Context. Since the first extra-solar planet discovery in 1995, several hundreds of these planets have been discovered. Most are hot Jupiters, i.e. massive planets orbiting close to their star. These ...planets may be powerful radio emitters. Aims. We simulate the radio dynamic spectra resulting from various interaction models between an exoplanet and its parent star, i.e. exoplanet-induced stellar emission and three variants of the exoplanet’s magnetospheric auroral radio emission (full auroral oval, active sector fixed in longitude, and active sector fixed in local time). Methods. We show the physical information about the system that can be drawn from radio observations, and how this can be achieved. This information includes the magnetic field strength and the rotation period of the emitting body (planet or star), the orbital period, the orbit’s inclination, and the magnetic field tilt relative to the rotation axis or offset relative to the center of the planet. For most of these parameters, radio observations provide a unique means of measuring them. Results. Our results should provide the proper framework of analysis and interpretation for future detections of radio emissions from exoplanetary systems – or from magnetic white dwarf-planet or white dwarf-brown dwarf systems –, that are expected to commence soon as part of extensive programs at large radiotelescopes such as LOFAR, UTR2 or the GMRT. Our methodology can be easily adapted to simulate specific observations, once effective detection is achieved.
The observations from the Juno spacecraft in polar orbit of Jupiter provide for the first time a complete view of Jupiter's radio emissions from all latitudes. Characterizing the latitudinal ...distribution of radio emissions' occurrence and intensity is a useful step for elucidating their origin. Here, we analyze for that purpose the first 3 years of observations from the Waves experiment on the Juno spacecraft (mid‐2016 to mid‐2019). Two prerequisites for the construction of the latitudinal distribution of intensities for each Jovian radio component are (a) to work with absolute flux densities and (b) to be able to associate each radio measurement with a specific radio component. Accordingly, we develop a method to convert the Juno/Waves data in flux densities and then we build a catalog of all Jovian radio components over the first 3 years of Juno's orbital mission. From these, we derive occurrence and intensity distributions versus observer's latitude and frequency for each component; these will be the basis for future detailed studies and interpretations of each component's characteristics and origin.
Key Points
We build a processing pipeline of Juno/Waves data that include conversion to absolute flux densities
We build a catalog of all Jovian radio components over the first 3 years of Juno's orbital mission
We derive occurrence and intensity distributions versus observer's latitude and frequency for each component
Context. The habitability of exoplanets hosted by M dwarf stars dramatically depends on the space weather, where the magnetic and ram pressure of the stellar wind, and the exoplanet magnetic field ...are the three main players. These three parameters also likely drive the radio emission arising close to the planet. Aims. Our aim is to characterize the magneto-plasma environment and thus the habitability of the Earth-like planet Proxima b, which is inside the habitable zone of its host M dwarf star Proxima, when it is subject to average calm space weather conditions, and to more extreme space weather conditions, for example a coronal mass ejection (CME) event. We study the role of the stellar wind and planetary magnetic field, and their mutual orientation. We also determine the radio emission arising from the interaction between the stellar wind of Proxima and the magnetosphere of its planet Proxima b, which is relevant to guiding radio observations aimed at unveiling planets. Methods. We used the PLUTO code to run a set of 3D magneto-hydrodynamic simulations focused on the space weather around planet Proxima b. We considered both calm and space weather conditions for Proxima b, under three different scenarios: (a) Proxima b subject to calm space weather in a sub-Alfvénic regime, where the stellar wind magnetic pressure dominates over the wind’s ram pressure; (b) Proxima b subject to calm space weather in a super-Alfvénic regime, where the ram pressure of the wind dominates, and a bow shock is formed; and (c) Proxima b subject to a coronal mass ejection event, when the dynamical and magnetic pressure of the stellar wind from its host star are increased enormously for a short period of time. Results. We find that if Proxima b has a magnetic field similar to that of the Earth ( B p = B ⊕ ≈ 0.32 G) or larger, the magnetopause standoff distance is large enough to shield the surface from the stellar wind for essentially any planetary tilt but the most extreme values (close to 90°) under a calm space weather. Even if Proxima b is subject to more extreme space weather conditions, for example a CME event from its host star, the planet is well shielded by an Earth-like magnetosphere ( B p ≈ B ⊕ ; i ≈ 23.5°), or if it has a tilt smaller than that of the Earth. Otherwise, the planetary magnetic field must be larger to shield the planet from particle precipitation on the surface. For calm space weather conditions, the radio emission caused by the day-side reconnection regions can be as high as 7×10 19 erg s −1 in the super-Alfvénic regime, and is on average almost an order of magnitude larger than the radio emission in the sub-Alfvénic cases, due to the much larger contribution of the bow shock, which is not formed in the sub-Alfvénic regime. We also find that the energy dissipation at the bow shock is essentially independent of the angle between the planet’s magnetic dipole and the incident stellar wind flow. If Proxima b is subject to extreme space weather conditions, the radio emission is more than two orders of magnitude larger than when under calm space weather conditions. This result yields expectations for a direct detection (from Earth) in radio of giant planets in close-in orbits as they are expected to have magnetic fields large enough, so that their electron-cyclotron frequency exceeds the ionosphere cutoff.
Since the radio-frequency emission from planets is expected to be strongly influenced by their interaction with the magnetic field and corona of the host star, the physics of this process can be ...effectively constrained by making sensitive measurements of the planetary radio emission. Up to now, however, numerous searches for radio emission from extrasolar planets at radio wavelengths have only yielded negative results. Here we report deep radio observations of the nearby Neptune-mass extrasolar transiting planet HAT-P-11b at 150 MHz, using the Giant Meterwave Radio Telescope (GMRT). On July 16, 2009, we detected a 3σ emission whose light curve is consistent with an eclipse when the planet passed behind the star. This emission is at a position 14′′ from the transiting exoplanet’s coordinates; thus, with a synthetized beam of FWHM ~ 16′′, the position uncertainty of this weak radio signal encompasses the location of HAT-P-11. We estimate a 5% false positive probability that the observed radio light curve mimics the planet’s eclipse light curve. If the faint signature is indeed a radio eclipse event associated with the planet, then its flux would be 3.87 mJy ± 1.29 mJy at 150 MHz. However, our equally sensitive repeat observations of the system on November 17, 2010 did not detect a significant signal in the radio light curve near the same position. This lack of confirmation leaves us with the possibility of either a variable planetary emission, or a chance occurrence of a false positive signal in our first observation. Deeper observations are required to confirm this hint of 150 MHz radio emission from HAT-P-11b.
ABSTRACT The habitability of an exoplanet depends on many factors. One such factor is the impact of stellar eruptive events on nearby exoplanets. Currently this is poorly constrained due to heavy ...reliance on solar scaling relationships and a lack of experimental evidence. Potential impacts of coronal mass ejections (CMEs), which are the large eruption of magnetic field and plasma from a star, are space weather and atmospheric stripping. A method for observing CMEs as they travel though the stellar atmosphere is the type II radio burst, and the new Low Frequency Array (LOFAR) provides a means of detection. We report on 15 hr of observation of YZ Canis Minoris (YZ CMi), a nearby M dwarf flare star, taken in LOFAR's beam-formed observation mode for the purposes of measuring transient frequency-dependent low-frequency radio emission. The observations utilized the Low Band Antenna (10-90 MHz) or High Band Antenna (110-190 MHz) for five three-hour observation periods. In this data set, there were no confirmed type II events in this frequency range. We explore the range of parameter space for type II bursts constrained by our observations. Assuming the rate of shocks is a lower limit to the rate at which CMEs occur, no detections in a total of 15 hr of observation places a limit of shocks/hr ≤ CME for YZ CMi due to the stochastic nature of the events and the limits of observational sensitivity. We propose a methodology to interpret jointly observed flares and CMEs which will provide greater constraints to CMEs and test the applicability of solar scaling relations.
Direct Radio Discovery of a Cold Brown Dwarf Vedantham, H. K.; Callingham, J. R.; Shimwell, T. W. ...
Astrophysical journal. Letters,
11/2020, Letnik:
903, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Magnetospheric processes seen in gas giants such as aurorae and circularly polarized cyclotron maser radio emission have been detected from some brown dwarfs. However, previous radio observations ...targeted known brown dwarfs discovered via their infrared emission. Here we report the discovery of BDR J1750+3809, a circularly polarized radio source detected around 144 MHz with the Low-Frequency Array (LOFAR) telescope. Follow-up near-infrared photometry and spectroscopy show that BDR J1750+3809 is a cold methane dwarf of spectral type T6.5 1 at a distance of . The quasi-quiescent radio spectral luminosity of BDR J1750+3809 is 5 × 1015 erg s−1 Hz−1, which is over two orders of magnitude larger than that of the known population of comparable spectral type. This could be due to a preferential geometric alignment or an electrodynamic interaction with a close companion. In addition, as the emission is expected to occur close to the electron gyrofrequency, the magnetic field strength at the emitter site in BDR J1750+3809 is B 25 G, which is comparable to planetary-scale magnetic fields. Our discovery suggests that low-frequency radio surveys can be employed to discover substellar objects that are too cold to be detected in infrared surveys.
The determination of the internal magnetic field of Jupiter has been the object of many studies and publications. These models have been computed from the Pioneer, Voyager, and Ulysses measurements. ...Some models also use the position of the Io footprints as a constraint: the magnetic field lines mapping to the footprints must have their origins along Io's orbit. The use of this latter constraint to determine the internal magnetic field models greatly improved the modeling of the auroral emissions, in particular the radio ones, which strongly depends on the magnetic field geometry. This constraint is, however, not sufficient for allowing a completely accurate modeling. The fact that the footprint field line should map to a longitude close to Io's was not used, so that the azimuthal component of the magnetic field could not be precisely constrained. Moreover, a recent study showed the presence of a magnetic anomaly in the northern hemisphere, which has never been included in any spherical harmonic decomposition of the internal magnetic field. We compute a decomposition of the Jovian internal magnetic field into spherical harmonics, which allows for a more accurate mapping of the magnetic field lines crossing Io, Europa, and Ganymede orbits to the satellite footprints observed in UV. This model, named VIPAL, is mostly constrained by the Io footprint positions, including the longitudinal constraint, and normalized by the Voyager and Pioneer magnetic field measurements. We show that the surface magnetic fields predicted by our model are more consistent with the observed frequencies of the Jovian radio emissions than those predicted by previous models.
Key Points
A more accurate magnetic field model can be computed from the mapping of Io's fo
This model is consistent with the magnetometer measurements
This model permits a better description of the satellite‐related aurorae
We present first results from a LOFAR census of non-recycled pulsars. The census includes almost all such pulsars known (194 sources) at declinations Dec > 8° and Galactic latitudes |Gb| > 3°, ...regardless of their expected flux densities and scattering times. Each pulsar was observed for ≥20 min in the contiguous frequency range of 110–188 MHz. Full-Stokes data were recorded. We present the dispersion measures, flux densities, and calibrated total intensity profiles for the 158 pulsars detected in the sample. The median uncertainty in census dispersion measures (1.5 × 10-3 pc cm-3) is ten times smaller, on average, than in the ATNF pulsar catalogue. We combined census flux densities with those in the literature and fitted the resulting broadband spectra with single or broken power-law functions. For 48 census pulsars such fits are being published for the first time. Typically, thechoice between single and broken power-laws, as well as the location of the spectral break, were highly influenced by the spectral coverage of the available flux density measurements. In particular, the inclusion of measurements below 100 MHz appears essential for investigating the low-frequency turnover in the spectra for most of the census pulsars. For several pulsars, we compared the spectral indices from different works and found the typical spread of values to be within 0.5–1.5, suggesting a prevailing underestimation of spectral index errors in the literature. The census observations yielded some unexpected individual source results, as we describe in the paper. Lastly, we will provide this unique sample of wide-band, low-frequency pulse profiles via the European Pulsar Network Database.
By analysing a database of 26 yr of observations of Jupiter with the Nançay Decameter Array, we unambiguously identify the radio emissions caused by the Ganymede–Jupiter interaction. We study the ...energetics of these emissions via the distributions of their intensities, duration, and power, and compare them to the energetics of the Io–Jupiter radio emissions. This allows us to demonstrate that the average emitted radio power is proportional to the Poynting flux from the rotating Jupiter’s magnetosphere intercepted by the obstacle. We then generalize this result to the radio-magnetic scaling law that appears to apply to all plasma interactions between a magnetized flow and an obstacle, magnetized or not. Extrapolating this scaling law to the parameter range corresponding to hot Jupiters, we predict large radio powers emitted by these objects, that should result in detectable radio flux with new-generation radiotelescopes. Comparing the distributions of the durations of Ganymede–Jupiter and Io–Jupiter emission events also suggests that while the latter results from quasi-permanent Alfvén wave excitation by Io, the former likely results from sporadic reconnection between magnetic fields Ganymede and Jupiter, controlled by Jupiter’s magnetic field geometry and modulated by its rotation.
The narrowband kilometric radiation (nKOM) is a Jovian low‐frequency radio component identified as a plasma emission produced in the region of the Io plasma torus. Measurements from the Waves ...instrument onboard the Juno spacecraft permitted to establish the distribution of nKOM occurrence and intensity as a function of frequency and latitude. We have developed a 3D geometrical model that can simulate at large scale the plasma emissions occurrence observed by a spacecraft based on an internal Jovian magnetic field model and a diffusive equilibrium model of the plasma density in Jupiter's inner magnetosphere. With this model, we propose a new method to discriminate the generation mechanism, wave mode, beaming and radio source location of plasma emissions. Here, this method is applied to the study of the nKOM observed from all latitudes by the Juno/Waves experiment to identify which conditions reasonably reproduce the observed occurrence distribution versus frequency and latitude. The results allow us to exclude the two main nKOM models published so far, and to show that the emission is produced at the local plasma frequency and then beamed anti‐parallel to the local density gradient into free space. We also propose that depending on its latitude, Juno observes two distinct kinds of nKOM: the low frequency nKOM in ordinary mode at high latitudes, and the high frequency nKOM on extraordinary mode at low latitudes. Both radio source locations are found to be distributed near the centrifugal equator ranging from the outer edge to the inner edge of the Io plasma torus.
Plain Language Summary
This paper investigates a specific type of Jupiter's natural radio emissions called “narrowband kilometric radiation” (nKOM). The nKOM is produced within the region of Jupiter's Io plasma torus. Using data collected by the Waves instrument on the Juno spacecraft, we analyze when and how the nKOM occurs across different frequencies and latitudes. To better understand these radio emissions, we have developed a 3D model that simulates the occurrence of nKOM. Additionally, we introduce a novel methodology for determining key characteristics of nKOM, including its generation mechanism, propagation mode, beaming direction, and the source location of the radio emissions. Applying this model to Juno's observations, we found that the previously proposed theoretical models for nKOM generation do not match the observed data. Instead, the study suggests that nKOM is generated at the local plasma frequency and is beamed along its local gradient, in the direction of decreasing frequencies. The study also revealed two distinct types of nKOM depending on Juno's latitude: low‐frequency nKOM in ordinary mode at high latitudes and high‐frequency nKOM in extraordinary mode at low latitudes. Both types of nKOM are found to originate near Jupiter's centrifugal equator.
Key Points
We developed a 3D modeling method to test the generation mechanism and beaming of plasma emissions from Jupiter's inner magnetosphere
Narrowband kilometric radiation (nKOM) occurrence distribution is reproduced with plasma emissions at fpe beamed anti‐parallel to the density gradient
nKOM is compatible with O‐mode at high latitudes and X‐mode at low latitudes, from radio sources distributed near the centrifugal equator
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