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 K and Ca ii 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 N Ca ii -parallax relation with a function of the form π = 1/(a · N Ca ii + 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. 1.3, we obtain the following approximate formula for the distance: D Ca ii = 77 + (2.78 + Results. For lines of sight with EW(K)/EW(H) > EW(K) EW(H) −0.932 )EW(H), where the equivalent widths EW(K) and EW(H) are in mÅ, and the distance D Ca ii in parsecs.The errors in D Ca ii , 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 D Ca ii /D assoc appears to depend on the Galactic longitude, being highest in the range 70 • < l < 120 • and lowest for 200 • < l < 300 • . This effect may be due to large-scale structure being present in the Ca ii layer, or to the nonmember confusion being enhanced in these directions.

We show that the equivalent widths of the well-known interstellar Ca ii H and K lines can be used to determine the distances to OB stars in our Galaxy. The equivalent widths, measured in the spectra of 147 early-type stars, are strongly related to the Hipparcos parallaxes of those objects. The lines fitted to the parallax-equivalent width data are given by the formulaewhere is in arcseconds and EW is in milliangstroms. The form of the formulae, yielding a finite parallax even for zero absorption, shows that space within %100 pc of the Sun contains very little Ca ii, which is in agreement with the known dimensions of the Local Bubble. Using Ca ii lines for distance determination does not require the knowledge of the absolute magnitude of the object; it is thus well suited for targets for which the absolute calibration is either not precise (OB supergiants) or not available at all (peculiar objects). We also demonstrate that neither the reddening E (B À V ) nor the equivalent widths of interstellar K i and CH lines are suitable candidates for distance estimation, their relation with parallaxes being far less tight than for Ca ii.

ABSTRACT. The rotation curve of the Galaxy is generally thought to be flat. However, using radial velocities from interstellar molecular clouds, as is common in rotation curve determination, seems to be incorrect and may lead to wrongly inferring that the rotation curve is flat indeed. Tests based on photometric and spectral observations of bright stars may also be misleading. The rotation tracers (OB stars) are affected by motions around local gravity centers and by pulsation effects seen in such early-type objects. To get rid of the latter involves extensive observing work. We introduce a method of studying the kinematics of the thin disk of our Galaxy outside the solar orbit in a way that avoids these problems. We propose a test based on observations of interstellar Ca II H and K lines that determines both radial velocities and distances. We implemented the test using stellar spectra of thin disk stars at Galactic longitudes of 135°and 180°. Using this method, we constructed the rotation curve of the thin disk of the Galaxy. The test leads to the obvious conclusion that the rotation curve of the thin gaseous galactic disk, represented by the Ca II lines, is Keplerian outside the solar orbit rather than flat.

Context. The system V383 Sco was discovered to be an eclipsing binary star at the beginning of the twentieth century. This system has one of the longest orbital periods known (13.5 yr) and was initially classified as a ζAur-type eclipsing variable. It was then forgotten about for decades, with no progress made in understanding it. Aims. This study provides a detailed look at the system V383 Sco, using new data obtained before, during, and after the last eclipse, which occurred in 2007/8. There was a suspicion that this system could be similar to eclipsing systems with extensive dusty disks like EE Cep and ε Aur. This and other, alternative hypotheses are considered here. Methods. The All Sky Automated Survey (ASAS-3) V and I light curves have been used to examine apparent magnitude and colour changes. Low-and high-resolution spectra have been obtained and used for spectral classification, to analyse spectral line profiles, as well as to determine the reddening, radial velocities and the distance to the system. The spectral energy distribution (SED) was analysed using all available photometric and spectroscopic data. Using our own original numerical code, we performed a very simplified model of the eclipse, taking into account the pulsations of one of the components. Results. The low-resolution spectrum shows apparent traces of molecular bands, characteristic of an M-type supergiant. The presence of this star in the system is confirmed by the SED, by a strong dependence of the eclipse depth on the photometric bands, and by the nature of pulsational changes. The presence of a very low excitation nebula around the system has been inferred from [O i] 6300 Å emission in the high-resolution spectrum. Analysis of the radial velocities, reddening, and period-luminosity relation for Mira-type stars imply a distance to the V383 Sco system of 8.4 ± 0.6 kpc. The distance to the nearby V381 Sco is 6.4 ± 0.8 kpc. The very different and oppositely directed radial velocities of these two systems (89.8 km s −1 vs. −178.8 km s −1 ) seem to be in agreement with a bulge/bar kinematic model of the Galactic centre and inconsistent with purely circular motion. Conclusions. We have found strong evidence for the presence of a pulsating M-type supergiant in the V383 Sco system. This supergiant periodically obscures the much more luminous F0 I-type star, causing the deep (possibly total) eclipses which vary in duration and shape.

No abstract

The main aim of this paper is (i) to examine the available metallicity data of open clusters, (ii) to gather a large set of homogeneous estimations of the characteristics, and (iii) to check whether open cluster metallicities are related to spatial and age distribution of these objects.Key words: open clusters -metallicity AAA subject classijkation: 153 IntroductionThe open clusters offer probably the best observational tests to a variety of astrophysical and galactic studies. In all these applications the one of the most crucial characteristics of an open cluster, apart from the age and distance, is its metallicity. The metallicity is a rough global characteristic of chemical properties of a stellar atmosphere in a form of relative abundance of elements heavier than the helium to hydrogen. In the high-dispersion spectroscopic methods of determining this quantity it is usually related to the iron-to-hydrogen ratio. However, many of the features included in low-resolution spectroscopic and photometric abundance indices are actually also due to other, than the Fe, species such as Mg, Si, Ca, Ti, CN and/or CH being the main donors of electrons in stellar atmospheres and thus determining its structural properties. This could indicate that Fe abundances as derived from the high dispersion methods and photometric metallicities may be quite distinct and could explain, at least partly, the discrepancies between spectroscopic and photometric metallicities. To make the both metallicity determinations more comparable GEISLER (1984) has proposed average abundances of the few primary electron donors i.e. Mg, Si and Fe (instead of Fe alone) as the spectroscopic metallicity characteristic. However, these elements can be uncoupled (e.g. Mg and Si can be enhanced relative to Fe in some stars) which can also be responsible for the abundance scale controversy.In some cases the metallicity is, for purely observational reasons, identified with the CN and/or CH abundances. Such approach arises the question of the possible differences between the abundances of Fe and evolutionary sensitive C, N, 0 elements in the stellar atmospheres. This question is close to the chemical homogeneity problem considered little below.The metallicity of open clusters is important in several respects both galactic as well as astrophysical. It can help in determining the correct model for the large scale structure, formation and evolution of the Galactic disk by supplying information on the dependence of stellar abundances on the place and time of formation. Open clusters are very useful in this respect because their ages and distances can be determined fairly reliably from their C-M diagrams. Furthermore, spatial abundance gradients in the Galaxy can be established, providing constraints on the galactic collapse model and on diffusion processes carrying objects out of the galactic plane. The metallicity itself affects strongly the observational C-M diagrams of the open clusters. If determined in an independent way, metallicity helps us to decode the C-M d...

Abstract. Some of the close O and B dwarfs appear to be fainter than indicated by their Hipparcos distances, intrinsic absolute magnitudes attributed to their spectral types, and estimated selective interstellar extinction. This discrepancy is explained in the paper by the grey (neutral) interstellar extinction in the visual range of spectrum. The measure of such an effect is related to discrete features of the interstellar matter. Key words. ISM: dust -extinction IntroductionInterstellar dust significantly influences estimates of stellar global characteristics. It changes not only brightness, but also colors of stars. The dependency of the interstellar extinction (comprising both, the absorption and the scatter of light), illustrated by so called Extinction Curve, and discovered by Trumpler (1930a,b), is probably caused by small interstellar dust particles ("Mie" particles).The absorbing interstellar medium is most likely made of particles of various sizes. The interstellar dust model of Li & Greenberg (1997), based on observed extinction curves, counts 3-4 population of grains responsible for the shape of extinction curve at the UV range, but for the extinction at the visual range it proposes only one population -so called large particles of the size about 10 −8 m. These large particles cause selective extinction at the visual range, but appear to be "grey" at the UV range. For very large particles light obstruction may take place, which will affect all wavelengths alike and cause "neutral" interstellar extinction at both, the visual and UV spectral ranges. We may refer to "grey" or neutral extinction as nonselective extinction.Since the Trumpler's discovery, it was usually assumed that the total interstellar extinction is proportional to the selective extinction, and that if there is any nonselective component of this extinction, its amount is strictly proportional to the selective as well (Aller & Trumpler 1939). This has led to the commonly accepted direct proportionality of the total interstellar Send offprint requests to: A. Strobel, e-mail: strobel@astri.uni.torun.pl extinction to the measure of its selective effect, the color excess: A v = R * E(B − V), with R being, at least for a given direction in the sky, constant.Although small particles, responsible for the selective extinction probably predominate in the interstellar space, some regions may offer conditions which allow creation or survival of very large particles as well. The possible presence of such particles was announced and discussed in literature during the last few decades. These were already considered by Trumpler. His early paper (Trumpler 1930b) containes the following statement: "our Milky Way system seems to contain a considerable amount of finely divided matter, noticeable by its absorption of light. This matter appears to be made up mainly of: 1. Free atoms..., 2. Free electrons..., 3. Fine cosmic dust particles of various sizes ... producing the observed selective absorption..., and 4. Perhaps ... also larger meteoric bodies, obstr...

We have applied the method of investigating extinction curves using statistically meaningful samples that was proposed by us 25 years ago. The extensive data sets of the ANS (Astronomical Netherlands Satellite) and 2MASS (Two Micron All Sky Survey) were used, together with UBV photometry to create average extinction curves for samples of OB stars. Our results demonstrate that in the vast majority of cases the extinction curves are very close to the mean galactic extinction curve. Only a few objects were found to be obviously discrepant from the average. The latter phenomenon may be related to nitrogen chemistry in translucent interstellar clouds.