ALMA Observes Gas Emission in a Protoplanetary Disk

Astronomers Obtain the First Spatially Resolved Observations of Gas Emission in a Protoplanetary Disk

A Hubble image of the protoplanetary disk around TW Hydra. Submillimeter wavelength observations have been able to measure the distribution of the material in the disk and its implication for the growth of gas giant planets. NASA/Hubble Space Telescope

Using the ALMA facility, a team of astronomers is the first to obtain spatially resolved observations of gas emission in the protoplanetary disk around the star TW Hydrae.

The planets in our Solar System formed in orbits that depended on the initial distribution of the matter in solar nebula. In particular, the most popular theory for the formation of giant gas planets argues that their rock-and-ice cores formed gradually through coagulation of smaller planetesimals until they were massive enough to accrete gaseous envelopes. The spatial distribution of gas in a primitive nebula is therefore critical not only to the accretion of the atmosphere of its giant planets but also to the formation of these early planetesimals. Many young stars are ringed by disks of dust from which new planets will form. Since that dust emits in the infrared, astronomers have been studying the rings in the infrared to constrain the models of solar system evolution. Only about one percent of the matter is in the form of dust however; the bulk is in gas, which is much harder to detect. Astronomers have tried, but so far have primarily only been able to detect the surface layer of the gas lying above the bulk mass reservoir.

CfA astronomer Ilse Cleeves and her colleagues used the ALMA facility to obtain the first spatially resolved observations of gas emission in a protoplanetary disk, the closest one to us around the star TW Hydrae. They observed it in a relatively rare isotopic species of CO that enabled them to probe the full thickness of the disk. By combining their results with other datasets, they were able to constrain the temperature, gas and dust abundances throughout the disk. They were also able to measure how these quantities vary with distance from the star, in particular in the key zone from about five to twenty astronomical units where giant planets are expected to form (one AU is the average distance of the Earth from the Sun).

Writing in Nature Astronomy, the scientists report that the ratio of gas mass to dust grain mass (in the millimeter-sized grains) is about 140. Since the nominal mass ratio of gas to dust is about 100, they suggest that at least 2.4 Earth-masses of dust has already aggregated into large sizes, a centimeter or more, that are too big to be seen by ALMA. They find that the radial dis­tribution of gas is much flatter than the nominal expectations for our solar system, although it is consistent with some other theoretical models. If the solar nebula mass distribution was like that in the TW Hyd disk, the results imply that the formation of the giant planets must have been independent of the gas mass distribution.

Reference: “Mass Inventory of the Giant-Planet Formation Zone in a Solar Nebula Analogue,” Ke Zhang, Edwin A. Bergin, Geoffrey A. Blake, L. Ilsedore Cleeves and Kamber R. Schwarz, Nature Astronomy, 1, 0130, 2017.

Harvard-Smithsonian Center for Astrophysics

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