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The last two plates, with slit upon the major axis of the image (9" X 7"), were exposed for possible rotation effects. The N, and N2 lines on both spectrograms show some inclination; the amount and the corresponding relative velocity east — west are given below from measures by Mr. Moore.
Slit Along Major Axis
Means, 8.4 +5.5
The results indicate a relative velocity of 5.5 km./sec. of recession for a point on the major axis about 4 seconds of arc east of the center, with respect to a similar point at the same distance west of the center of the nebula.
Jonckheeee 900. o = 6h 20ml; 8 = +17° 51'; A = 94° 48'; /3 = —5° 31' The discovery of this nebula was announced by Jonckheere in 1912, and Curtis's Crossley photographs of it enabled us to combine radial velocity determinations with search for rotation.
The first two plates were obtained with the slit placed on the long axis of the bright oval (775 X 6"). They show no inclination of the nebular lines. On the spectrogram of November 23, taken with the slit crossing the bright lobes of the nebula, the N, line appeared slightly inclined, but this observation is not confirmed by the last two observations secured with the slit in the same position.
N.G.C. 2392. a = 7h 23m3;8 = +21° 7'; A = 109° 22'; /3 = — 0° 57' The first four spectrograms, obtained with 1-prism dispersion in 1913-14 for radial velocity, were a little disappointing as to accuracy, but in the state of the development of the problem at that epoch we were disposed to be satisfied with the results. In the winter of 1915-16 this nebula was placed upon our rather long list of nebulae to test for rotation; and on April 7, 1916, we made detailed plans to observe it that evening with the 3-prism 16-inch camera spectrograph. After the instrument had been made ready, and only a few hours before the exposure was to
begin, Mr. Wright informed us that his one-prism spectrogram of this nebula obtained five days earlier "showed curved nebular lines, convex toward the red, indicative of a high positive velocity of the central part of the nebula with respect to the ring." We desire herewith to acknowledge the information. Proceeding with our investigation, as indicated by the last six exposures listed in the table below, we found the nebular lines to be of very complicated structure, their forms depending to a marked degree upon the position of the slit with reference to the major and minor axes of the nebular image; and rotational effects were also evident. The velocities of radial translation deduced from the measures of the 3-prism plates depend somewhat upon assumptions as to the proper points to measure in the structure of the bright lines, and this fact modifies the apparent accuracy of the results. N, is very bright in this spectrum, being about equal to N2.
The complex forms of the N, and N- lines obtained when the slit was placed along the shorter and longer axes of figure of the bright inner ring (19" X 15") are shown in the accompanying illustrations. Plate XXX, figure 28, is a direct enlargement of the N, line, (a) on the spectrogram of January 5, 1917, with the slit on the minor axis, and (6) on the plate of December 17, 1916, with the slit along the major axis. The comparison line on the right is the helium line 5015A. In figure 29 (a) and (b), plate XXXI, are reproduced drawings of the N, line from these same spectrograms, which bring out more clearly some of the details lost in the enlargement. The vertical scale indicates the distance from the nebular nucleus, in seconds of arc, to which each portion of the line corresponds. The width of a monochromatic line is shown in the upper right edge of the drawing.
Considered as a whole, there is some similarity in the general form of the lines with the slit in the two positions. The ends of the lines originating in the outer portions of the nebula, about 10 to 20 seconds of arc from the nucleus, appear to be monochromatic. At the outer edge of the bright ring the lines become wider, with a rapid increase in width for the portion whose origin is the bright ring. In the ring (some 5" to 7" east, 4" to 8" west, on the minor axis, plate XXXI, figure 29a; and 5" to 7" south, 5" to 9" north, on the major axis, figure 296) the line is distinctly separated into two components. For the brightest part of the ring east and south of the center the violet component is relatively much stronger than the red component; in fact, its intensity is greater than that of any other portion of the line. In the west and south parts of the ring the red component is the stronger. The narrow dark spaces separating the components appear so sharp that one's first impression is that they may be photographic defects. As they are distinctly shown on several of the best spectrograms, and with the same curved appearance indicated in the drawing, their existence is real.
The portion of the line which has its origin in the region of the nebula between the nucleus and the ring consists of two fairly narrow components having the appearance of being extensions of the outer edges of the wide lines proceeding from the ring itself. They show a separation between their centers of about 1.7 A. With the slit on the minor axis the strong red component and the faint violet one maintain about the same intensity across the central section, while along the major axis the red component extending north and the violet one south from the center are extremely faint. Considerable similarity in detail is noticeable for the lines in the south and east sections of the ring, but a marked change in form is shown in the components from the west and north sections. The faint continuous spectrum of the nucleus is recorded on several of the spectrograms.
Measures of radial velocities in broad lines of this type are evidently subject to some uncertainty. One is at a loss, amid such complexity of form, as to the particular portion of the line to select for such measurements. Tables I and II contain the results of Mr. Moore's measures of the best spectrograms secured with slit coincident respectively with the major and minor axes. The values are expressed as the radial velocities in kilometer per second at the points indicated by letters on the drawings. The notation r, v, and a with subscripts refer respectively to the red and violet components and absorption lines.
1916 1917 from from center
April 26 January 5 Mean center of nebula
w +102 k. +109 k. +107 k. 0.67 mm. 14" West
T3 +108 +108 0.28 6
a, +75 +75 0.28 6
v„ [+ 60]* +46 +46 0.28 6
r„ +136 +135 +135 0.00 0
v„ +27 +26 + 26 0.00 0
r, +115 +115 0.28 6
a, +94 +94 0.28 6
\t +55 +59 + 58 0.28 6
e +63 +72 +69 0.86 18 East
* On this spectrogram the ra and vs components are not separated. The measure is therefore of the blend of the two.
On the two spectrograms, December 7 and 17, the measures for the bright parts of the line at r„ r2, v„ and v2 are of the N, and N2 lines. All of the other measures in tables I and II refer to N, alone.
The values from the spectrogram of April 14, Table I, and of April 26, Table II, have been given half weight in the means.
It will be noticed that the measures for the outer ends of the lines indicate a relative velocity for the points w and e of w — e = +38 km./sec., and for the points n and s of n — s = +33 km./sec. If we attempt an interpretation of this as a rotational effect it would be necessary to assume