140 results on '"Saar, A."'
Search Results
2. X-Ray, UV, and Optical Observations of Proxima Centauri’s Stellar Cycle
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Bradford J. Wargelin, Steven H. Saar, Zackery A. Irving, Jonathan D. Slavin, Peter Ratzlaff, and José-Dias do Nascimento Jr
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Stellar activity ,Late-type dwarf stars ,M dwarf stars ,Astrophysics ,QB460-466 - Abstract
Proxima Cen (GJ 551; dM5.5e) is one of only about a dozen fully convective stars known to have a stellar cycle, and the only one to have long-term X-ray monitoring. A previous analysis found that X-ray and mid-UV observations, particularly two epochs of data from Swift, were consistent with a well-sampled ∼7 yr optical cycle seen in All Sky Automated Survey project (ASAS) data, but not convincing by themselves. The present work incorporates several years of new ASAS-SN optical data and an additional 5 yr of Swift XRT and UVOT observations, with Swift observations now spanning 2009–2021 and optical coverage from late 2000. X-ray observations by XMM-Newton and Chandra are also included. Analysis of the combined data, which includes modeling and adjustments for stellar contamination in the optical and UV, now reveals clear cyclic behavior in all three wavebands with a period of 8.0 yr. We also show that UV and X-ray intensities are anticorrelated with optical brightness variations caused by the cycle and by rotational modulation, discuss possible indications of two coronal mass ejections, and provide updated results for the previous finding of a simple correlation between X-ray cycle amplitude and Rossby number over a wide range of stellar types and ages.
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- 2024
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3. A Hale-like Cycle in the Solar Twin 18 Scorpii
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J.-D. do Nascimento Jr., S. A. Barnes, S. H. Saar, G. F. Porto de Mello, J. C. Hall, F. Anthony, L. de Almeida, E. N. Velloso, J. S. da Costa, P. Petit, A. Strugarek, B. J. Wargelin, M. Castro, K. G. Strassmeier, and A. S. Brun
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Solar cycle ,Stellar evolution ,Stellar magnetic fields ,Solar analogs ,Spectropolarimetry ,Astrophysics ,QB460-466 - Abstract
Characterizing the cyclic magnetic activity of stars that are close approximations of our Sun offers our best hope for understanding our Sun’s current and past magnetism, the space weather around solar-type stars, and more generally, the dynamos of other cool stars. The nearest current approximation to the Sun is the solar twin 18 Scorpii, a naked-eye Sun-like star of spectral type G2 Va. However, while 18 Scorpii’s physical parameters closely match those of the Sun, its activity cycle is about 7 yr, and shorter than the solar cycle. We report the measurement of a periodicity of 15 yr that corresponds to a longer activity cycle for 18 Scorpii based on observations extending to the last three decades. The global magnetic geometry of 18 Scorpii changes with this 15 yr cycle and appears to be equivalent to the solar 22 yr magnetic polarity cycle. These results suggest that 18 Scorpii is also a magnetic proxy for a younger Sun, adding an important new datum for testing dynamo theory and magnetic evolution of low-mass stars. The results perturb our understanding of the relationship between cycle and rotation, constrain the Sun’s magnetism and the Sun–Earth connection over the past billion years, and suggest that solar Schwabe and Hale cycle periods have increased over that time span.
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- 2023
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4. X-Ray Activity Variations and Coronal Abundances of the Star–Planet Interaction Candidate HD 179949
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Anshuman Acharya, Vinay L. Kashyap, Steven H. Saar, Kulinder Pal Singh, and Manfred Cuntz
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F stars ,Exoplanets ,Hot Jupiters ,Star-planet interactions ,Astrophysics ,QB460-466 - Abstract
We carry out detailed spectral and timing analyses of the Chandra X-ray data of HD 179949, a prototypical example of a star with a close-in giant planet with possible star–planet interaction (SPI) effects. We find a low coronal abundance A (Fe)/ A H) ≈ 0.2 relative to the solar photospheric baseline of Anders & Grevesse, and significantly lower than the stellar photosphere as well. We further find low abundances of high first ionization potential (FIP) elements A (O)/ A (Fe) ≲ 1, A (Ne)/ A (Fe) ≲ 0.1, but with indications of higher abundances of A (N)/ A (Fe) ≫ 1, A (Al)/ A (Fe) ≲ 10. We estimate a FIP bias for this star in the range ≈ − 0.3 to −0.1, larger than the ≲ −0.5 expected for stars of this type, but similar to stars hosting close-in hot Jupiters. We detect significant intensity variability over timescales ranging from 100 s to 10 ks, and also evidence for spectral variability over timescales of 1–10 ks. We combine the Chandra flux measurements with Swift and XMM-Newton measurements to detect periodicities, and determine that the dominant signal is tied to the stellar polar rotational period, consistent with expectations that the corona is rotational-pole dominated. We also find evidence for periodicity at both the planetary orbital frequency and at its beat frequency with the stellar polar rotational period, suggesting the presence of a magnetic connection between the planet and the stellar pole. If these periodicities represent an SPI signal, it is likely driven by a quasi-continuous form of heating (e.g., magnetic field stretching) rather than sporadic, hot, impulsive flare-like reconnections.
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- 2023
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5. Stellar Cycles in Fully Convective Stars and a New Interpretation of Dynamo Evolution
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Zackery A. Irving, Steven H. Saar, Bradford J. Wargelin, and José-Dias do Nascimento Jr
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M dwarf stars ,Stellar activity ,Stellar magnetic fields ,Stellar rotation ,Astrophysics ,QB460-466 - Abstract
An α Ω dynamo, combining shear and cyclonic convection in the tachocline, is believed to generate the solar cycle. However, this model cannot explain cycles in fast rotators (with minimal shear) or in fully convective stars (no tachocline); an analysis of these stars could therefore provide key insights into how these cycles work. We reexamine ASAS data for 15 M dwarfs, 11 of which are presumed fully convective; the addition of newer ASAS-SN data confirms cycles in roughly 12 of them, while presenting new or revised rotation periods for 5 stars. The amplitudes and periods of these cycles follow ${A}_{\mathrm{cyc}}\propto {P}_{\mathrm{cyc}}^{0.94\pm 0.11}$ , with P _cyc / P _rot ∝ Ro ^−1.02±0.06 (where Ro is the Rossby number), very similar to P _cyc / P _rot ∝ Ro ^−0.81±0.17 that we find for 40 previously studied FGK stars, although P _cyc / P _rot and α are a factor of ∼20 smaller in the M stars. The very different P _cyc / P _rot – Ro relation seen here compared to previous work suggests that two types of dynamo, with opposite Ro dependences, operate in cool stars. Initially, a (likely α ^2 or α ^2 Ω) dynamo operates throughout the convective zone in mid- to late-M and fast-rotating FGK stars, but once magnetic breaking decouples the core and convective envelope, a tachocline α Ω dynamo begins and eventually dominates in older FGK stars. A change in α in the tachocline dynamo generates the fundamentally different P _cyc / P _rot – Ro relation.
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- 2023
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6. A Hale-like Cycle in the Solar Twin 18 Scorpii
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do Nascimento, J.-D., primary, Barnes, S. A., additional, Saar, S. H., additional, de Mello, G. F. Porto, additional, Hall, J. C., additional, Anthony, F., additional, de Almeida, L., additional, Velloso, E. N., additional, da Costa, J. S., additional, Petit, P., additional, Strugarek, A., additional, Wargelin, B. J., additional, Castro, M., additional, Strassmeier, K. G., additional, and Brun, A. S., additional
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- 2023
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7. Stellar Cycles in Fully Convective Stars and a New Interpretation of Dynamo Evolution
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Irving, Zackery A., primary, Saar, Steven H., additional, Wargelin, Bradford J., additional, and do Nascimento, José-Dias, additional
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- 2023
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8. Magnetic and Rotational Evolution of ρ CrB from Asteroseismology with TESS
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Travis S. Metcalfe, Jennifer L. van Saders, Sarbani Basu, Derek Buzasi, Jeremy J. Drake, Ricky Egeland, Daniel Huber, Steven H. Saar, Keivan G. Stassun, Warrick H. Ball, Tiago L. Campante, Adam J. Finley, Oleg Kochukhov, Savita Mathur, Timo Reinhold, Victor See, Sallie Baliunas, and Willie Soon
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- 2021
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9. Unsigned Magnetic Flux as a Proxy for Radial-velocity Variations in Sun-like Stars
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Haywood, R. D., primary, Milbourne, T. W., additional, Saar, S. H., additional, Mortier, A., additional, Phillips, D., additional, Charbonneau, D., additional, Cameron, A. Collier, additional, Cegla, H. M., additional, Meunier, N., additional, and III, M. L. Palumbo, additional
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- 2022
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10. Magnetic and Rotational Evolution of ρ CrB from Asteroseismology with TESS
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Metcalfe, Travis S., primary, van Saders, Jennifer L., additional, Basu, Sarbani, additional, Buzasi, Derek, additional, Drake, Jeremy J., additional, Egeland, Ricky, additional, Huber, Daniel, additional, Saar, Steven H., additional, Stassun, Keivan G., additional, Ball, Warrick H., additional, Campante, Tiago L., additional, Finley, Adam J., additional, Kochukhov, Oleg, additional, Mathur, Savita, additional, Reinhold, Timo, additional, See, Victor, additional, Baliunas, Sallie, additional, and Soon, Willie, additional
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- 2021
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11. Estimating Magnetic Filling Factors from Simultaneous Spectroscopy and Photometry: Disentangling Spots, Plage, and Network
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Milbourne, T. W., primary, Phillips, D. F., additional, Langellier, N., additional, Mortier, A., additional, Haywood, R. D., additional, Saar, S. H., additional, Cegla, H. M., additional, Collier Cameron, A., additional, Dumusque, X., additional, Latham, D. W., additional, Malavolta, L., additional, Maldonado, J., additional, Thompson, S., additional, Vanderburg, A., additional, Watson, C. A., additional, Buchhave, L. A., additional, Cecconi, M., additional, Cosentino, R., additional, Ghedina, A., additional, Gonzalez, M., additional, Lodi, M., additional, López-Morales, M., additional, Sozzetti, A., additional, and Walsworth, R. L., additional
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- 2021
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12. When Do Stalled Stars Resume Spinning Down? Advancing Gyrochronology with Ruprecht 147
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Curtis, Jason Lee, primary, Agüeros, Marcel A., additional, Matt, Sean P., additional, Covey, Kevin R., additional, Douglas, Stephanie T., additional, Angus, Ruth, additional, Saar, Steven H., additional, Cody, Ann Marie, additional, Vanderburg, Andrew, additional, Law, Nicholas M., additional, Kraus, Adam L., additional, Latham, David W., additional, Baranec, Christoph, additional, Riddle, Reed, additional, Ziegler, Carl, additional, Lund, Mikkel N., additional, Torres, Guillermo, additional, Meibom, Søren, additional, Aguirre, Victor Silva, additional, and Wright, Jason T., additional
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- 2020
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13. A Study of Equatorial Coronal Holes during the Maximum Phase of Four Solar Cycles
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Karna, Mahendra Lal, primary, Karna, Nishu, additional, Saar, Steven H., additional, Pesnell, W. Dean, additional, and DeLuca, Edward E., additional
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- 2020
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14. Long-term Periodicities in Kepler Photometry of Open Cluster NGC 6811
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Velloso, E. N., primary, Nascimento, Jr., J.-D. do, additional, and Saar, S. H., additional
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- 2020
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15. Rotation of Solar Analogs Crossmatching Kepler and Gaia DR2
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Nascimento Jr., J.-D. do, primary, Almeida, L. de, additional, Velloso, E. N., additional, Anthony, F., additional, Barnes, S. A., additional, Saar, S. H., additional, Meibom, S., additional, Costa, J. S. da, additional, Castro, M., additional, Galarza, J. Y., additional, Lorenzo-Oliveira, D., additional, Beck, P. G., additional, and Meléndez, J., additional
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- 2020
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16. Testing the Spectroscopic Extraction of Suppression of Convective Blueshift
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Miklos, M., primary, Milbourne, T. W., additional, Haywood, R. D., additional, Phillips, D. F., additional, Saar, S. H., additional, Meunier, N., additional, Cegla, H. M., additional, Dumusque, X., additional, Langellier, N., additional, Maldonado, J., additional, Malavolta, L., additional, Mortier, A., additional, Thompson, S., additional, Watson, C. A., additional, Cecconi, M., additional, Cosentino, R., additional, Ghedina, A., additional, Li, C-H., additional, López-Morales, M., additional, Molinari, E., additional, Poretti, Ennio, additional, Sasselov, D., additional, Sozzetti, A., additional, and Walsworth, R. L., additional
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- 2020
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17. A Study of Equatorial Coronal Holes during the Maximum Phase of Four Solar Cycles
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Nishu Karna, Steven H. Saar, W. Dean Pesnell, Edward E. DeLuca, and Mahendra Karna
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Physics ,Solar storm of 1859 ,Solar wind ,Space and Planetary Science ,Maximum phase ,Coronal hole ,Astronomy ,Astronomy and Astrophysics ,Solar cycle - Published
- 2020
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18. Long-term Periodicities in Kepler Photometry of Open Cluster NGC 6811
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J. D. do Nascimento, Steven H. Saar, and Eduardo Nunes Velloso
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Physics ,Photometry (astronomy) ,Space and Planetary Science ,Astronomy ,Astronomy and Astrophysics ,Light curve ,Kepler ,Open cluster - Published
- 2020
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19. Rotation of Solar Analogs Crossmatching Kepler and Gaia DR2
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Francys Anthony, Paul G. Beck, Eduardo Nunes Velloso, Diego Lorenzo-Oliveira, Jhon Yana Galarza, Sydney A. Barnes, Steven H. Saar, J. D. do Nascimento, M. Castro, Jorge Melendez, S. Meibom, J. S. da Costa, and L. de Almeida
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Earth and Planetary Astrophysics (astro-ph.EP) ,Physics ,Stellar rotation ,Starspot ,FOS: Physical sciences ,Astronomy ,Astronomy and Astrophysics ,Rotation ,Kepler ,Astrophysics - Solar and Stellar Astrophysics ,Space and Planetary Science ,Stellar evolution ,Solar and Stellar Astrophysics (astro-ph.SR) ,Astrophysics - Earth and Planetary Astrophysics - Abstract
A major obstacle to interpreting the rotation period distribution for main-sequence stars from Kepler mission data has been the lack of precise evolutionary status for these objects. We address this by investigating the evolutionary status based on Gaia Data Release 2 parallaxes and photometry for more than 30,000 Kepler stars with rotation period measurements. Many of these are subgiants, and should be excluded in future work on dwarfs. We particularly investigate a 193-star sample of solar analogs, and report newly-determined rotation periods for 125 of these. These include 54 stars from a prior sample, of which can confirm the periods for 50. The remainder are new, and 10 of them longer than solar rotation period, suggesting that sun-like stars continue to spin down on the main sequence past solar age. Our sample of solar analogs could potentially serve as a benchmark for future missions such as PLATO, and emphasizes the need for additional astrometric, photometric, and spectroscopic information before interpreting the stellar populations and results from time-series surveys., Comment: 10 pages, 8 Figures, 2 tables. accepted to be published on The Astrophysical Journal (June 8, 2020)
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- 2020
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20. Turbulent Diffusion Derived from the Motions of SDO/AIA Coronal Bright Points
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Skokić, I., primary, Brajša, R., additional, Sudar, D., additional, Ruždjak, D., additional, and Saar, S. H., additional
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- 2019
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21. HARPS-N Solar RVs Are Dominated by Large, Bright Magnetic Regions
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Milbourne, T. W., primary, Haywood, R. D., additional, Phillips, D. F., additional, Saar, S. H., additional, Cegla, H. M., additional, Cameron, A. C., additional, Costes, J., additional, Dumusque, X., additional, Langellier, N., additional, Latham, D. W., additional, Maldonado, J., additional, Malavolta, L., additional, Mortier, A., additional, III, M. L. Palumbo, additional, Thompson, S., additional, Watson, C. A., additional, Bouchy, F., additional, Buchschacher, N., additional, Cecconi, M., additional, Charbonneau, D., additional, Cosentino, R., additional, Ghedina, A., additional, Glenday, A. G., additional, Gonzalez, M., additional, Li, C-H., additional, Lodi, M., additional, López-Morales, M., additional, Lovis, C., additional, Mayor, M., additional, Micela, G., additional, Molinari, E., additional, Pepe, F., additional, Piotto, G., additional, Rice, K., additional, Sasselov, D., additional, Ségransan, D., additional, Sozzetti, A., additional, Szentgyorgyi, A., additional, Udry, S., additional, and Walsworth, R. L., additional
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- 2019
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22. Turbulent Diffusion Derived from the Motions of SDO/AIA Coronal Bright Points
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I. Skokić, Domagoj Ruždjak, Steven H. Saar, Davor Sudar, and Roman Brajša
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Physics ,Convection ,Turbulent diffusion ,Space and Planetary Science ,Coronal plane ,Physics::Space Physics ,Astrophysics::Solar and Stellar Astrophysics ,Astronomy and Astrophysics ,Astrophysics::Earth and Planetary Astrophysics ,Astrophysics ,Diffusion (business) ,convection ,diffusion ,Sun: corona ,Sun: UV radiation - Abstract
Diffusion of magnetic elements on the Sun has an important role in current solar dynamo models as a part of the mechanism for redistribution of the magnetic field and as an important part for maintaining the solar activity cycle. The main goal is to determine the character of solar magnetic diffusivity and a value of the diffusion coefficient by analyzing the motions of coronal bright points (CBPs) within the frame of the random walk model. We tracked positions of CBPs in Solar Dynamics Observatory/Atmospheric Imaging Assembly images for a period of 5 months and examined their displacement spectrum. We calculated spectral index and diffusion coefficient from the spectrum and investigated their variation with temporal and spatial scale. For the first time, variations of the spectral index with heliographic latitude and time were analyzed. Our results indicate subdiffusion with the spectral index γ = 0.70 ± 0.01 and the corresponding diffusion coefficient with a value decreasing from 400 to 100 km^2 s^{;‑1}; for temporal scales of 10^3–10^5 s and spatial scales of (1.5–7) × 10^3 km. Seemingly random variations around the mean value of spectral index were found, with peak-to-peak amplitudes
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- 2019
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23. Stellar Surface Magneto-convection as a Source of Astrophysical Noise. II. Center-to-limb Parameterization of Absorption Line Profiles and Comparison to Observations
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Cegla, H. M., primary, Watson, C. A., additional, Shelyag, S., additional, Chaplin, W. J., additional, Davies, G. R., additional, Mathioudakis, M., additional, Palumbo III, M. L., additional, Saar, S. H., additional, and Haywood, R. D., additional
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- 2018
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24. ATMOSPHERIC IMAGING ASSEMBLY MULTITHERMAL LOOP ANALYSIS: FIRST RESULTS
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J. A. Kimble, B. T. Worley, D. J. Anderson, B. S. Jenkins, S. Pathak, Steven H. Saar, and J. T. Schmelz
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Physics ,Astronomy and Astrophysics ,Plasma ,Coronal loop ,Astrophysics ,Wake ,Measure (mathematics) ,Isothermal process ,Space and Planetary Science ,Extreme ultraviolet ,Physics::Space Physics ,Astrophysics::Solar and Stellar Astrophysics ,Image resolution ,Spectrograph - Abstract
The Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory has state-of-the-art spatial resolution and shows the most detailed images of coronal loops ever observed. The series of coronal filters peak at different temperatures, which span the range of active regions. These features represent a significant improvement over earlier coronal imagers and make AIA ideal for multithermal analysis. Here, we targeted a 171 A coronal loop in AR 11092 observed by AIA on 2010 August 3. Isothermal analysis using the 171-to-193 ratio gave a temperature of log T ≈ 6.1, similar to the results of Extreme ultraviolet Imaging Spectrograph (EIT) and TRACE. Differential emission measure analysis, however, showed that the plasma was multithermal, not isothermal, with the bulk of the emission measure at log T > 6.1. The result from the isothermal analysis, which is the average of the true plasma distribution weighted by the instrument response functions, appears to be deceptively low. These results have potentially serious implications: EIT and TRACE results, which use the same isothermal method, show substantially smaller temperature gradients than predicted by standard models for loops in hydrodynamic equilibrium and have been used as strong evidence in support of footpoint heating models. These implications may have to be re-examined in the wake of new results from AIA.
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- 2010
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25. Stellar Surface Magneto-convection as a Source of Astrophysical Noise. II. Center-to-limb Parameterization of Absorption Line Profiles and Comparison to Observations
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William J. Chaplin, Steven H. Saar, Heather M. Cegla, Guy R. Davies, Christopher A. Watson, Raphaëlle D. Haywood, M. Palumbo, Mihalis Mathioudakis, and Sergiy Shelyag
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Convection ,010504 meteorology & atmospheric sciences ,F300 ,detection [planets and satellites] ,FOS: Physical sciences ,Astrophysics ,01 natural sciences ,Spectral line ,OF-STATE TABLES ,ASYMMETRIES ,profiles [line] ,0103 physical sciences ,Radiative transfer ,Astrophysics::Solar and Stellar Astrophysics ,low-mass [stars] ,SOLAR ,010303 astronomy & astrophysics ,Solar and Stellar Astrophysics (astro-ph.SR) ,GRANULATION ,0105 earth and related environmental sciences ,Line (formation) ,Earth and Planetary Astrophysics (astro-ph.EP) ,Physics ,Line-of-sight ,activity [stars] ,radial velocities [techniques] ,Astronomy and Astrophysics ,Exoplanet ,Radial velocity ,Astrophysics - Solar and Stellar Astrophysics ,13. Climate action ,Space and Planetary Science ,WAVELENGTH SHIFTS ,Astrophysics::Earth and Planetary Astrophysics ,granulation [Sun] ,Noise (radio) ,PHOTOSPHERIC LINES ,Astrophysics - Earth and Planetary Astrophysics - Abstract
Manifestations of stellar activity (such as star-spots, plage/faculae, and convective flows) are well known to induce spectroscopic signals often referred to as astrophysical noise by exoplanet hunters. For example, setting an ultimate goal of detecting true Earth-analogs demands reaching radial velocity (RV) precisions of ~9 cm/s. While this is becoming technically feasible with the latest generation of highly stabilised spectrographs, it is astrophysical noise that sets the true fundamental barrier on attainable RV precisions. In this paper we parameterise the impact of solar surface magneto-convection on absorption line profiles, and extend the analysis from the solar disc centre (Paper I) to the solar limb. Off disc-centre, the plasma flows orthogonal to the granule tops begin to lie along the line-of-sight and those parallel to the granule tops are no longer completely aligned with the observer. Moreover, the granulation is corrugated and the granules can block other granules, as well as the intergranular lane components. Overall, the visible plasma flows and geometry of the corrugated surface significantly impact the resultant line profiles and induce centre-to-limb variations in shape and net position. We detail these herein, and compare to various solar observations. We find our granulation parameterisation can recreate realistic line profiles and induced radial velocity shifts, across the stellar disc, indicative of both those found in computationally heavy radiative 3D magnetohydrodynamical simulations and empirical solar observations., 17 pages, 14 figures, accepted to ApJ
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- 2018
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26. MULTI-STRANDED AND MULTI-THERMAL SOLAR CORONAL LOOPS: EVIDENCE FROMHINODEX-RAY TELESCOPE AND EUV IMAGING SPECTROMETER DATA
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J. T. Schmelz, Leon Golub, Edward E. DeLuca, Steven H. Saar, K. Nasraoui, Vinay L. Kashyap, and Mark Weber
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Physics ,Spectrometer ,Flux tube ,Imaging spectrometer ,Astronomy and Astrophysics ,X-ray telescope ,Astrophysics ,Coronal loop ,law.invention ,Loop (topology) ,Telescope ,Transverse plane ,Space and Planetary Science ,law ,Astrophysics::Solar and Stellar Astrophysics - Abstract
Data from the X-Ray Telescope (XRT) and the EUV Imaging Spectrometer (EIS) on the Japanese/USA/UK Hinode spacecraft were used to investigate the spatial and thermal properties of an isolated quiescent coronal loop. We constructed differential emission measure (DEM) curves using Monte Carlo based, iterative forward fitting algorithms. We studied the loop as a whole, in segments, in transverse cuts, and point-by-point, always with some form of background subtraction, and find that the loop DEM is neither isothermal nor extremely broad, with approximately 96% of the EM between 6.2 {
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- 2010
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27. INTERACTIONS OF THE MAGNETOSPHERES OF STARS AND CLOSE-IN GIANT PLANETS
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Steven H. Saar, Tamas I. Gombosi, Igor V. Sokolov, Jeremy J. Drake, Vinay Kashyap, Ward B. Manchester, Ofer Cohen, and Kenneth C. Hansen
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Earth and Planetary Astrophysics (astro-ph.EP) ,Physics ,Gas giant ,Stellar magnetic field ,FOS: Physical sciences ,Magnetosphere ,Astronomy and Astrophysics ,Astrophysics ,Planetary system ,Luminosity ,Stars ,Astrophysics - Solar and Stellar Astrophysics ,Space and Planetary Science ,Planet ,Physics::Space Physics ,Orbital motion ,Astrophysics::Solar and Stellar Astrophysics ,Astrophysics::Earth and Planetary Astrophysics ,Solar and Stellar Astrophysics (astro-ph.SR) ,Astrophysics - Earth and Planetary Astrophysics - Abstract
Since the first discovery of an extrasolar planetary system more than a decade ago, hundreds more have been discovered. Surprisingly, many of these systems harbor Jupiter-class gas giants located close to the central star, at distances of 0.1 AU or less. Observations of chromospheric 'hot spots' that rotate in phase with the planetary orbit, and elevated stellar X-ray luminosities,suggest that these close-in planets significantly affect the structure of the outer atmosphere of the star through interactions between the stellar magnetic field and the planetary magnetosphere. Here we carry out the first detailed three-dimensional MagnetoHydroHynamics (MHD) simulation containing the two magnetic bodies and explore the consequences of such interactions on the steady-state coronal structure. The simulations reproduce the observable features of 1) increase in the total X-ray luminosity, 2) appearance of coronal hot spots, and 3) phase shift of these spots with respect to the direction of the planet. The proximate cause of these is an increase in the density of coronal plasma in the direction of the planet, which prevents the corona from expanding and leaking away this plasma via a stellar wind. The simulations produce significant low temperature heating. By including dynamical effects, such as the planetary orbital motion, the simulation should better reproduce the observed coronal heating.
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- 2009
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28. SOME LIKE IT HOT: CORONAL HEATING OBSERVATIONS FROMHINODEX-RAY TELESCOPE ANDRHESSI
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Edward E. DeLuca, Vinay L. Kashyap, Brian R. Dennis, Steven H. Saar, Mark Weber, L. Lin, Gordon D. Holman, J. T. Schmelz, P. C. Grigis, and Leon Golub
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Physics ,Gamma ray ,Astronomy ,Flux ,Astronomy and Astrophysics ,Observable ,X-ray telescope ,Astrophysics ,Approx ,law.invention ,Telescope ,Stars ,Space and Planetary Science ,law ,Main sequence - Abstract
We have used Hinode X-Ray Telescope observations and RHESSI upper limits together to characterize the differential emission measure (DEM) from a quiescent active region. We find a relatively smooth DEM curve with the expected active region peak at log T = 6.4. We also find a high-temperature component with significant emission measure at log T approx> 7. This curve is consistent with previous observations of quiescent active regions in that it does not produce observable Fe XIX lines. It is different from that generated with X-Ray Telescope (XRT) data alone-RHESSI rules out the possibility of a separate high-temperature component with a peak of approximately log T = 7.4. The strength and position of the high-temperature peak in this XRT-only analysis was, however, poorly determined; adding RHESSI flux upper limits in the 4-13 keV energy range provide a strong high-temperature constraint which greatly improves the multi-thermal findings. The results of the present work as well as those from a growing number of papers on this subject imply that our previous understanding of the temperature distribution in active regions has been limited. Hot plasma (log T approx 7) appears to be prevalent, although in relatively small quantities as predicted by nanoflare models.more » Other models may need to be adjusted or updated to account for these new results.« less
- Published
- 2009
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29. RELIABILITY OF THE DETECTION OF THE BARYON ACOUSTIC PEAK
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Enn Saar, Silvestre Paredes, Pablo de la Cruz, María Jesús Pons-Bordería, Vicent J. Martínez, Elmo Tempel, Pablo Arnalte-Mur, and Alberto Fernández-Soto
- Subjects
Physics ,media_common.quotation_subject ,Astrophysics (astro-ph) ,FOS: Physical sciences ,Astronomy and Astrophysics ,Astrophysics::Cosmology and Extragalactic Astrophysics ,Astrophysics ,Correlation function (astronomy) ,Redshift survey ,Galaxy ,Redshift ,Baryon ,Distribution (mathematics) ,Space and Planetary Science ,Sky ,Baryon acoustic oscillations ,Astrophysics::Galaxy Astrophysics ,media_common - Abstract
The correlation function of the distribution of matter in the universe shows, at large scales, baryon acoustic oscillations, which were imprinted prior to recombination. This feature was first detected in the correlation function of the luminous red galaxies (LRG) of the Sloan Digital Sky Survey (SDSS). The final release (DR7) of the SDSS has been recently made available, and the useful volume is about two times bigger than in the old sample. We present here, for the first time, the redshift space correlation function of this sample at large scales together with that for one shallower, but denser volume-limited subsample drawn from the 2dF redshift survey. We test the reliability of the detection of the acoustic peak at about 100 Mpc/h and the behaviour of the correlation function at larger scales by means of careful estimation of errors. We confirm the presence of the peak in the latest data although broader than in previous detections., Comment: 6 pages, 3 figures, 1 table. Version accepted for publication in The Astrophysical Journal Letters. Numerical values of the correlation functions plotted in Fig. 3 can be downloaded from http://www.uv.es/martinez . The revised version corrects an error in the magnitudes printed in Table 1. The galaxy sample itself has not changed
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- 2009
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30. Toward Understanding Rich Superclusters
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M. Gramann, M. Einasto, P. Nurmi, Erik Tago, Jaan Einasto, G. Huetsi, P. Hein\\'am\\'aki, Silvestre Paredes, Volker Mueller, Vicent J. Martínez, Enn Saar, L. J. Liivam\\'agi, and Jean-Luc Starck
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Physics ,2dF Galaxy Redshift Survey ,Minkowski functional ,Star formation ,Astrophysics (astro-ph) ,FOS: Physical sciences ,Astronomy and Astrophysics ,Astrophysics ,Galaxy ,Space and Planetary Science ,Supercluster ,Galaxy group ,Cluster (physics) ,Substructure - Abstract
We present a morphological study of the two richest superclusters from the 2dF Galaxy Redshift Survey (SCL126, the Sloan Great Wall, and SCL9, the Sculptor supercluster). We use Minkowski functionals, shapefinders, and galaxy group information to study the substructure of these superclusters as formed by different populations of galaxies. We compare the properties of grouped and isolated galaxies in the core region and in the outskirts of superclusters. The fourth Minkowski functional $V_3$ and the morphological signature $K_1$- $K_2$ show a crossover from low-density morphology (outskirts of supercluster) to high-density morphology (core of supercluster) at mass fraction $m_f \approx 0.7$. The galaxy content and the morphology of the galaxy populations in supercluster cores and outskirts is different. The core regions contain a larger fraction of early type, red galaxies, and richer groups than the outskirts of superclusters. In the core and outskirt regions the fine structure of the two prominent superclusters as delineated by galaxies from different populations also differs. Our results suggest that both local (group/cluster) and global (supercluster) environments are important in forming galaxy morphologies and colors (and determining the star formation activity). The differences between the superclusters indicate that these superclusters have different evolutional histories (Abridged)., Comment: 23 pages, accepted for publication in The Astrophysical Journal
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- 2008
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31. The Outer Solar Atmosphere during the Maunder Minimum: A Stellar Perspective
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Philip G. Judge and Steven H. Saar
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Physics ,Solar minimum ,Sunspot ,Astronomy ,Astronomy and Astrophysics ,Astrophysics ,Corona ,Solar cycle ,Stars ,Space and Planetary Science ,Physics::Space Physics ,Radiative transfer ,Astrophysics::Solar and Stellar Astrophysics ,Astrophysics::Earth and Planetary Astrophysics ,Solar dynamo ,Chromosphere - Abstract
By comparing solar data with data of ‘‘flat activity’’ stars at UV and soft X-ray wavelengths, we estimate solar chromospheric, transition region, and coronal emission as it might have existed during the Maunder minimum (1645–1715),themostrecentofseveralepisodesofpersistentlyweaksunspotactivity.Severalflatactivity starsfrom theMountWilsonS-indexsurveyhavebeenobservedwiththeHubbleSpaceTelescopeandROSAT.Drawingonpublished data, we reassess the criteria by which a star may be considered as a Maunder minimum candidate. Of our targets, HD 10700 and HD 143761 are the most likely such candidates. Solar UV spectra from the SOHO and UARS spacecraft, and soft X-ray data from the SNOE spacecraft are compared with the stellar UV and X-ray data. The comparison suggests that the radiative output of the Maunder minimum chromosphere, transition region, and corona were similar to (or at least not much less than) those observed under conditions close to current solar minima. In turn, this suggests that the emitting structures (which on the Sun at sunspot minimum are small scale,TR� ), including the magneticnetwork,werealsosimilar.Theseresultshaveimplicationsforthenatureofthesurfacemagneticfieldsandirradiance during the Maunder and other magnetic minima, and for the solar dynamo. Subject headingg stars: activity — stars: chromospheres — stars: coronae — stars: individual (the Sun)
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- 2007
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32. Testing the Reality of Strong Magnetic Fields on T Tauri Stars: The Naked T Tauri Star Hubble 4
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Christopher M. Johns-Krull, Steven H. Saar, and Jeff A. Valenti
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Physics ,Zeeman effect ,Field (physics) ,010308 nuclear & particles physics ,Infrared ,Astrophysics (astro-ph) ,FOS: Physical sciences ,Astronomy and Astrophysics ,Field strength ,Astrophysics ,01 natural sciences ,Spectral line ,Magnetic field ,T Tauri star ,symbols.namesake ,Mean field theory ,Space and Planetary Science ,0103 physical sciences ,symbols ,Astrophysics::Solar and Stellar Astrophysics ,Astrophysics::Earth and Planetary Astrophysics ,010303 astronomy & astrophysics ,Astrophysics::Galaxy Astrophysics - Abstract
High resolution optical and infrared (IR) echelle spectra of the naked (diskless) T Tauri star Hubble 4 are presented. The K band IR spectra include 4 Zeeman sensitive Ti I lines along with several magnetically insensitive CO lines. Detailed spectrum synthesis combined with modern atmospheric models is used to fit the optical spectra of Hubble 4 in order to determine its key stellar parameters: Teff = 4158 +/- 56 K; log(g) = 3.61 +/- 0.50; [M/H] = -0.08 +/- 0.05; vsini = 14.6 +/- 1.7 km/s . These stellar parameters are used to synthesize K band spectra to compare with the observations. The magnetically sensitive Ti I lines are all significantly broadened relative to the lines produced in the non-magnetic model, while the magnetically insensitive CO lines are well matched by the basic non-magnetic model. Models with magnetic fields are synthesized and fit to the Ti I lines. The best fit models indicate a distribution of magnetic field strengths on the stellar surface characterized by a mean magnetic field strength of 2.51 +/- 0.18 kG. The mean field is a factor of 2.0 greater than the maximum field strength predicted by pressure equipartition arguments. To confirm the reality of such strong fields, we attempt to refit the observed profiles using a two component magnetic model in which the field strength is confined to the equipartition value representing plage-like regions in one component, and the field is allowed to vary in a cooler component representing spots. It is shown that such a model is inconsistent with the optical spectrum of the TiO bandhead at 7055 Angstroms., 31 pages, including 5 figures accepted for publication in The Astrophysical Journal
- Published
- 2004
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33. A Comparison of the Outer Atmosphere of the 'Flat Activity' Star τ Ceti (G8 V) with the Sun (G2 V) and α Centauri A (G2 V)
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Mats Carlsson, Thomas R. Ayres, Philip G. Judge, and Steven H. Saar
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Physics ,Dwarf star ,Stellar atmosphere ,Astronomy ,Astronomy and Astrophysics ,Astrophysics::Cosmology and Extragalactic Astrophysics ,Astrophysics ,Redshift ,Spectral line ,Magnetic field ,Atmosphere ,Stars ,Space and Planetary Science ,Physics::Space Physics ,Astrophysics::Solar and Stellar Astrophysics ,Astrophysics::Earth and Planetary Astrophysics ,Chromosphere ,Astrophysics::Galaxy Astrophysics - Abstract
Driven by the desire to understand the roles of acoustic and magnetic mechanisms in heating the outer atmospheres of Sun-like stars, we compare solar UV spectra with archival STIS spectra from the Hubble Space Telescope of α Cen A (G2 V) and new STIS spectra of the extremely inactive dwarf star τ Cet (G8 V, V = 3.5). The activity of τ Cet shows occasional rotational modulations but no long-term cyclic variation. It may be in a phase analogous to the solar Maunder minimum. Solar disk center intensities from both the HRTS instrument and the SUMER instrument on SOHO were converted to Sun-as-a-star fluxes by using center-to-limb data from Dammasch and colleagues. The derived solar flux spectrum represents conditions near the minimum of the solar magnetic activity cycle. We find that the τ Cet line profiles differ systematically from those of the Sun and α Cen A. While lines formed in the middle chromospheres appear similar, the entire emission from the upper chromosphere to the middle transition region of τ Cet has lower flux densities by factors of ≈2, the line widths are significantly narrower, and, uniquely, the transition region lines are not significantly redshifted. The soft X-ray surface flux of τ Cet, measured between 0.1 and 2.4 keV, is ≈9 × 103 ergs cm-2 s-1, several times smaller than the median solar value. We also find that the UV spectrum of α Cen serves as a proxy for a Sun-as-a-star spectrum when the Sun is in an intermediate phase of its activity cycle but that its coronal emission may be somewhat smaller. We discuss the implications of these results for magnetic fields and heating mechanisms in the stars and speculate that τ Cet's UV spectrum might represent the solar spectrum during a grand minimum phase.
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- 2004
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34. On Stellar Activity Enhancement Due to Interactions with Extrasolar Giant Planets
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Steven H. Saar, Manfred Cuntz, and Zdzislaw E. Musielak
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Physics ,Brown dwarf ,Astronomy ,Astronomy and Astrophysics ,Astrophysics::Cosmology and Extragalactic Astrophysics ,Astrophysics ,Planetary system ,Exoplanet ,Jupiter ,Space and Planetary Science ,Planet ,Physics::Space Physics ,Hot Jupiter ,Astrophysics::Solar and Stellar Astrophysics ,Astrophysics::Earth and Planetary Astrophysics ,Planetary mass ,Astrophysics::Galaxy Astrophysics ,Jupiter mass - Abstract
We present a first attempt to identify and quantify possible interactions between recently discovered extrasolar giant planets (and brown dwarfs) and their host stars, resulting in activity enhancement in the stellar outer atmospheres. Many extrasolar planets have masses comparable to or larger than Jupiter and are within a distance of 0.5 AU, suggesting the possibility of their significant influence on stellar winds, coronae, and even chromospheres. Beyond the well-known rotational synchronization, the interactions include tidal effects (in which enhanced flows and turbulence in the tidal bulge lead to increased magnetoacoustic heating and dynamo action) and direct magnetic interaction between the stellar and planetary magnetic fields. We discuss relevant parameters for selected systems and give preliminary estimates of the relative interaction strengths.
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- 2000
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35. Time Evolution of the Magnetic Activity Cycle Period. II. Results for an Expanded Stellar Sample
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Steven H. Saar and Axel Brandenburg
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Physics ,T Tauri star ,Stars ,Space and Planetary Science ,K-type main-sequence star ,Stellar mass loss ,Stellar collision ,Flare star ,Astronomy ,Astronomy and Astrophysics ,Astrophysics ,Blue straggler ,Superflare - Abstract
We further explore nondimensional relationships between the magnetic dynamo cycle period Pcyc, the rotational period Prot, the activity level (as observed in Ca II HK), and other stellar properties by expanding the stellar sample studied in the first paper in this series. We do this by adding photometric and other cycles seen in active stars and the secondaries of CV systems and by selectively adding less certain cycles from the Mount Wilson HK survey; evolved stars, long-term HK trends and secondary Pcyc are also considered. We confirm that most stars with age t 0.1 Gyr occupy two roughly parallel branches, separated by a factor of ~6 in Pcyc, with the ratio of cycle and rotational frequencies ωcyc/Ω Ro-0.5, where Ro is the Rossby number. Using the model of the first paper in this series, this result implies that the α effect increases with mean magnetic field (contrary to the traditional α-quenching concept) and that α and ωcyc decrease with t. Stars are not strictly segregated onto one or the other branch by activity level, though the high-ωcyc/Ω branch is primarily composed of inactive stars. The expanded data set suggests that for t 1 Gyr, stars can have cycles on one or both branches, though among older stars, those with higher (lower) mass tend to have their primary Pcyc on the lower (upper) ωcyc/Ω branch. The Sun's ~80 yr Gleissberg cycle agrees with this scenario, suggesting that long-term activity trends in many stars may be segments of long (Pcyc ~ 50-100 yr) cycles not yet resolved by the data. Most very active stars (Prot < 3 days) appear to occupy a new, third branch with ωcyc/Ω Ro0.4. Many RS CVn variables lie in a transition region between the two most active branches. We compare our results with various models, discuss their implications for dynamo theory and evolution, and use them to predict Pcyc for three groups: stars with long-term HK trends, stars in young open clusters, and stars that may be in Maunder-like magnetic minima.
- Published
- 1999
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36. Two‐Component Theoretical Chromosphere Models for K Dwarfs of Different Magnetic Activity: Exploring the Ca<scp>ii</scp>Emission–Stellar Rotation Relationship
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Zdzislaw E. Musielak, Steven H. Saar, W. Rammacher, Manfred Cuntz, and Peter Ulmschneider
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Physics ,Magnetic energy ,Stellar rotation ,Astronomy ,Astronomy and Astrophysics ,Astrophysics ,Rotation ,Stars ,Space and Planetary Science ,Astrophysics::Solar and Stellar Astrophysics ,Astrophysics::Earth and Planetary Astrophysics ,Emission spectrum ,Magnetohydrodynamics ,Chromosphere ,Line (formation) - Abstract
We compute two-component theoretical chromosphere models for K2 V stars with diUerent levels of magnetic activity. The two components are a nonmagnetic component heated by acoustic waves and a magnetic component heated by longitudinal tube waves. The —lling factor for the magnetic component is determined from an observational relationship between the measured magnetic area coverage and the stellar rotation period. We consider stellar rotation periods between 10 and 40 days. We investigate two diUerent geometrical distributions of magnetic —ux tubes: uniformly distributed tubes, and tubes arranged as a chromospheric network embedded in the nonmagnetic region. The chromosphere models are constructed by performing state-of-the-art calculations for the generation of acoustic and magnetic energy in stellar convection zones, the propagation and dissipation of this energy at the diUerent atmo- spheric heights, and the formation of speci—c chromospheric emission lines that are then compared to the observational data. In all these steps, the two-component structure of stellar photospheres and chromospheres is fully taken into account. We —nd that heating and chromospheric emission is signi—- cantly increased in the magnetic component and is strongest in —ux tubes that spread the least with height, expected to occur on rapidly rotating stars with high magnetic —lling factors. For stars with very slow rotation, we are able to reproduce the basal —ux limit of chromospheric emission previously identi- —ed with nonmagnetic regions. Most importantly, however, we —nd that the relationship between the Ca II H)K emission and the stellar rotation rate deduced from our models is consistent with the relationship given by observations. Subject headings: line: formationMHDstars: activitystars: chromospheres ¨ stars: late-typestars: rotation
- Published
- 1999
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37. Parent Stars of Extrasolar Planets. IV. 14 Herculis, HD 187123, and HD 210277
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George Wallerstein, Steven H. Saar, and Guillermo Gonzalez
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Physics ,Stars ,Super-Earth ,Space and Planetary Science ,Planet ,K-type main-sequence star ,Astronomy ,Astronomy and Astrophysics ,Astrophysics ,Planetary system ,A-type main-sequence star ,O-type main-sequence star ,Exoplanet - Abstract
Spectroscopic analyses of 14 Her, HD 187123, and HD 210277, recently reported to harbor planets, reveal that these stars are metal rich. We find [Fe/H]=0.50 ± 0.05, 0.16 ± 0.05, and 0.24 ± 0.05 for 14 Her, HD 187123, and HD 210277, respectively. This is the first spectroscopic analysis of HD 187123; our results for 14 Her and HD 210277 are in agreement with published studies. It is shown that 14 Her and ρ1 Cnc are nearly identical in their bulk physical characteristics. This result, combined with their extreme metallicities, suggests that their physical parameters have been affected by the process that formed their planets. These two stars join a group of about half a dozen stars in the solar neighborhood with [Fe/H] ≥ 0.4. It is also shown that 51 Peg and HD 187123, which have companions with similar orbital periods and masses, are nearly identical. We find v sin i ≈ 2.0 km s-1 for HD 210277 from a high-resolution spectrum.
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- 1999
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38. GIANT CORONAL LOOPS DOMINATE THE QUIESCENT X-RAY EMISSION IN RAPIDLY ROTATING M STARS
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Cohen, O., primary, Yadav, R., additional, Garraffo, C., additional, Saar, S. H., additional, Wolk, S. J., additional, Kashyap, V. L., additional, Drake, J. J., additional, and Pillitteri, I., additional
- Published
- 2016
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39. Measurements of Starspot Parameters on Active Stars using Molecular Bands in Echelle Spectra
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James E. Neff, Douglas O'Neal, and Steven H. Saar
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Physics ,Photosphere ,Photometry (astronomy) ,Stars ,Space and Planetary Science ,Subgiant ,Starspot ,Astronomy ,Astronomy and Astrophysics ,Emission spectrum ,Astrophysics ,Atmospheric temperature range ,Spectral line - Abstract
We present results from a study of starspot areas (fS) and temperatures (TS), primarily on active, single-lined spectroscopic binaries, determined using molecular absorption bands. Expanding upon our previous studies, we have analyzed multiorder echelle spectra of eight systems to simultaneously measure several different molecular bands and chromospheric emission lines. We determined starspot parameters by fitting the molecular bands of interest, using spectra of inactive G and K stars as proxies for the nonspotted photosphere of the active stars, and using spectra of M stars as proxies for the spots. At least two bands with different Teff sensitivities are required. We found that fitting bands other than the TiO 7055 and 8860 A features does not greatly extend the temperature range or sensitivity of our technique. The 8860 A band is particularly important because of its sharply different temperature sensitivity. We did not find any substantial departures from fS or TS that we have measured previously based on single-order spectra. We refined our derived spot parameters using contemporaneous photometry where available. We found that using M giants as spot proxies for subgiant active stars often underestimates fS needed to fit the photometry; this is presumably due to the increase in strength of the TiO bands with decreasing gravity. We also investigated correlations between fS and chromospheric emission, and we developed a simple method to measure nonspot temperature (TQ) solely from our echelle spectra.
- Published
- 1998
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40. Magnetic Activity–related Radial Velocity Variations in Cool Stars: First Results from the Lick Extrasolar Planet Survey
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Steven H. Saar, Geoffrey W. Marcy, and R. Paul Butler
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Physics ,Star (game theory) ,Starspot ,Astronomy and Astrophysics ,Astrophysics::Cosmology and Extragalactic Astrophysics ,Astrophysics ,Type (model theory) ,Rotation ,Exoplanet ,Radial velocity ,Stars ,Space and Planetary Science ,Planet ,Astrophysics::Solar and Stellar Astrophysics ,Astrophysics::Earth and Planetary Astrophysics ,Astrophysics::Galaxy Astrophysics - Abstract
The discovery of the radial velocity (v -->r) signatures of planets around several solar-like stars highlights the importance of exploring the sources of v -->r variations intrinsic to the stars themselves. We study the stars in the Lick planetary survey for v -->r variations related to stellar activity: the rotation of starspots and convective inhomogeneities and their temporal evolution. We study the relationships between the weighted v -->r dispersion, ?'v (which has first been corrected for the orbital contribution from known planets and the mean internal error), and spectral type, rotation, and activity (as measured by Ca II H and K). We find that the largest ?'v values occur among both the coolest (dMe) and the warmest (active F) stars. Values of ?'v increase with H and K emission and scale proportional to vsin${r sin}$ -->${r sin}$ -->i in G and K stars and proportional to (vsin${r sin}$ -->${r sin}$ -->i) -->1.3 in F stars. For a G star with vsin${r sin}$ -->${r sin}$ -->i ? 8-10 km s -->?1 (age ~0.3 Gyr), for example, 20 m s -->?1 ?'v 45 m s -->?1, roughly consistent with the predicted ?'v levels due to magnetic activity (Saar & Donahue). All the stars with proposed planetary companions show ?'v values typical for their spectral type, activity, and/or rotation. However, before the planetary v -->r perturbations are removed, these stars show significantly enhanced ?'v values. We develop a simple model that can predict the ?'v expected for a given star (within ? 40%) as a function of vsin${r sin}$ -->${r sin}$ -->i, spectral type, photometric variability, and macroturbulent velocity. The implications for extrasolar planet searches are discussed.
- Published
- 1998
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41. Time Evolution of the Magnetic Activity Cycle Period
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Steven H. Saar, Axel Brandenburg, and Christen R. Turpin
- Subjects
Physics ,Rossby number ,Rotation period ,T Tauri star ,Stars ,Space and Planetary Science ,K-type main-sequence star ,Time evolution ,Astronomy and Astrophysics ,Astrophysics ,Dynamo ,Magnetic field - Abstract
We propose a new interpretation of the relationships between the dynamo cycle period (Pcyc) as observed in Ca II H and K, the rotational period (Prot), the activity level, and other stellar properties. Viewed within this framework, the data suggest that the dynamo α-parameter increases with magnetic field strength, contrary to the conventional idea of α-quenching. The data also suggest a possibly discontinuous dependence of the ratio of cycle to rotation frequency, ωcyc/Ω, as a function of Rossby number, Ro (or equivalently, activity or age). Stars evolve with ωcyc/Ω ∝ t−0.35 (or Ro-0.7), until age t ≈ 2-3 Gyr (roughly at the Vaughan-Preston gap), where a sharp transition occurs, in which ωcyc/Ω increases by a factor of ≈ 6. Thereafter, evolution with ωcyc/Ω ∝ t−0.35 continues. The age at which transition occurs may be mass dependent, with K stars making the transition first.
- Published
- 1998
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42. Activity‐Related Radial Velocity Variation in Cool Stars
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Steven H. Saar and Robert A. Donahue
- Subjects
Physics ,K-type main-sequence star ,Starspot ,Astronomy ,Astronomy and Astrophysics ,Astrophysics ,Planetary system ,Exoplanet ,Radial velocity ,Stars ,Amplitude ,Space and Planetary Science ,Planet ,Astrophysics::Solar and Stellar Astrophysics ,Astrophysics::Earth and Planetary Astrophysics ,Astrophysics::Galaxy Astrophysics - Abstract
Planets have been detected orbiting several solar-type stars with the use of high-precision radial velocity (vr) measurements. While changes in vr can be measured with an accuracy of a few meters per second, there has been relatively little study of how other astrophysical processes, such as magnetic activity, may affect the observed velocities. In this paper, we use published data and simple models to explore the contributions to vr from two activity-related sources, starspots and convective inhomogeneities, as these features rotate across the disk and evolve in time. Radial velocity perturbations due to both of these sources increase with rotation and the level of surface activity. Our models indicate that for solar-age G stars, the amplitude of perturbations due to spots is AS 5 m s-1, increasing to AS ~ 30-50 m s-1 for Hyades-age G stars. If fS is the starspot area coverage, we find that A -->Sf -->0.9S v sin i. The effects of convective inhomogeneities (as observed in line bisector variations) appear to depend on both rotation and spectral type. Young (active) F and G dwarfs can have convective vr perturbations with amplitudes AC 50 m s-1, while vr amplitudes are reduced for stars with lower v sin i and cooler Teff. We show that vr data from the literature display similar trends with v sin i and Teff. AS and AC will be strongest at or near timescales related to magnetic activity variations: rotation, active region growth and decay, and activity cycles. Thus, knowledge of these timescales and typical AS and AC values are important in searching for extrasolar planets, especially those around younger, more active stars or those with small vr reflex amplitudes (i.e., 20 m s-1). We discuss implications of our results for current planet detections and planet search strategies.
- Published
- 1997
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43. The Emergence of Magnetic Flux Loops in Sunlike Stars
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Yuhong Fan, Steven H. Saar, and Edward E. DeLuca
- Subjects
Physics ,Flux tube ,Starspot ,Astronomy and Astrophysics ,Astrophysics ,Magnetic flux ,Stars ,T Tauri star ,Convection zone ,Space and Planetary Science ,Physics::Space Physics ,Astrophysics::Solar and Stellar Astrophysics ,Differential rotation ,Astrophysics::Earth and Planetary Astrophysics ,Magnetohydrodynamics - Abstract
We explore the latitude of emergence of flux tubes at the surface of G stars as a function of the rotation rate, magnetic flux, and injection latitude at the bottom of the convective zone. Our analysis is based on a thin flux tube evolution code that has been developed to study the emergence of magnetic flux in the Sun and is well calibrated by detailed comparisons with solar observations. We study solar models with rotation rates between $f {1}{3} $ --> and 10 times solar, injection latitudes I between 1° and 40°, and tubes with a range of field strengths, B0, and fluxes. For our range of input parameters, we find that the mean latitude of emergence, E, increases and its range decreases with higher rotation rates, that E ≤ 45° for stars with rotational periods ≥27 days, that E increases with B0 in rapid rotators, while the reverse is true for slow rotators, that the dependence of E on B0 decreases with increasing I, that tubes with higher flux emerge at larger E, and that the footpoint separation depends linearly on B0. We compare our results to other calculations and with observations of magnetic features on stars and suggest future observations and extensions of this research. Our results suggest that for near-polar starspots to occur, either active stars must have a larger range of I than inferred for the Sun, or differential rotation and meridional flows are more important for magnetic flux redistribution in these stars. Our models also imply that flux appearing near the equator of active stars may be generated by a distributed, rather than a boundary layer, dynamo.
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- 1997
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44. Properties of Sun‐like Stars with Planets: 51 Pegasi, 47 Ursae Majoris, 70 Virginis, and HD 114762
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Robert A. Donahue, Gregory W. Henry, Sallie L. Baliunas, Steven H. Saar, and Willie Soon
- Subjects
Physics ,Brightness ,K-type main-sequence star ,Astronomy ,Astronomy and Astrophysics ,Astrophysics::Cosmology and Extragalactic Astrophysics ,Astrophysics ,Planetary system ,Spectral line ,Radial velocity ,Stars ,Space and Planetary Science ,Planet ,Astrophysics::Solar and Stellar Astrophysics ,Astrophysics::Earth and Planetary Astrophysics ,Transit (astronomy) ,Astrophysics::Galaxy Astrophysics - Abstract
Radial velocity variations have revealed planets orbiting 51 Peg, 47 UMa, and 70 Vir, and a low-mass companion orbiting HD 114762. We analyze parallel records of photometric measurements in Stromgren b and y and Johnson V, R, and I passbands and Ca II H and K fluxes in those stars. In the case of 51 Peg, the high precision of the differential photometric measurements made by the 0.75 m Automatic Photoelectric Telescope and the nonvariability of the star would allow the detection of a transit of a planet as small as Earth (corresponding to an amplitude of 0.0001 mag) if its orbit were nearly coplanar with our line of sight. No transits were observed. For 51 Peg and 70 Vir, the upper limit of nondetection of photometric variability at their companion's orbital periods is Δ(b + y)/2 < 0.0002 ± 0.0002 mag. For HD 114762, it is ΔV < 0.0007 ± 0.0004 mag. Such small amplitudes of photometric variability seem to eliminate periodic velocity variations expected from p-mode oscillations. All four stars are magnetically quiet; that is, they lack the typical Ca II and photometric variability due to rotation and activity cycles expected from surface magnetic activity in solar-type stars. Such quiescence produces an interesting observational bias that favors the detection of planets from low-amplitude radial velocity or photometric variations by minimizing the contribution from intrinsic stellar variability. We discuss the circumstances for which the probability of planet detections is improved by the reduced level of variability from surface magnetic activity in G and K stars. Stars with low variability in surface activity should be the best candidates for planet searches using radial velocity and photometric techniques. Searches for planets around younger, more active stars will be impeded by variations in velocity or brightness caused by time-varying surface features. The Ca II H and K fluxes indicate that all four stars are older than 5 Gyr. Ages were estimated from the average levels of Ca II H and K fluxes and an existing relationship of the decrease of Ca II fluxes with age on the lower main sequence and were drawn from previous results based on theoretical isochrone fitting. Values of the projected rotational velocity, v sin i, are determined for 70 Vir and 47 UMa from high-resolution spectra.
- Published
- 1997
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45. GIANT CORONAL LOOPS DOMINATE THE QUIESCENT X-RAY EMISSION IN RAPIDLY ROTATING M STARS
- Author
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Cecilia Garraffo, Steven H. Saar, Scott J. Wolk, Rakesh K. Yadav, Vinay Kashyap, Jeremy J. Drake, Ofer Cohen, and Ignazio Pillitteri
- Subjects
Physics ,010504 meteorology & atmospheric sciences ,Astrophysics::High Energy Astrophysical Phenomena ,X-ray ,FOS: Physical sciences ,Astronomy and Astrophysics ,Astrophysics::Cosmology and Extragalactic Astrophysics ,Coronal loop ,Astrophysics ,01 natural sciences ,Corona ,Magnetic field ,Stars ,Astrophysics - Solar and Stellar Astrophysics ,Space and Planetary Science ,0103 physical sciences ,Astrophysics::Solar and Stellar Astrophysics ,Astrophysics::Earth and Planetary Astrophysics ,010303 astronomy & astrophysics ,Solar and Stellar Astrophysics (astro-ph.SR) ,Astrophysics::Galaxy Astrophysics ,0105 earth and related environmental sciences ,Dynamo - Abstract
Observations indicate that magnetic fields in rapidly rotating stars are very strong, on both small and large scales. What is the nature of the resulting corona? Here we seek to shed some light on this question. We use the results of an anelastic dynamo simulation of a rapidly rotating fully-convective M-star to drive a physics-based model for the stellar corona. We find that due to the several kilo Gauss large-scale magnetic fields at high latitudes, the corona and its X-ray emission are dominated by star-size large hot loops, while the smaller, underlying colder loops are not visible much in the X-ray. Based on this result we propose that, in rapidly rotating stars, emission from such coronal structures dominates the quiescent, cooler but saturated X-ray emission., 9 pages, 4 figures, accepted to ApJ
- Published
- 2016
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46. ULTRA-LUMINOUS X-RAY SOURCES IN HARO 11 AND THE ROLE OF X-RAY BINARIES IN FEEDBACK IN LyαEMITTING GALAXIES
- Author
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Prestwich, A. H., primary, Jackson, F., additional, Kaaret, P., additional, Brorby, M., additional, Roberts, T. P., additional, Saar, S. H., additional, and Yukita, M., additional
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- 2015
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47. LINKING STELLAR CORONAL ACTIVITY AND ROTATION AT 500 MYR: A DEEPCHANDRAOBSERVATION OF M37
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Núñez, Alejandro, primary, Agüeros, Marcel A., additional, Covey, Kevin R., additional, Hartman, Joel D., additional, Kraus, Adam L., additional, Bowsher, Emily C., additional, Douglas, Stephanie T., additional, López-Morales, Mercedes, additional, Pooley, David A., additional, Posselt, Bettina, additional, Saar, Steven H., additional, and West, Andrew A., additional
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- 2015
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48. INTERNETWORK CHROMOSPHERIC BRIGHT GRAINS OBSERVED WITH IRIS AND SST
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Martínez-Sykora, Juan, primary, Voort, Luc Rouppe van der, additional, Carlsson, Mats, additional, Pontieu, Bart De, additional, Pereira, Tiago M. D., additional, Boerner, Paul, additional, Hurlburt, Neal, additional, Kleint, Lucia, additional, Lemen, James, additional, Tarbell, Ted D., additional, Title, Alan, additional, Wuelser, Jean-Pierre, additional, Hansteen, Viggo H., additional, Golub, Leon, additional, McKillop, Sean, additional, Reeves, Kathy K., additional, Saar, Steven, additional, Testa, Paola, additional, Tian, Hui, additional, Jaeggli, Sarah, additional, and Kankelborg, Charles, additional
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- 2015
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49. HOMOLOGOUS HELICAL JETS: OBSERVATIONS BYIRIS,SDO, ANDHINODEAND MAGNETIC MODELING WITH DATA-DRIVEN SIMULATIONS
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Cheung, Mark C. M., primary, Pontieu, B. De, additional, Tarbell, T. D., additional, Fu, Y., additional, Tian, H., additional, Testa, P., additional, Reeves, K. K., additional, Martínez-Sykora, J., additional, Boerner, P., additional, Wülser, J. P., additional, Lemen, J., additional, Title, A. M., additional, Hurlburt, N., additional, Kleint, L., additional, Kankelborg, C., additional, Jaeggli, S., additional, Golub, L., additional, McKillop, S., additional, Saar, S., additional, Carlsson, M., additional, and Hansteen, V., additional
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- 2015
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50. ULTRA-LUMINOUS X-RAY SOURCES IN HARO 11 AND THE ROLE OF X-RAY BINARIES IN FEEDBACK IN LyαEMITTING GALAXIES
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M. Yukita, M. Brorby, P. Kaaret, Timothy P.L. Roberts, F. E. Jackson, Andrea Prestwich, and S. H. Saar
- Subjects
High Energy Astrophysical Phenomena (astro-ph.HE) ,Physics ,Supermassive black hole ,Active galactic nucleus ,Astrophysics::High Energy Astrophysical Phenomena ,FOS: Physical sciences ,Astronomy and Astrophysics ,Starburst region ,Astrophysics::Cosmology and Extragalactic Astrophysics ,Astrophysics ,Astrophysics - Astrophysics of Galaxies ,Galaxy ,Black hole ,Interstellar medium ,Space and Planetary Science ,Intermediate-mass black hole ,Astrophysics of Galaxies (astro-ph.GA) ,Astrophysics::Solar and Stellar Astrophysics ,Astrophysics - High Energy Astrophysical Phenomena ,Astrophysics::Galaxy Astrophysics ,Galaxy cluster - Abstract
Lyman Break Analogs (LBA) are local proxies of high-redshift Lyman Break Galaxies (LBG). Studies of nearby starbursts have shown that Lyman continuum and line emission are absorbed by dust and that the Lyman-alpha is resonantly scattered by neutral hydrogen. A source of feedback is required to prevent scattering and allow the Lyman-alpha emission to escape. There are two X-ray point sources embedded in the Lyman Break Analog (LBA) galaxy Haro 11. Haro 11 X-1 is an extremely luminous (L$_{X} \sim 10^{41}$ ergs s$^{-1}$), spatially compact source with a hard X-ray spectrum. Haro 11 X-1 is similar to the extreme Black Hole Binary (BHB) M82 X-1. The hard X-ray spectrum indicates Haro 11 X-1 may be a Black Hole Binary (BHB) in a low accretion state. The very high X-ray luminosity suggests an intermediate mass black hole that could be the seed for formation of a supermassive black hole. Source Haro 11 X-2 has an X-ray luminosity L$_{X} \sim 5\times10^{40}$ ergs s$^{-1}$ and a soft X-ray spectrum. This strongly suggests that Haro 11 X-2 is an X-ray binary in the ultra luminous state. Haro 11 X-2 is coincident with the star forming knot that is the source of the Lyman-alpha emission, raising the possibility that strong winds from X-ray binaries play an important part in injecting mechanical power into the Interstellar Medium (ISM), thus blowing away neutral material from the starburst region and allowing the Lyman-alpha to escape. We suggest that feedback from X-ray binaries may play a significant role in allowing Lyman-alpha emission to escape from galaxies in the early universe., Comment: Accepted for publication in Astrophysical Journal
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- 2015
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