University of Cologne, Germany | 2014 Nov 17
An international research team led by scientists from the Coordinated Research Center (CRC) 956 “Conditions and Impact of Star Formation” at the University of Cologne has used observations made with the GREAT instrument on board the SOFIA aircraft observatory and the APEX telescope to date the core of an interstellar cloud that is forming a group of Sun-like stars. This work, to which scientists from the University of Helsinki as well as from the Max-Planck-Institutes for Radio Astronomy (MPIfR) and Extraterrestrial Physics (MPE) contributed, is published in this week’s Nature journal.
Stars like our Sun and their planetary systems are born inside clouds consisting of dust and molecular gas. Stellar evolution begins with the contraction of dense material in these stellar nurseries until an embryonic star, the protostar, is formed. How this collapse happens exactly, and on what timescales, is not very well understood. Is the gas “free-falling” towards the center due to gravity or is the collapse slowed down by other factors? “Since this process takes much longer than human history, it cannot just be followed as a function of time. Instead, one needs to find an internal clock that allows to read off the age of a particular star forming cloud,” says the leading author Sandra Brünken from the University of Cologne.
The hydrogen molecule (H2), by far the most abundant molecule in space, could act as such an internal, “chemical” clock. Molecular hydrogen exists in two different forms, called ortho and para, which correspond to different orientations of the spins of the two hydrogen nuclei. In the cold and dense molecular clouds out of which stars are formed, the relative abundance of these two forms changes continuously with time by chemical exchange reactions. Therefore the current abundance ratio observed is a measure of the time elapsed since the formation of H2, and thereby the molecular cloud itself. Unfortunately, H2 cannot be directly detected in the very cold interstellar “breeding grounds” of stars. However, H2D+, an ionized variant in which a deuteron particle is attached to the H2 molecule, can be observed. Indeed, the ortho and para forms of H2D+ emit and absorb at characteristic wavelengths, forming “spectral lines” that are observable with different telescopes. “We knew from our own laboratory experiments and from theory that H2D+ has a very close chemical relation to H2,” says Stephan Schlemmer from the University of Cologne who proposed the observations. “For the first time we could now observe both variants of H2D+, which allowed us to indirectly determine the ratio of ortho H2 to para H2. Reading this chemical clock we find an age of at least one million years for the parental cloud that is right now giving birth to Sun-like stars.” This result is questioning theories of rapid star formation. ...
H2D+ observations give an age of at least one million years for a cloud core forming Sun-like stars - Sandra Brünken et al
- Nature (online 17 Nov 2014) DOI: 10.1038/nature13924