| 1. |
- Allard, F, et al.
(författare)
-
Model atmospheres and spectra: The role of dust
- 2003
-
Ingår i: Proceedings of the International Astronomical Union (Brown Dwarfs). - 0074-1809. - 158381132X ; 211, s. 325-332
-
Konferensbidrag (refereegranskat)abstract
- Brown dwarf atmospheres form molecules, then high temperature condensates (corundum, titanates, silicates, and iron compounds), and then low temperature condensates (ices) as they cool down over time. These produce large opacities which govern entirely their spectral energy distribution. Just as it is important to know molecular opacities (TiO, H2O, CH4, etc.) with accuracy, it is imperative to understand the interplay of processes (e.g. condensation, sedimentation, coagulation, convection) that determines the radial and size distribution of grains. Limiting case models have shown that young, hot brown (L) dwarfs form dust mostly in equilibrium, while at much cooler stages (late T dwarfs) all high temperature condensates have sedimented out of their photospheres. But this process is gradual and all intermediate classes of brown dwarfs can partly be understood in terms of partial sedimentation of dust. With new models accounting for these processes, we describe the effects they may have upon brown dwarf spectral properties.
|
|
| 2. |
- Aufdenberg, JP, et al.
(författare)
-
On the limb darkening, spectral energy distribution, and temperature structure of procyon
- 2005
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 633:1, s. 424-439
-
Tidskriftsartikel (refereegranskat)abstract
- We have fit synthetic visibilities from three-dimensional ((COBOLD)-B-5+PHOENIX) and one-dimensional (PHOENIX, ATLAS 12) model stellar atmospheres of Procyon (F5 IV) to high-precision interferometric data from the VLT Interferometer (K band) and from the Mark III interferometer (500 and 800 nm). These data sets provide a test of theoretical wavelength-dependent limb-darkening predictions. The work of Allende Prieto et al. has shown that the temperature structure from a spatially and temporally averaged three-dimensional hydrodynamic model produces significantly less limb darkening at 500 nm relative to the temperature structure of a one-dimensional MARCS model atmosphere with a standard mixing-length approximation for convection. Our direct fits to the interferometric data confirm this prediction. A one-dimensional ATLAS 12 model with " approximate overshooting" provides the required temperature gradient. We show, however, that one-dimensional models cannot reproduce the ultraviolet spectrophotometry below 160 nm with effective temperatures in the range constrained by the measured bolometric flux and angular diameter. We find that a good match to the full spectral energy distribution can be obtained with a composite model consisting of a weighted average of 12 one-dimensional model atmospheres based on the surface intensity distribution of a three-dimensional granulation simulation. We emphasize that one-dimensional models with overshooting may realistically represent the mean temperature structure of F-type stars such as Procyon, but the same models will predict redder colors than observed because they lack the multicomponent temperature distribution expected for the surfaces of these stars.
|
|
| 3. |
- Aufdenberg, J. P., et al.
(författare)
-
Procyon: Constraining Its Temperature Structure with High-Precision Interferometry and 3-D Model Atmospheres
- 2004
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We have fit synthetic visibilities from 3-D (CO5BOLD +PHOENIX) and 1-D (PHOENIX, ATLAS12) model stellar atmospheres forProcyon (F5 IV) to high-precision interferometric data from the VINCIinstrument at the VLT Interferometer (K-band) and from the Mark IIIinterferometer (500 nm, 800 nm). These data provide a test oftheoretical wavelength-dependent limb-darkening predictions, andtherefore Procyon's atmospheric temperature structure. Earlier work(Allende Prieto et al. 2002 ApJ 567, 544) has shown that the temperaturestructure from a spatially and temporally averaged 3-D hydrodynamicalmodel produces significantly less limb darkening at 500 nm relative tothe temperature structure from a 1-D MARCS model atmosphere which uses amixing-length approximation for convective flux transport. Our directfits to the interferometric data confirm this prediction, however wefind that not all 1-D models fail to reproduce the observations. The keyto matching the interferometric data is a shallower temperature gradientthan provided by the standard 1-D mixing-length approximation. We findthat in addition to our best fitting 3-D hydrodynamical model, a 1-DATLAS12 model, with an additional free parameter for ``approximateovershooting'', provides the required temperature gradient. We estimatethat an interferometric precision better than 0.1% will be required todistinguish between the 3-D model and the ATLAS12 model. Thisovershooting approximation has been shown to match Solar limb-darkeningobservations reasonably well (Castelli et al 1997 A&A 324, 432), howeverpublished work since using Strömgren photometry of solar-type starshas cast doubt on the importance of overshooting. We have also comparedsynthetic spectral energy distributions for Procyon to ultraviolet,optical and near-infrared spectrophotometry and find differences fromcomparisons to Strömgren photometry alone.This work was performed in part contract with the Jet PropulsionLaboratory (JPL) funded by NASA through the Michelson FellowshipProgram. JPL is managed for NASA by the California Institute ofTechnology.
|
|
| 4. |
- Christleib, Norbert, et al.
(författare)
-
4MOST Consortium Survey 2: The Milky Way Halo High-Resolution Survey
- 2019
-
Ingår i: Messenger. - 0722-6691. ; 175, s. 26-29
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We will study the formation history of the Milky Way, and the earliest phases of its chemical enrichment, with a sample of more than 1.5 million stars at high galactic latitude. Elemental abundances of up to 20 elements with a precision of better than 0.2 dex will be derived for these stars. The sample will include members of kinematically coherent substructures, which we will associate with their possible birthplaces by means of their abundance signatures and kinematics, allowing us to test models of galaxy formation. Our target catalogue is also expected to contain 30 000 stars at a metallicity of less than one hundredth that of the Sun. This sample will therefore be almost a factor of 100 larger than currently existing samples of metal-poor stars for which precise elemental abundances are available (determined from high-resolution spectroscopy), enabling us to study the early chemical evolution of the Milky Way in unprecedented detail.
|
|
| 5. |
- Cukanovaite, Elena, et al.
(författare)
-
3D spectroscopic analysis of helium-line white dwarfs
- 2021
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 501:4, s. 5274-5293
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, we present corrections to the spectroscopic parameters of DB and DBA white dwarfs with -10.0 <= log(H/He) <= -2.0, 7.5 <= log g <= 9.0, and less than or similar to T-eff less than or similar to 34 000 K, based on 282 3D atmospheric models calculated with the (COBOLD)-B-5 radiation-hydrodynamics code. These corrections arise due to a better physical treatment of convective energy transport in 3D models when compared to the previously available 1D model atmospheres. By applying the corrections to an existing Sloan Digital Sky Survey (SDSS) sample of DB and DBA white dwarfs, we find significant corrections both for effective temperature and surface gravity. The 3D log g corrections are most significant for T-eff less than or similar to 18 000 K, reaching up to -0.20 dex at log g = 8.0. However, in this low effective temperature range, the surface gravity determined from the spectroscopic technique can also be significantly affected by the treatment of the neutral van der Waals line broadening of helium and by non-ideal effects due to the perturbation of helium by neutral atoms. Thus, by removing uncertainties due to 1D convection, our work showcases the need for improved description of microphysics for DB and DBA model atmospheres. Overall, we find that our 3D spectroscopic parameters for the SDSS sample are generally in agreement with Gaia Data Release 2 absolute fluxes within 1 sigma-3 sigma for individual white dwarfs. By comparing our results to DA white dwarfs, we determine that the precision and accuracy of DB/DBA atmospheric models are similar. For ease of user application of the correction functions, we provide an example PYTHON code.
|
|
| 6. |
- Cunningham, Tim, et al.
(författare)
-
Horizontal spreading of planetary debris accreted by white dwarfs
- 2021
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 503:2, s. 1646-1667
-
Tidskriftsartikel (refereegranskat)abstract
- White dwarfs with metal-polluted atmospheres have been studied widely in the context of the accretion of rocky debris from evolved planetary systems. One open question is the geometry of accretion and how material arrives and mixes in the white dwarf surface layers. Using the three-dimensional (3D) radiation hydrodynamics code (COBOLD)-B-5, we present the first transport coefficients in degenerate star atmospheres that describe the advection-diffusion of a passive scalar across the surface plane. We couple newly derived horizontal diffusion coefficients with previously published vertical diffusion coefficients to provide theoretical constraints on surface spreading of metals in white dwarfs. Our grid of 3D simulations probes the vast majority of the parameter space of convective white dwarfs, with pure-hydrogen atmospheres in the effective temperature range of 6000-18 000K and pure-helium atmospheres in the range of 12 000-34 000 K. Our results suggest that warm hydrogen-rich atmospheres (DA; greater than or similar to 13 000 K) and helium-rich atmospheres (DB and DBA; greater than or similar to 30 000 K) are unable to efficiently spread the accreted metals across their surface, regardless of the time dependence of accretion. This result may be at odds with the current non-detection of surface abundance variations in white dwarfs with debris discs. For cooler hydrogen- and helium-rich atmospheres, we predict a largely homogeneous distribution of metals across the surface within a vertical diffusion time-scale. This is typically less than 0.1 per cent of disc lifetime estimates, a quantity that is revisited in this paper using the overshoot results. These results have relevance for studies of the bulk composition of evolved planetary systems and models of accretion disc physics.
|
|
| 7. |
- Dorch, S.B.F., et al.
(författare)
-
Dynamo action in M-dwarfs
- 2005
-
Ingår i: ESA SP (Special Publication). - 1609-042X .- 0379-6566. ; 560, s. 515-517
-
Konferensbidrag (refereegranskat)
|
|
| 8. |
- Dorch, S B F, et al.
(författare)
-
Small-scale magnetic fields on late-type M-dwarfs
- 2002
-
Ingår i: Astronomical Notes - Astronomische Nachrichten. - 0004-6337. ; 323:3-4, s. 402-406
-
Tidskriftsartikel (refereegranskat)abstract
- We performed kinematic studies of the evolution of small-scale magneticfields in the surface layers of M-dwarfs. We solved the inductionequation for a prescribed velocity field, magnetic Reynolds number ReM,and boundary conditions in a Cartesian box, representing a volumecomprising the optically thin stellar atmosphere and the uppermost partof the optically thick convective envelope. The velocity field isspatially and temporally variable, and stems from detailedradiation-hydrodynamics simulations of convective flows in aproto-typical late-type M-dwarf (Teff =2800pun {K}, logg =5.0, solarchemical composition, spectral type ~M6). We find dynamo action for ReM>= 400. Growth time scales of the magnetic field are comparable tothe convective turn-over time scale (~ 150pun {sec}). The convectivevelocity field concentrates the magnetic field in sheets and tubularstructures in the inter-granular downflows. Scaling from solarconditions suggests that field strengths as high as 20pun{kG} might bereached locally. Perhaps surprisingly, ReM is of order unity in thesurface layers of cooler M-dwarfs, rendering the dynamo inoperative. Inall studied cases we find a rather low spatial filling factor of themagnetic field.
|
|
| 9. |
|
|
| 10. |
- Dravins, Dainis, et al.
(författare)
-
Solar photospheric spectrum microvariability : I. Theoretical searches for proxies of radial-velocity jittering
- 2023
-
Ingår i: Astronomy and Astrophysics. - 0004-6361. ; 679
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Extreme precision radial-velocity spectrometers enable extreme precision in stellar spectroscopy. Searches for low-mass exoplanets around solar-type stars are limited by various types of physical variability in stellar spectra, such as the short-term jittering of apparent radial velocities on levels of ∼2 m s-1. Aims. To understand the physical origins of radial-velocity jittering, the solar spectrum is assembled, as far as possible, from basic principles. Solar surface convection is modeled with time-dependent 3D hydrodynamics, followed by the computation of high-resolution spectra during numerous instances of the simulation sequence. The behavior of different classes of photospheric spectral lines is monitored throughout the simulations to identify commonalities or differences between different classes of lines: weak or strong, neutral or ionized, high or low excitation, atomic or molecular. Methods. Synthetic spectra were examined. With a wavelength sampling λ/Δλ ∼ 1 000 000, the changing shapes and wavelength shifts of unblended and representative FeI and FeII lines were followed during the simulation sequences. The radial-velocity jittering over the small simulation area typically amounts to ±150 m s-1, scaling to ∼2 m s-1 for the full solar disk. Flickering within the G-band region and in photometric indices of the Strömgren uvby system were also measured, and synthetic G-band spectra from magnetic regions are discussed. Results. Most photospheric lines vary in phase, but with different amplitudes among different classes of lines. Amplitudes of radial-velocity excursions are greater for stronger and for ionized lines, decreasing at longer wavelengths. Matching precisely measured radial velocities to such characteristic patterns should enable us to remove a significant component of the stellar noise originating in granulation. Conclusions. The granulation-induced amplitudes in full-disk sunlight amount to ∼2 m s-1; the differences between various line groups are an order of magnitude less. To mitigate this jittering, a matched filter must recognize dissimilar lineshifts among classes of diverse spectral lines with a precision of ∼10 cm s-1 for each line group. To verify the modeling toward the filter, predictions of center-to-limb dependences of jittering amplitudes for different classes of lines are presented, testable with spatially resolving solar telescopes connected to existing radial-velocity instruments.
|
|
