Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Birth of a Be star: an APOGEE search for Be stars forming through binary
  mass transfer   Motivated by recent suggestions that many Be stars form through binary mass
transfer, we searched the APOGEE survey for Be stars with bloated, stripped
companions. From a well-defined parent sample of 297 Be stars, we identified
one mass-transfer binary, HD 15124. The object consists of a main-sequence Be
star ($M_{\rm Be}=5.3\pm 0.6 \,M_{\odot}$) with a low-mass ($M_{\rm
donor}=0.92\pm 0.22\,M_{\odot}$), subgiant companion on a 5.47-day orbit. The
emission lines originate in an accretion disk caused by ongoing mass transfer,
not from a decretion disk as in classical Be stars. Both stars have surface
abundances bearing imprint of CNO processing in the donor's core: the surface
helium fraction is $Y_{\rm He}\approx 0.6$, and the nitrogen-to-carbon ratio is
1000 times the solar value. The system's properties are well-matched by binary
evolution models in which mass transfer begins while a $3-5\,M_{\odot}$ donor
leaves the main sequence, with the secondary becoming the Be star. These models
predict that the system will soon become a detached Be + stripped star binary
like HR 6819 and LB-1, with the stripped donor eventually contracting to become
a core helium-burning sdOB star. Discovery of one object in this short-lived
($\sim$1 Myr) evolutionary phase implies the existence of many more that have
already passed through it and are now Be + sdOB binaries. We infer that
$(28_{-16}^{+27})\,\%$ of Be stars have stripped companions, most of which are
faint. Together with the dearth of main-sequence companions to Be stars and
recent discovery of numerous Be + sdOB binaries in the UV, our results imply
that binarity plays an important role in the formation of Be stars.

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The categories of this paper are: cmp-lg
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The categories of this paper are: dg-ga
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The categories of this paper are: eess
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The categories of this paper are: hep-ex
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The categories of this paper are: supr-con
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The categories of this paper are: quant-ph
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The categories of this paper are: math-ph
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The categories of this paper are: hep-ph
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The categories of this paper are: adap-org
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The categories of this paper are: ao-sci
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The categories of this paper are: solv-int
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The categories of this paper are: comp-gas
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The categories of this paper are: funct-an
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The categories of this paper are: math
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The categories of this paper are: hep-th
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The categories of this paper are: chao-dyn