JWST COMPASS Program: The 3--5$\mu$m transmission spectrum of LTT 1445 A b

TL;DR

This study presents the first JWST transmission spectrum of the rocky exoplanet LTT 1445 A b, achieving high precision measurements that place strong limits on its atmospheric metallicity and composition.

Contribution

It provides the initial JWST NIRSpec/G395H transmission spectrum of LTT 1445 A b with detailed data reduction and analysis, constraining atmospheric properties.

Findings

No significant spectral features detected at 23-36 ppm precision.

Limits on atmospheric metallicity are set at >350x Solar with JWST data alone.

Combined JWST and HST data extend metallicity limits to >500x Solar.

Abstract

The search for an atmosphere on the closest rocky M dwarf planet, LTT 1445 A b, has been the subject of intense investigation from both the ground and space. Here, we present the first JWST transmission spectrum of LTT 1445 A b using a single visit spanning 3-5~$\mu$m using NIRSpec/G395H. We conduct two independent reductions of the data using both the Eureka! and ExoTiC-JEDI pipelines. Overall, we measure the NRS1 transit depths to a median precision of $\sim23$~ppm in 41 spectroscopic channels with uniform widths of 30 pixels ($\sim$ 0.02 $\mu$m), and the NRS2 transit depths to $\sim36$~ppm precision in 65 spectroscopic channels, also with uniform widths of 30 pixels. We rule out any statistically significant spectral features at this precision and place limits on atmospheric metallicity using a grid of chemical equilibrium models with grey opaque clouds. Using NIRSpec/G395H alone, we can place limits on the atmospheric metallicity of $\gtrsim350~\times$ Solar when the opaque pressure level is greater than 0.01~bars. We also conduct a combined analysis of JWST/NIRSpec and HST/WFC3 transmission data and find our atmospheric limits can be extended $\gtrsim500~\times$ Solar when considering both datasets. Future analyses both in transit and emission will uncover whether there are detectable atmospheric features.