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Lots of Small Stars Born in Starburst Region

October 13, 1999

The present research programme was granted observing time with VLT ANTU in April 1999. Its general aim is to investigate collective, massive star formation, in particular the coalescence of high- and low-mass stars in the violent environments of starburst regions. These are areas in which the processes that lead to the birth of new stars are particularly active just now.

Several fundamental questions arise in this context. A very basic one is whether low-mass stars form at all in such environments. And if so, do they form together with the most massive stars in a starburst event or do they form at different times, before or after or perhaps on different timescales? Are low-mass stars born with any "preferred" mass that may possibly give further clues to the ongoing processes?

The new VLT observations are the key part of a larger research programme that also includes observations of the stellar cluster in the famous Tarantula Nebula in the Large Magellanic Cloud (LMC) with the NICMOS instrument on the Hubble Space Telescope (HST), as well as adaptive optics observations with ground-based telescopes of more quiescent, star-forming regions in the Galaxy.

Images of the NGC 3603 region were obtained in three near-IR filter bands with the ISAAC instrument at the ANTU telescope. The observations were made in "service" mode on April 4 - 6 and 9, 1999, during selected periods when the (optical) seeing was equal to or better than 0.4 arcsec. This was a most essential requirement in order to achieve sufficient angular resolution (image sharpness) that would allow to do accurate photometric measurements of individual stars in this crowded cluster. This particular observing mode, during which ESO observers at ANTU kept careful track of the actual atmospheric conditions, contributed greatly to securing the very high quality images needed for this programme.

The new VLT observations are the most sensitive ones made to date of this densely packed starburst region. They allowed the team to investigate in unprecedented detail the low-mass stellar population in this area. For the first time, it was possible to reach the necessary angular resolution and sensitivity to study a starburst region on a star-by-star basis down this a mass limit of about 0.1 solar masses.

A most important conclusion of this study is that there are lots of sub-solar mass stars in NGC 3603, i.e., contrary to several theoretical predictions, these low-mass stars do form in violent starbursts. The overall age of stars in the contraction phase that are located in the innermost region of NGC 3603 was found to be 300,000 - 1,000,000 years.



European Southern Observatory (ESO)