Dense Star Research Articles

Constraining the properties of dense neutron star cores: the case of the low-mass X-ray binary HETE J1900.1–2455

ABSTRACT Measuring the time evolution of the effective surface temperature of neutron stars can provide invaluable information on the properties of their dense cores. Here, we report on a new Chandra observation of the transient neutron star low-mass X-ray binary HETE J1900.1–2455, which was obtained ≈2.5 yr after the end of its ≈10-yr long accretion outburst. The source is barely detected during the observation, collecting only six net photons, all below 2 keV. Assuming that the spectrum is shaped as a neutron star atmosphere model, we perform a statistical analysis to determine a 1σ confidence upper range for the neutron star temperature of ≈30–39 eV (for an observer at infinity), depending on its mass, radius, and distance. Given the heat injected into the neutron star during the accretion outburst, estimated from data provided by all-sky monitors, the inferred very low temperature suggests that the core either has a very high heat capacity or undergoes very rapid neutrino cooling. While the present data do not allow us to disentangle these two possibilities, both suggest that a significant fraction of the dense core is not superfluid/superconductor. Our modelling of the thermal evolution of the neutron star predicts that it may still cool further, down to a temperature of ≃15 eV. Measuring such a low temperature with a future observation may provide constraints on the fraction of baryons that is paired in the stellar core.

Merging stellar and intermediate-mass black holes in dense clusters: implications for LIGO, LISA, and the next generation of gravitational wave detectors

Context.The next generation of gravitational wave (GW) observatories would enable the detection of intermediate-mass black holes (IMBHs), an elusive type of BH expected to reside in the centres of massive clusters, dwarf galaxies, and possibly the accretion discs of active galactic nuclei. Intermediate-mass ratio inspirals (IMRIs), which are composed of an IMBH and a compact stellar object, constitute one promising source of GWs detectable by this new generation of instruments.Aims.We study the formation and evolution of IMRIs triggered by interactions between two stellar BHs and an IMBH inhabiting the centre of a dense star cluster, with the aim of placing constraints on the formation rate and detectability of IMRIs.Methods.We exploit directN-body models varying the IMBH mass, the stellar BH mass spectrum, and the star cluster properties. Our simulations take into account the host cluster gravitational field and general relativistic effects via post-Newtonian terms up to order 2.5. These simulations are coupled with a semi-analytic procedure to characterise the evolution of the remnant IMBH after the IMRI phase.Results.Generally, the IMRI formation probability attains values of ∼5−50%, with larger values corresponding to larger IMBH masses. Merging IMRIs tend to map out the stellar BH mass spectrum, suggesting that IMRIs could be used to unravel the role of dynamics in shaping BH populations in star clusters harbouring an IMBH. After the IMRI phase, an initially almost maximal(almost non-rotating) IMBH tends to significantly decrease(increase) its spin. Under the assumption that IMBHs grow mostly via repeated IMRIs, we show that only sufficiently massive (Mseed > 300 M⊙) IMBH seeds can grow up toMIMBH > 103 M⊙in dense globular clusters (GCs). Assuming that these seeds form at a redshift ofz ∼ 2−6, we find that around 1−5% of them would reach typical masses of ∼500−1500 M⊙at redshiftz = 0 and would exhibit low spins, generallySIMBH < 0.2. Measuring the mass and spin of IMBHs involved in IMRIs could help to unravel their formation mechanism. We show that LISA can detect IMBHs in Milky Way GCs with a signal-to-noise ratioS/N = 10−100, or in the Large Magellanic Cloud, for which we get aS/N = 8−40. More generally, we provide the IMRI merger rate for different detectors, namely LIGO (ΓLIGO = 0.003−1.6 yr−1), LISA (ΓLISA = 0.02−60 yr−1), ET (ΓET = 1−600 yr−1), and DECIGO (ΓDECIGO = 6−3000 yr−1).Conclusions.Our simulations explore one possible channel for IMBH growth, namely via merging with stellar BHs in dense clusters. We find that the mass and spin of the IMRI components and the merger remnant encode crucial information about the mechanisms that regulate IMBH formation. Our analysis suggests that the future synergy among GW detectors will enable us to fully unravel IMBH formation and evolution.

Neutron star cooling in modified gravity theories

Abstract We study the thermal evolution of isolated neutron stars in scalar–tensor theories for the first time. Whether rapid cooling due to the direct Urca process occurs or not is an interesting question from the viewpoint of the temperature observation of isolated neutron stars. Moreover, investigation of the cooling effect of nucleon superfluidity also has large uncertainties, though it is important in modern cooling theory. The cooling effect is typically influenced by the proton fraction and the central density. If a fifth force is mediated due to a modification of gravity, the relation between the central density and mass of neutron stars differs from that in general relativity, and the cooling curve is also naively expected to vary. We find that an unscreened fifth force near the surface of neutron stars changes the mass–central density relation, and the direct Urca process can be triggered even for neutron stars with smaller mass. We also present cooling curves including nucleon superfluidity under the scalar–tensor theory. These results show that it might be useful to test gravitational theories with cooling observations of neutron stars.

Probing the relaxed relaxion and Higgs portal scenarios with XENON1T scintillation and ionization data

We study the recent XENON1T excess in the context of solar scalar, specifically in the framework of Higgs portal and the relaxion model. We show that ${m}_{\ensuremath{\phi}}=1.9\text{ }\text{ }\mathrm{keV}$ and ${g}_{\ensuremath{\phi}e}=2.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ can explain the observed excess in science run 1 (SR1) analysis in the 1--7 keV range. In the minimal scenarios we consider, the best-fit parameters are in tension with stellar cooling bounds. Despite this fact, the excess represents an example bringing attention to two interesting effects of general relevance. First, the scalar-Higgs mixing angle reproducing the excess, $\mathrm{sin}\ensuremath{\theta}\ensuremath{\simeq}{10}^{\ensuremath{-}8}$, is intriguingly close to the maximum value of mixing angle for the technical naturalness of the scalar mass. While finding a parameter value very close to its theoretical limit may naively seem an unlikely coincidence, we demonstrate that there exists a class of models which generically saturate the mixing naturalness bound. Secondly, we discuss a possibility that a large density of red giant stars may trigger a phase transition, resulting in a local scalar mass increase suppressing the stellar cooling. For the particular case of minimal relaxion scenarios, we find that such type of chameleon effects is automatically present but they can not ease the cooling bounds. They are however capable of triggering a catastrophic phase transition in the entire Universe. Following this observation we derive a new set of bounds on the relaxed relaxion parameter space. Finally, we present two nonminimal models that demonstrate how the cooling bounds can be relaxed as a result of high density effects.

Collision between molecular clouds – III: the effects of cloud initial density profile on head-on collisions

ABSTRACT In this third paper of our cloud collision series, we present the results from simulations of head-on collisions with a strongly centrally condensed initial density profile of ρ ∝ R−2. We investigate the impact of these density profiles on the overall evolution of the simulations: the structures formed, their dynamical evolution, and their star formation activity. We consider clouds that are globally bound and globally unbound, leading to three different scenarios – the collision of a bound cloud with a bound cloud, the collision of two unbound clouds, or the collision of one cloud of each type. In all the simulations, dense star clusters form before the collisions occur, and we find that star formation remains confined to these systems and is little affected by the collisions. If the clouds are both initially bound, the collision forms a filamentary structure, but otherwise this does not occur. We observe that rotating structures form around the clusters, but they also form in our non-colliding control simulations, so are not a consequence of the collisions. The dissipation of kinetic energy in these simulations is inefficient because of the substructure created in the clouds by turbulence before the collisions. As a result, although some gas is left bound in the COM frame, the star clusters formed in the simulations do not become bound to each other.

The density structure of supersonic self-gravitating turbulence

ABSTRACT We conduct numerical experiments to determine the density probability distribution function (PDF) produced in supersonic, isothermal, self-gravitating turbulence of the sort that is ubiquitous in star-forming molecular clouds. Our experiments cover a wide range of turbulent Mach number and virial parameter, allowing us for the first time to determine how the PDF responds as these parameters vary, and we introduce a new diagnostic, the dimensionless star formation efficiency versus density [ϵff(s)] curve, which provides a sensitive diagnostic of the PDF shape and dynamics. We show that the PDF follows a universal functional form consisting of a lognormal at low density with two distinct power-law tails at higher density: the first of these represents the onset of self-gravitation, and the second reflects the onset of rotational support. Once the star formation efficiency reaches a few per cent, the PDF becomes statistically steady, with no evidence for secular time evolution at star formation efficiencies from about 5 to 20 per cent. We show that both the Mach number and the virial parameter influence the characteristic densities at which the lognormal gives way to the first power law, and the first to the second, and we extend (for the former) and develop (for the latter) simple theoretical models for the relationship between these density thresholds and the global properties of the turbulent medium.

The heavy-ion program at the upgraded baryonic matter@nuclotron experiment at NICA

The future gold beams with energies of up to 3.8A[Formula: see text]GeV from the Nuclotron at JINR in Dubna are well suited to study the equation of state of dense baryonic matter, and to explore the microscopic degrees-of-freedom emerging at neutron star densities. The relevant observables in heavy-ion collisions at these energies include yield and multidifferential distributions of (multi-) strange particles, collective flow of identified particles, fluctuation of conserved quantities and hypernuclei. In order to measure these observables in Au [Formula: see text] Au collisions with rates of up to 50[Formula: see text]kHz, the Baryonic Matter@Nuclotron setup will be upgraded with a highly granulated and fast hybrid tracking system, and a forward calorimeter for event plane determination. The physics program, the detector upgrades and physics performance studies will be presented.

Variable stars in the VVV globular clusters

Context.The Galactic globular clusters (GGCs) located in the inner regions of the Milky Way suffer from high extinction that makes their observation challenging. High densities of field stars in their surroundings complicate their study even more. The VISTA Variables in the Via Lactea (VVV) survey provides a way to explore these GGCs in the near-infrared where extinction effects are highly diminished.Aims.We conduct a search for variable stars in several inner GGCs, taking advantage of the unique multi-epoch, wide-field, near-infrared photometry provided by the VVV survey. We are especially interested in detecting classical pulsators that will help us constrain the physical parameters of these GGCs. In this paper, the second of a series, we focus on NGC 6656 (M 22), NGC 6626 (M 28), NGC 6569, and NGC 6441; these four massive GGCs have known variable sources, but quite different metallicities. We also revisit 2MASS-GC 02 and Terzan 10, the two GGCs studied in the first paper of this series.Methods.We present an improved method and a new parameter that efficiently identify variable candidates in the GGCs. We also use the proper motions of those detected variable candidates and their positions in the sky and in the color-magnitude diagrams to assign membership to the GGCs.Results.We identify and parametrize in the near-infrared numerous variable sources in the studied GGCs, cataloging tens of previously undetected variable stars. We recover many known classical pulsators in these clusters, including the vast majority of their fundamental mode RR Lyrae. We use these pulsators to obtain distances and extinctions toward these objects. Recalibrated period–luminosity–metallicity relations for the RR Lyrae bring the distances to these GGCs to a closer agreement with those reported byGaia, except for NGC 6441, which is an uncommon Oosterhoff III GGC. Recovered proper motions for these GGCs also agree with those reported byGaia, except for 2MASS-GC 02, the most reddened GGC in our sample, where the VVV near-infrared measurements provide a more accurate determination of its proper motions.

On the Random Motion of Nuclear Objects in a Fuzzy Dark Matter Halo

Abstract Fuzzy dark matter (FDM), consisting of ultralight bosons ( m b ∼ 10 − 22 eV ), is an intriguing alternative to cold dark matter. Numerical simulations that solve the Schrödinger–Poisson (SP) equation show that FDM halos consist of a central solitonic core, which is the ground state of the SP equation, surrounded by an envelope of interfering excited states. These excited states also interfere with the soliton, causing it to oscillate and execute a confined random walk with respect to the halo center of mass. Using high-resolution numerical simulations of a 6.6 × 10 9 M ⊙ FDM halo with m b = 8 × 10 − 23 eV in isolation, we demonstrate that the wobbling, oscillating soliton gravitationally perturbs nuclear objects, such as supermassive black holes or dense star clusters, causing them to diffuse outwards. In particular, we show that, on average, objects with mass ≲0.3% of the soliton mass ( M sol ) are expelled from the soliton in ∼3 Gyr , after which they continue their outward diffusion due to gravitational interactions with the soliton and the halo granules. More massive objects (≳ 1 % M sol ), while executing a random walk, remain largely confined to the soliton due to dynamical friction. We also present an effective treatment of the diffusion, based on kinetic theory, that accurately reproduces the outward motion of low-mass objects and briefly discuss how the observed displacements of star clusters and active galactic nuclei from the centers of their host galaxies can be used to constrain FDM.

STATIC GENERAL SOLUTION TO EINSTEIN’S FIELD EQUATIONS

We consider the Vaidya Tikekar metric; 3- dimensional space with time (t) being constant, in a super dense star which is spheroidal. We report a static general solution (in terms of hyper-geometric series) to Einstein's field equations using Vaidya-Tikekar metric.

Formulation of radiation temperature gradient equations with non-homogeneous density in stars

The interior of a star consists of the convection region and radiation. The energy transfer process in the convection region and radiation is different which indicates the energy transfer process in the star. The energy transfer is affected by the density of the star. The radiation energy transfer equation with non-homogeneous density has been formulated. The density of a non-homogeneous star is the density that only depends on the distance. It has been obtained the formulation of an energy transfer equation in radiation with a non-homogeneous density in the form of a star temperature gradient. This equation is obtained by formulating the relationship between the specific intensity of the beam, the flux, and the luminosity of the star. The density of a non-homogeneous star affects the luminosity and flux of the star. Meanwhile, the star temperature gradient equation does not depend on the density of non-homogeneous stars.

Minimum Orbital Periods of H-rich Bodies

Abstract In this work we derive the minimum allowed orbital periods of H-rich bodies ranging in mass from Saturn’s mass to 1 M ⊙, emphasizing gas giants and brown dwarfs (BDs) over the range 0.0003–0.074 M ⊙. Analytic fitting formulae for as a function of the mass of the body and as a function of the mean density are presented. We assume that the density of the host star is sufficiently high so as not to limit the minimum period. In many instances this implies that the host star is a white dwarf. This work is aimed, in part, toward distinguishing BDs from planets that are found transiting the host white dwarf without recourse to near-infrared or radial velocity measurements. In particular, orbital periods of ≲100 minutes are very likely to be BDs. The overall minimum period over this entire mass range is ≃37 minutes.

Hybrid stars in the light of the merging event GW170817

We study quark-hadron hybrid stars with sharp phase transitions assuming that phase conversions at the interface are slow. Hadronic matter is described by a set of equations of state (EoS) based on the chiral effective field theory and quark matter by a generic bag model. Due to slow conversions at the interface, there is an extended region of stable hybrid stars with central densities above the density of the maximum mass star. We explore systematically the role of the transition pressure and the energy-density jump Δϵ at the interface on some global properties of hybrid stars, such as the maximum mass, the last stable configuration, and tidal deformabilities. We find that for a given transition pressure, the radius of the last stable hybrid star decreases as Δϵ raises resulting in a larger extended branch of stable hybrid stars. Contrary to purely hadronic stars, the tidal deformability Λ can be either a decreasing or an increasing function of the stellar mass M and for large values of the transition pressure has a very weak dependence on M. Finally, we analyze the tidal deformabilities Λ1 and Λ2 for a binary system with the same chirp mass as GW170817. In the scenario where at least one of the stars in the binary is hybrid, we find that models with low enough transition pressure are inside the 90 % credible region of GW170817. However, these models have maximum masses below 2 M⊙, in disagreement with observations. We also find that the LIGO/Virgo constrain (at 90% level) and the 2 M⊙ requirement can be simultaneously fulfilled in a scenario where all hybrid configurations have masses larger than 1.6 M⊙ and the hadronic EoS is not too stiff, such as several of our hybrid models involving a hadronic EoS of intermediate stiffness. In such scenario hybrid stars may exist in Nature but both objects in GW170817 were hadronic stars.

A New Identity Card for the Bulge Globular Cluster NGC 6440 from Resolved Star Counts*

Abstract We present a new identity card for the cluster NGC 6440 in the Galactic Bulge. We have used a combination of high-resolution Hubble Space Telescope images, wide-field ground-based observations performed with the ESO-FORS2, and the public survey catalog Pan-STARRS to determine the gravitational center, projected density profile, and structural parameters of this globular from resolved star counts. The new determination of the cluster center differs by ∼2″ (corresponding to 0.08 pc) from the previous estimate, which was based on the surface brightness peak. The star density profile, extending out to 700″ from the center and suitably decontaminated from the Galactic field contribution, is best fit by a King model with a significantly higher concentration (c = 1.86 ± 0.06) and smaller core radius (r c = 6.″4 ± 0.″3) with respect to the literature values. By taking advantage of high-quality optical and near-IR color–magnitude diagrams, we also estimated the cluster age, distance, and reddening. The luminosity of the red giant branch bump was also determined. This study indicates that the extinction coefficient in the bulge in the direction of the cluster has a value (R V = 2.7) that is significantly lower than that traditionally used for the Galaxy (R V = 3.1). The corresponding best-fit values of the age, distance, and color excess of NGC 6440 are 13 Gyr, 8.3 kpc, and E(B − V) ∼ 1.27. These new determinations also allowed us to update the values of the central (t rc = 2.5 107 yr) and half-mass (t rh = 109 yr) relaxation times, suggesting that NGC 6440 is in a dynamically evolved stage.

The mass budget for intermediate-mass black holes in dense star clusters

ABSTRACT Intermediate-mass black holes (IMBHs) could form via runaway merging of massive stars in a young massive star cluster (YMC). We combine a suite of numerical simulations of YMC formation with a semi-analytic model for dynamical friction and merging of massive stars and evolution of a central quasi-star, to predict how final quasi-star and relic IMBH masses scale with cluster properties (and compare with observations). The simulations argue that inner YMC density profiles at formation are steep (approaching isothermal), producing some efficient merging even in clusters with relatively low effective densities, unlike models that assume flat central profiles resembling those of globular clusters after central relaxation. Our results can be approximated by simple analytic scalings, with $M_{\rm IMBH} \propto v_{\rm cl}^{3/2}$ where $v_{\rm cl}^{2} = G\, M_{\rm cl}/r_{\rm h}$ is the circular velocity in terms of initial cluster mass Mcl and half-mass radius rh. While this suggests IMBH formation is possible even in typical clusters, we show that predicted IMBH masses for these systems are small, $\sim \! 100-1000\, {\rm M}_{\odot }$ or $\sim \! 0.0003\, M_{\rm cl}$, below even the most conservative observational upper limits in all known cases. The IMBH mass could reach $\gtrsim 10^{4}\, {\rm M}_{\odot }$ in the centres nuclear star clusters, ultra-compact dwarfs, or compact ellipticals, but in all these cases the prediction remains far below the present observed supermassive BH masses in these systems.

Decoherence from general relativity

It is of great interest to explore matter in nontrivial quantum arrangements, including Schrodinger cat-like states. Such states are sensitive to decoherence from their environment. Recently, in Ref. [1] we computed the rate of decoherence of a piece of superposed matter that primarily only interacts gravitationally, a dark-matter-Schrodinger-cat-state (DMSCS), within the nonrelativistic approximation. In this work we improve this to a general relativistic analysis. We firstly derive a single particle relativistic Schrodinger equation for a probe particle that passes through the DMSCS; the interaction is provided by the weak field metric of general relativity from the source. For a static DMSCS we find a neat generalization of our previous results. We then turn to the interesting new case of a time dependent DMSCS, which can be provided by a coherently oscillating axion field leading to superposed time dependent oscillations in the metric; a truly quantum-general relativistic phenomenon. We use scattering theory to derive the decoherence rate in all these cases. When the DMSCS is in a superposition of distinct density profiles, we find that the decoherence rate can be appreciable. We then consider the novel special case in which the density is not in a superposition, but the phase of its field oscillation is; this is a property that cannot be decohered within the nonrelativistic framework. We find that if the probe particle and/or the DMSCS's velocity dispersion is slow, then the rate of decoherence of the phase is exponentially suppressed. However, if both the probe and the DMSCS's velocity dispersion are relativistic, then the phase can decohere more rapidly. As applications, we find that diffuse galactic axions with superposed phases are robust against decoherence, while dense boson stars and regions near black hole horizons are not, and we discuss implications for experiment.

Effect of mass-loss due to stellar winds on the formation of supermassive black hole seeds in dense nuclear star clusters

ABSTRACT The observations of high-redshifts quasars at z ≳ 6 have revealed that supermassive black holes (SMBHs) of mass $\sim 10^9\, \mathrm{M_{\odot }}$ were already in place within the first ∼Gyr after the big bang. Supermassive stars (SMSs) with masses $10^{3-5}\, \mathrm{M_{\odot }}$ are potential seeds for these observed SMBHs. A possible formation channel of these SMSs is the interplay of gas accretion and runaway stellar collisions inside dense nuclear star clusters (NSCs). However, mass-loss due to stellar winds could be an important limitation for the formation of the SMSs and affect the final mass. In this paper, we study the effect of mass-loss driven by stellar winds on the formation and evolution of SMSs in dense NSCs using idealized N-body simulations. Considering different accretion scenarios, we have studied the effect of the mass-loss rates over a wide range of metallicities Z* = [.001–1]Z⊙ and Eddington factors $f_{\rm Edd}=L_\ast /L_{\mathrm{Edd}}=0.5,0.7,\, \,\mathrm{ and}\, 0.9$. For a high accretion rate of $10^{-4}\, \mathrm{M_{\odot }yr^{-1}}$, SMSs with masses $\gtrsim 10^3\, \mathrm{M_{\odot }yr^{-1}}$ could be formed even in a high metallicity environment. For a lower accretion rate of $10^{-5}\, \mathrm{M_{\odot }yr^{-1}}$, SMSs of masses $\sim 10^{3-4}\, \mathrm{M_{\odot }}$ can be formed for all adopted values of Z* and fEdd, except for Z* = Z⊙ and fEdd = 0.7 or 0.9. For Eddington accretion, SMSs of masses $\sim 10^3\, \mathrm{M_{\odot }}$ can be formed in low metallicity environments with Z* ≲ 0.01 Z⊙. The most massive SMSs of masses $\sim 10^5\, \mathrm{M_{\odot }}$ can be formed for Bondi–Hoyle accretion in environments with Z* ≲ 0.5 Z⊙. An intermediate regime is likely to exist where the mass-loss from the winds might no longer be relevant, while the kinetic energy deposition from the wind could still inhibit the formation of a very massive object.

Close stellar encounters kicking planets out of habitable zone in various stellar environments

Continuous habitability of a planet is a critical condition for advanced forms of life to appear, but it can be endangered by astronomical events such as stellar encounters. The purpose of this study is to analyze close stellar encounters able to change planetary orbits initially in circumstellar habitable zones and to investigate the expected encounter rates in a variety of stellar environments. Using gravitational simulations for three-body systems, this study analyzed the dependencies of encounter impact-parameters with kinematic, geometric, and habitability parameters of the system. We also used kinematic properties of various stellar regions and estimated encounter rates of the events. The expected number of threatening stellar encounters in the Solar neighborhood is [Formula: see text] in 4 billion years, while for the Galactic bulge environment, we expect approximately 5.5 times the value. The encounter rates for other stellar environments are calculated and spheroidal dwarf galaxies and globular clusters encounter rates are estimated. The results show that in contrast with the solar neighborhood, close stellar encounters can play a significant role in the expected number of planets with continuous habitability in dense stellar environments. Another notable result shows that threatening stellar encounter rate follows the number density of stars, and is not strongly dependent of the region’s velocity dispersion. Further investigations are needed to study long-term multiple planetary systems and how they can change the overall expected value of continuously habitable planets.

Expansion of intertidal mussel beds following disease-driven reduction of a keystone predator

Disease shapes community composition by removing species with strong interactions. To test whether the absence of keystone predation due to disease produced changes to the species composition of rocky intertidal communities, we leverage a natural experiment involving mass mortality of the keystone predator Pisaster ochraceus from Sea Star Wasting Syndrome. Over four years, we measured dimensions of mussel beds, sizes of Mytilus californianus, mussel recruitment, and species composition on vertical rock walls at six rocky intertidal sites on the central California coast. We also assessed the relationship between changes in mussel cover and changes in sea star density across 33 sites along the North American Pacific coast using data from long-term monitoring. After four years, the lower boundary of the central California mussel beds shifted downward toward the water 18.7 ± 15.8 cm (SD) on the rock and 11.7 ± 11.0 cm in elevation, while the upper boundary remained unchanged. In central California, downward expansion and total area of the mussel bed were positively correlated with mussel recruitment but were not correlated with pre-disease sea star density or biomass. At a multi-region scale, changes in mussel percent cover were positively correlated with pre-disease sea star densities but not change in densities. Species composition of primary substrate holders and epibionts below the mussel bed remained similar across years. Extirpation of the community below the bed did not occur. Instead, this community became limited to a smaller spatial extent while the mussel bed expanded.

Post-merger evolution of double helium white dwarfs and distribution of helium-rich hot subdwarfs

ABSTRACT The mergers of double helium white dwarfs are believed to form isolated helium-rich hot subdwarfs. Observations show that helium-rich hot subdwarfs can be divided into two subgroups based on whether the surface is carbon-rich or carbon-normal. However, it is not clear whether this distribution directly comes from binary evolution. We adopt the binary population synthesis to obtain the population of single helium-rich hot subdwarfs according to the merger channel of double helium white dwarfs. We find that the merger channel can represent the two subgroups in the Teff−log g plane related to different masses of progenitor helium white dwarfs. For Z = 0.02, the birth rate and local density of helium-rich hot subdwarf stars from the merger of two helium white dwarfs are $\sim 4.82 \times 10^{-3}\, \rm yr^{-1}$ and ∼ 290.0 $\rm kpc^{-3}$ at 13.7 Gyr in our Galaxy, respectively. The proportions of carbon-rich and carbon-normal helium-rich hot subdwarfs are 32 per cent and 68 per cent, respectively.