We present 3 mm ALMA continuum and line observations at resolutions of 6.5 au and 13 au, respectively, toward the Class 0 system IRAS 16293-2422 A. The continuum observations reveal two compact ...sources toward IRAS 16293-2422 A, coinciding with compact ionized gas emission previously observed at radio wavelengths (A1 and A2), confirming the long-known radio sources as protostellar. The emission toward A2 is resolved and traces a dust disk with an FWHM size of ∼12 au, while the emission toward A1 sets a limit to the FWHM size of the dust disk of ∼4 au. We also detect spatially resolved molecular kinematic tracers near the protostellar disks. Several lines of the J = 5−4 rotational transition of HNCO, NH2CHO, and t-HCOOH are detected, with which we derived individual line-of-sight velocities. Using these together with the CS (J = 2−1), we fit Keplerian profiles toward the individual compact sources and derive masses of the central protostars. The kinematic analysis indicates that A1 and A2 are a bound binary system. Using this new context for the previous 30 yr of Very Large Array observations, we fit orbital parameters to the relative motion between A1 and A2 and find that the combined protostellar mass derived from the orbit is consistent with the masses derived from the gas kinematics. Both estimations indicate masses consistently higher (0.5 M1 M2 2 ) than previous estimations using lower-resolution observations of the gas kinematics. The ALMA high-resolution data provides a unique insight into the gas kinematics and masses of a young deeply embedded bound binary system.
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
. These ...structures are often interpreted as evidence of planet formation
, with planetary-mass bodies carving rings and gaps in the disk
. 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
. Only within the past decade have detailed properties of disks in the earliest star-forming phases been observed
. 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)
protostar IRS 63. IRS 63 is a single class I source located in the nearby Ophiuchus molecular cloud at a distance of 144 parsecs
, 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)
. 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
.
The determination of the specific angular momentum radial profile, j(r), in the early stages of star formation is crucial to constrain star and circumstellar disk formation theories. The specific ...angular momentum is directly related to the largest Keplerian disk possible, and it could constrain the angular momentum removal mechanism. We determine j(r) toward two Class 0 objects and a first hydrostatic core candidate in the Perseus cloud, which is consistent across all three sources and well fit with a single power-law relation between 800 and 10,000 au: . This power-law relation is in between solid body rotation (∝r2) and pure turbulence (∝r1.5). This strongly suggests that even at 1000 au, the influence of the dense core's initial level of turbulence or the connection between core and the molecular cloud is still present. The specific angular momentum at 10,000 au is 3× higher than previously estimated, while at 1000 au, it is lower by 2×. We do not find a region of conserved specific angular momentum, although it could still be present at a smaller radius. We estimate an upper limit to the largest Keplerian disk radius of 60 au, which is small but consistent with published upper limits. Finally, these results suggest that more realistic initial conditions for numerical simulations of disk formation are needed. Some possible solutions include: (a) using a larger simulation box to include some level of driven turbulence or connection to the parental cloud or (b) incorporating the observed j(r) to set up the dense core kinematics initial conditions.
We present the full disk-fit results VANDAM survey of all Class 0 and I protostars in the Perseus molecular cloud. We have 18 new protostellar disk candidates around Class 0 and I sources, which are ...well described by a simple, parametrized disk model fit to the 8 mm VLA dust continuum observations. 33% of Class 0 protostars and just 11% of Class I protostars have candidate disks, while 78% of Class 0 and I protostars do not have signs of disks within our 12 au disk diameter resolution limit, indicating that at 8 mm most disks in the Class 0 and I phases are <10 au in radius. These small radii may be a result of surface brightness sensitivity limits. Modeled 8 mm radii are similar to the radii of known Class 0 disks with detected Keplerian rotation. Since our 8 mm data trace a population of larger dust grains that radially drift toward the protostar and are lower limits on true disk sizes, large disks at early times do not seem to be particularly rare. We find statistical evidence that Class 0 and I disks are likely drawn from the same distribution, meaning disk properties may be defined early in the Class 0 phase and do not undergo large changes through the Class I phase. By combining our candidate disk properties with previous polarization observations, we find a qualitative indication that misalignment between inferred envelope-scale magnetic fields and outflows may indicate disks on smaller scales in Class 0 sources.
Abstract
Characterizing the physical conditions at disk scales in class 0 sources is crucial for constraining the protostellar accretion process and the initial conditions for planet formation. We ...use ALMA 1.3 and 3 mm observations to investigate the physical conditions of the dust around the class 0 binary IRAS 16293–2422 A down to ∼10 au scales. The circumbinary material’s spectral index,
α
, has a median of 3.1 and a dispersion of ∼0.2, providing no firm evidence of millimeter-sized grains therein. Continuum substructures with brightness temperature peaks of
T
b
∼ 60–80 K at 1.3 mm are observed near the disks at both wavelengths. These peaks do not overlap with strong variations of
α
, indicating that they trace high-temperature spots instead of regions with significant optical depth variations. The lower limits to the inferred dust temperature in the hot spots are 122, 87, and 49 K. Depending on the assumed dust opacity index, these values can be several times higher. They overlap with high gas temperatures and enhanced complex organic molecular emission. This newly resolved dust temperature distribution is in better agreement with the expectations from mechanical instead of the most commonly assumed radiative heating. In particular, we find that the temperatures agree with shock heating predictions. This evidence and recent studies highlighting accretion heating in class 0 disks suggest that mechanical heating (shocks, dissipation powered by accretion, etc.) is important during the early stages and should be considered when modeling and measuring properties of deeply embedded protostars and disks.
Abstract
High-resolution, millimeter observations of disks at the protoplanetary stage reveal substructures such as gaps, rings, arcs, spirals, and cavities. While many protoplanetary disks host such ...substructures, only a few at the younger protostellar stage have shown similar features. We present a detailed search for early disk substructures in Atacama Large Millimeter/submillimeter Array 1.3 and 0.87 mm observations of ten protostellar disks in the Ophiuchus star-forming region. Of this sample, four disks have identified substructure, two appear to be smooth disks, and four are considered ambiguous. The structured disks have wide Gaussian-like rings (
σ
R
/
R
disk
∼ 0.26) with low contrasts (
C
< 0.2) above a smooth disk profile, in comparison to protoplanetary disks where rings tend to be narrow and have a wide variety of contrasts (
σ
R
/
R
disk
∼ 0.08 and
C
ranges from 0 to 1). The four protostellar disks with the identified substructures are among the brightest sources in the Ophiuchus sample, in agreement with trends observed for protoplanetary disks. These observations indicate that substructures in protostellar disks may be common in brighter disks. The presence of substructures at the earliest stages suggests an early start for dust grain growth and, subsequently, planet formation. The evolution of these protostellar substructures is hypothesized in two potential pathways: (1) the rings are the sites of early planet formation, and the later observed protoplanetary disk ring–gap pairs are secondary features, or (2) the rings evolve over the disk lifetime to become those observed at the protoplanetary disk stage.
We present subarcsecond (~0".35) resolved observations of the 1.3 mm dust polarization from the edge-on circumstellar disk around the Class 0 protostar L1527. The inferred magnetic field is ...consistent with a dominantly toroidal morphology; there is no significantly detected vertical poloidal component to which observations of an edge-on disk are most sensitive. This suggests that angular momentum transport in Class 0 protostars (when large amounts of material are fed down to the disk from the envelope and accreted onto the protostar) is driven mainly by magnetorotational instability rather than magnetocentrifugal winds at 50 AU scales. In addition, with the data to date there is an early, tentative trend that R> 30 AU disks have so far been found in Class 0 systems with average magnetic fields on the 1000 AU scale strongly misaligned with the rotation axis. The absence of such a disk in the aligned case could be due to efficient magnetic braking that disrupts disk formation. If this is the case, this implies that candidate Class 0 disk systems could be identified by the average magnetic field direction at ~1000 AU spatial scales.
The Reservoir of the Per-emb-2 Streamer Taniguchi, Kotomi; Pineda, Jaime E.; Caselli, Paola ...
The Astrophysical journal,
04/2024, Letnik:
965, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract Streamers bring gas from outer regions to protostellar systems and could change the chemical composition around protostars and protoplanetary disks. We have carried out mapping observations ...of carbon-chain species (HC 3 N, HC 5 N, CCH, and CCS) in the 3 mm and 7 mm bands toward the streamer flowing to the Class 0 young stellar object (YSO) Per-emb-2 with the Nobeyama 45 m radio telescope. A region with a diameter of ∼0.04 pc is located to the north at a distance of ∼20,500 au from the YSO. The streamer connects to this northern region, which is its origin. The reservoir has high density and low temperature ( n H 2 ≈ 1.9 × 10 4 cm −3 , T kin = 10 K), which are similar to those of early-stage starless cores. By comparison of the observed abundance ratios of CCS/HC 3 N to the chemical simulations, the reservoir and streamer are found to be chemically young. The total mass available for the streamer is derived to be 24–34 M ⊙ . It has been estimated that, if all of the gas in the reservoir were to accrete onto the Per-emb-2 protostellar system, the lifetime of the streamer would be (1.1–3.2) × 10 5 yr, suggesting that the mass accretion via the streamer would continue until the end of the Class I stage.
ABSTRACT We present the first dust emission results toward a sample of seven protostellar disk candidates around Class 0 and I sources in the Perseus molecular cloud from the VLA Nascent Disk and ...Multiplicity (VANDAM) survey with ∼0 05 or 12 AU resolution. To examine the surface brightness profiles of these sources, we fit the Ka-band 8 mm dust-continuum data in the u, v-plane to a simple, parametrized model based on the Shakura-Sunyaev disk model. The candidate disks are well-fit by a model with a disk-shaped profile and have masses consistent with known Class 0 and I disks. The inner-disk surface densities of the VANDAM candidate disks have shallower density profiles compared to disks around more evolved Class II systems. The best-fit model radii of the seven early-result candidate disks are Rc > 10 AU; at 8 mm, the radii reflect lower limits on the disk size since dust continuum emission is tied to grain size and large grains radially drift inwards. These relatively large disks, if confirmed kinematically, are inconsistent with theoretical models where the disk size is limited by strong magnetic braking to <10 AU at early times.
Spectral lines of ammonia, NH3, are useful probes of the physical conditions in dense molecular cloud cores. In addition to advantages in spectroscopy, ammonia has also been suggested to be resistant ...to freezing onto grain surfaces, which should make it a superior tool for studying the interior parts of cold, dense cores. Here we present high-resolution NH3 observations with the Very Large Array and Green Bank Telescope toward a prestellar core. These observations show an outer region with a fractional NH3 abundance of X(NH3) = (1.975 ± 0.005) × 10−8 (±10% systematic), but it also reveals that, after all, the X(NH3) starts to decrease above a H2 column density of ≈2.6 × 1022 cm−2. We derive a density model for the core and find that the break point in the fractional abundance occurs at the density n(H2) ∼ 2 × 105 cm−3, and beyond this point the fractional abundance decreases with increasing density, following the power law n−1.1. This power-law behavior is well reproduced by chemical models where adsorption onto grains dominates the removal of ammonia and related species from the gas at high densities. We suggest that the break-point density changes from core to core depending on the temperature and the grain properties, but that the depletion power law is anyway likely to be close to n−1 owing to the dominance of accretion in the central parts of starless cores.
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