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Telescopes and space missions

Telescopes and space missions

Evidence for recent star formation seen at Milky Way’s centre

17 Mar 2015
Star-maker: infrared image of the centre of the Milky Way

 

Stars could be forming in the inhospitable environment near Sagittarius A*, which is the supermassive black hole at the heart of the Milky Way. That is the conclusion of an international team of astronomers that has discovered a possible signature of low-mass star formation just two light-years from the centre of our galaxy – a region that was previously thought to be too hostile for such activity. If confirmed, these observations identify a “laboratory” where astronomers can study star formation – and even possible planetary formation – near a supermassive black hole.

Stars form when a cloud of gas becomes dense enough to collapse in on itself under the influence of its own gravity. This process is affected by the environment surrounding the cloud. Close to a supermassive black hole, the self-gravity of the cloud will be countered by “tidal shear”, which is the stretching force that results from the intense gravitational pull of the black hole. Forming stars near a supermassive black hole is therefore expected to be very difficult, because the self-gravity of a cloud must be strong enough to overcome the tidal shear. Indeed, a star-forming cloud would have to be unusually dense to avoid being torn apart.

Searching for signatures

Understanding star formation near supermassive black holes is important because these objects are known to reside at the heart of most large galaxies. Previous observations of the central few light-years of the Milky Way had focused on a population of about 200 massive, young and very bright stars in tight orbits around Sagittarius A*. These stars are only a few million years old and prompted scientists to wonder whether they somehow manage to form in their current locations in spite of their close proximity to the black hole, or whether they form further from the black hole and then migrate in?

Motivated by this mystery, Farhad Yusef-Zadeh of Northwestern University and collaborators looked for evidence of even younger stars close to Sagittarius A*, which would demonstrate that star formation in the area is an ongoing process. “We have been searching for signatures of more recent star formation within a few light-years of the black hole for some time,” Yusef-Zadeh says.

Looking for signs of star formation in this region is difficult because the Earth lies in the disc of the Milky Way, and our view of the galactic nucleus is obscured by interstellar dust particles. Scientists therefore rely on telescopes that use non-optical wavelengths, such as radio telescopes, to peer through the dust and probe activity at the galactic centre. New capabilities of one such telescope, the Very Large Array (VLA) in New Mexico, were key in allowing Yusef-Zadeh and collaborators to make their recent discovery of small sources in one arm of activity near Sagittarius A*.

Heated discs

The team identified these small sources as candidate photoevaporative protoplanetary discs – “proplyds” – which are areas of dense, ionized gas and dust surrounding young, newly formed stars. The proplyd candidates are between 10,000 and 100,000 years old, and they lie along the edge of a large molecular cloud. It is likely that this cloud produced the discs by providing a reservoir of gas to feed the star-formation activity.

The region surrounding these proplyds is blasted with harsh ultraviolet radiation streaming from hot stars orbiting close to Sagittarius A*. The gas of the proplyds is heated and stripped away by the radiation from these stars and characteristic shock waves are formed around the discs on the side facing the galactic centre. Both the proplyds themselves and the “bow shocks” surrounding them are visible in Yusef-Zadeh’s observations.

Planetary possibilities

Unlike the young massive stars that have previously been identified in the galactic centre, the proplyd candidates in this study are associated with low-mass stars – objects of less than about one solar mass. The analysis by Yusef-Zadeh’s team has led the researchers to speculate that it may in fact be easier for low-mass stars to form in the hostile surroundings of the black hole than it is for them to form elsewhere in the Milky Way. In addition, the rate at which material is lost from such proplyds is expected to be low, so there is a chance for the disc to eventually form planets. With that comes the tantalizing possibility that as telescope resolution and data-analysis techniques improve, we may even be able to watch planet formation occur near Sagittarius A*.

“The inner few light-years of the galaxy is clearly a unique environment,” says Yusef-Zadeh. Determining exactly what role this environment plays in the formation of stars is the challenge. Further work is necessary to determine whether the extremes of these surroundings hinder or, in fact, help star formation, but the observations in this study are an important step in the direction of better understanding.

This research is described in Astrophysical Journal Letters.

  • The following video abstract shows the locations of the proplyd candidates relative to Sagittarius A*, and provides more information about how the observations were made and analysed
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