arXiv subjects

Michael J. Barlow

Publications and source records attributed to Michael J. Barlow.

24 records · Page 2Linked to original sources

arXiv2011

A 3D Photoionization Model of the Extreme Planetary Nebula NGC 6302

We present a 3D photoionization model of the PN NGC 6302, one of the most complex objects of its kind. Our Mocassin model is composed of an extremely dense circumstellar disk and a large pair of diffuse bipolar lobes, a combination necessary to reproduce the observed spectrum. The masses of these components gives a total nebular mass of 4.7Mo. Discrepancies between our model fit and the observations are attributed to complex density inhomogeneities in the nebula. The potential to resolve such discrepancies with more complex models is confirmed by a range of models introducing small-scale structures. Compared to solar abundances He is enhanced by 50%, C is slightly subsolar, O is solar, and N is enhanced by a factor of 6. These imply a significant 3rd dredge-up coupled with hot-bottom burning CN-cycle conversion of dredged-up C to N. The central star is partly obscured by the edge-on circumstellar disk and its properties are not well constrained. Emission from a number of high-ionization `coronal' lines provides constraints on the form of the high-energy ionizing flux. Using a solar abundance stellar atmosphere we are unable to fit all of the observed line fluxes, but a substantially better fit was obtained using a 220,000K H-deficient stellar atmosphere with L*=14,300 Lo. The H-deficient nature of the central star suggests it has undergone a late thermal pulse, and fits to evolutionary tracks imply a central star mass of 0.73-0.82Mo. Timescales for these tracks suggest the object left the top of the AGB ~2100 years ago, in agreement with studies of the recent mass-loss event that formed the bipolar lobes. Based on the modelled nebular and central star masses we estimate the initial mass of the central star to be 5.5Mo, in agreement with that derived from evolutionary tracks. (Abstract truncated)

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arXiv2011

A mid-infrared imaging catalogue of post-AGB stars

Post-AGB stars are key objects for the study of the dramatic morphological changes of low- to intermediate-mass stars on their evolution from the Asymptotic Giant Branch (AGB) towards the Planetary Nebula stage. There is growing evidences that binary interaction processes may very well have a determining role in the shaping process of many objects, but so far direct evidence is still weak. We aim at a systematic study of the dust distribution around a large sample of Post-AGB stars as a probe of the symmetry breaking in the nebulae around these systems. We used imaging in the mid-infrared to study the inner part of these evolved stars to probe direct emission from dusty structures in the core of Post-AGB stars in order to better understand their shaping mechanisms. We imaged a sample of 93 evolved stars and nebulae in the mid-infrared using VISIR/VLT, T-Recs/Gemini South and Michelle/Gemini North. We found that all the the Proto-Planetary Nebulae we resolved show a clear departure from spherical symmetry. 59 out of the 93 observed targets appear to be non resolved. The resolved targets can be divided in two categories. The nebulae with a dense central core, that are either bipolar and multipolar. The nebulae with no central core have an elliptical morphology.The dense central torus observed likely host binary systems which triggered fast outflows that shaped the nebulae.

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arXiv2009

IPHAS A-type Stars with Mid-IR Excesses in Spitzer Surveys

We have identified 17 A-type stars in the Galactic Plane that have mid-IR excesses at 8 micron. From the observed colors in the (r'-H_alpha)-(r'-i') plane, we first identified 23050 early A-type main sequence (MS) star candidates in the Isaac Newton Photometric H-Alpha Survey (IPHAS) point source database that are located in Spitzer GLIMPSE Galactic Plane fields. Imposing the requirement that they be detected in all seven 2MASS and IRAC bands led to a sample of 2692 candidate A-type stars with fully sampled 0.6 to 8 micron SEDs. Optical classification spectra of 18 of the IPHAS candidate A-type MS stars showed that all but one could be well fitted using main sequence A-type templates, with the other being an A-type supergiant. Out of the 2692 A-type candidates 17 (0.6%) were found to have 8-micron excesses above the expected photospheric values. Taking into account non-A-Type contamination estimates, the 8-micron excess fraction is adjusted to ~0.7%. The distances to these sources range from 0.7-2.5 kpc. Only 10 out of the 17 excess stars had been covered by Spitzer MIPSGAL survey fields, of which 5 had detectable excesses at 24 micron. For sources with excesses detected in at least two mid-IR wavelength bands, blackbody fits to the excess SEDs yielded temperatures ranging from 270 to 650 K, and bolometric luminosity ratios L_IR/L* from 2.2x10^{-3}-1.9x10^{-2}, with a mean value of 7.9x10^{-3} (these bolometric luminosities are lower limits as cold dust is not detectable by this survey). Both the presence of mid-IR excesses and the derived bolometric luminosity ratios are consistent with many of these systems being in the planet-building transition phase between the early protoplanetary disk phase and the later debris disk phase.

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arXiv2008

Prospects for Studies of Stellar Evolution and Stellar Death in the JWST Era

I review the prospects for studies of the advanced evolutionary stages of low-, intermediate- and high-mass stars by the JWST and concurrent facilities, with particular emphasis on how they may help elucidate the dominant contributors to the interstellar dust component of galaxies. Observations extending from the mid-infrared to the submillimeter can help quantify the heavy element and dust species inputs to galaxies from AGB stars. JWST's MIRI mid-infrared instrument will be so sensitive that observations of the dust emission from individual intergalactic AGB stars and planetary nebulae in the Virgo Cluster will be feasible. The Herschel Space Observatory will enable the last largely unexplored spectral region, the far-IR to the submillimeter, to be surveyed for new lines and dust features, while SOFIA will cover the wavelength gap between JWST and Herschel, a spectral region containing important fine structure lines, together with key water-ice and crystalline silicate bands. Spitzer has significantly increased the number of Type II supernovae that have been surveyed for early-epoch dust formation but reliable quantification of the dust contributions from massive star supernovae of Type II, Type Ib and Type Ic to low- and high-redshift galaxies should come from JWST MIRI observations, which will be able to probe a volume over 1000 times larger than Spitzer.

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arXiv2006

Far-IR Excited OH Lines from Orion KL Outflows

As part of the first far-IR line survey towards Orion KL, we present the detection of seven new rotationally excited OH Lambda-doublets (at 48, 65, 71, 79, 98 and 115 um). Observations were performed with the Long Wavelength Spectrometer (LWS) Fabry-Perots on board the Infrared Space Observatory (ISO). In total, more than 20 resolved OH rotational lines, with upper energy levels up to 620 K, have been detected at an angular and velocity resolutions of 80$'' and 33 km s^-1 respectively. OH line profiles show a complex behavior evolving from pure absorption, P-Cygni type to pure emission. We also present a large scale 6' declination raster in the OH ^2Π_3/2 J=5/2^+-3/2^- and ^2Π_3/2 J=7/2^-5/2^+ lines (at 119.441 and 84.597 um) revealing the decrease of excitation outside the core of the cloud. From the observed profiles, mean intrinsic line widths and velocity offsets between emission and absorption line peaks we conclude that most of the excited OH arises from Orion outflow(s), i.e. the ``plateau'' component. We determine an averaged OH abundance relative to H_2 of X(OH)=(0.5-1.0)x10^-6, a kinetic temperature of 100 K and a density of n(H_2)=5x10^5 cm^-3. Even with these conditions, the OH excitation is heavily coupled with the strong dust continuum emission from the inner hot core regions and from the expanding flow itself.

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arXiv2005

"PAH" emission bands in selected planetary nebulae: a study of the behaviour with gas phase C/O ratio

Airborne and space-based low-resolution spectroscopy in the 1980s discovered tantalizing quantitative relationships between the gas phase C/O abundance ratio in planetary nebulae (PNe) and the fractions of total far-infrared luminosity radiated by the 7.7 and 11.3 micron bands (the C=C stretch and C-H bend, respectively), of polycyclic aromatic hydrocarbons. Only a very small sample of nebulae was studied in this context, limited by airborne observations of the 7.7 micron band, or the existence of adequate IRAS Low Resolution Spectrometer data for the 11.3 micron band. To investigate these trends further, we have expanded the sample of planetaries available for this study using Infrared Space Observatory (ISO) low-resolution spectra secured with the Short Wavelength Spectrometer (SWS) and the Long Wavelength Spectrometer (LWS). The new sample of 43 PNe, of which 17 are detected in PAH emission, addresses the range from C/O=0.2-13 with the objective of trying to delineate the pathways by which carbon dust grains might have formed in planetaries. For the 7.7-micron and 11.3- micron bands, we confirm that the ratio of band strength to total infrared luminosity is correlated with the nebular C/O ratio. Expressed in equivalent width terms, the cut-on C/O ratio for the 7.7-micron band is found to be 0.6(+0.2,-0.4), in good accord with that found from sensitive ground-based measurements of the 3.3-micron band too.

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