Browsing by Author "Tang, Shih-Yun"

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    Measuring the Spot Variability of T Tauri Stars Using Near-infrared Atomic Fe and Molecular OH Lines
    (IOP Publishing, 2024) Tang, Shih-Yun; Johns-Krull, Christopher M.; Prato, L.; Stahl, Asa G.
    As part of the Young Exoplanets Spectroscopic Survey, this study explores the spot variability of 13 T Tauri Stars (TTSs) in the near-infrared H band, using spectra from the Immersion GRating INfrared Spectrometer. By analyzing effective temperature (T eff) sensitive lines of atomic Fe i at ∼1.56259 μm and ∼1.56362 μm, and molecular OH at ∼1.56310 and ∼1.56317 μm, we develop an empirical equivalent width ratio (EWR) relationship for T eff in the range of 3400–5000 K. This relationship allows for precise relative T eff estimates to within tens of Kelvin and demonstrates compatibility with solar metallicity target models. However, discrepancies between observational data and model predictions limit the extension of the T eff–EWR relationship to a broader parameter space. Our study reveals that both classical and weak-line TTSs can exhibit T eff variations exceeding 150 K over a span of 2 yr. The detection of a quarter-phase delay between the EWR and radial velocity phase curves in TTSs indicates spot-driven signals. A phase delay of 0.06 ± 0.13 for CI Tau, however, suggests additional dynamics, potentially caused by planetary interaction, inferred from a posited 1:1 commensurability between the rotation period and orbital period. Moreover, a positive correlation between T eff variation amplitude and stellar inclination angle supports the existence of high-latitude spots on TTSs, further enriching our understanding of stellar surface activity in young stars.
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    TESS Hunt for Young and Maturing Exoplanets (THYME). VI. An 11 Myr Giant Planet Transiting a Very-low-mass Star in Lower Centaurus Crux
    (IOP Publishing, 2022) Mann, Andrew W.; Wood, Mackenna L.; Schmidt, Stephen P.; Barber, Madyson G.; Owen, James E.; Tofflemire, Benjamin M.; Newton, Elisabeth R.; Mamajek, Eric E.; Bush, Jonathan L.; Mace, Gregory N.; Kraus, Adam L.; Thao, Pa Chia; Vanderburg, Andrew; Llama, Joe; Johns-Krull, Christopher M.; Prato, L.; Stahl, Asa G.; Tang, Shih-Yun; Fields, Matthew J.; Collins, Karen A.; Collins, Kevin I.; Gan, Tianjun; Jensen, Eric L. N.; Kamler, Jacob; Schwarz, Richard P.; Furlan, Elise; Gnilka, Crystal L.; Howell, Steve B.; Lester, Kathryn V.; Owens, Dylan A.; Suarez, Olga; Mekarnia, Djamel; Guillot, Tristan; Abe, Lyu; Triaud, Amaury H. M. J.; Johnson, Marshall C.; Milburn, Reilly P.; Rizzuto, Aaron C.; Quinn, Samuel N.; Kerr, Ronan; Ricker, George R.; Vanderspek, Roland; Latham, David W.; Seager, Sara; Winn, Joshua N.; Jenkins, Jon M.; Guerrero, Natalia M.; Shporer, Avi; Schlieder, Joshua E.; McLean, Brian; Wohler, Bill
    Mature super-Earths and sub-Neptunes are predicted to be ≃ Jovian radius when younger than 10 Myr. Thus, we expect to find 5–15 R ⊕ planets around young stars even if their older counterparts harbor none. We report the discovery and validation of TOI 1227b, a 0.85 ± 0.05 R J (9.5 R ⊕) planet transiting a very-low-mass star (0.170 ± 0.015 M ⊙) every 27.4 days. TOI 1227's kinematics and strong lithium absorption confirm that it is a member of a previously discovered subgroup in the Lower Centaurus Crux OB association, which we designate the Musca group. We derive an age of 11 ± 2 Myr for Musca, based on lithium, rotation, and the color–magnitude diagram of Musca members. The TESS data and ground-based follow-up show a deep (2.5%) transit. We use multiwavelength transit observations and radial velocities from the IGRINS spectrograph to validate the signal as planetary in nature, and we obtain an upper limit on the planet mass of ≃0.5 M J. Because such large planets are exceptionally rare around mature low-mass stars, we suggest that TOI 1227b is still contracting and will eventually turn into one of the more common <5 R ⊕ planets.