HIFI opened up new window on Molecular Universe
Today, after more than 3,5 years in space, the liquid helium coolant on board the Herschel space observatory (ESA) has finally run out. This means that the Dutch molecule hunter HIFI – one of its three instruments – has ceased to collect spectra. But the scientific heritage is impressive already. Thanks to HIFI, scientists now much better understand the cosmic cycle of gas which leads to for instance the birth of stars and planets, and the role (water) molecules play.
Our Milky Way is far from empty. The space between the stars we see – individually or in the glow of the Milky Way – is filled with clouds of gas and dust. It is in these remnants of previous stellar generations that new stars and planets form – stars like our Sun and planets like in our Solar System. Through all the information that is enclosed in the spectral lines that HIFI measures, we now much better understand the details of the cosmic cycle of gas and dust.
HIFI has extracted a wealth of unique information from very different environments – from shells of dying stars to galaxy cores and comets. The Dutch Molecule Hunter was able to do this because of an unbeatable combination of uninterrupted spectral coverage, high spectral resolution and calibration accuracy. This combination may not be available for another 40 years.
Frank Helmich (SRON), HIFI’s Principal Investigator: “Water has been one of the most important targets for HIFI because of its rich spectrum and high abundance, and because it plays such an important role in star formation. Because of the high water vapor content of our own atmosphere, the data from space obtained with HIFI are a true legacy for decades to come.”
Water was found in the tenuous torus of gas released from Saturn’s moon Enceladus, in gas streaming off comets, in planetary atmospheres, external galaxies and around evolved stars. Water has been used to trace new types of shock waves present in the outflows that are part of the formation processes of stars . It has also been used to detect material flowing onto a star in the very first stages of its formation and to probe the cold water reservoir in the outer regions of planet-forming disks, with the water released from ice by a weak UV radiation field triggered by cosmic rays. Together, these data will allow to put together the water trail from collapsing clouds to planetary systems.
SRON researcher Floris van der Tak: “HIFI also found OH+ and H2O+ closely related to water. The discovery of these ionized molecules was a complete surprise, but their ubiquitous presence in our own Galaxy and beyond gives us a new handle on the number of high energy particles travelling through galaxies with speeds close to that of light and on the molecule-poor phase of the interstellar medium. OH+ en H2O+ are formed in a phase with an abundance of atomic hydrogen waterstof H instead of molecular hydrogen H2. Other molecules provide insight in the way molecular material reacts to ultraviolet radiation of hot stars.”
Cosmic rays were not only detected through the ionizing influence they have on interstellar gas. Several times HIFI was interrupted by a hit of such a high energy particle in the electronics of HIFI (local cosmic rays). Every time HIFI was successfully restarted.
The clouds in Herschel pictures look quite like clouds in our own atmosphere. They are dynamic and turbulent. Based on HIFI measurements of molecules like CH+ scientists have been able to see and quantify this turbulence. There are also areas where the gas is so tenuous that previous telescopes could not observe it. This ‘CO-dark’ gas has been successfully probed by ionized carbon observations of HIFI and provides the basis for future missions (which will take the study of star formation and galactic evolution one step further.
Besides observations of single molecules, HIFI also excels in making complete spectral scans. Such scans contain tens of thousands of lines from, in some cases, over 50 molecular species arising from within water and organic rich interstellar gas clouds. These observations directly characterize the chemical composition of star-forming gas with unprecedented accuracy while offering the unprecedented ability to probe gas physics with hundreds to thousands of lines of a single molecule.
Helmich: “With HIFI scientist have been able to decrypt the physical information about gas in our Solar System and in our Milky Way. The complexity that nature has shown us so far is baffling and will lead to many follow-up studies in the coming decade based on the HIFI data available in the Herschel Science Archive. The HIFI-Instrument Control Center will in the next three years prepare the final data products and the knowledge gained with Herschel/HIFI will pave the way for missions like SPICA (with the SAFARI instrument on board), aiming to extend our knowledge to earlier periods in the universe, rewriting the history of the universe.”
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. HIFI was built and developed by a consortium led by SRON Netherlands Institute for Space Research, with important contributions from TNO and small and medium-sized enterprises like Mecon.