2003ApJ...595..858L

We present Far Ultraviolet Spectroscopic Explorer observations of the interstellar gas toward 30 white dwarf stars and one subdwarf (SdO) star. These sight lines probe the Local Bubble (LB) and the local interstellar medium (LISM) near the LB. We systematically measure the column densities for the following species: C II, C II*, C III, N I, N II, O I, Ar I, Si II, P II, Fe II, Fe III, and H₂. Our survey detected only diffuse H₂molecular clouds (f_H2≲10^–4) along six sight lines. There is no evidence from this study that H₂exists well inside the perimeter of the LB. The kinematical temperature for H₂is less than the usual temperature observed in the local interstellar clouds, implying different gas phases in the LISM. The relative abundance ratios of Si II, P II, and Fe II give insight about the dust content. These ratios vary but are similar to the depletion patterns observed in warm and halo diffuse clouds in more distant sight lines in the Galaxy. The N I/O I and Ar I/O I ratios are significantly subsolar within the LB. Outside the LB a larger scatter is observed from subsolar to solar. Because Ar and N are only weakly depleted into dust grains if at all, the deficiencies of their neutral forms are likely due to photoionization. The evidence for significant ionization of N (and hence Ar) is strengthened by the detection and measurement of N II, which is a dominant ion for this element toward many sight lines. C III appears to be ubiquitous in the LISM toward our sight lines, but C II remains the dominant ionization stage of C. The limits on Fe III/Fe II imply that Fe II is the dominant ion. These observations imply that photoionization is the main ionization mechanism in the LISM and do not support the existence of a highly ionized condition in the past. In view of the variations observed in the different atomic and ionic ratios, the photoionization conditions vary significantly in the LB and the LISM. The cooling rate in the LISM, l_c[in ergs/s (H I atom)^–1], derived from the emission of the C II 157.7 µm line has a mean value of logl_c=-25.59±0.32 dex, very similar to previous determinations.