Stellar Birth in a Dark Cloud

Dear subscribers,

please find below the extended caption to a new, splendid
astronomical
photo from La Silla. It is a remarkable wide-field infrared view of
the
inner regions of the starforming region RCW 108 in the Southern
Milky
Way. The original URL is:

http://www.eso.org/outreach/press-rel/pr-2000/phot-30-00.html

Kind regards,

The ESO EPR Dept.

Information from the European Southern
Observatory

For immediate release

PR Photo 30a/00 shows a unique,
wide-field infrared view of the region around IRAS 16362-4845,
an "emission" nebula that shines by its own light. It is
situated within a dark cloud in a Milky Way region (the "RCW
108 complex") at a distance of about 4,000 light-years in
the direction of the Southern constellation of Ara (the Altar). The
dark cloud is seen silhouetted against the rich background of stars
in the Milky Way, whose light is blocked by the dust particles in the
cloud. The photo is based on approx. 600 individual exposures with
the SOFI instrument at the 3.58-m ESO New Technology
Telescope (NTT). The field-of-view measures about 13 x 13
arcmin2, coresponding to 17 x 17 light-years2. PR Photo 30b/00
shows a smaller area, near the centre. More technical details are
found below.

The above infrared view (PR Photo 30a/00) of an
interesting region in the Southern Milky Way, full of stars and
nebulae and known as the RCW 108 complex, was obtained in
February 2000. It provides a deep look into a star-forming region,
where stars are in the process of being born from dense clouds of
dust and gas.

It is particularly interesting to compare the appearance of the
area in this infrared view (SOFI) with that obtained earlier at
visible wavelengths (WFI), cf. ESO PR Photos 21a-b/99. As they look quite different, such
a comparison is greatly facilitated by referring to the positions of
the same brighter objects in the two photos.

In visible light, the Eastern edge of the cloud appears as a
bright rim; the same edge is present, although less prominent to the
left in the infrared image. This is where the outer surface of the
cloud is eroded and later completely dissolved by the strong
ultraviolet light of a nearby cluster of stars, that lies beyond the
edge of the infrared image.

The bright nebula IRAS 16362-4845 at the center of the
infrared picture is much more prominent here (see PR Photo
30b/00) than in the visible one. This is because it is deeply
embedded in the cloud and its visible light is heavily obscured by
the dust particles. This effect is much smaller at longer (infrared)
wavelengths, so the SOFI image provides a more detailed view of
IRAS 16362-4845.

It is exactly this greater transparency of interstellar dust
clouds in the infrared spectral region that is one of the main
reasons why studies of very young stars are best carried out at
infrared wavelengths. Such stars are often still surrounded by the
remnants of their parental gas and dust clouds.

Dust not only dims the light of objects located behind the
cloud, but also "reddens" it. This is because
shorter-wavelength (e.g., blue) light is scattered out of the
line-of-sight more efficiently than is longer-wavelength (e.g., red)
light.

This effect is clearly seen in the SOFI image, where stars
located within the cloud area on the average appear much redder than
those that are outside. There are a few exceptions, however: some
stars that are seen "in" the darkest part of the cloud
appear to be blue. This is because they are located in front of the
cloud or are only slightly embedded in it.

In fact, since the "real" (i.e., intrinsic)
"infrared colours" of most stars are rather similar, the
different colours we see in the SOFI picture are mostly due to
different degrees of reddening by foreground dust. The colour of a
star in this area is therefore mostly a measure of how deeply it is
embedded in the nebula.

This SOFI image of RCW 108 was obtained by ESO astronomer
Fernando Comeron for one of his current research projects. He
also performed the related, very extensive image processing.

PR Photo 30a/00 was obtained in February
2000 by means of SOFI, the infrared camera and spectrograph at
the ESO 3.58-m New Technology Telescope (NTT). The image is a
mosaic that was prepared by combining 33 overlapping frames, taken in
each of the J (wavelength 1.25 µm; here rendered as blue), H (1.65
µm; green) and K (2.2 µm; red) infrared filters. Each individual
frame is in turn a stack of six frames totaling 12 sec exposure time,
with the exception of the one covering the center of the region which
is 15 times longer in order to increase the sensitivity around the
central emission nebula (an HII region). The total area imaged is
12.8 x 12.8 arcmin2, using SOFI in its normal resolution mode with
0.29 arcsec pixels and a field of about 5 arcmin. Thus the field
shown here is about 6.6 times the field-of-view in a single SOFI
frame. Five-sigma detection limits in the least-exposed areas of the
image are typically J=18.6, H=17.7, K=17.2 mag. The image quality is
between 0.6 and 0.7 arcsec for the different filters over most of the
field. PR Photo 30b/00 is a smaller area near the centre of
the field; it measures approx. 6.3 x 6.6 arcmin2. North is up and
East is left.

This is the caption to ESO PR Photos 30a-b/00. They may
be reproduced, if credit is given to the European Southern
Observatory.