Unique Infrared SOFI Images of Barnard 68
Probe the Very First Stages of Star Formation
Secrets of a Dark Cloud, 2 July 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 B68 because of its very sensitive infrared detector and unrivalled large field-of-view.
The new SOFI images of B68 are now being studied by ESO astronomer João Alves and his collaborators, in particular Charles Lada (Harvard-Smithsonian Center for Astrophysics, Mass., USA) and Elizabeth Lada (University of Florida, USA). Several interesting conclusions can be drawn already.
Through careful measurements of the colour of the background stars that are seen through the cloud, it is now possible to determine the total amount of obscuration at the center of the cloud. It turns out to be no less than 35 magnitudes in the V-band at wavelength 0.55 µm. This number corresponds to a dimming of the starlight of a factor of no less than 1014
If, in a thought experiment, a sheet of dust with this high degree of obscuration were placed in front of the Sun, there would be eternal darkness on the Earth. Our central star would then shine with magnitude 9 only, i.e. it would be about 15 times too faint to be observable with the naked eye!
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, of the order of hundreds of magnitudes. Moreover, the observed distribution of matter inside B68 provides us a first glimpse of how nature begins to form stars. These outstanding observations will now be used to test current theories of protostellar collapse.
The total mass of the dust in B68 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.