arXiv subjects

Barbara Ercolano

Publications and source records attributed to Barbara Ercolano.

At least 109 records · Page 6Linked to original sources

arXiv2009

Multiple ionisation sources in HII regions and their effect on derived nebular abundances

We present a theoretical investigation of the effect of multiple ionisation sources in HII regions on the total elemental abundances derived from the analysis of collisionally excited emission lines. We focus on empirical methods based on direct temperature measurements that are commonly employed in cases when the temperature of the nebular gas can be determined from the ratio of nebular to auroral lines of (e.g.) doubly ionised oxygen. We find that direct temperature methods that employ a two-temperature zone approach (DT2T methods) are very robust against the spatial distribution of sources. Errors smaller than 0.15 dex are estimated for regions where the metallicity is twice solar and errors below 0.05 dex for solar metallicities and below. The biases introduced by the spatial distribution of the ionisation sources are thus much smaller for DT2T methods than for strong line methods, previously investigated by Ercolano, Bastian & Stasinska. Our findings are in agreement with the recent study of HII regions in NGC 300 by Bresolin et al.

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arXiv2009

Can X-rays provide a solution to the abundance discrepancy problem in photoionised nebulae?

We re-examine the well-known discrepancy between ionic abundances determined via the analysis of recombination lines (RLs) and collisionally excited lines (CELs). We show that abundance variations can be mimicked in a {\it chemically homogeneous} medium by the presence of dense X-ray irradiated regions which present different ionisation and temperature structures from those of the more diffuse medium they are embedded in, which is predominantly ionised by extreme-ultraviolet radiation. The presence of X-ray ionised dense clumps or filaments also naturally explains the lower temperatures often measured from O {\sc ii} recombination lines and from the Balmer jump when compared to temperatures determined by CELs. We discuss the implications for abundances determined via the analysis of CELs and RLs and provide a simple analytical procedure to obtain upwards corrections for CEL-determined abundance. While we show that the abundance discrepancy factor (ADF) and the Balmer Jump temperature determined from observations of the Orion Nebula can simultaneously be reproduced by this model (implying upward corrections for CELs by a factor of 1.15), we find that the required X-ray fluxes exceed the known Orion's stellar and diffuse X-ray budget, if we assume that the clumps are located at the edge of the blister. We propose, however, that spatially resolved observations may be used to empirically test the model, and we outline how the framework developed in this letter may be applied in the future to objects with better constrained geometries (e.g. planetary nebulae).

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arXiv2009

X-ray Photoevaporation-starved T Tauri Accretion

X-ray luminosities of accreting T Tauri stars are observed to be systematically lower than those of non-accretors. There is as yet no widely accepted physical explanation for this effect, though it has been suggested that accretion somehow suppresses, disrupts or obscures coronal X-ray activity. Here, we suggest that the opposite might be the case: coronal X-rays modulate the accretion flow. We re-examine the X-ray luminosities of T Tauri stars in the Orion Nebula Cluster and find that not only are accreting stars systematically fainter, but that there is a correlation between mass accretion rate and stellar X-ray luminosity. We use the X-ray heated accretion disk models of Ercolano et al. to show that protoplanetary disk photoevaporative mass loss rates are strongly dependent on stellar X-ray luminosity and sufficiently high to be competitive with accretion rates. X-ray disk heating appears to offer a viable mechanism for modulating the gas accretion flow and could be at least partially responsible for the observed correlation between accretion rates and X-ray luminosities of T Tauri stars.

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arXiv2009

X-ray irradiated protoplanetary disk atmospheres II: Predictions from models in hydrostatic equilibrium

We present new models for the X-ray photoevaporation of circumstellar discs which suggest that the resulting mass loss (occurring mainly over the radial range 10-40 AU) may be the dominant dispersal mechanism for gas around low mass pre-main sequence stars, contrary to the conclusions of previous workers. Our models combine use of the MOCASSIN Monte Carlo radiative transfer code and a self-consistent solution of the hydrostatic structure of the irradiated disc. We estimate the resulting photoevaporation rates assuming sonic outflow at the surface where the gas temperature equals the local escape temperature and derive mass loss rates of ~10^{-9} M_sun/yr, typically a factor 2-10 times lower than the corresponding rates in our previous work (Ercolano et al., 2008) where we did not adjust the density structure of the irradiated disc. The somewhat lower rates, and the fact that mass loss is concentrated towards slightly smaller radii, result from the puffing up of the heated disc at a few AU which partially screens the disc at tens of AU. (.....) We highlight the fact that X-ray photoevaporation has two generic advantages for disc dispersal compared with photoevaporation by extreme ultraviolet (EUV) photons that are only modestly beyond the Lyman limit: the demonstrably large X-ray fluxes of young stars even after they have lost their discs and the fact that X-rays are effective at penetrating much larger columns of material close to the star (abridged).

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arXiv2009

The Effect of Line of Sight Temperature Variation and Noise on Dust Continuum Observations

We investigate the effect of line of sight temperature variations and noise on two commonly used methods to determine dust properties from dust continuum observations of dense cores. One method employs a direct fit to a modified blackbody SED; the other involves a comparison of flux ratios to an analytical prediction. Fitting fluxes near the SED peak produces inaccurate temperature and dust spectral index estimates due to the line of sight temperature (and density) variations. Longer wavelength fluxes in the Rayleigh-Jeans part of the spectrum (>~ 600 micron for typical cores) may more accurately recover the spectral index, but both methods are very sensitive to noise. The temperature estimate approaches the density weighted temperature, or "column temperature," of the source as short wavelength fluxes are excluded. An inverse temperature - spectral index correlation naturally results from SED fitting, due to the inaccurate isothermal assumption, as well as noise uncertainties. We show that above some "threshold" temperature, the temperatures estimated through the flux ratio method can be highly inaccurate. In general, observations with widely separated wavelengths, and including shorter wavelengths, result in higher threshold temperatures; such observations thus allow for more accurate temperature estimates of sources with temperatures less than the threshold temperature. When only three fluxes are available, a constrained fit, where the spectral index is fixed, produces less scatter in the temperature estimate when compared to the estimate from the flux ratio method.

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arXiv2009

The transition disc frequency in M stars

We re-examine the recent suggestion of a high fraction of transition discs (i.e. those with a cleared inner hole) in M stars, motivated by the fact that we expect that, for M stars, even discs without inner holes should exhibit very weak excess shortward of around 10um. Our analysis of spectral energy distribution models suggest that this indeed means that M stars where a detectable excess begins at around 6um may be mis-classified as transition discs when in fact they have optically thick dust extending in to the dust sublimation radius. Consequently, we estimate that the transition disc fraction among M stars in the Coronet cluster is ~15 +/-10 % (rather than the recently claimed value of 50%). This revised figure would imply that the transition disc fraction is not after all markedly higher in later type stars. We suggest that for M stars, transition discs can only be readily identified if they have emission that is close to photospheric out to > 10um.

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arXiv2008

The Spatial Evolution of Stellar Structures in the LMC

We present an analysis of the spatial distribution of various stellar populations within the Large Magellanic Cloud. We combine mid-infrared selected young stellar objects, optically selected samples with mean ages between ~9 and ~1000 Myr, and existing stellar cluster catalogues to investigate how stellar structures form and evolve within the LMC. For the analysis we use Fractured Minimum Spanning Trees, the statistical Q parameter, and the two-point correlation function. Restricting our analysis to young massive (OB) stars we confirm our results obtained for M33, namely that the luminosity function of the groups is well described by a power-law with index -2, and that there is no characteristic length-scale of star-forming regions. We find that stars in the LMC are born with a large amount of substructure, consistent with a 2D fractal distribution with dimension ~1.8 and evolve towards a uniform distribution on a timescale of ~175 Myr. This is comparable to the crossing time of the galaxy and we suggest that stellar structure, regardless of spatial scale, will be eliminated in a crossing time. This may explain the smooth distribution of stars in massive/dense young clusters in the Galaxy, while other, less massive, clusters still display large amounts of structure at similar ages. By comparing the stellar and star cluster distributions and evolving timescales, we show that infant mortality of clusters (or 'popping clusters') have a negligible influence on galactic structure. Finally, we quantify the influence of the elongation, differential extinction, and contamination of a population on the measured Q value.

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arXiv2008

Fe K alpha and hydrodynamic loop model diagnostics for a large flare on II Peg

The observation by the Swift X-ray Telescope of the Fe K alpha_1, alpha_2 doublet during a large flare on the RS CVn binary system II Peg represents one of only two firm detections to date of photospheric Fe K alpha from a star other than our Sun. We present models of the Fe K alpha equivalent widths reported in the literature for the II Peg observations and show that they are most probably due to fluorescence following inner shell photoionisation of quasi-neutral Fe by the flare X-rays. Our models constrain the maximum height of flare the to 0.15 R_* assuming solar abundances for the photospheric material, and 0.1 R_* and 0.06 R_* assuming depleted photospheric abundances ([M/H]=-0.2 and [M/H]=-0.4, respectively). Accounting for an extended loop geometry has the effect of increasing the estimated flare heights by a factor of ~3. These predictions are consistent with those derived using results of flaring loop models, which are also used to estimate the flaring loop properties and energetics. From loop models we estimate a flare loop height of 0.13 R_*, plasma density of ~4 * 10^12 cm^-3 and emitting volume of ~6 * 10^30 cm^3. Our estimates for the flare dimensions and density allow us to estimate the conductive energy losses to E_cond <= 2 * 10^36 erg, consistent with upper limits previously obtained in the literature. Finally, we estimate the average energy output of this large flare to be ~10^33 erg sec^-1, or 1/10th of the stellar bolometric luminosity.

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arXiv2008

X-ray irradiated protoplanetary disk atmospheres I: Predicted emission line spectrum and photoevaporation

We present MOCASSIN 2D photoionisation and dust radiative transfer models of a prototypical T Tauri disk irradiated by X-rays from the young pre-main sequence star. The calculations demonstrate a layer of hot gas reaching temperatures of ~10^6 K at small radii and ~10^4 K at a distance of 1 AU. The gas temperatures decrease sharply with depth, but appear to be completely decoupled from dust temperatures down to a column depth of ~5*10^21 cm^-2. We predict that several fine-structure and forbidden lines of heavy elements, as well as recombination lines of hydrogen and helium, should be observable with current and future instrumentation, although optical lines may be smothered by the stellar spectrum. Predicted line luminosities are given for the the brightest collisionally excited lines (down to ~10^-8L_sun, and for recombination transitions from several levels of HI and HeI. The mass loss rate due to X-ray photoevaporation estimated from our models is of the order of 10^-8 M_sun yr^-1, implying a dispersal timescale of a few Myr for a disk of mass 0.027 M_sun, which is the mass of the disk structure model we employed. We discuss the limitations of our model and highlight the need for further calculations that should include the simultaneous solution of the 2D radiative transfer problem and the 1D hydrostatic equilibrium in the polar direction.

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arXiv2008

On the Detectability of Oxygen X-ray Fluorescence and its Use as a Solar Photospheric Abundance Diagnostic

Monte Carlo calculations of the O Kalpha line fluoresced by coronal X-rays and emitted just above the temperature minimum region of the solar atmosphere have been employed to investigate the use of this feature as an abundance diagnostic. While quite weak, we estimate line equivalent widths in the range 0.02-0.2 AA, depending on the X-ray plasma temperature. The line remains essentially uncontaminated by blends for coronal temperatures T =< 3e6 K and should be quite observable, with a flux >~ 2 ph/s/arcmin^2. Model calculations for solar chemical mixtures with an O abundance adjusted up and down by a factor of 2 indicate 35-60% changes in O Kalpha line equivalent width, providing a potentially useful O abundance diagnostic. Sensitivity of equivalent width to differences between recently recommended chemical compositions with ``high'' and ``low'' complements of the CNO trio important for interpreting helioseismological observations is less accute, amounting to 20-26% at coronal temperatures T ~< 2e6 K. While still feasible for discriminating between these two mixtures, uncertainties in measured line equivalent widths and in the models used for interpretation would need to be significantly less than 20%. Provided a sensitive X-ray spectrometer with resolving power >= 1000 and suitably well-behaved instrumental profile can be built, X-ray fluorescence presents a viable means for resolving the solar ``oxygen crisis''.

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arXiv2007

X-ray enabled MOCASSIN: a 3D code for photoionized media

We present a new version of the fully 3D photoionization and dust radiative transfer code, MOCASSIN, that uses a Monte Carlo approach for the transfer of radiation. The X-ray enabled MOCASSIN allows a fully geometry independent description of low-density gaseous environments strongly photoionized by a radiation field extending from radio to gamma rays. The code has been thoroughly benchmarked against other established codes routinely used in the literature, using simple plane parallel models designed to test performance under standard conditions. We show the results of our benchmarking exercise and discuss applicability and limitations of the new code, which should be of guidance for future astrophysical studies with MOCASSIN.

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arXiv2007

X-ray Fluorescent Fe Kalpha Lines from Stellar Photospheres

X-ray spectra from stellar coronae are reprocessed by the underlying photosphere through scattering and photoionization events. While reprocessed X-ray spectra reaching a distant observer are at a flux level of only a few percent of that of the corona itself, characteristic lines formed by inner shell photoionization of some abundant elements can be significantly stronger. The emergent photospheric spectra are sensitive to the distance and location of the fluorescing radiation and can provide diagnostics of coronal geometry and abundance. Here we present Monte Carlo simulations of the photospheric Kalpha doublet arising from quasi-neutral Fe irradiated by a coronal X-ray source. Fluorescent line strengths have been computed as a function of the height of the radiation source, the temperature of the ionising X-ray spectrum, and the viewing angle. We also illustrate how the fluorescence efficiencies scale with the photospheric metallicity and the Fe abundance. Based on the results we make three comments: (1) fluorescent Fe lines seen from pre-main sequence stars mostly suggest flared disk geometries and/or super-solar disk Fe abundances; (2) the extreme ~1400 mA line observed from a flare on V1486 Ori can be explained entirely by X-ray fluorescence if the flare itself were partially eclipsed by the limb of the star; and (3) the fluorescent Fe line detected by Swift during a large flare on II Peg is consistent with X-ray excitation and does not require a collisional ionisation contribution. There is no convincing evidence supporting the energetically challenging explanation of electron impact excitation for observed stellar Fe Kalpha lines.

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arXiv2007

The Detectability of Neon Fluorescence and Measurement of the Solar Photospheric Neon Abundance

Monte Carlo calculations of the Ne K$α$ line fluoresced by coronal x-rays and emitted near the temperature minimum region of the solar atmosphere have been employed to investigate the use of this feature to measure directly the solar photospheric Ne abundance. Though very weak, comparison with spectral line databases indicates that at plasma temperatures typical of the quiet Sun and cool active regions ($\leq 2\times 10^6$ K) the line is isolated and unblended. A canonical solar chemical composition yields an equivalent width of $\sim 6$ mÅ(0.3 eV) when observed at heliocentric angles $\sim 0$. For a 1 arcmin field of view, photon fluxes at Earth are of order 0.2 ph s$^{-1}$ for the quiet Sun, rendering the Ne K$α$ fluorescent line a quite feasible means for determining the solar photospheric Ne content.

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arXiv2007

A new algorithm for modelling photoionising radiation in smoothed particle hydrodynamics

We present a new fast algorithm which allows the simulation of ionising radiation emitted from point sources to be included in high-resolution three-dimensional smoothed particle hydrodynamics simulations of star cluster formation. We employ a Strömgren volume technique in which we use the densities of particles near the line-of-sight between the source and a given target particle to locate the ionisation front in the direction of the target. Along with one--dimensional tests, we present fully three--dimensional comparisons of our code with the three--dimensional Monte-Carlo radiative transfer code, MOCASSIN, and show that we achieve good agreement, even in the case of highly complex density fields.

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arXiv2006

Theoretical calculations of the HI, HeI and HeII free-bound continuou emission spectra

We present coefficients for the calculation of the continuous emission spectra of HI, HeI and HeII due to electron-ion recombination. Coefficients are given for photon energies from the first ionization threshold for each ion to the n=20 threshold of hydrogen (36.5um), and for temperatures 100 K <= Te <=10^5 K. The emission coefficients for HeI are derived from accurate ab initio photoionization data. The coefficients are scaled in such a way that they may be interpolated by a simple scheme with uncertainties less than 1% in the whole temperature and wavelength domain. The data are suitable for incorporation into photoionisation/plasma codes and should aid with the interpretation of spectra from the very cold ionised gas phase inferred to exist in a number of gaseous clouds.

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arXiv2006

Massive-Star Supernovae as Major Dust Factories

We present late-time optical and mid-infrared observations of the Type-II supernova 2003gd in NGC 628. Mid-infrared excesses consistent with cooling dust in the ejecta are observed 499-678 days after outburst, and are accompanied by increasing optical extinction and growing asymmetries in the emission-line profiles. Radiative-transfer models show that up to 0.02 solar masses of dust has formed within the ejecta, beginning as early as 250 days after outburst. These observations show that dust formation in supernova ejecta can be efficient and that massive-star supernovae can be major dust producers throughout the history of the Universe.

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arXiv2006

3D photoionisation and dust RT modelling with MOCASSIN: geometry effects on the emission line spectra of star-forming regions

Emission line spectra from HII regions are often used to study properties of the gas in star-forming regions, as well as temperatures and luminosities of the ionising sources. Empirical diagnostics for the interpretation of observational data must often be calibrated with the aid of photoionisation models. Most studies so far have been carried out by assuming spherical or plane-parallel geometries, with major limitations on allowed gas and dust density distributions and with the spatial distribution of multiple, non-centrally-located ionising sources not being accounted for. We present the first results of our theoretical study of geometric effects, via the construction of a number of 3D photoionisation models using the MOCASSIN code for a variety of spatial configurations and ionisation sources. We compare integrated emission line spectra from such configurations and show evidence of systematic errors caused by the simplifying assumption of a single, central location for all ionising sources.

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arXiv2006

New advances in nebular photoionisation modelling

The study of photoionised gas in planetary nebulae (PNe) has played a major role achieving, over the years, a better understanding of a number of physical processes pertinent to a broader range of fields than just PNe studies, ranging from atomic physics to stellar evolution. Whilst empirical techniques are routinely employed for the analysis of the emission line spectra of such objects, the accurate interpretation of the observational data often requires the solution of the radiative transfer (RT) problem in the nebula, via the application of a photoionisation code.The development of the codes has been driven by the observational constraints available, but also compromised by the available computer power. Modern codes are faster and more flexible, with the ultimate goal being the achievement of a description of the observations relying on the smallest number of parameters possible. In this light, recent developments have been focused on the inclusion of new atomic data, the inclusion of a realistic treatment for dust grains mixed in the ionised and photon dominated regions (PDRs) and the expansion of some codes to PDRs with the inclusion of chemical reaction networks. Furthermore the last few years have seen the development of fully 3D photoionisation codes based on the Monte Carlo method. A review of the photoionisation codes currently in use is given here, with emphasis on recent developments, including the expansion of the models to the 3D domain, the identification of new observational constraints and how these can be used to extract useful information from realistic models.[Abridged]

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