Keywords

Magnetic fields are significant in the structure and evolution of stars. We present a comprehensive catalog of 1784 known magnetic stars, detailing their identifications, HD numbers, precise locations, spectral types and averaged quadratic effective magnetic fields among other important information. The group comprises 177 O-type stars, 551 B-type stars, 520 A-type stars, 91 F-type stars, 53 G-type stars, 61 K-type stars, 31 M-type stars and an additional 300 stars whose spectral classification remains indeterminate. Our analysis examines the statistical properties of these magnetic stars. The relative integrated distribution function and number distribution function for all magnetic stars of the same spectral type can be effectively approximated using an exponential function of the averaged quadratic effective magnetic field. The analysis further reveals that A and B-type stars possess the strongest mean magnetic fields, indicating an easier detection of their magnetic fields.

The braking indices of pulsars may contain important information about the internal physics of neutron stars (NSs), such as neutron superfluidity and internal magnetic fields. As a subsequent paper of Cheng et al., we perform the same analysis as that done in the previous paper to other young pulsars with a steady braking index, n. Combining the timing data of these pulsars with the theory of magnetic field decay, and using their measured magnetic tilt angles, we can set constraints on the number of precession cycles, ξ, which represents the interactions between superfluid neutrons and other particles in the NS interior. For the pulsars considered in this paper, the results show that ξ is within the range of a few ×10³ to a few ×10⁶. Interestingly, for the Crab and Vela pulsars, the constraints on ξ obtained with our method are generally consistent with that derived from modeling of the glitch rise behaviors of the two pulsars. Furthermore, we find that the internal magnetic fields of pulsar with n < 3 may be dominated by the toroidal components. Our results may not only help to understand the interactions between the superfluid neutrons and other particles in the interior of NSs but also be important for the study of continuous gravitational waves from pulsars.

The observed electromagnetic radiation from some long and short gamma-ray bursts, and neutron stars (NSs), and the theoretical models proposed to interpret these observations together point to a very interesting but confusing problem, namely, whether fall-back accretion could lead to dipole field decay of newborn NSs. In this paper, we investigate the gravitational wave (GW) radiation of newborn magnetars with a fall-back disk formed in both the core-collapse of massive stars and the merger of binary NSs. We make a comparison of the results obtained with and without fall-back accretion-induced dipole-field decay (FADD) involved. Depending on the fall-back parameters, initial parameters of newborn magnetars, and models used to describe FADD, FADD may indeed occur in newborn magnetars. Because of the low dipole fields caused by FADD, the newborn magnetars will be spun up to higher frequencies and have larger masses in comparison with the non-decay cases. Thus the GW radiation of newborn accreting magnetars would be remarkably enhanced. We propose that observation of GW signals from newborn magnetars using future GW detectors may help to reveal whether FADD could occur in newborn accreting magnetars. Our model is also applied to the discussion of the remnant of GW170817. From the post-merger GW searching results of Advanced LIGO and Advanced Virgo we cannot confirm the remnant is a low-dipole-field long-lived NS. Future detection of GWs from GW170817-like events using more sensitive detectors may help to clarify the FADD puzzle.

We first present the multicolor photometry results of the rapidly rotating magnetic star HD 345439 using the Nanshan One-meter Wide-field Telescope. From the photometric observations, we derive a rotational period of 0.7699 ± 0.0014 day. The light curves of HD 345439 are dominated by the double asymmetric S-wave feature that arises from the magnetic clouds. Pulsating behaviors are not observed in Sector 41 of the Transiting Exoplanet Survey Satellite. No evidence is found of the occurrence of centrifugal breakout events neither in the residual flux nor in the systematic variations at the extremum of the light curve. Based on the hypothesis of the Rigidly Rotating Magnetosphere model, we restrict the magnetic obliquity angle β and the rotational inclination angle i so that they satisfy the approximate relation β + i ≈ 105°. The color excess, extinction, and luminosity are determined to be E_(B−V) = 0.745 ± 0.016 mag, A_V = 2.31 ± 0.05 mag, and $\mathrm{log}(L/{L}_{\odot })=3.82\pm 0.1$ dex, respectively. Furthermore, we derive the effective temperature as T_eff = 22 ± 1 kK and the surface gravity as log g = 4.00 ± 0.22. The mass $M={7.24}_{-1.24}^{+1.75}\,{M}_{\odot }$, radius $R={4.44}_{-1.93}^{+2.68}\,{R}_{\odot }$, and age $\,{\tau }_{\mathrm{age}}=23.62{\,}_{-21.97}^{+4.24}\,$ Myr are estimated from the Hertzsprung–Russell diagram.

This paper proposes the idea that the observed dependence of stellar activity cycles on rotation rate can be a manifestation of a stronger dependence on the effective temperature. Observational evidence is recalled and theoretical arguments are given for the presence of cyclic activity in the case of sufficiently slow rotation only. Slow rotation means proximity to the observed upper bound on the rotation period of solar-type stars. This maximum rotation period depends on temperature and shortens for hotter stars. The maximum rotation period is interpreted as the minimum rotation rate for operation of a large-scale dynamo. A combined model for differential rotation and the dynamo is applied to stars of different mass rotating with a rate slightly above the threshold rate for the dynamo. Computations show shorter dynamo cycles for hotter stars. As the hotter stars rotate faster, the computed cycles are also shorter for faster rotation. The observed smaller upper bound for rotation period of hotter stars can be explained by the larger threshold amplitude of the α-effect for onset of their dynamos: a larger α demands faster rotation. The amplitude of the (cycling) magnetic energy in the computations is proportional to the difference between the rotation period and its upper bound for the dynamo. Stars with moderately different rotation rates can differ significantly in super-criticality of their dynamos and therefore in their magnetic activity, as observed.

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) completed its commissioning and began the Commensal Radio Astronomy FasT Survey (CRAFTS), a multi-year survey to cover 60% of the sky, in 2020. We present predictions for the number of radio-flaring ultracool dwarfs (UCDs) that are likely to be detected by CRAFTS. Based on the observed flaring UCDs from a number of unbiased, targeted radio surveys in the literature, we derive a detection rate of ≥3%. Assuming a flat radio spectrum νL_ν ∝ ν^β+1 with β = −1.0 for UCD flares, we construct a flare luminosity function ${dN}/{dL}\propto {L}^{-1.96\pm 0.45}$ (here L = νL_ν). CRAFTS is found to be sensitive enough for flares from UCDs up to ∼180 pc. Considering the Galactic thin disk, we carry out a 3D Monte Carlo simulation of the UCD population, which is then fed to mock CRAFTS observations. We estimate that ∼170 flaring UCDs would be detected through transient searches in circular polarization. Though only marginally sensitive to the scale height of UCDs, the results are very sensitive to the assumed spectral index β. For β from 0 to −2.5, the number of expected detections increases dramatically from ∼20 to ∼3460. We also contemplate the strategies for following up candidates of flaring UCDs, and discuss the implications of survey results for improving our knowledge of UCD behavior at L band and dynamos.

New CCD photometric observations of G-type contact binary UV Lyn were obtained in 2006 and 2020, when the light curves (LCs) showed positive O'Connell effect and negative O'Connell effect, respectively. From the previous studies, the LCs by other ground-based telescope are variable from 1973 to 2020, particularly the magnitude difference between the two maxima. These phenomena indicate that the component has been active in the past 47 yr. In addition, during monitoring by the space telescope Transiting Exoplanet Survey Satellite (TESS) from January to March 2020, we fortunately found continuous variations from the O'Connell effect in every cycle for the first time. The analysis also shows that in a short time, the positive O'Connell effect has been transformed into the negative one, which demonstrates that there are stronger magnetic activities on the surface of the component. By using the Wilson-Devinney code with a spot model, these photometric solutions confirm UV Lyn is a shallow W-subtype contact binary with a cool equatorial spot on the less massive component. The successive variability of the O'Connell effect possibly results from one equatorial cool spot shifting gradually along with time. We also investigated its O − C curve from these continuous LCs, and there is no apparent variation in such a short time. However, regarding the O'Connell effect as the indicator of magnetic activity indicates the system is possibly undergoing a periodic trend with a period of nearly 38 days. Comparing with the trend of the O − C curve, we could not find any relation between the period variation and magnetic activity.

Main-sequence low-mass stars are known to spin down as a consequence of their magnetized stellar winds. However, estimating the precise rate of this spin-down is an open problem. The mass-loss rate, angular momentum loss rate, and magnetic field properties of low-mass stars are fundamentally linked, making this a challenging task. Of particular interest is the stellar magnetic field geometry. In this work, we consider whether non-dipolar field modes contribute significantly to the spin-down of low-mass stars. We do this using a sample of stars that have all been previously mapped with Zeeman–Doppler imaging. For a given star, as long as its mass-loss rate is below some critical mass-loss rate, only the dipolar fields contribute to its spin-down torque. However, if it has a larger mass-loss rate, higher-order modes need to be considered. For each star, we calculate this critical mass-loss rate, which is a simple function of the field geometry. Additionally, we use two methods of estimating mass-loss rates for our sample of stars. In the majority of cases, we find that the estimated mass-loss rates do not exceed the critical mass-loss rate; hence, the dipolar magnetic field alone is sufficient to determine the spin-down torque. However, we find some evidence that, at large Rossby numbers, non-dipolar modes may start to contribute.

The effects of inhomogeneous magnetic fields on the propagation of magnetohydrodynamical (MHD) laminar flame fronts are investigated. This investigation is motivated by the occurrence of magnetized thermonuclear combustion in astrophysical systems. Magnetized thermonuclear burning occurs on the surfaces of neutron stars during Type I X-ray bursts, within the interiors of white dwarfs during SNe Ia, and during classical novae. Thermonuclear flames that propagate in these systems are expected to travel through inhomogeneous magnetic fields. We present the results of a series of 1.5-dimensional numerical simulations of magnetized flame propagation. A simplified flame model is used with one-step Arrhenius kinetics, an ideal gas equation of state, and constant thermal conductivity coefficients. Although idealized, the model allows for the opportunity to study the physics of the problem without the complexities of the nuclear kinetics of thermonuclear burning. We simulate the propagation of laminar flames through inhomogeneous magnetic media. A changing magnetic medium significantly alters the structure of the flame through the generation of an electric current. The electric current rotates the direction of the magnetic field within the flame and produces strong shear flows. Furthermore, for flames that conduct heat anisotropically and that propagate at an angle 0 < ψ ≲ π/2 to the magnetic field, the flame speed increases due to the nonuniform magnetic field. Naturally occurring flames in astrophysical systems may experience similar changes to their structure and speed that would influence the observational properties of these systems.

We analyze the light curve of 1740 flare stars to study the relationship between the magnetic feature characteristics and the identified flare activity. Coverage and stability of magnetic features are inspired by rotational modulation of light-curve variations and flare activity of stars are obtained using our automated flare detection algorithm. The results show that: (i) the flare time occupation ratio (or flare frequency) and the total power of flares increase by increasing relative magnetic feature coverage and contrast in F–M-type stars; (ii) magnetic feature stability is highly correlated with the coverage and the contrast of the magnetic structures, as this is the case for the Sun; and (iii) stability, coverage, and contrast of the magnetic features, time occupation ratio, and total power of flares increases for G-, K-, and M-type stars by decreasing the Rossby number due to the excess of the produced magnetic field from dynamo procedure until reaching to the saturation level.