# Asymmetric mid-plane gas in ALMA images of HD~100546

## TL;DR

This study presents new ALMA observations of HD 100546's transitional disc, revealing azimuthal asymmetries, mass estimates, and grain growth evidence, advancing understanding of planet-forming environments.

## Contribution

First detection of C$^{18}$O in HD 100546's disc and detailed analysis of asymmetries, mass, and grain growth in a proto-planet hosting disc.

## Key findings

- Azimuthal asymmetry in gas and dust regions.
- Mass estimates of dust and gas in the disc.
- Evidence of grain growth to cm scales inward.

## Abstract

In this paper we present new ALMA observations towards the proto-planet hosting transitional disc of Herbig Ae/Be star HD 100546. This includes resolved 1.3 mm continuum, $^{13}$CO and the first detection of C$^{18}$O in this disc, which displays azimuthal asymmetry in regions spatially coincident with structures previously identified in HST images related to spiral arms. The lower limit on the mass of the dust disc is calculated to be 9.6x10$^{-4}$M$_\odot$. A firm lower-limit on the total gas mass calculated from optically thin, mid-plane tracing C$^{18}$O (2-1) emission is 0.018M$_\odot$ assuming ISM abundances. These mass estimates provide an estimate of gas-to-dust ratio in the disc of 19, the ratio will increase if C$^{18}$O is relatively under-abundant in the disc compared to CO and H2. Through deprojection and azimuthal averaging of the image plane we detect 1.3 mm continuum emission out to 290+/-10 au,$^{13}$CO to 390+/-10 au and C$^{18}$O to 300+/-10au. We measure a radially increasing millimetre spectral index between wavelengths of 867$\mu$m and 1.3 mm, which shows that grain sizes increase towards the star, with solid particles growing to cm scales in the inner disc.