# The ALMA view of MP Mus (PDS 66): a protoplanetary disk with no visible gaps down to 4 au scales

**Authors:** Á. Ribas, E. Macías, P. Weber, S. Pérez, N. Cuello, R. Dong, A. Aguayo, C. Cáceres, J. Carpenter, W. R. F. Dent, I. de Gregorio-Monsalvo, G. Duchêne, C. C. Espaillat, P. Riviere-Marichalar, M. Villenave  
**arXiv:** [2302.11592](https://arxiv.org/abs/2302.11592)  
**Date:** 2023-02-22  
**Summary page:** https://www.summarizepaper.com/en/paper/2302.11592v1/

## Key points

- Protoplanetary disk around MP Mus observed using ALMA at different wavelengths and resolutions
- Disk appears smooth down to the resolution limit of the observations at 4 au, which is unusual for disks observed at comparable spatial scales
- Dust disk has a radius of approximately 60±5 au, a dust mass of $0.14_{-0.06}^{+0.11} M_{\rm Jup}$, and a millimeter spectral index less than two in the inner region within a radius of about 30 au
- Several molecular gas lines detected extending up to approximately130±15 au, similar to small grains traced by scattered light observations
- Gas mass range between $0.1-1 M_{\rm Jup}$, implying a gas-to-dust ratio between one to ten for the system
- Dynamical stellar mass measurement yields $M_{\rm dyn}=1.30±0.08 M_\odot$, while an age estimate for the system is derived as being between seven to ten million years old.
- Lack of substructures places upper limits to the masses of planets in the disk as low as 2 $M_\oplus$-0.06 $M_{\rm Jup}$ at radii greater than 40 au
- No millimeter emission detected beyond 60 au which suggests that any gap seen in scattered light between 30-80 au could be due to shadowing from an inner puffed up disk rather than an actual decrease in density within this region
- ALMA observations provide invaluable insights into understanding protoplanetary disks around young solar analogs and shed light on planet formation processes

## Summary

The protoplanetary disk around MP Mus (PDS 66), a young solar analog located at a distance of approximately 100 parsecs, has been observed using the Atacama Large Millimeter/submillimeter Array (ALMA) at wavelengths of 0.89 mm, 1.3 mm, and 2.2 mm with angular resolutions of approximately 1", 0.05", and 0.25", respectively. The observations provide unprecedented detail and sensitivity in probing the dust and gas in the system. The disk appears smooth down to the resolution limit of the observations at 4 au, which is in contrast with most disks observed at comparable spatial scales. The dust disk has a radius of approximately 60±5 au, a dust mass of $0.14_{-0.06}^{+0.11} M_{\rm Jup}$, and a millimeter spectral index less than two in the inner region within a radius of about 30 au, suggesting optically thick emission from grains with high albedo in this region. Several molecular gas lines are also detected extending up to approximately130±15 au, similar to small grains traced by scattered light observations. Comparing the fluxes of different CO isotopologues with previous models yields a gas mass range between $0.1-1 M_{\rm Jup}$, implying a gas-to-dust ratio between one to ten for the system. A dynamical stellar mass measurement yields $M_{\rm dyn}=1.30±0.08 M_\odot$, while an age estimate for the system is derived as being between seven to ten million years old. The survival of large grains in an evolved disk without gaps or rings is surprising; it is possible that existing substructures remain undetected due to optically thick emission at 1.3 mm or that small structures may still remain unresolved with the current observations. Based on simple scaling relations for gap-opening planets and gap widths, this lack of substructures places upper limits to the masses of planets in the disk as low as 2 $M_\oplus$-0.06 $M_{\rm Jup}$ at radii greater than 40 au; furthermore, there is no millimeter emission detected beyond 60 au which suggests that any gap seen in scattered light between 30-80 au could be due to shadowing from an inner puffed up disk rather than an actual decrease in density within this region . The ALMA observations provide invaluable insights into understanding protoplanetary disks around young solar analogs; they shed light on planet formation processes and inform future studies in this field significantly more accurately than before these data were available..