Deep in our galaxy

Continue the way towards ever deeper knowledge of planetary systems in our galaxy. A new observational strategy and very detailed measures are described in the new article in Astronomy & Astrophysics.
Within the program GAPS (Global Architecture of Planetary Systems), which deals with the characterization of planetary systems thanks to the HARPS spectrograph mounted at the N-Telescopio Nazionale Galileo (TNG), a team of astronomers led by Francesco Stock Exchange, of INAF in Brera, has “paid a visit to an old friend.”


Credit: David Aguilar / Harvard-Smithsonian Center for Astrophysics

This is the planetary system, still identified in 1996, Tau Bootis, located 50 light-years away and consisting of a binary star system (Tau Bootis A and Tau Bootis B) and a planet (Tau Bootis b) that revolves around the A component of the system.
The planet in question is, in the family of exoplanets, defined as a super-hot Jupiter. It has, in fact, a mass about six times that of Jupiter rotates in about 3 days just around the host star, at a distance from it equal to about one-twentieth of the Earth-Sun distance. Astronomers are, in fact, far from having a complete picture on the formation of planetary systems, it is a puzzle in which many pieces are still missing, and it is essential to investigate more and better systems already known and search for new ones to have a case studies complete.
Collect detailed information on all types of systems so far observed is therefore an ongoing challenge to be faced with advanced instruments like HARPS-N, but also with new observational strategies. And that is what has made the research group GAPS, whose leader Francesco Exchange explained that the most important aspect of this new work is just to have developed a method of observation that consists of adding many consecutive spectra of the same object obtained from very short exposure. This can be done because the object is very bright and, at the same time, the brevity of individual exposures allowed to have a series of spectra is not saturated, which would have happened instead with prolonged exposure. At the end of this process are obtained very accurate data and temporally very close together, the rich information on the star, with a very high signal to noise ratio in all regions of the spectrum.
It is thought that planets like Tau Bootis b, very massive and very tight orbits, must somehow influence the stellar activity. As the planet’s orbital period of about three days it was expected then to detect a change in the activity of the magnetic star with the same frequency. This can be done by measuring the variation of the emission of a few lines of ionized calcium. As explained by Gaetano Scandariato, INAF Catania, co-author of the article, the change seems to be, but it is very small, and a comparison with previous studies does not establish with certainty whether the observed variation is related to the interaction with the planet. The technique developed has, however, proved to be effective, and can then be extended to systems with less brilliant but eccentric orbits.


Credit: David Aguilar / Harvard-Smithsonian Center for Astrophysics

The spectra temporally very close together were used to make asteroseismology, ie the field of astrophysics, which observes and interprets the change in the brightness of stars caused by the propagation of waves “seismic” inside them and on their surface. This, similarly to the geologists who derive from earthquakes information about the internal structure of the Earth, allows astronomers to obtain important data about the internal structure of stars. Thanks to the results obtained was also possible to develop models that were allowed to have, for the first time, a good estimate of the age of the star, which happens to be young having “only” 900 million years. The age of the stars is a key parameter for understanding the evolution of planetary systems.
It is also greatly improved the knowledge of other parameters of the main star, such as its mass, radius, temperature, and still others: this allowed, consequently, also to improve the knowledge of the parameters of the planet, coming to obtain a measure of its mass with an accuracy of 3%. Another important result of this work is the determination of the acceleration of the companion star of Tau Bootis A, which is moving on a highly eccentric orbit, and that will reach the minimum distance from the companion around 2030. This work, therefore, highlights an aspect important, more general and not so obvious to the “non-experts”, this young field of astrophysics.
Acquire more and more knowledge of the host stars, with values more accurate and innovative methodologies as in this case, to characterize always better planetary systems known. Then there is also the fact that such systems be able to observe at a distance of years (almost 20 in this case), allows comparisons and tests are important for understanding the phenomena that occur in time scale so long that it cannot in any way be reproduced in the laboratory .
The road is still long, but then, thanks to projects like observational GAPS project, we are making important steps towards a thorough understanding of planetary systems, their formation and evolution. That the work continues!

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