Zanstra Temperatures Research Articles

PoWR grids of non-LTE model atmospheres for OB-type stars of various metallicities

The study of massive stars in different metallicity environments is a central topic of current stellar research. The spectral analysis of massive stars requires adequate model atmospheres. The computation of such models is difficult and time-consuming. Therefore, spectral analyses are greatly facilitated if they can refer to existing grids of models. Here we provide grids of model atmospheres for OB-type stars at metallicities corresponding to the Small and Large Magellanic Clouds, as well as to solar metallicity. In total, the grids comprise 785 individual models. The models were calculated using the state-of-the-art Potsdam Wolf-Rayet (PoWR) model atmosphere code. The parameter domain of the grids was set up using stellar evolution tracks. For all these models, we provide normalized and flux-calibrated spectra, spectral energy distributions, feedback parameters such as ionizing photons, Zanstra temperatures, and photometric magnitudes. The atmospheric structures (the density and temperature stratification) are available as well. All these data are publicly accessible through the PoWR website.

OUP accepted manuscript

PHR 1315-6555 is a rare case of a Galactic Planetary Nebula that is a proven member of the Open Cluster AL 1. This allows its distance to be defined with precision and thus the accurate measurement of its physical characteristics along with the parameters of its Central Star (CS). In this work we use HST to detect this unique CS and constrain the cluster's physical parameters. Our results suggest that the cluster rests at a distance of $\sim$12 kpc, is highly reddened, and has an age of around 0.66 Gyrs and a turn-off mass of $\sim$2.2 M$_\odot$. Our deep Colour Magnitude Diagram (CMD) suggests that the metallicity of the cluster is subsolar (Z=0.006). Our photometric measurements indicate that the PN's core is a faint blue star close to the nebular apparent centre, with an observed dereddened visual VEGA magnitude of 21.82 $\pm$ 0.60. A significant contribution from any possible binary companion is unlikely but possible. Our results show that the CS has an effective Zanstra temperature of around 113 kK and a mass of 0.58 M$_\odot$ providing a unique additional point to the fundamental White Dwarf Initial-to-Final-Mass Relation.

A catalogue of integrated Hα fluxes for 1258 Galactic planetary nebulae

We present a catalogue of new integrated H-alpha fluxes for 1258 Galactic planetary nebulae (PNe), with the majority, totalling 1234, measured from the Southern H-Alpha Sky Survey Atlas (SHASSA) and/or the Virginia Tech Spectral-line Survey (VTSS). Aperture photometry on the continuum-subtracted digital images was performed to extract H-alpha + [NII] fluxes in the case of SHASSA, and H-alpha fluxes from VTSS. The [NII] contribution was then deconvolved from the SHASSA flux using spectrophotometric data taken from the literature or derived by us. Comparison with previous work shows that the flux scale presented here has no significant zero-point error. Our catalogue is the largest compilation of homogeneously derived PN fluxes in any waveband yet measured, and will be an important legacy and fresh benchmark for the community. Amongst its many applications, it can be used to determine statistical distances for these PNe, determine new absolute magnitudes for delineating the faint end of the PN luminosity function, provide baseline data for photoionization and hydrodynamical modelling, and allow better estimates of Zanstra temperatures for PN central stars with accurate optical photometry. We also provide total H-alpha fluxes for another 75 objects which were formerly classified as PNe, as well as independent reddening determinations for ~270 PNe, derived from a comparison of our H-alpha data with the best literature H-beta fluxes. In an appendix, we list corrected H-alpha fluxes for 49 PNe taken from the literature, including 24 PNe not detected on SHASSA or VTSS, re-calibrated to a common zero-point.

Peculiar CNO photospheric abundances in the central star of NGC 2392

AbstractUsing new, high signal-to-noise CFHT ESPaDOnS visual spectrograms, and archive IUE and FUSE UV spectrograms, together with state-of-the-art non-LTE hydrodynamical model atmospheres, we have obtained accurate He, C, N, O photospheric abundance determinations in the central stars of NGC 2392, IC 4593, and NGC 6826. We compare with the corresponding nebular abundances, taken from the literature. The central star of NGC 2392 shows high He, N, and very low C, O abundances. We propose that these peculiar abundances must have originated in a common-envelope phase of interaction with a close binary companion. If we assume that the companion is more evolved than the visible central star, this offers a way of solving the old mystery of the discrepant He II Zanstra temperature of NGC 2392.

The importance of soft X-rays for the excitation of H2 emission in planetary nebulae

We note that H 2 emitting planetary nebulae tend to have Zanstra temperatures T z (HeII) > 90 kK. This is shown to be consistent with a large evolutionary lifetime, and the kinematic ages of the envelopes. Non-local thermodynamic equilibrium stellar atmospheric modelling also shows that levels of soft X-ray emission increase more rapidly than has previously been assumed, and are preferentially large in H 2 emitting sources. It is suggested that this may hold the key to explaining the strengths of the H 2 transitions.

Synthetic Spectral Analysis of the Hot Component in the S-Type Symbiotic Variable EG Andromedae

We have applied grids of non-LTE (NLTE) high-gravity model atmospheres and optically thick accretion disk models for the first time to archival IUE and FUSE spectra of the S-type symbiotic variable EG Andromedae taken at superior spectroscopic conjunction, when Rayleigh scattering should be minimal and the hot component is viewed in front of the red giant. For EG And's widely accepted, published hot-component mass, orbital inclination, and distance from the Hipparcos parallax, we find that hot, high-gravity, NLTE photosphere model fits to the IUE spectra yield distances from the best-fitting models that agree with the Hipparcos parallax distance, but at temperatures substantially lower than the modified Zanstra temperatures. NLTE fits to an archival FUSE spectrum taken at the same orbital phase as the IUE spectra yield the same temperature as the IUE temperature (50,000 K). However, for the same hot-component mass, inclination, and parallax-derived distance, accretion disk models at moderately high inclinations, ~60°–75°, with accretion rates from 1 × 10-8 to 1 × 10-9 M⊙ yr-1 for white dwarf masses Mwd = 0.4 M⊙ yield distances grossly smaller than the distance from the Hipparcos parallax. Therefore, we rule out an accretion disk as the dominant source of the far-UV flux. Our findings support a hot bare white dwarf as the dominant source of far-UV flux.

The statistical relation between Zanstra temperatures and emission-line ratios

A knowledge of the H I and He II Zanstra temperatures T Z (He II) and T Z (H I), and the Zanstra temperature ratio TR = T Z (He II)/T Z (H I) is of interest in evaluating the properties of planetary nebula (PNe) shells and their central stars. These parameters are only known for a small proportion of PNe however, and are very difficult to assess for more distant sources. It would therefore be of use if T Z (He II) and TR could be assessed using line ratios alone. It has been suggested that Stoy temperatures T STOY are related to the line ratios [O III]λ5007/Hβ and [Ne III]λ3868/Hβ, whilst Zanstra temperatures depend upon the level of nebular excitation. It has also been noted that TR may be correlated with [O II]λ3727/Hβ, He IIλ4686/Hβ and [N II]λ6548/Hα. It is therefore possible that line ratios may help to determine T Z (He II) and TR where normal procedures are inapplicable, albeit with lower levels of precision than is normally the case. We have, in the following, undertaken an investigation of the statistical relation between Zanstra temperatures and line ratios. This is used to evaluate the levels of precision to which TR and T Z (He II) can be determined using emission-line ratios alone. We find that the expected correlation between [O III]/Hβ and T Z (He II) is not borne out in the present analysis. It is also found that neither [N II]/Hα nor [O II]/Hδ is of use in defining the Zanstra temperature ratio TR. On the other hand, it is found that T Z (He II) can be determined using He IIλ4686/Hβ and a variety of other more complex line ratios, providing that log[T Z (He II)] < 5.2. The level of precision is of order σ{log[T Z (He II)]} ≅ 0.074 → 0.1. Similarly, ratios He IIλ4686/Hβ and He IIλ4686/He Iλ4471 can be used to determine TR to an accuracy of ∼0.25, providing that T Z (He II) < 8 × 10 4 K. It is noted that this latter parameter may permit estimates of shell Lyman continuum optical depth τ LyC , although the relation between TR and τ LyC is more complicated than has previously been supposed. All of these relations apply for metallicities similar to those encountered in the Galactic disc. They may not, however, be applicable where metallicities vary strongly from this, such as is the case in certain other extragalactic systems.

The correlation between planetary nebula morphology and radio brightness temperatures

We have used model central star evolutionary tracks to evaluate the expected variation of 5 GHz brightness tempera- tures TB in planetary nebulae. Using these results, and previously derived central star mass functions, we find that there should be a correlation between TB and the mean central star massMCS� , and also between TB and nebular morphology. An analysis of the available observational results suggests that such a correlation between TB and nebular morphology does indeed exist. Bipolar nebulae appear to be concentrated towards high brightness temperatures, whilst circular sources tend to possess lower brightness temperature shells. Elliptical sources are present (and common) at all values of TB .T here are differences however between the observed and deduced morphological variations, and these enable us to place constraints upon the progenitor masses of the nebulae. In particular, it is clear that higher mass progenitors are likely to give rise to both bipolar and elliptical outflows; a result which is consistent with analyses of nebular abundances, Zanstra temperatures, and line excitations.

The relation between Zanstra temperature and morphology in planetary nebulae

We have created a master list of Zanstra temperatures for 373 galactic planetary nebulae based upon a compilation of 1575 values taken from the published literature. These are used to evaluate mean trends in temperature for differing nebular morphologies. Among the most prominent results of this analysis is the tendency for η=TZ(He ii)/TZ(He i) to increase with nebular radius, a trend which is taken to arise from the evolution of shell optical depths. We find that as many as 87 per cent of nebulae may be optically thin to H ionizing radiation where radii exceed ∼0.16 pc. We also note that the distributions of values η and TZ(He ii) are quite different for circular, elliptical and bipolar nebulae. A comparison of observed temperatures with theoretical H-burning tracks suggests that elliptical and circular sources arise from progenitors with mean mass 〈MPG〉≅ 1 M⊙ (although the elliptical progenitors are probably more massive). Higher-temperature elliptical sources are likely to derive from progenitors with mass ≅2 M⊙, however, implying that these nebulae (at least) are associated with a broad swathe of progenitor masses. Such a conclusion is also supported by trends in mean galactic latitude. It is found that higher-temperature elliptical sources have much lower mean latitudes 〈b〉 than those with smaller TZ(He ii), a trend which is explicable where there is an increase in 〈MPG〉 with increasing TZ(He ii). This latitude–temperature variation also applies for most other sources. Bipolar nebulae appear to have mean progenitor masses ≅2.5 M⊙, whilst jets, Brets and other highly collimated outflows are associated with progenitors at the other end of the mass range (〈MPG〉∼ 1 M⊙). Indeed it is possible, given their large mean latitudes and low peak temperatures, that the latter nebulae are associated with the lowest-mass progenitors of all. The present results appear fully consistent with earlier analyses based upon nebular scale heights, shell abundances and the relative proportions of differing morphologies, and offer further evidence for a link between progenitor mass and morphology.

The Zanstra Temperatures Revisited

The temperature of the central stars in planetary nebulae is generally obtained by the Zanstra method. This method provides two values for the temperature of a single star: one obtained from the H recombination lines, TZ(H), and another from the He recombination lines, TZ(He II). The ratio (ZR) of these two values can be different from unit, leading to the well-known “Zanstra discrepancy”. The discrepancy is higher for lower temperature stars and the method does not reproduce the high values of stellar temperatures suggested by stellar evolutionary models. In this work a careful analysis of the effect of the optical depth on the determination of the Zanstra temperature is made, using photoionization models. The effects due to deviations from a blackbody spectrum, as well as to the He abundance in the nebulae, are also discussed. We show, quantitatively, that the details of the distribution of the H and He II Zanstra temperatures are mainly explained by an optical depth effect; in particular, the fact that the discrepancy is larger for low stellar temperatures. The results also show that for high stellar temperatures both Zanstra temperatures underestimate the stellar temperature, even for high optical depths. The stellar temperature, as well as the optical depth, can be obtained from a plot of ZR vs. TZ(He II). Since for nebulae of very low optical depth and/or high stellar temperature this plot only provides lower limits for T*, we propose the use of the line intensity ratio He II/He I vs. TZ(He II) diagram for obtaining the stellar temperature, as well as the nebular optical depth. The results of this work has been published in ApJ 543, 889 (2000).

The Correlations between Planetary Nebula Morphology and Central Star Evolution: Analysis of the Northern Galactic Sample

Northern Galactic planetary nebulae (PNs) are studied to disclose possible correlations between the morphology of the nebulae and the evolution of the central stars (CSs). To this end, we have built the best database available to date, accounting for homogeneity and completeness. We use updated statistical distances and an updated morphological classification scheme, and we calculate Zanstra temperatures for a large sample of PNs. With our study we confirm that round, elliptical, and bipolar PNs have different spatial distributions within the Galaxy, with average absolute distances to the Galactic plane of 0.73, 0.38, and 0.21 kpc, respectively. We also find evidence that the distributions of the CS masses are different across these morphological groups, although we do not find that CSs hosted by bipolar PNs are hotter, on average, than CSs within round and elliptical PNs. Our results are in broad agreement with previous analyses, indicating that round, elliptical, and bipolar PNs evolve from progenitors in different mass ranges and might belong to different stellar populations, as also indicated by the helium and nitrogen abundances of PNs of different morphology.

The Internal Extinction Curve of NGC 6302 and its Extraordinary Spectrum

AbstractIn this paper we present a new method for obtaining the optical wavelength-dependent reddening function of planetary nebulae (PN), using the nebular and stellar continuum. The data used was a spectrum of NGC 6302 obtained using the Double Beam Spectrograph on the 2.3 m telescope at Siding Springs Observatory over three nights. This resulted in a spectrum covering a wavelength range 3300–8600 Å with a large dynamical range and a mean signal to noise of &gt;102 Å−1 in the nebular continuum. With such a high S/N the continuum can be accurately compared with a theoretical model of nebular plus stellar continuum. The nebular electron temperature and density used in the model are determined using ratios of prominent emission lines. The reddening function can then be obtained from the ratio of the theoretical and the observed continuum. In the case of NGC 6302, it is known that much of the reddening arises from dust within or around the nebula, so that any differences between the measured reddening law and the 'standard' interstellar reddening law will reflect differences in the nebular grain size distribution or composition. We find that for NGC 6302, the visible to IR extinction law is indistinguishable from 'standard' interstellar reddening, but that the UV extinction curve is much steeper than normal, suggesting that more small dust grains had been ejected into the nebula by the PN central star. We have detected the continuum from the central star and determined its Zanstra temperature to be of order 150,000 K. Finally, using the extinction law that we have determined, we present a complete dereddened line list of nearly 600 emission lines, and report on the detection of the He(2–10) and He(2–8) Raman features at λ4331 Å and λ4852 Å, and the detection of Raman scattered O VI features at λ6830 Å and λ7087 Å. We believe this to be the first detection of this process in a PN.

Temperature of the Central Stars of Planetary Nebulae and the Effect of the Nebular Optical Depth

The effect of the nebula optical depth on the determination of the temperature (T*) of the central stars in planetary nebulae is discussed. Based on photoionization models for planetary nebulae with different optical depths, we show, quantitatively, that the details of the distribution of the H and He II Zanstra temperatures are mainly explained by an optical depth effect; in particular, that the discrepancy is larger for low stellar temperatures. The results also show that for high stellar temperatures the He II Zanstra temperature underestimates the stellar temperature, even for high optical depths. The stellar temperature, as well as the optical depth, can be obtained from a Zanstra temperature ratio (ZR) plot ZR = Tz(He II)/Tz(H) versus Tz(He II). The effects of departures from a blackbody spectrum, as well as of the He abundance in the nebulae, are also discussed. For nebulae of very low optical depth and/or high stellar temperature the distribution ZR versus Tz(He II) only provides lower limits for T*. In order to obtain better values for the optical depth and T*, we propose the use of the line intensity ratio He II/He I versus Tz(He II) diagram.

ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] Imaging of Central Stars of High-Excitation Planetary Nebulae with WFC and WFPC2

We present observations of four high excitation planetary nebulae (IC 2165, Me 2-1, NGC 2440, NGC 7027) taken with the Wide Field Camera and the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The high angular resolution of the instruments allows for the unambiguous identi—ca- tion of the central star, which was not found to be the case in most previous studies. In addition, the panchromatic nature of the data, which include several ultraviolet bandpasses, permits us to directly determine the amount of dust extinction present along the sight line to the central star. The combination of these two characteristics make it possible to delineate the spectral energy distribution of the central stars. Assuming a Planck function spectrum and a standard interstellar extinction law with the R V 3.1, adopted values are 0.40 (IC 2165), 0.15 (Me 2-1), 0.15 (NGC 2440), and 1.10 (NGC 7027). The E B~V wavelength coverage of our measurements is not particularly sensitive to temperature values in the range 150,000¨400,000 K. However, we do —nd that 200,000 K produces the best —ts. Again using a Planck function to represent the wavelength dependence of the stellar —ux, we determine V magnitudes of 17.47 ^ 0.06 (IC 2165), 18.40 ^ 0.05 (Me 2-1), 17.63 ^ 0.16 (NGC 2440), and 16.04 ^ 0.04 (NGC 7027). Combining these results with literature values for the emission-line —uxes and distances, we derive the Zanstra temperature, crossover magnitude, radius, and luminosities for each object. The implications of these measurements for the mass and the evolutionary state of each object is brie—y discussed.

CCD spectrophotometry of extended PN: Observations at Wendelstein Observatory

Most of the existing flux data on galactic PN is based on measurements with scanners or photoelectric aperture photometers. Also, true global fluxes are not even available for many PNe in lines other than Hβ, and one has to rely on line intensities which have been derived from slit spectrometers. For extended, low surface brightness objects all of these instruments are not appropriate to derive reliable fluxes (see e.g. remark on NGC 7293 by O'Dell, this conference). As part of a PhD thesis a dedicated CCD camera for the Wendelstein Observatory 0.8m telescope has been developed (Roth 1990, 1993), optimized for flux-calibrated imaging through narrow-band interference filters with bandpasses down to 8å FWHM and a thermal control unit for central wavelength tuning. As a test, nebular and central star fluxes have been measured for NGC 2392, NGC 3242, NGC4361, and NGC 6210 in Hβ, HeII 4686, [OIII] 5007, and a continuum passband. The results for these bright objects are in agreement with the best references as compiled from the literature. A detailed comparison shows that the spatial extent of the nebula is now always considered correctly, based on the CCD image information and the curve-of-growth convergence of the surface photometry. The central star photometry is largely improved, in particular for high surface brightness PNe. Another advantage (which however was not important for the test objects here) is the technically easy removal of disturbing field stars within the radius of a faint, extended nebula. The use of very narrow bandwidth, thermally controlled filters proved to be beneficial in substantially reducing the sky and nebular continuum background contribution. From the nebular and central star fluxes, Zanstra temperatures have been derived. The cause of the observed Zanstra discrepancy (which is in accordance with several other determinations for the investigated objects) cannot be attributed to observational uncertainties. From the discussion of Unified NLTE model atmospheres (Gabler et al. 1991) and model fits to the central stars of NGC2392, NGC3242, NGC4361 (Méndez et al. 1988), and NGC6210 (McCarthy et al. 1990) the most likely explanation is a combination of two effects: EUV excess over currently predicted central star flux distributions, and leakage of UV photons from the nebula (see also Méndez et al. 1992).

Properties of planetary nebulae with WR central stars

A review of the observational properties of the Wolf-Rayet central stars is given. It is a great puzzle why some of the central stars of planetary nebulae show a WR spectrum. It does not seem to be a temporary stage in the evolution of the central star, because WR stars are seen with the complete range of Zanstra temperatures which are observed in non-WR central stars. In this review we will present the observational material available for the nebulae with WR central stars, in an attempt to see where WR nebulae differ from non-WR nebulae, and in this way we will hope to find a clue as to the origin of the WR phenomenon. About 25 % of all central stars with an observed spectrum, show an emission line spectrum. They are not all classified as WR stars however. For example, Tylenda et al. (1993) divide the emission line stars in two groups: the WR stars and the "weak emission line" stars (usually called Of-WR stars). The difference between the two groups, as the name implies, is suggested to be in the strength of the emission lines. Unfortunately there is no atlas of the spectra of individual stars, so that a

Two-temperature X-ray emission from the planetary nebula NGC 7293

ROSAT Position Sensitive Proportional Counter (PSPC) observations of the planetary nebula NGC are reported here. This planetary nebula is here the first discovered to show clearly two components in its X-ray spectrum. A two-component model consisting of a blackbody and a Raymond-Smith thermal plasma is fitted to the observed ROSAT PSPC spectrum. This results in a temperature of T(sub 1) = 1.4 x 10(exp 5) K for the blackbody component and a temperature T(sub 2) = 8.7 x 10(exp 6) K for the hot plasma component, at a hydrogen column density N(sub H) = 1.4 x 10(exp 20)/sq cm. The temperature of the blackbody component is consistent with the helium Zanstra temperature of the central star, indicating that it may be attributed to the photosphere of the central star. The high-temperature component is possibly from a corona around the central star, which may be related to a strong convection in the star. An alternative explanation is that the hot plasma resides in a hot bubble predicted by the interacting wind model. A lower limit of the electron density in the hot plasma is found to be approximately 10/cu cm.

Diffuse X-ray emission from the Dumbbell Nebula?

We have analyzed ROSAT Position Sensitive Proportional Counter pointed observations of the Dumbbell Nebula and find that the previously reported 'extended' X-ray emission is an instrumental electronic ghost image at the softest energy band. At slightly higher energy bands, the image of the Dumbbell is not very different from that of the white dwarf HZ43. We conclude that the X-ray emission of the Dumbbell Nebula comes from its central star. A blackbody model is fitted to the spectrum and the best-fit temperature of not greater than 136,000 +/- 10,000 K is in excellent agreement with the Zanstra temperatures.

The nebular remnant and quiescent spectrum of Nova GK Persei

CCD images of the nebular shell of the old nova GK Per in [N II] and [O III] emission lines, and also the quiescent spectrum in the optical region, are presented. Proper motion measurements of the knots in [N II] over the baseline 1984–90 give a mean expansion rate of 0.31 ± 0.07 arcsec yr -1 . This value agrees with models of the expansion of the ejecta into the ambient circumstellar medium. A correction for projection effects indicates that the ratio of the mass of the shell to the density of the circumstellar medium is a factor of 2 – 3 higher than estimated previously. The optical spectrum is decomposed into those of the secondary and the accretion disc. The estimate of M V = + 4.9 for the accretion disc implies a mass transfer rate of 3.5 × 10 -9 M☉ yr -1 in the steady state. The H and He emission-line fluxes are used to infer that (i) the line-emitting region has a high density, (ii) the He/H abundance is near-solar, and (iii) the ionizing source has a Zanstra temperature of 1.3 × 10 5 K and an effective radius of 0.01 R☉, which are consistent with its being a white dwarf.

The Wind Temperature and C/He and O/He Ratios of the WC10 Central Star CPD-56°8032

We have analysed absolutely calibrated 3500–8900Å AAT spectrophotometry of the WC10 central star CPD–56°8032. After dereddening by E(B–V) = 0.68, the few line-free portions of the stellar continuum are well fitted by the 18000K blackbody which corresponds to the nebular H I Zanstra temperature. The Wolf-Rayet wind terminal velocity is sufficiently low (210–230 km s−1) that it is possible to resolve virtually every fine structure component of every multiplet of C II, C III and O I listed by the NBS Multiplet Tables for this wavelength region.