Abstract
Upcoming James Webb Space Telescope observations will allow us to study exoplanet and brown dwarf atmospheres in great detail. The physical interpretation of these upcoming high ...signal-to-noise observations requires precise atmospheric models of exoplanets and brown dwarfs. While several 1D and 3D atmospheric models have been developed in the past three decades, these models have often relied on simplified assumptions like chemical equilibrium and are also often not open-source, which limits their usage and development by the wider community. We present a Python-based 1Dl atmospheric radiative-convective equilibrium (RCE) model. This model has heritage from the Fortran-based code, which has been widely used to model the atmospheres of solar system objects, brown dwarfs, and exoplanets. In short, the basic capability of the original model is to compute the atmospheric state of the object under RCE given its effective or internal temperature, gravity, and host-star properties (if relevant). In the new model, which has been included within the well-utilized code-base
PICASO
, we have added these original features as well as the new capability of self-consistently treating disequilibrium chemistry. This code is widely applicable to hydrogen-dominated atmospheres (e.g., brown dwarfs and giant planets).
Abstract
We reanalyze the near-infrared spectra of the young extrasolar giant planet 51 Eridani b, which was originally presented in Macintosh et al. and Rajan et al. using modern atmospheric models, ...including a self-consistent treatment of disequilibrium chemistry due to turbulent vertical mixing. In addition, we investigate the possibility that significant opacity from micrometeors or other impactors in the planet’s atmosphere may be responsible for shaping the observed spectral energy distribution (SED). We find that disequilibrium chemistry is useful for describing the mid-infrared colors of the planet’s spectra, especially in regard to photometric data at the
M
band around 4.5
μ
m, which is the result of superequilibrium abundances of carbon monoxide, while the micrometeors are unlikely to play a pivotal role in shaping the SED. The best-fitting, micrometeoroid dust–free, disequilibrium chemistry, patchy cloud model has the following parameters: effective temperature
T
eff
= 681 K with clouds (or without clouds, i.e., the grid temperature
T
grid
= 900 K), surface gravity
g
= 1000 m s
−2
, sedimentation efficiency
f
sed
= 10, vertical eddy diffusion coefficient
K
zz
= 10
3
cm
2
s
−1
, cloud hole fraction
f
hole
= 0.2, and planet radius
R
planet
= 1.0
R
Jup
.
Abstract Disequilibrium chemistry due to vertical mixing in the atmospheres of many brown dwarfs and giant exoplanets is well established. Atmosphere models for these objects typically parameterize ...mixing with the highly uncertain K zz diffusion parameter. The role of mixing in altering the abundances of C-N-O-bearing molecules has mostly been explored for atmospheres with a solar composition. However, atmospheric metallicity and the C/O ratio also impact atmospheric chemistry. Therefore, we present the Sonora Elf Owl grid of self-consistent cloud-free 1D radiative-convective equilibrium model atmospheres for JWST observations, which includes a variation in K zz across several orders of magnitude and also encompasses subsolar to supersolar metallicities and C/O ratios. We find that the impact of K zz on the T ( P ) profile and spectra is a strong function of both T eff and metallicity. For metal-poor objects, K zz has large impacts on the atmosphere at significantly higher T eff than in metal-rich atmospheres, where the impact of K zz is seen to occur at lower T eff . We identify significant spectral degeneracies between varying K zz and metallicity in multiple wavelength windows, in particular, at 3–5 μ m. We use the Sonora Elf Owl atmospheric grid to fit the observed spectra of a sample of nine early to late T-type objects from T eff = 550–1150 K. We find evidence for very inefficient vertical mixing in these objects, with inferred K zz values lying in the range between ∼10 1 and 10 4 cm 2 s −1 . Using self-consistent models, we find that this slow vertical mixing is due to the observations, which probe mixing in the deep detached radiative zone in these atmospheres.
Abstract
We present the first JWST spectral energy distribution of a Y dwarf. This spectral energy distribution of the Y0 dwarf WISE J035934.06−540154.6 consists of low-resolution (
λ
/Δ
λ
∼100) ...spectroscopy from 1–12
μ
m and three photometric points at 15, 18, and 21
μ
m. The spectrum exhibits numerous fundamental, overtone, and combination rotational–vibrational bands of H
2
O, CH
4
, CO, CO
2
, and NH
3
, including the previously unidentified
ν
3
band of NH
3
at 3
μ
m. Using a Rayleigh–Jeans tail to account for the flux emerging at wavelengths greater than 21
μ
m, we measure a bolometric luminosity of 1.523 ± 0.090 × 10
20
W. We determine a semiempirical effective temperature estimate of
467
−
18
+
16
K using the bolometric luminosity and evolutionary models to estimate a radius. Finally, we compare the spectrum and photometry to a grid of atmospheric models and find reasonably good agreement with a model having
T
eff
= 450 K, log
g
= 3.25 cm s
−2
, and M/H = −0.3. However, the low surface gravity implies an extremely low mass of 1
M
Jup
and a very young age of 20 Myr, the latter of which is inconsistent with simulations of volume-limited samples of cool brown dwarfs.
Abstract
We observed HD 19467 B with JWST’s NIRCam in six filters spanning 2.5–4.6
μ
m with the long-wavelength bar coronagraph. The brown dwarf HD 19467 B was initially identified through a ...long-period trend in the radial velocity of the G3V star HD 19467. HD 19467 B was subsequently detected via coronagraphic imaging and spectroscopy, and characterized as a late-T type brown dwarf with an approximate temperature ∼1000 K. We observed HD 19467 B as a part of the NIRCam GTO science program, demonstrating the first use of the NIRCam Long Wavelength Bar coronagraphic mask. The object was detected in all six filters (contrast levels of 2 × 10
−4
to 2 × 10
−5
) at a separation of 1.″6 using angular differential imaging and synthetic reference differential imaging. Due to a guide star failure during the acquisition of a preselected reference star, no reference star data were available for post-processing. However, reference differential imaging was successfully applied using synthetic point-spread functions developed from contemporaneous maps of the telescope’s optical configuration. Additional radial velocity data (from Keck/HIRES) are used to constrain the orbit of HD 19467 B. Photometric data from TESS are used to constrain the properties of the host star, particularly its age. NIRCam photometry, spectra, and photometry from the literature, and improved stellar parameters are used in conjunction with recent spectral and evolutionary substellar models to derive the physical properties of HD 19467 B. Using an age of 9.4 ± 0.9 Gyr inferred from spectroscopy, Gaia astrometry, and TESS asteroseismology, we obtain a model-derived mass of 62 ± 1
M
J
, which is consistent within 2
σ
with the dynamically derived mass of
81
−
12
+
14
M
J
.
Abstract
Wolf 359 (CN Leo, GJ 406, Gaia DR3 3864972938605115520) is a low-mass star in the fifth-closest neighboring system (2.41 pc). Because of its relative youth and proximity, Wolf 359 offers a ...unique opportunity to study substellar companions around M stars using infrared high-contrast imaging and radial velocity monitoring. We present the results of
Ms
-band (4.67
μ
m) vector vortex coronagraphic imaging using Keck-NIRC2 and add 12 Keck-HIRES and 68 MAROON-X velocities to the radial velocity baseline. Our analysis incorporates these data alongside literature radial velocities from CARMENES, the High Accuracy Radial velocity Planet Searcher, and Keck-HIRES to rule out the existence of a close (
a
< 10 au) stellar or brown dwarf companion and the majority of large gas giant companions. Our survey does not refute or confirm the long-period radial velocity candidate, Wolf 359 b (
P
∼ 2900 days), but rules out the candidate's existence as a large gas giant (>4
M
Jup
) assuming an age of younger than 1 Gyr. We discuss the performance of our high-contrast imaging survey to aid future observers using Keck-NIRC2 in conjunction with the vortex coronagraph in the
Ms
band and conclude by exploring the direct imaging capabilities with JWST to observe Jupiter- and Neptune-mass planets around Wolf 359.
Abstract Recent JWST eclipse spectra of the high-density hot Saturn HD 149026b between 2.35 and 5.08 μ m have allowed for in-depth study of its atmosphere. To understand its atmospheric properties, ...we have created a grid of 1D radiative-convective–thermochemical–equilibrium atmosphere models and spectra with PICASO 3.0 . In agreement with previous work, we find that the presence of gaseous TiO creates a thermal inversion, which is inconsistent with the data. The presence of gaseous VO, however, which condenses at temperatures 200 K cooler, does not cause such inversions but alters the temperature–pressure profile of the atmosphere. We estimate an atmospheric metallicity of 14 − 8 + 12 × solar without VO and 20 − 8 + 11 × solar with VO, a factor of ∼10 times smaller than previous work from Bean et al., who relied on atmosphere retrievals. We attribute this significant difference in metallicity to a larger temperature gradient at low pressures in radiative equilibrium models. Such models with lower metallicities readily fit the strong CO 2 feature at 4.3 μ m. Our lower estimated metallicity makes HD 149026b more consistent with the mass–metallicity relationship for other giant planets. We find a C/O ratio of 0.67 − 0.27 + 0.06 with and without VO. The best-fit heat redistribution factor without VO is 1.17, a very high value suggesting very little dayside energy transport and no energy transport to the nightside. The heat redistribution factor shrinks to a more plausible value of 0.91 − 0.05 + 0.05 , with VO, which we regard as circumstantial evidence for the molecule in the atmosphere of HD 149026b.
Abstract
Future space-based direct imaging missions will perform low-resolution (
R
< 100) optical (0.3–1
μ
m) spectroscopy of planets, thus enabling reflected spectroscopy of cool giants. Reflected ...light spectroscopy is encoded with rich information about the scattering and absorbing properties of planet atmospheres. Given the diversity of clouds and hazes expected in exoplanets, it is imperative that we solidify the methodology to accurately and precisely retrieve these scattering and absorbing properties that are agnostic to cloud species. In particular, we focus on determining how different cloud parameterizations affect resultant inferences of both cloud and atmospheric composition. We simulate mock observations of the reflected spectra from three top-priority direct imaging cool giant targets with different effective temperatures, ranging from 135 to 533 K. We perform retrievals of cloud structure and molecular abundances on these three planets using four different parameterizations, each with an increasing level of cloud complexity. We find that the retrieved atmospheric and scattering properties depend strongly on the choice of cloud parameterization. For example, parameterizations that are too simplistic tend to overestimate the abundances. Overall, we are unable to retrieve precise/accurate gravity beyond ±50%. Lastly, we find that even reflected light spectroscopy with a low signal-to-noise ratio of 5 and low
R
= 40 gives cursory zeroth-order insights into the position of the cloud deck relative to the molecular and Rayleigh optical depth level.
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.
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