We use chemically anomalous carbon-enhanced metal-poor (CEMP) stars to investigate the binary properties of very metal-poor stars in the inner Galaxy. One of the main sub-classes of CEMP stars are those enhanced in s-process elements, these are expected to form through binary interaction with a former AGB companion (analogous to Ba- and CH-stars at higher metallicities). In the Galactic halo, the fraction of CEMP-s stars among very metal-poor (-3.0 < [Fe/H] < -2.0) field stars is ~10%. In absence of e.g. large-scale radial velocity monitoring, the fraction of CEMP-s stars can be used to investigate variations in binary fraction among very metal-poor stars in different Galactic environments — for example in the inner Galaxy. The occurrence of CEMP-s stars in the inner Galaxy has not been studied previously, because it is extremely challenging to even find very metal-poor stars in the Galactic bulge due to the high density of (mainly metal-rich) stars, and severe foreground extinction. The recent Pristine Inner Galaxy Survey (PIGS), however, has been very successful at uncovering such stars, efficiently selecting very metal-poor candidates for spectroscopic follow-up using metallicity-sensitive narrow-band CaHK photometry. I will present recent PIGS results on carbon-rich stars in our low-resolution spectroscopic follow-up sample. The fraction of CEMP-s stars in PIGS appears to be lower than in the Galactic halo, which may indicate a lower binary fraction among very metal-poor stars in the inner Galaxy. 

What is the impact of stellar multiplicity on planet formation? This question is motivated by the fact that the process of stellar formation leads to a very high fraction of multiplicity. On top of this, planet formation occurs early on around young stellar objects. This leads to an unavoidable conclusion: planetary cradles (a.k.a protoplanetary discs) are deeply affected by the presence of nearby stars. Here, I will present some recent works on disc dynamics and planet stability in multiple stellar systems. In particular, I will focus on binary and triple stellar systems, as opposed to single stars. First, I will present a series of observed discs in multiple stellar systems and their corresponding modelling through 3D SPH hydrodynamical simulations. Among the systems of interest: AB Aur, HD142527, HD100453, HD143006, HD98800 and GG Tau. This will allow us to obtain a comprehensive view on discs dynamics in multiple stellar systems. Then, I will briefly talk about the stability of planetary systems in these multiple systems, which is key to interpret the emerging population of exoplanets. Finally, I will discuss the deep implications of stellar multiplicity on planet formation and the current open questions in the field.

Stellar multiplicity has been recognized as a ubiquitous feature: stars seldom live an effectively single life. In the late stellar evolutionary stages, mass loss plays a major role and interaction with an orbiting companion can leave remarkable imprints in the wind up to distances much larger than the orbital separation. High spatial and spectral resolution instruments have shed unprecedented light on the complexity of the astrochemistry at work in the outflows from cool evolved stars like red giant branch and asymptotic giant branch stars. Although still largely unconstrained, the wind launching mechanism is known to be sensitive to the chemical content of the dust since it determines its opacity and thus its capacity to be accelerated by the stellar radiative field and impacted by the presence of a companion.

In this talk, I will present simulations of dust-driven winds in detached binaries where the donor star is a carbon or oxygen-rich asymptotic giant branch star. With the mesh-based radiative-hydrodynamics MPI-AMRVAC code, we designed a versatile 3D setup suitable to capture the expansion of dust-driven winds from the dust condensation radius up to several 10 orbital separations at an affordable computational cost. We explored a wide variety of orbital and stellar configurations and reproduced typical non-spherical features such as arcs, spirals, petals and orbital density enhancements. We evaluated the influence of the dust chemical content on the mass loss rate and on the morphology of the circumbinary envelope. Mass transfer from the asymptotic giant branch star to the secondary also redistributes angular momentum and can lead to orbital inspiral and wind-captured disks around the secondary. Finally, I will show how synthetic multi-channel molecular line emission maps can be obtained from these simulations for different orbital inclinations, to be compared to observations.

Combining absolute astrometry with direct imaging or radial velocities allows to better constrain the masses of binary systems. The absolute astrometry data from the Hipparcos (1990-1993) and Gaia (2014-) satellites allow us to use the long period of time between the two missions to accurately characterize orbits. Moreover, the exploitation of the Hipparcos data prepares us to use the future Gaia data in the study of binary systems. I will present here a new tool that combines the data from these two satellites rigorously, together with direct imaging and radial velocities. This modular tool can use the Transit Data from Hipparcos, which should be used in cases where the secondary light affects the observations. I will also present some typical test cases.

Stars form in clusters and associations; some of the multiple systems born in such environments will remain gravitationally bound for their entire life. Multiplicity is thus an inherent property of stellar populations. Recent large spectroscopic surveys have harvested many spectroscopic multiple systems with two and three components but those with four components (SB4) remain comparatively very rare. Here we report on the properties of the first SB4 found within a cluster: HD 74438, located in the close-by young open cluster IC 2391. It is the youngest (43 My) SB4 discovered so far. Identified in the context of the Gaia-ESO Survey, this system benefited from HRS/SALT and HERCULES/UCMJO spectroscopic follow-up. We show that this system consists of a 2+2 bound hierarchical stellar system, the two inner orbits being non-coplanar. We derive orbital solutions for the two inner pairs, while the wide pair is characterized by a preliminary orbit of about 6 y and the lowest mass ratio discovered so far (0.58 ± 0.11) in this context. The well-constrained inner periods, individual component masses and mass ratios of this remarkable system provide information on star formation, while the non-coplanarity of the two inner pairs sheds light on secular evolution of quadruple systems through Kozai-Lidov oscillations.

Binary stars evolve into chemically-peculiar objects or binary compact objects, releasing significant amounts of enriched gas, making them the main driver of the Galactic enrichment in heavy elements (and for some of those, the only driver). During their evolution, they go through a series of interactions, among which tides are the most common kind.

To pin down tidal interactions and the evolution of binary stars, we implement an updated prescription in the stellar population code binary_c. Relying on Zahn's theory of tides, and relying on grids of MESA main-sequence models covering an extensive mass range, we use detailed structural quantities to derive the parameters lambda and E2, that rule equilibrium and dynamical tides respectively. We then replace the ubiquitous BSE implementation in the binary_c code.

I will present a few applications of this new determination of circularisation and synchronisation timescales, while the comparison between the derived and observed period-eccentricity diagrams for clusters provides an accurate estimate of their ages.

Todo