A group of European astronomers led by Laboratoire d’Astrophysique de Bordeaux scientist Stephane Guilloteau has measured the temperature of large dust grains in the outer parts of a protoplanetary disc around an infant star known as J1628.
This image shows part of the Rho Ophiuchi region; a much enlarged close-up infrared view of the young star J1628 from the Hubble Space Telescope is shown as an insert. Image credit: NASA / ESA / Digitized Sky Survey 2.
J1628 (full name 2MASS J16281370-2431391) is located in the Rho Ophiuchi star formation region, approximately 400 light-years away.
The star’s dust disk is seen nearly edge-on, and its appearance in visible light pictures has led to its being nicknamed the Flying Saucer.
Dr. Guilloteau and his colleagues from France and Germany used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the glow coming from molecules of carbon monoxide (CO) in J1628’s disk.
They were able to create sharp images and found something strange – in some cases they saw a negative signal.
“This disc is not observed against a black and empty night sky,” said Dr. Guilloteau, who is the first author of a paper in the journal Astronomy & Astrophysics Letters (arXiv.org preprint).
“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.”
The astronomers then combined the ALMA measurements of the J1628’s disc with observations of the background glow made with the IRAM 30-m telescope in Spain.
They derived a disc dust grain temperature of minus 446.8 Fahrenheit (minus 266 degrees Celsius, or 7 Kelvin) at a distance of about 15 billion km from the central star.
“J1628’s dust disk absorbs the radiation from the CO clouds at several velocities. We derive very low dust temperatures, 5 to 7 K at radii around 100 astronomical units, which is much lower than most model predictions,” the scientists said.
To resolve the discrepancy, the large dust grains must have different properties than those currently assumed, to allow them to cool down to such low temperatures.
“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,” said team member Dr. Emmanuel di Folco, of the Laboratoire d’Astrophysique de Bordeaux.
“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.”
Further observations are needed, but it seems that the cooler dust found by the team may have significant consequences for the understanding of planet-forming discs.
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S. Guilloteau et al. 2016. The shadow of the Flying Saucer: A very low temperature for large dust grains. A&A 586, L1; doi: 10.1051/0004-6361/201527620
