arXiv subjects

V. Graham

Publications and source records attributed to V. Graham.

3 recordsLinked to original sources

arXiv2021

Cloud property trends in hot and ultra-hot giant gas planets (WASP-43b, WASP-103b, WASP-121b, HAT-P-7b, and WASP-18b)

Ultra-hot Jupiters are the hottest exoplanets discovered so far. Observations begin to provide insight into the composition of their extended atmospheres and their chemical day/night asymmetries. Both are strongly affected by cloud formation. We explore trends in cloud properties for a sample of five giant gas planets: WASP-43b, WASP-18b, HAT-P-7b, WASP-103b, and WASP-121b. This provides a reference frame for cloud properties for the JWST targets WASP-43b and WASP-121b. We further explore chemically inert tracers to observe geometrical asymmetries, and if the location of inner boundary of a 3D GCM matters for the clouds that form. The large day/night temperature differences of ultra-hot Jupiters cause large chemical asymmetries: cloud-free days but cloudy nights, atomic vs. molecular gases and respectively different mean molecular weights, deep thermal ionospheres vs. low-ionised atmospheres, undepleted vs enhanced C/O. WASP-18b, as the heaviest planet in the sample, has the lowest global C/O. The global climate may be considered as similar amongst ultra-hot Jupiters, but different to that of hot gas giants. The local weather, however, is individual for each planet since the local thermodynamic conditions, and hence the local cloud and gas properties, differ. The morning and the evening terminator of ultra-hot Jupiters will carry signatures of their strong chemical asymmetry such that ingress/egress asymmetries can be expected. An increased C/O ratio is a clear sign of cloud formation, making cloud modelling a necessity when utilizing C/O (or other mineral ratios) as tracer for planet formation. The changing geometrical extension of the atmosphere from the day to the nightside may be probed through chemically inert species like helium. Ultra-hot Jupiters are likely to develop deep atmospheric ionospheres which may impact the atmosphere dynamics through MHD processes.

astro-ph.EP↗

arXiv2020

Mineral cloud and hydrocarbon haze particles in the atmosphere of the hot Jupiter JWST target WASP-43b

Having a short orbital period and being tidally locked makes WASP-43b an ideal candidate for JWST observations. Phase curve observations of an entire orbit will enable the mapping of the atmospheric structure across the planet, with different wavelengths of observation allowing different atmospheric depths to be seen. We provide insight into the details of the clouds that may form on WASP-43b in order to prepare the forthcoming interpretation of the JWST and follow-up data. We utilize 3D GCM results as input for a kinetic, non-equilibrium model for mineral cloud particles, and for a kinetic model to study a photochemicaly-driven hydrocarbon haze component. Mineral condensation seeds form throughout the atmosphere of WASP-43b. This is in stark contrast to the ultra-hot Jupiters, like WASP-18b and HAT-P-7b. The dayside is loaded with few but large mineral cloud particles in addition to hydrocarbon haze particles of comparable abundance. Photochemically driven hydrocarbon haze appears on the dayside, but does not contribute to the cloud formation on the nightside. The geometrical cloud extension differs across the globe due to the changing thermodynamic conditions. Day and night differ by 6000km in pressure scale height. As reported for other planets, the C/O is not constant throughout the atmosphere. The mean molecular weight is approximately constant in a H2-dominated WASP-43b. WASP-43b is expected to be fully covered in clouds which are not homogeneously distributed throughout the atmosphere. The dayside and the terminator clouds will be a combination of mineral particles of locally varying size and composition, and of hydrocarbon hazes. The optical depth of hydrocarbon hazes is considerably lower than that of mineral cloud particles such that a wavelength-dependent radius measurement of WASP-43b would be determined by the mineral cloud particles but not by hazes.

astro-ph.EP↗

arXiv2020

Optical/near-infrared observations of the Fried Egg Nebula: Multiple shell ejections on a 100 yr timescale from a massive yellow hypergiant

Context. The fate of a massive star during the latest stages of its evolution is highly dependent on its mass-loss rate/geometry and therefore knowing the geometry of the circumstellar material close to the star and its surroundings is crucial. Aims. We aim to study the nature (i.e. geometry, rates) of mass-loss episodes. In this context, yellow hypergiants are great targets. Methods. We analyse a large set of optical/near-infrared data, in spectroscopic and photometric (X-shooter/VLT), spectropolarimetric (ISIS/WHT), and interferometric GRAVITY-AMBER/VLTI) modes, toward the yellow hypergiant IRAS 17163-3907. We present the first model-independent reconstructed images of IRAS 17163-3907 at these wavelengths at milli-arcsecond scales. Lastly, we apply a 2D radiative transfer model to fit the dereddened photometry and the radial profiles of published VISIR images at 8.59 {\mu}m, 11.85 {\mu}m and 12.81 {\mu}m simultaneously, adopting the revised Gaia distance (DR2). Results. The interferometric observables around 2 {\mu}m show that the Br{\gamma} emission is more extended and asymmetric than the Na i and the continuum emission. In addition to the two known shells surrounding IRAS 17163-3907 we report on the existence of a third hot inner shell with a maximum dynamical age of only 30 yr. Conclusions. The interpretation of the presence of Na i emission at closer distances to the star compared to Br{\gamma} has been a challenge in various studies. We argue that the presence of a pseudophotosphere is not needed, but it is rather an optical depth effect. The three observed distinct mass-loss episodes are characterised by different mass-loss rates and can inform the theories on mass-loss mechanisms, which is a topic still under debate. We discuss these in the context of photospheric pulsations and wind bi-stability mechanisms.

astro-ph.SR↗