Ivan S. Bojičić

The Hα surface brightness–radius relation: a robust statistical distance indicator for planetary nebulae

Measuring the distances to Galactic planetary nebulae (PNe) has been an intractable problem for many decades. We have now established a robust optical statistical distance indicator, the Hα surface brightness–radius or SHα–r relation, which addresses this problem. We developed thisrelationfromacriticallyevaluatedsampleofprimarycalibratingPNe.Therobustnatureof the method results from our revised calibrating distances with significantly reduced systematic uncertainties, and the recent availability of high-quality data, including updated nebular diameters and integrated Hα fluxes. The SHα–r technique is simple in its application, requiring only an angular size, an integrated Hα flux, and the reddening to the PN. From these quantities, an intrinsic radius is calculated, which when combined with the angular size, yields the distance directly. Furthermore, we have found that optically thick PNe tend to populate the upper bound of the trend, while optically thin PNe fall along the lower boundary in the SHα–r plane. This enables sub-trends to be developed which offer even better precision in the determination of distances, as good as 18 per cent in the case of optically thin, high-excitation PNe. This is significantly better than any previous statistical indicator. We use this technique to create a catalogue of statistical distances for over 1100 Galactic PNe, the largest such compilation in the literature to date. Finally, in an appendix, we investigate both a set of transitional PNe and a range of PN mimics in the SHα–r plane, to demonstrate its use as a diagnostic tool. Interestingly, stellar ejecta around massive stars plot on a tight locus in SHα–r space with the potential to act as a separate distance indicator for these objects.

A catalogue of integrated Hα fluxes for 1258 Galactic planetary nebulae

We present a catalogue of new integrated H-alpha fluxes for 1258 Galactic planetary nebulae (PNe), with the majority, totalling 1234, measured from the Southern H-Alpha Sky Survey Atlas (SHASSA) and/or the Virginia Tech Spectral-line Survey (VTSS). Aperture photometry on the continuum-subtracted digital images was performed to extract H-alpha + [NII] fluxes in the case of SHASSA, and H-alpha fluxes from VTSS. The [NII] contribution was then deconvolved from the SHASSA flux using spectrophotometric data taken from the literature or derived by us. Comparison with previous work shows that the flux scale presented here has no significant zero-point error. Our catalogue is the largest compilation of homogeneously derived PN fluxes in any waveband yet measured, and will be an important legacy and fresh benchmark for the community. Amongst its many applications, it can be used to determine statistical distances for these PNe, determine new absolute magnitudes for delineating the faint end of the PN luminosity function, provide baseline data for photoionization and hydrodynamical modelling, and allow better estimates of Zanstra temperatures for PN central stars with accurate optical photometry. We also provide total H-alpha fluxes for another 75 objects which were formerly classified as PNe, as well as independent reddening determinations for ~270 PNe, derived from a comparison of our H-alpha data with the best literature H-beta fluxes. In an appendix, we list corrected H-alpha fluxes for 49 PNe taken from the literature, including 24 PNe not detected on SHASSA or VTSS, re-calibrated to a common zero-point.

Multifrequency study of the Large Magellanic Cloud supernova remnant (SNR) B0513-692 and new SNR candidate J051327-6911

We present a new multiwavelength study of supernova remnant (SNR) B0513−692 in the Large Magellanic Cloud (LMC). The remnant also has a strong, superposed, essentially unresolved, but unrelated radio source at its north-western edge, J051324−691049. This is identified as a likely compact H ii region based on related optical imaging and spectroscopy. We use the Australia Telescope Compact Array (ATCA) at 4790 and 8640 MHz (λ≃ 6 cm and λ≃ 3.5 cm) to determine the large-scale morphology, spectral index and polarization characteristics of B0513−692 for the first time. We detect a strongly polarized region (49 per cent) in the remnants southern edge (λ≃ 6 cm) . Interestingly, we also detect a small (∼40 arcsec) moderately bright, but distinct optical, circular shell in our Hα imagery which is adjacent to the compact H ii region and just within the borders of the north-eastern edge of B0513−692. We suggest that this is a separate new SNR candidate based on its apparently distinct character in terms of optical morphology in three imaged emission lines and indicative SNR optical spectroscopy (including enhanced optical [S ii] emission relative to Hα).

First release of the IPHAS catalogue of new extended planetary nebulae

We present the first results of our search for new, extended planetary nebulae (PNe) based on careful, systematic, visual scrutiny of the imaging data from the Isaac Newton Telescope Photometric Hα Survey of the Northern Galactic plane (IPHAS). The newly uncovered PNe will help to improve the census of this important population of Galactic objects that serve as key windows into the late-stage evolution of low- to intermediate-mass stars. They will also facilitate study of the faint end of the ensemble Galactic PN luminosity function. The sensitivity and coverage of IPHAS allows PNe to be found in regions of greater extinction in the Galactic plane and/or those PNe in a more advanced evolutionary state and at larger distances compared to the general Galactic PN population. Using a set of newly revised optical diagnostic diagrams in combination with access to a powerful, new, multiwavelength imaging data base, we have identified 159 true, likely and possible PNe for this first catalogue release. The ability of IPHAS to unveil PNe at low Galactic latitudes and towards the Galactic Anticentre, compared to previous surveys, makes this survey an ideal tool to contribute to the improvement of our knowledge of the whole Galactic PN population.

Discovery of planetary nebulae using predictive mid-infrared diagnostics

We demonstrate a newly developed mid-infrared (MIR) planetary nebula (PN) selection technique. It is designed to enable ecient searches for obscured, previously unknown, PN candidates present in the photometric source catalogues of Galactic plane MIR sky surveys. Such selection is now possible via new, sensitive, high-to-medium resolution, MIR satellite surveys such as those from the Spitzer Space Telescope and the all-sky Wide-Field Infrared Survey Explorer (WISE) satellite missions. MIR selection is based on how dierent colour-colour planes isolate zones (sometimes overlapping) that are predominately occupied by dierent astrophysical object types. These techniques depend on the reliability of the available MIR source photometry. In this pilot study we concentrate on MIR point source detections and show that it is dangerous to take the MIR GLIMPSE (Galactic Legacy Infrared Mid-Plane Survey Extraordinaire) photometry from Spitzer for each candidate at face value without examining the actual MIR image data. About half of our selected sources are spurious detections due to the applied source detection algorithms being aected

The planetary nebula Abell 48 and its [WN] nucleus

We have conducted a detailed multi-wavelength study of the peculiar nebula Abell 48 and its central star. We classify the nucleus as a helium-rich, hydr ogen-deficient star of type [WN4‐ 5]. The evidence for either a massive WN or a low-mass [WN] interpretation is critically examined, and we firmly conclude that Abell 48 is a planetary n ebula (PN) around an evolved low-mass star, rather than a Population I ejecta nebula. Importantly, the surrounding nebula has a morphology typical of PNe, and is not enriched in nitrogen, and thus not the ‘peeled atmosphere’ of a massive star. We estimate a distance of 1.6 k pc and a reddening, E(B V ) = 1.90 mag, the latter value clearly showing the nebula lies on the near side of the Galactic bar, and cannot be a massive WN star. The ionized mass (�0.3 M⊙) and electron density (700 cm −3 ) are typical of middle-aged PNe. The observed stellar spectrum was compared to a grid of models from the Potsdam Wolf-Rayet (PoWR) grid. The best fit temperature is 71 kK, and the atmospheric composition is dominated by helium with an upper limit on the hydrogen abundance of 10 per cent. Our results are in very good agreement with the recent study of Todt et al., who determined a hydrogen fraction of 10 per cent and an unusually large nitrogen fraction of �5 per cent. This fraction is higher than any other low-mass H-deficient star, and is not readily explained by current post-AGB models. We give a discussion of the implications of this discovery for the late-stage evolutio n of intermediate-mass stars. There is now tentative evidence for two distinct helium-dominated post-AGB lineages, separate to the helium and carbon dominated surface compositions produced by a late thermal pulse. Further theoretical work is needed to explain these recent discoveries.

Flux calibration of the AAO/UKST SuperCOSMOS Hα Survey

The AAO/UKST SuperCOSMOS Hα Survey (SHS) was, when completed in 2003, a powerful addition to extant wide-field surveys. The combination o f areal coverage, spatial resolution and sensitivity in a narrow imaging band, still marks it out t oday as an excellent resource for the astronomical community. The 233 separate fields are avai lable online in digital form, with each field covering 25 square degrees. The SHS has been th e motivation for equivalent surveys in the north, and new digital Hα surveys now beginning in the south such as VPHAS+. It has been the foundation of many important discovery projects with the Macquarie/AAO/Strasbourg Hα planetary nebula project being a particularly successful e xample. However, the full potential of the SHS has been hampered by lack of a clear route to acceptable flux calibration from the base photographic data. We have det ermined the calibration factors for 170 individual SHS fields, and present a direct pathway to the measurement of integrated Hα fluxes and surface brightnesses for resolved nebulae detect ed in the SHS. We also include a catalogue of integrated Hα fluxes for >100 planetary and other nebulae measured from the SHS, and use these data to show that fluxes, accurate to ± 0.10 ‐ 0.14 dex (�25‐35 per cent), can be obtained from these fields. For the remaining 63 fields, a mean calibration factor of 12.0 counts pix 1 R 1 can be used, allowing the determination of reasonable integrated fluxes accurate to better than ±0.2 dex (�50 per cent). We outline the procedures involved and the caveats that need to be appreciated in achieving such flux measurements. This paper forms a handy reference source that will significantly incre ase the scientific utility of the SHS.

Radio‐continuum detections of Galactic Planetary Nebulae – I. MASH PNe detected in large‐scale radio surveys

We present an updated and newly compiled radio-continuum data base for Macquarie/AAO/Strasbourg Hα (MASH) planetary nebulae (PNe) detected in the extant large-scale ‘blind’ radio-continuum surveys [NRAO VLA Sky Survey (NVSS), Sydney University Molonglo Sky Survey/Molonglo Galactic Plane Surveys (SUMSS/MGPS-2) and Parkes-MIT-NRAO (PMN)] and, for a small number of MASH PNe, observed and detected in targeted radio-continuum observations. We found radio counterparts for approximately 250 MASH PNe. In comparison with the percentage of previously known Galactic PNe detected in the NVSS and MGPS-2 radio-continuum surveys and according to their position on the flux density angular diameter and the radio brightness temperature evolutionary diagrams we conclude, unsurprisingly, that the MASH sample presents the radio-faint end of the known Galactic PNe population. Also, we present radio-continuum spectral properties of a small sub-sample of MASH PNe located in the strip between declinations −30° and −40°, that are detected in both the NVSS and MGPS-2 radio surveys.

Multiwavelength study of a new Galactic SNR G332.5-5.6

We present compelling evidence for confirmation of a Galactic supernova remnant (SNR) candidate, G332.5−5.6, based initially on identification of new, filamentary, optical emission-line nebulosity seen in the arcsecond resolution images from the Anglo Australian Observatory (AAO)/United Kingdom Schmidt Telescope (UKST) Hα survey. The extant radio observations and X-ray data which we have independently re-reduced, together with new optical spectroscopy of the large-scale fragmented nebulosity, confirm the identification. Optical spectra, taken across five different, widely separated nebula regions of the remnant as seen in the Hα images, show average ratios of [N II]/Hα = 2.42, [S II]/Hα = 2.10 and [S II] 6717/6731 = 1.23, as well as strong [O I] 6300, 6364 A and [O II] 3727 A emission. These ratios are firmly 3within those typical of SNRs. Here, we also present the radio-continuum detection of the SNR at 20/13 cm from observations with the Australia Telescope Compact Array (ATCA). Radio emission is also seen at 4850 MHz, in the Parkes‐MIT‐NRAO (PMN) survey and at 843 MHz from the Sydney University Molonglo Sky Survey (SUMSS) survey. We estimate an angular diameter of ∼30 arcmin and obtain an average radio spectral index of α =− 0.6 ± 0.1 which indicates the non-thermal nature of G332.5−5.6. Fresh analysis of existing ROSAT X-ray data in the vicinity also confirms the existence of the SNR. The distance to G332.5−5.6 has been independently estimated by Reynoso and Green as 3.4 kpc based on measurements of the H I λ21-cm line seen in absorption against the continuum emission. Our cruder estimates via assumptions on the height of the dust layer (3.1 kpc) and using the � ‐D relation (4 kpc) are in good agreement.

Radio planetary nebulae in the Magellanic Clouds

We report the extragalactic radio-continuum detection of 15 planetary nebulae (PNe) in the Magellanic Clouds (MCs) from recent Australia Telescope Compact Array+Parkes mosaic surveys. These detections were supplemented by new and high-resolution radio, optical and infrared observations which helped to resolve the true nature of the objects. Four of the PNe are located in the Small Magellanic Cloud (SMC) and 11 are located in the Large Magellanic Cloud (LMC). Based on Galactic PNe the expected radio flux densities at the distance of the LMC/SMC are up to ∼2.5 and ∼2.0 mJy at 1.4 GHz, respectively. We find that one of our new radio PNe in the SMC has a flux density of 5.1 mJy at 1.4 GHz, several times higher than expected. We suggest that the most luminous radio PN in the SMC (N S68) may represent the upper limit to radio-peak luminosity because it is approximately three times more luminous than NGC 7027, the most luminous known Galactic PN. We note that the optical diameters of these 15 Magellanic Clouds (MCs) PNe vary from very small (∼0.08 pc or 0.32 arcsec; SMP L47) to very large (∼1 pc or 4 arcsec; SMP L83). Their flux densities peak at different frequencies, suggesting that they may be in different stages of evolution. We briefly discuss mechanisms that may explain their unusually high radio-continuum flux densities. We argue that these detections may help solve the ‘missing mass problem’ in PNe whose central stars were originally 1–8 M� . We explore the possible link between ionized haloes ejected by the central stars in their late evolution and extended radio emission. Because of their higher than expected flux densities, we tentatively call this PNe (sub)sample – ‘Super PNe’.