Researchers from the European Southern Observatory (ESO) have used the Atacama Large Millimeter/submillimeter Array (ALMA) and IRAM telescopes to make the first ever direct measurement of the large dust grains that lie in the outer regions of the planet-forming disc around a budding star; the star, 2MASS J16281370-2431391, is located in the Rho Ophiuchi star formation region, which is located approximately 400 light-years from Earth. Using observations of the "Flying Saucer," which refers to the disc of this particular star, the team discovered that the grains are approximately minus 266 degrees Celsius, much colder than expected by current models.
Due to the particular brightness of the disc of gas and dust surrounding 2MASS J16281370-2431391, which represents the early stages of planetary system creation, astronomers refer to it as the Flying Saucer. Using the ALMA telescope to observe the glow originating from the carbon monoxide molecules in the disc, the team was able to create sharp images that revealed a negative signal, which is typically impossible. However, lead author Stephane Guilloteau has an explanation for the finding.
"This disc is not observed against a black and empty night sky," he said in a press release. "Instead it's seen in silhouette in front of the glow of the Rho Ophiuchi Nebula. This diffuse glow is too extended to be detected by ALMA, but the disc absorbs it. The resulting negative signal means that parts of the disc are colder than the background. The Earth is quite literally in the shadow of the Flying Saucer!"
Using the ALMA measurements of the disc in combination with the background glow created using the IRAM telescope, the team calculated a disc dust grain temperature of minus 266 degrees Celsius. This finding contradicts the predictions of current models, which point to a temperature of minus 258 to minus 253 degrees Celsius. The solution to this discrepancy lies in the properties of the large dust grains, which are likely different than those currently assumed by modern models.
"To work out the impact of this discovery on disc structure, we have to find what plausible dust properties can result in such low temperatures. We have a few ideas - for example the temperature may depend on grain size, with the bigger grains cooler than the smaller ones. But it is too early to be sure," said study co-author Emmanuel di Folco.
If the low dust temperatures observed in the current study apply to other protoplanetary discs, our understanding of how they form and evolve may be altered.
The findings were published in the Jan. 19 issue of Astronomy & Astrophysics Letters.
