J. Chris Blades

A sample of 27 planetary nebulae (PNs) in the Small Magellanic Cloud (SMC) have been observed with the Hubble Space Telescope Imaging Spectrograph (HST/STIS) to determine their morphology, size, and the spatial variation of the ratios of bright emission lines. The morphologies of SMC PNs are similar to those of LMC and Galactic PNs. However, only a third of the resolved SMC PNs are asymmetric, compared to half of those in the LMC. The low-metallicity environment of the SMC seems to discourage the onset of bipolarity in PNs. We measured the line intensity, average surface brightness, and photometric radius of each nebula in Hα, Hβ, [O III] λλ4959 and 5007, [N II] λλ6548 and 6584, [S II] λλ6716 and 6731, He I λ6678, and [O I] λλ6300 and 6363. We show that the surface brightness-to-radius relationship is the same as in LMC PNs, indicating its possible use as a distance scale indicator for Galactic PNs. We determine the electron densities and the ionized masses of the nebulae where the [S II] lines were measured accurately, and we find that the SMC PNs are denser than the LMC PNs by a factor of 1.5. The average ionized mass of the SMC PNs is 0.3 M_☉. We also found that the median [O III]/Hβ intensity ratio in the SMC is about half that of the corresponding LMC median. We use CLOUDY to model the dependence of the [O III]/Hβ ratio on the oxygen abundance. Our models encompass very well the average observed physical quantities. We suggest that the SMC PNs are principally cooled by the carbon lines, making it hard to study their excitation based on the optical lines at our disposal.

The study of the formation and evolution of planetary nebulae (PNs) has been a subject of active investigation for several decades. In the past 15 years the relationship between nebular morphology and nebular/stellar evolution has been investigated in some detail. Although important insights have been gained, the connection between PN formation and even basic morphological features of the evolving nebula is far from clear. One of the most vexing problems to overcome is the difficulty of obtaining, at least for Galactic PNs where statistical distances must be adopted, reliable dimensions, ages, luminosities, and other physical quantities that are essential for understanding the evolutionary state of individual nebulae. Our emphasis of late has been to obtain high-resolution images of a large sample of PNs in the Magellanic Clouds, where uncertainties in the distances are minimal and selection effects due to, e. g., dust absorption in the Galactic plane do not apply.

HST STIS slitless spectroscopy of LMC PNs is the ideal tool to study their morphology and their ionization structures at once. We present the results from a group of 29 PNs that have been spatially resolved, for the first time, in all the major optical lines. Images in the light of \ha, \nii, and \oiii are presented, together with line intensities, measured from the extracted 1D and 2D spectra. A study on the surface brightness in the different optical lines, the electron densities, the ionized masses, the excitation classes, and the extinction follows, illustrating an ideal consistence with the previous results found by us on LMC PNs. In particular, we find the surface brightness decline with the photometric radius to be the same in most emission lines. We find that asymmetric PNs form a well defined cooling sequence in the excitation -- surface brightness plane, confirming their different origin, and larger progenitor mass.

We report initial results from an analysis of Hubble Space Telescope/Space Telescope Imaging Spectrograph echelle spectra of interstellar absorption toward Sk 155, located in the "wing" region of the SMC. There are at least 12 Milky Way (disk and halo) and at least 25 SMC components discernible in the profiles of various neutral and singly ionized species. Fits to the line profiles yield column densities and relative elemental abundances for the individual components. In the SMC components, the depletions of Fe and Ni range from mild ([Fe, Ni/Zn] ~ -0.3 to -0.8 dex) to severe ([Fe, Ni/Zn] ~ -1.7 dex); Mg and Si, however, appear to be essentially undepleted ([Mg, Si/Zn] ~ -0.1 to +0.2 dex) throughout. The combination of severe depletion for Cr, Mn, Fe, and Ni and negligible depletion for Mg and Si has not been seen in any Galactic sight line. If Si is generally undepleted in the SMC, then current models of the SMC dust—which rely heavily on silicates—will have to be reconsidered; oxides and/or metallic grains may dominate. We briefly discuss possible the implications of these SMC depletions for understanding the elemental abundances in QSO absorption-line systems.

The LMC is ideal for studying the coevolution of planetary nebulae (PNs) and their central stars in that the debilitating uncertainties of the Galactic PN distance scale and selection biases from attenuation by interstellar dust do not apply. We present images and analyze slitless spectra that were obtained in a survey of Large Magellanic Cloud PNs. These data on 29 targets were obtained with the Hubble Space Telescope (HST) using the Space Telescope Imaging Spectrograph. The data permit us to determine the nebular dimensions and morphology in the monochromatic light of several emission lines, including those that have traditionally been used for morphological studies in the Galaxy: Hα, [N II] λ6583, and [O III] λ5007, plus others of varying ionization including [O I], He I, and [S II]. Together with the 31 resolved LMC PNs for which monochromatic images exist in the HST archive, these data show that the incidence of nonsymmetric nebulae, including bipolar nebulae (which is an indicator of Population I ancestry in the Galaxy), is significantly higher than that reported for the Galaxy. The onset of asymmetric features appears even in very young nebulae (with dynamical ages of ~1400 yr), suggesting that at least the gross features of the nebular morphology may be more closely tied to PN formation and that subsequent shaping of the expanding envelope by the radiation field and wind from the central star may play the lesser role of amplifying these gross features. There is some evidence of evolution between two morphological types in the sense that bipolar core nebulae may evolve to pure bipolars late in the PN lifetime.

The LMC is ideal for studying the co-evolution of planetary nebulae (PNe) and their central stars, in that the debilitating uncertainties of the Galactic PN distance scale and selection biases from attenuation by interstellar dust do not apply. We present images and analyze slit-less spectra which were obtained in a survey of Large Magellanic Cloud PNe. These data on 29 targets were obtained with HST using the Space Telescope Imaging Spectrograph. The data permit us to determine the nebular dimensions and morphology in the monochromatic light of several emission lines, including those that have traditionally been used for morphological studies in the Galaxy: H-alpha, [N II] 6583 and [O III] 5007, plus others of varying ionization, including [O I], He I, and [S II]. Together with the 31 resolved LMC PNe for which monochromatic images exist in the HST archive, these data show that the incidence of non-symmetric nebulae, including bipolar nebulae (which is an indicator of Population I ancestry in the Galaxy), is significantly higher than that reported for the Galaxy. The onset of asymmetric features appears even in very young nebulae (with dynamical ages of ~1400 yr), suggesting that at least the gross features of the nebular morphology may be more closely tied to PN formation, and that subsequent shaping of the expanding envelope by the radiation field and wind from the central star may play the lesser role of amplifying these gross features. There is some evidence of evolution between two morphological types, in the sense that bipolar core (BC) nebulae may evolve to pure bipolars late in the PN lifetime.

We have observed SN 1998S which exploded in NGC 3877, with the Utrecht Echelle Spectrograph (6-7 km s^-1 FWHM) at the William Herschel Telescope and with the E230M echelle of the Space Telescope Imaging Spectrograph (8 km s^-1 FWHM) aboard the Hubble Space Telescope. Both data sets were obtained at two epochs, separated by 19 (optical) and 7 days (UV data). From our own Galaxy we detect interstellar absorption lines of Ca II K, Fe II λλ2600, 2586, 2374, 2344, Mg I λ2852, and probably Mn II λ2576, at v_LSR = -95 km s^-1 arising from the outer edge of the High Velocity Cloud Complex M. We derive gas-phase abundances of [Fe/H] = -1.4 and [Mn/H] = -1.0, values which are very similar to warm disk clouds found in the local ISM. This is the first detection of manganese from a Galactic HVC, and we believe that the derived gas-phase abundances argue against the HVC material having an extragalactic origin. At the velocity of NGC 3877 we detect interstellar Mg I λ2852, Mn II λλ2576, 2594, 2606, Ca II K and Na I D2, D1 absorption lines, spanning a velocity range of -102 to +9 km s^-1 from the systemic velocity of the galaxy (910 km s^-1). Surprisingly, the component at -102 km s^-1 is seen to increase by a factor of 1 dex in N(Na I) between 1998 March 20 and April 8, and in N(Mg I) between 1998 April 4 and April 11. Unusually, our data also show narrow Balmer, He I, and metastable UV Fe II P Cygni profiles, with a narrow absorption component superimposed on the bottom of the profile's absorption trough. Both the broad and narrow components of the optical lines are seen to increase substantially in strength between March 20 and April 8. The broad absorption covers ~350 km s^-1 and is seen in Mg II λλ2796, 2803 absorption as well, although there is no evidence of narrow Mg II emission forming a P Cygni profile. There is some suggestion that this shelf has decreased in strength over 7 days between April 4 and April 11. Most of the low-ionization absorption can be understood in terms of gas corotating with the disk of NGC 3877, provided the supernova is at the back of the disk as we observe it, and the H I disk is of a similar thickness to our own Galaxy. However, the variable component seen in all the other lines, and the accompanying emission which forms the classic P Cygni profiles, most likely arise in slow-moving circumstellar outflows originating from the red supergiant progenitor of SN 1998S.

Planetary nebulae (PNe) in the Large Magellanic Cloud (LMC) offer the unique opportunity to study both the population and evolution of low- and intermediate-mass stars, by means of the morphological type of the nebula. Using observations from our LMC PN morphological survey, and including images available in the Hubble Space Telescope Data Archive and published chemical abundances, we find that asymmetry in PNe is strongly correlated with a younger stellar population, as indicated by the abundance of elements that are unaltered by stellar evolution (Ne, Ar, and S). While similar results have been obtained for Galactic PNe, this is the first demonstration of the relationship for extragalactic PNe. We also examine the relation between morphology and abundance of the products of stellar evolution. We found that asymmetric PNe have higher nitrogen and lower carbon abundances than symmetric PNe. Our two main results are broadly consistent with the predictions of stellar evolution if the progenitors of asymmetric PNe have on average larger masses than the progenitors of symmetric PNe. The results bear on the question of formation mechanisms for asymmetric PNe—specifically, that the genesis of PNe structure should relate strongly to the population type, and by inference the mass, of the progenitor star and less strongly on whether the central star is a member of a close binary system.

We examine the clustering properties of low-redshift Lyα and heavy-element QSO absorption line systems seen in the spectra of 13 QSOs at the Galactic poles. This is the densest sample of ~1° separated QSOs observed spectroscopically with the Hubble Space Telescope to date. At the median redshift of the Lyα sample (z ≈ 0.7), the QSO lines of sight are separated on transverse scales from about 15 to 200 h^-1 Mpc (q₀=0.5, H=100 h km s^-1 Mpc^-1), allowing the three-dimensional clustering of the absorbers to be examined on those scales. The Galactic poles are also regions where relatively deep and wide-field galaxy redshift surveys have taken place, so the distributions of galaxies and Lyα systems can be compared within the same volume of space. There are 545 total absorption lines detected in the complete sample from 13 QSOs. We identify 307 Lyα systems, of which 18 contain heavy-element lines. We confirm the relatively slow redshift number density evolution for Lyα systems at z1. There are also five likely C IV doublets in our sample, for which the Lyα line is not accessible. The main results are summarized as follows: 1. Most, but not all, of the very low-z (zh^-1 Mpc, the line-of-sight correlation function shows a positive and significant clustering amplitude, ξ=0.95±0.28, for systems with rest equivalent width, W, exceeding 0.24 Å. Most of this signal can be accounted for by a small number of Lyα "clumps," which typically contain about five systems spanning ~20 h^-1 Mpc. There is qualitative evidence that the strength of the correlation function rises with the limiting Lyα rest equivalent width. 3. Over half of the heavy-element systems are either in groups or have a nearby "clump" of Lyα lines. However, there is only weak evidence for a significant heavy-element/Lyα cross-correlation function. 4. There is no clear evidence that the Lyα systems are distributed in redshift "peaks" and "voids" as is characteristic for the galaxies. In particular, there is no significant periodicity detected on the scale of 128 h^-1 Mpc, which had been detected for the galaxies located in the same direction of the sky. There is marginal evidence for a 41.5 h^-1 Mpc "periodicity" in the distribution, which is seen for all equivalent width-limited samples. 5. The three-dimensional Lyα radial correlation function shows no significant departures from a random sample at any scale or using any limiting equivalent width. This indicates that if there are sheetlike structures spanning separations on the order of 30 h^-1 Mpc or more, they are not clearly traced by Lyα systems. 6. The fraction of Lyα lines with W>0.24 Å, which may be clustered in the same way as the heavy-element systems detected mostly at higher redshifts, is about 39%±6%. This value is likely to depend on the limiting equivalent width and redshift of the sample.