For some things in life, the temperature needs to be just right. Take butter, for example. If it’s too cold, butter almost freezes, and it becomes very hard to put it on your slice of bread. But if it’s too hot, the butter almost becomes a liquid, which isn’t good either.
In space, the same is true for the formation of certain molecules, like DCO+. DCO+ is a molecule that consist of three atoms: deuterium, carbon and oxygen – that’s what the three letters stand for. (The plus-sign denotes that the molecule has lost one of its electrons, giving it a positive electrical charge.)
Astronomers know how DCO+ is made in the disks of gas and dust that surround new-born stars. Carbon monoxide gas (CO) first combines with hydrogen (H) to form HCO+. Then, if the temperature in the disk is not too high, hydrogen atoms can be replaced by heavier deuterium atoms, creating DCO+.
So in regions of the disk close to the star, where the temperature is too high for this ‘swapping process’ to occur, DCO+ cannot form. But at larger distances from the star, where temperatures are too low, the molecule can’t form either. That’s because the necessary carbon monoxide gas starts to freeze out.
Indeed, ALMA observations of the young star IM Lupi have revealed the existence of DCO+ molecules in a ring around the star – a relatively narrow region where temperatures are just right. No surprise there. The big surprise, however, is that ALMA found a second ring of DCO+ molecules, further away from the star.
How is this possible? Apparently, at larger distances, the disk becomes thinner and more tenuous. As a result, starlight is able to penetrate much deeper into the disk, raising temperatures there high enough to evaporate carbon monoxide ice into gas again. As soon as there is a new supply of carbon monoxide gas, HCO+ and DCO+ will again be able to form.
At first sight, this result doesn’t seem to be too impressive. But to astronomers, it’s important. It tells them that heavy molecules can form at places where you wouldn’t expect it to happen. Eventually, this may shed light on the formation and distribution of molecules in our own solar system, which formed from a disk quite similar to the one surrounding the star IM Lupi.
IM Lupi is a new-born star in the southern constellation Lupus the Wolf, at a distance of some 620 light-years. The star is only about a million years old – it is still surrounded by a thick disk of gas and dust from which planets may form in the future. IM Lupi has about the same mass as our sun, but it has a lower surface temperature of some 3,600 degrees Centigrade. However, since the star is almost three times as large as the sun, it produces more radiation: its energy output is 90 percent higher than the luminosity of the sun.
The ALMA observations of IM Lupi were carried out by an international team of astronomers led by Karin Öberg of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Karin worked together with colleagues at her own institution and with astronomers at the Center for Computational Sciences at the University of Tsukuba in Japan and of Leiden Observatory in the Netherlands. The astronomers published their result in the 4 September 2015 issue of The Astrophysical Journal.