H II regions in the Sco OB 1 association.

Two bright nebulae are related to the Sco OB 1 association. RCW 113 is situated in the southern part, IC 4628 in the northern part. The purpose of this study is to know whether the two observed H H regions originated at the same time from the same initial gas cloud. Optical and radio-astronomical data have been used. For IC 4628, the brighter emitting nebula, the optical results obtained from Hoc photographs and FabryPerot interferograms cover the largest area of emission, while the radio astronomical study is concerned with the densest part of the nebula. Kinematic results are in good agreement and the radial velocity discrepancies appear both in dense sources and in peripheral regions; this shows that the optical and radio emissions originate in the same region and that across the nebula there is a continuity of motion. The optical mean radial velocity of the field is: -24.216.3 km -l which, using the Schmidt model corresponds to the distance photometrically deduced from the member stars of the association. This velocity is not constant over the surface of the nebulosity; we can distinguish: IC 4628 a: - 24.2 1 3.0 km -l for 34407 <1 34503 IC 4628 b: - 14.9 1 3.3 km -1 34309 < 1 34407 RCW 113: - 29.7 + 2.7 km -l 34200 < 1 34309 Mter having examined the agreement of the observed electron density with the expected one, this velocity variation may be interpreted as an expansion effect. Radio sources of high electron density have been found in IC 4628. Relative to the paucity of H H sources and the faint Hoc emissivity in RCW 113, they indicate a very young age for the northern nebula. This disproportion suggests the presence of two subsystems within the association; this suggestion will be supported by the stellar study in paper H. Key words: association - H H regions