A unique Galactic planetary nebula with a [WN] central star

We report the discovery of the first probable Galactic [WN] central star of a planetary nebula (CSPN). The planetary nebula candidate was found during our systematic scans of the AAO/UKST Hα Survey of the Milky Way. Subsequent confirmatory spectroscopy of the nebula and central star reveals the remarkable nature of this object. The nebular spectrum shows emission lines with large expansion velocities exceeding 150 km s⁻¹, suggesting that perhaps the object is not a conventional planetary nebula. The central star itself is very red and is identified as being of the [WN] class, which makes it unique in the Galaxy. A large body of supplementary observational data supports the hypothesis that this object is indeed a planetary nebula and not a Population I Wolf–Rayet star with a ring nebula.     

Tracing the star stream through M31 using planetary nebula kinematics

We present a possible orbit for the Southern Stream of stars in M31, which connects it to the Northern Spur. Support for this model comes from the dynamics of planetary nebulae (PNe) in the disc of M31: analysis of a new sample of 2611 PNe obtained using the Planetary Nebula Spectrograph reveals ∼20 objects with kinematics inconsistent with the normal components of the galaxy, but which lie at the right positions and velocities to connect the two photometric features via this orbit. The satellite galaxy M32 is coincident with the stream both in position and velocity, adding weight to the hypothesis that the stream comprises its tidal debris.     

The birth of a planetary nebula around the carbon star IRC+10216

Our current understanding of the evolution of solar-type stars suggests that after a period as a red giant star, during which mass loss occurs continuously in the form of a stellar wind, a period of intense mass loss known as a superwind occurs, during which a significant fraction of the envelope of the star is ejected into space, forming the material from which a planetary nebula (PN) will be constructed. It has been suggested that this superwind ejects material from the star in a toroidal or disc-like fashion, rather than isotropically. Here we present Hubble Space Telescope optical images of a toroidal superwind caught in the act: our images of the carbon star IRC+10216, which is believed to be in the final stages of red giant evolution, show that most of its optical emission is a bipolar reflection nebula. We show that the full spectral energy distribution and these images can be modelled as an equatorially enhanced dusty superwind, providing the first direct observational support for the toroidal superwind model, and supporting the 'interacting winds' model of PN formation.     

M4–18: the planetary nebula and its WC10 central star

We present a detailed analysis of the planetary nebula M4–18 (G146.7+07.6) and its WC10-type Wolf–Rayet (WR) central star, based on high‐quality optical spectroscopy (WHT/UES, INT/IDS, WIYN/DensPak) and imaging ( HST /WFPC2). From a non-LTE model atmosphere analysis of the stellar spectrum, we derive T _eff=31 kK,     v _∞=160 km s⁻¹ and abundance number ratios of H/He<0.5, C/He=0.60 and O/He=0.10. These parameters are remarkably similar to those of He 2–113 ([WC10]). Assuming an identical stellar mass to that determined by De Marco et al. for He 2–113, we obtain a distance of 6.8 kpc to M4–18 [ E ( B−V )=0.55 mag from nebular and stellar techniques]. This implies that the planetary nebula of M4–18 has a dynamical age of ∼3100 yr, in contrast to ≥270 yr for He 2–113. This is supported by the much higher electron density of the latter. These observations may be reconciled with evolutionary predictions only if [WC]-type stars exhibit a range in stellar masses.
Photoionization modelling of M4–18 is carried out using our stellar WR flux distribution, together with blackbody and Kurucz energy distributions obtained from Zanstra analyses. We conclude that the ionizing energy distribution from the WR model provides the best consistency with the observed nebular properties, although discrepancies remain.     

Recent observations of the WR central star of the planetary nebula LMC-N66

The recent evolution of the central star of the planetary nebula LMC-N66 is presented. Before 1987, it showed a weak continuum with aT _eff120 000 K andL _bol25 000L and in a few years it developed strong WR features (P Cygni line profiles in N v at 124.0 nm and C IV at 155.0 nm, wide Heii emission, etc.) typical of a WN 4.5. Additionally the stellar continuum increased by a large factor and the absolute visual magnitude of the star changed from + 1.24 in 1987 to–2.57 in January, 1995. The WR features and enhanced continuum, evidencing a powerful mass-loss event remained with small variations for more than 5 years. Recent ultraviolet and optical data shows that the mass-loss seems to have diminished abruptly in the last three months.     

Spectral manifestations of the stellar wind of the central star of a planetary nebula

Leningrad State University; Institute of Astrophysics and Physics of the Atmosphere, Estonian Academy of Sciences. Translated from Astrofizika, Vol. 30, No. 1, pp. 151–157, January–February, 1989.     

The disappearance of eclipses in the central star of the planetary nebula NGC 2346

This paper gives the result of photographic photography of the central star AGK3-0° 695 of the planetary nebula NGC 2346 made from 1981 through 1987. From it we see that after the large-amplitude eclipses which had started at the end of 1981 had continued for several years, the amplitude began to decrease rapidly in 1986 to ˜1.1 mag from ˜4 mag in 1984. In 1987, only irregular fluctuations with amplitudes ˜0.4 mag were present in the light curve and not longer any clear periodicity. A preliminary analysis of this phenomenon is carried out here.     

Photometric observations of the central star of the planetary nebula NGC 2346 in 1991/1992

Photoelectric UBV observations of the central star of the planetary nebula NGC 2346 obtained during 60 nights between October 1991 and February 1992 are presented (Tables 1 and 2). Four minima have been stated and can be interpreted in terms of occulting dust clouds, probably representing dense condensations of the planetary nebula. We derived R = A_V/E_B—V = 4.0.     

Evolutionary status of WR-type planetary nebula nuclei

In this paper we review the evolutionary paths that lead to the different types of planetary nebulae nuclei, including hydrogen and helium-burning central stars. Starting from the empirical definition of WR central stars, based on stellar spectra, we examine the theoretical work produced to account for the observed configurations. Then we discuss the constraints provided by recent galactic and extragalactic observations, and finally state the possible developments in this field to achieve a better understanding of the subject.     

Morphological and kinematic signatures of a binary central star in the planetary nebula Hu 2-1

We present H α , [N  ii ] and [O  iii ] ground-based and HST archive images, VLA–A 3.6-cm continuum and H92 α emission-line data and high-resolution long-slit [N  ii ] spectra of the planetary nebula Hu 2-1. A large number of structural components are identified in the nebula: an outer bipolar and an inner shell, two pairs of collimated bipolar structures at different directions, monopolar bow-shock-like structures, and an extended equatorial structure within a halo. The formation of Hu 2-1 appears to be dominated by anisotropic mass ejection during the late-AGB stage of the progenitor and by variable, 'precessing' collimated bipolar outflows during the protoplanetary nebula and/or early planetary nebula phases. Different observational results strongly support the existence of a binary central star in Hu 2-1, among them (1) the observed point-symmetry of the bipolar lobes and inner shell, and the departures from axial symmetry of the bipolar lobes, (2) the off-centre position of the central star, (3) the detection of mass ejection towards the equatorial plane, and (4) the presence of 'precessing' collimated outflows. In addition, (5) an analysis of the kinematics shows that the systemic velocity of the bipolar outflows does not coincide with the systemic velocity of the bipolar shell. We propose that this velocity difference is a direct evidence of orbital motion of the ejection source in a binary system. From a deduced orbital velocity of ∼10 km s⁻¹, a semimajor axis of ∼ 9–27 au and period of ∼ 25–80 yr are obtained, assuming a reasonable range of masses. These parameters are used to analyse the formation of Hu 2-1 within current scenarios of planetary nebulae with binary central stars.     

Spectrophotometry of the planetary nebula KjPn 8

Flux-calibrated low-resolution spectra covering the optical wavelength range from 3400 to 7500 Å have been obtained over the central region and the surroundings of the extraordinary planetary nebula (PN) KjPn 8 (PNG 112.5-00.1). The spectrum from the core is of low excitation with T _e(N  II ) = 8000 K and n _e(S  II ) = 550 cm⁻³. KjPn 8 is found to be a Type I PN according to the original classification scheme of Peimbert & Torres-Peimbert, with enriched He/H and N/O ratios with respect to mean values for PN. Increased O/H, Ne/H and Ar/H ratios over those of average PN reflect the possible metal-rich environment from which the progenitor star formed, and also are similar to those found in the extreme Type I PN He 2-111. The N/H ratio is found to be only moderately high compared to the average PN and consequently, the large O abundance pulls the N/O ratio towards the lower limit of the criterion for Type I planetary nebulae (PNe) in this case. In addition, the spectra of some knots and faint regions in the KjPn 8 surroundings are presented, which show only a few spectral lines. Low electron densities ranging from 100 to 300 cm⁻³ have been derived in these outer regions.     

Why every bipolar planetary nebula is 'unique'

We present the many evolutionary routes that progenitors of bipolar planetary nebulae (BPNe) can take. Overall, there are about a hundred qualitatively different evolutionary routes, hence about a hundred qualitatively different types of BPNe. Within each type there are quantitative differences as well. Adding the dependence of the appearance on inclination, we find that the number of different apparent structures of BPNe is about equal to, or even larger than, the number of known BPNe and proto-BPNe. Accordingly we argue that every BPN is a 'unique' object in its appearance, but all can be explained within the binary model paradigm. Therefore, we request a stop to the attaching of adjectives such as 'unique', 'peculiar', and 'unusual' to BPNe and proto-BPNe, thereby removing the need to invoke a new model for almost every 'unusual' BPN. As a case study we try to build a binary model for the proto-BPN OH 231.8+4.2. In our preferred model the AGB Mira-type star has a main sequence companion of mass ∼1 M_⊙, orbital period of ∼5 yr, and eccentricity of ≳0.1.     

Generalized energy balance method for the determination of central star parameters and optical thickness of a planetary nebula

A method is proposed allowing a quick self-consistent determination of both the central star parameters (effective temperature, surface gravity, stellar massetc.) and the optical thickness of a planetary nebula (PN). The method is a generalization of the well-known energy balance method. The method has been calibrated and tested using a photoionization model grid computed for this purpose. The internal accuracy of the method is estimated as 0.038dex for the effective temperature of central star and 0.076dex for the surface gravity.The problem of determination of overall energy losses in the nebula required by any kind of energy balance method is considered thoroughly. Approximate expressions are obtained, relating the overall energy losses to the sum of intensities of collisionally excited lines in the optical and ultraviolet spectral ranges and to some other nebular parameters. It is shown that neglecting the energy losses caused by directly unobservable collisional excitation of neutral hydrogen and helium may underestimate the central star temperature by 0.2 or even 0.5dex. Generalized energy balance method is applied to a sample of 41 PN. Central star temperaturesT _GB found by this method show an agreement withHeII Zanstra temperaturesT _z (HeII) whereasT _z (HI) is always less thanT _GB or equal to it within the accuracy of the method. So, we confirm the explanation that the well-known Zanstra discrepancy is caused merely by low optical thickness of many PN in the Lyman continuum of hydrogen. The value ofT _z (HeII) found with modern model atmospheres can be used as good approximation toT _ef for central stars of overwhelming majority of PN whileT _z (HI) is usually close toT _ef for young nebulae only.