47,866 research outputs found
Cataclysmic Variables with Evolved Secondaries and the Progenitors of AM CVn Stars
We present the results of a systematic study of cataclysmic variables (CVs)
and related systems, combining detailed binary-population synthesis (BPS)
models with a grid of 120 binary evolution sequences calculated with a
Henyey-type stellar evolution code. In these sequences, we used 3 masses for
the white dwarf (0.6, 0.8, 1.0 Msun) and seven masses for the donor star in the
range of 0.6-1.4 Msun. The shortest orbital periods were chosen to have
initially unevolved secondaries, and the longest orbital period for each
secondary mass was taken to be just longer than the bifurcation period (16 - 22
hr), beyond which systems evolve towards long orbital periods. These
calculations show that systems which start with evolved secondaries near the
end or just after their main-sequence phase become ultra-compact systems with
periods as short as 7 min. These systems are excellent candidates for AM CVn
stars. Using a standard BPS code, we show how the properties of CVs at the
beginning of mass transfer depend on the efficiency for common-envelope (CE)
ejection and the efficiency of magnetic braking. In our standard model, where
CE ejection is efficient, some 10 per cent of all CVs have initially evolved
secondaries (with a central hydrogen abundance X_c < 0.4) and ultimately become
ultra-compact systems (implying a Galactic birthrate for AM CVn-like stars of
10^{-3} yr^{-1}). Almost all CVs with orbital periods longer than 5 hr are
found to have initially evolved or relatively massive secondaries. We show that
their distribution of effective temperatures is in good agreement with the
distribution of spectral types obtained by Beuermann et al. (1998).Comment: 16 pages, 6 figures (Fig. 4 in reduced format). Submitted to MNRA
Coauthor prediction for junior researchers
Research collaboration can bring in different perspectives and generate more productive results. However, finding an appropriate collaborator can be difficult due to the lacking of sufficient information. Link prediction is a related technique for collaborator discovery; but its focus has been mostly on the core authors who have relatively more publications. We argue that junior researchers actually need more help in finding collaborators. Thus, in this paper, we focus on coauthor prediction for junior researchers. Most of the previous works on coauthor prediction considered global network feature and local network feature separately, or tried to combine local network feature and content feature. But we found a significant improvement by simply combing local network feature and global network feature. We further developed a regularization based approach to incorporate multiple features simultaneously. Experimental results demonstrated that this approach outperformed the simple linear combination of multiple features. We further showed that content features, which were proved to be useful in link prediction, can be easily integrated into our regularization approach. © 2013 Springer-Verlag
Stellar adiabatic mass loss model and applications
Roche-lobe overflow and common envelope evolution are very important in
binary evolution, which is believed to be the main evolutionary channel to hot
subdwarf stars. The details of these processes are difficult to model, but
adiabatic expansion provides an excellent approximation to the structure of a
donor star undergoing dynamical time scale mass transfer. We can use this model
to study the responses of stars of various masses and evolutionary stages as
potential donor stars, with the urgent goal of obtaining more accurate
stability criteria for dynamical mass transfer in binary population synthesis
studies. As examples, we describe here several models with the initial masses
equal to 1 Msun and 10 Msun, and identify potential limitations to the use of
our results for giant-branch stars.Comment: 7 pages, 5 figures,Accepted for publication in AP&SS, Special issue
Hot Sub-dwarf Stars, in Han Z., Jeffery S., Podsiadlowski Ph. ed
The response of stratified swirling flames to acoustic forcing: Experiments and comparison to model
The gradient of local equivalence ratio in reacting mixtures significantly affects the flame structure and their corresponding response to acoustic velocity perturbations. We study the effect of acoustic velocity fluctuations on flames created by two co-annular, swirling streams with different equivalence ratios to simulate the effects of pilot-mains split. The flames are stabilized both by a bluff body and by swirl. The flame responses were measured via chemiluminescence as a function of frequency, in the linear perturbation range. A linearized version of the G-equation model is employed to describe the flame dynamics, combined with effects of axial and azimuthal velocity perturbations downstream of the swirlers. The model accounts for the phase shift between the main acoustic and swirler vortical perturbations, which propagate at different speeds. The very different flame structures generated by different fuel splits lead to different flame responses. Models based on time delay of vortical disturbances are able to capture the behaviour reasonably well for the case of outer fuel enrichment, but offer limited agreement for the case of the inner enriched flame, particularly under higher mean equivalence ratios.The authors acknowledge the support provided by the Cambridge Overseas Trust and China Scholarship Council. Additional funding was provided by Rolls-Royce plc for the initial set up of the experiments.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.proci.2014.05.04
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