Credit: DSS and ESO

Observations with the VLT of a star-forming cloud have revealed, for the first time, a ring of infrared light around a nascent star. The images also show the presence of jets that emanate from the young object and collide with the surrounding cloud.

A small and dark interstellar cloud with the rather cryptic name of DC303.8-14.2 is located in the inner part of the Milky Way galaxy. It is seen in the southern constellation Chamaeleon and consists of dust and gas. Astronomers classify it as a typical example of a "globule".

As many other globules, this cloud is also giving birth to a star. Some years ago, observations in the infrared spectral region with the ESA IRAS satellite observatory detected the signature of a nascent star at its centre. Subsequent observations with the Swedish ESO Submillimetre Telescope (SEST) at La Silla (Chile) were carried out by Finnish astronomer Kimmo Lehtinen. He revealed that DC303.8-14.2 is collapsing under its own gravity, a process which will ultimately result in the birth of a new star from the gas and dust in this cloud.

Additional SEST observations of the millimetre emission of carbon monoxide (CO) molecules demonstrated a strong outflow from the nascent star. A small part of the gas that falls inward onto the central object is re-injected into the surrounding via this outward-bound "bipolar stream".

The left panel in above photo shows the DC303.8-14.2 globule as it looks in red light. This image was obtained at wavelength 700 nm and has been reproduced from the Digitized Sky Survey (DSS). It covers a sky region of 20 x 20 arcmin2 , or about 50% of the area of the full moon. The dust particles in the cloud reflect the light from stars, causing the cloud to appear brighter than the adjacent sky.

The brightness distribution over the cloud depends mostly on three factors connected to the dust. The first is the distribution of dust grains in the cloud, the way the dust density changes with the distance from the centre of the cloud. The second is the relative amount of light that is reflected by the dust particles. The third indicates the dominant direction in which the dust particles scatter light; this is dependent on the geometry of the grains and their preferred spatial alignment. Accurate observations of the brightness distribution over the surface of a globule allow an investigation of these properties and thus to learn more about the structure and composition of the cloud.

From the image obtained in red light it appears, somewhat surprisingly, that the brightest area of DC303.8-14.2 is not where there is most dust. Instead, it takes the form of a bright ring around the centre. This rim corresponds to a region where the intensity of the light from stars behind the cloud is reduced by a moderate factor of 3 to 5 when passing through the cloud and where the light-scattering efficiency of the dust grains in the cloud is the highest.

In order to study the structure of DC303.8-14.2 in more detail, Kimmo Lehtinen and his team of Finnish and Danish astronomers used the near-infrared imaging capabilities of the ISAAC multi-mode instrument on the 8.2-m VLT ANTU telescope at the ESO Paranal Observatory (Chile). Under good observing conditions, they obtained a mosaic image of this cloud in several near-IR wavelength bands, including the J- (centered at wavelength 1.25 micrometers), H- (1.65) and Ks-bands (2.17). These exposures were combined to produce images of DC303.8-14.2, two of which are shown in PR Photo 26a03 (middle and right panels).

The middle image shows the central part of the globule in the H-band. A bright rim is clearly detected - this is the first time such a ring is seen in infrared light around a globule.

This rim has a smaller size in infrared than in visible light. This is because the absorption of infrared light by dust particles is smaller than the absorption of visible light. More dust is then needed to produce the same amount of scattering and to show a rim in infrared light. The infrared rim will therefore show up in an area where the dust density is higher, i.e. closer to the centre of the cloud, than the visible-light rim.

Similar rings were also detected in the J- and Ks-band images and, as expected, of different sizes. Thus the mere observation of the size (and shape) of a bright rim already provides information about the internal structure of the cloud. In the case of DC303.8-14.2, a detailed evaluation shows that the dust density of the centre is so high that any visible light from the nascent star in there would be dimmed at least 1000 times before it emerges from the cloud.

As an unexpected and welcome bonus, the astronomers also detected several jet- and knot-like structures in the Ks-band image (right panel in PR Photo 26a03), near the IRAS source. The area shown represents the innermost region of the cloud (65 x 50 arcsec2 , or just 1500 of the area of the DSS image to the left).

Several knot-like structures on a line like a string of beads are clearly seen. They are most probably regions where the gas ejected by the young stellar object rams into the surrounding medium, creating zones of compressed and hot molecular hydrogen. Such structures are known by astronomers as "Herbig-Haro objects".