Drifters on the edge of town: $λ$ Boötis stars in clusters

TL;DR

This work assesses whether the interstellar medium accretion scenario for λ Boötis stars remains viable within the dynamical environments of star clusters. Using N-body simulations of substructured clusters plus post-processing Bondi-Hoyle-Lyttleton accretion and disc photoevaporation modeling, it shows that A-stars can travel beyond the tidal radius and accrete pristine gas at rates compatible with λ Boo abundances, especially in smaller clusters where discs survive longer. Radiation fields reduce the number of viable cases, but a non-negligible fraction still meet the criteria, and the formation of λ Boo stars in binary systems via capture is a robust prediction that could explain abundance differences between binary components. The results offer a dynamical pathway for λ Boo formation in clusters and provide observational tests, such as searching for λ Boo stars in wide binaries with differing abundances and examining clusters of differing richness and radiation environments.

Abstract

$λ$ Boötis stars are a subset of chemically peculiar A-stars that display Solar abundances in lighter elements (C, N, O, S, etc.) but a deficiency in Iron-peak elements. This difference has been attributed to the A-stars accreting pristine (metal deficient) gas from the Interstellar Medium. However, the recent discovery of $λ$ Boötis stars in clusters challenges this theory, due to the presence of ionising radiation from intermediate/massive ($>$5 M$_\odot$) stars, which could prevent accretion of pristine ISM gas. We use $N$-body simulations to track the dynamical histories of A-stars during the evolution of a star cluster. We find that some stars leave the confines of the cluster and travel beyond the tidal radius, where they may be able to accrete pristine ISM gas. These A-stars then sometimes move back into the inner regions of the cluster, but the photoionising radiation flux they receive is not high enough to prevent $λ$ Boötis abundances from occurring in these A-stars. We find that A-stars can develop $λ$ Boötis abundances and subsequently form a wide ($>100$ au) binary system, meaning that observations of binary systems that have different abundances between the component stars would not rule out the ISM accretion scenario. Whilst we have shown that $λ$ Boötis stars can reside in and around star clusters, further research is required to assess the validity of the accretion rates required to explain their abundance patterns.

Figures (6)

• Figure 1: Evolution of a representative simulation of a star-forming region with $N_\star = 500$ stars. The A-stars are shown by the coloured triangles. The grey dashed circle shows the position of the Jacobi radius, as defined in Eqn. \ref{['eq:jacobi_rad']}. As the star-forming region evolves and expands, stars (including some of the A-type stars) move outside the Jacobi radius. Note that the position of the Jacobi radius remains constant, but we have changed the axes scales to accomodate the expansion of the star-forming region. We show the region before any dynamical evolution (0 Myr, panel (a)), then at 11 Myr (panel b) and after 40 Myr (panel c).
• Figure 2: The distance of each A-star from the centre of the star-forming region over the run-time of a simulation with $N_\star = 500$ stars. Each coloured line represents the dynamical history of an A-star (the colours are matched to the stars shown in Fig. \ref{['fig:positions']}). The horizontal grey dashed line shows the distance of the Jacobi radius, as defined in Eq. \ref{['eq:jacobi_rad']}.
• Figure 3: The Bondi-Hoyle-Lyttleton accretion rates as defined by Eqn. \ref{['eq:mdot']} for each A-star in our $N_\star = 500$ simulation. We show the accretion rates of each of the 12 stars when they travel beyond the Jacobi radius. Some of these stars are ejected and never return to the cluster, but several (e.g. the cyan star) are on cluster-centric orbits and return inside the Jacobi radius during the cluster's evolution.
• Figure 4: The Far Ultra violet radiation flux recieved by each A-star as a function of time in our $N_\star = 500$ simulation. Each coloured line represents the FUV flux (in terms of the Habing68$G_0$ unit) for each A-star, where the colours are the same as the stars in Figs. \ref{['fig:positions']} and \ref{['fig:clus_cen']}. If the A-star still retains a gaseous component to its disc at a given time, we plot a filled square on the line.
• Figure 5: Histograms of the maximum velocities of individual A-stars in our simulations. In each panel the open histogram shows the maximum velocity distribution for all A-stars, the hashed histogram shows the distribution of maximum velocities for A-stars that could accrete pristine gas (i.e. those that move beyond the Jacobi radius), and the filled histogram is the distribution of maximum velocities for A-stars that move beyond the Jacobi radius but also retain a circumstellar disc after 10 Myr (no discs survive beyond 10 Myr in the $N_\star = 3000$ simulations).