A Chandra Observation of Supernova Remnant G350.1-0.3 and Its Central Compact Object

We present a new Chandra observation of supernova remnant (SNR) G350.1-0.3. The high resolution X-ray data reveal previously unresolved filamentary structures and allow us to perform detailed spectroscopy in the diffuse regions of this SNR. Spectral analysis demonstrates that the region of brightest emission is dominated by hot, metal-rich ejecta while the ambient material along the perimeter of the ejecta region and throughout the remnant's western half is mostly low-temperature, shocked interstellar/circumstellar medium (ISM/CSM) with solar-type composition. The data reveal that the emission extends far to the west of the ejecta region and imply a lower limit of 6.6 pc on the diameter of the source (at a distance of 4.5 kpc). We show that G350.1-0.3 is likely in the free expansion (ejecta-dominated) stage and calculate an age of 600-1200 years. The derived relationship between the shock velocity and the electron/proton temperature ratio is found to be entirely consistent with that of other SNRs. We perform spectral fits on the X-ray source XMMU J172054.5-372652, a candidate central compact object (CCO), and find that its spectral properties fall within the typical range of other CCOs. We also present archival 24 um data of G350.1-0.3 taken with the Spitzer Space Telescope during the MIPSGAL galactic survey and find that the infrared and X-ray morphologies are well-correlated. These results help to explain this remnant's peculiar asymmetries and shed new light on its dynamics and evolution

Turbulent Mixing in the Interstellar Medium -- an application for Lagrangian Tracer Particles

We use 3-dimensional numerical simulations of self-gravitating compressible turbulent gas in combination with Lagrangian tracer particles to investigate the mixing process of molecular hydrogen (H2) in interstellar clouds. Tracer particles are used to represent shock-compressed dense gas, which is associated with H2. We deposit tracer particles in regions of density contrast in excess of ten times the mean density. Following their trajectories and using probability distribution functions, we find an upper limit for the mixing timescale of H2, which is of order 0.3 Myr. This is significantly smaller than the lifetime of molecular clouds, which demonstrates the importance of the turbulent mixing of H2 as a preliminary stage to star formation.Comment: 10 pages, 5 figures, conference proceedings "Turbulent Mixing and Beyond 2007

A Numerical Study of Brown Dwarf Formation via Encounters of Protostellar Disks

The formation of brown dwarfs (BDs) due to the fragmentation of proto-stellar disks undergoing pairwise encounters was investigated. High resolution allowed the use of realistic initial disk models where both the vertical structure and the local Jeans mass were resolved. The results show that objects with masses ranging from giant planets to low mass stars can form during such encounters from initially stable disks. The parameter space of initial spin-orbit orientations and the azimuthal angles for each disk was explored. An upper limit on the initial Toomre Q value of ~2 was found for fragmentation to occur. Depending on the initial configuration, shocks, tidal-tail structures and mass inflows were responsible for the condensation of disk gas. Retrograde disks were generally more likely to fragment. When the interaction timescale was significantly shorter than the disks' dynamical timescales, the proto-stellar disks tended to be truncated without forming objects. The newly-formed objects had masses ranging from 0.9 to 127 Jupiter masses, with the majority in the BD regime. They often resided in star-BD multiples and in some cases also formed hierarchical orbiting systems. Most of them had large angular momenta and highly flattened, disk-like shapes. The objects had radii ranging from 0.1 to 10 AU. The disk gas was assumed to be locally isothermal, appropriate for the short cooling times in extended proto-stellar disks, but not for condensed objects. An additional case with explicit cooling that reduced to zero for optically thick gas was simulated to test the extremes of cooling effectiveness and it was still possible to form objects in this case. Detailed radiative transfer is expected to lengthen the internal evolution timescale for these objects, but not to alter our basic results.Comment: 18 pages, 12 figures and 2 tables. Accepted for publication in MNRA

Star Forming Dense Cloud Cores in the TeV {\gamma}-ray SNR RX J1713.7-3946

RX J1713.7-3946 is one of the TeV {\gamma}-ray supernova remnants (SNRs) emitting synchrotron X rays. The SNR is associated with molecular gas located at ~1 kpc. We made new molecular observations toward the dense cloud cores, peaks A, C and D, in the SNR in the 12CO(J=2-1) and 13CO(J=2-1) transitions at angular resolution of 90". The most intense core in 13CO, peak C, was also mapped in the 12CO(J=4-3) transition at angular resolution of 38". Peak C shows strong signs of active star formation including bipolar outflow and a far-infrared protostellar source and has a steep gradient with a r^{-2.2$\pm$0.4} variation in the average density within radius r. Peak C and the other dense cloud cores are rim-brightened in synchrotron X rays, suggesting that the dense cloud cores are embedded within or on the outer boundary of the SNR shell. This confirms the earlier suggestion that the X rays are physically associated with the molecular gas (Fukui et al. 2003). We present a scenario where the densest molecular core, peak C, survived against the blast wave and is now embedded within the SNR. Numerical simulations of the shock-cloud interaction indicate that a dense clump can indeed survive shock erosion, since shock propagation speed is stalled in the dense clump. Additionally, the shock-cloud interaction induces turbulence and magnetic field amplification around the dense clump that may facilitate particle acceleration in the lower-density inter-clump space leading to the enhanced synchrotron X rays around dense cores.Comment: 22 pages, 7 figures, to accepted in The Astrophysical Journal. A full color version with higher resolution figures is available at http://www.a.phys.nagoya-u.ac.jp/~sano/ApJ10/ms_sano.pd

Computational Eulerian Hydrodynamics and Galilean Invariance

Eulerian hydrodynamical simulations are a powerful and popular tool for modeling fluids in astrophysical systems. In this work, we critically examine recent claims that these methods violate Galilean invariance of the Euler equations. We demonstrate that Eulerian hydrodynamics methods do converge to a Galilean-invariant solution, provided a well-defined convergent solution exists. Specifically, we show that numerical diffusion, resulting from diffusion-like terms in the discretized hydrodynamical equations solved by Eulerian methods, accounts for the effects previously identified as evidence for the Galilean non-invariance of these methods. These velocity-dependent diffusive terms lead to different results for different bulk velocities when the spatial resolution of the simulation is kept fixed, but their effect becomes negligible as the resolution of the simulation is increased to obtain a converged solution. In particular, we find that Kelvin-Helmholtz instabilities develop properly in realistic Eulerian calculations regardless of the bulk velocity provided the problem is simulated with sufficient resolution (a factor of 2-4 increase compared to the case without bulk flows for realistic velocities). Our results reiterate that high-resolution Eulerian methods can perform well and obtain a convergent solution, even in the presence of highly supersonic bulk flows.Comment: Version accepted by MNRAS Oct 2, 2009. Figures degraded. For high-resolution color figures and movies of the numerical simulations, please visit http://www.astro.caltech.edu/~brant/Site/Computational_Eulerian_Hydrodynamics_and_Galilean_Invariance.htm

Polymeric Nanoparticle PET/MR Imaging Allows Macrophage Detection in Atherosclerotic Plaques

Author Manuscript 2013 March 02.Rationale: Myeloid cell content in atherosclerotic plaques associates with rupture and thrombosis. Thus, imaging of lesional monocytes and macrophages could serve as a biomarker of disease progression and therapeutic intervention. Objective: To noninvasively assess plaque inflammation with dextran nanoparticle (DNP)-facilitated hybrid positron emission tomography/magnetic resonance imaging (PET/MRI). Methods and Results: Using clinically approved building blocks, we systematically developed 13-nm polymeric nanoparticles consisting of cross-linked short chain dextrans, which were modified with desferoxamine for zirconium-89 radiolabeling ([superscript 89]Zr-DNP) and a near-infrared fluorochrome (VT680) for microscopic and cellular validation. Flow cytometry of cells isolated from excised aortas showed DNP uptake predominantly in monocytes and macrophages (76.7%) and lower signal originating from other leukocytes, such as neutrophils and lymphocytes (11.8% and 0.7%, P<0.05 versus monocytes and macrophages). DNP colocalized with the myeloid cell marker CD11b on immunohistochemistry. PET/MRI revealed high uptake of [superscript 89]Zr-DNP in the aortic root of apolipoprotein E knock out (ApoE[superscript −/−]) mice (standard uptake value, ApoE[superscript −/−] mice versus wild-type controls, 1.9±0.28 versus 1.3±0.03; P<0.05), corroborated by ex vivo scintillation counting and autoradiography. Therapeutic silencing of the monocyte-recruiting receptor C-C chemokine receptor type 2 with short-interfering RNA decreased [superscript 89]Zr-DNP plaque signal (P<0.05) and inflammatory gene expression (P<0.05). Conclusions: Hybrid PET/MRI with a 13-nm DNP enables noninvasive assessment of inflammation in experimental atherosclerotic plaques and reports on therapeutic efficacy of anti-inflammatory therapy.National Heart, Lung, and Blood InstituteNational Institutes of Health (U.S.). Dept. of Health and Human Services (HHSN268201000044C)National Institutes of Health (U.S.). Dept. of Health and Human Services (R01-HL096576)National Institutes of Health (U.S.). Dept. of Health and Human Services (R01-HL095629)National Institutes of Health (U.S.). Dept. of Health and Human Services (T32-HL094301

The stellar mass spectrum from non-isothermal gravoturbulent fragmentation

Identifying the processes that determine the initial mass function of stars (IMF) is a fundamental problem in star formation theory. One of the major uncertainties is the exact chemical state of the star forming gas and its influence on the dynamical evolution. Most simulations of star forming clusters use an isothermal equation of state (EOS). However, theoretical predictions and observations suggest that the effective polytropic exponent gamma in the EOS varies with density. We address these issues and study the effect of a piecewise polytropic EOS on the formation of stellar clusters in turbulent, self-gravitating molecular clouds using three-dimensional, smoothed particle hydrodynamics simulations. To approximate the results of published predictions of the thermal behavior of collapsing clouds, we increase the polytropic exponent gamma from 0.7 to 1.1 at some chosen density n_c, which we vary. The change of thermodynamic state at n_c selects a characteristic mass scale for fragmentation M_ch, which we relate to the peak of the observed IMF. Our investigation generally supports the idea that the distribution of stellar masses depends mainly on the thermodynamic state of the star-forming gas. The thermodynamic state of interstellar gas is a result of the balance between heating and cooling processes, which in turn are determined by fundamental atomic and molecular physics and by chemical abundances. Given the abundances, the derivation of a characteristic stellar mass can thus be based on universal quantities and constants.Comment: 13 pages, 7 figures, accepted by A&

Understanding hadronic gamma-ray emission from supernova remnants

We aim to test the plausibility of a theoretical framework in which the gamma-ray emission detected from supernova remnants may be of hadronic origin, i.e., due to the decay of neutral pions produced in nuclear collisions involving relativistic nuclei. In particular, we investigate the effects induced by magnetic field amplification on the expected particle spectra, outlining a phenomenological scenario consistent with both the underlying Physics and the larger and larger amount of observational data provided by the present generation of gamma experiments, which seem to indicate rather steep spectra for the accelerated particles. In addition, in order to study to study how pre-supernova winds might affect the expected emission in this class of sources, the time-dependent gamma-ray luminosity of a remnant with a massive progenitor is worked out. Solid points and limitations of the proposed scenario are finally discussed in a critical way.Comment: 30 pages, 5 figures; Several comments, references and a figure added. Some typos correcte

MHD models of Pulsar Wind Nebulae

Pulsar Wind Nebulae (PWNe) are bubbles or relativistic plasma that form when the pulsar wind is confined by the SNR or the ISM. Recent observations have shown a richness of emission features that has driven a renewed interest in the theoretical modeling of these objects. In recent years a MHD paradigm has been developed, capable of reproducing almost all of the observed properties of PWNe, shedding new light on many old issues. Given that PWNe are perhaps the nearest systems where processes related to relativistic dynamics can be investigated with high accuracy, a reliable model of their behavior is paramount for a correct understanding of high energy astrophysics in general. I will review the present status of MHD models: what are the key ingredients, their successes, and open questions that still need further investigation.Comment: 18 pages, 5 figures, Invited Review, Proceedings of the "ICREA Workshop on The High-Energy Emission from Pulsars and their Systems", Sant Cugat, Spain, April 12-16, 201