The radio spectrum and physical parameters of the W80 Hii complex

The radio continuum spectrum of the W80 nebular complex (NGC 7000 and IC 5070 nebulae mainly) is established over a wide range of frequencies. The resultant spectrum shows a thermal shape and the spectral index established for the optically thin part of it is –0.04. Certain physical parameters of the complex (rms densities, mass, emission measure, excitation parameter) are derived for adopted models. The possible exciting star is located. Limitations of the models involved and uncertainties of the results are also discussed.     

Internal radial velocities of some selected small diameterHii regions

A photographic Fabry-Pérot interferometer was built for the University of Maryland's 20-in. telescope for the study of the radial velocities of diffuse emission nebulae. Observations of S101, S112, IC5146, NGC7380, NGC281, and IC1795 were made and radial velocities at many points within each nebula were determined with an estimated error of ±3 km/sec. Average heliocentric radial velocities were obtained for each nebula and they agree well with those found by other observers.For S112 and IC5146, a systematic difference of 6–8 km/sec in the radial velocities between the east and west sides of the nebulae was observed. This phenomenon is attributed to the braking effect of cold gas which appears to be associated with the western side of both nebulae.Ring half-widths were obtained for S112, IC5146 and NGC281, and found to be consistent with expansion velocities of 5 km/sec or less. For IC5146 the average Doppler temperature is only 5500±1000 K, which indicates an even lower electron temperature.     

Infrared photometry of eight planetary nebulae

We discuss the infrared (IR) (1.25–5 µm) photometry of eight planetary nebulae performed in 1999–2006. For all of the nebulae under study, we have firmly established IR brightness and color variations on time scales shorter than one year and up to 6–8 years. The greatest IR brightness variations were observed in IC 2149, IC 4997, and NGC 7662. Their J magnitudes varied within 0 _. ^m 2–0 _. ^m 25. In the remaining objects, the J magnitude variations did not exceed 0 _. ^m 15. All of the planetary nebulae under study exhibited IR color variations. Based on the IR photometry, we have classified the central regions of the planetary nebula NGC 1514 and of the northern part of NGC 7635 seen through a 12″ aperture as a B(3–7) main-sequence star (NGC 1514) and a ～O9.5 upper-main-sequence star (NGC 7635). The nebulae IC 4997 and NGC 7027 exhibited an excess emission (with respect to the emission from a hot source) at λ > 2.5 µm.     

Thermal emission spectra of silicates from planetary nebulae

Calculations of the grain equilibrium temperature and of the expected infrared spectra of IC 418, BD+30° 3639, NGC 6572 and NGC 7027 have been performed using dielectric constants of lunar silicates. The results have been compared with previous work on pure graphite and ice-mantle grains. Lα heating of dust followed by thermal re-emission is consistent with the large infrared excesses detected in planetary nebulae. An extra source of heating is, nevertheless, necessary to fit correctly the experimental results. It appears from the calculations that, for each object, it is possible to define theoretically the most probable nature of the emitting dust.     

Low-ionization pairs of knots in planetary nebulae: physical properties and excitation

We obtained optical long-slit spectra of four planetary nebulae (PNe) with low-ionization pair of knots, namely He 1-1, IC 2149, KjPn 8 and NGC 7662.
These data allow us to derive the physical parameters and excitation of the pairs of knots, and those of higher ionization inner components of the nebulae, separately.
Our results are as follows. (1) The electron temperatures of the knots are within the range 9500–14 500 K, similar to the temperatures of the higher ionization rims/shells. (2) Typical knots' densities are 500–2000 cm⁻³. (3) Empirical densities of the inner rims/shells are higher than those of the pairs of knots, by up to a factor of 10. Theoretical predictions, at variance with the empirical results, suggest that knots should be denser than the inner regions, by at least a factor of 10. (4) Empirical and theoretical density contrasts can be reconciled if we assume that at least 90 per cent of the knots' gas is neutral (likely composed of dust and molecules). (5) By using the new Raga et al. shock modelling and diagnostic diagrams appropriated for spatially resolved PNe, we suggest that high-velocity shocked knots travelling in the photoionized outer regions of PNe can explain the emission of the pairs of knots analysed in this paper.     

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.     

Conjugate excitation of atomic oxygen emissions in the far ultraviolet

Thirty four objects appearing in the SharplessCatalogue of H ii Regions with radio flux densities below 0.1 fu^* at 1400 MHz, are studied. It is shown that two of them are planetary nebulae, two are reflection nebulae and two weak emission-reflection nebulae. From statistical arguments it is concluded that six may be evolved planetaries while the remaining are faint nearby Hii regions excited by stars of relatively advanced spectral type.     

Magnetic fields in planetary nebulae and post-AGB nebulae

Magnetic fields are an important but largely unknown ingredient of planetary nebulae. They have been detected in oxygen-rich asymptotic giant branch (AGB) and post-AGB stars, and may play a role in the shaping of their nebulae. Here we present SCUBA submillimetre polarimetric observations of four bipolar planetary nebulae and post-AGB stars, including two oxygen-rich and two carbon-rich nebulae, to determine the geometry of the magnetic field by dust alignment. Three of the four sources (NGC 7027, 6537 and 6302) present a well-defined toroidal magnetic field oriented along their equatorial torus or disc. NGC 6302 may also show field lines along the bipolar outflow. CRL 2688 shows a complex field structure, where part of the field aligns with the torus, whilst an other part approximately aligns with the polar outflow. It also presents marked asymmetries in its magnetic structure. NGC 7027 shows evidence for a disorganized field in the south-west corner, where the SCUBA shows an indication for an outflow. The findings show a clear correlation between field orientation and nebular structure.     

Observations of photometric profiles of two planetary nebulae

It is noted that the Abel integral of a Gaussian function is a Gaussian function. This special property is used in order to develop an analytic method for studying photometric profile in Planetary Nebulae. Monochromatic observations have been made at the Haute-Provence Observatory using the RCA-CCD (320×512 pixels) mounted at theF/6 Newtonian focus of the 1.2 m telescope. Among the observed nebulae, two of them (NGC 6778 and NGC 7354) are regular enough to test the photometric profile method. The nebular emissivity per volume unit is then obtained for H⁺ and O⁺⁺ ions.     

The structure of the Cygnus loop at 34.5 MHz

We have observed the large supernova remnant Cygnus Loop at 34.5 MHz with the low frequency radio telescope at Gauribi-danur, India. A radio map of the region with a resolution of 26 arcmin × 40 arcmin (α × δ) is presented. The integrated flux density of the Cygnus Loop at this frequency is 1245 ± 195 Jy. The radio fluxes of different parts of the nebula at this frequency were also measured and used to construct their spectra. It is found that the spectrum of the region associated with the optical nebulosity NGC 6992/5 is not flat at low frequencies, and also exhibits a break at a frequency around 400 MHz. The spectrum of the region associated with NGC 6960 also shows a break but around 1000 MHz, while the spectrum of the region associated with NGC 6974 is straight in the entire frequency range 25 to 5000 MHz. The implication of these results on the basis of existing theories of the origin of radio emission from supernova remnants is discussed.     

H i content and star formation in the interacting galaxy Arp86

We present the results of Giant Metrewave Radio Telescope (GMRT) observations of the interacting system Arp86 in both neutral atomic hydrogen, H  i , and in radio continuum at 240 606 and 1394 MHz. In addition to H  i emission from the two dominant galaxies, NGC 7752 and NGC 7753, these observations show a complex distribution of H  i tails and bridges due to tidal interactions. The regions of highest column density appear related to the recent sites of intense star formation. H  i column densities  ∼1–1.5 × 10²¹ cm⁻²  have been detected in the tidal bridge which is bright in Spitzer image as well. We also detect H  i emission from the galaxy 2MASX J23470758+2926531, which is shown to be a part of this system. We discuss the possibility that this could be a tidal dwarf galaxy. The radio continuum observations show evidence of a non-thermal bridge between NGC 7752 and NGC 7753, and a radio source in the nuclear region of NGC 7753 consistent with it having a low-ionization nuclear emission region nucleus.     

The electron temperature distributions and internal kinematics of seven diffuse nebulae

A Fabry-Pérot spectrophotometer is used to derive values of the intensity ratio H/[Nii] at 98 points in the seven bright diffuse nebulae M8, M20, M16, M17, NGC7000, M42, IC434. The fraction of nitrogen in the singly ionized state is estimated in the different objects, and is found to be sufficiently constant within any one nebula so that the above intensity ratio may be used to derive accurate electron temperature distributions. The position of the peak of the nebular line, its excess non-thermal width, its shape and relative intensity are used to derive kinematical models of these objects.It is found that values of H/[Nii]1 are representative of the bright central cores of these nebulae. Temperatures between 7000K and 12000K were derived in the different objects. Although some of this apparent variation is due to the different conditions of excitation in the various nebulae, it is shown that a convincing progression of temperature in M8, M16, M17 is supported by radio recombination line results. The temperature variation within any one object was generally significantly less than 1500K.No evidence was found for velocities of mass motion at more than twice the speed of sound. Relative radial velocities of generally less than 15 km sec^–1 characterized the velocity fields of M8, M20, M16, M42. The velocities in M17 were measured as about 20 km sec^–1. Motions in NGC 7000 and IC 434 were much lower (5 km sec^–1) although here the number of points taken was too small to construct meaningful kinematical models.It is concluded that the internal motions of radiatively ionizedHii regions of Pop. I will not significantly affect the results of existing surveys for determining the rotation of the galaxy with radial velocities deduced from nebular emission lines.     

Hα line measurements from ten diffuse galactic sources using the DEFPOS facility

The hydrogen Balmer‐α emission line spectrum of ten diffuse ionization sources in the Milk Way – NGC 40 (WC8), NGC 2022, NGC 6210, NGC 6618 (M17, Sh2‐45), NGC 6720 (M57), NGC 6781, NGC 6888 (Sh2‐105), NGC 6992 (Sh2‐103), NGC 7635 (Sh2‐162,) and IC 1848 (Sh2‐199) – has been investigated using a dual etalon Fabry‐Pérot optical spectrometer (DEFPOS) aatached to the 150 cm RTT150 telescope at TUBITAK National Observatory (TUG, Antalya, Turkey: 36° 51′ N; 30° 20′ E; elevation: 2547 m). All of our galactic Hα observations discussed in this paper were carried out during the nights of 2013 June 21–24 with exposure time of 3600 s. As main results the intensity, the full width at half maximum, and the radial velocity with respect to the LSR have been determined for each data set. The intensities, the radial velocities, and the line widths of the Hα emission line vary from 59.15 to 8923.44 R, –46.72 to +54.07 km s^–1, and 31.4 to 48.01 km s^–1, respectively. The radial velocities and the half‐widths of the Hii regions and planetary nebulae determined from our measurements are found tobe consistent with values given in literature, especially with those in Schneider et al. (1983) and Fich et al. (1990). (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Planetary Nebulae

This review summarizes spectroscopic and imaging results obtained on board the Infrared Space Observatory (ISO) on well-known planetary nebulae: the young nebula NGC 7027, and two fully evolved nearby nebulae, the Dumbbell and the Helix. This revised version was published online in August 2006 with corrections to the Cover Date.     

Instability of a thin photoevaporable circumstellar envelope

We investigate the stability of a dense neutral shell that is accelerated outward by the hot-gas pressure and that loses its mass through photoionization by radiation from the central star. We assume the H I shell to be thin and use the Lagrangian coordinates to describe its motion. We show that a flow accompanied by cumulative effects emerges during the nonlinear development of the instability. We estimate the influence of the radiative cooling rate on the motion and determine parameters of the gas in the cumulative region. The results obtained are compared with the observations of the nebulae NGC 7293 and NGC 2392.     

Companions and interactions in the NGC 6946 system

We report the results of a survey of the H  i environment of NGC 6946 in a search for gas-rich companions to the galaxy. Such gas-rich companions could include material left over from the galaxy assembly process which could persist into the current day around an isolated galaxy such as NGC 6946. NGC 6946 is prolifically forming stars, has a nuclear starburst, and has widespread high-velocity clouds associated with the disc. All of these features could be explained by the accretion of low-mass H  i clouds by NGC 6946. Our survey recovered two previously detected dwarf galaxies associated with NGC 6946, but otherwise found no signatures of interactions in the NGC 6946 system. The companions are small enough, and distant enough from NGC 6946 that they should have minimal effect on the main galaxy. Some tidal debris may be expected due to interaction between the two dwarf galaxies, but none is observed. This could be because it is at low column densities, or because the dwarf galaxies are more separated than they appear on the sky. This study of the system suggests that NGC 6946 is a gravitationally bound system with two dwarf galaxies in stable orbits about the larger primary galaxy.     

Further 2MASS mapping of hot dust in planetary nebulae

We have used 2 Micron All Sky Survey (2MASS) mapping results to investigate the distribution of hot dust continua in 12 planetary nebulae (PNe). The nature of this emission is unclear, but it is possible that where the continuum is extended, as is the case for M 1-12 and NGC 40, then the grains concerned may be very small indeed. The absorption of individual photons by such grains may lead to sharp spikes in temperature, as has previously discussed for several other such outflows. Other sources (such as MaC 1-4, He 2-25, B1 2-1 and K 3-15) appear to be relatively compact, and the high temperatures observed are understandable in terms of more normal heating processes. It is possible that the grains in these cases are experiencing high radiant flux levels.
Finally, it is noted that whilst the core of M 2-2 appears to show hot grain emission, this is less the case for its more extended envelope. The situation may, in this case, be similar to that of NGC 2346, in which much of the emission is located within an unresolved nucleus. Similarly, it is noted that in addition to hot dust and gas thermal continua, the emission in the interior of NGC 40 may be enhanced through rotational–vibrational transitions of H₂, and/or the 2p³P⁰–2s³S transition of He  i .     

Gas-phase models for the evolved planetary nebulae NGC 6781, M4-9 and NGC 7293

We have studied the chemistry of the molecular gas in evolved planetary nebulae. Three pseudo-time-dependent gas-phase models have been constructed for dense (10⁴–10⁵ cm⁻³) and cool ( T ∼15 K) clumpy envelopes of the evolved nebulae NGC 6781, M4-9 and NGC 7293. The three nebulae are modelled as carbon-rich stars evolved from the asymptotic giant branch to the late planetary nebula phase. The clumpy neutral envelopes are subjected to ultraviolet radiation from the central star and X-rays that enhance the rate of ionization in the clumps. With the ionization rate enhanced by four orders of magnitude over that of the ISM, we find that resultant abundances of the species HCN, HNC, HC₃N and SiC₂ are in good agreement with observations, while those of CN, HCO⁺, CS and SiO are in rough agreement. The results indicate that molecular species such as CH, CH₂, CH₂⁺ , HCl, OH and H₂O are anticipated to be highly abundant in these objects.     

Astrophysical properties of red giants in three open clusters older than the hyades

Results fromCMT ₁T₂ 1T2 broad-band and DDO intermediate-band photometry are presented for G and K giants in the old open clusters NGC. 2482, NGC 3680, and IC 4651. Two independent photometric criteria have been used to separate red field stars from the physical members of the clusters. Recent calibrations of the DDO andCMT ₁T₂ systems have been used to derive reddening, distance moduli, metallicities, effective temperatures, and surface gravities. Rough estimates of masses have also been made. The giants of NGC 2482 and IC 4651 have CN strengths nearly identical to the Hyades giants, while those of NGC 3680 are slightly richer in CN than the nearby K giants.CMT1T2 abundance analysis in NGC 2482 and NGC 3680 yield [Fe/H]_MT = - 0.1 ± 0.1 as derived from the iron lines, while abundances derived from the CNO - contaminated (C - M) index are 0.4 dex higher. BothCMT ₁T₂ and DDO data support the conclusion that 1C 4651, with [Fe/H] = + 0.2 ± 0.1, is on the metalrich side of the distribution of intermediate and old open clusters. Finally, the mass results suggest that the clump stars in NGC 3680 and. IC 4651 could have undergone mass loss before reaching their helium core burning phase of evolution. Supported in part by the Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET) of Argentina. Visiting astronomer of Cerro Tololo Inter-American Observatory supported by the National Science Foundation under contract No. AST 74-04128.     

The dynamics of double-envelope planetary nebulae

A number of criteria are elaborated based on the careful analysis of nebulae images which confirm the version of the origin of double-envelope planetary nebulae by means of dynamical separation but not by multiple ejection. The importance of stellar winds in the origin of the gigantic halos around double envelope nebulae is outlined. The problem concerning the existence of two types of filamentary planetary nebulae (Figure 8) is raised: type A, filaments are the fragmentations of the Rayleigh-Taylor instability, the result of dynamical interaction of the envelope with the outer interstellar matter (NGC 6543), and type B, the origin of filaments is connected with the Magnetic field fluctuations (A 43, A 72).The possibility of the origin of three-envelope nebulae in the framework of the dynamic separation version is discussed (NGC 7293). Attention is paid to the impossibility of outbursts with low velocities (20–30 km s^–1) from hot stars, i.e., the nuclei of nebulae.