arXiv subjects

M. G. Edmunds

Publications and source records attributed to M. G. Edmunds.

13 recordsLinked to original sources

arXiv2009

Submillimetre Variability of Eta Carinae: cool dust within the outer ejecta

Previous submillimetre (submm) observations detected 0.7 solar masses of cool dust emission around the Luminous Blue Variable (LBV) star Eta Carinae. These observations were hindered by the low declination of Eta Carinae and contamination from free-free emission orginating from the stellar wind. Here, we present deep submm observations with LABOCA at 870um, taken shortly after a maximum in the 5.5-yr radio cycle. We find a significant difference in the submm flux measured here compared with the previous measurement: the first indication of variability at submm wavelengths. A comparison of the submm structures with ionised emission features suggests the 870um is dominated by emission from the ionised wind and not thermal emission from dust. We estimate 0.4 +/- 0.1 solar masses of dust surrounding Eta Carinae. The spatial distribution of the submm emission limits the mass loss to within the last thousand years, and is associated with mass ejected during the great eruptions and the pre-outburst LBV wind phase; we estimate that Eta Carinae has ejected > 40 solar masses of gas within this timescale.

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arXiv2007

The James Clerk Maxwell Telescope Legacy Survey of Nearby Star-forming Regions in the Gould Belt

This paper describes a James Clerk Maxwell Telescope (JCMT) legacy survey that has been awarded roughly 500 hrs of observing time to be carried out from 2007 to 2009. In this survey we will map with SCUBA-2 (Submillimetre Common User Bolometer Array 2) almost all of the well-known low-mass and intermediate-mass star-forming regions within 0.5 kpc that are accessible from the JCMT. Most of these locations are associated with the Gould Belt. From these observations we will produce a flux-limited snapshot of star formation near the Sun, providing a legacy of images, as well as point-source and extended-source catalogues, over almost 700 square degrees of sky. The resulting images will yield the first catalogue of prestellar and protostellar sources selected by submillimetre continuum emission, and should increase the number of known sources by more than an order of magnitude. We will also obtain CO maps with the array receiver HARP (Heterodyne Array Receiver Programme), in three CO isotopologues, of a large typical sample of prestellar and protostellar sources. We will then map the brightest hundred sources with the SCUBA-2 polarimeter (POL-2), producing the first statistically significant set of polarization maps in the submillimetre. The images and source catalogues will be a powerful reference set for astronomers, providing a detailed legacy archive for future telescopes, including ALMA, Herschel and JWST.

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arXiv2006

Submillimetre Emission from Eta Carinae

We present critical, long-wavelength observations of Eta Carinae in the submillimetre using SCUBA on the JCMT at 850 and 450 um to confirm the presence of a large mass of warm dust around the central star. We fit a two-component blackbody to the IR-submm spectral energy distribution and estimate between 0.3-0.7 solar masses of dust exists in the nebula depending on the dust absorption properties and the extent of contamination from free-free emission at the SCUBA wavelengths. These results provide further evidence that Eta Carinae's circumstellar nebula contains > 10 solar masses of gas, although this may have been ejected on a longer timescale than previously thought.

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arXiv2003

Modification of Dust Grain Structure by Sputtering

We have applied the SRIM computer code to study the sputtering of some likely astrophysical grain materials, and we have shown that selective embedding of metallic projectiles offers a partial explanation of gas-phase depletions. We show that supernova shockwaves sweep a significantly larger mass of interstellar gas per unit time than the shockwaves generated by outflows in star-forming regions. We apply our sputtering model to the bombardment levels expected in a supernova shock, and show that net embedding may dominate over net sputtering, leading to grain growth under some circumstances, particularly when the bombarding gas is enriched with metals from the supernova progenitor star. A combination of short cooling times and net embedding mean that it is possible for a type II supernova to generate more dust that it destroys, and we conclude that, in general, the sputtering process often leads to a compositional change in the grain material rather than simply to grain erosion.

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arXiv2003

Dust obscuration studies along quasar sight lines using simulated galaxies

We use the results of a set of three-dimensional SPH-Treecode simulations which model the formation and early evolution of disk galaxies, including the generation of heavy elements by star formation, to investigate the effects of dust absorption in quasar absorption line systems. Using a simple prescription for the production of dust, we have compared the column density, zinc abundance and optical depth properties of our models to the known properties of Damped Lyman alpha systems. We find that a significant fraction of our model galaxy disks have a higher column density than any observed DLA system. We are also able to show that such parts of the disk tend to be optically thick, implying that any background quasar would be obscured through much of the disk. This would produce the selection effect against the denser absorption systems thought to be present in observations.

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arXiv2003

Cold Dust in Kepler's Supernova Remnant

The timescales to replenish dust from the cool, dense winds of Asymptotic Giant Branch stars are believed to be greater than the timescales for dust destruction. In high redshift galaxies, this problem is further compounded as the stars take longer than the age of the Universe to evolve into the dust production stages. To explain these discrepancies, dust formation in supernovae (SNe) is required to be an important process but until very recently dust in supernova remnants has only been detected in very small quantities. We present the first submillimeter observations of cold dust in Kepler's supernova remnant (SNR) using SCUBA. A two component dust temperature model is required to fit the Spectral Energy Distribution (SED) with $T_{warm} \sim 102$K and $T_{cold} \sim 17$K. The total mass of dust implied for Kepler is $\sim 1M_{\odot}$ - 1000 times greater than previous estimates. Thus SNe, or their progenitors may be important dust formation sites.

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arXiv2003

A census of metals at high and low redshifts and the connection between submillimetre sources and spheroid formation

Deep surveys in many wavebands have shown that the rate at which stars were forming was at least a factor of 10 higher at z > 1 than today. Heavy elements (metals) are produced by stars, and the star formation history deduced by these surveys implies that a significant fraction of metals in the universe today should already exist at z~2-3. However, only 10% of the total metals expected to exist at this epoch have so far been accounted for (in DLAs and the Lyman forest). In this paper, we use the results of submillimetre surveys of the local and high redshift universe to show that there was much more dust in galaxies in the past. We find that a large proportion of the missing metals are traced by this dust, bringing the metals implied from the star formation history and observations into agreement. We also show that the observed distribution of dust masses at high redshift can be reproduced remarkably well by a simple model for the evolution of dust in spheroids, suggesting that the descendants of the dusty galaxies found in the deep submm surveys are the relatively dust-free spiral bulges and ellipticals in the universe today.

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arXiv2002

SCUBA observations of galaxies with metallicity measurements: a new method for determining the relation between submm luminosity and dust mass

Using a new technique we have determined a value for the constant of proportionality between submillimetre emission and dust mass, the dust mass absorption coefficient at 850 microns. Our method has an advantage over previous methods in that we avoid assumptions about the properties of dust in the interstellar medium. Our only assumption is that the fraction of metals incorporated in the dust is a universal constant. To implement our method we require objects that have submillimetre and far-infrared flux measurements as well as gas mass and metallicity estimates. We present data for all the galaxies with suitable measurements, including new submillimetre maps for five galaxies. We find kappa(850 microns)=0.07 m^2/kg

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arXiv2001

On The Cosmic Origins Of Carbon & Nitrogen

We employ analytical and numerical chemical evolution models to study observed trends in abundance ratios involving carbon, nitrogen, and oxygen. Several sets of published stellar yields for both intermediate-mass and massive stars are considered, and the most appropriate sets are selected through the use of analytical models. These yields are then used in the numerical models to match observed data trends in C/O, N/O, and O/H. We conclude that the principal production site for carbon is massive stars, while that for nitrogen is intermediate-mass stars.

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arXiv2000

The Chemical Evolution of the Universe I: High Column Density Absorbers

We construct a simple, robust model of the chemical evolution of galaxies from high to low redshift, and apply it to published observations of damped Lyman-alpha quasar absorption line systems (DLAs). The elementary model assumes quiescent star formation and isolated galaxies (no interactions, mergers or gas flows). We consider the influence of dust and chemical gradients in the galaxies, and hence explore the selection effects in quasar surveys. We fit individual DLA systems to predict some observable properties of the absorbing galaxies, and also indicate the expected redshift behaviour of chemical element ratios involving nucleosynthetic time delays. Despite its simplicity, our `monolithic collapse' model gives a good account of the distribution and evolution of the metallicity and column density of DLAs, and of the evolution of the global star formation rate and gas density below redshifts z 3. However, from the comparison of DLA observations with our model, it is clear that star formation rates at higher redshifts (z>3) are enhanced. Galaxy interactions and mergers, and gas flows very probably play a major role.

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arXiv2000

On The Cosmic Origins Of Carbon And Nitrogen

We analyze the behavior of N/O and C/O abundance ratios as a function of metallicity as gauged by O/H in large, extant Galactic and extragalactic H II region abundance samples. Numerical chemical evolution models are computed using published stellar yields implied by comparing analytical models to the observations. Our results suggest that carbon and nitrogen originate from separate production sites and are decoupled from one another. Massive stars (M>8M_sun) dominate the production of carbon, while intermediate-mass stars between 4 and 8 solar masses, with a characteristic ejection delay time of 250 Myr after their formation, dominate nitrogen production. Carbon production is positively sensitive to metallicity through mass loss processes in massive stars and has a pseudo-secondary character. Nitrogen production in intermediate mass stars is primary at low metallicity, but clearly secondary (and perhaps tertiary) when 12+log(O/H)>8.3. The observed flat behavior of N/O versus O/H in metal-poor galaxies is explained by invoking low star formation rates which flatten the age-metallicity relation and thereby allow N/O to rise to observed levels at low metallicities. The observed scatter and distribution of data points for N/O challenge the popular idea that intermittent polluting by oxygen from massive stars is occurring following star bursts. The effect of inflow of gas into galactic systems on secondary production of nitrogen from carbon may introduce some scatter into N/O ratios at high metallicities.

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arXiv2000

The SCUBA Local Universe Galaxy Survey I: First Measurements of the Submillimetre Luminosity and Dust Mass Functions

We have used SCUBA to observe a complete sample of 104 galaxies selected at 60 microns from the IRAS BGS and we present here the 850 micron measurements. Fitting the 60,100 and 850 micron fluxes with a single temperature dust model gives the sample mean temperature T=36 K and beta = 1.3. We do not rule out the possibility of dust which is colder than this, if a 20 K component was present then our dust masses would increase by factor 1.5-3. We present the first measurements of the luminosity and dust mass functions, which were well fitted by Schechter functions (unlike those 60 microns). We have correlated many global galaxy properties with the submillimetre and find that there is a tendancy for less optically luminous galaxies to contain warmer dust and have greater star formation efficiencies (cf. Young 1999). The average gas-to-dust ratio for the sample is 581 +/- 43 (using both atomic and molecular hydrogen), significantly higher than the Galactic value of 160. We believe this discrepancy is due to a cold dust component at T < 20 K. There is a suprisingly tight correlation between dust mass and the mass of molecular hydrogen as estimated from CO measurements, with an intrinsic scatter of ~50%.

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arXiv1999

The Chemical Evolution of the Universe

We are encouraged by the improving abundance measurements for quasar damped absorption line systems to start a new study of galaxy chemical evolution from high to low redshifts. Our goal is a simple, robust model based on a synthesis of our best understanding of the relevant physics, such as cosmology, large-scale structure formation, galaxy collapse, star formation and chemical evolution. Our initial model assumes continuous quiescent star formation, and no galaxy mergers. We use this galaxy chemical evolution model to study the properties high column density gas out to high redshifts, as revealed by damped Ly-alpha absorption (DLA). We find that there is a significant population of dwarf galaxy DLA systems at moderate redshift (z ~ 2.5, which would not have been accessible to previous imaging surveys around DLA quasars. Our model also provides a reasonable prediction of the global star formation history of the Universe as measured from the UV luminosity density. The next theoretical step is clearly to allow galaxy mergers and hence, model the effects of major bursts of (probably dust-enshrouded) star formation. We are also using time-delayed element production to probe stellar populations in DLA galaxies.

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