FEEDBACK - Program advances findings on star formation

Apart from helium and hydrogen, most heavier chemical elements in our cosmos up to iron are formed by the fusion of atomic nuclei inside stars. The most massive, hottest and brightest of these are the driving forces. They influence their immediate surroundings through the emission of large amounts of ionizing UV radiation and their strong stellar winds. They burn hydrogen and helium into heavier elements faster than any of the lighter stars, explode after a relatively short time - by astronomical standards - of only a few million years and enrich the surrounding gas with heavy elements. More so, as the enormous energies released by the supernova explosions lead to the additional formation of elements even heavier than iron. This material is then available to form new solar systems such as ours with its variety of chemical substances.

In order to decipher what proportion of a dense interstellar cloud of such composition will be converted back into stars and on what time scales, a team led by Cornelia Pabst from Leiden University and Consejo Superior de Investigaciones Científicas in Madrid has combined data from a research program called FEEDBACK with new radio data from the 40m Yebes Observatory and the IRAM 30m telescope. FEEDBACK is a long-term project of SOFIA, the Stratospheric Observatory for Infrared Astronomy, jointly operated by the German and US-American space agencies, DLR and NASA. For her study, Pabst and her colleagues examined ten regions in the Orion Nebula in detail, which represent different evolutional phases of star formation.

The results show the extreme conditions - by earthly standards - under which stars and thus the building blocks for life are formed. The respective paper was recently published in the journal Astronomy & Astrophysics. The German SOFIA Institute (DSI) at the University of Stuttgart coordinates the SOFIA activities on the German side.

The formation of new stars, the effect of this process on their surroundings and the material they were formed from - the so-called interstellar medium (ISM) - is crucial for the development of galaxies: When do stars that just formed prevent the formation of further generations? Under what circumstances does this feedback stimulate the formation of new stars? Which processes dominate? And how does this affect the large-scale properties of the ISM and the further evolution of galaxies? Massive, hot, bright stars that emit extremely energetic ultraviolet radiation are relatively rare. Thus, even in the Orion Nebula only a few of them are responsible for heating and ionizing the ISM. The brightest of them is 33 times as heavy and 20,000 times as bright as our sun. A bubble of ionized gas has formed around these stars, that is 10,000 degrees Kelvin hot and contains between 100 and 100,000 gas particles in one cubic centimeter.

Seen from our direction, behind this bright group of stars, there is a comparatively dense and very cold molecular cloud with up to ten million particles per cubic centimeter, and only 30 degrees Celsius above absolute zero (30 Kelvin), as previous radio observations by IRAM and Yebes have shown. The cloud is still successfully resisting destruction by the intense radiation emitted by its hot neighborhood. Meanwhile, a gigantic plasma bubble with a radius of up to 2 parsecs (1 pc = 3.26 light years) has expanded, compressing the displaced gas into a thin surrounding shell. The intense radiation and the winds of the stellar giants accelerate this plasma and heat it to a temperature of one million Kelvin. It has an extremely low density of only one particle per cubic centimeter and yet continues to expand the envelope of matter at a speed of 13 km/s, as shown by SOFIA’s high-resolution spectra.

Such an extreme vacuum cannot even be approximated in a terrestrial laboratory. The best achievable values are between 10,000 and 100,000 particles per cubic centimeter. This is still very good compared to the Earth's atmosphere. At normal pressure and 0 degrees Celsius, the latter contains much more, around 27 billion billion particles per cubic centimeter. The size of the expanding envelope in Orion is also astonishing. With a diameter of 4 parsecs, it takes around 13 years for light to pass through. It continues to expand and observations of similar bubbles suggest that sooner or later it will break up in some places and disperse its material into its surroundings. Thus it will no longer be available for the formation of further stars for the time being. Based on the size and expansion speed of this bubble, scientists can estimate how efficiently massive stars pump energy into the surrounding ISM and how much mass is lost for future star formation. However, such an expanding shell can also trigger the formation of further stars if, for example, it encounters another molecular cloud on its way and compresses its matter through this collision to such an extent that it collapses under its own gravity.

"We are currently experiencing exciting times in astrophysics," says Cornelia Pabst. There are the latest findings about the very early universe and its development with the Planck satellite; the ALMA radio observatory and the JWST space telescope are providing more and more insights into the earliest development of galaxies; the hunt for exoplanets has picked up full speed; high-energy phenomena, gravitational waves, ultra-fast radio bursts and extreme X-ray luminosity are gradually revealing their secrets. "Nevertheless, the study of "nearby" star formation in our galaxy remains an important and fundamental topic, allowing us to understand the evolution of other galaxies in the history of the universe and their enrichment with heavy elements," says Pabst.
The high spectral resolution and sensitivity of upGREAT, the upgraded German REceiver for Astronomy at Terahertz Frequencies, on board SOFIA enabled Pabst's team to determine the movements and physical conditions of the interstellar gas using the fine structure line [CII] 158 µm of ionized carbon. To continue the long-term FEEDBACK project, despite the end of the SOFIA mission, Cornelia Pabst's team performed new radio observations of the star-forming regions in Orion. The combination of these radio observations with the SOFIA archive data allows in particular a determination of the above-mentioned matter densities and temperatures. This provides for a better understanding of the complex processes in the various regions of the Orion Nebula that determine the speed and efficiency of star formation.
Even after SOFIA finished observational operations in September 2022, the data from the FEEDBACK Legacy program will continue to provide answers to these questions. "Over the next 5 years, we will establish the SOFIA Data Center (SDC) at the University's Institute of Space Systems (IRS), improve data processing and support researchers in the further use of the SOFIA data archive," announces Bernhard Schulz, former SOFIA Science Mission Deputy Director and new Project Scientist of the SDC. "In this way, we can preserve the scientific heritage of SOFIA for further research."

Original Publication:

• Multiline observations of hydrogen, helium, and carbon radio-recombination lines toward Orion A: A detailed dynamical study and direct determination of physical conditions, C.H.M. Pabst, J.R. Goicoechea, S. Cuadrado, P. Salas, A.G.G.M. Tielens, N. Marcelino, A&A, in press
  DOI https://doi.org/10.1051/0004-6361/202347574
• Als Preprint auf arXiv/astro-ph

Contact:
Cornelia Pabst (pabst@strw.leidenuniv.nl)
Dörte Mehlert (mehlert@dsi.uni-stuttgart.de)

Further SOFIA Links:

• SOFIA Science Center beim NASA/IPAC Infrared Science Archive
• NASA SOFIA Missionsseite
• NASA SOFIA blog
• SOFIA Missionsseite der DLR Raumfahrtagentur    
• DLR Blogs zu SOFIA
• Zur SOFIA-Bildergalerie
• Zur News-Übersicht
• Zur Projektchronik

SOFIA, the "Stratospheric Observatory for Infrared Astronomy" is a joint project of the Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR; German Aerospace Center, grant: 50OK0901, 50OK1301, 50OK1701, and 50OK2002) and the National Aeronautics and Space Administration (NASA). It is funded on behalf of DLR by the Federal Ministry for Economic Affairs and Climate Action based on legislation by the German Parliament, the State of Baden-Württemberg and the University of Stuttgart. SOFIA activities are coordinated on the German side by the German Space Agency at DLR and carried out by the German SOFIA Institute (DSI) at the University of Stuttgart, and on the U.S. side by NASA and the Universities Space Research Association (USRA). The development of the German instruments was funded by the Max Planck Society (MPG), the German Research Foundation (DFG) and DLR.

upGREAT is a further development of the German Receiver for Astronomy at Terahertz Frequencies (GREAT), a far-infrared spectrometer that has been used to carry out numerous successful scientific flights on board SOFIA since 2011. The instrument was developed and built by a consortium of German research institutes – the Max Planck Institute for Radio Astronomy and the Cologne Observatory for SubMillimeter Astronomy (KOSMA) at the University of Cologne, in collaboration with the DLR Institute of Optical Sensor Systems in Berlin.