New window into escaping exoplanet atmospheres: He 10830 Å line

Update (May 2018): Evidence for absorption in the He 10830 Å line from an exoplanet atmosphere has been found for the first time. The result is based on the analysis of the Hubble Space Telescope transit observations of WASP 107b, led by Jessica Spake. Nature 557, 68 (2018), arXiv:1805.01298 , Behind the paper blog post.

  Atmospheric mass loss is an important process in the formation and evolution of exoplanets, especially those orbiting very close to their host stars. An extended cloud of gas escaping from a planet can cause large transit depths at specific wavelengths associated with key atomic transitions. Most such observations so far have been focused on strong UV transitions, particulary the hydrogen Lyman-α line. While very interesting, Lyα transit observations are challenging because they can only be performed from space and the spectral information in the core of the line is lost to us due to the ISM absorption along the line of sight. The goal of this project is to identify other atomic transitions that cause large transit depths and could therefore be used as good tracers of atmospheric escape, but whose spectral lines are more easily accessible to observations from the ground and are not as sensitive to the ISM extinction as Lyα.
  One of the most promising candidates is the He 10830 Å line originating from a metastable excited state of helium.

Left: schematic diagram of the helium atomic structure. The transition shown in red depicts the 10830 Å absorption line. Right: calculated transmission spectra for planets with properties similar to GJ 436b and HD 209458b. Solid lines show the absorption caused by the metastable helium within the planetary Roche lobe and the dashed lines show the absorption caused by gas at all altitudes. Dotted lines mark the optical transit depths.
Oklopčić and Hirata (2018)

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 our first paper on this topic, 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 the second paper, 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 the most prominent Raman features in a given wavelength range.

Raman scattering in a planetary atmosphere produces spectral features (peaks and throughs) in the albedo spectrum of the planet that carry information about the propeties of its atmosphere.

Oklopčić, Hirata and Heng (2016); Oklopčić, Hirata and Heng (2017)

Clumpy galaxies at high redshifts

  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 our paper, 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.

Left: Clumpy distribution of the gas surface denisty in a star forming galaxy at z = 2. Right: Lifetime of individual clumps as a function of clump mass. Even the most massive clumps get destroyed on time scales shorter than ~20 Myr.
Oklopčić et al. (2017)

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 show that the length-scale at which this happens is much smaller than the IGM Jeans scale.

Oklopčić and Hirata (2013)

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 a separate paper, 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.

Left: Voronoi tessellation of the area around the wide-angle tail radio galaxy CWAT-02, overlaid with radio contours. Filled dots represent high-density galaxies that are less than 500 kpc away from CWAT-02 and belong to its galaxy group. Right: 1.4 GHz radio image of CWAT-02.
Oklopčić et al. (2010)