The Internal Extinction Curve of NGC 6302 and its Extraordinary Spectrum

In this paper we present a new method for obtaining the optical wavelength-dependent reddening function of planetary nebulae, using the nebular and stellar continuum. The data used was a spectrum of NGC 6302 obtained with a mean signal to noise of >10^2 A^-1 in the nebular continuum. With such a high S/N the continuum can be accurately compared with a theoretical model nebular plus stellar continuum. The nebular electron temperature and density used in the model are determined using ratios of prominent emission lines. The reddening function can then be obtained from the ratio of the theoretical and the observed continuum. We find that for NGC 6302, the visible to IR extinction law is indistinguishable from `standard' interstellar reddening, but that the UV extinction curve is much steeper than normal, suggesting that more small dust grains had been ejected into the nebula by the PN central star. Finally, using the extinction law that we have determined, we present a complete de--reddened line list of nearly 600 emission lines, and report on the detection of the He(2-10) and He(2-8) Raman Features at 4331 A and 4852 A, and the detection of Raman-Scattered OVI features at 6830 and 7087 AA.Comment: 32 pages, 7 figures, to appear in PASA 2002, 1

NGC 2579 and the carbon and oxygen abundance gradients beyond the solar circle

We present deep echelle spectrophotometry of the Galactic HII region NGC 2579. The data have been taken with the Very Large Telescope Ultraviolet-Visual Echelle Spectrograph in the 3550--10400 \AA\ range. This object, which has been largely neglected, shows however a rather high surface brightness, a high ionization degree and is located at a galactocentric distance of 12.4 $\pm$ 0.7 kpc. Therefore, NGC 2579 is an excellent probe for studying the behaviour of the gas phase radial abundance gradients in the outer disc of the Milky Way. We derive the physical conditions of the nebula using several emission line-intensity ratios as well as the abundances of several ionic species from the intensity of collisionally excited lines. We also determine the ionic abundances of C$^{2+}$, O$^+$ and O$^{2+}$ -- and therefore the total O abundance -- from faint pure recombination lines. The results for NGC 2579 permit to extend our previous determinations of the C, O and C/O gas phase radial gradients of the inner Galactic disc (Esteban etal. 2005) to larger galactocentric distances. We find that the chemical composition of NGC 2579 is consistent with flatten gradients at its galactocentric distance. In addition, we have built a tailored chemical evolution model that reproduces the observed radial abundance gradients of O, C and N and other observational constraints. We find that a levelling out of the star formation efficiency about and beyond the isophotal radius can explain the flattening of chemical gradients observed in the outer Galactic disc.Comment: 10 pages, 5 figures. Accepted for publication in Monthly Notices of the Royal Astronomical Societ

Unveiling the structure of the planetary nebula M 2-48: Kinematics and physical conditions

The kinematics and physical conditions of the bipolar planetary nebula M 2-48 are analysed from high and low dispersion long-slit spectra. Previous CCD narrow-band optical observations have suggested that this nebula is mainly formed by a pair of symmetric bow-shocks, an off-center semi-circular shell, and an internal bipolar structure. The bipolar outflow has a complex structure, characterised by a series of shocked regions located between the bright core and the polar tips. There is an apparent kinematic discontinuity between the bright bipolar core and the outer regions. The fragmented ring around the bright bipolar region presents a low expansion velocity and could be associated to ejection in the AGB-PN transition phase, although its nature remains unclear. The chemical abundances of the central region are derived, showing that M 2-48 is a Type I planetary nebula (PN)

Early evolution of the extraordinary Nova Del 2013 (V339 Del)

We determine the temporal evolution of the luminosity L(WD), radius R(WD) and effective temperature Teff of the white dwarf (WD) pseudophotosphere of V339 Del from its discovery to around day 40. Another main objective was studying the ionization structure of the ejecta. These aims were achieved by modelling the optical/near-IR spectral energy distribution (SED) using low-resolution spectroscopy (3500 - 9200 A), UBVRcIc and JHKLM photometry. During the fireball stage (Aug. 14.8 - 19.9, 2013), Teff was in the range of 6000 - 12000 K, R(WD) was expanding non-uniformly in time from around 66 to around 300 (d/3 kpc) R(Sun), and L(WD) was super-Eddington, but not constant. After the fireball stage, a large emission measure of 1.0-2.0E+62 (d/3 kpc)**2 cm**(-3) constrained the lower limit of L(WD) to be well above the super-Eddington value. The evolution of the H-alpha line and mainly the transient emergence of the Raman-scattered O VI 1032 A line suggested a biconical ionization structure of the ejecta with a disk-like H I region persisting around the WD until its total ionization, around day 40. It is evident that the nova was not evolving according to the current theoretical prediction. The unusual non-spherically symmetric ejecta of nova V339 Del and its extreme physical conditions and evolution during and after the fireball stage represent interesting new challenges for the theoretical modelling of the nova phenomenon.Comment: 14 pages, 9 figures, 3 tables, accepted for Astronomy and Astrophysic

The Planetary Nebula population of the Sagittarius Dwarf Spheroidal Galaxy

The identification of two new Planetary Nebulae in the Sagittarius Dwarf Spheroidal Galaxy (Sgr) is presented. This brings the total number to four. The first, StWr 2-21, belongs to the main body of Sgr. The second, the halo PN BoBn 1, has a location, distance and velocity in agreement with the leading tidal tail of Sgr. We estimate that 10 per cent of the Galactic halo consists of Sgr debris. The specific frequency of PNe indicates a total luminosity of Sgr, including its tidal tails, of M_V=-14.1. StWr 2-21 shows a high abundance of [O/H]=-0.23, which confirms the high-metallicity population in Sgr uncovered by Bonaficio et al. (2004). The steep metallicity--age gradient in Sgr is due to ISM removal during the Galactic plane passages, ISM reformation due to stellar mass loss, and possibly accretion of metal-enriched gas from our Galaxy. The ISM re-formation rate of Sgr, from stellar mass loss, is 5 X 10^-4 M_sun yr^-1, amounting to ~10^6 M_sun per orbital period. HST images reveal well-developed bipolar morphologies, and provide clear detections of the central stars. All three stars with deep spectra show WR-lines, suggesting that the progenitor mass and metallicity determines whether a PN central star develops a WR spectrum. One Sgr PN belongs to the class of IR-[WC] stars. Expansion velocities are determined for three nebulae. Comparison with hydrodynamical models indicates an initial density profile of rho ~ r^-3. This is evidence for increasing mass-loss rates on the AGB. Peak mass-loss rates are indicated of ~ 10^-4 M_sun yr^-1. The IR-[WC] PN, He 2-436, provides the sole direct detection of dust in a dwarf spheroidal galaxy, to date.Comment: 16 pages. MNRAS, accepted for publicatio

Detection of [O I] 63 <i>μ</i>m in absorption toward Sgr B2

A high signal-to-noise 52-90 μm spectrum is presented for the central part of the Sagittarius B2 complex. The data were obtained with the Long Wavelength Spectrometer on board the Infrared Space Observatory (ISO). The [O I] 63 μm line is detected in absorption even at the grating spectral resolution of 0.29 μm. A lower limit for the column density of atomic oxygen of the order of 1019 cm-2 is derived. This implies that more than 40% of the interstellar oxygen must be in atomic form along the line of sight toward the Sgr B2 molecular cloud

The Iron abundance in Galactic Planetary Nebulae

We constrain the iron abundance in a sample of 33 low-ionization Galactic planetary nebulae (PNe) using [Fe III] lines and correcting for the contribution of higher ionization states with ionization correction factors (ICFs) that take into account uncertainties in the atomic data. We find very low iron abundances in all the objects, suggesting that more than 90% of their iron atoms are condensed onto dust grains. This number is based on the solar iron abundance and implies a lower limit on the dust-to-gas mass ratio, due solely to iron, of M_dust/M_gas>1.3x10^{-3} for our sample. The depletion factors of different PNe cover about two orders of magnitude, probably reflecting differences in the formation, growth, or destruction of their dust grains. However, we do not find any systematic difference between the gaseous iron abundances calculated for C-rich and O-rich PNe, suggesting similar iron depletion efficiencies in both environments. The iron abundances of our sample PNe are similar to those derived following the same procedure for a group of 10 Galactic H II regions. These high depletion factors argue for high depletion efficiencies of refractory elements onto dust grains both in molecular clouds and AGB stars, and low dust destruction efficiencies both in interstellar and circumstellar ionized gas.Comment: Accepted for publication in ApJ. 15 pages, 4 Postscript figures, corrected typos, Tables 2 and 3 correcte

Atomic Processes in Planetary Nebulae and H II Regions

Spectroscopic studies of Planetary Nebulae (PNe) and H {\sc ii} regions have driven much development in atomic physics. In the last few years the combination of a generation of powerful observatories, the development of ever more sophisticated spectral modeling codes, and large efforts on mass production of high quality atomic data have led to important progress in our understanding of the atomic spectra of such astronomical objects. In this paper I review such progress, including evaluations of atomic data by comparisons with nebular spectra, detection of spectral lines from most iron-peak elements and n-capture elements, observations of hyperfine emission lines and analysis of isotopic abundances, fluorescent processes, and new techniques for diagnosing physical conditions based on recombination spectra. The review is directed toward atomic physicists and spectroscopists trying to establish the current status of the atomic data and models and to know the main standing issues.Comment: 9 pages, 1 figur

A Survey of Molecular Hydrogen in the Crab Nebula

We have carried out a near-infrared, narrow-band imaging survey of the Crab Nebula, in the H2 2.12 micron and Br-gamma 2.17 micron lines, using the Spartan Infrared camera on the SOAR Telescope. Over a 2.8' x 5.1' area that encompasses about 2/3 of the full visible extent of the Crab, we detect 55 knots that emit strongly in the H2 line. We catalog the observed properties of these knots. We show that they are in or next to the filaments that are seen in optical-passband emission lines. Comparison to HST [S II] and [O III] images shows that the H2 knots are strongly associated with compact regions of low-ionization gas. We also find evidence of many additional, fainter H2 features, both discrete knots and long streamers following gas that emits strongly in [S II]. A pixel-by-pixel analysis shows that about 6 percent of the Crab's projected surface area has significant H2 emission that correlates with [S II] emission. We measured radial velocities of the [S II] lambda6716 emission lines from 47 of the cataloged knots and find that most are on the far (receding) side of the nebula. We also detect Br-gamma emission. It is right at the limit of our survey, and our Br-gamma filter cuts off part of the expected velocity range. But clearly the Br-gamma emission has a quite different morphology than the H2 knots, following the long linear filaments that are seen in H-alpha and in [O III] optical emission lines.Comment: Accepted for publication in the ApJ