Protoplanetary gas disks in the far infrared

TL;DR

This paper discusses the importance of far-infrared spectroscopy in studying the gas and water ice in protoplanetary disks, highlighting the capabilities of upcoming instruments like SPICA/SAFARI to advance understanding of planet formation.

Contribution

It emphasizes the limitations of Herschel in detecting gas in evolved disks and advocates for SPICA/SAFARI's enhanced sensitivity to study gas and water ice in diverse protoplanetary systems.

Findings

Herschel cannot detect gas in less massive or evolved disks.

SPICA/SAFARI will detect water ice features at 44 and 62 micrometers.

Spectroscopy of gas lines reveals disk evolution stages.

Abstract

The physical and chemical conditions in young protoplanetary disks set the boundary conditions for planet formation. Although the dust in disks is relatively easily detected as a far-IR photometric ``excess'' over the expected photospheric emission, much less is known about the gas phase. It seems clear that an abrupt transition from massive optically thick disks (gas-rich structures where only ~1% of the total mass is in the form of dust) to tenuous debris disks almost devoid of gas occurs at ~10^7 years, by which time the majority of at least the giant planets must have formed. Indeed, these planets are largely gaseous and thus they must assemble before the gas disk dissipates. Spectroscopic studies of the disk gas content at different evolutive stages are thus critical. Far-IR water vapor lines and atomic fine structure lines from abundant gas reservoirs (e.g., [OI]63um, [SI]56um, [SiII]34um) are robust tracers of the gas in disks. Spectrometers on board Herschel will detect some of these lines toward the closest, youngest and more massive protoplanetary disks. However, according to models, Herschel will not reach the required sensitivity to (1) detect the gas residual in more evolved and tenuous transational disks that are potentially forming planets and (2) detect the gas emission from less massive protoplanetary disks around the most numerous stars in the Galaxy (M-type and cooler dwarfs). Both are unique goals for SPICA/SAFARI. Besides, SAFARI will be able to detect the far-IR modes of water ice at ~44 and ~62um, and thus allow water ice to be observed in many protoplanetary systems and fully explore its impact on planetary formation and evolution.