Excitation Mechanisms of the Nitrogen First‐Positive and First‐Negative Radiation at High Temperature

The kinetic mechanisms responsible for the excitation of the first-positive and first-negative emission of nitrogen have been investigated in a re-examination of previously reported shock-tube measurements of the nonequilibrium radiation for these systems. The rate coefficients of the collisional quenching reactions, N_2(A^(3)Σ^(+)_u)(^(k^(N)_(-2))⇒) N_2(X^(1)Σ^(+)_g) + N(^(4)S) and N^(+)_2(B^(2)Σ^(+)_u) + N_2(X^(1)Σ^(+)_g)(^(k^(N)_(q))⇒) N^(+)_2(X^(2)Σ^(+)_g) or N^(+)_2(A(^(2)II_u)+N_2(X^(1)Σ^(+)_g) were found to be given by the empirical expressions, k_(2^(N))=5.1x10^(-3)T^(-2.23) cm^3 sec^(-1) and k_(q^(N_2))=1.9x10^(-2)T^(-2.33)cm^3 sec^(-1), respectively, over the approximate temperature range 6000-14 000°K

A Study of Heating and Cooling of the ISM in NGC 1097 with Herschel-PACS and Spitzer-IRS

NGC 1097 is a nearby Seyfert 1 galaxy with a bright circumnuclear starburst ring, a strong large-scale bar, and an active nucleus. We present a detailed study of the spatial variation of the far-infrared (FIR) [C II]158 μm and [O I]63 μm lines and mid-infrared H_2 emission lines as tracers of gas cooling, and of the polycyclic aromatic hydrocarbon (PAH) bands as tracers of the photoelectric heating, using Herschel-PACS and Spitzer-IRS infrared spectral maps. We focus on the nucleus and the ring, and two star-forming regions (Enuc N and Enuc S). We estimated a photoelectric gas heating efficiency ([C II]158 μm+[O I]63 μm)/PAH in the ring about 50% lower than in Enuc N and S. The average 11.3/7.7 μm PAH ratio is also lower in the ring, which may suggest a larger fraction of ionized PAHs, but no clear correlation with [C II]158 μm/PAH(5.5-14 μm) is found. PAHs in the ring are responsible for a factor of two more [C II]158 μm and [O I]63 μm emission per unit mass than PAHs in the Enuc S. spectral energy distribution (SED) modeling indicates that at most 25% of the FIR power in the ring and Enuc S can come from high-intensity photodissociation regions (PDRs), in which case G_0 ~ 10^(2.3) and n_H ~ 10^(3.5) cm^(–3) in the ring. For these values of G_0 and n_H, PDR models cannot reproduce the observed H2 emission. Much of the H2 emission in the starburst ring could come from warm regions in the diffuse interstellar medium that are heated by turbulent dissipation or shocks

Soot oxidation rates in gas turbine engines

A basis is proposed for extrapolating soot oxidation rate measurements obtained in laboratory flames to the more extreme operating conditions of gas turbine combustion chambers. The proposal is based on the observation that, within probable experimental uncertainty, the limited soot oxidation measurements correlate with the more extensive measurements of the surface oxidation rates of macroscopic samples of pyrographite. The soot oxidation rates thus determined for the conditions of a typical gas turbine combustion chamber are considerably lower than estimates which were based on simple extrapolations of the flame data

A stochastic model of turbulent mixing with chemical reaction: Nitric oxide formulation in a plug-flow burner

A stochastic model of turbulent mixing was developed for a reactor in which mixing is represented by n-body fluid particle interactions. The model was used to justify the assumption (made in previous investigations of the role of turbulent mixing on burner generated thermal nitric oxide and carbon monoxide emissions) that for a simple plug flow reactor, composition nonuniformities can be described by a Gaussian distribution function in the local fuel:air equivalence ratio. Recent extensions of this stochastic model to include the combined effects of turbulent mixing and secondary air entrainment on thermal generation of nitric oxide in gas turbine combustors are discussed. Finally, rate limited upper and lower bounds of the nitric oxide produced by thermal fixation of molecular nitrogen and oxidation of organically bound fuel nitrogen are estimated on the basis of the stochastic model for a plug flow burner; these are compared with experimental measurements obtained using a laboratory burner operated over a wide range of test conditions; good agreement is obtained

Shock-tube measurements of carbon to oxygen atom ratios for incipient soot formation with C2H2, C2H4 and C2H6 fuels

The critical atomic carbon to oxygen ratios, Phi sub C, for incipient soot formation in shock heated acetylene, ethylene, ethane/oxygen/ argon mixtures was measured over the temperature range 2000 K to 2500 K for reactant partial pressures between 0.1 and 0.4 atoms. Absorption of light from a He-Ne laser at 6328A was was used to detect soot. It was observed that the values of Phi sub C for all three fuels increased uniformly with temperature such that at the highest temperatures Phi sub C was considerably greater than unity, i.e. greater than the value of about unity at which solid carbon should have been precipitated on a thermochemical equilibrium basis. Observations were made over periods extending up to about one millisecond, which was well in excess of the time required for the major heat release of the combustion reactions. The relevance of these experimental findings to the problem of soot formation in gas turbine combustion chambers is discussed

H2 formation and excitation in the Stephan's Quintet galaxy-wide collision

Context. The Spitzer Space Telescope has detected a powerful (L(H2)~10^41 erg s-1) mid-infrared H2 emission towards the galaxy-wide collision in the Stephan's Quintet (SQ) galaxy group. This discovery was followed by the detection of more distant H2-luminous extragalactic sources, with almost no spectroscopic signatures of star formation. These observations set molecular gas in a new context where one has to describe its role as a cooling agent of energetic phases of galaxy evolution. Aims. The SQ postshock medium is observed to be multiphase, with H2 gas coexisting with a hot (~ 5 10^6 K), X-ray emitting plasma. The surface brightness of H2 lines exceeds that of the X-rays and the 0-0 S(1) H2 linewidth is ~ 900 km s-1, of the same order of the collision velocity. These observations raise three questions we propose to answer: (i) Why H2 is present in the postshock gas ? (ii) How can we account for the H2 excitation ? (iii) Why H2 is a dominant coolant ? Methods. We consider the collision of two flows of multiphase dusty gas. Our model quantifies the gas cooling, dust destruction, H2 formation and excitation in the postshock medium. Results. (i) The shock velocity, the post-shock temperature and the gas cooling timescale depend on the preshock gas density. The collision velocity is the shock velocity in the low density volume filling intercloud gas. This produces a ~ 5 10^6 K, dust-free, X-ray emitting plasma. The shock velocity is smaller in clouds. We show that gas heated to temperatures less than 10^6 K cools, keeps its dust content and becomes H2 within the SQ collision age (~ 5 10^6 years). (ii) Since the bulk kinetic energy of the H2 gas is the dominant energy reservoir, we consider that the H2 emission is powered by the dissipation of kinetic turbulent energy. (Abridged)Comment: 19 pages, 12 figures. Accepted for publication in Astronomy & Astrophysics Minor editing and typo

H_2 formation and excitation in the Stephan's Quintet galaxy-wide collision

Context. The Spitzer Space Telescope has detected a powerful (L_(H_2) ~ 10^(41) erg s^(-1)) mid-infrared H_2 emission towards the galaxy-wide collision in the Stephan's Quintet (henceforth SQ) galaxy group. This discovery was followed by the detection of more distant H_2-luminous extragalactic sources, with almost no spectroscopic signatures of star formation. These observations place molecular gas in a new context where one has to describe its role as a cooling agent of energetic phases of galaxy evolution. Aims. The SQ postshock medium is observed to be multiphase, with H_2 gas coexisting with a hot (~5 × 10^6 K), X-ray emitting plasma. The surface brightness of H_2 lines exceeds that of the X-rays and the 0-0 S(1)H_2 linewidth is ~900 km  s^(-1), of the order of the collision velocity. These observations raise three questions we propose to answer: (i) why is H_2 present in the postshock gas? (ii) How can we account for the H_2 excitation? (iii) Why is H_2 a dominant coolant? Methods. We consider the collision of two flows of multiphase dusty gas. Our model quantifies the gas cooling, dust destruction, H_2 formation and excitation in the postshock medium. Results. (i) The shock velocity, the post-shock temperature and the gas cooling timescale depend on the preshock gas density. The collision velocity is the shock velocity in the low density volume-filling intercloud gas. This produces a ~5 × 10^6 K, dust-free, X-ray emitting plasma. The shock velocity is lower in clouds. We show that gas heated to temperatures of less than 10^6 K cools, keeps its dust content and becomes H_2 within the SQ collision age (~5 × 10^6 years). (ii) Since the bulk kinetic energy of the H_2 gas is the dominant energy reservoir, we consider that the H_2 emission is powered by the dissipation of kinetic turbulent energy. We model this dissipation with non-dissociative MHD shocks and show that the H_2 excitation can be reproduced by a combination of low velocities shocks (5-20 km s^(-1)) within dense (n_H > 10^3 cm^(-3)) H_2 gas. (iii) An efficient transfer of the bulk kinetic energy to turbulent motion of much lower velocities within molecular gas is required to make H_2 a dominant coolant of the postshock gas. We argue that this transfer is mediated by the dynamic interaction between gas phases and the thermal instability of the cooling gas. We quantify the mass and energy cycling between gas phases required to balance the dissipation of energy through the H_2 emission lines. Conclusions. This study provides a physical framework to interpret H_2 emission from H_2-luminous galaxies. It highlights the role that H_2 formation and cooling play in dissipating mechanical energy released in galaxy collisions. This physical framework is of general relevance for the interpretation of observational signatures, in particular H_2 emission, of mechanical energy dissipation in multiphase gas

Shock-tube measurements of the vibration- vibration energy exchange probability for the CO-N2 system

Measuring vibration-vibration energy exchange probability in nitrogen-carbon dioxide-argon mixtures in shock tube