Molecular hydrogen in the galaxy and galactic gamma rays

Recent surveys of 2.6 mm CO emission and 100 MeV gamma-radiation in the galactic plane reveal a striking correlation suggesting that both emissions may be primarily proportional to the line-of-sight column density of H2 in the inner galaxy. Both the gamma ray and CO data suggest a prominent ring or arm consisting of cool clouds of H2 at a galactocentric distance of approximately 5 kpc with a mean density of approximately 4 atoms/cu cm. The importance of H2 in understanding galactic gamma ray observations is also reflected in the correlation of galactic latitude distribution of gamma rays and dense dust clouds. A detailed calculation of the gamma ray flux distribution in the 0 deg to 180 deg range using the CO data to obtain the average distribution of molecular clouds in the galaxy shows that most of the enhancement in the inner galaxy is due to pion-decay radiation and the 5 kpc ring plays a major role. Detailed agreement with the gamma ray data is obtained with the additional inclusion of contributions from bremsstrahlung and Compton radiation of secondary electrons and Compton radiation from the intense radiation field near the galactic center

Molecular clouds in the Galaxy

The J = 1 → 0 emission of CO has been surveyed in the galactic plane between ℓ = -10° and +90° with a U beam sampling every degree. Molecular clouds emitting in the CO line are plentiful over the inner region of the Galaxy. Their greatest number occurs in the galactic nucleus and at a radius of 5.5 kpc—a distribution similar to radio H II regions and galactic γ-ray emission but very different from that previously derived for atomic hydrogen. The total mass in molecular clouds is found to be 1-3 x 10^9 and each typically has 10^5 M_⊙, an H_2 density of 700 cm^(-3), and a temperature near 7 K. These results suggest that most of the interstellar medium in the interior of the Galaxy is molecular H_2

Molecular Clouds in W49 and W51

Radio observations of six molecular lines have been obtained in the W49 and W51 H ii region sources as part of an investigation of the physical conditions in molecular clouds and the relationship of these clouds to the H ii regions. The principal observations are maps with 4' spacing of the 6-cm formaldehyde (H_2CO) absorption and strip maps with 1' spacing of carbon monoxide (CO, J = 1 → 0) emission at 2.6 mm. A few selected positions were also observed in ^(13)CO and C^(18)O as well as the carbon monosulfide lines (CS, 3 → 2 and 2 → 1) at 2 and 3 mm. Seven distinct clouds are found, and five of these are associated with or near H ii regions. The molecular clouds are all much larger than the H ii regions; one near W51 (at 65 km s^(-1)) extends over an area containing at least three H ii regions and appears as a self-absorption feature in CO. The mass of three of these molecular clouds is estimated to be at least 10^4 - 10^5 M_⊙, 10 times as great as the ionized regions. Average hydrogen molecule densities, over the entire clouds, are about 200 - 1000 cm^(-3); but near the H ii regions there are probably compact, high-density (≳ 10^6 cm^(-3)) molecular regions which are responsible for the high-excitation lines of CS. The excitation temperature of the 6-cm H2_CO transition in two clouds in the direction of W49A is estimated to be 1.76 ± 1.2° K. The large velocity difference between these clouds and the H ii region and the narrowness of the lines suggest that the clouds are dark nebulae and unrelated to W49A; however, there is H_2O maser emission at the velocities of the dark cloud lines suggesting that some of the maser emission may be from foreground objects

Radiative Transfer, Excitation, and Cooling of Molecular Emission Lines (CO and CS)

We consider the radiative transfer of molecular lines in interstellar clouds having flow velocities large compared with random motions. The equilibrium level populations of CO and CS are calculated including the effects of both self-radiation (radiative trapping) and collisions with hydrogen molecules (using recently measured cross-sections). Analytic expressions are also developed for the excitation of a two-level molecule. Because of the velocity gradients in the cloud, the observed emission will originate not only from the near boundary but also from interior regions where excitation is greatly enhanced by scattered photons. This ability to see into the clouds qualitatively accounts for the general similarities of the different line profiles in individual clouds and considerably reduces the density of collisional particles needed to account for the observed excitation. Remarkably, even if A > C but τ > 1, the excitation temperature (and the observed intensity) depends on the molecular density but is totally independent of the spontaneous rate A. The rate of gas cooling by CO molecules in clouds of moderate density (n_(H_2)) ~ 10^3 cm^(-3)) is high (~10^(-22) ergs s^(-1)) even when the important cooling transitions (J = 3 → 2 and higher) are optically thick. These results are applicable to either cloud collapse or expansion

Survey of Molecular Lines Near the Galactic Center. I. 6-Centimeter Formaldehyde Absorption in Sagittarius a, Sagittarius B2, and the Galactic Plane from l^(II) = 359°.4 to l^(II) = 2°.2

The 1_(11)→ 1_(10) 4830-MHz transition of has been mapped in Sgr A, Sgr B2, and along the galactic equator from l^(II) = 359°.4 to 112 = 2°.2 with a 6' beam and velocity resolution of 1 km s^(-1). That this line is observed in absorption in all locations indicates a low 6-cm excitation temperature for all molecular clouds. Comparison with 21-cm data indicates that the H_2CO is concentrated in distinct clouds to a much greater extent than atomic hydrogen, with many strong H_2CO features occurring at velocities with weak or missing hydrogen lines. This suggests that most of the mass in these clouds is in the form of molecular hydrogen. There are four dominant clouds occurring at l^(II) = 0°.0, 0°.7, 0°.9, and 1°.7. In contrast with atomic hydrogen, the strongest formaldehyde features occur at [V_(LSR)] > 40 km s^(-1) and are apparently associated with the galactic nucleus. Estimates of the hydrogen density in these clouds, based on the assumptinn of stability and also on a comparison with local dark clouds, indicate a number density in the range of 10^3 - 10^4 cm^(-3) and a mass of approximately 5 X 10^5 M_⊙

Thermal design optimization of novel modular power converter assembly enabling higher performance, reliability and availability

An alternative integration scheme for a half-bridge switch using 70 μm thin Si IGBTs and diodes is presented. This flat switch, which is designed for high-frequency application with high power density, exhibits high strength, high toughness, low parasitic inductance and high thermal conductivity. Such a novel assembly approach is suitable to optimize performance, reliability and availability of the power system in which it is used. The paper focuses on the thermal performance of this assembly at normal and extreme operating conditions, studied by means of FEM thermo-fluidynamic simulations of the module integrated with connectors and liquid cooler, and thermal measurement performed on an early prototype. Improved solutions are also investigated by the FE model

The galactic distribution (in radius and Z) of interstellar molecular hydrogen

Observations of the galactic longitude and latitude distributions of lambda = 2.6 mm CO emission are presented. Analysis of these spectral-line data yields the large-scale distribution of molecular clouds in the galactic disk and their z-distribution out of the disk. Strong maxima in the number of molecular clouds occur in the galactic nucleus and at galactic radii 4 to 8 kpc. The peak at 4 to 8 kpc correlates well with a region of enhanced 100-MeV γ-ray emissivity. This correlation strongly supports the conclusion that the γ-rays are produced as a result of cosmic ray interactions in molecular H_2 clouds rather than in H(I). The width of the cloud layer perpendicular to the galactic plane between half-density points is 105 ± 15 pc near the 5.5-kpc peak. The total mass of molecular gas in the interior of the galaxy exceeds that of atomic hydrogen and is 3·10^9 M⊙ based on these observations

Modular assembly of a single phase inverter based on integrated functional block

This paper presents an original modular plug-in type assembly approach for a single phase-inverter. The main focus here is, indicatively, on the power range 1-20 kW, but the methodology can be transferred to higher power levels, too. At the core of the inverter lies a power-dense double-sided-cooled half-bridge power switch architecture with integrated cooler, which is interconnected to filter elements, gate-driver and control circuitry by means of compact flat connectors. The integration exercise targets, on the one hand, the optimization of the power switch performance and reliability, as well as the reduction of circuit parasitic elements; on the other, the production of a system compatible with maintenance and repairing, featuring minimized impact of single component failure on the system maintenance and repair cost and thus on its availability. Preliminary experimental tests demonstrate the nominal functionality of the inverter