arXiv subjects

D. Homeier

Publications and source records attributed to D. Homeier.

At least 55 records · Page 3Linked to original sources

arXiv2013

The near-infrared spectral energy distribution of β Pictoris b

A gas giant planet has previously been directly seen orbiting at 8-10 AU within the debris disk of the ~12 Myr old star β Pictoris. The β Pictoris system offers the rare opportunity to study the physical and atmospheric properties of an exoplanet placed on a wide orbit and to establish its formation scenario. We obtained J (1.265 μm), H (1.66 μm), and M' (4.78 μm) band angular differential imaging of the system between 2011 and 2012. We detect the planetary companion in our four-epoch observations. We estimate J = 14.0 +- 0.3, H = 13.5 +- 0.2, and M' = 11.0 +- 0.3 mag. Our new astrometry consolidates previous semi-major axis (sma=8-10 AU) and excentricity (e <= 0.15) estimates of the planet. These constraints, and those derived from radial velocities of the star provides independent upper limits on the mass of β Pictoris b of 12 and 15.5 MJup for semi-major axis of 9 and 10 AU. The location of β Pictoris b in color-magnitude diagrams suggests it has spectroscopic properties similar to L0-L4 dwarfs. This enables to derive Log10(L/Lsun) = -3.87 +- 0.08 for the companion. The analysis with 7 PHOENIX-based atmospheric models reveals the planet has a dusty atmosphere with Teff = 1700 +- 100 K and log g = 4.0+- 0.5. "Hot-start" evolutionary models give a new mass of 10+3-2 MJup from Teff and 9+3-2 MJup from luminosity. Predictions of "cold-start" models are inconsistent with independent constraints on the planet mass. "Warm-start" models constrain the mass to M >= 6MJup and the initial entropies to values (Sinit >= 9.3Kb/baryon), intermediate between those considered for cold/hot-start models, but likely closer to those of hot-start models.

astro-ph.EP↗

arXiv2012

CFBDSIR2149-0403: a 4-7 Jupiter-mass free-floating planet in the young moving group AB Doradus ?

Using the CFBDSIR wide field survey for brown dwarfs, we identified CFBDSIRJ214947.2-040308.9, a late T dwarf with atypically red J-Ks colour. We obtained an X-Shooter spectra, with signal detectable from 0.8 to 2.3 micron, which confirmed a T7 spectral type with an enhanced Ks-band flux indicative of a potentially low-gravity, young, object. The comparison of our near infrared spectrum with atmosphere models, for solar metallicity, shows that CFBDSIRJ214947.2-040308.9 is probably a 650-750 K, log g=3.75-4.0 substellar object. Using evolution models, this translates into a planetary mass object, with an age in the 20-200 Myr range. An independent Bayesian analysis from proper motion measurements results in a 87% probability that this free-floating planet is a member of the 50-120 Myr old AB Doradus moving group, which strengthens the spectroscopic youth diagnosis. By combining our atmospheric characterisation with the age and metallicity constraints arising from the probable membership to the AB Doradus moving group, we find that CFBDSIRJ214947.2-040308.9 is probably a 4-7 Jupiter masses free-floating planet with an effective temperature of ~700K and a log g of ~4.0, typical of the late T-type exoplanets that are targeted by direct imaging. We stress that this object could be used as a benchmark for understanding the physics of the similar T-type exoplanets that will be discovered by the upcoming high contrast imagers.

astro-ph.SR↗

arXiv2012

The very low mass multiple system LHS\,1070 -- a testbed for model atmospheres for the lower end of the main sequence

LHS1070 is a nearby multiple system of low mass stars. It is an important source of information for probing the low mass end of the main sequence, down to the hydrogen-burning limit. The primary of the system is a mid-M dwarf and two components are late-M to early L dwarfs, at the star-brown dwarf transition. Hence LHS1070 is a valuable object to understand the onset of dust formation in cool stellar atmospheres.This work aims at determining the fundamental stellar parameters of LHS1070 and to test recent model atmospheres: BT-Dusty,BT-Settl, DRIFT, and MARCS models.Unlike in previous studies, we have performed a chi^2-minimization comparing well calibrated optical and infrared spectra with recent cool star synthetic spectra leading to the determination of the physical stellar parameters Teff, radius, and log g for each of the three components of LHS1070.

astro-ph.SR↗

arXiv2012

Discovery of the benchmark metal poor T8 dwarf BD+01 2920B

We have searched the WISE first data release for widely separated (<10,000AU) late T dwarf companions to Hipparcos and Gliese stars. We have discovered a new binary system containing a K-band suppressed T8p dwarf WISEP J1423+0116 and the mildly metal poor ([Fe/H]=-0.38+-0.06) primary BD+01 2920 (Hip 70319), a G1 dwarf at a distance of 17.2pc. This new benchmark has Teff=680+-55K and a mass of 20-50 Mjup. Its spectral properties are well modelled except for known discrepancies in the Y and K bands. Based on the well determined metallicity of its companion, the properties of BD+01 2920B imply that the currently known T dwarfs are dominated by young low-mass objects. We also present an accurate proper motion for the T8.5 dwarf WISEP J075003.84+272544.8.

astro-ph.SR↗

arXiv2012

The Properties of the 500 K Dwarf UGPS J072227.51-054031.2, and a Study of the Far-Red Flux of Cold Brown Dwarfs

We present i and z photometry for 25 T dwarfs and one L dwarf. Combined with published photometry, the data show that the i - z, z - Y and z - J colors of T dwarfs are very red, and continue to increase through to the late-type T dwarfs, with a hint of a saturation for the latest types with T_eff ~ 600 K. We present new 0.7-1.0 um and 2.8-4.2 um spectra for the very late-type T dwarf UGPS J072227.51-054031.2, as well as improved astrometry for this dwarf. Examination of the spectral energy distribution using the new and published data, with Saumon & Marley models, shows that the dwarf has T_eff = 505 +/- 10 K, a mass of 3-11 M_Jupiter and an age between 60 Myr and 1 Gyr. This young age is consistent with the thin disk kinematics of the dwarf. The mass range overlaps with that usually considered to be planetary, despite this being an unbound object discovered in the field near the Sun. This apparently young rapid rotator is also undergoing vigorous atmospheric mixing, as determined by the IRAC and WISE-2 4.5 um photometry and the Saumon & Marley models. The optical spectrum for this 500 K object shows clearly detected lines of the neutral alkalis Cs and Rb, which are emitted from deep atmospheric layers with temperatures of 900-1200 K.

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arXiv2011

The properties of the T8.5p dwarf Ross 458C

We present near-infrared photometry and spectroscopy, and warm-Spitzer IRAC photometry of the young very cool T dwarf Ross 458C, which we have typed as T8.5p. By applying the fiducial age constraints (< 1Gyr) imposed by the properties of the active M dwarf Ross 458A, we have used these data to determine that Ross 458C has Teff = 695 \pm 60K, log g = 4.0 - 4.7 and an inferred mass of 5-20 MJup. We have compared fits of the near-infrared spectrum and IRAC photometry to the BT Settl and Saumon & Marley model grids, and have found that both sets provide best fits that are consistent with our derived properties, whilst the former provide a marginally closer match to the data for all scenarios explored here. The main difference between the model grids arises in the 4.5μm region, where the BT Settl models are able to better predict the flux through the IRAC filter, suggesting that non-equilibrium effects on the CO-CO2 ratio are important for shaping the mid-infrared spectra of very cool T dwarfs. We have also revisited the issue of dust opacity in the spectra of Ross 458C that was raised by Burgasser et al (2010). We have found that the BT Settl models which also incorporate a condensate cloud model, provide a better match to the near-infrared spectrum of this target than the Saumon & Marley model with fsed = 2, and we briefly discuss the influence of condensate clouds on T dwarf spectra.

astro-ph.SR↗

arXiv2011

The discovery of the T8.5 dwarf UGPS J0521+3640

We have carried out a search for late-type T dwarfs in the UKIDSS Galactic Plane Survey 6th Data Release. The search yielded two persuasive candidates, both of which have been confirmed as T dwarfs. The brightest, UGPS J0521+3640 has been assigned the spectral type T8.5 and appears to lie at a distance of 7-9 pc. The fainter of the two, UGPS J0652+0324, is classified as a T5.5 dwarf, and lies at an estimated distance of 28-37 pc. Warm-Spitzer observations in IRAC channels 1 and 2, taken as part of the GLIMPSE360 Legacy Survey, are available for UGPS J0521+3640 and we used these data with the near-infrared spectroscopy to estimate its properties. We find best fitting solar metallicity BT-Settl models for Teff = 600K and 650K and log g = 4.5 and 5.0. These parameters suggest a mass of between 14 and 32 MJup for an age between 1 and 5 Gyr. The proximity of this very cool T dwarf, and its location in the Galactic plane makes it an ideal candidate for high resolution adaptive optics imaging to search for cool companions.

astro-ph.SR↗

arXiv2010

Aspects of Multi-Dimensional Modelling of Substellar Atmospheres

Theoretical arguments and observations suggest that the atmospheres of Brown Dwarfs and planets are very dynamic on chemical and on physical time scales. The modelling of such substellar atmospheres has, hence, been much more demanding than initially anticipated. This Splinter (http://star-www.st-and.ac.uk/~ch80/CS16/MultiDSplinter_CS16.html) has combined new developments in atmosphere modelling, with novel observational techniques, and new challenges arising from planetary and space weather observations.

astro-ph.SR↗

arXiv2010

Model Atmospheres From Very Low Mass Stars to Brown Dwarfs

Since the discovery of brown dwarfs in 1994, and the discovery of dust cloud formation in the latest Very Low Mass Stars (VLMs) and Brown Dwarfs (BDs) in 1996, the most important challenge in modeling their atmospheres as become the understanding of cloud formation and advective mixing. For this purpose, we have developed radiation hydrodynamic 2D model atmosphere simulations to study the formation of forsterite dust in presence of advection, condensation, and sedimentation across the M-L-T VLMs to BDs sequence (Teff = 2800 K to 900 K, Freytag et al. 2010). We discovered the formation of gravity waves as a driving mechanism for the formation of clouds in these atmospheres, and derived a rule for the velocity field versus atmospheric depth and Teff , which is relatively insensitive to gravity. This rule has been used in the construction of the new model atmosphere grid, BT-Settl, to determine the microturbulence velocity, the diffusion coefficient, and the advective mixing of molecules as a function of depth. This new model grid of atmospheres and synthetic spectra has beencomputedfor100,000K>Teff >400K,5.5>logg>-0.5,and[M/H]=+0.5to -1.5, and the reference solar abundances of Asplund et al. (2009). We found that the new solar abundances allow an improved (close to perfect) reproduction of the photo- metric and spectroscopic VLMs properties, and, for the first time, a smooth transition between stellar and substellar regimes -- unlike the transition between the NextGen models from Hauschildt et al. 1999a,b, and the AMES-Dusty models from Allard et al. 2001). In the BDs regime, the BT-Settl models propose an improved explanation for the M-L-T spectral transition. In this paper, we therefore present the new BT-Settl model atmosphere grid, which explains the entire transition from the stellar to planetary mass regimes.

astro-ph.SR↗

arXiv2010

Near-infrared integral-field spectra of the planet/brown dwarf companion AB Pic b

Chauvin et al. 2005 imaged a co-moving companion at ~260 AU from the young star AB Pic A. Evolutionary models predictions based on J H K photometry of AB Pic b suggested a mass of ~13 - 14 MJup, placing the object at the deuterium-burning boundary. We used the adaptive-optics-fed integral field spectrograph SINFONI to obtain high quality medium-resolution spectra of AB Pic b (R = 1500-2000) over the 1.1 - 2.5 microns range. Our analysis relies on the comparison of our spectra to young standard templates and to the latest libraries of synthetic spectra developed by the Lyon's Group. AB Pic b is confirmed to be a young early-L dwarf companion. We derive a spectral type L0-L1 and find several features indicative of intermediate gravity atmosphere. A comparison to synthetic spectra yields Teff = 2000+100-300 K and log(g) = 4 +- 0.5 dex. The determination of the derived atmospheric parameters of AB Pic b is limited by a non-perfect match of current atmosphere spectra with our near-infrared observations of AB Pic b. The current treatment of dust settling and missing molecular opacity lines in the atmosphere models could be responsible. By combining the observed photometry, the surface fluxes from atmosphere models and the known distance of the system, we derive new mass, luminosity and radius estimates of AB Pic b. They confirm independently the evolutionary model predictions. We finally review the current methods used to characterize planetary mass companions and discuss them in the perspective of future planet deep imaging surveys.

astro-ph.EP↗

arXiv2009

Monte Carlo approach to turbulence

The behavior of the one-dimensional random-force-driven Burgers equation is investigated in the path integral formalism on a discrete space-time lattice. We show that by means of Monte Carlo methods one may evaluate observables, such as structure functions, as ensemble averages over different field realizations. The regularization of shock solutions to the zero-viscosity limit (Hopf-equation) eventually leads to constraints on lattice parameters required for the stability of the simulations. Insight into the formation of localized structures (shocks) and their dynamics is obtained.

hep-lat↗

arXiv2009

High-resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia Progenitor Survey. III. DA white dwarfs

The ESO Supernova Ia Progenitor Survey (SPY) took high-resolution spectra of more than 1000 white dwarfs and pre-white dwarfs. About two thirds of the stars observed are hydrogen-dominated DA white dwarfs. Here we present a catalog and detailed spectroscopic analysis of the DA stars in the SPY. Atmospheric parameters effective temperature and surface gravity are determined for normal DAs. Double-degenerate binaries, DAs with magnetic fields or dM companions, are classified and discussed. The spectra are compared with theoretical model atmospheres using a chi^2 fitting technique. Our final sample contains 615 DAs, which show only hydrogen features in their spectra, although some are double-degenerate binaries. 187 are new detections or classifications. We also find 10 magnetic DAs (4 new) and 46 DA+dM pairs (10 new).

astro-ph.SR↗

arXiv2009

Spectral synthesis of circumstellar disks - application to white dwarf debris disks

Gas and dust disks are common objects in the universe and can be found around various objects, e.g. young stars, cataclysmic variables, active galactic nuclei, or white dwarfs. The light that we receive from disks provides us with clues about their composition, temperature, and density. In order to better understand the physical and chemical dynamics of these disks, self-consistent radiative transfer simulations are inevitable. Therefore, we have developed a 1+1D radiative transfer code as an extension to the well-established model atmosphere code \verb!PHOENIX!. We will show the potential of the application of our code to model the spectra of white dwarf debris disks.

astro-ph.SR↗

arXiv2009

Radiative transfer in circumstellar disks - I. 1D models for GQ Lupi

We present a new code for the calculation of the 1D structure and synthetic spectra of accretion disks. The code is an extension of the general purpose stellar atmosphere code PHOENIX and is therefore capable of including extensive lists of atomic and molecular lines as well as dust in the calculations. We assume that the average viscosity can be represented by a critical Reynolds number in a geometrically thin disk and solve the structure and radiative transfer equations for a number of disk rings in the vertical direction. The combination of these rings provides the total disk structure and spectrum. Since the warm inner regions of protoplanetary disks show a rich molecular spectrum, they are well suited for a spectral analysis with our models. In this paper we test our code by comparing our models with high-resolution VLT CRIRES spectra of the T Tauri star GQ Lup.

astro-ph.SR↗

arXiv2009

On the possibility of detecting extrasolar planets' atmospheres with the Rossiter-McLaughlin-effect

The detection of extrasolar planets' atmospheres requires very demanding observations. For planets that can not be spatially separated from their host stars, i.e. the vast majority of planets, the transiting planets are the only ones allowing to probe their atmospheres. This is possible from transmission spectroscopy or from measurements taken during secondary eclipse. An alternative is the measurement of the Rossiter-McLaughlin-effect, which is sensitive to the size of the planetary radius. Since the radius is wavelength-dependent due to contributions of strong planetary absorption lines, this opens a path to probe planetary atmospheres also with ground-based high-resolution spectroscopy. The major goal of our numerical simulations is to provide a reliable estimate of the amplitude of the wavelength-dependent RM-effect. Our numerical simulations provide phase resolved synthetic spectra modeling the partly eclipsed stellar surface during the transit in detail. Using these spectra we can obtain RM-curves for different wavelength regions and for a wavelength-dependent planetary radius. Curves from regions with high and low contributions of absorption lines within the planetary atmosphere can be compared. From these differential effects observable quantities are derived. We applied our simulations to HD209458. Our numerical simulations show that a detailed treatment of the limb darkening for the synthetic spectra is important for a precise analysis. Compared to a parameterized limb darkening law, systematic errors of 6 m/sec occur. The wavelength dependency of the planetary atmospheres over the NaD-doublet produce a differential effect in the RM-curve of 1.5 m/sec for a star with a rotation velocity of 4.5 km/sec which increases to 4 m/sec for twice the rotation velocity.

astro-ph.EP↗

arXiv2009

The discovery of an M4+T8.5 binary system

We report the discovery of a T8.5 dwarf, which is a companion to the M4 dwarf Wolf 940. At a distance of 12.50 (+0.75,-0.67) pc, the angular separation of 32arcsec corresponds to a projected separation of 400 AU. The M4 primary displays no Halpha emission, and we apply the age-activity relations of West et al. to place a lower limit on the age of the system of 3.5 Gyr. Weak Halpha absorption suggests some residual activity and we estimate an upper age limit of 6 Gyr. We apply the relations of Bonfils et al for V-Ks and M_Ks to determine the metallicity, [Fe/H] = -0.06 +/- 0.20 for Wolf~940A, and by extension the T8.5 secondary, Wolf 940B. We have obtained JHK NIRI spectroscopy and JHKL' photometry of Wolf 940B, and use these data, in combination with theoretical extensions, to determine its bolometric flux, Fbol = 1.75 +/- 0.18 E-16 Wm^-2 and thus its luminosity log(L*/Lsun) = -6.07 +/- 0.04. Using the age constraints for the system, and evolutionary structural models of Baraffe et al. we determine Teff = 570 +/- 25K and log g = 4.75-5.00 for Wolf940B, based on its bolometric luminosity. This represents the first determination of these properties for a T8+ dwarf that does not rely on the fitting of T-dwarf spectral models. This object represents the first system containing a T8+ dwarf for which fiducial constraints on its properties are available, and we compare its spectra with those of the latest very cool BT-Settl models. This clearly demonstrates that the use of the (W_J,K/J) spectral ratios (used previously to constrain Teff and log g) would have over-estimated Teff by ~100K.

astro-ph.SR↗

arXiv2008

Monte Carlo simulations of the randomly forced Burgers equation

The behaviour of the one--dimensional random--forced Burgers equation is investigated in the path integral formalism, using a discrete space--time lattice. We show that by means of Monte Carlo methods one may evaluate observables, such as structure functions, as ensemble averages over different field realizations. The regularization of shock solutions to the zero--viscosity limit (Hopf-eq.) eventually leads to constraints on lattice parameters, required for the stability of the simulations. Insight into the formation of localized structures (shocks) and their dynamics is obtained.

hep-lat↗

arXiv2008

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 < Δm < 0.875 (0.1 < Δm 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.)

astro-ph↗