An interferometric study of B star multiplicity

TL;DR

This study uses high-angular-resolution VLTI/PIONIER interferometry to quantify the multiplicity of 32 B-type stars over separations of $0.5-35$ au, providing a detailed census through modeling of visibilities and closure phases. By combining interferometric results with literature on spectroscopic and Gaia-detected companions, the authors derive an interferometric companion fraction of $f_c = 1.88 ± 0.24$ and a multiplicity fraction of $f_m = 0.72 ± 0.08$; incorporating additional companions yields a complete multiplicity of $f_m = 0.88 ± 0.06$ and a companion fraction of $f_c = 2.31 ± 0.27$, with a notable share of systems becoming hierarchical triples. The analysis shows that binarity and higher-order multiplicity are common among B stars, with environment, luminosity class, and spectral subtype modulating the multiplicity statistics. The results, consistent with trends seen in O-type stars, highlight the importance of multiplicity in the evolution of massive stars and emphasize the need to account for observational biases when combining interferometric, spectroscopic, and Gaia-based detections. Overall, the work provides a comprehensive, bias-conscious view of B-star multiplicity across a broad separation range, informing population synthesis and massive-star formation scenarios.

Abstract

Massive stars can have extreme effects on their environments from local to galactic scales. While O star multiplicity has been studied over a broad separation range (to the point where absolute masses of these systems have been determined and investigations into multiple system formation and interactions have been performed), studies of B star multiplicity are lacking. Using interferometry, we investigated the multiplicity of a statistically significant sample of B stars over a range of separations (~0.5-35 au, given that the average distance to our sample is 412 pc). We analysed high angular resolution interferometric data taken with VLTI/PIONIER for a sample of 32 B stars. Using parametric modelling of the closure phases and visibilities, we determined best-fitting models to each of the systems and investigated whether each source was best represented by a single star or a higher-order system. The detection limits were calculated for companions to determine whether they were significant. We then combined our findings from the interferometric data with results from a literature search to determine whether other companions were reported at different separation ranges. Within the interferometric range 72+/-8% of the B stars are resolved as multiple systems. The most common type of system is a binary system, followed by single stars, triple systems, and quadruple systems. The interferometric companion fraction derived for the sample is 1.88+/-0.24. When we accounted for spectroscopic companions that have been confirmed in the literature and wide companions inferred from Gaia data in addition to the companions we found with interferometry, we obtain multiplicity and companion fractions of 0.88+/-0.06 and 2.31+/-0.27, respectively, for our sample. The number of triple systems increases to the second-most populous type of system when accounting for spectroscopic companions.

Figures (130)

• Figure 1: A typical fit to the data of one of our sources, HD 3379. On the left, we show the $u-v$ coverage of the PIONIER observations. The different colours correspond to the telescope pair with which the particular data were acquired at the VLTI. In the middle, we show the closure phase (T3PHI) fit and residuals in terms of the spatial frequency (B$_{avg}$/$\lambda$; written as Bavg/wl on the axes). On the right, we show the fit to the squared visibilities (V2) and the associated residuals, again in terms of spatial frequency. The data are represented as points in the fits, whilst the model fits are shown as continuous lines.
• Figure 2: Left: Model image created based on the best-fit model to the data shown in Figure \ref{['fit']}, showing the primary (1) and secondary (2) stars needed in the model to reproduce the interferometric observables. The colour bar displays the arbitrary flux of the star. Right: The spectral energy distribution displaying the flux ratios of the stars in the model.
• Figure 3: Bootstrapping plot showing the error determination for the dataset fit in Figure \ref{['fit']}. The fit to all data is shown in orange, and blue corresponds to the bootstrap fit.
• Figure 4: Example of a grid fit, in this case corresponding to Figure \ref{['fit']}. On the left, we show the grid of potential companion positions that we searched in terms of goodness of fit, as quantified by the $\chi^{2}$. The primary star, which was fixed at position (0,0) during the fit, is shown by a blue star. The black circle shows the (x,y) position of the companion in the best-fitting model. On the right, we show the grid in terms of companion significance. This allowed us to determine whether a companion determined by the best-fitting model was significant in terms of its flux.
• Figure 5: Percentage of the different types of multiple systems detected with interferometry in our B-star sample.