points the lines become faint and hazy, making the settings on these sections of the line somewhat uncertain. This difficulty is further increased by the gradation in intensity from one edge of the line to the other. The effect of this is particularly noticeable upon the last two measures, corresponding to points 10" and 12" west of the nucleus (Table II). No inclination of the lines is shown by the measures of the plate of November 22, 1915 (Table I), which was obtained with the slit on the minor axis. Dr. Green's results with higher dispersion, for the same position of the slit (Table II, sixth column), indicate a relative velocity of 4 or 5 km./sec. between points 8" to 10" north and south of the center of the nebula. No inclination of the line, however, is shown by the section corresponding to the central portion of the nebula whose diameter is about 6 or 7 seconds of arc. A comparison of the values in Table II with a photograph of the nebular image (see drawing by Curtis, plate XXII, fig. 70) shows that the radial velocity results for the portion of the nebula within the bright inner ring (whose dimensions are given above) are explained by a rotation of this central mass about an axis whose position angle is approximately 169°. The parts of the lines whose origin is in the bright ring formation show the peculiar curved appearance and gradation of intensities similar to those which we have described earlier for N.G.C. 2392 and certain other nebulae, and to this effect undoubtedly the apparent anomalies in the radial velocities for this portion of the nebula. are to be ascribed. In plate XXXVII, figure 59, is reproduced a drawing of the N, line, on the spectrogram of August 20, 1916, obtained with the slit coincident with the major axis of the nebula. The inclination of the central portion of the line, the curved structure of the parts originating in the rings, and the bending back of the outer ends of the line are easily recognized in the drawing. It will be seen that the line has the same features which characterize those in which the nebular line is separated into two components, and it appears probable that in the present case such resolution would be effected by employing a narrow slit and a fine-grained plate. N.G.C. 7026. 21h 2m9; 8 α= +47° 27'; λ 343° 53′; B +59° 44′ This is apparently the most elongated of all the planetary nebulae observed. It was consequently no surprise to find that the lines are strongly inclined on all spectrograms secured with the slit parallel to the major axis of figure. This object, though fairly bright, is therefore not well adapted to accurate velocity determinations. Fig. 58-The N, line in N.G.C. 6825, 1916 June 1. Fig. 59-Drawing of the N, line in N.G.C. 7009 (Moore). Major axis, 1916 Aug. 20. Fig. 61 The image of the nebula is an elongated, somewhat irregular-shaped formation (about 9" X 25"), with two bright masses at the extremes of the short diameter. In the direction of the long axis (in position angle approximately 10°) the central portion of the nebula is very faint. Two of the above spectrograms were secured with the slit along the short axis, i.e., on the line joining the two bright lobes and passing through the nucleus. The nebular lines on these plates show no inclination of the lines as a whole, but their appearance indicates the presence in this nebula of the phenomenon of which N.G.C. 2392 supplies the typical example. Three spectrograms were taken with the slit placed on the long diameter of the image, and half way between the two bright lobes. The appearance of the line with the slit in this position is shown in plate XXXVII, figure 60, which is a drawing of the N, line on the spectrogram of October 24, 1916. A direct enlargement of this line from the same spectrogram is reproduced in plate XXXVII, figure 61. A very strong inclination of the bright central section of the line is apparent, while the outer portions of the line are inclined to a smaller degree. The central bright section seems to be practically monochromatic, and the faint outer parts to be somewhat widened. There is here also some evidence of the curved and shaded appearance referred to above for the plates taken along the short axis. In the table are measures, by Campbell and Moore, of the three spectrograms obtained with the slit on the major axis. The values are expressed as the relative velocity of a point 3 to 4 seconds of are south of the nebular center with reference to that of the corresponding point. north of the center. Mr. Campbell's measures were made in the usual manner, whereas Mr. Moore measured the angle of inclination. The mean inclination of this bright central section is 4:64, the greatest value we have found for any of the planetaries. The last two spectrograms were taken with the slit parallel to the major axis of figure and set respectively upon the bright east and west lobes of the nebula. Measures on the N, line for relative velocity of a point south of the center of the lobe with reference to its corresponding point north of the center yield the following results: The spectrogram of November 22 is under-exposed and the line very difficult to measure, and the result for that date is entitled to little or no weight. We have very strong evidence that this planetary is in rapid rotation about an axis approximately coincident with the shorter axis of figure. B: N.G.C. 7027. a = 21h 3m3; 8=4-41° 50′; λ=338° 28'; p= +54° 56′ Keeler's radial velocity of this brilliant nebula is +10.1 ±1.9 km. The 3-prism plates of 1915 were obtained for the purpose of determining the accurate wave-lengths of N, and N., and the 32-inch spectrograms, in 1916, sought for evidence on the subject of internal motions. |
