{"status":"public","file_date_updated":"2026-09-09T13:08:37Z","day":"01","language":[{"iso":"eng"}],"author":[{"first_name":"Johanna","last_name":"Müller-Horn","full_name":"Müller-Horn, Johanna"},{"first_name":"Varsha","last_name":"Ramachandran","full_name":"Ramachandran, Varsha"},{"first_name":"Kareem","last_name":"El-Badry","full_name":"El-Badry, Kareem"},{"last_name":"Sander","full_name":"Sander, Andreas A.C.","first_name":"Andreas A.C."},{"first_name":"Julia K","last_name":"Bodensteiner","id":"48162413-c59a-11ee-a1af-dbba36c8a756","full_name":"Bodensteiner, Julia K"},{"first_name":"Douglas R.","full_name":"Gies, Douglas R.","last_name":"Gies"},{"last_name":"Götberg","id":"d0648d0c-0f64-11ee-a2e0-dd0faa2e4f7d","full_name":"Götberg, Ylva Louise Linsdotter","first_name":"Ylva Louise Linsdotter","orcid":"0000-0002-6960-6911"},{"first_name":"Thomas","last_name":"Rivinius","full_name":"Rivinius, Thomas"},{"first_name":"Tomer","full_name":"Shenar, Tomer","last_name":"Shenar"},{"last_name":"Schösser","full_name":"Schösser, Elisa C.","first_name":"Elisa C."},{"first_name":"Luqian","last_name":"Wang","full_name":"Wang, Luqian"},{"first_name":"Allyson","last_name":"Bieryla","full_name":"Bieryla, Allyson"},{"first_name":"Lars A.","full_name":"Buchhave, Lars A.","last_name":"Buchhave"},{"first_name":"David W.","last_name":"Latham","full_name":"Latham, David W."}],"arxiv":1,"external_id":{"arxiv":["2601.14403"]},"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."}],"OA_type":"diamond","user_id":"2DF688A6-F248-11E8-B48F-1D18A9856A87","article_number":"A187","publisher":"EDP Sciences","researchdata_availability":"no","das_tickbox":"1","citation":{"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>","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).","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>.","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.","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>","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>.","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."},"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).","_id":"22816","supplementarymaterial":"no","department":[{"_id":"YlGo"}],"scopus_import":"1","doi":"10.1051/0004-6361/202557960","date_published":"2026-04-01T00:00:00Z","OA_place":"publisher","has_accepted_license":"1","oa_version":"Published Version","article_type":"original","ddc":["520"],"tmp":{"image":"/images/cc_by.png","legal_code_url":"https://creativecommons.org/licenses/by/4.0/legalcode","short":"CC BY (4.0)","name":"Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)"},"quality_controlled":"1","license":"https://creativecommons.org/licenses/by/4.0/","oa":1,"month":"04","article_processing_charge":"No","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","title":"Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656","intvolume":"       708","DOAJ_listed":"1","volume":708,"type":"journal_article","year":"2026","date_updated":"2026-09-09T13:11:09Z","PlanS_conform":"1","publication":"Astronomy & Astrophysics","publication_status":"published","publication_identifier":{"eissn":["1432-0746"],"issn":["0004-6361"]},"date_created":"2026-09-06T22:01:57Z","fulldoi":"https://doi.org/10.1051/0004-6361/202557960","file":[{"file_id":"22891","file_size":5726333,"checksum":"58213b981fe02219f3c9b77ad76abd8e","access_level":"open_access","date_created":"2026-09-09T13:08:37Z","date_updated":"2026-09-09T13:08:37Z","success":1,"file_name":"2026_AstronomyAstrophysics_MuellerHorn.pdf","content_type":"application/pdf","creator":"dernst","relation":"main_file"}]}