# Can close binaries have a very eccentric orbit?

For example, are there binaries with an orbital period < 5 day and that are very eccentric?

What kind of factors could slow down orbital circularisation?

Are there examples of binaries with p<5day and e>0.1?

• Do you have any examples of eccentric binaries with orbital periods less than 5 days? I'm not sure I understand the question. Eccentric binaries generally have longer periods. – ProfRob Apr 26 '15 at 18:05
• @rob I have revise the question. Do you have any idea now? – questionhang Apr 27 '15 at 1:13

There are surprisingly many examples of short period, eccentric binry systems.

A very incomplete list would include

KIC4544587 P=2.19 d e=0.288 +/- 0.026 Hambleton et al. (2013)

HD174884 P=3.66 d e=0.2939 +/- 0.0005 Maceroni et al. (2010)

CoRoT 102918586 P=4.39 d e=0.249 +/- 0.005 Maceroni et al. (2013)

HD313926 P=2.27d e=0.209 +/-0.001 Rucinski et al. (2007)

In each of these cases, the stars are relatively warm, with thin (if any) subphotospheric convection zones. Tidal circularisation is much more effective in stars with thick convection zones (giants, or cooler main sequence stars). Therefore it is possible that all of these objects (and, as I say, there are many other examples) are too young to have circularised.

Some theory is presented by Zahn et al. (2005). Indeed, circularisation times are very short - basically you expect everything with a solar mass or below to be circularised if its period is less than 6 days, but the process is less efficient for higher mass stars. Circularisation will essentilly take place during pre-main-sequence evolution. Since a circular orbit represents the minimum energy configuration, the only way you are going to get non-circular orbits in solar-type and lower mass stars is if they are very young (less than some tens of millions of years), the binary system formed after their pre-main-sequence evolution or if there is an injection of energy via a dynamical interaction or an eccentric third body in a much wider orbit.

• you are very professional. How can you find them so quickly? There are two binaries which are not so young, adsabs.harvard.edu/abs/2014ApJ...788..134K and adsabs.harvard.edu/abs/2015IBVS.6138....1C, especially the 2rd one. Kozai mechanism is suppressed for very short period binaries. Could you please say more about "an injection of energy via a dynamical interaction"? – questionhang May 3 '15 at 11:15
• The first paper is behind a paywall and I'm not at work. The second is an example of a triple system. All I mean by injection of energy is that if a third body interacts with a binary it can result in an increase in eccentricity. This is an endothermic reaction, the energy comes from the third body. – ProfRob May 3 '15 at 12:02
• I have sent the right pdf to you. – questionhang May 3 '15 at 12:27