Degenerate Core Research Articles

Younger age for the oldest magnetic white dwarfs

Abstract Sufficiently old white dwarfs cool down through a convective envelope that directly couples their degenerate cores to the surface. Magnetic fields may inhibit this convection by stiffening the criterion for convective instability. We consistently implemented the modified criterion in the stellar evolution code mesa, and computed the cooling of white dwarfs as a function of their mass and magnetic field B. In contrast to previous estimates, we find that magnetic fields can significantly change the cooling time t even if they are relatively weak $B^2\ll 8\pi P$, where P is the pressure at the edge of the degenerate core. Fields B ≳ 1 MG open a radiative window that decouples the core from the convective envelope, effectively lowering the luminosity to that of a fully radiative white dwarf. We identified a population of observed white dwarfs that are younger by Δt ∼ Gyr than currently thought due to this magnetic inhibition of convective energy transfer – comparable to the cooling delay due to carbon–oxygen phase separation. In volume-limited samples, the frequency and strength of magnetic fields increase with age. Accounting for magnetic inhibition is therefore essential for accurate cooling models for cosmic chronology and for determining the origin of the magnetic fields.

Accretion discs onto supermassive compact objects: A portal to dark matter physics in active galaxies

Context. The study of the physics of the accretion discs that develop around supermassive black hole (BH) candidates provides essential theoretical tools to test their nature. Aims. Here, we study the accretion flow and associated emission using generalised α-discs accreting onto horizonless dark compact objects in order to make comparisons with the traditional BH scenario. The BH alternative proposed here consists in a dense and highly degenerate core made of fermionic dark matter (DM) and surrounded by a more diluted DM halo. This dense core–diluted halo DM configuration is a solution of Einstein’s equation of general relativity (GR) in spherical symmetry, which naturally arises once the quantum nature of the DM fermions is duly accounted for. Methods. The methodology followed in this work consists in first generalising the theory of α-discs to work in the presence of regular and horizonless compact objects, and then applying it to the case of core–halo DM profiles typical of active-like galaxies. Results. The fact that the compactness of the dense and transparent DM core scales with particle mass allows the following key findings of this work: (i) There is always a given core compacity – corresponding particle mass – that produces a luminosity spectrum that is almost indistinguishable from that of a Schwarzschild BH of the same mass as the DM core. (ii) The disc can enter deep inside the non-rotating DM core, allowing accretion-powered efficiencies of as high as 28%, which is comparable to that of a highly rotating Kerr BH. Conclusions. These results, together with the existence of a critical DM core mass of collapse into a supermassive BH, open new avenues of research for two seemingly unrelated topics: AGN phenomenology and dark matter physics.

Thermonuclear explosions as Type II supernovae

We consider a binary stellar system in which a low-mass (0.6 ) carbon-oxygen white dwarf (WD) merges with the degenerate helium core (0.4 ) of a red giant. We analyse the outcome of a merger within a common envelope (CE). We predict the observational properties of the resulting transient. We find that the double detonation of the WD, being a pure thermonuclear explosion and embedded into the hydrogen-rich CE, has a light curve with the distinct plateau shape of a supernova (SN) Type\,IIP, with a duration of about 40 days. We find five observed SNe IIP (SN 2004dy, SN 2005af, SN 2005hd, SN 2007aa, and SN 2008bu) that match the $V$-band light curve of our models. Hence, we show that a thermonuclear explosion within a CE might be mistakenly identified as a SN IIP, believed to be an outcome of a core-collapse neutrino-driven explosion of a massive star. We discuss a number of diagnostics that may help to distinguish this kind of thermonuclear explosion from a core-collapse SN.

Baryon-induced Collapse of Dark Matter Cores into Supermassive Black Holes

Nonlinear structure formation for fermionic dark matter particles leads to dark matter density profiles with a degenerate compact core surrounded by a diluted halo. For a given fermion mass, the core has a critical mass that collapses into a supermassive black hole (SMBH). Galactic dynamics constraints suggest a ∼100 keV/c 2 fermion, which leads to ∼107 M ⊙ critical core mass. Here, we show that baryonic (ordinary) matter accretion drives an initially stable dark matter core to SMBH formation and determines the accreted mass threshold that induces it. Baryonic gas density ρ b and velocity v b inferred from cosmological hydrosimulations and observations produce sub-Eddington accretion rates triggering the baryon-induced collapse in less than 1 Gyr. This process produces active galactic nuclei in galaxy mergers and the high-redshift Universe. For TXS 2116–077, merging with a nearby galaxy, the observed 3 × 107 M ⊙ SMBH, for Qb=ρb/vb3=0.125M⊙/(100kms−1pc)3 , forms in ≈0.6 Gyr, consistent with the 0.5–2 Gyr merger timescale and younger jet. For the farthest central SMBH detected by the Chandra X-ray satellite in the z = 10.3 UHZ1 galaxy observed by the James Webb Space Telescope (JWST), the mechanism leads to a 4 × 107 M ⊙ SMBH in 87–187 Myr, starting the accretion at z = 12–15. The baryon-induced collapse can also explain the ≈107–108 M ⊙ SMBHs revealed by JWST at z ≈ 4–6. After its formation, the SMBH can grow to a few 109 M ⊙ in timescales shorter than 1 Gyr via sub-Eddington baryonic mass accretion.

Modal stability of low-frequency nucleus-acoustic waves in completely degenerate white dwarf cores and their nearly degenerate surroundings

Modal stability of low-frequency nucleus-acoustic waves in completely degenerate white dwarf cores and their nearly degenerate surroundings

Modelling of dislocations, twins and crack-tips in HCP and BCC Ti

Ti exhibits complex plastic deformation controlled by active dislocation and twinning systems. Understandings on dislocation cores and twin interfaces are currently not complete or quantitative, despite extensive experimental and simulation studies. Here, we determine the dislocation core, twin, and crack properties in both HCP and BCC Ti using a Deep Potential (DP), DFT and linear elastic fracture mechanics. We compute the core structures, critical resolved shear stresses and mobilities of the 〈a〉 , 〈c+a〉 , 〈c〉 dislocations in HCP and the 〈111〉/2 dislocations in BCC Ti. The 〈a〉 slip consists of slow core migration on pyramidal-I planes and fast migration on prism-planes, and is kinetically limited by cross-slips among them. This behaviour is consistent with “locking–unlocking” phenomena in TEM and is likely an intrinsic property. Large-scale DFT calculations provide a peek at the screw 〈c+a〉 core and glide behaviour, which is further quantified using DP-Ti. The screw 〈c+a〉 is unstable on pyramidal-II planes. The mixed 〈c+a〉 is nearly sessile on pyramidal-I planes, consistent with observations of long dislocations in this orientation. The edge and mixed 〈c+a〉 are unstable against a pyramidal-to-basal (PB) transition and become sessile at high temperatures, corroborate the difficulties in 〈c〉 -axis compression of Ti. Cracks on basal planes are predicted and shown to be intrinsically brittle, consistent with cleavage facets primarily on this plane in experiments. Twin boundary energies vary considerably but all twin boundary structures possess a mirror reflection symmetry. Finally, in BCC Ti, the 〈111〉/2 screw has a degenerate core with average glide on {112} planes; the 〈111〉/2 edge and mixed dislocations have non-dissociated cores on {110} planes. This work paints a self-consistent, complete picture on all dislocations in Ti, rationalises previous experimental observations on Ti plasticity and fracture, and points to future HRTEM examinations of unusual dislocations such as the mixed and PB transformed 〈c+a〉 cores.

Transient Stellar Collisions as Multimessenger Probes: Nonthermal, Gravitational-wave Emission and the Cosmic Ladder Argument

In dense stellar clusters like galactic nuclei and globular clusters, stellar densities are so high that stars might physically collide with each other. In galactic nuclei the energy and power output can be close to, and even exceed, those from supernovae events. We address the event rate and the electromagnetic characteristics of collisions of main-sequence stars (MS) and red giants (RGs). We also investigate the case in which the cores form a binary and emit gravitational waves. In the case of RGs, this is particularly interesting because the cores are degenerate. We find that MS event rate can be as high as tens per year, and that of RGs 1 order of magnitude larger. The collisions are powerful enough to mimic supernovae or tidal disruptions events. We find Zwicky Transient Facility observational data that seem to exhibit the features we describe. The cores embedded in the gaseous debris experience a friction force that has an impact on the chirping mass of the gravitational wave. As a consequence, the two small cores in principle mimic two supermassive black holes merging. However, their evolution in frequency along with the precedent electromagnetic burst and the ulterior afterglow are efficient tools to reveal the impostors. In the particular case of RGs, we derive the properties of the degenerate He cores and their H-burning shells to analyze the formation of the binaries. The merger is such that it can be misclassified with SN Ia events. Because the masses and densities of the cores are so dissimilar in values depending on their evolutionary stage, the argument about standard candles and cosmic ladder should be reevaluated.

Lithium Abundances in Giants as a Function of Stellar Mass: Evidence for He Flash as the Source of Li Enhancement in Low-mass Giants

In this work, we studied the distribution of lithium abundances in giants as a function of stellar mass. We used a sample of 1240 giants common among Kepler photometric and LAMOST medium-resolution (R ≈ 7500) spectroscopic survey fields. The asteroseismic ΔP–Δν diagram is used to define core He-burning red clump giants and red giant branch stars with an inert He core. Li abundances have been derived using spectral synthesis for all sample stars. Directly measured values of asteroseismic parameters ΔP (or ΔΠ1) and Δν are either taken from the literature or measured in this study. Of the 777 identified red clump giants, we found 668 low-mass (≤2 M ⊙) primary red clump giants and 109 high-mass (>2 M ⊙) secondary red clump giants. Observed Li abundances in secondary red clump giants agree with the theoretical model predictions. The lack of Li-rich giants among secondary red clump giants and the presence of Li-rich, including super Li-rich, giants among primary red clump stars reinforces the idea that helium flash holds the key for Li enrichment among low-mass giants. The results will further constrain theoretical models searching for a physical mechanism for Li enhancement among low-mass red clump giants. Results also serve as observational evidence that only giants with mass less than ≈2 M ⊙ develop a degenerate He core and undergo He flash.

Thermonuclear and electron-capture supernovae from stripped-envelope stars

Context. When stripped from their hydrogen-rich envelopes, stars with initial masses between ∼7 and 11 M⊙ may develop massive degenerate cores and collapse. Depending on the final structure and composition, the outcome can range from a thermonuclear explosion, to the formation of a neutron star in an electron-capture supernova (ECSN). It has recently been demonstrated that stars in this mass range may be more prone to disruption than previously thought: they may initiate explosive oxygen burning when their central densities are still below ρc ≲ 109.6 g cm−3. At the same time, their envelopes expand significantly, leading to the complete depletion of helium. This combination makes them interesting candidates for type Ia supernovae–which we call (C)ONe SNe Ia–and this might have broader implications for the formation of neutron stars via ECSNe. Aims. To constrain the observational counterparts of (C)ONe SNe Ia and the key properties that enable them, it is crucial to constrain the evolution, composition, and precollapse structure of their progenitors, as well as the evolution of these quantities with cosmic time. In turn, this requires a detailed investigation of the final evolutionary stages preceding the collapse, and their sensitivity to input physics. Methods. Here, we modeled the evolution of 252 single, nonrotating helium stars covering the initial mass range 0.8 − 3.5 M⊙, with metallicities between Z = 10−4 and 0.02, and overshoot efficiency factors from fOV = 0.0 to 0.016 across all convective boundaries. We used these models to constrain several properties of these stars, including their central densities, compositions, envelope masses, and radii at the onset explosive oxygen ignition, as well as the final outcome as a function of initial helium star mass. We further investigate the sensitivity of these properties to mass loss rate assumptions using an additional grid of 110 models with varying wind efficiencies. Results. We find that helium star models with masses between ∼1.8 and 2.7 M⊙ are able to evolve onto 1.35−1.37 M⊙ (C)ONe cores that initiate explosive burning at central densities between log10(ρc/g cm−3) ∼ 9.3 and 9.6. We constrained the amount of residual carbon retained after core carbon burning as a function of initial conditions, and conclude that it plays a critical role in determining the final outcome: Chandrasekhar-mass degenerate cores that retain more than approximately 0.005 M⊙ of carbon result in (C)ONe SNe Ia, while those with lower carbon mass become ECSNe. We find that (C)ONe SNe Ia are more likely to occur at high metallicities, whereas at low metallicities ECSNe dominate. However, both SN Ia and ECSN progenitors expand significantly during the final evolutionary stages, so that for the most extended models, a further binary interaction may occur. We constrain the relative ratio between (C)ONe SNe Ia and SNe Ib/c to be 0.17−0.30 at Z = 0.02, and 0.03−0.13 at Z ≤ 10−3. Conclusions. We conclude with a discussion on potential observational properties of (C)ONe SNe Ia and their progenitors. In the few thousand years leading to the explosion, at least some progenitors should be identifiable as luminous metal-rich super-giants, embedded in hydrogen-free circumstellar nebulae.

Red-giant branch stellar cores as macroscopic dark matter detectors

We show that macroscopic dark matter (DM) impacts on the degenerate helium cores of red-giant branch (RGB) stars can ignite helium fusion via DM-baryon elastic scattering. The onset of helium burning leads to a characteristic drop in luminosity and rise in temperature that marks the transition to a horizontal branch star. We show that such impacts can alter the RGB luminosity function of globular clusters (GCs), focusing in particular on the GC M15. Using models of M15 stars constructed with the stellar simulation code MESA, we compute the expected DM-ignition event rates and the theoretical RGB luminosity functions under the null and signal hypotheses. We constrain DM with masses ${10}^{17}\text{ }\text{ }\mathrm{g}\ensuremath{\lesssim}{m}_{\ensuremath{\chi}}\ensuremath{\lesssim}{10}^{20}\text{ }\text{ }\mathrm{g}$ and geometric cross sections ${10}^{2}\text{ }\text{ }{\mathrm{cm}}^{2}\ensuremath{\lesssim}{\ensuremath{\sigma}}_{\ensuremath{\chi}n}\ensuremath{\lesssim}{10}^{7}\text{ }\text{ }{\mathrm{cm}}^{2}$ assuming that the DM in M15 is sourced by the background Milky Way halo. We also place more stringent constraints assuming that M15 formed in a DM subhalo that survives today.

Seismic signature of electron degeneracy in the core of red giants: Hints for mass transfer between close red-giant companions

The detection of mixed modes in red giants with space missions COROT and Kepler has revealed their deep internal structure. These modes allow us to characterize the pattern of pressure modes (through the measurement of their asymptotic frequency separation Δν) and the pattern of gravity modes (through the determination of their asymptotic period spacing ΔΠ1). It has been shown that red giant branch (RGB) stars regroup on a well-defined sequence in the Δν − ΔΠ1 plane. Our first goal is to theoretically explain the features of this sequence and understand how it can be used to probe the interiors of red giants. Using a grid of red giant models computed with MESA, we demonstrate that red giants join the Δν − ΔΠ1 sequence whenever electron degeneracy becomes strong in the core. We argue that this can be used to estimate the central densities of these stars, and potentially to measure the amount of core overshooting during the main sequence part of the evolution. We also investigate a puzzling subsample of red giants that are located below the RGB sequence, in contradiction with stellar evolution models. After checking the measurements of the asymptotic period spacing for these stars, we show that they are mainly intermediate-mass red giants. This is doubly peculiar because these stars should have nondegenerate cores and they are expected to be located well above the RGB sequence. We show that these peculiarities are well accounted for if these stars result from the interaction between two low-mass (M ≲ 2 M⊙) close companions during the red giant branch phase. If the secondary component has already developed a degenerate core before mass transfer begins, it becomes an intermediate-mass giant with a degenerate core. The secondary star is then located below the degenerate sequence, which is in agreement with the observations.

Forbidden electron capture on Na24 and Al27 in degenerate oxygen-neon stellar cores

Background: Stars with an initial mass of $\ensuremath{\approx}7$--11 solar masses form degenerate oxygen-neon cores following carbon burning. Electron captures in such cores can trigger runaway oxygen burning, resulting in either a collapse or a thermonuclear explosion. Previous work constrained the contribution of the forbidden ${0}_{\text{g.s.}}^{+}\ensuremath{\rightarrow}{2}_{\text{g.s.}}^{+}$ transition to the $^{20}\mathrm{Ne}({e}^{\ensuremath{-}},{\ensuremath{\nu}}_{e})^{20}\mathrm{F}$ rate and discussed its significance for the evolution of the core.Purpose: We provide a detailed description of the formalism used in previous work and apply it to two further forbidden transitions that are relevant to degenerate oxygen-neon cores: the ${4}_{\text{g.s.}}^{+}\ensuremath{\rightarrow}{2}_{1}^{+}$ transition in $^{24}\mathrm{Na}({e}^{\ensuremath{-}},{\ensuremath{\nu}}_{e})^{24}\mathrm{Ne}$ and the $5/{2}_{\text{g.s.}}^{+}\ensuremath{\rightarrow}1/{2}_{\text{g.s.}}^{+}$ transition in $^{27}\mathrm{Al}({e}^{\ensuremath{-}},{\ensuremath{\nu}}_{e})^{27}\mathrm{Mg}$.Method: The relevant nuclear matrix elements are determined through shell model calculations and constraints from the conserved vector current (CVC) theory. We then investigate the astrophysical impact using the stellar evolution code mesa (Modules for Experiment in Stellar Astrophysics) and through timescale arguments.Results: In the relevant temperature range, the forbidden transitions substantially reduce the threshold densities for $^{24}\mathrm{Na}({e}^{\ensuremath{-}},{\ensuremath{\nu}}_{e})^{24}\mathrm{Ne}$ and $^{27}\mathrm{Al}({e}^{\ensuremath{-}},{\ensuremath{\nu}}_{e})^{27}\mathrm{Mg}$. In the mesa models, $^{24}\mathrm{Na}({e}^{\ensuremath{-}},{\ensuremath{\nu}}_{e})^{24}\mathrm{Ne}$ now occurs immediately following the onset of $^{24}\mathrm{Mg}({e}^{\ensuremath{-}},{\ensuremath{\nu}}_{e})^{24}\mathrm{Na}$. The impact on the overall evolution is uncertain; this is due to known difficulties in accounting for convective instabilities triggered by the $A=24$ electron captures. The transition between $^{27}\mathrm{Al}$ and $^{27}\mathrm{Mg}$ may have a minor effect on the early evolution but is unlikely to affect the outcome.Conclusions: The studied transitions should be included when calculating weak interaction rates between $^{24}\mathrm{Na}$ and $^{24}\mathrm{Ne}$ for temperatures ${log}_{10}(T[\text{K}])\ensuremath{\lesssim}8.5$ and between $^{27}\mathrm{Al}$ and $^{27}\mathrm{Mg}$ for ${log}_{10}(T[\text{K}])\ensuremath{\lesssim}8.8$.

Evolution of Highly Magnetic White Dwarfs by Field Decay and Cooling: Theory and Simulations

Abstract We investigate the luminosity suppression and its effect on the mass–radius relation and cooling evolution of highly magnetized white dwarfs. Based on the effect of magnetic field relative to gravitational energy, we suitably modify our treatment of the radiative opacity, magnetostatic equilibrium, and degenerate core equation of state to obtain the structural properties of these stars. Although the Chandrasekhar mass limit is retained in the absence of magnetic field and irrespective of the luminosity, strong central fields of about 1014 G can yield super-Chandrasekhar white dwarfs with masses ∼2.0 M ⊙. Smaller white dwarfs tend to remain super-Chandrasekhar for sufficiently strong central fields even when their luminosity is significantly suppressed to 10−16 L ⊙. Nevertheless, owing to the cooling evolution and simultaneous field decay over 10 Gyr, the limiting masses of small magnetized white dwarfs can fall to 1.5 M ⊙ over time. However, the majority of these systems still remain practically hidden throughout their cooling evolution because of their high fields and correspondingly low luminosities. Utilizing the stellar evolution code stars, we obtain close agreement with the analytical mass limit estimates, which suggests that our analytical formalism is physically motivated. Our results argue that super-Chandrasekhar white dwarfs born as a result of strong-field effects may not remain so forever. This explains their apparent scarcity, in addition to making them hard to detect because of their suppressed luminosities.

Carnegie Supernova Project: The First Homogeneous Sample of Super-Chandrasekhar-mass/2003fg-like Type Ia Supernovae

Abstract We present a multiwavelength photometric and spectroscopic analysis of 13 super-Chandrasekhar-mass/2003fg-like Type Ia supernovae (SNe Ia). Nine of these objects were observed by the Carnegie Supernova Project. The 2003fg-like SNe have slowly declining light curves (Δm 15(B) < 1.3 mag), and peak absolute B-band magnitudes of −19 < M B < −21 mag. Many of the 2003fg-like SNe are located in the same part of the luminosity–width relation as normal SNe Ia. In the optical B and V bands, the 2003fg-like SNe look like normal SNe Ia, but at redder wavelengths they diverge. Unlike other luminous SNe Ia, the 2003fg-like SNe generally have only one i-band maximum, which peaks after the epoch of the B-band maximum, while their near-IR (NIR) light-curve rise times can be ≳40 days longer than those of normal SNe Ia. They are also at least 1 mag brighter in the NIR bands than normal SNe Ia, peaking above M H = −19 mag, and generally have negative Hubble residuals, which may be the cause of some systematics in dark-energy experiments. Spectroscopically, the 2003fg-like SNe exhibit peculiarities such as unburnt carbon well past maximum light, a large spread (8000–12,000 km s−1) in Si ii λ6355 velocities at maximum light with no rapid early velocity decline, and no clear H-band break at +10 days. We find that SNe with a larger pseudo-equivalent width of C ii at maximum light have lower Si ii λ6355 velocities and more slowly declining light curves. There are also multiple factors that contribute to the peak luminosity of 2003fg-like SNe. The explosion of a C–O degenerate core inside a carbon-rich envelope is consistent with these observations. Such a configuration may come from the core-degenerate scenario.

ASASSN-15hy: An Underluminous, Red 03fg-like Type Ia Supernova

Abstract We present photometric and spectroscopic observations of the 03fg-like Type Ia supernova (SN Ia) ASASSN-15hy from the ultraviolet (UV) to the near-infrared (NIR). ASASSN-15hy shares many of the hallmark characteristics of 03fg-like SNe Ia, previously referred to as “super-Chandrasekhar” SNe Ia. It is bright in the UV and NIR, lacks a clear i-band secondary maximum, shows a strong and persistent C ii feature, and has a low Si ii λ6355 velocity. However, some of its properties are also extreme among the subgroup. ASASSN-15hy is underluminous (M B,peak = − 19.14 − 0.16 + 0.11 mag), red ( ( B − V ) B max = 0.18 − 0.03 + 0.01 mag), yet slowly declining (Δm 15(B) = 0.72 ± 0.04 mag). It has the most delayed onset of the i-band maximum of any 03fg-like SN. ASASSN-15hy lacks the prominent H-band break emission feature that is typically present during the first month past maximum in normal SNe Ia. Such events may be a potential problem for high-redshift SN Ia cosmology. ASASSN-15hy may be explained in the context of an explosion of a degenerate core inside a nondegenerate envelope. The explosion impacting the nondegenerate envelope with a large mass provides additional luminosity and low ejecta velocities. An initial deflagration burning phase is critical in reproducing the low 56Ni mass and luminosity, while the large core mass is essential in providing the large diffusion timescales required to produce the broad light curves. The model consists of a rapidly rotating 1.47 M ⊙ degenerate core and a 0.8 M ⊙ nondegenerate envelope. This “deflagration core-degenerate” scenario may result from the merger between a white dwarf and the degenerate core of an asymptotic giant branch star.

Asteroseismic fingerprints of stellar mergers

ABSTRACT Stellar mergers are important processes in stellar evolution, dynamics, and transient science. However, it is difficult to identify merger remnant stars because they cannot easily be distinguished from single stars based on their surface properties. We demonstrate that merger remnants can potentially be identified through asteroseismology of red giant stars using measurements of the gravity mode period spacing together with the asteroseismic mass. For mergers that occur after the formation of a degenerate core, remnant stars have overmassive envelopes relative to their cores, which is manifested asteroseismically by a g-mode period spacing smaller than expected for the star’s mass. Remnants of mergers that occur when the primary is still on the main sequence or whose total mass is less than $\approx \! 2 \, {\rm M}_\odot$ are much harder to distinguish from single stars. Using the red giant asteroseismic catalogues of Vrard, Mosser & Samadi and Yu et al., we identify 24 promising candidates for merger remnant stars. In some cases, merger remnants could also be detectable using only their temperature, luminosity, and asteroseismic mass, a technique that could be applied to a larger population of red giants without a reliable period spacing measurement.

New Bounds for the Mass of Warm Dark Matter Particles Using Results from Fermionic King Model

After reviewing several aspects about the thermodynamics of self-gravitating systems that undergo the evaporation (escape) of their constituents, some recent results obtained in the framework of fermionic King model are applied here to the analysis of galactic halos considering warm dark matter (WDM) particles. According to the present approach, the reported structural parameters of dwarf galaxies are consistent with the existence of a WDM particle with mass in the keV scale. Assuming that the dwarf galaxy Willman 1 belongs to the region III of fermionic King model (whose gravothermal collapse is a continuous phase transition), one obtains the interval 1.2 keV ≤ m ≤ 2.6 keV for the mass of WDM particle. This analysis improves previous estimates by de Vega and co-workers [Astropart. Phys. 46 (2013) 14–22] considering both the quantum degeneration and the incidence of the constituents evaporation. This same analysis evidences that most of galaxies are massive enough to undergo a violent gravothermal collapse (a discontinuous microcanonical phase transition) that leads to the formation of a degenerate core of WDM particles. It is also suggested that quantum-relativistic processes governing the cores of large galaxies (e.g., the formation of supermassive black holes) are somehow related to the gravothermal collapse of the WDM degenerate cores when the total mass of these systems are comparable to the quantum-relativistic characteristic mass Mc=ℏc/G3/2m−2≃1012M⊙ obtained for WDM particles with mass m in the keV scale. The fact that a WDM particle with mass in the keV scale seems to be consistent with the observed properties of dwarf and large galaxies provides a strong support to this dark matter candidate.

WD mass and orbital period relation of sdB + He WD binaries

ABSTRACT Most subdwarf B (sdB) + Helium white dwarf (He WD) binaries are believed to be formed from a particular channel. In this channel, the He WDs are produced first from red giants (RGs) with degenerate cores via stable mass transfer and sdB stars are produced from RGs with degenerate cores via common-envelope (CE) ejection. They are important for the studies of CE evolution, binary evolution, and binary population synthesis. However, the relation between WD mass and orbital period of sdB + He WD binaries has not been specifically studied. In this paper, we first use a semi-analytic method to follow their formation and find a WD mass and orbital period relation. Then we use a detailed stellar evolution code to model their formation from main-sequence binaries. We find a similar relation between the WD mass and orbital period, which is in broad agreement with observations. For most sdB + He WD systems, if the WD mass (orbital period) can be determined, the orbital period (WD mass) can be inferred with this relation and then the inclination angle can be constrained with the binary mass function. In addition, we can also use this relation to constrain the CE ejection efficiency and find that a relative large CE ejection efficiency is favoured. If both the WD and sdB star masses can be determined, the critical mass ratios of dynamically unstable mass transfer for RG binaries can also be constrained.

Nonplanar core structure of 1/2 screw dislocations: An anisotropic Peierls-Nabarro model

The nonplanar core of the 1/2⟨111⟩ screw dislocation for body-centered cubic metals (groups VB, VIB metals and Fe) has been studied in the framework of the Peierls-Nabarro model, with special interest focused on the effect of crystal anisotropy. By the dislocation interaction model based on the anisotropic elasticity theory, the improved Peierls-Nabarro (P–N) model for nonplanar dislocation core structures is obtained from the variational principle. Full γ-surfaces of the {110} plane and elastic properties employed in this anisotropic P–N model are all achieved by density functional theory (DFT) calculations. The dislocation core structure and its associated dislocation deformation fields can be solved when taking the anisotropy of BCC crystals into account. It is found that with the anisotropic elasticity considered, the proposed nondegenerate nonplanar structure has the lowest energy state, compared to those for both planar and degenerate core structures. The effect of crystalline anisotropy on the nondegenerate core energy cannot be ignored in Cr, Fe, V and Nb. Furthermore, considering the crystal anisotropy, the Burgers vector distribution, the dislocation core energy state and the stress field are clearly distinguished from results predicted by the isotropic model. The crystal anisotropy plays an important role in long-range dislocations stress field especially in Nb and V. The effect of anisotropic elasticity on both long-range and short-range interactions between dislocations for above BCC materials cannot be ignored. The present results can help to build a deeper understanding on the combined effect of the anisotropic elasticity and nonplanar core structures on the dislocation properties.

The pulsational properties of ultra-massive DB white dwarfs with carbon-oxygen cores coming from single-star evolution

Context. Ultra-massive white dwarfs are relevant for many reasons: their role as type Ia supernova progenitors, the occurrence of physical processes in the asymptotic giant branch phase, the existence of high-field magnetic white dwarfs, and the occurrence of double white dwarf mergers. Some hydrogen-rich ultra-massive white dwarfs are pulsating stars and, as such, they offer the possibility of studying their interiors through asteroseismology. On the other hand, pulsating helium-rich ultra-massive white dwarfs could be even more attractive objects for asteroseismology if they were found, as they should be hotter and less crystallized than pulsating hydrogen-rich white dwarfs, something that would pave the way for probing their deep interiors. Aims. We explore the pulsational properties of ultra-massive helium-rich white dwarfs with carbon-oxygen and oxygen-neon cores resulting from single stellar evolution. Our goal is to provide a theoretical basis that could eventually help to discern the core composition of ultra-massive white dwarfs and the scenario of their formation through asteroseismology, anticipating the possible future detection of pulsations in helium-rich ultra-massive white dwarfs. Methods. We focus on three scenarios for the formation of helium-rich ultra-massive white dwarfs. First, we consider stellar models coming from two recently proposed single-star evolution scenarios for the formation of ultra-massive white dwarfs with carbon-oxygen cores that involve the rotation of the degenerate core after core helium burning and reduced mass-loss rates in massive asymptotic giant branch stars. Finally, we contemplate ultra-massive oxygen-neon core white-dwarf models resulting from standard single-star evolution. We compute the adiabatic pulsation gravity-mode periods for models in a range of effective temperatures, embracing the instability strip of average-mass pulsating helium-rich white dwarfs, and we compare the characteristics of the mode-trapping properties for models of different formation scenarios through the analysis of the period spacing. Results. Given that the white dwarf models coming from the three scenarios considered are characterized by distinct core chemical profiles, we find that their pulsation properties are also different, thus leading to distinctive signatures in the period-spacing and mode-trapping properties. Conclusions. Our results indicate that in the case of an eventual detection of pulsating ultra-massive helium-rich white dwarfs, it would be possible to derive valuable information encrypted in the core of these stars in connection with the origin of such exotic objects. This is of the utmost importance regarding recent evidence for the existence of a population of ultra-massive white dwarfs with carbon-oxygen cores. There will soon be many opportunities to detect pulsations in these stars through observations collected with ongoing space missions.