A possible formation channel for blue hook stars in globular cluster – II. Effects of metallicity, mass ratio, tidal enhancement efficiency and helium abundance

Lei, Zhenxin; Zhao, Gang; Zeng, A.; Shen, L. C.; Lan, Zhongjian; Jiang, Dengkai; Han, Zhanwen

doi:10.1093/mnras/stw2242

Cited by 16 publications

(15 citation statements)

References 68 publications

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“…Using MESA, Vos et al (2015) and Lei et al (2015Lei et al ( , 2016 obtained several sdBs with long orbital periods through stable mass transfer or tidally enhanced stellar wind. These sdBs with long orbital periods are also divided into three classes: flash-mixing sdBs, shallow-mixing sdBs, and canonical sdBs.…”

Section: Comparison With Sdbs With Long Orbital Periodsmentioning

confidence: 99%

Subdwarf B stars from the common envelope ejection channel

Xiong

Chen

Podsiadlowski

et al. 2017

A&A

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Context. Subdwarf B stars (sdB) are important to stellar evolutionary theory and asteroseismology, and they are crucial to our understanding of the structure and evolution of the Galaxy. According to the canonical binary scenario, the majority of sdBs are produced from low-mass stars with degenerate cores where helium is ignited in a way of flashes. Owing to numerical difficulties, the models of produced sdBs are generally constructed from more massive stars with non-degenerate cores. This leaves several uncertainties on the exact characteristics of sdB stars. Aims. The purpose of this paper is to study the characteristics of sdBs produced from the common envelope (CE) ejection channel. Methods. We used the stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA), which can resolve flashes during stellar evolution. To mimic the CE ejection process, we first evolved a single star to a position near the tip of the red giant branch, then artificially removed its envelope with a very high mass-loss rate until the envelope began to shrink. Finally, we followed the evolution of the remnant until it became a helium or a carbon-oxygen white dwarf. Results. The sdB stars produced from the CE ejection channel appear to form two distinct groups on the effective temperature-gravity diagram. One group, referred to as the flash-mixing sdBs, almost has no H-rich envelope and crowds at the hottest temperature end of the extreme horizontal branch (EHB), while the other group, called the canonical sdBs, has significant H-rich envelope and is spread throughout the entire canonical EHB region. The key factor for the dichotomy of the sdB properties is the development of convection during the first helium flash, that is, that the convection region penetrates the H-rich envelope in the case of the flash-mixing sdBs, but fails to do this in the case of the canonical sdBs. Conclusions. The dichotomy of the sdB properties from the CE ejection channel is intrinsic and caused by the interior structure of the star after the CE ejection. The modelling of the CE ejection process will greatly change the parameter space for the two typical groups of sdB stars. If the CE ejection stops early for a given initial stellar mass and a given core mass at the onset of the CE, then the star has a relatively massive H-rich envelope, which generally results in a canonical sdB. Observationally, only a few sdB binaries with short orbital periods are located in the flash-mixing sdB region, and there is a lack of He-rich sdBs in binaries with short orbital periods. This indicates that flash mixing is not very frequent in products of the CE ejection. A falling-back process after the CE ejection, similar to what occurs in nova, is an appropriate way of increasing the envelope mass, and it then prevents flash mixing.

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Section: Comparison With Sdbs With Long Orbital Periodsmentioning

confidence: 99%

Subdwarf B stars from the common envelope ejection channel

Xiong

Chen

Podsiadlowski

et al. 2017

A&A

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“…The main deficiency of both scenarios is that they do not explain the physical mechanism behind the enhanced mass loss required on the RGB. Lei et al (2013Lei et al ( , 2015Lei et al ( , 2016 investigated possible mechanisms and showed that if the star is initially part of a wide-binary system (P ∼ 900−4600 d), the mass loss on the RGB can be tidally enhanced sufficiently to delay the helium-flash. Lei et al (2016) estimated that binaries with periods below 1400 d could have survived dynamical encounters during the evolution of ω Cen.…”

Section: Introductionmentioning

confidence: 99%

“…Lei et al (2013Lei et al ( , 2015Lei et al ( , 2016 investigated possible mechanisms and showed that if the star is initially part of a wide-binary system (P ∼ 900−4600 d), the mass loss on the RGB can be tidally enhanced sufficiently to delay the helium-flash. Lei et al (2016) estimated that binaries with periods below 1400 d could have survived dynamical encounters during the evolution of ω Cen. Dynamical encounters could themselves produce tidal stripping through Roche-Lobe overflow (Pasquato et al 2014).…”

Section: Introductionmentioning

confidence: 99%

Shotglas

Latour

Randall²,

Calamida

et al. 2018

A&A

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The presence of extreme horizontal branch (EHB) and blue hook stars in some Galactic globular clusters (GGCs) constitutes one of the remaining mysteries of stellar evolution. While several evolutionary scenarios have been proposed to explain the characteristics of this peculiar population of evolved stars, their observational verification has been limited by the availability of spectroscopic data for a statistically significant sample of such objects in any single GGC. We recently launched the SHOTGLAS project with the aim of providing a comprehensive picture of this intriguing stellar population in terms of spectroscopic properties for all readily accessible GGCs hosting an EHB. In this first paper, we focus on ω Cen, a peculiar, massive GGC that hosts multiple stellar populations. We use non-LTE model atmospheres to derive atmospheric parameters (Teff, log g and N(He)/N(H)) and spectroscopic masses for 152 EHB stars in the cluster. This constitutes the largest spectroscopic sample of EHB stars ever analyzed in a GGC and represents ≈20% of the EHB population of ω Cen. We also search for close binaries among these stars based on radial velocity variations. Our results show that the EHB population of ω Cen is divided into three spectroscopic groups that are very distinct in the Teff − helium abundance plane. The coolest sdB-type stars (Teff ≲ 30 000 K) have a hydrogen-rich atmosphere, populate the theoretical EHB region in the Teff − log g plane, and form 26% of our sample. The hottest sdO-type stars (Teff ≳ 42 000 K) make up 10% of the sample, have a hydrogen-rich atmosphere and are thought to be in a post-EHB evolutionary phase. The majority of our sample is found at intermediate temperatures and consists of sdOB stars that have roughly solar or super-solar atmospheric helium abundances. It is these objects that constitute the blue hook at V > 18.5 mag in the ω Cen color-magnitude diagram. Interestingly, the helium-enriched sdOBs do not have a significant counterpart population in the Galactic field, indicating that their formation is dependent on the particular environment found in ω Cen and other select GGCs. Another major difference between the EHB stars in ω Cen and the field is the fraction of close binaries. From our radial velocity survey we identify two binary candidates, however no orbital solutions could be determined. We estimate an EHB close binary fraction of ≈5% in ω Cen. This low fraction is in line with findings for other GGCs, but in sharp contrast to the situation in the field, where around 50% of the sdB stars reside in close binaries. Finally, the mass distribution derived is very similar for all three spectroscopic groups, however the average mass (0.38 M⊙) is lower than that expected from stellar evolution theory. While this mass conundrum has previously been noted for EHB stars in ω Cen, it so far appears to be unique to that cluster.

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“…Hot subdwarfs with pulsations can be used for asteroseismic analyses (Charpinet et al 2011). Lei et al (2015Lei et al ( , 2016 used Hot subdwarfs to study the extended horizontal branch morphology of globular clusters.…”

Section: Introductionmentioning

confidence: 99%

A Robust Hot Subdwarfs Identification Method Based on Deep Learning

Tan¹,

Mei²,

Liu³

et al. 2022

Preprint

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Hot subdwarf star is a particular type of star that is crucial for studying binary evolution and atmospheric diffusion processes. In recent years, identifying Hot subdwarfs by machine learning methods has become a hot topic, but there are still limitations in automation and accuracy. In this paper, we proposed a robust identification method based on the convolutional neural network (CNN). We first constructed the dataset using the spectral data of LAMOS DR7-V1. We then constructed a hybrid recognition model including an 8-class classification model and a binary classification model. The model achieved an accuracy of 96.17% on the testing set. To further validate the accuracy of the model, we selected 835 Hot subdwarfs that were not involved in the training process from the identified LAMOST catalog (2428, including repeated observations) as the validation set. An accuracy of 96.05% was achieved. On this basis, we used the model to filter and classify all 10,640,255 spectra of LAMOST DR7-V1, and obtained a catalog of 2393 Hot subdwarf candidates, of which 2067 have been confirmed. We found 25 new Hot subdwarfs among the remaining candidates by manual validation. The overall accuracy of the model is 87.42%. Overall, the model presented in this study can effectively identify specific spectra with robust results and high accuracy, and can be further applied to the classification of large-scale spectra and the search of specific targets.

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A possible formation channel for blue hook stars in globular cluster – II. Effects of metallicity, mass ratio, tidal enhancement efficiency and helium abundance

Cited by 16 publications

References 68 publications

Subdwarf B stars from the common envelope ejection channel

Subdwarf B stars from the common envelope ejection channel

Shotglas

A Robust Hot Subdwarfs Identification Method Based on Deep Learning

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