Using data from the DIGIT Herschel Key program, scientists have studied the protoplanetary disc surrounding a pre-main sequence star to find that around 80 percent of oxygen atoms are locked up in a large reservoir of crystalline water ice – material that is crucial for facilitating the rapid formation of planetesimals, that in turn decreases the time scale for giant planet core accretion.
DIGIT stands for Dust, Ice, and Gas In Time and is an Open Time Key project using data gathered from instruments aboard the Herschel Space Observatory to study how multi-planet systems beyond our own Solar System formed. by performing full spectral scans of young stellar objects embedded in protoplanetary discs. The young stellar object in question is HD 142527, a Herbig Ae/Be star located 145 parsecs away. Herbig stars are typically young stars (less than 10Myr) that are approaching main sequence status, I.e. they have yet to initiate hydrogen burning in their cores. The large protoplanetary disc surrounding HD 142527 extends 145 AU (astronomical units) and has a heavy depleted region inside, which is postulated to be the result of a multi-planet system carving gaps in the disc.
Studies have shown that ice is expected to be an integral part of planet formation, as ice grains stick together more easily than other types of grains. This can lead to an increase in the surface density of solid material, compared with regions that are low in or devoid of ice. Crystalline ice has already been observed in HD 142527 in previous studies, however the results of this analysis have been disputed in a paper recently submitted to Astronomy and Astrophysics journal by lead author M.Min from the SRON Netherlands Institute for Space Research.
This recent research suggests that the amount of water ice detected in the outer disk of HD 142527 requires around 80 percent of the oxygen atoms, a result that is much higher than previous studies of the same disc. The team state that their findings are comparable to the water ice abundance found in comets and in dense interstellar clouds in the outer regions of our own solar system. Furthermore, it is not only the ice content that is similar to our own Solar System, as the team re-analyse abundances of other previously identified minerals in the spectra (derived from the dust/ice mixture) and conclude that these values are also consistent with Solar abundance constraints.
Surprisingly, the team also found that despite finding a high abundance of crystalline water ice, temperatures at the location in the disk where it was found, are too low to form crystalline ice in situ. This could be due to the collision of icy planetesimals inside the large cavity in the disc, which preserves their crystalline structure from evaporation and photo destruction, before migrating outwards into the outer disc. Another possibility is the crystallisation of ice through extreme accretion events freeing enough energy to heat up and crystallise the ice in the outer regions. The team state that ‘whatever the formation mechanism, the detection of crystalline water ice is consistent with observations on solar system satellites, where most are detected to have crystalline ice at least at the surface.’
More information on this research can be found at http://arxiv.org/pdf/1606.07266.pdf