article image

ALMA records a young star's explosive birth

Paul Sutherland, Feature writer
Dec 5, 2013, 8:00 UTC

Sen—Europe’s powerful new ALMA observatory in Chile has made another exciting observation by discovering that a newborn star had an explosive start to life.

Studies show that the young star, which formed within the last 100,000 years in our Milky Way galaxy, was initially around 100 times brighter than it is now.

The star, which is still hidden in gas and dust, but can be “seen” by ALMA - the Atacama Large Millimeter/submillimeter Array - at its longer wavelengths. It lies about 500 light-years from Earth in a molecular cloud in the constellation of Lupus, the Wolf.

Like all stars, it formed as the cloud of gas and dust condensed. A point was reached where the material became so dense that it collapsed into a ball of gas that became heated under pressure until it started to glow.

It was detected by an international team led by astrophysicist Jes Jørgensen, Associate Professor at the Niels Bohr Institute and the Centre for Star and Planet Formation at the University of Copenhagen, Denmark. Their discovery is revealed in the scientific journal, Astrophysical Journal Letters.

Dr Jørgensen told Sen: “The star is very young and therefore deeply embedded in its cold envelope of cold gas and dust obscuring the light from the central star. It is therefore is not observable at visible wavelengths but only at longer - infrared and (sub)millimetre - wavelengths.”

ALMA map

The picture shows the young protostar at the centre surrounded by the gas and dust cloud. The blue colour shows the HCO+ molecule with a clear extended ring-structure. The inner yellow ring indicates where the temperature is 100 degrees above the absolute zero (-173 C) with the current luminosity of the star while the outer yellow ring shows where this temperature was reached when the star was a hundred times brighter. Credit: Jes Jørgensen (Niels Bohr Institute)

“As it is not detected at visible wavelengths one can not talk about the spectral type or colour of the star in the usual fashion. It is quite typical of what we expect for solar-type protostars in terms of its total luminosity and the mass of the cloud around it, though, so it is likely that it will evolve into a star with a mass roughly similar to our own Sun eventually.”

Describing the observations, Dr Jørgensen said: “We studied the chemistry of the gas and dust cloud surrounding the early protostar. In this dense cloud, a chemical reaction takes place that enables the formation of several kinds of complex molecules, including methanol. One would expect that all of the molecules would be near the star, but with one of them we saw a clear ring structure. Something had removed a certain molecule, HCO+, from a wide area around the protostar.”

Dr Jørgensen added: “From the area where the HCO+ molecule has been dissolved by water vapour we can now calculate how bright the young star has been. It turns out that that the area is much greater than expected compared to the star’s current brightness. The protostar has been up 100 times brighter than the star is now. From the chemistry we can also say that this change happened within the last 100 to 1,000 years - that is to say, very recently from an astronomical point of view.”

The team say that an eruption such as the star underwent can explain why it contains condensed methanol plus its high content of molecules with carbon, as found in the gas cloud. This could also have a great influence on the chemical processes that lead to the formation of complex organic molecules that can later be incorporated into planetary systems.

Dr Jørgensen believes that there has not only been a single burst of light and heat radiation, but that it could happen several times during the formation process. He said: “One of the major questions if we take a long view, is whether this is a common phenomenon - whether all young stars undergo similar ‘eruptions’ and if so, how often.”

Dr Jørgensen told Sen: “Similar eruptions or bursts are seen in slightly more evolved young stars - the so-called FU Orionis objects. Also statistical arguments have been made that similar bursts may occur in the early evolutionary stages - but this is, as far as I am aware, the first case where there are indications for such a burst in a specific young star.”