Exploring the binary origin of B and Be rapid rotators

TL;DR

The study investigates the binary origin of Be/n rapid rotators, arguing that binary mass transfer contributes substantially to their spin-up across spectral types. Using BPASS population synthesis, it maps post-interaction pathways and the expected locations of stripped companions (sdOB, sdB, bloated, WD) on the HR diagram, comparing to observed Be/n binaries. The authors identify correlative trends between donor and gainer masses, note tensions in predicted mass-transfer efficiency, and stress the need for UV spectroscopy and interferometry to detect faint stripped companions. These insights have broad implications for ionizing UV flux, supernova outcomes, and potential gravitational-wave progenitors. Overall, the work strengthens the case that binary interaction is a key driver of rapid rotation in Be/n systems and outlines concrete observational routes to fully characterize the Be/n+stripped population.

Abstract

Observational evidence has continued to mount that a significant fraction of rapidly rotating early-B type stars are products of binary mass transfer. However, very few mid- and late-type B stars with rapid rotation have been demonstrated to be post-interaction products, despite a growing sample of SB1 binaries among stars within this range of spectral types. By considering the currently available information over the entire range of rapidly rotating B-type binaries, we argue that a significant fraction of the mid- and late-type rapid rotators found in binaries are also likely the result of past mass transfer episodes. The observed properties of this sample are compared to the predictions from the Binary Population and Spectral Synthesis code (BPASS), with attention given to the expected evolutionary pathways of stripped stars and the stellar and binary properties of both components of post-interaction systems across a range of initial conditions. Prospects for directly detecting and characterizing the stripped cores of the previous mass donors in such systems are described, and the implications for the role of binary interaction in causing rapid rotation are discussed. An accurate description of prevalence of binary interaction, the physics of mass transfer, and the post-interaction configuration of systems over a range of initial conditions has far-reaching implications including double-degenerate binaries and their eventual mergers, the output of ionizing UV flux of stellar populations, and the supernova explosions that can arise from stripped or rapidly-rotating progenitors.

Figures (12)

• Figure 1: Eccentricity (left) and radial velocity (right) vs. orbital period for main sequence B-type stars from the SB9 catalog. Early-type and late-type Be/n stars are indicated as in the legend. The two early-type outliers in the left panel are 59 Cyg and 60 Cyg, and the two late-type outliers are Pleione and 88 Her (see Appendix \ref{['secA1']}). Of these, 88 Her is the only case where the eccentricity may be unreliable.
• Figure 2: Left: Histogram of orbital periods for early- (blue) and late-type (red) binaries with Be/n star primaries, including both SB1 systems and those containing confirmed stripped stars. Right: Histogram for Be/n binaries confirmed as post mass-transfer (with directly detected stripped stars, blue) and for SB1 Be/n binaries (red). The cumulative distribution function (CDF) is plotted as solid lines for these two samples. The $p$ value of the two-sided Kolmogorov-Smirnov comparing each pair of samples is given in both panels.
• Figure 3: Top-left: Comparison of the mass of the primary Be/n stars (M1) to the orbital period. Top-right: Comparison of the mass of the lower-mass secondary star (M2) to the orbital period. Bottom: Comparison of the secondary to primary mass. In the bottom panel, the dotted lines correspond to mass ratios of 0.2 and 0.05, and the dashed line to 0.1. In all panels, systems containing secondaries whose nature is unknown (SB1), and secondaries confirmed as being normal stripped stars, bloated stripped stars, and high mass stripped stars are indicated with different symbols. We further note that B4 and later types correspond to stars of $\sim$ 5 M$_{\odot}$ and lower. Two systems, 2dFS 163 ($q = 0.4$) and 2dFS 2553 ($q = 0.5$), both containing a high-mass stripped star and an early-type Be/n star ram24 are not shown in these figures.
• Figure 4: Left: Histogram of mass ratios (M$_{\rm stripped}$/M$_{\rm Be}$) for the early- and late-type binaries, and their sum (black solid line). The mean and standard deviation for the early- and late-type mass ratios are plotted, with values of q = 0.10 $\pm$0.04 (early), and q = 0.10 $\pm$0.03 (late). Right: The same, but divided into the four different categories of binary systems as indicated in the legend, but without regard to spectral type.
• Figure 5: Observational Hertzsprung–Russell diagram for known Be/n binaries. The filled circles represent the near-MS rapidly-rotating B-type stars, and the star symbols represent stripped stars. Whenever the luminosity and temperature of both components for a Be/n+stripped binary are known, the location of the MS star and stripped star are connected by a solid line. Different colors represent different classes of stripped stars, as represented in the legend. SB1 Be/n stars are indicated by light gray circles, and the SB1 systems that are $\gamma$ Cas analogs are indicated by dark gray circles. MS evolutionary tracks from the Geneva stellar models geo13 from 3 -- 15 M$_{\odot}$, with rotation rates at 95% of critical (here defined as $\Omega$/$\Omega_{\rm crit}$) and Z = 0.014 are plotted as gray lines. All high-mass sdO stars are from the Magellanic clouds, with the remainder of systems being Galactic.