Helium recombination spectra as temperature diagnostics for planetary nebulae

Electron temperatures derived from the He  i recombination line ratios, designated T _e(He  i ), are presented for 48 planetary nebulae (PNe). We study the effect that temperature fluctuations inside nebulae have on the T _e(He  i ) value. We show that a comparison between T _e(He  i ) and the electron temperature derived from the Balmer jump of the H  i recombination spectrum, designated T _e(H  i ), provides an opportunity to discriminate between the paradigms of a chemically homogeneous plasma with temperature and density variations, and a two-abundance nebular model with hydrogen-deficient material embedded in diffuse gas of a 'normal' chemical composition (i.e. ∼solar), as the possible causes of the dichotomy between the abundances that are deduced from collisionally excited lines and those deduced from recombination lines. We find that T _e(He  i ) values are significantly lower than T _e(H  i ) values, with an average difference of  〈 T _e(H  i ) − T _e(He  i )〉= 4000 K  . The result is consistent with the expectation of the two-abundance nebular model but is opposite to the prediction of the scenarios of temperature fluctuations and/or density inhomogeneities. From the observed difference between T _e(He  i ) and T _e(H  i ), we estimate that the filling factor of hydrogen-deficient components has a typical value of 10⁻⁴. In spite of its small mass, the existence of hydrogen-deficient inclusions may potentially have a profound effect in enhancing the intensities of He  i recombination lines and thereby lead to apparently overestimated helium abundances for PNe.     

The coronal line regions of planetary nebulae NGC 6302 and 6537: 3–13 μm grating and echelle spectroscopy

We report on advances in the study of the cores of NGC 6302 and 6537 using infrared grating and echelle spectroscopy. In NGC 6302, emission lines from species spanning a large range of ionization potential, and in particular [Si  ix ] 3.934 μm, are interpreted using photoionization models (including cloudy ), which allow us to re-estimate the temperature of the central star to be about 250 000 K. All of the detected lines are consistent with this value, except for [Al  v ] and [Al  vi ]. Aluminium is found to be depleted to one hundredth of the solar abundance, which provides further evidence for some dust being mixed with the highly ionized gas (with photons harder than 154 eV). A similar depletion pattern is observed in NGC 6537. Echelle spectroscopy of IR coronal ions in NGC 6302 reveals a stratified structure in ionization potential, which confirms photoionization to be the dominant ionization mechanism. The lines are narrow (<22 km s⁻¹ FWHM), with no evidence of the broad wings found in optical lines from species with similar ionization potentials, such as [Ne  v ] 3426 Å. We note the absence of a hot bubble, or a wind-blown bipolar cavity filled with a hot plasma, at least on 1 arcsec and 10 km s⁻¹ scales. The systemic heliocentric velocities for NGC 6302 and 6537, measured from the echelle spectra of IR recombination lines, are found to be −34.8±1 km s⁻¹ and −17.8±3 km s⁻¹. We also provide accurate new wavelengths for several of the infrared coronal lines observed with the echelle.     

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.     

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.     

The enigma of the oldest 'nova': the central star and nebula of CK Vul

CK Vul is classified as, amongst others, the slowest known nova, a hibernating nova or a very late thermal pulse object. Following its eruption in ad 1670, the star remained visible for 2 yr. A 15-arcsec nebula was discovered in the 1980s, but the star itself has not been detected since the eruption. We here present radio images which reveal a 0.1-arcsec radio source with a flux of 1.5 mJy at 5 GHz. Deep Hα images show a bipolar nebula with a longest extension of 70 arcsec, with the previously known compact nebula at its waist. The emission-line ratios show that the gas is shock-ionized, at velocities  >100 km s⁻¹  . Dust emission yields an envelope mass of  ∼5 × 10⁻² M_⊙  . Echelle spectra indicate outflow velocities up to 360 km s⁻¹. From a comparison of images obtained in 1991 and 2004 we find evidence for expansion of the nebula, consistent with an origin in the 1670 explosion; the measured expansion is centred on the radio source. No optical or infrared counterpart is found at the position of the radio source. The radio emission is interpreted as thermal free–free emission from gas with   T _e∼ 10⁴ K  . The radio source may be due to a remnant circumbinary disc, similar to those seen in some binary post-AGB stars. We discuss possible classifications of this unique outburst, including that of a sub-Chandrasekhar mass supernova, a nova eruption on a cool, low-mass white dwarf or a thermal pulse induced by accretion from a circumbinary disc.     

Precessing collimated outflows in the planetary nebula IC 4846

We present [N  ii ] and H α 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 the properties of which indicate a high-velocity, bipolar precessing jet. A difference of ≃10 km s⁻¹ is found between the systemic velocity of the precessing jets and the centroid velocity of the nebula, as recently reported for Hu 2-1. We propose that this difference is as a result of orbital motion of the ejection source in a binary central star. The orbital separation of 30 au and period 100 yr estimated for the binary are similar to those in 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.     

Radio planetary nebulae in the Magellanic Clouds

We report the extragalactic radio-continuum detection of 15 planetary nebulae (PNe) in the Magellanic Clouds (MCs) from recent Australia Telescope Compact Array+Parkes mosaic surveys. These detections were supplemented by new and high-resolution radio, optical and infrared observations which helped to resolve the true nature of the objects. Four of the PNe are located in the Small Magellanic Cloud (SMC) and 11 are located in the Large Magellanic Cloud (LMC). Based on Galactic PNe the expected radio flux densities at the distance of the LMC/SMC are up to ∼2.5 and ∼2.0 mJy at 1.4 GHz, respectively. We find that one of our new radio PNe in the SMC has a flux density of 5.1 mJy at 1.4 GHz, several times higher than expected. We suggest that the most luminous radio PN in the SMC (N S68) may represent the upper limit to radio-peak luminosity because it is approximately three times more luminous than NGC 7027, the most luminous known Galactic PN. We note that the optical diameters of these 15 Magellanic Clouds (MCs) PNe vary from very small (∼0.08 pc or 0.32 arcsec; SMP L47) to very large (∼1 pc or 4 arcsec; SMP L83). Their flux densities peak at different frequencies, suggesting that they may be in different stages of evolution. We briefly discuss mechanisms that may explain their unusually high radio-continuum flux densities. We argue that these detections may help solve the 'missing mass problem' in PNe whose central stars were originally  1–8 M_⊙  . We explore the possible link between ionized haloes ejected by the central stars in their late evolution and extended radio emission. Because of their higher than expected flux densities, we tentatively call this PNe (sub)sample –'Super PNe'.     

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.     

The Planetary Nebula Spectrograph elliptical galaxy survey: the dark matter in NGC 4494

We present new Planetary Nebula Spectrograph observations of the ordinary elliptical galaxy NGC 4494, resulting in positions and velocities of 255 planetary nebulae out to seven effective radii (25 kpc). We also present new wide-field surface photometry from MMT/Megacam, and long-slit stellar kinematics from VLT/FORS2. The spatial and kinematical distributions of the planetary nebulae agree with the field stars in the region of overlap. The mean rotation is relatively low, with a possible kinematic axis twist outside  1 R _e  . The velocity dispersion profile declines with radius, though not very steeply, down to  ∼70 km s⁻¹  at the last data point.
We have constructed spherical dynamical models of the system, including Jeans analyses with multi-component Λ cold dark matter (CDM) motivated galaxies as well as logarithmic potentials. These models include special attention to orbital anisotropy, which we constrain using fourth-order velocity moments. Given several different sets of modelling methods and assumptions, we find consistent results for the mass profile within the radial range constrained by the data. Some dark matter (DM) is required by the data; our best-fitting solution has a radially anisotropic stellar halo, a plausible stellar mass-to-light ratio and a DM halo with an unexpectedly low central density. We find that this result does not substantially change with a flattened axisymmetric model.
Taken together with other results for galaxy halo masses, we find suggestions for a puzzling pattern wherein most intermediate-luminosity galaxies have very low concentration haloes, while some high-mass ellipticals have very high concentrations. We discuss some possible implications of these results for DM and galaxy formation.     

A high-resolution radio study of neutral gas in the starburst galaxy NGC 520

We present subarcsec angular resolution observations of the neutral gas in the nearby starburst galaxy NGC 520. The central kpc region of NGC 520 contains an area of significantly enhanced star formation. The radio continuum structure of this region resolves into ∼10 continuum components. By comparing the flux densities of the brightest of these components at 1.4 GHz with published 15-GHz data we infer that these components detected at 1.4 and 1.6 GHz are related to the starburst and are most likely to be collections of several supernova remnants within the beam. None of these components is consistent with emission from an active galactic nuclei. Both neutral hydrogen (H  i ) and hydroxyl (OH) absorption lines are observed against the continuum emission, along with a weak OH maser feature probably related to the star formation activity in this galaxy. Strong H  i absorption  ( N _H∼ 10²² atoms cm⁻²)  traces a velocity gradient of 0.5 km s⁻¹ pc⁻¹ across the central kpc of NGC 520. The H  i absorption velocity structure is consistent with the velocity gradients observed in both the OH absorption and in CO emission observations. The neutral gas velocity structure observed within the central kpc of NGC 520 is attributed to a kpc-scale ring or disc. It is also noted that the velocity gradients observed for these neutral gas components appear to differ with the velocity gradients observed from optical ionized emission lines. This apparent disagreement is discussed and attributed to the extinction of the optical emission from the actual centre of this source hence implying that optical ionized emission lines are only detected from regions with significantly different radii to those sampled by the observations presented here.     

An asteroseismic study of the β Cephei star θ Ophiuchi: spectroscopic results

The central stars of highly evolved planetary nebulae (PNe) are expected to have closely similar absolute visual magnitudes M_V . This enables us to determine approximate distances to these sources where one knows their central star visual magnitudes, and levels of extinction. We find that such an analysis implies values of D which are similar to those determined by Phillips; Cahn, Kaler & Stanghellin; Acker, and Daub. However, our distances are very much smaller than those of Zhang; Bensby & Lundstrom, and van de Steene & Zijlstra. The reasons for these differences are discussed, and can be traced to errors in the assumed relation between brightness temperature and radius.
Finally, we determine that the binary companions of such stars can be no brighter than   M_V ∼ 6 mag  , implying a spectral type of K0 or later in the case of main-sequence stars.     

A new population of planetary nebulae discovered in the Large Magellanic Cloud – I. Preliminary sample

We report our initial discovery of 73 new planetary nebulae (PNe) in the Large Magellanic Cloud (LMC) following confirmatory 2dF spectroscopy on the Anglo-Australian Telescope. Preliminary candidate sources come from a 10 per cent sub-area of our new deep, high-resolution Hα map of the central 25 deg² of the LMC obtained with the UK Schmidt Telescope. The depth of the high-resolution map was extended to   R _equiv∼ 22  for  Hα (4.5 × 10⁻¹⁷ erg cm⁻² s⁻¹Å⁻¹)  by a process of multi-exposure median co-addition of a dozen 2-h Hα exposures. The resulting map is at least 1-mag deeper than the best wide-field narrow-band LMC images currently available. This depth, combined with our selection technique, has also led to the discovery of extended asymptotic giant branch (AGB) haloes around many new and previously known LMC PNe for the first time. Once complete, our new survey is expected to triple the LMC PN population and have significant implications for the LMC PN luminosity function, kinematics, abundance gradients, chemical evolution and, via study of the AGB haloes, the initial to final mass relation for low- to intermediate-mass stars.     

Detecting planets in planetary nebulae

We examine the possibility of detecting signatures of surviving Uranus/Neptune-like planets inside planetary nebulae. Planets that are not too close to the stars (orbital separation larger than ∼5 au) are likely to survive the entire evolution of the star. As the star turns into a planetary nebula, it has a fast wind and strong ionizing radiation. The interaction of the radiation and wind with a planet may lead to the formation of a compact condensation or tail inside the planetary nebula, which emits strongly in H α , but not in [O  iii ]. The position of the condensation (or tail) will change over a time-scale of ∼10 yr. Such condensations might be detected with currently existing telescopes.     

Towards a spectral technique for determining material geometry around evolved stars: application to HD 179821

HD 179821 is an evolved star of unknown progenitor mass range (either post-asymptotic giant branch or post-red supergiant) exhibiting a double-peaked spectral energy distribution (SED) with a sharp rise from ∼8 to  20 μm  . Such features have been associated with ejected dust shells or inwardly truncated circumstellar discs. In order to compare SEDs from both systems, we employ a spherically symmetric radiative transfer code and compare it to a radiative, inwardly truncated disc code. As a case study, we model the broad-band SED of HD 179821 using both codes. Shortward of 40 μm, we find that both models produce equivalent fits to the data. However, longward of 40 μm, the radial density distribution and corresponding broad range of disc temperatures produce excess emission above our spherically symmetric solutions and observations. For HD 179821, our best fit consists of a   T _eff= 7000 K  central source characterized by  τ _V ∼ 1.95  and surrounded by a radiatively driven, spherically symmetric dust shell. The extinction of the central source reddens the broad-band colours so that they resemble a   T _eff= 5750 K  photosphere. We believe that HD 179821 contains a hotter central star than previously thought. Our results provide an initial step towards a technique to distinguish geometric differences from spectral modelling.     

Spectroscopy of six highly evolved Abell planetary nebulae

We have undertaken visual spectroscopy of the highly evolved planetary nebulae (PNe) A8, A13, A62, A72, A78 and A83 over a wavelength range  4330 < λ < 6830 Å  . This permits us to specify relative line intensities in various sectors of the nebular shells, and to investigate the variation of emission as a function of radius. We determine that the spectrum of the central star of A78 has varied appreciably over a period of 25 yr. There is now evidence for strong P Cygni absorption in the λ4589 and λ5412 transitions of He  ii , implying terminal velocities of the order of   V _∞≅ 3.83 × 10³ km s⁻¹  . We also note that the emission-line profiles of the sources can be used to investigate their intrinsic emission structures. We find that most PNe show appreciable levels of emission throughout their volumes; only one source (A13) possesses a thin-shell structure. Such results are in conformity with evolutionary theory, and probably reflect the consequences of adiabatic cooling in highly evolved outflows.     

Upper limits on the hot Jupiter fraction in the field of NGC 7789

We describe a method of estimating the abundance of short-period extra-solar planets based on the results of a photometric survey for planetary transits. We apply the method to a 21-night survey with the 2.5-m Isaac Newton Telescope of ∼32 000 stars in a ∼0.5 × 0.5 deg² field including the open cluster NGC 7789. From the colour–magnitude diagram, we estimate the mass and radius of each star by comparison with the cluster main sequence. We search for injected synthetic transits throughout the light curve of each star in order to determine their recovery rate, and thus calculate the expected number of transit detections and false alarms in the survey. We take proper account of the photometric accuracy, time sampling of the observations and criteria (signal-to-noise ratio and number of transits) adopted for transit detection. Assuming that none of the transit candidates found in the survey will be confirmed as real planets, we place conservative upper limits on the abundance of planets as a function of planet radius, orbital period and spectral type.     

Deep echelle spectrophotometry of S 311, a Galactic H ii region located outside the solar circle

We present echelle spectrophotometry of the Galactic H  ii region S 311. The data have been taken with the Very Large Telescope Ultraviolet-Visual Echelle Spectrograph in the 3100–10 400 Å range. We have measured the intensities of 263 emission lines; 178 are permitted lines of H⁰, D⁰ (deuterium), He⁰, C⁰, C⁺, N⁰, N⁺, O⁰, O⁺, S⁺, Si⁰, Si⁺, Ar⁰ and Fe⁰; some of them are produced by recombination and others mainly by fluorescence. Physical conditions have been derived using different continuum- and line-intensity ratios. We have derived He⁺, C⁺⁺ and O⁺⁺ ionic abundances from pure recombination lines as well as abundances from collisionally excited lines for a large number of ions of different elements. We have obtained consistent estimations of t ² applying different methods. We have found that the temperature fluctuations paradigm is consistent with the T _e(He  i ) versus T _e(H  i ) relation for H  ii regions, in contrast with what has been found for planetary nebulae. We report the detection of deuterium Balmer lines up to Dδ in the blue wings of the hydrogen lines, whose excitation mechanism seems to be continuum fluorescence.     

Limits on the 2.2-μm contrast ratio of the close-orbiting planet HD 189733b

We obtained 238 spectra of the close-orbiting extrasolar giant planet HD 189733b with resolution   R ∼ 15 000  during one night of observations with the Near-Infrared High-Resolution Spectrograph (NIRSPEC), at the Keck II Telescope. We have searched for planetary absorption signatures in the  2.0–2.4 μm  region where H₂O and CO are expected to be the dominant atmospheric opacities. We employ a phase-dependent orbital model and tomographic techniques to search for the planetary absorption signatures in the combined stellar and planetary spectra. Because potential absorption signatures are hidden in the noise of each single exposure, we use a model list of lines to apply a spectral deconvolution. The resulting mean profile possesses a signal-to-noise ratio (S/N) that is 20 times greater than that found in individual lines. Our spectral time series thus yields spectral signatures with a mean S/N = 2720. We are unable to detect a planetary signature at a contrast ratio of  log₁₀( F _p/ F _*) =−3.40  , with 63.8 per cent confidence. Our findings are not consistent with model predictions which nevertheless give a good fit to mid-infrared observations of HD 189733b. The 1σ result is a factor of 1.7 times less than the predicted 2.185-μm planet/star flux ratio of  log₁₀( F _p/ F _*) ∼−3.16  .     

The emission and kinematic structures of the bipolar planetary nebula M 1-8

We have undertaken echelle spectroscopy and narrow-band line imaging of the bipolar planetary nebula M 1-8. This has permitted us to map the outflow in [N  ii ]λλ 6548+6583 Å, Hα, and in the v = 1–0 S(1) transition of H₂ at λ 2.122 μm. It has also permitted us to acquire high-resolution spectra for [N  ii ]λ 6583 Å, Hα and He  ii λ 6560 Å. Our observations support the results of a previous 2MASS analysis by two of the authors (J. P. Phillips and G. Ramos-Larios), and confirm that there is strong H₂ emission outside of the ionized zone, as well as along the major axis of the outflow. Finally, we have investigated the spatial structure of the outflow in low and high excitation lines, and noted evidence for strong ionization stratification within the envelope of the source. We also note that major axis spectra show asymmetries attributable to outflow along the lobes, oriented at an angle i ∼ 35°–40° to the line of sight. Asymmetries along the minor axis, by contrast, appear to be associated with the central collimating disc, and may be interpretable in terms of asymmetries in disc structure, or rotation at an angular velocity of Ω∼ 1.4 10⁻¹² rad s⁻¹. If the disc arises due to common-envelope evolution, then it seems that angular momentum constraints must be relatively tight, and can only be satisfied given fairly extreme physical assumptions (such as low disc mass, high primary star mass, a low distance to the source and so forth).     

Evolutionary history of the elliptical galaxy NGC 1052

We have obtained Keck spectra for 16 globular clusters (GCs) associated with the merger remnant elliptical NGC 1052, as well as a long-slit spectrum of the galaxy. We derive ages, metallicities and abundance ratios from simple stellar population models using the recently published methods of Proctor & Sansom , applied to extragalactic GCs for the first time. A number of GCs indicate the presence of strong blue horizontal branches that are not fully accounted for in the current stellar population models. We find all of the GCs to be ∼13 Gyr old according to simple stellar populations, with a large range of metallicities. From the galaxy spectrum we find NGC 1052 to have a luminosity-weighted central age of ∼2 Gyr and metallicity of  [Fe/H]∼+0.6  . No strong gradients in either age or metallicity were found to the maximum radius measured  (0.3  r _e≃ 1 kpc)  . However, we do find a strong radial gradient in α-element abundance, which reaches a very high central value. The young central starburst age is consistent with the age inferred from the H  i tidal tails and infalling gas of ∼1 Gyr. Thus, although NGC 1052 shows substantial evidence for a recent merger and an associated starburst, it appears that the merger did not induce the formation of new GCs, perhaps suggesting that little recent star formation occurred. This interpretation is consistent with 'frosting' models for early-type galaxy formation.