---
DOAJ_listed: '1'
OA_place: publisher
OA_type: diamond
PlanS_conform: '1'
_id: '22816'
abstract:
- lang: eng
  text: "The Galactic Be star binary MWC 656 was long considered the only known Be
    star+black hole (BH) system, making it a critical\r\nbenchmark 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\r\nHubble Space Telescope,
    combined with high-resolution optical spectra, to reassess the nature of the companion.
    The far-ultraviolet\r\nspectra reveal high-ionisation features – including prominent
    N v and He ii lines – which are absent in the spectra of normal Be stars\r\nand
    are indicative of a hot, luminous companion. Spectral modelling shows that these
    features cannot originate from the Be star or\r\nfrom an accretion disc around
    a compact object. Instead, we find that the data are best explained by a hot (Teff
    ≈ 85 kK), compact,\r\nhydrogen-deficient star with strong wind signatures, consistent
    with an intermediate-mass stripped star. Our revised orbital solution\r\nand composite
    spectroscopic modelling yield a companion mass of M2 = 1.48+0.55\r\n−0.46 M\f,
    definitively ruling out a BH and disfavouring a\r\nwhite dwarf. MWC 656 thus joins
    the growing class of Be + stripped star binaries. The system’s unusual properties
    – including a high\r\ncompanion temperature and wind strength – extend the known
    parameter space of such binaries. The continued absence of confirmed\r\nOBe+BH
    binaries in the Galaxy highlights a growing tension with population synthesis
    models."
acknowledgement: "Open access funding provided by Max Planck Society. The authors
  sincerely thank the anonymous referee for\r\ntheir thorough review and thoughtful
  and constructive comments. The authors\r\nthank Howard Isaacson and Andrew W. Howard
  for their help in obtaining five HIRES optical spectra, which were used in the orbital
  analysis, and\r\nKevin Burdge and Charlie Conroy for their help in obtaining the
  HST spectra. J. Müller-Horn thanks Hans-Walter Rix, Sahar Shahaf, Pablo Marchant,\r\nAnnachiara
  Picco, Anna F. Pala, and Dominic Bowman for helpful discussions and suggestions.
  J. Müller-Horn acknowledges support from the European\r\nResearch Council for the
  ERC Advanced Grant [101054731]. V. Ramachandran and A.A.C. Sander are supported
  by the German Deutsche Forschungsgemeinschaft, DFG in the form of an Emmy Noether
  Research Group –\r\nProject-ID 445674056 (SA4064/1-1, PI Sander) and acknowledge
  financial\r\nsupport by the Federal Ministry for Economic Affairs and Energy (BMWE)\r\nvia
  the Deutsches Zentrum für Luft- und Raumfahrt (DLR) grant 50 OR\r\n2509 (PI Sander).
  V. Ramachandran, A.A.C. Sander, and E.C. Schösser further acknowledge financial
  support by the BMWE via the DLR grant 50 OR\r\n2306 (PI Ramachandran/Sander). This
  project was co-funded by the European Union (Project 101183150 – OCEANS). This work
  has made use of\r\ndata from the European Space Agency (ESA) mission Gaia (https://www.\r\ncosmos.esa.int/gaia),
  processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/\r\nconsortium).
  Funding for the DPAC has been provided by national institutions,\r\nin particular
  the institutions participating in the Gaia Multilateral Agreement.\r\nTS acknowledges
  support from the Israel Science Foundation (ISF) under grant\r\nnumber 0603225041
  and from the European Research Council (ERC) under the\r\nEuropean Union’s Horizon
  2020 research and innovation program (grant agreement 101164755/METAL)."
article_number: A187
article_processing_charge: No
article_type: original
arxiv: 1
author:
- first_name: Johanna
  full_name: Müller-Horn, Johanna
  last_name: Müller-Horn
- first_name: Varsha
  full_name: Ramachandran, Varsha
  last_name: Ramachandran
- first_name: Kareem
  full_name: El-Badry, Kareem
  last_name: El-Badry
- first_name: Andreas A.C.
  full_name: Sander, Andreas A.C.
  last_name: Sander
- first_name: Julia K
  full_name: Bodensteiner, Julia K
  id: 48162413-c59a-11ee-a1af-dbba36c8a756
  last_name: Bodensteiner
- first_name: Douglas R.
  full_name: Gies, Douglas R.
  last_name: Gies
- first_name: Ylva Louise Linsdotter
  full_name: Götberg, Ylva Louise Linsdotter
  id: d0648d0c-0f64-11ee-a2e0-dd0faa2e4f7d
  last_name: Götberg
  orcid: 0000-0002-6960-6911
- first_name: Thomas
  full_name: Rivinius, Thomas
  last_name: Rivinius
- first_name: Tomer
  full_name: Shenar, Tomer
  last_name: Shenar
- first_name: Elisa C.
  full_name: Schösser, Elisa C.
  last_name: Schösser
- first_name: Luqian
  full_name: Wang, Luqian
  last_name: Wang
- first_name: Allyson
  full_name: Bieryla, Allyson
  last_name: Bieryla
- first_name: Lars A.
  full_name: Buchhave, Lars A.
  last_name: Buchhave
- first_name: David W.
  full_name: Latham, David W.
  last_name: Latham
citation:
  ama: Müller-Horn J, Ramachandran V, El-Badry K, et al. Ultraviolet spectroscopy
    reveals a hot and luminous companion to the Be star+black hole candidate MWC 656.
    <i>Astronomy &#38; Astrophysics</i>. 2026;708. doi:<a href="https://doi.org/10.1051/0004-6361/202557960">10.1051/0004-6361/202557960</a>
  apa: Müller-Horn, J., Ramachandran, V., El-Badry, K., Sander, A. A. C., Bodensteiner,
    J. K., Gies, D. R., … Latham, D. W. (2026). Ultraviolet spectroscopy reveals a
    hot and luminous companion to the Be star+black hole candidate MWC 656. <i>Astronomy
    &#38; Astrophysics</i>. EDP Sciences. <a href="https://doi.org/10.1051/0004-6361/202557960">https://doi.org/10.1051/0004-6361/202557960</a>
  chicago: Müller-Horn, Johanna, Varsha Ramachandran, Kareem El-Badry, Andreas A.C.
    Sander, Julia K Bodensteiner, Douglas R. Gies, Ylva Louise Linsdotter Götberg,
    et al. “Ultraviolet Spectroscopy Reveals a Hot and Luminous Companion to the Be
    Star+black Hole Candidate MWC 656.” <i>Astronomy &#38; Astrophysics</i>. EDP Sciences,
    2026. <a href="https://doi.org/10.1051/0004-6361/202557960">https://doi.org/10.1051/0004-6361/202557960</a>.
  ieee: J. Müller-Horn <i>et al.</i>, “Ultraviolet spectroscopy reveals a hot and
    luminous companion to the Be star+black hole candidate MWC 656,” <i>Astronomy
    &#38; Astrophysics</i>, vol. 708. EDP Sciences, 2026.
  ista: Müller-Horn J, Ramachandran V, El-Badry K, Sander AAC, Bodensteiner JK, Gies
    DR, Götberg YLL, Rivinius T, Shenar T, Schösser EC, Wang L, Bieryla A, Buchhave
    LA, Latham DW. 2026. Ultraviolet spectroscopy reveals a hot and luminous companion
    to the Be star+black hole candidate MWC 656. Astronomy &#38; Astrophysics. 708,
    A187.
  mla: Müller-Horn, Johanna, et al. “Ultraviolet Spectroscopy Reveals a Hot and Luminous
    Companion to the Be Star+black Hole Candidate MWC 656.” <i>Astronomy &#38; Astrophysics</i>,
    vol. 708, A187, EDP Sciences, 2026, doi:<a href="https://doi.org/10.1051/0004-6361/202557960">10.1051/0004-6361/202557960</a>.
  short: J. Müller-Horn, V. Ramachandran, K. El-Badry, A.A.C. Sander, J.K. Bodensteiner,
    D.R. Gies, Y.L.L. Götberg, T. Rivinius, T. Shenar, E.C. Schösser, L. Wang, A.
    Bieryla, L.A. Buchhave, D.W. Latham, Astronomy &#38; Astrophysics 708 (2026).
das_tickbox: '1'
dataavailabilitystatement: "The newly calculated PoWR model spectra for the Be star
  and\r\nthe companion will be provided as supplementary material on\r\nZenodo. The
  HST UV spectra used in this work are publicly\r\naccessible and can be obtained
  from MAST. Optical TRES and\r\nHIRES spectra are available at the CDS via https://cdsarc.\r\ncds.unistra.fr/viz-bin/cat/J/A+A/708/A187"
date_created: 2026-09-06T22:01:57Z
date_published: 2026-04-01T00:00:00Z
date_updated: 2026-09-16T08:56:14Z
day: '01'
ddc:
- '520'
department:
- _id: YlGo
doi: 10.1051/0004-6361/202557960
external_id:
  arxiv:
  - '2601.14403'
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fulldoi: https://doi.org/10.1051/0004-6361/202557960
has_accepted_license: '1'
intvolume: '       708'
language:
- iso: eng
month: '04'
oa: 1
oa_version: Published Version
publication: Astronomy & Astrophysics
publication_identifier:
  eissn:
  - 1432-0746
  issn:
  - 0004-6361
publication_status: published
publisher: EDP Sciences
quality_controlled: '1'
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    relation: research_data
    status: public
researchdata_availability: no
scopus_import: '1'
status: public
supplementarymaterial: yes
title: Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black
  hole candidate MWC 656
tmp:
  image: /images/cc_by.png
  legal_code_url: https://creativecommons.org/licenses/by/4.0/legalcode
  name: Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)
  short: CC BY (4.0)
type: journal_article
user_id: 317138e5-6ab7-11ef-aa6d-ffef3953e345
volume: 708
year: '2026'
...