arXiv subjects

Jian-Ning Fu

Publications and source records attributed to Jian-Ning Fu.

36 records · Page 2Linked to original sources

arXiv2021

LTD064402+245919: A Subgiant with a 1-3 M$_{\odot}$ Undetected Companion Identified from LAMOST-TD Data

Single-line spectroscopic binaries recently contribute to the stellar-mass black hole discovery, independently of the X-ray transient method. We report the identification of a single-line binary system LTD064402+245919, with an orbital period of 14.50 days. The observed component is a subgiant with a mass of 2.77$\pm$0.68M$_{\odot}$, radius 15.5$\pm$2.5R$_{\odot}$, effective temperature $T_{\rm eff}$ 4500$\pm$200K, and surface gravity log\emph{g} 2.5$\pm$0.25dex. The discovery makes use of the LAMOST time-domain (LAMOST-TD) and ZTF survey. Our general-purpose software pipeline applies the Lomb-Scargle periodogram to determine the orbital period and uses machine-learning to classify the variable type from the folded light curves. We apply a combined model to estimate the orbital parameters from both the light and radial velocity curves, taking constraints on the primary star mass, mass function, and detection limit of secondary luminosity into consideration. We obtain a radial velocity semi-amplitude of 44.6$\pm$1.5 km s$^{-1}$, mass ratio of 0.73$\pm$0.07, and an undetected component mass of 2.02$\pm$0.49M$_{\odot}$ when the type of the undetected component is not set. We conclude that the inclination is not well constrained, and that the secondary mass is larger than 1M$_{\odot}$ when the undetected component is modelled as a compact object. According to our investigations using an MCMC simulation, increasing the spectra SNR by a factor of 3 would enable the secondary light to be distinguished (if present). The algorithm and software in this work are able to serve as general-purpose tools for the identification of compact objects quiescent in X-rays.

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arXiv2021

LAMOST Time-Domain Survey: First Results of four $K$2 plates

From Oct. 2019 to Apr. 2020, LAMOST performs a time-domain spectroscopic survey of four $K$2 plates with both low- and med-resolution observations. The low-resolution spectroscopic survey gains 282 exposures ($\approx$46.6 hours) over 25 nights, yielding a total of about 767,000 spectra, and the med-resolution survey takes 177 exposures ($\approx$49.1 hours) over 27 nights, collecting about 478,000 spectra. More than 70%/50% of low-resolution/med-resolution spectra have signal-to-noise ratio higher than 10. We determine stellar parameters (e.g., $T_{\rm eff}$, log$g$, [Fe/H]) and radial velocity (RV) with different methods, including LASP, DD-Payne, and SLAM. In general, these parameter estimations from different methods show good agreement, and the stellar parameter values are consistent with those of APOGEE. We use the $Gaia$ DR2 RV data to calculate a median RV zero point (RVZP) for each spectrograph exposure by exposure, and the RVZP-corrected RVs agree well with the APOGEE data. The stellar evolutionary and spectroscopic masses are estimated based on the stellar parameters, multi-band magnitudes, distances and extinction values. Finally, we construct a binary catalog including about 2700 candidates by analyzing their light curves, fitting the RV data, calculating the binarity parameters from med-resolution spectra, and cross-matching the spatially resolved binary catalog from $Gaia$ EDR3. The LAMOST TD survey is expected to get breakthrough in various scientific topics, such as binary system, stellar activity, and stellar pulsation, etc.

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arXiv2021

Double-, triple-line spectroscopic candidates in the LAMOST Medium-Resolution Spectroscopic Survey

The LAMOST Medium-Resolution Spectroscopic Survey (LAMOST-MRS) provides an unprecedented opportunity for detecting multi-line spectroscopic systems. Based on the method of Cross-Correlation Function (CCF) and successive derivatives, we search for spectroscopic binaries and triples and derive their radial velocities (RVs) from the LAMOST-MRS spectra. A Monte-Carlo simulation is adopted to estimate the RV uncertainties. After examining over 1.3 million LAMOST DR7 MRS blue arm spectra, we obtain 3,133 spectroscopic binary (SB) and 132 spectroscopic triple (ST) candidates, which account for 1.2% of the LAMOST-MRS stars. Over 95% of the candidates are newly discovered. It is found that all of the ST candidates are on the main sequence, while around 10% of the SB candidates may have one or two components on the red giant branch.

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arXiv2021

A triplet of the only pulsation mode detected in the DAV star G132-12

Hydrogen atmosphere pulsating white dwarfs, also known as DAV stars, are the most abundant type of pulsating white dwarfs. High-temperature DAV stars exhibit in general a small number of pulsation modes and stable frequencies. G132-12 is one of the pulsating hydrogen atmosphere white dwarf stars which lies close to the blue edge of the instability strip. Previous researches reported that G132-12 might have only one pulsation mode with the period of 212.69 s. To study the pulsation properties of G132-12 in detail, we carried out a bi-site observation campaign in October 2019. Time series photometric data were collected during around 154 hours in total. A Fourier Analysis reveals 3 frequencies which are identified as the triplet of a $l = 1$ g-mode pulsation with the period of 212.499 s. The rotational period is derived as $P_{rot} = 35.0\pm6.7$ hours and the inclination of the rotational axis to the line of sight is $70^{\circ}$. G132-12 could be an ideal target for measuring the cooling scale of this white dwarf star with only one excited pulsation mode detected.

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arXiv2021

Self-consistent Stellar Radial Velocities from LAMOST Medium-Resolution Survey (MRS) DR7

Radial velocity (RV) is among the most fundamental physical quantities obtainable from stellar spectra and is rather important in the analysis of time-domain phenomena. The LAMOST Medium-Resolution Survey (MRS) DR7 contains 5 million single-exposure stellar spectra at spectral resolution $R\sim7\,500$. However, the temporal variation of the RV zero-points (RVZPs) of the MRS survey, which makes the RVs from multiple epochs inconsistent, has not been addressed. In this paper, we measure the RVs of the 3.8 million single-exposure spectra (for 0.6 million stars) with signal-to-noise ratio (SNR) higher than 5 based on cross-correlation function (CCF) method, and propose a robust method to self-consistently determine the RVZPs exposure-by-exposure for each spectrograph with the help of \textit{Gaia} DR2 RVs. Such RVZPs are estimated for 3.6 million RVs and can reach a mean precision of $\sim 0.38\,\mathrm{km\,s}^{-1}$. The result of the temporal variation of RVZPs indicates that our algorithm is efficient and necessary before we use the absolute RVs to perform time-domain analysis. Validating the results with APOGEE DR16 shows that our absolute RVs can reach an overall precision of 0.84/0.80 $\mathrm{km\,s}^{-1}$ in the blue/red arm at $50<\mathrm{SNR}<100$, while 1.26/1.99 $\mathrm{km\,s}^{-1}$ at $5<\mathrm{SNR}<10$. The cumulative distribution function (CDF) of the standard deviations of multiple RVs ($N_\mathrm{obs}\geq 8$) for 678 standard stars reach 0.45/0.54, 1.07/1.39, and 1.45/1.86 $\mathrm{km\,s}^{-1}$ in the blue/red arm at 50\%, 90\%, and 95\% levels, respectively. The catalogs of the RVs, RVZPs, and selected candidate RV standard stars are available at \url{https://github.com/hypergravity/paperdata}.

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arXiv2020

LAMOST Observations in 15 \textit{K}2 Campaigns: I. Low resolution spectra from LAMOST DR6

The LAMOST-\textit{K}2 (L\textit{K}2) project, initiated in 2015, aims to collect low-resolution spectra of targets in the \textit{K}2 campaigns, similar to LAMOST-\textit{Kepler} project. By the end of 2018, a total of 126 L\textit{K}2 plates had been observed by LAMOST. After cross-matching the catalog of the LAMOST data release 6 (DR6) with that of the \textit{K}2 approved targets, we found 160,619 usable spectra of 84,012 objects, most of which had been observed more than once. The effective temperature, surface gravity, metallicity, and radial velocity from 129,974 spectra for 70,895 objects are derived through the LAMOST Stellar Parameter Pipeline (LASP). The internal uncertainties were estimated to be 81 K, 0.15 dex, 0.09 dex and 5 kms$^{-1}$, respectively, when derived from a spectrum with a signal-to-noise ratio in the $g$ band (SNR$_g$) of 10. These estimates are based on results for targets with multiple visits. The external accuracies were assessed by comparing the parameters of targets in common with the APOGEE and GAIA surveys, for which we generally found linear relationships. A final calibration is provided, combining external and internal uncertainties for giants and dwarfs, separately. We foresee that these spectroscopic data will be used widely in different research fields, especially in combination with \textit{K}2 photometry.

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arXiv2020

Most Lithium-rich Low-mass Evolved Stars Revealed as Red Clump stars by Asteroseismology and Spectroscopy

Lithium has confused scientists for decades at almost each scale of the universe. Lithium-rich giants are peculiar stars with lithium abundances over model prediction. A large fraction of lithium-rich low-mass evolved stars are traditionally supposed to be red giant branch (RGB) stars. Recent studies, however, report that red clump (RC) stars are more frequent than RGB. Here, we present a uniquely large systematic study combining the direct asteroseismic analysis with the spectroscopy on the lithium-rich stars. The majority of lithium-rich stars are confirmed to be RCs, whereas RGBs are minor. We reveal that the distribution of lithium-rich RGBs steeply decline with the increasing lithium abundance, showing an upper limit around 2.6 dex, whereas the Li abundances of RCs extend to much higher values. We also find that the distributions of mass and nitrogen abundance are notably different between RC and RGB stars. These findings indicate that there is still unknown process that significantly affects surface chemical composition in low-mass stellar evolution.

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arXiv2020

Phase II of the LAMOST-Kepler/K2 survey. I. Time series of medium-resolution spectroscopic observations

Phase \RNum{2} of the LAMOST-{\sl Kepler/K}2 survey (LK-MRS), initiated in 2018, aims at collecting medium-resolution spectra ($R\sim7,500$; hereafter MRS) for more than $50,000$ stars with multiple visits ($\sim60$ epochs) over a period of 5 years (2018 September to 2023 June). We selected 20 footprints distributed across the {\sl Kepler} field and six {\sl K}2 campaigns, with each plate containing a number of stars ranging from $\sim2,000$ to $\sim 3,000$. During the first year of observations, the LK-MRS has already collected $\sim280,000$ and $\sim369,000$ high-quality spectra in the blue and red wavelength range, respectively. The atmospheric parameters and radial velocities for $\sim259,000$ spectra of $21,053$ targets were successfully calculated by the LASP pipeline. The internal uncertainties for the effective temperature, surface gravity, metallicity, and radial velocity are found to be $100$\,K, $0.15$\,dex, $0.09$\,dex, and $1.00$\,km\,s$^{-1}$, respectively. We found $14,997$, $20,091$, and $1,514$ stars in common with the targets from the LAMOST low-resolution survey (LRS), GAIA and APOGEE, respectively, corresponding to a fraction of $\sim70\%$, $\sim95\%$ and $\sim7.2\%$. In general, the parameters derived from LK-MRS spectra are consistent with those obtained from the LRS and APOGEE spectra, but the scatter increases as the surface gravity decreases when comparing with the measurements from APOGEE. A large discrepancy is found with the GAIA values of the effective temperature. The comparisons of radial velocities of LK-MRS to GAIA and LK-MRS to APOGEE nearly follow an Gaussian distribution with a mean $μ\sim1.10$ and $0.73$\,km\,s$^{-1}$, respectively.

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arXiv2019

Radial velocity measurements from LAMOST medium-resolution spectroscopic observations: A pointing towards the Kepler field

Radial velocity is one of key measurements in understanding the fundamental properties of stars, stellar clusters and the Galaxy. A plate of stars in the Kepler field were observed in May of 2018 with the medium-resolution spectrographs of LAMOST, aiming to test the performance of this new system which is the upgraded equipment of LAMOST after the first five-year regular survey.We present our analysis on the radial velocity measurements (RVs) derived from these data. The results show that slight and significant systematic errors exist among the RVs obtained from the spectra collected by different spectrographs and exposures, respectively. After correcting the systematic errors with different techniques, the precision of RVs reaches ~1.3, ~1.0, ~0.5 and ~0.3 km/s at S/Nr = 10, 20, 50, and 100, respectively. Comparing with the RVs of the standard stars of the APOGEE survey, our RVs are calibrated with a zero-point shift of ~7 km/s. The results indicate that the LAMOST medium-resolution spectroscopic system may provide RVs in a reasonable accuracy and precision for the selected targets.

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arXiv2018

NGC 1893: a young open cluster rich in multi-type variable stars

In this work, we have studied the variable stars in the young open cluster NGC 1893 based on a multi-year photometric survey covering a sky area around the cluster up to $31' \times 31'$ wide. More than 23\,000 images in the $V$ band taken from January 2008 to February 2017 with different telescopes, complemented with 90 images in the $B$ band in 2014 and 2017, were reduced, and light curves were derived in $V$ for 5653 stars. By analyzing these light curves, we detected 147 variable stars (85 of them being new discoveries), including 110 periodic variables, 15 eclipsing binaries and 22 non-periodic variables. Proper motions, radial velocities, color-magnitude and two-color diagrams were used to identify the cluster membership of these variable stars, resulting in 84 members. Periodic variable members were then classified into different variability types, mainly according to their magnitudes and to their periods of variability, as well as to their positions in the Hertzsprung-Russell diagram for the early-type stars. As a result, among main-sequence periodic variable members, we identified five $β$ Cep candidates, seven slowly pulsating B-type candidates, and thirteen fast-rotating pulsating B-type (FaRPB) candidates (one of which is a confirmed classical Be star). While most of the FaRPB stars display properties similar to the ones discovered in NGC 3766 by Mowlavi et al. (2013), five of them have periods below 0.1~d, contrary to expectations. Additional observations, including spectroscopic, are called for to further characterize these stars. We also find a binary candidate harboring a $δ$-Scuti candidate.

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arXiv2018

LAMOST observations in the Kepler field. II. Database of the low-resolution spectra from the five-year regular survey

The LAMOST-Kepler (LK-) project was initiated to use the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) to make spectroscopic follow-up observations for the targets in the field of the Kepler mission. The Kepler field is divided into 14 subfields that are adapted to the LAMOST circular field with a diameter of 5 degrees. During the regular survey phase of LAMOST, the LK-project took data from 2012 June to 2017 June and covered all the 14 subfields at least twice. In particular, we describe in this paper the second Data Release of the LK-project, including all spectra acquired through 2015 May to 2017 June together with the first round observations of the LK-project from 2012 June to 2014 September. The LK-project now counts 227 870 spectra of 156 390 stars, among which we have derived atmospheric parameters (log g, T eff and [Fe/H]) and heliocentric radial velocity (RV) for 173 971 spectra of 126 172 stars. These parameters were obtained with the most recent version of the LAMOST Stellar Parameter Pipeline v 2.9.7. Nearly one half, namely 76 283 targets, are observed both by LAMOST and Kepler telescopes. These spectra, establishing a large spectroscopy library, will be useful for the entire astronomical community, particularly for planetary science and stellar variability on Kepler targets.

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arXiv2018

Asteroseismology of the Double-Mode High-Amplitude $δ$ Scuti Star VX Hydrae

Bi-site time-series photometric and high-resolution spectroscopic observations were made for the double-mode high-amplitude $δ$ Scuti star VX\ Hya. The fundamental frequency $f_{0}=4.4763\ \rm{c\ days^{-1}}$, the first overtone $f_{1}=5.7897\ \rm{c\ days^{-1}}$ and 23 harmonics and linear combinations of $f_{0}$ and $f_{1}$ are detected by pulsation analysis. From the spectroscopic data, we get $\rm{[Fe/H] = -0.2\pm0.1\ dex}$. The period change rate of the fundamental mode is obtained by using the Fourier-phase diagram method, which gives the value of $(1/P_{0})(dP_{0}/dt)=(1.81\pm0.09) \times 10^{-7}\ \rm{yr}^{-1}$. With these results from the observations, we perform theoretical explorations with the stellar evolution code MESA, and constrain the models by fitting $f_{0}$, $f_{1}$, and $(1/P_{0})(dP_{0}/dt)$ within $3σ$ deviations. The results show that the period change of VX\ Hya could be ascribed to the evolutionary effect. The stellar parameters of VX\ Hya could be derived as: the mass $2.385\pm0.025\ M_{\odot}$, the luminosity $\log(L/L_{\odot})=1.93\pm0.02$ and the age $(4.43\pm0.13)\times 10^8$ years. VX\ Hya is found to locate at the post-main-sequence stage with a helium core and a hydrogen-burning shell on the H${-}$R diagram.

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arXiv2017

AE Ursae Majoris -- a $δ$ Scuti Star in the Hertzsprung Gap

We analyze the photometric data and spectroscopic data that collect on the $δ$ Scuti star AE UMa. The fundamental and the first overtone frequencies are confirmed as $f_{0} = 11.62560$\,c\,d$^{-1}$ and $f_{1} = 15.03124$\,c\,d$^{-1}$, respectively, from the frequency content by analyzing of the 40 nights light curve spanning from 2009 to 2012. Additionally, another 37 frequencies are identified as either the harmonics or the linear combinations of the fundamental and the first overtone frequencies, among which 25 are newly detected. The rate of period change of the fundamental mode is determined as $(1/P_{0})(dP_{0}/dt) = 5.4(\pm 1.9) \times 10^{-9}$\,yr$^{-1}$ as revealed from the $O-C$ diagram based on the 84 newly determined times of maximum light combined with those derived from the literature. The spectroscopic data suggests that AE UMa is a population\,I $δ$ Scuti star. With these physical properties, we perform theoretical explorations based on the stellar evolution code MESA on this target, considering that the variation of pulsation period is caused by secular evolutionary effects. We finally constraint the AE UMa with the physical parameters as: the mass of $1.805 \pm 0.055\ M_{\odot}$, the radius of $1.647 \pm 0.032 \times 10^{11}$\,cm, the luminosity of $1.381 \pm 0.048~(\log L/L_{\odot})$ and the age of $1.055 \pm 0.095 \times 10^{9}$\,yr. AE UMa can be the (Pop. I) $δ$ Scuti star that locates just after the second turn-off of its evolutional track leaving the main sequence, a star in the phase of the Hertzsprung Gap with a helium core and a hydrogen-burning shell.

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arXiv2016

Understanding the residual patterns of timing solutions of radio pulsars with a model of magnetic field oscillation

We explain some phenomena existing generally in the timing residuals: amplitude and sign of the second derivative of a pulsar's spin-frequency ($\ddotν$), some sophisticated residual patterns, which also change with the time span of data segments. The sample is taken from Hobbs et al.\,(2010), in which the pulsar's spin-frequency and its first derivative have been subtracted from the timing solution fitting. We first classify the timing residual patterns into different types based on the sign of $\ddotν$. Then we use the magnetic field oscillation model developed in our group \citep{zhang12a} to fit successfully the different kinds of timing residuals with the Markov Chain Monte Carlo method. Finally, we simulate the spin evolution over 20 years for a pulsar with typical parameters and analyze the data with the conventional timing solution fitting. By choosing different segments of the simulated data, we find that most of the observed residual patterns can be reproduced successfully. This is the first time that the observed residual patterns are fitted by a model and reproduced by simulations with very few parameters. From the distribution of the different residual patterns in the $P-\dot P$ diagram, we argue that (1) a single magnetic field oscillation mode exists commonly in all pulsars throughout their lifetimes; (2) there may be a transition period over the lifetimes of pulsars, in which multiple magnetic field oscillation modes exist.

astro-ph.HE↗

arXiv2015

Asteroseismic based estimation of the surface gravity for the LAMOST giant stars

Asteroseismology is one of the most accurate approaches to estimate the surface gravity of a star. However, most of the data from the current spectroscopic surveys do not have asteroseismic measurements, which is very expensive and time consuming. In order to improve the spectroscopic surface gravity estimates for a large amount of survey data with the help of the small subset of the data with seismic measurements, we set up a support vector regression model for the estimation of the surface gravity supervised by 1,374 LAMOST giant stars with Kepler seismic surface gravity. The new approach can reduce the uncertainty of the estimates down to about 0.1 dex, which is better than the LAMOST pipeline by at least a factor of 2, for the spectra with signal-to-noise ratio higher than 20. Compared with the logg estimated from the LAMOST pipeline, the revised logg values provide a significantly improved match to the expected distribution of red clump and RGB stars from stellar isochrones. Moreover, even the red bump stars, which extend to only about 0.1 dex in logg, can be discriminated from the new estimated surface gravity. The method is then applied to about 350,000 LAMOST metal-rich giant stars to provide improved surface gravity estimates. In general, the uncertainty of the distance estimate based on the SVR surface gravity can be reduced to about 12% for the LAMOST data.

astro-ph.IM↗

arXiv2015

Photometric Variability in the CSTAR Field: Results From the 2008 Data Set

The Chinese Small Telescope ARray (CSTAR) is the first telescope facility built at Dome A, Antarctica. During the 2008 observing season, the installation provided long-baseline and high-cadence photometric observations in the i-band for 18,145 targets within 20 deg2 CSTAR field around the South Celestial Pole for the purpose of monitoring the astronomical observing quality of Dome A and detecting various types of photometric variability. Using sensitive and robust detection methods, we discover 274 potential variables from this data set, 83 of which are new discoveries. We characterize most of them, providing the periods, amplitudes and classes of variability. The catalog of all these variables is presented along with the discussion of their statistical properties.

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arXiv2015

Whether the Classical O-C Diagram can be used to Multi-mode Pulsators

O-C diagram is a useful technique to analyse the period changes of a pulsator by using the maximum (or minimum) value points which have been obtained from the historical data. But if an object is a double-mode or multi-mode pulsator, the extreme value points are the results of all the modes other than just the fundamental mode. We discussed these situations and give out some criteria to judge whether the O-C diagram can be used in these situations.

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arXiv2013

Pulsation Analysis of the High Amplitude δ Scuti Star CW Serpentis

Time-series photometric observations were made for the High Amplitude $δ$ Scuti star CW Ser between 2011 and 2012 at the Xinglong Station of National Astronomical Observatories of China. After the frequency analysis of the light curves, we confirmed the fundamental frequency of $f = 5.28677\ cd^{-1}$, together with 8 harmonics of the fundamental frequency, 7 of which are newly detected. No additional frequencies were detected. The O-C diagram, produced with the 21 newly determined times of maximum light combined with those provided in the literature, helps to obtain a new ephemeris formula of the times of maximum light with the pulsation period of $0.189150355 \pm 0.000000003\ days$.

astro-ph.SR↗