Atmospheric Escape in Exoplanet Atmospheres
Atmospheric escape and mass loss is an important process in the formation and evolution of exoplanets, especially those orbiting very close to their host stars. These planets are exposed to extreme levels of stellar irradiation causing their atmospheres to heat up, expand, and eventually escape the gravitational influence of the planet. The escaping material forms an extended envelope around the planet, which can be detected in transit observations at specific wavelengths associated with strong atomic transitions. Most observations of escaping atmospheres so far have been focused on spectral signatures in the UV wavelength range, particularly in the hydrogen Lyman-α line.
I am interested in investigating what other atomic lines could be used to study the escaping exoplanet atmospheres, with an emphasis on atomic lines accessible to ground-based observations. In Oklopčić and Hirata (2018), we show that the helium line at 1083 nm could be an excellent tracer of extended atmospheres. This line originates from neutral helium atoms in an excited triplet state which is radiatively decoupled from the ground state and hence metastable. The exceptionally long lifetime of this state enables a significant population of excited helium atoms to be built up in the upper layers of a planetary atmosphere, causing strong absorption and transit depths at 1083 nm. Shortly thereafter, this theoretical prediction was confirmed by observations by Spake et al. (2018). Transit spectra of WASP-107b obtained with the Hubble Space Telescope show evidence of enhanced absorption in the helium 1083 nm line. To find out more about this discovery, read the Behind the paper blog post by the lead author, Jessica Spake.
Raman Scattering in Exoplanet Atmospheres
Raman scattering on molecules in a planetary atmosphere imprints specific features in the geometric albedo spectrum of the planet which can be used to probe the composition and the physical conditions in the exoplanet atmosphere. In Oklopčić, Hirata and Heng (2016), we show how Raman features can be used to constrain the presence and the altitude of clouds, measure the atmospheric temperature, and spectroscopically identify the composition of the bulk of the atmosphere, even when it is made up of spectrally inactive gases like hydrogen or nitrogen.
In Oklopčić, Hirata and Heng (2017), we extend the investigation of the Raman effect to atmospheres irradiated by different types of stellar spectra. The intensity of Raman features depends on both the properties of the atmosphere and the structure of lines in the stellar spectrum that produce them. Otherwise identical planetary atmospheres can produce a diverse range of albedo spectra depending on the spectral type of the host star. We explore this diversity in order to identify what types of stars host planets that show most prominent Raman features.
Clumpy Galaxies at High Redshift
Massive, star-forming galaxies at redshift z ~ 2 have much more irregular structure compared to galaxies of similar properties in the local Universe. Their morphology is often dominated by several giant star forming clumps. These clumps are believed to form via gravitational instabilities in gas-rich disks. Previous studies have proposed that giant clumps could have important repercussions on the morphology and evolution of their host galaxy—if clumps migrate radially inwards via dynamical friction, they can sink to the center of the galaxy and help build up a bulge. This picture holds if clumps can survive long enough to reach the center without being destroyed first by stellar feedback caused by their own intense star formation.
In Oklopčić et al. (2017), we study the results of a cosmological hydrodynamic galaxy evolution simulation that is part of the FIRE project. We find that giant clumps get destroyed by feedback on a short time scale of ~20 Myr, and we do not see evidence for systematic inward radial migration of clumps. Our results suggest that giant clumps are not the dominant contributor to the bulge growth.
Lyman-α heating of inhomogeneous high-redshift IGM
At the end of the Dark Ages (z ~ 20), a background of UV radiation coming from the early generation of stars interacts with the still mostly neutral intergalactic medium (IGM). Resonant scattering of Lyα photons increases the temperature of the IGM due to atomic recoil upon scattering. However, Lyα photons are inefficient at heating a homogeneous IGM because they quickly equilibrate with the gas. In this paper we investigate what happens in a more realistic scenario—an inhomogeneous IGM with temperature and density fluctuations, in which Lyα photons can cause more significant effects if they leak into regions where the gas temperature is different from theirs. This thermal conduction via Lyα photons can erase the fluctuations on scales comparable to the photon diffusion length. Our calculations, described in Oklopčić and Hirata (2013), show that the length-scale at which this happens is much smaller than the IGM Jeans scale.
Wide-angle tail radio galaxies in the COSMOS field
For my Master's thesis, I analyzed a sample of a dozen wide-angle tail (WAT) radio galaxies observed in the COSMOS surey. WATs are peculiar-looking radio galaxies usually found in dense environments, such as galaxy clusters and groups. Their radio jets are bent, forming a wide C shape that tails the core of the galaxy.
In Oklopčić et al. (2010), I focused on one particular WAT galaxy in the COSMOS field (called CWAT-02) and its host galaxy group at z=0.53. I performed an in-depth study of the galaxy's environment, morphology, and velocity structure, and found evidence for an unrelaxed state of the host group, possibly caused by a galaxy group merger. This result is consistent with the idea that WAT galaxies can be used as good tracers of dynamically young, unrelaxed systems. The analysis of radio-energy outflows suggests that they may be powerful enough to expel gas from the group, over the lifetime of the host galaxy.