1varSigma ^+ system, an existing program was extended and a polynomial regression correlation was developed for signals with low signal-to-noise ratios. After the first one-dimensional temperature measurements using CO-TALIF in the free-stream to quantify the radial temperature profile for three conditions in the medium- to high- enthalpy range, respectively, two-dimensional measurements in a steady state test chamber at room temperature were carried out. For the two-dimensional investigations, a light sheet optic was designed to generate a light sheet free of divergence in width and thickness. The results of the one- and two-dimensional temperature measurements performed at medium- to high-enthalpy conditions provided plausible results concerning both the values and the course in radial and axial direction, the latter being, to the authors’ knowledge, the first two-dimensional temperature measurements under such conditions using CO-TALIF.

Vibrational quenching by water in a CO2 glow discharge measured using quantum cascade laser absorption spectroscopy

In situ quantum cascade laser (QCL) absorption spectroscopy is used to investigate the effect of admixed water in a pulsed CO2 glow discharge on the vibrational excitation of CO2 and CO and the conversion of CO2. Time-resolved transmittance spectra of the non-equilibrium CO2 plasma are measured with a 100 μs time resolution. A custom fitting routine is used to extract the time evolution of the gas temperature, rotational temperature and vibrational temperatures of CO2 and CO, while the CO2 conversion is determined from measured CO2 and CO number densities. Rotational Raman scattering is additionally performed in the centre of the reactor to verify measured rotational and vibrational temperatures from line-of-sight absorption spectroscopy. The plasma is operated at 6.7 mbar, with up to 10% water admixed, and is pulsed with a 5–10 ms on-off cycle, with a current of 50 mA supplied during the plasma on-time. Vibrational temperatures and CO2 conversion are not significantly affected by water admixtures below 0.5%. However, the asymmetric stretch temperature of CO2 (T 3) shows considerable quenching upon admixing 10% water vapour, with the maximum elevation above the rotational temperature (T rot) decreasing from 580 ± 86 K to 230 ± 63 K. For the vibrational temperature of CO (T CO), a similar trend is measured. However, the slopes of T 3 and T CO within the first few hundred μs after the start of the plasma remain unchanged, even when admixing 10% water vapour, suggesting equal excitation of the vibrational modes through e–V and V–V interactions. The conversion decreases by almost a factor of 4 when admixing 10% water. We argue that vibrational quenching of CO2 by water can explain part of the decrease. Changes in electron density and temperature and reactions between CO and OH can also play a role.

Absolute CO number densities measured using TALIF in a non-thermal plasma environment

We report first measurements of time-resolved absolute CO number densities and rotational temperatures in a non-thermal CO2 plasma environment using TALIF with a nanosecond pulsed laser. Two-photon excitation spectra from the (v′ = 0) ← (v″ = 0) Q-branch are recorded and fitted to extract rotational temperatures. Absolute number densities are determined from the frequency-integrated excitation spectrum. The plasma under investigation is a pulsed glow discharge operated at a frequency of 60 Hz with an plasma-on time of 5 ms per plasma cycle, 50 mA plasma current and a pressure of 6.67 mbar. CO number densities range from m−3 to m−3, while rotational temperatures range from 370 ± 40 K to 700 ± 70 K at the beginning and end of the plasma-on phase, respectively. Our results show fair agreement with literature data.

Experimental and Numerical Study of Carbon-Dioxide Dissociation for Mars Atmospheric Entry

The mechanism of dissociation during entry in the Mars atmosphere is experimentally investigated. A hydrogen–oxygen combustion-driven shock tube is used to simulate physical and chemical conditions in a mixture. Two shock velocity/initial pressure conditions are studied: at 100 Pa (called the low-pressure condition) and at 300 Pa (called the high-pressure condition). The temperature behind the shock wave is obtained by analyzing the high-temporal-resolution and high-spatial-resolution experimental spectra of the CN violet (, ) system. The CO number density is derived using a tunable diode laser absorption spectroscopy system based on CO absorption near . Moreover, a numerical code is developed to reproduce the experimental results (temperatures and species densities). The kinetic code in this work is based on Park’s two-temperature model. Comparisons between experiments and calculations are presented. Such a relatively simple two-temperature model fails to accurately describe the nonequilibrium temperature and CO number density but is suitable for equilibrium temperature predictions.

Optical emission spectroscopic studies and comparisons of CH3F/CO2 and CH3F/O2 inductively coupled plasmas

A CH3F/CO2 inductively coupled plasma (ICP), sustained in a compact plasma reactor, was investigated as a function of power (5–400 W) and feed gas composition, at a pressure of 10 mTorr, using optical emission spectroscopy and rare gas actinometry. Number densities of H, F, and O increased rapidly between 74% and 80% CO2, ascribed to the transition from polymer-covered to polymer-free reactor walls, similar to that found previously in CH3F/O2 ICPs at 48% O2. Below 40% O2 or CO2, relative emission intensity ratios were almost identical for most key species in CH3F/O2 and CH3F/CO2 ICPs except for higher OH/Xe (a qualitative measure of OH and H2O densities) over the full range of CH3F/O2 composition. The number density of H, F, and O increased with power in CH3F/CO2 (20%/80%) plasmas (polymer-free walls), reaching 4.0, 0.34, and 1.6 × 1013/cm3, respectively, at 300 W. The CO number density increased with power and was estimated, based on self-actinometry, to be 8.8 × 1013/cm3 at 300 W. The CO2 number density was independent of power below 40 W (where very little decomposition occurred), and then decreased rapidly with increasing power, reaching 2.8 × 1013/cm3 at 300 W, corresponding to 83% dissociation. Films deposited on p-Si, 10 cm from the open, downstream end of the plasma reactor, were analyzed by x-ray photoelectron spectroscopy. Between 10% and 40% CO2 or O2 addition to CH3F, film deposition rates fell and O content in the films increased. Faster deposition rates in CH3F/CO2 plasmas were ascribed mainly to a larger thermodynamic driving force to form solid carbon, compared with CH3F/O2 plasmas. Oxygen content in the films increased with increasing CO2 or O2 addition, but for the same deposition rate, no substantial differences were observed in the composition of the films.

Measurement of absolute CO number densities in CH3F/O2 plasmas by optical emission self-actinometry

CH3F/O2 inductively coupled plasmas at 10 mTorr were investigated using optical emission spectroscopy. A ‘self-actinometry’ method was developed to measure the absolute number density of CO that formed in reactions following dissociation of CH3F and O2 in the plasma. In this method, small amounts of CO were added to the plasma, leading to small increases in the CO emission intensity. By carefully accounting for small perturbations to the plasma electron density and/or electron energy distribution, and by showing that very little of the CO added to the plasma was decomposed by electron impact or other reactions, it was possible to derive absolute number densities for the CO content of the plasma. With equal fractions (0.50) of CH3F and O2 in the feed gas, the CO mole fraction as a function of plasma power saturated at a value of 0.20–0.25. As O2 in the feed gas was varied at a constant power of 100 W, the CO mole fraction went through a maximum of about 0.25 near an O2 feed gas fraction of 0.5. The relative CO number densities determined by ‘standard’ actinometry followed the same functional dependence as the absolute mole fractions determined by self-actinometry, aided by the fact that electron temperature did not change appreciably with power or feed gas composition.

Nonequilibrium CO Chemistry in the Solar Atmosphere

Investigating the reliability of the assumption of instantaneous chemical equilibrium (ICE) for calculating the CO number density in the solar atmosphere is of crucial importance for the resolution of the long-standing controversy over the existence of ‘cool clouds’ in the chromosphere, and for determining whether the cool gas owes its existence to CO radiative cooling or to a hydrodynamical process. Here we report the first results of such an investigation in which we have carried out time-dependent gas-phase chemistry calculations in radiation hydrodynamical simulations of solar chromospheric dynamics. We show that while the ICE approximation turns out to be suitable for modeling the observed infrared CO lines at the solar disk center, it may substantially overestimate the ‘heights of formation’ of strong CO lines synthesized close to the edge of the solar disk, especially concerning vigorous dynamic cases resulting from relatively strong photospheric disturbances. This happens because during the cool phases of the hydrodynamical simulations the CO number density in the outer atmospheric regions is smaller than what is stipulated by the ICE approximation, resulting in decreased CO opacity in the solar chromosphere. As a result, the cool CO-bearing gas which produces the observed molecular lines must be located at atmospheric heights not greater than 700 km, approximately. We conclude that taking into account the non-equilibrium chemistry improves the agreement with the available on-disk and off-limb observations, but that the hydrodynamical simulation model has to be even cooler than anticipated by the ICE approximation, and this has to be the case at the ‘new’ (i.e. deeper) formation regions of the rovibrational CO lines. Subject headings: astrochemistry—molecular processes—radiative transfer—Sun: chromosphere

Observations of ozone and related species in the northeast Pacific during the PHOBEA campaigns: 1. Ground‐based observations at Cheeka Peak

As part of the Photochemical Ozone Budget of the Eastern North Pacific Atmosphere (PHOBEA) project, we made observations of CO, O3, NOx, peroxyacetyl nitrate (PAN), nonmethane hydrocarbons (NMHC), Rn, aerosol scattering, aerosol absorption, and aerosol number density during the springs of 1997 and 1998 at the Cheeka Peak Observatory (CPO) on the western tip of Washington State. The data have been segregated to quantify the mixing ratio of these species in the Pacific marine atmosphere. However, even in these marine air masses, there are occasionally substantial enhancements of NOx and aerosols, but not CO, which we attribute to diesel exhaust from ship traffic to and from major ports in the region. The marine air masses were further classified into four categories based on 10‐day back isentropic trajectories; high, mid, and low latitude and those which had crossed over the Asian industrial region. Mean marine mixing ratios in 1998 were significantly higher than the 1997 values for CO (1997 mean equal to 151, 1998 mean equal to 170 ppbv), ethane (1771, 1968 parts per trillion by volume (pptv)), and ethyne (306, 452 pptv). Also, segregation of the 1998 data by air mass origin produced smaller differences in the mixing ratios for most species when comparing different source regions. We attribute both of these results to elevated emissions associated with unusually large areas of biomass burning which took place in Indonesia and Siberia dunng late 1997 and 1998. The relative enhancements of CO, ethane, ethyne, and propane we observed at CPO are consistent with enhanced biomass burning and industrial sources in the spring of 1998, relative to the spring of 1997.

Transition Metal Bimetallic Oxycarbides: Synthesis, Characterization, and Activity Studies

A new family of bimetallic oxycarbide compounds MI–MII–O–C (MI=Mo, W; MII=V, Nb, Cr, Fe, Co, Ni) has been synthesized by carburizing bimetallic oxide precursors using a temperature-programmed method. The oxide precursors are prepared by conventional solid-state reaction between two appropriate monometallic oxides. The synthesis involves passing a 20 mol% CH4 in H2 mixture over the oxide precursors while raising the temperature at a linear rate of 8.3×10−2 K s−1 (5 K/min) to a final temperature (Tmax) which is held for a period of time (thold). The synthesis, chemisorption properties, and reactivation of the materials indicate that the compounds can be divided into two groups of different reducibility (high and low). Their surface activity and surface area are evaluated based on CO chemisorption and N2 physisorption measurements. It is found that the CO number density correlates with the reducibility of the compounds.The catalysts were evaluated for hydroprocessing in a three-phase trickle-bed reactor operated at 3.1 MPa and 643 K. The feed was a model liquid mixture containing 3000 ppm sulfur (dibenzothiophene), 2000 ppm nitrogen (quinoline), 500 ppm oxygen (benzofuran), 20 wt% aromatics (tetralin), and balance aliphatics (tetradecane). The bimetallic oxycarbides had moderate activity for HDN of quinoline, with Nb–Mo–O–C showing higher HDN than a commercial sulfided Ni–Mo/Al2O3 catalyst tested at the same conditions. X-ray diffraction of the spent catalysts indicated that the oxycarbides of the early transition metals were tolerant of sulfur, while those involving the late transition metals showed bulk sulfide phases.

New Catalysts for Hydroprocessing: Bimetallic Oxynitrides MI–MII–O–N (MI, MII=Mo, W, V, Nb, Cr, Mn, and Co): Part I. Synthesis and Characterization

A new family of bimetallic oxynitride compounds, MI–MII–O–N (MI, MII=Mo, W, V, Nb, Cr, Mn, and Co), has been synthesized by nitriding bimetallic oxide precursors with ammonia gas via a temperature programmed reaction. The oxide precursors are prepared by conventional solid state reaction between two appropriate monometallic oxides. The synthesis involves passing NH3gas over the oxide precursors at a flow rate of 6.80×102μmol s−1(1000 cm3/min) and raising the temperature at a heating rate of 8.3×10−2K s−1(5 K/min) to a final temperature (Tf) which is held constant for a short period of time (thold). The oxynitrides thus obtained are pyrophoric and need to be passivated before exposing them to air. All these new bimetallic oxynitrides have a face centered cubic crystal structure and high values of surface area. The surface reactivation and the thermal stability of the materials are studied by temperature programmed reduction and this indicates that the compounds can be divided into three groups of different reducibility (high, medium, and low). Their surface activity and surface area are evaluated based on CO chemisorption and N2physisorption measurements. It is found that the chemisorbed CO number density correlates with the reducibility of the compounds.

Basalt Oxidation and the Formation of Hematite on the Surface of Venus

We present experimental results showing that basalt is oxidized in gas mixtures with CO number densities approximately equal to those at the surface of Venus. Although the gas mixtures have CO/CO2 ratios falling inside the magnetite stability field, Mössbauer spectroscopy shows that hematite and Fe3+ in pyroxene are produced in the oxidized basalt. The results suggest that the red color observed at several Venera landing sites is due to sub-aerial oxidation of Fe2+-bearing basalt on the surface of Venus, and that hematite, instead of magnetite, is present on the surface of Venus.

The Rate of Pyrite Decomposition on the Surface of Venus

We report the results of a detailed experimental study of the kinetics and mechanism of pyrite (FeS2) chemical weathering under Venus surface conditions. Pyrite is thermodynamically unstable on the surface of Venus and will spontaneously decompose to pyrrhotite (Fe7S8) because the observed S2 partial pressure in the lower atmosphere of Venus is lower than the S2 vapor pressure over coexisting pyrite and pyrrhotite. Pyrite decomposition kinetics were studied in pure CO2 and CO2 gas mixtures (CO-CO2, Ar-CO2, H2-CO2, CO-CO2-SO2) along five isotherms in the temperature range 390-531°C. In all gas mixtures studied, pyrite thermally decomposes to pyrrhotite (Fe7S8), which on continued heating loses sulfur to form more Fe-rich pyrrhotites. During this process the pyrrhotites are also being oxidized to form magnetite (Fe3O4), which converts to maghemite (γFe2O3), and then to hematite (α-Fe2O3). This reaction sequence is supported by X-ray diffraction data, Mössbauer spectra, optical microscopy, and prior literature studies. The reaction rates for pyrite thermal decomposition to pyrrhotite were determined by measuring the weight loss. The thickness of the unreacted pyrite in the samples provided a second independent reaction rate measurement. Finally, Mössbauer spectra done on 42 of the 115 experimental samples provided a third set of independent reaction rate data. All three independent methods give the same reaction rate within experimental uncertainties. Pyrite decomposition follows zero-order kinetics and is independent of the amount of pyrite present. The rate of pyrite decomposition is apparently independent of the gas compositions used and of the CO2 number density over a range of a factor of 40. The derived activation energy of ∼150 kJ mole-1 is the same in pure CO2, two different CO-CO2 mixtures, and a ternary CO-SO2-CO2 mixture. Based on data for a CO-CO2-SO2 gas mixture with a CO number density ∼10 times higher than at the surface of Venus and a SO2 number density approximately equal to that at the surface of Venus, the rate of pyrite destruction on the surface of Venus varies from about 1225 ± 238 days/cm at the top of Maxwell Montes (∼660 K) to about 233 ± 133 days/cm in the plains of Venus (∼740 K.) These lifetimes are very short on a geological time scale and show that pyrite cannot exist on the surface of Venus for any appreciable length of time.

Solar Cycle Variations of the Hydrogen Escape Rate and the CO Mixing Ratio on Mars

A one-dimensional photochemical model of the Martian atmosphere has been developed to study variations in the atmospheric composition with solar activity. The low sensitivity of O and CO number densities near the ionospheric peak to solar activity suggests that eddy mixing on Mars is proportional to the solar activity index (K(z) = (F10.7 cm/30) × 1013n-1/2). Due to the higher escape rate and the stronger eddy diffusion at solar maximum the H2 densities above the ionospheric peak are smaller by a factor of 6 than those at the solar minimum. The solar cycle variation of CO+2 by a factor of 2.2 cannot compensate for the variation of H2, and the atomic hydrogen escape flux at solar minimum is larger by a factor of 1.7 than that at solar maximum (the variation of the H number density is a factor of 30). The total hydrogen (H + 2H2) escape flux is stable (though not diffusion limited) and equal to 2.5 × 108 cm-2 sec-1. If eddy diffusion does not depend on solar activity, then the H escape is stable while the total escape flux varies by a factor of 2. The main reason for the hydrogen escape rate being close to the diffusion limit in the early models is that the rate coefficient of the H2 dissociation reaction was larger in the early models by a factor of 3 than the current value. Solar cycle variation of the lower and middle atmosphere is also studied. The CO mixing ratio in the lower and middle atmosphere would vary by a factor of 4 if the species lifetime were a few months. A mechanism of this variation is analyzed. However, since the real lifetime is 6 years the variation is only a factor of 1.35, with a time lag of 2 years.

Spectroscopic observations of methane-pulverized coal flames

A premixed laminar flame burner was used to study physical and chemical effects of burning methane and pulverized coal simultaneously. Spectral emissions obtained as a function of height in the flame, for OH, CH, C 2, and CO were used to calculate excited state species number densities. Premixed methane-air flames with a number of equivalence ratios and methane-air-coal dust flames with three equivalence ratios were studied and differences were noted. Spatial temperatures were measured in all flames by means of a thermocouple probe. The introduction of a pulverized coal additive (30mg/min) leads to changes in OH, Ch, C 2, and CO number densities and temperature at various heights, which depend on the stoichiometry of the methane-air flame. Possible chemical and physical mechanisms responsible for these changes are discussed qualitatively. The data pose a severe test for any quantitative model which includes the fluid dynamics, gas-phase chemistry, heterogeneous processes, and radiation transport.