Fig. 62-Drawings of the N, line in N.G.C. 7027 (Moore). (a) 1916 Aug. 19, pos. ang. 180°; (b) 1916 Sept. 15, pos. ang. 135°; (c) 1916 Aug. 19, pos. ang. 90°; (d) 1916 Aug. 15, pos. ang. 65°; (e) 1916 Oct. 13, pos. ang. 225°; (f) 1916 Aug. 9, pos. ang. 70°. Wright in 1901, comparing the Hẞ nebular line with the Hẞ line from the hydrogen tube, on a 3-prism spectrogram, obtained a radial velocity of +12.3 km. per second; and from two 1-prism spectrograms, +6 km.47 Hartmann in 1901 obtained +4.9 km. from one 1-prism spectrogram." 48 The image of this planetary is an irregular oblong (about 18" X 11") containing three condensations, of which the northeastern one is much the stronger. Its form appears to be very different from that of the other planetaries which we have investigated for internal motion, and it gives no clear indication as to the direction in which the slit should be placed across the nebula in order to obtain possible rotational effects. Our spectrograms were taken with the slit placed centrally on the northeastern and brighter condensation, and successively along the axes indicated by the position angles, 45°, 65°, 70°, 90°, 135°, 140°, and 180°. Two spectrograms were obtained with the slit central on the southeastern nucleus and in position angle 70°. All of these excepting two were taken with the 3-prism 32-inch spectrograph. The spectra are difficult to interpret, as the lines are not monochromatic and show considerable complexity, and in particular the peculiar variation in intensities from one edge of the line to the other which we have previously described for other nebulae. A typical case has been selected from each of the above positions of the slit across the brighter condensation, and a drawing of the N, line as shown on the best spectrogram, corresponding to each position, is given in plate XXXVIII, figure 62, a to e. The direction of the slit for each of these nebular lines is indicated on the drawing of the nebula, made by Dr. Curtis, and reproduced in plate XXXIX, figure 63. Measure of relative velocities in different parts of the line seem to have little or no meaning, as one is uncertain whether the settings should be made by bisecting the whole width of the line. or with reference to the details within the line. It will be noted that in the lines, corresponding to the five positions of the slit across the northern bright nucleus (fig. 62, a, b, c, d, and e), the red edge of the central section is conspicuously stronger than the corresponding violet edge. With the slit placed in position angle 70° and across the southern fainter condensation, the violet edge of the line is stronger than the red edge. The appearance of the N, line, on the spectrogram of August 9, 1916, obtained with the slit in this position, is shown in figure 62, f. This effect is also observed in the line of the spectrograms secured with the slit in position angles, 135°, 140°, 315°, and 320°, and central on the bright northern nucleus, but including near the extremity of the slit the fainter southern condensation. The N, line on the spectrogram of September 15 is taken as representative of these plates, and is shown in figure 62, b. On the plates taken in this position of the slit, but with longer effective exposures, the violet edge of the line whose origin is the northern nucleus increases in intensity, always however, remaining fainter and narrower than the red component. The separation of the two components on the above spectrogram amounts to about 0.8 A. Long exposures widen the lines from the principal nucleus more on the faint violet edge than on the strong red edge, while the reverse is true for the southern condensation. Due to this effect, measures of the relative velocity of the two components are to some extent dependent upon the effective exposure which the spectrogram has received. The mean of the values obtained on four spectrograms representing the conditions of short, medium, and long exposures give a relative velocity of recession of about 8 km./sec. for the faint southern condensation with reference to the bright nucleus. All of the plates secured in this position of the slit, which corresponds approximately with the longer diameter of the oblong nebular image, show marked inclination of the N, and N, lines, indicating a velocity of approach of about 20 km./sec. for a point in the extreme northwestern edge of the nebula, some 3 to 4" from the center of the bright nucleus, relative to that of a point 7 to 8" southeast of the center. Some inclination of the line as a whole is shown for the other positions of the slit across the bright nucleus, although much less in amount than that obtained along the major axis of figure. 47 Lick Obs. Bull. 1, 155, 1902. 48 Astroph. Jour., 15, 294, 1902. 2 On the spectrograms of September 22 and October 13 (fig. 62, e) for which the slit was placed perpendicular to the long axis of the nebula the lines still show noticeable inclination. In the appearance and the inclination of the lines, this nebula bears a strong resemblance to N.G.C. 6210; and, as in the former case, the results obtained for the relative velocities of different parts of the nebula do not appear to be interpretable upon a simple hypothesis of rotation. To what extent rotational effects, if present, are concealed by the phenomenon which gives rise to the peculiar curved strong red component and the somewhat fainter violet one is difficult to determine. This latter effect is undoubtedly the same as that which we have observed in several nebulae. N.G.C., 5117. a = 21h 28m7; 8 = +44° 10′; λ=347° 26'; B=+54° 31′ The hydrogen radiations seem to be weaker than normal in this nebula. The image of this nebula is indistinguishable from a star on Crossley photographs. On the spectrogram of August 5, 1916, the N, line appears to be slightly inclined, but the poor quality of the line makes such evidence very uncertain. The N, and N, lines on the last plate are very short and stubby. Our observations show no definite evidence of rotational effects in this object. Of 79 small nebulae observed by Curtis with a slitless spectroscope attached to the Crossley reflector, this is the only one which showed a bright-line spectrum. Campbell and Paddock observed the nebula with a 1-prism spectroscope attached to the 36-inch refractor, and were able to see bright N, and N. However, these lines are too faint to photograph for radial velocity determination under present conditions; an exposure of 35 hours, more or less, would be required to record N, in measurable strength with the 3-pr. 6-in. combination. This nebula is a bright oval (75 X 6") with the major axis in position angle about 0°. The last two spectrograms were obtained with the slit along the major axis of the oval, but the nebular lines on these plates show little if any inclination. |