NGC 1850 BH1 is another stripped-star binary masquerading as a black hole

El-Badry, Kareem; Burdge, Kevin

doi:10.48550/arxiv.2111.07925

Cited by 7 publications

(8 citation statements)

References 20 publications

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“…Their evolutionary history is qualitatively quite similar to the models explored to explain the HR 6819, LB-1, and NGC 1805 BH1 systems (e.g. Eldridge et al 2020;Bodensteiner et al 2020b;El-Badry & Quataert 2021;El-Badry & Burdge 2021;Stevance et al 2021). In all the selected models, mass transfer begins while the donor is still on the main sequence ("case A").…”

Section: Bpass Library Searchsupporting

confidence: 58%

“…All of these systems were initially interpreted as containing a main-sequence B star orbiting a black hole, but subsequent studies (e.g. Irrgang et al 2020;Shenar et al 2020;Bodensteiner et al 2020b;El-Badry & Quataert 2021;El-Badry & Burdge 2021) have shown that the B stars are not on the main sequence, but appear to be undermassive (≈ 0.5−1.5 𝑀 ) stripped products of binary evolution that are currently contracting to become core helium burning sdOB stars. Because the current evolutionary state of these objects is short-lived, their discovery implies that there may be many other systems with similar evolutionary histories yet to be discovered.…”

Section: Introductionmentioning

confidence: 99%

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Birth of a Be star: an APOGEE search for Be stars forming through binary mass transfer

El-Badry¹,

Conroy²,

Quataert³

et al. 2022

Preprint

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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 Be = 5.3 ± 0.6 𝑀 ) with a low-mass (𝑀 donor = 0.92 ± 0.22 𝑀 ), 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 He ≈ 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 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 (∼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 +27 −16 ) % 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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Section: Bpass Library Searchsupporting

confidence: 58%

Section: Introductionmentioning

confidence: 99%

Birth of a Be star: an APOGEE search for Be stars forming through binary mass transfer

El-Badry¹,

Conroy²,

Quataert³

et al. 2022

Preprint

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“…The mass function yields a Roche radius of approximately 10 R , giving a fill factor of only 0.66. This system cannot reproduce the observed light curve as it would generate maximum (i.e., sini=1) ellipsoidal light variations with an amplitude of only ∼4-5%, significantly smaller than the observed light curve constraint of ∼10-12%, in broad agreement with El-Badry & Burdge (2021). Reducing the mass of the primary we find that its Roche radius matches the stellar radius for a mass of ∼1.8 M , the light curve then implying i∼47°with a BH mass of ∼5.8 M .…”

Section: Conclusion and Caveatsmentioning

confidence: 47%

The VLT-FLAMES survey of massive stars. NGC2004#115: A triple system hosting a possible short period B+BH binary

Lennon,

Dufton,

Villaseñor

et al. 2021

Preprint

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Context. NGC 2004#115 was classified as a single lined (SB1) Be spectroscopic binary in the Large Magellanic Cloud. Its Hα morphology is reminiscent of the Galactic systems LB-1 and HR 6819, both of which are proposed as either Be+BH or Be+stripped He-star systems. Aims. Multi-epoch optical spectra of NGC 2004#115 are used to determine if this binary can be explained by either of these two scenarios, and hence shed additional light on these interesting systems. Methods. VLT-FLAMES and SALT-HRS data covering a baseline of ∼20 years were analyzed to determine radial velocities and orbital parameters, while non-LTE model atmospheres were used to determine stellar parameters and the relative brightness of the system components. Archive MACHO, Gaia, and XMM-Newton data provide additional constraints on the system. Results. NGC 2004#115 is found to be a triple system consisting of an inner binary with a period P=2.92 d, eccentricity e ∼0.0, and mass function f =0.07 M . The only firmly detected star in this inner binary is a B2 star, the primary, with a projected rotational velocity (v e sin i) of 10 km s −1 and a luminosity of log L/L =3.87. It contributes ∼ 60% of the total V-band light, with the tertiary contributing the other ∼ 40% of the light, while the secondary is not detected in the optical spectrum. The possibility that the primary is a low mass inflated stripped star is excluded since its Roche radius would be smaller than its stellar radius in such a compact system. A main sequence star of mass 8.6 M is therefore inferred; however, the assumption of synchronous rotation leads to a secondary mass in excess of 25 M , which would therefore be a black hole. The tertiary is detected as a fainter blended component to the hydrogen and helium lines, which is consistent with a slightly less massive B-type star, though with v e sin i∼300 km s −1 . The data do not permit the characterization of the outer period, though it likely exceeds 120 days and is therefore in a stable configuration. The disk-like emission is variable, but may be associated with the inner binary rather than the rapidly rotating tertiary. XMM-Newton provides an upper limit of 5 × 10 33 ergs s −1 in the 0.2-12 keV band which is consistent with, though not constraining, the system hosting a quiescent B+BH binary. A number of caveats to this scenario are discussed.

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“…Given that these systems can be well described by models in which an undermassive stripped star orbits a Be star (e.g. Shenar et al 2020;Bodensteiner et al 2020;El-Badry & Burdge 2021), it is worth considering carefully whether such a scenario could also explain NGC 2004 #115. An attractive feature of the stripped star scenario is that it simultaneously explains the slow rotation of the B star and the rapid rotation of the Be star.…”

Section: Is a Stripped Star Scenario Possible?mentioning

confidence: 99%

NGC 2004 #115: a black hole imposter containing three luminous stars

El-Badry,

Burdge,

Mróz

2021

Preprint

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NGC 2004 #115 is a recently identified black hole (BH) candidate in the Large Magellanic Cloud (LMC) containing a B star orbiting an unseen companion in a 2.9 day orbit and Be star tertiary. We show that the unseen companion is not a 25 𝑀 BH, but a (2 − 3) 𝑀 luminous star. Analyzing the OGLE and MACHO light curves of the system, we detect ellipsoidal variability with amplitude 10 times larger than would be expected if the companion were a 25 𝑀 BH, ruling out the low inclination required for a massive companion. The light curve also shows a clear reflection effect that is well-modeled with a 2.5 𝑀 main-sequence secondary, ruling out a lower-mass BH or neutron star companion. We consider and reject models in which the system is a binary containing a stripped star orbiting the Be star: only a triple model with an outer Be star can explain both the observed light curve and radial velocities. Our results imply that the B star, whose slow projected rotation velocity and presumed tidal synchronization were interpreted as evidence for a low inclination (and thus a high companion mass), is far from being tidally synchronized: despite being in a 2.9 day orbit that is fully or nearly circularized (𝑒 < 0.04), its surface rotation period appears to be at least 20 days. We offer cautionary notes on the interpretation of dormant BH candidates in binaries.

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NGC 1850 BH1 is another stripped-star binary masquerading as a black hole

Cited by 7 publications

References 20 publications

Birth of a Be star: an APOGEE search for Be stars forming through binary mass transfer

Birth of a Be star: an APOGEE search for Be stars forming through binary mass transfer

The VLT-FLAMES survey of massive stars. NGC2004#115: A triple system hosting a possible short period B+BH binary

NGC 2004 #115: a black hole imposter containing three luminous stars

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