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 μJy 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 α1−3mm=2.23±0.06, in all cases α1−3mm<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 α1−3mm≤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.
The first ALMA survey of protoplanetary discs at 3~mm : demographics of grain growth in the Lupus region / M. Tazzari, L. Testi, A. Natta, J. P Williams, M. Ansdell, J. M Carpenter, S. Facchini, G. Guidi, M. Hogherheijde, C. F Manara, A. Miotello, N. van der Marel. - In: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY. - ISSN 0035-8711. - 506:4(2021), pp. 5117-5128. [10.1093/mnras/stab1912]
|Titolo:||The first ALMA survey of protoplanetary discs at 3~mm : demographics of grain growth in the Lupus region|
|Parole Chiave:||accretion, accretion discs, planets and satellites: formation, protoplanetary discs, circumstellar matter, stars: pre-main-sequence, submillimetre: planetary systems|
|Settore Scientifico Disciplinare:||Settore FIS/05 - Astronomia e Astrofisica|
|Data di pubblicazione:||2021|
|Digital Object Identifier (DOI):||http://dx.doi.org/10.1093/mnras/stab1912|
|Appare nelle tipologie:||01 - Articolo su periodico|