Planetary nebula with a neutral envelope?

IUE ultraviolet spectral recording for a low excitating planetary nebula NGC 6369 is obtained. The very strong doublet 2800 Mgii in emission as well as not less strong absorption line 2852 Mgi are discovered in the spectrum of this nebula. It is shown that the resonance line 2852 Mgi may originate only in a neutral envelope, around the nebula, consisting of neutral hydrogen, neutral magnesium, and dust particles (Hi+Mgi). The importance of this absorption line as a powerful indicator of the discovery of neutral envelopes around the planetary nebulae is outlined.The possibility of the existence of one more envelope—transition zone—immediately contacting with the bright that is ionized part of nebula (Hii+Mgii) is also shown. The transition zone consists of neutral hydrogen, ionized magnesium, and dust particles (Hi+Mgii), main parameters of this zone are also obtained (Table IV).The temperature of the central star of this nebula is obtained for the first time:T _*=48000 K. Continuous background in the interval 2600–3000 Å is identified with Balmer continuum with electron temperatureT _e =12500 K.     

Periodicity in spectral variations of IC 4997?

The emission lines in the spectra of some planetary nebula experience variations with the time. These variations (due to evolution effects and by their nature) differ from the spectral changes which we usually observe in stars. It appears that the long-scale and systematic observations on the intensity behaviour of different emission lines of nebula may be used as an independent indicator to receive a principally new and quite unpredicted category of information related to the physics of nebulae and their nuclei. Particularly, the analysis of long standing observations carried out in relation to the forbidden lines 4363 [Oiii] andN ₁+N ₂ [Oiii] of double-ionized oxygen lead to a conception about the possible existence ofrelativistic electrons of moderate energy in planetary nebulae and the generation oftransition radiation as a result of electrodynamic interaction of these electrons with the dust particles in nebula (Gurzadyan, 1991). Just on the basis of the same observational material, the long-scale periodicity in the activity or variations of a star-like planetary nebula IC 4997 is discovered about which is the present note.     

Planetary nebula with a peculiar spectrum in the ultraviolet: CN 3-1

We have obtained IUE ultraviolet spectra of the low-excitation planetary nebula CN 3-1. The recordings show the well-known doublet 2800 MgII and resonance line 2852 MgI as strong absorption lines. We show that these lines cannot be of interstellar origin and that they may be formed in two envelopes surrounding the main nebula: in the transition zone (doublet 2800 MgII) and in the neutral envelope (line 2852 MgI). These envelopes possess an important property: they contain dust particles, and even a moderate amount of such particles may influence the strength of the absorption lines of MgII and MgI.The emission line 4686 HeII has no relation to the nebula CN 3-1 and belongs to its nucleus, a star of type WR. It is very probable that the nucleus of this nebula is a binary system with a WR component (T _*=50 000 K), exciting the helium lines, and a star of B0 or B2 type (T _*=26 000 K) exciting the nebular linesN ₁+N ₂ [OIII], 3727 [OII], hydrogen lines, etc.     

Pseudo-resonance absorption lines in planetary nebulae: Discovery?

The possibility of the formation of pseudo-resonance absorption lines in planetary nebulae is predicted only theoretically. However, this has not been confirmed by direct observations. In the present article an attempt has been made to show, as a result of careful analysis of IUE spectral recordings for a group of planetary nebulae, that the existence of one pseudo-resonance line 1300 SiIII is without doubt at least in spectra of three planetary nebulae: NGC 2610, NGC 3587, and NGC 6891. The presence of this line in the spectra of the other three planetary nebulae, NGC 6210, IC 3568, and IC 4776, seems to be probable. The role of the interstellar selective absorption, the blending effect by the resonance lines of SiII,Oi, andSi in the formation of the pseudo-resonance line 1300 SiIII as well as the possibility of formation of this line in the photosphere of central star of nebula were analysed.     

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.     

CCD photometry inVRI bands of the galactic cluster NGC 2818

bdAbstract The open cluster NGC 2818 containing a planetary nebula has been observed inVRI bands using the CCD system at prime focus of the 2.3-metre Vainu Bappu Telescope. The study extending to starsV ∼ 21 magnitude establishes the distance modulus as(m-M) ₀ = 12.9 ±0.1 for the cluster. Based on the fitting of theoretical isochrones computed for solar metallicity, an age of 5(±1) × 10₈ years has been assigned to the cluster. Association of the planetary nebula with the cluster indicates that the progenitor mass of the planetary nebula on the main sequence is ≥2.5M_⊙ Based on observations obtained with the Vainu Bappu Telescope.     

A Simulation for a Planetary Nebular Plasma

A plasma simulation code is applied to interpret the instabilities in an expanding planetary nebula. The temperature of the central star of a planetary nebula is assumed as above 50,000 K. Most of the atoms are ionized at this temperature. Since ionization cannot be neglected for such a hot plasma, the electrostatic instability should be taken into account. In the one dimensional electrostatic simulation, Maxwell and Vlasov equations are used and the fast Fourier transform is applied. The calculated drift velocity in the simulation is found comparable with the expansion velocity of a planetary nebula. The linear and non-linear behaviors of the simulated nebular plasma have been investigated in phase space; the simulation results agree with the theory. This revised version was published online in September 2006 with corrections to the Cover Date.     

Origin of the atmospheres of the terrestrial planets

The monotonic decrease in the atmospheric abundance of ³⁶Ar per gram of planet in the sequence, Venus, Earth, and Mars has been assumed to reflect some conditions in the primitive solar nebula at the time of formation of the planetary atmospheres, having to do either with the composition of the nebula itself or the composition of the trapped gases in small solid bodies in the nebula. Behind such hypotheses lies the assumption that planetary atmospheres steadily gain components. However, not only can gases enter atmospheres; they may also be lost from atmospheres both by adsorption into the planetary interior and by loss into space as a result of collisions with minor and major planetesimals. In this paper a necessarily qualitative discussion is given of the problem of collisions with minor planetesimals, a process called atmospheric cratering or atmospheric erosion, and a discussion is given of atmospheric loss accompanying collision of a planet with a major planetesimal, such as may have produced the Earth's Moon.     

Planetary nebula chemical abundances: Z-distribution and connection with the central star masses

Dependencies of galactic planetary nebula chemical abundances and their central star masses on the distance from the galactic plane are discussed.Z-dependencies of He/H, N/H, N/O and Ar/H and dependencies of He/H, N/H, N/O, Ne/H and Ar/H on central star mass are found. Three galactic planetary nebula distance scale samples are used and it is shown that the distance scale system (where distances of each planetary nebula mass class are determined with the separate scale) is the most reliable. The correlations obtained for the Magellanic Cloud planetary nebulae are used for comparison.     

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.     

Accumulation processes in the primitive solar nebula

Particle accumulation processes are discussed for a variety of physical environments, ranging from the collapse phase of an interstellar cloud to the different parts of the models of the primitive solar nebula constructed by Cameron and Pine. Because of turbulence in the collapsing interstellar gas, it is concluded that interstellar grains accumulate into bodies with radii of a few tens of centimeters before the outer parts of the solar nebula are formed. These bodies can descend quite rapidly through the gas toward midplane of the nebula, and accumulation to planetary size can occur in a few thousand years. Substantial modifications of these processes take place in the outer convection zone of the solar nebula, but again it is concluded that bodies in that zone can grow to planetary size in a few thousand years.From the discussion of the interstellar collapse phase it is concluded that the angular momentum of the primitive solar nebula was predominantly of random turbulent origin, and that it is plausible that the primitive solar nebula should have possessed satellite nebulae in highly elliptical orbits. It is proposed that the comets were formed in these satellite nebulae.A number of other detailed conclusions are drawn from the analysis. It is shown to be plausible that an iron-rich planet should be formed in the inner part of the outer nebular convection zone. Discussions are given of the processes of planetary gas accretion, the formation of satellites, the T Tauri solar wind, and the dissipation of excess condensed material after the nebular gases have been removed by the T Tauri solar wind. It is shown that the present radial distances of the planets (but not Bode's Law) should be predicted reasonably well by a solar nebula model intermediate between the uniform and linear cases of Cameron and Pine.     

Hydrodynamic instability of the solar nebula in the presence of a planetary core

When a planetary core composed of condensed matter is accumulated in the primitive solar nebula, the gas of the nebula becomes gravitationally concentrated as an envelope surrounding the planetary core. Models of such gaseous envelopes have been constructed subject to the assumption that the gas everywhere is on the same adiabat as that in the surrounding nebula. The gaseous envelope extends from the surface of the core to the distance at which the gravitational attraction of core plus envelope becomes equal to the gradient of the gravitational potential in the solar nebula; at this point the pressure and temperature of the gas in the envelope are required to attain the background values characteristics of the solar nebula. In general, as the mass of the condensed core increases, increasing amounts of gas became concentrated in the envelope, and these envelopes are stable against hydrodynamic instabilities. However, the core mass then goes through a maximum and starts to decrease. In most of the models tested, the envelopes were hydrodynamically unstable beyond the peak in the core mass. An unstable situation was always created if it was insisted that the core mass contain a larger amount of matter than given by these solutions. For an initial adiabat characterized by a temperature of 450°K and a pressure of 5 × 10^?6 atm, the maximum core mass at which instability occurs is approximately 115 earth masses; this value is rather insensitive to the position in the solar nebula or to the background pressure of the solar nebula. However, if the adiabat is lowered, then the core mass corresponding to instability is decreased. Since the core masses found by Podolak and Cameron for the giant planets are significantly less than the critical core mass corresponding to the initial solar nebula adiabat, we conclude that the giant planets obtained their large amounts of hydrogen and helium by a hydrodynamic collapse process in the solar nebula only after the nebula had been subjected to a considerable period of cooling.     

Observations of the planetary nebula NGC 6853 at [Nii] 6584 Å

New Observations of the [Nii]6584 Å line have been made over the surface of the Dumbell nebula (NGC 6853). The observed lines at the centre of the nebula disc exhibited line splitting of 54.3 km s^–1. The lines appeared double at the centre of the nebula and became single at the boundary. These observations are discussed and compared with those obtained by previous workers.     

The case of the fastest stellar wind in central stars of planetary nebulae

An exceptionally fast wind (8500 km/s) was suggested to occur in the central star of the planetary nebula K1-16, belonging to the class of the PG 1159 H-deficient pre-white dwarfs. To ascertain the reality of such a fast wind this star has been observed with the HST telescope using the GHRS in the zone of theCiv 155.0 nm doublet. The HST data and tests made using synthetic stellar spectra support the existence of a stellar wind with a terminal velocity of 3800 km/s and a mass loss rate lower thanM<2 · 10^–11 M per year. Possibly it is no longer the fastest stellar wind so far observed but it is still among the fastest.     

Discovery of a large-structure emission nebula near PN G200.7+08.4 (A 19)

We report narrowband (Δλ ≤ 10Å) imaging data obtained for the planetary nebula A 19 (PN G200.7+0.84) in continuing our programme of morphological study of planetary nebulae (PNe). Our deep survey through the Hα filter reveals a new nebula – probably a huge HII region –centred at R.A.(2000) = 6h59mn, DEC.(2000) = +14^°40′.     

High-speed knots in the hourglass-shaped planetary nebula Hubble 12

We present a detailed kinematical analysis of the young compact hourglass-shaped planetary nebula Hb 12. We performed optical imaging and long-slit spectroscopy of Hb 12 using the Manchester echelle spectrometer with the 2.1-m San Pedro Mártir telescope. We reveal, for the first time, the presence of end caps (or knots) aligned with the bipolar lobes of the planetary nebula shell in a deep [N  ii ]λ6584 image of Hb 12. We measured from our spectroscopy radial velocities of  ∼120 km s⁻¹  for these knots.
We have derived the inclination angle of the hourglass-shaped nebular shell to be ∼65° to the line of sight. It has been suggested that Hb 12's central star system is an eclipsing binary which would imply a binary inclination of at least 80°. However, if the central binary has been the major shaping influence on the nebula, then both nebula and binary would be expected to share a common inclination angle.
Finally, we report the discovery of high-velocity knots with Hubble-type velocities, close to the core of Hb 12, observed in Hα and oriented in the same direction as the end caps. Very different velocities and kinematical ages were calculated for the outer and inner knots showing that they may originate from different outburst events.     

Using slitless spectroscopy to study the kinematics of the planetary nebula population in M94

The planetary nebula populations of relatively nearby galaxies can be easily observed and provide both a distance estimate and a tool with which dynamical information can be obtained. Usually the requisite radial velocities are obtained by multi-object spectroscopy once the planetary nebulae have been located by direct imaging. Here we report on a technique for measuring planetary nebula kinematics using the double-beam ISIS spectrograph at the William Herschel Telescope in a novel slitless mode, which enables the detection and radial velocity measurements to be combined into a single step. The results on our first target, the Sab galaxy NGC 4736, allow the velocity dispersion of the stellar population in a disc galaxy to be traced out to four scalelengths for the first time and are consistent with a simple isothermal sheet model.     

Is gas in the Orion nebula depleted?

Depletion of heavy elements has been recognized to be important in the understanding of the chemical composition of the interstellar medium. This problem is also relevant to the study of Hii regions. In this paper we investigate the gaseous depletion in the physical conditions of the Orion nebula. We reach the conclusion that very probably no depletion of heavy elements, due to sticking on dust grains, took place during the lifetime of the Orion nebula.     

Fabry-Pérot observations of the Rosette Nebula

The results of observations of the Rosette Nebula using the equipment described byMeaburn andSmith (1968) and the reduction procedures outlined inSmith andWalker (1968) are reported. The results and conclusions differ markedly from an earlier report (Flynn, 1965), of detailed radial-velocity observations of this nebula. The peak positions of the H-line profiles have now been shown not to vary greatly. The mean of 37 measurements at different points gives, for the heliocentric velocity of the Rosette nebula, +35±5 km sec^–1. The widths of the lines are consistent with a model of randomly directed streaming motions of order 15 to 25 km sec^–1.     

Symbiotic stars with Wolf-Rayet spectra as protoplanetary nebulae?

Symbiotic systems, in particular symbiotic novae, have been suggested to be very early stages of planetary nebulae. Some of them have been described as going through a Wolf-Rayet phase. We argue that there may be a direct relation between symbiotic objects and planetary nebulae, and that the Wolf-Rayet phase is connected to an active spell of the hot companion. Symbiotic stars could lead to planetary nebulae with two central stars with different radiation temperatures and luminosities, where each has the power to ionize a planetary nebula on its own.