Raising the Dead: Clues to Type Ia Supernova Physics from the Remnant 0509-67.5

We present Chandra X-ray observations of the young supernova remnant (SNR) 0509-67.5 in the Large Magellanic Cloud (LMC), believed to be the product of a Type Ia supernova (SN Ia). The remnant is very round in shape, with a distinct clumpy shell-like structure. Our Chandra data reveal the remnant to be rich in silicon, sulfur, and iron. The yields of our fits to the global spectrum confirm that 0509-67.5 is the remnant of an SN Ia and show a clear preference for delayed detonation explosion models for SNe Ia. We study the spectrum of the single brightest isolated knot in the remnant and find that it is enhanced in iron by a factor of roughly two relative to the global remnant abundances. This feature, along with similar knots seen in Tycho's SNR, argues for the presence of modest small-scale composition inhomogeneities in SNe Ia. The presence of both Si and Fe, with abundance ratios that vary from knot to knot, indicates that these came from the transition region between the Si- and Fe-rich zones in the exploded star, possibly as a result of energy input to the ejecta at late times due to the radioactive decay of 56Ni and 56Co. Two cases for the continuum emission from the global spectrum were modeled: one where the continuum is dominated by hydrogen thermal bremsstrahlung radiation; another where the continuum arises from non-thermal synchrotron radiation. The former case requires a relatively large value for the ambient density (~1 cm^-3). Another estimate of the ambient density comes from using the shell structure of the remnant in the context of dynamical models. This requires a much lower value for the density (<0.05 cm^-3) which is more consistent with other evidence known about 0509-67.5. We therefore conclude that the bulk of the continuum emission from 0509-67.5 has a non-thermal origin.Comment: 34 pages, 9 figures (1 color), accepted to ApJ (10 June 2004 issue); correction made to calculation of magnetic field, small sentence change

Revealing New Physical Structures in the Supernova Remnant N63A through Chandra Imaging Spectroscopy

We present Chandra X-ray observations of the supernova remnant (SNR) N63A in the Large Magellanic Cloud (LMC). N63A, one of the brightest LMC remnants, is embedded in an H II region and probably associated with an OB association. The optical remnant consists of three lobes of emission contained within the approximately three times larger X-ray remnant. Our Chandra data reveal a number of new physical structures in N63A. The most striking of these are the several ``crescent''-shaped structures located beyond the main shell that resemble similar features seen in the Vela SNR. In Vela, these have been interpreted as arising from high speed clumps of supernova ejecta interacting with the ambient medium. Another distinct feature of the remnant is a roughly triangular ``hole'' in the X-ray emission near the location of the optical lobes and the brightest radio emission. X-ray spectral analysis shows that this deficit of emission is a result of absorption by an intervening dense cloud with a mass of ~450 M_sun that is currently being engulfed by the remnant's blast wave. We also find that the rim of the remnant, as well as the crescent-shaped features, have considerably softer X-ray spectra than the interior. Limits on hard X-ray emission rule out a young, energetic pulsar in N63A, but the presence of an older or less active one, powering a wind nebula with a luminosity less than ~4e10^34 erg/s, is allowed.Comment: 18 pages, 5 figures (2 color), accepted for publication in Ap

Stellar Collisions and the Interior Structure of Blue Stragglers

Collisions of main sequence stars occur frequently in dense star clusters. In open and globular clusters, these collisions produce merger remnants that may be observed as blue stragglers. Detailed theoretical models of this process require lengthy hydrodynamic computations in three dimensions. However, a less computationally expensive approach, which we present here, is to approximate the merger process (including shock heating, hydrodynamic mixing, mass ejection, and angular momentum transfer) with simple algorithms based on conservation laws and a basic qualitative understanding of the hydrodynamics. These algorithms have been fine tuned through comparisons with the results of our previous hydrodynamic simulations. We find that the thermodynamic and chemical composition profiles of our simple models agree very well with those from recent SPH (smoothed particle hydrodynamics) calculations of stellar collisions, and the subsequent stellar evolution of our simple models also matches closely that of the more accurate hydrodynamic models. Our algorithms have been implemented in an easy to use software package, which we are making publicly available (see http://vassun.vassar.edu/~lombardi/mmas/). This software could be used in combination with realistic dynamical simulations of star clusters that must take into account stellar collisions.Comment: This revised version has 37 pages, 13 figures, 4 tables; submitted to ApJ; for associated software package, see http://vassun.vassar.edu/~lombardi/mmas/ This revised version presents additional comparisons with SPH results and slightly improved merger recipe

The Chandra View of the Supernova Remnant 0506-68.0 in the Large Magellanic Cloud

A new Chandra observation of SNR 0506-68.0 (also called N23) reveals a complex, highly structured morphology in the low energy X-ray band and an isolated compact central object in the high energy band. Spectral analysis indicates that the X-ray emission overall is dominated by thermal gas whose composition is consistent with swept-up ambient material. There is a strong gradient in ambient density across the diameter of the remnant. Toward the southeast, near a prominent star cluster, the emitting density is 10 - 23 cm^{-3} while toward the northwest it has dropped to a value of only 1 cm^{-3}. The total extent of the X-ray remnant is 100" by 120" (24 pc x 29 pc for a distance of 50 kpc), somewhat larger than previously known. The remnant's age is estimated to be ~4600 yr. One part of the remnant shows evidence for enhanced O, Ne, and perhaps Mg abundances, which is interpreted as evidence for ejecta from a massive star core collapse supernova. The compact central object has a luminosity of a few times 10^{33} ergs/s and no obvious radio or optical counterpart. It does not show an extended nebula or pulsed emission as expected from a young energetic pulsar, but resembles the compact central objects seen in other core collapse SNe, such as Cas A.Comment: 5 pages, including 3 postscript figs, LaTeX, accepted to appear in ApJ Letter

Cosmic Ray Acceleration at the Forward Shock in Tycho's Supernova Remnant: Evidence from Chandra X-ray Observations

We present evidence for cosmic ray acceleration at the forward shock in Tycho's supernova remnant (SNR) from three X-ray observables: (1) the proximity of the contact discontinuity to the forward shock, or blast wave, (2) the morphology of the emission from the rim of Tycho, and (3) the spectral nature of the rim emission. We determine the locations of the blast wave (BW), contact discontinuity (CD), and reverse shock (RS) around the rim of Tycho's supernova remnant using a principal component analysis and other methods applied to new Chandra data. The azimuthal-angle-averaged radius of the BW is 251". For the CD and RS we find average radii of 241" and 183", respectively. Taking account of projection effects, we find ratios of 1:0.93:0.70 (BW:CD:RS). We show these values to be inconsistent with adiabatic hydrodynamical models of SNR evolution. The CD:BW ratio can be explained if cosmic ray acceleration of ions is occurring at the forward shock. The RS:BW ratio, as well as the strong Fe Ka emission from the Tycho ejecta, imply that the RS is not accelerating cosmic rays. We also extract radial profiles from ~34% of the rim of Tycho and compare them to models of surface brightness profiles behind the BW for a purely thermal plasma with an adiabatic shock. The observed morphology of the rim is much more strongly peaked than predicted by the model, indicating that such thermal emission is implausible here. Spectral analysis also implies that the rim emission is non-thermal in nature, lending further support to the idea that Tycho's forward shock is accelerating cosmic rays.Comment: 39 pages, 10 figures, accepted by Ap

Accurate simulation of direct laser acceleration in a laser wakefield accelerator

In a laser wakefield accelerator (LWFA), an intense laser pulse excites a plasma wave that traps and accelerates electrons to relativistic energies. When the pulse overlaps the accelerated electrons, it can enhance the energy gain through direct laser acceleration (DLA) by resonantly driving the betatron oscillations of the electrons in the plasma wave. The particle-in-cell (PIC) algorithm, although often the tool of choice to study DLA, contains inherent errors due to numerical dispersion and the time staggering of the electric and magnetic fields. Further, conventional PIC implementations cannot reliably disentangle the fields of the plasma wave and laser pulse, which obscures interpretation of the dominant acceleration mechanism. Here, a customized field solver that reduces errors from both numerical dispersion and time staggering is used in conjunction with a field decomposition into azimuthal modes to perform PIC simulations of DLA in an LWFA. Comparisons with traditional PIC methods, model equations, and experimental data show improved accuracy with the customized solver and convergence with an order-of-magnitude fewer cells. The azimuthal-mode decomposition reveals that the most energetic electrons receive comparable energy from DLA and LWFA.Comment: 10 pages, 5 figures, to submit to Physics of Plasma

Northeast Indian stalagmite records Pacific decadal climate change: Implications for moisture transport and drought in India

This is the final version. It is currently under embargo. It was first published by Wiley at http://onlinelibrary.wiley.com/doi/10.1002/2015GL063826/full.Two types of El Niño events are distinguished by sea surface temperature (SST) anomalies centered in the central or eastern equatorial Pacific. The Central Pacific El Niño events (CP-El Niño) are more highly correlated with weakening of the central Indian Summer Monsoon and linked to decadal Pacific climate variability. We present a 50 year, subannually resolved speleothem δ18O record from northeast India that exhibits a significant correlation with northern Pacific decadal variability and central equatorial Pacific SSTs. Accordingly, we suggest that δ18O time series in similar northeast Indian speleothems are effective tools for investigating preinstrumental changes in Pacific climate, including changes in El Niño dynamics. In contrast to central India, rainfall amounts in northeast India are relatively unaffected by El Niño. However, back trajectory analysis indicates that during CP-El Niño events moisture transport distance to northeast India is reduced, suggesting that variations in moisture transport primarily control δ18O in the region.This work was supported through the BanglaPIRE project (NSF OISE-0968354), an award from the Vanderbilt International Office to JLO and SFMB, and awards from the Cave Research Foundation and the Geological Society of America to CGM. SFMB received financial support from the Schweizer National Fond (SNF), Sinergia grant CRSI22 132646/1

Discovery of a Gas-Rich Companion to the Extremely Metal-Poor Galaxy DDO 68

We present HI spectral-line imaging of the extremely metal-poor galaxy DDO 68. This system has a nebular oxygen abundance of only 3% Z$_{\odot}$, making it one of the most metal-deficient galaxies known in the local volume. Surprisingly, DDO 68 is a relatively massive and luminous galaxy for its metal content, making it a significant outlier in the mass-metallicity and luminosity-metallicity relationships. The origin of such a low oxygen abundance in DDO 68 presents a challenge for models of the chemical evolution of galaxies. One possible solution to this problem is the infall of pristine neutral gas, potentially initiated during a gravitational interaction. Using archival HI spectral-line imaging obtained with the Karl G. Jansky Very Large Array, we have discovered a previously unknown companion of DDO 68. This low-mass (M$_{\rm HI}$ $=$ 2.8$\times$10$^{7}$ M$_{\odot}$), recently star-forming (SFR$_{\rm FUV}$ $=$ 1.4$\times$10$^{-3}$ M$_{\odot}$ yr$^{-1}$, SFR$_{\rm H\alpha}$ $<$ 7$\times$10$^{-5}$ M$_{\odot}$ yr$^{-1}$) companion has the same systemic velocity as DDO 68 (V$_{\rm sys}$ $=$ 506 km s$^{-1}$; D $=$ 12.74$\pm$0.27 Mpc) and is located at a projected distance of 42 kpc. New HI maps obtained with the 100m Robert C. Byrd Green Bank Telescope provide evidence that DDO 68 and this companion are gravitationally interacting at the present time. Low surface brightness HI gas forms a bridge between these objects.Comment: Accepted for publication in the Astrophysical Journal Letter

Detection of Magnesium-Rich Ejecta in the Middle-Aged Supernova Remnant N49B

The middle-aged supernova remnant (SNR) N49B in the Large Magellanic Cloud has been observed with the {\it Chandra X-Ray Observatory}. The superb angular resolution of {\it Chandra} resolves the complex structure of X-ray emitting filaments across the SNR. All observed features are soft ($E <$ 3 keV) and we find no evidence for either point-like or extended hard emission within the SNR. Spectral lines from O, Ne, Mg, Si, S, and Fe are present. Equivalent width images for the detected elemental species and spatially-resolved spectral analysis reveal the presence of Mg-rich ejecta within the SNR. We find no such enrichment in O or Ne, which may reflect details of the nucleosynthesis process or the heating and cooling of the ejecta as it evolved. The bright circumferential filaments are emission from the shocked dense interstellar medium (ISM). We detect faint diffuse X-ray emission that extends beyond the X-ray bright filaments toward the west and southeast. These features appear to be the blast wave shock front expanding into lower density portions of the ISM seen in projection. We set an upper limit of $\sim$$2\times 10^{33}$ ergs s$^{-1}$ on the 0.5 $-$ 5 keV band X-ray luminosity of any embedded compact object.Comment: 3 text pages (ApJ emulator style), 3 figures, 1 table, Accepted for the publication in Ap J Letter

Comprehension of acoustically degraded speech in Alzheimer's disease and primary progressive aphasia

Successful communication in daily life depends on accurate decoding of speech signals that are acoustically degraded by challenging listening conditions. This process presents the brain with a demanding computational task that is vulnerable to neurodegenerative pathologies. However, despite recent intense interest in the link between hearing impairment and dementia, comprehension of acoustically degraded speech in these diseases has been little studied. Here we addressed this issue in a cohort of 19 patients with typical Alzheimer's disease and 30 patients representing the three canonical syndromes of primary progressive aphasia (nonfluent/agrammatic variant primary progressive aphasia; semantic variant primary progressive aphasia; logopenic variant primary progressive aphasia), compared to 25 healthy age-matched controls. As a paradigm for the acoustically degraded speech signals of daily life, we used noise-vocoding: synthetic division of the speech signal into frequency channels constituted from amplitude-modulated white noise, such that fewer channels convey less spectrotemporal detail thereby reducing intelligibility. We investigated the impact of noise-vocoding on recognition of spoken three-digit numbers and used psychometric modelling to ascertain the threshold number of noise-vocoding channels required for 50% intelligibility by each participant. Associations of noise-vocoded speech intelligibility threshold with general demographic, clinical and neuropsychological characteristics and regional grey matter volume (defined by voxel-based morphometry of patients' brain images) were also assessed. Mean noise-vocoded speech intelligibility threshold was significantly higher in all patient groups than healthy controls, and significantly higher in Alzheimer's disease and logopenic variant primary progressive aphasia than semantic variant primary progressive aphasia (all p < 0.05). In a receiver-operating-characteristic analysis, vocoded intelligibility threshold discriminated Alzheimer's disease, non-fluent variant and logopenic variant primary progressive aphasia patients very well from healthy controls. Further, this central hearing measure correlated with overall disease severity but not with peripheral hearing or clear speech perception. Neuroanatomically, after correcting for multiple voxel-wise comparisons in pre-defined regions of interest, impaired noise-vocoded speech comprehension across syndromes was significantly associated (p < 0.05) with atrophy of left planum temporale, angular gyrus and anterior cingulate gyrus: a cortical network that has previously been widely implicated in processing degraded speech signals. Our findings suggest that the comprehension of acoustically altered speech captures an auditory brain process relevant to daily hearing and communication in major dementia syndromes, with novel diagnostic and therapeutic implications