Magnetic field strengths of hot Jupiters from signals of star-planet interactions (1907.09068v1)

Abstract: Evidence of star-planet interactions in the form of planet-modulated chromospheric emission has been noted for a number of hot Jupiters. Magnetic star-planet interactions involve the release of energy stored in the stellar and planetary magnetic fields. These signals thus offer indirect detections of exoplanetary magnetic fields. Here we report the derivation of the magnetic field strengths of four hot Jupiter systems using the power observed in Ca II K emission modulated by magnetic star-planet interactions. By approximating the fractional energy released in the Ca II K line we find that the surface magnetic field values for the hot Jupiters in our sample range from 20 G to 120 G, ~10-100 times larger than the values predicted by dynamo scaling laws for planets with rotation periods of ~2 - 4 days. On the other hand, these value are in agreement with scaling laws relating the magnetic field strength to the internal heat flux in giant planets. Large planetary magnetic field strengths may produce observable electron-cyclotron maser radio emission by preventing the maser from being quenched by the planet's ionosphere. Intensive radio monitoring of hot Jupiter systems will help confirm these field values and inform on the generation mechanism of magnetic fields in this important class of exoplanets.

Magnetic Field Strengths of Hot Jupiters from Star-Planet Interaction Signals

This paper presents an analytical paper of the magnetic fields of hot Jupiters, utilizing the phenomenon of star-planet interactions (SPI) to indirectly measure magnetic field strength. The researchers examined signals from planet-modulated chromospheric emissions, specifically Ca II K line variations, which are indicative of magnetic SPI. The paper focuses on four distinct systems, revealing surface magnetic field strengths ranging from 20 to 120 Gauss—substantially exceeding predictions made by models based solely on dynamo scaling laws tied to planetary rotation periods. These values are instead consistent with models linking magnetic field strength to internal heat flux, providing important insights into the magnetic dynamics of giant planets.

Key Findings and Implications

1. Signal Analysis and Calibration: The paper describes an innovative calibration method for high-resolution spectroscopic data. By carefully converting observed SPI signals into physical powers, the researchers estimated the total power generated in the Ca II K line emission. This meticulous calibration allows for a more accurate derivation of magnetic field strengths across the studied systems.
2. Magnetic Field Modeling: Utilizing established SPI models, the planetary magnetic field strengths were derived from observed emissions. The results indicate a clear disparity between empirical observations and predictions based solely on rotational dynamics. Instead, the internal heat flux model, accounting for the energy deposited by host stars into planetary interiors, aligns closely with the observed magnetic properties.
3. Comparative Analysis: This research contrasts SPI-derived magnetic fields with other estimation models, including Lyman-alpha absorption methods. The findings challenge previous assumptions, suggesting that the magnetic moments derived from less direct methods are potentially underestimating the actual strength of planetary magnetic fields.
4. Practical Implications for Radio Observations: The enhanced magnetic field strengths have implications for the detection of radio emissions generated via electron-cyclotron maser instability. The increased field strengths make conditions favorable for the escape and observation of radio waves, aiding future detection efforts through arrays like the Low-Frequency Array (LOFAR).

Theoretical and Future Perspectives

This paper underscores the necessity of integrating internal heat flux considerations into models of magnetic field generation in exoplanets, especially hot Jupiters. The compelling alignment between SPI-derived measurements and these models suggests new pathways for exploring magnetic properties in exoplanetary science.

Future efforts should focus on expanding flux-calibrated SPI signal monitoring across other systems, particularly transiting planets where dimensions are well known, thereby refining comparative analysis further. Enhanced observational strategies, including targeted radio wave measurements at identified frequencies, will likely provide richer data sets and unveil intricate details of magnetic field generation mechanisms.

By advancing our understanding of how stellar interactions influence planetary magnetic fields, this paper offers vital insights into the shielding and atmospheric retention capacities of exoplanets, with broader implications for studies of habitability and planetary dynamics across diverse celestial contexts.