arXiv subjects

Luis Welbanks

Publications and source records attributed to Luis Welbanks.

96 records · Page 6Linked to original sources

arXiv2020

The interior and atmosphere of the habitable-zone exoplanet K2-18b

Exoplanets orbiting M dwarfs present a valuable opportunity for their detection and atmospheric characterisation. This is evident from recent inferences of H$_2$O in such atmospheres, including that of the habitable-zone exoplanet K2-18b. With a bulk density between Earth and Neptune, K2-18b may be expected to possess a H/He envelope. However, the extent of such an envelope and the thermodynamic conditions of the interior remain unexplored. In the present work, we investigate the atmospheric and interior properties of K2-18b based on its bulk properties and its atmospheric transmission spectrum. We constrain the atmosphere to be H$_2$-rich with a H$_2$O volume mixing ratio of $0.02-14.8$%, consistent with previous studies, and find a depletion of CH$_4$ and NH$_3$, indicating chemical disequilibrium. We do not conclusively detect clouds/hazes in the observable atmosphere. We use the bulk parameters and retrieved atmospheric properties to constrain the internal structure and thermodynamic conditions in the planet. The constraints on the interior allow multiple scenarios between rocky worlds with massive H/He envelopes and water worlds with thin envelopes. We constrain the mass fraction of the H/He envelope to be $\lesssim 6$%; spanning $\lesssim 10^{-5}$ for a predominantly water world to $\sim6$% for a pure iron interior. The thermodynamic conditions at the surface of the H$_2$O layer range from the super-critical to liquid phases, with a range of solutions allowing for habitable conditions on K2-18b. Our results demonstrate that the potential for habitable conditions is not necessarily restricted to Earth-like rocky exoplanets.

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arXiv2020

Detection of neutral atomic species in the ultra-hot jupiter WASP-121b

The class of ultra-hot Jupiters comprises giant exoplanets undergoing intense irradiation from their host stars. They have proved to be a particularly interesting population for their orbital and atmospheric properties. One such planet, WASP-121~b, is in a highly misaligned orbit close to its Roche limit, and its atmosphere exhibits a thermal inversion. These properties make WASP-121~b an interesting target for additional atmospheric characterization. In this paper, we present analysis of archival high-resolution optical spectra obtained during transits of WASP-121~b. Artifacts from the Rossiter-McLaughlin effect and Center-to-Limb Variation are deemed negligible. However, we discuss scenarios where these effects warrant more careful treatment by modeling the WASP-121 system and varying its properties. We report a new detection of atmospheric absorption from H$α$ in the planet with a transit depth of $1.87\pm0.11\%$. We further confirm a previous detection of the Na I doublet, and report a new detection of Fe I via cross-correlation with a model template. We attribute the H$α$ absorption to an extended Hydrogen atmosphere, potentially undergoing escape, and the Fe I to equilibrium chemistry at the planetary photosphere. These detections help to constrain the composition and chemical processes in the atmosphere of WASP-121~b.

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arXiv2019

Mass-Metallicity Trends in Transiting Exoplanets from Atmospheric Abundances of H$_2$O, Na, and K

Atmospheric compositions can provide powerful diagnostics of formation and migration histories of planetary systems. We investigate constraints on atmospheric abundances of H$_2$O, Na, and K, in a sample of transiting exoplanets using latest transmission spectra and new H$_2$ broadened opacities of Na and K. Our sample of 19 exoplanets spans from cool mini-Neptunes to hot Jupiters, with equilibrium temperatures between $\sim$300 and 2700 K. Using homogeneous Bayesian retrievals we report atmospheric abundances of Na, K, and H$_2$O, and their detection significances, confirming 6 planets with strong Na detections, 6 with K, and 14 with H$_2$O. We find a mass-metallicity trend of increasing H$_2$O abundances with decreasing mass, spanning generally substellar values for gas giants and stellar/superstellar for Neptunes and mini-Neptunes. However, the overall trend in H$_2$O abundances, from mini-Neptunes to hot Jupiters, is significantly lower than the mass-metallicity relation for carbon in the solar system giant planets and similar predictions for exoplanets. On the other hand, the Na and K abundances for the gas giants are stellar or superstellar, consistent with each other, and generally consistent with the solar system metallicity trend. The H$_2$O abundances in hot gas giants are likely due to low oxygen abundances relative to other elements rather than low overall metallicities, and provide new constraints on their formation mechanisms. The differing trends in the abundances of species argue against the use of chemical equilibrium models with metallicity as one free parameter in atmospheric retrievals, as different elements can be differently enhanced.

astro-ph.EP↗

arXiv2019

On degeneracies in retrievals of exoplanetary transmission spectra

Accurate estimations of atmospheric properties of exoplanets from transmission spectra require understanding of degeneracies between model parameters and observations that can resolve them. We conduct a systematic investigation of such degeneracies using a combination of detailed atmospheric retrievals and a range of model assumptions, focusing on H$_2$-rich atmospheres. As a case study, we consider the well-studied hot Jupiter HD 209458 b. We perform extensive retrievals with models ranging from simple isothermal and isobaric atmospheres to those with full pressure-temperature profiles, inhomogeneous cloud/haze coverage, multiple molecular species, and data in the optical-infrared wavelengths. Our study reveals four key insights. First, we find that a combination of models with minimal assumptions and broadband transmission spectra with current facilities allow precise estimates of chemical abundances. In particular, high-precision optical and infrared spectra along with models including variable cloud coverage and prominent opacity sources, Na and K being important in optical, provide joint constraints on cloud/haze properties and chemical abundances. Second, we show that the degeneracy between planetary radius and its reference pressure is well characterised and has little effect on abundance estimates, contrary to previous claims using semi-analytic models. Third, collision induced absorption due to H$_2$-H$_2$ and H$_2$-He interactions plays a critical role in correctly estimating atmospheric abundances. Finally, our results highlight the inadequacy of simplified semi-analytic models with isobaric assumptions for reliable retrievals of transmission spectra. Transmission spectra obtained with current facilities such as HST and VLT can provide strong constraints on atmospheric abundances of exoplanets.

astro-ph.EP↗

arXiv2016

The Superluminous (Type I) Supernova ASASSN-15lh : A case for a Quark-Nova inside an Oxygen-type Wolf-Rayet supernova remnant

We show that a Quark-Nova (QN; the explosive transition of a neutron star to a quark star) occurring a few days following the supernova explosion of an Oxygen-type Wolf-Rayet (WO) star can account for the intriguing features of ASASSN-15lh, including its extreme energetics, its double-peaked light-curve and the evolution of its photospheric radius and temperature. A two-component configuration of the homologously expanding WO remnant (an extended envelope and a compact core) is used to harness the kinetic energy (>10^52 ergs) of the QN ejecta. The delay between the WO SN and the QN yields a large (~ 10^4 Rsun) envelope which when energized by the QN ejecta/shock gives the first peak in our model. As the envelope's photosphere recedes into the slowly expanding, hot and insulated, denser core (initially heated by the QN shock) a second hump emerges. The spectrum in our model should reflect the composition of an WO SN remnant re-heated by a QN going off in its wake.

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arXiv2015

The Burn-UD code for the numerical simulations of the Hadronic-to-Quark-Matter phase transition

Burn-UD is a hydrodynamic combustion code used to model the phase transition of hadronic to quark matter with particular application to the interior of neutron stars. Burn-UD models the flame micro-physics for different equations of state (EoS) on both sides of the interface, i.e. for both the ash (up-down-strange quark phase) and the fuel (up-down quark phase). It also allows the user to explore strange quark seeding produced by different processes including DM annihilation inside neutron stars. The simulations provide a physical window to diagnose whether the combustion process will simmer quietly and slowly, lead to a transition from deflagration to detonation or a (quark) core-collapse explosion. Such an energetic phase transition (a Quark-Nova) would have consequences in high-energy astrophysics and could aid in our understanding of many still enigmatic astrophysical transients. Furthermore, having a precise understanding of the phase transition dynamics for different EoSs could aid further in constraining the nature of the non-perturbative regimes of QCD in general. We hope that Burn-UD will evolve into a platform/software to be used and shared by the QCD community exploring the phases of Quark Matter and astrophysicists working on Compact Stars.

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