We present estimates of stellar age and mass for 0.93 million Galactic disk main sequence turn-off and sub-giant stars from the LAMOST Galactic Spectroscopic Surveys. The ages and masses are determined by matching with stellar isochrones using Bayesian algorithm, utilizing effective temperature T eff , absolute magnitude M V , metallicity Gyr exhibit both the thin and thick disk sequences, while younger (older) stars show only the thin (thick) disk sequence, indicating that the thin disk became prominent 8-10 Gyr ago, while the thick disk formed earlier and almost quenched 8 Gyr ago. Stellar ages exhibit positive vertical and negative radial gradients across the disk, and the outer disk of R 9 kpc exhibits a strong flare in stellar age distribution.

Five fast radio bursts (FRBs), including three apparently non-repeating ones, FRB 180924, FRB 181112, and FRB 190523, and two repeaters, FRB 121102 and FRB 180916.J0158+65, have already been localized so far. We apply a method developed recently by us to these five localized FRBs to give a cosmology-insensitive estimate of the fraction of baryon mass in the intergalactic medium, fIGM. Using the measured dispersion measure (DM) and luminosity distance dL data (inferred from the FRB redshifts and dL of Type Ia supernovae at the same redshifts) of the five FRBs, we constrain the local $f_{\rm IGM} = 0.84^{+0.16}_{-0.22}$ with no evidence of redshift dependence. This cosmology-insensitive estimate of fIGM from FRB observations is in excellent agreement with previous constraints using other probes. Moreover, using the three apparently non-repeating FRBs only we get a little looser but consistent result: $f_{\rm IGM} = 0.74^{+0.24}_{-0.18}$. In these two cases, reasonable estimations for the host galaxy DM contribution (DMhost) can be achieved by modelling it as a function of star formation rate. The constraints on both fIGM and DMhost are expected to be significantly improved with the rapid progress in localizing FRBs.

Coherent curvature radiation as the radiation mechanism for fast radio bursts (FRBs) has been discussed since FRBs were discovered. We study the spectral and polarization properties of repeating FRBs within the framework of coherent curvature radiation by charged bunches in the magnetosphere of a highly magnetized neutron star. The spectra can be generally characterized by multisegmented broken power laws, and evolve as bunches move and the line of sight sweeps. Emitted waves are highly linear polarized and polarization angles are flat across the burst envelopes, if the line of sight is confined to the beam within an angle of 1/γ, while a circular polarization fraction becomes strong for off-beam cases. The spectro-temporal pulse-to-pulse properties can be a natural consequence due to the magnetospheric geometry. We investigate the relationship between drift rate, central frequency, and temporal duration. The radius-to-frequency mapping is derived and simulated within the assumptions of both dipolar and quadrupolar magnetic configurations. The geometric results show that FRBs are emitted in field lines more curved than open field lines for a dipolar geometry. This suggests that there are most likely existing multipolar magnetic configurations in the emission region.

No abstract