The idea of organizing a joint discussion on the impact of ultraviolet observations on spectral classification arose during the IAU symposium n»72 held at Lausanne, in the honour of W.W. Morgan. It became evident at that time that the wealth of spectroscopic and spectrometric data already obtained from various orbiting telescopes should shed some new light on the problem of spectral classification. I whish to thank here the members of the Organizing Committee who planned this Joint Discussion: W.P. Bidelman, A.D. Code, C. Jaschek, and K. Nandy. I also thank the authors of invited reports and contributed papers, as well as those who participated in the discussions.

A spectral classification system is a morphological device for arranging in order a finite series of observations of individual objects, One has thus three elements: an “order”, i.e. an abstract discontinuous scheme, a series of objects and a certain type of observation (spectrogram) that is performed on all objects of the sample. The “order” is in principle arbitrary; one could think for instance of an order based on richness of lines, or on the presence of hydrogen lines, etc. However such an idealization is clearly simplistic, because it would imply that the classifier is completely unaware of theory -but he is not, if for no other reason than simply that he would never get his PhD. Thus one should add the constraint that the scheme one adopts be physically sound.

The methods of spectral classification from the low dispersion ultraviolet spectra obtained with the S2/68 experiment in the TD1 satellite have been described. The bright stars, the spectra of which are photometrically accurate, can be divided into natural groups according to the spectral appearance of the features. These features vary in strength with spectral type and luminosity, and enable separation between main sequence and luminous stars. The limits for these stars are at BO to at AO. For fainter stars the spectral data have been combined to obtain narrow band magnitudes at several wavelengths. These photometric bands have an effective width of 100 A. An ultraviolet photometric system which enables determinations of spectral type and luminosity of early type stars is described and the results for about 3000 stars is presented. The photometric system considered here consists of the ultraviolet colour indices (m2740−V,) (m2190−V) and (m1490−V).

The experiment S2/68 has supplied a large number of ear type spectra in the wavelength region 1350–2730 A. The resolution is 37 A, which is about equivalent to a reciprocal dispersion of 1850 A/mm. We had at our disposal about one thousand spectra of stars brighter than 6m5.

The availability of such material has lead us to establish a spectral classification system based only upon features visible in the spectrum. A second step was to compare this system to the MK classifications.

It turned out to be easy to establish a temperature sequence based upon convenient intensity ratios of line features and somewhat more difficult to establish luminosity criteria. Among the B type stars it is possible to distinguish main sequence objects, supergiants and intermediate objects.

Ultraviolet low resolution spectra of most of the B stars observed with the S2/68 experiment onboard the TD1 satellite in the spectral range 1350 A - 2500 A are computer-investigated in order to define a grid of standard behaviors (J.M. Vreux and J.P. Swings, 1976 Astron. Astrophys., in press). The behavior of the relative intensities of the Balmer and Paschen continua is studied as a function of the spectral type and the visible photometric index Q: both relations are shown to be luminosity dependent. A comparison to theoretical predictions and to previous studies is also presented. A measure of the slope of the pseudo-continuum drawn between ~ λ 1660 A and ~ λ 2550 A is studied in the same way: the effect of luminosity is discussed in connection with recently proposed temperature scales for super-giants. The behavior of the depths of the absorption features at λλ 1550 A, 1940 A, 2000 A, 2055 A, 2105 A, 2340 A and 2395 A with respect to the spectral type, the Q index and the luminosity class is also briefly presented. Special emphasis is given to the absorption features at λ 1550 A in connection to the discussion of the Felll lines in the ultraviolet spectra of early B stars by J.P. Swings, M. Klutz, J.M. Vreux and E. Peytremann (1976, Astron. Astrophys. Suppl. 25, 193). Preliminary results of a study of high resolution Copernicus spectra in the A 1550 A region are also presented: it is shown that the role of Felll remains predominant for stars of spectral type as early as about BIII - BIIII (J.P. Swings and J.M. Vreux, to be published).

In this contribution we will discuss the extension of visual photoelectrical photometry into the ultraviolet and its potential impact on stellar classification. As is the case in the visual, classification by photoelectrical photometry in the ultraviolet has some important advantages over classification by inspection of individual spectra. Studies that require global characteristics of stellar properties -such as studies of stellar distribution, extinction properties of interstellar dust, galactic structure - often require large numbers of observations to derive statistical properties. In photoelectric photometry such a large number of observations can usually be obtained in a reasonable amount of time and relatively conveniently processed without subjective criteria being applied to the data. Comparatively, the classical way to obtain stellar classification is a complex process that requires many steps.

During the Skylab I, II and III missions, ultraviolet spectra were obtained in 188 fields with a. 15 cm aperture objective-prism spectrograph. The instrument has been described by Henize, Wray, Parsons, Benedict, Bruhweiler, Rybski and O’Callaghan (1975; hereafter referred to as Paper I).

The spectra cover the wavelength region from 1300 to 5000 Å and have a resolution of 2 Å at 1400 Å and 12 Å at 2000 Å. Absorption and/or emission lines of C II λ 1335, Si IV λ 1394, 1403 and C IV λ 1549 are visible in more than one hundred stars. The lines of Si IV and C IV are found to be particularly sensitive to stellar temperature and luminosity. Since these lines are visible in spectra of moderate to low resolution it is clear that they should be of special interest in any UV classification system for faint stars. This paper investigates the correlation of the intensities of the C IV and Si IV lines with MK spectral type, and presents a preliminary classification scheme for 04 to B2 stars based on these lines.

Ultraviolet stellar fluxes have been presented for 140 bright stars in the spectral region from 1200 A to 3600 Å in a graphical and tabular form (A. D. Code and M. R. Meade, 1976). The spectra represent a subset of 0A0-2 spectrometer data on file at the National Space Science Data Center. The monochromatic flux is given in units of ergs cm−2 sec−1 Å−1 with a spectral resolution of about 22 Angstroms in the region from 3600 Å to 1850 Å and approximately 12 Angstroms from 1850 Å to 1160 Å.

The monochromatic flux, Fλ, is based upon the absolute calibration described by Bless, Code and Fairchild (1976).

The empirical temperature scale of Code, Davis, Bless, and Hanbury Brown has been recalibrated in terms of an ultraviolet color derived from OAO-2 photometry, and mean temperatures have been computed for MK spectral types from O9 to A0. The dereddened color (1910-V)o was chosen as a temperature indicator because it is not strongly affected by lines or continuum edges. This parameter was computed for 16 stars with spectral types earlier than A2, luminosity classes III-V, and normal spectra, whose temperatures had been measured by Code et al. Plotting 8eff against (1910-V)o, we obtained the relation.θeff = 0.111 (1910-V)o + 0. 565. Next, the quantity (1910-V) was computed for over 150 stars in the same spectral-type range which had been observed by OAO-2, and a mean value was obtained for each spectral type. (Class III stars were excluded for types B5 and later because they were found to be systematically redder than classes IV and V - possibly an evolutionary effect.) Finally, the mean values of (1910-V)o were converted to effective temperatures using the formula given above. The resulting temperature scale is in good agreement with the scales of Morton and Adams, Schild, Peterson and Oke, and Code et al. for types B3 and later, but between types B0.5 and B2 the new scale and the Code scale are significantly hotter than the others. A particular application of the new calibration is the determination of small temperature changes occurring in early-type variable stars, since in this case the large and uncertain reddening correction drops out. For the β Cephei variables δ Cet and γ Peg, we obtained ΔT = 550°K and 185°K, respectively. This work was supported by the National Aeronautics and Space Administration under grants NSG 5004 and NSG 5069.

Observations shortward of the hydrogen Lyman limit provide sensitive determinations of stellar temperatures and interstellar absorption. Such data are of particular value in studies of hot white dwarfs, for which a large fraction of the emission occurs in the extreme ultraviolet band (100-1000 Å). Observations of HZ 43 and Feige 24 have been obtained with the Apollo-Soyuz extreme ultraviolet telescope; both stars are copious EUV emitters, with 4 × 10−9 and 3 × 10−9 erg/cm2 sec in the 170-620 A band respectively. The EUV data combined with optical spectrophotometry, allow their temperatures to be estimated as 80, 000 and 60, 000 K respectively. The corresponding interstellar neutral hydrogen column densities are ~ 4 × 1018 cm−2.

The Utrecht Orbiting Ultraviolet Stellar Spectrophotometer S59 on board the ESRO TD-1A satellite has observed ultraviolet spectra of about 200 stars in the wavelength regions 2060–2160 Å. 2490–2590 Å and 2770–2870 Å with a resolution of 1.8 Å (cf. de Jager et al., 1974). The spectra are analyzed in order to find UV criteria for stellar classification. Particular attention has been given to Fe III lines since no strong lines of this ion occur in the visible part of the spectrum. As one of the most striking results, it is found that the feature at 2078 Å, which is mainly due to Fe III, is very sensitive to luminosity.

Of the nearly 300 stars which have been observed in the ultraviolet with Copernicus, 60 have been chosen for publication of their complete intermediate-resolution spectra, which consist of scans made with photomultiplier U2 at a nominal resolution and step length of 0.2 Å. The spectra cover the wavelength range 1000 to 1450 Å and are expressed in the form of direct numerical tabulations, compressed-scale plots, and synthetic photographic spectrograms. These three modes of presentation are expected to satisfy the needs of various types of research on stellar spectra, ranging from detailed and rather specialized analyses of line profiles, which require accurate numerical results, to broad, comparative studies of various qualitative features over different spectral types, for which the plots or photographs are best suited. From the qualitative comparisons one might expect to synthesize more exacting criteria for spectral classifications of normal stars, using ultraviolet instead of visible spectra. The catalogue has been submitted for publication in the Astrophysical Journal Supplements.

The suitability of Copernicus U2 spectrometer scans (resolution 0.2 Å, spectral range roughly 1000–1450 Å) for purposes of spectral classification is discussed. The main conclusion is that while many features in the spectra complicate the situation (numerous interstellar features and those related to the phenomenon of mass loss), the general behavior of the lines in the spectra of most of the 22 stars studied is entirely consistent with the presently accepted classifications of these objects. Whether a significant increase in the accuracy of determination of all of the relevant classification parameters can be obtained from this material is not yet certain. It is emphasized that study of the high-resolution Copernicus scans should be of great value for forthcoming classification programs necessarily utilizing spectral data of considerably lower resolution.

Selected regions of the ultraviolet spectrum of Beta Lyrae were observed in 1973 and 1974 at the epochs of the two eclipses and of the two quadratures with the Copernicus Princeton University spectrometer. The results were published in the Astrophysical Journal (Hack et al. 1975, 1976). Here we summarize the results of a third series of observations with the same instrument covering the whole period of 12.93 days. The following spectral regions were observed in June 1975 during 13 consecutive days: λ 1036–1060; 1300–1326; 1398–1416; 2050–2098; 2580–2632; 2777–2812 in the low resolution mode; λ 1172–1177 and 2795–2799 in the high resolution mode. The main results are the following: 1) The radial velocity curve for λ 1175 C III is almost 180° out of phase with the orbital velocity curve of the primary star and the mean velocity is - 240 km s−1; K = 70 km s−1. This suggests that λ 1175 C III is formed in an expanding envelope associated with the secondary. The mass ratio can be determined: m2 /m1 = 2.7 −1.1/+0.2 .2) The continuum variation confirms the results obtained with 0A0-2. At λ 2100 we observe the light of a continuum indicating that the temperature of the body eclipsing the primary star is lower than that of the primary and of about 9000 K. The light curves at λ 1900, 2100 can be explained with the contribution of Fe III emission in the circumbinary plasma. On the contrary, the depth of the secondary minimum at λ 1311–1326 and at λ 1410 cannot be explained with the Si III and Si IV emissions. A possible explanation is the presence of a hot spot in the region of the eclipsing body (probably a disk surrounding the companion) where the stream from the primary impinges on the disk. The hot spot can also explain the behavior of the infrared light curves observed by Jameson and Longmore (1976).

The Mg II resonance lines at 2795.523 and 2802.698Å and their respective subordinate lines at 2797.989 and 2790.768Å are probably among the most prominent and interesting spectral features in the ultraviolet; they are perhaps the most significant in the mid-ultraviolet. They are also observable in one form or the other in stars of practically all spectral types. We shall discuss relatively high (about 0.4Å) resolution observations of these features.

In the early spectral types both the resonance and subordinate lines are seen primarily as absorption lines. An interstellar absorption of varying strengths is superimposed on the photospheric absorption of the resonance lines. The strengths of the photospheric resonance and subordinate lines increase from 0 to B, e.g., Lamers et al. (1973) and Kondo et al. (1975). The subordinate lines begin to merge with the resonance lines in late-B stars. In mid-A type stars, the resonance and subordinate absorption strengths become maximum. In F-type stars, the photospheric absorption strenghts continue to decrease. Chromospheric emissions become definitely detectable in F-type stars (Kondo et al. 1972). In a G2 V star, the sun, the chromospheric emission is fairly prominent at the core of relatively weak photospheric absorption. In K and M-type stars, this region is presumably dominated by the chromospheric emissions of Mg II resonance lines with the photospheric absorption becoming negligible in late-K stars; the only extant observation in this region is that of e Eri (K2 V) (McClintock et al. 1975).

We have had today for the first time a confrontation of the views of many astronomers about the use of ultraviolet spectroscopic and photometric data in the field of spectral classification.

Dr. Jaschek recalled us that much work has still to be done from the ground since only 14% of the stars up to the tenth visual magnitude have M.K. spectral types. The current accuracy is around one tenth of a spectral type and 0.6 of a luminosity class. Jaschek notes that the luminosity classification from spectral inspection is superior to what is obtained from photometric data, a fact which is not surprising if we recall that lines are much more sensitive to pressure effects than the continuum. However, the introduction of a third parameter seems necessary, although it is not clear what this parameter should be.