1997A&A...324.1105L

The ambient interstellar plasma density (i.e. the plasma density of the interstellar medium surrounding the Sun) directly governs the structure and the size of our heliosphere. Information on this density can be derived from the ionization states of the interstellar species which can be detected in absorption along the paths to the nearby stars, and which can be shown to belong to the Local Interstellar Cloud (LIC). Echelle spectra around the resonance lines of neutral and singly ionized magnesium have been obtained for the nearby star δ Cas with the Goddard High-Resolution Spectrograph (GHRS) on board the Hubble Space Telescope. While apparently a unique velocity component (a unique cloudlet) is detected in both lines of the MgII λλ2800 resonance doublet, at the expected Doppler shift for the LIC, an extremely small λλ2853 Mg line is also detected at a Doppler shift compatible with the LIC motion, allowing a measurement of the LIC MgII/MgI ratio, here found to be 400 (-130,+190). This ratio implies a mean electron density of about 0.28 (-0.14, +0.34)cm^–3 along this line-of-sight, if equilibrium conditions prevail, and if T=7000K, when using the most recent recombination and charge-exchange rates. This MgII/MgI ratio is larger than for Sirius (R≃220), which lies at 110^o from δ Cas, providing some evidence for an ionization gradient in the local cloud. Such an electron density implies a surprisingly large ionization degree, and the upper range of the interval is incompatible with the minimum size of our heliosphere. A second and independent way to derive the electron density along the path to δ Cas uses the simplicity of the LIC geometry in the sky region surrounding the star, which allows an estimate of the H column-density to the star, as well as previous ground-based CaII data, and the measured NaI/CaII ratio and calcium depletion in the LIC. The resulting most probable electron density at 7000K, 0.05cm^–3, provides a new evidence for a significant ionization degree of the LIC, but is a factor of four to five smaller than the value based on magnesium. The upper limit of 0.19cm^–3 remains consistent with the minimum size of our heliosphere. The existence of a common interval to the two determinations: n_e=0.14-0.19cm^–3 implies that ionization equilibrium within the LIC is not totally precluded. However, the lack of a real convergence deserves further observations, involving other interstellar species. The common interval is compatible with the result of Frisch (1994Sci...265.1443F), from anomalous C and O cosmic rays abundances, if carbon is not too much filtered at the heliospheric interface. On the other hand, the sodium-based most probable value is in agreement with neutral hydrogen deceleration at the heliospheric interface for the Baranov two-shocks model, as well as with the ionization degree of hydrogen implied by local EUV sources.