No abstract

The binding energy parameter λ plays an important role in common envelope (CE) evolution. Previous works have already pointed out that λ varies throughout the stellar evolution, though it has been adopted as a constant in most of the population synthesis calculations. We have systematically calculated the binding energy parameter λ for both Population I and Population II stars of masses 1 − 20M ⊙ , taking into account the contribution from the internal energy of stellar matter. We present fitting formulae for λ that can be incorporated into future population synthesis investigations. We also briefly discuss the possible applications of the results in binary evolutions.

The properties of the Galactic Ridge X-ray Emission (GRXE) observed in the 2-10 keV band place fundamental constraints on various types of X-ray sources in the Milky Way. Although the primarily discrete origin of the emission is now well established, the responsible populations of these sources remain uncertain, especially at relatively low fluxes. To provide insights into this issue, we systematically characterize the Fe emission line properties of the candidate types of the sources in the solar neighborhood and compare them with those measured for the GRXE. Our source sample includes 6 symbiotic stars (SSs), 16 intermediate polars (IPs), 3 polars, 16 quiescent dwarf novae (DNe) and 4 active binaries (ABs); they are all observed with the Suzaku X-ray Observatory. The data of about 1/4 of these sources are analyzed for the first time. We find that the mean equivalent width (EW 6.7 ) of the 6.7-keV line and the mean 7.0/6.7-keV line ratio are 107 ± 16.0 eV and 0.71 ± 0.04 for intermediate polars and 221 ± 135 eV and 0.44 ± 0.14 for polars, respectively, which are all substantially different from those (490 ± 15 eV and 0.2 ± 0.08) for the GRXE. Instead, the GRXE values are better agreed by the EW 6.7 (438 ± 84.6 eV) and the ratio (0.27 ± 0.06) observed for the DNe. We further find that the EW 6.7 is strongly correlated with the 2-10-keV luminosity of the DNe, which can be characterized by the relation EW 6.7 = (438 ± 95 eV)(L/10 31 ergs s −1 ) (−0.31±0.15) . Accounting for this correlation, the agreement can be improved further, especially when the contributions from other classes sources to the GRXE are considered, which all have low EW 6.7 values. We conclude that the GRXE mostly consists of typically faint, but numerous DNe, plus ABs, while magnetic CVs are probably mainly the high-flux representatives of the responsible populations and dominate the GRXE only in harder energy bands.

Based on archival Chandra data, we have carried out an X-ray survey of 69, or nearly half the known population of, Milky Way globular clusters (GCs), focusing on weak X-ray sources, mainly cataclysmic variables (CVs) and coronally active binaries (ABs). Using the cumulative X-ray luminosity per unit stellar mass (i.e., X-ray emissivity) as a proxy of the source abundance, we demonstrate a paucity (lower by 41% ± 27% on average) of weak X-ray sources in most GCs relative to the field, which is represented by the Solar neighborhood and Local Group dwarf elliptical galaxies. We also revisit the mutual correlations among the cumulative X-ray luminosity (L X ), cluster mass (M ) and stellar encounter rate (Γ), finding L X ∝ M 0.74±0.13 , L X ∝ Γ 0.67±0.07 and Γ ∝ M 1.28±0.17 . The three quantities can further be expressed as L X ∝ M 0.64±0.12 Γ 0.19±0.07 , which indicates that the dynamical formation of CVs and ABs through stellar encounters in GCs is less dominant than previously suggested, and that the primordial formation channel has a substantial contribution. Taking these aspects together, we suggest that a large fraction of primordial, soft binaries have been disrupted in binary-single or binary-binary stellar interactions before they can otherwise evolve into X-ray-emitting close binaries, whereas the same interactions also have led to the formation of new close binaries. No significant correlations between L X /L K and cluster properties, including dynamical age, metallicity and structural parameters, are found.

Using archival Chandra observations with a total exposure of 510 ks, we present an updated catalog of point sources for Globular Cluster 47 Tucanae. Our study covers an area of ∼ 176.7 arcmin 2 (i.e., with R 7.5 ′ ) with 537 X-ray sources. We show that the surface density distribution of X-ray sources in 47 Tuc is highly peaked in cluster center, rapidly decreases at intermediate radii, and finally rises again at larger radii, with two distribution dips at R ∼ 100 ′′ and R ∼ 170 ′′ for the faint (L X 5.0 × 10 30 erg s −1 ) and bright (L X 5.0 × 10 30 erg s −1 ) groups of X-ray sources, separately. These distribution features are similar to those of Blue Straggler Stars (BSS), where the distribution dip is located at R ∼ 200 ′′ ). By fitting the radial distribution of each group of sources with a "generalized King model", we estimated an average mass of 1.51 ± 0.17 M ⊙ , 1.44 ± 0.15 M ⊙ and 1.16 ± 0.06 M ⊙ for the BSS, bright and faint X-ray sources, respectively. These results are consistent with the mass segregation effect of heavy objects in GCs, where more massive objects drop to the cluster center faster and their distribution dip propagates outward further. Besides, the peculiar distribution profiles of X-ray sources and BSS are also consistent with the mass segregation model of binaries in GCs, which suggests that in addition to the dynamical formation channel, primordial binaries are also a significant contributor to the X-ray source population in GCs.