Browsing by Subject "Stellar associations"

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Mapping nearby young stellar structures and their star formation histories
(2021-12-02) Kerr, Ronan Murdoch Paterson; Kraus, Adam L.
Young stellar associations hold a star formation record that can persist for millions of years, revealing the progression of star formation long after the dispersal of the natal cloud. To identify nearby young stellar populations that trace this progression, we have designed a comprehensive framework for the identification of young stars, and use it to identify ~3x10⁴ candidate young stars within a distance of 333 pc using Gaia DR2. Applying the HDBSCAN clustering algorithm to this sample, we identify 27 top-level groups, nearly half of which have little to no presence in previous literature. Ten of these groups have visible substructure, including notable young associations such as Orion, Perseus, Taurus, and Sco-Cen. We provide a complete subclustering analysis on all groups with substructure, using age estimates to reveal each region's star formation history. The patterns we reveal include an apparent star formation origin for Sco-Cen along a semicircular arc, as well as clear evidence for sequential star formation moving away from that arc with a propagation speed of ~4 km s⁻¹ (~4 pc Myr⁻¹). We also identify earlier bursts of star formation in Perseus and Taurus that predate current, kinematically identical active star-forming events, suggesting that the mechanisms that collect gas can spark multiple generations of star formation, punctuated by gas dispersal and cloud regrowth. The large spatial scales and long temporal scales on which we observe star formation offer a bridge between the processes within individual molecular clouds and the broad forces guiding star formation at galactic scales.
Revealing star and planet formation with stellar multiplicity
(2023-08-07) Sullivan, Kendall; Kraus, Adam L.; Herczeg, Gregory; Offner, Stella; Morley, Caroline; Hawkins, Keith
Studies of star and planet formation work to understand the processes that produced the Solar System and the many other systems now known to host exoplanets. Understanding star and planet formation requires measurement of accurate stellar properties at all evolutionary stages of stellar and planetary systems. These stellar properties include age, mass, effective temperature (T [subscript eff]), stellar radius, and stellar multiplicity. Binary stars and higher-order multiples comprise about half of the population of main-sequence solar-type stars, and stellar multiplicity impacts the observed properties of stars across their lifetimes. Because exoplanet and stellar demographics are typically inferred from stellar properties, incorrect stellar characterization because of binaries feeds into biases and errors in stellar populations and exoplanet demographics. In this dissertation, I explored the impact of binary stars in the two scientific contexts of young stellar associations and binary stars that host exoplanets. In my studies of young stellar associations, I developed a simulation suite to perform synthetic spectroscopic surveys. I implemented mass-dependent binary properties to explore the origins of apparent mass-dependent age gradients previously observed in star-forming regions. My subsequent work added starspots to the simulation. I found that although binary stars can explain mass-dependent age gradients, starspots become the dominant contributor to the gradient in populations with Gaia distances. I also explored the nature of the relationship between accretion and circumstellar disks in young stars and found that the inner disks of binaries and single stars are probably similar, and that the inner rim of the dust disk is related to the accretion rate as a result of mass transfer through the disk. These studies demonstrated the importance of considering binary stars when attempting to measure ages or understand star formation histories in young stellar associations. In my studies of main sequence binary star exoplanet hosts, I developed an algorithm to accurately characterize the individual components of binary stars that are unresolved in most observations. As an initial step, I tested this code with an archival sample of M stars. Then, I performed a spectroscopic survey of binary stars from the Kepler sample using the Hobby-Eberly Telescope, and carried out two targeted studies of subsamples from the survey. The first study explored binary stars that supposedly host rocky Earth-analog planets and found that most of them are actually gaseous planets, which has implications for exoplanet demographics and attempts to measure the frequency of Earth analogs. The second study explored the radius distribution of small exoplanets and found that the gap in the radius distribution separating rocky and gaseous exoplanets in single systems was not present in binary stars. This result suggested that the location of the gap may be binary-separation-dependent and therefore “blurred out” by a range of stellar separations in the sample. This series of papers has demonstrated the power of using binary stars that host planets as a laboratory for controlled experiments in planet formation and evolution, because the binary properties leave a record of the planet-forming environment. The work presented in this dissertation has shown the ability of binary stars to influence observations of young stars and exoplanet hosts, and has demonstrated the potential of binary stars to provide a direct link between formation environment and exoplanet properties for the first time.