Search

Characteristics of the flares of gamma rays detected from the Crab Nebula by the AGILE and Fermi-LAT satellite instruments are compared with those of a gamma ray burst recorded by several air shower arrays on February 23, 1989 and with one recent observation made by the ARGO-YBJ array. It is demonstrated that though pulsar-periodicity and energy spectra of emissions at 100 MeV (satellite gamma ray telescopes) and 100 TeV (EAS arrays) are different, their time structures seem to be similar. Moreover, maybe the difference between “flares” and “waves” recently found in the Crab Nebula emission by the AGILE team also exists at ultra-high energies.

Context. A handful of young (a few thousand years) supernova remnants (SNRs) host point-like X-ray sources, dubbed central compact objects (CCOs), which are thought to be radio-silent isolated neutron stars formed by the supernova explosion. So far, no CCO has been firmly detected at other wavelengths. However, ground-based observation in the H α band detected a nebula around CXO J085201.4-461753, the CCO in the Vela Jr. SNR. The nebula has also been detected in deep R -band observations performed with the Very Large Telescope ( VLT ). Interestingly, both its extension and its flux in the R band are consistent with those measured in H α , suggesting that the nebula spectrum is dominated by line emission, possibly produced by a velocity-driven bow-shock in the interstellar medium (ISM) or by its photo-ionisation from the neutron star. Aims. The aim of this work is to resolve the morphology of the H α nebula around the CCO to verify the proposed interpretations. Methods. We performed high-resolution imaging observations of the nebula with the Wide Field Planetary Camera 2 ( WFPC2 ) of the Hubble Space Telescope ( HST ) through the 656N filter, almost exactly centred on the rest wavelength of the H α line. Results. Surprisingly enough, we did not detect the nebula in our WFPC2 image down to a $3 \sigma$ flux limit of ~3 $\times$ 10 -15 erg cm -2 s -1 . This limit is a factor of 10 fainter than the nebula flux measured in the discovery ground-based observations which were, however, performed with redder and broader H α filters. Conclusions. The non-detection of the nebula in the narrower and bluer WFPC2 656N filter suggests that the peak of the emission might actually be at longer wavelengths. One possibility, compatible with the bow-shock scenario only, is that the H α line is red-shifted by ~$10{-}60$ A due to the neutron star motion with a radial velocity 450 $\la$$ V_{\mathrm r}$ $\la$ 2700 km s -1 . The other possibility is that the nebula is a knot of [NII] emission ( λ = 6583.6 A) unrelated to CXO J085201.4-461753 and possibly associated with the nearby star Wray 16-30. High-resolution spectroscopy observations will help to settle the nature of the nebula.

We present multi-wavelength radio observations in the direction of the spiral galaxy IC 2497 and the neighbouring emission nebula known as "Hanny's Voorwerp". Our WSRT continuum observations at 1.4 GHz and 4.9 GHz, reveal the presence of extended emission at the position of the nebulosity, although the bulk of the emission remains unresolved at the centre of the galaxy. e-VLBI 1.65 GHz observations show that on the milliarcsecond-scale a faint central compact source is present in IC 2497 with a brightness temperature in excess of 4E5 K. With the WSRT, we detect a large reservoir of neutral hydrogen in the proximity of IC 2497. One cloud complex with a total mass of 5.6E9 Msol to the South of IC 2497, encompasses Hanny's Voorwerp. Another cloud complex is located at the position of a small galaxy group ~100 kpc to the West of IC 2497 with a mass of 2.9E9 Msol. Our data hint at a physical connection between both complexes. We also detect HI in absorption against the central continuum source of IC 2497. Our observations strongly support the hypothesis that Hanny's Voorwerp is being ionised by an AGN in the centre of IC 2497. In this scenario, a plasma jet associated with the AGN, clears a path through the ISM/IGM in the direction towards the nebulosity. The large-scale radio continuum emission possibly originates from the interaction between this jet and the large cloud complex that Hanny's Voorwerp is embedded in. The HI kinematics do not fit regular rotation, thus the cloud complex around IC 2497 is probably of tidal origin. From the HI absorption against the central source, we derive a lower limit of 2.8E21 +- 0.4E21 atoms/sqcm to the HI column density. However, assuming non-standard conditions for the detected gas, we cannot exclude the possibility that the AGN in the centre of IC 2497 is Compton-thick., Comment: 4 pages, 5 figures, A&A, letter to the editor, accepted for publication

The Antares nebula is a peculiar emission nebula seen in numerous Fe II lines and in radio free-free emission, probably associated with the H II region caused by alpha Sco B in the wind of alpha Sco A. High-resolution spectra with spatial resolution were used to study the emission line spectrum, the physical nature of the nebula and to determine the mass-loss rate of the M supergiant alpha Sco A.

Aims. We determined the atmospheric parameters and evolutionary state of the cool stellar component of the peculiar planetary nebula Me 1-1 and analyzed its spatio-kinematic structure. Methods. High-resolution optical spectroscopy was used in an abundance analysis of the cool component in Me 1-1. Narrow-band images were used to study the nebula. Results. The stellar parameters derived for the cool star in Me 1-1 are Teff = 4500 K and log g = 2.0, which correspond to a spectral type of K(1-2) II. This star is rapidly rotating with a projected rotational velocity of Vrot = 90 km s −1 . Its estimated mass is 3.5 M� with a distance of 6 kpc and a luminosity of 370 L� . The cool star in Me 1-1 has nearly solar iron and calcium abundances. Barium, a monitor of s-process nucleosynthesis, is not enriched. We show that this cool star has reached the base of the RGB and its accreted matter has been diluted by the deepening convective envelope. The images strongly suggest that Me 1-1 is an edge-on ring with incipient bipolar lobes. The central star is displaced ∼400 AU from the center of the nebula. The expansion velocity of the ring ranges from 10 and 13 km s −1 in the different emission lines. Broad (� 600 km s −1 ) wings are detected in the Hα emission, probably with a stellar origin. A comparison of Me 1-1 with other ring-like planetary nebula shows that most of them host off-center binary central stars.

Context. ISOPHOT multi-aperture photometry is an efficient method to resolve compact sources or to detect extended emission down to relatively faint levels with single detectors in the wavelength range 3 to 100 μm. Aims. Using ISOPHOT multi-aperture photometry and complementary ISO spectra and IR spectral energy distributions we discuss the nature of the extended IR emission of the two PNe NGC 6543 and NGC 7008. Methods. In the on-line appendix we describe the data reduction, calibration and interpretation methods based on a simultaneous determination of the IR source and background contributions from the on-source multi-aperture sequences. Normalized profiles enable direct comparison with point source and flat-sky references. Modelling the intensity distribution offers a quantitative method to assess source extent and angular scales of the main structures and is helpful in reconstructing the total source flux, if the source extends beyond a radius of 1 arcmin. The photometric calibration is described and typical accuracies are derived. General uncertainty, quality and reliability issues are addressed, too. Transient fitting to non-stabilised signal time series, by means of combinations of exponential functions with different time constants, improves the actual average signals and reduces their uncertainty. Results. The emission of NGC 6543 in the 3.6μm band coincides with the core region of the optical nebula and is homogeneously distributed. It is comprised of 65% continuum and 35% atomic hydrogen line emission. In the 12μm band a resolved but compact double source is surrounded by a fainter ring structure with all emission confined to the optical core region. Strong line emission of [ArIII] at 8.99 μm and in particular [SIV] at 10.51μm shapes this spatial profile. The unresolved 60μm emission originates from dust. It is described by a modified (emissivity index β = 1.5) blackbody with a temperature of 85 K, suggesting that warm dust with a mass of 6.4 × 10 -4 M Θ is mixed with the ionised gas. The gas-to-dust mass ratio is about 220. The 25 μm emission of NGC 7008 is characterised by a FWHM of about 50" with an additional spot-like or ring-like enhancement at the bright rim of the optical nebula. The 60μm emission exhibits a similar shape, but is about twice as extended. Analysis of the spectral energy distribution suggests that the 25 μm emission is associated with 120 K warm dust, while the 60μm emission is dominated by a second dust component with 55 K. The dust mass associated with this latter component amounts to 1.2 x 10 -3 M Θ , significantly higher than previously derived. The gas-to-dust mass ratio is 59 which, compared to the average value of 160 for the Milky Way, hints at dust enrichment by this object.

We present a new isolated interstellar nebula (RA = 08 h 27 m ,D ec =+ 25 ◦ 54 � (J2000); � = 197. 8, b =+ 31. ◦ 6) found at 100 µm and 60 µm on IRAS maps. It has dimensions of ∼140 � × 70 � and a cone-like shape, suggesting interaction with ambient matter or external radiation. The nebula contains 2 bright condensations (“nuclei”) and several prominent filaments, the latter being approximately parallel to each other. We carried out preliminary studies based on IRAS data and the Palomar Observatory Sky Survey and found that there are no hints of star formation in the nebula. The nebula’s long axis is almost parallel to the Galactic plane. A shaping due to the nebula’s motion through the interstellar medium might be not a sufficient reason in this case. We have discussed this possibility by applying results of Reynolds number hydrodynamics. During a search for possible stellar candidates for shaping this nebula we noted a remarkable position of one of the very few nearby (D ≈ 0.36 kpc) pulsars known at about this latitude: PSR B0823+26. This pulsar is projected at the tip of the nebula and its proper motion is approaching along the long axis (∼180 ◦ to the direction of the cone) of the nebula. The true cause for the shaping of the nebula, which was coined Skeleton Nebula by us, however remains unknown.

We present visible and near-infrared observations of the G84.0+0.8 HII region, a bright compact knot projected within the boundaries of the W80 complex dominated by the North America and Pelican nebulae. The spectrum of the nebula indicates a temperature of the ionizing stellar spectrum $T_* \simeq 40\,000{-}45\,000$ K (corresponding to a O7–O5 star) and a density of the HII region $n \simeq 460$ cm -3 , with a foreground extinction of $A_{\rm V} \simeq 5.9$ mag. A comparison of narrow-band near-infrared images through the Br γ and the H 2 $S(1)~v=1 \rightarrow 0$ filters shows that G84.0+0.8 consists of a fan-shaped cavity in a molecular cloud at least partly bounded by a photodissociation region, filled with Br γ -emitting ionized gas, and with a compact cluster at the tip of the fan. The brightest star at the position of the cluster is found to be a late G-type interloper. While membership of G84.0+0.8 in the local arm is well established from existing radial velocity measurements of the ionized gas, we find that the ionizing flux estimated from the size and density of the nebula on the one hand, and the radio continuum properties of the nebula on the other hand, are well below the expected ionizing flux of a mid, or even late, O-type star. We consider the possibility that G84.0+0.8 might be externally ionized by a nearby mid-O star. Currently available observations do not definitely confirm or reject the membership of G84.0+0.8 in the W80 complex, although a larger distance seems favored by the available data. Nevertheless, we can firmly rule out the possibility that it represents a massive star forming site in that complex, as its appearance as a compact HII region containing an embedded cluster may lead one to think.

We present the results of a search for the ionizing star of the North America (NGC 7000) and the Pelican (IC 5070) nebulae complex. The application of adequate selection criteria to the 2MASS JHKS broad-band photometry allows us to narrow the search down to 19 preliminary candidates in a circle of 0 ◦ 5 radius containing most of the L935 dark cloud that separates both nebulae. Follow-up near-infrared spectroscopy shows that most of these objects are carbon stars and mid-to-late-type giants, including some AGB stars. Two of the three remaining objects turn out to be later than spectral type B and thus cannot account for the ionization of the nebula, but a third object, 2MASS J205551.25+435224.6, has infrared properties consistent with it being a mid O-type star at the distance of the nebulae complex and reddened by AV � 9.6. We confirm its O5V spectral type by means of visible spectroscopy in the blue. This star has the spectral type required by the ionization conditions of the nebulae and photometric properties consistent with the most recent estimates of their distance. Moreover, it lies close to the geometric center of the complex that other studies have proposed as the most likely location for the ionizing star, and is also very close to the position inferred from the morphology of cloud rims detected in radio continuum. Given the fulfillment of all the conditions and the existence of only one star in the whole search area that satisfies them, we thus propose 2MASS J205551.25+435224.6 as the ionizing star of the North America/Pelican complex.

In the present paper we review some basic facts regarding the similarities and differences between Galactic planetary nebulae (PN) and Galactic Wolf-Rayet (WR) nebulae within the scope of stellar mass-loss history and its subsequent impact on the nebular dynamics and morphology. The case of planetary nebulae with [WR] nucleus, which allows one to perform a more direct comparison with WR nebulae, is emphasized. In particular, we review selected studies on the PN NGC 40 and on the WR nebula M 1-67. We describe the apparently ubiquitous turbulent-like phenomena originating in WR/[WR] stellar atmospheres and the surrounding nebulae, and discuss the possible impact of turbulence on ejected nebula studies. The properties of turbulence observed in ejected nebulae (apparently supersonic) are at odds with the ones related to Kolmogorov's subsonic, incompressible turbulence. The large compressibility of the nebular gases and the possible multiplicity of turbulence energy sources are likely the main explanations for that difference. The Universal Multifractal approach of Lovejoy & Schertzer is presented and constitutes a physically realistic and natural way of modeling and quantifying nebular turbulence data with only three independent parameters, including the key C1 intermittence parameter.

We present the results of imaging and scanning Fabry-Perot interferometry of the cometary nebula GM 3-12 (RNO 124) and Herbig-Haro object HH 376A. The nebula is cone-shaped with the star IRAS 20359+6745 at the apex, straight walls and two probable helical arms. HH 376A is the bow shock structure in the collimated high velocity flow, directed by the axis of the cone nebula, with shock velocity about 70-80 km s - 1 . The spatial and kinematical separation between bow shock and Mach disk in HH 376A was not detected, which makes this object similar to HH 111. The Ha profiles in the vicinity of the source are double peaked. The high velocity component can be attributed to the collimated flow.

Near-infrared images of the star forming region S87 in the JHK broad-band filters and H-2 v = 1-0 S(1) narrow-band filter are presented. In the eastern part of the region, the S87E nebula shows a quite complex structure. In the optically thin southern part of the nebula, there is an elongated cavity in the direction aligned with the blue-shifted molecular outflow. A subcluster of twelve young near-infrared sources has been detected in the optically thick northern part of the nebula, where a cone-shaped opaque patch is seen. In the 90" x 90" region around S87E, we detected I I I sources with m(K), > 15.2 mag. From analysis of the JHK' colors of the stars inside S87E, we find that after adjusting for background contamination, more than 40% of all the sources associated with the cluster display infrared excesses. This high fraction of infrared excess sources, along with the evidence of molecular outflow activity near the cluster, suggests that the S87E cluster is very young(similar to3-4 x 10(6) yr). We construct the differential K' luminosity function for S87E and find the overall shape and structure are somewhat different to that of other young clusters. Besides a common turnover at 14 mag, there is a prominent peak and still more cluster members with luminosities fainter than 14th magnitude at K' band. Five H-2 regions were also discovered and three of them are associated with the S87E nebula.

The planetary nebula IRAS 07027–7934 has a compact ionized core surrounded by a large cloud of matter believed to be of neutral gas and dust particles. A photoionization model of this ionized core is presented in this paper. The parameters T eff , $R_{\ast}$, and distance of the central star along with the radius of the nebula are derived by photoionization modeling. The nebula is found to have a much lower electron temperature than was estimated earlier but the electron density is very high, in broad agreement with previous result. The abundances derived for the first time point to a C/O ratio of > 5 for the nebula, and the internal extinction caused by the ionized core, $E(B-V)$, is 2.

We present high-resolution images obtained with the WFPC2, on board the HST, of the protoplanetary nebula (PPN) OH 231.8+4.2. H and NII line emission and scattered light in the continuum at 6750 and 7910 A were observed. We also discuss NIR NICMOS images from the HST archive. The images show with high accuracy the shape and excitation state of the shocks developed in the nebula. Our high-resolution images (and data from other works) allow a very detailed and quantitative description of the dierent nebular components and of the physical conditions in them. We interpret specic structures identied in our images using existing models of shock interaction. In the center of the nebula, there is a dense torus- or disk-like condensation continued by an hourglass-like structure, with relatively high densities (10 5 {10 6 cm 3 ) and temperatures (30 K). Inside this torus we have identied the location of the central star, from SiO maser observations. Two shock regions are detected from the optical line emission images, respectively in the north and south lobes. In both regions, a forward and a backward shock are identied. The densities of this hot gas vary between 40 and 250 cm 3 ,w ith the densest clumps being placed in the reverse shocks. The total mass of the shocked hot gas is210 3 M, both lobes showing similar masses in spite of their dierent extents. The relatively collimated jet that impinges on an originally slow shell, so producing the shocks, is identied from the scattered light images and in CO maps. This flow is signicantly denser and cooler than the shocked H regions. Its density decreases with the distance to the star, with typical values10 5 {10 4 cm 3 , and its temperature ranges between about 25 and 8 K. We explain the high H emission of the backward shock assuming that it propagates in a diuse gas component, entrained by the observed collimated flow and sharing its axial movement. The existence of shocks also in the collimated densest flow is suggested by the high abundance of some molecules like HCO + and its structure and kinematics in certain regions, but they are not seen in H emission, probably because of the absence of (well developed) hot components in this dense flow. We think that the exceptionally detailed and quantitative image derived for the wind interaction regions in OH 231.8+4.2 is a challenge to check and improve hydrodynamical models of wind interaction in PPNe.

We report on the discovery of the proper motion of the central star (CS) of the Planetary Nebula (PN) Sh 2–68 (PN G030.6+06.2). Sh 2–68 is a PN interacting with the interstellar medium (ISM), which impedes and finally stops the expansion of the nebula itself while the galactic motion of the CS leads to it moving out of the geometric center of the PN. Our observation is a direct confirmation of this process and combined with imaging data suggests that the ISM's magnetic field plays an important role in shaping and destroying evolved PNe. The proper motion of 53.2 ± 5.5 mas yr -1 is the largest value for a PN found from ground based observations.

International audience; Context. Medium-resolution échelle spectra of the Red Square Nebula surrounding the star MWC 922 are presented. The spectra have been obtained in 2010 and 2012 using the X-shooter spectrograph mounted on the Very Large Telescope (VLT) in Paranal, Chile. The spectrum covers a wavelength range between 300 nm-2.5 μm and shows that the nebula is rich in emission lines. Aims: We aim to identify the emission lines and use them as a tool to determine the physical and chemical characteristics of the nebula. The emission lines are also used to put constraints on the structure of the nebula and on the nature of the central stars. Methods: We analyzed and identified emission lines that indicated that the Red Square Nebula consists of a low density bipolar outflow, eminent in the broad emission component seen in [Fe II], as well as in P Cygni line profiles indicative of fast outflowing material. The narrow component in the [Fe II] lines is most likely formed in the photosphere of a surrounding disk. Some of the emission lines show a pronounced double peaked profile, such as Ca II, indicating an accretion disk in Keplerian rotation around the central star. [O I] emission lines are formed in the neutral atomic zone separating the ionized disk photosphere from the molecular gas in the interior of the disk, which is prominent in molecular CO emission in the near-IR. [N II] and [S II] emission clearly originates in a low density but fairly hot (7 000-10 000 K) nebular environment. H I recombination lines trace the extended nebula as well as the photosphere of the disk. Results: These findings put constraints on the evolution of the central objects in MWC 922. The Red Square shows strong similarities to the Red Rectangle Nebula, both in morphology and in its mid-IR spectroscopic characteristics. As for the Red Rectangle, the observed morphology of the nebula reflects mass-loss in a binary system. Specifically, we attribute the biconical morphology and the associated rung-like structure to the action of intermittent jets blown by the accreting companion in a dense shell, which has been created by the primary. We stress, though, that despite the morphological similarities, these two objects represent very different classes of stellar objects. The data-reduced spectra are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/601/A69

We present spectroscopy and Hα imaging of a faint nebula surrounding the X-ray bright, nearby neutron star RX J1856.5-3754 . The nebula shows no strong lines other than the hydrogen Balmer lines and has a cometary-like morphology, with the apex being approximately 1" ahead of the neutron star, and the tail extending up to at least 25" behind it. We find that the current observations can be satisfactorily accounted for by two different models. In the first, the nebula is similar to "Balmer-dominated" cometary nebulae seen around several radio pulsars, and is due to a bow shock in the ambient gas arising from the supersonic motion of a neutron star with a relativistic wind. In this case, the emission arises from shocked ambient gas; we find that the observations require an ambient neutral hydrogen number density n_(H^0) ≃ 0.8 cm^(-3) and a rotational energy loss Ė ≃ 6 × 10^(31) erg s^(-1). In the second model, the nebula is an ionisation nebula, but of a type not observed before (though expected to exist), in which the ionisation and heating are very rapid compared to recombination and cooling. Because of the hard ionising photons, the plasma is heated up to ~70 000 K and the emission is dominated by collisional excitation. The cometary morphology arises naturally as a consequence of the lack of emission from the plasma near and behind the neutron star (which is ionised completely) and of thermal expansion. We confirm this using a detailed hydrodynamical simulation. We find that to reproduce the observations for this case, the neutral hydrogen number density should be n_(H^0) ≃ 3 cm^(-3) and the extreme ultraviolet flux of the neutron star should be slightly in excess, by a factor ~1.7, over what is expected from a black-body fit to the optical and X-ray fluxes of the source. For this case, the rotational energy loss is less than 2 × 10^(32) erg s^(-1). Independent of the model, we find that RX J1856.5-3754 is not kept hot by accretion. If it is young and cooling, the lack of pulsations at X-ray wavelengths is puzzling. Using phenomenological arguments, we suggest that RX J1856.5-3754 may have a relatively weak, few 10^(11) G, magnetic field. If so, it would be ironic that the two brightest nearby neutron stars, RX J1856.5-3754 and RX J0720.4-3125 , may well represent the extreme ends of the neutron star magnetic field distribution, one a weak field neutron star and another a magnetar.

In the vicinity of the southern hemisphere giant H II region NGC 3603 we discovered a new planetary nebula: PN G291.4-00.3 located at RA J2000.0 = 11h14m32$.^{\rm s}$1 ± 0$.^{\rm s}$3, DEC J2000.0 = -61°00′02″ ± 1″. Monochromatic images reveal a central ring-like structure accompanied by onsets of arc-like filaments which might outline a bipolar outflow. Analysis of the nebula spectrum shows that helium and nitrogen are enriched, while oxygen, sulphur and argon abundances seem to be solar. The central star must be hot (above 100 kK) and having a luminosity of more than 3000 $L_{\odot}$. All evidence shows that PN G291.4-00.3 is a high-excitation, density bounded planetary nebula of the type I, coming from the high-mass end of the intermediate mass stars.

In a search for emission line objects made in a southern region of M 33 using narrow band filters centered on [O III], H α and adjacent continua, we detect some known and new objects that can be classified as planetary nebulae candidates (PNc). This search, the deepest one made up to now in M 33, has allowed us to detect 48 PNc, from which 14 have a ratio of [O III]-to-H α fluxes greater than 1. We compare our findings with a previous search and present the list and finding charts of these interesting objects.

The rst optical spectra of Th4{4, obtained by the author in 1970, showed strong continuum and broad HI emission lines. In other words, at this time the object looked like a Be star. Some years later, (OIII) emission lines appeared in the spectrum indicating the existence of a gaseous envelope. Today the optical spectrum of Th4{4 is quite similar to that of a moderate excitation planetary nebula. The magnitude of the object has dropped from V =1 4: 4t o 16: m 6, and the eective temperature of the central star has increased from 22 000 K up to 40 000 K

New, high-sensitivity interferometric CO observations of BV 5-1 show that the molecular gas in this bipolar planetary nebula is distributed along a very inhomogeneous ring made of clumps. The individual masses of such molecular clumps are10 4 M. The BV 5-1 molecular ring is seen edge-on, it is nearly perpendicular to the axis of bipolarity of the nebula, and it is expanding with an expansion velocity of 9k m s 1 .B V 5-1 is conrmed to be a relatively evolved nebula, as indicated by both its kinematic age (2:4 10 4 yrs) and its molecular to ionized mass ratio (0.3). The structure of the BV 5-1 molecular ring, as well as that of other rings previously observed in a few additional nebulae, underscore the importance of strongly asymmetrical mass loss processes starting early at the asymptotic giant branch phase.

The planetary nebula NGC 6781 was imaged in major optical emission lines. These lines allow us to construct maps of the projected, two dimensional Balmer decrement, electron density, electron temperature, ionization and abundance structure. The average electron density, determined from the (S ii) lines, is500 cm 3 , while the electron temperature distribution, determined from the (N ii) lines, is flat at 10 000 K. The Balmer decrement map shows that there are variations in extinction between the north and south areas of the planetary nebula. The higher extinction observed to the north of the central star is probably caused by dust spatially associated with CO emission at blue{shifted velocities. The (N ii) image reveals the known optical halo, at a flux level of0.2% of the strong shell emission in the east, but now the angular extent of 216 00 190 00 is much larger than previous measurements. The halo is also present in (O iii), where we measure an extent of 190 00 162 00 .T he ionization maps indicate substantial ionization along the caps of the ellipsoid as well as in the halo. The maps also show a sharp decrease in ionization along the outer edge of the shell in the west and the east, south{east. The typical log abundances measured for He, N, O and S are 10.97, 8.14, 8.72 and 6.90, respectively.

Vilnius seven-color photometry has been obtained for 238 stars down to ~13 mag in the area of the California Nebula in Perseus. For nearly all of the stars, photometric spectral classes, luminosity classes, absolute magnitudes, interstellar reddenings, extinctions and distances are determined. The "extinction versus distance" diagrams give evidence for the presence of one dust layer at ~160 pc distance in the direction of the California Nebula and its nearest surroundings and of two dust layers at distances of 160 pc and 300 pc north-west of the nebula, in the direction of the dark clouds L 1449 and L 1456. The front layer contributes extinction between 0.3 and 1.3 mag, and the second layer gives about 1 mag of additional extinction. It appears likely that the front dust layer is the extension of the Taurus dark clouds. The second dust layer probably belongs to the complex of dark clouds found in other areas of the southern part of Perseus (in the directions of the open cluster IC 348, the reflection nebula NGC 1333, etc.). Both cloud complexes run more or less parallel to the Galactic plane.

Deep narrow band interference filters observations are presented for 7 type I galactic planetary nebulae (PNe). Except for the nearly circular planetary nebula A 39, the observations show overall dimensions appreciably larger than those quoted so far. The observed PN (NGC 6302 excepted) could have linear diameters larger than 1 pc. Our new observations imply ionized masses often larger than , suggesting that much of the mass ejected by the PN precursor is contained in faint outer extensions. This might contribute to the solution of the "mass deficit" in PNe. We have also provided calibrated fluxes for some specific emission lines. In spite of their apparent morphological differences, our observations suggest that these type I PNe possess the same intrinsic structure as that displayed by NGC 650-1, the Ring Nebula, and even Sh 1-89 with its elongated jet-like feature, but viewed under different angles.

The major purpose of this paper was to prove or disprove the nature of planetary nebula (PN) candidates in the northern hemisphere, taken from an internal list. We present spectroscopic observations and imaging of fteen PNe, twelve of them identied for the rst time. Another two candidates turned out to be an emission{line galaxy and an H ii region. All observed PNe represent evolved stages, their angular diameter ranging from 8 00 to 90 00 , and exhibit very low surface brightnesses. The three largest and faintest nebulae each host a central star of magnitude 18 to 20 m .

Context. Most spectroscopic studies of planetary nebulae (PNe) are based on data from specific parts of the objects. Considering the high complexity of their morphological structures, integrated or average values across an observed region can only provide limited information of their ionic, density, and temperature structures. Aims. We investigate the spatial variation of the physical and chemical properties of NGC 6302, a high-ionization PNe, with a complex bipolar structure. Methods. Spatially resolved long-slit spectroscopic data were obtained from 11 parallel and equally spaced directions over the nebula. Maps of emission line fluxes, line ratios, electron temperature T e , electron density N e , and ionic abundances (He+ , He2+ , N0 , N+ , O0 , O+ , O2+ , S+ , S2+ , Ne2+ , Ar2+ , Ar3+ , and Ar4+ relative to H+ ) were constructed with a spatial resolution of 1.45″ × 1″ for a large portion of the nebula over an area of 11″ × 250″. Electron densities were estimated from the [S ii] λ 6716/λ 6731 and [Ar iv] λ 4711/λ 4740 line ratios, and electron temperatures were estimated from the [O iii] (λ 4959 + λ 5007)/λ 4363 and [N ii] (λ 6548 + λ 6584)/λ 5755 ratios. Integrated spectra were also obtained and 142 lines were identified. Results. The electron density maps show a peaked distribution, with the densest area at the circumstellar region, reaching N e ≈ 40 000 cm-3 , and decreasing to N e ≤ 1000 cm-3 at the lobes. Knots were observed in the N e (S ii) map, with 2000 cm-3 e . The N e (Ar iv) map suggests the presence of an inner region with N e ≈ 20 000 cm-3 that extends from the center through the east lobe. A homogeneous temperature distribution was found for both T e (O iii) and T e (N ii). Small temperature fluctuations on the plane of the sky of and were estimated. Conclusions. In general, the total abundances obtained are similar to the maximum found in the ionic abundance maps of the dominant ionic species. This demonstrates the potential of spatially resolved abundance studies for establishing lower limits of the total abundances based on ionic abundances, for checking the ionization correlation factors, and in some cases, for determining the total abundances directly from maps of ionic abundances.

We present the high-dispersion spectra of the elliptical ring shaped planetary nebula NGC 6803, secured with the Hamilton Echelle Spectrograph attached to the 3-m Shane telescope of Lick Observatory. Numerous lines from neutral to quadruply ionized ions are presented in the wavelength region from 3650 to 9900 A. We also use the low dispersion UV spectral data obtained with the 60-cm interstellar ultraviolet explorer. In spite of its simplistic symmetrical bilateral shape, the diagnostics imply that the physical condition of the nebular shell is very complex with a huge density range of 1300−80 000 cm −3 . A comparison of the 1995 and 2001 [Ar iv ]d ata suggests that the density increase occurred near the inner shell boundary. In spite of a huge ionization potential range, the average electron temperature indicated by primary diagnostic lines is relatively low, i.e., Te ≤ 9500 K, except for [Cl iv], from which we derive a temperature that is around 11 500 K. We derived the chemical abundances of He, C, N, O, Ne, S, Ar, Cl, and K, based on the physical condition suggested by diagnostics and photo-ionization analysis. The chemical abundances of NGC 6803 are mostly enhanced when compared with the average Galactic planetary nebula. The effective temperature of its central star appears to be about 90 000 K and its luminosity about 2400 L� , assuming a distance of 3000 pc. The evolutionary track implies that NGC 6803 might ...

Analysis of data from the Chandra X-ray Observatory for the double nucleus ULIRG Mrk 273 reveals an absorbed hard X-ray source coincident with the southwest nucleus, implying that this unresolved, near-infrared source is where an active nucleus resides, while the northern nuclear region contains a powerful starburst that dominates the far infrared luminosity. There is evidence of a slight image extension in the 6–7 keV band, where an Fe K line is present, towards the northern nucleus. A large-scale, diffuse emission nebula detected in soft X-rays contains a dark lane that spatially coincides with a high surface-brightness tidal tail extending ~50 arcsec (40 kpc) to the south. The soft X-ray source is likely located behind the tidal tail, which absorbs X-ray photons along the line of sight. The estimated column density of cold gas in the tidal tail responsible for shadowing the soft X-rays is N_H ≥ 6 × 10^(21) cm^(-2), consistent with the tidal tail having an edge-on orientation.

The reddening in and around emission nebulae is characterized by the extinction AV and the ratio R of the absolute-to-selective absorption. Both are usually derived from the photometry of a single star or cluster stars that are associated with an emission nebula. Using the parameterized reddening relation published by Cardelli et al. (1989, ApJ, 345, 245), we show that AV and R can be derived with good precision from the observation of a set of common upper level Paschen-Balmer hydrogen line ratios. The use of common upper level line ratios has the advantage of being nearly independent of the excitation condition of the nebula (ne, Te). The line ratio method can be applied in regions where no stars are available for photometry.

[abridged] From a sample of Ly-alpha emitters in the GOODS-S field with uncontaminated photometry and optical (red) spectroscopy, we select a spatially compact object at a redshift of 5.563 (Ly-alpha) that shows a second emission line, identified as N IV] 1486 A. The SED is modelled in a way that accounts for both the N IV] line emission and the photometry in a self-consistent way. The photoionization code CLOUDY is used to calculate a range of nebular models as a function of stellar ionizing source temperature, ionization parameter, density and nebular metallicity. We compare the theoretical and observed magnitudes and search for the model parameters that also reproduce the observed N IV] luminosity and equivalent width. A nebular model with a hot blackbody ionizing source of around 100 kK and a nebular metallicity of ~5% of solar is able to fit the observed SED and, in particular, explain the large apparent Balmer break which is inferred from the pure stellar population model fitting conventionally applied to multi-band photometric observations. In our model, an apparent spectral break is produced by strong [O III] 4959, 5007 A emission falling in one of the IR bands (IRAC1 in this case). A lower limit on the total baryonic mass of a model of this type is 3.2 x 10^8 Msun . It is argued that objects with Ly-alpha emission at high redshift that show an apparent Balmer break may have their SED dominated by nebular emission and so could possibly be identified with very young starbursting galaxies rather than massive evolved stellar populations. Detailed studies of these emission nebulae with large telescopes will provide a unique insight into very early chemical evolution., Comment: 9 pages, 6 figures, A & A accepted