What does a measurement of mass and/or radius of a neutron star constrain: Equation of state or gravity?

Neutron stars (NSs) are thought to be excellent laboratories for determining the equation of state (EoS) of cold dense matter. Their strong gravity suggests that they can also be used to constrain gravity models. The mass and radius (M-R) of a NS both depend on the choice of EoS and gravity, meaning that NSs cannot be simultaneously good laboratories for both of these questions. A measurement of M-R would constrain the less well known physics input. The assumption that M-R measurements can be used to constrain EoS-presumes general relativity (GR) is the ultimate model of gravity in the classical regime. We calculate the radial profile of compactness and curvature (square root of the full contraction of the Weyl tensor) within a NS and determine the domain not probed by the Solar System tests of GR. We find that, except for a tiny sphere of radius less than a millimeter at the center, the curvature is several orders of magnitude above the values present in Solar System tests. The compactness is beyond the solar surface value for r>10 m, and increases by 5 orders of magnitude towards the surface. With the density being only an order of magnitude higher than that probed by nuclear scattering experiments, our results suggest that the employment of GR as the theory of gravity describing the hydrostatic equilibrium of NSs is a rather remarkable extrapolation from the regime of tested validity, as opposed to that of EoS models. Our larger ignorance of gravity within NSs suggests that a measurement of M-R constrains gravity rather than EoS, and given that EoS has yet to be determined by nucleon scattering experiments, M-R measurements cannot tightly constrain the gravity models either. Near the surface the curvature and compactness attain their largest values, while EoS in this region is fairly well known. This renders the crust as the best site to look for deviations from GR.Comment: Phys.Rev. D published, typos corrected to match the published versio

Dünya öğretmeni Hasan Ali Yücel

Taha Toros Arşivi, Dosya No: 109-Hasan Ali-Can Yüce

Spin polarized tunneling in large area mesas of superconducting Bi2Sr2CaCu2O8+d for terahertz emission

Thesis (Master)--Izmir Institute of Technology, Physics, Izmir, 2010Includes bibliographical references (leaves: 70-77)Text in English; Abstract: Turkish and Englishxi, 77 leavesThere is an increasing interest in science and technology of electromagnetic waves in terahertz frequency range (0.1-10 THz) because of their emerging application areas including physics, biology, chemistry, astronomy, medicine etc. The observation on generation of THz radiation emitted from lateral dimension of high temperature superconductor (HTS) Bi2Sr2CaCu2O8+.. (Bi2212) and responses to THz waves increase the importance of these HTSs. Single crystal of HTS Bi2212 forms natural superconductor-insulator-superconductor (SIS) layered junctions, which are called intrinsic Josephson junctions (IJJ). The stacks of IJJs in Bi2212 can be used such a voltage-frequency converter and their large energy gap allows the emissions at THz frequency range. Recently, it has been demonstrated that rectangular IJJ mesa structures of Bi2212 can be used as a source of continuous, coherent and polarized THz radiation. It was shown that all THz emitting mesas are below a certain underdoped level, which has relatively small critical current in contrast to optimally doped and overdoped Bi2212. In this work, rectangular Au/Co/Au/Bi2212 mesa structures with large areas and high thicknesses were fabricated on as-grown Bi2212 single crystals using standard photolithograph and Ar ion beam etching techniques. In order to characterize the mesas, c-axis resistance versus temperature (R-T) and current-voltage (I-V) characteristics were investigated. During I-V characterization, Si composite bolometer was used to detect the emission. We obtained small critical current from as-grown mesas due to injection of spin polarized current. We observed THz emission peak for one of the mesas which has low quasiparticle conductivity and low dissipation due to its small critical current density. It means that the adjustment of doping level can be eliminated for THz emission by the injection of spin polarized current through the c-axes of the asgrown mesas

Determination of the antioxidant properties of ethanol and water extracts from different parts of Teucrium parviflorum Schreber

This study was designed to examine in vitro antioxidant of extracts of Teucrium parviflorum Schreber. The plants were extracted successively in ethanol and water using soxhlet-apparatus. They were screened for possible antioxidant activity using the ABTS• (2,2'-azino-bis-(3-ethylbenzothiazoline-6- sulphonic acid)) radical scavenging capacity, DPPH• (2,2-diphenyl-1-picrylhydrazyl) radical scavenging capacity, superoxide anion radical scavenging capacity, hydrogen peroxide scavenging capacity, reducing power and metal chelating activities. These various antioxidant activities were compared with standard antioxidants such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT) and α- tocopherol. The extracts of T. parviflorum showed strong antioxidant activity and effective antioxidant assay. This antioxidant property depends on concentration and increase with increased amount of sample. In addition, total phenolic compounds in the extracts of T. parviflorum were determined as pyrocatecol equivalents. The results obtained in the present study indicated that T. parviflorum is a potential source of natural antioxidant.Keywords: Teucrium parviflorum, antioxidant activity, medicinal plants, extract, flower, leaves