Secondary Star Research Articles

Identification, mass, and age of primary red clump stars from spectral features derived with the LAMOST DR7

ABSTRACT Although red clump (RC) stars are easy to identify due to their stability of luminosity and colour, about 20–50 per cent are actually red giant branch (RGB) stars in the same location on the HR diagram. In this paper, a sample of 210 504 spectra for 184 318 primary RC (PRC) stars from the LAMOST DR7 is identified, which has a purity of higher than 90 per cent. The RC and the RGB stars are successfully distinguished through LAMOST spectra (R ∼ 1800 and signal-to-noise ratio >10) by adopting the XGBoost ensemble learning algorithm, and the secondary RC stars are also removed. The SHapley Additive exPlanations (SHAP) value is used to explain the top features that the XGBoost model selected. The features are around Fe5270, MgH & Mg Ib, Fe4957, Fe4207, Cr5208, and CN, which can successfully distinguish RGB and RC stars. The XGBoost is also used to estimate the ages and masses of PRC stars by training their spectra with Kepler labelled asteroseismic parameters. The uncertainties of mass and age are 13 and 31 per cent, respectively. Verifying the feature attribution model, we find that the age-sensitive element XGBoost is consistent with the literature. Distance of the PRC stars is derived by KS absolute magnitude calibrated by Gaia EDR3, which has an uncertainty of about 6 per cent and shows the stars mainly located at the Galactic disc. We also test the XGBoost with R ∼ 250, which is the resolution of the Chinese Space Station Telescope under construction; it is still capable of finding sensitive features to distinguish RC and RGB.

Mass Transfer as an Explanation for the Lifetime Travel Time Discrepancy in IT Librae

Abstract The eclipsing binary IT Librae is an unusual system of two B-type stars that is situated about 1 kpc above the Galactic plane. The binary was probably ejected from its birthplace in the disk, but the implied time of flight to its current location exceeds the evolutionary lifetime of the primary star. Here we present a study of new high-dispersion spectroscopy and an exquisite light curve from the Kepler K2 mission in order to determine the system properties and resolve the timescale discrepancy. We derive a revised spectroscopic orbit from radial-velocity measurements and determine the component effective temperatures through comparison of reconstructed and model spectra (T 1 = 23.8 ± 1.8 kK, T 2 = 13.7 ± 2.5 kK). We use the Eclipsing Light Curve code to model the K2 light curve, and from the inclination of the fit we derive the component masses (M 1 = 9.6 ± 0.6 M ⊙, M 2 = 4.2 ± 0.2 M ⊙) and mean radii (R 1 = 6.06 ± 0.16 R ⊙, R 2 = 5.38 ± 0.14 R ⊙). The secondary star is overluminous for its mass and appears to fill its Roche lobe. This indicates that IT Librae is a post-mass-transfer system in which the current secondary was the mass donor star. The current primary star was rejuvenated by mass accretion, and its evolutionary age corresponds to the time since the mass transfer stage. Consequently, the true age of the binary is larger than the ejection time of flight, thus resolving the timescale discrepancy.

The white dwarf binary pathways survey – VI. Two close post-common envelope binaries with TESS light curves

Abstract Establishing a large sample of post-common-envelope binaries (PCEBs) that consist of a white dwarf plus an intermediate mass companion star of spectral type AFGK, offers the potential to provide new constraints on theoretical models of white dwarf binary formation and evolution. Here, we present a detailed analysis of two new systems, TYC 110-755-1 and TYC 3858-1215-1. Based on radial velocity measurements, we find the orbital periods of the two systems to be ∼0.85 and ∼1.64 d, respectively. In addition, HST spectroscopy of TYC 110-755-1 allowed us to measure the mass of the white dwarf in this system (0.78 M⊙). We furthermore analysed TESS high-time-resolution photometry and find both secondary stars to be magnetically extremely active. Differences in the photometric and spectroscopic periods of TYC 110-755-1 indicate that the secondary in this system is differentially rotating. Finally, studying the past and future evolution of both systems, we conclude that the common envelope efficiency is likely similar in close white dwarf plus AFGK binaries and PCEBs with M-dwarf companions and find a wide range of possible evolutionary histories for both systems. While TYC 3858-1215-1 will run into dynamically unstable mass transfer that will cause the two stars to merge and evolve into a single white dwarf, TYC 110-755-1 is a progenitor of a cataclysmic variable system with an evolved donor star.

The peculiar chemical abundance of the transitional millisecond pulsar PSR J1023+0038 – Li enhancement

ABSTRACT Using high-resolution optical spectroscopy we determine the chemical abundance of the secondary star in the binary millisecond pulsar PSR J1023+0038. We measure a metallicity of [Fe/H] = 0.48 ± 0.04 which is higher than the Solar value and in general find that the element abundances are different compared to the secondary stars in X-ray binaries and stars in the solar neighbourhood of similar Fe content. Our results suggest that the pulsar was formed in a supernova explosion. We find that supernova models, where matter that has been processed in the supernova is captured by the secondary star leading to abundance anomalies, qualitatively agree with the observations. We measure Li abundance of A(Li) = 3.66 ± 0.20, which is anomalously high compared to the Li abundance of stars with the same effective temperature, irrespective of the age of the system. Furthermore, the Li abundance in PSR J1023+0038 is higher than the Cosmic value and what is observed in young Population I stars and so provides unambiguous evidence for fresh Li production. The most likely explanation is the interaction of high-energy gamma-rays or relativistic protons from the pulsar wind or intrabinary shock with the CNO nuclei in the secondary star’s atmosphere via spallation which leads to substantial Li enrichment in the secondary star’s atmosphere.

Seismic signature of electron degeneracy in the core of red giants: Hints for mass transfer between close red-giant companions

The detection of mixed modes in red giants with space missions COROT and Kepler has revealed their deep internal structure. These modes allow us to characterize the pattern of pressure modes (through the measurement of their asymptotic frequency separation Δν) and the pattern of gravity modes (through the determination of their asymptotic period spacing ΔΠ1). It has been shown that red giant branch (RGB) stars regroup on a well-defined sequence in the Δν − ΔΠ1 plane. Our first goal is to theoretically explain the features of this sequence and understand how it can be used to probe the interiors of red giants. Using a grid of red giant models computed with MESA, we demonstrate that red giants join the Δν − ΔΠ1 sequence whenever electron degeneracy becomes strong in the core. We argue that this can be used to estimate the central densities of these stars, and potentially to measure the amount of core overshooting during the main sequence part of the evolution. We also investigate a puzzling subsample of red giants that are located below the RGB sequence, in contradiction with stellar evolution models. After checking the measurements of the asymptotic period spacing for these stars, we show that they are mainly intermediate-mass red giants. This is doubly peculiar because these stars should have nondegenerate cores and they are expected to be located well above the RGB sequence. We show that these peculiarities are well accounted for if these stars result from the interaction between two low-mass (M ≲ 2 M⊙) close companions during the red giant branch phase. If the secondary component has already developed a degenerate core before mass transfer begins, it becomes an intermediate-mass giant with a degenerate core. The secondary star is then located below the degenerate sequence, which is in agreement with the observations.

DI Herculis Revisited: Starspots, Gravity Darkening, and 3D Obliquities

Abstract DI Herculis is an eclipsing binary famous for a longstanding disagreement between theory and observation of the apsidal precession rate, which was resolved when both stars were found to be severely misaligned with the orbit. We used data from the Transiting Exoplanet Survey Satellite (TESS) to refine our knowledge of the stellar obliquities and sharpen the comparison between the observed and theoretical precession rates. The TESS data show variations with a 1.07 day period, which we interpret as rotational modulation from starspots on the primary star. This interpretation is supported by the detection of photometric anomalies during primary eclipses consistent with starspot crossings. The secondary eclipse light curve shows a repeatable asymmetry that we interpret as an effect of gravity darkening. By combining the TESS data with previously obtained data, we determined the three-dimensional spin directions of both stars. Using this information, the updated value of the theoretical apsidal precession rate (including the effects of tides, rotation, and general relativity) is 1.35 − 0.50 + 0.58 arcsec cycle−1. The updated value of the observed rate (after including new TESS eclipse times) is 1.41 − 0.28 + 0.39 arcsec cycle−1. Given the agreement between the observed and theoretical values, we fitted all the relevant data simultaneously, assuming the theory is correct. This allowed us to place tighter constraints on the stellar obliquities, which are 75 − 3 + 3 and 80 − 3 + 3 degrees for the primary and secondary stars, respectively.

Discovery of a Short-period and Unusually Helium-deficient Dwarf Nova KSP-OT-201701a by the KMTNet Supernova Program

Abstract We present the first ever discovery of a short-period and unusually helium-deficient dwarf nova KSP-OT-201701a by the Korea Microlensing Telescope Network Supernova Program. The source shows three superoutbursts, each led by a precursor outburst, and several normal outbursts in BVI during the span of ∼2.6 yr with supercycle and normal cycle lengths of about 360 and 76 days, respectively. Spectroscopic observations near the end of a superoutburst reveal the presence of strong double-peaked H i emission lines together with weak He i emission lines. The helium-to-hydrogen intensity ratios measured by He i λ5876 and Hα lines are 0.10 ± 0.01 at a quiescent phase and 0.26 ± 0.04 at an outburst phase, similar to the ratios found in long-period dwarf novae, while significantly lower than those in helium cataclysmic variables (He CVs). Its orbital period of 51.91 ± 2.50 minutes, which is estimated based on time-series spectroscopy, is a bit shorter than the superhump period of 56.52 ± 0.19 minutes, as expected from the gravitational interaction between the eccentric disk and the secondary star. We measure its mass ratio to be 0.37 − 0.21 + 0.32 using the superhump period excess of 0.089 ± 0.053. The short orbital period, which is under the period minimum, the unusual helium deficiency, and the large mass ratio suggest that KSP-OT-201701a is a transition object evolving to an He CV from a long-period dwarf nova with an evolved secondary star.

Bright spectroscopic binaries: II . A study of five systems with orbital periods of days

We present a detailed analysis of five bright spectroscopic binary systems (HD 18665, HD 27131, HD 171852, HD 215550, HD 217427) that have orbital periods of P < 500 days. We determined the complete set of orbital parameters using the toolkit RadVel by analyzing the observed radial velocity curves. To study the properties of the five systems, we also analyzed the intermediate resolution spectra (R = 20,000) observed with the TIGRE telescope and determined the stellar parameters of the primary stars using the toolkit iSpec. With Gaia Early Data Release 3 parallaxes, a correction for interstellar extinction using the 3D dust map, and bolometric corrections, we placed the stars in the Hertzsprung-Russell diagram and compared the positions with stellar evolution tracks calculated with the Eggleton code to determine the masses and ages of the primary stars. They have all evolved to the giant phase. Finally, we were able to determine the masses of the secondary stars and to estimate the orbital inclinations i of the binary systems.

J1832.4-1627, the first eclipsing stream-fed intermediate polar

We present a photometric study of the newly discovered eclipsing intermediate polar J183221.56-162724.25 (in short J1832) with an orbital period of 8.87 h. The system features a box-like deep eclipse with a full width at 50% depth of 1970 ± 2 s and a large-amplitude coherent pulsation with Pobs = 65.18 min, which represents either the synodic (beat) period or the spin period of the white dwarf (WD). The period ratio is either Pspin/Porb = 0.1091 or 0.1225, respectively. The eclipsed light originates almost entirely from the two accretion spots and columns on the WD, with characteristics indicative of pole flipping. There is no evidence for an accretion disk, and we identify J1832 as the first deeply eclipsing stream-fed intermediate polar. Our grizy photometry in eclipse yielded an i-band AB magnitude of the Roche-lobe-filling secondary star of 18.98(3), an extinction EB − V = 0.54 ± 0.17, and a spectral type ∼K6. Dynamic models, fitting the photometry, limit the distance to between 1270 and 2500 pc for masses of the secondary star, M2, between 0.16 and 1.0 M⊙, well within the Gaia EDR3 confidence limits. Employing a luminosity selection inspired by binary population studies yields a mean M2 = 0.32 M⊙ with a 2σ upper limit of 0.60 M⊙ and a mean distance d = 1596 pc with a 2σ upper limit of 1980 pc. The secondary star is located in its Hertzsprung-Russell diagram at a mean Teff, 2 = 4120 K and log(L2/L⊙) = − 0.92, from where the binary can evolve into either a polar or an ultracompact binary with a highly magnetic primary. The system displays a variable accretion rate, lapses repeatedly into short-lived low states of negligible accretion, and currently displays an orbital period that decreases on a timescale of τ ∼ 3 × 105 yr. X-ray observations, optical spectroscopy, and spectropolarimetry have a high potential for studies of the properties of J1832 as an individual object and of stream-fed accretion in general.

Spot and Facula Activity Variations of the Eccentric Detached Eclipsing Binary KIC 8098300 Based on the Time-series Orbital Solutions

The LAMOST spectra and Kepler light curves are combined to investigate the detached eclipsing binary KIC 8098300, which shows the O’Connell effect caused by spot/facula modulation. The radial velocity (RV) measurements are derived by using the tomographic spectra disentangling technology. The mass ratio q = K 1/K 2 = 0.812 ± 0.007, and the orbital semimajor axis are obtained by fitting the RV curves. We optimize the binary model concerning the spot/facula activity with the code PHOEBE and obtain precise parameters of the orbit including the eccentricity e = 0.0217 ± 0.0008, the inclination i = 87.°71 ± 0.°04, and the angle of periastron ω = 284.°1 ± 0.°5. The masses and radii of the primary and secondary star are determined as M 1 = 1.3467 ± 0.0001 M ⊙, R 1 = 1.569 ± 0.003 R ⊙, and M 2 = 1.0940 ± 0.0001 M ⊙, R 2 = 1.078 ± 0.002 R ⊙, respectively. The ratio of temperatures of the two component stars is r teff = 0.924 ± 0.001. We also obtain the periastron precession speed of 0.000024 ± 0.000001 day cycle−1. The residuals of out-of-eclipse are analyzed using the auto-correlation function and the discrete Fourier transform. The spot/facula activity is relatively weaker, but the lifetime is longer than that of most single main sequence stars in the same temperature range. The average rotation period of the spots P rot = 4.32 days is slightly longer than the orbital period, which may be caused by either the migration of spots/faculae along the longitude or the latitudinal differential rotation. The activity may be spot-dominated for the secondary star and facula-dominated for the primary star. The method of this work can be used to analyze more eclipsing binaries with the O’Connell effect in the Kepler field to obtain the precise parameters and investigate the difference of spot behavior between binaries and single stars. It is helpful for a deeper understanding of the stellar magnetic activity and dynamo theory.

The influence of initial orbital period on helium and carbon-oxygen core masses in massive case a binary systems with low accretion efficiency

The evolutionary models of 33 massive Case A binary systems in mass range from 30M? to 40M? with initial orbital periods of 3, 4 and 5 days, accretion efficiency of 10% and at the solar metallicity are presented. The models are obtained with the MESA (Modules for Experiments in Stellar Astrophysics) numerical code. The evolution is followed from a double O-type star, through Case A and Case mass transfer, to the formation of a carbon-oxygen (CO) core in the primary. The evolution of the secondary star in each binary system is further modeled with the same numerical code in an approximation of a single star, also to the formation of a carbon-oxygen core. The resulting helium core masses are in the range of 7.94M? - 13.19M? and 12.30M? and 19.12M? for primary and secondary stars, respectively. The carbon-oxygen core masses are between 5.26M? and 10M? for primaries and between 8.96M? and 15.32M? for secondaries. A clear influence of the initial orbital period on the resulting helium and CO core masses is demonstrated: primary stars in binary systems with initial orbital periods of 3, 4 and 5 days have on average about 15%, 8% and 2.5% smaller CO cores than single stars with the same initial masses. On the other hand, it was found that the correlation between the CO and helium core mass does not depend on the initial orbital period and can be approximated with the same linear fit for all binary systems. The CO/helium core mass ratio is found to be larger in binary systems than for single stars. It is also shown that the black hole formation limit for primary stars depends on the initial orbit and is between 33M? - 34M?, 32M? - 33M? and 30M? - 31M?, for the initial orbital periods of 3, 4 and 5 days, respectively. The resulting double compact objects are of two types: mixed neutron star - black hole systems (6 models) and double black holes (27 models). The resulting black hole masses are estimated to be in the range of 5M? to 17M?.

A Stellar Constraint on Eddington-inspired Born–Infeld Gravity from Cataclysmic Variable Binaries

Abstract Eddington-inspired Born–Infeld gravity is an important modification of Einstein’s general relativity, which can give rise to nonsingular cosmologies at the classical level, and avoid the end-stage singularity in a gravitational collapse process. In the Newtonian limit, this theory gives rise to a modified Poisson’s equation, as a consequence of which stellar observables acquire model dependent corrections, compared to the ones computed in the low energy limit of general relativity. This can in turn be used to establish astrophysical constraints on the theory. Here, we obtain such a constraint using observational data from cataclysmic variable binaries. In particular, we consider the tidal disruption limit of the secondary star by a white dwarf primary. The Roche lobe filling condition of this secondary star is used to compute stellar observables in the modified gravity theory in a numerical scheme. These are then contrasted with the values obtained by using available data on these objects, via a Monte Carlo error progression method. This way, we are able to constrain the theory within the 5σ confidence level.

Planet–planet scattering in presence of a companion star

ABSTRACT Planet–planet (P–P) scattering is a leading dynamical mechanism invoked to explain the present orbital distribution of exoplanets. Many stars belong to binary systems; therefore, it is important to understand how this mechanism works in the presence of a companion star. We focus on systems of three planets orbiting the primary star and estimate the time-scale for instability, finding that it scales with the Keplerian period for systems that have the same ratio between inner planet and binary semimajor axes. An empirical formula is also derived from simulations to estimate how the binary eccentricity affects the extent of the stability region. The presence of the secondary star affects the P–P scattering outcomes, causing a broadening of the final distribution in semimajor axis of the inner planet as some of the orbital energy of the planets is absorbed by the companion star. Repeated approaches to the secondary star also cause a significant reduction in the frequency of surviving two-planet systems in particular for larger values of the inner planet semimajor axis. The formation of Kozai states with the companion star increases the number of planets that may be tidally circularized. To predict the possible final distribution of planets in binaries, we have performed a large number of simulations where the initial semimajor axis of the inner planets is chosen randomly. For small values of the binary semimajor axis, the higher frequency of collision alters the final planet orbital distributions that, however, beyond 50 au appear to be scalable to wider binary separations.

Light curve analysis of the eclipsing binary system V781 Tau

V781 Tau is one of W UMa eclipsing binary systems whose orbital period is 0.34 days. The 0.7-meter telescope with CCD photometric system in B and V filters was conducted at the Regional Observatory for the Public, Chachoengsao, Thailand during December 2018, UT. The Wilson-Devinney Technique was used for calculating the physical properties of V781 Tau. The results showed the inclination of their orbital is 66.140°±0.14. The effective temperature of the primary and secondary star is 6,060 and 5,881 K, respectively and the degree of contact is 4.38 %

APOGEE Chemical Abundance Patterns of the Massive Milky Way Satellites

Abstract The SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey has obtained high-resolution spectra for thousands of red giant stars distributed among the massive satellite galaxies of the Milky Way (MW): the Large and Small Magellanic Clouds (LMC/SMC), the Sagittarius Dwarf Galaxy (Sgr), Fornax (Fnx), and the now fully disrupted Gaia Sausage/Enceladus (GSE) system. We present and analyze the APOGEE chemical abundance patterns of each galaxy to draw robust conclusions about their star formation histories, by quantifying the relative abundance trends of multiple elements (C, N, O, Mg, Al, Si, Ca, Fe, Ni, and Ce), as well as by fitting chemical evolution models to the [α/Fe]–[Fe/H] abundance plane for each galaxy. Results show that the chemical signatures of the starburst in the Magellanic Clouds (MCs) observed by Nidever et al. in the α-element abundances extend to C+N, Al, and Ni, with the major burst in the SMC occurring some 3–4 Gyr before the burst in the LMC. We find that Sgr and Fnx also exhibit chemical abundance patterns suggestive of secondary star formation epochs, but these events were weaker and earlier (∼5–7 Gyr ago) than those observed in the MCs. There is no chemical evidence of a second starburst in GSE, but this galaxy shows the strongest initial star formation as compared to the other four galaxies. All dwarf galaxies had greater relative contributions of AGB stars to their enrichment than the MW. Comparing and contrasting these chemical patterns highlight the importance of galaxy environment on its chemical evolution.

Eta Carinae: A Tale of Two Periastron Passages

Abstract Since 2002, the far-ultraviolet (FUV) flux (1150–1680 Å) of Eta Carinae, monitored by the Hubble Space Telescope/Space Telescope Imaging Spectrograph, has increased by an order of magnitude. This increase is attributed to partial dissipation of a line-of-sight (LOS) occulter that blocks the central core of the system. Across the 2020 February periastron passage, changes in the FUV emission show a stronger wavelength dependence than occurred across the 2003 July periastron passage. Across both periastron passages, most of the FUV spectrum dropped in flux then recovered a few months later. The 2020 periastron passage included enhancements of FUV flux in narrow spectral intervals near periastron followed by a transient absorption and recovery to pre-periastron flux levels. The drop in flux is due to increased absorption by singly ionized species as the secondary star plunges deep into the wind of the primary star, which blocks the companion’s ionizing radiation. The enhanced FUV emission is caused by the companion’s wind-blown cavity briefly opening a window to deeper layers of the primary star. This is the first time transient brightening has been seen in the FUV comparable to transients previously seen at longer wavelengths. Changes in resonance line-velocity profiles hint that the dissipating occulter is associated with material in LOS moving at −100 to −300 km s−1, similar in velocity of structures previously associated with the 1890s lesser eruption.

Follow-up Photometry in Another Band Helps to Reduce Kepler’s False-positive Rates

Abstract The Kepler mission’s single-band photometry suffers from astrophysical false positives, most commonly of background eclipsing binaries (BEBs) and companion transiting planets (CTPs). Multicolor photometry can reveal the color-dependent depth feature of false positives and thus exclude them. In this work, we aim to estimate the fraction of false positives that cannot be classified by Kepler alone but can be identified from their color-dependent depth feature if a reference band (z, K s , and Transiting Exoplanet Survey Satellite (TESS)) is adopted in follow-up observation. We construct physics-based blend models to simulate multiband signals of false positives. Nearly 65%–95% of the BEBs and more than 80% of the CTPs that host a Jupiter-sized planet will show detectable depth variations if the reference band can achieve a Kepler-like precision. The K s band is most effective in eliminating BEBs exhibiting features of any depth, while the z and TESS bands are better for identifying giant candidates, and their identification rates are more sensitive to photometric precision. Given the radius distribution of planets transiting the secondary star in binary systems, we derive a formalism to calculate the overall identification rate for CTPs. By comparing the likelihood distribution of the double-band depth ratio for BEB and planet models, we calculate the false-positive probability (FPP) for typical Kepler candidates. Additionally, we show that the FPP calculation helps distinguish the planet candidate’s host star in an unresolved binary system. The framework of the analysis in this paper can be easily adapted to predict the multicolor photometric yield for other transit surveys, especially TESS.

TIC 172900988: A Transiting Circumbinary Planet Detected in One Sector of TESS Data

Abstract We report the first discovery of a transiting circumbinary planet detected from a single sector of Transiting Exoplanet Survey Satellite (TESS) data. During Sector 21, the planet TIC 172900988b transited the primary star and then five days later it transited the secondary star. The binary is itself eclipsing, with a period P ≈ 19.7 days and an eccentricity e ≈ 0.45. Archival data from ASAS-SN, Evryscope, KELT, and SuperWASP reveal a prominent apsidal motion of the binary orbit, caused by the dynamical interactions between the binary and the planet. A comprehensive photodynamical analysis of the TESS, archival and follow-up data yields stellar masses and radii of M 1 = 1.2384 ±0.0007 M ⊙ and R 1 = 1.3827 ± 0.0016 R ⊙ for the primary and M 2 = 1.2019 ± 0.0007 M ⊙ and R 2 = 1.3124 ±0.0012 R ⊙ for the secondary. The radius of the planet is R 3 = 11.25 ± 0.44 R ⊕ (1.004 ± 0.039R Jup). The planet’s mass and orbital properties are not uniquely determined—there are six solutions with nearly equal likelihood. Specifically, we find that the planet’s mass is in the range of 824 ≲ M 3 ≲ 981 M ⊕ (2.65 ≲ M 3 ≲ 3.09M Jup), its orbital period could be 188.8, 190.4, 194.0, 199.0, 200.4, or 204.1 days, and the eccentricity is between 0.02 and 0.09. At V = 10.141 mag, the system is accessible for high-resolution spectroscopic observations, e.g., the Rossiter–McLaughlin effect and transit spectroscopy.

The relative calibration of radial velocity for LAMOST medium resolution stellar spectra

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) started a median-resolution spectroscopic (MRS, R ∼7500) survey since October 2018. The main scientific goals of MRS, including binary stars, pulsators and other variable stars, were 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 on different nights can be corrected well for LAMOST MRS in each spectrograph. In addition, the precision of radial velocity zero-point of multi-epoch time-domain observations reaches below 0.5 km s−1. 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.

Photometric Analysis of the TESS Light Curve for the Asynchronous Polar V1500 Cyg

After 40 yr of nova outburst, there are still some difficulties in completely understanding V1500 Cyg. The faintness of V1500 Cyg and its crowded field pose a challenge for high-precision photometry by TESS, while long-term intensive data from TESS provide the first opportunity to accurately model this binary system. The best-fitting model reveals that the high-temperature white dwarf illuminates the surface of its companion star, a low-mass secondary (M 2 ∼ 0.22 M ⊙), and the emission from the heated secondary star still dominates the luminosity of the system. The binary model has determined the white dwarf’s mass to be M 1 ∼ 1.4 M ⊙ and its orbital inclination i ∼ 60°. The residuals after the removal of the fitted model curve present variations in both the white dwarf’s spin period and the binary orbital period, confirming the previous claim that V1500 Cyg is still an asynchronous polar, and the observed spin period demonstrates that V1500 Cyg is approaching synchronicity with the orbital period following the prediction of the polarization ephemeris. The phasing of the spin humps indicates their cyclotron origin, and the residual orbital humps may be produced by the emission from the accretion stream. There is no evidence for the pole-switching effect, while the double-humped profile of the spin modulation suggests that the white dwarf may be accreting matter from a “ring” around it. Further simultaneous polarization and X-ray observations are urged to explore the accretion geometry of V1500 Cyg in detail.