A Complete ALMA Map of the Fomalhaut Debris Disk (1705.05867v1)

Abstract: We present ALMA mosaic observations at 1.3 mm (223 GHz) of the Fomalhaut system with a sensitivity of 14 $\mu$Jy/beam. These observations provide the first millimeter map of the continuum dust emission from the complete outer debris disk with uniform sensitivity, enabling the first conclusive detection of apocenter glow. We adopt a MCMC modeling approach that accounts for the eccentric orbital parameters of a collection of particles within the disk. The outer belt is radially confined with an inner edge of $136.3\pm0.9$ AU and width of $13.5\pm1.8$ AU. We determine a best-fit eccentricity of $0.12\pm0.01$. Assuming a size distribution power law index of $q=3.46\pm 0.09$, we constrain the dust absorptivity power law index $\beta$ to be $0.9<\beta<1.5$. The geometry of the disk is robustly constrained with inclination $65.!!^\circ6\pm0.!!^\circ3$, position angle $337.!!^\circ9\pm0.!!^\circ3$, and argument of periastron $22.!!^\circ5\pm4.!!^\circ3$. Our observations do not confirm any of the azimuthal features found in previous imaging studies of the disk with HST, SCUBA, and ALMA. However, we cannot rule out structures $\leq10$ AU in size or which only affect smaller grains. The central star is clearly detected with a flux density of $0.75\pm0.02$ mJy, significantly lower than predicted by current photospheric models. We discuss the implications of these observations for the directly imaged Fomalhaut b and the inner dust belt detected at infrared wavelengths.

Insights into the Fomalhaut Debris Disk from ALMA Observations

The paper presents an in-depth analysis of the Fomalhaut system, specifically focusing on its debris disk, as observed by the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.3 mm (223 GHz). With high sensitivity observations yielding a millimeter map of the dust continuum emission, significant insights have been obtained regarding the geometric and physical properties of the outer disk.

Observational Findings and Modeling

The primary result includes the complete imaging of the outer debris disk, revealing a radially confined, narrow, and eccentric ring. Through the application of Markov Chain Monte Carlo (MCMC) modeling, the authors determined key parameters of the disk, including an inner edge radius of $136.3\pm0.9$ AU and a width of $13.5\pm1.8$ AU. The disk's best-fit eccentricity was established as $0.12\pm0.01$, indicating pronounced eccentricity, a critical parameter in understanding the disk's dynamics and potential planetary interactions.

Moreover, a notable outcome of the paper is the detection of apocenter glow, characterized by a brightness asymmetry due to increased surface density at apocenter. This observational evidence aligns with theoretical predictions about the influence of orbital eccentricity on particle distribution within debris disks. Flux measurements established a power law index for the dust size distribution as $q=3.46\pm0.09$, which is consistent with previous findings, and provided constraints on the dust absorptivity index, $\beta$, within the range $0.9<\beta<1.5$.

Implications and Discussions

Data revealed no significant azimuthal features previously reported by HST and SCUBA, suggesting that certain features may be due to shadowing effects rather than true density variations. The implications for understanding the spatial distribution of solid bodies and underlying planetary dynamics are profound, as detecting these structures helps infer the presence and characteristics of unseen planetary objects.

The paper also assessed the implications for Fomalhaut b, previously imaged through Hubble Space Telescope (HST), concluding that the upper limit on dust mass from non-detection at millimeter wavelengths aligns with the hypothesis of Fomalhaut b being a transient dust concentration rather than a massive planet.

Additionally, the discrepancy between measured millimeter wavelength fluxes and predicted photospheric models for the central star suggests interesting stellar atmospheric phenomena, such as potential chromospheric activity, which could significantly impact interpretations of infrared excess associated with the system's inner dust belt.

Conclusion and Future Work

The detailed and uniform sensitivity map of the Fomalhaut debris disk provided by ALMA has clarified many facets of its structure and complex dynamics. Going forward, increased resolution and sensitivity in millimeter and submillimeter observations could uncover finer details of the debris disk, constrain particle dynamics more tightly, and help resolve the role of potential planets in shaping these structures.

Ultimately, this paper enriches our understanding of debris disks (particularly the interplay of planetary formation) and presents a methodological approach that can be useful for observing other similar systems. Continued exploration using ALMA and similar arrays promises further breakthroughs in planetary system formation and the evolution of circumstellar environments, contributing to a wider comprehension of planetary disks both near and distant.