1 to 2.4 micron Near-IR spectrum of the Giant Planet $β$ Pictoris b obtained with the Gemini Planet Imager (1703.00011v1)

Abstract: Using the Gemini Planet Imager (GPI) located at Gemini South, we measured the near-infrared (1.0-2.4 micron) spectrum of the planetary companion to the nearby, young star $\beta$ Pictoris. We compare the spectrum obtained with currently published model grids and with known substellar objects and present the best matching models as well as the best matching observed objects. Comparing the empirical measurement of the bolometric luminosity to evolutionary models, we find a mass of $12.9\pm0.2$ $\mathcal{M}\mathrm{Jup}$, an effective temperature of $1724\pm15$ K, a radius of $1.46\pm0.01$ $\mathcal{R}\mathrm{Jup}$, and a surface gravity of $\log g = 4.18\pm0.01$ dex. The stated uncertainties are statistical errors only, and do not incorporate any uncertainty on the evolutionary models. Using atmospheric models, we find an effective temperature of $1700-1800$ K and a surface gravity of $\log g = 3.5$-$4.0$ [dex] depending upon model. These values agree well with other publications and with "hot-start" predictions from planetary evolution models. Further, we find that the spectrum of $\beta$ Pic b best matches a low-surface gravity L2$\pm$1 brown dwarf. Finally comparing the spectrum to field brown dwarfs we find the the spectrum best matches 2MASS J04062677-381210 and 2MASS J03552337+1133437.

• The paper presents a comprehensive 1–2.4 μm near-IR spectrum of β Pictoris b using the Gemini Planet Imager combined with adaptive optics and advanced data reduction techniques.
• By comparing the spectrum with atmospheric and evolutionary models, the study estimates key parameters including a mass of ~12.9 M_J, an effective temperature of ~1724 K, and a radius of ~1.46 R_J.
• The spectral similarity to low-surface gravity brown dwarfs endorses a 'hot-start' formation model and offers valuable insights into the atmospheric characteristics of young exoplanets.

Analysis of Near-Infrared Spectrum for $\beta$ Pictoris \MakeLowercase{b}

The paper by Chilcote et al. focuses on obtaining a comprehensive near-infrared (1 to 2.4 μm) spectrum of $\beta$ Pictoris b using the Gemini Planet Imager (GPI). Positioned on a nearby young star known as $\beta$ Pictoris, this directly imaged giant planet provides a unique opportunity to paper a young exoplanetary atmosphere and its evolutionary characteristics.

Methodology and Observations

The spectrum was achieved with GPI on the Gemini South telescope over several observing periods from 2013 to 2016. Using adaptive optics and meticulous data reduction techniques, including the Karhunen-Loève Image Projection forward modeling, the paper effectively isolates $\beta$ Pic b from the surrounding stellar light. The resulting data provides a high signal-to-noise ratio across the Y, J, H, and K bands, forming a near-continuous profile across these wavelengths.

Empirical Findings

By comparing the observed spectrum with atmospheric and evolutionary models, the authors inferred physical characteristics for $\beta$ Pic b. The bolometric luminosity determines empirical parameters such as a mass of approximately 12.9 M, effective temperature around 1724 K, and a radius of approximately 1.46 R with a surface gravity $\log g$ of 4.18 dex. Atmospheric model fits suggested effective temperatures ranging from 1700 to 1800 K and surface gravities between $\log g$ = 3.5 and 4.0 dex, aligning with existing evolutionary model predictions.

Comparative Spectral Analysis

The spectrum of $\beta$ Pic b shows significant parallels with low-surface gravity brown dwarfs, particularly matching well with the object 2MASS J03552337+1133437, an L3 VL-G type brown dwarf member of the AB Doradus moving group, suggesting similar atmospheric characteristics. This comparison provides indirect evidence for $\beta$ Pic b’s low surface gravity, implying youth and a relatively uncompressed atmosphere.

Implications and Theoretical Considerations

The resemblance to young isolatable brown dwarfs raises questions about shared formation pathways and atmospheric compositions between directly imaged giant planets and brown dwarfs. The bolometric luminosity corroborates a "hot-start" formation scenario, hypothesizing high initial entropy conditions consistent with gravitational instability during formation.

Future Prospects

$\beta$ Pic b is a prime candidate for further observation as it approaches maximum elongation, making it accessible for more advanced instruments like the James Webb Space Telescope. These observations could refine physical and atmospheric models, elucidate exoplanetary formation processes, and potentially challenge existing theoretical frameworks by providing rich datasets for retrieval methodologies.

In conclusion, the insights gleaned from the GPI spectrum of $\beta$ Pictoris b underscore the capability of direct imaging and spectroscopic analyses to unravel the complex characteristics of exoplanetary bodies in nearby stellar systems. These findings pave the way for improving models of planetary atmospheres and understanding the intricacies of planetary formation and evolution.