There are many outstanding questions which improved age and mass estimates can answer about the nature of A-type stars and the systems they are in. Here, we summarize some of these questions and outline how improved estimates can contribute to their answers.
1.3.1 Observational Test of Evolution Models that Account for Rapid Rotation
Rapid rotation doesn’t only affect the observed properties of the star (e.g., von Zeipel 1924a,b, Section 1.1.1), it also affects how the star evolves (Sackmann 1970). The meridional flows that result from rapid rotation cycle hydrogen into the core, effectively giving a rapid rotator a longer main sequence lifespan than a more slowly rotating star of the same mass (Paxton et al. 2013). Figure 1.1 illustrates this; it is especially noticeable in the 2.0 and 2.5
M mass tracks that the properties of the modeled rapid rotators evolve less over 500 Myr
than do those for modeled slow rotators. It is only recently that sufficiently sophisticated evolutionary models have been developed that account for this effect (e.g, Maeder & Meynet 2010; Paxton et al. 2011, 2013). Observations of both rapidly and slowly rotating stars in clusters and moving groups (see Section 3.3.1) as well as observations of stars in wide bi- nary systems can potentially provide a self-consistency check between ages inferred for both rapidly and slowly rotating stars.
1.3.2 Evolutionary Snapshot of Disk and Exoplanetary Systems
The measured and inferred properties of debris disks require knowledge of the accurate stellar properties as the estimated temperatures and sizes of dust grains are dependent on the properties of the host star. Furthermore, having better age estimates will help constrain the timescales and mechanisms of disk dissipation and the transition from remnant to debris disks, placing much needed constraints on when and how planetesimals form. Since debris disks are believed to result from recent collisions of planetesimals (e.g., Johansen et al. 2015), better ages would constrain our understanding of when this occurs as well as the frequency of these events versus age. Better ages may also help determine how common episodes analogous to the late heavy bombardment (Gomes et al. 2005) are in other systems.
1.3.3 The Stellar Properties of Directly Imaged Planet Hosts
One common scientific driver for new observatories and instruments is to directly image and spectroscopically study the disks and planets of nearby stars. The known disks and planets
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of nearby A-stars make them preferred targets for this work (e.g., Brandt et al. 2014). The success of high-contrast imaging techniques such as ‘Extreme Adaptive Optics’,‘Nulling Interferometry’ or ‘Coronographic Imaging’ requires knowing the sizes and shapes of the target stars (e.g, Crepp et al. 2009).
As more directly imaged planets are bound to be discovered around nearby A-stars, having improved stellar properties and especially ages will be beneficial in several respects. Unlike more common planet discovery techniques, the mass of a directly imaged planet can- not be easily inferred from the observations that led to its discovery. Planetary cooling models (e.g., Baraffe et al. 2003; Baraffe et al. 2015) must be used to estimate a mass based on the measured luminosity or temperature of the star and its age. As a consequence, a more accurate estimate of the age of a star which harbors a directly imaged planet will lead to a more accurate estimation of the planet’s mass. In addition, ages in combination with basic orbital properties can help distinguish between proposed scenarios for migration, such as planet-disk interactions that must occur before the disk dissipates (e.g., Goldreich & Tremaine 1980; Lin et al. 1996), or interactions with other planets (e.g., Adams & Laughlin 2003) that can occur much later (Quinn et al. 2014). As these imaged planet populations grow, their ensemble properties can potentially distinguish between the proposed formation scenarios of core accretion and disk instabilities, which likely yield gas giant planets with distinctly different observable properties up to 1 Gyr (e.g., Fortney et al. 2008). Finally, the modeled mass of directly imaged planet host stars can be used to constrain the astrometri- cally determined orbits of the planets (e.g., Konopacky et al. 2016).
1.3.4 Evolutionary Snapshot of Chemically Peculiar Stars
With accurate age estimates of nearby A-stars, it is possible to address some of the questions that remain open about chemically peculiar stars. Is the λ Boo chemical peculiarity caused by disk material accreting onto a young star (Venn & Lambert 1990) or is it a manifestation of a diffusion process like the peculiarities of Am and Ap stars (Michaud & Charland 1986)? There are some early A-stars that appear to rotate slowly but contrary to the diffusion hypothesis don’t show any signs of chemical peculiarity. There are sufficient numbers of these systems that it is unlikely that they are rapid rotators oriented pole-on like Vega is (Royer et al. 2014). It is possible that these stars are truly slow rotators, but are young enough that there hasn’t been enough time for the diffusion process that typically leads to chemical peculiarity to show an effect. Accurate ages for chemically peculiar stars (and stars that ‘should’ be chemically peculiar but are not) will enlighten these still open questions about the nature of chemical peculiarity in A-stars.