Brown Bag Colloquium

Monday, Feb 24, 2014
12:00 pm — 1:00 pm

Location: Kuiper Space Sciences
Room: 309

Dr. Gilda Ballester
Associate Staff Scientist
U of A Lunar and Planetary Laboratory

Extended upper atmospheres of hot Jupiters from the perspective of UV transit observations with HST

The hot-Jupiter HD 209458b was the first exoplanet on which an extended upper atmosphere was discovered in H I reaching out to Roche-lobe distances. Many theoretical studies have followed that can explain the H I Lyman-alpha line transit absorption: the immense stellar extreme-UV and X-ray input should heat and drive a hydrodynamic outflow in the upper atmosphere, and the effects of magnetic-field plasma confinement and stellar-wind interaction have also been explored. Detection of other species can provide further constrains to our understanding of the system, and the heavy and abundant species of O I and C II, which should be dragged along by the lighter but dominant H I and H II gas, have also been observed. Their measured absorption depths require either heating by a higher stellar XUV flux than minimum solar values, additional super-solar abundances, and/or super-thermal processes broadening the absorption line profiles. HST/COS observations have been reported resolving a broad C II line absorption, but, unfortunately, we will show these results to be uncertain because the reference off-transit stellar data were not consecutive to the in-transit observation and suffer from significant stellar flux variations.

We will also show new evidence of an unexpectedly low activity on the star HD 209458 that is also long-term, revealing a significantly lower XUV flux to the planet than assumed in all previous models. This requires a re-evaluation of the energetics and super-thermal processes at play (and of the possible abundances). We will also present a new detection of Mg I in the upper atmosphere of the planet, but not of Mg II as would be expected. This detection provides new constrains and requires further study of the recombination rates that would be expected from current models. It also supports the previously proposed notion that Si and Mg are not effectively removed by condensation into silicates in the lower atmosphere of HD 209458b (in agreement with optical transit data). However, contrary to a previous claim of detection of Si III based on the above COS observations, our re-analysis of that data as well as of archival HST/STIS data shows a negative detection in these ions. The silicon may be present but at a different ionization state.

We will finish by presenting HST FUV observations of HD 189733b, a hot Jupiter that orbits close to a very active star. H I has been detected on the hotter upper atmosphere of this planet, and more recently OI has also been detected with COS. These neutrals are not as extended as on HD 209458b because of the larger photo-ionization rates. A great variation is seen in H I, which should be related to the large variability of the active star. A tentative C II detection shows an apparent early-ingress absorption, potentially revealing a magnetospheric signature, but the detection requires further confirmation due to the possibility of a stellar variation during the observations.

Clearly, the parameter space characterizing the upper atmospheres of hot Jupiters is vast, and new observations will be critical in our understanding of these systems.