Echelle spectra of the double-lined spectroscopic binary HD 204827 were obtained on five nights, at a resolving power R ¼ 38;000 and with a S/N ¼ 750 near 6000 8 in the final, combined spectrum. The stars show E(B À V ) ¼ 1:11 and spectral types near O9.5 V and B0.5 III. A catalog is presented of 380 diffuse interstellar bands (DIBs) measured between 3900 and 8100 8 in the stars' spectrum. The central wavelengths, the widths (FWHM ), and the equivalent widths of nearly all of the bands are tabulated, along with the minimum uncertainties in the latter. The reliable removal of very weak stellar lines from the catalog, and of some stellar lines from the less severe blends with DIBs, is made generally easy by the highly variable radial velocities of both stars. The principal result of this investigation is that the great majority of the bands in the catalog are very weak and relatively narrow. Typical equivalent widths amount to a few m8, and the bandwidths (FWHM ) are most often near 0.55 8. Therefore, most of these DIBs can be detected only in spectra obtained at a resolving power and a S/ N at least comparable to those used here. In addition, the anomalous interstellar reddening and the very high value of the ratio N(C 2 )/E(B À V ) seen toward HD 204827 indicate that the physical conditions in one or more of the several interstellar clouds seen in this direction differ significantly from those found toward the prototypical DIB target HD 183143, for example. Probably primarily for these reasons, 113 of the 380 bands (30%) were not detected in four previous modern surveys of the DIBs seen in the spectra of stars other than HD 204827. No preferred wavenumber spacings among the 380 bands are reliably identified which could provide clues to the identities of the large molecules thought to cause the DIBs.

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We establish correlations between equivalent widths of eight diffuse interstellar bands (DIBs), and examine their correlations with atomic hydrogen, molecular hydrogen, and E B−V . The DIBs are centered at λλ 5780. 5, 6204.5, 6283.8, 6196.0, 6613.6, 5705.1, 5797.1, and 5487.7, in decreasing order of Pearson's correlation coefficient with N(H) (here defined as the column density of neutral hydrogen), ranging from 0.96 to 0.82. We find the equivalent width of λ5780.5 is better correlated with column densities of H than with E B−V or H 2 , confirming earlier results based on smaller datasets. We show the same is true for six of the seven other DIBs presented here. Despite this similarity, the eight strong DIBs chosen are not well enough correlated with each other to suggest they come from the same carrier. We further conclude that these eight DIBs are more likely to be associated with H than with H 2 , and hence are not preferentially located in the densest, most UV shielded parts of interstellar clouds. We suggest they arise from different molecules found in diffuse H regions with very little H 2 (molecular

Echelle spectra of HD 183143 [B7Iae, E(B-V) = 1.27] were obtained on three nights, at a resolving power R = 38,000 and with a S/N ratio ≈ 1000 at 6400 Å in the final, combined spectrum. A catalog is presented of 414 diffuse interstellar bands (DIBs) measured between 3900 and 8100 Å in this spectrum. The central wavelengths, the widths (FWHM), and the equivalent widths of nearly all of the bands are tabulated, along with the minimum uncertainties in the latter. Among the 414 bands, 135 (or 33%) were not reported in four previous, modern surveys of the DIBs in the spectra of various stars, including HD 183143. The principal result of this study is that the great majority of the bands in the catalog are very weak and fairly narrow. Typical equivalent widths amount to a few mÅ, and the band widths (FWHM) are most often near 0.7 Å. No preferred wavenumber spacings among the 414 bands are identified which could provide clues to the identities of the large molecules thought to cause the DIBs. At generally comparable detection limits in both spectra, the population of DIBs observed toward HD 183143 is systematically redder, broader, and stronger than that seen toward HD 204827 (Paper II). In addition, interstellar lines of C 2 molecules have not been detected toward HD 183143, while a very high value of N(C 2 )/E(B-V) is observed toward HD 204827. Therefore, either the abundances of the large molecules presumed to give rise to the DIBs, or the physical conditions in the absorbing clouds, or both, must differ significantly between the two cases.Subject Headings: ISM: lines and bands ---ISM: molecules ---stars: individual (HD 183143)

We present high-resolution (FWHM D0.4È1.8 km s~1) spectra, obtained with the AAT UHRF, the McDonald Observatory 2.7 m spectrograph, and/or the KPNO feed, of interstellar K I coude coude absorption toward 54 Galactic stars. These new K I spectra reveal complex structure and narrow, closely blended components in many lines of sight. Multicomponent Ðts to the line proÐles yield estimates for the column densities, line widths, and velocities for 319 individual interstellar cloud components. The median component width (FWHM) and the true median separation between adjacent components are both km s~1. The median and maximum individual component K I column densities, about [1.2 4 ] 1010 and 1012 cm~2, correspond to individual component hydrogen column densities of about 2 ] 1020 and 1021 cm~2 and E(B[V ) D 0.03 and 0.17, respectively. If T is typically D100 K, then at least half the individual components have subsonic internal turbulent velocities. We also reexamine the relationships between the column densities of K I, Na I, C I, Li I, and CH. The four trace H tot , H 2 , neutral species exhibit essentially linear relationships with each other over wide ranges in overall column density. If C is uniformly depleted by 0.4 dex, then Li, Na, and K are each typically depleted by 0.6È0.7 dex. The total line of sight values for N(K I) and N(Na I) show roughly quadratic dependences on but the relationships for the ensemble of individual clouds could be signiÐcantly steeper. These N(H tot ), quadratic (or steeper) dependences appear to rule out signiÐcant contributions to the ionization from cosmic rays, X-rays, and/or charge exchange with C II in most cases. Charge exchange with negatively charged large molecules may often be more important than radiative recombination in neutralizing most singly ionized atomic species in cool H I clouds, howeverÈsuggesting that the true and thermal n e , n H , pressures may be signiÐcantly smaller than the values estimated by considering only radiative recombination. Both N(CH) and are nearly linearly proportional to N(K I) and N(Na I) [except for 1015 N(H 2 ) cm~2, over which makes the transition to the self-shielded regime]. Those cm~2 [ N(H 2 ) [ 1019 H 2 relationships appear also to hold for many individual components and component groups, suggesting that high-resolution spectra of K I and Na I can be very useful for interpreting lower resolution molecular data. The scatter about all these mean relationships is generally small dex), if certain con-([0.1È0.2 sistently "" discrepant ÏÏ sight lines are excludedÈsuggesting that both the relative depletions and the relative ionization of Li, C, Na, and K are generally within factors of 2 of their mean values. Di †erences noted for sight lines in Sco-Oph, in the Pleiades, near the Orion Trapezium, and in the LMC and SMC may be due to di †erences in the strength and/or shape of the ambient radiation Ðelds, perhaps ampliÐed by the e †ects of charge transfer with large molecules.

Spectra obtained with the Hubble Space T elescope Goddard High Resolution Spectrograph are combined with high-resolution optical spectra and UV spectra from Copernicus to study the abundances and physical conditions in the di †use interstellar clouds seen along the line of sight to the star 23 Ori. Multiple absorption components are present for each of several distinct types of gas, which are characterized by di †erent relative abundance and depletion patterns and physical conditions. Strong low-velocity (SLV) absorption, due to cool, moderately dense neutral gas and representing about 92% of the total N(H I), is seen for various neutral and singly ionized species at ]20 km s~1 km s~1. Most typically severely depleted species are less depleted by factors of 2È4, com-

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