The Blob  


The remarkable discovery of a dust cloud next to a luminous hot star

In December 2002, we discovered an extended object only a few arcseconds away from the naked-eye star sigma Orionis. The new object, dubbed sigma Orionis IRS-1, shines brightly in the infrared. Its spectral energy distribution ("colours") and a strong spectral emission feature around a wavelength of 10 micron due to silicate grains imply that it is a dust cloud. The cloud is spatially resolved: it has a fan shape, pointing away from the hot star sigma Orionis. With an estimated age of about 5 million years, the stars belonging to the sigma Orionis cluster have only recently formed. Yet it is surprising how a dust cloud could have survived so long so near to sigma Orionis. It is not yet clear whether the dust cloud harbours an infant star, or whether it is a "leftover" from the molecular cloud in which sigma Orionis formed.

Drake, in 1990, actually discovered this object at radio wavelengths but at the time attributed the difference in position with sigma Orionis to astrometric inaccuracies of the latter. The radio source is located closer to IRS-1 but at the side facing sigma Orionis, suggesting it is the part of the dust cloud which is directly irradiated and ionized by sigma Orionis. This confirms the physical interaction between the two objects and their close proximity to one another.

The observations were performed with the TIMMI-2 imager/spectrograph at the 3.6 metre telescope at the European Southern Observatory, La Silla, Chile, and have been published as a Letter to Astronomy and Astrophysics.