Measuring titanium isotope ratios in exoplanet atmospheres

¹ Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
e-mail: serindag@strw.leidenuniv.nl
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 3 August 2021
Accepted: 30 September 2021

Abstract

Context. Measurements of relative isotope abundances can provide unique insights into the formation and evolution histories of celestial bodies, tracing various radiative, chemical, nuclear, and physical processes. In this regard, the five stable isotopes of titanium are particularly interesting. They are used to study the early history of the Solar System, and their different nucleosynthetic origins help constrain Galactic chemical models. Additionally, titanium’s minor isotopes are relatively abundant compared to those of other elements, making them more accessible for challenging observations, such as those of exoplanet atmospheres.

Aims. We aim to assess the feasibility of performing titanium isotope measurements in exoplanet atmospheres. Specifically, we are interested in understanding whether processing techniques used for high-resolution spectroscopy, which remove continuum information about the planet spectrum, affect the derived isotope ratios. We also want to estimate the signal-to-noise requirements for future observations.

Methods. We used an archival high-dispersion CARMENES spectrum of the M-dwarf GJ 1002 as a proxy for an exoplanet observed at very high signal-to-noise. Both a narrow (7045–7090 Å) and wide (7045–7500 Å) wavelength region were defined for which spectral retrievals were performed using petitRADTRANS models, resulting in isotope ratios and uncertainties. These retrievals were repeated on the spectrum with its continuum removed to mimic typical high-dispersion exoplanet observations. The CARMENES spectrum was subsequently degraded by adding varying levels of Gaussian noise to estimate the signal-to-noise requirements for future exoplanet atmospheric observations.

Results. The relative abundances of all minor Ti isotopes are found to be slightly enhanced compared to terrestrial values. A loss of continuum information from broadband filtering of the stellar spectrum has little effect on the isotope ratios. For the wide wavelength range, a spectrum with a signal-to-noise of 5 is required to determine the isotope ratios with relative errors ≲10%. Super Jupiters at large angular separations from their host star are the most accessible exoplanets, requiring about an hour of observing time on 8-meter-class telescopes, and less than a minute of observing time with the future Extremely Large Telescope.