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Galactic HI (neutral hydrogen) shells are central to our understanding of the interstellar medium (ISM), which plays a key role in the development and evolution of galaxies, including our own. Several models involving supernovae and stellar winds have contributed to our broad understanding, but a complete, detailed picture remains elusive. To extend existing Galactic shell catalogs, we visually examined the SETHI (Search for Extraterrestrial HI) database to identify shell-like structures. This high-sensitivity 21-cm radio survey covering the Arecibo sky uniquely provides high-resolution data on shells at a wide range of Galactic latitudes. We present basic information (location, radial velocity, angular size, shape) for 74 previously unidentified HI shells. Due to limitations of coverage and data quality, and the biases inherent in search techniques, our catalog is not a complete sample of Galactic shells. We discuss the catalog completeness, and comment on the new shells' relationship with known interstellar structure as warranted. Unlike many previous catalogs, this sample is not biased towards expanding shells. Where possible we also estimate the kinematic distances, physical sizes, expansion velocities, and energies of these shells. Overall, they are relatively large and old, each the result of multiple supernovae. Unlike previous surveys, we do not find that the shells in our sample are preferentially aligned relative to the Galactic plane., Comment: 17 pages, 7 figures, 6 tables, Published in The Astronomical Journal. This revised version fixes typographical error in equation in third paragraph of Section 6.2

We analyze potential effects of an extraterrestrial civilization's use of orbiting mirrors to illuminate the dark side of a synchronously rotating planet on planetary transit light curves. Previous efforts to detect civilizations based on side effects of planetary-scale engineering have focused on structures affecting the host star output (e.g. Dyson spheres). However, younger civilizations are likely to be less advanced in their engineering efforts, yet still capable of sending small spacecraft into orbit. Since M dwarfs are the most common type of star in the solar neighborhood, it seems plausible that many of the nearest habitable planets orbit dim, low-mass M stars, and will be in synchronous rotation. Logically, a civilization evolving on such a planet may be inspired to illuminate their planet's dark side by placing a single large mirror at the L2 Lagrangian point, or launching a fleet of small thin mirrors into planetary orbit. We briefly examine the requirements and engineering challenges of such a collection of orbiting mirrors, then explore their impact on transit light curves. We incorporate stellar limb darkening and model a simplistic mirror fleet's effects for transits of Earth-like (R = 0.5 to 2 R_Earth) planets which would be synchronously rotating for orbits within the habitable zone of their host star. Although such an installation is undetectable in Kepler data, JWST will provide the sensitivity necessary to detect a fleet of mirrors orbiting Earth-like habitable planets around nearby stars., Comment: 14 pages, 12 figures, some color, rev 3 corrects arithmetic error in cost of launching a mirror fleet

We analyze potential effects of an extraterrestrial civilization's use of orbiting mirrors to illuminate the dark side of a synchronously rotating planet on planetary transit light curves. Previous efforts to detect civilizations based on side effects of planetary-scale engineering have focused on structures affecting the host star output (e.g. Dyson spheres). However, younger civilizations are likely to be less advanced in their engineering efforts, yet still capable of sending small spacecraft into orbit. Since M dwarfs are the most common type of star in the solar neighborhood, it seems plausible that many of the nearest habitable planets orbit dim, low-mass M stars, and will be in synchronous rotation. Logically, a civilization evolving on such a planet may be inspired to illuminate their planet's dark side by placing a single large mirror at the L2 Lagrangian point, or launching a fleet of small thin mirrors into planetary orbit. We briefly examine the requirements and engineering challenges of such a collection of orbiting mirrors, then explore their impact on transit light curves. We incorporate stellar limb darkening and model a simplistic mirror fleet's effects for transits of Earth-like (R = 0.5 to 2 R_Earth) planets which would be synchronously rotating for orbits within the habitable zone of their host star. Although such an installation is undetectable in Kepler data, JWST will provide the sensitivity necessary to detect a fleet of mirrors orbiting Earth-like habitable planets around nearby stars., Comment: 14 pages, 12 figures, some color, rev 3 corrects arithmetic error in cost of launching a mirror fleet

Galactic HI (neutral hydrogen) shells are central to our understanding of the interstellar medium (ISM), which plays a key role in the development and evolution of galaxies, including our own. Several models involving supernovae and stellar winds have contributed to our broad understanding, but a complete, detailed picture remains elusive. To extend existing Galactic shell catalogs, we visually examined the SETHI (Search for Extraterrestrial HI) database to identify shell-like structures. This high-sensitivity 21-cm radio survey covering the Arecibo sky uniquely provides high-resolution data on shells at a wide range of Galactic latitudes. We present basic information (location, radial velocity, angular size, shape) for 74 previously unidentified HI shells. Due to limitations of coverage and data quality, and the biases inherent in search techniques, our catalog is not a complete sample of Galactic shells. We discuss the catalog completeness, and comment on the new shells' relationship with known interstellar structure as warranted. Unlike many previous catalogs, this sample is not biased towards expanding shells. Where possible we also estimate the kinematic distances, physical sizes, expansion velocities, and energies of these shells. Overall, they are relatively large and old, each the result of multiple supernovae. Unlike previous surveys, we do not find that the shells in our sample are preferentially aligned relative to the Galactic plane., Comment: 17 pages, 7 figures, 6 tables, Published in The Astronomical Journal. This revised version fixes typographical error in equation in third paragraph of Section 6.2

We used the FUSE (Far Ultraviolet Spectroscopic Explorer) satellite to observe OVI emission along two sightlines towards the edge of the interaction zone (IZ) between the Loop I superbubble and the Local Bubble. One sightline was chosen because material in the interaction zone blocks distant X-ray emission, and should thus do the same for non-local OVI emission. We measured an OVI intensity of I_shadowed = 2750 +- 550 L.U. along this `Shadowed' sightline, and I_unshadowed = 10800 +- 1200 L.U. along the other sightline. Given these results, very little (< 800 L.U.) of the emission arises from the near side of the interaction zone, which likely has an HI column density of about 4e+20 cm-2 along the `Shadowed' sightline. The OVI emission arising within Loop I (~1e+4 L.U.) is probably associated with gas of n_e ~ 0.1 cm-3 and an emitting pathlength of ~1.2 pc, suggesting it arises at interfaces rather than from gas filling Loop I. In contrast, the CIII emission is similar along both sightlines, indicating that much of the emission likely arises on the near side of the interaction zone., Comment: 29 pages, 3 EPS figures, 2 tables. To appear in ApJ 682, 2008 July 20

We used the FUSE (Far Ultraviolet Spectroscopic Explorer) satellite to observe OVI emission along two sightlines towards the edge of the interaction zone (IZ) between the Loop I superbubble and the Local Bubble. One sightline was chosen because material in the interaction zone blocks distant X-ray emission, and should thus do the same for non-local OVI emission. We measured an OVI intensity of I_shadowed = 2750 +- 550 L.U. along this `Shadowed' sightline, and I_unshadowed = 10800 +- 1200 L.U. along the other sightline. Given these results, very little (< 800 L.U.) of the emission arises from the near side of the interaction zone, which likely has an HI column density of about 4e+20 cm-2 along the `Shadowed' sightline. The OVI emission arising within Loop I (~1e+4 L.U.) is probably associated with gas of n_e ~ 0.1 cm-3 and an emitting pathlength of ~1.2 pc, suggesting it arises at interfaces rather than from gas filling Loop I. In contrast, the CIII emission is similar along both sightlines, indicating that much of the emission likely arises on the near side of the interaction zone., Comment: 29 pages, 3 EPS figures, 2 tables. To appear in ApJ 682, 2008 July 20

We used FUSE to observe ultraviolet emission from diffuse O VI in the hot gas in the Galactic halo. By comparing our result with another, nearby observation blocked by an opaque cloud at a distance of 230 pc, we could subtract off the contribution from the Local Bubble, leading to an apparent halo intensity of I_{OVI} = 4680^{+570}_{-660} photons/cm^2/s/sr. A correction for foreground extinction leads to an intrinsic intensity that could be as much as twice this value. Assuming T ~ 3 x 10^5 K, we conclude that the electron density, n_e, is 0.01-0.02 /cm^3, the thermal pressure, p/k, is 7000-10,000 K/cm^3, and that the hot gas is spread over a length of 50-70 pc, implying a small filling factor for O VI-rich gas. ROSAT observations of emission at 1/4 keV in the same direction indicate that the X-rays are weaker by a factor of 1.1 to 4.7, depending on the foreground extinction. Simulated supernova remnants evolving in low density gas have similar O VI to X-ray ratios when the remnant plasma is approaching collisional ioinizational equilibrium and the physical structures are approaching dynamical ``middle age''. Alternatively, the plasma can be described by a temperature power-law. Assuming that the material is approximately isobaric and the length scales according to T^(beta) d(ln T), we find beta = 1.5+/-0.6 and an upper temperature cutoff of 10^{6.6(+0.3,-0.2)} K. The radiative cooling rate for the hot gas, including that which is too hot to hold O VI, is 6 x 10^{38} erg/s/kpc^2. This rate implies that ~70% of the energy produced in the disk and halo by SN and pre-SN winds is radiated by the hot gas in the halo., Comment: Accepted by the Astrophysical Journal, AASTEX, 39 pages, including 6 figures and 4 tables (manuscript replaced in order to re-typeset Table 3 and correct submission entry)

We used FUSE to observe ultraviolet emission from diffuse O VI in the hot gas in the Galactic halo. By comparing our result with another, nearby observation blocked by an opaque cloud at a distance of 230 pc, we could subtract off the contribution from the Local Bubble, leading to an apparent halo intensity of I_{OVI} = 4680^{+570}_{-660} photons/cm^2/s/sr. A correction for foreground extinction leads to an intrinsic intensity that could be as much as twice this value. Assuming T ~ 3 x 10^5 K, we conclude that the electron density, n_e, is 0.01-0.02 /cm^3, the thermal pressure, p/k, is 7000-10,000 K/cm^3, and that the hot gas is spread over a length of 50-70 pc, implying a small filling factor for O VI-rich gas. ROSAT observations of emission at 1/4 keV in the same direction indicate that the X-rays are weaker by a factor of 1.1 to 4.7, depending on the foreground extinction. Simulated supernova remnants evolving in low density gas have similar O VI to X-ray ratios when the remnant plasma is approaching collisional ioinizational equilibrium and the physical structures are approaching dynamical ``middle age''. Alternatively, the plasma can be described by a temperature power-law. Assuming that the material is approximately isobaric and the length scales according to T^(beta) d(ln T), we find beta = 1.5+/-0.6 and an upper temperature cutoff of 10^{6.6(+0.3,-0.2)} K. The radiative cooling rate for the hot gas, including that which is too hot to hold O VI, is 6 x 10^{38} erg/s/kpc^2. This rate implies that ~70% of the energy produced in the disk and halo by SN and pre-SN winds is radiated by the hot gas in the halo., Comment: Accepted by the Astrophysical Journal, AASTEX, 39 pages, including 6 figures and 4 tables (manuscript replaced in order to re-typeset Table 3 and correct submission entry)