You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
# 1 # 1    the existence of a neutron stars have been confirmed observationally , and the measured masses and radii of neutron stars have been used to partially constrain the physical properties of the dense nuclear matter .
so far , about 70 nss have been observed and their distribution in mass has been accumulated according to their binary types @xcite . in the x - ray / optical binaries ,
the mean and error - weighted mean of ns mass distribution are @xmath4 and @xmath5 , respectively , which implies that the measurement errors are relatively large .
the ns - white dwarf binaries have similar precision in the mass distribution to that of the x - ray / optical binaries ; the mean and error - weighted mean are @xmath6 and @xmath7 , respectively .
on the other hand , the ns - ns binaries show relatively stable statistics ; the mean and error - weighted mean are @xmath8 and @xmath9 , respectively . in all of these binaries , the mean masses do not exceed @xmath10 , and one may conclude that most of ns masses lie in the range of @xmath11 , which can be regarded as a canonical range of ns masses . note that nss with larger masses have relatively larger measurement errors except for a few exceptions for the large ns masses , which were measured with high accuracy , e.g. , psr j1614 - 2230 , @xmath12 @xcite and psr j0348 + 0432 , @xmath13 @xcite .    by solving the tolman - oppenheimer - volkoff ( tov ) equation
, one can see that the ns with large mass can have a high - density region in the core , providing a room for the existence of non - nucleonic degrees of freedom such as hyperons , bose - einstein condensation , free quark phase and etc .
these exotic states relieve the pressure exerted by the fermionic nature of nucleons , make the equation of state ( eos ) soft , and eventually reduce the mass of ns . among diverse possibilities ,
hyperons have been playing a key role in reducing the maximum mass of ns to the canonical range .
however , recent observations of nss with @xmath12 and @xmath13 initiate a challenge to the existence of hyperons at the core of ns ( so called  hyperon puzzle " ) .    in order to solve the hyperon puzzle ,
some studies based upon both relativistic and non - relativistic mean field ( rmf and nrmf , respectively ) models have been conducted so far .
the modern version of these models shows great accuracy in reproducing the properties of known nuclei .
however , the maximum mass of ns calculated with these models shows drastic fluctuations , ranging from @xmath14 to @xmath15 even without including any hyperon yet @xcite .
main uncertainty comes from the extrapolation of models to the high density where no experimental data available .
therefore , the measurement of the @xmath16 ns from psr j1614 - 2230 and psr j0348 + 0432 can rule out some of rmf or nrmf models that predict the maximum mass less than @xmath3 .
when the effect of hyperons is included in the rmf models , the coupling constants of hyperons are fitted to the depth of hyperon - nucleus optical potentials at the nuclear saturation density in most of the rmf models .
( note that the hyperon - nucleus optical potentials are obtained from the single - strangeness hypernuclei . ) with this method , the rmf models show the similar effect of hyperons on ns mass regardless of the differences among various rmf models ; the mass of ns is depending on the background nuclear models @xcite . in order to solve the hyperon puzzle , some recent works introduced repulsive forces caused by the vector - meson exchange in the rmf models @xcite . in ref .
@xcite , by assuming a hard core of hyperons , one of the authors ( chh ) obtained the maximum mass of ns close to @xmath3 .
however , the parameterization used in these approaches is not directly compatible with hypernuclear data and relies partly on the theoretical arguments .
dealing with the interactions between hyperons is in a worse situation because of statistically insufficient data from the double - hypernuclei experiments . in most cases ,
the coupling constants between hyperons are determined from the symmetry arguments .
the coupling constants obtained in this manner show very minor effects on the mass of ns @xcite , so in many works the interactions between hyperons are justified to be omitted for simplicity .
the purpose of this work is to explore the effect of the @xmath1 interactions on the eos and the resulting bulk properties of ns by employing a few skyrme - type models and a finite - range model that reproduce the empirical binding energies of the double-@xmath17 hypernuclei . for the nucleon and single-@xmath17 interactions and the nucleon - nucleon interactions
, we also use the skyrme - type models that are consistent with the experimental data .
we find that our results with these models turn out to be quite different from those of the rmf models , suggesting a possible solution to the hyperon puzzle .
we find that some of our models both predict the maximum mass of @xmath3 and are in good agreement with the observationally - constrained mass - radius relation @xcite .
we organize this paper as follows .
ii explains the skyrme - type models for the nucleon - nucleon ( @xmath18 ) interactions and the nucleon-@xmath17 ( @xmath0 ) interactions .
as mentioned above , we use both the skyrme - type models and a finite - range force model for the @xmath1 interactions . sect .
ii also contains some details of these models . sect .
iii is devoted to the results about the properties of ns matter , and the consequent mass - radius relation of ns .
we summarize our work and give some discussions in sect .
we employ the standard skyrme - type forces for the @xmath18 interactions given in the form @xmath19 \nonumber \\ & & + t_2(1+x_2 p_\sigma ) \bm{k}'_{ij } \cdot \delta(\bm{r}_{ij } ) \bm{k}_{ij } + \frac{1}{6 } t_3(1+x_3
p_\sigma ) \rho_n^\epsilon ( \bm{r } ) \delta(\bm{r}_{ij } ) \nonumber \\ & &   + i w_0 \bm{k}'_{ij } \cdot \delta(\bm{r}_{ij } ) ( \bm{\sigma}_i + \bm{\sigma}_j )   \times \bm{k}_{ij},\end{aligned}\ ] ] where @xmath20 , @xmath21 , @xmath22 , @xmath23 , the spin exchange operator @xmath24 , and the nucleon density @xmath25 .
we choose three models sly4 , ski4 , and sgi for the parameters of @xmath18 interactions . as shown in ref .
@xcite , these models are in reasonable agreement with the mass - radius constraints of the observed ns presented in ref .
the basic properties of the models are summarized in table [ tab : nnforce ] .
we note that the models also satisfy the maximum mass constraints given by psr j1614 - 2230 or psr j0348 + 0432 .
.nuclear matter properties and the maximum mass of ns calculated from the @xmath18 skyrme force models .
@xmath26 : saturation density in unit of @xmath27 , @xmath28 : binding energy of the symmetric nuclear matter in unit of mev , @xmath29 : symmetry energy at the saturation density in unit of mev , @xmath30 : slope of the symmetry energy at the saturation density in unit of mev , @xmath31 : compression modulus of the symmetric matter at the saturation density in unit of mev , @xmath32 : ratio of the effective mass of the nucleon at the saturation density ( @xmath33 ) to the free mass of the nucleon ( @xmath34 ) , and @xmath35 : maximum mass of ns in unit of the solar mass ( @xmath36 ) . [ cols="<,^,^,^,^,^,^,^",options="header " , ]     in order to see the effect of inconsistent combination of interaction models , we evaluate the difference of ns maximum mass with and without @xmath1 forces .
table  [ tab : diff ] summarizes the result where @xmath37 ( @xmath38 ) is defined as the difference of ns maximum mass between with and without s@xmath39 ( s@xmath40 ) interactions .
parentheses denote the ns maximum mass for given @xmath18 and @xmath41 interactions .
all the values in the table are in the unit of solar mass @xmath36 .
@xmath18 force models give significant variation for the ns maximum mass , and the selected @xmath0 forces show the reduction of the maximum mass in the range of @xmath42 .
inspite of this strong dependence on the @xmath18 and @xmath0 interactions , @xmath43 and @xmath44 are lesser sensitive to the @xmath1 force models .
we note that the s@xmath39 models are fitted to skm@xmath45 and ybz5 models for @xmath18 and @xmath0 interactions , respectively , so all the @xmath37 values are results of inconsistent model combinations .
nevertheless , the contribution of @xmath1 interaction is not much affected by the inconsistency .
right part of the table presents the result of @xmath38 . in ref .
@xcite , skm@xmath45 and ybz4 models are used to fit the @xmath1 interactions , so @xmath38 values in the column of skm@xmath45 are the results of consistent use of interaction models .
similar to @xmath37 , ns maximum mass shows dependence on the @xmath1 interaction at the order of @xmath46 .
the reason for this weak dependence on the @xmath1 interaction can be found from the energy density in eq .
( [ eq : endenll ] ) , and the fraction of @xmath17 hyperons in fig .
[ fig : frac ] .
first , since @xmath47 in the @xmath1 forces , the contribution of @xmath1 interactions to the eos is determined by @xmath48 , @xmath49 and the density of @xmath17 hyperon @xmath50 .
second , as can be seen in fig .
[ fig : frac ] , fraction of @xmath17 hyperons at the maximum mass density ( denoted by circles in the figure ) is similar to each other among different @xmath41 interactions . with similar @xmath17 hyperon fraction at the center of ns where tov equation is started to be integrated , the contribution of @xmath1 interaction is dominantly controlled by @xmath48 and @xmath49 values .
therefore , a @xmath1 interaction model gives similar contribution to the ns maximum mass with little dependence on the @xmath18 and @xmath0 interactions .
some errors are inevitable due to the inconsistent combination of @xmath18 , @xmath0 and @xmath1 interactions , but it is unlikely to affect or change the conclusion of this work significantly .
another limitation of the current work arises from the omission of @xmath51 and @xmath52 hyperons .
some recent works @xcite consider the role of these hyperons , and the results turn out to be sensitive to the hyperon - nucleon potentials .
due to the lack of sufficient experimental data , the contributions of @xmath51 and @xmath52 hyperons are more uncertain than the those of @xmath17 hyperon . though our investigation is not complete at this moment in the aspect of the self - consistent application of models and the full account of exotic degrees of freedom , we expect that these two effects are not large enough to alter the conclusion of this work . however , it is still worth continuing to investigate them in the future studies .
yl was supported by the rare isotope science project of institute for basic science funded by ministry of science , ict and future planning and national research foundation of korea ( 2013m7a1a1075766 ) .
chh is grateful to the institute for basic science , where part of the work was completed .
work of chh was supported by basic science research program through the national research foundation of korea ( nrf ) funded by the ministry of education ( nrf-2014r1a1a2054096 ) .
kk was supported by basic science research program through the national research foundation of korea ( nrf ) funded by the ministry of science , ict , and future planning ( nrf-2014m1a7a1a03029872 ) .
chl was supported by the national research foundation of korea ( nrf ) grant funded by the korea government ( msip ) ( no .
nrf-2015r1a2a2a01004238 ) .
we consider the so called hyperon puzzle of neutron star ( ns ) . 
 we employ skyrme force models for the description of in - medium nucleon - nucleon , nucleon - lambda hyperon ( @xmath0 ) , and lambda - lambda ( @xmath1 ) interactions . a phenomenological finite - range force for the @xmath1 interaction 
 is considered as well . 
 equation of state ( eos ) of ns matter is obtained in the framework of density functional theory , and tolman - oppenheimer - volkoff equations are solved to obtain the mass - radius relations of nss . 
 it has been generally known that the existence of hyperons in the ns matter is not well supported by the recent discovery of large - mass nss ( @xmath2 ) since hyperons make the eos softer than the one without them . for the selected interaction models , @xmath0 interactions 
 reduce the maximum mass of ns by about 30  % , while @xmath1 interactions can give about 10  % enhancement . 
 consequently , we find that some skyrme force models predict the maximum mass of ns consistent with the observation of @xmath3 nss , and at the same time satisfy observationally constrained mass - radius relations .