The researchers looked at 30 discs of gas and dust around Sun-like stars to measure gas disc masses at different ages. They used the powerful Atacama Large Millimeter/submillimeter Array (ALMA), a radio telescope consisting of 66 massive antennas spread across the Chilean Andes.
Their observations, published in 12 papers in a focus issue of the Astrophysical Journal, change our understanding of the evolution of gas in the birthplace of exoplanets, providing the most accurate measurement yet of the gas swirling around young stars and how their mass changes over time.
A protoplanetary disk surrounds its host star for several million years as its gas and dust evolve and dissipate, setting the timescale for giant planets to form.
The disc’s initial mass and size, as well as its angular momentum, has a profound influence on the type of planet it could form (gas giants, icy giants, or mini-Neptunes) and migration paths of planets.
The lifetime of the gas within the disk determines the timescale for the growth of dust particles to an object the size of an asteroid, the formation of a planet, and finally the planet’s migration from where it was born.
Prior ALMA observations have examined the evolution of dust in disks. The new results, part of an ALMA large programme called the ALMA Survey of Gas Evolution of PROtoplanetary Disks, or AGE-PRO, traces the evolution of gas for the first time.
Co-author Dr Anibal E. Sierra Morales, of the Mullard Space Science Laboratory at UCL, said: "These studies have revealed how protoplanetary discs evolve over time. The extraordinary results are an essential step toward understanding the initial conditions that lead to the formation of Earth-like planets."
Co-author Dr Paola Pinilla, also of the Mullard Space Science Laboratory at UCL, said: "AGE-PRO reveals that the median of the gas disk mass goes from several Jupiter masses in the early ages (less than one million years) to less than a Jupiter mass in the first one to three million years.
"This means that discs have the reservoir to form giant planets in the young discs, but as they mature, the fuel for forming giant planets significantly decreases. However, it is surprising that the discs that survive longer (two to three million years) remain with a very similar gas disk mass to those aged one to three million years old."
ALMA’s unique sensitivity allowed researchers to use faint molecular lines to study the cold gas in these disks. The survey observed 30 disks from less than one million years old to over five million years old, in three star-forming regions: Ophiuchus, Lupus, and Upper Scorpius.
Using ALMA, AGE-PRO obtained observations of key tracers of gas and dust masses in disks spanning crucial stages of their evolution, from their earliest formation to their eventual dispersal. This ALMA data will serve as a comprehensive legacy library of spectral line observations for a large sample of disks at different evolutionary stages.
Carbon monoxide (CO) is the most widely used chemical tracer in protoplanetary disks, but to thoroughly measure the mass of gas in a disk, additional molecular tracers are needed. AGE-PRO used N2H+ (diazenylium) as an additional gas tracer to significantly improve the accuracy of measurements. ALMA’s detections were also set up to receive serendipitous spectral lines, including H2CO ( formaldehyde), DCN (deuterated hydrogen cyanide) and DCO+ (deuterated formyl ion).
AGE-PRO results indicate that as discs age, their gas and dust are consumed at different rates, and undergo a "swing" in gas-to-dust mass ratio as the discs evolve.
ALMA’s ability to detect faint molecular lines provided a window into the detailed processes of gas evolution in discs. By comparing AGE-PRO’s measurements of gas masses and disc sizes with prior studies mapping the same characteristics of dust particles, the team were able to piece together the interrelationships between mass, size, angular momentum transport, and environmental factors like photoevaporation.
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Mark Greaves
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