The earth and other planets in the solar system owe some of their warmth to the decay of radioactive elements inside them. Now a look into space provides new clues as to how these partly short-lived nuclides once got into our planet. The telescope observations reveal that there is a stream of radioactive aluminum in the star cradle Rho-Ophiuchi, which is a good 420 light-years away, which leads from a cluster of massive stars to the star-forming regions. According to a supplementary model, this radioactive material most likely originates from supernovae of these short-lived stars. This confirms theories that the solar system also got a large part of its radioactive heavy elements from such nearby stellar explosions.
Where did the earth and its neighboring planets get their heavy elements from, and especially the radionuclides that are still heating their interior today? They cannot come from the sun, because it only started nuclear fusion at that time, and their mass is not large enough to produce atoms heavier than carbon. The theory has therefore been around for a long time that young planetary systems received their dose of heavy elements from nearby supernova explosions. This is because very massive stars can release various metals and short-lived radioactive nuclides in their final explosions. If these radionuclides are blown into a nearby star cradle, these elements “contaminate” the primeval clouds of the emerging planetary systems and then accumulate inside the new planets – according to the theory. So far, however, there has been no clear evidence of this scenario.
A stream of radioactive aluminum
Now the star cradle Rho-Ophiuchi, only 427 light-years away, provides indications that the supernova theory could be correct. According to previous observations, this region of star formation, shrouded in dark dust, contains a good 400 infrared signatures emanating from protostellar and protoplantar disks as well as young stars. In the immediate vicinity of this cradle of stars lies the Upper Scorpius group of stars, a collection of very massive and short-lived stars, between which traces of past supernovae are visible. Astronomers working with John Forbes from the Flatiron Institute in New York have now examined the cradle of the stars and the Upper Scorpius cluster for possible connections, for example in the form of gas flows. To do this, they used telescopes that covered wavelength ranges from millimeter waves to gamma radiation.
In fact, the observations revealed that a cloud of radioactive aluminum-26 emanated from Upper Scorpius. This is moving towards us and thus also towards the cradle of stars in Rho-Ophiuchi, as the team determined. The shape of the gas clouds in the star cradle also showed that they were being blown by a current from the direction of the cluster of massive stars. “Our multi-wavelength observations demonstrate that Ophiuchus, with its many pre-stellar nuclei, is literally inundated with aluminum-26 from the neighboring Upper Scorpius association,” the researchers report. Because the observations alone did not make it clear whether this influx of radioactive aluminum originates from supernovae or strongly radiating Wolf-Rayet stars in Upper Scorpius, the astronomers checked this with a supplementary model. Wolf-Rayet stars are the remains of very massive stars that release large amounts of material into their surroundings through their strong stellar winds.
Origin primarily in supernovae
Based on the observation data and their model, the researchers succeeded in calculating the probability of the aluminum-26 emission coming from supernovae or Wolf-Rayet stars. According to this, a scenario in which at least 90 percent of the radioactive aluminum comes from supernovae has an average probability of 59 percent. “A scenario dominated by Wolf-Rayet stars, on the other hand, only occurs in 27 percent of the model runs,” report Forbes and his colleagues. “Supernovae are therefore the more likely originators of the radioactive elements, even if an origin in Wolf-Rayet stars cannot be completely ruled out.” According to the astronomers, these results fit very well with the theories about the origin of the radionuclides in our own solar system. “The enrichment process we see in Ophiuchus fits in well with what happened when the solar system was formed five billion years ago,” says Forbes.
However, the model also shows that the amount of short-lived radionuclides absorbed by the newly emerging star systems can vary greatly. “Many of the new planetary systems are born with as much aluminum-26 as our solar system, but the range of variation is huge and extends over several orders of magnitude,” explains Forbes. This is important information because aluminum-26 is an important source of heat for young planets and can therefore have a significant impact on their future fate.
Source: John Forbes (Flatiron Institute, New York) et al., Nature Astronomy, doi: 10.1038 / s41550-021-01442-9