Wolf-Rayet Nebulae Research Articles

Ring nebulae around Wolf–Rayet stars in M33 as seen by SITELLE

ABSTRACT We have conducted an analysis of nebulae around Wolf–Rayet (WR) stars in M33 using data collected by the imaging Fourier transform spectrometer SITELLE at the Canada–France–Hawaii telescope as part of the SIGNALS Large Program. Of the 211 known WR stars in M33, 178 are located in the fields observed in this study. We present the results of this analysis in the form of a comprehensive summary of all nebulae found around the observed WR stars. Based on three criteria we find to be the most effective for their detection, we detect a clear association with a circumstellar bubble around 33 of them (19 per cent). Our results show that the presence of bubbles does not correlate with the spectral type of the central star. The mean diameter of the WR nebulae we have found is 21 parsec.

On the bipolarity of Wolf–Rayet nebulae

ABSTRACT Wolf–Rayet stars are amongst the rarest but also most intriguing massive stars. Their extreme stellar winds induce famous multiwavelength circumstellar gas nebulae of various morphologies, spanning from circles and rings to bipolar shapes. This study is devoted to the investigation of the formation of young, asymmetric Wolf–Rayet gas nebulae and we present a 2.5-dimensional magneto-hydrodynamical toy model for the simulation of Wolf–Rayet gas nebulae generated by wind–wind interaction. Our method accounts for stellar wind asymmetries, rotation, magnetization, evolution, and mixing of materials. It is found that the morphology of the Wolf–Rayet nebulae of blue supergiant ancestors is tightly related to the wind geometry and to the stellar phase transition time interval, generating either a broadened peanut-like or a collimated jet-like gas nebula. Radiative transfer calculations of our Wolf–Rayet nebulae for dust infrared emission at $24\, \mu \rm m$ show that the projected diffuse emission can appear as oblate, bipolar, ellipsoidal, or ring structures. Important projection effects are at work in shaping observed Wolf–Rayet nebulae. This might call a revision of the various classifications of Wolf–Rayet shells, which are mostly based on their observed shape. Particularly, our models question the possibility of producing pre-Wolf–Rayet wind asymmetries, responsible for bipolar nebulae like NGC 6888, within the single red supergiant evolution channel scenario. We propose that bipolar Wolf–Rayet nebulae can only be formed within the red supergiant scenario by multiple/merged massive stellar systems, or by single high-mass stars undergoing additional, e.g. blue supergiant, evolutionary stages prior to the Wolf–Rayet phase.

A Multiwavelength Survey of Wolf–Rayet Nebulae in the Large Magellanic Cloud

Abstract Surveys of Wolf–Rayet (WR) stars in the Large Magellanic Cloud (LMC) have yielded a fairly complete catalog of 154 known stars. We have conducted a comprehensive, multiwavelength study of the interstellar/circumstellar environments of WR stars, using the Magellanic Cloud Emission Line Survey images in the Hα, [O iii], and [S ii] lines; Spitzer Space Telescope 8 and 24 μm images; Blanco 4 m Telescope Hα CCD images; and Australian Telescope Compact Array + Parkes Telescope H i data cube of the LMC. We have also examined whether the WR stars are in OB associations, classified the H ii environments of WR stars, and used this information to qualitatively assess the WR stars’ evolutionary stages. The 30 Dor giant H ii region has active star formation and hosts young massive clusters, thus we have made statistical analyses for 30 Dor and the rest of the LMC both separately and altogether. Due to the presence of massive young clusters, the WR population in 30 Dor is quite different from that from elsewhere in the LMC. We find small bubbles (<50 pc diameter) around ∼12% of WR stars in the LMC, most of which are WN stars and not in OB associations. The scarcity of small WR bubbles is discussed. Spectroscopic analyses of abundances are needed to determine whether the small WR bubbles contain interstellar medium or circumstellar medium. Implications of the statistics of interstellar environments and OB associations around WR stars are discussed. Multiwavelength images of each LMC WR star are presented.

Magnetised gas nebulae of evolved massive stars

AbstractMassive stars are amongst the rarest but also most intriguing stars. Their extreme, magnetised stellar winds induce, by wind-ISM interaction, famous multi-wavelengths circumstellar gas nebulae of various morphologies, spanning from large-scale wind bubbles to stellar wind bow shocks, rings and bipolar shapes. We present two- and three-dimensional magneto-hydrodynamical (MHD) simulations of the circumstellar medium of such massive stars at different phase of their evolution. Particularly, we investigate the stability properties of 3D MHD bow shock nebulae around the runaway red supergiant stars IRC-10414 and Betelgeuse. Our results show that their astrospheres are stabilised by an organised, non-parallel ambient magnetic field. These findings suggest that Betelgeuse’s bar is of interstellar origin. Last, we explore the circular aspect of the young nebula around the Wolf-Rayet stars. It is found that Wolf-Rayet nebulae are not affected by the ISM gas distribution in which the stellar objects lie, even in the case of fast stellar motion: as testifies the ring-like surroundings of the Milky Way’s fastest Wolf-Rayet star, WR124. The morphology of these nebulae is tightly related to their pre-Wolf-Rayet wind geometry and to their phase evolution transition properties, which can generate bipolar shapes. We will further discuss their diffuse projected emission by means of radiative transfer calculations and show that the projected diffuse emission can appear as bipolar structures as in NGC6888.

Spectroscopic Observation of Wolf-Rayet Stars: Study of Expansion Velocity and Mass Loss as Contributors of Interstellar Matter Enrichment

Wolf-Rayet (WR) stars exhibit broad emission lines in their spectra. Based on the featured lines, they are classified into three types: nitrogen-rich WN, carbon-rich WC, and oxygen-rich WO. Broad emission lines indicate high expansion velocity and also previous studies showed that WR stars possessed higher mass-loss rates compared with another type of stars. Thus, it is suggested that the expansion velocity and the mass loss of WR stars play a role in the enrichment of interstellar matter. Our observations were conducted between July 2018 and June 2019 using NEO-R1000 (R∼1000) spectrograph with spectral range 4500-7000Å at the Bosscha Observatory and consisted of WN and WC type stars. Data reduction and expansion velocities determination were carried out using IRAF with longslit-transform routine and Gaussian fitting respectively. To obtain mass-loss rates, we compared our observed spectra with models provided by Potsdam Wolf-Rayet Models (PoWR). In several cases, the contribution of WR stars can be directly observed as Wolf-Rayet nebulae, whose structures are influenced by the high expansion velocity of the nearby WR stars. Assuming that the total of WR stars in the Milky Way is ∼ 2600, we estimated that the population release ∼ 0.507 M⊙ of material annually into Galactic environment.

On the X-ray temperature of hot gas in diffuse nebulae

X-ray emitting diffuse nebulae around hot stars are observed to have soft-band temperatures in the narrow range [1-3]$\times10^{6}$ K, independent of the stellar wind parameters and the evolutionary stage of the central star. We discuss the origin of this X-ray temperature for planetary nebulae (PNe), Wolf-Rayet nebulae (WR) and interstellar wind bubbles around hot young stars in our Galaxy and the Magellanic Clouds. We calculate the differential emission measure (DEM) distributions as a function of temperature from previously published simulations and combine these with the X-ray emission coefficient for the 0.3-2.0 keV band to estimate the X-ray temperatures. We find that all simulated nebulae have DEM distributions with steep negative slopes, which is due to turbulent mixing at the interface between the hot shocked stellar wind and the warm photoionised gas. Sharply peaked emission coefficients act as temperature filters and emphasize the contribution of gas with temperatures close to the peak position, which coincides with the observed X-ray temperatures for the chemical abundance sets we consider. Higher metallicity nebulae have lower temperature and higher luminosity X-ray emission. We show that the second temperature component found from spectral fitting to X-ray observations of WR nebulae is due to a significant contribution from the hot shocked stellar wind, while the lower temperature principal component is dominated by nebular gas. We suggest that turbulent mixing layers are the origin of the soft X-ray emission in the majority of diffuse nebulae.

Wolf-Rayet nebulae

Since the discovery of nebulae around Wolf-Rayet (WR) stars in the 1960s, it has been established that WR stars are massive stars at advanced evolutionary stages and that their surrounding nebulae result from the interactions between the stellar mass loss and the ambient interstellar medium. Surveys of WR nebulae have been made in the Galaxy, Magellanic Clouds, and other nearby galaxies in the Local Group. Some WR nebulae exhibit He II λ4686 line emission, indicating stellar effective temperatures of 90 — 100 x 103 K. The shocked fast stellar winds from WR nebulae have been detected in soft X-rays, but theoretical models have not been able to reproduce the observed X-ray spectral properties. Elemental abundances of WR nebulae consisting of synthesized stellar material can constrain stellar evolution models, but high-dispersion spectra are needed to kinematically separate the expanding shell of a WR nebula and the background interstellar medium for accurate abundance analyses.

WISE morphological study of Wolf-Rayet nebulae

We present a morphological study of nebulae around Wolf-Rayet (WR) stars using archival narrow-band optical and Wide-field Infrared Survey Explorer (WISE) infrared images. The comparison among WISE images in different bands and optical images proves to be a very efficient procedure to identify the nebular emission from WR nebulae, and to disentangle it from that of the ISM material along the line of sight. In particular, WR nebulae are clearly detected in the WISE W4 band at 22 $\mu$m. Analysis of available mid-IR Spitzer spectra shows that the emission in this band is dominated by thermal emission from dust spatially coincident with the thin nebular shell or most likely with the leading edge of the nebula. The WR nebulae in our sample present different morphologies that we classified into well defined WR bubbles (bubble ${\cal B}$-type nebulae), clumpy and/or disrupted shells (clumpy/disrupted ${\cal C}$-type nebulae), and material mixed with the diffuse medium (mixed ${\cal M}$-type nebulae). The variety of morphologies presented by WR nebulae shows a loose correlation with the central star spectral type, implying that the nebular and stellar evolutions are not simple and may proceed according to different sequences and time-lapses. We report the discovery of an obscured shell around WR35 only detected in the infrared.

The spectroscopic properties of the nebulae around seven Galactic and LMC Wolf-Rayet stars

We have obtained spectroscopic observations of the nebulae around seven Wolf-Rayet (WR) stars, four in the Milky Way (WR 8, 16, 18 and 40) and three in the LMC (BAT99-2, -11 and -38). They were observed using the ESO NTT with the EFOSC-2 instrument, except for one nebula, NGC 3199, which was observed using the UVES echelle spectrometer on the VLT. The aims of these observations were to (a) quantify the degree of chemical enrichment in WR nebulae which had previously been suggested to have abundances reflecting nucleosynthetic processing; and (b) to attempt to detect the far-red lines of neutral carbon (e.g. [C I] 9850A) in the nebulae around WC stars. Nebular densities, temperatures and elemental abundances were derived using standard emission line diagnostics. Our high spectral resolution UVES dataset for NGC 3199 allowed the determination of line broadening temperatures using lines from several different species. These showed consistent patterns and fair agreement with the nebular temperatures derived from diagnostic line ratios. Amongst the Galactic WR nebulae, the nebulae around WR 8, 16 and 40 were found to be strongly nitrogen-enriched while NGC 3199 displayed abundances which were similar to those of Galactic H II regions. Amongst the LMC WR nebulae, none displayed an N/O ratio that significantly exceeded LMC H II region values. The far-red [C I] lines were detected from NGC 3199, with the strength of the 8727A line indicating that at least part of it was produced by C+ recombination rather than collisional excitation. The line widths of the [C I] 9824 and 9850A lines were however the same as those of collisionally excited nebular lines, indicating that those lines originated largely from ionized regions within the nebula.

A search for ejecta nebulae around Wolf-Rayet stars using the SHS Hα survey

Recent large scale Galactic Plane H$\alpha$ surveys allow a re-examination of the environs of Wolf-Rayet (WR) stars for the presence of a circumstellar nebula. Using the morphologies of WR nebulae known to be composed of stellar ejecta as a guide, we constructed ejecta nebula criteria similar to those of Chu (1991) and searched for likely WR ejecta nebula in the SHS H$\alpha$ survey. A new Wolf-Rayet ejecta nebula around WR 8 is found and its morphology discussed. The fraction of WR stars with ejecta type nebulae is roughly consistent between the MilkyWay (MW) and LMC at around 5-6%, with the MW sample dominated by nitrogen rich WR central stars (WN type) and the LMC stars having a higher proportion of carbon rich WR central stars (WC type). We compare our results with those of previous surveys, including those of Marston (1997) and Miller & Chu (1993), and find broad consistency. We investigate several trends in the sample: most of the clear examples of ejecta nebulae have WNh central stars; and very few ejecta nebulae have binary central stars. Finally, the possibly unique evolutionary status of the nebula around the binary star WR 71 is explored.

Towards an explanation for the 30 Dor (LMC) Honeycomb nebula - the impact of recent observations and spectral analysis

The unique Honeycomb nebula, most likely a peculiar supernova remnant, lies in 30 Doradus in the Large Magellanic Cloud. Due to its proximity to SN1987A, it has been serendipitously and intentionally observed at many wavelengths. Here, an optical spectral analysis of forbidden line ratios is performed in order to compare the Honeycomb high-speed gas with supernova remnants in the Galaxy and the LMC, with galactic Wolf-Rayet nebulae and with the optical line emission from the interaction zone of the SS433 microquasar and W50 supernova remnant system. An empirical spatiokinematic model of the images and spectra for the Honeycomb reveals that its striking appearance is most likely due to a fortuitous viewing angle. The Honeycomb nebula is more extended in soft X-ray emission and could in fact be a small part of the edge of a giant LMC shell revealed for the first time in this short wavelength domain. It is also suggested that a previously unnoticed region of optical emission may in fact be an extension of the Honeycomb around the edge of this giant shell. A secondary supernova explosion in the edge of a giant shell is considered for the creation of the Honeycomb nebula. A microquasar origin of the Honeycomb nebula as opposed to a simple supernova origin is also evaluated.

Mass-loss in central stars of planetary nebulae and in Wolf-Rayet stars

In the present paper we review some basic facts regarding the similarities and differences between Galactic planetary nebulae (PN) and Galactic Wolf-Rayet (WR) nebulae within the scope of stellar mass-loss history and its subsequent impact on the nebular dynamics and morphology. The case of planetary nebulae with [WR] nucleus, which allows one to perform a more direct comparison with WR nebulae, is emphasized. In particular, we review selected studies on the PN NGC 40 and on the WR nebula M 1-67. We describe the apparently ubiquitous turbulent-like phenomena originating in WR/[WR] stellar atmospheres and the surrounding nebulae, and discuss the possible impact of turbulence on ejected nebula studies. The properties of turbulence observed in ejected nebulae (apparently supersonic) are at odds with the ones related to Kolmogorov's subsonic, incompressible turbulence. The large compressibility of the nebular gases and the possible multiplicity of turbulence energy sources are likely the main explanations for that difference. The Universal Multifractal approach of Lovejoy & Schertzer is presented and constitutes a physically realistic and natural way of modeling and quantifying nebular turbulence data with only three independent parameters, including the key C1 intermittence parameter.

Planetary Nebulae and Wolf-Rayet Nebulae

In the present paper I review some important facts regarding the similarities and differences between Galactic planetary nebulae and Galactic Wolf-Rayet (WR) nebulae within the scope of stellar mass-loss history and its subsequent impact on the nebular dynamics and morphology. The case of planetary nebulae with [WR] nuclei, which allows one to perform a more direct comparison with WR nebulae, is emphasized. In particular, I describe the apparently ubiquitous turbulent-like phenomena originating in [WR] stellar atmospheres and the surrounding nebulae, and discuss the possible impact of turbulence on planetary nebula studies.

Empirical Bounds for the Ionizing Fluxes of Wolf‐Rayet Stars

Hα photometry and spectroscopic data have been obtained for 10 Wolf-Rayet nebulae, representing a wide variety of WN spectral types. We use these data to constrain the ionizing flux of the exciting Wolf-Rayet star, calculating lower bounds for the Lyman continuum flux (Q0) and for the He0- and He+-ionizing fluxes (Q1 and Q2). We find that Q0 appears independent of WN spectral type, and lower bound estimates tend to cluster around 48 dex. Finally, we discuss the effects of potential shock excitation and density bounding on these nebulae and compare our results to recent models. Our results are consistent with the predictions of line-blanketed ISA-wind models and nonblanketed CMFGEN models but are consistent with only some of the line-blanketed CMFGEN models.

The AG Carinae nebula: abundant evidence for a red supergiant progenitor?

AG Carinae is a massive, evolved supergiant which is thought to be in transition from an O star to a Wolf-Rayet (WR) star and is currently identified as a luminous blue variable (LBV) with log L/L⊙ = 6.0. We present an abundance study of the éjecta nebula surrounding AG Car with the aim of elucidating the evolutionary history of the central star. Physical parameters and abundances are derived for five regions across the nebula from high spatial resolution spectroscopy obtained at the Anglo-Australian Telescope (AAT). We derive an average Te of 6350 ± 400 K, an ne of 820 ± 170 cm−3, and find that nitrogen (N) is enhanced by a factor of 4.5 ±1.3 and that oxygen (O) is deficient by a factor of 15.1 ± 7.2. The derived abundances are compared with those determined for ejecta-type nebulae around WR stars and those predicted by hydrodynamical calculations and stellar evolutionary models. We find that the AG Car nebula is composed of mildly processed material that has not reached the CNO-equilibrium abundances predicted for LBV nebulae. The similarity of the AG Car nebular N abundance to WR nebulae leads us to suggest that the nebulae were ejected at the same evolutionary point, and have undergone no further chemical modification. For AG Car, this point appears to have occurred before the LBV phase because of the observed low N enrichment. Comparison of the observed N abundance with evolutionary model predictions indicates that the AG Car nebula may represent the hydrogen-rich (H-rich) envelope of a red supergiant (RSG). The problem of an RSG progenitor for AG Car is discussed and it is found that the LBV model of Stothers & Chin, incorporating a brief unstable RSG phase, is capable of explaining the observations. We conclude that despite its high luminosity, AG Car has probably experienced a brief RSG phase where it ejected its outer layers to form the currently observed nebula.

Properties of hot massive stars from studies of their ring nebulae

Wolf-Rayet (WR) stars and their possible precursors, the Luminous Blue Variables (LBVs), are often surrounded by ring nebulae. It is believed that these nebulae are formed by the action of the stellar wind, matter being ejected from the star in the past, or a combination of these two processes. The various research applications of LBV and WR nebulae are reviewed with regard to the information they provide on the properties of the central stars. They are very useful probes of stellar evolution since the details of the previous evolutionary phases of the central stars are contained in the nebulae. In particular, abundance studies can provide insights into the chemical processes operating during the precursor phases. The nebulae can also be used as probes of the far-UV ionizing flux distribution of the central WR stars by comparing the observed levels of nebular ionization with those produced with non-LTE WR model flux distributions. Comparisons of stellar effective temperatures and luminosities derived using this technique with those determined by modelling stellar emission lines can identify deficiencies such as the lack of line-blanketing in WR model atmospheres. Very hot WR stars can also be identified by searching for nebular HeII emission. Studies of bipolar structures in LBV and WR nebulae provide valuable clues on wind asymmetries in the central stars.

Narrow-band photometry of Wolf-Rayet stars and planetary nebulae

We present observational data in the (16)Λ(9)Ψ(25)-photometric system for 11 Wolf-Rayet stars and 7 planetary nebulae. The results show anomalous α(16)Λ(9)-indices for these emission objects. Six W-R stars show possible variations in the strength of the HeII line at λ6560 A, perhaps none in the strength of the HeI line at λ10830 A. In spite of these misconstrued α(16) and Λ(9)-indices, most WC stars are separated from WN stars in the α(16)Λ(9)-array; and PN, in this diagram lie far apart from all kind of stellar objects. The results, yielded by the Ψ(25)-index, indicate that the HeI and HeII lines at λ10830 A and λ10124 A, respectively, are probably well suited for photometric examination in W-R stars, PN and other peculiar objects.

Wolf-Rayet nebulae - Enrichment in He and N and effective temperatures of Wolf-Rayet stars

Nebulae surrounding isolated Pop I WR stars provide observational constraints on as yet poorly determined chemical surface abundances and FUV energy distributions of their central stars. An integral of the mass loss history and the chemical evolution is stored in those parts of the nebulae that have suffered only very little mixing with the ISM (cf. Kwitter 1984). Effective temperatures in the Lyman continuum region are reflected in the ionization structure of oxygen and sulfur (cf. Mathis 1982). For intrinsic problems involved refer to the papers cited above.