We present a multiplicity study of all known protostars (94) in the Perseus molecular cloud from a Karl G. Jansky Very Large Array (VLA) survey at Ka-band (8 mm and 1 cm) and C-band (4 cm and 6.6 cm). The observed sample has a bolometric luminosity range between 0.1 L and ∼33 L , with a median of 0.7 L . This multiplicity study is based on the Ka-band data, having a best resolution of ∼0. 065 (15 AU) and separations out to ∼43 (10000 AU) can be probed. The overall multiplicity fraction (MF) is found to be of 0.40±0.06 and the companion star fraction (CSF) is 0.71±0.06. The MF and CSF of the Class 0 protostars are 0.57±0.09 and 1.2±0.2, and the MF and CSF of Class I protostars are both 0.23±0.08. The distribution of companion separations appears bimodal, with a peak at ∼75 AU and another peak at ∼3000 AU. Turbulent fragmentation is likely the dominant mechanism on >1000 AU scales and disk fragmentation is likely to be the dominant mechanism on <200 AU scales. Toward three Class 0 sources we find companions separated by <30 AU. These systems have the smallest separations of currently known Class 0 protostellar binary systems. Moreover, these close systems are embedded within larger (50 AU to 400 AU) structures and may be candidates for ongoing disk fragmentation.

We present a high angular resolution millimeter-wave dust continuum imaging survey of circumstellar material associated with the individual components of 23 multiple star systems in the Taurus-Auriga young cluster. Combined with previous measurements in the literature, these new data permit a comprehensive look at how the millimeter luminosity (a rough tracer of disk mass) relates to the separation and mass of a stellar companion. Approximately one third (28-37%) of the individual stars in multiple systems have detectable millimeter emission, an incidence rate half that for single stars (∼62%) which does not depend on the number of companions. There is a strong, positive correlation between the luminosity and projected separation (a p ) of a stellar pair. Wide pairs (a p > 300 AU) have a similar luminosity distribution as single stars, medium pairs (a p ≈ 30-300 AU) are a factor of 5 fainter, and close pairs (a p < 30 AU) are ∼5× fainter yet (aside from a small, but notable population of bright circumbinary disks). In most cases, the emission is dominated by a disk around the primary (or a wide tertiary in hierarchical triples), but there is no clear relationship between luminosity and stellar mass ratio. A direct comparison of resolved disk sizes with predictions from tidal truncation models yields mixed results; some disks are much larger than expected given the projected distances of their companions. We suggest that the presence of a stellar companion impacts disk properties at a level comparable to the internal evolution mechanisms that operate in an isolated system, with both the multiple star formation process itself and star-disk tidal interactions likely playing important roles in the evolution of circumstellar material. From the perspective of the mass content of the disk reservoir, we expect that (giant) planet formation is inhibited around the components of close pairs or secondaries, but should be as likely as for single stars around the primaries (or wide tertiaries in hierarchical triples) in more widely-separated multiple star systems.

Binary and multiple star systems are a frequent outcome of the star formation process 1;2 , and as a result, almost half of all sun-like stars 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 >1000 AU 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 candidate to search for evidence of disk fragmentation. L1448 IRS3B is in an early phase of the star formation process, likely less than 150,000 years in age 14 , and all protostars in the system are separated by <200 AU. Here we report observations of dust and molecular gas emission that reveal a disk with spiral structure surrounding the three protostars. Two protostars near the center of the disk are separated by 61 AU, and a tertiary protostar is coincident with a spiral arm in the outer disk at a 183 AU separation 13 . The inferred mass of the central pair of protostellar objects is ⇠1 M , while the disk surrounding the three protostars has a total mass of ⇠0.30 M . The tertiary protostar itself has a minimum mass of ⇠0.085 M . We demonstrate that the disk around L1448 IRS3B appears susceptible to disk fragmentation at radii between 150 AU and 320 AU, 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.L1448 IRS3B is located in the Perseus molecular cloud at a distance of ⇠230 pc 15 and contains three protostars out of the six that collectively make up L1448 IRS3 13;14 , spanning 0.05 pc. L1448 IRS3B is a Class 0 protostar system 16 , which signifies an early phase of the star formation process when the protostars are deeply enshrouded in an envelope of accreting material 17 . The three protostars in L1448 IRS3B (denoted -a, -b, and -c) have a hierarchical configuration; the central-most protostar, IRS3B-a, has projected separations from IRS3B-b and IRS3B-c of 61 AU and 183 AU, respectively 13 . The new observations of L1448 IRS3B conducted with the Atacama Large Millimeter/submillimeter Array (ALMA) at a resolution of 0. 00 27⇥0. 00 16 (62 AU ⇥ 37 AU) provide images at 1.3 mm of the dust and gas emission surrounding the three protostars with 10⇥ higher sensitivity and 2⇥ higher resolution than previous studies.The ALMA 1.3 mm image of L1448 IRS3B is shown in Figure 1, revealing dust emission toward each of the three distinct protostars identified in previous Karl G. Jansky Very Large Array (VLA) observations 13 . The ALMA images also reveal a disk with substructure sur...

We present dust continuum observations of the protoplanetary disk surrounding the pre-main sequence star AS 209, spanning more than an order of magnitude in wavelength from 0.88 to 9.8 mm. The disk was observed with sub-arcsecond angular resolution (0.2 ′′ − 0.5 ′′ ) to investigate radial variations in its dust properties. At longer wavelengths, the disk emission structure is notably more compact, providing model-independent evidence for changes in the grain properties across the disk. We find that physical models which reproduce the disk emission require a radial dependence of the dust opacity κ ν . Assuming that the observed wavelength-dependent structure can be attributed to radial variations in the dust opacity spectral index (β), we find that β(R) increases from β < 0.5 at ∼ 20 AU to β > 1.5 for R 80 AU, inconsistent with a constant value of β across the disk (at the 10σ level). Furthermore, if radial variations of κ ν are caused by particle growth, we find that the maximum size of the particle-size distribution (a max ) increases from sub-millimeter-sized grains in the outer disk (R 70 AU) to millimeter and centimeter-sized grains in the inner disk regions (R 70 AU). We compare our observational constraint on a max (R) with predictions from physical models of dust evolution in proto-planetary disks. For the dust composition and particle-size distribution investigated here, our observational constraints on a max (R) are consistent with models where the maximum grain size is limited by radial drift.

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.Spiral density waves are expected to be excited in the midplane of protoplanetary disks by the action of gravitational forces, generated for example by planet-disk interactions (1) or by gravitational instabilities (2). These waves give rise to spiral structure whose observable characteristics the number and location of arms, their amplitudes and pitch angles depend on the driving mechanism and the disk physical properties (1,3-5). Theoretical predictions agree that these spiral features can be very prominent and thus more easily observable than the putative embedded planets or instabilities driving such waves (6,7). Spiral-like patterns have been observed in evolved protoplanetary disks with depleted inner regions, in optical scattered light (8-13) or gas spectral lines (14,15). However, at the wavelength of such observations the emission is optically thick and scattered light only traces the tenuous surface layers of these disks rather than their midplane densities. This makes it impossible to disentangle between minute perturbations near the disk surface and true density enhancements over the disk column density due to spiral density waves (16,5). To probe the disk density structure, particularly the disk midplane that contains most of the mass and where planets form, observations of optically thin emission are necessary.We used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the protoplanetary disk around the young star Elias 2-27 at a wavelength of 1.3 mm. Our spatially resolved image (Fig. 1) shows two symmetric spiral arms extending from an elliptical emission ring. To emphasize the spirals and the dark ring of attenuated emission seen at ≈ 70 AU radius, we applied an unsharp masking filter (17) to increase significantly the image contrast (Fig. 1B).The young star Elias 2-27 (18) is a member of the ρ-Ophiuchus star-forming complex at a distance of 139 pc (19) and is classified as a Class II young stellar object from analysis of its spectral energy distribution (SED,20,21). Although the star is only 50-60% of the Sun's mass (M ) (20,22) it is known to harbor an unusually massive (0.04-0.14 M , 20,23,24) protoplanetary disk. The star, obscured by 15 magnitudes of extinction at optical wavelengths by the parent molecular clou...