Late WN Stars Research Articles

A radio census of the massive stellar cluster Westerlund 1

Context. Massive stars and their stellar winds are important for a number of feedback processes. The mass lost in the stellar wind can help determine the end-point of the star as a neutron star (NS) or a black hole (BH). However, the impact of mass loss on the post-main sequence evolutionary stage of massive stars is not well understood. Westerlund 1 is an ideal astrophysical laboratory in which to study massive stars and their winds in great detail over a large range of different evolutionary phases. Aims. We aim to study the radio emission from Westerlund 1, in order to measure radio fluxes from the population of massive stars, and determine mass-loss rates and spectral indices where possible. Methods. Observations were carried out in 2015 and 2016 with the Australia Telescope Compact Array (ATCA) at 5.5 and 9 GHz using multiple configurations, with maximum baselines ranging from 750 m to 6 km. Results. Thirty stars are detected in the radio from the fully concatenated dataset, ten of which are Wolf-Rayet stars (WRs) (predominantly late type WN stars), five yellow hypergiants (YHGs), four red supergiants (RSGs), one luminous blue variable (LBV), the sgB[e] star W9, and several OB supergiants. New source detections in the radio are found for five WR stars, and five OB supergiants. These detections lead to evidence for three new OB supergiant binary candidates, which is inferred from derived spectral index limits. Conclusions. Spectral indices and index limits were determined for massive stars in Westerlund 1. For cluster members found to have partially optically thick emission, mass-loss rates were calculated. Under the approximation of a thermally emitting stellar wind and a steady mass-loss rate, clumping ratios were then estimated for eight WRs. Diffuse radio emission was detected throughout the cluster. Detections of knots of radio emission with no known stellar counterparts indicate the highly clumped structure of this intra-cluster medium, likely shaped by a dense cluster wind.

Westerlund 1 is a Galactic Treasure Chest: The Wolf-Rayet Stars

AbstractThe Westerlund 1 Galactic cluster hosts an eclectic mix of coeval massive stars. At a modest distance of 4–5 kpc, it offers a unique opportunity to study the resolved stellar content of a young (~5 Myr) high mass (5·104M⊙) star cluster. With the aim of testing single-star evolutionary predictions, and revealing any signatures of binary evolution, we discuss on-going analyses of NTT/SOFI near-IR spectroscopy of Wolf-Rayet stars in Westerlund 1. We find that late WN stars are H-poor compared to their counterparts in the Milky Way field, and nearly all are less luminous than predicted by single-star Geneva isochrones at the age of Westerlund 1.

The distance to the young open cluster Westerlund 2

A new X-ray, {\it UBVR}$I_c$, and {\it JHK$s$} study of the young cluster Westerlund 2 was undertaken to resolve discrepancies tied to the cluster's distance. Existing spectroscopic observations for bright cluster members and new multi-band photometry imply a reddening relation towards Westerlund~2 described by $E_{U-B}/E_{B-V}=0.63 + 0.02\;E_{B-V}$. Variable-extinction analyses for Westerlund~2 and nearby IC 2581 based upon spectroscopic distance moduli and ZAMS fitting yield values of $R_V=A_V/E_{B-V}=3.88\pm0.18$ and $3.77\pm0.19$, respectively, and confirm prior assertions that anomalous interstellar extinction is widespread throughout Carina (e.g., Turner 2012). The results were confirmed by applying the color difference method to {\it UBVR$I_c$JH$K_s$} data for 19 spectroscopically-observed cluster members, yielding $R_V=3.85\pm0.07$. The derived distance to Westerlund~2 of $d=2.85\pm0.43$ kpc places the cluster on the far side of the Carina spiral arm. The cluster's age is no more than $\tau \sim2\times10^6$ yr as inferred from the cluster's brightest stars and an X-ray (Chandra) cleaned analysis of its pre-main-sequence demographic. Four Wolf-Rayet stars in the cluster core and surrounding corona (WR20a, WR20b, WR20c, and WR20aa) are likely cluster members, and their inferred luminosities are consistent with those of other late-WN stars in open clusters. The color-magnitude diagram for Westerlund~2 also displays a gap at spectral type B0.5 V with associated color spread at higher and lower absolute magnitudes that might be linked to close binary mergers. Such features, in conjunction with the evidence for mass loss from the WR stars, may help to explain the high flux of $\gamma$ rays, cosmic rays, and X-rays from the direction towards Westerlund~2.

A new Wolf-Rayet star and its circumstellar nebula in Aquila★

We report the discovery of a new Wolf–Rayet star in Aquila via detection of its circumstellar nebula (reminiscent of ring nebulae associated with late WN stars) using the Spitzer Space Telescope archival data. Our spectroscopic follow-up of the central point source associated with the nebula showed that it is a WN7h star (we named it WR 121b). We analysed the spectrum of WR 121b by using the Potsdam Wolf–Rayet model atmospheres, obtaining a stellar temperature of ≃50 kK. The stellar wind composition is dominated by helium with ∼20 per cent of hydrogen. The stellar spectrum is highly reddened [E(B−V) = 2.85 mag]. Adopting an absolute magnitude of Mv=−5.7, the star has a luminosity of log L/L⊙= 5.75 and a mass-loss rate of 10−4.7 M⊙ yr−1, and resides at a distance of 6.3 kpc. We searched for a possible parent cluster of WR 121b and found that this star is located at ≃1° from the young star cluster embedded in the giant H ii region W43 (containing a WN7+a/OB? star – WR 121a). We also discovered a bow shock around the O9.5III star ALS 9956, located at from the cluster. We discuss the possibility that WR 121b and ALS 9956 are runaway stars ejected from the cluster in W43.

Discovery of a new Wolf���Rayet star and its ring nebula in Cygnus

We report the serendipitous discovery of a ring nebula around a candidate Wolf-Rayet (WR) star, HBHA 4202-22, in Cygnus using the Spitzer Space Telescope archival data. Our spectroscopic follow-up observations confirmed the WR nature of this star (we named it WR138a) and showed that it belongs to the WN8-9h subtype. We thereby add a new example to the known sample of late WN stars with circumstellar nebulae. We analyzed the spectrum of WR138a by using the Potsdam Wolf-Rayet (PoWR) model atmospheres, obtaining a stellar temperature of 40 kK. The stellar wind composition is dominated by helium with 20 per cent of hydrogen. The stellar spectrum is highly reddened and absorbed (E_{B-V} = 2.4 mag, A_V = 7.4 mag). Adopting a stellar luminosity of log L/Lsun = 5.3, the star has a mass-loss rate of 10^{-4.7} Msun yr^{-1}, and resides in a distance of 4.2 kpc. We measured the proper motion for WR138a and found that it is a runaway star with a peculiar velocity of ~50 km/s. Implications of the runaway nature of WR138a for constraining the mass of its progenitor star and understanding the origin of its ring nebula are discussed.

Massive star populations in Wolf-Rayet galaxies

We analyze longslit spectral observations of fourteen Wolf-Rayet galaxies from the sample of Schaerer, Contini & Pindao (1999). All 14 galaxies show broad Wolf-Rayet emission in the blue region of the spectrum, consisting of a blend of NIII�4640, CIII�4650, CIV�4658, and HeII�4686 emission lines, which is a spectral characteristic of WN stars. Broad CIV�5808 emission, termed the red bump, is detected in 9 galaxies and CIII�5996 is detected in 6 galaxies. These emission features are due to WC stars. We derive the numbers of late WN and early WC stars from the luminosity of the blue and red bumps, respectively. � (. . :.

An exceptional population of late-type WC stars in the metal-rich spiral galaxy M 83

We have surveyed the metal-rich spiral galaxy M 83 (NGC 5236) for its Wolf-Rayet population using VLT-FORS2 narrow-band imaging and follow-up spectroscopy. From a total of 280 candidates identified using He  λ4686 imaging, Multi Object Spectroscopy of 198 sources was carried out, revealing 132 objects containing bona-fide Wolf-Rayet features. From this sample, an exceptional W-R content of ∼1030 is inferred, with N(WC)/N(WN) ∼ 1.3, continuing the trend to larger values at higher metallicity amongst Local Group galaxies. More dramatic is the dominance of late-type WC stars in M 83 with N(WC8−9)/N(WC4−7) = 9 which we attribute to the sensitivity of the classification line C  λ5696 to mass-loss, providing the strength of WC winds scale with metallicity. One young massive compact cluster, #74 in our catalogue, hosts 20% of the entire galactic population, namely ∼180 late WC stars and ∼50 late WN stars.

A Spectroscopic Study of a Large Sample Of Wolf‐Rayet Galaxies

We analyze long-slit spectral observations of 39 Wolf-Rayet (WR) galaxies with heavy element mass fraction ranging over 2 orders of magnitude, from Z☉/50 to 2Z☉. Nearly all galaxies in our sample show broad WR emission in the blue region of the spectrum (the blue bump) consisting of an unresolved blend of N III λ4640, C III λ4650, C IV λ4658, and He II λ4686 emission lines. Broad C IV λ5808 emission (the red bump) is detected in 30 galaxies. Additionally, weaker WR emission lines are identified, most often the N III λ4512 and Si III λ4565 lines, which have very rarely or never been seen and discussed before in WR galaxies. These emission features are characteristic of WN7-WN8 and WN9-WN11 stars, respectively. We derive the numbers of early WC (WCE) and late WN (WNL) stars from the luminosities of the red and blue bumps, and the number of O stars from the luminosity of the Hβ emission line. Additionally, we propose a new technique for deriving the numbers of WNL stars from the N III λ4512 and Si III λ4565 emission lines. This technique is potentially more precise than the blue-bump method because it does not suffer from contamination of WCE and early WN (WNE) stars and nebular gaseous emission. It is found that the relative number of WR stars N(WR)/N(O+WR) decreases with decreasing metallicity, in agreement with predictions of evolutionary synthesis models. The relative number ratios N(WC)/N(WN) and the equivalent widths of the blue bump EW(λ4650) and of the red bump EW(λ5808) derived from observations are also in satisfactory agreement with theoretical predictions, except for the most metal-deficient WR galaxies. A possible source of disagreement is too low a line emission luminosity adopted for a single WCE star in low-metallicity models. We assemble a sample of 30 H II regions with detected He II λ4686 nebular emission to analyze the possible connection of this emission with the hard UV radiation of the WR stars. The theoretical predictions satisfactorily reproduce the observed intensities and equivalent widths of the He II λ4686 nebular emission line. However, galaxies with nebular He II λ4686 emission do not always show WR emission. Therefore, in addition to the ionization of He+ in the H II region by WR stars, other mechanisms for the origin of He II λ4686 such as radiative shocks probably need to be invoked.

The OB Zoo: A Digital Atlas of Peculiar Spectra

ABSTRACTA digital atlas of 20 high‐luminosity, peculiar OB spectra in the 3800–4900 Å range is presented. The atlas is organized and discussed in terms of the following four categories: WN‐A or WNL stars, OB Iape or very late WN (WNVL) stars, iron stars, and B‐supergiant luminous blue variables (LBVs). Several objects in the earlier categories are also active or quiescent LBVs. Some (but not all) of these objects have been well studied, and extensive references are provided, as are comprehensive spectral‐line identifications. Several new morphological relationships among the objects have been recognized through this presentation. In particular, attention is drawn to the occurrence of spatial pairing between nearly identical, unusual spectra, which may have implications for a particular mode of massive‐star formation. This small sample includes one or both members of at least five such pairs. Physical explanations of these peculiar, likely transitional spectra and the relationships among them are essential for a complete understanding of massive stellar evolution.

Quantitative spectroscopy of Wolf—Rayet stars in HD 97950 and R136a — the cores of giant H ii regions

We present quantitative analyses of Wolf–Rayet stars in the cores of two giant H ii regions — HD 97950 in NGC 3603 and R136a in 30 Doradus — based on archive Hubble Space Telescope (HST) spectroscopy. We confirm previous WN6h+abs classifications for components A1, B and C in HD 97950, while classifications for R136a1–3 are revised from O3 If*/WN6 to WN5h. From detailed non-local thermodynamic equilibrium analyses, we find that all Wolf–Rayet stars exhibit products of CNO-processed material at their surface since they are rich in both helium (H/He ≈ 3–6, by number) and nitrogen (N/He ≈ 0.002–0.006). Their luminosities, log (L/L⊙) = 6.0–6.3, are amongst the highest known for Wolf–Rayet stars. Consequently they are very massive stars (Minit ≥ 100 M⊙) at a relatively low age (∼ 2 Myr), reminiscent of the late WN stars in the Carina Nebula. We obtain a revised distance modulus of 15.03 mag (= 10.1 kpc) to NGC 3603 based on available photometry, an updated Mv calibration for early O stars and a reddening of E (B − V) = 1.23 mag towards its core. From a census of the massive stellar content of the two central clusters we conclude that their global properties are comparable. We evaluate the contribution made by Wolf–Rayet stars to the total Lyman continuum ionizing flux and kinetic energy released into the ISM. We discuss how simple calibrations can be used to estimate stellar luminosities, ionizing fluxes and mass-loss rates of luminous OB stars. Wolf–Rayet stars provide ∼ 20 per cent of the total ionizing flux (∼ 1.3 × 1051 Ly photon s−1) within 0.5 pc of their cores, and ∼ 60 per cent of the total kinetic energy injected into the ISM (5–6 × 1038 erg s−1), despite representing only 10 per cent of the massive stellar population. For the larger R136 cluster in 30 Doradus (r ≤ 10 pc), 117 massive stars provide a total ionizing flux of 4 × 1051 Ly photon s−1 and release a total kinetic energy of 1.6 × 1039 erg s−1 into the ISM, the latter being dominated by nine WR (43 per cent) and six O3 If*/WN (29 per cent) stars.

Radio Continuum Measurements of Southern Early‐Type Stars. II. A Distance‐limited Sample of Wolf‐Rayet Stars

A distance-limited sample of southern Wolf-Rayet stars within 3 kpc of the Sun has been observed with the Australia Telescope Compact Array at 8.64 and 4.80 GHz. Radio continuum flux densities at one or both frequencies were obtained for 10 sources and upper limits for 20; four sources are found to be thermal emitters on the basis of the observed spectral index. Five sources are classified as nonthermal. One source could not be classified. We derive mass-loss rates for the thermal sources. After combining them with all existing radio mass-loss rates of Wolf-Rayet stars in the northern and southern hemisphere, we perform a comparison with mass-loss rates derived from optical emission lines. The two methods lead to consistent results, which suggests either that the assumption of a spherically symmetric, stationary, homogeneous stellar wind is correct or that deviations from this assumption affect both methods in the same way. Wolf-Rayet mass-loss rates are surprisingly uniform across spectral type. We find an average mass-loss rate of 4 × 10-5 M☉ yr-1 for all types observed, except for WC9 stars, which have rates that are lower by at least a factor of 2. An alternative explanation could be partial recombination of helium from He+ to He0 in the radio region, which would lead to a reduced number of free electrons, and therefore reduced radio flux for WC9 stars. Mass-loss rates of 8 × 10-5 M☉ yr-1 for late WN stars favored in recent stellar evolution models disagree with the observations of these subtypes. The results of this survey suggest that ~40% of all Wolf-Rayet stars with measured spectral index are nonthermal emitters at centimeter wavelengths. This percentage is nearly twice as high as that of nonthermal emitters among OB stars and is higher than that previously estimated for WR stars. The nature of the nonthermal emission is still not fully understood. Possible causes of nonthermal emission are discussed. In particular, we speculate that nonthermal emission may arise from an interaction between a thermal WR wind and surrounding material owing to a shell ejected during a previous evolutionary stage or owing the wind of a companion star.

The evolutionary status of WNL stars

We discuss the evolutionary and mass loss implications of recent non-LTE analyses of late WN (WNL) stars in the Galaxy, LMC and M33 using the Wolf-Rayet standard model and address the observed dichotomy of WNL stars discussed by Moffat (1989). Individual subtypes belong to two distinct groups. We find that the single WNL+abs and WN7 stars evolve directly from very massive O stars (60–100 M⊙) with the former intimately related to extreme Of stars and found exclusively in the youngest clusters in our Galaxy. Conversely, the observational properties and chemistries of WN8-10 stars suggest that they are descended from lower initial mass progenitors (∼25–60 M⊙). These stars are either dormant LBVs (= WN9–10) or at a phase immediately after this stage (= WN8) although a previous RSG phase cannot be excluded. Included in this subgroup are the LMC and M33 Ofpe/WN9 stars, re-classified as WN9–10 since they appear to be genuine WN stars.

The fundamental parameters of the central stars of eight WR ring nebulae

We describe work that has recently been completed on deriving the fundamental parameters of eight WR stars through the photoionization modelling of their surrounding nebulae using non-LTE WR flux distributions. The resulting effective temperatures range from 57 000–71 000 K for the WN4-5 stars and <30 000–42 000 K for the WN6-8 stars. The derived stellar parameters are compared with those obtained from stellar emission line modelling. We find good agreement for the hot early WN stars, indicating that the non-LTE WR flux distributions have essentially the correct shape in the crucial far-UV region. We find lower temperatures for the four cooler late WN stars, particularly for the two WN6 stars. For the nebulae surrounding these stars, we find that the model flux distributions produce too much nebular ionization. We suggest that these discrepancies arise because of the lack of line-blanketing in the WR atmospheres. For the WO1 central star of G2.4+1.4, with strong nebular He II 4686 A emission, we derive a temperature of 105 000 K, somewhat less than previous estimates. The positions of our eight WR stars on the H-R diagram are compared with the evolutionary tracks of Maeder (1990) for solar metallicity. In common with previous workers, we find that our derived luminosities are too low, giving an initial mass range of 25–40 M⊙, below that expected for the majority of WR stars.

LSS 4005 and NaSt1, two Galactic Ofpe/WN or B[e] objects

Optical and IR spectroscopy of two galactic objects previously considered to be extreme late WN stars (van der Hucht et al., 1984), LSS4005 (formerly tentatively listed as WR85a) and NaStl (WR122), show both objects not to be WR stars, but Ofpe/WN- or B[e]-like objects.

The Spectral Evolution of the LMC S Dor Variable R127 During Outburst

The LMC star R127 was classified as O Iafpe extr. by Walborn (1977). Later, Walborn (1982) noted the spectral similarity to late WN stars and classified R127 as Ofpe/WN9 star. Stahl et al. (1983) discovered an S Dor outburst of R127. It has now been monitored photometrically almost continuously for more than four years. In addition, several high-dispersion and high signal-to-noise spectra were taken with the ESO CASPEC and CES spectrographs. In 1986 R127 became the visually brightest star of the LMC until it was surpassed by SN1987A in Feb. 1987.

X-ray observations of Wolf-Rayet stars

X-ray observations have been made of a very luminous O8f star, four late WN stars, and eight O VI Wolf-Rayet stars. The X-ray pulse height distributions of the O8f star is analyzed to obtain information on source temperature, emission measure, and location relative to the stellar wind. Only upper limits are obtained for the X-rays from the Wolf-Rayet stars, but for the late WN stars the upper limits are well below the maximum Lx/Lbol detected in earlier surveys. For the O VI stars, the limits, used in conjunction with models for the production of the optical O VI lines, supply information about the excitation conditions in the winds.

A new search for nebulae surrounding Wolf-Rayet stars

view Abstract Citations (75) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A new search for nebulae surrounding Wolf-rayet stars. Heckathorn, J. N. ; Bruhweiler, F. C. ; Gull, T. R. Abstract A comprehensive narrow band emission line survey of the Milky Way is used in searching for nebulosities surrounding Wolf-Rayet stars. Fifteen ring nebulae are definitely identified, including five previously unreported shell structures. An additional 30 nebulosities are classified as 'probable' or 'possible' ring nebulae. Angular diameter, sharpness or diffuseness, and level of brightness in three emission line bandpasses are determined for each nebulosity detected. Five selected shell structures are discussed in detail. Analysis of these data reveals a tendency for nebulae surrounding early WN stars to be brighter in the forbidden O III than in H-alpha plus the forbidden N II, whereas nebulae surrounding late WN stars tend to be brighter in H-alpha plus the forbidden N II than in the forbidden O III. Publication: The Astrophysical Journal Pub Date: January 1982 DOI: 10.1086/159550 Bibcode: 1982ApJ...252..230H Keywords: Emission Spectra; Line Spectra; Milky Way Galaxy; Nebulae; Stellar Spectrophotometry; Wolf-Rayet Stars; Forbidden Bands; Gas Ionization; H Alpha Line; Interstellar Gas; Nitrogen Ions; Oxygen Ions; Photoionization; Stellar Envelopes; Stellar Evolution; Sulfur; Astronomy full text sources ADS | data products SIMBAD (38)

HD 192641 and HD 193077: Two Single, Hydrogen-Rich Wolf-Rayets?

As recently as six years ago it was generally agreed that all Wolf-Rayet (WR) stars were the product of mass loss induced by Roche lobe over-flow (RLOF), and that thus they were all members of binary systems. In particular, the presence of absorption lines in the spectrum of a WR star was taken as a definitive indicator that the star was a binary, as it was well known that emission alone (other than the absorption present in P Cygni profiles) was a WR characteristic. However, Niemela (1973) showed that in the WN 7 binary HD 92740 that the absorption and emission lines move in phase, proving that in at least one case the absorption spectrum originates in the WR star itself. Conti (1976) has meanwhile suggested that WR stars could form by mass loss due to a stellar wind rather than RLOF. Obviously it is well worth examining the belief that all (or even most) WR stars are binaries. The currently popular feeling is that the late WN stars (WN 7, 8, and 9) are the only possible exceptions to the traditional picture, while the members of the other subclasses are all hydrogen-poor and probably members of binaries. I hope to convince you today that this is not the complete story.

Evolution of O stars and the WR connection

The stellar wind mass loss rates of at least some single Of type stars appear to be sufficient to remove much if not all of the hydrogen-rich envelope such that nuclear processed material is observed at the surface. This highly evolved state can then be naturally associated with classic Population I WR stars that have properties of high luminosity for their mass, helium enriched composition, and nitrogen or carbon enhanced abundances. If stellar wind mass loss is the dominant process involved in this evolutionary scenario, then stars with properties intermediate between Of and WR types should exist. The stellar parameters of luminosity, temperature, mass and composition are briefly reviewed for both types. All late WN stars so far observed are relatively luminous like Of stars, and also contain hydrogen. All early WN stars, and WC stars, are relatively faint and contain little or no hydrogen. The late WN stars seem to have the intermediate properties required if a stellar wind is the dominant mass loss mechanism that transforms an Of star to a WR type.