Toward understanding the B[e] phenomenon: the curious case of IRAS 17449+2320

The existence of classical Be stars, stars that show emission lines due to a disk-like structure around the central object, has been recognize for more than one century. In contrast the history of the B[e] stars is much shorter. Conti suggested calling B-type stars that show low ionization forbidden emission lines as B[e]. Nowadays, we know that B[e] stars are an inhomogeneous group of objects so it is uncorrect to speak about a class of stars but it is better to talk about the B[e] phenomenon. The B[e] phenomenon appears in early-type emission lines stars with low-excitation lines, forbidden lines, and hot dust detected by an IR excess. The phenomenon is important because appears in very different evolutionary stages of stars (in the intermediate mass pre-main sequence stars HAeB[e], in the main sequence stars MSB[e], in the evolved high-mass stars sgB[e], evolved low-mass stars cPNB[e], symbiotic stars symB[e] etc.). Studying this phenomenon we can better understand the physical mechanisms that appear during the life of a star and correlations between dynamical mechanisms (wind, rotation, eruptions, non radial pulsations, binarity etc.) and evolution. It is important that B[e] objects harbor dust in a circumstellar disk and may be responsible for at least few percent of the Galactic circumstellar dust. Dust forming B[e] stars exist in the lower metallicity environments of LMC and SMC so it is reasonable to suggest that they might have been important dust producers in the earlier Universe. It is difficult to study these stars beacause they are embedded in a geometrically and optically thick circumstellar envelope, but, we can try to infer the physical properties by studying the interaction between the star and its environment. The object of my analysis is IRAS 17449+2320, it is a 10 mag B[e] object situated in the Hercules constellation at a distance of 700 pc from the Sun. IRAS 17449+2320 is catalogued as a A0V star and a possible FS CMa object, a subclass of the B[e] phenomenon. There are observational and physical criteria for the identification of a FS CMa star. Observational criteria: - Emission line spectra contain H lines stronger than in Be, HAeBe, and normal supergiants; Fe II and [O I]; sometimes [Fe II] and weak [O III] lines. -Large IR excess that peaks at 10-30 micrometer and sharply decreases longward. -Location outside of star forming regions. -When presents, a secondary companion (i) is typically fainter and cooler (if it is a normal star) than the primary or (ii) is degenerate. Physical criteria: -T_eff of the hot star/companion of about 9000- 30000 K (O9 - A2 type stars). -Luminosity of the hot star/companion log (L/L_sun) approx 2.5 - 4.5 V_pri -V_sec > 2 mag This is an observational thesis, so a special focus is on the data analysis. I will explain how to reduce spectral data, both slit and échelle, with IRAF software, then I will analyze the spectra taken from the Ondrejov Observatory, the Observatorio Astronomico Nacional San Pedro Martir , McDonald Observatory and the Canada France Hawaii Telescope. In particular, I will analyze line profile variations, expecially for the Balmer lines. Then, theoretical chapters follow, trying to describe the phenomenology behind the physical mechanisms inferred by the spectroscopic studies. The Balmer lines have broad wings in absorption wide 3600 km/s (FWZI) but the core is composite by emission and absorption features. The variation of Balmer lines is only in the core and in the range of velocity of [-300,300] km/s, this means that the dynamical phenomena that are affecting these lines are playing at a distance lesser than an AU. The wings of Fe II, Na I D, O I and Ca II are broad as the range of velocity where the dynamical phenomena are affecting the Balmer lines. This can suggest that these lines are forming in the same region of the perturbations. Instead the width of [O I] emission lines suggest that [O I] are forming in a wider low density region and it is possible to use the doppler shift of these lines as an indicator of the radial velocity of the system that is RV=(-16\pm 2) km/s. I found in the Balmer lines, the presence of composite absorption features moving from the blue to the red side with a time scale of few days, changing their velocity of 200 km/s. This behaviour be explained by the presence of coherent but temporary structures that are orbiting around the star at about 0.2 AU where the radiation has a temperature of about approx 2900 K. The origin of this structure is not known, but some scenarios are the presence of global one-armed (i.e. azimuthal wave number m = 1) oscillations in the equatorial discs and the evaporation of a planet forming a multi-cometary tail. Line variations are found on a range of timescales: few days for Balmer lines, and month or years for the other lines. The presence of He I lines suggests that the star is earlier than thought by Miroshnichenko but not earlier than a B8V. The wings of Balmer lines compared with the ones obtained rotating the spectrum of Vega gives an estimation of the projected rotational velocity. The best match is assuming v_{eq} sin(theta) approx 200 km/s, hence the star is a fast rotator. FS CMa objects sometimes show binarity, in this case there is no clear evidence of a companion but binarity cannot be ruled out since a brown dwarf or a white dwarf do not produce detectable spectra.

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