* Astronomy

Members Login
Post Info TOPIC: GJ436b


L

Posts: 131433
Date:
RE: GJ436b
Permalink  
 


Title: HST/STIS Lyman-alpha observations of the quiet M dwarf GJ436: Predictions for the exospheric transit signature of the hot neptune GJ436b
Authors: David Ehrenreich, Alain Lecavelier des Etangs, Xavier Delfosse

Lyman-alpha (Lya) emission of neutral hydrogen (1215.67 Angstrom) is the main contributor to the ultraviolet flux of low-mass stars such as M dwarfs. It is also the main light source used in studies of the evaporating upper atmospheres of transiting extrasolar planets with ultraviolet transmission spectroscopy. However, there are very few observations of the Lya emissions of quiet M dwarfs, and none exist for those hosting exoplanets. Here, we present Lya observations of the hot-neptune host star GJ436 with the Hubble Space Telescope Imaging Spectrograph (HST/STIS). We detect bright emission in the first resolved and high quality spectrum of a quiet M dwarf at Lya. Using an energy diagram for exoplanets and an N-body particle simulation, this detection enables the possible exospheric signature of the hot neptune to be estimated as a ~11% absorption in the Lya stellar emission, for a typical mass-loss rate of 10^10 g/s. The atmosphere of the planet GJ436b is found to be stable to evaporation, and should be readily observable with HST. We also derive a correlation between X-ray and Lya emissions for M dwarfs. This correlation will be useful for predicting the evaporation signatures of planets transiting other quiet M dwarfs.

Read more (162kb, PDF)

__________________


L

Posts: 131433
Date:
Permalink  
 

Title: Atmospheric Circulation of Eccentric Hot Neptune GJ436b
Authors: Nikole K. Lewis, Adam P. Showman, Jonathan J. Fortney, Mark S. Marley, Richard S. Freedman, Katharina Lodders

GJ436b is a unique member of the transiting extrasolar planet population being one of the smallest and least irradiated and possessing an eccentric orbit. Because of its size, mass and density, GJ436b could plausibly have an atmospheric metallicity similar to Neptune (20-60 times solar abundances), which makes it an ideal target to study the effects of atmospheric metallicity on dynamics and radiative transfer in an extrasolar planetary atmosphere. We present three-dimensional atmospheric circulation models that include realistic non-grey radiative transfer for 1, 3, 10, 30, and 50 times solar atmospheric metallicity cases of GJ436b. Low metallicity models (1 and 3 times solar) show little day/night temperature variation and strong high-latitude jets. In contrast, higher metallicity models (30 and 50 times solar) exhibit day/night temperature variations and a strong equatorial jet. Spectra and light curves produced from these simulations show strong orbital phase dependencies in the 50 times solar case and negligible variations with orbital phase in the 1 times solar case. Comparisons between the predicted planet/star flux ratio from these models and current secondary eclipse measurements support a high metallicity atmosphere (30-50 times solar abundances) with disequilibrium carbon chemistry at play for GJ436b. Regardless of the actual atmospheric composition of GJ436b, our models serve to illuminate how metallicity influences the atmospheric circulation for a broad range of warm extrasolar planets.

Read more (2017kb, PDF)

__________________


L

Posts: 131433
Date:
Permalink  
 

Title: Methane in the atmosphere of the transiting hot Neptune GJ436b?
Authors: J.-P. Beaulieu, G. Tinetti, D. M. Kipping, I. Ribas, R. J. Barber, J. Y-K. Cho, I. Polichtchouk, J. Tennyson, S. N. Yurchenko, C. A. Griffith, V. Batista, I. Waldmann, S. Miller, S. Carey, O. Mousis, S. J. Fossey, A. Aylward

We present an analysis of seven primary transit observations of the hot Neptune GJ436b at 3.6, 4.5 and microns obtained with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. After correcting for systematic effects of the instrument, we fitted the light curves - including limb darkening effects - using the Markov Chain Monte Carlo technique. Combining these new data with the EPOXI, HST and ground-based V, I, H and K_s observations available in the literature, the range 0.5-10 microns can be covered.
The temperature distribution of the planet was estimated by using a three-dimensional, pseudo-spectral general circulation model with idealised thermal forcing. Transmission spectra of GJ436b were generated using line-by-line radiative transfer models including the opacities of the molecular species expected to be present in such planetary atmosphere, namely water vapour, methane, ammonia, carbon monoxide and dioxide, and hydrogen sulphide. In particular, a new, ab-initio calculated, linelist for hot ammonia has been used for the first time. The photometric data observed at multiple wavelengths can be interpreted with methane being the dominant species after molecular hydrogen, possibly with minor contributions from ammonia, water and other molecules. No clear evidence of carbon monoxide and dioxide is found from transmission photometry. We discuss this result in the light of a recent paper where photochemical disequilibrium is hypothesised to interpret secondary transit photometric data. In particular we show that the emission photometric data are not incompatible with the presence of abundant methane, but further spectroscopic data are desirable to confirm this scenario.

Read more (723kb, PDF)

__________________
Page 1 of 1  sorted by
Quick Reply

Please log in to post quick replies.



Create your own FREE Forum
Report Abuse
Powered by ActiveBoard