# Search for Interstellar LiH in the Milky Way

## TL;DR

This study conducted a sensitive search for interstellar LiH molecules in two Milky Way clouds using the APEX telescope, setting new upper limits on LiH abundance that challenge previous tentative detections.

## Contribution

It provides the first sensitive upper limits on interstellar LiH in the Milky Way, refining our understanding of lithium hydride abundance in different interstellar environments.

## Key findings

- Upper limit of 1.9 x 10^-11 on LiH abundance in W49N
- LiH abundance in Sgr B2 (Main) less than 3.6 x 10^-13
- Results are significantly lower than previous tentative detections

## Abstract

We report the results of a sensitive search for the 443.952902 GHz $J=1-0$ transition of the LiH molecule toward two interstellar clouds in the Milky Way, W49N and Sgr B2 (Main), that has been carried out using the Atacama Pathfinder Experiment (APEX) telescope. The results obtained toward W49N place an upper limit of $1.9 \times 10^{-11}\, (3\sigma)$ on the LiH abundance, $N({\rm LiH})/N({\rm H}_2)$, in a foreground, diffuse molecular cloud along the sight-line to W49N, corresponding to 0.5% of the solar system lithium abundance. Those obtained toward Sgr B2 (Main) place an abundance limit $N({\rm LiH})/N({\rm H}_2) < 3.6 \times 10^{-13} \,(3\sigma)$ in the dense gas within the Sgr B2 cloud itself. These limits are considerably smaller that those implied by the tentative detection of LiH reported previously for the $z=0.685$ absorber toward B0218+357.