White dwarf-main sequence binaries from LAMOST: the DR1 catalogue

Context. White dwarf-main sequence (WDMS) binaries are used to study several different important open problems in modern astrophysics. Aims. The Sloan Digital Sky Survey (SDSS) identified the largest catalogue of WDMS binaries currently known. However, this sample is seriously affected by selection effects and the population of systems containing cool white dwarfs and early-type companions is under-represented.Here we search for WDMS binaries within the spectroscopic data release 1 of the LAMOST (Large sky Area Multi-Object fiber Spectroscopic Telescope) survey. LAMOST and SDSS follow different target selection algorithms. Hence, LAMOST WDMS binaries may be drawn from a different parent population and thus help in overcoming the selection effects incorporated by SDSS on the current observed population. Methods. We develop a fast and efficient routine based on the wavelet transform to identify LAMOST WDMS binaries containing a DA white dwarf and a M dwarf companion, and apply a decomposition/fitting routine to their LAMOST spectra to estimate their distances and measure their stellar parameters, namely the white dwarf effective temperatures, surface gravities and masses, and the secondary star spectral types. Results. We identify 121 LAMOST WDMS binaries, 80 of which are new discoveries, and estimate the sample to be \sim90 per cent complete. The LAMOST and SDSS WDMS binaries are found to be statistically different. However, this result is not due to the different target selection criteria of both surveys, but likely a simple consequence of the different observing conditions. Thus, the LAMOST population is found at considerably shorter distances (\sim50-450 pc) and is dominated by systems containing early-type companions and hot white dwarfs. (abridged)Comment: 14 pages, 8 figures, accepted for publication in A&

The Milky Way's rotation curve out to 100 kpc and its constraint on the Galactic mass distribution

The rotation curve (RC) of the Milky Way out to $\sim$ 100 kpc has been constructed using $\sim$ 16,000 primary red clump giants (PRCGs) in the outer disk selected from the LSS-GAC and the SDSS-III/APOGEE survey, combined with $\sim$ 5700 halo K giants (HKGs) selected from the SDSS/SEGUE survey. To derive the RC, the PRCG sample of the warm disc population and the HKG sample of halo stellar population are respectively analyzed using a kinematical model allowing for the asymmetric drift corrections and re-analyzed using the spherical Jeans equation along with measurements of the anisotropic parameter $\beta$ currently available. The typical uncertainties of RC derived from the PRCG and HKG samples are respectively 5-7 km/s and several tens km/s. We determine a circular velocity at the solar position, $V_c (R_0)$ = 240 $\pm$ 6 km/s and an azimuthal peculiar speed of the Sun, $V_{\odot}$ = 12.1 $\pm$ 7.6 km/s, both in good agreement with the previous determinations. The newly constructed RC has a generally flat value of 240 km/s within a Galactocentric distance $r$ of 25 kpc and then decreases steadily to 150 km/s at $r$ $\sim$ 100 kpc. On top of this overall trend, the RC exhibits two prominent localized dips, one at $r$ $\sim$ 11 kpc and another at $r$ $\sim$ 19 kpc. From the newly constructed RC, combined with other constraints, we have built a parametrized mass model for the Galaxy, yielding a virial mass of the Milky Way's dark matter halo of $0.90^{+0.07}_{-0.08} \times 10^{12}$ ${\rm M}_{\odot}$ and a local dark matter density, $\rho_{\rm \odot, dm} = 0.32^{+0.02}_{-0.02}$ GeV cm$^{-3}$.Comment: MNRAS accepted, 18 pages, 15 figures, 4 table

Understanding Turn-On Transients of SiC High-Power Modules: Drain-Source Voltage Plateau Characteristics

The SiC (silicon carbide) high-power module has great potential to replace the IGBT (insulated gate bipolar transistor) power module in high-frequency and high-power applications, due to the superior properties of fast switching and low power loss, however, when the SiC high-power module operates under inappropriate conditions, the advantages of the SiC high-power module will be probably eliminated. In this paper, four kinds of SiC high-power modules are fabricated to investigate fast switching performance. The variations in characteristics of drain-source voltage at turn-on transient under the combined conditions of multiple factors are studied. A characteristic of voltage plateau is observed from the drain-source voltage waveform at turn-on transient in the experiments, and the characteristic is reproduced by simulation. The mechanism behind the voltage plateau is studied, and it is revealed that the characteristic of drain-source voltage plateau is a reflection of the miller plateau effect of gate-source voltage on drain-source voltage under the combined conditions of fast turn-on speed and low DC bus voltage, while the different values of drain-source voltage plateau are attributed to the discrepancy of structure between upper-side and lower-side in the corresponding partial path of the drain circuit loop inside the module, with the standard 62 mm package outline