Miriam Peña

Ammonia downstream from HH 80 North

HH 80-81 are two optically visible Herbig-Haro (HH) objects located about 5 minutes south of their exciting source IRAS 18162-2048. Displaced symmetrically to the north of this luminous IRAS source, a possible HH counterpart was recently detected as a radio continuum source with the very large array (VLA). This radio source, HH 80 North, has been proposed to be a member of the Herbig-Haro class since its centimeter flux density, angular size, spectral index, and morphology are all similar to those of HH 80. However, no object has been detected at optical wavelengths at the position of HH 80 North, possibly because of high extinction, and the confirmation of the radio continuum source as an HH object has not been possible. In the prototypical Herbig-Haro objects HH 1 and 2, ammonia emission has been detected downstream of the flow in both objects. This detection has been intepreted as a result of an enhancement in the ammonia emission produced by the radiation field of the shock associated with the HH object. In this Letter we report the detection of the (1,1) and (2,2) inversion transitions of ammonia downstream HH 80 North. This detection gives strong suppport to the interpretation of HH 80 North as a heavily obscured HH object. In addition, we suggest that ammonia emission may be a tracer of embedded Herbig-Haro objects in other regions of star formation. A 60 micrometer IRAS source could be associated with HH 80 North and with the ammonia condensation. A tentative explanation for the far-infrared emission as arising in dust heated by their optical and UV radiation of the HH object is presented.

Faint recombination lines in Galactic PNe with a [WC] nucleus

Aims. We present spatially resolved high-resolution spectrophotometric data for the planetary nebulae PB 8, NGC 2867, and PB 6. We have analyzed two knots in NGC 2867 and PB 6 and one in PB 8. The three nebulae are ionized by [WC] type nuclei: early [WO] for PB 6 and NGC 2867 and [WC 5-6] in the case of PB 8. Our aim is to study the behavior of the abundance discrepancy problem (ADF) in this type of planetary nebula. Methods. We measured a large number of optical recombination (ORL) and collisionally excited lines (CEL), from different ionization stages (many more than in any previous work), thus, we were able to derive physical conditions from many different diagnostic procedures. We determined ionic abundances from the available collisionally excited and recombination lines. Based on both sets of ionic abundances, we derived total chemical abundances in the nebulae using suitable ionization correction factors. Results. From CELs, we have found abundances typical of Galactic disk planetary nebulae. Moderate ADF(O ++ ) were found for PB 8 (2.57) and NGC 2867 (1.63). For NGC 2867, abundances from ORLs are higher but still consistent with Galactic disk planetary nebulae. On the contrary, PB 8 presents a very high O/H ratio from ORLs. A high C/O was obtained from ORLs for NGC 2867; this ratio is similar to C/O obtained from CELs and with the chemical composition of the wind of the central star, indicating that there was no further C-enrichment in the star, relative to O, after the nebular material ejection. On the contrary, we found C/O < 1 in PB 8. Interestingly, we obtain (C/O) ORLs /(C/O) CELs < 1 in PB 8 and NGC 2867; this added to the similarity between the heliocentric velocities measured in [O III] and O II lines for our three objects argue against the presence of H-deficient metal-rich knots coming from a late thermal pulse event.

The chemical composition of planetary nebulae and HII regions in NGC 3109

Aims. We present deep spectrophotometry for a sample of 8 planetary nebulae (PNe) and 12 HII regions distributed throughout the dwarf irregular galaxy NGC 3109, in order to analyze the chemical composition of both types of nebulae. Methods. We describe the observations and data reduction, and present line intensities for the nebular emission lines detected. The physical conditions and the abundances of He, O, Ne, N, S and Ar are derived, using the classical Te-based method. We confirm our previous identification of PNe and HII regions based on photometry, except for one object, which we argue is a compact HII region rather than a planetary nebula. Results. We find that the chemical composition of the interstellar medium in NGC 3109, as sampled by its HII regions, is remarkably uniform. The oxygen abundance is log O/H + 12 = 7.77 ± 0.07 in this galaxy, as compared to 8.05 ± 0.09 for the Small Magellanic Cloud (for which we rederived the metallicity in a homogeneous way). PNe show significantly higher oxygen abundances in NGC 3109: log O/H + 12 = 8.16 ± 0.19. Similarly to what has been suggested for some of the PNe in the Magellanic Clouds and other metal-poor galaxies, we argue that oxygen in the PNe in NGC 3109 is affected by dredge up in their progenitors. This could also be the case for neon, although the uncertainties for this element are bigger. Conclusions. From our analysis, we conclude that oxygen and neon are not always a safe indicator of the chemical composition of the interstellar medium at low metallicities. An alternative to the O and Ne enrichment in PNe is that the low metallicity in HII regions has been caused by dilution of the interstellar medium due to an interaction with a neighboring galaxy about a Gyr ago. The excitation patterns of the PNe in NGC 3109 are very different from the excitation patterns of PNe in other galaxies. This issue needs to be investigated further, as it implies that the evolution of PNe depends upon the properties of their progenitor stellar populations, which vary from galaxy to galaxy. This should affect the planetary nebula luminosity function and its use as a distance indicator. Regarding individual objects, we find that the planetary nebula named PN 14 shows clear Wolf-Rayet features, very low excitation and high density. Thus, it is similar to some of the galactic PNe ionized by late [WC] stars.

Analysis of chemical abundances in planetary nebulae with [WC] central stars - I. Line intensities and physical conditions

Context. Planetary nebulae (PNe) around Wolf-Rayet [WR] central stars ([WR]PNe) constitute a particular photoionized nebula class that represents about 10% of the PNe with classified central stars. Aims. We analyse deep high-resolution spectrophotometric data of 12 [WR] PNe. This sample of [WR]PNe represents the most extensive analysed so far, at such high spectral resolution. We aim to select the optimal physical conditions in the nebulae to be used in ionic abundance calculations that will be presented in a forthcoming paper. Methods. We acquired spectra at Las Campanas Observatory with the 6.5-m telescope and the Magellan Inamori Kyocera (MIKE) spectrograph, covering a wavelength range from 3350 A to 9400 A. The spectra were exposed deep enough to detect, with signal-tonoise ratio higher than three, the weak optical recombination lines (ORLs) of O ii ,C ii, and other species. We detect and identify about 2980 emission lines, which, to date, is the most complete set of spectrophotometric data published for this type of objects. From our deep data, numerous diagnostic line ratios for Te and ne are determined from collisionally excited lines (CELs), ORLs, and continuum measurements (Hi Paschen continuum in particular). Results. Densities are closely described by the average of all determined values for objects with ne < 10 4 cm −3 , and by ne([Cl iii]) for the densest objects. For some objects, ne([Ariv]) is adopted as the characteristic density of the high ionization zone. For Te, we adopt a three-zone ionization scheme, where the low ionization zone is characterised by Te([N ii]), the medium ionization zone by Te([O iii]), and the highest ionization one by Te([Ar iv]) when available. We compute Te from the Hi Paschen discontinuity and from He i lines. For each object, Te(Hi) is, in general, consistent with Te derived from CELs, although it has a very large error. Values of Te(He i )a re systematically lower than the Te derived from CELs. When comparing Te(Hi )a ndTe(He i) it is unclear whether the behaviour of both temperatures agrees with the predictions of the temperature fluctuations paradigm, owing to the large errors in Te(Hi). We do not find any evidence of low-temperature, high-density clumps in our [WR]PNe from the analysis of faint O ii and N ii plasma diagnostics, although uncertainties dominate the observed line ratios in most objects. The behaviour of Te([O iii])/Te([N ii]), which is smaller for high ionization degrees, can be reproduced by a set of combined matter-bounded and radiation-bounded models, although, for the smallest temperature ratios, a too high metallicity seem to be required.

THE DOUBLE-DEGENERATE NUCLEUS OF THE PLANETARY NEBULA TS 01: A CLOSE BINARY EVOLUTION SHOWCASE

Original article can be found at: http://iopscience.iop.org/0004-637X/ Copyright American Astronomical Society. [Full text of this article is not available in the UHRA]

Chemical behavior of the dwarf irregular galaxy NGC6822. Its PN and HII region abundances

Aims. We aim to derive the chemical behavior of a significant sample of PNe and HII regions in the irregular galaxy NGC 6822. The selected objects are distributed in different zones of the galaxy. Our purpose is to obtain the chemical abundances of the present interstellar medium (ISM), represented by H ii regions, and the corresponding values at the time of formation of PNe. With these data the chemical homogeneity of NGC 6822 were tested and the abundance pattern given by H ii regions and PNe used as an observational constraint for computing chemical evolution models to infer the chemical history of NGC 6822. Methods. Due to the faintness of PNe and H ii regions in NGC 6822, to gather spectroscopic data with large telescopes is necessary. We obtained a well suited sample of spectra by employing VLT-FORS 2 and Gemini-GMOS spectrographs. Ionic and total abundances were calculated for the objects where electron temperatures could determined through the detection of [O iii] λ4363 or/and [N ii] λ5755 lines. A “simple” chemical evolution model was developed and the observed data were used to compute a model for NGC 6822 in order to infer a preliminary chemical history in this galaxy. Results. Confident determinations of He, O, N, Ne, S and Ar abundances were derived for a sample of 11 PNe and one H ii region. We confirm that the present ISM is chemically homogeneous, at least in the central 2 kpc of the galaxy, showing a value 12 + log O/H = 8.06 ± 0.04. From the abundance pattern of PNe, we identified two populations: a group of young PNe with abundances similar to H ii regions and a group of older objects with abundances a factor of two lower. A pair of extreme Type I PNe were found. No third dredge-up O enrichement was detected in PNe of this galaxy. The abundance determinations allow us to discuss the chemical behavior of the present and past ISM in NGC 6822. Our preliminary chemical evolution model predicts that an important

Mass-Loss Rate and He/H Abundance of the Erupting Component in the Small Magellanic Cloud System HD 5980

The binary Wolf-Rayet system HD 5980 in the Small Magellanic Cloud underwent a major luminous blue variable-type eruptive event in 1994. It is the first such recorded event in which the apparent precursor transits from WNE to WNL spectral types prior to the eruption. In this paper we analyze the spectrum of the system obtained when the outburst was declining (1994 December), but the dominant spectrum was that of the eruptor. From non-LTE analysis we obtain =10−3 M☉ yr-1, N[He]/N[H] = 0.43 (by number; Y = 0.63 by mass), v∞ = 600 km s-1, T* = 35,500 K, and Leruptor = 3 × 106L☉. A comparison of the He/H abundance derived here and an estimate obtained from published data of 20 yr ago leads us to conclude that the member of the binary system that underwent the eruption is the star formerly classified as an O7 supergiant. The considerable amount of He in this star indicates that it is in transition to becoming an H-poor W-R. By comparing the stellar parameters with single-star evolutionary tracks, we derive that the progenitor was more massive than 120 M☉, and that its current mass is close to 80 M☉.

Analysis of chemical abundances in planetary nebulae with [WC] central stars - II. Chemical abundances and the abundance discrepancy factor

Aims. We present the abundance analysis of 12 planetary nebulae ionized by [WC]-type stars and weak-emission-line stars (wels) obtained from high-resolution spectrophotometric data. Our main aims are to determine the chemical composition of the nebulae and to study the behaviour of the abundance discrepancy problem (ADF) in this type of planetary nebulae. Methods. The detection of a large number of optical recombination lines (ORLs) and collisionally excited lines (CELs) from different ions (O + ,O ++ ,C ++ ,C +3 and Ne ++ ) were presented previously. Most of the ORLs were reported for the first time in these PNe, which increased the sample of PNe with detected faint ORLs. Ionic abundances were determined from the available CELs and ORLs, using previously determined physical conditions. Based on these two sets of ionic abundances, we derived the total chemical abundances in the nebulae using suitable ionization correction factors (when available). Results. In spite of the [WC] nature of the central stars, moderate ADF(O ++ ) in the range from 1.2 to 4 were found for all the objects. We found that when the quality of the spectra is high enough, the ORLs O ++ /H + abundance ratios obtained from different multiplets excited mainly by recombination are very similar. Possible dependence of ADFs on some nebular characteristics such as surface brightness and nebular diameter were analysed, but we found no correlation. Abundances derived from CELs were corrected by determining the t 2 temperature fluctuation parameter. O abundances for PNe, derived from ORLs, are in general higher than the solar abundance. We derived the C/O ratio from ORLs and N/ Oa ndα-element/O ratios from CELs and found that these PNe are, on average, richer in N and C than the average of the large PN samples. About half of our sample is C-rich (C/O > 1). The growth of α-elements is correlated with the O abundance. Comparing the N/ Oa nd C/O ratios with those derived from stellar evolution models, we estimate that about half of our PNe have progenitors with initial masses similar to or larger than 4 M� . No correlation was found between the stellar [WC] type and the nebular chemical abundances. A rough O abundance gradient computed for our limited PN sample, compared with the gradient obtained for H ii regions, shows that there is a large dispersion in estimates of the PNe O abundance for a given Galactocentric distance. The PN gradient is flatter than that for H ii regions and at the solar distance and farther out, the PNe have a higher O abundance than H ii regions, similarly to what is found in other spiral galaxies. This fact has no convincing explanation so far.

The chemical composition of TS 01, the most oxygen-deficient planetary nebula - AGB nucleosynthesis in a metal-poor binary star

The planetary nebula TS 01 (also called PN G 135.9+55.9 or SBS 1150+599A) with its record-holding low oxygen abundance and its double degenerate close binary core (period 3.9 h) is an exceptional object located in the Galactic halo. We have secured observational data in a complete wavelength range to pin down the abundances of half a dozen elements in the nebula. The abundances are obtained via detailed photoionization modelling which takes into account all the observational constraints (including geometry and aperture effects) using the pseudo-3D photoionization code Cloudy_3D. The spectral energy distribution of the ionizing radiation is taken from appropriate model atmospheres. Incidentally we find from the new observational constraints that both stellar components contribute to the ionization: the “cool” one provides the bulk of hydrogen ionization, while the “hot” one is responsible for the presence of the most highly charged ions, which explains why previous attempts to model the nebula experienced difficulties. The nebular abundances of C, N, O, and Ne are found to be 1/3.5, 1/4.2, 1/70, and 1/11 of the solar value respectively, with uncertainties of a factor 2. Thus the extreme O deficiency of this object is confirmed. The abundances of S and Ar are less than 1/30 of solar. The abundance of He relative to H is 0.089 ± 0.009. Standard models of stellar evolution and nucleosynthesis cannot explain the abundance pattern observed in the nebula. To obtain an - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Spectrophotometric Data of the Central Star of the Large Magellanic Cloud Planetary Nebula N66: Quantitative Analysis of Its WN-Type Spectrum*

Hubble Space Telescope, IUE, and ground-based observations of the central star of the LMC planetary nebula N66 (CS N66), obtained in different epochs, are presented. Since 1990, CS N66 has displayed remarkable short- and long-term spectroscopic and photometric changes amounting to more than 3 mag in the optical. Expanding model atmospheres have been constructed to fit observations from different epochs. Fits provide the chemical composition, the fundamental stellar parameters L*, T*, R*, the mass-loss rate, and the wind velocity. From our best models we find that CS N66 is a very luminous He star (X/Y ≤ 0.1), with a small amount of N, undergoing a violent and unstable mass-loss event. The photospheric chemical abundances correspond to the equilibrium CNO nuclear burning values, while the nebula has a normal chemical composition. Models fitting data from different epochs indicate that the fundamental stellar parameters remain constant with time, with values log (L*/L☉) = 4.53 ± 0.10, T* = 93,300 K, and R* = 0.71 R☉. The short- and long-term stellar variations are produced by large changes in the mass-loss rate, which varies by large factors, from ≤ 8 × 10−7 M☉ yr-1 in 1983 (preoutburst epoch) to =2.5 × 10−5 M☉ yr-1 in early 1995 (maximum stellar brightness). No evidence was found to support the suggestion that the outburst was due to a late thermal pulse. We propose that the event taking place in CS N66 was produced by an atmospheric instability similar to that triggering the giant eruptions of Population I luminous blue variable stars. The possible mechanism causing the atmospheric instability is briefly discussed.