Count the Photons To “Zoom” On The Stars (Planetary Science)

A group of researchers from the National Institute of Astrophysics and the University of Padua demonstrated the feasibility of a method for the study of bright stars by counting one by one the photons coming from the star Vega with the two telescopes of 1.8 and 1, 2 meters of the Asiago Observatory. This technique can be used in the future by observers such as Astri Mini-Array to capture details on stellar surfaces with a resolution never reached in the optical band.

Stars are huge spheres of red-hot gas with diameters ranging from hundreds of thousands to billions of kilometers but, due to the immense distances involved, taking a picture of them is an exercise to the limit of the impossible even for the most powerful telescopes. An experiment carried out by networking the two telescopes of the Asiago Observatory , in the province of Vicenza, has now demonstrated the feasibility of a new technique that can be used in the future to circumvent this limit, revealing details never seen before on the surface of bright stars. .

“Apart from a few exceptional cases, it is not possible to photograph the stars directly because they are too small even for the resolution of the largest optical telescopes available today”, explains Luca Zampieri of the National Institute of Astrophysics, first author of the article published in Monthly Notices of the Royal Astronomical Society . “Using multiple telescopes working together, as is done with the phase interferometry technique, allows you to overcome this limitation and obtain a higher angular resolution, which improves the further away the telescopes are. In our case, we used a similar but slightly different technique: intensity interferometry.

The pilot experiment, carried out in 2019, used Vega , the brightest star in the Lyra constellation , as its target , where it shows off on summer nights right above our heads, near the zenith. Just 25 light years away, Vega is the fifth brightest star in the night sky and the second in the northern hemisphere: an excellent test bed for this technique, which requires very bright sources.

The Aqueye + instrument (Asiago Quantum Eye) mounted on the Copernicus Telescope. Credits: Inaf

L ‘ interferometry intensity , which was introduced in the fifties by astronomers Robert Hanbury Brown and Richard Q. Twiss , is a technique that is based on the principle of interference between two waves: observing a star with two distant telescopes and studying the correlation of the intensity of two signals, the angular size of the star in the sky can be measured. The team led by Zampieri has introduced an important innovation that exploits the quantum properties of light: instead of measuring the intensity of the signal with a conventional instrument, the researchers recorded the time of arrival of each single photon at the two telescopes.. This was possible thanks to the avant-garde instruments Aqueye + (Asiago Quantum Eye) and Iqueye (Italian Quantum Eye), developed starting from 2005 by Luca Zampieri of INAF and Giampiero Naletto and Cesare Barbieri of the University of Padua, also co-authors of the new study. The two instruments are now mounted respectively on the Copernicus Telescope (1.82 meters in diameter) and on the Galileo Telescope (1.22 meters in diameter) of the Asiago observatory, which are 4 kilometers apart.

“With this technique, it can be established whether the photons travel very close together and arrive at the two telescopes within a very short interval of time,” explains Zampieri. “We are able to determine the arrival of a photon at the telescope with an accuracy of 400 picoseconds, that is less than half a billionth of a second: in this tiny interval of time, light travels about 12 centimeters, and this length translates in the accuracy we need on the position of the two instruments ».

The Iqueye instrument (Italian Quantum Eye) mounted on the Galileo Telescope. Credits: Inaf

The Asiago experiment is the first example of a measurement of this type carried out with the acquisition and storage of the arrival times of the photons received by the two telescopes independently, and carrying out the analysis afterwards. “Our measurement does not yet allow us to construct an image”, explains Zampieri, “but it shows that with these instruments, with this setup , with this temporal accuracy, saving the data and analyzing them offline , the method works”.

This feasibility study opens up new perspectives for stellar imaging : in the future, by moving from two telescopes to an array with elements separated by about one kilometer, it will be possible to image bright stars with unprecedented resolution . A possible application could involve Astri Mini-Array , a project developed by INAF which involves the construction of a battery of nine Cherenkov telescopes in Tenerife, in the Canary Islands. Cherenkov telescopes study the universe through gamma raysat very high energy, but their technology is that of an optical telescope: in fact they hunt for emission in the visible wavelengths produced by gamma rays that interact with the earth’s atmosphere. On full moon nights, when this type of observation is not possible, the array of telescopes could be aimed at bright stars to observe details never seen on their surface.

“By applying this technique to an observatory like Astri Mini-Array”, Zampieri continues, “we expect to be able to solve an object of the size of Jupiter on the surface of a star that is at the distance of Vega: not a planet like Jupiter, however, because we need a very high brightness contrast, but we could identify bright spots on the surface of a star, caused for example by a magnetic hotspot , which would be extremely interesting. ‘

Featured image: Top view of the two observation stations located in Asiago: top left, the Cima Pennar station, which houses the Galileo Telescope, and bottom right, the Cima Ekar station, which houses the Copernicus Telescope. Credits: National Geoportal of the Ministry of the Environment

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