| 其他摘要 | Criterion of dynamically unstable mass transfer, common envelope (CE) evolution and angular momentum loss (AML) are three basic problems in close binary evolution. Solving these problems could help us to understand the formation of various related objects. However, CE evolution and the AML have been mainly developed in theoretical studies. Theoretical results can’t describe the CE phase and AML completely, and there are rare stringent observational constraint on the theoretical studies. The ideal sample of detached post common envelope binaries (PCEBs) provides a observational constraint on the CE evolution and AML in close binary. Short period white dwarf-main sequence (WDMS) binaries are candidates of the PCEBs, and the Sloan Digital Sky Survey (SDSS) offers the possibility of dramatically increasing the number of the WDMS binaries. So, we have developed a color selection criteria to diagnose the WDMS from SDSS, and hoped that completeness of the WDMS binary sample will be improved. Using the single white dwarf and main sequence models, we constructed a detached binary model consisting of a white dwarf plus a main sequence, and obtained model colors of this binary system. According to the model colors areas, we designed optical and near-infrared color selection criteria. Using the colorcriteria, 759 targets which have been observed spectroscopically were selected from the SDSS DR7. Among these, 512 objects are WDMS and 4 objects are WDMS candidates. Of these 516 WDMS and candidates, 86 have not been published so far. Combining the near-infrared with optical color-criteria (r−i > 0.3; II region), we have found 506 targets are the WDMS candidates from UKIRT Infrared Sky Survey (UKIDSS) DR5 cross-matched with the SDSS DR7. Of these 506 candidates, 13 have been identified as the WDMS binaries. By fitting the combined spectrum of the WDMS binaries using a χ2 minimizationtechnique, we derived independent parameter estimates, such as effective temperature, surface gravity, mass, and metallicity, for its components. Furthermore, we independently estimated the distances of the white dwarf and secondary star from us and determined the cooling age of white dwarf in our fitting procedure. According to the distribution of the cooling ages, we found that the mean cooling age t of white dwarf is ∼ 4.33×108 years, and the evolution of∼ 12% WDMS binaries have gone thought about 1.2 × 109 years. So, comparing with the current PCEBs sample, our sample is more extended and contains cooler and older white dwarf binaries. The sample also is uniform distribution. Accordingly, we provide a relatively complete candidate sample for identifying the short period PCEBs. |
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