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

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

Heavy elements in Galactic and Magellanic Cloud HII regions: recombination-line versus forbidden-line abundances

We have obtained deep optical, long-slit spectrophotometry of the Galactic HII regions M 17, NGC 3576 and of the Magellanic Cloud HII regions 30 Doradus, LMC N11B and SMC N66, recording the optical recombination lines (ORLs) of CII, NII and OII. Temperature-insensitive ORL C2+/O2+ and N2+/O2 ratios are obtained for all nebulae except SMC N66. The ORL C2+/O2+ ratios show remarkable agreement within each galactic system, while also being in agreement with the corresponding CEL ratios. For all five nebulae, the O2+/H+ abundance derived from multiple OII ORLs is found to be higher than the corresponding value derived from the strong [OIII] 4959, 5007A CELs, by factors of 1.8--2.7 for four of the nebulae. The LMC N11B nebula exhibits a more extreme discrepancy factor for the O2+ ion, ~5. Thus these HII regions exhibit ORL/CEL abundance discrepancy factors that are similar to those previously encountered amongst planetary nebulae. Our optical CEL O2+/H+ abundances agree to within 20-30 per cent with published O2+/H+ abundances that were obtained from observations of infrared fine-structure lines. Since the low excitation energies of the latter make them insensitive to variations about typical nebular temperatures, fluctuations in temperature are ruled out as the cause of the observed ORL/CEL O2+ abundance discrepancies. We present evidence that the observed OII ORLs from these HII regions originate from gas of very similar density (<3500 cm-3) to that emitting the observed heavy-element optical and infrared CELs, ruling out models that employ high-density ionized inclusions in order to explain the abundance discrepancy. We consider a scenario whereby much of the heavy-element ORL emission originates from cold (<=500 K) metal-rich ionized regions.Comment: 24 pages; 9 figures; accepted by 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

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

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

Chemical abundances in the protoplanetary disk LV2 (Orion): clues to the causes of the abundance anomaly in HII regions

Optical integral field spectroscopy of the archetype protoplanetary disk LV2 in the Orion Nebula is presented, taken with the VLT FLAMES/Argus fibre array. The detection of recombination lines of CII and OII from this class of objects is reported, and the lines are utilized as abundance diagnostics. The study is complemented with the analysis of HST Faint Object Spectrograph ultraviolet and optical spectra of the target contained within the Argus field of view. By subtracting the local nebula background the intrinsic spectrum of the proplyd is obtained and its elemental composition is derived for the first time. The proplyd is found to be overabundant in carbon, oxygen and neon compared to the Orion Nebula and the sun. The simultaneous coverage over LV2 of the CIII] 1908-A and [OIII] 5007-A collisionally excited lines (CELs) and CII and OII recombination lines (RLs) has enabled us to measure the abundances of C++ and O++ for LV2 with both sets of lines. The two methods yield consistent results for the intrinsic proplyd spectrum, but not for the proplyd spectrum contaminated by the generic nebula spectrum, thus providing one example where the long-standing abundance anomaly plaguing metallicity studies of HII regions has been resolved. These results would indicate that the standard forbidden-line methods used in the derivation of light metal abundances in HII regions in our own and other galaxies underestimate the true gas metallicity.Comment: Accepted by MNRAS November 8; 16 pages, 9 figs; typos corrected, error in FWHMs in table 4 corrected in this versio

Integrated spectrum of the planetary nebula NGC 7027

We present deep optical spectra of the archetypal young planetary nebula (PN) NGC 7027, covering a wavelength range from 3310 to 9160 A. The observations were carried out by uniformly scanning a long slit across the entire nebular surface, thus yielding average optical spectra for the whole nebula. A total of 937 emission features are detected. The extensive line list presented here should prove valuable for future spectroscopic analyses of emission line nebulae. The optical data, together with the archival IUE and ISO spectra, are used to probe the temperature and density structures and to determine the elemental abundances from lines produced by different excitation mechanisms. The C++/H+, N++/H+, O++/H+ and Ne++/H+ ionic abundance ratios derived from optical recombination lines (ORLs) are found to be only slightly higher than those derived from collisionally excited lines (CELs). We conclude that whatever mechanism is causing the BJ/CEL temperature discrepanies and the ORL/CEL abundance discrepancies that have been observed in many PNe, it has an insignificant effect on this bright young compact PN. The properties of the central star are also discussed. Based on the integrated spectrum and using the energy-balance method, we have derived an effective temperature of 219 000 K for the ionizing star. Finally, we report the first detection in the spectrum of this bright young PN of Raman-scattered O VI features at 6830 and 7088 A, pointing to the existence of abundant neutral hydrogen around the ionized regions. (abridged)Comment: 55 pages, 7 figures, accepted for publication in A&