Inverse-Chirp Imprint of Gravitational Wave Signals in Scalar Tensor Theory

The scalar tensor theory contains a coupling function connecting the quantities in the Jordan and Einstein frames, which is constrained to guarantee a transformation rule between frames. We simulate the supernovae core collapse with different choices of coupling functions defined over the viable region of the parameter space and find that a generic inverse-chirp feature of the gravitational waves in the scalar tensor scenario.Comment: 13 pages, 4 figures, revised version accepted by EPJ

Tidal Effects in Pre-merger Neutron Stars and Dynamics of Scalarized Compact Objects

In the first part of this thesis, we investigate some tidal phenomena in the pre-merger stage of coalescing binaries with at least one neutron star (NS) involved. In particular, during the last few minutes of coalescences, the tidal field exerted by the companion can force the primary strongly to excite certain quasi-normal modes thus resulting in various observable effects. Among other things, resonance of low frequency modes (e.g., $g$- and $i$-modes) may result in crustal fracture, whereby unleash the energy used to be stored in the cracked area, possibly constituting a pre-emission of short gamma-ray burst (SGRB) if the NS is highly magnetised. In particular, we find it possible to associate two pre-emissions of SGRB 090510 with the resonantly excited $g_1$- and $g_2$-modes. We present, in addition, that the inferred frequencies of these two $g$-modes provide a novel avenue to estimate the spin of the NS, which can be applied to any SGRB preceded by two or more precursors. This presumably, $g$-mode-related phenomenon can also benefit in constraining the equation of state (EOS) since the EOS candidates can be grouped in terms of $g$-mode frequency. On the other hand, $f$-mode excitation accelerates the merger course, leading to a ``tidal plunge'' phase; thereby, a phase shift is rendered in the associated gravitational waveform, which dictates the evolutionary track of the binary. Although the adiabatic tide attributes much more to the phase shift than the dynamical ones if the NS rotates slowly, the situation for fast spinning stars is different: a few hundred radiants of shift may be rendered. The second part of this thesis is dedicated to the study of the dynamics of compact objects, viz.~NSs and black holes (BHs), in alternative gravity theories in the strong gravity regime. In particular, we consider some theories involving scalar field(s) as additional mediator(s) of gravitational interaction such as the (multi-)scalar-tensor theory and scalar-Gauss-Bonnet theory. In the former theory, it can happen that the scalar field of static stars dies out in the power of $-2$ of the distance, suppressing the scalar dipole radiation thus not constrained by pulsar experiments. In addition, these solutions are of discrete topological types, characterised by topological charge. For the zero charge configurations, we show that up to three stable stars exist for a certain range of central energy density, and the stability is lost right at the occurrence of the most massive (either scalarized or non-scalarized) star. Accretions may therefore bring a stable scalarized NS into an unstable state, where a descalarization would be triggered, generating the gravitational phase transition (PT). This novel kind of PT leads to a sudden shrink in size of the star, mimicking well the traditional, material PT. However, the former transition will be accompanied by scalar-induced gravitational waves that are absent in material PT. In addition to the accreting process, we consider the spherically-symmetric core collapse for the scalar-tensor and the scalar-Gauss-Bonnet theories. Although a scalarized BH is absent in the former theory due to no-hair reason, we can construct one in the latter theory. In particular, we numerically demonstrate scalarization in a remnant BH behind stellar collapse, giving a first example on the production channel for scalarized BHs in the scalar-Guass-Bonnet theory. The scalar-induced gravitational waves generated along with (de)scalarization in both theories are also discussed

The Last Three Seconds: Packed Message delivered by Tides in Binary Neutron Star Mergers

It is known that the leading-order tidal effects in gravitational waveforms can be quantified by tidal deformability, while higher order terms, e.g., harmonic overtones of Love number and dynamical tides, have not been well-investigated yet. The concept of a ``form factor'', which is different from while resembles the effective tidal deformability, for the tidal interactions between neutron stars in coalescing binaries is illustrated here. The form factor effectively incorporates the contribution of dynamical tides. The dependence of tidal form factor on tidal deformability, spins, and inclination angles is modeled and expressed in a closed form.Comment: 8 pages, 2 figures, 3 tables, 1 appendix. Accepted for publication in PR

Measuring spin in coalescing binaries of neutron stars showing double precursors

Gamma-ray bursts resulting from binary neutron-star mergers are sometimes preceded by precursor flares. These harbingers may be ignited by quasi-normal modes, excited by orbital resonances, shattering the stellar crust of one of the inspiralling stars up to $\gtrsim10$ seconds before coalescence. In the rare case that a system displays two precursors, successive overtones of either interface- or $g$-modes may be responsible for the overstrainings. Since the free-mode frequencies of these overtones have an almost constant ratio, and the inertial-frame frequencies for rotating stars are shifted relative to static ones, the spin frequency of the flaring component can be constrained as a function of the equation of state, the binary mass ratio, the mode quantum numbers, and the spin-orbit misalignment angle. As a demonstration of the method, we find that the precursors of GRB090510 hint at a spin frequency range of $2 \lesssim \nu_{\star}/\text{Hz} \lesssim 20$ for the shattering star if we allow for an arbitrary misalignment angle, assuming $\ell=2$ $g$-modes account for the events.Comment: 11 pages, 6 figures, 2 tables, with an appendix containing 1 figur

General-relativistic treatment of tidal gg-mode resonances in coalescing binaries of neutron stars. II. As triggers for precursor flares of short gamma-ray bursts

In some short gamma-ray bursts, precursor flares occurring $\sim$ seconds prior to the main episode have been observed. These flares may then be associated with the last few cycles of the inspiral when the orbital frequency is a few hundred Hz. During these final cycles, tidal forces can resonantly excite quasi-normal modes in the inspiralling stars, leading to a rapid increase in their amplitude. It has been shown that these modes can exert sufficiently strong strains onto the neutron star crust to instigate yieldings. Due to the typical frequencies of $g$-modes being $\sim 100\text{ Hz}$, their resonances with the orbital frequency match the precursor timings and warrant further investigation. Adopting realistic equations of state and solving the general-relativistic pulsation equations, we study $g$-mode resonances in coalescing quasi-circular binaries, where we consider various stellar rotation rates, degrees of stratification, and magnetic field structures. We show that for some combination of stellar parameters, the resonantly excited $g_1$- and $g_2$-modes may lead to crustal failure and trigger precursor flares.Comment: 14 pages, 8 figures, 2 tables, submitted to MNRA

Binary neutron star mergers in massive scalar-tensor theory: Quasi-equilibrium states and dynamical enhancement of the scalarization

We study quasi-equilibrium sequences of binary neutron stars in the framework of Damour-Esposito-Farese-type scalar-tensor theory of gravity with a massive scalar field, paying particular attention to the case where neutron stars are already spontaneously scalarized at distant orbits, i.e., in the high coupling constant case. Although scalar effects are largely quenched when the separation $a$ is $\gtrsim 3$--$6$ times of the Compton length-scale that is defined by the scalar mass, we show that the interaction between the scalar fields of the two neutron stars generates a scalar cloud surrounding the binary at the price of orbital energy when $a \lesssim 3$--$6$ times of the Compton length-scale. This enables us to constrain the scalar mass $m_\phi$ from gravitational-wave observations of binary neutron star mergers by inspecting the dephasing due to such phenomenon. In particular, the event GW170817 is suggestive of a constraint of $m_\phi \gtrsim 10^{-11}$ eV and the coupling strength should be mild if the neutron stars in this system were spontaneously scalarized.Comment: 14 pages, 8 figures, 1 table. Submitted to PR

Modulating Microglia/Macrophage Activation by CDNF Promotes Transplantation of Fetal Ventral Mesencephalic Graft Survival and Function in a Hemiparkinsonian Rat Model

Parkinson's disease (PD) is characterized by the loss of dopaminergic neurons in substantia nigra pars compacta, which leads to the motor control deficits. Recently, cell transplantation is a cutting-edge technique for the therapy of PD. Nevertheless, one key bottleneck to realizing such potential is allogenic immune reaction of tissue grafts by recipients. Cerebral dopamine neurotrophic factor (CDNF) was shown to possess immune-modulatory properties that benefit neurodegenerative diseases. We hypothesized that co-administration of CDNF with fetal ventral mesencephalic (VM) tissue can improve the success of VM replacement therapies by attenuating immune responses. Hemiparkinsonian rats were generated by injecting 6-hydroxydopamine (6-OHDA) into the right medial forebrain bundle of Sprague Dawley (SD) rats. The rats were then intrastriatally transplanted with VM tissue from rats, with/without CDNF administration. Recovery of dopaminergic function and survival of the grafts were evaluated using the apomorphine-induced rotation test and smallanimal positron emission tomography (PET) coupled with [F-18] DOPA or [F-18] FE-PE2I, respectively. In addition, transplantation-related inflammatory response was determined by uptake of [F-18] FEPPA in the grafted side of striatum. Immunohistochemistry (IHC) examination was used to determine the survival of the grated dopaminergic neurons in the striatum and to investigate immune-modulatory effects of CDNF. The modulation of inflammatory responses caused by CDNF might involve enhancing M2 subset polarization and increasing fractal dimensions of 6-OHDA-treated BV2 microglial cell line. Analysis of CDNF-induced changes to gene expressions of 6-OHDA-stimulated BV2 cells implies that these alternations of the biomarkers and microglial morphology are implicated in the upregulation of protein kinase B signaling as well as regulation of catalytic, transferase, and protein serine/threonine kinase activity. The effects of CDNF on 6-OHDA-induced alternation of the canonical pathway in BV2 microglial cells is highly associated with PI3K-mediated phagosome formation. Our results are the first to show that CDNF administration enhances the survival of the grafted dopaminergic neurons and improves functional recovery in PD animal model. Modulation of the polarization, morphological characteristics, and transcriptional profiles of 6-OHDA-stimualted microglia by CDNF may possess these properties in transplantation-based regenerative therapies.Peer reviewe

Modulating Microglia/Macrophage Activation by CDNF Promotes Transplantation of Fetal Ventral Mesencephalic Graft Survival and Function in a Hemiparkinsonian Rat Model

Parkinson’s disease (PD) is characterized by the loss of dopaminergic neurons in substantia nigra pars compacta, which leads to the motor control deficits. Recently, cell transplantation is a cutting-edge technique for the therapy of PD. Nevertheless, one key bottleneck to realizing such potential is allogenic immune reaction of tissue grafts by recipients. Cerebral dopamine neurotrophic factor (CDNF) was shown to possess immune-modulatory properties that benefit neurodegenerative diseases. We hypothesized that co-administration of CDNF with fetal ventral mesencephalic (VM) tissue can improve the success of VM replacement therapies by attenuating immune responses. Hemiparkinsonian rats were generated by injecting 6-hydroxydopamine (6-OHDA) into the right medial forebrain bundle of Sprague Dawley (SD) rats. The rats were then intrastriatally transplanted with VM tissue from rats, with/without CDNF administration. Recovery of dopaminergic function and survival of the grafts were evaluated using the apomorphine-induced rotation test and small-animal positron emission tomography (PET) coupled with [18F] DOPA or [18F] FE-PE2I, respectively. In addition, transplantation-related inflammatory response was determined by uptake of [18F] FEPPA in the grafted side of striatum. Immunohistochemistry (IHC) examination was used to determine the survival of the grated dopaminergic neurons in the striatum and to investigate immune-modulatory effects of CDNF. The modulation of inflammatory responses caused by CDNF might involve enhancing M2 subset polarization and increasing fractal dimensions of 6-OHDA-treated BV2 microglial cell line. Analysis of CDNF-induced changes to gene expressions of 6-OHDA-stimulated BV2 cells implies that these alternations of the biomarkers and microglial morphology are implicated in the upregulation of protein kinase B signaling as well as regulation of catalytic, transferase, and protein serine/threonine kinase activity. The effects of CDNF on 6-OHDA-induced alternation of the canonical pathway in BV2 microglial cells is highly associated with PI3K-mediated phagosome formation. Our results are the first to show that CDNF administration enhances the survival of the grafted dopaminergic neurons and improves functional recovery in PD animal model. Modulation of the polarization, morphological characteristics, and transcriptional profiles of 6-OHDA-stimualted microglia by CDNF may possess these properties in transplantation-based regenerative therapies