Fig.1: Band-to-band (B2B) method tested to achieve the highest resolution with ALMA. With the B2B method, a calibrator located close to a target source is observed at a low frequency, while the target source is observed at a high frequency: observation errors of the target source are compensated for by using calibration data. The top right image encircled with the white line shows the ALMA image of R Leporis with the highest resolution of 5 milli-arcsec achieved using the Band 10 receivers and an array configuration with a maximum baseline length of 16 km with the B2B method. Orange and blue colors represent submillimeter-wave emission from the stellar surface and the hydrogen cyanide maser at 891 GHz. The top left image encircled with the white line shows the same but using another array configuration with a maximum baseline length of 1 km without the B2B method, resulting in a resolution of 75 milli-arcsec. The resolution is too coarse to specify the positions of each of the two emission components. (Credit: ALMA (ESO/NAOJ/NRAO), Y. Asaki et al.)

Background
ALMA is a millimeter/submillimeter array that consists of more than 50 parabolic antennas. Its ability to spatially resolve the observed sources is determined primarily by a combination of two factors—the observing frequency of electromagnetic waves and the maximum distance between the antennas (maximum baseline length). This means ALMA’s highest resolution is achieved when the array is configured to its maximum extent of 16 km baselines and the receivers have the highest observing frequency in Band 10 (up to 950 GHz). However, observations simultaneously satisfying these two conditions are extremely rigorous in terms of selection of weather conditions and observation error correction. In particular, observations at the highest observing frequencies required implementation of new observation techniques for the error correction.

Achievement
In a radio interferometry system like ALMA, signals received at each antenna are synthesized in the correlator. Since the signals from a celestial object (observation target source) pass through Earth’s atmosphere, they are affected by atmospheric fluctuations as observation errors. Therefore, in order to observe with the interferometer at its full performance, it is important to mitigate the influence from atmospheric fluctuations in the interferometric data of the target source. ALMA corrects the observation errors by alternating observations of the target and a nearby source (calibrator) and transferring calibration data from the calibrator to the target. For the calibrator, ALMA usually observes a compact quasar which is bright enough at millimeter wavelengths and whose structure is too compact to be resolved even with ALMA’s angular resolution. However, the higher the observing frequency, the fainter quasars become, making it difficult to use one as a calibrator at the Band 10 frequencies. Besides, the observation error is almost proportional to the observing frequency, so that the errors become larger at higher observing frequencies. Especially for long baselines, the atmosphere above the antennas is different, so the atmospheric fluctuations affect the observation errors more, therefore finding a close calibrator is more important at higher observing frequencies in long baseline array configurations. For the above reasons, it is necessary at high observing frequencies to overcome the difficulties in finding a nearby suitable calibrator close to the target source in ALMA observations with a combination of the highest frequency Band 10 receivers and the longest baselines. The team has been implementing and verifying an observation technique called “band-to-band (B2B),” in which a calibrator is observed using lower frequency band receivers (for example, Band 7 at 300 GHz), where more quasars are bright enough to use, to correct the observation errors of the target source observed with higher frequency band receivers (for example, Band 10 at 900 GHz) (Fig. 1). This method was originally developed in the 1990s at Nobeyama Radio Observatory of NAOJ for future millimeter/submillimeter interferometers. In ALMA’s designing and construction phases, hardware and preliminary software for B2B were considered and implemented. In the course of the B2B verification, various optimizations have been also implemented, such as a short time switching sequence between the target source and the calibrator to acquire the data before the atmospheric fluctuations fully affect the interferometric data of the target source. The team has been repeatedly testing the application of the B2B method for ALMA and successfully achieved a resolution of 7 milli-arcsec in 2020 using the Band 9 receivers in an array configuration with baselines of up to 14 km.

In the technical demonstration test with the Band 10 receivers and an array configuration with the 16 km longest baselines, the team first observed the compact quasar J2229-0832 and confirmed that the quasar could be imaged as a point source with a resolution of 5 milli-arcsec (this is the equivalent of being able to see a single human hair 4 km away) as expected. Following this success, the team observed R Leporis, a star in the Milky Way that is at the final stage of stellar evolution, approximately 1535 light-years away from Earth, with a combination of the Band 10 receivers and an array configuration with the maximum 16 km baselines, then successfully captured the submillimeter-wave emission from the stellar surface and the gas radiating a bright hydrogen cyanide maser (naturally-occurring radio analogue of lasers) with a resolution of 5 milli-arcsec. The observed image reveals that the stellar surface is surrounded by a ring-like structure of gas traced by the hydrogen cyanide maser and that the gas from the star is escaping to circumstellar space. By comparing the new results with the image obtained by a previous ALMA observation with the Band 10 receivers but in another array configuration with baselines up to 1 km with a resolution of 75 milli-arcsec, you can understand how tremendous the impact of the highest angular resolution achieved in this technical test is (Fig. 1).

Significance of this research achievement
ALMA has so far captured multiple protoplanetary disks with a high angular resolution, revealing the universality and diversity of these structures. It also succeeded in imaging the protoplanetary disk closest to Earth down to a separation from the star of as small as the size of Earth’s orbit (Fig. 2). Meanwhile, the number of protoplanetary disks that can be resolved down to the size of Earth’s orbit is extremely limited, with only about five objects.
It is notable that this research has improved the angular resolution, making observations at 5 milli-arcsec possible. It will enable detailed observation of distant objects and increase the number of objects that can be resolved down to the size of Earth’s orbit by approximately 100 times (Fig. 3).
By observing a large number of protoplanetary disks and imaging their detailed structures in a wide range of areas from regions near the central star (the size of Earth’s orbit) to distant regions, we can clarify the growth location of dust, which is a planet forming material, and the initial orbital distribution of the planets over a wide area of the disk. Research on the planetary formation process of numerous protoplanetary disks will lead to an understanding of the origin of the diversity of planetary systems. Furthermore, it is expected to detect signs of planetary formation, not only indicative of gas giant planets but also of rocky and icy planets like Earth, and to unveil the primordial physical environment of planets.

Fig.2: The protoplanetary disk around TW Hydrae imaged in optical (observed with the Hubble Space Telescope), near-infrared (Subaru Telescope), and radio (ALMA). In the optical and near-infrared images captured with the Hubble Space Telescope and the Subaru Telescope, we can see the dust disk but cannot see the disk region in the vicinity of the central star because the light from the central star, which is too bright compared to the disk, needs to be blocked. In this regard, ALMA is suitable for observing the region down to the size of Earth’s orbit. (Credit: NAOJ, ALMA (ESO/NAOJ/NRAO), Tsukagoshi et al., Andrews (Harvard-Smithsonian CfA))

Fig.3: The number of protoplanetary disks as a function of distance from Earth. Previous ALMA observations were able to achieve an angular resolution corresponding to Earth’s orbit only for a few objects within about 300 light-years from Earth. The improved angular resolution will increase the number of objects by approximately 100 times. (Credit: NAOJ)

These results will be published as
(1) Y. 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 on November 15, 2023 (DOI: 10.3847/1538-4357/acf619) and were published as
(2) L. 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 on July 20, 2023 (DOI: 10.3847/1538-4365/acd6f1).

These works were supported by JSPS KAKENHI grant JP16K05306 and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement No. 883867, project EXWINGS).

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI). ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

Links

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National Radio Astronomy Observatory

European Southern Observatory