Browsing by Author "Prato, L."

Now showing 1 - 7 of 7
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    A SEARCH FOR GIANT PLANET COMPANIONS TO T TAURI STARS
    (The American Astronomical Society, 2012) Crockett, Christopher J.; Mahmud, Naved I.; Prato, L.; Johns-Krull, Christopher M.; Jaffe, Daniel T.; Hartigan, Patrick M.; Beichman, Charles A.
    We present results from an ongoing multiwavelength radial velocity (RV) survey of the Taurus–Auriga star-forming region as part of our effort to identify pre-main-sequence giant planet hosts. These 1–3 Myr old T Tauri stars present significant challenges to traditional RV surveys. The presence of strong magnetic fields gives rise to large, cool star spots. These spots introduce significant RV jitter which can mimic the velocity modulation from a planet-mass companion. To distinguish between spot-induced and planet-induced RV modulation, we conduct observations at ∼6700Åand∼2.3μmand measure thewavelength dependence (if any) in theRVamplitude. CSHELL observations of the known exoplanet host Gl 86 demonstrate our ability to detect not only hot Jupiters in the near-infrared but also secular trends from more distant companions. Observations of nine very young stars reveal a typical reduction in RV amplitude at the longer wavelengths by a factor of ∼2–3. While we cannot confirm the presence of planets in this sample, three targets show different periodicities in the two wavelength regions. This suggests different physical mechanisms underlying the optical and the K-band variability.
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    Amplitude Modulation of Short-timescale Hot Spot Variability
    (IOP Publishing, 2021) Biddle, Lauren I.; Llama, Joe; Cameron, Andrew; Prato, L.; Jardine, Moira; Johns-Krull, Christopher M.
    Variability of Classical T Tauri stars (CTTS) occurs over a vast range of timescales. CTTS in particular are subject to variability caused by accretion shocks, which can occur stochastically, periodically, or quasi-periodically on timescales over a few days. The detectability of young planets within these systems is likely hampered by activity; therefore, it is essential that we understand the origin of young star variability over a range of timescales to help disentangle stellar activity from signatures of planetary origin. We present an analysis of the stochastic small-amplitude photometric variability in the K2 lightcurve of CI Tau occurring on timescales of  lesssim1 day. We find the amplitude of this variability exhibits the same periodic signatures as detected in the large-amplitude variability, indicating that the physical mechanism modulating these brightness features is the same. The periods detected are also in agreement with the rotation period of the star (~6.6 days) and the orbital period of the planet (~9.0 days) known to drive pulsed accretion onto the star.
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    High-resolution Millimeter Imaging of the CI Tau Protoplanetary Disk: A Massive Ensemble of Protoplanets from 0.1 to 100 au
    (IOP Publishing, 2018) Clarke, C.J.; Tazzari, M.; Juhasz, A.; Rosotti, G.; Booth, R.; Facchini, S.; Ilee, J.D.; Johns-Krull, C.M.; Kama, M.; Meru, F.; Prato, L.
    We present high-resolution millimeter continuum imaging of the disk surrounding the young star CI Tau, a system hosting the first hot Jupiter candidate in a protoplanetary disk system. The system has extended mm emission on which are superposed three prominent annular gaps at radii ~13, 39, and 100 au. We argue that these gaps are most likely to be generated by massive planets so that, including the hot Jupiter, the system contains four gas giant planets at an age of only 2 Myr. Two of the new planets are similarly located to those inferred in the famous HL Tau protoplanetary disk; in CI Tau, additional observational data enables a more complete analysis of the system properties than was possible for HL Tau. Our dust and gas dynamical modeling satisfies every available observational constraint and points to the most massive ensemble of exoplanets ever detected at this age, with its four planets spanning a factor 1000 in orbital radius. Our results show that the association between hot Jupiters and gas giants on wider orbits, observed in older stars, is apparently in place at an early evolutionary stage.
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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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    Radial Velocity Monitoring of the Young Star Hubble 4: Disentangling Star-spot Lifetimes from Orbital Motion
    (IOP Publishing, 2021) Carvalho, Adolfo; Johns-Krull, Christopher M.; Prato, L.; Anderson, Jay
    We studied the weak-lined T Tauri star Hubble 4, a known long-period binary, and its star-spot phenomena. We used optical radial velocity (RV) data taken over a span of 14 yr (2004–2010, 2017–2019) at the McDonald Observatory 2.7 m Harlan J. Smith Telescope and single epoch imaging from the Hubble Space Telescope (HST)/Wide Field Camera 3 instrument. The observed and apparent RV variations show contributions, respectively, from the binary motion as well as from a large spot group on one of the stars, presumed to be the primary. Fitting and removing the orbital signal from the RVs, we found the lower bound on the lifetime of a previously identified large spot group on the surface of the star to be at least 5.1 yr. An ~5 yr lower limit is a long, but not unprecedented, duration for a single spot group. The later epoch data indicate significant spot evolution has occurred, placing an upper bound on the spot group lifetime at 12 yr. We find that pre-main-sequence evolutionary models for the age of Taurus (~2 Myr), combined with component mass estimates from the literature, permit us to reproduce the HST relative photometry and the binary-induced contribution to the apparent RV variations. The long-lived star spot we find on Hubble 4 has significant implications for dynamo models in young stars, as it adds evidence for long lifetimes of magnetic field topologies. There are also significant implications for young star exoplanet searches, as long-lived coherent RV signals may be spot induced and not the result of planetary motion.
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    A search for giant planet companions to t tauri stars
    (The American Astronomical Society, 2012-12-05) Crockett, Christopher J.; Mahmud, Naved I.; Prato, L.; Johns-Krull, Christopher M.; Jaffe, Daniel T.; Hartigan, Patrick M.; Beichman, Charles A.
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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.