Physical and Chemical Characterization of GY 91's Multi-ringed Protostellar Disk with ALMA

TL;DR

This study uses ALMA Band 7 observations with multiple configurations to characterize GY 91, a heavily extincted Class I YSO, by combining dust continuum and a suite of molecular tracers (CS 6-5, N$_2$H$^+$ 3-2, C$^{18}$O 3-2, H$_2$CO, H$_2$CS). Through image- and visibility-domain dynamical modeling of Keplerian CS emission, the stellar mass is constrained to $M_\ast\approx0.58\,M_\odot$, while radiative transfer and thermochemical modeling of dust and gas yield disk masses of order $M_ ext{gas} \sim 0.008$–$0.013\,M_\odot$, with envelope contributions to continuum emission found to be small. The observations reveal azimuthal asymmetries in sulfur-bearing species and ring-like N$_2$H$^+$ emission near the CO snowline, alongside radially structured H$_2$CO emission beyond the dust continuum, suggesting a chemically evolving, relatively mature disk for its class. The work also assesses envelope contamination and identifies line tracers (e.g., CS, N$_2$H$^+$) that robustly trace disk kinematics in highly extincted environments, offering a practical roadmap for similar embedded-disk studies. Overall, GY 91 appears chemically similar to Class II disks, with a dynamically established mass and a gravitationally stable disk, informing the timescales and processes of early planet formation.

Abstract

GY 91, commonly categorized as a Class I young stellar object, is notable for disk dust substructures that have been hypothesized to trace early planet formation. Using the ALMA 12-m and ACA arrays, we present new Band 7 dust continuum and molecular line observations of GY 91 at an angular resolution of (~40 au). We report detections of CS $J=6-5$, N$_2$H$^+$ $J=3-2$, C$^{18}$O $J=3-2$, H$_2$CS $J_{K_a, K_c} = 8_{1,7}-7_{1,6}$, H$_2$CO $J_{K_a, K_c} = 4_{0,4}-3_{0,3}$, and H$_2$CO $J_{K_a, K_c} = 4_{2,3}-3_{2,2}$, as well as a tentative detection of $^{13}$C$^{18}$O $J=3-2$. We observe azimuthal asymmetry in CS and H$_2$CS emission, as well as radially structured H$_2$CO $4_{0,4}-3_{0,3}$ emission outside the dust continuum. C$^{18}$O and H$_2$CO 4$_{0,4}-3_{0,3}$ show significant cloud contamination, while CS and N$_2$H$^+$ are good tracers of Keplerian rotation originating from the disk. Envelope emission does not appear to contribute significantly either to the continuum or molecular line observations. GY 91's chemical properties appear in large part to resemble those of Class II disks, although observations of additional molecular probes should be obtained for a fuller comparison. With CS, we estimated a dynamical stellar mass of 0.58 $M_\odot$, which is higher than previous estimates from stellar evolutionary models (0.25 $M_\odot$). Using both radiative transfer modeling of the dust continuum and comparison of the C$^{18}$O and N$_2$H$^+$ fluxes to literature thermochemical models, we estimate a disk mass of $\sim0.01$ $M_\odot$.

Figures (13)

• Figure 1: Zeroth moment maps of molecular line emission and the 1.05 mm dust continuum image. The synthesized beam size is indicated in the lower right of each image. CS contours are at the 3, 5, 10, and 15$\sigma$ levels, where $\sigma$ is the rms listed in Table \ref{['tab:imaging']}. The N$_2$H$^+$, H$_2$CO $4_{0,4}-3_{0,3}$, and C$^{18}$O contour levels are at 3, 5, and 7$\sigma$. H$_2$CS, H$_2$CO $4_{2,3}-3_{2,2}$, and $^{13}$C$^{18}$O have contours at 3, 4, and 5$\sigma$. The dust continuum contours are at 10, 50, 100, and 200$\sigma$. A Keplerian mask was used to compute the zeroth-moment maps of H$_2$CO $4_{0,4}-3_{0,3}$ and C$^{18}$O. A 50 au scale bar is shown in the bottom left of each panel. The location of the continuum peak is marked with a cyan cross.
• Figure 2: Moment 1 maps of CS, N$_2$H$^+$, H$_2$CO 4$_{0,4}$-3$_{0,3}$, H$_2$CS, and C$^{18}$O. The synthesized beam is shown in the lower right corner of each panel.
• Figure 3: Deprojected, azimuthally-averaged radial profiles for each line and the 1.05 mm dust continuum. The shaded areas show the statistical $1\sigma$ uncertainty. The horizontal black lines show the scale of the synthesized beam.
• Figure 4: Subset of channels of H$_2$CO $4_{0,4}-3_{0,3}$ and C$^{18}$O where an annular gap in emission is visible. Ellipses are drawn with the same P.A. and inclination as the GY 91 disk to denote the radius at which the break in emission is observed. The semi-major axis of the H$_2$CO ellipse is 122 au, while that for C$^{18}$O is 118 au. Contours correspond to 3, 5, 10, and 15$\sigma$, where $\sigma$ is the rms listed in \ref{['tab:imaging']}.
• Figure 5: Left: CS moment 1 map generated from bettermoments without down-sampling. The synthesized beam is shown in the lower right corner. Middle: Model line-of-sight velocity map based on best-fit parameters from eddy. Right: Residuals from subtracting the model map in the center panel from the observed map in the left panel.