Extremely Large Telescope

Abstract:

ABSTRACT Substellar companions such as exoplanets and brown dwarfs exhibit changes in brightness arising from top-of-atmosphere inhomogeneities, providing insights into their atmospheric structure and dynamics. This variability can be measured in the light curves of high-contrast companions from the ground by combining differential spectrophotometric monitoring techniques with high-contrast imaging. However, ground-based observations are sensitive to the effects of turbulence in Earth’s atmosphere, and while adaptive optics (AO) systems and bespoke data processing techniques help to mitigate these, residual systematics can limit photometric precision. Here, we inject artificial companions to data obtained with an AO system and a vector Apodizing Phase Plate coronagraph to test the level to which telluric and other systematics contaminate such light curves, and thus how well their known variability signals can be recovered. We find that varying companions are distinguishable from non-varying companions, but that variability amplitudes and periods cannot be accurately recovered when observations cover only a small number of periods. Residual systematics remain above the photon noise in the light curves but have not yet reached a noise floor. We also simulate observations to assess how specific systematic sources, such as non-common path aberrations and AO residuals, can impact aperture photometry as a companion moves through pupil-stabilized data. We show that only the lowest order aberrations are likely to affect flux measurements, but that thermal background noise is the dominant source of scatter in raw companion photometry. Predictive control and focal-plane wavefront sensing techniques will help to further reduce systematics in data of this type.

Abstract:

<jats:title>Abstract</jats:title> <jats:p>We have carried out a detailed analysis of the 3.4 μm spectral feature arising from Polycyclic Aromatic Hydrocarbons (PAH), using JWST archival data. For the first time in an external galaxy (NGC 6240), we have identified two distinct spectral components of the PAH 3.4 μm feature: a shorter wavelength component at 3.395 μm, which we attribute to short aliphatic chains tightly attached to the aromatic rings of the PAH molecules; and a longer wavelength feature at 3.405 μm that arises from longer, more fragile, aliphatic chains that are weakly attached to the parent PAH molecule. These longer chains are more easily destroyed by far-ultraviolet photons (&amp;gt;5eV) and PAH thermal emission only occurs where PAH molecules are shielded from more energetic photons by dense molecular gas. We see a very strong correlation in the morphology of the PAH 3.395 μm feature with the PAH 3.3 μm emission, the latter arising from robust aromatic PAH molecules. We also see an equally strong correlation between the PAH 3.405 μm morphology and the warm molecular gas, as traced by H2 vibrational lines. We show that the flux ratio PAH 3.395/PAH 3.405 &amp;lt; 0.3 corresponds strongly to regions where the PAH molecules are shielded by dense molecular gas, so that only modestly energetic UV photons penetrate to excite the PAHs. Our work shows that PAH 3.405 μm and PAH 3.395 μm emission features can provide robust diagnostics of the physical conditions of the interstellar medium in external galaxies, and can be used to quantify the energies of the photon field penetrating molecular clouds.</jats:p>

Abstract:

LTT-9779,b is an inhabitant of the hot-Neptune desert and one of only a few planets with a measured high albedo. Characterising the atmosphere of this world is the key to understanding the processes that dominate in reducing the number of short-period intermediate-mass planets that create the hot-Neptune desert. We aim to characterise the reflected light of LTT-9779,b at high spectral resolution to break the degeneracy between clouds and atmospheric metallicity. This is key to interpreting its mass-loss history, which might illuminate how it kept its place in the desert. We used the high-resolution cross-correlation spectroscopy technique on four half-nights of ESPRESSO observations in 4-UT mode (16.4 m effective mirror) to constrain the reflected-light spectrum of łttb. We did not detect the reflected-light spectrum of łttb, although these data had the expected sensitivity at the level 100 ppm. Injection tests of the post-eclipse data indicated that TiO should have been detected for a range of different equilibrium chemistry models. Therefore, this non-detection suggests TiO depletion in the western hemisphere, but this conclusion is sensitive to temperature, which affects the chemistry in the upper atmosphere and the reliability of the line list. Additionally, we were able to constrain the top of the western cloud deck to P_ top, western bar and the top of the eastern cloud deck to P_ top, eastern bar, which is consistent with the predicted altitude of MgSiO_3 and Mg_2SiO_4 clouds from JWST NIRISS/SOSS. While we did not detect the reflected-light spectrum of łttb, we verified that this technique can be used in practice to characterise the reflected light of exoplanets at high spectral resolution when their spectra contain a sufficient number of deep spectral lines. Therefore, this technique may become an important cornerstone of exoplanet characterisation with the ELT and beyond.