arXiv subjects

Demosthenes Kazanas

Publications and source records attributed to Demosthenes Kazanas.

At least 37 records · Page 2Linked to original sources

arXiv2021

On the observed clustering of major bodies in solar and extrasolar subsystems

Major (exo)planetary and satellite bodies seem to concentrate at intermediate areas of the radial distributions of all the objects present in each (sub)system. We prove rigorously that the secular evolution of (exo)planets and satellites necessarily results in the observed intermediate accumulation of the massive objects in all such subsystems. We quantify a "middle" as the mean of mean motions (orbital angular velocities) of three or more massive objects involved. Orbital evolution is expected to be halted or severely diminished when the survivors settle near mean-motion resonances and substantial angular-momentum transfer between bodies ceases to occur (gravitational Landau damping). The dynamics is opposite in direction to what has been theorized for viscous and magnetized accretion disks in which gas spreads out and away from either side of any conceivable intermediate area. The results are bound to change the way we think about planet and moon formation and evolution.

astro-ph.EP↗

arXiv2020

A variable magnetic disc wind in the black hole X-ray binary GRS 1915+105?

GRS 1915+105 being one of the brightest transient black hole binary (BHB) in the X-rays, offers a unique test-bed for the study of the connection between accretion and ejection mechanisms in BHBs. In particular, this source can be used to study the accretion disc wind and how it depends on the state changes in BHBs. Our aim is to investigate the origin and geometry of the accretion disc wind in GRS 1915+105. We analysed the spectra of GRS 1915+105 in the soft $ϕ$ and hard $χ$ classes, using the high resolution spectroscopy offered by Chandra HETGS. In the soft state, we find a series of wind absorption lines that follow a non linear dependence of velocity width, velocity shift and equivalent width with respect to ionisation, indicating a multiple component or stratified outflow. In the hard state we find only a faint Fe XXVI absorption line. We model the absorption lines in both the states using a dedicated MHD wind model to investigate a magnetic origin of the wind and to probe the cause of variability in the observed lines flux between the two states. The MHD disc wind model provides a good fit for both states, indicating the possibility of a magnetic origin of the wind. The multiple ionisation components of the wind are well characterised as a stratification of the same magnetic outflow. We find that the observed variability in the lines flux between soft and hard states cannot be explained by photo-ionisation alone but it is most likely due to a large (three orders of magnitude) increase in the wind density. We find the mass outflow rate of the wind to be comparable to the accretion rate, suggesting a intimate link between accretion and ejection processes that lead to state changes in BHBs.

astro-ph.HE↗

arXiv2020

The Multipolar Magnetic Field of Millisecond Pulsar PSR J0030+0451

Modeling of the NICER X-ray waveform of the pulsar PSR J0030+0451, aimed to constrain the neutron star mass and radius, has inferred surface hot-spots (the magnetic polar caps) that imply significantly non-dipolar magnetic fields. To this end, we investigate magnetic field configurations that comprise offset dipole plus quadrupole components using static vacuum field and force-free global magnetosphere models. Taking into account the compactness and observer angle values provided by Miller et al. (2019) and Riley et al. (2019), we compute geodesics from the observer plane to the polar caps to compute the resulting X-ray light curve. We explore, through Markov chain Monte Carlo techniques, the detailed magnetic field configurations that can reproduce the observed X-ray light curve and have discovered degeneracies, i.e., diverse field configurations, which can provide sufficient descriptions to the NICER X-ray waveforms. Having obtained the force-free field structures, we then compute the corresponding synchronous gamma-ray light curves following Kalapotharakos et al. (2014) these we compare to those obtained by Fermi-LAT, to provide models consistent with both the X-ray and the gamma-ray data, thereby restricting further the multipole field parameters. An essential aspect of this approach is the proper computation of the relative phase between the synchronous X- and gamma-ray light curves. We conclude with a discussion of the broader implications of our study.

astro-ph.HE↗

arXiv2020

The Fast Radio Burst Luminosity Function and Death Line in the Low-Twist Magnetar Model

We explore the burst energy distribution of fast radio bursts (FRBs) in the low-twist magnetar model of Wadiasingh and Timokhin (2019). Motivated by the power-law fluence distributions of FRB 121102, we propose an elementary model for the FRB luminosity function of individual repeaters with an inversion protocol which directly relates the power-law distribution index of magnetar short burst fluences to that for FRBs. The protocol indicates the FRB energy scales virtually linearly with crust/field dislocation amplitude, if magnetar short bursts prevail in the magnetoelastic regime. Charge starvation in the magnetosphere during bursts (required in WT19) for individual repeaters implies the predicted burst fluence distribution is narrow, $\lesssim 3$ decades for yielding strains and oscillation frequencies feasible in magnetar crusts. Requiring magnetic confinement and charge starvation, we obtain a death line for FRBs which segregates magnetars from the normal pulsar population, suggesting only the former will host recurrent FRBs. We convolve the burst energy distribution for individual magnetars to define the distribution of luminosities in evolved magnetar populations. The broken power-law luminosity function's low energy character depends on the population model, while the high energy index traces that of individual repeaters. Independent of the evolved population, the broken power-law isotropic-equivalent energy/luminosity function peaks at $\sim10^{37}-10^{40}$ erg with a low-energy cutoff at $\sim 10^{37}$ erg. Lastly, we consider the local fluence distribution of FRBs, and find that it can constrain the subset of FRB-producing magnetar progenitors. Our model suggests that improvements in sensitivity may reveal flattening of the global FRB fluence distribution and saturation in FRB rates.

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arXiv2019

Generation of massive stellar black holes by rapid gas accretion in primordial dense clusters

Supernova theory suggests that black holes of a stellar origin cannot attain masses in the range of 50-135 solar masses in isolation. We argue here that this mass gap is filled in by black holes that grow by gas accretion in dense stellar clusters, such as protoglobular clusters. The accretion proceeds rapidly, during the first 10 megayears of the cluster life, before the remnant gas is depleted. We predict that binaries of black holes within the mass gap can be observed by LIGO.

astro-ph.GA↗

arXiv2019

Conformal Weyl gravity and perihelion precession

We investigate the perihelion shift of planetary motion in conformal Weyl gravity using the metric of the static, spherically symmetric solution discovered by Mannheim \& Kazanas (1989). To this end we employ a procedure similar to that used by Weinberg for the Schwarzschild solution, which has also been used recently to study the solar system effects of the cosmological constant $Λ$. We show that besides the general relativistic terms obtained earlier from the Schwarzschild - de Sitter solution, the expression for the perihelion shift includes a negative contribution which arises from the linear term $γr$ in the metric. Using data for perihelion shift observations we obtain constraints on the value of the constant $γ$ similar to that obtained earlier using galactic rotational curves.

gr-qc↗

arXiv2019

Ellis wormhole without a phantom scalar field

In this paper, we present an exact solution for $(3+1)$-dimensional Einstein-scalar-Gauss-Bonnet theory (EsGB) in electrovacuum. The solution is characterized by only one parameter, $Q$, which in general can be associated with the electromagnetic field and the scalar field. We show that the solution corresponds to a charged wormhole with throat at the region $r = |Q|$, and is also supported by a real scalar field having a positive kinetic term. We show that the solution belongs to the most general class of solutions known as Ellis wormholes but without the need for `exotic matter' or a phantom scalar field.

gr-qc↗

arXiv2019

A Fundamental Plane for Gamma-Ray Pulsars

We show that the $γ$-ray pulsar observables, i.e., their total $γ$-ray luminosity, $L_γ$, spectral cut-off energy, $ε_{\rm cut}$, stellar surface magnetic field, $B_{\star}$, and spin-down power $\dot{\mathcal{E}}$, obey a relation of the form $L_γ=f(ε_{\rm cut},B_{\star},\dot{\mathcal{E}})$, which represents a 3D plane in their 4D log-space. Fitting the data of 88 pulsars of the second Fermi pulsar catalog, we show this relation to be $L_γ\propto ε_{\rm cut}^{1.18\pm 0.24}B_{\star}^{0.17\pm 0.05}\dot{\mathcal{E}}^{0.41\pm 0.08}$, a pulsar fundamental plane (FP). We show that the observed FP is remarkably close to the theoretical relation $L_γ\propto ε_{\rm cut}^{4/3}B_{\star}^{1/6}\dot{\mathcal{E}}^{5/12}$ obtained assuming that the pulsar $γ$-ray emission is due to curvature radiation by particles accelerated at the pulsar equatorial current sheet just outside the light cylinder. Interestingly, the FP seems incompatible with emission by synchrotron radiation. The corresponding scatter about the FP is $\sim 0.35$dex and can only partly be explained by the observational errors while the rest is probably due to the variation of the inclination and observer angles. We predict also that $ε_{\rm cut}\propto \dot{\mathcal{E}}^{7/16}$ toward low $\dot{\mathcal{E}}$ for both young and millisecond pulsars implying that the observed death-line of $γ$-ray pulsars is due to $ε_{\rm cut}$ dropping below the Fermi-band. Our results provide a comprehensive interpretation of the observations of $γ$-ray pulsars, setting requirement for successful theoretical modeling.

astro-ph.HE↗

arXiv2019

Meta-analysis of Electron Cyclotron Resonance Absorption Features Detected in High-Mass X-ray Binaries

Using recent compilations of detailed X-ray observations and spectral models of exceptional quality, we record the electron cyclotron resonance absorption (ECRA) features that have been detected in 45 pulsating high-mass X-ray binaries (HMXBs) and ultraluminous X-ray (ULX) sources harboring neutron stars, although seven of these detections are still questionable and another 21 are single and/or not independently confirmed. From the comprehensive catalogs of Jaisawal \& Naik and Staubert et al. and from several additional recent observations, we produce two lists of HMXB ECRA sources: a list of 17 sources in which multiple ECRA lines or single very low-energy lines are seen, in which we can reasonably assume that the lowest energy reveals the fundamental cyclotron level for each source; and a `contaminated' list of 38 sources including the 21 detections of single ECRA lines that may (not) be higher-level harmonics. Both lists confirm a previous result that we have obtained independently by modeling the propeller lines of Magellanic HMXB pulsars: the surface dipolar magnetic fields $B_*$ of HMXB neutron stars are segregated around five distinct values with $B_* = 0.28\pm 0.08, 0.55\pm 0.11, 1.3\pm 0.37, 3.0\pm 0.68$, and $7.9\pm 3.1$, in units of TG. An explanation of this phenomenon is currently lacking. We have found no correlation between these $B_*$ values and the corresponding observed spin periods, spin period derivatives, orbital periods, maximum X-ray luminosities, neutron star masses, or companion star masses.

astro-ph.HE↗

arXiv2019

Universal expansion with spatially varying $G$

We calculate the expansion of the universe under the assumptions that $G$ varies in space and the radial size $r$ of the universe is very large (we call this the MOND regime of varying-$G$ gravity). The inferred asymptotic behavior turns out to be different than that found by McCrea & Milne in 1934 and our equations bear no resemblance to those of the relativistic case. In this cosmology, the scale factor $R(t)$ increases linearly with time $t$, the radial velocity is driven by inertia, and gravity is incapable of hindering the expansion. Yet, Hubble's law is borne out without any additional assumptions. When we include a repulsive acceleration $a_{\rm de}$ due to dark energy, the resulting universal expansion is then driven totally by this new term and the solutions for $a_{\rm de}\to 0$ do not reduce to those of the $a_{\rm de}\equiv 0$ case. This is a realization of a new Thom catastrophe: the inclusion of the new term destroys the conservation of energy and the results are not reducible to the previous case in which energy is conserved.

gr-qc↗

arXiv2019

On the formation of our solar system and many other protoplanetary systems observed by ALMA and SPHERE

In view of the many recent observations conducted by ALMA and SPHERE, it is becoming clear that protoplanetary disks form planets in narrow annular gaps at various distances from the central protostars before these protostars are actually fully formed and the gaseous disks have concluded their accretion/dispersal processes. This is in marked contrast to the many multi-planet exoplanetary systems that do not conform to this pristine picture. This major discrepancy calls for an explanation. We provide such an explanation in this work, based on analytical solutions of the cylindrical isothermal Lane-Emden equation with rotation which do not depend on boundary conditions. These ``intrinsic'' solutions of the differential equation attract the solutions of the Cauchy problem and force them to oscillate permanently. The oscillations create density maxima in which dust and planetesimals are trapped and they can form protoplanetary cores during the very early isothermal evolution of such protoplanetary nebulae. We apply this model to our solar nebula that formed in-situ a minimum of eleven protoplanetary cores that have grown to planets which have survived undisturbed to the present day. We are also in the process of applying the same model to the ALMA/DSHARP disks.

astro-ph.EP↗

arXiv2019

Conundrums and constraints concerning the formation of our solar system -- An alternative view

We have proposed an alternative model for the formation of our solar system that does not predict any mean-motion resonant interactions, planetary migrations, or self-gravitating instabilities in the very early isothermal solar nebula and before the protosun has formed. Within this context of nonviolent protoplanetary evolution over more than 10 million years, we examine some conundrums and constraints that have been discovered from studies of small bodies in the present-day solar system (Jupiter and Neptune's Trojans and their differences from Kuiper belt objects, the irregular satellites of gaseous giants, the stability of the main asteroid belt, and the Late Heavy Bombardment). These issues that have caused substantial difficulties to models of violent formation do not appear to be problematic for the alternative model, and the reason is the complete lack of violent events during the evolution of protoplanets.

astro-ph.EP↗

arXiv2019

Models of a protoplanetary disk forming in-situ the major Uranian satellites before the planet is formed

We fit an isothermal oscillatory density model of Uranus' protoplanetary disk to the present-day major satellites and we determine the radial scale length of the disk, the equation of state and the central density of the primordial gas, and the rotational state of the Uranian nebula. This disk does not at all look like the Jovian disk that we modeled previously. Its rotation parameter that measures centrifugal support against self-gravity is a lot smaller ($β_0=0.00507$), as is the radial scale length (only 27.6 km) and the size of the disk (only 0.60 Gm). On the other hand, the central density of the compact Uranian core is higher by a factor of 180 and its core's angular velocity is about 2.3 times that of Jupiter's core (a rotation period of 3.0 d as opposed to 6.8 d). Yet, the rotation of the disk is sufficiently slow to guarantee its long-term stability against self-gravity induced instabilities for millions of years.

astro-ph.EP↗

arXiv2019

Model of Neptune's protoplanetary disk forming in-situ its surviving regular satellites after Triton's capture and comparison of the protoplanetary disks of the four gaseous giants

We fit an isothermal oscillatory density model of Neptune's protoplanetary disk to the surviving regular satellites and its innermost ring and we determine the radial scale length of the disk, the equation of state and the central density of the primordial gas, and the rotational state of the Neptunian nebula. Neptune's regular moons suffered from the retrograde capture of Triton that disrupted the system. Some moons may have been ejected, while others may have survived inside their potential minima. For this reason, the Neptunian nebula does not look like any of the nebulae that we modeled previously. In particular, there must be two density maxima deep inside the core of the nebula where no moons or rings are found nowadays. Even with this strong assumption, the recent discovery of the minor moon N XIV complicates further the modeling effort. With some additional assumptions, the Neptunian nebula still shares many similarities with the Uranian nebula, as was expected from the relative proximity and similar physical conditions of the two systems. For Neptune's primordial disk, we find a steep power-law index ($k=-3.0$), needed to accommodate the arrangement of the outer moons Larissa, N XIV, and Proteus. The rotation parameter that measures centrifugal support against self-gravity is quite small ($β_0=0.00808$), as is its radial scale length (13.6 km). The extent of the disk ($R_{\rm max}=0.12$ Gm) is a lot smaller than that of Uranus ($R_{\rm max}=0.60$ Gm) and Triton appears to be responsible for the truncation of the disk. The central density of the compact Neptunian core and its angular velocity are higher than but comparable to those of Uranus' core. In the end, we compare the models of the protoplanetary disks of the four gaseous giants.

astro-ph.EP↗

arXiv2019

Models of Saturn's protoplanetary disk forming in-situ its regular satellites and innermost rings before the planet is formed

We fit an isothermal oscillatory density model of Saturn's protoplanetary disk to the present-day major satellites and innermost rings D/C and we determine the radial scale length of the disk, the equation of state and the central density of the primordial gas, and the rotational state of the Saturnian nebula. This disk does not look like the Jovian and Uranian disks that we modeled previously. Its power-law index is extremely steep ($k=-4.5$) and its radial extent is very narrow ($ΔR\lesssim 0.9$ Gm), its rotation parameter that measures centrifugal support against self-gravity is somewhat larger ($β_0=0.0431$), as is its radial scale length (395 km); but, as was expected, the size of the Saturnian disk, $R_{\rm max}=3.6$ Gm, takes just an intermediate value. On the other hand, the central density of the compact Saturnian core and its angular velocity are both comparable to that of Jupiter's core (density of $\approx 0.3$~g~cm$^{-3}$ in both cases, and rotation period of 5.0 d versus 6.8 d); and significantly less than the corresponding parameters of Uranus' core. As with the other primordial nebulae, this rotation is sufficiently slow to guarantee the disk's long-term stability against self-gravity induced instabilities for millions of years of evolution.

astro-ph.EP↗

arXiv2019

Theoretical models of the protostellar disks of AS 209 and HL Tau presently forming in-situ planets

We fit an isothermal oscillatory density model to two ALMA/DSHARP-observed disks, AS 209 and HL Tau, in which planets have presumably already formed and they are orbiting within the observed seven dark gaps in each system. These large disks are roughly similar to our solar nebula, albeit they exhibit milder radial density profiles and they enjoy lower centrifugal support. We find power-law density profiles with index $k=0.0$ (radial densities $ρ(R) \propto R^{-1}$) and centrifugal support against self-gravity so small that it guarantees dynamical stability for millions of years of evolution. The scale lengths of the models differ only by a factor of 1.9, but the inner cores of the disks are very different: HL Tau's core is 8.0 times larger and 3.6 times denser than the core of AS 209. This results in four dark gaps having formed within the core of HL Tau, whereas no dark gap is found in the core of AS 209. On the other hand, the Jeans frequencies and the angular velocities of the cores are comparable to within factors of 1.9 and 1.6, respectively.

astro-ph.EP↗

arXiv2019

Preliminary model of the outer disk of RU Lup presently showing only four dark gaps

Recent ALMA/DSHARP observations reveal that the disk of RU Lup shows presently only four dark gaps at large radii in which planets are presumably already orbiting. This gap arrangement is quite similar to the current structure of the outer planets in our solar system. Although there may be more undetected planets forming in the inner disk of RU Lup, the information obtained from the current observations prompted us to fit preliminary models with an isothermal oscillatory density profile to the outer disk of RU Lup. The best-fit model confirms that several physical properties of the disk of RU Lup are comparable to those of our solar nebula to within factors of 1.0-2.3; but there are some differences as well: central density ($ρ_0$), centrifugal support ($β_0$), and core angular velocity ($Ω_0$) in RU Lup are smaller by factors of 5.5, 8.0, and 18, respectively, whereas the inner core radius ($R_1$) of RU Lup is 25 times larger. The smaller values found for RU Lup are linked to the (apparently gap-empty) inner 10 AU of its disk, a region that is presently inadequately resolved.

astro-ph.EP↗

arXiv2019

Theoretical model of the outer disk of TW Hya presently forming in-situ planets and comparison with models of AS 209 and HL Tau

We fit an isothermal oscillatory density model to the outer disk of TW Hya in which planets have presumably already formed and they are orbiting within four observed dark gaps. At first sight, this 52 AU small disk does not appear to be similar to our solar nebula; it shows several physical properties comparable to those in HL Tau (size $R_{\rm max}=102$ AU) and very few similarities to AS 209 ($R_{\rm max}=144$ AU). We find a power-law density profile with index $k=-0.2$ (radial densities $ρ(R) \propto R^{-1.2}$) and centrifugal support against self-gravity so small that it virtually guarantees dynamical stability for millions of years of evolution to come. Compared to HL Tau, the scale length $R_0$ and the core size $R_1$ of TW Hya are smaller only by factors of $\sim$2, reflecting the disk's half size. On the opposite end, the Jeans frequency $Ω_J$ and the angular velocity $Ω_0$ of the smaller core of TW Hya are larger only by factors of $\sim$2. The only striking difference is that the central density ($ρ_0$) of TW Hya is 5.7 times larger than that of HL Tau, which is understood because the core of TW Hya is only half the size ($R_1$) of HL Tau and about twice as heavy ($Ω_J$). In the end, we compare the protostellar disks that we have modeled so far.

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