New Constraints on Protoplanetary Disk Gas Masses in Lupus

TL;DR

This study uses multi-molecular observations to refine gas mass estimates in Lupus protoplanetary disks, revealing that previous estimates are significantly underestimated and that CO isotopologues alone are insufficient for accurate mass determination.

Contribution

It introduces a multi-molecular approach to better constrain disk gas masses and chemical compositions, addressing uncertainties in previous CO-based estimates.

Findings

Gas mass estimates need to be increased by 10-100 times to match observations.

Line fluxes of HCO+, HCN, and N2H+ show tentative correlations.

CO isotopologues alone are inadequate for accurate disk mass measurements.

Abstract

Gas mass is a fundamental quantity of protoplanetary disks that directly relates to their ability to form planets. Because we are unable to observe the bulk H$_2$ content of disks directly, we rely on indirect tracers to provide quantitative mass estimates. Current estimates for the gas masses of the observed disk population in the Lupus star-forming region are based on measurements of isotopologues of CO. However, without additional constraints, the degeneracy between H$_2$ mass and the elemental composition of the gas leads to large uncertainties in such estimates. Here we explore the gas compositions of seven disks from the Lupus sample representing a range of CO-to-dust ratios. With Band 6 and 7 ALMA observations, we measure line emission for HCO$^+$, HCN, and N$_2$H$^+$. We find a tentative correlation among the line fluxes for these three molecular species across the sample, but no correlation with $^{13}$CO or sub-mm continuum fluxes. For the three disks where N$_2$H$^+$ is detected, we find that a combination of high disk gas masses and sub-interstellar C/H and O/H are needed to reproduce the observed values. We find increases of $\sim$10-100$\times$ previous mass estimates are required to match the observed line fluxes. This study highlights how multi-molecular studies are essential for constraining the physical and chemical properties of the gas in populations of protoplanetary disks and that CO isotopologues alone are not sufficient for determining the mass of many observed disks.