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Under Pressure: A New Technique for Measuring Gas Surface Density

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An image of the TW Hydrae disk, zoomed out far enough that the entire disk is visible and a set of concentric dark rings neatly surround the central star.
ALMA image of emission from TW Hydrae. [S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO); courtesy of the European Southern Observatory; CC BY 4.0]

The gorgeous, nearby, face-on TW Hydrae protoplanetary disk has proved challenging to quantify: just how much gas is sitting between those rings? A new study presents a possible answer and develops a new technique for addressing the question along the way.

Covert Disks

Much like babies cling to their blankets, young stars wrap themselves in disks of gas and dust when they’re very young. Although this state of infancy lasts only a short few million years, this temporary cosmic comforter is a profoundly important artifact of a star’s birth, at least from our earthly perspective. That’s because all planets, asteroids, and humans formed from this material: we are the worn remains of a structure long ago dispersed, clumps of matter that couldn’t get blown away in our Sun’s angsty teenage years when it scattered the rest of its protoplanetary disk.

Basic measurements such as how gas is distributed around young stars are therefore vital for our models of planet formation. Unfortunately, this measurement is notoriously tricky to make since the most dominant component of the disk, hydrogen gas, stubbornly refuses to emit light while at the temperatures in this environment. Most of astronomy relies on catching photons flung our way across the void, but if the hydrogen refuses to play catch, we can’t see it.

An image showing emission from the small region of sky surrounding TW Hydrae. Equal brightness contours appear as concentric rings centered on inner region of the disk.

Emission from dust in the TW Hydrae protoplanetary disk, colored by brightness temperature of the dust. Axes are in arcseconds. [Tsukagoshi et al. 2019, reproduced in Yoshida et al. 2022]

Luckily, although we cannot detect most of the disk, other components besides hydrogen are more amenable to electromagnetic communication. Dust, for example, glows even while embedded in the hydrogen, so we can measure its distribution and use it as a proxy for the gas distribution. However, this method relies on an assumed gas-to-dust ratio and profile, which isn’t always certain. Alternatively, various forms of CO also shine through the gas, but the ratio of CO to hydrogen is likely unique to each disk. As a result of these uncertainties, previous estimates of the total mass of the nearby TW Hydrae disk have spanned more than an order of magnitude.

 

Pressure Wings

A plot of flux vs. velocity shift. The data is a broad downward-opening V, while the comparison Gaussian profile is a much narrower V. The model closely follows the data.

An illustration of the pressure-broadened wings. The dashed line marks a typically used Gaussian shape, while the green line shows the best-fitting Voigt profile, which captures broadening. The data are shown in black, and the 3-sigma significance level is denoted by the dotted line. [Yoshida et al. 2022]

A new study led by Tomohiro C. Yoshida (National Astronomical Observatory of Japan; The Graduate University for Advanced Studies) develops a workaround to these challenges: rather than relying on a tracer, the team instead looked at the shape of CO lines at the inner region of the disk.

Typically, astronomers assume these lines follow a roughly Gaussian shape. However, Yoshida and collaborators found that in the inner regions of the disk, the “wings” of the lines were wider than expected. After considering various explanations for what might cause this broadening, they concluded that the wings are the result of higher pressures along the midplane of the disk.

This new method should be applicable to other disks moving forward, so astronomers can hope for similarly refined measurements in the near future.

Citation

“Discovery of Line Pressure Broadening and Direct Constraint on Gas Surface Density in a Protoplanetary Disk,” Tomohiro C. Yoshida et al 2022 ApJL 937 L14. doi:10.3847/2041-8213/ac903a