The first ALMA survey of protoplanetary discs at 3 mm: demographics of grain growth in the Lupus region

M Tazzari,A Miotello,J P Williams,S Facchini,A Natta,G Guidi,M Hogherheijde,M Ansdell,J M Carpenter,L Testi,C F Manara,N Van Der Marel

doi:10.1093/mnras/stab1912

Abstract

ABSTRACT We present the first ALMA survey of protoplanetary discs at 3 mm, targeting 36 young stellar objects in the Lupus star-forming region with deep observations (sensitivity 20–50 $\mu$Jy beam−1) at ∼0.35 arcsec resolution (∼50 au). Building on previous ALMA surveys at 0.89 and 1.3 mm that observed the complete sample of Class II discs in Lupus at a comparable resolution, we aim to assess the level of grain growth in the relatively young Lupus region. We measure 3 mm integrated fluxes, from which we derive disc-averaged 1–3 mm spectral indices. We find that the mean spectral index of the observed Lupus discs is $\alpha _\mathrm{1-3\, mm}=2.23\pm 0.06$, in all cases $\alpha _\mathrm{1-3\, mm}\lt 3.0$, with a tendency for larger spectral indices in the brightest discs and in transition discs. Furthermore, we find that the distribution of spectral indices in Lupus discs is statistically indistinguishable from that of the Taurus and Ophiuchus star-forming regions. Assuming the emission is optically thin, the low values $\alpha _\mathrm{1-3\, mm}\le 2.5$ measured for most discs can be interpreted with the presence of grains larger than 1 mm. The observations of the faint discs in the sample can be explained without invoking the presence of large grains, namely through a mixture of optically thin and optically thick emission from small grains. However, the bright (and typically large) discs do inescapably require the presence of millimetre-sized grains in order to have realistic masses. Based on a disc mass argument, our results challenge previous claims that the presence of optically thick substructures may be a universal explanation for the empirical millimetre size-luminosity correlation observed at 0.89 mm.

Highlights

Protoplanetary discs are the birth place of planets
Using the new 3 mm data presented here, we find that the mean spectral index in the Lupus discs is α0.89−3.1 mm = 2.23 ± 0.06, which corresponds to a nominal average dust opacity β = 0.23±0.06 according to Eq (2)
(2) By combining the new 3 mm observations with previous ALMA observations at 0.89 mm and a similar angular resolution, we find that all Lupus discs have spectral indices α0.89−3.1 mm < 3.0, with a tendency of larger values for transition discs

Summary

Introduction

Protoplanetary discs are the birth place of planets. In the coreaccretion scenario (Safronov 1972; Wetherill 1980), the first step to form a terrestrial planet is the growth of the typically micronsized interstellar medium (ISM) dust grains to millimeter and centimeter-sized pebbles. The measured disc fluxes and extents at 1 mm suggest that radial drift has to be effectively slowed down or even halted in most discs (Birnstiel et al 2010; Pinilla et al 2012a) Thanks to their ability to trap (or, at least, decelerate the inward drift of) dust grains, local maxima in the gaseous component of a protoplanetary disc are a ready solution to the radial drift problem. This scenario is supported by the recent high angular resolution ALMA observations (e.g., Isella et al 2016; Fedele et al 2018; Clarke et al 2018; Andrews et al 2018b; Long et al 2018, 2019) which showed a suggestive recurrence in the discs millimetre continuum emission of orderly and axisymmetric structures at small spatial scales. The origin of local maxima can be linked to a variety of physical mechanisms, such as the interaction with an embedded forming planet (e.g., Pinilla et al 2012b; Clarke et al 2018), zonal flows (Johansen & Klahr 2005; Flock et al 2015), and the presence of vortices (Barge & Sommeria 1995; Klahr & Henning 1997)

Objectives

Results

Discussion

Conclusion