Planet Engulfment Sample

Chapter IV: Planet Engulfment Detections are Rare According to Observa-

4.2 Planet Engulfment Sample

planet hosts. Assessing engulfment signatures in systems with existing planets is important for understanding the dynamical conditions that may give rise to planet engulfment, such as planet-planet scattering in multi-planet systems (Rasio and Ford, 1996; Weidenschilling and Marzari, 1996). To fill this gap, we carried out a survey with the Keck High Resolution Echelle Spectrometer (HIRES) of 36 confirmed planet host systems with stellar companions to investigate the role of engulfment in planetary system evolution, and shed light on which dynamical pathways may dominate. For more details on the sample, see Section 4.2. The abundance analysis and engulfment model used to derive mass measurements of engulfed material are presented in Sections 4.3 and 4.4, respectively. Our MESA analysis is outlined in Section 4.5. The results of our survey are presented in Section 4.6, and are compared to previously published results in Section 4.7. Implications for planet engulfment and chemical homogeneity in multi-star systems are discussed in Section 4.8. Finally, we summarize our findings in Section 4.9.

in Mugrauer (2019). These were determined from absolute G-bands magnitudes and the Baraffe et al. (2015) (sub)stellar evolution models assuming an age of 5 Gyr, which is the average age of systems in the Mugrauer (2019) sample. For the planet hosts, we used the most recently reported𝑇_efffrom theNASA Exoplanet Archive2. We foreshadow here thatSMEprovides more accurate𝑇_effmeasurements (typical errors are 7−14 K due to varying SNR levels, added in quadrature with an additional 25 K stemming from instrumental and stellar sources, e.g., the spectral line spread function (SLSF) or point spread function (PSF), telluric lines, and stellar activity Brewer and Fischer 2018), so this cut was redone after collecting spectra for our targets and running them throughSME. This eliminated a further seven systems, which is described in more detail below. However at this point, we were left with 35 systems. We augmented this sample by searching for stellar companions to planet hosts that met these criteria in the NASA Exoplanet Archive, which resulted in an additional two systems (HAT-P-4 and WASP-180). Eleven of the 37 planet host binaries qualify as stellar twins (Δ𝑇_eff < 200 K, Andrews et al. 2019), which are well suited to differential abundance analyses given their near-identical evolutionary states. All systems in our sample were verified to host confirmed planets according to the NASA Exoplanet Archive². Finally, we removed any systems that display evidence of spectroscopic binary contamination in their spectral cross-correlation;

such contamination will lead to inaccurate SME abundance predictions. This was the case for𝜓¹Dra, leaving 36 systems.

The final sample of 36 systems contains 28 binaries and eight triples. Though four of the eight triples are hierarchical, we determined that the spectra of individual stars in these systems are not blended with those of nearby companions using theReaMatch code (Kolbl et al., 2015). We also ensured that the planet hosts and their stellar companions have similar rotational velocities by checking that their𝑣sin𝑖 agree to within ∼10 km/s. Each of the triple systems has only one stellar companion that meets the𝑇_effand projected separation criteria. Thus, two stars were always analyzed per system. The equatorial coordinates, 𝑇_eff, log𝑔, 𝑀_∗, Gaia Early Data Release 3 (ER3)-sourced RVs, parallaxes, proper motions, and𝑉-band magnitudes of stars in the sample are listed in Table 4.2. Some sources are missing RV measurements because they do not meet the Gaia DR2/EDR3 RV criteria of𝐺-band magnitudes less than∼13, or were deemed inaccurate due to companion contamination (Boubert et al., 2019). Among the 36 systems, ten have existing high-precision abundance measurements (HAT-P-1, HD 20781-82, XO-2, WASP-94, HAT-P-4, HD 80606-07,

2https://exoplanetarchive.ipac.caltech.edu/

Hot/Warm Jupiters

Hot/Warm Sub-Saturns

Super-Earths/Sub-Neptunes Cold Jupiters

Figure 4.1: The radii vs. orbital period distribution for planets in our sample.

Hot/warm Jupiters are defined as planets with𝑅 >8𝑅_⊕and𝑃 <100 days, hot/warm sub-Saturns with 4 𝑅_⊕ < 𝑅 < 8 𝑅_⊕ and 𝑃 < 100 days, cold Jupiters with 𝑅 > 8 𝑅_⊕ and𝑃 >100 days, and super-Earths/sub-Neptunes with𝑅 <4 𝑅_⊕. Planets that share the same host star are connected by dashed lines.

16-Cygni, HD 133131, HD 106515, WASP-160; Table 3.1) derived from theMOOG spectral synthesis code (Sneden, 1973; Sobeck et al., 2011) that can be compared with predictions fromSME.

The engulfment sample systems span a wide range of planetary architectures that include super-Earths/sub-Neptunes, compact multi-planet systems, and giant planets at a range of orbital periods (Table 4.3). Figure 4.1 shows the radii versus rotation periods for all planets in the engulfment sample. For planets lacking reported radius measurements according to the NASA Exoplanet Archive², we derived radii from mass measurements with the following power-law mass-radius relation that assumes Earth-like compositions (Rubenzahl et al. in prep.):

𝑀 =𝐶 𝑅^𝛾 (4.1)

where the𝐶 and𝛾 were constrained to values of 0.83 and 3.52 using a sample of

122 confirmed exoplanets with Keck-HIRES spectra and precise radii measurements.

For planets massive enough to host gaseous envelopes greater than 1% by mass, the envelope mass was accounted for by assuming a gas density of 0.417 g cm⁻³ as constrained with the Rubenzahl et al. (in prep.) planet sample. We include errors bars on planet radii measurements in Figure 4.1 if they are reported in the NASA Exoplanet Archive².