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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.

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 Stardust samples collected from Comet 81P/Wild 2 indicate that large-scale mixing occurred in the solar nebula, carrying materials from the hot inner regions to cooler environments far from the Sun. Similar transport has been inferred from telescopic observations of protoplanetary disks around young stars. Models for protoplanetary disks, however, have difficulty explaining the observed levels of transport. Here I report the results of a new two-dimensional model that shows that outward transport of high-temperature materials in protoplanetary disks is a natural outcome of disk formation and evolution. This outward transport occurs around the midplane of the disk.

Although planets are being discovered around stars more massive than the Sun, information about the proto-planetary disks where such planets have built up is sparse. We have imaged mid-infrared emission from polycyclic aromatic hydrocarbons at the surface of the disk surrounding the young intermediate-mass star HD 97048 and characterized the disk. The disk is in an early stage of evolution, as indicated by its large content of dust and its hydrostatic flared geometry, indicative of the presence of a large amount of gas that is well mixed with dust and gravitationally stable. The disk is a precursor of debris disks found around more-evolved A stars such as β-Pictoris and provides the rare opportunity to witness the conditions prevailing before (or during) planet formation.

Field patterns in Orion Accretion disks are important building blocks in astrophysics and are found in a wide range of contexts, around protostars in star-forming regions and around supermassive black holes at the centre of galaxies. Magnetic fields are supposedly a key ingredient of these disks, yet no direct observations of these fields have been available to constrain existing disk models near the critical central regions. A new spectrograph/spectropolarimeter called ESPaDOnS, fitted on a 3.6-metre telescope on Mauna Kea in Hawaii, has now filled this gap. It has been used to detect a magnetic field in the core of FU Orionis, a variable star near Orion's ‘shoulder’. Models 1 , 2 , 3 , 4 , 5 predict that magnetic fields play a crucial role in the physics of astrophysical accretion disks and their associated winds and jets 6 , 7 . For example, the rotation of the disk twists around the rotation axis the initially vertical magnetic field, which responds by slowing down the plasma in the disk and by causing it to fall towards the central star. The magnetic energy flux produced in this process points away from the disk, pushing the surface plasma outwards, leading to a wind from the disk and sometimes a collimated jet. But these predictions have hitherto not been supported by observations. Here we report the direct detection of the magnetic field in the core of the protostellar accretion disk FU Orionis 8 . The surface field reaches strengths of about 1 kG close to the centre of the disk, and it includes a significant azimuthal component, in good agreement with recent models 5 . But we find that the field is very filamentary and slows down the disk plasma much more than models predict, which may explain why FU Ori fails to collimate its wind into a jet.

Keck adaptive optics imaging with a physical resolution of 0.4 astronomical units (AU) resolves the inner (15 to 80 AU) disk of AU Microscopii (AU Mic, GJ 803, HD 197481), the nearest known scattered light disk to Earth. The inner disk is asymmetric and possesses a sharp change in structure at 35 AU. The disk also shows spatially localized enhancements and deficits at 25- to 40-AU separations. The overall morphology points to the influence of unseen larger bodies and resembles structures expected from recent planet formation. AU Mic is coeval with the archetypical debris disk system β Pictoris, and the similarities between their two disks point to synchronous disk evolution. Multiple indications of substructure appear to be common in circumstellar disks at an age of ≈12 million years.

The isolated, young, sunlike star TW Hya and four other young stars in its vicinity are strong x-ray sources. Their similar x-ray and optical properties indicate that the stars make up a physical association that is on the order of 20 million years old and that lies between about 40 and 60 parsecs (between about 130 and 200 light years) from Earth. TW Hya itself displays circumstellar CO, HCN, CN, and HCO$^+$ emission. These molecules probably orbit the star in a solar-system-sized disk viewed more or less face-on, whereas the star is likely viewed pole-on. Being at least three times closer to Earth than any well-studied region of star formation, the TW Hya Association serves as a test-bed for the study of x-ray emission from young stars and the formation of planetary systems around sunlike stars.

The stellar Initial Mass Function (IMF) suggests that stars with sub-solar mass form in very large numbers. Most attractive places for catching low-mass star formation in the act are young stellar clusters and associations, still (half-)embedded in star-forming regions. The low-mass stars in such regions are still in their pre–main-sequence (PMS) evolutionary phase, i.e., they have not started their lives on the main-sequence yet. The peculiar nature of these objects and the contamination of their samples by the fore- and background evolved populations of the Galactic disk impose demanding observational techniques, such as X-ray surveying and optical spectroscopy of large samples for the detection of complete numbers of PMS stars in the Milky Way. The Magellanic Clouds, the metal-poor companion galaxies to our own, demonstrate an exceptional star formation activity. The low extinction and stellar field contamination in star-forming regions of these galaxies imply a more efficient detection of low-mass PMS stars than in the Milky Way, but their distance from us make the application of the above techniques unfeasible. Nonetheless, imaging with the Hubble Space Telescope within the last five years yield the discovery of solar and sub-solar PMS stars in the Magellanic Clouds from photometry alone. Unprecedented numbers of such objects are identified as the low-mass stellar content of star-forming regions in these galaxies, changing completely our picture of young stellar systems outside the Milky Way, and extending the extragalactic stellar IMF below the persisting threshold of a few solar masses. This review presents the recent developments in the investigation of the PMS stellar content of the Magellanic Clouds, with special focus on the limitations by single-epoch photometry that can only be circumvented by the detailed study of the observable behavior of these stars in the color-magnitude diagram. The achieved characterization of the low-mass PMS stars in the Magellanic Clouds allowed thus a more comprehensive understanding of the star formation process in our neighboring galaxies.

We are conducting a large-scale, multiepoch, optical photometric survey [Centro de Investigaciones de Astronomía-Quasar Equatorial Survey Team (CIDA-QUEST)] covering about 120 square degrees to identify the young low-mass stars in the Orion OB1 association. We present results for an area of 34 square degrees. Using photometric variability as our main selection criterion, as well as follow-up spectroscopy, we confirmed 168 previously unidentified pre-main sequence stars that are about 0.6 to 0.9 times the mass of the sun ($M_\\odot$), with ages of about 1 million to 3 million years (Ori OB1b) and about 3 million to 10 million years (Ori OB 1a). The low-mass stars are spatially coincident with the high-mass (at least$3\\ M_\\odot$) members of the associations. Indicators of disk accretion such as Hα emission and near-infrared emission from dusty disks fall sharply from Ori OB1b to Ori OB1a, indicating that the time scale for disk dissipation and possibly the onset of planet formation is a few million years.

Think inside the envelope The accretion by a protoplanetary disk of material from its surrounding natal envelope has been observed for the first time in the Class 0 protostar NGC 1333–IRAS 4B. This is a crucial early step in the formation of stars and planetary systems, through which all such systems are thought to go. Observations with the Spitzer Space Telescope reveal a rich emission-line mid-infrared spectrum from water vapour, which indicates an origin in an extremely dense disk surface, heated by a shock from the infalling envelope material. Once a protoplanetary disk has formed, planetesimals are thought to develop as the products of collisions between dust grains form ever larger objects. But current theories fail at the point where metre-sized boulders are formed: theory has them falling into the central protostar too quickly to form kilometre-sized planetesimals. New computer simulations suggest that the interaction of the gas disk with the boulders creates extremely dense regions. There the boulders are so close to each other that their mutual gravity draws them together into solid objects of many kilometres in size, forming directly the planetesimals that serve as building blocks of planets. The youngest protostellar objects show many signs of rapid development from their initial, spheroidal configurations. Watson et al. find in NGC1333 — IRAS4B a rich emission spectrum of H 2 O, at wavelengths 20–37 mm, which indicates an origin in extremely dense, warm gas. They model the emission as infall from a protostellar envelope onto the surface of a deeply embedded, dense disc. This is the only example in a sample of 30 class 0 objects, perhaps arising from a favourable orientation or this may be an early and short-lived stage in the evolution of a protoplanetary disk. Class 0 protostars, the youngest type of young stellar objects, show many signs of rapid development from their initial, spheroidal configurations, and therefore are studied intensively for details of the formation of protoplanetary disks within protostellar envelopes. At millimetre wavelengths, kinematic signatures of collapse have been observed in several such protostars, through observations of molecular lines that probe their outer envelopes. It has been suggested that one or more components of the proto-multiple system NGC 1333–IRAS 4 (refs 1 , 2 ) may display signs of an embedded region that is warmer and denser than the bulk of the envelope 3 , 4 . Here we report observations that reveal details of the core on Solar System dimensions. We detect in NGC 1333–IRAS 4B a rich emission spectrum of H 2 O, at wavelengths 20–37 μm, which indicates an origin in extremely dense, warm gas. We can model the emission as infall from a protostellar envelope onto the surface of a deeply embedded, dense disk, and therefore see the development of a protoplanetary disk. This is the only example of mid-infrared water emission from a sample of 30 class 0 objects, perhaps arising from a favourable orientation; alternatively, this may be an early and short-lived stage in the evolution of a protoplanetary disk.