Milorad Stupar

New Galactic supernova remnants discovered with IPHAS

As part of a systematic search programme of a 10-degree wide strip of the Northern Galactic plane we present preliminary evidence for the discovery of four (and possibly five) new supernova remnants (SNRs). The pilot search area covered the 19-20 hour right ascension zone sampling from +20 to +55 degrees in declination using binned mosaic images from the INT Photometric H-alpha Survey (IPHAS). The optical identification of the candidate SNRs was based mainly on their filamentary and arc-like emission morphologies, their apparently coherent, even if fractured structure and clear disconnection from any diffuse neighbouring HII region type nebulosity. Follow-up optical spectroscopy was undertaken, sampling carefully across prominent features of these faint sources. The resulting spectra revealed typical emission line ratios for shock excited nebulae which are characteristic of SNRs, which, along with the latest diagnostic diagrams, strongly support the likely SNR nature of these sources: G038.7-1.3 (IPHASX J190640.5+042819); G067.6+0.9 (IPHASX J195744.9+305306); G066.0-0.0 (IPHASX J195749.2+290259) and G065.8-0.5 (IPHASX J195920.4+283740). A fifth possible younger, higher density nebula SNR candidate, G067.8+0.5 (IPHASX J200002.4+305035) was discovered about 5 arcmins to the west of IPHASX J195744.9+305306, and warrants further study. A multi-wavelength cross-check from available archived data in the regions of these candidates was also performed with a focus on possible radio counterparts.

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.

NON-THERMAL X-RAY EMISSION FROM THE NORTHWESTERN RIM OF THE GALACTIC SUPERNOVA REMNANT G266.2–1.2 (RX J0852.0-4622)

We present a detailed spatially resolved spectroscopic analysis of two observations (with a total integration time of 73280 s) made of the X-ray-luminous northwestern rim complex of the Galactic supernova remnant (SNR) G266.2–1.2 (RX J0852.0-4622) with the Chandra X-ray Observatory. G266.2–1.2 is a member of a class of Galactic SNRs which feature X-ray spectra dominated by non-thermal emission: in the cases of these SNRs, the emission is believed to have a synchrotron origin and studies of the X-ray spectra of these SNRs can lend insight into how SNRs accelerate cosmic-ray particles. The Chandra observations have clearly revealed fine structure in this rim complex (including a remarkably well-defined leading shock) and the spectra of these features are dominated by non-thermal emission. We have measured the length scales of the upstream structures at eight positions along the rim and derive lengths of 0.02-0.08 pc (assuming a distance of 750 pc to G266.2–1.2). We have also extracted spectra from seven regions in the rim complex (as sampled by the ACIS-S2, -S3, and -S4 chips) and fit these spectra with such models as a simple power law as well as the synchrotron models SRCUT and SRESC. We have constrained our fits to the latter two models using estimates for the flux densities of these filaments at 1 GHz as determined from radio observations of this rim complex made with the Australia Telescope Compact Array. Statistically acceptable fits to all seven regions are derived using each model: differences in the fit parameters (such as photon index and cutoff frequency) are seen in different regions, which may indicate variations in shock conditions and the maximum energies of the cosmic-ray electrons accelerated at each region. Finally, we estimate the maximum energy of cosmic-ray electrons accelerated along this rim complex to be approximately 40 TeV (corresponding to one of the regions of the leading shock structure assuming a magnetic field strength of 10 μG). We include a summary of estimated maximum energies for both Galactic SNRs as well as SNRs located in the Large Magellanic Cloud. Like these other SNRs, it does not appear that G266.2–1.2 is currently accelerating electrons to the knee energy (~3000 TeV) of the cosmic-ray spectrum. This result is not surprising, as there is some evidence that loss mechanisms which are not important for the accelerated cosmic-ray nucleons at energies just below the knee might cut off electron acceleration.

Multifrequency study of SNR J0533-7202, a new supernova remnant in the LMC

We present a detailed study of Australia Telescope Compact Array (ATCA) observations of a newly discovered Large Magellanic Cloud (LMC) supernova remnant (SNR), SNR J0533-7202. This object follows a horseshoe morphology, with a size 37 pc x 28 pc (1-pc uncertainty in each direction). It exhibits a radio spectrum with the intrinsic synchrotron spectral index of alpha= -0.47+-0.06 between 73 and 6 cm. We report detections of regions showing moderately high fractional polarisation at 6 cm, with a peak value of 36+-6% and a mean fractional polarisation of 12+-7%. We also estimate an average rotation measure across the remnant of -591 rad m^-2. The current lack of deep X-ray observation precludes any conclusion about high-energy emission from the remnant. The association with an old stellar population favours a thermonuclear supernova origin of the remnant.

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.

First detection of optical light from SNR G279.0+1.1

This is the initial paper in a series presenting the first optical detections and subsequent follow-up spectroscopy of known southern Galactic supernova remnants (SNRs) previously discovered in the radio. These new detections come from the Anglo-Australian Observatory (AAO)/United Kingdom Schmidt Telescope Hα survey of the southern Galactic plane which has opened up fresh opportunities to study Galactic remnants. Here, we present the first optical imaging and follow-up spectra of Galactic SNR G279.0+1.1 where a series of 14 small-scale fragmented groups of Hα filaments have been discovered in a ∼2. ◦ 3 area centred on G279.0+1.1. Individually they are somewhat inconspicuous but collectively they are completely enclosed within the overall radio contours of this known SNR. Three of these filamentary groupings are particularly prominent and optical spectra have been obtained across two of them. Their morphological structure and spectral characteristics are typical of optically detected SNR filaments. A very strong [S II] emission relative to Hα has been detected with [S II]/H α> 0.7 and 1.1, confirming strong, shock-heated emission. This is sufficient to classify these filaments in the likely SNR domain and therefore indicating a direct connection with the radio remnant. Other typical SNR emission lines such as [O II] at 3727 A, Hβ ,[ OIII] at 4959 and 5007 A, Hα and [N II] at 6548 and 6584 A were also detected, lending strong support to an SNR origin of these optical filaments. The value and insights that these optical data can provide for known remnants are discussed along with their relevance to the Galactic nitrogen abundance. A serendipitous discovery of an adjacent H II region is also briefly described.

Multifrequency study of a new Fe-rich supernova remnant in the Large Magellanic Cloud, MCSNR J0508−6902

We present a detailed radio, X-ray and optical study of a newly discovered Large Mag- ellanic Cloud (LMC) supernova remnant (SNR) which we denote MCSNR J0508-6902. Observations from the Australian Telescope Compact Array (ATCA) and the XMM- Newton X-ray observatory are complemented by deep Himages and Anglo Aus- tralian Telescope AAOmega spectroscopic data to study the SNR shell and its shock- ionisation. Archival data at other wavelengths are also examined. The remnant follows a filled-in shell type morphology in the radio-continuum and has a size of ∼74 pc × 57 pc at the LMC distance. The X-ray emission exhibits a faint soft shell morphology with Fe-rich gas in its interior - indicative of a Type Ia origin. The remnant appears to be mostly dissipated at higher radio-continuum frequencies leaving only the south- eastern limb fully detectable while in the optical it is the western side of the SNR shell that is clearly detected. The best-fit temperature to the shell X-ray emission (kT = 0.41 +0.05 0.06 keV) is consistent with other large LMC SNRs. We determined an O/Fe ratio of < 21 and an Fe mass of 0.5-1.8 M⊙ in the interior of the remnant, both of which are consistent with the Type Ia scenario. We find an equipartition magnetic field for the remnant of ∼28 µG, a value typical of older SNRs and consistent with other analyses which also infer an older remnant.

Catalogue of known Galactic SNRs uncovered in Hα light

During detailed searches for new Galactic supernova remnants (SNRs) in the Anglo-Australian Observatory/United Kingdom Schmidt Telescope (AAO/UKST) Hα survey of the southern Galactic plane, we also uncovered, for the first time, possible associated Hα emission in the vicinity of about 24 known Galactic SNRs previously known solely from radio or X-ray observations. The possible optical counterparts to these known SNR were detected due to the 1-arcsec resolution and 5 Rayleigh sensitivity of this Hα survey. The newly discovered emission frequently exhibits the typical filamentary form of other optically detected SNRs although sometimes the Hα emission clouds or fragmented filaments largely inside an SNR extend over the radio border. It is true that superposition of general diffuse and extended Galactic emission in the region of these remnants is a complicating factor, but for many optical candidates the Hα emission provides an excellent morphological and positional match to the observed radio emission so that an association seems clear. We have already published Hα images and confirmatory spectral observations for several of the best optical counterparts to known SNRs but for completeness and convenience we include them in our complete catalogue of previously known radio detected SNRs for which we have now uncovered Hα optical emission. For better visualization of the optical emissions from these faint supernova remnants and to enhance some low surface-brightness features we also present quotient images of the Hα data divided by the accompanying broad-band short-red data. Out of 274 Galactic SNRs currently catalogued and detected in the radio only ∼20 per cent had previous optical counterparts. We may have now increased this by a further third by adding a further 24 candidate optical counterparts.