We turn now from a consideration of the most global aspect of the ISM to a consideration of its properties in the immediate vicinity of the Sun (Cox & Reynolds 1987). As already mentioned, it is believed that the Sun is inside a local bubble of hot gas with about K and number density around . This gas is highly ionized and therefore provides no opacity for observations within 100 pc or so. Numerous investigations have shown, however, that there is a minimum column density of neutral atomic hydrogen of about toward all stars observed, and this minimum is all that is seen for stars in the local bubble. Thus, it appears that the Sun must be surrounded by a small, at least partially neutral cloud with approximately this column density. An atomic-hydrogen density of about 0.1 cm and a bubble size of 3-5 pc are consistent with the existing data.
There is completely independent evidence of partially neutral interstellar gas in the immediate vicinity of the Sun from observations of solar Lyman- and helium 584 Å resonance radiation scattered by the neutral gas (Cox & Reynolds 1987). Observations made with many spacecraft have indicated a neutral-helium density of about 0.01 cm in the gas flowing through the solar system and a neutral-hydrogen density about five times as large with a temperature of about 8,000 K. The low observed ratio of H to He compared with the nominal cosmic abundance ratio of 10 has usually been interpreted to indicate that the hydrogen is significantly ionized, whereas the helium is largely neutral. However, known sources of ionizing radiation in the solar neighborhood are insufficient to maintain such a high degree of hydrogen ionization in the local cloud.
There is a possible solution to this dilemma that does not require the interstellar hydrogen to be highly ionized. Ripken & Fahr (1983) proposed that the inflowing hydrogen from the ISM undergoes charge exchange with solar-wind protons, reducing the observed neutral-hydrogen density by a factor of 2 or so from its true value; but this suggestion has remained controversial. According to Ripken and Fahr's calculations, the mechanism requires that the local interstellar hydrogen be 10%-20% ionized. This degree of ionization is consistent with known sources of ionizing radiation, including the line emission from the coronal gas in the local bubble, which, somewhat surprisingly, produces a slightly higher level of ionization for the helium in the local cloud (Cheng & Bruhweiler 1990).
The sensitivity of HUT in the 400-900 Å portion of the extreme-UV band allows us to make a direct measurement of the neutral-hydrogen and neutral helium column densities in the local ISM. The nearby hot white dwarf G191-B2B, located within the bubble but beyond the local cloud, provides a suitable background source for this measurement. It is a DA white dwarf, meaning that it has a pure hydrogen atmosphere whose properties are readily calculated, and is hot enough (60,000 K) to provide a strong signal at 500 Å. Furthermore, the absence of atmospheric helium in such a star avoids potential confusion with interstellar helium.
Absorption by interstellar hydrogen reduces the flux from this star just below 912 Å to an undetectable level, but the decreasing photoelectric cross-section at shorter wavelengths transmits an easily detected flux at 500 Å. The helium photoionization edge at 504 Å produces a discontinuity in the spectrum, whose strength is directly proportional to the neutral-helium column density. Meanwhile, comparison of the observed spectrum with the theoretical stellar atmosphere for this star permits determination of the hydrogen column density as well.
Isolating the 400-700 Å region of the spectrum with its aluminum filter, HUT observed G191-B2B on the Astro-1 mission (Figure 1). We found a hydrogen column density of and a helium column density of , giving a HI/HeI ratio of . If the total abundance ratio is the usually presumed cosmic value of 10, this ratio of neutral species indicates that helium is preferentially ionized compared with hydrogen, as predicted by Cheng & Bruhweiler (1990). All the data are consistent if hydrogen is 10% to 20% ionized and helium is 10% to 35% ionized and if charge exchange with the solar wind reduces the neutral-hydrogen fraction of the interstellar gas flowing into the solar system. Only if the helium abundance is less than the primordial value produced in the Big Bang (a situation that is, of course, not expected) can the ionized fraction of hydrogen significantly exceed that of helium. If the helium abundance in the cloud exceeds the normal cosmic value, our results would require the helium to be much more highly ionized than the hydrogen.
The local cloud is found to have a total hydrogen density of and an electron density of . With a total column density of , the local cloud has an extent of about 5 pc (15 light-years) in the direction of G191-B2B. If this dimension is typical, the total mass of the cloud is about 0.4 , where is the mass of the Sun. Of course, it would be interesting to determine the hydrogen and helium column densities and ionized fractions along several other lines of sight, too. This can be done on the Astro-2 mission.
The survival of the local cloud in the environment of the hot local bubble is problematical. If it predated the occurrence of a nearby supernova whose blast wave might have created the local bubble, it should not have survived the blast. Even in the current environment, such a cloud must have a short lifetime because of evaporation by the hot bubble gas, and it is therefore tempting to conclude that the cloud is fairly young. Perhaps it is the remnant of a planetary nebula shed by a nearby star that has recently become a white dwarf (Cox & Reynolds 1987).