SOFIA Observations Help Determine the Age of a Star Nursery

Our own solar system formed some billion years ago when a dark

interstellar cloud started to contract to form the protostar that later became
the Sun. The duration of this first step in stellar evolution has now been
determined to last at least 1 million years in the similar system IRAS
16293-2422, a collection of protostars ~400 light years away in the
constellation Ophiuchus (background image). This has been achieved by
using molecular hydrogen, H₂, as a chemical clock. As hydrogen is not
directly detectable, the chemically closely related species H₂D⁺ has been

observed instead in the far-infrared and sub-millimeter wavelength ranges,
respectively using the airborne observatory SOFIA and the ground-based
telescope APEX in the Chilean Andes. (Figure: Martina Markus &
Oskar Asvany, Credits: NASA/Carla Thomas, C. Durán/ESO/APEX
(MPIfR/ESO/OSO), ESO/Digitized Sky Survey 2/Davide De Martin,
ESO/ L. Calçada, Bill Saxton, NRAO/AUI/NSF)

An international scientific team led by scientists of the Coordinated Research Center (CRC) 956 at the University of Cologne, Germany, applied a new method of age determination to a combination of data from SOFIA and other observatories to make a surprising discovery: The star forming cloud IRAS 16293-2422, located at a distance of about 400 light years in the direction of the constellation Ophiuchus, is at least 1 million years old yet is still making sun-like stars. This is in conflict with current models, which predict star formation should proceed much more rapidly. That result is published in this week’s volume of Nature magazine by researchers from Cologne plus the University of Helsinki and the Max Planck Institutes for Radio Astronomy (MPIfR; Bonn) and Extraterrestrial Physics (MPE; Garching).

Stars like our sun and their planetary systems form from cold and dense interstellar gas and dust clouds that collapse under their own weight. In the first step the material condenses into stellar “embryos” called protostars. Details of how such condensations occur, and on what timescales, are not very well understood. For example, do the clouds “free-fall” toward their respective centers solely under the influence of gravity, or is the collapse significantly slowed 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 us to read off the age of a particular star forming cloud,” says lead author Sandra Brünken.

The GREAT far-infrared spectrometer onboard SOFIA. The instrument is
shown mounted on the telescope flange inside the pressurized cabin.
© R. Güsten

Here is where SOFIA flies in to help: The molecule H₂D⁺ (a combination or two atoms of ordinary ‘ light’ hydrogen plus one atom of ‘heavy’ hydrogen, deuterium) is enriched in dense and cold star forming regions. The spin axes of the two H atom nuclei within each molecule flip their relative orientations at a known rate. Molecules with one nuclear spin orientation emit and absorb a spectral line at a far-infrared wavelength of 219 microns (1.37 THz, ‘para’ state, anti-parallel spins). Molecules with the opposite spin emit and absorb at a radio wavelength of 0.806 millimeters (372 GHz, ‘ortho’ state, parallel spins). Because Earth’s atmosphere absorbs all far-infrared radiation from celestial sources, the only observatory able to detect the 219-micron line is SOFIA, operating at an altitude of about 14 km, carrying the GREAT (German Receiver for Astronomy at THz Frequencies) spectrometer. Another advantage is that – in contrast to satellites - the newest technology could be implemented on SOFIA on short time scales; until recently, no instrument was available that could detect the critical range of wavelength for this study. Complementary observations of the millimeter-wavelength line were obtained using the ground-based APEX (Atacama Pathfinder EXperiment) telescope located in the Chilean Andes at an altitude of 5100 meters (16,700 feet). In their Nature publication the team around Stephan Schlemmer at the University of Cologne explains why the ratio of the ortho (APEX) to para (SOFIA) states of H₂D⁺ in cold and dense gas clouds allows an accurate age estimation of Sun-like star nursery. Reading this chemical clock for IRAS 16293-2422 yields an age of at least 1 Million years.