105A . 97 THE STRUCTURE OF INTERSTELLAR H AND K LINES IN FIFTY STARS* Walter S. Adams 1943ApJ Mount Wilson Observatory Received December 22, 1942 ABSTRACT The structure of the interstellar H and K lines of Ca n appearing in the spectra of 50 stars of early type has been studied in the second order of the grating spectrograph at the coudé focus of the 100-inch telescope. The camera has a focal length of 114 inches and gives a linear scale of 2.9 A/mm. More than 80 per cent of the stars show complex lines consisting of two to five components. Stars in various regions show differences in complexity, those in Perseus and Scorpius as a rule having single lines or at most lines with a faint component, while those in Orion, Sagittarius, and Cygnus usually show more intricate structure. The intensities of the components have been estimated. Measurements of their radial velocities provide a means of distinguishing individual interstellar clouds in various portions of the sky. Many of the clouds are found to extend over great areas. The narrowness of the components affords strong evidence for the discrete character of the clouds and the absence of any considerable tur- bulence. The spectra have been examined for the additional narrow interstellar lines discovered and identified during recent years, and their intensities have been estimated. Lines of Ca I and Fe i seem to appear only in stars which show very strong H and K lines, while those of the diatomic molecules CN, CH i, and CH n may occur when H and K are relatively weak. The fines of CH i and CH n showed marked differences of intensity in various stars. The radial velocities from some of these additional fines in a few selected stars have been compared with the velocities from the components of H and K in the same stars. In this way it has been possible to distinguish the interstellar clouds in which the various fines are formed. Since the original discovery by C. S. Beals1 of the doubling of the interstellar H and K lines of ionized calcium in the spectra of several stars, a considerable number of addition- al stars showing similar effects have been found, largely through the work of R. F. San- ford.2 Most of these spectra have been studied with the moderate dispersion best adapted for the investigation of the broad and hazy stellar lines usually characteristic of early- type stars. The use of very high dispersion, however, is especially favorable for the study of interstellar lines because of their remarkable sharpness and narrowness; and an addi- tional advantage is the fact that stellar H and K, if present, are widened and made so diffuse that the interstellar lines stand out clearly upon them and may be recognized without difficulty. The 50 stars considered in this investigation range in spectral type from 06 to cApe (v Sagittarii) and cA2 (a Cygni) and in apparent magnitude from 1.8 to 6.1. The spectra have all been taken in the second order of the grating spectrograph at the coudé focus of the 100-inch telescope. The camera has a focal length of 114 inches with a concave mirror 36 inches in diameter. No Schmidt correcting plate is necessary because of the relatively small ratio of aperture to focal length,3 and hence there is no glass in the entire optical train. The photographic plates consist of two strips, each 10 inches long, placed end to end upon the slightly curved rails of the plateholder. The region covered on a well- exposed spectrogram usually extends from about X 3600 to X 4700. The grating has sometimes been rotated to include the region near Hß. The linear scale of the spectra is closely 1 mm = 2.9 A, becoming slightly larger toward longer wave lengths. * Contributions from the Mount Wilson Observatory, Carnegie Institution of Washington, No. 673. 1 M.N., 96, 661, 1936. 2 Pub. A.S.P., 51, 238, 1939. 3 The concave mirror is large in order to provide full illumination over a long plate. 105 © American Astronomical Society • Provided by the NASA Astrophysics Data System 105A . 97 106 WALTER S. ADAMS The photographic emulsions used have been chiefly Cramer Hi-Speed Special and Eastman Experimental 103a-O. The latter has proved to be the most rapid emulsion so 1943ApJ far employed and gives excellent results for fairly wide spectra in spite of the coarseness of its grain. A few spectrograms of exceptionally interesting stars have been made on Cramer Contrast plates, with a marked gain in resolution and visibility of faint lines; but, in general, the necessary exposure times have proved prohibitive. Most of the spectro- grams obtained in the course of the search for new interstellar lines have been equally useful for the present study. Table 1 gives the results for the 50 stars so far investigated. Single spectrograms have been used for about half the stars, and from two to seven for the remainder. The first seven columns give the constellation designations or Henry Draper numbers, right ascen- sions, declinations, magnitudes, and spectral types, and galactic latitudes and longitudes. The types are* taken directly from the list of stars investigated for interstellar lines by P. W. Merrill and others,4 with the addition of a few from the Henry Draper Catalogue, which are given in parenthesis. The eighth and ninth columns give the intensity of each component observed in the H and K lines, estimated upon an arbitrary scale. Faint components are always better seen in K than in H, partly because of greater intensity and partly because of occasional interference of the wings oí He with H. The faintest line seen with certainty is given intensity 1 ; the few lines marked < 1 are not certain but probable. The relative intensi- ties should be reasonably reliable in the lower range but are much more doubtful when the intensity is high. Column 10 contains the radial velocities in km/sec of the individ- ual components reduced to the sun. These values are the combined results for K and H. The wave lengths used for H and K are those in the sun given in the Revision of Row- land’s Preliminary Table, X 3933.684 for K and X 3968.494 for H. Since the limit of resolution of lines of moderate intensity upon the spectrograms is about 0.10 A, differ- ences of velocity of less than 8 km/sec are subject to appreciable uncertainty. The ac- curacy in the measurement of well-separated components, however, should be high, dif- ferences between K and H, or between various spectrograms of the same star, rarely exceeding 2 km/sec. Although the usual correction for a solar motion of 20 km/sec directed toward R.A. 270°, Dec. +30°, is probably not strictly applicable to stars of spectral types O and B, comparatively little error is introduced by its use. Accordingly, the radial velocities given in the final column of Table 1, the “residual velocities,” are those of the preceding column corrected upon this basis. Finally, some notes are given usually dealing with the character of the lines and the probability of unresolved components. A simple inspection of the results in Table 1 leads to several conclusions : 1. More than 80 per cent of the stars observed show double or multiple interstellar lines. 2. Areas in the sky show marked differences in complexity of interstellar lines. Stars in the direction of Perseus and Scorpius generally show single lines, while those in Orion, Sagittarius, Cygnus, and Lacerta are usually complex. In general, the lines in the stars nearest the galactic plane show the greatest complexity, although there are some notable exceptions, such as p Leonis, galactic latitude +54°, with triple lines, and tt Aquarii, latitude —46°, with double lines. In both stars, however, the interstellar lines are of very moderate intensity and indicate relatively thin absorbing clouds. 3. Stars close to one another in the sky usually show much the same structure in the interstellar lines and similar relative intensities in the components. Illustrations are a and f Orionis, v and $ Orionis, a?1 and v Scorpii, 67 Ophiuchi and HD 165174, /z and 15 Sagittarii, and the stars in Lacerta. A striking exception is formed by 66 and 67 Ophiu- chi, in which the intensities of the two components are reversed. 4 Merrill, Sanford, Wilson, and Burwell, Mt. W. Contr., No. 576; Ap. 86, 274, 1937. © American Astronomical Society • Provided by the NASA Astrophysics Data System 105A . 97 PLATE VIII 1943ApJ Interstellar Lines with High Dispersion Calcium K {left) and H {right). From top to bottom: k Orionis, p Leonis, 10 Lacertae, e Orionis, X1 Orionis. Relative displacements from star to star correspond to the radial velocities at time of ob- servation. © American Astronomical Society Provided by the NASA Astrophysics Data System 105A . 97 PLATE IX 1943ApJ Interstellar Lines with High Dispersion Calcium K {left) and H {right). From top to bottom: k Aquilae, HD 167264, x Aurigae, u Sagittarii, HD 199478. © American Astronomical Society • Provided by the NASA Astrophysics Data System 105A . 97 PLATE X 1943ApJ Interstellar Lines with High Dispersion From top to bottom: i* Ophiuchi: left, K; right, X 3957 and H P Cygni: left, K; right, X 3957 and H v Sagittarii: left, K; right, H a Cygni: left, K; right, H v Sagittarii : X 4232 © American Astronomical Society • Provided by the NASA Astrophysics Data System 105A . 97 STRUCTURE OF INTERSTELLAR LINES 107 TABLE 1 Interstellar H and K Lines 1943ApJ 1900 Intensity Name or HD No.