Calibrations of the uvby, β photometric systems have been made by Crawford (1975, 1978, 1979) for F, B and late A-type stars of luminosity class V-III; the calibration for early A-type stars has been made by Hilditch et al. (1983). One of the remaining areas of the HR diagram to be calibrated is that of the bright giant to supergiant region (luminosity class II-I). We have attempted to obtain a calibration in terms of luminosity for supergiant F-type stars. To this end we have used the statistical method of multiple stepwise regression analysis. Before doing the calibration for supergiants, we show the usefulness of this method by a comparison of the results obtained for dwarf F-type stars with the previous calibration given by Crawford (1975).

We have carried out a preliminary investigation of a method of observation and reduction which leads to a large improvement in the accuracy of the color transformation of photometric measurements. This method involves the use of extra filters for some of the observations. It is argued that the better precision obtained is well worth the additional telescope work.

The applicability of the Strömgren system for abundance determination of late-type stars has been shown by several authors (e.g. Eggen 1978b, Bond 1980, Ardeberg and Lindgren 1981). The aim of our contribution is not to present a completely new calibration. Rather we want to discuss the (b-y)-m1 diagram with an increased sample of spectroscopically analysed stars, hoping to eliminate some unsatisfying properties of previous approaches and to make improvements in terms of generality and reliability.

A method of analysis of Strömgren photometry and low-resolution spectrophotometry of B stars is being developed in collaboration with J.P. Kaufmann (Technische Universität, Berlin). A full account will be published elsewhere.

The recent analysis by Olsen (1983) of uvby data demonstrates the difficulty of comparing observations obtained with different equipment. He detected systematic errors in the standard stars of Crawford and Barnes (1970), who merged several sources. The introduction of new photometers with quasi-rectangular passbands (Nielsen 1983), which give theoretically nontransformable output, (Young 1974), could make the problem more acute.

An attempt has been made to calibrate the indices of the uvby photometric system in terms of MK classes equal to and later than that of the Sun. Results are presented for stars of luminosity classes V and III; b-y and c1 data are given. For stars on the main sequence, the relation between m1 and b-y is discussed for stars of solar type.

The large data base of photoelectric measurements on the Johnson UBV system has been a primary source of information in many fields of astrophysical interest. The availability of UBV data for virtually all types of stars known to make up the stellar populations in galaxies requires a continued effort toward establishing a fully physical calibration of these data in order to propagate effectively our improving knowledge on stellar evolution and stellar atmospheres (i.e., the HR diagram) through the observations relevant to the structure and evolution of the galaxies. One of the major links in this long chain of scientific progress is provided by the synthesis of stellar photometric properties from theoretical model atmospheres. This paper will briefly address some of the basic problems involved in the synthetic color calculations and discuss the theoretical calibration of UBV photometry as obtained from various grids of model atmospheres covering a large range of stellar types.

The atlas of stellar spectrophotometric data published by Gunn and Stryker (GS, 1983) constitutes an extremely valuable tool for the evaluation and calibration of photometric systems. The resolution of the scanner fluxes (10 or 20 Å), given for a broad wavelength range (λλ 3160–10620 Å) and the systematic coverage of stellar types are (almost) ideal for calculating synthetic colors for Becker's (1946) photographic RGU system which has been extensively applied in galactic structure work (cf. Buser 1981). Since RGU photometry is tightly linked to UBV data (Steinlin 1968, Buser 1978b), the Gunn-Stryker atlas has been used in the present paper to evaluate these two systems and subsequently investigate the resulting calibration of the RGU colors in terms of MK spectral classification.

Angular diameters of 593 B5-F5 main sequence stars listed in the “Catalogue of apparent diameters and absolute radii of stars” (CADARS; Fracassini et al. 1981) have been analysed in order to improve the precision of the visual surface brightness Sv. The new relations between this quantity and the color index (B-V)o turn out to be in good agreement with those found with the interferometric method (Barnes et al. 1978). Moreover, the results suggest that surface gravity effects may bias the Sv-(B-V)o relations.

By comparing the numerical data for the response functions of the UBV color system originally given by Matthews and Sandage (1963) with the improved functions published by Ažusienis and Straižys (1969), taking into account of the extinction values given by Melbourne (1960), the response function of U and B is found corrected for a printing error and other mistakes.

Observational evaluation of intrinsic energy distributions in OB spectra is difficult as these stars are usually reddened and any dereddening procedure may change from place to place in the Galaxy. We compare two-color diagrams (UV vs. visual) from two stellar complexes of the same Sp/L class. The observed differences may be caused by additional far-UV reddening of distant objects as well as by intrinsic differences between stars belonging to different complexes. We argue in favor of the first possibility which would allow one to use the same standards in all of the Galaxy.

The determination of the effective temperature of each individual star requires observations of the flux distribution in the largest possible wavelength range. Unfortunately, owing to observational constraints, this requirement severely limits the number of stars for which the direct determination of Teff can be achieved. However, an estimate of Teff for a very large number of stars can be obtained through a calibration of a photometric index versus the stellar effective temperature. The UV-Visual dereddened index, R=log(F 1965/F 5445), has been shown to be suitable for “normal” non-supergiant stars in the spectral type range B5 - F7 (Malagnini et al. 1984). The analysis, with the method described in Malagnini et al. (1983), of stars in the spectral type range B5 - B0, enables us to extend and to refine the already proposed calibration of R versus Teff. By using the complete calibration, the determination of Teff for all the “normal” non-supergiant stars sampled from the 31215 objects in the Catalogue of Stellar Ultraviolet Fluxes (Thompson et al. 1978) is being carried out.

A systematic analysis, as complete as possible, of early type standard stars has been undertaken at the Astronomical Observatory of Trieste, in order to obtain a sequence of carefully determined spectroscopic data to be used in the comparison of “normal” and “chemically peculiar” stars. As an intermediate and necessary step, effective temperatures are derived. The determination of Teff is performed either by comparing observed and computed flux distributions, or by using a calibration of UV-visual photometric index versus Teff. The results for a sample of stars in the spectral type range B2-F8 are presented, and an analysis of the influence of the adopted value of log g on the derived Teff values is reported. As a check on the validity of the results, the Teff and log g values are used to construct a synthetic spectrum which is compared with the IUE high resolution observations of the stars in our list.

The temperature calibration for cool stars and in particular the Miras continues to be contentious. Lunar occultations have provided radii for many K and M stars and a good temperature calibration has been derived for the hotter non-variable M stars (Ridgway et al. 1980). The situation for the Miras and carbon stars and the metal-rich and metal-poor M stars is, however, not so clear cut. Observations are generally made in some broad-band color such as (R-I), (V-K) or (J-K) and a temperature derived using either the Ridgway et al. (1980) empirical scale or a black-body scale; differences can amount to several hundred degrees. We decided to theoretically explore the effects that extension, metallicity and pulsation could have on colors.

This investigation of nearby Population I stars was motivated by our need for accurate effective temperatures in order to be able to obtain reliable N abundances from lines belonging to the CN red system. These determinations are based on narrowband photometry calibrated by means of synthetic spectra. It is believed that the temperatures are accurate to within 100 K. The results show that recent calculations of evolutionary tracks of G and K subgiants predict the observations quite well. However, a value for the mixing length parameter 1/Hp close to 1.6 is required.

Photometric errors due to scintillation are considered in detail. Given noise characteristics, the standard deviations, as deduced from observations, are computed for the quantities measured at successive steps of the photometric procedure. This allows us to understand better the errors computed on-line in observatories, and to understand better the overall error. The latter can be minimized by suitable changes in the time sequence of the measurement.

The 13 color medium-narrow band photometric system (Johnson and Mitchell 1975) potentially contains a great deal of information, but so far only a little work has been done in order to recover some physical parmeters from its color indices. Our aim is to obtain relations to be used in calibrating this system in terms of physical quantities, making use of the information given by this system alone.

A spectrophotometer was originally designed by Barbier (1960) for the study of early-type stars. The band passes were chosen to measure the features of the hydrogen spectrum. To the first definition of the system (Barbier and Morguleff 1964), two more bands were added, specifically devoted to the peculiar stars of type Ap and Am (Gerbaldi 1972). The interpretation of these observations has already been given separately. We will concentrate here on the calibration of this spectrophotometric system in terms of Teff, log g and electron density ne.

Lockwood's measurements in five-color narrow-band photometry of spectroscopic standard M stars is used to define an estimator of temperature. The TiO and VO indices deduced from this photometry depend in the same way on the metallicity but vary in a different way with the temperature: their ratio is only influenced by the temperature. A relation between T/V and the temperature is calibrated for M giants between 2000 and 3000° K. Both the T and V indices are indicators of metallicity and gravity which allow us to check that the relation (T/V)/temperature is independent of the metallicity. This relation leads to a precise determination of the temperature and can be applied to a wide variety of M giants, independently of their reddening and chemical composition.

Catalogs of averaged photometric data have been published for several photometric systems. The homogenizing procedures used to produce them are not without pitfalls. We question the accuracy of these methods with the available data.