1,770 research outputs found
A comparison of chemistry and dust cloud formation in ultracool dwarf model atmospheres
The atmospheres of substellar objects contain clouds of oxides, iron,
silicates, and other refractory condensates. Water clouds are expected in the
coolest objects. The opacity of these `dust' clouds strongly affects both the
atmospheric temperature-pressure profile and the emergent flux. Thus any
attempt to model the spectra of these atmospheres must incorporate a cloud
model. However the diversity of cloud models in atmospheric simulations is
large and it is not always clear how the underlying physics of the various
models compare. Likewise the observational consequences of different modeling
approaches can be masked by other model differences, making objective
comparisons challenging. In order to clarify the current state of the modeling
approaches, this paper compares five different cloud models in two sets of
tests. Test case 1 tests the dust cloud models for a prescribed L, L--T, and
T-dwarf atmospheric (temperature T, pressure p, convective velocity
vconv)-structures. Test case 2 compares complete model atmosphere results for
given (effective temperature Teff, surface gravity log g). All models agree on
the global cloud structure but differ in opacity-relevant details like grain
size, amount of dust, dust and gas-phase composition. Comparisons of synthetic
photometric fluxes translate into an modelling uncertainty in apparent
magnitudes for our L-dwarf (T-dwarf) test case of 0.25 < \Delta m < 0.875 (0.1
< \Delta m M 1.375) taking into account the 2MASS, the UKIRT WFCAM, the Spitzer
IRAC, and VLT VISIR filters with UKIRT WFCAM being the most challenging for the
models. (abr.)Comment: 22 pages, 17 figures, MNRAS 2008, accepted, (minor grammar/typo
corrections
Comparison of cloud models for Brown Dwarfs
A test case comparison is presented for different dust cloud model approaches
applied in brown dwarfs and giant gas planets. We aim to achieve more
transparency in evaluating the uncertainty inherent to theoretical modelling.
We show in how far model results for characteristic dust quantities vary due to
different assumptions. We also demonstrate differences in the spectral energy
distributions resulting from our individual cloud modelling in 1D substellar
atmosphere simulationsComment: 5 pages, Proceeding to "Exoplantes: Detection, Formation, Dynamics",
eds. Ferraz-Mello et
Detectability of dirty dust grains in brown dwarf atmospheres
Dust clouds influence the atmospheric structure of brown dwarfs, and they
affect the heat transfer and change the gas-phase chemistry. However, the
physics of their formation and evolution is not well understood. In this
letter, we predict dust signatures and propose a potential observational test
of the physics of dust formation in brown dwarf atmosphere based on the
spectral features of the different solid components predicted by dust formation
theory. A momentum method for the formation of dirty dust grains (nucleation,
growth, evaporation, drift) is used in application to a static brown dwarf
atmosphere structure to compute the dust grain properties, in particular the
heterogeneous grain composition and the grain size. Effective medium and Mie
theory are used to compute the extinction of these spherical grains. Dust
formation results in grains whose composition differs from that of grains
formed at equilibrium. Our kinetic model predicts that solid amorphous SiO2[s]
(silica) is one of the most abundant solid component followed by amorphous
MgSiO4[s] and MgSiO3[s], while SiO2[s] is absent in equilibrium models
because it is a metastable solid. Solid amorphous SiO2[s] possesses a strong
broad absorption feature centered at 8.7mum, while amorphous
Mg2SiO4[s]/MgSiO3[s] absorb at 9.7mum beside other absorption features at
longer wavelength. Those features at lambda < 15mum are detectable in
absorption if grains are small (radius < 0.2mum) in the upper atmosphere as
suggested by our model. We suggest that the detection of a feature at 8.7mum in
deep infrared spectra could provide evidence for non-equilibrium dust formation
that yields grains composed of metastable solids in brown dwarf atmospheres.
This feature will shift towards 10mum and broaden if silicates (e.g. fosterite)
are much more abundant.Comment: A&A Letter, accepte
Emission lines in the atmosphere of the irradiated brown dwarf WD0137â349B
ESL acknowledges the support of STFC studentship. SLC acknowledges support from the University of Leicester College of Science and Engineering. CH highlights the financial support of the European community under the FP7 ERC starting grant 257431. This work was supported by the Science and Technology Facilities Council [ST/M001040/1].We present new optical and near-infrared spectra of WD0137â349; a close white dwarfâbrown dwarf non-interacting binary system with a period of â114âmin. We have confirmed the presence of Hâα emission and discovered He, Na, Mg, Si, K, Ca, Ti and Fe emission lines originating from the brown-dwarf atmosphere. This is the first brown-dwarf atmosphere to have been observed to exhibit metal emission lines as a direct result of intense irradiation. The equivalent widths of many of these lines show a significant difference between the day-side and night-side of the brown dwarf. This is likely an indication that efficient heat redistribution may not be happening on this object, in agreement with models of hot Jupiter atmospheres. The Hâα line strength variation shows a strong phase dependency as does the width. We have simulated the Caâii emission lines using a model that includes the brown-dwarf Roche geometry and limb darkening, and we estimate the mass ratio of the system to be 0.135 ± 0.004. We also apply a gas-phase equilibrium code using a prescribed drift-phoenix model to examine how the chemical composition of the brown-dwarf upper atmosphere would change given an outward temperature increase, and discuss the possibility that this would induce a chromosphere above the brown-dwarf atmosphere.Publisher PDFPeer reviewe
Transmission spectroscopy of the sodium 'D' doublet in WASP-17b with the VLT
The detection of increased sodium absorption during primary transit implies
the presence of an atmosphere around an extrasolar planet, and enables us to
infer the structure of this atmosphere. Sodium has only been detected in the
atmospheres of two planets to date - HD189733b and HD209458b. WASP-17b is the
least dense planet currently known. It has a radius approximately twice that of
Jupiter and orbits an F6-type star. The transit signal is expected to be about
five times larger than that observed in HD209458b. We obtained 24 spectra with
the GIRAFFE spectrograph on the VLT, eight during transit. The integrated flux
in the sodium doublet at wavelengths 5889.95 and 5895.92 {\AA} was measured at
bandwidths 0.75, 1.5, 3.0, 4.0, 5.0, and 6.0 {\AA}. We find a transit depth of
0.55 \pm 0.13 per cent at 1.5 {\AA}. This suggests that, like HD209458b,
WASP-17b has an atmosphere depleted in sodium compared to models for a
cloud-free atmosphere with solar sodium abundance. We observe a sharp cut-off
in sodium absorption between 3.0 and 4.0 {\AA} which may indicate a layer of
clouds high in the atmosphere.Comment: Amended for typographic conventions following publicatio
Numerical simulations of surface convection in a late M-dwarf
Based on detailed 2D and 3D numerical radiation-hydrodynamics (RHD)
simulations of time-dependent compressible convection, we have studied the
dynamics and thermal structure of the convective surface layers of a
prototypical late-type M-dwarf (Teff~2800K log(g)=5.0, solar chemical
composition). The RHD models predict stellar granulation qualitatively similar
to the familiar solar pattern. Quantitatively, the granular cells show a
convective turn-over time scale of ~100s, and a horizontal scale of 80km; the
relative intensity contrast of the granular pattern amounts to 1.1%, and
root-mean-square vertical velocities reach 240m/s at maximum. Deviations from
radiative equilibrium in the higher, formally convectively stable atmospheric
layers are found to be insignificant allowing a reliable modeling of the
atmosphere with 1D standard model atmospheres. A mixing-length parameter of
alpha=2.1 provides the best representation of the average thermal structure of
the RHD model atmosphere while alternative values are found when fitting the
asymptotic entropy encountered in deeper layers of the stellar envelope
alpha=1.5, or when matching the vertical velocity field alpha=3.5. The close
correspondence between RHD and standard model atmospheres implies that
presently existing discrepancies between observed and predicted stellar colors
in the M-dwarf regime cannot be traced back to an inadequate treatment of
convection in the 1D standard models. The RHD models predict a modest extension
of the convectively mixed region beyond the formal Schwarzschild stability
boundary which provides hints for the distribution of dust grains in cooler
(brown dwarf) atmospheres.Comment: 19 pages, 16 figures, accepted for publication in A&
F stars, metallicity, and the ages of red galaxies at z > 1
We explore whether the rest-frame near-UV spectral region, observable in
high-redshift galaxies via optical spectroscopy, contains sufficient
information to allow the degeneracy between age and metallicity to be lifted.
We do this by testing the ability of evolutionary synthesis models to reclaim
the correct metallicity when fitted to the near-UV spectra of F stars of known
(sub-solar and super-solar) metallicity. F stars are of particular interest
because the rest-frame near-UV spectra of the oldest known elliptical galaxies
at z > 1 appear to be dominated by F stars near to the main-sequence turnoff.
We find that, in the case of the F stars, where the HST ultraviolet spectra
have high signal:noise, model-fitting with metallicity allowed to vary as a
free parameter is rather successful at deriving the correct metallicity. As a
result, the estimated turnoff ages of these stars yielded by the model fitting
are well constrained. Encouraged by this we have fitted these same variable-
metallicity models to the deep, optical spectra of the z \simeq 1.5 mJy radio
galaxies 53W091 and 53W069 obtained with the Keck telescope. While the
age-metallicity degeneracy is not so easily lifted for these galaxies, we find
that even when metallicity is allowed as a free parameter, the best estimates
of their ages are still \geq 3 Gyr, with ages younger than 2 Gyr now strongly
excluded. Furthermore, we find that a search of the entire parameter space of
metallicity and star formation history using MOPED (Heavens et al., 2000) leads
to the same conclusion. Our results therefore continue to argue strongly
against an Einstein-de Sitter universe, and favour a lambda-dominated universe
in which star formation in at least these particular elliptical galaxies was
completed somewhere in the redshift range z = 3 - 5.Comment: 10 pages, LaTeX, uses MNRAS style file, incorporates 14 postscript
figures, submitted to MNRAS. Changes include: inclusion of single stellar
atmosphere model fits; more rigorous calculation of confidence regions; some
re-structurin
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