FAUST XXX: Dust enhancement in the young binary L1551 IRS 5

TL;DR

This paper investigates dust enhancement at the inner edge of the circumbinary disc around L1551 IRS 5 using ALMA FAUST data and tailored hydrodynamical and radiative transfer modelling. The authors perform 3D SPH simulations of a mildly eccentric, nearly coplanar binary with masses 0.8 and 0.3 solar masses and map the outputs to synthetic observables using MCFOST, enabling direct comparison with the data. The simulations predict a long-lived inner-edge overdensity driven by binary–disc interaction, and the synthetic continuum and C18O maps reproduce the northern overdensity observed, supporting the dust concentration scenario. The work highlights the role of envelope dynamics, and multi-wavelength data will be required to constrain the orbit and fully characterize dust growth and the potential for planetesimal formation in these young systems.

Abstract

Young binary stars with discs provide unique laboratories to study the earliest stages of planet formation in star-forming environments. The detection of substructure in discs around Class I protostars challenges current models of disc evolution, suggesting that planets may form earlier than previously expected ($<1$ Myr). In the context of the ALMA Large Program FAUST, we present observations of the circumbinary disc (CBD) around the young binary system L1551 IRS 5. The CBD exhibits two prominent over-densities in the continuum emission at the edge of the cavity, with the Northern over-density being about 20% brighter than the Southern one. By analysing the disc morphology and kinematics of L1551 IRS 5, we delineate dynamical constraints on the binary's orbital parameters. Additionally, we present 3D hydrodynamical models of the CBD to predict both the dust and the gas surface densities. Then, we compare the resulting synthetic observations with ALMA observations of the continuum emission at 1.3 mm and the C$^{18}$O line emission. Our analysis suggests that the density enhancements observed with ALMA in L1551 IRS 5 can be caused by interactions between the binary stars and the CBD, leading to dust concentration within the disc. We conclude that the observed over-density corresponds to a location where could potentially grow under favourable conditions.

Figures (5)

• Figure 1: Left: Continuum emission at 1.33 mm in colour scale (see Sec. \ref{['sec:observations']}). Contours indicate the protostellar discs, subtracted from the image during the cleaning process. First contours and steps are 30$\sigma$ (13 mJy beam$^{-1}$) and 100$\sigma$, respectively. The black stars indicate the positions of the N and S protostars. Middle: Moment 0 map of the C$^{18}$O emission (integrated between -10 km s$^{-1}$ and +20 km s$^{-1}$) in colour scale superposed to 1.3 mm dust continuum emission in grey contours (from Bianchi+2020). First contours and steps are 10$\sigma$ (1.8 mJy beam$^{-1}$) and 100$\sigma$, respectively. Right: Moment 1 map of the C$^{18}$O emission in colour scale, 1.3 mm dust continuum emission in grey contours. The moment 1 map is generated using a threshold of 5$\sigma$ (20 mJy beam$^{-1}$). The source systemic velocity ($v_{\rm sys}$) is 6.4 ${\rm km\,s^{-1}}$Mercimek+2022
• Figure 2: Top row:Phantom hydrodynamical models of L1551 IRS 5, gas surface density (left) and dust 100 $\mu$m-sized dust particles surface density (right). Bottom row:mcfost synthetic observations, C$^{18}$O moment 1 map (left) and continuum emission (right). Snapshots after 13 100 years, which correspond to more than 50 binary orbits. All the images are shown using the same scale (top in au and bottom in arcsec).
• Figure 3: Gas surface density for different evolutionary stages for our nominal Phantom hydrodynamical model. Time moves forward from top to bottom: 0, 10, 30, 50 binary orbits.
• Figure 4: Dust-to-gas ratio after t=13 100 years. The homogeneous distribution indicates that the dust over-density corresponds to a dust concentration rather than a dust trap for which the ratio should increase at the over-density location.
• Figure 5: Comparison between two different initial conditions with $R_{\rm in}=60$ au (left panel) and $R_{\rm in}=80$ au (right panel) after t=13 100 years. We do not see noticeable differences showing that the over-density is not a direct result of the initial condition.