Cold brown dwarfs are excellent analogs of widely separated, gas giant exoplanets, and provide insight into the potential atmospheric chemistry and physics we may encounter in objects to be ...discovered by future direct imaging surveys. We present a low-resolution, R ∼ 300, M-band spectroscopic sequence of seven brown dwarfs with effective temperatures between 750 and 250 K along with Jupiter. These spectra reveal disequilibrium abundances of carbon monoxide (CO) produced by atmospheric quenching. We use the eddy diffusion coefficient (Kzz) to estimate the strength of vertical mixing in each object. The Kzz values of cooler gaseous objects are close to their theoretical maximum, and warmer objects show weaker mixing, likely due to less efficient convective mixing in primarily radiative layers. The CO-derived Kzz values imply that disequilibrium phosphine (PH3) should be easily observable in all of the brown dwarfs, but none as yet show any evidence for PH3 absorption. We find that ammonia is relatively insensitive to atmospheric quenching at these effective temperatures. We are able to improve the fit to WISE 0855's M-band spectrum by including both CO and water clouds in the atmospheric model.
Abstract
Evidence of disequilibrium chemistry due to vertical mixing in the atmospheres of many T- and Y-dwarfs has been inferred due to enhanced mixing ratios of CO and reduced NH
3
. Atmospheric ...models of planets and brown dwarfs typically parameterize this vertical mixing phenomenon with the vertical eddy diffusion coefficient,
K
zz
. While
K
zz
can perhaps be approximated in the convective regions in the atmosphere with mixing length theory, in radiative regions, the strength of vertical mixing is uncertain by many orders of magnitude. With a new grid of self-consistent 1D model atmospheres from
T
eff
of 400–1000 K, computed with a new radiative-convective equilibrium python code
PICASO 3.0
, we aim to assess how molecular abundances and corresponding spectra can be used as a probe of depth-dependent
K
zz
. At a given surface gravity, we find nonmonotonic behavior in the CO abundance as a function of
T
eff
, as chemical abundances are sometimes quenched in either of two potential atmospheric convective zones, or quenched in either of two possible radiative zones. The temperature structure and chemical quenching behavior also change with gravity. We compare our models with available near-infrared and
M
-band spectroscopy of several T- and Y-dwarfs and assess their atmospheric vertical mixing profiles. We also compare to color–magnitude diagrams and make predictions for James Webb Space Telescope spectra. This work yields new constraints, and points the way to significant future gains, in determining
K
zz
, a fundamental atmospheric parameter in substellar atmospheres, with significant implications for chemistry and cloud modeling.
ABSTRACT
The James Webb Space Telescope (JWST), currently scheduled to launch in 2021, will dramatically advance our understanding of exoplanetary systems with its ability to directly image and ...characterize planetary-mass companions at wide separations through coronagraphy. Using state-of-the-art simulations of JWST performance, in combination with the latest evolutionary models, we present the most sophisticated simulated mass sensitivity limits of JWST coronagraphy to date. In particular, we focus our efforts towards observations of members within the nearby young moving groups β Pictoris and TW Hya. These limits indicate that whilst JWST will provide little improvement towards imaging exoplanets at short separations, at wide separations the increase in sensitivity is dramatic. We predict JWST will be capable of imaging sub-Jupiter mass objects beyond ∼30 au, sub-Saturn mass objects beyond ∼50 au, and that beyond ∼100 au, JWST will be capable of directly imaging companions as small as 0.1 MJ − at least an order of magnitude improvement over the leading ground-based instruments. Probing this unexplored parameter space will be of immediate value to modelling efforts focused on planetary formation and population synthesis. JWST will also serve as an excellent complement to ground-based observatories through its unique ability to characterize previously detected companions across the near- to mid-infrared for the first time.
Abstract
Understanding the physical processes sculpting the appearance of young gas-giant planets is complicated by degeneracies confounding effective temperature, surface gravity, cloudiness, and ...chemistry. To enable more detailed studies, spectroscopic observations covering a wide range of wavelengths are required. Here we present the first L-band spectroscopic observations of HR 8799 d and e and the first low-resolution wide-bandwidth L-band spectroscopic measurements of HR 8799 c. These measurements were facilitated by an upgraded LMIRCam/ALES instrument at the Large Binocular Telescope, together with a new apodizing phase plate coronagraph. Our data are generally consistent with previous photometric observations covering similar wavelengths, yet there exists some tension with narrowband photometry for HR 8799 c. With the addition of our spectra, each of the three innermost observed planets in the HR 8799 system has had its spectral energy distribution measured with integral field spectroscopy covering ∼0.9–4.1
μ
m. We combine these spectra with measurements from the literature and fit synthetic model atmospheres. We demonstrate that the bolometric luminosity of the planets is not sensitive to the choice of model atmosphere used to interpolate between measurements and extrapolate beyond them. Combining luminosity with age and mass constraints, we show that the predictions of evolutionary models are narrowly peaked for effective temperature, surface gravity, and planetary radius. By holding these parameters at their predicted values, we show that more flexible cloud models can provide good fits to the data while being consistent with the expectations of evolutionary models.
We present the Vortex Image Processing (VIP) library, a python package dedicated to astronomical high-contrast imaging. Our package relies on the extensive python stack of scientific libraries and ...aims to provide a flexible framework for high-contrast data and image processing. In this paper, we describe the capabilities of VIP related to processing image sequences acquired using the angular differential imaging (ADI) observing technique. VIP implements functionalities for building high-contrast data processing pipelines, encompassing pre- and post-processing algorithms, potential source position and flux estimation, and sensitivity curve generation. Among the reference point-spread function subtraction techniques for ADI post-processing, VIP includes several flavors of principal component analysis (PCA) based algorithms, such as annular PCA and incremental PCA algorithms capable of processing big datacubes (of several gigabytes) on a computer with limited memory. Also, we present a novel ADI algorithm based on non-negative matrix factorization, which comes from the same family of low-rank matrix approximations as PCA and provides fairly similar results. We showcase the ADI capabilities of the VIP library using a deep sequence on HR 8799 taken with the LBTI/LMIRCam and its recently commissioned L-band vortex coronagraph. Using VIP, we investigated the presence of additional companions around HR 8799 and did not find any significant additional point source beyond the four known planets. VIP is available at http://github.com/vortex-exoplanet/VIP and is accompanied with Jupyter notebook tutorials illustrating the main functionalities of the library.
An L Band Spectrum of the Coldest Brown Dwarf Morley, Caroline V.; Skemer, Andrew J.; Allers, Katelyn N. ...
The Astrophysical journal,
05/2018, Letnik:
858, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The coldest brown dwarf, WISE 0855, is the closest known planetary-mass, free-floating object and has a temperature nearly as cold as the solar system gas giants. Like Jupiter, it is predicted to ...have an atmosphere rich in methane, water, and ammonia, with clouds of volatile ices. WISE 0855 is faint at near-infrared wavelengths and emits almost all its energy in the mid-infrared. Skemer et al. presented a spectrum of WISE 0855 from 4.5-5.1 m (M band), revealing water vapor features. Here, we present a spectrum of WISE 0855 in the L band, from 3.4-4.14 m. We present a set of atmosphere models that include a range of compositions (metallicities and C/O ratios) and water ice clouds. Methane absorption is clearly present in the spectrum. The mid-infrared color can be better matched with a methane abundance that is depleted relative to solar abundance. We find that there is evidence for water ice clouds in the M band spectrum, and we find a lack of phosphine spectral features in both the L and M band spectra. We suggest that a deep continuum opacity source may be obscuring the near-infrared flux, possibly a deep phosphorous-bearing cloud, ammonium dihyrogen phosphate. Observations of WISE 0855 provide critical constraints for cold planetary atmospheres, bridging the temperature range between the long-studied solar system planets and accessible exoplanets. The James Webb Space Telescope will soon revolutionize our understanding of cold brown dwarfs with high-precision spectroscopy across the infrared, allowing us to study their compositions and cloud properties, and to infer their atmospheric dynamics and formation processes.
Abstract
The photometric and spectral variability of brown dwarfs probes heterogeneous temperature and cloud distributions and traces the atmospheric circulation patterns. We present a new 42 hr ...Hubble Space Telescope (HST) Wide Field Camera 3 G141 spectral time series of VHS 1256-1257 b, a late L-type planetary-mass companion that has been shown to have one of the highest variability amplitudes among substellar objects. The light curve is rapidly evolving and best fit by a combination of three sine waves with different periods and a linear trend. The amplitudes of the sine waves and the linear slope vary with the wavelength, and the corresponding spectral variability patterns match the predictions by models invoking either heterogeneous clouds or thermal profile anomalies. Combining these observations with previous HST monitoring data, we find that the peak-to-valley flux difference is 33% ± 2% with an even higher amplitude reaching 38% in the
J
band, the highest amplitude ever observed in a substellar object. The observed light curve can be explained by maps that are composed of zonal waves, spots, or a mixture of the two. Distinguishing the origin of rapid light curve evolution requires additional long-term monitoring. Our findings underscore the essential role of atmospheric dynamics in shaping brown-dwarf atmospheres and highlight VHS 1256-1257 b as one of the most favorable targets for studying the atmospheres, clouds, and atmospheric circulation of planets and brown dwarfs.
Abstract
Gravitational microlensing has the potential to provide direct gravitational masses of single, free-floating brown dwarfs, independent of evolutionary and atmospheric models. The proper ...motions and parallaxes of nearby brown dwarfs can be used to predict close future alignments with distant background stars that cause a microlensing event. Targeted astrometric follow up of the predicted microlensing events permits the brown dwarf’s mass to be measured. Predicted microlensing events are typically found via searching for a peak threshold signal using an estimate of the lens mass. We develop a novel method that finds predicted events that instead will lead to a target lens-mass precision. The main advantage of our method is that it does not require a lens-mass estimate. We use this method to search for predicted astrometric microlensing events occurring between 2014 and 2032 using a catalog of 1225 low-mass star and brown-dwarf lenses in the Solar Neighborhood of spectral type M6 or later and a background source catalog from DECaLS Data Release 9. The background source catalog extends to
g
= 23.95, providing a more dense catalog compared to Gaia. Our search did not reveal any upcoming microlensing events. We estimate the rate of astrometric microlensing event for brown dwarfs in the Legacy Survey and find it to be low ∼10
−5
yr
−1
. We recommend carrying out targeted searches for brown dwarfs in front of the Galactic Bulge and Plane to find astrometric microlensing events that will allow the masses of single, free-floating brown dwarfs to be measured.
THE FIRST SPECTRUM OF THE COLDEST BROWN DWARF Skemer, Andrew J.; Morley, Caroline V.; Allers, Katelyn N. ...
Astrophysical journal. Letters,
08/2016, Letnik:
826, Številka:
2
Journal Article
Recenzirano
Odprti dostop
ABSTRACT The recently discovered brown dwarf WISE 0855 presents the first opportunity to directly study an object outside the solar system that is nearly as cold as our own gas giant planets. ...However, the traditional methodology for characterizing brown dwarfs-near-infrared spectroscopy-is not currently feasible, as WISE 0855 is too cold and faint. To characterize this frozen extrasolar world we obtained a 4.5-5.2 m spectrum, the same bandpass long used to study Jupiter's deep thermal emission. Our spectrum reveals the presence of atmospheric water vapor and clouds, with an absorption profile that is strikingly similar to Jupiter's. The spectrum quality is high enough to allow for the investigation of dynamical and chemical processes that have long been studied in Jupiter's atmosphere, but now on an extrasolar world.
We present detections of methane in R ∼ 1300, L-band spectra of VHS 1256 b and PSO 318.5, two low-gravity, red, late L dwarfs that share the same colors as the HR 8799 planets. These spectra reveal ...shallow methane features, which indicate VHS 1256 b and PSO 318.5 have photospheres that are out of chemical equilibrium. Directly imaged exoplanets usually have redder near-infrared colors than the field-age population of brown dwarfs on a color-magnitude diagram. These objects along the L-to-T transition show reduced methane absorption and evidence of photospheric clouds. Compared to the H- and K-bands, L-band (3 m-4 m) spectroscopy provides stronger constraints on the methane abundances of brown dwarfs and directly imaged exoplanets that have similar effective temperatures to L-to-T transition objects. When combined with near-infrared spectra, the L-band extends our conventional wavelength coverage, increasing our understanding of atmospheric cloud structure. Our model comparisons show that relatively strong vertical mixing and photospheric clouds can explain the molecular absorption features and continua of VHS 1256 b and PSO 318.5. We also discuss the implications of this work for future exoplanet-focused instruments and observations with the James Webb Space Telescope.
PDF
