Several interstellar environments produce anomalous microwave emission (AME), with brightness peaks at tens-of-gigahertz frequencies1. The emission’s origins are uncertain; rapidly spinning nanoparticles could emit electric-dipole radiation2, but the polycyclic aromatic hydrocarbons that have been proposed as the carrier are now found not to correlate with Galactic AME signals3,4. The difficulty is in identifying co-spatial sources over long lines of sight. Here, we identify AME in three protoplanetary disks. These are the only known systems that host hydrogenated nanodiamonds5, in contrast with the very common detection of polycyclic aromatic hydrocarbons6. Using spectroscopy, the nanodiamonds are located close to the host stars, at physically well-constrained temperatures7. Developing disk models8, we reproduce the emission with diamonds 0.75–1.1 nm in radius, holding ≤1–2% of the carbon budget. Ratios of microwave emission to stellar luminosity are approximately constant, allowing nanodiamonds to be ubiquitous, but emitting below the detection threshold in many star systems. This result is compatible with the findings of similar-sized diamonds within Solar System meteorites9. As nanodiamond spectral absorption is seen in interstellar sightlines10, these particles are also a candidate for generating galaxy-scale3 AME.
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The NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities. Infrared spectra are presented from the processed data archives of ESA’s ISO and NASA’s Spitzer Space Telescope. A. Avison at JBCA reduced the Atacama Large Millimeter/submillimeter Array observations of HD 97048. A.M.M.S. gratefully acknowledges support from the European Research Council under grant ERC-2012-StG-307215 LODESTONE. We thank the staff of the Lord’s Bridge Observatory for their assistance in the operation of the AMI. The AMI is supported by the University of Cambridge and the STFC.
The authors declare no competing interests.
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Greaves, J.S., Scaife, A.M.M., Frayer, D.T. et al. Anomalous microwave emission from spinning nanodiamonds around stars. Nat Astron 2, 662–667 (2018). https://doi.org/10.1038/s41550-018-0495-z
Astrophysics and Space Science (2020)