Smoothed Particle Hydrodynamics simulations of white dwarf ... · white dwarf. Direct collisions...

MotivationPhysical scenario

Numerical modellingResults

Smoothed Particle Hydrodynamicssimulations of white dwarf collisions and

close encounters

P. Lorén-Aguilar, J. Isern & E. García–Berro

September 29, 2009

P. Lorén-Aguilar, J. Isern & E. García–Berro SPH simulations of white dwarf collisions and close encounters



Outline

1 Motivation

2 Physical scenario

3 Numerical modelling

4 ResultsOutcomesStructure of the remnantsObservational signatures




Motivation (1)

The study of stellar collisions in dense stellar systems hasattracted much interest in recent years.

One of the reasons for this is the relatively high frequencyof such events expected in these environments (Hills & Day1976).

It has been predicted that up to 10% of the stars in the coreof typical globular clusters have undergone a collision atsome point during the lifetime of the cluster.




Motivation (2)

The collisions of two white dwarfs deserves study for vari-ous reasons.

First of all, the collision of two white dwarfs might produce aType Ia supernova. It has been predicted that the whitedwarf merger rate leading to super Chandrasekhar rem-nants will be increased by an order of magnitude throughdynamical interactions (Shara & Hurley 2002).

Consequently, collisions of two white dwarfs could explainsupernovae occuring in the nuclei of galaxies.




Motivation (3)

It has also been recently suggested that such a processcould lead to the formation of a magnetar (King, Pringle &Wickramasinghe 2001). This could explain the main char-acteristics of some soft gamma-ray repeaters and anoma-lous X-ray pulsars like 1E2259+586.

Also, dynamical interactions in globular clusters can formeccentric double white dwarfs which could be powerful sourcesof gravitational radiation (Willems et al. 2007).

It has also to be noted that due to the high temperaturesachieved during these collisions, we expect that some nu-clearly processed material would be ejected, leading to apollution of the environment.




Outline

1 Motivation

2 Physical scenario






Physical scenario (1)

A close encounter between two stars might produce severaldifferent outcomes.

If stars get close enough, kinetic energy might be dissipatedin several ways and an eccentric binary system might beformed.

Typical dispersion velocities in globular clusters are vd ≈ 10km/s, while typical relative velocities in close encounters arevc ≈ (2G(M1 + M2)/dc)

1/2 ∼ 100 km/s




Physical scenario (2)

Thus, only ∼ (vd/vc)2 . 0.01 of the kinetic energy avail-

able at closest approach needs to be dissipated in order tobound the system (Fabian et al. 1975).

This fraction is small enough to expect a relatively high for-mation rate of this type of systems (Lee & Ostriker 1986).

If enough kinetic energy is dissipated from the system, masstransfer might begin and an a stellar merger might occur.




Outline

1 Motivation

2 Physical scenario






Numerical modelling (1)

We have simulated a set of close encounters between whitedwarfs. As a first step towards a complete understandingof this processes we have decided to focus on the post-capture configuration.

Our objective is to compute the set of physical parametersleading to the different possible outcomes — that is, eccen-tric binary systems and stellar mergers.

To do so we use an SPH code (Guerrero et al. 2004, Lorén–Aguilar et al. 2005), which is very well suited for treatingfully three-dimensional hydrodynamical problems.




Numerical modelling (2)

We have performed 10 simulations of the close encountersof a 0.6 and a 0.8 M� CO white dwarfs.

Our free parameters have been the initial relative velocitiesof the stars, ranging from 50 to 200 km/s, and the impactparameter b, ranging from 0.3 to 0.9 R�.




OutcomesStructure of the remnantsObservational signatures

Outline

1 Motivation

2 Physical scenario







Outcomes (1)

We have found three different types of outcomes: eccentricbinary systems, lateral collisions, and direct collisions.Summary of the main properties:

Run b v Outcome E L rmax rmin ε

(R�) (km/s) (1049 erg) (1051 erg s) (0.1 R�) (0.1 R�)1 0.8 100 O −0.21 0.78 8.26 0.52 0.8812 0.5 150 O −0.25 0.74 6.85 0.50 0.8643 0.3 200 LC −0.12 0.58 3.74 0.30 0.8524 0.3 175 LC −0.46 0.50 3.70 0.22 0.8865 0.3 150 LC −0.40 0.37 4.36 0.12 0.9476 0.3 100 DC −0.50 0.29 3.63 0.07 0.9627 0.5 50 DC −0.27 0.25 6.72 0.05 0.9848 0.1 200 DC −0.78 0.19 2.36 0.03 0.9749 0.1 150 DC −0.80 0.16 2.30 0.02 0.985

10 0.1 120 DC −0.21 0.06 2.28 0.01 0.989





Outcomes (2)

Some examples of the different types of outcomesMovies will be available at the conference program page





Outcomes (3)

Using the results of the simu-lations the different outcomescan be studied. We have foundthat rmin ' 0.030 and 0.012 R�separate the regimes.

In the classical two-body prob-lem the relation between thedistance at periastron and theorbital energy and angular mo-mentum is given by rmin =

k2|E |

(1−

√1 + 2EL2

µk

).





Structure of the remnants (1)

The structure of the remnant isnot very different from the onewe found in classical simula-tions of the merger of a binarywhite dwarf.

Direct collisions show a spher-ical mass distribution aroundthe primary star in comparisonwith the disk-like structure ob-tained in lateral collisions.





Structure of the remnants (2)

The temperatures are not highenough to ignite representativecarbon reactions.

We do not obtain a hot enve-lope surrounding the primarystar. Different long-term evolu-tion of the remnant when com-pared with a classical merger?





Observational signatures (1)

We have computed the gravita-tional wave emission of thesesystems.

Emission corresponding toeach one of the previouslystudied outcomes.






It has been argued that theformation of eccentric systemsmight be a powerful source ofgravitational radiation.

The frequency and amplitudeof this systems lies out ofthe range of LISA’s sensitivitycurve.

Orbital circularization will movethe emission of these systemsabove the confussion noise,giving us a chance to detectthem.






In the case of mergers, a pos-sible observational signaturecould be its electromagneticemission

We followed the model ofRosswog (2007) in order tostudy which would be the pos-sible signature of a recentmerger.





Conclusions

We have performed a set of simulations of close encountersof white dwarfs in dense stellar systems.

We have determined the regions in the E − L plane thatdetermine the type of outcome of the close encounter.

We have shown that the detection of gravitational wavesarising from such eccentric systems does not seem to befeasible with LISA. Possibly, after orbital circularization thegravitational wave emission might be detected.





Smoothed Particle Hydrodynamicssimulations of white dwarf collisions and

close encounters

P. Lorén-Aguilar, J. Isern & E. García–Berro

September 29, 2009


Smoothed Particle Hydrodynamics simulations of white dwarf ... · white dwarf. Direct collisions...

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