Elemental abundances for a sample of southern galctic planetary nebulae

We present spectrophotometric observations of a sample of 80 southern galactic planetary nebulae (PN), and derive elemental abundances for 68 objects, supplementing the optical observations with UV data in 25 cases. We define Type I PN as those objects that have experienced envelope-burning conversion to nitrogen of dredgedup primary carbon. Such nebulae are recognised by their having nitrogen abundances that exceed the total C + N abundance of H II regions in the same galaxy. In our own galaxy, this criterion corresponds to N/O > 0.8. In the current sample, 11 nebulae having N/O > 0.8 are thereby classified as Type I. For these Type I PN, no evidence is found for oxygen depletion, compared with non-Type I PN. No trend is found between the N/O and O/H ratios for the entire sample, and the mean O/H ratios for the non-Type I and Type I PN are the same within the errors; O/H=(4.93±2.22)×10−4 by number for 42 non-Type I PN and O/H=(4.42±1.44)×10−4 for 11 Type I PN. Also, no difference is found between the oxygen abundances in the PN in this sample and the oxygen abundances in galactic H II regions. Hence we find no evidence for the ON cycle (which is predicted to operate during the second dredge-up) to have significantly altered the surface abundances of the progenitor stars, even for the Type I PN. The helium abundances derived for the non-Type I PN are in accord with those predicted by Becker&Iben for the first and third dredge-up phases. A comparison between the nitrogen abundances in the PN and the carbon+ nitrogen abundances in galactic H II regions indicates that roughly 36 per cent of the initial carbon is converted into nitrogen in the case of the non-Type I PN, consistent with predictions for the first dredge-up by Becker&Iben. In order to explain the high nitrogen abundances derived for the Type I PN, however, envelope-burning of dredged-up carbon into nitrogen, following the third dredge-up, is definitely required. Total C + N + O abundances are found to be correlated with C/H for the combined nonType I and Type I sample; the carbon has been enhanced by He-burning processed material brought up by the third dredge-up

Distances for galactic planetary nebulae using mean forbidden [O II] doublet ratio electron densities

We present [O II| 3726, 3729-Å doublet ratios and electron densities for 68 galactic planetary nebulae (PN). For 45 of the objects, the doublet ratios represent integrations over the whole of the nebula. Distances have been derived for the majority of the nebulae, using calibrations recently derived from Magellanic Cloud PN. For PN which are optically thin in the hydrogen Lyman continuum, we have derived distances using a variant of the Shklovsky method (constant ionized hydrogen mass) which uses the mean [O II] electron density and the measured radio flux and which does not require knowledge of the filling factor or nebular angular radius. For PN which are optically thick in the Lyman continuum, the constant Hβ flux method was used to derive distances. The typical [O II] density at the transition point between an optically thick and thin nebula is 4500 cm−3. Since the optically thin and thick methods both over-estimate the distance when applied to inappropriate nebulae, the smaller of the two distance estimates is adopted for each nebula. An extensive comparison is made between the distances derived here and previously published distances and distance scales. It is shown that the present distances, based on Magellanic Cloud calibrations, give consistency with independent distance estimates. They also yield much greater self-consistency between central star masses derived from luminosity versus Teff comparisons on the one hand, and from absolute magnitude versus evolutionary age comparisons on the other hand. For the PN in our sample, rms electron densities, filling factors and absolute radii have also been derived. The derived filling factors are found to decrease with increasing absolute angular radius, but we argue that this effect can be attributed entirely to the effects of measurement uncertainties in the adopted angular radii

DR 1: a WO3 star in IC 1613 and its surrounding nebula, S3

We present an analysis of the WO3 star, DR 1, which is located in the dwarf irregular galaxy IC 1613, and is surrounded by an HII region which shows nebular HeII 4686 A in emission, a rare phenomenon in nebulae surrounding Wolf-Rayet stars. We have derived E(B-V)=0.07 via a comparison of the observed Balmer line ratios to those predicted by theory, using the electron temperature of T_e_=17100K derived from our nebular analysis. We find O/H=4.99x10^-5^ by number for the nebula, in agreement with the O/H ratios found for other emission-line regions in IC 1613. We derive the following nebular mass fractions: X=0.761, Y=0.238 and Z=0.00091. After allowance for the contribution by the nebular continuum, we have derived a stellar absolute magnitude of M_V_=-3.6 for DR 1, a stellar effective temperature of T_*_=75000K via a HI and HeII Zanstra analysis, and a stellar luminosity of 10^6^Lsun_. A terminal wind velocity of vinfinity_=2850km/s is derived for DR 1 from the width of the strongest stellar emission lines. We also performed an abundance analysis of the stellar wind via a recombination theory analysis of the stellar emission-line features, and derive X(C)=0.48, X(O)=0.27 and X(He)=0.25. These values are within the range found for other WO stars by Kingsburgh et al. (1994) and agree with those predicted by the Z=0.004 massive star evolutionary models of Meynet et al. (1994), but not with their Z=0.001 models. Our observations confirm the prediction that WO stars in low-metallicity galaxies should be much more luminous than their counterparts in higher metallicity galaxies

Properties of the WO Wolf-Rayet stars

We present optical spectrophotometry for five WO Wolf-Rayet stars, three of them in our own Galaxy and one in each of the SMC and LMC. IUE ultraviolet spectrophotometry has also been obtained for the two Magellanic Cloud WO stars, including a high resolution spectrum for one of them. Quantitative spectral typing criteria are defined for WO subtypes spanning WO1 to WO5 and, for the case of the two WO stars in spectroscopic binaries, spectral types for the O-type primaries are derived. From our spectrophotometry we derive reddenings and magnitudes for each star. Absolute visual magnitudes of -2.5 and -1.8 are derived for a WO4 star and for a WO2 star, respectively, each star lying at a known distance. Wind terminal velocities ranging from 4200km/s to 5500km/s are derived from the black absorption edge of an ultraviolet P Cygni profile and from the FWZI of strong optical emission lines. The relative abundances of helium, carbon and oxygen in the winds of each of the WO stars are derived using a recombination theory analysis of selected ultraviolet and optical emission lines to determine the ionic abundances of He^2+^, C^4+^, O^4+^, O^5+^ and O^6+^. The derived abundance ratios show relatively narrow ranges. C/He number ratios of 0.51-0.52 are derived for two Galactic WO stars and one LMC WO star, with their C/O ratios ranging between 4.6 and 5.2, and their (C+O)/He ratios equal to 0.62. The one SMC WO star has a C/He ratio of 0.81, a C/O ratio of 2.7 and a (C+O)/He number ratio of 1.10. These abundance ratios are broadly consistent with evolutionary models for the advanced stages of massive stars, and promising agreement as a function of initial metallicity is found with the most recent evolutionary models

A large radio nebula around P Cygni

We present a large set of radio observations of the luminous blue variable P Cygni. These include two 6-cm images obtained with MERLIN which spatially resolve the 6-cm photosphere, monitoring observations obtained at Jodrell Bank every few days over a period of two months, and VLA observations obtained every month for seven years. This combination of data shows that the circumstellar environment of P Cyg is highly inhomogeneous, that there is a radio nebula extending to almost an arcminute from the star at 2 and 6 cm, and that the radio emission is variable on a time-scale no longer than one month, and probably as short as a few days. This short-time-scale variability is difficult to explain. We present a model for the radio emission with which we demonstrate that the star has probably been losing mass at a significant rate for at least a few thousand years, and that it has undergone at least two major outbursts of increased mass loss during the past two millenia

Phytoplankton communities and acclimation in a cyclonic eddy in the southwest Indian Ocean

A study of phytoplankton in a cyclonic eddy was undertaken in the Mozambique Basin between Madagascar and southern Africa during austral winter. CHEMTAX analysis of pigment data indicated that the community comprised mainly haptophytes and diatoms, with Prochlorococcus, prasinophytes and pelagophytes also being prominent to the east and west of the eddy. There was little difference in community structure, chlorophyll-specific absorption [a*ph(440)] and pigment:TChla ratios between the surface and the sub-surface chlorophyll maximum (SCM), reflecting acclimation to fluctuating light conditions in a well mixed upper layer. Values for a*ph(440) were low for diatom dominance, high where prokaryote proportion was high, and intermediate for flagellate dominated communities. Chlorophyll c and fucoxanthin:TChla ratios were elevated over most of the eddy, while 19′-hexanoyloxyfucoxanthin ratios increased in the eastern and western sectors. In a community comprising mainly flagellates and Prochlorococcus to the west of the eddy, there was high a*ph(440) at the surface and elevated ratios for divinyl chlorophyll a, chlorophyll b and 19′-hexanoyloxyfucoxanthin at the SCM. An increase in diadinoxanthin:TChla ratios and a decline in the quantum efficiency of photochemistry in PSII under high light conditions, indicated some photoprotection and photoinhibition at the surface even in a well mixed environment. Diadinoxanthin was the main photoprotective carotenoid within the eddy, while zeaxanthin was the dominant photoprotective pigment outside the eddy. The results of this study will be useful inputs into appropriate remote sensing models for estimating primary production and the size class distribution of phytoplankton in eddies in the southwest Indian Ocean

Herschel observations of NGC 7027

We present our analysis of the spectra of NGC 7027 taken with the PACS and SPIRE instruments of the Herschel satellite

The role of input noise in transcriptional regulation

Even under constant external conditions, the expression levels of genes fluctuate. Much emphasis has been placed on the components of this noise that are due to randomness in transcription and translation; here we analyze the role of noise associated with the inputs to transcriptional regulation, the random arrival and binding of transcription factors to their target sites along the genome. This noise sets a fundamental physical limit to the reliability of genetic control, and has clear signatures, but we show that these are easily obscured by experimental limitations and even by conventional methods for plotting the variance vs. mean expression level. We argue that simple, global models of noise dominated by transcription and translation are inconsistent with the embedding of gene expression in a network of regulatory interactions. Analysis of recent experiments on transcriptional control in the early Drosophila embryo shows that these results are quantitatively consistent with the predicted signatures of input noise, and we discuss the experiments needed to test the importance of input noise more generally.Comment: 11 pages, 5 figures minor correction