Neutron Star Mass Research Articles

The nuclear symmetry energy and the neutron skin thickness in nuclei

IntroductionWe investigate possible correlations between the stiffness of the symmetry energy at saturation density, the so-called L parameter, and the neutron skin thickness of 48Ca and 208Pb, for which the recent measurements from the CREX and PREX I + II experiments at the Thomas Jefferson Laboratory became available.MethodsWe choose an ensemble of nucleonic equations of state (EoS) derived within microscopic (BHF, Variational, AFDMC) and phenomenological (Skyrme, RMF, DD-RMF) approaches. They are all compatible with the laboratory nuclear collisions data and with current observations of neutron stars (NS) mass and the tidal polarizability of a 1.4 M⊙ NS, as deduced from the GW170817 event.ResultsWe find some degree of correlation between the L parameter and the neutron skin thickness whereas a much weaker correlation does exist with the tidal polarizability and the symmetry energy at saturation density. However, some EoS which are able to explain the CREX experimental data, are not compatible with the PREX I + II data, and viceversa.ConclusionWe confirm the results previously obtained with a different set of EoS models, and find a possible tension between the experimental data and the current understanding of the nuclear EoS.

Constraining neutron star properties and dark matter admixture with the NITR-I equation of state: Insights from observations and universal relations

A recent observational study has set a constraint on the maximum mass of neutron stars (NSs), specifically focusing on PSR J0952-0607 and the compact star remnant HESS J1731-347, particularly within the low-mass regime. In our recent study, Ref. 1 , we developed an energy density functional named NITR, which successfully produced the mass limit of the aforementioned pulsar but did not fully meet other observational constraints, such as those from NICER+XMM and GW170817. In this study, we introduce a new EDF named “NITR-I”, which not only reproduces the mass limit of PSR J0952-0607 but also aligns its canonical radius with NICER+XMM data, and its canonical dimensionless tidal deformability is consistent with the GW170817 event, thereby demonstrating the robustness of our model. The low-mass constraint associated with HESS J1731-347 suggests various possible compositions for the NS. The NITR-I model alone does not satisfy the HESS J1731-347 constraint; thus, we explore the possibility of incorporating dark matter (DM) inside the NS to meet this constraint. This approach proves successful when a specific value of Fermi momentum is considered. We also examine the impact of DM with varying Fermi momentum on different NS properties, such as tidal deformability and nonradial f-mode oscillation, using various relativistic mean-field (RMF) models. For the NITR-I EOS, the f-mode frequency is about 2.15[Formula: see text]kHz at 1.4 [Formula: see text] when [Formula: see text][Formula: see text]GeV, and it slightly increases to around 2.32[Formula: see text]kHz with [Formula: see text][Formula: see text]GeV. This increase in frequency due to DM suggests a possible reduction in tidal deformability, indicating that neutron stars with higher DM content are less susceptible to deformation by tidal forces which could be detectable in gravitational wave signals from neutron star mergers. Finally, we explore various universal relations (URs) for DM-admixed NSs, such as the relation between compactness and tidal deformability, the f-mode frequency and tidal deformability, and estimate the canonical values corresponding to both compactness and f-mode frequency using the GW170817 data.

Origin of the black hole spin in lower-mass-gap black hole-neutron star binaries

During the fourth observing run, the LIGO-Virgo-KAGRA Collaboration reported the detection of a coalescing compact binary (GW230529$_ $181500) with component masses estimated at $2.5-4.5\, M_ and $1.2-2.0\, M_ with 90<!PCT!> credibility. Given the current constraints on the maximum neutron star (NS) mass, this event is most likely a lower-mass-gap (LMG) black hole-neutron star (BHNS) binary. The spin magnitude of the BH, especially when aligned with the orbital angular momentum, is critical in determining whether the NS is tidally disrupted. An LMG BHNS merger with a rapidly spinning BH is an ideal candidate for producing electromagnetic counterparts. However, no such signals have been detected. In this study, we employ a detailed binary evolution model that incorporates new dynamical tide implementations to explore the origin of BH spin in an LMG BHNS binary. If the NS forms first, the BH progenitor (He-rich star) must begin in orbit shorter than 0.35 days to spin up efficiently, potentially achieving a spin magnitude of $ BH > 0.3$. Alternatively, if a nonspinning BH (e.g., $M_ BH = 3.6\, M_ forms first, it can accrete up to $ 0.2\, M_ via case BA mass transfer (MT), reaching a spin magnitude of $ BH 0.18$ under Eddington-limited accretion. With a higher Eddington accretion limit (i.e., 10.0 $ M Edd $), the BH can attain a significantly higher spin magnitude of $ BH by accreting approximately $1.0\, M_ during case BA MT phase.

The impact of astrophysical priors on parameter inference for GW230529

ABSTRACT We investigate the effects of prior selection on the inferred mass and spin parameters of the neutron star–black hole merger GW230529_181500. Specifically, we explore models motivated by astrophysical considerations, including massive binary and pulsar evolution. We examine mass and spin distributions of neutron stars constrained by radio pulsar observations, alongside black hole spin observations from previous gravitational-wave detections. We show that the inferred mass distribution highly depends upon the spin prior. Specifically, under the most restrictive, binary stellar evolution models, we obtain narrower distributions of masses with a black hole mass of $4.3^{+0.1}_{-0.1}\ {\rm M}_{\odot }$ and neutron star mass of $1.3^{+0.03}_{-0.03}\ {\rm M}_{\odot }$ where, somewhat surprisingly, it is the prior on component spins that has the greatest impact on the inferred mass distributions. Re-weighting using neutron star mass and spin priors from observations of radio pulsars, with black hole spins from observations of gravitational waves, yields the black hole and the neutron star masses to be $3.8^{+0.5}_{-0.6}$ and $1.4^{+0.2}_{-0.1} \ \mathrm{ M}_\odot$, respectively. The sequence of compact object formation – whether the neutron star or the black hole formed first – cannot be determined at the observed signal-to-noise ratio. However, there is no evidence that the black hole was tidally spun up.

A NICER View of PSR J1231−1411: A Complex Case

Recent constraints on neutron star mass and radius have advanced our understanding of the equation of state (EOS) of cold dense matter. Some of them have been obtained by modeling the pulses of three millisecond X-ray pulsars observed by the Neutron Star Interior Composition Explorer (NICER). Here, we present a Bayesian parameter inference for a fourth pulsar, PSR J1231−1411, using the same technique with NICER and XMM-Newton data. When applying a broad mass-inclination prior from radio timing measurements and the emission region geometry model that can best explain the data, we find likely converged results only when using a limited radius prior. If limiting the radius to be consistent with the previous observational constraints and EOS analyses, we infer the radius to be 12.6 ± 0.3 km and the mass to be 1.04−0.03+0.05 M ⊙, each reported as the posterior credible interval bounded by the 16% and 84% quantiles. If using an uninformative prior but limited between 10 and 14 km, we find otherwise similar results, but Req=13.5−0.5+0.3 km for the radius. In both cases, we find a nonantipodal hot region geometry where one emitting spot is at the equator or slightly above, surrounded by a large colder region, and where a noncircular hot region lies close to southern rotational pole. If using a wider radius prior, we only find solutions that fit the data significantly worse. We discuss the challenges in finding the better fitting solutions, possibly related to the weak interpulse feature in the pulse profile.

Occurrence of Gravitational Collapse in the Accreting Neutron Stars of Binary-driven Hypernovae

The binary-driven hypernova (BdHN) model proposes long gamma-ray bursts (GRBs) originate in binaries composed of a carbon–oxygen (CO) star and a neutron star (NS) companion. The CO core collapse generates a newborn NS and a supernova that triggers the GRB by accreting onto the NSs, rapidly transferring mass and angular momentum to them. This article aims to determine the conditions under which a black hole (BH) forms from NS collapse induced by the accretion and the impact on the GRB’s observational properties and taxonomy. We perform three-dimensional, smoothed particle hydrodynamics simulations of BdHNe using up-to-date NS nuclear equations of state, with and without hyperons, and calculate the structure evolution in full general relativity. We assess the binary parameters leading either NS in the binary to the critical mass for gravitational collapse into a BH and its occurrence time, t col. We include a nonzero angular momentum of the NSs and find that t col ranges from a few tens of seconds to hours for decreasing NS initial angular momentum values. BdHNe I are the most compact (about 5 minute orbital period), promptly form a BH, and release ≳1052 erg of energy. They form NS–BH binaries with tens of kiloyears merger timescales by gravitational-wave emission. BdHNe II and III do not form BHs, and release ∼1050–1052 erg and ≲1050 erg of energy, respectively. They form NS–NS binaries with a range of merger timescales larger than for NS–BH binaries. In some compact BdHNe II, either NS can become supramassive, i.e., above the critical mass of a nonrotating NS. Magnetic braking by a 1013 G field can delay BH formation, leading to BH–BH or NS–BH with tens of kiloyears merger timescales.

Stable case BB/BC mass transfer to form GW190425-like massive binary neutron star mergers

Context. On April 25, 2019, the LIGO-Virgo Collaboration discovered a gravitational-wave (GW) signal from a binary neutron star (BNS) merger, that is, GW190425. Due to the inferred large total mass, the origin of GW190425 remains unclear. Aims. Assuming GW190425 originated from the standard isolated binary evolution channel, its immediate progenitor is considered to be a close binary system, consisting of a He-rich star and a NS just after the common envelope phase. We aim to study the formation of GW190425 in a solar-like environment by using the detailed binary evolution code MESA. Methods. We perform detailed stellar structure and binary evolution calculations that take into account mass loss, internal differential rotation, and tidal interactions between a He-rich star and a NS companion. We explore the parameter space of the initial binary properties, including initial NS and He-rich masses and initial orbital period. Results. We find that the immediate post-common-envelope progenitor system, consisting of a primary ∼2.0 M⊙ (∼1.7 M⊙) NS and a secondary He-rich star with an initial mass of ∼3.0 − 5.5 M⊙ (∼5.5 − 6.0 M⊙) in a close binary with an initial period of ∼0.08 − 0.5 days (∼0.08 − 0.4 days), that experiences stable Case BB/BC mass transfer (MT) during binary evolution, can reproduce the formation of GW190425-like BNS events. Our studies reveal that the secondary He-rich star of the GW190425’s progenitor before its core collapse can be efficiently spun up through tidal interaction, finally remaining as a NS with rotational energy even reaching ∼1052 erg, which is always much higher than the neutrino-driven energy of the supernova (SN) explosion. If the newborn secondary NS is a magnetar, we expect that GW190425 can be the remnant of a magnetar-driven SN, namely a magnetar-driven ultra-stripped SN, a superluminous SN, or a broad-line Type Ic SN. Conclusions. Our results show that GW190425 could be formed through the isolated binary evolution, which involves a stable Case BB/BC MT just after the common envelope phase. On top of that, we show the He-rich star can be tidally spun up, potentially forming a spinning magnetized NS (magnetar) during the second SN explosion.

Interpreting Mass and Radius Measurements of Neutron Stars with Dark Matter Halos

The high densities of neutron stars (NSs) could provide astrophysical locations for dark matter (DM) to accumulate. Depending on the DM model, these DM admixed NSs (DANSs) could have significantly different properties than pure baryonic NSs, accessible through X-ray observations of rotation-powered pulsars. We adopt the two-fluid formalism in general relativity to numerically simulate stable configurations of DANSs, assuming a fermionic equation of state (EOS) for the DM with repulsive self-interaction. The distribution of DM in the DANS as a halo affects the path of X-rays emitted from hot spots on the visible baryonic surface, causing notable changes in the pulse profile observed by telescopes such as NICER, compared to pure baryonic NSs. We explore how various DM models affect the DM mass distribution, leading to different types of dark halos. We quantify the deviation in observed X-ray flux from stars with each of these halos. We identify the pitfalls in interpreting mass and radius measurements of NSs inferred from electromagnetic radiation and constraining the baryonic matter EOS if these dark halos exist.

Inferring system parameters from the bursts of the accretion-powered pulsar IGR J17498–2921

ABSTRACT Thermonuclear (type-I) bursts exhibit properties that depend both on the local surface conditions of the neutron stars on which they ignite, as well as the physical parameters of the host binary system. However, constraining the system parameters requires a comprehensive method to compare the observed bursts to simulations. We have further developed the beansp code for this purpose and analysed the bursts observed from IGR J17498$-$2921, a 401-Hz accretion-powered pulsar, discovered during it’s 2011 outburst. We find good agreement with a model having H-deficient fuel with $X=0.15\pm 0.4$, and CNO metallicity $Z=0.0014^{+0.0004}_{-0.0003}$, about a tenth of the solar value. The model has the system at a distance of $5.7^{+0.6}_{-0.5}$ kpc, with a massive (${\approx} 2\ \mathrm{ M}_\odot$) neutron star and a likely inclination of $60^\circ$. We also re-analysed the data from the 2002 outburst of the accretion-powered millisecond pulsar SAX J1808.4$-$3658. For that system we find a substantially closer distance than previously inferred, at $2.7\pm 0.3$ kpc, likely driven by a larger degree of burst emission anisotropy. The other system parameters are largely consistent with the previous analysis. We briefly discuss the implications for the evolution of these two systems.

Differential rotation in neutron stars at finite temperatures

IntroductionThis paper investigates the impact of differential rotation on the bulk properties and onset of rotational instabilities in neutron stars at finite temperatures up to 50 MeV.MethodsUtilizing the relativistic Brueckner-Hartree-Fock (RBHF) formalism in full Dirac space, the study constructs equation of state (EOS) models for hot neutron star matter, including conditions relevant for high temperatures. These finite-temperature EOS models are applied to compute the bulk properties of differentially rotating neutron stars with varying structural deformations.ResultsThe findings demonstrate that the stability of these stars against bar-mode deformation, a key rotational instability, is only weakly dependent on temperature. Differential rotation significantly affects the maximum mass and radius of neutron stars, and the threshold for the onset of bar-mode instability shows minimal sensitivity to temperature changes within the examined range.DiscussionThese findings are crucial for interpreting observational data from neutron star mergers and other high-energy astrophysical events. The research underscores the necessity of incorporating differential rotation and finite temperature effects in neutron star models to predict their properties and stability accurately.

Mass and spin measurements of black hole and neutron stars X-ray binaries through axisymmetric oscillation modes of thick torus

ABSTRACT This paper examines the oscillatory behaviour of relativistic, non-self-gravitating, charged-fluid toroidal structures within the context of the Kerr metric. The primary objective is to explore how thick accretion discs influence the mass and spin measurements of black holes and neutron stars in low-mass X-ray binaries (LMXBs) through quasi-periodic oscillations (QPOs) models. To achieve this, we conduct a local analysis within a general relativistic framework, determining the radial epicyclic and orbital frequencies in a perfect fluid disc. The tori are modelled using a non-Keplerian distribution of specific angular momentum, and we analyse how the oscillation properties depend on the model’s angular momentum distribution parameters. Subsequently, we connect these oscillatory frequencies to high-frequency QPOs observed in LMXBs, enabling us to calculate the optimal mass and spin values for each studied source.

Probing kaon meson condensations through gravitational waves during neutron star inspiral phases

Gravitational waves from binary neutron star merger events have shown unparalleled advantages in deciphering the stellar internal structure, yet it remains underexplored to seek clues about kaon meson condensations inside massive neutron stars through these events. To address this gap, this study examines the binary star inspiral processes with kaon meson condensations, employing various kaon meson potential wells to probe unique imprints in the inspiral gravitational waves. Our analysis reveals that the inspiral process of kaon meson condensation binary stars exhibits lower gravitational wave frequencies and shorter retarded times compared to traditional neutron stars. Furthermore, a deeper kaon meson potential well in these systems further shortens the inspiral duration time, resulting in distinctive gravitational waveforms and notable orbital phase deviations. These findings underscore the pivotal role of kaon meson condensations in modulating the inspiral gravitational waves, suggesting the inspiral gravitational waves could serve as a powerful tool for probing potential signals of kaon mesons within neutron stars.

Role of local anisotropy in hybrid stars

Using the Bower–Liang model, we discuss how pressure anisotropies affect the microscopic and macroscopic properties of hybrid stars. We find that anisotropies affect the maximum mass, central density, and radius of the canonical stars. Anisotropies also affect the minimum neutron star mass that presents quarks in their core, as well as the total amount of quarks for the maximally massive stars. We also confront our results with standard constraints, such as the radius and the tidal parameter of the canonical star, as well as the mass and radius of the PSR J0740+6620 pulsar. We observe that moderate values for anisotropies could fulfill these constraints simultaneously. On the other hand, within more extreme degrees of anisotropies, more speculative constraints such as black widow pulsars PSR J0952-0607 and the mass-gap object in the GW190814 event can be explained as hybrid stars. We also investigate the role of anisotropies in the neutron stars’ moment of inertia.

Long-term evolution of spin and other properties of neutron star low-mass X-ray binaries: implications for millisecond X-ray pulsars

ABSTRACT A neutron star (NS) accreting matter from a companion star in a low-mass X-ray binary (LMXB) system can spin up to become a millisecond pulsar (MSP). Properties of many such MSP systems are known, which is excellent for probing fundamental aspects of NS physics when modelled using the theoretical computation of NS LMXB evolution. Here, we systematically compute the long-term evolution of NS, binary, and companion parameters for NS LMXBs using the stellar evolution code mesa. We consider the baryonic to gravitational mass conversion to calculate the NS mass evolution and show its cruciality for the realistic computation of some parameters. With computations using many combinations of parameter values, we find the general nature of the complex NS spin frequency ($\nu$) evolution, which depends on various parameters, including accretion rate, fractional mass-loss from the system, and companion star magnetic braking. Further, we utilize our results to precisely match some main observed parameters, such as $\nu$, orbital period ($P_{\rm orb}$), etc., of four accreting millisecond X-ray pulsars (AMXPs). By providing the $\nu$, $P_{\rm orb}$, and the companion mass spaces for NS LMXB evolution, we indicate the distribution and plausible evolution of a few other AMXPs. We also discuss the current challenges in explaining the parameters of AMXP sources with brown dwarf companions and indicate the importance of modelling the transient accretion in LMXBs as a possible solution.

The Intrabinary Shock and Companion Star of Redback Pulsar J2215+5135

PSR J2215+5135 (J2215) is a “redback” spider pulsar, where the intrabinary shock (IBS) wraps around the pulsar rather than the stellar-mass companion. Spider orbital light curves are modulated, dominated by their binary companion thermal emission in the optical bands and by IBS synchrotron emission in the X-rays. We report on new XMM-Newton X-ray and U-band observations of J2215. We produce orbital light curves and use them to model the system properties. Our best-fit optical light model gives a neutron star mass M NS = 1.98 ± 0.08 M ⊙, lower than previously reported. However, uncertainty in the stellar atmosphere metallicity, a parameter to which J2215 is unusually sensitive, requires us to consider an acceptable systematic plus statistical range of M NS ∼ 1.85–2.3 M ⊙. From the X-ray analysis, we find that the IBS wraps around the pulsar but with a pulsar-wind-to-companion-wind-momentum ratio unusually close to unity, implying a flatter IBS geometry than seen in other spiders. Estimating the companion wind momentum and speed from the X-ray light curve, we find a companion mass-loss rate of Ṁc≳10−10M⊙ yr−1 so that J2215 may become an isolated millisecond pulsar in ∼1 Gyr. Our X-ray analyses place constraints on the magnetization and particle density of the pulsar wind and support models of magnetic reconnection and particle acceleration in the highly magnetized relativistic IBS.

Effect of dark matter interaction on hybrid star in the light of the recent astrophysical observations

We have explored the effect of dark matter interaction on hybrid star (HS) in the light of recent astrophysical observational constraints. The presence of dark matter is assumed to be there in both the hadron as well as the quark sector. The dark matter particle interacts with both hadron and quark matter through the exchange of a scalar as well as a vector meson. The equation of state (EOS) of the hadron part is computed using the NL3 version of the relativistic mean field(RMF) model, whereas the quark part is taken care of using the well-known MIT Bag model with the vector interaction. We investigate the effect of the dark matter density and the mass of the dark matter particle on various observables like mass, radius, tidal deformability of the dark matter admixed hybrid star(DMAHS).In this study, we have noted an intriguing aspect that is the speed of sound in the DMAHS is insensitive to both the mass as well as the density of dark matter. We also observe a striking similarity in the variation of transition mass and its corresponding radius, as well as the maximum mass of neutron stars, with dark matter density and mass. We employ observational constraints from neutron stars to narrow down the allowed range of the parameters of dark matter.

Two black widow pulsars in the optical and X-rays

Context. Two millisecond pulsars, PSR J1513−2550 and PSR J2017−1614, with spin periods of about 2.1 and 2.3 ms were recently discovered in the radio and γ-rays and classified as black widow pulsars in tight binary stellar systems with orbital periods of about 4.3 and 2.3 h. Aims. Our goals are to reveal the fundamental parameters of both systems and their binary components using multi-wavelength observations. Methods. We carried out the first time-series multi-band optical photometry of the objects with the 2.1-m telescope of the Observatorio Astronómico Nacional San Pedro Mártir, the 6.5-m Magellan-1 telescope, and the 10.4-m Gran Telescopio Canarias. To derive the parameters of both systems, we fitted the obtained light curves with a model assuming heating of the companion by the pulsar. We also analysed archival X-ray data obtained with the XMM-Newton observatory. Results. For the first time, we firmly identified J1513−2550 in the optical and both pulsars in X-rays. The optical light curves of both systems have a single peak per orbital period with a peak-to-peak amplitude of ≳2 magnitudes. The J2017−1614 light curves are symmetric, while J1513−2550 demonstrates strong asymmetry whose nature remains unclear. Conclusions. We constrained the orbital inclinations, pulsar masses, companion temperatures and masses, and the distances to both systems. We also conclude that J2017−1614 may contain a massive neutron star of 2.4 ± 0.6 M⊙. The X-ray spectra of both sources can be fitted by power laws with parameters typical of black widow systems.

Upper Limit of Sound Speed in Nuclear Matter: A Harmonious Interplay of Transport Calculation and Perturbative Quantum Chromodynamic Constraint

Very recently, it has been shown that there is an upper bound on the squared sound speed of nuclear matter from the transport, which reads cs2≤0.781 . In this work, we demonstrate that this upper bound is corroborated by the reconstructed equation of state (EOS; modeled with a nonparametric method) for ultradense matter. The reconstruction integrates multimessenger observation for neutron stars, in particular, the latest radius measurements for PSR J0437–4715 ( 11.36−0.63+0.95 km), PSR J0030+0451 ( 11.71−0.83+0.88 km, in the ST+PDT model), and PSR J0740+6620 ( 12.49−0.88+1.28 km) by NICER have been adopted. The result shows in all cases, the cs2≤0.781 upper limit for EOS will naturally yield the properties of matter near the center of the massive neutron star consistent with the causality-driven constraint from pQCD, where, in practice, the density in implementing the pQCD likelihood (n L) is applied at 10ns (where ns is the nuclear saturation density). We also note that there is a strong correlation for the maximum c s 2 with n L, and cs2≤0.781 is somehow violated when n L = n c,TOV. The result indicates that a higher n L, even considering the uncertainties from statistics, is more natural. Moreover, the remarkable agreement between the outcomes derived from these two distinct and independent constraints (i.e., the transport calculation and pQCD boundary) lends strong support to their validity. In addition, the latest joint constraint for R 1.4, R 2.0, R 1.4 − R 2.0, and M TOV are 11.94−0.68+0.77 km, 11.99−0.67+0.88 km, −0.1−0.27+0.42 km, and 2.24−0.10+0.13M⊙ (at 90% credible level), respectively.

The Orbit and Companion of PSR J1622-0315: Variable Asymmetry and a Massive Neutron Star

The companion to PSR J1622-0315, one of the most compact known redback millisecond pulsars, shows extremely low irradiation despite its short orbital period. We model this system to determine the binary parameters, combining optical observations from the New Technology Telescope in 2017 and the Nordic Optical Telescope in 2022 with the binary modeling code ICARUS. We find a best-fit neutron star mass of 2.3 ± 0.4 M ⊙, and a companion mass of 0.15 ± 0.02 M ⊙. We detect for the first time low-level irradiation from asymmetry in the minima as well as a change in the asymmetry of the maxima of its light curves over five years. Using starspot models, we find better fits than those from symmetric direct heating models, with consistent orbital parameters. We discuss an alternative scenario where the changing asymmetry is produced by a variable intrabinary shock. In summary, we find that PSR J1622-0315 combines low irradiation with variable light-curve asymmetry and a relatively high neutron star mass.

Black hole-neutron star mergers with massive neutron stars in numerical relativity

We study the merger of black hole-neutron star (BH-NS) binaries in numerical relativity, focusing on the properties of the remnant disk and the ejecta, varying the mass of compactness of the NS and the mass and spin of the BH. We find that within the precision of our numerical simulations, the remnant disk mass and ejecta mass normalized by the NS baryon mass (M^rem and M^eje, respectively), and the cutoff frequency fcut normalized by the initial total gravitational mass of the system at infinite separation approximately agree among the models with the same NS compactness CNS=MNS/RNS, mass ratio Q=MBH/MNS, and dimensionless BH spin χBH irrespective of the NS mass MNS in the range of 1.092−1.691M⊙. This result shows that the merger outcome depends sensitively on Q, χBH, and CNS but only weakly on MNS. This justifies the approach of studying the dependence of NS tidal disruptions on the NS compactness by fixing the NS mass but changing the EOS. We further perform simulations with massive NSs of MNS=1.8M⊙, and compare our results of M^rem and M^eje with those given by existing fitting formulas to test their robustness for more compact NSs. We find that the fitting formulas obtained in the previous studies are accurate within the numerical errors assumed, while our results also suggest that further improvement is possible by systematically performing more precise numerical simulations. Published by the American Physical Society 2024