Experimental demonstration of a W-band gyroklystron amplifier

The experimental demonstration of a four cavity W-band (93 GHz) gyroklystron amplifier is reported. The gyroklystron has produced 67 kW peak output power and 28% efficiency in the TE011 mode using a 55 kV, 4.3 A electron beam. The full width at half maximum instantaneous bandwidth is greater than 460 MHz, a significant increase over the bandwidth demonstrated in previous W-band gyroklystron amplifier experiments. The amplifier is unconditionally stable at this operating point. Experimental results are in good agreement with theoretical predictions

High Velocity Rain: The Terminal Velocity of Model of Galactic Infall

A model is proposed for determining the distances to falling interstellar clouds in the galactic halo by measuring the cloud velocity and column density and assuming a model for the vertical density distribution of the Galactic interstellar medium. It is shown that falling clouds with $N(H I) < \sim 10^{19} cm^{-2}$ may be decelerated to a terminal velocity which increases with increasing height above the Galactic plane. This terminal velocity model correctly predicts the distance to high velocity cloud Complex M and several other interstellar structures of previously determined distance. It is demonstrated how interstellar absorption spectra alone may be used to predict the distances of the clouds producing the absorption. If the distances to the clouds are already known, we demonstrate how the model may be used to determine the vertical density structure of the ISM. The derived density distribution is consistent with the expected density distribution of the warm ionized medium, characterized by Reynolds. There is also evidence that for $z >\sim 0.4 kpc$ one or more of the following occurs: (1) the neutral fraction of the cloud decreases to $\sim 31 \pm 14$, (2) the density drops off faster than characterized by Reynolds, or (3) there is a systematic decrease in drag coefficient with increasing z.Comment: ApJ, in pres

Global properties of the HI high velocity sky, a statistical investigation based on the LAB survey

We study the properties of all major HVC complexes from a sample compiled 1991 by Wakker & van Woerden (WvW). We use the Leiden/Argentine/Bonn all sky 21-cm line survey and decompose the profiles into Gaussian components. We find a well defined multi-component structure for most of the HVC complexes. The cold HVC phase has lines with typical velocity dispersions of sigma = 3 km/s and exists only within more extended broad line regions, typically with sigma = 12 km/s. The motions of the cores relative to the envelopes are characterized by Mach numbers M = 1.5. The center velocities of the cores within a HVC complex have typical dispersions of 20 km/s. Remarkable is the well defined two-component structure for some prominent HVC complexes in the outskirts of the Milky Way: Complex H, the Magellanic Stream and the Leading Arm. There might be some indications for an interaction between HVCs and disk gas at intermediate velocities. This is possible for complex H, M, C, WB, WD, WE, WC, R, G, GCP, and OA, but not for complex A, MS, ACVHV, EN, WA, and P. Conclusions: The line widths, determined by us, imply that estimates of HVC masses, as far as derived from the WvW database, need to be scaled up by a factor 1.4. Correspondingly, guesses for the external pressure of a confining coronal gas need to be revised upward by a factor of 2. The HVC multi-phase structure implies in general that currently the halo pressure is significantly underestimated. In consequence, the HVC multi-phase structure may indicate that most of the complexes are circum-galactic. HVCs have turbulent energy densities which are an order of magnitude larger than that of comparable clumps in the Galactic disk.Comment: Accepted for publication in A&

Distances and Metallicities of High- and Intermediate-Velocity Clouds

A table is presented that summarizes published absorption line measurements for the high- and intermediate velocity clouds (HVCs and IVCs). New values are derived for N(HI) in the direction of observed probes, in order to arrive at reliable abundances and abundance limits (the HI data are described in Paper II). Distances to stellar probes are revisited and calculated consistently, in order to derive distance brackets or limits for many of the clouds, taking care to properly interpret non-detections. The main conclusions are the following. 1) Absolute abundances have been measured using lines of SII, NI and OI, with the following resulting values: ~0.1 solar for one HVC (complex C), ~0.3 solar for the Magellanic Stream, ~0.5 solar for a southern IVC, and ~ solar for two northern IVCs (the IV Arch and LLIV Arch). Finally, approximate values in the range 0.5-2 solar are found for three more IVCs. 2) Depletion patterns in IVCs are like those in warm disk or halo gas. 3) Most distance limits are based on strong UV lines of CII, SiII and MgII, a few on CaII. Distance limits for major HVCs are >5 kpc, while distance brackets for several IVCs are in the range 0.5-2 kpc. 4) Mass limits for major IVCs are 0.5-8x10^5 M_sun, but for major HVCs they are >10^6 M_sun. 5) The CaII/HI ratio varies by up to a factor 2-5 within a single cloud, somewhat more between clouds. 6) The NaIHI ratio varies by a factor >10 within a cloud, and even more between clouds. Thus, CaII can be useful for determining both lower and upper distance limits, but NaI only yields upper limits.Comment: To appear in the "Astrophysical Journal Supplement"; 82 pages; figures 6, 9 and 10 are in color; degraded figures (astro-ph restriction) - ask for good version

High-resolution observations of interstellar Na I and Ca II towards the southern opening of the 'Local Interstellar Chimney': probing the disc—halo connection

We present high-resolution (R = 400 000) observations of interstellar Ca II and Na I absorption lines towards seven stars in the direction of the southern opening of the recently identified Local Interstellar Chimney. These lines of sight probe the lower Galactic halo (0.3 ≲∣z∣≲ 2.5 kpc), without the complication of sampling dense foreground interstellar material. In addition to components with velocities expected from Galactic rotation, these stars also exhibit components with negative local standard of rest velocities, which are contrary to the sense of Galactic rotation for the sightlines observed. After a discussion of possible origins for these peculiar velocities, we conclude that at least some of them result from gas falling towards the Galactic plane from distances of ∣z∣≳ 300 pc. The narrow linewidths are generally inconsistent with temperatures as high as the ∼6000 K generally assumed for the so-called Lockman layer. Rather, the picture that emerges is one of a scattered, generally infalling, population of high-∣z∣ diffuse clouds, seemingly not very different from those encountered in the local interstellar medium. Overall, we argue that our results are most consistent with a ‘Galactic fountain’ model