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Register a new userImproved prospects for the detection of new Large Magellanic Cloud planetary nebulae
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The Large Magellanic Cloud (LMC) contains the nearest large extragalactic population of planetary nebulae (PNe). A shallow viewing angle and low interstellar reddening towards the LMC potentially means a larger, more complete flux-limited population can be assembled than for any other galaxy. These advantages appear to be reflected by the small gap between the catalogued ($sim$700 PNe) and estimated ($1000pm250$ PNe) population size. With more detailed multi-wavelength studies the catalogued number of LMC PNe may fall, potentially widening this gap. We demonstrate here that the gap can be further bridged with improved optical and near-infrared imaging surveys. We present three [O III]-selected PNe discovered from ESO WFI observations of the 30 Doradus region and one serendipitous discovery from near-infrared Vista Magellanic Cloud (VMC) survey observations. The WFI PNe have resolved [O III] and H$alpha$ nebulae that verify their PN nature and their [O III] fluxes place them 6--7 mag ($m_{5007}=20$--21 mag) fainter than the bright-end of the planetary nebula luminosity function (PNLF). Their faintness, small angular size and surrounding complex emission-line background explains why previous H$alpha$ surveys of the region did not select them. We estimate there may be as many as 50--75 similar PNe awaiting discovery in the central $5times5$ degrees of the LMC. The VMC survey routinely detects PNe as red resolved nebulae that may allow some of this expected population to be recovered without traditional narrow-band imaging surveys. We demonstrate this potential with the first new VMC-selected PN which has a rare Wolf-Rayet [WC9]--[WC11] central star.
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We present 21 new radio-continuum detections at catalogued planetary nebula (PN) positions in the Large Magellanic Cloud (LMC) using all presently available data from the Australia Telescope Online Archive at 3, 6, 13 and 20 cm. Additionally, 11 previously detected LMC radio PNe are re-examined with $ 7 $ detections confirmed and reported here. An additional three PNe from our previous surveys are also studied. The last of the 11 previous detections is now classified as a compact HII region which makes for a total sample of 31 radio PNe in the LMC. The radio-surface brightness to diameter ($Sigma$-D) relation is parametrised as $Sigma propto {D^{ - beta }}$. With the available 6~cm $Sigma$-$D$ data we construct $Sigma$-$D$ samples from 28 LMC PNe and 9 Small Magellanic Cloud (SMC) radio detected PNe. The results of our sampled PNe in the Magellanic Clouds (MCs) are comparable to previous measurements of the Galactic PNe. We obtain $beta=2.9pm0.4$ for the MC PNe compared to $beta = 3.1pm0.4$ for the Galaxy. For a better insight into sample completeness and evolutionary features we reconstruct the $Sigma$-$D$ data probability density function (PDF). The PDF analysis implies that PNe are not likely to follow linear evolutionary paths. To estimate the significance of sensitivity selection effects we perform a Monte Carlo sensitivity simulation on the $Sigma$-$D$ data. The results suggest that selection effects are significant for values larger than $beta sim 2.6$ and that a measured slope of $beta=2.9$ should correspond to a sensitivity-free value of $sim 3.4$.
We present IRAC and MIPS images and photometry of a sample of previously known planetary nebulae (PNe) from the SAGE survey of the Large Magellanic Cloud (LMC) performed with the Spitzer Space Telescope. Of the 233 known PNe in the survey field, 185 objects were detected in at least two of the IRAC bands, and 161 detected in the MIPS 24 micron images. Color-color and color-magnitude diagrams are presented using several combinations of IRAC, MIPS, and 2MASS magnitudes. The location of an individual PN in the color-color diagrams is seen to depend on the relative contributions of the spectral components which include molecular hydrogen, polycyclic aromatic hydrocarbons (PAHs), infrared forbidden line emission from the ionized gas, warm dust continuum, and emission directly from the central star. The sample of LMC PNe is compared to a number of Galactic PNe and found to not significantly differ in their position in color-color space. We also explore the potential value of IR PNe luminosity functions (LFs) in the LMC. IRAC LFs appear to follow the same functional form as the well-established [O III] LFs although there are several PNe with observed IR magnitudes brighter than the cut-offs in these LFs.
We present near-infrared spectra of ten planetary nebulae (PNe) in the Large and Small Magellanic Clouds (LMC and SMC), acquired with the FIRE and GNIRS spectrometers on the 6.5-m Baade and 8.1-m Gemini South Telescopes, respectively. We detect Se and/or Kr emission lines in eight of these objects, the first detections of n-capture elements in Magellanic Cloud PNe. Our abundance analysis shows large s-process enrichments of Kr (0.6-1.3 dex) in the six PNe in which it was detected, and Se is enriched by 0.5-0.9 dex in five objects. We also estimate upper limits to Rb and Cd abundances in these objects. Our abundance results for the LMC are consistent with the hypothesis that PNe with 2--3 M$_{odot}$ progenitors dominate the bright end of the PN luminosity function in young gas-rich galaxies. We find no significant correlations between s-process enrichments and other elemental abundances, central star temperature, or progenitor mass, though this is likely due to our small sample size. We determine S abundances from our spectra and find that [S/H] agrees with [Ar/H] to within 0.2 dex for most objects, but is lower than [O/H] by 0.2-0.4 dex in some PNe, possibly due to O enrichment via third dredge-up. Our results demonstrate that n-capture elements can be detected in PNe belonging to nearby galaxies with ground-based telescopes, allowing s-process enrichments to be studied in PN populations with well-determined distances.
We present ten new Spitzer detections of fullerenes in Magellanic Cloud Planetary Nebulae, including the first extragalactic detections of the C70 molecule. These new fullerene detections together with the most recent laboratory data permit us to report an accurate determination of the C60 and C70 abundances in space. Also, we report evidence for the possible detection of planar C24 in some of our fullerene sources, as indicated by the detection of very unusual emission features coincident with the strongest transitions of this molecule at ~6.6, 9.8, and 20 um. The infrared spectra display a complex mix of aliphatic and aromatic species such as hydrogenated amorphous carbon grains (HACs), PAH clusters, fullerenes, and small dehydrogenated carbon clusters (possible planar C24). The coexistence of such a variety of molecular species supports the idea that fullerenes are formed from the decomposition of HACs. We propose that fullerenes are formed from the destruction of HACs, possibly as a consequence of shocks driven by the fast stellar winds, which can sometimes be very strong in transition sources and young PNe. This is supported by the fact that many of our fullerene-detected PNe show altered [NeIII]/[NeII] ratios suggestive of shocks as well as P-Cygni profiles in their UV lines indicative of recently enhanced mass loss.
We have investigated the light variability in a sample of 22 carbon-rich post-AGB stars in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), based primarily on photometric data from the OGLE survey. All are found to vary. Dominant periods are found in eight of them; these periods range from 49 to 157 days, and most of these stars have F spectral types. These eight are found to be similar to the Milky Way Galaxy (MWG) carbon-rich proto-planetary nebulae (PPNs) in several ways: (a) they are in the same period range of ~38 to ~160 days, (b) they have similar spectral types, (c) they are (all but one) redder when fainter, (d) they have multiple periods, closely spaced in time, with a average ratio of secondary to primary period of ~1.0, and as an ensemble, (e) they show a trend of decreasing period with increasing temperature, and (f) they show a trend of decreasing amplitude with decreasing period. However, they possibly differ in that the decreasing trend of period with temperature may be slightly offset from that of the MWG. These eight are classified as PPNs. The other 14 all show evidence of variability on shorter timescales. They are likely hotter PPNs or young planetary nebulae. However, in the MWG the numbers of PPNs peak in the F-G spectral types, while it appears that in the LMC they peak at a hotter B spectral type. One of the periodic ones shows a small, R Coronae Borealis-type light curve drop.
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