Host Star Metallicity of Directly Imaged Wide-orbit Planets: Implications for Planet Formation

Abstract

Directly imaged planets (DIPs) are self-luminous companions of pre-main-sequence and young main-sequence stars. They reside in wider orbits (∼tens to thousands of astronomical units) and generally are more massive compared to the close-in (≲10 au) planets. Determining the host star properties of these outstretched planetary systems is important to understand and discern various planet formation and evolution scenarios. We present the stellar parameters and metallicity ([Fe/H]) for a subsample of 18 stars known to host planets discovered by the direct imaging technique. We retrieved the high-resolution spectra for these stars from public archives and used the synthetic spectral fitting technique and Bayesian analysis to determine the stellar properties in a uniform and consistent way. For eight sources, the metallicities are reported for the first time, while the results are consistent with the previous estimates for the other sources. Our analysis shows that metallicities of stars hosting DIPs are close to solar with a mean [Fe/H] = −0.04 ± 0.27 dex. The large scatter in metallicity suggests that a metal-rich environment may not be necessary to form massive planets at large orbital distances. We also find that the planet mass–host star metallicity relation for the directly imaged massive planets in wide orbits is very similar to that found for the well-studied population of short-period (≲1 yr) super-Jupiters and brown dwarfs around main-sequence stars.

The SOPHIE search for northern extrasolar planets. XVII. A wealth of new objects: Six cool Jupiters, three brown dwarfs, and 16 low-mass binary stars

“…from a single population. This reinforces the previous observations that the average metallicity of the host star decreases as the mass of the planet increases (Santos et al 2017;Narang et al 2018;Swastik et al 2021). A possible interpretation is that the population of giant planets is bimodal, with giant planets (M p ≤ 4 M J ) forming via core accretion and more massive giant planets (M p > 4 M J ) forming via the disk gravitational instability.…”

The SOPHIE search for northern extrasolar planets -- XVII. A wealth of new objects: Six cool Jupiters, three brown dwarfs, and 16 low-mass binary stars

“…Figure 7 shows the cumulative metallicity distribution for host stars of these two samples, which a Kolmogorov-Smirnov (K-S) test 5 shows has a very low p-value of 9.53 ×10 −8 to be drawn from a single population. This reinforces the previous observations that the average metallicity of the host star decreases as the mass of the planet increases (Santos et al 2017;Narang et al 2018;Swastik et al 2021). A possible interpretation is that the population of giant planets is bimodal, with giant planets (m p ≤ 4 M J ) forming via core accretion and more massive giant planets (m p > 4 M J ) forming via the disk gravitational instability.…”

The GAPS Programme with HARPS-N at TNG. XXXV. Fundamental properties of transiting exoplanet host stars

“…Moreover, several studies have pointed out the existence of correlations between characteristics of the host stars and properties/frequencies of their planetary systems, in particular for giant transiting planets, for which the present work is focused on. As a result, the evidences supporting the correlation between stellar metallicity and occurrence rate of giant planets (e.g., Santos et al 2004;Valenti et al 2005) and the weakening of this correlation towards lower regimes of planetary mass (e.g., Sousa et al 2008;Ghezzi et al 2010) or for wide-orbit planets (Swastik et al 2021), the connection between planet radius and stellar metallicity (Buchhave et al 2014), the correlation between stellar metallicity and planetary heavy-metal content (e.g., Guillot et al 2006), the trend between orbit eccentricity and star metallicity (Dawson & Murray-Clay 2013), the role of the abundances of α-elements (Robinson et al 2006;Gonzalez 2009;Adibekyan et al 2012cAdibekyan et al , 2015 or of the Li depletion observed for massive planet-hosting stars (e.g., Delgado are becoming more and more numerous thanks to the constant increase of new planet discoveries.…”