A CRUCIAL TEST FOR ASTRONOMICAL SPECTROGRAPH CALIBRATION WITH FREQUENCY COMBS

How common are planets that resemble Earth? How different can planetary systems be? Knowing what is happening on planets orbiting other stars - specially, planets similar to ours - has become a viable expectation with the development of a Laser Frequency Combs, or LFC, for astronomy applications.

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The LFC derives from studies on high-precision spectroscopy (Udem Th., Holzwarth R., Hansch T. W., 2002, Nat, 416, 233) - a 2005 Nobel laureate in physics research - and in practice constitutes a radiation spectrum generated by a laser. The projection forms a series of thin, discrete and equally spaced lines, resembling the image of a hair comb. The central idea for the LFC applications in astronomy is to use it as a kind of ruler, with which you can measure astronomical spectra with unprecedented precision.

Currently, the most widely used method for finding exoplanets is to measure the variation in the speed with which a star moves relative to the observer - the Doppler effect of light, as it were (Mayor, M. & Queloz, D., 1995, Nat, 378, 355 – a 2019 Nobel laureate in physics research). These variations are caused by the gravitational interaction between the star itself and the planets in its orbit. Based on this data, scientists get information about the presence and characteristics of the planets even without being able to observe them directly. 

The LFC device was recently installed at the La Silla Observatory in Chile. It is now part of the HARPS spectrometer (acronym for High Accuracy Radial Velocity Planet Searcher), one of the fundamental instruments of the European Southern Observatory (ESO), considered to be the leading Earth-based 'planet hunter'. The addition of a LFC to the HARPS spectrometer represents a significant advance in the search for new planets, as it allows to detect lower frequencies and amplitudes of oscillations around stars, more similar to the frequency generated by Earth.  

Photo of an LFC's echelle spectrum. The dots are the spectral lines of the LFC, which are arranged in diffraction orders of an echelle grating. The spectrum was projected on a laboratory wall with a small home-built spectrograph (Appl. Phys. B, 2017, 123, 76) and photographed.

In a previous commissiong, other people had previously measured 2 cm/s stability with an LFC, but they had no sufficiently precise reference relative to which they could measure the LFC. This is why in April 2015, a team of Engineers and Scientists participated of the testing phase of the LFC and brought along a second LFC, borrowed from the University Observatory Munich (mention the USM among the participating institutions). Not only could we verify the previous performance, we could even go down to 1 cm/s. After the campaign, the first LFC remained at the observatory to work with HARPS. The other one was shipped back to Germany and is now operating on Mt. Wendelstein with the FOCES spectrograph. To carry out the project, the following institutions worked together: the Universidade Federal do Rio Grande do Norte (Brazil), the Instituto de Astrofísica de Canárias (Spain), the Max Planck Institute of Quantum Optics  (Germany), the Menlo Systems GmbH (Germany), and the European Southern Observatory “(headquartered in Germany and Chile). 

The LFC team of Engineers and Scientists participated of the testing phase, at ESO, La Silla.

Despite the seriousness of the work at the observatory, during the tests in La Silla, something funny always happens. One of the participants had a severe allergic crisis, with a cough that upset the whole team and, in the absence of anything better, the solution was to give him the Asterix Magic Potion. Result ... the cough is gone! Another funny case was that one of the Team members (or the same one?)  was so excited by the results of the Commissioning that he lost his coat in La Serena airport! Also, three of the engineers no longer trusted their senses after driving by a garden gnome that someone had placed in the desert on the way to the telescope building. 

In addition to searching for new planets, LFC has applications in other fields of science with a wide range of probing the distribution of dark matter in our galaxy, measuring the acceleration of the cosmic expansion in real time, possibly finding variations in fundamental constants on cosmic scales.

The LFC in action. Device makes it feasible to detect planets similar to Earth outside our solar system.  

An advantage of the new device is that, the LFC reduces many difficulties in characterizing remote spectra. It is an absolute calibrator, which is directly linked to the SI second via an atomic clock and GPS. We have shown the power of the method by accurately measuring the absolute Doppler velocity of the dwarf planet Ceres through its reflected sunlight. Looking at more and more resolving spectra and defining smaller and smaller frequency deviations also allows us to measure possible variations in physical constants that would be linked to the expansion of the universe.

Bruno Leonardo Canto Martins

Professor, Federal University of Rio Grande do Norte

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