arXiv subjects

K. A. Marsh

Publications and source records attributed to K. A. Marsh.

35 records · Page 2Linked to original sources

arXiv2017

Properties of Hi-GAL clumps in the inner Galaxy]{The Hi-GAL compact source catalogue. I. The physical properties of the clumps in the inner Galaxy ($-71.0^{\circ}< \ell < 67.0^{\circ}$)

Hi-GAL is a large-scale survey of the Galactic plane, performed with Herschel in five infrared continuum bands between 70 and 500 $μ$m. We present a band-merged catalogue of spatially matched sources and their properties derived from fits to the spectral energy distributions (SEDs) and heliocentric distances, based on the photometric catalogs presented in Molinari et al. (2016a), covering the portion of Galactic plane $-71.0^{\circ}< \ell < 67.0^{\circ}$. The band-merged catalogue contains 100922 sources with a regular SED, 24584 of which show a 70 $μ$m counterpart and are thus considered proto-stellar, while the remainder are considered starless. Thanks to this huge number of sources, we are able to carry out a preliminary analysis of early stages of star formation, identifying the conditions that characterise different evolutionary phases on a statistically significant basis. We calculate surface densities to investigate the gravitational stability of clumps and their potential to form massive stars. We also explore evolutionary status metrics such as the dust temperature, luminosity and bolometric temperature, finding that these are higher in proto-stellar sources compared to pre-stellar ones. The surface density of sources follows an increasing trend as they evolve from pre-stellar to proto-stellar, but then it is found to decrease again in the majority of the most evolved clumps. Finally, we study the physical parameters of sources with respect to Galactic longitude and the association with spiral arms, finding only minor or no differences between the average evolutionary status of sources in the fourth and first Galactic quadrants, or between "on-arm" and "inter-arm" positions.

astro-ph.GA↗

arXiv2016

Evidence that widespread star formation may be underway in G0.253+016, "The Brick"

Image cubes of differential column density as a function of dust temperature are constructed for Galactic Centre molecular cloud G0.253+0.016 ("The Brick") using the recently described PPMAP procedure. The input data consist of continuum images from the Herschel Space Telescope in the wavelength range 70-500 $μ$m, supplemented by previously published interferometric data at 1.3 mm wavelength. While the bulk of the dust in the molecular cloud is consistent with being heated externally by the local interstellar radiation field, our image cube shows the presence, near one edge of the cloud, of a filamentary structure whose temperature profile suggests internal heating. The structure appears as a cool ($\sim 14$ K) tadpole-like feature, $\sim 6$ pc in length, in which is embedded a thin spine of much hotter ($\sim$ 40-50 K) material. We interpret these findings in terms of a cool filament whose hot central region is undergoing gravitational collapse and fragmentation to form a line of protostars. If confirmed, this would represent the first evidence of widespread star formation having started within this cloud.

astro-ph.GA↗

arXiv2016

A census of dense cores in the Taurus L1495 cloud from the Herschel Gould Belt Survey

We present a catalogue of dense cores in a $\sim 4^\circ\times2^\circ$ field of the Taurus star-forming region, inclusive of the L1495 cloud, derived from Herschel SPIRE and PACS observations in the 70 $μ$m, 160 $μ$m, 250 $μ$m, 350 $μ$m, and 500 $μ$m continuum bands. Estimates of mean dust temperature and total mass are derived using modified blackbody fits to the spectral energy distributions. We detect 525 starless cores of which $\sim10$-20% are gravitationally bound and therefore presumably prestellar. Our census of unbound objects is $\sim85$% complete for $M>0.015\,M_\odot$ in low density regions ($A_V\stackrel{<}{_\sim}5$ mag), while the bound (prestellar) subset is $\sim85$% complete for $M>0.1\,M_\odot$ overall. The prestellar core mass function (CMF) is consistent with lognormal form, resembling the stellar system initial mass function, as has been reported previously. All of the inferred prestellar cores lie on filamentary structures whose column densities exceed the expected threshold for filamentary collapse, in agreement with previous reports. Unlike the prestellar CMF, the unbound starless CMF is not lognormal, but instead is consistent with a power-law form below $0.3\,M_\odot$ and shows no evidence for a low-mass turnover. It resembles previously reported mass distributions for CO clumps at low masses ($M\stackrel{<}{_\sim}0.3\,M_\odot$). The volume density PDF, however, is accurately lognormal except at high densities. It is consistent with the effects of self-gravity on magnetized supersonic turbulence. The only significant deviation from lognormality is a high-density tail which can be attributed unambiguously to prestellar cores.

astro-ph.GA↗

arXiv2015

Evidence for high-energy and low-emittance electron beams using ionization injection of charge in a plasma wakefield accelerator

Ionization injection in a plasma wakefield accelerator was investigated experimentally using two lithium plasma sources of different lengths. The ionization of the helium gas, used to confine the lithium, injects electrons in the wake. After acceleration, these injected electrons were observed as a distinct group from the drive beam on the energy spectrometer. They typically have a charge of tens of pC, an energy spread of a few GeV, and a maximum energy of up to 30 GeV. The emittance of this group of electrons can be many times smaller than the initial emittance of the drive beam. The energy scaling for the trapped charge from one plasma length to the other is consistent with the blowout theory of the plasma wakefield.

physics.acc-ph↗

arXiv2015

Self-modulated laser wakefield accelerators as x-ray sources

The development of a directional, small-divergence, and short-duration picosecond x-ray probe beam with an energy greater than 50 keV is desirable for high energy density science experiments. We therefore explore through particle-in-cell (PIC) computer simulations the possibility of using x-rays radiated by betatron-like motion of electrons from a self-modulated laser wakefield accelerator as a possible candidate to meet this need. Two OSIRIS 2D PIC simulations with mobile ions are presented, one with a normalized vector potential a0 = 1.5 and the other with an a0 = 3. We find that in both cases direct laser acceleration (DLA) is an important additional acceleration mechanism in addition to the longitudinal electric field of the plasma wave. Together these mechanisms produce electrons with a continuous energy spectrum with a maximum energy of 300 MeV for a0 = 3 case and 180 MeV in the a0 = 1.5 case. Forward-directed x-ray radiation with a photon energy up to 100 keV was calculated for the a0 = 3 case and up to 12 keV for the a0 = 1.5 case. The x-ray spectrum can be fitted with a sum of two synchrotron spectra with critical photon energy of 13 and 45 keV for the a0 of 3 and critical photon energy of 0.3 and 1.4 keV for a0 of 1.5 in the plane of polarization of the laser. The full width at half maximum divergence angle of the x-rays was 62 x 1.9 mrad for a0 = 3 and 77 x 3.8 mrad for a0 = 1.5.

physics.plasm-ph↗

arXiv2015

Temperature as a third dimension in column-density mapping of dusty astrophysical structures associated with star formation

We present PPMAP, a Bayesian procedure that uses images of dust continuum emission at multiple wavelengths to produce resolution-enhanced image cubes of differential column-density as a function of dust temperature and position. PPMAP is based on the generic 'point process' formalism, whereby the system of interest (in this case, a dusty astrophysical structure such as a filament or prestellar core) is represented by a collection of points in a suitably defined state space. It can be applied to a variety of observational data, such as Herschel images, provided only that the image intensity is delivered by optically thin dust in thermal equilibrium. PPMAP takes full account of the instrumental point spread functions and does not require all images to be degraded to the same resolution. We present the results of testing using simulated data for a prestellar core and a fractal turbulent cloud, and demonstrate its performance with real data from the Hi-GAL survey. Specifically, we analyse observations of a large filamentary structure in the CMa OB1 giant molecular cloud. Histograms of differential column-density indicate that the warm material (T > 13 K) is distributed log-normally, consistent with turbulence, but the column-densities of the cooler material are distributed as a high density tail, consistent with the effects of self-gravity. The results illustrate the potential of PPMAP to aid in distinguishing between different physical components along the line of sight in star-forming clouds, and aid the interpretation of the associated PDFs of column density.

astro-ph.IM↗

arXiv2015

Estimation of direct laser acceleration in laser wakefield accelerators using particle-in-cell simulations

Many current laser wakefield acceleration (LWFA) experiments are carried out in a regime where the laser pulse length is on the order of or longer than the wake wavelength and where ionization injection is employed to inject electrons into the wake. In these experiments, the trapped electrons will co-propagate with the longitudinal wakefield and the transverse laser field. In this scenario, the electrons can gain a significant amount of energy from both the direct laser acceleration (DLA) mechanism as well as the usual LWFA mechanism. Particle-in-cell (PIC) codes are frequently used to discern the relative contribution of these two mechanisms. However, if the longitudinal resolution used in the PIC simulations is inadequate, it can produce numerical heating that can overestimate the transverse motion, which is important in determining the energy gain due to DLA. We have therefore carried out a systematic study of this LWFA regime by varying the longitudinal resolution of PIC simulations from the standard, best-practice resolution of 30 points per laser wavelength to four times that value and then examining the energy gain characteristics of both the highest-energy electrons and the bulk electrons. By calculating the contribution of DLA to the final energies of the electrons produced from the LWFA, we find that although the transverse momentum and oscillation radii are over-estimated in the lower-resolution simulations, this over-estimation does not lead to artificial energy gain by DLA. Rather, the DLA contribution to the highest-energy electrons is larger in the higher-resolution cases because the DLA resonance is better maintained. Thus, even at the highest longitudinal resolutions, DLA contributes a significant portion of the energy gained by the highest-energy electrons and also contributes to accelerating the bulk of the charge in the electron beam produced by the LWFA.

physics.plasm-ph↗

arXiv2015

Satisfying the Direct Laser Acceleration Resonance Condition in a Laser Wakefield Accelerator

In this proceeding, we show that when the drive laser pulse overlaps the trapped electrons in a laser wakefield accelerator (LWFA), those electrons can gain energy from direct laser acceleration (DLA) over extended distances despite the evolution of both the laser and the wake. Through simulations, the evolution of the properties of both the laser and the electron beam is quantified, and then the resonance condition for DLA is examined in the context of this change. We find that although the electrons produced from the LWFA cannot continuously satisfy the DLA resonance condition, they nevertheless can gain a significant amount of energy from DLA.

physics.plasm-ph↗

arXiv2015

Sub-femtosecond electron bunches created by direct laser acceleration in a laser wakefield accelerator with ionization injection

In this work, we will show through three-dimensional particle-in-cell simulations that direct laser acceleration in laser a wakefield accelerator can generate sub-femtosecond electron bunches. Two simulations were done with two laser pulse durations, such that the shortest laser pulse occupies only a fraction of the first bubble, whereas the longer pulse fills the entire first bubble. In the latter case, as the trapped electrons moved forward and interacted with the high intensity region of the laser pulse, micro-bunching occurred naturally, producing 0.5 fs electron bunches. This is not observed in the short pulse simulation.

physics.plasm-ph↗

arXiv2014

Properties of Starless and Prestellar Cores in Taurus Revealed by Herschel SPIRE/PACS Imaging

The density and temperature structures of dense cores in the L1495 cloud of the Taurus star-forming region are investigated using Herschel SPIRE and PACS images in the 70 $μ$m, 160 $μ$m, 250 $μ$m, 350 $μ$m and 500 $μ$m continuum bands. A sample consisting of 20 cores, selected using spectral and spatial criteria, is analysed using a new maximum likelihood technique, COREFIT, which takes full account of the instrumental point spread functions. We obtain central dust temperatures, $T_0$, in the range 6-12 K and find that, in the majority of cases, the radial density falloff at large radial distances is consistent with the $r^{-2}$ variation expected for Bonnor-Ebert spheres. Two of our cores exhibit a significantly steeper falloff, however, and since both appear to be gravitationally unstable, such behaviour may have implications for collapse models. We find a strong negative correlation between $T_0$ and peak column density, as expected if the dust is heated predominantly by the interstellar radiation field. At the temperatures we estimate for the core centres, carbon-bearing molecules freeze out as ice mantles on dust grains, and this behaviour is supported here by the lack of correspondence between our estimated core locations and the previously-published positions of H$^{13}$CO$^+$ peaks. On this basis, our observations suggest a sublimation-zone radius typically $\sim 10^4$ AU. Comparison with previously-published N$_2$H$^+$ data at 8400 AU resolution, however, shows no evidence for N$_2$H$^+$ depletion at that resolution.

astro-ph.GA↗

arXiv2012

New brown dwarf disks in Upper Scorpius observed with WISE

We present a census of the disk population for UKIDSS selected brown dwarfs in the 5-10 Myr old Upper Scorpius OB association. For 116 objects originally identified in UKIDSS, the majority of them not studied in previous publications, we obtain photometry from the WISE database. The resulting colour-magnitude and colour-colour plots clearly show two separate populations of objects, interpreted as brown dwarfs with disks (class II) and without disks (class III). We identify 27 class II brown dwarfs, 14 of them not previously known. This disk fraction (27 out of 116 or 23%) among brown dwarfs was found to be similar to results for K/M stars in Upper Scorpius, suggesting that the lifetimes of disks are independent of the mass of the central object for low-mass stars and brown dwarfs. 5 out of 27 disks (19%) lack excess at 3.4 and 4.6 microns and are potential transition disks (i.e. are in transition from class II to class III). The transition disk fraction is comparable to low-mass stars. We estimate that the timescale for a typical transition from class II to class III is less than 0.4 Myr for brown dwarfs. These results suggest that the evolution of brown dwarf disks mirrors the behaviour of disks around low-mass stars, with disk lifetimes on the order of 5-10 Myr and a disk clearing timescale significantly shorter than 1 Myr.

astro-ph.SR↗

arXiv2012

Variability Flagging in the WISE Preliminary Data Release

The Wide-field Infrared Survey Explorer Preliminary Data Release Source Catalog contains over 257 million objects. We describe the method used to flag variable source candidates in the Catalog. Using a method based on the chi- square of single-exposure flux measurements, we generated a variability flag for each object, and have identified almost 460,000 candidates sources that exhibit significant flux variability with greater than \sim 7σ confidence. We discuss the flagging method in detail and describe its benefits and limitations. We also present results from the flagging method, including example light curves of several types of variable sources including Algol-type eclipsing binaries, RR Lyr, W UMa, and a blazar candidate.

astro-ph.IM↗

arXiv2007

Highly Collimated Jets and Wide-Angle Outflows in HH46/47: New Evidence from Spitzer IR Images

We present new details of the structure and morphology of the jets and outflows in HH46/47 as seen in Spitzer infrared images from IRAC and MIPS, reprocessed using the ``HiRes'' deconvolution technique. HiRes improves the visualization of spatial morphology by enhancing resolution (to sub-arcsec levels in IRAC bands) and removing the contaminating side lobes from bright sources. In addition to sharper views of previously reported bow shocks, we have detected: (i) the sharply-delineated cavity walls of the wide-angle biconical outflow, seen in scattered light on both sides of the protostar, (ii) several very narrow jet features at distances 400 AU to 0.1 pc from the star, and, (iii) compact emissions at MIPS 24 micron coincident with the jet heads, tracing the hottest atomic/ionic gas in the bow shocks.

astro-ph↗

arXiv2006

Point Process Algorithm: A New Bayesian Approach for Planet Signal Extraction with the Terrestrial Planet Finder

The capability of the Terrestrial Planet Finder Interferometer (TPF-I) for planetary signal extraction, including both detection and spectral characterization, can be optimized by taking proper account of instrumental characteristics and astrophysical prior information. We have developed the Point Process Algorithm (PPA), a Bayesian technique for extracting planetary signals using the sine-chopped outputs of a dual nulling interferometer. It is so-called because it represents the system being observed as a set of points in a suitably-defined state space, thus providing a natural way of incorporating our prior knowledge of the compact nature of the targets of interest. It can also incorporate the spatial covariance of the exozodi as prior information which could help mitigate against false detections. Data at multiple wavelengths are used simultaneously, taking into account possible spectral variations of the planetary signals. Input parameters include the RMS measurement noise and the a priori probability of the presence of a planet. The output can be represented as an image of the intensity distribution on the sky, optimized for the detection of point sources. Previous approaches by others to the problem of planet detection for TPF-I have relied on the potentially non-robust identification of peaks in a "dirty" image, usually a correlation map. Tests with synthetic data suggest that the PPA provides greater sensitivity to faint sources than does the standard approach (correlation map + CLEAN), and will be a useful tool for optimizing the design of TPF-I.

astro-ph↗

arXiv2006

Images of Vega Dust Ring at 350 and 450 microns: New Clues to the Trapping of Multiple-Sized Dust Particles in Planetary Resonances

We have used the SHARC II camera at Caltech Submillimeter Observatory to make 350 and 450 micron images of the Vega dust disk at spatial resolutions (FWHM) of 9.7" and 11.1", respectively. The images show a ring-like morphology (radius \~ 100 AU) with inhomogeneous structure that is qualitatively different from that previously reported at 850 microns and longer wavelengths. We attribute the 350/450 micron emission to a grain population whose characteristic size (~ 1 mm) is intermediate between that of the cm-sized grains responsible for emission longward of 850 microns and the much smaller grains (< 18 microns) in the extensive halo, visible at 70 microns, discussed by Su et al. (2005). We have combined our submillimeter images with Spitzer data at 70 microns to produce 2-d maps of line-of-sight optical depth (relative column density). These "tau maps" suggest that the mm-sized grains are located preferentially in three symmetrically-located concentrations. If so, then this structure could be understood in terms of the Wyatt (2003) model in which planetesimals are trapped in the mean motion resonances of a Neptune-mass planet at 65 AU, provided allowance is made for the spatial distribution of dust grains to differ from that of the parent planetesimals. The peaks of the tau maps are, in fact, located near the expected positions corresponding to the 4:3 resonance. If this identification is confirmed by future observations, it would resolve an ambiguity with regard to the location of the planet.

astro-ph↗

arXiv2005

Image of Fomalhaut Dust Ring at 350 Microns: Relative Column Density Map Shows Pericenter-Apocenter Asymmetry

We have imaged the circumstellar disk of Fomalhaut at 350 microns wavelength, using SHARC II at the Caltech Submillimeter Observatory. The spatial resolution of the raw images (9 arcsec) has been enhanced by a factor of three using the HiRes deconvolution procedure. We find that at this wavelength and signal to noise ratio (~ 12), the observed morphology is that of a simple inclined ring (i ~ 70 deg), with little or no other apparent structure; this is the first observation that shows clearly the ring morphology of the disk. We have combined our 350 micron data with Spitzer Space Telescope images at 24, 70, and 160 microns in order to estimate the 2-dimensional spatial variation of relative column density ("tau map") using our DISKFIT procedure. The tau map is based on the following physical assumptions: (1) the wavelength variation of opacity is the same throughout the disk, (2) the radial variation of dust temperature is dictated by the energy balance of individual grains in the stellar radiation field, and (3) the vertical scale height of the disk follows a power-law radial variation. The results confirm the ring-like morphology, but also show that the geometric center is displaced from the star by about 8 AU and that the ring has an apocentric enhancement of approximately 14% in integrated column density. If we interpret the displacement in terms of elliptical orbital motion due to gravitational perturbation by an unseen planet, then the implied forced eccentricity is ~ 0.06; dynamical modeling then predicts an apocentric density enhancement consistent with that inferred from the tau map.

astro-ph↗

arXiv1998

High Resolution Mid-Infrared Imaging of the Nucleus of NGC 1068

We have obtained mid-infrared images of the nucleus of NGC 1068 from the Hale 5 m telescope at Mt. Palomar with diffraction-limited resolution and high sensitivity at 8.8, 10.3, and 12.5 microns. Deconvolved images show that the infrared emission extends north to south in the inner 2", consisting of a central peak, a component extending 1" north of the central source, a component extending 1" south of the central source, and several smaller structures located 1" to the northeast. The central peak is extended 0.4" N-S and unresolved (<= 0.2") E-W. We find that 50 +/- 5% of the flux emerges from the central 0.4" and that a single unresolved point source can account for only 27 +/- 5% of the total flux. However, if the central peak arises from optically thick emission, we estimate that the emitting region has a projected area >= 2 pc^2, and may thus contain a compact source such as a parsec-scale torus. We observe a correspondence between the northern extension and the northeastern sources appearing on the mid-infrared images and the [OIII] clouds A-C & E. We interpret the faint optical counterpart to the mid-infrared southern extension as being due to partial obscuration by the intervening disk of the host galaxy. The N-S extension of the mid-infrared emission coincides with one wall of the conical narrow line region and aligns with the N-S orientation of the radio jet close to the nucleus. We interpret the infrared emission as arising from optically thick dust lining the walls of the low density cavity formed by the radio jet and heated by radiation from the central source.

astro-ph↗