Weakly ionized cosmic gas: ionization and characterization

Since collective plasma behavior may determine important transport processes (e.g., plasma diffusion across a magnetic field) in certain cosmic environments, it is important to delineate the parameter space in which weakly ionized cosmic gases may be characterized as plasmas. In this short note, we do so. First, we use values for the ionization fraction given in the literature, wherein the ionization is generally assumed to be due primarily to ionization by cosmic rays. We also discuss an additional mechanism for ionization in such environments, namely, the photoelectric emission of electrons from cosmic dust grains in an interstellar FUV radiation field. Simple estimates suggest that under certain conditions this mechanism may dominate cosmic ray ionization, and possibly also the photoionization of metal atoms by the interstellar FUV field, and thereby lead to an enhanced ionization level.     

The impact of fragment “L” of comet SL-9 on Jupiter

Filtergrams of high spatial and temporal resolution were obtained in the methane band centred at 892 nm during the impact of fragment L of comet Shoemaker-Levy 9 on Jupiter. The light curve shows two maxima of an emission ball observed above the limb shortly after the impact. The second maximum was the brightest and had a short life time of about 90 seconds. During it's life, the apparent height of the emission ball declined towards the surface of Jupiter; the amount of displacement is larger than the expected effect caused by Jupiter's rotation. About half an hour after the impact, a domelike feature became visible when the location of the impact rotated into the illuminated hemisphere of Jupiter.     

Possible consequences of absence of “jupiters” in planetary systems

The formation of the gas giant planets Jupiter and Saturn probably required the growth of massive 15 Earth-mass cores on a time scale shorter than the 10⁷ time scale for removal of nebular gas. Relatively minor variations in nebular parameters could preclude the growth of full-size gas giants even in systems in which the terrestrial planet region is similar to our own. Systems containing failed Jupiters, resembling Uranus and Neptune in their failure to capture much nebular gas, would be expected to contain more densely populated cometary source regions. They will also eject a smaller number of comets into interstellar space. If systems of this kind were the norm, observation of hyperbolic comets would be unexpected. Monte Carlo calculations of the orbital evolution of region of such systems (the Kuiper belt) indicate that throughout Earth history the cometary impact flux in their terrestrial planet regions would be 1000 times greater than in our Solar System. It may be speculated that this could frustrate the evolution of organisms that observe and seek to understand their planetary system. For this reason our observation of these planets in our Solar System may tell us nothing about the probability of similar gas giants occurring in other planetary systems. This situation can be corrected by observation of an unbiased sample of planetary systems.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

WKB estimates for the onset of ideal MHD instabilities in solar coronal loops

A WKB approach, based on the method of Connor, Hastie, and Taylor (1979), is used to obtain simple estimates of the critical conditions for the onset of ideal MHD instabilities in line-tied solar coronal loops. The method is illustrated for the constant twist, Gold-Hoyle (1960) field, and the critical conditions are compared with previous and new numerical results. For the force-free case, the WKB estimate for the critical loop length reduces to . For the sufficiently non-force-free case the critical length can be expressed in the forml ₀ +l ₁/m. The results confirm the findings of De Bruyne and Hood (1992) that for force-free fields them = 1 mode is the first mode to become unstable but for the sufficiently strong non-force-free case this reverses with them → ∞ mode being excited first.     

Photospheric variability of O stars

The characteristics of the line profile variations observed in optical transitions of O-type stars are reviewed. For a few well-observed stars, there is compelling evidence that the variations are due to photospheric velocity fields from one or more modes of nonradial pulsation. However, the origin of the line profile variations observed in most O stars is not yet established. To date, there is little empirical evidence to suggest that the variability in optical absorption lines of O stars is causally linked to the stellar wind variability commonly observed in their UV resonance lines.     

Mid-infrared imaging of ultracompact H II regions

We discuss preliminary results of an 11.7 m imaging survey of ultracompact H II regions from the Wood and Churchwell radio survey. We find that that the morphologies of ionized gas and warm dust are often significantly different, indicating that an H II region classification scheme should be based on more than radio data.