Stellar and Planetary Characterization of the Ross 128 Exoplanetary System from APOGEE Spectra

TL;DR

This study provides a detailed chemical and physical characterization of the Ross 128 star and its exoplanet Ross 128b, revealing its composition, potential core size, and habitability prospects based on high-resolution infrared spectra.

Contribution

First detailed chemical abundance analysis of Ross 128 using APOGEE spectra, linking stellar composition to planetary structure and habitability.

Findings

Ross 128 has near solar metallicity ([Fe/H] = +0.03).

Ross 128b likely has a mixed rock and iron composition with a larger core.

Ross 128b is a temperate exoplanet near the habitable zone edge.

Abstract

The first detailed chemical abundance analysis of the M dwarf (M4.0) exoplanet-hosting star Ross 128 is presented here, based upon near-infrared (1.5--1.7 \micron) high-resolution ($R$$\sim$22,500) spectra from the SDSS-APOGEE survey. We determined precise atmospheric parameters $T_{\rm eff}$=3231$\pm$100K, log$g$=4.96$\pm$0.11 dex and chemical abundances of eight elements (C, O, Mg, Al, K, Ca, Ti, and Fe), finding Ross 128 to have near solar metallicity ([Fe/H] = +0.03$\pm$0.09 dex). The derived results were obtained via spectral synthesis (1-D LTE) adopting both MARCS and PHOENIX model atmospheres; stellar parameters and chemical abundances derived from the different adopted models do not show significant offsets. Mass-radius modeling of Ross 128b indicate that it lies below the pure rock composition curve, suggesting that it contains a mixture of rock and iron, with the relative amounts of each set by the ratio of Fe/Mg. If Ross 128b formed with a sub-solar Si/Mg ratio, and assuming the planet's composition matches that of the host-star, it likely has a larger core size relative to the Earth. The derived planetary parameters -- insolation flux (S$_{\rm Earth}$=1.79$\pm$0.26) and equilibrium temperature ($T_{\rm eq}$=294$\pm$10K) -- support previous findings that Ross 128b is a temperate exoplanet in the inner edge of the habitable zone.