arXiv subjects

Jiao Li

Publications and source records attributed to Jiao Li.

70 records · Page 4Linked to original sources

arXiv2021

The Relative Calibration of Radial Velocity for LAMOST Medium Resolution Stellar Spectra

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) started median-resolution spectroscopic (MRS, R$\sim$7500) survey since October 2018. The main scientific goals of MRS, including binary stars, pulsators, and other variable stars are launched with a time-domain spectroscopic survey. However, the systematic errors, including the bias induced from wavelength calibration and the systematic difference between different spectrographs have to be carefully considered during radial velocity measurement. In this work, we provide a technique to correct the systematics in the wavelength calibration based on the relative radial velocity measurements from LAMOST MRS spectra. We show that, for the stars with multi-epoch spectra, the systematic bias which is induced from the exposures of different nights can be well corrected for LAMOST MRS in each spectrograph. And the precision of radial velocity zero-point of multi-epoch time-domain observations reaches below 0.5 km/s . As a by-product, we also give the constant star candidates, which can be the secondary radial-velocity standard star candidates of LAMOST MRS time-domain surveys.

astro-ph.SR↗

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}.

astro-ph.SR↗

arXiv2021

CODER: Knowledge infused cross-lingual medical term embedding for term normalization

This paper proposes CODER: contrastive learning on knowledge graphs for cross-lingual medical term representation. CODER is designed for medical term normalization by providing close vector representations for different terms that represent the same or similar medical concepts with cross-lingual support. We train CODER via contrastive learning on a medical knowledge graph (KG) named the Unified Medical Language System, where similarities are calculated utilizing both terms and relation triplets from KG. Training with relations injects medical knowledge into embeddings and aims to provide potentially better machine learning features. We evaluate CODER in zero-shot term normalization, semantic similarity, and relation classification benchmarks, which show that CODERoutperforms various state-of-the-art biomedical word embedding, concept embeddings, and contextual embeddings. Our codes and models are available at https://github.com/GanjinZero/CODER.

cs.CL↗

arXiv2020

591 high velocity stars in the Galactic halo selected from LAMOST DR7 and Gaia DR2

In this paper, we report 591 high velocity star candidates (HiVelSCs) selected from over 10 million spectra of the data release seven (DR7) of the Large Sky Area Multi-object Fiber Spectroscopic Telescope and the second Gaia data release, with three-dimensional velocities in the Galactic rest-frame larger than 445 km/s. We show that at least 43 HiVelSCs are unbound to the Galaxy with escape probabilities larger than 50%, and this number decreases to eight if the possible parallax zero-point error is corrected. Most of these HiVelSCs are metal-poor and slightly alpha-enhanced inner halo stars. Only 14% of them have [Fe/H] > -1, which may be the metal-rich "in situ" stars in halo formed in the initial collapse of the Milky Way or metal-rich stars formed in the disk or bulge but kinematically heated. The low ratio of 14% implies that the bulk of stellar halo was formed from the accretion and tidal disruption of satellite galaxies. In addition, HiVelSCs on the retrograde orbits have a slightly lower metallicities on average compared with these on the prograde orbits, meanwhile metal-poor HiVelSCs with [Fe/H] < -1 have an even faster mean retrograde velocity compared with metal-rich HiVelSCs. To investigate the origins of HiVelSCs, we perform orbit integrations and divide them into four types, i.e., hypervelocity stars, hyper-runaway stars, runaway stars and fast halo stars. A catalog for these 591 HiVelSCs, including radial velocities, atmospheric parameters, Gaia astrometric parameters, spatial positions, and velocities, etc., is available at http://paperdata.chinavo.org/LYB/lamostdr7_gaiadr2_hvs_591.csv.

astro-ph.GA↗

arXiv2020

Blue Large-Amplitude Pulsators: the Possible Surviving Companions of Type Ia Supernovae

The single degenerate (SD) model, one of the leading models for the progenitors of Type Ia supernovae (SNe Ia), predicts that there should be binary companions that survive the supernova explosion which, in principle, should be detectable in the Galaxy. The discovery of such surviving companions could therefore provide conclusive support for the SD model. Several years ago, a new type of mysterious variables was discovered, the so-called blue large-amplitude pulsators (BLAPs). Here we show that all the properties of BLAPs can be reasonably well reproduced if they are indeed such surviving companions, in contrast to other proposed channels. This suggests that BLAPs could potentially be the long-sought surviving companions of SNe Ia. Our model also predicts a new channel for forming single hot subdwarf stars, consistent with a small group in the present hot-subdwarf-star sample.

astro-ph.SR↗

arXiv2020

LAMOST Medium-Resolution Spectroscopic Survey (LAMOST-MRS): Scientific goals and survey plan

Since September 2018, LAMOST starts a new 5-year medium-resolution spectroscopic survey (MRS) using bright/gray nights. We present the scientific goals of LAMOST-MRS and propose a near optimistic strategy of the survey. A complete footprint is also provided. Not only the regular medium-resolution survey, but also a time-domain spectroscopic survey is being conducted since 2018 and will be end in 2023. According to the detailed survey plan, we expect that LAMOST-MRS can observe about 2 million stellar spectra with ~7500 and limiting magnitude of around G=15 mag. Moreover, it will also provide about 200 thousand stars with averagely 60-epoch observations and limiting magnitude of G~14 mag. These high quality spectra will give around 20 elemental abundances, rotational velocities, emission line profiles as well as precise radial velocity with uncertainty less than 1 km/s. With these data, we expect that LAMOST can effectively leverage sciences on stellar physics, e.g. exotic binary stars, detailed observation of many types of variable stars etc., planet host stars, emission nebulae, open clusters, young pre-main-sequence stars etc.

astro-ph.SR↗

arXiv2020

Three New Late-type Hypervelocity Star Candidates from Gaia DR2 with Refined Selection Criteria

Several dozen hypervelocity star (HVS) candidates have been reported based on the second data release of Gaia (Gaia DR2). However, it has been proven that the radial velocities of some Gaia HVS candidates are not reliable. In this paper, we employ refined astrometric criteria to re-examine Gaia DR2, arriving at a more reliable sample of HVS and high velocity star candidates than those found by previous authors.We develop a method called Binary Escape Probability Analysis to identify some HVS candidates. This method allows us to work with stars having only two epochs of measured radial velocity. These stars were usually discarded in previous similar studies. A scrutiny of our final results sheds light on selection effects present in our studies, which we propose to be the focus of future studies. In total, we find three late-type (2 G-type and 1 K-type) HVS and 21 high velocity star candidates, 3 and 11 of which are new, respectively. Judging by their historical trajectories, which we calculate, all three HVS candidates could not have had Galactic center origins. Further monitoring is required to confirm their status.

astro-ph.SR↗

arXiv2020

Gravitational Wave Radiation of Double Degenerates with Extremely low-mass WD companions

Double Degenerate systems (DDs) are supposed to be significant gravitational wave (GW) sources for future space-based gravitational-wave detectors, e.g., Laser Interferometer Space Antenna (LISA). Recently, one type of DDs with Extremely low-mass WD (ELM WD; $\leq 0.30\; M_\odot$) companions has been largely found in the ELM Survey. They have very short orbital periods and are therefore important sources for LISA detection. Besides, due to the thick envelope of ELM WDs compared with massive WDs (e.g. CO WDs), they are much easier to be found by the combination of electromagnetic (EM) and GW observations. In this paper, we first obtain the population of ELM WDs in DDs with considering the detailed evolutionary tracks of ELM WDs, and then analyse the GW radiation of these systems. We found that about $6\times10^3$ sources could be solely detected by LISA, including $\sim2\times10^3$ chirping sources, and $\sim13$ ($\sim107$) more sources are expected to be detected by both LISA and ELM Survey (Gaia).

astro-ph.SR↗

arXiv2020

$(p,q)$-frames in shift-invariant subspaces of mixed Lebesgue spaces $L^{p,q}(\mathbf{R}\times \mathbf{R}^{d})$

In this paper, we mainly discuss the $(p,q)$-frame in shift-invariant subspace \begin{equation*} V_{p,q}(Φ)=\left\{\sum\limits_{i=1}^{r}\sum\limits_{j_{1}\in \mathbf{Z}}\sum\limits_{j_{2}\in \mathbf{Z}^{d}}d_{i}(j_{1},j_{2})ϕ_{i}(\cdot-j_{1},\cdot-j_{2}):\Big(d_{i}(j_{1},j_{2})\Big)_{(j_{1},j_{2})\in \mathbf{Z}\times\mathbf{Z}^{d}}\in \ell^{p,q}(\mathbf{Z}\times\mathbf{Z}^d)\right\} \end{equation*} of mixed Lebesgue space $L^{p,q}(\mathbf{R}\times \mathbf{R}^{d})$. Some equivalent conditions for $\{ϕ_{i}(\cdot-j_{1},\cdot-j_{2}):(j_{1},j_{2})\in\mathbf{Z}\times\mathbf{Z}^d,1\leq i\leq r\}$ to constitute a $(p,q)$-frame of $V_{p,q}(Φ)$ are given. Moreover, the result shows that $V_{p,q}(Φ)$ is closed under these equivalent conditions of $(p,q)$-frame for the family $\{ϕ_{i}(\cdot-j_{1},\cdot-j_{2}):(j_{1},j_{2})\in\mathbf{Z}\times\mathbf{Z}^d,1\leq i\leq r\}$, although the general result is not correct.

math.FA↗

arXiv2019

High Velocity Runaway Binaries from Supernovae in Triple Systems

Recent studies on hypervelocity stars (HVSs) have generated a need to understand the high velocity limits of binary systems. If runaway binary systems with high movement speeds well in excess of 200km/s were to exist, it would have implications on how HVS candidates are selected, and our current understanding of how they form needs to be reinforced. In this paper, we explore the possibility that such high velocity runaway binaries (HVRBs) can be engendered by supernova explosions of the tertiary in close hierarchical triple systems. We find that such explosions can lead to significant remnant binary velocities, and demonstrate via constraining the velocity distribution of such HVRBs that this mechanism can lead to binaries with centre of mass velocities of 350 km/s or more, relative to the original centre of mass of the progenitor triple system. This translates into potential observations of binaries with velocities high enough to escape the Galaxy, once the Galactic rotational velocity and objects of Large Magellanic Cloud origins are considered.

astro-ph.SR↗

arXiv2018

Subdwarf B stars as possible surviving companions in Type Ia supernova remnants

Although type Ia supernovae (SNe Ia) are so important in many astrophysical fields, a debate on their progenitor model is still endless. Searching the surviving companion in a supernova remnant (SNR) may distinguish different progenitor models, since a companion still exists in the remnant for the single-degenerate (SD) model, but does not for the double degenerate (DD) model. However, some recent surveys do not discover the surviving companions in the remnant of SN 1006 and Kepler's supernova, which seems to disfavor the SD model. Such a result could be derived from an incorrect survey target. Here, based on the common-envelope wind SD model for SNe Ia, in which the initial binary system consists of a white dwarf (WD) and a main-sequence (MS) star, we found that the companion at the moment of supernova explosion may be a MS, red-giant (RG) or subdwarf B (sdB) star if a spin-down timescale of less than $10^{\rm 7}$ yr is assumed. We show the properties of the companions at the moment of supernova explosion, which are key clues to search the surviving companions in SNRs. Here, we suggest that the sdB star may be the surviving companion in some SNRs, even if the progenitor systems are the WD + MS systems. The SNe Ia with the sdB companions may contribute to all SNe Ia as much as 22\%.

astro-ph.HE↗

arXiv2018

Optimal Timing to Trade Along a Randomized Brownian Bridge

This paper studies an optimal trading problem that incorporates the trader's market view on the terminal asset price distribution and uninformative noise embedded in the asset price dynamics. We model the underlying asset price evolution by an exponential randomized Brownian bridge (rBb) and consider various prior distributions for the random endpoint. We solve for the optimal strategies to sell a stock, call, or put, and analyze the associated delayed liquidation premia. We solve for the optimal trading strategies numerically and compare them across different prior beliefs. Among our results, we find that disconnected continuation/exercise regions arise when the trader prescribe a two-point discrete distribution and double exponential distribution.

q-fin.MF↗

arXiv2018

Building a Controlled Vocabulary for Standardizing Precision Medicine Terms

Rapid advances of technology and development of research in precision medicine domain have led to the production of different types of biomedical data. Standard medical vocabularies were shown to be limited in dealing with such heterogeneous data and consequently, new controlled vocabulary for data integration and normalization has been proposed. In this study, the precision medicine vocabulary (PMV), which is a controlled vocabulary for terms used in precision medicine, is built based on the method of data integration in Unified Medical Language System (UMLS). It now covers ten top semantic types of disease, drug, gene, gene variation and so on. In total of 1,372,967 concepts and 4,567,208 terms have been integrated from widely used databases related with precision medicine.

cs.CY↗

arXiv2016

Trading VIX Futures under Mean Reversion with Regime Switching

This paper studies the optimal VIX futures trading problems under a regime-switching model. We consider the VIX as mean reversion dynamics with dependence on the regime that switches among a finite number of states. For the trading strategies, we analyze the timings and sequences of the investor's market participation, which leads to several corresponding coupled system of variational inequalities. The numerical approach is developed to solve these optimal double stopping problems by using projected-successive-over-relaxation (PSOR) method with Crank-Nicolson scheme. We illustrate the optimal boundaries via numerical examples of two-state Markov chain model. In particular, we examine the impacts of transaction costs and regime-switching timings on the VIX futures trading strategies.

q-fin.CP↗

arXiv2016

Speculative Futures Trading under Mean Reversion

This paper studies the problem of trading futures with transaction costs when the underlying spot price is mean-reverting. Specifically, we model the spot dynamics by the Ornstein-Uhlenbeck (OU), Cox-Ingersoll-Ross (CIR), or exponential Ornstein-Uhlenbeck (XOU) model. The futures term structure is derived and its connection to futures price dynamics is examined. For each futures contract, we describe the evolution of the roll yield, and compute explicitly the expected roll yield. For the futures trading problem, we incorporate the investor's timing option to enter or exit the market, as well as a chooser option to long or short a futures upon entry. This leads us to formulate and solve the corresponding optimal double stopping problems to determine the optimal trading strategies. Numerical results are presented to illustrate the optimal entry and exit boundaries under different models. We find that the option to choose between a long or short position induces the investor to delay market entry, as compared to the case where the investor pre-commits to go either long or short.

q-fin.MF↗

arXiv2015

The first symbiotic stars from the LAMOST survey

Symbiotic stars are interacting binary systems with the longest orbital periods. They are typically formed by a white dwarf, a red giant and a nebula. These objects are natural astrophysical laboratories for studying the evolution of binaries. Current estimates of the population of Milky Way symbiotic stars vary from 3000 up to 400000. However, the current census is less than 300. The Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST) survey can obtain hundreds of thousands of stellar spectra per year, providing a good opportunity to search for new symbiotic stars. In this work we detect 4 of such binaries among 4,147,802 spectra released by the LAMOST, of which two are new identifications. The first is LAMOST J12280490-014825.7, considered to be an S-type halo symbiotic star. The second is LAMOST J202629.80+423652.0, a D-type symbiotic star.

astro-ph.GA↗