RCW-108



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 arcmin2.

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.

 PR Photo 21a/99 displays (very nearly) the full extent of the WFI
 field, while PR Photo 21b/99 shows a smaller area in more detail.

 In PR Photo 21a/99, the resolution has been degraded by reducing the
 number of pixels in one direction from about 8000 to 3000 in the
 "High-Resolution version", in order to make the image transportable
 over the web without incurring completely unacceptable transfer
 times. Still it is very large, even in the highly compressed
 jpeg-format, reflecting the great amount of details visible. The
 "Normal" and "Preview" versions are smaller and may be acquired much
 faster, but with a corresponding loss of detail. The
 "Full-Resolution" version of PR Photo 21b/99 retains the original
 pixel structure and image sharpness. It covers an area that
 corresponds to about 1/7 of the full WFI field.


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.