PALANTIR: An updated prediction tool for exoplanetary radio emissions

In the past two decades, it has been convincingly argued that magnetospheric radio emissions, of cyclotron maser origin, can occur for exoplanetary systems, similarly as solar planets, with the same periodicity as the planetary orbit. These emissions are primarily expected at low frequencies (usually below 100 MHz, c.f. Farrell et al., 1999; Zarka, 2007). The radio detection of exoplanets will considerably expand the field of comparative magnetospheric physics and star-planet plasma interactions (Hess & Zarka, 2011). We have developped a prediction code for exoplanetary radio emissions, PALANTIR: "Prediction Algorithm for star-pLANeT Interactions in Radio". This code has been developed for the construction of an up-to-date and evolutive target catalog, based on observed exoplanet physical parameters, radio emission theory, and magnetospheric physics embedded in scaling laws. It is based on, and extends, previous work by Grießmeier et al. (2007b). Using PALANTIR, we prepared an updated list of targets of interest for radio emissions. Additionally, we compare our results with previous studies conducted with similar models (Griessmeier, 2017). For the next steps, we aim at improving this code by adding new models and updating those already used

The search for radio emission from the exoplanetary systems 55 Cancri, υ Andromedae, and τ Boötis using LOFAR beam-formed observations

International audienceContext. The detection of radio emissions from exoplanets will open up a vibrant new research field. Observing planetary auroral radio emission is the most promising method to detect exoplanetary magnetic fields, the knowledge of which will provide valuable insights into the planet’s interior structure, atmospheric escape, and habitability. Aims. We present LOFAR (LOw-Frequency ARray) Low Band Antenna (LBA: 10–90 MHz) circularly polarized beamformed observations of the exoplanetary systems 55 Cancri, υ Andromedae, and τ Boötis. All three systems are predicted to be good candidates to search for exoplanetary radio emission. Methods. We applied the BOREALIS pipeline that we have developed to mitigate radio frequency interference and searched for both slowly varying and bursty radio emission. Our pipeline has previously been quantitatively benchmarked on attenuated Jupiter radio emission. Results. We tentatively detect circularly polarized bursty emission from the τ Boötis system in the range 14–21 MHz with a flux density of ~890 mJy and with a statistical significance of ~3 σ . For this detection, we do not see any signal in the OFF-beams, and we do not find any potential causes which might cause false positives. We also tentatively detect slowly variable circularly polarized emission from τ Boötis in the range 21–30 MHz with a flux density of ~400 mJy and with a statistical significance of >8 σ . The slow emission is structured in the time-frequency plane and shows an excess in the ON-beam with respect to the two simultaneous OFF-beams. While the bursty emission seems rather robust, close examination casts some doubts on the reality of the slowly varying signal. We discuss in detail all the arguments for and against an actual detection, and derive methodological tests that will also apply to future searches. Furthermore, a ~2 σ marginal signal is found from the υ Andromedae system in one observation of bursty emission in the range 14–38 MHz and no signal is detected from the 55 Cancri system, on which we placed a 3 σ upper limit of 73 mJy for the flux density at the time of the observation. Conclusions. Assuming the detected signals are real, we discuss their potential origin. Their source probably is the τ Boötis planetary system, and a possible explanation is radio emission from the exoplanet τ Boötis b via the cyclotron maser mechanism. Assuming a planetary origin, we derived limits for the planetary polar surface magnetic field strength, finding values compatible with theoretical predictions. Further observations with LOFAR-LBA and other low-frequency telescopes, such as NenuFAR or UTR-2, are required to confirm this possible first detection of an exoplanetary radio signal

Pulsar scintillation studies with LOFAR. I. The census

Wu Z, Verbiest J, Main RA, et al. Pulsar scintillation studies with LOFAR. I. The census. arXiv:2203.10409. 2022.Context. Interstellar scintillation (ISS) of pulsar emission can be used both as a probe of the ionised interstellar medium (IISM) and cause corruptions in pulsar timing experiments. Of particular interest are so-called scintillation arcs which can be used to measure time-variable interstellar scattering delays directly, potentially allowing high-precision improvements to timing precision. Aims. The primary aim of this study is to carry out the first sizeable and self-consistent census of diffractive pulsar scintillation and scintillation-arc detectability at low frequencies, as a primer for larger-scale IISM studies and pulsar-timing related propagation studies with the LOw-Frequency ARray (LOFAR) High Band Antennae (HBA). Results. In this initial set of 31 sources, 15 allow full determination of the scintillation properties; nine of these show detectable scintillation arcs at 120-180 MHz. Eight of the observed sources show unresolved scintillation; and the final eight don't display diffractive scintillation. Some correlation between scintillation detectability and pulsar brightness and dispersion measure is apparent, although no clear cut-off values can be determined. Our measurements across a large fractional bandwidth allow a meaningful test of the frequency scaling of scintillation parameters, uncorrupted by influences from refractive scintillation variations. Conclusions. Our results indicate the powerful advantage and great potential of ISS studies at low frequencies and the complex dependence of scintillation detectability on parameters like pulsar brightness and interstellar dispersion. This work provides the first installment of a larger-scale census and longer-term monitoring of interstellar scintillation effects at low frequencies

Pulsar scintillation studies with LOFAR, I. The census

Wu Z, Verbiest J, Main RA, et al. Pulsar scintillation studies with LOFAR, I. The census. Astronomy & Astrophysics. 2022;663: A116.Context.Interstellar scintillation (ISS) of pulsar emission can be used both as a probe of the ionized interstellar medium (IISM) and cause corruptions in pulsar timing experiments. Of particular interest are so-called scintillation arcs which can be used to measure time-variable interstellar scattering delays directly, potentially allowing high-precision improvements to timing precision.Aims.The primary aim of this study is to carry out the first sizeable and self-consistent census of diffractive pulsar scintillation and scintillation-arc detectability at low frequencies, as a primer for larger-scale IISM studies and pulsar-timing related propagation studies with the LOw-Frequency ARray (LOFAR) High Band Antennae (HBA).Methods.We use observations from five international LOFAR stations and the LOFAR core in the Netherlands. We analyze the 2D auto-covariance function of the dynamic spectra of these observations to determine the characteristic bandwidth and timescale of the ISS toward the pulsars in our sample and investigate the 2D power spectra of the dynamic spectra to determine the presence of scintillation arcs.Results.In this initial set of 31 sources, 15 allow for the full determination of the scintillation properties; nine of these show detectable scintillation arcs at 120–180 MHz. Eight of the observed sources show unresolved scintillation; and the final eight do not display diffractive scintillation. Some correlation between scintillation detectability and pulsar brightness and a dispersion measure is apparent, although no clear cut-off values can be determined. Our measurements across a large fractional bandwidth allow a meaningful test of the frequency scaling of scintillation parameters, uncorrupted by influences from refractive scintillation variations.Conclusions.Our results indicate the powerful advantage and great potential of ISS studies at low frequencies and the complex dependence of scintillation detectability on parameters such as pulsar brightness and interstellar dispersion. This work provides the first installment of a larger-scale census and longer-term monitoring of ISS effects at low frequencies

Comet-like tail-formation of exospheres of hot rocky exoplanets: Possible implications for CoRoT-7b

International audienceIn this study, the interaction of stellar wind plasma with the exosphere and possibly with the planetary magnetospheric environment of close-in rocky exoplanets is investigated. In particular, we focus on the "super-Earth" CoRoT-7b, which has been recently discovered by the CoRoT space observatory. The physical properties of such a planet, with an orbital distance of about 0.017 AU from its host star, may most likely resemble a big and more massive Mercury-type planet in the sense that it most likely releases its surface elements into space. Based on the present knowledge of CoRoT-7b and drawing on the analogy to Solar System planets, we use numerical models to simulate exospheric and magnetospheric distributions of different particle populations, among which are neutral sodium and ionised calcium and magnesium. We find that, for most species, the atmospheric loss rate in such an extreme environment can be very high, so that a neutral and an ionised tail of escaping particles will form. Depending on the planetary composition we postulate the presence of a sodium tail, similar to that of Mercury but shorter due to the shorter Na lifetime, and of an extended magnetospheric distribution of ionised calcium or magnesium. The feasibility of observation of such populations is also discussed

Pulsar Scintillation Studies with LOFAR: II. Dual-frequency scattering study of PSR J0826+2637 with LOFAR and NenuFAR

Wu Z, Coles WA, Verbiest J, et al. Pulsar Scintillation Studies with LOFAR: II. Dual-frequency scattering study of PSR J0826+2637 with LOFAR and NenuFAR. 2023.Interstellar scattering (ISS) of radio pulsar emission can be used as a probe of the ionised interstellar medium (IISM) and causes corruptions in pulsar timing experiments. Two types of ISS phenomena (intensity scintillation and pulse broadening) are caused by electron density fluctuations on small scales (< 0.01 AU). Theory predicts that these are related, and both have been widely employed to study the properties of the IISM. Larger scales ($\sim$1-100\,AU) cause measurable changes in dispersion and these can be correlated with ISS observations to estimate the fluctuation spectrum over a very wide scale range. IISM measurements can often be modeled by a homogeneous power-law spatial spectrum of electron density with the Kolmogorov ($-11/3$) spectral exponent. Here we aim to test the validity of using the Kolmogorov exponent with PSR~J0826+2637. We do so using observations of intensity scintillation, pulse broadening and dispersion variations across a wide fractional bandwidth (20 -- 180\,MHz). We present that the frequency dependence of the intensity scintillation in the high frequency band matches the expectations of a Kolmogorov spectral exponent but the pulse broadening in the low frequency band does not change as rapidly as predicted with this assumption. We show that this behavior is due to an inhomogeneity in the scattering region, specifically that the scattering is dominated by a region of transverse size $\sim$40\,AU. The power spectrum of the electron density, however, maintains the Kolmogorov spectral exponent from spatial scales of 5$\times10^{-6}$\,AU to $\sim$100\,AU