arXiv subjects

Mark W. Phillips

Publications and source records attributed to Mark W. Phillips.

At least 19 recordsLinked to original sources

arXiv2026

The Companions to B and A Stars Snapshot (C-BASS) Survey: I. Discovery of a Young Brown Dwarf Companion to HIP 17453

We report the detection of a new brown dwarf companion to HIP 17453 A, a chemically peculiar A0V star located at a distance of 81 pc. HIP 17453 A was observed with high-resolution adaptive optics imaging using the Near-Infrared Camera 2 on the Keck II telescope as part of the Companions to B and A Stars Snapshot (C-BASS) survey over a ten-year baseline, revealing the presence of a companion with proper motion consistent with the primary. We estimate the age of the HIP 17453 system as 280 $\pm$ 125 Myr, and with follow-up intermediate resolution (R~1800) spectroscopic observations with the Gemini Near Infra-Red Spectrograph (GNIRS) on the Gemini-North telescope, we found the spectrum of HIP 17453 B to be consistent with a spectral type of L2 $\pm$ 1. Through interpolation of Sonora Diamondback evolutionary models, we calculate an effective temperature of $1953^{+84}_{-78}$ K and mass of $53^{+10}_{-8}$ $M_{Jup}$ for HIP 17453 B, which corresponds to a mass ratio of $q = 0.024 \pm 0.004$ for the HIP 17453 system. With its intermediate mass and young age, HIP 17453 B joins a small set of benchmark brown dwarf companions around early-type stars that are suitable for follow-up atmospheric and evolutionary studies.

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arXiv2026

Panchromatic View of the Frigid Jovian Exoplanet COCONUTS-2 b

We use a high signal-to-noise MIRI-LRS spectrum (5.45 - 11 $\mu$m, R$_\lambda$ $\sim100$) of COCONUTS-2~b revealing prominent molecular features of H$_2$O, CH$_4$ and NH$_3$. This dataset is combined with spectra from Gemini/FLAMINGOS-2 and JWST/NIRSpec (G395H), as well as photometry from WISE and Spitzer, resulting in almost continuous wavelength coverage from 1 to 15 $\mu$m. We analyze the data using five grids of self-consistent atmospheric models, spanning a wide range of T$_\text{eff}$, log(g), and [M/H]. We also investigate the use of Gaussian Processes to account for correlated noise either caused by the spectrograph or by systematic departures of models in the inversion framework. All models manage to fit the overall combined observations but predict fainter flux in Y- and N-bands. Classical model comparison suggests that the ATMO2020++ synthetic specra (with and without PH$_3$) are statistically preferred. Fitting for the correlated noise of the three spectroscopic instruments, ATMO2020++ models yields constraints consistent with previous studies and evolutionary models predictions: T$_\text{eff}$ $=496^{+5}_{-3}$ K, log(g) $=4.30^{+0.04}_{-0.02}$ dex, [M/H] $=-0.02^{+0.03}_{-0.02}$ dex, and R $=1.03^{+0.01}_{-0.02}$ R$_\text{jup}$. The extended wavelength coverage provided by MIRI (accounting for 41% of the bolometric flux) completes the SED, yielding a precise luminosity estimation of log(L/L$_{\odot}$) $=-6.166\pm0.002$ dex. Combined with a previous estimate of the system age ($414\pm23$ Myr), cooling models predict a mass of M $=7.3\pm0.3$ M$_\text{jup}$.

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arXiv2026

Keck Observations in the INfrared of Taurus and $\rho$ Oph Exoplanets And Ultracool dwarfs (KOINTREAU) II: Two Young Bound Companions to Ophiuchus Stars

We present the second set of discoveries from Keck Observations in the INfrared of Taurus and $\rho$ Oph Exoplanets And Ultracool dwarfs (KOINTREAU), an adaptive optics survey of young stars in the Taurus and $\rho$ Oph star-forming regions using Keck/NIRC2 in conjunction with the Keck infrared pyramid wavefront sensor. We have discovered two faint comoving companions to young stars ISO-Oph 96 and 2MASS J16262785-2625152. The companion to ISO-Oph 96, KOINTREAU-3b, is at a projected separation of 340 au (2.49"). Using our NIRC2 photometry and evolutionary models, and assuming that the companion has the same extinction as its host star, we infer that KOINTREAU-3b has a mass of $3.4\pm0.7$ M$_{\rm Jup}$. The companion to 2MASS J16262785-2625152, KOINTREAU-4b, has a projected separation of 180 au (1.25") and could have a mass of either $11.5^{+1.2}_{-1.6}$ M$_{\rm Jup}$ or $15.3^{+0.7}_{-0.8}$ M$_{\rm Jup}$, depending on whether the host star is a member of $\rho$ Oph or Upper Sco.

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arXiv2026

The Y Dwarf Population with HST: unlocking the secrets of our coolest neighbours -- III. Near-Infrared Photometry

Y dwarfs represent the coldest class of brown dwarfs, with effective temperatures below 500K, and provide unique analogues to cold giant exoplanets. We present a large compilation of uniform near-infrared photometry from the Hubble Space Telescope for 21 Y dwarfs across multiple WFC3/IR filters, including the F105W, F125W and F160W bands. We employed refined PSF-fitting and calibration procedures to reach photometric uncertainties at the 0.02-0.05 mag level for most targets. Combined with precise parallax measurements, our data reveal well-defined Y-dwarf sequences in near-infrared colour-magnitude diagrams, observed with a markedly improved tightness. Known photometric trends emerge with minimal scatter, including the continuous redward progression in F125W-F160W with decreasing temperature, and the blueward trend in F105W-F125W with possible hints of a reversal around 350K. Comparisons to the ATMO, Sonora Elf Owl, and Lacy & Burrows atmospheric models highlight systematic discrepancies, in particular F105W-F125W and F105W-F160W colours predicted to be too red. Low-metallicity grids provide the best fits to the global Y-dwarf population, although closer inspection across wavelengths shows that these preferences likely reflect compensating effects in missing or incomplete physics rather than true population-level abundances. While some atmospheric diversity is expected among Y dwarfs, their tight observational sequences and systematic offsets from model predictions reveal that key physical and chemical processes remain inadequately captured in current grids. Our results underscore the importance of high-precision, internally consistent datasets in robustly tracing the Y-dwarf cooling sequence and providing the empirical constraints needed to advance theoretical models at the lowest temperatures.

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arXiv2025

Keck Observations in the INfrared of Taurus and $\rho$ Oph Exoplanets And Ultracool dwarfs (KOINTREAU) I: A Planetary-Mass Companion and a Disk-Obscured Stellar Companion Discovered in Taurus

We present the first discoveries from Keck Observations in the INfrared of Taurus and $\rho$ Oph Exoplanets And Ultracool dwarfs (KOINTREAU), an adaptive optics imaging survey of young stars in the Taurus and $\rho$ Oph star-forming regions using the Keck infrared pyramid wavefront sensor (PyWFS). We have found two faint ($\Delta$K~7 mag), wide-separation companions to two ~3-Myr-old Taurus members. Relative astrometry for these systems show that both companions are bound to their host stars. We obtained near-infrared spectra of these companions using IRTF/SpeX (R~100) and Gemini/GNIRS (R~1000-2000), and combine these with photometry from our NIRC2 imaging, the Pan-STARRS survey, and Spitzer/IRAC archival imaging to constrain their properties. One companion, KOINTREAU-1b (at a projected separation of 690 au), has an average near-IR spectral type of M9$\pm$2, a gravity classification of VL-G, and a changing spectral type between the SpeX (M7) and GNIRS (L1) observations. We estimate this object's mass to be $10.6^{+2.5}_{-2.3}$ M$_{\rm Jup}$, making KOINTREAU-1b the fifth planetary-mass companion found in Taurus. The other companion, KOINTREAU-2b (projected separation 560 au), has a spectral type of M4.5$\pm$1 but is ~4 magnitudes underluminous relative to other Taurus stars of the same spectral type. We detect exceptionally strong He I 1.083 micron emission from this object, indicative of outflows driven by ongoing accretion, but with a conspicuous lack of accompanying H emission. We conclude that KOINTREAU-2b is a young star obscured by an edge-on disk and observed in scattered light. Finally, we derive a distortion solution for NIRC2 imaging which shows a 0.118{\deg} difference in position angle from the previous distortion solution.

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arXiv2025

Silicate Precursor Silane detected in Cold Low-Metallicity Brown Dwarf

Within 20 pc of the Sun there are currently 29 known cold brown dwarfs, sources with measured distances and an estimated effective temperature between that of Jupiter (170K) and ~500K. These sources are almost all isolated and are the closest laboratories we have for detailed atmospheric studies of giant planets formed outside the solar system. Here we report JWST observations of one such source, WISEA J153429.75-104303.3 (W1534), which we confirm is a substellar mass member of the Galactic halo with a metallicity <0.01xsolar. Its spectrum reveals methane (CH4), water (H2O), and silane (SiH4) gas. Although SiH4 is expected to serve as a key reservoir for the cloud-forming element Si in gas giant worlds, it eluded detection until now because it is removed from observable atmospheres by the formation of silicate clouds at depth. These condensates are favored with increasing metallicity, explaining why SiH4 remains undetected on well studied, metal-rich solar system worlds like Jupiter and Saturn. On the metal-poor world W1534, we detect a clear signature of SiH4 centered at ~4.55 microns with an abundance of 19+/-2 parts per billion (ppb). Our chemical modelling suggests that this SiH4 abundance may be quenched at ~kilobar levels just above the silicate cloud layers, whereupon vertical atmospheric mixing can transport SiH4 to the observable photosphere. The formation and detection of SiH4 demonstrates key coupled relationships between composition, cloud formation, and atmospheric mixing in cold brown dwarf and planetary atmospheres.

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arXiv2025

Exploration of the Near-Infrared Colors of Cold Y Dwarfs from the Ground and Space

JWST has provided critical mid-infrared data for cold brown dwarfs. It has also provided low-resolution near-infrared spectra, and for faint sources these are the first spectra at these wavelengths. We use these data and other literature sources to synthesize near-infrared photometry on the MKO system for 19 T and Y dwarfs, on the Euclid system for 44 T and Y dwarfs, and on the Roman system for 48 T and Y dwarfs. We also synthesize Euclid I_E magnitudes for 15 T and Y dwarfs. Using the Beiler et al. 2024 observational effective temperatures (T_eff), together with ATMO 2020++ model colors, we show that the absolute 4.6um magnitude can be used as a proxy for T_eff. We present a polynomial fit to the M_W2:T_eff relationship for cool dwarfs with 250 < T_eff K < 1000. We select five Y dwarfs with 275 < T_eff K < 400 which have a range in near- to mid-infrared colors. Comparison of the JWST spectral energy distribution to ATMO 2020++ models indicate that Y dwarfs which are bluer in J - W2 are lower gravity or more metal-rich than their redder counterparts, with stronger CO and CO_2 absorption at 4.2 < lambda um < 4.9. The near-infrared color diagrams show significant scatter, with complex dependencies on T_eff, metallicity and gravity. In order to disentangle these effects, opacity sources for cool model atmospheres need to be more complete at lambda < 1um.

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arXiv2024

Disequilibrium Chemistry, Diabatic Thermal Structure, and Clouds in the Atmosphere of COCONUTS-2b

Located 10.888 pc from Earth, COCONUTS-2b is a planetary-mass companion to a young (150-800 Myr) M3 star, with a wide orbital separation (6471 au) and a low companion-to-host mass ratio ($0.021\pm0.005$). We have studied the atmospheric properties of COCONUTS-2b using newly acquired 1.0-2.5 $\mu$m spectroscopy from Gemini/Flamingos-2. The spectral type of COCONUTS-2b is refined to T$9.5 \pm 0.5$ based on comparisons with T/Y dwarf spectral templates. We have conducted an extensive forward-modeling analysis, comparing the near-infrared spectrum and mid-infrared broadband photometry with sixteen state-of-the-art atmospheric model grids developed for brown dwarfs and self-luminous exoplanets near the T/Y transition. The PH$_{3}$-free ATMO2020++, ATMO2020++, and Exo-REM models best match the specific observations of COCONUTS-2b, regardless of variations in the input spectrophotometry. This analysis suggests the presence of disequilibrium chemistry, along with a diabatic thermal structure and/or clouds, in the atmosphere of COCONUTS-2b. All models predict fainter $Y$-band fluxes than observed, highlighting uncertainties in the alkali chemistry models and opacities. We determine a bolometric luminosity of $\log{(L_{\rm bol}/L_{\odot})}=-6.18$ dex, with a 0.5 dex-wide range of $[-6.43,-5.93]$ dex that accounts for various assumptions of models. Using thermal evolution models, we derive an effective temperature of $T_{\rm eff}=483^{+44}_{-53}$ K, a surface gravity of $\log{(g)}=4.19^{+0.18}_{-0.13}$ dex, a radius of $R=1.11^{+0.03}_{-0.04}$ R$_{\rm Jup}$, and a mass of $M=8 \pm 2$ M$_{\rm Jup}$. Various atmospheric model grids consistently indicate that COCONUTS-2b's atmosphere has sub- or near-solar metallicity and C/O. These findings provide valuable insights into COCONUTS-2b's formation history and the potential outward migration to its current wide orbit.

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arXiv2023

The Carbon-to-Oxygen Ratio in Cool Brown Dwarfs and Giant Exoplanets. I. The Benchmark Late-T dwarfs GJ 570D, HD 3651B and Ross 458C

Measurements of the C/O ratio in brown dwarfs are lacking, in part due to past models adopting solar C/O only. We have expanded the ATMO 2020 atmosphere model grid to include non-solar metallicities and C/O ratios in the T dwarf regime. We change the C/O ratio by altering either the carbon or oxygen elemental abundances, and we find that non-solar abundances of these elements can be distinguished based on the shapes of the $H$- and $K$- bands. We compare these new models with medium-resolution ($R\approx1700$), near-infrared ($0.8-2.4\,μ$m) Gemini/GNIRS spectra of three benchmark late-T dwarfs, GJ 570D, HD 3651B, and Ross 458C. We find solar C/O ratios and best-fitting parameters ($T_\mathrm{eff}$, $\log(g)$, $Z$) broadly consistent with other analyses in the literature based on low-resolution ($R\sim100$) data. The model-data discrepancies in the near-infrared spectra are consistent across all three objects. These discrepancies are alleviated when fitting the Y, J, H and K bands individually, but the resulting best-fit parameters are inconsistent and disagree with the results from the full-spectrum. By examining the model atmosphere properties we find this is due to the interplay of gravity and metallicity on $\mathrm{H_2-H_2}$ collisionally induced absorption. We therefore conclude that there are no significant issues with the molecular opacity tables used in the models at this spectral resolution. Instead, deficiencies are more likely to lie in the model assumptions regarding the thermal structures. Finally, we find a discrepancy between the GNIRS, SpeX, and other near-infrared spectra in the literature of Ross 458C, indicating potential spectroscopic variability.

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arXiv2023

On the Masses, Age, and Architecture of the VHS J1256-1257AB b System

VHS J1256$-$1257 AB is an ultracool dwarf binary that hosts a wide-separation planetary-mass companion that is a key target of the {\sl JWST} Exoplanet Early Release Science (ERS) program. Using Keck adaptive optics imaging and aperture masking interferometry, we have determined the host binary's orbit, $a=1.96\pm0.03$ au, $P=7.31\pm0.02$ yr, $e=0.883\pm0.003$, and measured its dynamical total mass, $0.141\pm0.008$ $M_{\odot}$. This total mass is consistent with VHS J1256$-$1257 AB being a brown dwarf binary or pair of very low-mass stars. In addition, we measured the orbital motion of VHS J1256$-$1257 b with respect to the barycenter of VHS J1256$-$1257 AB, finding that the wide companion's orbit is also eccentric, $e=0.68^{+0.11}_{-0.10}$, with a mutual inclination of $115\pm14^{\circ}$ with respect to the central binary. This orbital architecture is consistent with VHS J1256$-$1257 b attaining a significant mutual inclination through dynamical scattering and thereafter driving Kozai-Lidov cycles to pump the eccentricity of VHS J1256$-$1257 AB. We derive a cooling age of $140\pm20$ Myr for VHS J1256$-$1257 AB from low-mass stellar/substellar evolutionary models. At this age, the luminosity of VHS J1256$-$1257 b is consistent with both deuterium-inert and deuterium-fusing evolutionary tracks. We thus find a bimodal probability distribution for the mass of VHS J1256$-$1257 b, either $12.0\pm0.1$ $M_{\rm Jup}$ or $16\pm1$ $M_{\rm Jup}$, from these models. Future spectroscopic data to measure isotopologues such as HDO and CH$_3$D could break this degeneracy and provide a strong test of substellar models at the deuterium-fusion mass boundary.

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arXiv2023

Retrieval study of cool, directly imaged exoplanet 51 Eri b

Retrieval methods are a powerful analysis technique for modelling exoplanetary atmospheres by estimating the bulk physical and chemical properties that combine in a forward model to best-fit an observed spectrum, and they are increasingly being applied to observations of directly-imaged exoplanets. We have adapted TauREx3, the Bayesian retrieval suite, for the analysis of near-infrared spectrophotometry from directly-imaged gas giant exoplanets and brown dwarfs. We demonstrate TauREx3's applicability to sub-stellar atmospheres by presenting results for brown dwarf benchmark GJ 570D which are consistent with previous retrieval studies, whilst also exhibiting systematic biases associated with the presence of alkali lines. We also present results for the cool exoplanet 51 Eri b, the first application of a free chemistry retrieval analysis to this object, using spectroscopic observations from GPI and SPHERE. While our retrieval analysis is able to explain spectroscopic and photometric observations without employing cloud extinction, we conclude this may be a result of employing a flexible temperature-pressure profile which is able to mimic the presence of clouds. We present Bayesian evidence for an ammonia detection with a 2.7$σ$ confidence, the first indication of ammonia in an exoplanetary atmosphere. This is consistent with this molecule being present in brown dwarfs of a similar spectral type. We demonstrate the chemical similarities between 51 Eri b and GJ 570D in relation to their retrieved molecular abundances. Finally, we show that overall retrieval conclusions for 51 Eri b can vary when employing different spectral data and modelling components, such as temperature-pressure and cloud structures.

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arXiv2021

Improved Dynamical Masses for Six Brown Dwarf Companions Using Hipparcos and Gaia EDR3

We present comprehensive orbital analyses and dynamical masses for the substellar companions Gl~229~B, Gl~758~B, HD~13724~B, HD~19467~B, HD~33632~Ab, and HD~72946~B. Our dynamical fits incorporate radial velocities, relative astrometry, and most importantly calibrated Hipparcos-Gaia EDR3 accelerations. For HD~33632~A and HD~72946 we perform three-body fits that account for their outer stellar companions. We present new relative astrometry of Gl~229~B with Keck/NIRC2, extending its observed baseline to 25 years. We obtain a $<$1\% mass measurement of $71.4 \pm 0.6\,M_{\rm Jup}$ for the first T dwarf Gl~229~B and a 1.2\% mass measurement of its host star ($0.579 \pm 0.007\,M_{\odot}$) that agrees with the high-mass-end of the M dwarf mass-luminosity relation. We perform a homogeneous analysis of the host stars' ages and use them, along with the companions' measured masses and luminosities, to test substellar evolutionary models. Gl~229~B is the most discrepant, as models predict that an object this massive cannot cool to such a low luminosity within a Hubble time, implying that it may be an unresolved binary. The other companions are generally consistent with models, except for HD~13724~B that has a host-star activity age 3.8$σ$ older than its substellar cooling age. Examining our results in context with other mass-age-luminosity benchmarks, we find no trend with spectral type but instead note that younger or lower-mass brown dwarfs are over-luminous compared to models, while older or higher-mass brown dwarfs are under-luminous. The presented mass measurements for some companions are so precise that the stellar host ages, not the masses, limit the analysis.

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arXiv2021

Optimized modeling of Gaia-Hipparcos astrometry for the detection of the smallest cold Jupiter and confirmation of seven low mass companions

To fully constrain the orbits of low mass circumstellar companions, we conduct combined analyses of the radial velocity data as well as the Gaia and Hipparcos astrometric data for eight nearby systems. Our study shows that companion-induced position and proper motion differences between Gaia and Hipparcos are significant enough to constrain orbits of low mass companions to a precision comparable with previous combined analyses of direct imaging and radial velocity data. We find that our method is robust to whether we use Gaia DR2 or Gaia EDR3, as well as whether we use all of the data, or just proper motion differences. In particular, we fully characterize the orbits of HD 190360 b and HD 16160 C for the first time. With a mass of 1.8$\pm$0.2$m_{\rm Jup}$ and an effective temperature of 123-176 K and orbiting around a Sun-like star, HD 190360 b is the smallest Jupiter-like planet with well-constrained mass and orbit, belonging to a small sample of fully characterized Jupiter analogs. It is separated from its primary star by 0.25$''$ and thus may be suitable for direct imaging by the CGI instrument of the Roman Space Telescope.

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arXiv2021

Measuring and Replicating the 1-20 um Energy Distributions of the Coldest Brown Dwarfs: Rotating, Turbulent and Non-Adiabatic Atmospheres

Cold, low-mass, field brown dwarfs are important for constraining the terminus of the stellar mass function, and also for optimizing atmospheric studies of exoplanets. In 2020 new model grids for such objects were made available: Sonora-Bobcat and ATMO 2020. Also, new candidate cold brown dwarfs were announced, and new spectroscopic observations at lambda ~4.8 um were published. In this paper we present new infrared photometry for some of the coldest brown dwarfs, and put the new data and models together to explore the properties of these objects. We reconfirm the importance of mixing in these atmospheres, which leads to CO and NH_3 abundances that differ by orders of magnitude from chemical equilibrium values. We also demonstrate that the new models retain the known factor >~3 discrepancy with observations at 2 <~ lambda um <~ 4, for brown dwarfs cooler than 600 K. We show that the entire 1 <~ lambda um <~ 20 energy distribution of six brown dwarfs with 260 <= T_eff K <= 475 can be well reproduced, for the first time, by model atmospheres which include dis-equilibrium chemistry as well as a photospheric temperature gradient which deviates from the standard radiative/convective equilibrium value. This change to the pressure-temperature profile is not unexpected for rotating and turbulent atmospheres which are subject to diabatic processes. A limited grid of modified-adiabat model colors is generated, and used to estimate temperatures and metallicities for the currently known Y dwarfs. A compilation of the photometric data used here is given in the Appendix.

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arXiv2021

Direct imaging of sub-Jupiter mass exoplanets with James Webb Space Telescope coronagraphy

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 characterise 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 $M_\textrm{J}$ - 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 characterise previously detected companions across the near- to mid-infrared for the first time.

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arXiv2021

Direct imaging and spectroscopy of exoplanets with the ELT/HARMONI high-contrast module

Combining high-contrast imaging with medium-resolution spectroscopy has been shown to significantly boost the direct detection of exoplanets. HARMONI, one of the first-light instruments to be mounted on ESO's ELT, will be equipped with a single-conjugated adaptive optics system to reach the diffraction limit of the ELT in H and K bands, a high-contrast module dedicated to exoplanet imaging, and a medium-resolution (up to R = 17 000) optical and near-infrared integral field spectrograph. Combined together, these systems will provide unprecedented contrast limits at separations between 50 and 400 mas. In this paper, we estimate the capabilities of the HARMONI high-contrast module for the direct detection of young giant exoplanets. We use an end-to-end model of the instrument to simulate observations based on realistic observing scenarios and conditions. We analyze these data with the so-called "molecule mapping" technique combined to a matched-filter approach, in order to disentangle the companions from the host star and tellurics, and increase the S/N of the planetary signal. We detect planets above 5-sigma at contrasts up to 16 mag and separations down to 75 mas in several spectral configurations of the instrument. We show that molecule mapping allows the detection of companions up to 2.5 mag fainter compared to state-of-the-art high-contrast imaging techniques based on angular differential imaging. We also demonstrate that the performance is not strongly affected by the spectral type of the host star, and that we reach close sensitivities for the best three quartiles of observing conditions at Armazones, which means that HARMONI could be used in near-critical observations during 60 to 70% of telescope time at the ELT. Finally, we simulate planets from population synthesis models to further explore the parameter space that HARMONI and its high-contrast module will soon open.

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arXiv2020

The ExoMolOP Database: Cross-sections and k-tables for Molecules of Interest in High-Temperature Exoplanet Atmospheres

A publicly available database of opacities for molecules of astrophysical interest, ExoMolOP, has been compiled for over 80 species, based on the latest line list data from the ExoMol, HITEMP and MoLLIST databases. These data are generally suitable for characterising high temperature exoplanet or cool stellar/substellar atmospheres, and have been computed at a variety of pressures and temperatures, with a few molecules included at room-temperature only from the HITRAN database. The data are formatted in different ways for four different exoplanet atmosphere retrieval codes; ARCiS, TauREx, NEMESIS and petitRADTRANS, and include both cross-sections (at R~=~$\fracλ{Δλ}$~=~15,000) and k-tables (at R~=~$\fracλ{Δλ}$~=~1000) for the 0.3~-~50$μ$m wavelength region. Opacity files can be downloaded and used directly for these codes. Atomic data for alkali metals Na and K are also included, using data from the NIST database and the latest line shapes for the resonance lines. Broadening parameters have been taken from the literature where available, or from those for a known molecule with similar molecular properties where no broadening data are available. The data are available from www.exomol.com.

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arXiv2020

A Library of Self-Consistent Simulated Exoplanet Atmospheres

We present a publicly available library of model atmospheres with radiative-convective equilibrium Pressure-Temperature ($P$-$T$) profiles fully consistent with equilibrium chemical abundances, and the corresponding emission and transmission spectrum with R$\sim$5000 at 0.2 $μ$m decreasing to R$\sim$35 at 30 $μ$m, for 89 hot Jupiter exoplanets, for four re-circulation factors, six metallicities and six C/O ratios. We find the choice of condensation process (local/rainout) alters the $P$-$T$ profile and thereby the spectrum substantially, potentially detectable by JWST. We find H$^-$ opacity can contribute to form a strong temperature inversion in ultra-hot Jupiters for C/O ratios $\geq$ 1 and can make transmission spectra features flat in the optical, alongside altering the entire emission spectra. We highlight how adopting different model choices such as thermal ionisation, opacities, line-wing profiles and the methodology of varying the C/O ratio, effects the $P$-$T$ structure and the spectrum. We show the role of Fe opacity to form primary/secondary inversion in the atmosphere. We use WASP-17b and WASP-121b as test cases to demonstrate the effect of grid parameters across their full range, while highlighting some important findings, concerning the overall atmospheric structure, chemical transition regimes and their observables. Finally, we apply this library to the current transmission and emission spectra observations of WASP-121b, which shows H$_2$O and tentative evidence for VO at the limb, and H$_2$O emission feature indicative of inversion on the dayside, with very low energy redistribution, thereby demonstrating the applicability of library for planning and interpreting observations of transmission and emission spectrum.

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