The formation and evolution of protoplanetary disks are fundamental ingredients in the process of low-mass star and planet formation. In particular, Class I sources connect the deeply embedded Class 0 protostars with the more evolved Class II disks, and are thought to be associated with the first steps of planet formation. The study of the chemistry related to Class I sources allow us to infer the main physical processes that are taking place. Disk tracers, such as CO isotopologues, allow us to infer the protostellar mass and estimate the mass accretion rate from the disk to the protostar. We find an average value of 2.4×10-7 M⊙/year for the mass accretion rate of a sample of 14 Class I sources, supporting the scenario of episodic accretion bursts and a variable accretion rate. The sources with the highest mass accretion rate show emission of SO2, a well-known shock tracer. SO2 seems to be tracing accretion shocks at the interface between the inner envelope and the disk surface. Therefore, a connection seems to exist between the accretion shocks at the disk surface and the highest mass accretion rates from the disk to the protostar, linking together shock and disk tracers. The mass accretion rates found for a sample of Class I sources is higher than the value of 10-8 M⊙/year, commonly used for models of planet formation towards Class II disks. Future models of planet formation within embedded disks need to take into account higher mass accretion rates and, thus, a higher degree of turbulence.

GGD 27 is a reflection nebula illuminated by a massive B0-type protostar that powers a spectacular, fast and highly collimated radio jet associated with HH 80-81. The detection of linearly polarized synchrotron emission due to relativistic electrons indicates the presence of magnetic field aligned with the jet, which suggests that the jet is being launched from an accretion disk. I will present ALMA observations at 1.14 mm toward the GGD 27 protocluster with an unprecedented angular resolution (40 mas) and sensitivity (16 microJy/beam), which clearly resolve the disk oriented perpendicular to the radio jet around the massive protostar and  allowed us to discover a cluster of 25 continuum sources. In this talk, I will present the physical properties of the disk around the massive protostar obtained from a self-consistent alpha-accretion flare disk model and from dust polarization. The disk is compact (Rdisk~170 au) and massive (~5 Msun) and the dust polarization pattern indicate the presence of relatively large grains all along the disk. Finally, I will discuss the properties of the disk population from the low-mass cluster members and the comparison with disks in nearby star-forming regions.

While it is widely accepted that planets are formed in protoplanetary disks, there is still much debate on when this process happens. In a few cases protoplanets have been directly imaged, but for the vast majority of systems, disk gaps and cavities -- seen especially in dust continuum observations -- have been the strongest evidence of recent or on-going planet formation. I will present ALMA observations of a nearly edge-on disk containing a giant gap seen in dust. Inside the gap, the molecular gas has a warm (100 K) component coinciding in position with a tentative free-free emission excess observed with the VLA. Using 1D hydrodynamic models, we find the structure of the gap is consistent with being carved by a planet with 4-70 Jupiter masses, and these other lines of evidence point to the planet being very young and/or still accreting. In addition, the CO observations reveal large scale filaments aligned with the disk major axis and quasi-velocity coherent with the disk gas that we interpret as ongoing gas infall from the local ISM. Therefore this system appears to be an interesting case where both a star (from the environment and the disk) and a planet (from the disk) are growing in tandem.

Constraining the dynamics of multiple stellar systems during the earliest evolutionary stages provides unique information about stellar masses while accretion is still ongoing. I will show ALMA 3 mm observations with a resolution of 6.5 au that confirm one of the most well-known Class 0 systems (IRAS 16293-2422 A) as a tight binary, currently separated by 54 au. The compact (<12 au) dust 3 mm emission towards A1 and A2 coincides with compact ionized gas emission previously observed at radio wavelengths finally establishing that the long-known radio sources were tracing the location of two protostars with compact dust disks that are misaligned with the extended circumbinary disk-like structure. With molecular tracers of dense gas (CS, HNCO, NH2CHO, t-HCOOH) we analyze gas motion down to 13 au scales to measure the A1 and A2 masses and show that the pair is bound. Using this new context for the previous 30 years of observations, we fit orbital parameters and find that the combined protostellar+compact disk mass derived from the orbit is consistent with the masses derived from gas motion. Both measurements indicate that previous estimates using lower resolution observations underestimated the protostellar mass towards IRAS 16293 A. The ALMA high-resolution data provides a unique insight into the gas kinematics and masses of a deeply embedded bound binary system.

Earlier, protostellar accretion and disk evolution were imagined as smooth processes that happen over millions of years. According to our current paradigm, however, accretion is inhomogeneous in space and variable in time. Young eruptive stars are observational evidences of episodic accretion. Using ALMA we performed a multi-scale study, down to a resolution of 0.15", of the dust component around a large sample of FU Orionis-type young eruptive stars. We resolved the disks and fitted them with radiative transfer and analytical disk models. We found the disks to be mostly featureless, rather massive and relatively small, compared to the disks of regular T Tauri stars. As much as three quarters of our sample may host gravitationally unstable disks, but some have surprisingly low masses, pointing to different outburst mechanisms.

(Sub)millimeter dust opacities are required for converting the observable dust continuum emission to the mass, but their values have long been uncertain, especially in disks around young stellar objects. I present a method to constrain the opacity for marginally gravitationally unstable edge-on disks. The Class 0 source, HH 212 mms, is well suited for this application because there is high angular resolution Band 7, 6, and 3 observations from ALMA. The model can also probe the temperature structure of an edge-on disk naturally because different wavelengths probe different regions of the disk. The derived opacity supports the conventional opacity prescription by Beckwith et al. 1990 and the temperature is found to be warm enough such that CO freeze-out is unlikely.

We present ALMA observations of complex organic molecules toward three Class 0 sources in the Serpens Molecular Cloud, which were previously postulated as candidates to also harbor a protostellar disk. The organic emission is located in a compact, barely resolved region around the continuum peak, and the estimated rotational temperatures are well above 100 K (the sublimation temperature of ice mantles on top of the dust grains) confirming the presence of hot coring chemistry in these sources.
The number of detected hot coring sources has doubled in the last couple of years, with half of the sources also known to either harbor a protostellar disk, or at least being a protostellar disk candidate. This is a high occurrence rate given the sparsity of hot coring detection, and suggests that the formation of hot corinos and protostellar disks during the earlier stages of star formation could be related.

A disk/envelope system and an outflow are deeply related to each other in the earliest stage of protostellar evolution. Characterization of an outflow therefore provide us with important information to investigate formation processes of a disk structure in which a planetary system is formed. Such studies are particularly important, if the planet formation begins in embedded stages. IRAS 15398-3359 is a low-mass Class 0 protostellar source (Tbol=44 K) located in the Lupus 1 molecular cloud (d=155 pc). A bipolar outflow along the northeast to southwest axis is reported (Oya et al. 2014). Its dynamical timescale is~10^3 yr, indicating that this source is in the earliest stage of protostellar evolution. A Keplerian disk perpendicular to the outflow is associated with the protostar (Okoda et al. 2018). We have conducted observations toward this source on scales from 50 au to 1800 au as a part of the ALMA large program FAUST (Fifty AU STudy of the chemistry in the disk/envelope system of Solar-like protostars). We have found an interesting collimated feature in the H2CO, SO, SiO, and CH3OH emission lines which extends along a direction almost disk/envelope system. These molecular line emissions show a shell-like structure apart from the protostar by 1200 au in the southeastern part. The H2CO emission shows a gradient of 1.2 km s^(-1)/1200 au from the protostar to the shell-like structure, so that the above extended distribution can be explained by an outflow motion. Since SO, SiO, and CH3OH are known as shock tracers, the shell-like structure is most likely a shocked region caused by the newly found outflow. This result implies a change in the outflow direction caused by episodic accretion, although a possibility of an unresolved close companion (<30 au) cannot be ruled out. Our observation demonstrates a dynamic feature of the protostellar evolution in its earliest phase, which would possibly affect the nature of a disk structure being formed around the protostar.

Ringed protoplanetary disks, in the Class II phase of low-mass star formation when the envelope has mostly dispersed, have been found in abundance in recent years with high-resolution ALMA observations. These ringed disks have been often interpreted as evidence of planet formation, caused by planetesimal-disk interactions. I will present ALMA observations of a younger embedded Class I protostar which has a ringed dust disk (5 au resolution data) as well as a larger-scale infalling streamer of gas 1500 au in length from the envelope to the disk (100 au resolution data). The dust rings are the least-evolved example of rings in a protostellar disk known to date, indicating that stable zones of grain growth---required for the first steps of planet formation---are already in place during the early embedded phase of star formation.  Further, these first steps of planet formation occur while the envelope continues to accrete onto the disk via the 1500 au gas streamer, likely influencing the disk composition and dynamics until the envelope is fully dissipated. Our work highlights the need for both disk and envelope scale observations to understand disk dynamics, structures, and evolution in the young embedded phases of star formation.

ALMA data for protoplanetary disks such as HL Tau show that planet formation seems well underway at 0.5 – 1 Myr, suggesting that planet formation must start even earlier, during the protostar phase at 0.1 Myr. Following gravitational collapse motions from the infalling envelope to the disk further means that we can trace chemical inheritance from the ISM to the disk, and to the forming planets. We can therefore use ALMA to trace gas motions if we have good theoretical models that include chemistry. We present the RadChemT package to model protostar dynamics with chemistry and compare with ALMA data. RadChemT includes 2D collapse, radiative transfer, & chemistry, generating synthetic images & spectral lines for 292 chemical species simultaneously. A benchmark study of protostar L1527 validates RadChemT against ALMA & CARMA data, showing a good match for CO, C18O and N2H+ data. In line with previous work, strong CO freezeout & enhanced N2H+ is seen at ~1000au. The predicted dynamics in the C18O Position-Velocity diagram traces both envelope and disk motion, and shows striking similarity to the data. The RadChemT package is a promising tool to investigate dynamics and chemistry in protostars.

FUor objects (a class of outbursting, young, pre-main sequence stars) have been shown to be variable at millimeter wavelengths on timescale of ~years. We find that the object V1735 Cyg exhibited a ~80% increase in 2.7 mm flux between 2014 and 2017. It does not show corresponding variability at optical/NIR wavelengths, but shows a decrease in the millimeter spectral index. This implies that the millimeter flux increase may be due to an accretion event which increased the contribution from free-free emission in the jet. This result shows that an FUor disk mass obtained from millimeter flux may be overestimated if the object has recently undergone recent accretion activity.