M. J. Barlow

The Herschel Reference Survey

The Herschel Reference Survey is a Herschel guaranteed time key project and will be a benchmark study of dust in the nearby universe. The survey will complement a number of other Herschel key projects including large cosmological surveys that trace dust in the distant universe. We will use Herschel to produce images of a statistically-complete sample of 323 galaxies at 250, 350, and 500 μm. The sample is volume-limited, containing sources with distances between 15 and 25 Mpc and flux limits in the K band to minimize the selection effects associated with dust and with young high-mass stars and to introduce a selection in stellar mass. The sample spans the whole range of morphological types (ellipticals to late-type spirals) and environments (from the field to the center of the Virgo Cluster) and as such will be useful for other purposes than our own. We plan to use the survey to investigate (i) the dust content of galaxies as a function of Hubble type, stellar mass, and environment; (ii) the connection between the dust content and composition and the other phases of the interstellar medium; and (iii) the origin and evolution of dust in galaxies. In this article, we describe the goals of the survey, the details of the sample and some of the auxiliary observing programs that we have started to collect complementary data. We also use the available multifrequency data to carry out an analysis of the statistical properties of the sample.

mocassin: a fully three-dimensional Monte Carlo photoionization code

The study of photoionized environments is fundamental to many astrophysical problems. Up to the present most photoionization codes have numerically solved the equations of radiative transfer by making the extreme simplifying assumption of spherical symmetry. Unfortunately very few real astronomical nebulae satisfy this requirement. To remedy these shortcomings, a self-consistent, three-dimensional radiative transfer code has been developed using Monte Carlo techniques. The code, MOCASSIN, is designed to build realistic models of photoionized nebulae having arbitrary geometry and density distributions, with both the stellar and diffuse radiation fields treated self-consistently. In addition, the code is capable of treating one or more exciting stars located at non-central locations. The gaseous region is approximated by a cuboidal Cartesian grid composed of numerous cells. The physical conditions within each grid cell are determined by solving the thermal equilibrium and ionization balance equations. This requires a knowledge of the local primary and secondary radiation fields, which are calculated self-consistently by locally simulating the individual processes of ionization and recombination. The structure and the computational methods used in the MOCASSIN code are described in this paper. MOCASSIN has been benchmarked against established one-dimensional spherically symmetric codes for a number of standard cases, as defined by the Lexington/Meudon photoionization workshops: at Meudon in 1985 and at Lexington in 1995 and 2000. The results obtained for the benchmark cases are satisfactory and are presented in this paper. A performance analysis has also been carried out and is discussed here.

The dusty mocassin: fully self-consistent 3D photoionization and dust radiative transfer models

We present the first 3D Monte Carlo (MC) photoionization code to include a fully self-consistent treatment of dust radiative transfer (RT) within a photoionized region. This is the latest development (version 2.0) of the gas-only photoionization code MOCASSIN and employs a stochastic approach to the transport of radiation, allowing both the primary and secondary components of the radiation field to be treated self-consistently, whilst accounting for the scattering of radiation by dust grains mixed with the gas, as well as the absorption and emission of radiation by both the gas and the dust components. An escape probability method is implemented for the transfer of resonance lines that may be absorbed by the grains, thus contributing to their energy balance. The energetics of the co-existing dust and gas components must also take into account the effects of dust‐gas collisions and photoelectric emission from the dust grains, which are dependent on the grain charge. These are included in our code using the average grain potential approximation scheme. A set of rigorous benchmark tests have been carried out for dust-only spherically symmetric geometries and 2D disc configurations. The results of MOCASSIN are found to be in agreement with those obtained by well-established dust-only RT codes that employ various approaches to the solution of the RT problem. A model of the dust and of the photoionized gas components of the planetary nebula NGC 3918 is also presented as a means of testing the correct functioning of the RT procedures in a case where both gas and dust opacities are present. The two components are coupled via the heating of dust grains by the absorption of both UV continuum photons and resonance line photons emitted by the gas. The MOCASSIN results show agreement with those of a 1D dust and gas model of this nebula published previously, showing the reliability of the new code, which can be applied to a variety of astrophysical environments. Ke yw ords: radiative transfer ‐ dust, extinction ‐ H II regions ‐ planetary nebulae: general.

Heavy elements in Galactic and Magellanic Cloud H ii regions: recombination‐line versus forbidden‐line abundances

We have obtained deep optical, long-slit spectrophotometry of the Galactic HII regions M 17, NGC 3576 and of the Magellanic Cloud HII regions 30 Doradus, LMC N11B and SMC N66, recording the optical recombination lines (ORLs) of CII, NII and OII. Temperature-insensitive ORL C2+/O2+ and N2+/O2 ratios are obtained for all nebulae except SMC N66. The ORL C2+/O2+ ratios show remarkable agreement within each galactic system, while also being in agreement with the corresponding CEL ratios. For all five nebulae, the O2+/H+ abundance derived from multiple OII ORLs is found to be higher than the corresponding value derived from the strong [OIII] 4959, 5007A CELs, by factors of 1.8--2.7 for four of the nebulae. The LMC N11B nebula exhibits a more extreme discrepancy factor for the O2+ ion, ~5. Thus these HII regions exhibit ORL/CEL abundance discrepancy factors that are similar to those previously encountered amongst planetary nebulae. Our optical CEL O2+/H+ abundances agree to within 20-30 per cent with published O2+/H+ abundances that were obtained from observations of infrared fine-structure lines. Since the low excitation energies of the latter make them insensitive to variations about typical nebular temperatures, fluctuations in temperature are ruled out as the cause of the observed ORL/CEL O2+ abundance discrepancies. We present evidence that the observed OII ORLs from these HII regions originate from gas of very similar density (<3500 cm-3) to that emitting the observed heavy-element optical and infrared CELs, ruling out models that employ high-density ionized inclusions in order to explain the abundance discrepancy. We consider a scenario whereby much of the heavy-element ORL emission originates from cold (<=500 K) metal-rich ionized regions.

A deep survey of heavy element lines in planetary nebulae – II. Recombination-line abundances and evidence for cold plasma

In our Paper 1, we presented deep optical observations of the spectra of 12 Galactic planetary nebulae (PNe) and three Magellanic Cloud PNe, carrying out an abundance analysis using the collisionally excited forbidden lines. Here, we analyse the relative intensities of faint optical recombination lines (ORLs) from ions of carbon, nitrogen and oxygen in order to derive the abundances of these ions relative to hydrogen. The relative intensities of four high-l C II recombination lines with respect to the well-known 3d-4f lambda4267 line are found to be in excellent agreement with the predictions of recombination theory, removing uncertainties about whether the high C2+ abundances derived from the lambda4267 line could be due to non-recombination enhancements of its intensity.We define an abundance discrepancy factor (ADF) as the ratio of the abundance derived for a heavy element ion from its recombination lines to that derived for the same ion from its ultraviolet, optical or infrared collisionally excited lines (CELs). All of the PNe in our sample are found to have ADFs that exceed unity. Two of the PNe, NGC 2022 and LMC N66, have O2+ ADFs of 16 and 11, respectively, while the remaining 13 PNe have a mean O2+ ADF of 2.6, with the smallest value being 1.8.Garnett and Dinerstein found that for a sample of about 12 PNe the magnitude of the O2+ ADF was inversely correlated with the nebular Balmer line surface brightness. We have investigated this for a larger sample of 20 PNe, finding weak correlations with decreasing surface brightness for the ADFs of O2+ and C2+. The C2+ ADFs are well correlated with the absolute radii of the nebulae, although no correlation is present for the 021 ADFs. We also find both the C2+ and O2+ ADFs to be strongly correlated with the magnitude of the difference between the nebular [O III] and Balmer jump electron temperatures (DeltaT), corroborating a result of Liu et al. for the O2+ ADF. AT is found to be weakly correlated with decreasing nebular surface brightness and increasing absolute nebular radius.There is no dependence of the magnitude of the ADF upon the excitation energy of the ultraviolet, optical or infrared CEL transition used, indicating that classical nebular temperature fluctuations - i.e. in a chemically homogeneous medium - are not the cause of the observed abundance discrepancies. Instead, we conclude that the main cause of the discrepancy is enhanced ORL emission from cold ionized gas located in hydrogen-deficient clumps inside the main body of the nebulae, as first postulated by Liu et al. for the high-ADFPN, NGC 6153. We have developed a new electron temperature diagnostic, based upon the relative intensities of the O II 4f-3d lambda4089 and 3p-3s lambda4649 recombination transitions. For six out of eight PNe for which both transitions are detected, we derive O2+ ORL electron temperatures of less than or equal to300 K, very much less than the O2+ forbidden-line and H+ Balmer jump temperatures derived for the same nebulae. These results provide direct observational evidence for the presence of cold plasma regions within the nebulae, consistent with gas cooled largely by infrared fine-structure transitions; at such low temperatures, recombination transition intensities will be significantly enhanced due to their inverse power-law temperature dependence, while ultraviolet and optical CELs will be significantly suppressed.

A deep survey of heavy element lines in planetary nebulae – I. Observations and forbidden-line densities, temperatures and abundances

We present deep optical spectrophotometry of 12 Galactic planetary nebulae (PNe) and three Magellanic Cloud PNe. Nine of the Galactic PNe were observed by scanning the slit of the spectrograph across the nebula, yielding relative line intensities for the entire nebula that are suitable for comparison with integrated nebular fluxes measured in other wavelength regions. In this paper we use the fluxes of collisionally excited lines (CELs) from the nebulae to derive electron densities and temperatures, and ionic abundances. We find that the nebular electron densities derived from optical CEL ratios are systematically higher than those derived from the ratios of the infrared (IR) fine-structure (FS) lines of [O III]. The latter have lower critical densities than the typical nebular electron densities derived from optical CELs, indicating the presence of significant density variations within the nebulae, with the IR CELs being biased towards lower density regions. We find that for several nebulae the electron temperatures obtained from [O II] and [N II] optical CELs are significantly affected by recombination excitation of one or more of the CELs. When allowance is made for recombination excitation, much better agreement is obtained with the electron temperatures obtained from optical [O III] lines. We also compare electron temperatures obtained from the ratio of optical nebular to auroral [O III] lines with temperatures obtained from the ratio of [O III] optical lines to [O III] IR FS lines. We find that when the latter are derived using electron densities based on the [O III ]5 2µm/88 µm line ratio, they yield values that are significantly higher than the optical [O III] electron temperatures. In contrast to this, [O III] optical/IR temperatures derived using the higher electron densities obtained from optical [Cl III] λ5517/λ5537 ratios show much closer agreement with optical [O III] electron temperatures, implying that the observed [O III] optical/IR ratios are significantly weighted by densities in excess of the critical densities of both [O III] FS lines. Consistent with this, ionic abundances derived from [O III] and [N III] FS lines using electron densities from optical CELs show much better agreement with abundances derived for the same ions from optical and ultraviolet CELs than do abundances derived from the FS lines using the lower electron densities obtained from the observed [O III ]5 2µm/88 µm ratios. The behaviour of these electron temperatures, obtained making use of the temperatureinsensitive [O III] IR FS lines, provides no support for significant temperature fluctuations within the nebulae being responsible for derived Balmer jump electron temperatures that are lower than temperatures obtained from the much more temperature sensitive [O III] optical lines.

An Overview of the Dwarf Galaxy Survey

The Dwarf Galaxy Survey (DGS) program is studying low-metallicity galaxies using 230 hr of farinfrared (FIR) and submillimetre (submm) photometric and spectroscopic observations of the Herschel Space Observatory and draws from this a rich database of a wide range of wavelengths tracing the dust, gas and stars. This sample of 50 galaxies includes the largest metallicity range achievable in the local Universe including the lowest metallicity (Z) galaxies, 1/50 Z., and spans four orders of magnitude in star formation rates. The survey is designed to get a handle on the physics of the interstellar medium (ISM) of low metallicity dwarf galaxies, especially their dust and gas properties and the ISM heating and cooling processes. The DGS produces PACS and SPIRE maps of low-metallicity galaxies observed at 70, 100, 160, 250, 350, and 500 mu m with the highest sensitivity achievable to date in the FIR and submm. The FIR fine-structure lines, [CII] 158 mu m, [OI] 63 mu m, [OI] 145 mu m, [OIII] 88 mu m, [NIII] 57 mu m, and [NII] 122 and 205 mu m have also been observed with the aim of studying the gas cooling in the neutral and ionized phases. The SPIRE FTS observations include many CO lines (J = 4-3 to J = 13-12), [NII] 205 mu m, and [CI] lines at 370 and 609 mu m. This paper describes the sample selection and global properties of the galaxies and the observing strategy as well as the vast ancillary database available to complement the Herschel observations. The scientific potential of the full DGS survey is described with some example results included.

Warm water vapour in the sooty outflow from a luminous carbon star

The detection of circumstellar water vapour around the ageing carbon star IRC +10216 challenged the current understanding of chemistry in old stars, because water was predicted to be almost absent in carbon-rich stars. Several explanations for the water were postulated, including the vaporization of icy bodies (comets or dwarf planets) in orbit around the star, grain surface reactions, and photochemistry in the outer circumstellar envelope. With a single water line detected so far from this one carbon-rich evolved star, it is difficult to discriminate between the different mechanisms proposed. Here we report the detection of dozens of water vapour lines in the far-infrared and sub-millimetre spectrum of IRC +10216 using the Herschel satellite. This includes some high-excitation lines with energies corresponding to ∼1,000 K, which can be explained only if water is present in the warm inner sooty region of the envelope. A plausible explanation for the warm water appears to be the penetration of ultraviolet photons deep into a clumpy circumstellar envelope. This mechanism also triggers the formation of other molecules, such as ammonia, whose observed abundances are much higher than hitherto predicted.

Physical conditions in the planetary nebula Abell 30

We have analysed optical spectra of two of the hydrogen-deficient knots (J1 and J3) in the born-again planetary nebula Abell 30, together with ultraviolet (UV) spectra of knots J3 and J4. We determine electron temperatures in the knots based on several diagnostics. The [O III] nebular-to-auroral transition ratio yields temperatures of the order of 17 000 K. The weak temperature-dependence of the ratios of helium lines λ4471, λ5876 and λ6678 is used to derive a temperature of 8850 K for knot J3 and 4600 K for knot J1. Ratios of O II recombination lines, which directly measure the temperature in the coldest regions of the knots, are used to derive temperatures of 2500 K for knot J3, and just 500 K for knot J1. We calculate abundances both from collisionally excited lines and from the well-observed recombination spectra of C, N, O and Ne ions. The forbidden line abundances agree well with previous determinations, but the recombination line abundances are several hundred times higher. These results confirm the scenario proposed by Harrington & Feibelman, in which the knots contain a cold core highly enriched in heavy elements. Forbidden lines are almost entirely emitted by the hot outer part of the knot, while recombination lines are emitted predominantly from the cold core. The C/O ratios we derive imply that the knots are oxygen-rich, contrary to theoretical predictions for born-again nebulae.

Integrated spectrum of the planetary nebula NGC 7027

We present deep optical spectra of the archetypal young planetary nebula (PN) NGC 7027, covering a wavelength range from 3310 to 9160 A. The observations were carried out by uniformly scanning a long slit across the entire nebular surface, thus yielding average optical spectra for the whole nebula. A total of 937 emission features are detected. The extensive line list presented here should prove valuable for future spectroscopic analyses of emission line nebulae. The optical data, together with the archival IUE and ISO spectra, are used to probe the temperature and density structures and to determine the elemental abundances from lines produced by different excitation mechanisms. Electron temperatures have been derived from the hydrogen recombination Balmer jump (BJ), from ratios of He  optical recombination lines (ORLs) and from a variety of diagnostic ratios of collisionally excited lines (CELs). Electron densities have been determined from the intensities of high-order H  Balmer lines and of He  Pfund lines, as well as from a host of CEL diagnostic ratios. CEL and ORL diagnostics are found to yield compatible results. Adopting respectively electron temperatures of Te = 12 600 and 15 500 K for ions with ionization potentials lower or higher than 50 eV and a constant density of Ne = 47 000 cm −3 , elemental abundances have been determined from a large number of CELs and ORLs. The C 2+ /H + , N 2+ /H + ,O 2+ /H + and Ne 2+ /H + ionic abundance ratios derived from ORLs are found to be only slightly higher than the corresponding CEL values. We conclude that whatever mechanism is causing the BJ/CEL temperature discrepanies and the ORL/CEL abundance discrepancies that have been observed in many PNe, it has an insignificant effect on this bright young compact PN. The properties of the central star are also discussed. Based on the integrated spectrum and using the energy-balance method, we have derived an effective temperature of 219 000 K for the ionizing star. Finally, we report the first detection in the spectrum of this bright young PN of Raman-scattered O  features at 6830 and 7088 A, pointing to the existence of abundant neutral hydrogen around the ionized regions.