1. Amanda Truitt & Patrick A. Young
Arizona State University, School of Earth and Space Exploration
Using TYCHO [13] we have modeled the evolutionary tracks for a total of 1232 stars, each with a
different combination of mass, scaled metallicity, and speciLic elemental composition. TYCHO is a
1D stellar evolution code with a hydrodynamic formulation of the stellar evolution equations. It
uses OPAL opacities [14, 15, 16], and a combined OPAL and Timmes equation of state [17]. TYCHO
outputs data on stellar surface quantities for each time-step of the evolution, which we use to
produce a predicted radius for the inner and outer edges of the HZ as a function of the star’s age.
To calculate the HZ for an individual star at each point in its evolution, we use equations from [6],
combined with TYCHO outputs. Our program CHAD (Calculating HAbitable Distances) then
determines the inner and outer HZ boundaries, discussed in [7]. The original grid of 376 models
[18] includes “solar-type” stars, with masses ranging from 0.5–1.2 M¤. Models are also calculated
for scaled metallicity values of 0.1–1.5 Z¤, and four models for oxygen abundance values (0.44,
0.67, 1.28, and 2.28 O/Fe¤) calculated at Z¤. The end-member oxygen models are also calculated at
each scaled Z value. The grid is complete for the MS until hydrogen is exhausted in the core.
Since it has been determined observationally that carbon and magnesium make the most
difference (after oxygen) to the stellar evolution [19, 20, 21], we have simulated tracks to
represent variations in these elements as well. The spread in values we use reLlects the diversity in
abundances that have been directly measured in nearby stars [12, 22, 23, 24, 25]. This more recent
grid includes a total of 480 models: 240 models each for variations in carbon and magnesium
(ranging from 0.58–1.72 C/Fe¤ and 0.54–1.84 Mg/Fe¤) [paper in prep].
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Evolutionary_Tracks_Database.html
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We are working to understand how stars of different mass and composition evolve, and how
stellar evolution directly inOluences the location of the habitable zone (HZ) around a star. It
is now estimated that more than 20% of all Sun-like stars and 50% of M-dwarfs may host a planet
in the HZ [1, 2, 3, 4, 5], the latter indicated by recent results from NASA’s Kepler mission. We have
created a large catalog of stellar evolution models for FGKM-type stars with variable compositions,
including the time dependent evolution of HZ boundaries using the prescriptions of [6, 7, 8, 9]. We
want to determine what kind of star could host a planet that would remain “continuously
habitable” for at least 2 billion years (CHZ_2). This is roughly the time it took for life on Earth to
change the atmosphere [10] such that it would be detectable with the kind of space missions
recommended in the most recent Decadal Review.
It is extremely important to consider the composition of a host star when we want to
determine the habitability potential of a planetary system. The models we have created for
this catalog are valuable for any work that requires accurate stellar evolution predictions. [26]
utilized TYCHO models to gauge the habitability potential for the Tau Ceti planetary system. We
have made our entire catalog of stellar models available in an online database [27], which
includes an interactive interpolation tool for plotting any track within our grid boundaries. This
work will also potentially help us identify likely host stars for habitable Earth-like planets.
We are working to update TYCHO to incorporate a new minimalist coupled evolution model for
estimating stellar X-ray activity, rotation, mass loss, and magnetic Lields [28]. We want to enable
the code to calculate the stellar activity changes with increasing age (for any of the stars in our
catalog and all future evolutionary track simulations). We also want to assign a spectral type to
each star in our catalog so that we can understand how spectral type changes with age and how it
correlates with stellar activity. This can be difLicult since a rather wide range of stellar masses
and compositions can look like the same spectral type without detailed observations.
We are also in the process of creating an additional grid of evolutionary tracks for M-dwarf
stars, essentially duplicating the original grid of 376 models with varying oxygen abundance
ratios, but with stellar masses from 0.1–0.45 M¤, in 0.05 M¤ increments.
Figure 1: Variations to the elemental abundance ratios directly impact both the MS lifetime, as well as
the location of the HZ. It is clear that for stars with higher compositions, the HZ is closer to the star, and
the star will live longer than a star of the same mass with lower opacity. Oxygen makes the largest
difference, followed by magnesium and carbon. The inner and outer HZ boundaries are deLined by the
“Runaway Greenhouse” and “Maximum Greenhouse” cases discussed in [6].
Figure 2: The “Continuously Habitable Zone” – (1) The bright green (vertical) shaded region shows the
overlap between ZAMS and TAMS, or the location around a star that a planet could remain habitable
for the entire MS lifetime. However, we need to deLine the CHZ more carefully. (2) The blue-green
(horizontal) shaded region is the CHZ_2, the range of orbital distances where an exoplanet would
remain in the liquid-water habitable zone for at least 2 Gyr. The inner edge is deLined as 2 Gyr after
ZAMS, and the outer edge is deLined as 2 Gyr before TAMS. Because of the much longer MS lifetimes of
the low-mass stars, a higher proportion of them meet the 2 Gyr criterion, and are therefore much more
likely to host a long-term habitable planet (also exempliLied by Table 2).
Table 1: Main Sequence lifetimes (Gyr) for each oxygen case of interest. It is clear that variations in
the speciLic abundance ratios alone will have a signiLicant impact on the stellar evolution.
Table 2: The fraction (%) of time a planet would spend in the CHZ(2) vs. time it would spend in the
HZ over the host star’s entire MS lifetime, for each carbon case of interest. This information may
help to quantify stars we should focus on in the continued search for habitable exoplanets.
Fig. 2: CHZ
For solar composition,
at all masses in original grid
Fig.1: HZ Distance vs. Stellar Age
(For solar mass, 1 AU line drawn for reference)
Figure 3: Synthetic spectrum for a 3400 K
M-type star. We generated this spectrum
using the PHOENIX (BT-Settl) simulator, in
which we have the ability to input mass,
luminosity, and any combination of speciLic
elemental abundances of interest. Ultimately,
we would like to use this kind of stellar
spectrum as an input for exoplanetary
atmospheric climate models, from which we
can generate transmission spectra that we
might expect to observe from planets that
orbit the kinds of stars in our catalog.
How can we use stellar evolution models to identify the best candidates for potentially
habitable exoplanet host stars? This is complicated by two factors: (1) the HZ is an ever-
changing entity because stars evolve over time; and (2) chemical abundances within stars are
measurably different, so two stars of identical mass may have divergent evolutionary paths
depending on speciLic composition [11]. We also want to understand how the speciOic elemental
composition (not just the total scaled metallicity) affects the stellar evolution. In stellar
modeling, it is typically only the iron abundance that is measured, and the abundances of all other
elements are assumed to scale in the same proportions as observed in the Sun; however, the
speciLic abundance ratios in real stars have been shown to vary substantially [12].
Fig. 3