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Molecular jets are seen coming from the youngest protostars in the early phase of low-mass star formation. They are detected in CO, SiO, and SO at (sub)millimeter wavelengths down to the innermost regions, where their associated protostars and accretion disks are deeply embedded and where they are launched and collimated. They are not only the fossil records of accretion history of the protostars but also are expected to play an important role in facilitating the accretion process. Studying their physical properties (e.g., mass-loss rate, velocity, rotation, radius, wiggle, molecular content, shock formation, periodical variation, magnetic field, etc) allows us to probe not only the jet launching and collimation, but also the disk accretion and evolution, and potentially binary formation and planetary formation in the disks. Here, the recent exciting results obtained with high-spatial and high-velocity resolution observations of molecular jets in comparison to those obtained in the optical jets in the later phase of star formation are reviewed. Future observations of molecular jets with a large sample at high spatial and velocity resolution with ALMA are expected to lead to a breakthrough in our understanding of jets from young stars.

In the earliest stage of star formation, protostars accrete mass from their surrounding envelopes through circumstellar disks; observations of the protostar L1527 IRS find a large, rotating proto-planetary disk from which the protostellar mass is measured to be 0.19 solar masses, with a protostar-to-envelope mass ratio of about 0.2. A ringside seat at the birth of a solar system This paper reports the use of submillimetre interferometry to obtain the first detection of a large Keplerian disk around a protostar in the earliest phase of evolution, the class 0 phase. Hitherto the smallest observed protostar-to-envelope mass ratio was about 2.1. The newly discovered protostar, L1527 IRS, has a mass of approximately 0.2 solar masses and a protostar-to-envelope mass ratio of about 0.2. L1527 already has a proto-planetary disk of at least seven Jupiter masses, similar to presumed planet-forming disks, so it appears to have all the elements of a solar system in the making. In their earliest stages, protostars accrete mass from their surrounding envelopes through circumstellar disks. Until now, the smallest observed protostar-to-envelope mass ratio was about 2.1 (ref. 1 ). The protostar L1527 IRS is thought to be in the earliest stages of star formation 2 . Its envelope contains about one solar mass of material within a radius of about 0.05 parsecs (refs 3 , 4 ), and earlier observations suggested the presence of an edge-on disk 5 . Here we report observations of dust continuum emission and 13 CO (rotational quantum number J = 2 → 1) line emission from the disk around L1527 IRS, from which we determine a protostellar mass of 0.19 ± 0.04 solar masses and a protostar-to-envelope mass ratio of about 0.2. We conclude that most of the luminosity is generated through the accretion process, with an accretion rate of about 6.6 × 10 −7  solar masses per year. If it has been accreting at that rate through much of its life, its age is approximately 300,000 years, although theory suggests larger accretion rates earlier 6 , so it may be younger. The presence of a rotationally supported disk is confirmed, and significantly more mass may be added to its planet-forming region as well as to the protostar itself in the future.

IRAS 04368+2557 is a solar-type (low-mass) protostar embedded in a protostellar core (L1527) in the Taurus molecular cloud (1,2), which is only 140 parsecs away from Earth, making it the closest large starforming region. The protostellar envelope has a flattened shape with a diameter of a thousand astronomical units (1 au is the distance from Earth to the Sun), and is infalling and rotating (3-5). It also has a protostellar disk with a radius of 90 au (ref. 6), from which a planetary system is expected to form (7,8). The interstellar gas, mainly consisting of hydrogen molecules, undergoes a change in density of about three orders of magnitude as it collapses from the envelope into the disk, while being heated from 10 kelvin to over 100 kelvin in the midplane, but it has hitherto not been possible to explore changes in chemical composition associated with this collapse. Here we report that the unsaturated hydrocarbon molecule cyclic-[C.sub.3][H.sub.2] resides in the infalling rotating envelope, whereas sulphur monoxide (SO) is enhanced in the transition zone at the radius of the centrifugal barrier (100 ± 20 au), which is the radius at which the kinetic energy of the infalling gas is converted to rotational energy. Such a drastic change in chemistry at the centrifugal barrier was not anticipated, but is probably caused by the discontinuous infalling motion at the centrifugal barrier and local heating processes there.

Gravitational forces are expected to excite spiral density waves in protoplanetary disks, disks of gas and dust orbiting young stars. However, previous observations that showed spiral structure were not able to probe disk midplanes, where most of the mass is concentrated and where planet formation takes place. Using the Atacama Large Millimeter/submillimeter Array, we detected a pair of trailing symmetric spiral arms in the protoplanetary disk surrounding the young star Elias 2-27. The arms extend to the disk outer regions and can be traced down to the midplane. These millimeter-wave observations also reveal an emission gap closer to the star than the spiral arms. We argue that the observed spirals trace shocks of spiral density waves in the midplane of this young disk.

Annular structures (rings and gaps) in disks around pre-main-sequence stars have been detected in abundance towards class II protostellar objects that are approximately 1,000,000 years old 1 . These structures are often interpreted as evidence of planet formation 1 – 3 , with planetary-mass bodies carving rings and gaps in the disk 4 . This implies that planet formation may already be underway in even younger disks in the class I phase, when the protostar is still embedded in a larger-scale dense envelope of gas and dust 5 . Only within the past decade have detailed properties of disks in the earliest star-forming phases been observed 6 , 7 . Here we report 1.3-millimetre dust emission observations with a resolution of five astronomical units that show four annular substructures in the disk of the young (less than 500,000 years old) 8 protostar IRS 63. IRS 63 is a single class I source located in the nearby Ophiuchus molecular cloud at a distance of 144 parsecs 9 , and is one of the brightest class I protostars at millimetre wavelengths. IRS 63 also has a relatively large disk compared to other young disks (greater than 50 astronomical units) 10 . Multiple annular substructures observed towards disks at young ages can act as an early foothold for dust-grain growth, which is a prerequisite of planet formation. Whether or not planets already exist in the disk of IRS 63, it is clear that the planet-formation process begins in the initial protostellar phases, earlier than predicted by current planet-formation theories 11 . Dust-emission observations of the young (<500,000 years old) protostar IRS 63 show evidence of rings and gaps in its disk, a prerequisite of planet formation.

Observations of the triple protostar system L1448 IRS3B support the hypothesis that companion stars can form because of gravitational instability in a protostellar disk. Gravitational instability in a triple-star system Multiple stars that orbit each other, a common occurrence across the Universe, result from the early-stage fragmentation of molecular clouds. John Tobin et al . report observations of the triple protostar system L1448 IRS3B, which is in an early phase of the star formation process, making it an ideal candidate for the search for evidence of disk fragmentation. They find dust and molecular gas emissions that indicate a disk with a spiral structure surrounding the three protostars. They demonstrate that this disk is susceptible to fragmentation, supporting the hypothesis that companion stars can form as a result of gravitational instability in a protostellar disk. Binary and multiple star systems are a frequent outcome of the star formation process 1 , 2 and as a result almost half of all stars with masses similar to that of the Sun have at least one companion star 3 . Theoretical studies indicate that there are two main pathways that can operate concurrently to form binary/multiple star systems: large-scale fragmentation of turbulent gas cores and filaments 4 , 5 or smaller-scale fragmentation of a massive protostellar disk due to gravitational instability 6 , 7 . Observational evidence for turbulent fragmentation on scales of more than 1,000 astronomical units has recently emerged 8 , 9 . Previous evidence for disk fragmentation was limited to inferences based on the separations of more-evolved pre-main sequence and protostellar multiple systems 10 , 11 , 12 , 13 . The triple protostar system L1448 IRS3B is an ideal system with which to search for evidence of disk fragmentation as it is in an early phase of the star formation process, it is likely to be less than 150,000 years old 14 and all of the protostars in the system are separated by less than 200 astronomical units. Here we report observations of dust and molecular gas emission that reveal a disk with a spiral structure surrounding the three protostars. Two protostars near the centre of the disk are separated by 61 astronomical units and a tertiary protostar is coincident with a spiral arm in the outer disk at a separation of 183 astronomical units 13 . The inferred mass of the central pair of protostellar objects is approximately one solar mass, while the disk surrounding the three protostars has a total mass of around 0.30 solar masses. The tertiary protostar itself has a minimum mass of about 0.085 solar masses. We demonstrate that the disk around L1448 IRS3B appears susceptible to disk fragmentation at radii between 150 and 320 astronomical units, overlapping with the location of the tertiary protostar. This is consistent with models for a protostellar disk that has recently undergone gravitational instability, spawning one or two companion stars.

The majority of stars are part of gravitationally bound stellar systems, such as binaries. Observations of protobinary systems constrain the conditions that lead to stellar multiplicity and subsequent orbital evolution. We report high–angular resolution observations of the circumbinary disk around [BHB2007] 11, a young binary protostar system. The two protostars are embedded in circumstellar disks that have radii of 2 to 3 astronomical units and probably contain a few Jupiter masses. These systems are surrounded by a complex structure of filaments connecting to the larger circumbinary disk. We also observe accretion and radio jets associated with the protobinary system. The accretion is preferentially onto the lower-mass protostar, consistent with theoretical predictions.

Synchrotron emission is commonly found in relativistic jets from active galactic nuclei (AGNs) and microquasars, but so far its presence in jets from young stellar objects (YSOs) has not been proved. Here, we present evidence of polarized synchrotron emission arising from the jet of a YSO. The apparent magnetic field, with strength of approximately 0.2 milligauss, is parallel to the jet axis, and the polarization degree increases toward the jet edges, as expected for a confining helical magnetic field configuration. These characteristics are similar to those found in AGN jets, hinting at a common origin of all astrophysical jets.

The origin of the supermassive black holes that inhabit the centres of massive galaxies remains unclear 1 , 2 . Direct-collapse black holes—remnants of supermassive stars, with masses around 10,000 times that of the Sun—are ideal seed candidates 3 – 6 . However, their very existence and their formation environment in the early Universe are still under debate, and their supposed rarity makes modelling their formation difficult 7 , 8 . Models have shown that rapid collapse of pre-galactic gas (with a mass infall rate above some critical value) in metal-free haloes is a requirement for the formation of a protostellar core that will then form a supermassive star 9 , 10 . Here we report a radiation hydrodynamics simulation of early galaxy formation 11 , 12 that produces metal-free haloes massive enough and with sufficiently high mass infall rates to form supermassive stars. We find that pre-galactic haloes and their associated gas clouds that are exposed to a Lyman–Werner intensity roughly three times the intensity of the background radiation and that undergo at least one period of rapid mass growth early in their evolution are ideal environments for the formation of supermassive stars. The rapid growth induces substantial dynamical heating 13 , 14 , amplifying the Lyman–Werner suppression that originates from a group of young galaxies 20 kiloparsecs away. Our results strongly indicate that the dynamics of structure formation, rather than a critical Lyman–Werner flux, is the main driver of the formation of massive black holes in the early Universe. We find that the seeds of massive black holes may be much more common than previously considered in overdense regions of the early Universe, with a co-moving number density up to 10 −3 per cubic megaparsec. Simulations of early galaxy formation suggest that the dynamics of structure formation, rather than the Lyman–Werner flux, drives the formation of massive black holes in the early Universe.

Observations of the outflow associated with the TMC1A protostellar system reveal that the ‘disk wind’ model correctly explains how material is ejected from protostars. A young star turns on the gas Observations of the young, solar-type protostar TMC1A, taken with the Atacama Large Millimeter/submillimetre Array (ALMA) in high-angular-resolution mode, provide new data on the outflows of molecular gas associated with such systems. Per Bjerkeli et al . report images of carbon monoxide gas that is ejected from a region extending up to a radial distance of 25 astronomical units from the central protostar. Their data also show that angular momentum is removed from an extended region of the disk. These findings are consistent with the 'disk wind' model, in which the outflowing gas is launched by an extended disk wind from a Keplerian disk. Young stars are associated with prominent outflows of molecular gas 1 , 2 . The ejection of gas is believed to remove angular momentum from the protostellar system, permitting young stars to grow by the accretion of material from the protostellar disk 2 . The underlying mechanism for outflow ejection is not yet understood 2 , but is believed to be closely linked to the protostellar disk 3 . Various models have been proposed to explain the outflows, differing mainly in the region where acceleration of material takes place: close to the protostar itself (‘X-wind’ 4 , 5 , or stellar wind 6 ), in a larger region throughout the protostellar disk (disk wind 7 , 8 , 9 ), or at the interface between the two 10 . Outflow launching regions have so far been probed only by indirect extrapolation 11 , 12 , 13 because of observational limits. Here we report resolved images of carbon monoxide towards the outflow associated with the TMC1A protostellar system. These data show that gas is ejected from a region extending up to a radial distance of 25 astronomical units from the central protostar, and that angular momentum is removed from an extended region of the disk. This demonstrates that the outflowing gas is launched by an extended disk wind from a Keplerian disk.