Planetary Nebula IC Research Articles

Spectroscopic study of the variable planetary nebula IC 4997

Based on our spectroscopic observations of the variable planetary nebula IC 4997 in 2003–2009, we have obtained the relative fluxes in optical emission lines. The interstellar extinction c = 0.35 has been found from the Balmer decrement by taking into account the effect of self-absorption in hydrogen lines in dense nebular regions. The variations in the Balmer decrement point to variability of the self-absorption and circumstellar extinction. We have investigated the variations in the relative intensities of some spectral lines and their ratios with time. The drop in the ratios F(λ4363)/F(Hγ) and F(λ363)/F(λ4959) that began back in 1990–1995 has continued, suggesting a decrease in the electron density and temperature in the central nebular region. The ratio F(λ6731)/F(λ6717) has remained constant. It gives an estimate for the electron density in the outer regions of IC 4997, N e ∼ 104 cm−3.

A many-year photometric and spectroscopic variability study of planetary nebulae (1968–2008). The young planetary nebula IC 4997

We present the results of our many-year (1968–2008) photoelectric photometry and absolute spectrophotometry of the young variable planetary nebula IC 4997, performed in uniform systems. Integrated line energy fluxes in the range 3727–5007 A are tabulated, along with the integrated (star + nebula) continuum flux at 4500 A, and time variations investigated. We study the time behavior of fluxes in the hydrogen, HeI, [OIII], [OII], [NeIII] emission lines. Variations of the line intensity ratio R = F([OIII]4363 A)/F(Hγ) between 1938 and 2008 are presented. We estimate the nebula’s electron density Ne and electron temperature Te for 1972–1992: the mean nebula Ne increased by the factor of five, from ∼4 × 105 to ∼2 × 106 cm−3, while the mean Te increased from 12 000 to 14 000 K. The color temperature of the exciting star, measured from the 4000–4900 A continuum, increased from 37 000–40 000 K to 47 000 K during the time interval covered by our observations, as is confirmed by the growth of ionization of HeI, NeIII, and OIII (from the 4363 A line). V-band variations of the object’s integrated (nebula + star) light with an amplitude about 0.5m can essentially be fully explained as being due to changing contributions from variable fluxes in the [OIII] 5007 and 4959 A lines. The V magnitude in 2009 (after 40 years) happened to be the same as at the start of our observations in 1968. A period of the order of 50 years can also be noted in log R(t). This may provide evidence for binarity of the central star and be related to its orbital period.

A self-consistent stellar and 3D nebular model of planetary nebula IC 418

We present a coherent stellar and nebular model reproducing the observations of the Planetary Nebula IC418. We want to test whether a stellar model obtained by fitting the stellar observations is able to satisfactory ionize the nebula and reproduce the nebular observations, which is by no mean evident. This allows us to determine all the physical parameters of both the star and the nebula, including the abundances and the distance. We used all the observational material available (FUSE, IUE, STIS and optical spectra) to constrain the stellar atmosphere model performed using the CMFGEN code. The photoionization model is done with Cloudy_3D, and is based on CTIO, Lick, SPM, IUE and ISO spectra as well as HST images. More than 140 nebular emission lines are compared to the observed intensities. We reproduce all the observations for the star and the nebula. The 3D morphology of the gas distribution is determined. The effective temperature of the star is 36.7kK. Its luminosity is 7700 solar luminosity. We describe an original method to determine the distance of the nebula using evolutionary tracks. No clumping factor is need to reproduce the age-luminosity relation. The distance of 1.25 kpc is found in very good agreement with recent determination using parallax method. The chemical composition of both the star and the nebula are determined. Both are Carbon-rich. The nebula presents evidence of depletion of elements Mg, Si, S, Cl (0.5 dex lower than solar) and Fe (2.9 dex lower than solar). This is the first self-consistent stellar and nebular model for a Planetary Nebula that reproduces all the available observations ranging from IR to UV, showing that the combined approach for the modeling process leads to more restrictive constraints and, in principle, more trustworthy results.

EXPANSION PARALLAX OF THE PLANETARY NEBULA IC 418

In this paper, we present radio continuum observations of the planetary nebula IC 418 obtained at two epochs separated by more than 20 years. These data allow us to show that the angular expansion rate of the ionization front in IC 418 is 5.8 $\pm$ 1.5 mas yr$^{-1}$. If the expansion velocity of the ionization front is equal to the expansion velocity of the gas along the line of sight as measured by optical spectroscopy, then the distance to IC 418 must be 1.1 $\pm$ 0.3 kpc. Recent theoretical predictions appropriate for the case of IC 418, however, suggest that the ionization front may be expanding about 20% faster than the material. Under this assumption, the distance to IC 418 would increase to 1.3 $\pm$ 0.4 kpc.

Emission line profiles as a probe of physical conditions in planetary nebulae

We present an analysis of physical conditions in planetary nebulae (PNe) in terms of collisionally-excited line (CEL) and optical-recombination line (ORL) profiles. We aim to investigate whether line profiles could be used to study the long-standing CEL/ORL abundance-discrepancy problem in nebular astrophysics. Using 1D photoionization models and their assumed velocity fields, we simulate the line profiles of various ionic species. We attempt to use our model to account for the observed CEL and ORL profiles. As a case study we present a detailed study of line profiles of the low-excitation planetary nebula (PN) IC 418. Our results show that the profiles of classical temperature and density diagnostic lines, such as [O III] 4363,5007, [S II] 6716,6731, and [Ar IV] 4711,4740, provide a powerful tool to study nebular temperature and density variations. The method enables the CEL/ORL abundance-discrepancy problem to be studied more rigorously than before. A pure photoionization model of a chemically-homogeneous nebula seems to explain the observed disagreements in the profiles for the [O III] 4363 and the 5007 lines, but cannot account for the differences between the [O III] CELs and the O II ORLs. We also investigate the temperature and density variations in the velocity space of a sample of PNe, which are found to be insignificant.

Multiple and Precessing Collimated Outflows in the Planetary Nebula IC 4634

With its remarkable double-S shape, IC 4634 is an archetype of point-symmetric planetary nebulae (PNe). In this paper, we present a detailed study of this PN using archival HST WFPC2 and ground-based narrowband images to investigate its morphology, and long-slit spectroscopic observations to determine its kinematics and to derive its physical conditions and excitation. The data reveal new structural components, including a distant string of knots distributed along an arclike feature 40 -->''-60 -->'' from the center of the nebula, a skin of enhanced [O III]/Hα ratio enveloping the inner shell and the double-S feature, and a triple-shell structure. The spatio-kinematical study also finds an equatorial component of the main nebula that is kinematically independent of the bright inner S-shaped arc. We have investigated in detail the bow shock-like features in IC 4634 and found that their morphological, kinematical, and emission properties are consistent with the interaction of a collimated outflow with surrounding material. Indeed, the morphology and kinematics of some of these features can be interpreted using a three-dimensional numerical simulation of a collimated outflow precessing at a moderate, time-dependent velocity. Apparently, IC 4634 has experienced several episodes of point-symmetric ejections oriented in different directions, with the outer S-shaped feature being related to an earlier point-symmetric ejection, and the outermost arclike string of knots being the relic of a still much earlier point-symmetric ejection. There is tantalizing evidence that the action of these collimated outflows has also taken part in the shaping of the innermost shell and inner S-shaped arc of IC 4634.

Distant future of the Sun and Earth revisited

We revisit the distant future of the Sun and the solar system, based on stellar models computed with a thoroughly tested evolution code. For the solar giant stages, mass-loss by the cool (but not dust-driven) wind is considered in detail. Using the new and well-calibrated mass-loss formula of Schroder & Cuntz (2005, 2007), we find that the mass lost by the Sun as an RGB giant (0.332 M_Sun, 7.59 Gy from now) potentially gives planet Earth a significant orbital expansion, inversely proportional to the remaining solar mass. According to these solar evolution models, the closest encounter of planet Earth with the solar cool giant photosphere will occur during the tip-RGB phase. During this critical episode, for each time-step of the evolution model, we consider the loss of orbital angular momentum suffered by planet Earth from tidal interaction with the giant Sun, as well as dynamical drag in the lower chromosphere. We find that planet Earth will not be able to escape engulfment, despite the positive effect of solar mass-loss. In order to survive the solar tip-RGB phase, any hypothetical planet would require a present-day minimum orbital radius of about 1.15 AU. Furthermore, our solar evolution models with detailed mass-loss description predict that the resulting tip-AGB giant will not reach its tip-RGB size. The main reason is the more significant amount of mass lost already in the RGB phase of the Sun. Hence, the tip-AGB luminosity will come short of driving a final, dust-driven superwind, and there will be no regular solar planetary nebula (PN). But a last thermal pulse may produce a circumstellar (CS) shell similar to, but rather smaller than, that of the peculiar PN IC 2149 with an estimated total CS shell mass of just a few hundredths of a solar mass.

TheSpitzerIRS Infrared Spectrum and Abundances of the Planetary Nebula IC 2448

We present the mid-infrared spectrum of the planetary nebula IC 2448. In order to determine the chemical composition of the nebula, we use the infrared line fluxes from the Spitzer spectrum along with optical line fluxes from the literature and ultraviolet line fluxes from archival IUE spectra. We determine an extinction of C(H-beta) = 0.27 from hydrogen recombination lines and the radio to H-beta ratio. Forbidden line ratios give an electron density of 1860 cm-3 and an average electron temperature of 12700 K. The use of infrared lines allows us to determine more accurate abundances than previously possible because abundances derived from infrared lines do not vary greatly with the adopted electron temperature and extinction, and additional ionization stages are observed. Elements left mostly unchanged by stellar evolution (Ar, Ne, S, and O) all have subsolar values in IC 2448, indicating that the progenitor star formed out of moderately metal deficient material. Evidence from the Spitzer spectrum of IC 2448 supports previous claims that IC 2448 is an old nebula formed from a low mass progenitor star.

IC 5217 as a double-shell, point-symmetric planetary nebula with a very narrow waist

Aims. Identification of the structural components and analysis of the internal kinematics in the planetary nebula IC 5217. Methods. Narrow-band images and high resolution long-slit spectra in the Hα ,[ Nii ]a nd [Oiii] emission lines, and VLA 6 cm continuum data. Results. IC 5217 is composed of a very bright equatorial ring, open bipolar lobes, off-axis point-symmetric features, on-axis distant faint regions, and an off-axis very elongated bipolar structure. The ring, open lobes, point-symmetric features and distant faint regions appear as elements of a single point-symmetric bipolar shell with a very narrow waist. This shell presents an axis ratio of ∼37 and an aspect ratio of ∼5. The 6 cm data show that the ring is an extremely flat disk with a central hole. Expansion velocity in the ring ranges from ≤10 km s −1 in He ii up to � 27 km s −1 in [N ii], whereas a velocity of ∼460 km s −1 is estimated for the polar regions of the bipolar shell. Strong acceleration of the outer regions of the ring is observed. The elongated bipolar structure probably represents a highly collimated (aspect ratio ∼12), high velocity cylindrical-like shell. A collimated agent (wind or jet) would account for the shaping of the bipolar shell if this agent has operated in the direction perpendicular to the equatorial disk. The point-symmetric features and cylindrical shell are probably related to collimated ejections but that occurred when the basic nebular shape had already been established.

Raman‐scattered Heiiλλ4850, 6545 in the Young and Compact Planetary Nebula IC 5117

We report the discovery of He II λλ4850 and 6545 Raman-scattered by atomic hydrogen in the high-resolution spectrum of the very compact and young planetary nebula IC 5117 obtained with the ESPaDOnS (Echelle Spectropolarimetric Device for the Observation of Stars) installed on the 3.6 m Canada-France-Hawaii Telescope. By adopting case B recombination atomic data and considering all the fine-structure emission components, we calculate the weighted averages for the line centers of the Raman-scattered features, as well as those for the emission lines He II λλ4859 and 6560 to determine redward velocity shifts of Δv4850 = +29 km s-1 and Δv6545 = +24 km s-1. We compute the Raman conversion efficiency by a Monte Carlo technique adopting a simple scattering geometry, in which the hot central star is surrounded by a hollow cylindrical H I region characterized by thickness T that is in turn parameterized by the H I column density. It is proposed that the central far-UV emission region is covered significantly by the neutral scattering region characterized by the H I column density N 4 × 1021 cm-2. The H I column density is higher by an order of magnitude than the value proposed from 21 cm radio observations. We briefly discuss the importance of Raman scattering as a probe of neutral material around a newly formed hot white dwarf and the mass-loss process occurring in the late stage of stellar evolution.

Identification and Characterization of Faint Emission Lines in the Spectrum of the Planetary Nebula IC 418

We present high signal-to-noise ratio echelle spectra of the compact high surface brightness, low-ionization planetary nebula (PN) IC 418. These reveal 807 emission lines down to intensities less than 10-5 that of Hβ for which we determine widths and relative intensities. We show that line profiles are a valuable parameter for making line identifications and in constraining the excitation mechanism of the lines. We present evidence that indicates that many supposed high-level recombination lines may in fact be excited by a process other than recombination. We contend from the detection of dielectronic recombination lines that their relatively low intensities argue against their making a significant contribution to level populations of the heavy ions in this object. Following similar analyses of other PNe we find that IC 418 shows a small discrepancy in ion abundances derived from forbidden versus recombination lines of the heavy elements.

Introducing EMILI: Computer‐aided Emission Line Identification

The identification of spectral lines can be a tedious process requiring the interrogation of large spectroscopic databases, but it does lend itself to software algorithms that can determine the characteristics of candidate line identifications. We present here criteria used for the identification of lines and a logic developed for a line identification software package called EMILI, which uses the v2.04 Atomic Line List as the basic line database. EMILI considers all possible database transitions within the wavelength uncertainties for observed optical emission lines and computes an approximate intensity for each candidate line. It searches for other multiplet members that are expected to be seen with each candidate line, and rank-orders all of the tentative line identifications for each observed line based on a set of criteria. When applied to the spectra of the Orion Nebula and the planetary nebula IC 418, EMILI's recommended line IDs agree well with those of previous traditional manual line assignments. The existence of a semiautomated procedure should give impetus to the study of very high signal-to-noise spectra, enabling the identification of previously unidentified spectral lines to be handled with ease and consistency.

VLA observations at 7 mm of the planetary nebula IC 4997

We present high angular resolution (0.1 arcsec) VLA observations at 7-mm wavelength towards the bipolar planetary nebula IC 4997. The 7-mm emission in IC 4997 is dominated by a bright, compact core, the flux density of which at this wavelength is about six times brighter than at 2.0 and 3.6 cm. We model the centimetre and millimetre continuum spectrum in terms of free–free emission from an ionized core–halo source. In this model a dense (2.6 × 106 cm−3), optically thick inner core dominates the millimetre emission, while a low-density (1 × 104 cm−3), optically thin halo dominates the centimetre emission. The spectral index of the core (between 2 cm and 7 mm) is α≃ 1.6 ± 0.1(Sν∝να), which is consistent with optically thick free–free emission. An upper limit for the mass-loss rate that created this inner core is ∼ 3 × 10−6 M⊙ yr−1, suggesting that this core may have been formed at the end of the superwind phase.

Multiwavelength Observations of the Peculiar Planetary Nebula IC 2149

We report high- and low-dispersion spectroscopy, optical imaging, and high-resolution Very Large Array-A λ3.6 cm continuum observations of the peculiar planetary nebula IC 2149. These observations show that "bipolar" is a suitable morphological classification for IC 2149. Most nebular material is concentrated in a knotty, bright ring seen edge-on, embedded in an apparently oblate ellipsoidal shell from which remnant or incipient bipolar lobes emerge. We confirm the previously reported depletion in heavy elements and deduce a very low ejected nebular mass ≲0.03 M☉. All this information indicates that the formation of IC 2149 is the result of the evolution of a low-mass central star.

Optical Spectrum of the Compact Planetary Nebula IC 5117

High resolution spectroscopic data of the very compact planetary nebula IC 5117 are obtained in the optical wavelengths, 3700A - 10050A, with the Hamilton Echelle Spectrograph at Lick Observatory, and which have been analyzed along with the International Ultraviolet Explorer (IUE) UV archive data. Although a diagnostic diagram shows significant density and temperature fluctuations, our analysis indicates that the nebular gas may be represented by a homogeneous shell of extremely high density gas, N(sub epsilon) approx. 90 000 /cu cm. The average electron temperatures, e.g. indicated by the [OIII] diagnostics, are around 12 000 K. We construct a photoionization model to represent most of the observed line intensities, and the physical condition of this compact nebulosity. Based on the semi-empirical ionization correction approach, and model indications, we derived the elemental abundances: He, C, N, O, Ne, and Ar appear to be normal or marginally depleted compared to the average planetary nebula, while the remaining elements, S, Cl, and K appear to be enhanced. IC 5117 is perhaps a very young compact planetary nebula, slightly more evolved than the other well-known compact planetary nebula IC 4997. The central stellar temperature is likely to be around 120 000 K, evolved from a C-rich AGB progenitor.

Spectroscopic Observation of the Planetary Nebula IC 4846

ABSTRACTA high‐dispersion optical (3700–10100 Å) spectrum was secured with the Hamilton echelle spectrograph at the coudé focus of the 120 inch (3 m) Shane telescope at Lick Observatory, with the goal of deriving chemical abundances for the compact planetary nebula IC 4846. We also remeasured the UV spectra from the IUE archive. Diagnostics indicate that the entire planetary nebula may be represented by an electron density Nϵ≃9000 cm−3 and electron temperature Tϵ≃10,500 K. However, diagnostics seem to imply that the [O ii] and [S ii] zone electron temperatures are higher than the [O iii] radiating strata. The electron density of the [O ii] and [S ii] zone might be higher as well (for which Nϵ≃20,000 cm−3). The photoionization model, which represents most of the observed line intensities, seems to confirm this physical condition, although the actual situation may be much more complicated. On the basis of the semiempirical ionization correction method and the photoionization model calculation, we derived the chemical abundances in this planetary nebula. Our optical data were compared with the image‐tube scanner data from Aller & Czyzak. Apparently, there is a long‐term spectral line variation in IC 4846. Chemical abundances derived in the present study agree, within 30%, with those previously estimated in Aller & Czyzak, except for carbon; the C, N, O, and Ne ratios are lower than both the average planetary nebula and the solar abundance. IC 4846 is a metal‐deficient planetary nebula that may have evolved from a Population II progenitor.

Theoretical emission line ratios for [Fe III] and [Fe VII] applicable to the optical and infrared spectra of gaseous nebulae.

Recent calculations of electron impact excitation rates and Einstein A-coefficients for transitions among the 3d(6) levels of Fe III and among the 3d(2) levels of Fe VII are used to derive theoretical emission line ratios applicable to the optical and infrared spectra of gaseous nebulae. Results for [Fe III] are generated for electron temperatures T(e) = 7,000-20,000 K and densities N(e) = 10(2)-10(8) cm(-3), whereas those for [Fe VII] are provided for T(e) = 10,000-30,000 K and N(e) = 10(2)-10(8) cm(-3). The theoretical line ratios are significantly different in some instances from earlier calculations and resolve discrepancies between theory and observation found for the planetary nebulae IC 4997 and NGC 7027.

Abundances in the Planetary Nebula IC 5217

High resolution optical wavelength spectroscopic data were secured in the optical wavelengths, 3700A - 10,050A, for the planetary nebula IC 5217 with the Hamilton Echelle Spectrograph at Lick Observatory. These optical spectra have been analyzed along with the near-UV and UV archive data. Diagnostic analyses indicate a nebular physical condition with electron temperature of about 10,700 K (from the [O III] lines) and the density of N(sub epsilon) = 5000/cm. Ionic concentrations have been derived with the representative diagnostics, and with the aid of a photoionization model construction, we derived the elemental abundances. Contrary to the previous studies found in the literature, He and C appear to be depleted compared to the average planetary nebula and to the Sun (and S marginally so), while the remaining elements appear to be close to the average value. IC 5217 may have evolved from an O-rich progenitor and the central star temperature of IC 5217 is likely to be 92,000 K.

Precessing collimated outflows in the planetary nebula IC 4846

We present [N II] and H-alpha images and high resolution long-slit spectra of the planetary nebula IC 4846, which reveal, for the first time, its complex structure and the existence of collimated outflows. The object consists of a moderately elongated shell, two (and probably three) pairs of collimated bipolar outflows at different orientations, and an attached circular shell. One of the collimated pairs is constituted by two curved, extended filaments whose properties indicate a high velocity, bipolar precessing jet. A difference of \~10 km/s is found between the systemic velocity of the precessing jets and the centroid velocity of the nebula, as recently report for Hu 2-1. We propose that this difference is due to orbital motion of the ejection source in a binary central star. The orbital separation and period estimates for the binary star are less than or equal to 30 AU and 100 yr, respectively. These are similar to those previously estimated for Hu 2-1, linking the central stars of both planetary nebulae to interacting binaries. Extraordinary similarities also exist between IC 4846 and the bewildering planetary nebula NGC 6543, suggesting a similar formation history for both objects.

The Near-Infrared Spectrum of the Planetary Nebula IC 5117

Infrared spectroscopy from 0.8 to 2.5 μm is presented for the planetary nebula IC 5117. The emission lines of IC 5117 span a wide range of ionization that includes He II, [S III], [S II], [N I], and H2. The reddening measured from the hydrogen lines is E(B-V) = 0.79, most of which is probably interstellar in origin. The He/H abundance ratio is 0.113 ± 0.015, with approximately 10% of the helium being doubly ionized. Using our measurements of [S II] and [S III] lines and published observations of [S IV], we find a sulfur abundance, relative to hydrogen, of N(S)/N(H) = 7.8 × 10-6, or approximately half the solar value. Fluxes and flux limits for several lines of molecular hydrogen are presented. Measurements of 1–0 transitions, together with the limits on 2–1 transitions, indicate Tvib ~ Trot = 1900 K, suggesting a purely collisional excitation mechanism. The ortho-to-para ratio is ~3, a value that is also indicative of collisional excitation. The presence of [C I] λ9850 is consistent with previous studies of IC 5117 that indicated carbon is more abundant than oxygen. IC 5117 follows the trend of planetary nebulae that display bipolar outflows and H2 emission to be carbon-rich. We confirm the results of Zhang & Kwok, who reported infrared continuum emission substantially in excess of that produced by the ionized gas. This emission is most likely due to hot dust (T ~ 1300 K) and accounts for roughly half of the continuum between 1.5 and 2 μm.