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${}^{12}$CO Ro-vibrational Spectroscopy of AB Aurigae -- A Potential Point Source is Present

Janus Kozdon, Jeffrey Fung, Sean D. Brittain, Stanley Jensen, Josh Kern, Cory Padgett, Yasuhiro Hasegawa

Abstract

The Herbig Ae star AB Aurigae hosts a vast, low-inclination protoplanetary disk that exhibits a plethora of substructures, including the protoplanet candidate AB Aur b. We present M-band spectroscopic data taken with NASA IRTF from Feb 2024 covering multiple position angles that captured emission from an off-centered, low temperature, and compact source. Analysis of the ${}^{12}$CO $ν=$1-0 low-J ro-vibrational emission line profiles and spectroastrometric signals localizes the source at around an orbital radius of 65 au and a position angle of 143$^\circ$. These coordinates are distinctly different from those of AB Aur b, which was not detected. Although there is no obvious explanation for the detected source, if we assume it was a circumplanetary disk, then its maximum temperature would be about 550 K and its maximum radius would be about 5 au. Our results alludes to a previously unknown companion that may be residing in the AB Aurigae system.

${}^{12}$CO Ro-vibrational Spectroscopy of AB Aurigae -- A Potential Point Source is Present

Abstract

The Herbig Ae star AB Aurigae hosts a vast, low-inclination protoplanetary disk that exhibits a plethora of substructures, including the protoplanet candidate AB Aur b. We present M-band spectroscopic data taken with NASA IRTF from Feb 2024 covering multiple position angles that captured emission from an off-centered, low temperature, and compact source. Analysis of the CO 1-0 low-J ro-vibrational emission line profiles and spectroastrometric signals localizes the source at around an orbital radius of 65 au and a position angle of 143. These coordinates are distinctly different from those of AB Aur b, which was not detected. Although there is no obvious explanation for the detected source, if we assume it was a circumplanetary disk, then its maximum temperature would be about 550 K and its maximum radius would be about 5 au. Our results alludes to a previously unknown companion that may be residing in the AB Aurigae system.
Paper Structure (15 sections, 18 equations, 12 figures)

This paper contains 15 sections, 18 equations, 12 figures.

Table of Contents

  1. Introduction
  2. Observations and Reductions
  3. Results
  4. High-J Lines vs. Low-J Lines
  5. Hydrogen Line
  6. Emission Line Profile Analysis
  7. Protoplanetary Disk Model (High-J Fit)
  8. Point Source Model (Low-J Fit)
  9. Circumplanetary Disk Model (Low-J Fit)
  10. Hydrodynamic Simulation
  11. Discussion
  12. It is not AB Aur b
  13. Summary and Conclusions
  14. Acknowledgments
  15. The Multigrid Setup

Figures (12)

  • Figure 1: Schematic of the slit observations (teal) presented in Table \ref{['tab:epochs']} (Sec. \ref{['sec:OBS']}). The overlay uses the CHARIS image from Currie2022 to illustrate the attained coverage. This slit arrangement allows for the analysis of emission from outer disk sources (Sec. \ref{['sec:LINES_LO']}), like the protoplanet candidate AB Aur b.
  • Figure 2: The average spectrum of the observations (Table \ref{['tab:epochs']}; Sec. \ref{['sec:OBS']}). The CO 5=1-0 lines (red) and the hydrogen Pf$\beta$ transition (blue) are analyzed in Sec. \ref{['sec:RESULTS']}). A select few CO 5=2-1 lines (brown) are labeled and their presence indicates a high temperature (Sec. \ref{['sec:LINES_HI']}). Also pointed out is the Hydrogen Hu$\epsilon$ transition.
  • Figure 3: Rotational diagram for the transitions labeled in Fig. \ref{['fig:Spectrum']}. In order the data points are R(2), P(3), P(8), P(18), P(19), P(25), P(26) and P(27). If the observed fluxes originates from an optically thin, isothermal source then the data points should follow a line, which it doesn't (Sec. \ref{['sec:RESULTS']}). This is an indication that the emission from AB Aur's protoplanetary disk has a gradient in optical depth and temperature.
  • Figure 4: The PA averages of the emission line profiles (bottom) and spectroastrometric signals (top) from AB Aur's high-J (left) and low-J (right) ${}^{12}$CO ro-vibrational 5 = 1-0 transitions (Sec. \ref{['sec:high_vs_low']}). Variable emission between PAs was captured in the low-J line profiles as well as a spatial offset in the spectroastrometric signals. This behaviour is not mirrored in the high-J equivalent and that can be an indication of a low-temperature substructure being present in the protoplanetary disk (Sec. \ref{['sec:LINES_LO']} and Sec. \ref{['sec:LINES_LOCPD']}). Also, the high-J line profiles are used for determining the thermophysical environment of the protoplanetary disk's emitting surface layer (Sec. \ref{['sec:LINES_HI']}).
  • Figure 5: The Hydrogen Pf$\beta$ transition of each observing epoch (Table \ref{['tab:epochs']}; Sec. \ref{['sec:RESULTS']}). No stellar variability was detected during the 1 week span of observations. The average profile (green) is utilized for determining the velocity spread and calculating a stellar accretion rate in Sec. \ref{['sec:hydrogen_transition']}. The dashed lines are where external features, like telluric corruption and CO lines, are present and were masked over via linear interpolation.
  • ...and 7 more figures