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A small elliptical ring with a very faint nucleus. The object is too faint to furnish intensity measurements of much value, the line H8, for instance, being barely discernible on the stronger plate.

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The nucleus gives a faint continuous spectrum with a broad emission band estimated roughly to extend from 4640 to 4700A.

I.C. 351. a = 3h 41ml; 8 = +34° 45'
DETAILS OF OBSERVATION

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A small slightly elliptical nebula. Direct photographs show an apparently uniform disc, but, as sometimes happens, this is due to the fitting together of component images of different forms and sizes. 4686A gives a small, almost stellar pattern not more than 2'.'5 in diameter at most, while the remaining images are ellipses showing condensations at the ends of the major axis. The distance between these condensations is about 5". In an integrated image, such as is secured by a direct photograph, the 4686 image fills in between these condensations and

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gives the general appearance of a uniform disc. The following are the lines with their relative photometric intensities:

NEBULAR LINES

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There is a very faint continuous nucleus spectrum which affords a suggestion of hroad emission bands.

N.G.C. 1535. a = 4h 9m6; 8 = —13° 0'

DETAILS OF OBSERVATION

Exposure Region of

Date Plate No. Instrument time Spectrum

1914 Dec. 7.83 683 a 4" 30m blue-violet

Dec. 22.76 684 i 5 0 blue-violet

1916 Sept. 26.98 777 g (slitless) 2 50 ultra-violet

A slightly elliptical ring lying on a uniformly illuminated circular disc. The outer diameter of the ring is about I8V2" ar"d that of the background 40". There is a very strong central star.

4686A gives an image about 10 or 15 per cent smaller than the hydrogen set, and, judging from the appearance of the lines 4363, 4712, and 4741 on the small scale slit spectrograms, these images are also small, though this is not so certain. The three last mentioned lines are faint and hardly record on the slitless spectrogram. The other images are of about the same size as those due to hydrogen.

NEBULAR LINES

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The above measurements were made on a single plate and the results are therefore only approximate.

The nucleus gives a continuous spectrum. The hydrogen lines are dark.

I.C. 418. a = 5h 22m8; 8 = —12° 46'
DETAILS OF OBSERVATION

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This is a slightly elliptical nebula with a bright central star. In the telescope its appearance, except for the star, is that of a uniformly illuminated disc.

The spectrum is shown in plates XLV and XLIX, figure 4. It is comparatively simple, the principal lines being 3727A, N„ N„, and those of hydrogen. The hydrogen image and the one at 3727 are not discs, but small rings, a fact which is not readily perceived, however, unless the photographic density is just right, because the images are traversed by the strong continuous spectrum of the nucleus. 3727A gives the largest image and the best defined ring. It is uniform and does not exhibit any mottling or curdling such as is sometimes found in the corresponding images of other nebulae. It is more elliptical than the other rings.

At first glance the hydrogen images appear to decrease in size from line to line with decreasing wave-length, but the difference is, I believe, only apparent, and due to difference in intensity. On comparing plates of unequal exposure so as to match, in intensity, the different images of the series, one with another, the apparent inequality disappears.

The images N,.- were discovered by Dr. Campbell, who observed the nebula in 1893,5 to be smaller than the image at H#. His measures of the three diameters are:

N, = 11", N, = 9", H/3 = 14".

The present investigation confirms this early observation with respect to the smaller size of the two nebular images as compared with that of 118, but not the difference in size of the first two images. While there is apparently such a difference on all of the plates, I am nevertheless inclined to think that the discs are. actually equal in diameter, and that the seeming difference is due to the character of the images. These lines do not give ring-shaped patterns, such as the hydrogen lines and 3727A do, but nebulous discs which fade in intensity from the center outward. On account of this fact, the measured diameter will depend on the brightness, and N, being the brighter line will appear to give the larger image. However that may be, the diameters on the photographs certainly vary with the exposure time. On plate No. 789, which was exposed for an hour, N2 is more intense and larger than N, is on 788 D, exposed for 15 minutes. The diameters as measured approximately on these two plates are as follows:

Line Diameter (E and W)*

1 hr. exp. 15 m. exp.

N, 1175 9"

N, 10.5 8

H/3t 13 (over-exposed) 12

* The difference in intensity between Ni and N2 is less p"otographically than it is visually, and one would expect less difference in the apparent diameters, when measured on a single plate than when determined directly by the eye. This seems to be the case. Photographic diameters from the same plate differ by only 1", while visually the difference appears to be about 2".

t Outside diameter of the ring.

The measures are inexact, but they serve to show the effect of increased density on diameter. From the appearance of the images I am inclined to think that with equal density N, would appear equal in size to N„ and that the two discs are equal. The matter is of some importance, as the inequality, if proved, would establish a difference in the behavior of these two lines which appear to belong together.

The three discs due to N„ N2, and H8 have been described by Campbell as being brightest at the center and fading gradually as the edges are approached, but that 1I8 is more uniform than the others. My own observations confirm this with respect to the first two discs, but the third shows the ring structure quite unmistakably.

A comparison of all of the plates indicates the following order in size of the images:

. 3727 > Hydrogen > 4471 (Helium) > N,.„.

The inequality in the last two members is uncertain on account of the difference in the character of the images. 4471A appears to have a definitely defined edge, while the N images are nebulous. The relative size will therefore vary with exposure time. The N discs are sensibly round, the hydrogen rings slightly elliptical, and the 3727 ring more elliptical still.

5 Astron. and Astrophysics, 13, 494, 1894.

The integrated image of the nebula as viewed in a telescope is a disc for the reason that the N^.o images fill in the H/3 ring. The same is true, in a less degree, of the nebula as photographed. The different images tend to nest one within another, but plates, insensitive to N,.„, such as the lantern slide, may give indications of a hollow interior.

NEBULAR LINES

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* The relative intensity of the components of the doublet, as estimated on slit spectrograms, is about 2 than 3729a.

1, 3726 being brighter

The intensities of the lines according to the system established by the slitless quartz spectrograph are given in the usual table. Wave-lengths as measured on the slit spectrograms are given in table 3. There are so many faint and uncertain lines among them that it seems preferable to give the measures for each plate, rather than the means. There are included some lines which do not appear in the intensity table. These are so faint that their strength cannot well be estimated.

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TABLE 3
I.C. 418. Wave-lengths

Remarks

long, strong

long, fairly strong

very faint

v. v. ft. extends out into nebula

long, strong, probably compound helium and hydrogen

v. v. ft.

long, strong

v. ft.

suspected v. v. ft. line

ft., long

v. ft., long

long, v. strong

v. v. ft., v. short

v. difficult dark line

v. v. ft., length uncertain

suspected v. v. ft. nucleus band

long, v. strong

v. v. ft.

v. v. ft., extends out

fairly strong, medium length

dark line

strong dark line

v. v. ft., only certain in nucleus

suspected v. ft. nucleus band

ft., nucleus only

nucleus line, v. ft. and difficult

long, v. strong

v. ft.

medium length, strong

suspected v. ft. nucleus band

medium length, strong

The spectrum of the nucleus is strong and continuous, with bright and dark bands, as indicated in table 3.

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The spectrum has been observed only for the purpose of measuring the wave-lengths of lines, no attempt having been made to study the line distribution through the nebula. That the nebula is not homogeneous is well known. Campbell was the first to prove it with respect to H/3 on the one hand and the two chief nebular lines on the other. Since then it has been found by Hartmann and others that the line at 3727A has a distribution peculiar to itself. The object affords an interesting subject for study, but the apparatus available in the present investigation is not suitable for the observation of anything so large. No attempt has been made, therefore, to extend the results of previous investigators with respect to the homogeniety of the nebula.

Wave-lengths of the lines, measured except as noted, with a single-prism spectrograph attached to the 36-ineh refractor, are given in table 11.

N.G.C. 2022. a = 5" 36m6; 8 = + 9° 2'

DETAILS OF OBSERVATION

Exposure
Date Plate No. Instrument time

1917 Dec. 13.86 898 g (slitless) 4ii 0m

An,elliptical ring nebula resembling, somewhat, in general appearance N.G.C. 7009 and N.G.C. 7662. Direct photographs show condensations at the extremities of the major axis of the ellipse. In the slitless spectrograms these condensations occur in the N„ N2, and 3869 images, being particularly strong in the first two. The other lines hardly appear to show them. The line 3727A, which usually surpasses all others in the depiction of these outlying condensations, is not present.

There is little difference in the size or shape of the images, except that the one at 4686 is noticeably less elliptical than the others, and the illumination within the space enclosed by this ring is relatively strong. The ultra-violet line at 3426 is rather faint, and it is not possible to form an accurate estimate of its relative size nor of the intensity distribution within it.

The spectrum of the nucleus is continuous, and, as is the case with the spectra of all of these nuclei, it extends with almost undiminished intensity far into the ultra-violet. Although faint throughout its length, it can readily be traced to 3450A.

The spectrogram is hardly suitable for measurement with the photometer. The lines in the order of their estimated intensities are:

N„ 4686, H7, H/ 3 = N2 = 3869, 118= (He + 3967), 3426.
I.C. 2149. a = 5h 48^9; 8= +46° 6'

DETAILS OF OBSERVATION

Exposure
Date Plate No. Instrument time

1916 Oct. 26.90 791 g (slitless) 3" 0m

Oct. 27.00 792 g (slitless) 1 0

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