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Register a new userDisk wind feedback from high-mass protostars
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We perform a sequence of 3D magnetohydrodynamic (MHD) simulations of the outflow-core interaction for a massive protostar forming via collapse of an initial cloud core of $60~{M_odot}$. This allows us to characterize the properties of disk wind driven outflows from massive protostars, which can allow testing of different massive star formation theories. It also enables us to assess quantitatively the impact of outflow feedback on protostellar core morphology and overall star formation efficiency. We find that the opening angle of the flow increases with increasing protostellar mass, in agreement with a simple semi-analytic model. Once the protostar reaches $sim24~{M_odot}$ the outflows opening angle is so wide that it has blown away most of the envelope, thereby nearly ending its own accretion. We thus find an overall star formation efficiency of $sim50%$, similar to that expected from low-mass protostellar cores. Our simulation results therefore indicate that the MHD disk wind outflow is the dominant feedback mechanism for helping to shape the stellar initial mass function from a given prestellar core mass function.
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Star-formation feedback onto the parent cloud is conventionally examined through the study of molecular outflows. Little is however known on the effect that atomic ejecta, tracing fast shocks, can have on the global cloud properties. In this study we employ Herschel/PACS [OI] and [CII] spectral line maps of the NGC 1333 star-forming region to assess the relative influence of atomic jets onto the star-formation process. Atomic line maps are compared against molecular outflow tracers and atomic ejecta are associated to individual driving sources. We study the detailed morphology and velocity distribution of [OI] line using channel and line-centroid maps and derive the momentum, energy, and mass flux for all the bipolar jets traced by [OI] line emission. We find that the line-centroid maps can trace velocity structures down to 5 km s$^{-1}$ which is a factor of $sim$20 beyond the nominal velocity resolution reached by Herschel/PACS. These maps reveal an unprecedented degree of details that assist significantly in the association and characterization of jets and outflows. Comparisons of the dynamical and kinematical properties shows that [OI] momentum accounts for only $sim$1% of the momentum carried by the large scale CO outflows but the energy released through the jets corresponds to 50 - 100% of the energy released in outflows. The estimated ratios of the jet to the outflow momenta and energies are consistent with the results of two-component, nested jet/outflow simulations, where jets are associated to episodic accretion events. Under this scenario, the energy from atomic jets to the cloud is as important as the energy output from outflows in maintaining turbulence and dissipating the cloud gas.
Due to instrumental limitations and a lack of disk detections, the structure between the envelope and the rotationally supported disk has been poorly studied. This is now possible with ALMA through observations of CO isotopologs and tracers of freezeout. Class 0 sources are ideal for such studies given their almost intact envelope and young disk. The structure of the disk-envelope interface of the prototypical Class 0 source, VLA1623A which has a confirmed Keplerian disk, is constrained from ALMA observations of DCO+ 3-2 and C18O 2-1. The physical structure of VLA1623 is obtained from the large-scale SED and continuum radiative transfer. An analytic model using a simple network coupled with radial density and temperature profiles is used as input for a 2D line radiative transfer calculation for comparison with the ALMA Cycle 0 12m array and Cycle 2 ACA observations of VLA1623. DCO+ emission shows a clumpy structure bordering VLA1623As Keplerian disk, suggesting a cold ring-like structure at the disk-envelope interface. The radial position of the observed DCO+ peak is reproduced in our model only if the regions temperature is between 11-16K, lower than expected from models constrained by continuum and SED. Altering the density has little effect on the DCO+ position, but increased density is needed to reproduce the disk traced in C18O. The DCO+ emission around VLA1623A is the product of shadowing of the envelope by the disk. Disk-shadowing causes a drop in the gas temperature outside of the disk on >200AU scales, encouraging deuterated molecule production. This indicates that the physical structure of the disk-envelope interface differs from the rest of the envelope, highlighting the drastic impact that the disk has on the envelope and temperature structure. The results presented here show that DCO+ is an excellent cold temperature tracer.
Aims. Young stars interact vigorously with their surroundings, as evident from the highly rotationally excited CO (up to Eup=4000 K) and H2O emission (up to 600 K) detected by the Herschel Space Observatory in embedded low-mass protostars. Our aim is to construct a model that reproduces the observations quantitatively, to investigate the origin of the emission, and to use the lines as probes of the various heating mechanisms. Methods. The model consists of a spherical envelope with a bipolar outflow cavity. Three heating mechanisms are considered: passive heating by the protostellar luminosity, UV irradiation of the outflow cavity walls, and C-type shocks along the cavity walls. Line fluxes are calculated for CO and H2O and compared to Herschel data and complementary ground-based data for the protostars NGC1333 IRAS2A, HH 46 and DK Cha. The three sources are selected to span a range of evolutionary phases and physical characteristics. Results. The passively heated gas in the envelope accounts for 3-10% of the CO luminosity summed over all rotational lines up to J=40-39; it is best probed by low-J CO isotopologue lines such as C18O 2-1 and 3-2. The UV-heated gas and the C-type shocks, probed by 12CO 10-9 and higher-J lines, contribute 20-80% each. The model fits show a tentative evolutionary trend: the CO emission is dominated by shocks in the youngest source and by UV-heated gas in the oldest one. This trend is mainly driven by the lower envelope density in more evolved sources. The total H2O line luminosity in all cases is dominated by shocks (>99%). The exact percentages for both species are uncertain by at least a factor of 2 due to uncertainties in the gas temperature as function of the incident UV flux. However, on a qualitative level, both UV-heated gas and C-type shocks are needed to reproduce the emission in far-infrared rotational lines of CO and H2O.
Large scale spectral maps of star forming regions enable the comparative study of the gas excitation around an ensemble of sources at a common frame of reference, providing direct insights in the multitude of processes involved. In this paper we employ spectral-line maps to decipher the excitation, the kinematical and dynamical processes in NGC 1333 as revealed by a number of different emission lines, aiming to set a reference for the applicability of tracers in constraining diverse physical processes. We reconstruct line maps for H$_2$ , CO, H$_2$O and C$^+$ using data obtained with the Spitzer-IRS and Herschel HIFI-SPIRE. We compare the morphological features of the maps and derive the gas excitation for regions of interest employing LTE and non-LTE methods. We also calculate the kinematical and dynamical properties for each outflow tracer consistently for all outflows in NGC 1333. We finally measure the water abundance in outflows with respect to carbon monoxide and molecular hydrogen. CO and H$_2$ are highly excited around B-stars and at lower levels trace protostellar outflows. H$_2$O emission is dominated by a moderately fast component associated with outflows. Intermediate J CO lines appear brightest at the locations traced by a narrow H$_2$O component, indicating that beyond the dominating collisional processes, a secondary, radiative excitation component can also be active. The morphology, kinematics, excitation and abundance variations of water are consistent with its excitation and partial dissociation in shocks. Water abundance ranges between 5 x 10$^{-7}$ and 10$^{-5}$, with the lower values being more representative. Water is brightest and most abundant around IRAS 4A which is consistent with the latter hosting a hot corino source. Finally, the outflow mass flux is found highest for CO and decreases by one and two orders of magnitude for H$_2$ and H$_2$O, respectively.
Complex organic molecules (COMs) have been observed towards several low-mass young stellar objects (LYSOs). Small and heterogeneous samples have so far precluded conclusions on typical COM abundances, as well as the origin(s) of abundance variations between sources. We present observations towards 16 deeply embedded (Class 0/I) low-mass protostars using the IRAM 30m telescope. We detect CH$_2$CO, CH$_3$CHO, CH$_3$OCH$_3$, CH$_3$OCHO, CH$_3$CN, HNCO, and HC$_3$N towards 67%, 37%, 13%, 13%, 44%, 81%, and 75% of sources respectively. Median column densities derived using survival analysis range between 6.0x10$^{10}$ cm$^{-2}$ (CH$_3$CN) and 2.4x10$^{12}$ cm$^{-2}$ (CH$_3$OCH$_3$) and median abundances range between 0.48% (CH$_3$CN) and 16% (HNCO) with respect to CH$_3$OH. Column densities for each molecule vary by about one order of magnitude across the sample. Abundances with respect to CH$_3$OH are more narrowly distributed, especially for oxygen-bearing species. We compare observed median abundances with a chemical model for low-mass protostars and find fair agreement, although some modeling work remains to bring abundances higher with respect to CH$_3$OH. Median abundances with respect to CH$_3$OH in LYSOs are also found to be generally comparable to observed abundances in hot cores, hot corinos, and massive young stellar objects. Compared with comets, our sample is comparable for all molecules except HC$_3$N and CH$_2$CO, which likely become depleted at later evolutionary stages.
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