ALMA Demonstrates Highest Resolution Yet

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This time, the Band-to-band (B2B) method has shown that the highest resolution can be achieved with ALMA. In the B2B method, atmospheric fluctuations are compensated by observing a nearby calibrator in low-frequency radio waves, while the target is observed in high-frequency radio waves. The inset image at the top right shows the ALMA image of R Leporis at the highest resolution of 5 milliarcseconds. Submillimeter wave emissions from the star’s surface are shown in orange and hydrogen cyanide maser emissions at 891 GHz are shown in blue. The top left inset image shows a previous observation of the same star using a different array configuration with less antenna spacing and without the B2B method, resulting in a resolution of 75 milli-arcseconds. The previous resolution is too coarse to specify the positions of each of the two emission components. (Credit: ALMA (ESO/NAOJ/NRAO), Y. Asaki et al.)

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ALMA (Atacama Large Millimeter/submillimeter Array) has demonstrated the highest resolution yet of observations of an old star. The observations show that the star is surrounded by a ring-shaped gas structure and that gas escapes from the star into the surrounding space. Future observations at the newly demonstrated high resolution are expected to elucidate not only the end of a star’s life, but also the beginning, when planets are still forming.

ALMA is a radio interferometric array telescope, in which individual antennas work together to observe a celestial body. The resolution of ALMA, the ability to see small details, is determined by the maximum distance between the antennas and the frequency of the observed radio waves. In this study, an international team, consisting mainly of astronomers from the Joint ALMA Observatory, the National Astronomical Observatory of Japan (NAOJ), the National Radio Astronomy Observatory and the European Southern Observatory, used ALMA’s maximum antenna distance of 16 km and receivers with the highest frequency (known as Band 10, up to 950 GHz) to achieve the best possible resolution. Taking ALMA’s resolution to new limits also required a new calibration technique to correct fluctuations in the Earth’s atmosphere above the antennas. The calibration technique the team used, known as “band-to-band (B2B),” was originally tested in the 1990s at NAOJ’s Nobeyama Radio Observatory for future millimeter/submillimeter interferometers.

For their demonstration observations, the team chose R Leporis, a star in the final stages of stellar evolution, located about 1,535 light-years from Earth. The team managed to observe R Leporis at the best resolution ever, 5 milli-arcseconds, which is the equivalent of being able to see one human hair from two and a half miles away. The observations show the surface of the star and a ring of gas around the star. The team also confirmed that gas escapes from the star into surrounding space.

This recently demonstrated high-resolution capability can now be applied to young stars with protoplanetary disks in which planets are forming. Future high-resolution observations will provide new insights into how planets, especially Earth-like planets, form.

Detailed item(s)

ALMA achieves the highest resolution yet – Combining ALMA’s highest frequency Band 10 receivers and an array configuration with a baseline length of up to 16 km


Release information

Researcher(s) involved in this release

  • Yoshiharu Asaki (Associate Professor at the National Astronomical Observatory of Japan)

Coordinated release organization(s)

  • National Astronomical Observatory of Japan
  • Joint ALMA Observatory


  • Yoshiharu. Asaki et al. “ALMA High-frequency Long Baseline Campaign in 2021: Highest Angular Resolution Submillimeter Wave Images for the Carbon-Rich Star R Lep,” in The Astrophysical Journal, DOI: 10.3847/1538-4357/acf619
  • Luke T. Maud et al. “ALMA high-frequency long-baseline campaign in 2019: band 9 and 10 in-band and band-to-band observations using ALMA’s longest baselines” in The Astrophysical Journal Supplement Series, DOI: 10.3847/1538- 4365/acd6f1

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