Insight into the Formation of β Pic b through the Composition of Its Parent Protoplanetary Disk as Revealed by the β Pic Moving Group Member HD 181327

Abstract

It has been suggested that β Pic b has a supersolar metallicity and subsolar C/O ratio. Assuming solar carbon and oxygen abundances for the star β Pic and therefore the planet’s parent protoplanetary disk, β Pic b’s C/O ratio suggests that it formed via core accretion between its parent protoplanetary disk’s H2O and CO2 ice lines. However, β Pic b’s high metallicity is difficult to reconcile with its mass M p = 11.7 M Jup. Massive stars can present peculiar photospheric abundances that are unlikely to record the abundances of their former protoplanetary disks. This issue can be overcome for early-type stars in moving groups by inferring the elemental abundances of the FGK stars in the same moving group that formed in the same molecular cloud and presumably share the same composition. We infer the photospheric abundances of the F dwarf HD 181327, a β Pic moving group member that is the best available proxy for the composition of β Pic b’s parent protoplanetary disk. In parallel, we infer updated atmospheric abundances for β Pic b. As expected for a planet of its mass formed via core-accretion beyond its parent protoplanetary disk’s H2O ice line, we find that β Pic b’s atmosphere is consistent with stellar metallicity and confirm that it has superstellar carbon and oxygen abundances with a substellar C/O ratio. We propose that the elemental abundances of FGK dwarfs in moving groups can be used as proxies for the otherwise difficult-to-infer elemental abundances of early-type and late-type members of the same moving groups.

MIRI MRS Observations of β Pictoris. I. The Inner Dust, the Planet, and the Gas

“…The C/O ratio of HD 181327 was used as a proxy for β Pic since they likely formed out of the same molecular cloud. The C/O ratio of HD 181327 is 0.62 ± 0.08 (Reggiani et al 2024). The C/O ratio for β Pic b we got from the ATMO grid fitting is substellar and does not contain the stellar C/O in its uncertainty.…”

“…From our modeling with the ATMO grid of all the spectra and photometry in Figure 14 Using the C/O ratio of a planetary atmosphere to infer its formation history requires knowledge of the C/O ratio of the host star; however, there is not a published value for the C/O ratio of β Pic in the literature. Because of this, we compared to the C/O ratio measured from the β Pic moving group member HD 181327 as in Reggiani et al (2024). The C/O ratio of HD 181327 was used as a proxy for β Pic since they likely formed out of the same molecular cloud.…”

JWST-TST High Contrast: JWST/NIRCam Observations of the Young Giant Planet β Pic b

“…With the help of atmospheric forward models and spectral retrievals, they were able to derive a subsolar C/O ratio of ∼0.43 for the atmosphere of β Pic b, suggesting strong volatile enrichment during its formation process if the host star β Pic is assumed to be of similar composition as the Sun. We note that measuring the C/O abundance ratio for β Pic is difficult, but it can be done for other stars in the β Pic moving group, which can serve as a proxy for the chemical composition of β Pic (e.g., Reggiani et al 2024). While the C/O abundance ratio of the host star β Pic can hence be expected to be ∼solar or supersolar, the subsolar C/O ratio of β Pic b has recently been challenged by Kiefer et al (2024), who found that with a more careful treatment of telluric and stellar features, the SINFONI spectrum of β Pic b also supports a ∼solar C/O ratio for the planet.…”