Expanding the Catalog: Considering the Importance of Carbon, Magnesium, and Neon in the Evolution of Stars and Habitable Zones

Abstract

Building on previous work, we have expanded our catalog of evolutionary models for stars with variable composition; here we present models for stars of mass 0.5 -1.2 M ⊙ , at scaled metallicities of 0.1 -1.5 Z ⊙ , and specific C/Fe, Mg/Fe, and Ne/Fe values of 0.58 -1.72 C/Fe ⊙ , 0.54 -1.84 Mg/Fe ⊙ and 0.5 -2.0 Ne/Fe ⊙ , respectively. We include a spread in abundance values for carbon and magnesium based on observations of their variability in nearby stars; we choose an arbitrary spread in neon abundance values commensurate with the range seen in other low Z elements due to the difficult nature of obtaining precise measurements of neon abundances in stars. As indicated by the results of Truitt et al. (2015), it is essential that we understand how differences in individual elemental abundances, and not just the total scaled metallicity, can measurably impact a star's evolutionary lifetime and other physical characteristics. In that work we found that oxygen abundances significantly impacted the stellar evolution; carbon, magnesium, and neon are potentially important elements to individually consider due to their relatively high (but also variable) abundances in stars. We present 528 new stellar main sequence models, and we calculate the timedependent evolution of the associated habitable zone boundaries for each based on mass, temperature, and luminosity. We also reintroduce the 2 Gyr "Continuously Habitable Zone" (CHZ 2 ) as a useful tool to help gauge the habitability potential for a given planetary system.

A Flexible Bayesian Framework for Assessing Habitability with Joint Observational and Model Constraints

“…We used the stellar evolution code Tycho (Young & Arnett 2005) to create the catalog of evolutionary tracks discussed in detail in Truitt et al (2015) and Truitt & Young (2017), which are the basis for the model priors discussed here. The database currently contains models between 0.5-1.2 solar masses, with metallicities that fall between 0.1 − 1.5 of solar Z-value, so this is the parameter space we analyze here.…”

“…Though the measurements included in Hypatia may be inherently biased due to the requirements for obtaining highresolution abundance measurements, these are stars in the solar neighborhood that give us an idea of the representative distribution of elemental compositions. We used the stellar evolution code Tycho (Young & Arnett 2005) to create the catalog of evolutionary tracks discussed in detail in Truitt et al (2015) and Truitt & Young (2017), which are the basis for the model priors discussed here. The database currently contains models between 0.5-1.2 solar masses, with metallicities that fall between 0.1 − 1.5 of solar Z-value, so this is the parameter space we analyze here.…”

“…Life must be present for long enough to measurably alter the environment. We choose a time of 2 Gy as the minimum time for a planet to be in the HZ in order for life to possibly be detectable following Truitt et al (2015) and Truitt & Young (2017) and motivated by, e.g., Brocks et al (1999), Kopp et al (2005), and Crowe et al (2013). This limit can also easily be adjusted without compromising the framework presented here.…”

Orbital Dynamics and the Evolution of Planetary Habitability in the AU Mic System

“…The evolution of stars on the main sequence, particularly with regards to their luminosity and effective temperature, results in a subsequent evolution of the HZ (Underwood et al 2003;Ramirez & Kaltenegger 2014). This HZ evolution has been studied within the context of stellar masses and chemical compositions (Young et al 2012;Valle et al 2014;Truitt et al 2015;Truitt & Young 2017), and also with respect to stellar rotation and magnetic activity (Gallet et al 2017). The AU Mic system provides an opportunity to study the evolution of a planetary system within the context of the early luminosity environment of the host star as it joins the main sequence.…”

Continuous Habitable Zones: Using Bayesian Methods to Prioritize Characterization of Potentially Habitable Worlds

“…Previous work has shown that life on Earth made a detectable impact on the atmosphere after ∼1-2 Gy following the Earth's formation (Kasting et al 1993;Brocks et al 1999;Kopp et al 2005;Anbar et al 2007;Crowe et al 2013), with the Great Oxidation Event (GOE) occurring around 2 Gy following formation (Summons et al 1999;Kasting & Catling 2003;Holland 2006). Following the work of Truitt et al (2015), Truitt & Young (2017), and Truitt et al (2020), we therefore define the orbital area around a host star that will remain habitable for at least 2 Gy, here called the 2 Gy continuous habitable zone (CHZ 2 ), as a conservative estimate of the potential habitability of planets in the system across the entire main sequence. More importantly, for specific planets, it is necessary to determine whether they have already spent 2 Gy in the HZ.…”