The Distribution of Spiral Structure Tracers in the Region of the Galaxy Between 30 Degrees and 70 Degrees.
A programme of photoelectric and photographic UBV photometry and image-tube spectroscopy of luminous early-type stars in the northern Milky Way has been carried out in an effort to determine details of the local spiral structure of the Galaxy between longitudes 30(DEGREES) and 70(DEGREES). Distances and interstellar reddenings were determined for these stars (and for 60 other OB stars for which photometry and spectroscopy were available from the literature) and used to investigate the space distribution of interstellar dust and of spiral tracers between 30(DEGREES) and 70(DEGREES). The results may be summarised as follows: (1) The distribution of interstellar absorption at 30(DEGREES) to 70(DEGREES) is mainly local (within (TURNEQ)500 pc of the Sun), with essentially clear regions beyond, extending from 1 to 5 kpc from the Sun. Within 2(DEGREES) of the galactic plane obscuration is greatest between 30(DEGREES) and 60(DEGREES), and least near 28(DEGREES) and between 60(DEGREES) and 70(DEGREES). (2) The space distributions of the various spiral tracers share the following characteristics: (a) a heliocentric extent of up to 5 kpc, in contrast to earlier studies which had a limit of 1 - 2 kpc; (b) a small but significant number of optical tracers are found in the highly obscured region between 40(DEGREES) and 60(DEGREES), including a newly-discovered OB association with a probable 51-day Cepheid member at 1 = 55(DEGREES) and a distance of 3.5 kpc. (c) a tendency for all types of tracers to be found in "clumps". This clumpiness does not appear to be the result of selective obscuration. (d) the numbers of optical and radio tracers as a function of longitude show very good agreement, and in particular, show a marked minimum between (TURNEQ)35(DEGREES) and (TURNEQ)55(DEGREES) --a surprising result in view of the fact that the supposedly major Sagittarius spiral arm should be seen nearly tangentially in this direction. (3) A number of first-order tests of linear density-wave theory, based on radial velocities of (TURNEQ)75 OB stars and H II regions (some of which were obtained in this study) show no evidence for the sort of streaming motions or velocity differences in the stars and gas as have been observed in the Carina portion of the Galaxy.
- Pub Date:
- Physics: Astronomy and Astrophysics