Search results for "Stars"
showing 10 items of 798 documents
Zvaigžņotā Debess: 2018, Vasara (240)
2018
Jauna hipotēze par I tipa pārnovu izcelšanos ; No 108 minūtēm līdz 96 dienām ; Maiņzvaigznes (astronomija skolā) ; O.Dumbrājs. Nēteres teorēmai 100 gadu ; K.Švarcs. Jauni novērojumi tālā un tuvā kvazārā ; I.P. Īsumā: ALMA un VLT iegūst liecību par zvaigžņu veidošanos tikai 250 miljonus gadu pēc Lielā Sprādziena ; I.Pundure. Lagūnas miglājs – zvaigžņu bērnistaba Habla skatījumā ; I.Pundure. ALMA un APEX atklāj veidojošos galaktiku megasaplūšanu agrīnajā Visumā ; K.Salmiņš. LU Astronomijas institūts pirmais veicis nanopavadoņa S-NET lāzermērījumus ; J.Jaunbergs. Zeme atpakaļskata spogulī ; J.Jaunbergs. Sniegotā Helēne un mazais Polideiks ; R.Misa. Latvijas uzņēmums Eventech piedalīsies Luna-2…
Constraining the Equation of State of Neutron Stars with Genral Relativity
2005
When a radio pulsar breakes down by virtue of magnetodipole emission, its gravitational mass decreases accordingly. If the pulsar in hosted in a binary system, this mass loss will increase the orbital period of the system. We show that this relativistic effect can be indeed observable if the NS is fast and magnetized enough and that, if observed, it will help to put tight constraints on the equation of state of ultradense matter.
Is it possible to explore Peccei-Quinn axions from frequency dependence radiation dimming?
2011
Abstract We explore how the Peccei–Quinn (PQ) axion parameter space can be constrained by the frequency-dependence dimming of radiation from astrophysical objects. To do so we perform accurate calculations of photon–axion conversion in the presence of a variable magnetic field. We propose several tests where the PQ axion parameter space can be explored with current and future astronomical surveys: the observed spectra of isolated neutron stars, occultations of background objects by white dwarfs and neutron stars, the light-curves of eclipsing binaries containing a white dwarf. We find that the lack of dimming of the light-curve of a detached eclipsing white dwarf binary recently observed, l…
Study of the Ti44(α,p)V47 reaction and implications for core collapse supernovae
2014
The underlying physics triggering core collapse supernovae is not fully understood but observations of material ejected during such events helps to solve this puzzle. In particular, several satellite based γ-ray observations of the isotope 44Ti have been reported recently. Conveniently, the amount of this isotope in stellar ejecta is thought to depend critically on the explosion mechanism. The most influential reaction to the amount of 44Ti in supernovae is Ti44(α,p)V47. Here we report on a direct study of this reaction conducted at the REX-ISOLDE facility, CERN. The experiment was performed with a 44Ti beam at Elab = 2.16MeV/u, corresponding to an energy distribution, for reacting α-partic…
Stellar(n,γ)Cross Section ofNi62
2005
The 62Ni(n,gamma)63Ni(t(1/2)=100+/-2 yr) reaction plays an important role in the control of the flow path of the slow neutron-capture (s) nucleosynthesis process. We have measured for the first time the total cross section of this reaction for a quasi-Maxwellian (kT=25 keV) neutron flux. The measurement was performed by fast-neutron activation, combined with accelerator mass spectrometry to detect directly the 63Ni product nuclei. The experimental value of 28.4+/-2.8 mb, fairly consistent with a recent calculation, affects the calculated net yield of 62Ni itself and the whole distribution of nuclei with 62<A<90 produced by the weak s process in massive stars.
N22e(α,n)M25g: The Key Neutron Source in Massive Stars
2001
The excitation function of the reaction (22)Ne(alpha,n)(25)Mg, the key neutron source in the astrophysical s process in massive stars, has been determined from threshold at E(alpha) = 570 up to 1450 keV with an experimental sensitivity of 10(-11) b. For all resonances in this energy range new resonance parameters have been measured. For a possible resonance at about 635 keV a new upper limit omega gamma < 60 neV for the strength was obtained. Based on the new data, improved reaction rates were calculated as a function of temperature. The new uncertainty limits are considerably smaller than in previous determinations, ruling out the large enhancement factors, up to 500, assumed in some stell…
Nuclear-structure input to r-process calculations
1998
Abstract By comparing results from r-process parameter studies within the classical waiting-point approximation with the observed solar-system r-abundance distribution N r ,⊙ , in-formation on nuclear-structure phenomena on neutron-rich isotopes far off the stability line can be obtained. Such information is — to a large extent — experimentally not accessible. Investigations in the early 1990's, making use of QRPA β-decay properties and nuclear masses from the macroscopic-microscopic FRDM and ETFSI-1 models, showed r-abundance deficiencies around A ⋍115 and 180 , indicating overly strong N=82 and 126 shell strengths far from stability. Recent experiments in the A ⋍110–130 region as well as …
Plasma effects on neutrino-antineutrino synchrotron radiation in dense matter
2000
Abstract We consider plasma effects on ν ν synchroton radiation from ultrarelativistic, degenerate electrons in neutron stars with strong magnetic fields. Under typical conditions for such stars, a semiclassical calculation can be performed, in which electrons are considered to move along classical paths. We show that collective effects in the plasma lead to an essential enhancement (about three times) of the vector weak-current contributions to neutrino pair emissivity.
β-decay studies of r-process nuclei at NSCL
2008
Abstract Observed neutron-capture elemental abundances in metal-poor stars, along with ongoing analysis of the extremely metal-poor Eu-enriched sub-class provide new guidance for astrophysical models aimed at finding the r-process sites. The present paper emphasizes the importance of nuclear physics parameters entering in these models, particularly β -decay properties of neutron-rich nuclei. In this context, several r-process motivated β -decay experiments performed at the National Superconducting Cyclotron Laboratory (NSCL) are presented, including a summary of results and impact on model calculations.
β-Decay Half-Lives of 110 Neutron-Rich Nuclei across theN=82Shell Gap: Implications for the Mechanism and Universality of the AstrophysicalrProcess
2015
The $\ensuremath{\beta}$-decay half-lives of 110 neutron-rich isotopes of the elements from $_{37}\mathrm{Rb}$ to $_{50}\mathrm{Sn}$ were measured at the Radioactive Isotope Beam Factory. The 40 new half-lives follow robust systematics and highlight the persistence of shell effects. The new data have direct implications for $r$-process calculations and reinforce the notion that the second ($A\ensuremath{\approx}130$) and the rare-earth-element ($A\ensuremath{\approx}160$) abundance peaks may result from the freeze-out of an $(n,\ensuremath{\gamma})\ensuremath{\rightleftarrows}(\ensuremath{\gamma},n)$ equilibrium. In such an equilibrium, the new half-lives are important factors determining t…