[Users] The generaion of initial data of BNS system with large mass ratio using Lorene

[Bruno Giacomazzo]

producing NS-NS binaries with mass ratios different from one is a bit
problematic sometimes. There is this trick though that can help (if it was
not yet implemented in the current LORENE version):

In the file C++/Source/Binaire/binaire_orbite.C, there is a routine
fonc_binaire_axe on lines 799-854 that uses a secant method to determine
the values for x_rot, the rotation axis of the binary, and omega, the
angular momentum.  Unfortunately, the secant method that tries to set the
guesses for omega for each star equal to each other, om_star1=om_star2
calculates

0= om_star1 - om_star2

but either or both of these can yield a division by zero as they are
calculated.  We've found it helps stabilize things by solving

0 = 1.0/om_star1 - 1.0/om_star2

which can be done by modifying line 852 accordingly.  This particularly
helps with divergent mass ratios.  Note that this is a different file than
C++/Source/Binary/binary_orbite.C, which I believe has its own problems.


Otherwise the FUKA branch of KADATH is very robust:
https://bitbucket.org/fukaws/

Please note that I never tried to compute binaries with a mass ratio of 7.

Cheers,
Bruno


Il giorno mer 1 mar 2023 alle ore 12:14 Konrad Topolski <
topolski at itp.uni-frankfurt.de> ha scritto:

> Hi Jimmy
>
> The lower mass that you are asking Lorene for is not realized for neutron
> stars (lower mass limit >~1.0M_sun)
> and would rather approach the mass for a brown dwarf. This lower mass and
> the upper theoretical limit places an upper constraint
> on the mass ratio to be around q~2. For initial data generation, this
> limit is of course equation of state - dependent.
>
> Consistent with this fact and the theoretical constructions of the
> equations of state,
> such a mass ratio is not achievable in the context of neutron stars.
>
> Out of the most extreme configurations I've seen,
> FUKA initial data solver (publicly available) is known to achieve q~2.2 and
> at least q~2.1 is achieved for SGRID code,  LORENE q~1.82, judging
> from the CoRe database.
>
> I do not have much experience with Lorene, but it might not be
> straightforward to achieve even the
> theoretically admissible vicinity of q~2.0 due to domain decomposition
> issues.
> Perhaps someone with more experience with it could clarify what's
> realizable with publicly available version of Lorene.
>
> Best regards
> Konrad Topolski
>
> W dniu: Środa, Marzec 01, 2023 08:52 CET, 白济民 <beki-cat at sjtu.edu.cn>
> napisał(a):
>
>
> Hi everyone:
> I intend to use Lorene to generate an unequal mass binary NS system with
> large q (with mass 1.4 Mo vs 0.2 Mo), that is, a very light NS orbiting
> around a standard mass NS. However, Lorene code seems to fail to generate
> .resu file at such large q and I wonder how can that be possible within
> Lorene. Is it still under development or can it be done by varying
> parameter settings in the corresponding parcoal file in
> Lorene/Codes/Bin_star directory?
>
> I also wonder if there is a systematic procedure to adjust the parameter
> settings of parcoal to generate the initial data of a BNS system with small
> q?
> Thanks in advance!
> Best,
> Jimmy
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-- 

Prof. Bruno Giacomazzo
Department of Physics
University of Milano-Bicocca
Piazza della Scienza 3
20126 Milano
Italy

email: bruno.giacomazzo at unimib.it
phone: (+39) 02 6448 2321
web: http://www.brunogiacomazzo.org

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