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8 results on '"Christopher Johns-Krull"'

3. Follow-up of Young Stars Identified with BANYAN Σ: New Low-mass Members of Nearby Moving Groups

Author

Laura Flagg, Christopher Johns-Krull, and Asa Stahl

Subjects
Space and Planetary Science, Astronomy and Astrophysics
Abstract

The characterization of moving groups offers a powerful means to identify large populations of young stars. In this paper, we present a sample of follow-up observations for 56 systems that have previously been proposed as members of young stellar associations through the application of the BANYAN Σ kinematic classification tool. Our measurements, which probe seven different associations, provide a sample of 39 stellar systems that either are confirmed or appear consistent with being young members of their respective associations. Nineteen of these are single M dwarfs. This sample expands our knowledge of Upper Centaurus Lupus, Coma Berenices, and AB Doradus Major to cooler temperatures and also significantly increases the known population of the Carina-Near association. The young systems present excellent targets for future planet searches and would also be valuable for studies of star formation and evolution. Additionally, we find two stellar systems that show indications of being rare instances of late-stage circumstellar accretion. Lastly, our follow-up measurements serve as a test of BANYAN Σ, finding an overall contamination rate that is consistent with previous findings (29% for systems with RV measurements, 37% without).

Published
2022
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4. A CO-to-H2 Ratio of ≈10−5 toward the Herbig Ae Star HK Ori

Author

Gregory Herczeg, Christopher Johns-Krull, Paul Wilson Cauley, and KEVIN FRANCE

Subjects
Physics, Space and Planetary Science, Ultraviolet astronomy, Star formation, Astronomy and Astrophysics, Astrophysics, Star (graph theory), Circumstellar disk
Abstract

Measurements of gas mass in protoplanetary gas disks form the basis for estimating the conditions of planet formation. Among the most important constraints derived from disk diagnostics are the abundances of gas-phase species critical for understanding disk chemistry. Toward this end, we present direct line-of-sight measurements of H2 and CO, employing UV absorption spectroscopy from Hubble Space Telescope Cosmic Origins Spectrograph to characterize disk composition, molecular excitation temperatures, and spatial distribution in the circumstellar material around the Herbig Ae stars HK Ori and T Ori. We observe strong CO (N(CO) = 1015.5 cm−2; T rot(CO) = 19 K) and H2 (N(H2) = 1020.34 cm−2; T rot(H2) = 141 K) absorption toward HK Ori with a CO/H2 ratio of (≡N(CO)/N(H2)) = × 10−5. These measurements place direct empirical constraints on the CO-to-H2 conversion factor in the disk around a Herbig Ae star for the first time, although there is uncertainty concerning the exact viewing geometry of the disk. The spectra of T Ori show CO (N(CO) = 1014.9 cm−2; T rot(CO) = 124 K) absorption. Interestingly, we do not detect any H2 absorption toward this star (N(H2) < 1015.9 cm−2). We discuss a potential scenario for the detection of CO without H2, which deserves further investigation. The low abundance ratio measured around HK Ori suggests significant depletion of CO in the circumstellar gas, which conforms with the handful of other recent CO abundance measurements in protoplanetary disks.

Published
2021
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5. Observing Planetary Systems in the Making

Author

Andrea Isella, Luca Ricci, Sean Andrews, Clément Baruteau, Jean-Philippe Berger, Edwin Bergin, Til Birnstiel, Brendan Bowler, Crystal Brogan, Carlos Carrasco Gonzalez, Claire Chandler, Thayne Currie, Jeffrey Cuzzi, Angelo, Gennaro D., Ruobing Dong, Gaspard Duchene, Anne Dutrey, Barbara Ercolano, Catherine Espaillat, Paul Estrada, Mario Flock, Andras Gaspar, Greene, Thomas P., Jane Huang, Hannah Jang-Condell, Christopher Johns-Krull, Grant Kennedy, Serena Kim, J., Florian Kirchschlager, Stefan Kraus, Sebastiaan Krijt, Hui Li, Wladimir Lyra, Bruce Macintosh, John Monnier, Karin Oberg, Ilaria Pascucci, Laura Perez, Romain Petrov, Paola Pinilla, Stephen Ridgway, Keivan Stassun, Theo ten Brummelaar, Leonardo Testi, Turner, Neal J., Gerard van Belle, Nienke van Der Marel, Alycia Weinberger, Jacob White, Jonathan Williams, David Wilner, Alwyn Wootten, Ya-Lin Wu, Andrew Youdin, Ke Zhang, Zhaohuan Zhu, Astrophysique Interprétation Modélisation (AIM (UMR7158 / UMR_E_9005 / UM_112)), Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris Diderot - Paris 7 (UPD7)-Centre National de la Recherche Scientifique (CNRS), Harvard-Smithsonian Center for Astrophysics (CfA), Harvard University-Smithsonian Institution, McDonald Observatory, University of Texas at Austin [Austin], NASA Ames Research Center (ARC), Steward Observatory, University of Arizona, Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Observatoire des Sciences de l'Univers de Grenoble (OSUG ), Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes [2016-2019] (UGA [2016-2019])-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes [2016-2019] (UGA [2016-2019]), AMOR 2019, Laboratoire d'Astrophysique de Bordeaux [Pessac] (LAB), Université de Bordeaux (UB)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université de Bordeaux (UB)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Search for Extraterrestrial Intelligence Institute (SETI), Max-Planck-Institut für Radioastronomie (MPIFR), Department of Astrophysics, American Museum of Natural History (AMNH), Umeå University, Molecular Genetics Unit, Instituto de Salud Carlos III [Madrid] (ISC)-Carretera Pozuelo - Callejero de Majadahonda- Instituto de Investigación en Enfermedades Raras (IIER), Laboratoire Universitaire d'Astrophysique de Nice (LUAN), Université Nice Sophia Antipolis (1965 - 2019) (UNS), COMUE Université Côte d'Azur (2015-2019) (COMUE UCA)-COMUE Université Côte d'Azur (2015-2019) (COMUE UCA)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), National Optical Astronomy Observatory (NOAO), Center for High Angular Resolution Astronomy (CHARA), Georgia State University, University System of Georgia (USG)-University System of Georgia (USG), INAF - Osservatorio Astrofisico di Arcetri (OAA), Istituto Nazionale di Astrofisica (INAF), School of Mathematical and Statistical Sciences (Arizona, Tempe), Arizona State University [Tempe] (ASU), Centre Européen de Réalité Virtuelle (CERV), École Nationale d'Ingénieurs de Brest (ENIB), Flathead Lake Biological Station, University of Montana, Astrophysique Interprétation Modélisation (AIM (UMR_7158 / UMR_E_9005 / UM_112)), Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Paris Diderot - Paris 7 (UPD7), Smithsonian Institution-Harvard University [Cambridge], American Museum of Natural History, Université Nice Sophia Antipolis (... - 2019) (UNS), Rice University [Houston], Joseph Louis LAGRANGE (LAGRANGE), Université Côte d'Azur (UCA)-Université Nice Sophia Antipolis (... - 2019) (UNS), COMUE Université Côte d'Azur (2015-2019) (COMUE UCA)-COMUE Université Côte d'Azur (2015-2019) (COMUE UCA)-Observatoire de la Côte d'Azur, COMUE Université Côte d'Azur (2015-2019) (COMUE UCA)-Université Côte d'Azur (UCA)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Harvard University [Cambridge]-Smithsonian Institution, Laboratoire d'Astrophysique de Grenoble (LAOG), Université Joseph Fourier - Grenoble 1 (UJF)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Instituto de Salud Carlos III (ISC)-Carretera Pozuelo - Callejero de Majadahonda- Instituto de Investigación en Enfermedades Raras (IIER), and Université Côte d'Azur (UCA)-Université Côte d'Azur (UCA)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)

Subjects
[SDU.ASTR.SR]Sciences of the Universe [physics]/Astrophysics [astro-ph]/Solar and Stellar Astrophysics [astro-ph.SR], [SDU.ASTR.CO]Sciences of the Universe [physics]/Astrophysics [astro-ph]/Cosmology and Extra-Galactic Astrophysics [astro-ph.CO], Computer Science::Computer Vision and Pattern Recognition, Physics::Space Physics, [SDU.ASTR.EP]Sciences of the Universe [physics]/Astrophysics [astro-ph]/Earth and Planetary Astrophysics [astro-ph.EP], ComputingMethodologies_IMAGEPROCESSINGANDCOMPUTERVISION, Astrophysics::Earth and Planetary Astrophysics, GeneralLiterature_MISCELLANEOUS, Astrophysics::Galaxy Astrophysics, [SDU.ASTR.IM]Sciences of the Universe [physics]/Astrophysics [astro-ph]/Instrumentation and Methods for Astrophysic [astro-ph.IM]
Abstract

International audience; We discuss science cases to support the development of sub-au imaging capabilities to image forming planets in the terrestrial region of nearby proto-planetary disks.

Published
2019

6. NEAR-ULTRAVIOLET EXCESS IN SLOWLY ACCRETING T TAURI STARS: LIMITS IMPOSED BY CHROMOSPHERIC EMISSION

Author

Laura Ingleby, Nuria Calvet, Edwin Bergin, Gregory Herczeg, Alexander Brown, Richard Alexander, Suzan Edwards, Catherine Espaillat, Kevin France, Scott G. Gregory, Lynne Hillenbrand, Evelyne Roueff, Jeff Valenti, Frederick Walter, Christopher Johns-Krull, Joanna Brown, Jeffrey Linsky, Melissa McClure, David Ardila, Hervé Abgrall, Thomas Bethell, Gaitee Hussain, and Hao Yang

Subjects
Physics, Cosmic Origins Spectrograph, 010308 nuclear & particles physics, Astrophysics::High Energy Astrophysical Phenomena, Astronomy, Astronomy and Astrophysics, Astrophysics::Cosmology and Extragalactic Astrophysics, Astrophysics, 01 natural sciences, Photoevaporation, Accretion (astrophysics), Stars, T Tauri star, 13. Climate action, Space and Planetary Science, 0103 physical sciences, Astrophysics::Solar and Stellar Astrophysics, Astrophysics::Earth and Planetary Astrophysics, Emission spectrum, 010303 astronomy & astrophysics, Stellar evolution, Astrophysics::Galaxy Astrophysics, Space Telescope Imaging Spectrograph
Abstract

Young stars surrounded by disks with very low mass accretion rates are likely in the final stages of inner disk evolution and therefore particularly interesting to study. We present ultraviolet (UV) observations of the ~5-9 Myr old stars RECX-1 and RECX-11, obtained with the Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph on the Hubble Space Telescope, as well as optical and near-infrared spectroscopic observations. The two stars have similar levels of near-UV emission, although spectroscopic evidence indicates that RECX-11 is accreting and RECX-1 is not. The line profiles of Hα and He I λ10830 in RECX-11 show both broad and narrow redshifted absorption components that vary with time, revealing the complexity of the accretion flows. We show that accretion indicators commonly used to measure mass accretion rates, e.g., U-band excess luminosity or the Ca II triplet line luminosity, are unreliable for low accretors, at least in the middle K spectral range. Using RECX-1 as a template for the intrinsic level of photospheric and chromospheric emission, we determine an upper limit of 3 × 10^(–10) M_☉ yr–1 for RECX-11. At this low accretion rate, recent photoevaporation models predict that an inner hole should have developed in the disk. However, the spectral energy distribution of RECX-11 shows fluxes comparable to the median of Taurus in the near-infrared, indicating that substantial dust remains. Fluorescent H2 emission lines formed in the innermost disk are observed in RECX-11, showing that gas is present in the inner disk, along with the dust.

Published
2011
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7. The Near-ultraviolet Continuum Radiation in the Impulsive Phase of HF/GF-type dMe Flares. I. Data.

Author

Adam F. Kowalski, John P. Wisniewski, Suzanne L. Hawley, Rachel A. Osten, Alexander Brown, Cecilia Fariña, Jeff A. Valenti, Stephen Brown, Manolis Xilouris, Sarah J. Schmidt, and Christopher Johns-Krull

Subjects
SPECTROGRAPHS, WAVELENGTHS, HYDRODYNAMICS, COMPUTER simulation
Abstract

We present near-UV (NUV) flare spectra from the Hubble Space Telescope (HST)/Cosmic Origins Spectrograph during two moderate-amplitude U-band flares on the dM4e star GJ 1243. These spectra are some of the first accurately flux-calibrated, NUV flare spectra obtained over the impulsive phase in M dwarf flares. We observed these flares with a fleet of nine ground-based telescopes simultaneously, which provided broadband photometry and low-resolution spectra at the Balmer jump. An increase in the broadband continuum occurred with a signal-to-noise ratio >20 in the HST spectra, while numerous Fe ii lines and the Mg ii lines also increased but with smaller flux enhancements than the continuum radiation. These two events produced the most prominent Balmer line radiation and the largest Balmer jumps that have been observed to date in dMe flare spectra. A T = 9000 K blackbody underestimates the NUV continuum flare flux by a factor of two and is a poor approximation to the white light in these types of flare events. Instead, our data suggest that the peak of the specific continuum flux density is constrained to U-band wavelengths near the Balmer series limit. A radiative-hydrodynamic simulation of a very high energy deposition rate averaged over times of impulsive heating and cooling better explains the properties of the λ > 2500 Å flare continuum. These two events sample only one end of the empirical color–color distribution for dMe flares, and more time-resolved flare spectra in the NUV, U band, and optical from 2000 to 4200 Å are needed during more impulsive and/or more energetic flares. [ABSTRACT FROM AUTHOR]

Published
2019
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8. ACCRETION AND MAGNETIC RECONNECTION IN THE CLASSICAL T TAURI BINARY DQ TAU.

Author

Benjamin M. Tofflemire, Robert D. Mathieu, David R. Ardila, Rachel L. Akeson, David R. Ciardi, Christopher Johns-Krull, Gregory J. Herczeg, and Alberto Quijano-Vodniza

Subjects
BINARY stars, STELLAR evolution, GLOBULAR clusters, STARBURSTS, STELLAR magnetic fields, STELLAR structure
Abstract

The theory of binary star formation predicts that close binaries (a < 100 au) will experience periodic pulsed accretion events as streams of material form at the inner edge of a circumbinary disk (CBD), cross a dynamically cleared gap, and feed circumstellar disks or accrete directly onto the stars. The archetype for the pulsed accretion theory is the eccentric, short-period, classical T Tauri binary DQ Tau. Low-cadence (∼daily) broadband photometry has shown brightening events near most periastron passages, just as numerical simulations would predict for an eccentric binary. Magnetic reconnection events (flares) during the collision of stellar magnetospheres near periastron could, however, produce the same periodic, broadband behavior when observed at a one-day cadence. To reveal the dominant physical mechanism seen in DQ Tau’s low-cadence observations, we have obtained continuous, moderate-cadence, multiband photometry over 10 orbital periods, supplemented with 27 nights of minute-cadence photometry centered on four separate periastron passages. While both accretion and stellar flares are present, the dominant timescale and morphology of brightening events are characteristic of accretion. On average, the mass accretion rate increases by a factor of five near periastron, in good agreement with recent models. Large variability is observed in the morphology and amplitude of accretion events from orbit to orbit. We argue that this is due to the absence of stable circumstellar disks around each star, compounded by inhomogeneities at the inner edge of the CBD and within the accretion streams themselves. Quasiperiodic apastron accretion events are also observed, which are not predicted by binary accretion theory. [ABSTRACT FROM AUTHOR]

Published
2017
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