are cosmic engines that drive the evolution of their entire host galaxy.
Binary interaction products
are direct probes of the outcome and the physics of binary interactions.
serves as ingredients for numerous
other astrophysical disciples.
strongly impacts the evolution of all components of a multiple system.
Investigating the possible binary origin of Be stars
About 20% of the massive galactic B stars are classical Be stars, observationally classified as such because their B-star spectrum is dominated by strong emission lines. While it has been shown that the formation of a circumstellar decretion disk, in which the emission originates, is linked to fast rotation of the central star, the origin of the rapid rotation remains unclear.
Different channels for the origin of the Be phenomenon have been proposed. One of them based on binarity, proposing that Be stars are the mass gainers in post-mass-transfer binary systems. Apart from gaining mass, the star also gained angular momentum, spinning it up to almost critical rotation rates. If the binary channel is the dominating one in the formation of massive Be stars, two clearly testable predictions emerge:
there should be a lack of Be stars in close binary systems with main-sequence (MS) companions
Be stars in binaries should have exotic companions such as envelope-stripped Helium stars, neutron stars and black holes
In a literature study of a sample of ~300 massive Be stars in our the Milky Way, we find that there is a clear lack of reported Be+MS binaries. While the sample is inhomogeneous, this gives a first hint that the binary channel might be dominating in the formation of massive Be stars.
Image credit: Pablo Marchant
Hunting for post-interaction systems in the SMC cluster NGC 330
When the integral field spectrograph MUSE mounted at UT4 of the VLT, Chile was equipped with a state-of-the-art adaptive optics module in summer 2017, it was possible for the first time to resolve the dense core of open stellar clusters in our neighboring galaxies, the Magellanic Clouds, and obtain spectra for its entire massive star population.
Using MUSE spectroscopy for NGC 330, a young, massive cluster in the Small Magellanic Clouds, we are able to provide a spectroscopic consensus of the massive star content of NGC 330. We confirm that there is a surprisingly large number of classical Be stars, i.e., rapidly rotating stars that display characteristics Balmer emission lines arising in a circumstellar decretion disk. Based on their position in the color-magnitude diagram, they are prime candidates for binary interaction products. Using the multi-epoch nature of the MUSE observations, we identify binary systems in the cluster. We find that the current binary fraction in NGC 330 is significantly lower than in young stellar cluster.
The RGB image on the right (which serves as background of this entire web page) is constructed from the actual MUSE observations of NGC 330.