Orion’s salty star
A sprinkling of salt could allow astronomers to watch stars grow
Wherever stars are forming, astronomers have come to expect a disc of material to form too. Such discs eventually provide the site and the raw material for planets to grow, but they have another important role that starts long before that.
A protostar grows through accreting material from the disc. But there are powerful winds that sweep up material from the disc, taking it away from the growing star. Eventually this removal of material brings a halt to the accretion process.
Or at least, that’s what we assume. It’s hard to actually observe discs while stars are forming, especially for the most massive specimens where such a disc might exist only while the protostar is very young. To peer into the mysteries of star formation, astronomers normally turn to what’s known as the sub-millimetre region of the electromagnetic spectrum, where the molecules that form in the environment around young stars reveal themselves. (The sub-millimetre sits between radio and the infrared, in the same wavelength range a microwave uses.) The trouble is that most molecules you might think of observing also occur in the material around the star itself or – worse – throughout the interstellar medium, making it hard to pick out the subtle signature coming from the disc.
Researchers have now found a way to crack this problem using, of all things, salt. They employed the Atacama Large Millimeter/Submillimeter Array (ALMA) – sensitive, as the name suggests, in the sub-millimetre – to stare at the nearest massive protostar believed to have a disc, Orion Source I. Mysterious lines had previously been spotted in the spectrum of the growing star, but the ALMA observations managed to identify these as sodium chloride and potassium chloride – exactly the same chemicals that live in your tabletop salt shaker.
The attraction of salts in space is that they’re rare. They’re only seen in and around the disc itself, meaning WKDW IRU WKH UVW WLPH ZH FDQ DFFHVV LQIRUPDWLRQ DERXW the disc. But they bring a mystery with them – how did VDOWV FRPH WR EH WKHUH LQ WKH UVW SODFH" is an astrophysicist and co-presenter of
on BBC TV. He is also director of the Zooniverse project
The team aren’t sure. It seems likely that the salts form because of the effect of the harsh protostellar HQYLURQPHQW OOHG with high-energy particles thrown off in the star’s unstable stellar wind. Such particles could slam into GXVW SDUWLFOHV LQ WKH GLVF DQG LQ WKH RXW RZLQJ material that rushes away from it, liberating molecules from the dust grains and creating a gas where they can react – but at this point we can’t be sure.
Nor do we know how the salt molecules became excited enough to emit strongly enough to be detected by ALMA. And, most importantly, because this is only one star we don’t know whether the salty story of Orion Source I tells us a general lesson about how such stars form, or whether this is just a one-off. There’s a lot more staring at salt to be done if we’re to really get to the bottom of how massive stars like this one form.
“They identified sodium chloride and potassium chloride – exactly the same chemicals that live in your tabletop salt shaker”