Browsing by Subject "Planet formation"

Now showing 1 - 4 of 4

Exploring the formation and evolutionary pathways of young stars and planetary systems at high precision
(2022-08-12) Krolikowski, Daniel Milker; Kraus, Adam L.; Bowler, Brendan; Covey, Kevin; Hawkins, Keith; Morley, Caroline
The star and planet formation process is one continuous event on scales from a giant molecular cloud to an individual circumstellar disk. There are a multitude of competing theories for the dominant physical mechanisms acting at each stage of this process, ranging from cloud conditions to how planet characteristics change over time. The properties of young stellar associations, the stars within them, and their planetary systems are crucial direct tests of these competing models. However, finding and characterizing isolated young systems is hard. Our best opportunity is to measure the ensemble age of a coeval group of stars, and observe systems within them to study the inherently intertwined star and planet formation and evolution process. In my dissertation, I use precision observations of young stars and planetary systems to explore their formation and evolutionary pathways. I first present a comprehensive census of the Taurus star forming region to reconstruct its complex star forming history. I use Gaia astrometry to reveal its highly substructured nature, identifying subgroups with age spreads indicating a prolonged star forming event, and kinematics indicating a typical turbulent environment and early dynamical evolution. Taurus is likely connected to a long-lasting, larger-scale galactic star forming event that can only be uncovered in the Gaia era. I then discuss exoplanet-related projects using high precision NIR spectroscopy from the Habitable-zone Planet Finder. Young stars are highly active, which introduces significant noise in spectroscopic observations. In a sample of young transiting planet hosts, I characterize the NIR helium spectral feature, which is an important probe of atmospheric mass loss and conditions in the stellar chromosphere. Stellar helium variability decreases with age, reflecting the higher activity levels in youth, but the line strength is constant beyond 100 Myr implying similar line formation conditions across the sample. Stellar variability should not preclude detection of mass loss at young ages. With this same data set, I search for giant planets exterior to the known transiting planets to measure their occurrence rate and constrain the typical dynamical history of a planetary system. I find three candidate signals of long-period companions, although the occurrence rate remains largely unconstrained. My dissertation exemplifies the power that precision observations of young stars has to improve our understanding of the complicated and interrelated processes of star and planet formation and evolution.
Probing giant-planet forming zones around Solar-like stars with CO
(2017-08) Yu, Mo, Ph. D.; Kraus, Adam L.; Evans, Neal J.; Dodson-Robinson, Sarah; Willacy, Karen; Lacy, John; Jaffe, Daniel; Bergin, Edwin
Protoplanetary disks are dusty disks around young stars where planets are formed. The evolution and composition of protoplanetary disks determine the time, environments and materials available for planet formation. However fundamental properties of protoplanetary disks such as mass, composition, and the angular momentum transfer mechanism are poorly constrained by observations. In this dissertation, we discuss the thermal and chemical evolution of protoplanetary disks around Solar-type stars, and evaluate methods to measure two key parameters - disk mass and turbulent velocity in the framework of an evolving disk system. We first build a chemical evolution model based on an MRI-active disk around a Solar-type star, and discuss the chemical depletion of CO due to the formation of complex organic molecules (Chapter 2). We then investigate the challenges one faces when measuring disk masses with CO due to the chemical depletion of CO and optical depth effects (Chapter 3). We propose strategies to correct for the CO depletion effect and constrain the disk mass within factor of a few accuracy. We also investigate the possibility of constraining turbulent velocities with CO line profiles in Chapter 4. Peak-to-trough ratios of CO rotational lines have been proposed as a robust probe for turbulent velocity. However we show that the peak-to-trough ratio could vary by $25\%$ due uncertainties in effects of CO depletion. One would underestimate the degree of turbulence if the chemical depletion of CO is not properly accounted for.
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.
Tracing the CO “ice line'' in an MRI-active protoplanetary disk with rare CO isotopologues
(2013-08) Yu, Mo, Ph. D.; Dodson-Robinson, Sarah E.
The properties of planet-forming midplanes of protostellar disks remain largely unprobed by observations due to the high optical depth of common molecular lines and continuum. However, rotational emission lines from rare isotopologues may have optical depth near unity in the vertical direction, so that the lines are strong enough to be detected, yet remain transparent enough to trace the disk midplane. In this thesis, we present a chemical model of an MRI-active protoplanetary disk including different C, O isotopes and detailed photochemical reactions. The CO condensation front is found to be at 1.5 AU on the disk midplane around a solar like star, and its location remains almost unchanged during 3Myr of evolution. The optical depth of low-order rotational lines of C¹⁷O are around unity, which suggests it may be possible to see into the disk midplane using C¹⁷O. Such ALMA observations would provide estimates of the disk midplane temperature if the CO ice lines were spatially or spectrally resolved. With our computed C¹⁷O/H₂ abundance ratio, one would also be able to measure the disk masses by measuring the intensity of gas emission.