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Автор: Grafiati

Опубліковано: 10 грудня 2022

Оновлено: 28 січня 2023

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1

Rodríguez, Luis F. "Molecules and Neutral Hydrogen in Planetary Nebulae." Symposium - International Astronomical Union 131 (1989): 129–38. http://dx.doi.org/10.1017/s0074180900137714.

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Анотація:

Molecules and/or neutral hydrogen have been detected in a modest number of planetary nebulae. However, when detected, these indicators of a significant neutral component can provide fundamental information on the total mass of the envelope, its chemistry and kinematics, and on the morphology and evolutionary status of the planetary nebula. A review of recent results is presented, giving emphasis to CO (carbon monoxide), H2 (molecular hydrogen), OH (hydroxil), and HI (neutral hydrogen). A major development of the last few years has been the capability to map with considerable angular resolution these species in planetary nebulae. In the best studied cases, the neutral component appears to be located surrounding the “waist” of a bipolar planetary nebulae. These is evidence suggesting that molecules and neutral hydrogen are not as uncommon in planetary nebula as the present statistics suggest. Indeed, the outer parts of a considerable fraction of the known planetary nebulae could be neutral. It is possible that a combination of good selection criteria and long integrations with the best telescopes could increase largely the number of known cases.

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2

Pottasch, S. R., A. Mampaso, A. Manchado, and J. Menzies. "Hydrogen Deficient Planetary Nebulae: Preliminary Results." International Astronomical Union Colloquium 87 (1985): 359–65. http://dx.doi.org/10.1017/s0252921100090953.

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AbstractNew spectra of A78 and A58 at different positions in the nebulae are presented. An abundance gradient is found in A78, extending quite close to the center. Similarly the nebulous knot near the center of A58 has considerably higher heavy element abundances than the outer regions of this nebula. The ionization state is considerably lower in A58 than A78. In A78 most of the neon is in the form of Ne+3 and Ne+4, indicating that the standard ionization correction factor as used by Jacoby and Ford, is substantially in error. Finally, the very high infrared excesses found in this nebulae are discussed.

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3

Terzian, Yervant. "Radio Images of Planetary Nebulae." Symposium - International Astronomical Union 131 (1989): 17–28. http://dx.doi.org/10.1017/s0074180900137453.

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Анотація:

The continuum radio spectra of planetary nebulae are discussed, and the structure of these objects is examined from the observed aperture synthesis brightness distributions determined with the Very Large Array. The use of radio observations in determining distances to planetary nebulae is examined. The detection of atomic neutral hydrogen at λ21 cm associated with planetary nebulae, as well as the associated CO and OH components are discussed. An upper limit, of the nebular magnetic field associated with the neutral material, of 1mG is reported for NGC 6302.

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4

Liebert, James. "White Dwarfs and Planetary Central Stars." Symposium - International Astronomical Union 131 (1989): 545–54. http://dx.doi.org/10.1017/s0074180900139166.

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Studies of hot white dwarf samples constrain the properties and evolution of planetary nuclei and the nebulae. In particular, the white dwarf and planetary nebulae formation rates are compared. I discuss the overlap of the sequences of white dwarfs having hydrogen (DA) and helium-rich (DO) atmospheres with known central stars of high surface gravity. There is evidence that the hydrogen atmosphere nuclei have “thick” outer hydrogen layers (≳ 10−4 M⊙), but that DA white dwarfs may have surface hydrogen layers orders of magnitude thinner. Finally, a DA planetary nucleus is discussed (0950+139) which has undergone a late nebular ejection; this object may be demonstrating that a hydrogen layer can be lost even after the star has entered the white dwarf cooling sequence.

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5

Iben, Icko, and Alexander V. Tutukov. "Binary Stars and Planetary Nebulae." Symposium - International Astronomical Union 131 (1989): 505–22. http://dx.doi.org/10.1017/s0074180900139075.

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A non-negligible (∼ 15–20%) fraction of planetary nebulae is expected to be formed in close binaries in which one component fills its Roche lobe after the exhaustion of hydrogen or helium at its center. The nebula is ejected as a consequence of a frictional interaction between the stellar cores and a common envelope; the ionizing component of the central binary star may be a relatively high luminosity contracting star with a degenerate CO core, burning hydrogen or helium in a shell, or it may be a lower luminosity shell hydrogen-burning star with a degenerate helium core or a core helium-burning star. Even more exotic ionizing central stars are possible. Once the initial primary has become a white dwarf or neutron star, the secondary, after exhausting central hydrogen, will also fill its Roche lobe and eject a nebular shell in a common envelope event. The secondary becomes the ionizing star in a tight orbit with its compact companion. In all, there are roughly twenty different possibilities for the make-up of binary central stars, with the ionizing component being a post asymptotic giant branch star with a hydrogen- or helium-burning shell, a CO dwarf, a core helium-burning star, a shell helium-burning star with a degenerate CO core, a shell hydrogen-burning star with a degenerate helium core, or a helium degenerate dwarf, while its companion is a main sequence star, a CO degenerate dwarf, a helium star, a helium degenerate dwarf, or a neutron star. We estimate the occurrence frequency of several of these types and comment on the prior evolutionary history of 4 observed binary central stars.

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6

Akras, Stavros, Denise R. Gonçalves, Gerardo Ramos-Larios, and Isabel Aleman. "Molecular Hydrogen Microstructures in Planetary Nebulae." Galaxies 8, no. 2 (April 1, 2020): 30. http://dx.doi.org/10.3390/galaxies8020030.

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Molecular hydrogen (H 2 ) emission is commonly detected in planetary nebulae (PNe), specially in objects with bipolar morphologies. New studies showed that H 2 gas is also packed in microstructures embedded in PNe of any morphological type. Despite the presence of H 2 in cometary knots being known for years, only in the last five years, much deeper imagery of PNe have revealed that H 2 also exists in other types of low-ionisation microstructures (LISs). Significant differences are found between the host PNe of cometary knots and other types of LISs, such as nebula age, central star temperature (evolutionary stage) and the absolute sizes of the microstructure itself.

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7

Walton, N. A., M. J. Barlow, D. J. Monk, and R. E. S. Clegg. "Abundances and nebular and central star masses for Magellanic Cloud planetary nebulae." Symposium - International Astronomical Union 148 (1991): 334–36. http://dx.doi.org/10.1017/s0074180900200727.

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Анотація:

We present the results of a spectroscopic study of planetary nebulae (PN) in the Magellanic Clouds. The optical survey of He, N, O, and Ne abundances by Monk et al. (1988) has been updated by higher S/N AAT optical data. In addition, carbon and other elemental abundances have been derived from the IUE spectra of 38 PN. Ionized nebular masses have been derived for 80 PN. The ionised mass versus nebular electron density plot shows that planetary nebulae become optically thin when their electron densities drop below 4500 cm--3. Below this density, the mean nebular hydrogen mass found for non-Type I PN is 0.22±0.08 M⊙. Using Zanstra and energy-balance methods, the mean central star mass found for 14 SMC and LMC PN is 0.59±0.02 M⊙.

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8

Heller, R., R. Jacob, D. Schönberner, and M. Steffen. "Hot bubbles of planetary nebulae with hydrogen-deficient winds." Astronomy & Astrophysics 620 (December 2018): A98. http://dx.doi.org/10.1051/0004-6361/201832683.

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Context. The first high-resolution X-ray spectroscopy of a planetary nebula, BD +30° 3639, opened the possibility to study plasma conditions and chemical compositions of X-ray emitting “hot” bubbles of planetary nebulae in much greater detail than before. Aims. We investigate (i) how diagnostic line ratios are influenced by the bubble’s thermal structure and chemical profile, (ii) whether the chemical composition inside the bubble of BD +30° 3639 is consistent with the hydrogen-poor composition of the stellar photosphere and wind, and (iii) whether hydrogen-rich nebular matter has already been added to the bubble of BD +30° 3639 by evaporation. Methods. We applied an analytical, one-dimensional (1D) model for wind-blown bubbles with temperature and density profiles based on self-similar solutions including thermal conduction. We also constructed heat-conduction bubbles with a chemical stratification. The X-ray emission was computed using the well-documented CHIANTI code. These bubble models are used to re-analyse the high-resolution X-ray spectrum from the hot bubble of BD +30° 3639. Results. We found that our 1D heat-conducting bubble models reproduce the observed line ratios much better than plasmas with single electron temperatures. In particular, all the temperature- and abundance-sensitive line ratios are consistent with BD +30° 3639 X-ray observations for (i) an intervening column density of neutral hydrogen, NH = 0.20-0.10+0.05 × 1022cm−2, (ii) a characteristic bubble X-ray temperature of TX = 1.8 ± 0.1 MK together with (iii) a very high neon mass fraction of about 0.05, virtually as high as that of oxygen. For lower values of NH, we cannot exclude the possibility that the hot bubble of BD +30° 3639 contains a small amount of “evaporated” (or mixed) hydrogen-rich nebular matter. Given the possible range of NH, the fraction of evaporated hydrogen-rich matter cannot exceed 3% of the bubble mass. Conclusions. The diffuse X-ray emission from BD +30° 3639 can be well explained by models of wind-blown bubbles with thermal conduction and a chemical composition equal to that of the hydrogen-poor and carbon-, oxygen-, and neon-rich stellar surface.

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9

Sonneborn, George. "Far-Ultraviolet Spectra of Planetary Nebulae." Symposium - International Astronomical Union 209 (2003): 405–11. http://dx.doi.org/10.1017/s0074180900209224.

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The Far Ultraviolet Spectroscopic Explorer (FUSE)> satellite provides a unique opportunity to obtain high-resolution far-UV spectra of a wide variety of astronomical objects, including planetary nebulae. Most FUSE observations of PNe to date have concentrated on the hot central star, providing a very effective way to study the atmosphere of the central star, the surrounding nebula through the absorption features from circumstellar gas. FUSE has found evidence of hot molecular hydrogen in several planetary nebulae, including M27 and BD+30° 3639. Central star spectra also reveal new information about stellar winds, mass loss, and photospheric abundances.

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10

Pakull, Manfred W. "High-exitation nebulae around Magellanic Wolf-Rayet stars." Proceedings of the International Astronomical Union 4, S256 (July 2008): 437–42. http://dx.doi.org/10.1017/s1743921308028834.

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AbstractThe SMC harbours a class of hot nitrogen-sequence Wolf-Rayet stars (WNE) that display only relatively weak broad Heiiλ4686 emission indicative of their low mass-loss rates and which are therefore hard to detect. However, such stars are possible emitters of strong He+Lyman continua which in turn could ionize observable Heiiiregions, i.e. highly excited Hiiregions emitting nebular Heiiλ4686 emission. We here report the discovery of a second Heiiiregion in the SMC within OB association NGC 249 within which the weak-lined WN star SMC WR10 is embedded. SMC WR10 is of special importance since it is a single star showing the presence of atmospheric hydrogen. While analysing the spectrum in the framework of two popular, independent WR atmosphere models we found strongly discrepant predictions (by 1 dex) of the He+continuum for the same input parameters. A second interesting aspect of the work reported here concerns the beautiful MCELS images which clearly reveal a class of strongly Oiiiλ5007 emitting (blue-coded) nebulae. Not unexpectedly, most of the “blue” nebulae are known Wolf-Rayet bubbles, but new bubbles around a few WRs are also detected. Moreover, we report the existence of blue nebulae without associated known WRs and discuss the possibility that they reveal weak-wind WR stars with very faint stellar Heiiλ4686 emission. Alternatively, such nebulae might hint at the hitherto missing population of relatively low-mass, hot He stars predicted by massive binary evolution calculations. Such a binary system is probably responsible for the ionization of the unique Heiiλ4686-emitting nebula N 44C.

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11

Harrington, J. P., K. J. Borkowski, and Z. I. Tsvetanov. "Modeling hydrogen-deficient Planetary Nebulae." Symposium - International Astronomical Union 180 (1997): 235. http://dx.doi.org/10.1017/s0074180900130554.

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Анотація:

Hydrogen-deficient planetary nebulae have central regions devoid of hydrogen. (See Harrington 1996 for a review of these nebulae.) They are characterized by exceptionally strong collisionally excited lines of [O III] and [Ne III], but relatively weak or undetected recombination lines of He or other elements. Such nebulae cannot be modeled successfully unless we include some source of heating in addition to photoionization by the central star.

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12

Speck, A. K., M. Meixner, P. Knezek, and G. H. Jacoby. "High resolution molecular hydrogen imaging of the Ring Nebula." Symposium - International Astronomical Union 209 (2003): 271. http://dx.doi.org/10.1017/s0074180900208711.

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Анотація:

The relative morphologies and structures of molecular and ionized gas emission from planetary nebulae (PNe) allow a better understanding of the nature and evolution of these objects. The classical paradigm for the structure of PNe is that of an ionized gas bubble bounded by neutral gas and molecules. However, it has been shown that molecular gas exists within ionized regions, leading to a re-evaluation of the classic structure. In the Helix Nebula (NGC 7293) dense condensations known as cometary knots are known to exist in the main ionized nebula. The molecules in these knots are shielded from the ionizing radiation and thus survive within the ionized zone. Another PN in which H2 emission is seen to originate from within the ionized nebula in NGC 6720 (the Ring Nebula).

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13

Malkov, Yu F. "Self-consistent determination of distances and central star parameters for a large sample of Galactic Planetary Nebulae." Symposium - International Astronomical Union 180 (1997): 50. http://dx.doi.org/10.1017/s007418090012981x.

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Анотація:

The distances, central star parameters Tef, log g, L∗, M∗, and optical thicknesses of the envelopes in the Lyman limits of hydrogen and helium are determined in a self-consistent way for 156 planetary nebulae (PNe) in our Galaxy (129, 22, and 5 PNe in the galactic disk, bulge, and halo respectively). For each nebula, the distance is adjusted in such a way that the theoretical evolutionary age of the central star (CS) should be equal to the dynamical age of the expanding nebular envelope (Malkov, 1994). This method combines the individual approach to each nebula with the possibility of application to a large sample of PNe. Some improvements have been introduced in the method as compared with its original version. The evolutionary tracks and time scales for CS with masses of 0.605, 0.696, 0.836, and 0.940 M⊙ were taken from Blöcker (1995). The change of the nebular expansion velocity during the evolution of a PN was roughly taken into account. The effective temperature of the CS and related parameters were found by the generalized energy balance method (Malkov et al., 1995) or, in appropriate cases, by Zanstra methods. Observational data have been collected from 93 papers published in 1975–1995.

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14

Latter, William B., and Joseph L. Hora. "Investigating the near-IR properties of PN: Deep imaging of molecular hydrogen emission." Symposium - International Astronomical Union 180 (1997): 254. http://dx.doi.org/10.1017/s0074180900130724.

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Анотація:

We present the first results of a deep near-infrared narrowband imaging study of several planetary nebulae. The data were acquired using the University of Hawaii 2.2m telescope on Mauna Kea and the “QUIRC” 1024 × 1024 array camera. With these sensitive high spatial resolution imaging data, we are able to explore in detail where H2 is found relative to the ionized region in planetary nebulae at various stages of evolution (See Figure 1, for an example). The objects were selected from previous studies to have evidence for extended emission from H2, and they are at several differing phases of central star evolution. The presence of molecules in the UV–flooded environment of a planetary nebula cannot be explained without highly non-spherical geometries and high densities, or clumping. Although there can be an ambiguity in the excitation mechanism (UV photons or shocks), the location of H2 emission identifies regions of the nebula with sufficient dust shielding and density for the survival of molecular species. The emission morphology also provides information that helps identify which excitation mechanism is most likely. When combined with data provided by other molecular studies, we can examine how the photodissociation region evolves through the circumstellar envelope as the planetary nebula tracks across the top of the HR diagram. We can also consider how morphological evolution of the nebula, through interacting winds and shocks, might provide the conditions necessary for molecular survival.

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15

Schneider, Stephen E., Peter R. Silverglate, Daniel R. Altschuler, and Carlo Giovanardi. "Atomic hydrogen in planetary nebulae." Astrophysical Journal 314 (March 1987): 572. http://dx.doi.org/10.1086/165086.

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16

Bässgen, G., M. Bässgen, and M. Grewing. "Comparison of Different Treatments of the Radiation Transfer in Model Calculations of Planetary Nebulae." Symposium - International Astronomical Union 131 (1989): 227. http://dx.doi.org/10.1017/s0074180900138409.

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Анотація:

A method of Radiation Transfer treatment (Global Shell Model (GSM)) has been developed, different from the On the Spot Approximation (OSA), taking into account all diffuse reemission from radiative recombination of hydrogen and helium in the nebula itself. For optically thick and optically thin nebulae the method automatically reproduces the On the Spot Approximation and the models without any Radiation Transfer, respectively.

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17

Greenhouse, M. A., T. L. Hayward, and H. A. Thronson. "H2 and HI Emission Line Imaging of the Ring Nebula NGC 6720." Symposium - International Astronomical Union 131 (1989): 170. http://dx.doi.org/10.1017/s0074180900137787.

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Анотація:

We present infrared emission line images of the v = 1 → 0 S(1) transition of molecular hydrogen and Brγ recombination line of atomic hydrogen which cover the entire extent of NGC 6720, the Ring Nebula. The maps presented here are the highest angular resolution images of these transitions yet produced for this object, and have very low relative positional uncertainty. As a result, we clearly resolve the spatial stratification of the ionized and shocked molecular zones within the nebula discussed previously by Beckwith et al. (1978). The relative spatial distribution of molecular and ionized hydrogen we observe is typical of several planetaries which exhibit shocked H2 emission (eg. see Zuckerman and Gatley 1987), and is similar to that predicted by the interacting-stellar-winds model of planetary nebulae formation (see Volk and Kwok 1985 and references therein).

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18

Taylor, A. R., G. T. Gussie, and S. R. Pottasch. "Circumnebular neutral hydrogen in planetary nebulae." Astrophysical Journal 351 (March 1990): 515. http://dx.doi.org/10.1086/168489.

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19

Gvaramadze, V. V., A. Y. Kniazev, G. Gräfener, and N. Langer. "WR 72: a born-again planetary nebula with hydrogen-poor knots." Monthly Notices of the Royal Astronomical Society 492, no. 3 (January 10, 2020): 3316–22. http://dx.doi.org/10.1093/mnras/stz3639.

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Анотація:

ABSTRACT We report the discovery of a handful of optical hydrogen-poor (H-poor) knots in the central part of an extended infrared nebula centred on the [WO1] star WR 72, obtained by spectroscopic and imaging observations with the Southern African Large Telescope (SALT). Wide-field Infrared Survey Explorer (WISE) images of the nebula show that it is composed of an extended almost circular halo (of ≈6 arcmin or ≈2.4 pc in diameter) and an elongated and apparently bipolar inner shell (of a factor of six smaller size), within which the knots are concentrated. Our findings indicate that WR 72 is a new member of the rare group of H-poor planetary nebulae, which may be explained through a very late thermal pulse of a post-AGB star or by a merger of two white dwarfs.

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20

Kimeswenger, S., F. Kerber, and R. Weinberger. "Planetary nebulae with ISO: A58 and A78 — nebulae with hydrogen-poor ejecta." Monthly Notices of the Royal Astronomical Society 296, no. 3 (May 1998): 614–18. http://dx.doi.org/10.1046/j.1365-8711.1998.01381.x.

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21

Starrfield, Sumner, and Arthur N. Cox. "Nonradial Pulsational Analyses of the Pulsating Central Stars of Planetary Nebulae." Symposium - International Astronomical Union 131 (1989): 311. http://dx.doi.org/10.1017/s0074180900138598.

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Анотація:

We have performed nonradial pulsation analyses of the central star of the planetary nebula K1-16. K1-16 is a very unusual nebulae which appears to have ejected material that is very rich in helium. The central star shows no evidence for hydrogen in its spectrum and the helium and carbon lines are in emission. Grauer and Bond (Ap. J., 277, 211, 1984) discovered that it is pulsating with periods around 1700 sec. Although its spectral characteristics are similar to those of the PG1159-035 variables, it is pulsating in much longer periods than they are.

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22

Rodriguez, Luis F. "H Atom Observations in Near-Stellar Environments." Symposium - International Astronomical Union 150 (1992): 351–55. http://dx.doi.org/10.1017/s0074180900090355.

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Анотація:

Interferometric observations of the 21-cm line of atomic hydrogen resolve out the emission from extended, line-of-sight clouds and allow the detailed study of compact H I structures in the surroundings of some stars. These atomic hydrogen components most probably are the result of photodissociation of gas that originally was in molecular form. They have been observed in H II regions, reflection nebulae, and planetary nebulae. The study of this atomic hydrogen component is important to determine the mass and physical conditions of gas in the environment of luminous stars and to test our theoretical knowledge of photodissociation regions.

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23

Aller, Lawrence H. "Early days of Planetary Nebular theory." Symposium - International Astronomical Union 180 (1997): 3–9. http://dx.doi.org/10.1017/s0074180900129651.

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The spectra of gaseous nebulae differed strikingly from those of stars which were well understood since 1922 thanks to the work of Saha. Gaseous nebulae exhibited exotic bright line spectra characterized by strange emissions of unknown origin as well as familiar lines of hydrogen and helium. The strongest lines in most nebulae fell at 4959A and 5007A. They were originally attributed to an unknown element. First came the interpretation of the lines of H.

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24

Allen, L. E., M. C. B. Ashley, S. D. Ryder, J. W. V. Storey, Y.-S. Sun, and M. G. Burton. "IR Fabry-Perot imaging of NGC 3132." Symposium - International Astronomical Union 180 (1997): 205. http://dx.doi.org/10.1017/s0074180900130293.

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Анотація:

Near-infrared (1-3μm) emission lines of molecular and ionized hydrogen are excellent tools for investigating the morphology, energetics and kinematics of planetary nebulae, especially those PNe which contain large amounts of dust and are thus obscured at shorter wavelengths. The southern planetary nebula NGC 3132 was imaged with UNSWIRF (University of New South Wales Infrared Fabry-Perot) and IRIS on the 3.9m AAT Images in the H2 v=1-0 S(1) and H2 v=2-1 S(1) lines at 2.12μm and 2.25μm, and in Hii Brγ at 2.16μm are presented.

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25

Arias, Lorena, and Margarita Rosado. "NIR Fabry-Perot Imaging Spectroscopy of PNe." Symposium - International Astronomical Union 209 (2003): 257–58. http://dx.doi.org/10.1017/s0074180900208668.

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Анотація:

In this work we report the radial velocity field of the molecular hydrogen in five planetary nebulae, obtained from scanning Fabry-Perot interferometer observatins at the near-infrared vibrationally excited line H2 S(1) 1-0 at 2.122 μm. Direct images of the nebulae in both transitions of molecular hydrogen S(1) 1-0 and S(1) 2-1 are used in order to discriminate between shocks and fluorescence as the excitation mechanism of the H2. Finally, some physical parameters are derived.

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26

Napiwotzki, R. "Central Stars of Old Planetary Nebulae." Symposium - International Astronomical Union 155 (1993): 88. http://dx.doi.org/10.1017/s0074180900170184.

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Анотація:

During a program for the observation of central stars of old planetary nebulae 29 stars are classified until now. Most of them (22) are belonging to the hydrogen-rich sequence and resemble either high-gravity sdO stars or white dwarfs (14 DAOs, 3 DAs). 3 are hydrogen-deficient PG 1159 stars, and also 3 are hybrid type stars Further 3 CPN are close binaries, whose evolution has been certainly influenced by this circumstance. A complete list of so far observed objects is given in Napiwotzki (1992).

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27

Medina, Selene, and Miriam Peña. "High-resolution spectroscopic study of the planetary nebula Abell 78." Symposium - International Astronomical Union 193 (1999): 374–75. http://dx.doi.org/10.1017/s0074180900205809.

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Анотація:

High resolution optical spectra of the planetary nebula Abell 78 were gathered, covering the wavelength ranges 3500–6600 Å and 3358–7361 Å, with a spectral resolution of ∼0.1 Å. Two different regions of Abell 78 are analyzed: the strongest knot near the central star which is thought to be a mass-loaded wind from the central star, and the outer, hydrogen-rich envelope. Physical parameters and chemical composition of both regions are estimated. The inner knot appears to be cooler and denser than the hydrogen-rich envelope. The chemical composition of the inner knot shows that there is almost no hydrogen near the central star and the emission is dominated by helium and processed material which has been dredged-up from the inner layers of the star. The outer, hydrogen-rich envelope appears to be deficient in heavy elements, mainly in oxygen, compared to the average abundances that are found among galactic planetary nebulae.

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28

D'antona, Francesca, and Italo Mazzitelli. "Pre-White Dwarf Evolution Through Planetary Nebulae." Symposium - International Astronomical Union 145 (1991): 399–409. http://dx.doi.org/10.1017/s0074180900227538.

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Анотація:

In the present review we summarize the problems relative to the chemical composition of the inner and outer layers of white dwarfs as expected from stellar evolution. We point out that there is a contrast between standard predictions and the indications deriving from studies of white dwarfs, as “massive” hydrogen remnat layers seem not to be present on single white dwarfs. We discuss a previously neglected feature of stellar models in the phase of thermal pulses -which occurs when the outer hydrogen envelope becomes very small- by which the progenitors of low total mass may get rid of practically the whole hydrogen envelope during the final phases of asymptotic giant branch evolution. We finally propose a new global scheme for the pre-white dwarf evolution, which depends mainly on the initial progenitor mass.

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29

Payne, P. W., J. W. V. Storey, B. L. Webster, M. A. Dopita, and S. J. Meatheringham. "The Systematics and Distribution of Molecular Hydrogen in Planetary Nebulae." Symposium - International Astronomical Union 131 (1989): 208. http://dx.doi.org/10.1017/s0074180900138197.

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The infrared S(1) line of molecular hydrogen has been searched for in twenty-two planetary nebulae using the imaging mode of the Anglo-Australian Telescope. The line was detected and mapped in eleven objects. It has been demonstrated that all those with strong excited molecular hydrogen belong to a subclass of the Type I planetary nebulae, morphologically consisting of an equatorial toroid with faint bipolar extensions. Furthermore, nearly all planetaries with these characteristics have strong molecular hydrogen. The molecular line ratios in the 2.0 to 2.5 micron window are consistent with shock excitation. The observations suggest that the morphology of these planetaries has been controlled by a fast stellar wind interacting with a disc of gas concentrated in the equatorial plane.

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30

Guerrero, Martin A., Quentin A. Parker, and Noam Soker. "Preface of “Asymmetric Planetary Nebulae 8e”." Galaxies 10, no. 4 (July 8, 2022): 81. http://dx.doi.org/10.3390/galaxies10040081.

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Planetary nebulae (PNe) are the progeny of low- and intermediate-mass stars, at the exact time in their late evolution when they eject their hydrogen-rich envelopes and start their transition towards white dwarfs [...]

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31

Barkov, Maxim V., Maxim Lyutikov, and Dmitry Khangulyan. "Fast-moving pulsars as probes of interstellar medium." Monthly Notices of the Royal Astronomical Society 497, no. 3 (June 8, 2020): 2605–15. http://dx.doi.org/10.1093/mnras/staa1601.

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ABSTRACT Pulsars moving through interstellar medium (ISM) produce bow shocks detected in hydrogen H α line emission. The morphology of the bow shock nebulae allows one to probe the properties of ISM on scales ∼0.01 pc and smaller. We performed 2D relativistic magnetohydrodynamic modelling of the pulsar bow shock and simulated the corresponding H α emission morphology. We find that even a mild spatial inhomogeneity of ISM density, δρ/ρ ∼ 1, leads to significant variations of the shape of the shock seen in H α line emission. We successfully reproduce the morphology of the Guitar Nebula. We infer quasi-periodic density variations in the warm component of ISM with characteristic length of ∼0.1 pc. Structures of this scale might be also responsible for the formation of the fine features seen at the forward shock of Tycho supernova remnant (SNR) in X-rays. Formation of such short periodic density structures in the warm component of ISM is puzzling, and bow-shock nebulae provide unique probes to study this phenomenon.

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32

Todt, Helga, Miriam Peña, Julia Zühlke, Lida Oskinova, Wolf-Rainer Hamann, and Götz Gräfener. "Weak emission line central stars of planetary nebulae." Proceedings of the International Astronomical Union 7, S283 (July 2011): 510–11. http://dx.doi.org/10.1017/s1743921312012173.

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AbstractTo understand the evolution and morphology of planetary nebulae, a detailed knowledge of their central stars is required. Central stars that exhibit emission lines in their spectra, indicating stellar mass-loss allow to study the evolution of planetary nebulae in action. Emission line central stars constitute about 10 % of all central stars. Half of them are practically hydrogen-free Wolf-Rayet type central stars of the carbon sequence, [WC], that show strong emission lines of carbon and oxygen in their spectra. In this contribution we address the weak emission-lines central stars (wels). These stars are poorly analyzed and their hydrogen content is mostly unknown. We obtained optical spectra, that include the important Balmer lines of hydrogen, for four weak emission line central stars. We present the results of our analysis, provide spectral classification and discuss possible explanations for their formation and evolution.

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33

Sánchez Contreras, Carmen. "(Sub)mm-Wavelength Observations of Pre-Planetary Nebulae and Young Planetary Nebulae." Galaxies 8, no. 1 (March 10, 2020): 21. http://dx.doi.org/10.3390/galaxies8010021.

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This is a non-comprehensive review of observations of pre-Planetary Nebulae (pPNe) and young Planetary Nebulae (yPNe) at (sub)mm-wavelengths, a valuable window for probing multi-phased gas and dust in these objects. This contribution focuses on observations of molecular lines (from carbon monoxide—CO—and other species), and briefly at the end, on hydrogen radio recombination lines from the emerging H ii regions at the center of yPNe. The main goal of this contribution is to show the potential of (sub)mm-wavelength observations of pPNe/yPNe to help the community to devise and develop new observational projects that will bring us closer to a better understanding of these latest stages of the evolution of low-to-intermediate (∼0.8–8 M ⊙ ) mass stars.

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34

Rosado, Margarita, Etienne le Coarer та Yvon Georgelin. "Results of the Hα Survey of the Small Magellanic Cloud". International Astronomical Union Colloquium 149 (1995): 150–54. http://dx.doi.org/10.1017/s0252921100022843.

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AbstractWe present the results of a kinematic survey of the Small Magellanic Cloud, performed with a scanning FP interferometer mainly at Hα. These data allow us to study the overall dynamics of the ionized hydrogen of this galaxy as well as the kinematics of classes of objects. In this presentation we exemplify how the availability of data in several wavelengths can be very useful in establishing the nature of large nebular complexes of the Small Magellanic Cloud. Our kinematic study allows us to discriminate faint shocked nebulae (such as supernova remnants) inside bright HII regions. This is confirmed by recently available radio and X-ray data.

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35

Leisy, P., and M. Dennefeld. "A Detailed Study of the Galactic Planetary Nebula G 258-15.7." Symposium - International Astronomical Union 155 (1993): 384. http://dx.doi.org/10.1017/s0074180900171736.

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The galactic Planetary Nebula G 258-15.7 is a large, bright nebula well suited for a detailed study. Known since Wray (1966), its morphology presents several blobs and ansae, generally associated with type I nebulae, and could be described as “late-butterfly” type according to the classification by Balick (1989). The central star has been classified as hydrogen-deficient by Mendez et al. (1985). Spectroscopy of the two main blobs shows a clear overabundance in He and N, with a marginally significant difference between the two sides. The most striking feature is the jet-like structure appearing on the [OIII]/Halpha picture (Fig. 1), the “jets” being located within the main blobs seen on the monochromatic images. A detailed appraisal of all the data will be presented in a subsequent paper.

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36

Storey, J. W. V., B. L. Webster, P. Payne, and M. A. Dopita. "The Distribution of Molecular Hydrogen in Planetary Nebulae." Symposium - International Astronomical Union 120 (1987): 339–40. http://dx.doi.org/10.1017/s0074180900154270.

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A correlation has been found between strong molecular hydrogen emission and the morphological type of a planetary nebula. Those with an equatorial toroid and bipolar extensions have H2 1−0 S(1) stronger than Brackett γ. H2 maps of several objects, and NGC 2346 in particular, are consistent with a fast stellar wind interacting with an anisotropic medium.

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37

Escalante, V., and A. Góngora-T. "Photodissociation Regions in Planetary Nebulae." Symposium - International Astronomical Union 155 (1993): 220. http://dx.doi.org/10.1017/s0074180900170822.

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The photodissociation rate of H2 molecules by UV photons from PN central stars is generally several orders of magnitude larger than the rate produced by the average interstellar field (Sternberg 1988, Escalante et al. 1991). Thus, in neutral envelopes of PN's, H2 molecules are destroyed quickly, and a photodissociation region forms around ionization bounded nebulae. Observations of H2 in PN's reveal that not all the hydrogen is photodissociated, and it has been suggested that this is due to the existence of dense disks around the ionized region (Zuckerman and Gatley 1988).

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38

Schönberner, Detlef. "Evolutionary Tracks for Central Stars of Planetary Nebulae." Symposium - International Astronomical Union 131 (1989): 463–72. http://dx.doi.org/10.1017/s0074180900139038.

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Our understanding of the evolution of Central Stars of Planetary Nebulae (CPN) has made considerable progress during the last years. This was possible since consistent computations through the asymptotic giant branch (AGB), with thermal pulses and (in some cases) mass loss taken into account, became available (Schönberner, 1979, 1983; Kovetz and Harpaz, 1981; Harpaz and Kovetz, 1981; Iben, 1982, 1984; Wood and Faulkner, 1986). It turned out that the evolution depends very sensitively on the inital conditions on the AGB. More precisely, the evolution of an AGB remnant is a function of the phase of the thermal-pulse cycle during which this remnant was created on the tip of the AGB by the planetary-nebula (PN) formation process (Iben, 1984, 1987). This was first shown by Schönberner (1979), and then fully explored by Iben (1984). In short, two major modes of PAGB evolution to the white dwarf stage are possible, according to the two main phases of a thermally pulsing AGB star: the hydrogen-burning or helium-burning mode. If, for instance, the PN formation, i.e. the removal of the stellar envelope by mass loss, happens during a luminosity peak that follows a thermal pulse of the helium-burning shell, the remnant leaves the AGB while still burning helium as the main energy supplier (Härm and Schwarzschild, 1975). On the other hand, PN formation may also occur during the quiescent hydrogen-burning phase on the AGB, and the remnant continues then to burn mainly hydrogen on its way to becoming a white dwarf.

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39

McCray, Richard. "Understanding the Nebular Spectrum of SN 1987A." International Astronomical Union Colloquium 145 (1996): 223–34. http://dx.doi.org/10.1017/s0252921100008095.

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The nebular spectra of supernovae differ from those of better-known emission nebulae in that many of the emission lines are optically thick. Here we sketch the theory for interpreting such spectra, and show how it can be used to interpret prominent emission line systems in the spectrum of SN 1987A. As examples, we describe: (1) a simple method to infer the density of OI from observations of the evolution of the doublet ratio in [O I]λλ6300; (2) new kind of hydrogen recombination hne spectrum; (3) an analysis showing that the Call infrared emission lines must come from primordial, not newly-synthesized, calcium; (4) a theory for the Fe/Co/Ni emission lines that shows that the inner envelope of SN 1987A must have a foamy texture, in which low density radioactive bubbles of Fe/Co/Ni reside in a massive substrate of hydrogen, helium, and other elements.

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40

Todt, H., B. Miszalski, J. A. Toalá, and M. A. Guerrero. "[WN] central stars of planetary nebulae." Proceedings of the International Astronomical Union 12, S323 (October 2016): 174–78. http://dx.doi.org/10.1017/s1743921317000540.

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AbstractWhile most of the low-mass stars stay hydrogen-rich on their surface throughout their evolution, a considerable fraction of white dwarfs as well as central stars of planetary nebulae have a hydrogen-deficient surface composition. The majority of these H-deficient central stars exhibit spectra very similar to massive Wolf-Rayet stars of the carbon sequence, i.e. with broad emission lines of carbon, helium, and oxygen. In analogy to the massive Wolf-Rayet stars, they are classified as [WC] stars. Their formation, which is relatively well understood, is thought to be the result of a (very) late thermal pulse of the helium burning shell. It is therefore surprising that some H-deficient central stars which have been found recently, e.g. IC 4663 and Abell 48, exhibit spectra that resemble those of the massive Wolf-Rayet stars of the nitrogen sequence, i.e. with strong emission lines of nitrogen instead of carbon. This new type of central stars is therefore labelled [WN]. We present spectral analyses of these objects and discuss the status of further candidates as well as the evolutionary status and origin of the [WN] stars.

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41

Barlow, M. J., X. W. Liu, D. Péquignot, P. J. Storey, Y. G. Tsamis, and C. Morisset. "Recombination-line Abundances for the Third-Row Element, Magnesium." Symposium - International Astronomical Union 209 (2003): 373–74. http://dx.doi.org/10.1017/s0074180900209054.

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We have obtained optical recombination-line abundances for the third-row element magnesium in ten planetary nebulae. The nebulae in our sample include four (NGC 7009, NGC 6153, M 2–36 and M 1–42) for which we have previously found very large enhancements, relative to solar, in the recombination line abundances of the second-row elements carbon, nitrogen, oxygen and neon (Liu et al. 1995, Liu et al. 2000, Liu et al. 2001). Nebular temperature fluctuations appear unable to account for these effects. However, models that invoke high-density clumps, particularly clumps which are hydrogen-deficient, appear more viable as an explanation for the high recombination line abundances that are observed (see Liu et al. 2000 for more details). While it may be possible to appeal to astrophysical nucleosynthetic processes to enhance the CNONe second-row elements in AGB stars, such effects are not expected to modify the abundances of third-row elements such as magnesium. We have therefore measured and dereddened the intensities of the 4481 Å 4f-3d line of Mg II, relative to Hβ, in order to investigate whether magnesium recombination line abundances are enhanced or not.

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42

García-Hernández, D. A., A. Manchado, P. García-Lario, C. Domínguez-Tagle, G. M. Conway, and F. Prada. "Nature of the H2 Emission Around Planetary Nebulae Precursors." Symposium - International Astronomical Union 209 (2003): 135. http://dx.doi.org/10.1017/s0074180900208334.

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We present near-IR spectroscopy of a sample of 30 IRAS sources recently identified as late AGB stars, post-AGB stars or early PNe. The spectra obtained are centred at various wavelengths covering the molecular hydrogen v=1→0 S(1) 2.122 μm and v=2→1 S(1) 2.248 μm emission lines, the recombination lines of hydrogen Brγ 2.166 μm, Pfγ 3.741 μm and Brα 4.052 μm, and the CO[v=2→0] first overtone bandhead at 2.294 μm. As a result of these observations we have increased from 4 to 13 the total number of proto-PNe detected in H2 and we have confirmed that the onset of H2 emission takes place in the post-AGB phase. When the molecular hydrogen is fluorescence-excited the detection rate is found to be directly correlated with the evolutionary stage of the central star, rather than with the nebular morphology. In contrast, shocked-excited H2 is detected only in strongly bipolar proto-PNe, sometimes even at an early stage in the post-AGB phase. The strong correlation of shocked-excited H2 emission with bipolarity found confirms the result previously reported by Kastner et al. (1996) in evolved PNe. However, our results show that this correlation does not exist in the case of fluorescence-excited molecular hydrogen. (to be published in A&A).

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43

Andrews, H., E. Peeters, A. G. G. M. Tielens, and Y. Okada. "Whipping IC 63/IC 59." Astronomy & Astrophysics 619 (November 2018): A170. http://dx.doi.org/10.1051/0004-6361/201832808.

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Context. IC 63 and IC 59 are a pair of cometary-shaped nebulae in the vicinity of the star γCas (also known as Tsih, “the Whip”). Both nebulae have very different optical appearances, despite the fact that both objects lie at similar projected distances from the star: IC 63 shows bright rims and filaments, while IC 59 looks more homogeneous and faint. Aims. We aim to perform a general description of the two nebulae from an observational standpoint in order to study the physical conditions at the UV-illuminated surfaces of these objects. Methods. We used the available data on both nebulae taken with Spitzer and Herschel to study the infrared emission at the tip of both clouds, and derive the intensity of the UV radiation field. Using the photodetector array camera and spectrometer (PACS) in line spectroscopy mode, we obtained the intensity of the cooling lines [C II] 157 μm and [O I] 63 μm, and we used these to estimate the density in these regions. Excitation diagrams of molecular hydrogen were obtained to derive the gas temperature. We also used [C II] 157 μm velocity maps of IC 59 taken with German REceiver for Astronomy at Terahertz frequencies (GREAT) on board SOFIA to explore the gas dynamics in this nebula. Results. We find that the IR emission from polycyclic aromatic hydrocarbons (PAHs) is very similar at the tip of both nebulae. Even though it varies in intensity between the two, the derived PAH band ratios are remarkably similar. These ratios are similar to those found in the more shielded regions of other nebulae such as NGC 7023 and NGC 2023. Regarding the physical conditions, we get that while in IC 63 the intensity of the UV field, G0, is a factor of approximately ten higher than in IC 59, the density n at the tip of IC 59 is lower than in IC 63 by a similar factor. For both objects we derive G0 values significantly lower than what previous works have so far assumed. Comparison with other reflection nebulae photo-dissociation regions (PDRs) and known correlations support our claim that both IC 63 and IC 59 are low-UV irradiated environments. Conclusions. We conclude that the tips of IC 63 and IC 59 are about three and five times farther away from the star than their respective projected distances. The similarity of the mid-infrared emission between the two nebulae is consistent not only with both objects being overdensities within the same region around γCas, but it is also consistent with the similar G0/n and ionization parameters, which altogether rule the evolution of the hydrogenation and ionization level of the emitting population of PAHs. Finally, regarding the kinematics of the material in IC 59, we find evidence of photo-evaporation due to the incident radiation from γCas.

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44

Aleman, Isabel, and Ruth Gruenwald. "Molecular Hydrogen in the Ionized Region of Planetary Nebulae." Astrophysical Journal 607, no. 2 (June 2004): 865–72. http://dx.doi.org/10.1086/383562.

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45

Sandin, C., M. Steffen, D. Schönberner, and U. Rühling. "Hot bubbles of planetary nebulae with hydrogen-deficient winds." Astronomy & Astrophysics 586 (January 26, 2016): A57. http://dx.doi.org/10.1051/0004-6361/201527357.

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46

Wang, Mei-Yan, C. Muthumariappan, and Sun Kwok. "Molecular Hydrogen Jets, Filaments, and Haloes in Planetary Nebulae." Proceedings of the International Astronomical Union 2, S234 (April 2006): 537. http://dx.doi.org/10.1017/s1743921306004121.

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47

McNabb, Ian A., Xuan Fang, Xiao–Wei Liu, and Peter J. Storey. "Plasma diagnostics for planetary nebulae and H ii regions using N ii and O ii optical recombination lines." Proceedings of the International Astronomical Union 7, S283 (July 2011): 432–33. http://dx.doi.org/10.1017/s1743921312011787.

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AbstractWe carry out plasma diagnostic analysis for a number of planetary nebulae (PNe) and H ii regions. We use N ii and O ii optical recombination lines (ORLs) with new effective recombination coefficients calculated under the intermediate coupling scheme, for a range of electron temperatures (Te) and densities (Ne), and fitted against the most reliable measurements. Comparing Te derived from ORLs, collisionally excited lines (CELs), the hydrogen Balmer Jump, and/or He i if available, we find the relation Te (ORLs) < Te (He i) < Te (H i BJ) < Te (CELs), confirming the physical conditions in the bi-abundance model postulated by Liu et al., i.e. the nebula contains another cold, metal-rich and probably H-deficient component.

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48

Danehkar, A. "Physical and Chemical Properties of Wolf–Rayet Planetary Nebulae." Astrophysical Journal Supplement Series 257, no. 2 (December 1, 2021): 58. http://dx.doi.org/10.3847/1538-4365/ac2310.

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Abstract Wolf–Rayet ([WR]) and weak-emission-line (wels) central stars of planetary nebulae (PNs) have hydrogen-deficient atmospheres, whose origins are not well understood. In the present study, we have conducted plasma diagnostics and abundance analyses of 18 Galactic PNs surrounding [WR] and wels nuclei, using collisionally excited lines (CELs) and optical recombination lines (ORLs) measured with the Wide Field Spectrograph on the Australian National University 2.3 m telescope at the Siding Spring Observatory complemented with optical archival data. Our plasma diagnostics imply that the electron densities and temperatures derived from CELs are correlated with the intrinsic nebular Hβ surface brightness and excitation class, respectively. Self-consistent plasma diagnostics of heavy-element ORLs of N2+ and O2+ suggest that a small fraction of cool (≲7000 K), dense (∼104–105 cm−3) materials may be present in some objects, though with large uncertainties. Our abundance analyses indicate that the abundance discrepancy factors (ADFs ≡ ORLs/CELs) of O2+ are correlated with the dichotomies between forbidden-line and He i temperatures. Our results likely point to the presence of a tiny fraction of cool, oxygen-rich dense clumps within diffuse warm ionized nebulae. Moreover, our elemental abundances derived from CELs are mostly consistent with asymptotic giant branch models in the range of initial masses from 1.5 to 5 M ⊙. Further studies are necessary to understand better the origins of abundance discrepancies in PNs around [WR] and wels stars.

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49

Clegg, R. E. S., N. A. Walton, and M. J. Barlow. "Near–Infrared Imaging of Proto-Planetary Nebulae." Symposium - International Astronomical Union 155 (1993): 340. http://dx.doi.org/10.1017/s0074180900171293.

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It is not really known how low and intermediate mass stars eject mass to form PNs. We present preliminary results from a programme of near–IR imaging, in which we study a sequence of objects, from extreme AGB stars through proto–planetaries to young, compact PNs. We aim to study the sequence of morphologies, to see where the onset of bipolar shaping occurs, and to use the IR molecular hydrogen lines to map neutral regions around ionized nebulae.

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50

Kondratjeva, L. N. "Planetary Nebulae by the Strong [NII] Emission Lines." Symposium - International Astronomical Union 155 (1993): 195. http://dx.doi.org/10.1017/s0074180900170573.

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The sample of planetaries with the strong [NII] emission lines are analysed. All available parameters are discussed. The enrichments of all elements and the tendency to rather low surface brightness of ne bulae in Hβ flux are remarked. The deficiency of hydrogen in envelopes is proposed as the possible reason of observational spectra.

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