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 ...H ii 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 H ii regions and 2 were unable to be categorized. 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. 10 jets display non-thermal (synchrotron emission) spectra in their lobes, with an average spectral index of α = −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 × 104 yr. Typical mass-loss rates in the jet are found to be 1.4 × 10−5 M⊙ yr−1 with associated momentum rates of the order of (1–2) × 10−2 M⊙ km s−1 yr−1.
ABSTRACT
In conjunction with a previous southern-hemisphere work, we present the largest radio survey of jets from massive protostars to date with high-resolution (∼0.04 arcsec) Jansky Very Large ...Array observations towards two subsamples of massive star-forming regions of different evolutionary statuses: 48 infrared-bright, massive, young, stellar objects (MYSOs) and 8 infrared dark clouds (IRDCs) containing 16 luminous ($\hbox{$\, L_{\rm {bol}}$}\gt 10^3\hbox{${\rm \, L_{\odot }}$}$) cores. For 94 per cent of the MYSO sample, we detect thermal radio (α ≥ −0.1 whereby Sν ∝ να) sources coincident with the protostar, of which 84 per cent (13 jets and 25 candidates) are jet like. Radio luminosity is found to scale with $\hbox{$\, L_{\rm {bol}}$}$ similarly to the low-mass case supporting a common mechanism for jet production across all masses. Associated radio lobes tracing shocks are seen towards 52 per cent of jet-like objects and are preferentially detected towards jets of higher radio and bolometric luminosities, resulting from our sensitivity limitations. We find jet mass-loss rate scales with bolometric luminosity as $\hbox{$\dot{m}_{\rm jet}$}\propto \hbox{$\, L_{\rm {bol}}$}^{0.9\pm 0.2}$, thereby discarding radiative, line-driving mechanisms as the dominant jet-launching process. Calculated momenta show that the majority of jets are mechanically capable of driving the massive, molecular outflow phenomena since pjet > poutflow. Finally, from their physical extent we show that the radio emission cannot originate from small, optically thick H ii regions. Towards the IRDC cores, we observe increasing incidence rates/radio fluxes with age using the proxy of increasing luminosity-to-mass (L/M) and decreasing infrared flux ratios $({S_{70{\rm \, \mu m}}}/{S_{24{\rm \, \mu m}}})$. Cores with $({L}/{M})\,\lt \,40\, \hbox{${\rm \, L_{\odot }}$}\hbox{${\rm \, M_{\odot }}$}^{-1}$ are not detected above ($5.8{\rm \, GHz}$) radio luminosities of ${\sim}1{\rm \, mJy}{\rm \, kpc}^2$.
Abstract
Massive young stellar objects (MYSOs) have recently been shown to drive jets whose particles can interact with either the magnetic fields of the jet or ambient medium to emit non-thermal ...radiation. We report a search for non-thermal radio emission from a sample of 15 MYSOs to establish the prevalence of the emission in the objects. We used their spectra across the L, C, and Q bands along with spectral index maps to characterize their emission. We find that about 50 per cent of the sources show evidence for non-thermal emission with 40 per cent showing clear non-thermal lobes, especially sources of higher bolometric luminosity. The common or IRAS names of the sources that manifest non-thermal lobes are V645Cyg, IRAS 22134+5834, NGC 7538 IRS 9, IRAS 23262 + 640, AFGL 402d, and AFGL 490. All the central cores of the sources are thermal with corresponding mass-loss rates that lie in the range of ∼3 × 10−7 to $\mathrm{7 \times 10^{-6}\, M_\odot \, yr^{-1}}$. Given the presence of non-thermal lobes in some of the sources and the evidence of non-thermal emission from some spectral index maps, it seems that magnetic fields play a significant role in the jets of massive protostars. Also noted is that some of the sources show evidence of binarity and variability.
It is important to determine if massive stars form via disc accretion, like their low-mass counterparts. Theory and observation indicate that protostellar jets are a natural consequence of accretion ...discs and are likely to be crucial for removing angular momentum during the collapse. However, massive protostars are typically rarer, more distant and more dust enshrouded, making observational studies of their jets more challenging. A fundamental question is whether the degree of ionisation in jets is similar across the mass spectrum. Here we determine an ionisation fraction of ~5-12% in the jet from the massive protostar G35.20-0.74N, based on spatially coincident infrared and radio emission. This is similar to the values found in jets from lower-mass young stars, implying a unified mechanism of shock ionisation applies in jets across most of the protostellar mass spectrum, up to at least ~10 solar masses.
Abstract
Gravitational instabilities (GIs) are most likely a fundamental process during the early stages of protoplanetary disc formation. Recently, there have been detections of spiral features in ...young, embedded objects that appear consistent with GI-driven structure. It is crucial to perform hydrodynamic and radiative transfer simulations of gravitationally unstable discs in order to assess the validity of GIs in such objects, and constrain optimal targets for future observations. We utilize the radiative transfer code lime (Line modelling Engine) to produce continuum emission maps of a 0.17 M⊙ self-gravitating protosolar-like disc. We note the limitations of using lime as is and explore methods to improve upon the default gridding. We use casa to produce synthetic observations of 270 continuum emission maps generated across different frequencies, inclinations and dust opacities. We find that the spiral structure of our protosolar-like disc model is distinguishable across the majority of our parameter space after 1 h of observation, and is especially prominent at 230 GHz due to the favourable combination of angular resolution and sensitivity. Disc mass derived from the observations is sensitive to the assumed dust opacities and temperatures, and therefore can be underestimated by a factor of at least 30 at 850 GHz and 2.5 at 90 GHz. As a result, this effect could retrospectively validate GIs in discs previously thought not massive enough to be gravitationally unstable, which could have a significant impact on the understanding of the formation and evolution of protoplanetary discs.
In recent years, thanks to new facilities such as LOFAR that are capable of sensitive observations, much work has been done on the detection of stellar radio emission at low frequencies. Such ...emission has commonly been shown to be coherent emission, generally attributed to electron-cyclotron maser (ECM) emission, and has usually been detected from main-sequence M dwarfs. Here we report the first detection of coherent emission at low frequencies from T Tauri stars, which are known to be associated with high levels of stellar activity. Using LOFAR, we detect several bright radio bursts at 150 MHz from two weak-line T Tauri stars: KPNO-Tau 14 and LkCa 4. All of the bursts have high brightness temperatures (10
13
− 10
14
K) and high circular polarisation fractions (60–90%), indicating that they must be due to a coherent emission mechanism. This could be either plasma emission or ECM emission. Due to the exceptionally high brightness temperatures seen in at least one of the bursts (≥10
14
K), as well as the high circular polarisation levels, it seems unlikely that plasma emission could be the source; as such, ECM is favoured as the most likely emission mechanism. Assuming this is the case, the required magnetic field in the emission regions would be 40–70 G. We determine that the most likely method of generating ECM emission is plasma co-rotation breakdown in the stellar magnetosphere. There remains the possibility, however, that it could be due to an interaction with an orbiting exoplanet.
Abstract
We present sensitive (σ < 10 μJy beam− 1), radio continuum observations using the Australian Telescope Compact Array at frequencies of 6 and 9 GHz towards four massive young stellar objects ...(MYSOs). From a previous, less sensitive work, these objects are known to harbour ionized jets associated with radio lobes, which result from shock processes. In comparison with that work, further emission components are detected towards each MYSO. These include extended, direct, thermal emission from the ionized jet's stream, new radio lobes indicative of shocks close (<105 au) to the MYSO, three radio Herbig–Haro objects separated by up to 3.8 pc from the jet's launching site, and an IR-dark source coincident with CH3OH maser emission. No significant, integrated flux variability is detected towards any jets or shocked lobes, and only one proper motion is observed ($1806\pm 596{\rm {\rm \, km}{\rm \, s}^{-1}{\, }}$ parallel to the jet axis of G310.1420+00.7583A). Evidence for precession is detected in all four MYSOs with precession periods and angles within the ranges 66–15 480 yr and 6°–36°, respectively. Should precession be the result of the influence from a binary companion, we infer orbital radii of 30–1800 au.
ABSTRACT
We report the results of the Jansky Very Large Array (JVLA) observation of five massive protostars at 6 and 22.2 GHz. The aim of the study was to compare their current fluxes and positions ...with previous observations to search for evidence of variability. Most of the observed sources present the morphologies of a thermal core, hosting the protostar and exhibiting no proper motion, and associated non-thermal radio lobes that are characterized by proper motions and located away from the thermal core. Some of the protostars drive jets whose lobes have dissimilar displacement vectors, implying precession of the jets or the presence of multiple jet drivers. The jets of the protostars were found to have proper motions that lie in the range of 170 ≤ v ≤ 650 km s−1, and precessions of periods of 40 ≤ p ≤ 50 yr and angles of 2 ≤ α ≤ 10°, assuming that their velocities v = 500 km s−1. The core of one of the sources, S255 NIRS3, which was in outburst at the time of our observations, showed a significant change in flux compared to the other sources. Its spectral index decreased during the outburst, consistent with the model of an expanding gas bubble. Modelling the emission of the outburst as that of a new non-thermal lobe that is emerging from a thermal core whose emission enshrouds that of the lobe also has the potential to account for the increase in flux and a decrease in the spectral index of the source’s outburst.
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