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Edge-On Disk Study (EODS) II: HCO$^+$ and CO vertical stratification in the disk surrounding SSTTau042021

C. Foucher, A. Dutrey, V. Piétu, S. Guilloteau, E. Chapillon, O. Denis-Alpizar, E. Dartois, E. Di Folco, S. Gavino, U. Gorti, Th. Henning, Á. Kóspál, F. Le Petit, L. Majumdar, R. Meshaka, N. T. Phuong, M. Ruaud, D. Semenov, Y. W. Tang, S. Wolf

TL;DR

Edge-on disks offer direct access to vertical disk structure; this study combines ALMA CO isotopologue data with NOEMA HCO$^+$ 3-2 observations to map gas and dust temperatures and densities in SSTTau042021. Using tomographic reconstructions and DiskFit modeling, the authors derive a multi-layer thermal structure: a cold mid-plane of $T\sim$7–11 K, a dense molecular layer at $T\sim$16 K, and a CO atmospheric temperature around $T\sim$31 K at 100 au, with HCO$^+$ indicating densities above $3\times10^{6}$ cm$^{-3}$ at 100–200 au. They also find gas extending beyond the dust outer radius (≥300 au) and estimate a total disk mass of $\sim4.6\times10^{-2}$ M$_\odot$. The results provide direct evidence for vertical molecular stratification and reveal a disk wind component traced by CO, highlighting the complex, layered structure of protoplanetary disks important for planet formation theories. The combination of high-inclination geometry and multi-tracer analysis advances our understanding of gas-dust coupling, freeze-out, and wind processes in young disks.

Abstract

Context. Edge-on disks offer a unique opportunity to directly examine their vertical structure, providing valuable insights into planet formation processes. We investigate the dust properties, as well as the CO and HCO$^+$ gas properties, in the edge-on disk surrounding the T Tauri star 2MASS J04202144+281349 (SSTTau042021). Aims. We estimate the radial and vertical temperature and density profile for the gas and the dust. Methods. We use ALMA archival data of CO isotopologues and continuum emission at 2, 1.3 and 0.9 mm together with new NOEMA HCO$^+$ 3-2 observations. We retrieve the gas and dust disk properties using the tomographic method and the \textsc{DiskFit} model. Results. The vertical CO emission appears very extended, partly tracing the H$_2$ wind observed by JWST. C$^{18}$O, $^{13}$CO and HCO$^+$ emission characterize the bulk of the molecular layer. The dust and gas have a mid-plane temperatures of $\sim 7-11$ K. The temperature of the molecular layer (derived from $^{13}$CO and HCO$^+$) is on the order of 16 K. HCO$^+$ 3-2 being thermalized, we derive a lower limit for the H$_2$ volume density of $\sim 3 \times 10^6$ cm$^{-3}$ at radius 100-200 au between 1 and 2 scale heights. The atmosphere temperature of the CO gas is of the order $\sim$ 31 K at a radius of 100 au. We directly observe CO and HCO$^+$ gas onto the mid-plane beyond the dust outer radius ($\ge 300$ au). The (gas+dust) disk mass estimated up to a radius of 300 au is on the order of $4.6 \times 10^{-2} \mathrm{M}_\odot$. Conclusions. Thanks to the favorable disk inclination, we present the first quantitative evidence for vertical molecular stratification with direct observation of CO and HCO$^+$ gas along the mid-plane. We estimate the temperature profile with temperature of 7-11 K near the mid-plane, and 15-20 K in the dense part of the molecular layer up to $\sim$ 35 K above.

Edge-On Disk Study (EODS) II: HCO$^+$ and CO vertical stratification in the disk surrounding SSTTau042021

TL;DR

Edge-on disks offer direct access to vertical disk structure; this study combines ALMA CO isotopologue data with NOEMA HCO 3-2 observations to map gas and dust temperatures and densities in SSTTau042021. Using tomographic reconstructions and DiskFit modeling, the authors derive a multi-layer thermal structure: a cold mid-plane of 7–11 K, a dense molecular layer at 16 K, and a CO atmospheric temperature around 31 K at 100 au, with HCO indicating densities above cm at 100–200 au. They also find gas extending beyond the dust outer radius (≥300 au) and estimate a total disk mass of M. The results provide direct evidence for vertical molecular stratification and reveal a disk wind component traced by CO, highlighting the complex, layered structure of protoplanetary disks important for planet formation theories. The combination of high-inclination geometry and multi-tracer analysis advances our understanding of gas-dust coupling, freeze-out, and wind processes in young disks.

Abstract

Context. Edge-on disks offer a unique opportunity to directly examine their vertical structure, providing valuable insights into planet formation processes. We investigate the dust properties, as well as the CO and HCO gas properties, in the edge-on disk surrounding the T Tauri star 2MASS J04202144+281349 (SSTTau042021). Aims. We estimate the radial and vertical temperature and density profile for the gas and the dust. Methods. We use ALMA archival data of CO isotopologues and continuum emission at 2, 1.3 and 0.9 mm together with new NOEMA HCO 3-2 observations. We retrieve the gas and dust disk properties using the tomographic method and the \textsc{DiskFit} model. Results. The vertical CO emission appears very extended, partly tracing the H wind observed by JWST. CO, CO and HCO emission characterize the bulk of the molecular layer. The dust and gas have a mid-plane temperatures of K. The temperature of the molecular layer (derived from CO and HCO) is on the order of 16 K. HCO 3-2 being thermalized, we derive a lower limit for the H volume density of cm at radius 100-200 au between 1 and 2 scale heights. The atmosphere temperature of the CO gas is of the order 31 K at a radius of 100 au. We directly observe CO and HCO gas onto the mid-plane beyond the dust outer radius ( au). The (gas+dust) disk mass estimated up to a radius of 300 au is on the order of . Conclusions. Thanks to the favorable disk inclination, we present the first quantitative evidence for vertical molecular stratification with direct observation of CO and HCO gas along the mid-plane. We estimate the temperature profile with temperature of 7-11 K near the mid-plane, and 15-20 K in the dense part of the molecular layer up to 35 K above.

Paper Structure

This paper contains 23 sections, 4 equations, 8 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Observations and Data Reduction
  3. Observations
  4. NOEMA:
  5. ALMA:
  6. Data Reduction
  7. Proper Motions:
  8. Imaging:
  9. Tomographies
  10. Modeling
  11. Continuum data
  12. Molecular lines
  13. Discussion
  14. Dust disk
  15. A disk wind traced by CO?
  16. ...and 8 more sections

Figures (8)

  • Figure 1: Continuum observations: Left column (a): observation, middle column (b): best model, right column (c): difference (observation - model). The rms noise levels reach $0.1 \mathrm{K}$ at 146 GHz, $6.55 \times 10^{-3}\mathrm{K}$ at 225 GHz, and $1.33 \times 10^{-2}\mathrm{K}$ at 333 GHz. The peaks of the residuals correspond to $5.2\sigma$ at 146 GHz, $11.8\sigma$ at 225 GHz, and $8.7\sigma$ at 333 GHz.
  • Figure 2: On the left is an integrated intensity map of $^{12}$CO 2-1 (clipped at 22 mJy/beam.km/s), and on the right is an integrated intensity map of HCO$^{+}$ 3-2 (clipped at 8 mJy/beam.km/s). The maps are rotated by -73.5$^\circ$. The contours are defined from 0 to 0.1 Jy/beam.km/s in steps of 0.01 Jy/beam.km/s, i.e 3.36$\sigma$ for the $^{12}$CO 2-1 map, and from 0 to 0.5 Jy/beam.km/s in steps of 0.02 Jy/beam.km/s , i.e 2.30$\sigma$ for the HCO$^{+}$ 3-2 map.
  • Figure 3: Montage of JWST and interferometric data for SSTTau042021. MIRI MRS images from JWST reveal H$_{2}$ S(2) emission at 12.278 $\mu$m (Arulanantham+2024). White contours indicate the mid-infrared scattered-light continuum at 12.2 $\mu$m, and the white dashed line marks the rotation axis of the disk at PA = 73.5$^\circ$. From archival ALMA data and new NOEMA observations, cyan contours show molecular emission lines of HCO$^+$ 3–2, $^{13}$CO 2–1, C$^{18}$O 2–1, $^{12}$CO 2–1, and $^{12}$CO 3–2, while yellow contours represent the 0.85 mm continuum emission at 1 mJy/beam. Beige circles in the bottom left corner of each panel represent the average theoretical FWHM of the PSF for JWST data, calculated from the relationship reported in Law+2023, and the beam of the molecular line emission is shown as a cyan ellipse in the top-right corner of each panel. The contours for HCO$^+$ (3–2) range from 0.02 to 0.2 Jy/beam.km/s with a step of 0.02 Jy/beam.km/s, i.e 2.5 $\sigma$; for $^{13}$CO (2–1), they range from 0.02 to 0.08 Jy/beam.km/s with a step of 0.01 Jy/beam.km/s, i.e 3.5 $\sigma$; for C$^{18}$CO (2–1), they range from 0.008 to 0.016 Jy/beam.km/s with a step of 0.008 Jy/beam.km/s, i.e 3.5$\,\sigma$; for $^{12}$CO (2–1), from 0.01 to 0.08 Jy/beam.km/s with a step of 0.02 Jy/beam.km/s, i.e 6 $\sigma$; and for $^{12}$CO (3–2) , from 0.06 to 0.8 Jy/beam.km/s with a step of 0.06 Jy/beam.km/s , i.e 4.2$\sigma$.
  • Figure 4: From top to bottom: Tomographically Reconstructed Distribution (TRD) of $^{12}$CO 2-1, $^{12}$CO 3-2, $^{13}$CO 2-1, C$^{18}$O 2-1 and HCO$^+$ 3-2 : (a) TRD of the observations (b) TRD of the final result of the models (c) difference between the observations and models TRD. For $^{12}$CO (2–1), the residuals range from –2 to 2 K, while for the $^{12}$CO (3–2) transition, they extend from –3 to 2 K. In the case of $^{13}$CO (2–1), the differences are between 0 and 2 K. For C$^{18}$O (2–1), the deviations range from –1 to 2 K, and for HCO$^+$ (3–2), they span from –1 to 3 K. The vertical black line represents the effective resolution and two error bars show the widening of the beam as a function of radius caused by the TRD method. The contours are defined from 4 K to 20 K in steps of 2 K for observational and model maps, and from -3 K to 3 K in steps of 1 K for the residual maps.
  • Figure 5: As in Figure \ref{['fig:jwst']}, overlay of the JWST image from Arulanantham+2024 with contours of the $^{12}$CO (2-1) channel maps at 6.80 km/s and 8.23 km/s illustrating the large vertical extent around the H$_2$ emission. The contours are in step of $3.4\sigma$ and range from 10 to 40 mJy beam$^{-1}$.
  • ...and 3 more figures