عنوان مقاله [English]
The study of the existence of life or habitable zone somewhere in the universe, beyond the Earth, has been one of the important research areas in the field of astronomy and astrophysics in the last few decades. Countless studies have been done and are being done theoretically and experimentally.
Proxima Centauri () with visual magnitude of 11.01 and at a distance of 1.3 pc is the closest star to Earth after the Sun and is especially important for our knowledge of very cool stars. This M5.5V spectral type star is the faintest member of the Alpha Centauri ternary star system, located about 1400 astronomical units closer to Earth than the other members. The physical characteristics of this star, including radius (), mass (, rotational periodicity (1.5 35) and its age, which is about 4.85 billion years old, are well determined. Despite its old age, Proxima Centauri is an active star, and like the sun it has activity cycle (the activity cycle of the sun is about 11 years).
Generally, M-type stars are hard to study due to their optical faintness. But Studying Proxima Centauri can improve our knowledge of very cool stars as its proximity lets us to observe it with great accuracy. Moreover, its similarity to the sun and the possibility of having a system of planets around it and consequently the study of life on these planets is of particular importance.
This paper aims to determine the activity cycle of Proxima Centauri star using spectral line and to evaluate the generalized Lamb-Scargel periodogram technique (GLS) to determine the period of active dwarf stars, including Proxima Centauri.
The GLS is an extension to the Lomb-Scargle periodogram which takes into account the measurement of errors and also is more suitable for time series with non-zero average. GLS tries to fit the sinusoidal equation to the time series and find the power spectrum for frequencies. We consider a given periodogram peak, derived from GLS, significant when it exceeds the one present “false alarm probability” level (FAP), which means there is 99% confidence that it is real and could not be simulated by Gaussian noise. FAP levels are calculated by performing random permutations of the data with similar times of observations.
For this purpose, we used HARPS spectroscopic data over a period from 2004 to 2017. HARPS, the High Accuracy Radial velocity Planet Searcher at the European Southern Observatory La Silla 3.6m Cassegrain telescope is dedicated to the discovery of extrasolar planets. It is a fibre-fed high resolution echelle spectrograph. This instrument is used to accurately measure radial velocities of the order of 1 m/s in extrasolar planet research. The spectral area is 378-691 nm and its resolution 115,000. Therefore, from this point of view, we can say that our analysis is more accurate than others.The magnetic activity period of Proxima Centauri is obtained as 2349 days, which is in good agreement with the results obtained from other methods. Therefore, our results confirm the efficiency and superiority of the generalized Lamb-Scargel periodogram technique in determining the period of active cool dwarf stars.