White dwarf stars as strange quark matter detectors - III

We continue the study of the properties of non-radial pulsations of strange dwarfs. These stars are essentially white dwarfs with a strange quark matter (SQM) core. We have previously shown that the spectrum of oscillations should be formed by several, well-detached clusters of modes inside which the modes are almost evenly spaced. Here, we study the relation between the characteristics of these clusters and the size of the SQM core. We do so assuming that, for a given cluster, the kinetic energy of the modes is constant. For a constant amplitude of the oscillation at the stellar surface, we find that the kinetic energy of the modes is very similar for the cases of models with LogQSQM = -2, -3 and -4, while it is somewhat lower for LogQSQM = -5 (here QSQM = MSQM/M; M SQM and M are the masses of the SQM core and the star, respectively). Remarkably, the shape (amplitude of the modes versus period of oscillation) of the clusters of periods is very similar. However, the number of modes inside each cluster is strongly (and non-monotonously) dependent upon the size of the SQM core. The characteristics of the spectrum of oscillations of strange dwarf stars are very different from the ones corresponding to normal white dwarfs and should be, in principle, observable. Consequently, the stars usually considered as white dwarfs may indeed provide an interesting and affordable way to detect SQM in an astrophysical environment.Facultad de Ciencias Astronómicas y Geofísica

A coordinated x-ray and optical campaign of the nearest massive eclipsing binary, δ orionis Aa. III. Analysis of optical photometric (MOST) and spectroscopic (ground-based) variations

We report on both high-precision photometry from the Microvariability and Oscillations of Stars (MOST) space telescope and ground-based spectroscopy of the triple system δ Ori A, consisting of a binary O9.5II+early-B (Aa1 and Aa2) with P = 5.7 days, and a more distant tertiary (O9 IV years). This data was collected in concert with X-ray spectroscopy from the Chandra X-ray Observatory. Thanks to continuous coverage for three weeks, the MOST light curve reveals clear eclipses between Aa1 and Aa2 for the first time in non-phased data. From the spectroscopy, we have a well-constrained radial velocity (RV) curve of Aa1. While we are unable to recover RV variations of the secondary star, we are able to constrain several fundamental parameters of this system and determine an approximate mass of the primary using apsidal motion. We also detected second order modulations at 12 separate frequencies with spacings indicative of tidally influenced oscillations. These spacings have never been seen in a massive binary, making this system one of only a handful of such binaries that show evidence for tidally induced pulsations.La lista completa de autores que integran el documento puede consultarse en el archivo.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

White dwarf stars as strange quark matter detectors - II

We study the properties of the non-radial pulsations of strange dwarf stars. These objects are white dwarfs (WDs) with a compact core made up of strange quark matter (SQM). We show that the SQM core compresses the surrounding normal matter strongly enough to give rise to the occurrence of a sharp peak in the Brunt-Väisälä frequency. This, in turn, allows for the existence of a completely new resonant cavity for gravity (g-) modes, which is absent in standard WDs. We study the cases in which the mass of the SQM core is 10-2, 10-3, 10-4 and 10-5 of the total stellar mass, which have been added to a 0.525 M⊙ WD model adequate to account for the period structure of the DAV G117B15A, showing that this new resonant cavity is present for such a large range of core mass fractions. Due to the extremely short wavelength of g-modes in the new resonant cavity, we treat oscillations there with an asymptotic analysis up to an intermediate, evanescent zone (located at ≈10 per cent of the stellar radius). At such a point, we consider the asymptotic treatment as a boundary condition for a self-consistent numerical calculation of the g-mode spectrum of oscillations. In particular, we consider dipolar oscillations, which are currently identified with the observed oscillations in standard WDs. We find a very distinctive signal for the presence of a SQM core inside a WD: the difference of periods between two consecutive modes is far shorter than it is in standard WDs due to the oscillations in the new resonant cavity, being even shorter than a second. This confirms previous expectations based on very simplified calculations. Our calculations indicate that, while the period spacing between consecutive modes is a smooth function of the period, the square of the amplitude of the modes near the SQM core is a strongly varying function. While some modes will have large amplitude there, and thus large kinetic energy, others will have far lower energy. Then, if (as usual) we assume that the excited modes are those with low kinetic energy, we expect a very particular spectrum of dipolar oscillations of WDs with SQM cores. The spectrum should be characterized by several well-detached sets of a very large number of evenly (in period) spaced modes. This should be considered as a clearly distinctive, observable signature of the presence of SQM inside WDs.Facultad de Ciencias Astronómicas y Geofísica

White dwarf stars as strange quark matter detectors - II

We study the properties of the non-radial pulsations of strange dwarf stars. These objects are white dwarfs (WDs) with a compact core made up of strange quark matter (SQM). We show that the SQM core compresses the surrounding normal matter strongly enough to give rise to the occurrence of a sharp peak in the Brunt-Väisälä frequency. This, in turn, allows for the existence of a completely new resonant cavity for gravity (g-) modes, which is absent in standard WDs. We study the cases in which the mass of the SQM core is 10-2, 10-3, 10-4 and 10-5 of the total stellar mass, which have been added to a 0.525 M⊙ WD model adequate to account for the period structure of the DAV G117B15A, showing that this new resonant cavity is present for such a large range of core mass fractions. Due to the extremely short wavelength of g-modes in the new resonant cavity, we treat oscillations there with an asymptotic analysis up to an intermediate, evanescent zone (located at ≈10 per cent of the stellar radius). At such a point, we consider the asymptotic treatment as a boundary condition for a self-consistent numerical calculation of the g-mode spectrum of oscillations. In particular, we consider dipolar oscillations, which are currently identified with the observed oscillations in standard WDs. We find a very distinctive signal for the presence of a SQM core inside a WD: the difference of periods between two consecutive modes is far shorter than it is in standard WDs due to the oscillations in the new resonant cavity, being even shorter than a second. This confirms previous expectations based on very simplified calculations. Our calculations indicate that, while the period spacing between consecutive modes is a smooth function of the period, the square of the amplitude of the modes near the SQM core is a strongly varying function. While some modes will have large amplitude there, and thus large kinetic energy, others will have far lower energy. Then, if (as usual) we assume that the excited modes are those with low kinetic energy, we expect a very particular spectrum of dipolar oscillations of WDs with SQM cores. The spectrum should be characterized by several well-detached sets of a very large number of evenly (in period) spaced modes. This should be considered as a clearly distinctive, observable signature of the presence of SQM inside WDs.Facultad de Ciencias Astronómicas y Geofísica

A coordinated x-ray and optical campaign of the nearest massive eclipsing binary, δ orionis Aa. III. Analysis of optical photometric (MOST) and spectroscopic (ground-based) variations

We report on both high-precision photometry from the Microvariability and Oscillations of Stars (MOST) space telescope and ground-based spectroscopy of the triple system δ Ori A, consisting of a binary O9.5II+early-B (Aa1 and Aa2) with P = 5.7 days, and a more distant tertiary (O9 IV years). This data was collected in concert with X-ray spectroscopy from the Chandra X-ray Observatory. Thanks to continuous coverage for three weeks, the MOST light curve reveals clear eclipses between Aa1 and Aa2 for the first time in non-phased data. From the spectroscopy, we have a well-constrained radial velocity (RV) curve of Aa1. While we are unable to recover RV variations of the secondary star, we are able to constrain several fundamental parameters of this system and determine an approximate mass of the primary using apsidal motion. We also detected second order modulations at 12 separate frequencies with spacings indicative of tidally influenced oscillations. These spacings have never been seen in a massive binary, making this system one of only a handful of such binaries that show evidence for tidally induced pulsations.La lista completa de autores que integran el documento puede consultarse en el archivo.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

White dwarf stars as strange quark matter detectors - III

We continue the study of the properties of non-radial pulsations of strange dwarfs. These stars are essentially white dwarfs with a strange quark matter (SQM) core. We have previously shown that the spectrum of oscillations should be formed by several, well-detached clusters of modes inside which the modes are almost evenly spaced. Here, we study the relation between the characteristics of these clusters and the size of the SQM core. We do so assuming that, for a given cluster, the kinetic energy of the modes is constant. For a constant amplitude of the oscillation at the stellar surface, we find that the kinetic energy of the modes is very similar for the cases of models with LogQSQM = -2, -3 and -4, while it is somewhat lower for LogQSQM = -5 (here QSQM = MSQM/M; M SQM and M are the masses of the SQM core and the star, respectively). Remarkably, the shape (amplitude of the modes versus period of oscillation) of the clusters of periods is very similar. However, the number of modes inside each cluster is strongly (and non-monotonously) dependent upon the size of the SQM core. The characteristics of the spectrum of oscillations of strange dwarf stars are very different from the ones corresponding to normal white dwarfs and should be, in principle, observable. Consequently, the stars usually considered as white dwarfs may indeed provide an interesting and affordable way to detect SQM in an astrophysical environment.Facultad de Ciencias Astronómicas y Geofísica

Is There a Crisis in Neutron Star Physics?

We shall show in this report that the theoretical evolution of a particular class of systems containing a neutron star, the so-called black widow binaries, suggest masses above the∼2M⊙, a fact that would bring serious concerns about the right description of the equation of state above the saturation density. Moreover, so far the actual determinations of masses for these systems consistently give values above2M⊙, reinforcing the quandary. In this sense, and given that the confirmation of these ideas would create problems for the microphysical description, we argue that a crisis could be in the works in neutron star physic

Methods for computing giant planet formation and evolution

We present a numerical code for computing all stages of the formation and evolution of giant planets in the framework of the core instability mechanism. This code is a non-trivial adaption of the stellar binary evolution code and is based on a standard Henyey technique. To investigate the performance of this code we applied it to the computation of the formation and evolution of a Jupiter mass object from a half Earth core mass to ages in excess of the age of the Universe. We also present a new smoothed linear interpolation algorithm devised especially for the purpose of circumventing some problems found when some physical data (e.g. opacities, equation of state, etc.) are introduced into an implicit algorithm like the one employed in this work.Fil: Benvenuto, Omar Gustavo. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Pontificia Universidad Católica de Chile; ChileFil: Brunini, Adrian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentin

β Pictoris: its evolutionary status

Since the discovery of a large infrared excess in β Pic, this star has been intensively studied, and is currently considered as one of the best candidates for being the centre of an extrasolar planetary system. Such a star will be surrounded by an asymmetric, edge-on dust disc extending beyond 1(f au from the star. The suspected existence of a planetary system around β Pic has recently been strongly reinforced by the discovery of a central cleared zone about 40 au from the star. A disc warp has also been detected, the most straightforward explanation of which is the presence of at least a giant planet orbiting β Pic. Further evidence comes from the low gas-todust ratio, which may be due to the planetary system formation process. Redshifted and highly variable circumstellar absorption lines of several ions have been interpreted as resulting from the evaporation of comets falling on to β Pic. Recently, the very existence of the dust disc was employed to attribute a small age to β Pic. However, the amount of dust produced by the evaporating comets has been shown to be sufficient to replenish the disc, making the arguments in favour of small ages no longer meaningful. In the frame of the cometary hypothesis, we show that the analysis of the rate of events that would have been observed in the Solar system at different evolutionary stages argues in favour of a large age for β Pic. However, the estimation of stellar ages employing cometary fluxes should be treated with caution, on account of the diversity of possible planetary systems. We also present a new analysis of the evolutionary status of β Pic in the framework of stellar evolution, showing that, with the present uncertainties, there is, a continuum of possible ages for this star. However, employing the data deduced from the cometary flux, we obtain a consistent scenario of a large stellar age.Facultad de Ciencias Astronómicas y Geofísica

Binarity as the solution to the Stellar Evolution Enigma Posed by NGC 6791

Binary evolution is investigated as the source for the extreme horizontal branch (EHB) stars in the old and metal-rich open cluster NGC 6791. Employing an updated version of our binary stellar evolution code, we demonstrate that EHB stars naturally emerge from the common-envelope phase. In sum, the binary model reproduces the observed (Teff, log g) and temporal properties of the EHB overdensity tied to NGC 6971, without needing an ad hoc and anomalous mass-loss prescription.Instituto de Astrofísica de La Plat