| 1. |
- Brewer, John M., et al.
(författare)
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Accurate Gravities of F, G, and K Stars from High Resolution Spectra Without External Constraints
- 2015
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 805:2
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate a new procedure to derive accurate and precise surface gravities from high resolution spectra without the use of external constraints. Our analysis utilizes Spectroscopy Made Easy with robust spectral line constraints and uses an iterative process to mitigate degeneracies in the fitting process. We adopt an updated radiative transfer code, a new treatment for neutral perturber broadening, a line list with multiple gravity constraints and separate fitting for global stellar properties and abundance determinations. To investigate the sources of temperature dependent trends in determining log g noted in previous studies, we obtained Keck HIRES spectra of 42 Kepler asteroseismic stars. In comparison to asteroseismically determined log g our spectroscopic analysis has a constant offset of 0.01 dex with a rms scatter of 0.05 dex. We also analyzed 30 spectra which had published surface gravities determined using the a/R-* technique from planetary transits and found a constant offset of 0.06 dex and rms scatter of 0.07 dex. The two samples covered effective temperatures between 5000 and 6700 K with log g between 3.7 and 4.6.
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| 2. |
- Brewer, John M., et al.
(författare)
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Spectral Properties Of Cool Stars : Extended Abundance Analysis Of 1,617 Planet-Search Stars
- 2016
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 225:2
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Tidskriftsartikel (refereegranskat)abstract
- We present a catalog of uniformly determined stellar properties and abundances for 1,617 F, G, and K stars using an automated spectral synthesis modeling procedure. All stars were observed using the HIRES spectrograph at Keck Observatory. Our procedure used a single line list to fit model spectra to observations of all stars to determine effective temperature, surface gravity, metallicity, projected rotational velocity, and the abundances of 15 elements (C, N, O, Na, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Fe, Ni, and Y). Sixty percent of the sample had Hipparcos parallaxes and V-band photometry, which we combined with the spectroscopic results to obtain mass, radius, and luminosity. Additionally, we used the luminosity, effective temperature, metallicity and alpha-element enhancement to interpolate in the Yonsei-Yale isochrones to derive mass, radius, gravity, and age ranges for those stars. Finally, we determined new relations between effective temperature and macroturbulence for dwarfs and subgiants. Our analysis achieved precisions of 25 K in T-eff, 0.01. dex in [M/H], 0.028. dex for log g,. and 0.5 km s(-1) in v sin i based on multiple observations of the same stars. The abundance results were similarly precise, between similar to 0.01 and similar to 0.04. dex, though trends with respect to T-eff remained for which we derived empirical corrections. The trends, though small, were much larger than our uncertainties and are shared with published abundances. We show that changing our model atmosphere grid accounts for most of the trend in [M/H] between 5000 and 5500 K, indicating a possible problem with the atmosphere models or opacities.
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| 3. |
- Fischer, Debra A., et al.
(författare)
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M2K. II. A Triple-Planet System Orbiting Hip 57274
- 2012
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 745:1, s. 21-
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Tidskriftsartikel (refereegranskat)abstract
- Doppler observations from Keck Observatory have revealed a triple-planet system orbiting the nearby K4V star, HIP 57274. The inner planet, HIP 57274b, is a super-Earth with M sin i = 11.6 M-circle plus (0.036 M-Jup), an orbital period of 8.135 +/- 0.004 days, and slightly eccentric orbit e = 0.19 +/- 0.1. We calculate a transit probability of 6.5% for the inner planet. The second planet has M sin i = 0.4 M-Jup with an orbital period of 32.0 +/- 0.02 days in a nearly circular orbit (e = 0.05 +/- 0.03). The third planet has M sin i = 0.53 M-Jup with an orbital period of 432 +/- 8 days (1.18 years) and an eccentricity e = 0.23 +/- 0.03. This discovery adds to the number of super-Earth mass planets with M sin i < 12 M-circle plus that have been detected with Doppler surveys. We find that 56% +/- 18% of super-Earths are members of multi-planet systems. This is certainly a lower limit because of observational detectability limits, yet significantly higher than the fraction of Jupiter mass exoplanets, 20% +/- 8%, that are members of Doppler-detected, multi-planet systems.
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| 4. |
- Howard, Andrew W., et al.
(författare)
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The California planet survey. I. four new giant exoplanets
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 721:2, s. 1467-1481
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Tidskriftsartikel (refereegranskat)abstract
- We present precise Doppler measurements of four stars obtained during the past decade at Keck Observatory by the California Planet Survey (CPS). These stars, namely, HD 34445, HD 126614, HD 13931, and Gl 179, all show evidence for a single planet in Keplerian motion. We also present Doppler measurements from the Hobby-Eberly Telescope (HET) for two of the stars, HD 34445 and Gl 179, that confirm the Keck detections and significantly refine the orbital parameters. These planets add to the statistical properties of giant planets orbiting near or beyond the ice line, and merit follow-up by astrometry, imaging, and space-borne spectroscopy. Their orbital parameters span wide ranges of planetary minimum mass (M sin i = 0.38-1.9 M-Jup), orbital period (P = 2.87-11.5 yr), semimajor axis (a = 2.1-5.2 AU), and eccentricity (e = 0.02-0.41). HD 34445 b (P = 2.87 yr, M sin i = 0.79 MJup, e = 0.27) is a massive planet orbiting an old, G-type star. We announce a planet, HD 126614 Ab, and an M dwarf, HD 126614 B, orbiting the metal-rich star HD 126614 (which we now refer to as HD 126614 A). The planet, HD 126614 Ab, has minimum mass M sin i = 0.38 MJup and orbits the stellar primary with period P = 3.41 yr and orbital separation a = 2.3 AU. The faint M dwarf companion, HD 126614 B, is separated from the stellar primary by 489 mas (33 AU) and was discovered with direct observations using adaptive optics and the PHARO camera at Palomar Observatory. The stellar primary in this new system, HD 126614 A, has the highest measured metallicity ([ Fe/ H] = + 0.56) of any known planet-bearing star. HD 13931 b (P = 11.5 yr, M sin i = 1.88 MJup, e = 0.02) is a Jupiter analog orbiting a near solar twin. Gl 179 b (P = 6.3 yr, M sin i = 0.82 M-Jup, e = 0.21) is a massive planet orbiting a faint M dwarf. The high metallicity of Gl 179 is consistent with the planet-metallicity correlation among M dwarfs, as documented recently by Johnson & Apps.
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| 5. |
- Howard, Andrew W., et al.
(författare)
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The NASA-UC Eia-earth program. II. a planet orbiting HD 156668 with a minimum mass of four earth masses
- 2011
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 726:2, s. 73-
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Tidskriftsartikel (refereegranskat)abstract
- We report the discovery of HD 156668 b, an extrasolar planet with a minimum mass of M-P sin i = 4.15 M-circle plus. This planet was discovered through Keplerian modeling of precise radial velocities from Keck-HIRES and is the second super-Earth to emerge from the NASA-UC Eta-Earth Survey. The best-fit orbit is consistent with circular and has a period of P = 4.6455 days. The Doppler semi-amplitude of this planet, K = 1.89 m s(-1), is among the lowest ever detected, on par with the detection of GJ 581 e using HARPS. A longer period (P approximate to 2.3 years), low-amplitude signal of unknown origin was also detected in the radial velocities and was filtered out of the data while fitting the short-period planet. Additional data are required to determine if the long-period signal is due to a second planet, stellar activity, or another source. Photometric observations using the Automated Photometric Telescopes at Fairborn Observatory show that HD 156668 (an old, quiet K3 dwarf) is photometrically constant over the radial velocity period to 0.1 mmag, supporting the existence of the planet. No transits were detected down to a photometric limit of similar to 3 mmag, ruling out transiting planets dominated by extremely bloated atmospheres, but not precluding a transiting solid/liquid planet with a modest atmosphere.
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| 6. |
- Howard, Andrew W., et al.
(författare)
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THE NASA-UC ETA-EARTH PROGRAM : I. A SUPER-EARTH ORBITING HD 7924
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 696:1, s. 75-83
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Tidskriftsartikel (refereegranskat)abstract
- We report the discovery of the first low-mass planet to emerge from the NASA-UC Eta-Earth Program, a super-Earth orbiting the K0 dwarf HD 7924. Keplerian modeling of precise Doppler radial velocities reveals a planet with minimum mass M-P sin i = 9.26M(circle plus) in a P = 5.398 d orbit. Based on Keck-HIRES measurements from 2001 to 2008, the planet is robustly detected with an estimated false alarm probability of less than 0.001. Photometric observations using the Automated Photometric Telescopes at Fairborn Observatory show that HD 7924 is photometrically constant over the radial velocity period to 0.19 mmag, supporting the existence of the planetary companion. No transits were detected down to a photometric limit of similar to 0.5 mmag, eliminating transiting planets with a variety of compositions. HD 7924b is one of only eight planets detected by the radial velocity technique with M-P sini < 10 M-circle plus and as such is a member of an emerging family of low-mass planets that together constrain theories of planet formation.
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| 7. |
- Howard, Andrew W., et al.
(författare)
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THE NASA-UC ETA-EARTH PROGRAM. III. A SUPER-EARTH ORBITING HD 97658 AND A NEPTUNE-MASS PLANET ORBITING G1 785
- 2011
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 730:1, s. 10-
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Tidskriftsartikel (refereegranskat)abstract
- We report the discovery of planets orbiting two bright, nearby early K dwarf stars, HD 97658 and G1 785. These planets were detected by Keplerian modeling of radial velocities measured with Keck-HIRES for the NASA-UC Eta-Earth Survey. HD 97658 b is a close-in super-Earth with minimum mass M sin i = 8.2 +/- 1.2 M-circle plus, orbital period P = 9.494 +/- 0.005 days, and an orbit that is consistent with circular. G1 785 b is a Neptune-mass planet with M sin i = 21.6 +/- 2.0 M-circle plus, P = 74.39 +/- 0.12 days, and orbital eccentricity e = 0.30 +/- 0.09. Photometric observations with the T12 0.8 m automatic photometric telescope at Fairborn Observatory show that HD 97658 is photometrically constant at the radial velocity period to 0.09 mmag, supporting the existence of the planet.
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| 8. |
- Johns-Krull, Christopher M., et al.
(författare)
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Magnetically Controlled Accretion on the Classical T Tauri Stars GQ Lupi and TW Hydrae
- 2013
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 765:1, s. 11-
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Tidskriftsartikel (refereegranskat)abstract
- We present high spectral resolution (R approximate to 108,000) Stokes V polarimetry of the classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m telescope. We present data on both photospheric lines and emission lines, concentrating our discussion on the polarization properties of the He I emission lines at 5876 angstrom and 6678 angstrom. The He I lines in these CTTSs contain both narrow emission cores, believed to come from near the accretion shock region on these stars, and broad emission components which may come from either a wind or the large-scale magnetospheric accretion flow. We detect strong polarization in the narrow component of the two He I emission lines in both stars. We observe a maximum implied field strength of 6.05 +/- 0.24 kG in the 5876 angstrom line of GQ Lup, making it the star with the highest field strength measured in this line for a CTTS. We find field strengths in the two He I lines that are consistent with each other, in contrast to what has been reported in the literature on at least one star. We do not detect any polarization in the broad component of the He I lines on these stars, strengthening the conclusion that they form over a substantially different volume relative to the formation region of the narrow component of the He I lines.
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| 9. |
- Lavail, Alexis
(författare)
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Magnetic fields of cool stars from near-infrared spectropolarimetry
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Magnetic fields rule many physical processes in and around stars throughout their lifetime. All cool stars possess a magnetic field, likely generated by dynamo processes. In order to properly understand the evolution of cool stars, we need to understand their magnetism. Stellar magnetic fields can be directly observed through the imprint of the Zeeman effect in intensity and polarized spectra. In intensity spectra (Stokes I), spectral lines are broadened or split into several components by the magnetic field. Modelling this effect in high-resolution spectra allows us to determine the average unsigned magnetic field strength over the stellar surface. The magnetic field also induces circular (Stokes V) and linear polarization (Stokes QU) in spectral lines, according to its orientation. These polarization signals can be used to map the large-scale magnetic field at the surface of the star using tomographic techniques such as Zeeman Doppler imaging (ZDI). In this thesis, we investigated pre-main-sequence T Tauri stars and the active M dwarf AD Leo with the goal to understand their magnetic fields. We modelled the Zeeman broadening in high-resolution near-infrared spectra of low-mass and intermediate-mass T Tauri stars and derived their mean magnetic field strengths. In intermediate-mass T Tauri stars, we only found fields weaker than 2-3 kG. However, we found that low-mass T Tauri stars can have a wide range of magnetic field strength from relatively weak fields of 1.5 kG to fields as strong as 4.4 kG, and that their field strengths do not correlate with stellar parameters. Our observations of the M dwarf AD Leo led to the first detection of linear polarization in the spectral lines of an M dwarf. We also discovered that its Stokes V profiles, which were constant over many years, had changed in our observations. We mapped its global magnetic field using ZDI and found that it became concentrated into smaller areas on the stellar surface. Finally, we analyzed Stokes IV observations of the spectroscopic binary V1878 Ori. Both components of this system are intermediate-mass T Tauri stars with very similar properties. We determined stellar parameters by studying orbital motion of the components and comparing their disentangled spectra to theoretical models. We then mapped the global magnetic fields of the two stars simultaneously using ZDI. We found that their magnetic fields have radically different geometries and different strengths.
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| 10. |
- Makaganiuk, Vitalii, et al.
(författare)
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Magnetism, chemical spots, and stratification in the HgMn star ϕ Phoenicis
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 539, s. A142-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Mercury-manganese (HgMn) stars have been considered as non-magnetic and non-variable chemically peculiar (CP) starsfor a long time. However, recent discoveries of the variability in spectral line profiles have suggested an inhomogeneous surfacedistribution of chemical elements in some HgMn stars. From the studies of other CP stars it is known that magnetic field plays a keyrole in the formation of surface spots. All attempts to find magnetic fields in HgMn stars have yielded negative results.Aims. In this study, we investigate the possible presence of a magnetic field in ϕ Phe (HD 11753) and reconstruct surface distributionof chemical elements that show variability in spectral lines.We also test a hypothesis that a magnetic field is concentrated in chemicalspots and look into the possibility that some chemical elements are stratified with depth in the stellar atmosphere.Methods. Our analysis is based on high-quality spectropolarimetric time-series observations, covering a full rotational period ofthe star. Spectra were obtained with the HARPSpol at the ESO 3.6-m telescope. To increase the sensitivity of the magnetic fieldsearch, we employed the least-squares deconvolution (LSD) technique. Using Doppler imaging code INVERS10, we reconstructedsurface chemical distributions by utilising information from multiple spectral lines. The vertical stratification of chemical elementswas calculated with the DDAFit program.Results. Combining information from all suitable spectral lines, we set an upper limit of 4 G on the mean longitudinal magnetic field.For chemical spots, an upper limit on the longitudinal field varies between 8 and 15 G. We confirmed the variability of Y, Sr, and Tiand detected variability in Cr lines. Stratification analysis showed that Y and Ti are not concentrated in the uppermost atmosphericlayers.Conclusions. Our spectropolarimetric observations rule out the presence of a strong, globally-organised magnetic field in ϕ Phe.This implies an alternative mechanism of spot formation, which could be related to a non-equilibrium atomic diffusion. However, thetypical time scales of the variation in stratification predicted by the recent time-dependent diffusion models exceed significantly thespot evolution time-scale reported for ϕ Phe.
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