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Secrets of a Dark Cloud

July 02, 1999

SOFI (Son OF ISAAC) is a scaled-down copy of ISAAC, the major VLT instrument that has already produced spectacular observations. SOFI is a unique instrument for the study of extended objects like "Barnard 68 (B68)" because of its very sensitive infrared detector and unrivalled large field-of-view.

The new data are unique in the sense that they allow astronomers, for the first time, to see through the very centre of a dense molecular cloud, into the cold regions where stars like our Sun will form.

Dark clouds like are the coolest objects in the known Universe with temperatures around -263 °C, just ten degrees above the absolute zero. They are the nurseries of stars and planets. To understand them is to understand the processes that took place when the Solar System was formed about 4,500 million years ago.

A team of astronomers, lead by Joao Alves (ESO-Garching), has made careful measurements of the colour of the background stars that are seen through the cloud on the SOFI images and have mapped the obscuration over the cloud. It is now possible to determine the total amount of obscuration at the center - it turns out to be no less than 35 magnitudes in the V-band at wavelength 0.55 �m. This number corresponds to a dimming of the starlight of a factor of no less than 10^14, or 100 million million times!

The small-scale structure of B68 seems to be very smooth and
homogeneous. The SOFI observations rule out the presence of
"clumpy" structures inside the cloud, on nearly all scales.

The new data clearly show that B68 is now in the very early phase of collapse, on its way towards star formation. The duration of such a stage is relatively short, of the order of 100,000 years, and to catch a cloud in this phase is likely to be a rare occurrence. If the collapse had been going on for a little longer, it would not have been possible to see through this cloud today, since the obscuration would then have been much higher.

The total mass of the dust in B68 can be determined quite accurately from the obscuration map by adding over the entire area of the cloud. It comes to about 0.03 solar mass. If the gas-to-dust ratio in B68 is what is normally assumed, about 100, then the total mass of this cloud is about 3 solar masses. Accordingly, only a few stars will eventually form in this cloud.

A video clip (ESO Video Clip 05/99) based on images of "Barnard 68" in six wavebands from blue to near-infrared is available at the ESO website. It illustrates in a very direct way the diminishing obscuration and, hence, improved view into the cloud, towards longer wavelengths.

European Southern Observatory (ESO)

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