Direct confirmation of the radial-velocity planet $β$ Pic c (2010.04442v1)

Abstract: Methods used to detect giant exoplanets can be broadly divided into two categories: indirect and direct. Indirect methods are more sensitive to planets with a small orbital period, whereas direct detection is more sensitive to planets orbiting at a large distance from their host star. %, and thus on long orbital period. This dichotomy makes it difficult to combine the two techniques on a single target at once. Simultaneous measurements made by direct and indirect techniques offer the possibility of determining the mass and luminosity of planets and a method of testing formation models. Here, we aim to show how long-baseline interferometric observations guided by radial-velocity can be used in such a way. We observed the recently-discovered giant planet $\beta$ Pictoris c with GRAVITY, mounted on the Very Large Telescope Interferometer (VLTI). This study constitutes the first direct confirmation of a planet discovered through radial velocity. We find that the planet has a temperature of $T = 1250\pm50$\,K and a dynamical mass of $M = 8.2\pm0.8\,M_{\rm Jup}$. At $18.5\pm2.5$\,Myr, this puts $\beta$ Pic c close to a 'hot start' track, which is usually associated with formation via disk instability. Conversely, the planet orbits at a distance of 2.7\,au, which is too close for disk instability to occur. The low apparent magnitude ($M_{\rm K} = 14.3 \pm 0.1$) favours a core accretion scenario. We suggest that this apparent contradiction is a sign of hot core accretion, for example, due to the mass of the planetary core or the existence of a high-temperature accretion shock during formation.

Direct Confirmation of the Radial-Velocity Planet Beta Pictoris c

The paper "Direct Confirmation of the Radial-Velocity Planet Beta Pictoris c" presents pioneering research in the detection and characterization of exoplanets, specifically targeting the giant planet Beta Pictoris c (hereafter, ß Pic c). Utilizing the capability of the GRAVITY instrument mounted on the Very Large Telescope Interferometer (VLTI), this paper marks the first instance of a direct confirmation of a planet initially discovered through radial velocity methods.

Methodology and Observations

The research leverages the synergy between direct imaging and radial velocity techniques, which traditionally excel in different planetary environments. Radial velocity is adept at identifying planets in close proximity to their host stars, while direct imaging shines in spotting planets at broader separations. In this paper, ß Pic c was observed with the GRAVITY instrument over multiple nights in 2020, employing long-baseline interferometric observations.

The core advantage of the GRAVITY instrument lies in its ability to pinpoint a planet's position with exceptional precision, thereby overcoming the msin(i) degeneracy inherent in radial velocity measurements. The scientific strategy involved centering a science fiber on the expected location of ß Pic c to collect data on its astrometry and spectrum and utilize a fringe-tracking fiber to stabilize the star's light for phase referencing.

Results

The researchers successfully confirmed the existence of ß Pic c, directly imaging a planet that radial velocity had first detected. The spectral analysis provided important insights into its atmospheric characteristics, determining its temperature at $1250 \pm 50$ K and a dynamical mass of $8.2 \pm 0.8$ $M_{Jup}$. This data positions ß Pic c along a 'hot start' track in mass-luminosity diagrams traditionally associated with formation via disk instability — a mechanism thought to be improbable at its current 2.7 au orbital distance.

Contradictorily, the data implies that a core accretion scenario might be more plausible, supported by its relatively low apparent magnitude ($M_K = 14.3 \pm 0.1$). This ambiguity reflects nuanced interpretations of its formation, advocating a substantial consideration of core accretion possibly involving an inefficient cooling accretion shock during its formation, as posited by recent studies.

Implications and Future Directions

This research advances the understanding of planet formation and evolution by providing rare empirical evidence that bridges disparate theoretical formation models. The ability to simultaneously constrain orbital inclinations, masses, and luminosities allows for stringent testing against theoretical predictions.

Practically, this work underscores the potential of instruments like GRAVITY in refining exoplanet discovery and characterization strategies, heralding a significant directional shift. We can expect future endeavors to exploit this methodology to detect smaller, yet equally significant planets, enriching our comprehension of planetary systems in both mass and composition. There is also promise in extending radial velocity techniques to longer periods and younger stars, enhancing the detection spectrum.

As observational capabilities advance, particularly towards integrating these techniques seamlessly, this dual-approach can catalyze breakthroughs in our understanding of planet formation, specifically probing the complex interplay of processes like core accretion versus gravitational instability. This work sets a foundational benchmark, opening new frontiers in the direct observation of exoplanets, hinting at the upcoming era where theoretical models are directly informed by and reconciled with empirical data.