Testing protostellar disk formation models with ALMA observations

Abstract

Context. Recent simulations have explored different ways to form accretion disks around low-mass stars. However, it has been difficult to differentiate between the proposed mechanisms because of a lack of observable predictions from these numerical studies. Aims. We aim to present observables that can differentiate a rotationally supported disk from an infalling rotating envelope toward deeply embedded young stellar objects (M env > M disk ) and infer their masses and sizes. Methods. Two 3D magnetohydrodynamics (MHD) formation simulations are studied with a rotationally supported disk (RSD) forming in one but not the other (where a pseudo-disk is formed instead), together with the 2D semi-analytical model. We determine the dust temperature structure through continuum radiative transfer RADMC3D modeling. A simple temperature-dependent CO abundance structure is adopted and synthetic spectrally resolved submm rotational molecular lines up to J u = 10 are compared with existing data to provide predictions for future ALMA observations. Results. The 3D MHD simulations and 2D semi-analytical model predict similar compact components in continuum if observed at the spatial resolutions of 0.5-1 (70-140 AU) typical of the observations to date. A spatial resolution of ∼14 AU and high dynamic range (>1000) are required in order to differentiate between RSD and pseudo-disk formation scenarios in the continuum. The first moment maps of the molecular lines show a blue-to red-shifted velocity gradient along the major axis of the flattened structure in the case of RSD formation, as expected, whereas it is along the minor axis in the case of a pseudo-disk. The peak position-velocity diagrams indicate that the pseudo-disk shows a flatter velocity profile with radius than does an RSD. On larger scales, the CO isotopolog line profiles within large (>9 ) beams are similar and are narrower than the observed line widths of low-J (2-1 and 3-2) lines, indicating significant turbulence in the large-scale envelopes. However a forming RSD can provide the observed line widths of high-J (6-5, 9-8, and 10-9) lines. Thus, either RSDs are common or a higher level of turbulence (b ∼ 0.8 km s −1 ) is required in the inner envelope compared with the outer part (0.4 km s −1 ). Conclusions. Multiple spatially and spectrally resolved molecular line observations can differentiate between the pseudo-disk and the RSD much better than continuum data. The continuum data give a better estimate of disk masses, whereas the disk sizes can be estimated from the spatially resolved molecular lines observations. The general observable trends are similar between the 2D semi-analytical models and 3D MHD RSD simulations.

Possible Counterrotation between the Disk and Protostellar Envelope around the Class I Protostar IRAS 04169+2702

“…Furthermore, our modeling of protostellar envelopes and disks around the other protostellar sources as observed with ALMA reveals that geometrically-thin envelope / disk models reproduce the observed features reasonably well (e.g., Aso et al 2015;Yen et al 2017;Takakuwa et al 2017). The formation of so-called pseudo-disks is also expected from several MHD simulations (Machida et al 2011a;Harsono et al 2015;Tomida et al 2015).…”

“…We note, however, that there is likely a significant contamination from the outflow component to the 13 CO (2-1) emission (Figure 2), which makes this simple one-zone LTE analysis infeasible. On the other hand, the observed peak brightness temperature of the C 18 O (2-1) emission ( 1.6 K) is well below the anticipated gas temperature of the protostellar envelope (∼ 20 K) (Aso et al 2015;Harsono et al 2015), which suggests that the C 18 O (2-1) emission is likely optically thin.…”

Revealing the dynamics of Class 0 protostellar discs with ALMA

“…In contrast, when taking into account the large-scale emission Harsono et al (2015) find that disc masses inferred from millimetre observations agree rather well with the actual mass. They also investigate the dynamics of a Keplerian disc by means of synthetic CO line emission maps.…”

Magnetic diffusivities in 3D radiative chemo-hydrodynamic simulations of protostellar collapse

“…In spite of increased computational costs, the models such as presented in Paper I are self-consistent and can be used threefold: for matching the observations, for calculating nonideal MHD terms, and for the thermal chemical feedback (i.e. cooling with atomic or molecular lines) (Hincelin et al 2016;Harsono et al 2015;Gerin et al 2015). We leave the atomic/molecular lines cooling for the follow-up studies.…”