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Register a new userApparent disk-mass reduction and planetesimal formation in gravitationally unstable disks in Class 0/I YSOs
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We investigate the dust structure of gravitationally unstable disks undergoing mass accretion from the envelope envisioning the application to Class 0/I young stellar objects (YSOs) We find that the dust disk quickly settles into a steady state and that, compared to a disk with interstellar medium (ISM) dust-to-gas mass ratio and micron-sized dust, the dust mass in the steady-state decreases by a factor of 1/2 to 1/3, and the dust thermal emission decreases by a factor of 1/3 to 1/5. The latter decrease is caused by dust depletion and opacity decrease owing to dust growth. Our results suggest that the masses of gravitationally unstable disks in the Class 0/I YSOs are underestimated by a factor of 1/3 to 1/5 when calculated from the dust thermal emission assuming an ISM dust-to-gas mass ratio and micron-sized dust opacity, and that a larger fraction of disks in Class 0/I YSOs is gravitationally unstable than was previously believed. We also investigate the orbital radius $r_{rm P}$ within which planetesimals form via coagulation of porous dust aggregates and show that $r_{rm P}$ becomes $sim 20$ AU for a gravitationally unstable disk around a solar mass star. Because $r_{rm P}$ increases as the gas surface density increases and a gravitationally unstable disk has a maximum gas surface density, $r_{rm P}sim 20$ AU is the theoretical maximum radius. We suggest that planetesimals formation in the Class 0/I phase is preferable to that in the Class II phase because large gas surface density is expected and large amount of dust is supplied by envelope-to-disk accretion.
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We report on the first birds-eye view of the innermost accretion disk around the high-mass protostellar object G353.273+0.641, taken by Atacama Large Millimter/submillimeter Array long-baselines. The disk traced by dust continuum emission has a radius of 250 au, surrounded by the infalling rotating envelope traced by thermal CH$_3$OH lines. This disk radius is consistent with the centrifugal radius estimated from the specific angular momentum in the envelope. The lower-limit envelope mass is $sim$5-7 M$_{odot}$ and accretion rate onto the stellar surface is 3 $times$ 10$^{-3}$ M$_{odot}$ yr$^{-1}$ or higher. The expected stellar age is well younger than 10$^{4}$ yr, indicating that the host object is one of the youngest high-mass objects at present. The disk mass is 2-7 M$_{odot}$, depending on the dust opacity index. The estimated Toomres $Q$ parameter is typically 1-2 and can reach 0.4 at the minimum. These $Q$ values clearly satisfy the classical criteria for the gravitational instability, and are consistent with the recent numerical studies. Observed asymmetric and clumpy structures could trace a spiral arm and/or disk fragmentation. We found that 70$%$ of the angular momentum in the accretion flow could be removed via the gravitational torque in the disk. Our study has indicated that the dynamical nature of a self-gravitating disk could dominate the early phase of high-mass star formation. This is remarkably consistent with the early evolutionary scenario of a low-mass protostar.
Water is a key volatile that provides insights into the initial stages of planet formation. The low water abundances inferred from water observations toward low-mass protostellar objects may point to a rapid locking of water as ice by large dust grains during star and planet formation. However, little is known about the water vapor abundance in newly formed planet-forming disks. We aim to determine the water abundance in embedded Keplerian disks through spatially-resolved observations of H$_2^{18}$O lines to understand the evolution of water during star and planet formation. We present H$_2^{18}$O line observations with ALMA and NOEMA millimeter interferometers toward five young stellar objects. NOEMA observed the 3$_{1,3}$ - $2_{2,0}$ line (E$_{rm up}$ = 203.7 K) while ALMA targeted the $4_{1,4}$ - $3_{2,1}$ line (E$_{rm up}$ = 322.0 K). Water column densities are derived considering optically thin and thermalized emission. Our observations are sensitive to the emission from the known Keplerian disks around three out of the five Class I objects in the sample. No H$_2^{18}$O emission is detected toward any of our five Class I disks. We report upper limits to the integrated line intensities. The inferred water column densities in Class I disks are N < 10$^{15}$ cm$^{-2}$ on 100 au scales which include both disk and envelope. The upper limits imply a disk-averaged water abundance of $lesssim 10^{-6}$ with respect to H$_2$ for Class I objects. After taking into account the physical structure of the disk, the upper limit to the water abundance averaged over the inner warm disk with $T>$ 100 K is between 10$^{-7}$ up to 10$^{-5}$. Water vapor is not abundant in warm protostellar envelopes around Class I protostars. Upper limits to the water vapor column densities in Class I disks are at least two orders magnitude lower than values found in Class 0 disk-like structures.
We investigate the possibility of the growth of magnetorotational instability (MRI) in disks around Class 0 protostars. We construct a disk model and calculate the chemical reactions of neutral and charged atoms, molecules and dust grains to derive the abundance of each species and the ionization degree of the disk. Then, we estimate the diffusion coefficients of non-ideal magnetohydrodynamics effects such as ohmic dissipation, ambipolar diffusion and the Hall effect. Finally, we evaluate the linear growth rate of MRI in each area of the disk. We investigate the effect of changes in the strength and direction of the magnetic field in our disk model and we adopt four different dust models to investigate the effect of dust size distribution on the diffusion coefficients. Our results indicate that an MRI active region possibly exists with a weak magnetic field in a region far from the protostar where the Hall effect plays a role in the growth of MRI. On the other hand, in all models the disk is stable against MRI in the region within $<20$ au from the protostar on the equatorial plane. Since the size of the disks in the early stage of star formation is limited to $lesssim 10-$$20$ au, it is difficult to develop MRI-driven turbulence in such disks.
Sub-millimeter spectral line and continuum emission from the protoplanetary disks and envelopes of protostars are powerful probes of their structure, chemistry, and dynamics. Here we present a benchmark study of our modeling code, RadChemT, that for the first time uses a chemical model to reproduce ALMA C$^{18}$O (2-1) and CARMA $^{12}$CO (1-0) and N$_{2}$H$^{+}$ (1-0) observations of L1527, that allow us to distinguish the disk, the infalling envelope and outflow of this Class 0/I protostar. RadChemT combines dynamics, radiative transfer, gas chemistry and gas-grain reactions to generate models which can be directly compared with observations for individual protostars. Rather than individually fit abundances to a large number of free parameters, we aim to best match the spectral line maps by (i) adopting a physical model based on density structure and luminosity derived primarily from previous work that fit SED and 2D imaging data, updating it to include a narrow jet detected in CARMA and ALMA data near ($leq 75$au) the protostar, and then (ii) computing the resulting astrochemical abundances for 292 chemical species. Our model reproduces the C$^{18}$O and N$_{2}$H$^{+}$ line strengths within a factor of 3.0; this is encouraging considering the pronounced abundance variation (factor $> 10^3$) between the outflow shell and CO snowline region near the midplane. Further, our modeling confirms suggestions regarding the anti-correlation between N$_{2}$H$^{+}$ and the CO snowline between 400 au to 2,000 au from the central star. Our modeling tools represent a new and powerful capability with which to exploit the richness of spectral line imaging provided by modern submillimeter interferometers.
(abbreviated) We aim to determine the masses of the envelopes, disks, and central stars of young stellar objects (YSOs) in the Class I stage. We observed the embedded Class I objects IRS 63 and Elias 29 in the rho Ophiuchi star-forming region with the Submillimeter Array (SMA) at 1.1 mm. IRS 63 and Elias 29 are both clearly detected in the continuum, with peak fluxes of 459 resp. 47 mJy/beam. The continuum emission toward Elias 29 is clearly resolved, whereas IRS 63 is consistent with a point source down to a scale of 3 arcsec (400 AU). The SMA data are combined with single-dish data, and disk masses of 0.055 and >= 0.007 MSun and envelope masses of 0.058 and >= 0.058 MSun are empirically determined for IRS 63 and Elias 29, respectively. The disk+envelope systems are modelled with the axisymmetric radiative-transfer code RADMC, yielding disk and envelope masses that differ from the empirical results by factors of a few. HCO+ J = 3-2 is detected toward both sources, HCN J = 3-2 is not. The HCO+ position-velocity diagrams are indicative of Keplerian rotation. For a fiducial inclination of 30 degrees, we find stellar masses of 0.37 +/- 0.13 and 2.5 +/- 0.6 MSun for IRS 63 and Elias 29, respectively. We conclude that the sensitivity and spatial resolution of the SMA at 1.1 mm allow a good separation of the disks around Class I YSOs from their circumstellar envelopes and environments, and the spectral resolution makes it possible to resolve their dynamical structure and estimate the masses of the central stars. The ratios of the envelope and disk masses are found to be 0.2 and 6 for IRS 63 and Elias 29, respectively. This is lower than the values for Class 0 sources, which have Menv/Mdisk >= 10, suggesting that this ratio is a tracer of the evolutionary stage of a YSO.
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