Wide sdB binaries. I. Orbital and atmospheric parameters

TL;DR

This study expands the catalog of long-period sdB binaries with fully determined orbits by combining a ground-based, high-resolution spectroscopic program (HERMES and UVES) with Gaia NSS–based candidates. It jointly fits Keplerian orbits to the radial velocities of sdB primaries and their cool MS companions and derives atmospheric parameters for the companions via the GSSP framework, enabling a comprehensive view of orbital and atmospheric properties. The results support general agreement with contemporary binary-evolution models for the ground-based sample, while Gaia-derived systems reveal tensions in period and eccentricity distributions, suggesting either observational biases or additional population components. Together, these findings constrain formation channels (notably stable mass transfer via Roche-lobe overflow) and guide future mass-distribution analyses and model refinements, contributing to a more robust understanding of wide sdB binary evolution and Galactic population effects.

Abstract

Long-period binary systems containing a B-type hot subdwarf (sdB) and a main-sequence companion are thought to originate from binary interactions involving stable mass transfer from the red giant, the progenitor of the sdB, to the MS companion. However, despite the recent progress in modelling their population, some of their observed properties are not entirely understood. Because determining their orbits requires extended campaigns of high-resolution spectroscopic observations, only a limited number of long-period sdB binaries have been studied with completely determined orbital parameters. A sample of 32 wide binary systems containing sdB stars was selected for the analysis of the radial velocity curves of both companions. The dataset consisted of high-resolution spectra obtained with the HERMES and UVES spectrographs. The orbital parameters were derived by simultaneously fitting Keplerian orbits to the radial velocities of the sdB and its companion. The atmospheric parameters of the cool companions were determined using the GSSP code, which analyses the master spectra of the systems with a grid of LTE atmospheric models. An additional sample of wide sdB binaries was built up by cross-matching the Gaia NSS catalogue with catalogues of sdB candidates and spectroscopically confirmed systems reported by Culpan2022A&A catalogues. The outcomes from both samples were compared with existing theoretical models to assess their consistency with current formation and evolutionary scenarios.

Figures (13)

• Figure 1: Left: The color-magnitude diagram (CMD) presented by Pelisoli2020AA for their composite hot subdwarf sample is shown as dimmed red diamonds, representing systems whose light curves and rotation periods were studied by the authors. We have included the wide hot sdB binary Ground-based sample with fully solved orbital parameters as blue-filled circles. The Gaia-based sample is shown as green-filled squares. Interstellar extinctions for both, Ground- and Gaia-based samples, are not corrected. Stars catalogued by Geier2020AA as hot subdwarfs are represented as dark gray open circles. Main-sequence stars and stars in other evolutionary stages within the region by Lindengren2018AA are shown as pale gray dots. Right: An Aitoff projection displaying the Galactic coordinates of the wide hot sdB sample. Galactic longitude increases from the centre toward the left, with increments of 30º per tick. Same color-coded wide sdB systems.
• Figure 2: Radial velocity curves and residuals for PG1514+034. The RVs of the cool companion are plotted as green-filled circles, while those of the sdB companion are shown as blue-filled squares. RV error bars (1$\sigma$), obtained through MC simulations, are included. The best-fit Keplerian orbits, simultaneously fitted for both the MS and sdB companions, are represented by a solid line for the cool companion and a dotted line for the hot one.
• Figure 3: The observed normalized spectrum (black solid line) of PG1514+034 and the best-fitting GSSP model to the cool companion lines (red dotted) for the wavelength range of 6000-6260 Å, used to determine the stellar atmospheric parameters of the cool companion.
• Figure 4: The process of obtaining $v\sin{i}$ and its error interval for the cool companion of PG1514+034. The fitting of the polynomial function (in red) to the reduced $\chi^2$ coefficients (dark blue points) yields the outcome of the atmospheric parameter as the minimum, while the 1$\sigma$ cut-off (green bar) provides its error interval.
• Figure 5: The Galactic orbit of PG1514+034 projected onto the Galactic plane (left) and the vertical Galactic component versus the radius with respect to the Galactic center (right).