IRS5 Research Articles

A high HDO/H2O ratio in the Class I protostar L1551 IRS5

Context. Water is a very abundant molecule in star-forming regions. Its deuterium fractionation provides an important tool for understanding its formation and evolution during the star and planet formation processes. While the HDO/H2O abundance ratio has been determined toward several young Class 0 protostars and comets, the number of studies toward Class I protostars is limited. Aims. Our aim is to study the water deuteration toward the Class I protostellar binary L1551 IRS5 and to investigate the effect of evolutionary stage and environment on variations in the water D/H ratio. Methods. Observations were carried out toward L1551 IRS5 using the NOrthern Extended Millimeter Array (NOEMA) interferometer. The HDO 31, 2–22, 1 transition at 225.9 GHz and the H218O 31, 3–22, 0 transition at 203.4 GHz were covered with a spatial resolution of 0.5″× 0.8″, while the HDO 42, 2–42, 3 transition at 143.7 GHz was observed with a resolution of 2.0″ × 2.5″. We constrained the water D/H ratio using both local thermodynamic equilibrium (LTE) and non-LTE models. Results. The three transitions are detected. The line profiles display two peaks, one at ∼6 km s−1 and one at ∼9 km s−1. We derive an HDO/H2O ratio of (2.1 ± 0.8) × 10−3 for the redshifted component and a lower limit of > 0.3 × 10−3 for the blueshifted component. This lower limit is due to the blending of the blueshifted H218O component with redshifted CH3OCH3 emission. Conclusions. The HDO/H2O ratio in L1551 IRS5 is similar to the values in Class 0 isolated sources and in the disk of the Class I protostar V883 Ori, while it is significantly higher than in the previously studied clustered Class 0 sources and the comets. This result suggests that the chemistry of protostars that belong to molecular clouds with relatively low source densities, such as L1551, share more similarities with the isolated sources than the protostars of very dense clusters. If the HDO/H2O ratios in Class 0 protostars with few sources around are comparable to those found to date in isolated Class 0 objects, it would mean that there is little water reprocessing from the Class 0 to Class I protostellar stage.

Formation of Dust Clumps with Sub-Jupiter Mass and Cold Shadowed Region in Gravitationally Unstable Disk around Class 0/I Protostar in L1527 IRS

We have investigated the protostellar disk around a Class 0/I protostar, L1527 IRS, using multiwavelength observations of the dust continuum emission at λ = 0.87, 2.1, 3.3, and 6.8 mm, obtained by the Atacama Large Millimeter/submillimeter Array and the Jansky Very Large Array (VLA). Our observations achieved a spatial resolution of 3–13 au and revealed an edge-on disk structure with a size of ∼80–100 au. The emission at 0.87 and 2.1 mm is found to be optically thick, within a projected disk radius of r proj ≲ 50 au. The emission at 3.3 and 6.8 mm shows that the power-law index of the dust opacity (β) is β ∼ 1.7 around r proj ∼ 50 au, suggesting that grain growth has not yet begun. The dust temperature (T dust) shows a steep decrease with T dust ∝ r proj −2 outside the VLA clumps previously identified at r proj ∼ 20 au. Furthermore, the disk is gravitationally unstable at r proj ∼ 20 au, as indicated by a Toomre Q parameter value of Q ≲ 1.0. These results suggest that the VLA clumps are formed via gravitational instability, which creates a shadow on the outside of the substructure, resulting in the sudden drop in temperature. The derived dust masses for the VLA clumps are ≳0.1 M J. Thus, we suggest that Class 0/I disks can be massive enough to be gravitationally unstable, which may be the origin of gas giant planets in a 20 au radius. Furthermore, the protostellar disks could be cold due to shadowing.

Supersonic Expansion of the Bipolar H ii Region Sh2-106: A 3500 Year Old Explosion?

Abstract Multi-epoch narrowband Hubble Space Telescope images of the bipolar H ii region Sh2-106 reveal highly supersonic nebular proper motions that increase with projected distance from the massive young stellar object S106 IR, reaching over ∼30 mas yr−1 (∼150 km s−1 at D = 1.09 kpc) at a projected separation of ∼1.′4 (0.44 pc) from S106 IR. We propose that S106 IR experienced a ∼1047 erg explosion ∼3500 yr ago. The explosion may be the result of a major accretion burst or a recent encounter with another star, or a consequence of the interaction of a companion with the bloated photosphere of S106 IR as it grew from ∼10 through ∼15 M ⊙ at a high accretion rate. Near-IR images reveal fingers of H2 emission pointing away from S106 IR and an asymmetric photon-dominated region surrounding the ionized nebula. Radio continuum and Brγ emission reveal a C-shaped bend in the plasma, indicating either the motion of S106 IR toward the east, or the deflection of plasma toward the west by the surrounding cloud. The H ii region bends around a ∼1′ diameter dark bay west of S106 IR that may be shielded from direct illumination by a dense molecular clump. Herbig–Haro and Molecular Hydrogen Objects tracing outflows powered by stars in the Sh2-106 protocluster such as the Class 0 source S106 FIR are discussed.

Atomic Shocks in the Outflow of L1551 IRS 5 Identified with SOFIA-upGREAT Observations of [O i

Abstract We present velocity-resolved Stratospheric Observatory for Infrared Astronomy (SOFIA)/upgrade German REceiver for Astronomy at Terahertz Frequencies observations of [O i] and [C ii] lines toward a Class I protostar, L1551 IRS 5, and its outflows. The SOFIA observations detect [O i] emission toward only the protostar and [C ii] emission toward the protostar and the redshifted outflow. The [O i] emission has a width of ∼100 km s−1 only in the blueshifted velocity, suggesting an origin in shocked gas. The [C ii] lines are narrow, consistent with an origin in a photodissociation region. Differential dust extinction from the envelope due to the inclination of the outflows is the most likely cause of the missing redshifted [O i] emission. Fitting the [O i] line profile with two Gaussian components, we find one component at the source velocity with a width of ∼20 km s−1 and another extremely broad component at −30 km s−1 with a width of 87.5 km s−1, the latter of which has not been seen in L1551 IRS 5. The kinematics of these two components resemble cavity shocks in molecular outflows and spot shocks in jets. Radiative transfer calculations of the [O i], high-J CO, and H2O lines in the cavity shocks indicate that [O i] dominates the oxygen budget, making up more than 70% of the total gaseous oxygen abundance and suggesting [O]/[H] of ∼1.5 × 10−4. Attributing the extremely broad [O i] component to atomic winds, we estimate an intrinsic mass-loss rate of (1.3 ± 0.8) × 10−6 M ⊙ yr−1. The intrinsic mass-loss rates derived from low-J CO, [O i], and H i are similar, supporting the model of momentum-conserving outflows, where the atomic wind carries most momentum and drives the molecular outflows.

Evolution of the atomic component in protostellar outflows

Context. We present SOFIA/FIFI-LS observations of three Class 0 and one Class I outflows (Cep E, HH 1, HH 212, and L1551 IRS5) in the far-infrared [O I]63 μm and [O I]145 μm transitions. Spectroscopic [O I]63 μm maps enabled us to infer the spatial extent of warm (T ∼ 500−1200 K), low-excitation atomic gas within these protostellar outflows. Aims. Our main goal is to determine mass-loss rates from the obtained [OI]63 μm maps and compare these with accretion rates from other studies. Methods. The far-infrared [O I]63 μm emission line is predicted to be the main coolant of dense, dissociative J-shocks caused by decelerated wind or jet shocks. If proper shock conditions prevail, the instantaneous mass-ejection rate is directly connected to the [O I]63 μm luminosity. In order to unravel evolutionary trends, we analysed a set of 14 Class 0/I outflow sources that were spatially resolved in the [O I]63 emission. We compared these data with a sample of 72 Class 0/I/II outflow sources that have been observed with Herschel (WISH, DIGIT, WILL, GASPS surveys) without spatially resolving the [O I]63 μm line. Results. All our newly observed targets feature prominent [O I]63μm emission either close to the driving source (L1551 IRS5, HH 1, HH 212) or as extended jet-like or knotty emission region away from it (Cep E). The detected [O I]63 μm emission can mostly be attributed to dissociative shocks and photodissociation regions (PDRs). Flux values at 63 μm and 145 μm of all four associated continuum sources are presented. We calculated mass-loss rates connected to the low-excitation, atomic outflow component in the range of (5−50)×10−7 M⊙ yr−1. Estimated ratios between the mass loss in the outflow and the mass accretion onto the source (jet efficiency ratios) are largely in the range of Ṁout/Ṁacc ∼ 0.05 − 0.5 for the observed outflow sources, which are consistent with theoretical predictions and quoted Herschel data. Conclusions. Our new observations and a comparison with the 72 outflow sources observed with Herschel indicate that the bulk ejected material in outflows from Class 0 sources resides in the molecular component, that is mass-loss rates derived from the [O I]63 emission line significantly underestimate the total mass-loss rate during this and possibly also later phases of the star formation process.

FAUST I. The hot corino at the heart of the prototypical Class I protostar L1551 IRS5

ABSTRACT The study of hot corinos in solar-like protostars has been so far mostly limited to the Class 0 phase, hampering our understanding of their origin and evolution. In addition, recent evidence suggests that planet formation starts already during Class I phase, which therefore represents a crucial step in the future planetary system chemical composition. Hence, the study of hot corinos in Class I protostars has become of paramount importance. Here, we report the discovery of a hot corino towards the prototypical Class I protostar L1551 IRS5, obtained within the ALMA (Atacama Large Millimeter/submillimeter Array) Large Program FAUST (Fifty AU STudy of the chemistry in the disc/envelope system of solar-like protostars). We detected several lines from methanol and its isotopologues (13CH3OH and CH2DOH), methyl formate, and ethanol. Lines are bright towards the north component of the IRS5 binary system, and a possible second hot corino may be associated with the south component. The methanol lines' non-LTE analysis constrains the gas temperature (∼100 K), density (≥1.5 × 108 cm−3), and emitting size (∼10 au in radius). All CH3OH and 13CH3OH lines are optically thick, preventing a reliable measure of the deuteration. The methyl formate and ethanol relative abundances are compatible with those measured in Class 0 hot corinos. Thus, based on this work, little chemical evolution from Class 0 to I hot corinos occurs.

Circumbinary Disks of the Protostellar Binary Systems in the L1551 Region

Abstract We report Atacama Large Millimeter/submillimeter Array Cycle 4 observations of the Class I binary protostellar system L1551 IRS 5 in the 0.9 mm continuum emission, C18O (J = 3–2), OCS (J = 28–27), and four other Band 7 lines. At ∼0.″07 (=10 au) resolution in the 0.9 mm emission, two circumstellar disks (CSDs) associated with the binary protostars are separated from the circumbinary disk (CBD). The CBD is resolved into two spiral arms, one connecting to the CSD around the northern binary source, Source N, and the other to Source S. As compared to the CBD in the neighboring protobinary system L1551 NE, the CBD in L1551 IRS 5 is more compact (r ∼ 150 au) and the m = 1 mode of the spirals found in L1551 NE is less obvious in L1551 IRS 5. Furthermore, the dust and molecular-line brightness temperatures of CSDs and CBD reach >260 and >100 K, respectively, in L1551 IRS 5, much hotter than those in L1551 NE. The gas motions in the spiral arms are characterized by rotation and expansion. Furthermore, the transitions from the CBD to the CSD rotations at around the L2 and L3 Lagrangian points and gas motions around the L1 point are identified. Our numerical simulations reproduce the observed two spiral arms and expanding gas motion as a result of gravitational torques from the binary, transitions from the CBD to the CSD rotations, and the gas motion around the L1 point. The higher temperature in L1551 IRS 5 likely reflects the inferred FU Ori event.

Resolved ALMA Continuum Image of the Circumbinary Ring and Circumstellar Disks in the L1551 IRS 5 System

Abstract L1551 IRS 5 is a FUor-like object located in the Taurus star-forming region. We present Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm continuum observations using a wide range of baselines. The observations recovered the two circumstellar disks comprising the system and, for the first time, resolved the circumbinary ring. We determined the geometry and estimated lower mass limits for the circumstellar disks using simple models. We calculated lower limits for the total mass of both circumstellar disks. After subtracting the two circumstellar disk models from the image, the residuals show a clearly resolved circumbinary ring. Using a radiative transfer model, we show that geometrical effects can explain some of the brightness asymmetries found in the ring. The remaining features are interpreted as enhancements in the dust density.

Anatomy of the massive star-forming region S106

The central area (40″ × 40″) of the bipolar nebula S106 was mapped in the [O I] line at 63.2 μm (4.74 THz) with high angular (6″) and spectral (0.24 MHz) resolution, using the GREAT heterodyne receiver on board SOFIA. The spatial and spectral emission distribution of [O I] is compared to emission in the CO 16 →15, [C II] 158 μm, and CO 11 →10 lines, mm-molecular lines, and continuum. The [O I] emission is composed of several velocity components in the range from –30 to 25 km s−1. The high-velocity blue- and red-shifted emission (v = −30 to –9 km s−1 and 8 to 25 km s−1) can be explained as arising from accelerated photodissociated gas associated with a dark lane close to the massive binary system S106 IR, and from shocks caused by the stellar wind and/or a disk–envelope interaction. At velocities from –9 to –4 km s−1 and from 0.5 to 8 km s−1 line wings are observed in most of the lines that we attribute to cooling in photodissociation regions (PDRs) created by the ionizing radiation impinging on the cavity walls. The velocity range from –4 to 0.5 km s−1 is dominated by emission from the clumpy molecular cloud, and the [O I], [C II], and high-J CO lines are excited in PDRs on clump surfaces that are illuminated by the central stars. Modelling the line emission in the different velocity ranges with the KOSMA-τ code constrains a radiation field χ of a few times 104 and densities n of a few times 104 cm−3. Considering self-absorption of the [O I] line results in higher densities (up to 106 cm−3) only for the gas component seen at high blue- and red velocities. We thus confirm the scenario found in other studies that the emission of these lines can be explained by a two-phase PDR, but attribute the high-density gas to the high-velocity component only. The dark lane has a mass of ~275 M⊙ and shows a velocity difference of ~1.4 km s−1 along its projected length of ~1 pc, determined from H13CO+ 1 →0 mapping. Its nature depends on the geometry and can be interpreted as a massive accretion flow (infall rate of ~2.5 × 10−4 M⊙ yr−1), or the remains of it, linked to S106 IR/FIR. The most likely explanation is that the binary system is at a stage of its evolution where gas accretion is counteracted by the stellar winds and radiation, leading to the very complex observed spatial and kinematic emission distribution of the various tracers.

The origin of rotation profiles in star-forming clouds

Angular momentum distribution and its redistribution are of crucial importance in formation and evolution of circumstellar disks. Many molecular line observations toward young stellar objects indicate that radial distributions of the specific angular momentum $j$ are more or less universal. In small scales, typically $R\lesssim$ 100 AU, the specific angular momenta distribute like $j\propto r^{1/2}$, indicating existence of rotationally supported disk. In outer regions, $R\gtrsim$ 5000 AU, $j$ increases as the radius increases and the slope is steeper than unity, which is supposed to reflect the original angular momentum distributions in the maternal molecular clouds. And lastly there is a connecting region, 100 AU $\lesssim R \lesssim$ 5000 AU, in which $j$-distribution looks almost flat. While this is often interpreted as a consequence of conservation of the specific angular momentum, it actually is insufficient and requires a stronger condition that the initial distribution of $j$ must be spatially uniform. However, this requirement is unrealistic and inconsistent with observations. In this work, we propose a simple alternative explanation; the flat $j$ profile is produced by strong prolongation due to the large velocity gradient in the accreting flow no matter what the initial $j$ distribution is. We provide a simple analytic model for gravitational collapse of molecular clouds. This model can be used to estimate ages of protostars based solely on the observed rotation profile. We demonstrate its validity in comparison with hydrodynamic simulations, and apply the model to young stellar objects such as L1527 IRS, TMC-1A and B335.

A GMRT survey of regions towards the Taurus molecular cloud at 323 and 608 MHz

We present observations of three active sites of star formation in the Taurus Molecular Cloud complex taken at 323 and 608 MHz (90 and 50 cm, respectively) with the Giant Metrewave Radio Telescope (GMRT). Three pointings were observed as part of a pathfinder project, targeted at the young stellar objects (YSOs) L1551 IRS 5, T Tau and DG Tau (the results for these target sources were presented in a previous paper). In this paper, we search for other YSOs and present a survey comprising of all three fields; a by-product of the large instantaneous field of view of the GMRT. The resolution of the survey is of order 10 arcsec and the best rms noise at the centre of each pointing is of order $100\,\mu$Jy beam$^{-1}$ at 323 MHz and $50\,\mu$Jy beam$^{-1}$ at 608 MHz. We present a catalogue of 1815 and 687 field sources detected above $5\,\sigma_{\rm rms}$ at 323 and 608 MHz, respectively. A total of 440 sources were detected at both frequencies, corresponding to a total unique source count of 2062 sources. We compare the results with previous surveys and showcase a sample of extended extragalactic objects. Although no further YSOs were detected in addition to the target YSOs based on our source finding criteria, these data can be useful for targeted manual searches, studies of radio galaxies or to assist in the calibration of future observations with the Low Frequency Array (LOFAR) towards these regions.

GMRT detections of low-mass young stars at 323 and 608 MHz

We present the results of a pathfinder project conducted with the Giant Metrewave Radio Telescope (GMRT) to investigate protostellar systems at low radio frequencies. The goal of these investigations is to locate the break in the free-free spectrum where the optical depth equals unity in order to constrain physical parameters of these systems, such as the mass of the ionised gas surrounding these young stars. We detect all three target sources, L1551 IRS 5 (Class I), T Tau and DG Tau (Class II), at frequencies 323 and 608 MHz (wavelengths 90 and 50 cm, respectively). These are the first detections of low mass young stellar objects (YSOs) at such low frequencies. We combine these new GMRT data with archival information to construct the spectral energy distributions for each system and find a continuation of the optically thin free-free spectra extrapolated from higher radio frequencies to 323 MHz for each target. We use these results to place limits on the masses of the ionised gas and average electron densities associated with these young systems on scales of ~1000 au. Future observations with higher angular resolution at lower frequencies are required to constrain these physical parameters further.

SOFIA/FORCAST OBSERVATIONS OF WARM DUST IN S106: A FRAGMENTED ENVIRONMENT

We present mid-IR (19 - 37 microns) imaging observations of S106 from SOFIA/FORCAST, complemented with IR observations from Spitzer/IRAC (3.6 - 8.0 microns), IRTF/MIRLIN (11.3 and 12.5 microns), and Herschel/PACS (70 and 160 microns). We use these observations, observations in the literature, and radiation transfer modeling to study the heating and composition of the warm (~ 100 K) dust in the region. The dust is heated radiatively by the source S106 IR, with little contributions from grain-electron collisions and Ly-alpha radiation. The dust luminosity is >~ (9.02 +/- 1.01) x 10^4 L_sun, consistent with heating by a mid- to late-type O star. We find a temperature gradient (~ 75 - 107 K) in the lobes, which is consistent with a dusty equatorial geometry around S106 IR. Furthermore, the SOFIA observations resolve several cool (~ 65 - 70 K) lanes and pockets of warmer (~ 75 - 90 K) dust in the ionization shadow, indicating that the environment is fragmented. We model the dust mass as a composition of amorphous silicates, amorphous carbon, big grains, very small grains, and PAHs. We present the relative abundances of each grain component for several locations in S106.

SMA and ALMA studies of protoplanetary disk formation around low-mass protostars

AbstractWe report our systematic survey observations of protostellar sources with the SubMillimeter Array (SMA) and Atacama Large Millimeter/submillimeter Array (ALMA). The purpose of our survey is to investigate formation mechanism of protoplanetary disks, precursors of planetary systems, out of ~1000 AU-scale protostellar envelopes surrounding the protostars. We found that in the early protostars (B335, NGC1333 IRAS 4B), the envelopes do not show significant rotating motions but infalling motions toward the central protostars. In more evolved protostars (L1527 IRS, L1448-mm, L1551 IRS 5), the envelopes are infalling and rotating with the conserved specific angular momenta (that is, vrot ∝ r−1). In most evolved sources (L1551 NE, TMC-1A, L1489 IRS) large-scale (≳100 AU) disks in Keplerian rotation or protoplanetary disks are evident. These results demonstrate a systematic evolutionary trend of envelope gas motions toward the disk formation.

FIRST DETECTION OF [C I] 3 P 1 – 3 P 0 EMISSION FROM A PROTOPLANETARY DISK

We performed single point [C I] $^3$P$_1$-$^3$P$_0$ and CO J=4-3 observations toward three T Tauri stars, DM Tau, LkCa 15, and TW Hya, using the Atacama Large Millimeter/submillimeter Array (ALMA) Band 8 qualification model receiver installed on the Atacama Submillimeter Telescope Experiment (ASTE). Two protostars in the Taurus L1551 region, L1551 IRS 5 and HL Tau, were also observed. We successfully detected [C I] emission from the protoplanetary disk around DM Tau as well as the protostellar targets. The spectral profile of the [C I] emission from the protoplanetary disk is marginally single-peaked, suggesting that atomic carbon (C) extends toward the outermost disk. The detected [C I] emission is optically thin and the column densities of C are estimated to be <~10$^{16}$ cm$^{-2}$ and ~10$^{17}$ cm$^{-2}$ for the T Tauri star targets and the protostars, respectively. We found a clear difference in the total mass ratio of C to dust, $M$(C)/$M$(dust), between the T Tauri stars and protostellar targets; the $M$(C)/$M$(dust) ratio of the T Tauri stars is one order of magnitude smaller than that of the protostars. The decrease of the estimated $M$(C)/$M$(dust) ratios for the disk sources is consistent with a theoretical prediction that the atomic C can survive only in the near surface layer of the disk and C$^+$/C/CO transition occurs deeper into the disk midplane.

Establishment of model of serum-caused damage to pulmonary microvascular endothelial cells of mice with renal ischemia-reperfusion injury

Objective To establish the model of serum-caused damage to pulmonary microvascular endothelial cells (PMVECs) of mice with renal ischemia-reperfusion (I/R) injury. Methods Mice PMVECs were cultured to measure the standard trans-endothelial electrical resistance (TER) in the monolayer of PMVECs. When PMVECs were cultured and arranged in compact monolayer and TER was achieved, they were divided into 4 groups (n = 3 each) using a random number table: serum of normal mice group (NS group) and different concentrations (5%, 10% and 20%) of serum of mice with renal I/R injury groups (IRS5 group, IRS10group and IRS20 group). The PMVECs were cultured for 1 h in the serum-free endothelial culture medium. The 0.8 and 0.2 ml culture medium containing 20% serum of normal mice were then added to the upper and lower chambers, respectively, in group NS. The 0.8 and 0.2 ml culture medium containing 5%, 10% and 20% serum of mice with renal I/R injury were then added to the upper and lower chambers in IRS5, IRS10 and IRS20 groups, respectively.100 μg/ml FITC-BSA 100 μl was added to the upper chamber in the four groups. At 3, 6, 9, 12, 15, 18, 21 and 24 h of incubation, the PMVEC monolayer permeability (apparent permeability coefficient, Pa) was detected. Results Compared with NS group, the Pa was significantly increased at 12 and 15 h of incubation in IRS5 group, and the Pa was increased at 6-24 h of incubation in IRS10 and IRS20 groups. Compared with IRS5 group, the Pa at 21 and 24 h in IRS10 group and at 9-24 h in IRS20 group were significantly increased. Conclusion Both 10% and 20% serum of mice with renal I/R injury can successfully establish the model of damage to PMVECs, and 20% serum causes a more severe damage. Key words: Reperfusion injury; Kidney; Serum; Endothelial cells

FORMATION OF A KEPLERIAN DISK IN THE INFALLING ENVELOPE AROUND L1527 IRS: TRANSFORMATION FROM INFALLING MOTIONS TO KEPLER MOTIONS

We report Atacama Large Millimeter/submillimeter Array (ALMA) cycle 0 observations of C$^{18}$O ($J=2-1$), SO ($J_N= 6_5-5_4$) and 1.3mm dust continuum toward L1527 IRS, a class 0 solar-type protostar surrounded by an infalling and rotating envelope. C$^{18}$O emission shows strong redshifted absorption against the bright continuum emission associated with L1527 IRS, strongly suggesting infall motions in the C$^{18}$O envelope. The C$^{18}$O envelope also rotates with a velocity mostly proportional to $r^{-1}$, where $r$ is the radius, while the rotation profile at the innermost radius (54 AU) may be shallower than $r^{-1}$, suggestive of formation of a Keplerian disk around the central protostar of 0.3 Mo in dynamical mass. SO emission arising from the inner part of the C$^{18}$O envelope also shows rotation in the same direction as the C$^{18}$O envelope. The rotation is, however, rigid-body like which is very different from the differential rotation shown by C$^{18}$O. In order to explain the line profiles and the position-velocity (PV) diagrams of C$^{18}$O and SO observed, simple models composed of an infalling envelope surrounding a Keplerian disk of 54 AU in radius orbiting a star of 0.3 Mo are examined. It is found that in order to reproduce characteristic features of the observed line profiles and PV diagrams, the infall velocity in the model has to be smaller than the free-fall velocity yielded by a star of 0.3 Mo. Possible reasons for the reduced infall velocities are discussed.

ALMA OBSERVATIONS OF INFALLING FLOWS TOWARD THE KEPLERIAN DISK AROUND THE CLASS I PROTOSTAR L1489 IRS

We have conducted ALMA observations in the 1.3 mm continuum and 12CO (2–1), C18O (2–1), and SO (56–45) lines toward L1489 IRS, a Class I protostar surrounded by a Keplerian disk and an infalling envelope. The Keplerian disk is clearly identified in the 12CO and C18O emission, and its outer radius (∼700 AU) and mass (∼0.005 M☉) are comparable to those of disks around T Tauri stars. The protostellar mass is estimated to be 1.6 M☉ with the inclination angle of 66°. In addition to the Keplerian disk, there are blueshifted and redshifted off-axis protrusions seen in the C18O emission pointing toward the north and the south, respectively, adjunct to the middle part of the Keplerian disk. The shape and kinematics of these protrusions can be interpreted as streams of infalling flows with a conserved angular momentum following parabolic trajectories toward the Keplerian disk, and the mass infalling rate is estimated to be ∼5 × 10−7 M☉ yr−1. The specific angular momentum of the infalling flows (∼2.5 × 10−3 km s−1 pc) is comparable to that at the outer radius of the Keplerian disk (∼4.8 × 10−3 km s−1 pc). The SO emission is elongated along the disk major axis and exhibits a linear velocity gradient along the axis, which is interpreted to mean that the SO emission primarily traces a ring region in the flared Keplerian disk at radii of ∼250–390 AU. The local enhancement of the SO abundance in the ring region can be due to the accretion shocks at the centrifugal radius where the infalling flows fall onto the disk. Our ALMA observations unveiled both the Keplerian disk and the infalling gas onto the disk, and the disk can further grow by accreting material and angular momenta from the infalling gas.

UNVEILING THE EVOLUTIONARY SEQUENCE FROM INFALLING ENVELOPES TO KEPLERIAN DISKS AROUND LOW-MASS PROTOSTARS

We performed SMA observations in the C18O (2-1) emission line toward six Class 0 and I protostars, to study rotational motions of their surrounding envelopes and circumstellar material on 100 to 1000 AU scales. C18O (2-1) emission with intensity peaks located at the protostellar positions is detected toward all the six sources. The rotational velocities of the protostellar envelopes as a function of radius were measured from the Position-Velocity diagrams perpendicular to the outflow directions passing through the protostellar positions. Two Class 0 sources, B335 and NGC 1333 IRAS 4B, show no detectable rotational motion, while L1527 IRS (Class 0/I) and L1448-mm (Class 0) exhibit rotational motions with radial profiles of Vrot ~ r^{-1.0+/-0.2} and ~ r^{-1.0+/-0.1}, respectively. The other Class I sources, TMC-1A and L1489 IRS, exhibit the fastest rotational motions among the sample, and their rotational motions have flatter radial profiles of Vrot ~ r^{-0.6+/-0.1} and ~ r^{-0.5+/-0.1}, respectively. The rotational motions with the radial dependence of ~ r^{-1} can be interpreted as rotation with a conserved angular momentum in a dynamically infalling envelope, while those with the radial dependence of ~ r^{-0.5} can be interpreted as Keplerian rotation. These observational results demonstrate categorization of rotational motions from infalling envelopes to Keplerian-disk formation. Models of the inside-out collapse where the angular momentum is conserved are discussed and compared with our observational results.

A truly embryonic star

The discovery of what may be the best example yet of a forming star caught in the moments just before birth provides a missing link in our understanding of how giant gas clouds collapse to form fully fledged stars. See Letter p.83 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.