arXiv subjects

Licai Deng

Publications and source records attributed to Licai Deng.

At least 55 records · Page 3Linked to original sources

arXiv2019

Tidal-locking-induced stellar rotation dichotomy in the open cluster NGC 2287?

Stars spend most of their lifetimes on the `main sequence' (MS) in the Hertzsprung--Russell diagram. The obvious double MSs seen in the equivalent color--magnitude diagrams characteristic of Milky Way open clusters pose a fundamental challenge to our traditional understanding of star clusters as `single stellar populations.' The clear MS bifurcation of early-type stars with masses greater than $\sim1.6 M_\odot$ is thought to result from a range in the stellar rotation rates. However, direct evidence connecting double MSs to stellar rotation properties has yet to emerge. Here, we show through analysis of the projected stellar rotational velocities ($v\sin i$, where $i$ represents the star's inclination angle) that the well-separated double MS in the young, $\sim200Myr$-old Milky Way open cluster NGC 2287 is tightly correlated with a dichotomous distribution of stellar rotation rates. We discuss whether our observations may reflect the effects of tidal locking affecting a fraction of the cluster's member stars in stellar binary systems. We show that the slow rotators could potentially be initially rapidly rotating stars that have been slowed down by tidal locking by a low mass-ratio companion in a cluster containing a large fraction of short-period, low-mass-ratio binaries. This demonstrates that stellar rotation drives the split MSs in young, $\lessapprox 300$Myr-old star clusters. However, special conditions, e.g., as regards the mass-ratio distribution, might be required for this scenario to hold.

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arXiv2019

Hot subdwarf stars observed in Gaia DR2 and LAMOST DR5

Combing Gaia DR2 with LAMOST DR5, we spectroscopically identified 924 hot subdwarf stars, among which 32 stars exhibit strong double-lined composite spectra. We measured the effective temperature $T_{\rm eff}$, surface gravity $\log\,g$, helium abundance $y=n{\rm He}/n{\rm H}$, and radial velocities of 892 non-composite spectra hot subdwarf stars by fitting LAMOST observations with Tlusty/Synspec non-LTE synthetic spectra. We outlined four different groups in the $T_{\rm eff}-\log\,g$ diagram with our helium abundance classification scheme and two nearly parallel sequences in the $T_{\rm eff}-\log(y)$ diagram. 3D Galactic space motions and orbits of 747 hot subdwarf stars with $(G_{BP}-G_{RP})_{0}<-0.36$ mag were computed using LAMOST radial velocities and Gaia parallaxes and proper motions. Based on the $U-V$ velocity diagram, $J_{z}-$eccentricity diagram, and Galactic orbits, we derived Galactic population classifications and the fractional distributions of the four hot subdwarf helium groups in the halo, thin disk and thick disk. Comparisons with the predictions of binary population synthesis calculations (Han 2008) suggest that He-rich hot subdwarf stars with $\log(y)\ge0$ are from the double helium white dwarfs merger, He-deficient hot subdwarf stars with $-2.2\le\log(y)<-1$ from the common envelope ejection, and He-deficient hot subdwarf stars with $\log(y)<-2.2$ from the stable Roche lobe overflow channels. The relative number of He-rich hot subdwarf stars with $-1\le\log(y)<0$ and $\log(y)\ge0$ in the halo is more than twice the prediction of Zhang et al.(2017), even more than six times in the thin disk, which implies that the mergers of helium white dwarfs with low mass main sequence stars may not be the main formation channel of He-rich hot subdwarf stars with $-1\le\log(y)<0$, specially in younger environments.

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arXiv2019

Extended main-sequence turnoffs in the double cluster $h$ and $χ$ Persei: The complex role of stellar rotation

Using {\sl Gaia} Data Release 2 photometry, we report the detection of extended main-sequence turnoff (eMSTO) regions in the color--magnitude diagrams (CMDs) of the $\sim 14$ Myr-old double clusters $h$ and $χ$ Persei (NGC 869 and NGC 884). We find that stars with masses below $\sim$1.3 $M_{\odot}$ in both $h$ and $χ$ Persei populate narrow main sequences (MSs), while more massive stars define the eMSTO, closely mimicking observations of young Galactic and Magellanic Cloud clusters (with ages older than $\sim$30 Myr). Previous studies based on clusters older than $\sim$30 Myr find that rapidly rotating MS stars are redder than slow rotators of similar luminosity, suggesting that stellar rotation may be the main driver of the eMSTO. By combining photometry and projected rotational velocities from the literature of stars in $h$ and $χ$ Persei, we find no obvious relation between the rotational velocities and colors of non-emission-line eMSTO stars, in contrast with what is observed in older clusters. Similarly to what is observed in Magellanic Cloud clusters, most of the extremely rapidly rotating stars, identified by their strong H$α$ emission lines, are located in the red part of the eMSTOs. This indicates that stellar rotation plays a role in the color and magnitude distribution of MSTO stars. By comparing the observations with simulated CMDs, we find that a simple population composed of coeval stars that span a wide range of rotation rates is unable to reproduce the color spread of the clusters' MSs. We suggest that variable stars, binary interactions, and stellar rotation affect the eMSTO morphology of these very young clusters.

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arXiv2019

Stellar rotation and the extended main sequence turnoff in the open cluster NGC 5822

The origin of extended main sequence turnoffs (eMSTOs) in intermediate-age (1-3 Gyr) clusters is one of the most intriguing questions in current star cluster research. Unlike the split main sequences found in some globular clusters. which are caused by bimodal populations in age and/or chemical abundances, eMSTOs are believed to be owing to stellar rotation. We present a spectroscopic survey of MSTO stars in a nearby, intermediate-age (\unit[0.9]{Gyr}), low mass ($\sim 1.7\times10^3 M_\odot$) Galactic open cluster, NGC 5822. We derive a clean sample of member stars based on \textit{Gaia} proper motions and parallaxes and confirm the existence of an eMSTO. Using medium-resolution ($R\sim 4000$) Southern African Large Telescope (SALT) spectra, we derive the rotational velocities of 24 member stars (representing 20\% completeness around the eMSTO region) and find that the loci of the main sequence stars in the eMSTO region show a clear correlation with the projected rotational velocities in the sense that fast rotators are located on the red side of the eMSTO and slow rotators are found on the blue side. By comparison with a synthetic cluster model, we show that the stellar rotational velocities and the eMSTO of NGC 5822 can be well reproduced and we conclude that stellar rotation is the main cause of the eMSTO in NGC 5822.

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arXiv2019

Asteroseismology of the Hyades red giant and planet host epsilon Tauri

Asteroseismic analysis of solar-like stars allows us to determine physical parameters such as stellar mass, with a higher precision compared to most other methods. Even in a well-studied cluster such as the Hyades, the masses of the red giant stars are not well known, and previous mass estimates are based on model calculations (isochrones). The four known red giants in the Hyades are assumed to be clump (core-helium-burning) stars based on their positions in colour-magnitude diagrams, however asteroseismology offers an opportunity to test this assumption. Using asteroseismic techniques combined with other methods, we aim to derive physical parameters and the evolutionary stage for the planet hosting star epsilon Tau, which is one of the four red giants located in the Hyades. We analysed time-series data from both ground and space to perform the asteroseismic analysis. By combining high signal-to-noise (S/N) radial-velocity data from the ground-based SONG network with continuous space-based data from the revised Kepler mission K2, we derive and characterize 27 individual oscillation modes for epsilon Tau, along with global oscillation parameters such as the large frequency separation and the ratio between the amplitude of the oscillations measured in radial velocity and intensity as a function of frequency. The latter has been measured previously for only two stars, the Sun and Procyon. Combining the seismic analysis with interferometric and spectroscopic measurements, we derive physical parameters for epsilon Tau, and discuss its evolutionary status.

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arXiv2019

An intuitive 3D map of the Galactic warp's precession traced by classical Cepheids

The Milky Way's neutral hydrogen (HI) disk is warped and flared. However, a dearth of accurate HI-based distances has thus far prevented the development of an accurate Galactic disk model. Moreover, the extent to which our Galaxy's stellar and gas disk morphologies are mutually consistent is also unclear. Classical Cepheids, primary distance indicators with distance accuracies of 3-5%, offer a unique opportunity to develop an intuitive and accurate three-dimensional picture. Here, we establish a robust Galactic disk model based on 1339 classical Cepheids. We provide strong evidence that the warp's line of nodes is not oriented in the Galactic Center-Sun direction. Instead, it subtends a mean angle of 17.5 \pm 1 (formal) \pm 3 (systematic) and exhibits a leading spiral pattern. Our Galaxy thus follows Briggs' rule for spiral galaxies, which suggests that the origin of the warp is associated with torques forced by the massive inner disk. The stellar disk traced by Cepheids follows the gas disk in terms of their amplitudes; the stellar disk extends to at least 20 kpc. This morphology provides a crucial, updated map for studies of the kinematics and archaeology of the Galactic disk.

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arXiv2019

Frequent flare events on the short-period M-type eclipsing binary BX Tri

We present long-term, multi-color photometric monitoring and spectroscopic observations of the short-period M-type eclipsing binary BX Tri. Six flare events were recorded in 4 years from 2014 to 2017. Three of them were detected on one night within an orbital cycle. The strongest one was identified on December 23, 2014. With the amplitudes of $Δ$B=0.48 mag, $Δ$V=0.28 mag, $Δ$R=0.10 mag, $Δ$I=0.02 mag, the total energy due to the flare event was measured to be 4.08 ($\pm$0.24) $\times$$10^{34}$ erg, exceeding the superflare level ($10^{34}$). Based on the observations, the evolutionary status of the binary system as well as the long-term orbital period changes were analyzed. It reveals that BX Tri is probably a semi-detached system with the primary component filling its Roche lobe. The extremely high occurrence of flare events of the binary could be related to the rapid mass transfer between components.

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arXiv2018

Blue straggler stars beyond the Milky Way. IV. Radial distributions and dynamical implications

Blue straggler stars have been proposeed as powerful indicators to measure the dynamical state of Galactic globular clusters. Here we examine for the first time if this framework of blue straggler stars as dynamical clocks, which was specifically developed for Galactic globular clusters, may also hold for younger globular clusters in the Large Magellanic Cloud. Using Hubble Space Telescope observations, we study seven Large Magellanic Cloud star clusters with ages between ~700 Myr and 7 Gyr. We confirm that our sample clusters are consistent with the empirical correlation previously derived for Galactic globular clusters, within a modest tolerance. We suggest that to further examine if blue straggler stars can measure the dynamical state of their host clusters over an extended range of dynamical ages, more studies of dynamically older Magellanic Cloud clusters are required. We discuss the physical implications of our results in terms of their central, dimensionless King potential, as well as the initial retention fraction of black holes.

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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

Blue straggler populations beyond the Milky Way

Although the formation of blue straggler stars (BSSs) is routinely attributed to stellar interactions in binary systems, the relative importance of the direct collision and slow(er) stellar coalescence formation channels is still poorly understood. We selected a sample of 24 Magellanic Cloud star clusters for which multi-passband Hubble Space Telescope images are available to address this outstanding question. We compiled a BSS database, containing both traditional and evolved BSSs. We found a robust correlation between the number of BSSs in a cluster's core and its core mass, $N_{\rm BSS, core} \propto M_{\rm core}^{0.51 \pm 0.07}$, which supports the notion that BSS formation is linked to a population's binary fraction. At low stellar collision rates, the mass-normalised number of BSSs does not appear to depend on the collision rate, which implies that the coalescence-driven BSS formation channel dominates. Comparison with simulations suggests that stellar collisions contribute less than 20% to the total number of BSSs formed.

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arXiv2018

Asteroseismic modelling of the subgiant μ Herculis using SONG data: lifting the degeneracy between age and model input parameters

We model the oscillations of the SONG target $μ$ Herculis to estimate the parameters of the star. The $\ell$ = 1 mixed modes of $μ$ Her provide strong constraints on stellar properties. The mass and age given by our asteroseismic modelling are 1.10$^{+0.11}_{-0.06}$ M$_{\odot}$ and 7.55$^{+0.96}_{-0.79}$ Gyr. The initial helium abundance is also constrained at around $Y_{\rm{init}}$ = 0.28, suggesting a ratio in the elements enrichment law ($ΔY/ΔZ$) around 1.3, which is closed to the solar value. The mixing-length parameter converges to about 1.7, which is $\sim$ 10\% lower than the solar value and consistent with the results from hydrodynamic simulations. Our estimates of stellar mass and age agree very well with the previous modelling results with different input physics. Adding asteroseismic information makes these determinations less model-dependent than is typically the case when only surface information is available. Our studies of the model dependence (mass, initial helium and metallicity fractions, and the mixing length parameter) of the age determination indicate that accurate stellar ages ($\lesssim$ 10\%) can be expected from asteroseismic modelling for stars similar to $μ$ Her. The $\ell$ = 1 bumped modes, which are sensitive to the mean density of the helium core, provide a useful 'clock' that provides additional constraints on its age.

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arXiv2018

V1224 Cas - an EL CVn-type Eclipsing Binary Consisting of a Helium White Dwarf Precursor and a Delta Scuti Pulsator

We report the discovery of a new eclipsing EL CVn-type binary, consisting of a $δ$ Sct-type pulsator and a thermally bloated low-mass pre-He white dwarf (WD). Spectroscopy and time-series $BV$ photometry of V1224 Cas were carried out. The spectroscopy reveals a spectral type of A3 for the star. Light-curve modelling indicate that V1224 Cas is a short-period detached system containing a possible low-mass WD with an effective temperature of about 9516 K. Based on the effective temperature and the surface gravity of the A-type primary star from the spectroscopic results, the absolute parameters of the components were estimated as: $M_{P}$=2.16$\pm$0.22$M_{\sun}$, $R_{P}$=3.54$\pm$0.12$R_{\sun}$, $L_{P}$=55.9$\pm$6.9$L_{\sun}$, $M_{S}$=0.19$\pm$0.02$M_{\sun}$, $R_{S}$=0.97$\pm$0.04$R_{\sun}$, and $L_{S}$=6.9$\pm$0.6$L_{\sun}$. We therefore introduce V1224 Cas as a new EL CVn-type binary candidate. The light curves in both filters all show multi-periodic pulsations, superimposed on binary effects. We performed a preliminary frequency analysis of the light residuals after removing the synthetic eclipsing curve from the original observational data. The results suggest that the rapid light variations among the light curves could be attributed to the $δ$ Sct-type primary component. We therefore conclude that V1224 Cas is very likely a WD+$δ$ Sct binary.

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arXiv2018

Blue straggler stars beyond the Milky Way. III. Detection of evolved blue straggler candidates in Large Magellanic Cloud clusters

We analyze {\sl Hubble Space Telescope} observations of nine Large Magellanic Cloud star clusters with ages of 1--2 Gyr to search for evolved counterparts of blue straggler stars. Near the red clump regions in the clusters' color--magnitude diagrams, we find branches of evolved stars that are much brighter than normal evolved stars. We examine the effects of photometric artifacts, differential reddening, and field contamination. We conclude that these bright evolved stars cannot be explained by any of these effects. Our statistical tests show that the contributions of photometric uncertainties and crowding effects, as well as that owing to differential reddening, to these bright evolved stars are insufficient to fully explain the presence of these bright evolved stars. Based on isochrone fitting, we also ruled out the possibility that these bright evolved stars could be reproduced by an internal chemical abundance spread. The spatial distributions of the bright evolved stars exhibit clear concentrations that cannot be explained by homogeneously distributed field stars. This is further confirmed based on Monte Carlo-based tests. By comparing our observations with stellar evolution models, we find that the masses of most of bright evolved stars do not exceed twice the average mass of normal evolved stars. We suggest that these bright evolved stars are, in fact, evolved blue straggler stars.

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arXiv2018

Wide-field Infrared Survey Explorer (WISE) Catalog of Periodic Variable Stars

We have compiled the first all-sky mid-infrared variable-star catalog based on Wide-field Infrared Survey Explorer (WISE) five-year survey data. Requiring more than 100 detections for a given object, 50,282 carefully and robustly selected periodic variables are discovered, of which 34,769 (69%) are new. Most are located in the Galactic plane and near the equatorial poles. A method to classify variables based on their mid-infrared light curves is established using known variable types in the General Catalog of Variable Stars. Careful classification of the new variables results in a tally of 21,427 new EW-type eclipsing binaries, 5654 EA-type eclipsing binaries, 1312 Cepheids, and 1231 RR Lyraes. By comparison with known variables available in the literature, we estimate that the misclassification rate is 5% and 10% for short- and long-period variables, respectively. A detailed comparison of the types, periods, and amplitudes with variables in the Catalina catalog shows that the independently obtained classifications parameters are in excellent agreement. This enlarged sample of variable stars will not only be helpful to study Galactic structure and extinction properties, they can also be used to constrain stellar evolution theory and as potential candidates for the James Webb Space Telescope.

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arXiv2018

Blue straggler stars beyond the Milky Way. II. A binary origin for blue straggler stars in Magellanic Cloud clusters

We have analyzed populations of blue straggler stars (BSSs) in 24 Magellanic Cloud star clusters using multi-passband Hubble Space Telescope images. We compiled a homogeneous BSS database, containing both traditional and evolved BSSs. We uncovered a sub-linear correlation between the number of BSSs in the cluster cores and the clusters' core masses, characterized by a power-law index of $0.51\pm 0.07$. For low stellar collision rates, the mass-normalized number of BSSs depends only weakly (or perhaps not at all) on the collision rate, implying that the binary-driven BSS formation channel dominates. Comparison with simulations suggests that stellar collisions contribute less than 20\% to the total number of BSSs formed. Further tests, including analysis of the BSS specific frequencies and their population numbers at larger cluster radii, suggest that binary interactions may be their main formation channel, hinting at an anti-correlation between a cluster's binary fraction and its core mass.

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arXiv2018

Mapping the Milky Way with LAMOST III: Complicated spatial structure in the outer disc

We present {complexity} of the Galactic outer disc {by fitting the stellar volume densities of the red giant branch stars with a two-disc component model}. {The discs are confirmed to} extend to $R\sim19$\,kpc. The radial density profile of the discs shows {two breaks at $R\sim11$ and $\sim14$\,kpc, respectively, which} separate the radial profile into three segments with different scale lengths of $2.12\pm0.26$, $1.18\pm0.08$, and $2.72$\,kpc at $R<11$, $11\leq R\leq14$, and $R>14$\,kpc, respectively. The first {break} is likely due to the sudden drop in the radial profile of the thin disc, which may be an evidence of the radial migration. {Beyond $14$\,kpc, the thick disc becomes prominent and the transition from thin to thick disc leads to the second break.} This implies that the geometrically defined thick disc is more {radially} extended than the thin disc. This is also supported by the larger scale length of the thick disc than that of the thin disc. Meanwhile, {the scale height of the thicker component increases from $0.637_{-0.036}^{+0.056}$ at $R=8$ to $1.284_{-0.079}^{+0.086}$\,kpc at $R=19$\,kpc, showing an intensive flared disc}. Moreover, rich substructures are displayed in the residuals of the stellar density. Among them, the substructures $D14+2.0$ and $O14-1.5$ show a north-south asymmetry, which can be essentially explained by southward shifting of the thick disc. However, no significant overdensity is found for the Monoceros ring. Finally, the thick disc shows a ripple-like feature with unclear origin at $9<R<10.5$\,kpc.

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arXiv2018

An extremely low mid-infrared extinction law toward the Galactic Center and 4% distance precision to 55 classical Cepheids

Distances and extinction values are usually degenerate. To refine the distance to the general Galactic Center region, a carefully determined extinction law (taking into account the prevailing systematic errors) is urgently needed. We collected data for 55 classical Cepheids projected toward the Galactic Center region to derive the near- to mid-infrared extinction law using three different approaches. The relative extinction values obtained are AJ/AKs = 3.005, AH/AKs = 1.717, A[3.6]/AKs = 0.478, A[4.5]/AKs = 0.341, A[5.8]/AKs = 0.234, A[8.0]/AKs = 0.321, AW1/AKs = 0.506, and AW2/AKs = 0.340. We also calculated the corresponding systematic errors. Compared with previous work, we report an extremely low and steep mid-infrared extinction law. Using a seven-passband 'optimal distance' method, we improve the mean distance precision to our sample of 55 Cepheids to 4%. Based on four confirmed Galactic Center Cepheids, a solar Galactocentric distance of R_0 = 8.10\pm0.19\pm0.22 kpc is determined, featuring an uncertainty that is close to the limiting distance accuracy (2.8%) for Galactic Center Cepheids.

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arXiv2018

Optical-Mid-Infrared Period-Luminosity Relations for W UMa-type Contact Binaries Based on Gaia DR 1: 8% Distance Accuracy

W Ursa Majoris (W UMa)-type contact binary systems (CBs) are useful statistical distance indicators because of their large numbers. Here, we establish (orbital) period-luminosity relations (PLRs) in 12 optical-to-mid-infrared bands (GBV RIJHKsW1W2W3W4) based on 183 nearby W UMa-type CBs with accurate Tycho-Gaia parallaxes. The 1σ dispersion of the PLRs decreases from optical to near- and mid-infrared wavelengths. The minimum scatter, 0.16 mag, implies that W UMa-type CBs can be used to recover distances to 7% precision. Applying our newly determined PLRs to 19 open clusters containing W UMa-type CBs demonstrates that the PLR and open cluster CB distance scales are mutually consistent to within 1%. Adopting our PLRs as secondary distance indicators, we compiled a catalog of 55,603 CBs candidates, of which 80% have distance estimates based on a combination of optical, near-, and mid-infrared photometry. Using Fourier decomposition, 27,318 high-probability W UMa-type CBs were selected. The resulting 8% distance accuracy implies that our sam- ple encompasses the largest number of objects with accurate distances within a local volume with a radius of 3 kpc available to date. The distribution of W UMa-type CBs in the Galaxy suggests that in different environments, the CB luminosity function may be different: larger numbers of brighter (longer-period) W UMa-type CBs are found in younger environments.

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