Search results for "Stars"

Accretion disk coronae of intermediate polar cataclysmic variables

2017

Context. Intermediate polar cataclysmic variables (IPCV) contain a magnetic, rotating white dwarf surrounded by a magnetically truncated accretion disk. To explain their strong flickering X-ray emission, accretion has been successfully taken into account. Nevertheless, observations suggest that accretion phenomena might not be the only process behind it. An intense flaring activity occurring on the surface of the disk may generate a corona, contribute to the thermal X-ray emission, and influence the system stability. Aims. Our purposes are: investigating the formation of an extended corona above the accretion disk, due to an intense flaring activity occurring on the disk surface; studying t…

Accretion disk coronae of Intermediate Polar Cataclysmic Variables: 3D MagnetoHydroDynamic modeling and thermal X-ray emission

Intermediate Polar Cataclysmic Variables (IPCVs), also known as DQ Her stars after their prototype, are cataclysmic variable stars characterized by the presence of an accreting, magnetic, rapidly rotating white dwarf surrounded by an accretion disk magnetically truncated by the inner magnetosphere of the star. These objects exhibit a magnetic field of the order of few MG, able to disrupt the inner part of the disk but not strong enough to inhibit the formation of a disk. These stars show a strong X-ray emission with a stochastic pulsation that in some cases arise in the inner part of the disk. However, the observations of the X-ray luminosity for these objects do not match the expected valu…

Phase behavior of low-functionality, telechelic star block copolymers.

2010

We apply state-of-the-art, Grand Canonical Monte Carlo simulations to determine the self-organization and phase behavior of solutions of block copolymer stars. The latter consist of f AB-block copolymers with N monomers each, which contain a solvophilic block A and solvophobic block B, and which are tethered on a common center on their A-side. We vary the degree of polymerization N and the relative composition of the block copolymer arms and investigate the interplay between macrophase and microphase separation in the system. Preliminary results of the effect of increasing the number of arms, f of the stars are also presented.

Lithium in metal-poor red giants

2009

AbstractThe lithium abundance was calculated for five metal-poor red giant stars from Li i doublet at 6707 Å by fitting the observed high-resolution spectra with synthetic spectra. The lithium abundance was found to be low in all stars, logϵ(Li) ≤ 1.8, confirming lithium depletion on the red giant and asymptotic giant branch.

Star-Shaped Conjugated Systems

2010

The present review deals with the preparation and the properties of star-shaped conjugated compounds. Three, four or six conjugated arms are attached to cross-conjugated cores, which consist of single atoms (B, C+, N), benzene or azine rings or polycyclic ring systems, as for example triphenylene or tristriazolotriazine. Many of these shape-persistent [n]star compounds tend to π-stacking and self-organization, and exhibit interesting properties in materials science: Linear and non-linear optics, electrical conductivity, electroluminescence, formation of liquid crystalline phases, etc.

Dust Production and Particle Acceleration in Supernova 1987A Revealed with ALMA

2013

Supernova (SN) explosions are crucial engines driving the evolution of galaxies by shock heating gas, increasing the metallicity, creating dust, and accelerating energetic particles. In 2012 we used the Atacama Large Millimeter/Submillimeter Array to observe SN 1987A, one of the best-observed supernovae since the invention of the telescope. We present spatially resolved images at 450um, 870um, 1.4mm, and 2.8mm, an important transition wavelength range. Longer wavelength emission is dominated by synchrotron radiation from shock-accelerated particles, shorter wavelengths by emission from the largest mass of dust measured in a supernova remnant (>0.2Msun). For the first time we show unambig…

The Gaia-ESO Survey: Extracting diffuse interstellar bands from cool star spectra DIB-based interstellar medium line-of-sight structures at the kpc s…

2014

We study how diffuse interstellar bands (DIBs) measured toward distance-distributed target stars can be used to locate dense interstellar (IS) clouds in the Galaxy and probe a line-of-sight (LOS) kinematical structure, a potential useful tool when gaseous absorption lines are saturated or not available in the spectral range. Cool target stars are numerous enough for this purpose. We have devised automated DIB fitting methods appropriate to cool star spectra and multiple IS components. The data is fitted with a combination of a synthetic stellar spectrum, a synthetic telluric transmission, and empirical DIB profiles. In parallel, stellar distances and extinctions are estimated self-consisten…

Constraining the Galactic structure parameters with the XSTPS-GAC and SDSS photometric surveys

2016

Photometric data from the Xuyi Schmidt Telescope Photometric Survey of the Galactic Anticentre (XSTPS-GAC) and the Sloan Digital Sky Survey (SDSS) are used to derive the global structure parameters of the smooth components of the Milky Way. The data, which cover nearly 11,000 deg$^2$ sky area and the full range of Galactic latitude, allow us to construct a globally representative Galactic model. The number density distribution of Galactic halo stars is fitted with an oblate spheroid that decays by power law. The best-fit yields an axis ratio and a power law index $��=0.65$ and $p=2.79$, respectively. The $r$-band differential star counts of three dwarf samples are then fitted with a Galacti…

On the nature of a shell of young stars in the outskirts of the Small Magellanic Cloud

2019

Understanding the evolutionary history of the Magellanic Clouds requires an in-depth exploration and characterization of the stellar content in their outer regions, which ultimately are key to tracing the epochs and nature of past interactions. We present new deep images of a shell-like over-density of stars in the outskirts of the Small Magellanic Cloud (SMC). The shell, also detected in photographic plates dating back to the fifties, is located at ~1.9 degr from the center of the SMC in the north-east direction.The structure and stellar content of this feature were studied with multi-band, optical data from the Survey of the MAgellanic Stellar History (SMASH) carried out with the Dark Ene…

Swings between rotation and accretion power in a binary millisecond pulsar

2013

It is thought that neutron stars in low-mass binary systems can accrete matter and angular momentum from the companion star and be spun-up to millisecond rotational periods1, 2, 3. During the accretion stage, the system is called a low-mass X-ray binary, and bright X-ray emission is observed. When the rate of mass transfer decreases in the later evolutionary stages, these binaries host a radio millisecond pulsar4, 5 whose emission is powered by the neutron star’s rotating magnetic field6. This evolutionary model is supported by the detection of millisecond X-ray pulsations from several accreting neutron stars7, 8 and also by the evidence for a past accretion disc in a rotation-powered milli…