Search results for "Stars"
showing 10 items of 798 documents
Nonlinear dynamics of spinning bosonic stars: formation and stability
2019
We perform numerical evolutions of the fully non-linear Einstein-(complex, massive)Klein-Gordon and Einstein-(complex)Proca systems, to assess the formation and stability of spinning bosonic stars. In the scalar/vector case these are known as boson/Proca stars. Firstly, we consider the formation scenario. Starting with constraint-obeying initial data, describing a dilute, axisymmetric cloud of spinning scalar/Proca field, gravitational collapse towards a spinning star occurs, via gravitational cooling. In the scalar case the formation is transient, even for a non-perturbed initial cloud; a non-axisymmetric instability always develops ejecting all the angular momentum from the scalar star. I…
Astrophysical constraints on extended gravity models
2015
We investigate the propagation of gravitational waves in the context of fourth order gravity nonminimally coupled to a massive scalar field. Using the damping of the orbital period of coalescing stellar binary systems, we impose constraints on the free parameters of extended gravity models. In particular, we find that the variation of the orbital period is a function of three mass scales which depend on the free parameters of the model under consideration; we can constrain these mass scales from current observational data.
Merger of dark matter axion clumps and resonant photon emission
2020
A portion of light scalar dark matter, especially axions, may organize into gravitationally bound clumps (stars) and be present in large number in the galaxy today. It is therefore of utmost interest to determine if there are novel observational signatures of this scenario. Work has shown that for moderately large axion-photon couplings, such clumps can undergo parametric resonance into photons, for clumps above a critical mass $M^{\star}_c$ determined precisely by some of us in Ref. [1]. In order to obtain a clump above the critical mass in the galaxy today would require mergers. In this work we perform full 3-dimensional simulations of pairs of axion clumps and determine the conditions un…
Stellar structure models in modified theories of gravity: Lessons and challenges
2019
The understanding of stellar structure represents the crossroads of our theories of the nuclear force and the gravitational interaction under the most extreme conditions observably accessible. It provides a powerful probe of the strong field regime of General Relativity, and opens fruitful avenues for the exploration of new gravitational physics. The latter can be captured via modified theories of gravity, which modify the Einstein-Hilbert action of General Relativity and/or some of its principles. These theories typically change the stellar structure equations, thus having a large impact on the astrophysical properties of the corresponding stars and opening a new window to constrain these …
Lensing and dynamics of ultracompact bosonic stars
2017
Spherically symmetric bosonic stars are one of the few examples of gravitating solitons that are known to form dynamically, via a classical process of (incomplete) gravitational collapse. As stationary solutions of the Einstein--Klein-Gordon or the Einstein--Proca theory, bosonic stars may also become sufficiently compact to develop light rings and hence mimic, in principle, gravitational-wave observational signatures of black holes (BHs). In this paper, we discuss how these horizonless ultra-compact objects (UCOs) are actually distinct from BHs, both phenomenologically and dynamically. In the electromagnetic channel, the light ring associated phenomenology reveals remarkable lensing patter…
Temperature dependence of sodium and ionized calcium resonance lines perturbed by helium
2014
International audience; Traces of heavy metals in cool DZ white dwarf stars may be attributed to the accretion of circumstellar dust thought to originate from tidal disruption of rocky parent bodies. Spectra of such stars therefore provide a unique opportunity to study the composition of extrasolar planetary systems. The determination of metal abundances from stellar spectra depends on stellar atmospheric parameters and an accurate prior knowledge of the collision broadening of the line profiles by the most common constituents of the stellar atmosphere. For this purpose, we present theoretical absorption spectra of Na and Ca+ broadened by He for the conditions prevailing in cool white dwarf…
La voz de una autoridad heterodoxa sobre el amor en el primer franquismo. Los consultorios sentimentales de Amparo Rivelles
2021
Entre 1948 y 1952, el semanario cinematográfico Primer plano publicó un consultorio sentimental firmado por la actriz Amparo Rivelles. La estrella española más popular del momento se convertía así en consejera sobre la moral, el noviazgo y los usos amorosos entre los jóvenes; una cuestión que el Régimen consideraba como una pieza fundamental en la regulación de las relaciones de género. Se trata de un consultorio singular, alejado de la gravedad, el dramatismo y la grandilocuencia de otros, surgidos en diversos medios periodísticos a raíz de la buena acogida de público que estas secciones gozaron durante aquellos años. El presente artículo analiza las implicaciones de unos textos en los que…
Das Werden des Kosmos Von der Erfahrung der zeitlichen Dimension astronomischer Objekte im 18. Jahrhundert
1985
The Permanent ‘Becoming’ of the Cosmos: On Experiencing the Time Dimension of Astronomical Entities in the 18th Century. - This paper deals with two of the initial stages through which the dimension of time, in the sense of an irreversible development, found its way into astronomical-cosmological thinking. The one resulted from the first consequental application of Newtonian principles and laws to cosmic entities outside of our solar system found in the General Natural History or Theory of the Heavens of Immanuel Kant (1755): Endeavoring to explain through natural causes first the peculiarities of the solar system, no longer naturally explainable through the celestial mechanics of Isaac New…
Long-term X-ray variability of the microquasar system LS 5039/RX J1826.2−1450
2003
We report on the results of the spectral and timing analysis of a BeppoSAX observation of the microquasar system LS 5039/RX J1826.2-1450. The source was found in a low-flux state with Fx(1-10 keV)= 4.7 x 10^{-12} erg cm^{-2} s^{-1}, which represents almost one order of magnitude lower than a previous RXTE observation 2.5 years before. The 0.1--10 keV spectrum is described by an absorbed power-law continuum with photon-number spectral index Gamma=1.8+-0.2 and hydrogen column density of NH=1.0^{+0.4}_{-0.3} x 10^{22} cm^{-2}. According to the orbital parameters of the system the BeppoSAX observation covers the time of an X-ray eclipse should one occur. However, the 1.6-10 keV light curve does…
The second data release of the INT Photometric Hα Survey of the Northern Galactic Plane (IPHAS DR2)
2014
The INT/WFC Photometric H-Alpha Survey of the Northern Galactic Plane (IPHAS) is a 1800 square degrees imaging survey covering Galactic latitudes |b| < 5 deg and longitudes l = 30 to 215 deg in the r, i and H-alpha filters using the Wide Field Camera (WFC) on the 2.5-metre Isaac Newton Telescope (INT) in La Palma. We present the first quality-controlled and globally-calibrated source catalogue derived from the survey, providing single-epoch photometry for 219 million unique sources across 92% of the footprint. The observations were carried out between 2003 and 2012 at a median seeing of 1.1 arcsec (sampled at 0.33 arcsec/pixel) and to a mean 5\sigma-depth of 21.2 (r), 20.0 (i) and 20.3 (H-a…