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The variable stars in the region of NGC 7023.

1953; Institute of Physics; Volume: 58; Linguagem: Inglês

10.1086/106918

1538-3881

Edwin B. Weston,

Astronomical Observations and Instrumentation

The stars in the region of the reflection nebula NGC 7023 are being examined for their distribution in space, presence or absence of variability in light, proper motion, and the presence or absence of emission lines in their spectra. The plates used in this study were taken with the Curtis Schmidt telescope of the University of Michigan during the years 1950 to 1952 and with the Crossley reflector of the Lick Observatory between 1902 and 1950. Some of the Curtis Schmidt plates were taken with an objective prism. Within the region of obscuration caused by the nebulosity there are two apparent clusterings of stars. One of these is in the center of the nebula, and the other is located about 5' west of the center. The numbers of stars brighter than the nineteenth photographic magnitude (approximately), the incidence of variability among these stars, and the incidence of Ha emission among these that are bright enough to be examined on objective prism spectra have been compared for the stars in both apparent clusterings, in regions of very heavy obscuration, and in relatively clearer areas outside of the nebula. Stars were not classified as variable unless the observed light variation was one-half magnitude or more, and caution was exercised in the recognition of em ission lines. Hence, all variables of small amplitude and emission objects with weak lines were excluded. Light curves were determined for many of the tars. It was found that nearly all of the stars in the central clustering are irregular variables that resemble the RW Aurigae and T Tauri types. Some even show appreciable light variation during intervals of only a few hours. Nearly all of these show Ha emission, and in addition other emission lines of hydrogen, Ca ii, etc. were recognized in the spectra of a few of the brightest. However, few variables and emission-line objects were discovered in the apparent clustering to the west; one of the two variables discovered in this region appears to be an eclipsing variable. Evidence for only a small number of variables and emission-line objects was found for the regions of heaviest obscuration. Approximately normal incidences of variables and emission-line stars were indicated for the regions of relatively lower obscuration outside the neb a. The peculiar proper motions of all stars in the region of the nebula were found to be small, and there is no evidence of common proper moti$n groups among the stars in the apparent clusterings. The central clustering of stars thus appears to be a physical cluster, and its members to constitute a T-association of the form defined by Ambartsumian.1 On the other hand, the apparent clustering to the west appears to be neither a physical cluster nor a T-association. It is an apparent optical cluster produced by a window or lane of relatively high transparency between the individual clouds and filaments that form this nebula and through which we see the distant stars beyond the obscuration. As to the central cluster, the absence of common proper motion indicates that it is not a moving cluster that encountered the clouds and filaments composing the nebula, but rather that it has a genetic connection with the nebula in the sense that it may represent the birthplace of stars along the lines proposed by Ambartsumian for T-associations as well as 0-associations. However, it is not unlikely that some of the stars, particularly those in the outer regions of the nebula, are ordinary stars that have collided with the denser clouds and filaments that compose this nebula, and that the interaction of these stars with the interstellar matter has affected their spectra and constancy of light. This explanation of the T Tauri stars has been proposed by Green stein2 and others and, although it does not appear applicable to all of the members of this central cluster, it may well explain at least a few of the stars in the region of the nebula where there appears to be a deficiency of non-variable stars without emission lines. I.See Astronomical News Letter, No. 64 for a summary in English and for references to the original literature. 2.Pub. A. S. P. 62, 156, 1950. David Dunlap Observatory, Richmond Hill, Ontario.