Most stars and stellar systems go through evolutionary stages associated with strong mass loss and formation of circumstellar (CS) envelopes that may contain only gas or provide conditions for dust formation. Studying the CS matter and processes that lead to its existence provide understanding of a wide range of phenomena from evolution of galaxies to planet formation and refine our knowledge of adjacent evolutionary stages. Also, CS matter gets accreted by stars in mass-exchanging systems that may alter the course of stellar evolution (e.g., the Algol paradox). Although many stellar and CS phenomena have been successfully explained by the theory of stellar evolution, a number of them still remain puzzling even with the currently available wealth of data and sophisticated modeling methods. Such puzzles are often associated with complicated systems, whose true nature is hard to learn but which are crucial to study to make further progress in our understanding of the universe. To overcome the dificulties, it can be necessary to investigate together more than one group of objects with common features. This presents a unique opportunity to probe hidden pathways of evolution and creates a clearer view of the past, present, and future of the objects in question. One of these unexplained phenomena, the B[e] phenomenon, is the presence of permitted and forbidden emission lines in the spectra of intermediate-luminosity (log L/L_sun ~ 2.5---4.5) B-type stars. A second phenomenon is the existence of stable compact dusty disks around binary systems, such as post-AGB objects (e.g., van Winckel 2007). The third one is the presence of strong Li I lines in many rapidly rotating cool (G/K-type) giants. All of them are observed in a recently defined group of FSCMa objects. Their B-type spectra are dominated by extremely strong Balmer lines, the presence of lithium implies that they have cool components, and the evolutionary stage may range from late main-sequence to young Planetary Nebulae. There is growing evidence that the majority of intermediate-mass (~3-20 Msun) stars, which the hot components of FSCMa objects seem to be, are born at least in pairs. Binarity plays a key role in generating remarkable CS phenomena, such as long-lived gaseous disks around Be stars and a wide variety of Planetary Nebula shapes. It may explain the unusually abundant CS matter in FSCMa objects as well as the Li-excess and rapid rotation of the cool giants by mass transfer between the stellar components. However, details of evolution of the stars as well as mechanisms that drive matter to the CS environments are still unclear. Our project aims to reveal the processes that are responsible for the three above mentioned phenomena by determining the nature and evolutionary status of Galactic FSCMa objects, searching for binarity in a class of Li{rich cool giants, and studying connections between the FSCMa objects and post-AGB binaries with compact dusty disks. This will allow us to test models of binary evolution, address problems of mass loss and dust formation in stellar systems, improve CS matter modeling techniques, and test the role of planet accretion in lithium enrichment of cool giants in various systems.

The proposed project includes three interconnected studies. The first one is a spectroscopic and photometric study of the Galactic FSCMa objects (named after the prototype object of the class, Swings 2006) with an emphasis on those with detected cool secondary components. It will be aided by determination of detailed element abundances in a representative sample of the Li-rich cool giants. The latter will be spectroscopically monitored to search for binarity, which has been suggested to be a reason for the Li-excess (Barrado y Navascues et al. 1998), and serve as calibration sources for the abundance analysis in FSCMa objects. We will also comparatively study FSCMa objects and a class of post-AGB binaries with cooler (A/F{type) primary components and dusty disks. These two groups may either represent subsequent stages of post-AGB evolution or the FSCMa objects may be much less evolved and represent a new source of CS dust. The FSCMa objects are characterized by a central B-type star surrounded by a significant amount of CS gas, which powers low excitation forbidden lines and strong permitted lines, and CS dust, which is responsible for large IR excesses. They are important because they 1) manifest unusually high mass loss that is not currently accounted for in models of stellar evolution, 2) can contribute significantly to the Galactic interstellar dust, and 3) exhibit lines of lithium whose production mechanism in evolved stars is still unknown. The Li-excess in giants challenges the theory of stellar evolution that predicts a quick destruction of the initial lithium during the first dredge-up, when convection penetrates deep inside the star. Recent studies keep discovering Li-rich cool giants (e.g., Drake et al. 2002, Kumar et al. 2011) and show that many of them rotate rapidly. This may be due to transfer of angular momentum to the star by accreting CS gas or a planet that may partially inhibit convection. Studying these objects is important for understanding the stellar structure and evolution of planetary systems. The group of post-AGB binaries is the most studied of the three (de Ruyter et al. 2006). The presence of stable circumbinary dusty disks is considered to be their key signature, and energy exchange between the disk and the binary may delay the ejection of a Planetary Nebula (Dermine et al. 2012). This idea, still unexplored for this group, can also be applied to FSCMa objects and serve as a justification for a new evolutionary path toward the Planetary Nebula stage. This proposed project will address the following specific goals through a focused observational and interpretational program conducted by a strong international collaboration of experts in a wide range of fields of stellar astrophysics. 1. To study the impact of the CS matter spatial distribution and chemical composition on the observed properties of objects with dusty disks and refine their intrinsic properties. 2. To study the role of orbital parameters (e.g., periods, stellar/planetary masses) on the creation of the B[e] phenomenon and lithium excess through modeling of the observed properties and comparison with models of evolution of single stars and binary systems. 3. To constrain the nature and evolutionary status of FSCMa objects by analyzing intrinsic properties of their stars and CS environments through comparison with post-AGB binaries. The results will reveal new details of stellar structure and evolution, such as sources of lithium and its behavior in various types of radiation fields, diversity of evolutionary paths of low- and intermediate-mass stars, and importance of non-conservative mass loss in binary systems at different evolutionary stages. They also help developing new modeling techniques and extend evolutionary calculations to include predictions for parameters of the CS matter and emergent spectra, and refine the knowledge of CS dust formation.