Dust Around R Coronae Borealis Stars. I. Spitzer/Infrared Spectrograph Observations

Spitzer/infrared spectrograph (IRS) spectra from 5 to 37 mu m for a complete sample of 31 R Coronae Borealis stars (RCBs) are presented. These spectra are combined with optical and near-infrared photometry of each RCB at maximum light to compile a spectral energy distribution (SED). The SEDs are fitted with blackbody flux distributions and estimates are made of the ratio of the infrared flux from circumstellar dust to the flux emitted by the star. Comparisons for 29 of the 31 stars are made with the Infrared Astronomical Satellite (IRAS) fluxes from three decades earlier: Spitzer and IRAS fluxes at 12 mu m and 25 mu m are essentially equal for all but a minority of the sample. For this minority, the IRAS to Spitzer flux ratio exceeds a factor of three. The outliers are suggested to be stars where formation of a dust cloud or dust puff is a rare event. A single puff ejected prior to the IRAS observations may have been reobserved by Spitzer as a cooler puff at a greater distance from the RCB. RCBs which experience more frequent optical declines have, in general, a circumstellar environment containing puffs subtending a larger solid angle at the star and a quasi-constant infrared flux. Yet, the estimated subtended solid angles and the blackbody temperatures of the dust show a systematic evolution to lower solid angles and cooler temperatures in the interval between IRAS and Spitzer. Dust emission by these RCBs and those in the LMC is similar in terms of total 24 mu m luminosity and [8.0]-[24.0] color index.Spanish Ministry of Science and Innovation (MICINN) AYA-2007-64748NASA GO 50212, 1407Robert A. Welch Foundation of Houston, Texas F-634McDonald Observator

Local yield stress statistics in model amorphous solids

We develop and extend a method presented in [S. Patinet, D. Vandembroucq, and M. L. Falk, Phys. Rev. Lett., 117, 045501 (2016)] to compute the local yield stresses at the atomic scale in model two-dimensional Lennard-Jones glasses produced via differing quench protocols. This technique allows us to sample the plastic rearrangements in a non-perturbative manner for different loading directions on a well-controlled length scale. Plastic activity upon shearing correlates strongly with the locations of low yield stresses in the quenched states. This correlation is higher in more structurally relaxed systems. The distribution of local yield stresses is also shown to strongly depend on the quench protocol: the more relaxed the glass, the higher the local plastic thresholds. Analysis of the magnitude of local plastic relaxations reveals that stress drops follow exponential distributions, justifying the hypothesis of an average characteristic amplitude often conjectured in mesoscopic or continuum models. The amplitude of the local plastic rearrangements increases on average with the yield stress, regardless of the system preparation. The local yield stress varies with the shear orientation tested and strongly correlates with the plastic rearrangement locations when the system is sheared correspondingly. It is thus argued that plastic rearrangements are the consequence of shear transformation zones encoded in the glass structure that possess weak slip planes along different orientations. Finally, we justify the length scale employed in this work and extract the yield threshold statistics as a function of the size of the probing zones. This method makes it possible to derive physically grounded models of plasticity for amorphous materials by directly revealing the relevant details of the shear transformation zones that mediate this process

High-Resolution Optical Spectroscopy of DY Cen: Diffuse Interstellar Bands in a Proto-Fullerene Circumstellar Environment?

We search high-resolution and high-quality VLT/UVES optical spectra of the hot R Coronae Borealis star DY Cen for electronic transitions of the C-60 molecule and diffuse interstellar bands (DIBs). We report the non-detection of the strongest C-60 electronic transitions (e. g., those at similar to 3760, 3980, and 4024 angstrom). The absence of C-60 absorption bands may support recent laboratory results, which show that the similar to 7.0, 8.5, 17.4, and 18.8 mu m emission features seen in DY Cen-and other similar objects with polycyclic-aromatic-hydrocarbon-like dominated IR spectra-are attributable to proto-fullerenes or fullerene precursors rather than to C-60. DIBs toward DY Cen are normal for its reddening; the only exception is the DIB at 6284 angstrom (possibly also the 7223 angstrom DIB) which is found to be unusually strong. We also report the detection of a new broad (FWHM similar to 2 angstrom) and unidentified feature centered at similar to 4000 angstrom. We suggest that this new band may be related to the circumstellar proto-fullerenes seen at infrared wavelengths.Director's Discretionary Time (DDT) program 284.D-5048Spanish Ministry of Economy and Competitiveness AYA-2011-27754Robert A. Welch Foundation of Houston, Texas F-634McDonald Observator

Cno Abundances Of Hydrogen-Deficient Carbon And R Coronae Borealis Stars: A View Of The Nucleosynthesis In A White Dwarf Merger

We present high-resolution (R similar to 50,000) observations of near-IR transitions of CO and CN of the five known hydrogen-deficient carbon (HdC) stars and four R Coronae Borealis (RCB) stars. We perform an abundance analysis of these stars by using spectrum synthesis and state-of-the-art MARCS model atmospheres for cool hydrogen-deficient stars. Our analysis confirms reports by Clayton and colleagues that those HdC stars exhibiting CO lines in their spectrum and the cool RCB star SAps are strongly enriched in (18)O(with (16)O/(18)Oratios ranging from 0.3 to 16). Nitrogen and carbon are in the form of (14)N and (12)C, respectively. Elemental abundances for CNO are obtained from C I, Ci2, CN, and CO lines. Difficulties in deriving the carbon abundance are discussed. Abundances of Na from Na I lines and S from S I lines are obtained. Elemental and isotopic CNO abundances suggest that HdC and RCB stars may be related objects, and that they probably formed from a merger of an He white dwarf with a C-O white dwarf.Robert A. Welch Foundation of Houston, TexasSwedish Research CouncilGS-2006A-C-13GS-2007A-DD-1McDonald Observator

Evolved stars in the Local Group galaxies. I. AGB evolution and dust production in IC 1613

We used models of thermally-pulsing asymptotic giant branch (AGB) stars, that also describe the dust-formation process in the wind, to interpret the combination of near- and mid-infrared photometric data of the dwarf galaxy IC 1613. This is the first time that this approach is extended to an environment different from the Milky Way and the Magellanic Clouds (MCs). Our analysis, based on synthetic population techniques, shows a nice agreement between the observations and the expected distribution of stars in the colour-magnitude diagrams obtained with JHK and Spitzer bands. This allows a characterization of the individual stars in the AGB sample in terms of mass, chemical composition, and formation epoch of the progenitors. We identify the stars exhibiting the largest degree of obscuration as carbon stars evolving through the final AGB phases, descending from 1-1.25Msun objects of metallicity Z=0.001 and from 1.5-2.5Msun stars with Z=0.002. Oxygen-rich stars constitute the majority of the sample (65%), mainly low mass stars (<2Msun) that produce a negligible amount of dust (<10^{-7}Msun/yr). We predict the overall dust-production rate from IC 1613, mostly determined by carbon stars, to be 6x10^{-7}Msun/yr with an uncertainty of 30%. The capability of the current generation of models to interpret the AGB population in an environment different from the MCs opens the possibility to extend this kind of analysis to other Local Group galaxies.Comment: 14 pages, 6 figures, accepted for publication on MNRA