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Register a new userThe structure of the NGC1333-IRAS2 protostellar system on 500 AU scales: an infalling envelope, a circumstellar disk, multiple outflows, and chemistry
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J. K. Jorgensen
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This paper investigates small-scale structures of dense gas and dust around the low-mass protostellar binary NGC1333-IRAS2 using millimeter-wavelength aperture-synthesis observations from the OVRO and BIMA interferometers. The detected 3 mm continuum emission is consistent with models of the envelope around IRAS2A, based on previously reported submillimeter continuum images, down to the 3, or 500 AU, resolution of the interferometer data. Our data constrain the contribution of an unresolved point source to 22 mJy. Within the accuracy of the parameters describing the envelope model, the point source flux has an uncertainty by up to 25%. We interpret this point source as a cold disk of mass gtrsim 0.3 M_odot. The same envelope model also reproduces aperture-synthesis line observations of the optically thin isotopic species C34S and H13CO+. The more optically thick main isotope lines show a variety of components in the protostellar environment: N2H+ is closely correlated with dust concentrations as seen at submillimeter wavelengths and is particularly strong toward the starless core IRAS2C. We hypothesize that N2H+ is destroyed through reactions with CO that is released from icy grains near the protostellar sources IRAS2A and B. CS, HCO+, and HCN have complex line shapes apparently affected by both outflow and infall. In addition to the east-west jet from IRAS2A, a north-south velocity gradient near this source indicates a second, perpendicular outflow. This suggests the presence of a binary companion within 0.3 (65 AU) from IRAS2A as driving source of this outflow. Alternative explanations of the velocity gradient such as rotation in a circumstellar envelope or a single, wide-angle outflow are less likely.
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We present a radiative transfer model, which is applicable to the study of submillimetre spectral line observations of protostellar envelopes. The model uses an exact, non-LTE, spherically symmetric radiative transfer `Stenholm method, which numerically solves the radiative transfer problem by the process of `Lambda-iteration. We also present submillimetre spectral line data of the Class 0 protostars NGC1333-IRAS2 and Serpens SMM4. We examine the physical constraints which can be used to limit the number and range of parameters used in protostellar envelope models, and identify the turbulent velocity and tracer molecule abundance as the principle sources of uncertainty in the radiative transfer modelling. We explore the trends in the appearance of the predicted line profiles as key parameters in the models are varied. We find that the separation of the two peaks of a typical infall profile is dependent not on the evolutionary status of the collapsing protostar, but on the turbulent velocity dispersion in the envelope. We also find that the line shapes can be significantly altered by rotation. Fits are found for the observed line profiles of IRAS2 and SMM4 using plausible infall model parameters. The density and velocity profiles in our best fit models are inconsistent with a singular isothermal sphere model. We find better agreement with a form of collapse which assumes non-static initial conditions. We also find some evidence that the infall velocities are retarded from free-fall towards the centre of the cloud, probably by rotation, and that the envelope of SMM4 is rotationally flattened.
Context: The protostellar envelopes, outflow and large-scale chemistry of Class~0 and Class~I objects have been well-studied, but while previous works have hinted at or found a few Keplerian disks at the Class~0 stage, it remains to be seen if their presence in this early stage is the norm. Likewise, while complex organics have been detected toward some Class~0 objects, their distribution is unknown as they could reside in the hottest parts of the envelope, in the emerging disk itself or in other components of the protostellar system, such as shocked regions related to outflows. Aims: In this work, we aim to address two related issues regarding protostars: when rotationally supported disks form around deeply embedded protostars and where complex organic molecules reside in such objects. Methods: We observed the deeply embedded protostar, L483, using Atacama Large Millimeter/submillimeter Array (ALMA) Band~7 data from Cycles~1 and 3 with a high angular resolution down to $sim$~0.1$^{primeprime}$ (20~au) scales. Results: We find that the kinematics of CS~$J=7$--$6$ and H$^{13}$CN~$J=4$--$3$ are best fitted by the velocity profile from infall under conservation of angular momentum and not by a Keplerian profile. The spatial extents of the observed complex organics are consistent with an estimated ice sublimation radius of the envelope at $sim$~50~au, suggesting that the complex organics exist in the hot corino of L483. Conclusions: We find that L483 does not harbor a Keplerian disk down to at least $15$~au in radius. Instead, the innermost regions of L483 are undergoing a rotating collapse. This result highlights that some Class~0 objects contain only very small disks, or none at all, with the complex organic chemistry taking place on scales inside the hot corino of the envelope, in a region larger than the emerging disk.
We review recent advances in our understanding of the innermost regions of the circumstellar environment around young stars, made possible by the technique of long baseline interferometry at infrared wavelengths. Near-infrared observations directly probe the location of the hottest dust. The characteristic sizes found are much larger than previously thought, and strongly correlate with the luminosity of the central young stars. This relation has motivated in part a new class of models of the inner disk structure. The first mid-infrared observations have probed disk emission over a larger range of scales, and spectrally resolved interferometry has for the first time revealed mineralogy gradients in the disk. These new measurements provide crucial information on the structure and physical properties of young circumstellar disks, as initial conditions for planet formation.
We present the first observations of the circumstellar (CS) disk system 51 Ophiuchi with the Far Ultraviolet Spectroscopic Explorer. We detect several absorption lines arising from the unusual metastable atomic species NI (2D), NI (2P), and SII (2D). These levels lie 1.8 - 3.6 eV above the ground level and have radiative decay lifetimes of 2 days or less, indicating that the lines arise from warm CS gas. The high S/N FUSE spectra, obtained six days apart, also show time-variable absorption features arising from NI, NII, OI (1D), and FeIII, which are redshifted with respect to the stellar velocity. The resolved redshifted absorption extends over many tens of km/s (40 for NI, 100 for NII, 65 for OI, and 84 for FeIII). We calculate column densities for all the variable infalling CS gasses, using the apparent optical depth method. The FeIII and NII infalling gasses must be produced through collisional ionization, and the ionization fraction of nitrogen suggests a gas temperature between 20000 and 34000 K. The infalling gas shows a peculiar, non-solar composition, with nitrogen and iron more abundant than carbon. We also set upper limits on the line-of-sight column densities of molecular hydrogen and carbon monoxide. These observations strengthen the connection between 51 Oph and the older debris-disk system Beta Pictoris, and indicate that there may be infalling planetesimals in the 51 Oph system.
We have resolved for the first time the radial and vertical structure of the almost edge-on envelope/disk system of the low-mass Class 0 protostar L1527. For that, we have used ALMA observations with a spatial resolution of 0.25$^{primeprime}$$times$0.13$^{primeprime}$ and 0.37$^{primeprime}$$times$0.23$^{primeprime}$ at 0.8 mm and 1.2 mm, respectively. The L1527 dust continuum emission has a deconvolved size of 78 au $times$ 21 au, and shows a flared disk-like structure. A thin infalling-rotating envelope is seen in the CCH emission outward of about 150 au, and its thickness is increased by a factor of 2 inward of it. This radius lies between the centrifugal radius (200 au) and the centrifugal barrier of the infalling-rotating envelope (100 au). The gas stagnates in front of the centrifugal barrier and moves toward vertical directions. SO emission is concentrated around and inside the centrifugal barrier. The rotation speed of the SO emitting gas is found to be decelerated around the centrifugal barrier. A part of the angular momentum could be extracted by the gas which moves away from the mid-plane around the centrifugal barrier. If this is the case, the centrifugal barrier would be related to the launching mechanism of low velocity outflows, such as disk winds.
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