Molecules with ALMA at Planet-forming Scales (MAPS). IV. Emission Surfaces and Vertical Distribution of Molecules

The Astrophysical Journal Supplement Series · 2021 · 139 citations

• Astrophysics
• Physics
• Astronomy

Abstract The Molecules with ALMA at Planet-forming Scales (MAPS) Large Program provides a unique opportunity to study the vertical distribution of gas, chemistry, and temperature in the protoplanetary disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480. By using the asymmetry of molecular line emission relative to the disk major axis, we infer the emission height ( z ) above the midplane as a function of radius ( r ). Using this method, we measure emitting surfaces for a suite of CO isotopologues, HCN, and C 2 H. We find that 12 CO emission traces the most elevated regions with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>z</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi>r</mml:mi> <mml:mo>&gt;</mml:mo> <mml:mn>0.3</mml:mn> </mml:math> , while emission from the less abundant 13 CO and C 18 O probes deeper into the disk at altitudes of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>z</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi>r</mml:mi> <mml:mspace width="0.25em"/> <mml:mo>≲</mml:mo> <mml:mspace width="0.25em"/> <mml:mn>0.2</mml:mn> </mml:math> . C 2 H and HCN have lower opacities and signal-to-noise ratios, making surface fitting more difficult, and could only be reliably constrained in AS 209, HD 163296, and MWC 480, with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>z</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi>r</mml:mi> <mml:mspace width="0.25em"/> <mml:mo>≲</mml:mo> <mml:mspace width="0.25em"/> <mml:mn>0.1</mml:mn> </mml:math> , i.e., relatively close to the planet-forming midplanes. We determine peak brightness temperatures of the optically thick CO isotopologues and use these to trace 2D disk temperature structures. Several CO temperature profiles and emission surfaces show dips in temperature or vertical height, some of which are associated with gaps and rings in line and/or continuum emission. These substructures may be due to local changes in CO column density, gas surface density, or gas temperatures, and detailed thermochemical models are necessary to better constrain their origins and relate the chemical compositions of elevated disk layers with those of planet-forming material in disk midplanes. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

A Multifrequency ALMA Characterization of Substructures in the GM Aur Protoplanetary Disk

The Astrophysical Journal · 2020 · 95 citations

• Physics
• Astrophysics
• Astronomy

Abstract The protoplanetary disk around the T Tauri star GM Aur was one of the first hypothesized to be in the midst of being cleared out by a forming planet. As a result, GM Aur has had an outsized influence on our understanding of disk structure and evolution. We present 1.1 and 2.1 mm ALMA continuum observations of the GM Aur disk at a resolution of ∼50 mas (∼8 au), as well as HCO + J = 3 − 2 observations at a resolution of ∼100 mas. The dust continuum shows at least three rings atop faint, extended emission. Unresolved emission is detected at the center of the disk cavity at both wavelengths, likely due to a combination of dust and free–free emission. Compared to the 1.1 mm image, the 2.1 mm image shows a more pronounced “shoulder” near R ∼ 40 au, highlighting the utility of longer-wavelength observations for characterizing disk substructures. The spectral index α features strong radial variations, with minima near the emission peaks and maxima near the gaps. While low spectral indices have often been ascribed to grain growth and dust trapping, the optical depth of GM Aur’s inner two emission rings renders their dust properties ambiguous. The gaps and outer disk ( R &gt; 100 au) are optically thin at both wavelengths. Meanwhile, the HCO + emission indicates that the gas cavity is more compact than the dust cavity traced by the millimeter continuum, similar to other disks traditionally classified as “transitional.”