Compact disks. An explanation to faint CO emission in Lupus disks

A Miotello,J P Williams,M Ansdell,S Bruderer,S Facchini,G Rosotti,C F Manara

doi:10.1051/0004-6361/202140550

Abstract

Context. ALMA disk surveys have shown that a large fraction of observed protoplanetary disks in nearby star-forming regions (SFRs) are fainter than expected in CO isotopolog emission. Disks not detected in 13CO line emission are also faint and often unresolved in the continuum emission at an angular resolution of around 0.2 arcsec. Aims. Focusing on the Lupus SFR, the aim of this work is to investigate whether this population comprises radially extended and low-mass disks – as commonly assumed so far – or intrinsically radially compact disks, an interpretation that we propose in this paper. The latter scenario was already proposed for individual sources or small samples of disks, while this work targets a large population of disks in a single young SFR for which statistical arguments can be made. Methods. We ran a new grid of physical–chemical models of compact disks with the physical–chemical code DALI in order to cover a region of the parameter space that has not been explored before with this code. We compared these models with 12CO and 13CO ALMA observations of faint disks in the Lupus SFR, and report the simulated integrated continuum and CO isotopolog fluxes of the new grid of compact models. Results. Lupus disks that are not detected in 13CO emission and have faint or undetected 12CO emission are consistent with compact disk models. For disks with a limited radial extent, the emission of CO isotopologs is mostly optically thick and scales with the surface area, that is, it is fainter for smaller objects. The fraction of compact disks is potentially between roughly 50% and 60% of the entire Lupus sample. Deeper observations of 12CO and 13CO at a moderate angular resolution will allow us to distinguish whether faint disks are intrinsically compact or extended but faint, without the need to resolve them. If the fainter end of the disk population observed by ALMA disk surveys is consistent with such objects being very compact, this will either create a tension with viscous spreading or require MHD winds or external processes to truncate the disks.

Highlights

Thanks to its exquisite angular resolution and unprecedented sensitivity, the Atacama Large Millimeter/submillimeter Array (ALMA) has revolutionised the field of star and planet formation
Lupus disks that are not detected in 13CO emission and with faint or undetected 12CO emission are consistent with compact disk models
Results from a new grid of compact disk models, with critical radius Rc = 0.5, 1, 2, 5, 15 au, run with DALI are presented in this paper

Summary

Introduction

Thanks to its exquisite angular resolution and unprecedented sensitivity, the Atacama Large Millimeter/submillimeter Array (ALMA) has revolutionised the field of star and planet formation. Together with the very popular high angular resolution images (see e.g., ALMA Partnership et al 2015; Andrews et al 2018), ALMA has significantly enhanced the disk sample size by surveying disks at moderate resolution in many different nearby Star-Forming Regions (SFRs) Both dust and gas components have been traced through sub-mm continuum and CO isotopologues rotational line emission in the ∼ 1 Myr-old to ∼ 10 Myr-old Lupus, Chamaeleon I, Orion Nebula Cluster, Ophiuchus, IC348, Taurus, and Corona Australis, σ-Orionis, λOrionis, and Upper Scorpius regions (Ansdell et al 2016; Pascucci et al 2016; Eisner et al 2016; Cieza et al 2019; Long et al 2018; Cazzoletti et al 2019; Ansdell et al 2017; Barenfeld et al 2016; Ansdell et al 2020). Viscous evolution would predict large gaseous disks, and, in contrast, small outer radii could be explained by MHD winds or external processes that truncate the disks (see e.g., Clarke & Pringle 1991; Clarke et al 2007; Vincke et al 2015; Rosotti & Clarke 2018; Lesur 2020; Sellek et al 2020; Trapman et al 2020, Zagaria et al, in prep.)

Objectives

Results

Conclusion