Simulating Radio Emission from Low-mass Stars

Llama, Joe; Jardine, M.; Wood, Kenneth; Hallinan, Gregg; Morin, J.

doi:10.3847/1538-4357/aaa59f

Cited by 23 publications

(20 citation statements)

References 80 publications

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“…This radio emission occurs via the Cyclotron Maser instability (see Treumann 2006), wherein energetic electrons spiral through planetary magnetic field lines towards polar regions of strong magnetic field. This emission process has also been observed for low-mass stars (Bingham et al 2001;Llama et al 2018). The process for planets is powered by the dissipation of the magnetic flux of the stellar wind on the magnetosphere of the planet.…”

Section: Introductionsupporting

confidence: 67%

MOVES – II. Tuning in to the radio environment of HD189733b

Kavanagh

Vidotto

Fionnagáin

et al. 2019

Monthly Notices of the Royal Astronomical Society

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We present stellar wind modelling of the hot Jupiter host HD189733, and predict radio emission from the stellar wind and the planet, the latter arising from the interaction of the stellar wind with the planetary magnetosphere. Our stellar wind models incorporate surface stellar magnetic field maps at the epochs as boundary conditions. We find that the mass-loss rate, angular momentumloss rate, and open magnetic flux of HD189733 vary by 9%, 40%, and 19% over these three epochs. Solving the equations of radiative transfer, we find that from 10 MHz-100 GHz the stellar wind emits fluxes in the range of 10 −3 -5 µJy, and becomes optically thin above 10 GHz. Our planetary radio emission model uses the radiometric Bode's law, and neglects the presence of a planetary atmosphere. For assumed planetary magnetic fields of 1-10 G, we estimate that the planet emits at frequencies of 2-25 MHz, with peak flux densities of ∼ 10 2 mJy. We find that the planet orbits through regions of the stellar wind that are optically thick to the emitted frequency from the planet. As a result, unattenuated planetary radio emission can only propagate out of the system and reach the observer for 67% of the orbit for a 10 G planetary field, corresponding to when the planet is approaching and leaving primary transit. We also find that the plasma frequency of the stellar wind is too high to allow propagation of the planetary radio emission below 21 MHz. This means a planetary field of at least 8 G is required to produce detectable radio emission.

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Section: Introductionsupporting

confidence: 67%

MOVES – II. Tuning in to the radio environment of HD189733b

Kavanagh

Vidotto

Fionnagáin

et al. 2019

Monthly Notices of the Royal Astronomical Society

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“…This is an important factor when considering M-dwarf stars, which are known to potentially have extremely strong magnetic fields of up to few kG (Reiners & Basri 2010), and are much more magnetically active than the Sun. These strong stellar fields could potentially produce strong coronal radio emissions (such as in V374 Peg, Llama et al 2018). We repeat the simulations with a stellar dipole field of 100G, and the comparison is shown in Figure 7.…”

Section: The Effect Of the Stellar Magnetic Field Strengthmentioning

confidence: 99%

“…See et al (2015) have used Zeeman-Doppler imaging (ZDI) maps to estimate the temporal variations of radio emissions from exoplanets directly from the magnetic maps, and using some empirical estimation for the radio power (assuming planetary auroral emissions). Llama et al (2018) presented a more detailed calculation of the coronal radio emissions from V374 Peg using potential field approximation, and hydrostatic coronal density (non-MHD solution). However, both of these studies did not include an actual planet in their simulations.…”

Section: Introductionmentioning

confidence: 99%

Exoplanet Modulation of Stellar Coronal Radio Emission

Cohen

Moschou

Glocer

et al. 2018

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The search for exoplanets in the radio bands has been focused on detecting radio emissions produced by the interaction between magnetized planets and the stellar wind (auroral emission). Here we introduce a new tool, which is part of our MHD stellar corona model, to predict the ambient coronal radio emission and its modulations induced by a close planet. For simplicity, the present work assumes that the exoplanet is stationary in the frame rotating with the stellar rotation. We explore the radio flux modulations using a limited parameter space of idealized cases by changing the magnitude of the planetary field, its polarity, the planetary orbital separation, and the strength of the stellar field. We find that the modulations induced by the planet could be significant and observable in the case of hot Jupiter planets -above 100% modulation with respect to the ambient flux in the 10 − 100 M Hz range in some cases, and 2-10% in the frequency bands above 250 M Hz for some cases. Thus, our work indicates that radio signature of exoplanets might not be limited to low-frequency radio range. We find that the intensity modulations are sensitive to the planetary magnetic field polarity for shortorbit planets, and to the stellar magnetic field strength for all cases. The new radio tool, when applied to real systems, could provide predictions for the frequency range at which the modulations can be observed by current facilities.

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“…Metodieva et al 2017;Lynch et al 2016;Williams et al 2015b;Berger et al 2005) or ECMI (e.g. Llama et al 2018;Schlieder et al 2014;Hallinan et al 2008) processes. In any case, the presence of this radio emission reveals that these objects can generate strong, axisymmetric and active magnetic fields, in contrast to the expectations set by early fully-convective dynamo theory (Durney et al 1993).…”

Section: Introductionmentioning

confidence: 99%

“…For radio emission from the ECMI, low-frequency observations can give insights into electron motions driving the ECMI (Yu et al 2012), as well as probing regions of lower magnetic field strength. Using an input magnetospheric structure, low-frequency observations can therefore be used to constrain the coronal volume where conditions are favourable for the ECMI to take place (Llama et al 2018;Jaeger et al 2011). For gyrosynchrotron emission, the high-frequency regime is typically characterised by a power-law SED, where it is difficult but not impossible to uniquely constrain coronal magnetic field strength, electron density, and other coronal parameters (Metodieva et al 2017;Lynch et al 2016Lynch et al , 2015Ravi et al 2011;Burgasser & Putman 2005).…”

Section: Introductionmentioning

confidence: 99%

Low-frequency GMRT observations of ultra-cool dwarfs

Zic

Lynch

Murphy

et al. 2018

Monthly Notices of the Royal Astronomical Society

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Observations of radio emission in about 10 per cent of ultra-cool dwarfs (UCDs) indicate the presence of strong, persistent magnetic fields in these stars. These results are in contrast to early theoretical expectations on fully-convective dynamos, and to other tracers of magnetic activity, such as H α and X-ray luminosity. Radio-frequency observations have been key to physically characterising UCD magnetospheres, although explaining the diverse behaviour within them remains challenging. Most radio-frequency studies of UCDs have been conducted in the 4-8 GHz band, where traditional radio interferometers are typically most sensitive. Hence, the nature of UCD radio emission at low frequencies ( 1.4 GHz) remains relatively unexplored, but can probe optically thick emission, and regions of lower magnetic field strengths -regimes not accessible to higher-frequency observations. In this work, we present the results from Giant Metrewave Radio Telescope observations of nine UCDs taken at ∼ 610 and 1300 MHz. These are the first observations of UCDs in this frequency range to be published in the literature. Using these observations, we are able to constrain the coronal magnetic field strength and electron number density of one of the targets to 1 B 90 G and 4 log(N e ) 10, respectively. We do not detect the flaring emission observed at higher frequencies, to a limit of a few millijanskys. These results show that some UCDs can produce low-frequency radio emission, and highlights the need for simultaneous multi-wavelength radio observations to tightly constrain the coronal and magnetospheric properties of these stars.

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Simulating Radio Emission from Low-mass Stars

Cited by 23 publications

References 80 publications

MOVES – II. Tuning in to the radio environment of HD189733b

MOVES – II. Tuning in to the radio environment of HD189733b

Exoplanet Modulation of Stellar Coronal Radio Emission

Low-frequency GMRT observations of ultra-cool dwarfs

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