The First Detection of a Low-frequency Turnover in Nonthermal Emission from the Jet of a Young Star

Feeney-Johansson, A.; Purser, S. J. D.; Ray, T. P.; Eislöffel, J.; Hoeft, M.; Drabent, A.; Ainsworth, Rachael

doi:10.3847/2041-8213/ab4b56

Cited by 15 publications

(19 citation statements)

References 39 publications

(57 reference statements)

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“…The detection of such a low frequency turn-over would be important for an increased understanding of the source structure and, possibly, its formation. Feeney-Johansson et al (2019) established that a low-frequency turn-over is present in one, and possibly both, of the synchrotron emission regions in DG Tau. The implications for the strength of the magnetic field depend on the mechanism causing the turn-over.…”

Section: Introductionmentioning

confidence: 95%

“…It was shown in the previous section that observation of the synchrotron self-absorption frequency provides an estimate of the emitting surface (equation 15). In addition to synchrotron self-absorption, the most likely mechanisms to cause a low frequency turn-over are free-free absorption and the Razin effect (Feeney-Johansson et al 2019). The aim in this section is to deduce constraints on the size of the synchrotron source that can be obtained from either of these mechanisms.…”

Section: Implications Of a Low Frequency Turn-over In A Synchrotron S...mentioning

confidence: 99%

“…The implications for the strength of the magnetic field depend on the mechanism causing the turn-over. Synchrotron self-absorption is a possibility; however, as argued by Feeney-Johansson et al (2019), a more likely cause is the thermal plasma present in the outflow. In the latter case, either free-free absorption or the Razin effect is suggested.…”

Section: Introductionmentioning

confidence: 98%

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Implications of the low frequency turn-over in the spectrum of radio knot C in DG Tau

Bjornsson

2021

Preprint

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The synchrotron spectrum of radio knot C in the protostellar object DG Tau has a low frequency turn-over. This is used to show that its magnetic field strength is likely to be at least 10 mG, which is roughly two orders of magnitude larger than previously estimated. The earlier, lower value is due to an overestimate of the emission volume together with an omission of the dependence of the minimum magnetic field on the synchrotron spectral index. Since the source is partially resolved, this implies a low volume filling factor for the synchrotron emission. It is argued that the high pressure needed to account for the observations is due to shocks. In addition, cooling of the thermal gas is probably necessary in order to further enhance the magnetic field strength as well as the density of relativistic electrons. It is suggested that the observed spectral index implies that the energy of the radio emitting electrons is below that needed to take part in first order Fermi acceleration. Hence, the radio emission gives insights to the properties of its pre-acceleration phase. Attention is also drawn to the similarities between the properties of radio knot C and the shock induced radio emission in supernovae.

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Section: Introductionmentioning

confidence: 95%

Section: Implications Of a Low Frequency Turn-over In A Synchrotron S...mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 98%

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Implications of the low frequency turn-over in the spectrum of radio knot C in DG Tau

Bjornsson

2021

Preprint

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“…Due to inaccuracies in LOFAR beam models (Van Weeren et al 2016;Shimwell et al 2019), the derived flux density scale is often not accurate. To account for this, in Feeney-Johansson et al (2019) the integrated flux densities of several compact bright sources in the field were compared with their 1 https://github.com/lofar-astron/prefactor 2 https://github.com/lofar-astron/factor values in the TIFR GMRT Sky Survey (TGSS; Intema et al 2017). It was found that the average ratio of TGSS flux densities to LOFAR flux densities was 0.83 ± 0.17.…”

Section: Lofar Observationsmentioning

confidence: 99%

Detection of coherent low-frequency radio bursts from weak-line T Tauri stars

Feeney-Johansson

Purser

Ray

et al. 2021

A&A

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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 (1013 − 1014 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 (≥1014 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.

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“…Radiative shocks are not efficient particle accelerators, however, there is evidence of nonthermal emission. Synchrotron radio emission has been detected both in protostellar jets (e.g., Carrasco-González et al 2010;Feeney-Johansson et al 2019) and novae (e.g., Vlasov et al 2016), whereas gamma-rays have only been detected in the latter (Ackermann et al 2014a). Accelerating synchrotron radio emitting electrons is possible for almost any kind of shock.…”

Section: Introductionmentioning

confidence: 99%

Adiabatic-radiative shock systems in YSO jets and novae outflows

del Valle,

Araudo,

Suzuki-Vidal

2022

Preprint

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Context. The termination regions of non-relativistic jets in protostars and supersonic outflows in classical novae are nonthermal emitters. This has been confirmed by radio and gamma-ray detection, respectively. A two-shock system is expected to be formed in the termination region where the jet/outflow material and the ambient medium impact. Given the high densities in these systems, radiative shocks are expected to form. However, in the presence of high velocities, the formation of adiabatic shocks is also possible. A case of interest is when the two types of shocks occur simultaneously. Adiabatic shocks are more efficient at particle acceleration while radiative shocks strongly compress the gas. Furthermore, a combined adiabatic-radiative shock system is very prone to develop instabilities in the contact discontinuity leading to mixing, turbulence and density enhancement. Additionally, these dense non-relativistic jets/outflows are excellent candidates for laboratory experiments as demonstrated by magnetohydrodynamics scaling. Aims. We aim at studying the combination of adiabatic and radiative shocks in protostellar jets and novae outflows. We focus on determining the conditions under which this combination is feasible together with its physical implications. Methods. We perform an analytical study of the shocks in both types of sources for a set of parameters by comparing cooling times and propagation velocities. We also estimate the timescales for the growth of instabilities in the contact discontinuity separating both shocks. The hydrodynamical evolution of a jet colliding with an ambient medium is studied with 2D numerical simulations confirming our initial theoretical estimates. Results. We show that for a wide set of observationally constrained parameters the combination of an adiabatic and a radiative shock is possible at the working surface of the termination region in jets from young stars and novae outflows. We find that instabilities are developed at the contact discontinuity, mixing the shocked materials. Additionally, we explore the magnetohydrodynamic parameter scaling required for studying protostellar jets and novae outflows using laboratory experiments on laser facilities. Conclusions. The coexistence of an adiabatic and a radiative shock is expected at the termination region of protostellar jets and novae outflows. This scenario is very promising for particle acceleration and gamma-ray emission. The parameters for scaled laboratory experiments are very much in line with plasma conditions achievable in currently operating high-power laser facilities. This opens the door to new means for studying novae outflows never considered before.

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The First Detection of a Low-frequency Turnover in Nonthermal Emission from the Jet of a Young Star

Cited by 15 publications

References 39 publications

Implications of the low frequency turn-over in the spectrum of radio knot C in DG Tau

Implications of the low frequency turn-over in the spectrum of radio knot C in DG Tau

Detection of coherent low-frequency radio bursts from weak-line T Tauri stars

Adiabatic-radiative shock systems in YSO jets and novae outflows

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