Spiral arms in the proto-planetary disc HD100453 detected with ALMA: evidence for binary-disc interaction and a vertical temperature gradient (1911.00518v1)

Abstract: Scattered light high-resolution imaging of the proto-planetary disc orbiting HD100453 shows two symmetric spiral arms, possibly launched by an external stellar companion. In this paper we present new, sensitive high-resolution ($\sim$30 mas) Band 7 ALMA observations of this source. This is the first source where we find counterparts in the sub-mm continuum to both scattered light spirals. The CO J=3-2 emission line also shows two spiral arms; in this case they can be traced over a more extended radial range, indicating that the southern spiral arm connects to the companion position. This is clear evidence that the companion is responsible for launching the spirals. The pitch angle of the sub-millimeter continuum spirals ($\sim 6 ^{\circ}$) is lower than the one in scattered light ($\sim 16 ^{\circ}$). We show that hydrodynamical simulations of binary-disc interaction can account for the difference in pitch angle only if one takes into account that the midplane is colder than the upper layers of the disc, as expected for the case of externally irradiated discs.

• The paper identifies symmetric spiral arms in HD100453’s disc using high-resolution ALMA and CO J=3-2 emission data.
• The paper reveals binary-disc interaction by showing the southern spiral extends toward the known M-dwarf companion.
• The paper demonstrates a vertical temperature gradient affecting spiral pitch angles, supported by hydrodynamic simulations.

Overview of the Paper on Spiral Arms in HD100453

The paper conducted by G. P. Rosotti et al. investigates the intriguing spiral structures found in the proto-planetary disc surrounding the star HD100453. Using advanced observational techniques with the Atacama Large Millimeter/submillimeter Array (ALMA), the authors explore the hypothesis that the observed spiral arms are results of interactions with an external stellar companion. This paper combines state-of-the-art hydrodynamic simulations with empirical data from newly acquired high-resolution ALMA observations to advance our understanding of kinetic and thermal processes in proto-planetary disks.

Key Findings and Methodology

The paper's primary contribution is the detailed mapping of spiral structures in both the submillimeter continuum and CO emission spectra, providing new insights into the influence of orbital companions on disk morphology:

1. Observational Data: Utilizing Band 7 ALMA observations, the authors identify two symmetric spiral arms in the HD100453's disc that manifest both in submillimeter continuum and CO J=3-2 emission spectra. These structures corroborate earlier findings from scattered light imaging but also expand upon them by resolving spiral features all the way to the midplane of the disk.
2. Interaction with a Stellar Companion: One of the paper's critical insights is the identification of the southern spiral's continuation towards the star's known M-dwarf companion. This finding strengthens the hypothesis that spiral arms are gravitationally induced by binary interaction, providing a dynamic framework to understand the disc's non-axisymmetric features.
3. Vertical Temperature Gradient Analysis: By comparing the spiral arm structures in various observational wavelengths, the authors verify the existence of a vertical temperature gradient within the disk. The continuum traces are shown to have lower pitch angles compared to the observed scattered light signatures, aligning with theoretical predictions of temperature-driven layer differentiation in proto-planetary discs.
4. Hydrodynamical Simulations: The researchers conducted simulations of binary-disc interactions that align with the observed differential pitch angles, confirming that disc stratification (temperature gradients) influences the propagation and appearance of spiral density waves.

Implications and Future Directions

This research highlights significant theoretical and observational implications for our understanding of star formation and early planet dynamics:

• Gravitational Interaction Models: The confirmation that binary disc interactions can lead to observable spiral patterns in proto-planetary discs adds a vital tool for interpreting disk dynamics and inferring the presence of unseen planetary bodies or stellar companions influencing these structures.
• Disc Thermal Stratification: The paper emphasizes the necessity of accounting for vertical temperature gradients when modeling disc physics. This stratification is crucial for understanding the structure and evolution of proto-planetary discs and their capacity to form planets.
• Future Observational Campaigns: The paper suggests that future observations with higher spatial and spectral resolution could distinctively map these spirals' dynamics, possibly allowing for the precise identification of gravitational perturbers. Furthermore, the high angle of pitch in spirals could serve as an indicator of disc thermal conditions, aiding in the identification of planet-forming environments.

In conclusion, G. P. Rosotti et al.'s work provides pivotal empirical and theoretical underpinnings for the spiral structures in HD100453, marking a significant step forward in utilizing advanced observational tools and computational models to unravel complex proto-planetary disc dynamics. The implications of their findings will foster new lines of inquiry into how such dynamics can signal nascent planetary systems.