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N.G.C. 6514. a = IV 56m3; 8 = —23° 2'; A = 269° 9'; p = +0° 25' This is the Triftd nebula. A preliminary visual survey of the spectrum showed that the green lines were strongest in the region bordering the SW side of the dark lane, and the slit was accordingly set upon that area. The H/3 line is much brighter than N„ as Mr. Campbell announced in 1895.
N.G.C. 6523. a = 17h 57m6; 8-=—24° 23'; A = 269° 27'; / 3 = —0° 56' This object is the large nebula Messier 8, near the Trifid nebula. The slit was directed upon the brightest part of the nebula. The green lines are strong on all of the plates, and H/3, as in the Trifid spectrum, is about three times as intense as N,.
N.G.C. 6543. o = 17" 58*6; 8 = +66° 38'; A = 148° 31'; p = +89° 50' This nebula, in Draco, is the historic one in which nebular bright lines were first observed by Iluggins in 1864. Since that date it has appeared on many programmes relating to studies of the nebulae. Spectroscopic observations by Keeler on six nights in 1890-91 yielded a radial velocity of —64.7 d=2.9 km. per second. The five plates of November and December, 1915, with the 3-prism 16-inch spectrograph were used by us in studying the differential radial velocities supplied by different parts of the nebula ;24 and the nine spectrograms with 32-inch camera were secured and used by Dr. Green in a more detailed study of this object.25 H/3 is nearly as bright, photographically, as N-. The ratios of brightness are about N, :N- :H/3 :: 10 : 3 :2\.
Hartmann in 1901 obtained —65.8 km. from one 1-prism and one 3-prism spectrograms.26 The general form of the nebular image is elliptical in outline (about 22" X 16"), with the longer axis of figure in position angle approximately 40° and the shorter axis in position angle 130°. On the spectrograms secured with the slit placed along the major axis, the spectral lines have the form which we have previously described as that of the letter S, while on the plates obtained with the slit coincident with the minor axis the lines appear practically straight. When the slit is placed along intermediate axes, the lines are again of the broken form, but with decreasing inclination of the central portion of the S as we pass from the major to minor axis. In Tables I, II, and III are collected the measures of radial velocity made at different points along the nebular lines, for the spectrograms of November 29, 1915, to May 2, 1917, inclusive. The values are expressed as the relative velocities in kilometers per second for the different points, with reference to the midpoint of the central section of the line, which is assumed to correspond to the center of the nebula.
24 Lick Obs. Bull, 9, 2, 1916. ?5 Lick Obs. Bull, 9, 92, 1917. 2« Astroph. Jour., 15, 294, 1902.
Table I contains the mean of the measures of relative velocities made by Moore and Green, for four points on the nebulium lines, on the five spectrograms secured with the 3-prism 16-inch spectrograph in 1915. These values are taken from tables III and IV in Lick Observatory Bulletin, 9, 3 and 4, 1916. The results for the three positions of the slit on the nebular image are grouped in Table I under (1), (2), and (3), which refer respectively to the slit coincident with the major axis, with an axis inclined at an angle of 40° to the major axis, and with the shorter axis of figure. The first column gives the distance of the point measured in seconds of arc, from the assumed center of the nebula.
(2) November 29.604, pa = 0°
N 8'.'5 (+0.5)
S 9.3 (+4.5)
(3) December 6, pa = 130°
NW 779 +4.3
SE 7.9 —0.1
The results in Table I, (1) and (3), depend upon the measures of the N, and N2 lines, while those in Table I, (2), are for the N, line alone. The latter spectrogram is very much underexposed and the values for the extreme points are entitled to little or no weight.
In Table II are recorded the results of Dr. Green's measures of the N, and N2 lines on the spectrograms of March 31 to May 14, 1916, inclusive, obtained with the 3-prism 32-inch spectrograph. These values have been taken from the diagram of radial velocities in his paper in Lick Observatory Bulletin, 9, 94, 1916, where they are plotted (to the nearest kilometer) at approximately two-second27 intervals on an outline drawing of the nebula. The distance, corresponding to each plotted point, from the assumed center of the nebular image, is given in seconds of arc in the first column of Table II.
The spectrogram of May 2, 1917, was secured with the 3-prism 32-inch spectrograph, the slit being made very narrow and placed along the major axis of the nebula, in order to test for possible doubling of the lines in the central section. A drawing of the N, line as shown on this spectrogram is reproduced in plate XXXIV, figure 53. It will be seen that the central portion of the line is clearly separated into two components, for which the separation at the center, Ofiv is about 0.4 A, and that the relative intensities of the components on opposite sides of the nucleus are reversed, i.e., the red component is the stronger 2 to 5 seconds northeast of the center and the violet component the stronger at a corresponding distance southwest of the center. We have then very clear evidence of the existence in this nebula of the same phenomenon which is such a prominent feature of the spectrum of N.G.C. 2392, although in the present case the effect is so much smaller that it is shown only under high resolution.
In Table III are given the relative velocities derived from Mr. Moore's measures of different points of the N, line on the spectrogram of May 2, together with the corresponding distances of these points from the assumed center of line and nebula. The values are expressed as the velocities with reference to the midpoint of the line at the assumed center OtO-. The points on the line at which the measures were made are indicated on the drawing by the same letters used in the first column of the table.
A comparison of the measures made with the two spectrographs, in Tables I and II respectively, show a very satisfactory agreement, with the exception of those at the extreme points of the lines on the spectrogram of November 29, 1915, which, as we have already noted, are entitled to little or no weight. Small weight should also be given to the values for the extreme points in Table II, as at these points the lines become hazy and faint and are difficult of measurement. It will be noted that apparently considerable discrepancy exists between the measures given in Tables III for the points c, d, and e and those for the corresponding points in Tables I and II. This arises from the fact that the values for these points given in Table III refer to the bright component of the line, while the previous measures were made on the center of the blend of the two components, inasmuch as they were not separated on these plates. For this reason it would seem that the results for the central portion of the line, in Tables I and II, give a better representation of effects due to rotation alone.
Our measures indicate that the maximum relative velocity occurs for points on the major axis some 3 to 4 seconds of arc on opposite sides of the center of the nebula, in the sense that a point at this distance northeast of the nucleus is receding with a velocity of about 10 km./sec. relative to the corresponding point southwest of the center. No inclination of the central portion of the line is shown on the spectrograms secured along the minor axis, while intermediate values of the inclination are observed for those obtained along axes between the minor and major.
These results are in harmony with the hypothesis of a fairly rapid rotation of the central portion (about 8 seconds of arc in diameter) of the nebula, around an axis in position angle approximately 130°. For the outer portions of the nebula our measures become more difficult to interpret. Along the major axis the results for the region 4 to 8 seconds from the nucleus indicate that the outer strata are rotating more slowly than the central part of the nebula. This simple hypothesis meets with difficulty, however, when one attempts to interpret the measures obtained for the exterior portions along the other axes. Dr. Green,28 to explain the observed peculiarities of the motions of these exterior portions, offered the hypothesis that the axis of rotation of the outer strata is not coincident with that of the central portion.
2? Through an error in Green 's paper the interval of plotted points is called three seconds instead of two seconds.
N.G.C. 6537. a = 17h 59m3; 8 = —19° 51'; A = 269° 50'; / 3 = +3° 36' The lines recorded have about the intensity ratios N, :N2 : H/3 :N, :: 10 :3 :1 :1.
All the exposures were really too short for accurate results. This is the farthest south nebula on the Mount Hamilton programme.
The spectrogram of July 11, 1917, was obtained with the slit of the spectrograph placed along the major axis of the nebular image (an oval 8" X 10"). The N, line is under-exposed, hazy and irregular. Measures of this line indicate that the southern portion of the nebula some 3" from the center is receding with reference to the corresponding northern portion with n velocity of 7± km. per second.
The evidence justifies a strong suspicion of rotational effects in this nebula.
28 ;. C.