Investigating the lack of main-sequence companions to massive Be stars

Context. About 20% of all B-type stars are classical Be stars – stars whose spectra imply the presence of a circumstellar decretion disk. The disk phenomenon is strongly correlated with rapid rotation, the origin of which remains unclear. It may be rooted in single- or binary-star evolution. In the framework of the binary channel, the initially more massive star transfers mass and angular momentum to the original secondary, which becomes a Be star. The system then evolves into a Be binary with a post-main-sequence companion, which, depending on the companion mass, may later be disrupted in a supernova event. Hence, if the binary channel dominates the formation of Be stars, one may expect a strong lack of close Be binaries with main sequence (MS) companions. Aims. We want to test the prediction of the binary channel. Through an extensive, star-by-star review of the literature of a magnitude-limited sample of Galactic early-type Be stars, we investigate whether Be binaries with MS companions are known to exist. Methods. Our sample is constructed from the BeSS database and cross-matched with all available literature on the individual stars. Archival and amateur spectra are used to verify the existing literature when conflicting reports are found. Results. Out of an initial list of 505 Be stars, we compile a final sample of 287 Galactic Be stars earlier than B1.5 with V ≤ 12 mag. Out of those, 13 objects were reported as Be binaries with known post-MS companions (i.e., compact objects or helium stars) and 11 as binaries with unknown, uncertain or debated companions. We find no confirmed reports of Be binaries with MS companions. For the remaining 263 targets, no significant reports of multiplicity exist in the literature, implying that they are either Be binaries with faint companions, or truly single. Conclusions. The clear lack of reported MS companions to Be stars, which stands in contrast to the high number of detected B+B MS binaries, strongly supports the hypothesis that early-type Be stars are binary interaction products that spun up after mass and angular momentum transfer from a companion star. Taken at face value, our results may suggest that a large majority of the early-type Be stars have formed through binary mass-transfer.

Asymptotic Capacity Results on the Discrete-Time Poisson Channel and the Noiseless Binary Channel with Detector Dead Time

This paper studies the discrete-time Poisson channel and the noiseless binary channel where, after recording a 1, the channel output is stuck at 0 for a certain period; this period is called the “dead time.” The communication capacities of these channels are analyzed, with main focus on the regime where the allowed average input power is close to zero, either because the bandwidth is large, or because the available continuous-time input power is low.

Why binary interaction does not necessarily dominate the formation of Wolf-Rayet stars at low metallicity

Context. Classical Wolf-Rayet (WR) stars are massive, hydrogen-depleted, post main-sequence stars that exhibit emission-line dominated spectra. For a given metallicity Z, stars exceeding a certain initial mass MsingleWR(Z) can reach the WR phase through intrinsic mass-loss or eruptions (single-star channel). In principle, stars of lower masses can reach the WR phase via stripping through binary interactions (binary channel). Because winds become weaker at low Z, it is commonly assumed that the binary channel dominates the formation of WR stars in environments with low metallicity such as the Small and Large Magellanic Clouds (SMC, LMC). However, the reported WR binary fractions of 30−40% in the SMC (Z = 0.002) and LMC (Z = 0.006) are comparable to that of the Galaxy (Z = 0.014), and no evidence for the dominance of the binary channel at low Z could be identified observationally. Here, we explain this apparent contradiction by considering the minimum initial mass MspecWR(Z) needed for the stripped product to appear as a WR star. Aims. By constraining MspecWR(Z) and MsingleWR(Z), we estimate the importance of binaries in forming WR stars as a function of Z. Methods. We calibrated MspecWR using the lowest-luminosity WR stars in the Magellanic Clouds and the Galaxy. A range of MsingleWR values were explored using various evolution codes. We estimated the additional contribution of the binary channel by considering the interval [MspecWR(Z), MsingleWR(Z)], which characterizes the initial-mass range in which the binary channel can form additional WR stars. Results. The WR-phenomenon ceases below luminosities of log L ≈ 4.9, 5.25, and 5.6 [L⊙] in the Galaxy, the LMC, and the SMC, respectively, which translates to minimum He-star masses of 7.5, 11, 17 M⊙ and minimum initial masses of MspecWR = 18, 23, 37 M⊙. Stripped stars with lower initial masses in the respective galaxies would tend not to appear as WR stars. The minimum mass necessary for self-stripping, MsingleWR(Z), is strongly model-dependent, but it lies in the range 20−30, 30−60, and ≳40 M⊙ for the Galaxy, LMC, and SMC, respectively. We find that that the additional contribution of the binary channel is a non-trivial and model-dependent function of Z that cannot be conclusively claimed to be monotonically increasing with decreasing Z. Conclusions. The WR spectral appearance arises from the presence of strong winds. Therefore, both MspecWR and MsingleWR increase with decreasing metallicity. Considering this, we show that one should not a-priori expect that binary interactions become increasingly important in forming WR stars at low Z, or that the WR binary fraction grows with decreasing Z.

Enhanced Multiple-Object Tracking Using Delay Processing and Binary-Channel Verification

Tracking objects over time, i.e., identity (ID) consistency, is important when dealing with multiple object tracking (MOT). Especially in complex scenes with occlusion and interaction of objects this is challenging. Significant improvements in single object tracking (SOT) methods have inspired the introduction of SOT to MOT to improve the robustness, that is, maintaining object identities as long as possible, as well as helping alleviate the limitations from imperfect detections. SOT methods are constantly generalized to capture appearance changes of the object, and designed to efficiently distinguish the object from the background. Hence, simply extending SOT to a MOT scenario, which consists of a complex scene with spatially mixed, occluded, and similar objects, will encounter problems in computational efficiency and drifted results. To address this issue, we propose a binary-channel verification model that deeply excavates the potential of SOT in refining the representation while maintaining the identities of the object. In particular, we construct an integrated model that jointly processes the previous information of existing objects and new incoming detections, by using a unified correlation filter through the whole process to maintain consistency. A delay processing strategy consisting of the three parts—attaching, re-initialization, and re-claiming—is proposed to tackle drifted results caused by occlusion. Avoiding the fuzzy appearance features of complex scenes in MOT, this strategy can improve the ability to distinguish specific objects from each other without contaminating the fragile training space of a single object tracker, which is the main cause of the drift results. We demonstrate the effectiveness of our proposed approach on the MOT17 challenge benchmarks. Our approach shows better overall ID consistency performance in comparison with previous works.