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Internal Motions in the Orion Nebula

The Astrophysical Journal1969Vol. 157, pp. L111–L111

Abstract

Slit-spectroscopic observations of a bright region near the Trapezium in the Orion Nebula show evi- dence of matter with a mean radial velocity of approach of about 60 km sec' with respect to the nebula. Internal motions in the Orion Nebula have been extensively studied. The high surface brightness permits one to examine spectroscopically both the intrinsic widths of the nebular emission lines and the variation in radial velocity throughout a large volume of the nebula; such measurements are not generally available for other H ii regions. The most complete measures are those of Wilson et al. (1959). Radial velocities and line- splitting (if observed) were recorded over an area 4' X 4' around the Trapezium. A multislit was used at the coudé focus, and the lines [0 m] X5007, H'y, and [0 nI X3726 were studied. Sheglov (1968) has demonstrated the existence of high-velocity motions in the Orion Nebula by obtaining photographs through a narrow-pass-band tunable filter. He finds that a fraction of the matter in the nebula amounting to a few percent is moving with velocities up to ± 100 km sec'. Fisher and Wiffiamson (1969) have found a similar result. I have obtained a long-expdsure spectrogram of the Orion Nebula for a separate pro- gram. The purpose of this Letter is to call attention to a feature observed in this spectro- gram which would indicate a condensation in the Orion Nebula with a high radial velocity. Enlargements of two portions of this plate, obtained with the KPNO 84-inch coudé spectrograph, No. 2 camera, and an exposure time of 19 hours, are shown in Figure 1 (Plate L2). The plate was centered on the Balmer discontinuity, and the dis- persion is 13.7 A rnm'. On the first strip the two strong lines are [0 ii] XX3729 and 3727. To the violet of [0 nJ X3727, at the position indicated by the arrow, is a strong emission feature. Definitely present on the spectrogram are two features to the violet of [0 ii] X3729. There are similar emissions to the violet of 117 (X3970) and 11(5 and possibly near [0 m] X4363. The emissions at 117 and at H& are shown in the second strip of Figure 1, but their appearance is degraded in reproduction. The velocity of each feature has been measured and is listed in Table 1. The position of the condensation is approximately 43'~'5 from 01A Orionis in position angle 195°. Examination of Table 3b of Wilson et al. (1959) shows that in no case have they ob- served so large a velocity splitting. Caution must be exercised in deciding whether this feature is real. Kaler, Aller, and Bowen (1965) have carried out a spectrophotometric study of the spectrum of the Orion Nebula, listing wavelengths, intensities, and identi- fications for lines with strengths ~ 0.015 (Hf3 = 100). They show no lines at the meas- ured positions. Furthermore, Moore (1945) (Revised Multiplet Table) lists no likely identification at the observed positions. I believe that this is not an instrumental effect because (a) it is absent on focus plates taken immediately prior to and after the exposure and (b) it is not present near lines of other ions, even though the intensities might be nearly equal. It was not observed near [Ne iii] XX3967 and 3868 or at 117, probably be- cause of the intrinsic width of 117 and because Hy fell near the end of the plate. * Contri bulions from the Louisiana State University Observatory, No. 23; Contributions from the Kitt Peak National Observatory, No. 451

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