|
For their entire lives, stars do nothing but blast their surroundings with heat, but in the first phases of their birth there is a period, whose duration is uncertain, in which they actually produce water! Confirmation of this paradoxical phenomenon has come from work with the infrared space telescope Herschel, by a European group of astronomers who pointed the telescope at the object called L1448-MM, 750 light years away from Earth.
This is a protostar in our own galaxy, a star that is just condensing out of its parent cloud, which surrounds it in the gas and dust typical of this early phase. Penetrating the dense clouds that blanket the young star, Herschel has revealed two jets that emerge from the protostar's polar regions, ejecting the material in excess from that which the protostar accretes from the surrounding cloud.
This is a mechanism also seen elsewhere in the Universe, on very different scales, such as collapsed stars, quasars and radio galaxies. In all cases the common denominator is the elimination of material via polar jets, even though the exact point of origin and mechanism by which they are accelerated are unknown.
In the case of L1448-MM, as with all protostars of this mass in a similar evolutionary phase, the jets produce an enormous quantity of water, that astronomers estimate to be 100 million times the flux of the Amazon.
In actual fact the inside of the jets are at temperatures that reach 100,000 Kelvin, so the components of the water, hydrogen and oxygen, can only combine at some distance from the protostar, where the jets cool. The process is helped by the presence of dust in the protostellar cocoon, which acts as a catalyst.
It is just this water which is used to trace the behaviour of the jets.
The research team found that within the jets there are pulses of denser material that travel at velocities of about 50 km/s and last for about one year. As a pulse travels along the jet it slows, and may be caught by the successive pulse. When this happens a shock forms, creating something like a water "bullet". As well as water, Herschel also detected other molecules in the jets, like silicon oxide and carbon dioxide.
Considering that the jet phase can last for thousands of years and may be more, that a protostar will not be free of its parent cloud for at least a million years (up to 10 million years), and that most stars are expected to pass this phase, it is clear that this phase of stellar evolution is an important source of water in the Universe.
|
