arXiv subjects

Dengkai Jiang

Publications and source records attributed to Dengkai Jiang.

33 records · Page 2Linked to original sources

arXiv2021

The fractions of post-binary-interaction stars and evolved blue straggler stars on the red giant branch of globular clusters

The red giant branch (RGB) of globular clusters (GCs) is home to some exotic stars, which may provide clues on the formation of multiple stellar populations in GCs. It is well known that binary interactions are responsible for many exotic stars. Thus, it is important to understand what fraction of stars on the RGB of GCs is the result of binary interactions. In this paper, we performed a binary population synthesis study to track the number of post-binary-interaction (post-BI) stars that appear on the RGB, with particular emphasis on the evolved blue straggler stars (E-BSSs). Assuming an initial binary fraction of nearly 50%, we find that about half of the objects on the RGB (called giants) underwent the binary interactions, and that E-BSSs account for around 10% of the giants in our standard simulation. We also compare the properties of post-BI giants that evolved from different channels. We find that the initial orbital period and mass ratio distributions significantly affect the fraction of post-BI giants. Our results imply that the non-standard stars from binary interactions provide a non-negligible contribution to the RGB stars in GCs, which should be considered in future investigations of the origin of multiple stellar populations.

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arXiv2020

Mapping the Galactic disk with the LAMOST and $Gaia$ Red clump sample: V: On the origin of the "young" [$α$/Fe]-enhanced stars

Using a sample of nearly 140,000 primary red clump stars selected from the LAMOST and $Gaia$ surveys, we have identified a large sample of "young" [$α$/Fe]-enhanced stars with stellar ages younger than 6.0 Gyr and [$α$/Fe] ratios greater than 0.15 dex. The stellar ages and [$α$/Fe] ratios are measured from LAMOST spectra, using a machine learning method trained with common stars in the LAMOST-APOGEE fields (for [$α$/Fe]) and in the LAMOST-$Kepler$ fields (for stellar age). The existence of these "young" [$α$/Fe]-enhanced stars is not expected from the classical Galactic chemical evolution models. To explore their possible origins, we have analyzed the spatial distribution, and the chemical and kinematic properties of those stars and compared the results with those of the chemically thin and thick disk populations. We find that those "young" [$α$/Fe]-enhanced stars have distributions in number density, metallicity, [C/N] abundance ratio, velocity dispersion and orbital eccentricity that are essentially the same as those of the chemically thick disk population. Our results clearly show those so-called "young" [$α$/Fe]-enhanced stars are not really young but $genuinely$ $old$. Although other alternative explanations can not be fully ruled out, our results suggest that the most possible origin of these old stars is the result of stellar mergers or mass transfer.

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arXiv2020

Mass transfer of low-mass binaries and chemical anomalies among unevolved stars in globular clusters

While it is well known that mass transfer in binaries can pollute the surfaces of the accretors, it is still unclear whether this mechanism can reproduce the observed chemical inhomogeneities in globular clusters. We study the surface abundances of the accretors in low-mass binaries, as a first step towards understanding whether mass transfer in low-mass binaries is one of the potential origins of the aforementioned abundance anomalies in globular clusters. We use the Modules for Experiments in Stellar Astrophysics code to calculate binary evolutionary models with different initial donor masses between 0.9 and 1.9 $ \rm {M}_\odot$ for an initial metallicity of Z = 0.0034. The results show that in some low-mass binary systems, the accretors exhibit peculiar chemical patterns when they are still unevolved stars, e.g. C and O depletion; Na and N enhancement; constant Mg, Al and C+N+O. The abundance patterns of the accretors are significantly different from their initial abundances (or that of normal single stars), and can match the observed populations. These abundance patterns strongly depend not only on the initial parameters of binaries (donor mass, mass ratio, and orbital period), but also on the assumptions regarding mass-transfer efficiency and angular momentum loss. These results support the hypothesis that mass transfer in low-mass binaries is at least partly responsible for the unevolved anomalous stars in globular clusters. More work on binary evolutionary models and binary population synthesis is required to fully evaluate the contribution of this scenario.

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arXiv2019

Case AD, AR, and AS binary evolution and their possible connections with W UMa binaries

Close detached binaries were theoretically predicted to evolve into contact by three subtypes of case A binary evolution: case AD, AR, and AS, which correspond to the formation of contact during dynamic-, thermal-, and nuclear-timescale mass transfer phase, respectively. It is unclear, however, what is the difference between contact binaries in these subtypes, and whether all of these subtypes can account for the formation of observed W UMa binaries. Using Eggleton's stellar evolution code with the nonconservative assumption, I obtained the low-mass contact binaries produced by case AD, AR, and AS at the moment of contact, and their parameter spaces. The results support that the progenitors of low-mass contact binaries are detached binaries with orbital periods shorter than $\sim2-5\,$d, and their borderlines depend strongly on the primary mass. In addition, the period-colour relations for case AR and AS can be in better agreement with that for observed W UMa candidates, but case AD shows a significantly worse agreement. Moreover, case AR and AS can produce a short-period limit (corresponding to a low-mass limit) at almost any age, e.g. from young age ($\sim0.2\,$Gyr) to old age ($\sim13\,$Gyr), agreeing with observed W UMa binaries in star clusters, but no such limit occurs for case AD at any age. These results support that case AR and AS, as opposed to case AD, can lead to W UMa binaries (including young W UMa binaries).

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arXiv2018

The double blue straggler sequence in NGC 2173: a field contamination artefact?

Li et al. (2018) (hereafter L18) detected two apparently distinct populations of blue straggler stars (BSSs) in the young globular cluster NGC 2173, a similar feature as observed in numerous Galactic globular clusters (GCs). Recently, Dalessandro et al. (2018) (D18) compared the observed color--magnitude diagrams (CMDs) of both the cluster and a nearby reference field (although observed with the ACS/WFC instrument). They conclude that the bifurcated pattern of BSSs in NGC 2173 observed by L18 is a field contamination artefact. In this note, we explore the central concentration properties of the removed `field stars' identified by D18. Our purpose is to examine if these `field stars' are spatially homogeneously distributed. Employing a Monte Carlo-based approach, we have carefully studied the probability that any such central concentration may be caused by small number statistics. We find that, in most cases of, the `field stars' removed by D18 exhibit a clear central concentration, which cannot be explained on the basis of small number statistics alone. Therefore, we suggest that D18 may well have overestimated the field contamination level, implying that the bifurcated BSS pattern in NGC 2173 cannot, in fact, be explained by field contamination.

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arXiv2018

An unexpected detection of bifurcated blue straggler sequences in the young globular cluster NGC 2173

Bifurcated patterns of blue straggler stars in their color--magnitude diagrams have atracted significant attention. This type of special (but rare) pattern of two distinct blue straggler sequences is commonly interpreted as evidence of cluster core-collapse-driven stellar collisions as an efficient formation mechanism. Here, we report the detection of a bifurcated blue straggler distribution in a young Large MagellanicCloud cluster, NGC 2173. Because of the cluster's low central stellar number density and its young age, dynamical analysis shows that stellar collisions alone cannot explain the observed blue straggler stars. Therefore, binary evolution is instead the most viable explanation of the origin of these blue straggler stars. However, the reason why binary evolution would render the color--magnitude distribution of blue straggler stars bifurcated remains unclear.

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arXiv2017

Contribution of primordial binary evolution to the two blue-straggler sequences in globular cluster M30

Two blue-straggler sequences discovered in globular cluster M30 provide a strong constraint on the formation mechanisms of blue stragglers. We study the formation of the blue-straggler binaries through binary evolution, and find that binary evolution can contribute to the blue stragglers in both of the sequences. Whether a blue straggler is located in the blue sequence or red sequence depends on the contribution of the mass donor to the total luminosity of the binary, which is generally observed as a single star in globular clusters. The blue stragglers in the blue sequence have a cool white-dwarf companion, while the majority ($\sim 60\%$) of the objects in the red sequence are the binaries that are still experiencing mass transfer, but there are also some objects that the donors have just finished the mass transfer (the stripped-core stars, $\sim 10\%$) or the blue stragglers (the accretors) have evolved away from the blue sequence ($\sim 30\%$). Meanwhile, W UMa contact binaries found in both sequences may be explained by various mass ratios, that is, W UMa contact binaries in the red sequence have two components with comparable masses (e.g. mass ratio $q\sim$ 0.3-1.0), while those in the blue sequence have low mass ratio (e.g. $q<$ 0.3). However, the fraction of blue sequence in M30 cannot be reproduced by binary population synthesis if we assumed the initial parameters of a binary sample to be the same as that of the field. This possible indicates that dynamical effects on binary systems is very important in globular clusters.

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arXiv2016

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

Employing tidally enhanced stellar wind, we studied in binaries the effects of metallicity, mass ratio of primary to secondary, tidal enhancement efficiency and helium abundance on the formation of blue hook (BHk) stars in globular clusters (GCs). A total of 28 sets of binary models combined with different input parameters are studied. For each set of binary model, we presented a range of initial orbital periods that is needed to produce BHk stars in binaries. All the binary models could produce BHk stars within different range of initial orbital periods. We also compared our results with the observation in the Teff-logg diagram of GC NGC 2808 and ω Cen. Most of the BHk stars in these two GCs locate well in the region predicted by our theoretical models, especially when C/N-enhanced model atmospheres are considered. We found that mass ratio of primary to secondary and tidal enhancement efficiency have little effects on the formation of BHk stars in binaries, while metallicity and helium abundance would play important roles, especially for helium abundance. Specifically, with helium abundance increasing in binary models, the space range of initial orbital periods needed to produce BHk stars becomes obviously wider, regardless of other input parameters adopted. Our results were discussed with recent observations and other theoretical models.

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arXiv2014

Binary interactions as a possible scenario for the formation of multiple stellar populations in globular clusters

Observations revealed the presence of multiple stellar populations in globular clusters (GCs) that exhibit wide abundance variations and multiple sequences in Hertzsprung-Russell (HR) diagram. We present a scenario for the formation of multiple stellar populations in GCs. In this scenario, initial GCs are single-generation clusters, and our model predicts that the abundance anomalous stars observed in GCs are the merged stars and the accretor stars produced by binary interactions, which are rapidly rotating stars at the moment of their formation and are more massive than normal single stars in the same evolutionary stage. We find that due to their own evolution, these rapidly rotating stars have different surface abundances, effective temperatures and luminosities from normal single stars in the same evolutionary stage. The stellar population with binaries can reproduce two important observational evidences of multiple stellar populations, the Na-O anticorrelation and the multiple sequences in HR diagram. This suggests that binary interactions may be a possible scenario for the formation of multiple stellar populations in GCs.

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arXiv2013

The detached-binary channel for the formation of contact binaries

The detached-binary channel is an important channel for the formation of contact binaries, according to which a detached binary might evolve into contact by evolutionary expansion of the components, or angular momentum loss through the effect of magnetic braking (MB). We have carried out a detailed binary population synthesis (BPS) study of this channel, and obtained the parameter regions for detached binaries to evolve into contact. Combining the observations from the $\emph{Kepler}$ satellite with our results, we found that the ratio of the birth rate of the progenitors of contact binaries to that of contact binaries is greater than about 1.2. This suggests that for the detached-binary channel, the progenitors can be sufficient to produce observed contact binaries. In this channel, we find that the distribution of orbital period for contact binaries has a peak at about 0.25 days and a tail extending to longer periods, and the formation timescale of contact binaries has a large range from $\sim1$\,Myr to 15\,Gyr. These results show that the detached-binary channel could explain satisfactorily the main observational characteristics of contact binaries, in particular the distribution of orbital period shown by the $\emph{Kepler}$ observations and the existence of very young contact binaries.

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arXiv2013

On the minimum mass ratio of W UMa binaries

Using Eggleton's stellar evolution code, we study the minimum mass ratio ($q_{\rm min}$) of W Ursae Majoris (W UMa) binaries that have different primary masses. It is found that the minimum mass ratio of W UMa binaries decreases with increasing mass of the primary if the primary's mass is less than about 1.3$M_{\rm \odot}$, and above this mass the ratio is roughly constant. By comparing the theoretical minimum mass ratio with the observational data, it is found that the existence of low-$q$ systems can be explained by the different structure of the primaries with different masses. This suggests that the dimensionless gyration radius ($k_1^2$) and thus the structure of the primary is very important in determining the minimum mass ratio. In addition, we investigate the mass loss during the merging process of W UMa systems and calculate the rotation velocities of the single stars formed by the merger of W UMa binaries due to tidal instability. It is found that in the case of the conservation of mass and angular momentum, the merged single stars rotate with a equatorial velocity of about $588\sim819$ km s$^{-1}$, which is much larger than their break-up velocities ($v_{\rm b}$). This suggests that the merged stars should extend to a very large radius (3.7$\sim$5.3 times the radii of the primaries) or W UMa systems would lose a large amount of mass (21$\sim$33 per cent of the total mass) during the merging process. If the effect of magnetic braking is considered, the mass loss decreases to be 12$\sim$18 per cent of their total masses. This implies that the significant angular momentum and mass might be lost from W UMa systems in the course of the merging process, and this kind of mass and angular momentum loss might be driven by the release of orbital energy of the secondaries, which is similar to common-envelope evolution.

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arXiv2013

The binary merger channel for the progenitor of the fastest rotating O-type star VFTS102

VFTS102 has a projected rotational velocity ($>$500\,km s$^{-1}$) and would appear to be the fastest rotating O-type star. We show that its high rotational velocity could be understood within the framework of the binary merger. In the binary merger channel, the progenitor binary of VFTS102 would evolve into contact while two components are still on the main sequence, and then merge into a rapidly rotating single star. Employing Eggleton's stellar evolution code, we performed binary stellar evolution calculations and mapped out the initial parameters of the progenitor of VFTS102 in the orbital period-mass ratio ($P-q$) plane. We found that the progenitor binary of VFTS102 with initial mass ratio $q_0 \la 0.7$ should have an initial orbital period shorter than $3.76-4.25$\,days, while above this mass ratio it should have an initial orbital period shorter than $1.44-1.55$\,days. The progenitor of VFTS102 would evolve into contact during the rapid mass transfer phase or during the subsequent slow mass transfer phase, and might ultimately merge into a rapidly rotating massive star. In addition, we performed Monte Carlo simulations to investigate the binary merger channel. We estimated the fraction of binaries that would merge into single stars and the fraction of single stars that might be produced from the binary merger channel. It is found that about 8.7% of binaries would evolve into contact and merge into rapidly rotating single stars, and about 17.1% of single stars might be produced from the binary merger channel and should have similar properties to VFTS102. This suggests that the binary merger channel might be one of the main channels for the formation of rapidly rotating massive stars like VFTS102.

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arXiv2011

The short-period limit of contact binaries

The stability of mass transfer is important in the formation of contact binaries from detached binaries when the primaries of the initially detached binaries fill their Roche lobes. Using Eggleton's stellar evolution code, we investigate the formation and the short-period limit of contact binaries by considering the effect of the instability of mass transfer. It is found that with decreasing initial primary mass from 0.89M$_{\rm \odot}$ to 0.63M$_{\rm \odot}$, the range of the initial mass ratio decreases for detached binaries that experience stable mass transfer and evolve into contact. If the initial primary mass is less than 0.63M$_{\rm \odot}$, detached binaries would experience dynamically unstable mass transfer when the primaries of detached binaries fill their Roche lobes. These systems would evolve into a common envelope situation and probably then to a complete merger of two components on a quite short timescale. This results in a low mass limit at about 0.63M$_{\rm \odot}$ for the primary mass of contact binaries, which might be a main reason why the period distribution of contact binaries has a short limit of about 0.22 days. By comparing the theoretical period distribution of contact binaries with the observational data, it is found that the observed contact binaries are above the low mass limit for the primary mass of contact binaries and no observed contact binaries are below this limit. This suggests that the short-period limit of contact binaries can be explained by the instability of the mass transfer that occurs when the primaries of the initially detached binaries fill their Roche lobes.

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arXiv2009

The energy transfer and its effects on the secondaries in W Ursae Majoris-type contact binaries

The energy transfer of W UMa contact binaries and its effects on the secondary in W UMa contact binaries are investigated. Relations are given between the mass ratio (q) and the relative energy transfer rates, i.e. U1, the ratio of the transferred luminosity to the surface luminosity of the primary and U2, the ratio of the transferred luminosity to the nuclear luminosity of the secondary. The theoretical curves(U1 vs q and U2 vs q) are derived. Although these curves can reflect the distribution of U1 vs q and U2 vs q, some observational systems are significantly deviated from these curves. It is mainly resulted from the difference in the effective temperatures of the components in W UMa systems.The radius and the density of the secondary are related to the relative energy tran-sfer rate U2: the higher is U2, the greater is the expansion and the lower is the density of the secondaries in W UMa systems. The temperature difference of W UMa binary components is correlated with the relative energy transfer rate U1 and decreases with increasing U1. This might suggest that there is a thermal coupling between two components in W UMa contact binaries, and that the classif-ication of W UMa contact binaries into A- or W-types depends on the energy tran-sfer from the primary to the secondary. The temperature difference of W UMa bin-ary components is poorly correlated with the mass of the primary. This suggests that the properties of the common envelope of W UMa contact binaries might not have a significant effect on the energy transfer between two components.

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arXiv2008

The evolutionary status of W Ursae Majoris-type systems

Well-determined physical parameters of 130 W UMa systems have been collected from the literature. Based on these data, the evolutionary status and dynamical evolution of W UMa systems are investigated. It is found that there is no evolutionary difference between W- and A-type systems in $M-J$ diagram which is consistent with the results derived from the analysis of observed spectral type, $M-R$ and $M-L$ diagrams of W UMa systems. $M-R$ and $M-L$ diagrams of W- and A-type systems indicate that a large amount of energy should be transferred from the more massive to the less massive component so that they are not in thermal equilibrium and undergo thermal relaxation oscillation (TRO). Meanwhile, the distribution of angular momentum, together with the distribution of mass ratio, suggests that the mass ratio of the observed W UMa systems is decreased with the decrease of their total mass. This could be the result of the dynamical evolution of W UMa systems which suffer angular momentum loss (AML) and mass loss due to magnetic stellar wind (MSW). Consequently, the tidal instability forces these systems towards the lower q values and finally to fast rotating single stars.

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