Yanina Roxana Cochetti

Context . Long-term H α variability provides a direct probe of the dynamical evolution of Be-star circumstellar disks, but interpreting these changes in terms of physical disk properties remains challenging. Aims . We explore the entropy–complexity ( H–C ) plane as a compact diagnostic to link changes in the H α emission-line morphology to the physical evolution of Be-star disks. Methods . We applied entropy and López–Ruiz–Mancini–Calbet complexity diagnostics to H α line profiles from a grid of viscous decretion disk (VDD) models and from time-resolved observations of the late-type Be star β Psc. We compared their respective evolutionary pathways in the H–C plane. Results . The star exhibits an abrupt excursion and a subsequent secular recovery in the H–C plane, associated with rapid changes in the disk mass and apparent inclination within the adopted framework, before returning to a spectroscopic state similar to that observed prior to the major disk event around 2007. Conclusions . The H–C diagram provides a simple diagnostic that is capable of tracing long-term structural evolution and rapid transitions in Be-star disks using the morphology of the H α line profile alone.

Context . Binary systems with circumbinary molecular and dust rings are of great interest because they provide insights into the dynamics and evolution of stellar systems and the chemistry of the surrounding material. Aims . We aim to elucidate the nature of the B[e] binary system HD 327083 and constrain its orbital parameters and evolutionary status as well as the physical properties of its circumstellar and circumbinary envelopes. Methods . We present multi-epoch high-spectral resolution optical and near-infrared observations acquired at various observatories combined with VLTI/MIDI observations and ASAS-3 photometry. The data were analysed to obtain the orbital parameters, study individual members of the system, and derive the properties of the atomic and molecular gas and the dusty components of the complex environment of HD 327083. Results . We improved the orbital solution of HD 327083 and derived a period of P = 107.699 ± 0.005 d. Most optical lines display variations in shape and intensity when folded with the orbital motion. We assigned F6 II-III and B1 spectral types respectively to the cool and hot stellar components. A ring of molecular gas revolves around the system, and its appearance varies with the orbital phase. The MIDI flux distribution shows a silicate band in absorption at 9.7 µm. The interferometric data also indicate that the dust distribution has an elliptical shape, with its inner edge varying with orbital phase between 12.5 AU and 44 AU. Conclusions . We conclude that the B[e] binary system HD 327083 comprises a B-type massive accreting star and an F-type companion filling over 60% of its Roche lobe. The F-type companion is evolving towards the red supergiant stage. The deformed shape of the F-type star causes a broad minimum in the light curve. The hot companion (26 000 K, log g = 3.0) is hidden by a compact disc and shows a bi-polar outflow. The entire binary system presents an O-rich environment. It is surrounded by warm CO and SiO molecular rings, which are enclosed by an elliptical dust structure. All these components were formed by the evolved donor star during its evolution towards the red supergiant phase.

Context. It is already accepted that Be stars are surrounded by circumstellar envelopes, which are mostly compatible with a disc geometry in Keplerian rotation. Their infrared region is characterised by a moderate flux excess and the presence of hydrogen recombination lines that allow us to obtain information on the physical and dynamical structure of different regions inside the disc. Nevertheless, many of the infrared studies available in the literature show low-resolution spectra, or are restricted to a small object sample or describe just an individual object. Some others analyse reduced spectral ranges or just one infrared band. Aims. We aim to obtain a more complete characterisation of the properties of the circumstellar environment of Be stars that helps to constrain the theoretical models of the Be phenomenon. Methods. Throughout the last decade, we used the spectroscopic facilities at the Gemini and Las Campanas observatories to obtain quasi-simultaneous spectra in the J, H, K, and L bands of a sample of Be stars with medium resolution. Results. We present near-infrared, medium-resolution spectra of a sample of galactic Be stars with different spectral subtypes and luminosity classes. We measure different parameters of the hydrogen recombination lines from the Paschen, Brackett, Pfund, and Humphreys series, and use them to diagnose physical conditions in the circumstellar environment. We analysed the equivalent-width (EW) ratio between Brα and Brγ lines and different diagrams of flux ratios. We also identify lines from He i, C i, N i, O i, Na i, Mg i, Mg ii, Si i, Fe i, and Fe ii. Analysing the EW measurements of particular He i, Mg ii, Fe i, Fe ii and O i lines, we find that for some lines they correlate with the spectral type of the star. Particularly, the emission of the O i λ 1.3168 μm line decreases towards the later spectral types. Conclusions. We present an atlas of 22 Be stars, that covers a wide infrared (IR) spectral range with quasi-simultaneous observations. From a detailed analysis, we define new complementary criteria to Mennickent’s classification of Be stars according to their disc opacity. Some objects in our sample present compact thick envelopes, while in others the envelope is extended and optically thin. The correlation between the full widths at half maximum and the peak separation (ΔV) versus V sin i for the Br10, Brδ, and Hu14 lines reveals that the broadening mechanism is rotational. The Lyβ fluorescence is a key mechanism to explain the intensity of the emission of Mg ii and O i lines.

Massive stars typically undergo short-lived post-main-sequence evolutionary phases with strong mass loss and occasional mass eruptions. Many of such massive stars in transition phases have been identified based on their dusty envelopes. The ejected material often veils the stellar photospheres so that the central stars cannot be assigned proper spectral types and evolutionary stages. The infrared spectral range has proved to be ideal for the classification of evolved massive stars and for the characterization of their environments. To improve our knowledge on the central stars of four such dust-enshrouded objects, [GKF 2010] MN 83, [GKF 2010] MN 108, [GKF 2010] MN 109, and [GKF 2010] MN 112, we collect and present their first medium-resolution K-band spectra in the 2.3–2.47 μm region and discuss the location of the stars in the JHK color–color diagram. We find that the emission-line spectra of both MN 83 and MN 112 show characteristics typically seen in luminous blue variable (LBV) stars. In addition, we propose that the presence and strength of the newly reported Mg ii lines might be used as a new complementary criterion to identify LBV candidates. The spectra of the other two objects imply that MN 108 is an O-type supergiant, whereas MN 109 could be an LBV candidate in its active phase. We derive lower limits for the reddening toward the stars and find that three of all dereddened fall into the region of confirmed LBVs.

Context.A significant number of Be stars show a second Balmer discontinuity (sBD) attributed to an extended circumstellar envelope (CE). The fast rotational velocity of Be stars undoubtedly plays a significant role in the formation of the CE. However, Bn stars, which are also B-type rapidly rotating stars, do not all present clear evidence of being surrounded by circumstellar material.Aims.We aim to characterize the populations of Be and Bn stars, and discuss the appearance of the sBD as a function of the stellar parameters. We expect to find new indices characterizing the properties of CEs in Be stars and properties relating Be and Bn stars.Methods.We obtained low- and high-resolution spectra of a sample of Be and Bn stars, derived stellar parameters, characterized the sBD, and measured the emission in the Hαline.Results.Correlations of the aspect and intensity of the sBD and the emission in the Hαline with the stellar parameters and theVsiniare presented. Some Bn stars exhibit the sBD in absorption, which may indicate the presence of rather dense CEs. Six Bn stars show emission in the Hαline, so they are reclassified as Be stars. The sBD in emission appears in Be stars withVsini≲ 250 km s⁻¹, and in absorption in both Be and Bn stars withVsini≳ 50 km s⁻¹. Low-mass Be and Bn stars share the same region in the Hertzsprung-Russell diagram. The distributions of rotational to critical velocity ratios of Be and Bn stars corresponding to the current stellar evolutionary stage are similar, while distributions inferred for the zero-age main sequence have different skewness.Conclusions.We found emission in the Hαline and signs of a CE in some Bn stars, which motivated us to think that Bn and Be stars probably belong to the same population. It should be noted that some of the most massive Bn stars could display the Be phenomenon at any time. The similarities found among Be and Bn stars deserve to be more deeply pursued.

Be stars are rapid rotators surrounded by a gaseous disk envelope whose origin is still under debate. This envelope is responsible for observed emission lines and large infrared excess. To progress in the understanding of the physical processes involved in the disk formation, we estimate the disk parameters for a sample of Be stars and search for correlations between these parameters and stellar properties. We performed spectro-interferometric observations of 26 Be stars in the region of the Br$\gamma$ line to study the kinematical properties of their disks through the Doppler effect. Observations were performed at the Paranal observatory with the VLTI/AMBER interferometer. This instrument provides high spectral and high spatial resolutions. We modeled 18 Be stars with emission in the Br$\gamma$ line. The disk kinematic is described by a quasi-Keplerian rotation law, with the exception of HD28497 that presents a one-arm density-wave structure. Using a combined sample, we derived a mean value for the velocity ratio V/Vc=0.75, and found that rotation axes are probably randomly distributed in the sky. Disk sizes in the line component model are in the range of 2-13 stellar radii and do not correlate with the effective temperature or spectral type. However, we found that the maximum size of a stable disk correlates with the rotation velocity at the inner part of the disk and the stellar mass. We found that, on average, the Be stars of our combined sample do not rotate at their critical velocity. However, the centrifugal force and mass of the star defines an upper limit size for a stable disk configuration. For a given rotation, high-mass Be stars tend to have more compact disks than their low-mass counterparts. It would be interesting to follow up the evolution of the disk size in variable stars to better understand the formation and dissipation processes of their circumstellar disks.