Caracterização de estrelas do tipo Gamma Cassiopeiae

Abstract

The �� Cassiopeiae-like stars comprise a class of Be stars with variable hard thermal X- ray emission. For a long time, �� Cas was the odd man out among X-ray emitters. More recently, observations of stars with similar features identified by X-ray surveys created new possibilities to test and understand the origin of their peculiar X-ray emission. As of now, there are 12 known members of such class. Two different scenarios were put forward to explain the X-ray emission from these objects: magnetic interactions close to the Be star surface and accretion onto a white dwarf companion. Both scenarios have profound impact in our understanding of massive stars. �� Cas stars can be plausible progenitors of magnetars or account for the missing number of Be + white dwarf which is expected in our galaxy. The true nature of the �� Cassiopeiae-like stars can be revealed through a careful charac- terization of the parameter space of these systems. This work adds to this characterization with two main contributions: (i) investigation of the temporal evolution of fotometric and spectroscopic X-ray properties of the star HD161103 using two XMM-Newton observa- tions; and (ii) searching for signs of binarity and monitoring the stability of the circum- stellar disk of four members of the class (HD 161103, HD 110432, HD 119682, HD 157832) in the visible wavelengths, with a four-year long campaign carried out with the Coudé spectrograph at the 1,6m telescope of the Observatório do Pico dos Dias (Brazópolis, MG - Brazil). Optical spectra of �� Cassiopeiae-like stars does not differ significantly from others clas- sical Be stars. All known �� Cas stars present spectral types between B0e and B1.5e, a surprisingly narrow interval, and luminosity classes between III and V, compatible with an old main-sequence population. The observed spectra of our sample show strong Hy- drogen and Iron emission lines, which suggests the presence of a dense or large and stable circumstellar disk. In spite of climatic and instrumental duress, our spectra show signs of variability in radial velocity of at least ∼ 32 km/s. Of all the �� Cas stars, only �� Cas itself is known to be member of a binary system. The nature of its companion is still unknown and no relation between the relative position of the stars and the X-ray emission has been identified. The first XMM-Newton observation of HD161103, in 2004, exhibited an oscillation of 3.2 ks in its light-curve. If confirmed, this oscillation could be due to the spin period of a putative accreting companion associated with the X-ray emission. In a newer XMM obser- vation, carried out in 2012, the oscillation was not present. Our analysis did not revealed any periodicity with frequencies below 10.4Hz, – limit imposed by the satellite instru- mentation. On the other hand, the average X-ray luminosity in the new observation is two thirds of the one observed in 2004, revealing long scale variability. Also, the spectrum of the new observation shows higher temperature than previously (��T ≈ 9.9 keV instead of ��T ≈ 7.4 keV) and signs of sub-solar Iron abundance. Our analysis shows variations in the Iron abundance between observations and could indicate the presence of an inverse FIP (First Ionization Potential) effect as results from magnetic activities. Spectral and broad band timing analyses show that the spectral hardness of the source remains reasonably constant when comparing both observations and even for each observation separately. The spectral changes observed in X-rays are not expected in the accretion scenario and strengthens the interpretation of the X-ray emission being due to magnetic star-disk interactions. Based on the results of this work and evidences from the current literature, the magnetic scenario is the most likely to explain the origins of X-rays in HD 161103 and maybe for all �� Cassiopeiae-like stars.