Measuring diameters of distant stars

One more indicator that gamma-ray telescopes, and their scientists, are more than meets the eye.

High angular resolution observations at optical wavelengths provide valuable insights into stellar astrophysics. It also enables direct measurements of fundamental stellar parameters and the probing of stellar atmospheres, circumstellar disks, the elongation of rapidly rotating stars, and the pulsations of Cepheid variable stars.

In a new study, scientists implemented a stellar intensity interferometry system developed for the four VERITAS imaging atmospheric Cherenkov telescopes to measure the angular diameter of the two sub-milliarcsecond stars Canis Majoris and Orionis.

Originally demonstrated for the first time 50 years ago, the Stellar Intensity Interferometry technique could be a secondary use for other gamma-ray observatories as well, including the upcoming Cherenkov Telescope Array (CTA).

Nolan Matthews from the University of Utah said, “Interferometry has been widely successful in achieving the angular resolution needed to resolve stars spatially, and we’ve demonstrated the capability to perform optical intensity interferometry measurements with an array of many telescopes that in turn will help to improve our understanding of stellar systems.”

For the most part, the VERITAS telescopes monitor the sky for faint blue flashes of Cherenkov light that are delivered when gamma rays from the cosmos hit Earth’s atmosphere. In any case, these perceptions are constrained to dark moonless hours. The team used time during which VERITAS can’t perform its normal observations in December 2019.

Principal Investigator David Kieda from the University of Utah said, “Modern electronics allow us to combine light signals from each telescope computationally. The resulting instrument has the optical resolution of a football-field-sized reflector. This is the first demonstration of the original Hanbury Brown and Twiss technique using an array of optical telescopes.”

Scientists observed both stars for several hours. The measurements resulted in angular diameters of 0.523 milliarcseconds for Beta Canis Majoris and 0.631 milliarcseconds for Epsilon Orionis.

DESY scientist Tarek Hassan who was involved in the analysis of the VERITAS measurements, said, “The measured values for both stars are in good agreement with previous measurements with the same technique made with the Narrabri telescopes in the 1970s.”

Using modern electronics, scientists have proven that dozens of telescopes could be combined. This could prove an exciting option for the future Cherenkov Telescope Array. It will be the world’s most astronomical gamma-ray observatory. CTA will feature gamma-ray telescopes in three size classes, DESY is responsible for the medium-sized telescopes.

Hassan said“CTA will employ up to 99 telescopes with kilometer baseline in the southern hemisphere and 19 telescopes with several hundred-meter baselines in the Northern hemisphere. Performing Stellar Intensity Interferometry measurements with the future CTA would allow us to study stars with unparalleled angular resolution.”

The team led by astronomers from the Harvard & Smithsonian Center for Astrophysics (CfA) and the University of Utah and including scientists from DESY.

Journal Reference:
  1. A. U. Abeysekara et al. Demonstration of stellar intensity interferometry with the four VERITAS telescopes, Nature Astronomy (2020). DOI: 10.1038/s41550-020-1143-y

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