J. Krełowski

The interstellar Ca II distance scale

Aims. We attempt to extend the relation between the strengths of the interstellar Ca ii lines and the distances to early-type stars to objects beyond 1 kiloparsec, with the line saturation taken into account. Methods. We measure the Ca ii Ka nd Caii H equivalent widths, and compute Ca ii column densities for 262 lines of sight towards early-type stars with available Hipparcos parallaxes (π). The targets are located within a few hundred parsecs of the Galactic plane, and span all the range of Galactic longitudes. We fit the NCaii – parallax relation with a function of the form π = 1/(a · NCaii + b), using a maximum-likelihood approach to take account of errors in both variables. We use the resultant formula to estimate distances to stars in OB associations and clusters, and compare them to those found in the literature, usually estimated by spectrophotometric methods. Results. For lines of sight with EW(K)/EW(H) > 1.3, we obtain the following approximate formula for the distance: DCaii = 77 + (2.78 + 2.60 EW(K) EW(H) −0.932 )EW(H), where the equivalent widths EW(K )a ndEW(H) are in mA, and the distance D Caii in parsecs. The errors in DCaii, resulting from the uncertainty in the fit parameters and errors in the equivalent widths, are typically about 15% of the distance. We can also expect the equation not to hold for objects situated farther than a few hundred parsecs from the Galactic plane. We find several cases of significant column density differences between association or cluster members, especially notable in the Trumpler 16 cluster, indicating either a local contribution to the Ca ii column density, or background/foreground stars being confused with members. The ratio DCaii/Dassoc appears to depend on the Galactic longitude, being highest in the range 70 ◦ < l < 120 ◦ and lowest for 200 ◦ < l < 300 ◦ .T his effect may be due to large-scale structure being present in the Ca ii layer, or to the nonmember

Very high resolution profiles of 6196 Å and 6614 Å diffuse interstellar bands

We present a careful analysis of very high resolution (R = 220 000) proles of two well correlated diuse interstellar bands (DIBs): 6196 and 6614 A observed along 7 lines of sight free of the Doppler splitting in interstellar atomic lines. The high signal{to{noise ratio (S=N )o f the spectra (600{1000) allows us to draw denite conclusion that the ratio of equivalent widths of the two DIBs is not always exactly the same, i.e. they do not seem to originate at the same carrier. The DIB proles vary from object to object but the variations are dierent for every DIB. The width of 6196 DIB may change by up to 1.5 times lowest value while that of 6614 remains almost constant. The broadening of 6196 is not caused by the Doppler splitting, absent in atomic interstellar lines. Possibly this fact reflects a strong dependence of the carrier excitation pattern on small variations of physical parameters. The variations of substructure strength ratios inside the 6614 DIB prole, evident in our spectra, are apparently caused by a mechanism dierent from that which broadens DIB 6196.

The relation between CH and CN molecules and carriers of 5780 and 5797 diffuse interstellar bands

Optical absorption bands of the interstellar CN (near 3875 A) and CH molecules (the violet and blue ones near 4300 and 3886 A, respectively) were applied to determine the column densities of these two radicals in a statistically meaningful sample of 84 reddened OB stars. Equivalent widths of the major 5780 and 5797 diffuse bands (DIBs) were measured along the lines of sight toward the same stars in spectra acquired using four echelle spectrographs situated in both the northern and southern hemispheres. The mutual relation between abundances of CH and CN molecules shows a large scatter; and especially the CN molecule abundance varies strongly from cloud to cloud. The carriers of the major 5780 and 5797 DIBs seem to be spatially correlated with column densities of CH rather than of the CN molecule. This is most likely true in the case of a narrower feature: the 5797 DIB correlates with CH column density better than 5780 does. The correlations do suggest that the DIB carriers are likely hydrocarbons. They apparently occupy molecular clouds since the H 2 abundance is closely related to that of methylidyne (CH), as has already been demonstrated.

On the relation between diffuse bands and column densities of H~2, CH and CO molecules

Mutual relations between column densities of H 2 , CH and CO molecules as well as between the latter and strengths of the major 5780 and 5797 diffuse bands are presented and discussed. The CH radical seems to be a good H 2 tracer, possibly better than CO. It is also demonstrated that the molecular fraction of the H 2 molecule is correlated with an intensity ratio of 5797 and 5780 DIBs, suggesting the possible formation of narrow DIB carriers in denser clouds, dominated by molecular hydrogen and reasonably shielded from ionizing UV radiation by small dust grains.

HYDROXYL CATION IN TRANSLUCENT INTERSTELLAR CLOUDS

High-quality spectra acquired at the European Southern Observatory enabled us to discover a very weak spectral feature of the OH+ molecule, near 3584 A. The species likely shares environments with another molecular ion, CH+. Its abundance is by a factor of 30 lower than that of neutral OH.

The relation between column densities of interstellar OH and CH molecules

We present a new, close relation between column densities of OH and CH molecules based on 16 translucent sightlines (six of them new) and confirm the theoretical oscillator strengths of the OH A-X transitions at 3078 and 3082 A (0.001 05, 0.000 648) and CH B-X transitions at 3886 and 3890 A (0.003 20, 0.002 10), respectively. We also report no difference between observed and previously modelled abundances of the OH molecule.

A possible sets of diffuse bands originating at the same carrier

This paper discusses measurements of eight selected diffuse interstellar bands (DIBs):

The relation between interstellar OH and other simple molecules

lambdalambda

Relation between CH cation and neutral/molecular hydrogen (Research Note)

5793, 5809, 5819, 5828, 6196, 6397, 6614 and 6660 performed in high resolution, high

Abundances and rotational temperatures of the C2 interstellar molecule towards six reddened early-type stars

S/N