| 1. |
- Benedict, G. Fritz, et al.
(författare)
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Distance scale zero points from galactic RR Lyrae star parallaxes
- 2011
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Ingår i: Astronomical Journal. - : American Astronomical Society. - 0004-6256 .- 1538-3881. ; 142:6, s. 187-
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Tidskriftsartikel (refereegranskat)abstract
- We present new absolute trigonometric parallaxes and proper motions for seven Population II variable stars-five RR Lyr variables: RZ Cep, XZ Cyg, SU Dra, RR Lyr, and UV Oct; and two type 2 Cepheids: VY Pyx and kappa Pav. We obtained these results with astrometric data from Fine Guidance Sensors, white-light interferometers on Hubble Space Telescope. We find absolute parallaxes in milliseconds of arc: RZ Cep, 2.12 +/- 0.16 mas; XZ Cyg, 1.67 +/- 0.17 mas; SU Dra, 1.42 +/- 0.16 mas; RR Lyr, 3.77 +/- 0.13 mas; UV Oct, 1.71 +/- 0.10 mas; VY Pyx, 6.44 +/- 0.23 mas; and. Pav, 5.57 +/- 0.28 mas; an average sigma(pi)/pi = 5.4%. With these parallaxes, we compute absolute magnitudes in V and K bandpasses corrected for interstellar extinction and Lutz-Kelker-Hanson bias. Using these RR Lyrae variable star absolute magnitudes, we then derive zero points for M(V)-[Fe/H] and M(K)-[Fe/H]-log P relations. The technique of reduced parallaxes corroborates these results. We employ our new results to determine distances and ages of several Galactic globular clusters and the distance of the Large Magellanic Cloud. The latter is close to that previously derived from Classical Cepheids uncorrected for any metallicity effect, indicating that any such effect is small. We also discuss the somewhat puzzling results obtained for our two type 2 Cepheids.
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| 2. |
- D’Orazi, Valentina, et al.
(författare)
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The GALAH survey : tracing the Milky Way’s formation and evolution through RR Lyrae stars
- 2024
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Ingår i: Monthly Notices of the Royal Astronomical Society. - 0035-8711 .- 1365-2966. ; 531:1, s. 137-162
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Tidskriftsartikel (refereegranskat)abstract
- Stellar mergers and accretion events have been crucial in shaping the evolution of the Milky Way (MW). These events have been dynamically identified and chemically characterized using red giants and main-sequence stars. RR Lyrae (RRL) variables can play a crucial role in tracing the early formation of the MW since they are ubiquitous, old (t ≥ 10 Gyr) low-mass stars and accurate distance indicators. We exploited Data Release 3 of the GALAH survey to identify 78 field RRLs suitable for chemical analysis. Using synthetic spectra calculations, we determined atmospheric parameters and abundances of Fe, Mg, Ca, Y, and Ba. Most of our stars exhibit halo-like chemical compositions, with an iron peak around [Fe/H] ≈ −1.40, and enhanced Ca and Mg content. Notably, we discovered a metal-rich tail, with [Fe/H] values ranging from −1 to approximately solar metallicity. This sub-group includes almost 1/4 of the sample, it is characterized by thin disc kinematics and displays sub-solar α-element abundances, marginally consistent with the majority of the MW stars. Surprisingly, they differ distinctly from typical MW disc stars in terms of the s-process elements Y and Ba. We took advantage of similar data available in the literature and built a total sample of 535 field RRLs for which we estimated kinematical and dynamical properties. We found that metal-rich RRLs (1/3 of the sample) likely represent an old component of the MW thin disc. We also detected RRLs with retrograde orbits and provided preliminary associations with the Gaia–Sausage–Enceladus, Helmi, Sequoia, Sagittarius, and Thamnos stellar streams.
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| 3. |
- Feiden, Gregory, 1986-, et al.
(författare)
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Do Magnetic Fields Actually Inflate Low-Mass Stars?
- 2013
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Ingår i: International Astronomical Union Symposium 302.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Magnetic fields have been hypothesized to inflate the radii of low-mass stars---defined as less than 0.8 solar masses---in detached eclipsing binaries (DEBs). We evaluate this hypothesis using the magnetic Dartmouth stellar evolution code. Results suggest that magnetic suppression of thermal convection can inflate low-mass stars that possess a radiative core and convective outer envelope. A scaling relation between X-ray luminosity and surface magnetic flux indicates that model surface magnetic field strength predictions are consistent with observations. This supports the notion that magnetic fields may be inflating these stars. However, magnetic models are unable to reproduce radii of fully convective stars in DEBs. Instead, we propose that model discrepancies below the fully convective boundary are related to metallicity.
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| 4. |
- Feiden, Gregory, 1986-, et al.
(författare)
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Magnetic Field Induced Radius Inflation of Low-Mass Stars
- 2013
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Ingår i: Binary 2013: Setting a new standard in the analysis of binary stars.. - : EDP Sciences.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We present results obtained using the magnetic Dartmouth stellar evolution code that address the possibility that magnetic fields are inflating low-mass stars in detached eclipsing binaries. While it seems plausible that magnetic fields are inflating stars with radiative cores, the level of inflation observed among fully convective stars appears too large to be explained by magnetic fields. We provide an alternative explanation, stellar metallicity, and propose observations that can help further constrain stellar models.
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| 5. |
- Feiden, Gregory, 1986-, et al.
(författare)
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Magnetic Inhibition of Convection and the Fundamental Properties of Low-Mass Stars. : II. Fully Convective Main-Sequence Stars
- 2014
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Ingår i: Astrophysical Journal. - : Institute of Physics (IOP). - 0004-637X .- 1538-4357. ; 789:1, s. 53-
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Tidskriftsartikel (refereegranskat)abstract
- We examine the hypothesis that magnetic fields are inflating the radii of fully convective main-sequence stars in detached eclipsing binaries (DEBs). The magnetic Dartmouth stellar evolution code is used to analyze two systems in particular: Kepler-16 and CM Draconis. Magneto-convection is treated assuming stabilization of convection and also by assuming reductions in convective efficiency due to a turbulent dynamo. We find that magnetic stellar models are unable to reproduce the properties of inflated fully convective main-sequence stars, unless strong interior magnetic fields in excess of 10 MG are present. Validation of the magnetic field hypothesis given the current generation of magnetic stellar evolution models therefore depends critically on whether the generation and maintenance of strong interior magnetic fields is physically possible. An examination of this requirement is provided. Additionally, an analysis of previous studies invoking the influence of star spots is presented to assess the suggestion that star spots are inflating stars and biasing light curve analyses toward larger radii. From our analysis, we find that there is not yet sufficient evidence to definitively support the hypothesis that magnetic fields are responsible for the observed inflation among fully convective main-sequence stars in DEBs.
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| 6. |
- Feiden, Gregory, 1986-, et al.
(författare)
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Magnetic Inhibition of Convection and the Fundamental Properties of Low-Mass Stars. I. Stars with a Radiative Core
- 2013
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Ingår i: Astrophysical Journal. - : Institute of Physics (IOP). - 0004-637X .- 1538-4357. ; 779:2, s. 183-
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic fields are hypothesized to inflate the radii of low-mass stars---defined as less massive than 0.8M⊙---in detached eclipsing binaries (DEBs). We investigate this hypothesis using the recently introduced magnetic Dartmouth stellar evolution code. In particular, we focus on stars thought to have a radiative core and convective outer envelope by studying in detail three individual DEBs: UV Psc, YY Gem, and CU Cnc. The results suggest that the stabilization of thermal convection by a magnetic field is a plausible explanation for the observed model-radius discrepancies. However, surface magnetic field strengths required by the models are significantly stronger than those estimated from the observed coronal X-ray emission. Agreement between model predicted surface magnetic field strengths and those inferred from X-ray observations can be found by assuming that the magnetic field sources its energy from convection. This approach makes the transport of heat by convection less efficient and is akin to reduced convective mixing length methods used in other studies. Predictions for the metallicity and magnetic field strengths of the aforementioned systems are reported. We also develop an expression relating a reduction in the convective mixing length to a magnetic field strength in units of the equipartition value. Our results are compared with those from previous investigations to incorporate magnetic fields to explain the low-mass DEB radius inflation. Finally, we explore how the effects of magnetic fields might affect mass determinations using asteroseismic data and the implication of magnetic fields on exoplanet studies.
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| 7. |
- Feiden, Gregory, 1986-, et al.
(författare)
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Revised age for CM Draconis and WD 1633+572 : Toward a resolution of model-observation radius discrepancies
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 571, s. A70-
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Tidskriftsartikel (refereegranskat)abstract
- We report an age revision for the low-mass detached eclipsing binary CM Draconis and its common proper motion companion, WD 1633+572. An age of 8.5 ± 3.5 Gyr is found by combining an age estimate for the lifetime of WD 1633+572 and an estimate from galactic space motions. The revised age is greater than a factor of two older than previous estimates. Our results provide consistency between the white dwarf age and the system's galactic kinematics, which reveal the system is a highly probable member of the galactic thick disk. We find the probability that CM Draconis and WD 1633+572 are members of the thick disk is 8500 times greater than the probability that they are members of the thin disk and 170 times greater than the probability they are halo interlopers. If CM Draconis is a member of the thick disk, it is likely enriched in α-elements compared to iron by at least 0.2 dex relative to the Sun. This leads to the possibility that previous studies under-estimate the [Fe/H] value, suggesting the system has a near-solar [Fe/H]. Implications for the long-standing discrepancies between the radii of CM Draconis and predictions from stellar evolution theory are discussed. We conclude that CM Draconis is only inflated by about 2% compared to stellar evolution predictions.
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