Accreting Protoplanets in the LkCa 15 Transition Disk

Abstract: Exoplanet detections have revolutionized astronomy, offering new insights into solar system architecture and planet demographics. While nearly 1900 exoplanets have now been discovered and confirmed, none are still in the process of formation. Transition discs, protoplanetary disks with inner clearings best explained by the influence of accreting planets, are natural laboratories for the study of planet formation. Some transition discs show evidence for the presence of young planets in the form of disc asymmetries or infrared sources detected within their clearings, as in the case of LkCa 15. Attempts to observe directly signatures of accretion onto protoplanets have hitherto proven unsuccessful. Here we report adaptive optics observations of LkCa 15 that probe within the disc clearing. With accurate source positions over multiple epochs spanning 2009 - 2015, we infer the presence of multiple companions on Keplerian orbits. We directly detect H{\alpha} emission from the innermost companion, LkCa 15 b, evincing hot (~10,000 K) gas falling deep into the potential well of an accreting protoplanet.

• The paper presents robust evidence of accreting protoplanets in the LkCa 15 transition disk using adaptive optics and high-contrast imaging.
• The authors determined semimajor axes of ~14.7 AU and ~18.6 AU and estimated accretion rates around 3×10⁻⁶ M⊙²/yr from multi-epoch observations.
• Simulations suggest that resonant configurations among LkCa 15 b, c, and d can lead to stable planetary orbits, challenging conventional disc-interaction models.

Insights into Protoplanetary Formation in the LkCa 15 System

This paper presents detailed observations of the transitional disc surrounding the LkCa 15 system, with particular emphasis on the detection and analysis of accreting protoplanets within the disc's clearing. Employing state-of-the-art adaptive optics techniques, the authors provide compelling evidence for the presence of multiple planetary companions, shedding light on the dynamic processes governing planet formation.

The authors conducted high-contrast imaging using non-redundant masking (NRM) at the Large Binocular Telescope (LBT), along with observations in both the Ks and L bands, successfully identifying two planetary components, LkCa 15 b and c, and a potential third companion, LkCa 15 d. The detection of Hα emission from LkCa 15 b distinctly indicates the presence of hot gas accreting onto the protoplanet, with an estimated temperature around 10,000 K. The results span multiple epochs from 2009 to 2015, enabling robust orbital fits that suggest these bodies are on distinct Keplerian orbits.

A key numerical finding of the study is the calculation of semimajor axes and accretion rates for these protoplanets. The semi-major axes are measured to be approximately 14.7 AU for LkCa 15 b and 18.6 AU for LkCa 15 c, indicating significant orbital separation within the disc clearing that extends to around 56 AU. Additionally, the study estimates the combined mass and accretion rates, proposing values around $3 \times 10^{-6}$ $M_{\odot}^2$ $yr^{-1}$ for LkCa 15 b, inferred from its Hα and infrared emissions. These estimates align well with circumplanetary accretion disc models, contrasting with predictions from hot-start models that fail to fully replicate the observed emissions, particularly at longer wavelengths.

An intriguing aspect of the analysis is the debate on the dynamical stability of these protoplanets. The authors conducted extensive simulations, indicating that if LkCa 15 b, c, and d are in resonance or possess suitable lower masses, stable configurations are feasible. This assertion suggests possible resonant states that accommodate larger planetary masses than typically expected for such separations, challenging conventional models of planet-disc interaction.

Theoretical implications of these observations are profound. The direct detection of ongoing accretion onto these bodies provides a rare empirical benchmark for models of planet formation, particularly in transition discs. The observed properties, including the high luminosity in infrared and Hα, reflect significant accretion processes potentially characteristic of giant planet formation phases. Observations such as these could refine our understanding of the conditions necessary for planet formation and allow for calibration against theoretical models.

Practically, this study underscores the utility of advanced adaptive optics and high-contrast imaging techniques in revealing previously obscure details in protoplanetary environments. Future observational campaigns, particularly those utilizing ALMA for sub-millimetre analysis, could further elucidate the properties of circumplanetary discs and their role in planetary accretion processes.

In conclusion, this paper provides a substantial contribution to the study of planet formation within transition discs, offering valuable insights into the complex interplay of disc dynamics and protoplanetary evolution. Continued monitoring and analysis are essential for validating these initial findings and expanding our understanding of the formation and evolution of planetary systems.