Heavy Metal Rules. I. Exoplanet Incidence and Metallicity

Abstract

Discovery of only handful of exoplanets required to establish a correlation between giant planet occurrence and metallicity of their host stars. More than 20 years have already passed from that discovery, however, many questions are still under lively debate: What is the origin of that relation? what is the exact functional form of the giant planet -metallicity relation (in the metal-poor regime)?, does such a relation exist for terrestrial planets? All these question are very important for our understanding of the formation and evolution of (exo)planets of different types around different types of stars and are subject of the present manuscript. Besides making a comprehensive literature review about the role of metallicity on the formation of exoplanets, I also revisited most of the planet -metallicity related correlations reported in the literature using a large and homogeneous data provided by the SWEET-Cat catalog. This study lead to several new results and conclusions, two of which I believe deserve to be highlighted in the abstract: i) The hosts of sub-Jupiter mass planets (∼0.6 -0.9 M ) are systematically less metallic than the hosts of Jupiter-mass planets. This result might be related to the longer disk lifetime and higher amount of planet building materials available at high metallicities, which allow a formation of more massive Jupiter-like planets. ii) Contrary to the previous claims, our data and results do not support the existence of a breakpoint planetary mass at 4 M above and below which planet formation channels are different. However, the results also suggest that planets of the same (high) mass can be formed through different channels depending on the (disk) stellar mass i.e. environmental conditions.

Populating the brown dwarf and stellar boundary: Five stars with transiting companions near the hydrogen-burning mass limit

“…13 of transiting brown dwarfs and low-mass stars. Figure 13 shows the most common metallicity for brown dwarf host stars is at [Fe/H] ∼ 0, which is consistent with previous findings that brown dwarfs are not preferentially found around metal rich stars like hot Jupiters and are more consistent with metallicity distributions of stars without substellar companions or stars with low-mass planets (e.g., Ma & Ge 2014;Mata Sánchez et al 2014;Maldonado et al 2019;Adibekyan 2019). Maldonado & Villaver (2017) found that stars with less massive brown dwarfs tend to have higher metallicities than stars with more massive brown dwarfs, which is consistent with the interpretations of Ma & Ge (2014) and Maldonado et al (2019) that more massive brown dwarfs tend to form more like low-mass stars.…”

Investigating the planet–metallicity correlation for hot Jupiters

“…This result has evolved into the now well-known giant planet-metallicity correlation; that is, the higher the metallicity of a star, the more likely it is to host a giant planet (Santos et al 2004;Johnson et al 2010;Maldonado et al 2012;Mortier et al 2013;Schlaufman 2014). This result was recently reviewed by Adibekyan (2019), who reanalysed the giant planet-metallicity correlation using the homogeneous stellar parameters listed in the SWEET-Cat catalogue . They contrasted their sample of FGK dwarf star hosts (with planets discovered by the RV and transit methods) with a comparison sample of FGK stars hosting no planets from the HARPS GTO program (see Adibekyan et al 2012), which has stellar parameters derived using the same method as those in SWEET-Cat (thus making them directly comparable).…”

“…Returning to Adibekyan (2019), it is now worth noting that a KS test shows the hosts of their separate radial velocity and transiting planet samples have indistinguishable metallicity distributions, despite the planets having significantly different orbital period regimes. Their transiting sample has an average orbital period of 11 days, whereas the average for their RV sample is 1202 days.…”

Carbon, oxygen, and iron abundances in disk and halo stars

“…There is no apparent bias with planetary mass, apart from that stars with higher metallicities (here traced by [O/H]) tend to host more massive planets (e.g. Fischer & Valenti 2005;Johnson et al 2010;Adibekyan 2019). The mean rising trend in the plot is a result of the increasing production of carbon at later cosmic times (Sect.…”