Press Releases 1999

A Star-Forming Region in Ara

New Wide-Field Camera at La Silla Looks at RCW 108

The new Wide-Field Imager (WFI) at the MPG/ESO 2.2-m telescope at the La Silla observatory continues to obtain impressive images of the southern sky, see also ESO PR Photos 18a-d/99.

Recently, a series of images were obtained of areas in the Milky Way band, including some in which interstellar nebulae of gas and dust are seen. Each frame records 8184 x 8196, or over 67 million, pixels in a sky field of 32 x 32 arcmin 2.

The present photos show the RCW 108 complex of bright and dark nebulae in the southern association Ara OB1 , a star-forming region in the constellation Ara (The Altar), deep in the southern sky.

The Ara OB1 association contains many young and bright stars (of types O and B; hence the name). It is located at a distance of about 4000 light-years (1.3 kpc) from the Sun; the part shown in PR Photo 21a/99 covers an area of about 40 light-years across (approx. 12 x 12 pc) and includes most of RCW 108 .

RCW 108 is a molecular cloud that is in the process of being destroyed by intense ultraviolet radiation from heavy and hot stars in the nearby stellar cluster NGC 6193 , seen to the left in the photos. Most of this radiation comes from the bright object near the center of the image, which is actually a binary system composed of two O-type stars. The red glow that pervades the field is emission in the red H-alpha spectral line of hydrogen. It reveals a massive stream of gas that flows away from the molecular complex as it is being heated and ionized.

The small bright patch with several stars near the darkest part of the nebulosity, to the right in the photos, is the infrared source IRAS 16362-4845 . It marks a site where a small cluster of stars is being formed at present.

The designation RCW 108 refers to the inclusion of this object in "A catalogue of H-alpha emission regions in the southern Milky Way", published by three astronomers (A.W. Rodgers, C.T. Campbell and J.B. Whiteoak) in 1960.

Technical information: This colour picture is a composite made from 12 separate images, obtained with the WFI on 27 March 1999. The blue component corresponds to the B filter, the green to the V filter, and the red to the H-alpha filter. The images in each filter are the composite of 4 individual frames obtained with the telescope pointing at slightly different positions on the sky, so that the parts of the sky falling in the gaps between the 8 individual 2k x 4k CCDs in any given frame are recorded on the others. The monochromatic images are then produced by superimposing the individual frames, correcting for the telescope offsets; this ensures that the complete field is well covered. This procedure is not simple, as the observing conditions may change slightly from exposure to exposure, resulting in small differences. Finally, the combined images in each filter are aligned and colour-coded to produce the colour picture.

For the processing of this large photo (8k x 8k; 256 Mbytes), a minimum of contrast correction was made and very faint lines may still be perceived in some places where the individual frames were joined. It may also be noted that there is a slight misalignment of the individual colours in stellar images at the extreme corners of the large field. This is due to the effect of differential atmospheric refraction, i.e. light rays of different colours are bent differently in air.

The exposure time was 300 sec for each frame in H-alpha, and 60 sec in B and V. East is to the left and North to the top.

ESO Press Information

This is the caption to ESO PR Photos 21a-b/99 . They may be reproduced, if credit is given to the European Southern Observatory.