Compact Nebula Research Articles

X-RAY OBSERVATIONS OF THE SUPERNOVA REMNANT CTB 87 (G74.9+1.2): AN EVOLVED PULSAR WIND NEBULA

Pulsar wind nebulae (PWNe) studies with the Chandra X-ray Observatory have opened a new window to address the physics of pulsar winds, zoom on their interaction with their hosting supernova remnant (SNR) and interstellar medium, and identify their powering engines. We here present a new 70 ks, plus an archived 18 ks, Chandra ACIS observation of the SNR CTB 87 (G74.9+1.2), classified as a PWN with unusual radio properties and poorly studied in X-rays. We find that the peak of the X-ray emission is clearly offset from the peak of the radio emission by ~100" and located at the southeastern edge of the radio nebula. We detect a point source - the putative pulsar - at the peak of the X-ray emission and study its spectrum separately from the PWN. This new point source, CXOU J201609.2+371110, is surrounded by a compact nebula displaying a torus-like structure and possibly a jet. A more extended diffuse nebula is offset from the radio nebula, extending from the point source to the northwest for ~250" The spectra of the point source, compact nebula and extended diffuse nebula are all well described by a power law model with a photon index of 1.1 (0.7-1.6), 1.2 (0.9-1.4) and 1.7 (1.5-1.8), respectively, for a column density N_H = 1.38 (1.21-1.57) x 10^22 cm^-2 (90% confidence). The total X-ray luminosity of the source is ~1.6 x 10^34 erg s^-1 at an assumed distance of 6.1 kpc, with ~2% and 6% contribution from the point source and compact nebula, respectively. The observed properties suggest that CTB 87 is an evolved (~5-28 kyr) PWN, with the extended radio emission likely a `relic' PWN, as in Vela-X and G327.1-1.1. To date, however, there is no evidence for thermal X-ray emission from this SNR, and the SNR shell is still missing, suggesting expansion into a low-density medium (n_0 < 0.2 cm^-3, assuming D = 6.1 kpc), likely caused by a stellar wind bubble blown by the progenitor star.

On the carbonaceous carriers of infrared plateau and continuum emission

This study explores the molecular origins of plateaus and continua underlying the infrared and far-infrared (far-IR) bands emitted by compact nebulae, especially protoplanetary nebulae. Computational organic chemistry codes are used to deliver the vibrational integrated band intensities of various large, typical carbonaceous structures. These spectra are composed of a rather continuous distribution of weak modes from which emerge the fingerprints. The 6–18 μm region is interspersed with a great many weak lines, to which the plateaus are assigned. Similarly, the far-IR spectrum is ascribed to the phonon (skeletal) spectrum which is readily identified beyond 18 μm. The absorptivities and absorption cross-sections per interstellar H atom deduced from these spectra are comparable with those of laboratory dust analogues and astronomical measurements, respectively. Moreover, the 5–35 μm spectra of two typical protoplanetary nebulae were reasonably well simulated with combinations of molecules containing functional groups which carry the 21- and 30-μm bands, and molecules devoid of these but carrying strong phonon spectra. These results may help understand the emergence of plateaus, the origin of continua underlying far-IR bands, as well as the composition of circumstellar dust.

HUBBLE SPACE TELESCOPEIMAGING OF THE BINARY NUCLEUS OF THE PLANETARY NEBULA EGB 6

EGB 6 is an ancient, low-surface-brightness planetary nebula. The central star, also cataloged as PG 0950+139, is a very hot DAOZ white dwarf (WD) with an apparent M dwarf companion, unresolved from the ground but detected initially through excesses in the JHK bands. Its kinematics indicates membership in the Galactic disk population. Inside of EGB 6 is an extremely dense emission knot -- completely unexpected since significant mass loss from the WD should have ceased ~10^5 y ago. The electron density of the compact nebula is very high (2.2x10^6 cm^-3), as indicated by collisional de-excitation of forbidden emission lines. Hubble Space Telescope imaging and grism spectroscopy are reported here. These resolve the WD and apparent dM companion -- at a separation of 0."166, or a projected 96(+204,-45) AU at the estimated distance of 576(+1224, -271) pc (using the V magnitude). Much to our surprise, we found that the compact emission nebula is superposed on the dM companion, far from the photoionizing radiation of the WD. Moreover, a a striking mid-infrared excess has recently been reported in Spitzer/IRAC and MIPS bands, best fit with two dust shells. The derived ratio L_IR/L_WD = 2.7 x 10^-4 is the largest yet found for any WD or planetary nucleus. The compact nebula has maintained its high density for over three decades. We discuss two possible explanations for the origin and confinement of the compact nebula, neither of which is completely satisfactory. This leaves the genesis and confinement of the compact nebula an astrophysical puzzle, yet similar examples appear in the literature.

THE MAGELLAN/IMACS CATALOG OF OPTICAL SUPERNOVA REMNANT CANDIDATES IN M83

We present a new optical imaging survey of supernova remnants in M83, using data obtained with the Magellan I 6.5m telescope and IMACS instrument under conditions of excellent seeing. Using the criterion of strong [S II] emission relative to Halpha, we confirm all but three of the 71 SNR candidates listed in our previous survey, and expand the SNR candidate list to 225 objects, more than tripling the earlier sample. Comparing the optical survey with a new deep X-ray survey of M83 with Chandra, we find 61 of these SNR candidates to have X-ray counterparts. We also identify an additional list of 46 [O III] -selected nebulae for follow-up as potential ejecta-dominated remnants, seven of which have associated X-ray emission that makes them strong candidates. Some of the other [O III]-bright objects could also be normal ISM-dominated supernova remnants with shocks fast enough to doubly ionize oxygen, but with Halpha and [S II] emission faint enough to have been missed. A few of these objects may also be H II regions with abnormally high [O III] emission compared with the majority of M83 H II regions, compact nebulae excited by young Wolf-Rayet stars, or even background AGN. The supernova remnant Halpha luminosity function in M83 is shifted a factor of ~ 4.5x higher than for M33 supernova remnants, indicative of a higher mean ISM density in M83. We describe the search technique used to identify the supernova remnant candidates and provide basic information and finder charts for the objects.

Infrared imaging and polarimetric observations of the pulsar wind nebula in SNR G21.5-0.9

We present infrared observations of the supernova remnant G21.5-0.9 with the Very Large Telescope, the Canada-France-Hawaii Telescope and the Spitzer Space Telescope. Using the VLT/ISAAC camera equipped with a narrow-band [FeII] 1.64um filter the entire pulsar wind nebula in SNR G21.5-0.9 was imaged. This led to detection of iron line-emitting material in the shape of a broken ring-like structure following the nebula's edge. The detected emission is limb-brightened. We also detect the compact nebula surrounding PSR J1833-1034, both through imaging with the CFHT/AOB-KIR instrument (K' band) and the IRAC camera (all bands) and also through polarimetric observations performed with VLT/ISAAC (Ks band). The emission from the compact nebula is highly polarised with an average value of the linear polarisation fraction $P_{L}^{avg} \simeq 0.47$, and the swing of the electric vector across the nebula can be observed. The infrared spectrum of the compact nebula can be described as a power law of index $\alpha_{IR} = 0.7 \pm 0.3$, and suggests that the spectrum flattens between the infrared and X-ray bands.

Spectral study of the late nitrogen-sequence Wolf-Rayet star FSZ35 in M33

We study and analyse the low-resolution spectra of the unusual late nitrogen-sequence Wolf–Rayet (WN) star FSZ35 in M33. We classify the object as a hydrogen-rich WN8 star. Using the radiative transfer code cmfgen, we determine the physical parameters of this object and we compare these to the parameters of other WN8 stars, including the luminous blue variable V532 (also known as Romano’s star) during the minimum of its brightness. Unlike V532, the object is fairly stable, both spectrally and photometrically, which can be attributed to its more advanced evolutionary stage or lower luminosity. FSZ35 is shown to possess a compact nebula that produces a faint but detectable [O iii] emission.

The compact nebulae cn1 and cn2, coupled to infrared stellar clusters

This is a study of the compact nebulae cn1 and cn2 situated in the extended, bright nebula S235. 12CO observations reveal the presence of blue and red outflows (i.e., a bipolar outflow) from the molecular cloud in which these nebulae are embedded. cn1 and cn2 are shown to be coupled to IR clusters of young stars, some of which have dust disks or envelopes (these are so-called young stellar objects, YSOs), with the YSOs grouped around the center of the clusters. cn1 is coupled to the infrared point source IRAS05377 + 3548, whose IR colors are close to those of T Tau stars. A chain of objects emerging from S235 (which clearly implies they are coupled in terms of evolution) is studied. These are the compact nebulae S235 A, B, and C, and the Herbig-Haro objects GGD5 and GGD6. A group of IR stars associated with GGD6 is also studied.

CHANDRAOBSERVATION OF THE TeV SOURCE HESS J1834–087

Chandra ACIS observed the field of the extended TeV source HESS J1834-087 for 47 ks. A previous XMM-Newton EPIC observation of the same field revealed a point-like source (XMMU J183435.3-084443) and an offset region of faint extended emission. In the low-resolution, binned EPIC images the two appear to be connected. However, the high-resolution Chandra ACIS images do not support the alleged connection. Instead, in these images XMMU J183435.3-084443 is resolved into a point source, CXOU~J183434.9--084443 (L=2.5 x 10e33 ergs/s, for a distance of 4 kpc; photon index 1.1), and a compact (~20'') nebula with an isotropic morphology and a softer spectrum (L=4.2 x10e33 ergs/s, photon index 2.7). The nature of the nebula is uncertain. We discuss a dust scattering halo and a pulsar-wind nebula as possible interpretations. Based on our analysis of the X-ray data, we re-evaluate the previously suggested interpretations of HESS J1834-087 and discuss a possible connection to the Fermi LAT source 1FGL J1834.3-0842c. We also obtained an upper limit of 3 x 10e-14 ergs/s cm^2 on the unabsorbed flux of the SGR J1833--0832 (in quiescence), which happened to be in the ACIS field of view.

A MULTIWAVELENGTH STUDY OF STAR FORMATION IN THE VICINITY OF GALACTIC H II REGION Sh 2-100

We present multiwavelength investigation of morphology, physical-environment, stellar contents and star formation activity in the vicinity of star-forming region Sh 2-100. It is found that the Sh 2-100 region contains seven HII regions of ultracompact and compact nature. The present estimation of distance for three HII regions, along with the kinematic distance for others, suggests that all of them belong to the same molecular cloud complex. Using NIR photometry, we identified the most probable ionizing sources of six HII regions. Their approximate photometric spectral type estimates suggest that they are massive early-B to mid-O ZAMS stars and agree well with radio continuum observations at 1280 MHz. The morphology of the complex shows a non-uniform distribution of warm and hot dust, well mixed with the ionized gas, which correlates well with the variation of average visual extinction (~ 4.2 - 97 mag) across the region. We estimated the physical parameters of ionized gas with the help of radio continuum observations. We detected an optically visible compact nebula located to the south of the 850 micron emission associated with one of the HII regions and the diagnostic of the optical emission line ratios gives electron density and electron temperature of ~ 0.67 x 10^3 cm^-3 and ~ 10^4 K, respectively. The physical parameters suggest that all the HII regions are in different stages of evolution, which correlate well with the probable ages in the range ~ 0.01 - 2 Myr of the ionizing sources. The positions of IR excess stars, ultracompact and compact HII regions at the periphery of an HI shell, possibly created by a WR star, indicate that star formation in Sh 2-100 region might have been induced by an expanding HI shell.

Revealing evolved massive stars withSpitzer

Massive evolved stars lose a large fraction of their mass via copious stellar wind or instant outbursts. During certain evolutionary phases, they can be identified by the presence of their circumstellar nebulae. In this paper, we present the results of a search for compact nebulae (reminiscent of circumstellar nebulae around evolved massive stars) using archival 24-μm data obtained with the Multiband Imaging Photometer for Spitzer. We have discovered 115 nebulae, most of which bear a striking resemblance to the circumstellar nebulae associated with luminous blue variables (LBVs) and late WN-type (WNL) Wolf–Rayet (WR) stars in the Milky Way and the Large Magellanic Cloud (LMC). We interpret this similarity as an indication that the central stars of detected nebulae are either LBVs or related evolved massive stars. Our interpretation is supported by follow-up spectroscopy of two dozen of these central stars, most of which turn out to be either candidate LBVs (cLBVs), blue supergiants or WNL stars. We expect that the forthcoming spectroscopy of the remaining objects from our list, accompanied by the spectrophotometric monitoring of the already discovered cLBVs, will further increase the known population of Galactic LBVs. This, in turn, will have profound consequences for better understanding the LBV phenomenon and its role in the transition between hydrogen-burning O stars and helium-burning WR stars. We also report on the detection of an arc-like structure attached to the cLBV HD 326823 and an arc associated with the LBV R99 (HD 269445) in the LMC.

HIGH-RESOLUTION X-RAY OBSERVATIONS OF THE PULSAR WIND NEBULA ASSOCIATED WITH THE GAMMA-RAY SOURCE HESS J1640–465

We present a Chandra X-ray observation of the very high energy $\gamma$-ray source HESS$ $J1640-465. We identify a point source surrounded by a diffuse emission that fills the extended object previously detected by XMM Newton at the centroid of the HESS source, within the shell of the radio supernova remnant (SNR) G338.3-0.0. The morphology of the diffuse emission strongly resembles that of a pulsar wind nebula (PWN) and extends asymmetrically to the South-West of a point-source presented as a potential pulsar. The spectrum of the putative pulsar and compact nebula are well-characterized by an absorbed power-law model which, for a reasonable $N_{\rm H}$ value of $14\times 10^{22} \rm cm^{-2}$, exhibit an index of 1.1 and 2.5 respectively, typical of Vela-like PWNe. We demonstrate that, given the H$ $I absorption features observed along the line of sight, the SNR and the H$ $II surrounding region are probably connected and lie between 8 kpc and 13 kpc. The resulting age of the system is between 10 and 30 kyr. For a 10 kpc distance (also consistent with the X-ray absorption) the 2-10 keV X-ray luminosities of the putative pulsar and nebula are $L_{\rm PSR} \sim 1.3 \times 10^{33} d_{10 \rm kpc}^{2} \rm erg.s^{-1}$ and $L_{\rm PWN} \sim 3.9 \times 10^{33} d_{10}^{2} \rm erg.s^{-1}$ ($d_{10} = d / 10{\rm kpc}$). Both the flux ratio of $L_{\rm PWN}/L_{\rm PSR} \sim 3.4$ and the total luminosity of this system predict a pulsar spin-down power around $\dot{E} \sim 4 \times 10^{36} \rm erg s^{-1}$. We finally consider several reasons for the asymmetries observed in the PWN morphology and discuss the potential association with the HESS source in term of a time-dependent one-zone leptonic model.

Hubble Space Telescope observations of white dwarfs in detached binaries

I describe three programs of Hubble Space Telescope (HST) observations of white dwarfs (WDs) in detached binaries. (1) I summarize the HST UV spectroscopy of the Hyades eclipsing binary V471 Tauri, containing a dK main-sequence star and a hot WD. By phasing time-resolved STIS spectra on the 9.25-min rotation period of the WD, we have shown that there are two opposite spots on the WD where there is magnetic accretion from the K star's wind. (2) Several programs of direct imaging and FGS astrometry are yielding high-precision visual orbits and dynamical masses, including the famous systems of Sirius and Procyon, as well as three fainter binaries in which both components are DC WDs. (3) Finally I discuss the bizarre nucleus of the low-surface-brightness planetary nebula EGB 6. The central star is associated with a compact emission-line nebula, which HST imaging shows is associated with a resolved dM companion. Why there should be a compact nebula around the cool star is puzzling; we speculate that it could be an accretion disk of material captured from the outflow that produced the surrounding faint PN. In a recent development, Spitzer has detected a 24 μm excess from EGB 6, indicating a dust component at a temperature of ∼260 K; the location of this dust relative to the hot and cool stars and the compact nebula remains uncertain at present.

THE MOLECULAR PROPERTIES OF GALACTIC H II REGIONS

We derive the molecular properties for a sample of 301 Galactic H ii regions including 123 ultra compact (UC), 105 compact, and 73 diffuse nebulae. We analyze all sources within the BU-FCRAO Galactic Ring Survey (GRS) of 13 CO emission known to be H ii regions based upon the presence of radio continuum and cm-wavelength radio recombination line emission. Unlike all previous large area coverage 13 CO surveys, the GRS is fully sampled in angle and yet covers ∼ 75 deg 2 of the Inner Galaxy. The angular resolution of the GRS (46 �� ) allows us to associate molecular gas with H ii regions without ambiguity and to investigate the physical properties of this molecular gas. We find clear CO/H ii morphological associations in position and velocity for ∼ 80% of the nebular sample. Compact H ii region molecular gas clouds are on average larger than UC clouds: 2. � 2 compared to 1. � 7. Compact and UC H ii regions have very similar molecular properties, with ∼ 5 K line intensities and ∼ 4k m s −1 line widths. The diffuse H ii region molecular gas has lower line intensities, ∼ 3 K, and smaller line widths, ∼ 3. 5k m s −1 . These latter characteristics are similar to those found for quiescent molecular clouds in the GRS. Our sample nebulae thus show evidence for an evolutionary sequence wherein small, dense molecular gas clumps associated with UC H ii regions grow into older compact nebulae and finally fragment and dissipate into large, diffuse nebulae.

ChandraObservations of the Pulsar B1929+10 and Its Environment

We report on two Chandra observations of the 3 Myr old pulsar B1929+10, which reveal a faint compact (~9 -->'' × 5 -->'') nebula elongated in the direction perpendicular to the pulsar's proper motion, two patchy wings, and a possible short (~3 -->'') jet emerging from the pulsar. In addition, we detect a tail extending up to at least -->4' in the direction opposite to the pulsar's proper motion, aligned with the ~15 -->' long tail detected in ROSAT and XMM-Newton observations. The overall morphology of the nebula suggests that the shocked pulsar wind is confined by the ram pressure due to the pulsar's supersonic speed. The shape of the compact nebula in the immediate vicinity of the pulsar seems to be consistent with the current MHD models. However, since these models do not account yet for the change of the flow velocity at larger distances from the pulsar, they are not able to constrain the extent of the long pulsar tail. The luminosity of the whole nebula as seen by Chandra is -->LPWN ~ 1030 ergs s -->−1 in the 0.3-8 keV band, for the distance of 361 pc. Using the Chandra and XMM-Newton data, we found that the pulsar spectrum is composed of nonthermal (magnetospheric) and thermal components. The nonthermal component can be described by a power-law model with photon index -->Γ ≈ 1.7 and luminosity -->LnonthPSR ≈ 1.7 × 1030 ergs s -->−1 in the 0.3-10 keV band. The blackbody fit for the thermal component, which presumably emerges from hot polar caps, gives the temperature -->kT ≈ 0.3 keV and projected emitting area -->A⊥ ~ 3 × 103 m2, corresponding to the bolometric luminosity -->Lbol ~ (1–2) × 1030 ergs s -->−1.

M1–78: a nitrogen-rich Galactic compact H II region beyond the Perseus arm

Context. There is considerable controversy surrounding the nature of M1−78, a compact nebula located beyond the Perseus arm. It was first classified as a planetary nebula and is nowadays generally considered to be a compact H ii region. Aims. To investigate the nature of M1−78 further, we present a detailed spectroscopic study of M1−78 in the optical and near-infrared. Methods. We obtained long-slit, intermediate-resolution, optical spectroscopy with the ISIS spectrograph mounted on the William Herschel Telescope (WHT) at Roque de los Muchachos Observatory (La Palma, Spain). As a complement, we obtained long-slit, intermediate-resolution, near-infrared spectra using LIRIS, the near-infrared imager/spectrographer also installed at the WHT. Results. M1−78 is a high-density nebula with substantial physical differences between its two main morphological zones: a bright arc in the SW and a blob of emission in the NE. Specifically, the blob in the NE has a higher electron temperature (13 400 K) and visual extinction (about 9 mag) than the SW arc. The most important result, however, is the confirmation of a nitrogen enrichment in M1−78. This enrichment is stronger at the location of the NE blob and is correlated with a defficiency in the O abundance and a (dubious) He enrichment. Such an abundance pattern is typical of ejecta nebulae around evolved massive stars such as Wolf-Rayet and Luminous Blue Variable stars. The spatial variations in the physical conditions and chemical abundances and the presence of more than one possible ionizing source indicate, however, that M1−78 is better described as a combination of a compact H ii region + ejecta. This is confirmed by the He i 2.112 µm/Brγ line ratio, which indicates a hot (Teff > 40 000 K) O star in the SW arc. Finally, we detect H2 emission that extends over a large (∼30 �� ) area around the ionized nebula. Analysis of the near-infrared H2 lines indicates

The IPHAS catalogue of Hα emission-line sources in the northern Galactic plane

Abstract We present a catalogue of point-source Hα emission-line objects selected from the INT/WFC Photometric Hα Survey (IPHAS) of the northern Galactic plane. The catalogue covers the magnitude range 13 ≤r′≤ 19.5 and includes Northern hemisphere sources in the Galactic latitude range −5° &amp;lt; b &amp;lt; 5°. It is derived from ∼1500 deg2 worth of imaging data, which represents 80 per cent of the final IPHAS survey area. The electronic version of the catalogue will be updated once the full survey data become available. In total, the present catalogue contains 4853 point sources that exhibit strong photometric evidence for Hα emission. We have so far analysed spectra for ∼300 of these sources, confirming more than 95 per cent of them as genuine emission-line stars. A wide range of stellar populations are represented in the catalogue, including early-type emission-line stars, active late-type stars, interacting binaries, young stellar objects and compact nebulae. The spatial distribution of catalogue objects shows overdensities near sites of recent or current star formation, as well as possible evidence for the warp of the Galactic plane. Photometrically, the incidence of Hα emission is bimodally distributed in (r′−i′). The blue peak is made up mostly of early-type emission-line stars, whereas the red peak may signal an increasing contribution from other objects, such as young/active low-mass stars. We have cross-matched our Hα-excess catalogue against the emission-line star catalogue of Kohoutek &amp; Wehmeyer, as well as against sources in SIMBAD. We find that fewer than 10 per cent of our sources can be matched to known objects of any type. Thus IPHAS is uncovering an order of magnitude more faint (r′ &amp;gt; 13) emission-line objects than were previously known in the Milky Way.

A Half-Megasecond Chandra Observation of the Oxygen-rich Supernova Remnant G292.0+1.8

We report on our initial analysis of a deep 510 ks observation of the Galactic oxygen-rich supernova remnant (SNR) G292.0+1.8 with the Chandra X-Ray Observatory. Our new Chandra ACIS-I observation has a larger field of view and an order of magnitude deeper exposure than that of the previous Chandra observation, which allows us to cover the entire SNR and to detect new metal-rich ejecta features. We find a highly nonuniform distribution of thermodynamic conditions of the X-ray-emitting hot gas that correlates well with the optical [O III] emission, suggesting the possibility that the originating supernova explosion of G292.0+1.8 was itself asymmetric. We also reveal spectacular substructures of a torus, a jet, and an extended central compact nebula, all associated with the embedded pulsar J1124-5916.

NTT and VLT diffraction limited imaging of Trumpler 14: revealing a massive core-halo cluster

Aims. We present the deepest and highest resolution near-infrared imaging to date of cluster Trumpler 14 in Carina. Our goal is to identify and characterise the young stellar population of this massive cluster. Methods. We made use of deep and wide-field NIR images from NTT and VLT observations, that were sensitive enough to detect substellar sources at the distance to this cluster, and at high enough resolution (VLT diffraction limited) to fully resolve the core of the cluster crowded with O stars. Results. We find that Tr14 has a well-defined core-halo structure, where less than 30% of the cluster's members reside in the core. The core is well characterised by a King function with a core radius of $0\farcm17$ (0.14 pc at the adopted distance) and a constant baseline, the halo, of 125 sources/pc 2 . Despite the unusually large number of OB stars, the central number density at zero radius is ~7.3 $\times$ 10 3 pc -3 , which is loose in comparison with similar clusters. We find a normal reddening law towards the cluster and derive a global reddening of A v = 2.6 ± 0.3 mag. We find convincing evidence of a sparse foreground population (~5 sources/arcmin 2 ) reddened by about A v = 1.4 mag, which we suggest is not associated with Tr14 but is most likely an older population produced in the nearby young clusters of this complex. The colour–magnitude diagrams are compatible with ages between “zero” and ~5 Myr, although the sources from the core of the cluster appear to concentrate on the youngest isochrones, suggesting that the halo population is, on average, slightly older than the core population. Using a set of simplistic, fixed-age, mass-luminosity relations, we derive a mass of 10 4 $M_\odot$ for the cluster. From the NACO JHK s L ' data, we estimate a fraction of infrared-excess sources of 35%, although this is likely to be an underestimate given the bright completeness limits of the L' band. Finally, we argue that the formerly identified proplyd candidates that fall inside our survey are not proplyds but remnants of the disrupted molecular cloud that surround the cluster. We also find a series of interesting objects in our field that are worthy of future attention: a candidate photoionised proplyd best seen in the L' band, a compact nebula surrounding an early type star, and a tentative proplyd/small shock associated with a faint source.

The enigma of the oldest ‘nova’: the central star and nebula of CK Vul

CK Vul is classified as, amongst others, the slowest known nova, a hibernating nova or a very late thermal pulse object. Following its eruption in ad 1670, the star remained visible for 2 yr. A 15-arcsec nebula was discovered in the 1980s, but the star itself has not been detected since the eruption. We here present radio images which reveal a 0.1-arcsec radio source with a flux of 1.5 mJy at 5 GHz. Deep Hα images show a bipolar nebula with a longest extension of 70 arcsec, with the previously known compact nebula at its waist. The emission-line ratios show that the gas is shock-ionized, at velocities >100 km s−1. Dust emission yields an envelope mass of ∼5 × 10−2 M⊙. Echelle spectra indicate outflow velocities up to 360 km s−1. From a comparison of images obtained in 1991 and 2004 we find evidence for expansion of the nebula, consistent with an origin in the 1670 explosion; the measured expansion is centred on the radio source. No optical or infrared counterpart is found at the position of the radio source. The radio emission is interpreted as thermal free–free emission from gas with Te∼ 104 K. The radio source may be due to a remnant circumbinary disc, similar to those seen in some binary post-AGB stars. We discuss possible classifications of this unique outburst, including that of a sub-Chandrasekhar mass supernova, a nova eruption on a cool, low-mass white dwarf or a thermal pulse induced by accretion from a circumbinary disc.

Discovery of Extended Emission around the Pulsar B0355+54

PSR B0355+54 is one of the handful of pulsars that have been observed with both Chandra and XMM-Newton. The analysis of the archival data has revealed the pulsar and a ~30'' compact nebula surrounding it. XMM-Newton also has detected a trail that extends ~6' and, similar to the compact nebula, is also counteraligned with the proper motion of the pulsar. The spectra of both the pulsar and the extended emission are well described by an absorbed power-law model. The measured flux corresponds to an efficiency of converting the spin-down luminosity into X-rays in the 2-10 keV band of ~0.01% and ~1% for the pulsar and the extended emission, respectively. From the XMM-Newton data we have detected pulsations at the expected radio period. The energetics and the extent of the extended emission can be explained by a bow shock formed by the motion of the pulsar through the interstellar medium.