Browsing by Author "Johns-Krull, Christopher M"

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    Characterizing Young Stellar Systems And Their Suitability For Radial Velocity-Driven Exoplanet Detection
    (2023-03-15) Stahl, Asa; Johns-Krull, Christopher M
    The detection and characterization of planets around young stars presents a direct window into some of the most transformative and poorly understood processes of planet evolution, such as formation mechanisms, disk migration, and (perhaps) gap formation. Radial velocity detections can be particularly powerful for a number of reasons, not least of which is their ability to directly constrain planet masses and eccentricities. Yet despite RV analysis’ prolific track record when applied to main sequence stars, employing it to search for planets around young stars has proven exceptionally difficult because of stellar activity. This thesis is dedicated to the development and application of multiwavelength tools for RV planet searches as well as to enlarging the sample of young stars suitable for such searches. It presents work that forwards these causes in three ways: first, by presenting three software pipelines that measure RVs from reduced spectra taken with three different instruments; second, by validating a sample of young stars through follow-up spectroscopy and photometry; and third, by performing a multiwavelength RV planet search of several accreting and non-accreting T-Tauri stars.
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    Diagnosing Mass Flows Around Herbig Ae/Be Stars
    (2014-10-16) Cauley, Paul Wilson; Johns-Krull, Christopher M; Hartigan, Patrick M; Gustin, Michael
    Most stars form surrounded by a massive disk of dust and gas. As the star evolves, its interaction with the surrounding disk material has a significant impact on both the final evolutionary state of the star and the amount of material that is available in the disk to form planets, organic compounds, and, ultimately, life. Herbig Ae/Be stars (HAEBES) are recently formed intermediate mass (2-10 solar masses) pre--main sequence stars. Over 150 candidate HAEBES have been identified by various surveys. Although this number is large, it is relatively small compared to the number of identified classical T Tauri stars (CTTSs), the low mass (~1 solar mass) cousins of HAEBES. Although it is well established that both CTTSs and HAEBES are still evolving towards the main sequence, our understanding of how accretion and outflows operate around HAEBES compared to CTTSs is incomplete and there remains debate over the key launching mechanisms for the outflows in CTTSs. This is in large part due to the lack of comparisons of multi-wavelength mass flow diagnostics in large samples of HAEBES and CTTSs. In this thesis we attempt to address the gap in our knowledge of the driving mechanisms of accretion and outflows around HAEBES, and how they compare to those of CTTSs, by examining a wide variety of accretion and outflow diagnostics in the ultraviolet, optical, and near infrared with the goal of constraining the incidence of accretion and wind flows around these objects. A different incidence of accretion and outflow detections in HAEBES compared to CTTSs would indicate that the mechanisms governing the production of these flows in HAEBES differ from those in CTTSs and we can look to other key differences between these classes of stars to try to identify the mechanisms that control the flows. To accomplish this we analyze high resolution line profiles of a large number of spectral lines that are known to be good tracers of accretion and outflows around CTTSs. Our analysis reveals a significant difference between the occurrence of blue and red-shifted absorption features in HAEBES compared to CTTSs. This difference is largest for outflow signatures in the optical and is less significant in He I 10830, the near-IR diagnostic. We find that the incidence of red and blue-shifted absorption in HAEBES increases from the optical to the UV with intermediate rates being found in He I 10830. This suggests that hot (~100,000 K) mass flows are more common around HAEBES than cooler (~10,000 K) flows. We also find significant differences between the occurrence of red and blue--shifted absorption in HAe stars compared to HBe stars. In particular, our results support the idea of mangetospheric accretion occurring in HAe stars but provide more evidence for boundary layer accretion in HBe objects. In addition, we observe that the maximum red--shifted absorption velocities tracing infalling material in our sample are smaller fractions of the stellar escape velocity than is found for CTTSs. This is confirmed in both the optical diagnostics and at He I 10830. We suggest that this is a result of smaller magnetospheres mediating the accretion flows in HAEBES compared to CTTSs. We point out that smaller magnetospheres, and thus smaller infall velocities of material from the disk, will result in less energy being deposited onto the star which may in turn affect the launching of outflows. If winds from HAEBES are driven in part by accretion, which they appear to be in CTTSs, the smaller relative amount of energy deposited by the accretion flows onto HAEBES may result in less outflow activity, as observed. In particular, we observe a negligible fraction of objects that display simultaneous blue and red--shifted absorption signatures, a commonly observed trait in CTTSs. This supports the idea that accretion onto HAEBES is less efficient at driving outflows. Our results make it clear that the various mechanisms involved in controlling accretion and outflows in young stars do not, in general, operate in the same way around HAEBES and CTTSs.
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    Magnetic Fields and Accretion on T Tauri Stars
    (2015-04-20) Chen, Wei; Johns-Krull, Christopher M; Hartigan, Patrick M; Lenardic, Adrian
    In this thesis, we study magnetic properties of Classical T Tauri Stars (CTTSs) using diffferent analysis methods from different angles. We first implement a least-squares deconvolution (LSD) code to enhance the signal-to-noise ratio of the observed data and therefore the ability to detect weak magnetic signatures in the polarized spectra. We compare our code to another LSD code by analyzing the same spectra with the same line list, and the good agreement confirms that our code performs similarly with other state of the art LSD codes. We have also tested the instrument we use to collect the spectropolarimetric data presented in this thesis. The intrument do produce spurious signals but at a very low level, posing no problem in our analysis. The polarization efficiency of the instrument is also high. We then studied the CTTS BP Tau and found field configuration not consistent with other results in the literature, which may simply be due to the temporal variability given the observations were taken 8 years apart. Next, we detect extremely high levels of polarization on TW Hya in the 2013 epoch, which motivated us do a thorough magnetic variability study. We find that the field geometry does have changed, indicating a dynamo origin of these fields. We also try to use the rotational modulation of 6 CTTSs to locate the accretion spots. Some of the stars have shown definte change over the years while some remain similar.
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    On the Evolution of Planets: From Convective Bi-stability to Volcanic Edifice Instability
    (2015-11-24) Weller, Matt; Lenardic, Adrian; Morgan, Julia K; Sawyer, Dale S; McGovern, Patrick J; Johns-Krull, Christopher M
    The Eastern Olympus Mons Basal Scarp (EOMBS) is conditionally stable when the edifice contains pore fluid, and critically stable, or in failure, when the edifice contains a dipping-overpressured-confined aquifer and mechanical sublayer at depth. Failure of the fault bounded portion of the flank results in estimated volumes of material ranging from 5600–6900km3, or 32–39% of the estimated volume of the “East” Olympus Mons aureole lobe. We suggest that the EOMBS faults may be an expression of early stage flank collapse and aureole lobe formation. Ages of deformed volcano adjacent plains indicate that this portion of the edifice may have been tectonically active at <50Ma, and may be coeval with estimated ages of adjacent outflow channels, 25–40Ma. This observation suggests that conditions that favor flank failure, such as water at depth below the edifice, existed in the relatively recent past and potentially could drive deformation to the present day. Coupled 3D mantle convection and planetary tectonics models are used to explore the links between tectonic-regimes, the level of internal heating (Q) within the mantle, planetary surface-temperature, and planetary lithospheric-strength. At high and low values of Q, for moderate to high yield, hot and cold single-plate planets prevail. For intermediate Q, multiple stable tectonic-states exist. In this parameter space, the specific evolutionary path of the system has a dominant role in determining its tectonic state. For low to moderate lithospheric yield strength, mobile-lid behavior (a plate tectonic-like mode of convection) is attainable for high degrees of internal heating (i.e., early in a planet’s thermal evolution). This state is sensitive to climate driven changes in surface-temperatures. Relatively small increases in surface-temperature can be sufficient to usher in a transition from a mobile- to a stagnant-lid regime. Once stagnant, a return to mobile-lid is not attainable by a reduction of surface-temperatures alone. For lower levels of Q, the tectonic regime becomes less sensitive to surface-temperature changes. These results indicate that terrestrial planets can alternate between multiple tectonic-states over giga-year timescales. Within parameter space regions that allow for bi-stable behavior, any model-based prediction as to the current mode of tectonics is inherently non-unique in the absence of constraints on the geologic and climatic histories of a planet.