Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apjs..105..375v&link_type=abstract
Astrophysical Journal Supplement v.105, p.375
Astronomy and Astrophysics
Astronomy
16
Ism: Dust, Extinction, Ism: Abundances, Galaxies: Magellanic Clouds, Ism: Planetary Nebulae: General, Ultraviolet: Ism
Scientific paper
Ultraviolet grating spectra (λλ1150-2300) of planetary nebulae in the Magellanic Clouds, obtained with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST), are used to identify and measure nebular emission lines and continua, and in one case to detect the stellar wind of the central star. Eight nebulae are observed in the Large Magellanic Cloud (LMC) and four are observed in the Small Magellanic Cloud (SMC). The HST observations are normalized to the photometric scale of ground-based, visible-light spectra (λλ3400-8000) by means of FOS PRISM observations in the λλ1350-5100 range. There is an unexplained, systematic discrepancy between the average PRISM and ground fluxes of ~30%. Observed and dereddened UV emission-line intensities with respect to Hβ are tabulated. The published reddening determinations based on the Balmer decrement are used to deredden the data. In general, the reddening derived from the He II I(1640)/I(4686) emission-line ratio does not agree with that derived from the Balmer decrement. This discrepancy suggests that the two reddening determinations are truly not equal, or that the applied reddening law deviates from the published average LMC or SMC curves. The uncertainty in the relative PRISM to ground calibration also contributes to the discrepancy but is of less importance than the uncertainty associated with the adopted reddening. The reddening is also determined from the λ2200 interstellar absorption band, assuming the published average Galactic and Magellanic Cloud reddening curves and is found to agree with the reddening determined from the Balmer decrement, in most cases. Otherwise, the reddening is always determined to be less than the Balmer decrement value. The exception to this rule is SMC-SMP6, which is the most reddened object in the sample. An alternative calibration for the HST data is achieved by assuming that the reddening derived from the He II I(1640)/I(4686) ratio must equal the reddening derived from the Balmer decrement. However, this procedure can only be applied to seven of the 12 objects in the sample. alternative UV line intensities are also tabulated. Spectra of six of the nebulae are compared with archival IUE observations in the λλ1200-2000 range. There is satisfactory agreement between the FOS and IUE data for line intensities less than ~Hβ. For stronger lines, the FOS appears to overestimate the measured line fluxes by ~30%, which may be due to the uncertainty associated with the PRISM to ground calibration: the effect is not seen in the smaller fluxes because of larger measurement errors. The continuum fluxes agree to within 50%. Color temperatures, derived from the FOS continuum flux ratios, are uncertain by at least a factor of 2 and are not correlated with previously determined Zanstra temperatures. Nebular densities, derived from O IV] and Si III] UV emission-line ratios, are presented. They are larger than densities derived from visible light [O II] and [S II] line ratios, consistent with the premise that the higher ionization species are formed in denser regions, supposedly closer to the central star. Nebular temperatures are determined using the N+ [I(2139) + I(2143))/[I(6548) + I(6583)) ratio and are found to be in agreement with the previously published, solely optical N^+^ temperatures, when the PRISM to ground calibration is used. When the N II] fluxes are scaled with respect to the helium lines, the N^+^ temperatures reduce to ~11,000 K in all instances Nebular temperatures implied by the O^2^ + [I(1661) + I(1666))/I(5007) ratio are consistent with those derived from the I(4363)/I(5007) ratio if the PRISM to ground calibration is used. The high O^2^ + electron temperatures for SMC-SMP28 and LMC-SMP87 imply that photoionization is not the sole excitation process in the O^2^ + zone. However, if the UV line fluxes are rescaled such that the reddening from the Balmer decrement and the He II lines agree, then the optically derived temperatures for these two objects appear to be overestimated by ~10,000 K. Estimates of the C/O abundance ratio are made for most objects in the sample using the C III] λλ1907, 1909, O III] λλ1661, 1666, and [O III] λ5007 emission lines. The C/O ratio is anticorrelated with the previously published N/O values, suggesting that the hot bottom burning process operated during the asymptotic giant branch phase. There are four type I objects in the sample, which have the lowest C/O values. The Si/C ratio is also estimated solely from ultraviolet lines. Silicon emission is only detected in the four type I objects. P Cygni-type line profiles are detected in the spectrum for LMC-5MP76 at N V λλ1239, 1243, O V λ1371, and C IV λλ1548, 1551. The edge velocities derived from these lines are 1000-1500,800, and 2000 km s^-1^, respectively.
Bohlin Ralph Charles
Dopita Mike A.
Ford Holland C.
Harrington Patrick J.
Maran Stephen P.
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