X-Ray Emissivity of Old Stellar Populations: A Local Group Census

Ge, Chong; Li, Zhiyuan; Xu, Xiaojie; Gu, Qiusheng; Wang, Q. Daniel; Roberts, Shawn R.; Kraft, Ralph; Jones, C.; Forman, W.

doi:10.1088/0004-637x/812/2/130

Cited by 33 publications

(55 citation statements)

References 58 publications

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“…A direct consequence of the MLT++ treatment of convection is that the effective temperature becomes higher, as illustrated by the above figure. panding 4 whereas stars with radiative envelopes respond by adiabatically contracting. For that reason, mass transfer from stars with outer convective envelopes is more likely to become dynamically unstable, possibly leading to a CE evolution (Webbink 1984;Hjellming & Webbink 1987;Ivanova et al 2013;Ge et al 2015;Pavlovskii & Ivanova 2015). To take this into account, population synthesis codes need to assume different stability criteria depending on whether the donor has a radiative or convective envelope.…”

Section: Model Variationsmentioning

confidence: 99%

Massive donors in interacting binaries: effect of metallicity

Klencki

Nelemans

Istrate

et al. 2020

A&A

123

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Metallicity is known to significantly affect the radial expansion of a massive star: the lower the metallicity the more compact the star, especially during its post-MS evolution. Our goal is to study the impact of this effect in the context of binary evolution. Using the stellar-evolution code MESA, we compute evolutionary tracks of massive stars at six different metallicities between 1.0 Z and 0.01 Z . We explore variations of factors known to affect the radial expansion of massive stars (eg. semiconvection, overshooting, rotation). Using observational constraints, we find support for evolution in which already at metallicity Z ≈ 0.2 Z massive stars stay relatively compact (∼ 100 R ) during the Hertzprung-Gap phase (HG) and most of their expansion happens during core-helium burning (CHeB). Consequently, we show that metallicity has a strong influence on the type of mass transfer evolution in binary systems. At solar metallicity a case-B mass transfer is initiated shortly after the end of MS and a giant donor is almost always a rapidly-expanding HG star. However, at lower metallicity the parameter space for mass transfer from a more evolved, slowly-expanding CHeB star increases dramatically. This means that envelope stripping and formation of helium stars in low metallicity environments happens later during the evolution of the donor, implying a shorter duration of the Wolf-Rayet phase (even by an order of magnitude) and higher final core masses. This metallicity effect is independent of the impact of metallicity-dependent stellar winds. At metallicities Z ≤ 0.04 Z a significant fraction of massive stars in binaries with periods above 100 days engage in their first episode of mass transfer very late into their evolution, when they already have a well developed CO core. The remaining lifetime ( 10 4 yr) is unlikely to be enough to strip the entire H-rich envelope. Cases of unstable mass transfer leading to a merger would produce CO cores that are fast spinning at the moment of collapse. We find that the parameter space for mass transfer from massive donors (> 40 M ) with outer convective envelopes is extremely small or even non-existent. We briefly discuss this finding in the context of formation of binary black hole mergers.

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Section: Model Variationsmentioning

confidence: 99%

Massive donors in interacting binaries: effect of metallicity

Klencki

Nelemans

Istrate

et al. 2020

A&A

123

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“…Interestingly, in a broader comparison of X-ray emissivities of old stellar populations, Ge et al (2015) found that old open clusters not only have higher X-ray emissivities than globular clusters, but also than other old stellar populations without recent star formation, such as dwarf ellipticals, the outer bulge of M 31, and the solar neighbourhood. In those environments, the stellar density is much lower than in the cores of massive globular clusters, casting doubt on whether differences in density are solely responsible for the difference in X-ray output of old stellar populations.…”

Section: Comparison With Other Old Stellar Populationsmentioning

confidence: 99%

A Chandra X-Ray Census of the Interacting Binaries in Old Open Clusters—Collinder 261

Vats

Berg

2017

ApJ

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We present the first X-ray study of Collinder 261 (Cr 261), which at an age of 7 Gyr is one of the oldest open clusters known in the Galaxy. Our observation with the Chandra X-ray Observatory is aimed at uncovering the close interacting binaries in Cr 261, and reaches a limiting X-ray luminosity of L X ≈ 4 × 10 29 erg s −1 (0.3-7 keV) for stars in the cluster. We detect 107 sources within the cluster half-mass radius r h , and we estimate that among the sources with L X 10 30 erg s −1 , ∼26 are associated with the cluster. We identify a mix of active binaries and candidate active binaries, candidate cataclysmic variables, and stars that have "straggled" from the main locus of Cr 261 in the colour-magnitude diagram. Based on a deep optical source catalogue of the field, we estimate that Cr 261 has an approximate mass of 6500 M ⊙ , roughly the same as the old open cluster NGC 6791. The X-ray emissivity of Cr 261 is similar to that of other old open clusters, supporting the trend that they are more luminous in X-rays per unit mass than old populations of higher (globular clusters) and lower (the local neighbourhood) stellar density. This implies that the dynamical destruction of binaries in the densest environments is not solely responsible for the observed differences in X-ray emissivity.

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“…A key point of this paper is to address this question observationally, by considering the X-ray luminosity per unit mass in a range of environments of different stellar densities. We take advantage of a series of studies establishing the X-ray emissivity of a variety of old stellar populations (Sazonov et al 2006;Revnivtsev et al 2007Revnivtsev et al , 2008Warwick 2014;Ge et al 2015).…”

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The X-ray emissivity of low-density stellar populations

Heinke

Ivanov

Koch

et al. 2020

Monthly Notices of the Royal Astronomical Society

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The dynamical production of low-mass X-ray binaries and brighter cataclysmic variables (CVs) in dense globular clusters is well-established. We investigate how the X-ray emissivity of fainter X-ray binaries (principally CVs and coronally active binaries) varies between different environments. We compile calculations (largely from the literature) of the X-ray emissivity of old stellar populations, including open and globular clusters and several galaxies. We investigate three literature claims of unusual X-ray sources in low-density stellar populations. We show that a suggested quiescent neutron star in the open cluster NGC 6819 is a foreground M dwarf. We show that the suggested diffuse X-ray emission from an old nova shell in the globular cluster NGC 6366 is actually a background galaxy cluster. And we show that a suggested population of quiescent X-ray binaries in the Sculptor Dwarf Galaxy is mostly (perhaps entirely) background galaxies. We find that above densities of 10 4 M /pc 3 , the X-ray emissivity of globular clusters increases, due to dynamical production of X-ray emitting systems. Below this density, globular clusters have lower X-ray emissivity than the other populations, and we do not see a strong dependence of X-ray emissivity due to density effects. We find significant correlations between X-ray emissivity and binary fraction, metallicity, and density. Sampling these fits via bootstrap techniques gives less significant correlations, but confirms the effect of metallicity on low-density populations, and that of density on the full globular cluster sample.

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X-Ray Emissivity of Old Stellar Populations: A Local Group Census

Cited by 33 publications

References 58 publications

Massive donors in interacting binaries: effect of metallicity

Massive donors in interacting binaries: effect of metallicity

A Chandra X-Ray Census of the Interacting Binaries in Old Open Clusters—Collinder 261

The X-ray emissivity of low-density stellar populations

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