Revealing the lives of planet-forming disks

(Press-News.org) An international team of astronomers including researchers at the University of Arizona Lunar and Planetary Laboratory has unveiled groundbreaking findings about the disks of gas and dust surrounding nearby young stars, using the powerful Atacama Large Millimeter/submillimeter Array, or ALMA. 

The findings, published in 12 papers in a focus issue of the Astrophysical Journal, are part of an ALMA large program called the ALMA Survey of Gas Evolution of PROtoplanetary Disks, or AGE-PRO. AGE-PRO observed 30 planet-forming disks around sunlike stars to measure gas disk mass at different ages. The study revealed that gas and dust components in these disks evolve at different rates.

Prior ALMA observations have examined the evolution of dust in disks; AGE-PRO, for the first time, traces the evolution of gas, providing the first measurements of gas disk masses and sizes across the lifetime of planet-forming disks, according to the project's principal investigator, Ke Zhang of the University of Wisconsin-Madison. 

"Now we have both, the gas and the dust," said Ilaria Pascucci, a professor at planetary sciences at the U of A and one of three AGE-PRO co-principal investigators. "Observing the gas is much more difficult because it takes much more observing time, and that's why we have to go for a large program like this one to obtain a statistically significant sample." 

A protoplanetary disk swirls around its host star for several million years as its gas and dust evolve and dissipate, setting the timescale for giant planets to form. The disk's initial mass and size, as well as its angular momentum, have 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. 

In one of the survey's most surprising findings, the team discovered that as disks age, their gas and dust are consumed at different rates and undergo a shift in gas-to-dust mass ratio as the disks evolve: Unlike the dust, which tends to remain inside the disk over a longer time span, the gas disperses relatively quickly, then more slowly as the disk ages. In other words, planet-forming disks blow off more of their gas when they're young.

Zhang said the most surprising finding is that although most disks dissipate after a few million years, the ones that survive have more gas than expected. This would suggest that gaseous planets like Jupiter have less time to form than rocky planets. 

ALMA's unique sensitivity allowed researchers to use faint, so-called molecular lines to study the cold gas in these disks, characteristic wavelengths of a light spectrum that essentially act as "fingerprints," identifying different species of gas molecules. The first large-scale chemical survey of its kind, AGE-PRO targeted 30 planet-forming disks in three star-forming regions, ranging from 1 million to 6 million years in age: Ophiuchus (youngest), Lupus (1-3 million years old), and Upper Scorpius (oldest). 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. 

Dingshan Deng, a graduate student at LPL who is the lead author on one of the papers, provided the data reduction – essentially, the image analyses needed to get from radio signals to optical images of the disks – for the star-forming region in the constellation of Lupus (Latin for "wolf").

"Thanks to these new and long observations, we now have the ability to estimate and trace the gas masses, not only for the brightest and better studied disks in that region, but also the smaller and fainter ones," he said. "Thanks to the discovery of gas tracers in many disks where it hadn't been seen before, we now have a well-studied sample covering a wide range of disk masses in the Lupus star-forming region." 

"It took years to figure out the proper data reduction approach and analysis to produce the images used in this paper for the gas masses and in many other papers of the collaboration," Pascucci added. 

Carbon monoxide 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+, or diazenylium, an ion used as an indicator for nitrogen gas in interstellar clouds, as an additional gas tracer to significantly improve the accuracy of measurements. ALMA's detections were also set up to receive spectral light signatures from other molecules, including formaldehyde, methyl cyanide and several molecular species containing deuterium, a hydrogen isotope. 

"Another finding that surprised us was that the mass ratio between the gas and dust tends to be more consistent across disks of different masses than expected," Deng said. "In other words, different-size disks will share a similar gas-to-dust mass ratio, whereas the literature suggested that smaller disks might shed their gas faster."

Funding for this study was provided by the National Science Foundation, the European Research Council, the Alexander von Humboldt Foundation, FONDECYT (Chile) among other sources. For full funding information, see the research paper.

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