J/A+A/708/A187 Ultraviolet spectroscopy of MWC 656 (Mueller-Horn+, 2026)
Ultraviolet spectroscopy reveals a hot and luminous companion to the
Be star+ black hole candidate MWC 656.
Mueller-Horn J., Ramachandran V., El-Badry K., Sander A.A.C.,
Bodensteiner J., Gies D.R., Goetberg Y., Rivinius T., Shenar T.,
Schoesser E.C., Wang L., Bieryla A., Buchhave L.A., Latham D.W.
<Astron. Astrophys. 708, A187 (2026)>
=2026A&A...708A.187M 2026A&A...708A.187M (SIMBAD/NED BibCode)
ADC_Keywords: Binaries, spectroscopic ; Spectra, ultraviolet ; Stars, early-type
Keywords: binaries: spectroscopic - stars: black holes -
stars: emission line, Be - stars: individual: MWC 656
Abstract:
The Galactic Be star binary MWC 656 was long considered the only known
Be star +black hole (BH) system, making it a critical benchmark for
models of massive binary evolution and for the expected X-ray emission
of Be+BH binaries. However, recent dynamical measurements cast doubt
on the presence of a BH companion. We present new multi-epoch
ultraviolet spectroscopy from the Hubble Space Telescope, combined
with high-resolution optical spectra, to reassess the nature of the
companion. The far-ultraviolet spectra reveal high-ionisation features
-- including prominent NV and HeII lines -- which are absent in the
spectra of normal Be stars and are indicative of a hot, luminous
companion. Spectral modelling shows that these features cannot
originate from the Be star or from an accretion disc around a compact
object. Instead, we find that the data are best explained by a hot
(Teff∼85kK), compact, hydrogen-deficient star with strong wind
signatures, consistent with an intermediate-mass stripped star. Our
revised orbital solution and composite spectroscopic modelling yield a
companion mass of M2=1.48+0.55-0.46M☉, definitively ruling
out a BH and disfavouring a white dwarf. MWC 656 thus joins the
growing class of Be+stripped star binaries. The system's unusual
properties -- including a high companion temperature and wind strength
-- extend the known parameter space of such binaries. The continued
absence of confirmed OBe+BH binaries in the Galaxy highlights a
growing tension with population synthesis models.
Description:
This catalogue contains reduced optical echelle spectra of MWC 656
obtained with the TRES and HIRES spectrographs. The spectra are
wavelength calibrated and corrected to the barycentric reference
frame, but are not continuum- normalised. Wavelengths are given in air
Angstroem.
Each file corresponds to one observing epoch and is provided as an
ASCII table with comment lines (starting with '#') containing basic
metadata, followed by whitespace-separated data columns.
TRES spectra include the Echelle order number, wavelength, and
extracted flux. HIRES spectra include an additional column identifying
the instrumental setting (blue, green, red spectral range).
Objects:
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RA (2000) DE Designation(s)
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22 42 57.30 +44 43 18.3 MWC 656 = HD 215227
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 53 14 List of spectra
spt/* . 9 TRES spectra (one file per epoch)
sph/* . 5 HIRES spectra (one file per epoch)
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 19 A19 --- FileName Name of the spectrum file within subdirectory
22- 53 A32 --- Title Title
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Byte-by-byte Description of file (#): spt/*
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 3 I3 --- Order Echelle order number
5- 15 F11.6 0.1nm lambda Wavelength in air (in Angstroem)
17- 28 F12.6 --- Flux Extracted flux
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Byte-by-byte Description of file (#): sph/*
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 8 A8 --- Setting HIRES instrumental setting
9- 11 I3 --- Order Echelle order number
13- 23 F11.6 0.1nm lambda Wavelength in air (Angstroem)
25- 36 F12.6 --- Flux Extracted flux
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Acknowledgements:
Johanna Mueller-Horn, mueller-horn(at)mpia.de
(End) Patricia Vannier [CDS] 09-Feb-2026