Neutral carbon far-red forbidden line emission from planetary nubulae

The temperature-sensitive neutral carbon forbidden lines at 8727, 9824 and 9850 Å have been measured simultaneously for the first time from a planetary nebula. The nebulae NGC 2346, NGC 2440, NGC 3132 and IC 4406 were observed. Accurate rest wavelengths of these lines are obtained. The observed line ratios I(λ9824+λ9850)/I(λ8727) are consistent with collisional excitation by electron impacts. It is demonstrated that radiative recombination and stellar continuum fluorescence are unimportant in exciting the observed [CI] lines, with the possible exception of NGC 2440 where a contribution from the former process cannot be ruled out. For NGC 2346, NGC 3132 and IC 4406, the observed [C I]line ratios yield electron temperatures between 7400 and 8000 K, about 1800 to 2800 K lower than those deduced from the [N II], [S III] and [O III] line ratios that we also measured. Electron densities are derived from the observed [N I], [S II] and [Cl III] doublet ratios

Balmer Discontinuity Temperatures in the Orion Nebula

We have mapped the Balmer discontinuity temperature Te (BJ) along two long-slit positions in the Orion Nebula, using high-quality spectra obtained with the ESO 1.52 m telescope and with the 3.9 m AAT. One slit was oriented north-south and positioned 40” west of Ö1 C Ori. The second slit was oriented east-west, with its eastern end 10" west of Ö1 C Ori, identical to the slit position previously studied by Walter & Dufour (1994). For the NS slit, both the 1.52 m and the AAT data yield a constant temperature of about 9000 K, with variations of only a few hundred K over a total slit length of about 3#5. For the EW-slit, our data reveal two distinct regions of nearly constant temperatures of 8900 and 7200 K, respectively, possibly resulting from two separate H II regions. No evidence is found for the anomalously low temperatures reported by Walter and Dufour for the same region. © 1995, IOP Publishing Ltd

Chemical abundances of planetary nebulae from optical recombination lines - III. The Galactic bulge PN M 1-42 and M 2-36

We present deep, high-resolution optical spectra of two Galactic bulge planetary nebulae (PN), M 1-42 and M 2-36. The spectra show very prominent and rich optical recombination lines (ORLs) from C, N, O and Ne ions. Infrared spectra from graphic were also obtained using the Short and Long Wavelength Spectrometer (SWS and LWS) on board ISO. The optical and infrared spectra, together with archival IUE spectra, are used to study their density and thermal characteristics and to determine elemental abundances. We determine the optical and UV extinction curve towards these two bulge PN using observed H I and He II recombination line fluxes and the radio free–free continuum flux density. In the optical, the reddening curve is found to be consistent with the standard Galactic extinction law, with a total to selective extinction ratio graphic. However, the extinction in the UV is found to be much steeper, consistent with the earlier finding of Walton, Barlow & Clegg. The rich ORL spectra from C, N, O and Ne ions detected from the two nebulae have been used to determine the abundances of these elements relative to hydrogen. In all cases, the resultant ORL abundances are found to be significantly higher than the corresponding values deduced from collisionally excited lines (CELs). In M 2-36, the discrepancies are about a factor of 5 for all four elements studied. In M 1-42, the discrepancies reach a factor of about 20, the largest ever observed in a PN. M 1-42 also has the lowest Balmer jump temperature ever determined for a PN, graphic, 5660 K lower than its [O III] forbidden line temperature. We compare the observed intensities of the strongest O II ORLs from different electronic configurations, including λ4649 from graphic, λ4072 from graphic, λ4089 from graphic, and λ4590 and λ4190 from the doubly excited graphic and graphic configurations, respectively. In all cases, in spite of the fact that the ratios of the ORL to CEL ionic abundances span a wide range from ∼graphic, the intensity ratios of λ4649, λ4072, λ4590 and λ4190 relative to λ4089 are found to be nearly constant, apart from some small monotonic increase of these ratios as a function of electron temperature. Over a range of Balmer jump temperature from graphic, the variations amount to about 20 per cent for the graphic and graphic transitions and a factor of 2 for the primed transitions, and are consistent with the predictions of the current recombination theory. Our results do not support the claim by Dinerstein, Lafon & Garnett that the relative intensities of O II ORLs vary from nebula to nebula and that the scatter is largest in objects where the discrepancies between ORL and CEL abundances are also the largest. We find that the ORL to CEL abundance ratio is highly correlated with the difference between the temperatures yielded by the [O III] forbidden line ratio and by the H I Balmer jump, providing the strongest evidence so far that the two phenomena, i.e. the disparity between ORL and CEL temperature and abundance determinations, are closely related. However, temperature fluctuations of the type envisaged by Peimbert are unable to explain the low ionic abundances yielded by IR fine-structure lines. The very low Balmer jump temperature of M 1-42, coupled with its very low Balmer decrement density, may also be difficult to explain with a chemically inhomogeneous composite model of the type proposed by Liu et al. for NGC 6153

NGC 6153: a super-metal-rich planetary nebula?

We have obtained deep optical spectra of the planetary nebula NGC 6153, both along its minor axis and by uniformly scanning a long slit across the whole nebula. The scanned spectra, when combined with the nebular total Hβ flux, yield integrated fluxes for all the lines (∼400) in our spectra, which are rich in strong recombination lines from C, N, O and Ne ions A weak O VI λ 3811 emission line from the central star has been detected, suggesting that the nucleus of NGC 6153 has a hydrogen-deficient surface. The optical data, together with the ISO LWS 43-197 μm spectrum and the archival IUE and IRAS LRS spectra, are used to study the thermal and density structure and to derive the heavy-element abundances from lines produced by different excitation mechanisms. In all cases, the C2+/H+ N2+/H+, O2+/H+ and Ne2+/H+ abundances derived from multiple optical recombination lines (ORLs) are consistently higher, by about a factor of 10, than the corresponding values deduced from optical, UV or infrared (IR) collisionally excited lines (CELs). regardless of the excitation energies or critical densities of the latter. The agreement between the temperature-sensitive optical forbidden lines and the temperature-insensitive IR fine-structure lines rules out temperature fluctuations as the cause of the large difference between the ORL and CEL abundances. We present the results of a new calculation of recombination coefficients for [O II] which lead to good agreement between the observed and predicted [O II] λλ7320,7330 forbidden line intensities if these lines are solely excited by recombination at the Balmer jump temperature. Recombination excitation is also found to be important in exciting the [N II] λ5754 line, which, if unaccounted for, would lead to an overestimated [N II] temperature from the observed (λ6548 +λ6584)/λ5754 ratio. Analysis of a number of C II lines arising from levels as high as 7g in the recombination ladder reveals excellent agreement between their reddening-corrected relative intensities and those predicted by recombination theory. Spatial analysis of the long-slit spectra taken along the nebular minor axis yields a varying [O III] temperature, whereas the hydrogen Balmer jump temperature of 6000 K is approximately constant across the nebula, and is 2000-3000 K lower than the [O III] temperature The observed high-n Balmer line decrement indicates that the hydrogen lines arise from material having an electron density of 2000+2000-1000cm-3, consistent with the optical and IR forbidden-line density diagnostics, which yield average line-of-sight electron densities along the minor axis varying between 2000 and 4000 cm-3. While the He/H ratio mapped by He I and He II recombination lines is constant within 5 per cent across the nebula, the C2+/H+ and O2+/H+ recombination-line abundances decrease by a factor of 2-3 over a radius of 15 arcsec from the centre, pointing to the presence of abundance gradients. We consider a variety of hypotheses to account for the observed behaviour of the various thermal, density and abundance diagnostics. Empirical nebular models containing two components with differing densities and temperatures are able to account for many of the observed patterns, but only if one of the components is significantly hydrogen-deficient. One such model, which gives a good fit to the observed line intensities and patterns, has 500-K H-depleted material, presumed to be evaporating from dense neutral inclusions, embedded in 9500-K material with 'normal' abundances. An alternative model, which appears more physically plausible on a number of grounds, has high-density (2 × 106 cm-3), fully ionized, H-deficient knots embedded in the 'normal' component, although this model fails to account adequately for the observed low (6000 K) hydrogen Balmer jump temperature. However, the observed fact that the ORLs and CELs yield heavy-element abundance ratios that are identical within the uncertainties finds no obvious explanation in the context of H-deficient knot models

ISO LWS observations of planetary nebula fine-structure lines

We have obtained 43–198 μm far-infrared (IR) spectra for a sample of 51 Galactic planetary nebulae (PN) and protoplanetary nebulae (PPN), using the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory (ISO). Spectra were also obtained of the former PN candidate Lo 14. The spectra yield fluxes for the fine-structure lines [N II] 122 μm, [N III] 57 μm and [O III] 52 and 88 μm emitted in the ionized regions and the [O I] 63- and 146-μm and [C II] 158-μm lines from the photodissociation regions (PDRs), which have been used to determine electron densities and ionic abundances for the ionized regions and densities, temperatures and gas masses for the PDRs. The strong [N III] and [O III] emission lines detected in the LWS spectrum taken centred on Lo 14 could be associated with the nearby strong radio and infrared source G 331.5–0.1. We find that the electron densities yielded by the [O III] 88 μm/52 μm doublet ratio are systematically lower than those derived from the optical [Ar IV] λ4740/λ4711 and [Cl III] λ5537/λ5517 doublet ratios, which have much higher critical densities than the 52- and 88-μm lines, suggesting the presence of density inhomogeneities in the nebulae. Ionic abundances, N+/H+,N2+/H+ and O2+/H+, as well as the N2+/O2+ abundance ratio, which provides a good approximation to the N/O elemental abundance ratio, are derived. Although ionic abundances relative to H+ deduced from the far-IR fine-structure lines are sensitive to the adopted electron density and the presence of density inhomogeneities, the strong dependence on the nebular physical conditions is largely cancelled out when N2+/O2+ is calculated from the 57 μm/(52 μm+88 μm) flux ratio, owing to the similarity of the critical densities of the lines involved. The temperatures and densities of the PDRs around 24 PN have been determined from the observed [O I] and [C II] line intensity ratios. Except for a few objects, the deduced temperatures fall between 200 and 500 K, peaking around 250 K. The densities of the PDRs vary from 104–105 cm−3, reaching 3×105 cm−3 in some young compact PN. With a derived temperature of 1600 K and a density of 105 cm−3, the PDR of NGC 7027 is one of the warmest and at the same time one of the densest amongst the nebulae studied. For most of the PN studied, the [C II]-emitting regions contain only modest amounts of material, with gas masses ≲0.1 M⊙. Exceptional large PDR masses are found for a few nebulae, including NGC 7027, the bipolar nebulae M2-9 and NGC 6302, the young dense planetary nebulae BD+30°3639, IC 418 and NGC 5315, and the old, probably recombining, nebulae IC 4406 and NGC 6072

A test of general relativity from the three-dimensional orbital geometry of a binary pulsar

Binary pulsars provide an excellent system for testing general relativity because of their intrinsic rotational stability and the precision with which radio observations can be used to determine their orbital dynamics. Measurements of the rate of orbital decay of two pulsars have been shown to be consistent with the emission of gravitational waves as predicted by general relativity, providing the most convincing evidence for the self-consistency of the theory to date. However, independent verification of the orbital geometry in these systems was not possible. Such verification may be obtained by determining the orientation of a binary pulsar system using only classical geometric constraints, permitting an independent prediction of general relativistic effects. Here we report high-precision timing of the nearby binary millisecond pulsar PSR J0437-4715, which establish the three-dimensional structure of its orbit. We see the expected retardation of the pulse signal arising from the curvature of space-time in the vicinity of the companion object (the `Shapiro delay'), and we determine the mass of the pulsar and its white dwarf companion. Such mass determinations contribute to our understanding of the origin and evolution of neutron stars.Comment: 5 pages, 2 figure

Galactic and Extragalactic Samples of Supernova Remnants: How They Are Identified and What They Tell Us

Supernova remnants (SNRs) arise from the interaction between the ejecta of a supernova (SN) explosion and the surrounding circumstellar and interstellar medium. Some SNRs, mostly nearby SNRs, can be studied in great detail. However, to understand SNRs as a whole, large samples of SNRs must be assembled and studied. Here, we describe the radio, optical, and X-ray techniques which have been used to identify and characterize almost 300 Galactic SNRs and more than 1200 extragalactic SNRs. We then discuss which types of SNRs are being found and which are not. We examine the degree to which the luminosity functions, surface-brightness distributions and multi-wavelength comparisons of the samples can be interpreted to determine the class properties of SNRs and describe efforts to establish the type of SN explosion associated with a SNR. We conclude that in order to better understand the class properties of SNRs, it is more important to study (and obtain additional data on) the SNRs in galaxies with extant samples at multiple wavelength bands than it is to obtain samples of SNRs in other galaxiesComment: Final 2016 draft of a chapter in "Handbook of Supernovae" edited by Athem W. Alsabti and Paul Murdin. Final version available at https://doi.org/10.1007/978-3-319-20794-0_90-

Production of dust by massive stars at high redshift

The large amounts of dust detected in sub-millimeter galaxies and quasars at high redshift pose a challenge to galaxy formation models and theories of cosmic dust formation. At z > 6 only stars of relatively high mass (> 3 Msun) are sufficiently short-lived to be potential stellar sources of dust. This review is devoted to identifying and quantifying the most important stellar channels of rapid dust formation. We ascertain the dust production efficiency of stars in the mass range 3-40 Msun using both observed and theoretical dust yields of evolved massive stars and supernovae (SNe) and provide analytical expressions for the dust production efficiencies in various scenarios. We also address the strong sensitivity of the total dust productivity to the initial mass function. From simple considerations, we find that, in the early Universe, high-mass (> 3 Msun) asymptotic giant branch stars can only be dominant dust producers if SNe generate <~ 3 x 10^-3 Msun of dust whereas SNe prevail if they are more efficient. We address the challenges in inferring dust masses and star-formation rates from observations of high-redshift galaxies. We conclude that significant SN dust production at high redshift is likely required to reproduce current dust mass estimates, possibly coupled with rapid dust grain growth in the interstellar medium.Comment: 72 pages, 9 figures, 5 tables; to be published in The Astronomy and Astrophysics Revie