Chandra Observations of the Luminous, O-Rich SNR in the Irregular Galaxy NGC 4449

An analysis of a 29 ksec Chandra ACIS-S observation of the young, Cassiopeia-A like supernova remnant in the irregular galaxy NGC 4449 is presented. The observed 0.5-2.1 keV spectrum reveals the likely presence of several emission lines including O VIII at 0.65 keV and 0.77 keV, Ne X at 1.05 keV, Mg XI at 1.5 keV, and Si XIII at 1.85 keV. From the observed spectrum, we derive an N_H = 10^21 cm^-2 and an X-ray temperature of T = 9 * 10^6 K. A non-equilibrium ionization fit to the spectrum suggests an overabundance of oxygen around 20 times solar, consistent with the remnant's UV and optical emission-line properties. We discuss tht remnant's approximate X-ray derived elemental abundances and compare its X-ray spectrum and luminosity to other oxygen-rich remnants

Model Simulations of a Shock-Cloud Interaction in the Cygnus Loop

We present optical observations and 2D hydrodynamic modeling of an isolated shocked ISM cloud. H$\alpha$ images taken in 1992.6 and 2003.7 of a small optical emission cloud along the southwestern limb of the Cygnus Loop were used to measure positional displacements of $\sim$ 0 \farcs 1 yr$^{-1}$ for surrounding Balmer dominated emission filaments and 0\farcs025 - \farcs055 yr$^{-1}$ for internal cloud emission features. These measurements imply transverse velocities of $\simeq$ 250 km s$^{-1}$ and $\simeq$ 80 -- 140 km s$^{-1}$ for ambient ISM and internal cloud shocks respectively. The complex shock structure visible within the cloud indicates that the cloud's internal density distribution is two phased: a smoothly varying background density which is populated by higher density clumps. We present model results for a shock interacting with a non-uniform ISM cloud. We find that this cloud can be well modeled by a smoothly varying power law core surrounded by a low density envelope with a Lorentzian profile. The lack of sharp density gradients in such a model inhibits the growth of Kelvin-Helmholtz instabilities, consistent with the cloud's appearance. Our model results also suggest that cloud clumps have densities $\sim$ 10 times the ambient ISM density and account for $\sim$ 30% of the total cloud volume. Moreover, the observed spacing of internal cloud shocks and model simulations indicate that the distance between clumps is $\sim$ 4 clump radii.Comment: To be published in Ap

iPTF15eqv: Multi-wavelength Expos\'e of a Peculiar Calcium-rich Transient

The progenitor systems of the class of "Ca-rich transients" is a key open issue in time domain astrophysics. These intriguing objects exhibit unusually strong calcium line emissions months after explosion, fall within an intermediate luminosity range, are often found at large projected distances from their host galaxies, and may play a vital role in enriching galaxies and the intergalactic medium. Here we present multi-wavelength observations of iPTF15eqv in NGC 3430, which exhibits a unique combination of properties that bridge those observed in Ca-rich transients and Type Ib/c supernovae. iPTF15eqv has among the highest [Ca II]/[O I] emission line ratios observed to date, yet is more luminous and decays more slowly than other Ca-rich transients. Optical and near-infrared photometry and spectroscopy reveal signatures consistent with the supernova explosion of a < 10 solar mass star that was stripped of its H-rich envelope via binary interaction. Distinct chemical abundances and ejecta kinematics suggest that the core collapse occurred through electron capture processes. Deep limits on possible radio emission made with the Jansky Very Large Array imply a clean environment ($n <$ 0.1 cm$^{-3}$) within a radius of $\sim 10^{17}$ cm. Chandra X-ray Observatory observations rule out alternative scenarios involving tidal disruption of a white dwarf by a black hole, for masses > 100 solar masses). Our results challenge the notion that spectroscopically classified Ca-rich transients only originate from white dwarf progenitor systems, complicate the view that they are all associated with large ejection velocities, and indicate that their chemical abundances may vary widely between events.Comment: 24 pages, 16 figures. Closely matches version published in The Astrophysical Journa

Heavy‐Element Diffusion in Metal‐poor Stars

Stellar evolution models that include the effect of helium and heavy-element diffusion have been calculated for initial iron abundances of [Fe/H] = -2.3, -2.1, -1.9, and -1.7. These models were calculated for a large variety of masses and three separate mixing lengths, α = 1.50, 1.75, and 2.00 (with α = 1.75 being the solar calibrated mixing length). The change in the surface iron abundance for stars of different masses was determined for the ages of 11, 13, and 15 Gyr. Iron settles out of the surface convection zone on the main sequence ; this iron is dredged back up when the convection zone deepens on the giant branch. In all cases, the surface [Fe/H] abundance in the turnoff stars was at least 0.28 dex lower than the surface [Fe/H] abundance in giant branch stars of the same age. However, Gratton et al. recently found, based on high-dispersion spectra of stars in the globular cluster NGC 6397, that the turnoff and giant branch stars had identical (within a few percent) iron abundances of [Fe/H] = -2.03. These observations prove that heavy-element diffusion must be inhibited in the surface layers of metal- poor stars. When diffusion is inhibited in the outer layers of a stellar model, the predicted temperatures of the models are similar to those of models evolved without diffusion, while the predicted lifetimes are similar to those of stars in which diffusion is not inhibited. Isochrones constructed from the models in which diffusion is inhibited fall halfway between isochrones without diffusion and isochrones with full diffusion. As a result, absolute globular cluster ages based upon the absolute magnitude of the turnoff are 4% larger than ages inferred from full-diffusion isochrones and 4% smaller than ages inferred from non-diffusion isochrones

Evidence for past interaction with an asymmetric circumstellar shell in the young SNR Cassiopeia A

Context. Observations of the supernova remnant (SNR) Cassiopeia A (Cas A) show significant asymmetries in the reverse shock that cannot be explained by models describing a remnant expanding through a spherically symmetric wind of the progenitor star.Aims. We investigate whether a past interaction of Cas A with a massive asymmetric shell of the circumstellar medium can account for the observed asymmetries of the reverse shock.Methods. We performed three-dimensional (3D) (magneto)-hydrodynamic simulations that describe the remnant evolution from the SN explosion to its interaction with a massive circumstellar shell. The initial conditions (soon after the shock breakout at the stellar surface) are provided by a 3D neutrino-driven SN model whose morphology closely resembles Cas A and the SNR simulations cover approximate to 2000 yr of evolution. We explored the parameter space of the shell, searching for a set of parameters able to produce an inward-moving reverse shock in the western hemisphere of the remnant at the age of approximate to 350 yr, analogous to that observed in Cas A.Results. The interaction of the remnant with the shell can produce asymmetries resembling those observed in the reverse shock if the shell was asymmetric with the densest portion in the (blueshifted) nearside to the northwest (NW). According to our favorite model, the shell was thin (thickness sigma approximate to 0.02 pc) with a radius r(sh) approximate to 1.5 pc from the center of the explosion. The reverse shock shows the following asymmetries at the age of Cas A: (i) it moves inward in the observer frame in the NW region, while it moves outward in most other regions; (ii) the geometric center of the reverse shock is offset to the NW by approximate to 0.1 pc from the geometric center of the forward shock; and (iii) the reverse shock in the NW region has enhanced nonthermal emission because, there, the ejecta enter the reverse shock with a higher relative velocity (between 4000 and 7000 km s(-1)) than in other regions (below 2000 km s(-1)).Conclusions. The large-scale asymmetries observed in the reverse shock of Cas A can be interpreted as signatures of the interaction of the remnant with an asymmetric dense circumstellar shell that occurred between approximate to 180 and approximate to 240 yr after the SN event. We suggest that the shell was, most likely, the result of a massive eruption from the progenitor star that occurred between 10(4) and 10(5) yr prior to core-collapse. We estimate a total mass of the shell of the order of 2 M-circle dot

Ejection of the massive Hydrogen-rich envelope timed with the collapse of the stripped SN2014C

We present multi-wavelength observations of SN 2014C during the first 500 days. These observations represent the first solid detection of a young extragalactic stripped-envelope SN out to high-energy X-rays ~40 keV. SN 2014C shows ordinary explosion parameters (Ek ~ 1.8 × 10^(51) erg and M_(ej) ~ 1.7 M⊙). However, over an ~1 year timescale, SN 2014C evolved from an ordinary hydrogen-poor supernova into a strongly interacting, hydrogen-rich supernova, violating the traditional classification scheme of type-I versus type-II SNe. Signatures of the SN shock interaction with a dense medium are observed across the spectrum, from radio to hard X-rays, and revealed the presence of a massive shell of ~1 M⊙ of hydrogen-rich material at ~6 × 10^(16) cm. The shell was ejected by the progenitor star in the decades to centuries before collapse. This result challenges current theories of massive star evolution, as it requires a physical mechanism responsible for the ejection of the deepest hydrogen layer of H-poor SN progenitors synchronized with the onset of stellar collapse. Theoretical investigations point at binary interactions and/or instabilities during the last nuclear burning stages as potential triggers of the highly time-dependent mass loss. We constrain these scenarios utilizing the sample of 183 SNe Ib/c with public radio observations. Our analysis identifies SN 2014C-like signatures in ~10% of SNe. This fraction is reasonably consistent with the expectation from the theory of recent envelope ejection due to binary evolution if the ejected material can survive in the close environment for 10^3–10^4 years. Alternatively, nuclear burning instabilities extending to core C-burning might play a critical role