Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope have taken the first-ever image of a water snow-line within a protoplanetary disc.
This artist’s impression shows the water snow-line around the young star V883 Orionis. Image credit: A. Angelich / NRAO / AUI / NSF / ALMA / ESO / NAOJ.
Typically, heat from a young Sun-like star prevents water molecules from freezing within a radius of around 280 million miles (450 million km, or 3 AU), from the star.
Beyond that point, known as the snow-line, water condenses to form a layer of ice on dust grains and other particles.
An abrupt and powerful increase in the brightness of a star called V883 Orionis, however, has pushed the water snow-line out to about 3.7 billion miles (6 billion km, or 40 AU).
Even though V883 Orionis — which is approximately 1,350 light-years away in the constellation Orion — is only 1.3 times more massive than the Sun, it is currently 400 times more luminous and much hotter, thanks to its recent outburst triggered by material from the protoplanetary disk falling onto the surface of the star.
This event, combined with the revolutionary sensitivity and resolution of ALMA, has allowed a team of astronomers led by Dr. Lucas Cieza of the Universidad Diego Portales to make the first ever resolved observations of a water snow-line in a protoplanetary disc.
“The ALMA observations came as a surprise to us. Our observations were designed to look for disc fragmentation leading to planet formation,” Dr. Cieza said.
“We saw none of that; instead, we found what looks like a ring at 40 AU. This illustrates well the transformational power of ALMA, which delivers exciting results even if they are not the ones we were looking for.”
This image of the protoplanetary disc around the young star V883 Orionis was obtained by ALMA in long-baseline mode. The dark ring midway through the disc is the water snow-line, the point from the star where the temperature and pressure dip low enough for water ice to form. Image credit: ALMA / ESO / NAOJ / NRAO / L. Cieza.
The idea of snow orbiting in space is fundamental to planet formation. The presence of water ice regulates the efficiency of the coagulation of dust grains – the first step in planet formation.
Within the snow-line, where water is vaporized, terrestrial planets are believed to form.
Outside the water snow-line, the presence of water ice allows the rapid formation of cosmic snowballs, which eventually go on to form massive gaseous planets such as Jupiter.
“The distribution of water ice around a young star is fundamental to planet formation and even the development of life on Earth,” said team member Dr. Zhaohuan Zhu, an astronomer at Princeton University.
“ALMA’s observation sheds important light on how and where this happens in protoplanetary disks when young planets are still forming.”
“We now have direct evidence that a frosty region conducive to planet formation exists around other stars.”
Since water ice is more abundant than dust itself beyond the snowline, planets can aggregate more solid material and form bigger and faster there. In this way, gas giants can form before the protoplanetary disk is gone.
The discovery that star outbursts may blast the water snow-line to about ten times its typical radius is very significant to the development of reliable planetary formation models.
Such outbursts are believed to be a stage in the evolution of most planetary systems, so this may be the first observation of a common occurrence.
The results were published online this week in the journal Nature.
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Lucas A. Cieza et al. 2016. Imaging the water snow-line during a protostellar outburst. Nature 535, 258-261; doi: 10.1038/nature18612
