arXiv subjects

Gang Zhao

Publications and source records attributed to Gang Zhao.

At least 217 records · Page 12Linked to original sources

arXiv2018

Giant planets around FGK stars form probably through core accretion

We present a statistical study of the planet-metallicity (P-M) correlation, by comparing the 744 stars with candidate planets (SWPs) in the Kepler field which have been observed with LAMOST, and a sample of distance-independent, fake "twin" stars in the Kepler field with no planet reported (CKSNPs) yet. With the well-defined and carefully-selected large samples, we find for the first time a turn-off P-M correlation of Delta [Fe/H]_(SWPs-SNPs), which in average increases from ~0.00+-0.03 dex to 0.06+-0.03 dex, and to 0.12+-0.03 for stars with Earth, Neptune, Jupiter-sized planets successively, and then declines to ~-0.01+-0.03 dex for more massive planets or brown dwarfs. Moreover, the percentage of those systems with positive Delta[Fe/H] has the same turn-off pattern. We also find FG-type stars follow this general trend, but K-type stars are different. Moderate metal enhancement (~0.1-0.2 dex) for K-type stars with planets of radii between 2 to 4 Earth radius as compared to CKSNPs is observed, which indicates much higher metallicities are required for Super-Earths, Neptune-sized planets to form around K-type stars. We point out that the P-M correlation is actually metallicity-dependent, i.e., the correlation is positive at solar and super-solar metallicities, and negative at subsolar metallicities. No steady increase of Delta[Fe/H] against planet sizes is observed for rocky planets, excluding the pollution scenario as a major mechanism for the P-M correlation. All these clues suggest that giant planets probably form differently from rocky planets or more massive planets/brown dwarfs, and the core-accretion scenario is highly favoured, and high metallicity is a prerequisite for massive planets to form.

astro-ph.EP↗

arXiv2018

Broadband calibration-free cavity-enhanced complex refractive index spectroscopy using a frequency comb

We present broadband cavity-enhanced complex refractive index spectroscopy (CE-CRIS), a technique for calibration-free determination of the complex refractive index of entire molecular bands via direct measurement of transmission modes of a Fabry-Perot cavity filled with the sample. The measurement of the cavity transmission spectrum is done using an optical frequency comb and a mechanical Fourier transform spectrometer with sub-nominal resolution. Molecular absorption and dispersion spectra (corresponding to the imaginary and real parts of the refractive index) are obtained from the cavity mode broadening and shift retrieved from fits of Lorentzian profiles to the individual cavity modes. This method is calibration-free because the mode broadening and shift are independent of the cavity parameters such as the length and mirror reflectivity. In this first demonstration of broadband CE-CRIS we measure simultaneously the absorption and dispersion spectra of three combination bands of CO2 in the range between 1525 nm and 1620 nm and achieve good agreement with theoretical models. This opens up for precision spectroscopy of the complex refractive index of several molecular bands simultaneously.

physics.optics↗

arXiv2018

Two new Super Li-rich core He-burning giants: A new twist to the long tale of Li enhancement in K giants

In this paper we report two new super Li-rich K giants: KIC 2305930 and KIC 12645107 with Li abundances exceeding that of ISM (A(Li) $\geq$ 3.2 dex). Importantly, both the giants have been classified as core He-burning red clump stars based on asteroseismic data from Kepler mission. Also, both the stars are found to be low mass (M $\approx$ 1.0 M$_{\odot}$) which, together with an evidence of their evolutionary status of being red clump imply that the stars have gone through both the luminosity bump and He-flash during their RGB evolution. Stars' large Li abundance and evolutionary phase suggest that Li enrichment occurred very recently probably at the tip of RGB either during He-flash, an immediate preceding event on RGB, or by some kind of external event such as merger of RGB star with white dwarf. The findings will provide critical constraints to theoretical models for understanding of Li enhancement origin in RGB stars.

astro-ph.SR↗

arXiv2018

The Density Profile and Kinematics of the Milky Way with RR Lyrae Stars

Most of known RR Lyraes are type ab RR Lyraes (RRLab), and they are the excellent tool to map the Milky Way and its substructures. We find that 1148 RRLab stars determined by Drake et al.(2013) have been observed by spectroscopic surveys of SDSS and LAMOST. We derived radial velocity dispersion, circular velocity and mass profile from 860 halo tracers in our paper I. Here, we present the stellar densities and radial velocity distributions of thick disk and halo of the Milky Way. The 288 RRLab stars located in the thick disk have the mean metallicity of [Fe/H]$=-1.02$. Three thick disk tracers have the radial velocity lower than 215 km $\rm s^{-1}$. With 860 halo tracers which have a mean metallicity of [Fe/H]$=-1.33$, we find a double power-law of $n(r) \propto r^{-2.8}$ and $n(r) \propto r^{-4.8}$ with a break distance of 21 kpc to express the halo stellar density profile. The radial velocity dispersion at 50 kpc is around 78 km $\rm s^{-1}$.

astro-ph.GA↗

arXiv2018

Carbon stars identified from LAMOST DR4 using Machine Learning

In this work, we present a catalog of 2651 carbon stars from the fourth Data Release (DR4) of the Large Sky Area Multi-Object Fiber Spectroscopy Telescope (LAMOST). Using an efficient machine-learning algorithm, we find out these stars from more than seven million spectra. As a by-product, 17 carbon-enhanced metal-poor (CEMP) turnoff star candidates are also reported in this paper, and they are preliminarily identified by their atmospheric parameters. Except for 176 stars that could not be given spectral types, we classify the other 2475 carbon stars into five subtypes including 864 C-H, 226 C-R, 400 C-J, 266 C-N, and 719 barium stars based on a series of spectral features. Furthermore, we divide the C-J stars into three subtypes of CJ( H), C-J(R), C-J(N), and about 90% of them are cool N-type stars as expected from previous literature. Beside spectroscopic classification, we also match these carbon stars to multiple broadband photometries. Using ultraviolet photometry data, we find that 25 carbon stars have FUV detections and they are likely to be in binary systems with compact white dwarf companions.

astro-ph.SR↗

arXiv2018

The odd-isotope fractions of Barium in the strongly r-process enhanced (r-II) stars

We determined the \fodd\ values, $0.46\pm0.08$, $0.51\pm0.09$, $0.50\pm0.13$, $0.48\pm0.12$, which correspond to the r-contribution 100\% for four r-II stars, \cs, \hen, \hes\, and \het, respectively. Our results suggest that almost all of the heavy elements (in the range from Ba to Pb) in r-II stars have a common origin, that is, from a single r-process (the main r-process). We found that the \fodd\ has a intrinsic nature, and should keep constant value of about 0.46 in the main r-process yields, which is responsible for the heavy element enhancement of r-II stars and of our Galaxy chemical enhancement. In addition, except the abundance ratio [Ba/Eu] the \fodd\ is also an important indicator, which can be used to study the relative contributions of the r- and s-process during the chemical evolution history of the Milky Way and the enhancement mechanism in stars with peculiar abundance of heavy elements.

astro-ph.SR↗

arXiv2018

Stellar activity with LAMOST - II. Chromospheric activity in open clusters

We use the LAMOST spectra of member stars in Pleiades, M34, Praesepe, and Hyades to study how chromospheric activity vary as a function of mass and rotation at different age. We measured excess equivalent widths of H$α$, H$β$, and Ca~{\sc ii} K based on estimated chromospheric contributions from old and inactive field dwarfs, and excess luminosities are obtained by normalizing bolometric luminosity, for more than 700 late-type stars in these open clusters. Results indicate two activity sequences in cool spot coverage and H$α$ excess emission among GK dwarfs in Pleiades and M dwarfs in Praesepe and Hyades, paralleling with well known rotation sequences. A weak dependence of chromospheric emission on rotation exists among ultra fast rotators in saturated regime with Rossby number Ro$\lesssim0.1$. In the unsaturated regime, chromospheric and coronal emission show similar dependence on Ro, but with a shift toward larger Ro, indicating chromospheric emission gets easily saturated than coronal emission, and/or convective turnover time-scales based on X-ray data do not work well with chromospheric emission. More interestingly, our analysis show fully convective slow rotators obey the rotation-chromospheric activity relation similar to hotter stars, confirming the previous finding. We found correlations among H$α$, H$β$, and Ca~{\sc ii} K emissions, in which H$α$ losses are more important than Ca~{\sc ii} K for cooler and more active stars. In addition, a weak correlation is seen between chromospheric emission and photospheric activity that shows dependency on stellar spectral type and activity level, which provides some clues on how spot configuration vary as a function of mass and activity level.

astro-ph.SR↗

arXiv2017

Enormous Li-enhancement preceding red giant phases in low-mass stars in the Milky Way halo

Li abundances in the bulk of low-mass metal-poor stars are well reproduced by stellar evolution models adopting a constant initial abundance. However, a small number of stars have exceptionally high Li abundances, for which no convincing models have been established. We report on the discovery of 12 very metal-poor stars that have large excesses of Li, including an object having more than 100 times higher Li abundance than the values found in usual objects, which is the the largest excess in metal-poor stars known to date. The sample is distributed over a wide range of evolutionary stages, including five unevolved stars, showing no abundance anomaly in other elements. The results indicate the existence of an efficient process to enrich Li in a small fraction of low-mass stars at the main-sequence or subgiant phase. The wide distribution of Li-rich stars along the red giant branch could be explained by dilution of surface Li by mixing that occurs when the stars evolve into red giants. Our study narrows down the problem to be solved to understand the origins of Li-excess found in low-mass stars, suggesting the presence of unknown process that affects the surface abundances preceding red giant phases.

astro-ph.SR↗

arXiv2017

The Behavior of Selected Diffuse Interstellar Bands with Molecular Fraction in Diffuse Atomic and Molecular Clouds

We study the behavior of eight diffuse interstellar bands (DIBs) in different interstellar environments, as characterized by the fraction of hydrogen in molecular form [$f$(H$_2$)], with comparisons to the corresponding behavior of various known atomic and molecular species. The equivalent widths of the five "normal" DIBs ($λλ$5780.5, 5797.1, 6196.0, 6283.8, and 6613.6), normalized to $E(B-V)$, show a "Lambda-shaped" behavior: they increase at low $f$(H$_2$), peak at $f$(H$_2$) ~ 0.3, and then decrease. The similarly normalized column densities of Ca, Ca$^+$, Ti$^+$, and CH$^+$ also decline for $f$(H$_2$) > 0.3. In contrast, the normalized column densities of Na, K, CH, CN, and CO increase monotonically with $f$(H$_2$), and the trends exhibited by the three C$_2$ DIBs ($λλ$4726.8, 4963.9, and 4984.8) lie between those two general behaviors. These trends with $f$(H$_2$) are accompanied by cosmic scatter, the dispersion at any given $f$(H$_2$) being significantly larger than the individual errors of measurement. The Lambda-shaped trends suggest the balance between creation and destruction of the DIB carriers differs dramatically between diffuse atomic and diffuse molecular clouds; additional processes besides ionization and shielding are needed to explain those observed trends. Except for several special cases, the highest $W$(5780)/$W$(5797) ratios, characterizing the so-called "sigma-zeta effect", occur only at $f$(H$_2$) < 0.2. We propose a sequence of DIBs based on trends in their pair-wise strength ratios with increasing $f$(H$_2$). In order of increasing environmental density, we find the $λ$6283.8 and $λ$5780.5 DIBs, the $λ$6196.0 DIB, the $λ$6613.6 DIB, the $λ$5797.1 DIB, and the C$_2$ DIBs.

astro-ph.GA↗

arXiv2017

Stellar Stream Candidates in the Solar Neighborhood Found in the LAMOST DR3 and TGAS

We have cross-matched the LAMOST DR3 with the Gaia DR1 TGAS catalogs and obtained a sample of 166,827 stars with reliable kinematics. A technique based on the wavelet transform was applied to detect significant overdensities in velocity space among five subsamples divided by spatial position. In total, 16 significant overdensities of stars with very similar kinematics were identified. Among these, four are new stream candidates and the rest are previously known groups. Both the U-V velocity and metallicity distributions of the local sample show a clear gap between the Hercules structure and the Hyades-Pleiades structure. The U-V positions of these peaks shift with the spatial position. Following a description of our analysis, we speculate on possible origins of our stream candidates.

astro-ph.GA↗

arXiv2017

The Milky Way's circular velocity curve and its constraint on the Galactic mass with RR Lyrae stars

We present a sample of 1148 ab-type RR Lyrae (RRLab) variables identified from Catalina Surveys Data Release 1, combined with SDSS DR8 and LAMOST DR4 spectral data. We firstly use a large sample of 860 Galactic halo RRLab stars and derive the circular velocity distributions for the stellar halo. With the precise distances and carefully determined radial velocities (the center-of-mass radial velocities) by considering the pulsation of the RRLab stars in our sample, we can obtain a reliable and comparable stellar halo circular velocity curve. We take two different prescriptions for the velocity anisotropy parameter β in the Jeans equation to study the circular velocity curve and mass profile. We test two different solar peculiar motions in our calculation. Our best result with the adopted solar peculiar motion 1 of (U, V, W) = (11.1, 12, 7.2) km/s is that the enclosed mass of the Milky Way within 50 kpc is (3.75 +/- 1.33) *10^11Msun based on β= 0 and the circular velocity 180 +/- 31.92 (km/s) at 50 kpc. This result is consistent with dynamical model results, and it is also comparable to the previous similar works.

astro-ph.GA↗

arXiv2017

Sensitive and broadband measurement of dispersion in a cavity using a Fourier transform spectrometer with kHz resolution

Optical cavities provide high sensitivity to dispersion since their resonance frequencies depend on the index of refraction. We present a direct, broadband, and accurate measurement of the modes of a high finesse cavity using an optical frequency comb and a mechanical Fourier transform spectrometer with a kHz-level resolution. We characterize 16000 cavity modes spanning 16 THz of bandwidth in terms of center frequency, linewidth, and amplitude. We retrieve the group delay dispersion of the cavity mirror coatings and pure N${_2}$ with 0.1 fs${^2}$ precision and 1 fs${^2}$ accuracy, as well as the refractivity of the 3ν1+ν3 absorption band of CO${_2}$ with 5 x 10${^{-12}}$ precision. This opens up for broadband refractive index metrology and calibration-free spectroscopy of entire molecular bands.

physics.ins-det↗

arXiv2017

Chemical Composition of Young Stars in the Leading Arm of the Magellanic System

Chemical abundances of eight O- and B-type stars are determined from high-resolution spectra obtained with the MIKE instrument on the Magellan 6.5m Clay telescope. The sample is selected from 42 candidates of membership in the Leading Arm of the Magellanic System. Stellar parameters are measured by two independent grids of model atmospheres and analysis procedures, confirming the consistency of the stellar parameter results. Abundances of seven elements (He, C, N, O, Mg, Si, and S) are determined for the stars, as are their radial velocities and estimates of distances and ages. Among the seven B-type stars analyzed, the five that have radial velocities compatible with membership to the LA have an average [Mg/H] of $-0.42\pm0.16$, significantly lower than the average of the remaining two [Mg/H] = $-0.07\pm0.06$ that are kinematical members of the Galactic disk. Among the five LA members, four have individual [Mg/H] abundance compatible with that in the LMC. Within errors, we can not exclude the possibility that one of these stars has a [Mg/H] consistent with the more metal-poor, SMC-like material. The remaining fifth star has a [Mg/H] close to MW values. Distances to the LA members indicate that they are at the edge of the Galactic disk, while ages are of the order of $\sim 50-70$ Myr, lower than the dynamical age of the LA, suggesting a single star-forming episode in the LA. V$_{\rm LSR}$ the LA members decreases with decreasing Magellanic longitude, confirming the results of previous LA gas studies.

astro-ph.SR↗

arXiv2016

Nuclear Fusion in Laser-Driven Counter-Streaming Collisionless Plasmas

Nuclear fusion reactions are the most important processes in nature to power stars and produce new elements, and lie at the center of the understanding of nucleosynthesis in the universe. It is critically important to study the reactions in full plasma environments that are close to true astrophysical conditions. By using laser-driven counter-streaming collisionless plasmas, we studied the fusion D$+$D$\rightarrow n +^3$He in a Gamow-like window around 27 keV. The results show that astrophysical nuclear reaction yield can be modulated significantly by the self-generated electromagnetic fields and the collective motion of the plasma. This plasma-version mini-collider may provide a novel tool for studies of astrophysics-interested nuclear reactions in plasma with tunable energies in earth-based laboratories.

nucl-ex↗

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.

astro-ph.SR↗

arXiv2016

Stellar activity with LAMOST. I. Spot configuration in Pleiades

We use the spectra of Pleiades and field stars from LAMOST DR2 archive to study how spottedness and activity vary as a function of mass at young ages. We obtained standard TiO band strength by measuring TiO bands near 7050 $Å$ from LAMOST spectra (R$\approx$1800) for large sample of field GKM dwarfs with solar metallicity. Analysis show that active dwarfs, including late G- and early K-type, have extra TiO absorption compare to inactive counterparts, indicating the presence of cool spots on their surface. Active late K- and M-dwarfs show deeper TiO2 and shallower TiO4 compare to inactive stars at a given TiO5, which could be partly explained through cool spots. We estimated cool spot fractional coverage for 304 Pleiades candidates by modelling their TiO2 (&TiO5) band strength with respect to standard value. Results show that surface of large fraction of K- and M-type members have very large spot coverage ($\sim$50%). We analysed a correlation between spot coverage, rotation and the amplitude of light variation, and found spot coverage on slow rotators ($R_{o}$ > 0.1) increases with decreasing Rossby Number $R_{o}$. Interestingly, we detected a saturation-like feature for spot coverage in fast rotators with a saturation level of 40%-50%. In addition, spot distribution in hotter fast rotators show more symmetrical compare to slow rotators. More interestingly, we detected large spot coverage in many M type members with no or little light variation. In bigger picture, these results provide important constraints for stellar dynamo on these cool active stars.

astro-ph.SR↗

arXiv2016

The odd-isotope fractions of Barium in CEMP-r/s star HE 0338-3945 and r-II star CS 31082-001

We report the first measurement of the odd-isotope fractions for barium, \fodd\, in two extremely metal-poor stars: a CEMP-r/s star \he\ (\feh\,$=-2.42\pm0.11$) and an r-II star \cs\ (\feh\,$=-2.90\pm0.13$). The measured \fodd\ values are $0.23\pm0.12$ corresponding to $34.3\pm34.3$\% of the r-process contributions for \he\ and $0.43\pm0.09$ corresponding to $91.4\pm25.7$\% of the r-process contribution to Ba production for \cs. The high r-process signature of barium in \cs\ ($91.4\pm25.7\%$) suggests that the majority of the heavy elements in this star were synthesised via an r-process path, while the lower r-process value ($34.3\pm34.3\%$) found in \he\ indicates that the heavy elements in this star formed through a mix of s-process and r-process synthesis. These conclusions are consistent with studies based on AGB model calculations to fit their abundance distributions.

astro-ph.SR↗

arXiv2016

Carbon stars from LAMOST DR2 data

In this work, we present the new catalog of carbon stars from the LAMOST DR2 catalog. In total, 894 carbon stars are identified from multiple line indices measured from the stellar spectra. Combining the CN bands in the red end with \ctwo\ and other lines, we are able to identify the carbon stars. Moreover, we also classify the carbon stars into spectral sub-types of \ch, \CR, and \cn. These sub-types approximately show distinct features in the multi-dimensional line indices, implying that in the future we can use them to identify carbon stars from larger spectroscopic datasets. Meanwhile, from the line indices space, while the \cn\ stars are clearly separated from the others, we find no clear separation between \CR\ and \ch\ sub-types. The \CR\ and \ch\ stars seem to smoothly transition from one to another. This may hint that the \CR\ and \ch\ stars may not be different in their origins but look different in their spectra because of different metallicity. Due to the relatively low spectral resolution and lower signal-to-noise ratio, the ratio of $^{12}$C/$^{13}$C is not measured and thus the \cj\ stars are not identified.

astro-ph.SR↗