Chandra view of Kes 79: a nearly isothermal SNR with rich spatial structure

A 30 ks \chandra ACIS-I observation of Kes 79 reveals rich spatial structures, including many filaments, three partial shells, a loop and a ``protrusion''. Most of them have corresponding radio features. Regardless of the different results from two non-equilibrium ionization (NEI) codes, temperatures of different parts of the remnant are all around 0.7 keV, which is surprisingly constant for a remnant with such rich structure. If thermal conduction is responsible for smoothing the temperature gradient, a lower limit on the thermal conductivity of $\sim$ 1/10 of the Spitzer value can be derived. Thus, thermal conduction may play an important role in the evolution of at least some SNRs. No spectral signature of the ejecta is found, which suggests the ejecta material has been well mixed with the ambient medium. From the morphology and the spectral properties, we suggest the bright inner shell is a wind-driven shell (WDS) overtaken by the blast wave (the outer shell) and estimate the age of the remnant to be $\sim$ 6 kyr for the assumed dynamics. Projection is also required to explain the complicated morphology of Kes 79.Comment: 12 pages, 6 figures (3 in color), ApJ, in press, April 20, 200

Transformations: Suburban Cordoba During the Umayyad Caliphate, 929-1009

It has been noted how long it took for the Muslim presence in Iberia, starting in 711 BCE, to materialize into significant works of architecture. The continuous military campaigns, necessary to consolidate control of the Peninsula, were undertaken by a relatively small group of incoming Arab and Berber troops. This naturally limited the potential scope of construction to repairs of the Hispano-Roman infrastructure found in the conquered areas, mainly in the middle and the South of the Iberian Peninsula. The old walled city of Cordoba, locale of Roman and Visigoth rulers, served as the capital of the new emirate, with the reuse of its existing structures, bridge, walls, palaces, etc. It would ultimately be the political stability and economic prosperity of tenth-century al-Andalus[1] that would allow Abd al-Rahman III to declare himself Caliph in 929 CE and be able to undertake original architectural projects that would epitomize the centralization of his power and the legitimacy of his rulership. The development of the Cordoba suburbs, where a firmament of munyas, or residential villas, was already being built for affluent patrons in the Guadalquivir valley, was the prelude to the creation of the royal palatial city of Madinat al-Zahra, a large multi-use complex sited at the foothills of the Sierra Morena that would, for a brief moment from its inception in 941 CE until its destruction by fundamentalist Almohad troops in 1009 CE, serve as a center of power, administration and advancement of culture for the Cordoba Caliphate. The Umayyads brought their own Syrian traditions to Iberia. Yet the Muslim conquest of Iberia involved not only the Arabization of the local population, but also, the “Iberization” of the newcomers. Abd al-Rahman III had inherited the blond hair and blue eyes of his Christian mother. This physical fusion was symbolic of the population in al-Andalus: outwardly and culturally Arabic yet fundamentally more complex. This study will focus on the architectural and site planning developments of Suburban Cordoba during the reigns of Abd al-Rahman III (756–929 CE) and his son, al-Hakam II (912–961 CE). It will address how the architecture and planning of their new buildings combined Umayyad Syrian tradition with Roman classical concepts and local Visigoth methods and materials, adapting them to the specific characteristics of the new land they conquered while also incorporating features of contemporary Persian and Abbasid ceremonial culture. The innovations included not only construction techniques and decorative styles, but more importantly, new concepts in the planning of the buildings and the relationship of their interior and exterior spaces, reflecting the evolving concepts of sovereignty and legitimacy of the Cordoba Caliphate and the unusual nature of its court. [1] al-Andalus: Arabic name given to the geographical area of the Iberian Peninsula that came under Muslim control in the Middle Ages, from 711 CE to 1492 CE. The Oxford Dictionary of the Middle Ages, edited by Robert E. Bjork. Oxford University Press, 2010. www.oxfordreference.co

Are the Models for Type Ia Supernova Progenitors Consistent with the Properties of Supernova Remnants?

We explore the relationship between the models for progenitor systems of Type Ia supernovae and the properties of the supernova remnants that evolve after the explosion. Most models for Type Ia progenitors in the single degenerate scenario predict substantial outflows during the presupernova evolution. Expanding on previous work, we estimate the imprint of these outflows on the structure of the circumstellar medium at the time of the supernova explosion, and the effect that this modified circumstellar medium has on the evolution of the ensuing supernova remnant. We compare our simulations with the observational properties of known Type Ia supernova remnants in the Galaxy (Kepler, Tycho, SN 1006), the Large Magellanic Cloud (0509-67.5, 0519-69.0, N103B), and M31 (SN 1885). We find that optically thick outflows from the white dwarf surface (sometimes known as accretion winds) with velocities above 200 km/s excavate large low-density cavities around the progenitors. Such large cavities are incompatible with the dynamics of the forward shock and the X-ray emission from the shocked ejecta in all the Type Ia remnants that we have examined.Comment: To appear in ApJ. 17 pages, 10 figures, emulateap

Tails of the Unexpected: The Interaction of an Isothermal Shell with a Cloud

A new mechanism for the formation of cometary tails behind dense clouds or globules is discussed. Numerical hydrodynamical models show that when a dense shell of swept-up matter overruns a cloud, material in the shell is focussed behind the cloud to form a tail. This mode of tail formation is completely distinct from other methods, which involve either the removal of material from the cloud, or shadowing from a strong, nearby source of ionization. This mechanism is relevant to the cometary tails seen in planetary nebulae and to the interaction of superbubble shells with dense clouds.Comment: 6 pages, 6 figures, accepted for publication in MNRAS letter

Star clusters and the structure of the ISM. Tunnels and wakes in giant extragalactic HII regions

Several structures have been discovered embedded in regions of recent or ongoing star formation, which point to the importance of the interaction between fast moving wind-blowing stars and their environment. Using hydrodynamic simulations, we investigate the passage through the interstellar medium of a supersonic stellar wind source, and show how it can naturally lead to the formation of tubes, channels and swamps of globules as interfaces are crossed. The results are in excellent agreement with observation of 30 Doradus.Comment: 12 pages + 5 figures (GIF format) - Accepted for pub. in Astrophys. J. Letter

Initial Ionization of Compressible Turbulence

We study the effects of the initial conditions of turbulent molecular clouds on the ionization structure in newly formed H_{ii} regions, using three-dimensional, photon-conserving radiative transfer in a pre-computed density field from three-dimensional compressible turbulence. Our results show that the initial density structure of the gas cloud can play an important role in the resulting structure of the H_{ii} region. The propagation of the ionization fronts, the shape of the resulting H_{ii} region, and the total mass ionized depend on the properties of the turbulent density field. Cuts through the ionized regions generally show ``butterfly'' shapes rather than spherical ones, while emission measure maps are more spherical if the turbulence is driven on scales small compared to the size of the H_{ii} region. The ionization structure can be described by an effective clumping factor $\zeta=< n > \cdot /^2$, where $n$ is number density of the gas. The larger the value of $\zeta$, the less mass is ionized, and the more irregular the H_{ii} region shapes. Because we do not follow dynamics, our results apply only to the early stage of ionization when the speed of the ionization fronts remains much larger than the sound speed of the ionized gas, or Alfv\'en speed in magnetized clouds if it is larger, so that the dynamical effects can be negligible.Comment: 9 pages, 10 figures, version with high quality color images can be found in http://research.amnh.org/~yuexing/astro-ph/0407249.pd

Tuning the electronic, photophysical and charge transfer properties of small D-A molecules based on Thienopyrazine-terthienyls by changing the donor fragment: A DFT study

Indexación: Scopus.Four acceptor-donor organic conjugated molecules based on thieno[3,4-b]pyrazine-terthienyls were analyzed in order to explore the effect of the donor substituent on their molecular structures, electronic and optical properties. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD/DFT) calculations were carried out employing the B3LYP hybrid functional in combination with the 6-31G(d,p) basis set. The results suggests that the addition of electron-donating substituents to the conjugated molecules can diminish their energy gap value, which is beneficial to the photon harvesting. The lowest-lying absorption spectra of compounds substituted with electron donor groups exhibited a red-shift and a high oscillation factor compared with the unsubstituted molecule. Additionally, the ionization potential (IP), electron affinity (EA), reorganization energy (λ) and open-circuit voltage (Voc) of the molecules were evaluated. According to these values, the molecules show good photovoltaic properties, and efficient charge transfer for hole and electron and balanced charges.https://scielo.conicyt.cl/scielo.php?script=sci_arttext&pid=S0717-97072017000303637&lng=en&nrm=iso&tlng=e