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Register a new userMolecular jets from low-mass young protostellar objects
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C.-F. Lee
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Molecular jets are seen coming from the youngest protostars in the early phase of low-mass star formation. They are detected in CO, SiO, and SO at (sub)millimeter wavelengths down to the innermost regions, where their associated protostars and accretion disks are deeply embedded and where they are launched and collimated. They are not only the fossil records of accretion history of the protostars but also are expected to play an important role in facilitating the accretion process. Studying their physical properties (e.g., mass-loss rate, velocity, rotation, radius, wiggle, molecular content, shock formation, periodical variation, magnetic field, etc) allows us to probe not only the jet launching and collimation, but also the disk accretion and evolution, and potentially binary formation and planetary formation in the disks. Here I review recent exciting results obtained with high-spatial and high-velocity resolution observations of molecular jets in comparison to those obtained in the optical jets in the later phase of star formation. Future observations of molecular jets with a large sample at high spatial and velocity resolution with ALMA are expected to lead to a breakthrough in our understanding of jets from young stars.
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Jets and outflows are ubiquitous in the process of formation of stars since outflow is intimately associated with accretion. Free-free (thermal) radio continuum emission is associated with these jets. This emission is relatively weak and compact, and sensitive radio interferometers are required to study it. Observations in the cm range are most useful to trace the base of the ionized jets, close to the central protostar, where optical or near-IR imaging is made difficult by the high extinction present. Radio recombination lines in jets (in combination with proper motions) should provide their 3D kinematics at very small scale. Future instruments such as the Square Kilometre Array (SKA) and the Next Generation Very Large Array (ngVLA) will be crucial to perform this kind of sensitive observations. Thermal jets are associated with both high and low mass protostars and possibly even with substellar objects. The ionizing mechanism of these radio jets appears to be related to shocks in the associated outflows, as suggested by the observed correlation between the cm luminosity and the outflow momentum rate. Some protostellar jets show indications of non-thermal emission in their lobes. Linearly polarized synchrotron emission has been found in the jet of HH 80-81, allowing one to map the jet magnetic field, a key ingredient to determine the collimation and ejection mechanisms. As only a fraction of the emission is polarized, very sensitive observations such as those that will be feasible with the interferometers previously mentioned are required to perform studies in a large sample of sources. Jets are present in many kinds of astrophysical scenarios. Characterizing radio jets in young stars, where thermal emission allows one to determine their physical conditions, would also be useful in understanding acceleration and collimation mechanisms in all kinds of astrophysical jets.
We are carrying out multi-frequency radio continuum observations, using the Australia Telescope Compact Array, to systematically search for collimated ionized jets towards high-mass young stellar objects (HMYSOs). Here we report observations at 1.4, 2.4, 4.8 and 8.6 GHz, made with angular resolutions of about 7, 4, 2, and 1 arcsec, respectively, towards six objects of a sample of 33 southern HMYSOs thought to be in very early stages of evolution. The objects in the sample were selected from radio and infrared catalogs by having positive radio spectral indices and being luminous (L_bol > 20,000 L_sun), but underluminous in radio emission compared to that expected from its bolometric luminosity. This criteria makes the radio sources good candidates for being ionized jets. As part of this systematic search, two ionized jets have been discovered: one previously published and the other reported here. The rest of the observed candidates correspond to three hypercompact hii regions and two ultracompact hii regions. The two jets discovered are associated with two of the most luminous (70,000 and 100,000 Lsun) HMYSOs known to harbor this type of objects, showing that the phenomena of collimated ionized winds appears in the formation process of stars at least up to masses of ~ 20 M_sun and provides strong evidence for a disk-mediated accretion scenario for the formation of high-mass stars. From the incidence of jets in our sample, we estimate that the jet phase in high-mass protostars lasts for 40,000 yr.
We review some aspects of the bipolar molecular outflow phenomenon. In particular, we compare the morphological properties, energetics and velocity structures of outflows from high and low-mass protostars and investigate to what extent a common source model can explain outflows from sources of very different luminosities. Many flow properties, in particular the CO spatial and velocity structure, are broadly similar across the entire luminosity range, although the evidence for jet-entrainment is still less clear cut in massive flows than in low-mass systems. We use the correlation of flow momentum deposition rate with source luminosity to estimate the ratio f of mass ejection to mass accretion rate. From this analysis, it appears that a common driving mechanism could operate across the entire luminosity range. However, we stress that for the high-mass YSOs, the detailed physics of this mechanism and how the ejected wind/jet entrains ambient material remain to be addressed. We also briefly consider the alternative possibility that high-mass outflows can be explained by the recently proposed circulation models, and discuss several shortcomings of those models. Finally, we survey the current evidence on the nature of the shocks driven by YSOs during their pre-main-sequence evolution.
We present a study of hierarchical structure in the Perseus molecular cloud, from the scale of the entire cloud ($gtrsim$10 pc) to smaller clumps ($sim$1 pc), cores ($sim$0.05-0.1 pc), envelopes ($sim$300-3000 AU) and protostellar objects ($sim$15 AU). We use new observations from the Submillimeter Array (SMA) large project Mass Assembly of Stellar Systems and their Evolution with the SMA (MASSES) to probe the envelopes, and recent single-dish and interferometric observations from the literature for the remaining scales. This is the first study to analyze hierarchical structure over five scales in the same cloud complex. We compare the number of fragments with the number of Jeans masses in each scale to calculate the Jeans efficiency, or the ratio of observed to expected number of fragments. The velocity dispersion is assumed to arise either from purely thermal motions, or from combined thermal and non-thermal motions inferred from observed spectral line widths. For each scale, thermal Jeans fragmentation predicts more fragments than observed, corresponding to inefficient thermal Jeans fragmentation. For the smallest scale, thermal plus non-thermal Jeans fragmentation also predicts too many protostellar objects. However at each of the larger scales thermal plus non-thermal Jeans fragmentation predicts fewer than one fragment, corresponding to no fragmentation into envelopes, cores, and clumps. Over all scales, the results are inconsistent with complete Jeans fragmentation based on either thermal or thermal plus non-thermal motions. They are more nearly consistent with inefficient thermal Jeans fragmentation, where the thermal Jeans efficiency increases from the largest to the smallest scale.
Radio continuum observations using the Australia telescope compact array at 5.5, 9.0, 17.0 and 22.8 GHz have detected free-free emission associated with 45 of 49 massive young stellar objects and HII regions. Of these, 26 sources are classified as ionized jets (12 of which are candidates), 2 as ambiguous jets or disc winds, 1 as a disc-wind, 14 as HII regions and 2 were unable to be categorised. Classification as ionized jets is based upon morphology, radio flux and spectral index, in conjunction with previous observational results at other wavelengths. Radio-luminosity and momentum are found to scale with bolometric luminosity in the same way as low-mass jets, indicating a common mechanism for jet production across all masses. In 13 of the jets, we see associated non-thermal/optically-thin lobes resulting from shocks either internal to the jet and/or at working surfaces. Ten jets display non-thermal (synchrotron emission) spectra in their lobes, with an average spectral index of -0.55 consistent with Fermi acceleration in shocks. This shows that magnetic fields are present, in agreement with models of jet formation incorporating magnetic fields. Since the production of collimated radio jets is associated with accretion processes, the results presented in this paper support the picture of disc-mediated accretion for the formation of massive stars with an upper-limit on the jet phase lasting approximately $6.5 times 10^4 yr$. Typical mass loss rates in the jet are found to be $1.4 times 10^{-5} M_odot yr^{-1}$ with associated momentum rates of the order $(1-2) times 10^{-2} M_odot km s^{-1} yr^{-1}$.
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