March 2, 2017

Nucleosynthesis

To boost our understanding of our Galaxy, its stars and their history, Integral explored how the heavy elements of the Periodic table are produced. Iron, aluminium and titanium are the key elements chosen to observe nucleosynthesis (the nuclear reaction/process by which  chemical elements are created). But to catch a glimpse of their presence, human experiments are faced with a problem. In fact, the intensity of the signal emitted by these elements is so low that the exposure time needed is in the order of years. In iron's case (60Fe at 1173 and 1333 keV), two years were needed to reach the flux detection limit (satisfactory signal-to-noise ratio) so as to expand our knowledge of its progenitors. We already suspect both supernovae and the core of stars that are so bright, they blast off their outer layers into space. Such stars are named Wolf-Rayet and the energy of such stellar winds approximates that of a supernova.

What's more, Integral continue to refine the all-sky map of the aluminium (26Al at 1809 keV) in the Galaxy. SPI's results put forward the distinctive sign of a Doppler effect on the line, thus proving it not only belongs to the galactic disk but dances with the stars.

Besides, the comparative study of results between aluminium and iron combined to the results from probing our galaxy for titanium (44Ti) are getting us to rethink our models for nucleosynthesis: Integral showed that scientists have underestimated the role of very massive stars in such phenomena. Going against current predictions, the flux observed from titanium and iron is exceptionally low. In fact, a low flux would translate a lower maximum escape velocity from the environment. This favours hydrostatic nucleosynthesis, happening at the core or the surface of stars, versus explosive nucleosynthesis, from supernovae.

This is why Integral will use part of its extension to achieve greater exposure times resulting in refined maps and thus further study.

the Milky Way, our Galaxy, in the visible spectrum
Background: the Milky Way, our Galaxy, in the visible spectrum.
Overlaid is the distribution of radioactive aluminium 26Al (map OSSE/GRO).
Right: schematic decay of radioactive aluminium 26Al into
stable magnesium 26Mg producting a photon emission
with an energy of 1,809 keV (belonging to the gamma domain) which Integral is able to detect.
Left: distribution of the Doppler Effect throughout the Galaxy,
direct evidence of its rotation. (Credits: MPE, R. Diehl)

References:

  • Diehl, Roland. et al "Radioactive 26Al from Massive Stars in the Galaxy." Nature 439 (2006): 45-47
  • Wang, W. et al "SPI observations of the diffuse 60Fe emission in the Galaxy." Astronomy & Astrophysics 469 (2007): 1005-1012

More information:


Just like archaeologists, who rely on radioactive carbon to date the organic remains from past epochs, astronomers have exploited the radioactive decay of an isotope of aluminium to estimate the age of stars in the nearby Scorpius-Centaurus association, the closest group of young and massive stars to the Sun. New observations, obtained in the gamma-ray spectrum by ESA's Integral observatory provide evidence for recent ejections of matter from massive stars that took place only a few million years ago in our cosmic neighbourhood.

This dating procedure is possible because aluminium is one of the elements synthesized by massive stars during their late evolutionary stages, and its abundance in a stellar complex such as the Scorpius-Centaurus association varies strongly with time. One isotope of this element, namely aluminium-26 (26Al), is radioactive and decays with an exponential lifetime of about one million years. The decay process results in a stable isotope of the element magnesium (26Mg) and a number of by-products, including an extremely energetic photon observable in gamma rays at an energy of about 1.8 MeV.

"Conveniently for astronomers, the decay of 26Al involves a similar time scale to that spanned by the life time of massive stars, which is of the order of a few million years," explains Roland Diehl from the Max-Planck Institute for Extraterrestrial Physics in Germany, who led a recent study targeting the gamma-ray emission from this isotope in the Scorpius-Centaurus association. "As its decay time is 'just right', measuring the abundance of 26Al is an excellent tool to trace the presence of young and massive stars, and it allows us to directly estimate their age," he adds.

Earlier observations, conducted in the 1990s with the Comptel instrument on NASA's Compton gamma-ray observatory, revealed for the first time the emission of 26Al across the entire sky. Subsequent data collected by ESA's Integral mission confirmed these results, probing the global properties of this isotope throughout the plane of the Milky Way thanks to Integral's improved spectral resolution.

"At the characteristic energy of the 26Al line, Integral has a spectral resolution over 60 times better than Comptel's, enabling us to study the intensity and shape of this line across the Galaxy in much greater detail," comments Chris Winkler, Integral Project Scientist. "The data, gathered over five years, are so deep that it is now possible to isolate the contribution due to an individual, nearby stellar complex from the overall galactic 26Al emission," he adds.

The data analysed by Diehl's team focussed on the Scorpius-Centaurus association, which is located at a distance of about 100-150 parsec from the Sun, and revealed robust evidence for recent massive star formation therein. "The gamma-ray data show that the stars in the Upper Scorpius subgroup of the Scorpius-Centaurus association are only about 5 million years old," notes co-author Thomas Preibisch from the University Observatory Munich, also in Germany. "This is a direct estimate, in contrast to other procedures used to evaluate the ages of stars, which rely heavily on stellar evolution models. The very good agreement between these independent dating methods is an extremely reassuring result," he adds.

Via stellar winds and supernova explosions, the stars in the Scorpius-Centaurus association are currently enriching the surrounding interstellar medium with heavy elements, including aluminium, and from the shape of the emission line of 26Al it is possible to constrain the kinematics of such ejecta. "By investigating the details of these outflows of radioactive gas, streaming at velocities of about 100 km/s towards the Sun, we are starting to unravel the recent history of massive star formation in the Solar System's vicinity and its implications on our own cosmic environment," comments Diehl.

The new Integral data also allowed the astronomers to refine the estimate of the total content of 26Al in the Milky Way, which is  about 20 per cent lower than previous estimates. This is a critical step required to validate our understanding of the star formation and nucleosynthesis processes in our Galaxy and to predict the expected rate of supernova explosions.