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Probing the era of giant collisions: millimeter observations of the HD 166191 system

Kadin Worthen, Christine H. Chen, A. Meredith Hughes, Brandon C. Johnson, Isabel Rebollido, Diego E. Garcia, Jamar Kittling, Carey M. Lisse

TL;DR

This work targets the HD 166191 system to test whether giant collisions in the terrestrial planet zone occur within a disk that may still retain significant primordial gas. By combining SMA (2014) and ALMA Band 7 (2024) observations, the authors detect dust continuum and CO emission while constraining SiO abundance, revealing a compact disk (dust and CO within ≲20 au) and a high CO mass that suggests a gas-rich environment more akin to a transition or evolved protoplanetary disk than a typical debris disk. Radiative-transfer modeling of the dust and CO visibilities indicates both optically thin and modestly optically thick dust configurations can fit the data, with CO modeling yielding Rin ≈ 2.2 au, Rc ≈ 7.1 au, and M_CO ≈ 0.83 M_⊕, though these masses depend on vertical structure and excitation assumptions. The results imply that terrestrial-zone collisions can occur while substantial outer disk gas remains, making HD 166191 a key laboratory for studying the transition from protoplanetary to debris disks and the timing of collisions in planetary system evolution.

Abstract

We present non-simultaneous ALMA band 7 and SMA observations of the HD 166191 disk, which was recently thought to have a collision in its terrestrial planet zone. Both observations detect dust continuum emission and the ALMA observations detect the 12CO J=3-2 line from the circumstellar disk. We do not detect SiO, a potential indicator of giant collisions, but place a limit on the total SiO mass in the system. Unlike previously observed in the infrared, we do not find evidence for variability at millimeter wavelengths when comparing the ALMA continuum observations from 2024 to the pre-collision SMA observations from 2014. We perform modeling of the CO and continuum visibilities and find that both the CO and dust are marginally spatially resolved and are contained to within 20 au from the central star. The modeling of the CO suggests that the outer regions of the disk are gas rich, although further observations are needed to confirm the total gas mass. The evolutionary state of this system has been debated in the literature, and our observations, while not definitive, are generally consistent with the idea that this disk is similar to an evolved protoplanetary or transition/hybrid disk. This could suggest that collisions in the terrestrial planet zone of HD 166191 are occurring while the disk is in a transitional phase, where the inner few au are depleted of gas. This makes HD 166191 an important object for understanding the transition between protoplanetary and debris disks and the stages at which collisions occur.

Probing the era of giant collisions: millimeter observations of the HD 166191 system

TL;DR

This work targets the HD 166191 system to test whether giant collisions in the terrestrial planet zone occur within a disk that may still retain significant primordial gas. By combining SMA (2014) and ALMA Band 7 (2024) observations, the authors detect dust continuum and CO emission while constraining SiO abundance, revealing a compact disk (dust and CO within ≲20 au) and a high CO mass that suggests a gas-rich environment more akin to a transition or evolved protoplanetary disk than a typical debris disk. Radiative-transfer modeling of the dust and CO visibilities indicates both optically thin and modestly optically thick dust configurations can fit the data, with CO modeling yielding Rin ≈ 2.2 au, Rc ≈ 7.1 au, and M_CO ≈ 0.83 M_⊕, though these masses depend on vertical structure and excitation assumptions. The results imply that terrestrial-zone collisions can occur while substantial outer disk gas remains, making HD 166191 a key laboratory for studying the transition from protoplanetary to debris disks and the timing of collisions in planetary system evolution.

Abstract

We present non-simultaneous ALMA band 7 and SMA observations of the HD 166191 disk, which was recently thought to have a collision in its terrestrial planet zone. Both observations detect dust continuum emission and the ALMA observations detect the 12CO J=3-2 line from the circumstellar disk. We do not detect SiO, a potential indicator of giant collisions, but place a limit on the total SiO mass in the system. Unlike previously observed in the infrared, we do not find evidence for variability at millimeter wavelengths when comparing the ALMA continuum observations from 2024 to the pre-collision SMA observations from 2014. We perform modeling of the CO and continuum visibilities and find that both the CO and dust are marginally spatially resolved and are contained to within 20 au from the central star. The modeling of the CO suggests that the outer regions of the disk are gas rich, although further observations are needed to confirm the total gas mass. The evolutionary state of this system has been debated in the literature, and our observations, while not definitive, are generally consistent with the idea that this disk is similar to an evolved protoplanetary or transition/hybrid disk. This could suggest that collisions in the terrestrial planet zone of HD 166191 are occurring while the disk is in a transitional phase, where the inner few au are depleted of gas. This makes HD 166191 an important object for understanding the transition between protoplanetary and debris disks and the stages at which collisions occur.

Paper Structure

This paper contains 15 sections, 6 equations, 15 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Observations and Data Reduction
  3. SMA Observations
  4. ALMA Observations
  5. Results
  6. Dust
  7. SiO non-detection and CO Emission
  8. Radiative Transfer Modeling
  9. Dust Modeling
  10. CO Modeling
  11. Discussion
  12. What the SiO non-detection implies about the collision
  13. What is the evolutionary state of the system?
  14. How the collision fits into this system
  15. Conclusions

Figures (15)

  • Figure 1: 1.33 mm continuum SMA image of HD 166191. Contours are drawn in levels of 3 and 6 $\sigma$. The image is oriented such that north is up and east is left.
  • Figure 2: Continuum image of HD 166191 at 866 $\mu$m. The white ellipse in the bottom left corner shows the synthesized beam. The contours shown are at levels of 5, 15, 30, 60, 80, and 100$\sigma$ where $\sigma=$ 70 $\mu$Jy/beam.
  • Figure 3: Left: moment 0 map of the SiO cube integrated from -50 to 50 km/s. The red contours show the dust continuum and are at the same levels as in Figure \ref{['fig:Cont_image']}. There is no clear detection of SiO at the position of the continuum detection. Right: extracted spectrum from the SiO cube showing a non-detection of SiO emission towards HD 166191.
  • Figure 4: Left: moment 0 map of the CO J=3-2 line from the HD 166191 system. Contours are at levels of 3, 6, 10, 15, 25, 40, 50, 75, and 90 $\sigma$ of the moment 0 map where $\sigma=0.01$ Jy km/s/beam. The white oval represents the synthesized beam. Middle: moment 1 map showing the CO velocity around HD 166191. The contours are the same as the left figure. Right: Extracted CO spectra of HD 166191 showing a double peaked structure indicative of gas in Keplerian velocity around the star.
  • Figure 5: Moment 0 map of the entire field of view of the HD 166191 observations showing diffuse emission at the same velocities as the CO in the disk. The bright emission at the center of the image is from the disk. This moment 0 map is integrated from velocities of 7.5 to 11 km/s.
  • ...and 10 more figures