Peripatus Home Page  pix1Black.gif (807 bytes)  Astronomy Page >> Ofpe/WN9 Stars Updated: 12 Aug 2002 

Ofpe/WN9 Stars

Abstract

Ofpe/WN9 stars form a homogenous group of highly evolved objects which have CNO-cycle products at their surfaces, and are ejecting them into their circumstellar environments. Their properties are intermediate between those of Of stars on one hand, and B[e] or WNL stars on the other. They have slow winds with terminal velocities of about 400-500 km/s and high mass-loss rates of the order of 2 - 5 x 10-5 M¤/yr. Stellar temperatures and radii are in the range 30,000 - 39,000 K, and 19 - 39 R¤, respectively. Luminosities are between log(L/L¤) = 5.6 and 6.3. They occur in the evolutionary sequence:

O ® Of ® Ofpe/WN9 ® WR

for initial stellar masses less than ~100 M¤. Some of their properties are described here.

Keywords: Ofpe/WN9 stars, LBV, Of star, Wolf-Rayet, B[e] star, stellar evolution

Introduction

"A group of four unusual, hot emission-line spectra was isolated among the brightest OB supergiants in the Large Magellanic Cloud by Walborn (1977); their outstanding peculiarity is the combination of high-ionization Of (He II, N III) and low-ionization (He I, N II) emission features with comparable strengths. Despite their overall similarity, when examined in detail they divide into three subgroups, particularly in terms of the rich spectral morphology in the vicinity of Hd:

  • HDE 269858 and Sanduleak (Sk) -67º266 show exceptionally narrow absorption lines of Si IV l4089 and N III l4097;
  • HDE 269227 displays a striking configuration of partially overlapping P Cygni profiles; while
  • HDE 269445 has no absorption features except for a possible diffuse N III l4097"

(Walborn 1989, p. 27).
 
 

Related Topics

Further Reading

Related Pages

Other Web Sites

  •  
  
 
The class Ofpe/WN9 itself was introduced by Walborn 1982 to indicate that the stars could neither be classified as pure Of stars, nor as pure WNL stars.
Ofpe/WN9 stars have slow winds with terminal velocities of about 400 to 500 km/s and high mass-loss rates of the order of 2.4 - 6.6 x 10-5 M¤/yr (Pasquali et al. 1996). Stellar temperatures and radii are in the range 30,000 - 39,000 K, and 19 - 39 R¤, respectively; luminosities are between log(L/L¤) = 5.6 and 6.3.
"[S]pectroscopic observations have identified Ofpe/WN9 stars as a post-MS phase during which massive stars are turning into the WR configuration. In fact, Ofpe/WN9 stars display a hybrid spectral morphology characterized by the simultaneous presence of high (i.e., WR-like) and low (i.e., O-like) ionization emission lines at the same intensity (Bohannan & Walborn 1989). A detailed line profile and equivalent width analysis shows a smooth transition between Of and Ofpe/WN9 spectral types (Nota et al. 1996): spectral lines gradually change their absorption profile into a P-Cygni profile, increasing also their equivalent widths. The interpretation of this is an increasing mass loss rate" (Pasquali et al. 1996).
Ofpe/WN9 stars are intermediate between O and WR stars in several respects; their stellar properties place them in the evolutionary sequence: O ® Of ® H-rich WNL ® Ofpe/WN9, for initial stellar masses less than ~100 M¤.
Higher mass-loss rate and enhanced mixing between core and envelope are required in order to yield models compatible with the observed properties of Ofpe/WN9 stars. The emerging picture may be consistent with earlier evidence of Ofpe/WN9 stars being quiescent LBVs. This idea is further strengthened by the highly reduced surface H mass fractions of the Ofpe/WN9 stars. We derive Xs = 0.5 to 0.3, which still excludes Ofpe/WN9 stars from being core He burning objects, but is almost identical to the Xs values recently measured in LBVs.

Characteristics

Spectra
SiIV 1393, 1403 is characterized by a well developed P Cygni profile and CIV 1548, 1551 by a deep absorption profile. In most spectra an FeIV absorption line region is also seen developing redward of l = 1500Å (fig. 1).

Nebular lines from surrounding gas indicate enhanced nitrogen abundances, consistent with the very evolved nature of these objects.

"[S]pectroscopic observations have identified Ofpe/WN9 stars as a post-MS phase during which massive stars are turning into the WR configuration. In fact, Ofpe/WN9 stars display a hybrid spectral morphology characterized by the simultaneous presence of high (i.e., WR-like) and low (i.e., O-like) ionization emission lines at the same intensity (Bohannan & Walborn 1989). A detailed line profile and equivalent width analysis shows a smooth transition between Of and Ofpe/WN9 spectral types (Nota et al. 1996): spectral lines gradually change their absorption profile into a P-Cygni profile, increasing also their equivalent widths. The interpretation of this is an increasing mass loss rate.
lrgPasqualiFig1.gif (18741 bytes)

Fig. 1: Spectrum of Ofpe/WN9 star S61 between 1320Å and 1690Å (after Pasquali et al. 1996, fig. 1).
"These results suggest Ofpe/WN9 stars represent an evolutionary step beyond Of stars, when WR properties have already started to develop. Therefore, an evolutionary continuity seems to exist along the spectral types sequence O - Ofpe/WN9 - WR.
"Ofpe/WN9 stars appear also to be related to the LBV phenomenon. The best known examples of this connection are R127 and AG Carinae. R127 had been originally classified as an Ofpe/WN9 star till the '80s when Stahl et al. (1983) observed it undergoing a typical LBV outburst. AG Carinae is a proven LBV star which assumes an Ofpe/WN9-like spectral morphology during its quiescent periods (Stahl 1986). As a consequence of these observations Ofpe/WN9 stars are now suspected to be dormant LBVs.
"The above properties make Ofpe/WN9 stars ideal to verify stellar evolution models and to study the mass loss phenomenon. In particular, it is interesting to compare the mass loss parameters (such as mass loss rate, stellar radiation field and wind momentum) found for Ofpe/WN9 with the ones derived for O and WR stars. In this way, it becomes possible to compare the mass loss properties during the evolution from the main sequence to the early Wolf-Rayet stage" (Pasquali et al. 1996).

Occurrence

"A number of Ofpe/WN9 and LBV objects in the LMC are located in compact multiple systems which are most likely physically associated and coeval. The only case which has been investigated in detail so far is that of HDE 269227 by Schild (1987), who found that the associated stars are predominantly of type B and derived a likely initial mass for HDE 269227 of 25 M¤ ..." (Walborn 1989, p. 30).
Sk# HD/HDE# Other R.A. Dec Host Mag(V) #Components
— HDE 269858 R127 05:36:43.4 -69:29:46 LMC? 10.6 1
-68º73 HDE 269445 R99, S30, BE 261 05:22.8 -68:01 LMC? 11.46  
-69º79 HDE 269227 R84, S91, BE 543, Brey 18 05:13:54 -69:31.8 LMC? 11.78  
-69º142a HDE 269582 BE 294 05:27.9 -68:59 LMC? 11.88  
-69º175 HDE 269687 S119, BE 335, IRAS 05317-6907 05:31:25.3 -69:05:35 LMC? 11.90  
-67º266 — AL 418, S61, BE 153 05:45:53 -67:14.3 LMC? 12.01  
-69º249c HDE 269927c Brey 91 05:38:58.01 -69:29:18.4 LMC? 12.48  
-66º40 — AL 38, S9, Brey 10b 04:57.8 -66:33 ? 12.89  
— — BE 381, Brey 64 N/A N/A LMC? 13.26  
Table 1: Catalogue of Ofpe/WN9 stars with published information, ordered by descending magnitude.

Sk# = Sanduleak (1970) catalogue number, HD/HDE# = Henry Draper (Extension) catalogue number, Other = other catalogue references, R.A. = right ascension, Dec = declination, Host = host galaxy, Mag(V) = apparent visual magnitude, #Components = number of visual components in multiple systems (various sources).

Mass and Luminosity

Log10 (L/L¤) ranges from 5.83 (S9) to 6.19 (R99) (Pasquali et al. 1996, table 3).

Circumstellar Nebulae

"Another important property of several Ofpe/WN9 objects in the LMC, including HDE 269858, is their associated nitrogen-rich, nonspherical circumstellar nebulae, which were first detected spectroscopically by Walborn (1982). Stahl and Wolf (1986b) found evidence for similar structures in the spectra of a wider range of LMC emission-line supergiants, including the LBVs S Doradus and HDE 269006 = R71" (Walborn 1989, p. 29).

Variability/Mass Loss

"All the stars (except R99) exhibit a wind terminal velocity of 400-500 ± 50 km/s, a factor 4-5 less than the wind velocities measured for O and WR stars of comparable temperature. Uncertainties on v¥ are mainly due to the low dispersion of our spectra and to the SEI computational method which does not give a unique fit for each line. The wind of R99 is instead characterized by v¥ = 900 which is certainly unusual for this spectral type. ...

"Ofpe/WN9 stars exhibit mass loss properties which are intermediate between O and WR stars. In agreement with the suggestions by Nota et al. (1995), we find that also the wind physics indicates that a smooth transition from O to Ofpe/WN9 exists which may correspond to an evolutionary sequence: massive stars, evolving off the main sequence, would enter the Ofpe/WN9 phase during which they would begin to develop WR characteristics. Accurate determinations of the chemical abundances are needed to confirm this evolutionary link" (Pasquali et al. 1996).

Summary

Ofpe/WN9 stars have slow winds with terminal velocities of about 400 to 500 km/s and high mass-loss rates of the order of 2.4 - 6.6 x 10-5 M¤/yr (Pasquali et al. 1996). Stellar temperatures and radii are in the range 30,000 - 39,000 K, and 19 - 39 R¤, respectively; luminosities are between log(L/L¤) = 5.6 and 6.3.
Property Value Notes/References
M dot (M¤ yr-1) 2.4 - 6.6 x 10-5 Pasquali et al. 1996
v¥ (km s-1) 400 - 500 Pasquali et al. 1996
Lifetime (yr)    
Teff (K) 30,000 - 39,000  
R/R¤ 19 - 39  
L/L¤ 5.6 - 6.3  
Mbol    
Table 2: A comparison of some of the physical properties of Ofpe/WN9 stars.

Interpretation

Ofpe/WN9 stars form a homogenous group, with properties intermediate between Of and B[e] stars. "[A]t least for the Of-Ofpe/WN9 stars, this progression corresponds to an evolutionary sequence." In some Of stars, e.g. HDE313846 and HD152408, "the appearance of P Cygni profiles and strengthened emission components in both H and HeI lines seems to suggest they are in a transition phase between Of and Ofpe/WN9" (Nota et al. 1996, p. 383).
"Effective temperatures and stellar luminosities place Ofpe/WN9 stars in the upper HR diagram between the areas occupied by WR and LBV stars in agreement with early observational findings.... The link between Ofpe/WN9 and WR stars is also supported by the mass loss rates which are of the order of 10-5M¤yr-1 for both spectral types.
"These results suggest Ofpe/WN9 stars represent an evolutionary step beyond Of stars, when WR properties have already started to develop. Therefore, an evolutionary continuity seems to exist along the spectral types sequence O - Ofpe/WN9 - WR.
"Ofpe/WN9 stars appear also to be related to the LBV phenomenon. The best known examples of this connection are R127 and AG Carinae. R127 had been originally classified as an Ofpe/WN9 star till the '80s when Stahl et al. (1983) observed it undergoing a typical LBV outburst. AG Carinae is a proven LBV star which assumes an Ofpe/WN9-like spectral morphology during its quiescent periods (Stahl 1986). As a consequence of these observations Ofpe/WN9 stars are now suspected to be dormant LBVs" (Pasquali et al. 1996).

"This relationship has been further explored by Smith, Crowther, & Prinja (1994) who propose the some LBVs are the extension of the WN sequence towards later spectral types, hence unifying the classes of WNL, LBV, and Ofpe/WN9 stars" (Nota et al. 1996, p. 385).

"The above properties make Ofpe/WN9 stars ideal to verify stellar evolution models and to study the mass loss phenomenon. In particular, it is interesting to compare the mass loss parameters (such as mass loss rate, stellar radiation field and wind momentum) found for Ofpe/WN9 with the ones derived for O and WR stars. In this way, it becomes possible to compare the mass loss properties during the evolution from the main sequence to the early Wolf-Rayet stage" (Pasquali et al. 1996).

"In or about 1980, the Ofpe/WN9 prototype HDE 269858 = Radcliffe 127 unexpectedly entered an outburst phase (Stahl et al. 1983, Walborn 1984), during which its spectrum has evolved through an intermediate B-type to a peculiar supergiant A-type identical to that of S Doradus, while the bolometric luminosity has remained constant (Stahl and Wolf 1986a, Wolf et al. 1988). Thus HDE 269858 has firmly established its own LBV nature, as well as a connection between the Ofpe/WN9 and LBV classes. A connection in the opposite direction has been made by the recent discovery of Ofpe/WN9 characteristics during a light minimum of the galactic LBV AG Carinae (Stahl 1986), which at other times displays a B-type spectrum identical to that of P Cygni (Hutsemékers and Kohoutek 1988), and an A-type similar to S Doradus (Wolf and Stahl 1982).

"These remarkable observations indicate previously unsuspected relationships among the apparently disparate objects in question. Moreover, strong hypotheses are suggested that all Ofpe/WN9 objects may be quiescent LBVs, and conversely that many or perhaps all LBVs are of Ofpe/WN9 type at minimum. A corollary of the latter would be that the true minimum states of P Cygni and S Doradus have not yet been observed, and that they are in extended intermediate or maximum states" (Walborn 1989, p. 28).

Examples

R127 = HDE 269858 (LMC)

prototype Ofpe/WN9

A 'type 1' (exceptionally narrow absorption lines of Si IV l4089 and N III l4097) Ofpe/WN9 star, currently in outburst with a spectrum identical to that of S Doradus (a prototypical LBV) - R127 therefore now classified as a LBV.

Other 'type 1' Ofpe/WN9 stars include Sk -67° 266, Sk -66° 40, HDE269582, HDE269687, and Sk -69° 297.

BE381 (LMC)

(Bohannan-Epps 381)

Teff = 38,730; R/R¤ = 20; log L/L¤ = 5.91; M-dot = 2.4 x 10-5 M¤/yr.

"The Ha images of BE381 also reveal the presence of a faint nebulosity around the star; most of the nebular flux appears to be emitted by an arc of gas located to the east of BE381, while a much dimmer arc is detectable on the western side. The arcs delineate a shell of 13" in diameter, corresponding to a linear size of 3.2 pc, which appears to be expanding with a velocity of 14 km s-1. From the nebular emission lines we derive an electron density ranging between 30 cm-3 and 120 cm-3 (assuming Te = 10,000K), and a N+/S+ ratio between 1.5 and 2.3, which are typical of HII regions. We therefore conclude that the shell detected around BE381 is not of stellar origin and probably represents the relic of the interstellar bubble blown by BE381 during its O main-sequence phase." (Pasquali et al. 1999).

A 'type 2' (partially overlapping P Cygni profiles) Ofpe/WN9 star.

Other 'type 2' Ofpe/WN9 stars include HDE 269227 and HDE 269927C.

R99 (???)

Teff = 34,750; R/R¤ = 34; log L/L¤ = 6.19; M-dot = 4.8 x 10-5 M¤/yr.

"R99 shows several anomalies, such as a very strong infrared emission excess (see Stahl et al. 1984) and the lack of broad FeIV absorptions regions at l ³ 1500Å, possibly due to a lower metallicity or a higher ionization degree of its wind. The presence of the HeII l1640 line (with a strong P Cygni profile) supports this suggestion" (Pasquali et al. 1996.)

S61 (LMC)

Teff = 36,060; R/R¤ = 31; log L/L¤ = 6.18; M-dot = 2.6 x 10-5 M¤/yr.

"The nebula surrounding S 61 has the appearance of a shell with a mild central axisymmetry. The surface brightness is not uniform, and the northern region of the nebula is the brightest. The nebula exhibits a bipolar structure with an overall morphology very similar to nebulae around other LBVs or Opfe/WN9 stars, especially S119. The diameter of the shell is 7.3", corresponding to a linear size of 1.8 pc. From the profile of nebular emission lines we clearly detect an expansion motion with a velocity of 28 km s-1, which indicates a dynamical age of ~30,000 yrs. We find an electron density of 400 cm-3 and an electron temperature of 6120K. The nebula is similar to other LBV nebulae in that it is nitrogen enriched. The observed chemical and dynamical properties confirm that the nebula is associated with the central star and is of stellar origin. This result implies that S61 is likely to have undergone a LBV-type ouburst and, therefore, strenghtens the suggestion that Ofpe/WN9 stars are quiescent LBVs" (Pasquali et al. 1999).
type 3 (no absorption features except for a possible diffuse N III l4097): HDE269445
Star Teff (K) R/R¤ log (L/L¤) M-dot (M yr-1) B-V
BE294 32,140 37 6.11 6.6 ´ 10-5 -0.04
HDE 269927c 38,900 24 6.06 2.9 ´ 10-5 -0.11
R84 35,160 25 5.95 3.4 ´ 10-5 +0.22
S9 33,000 25 5.83 4.0 ´ 10-5 +0.03
S119 30,900 39 6.10 3.2 ´ 10-5 -0.07
Table 3: Physical data for some additional Ofpe/WN9 stars, after Pasquali et al. 1996, table 3, and Nota et al. 1996, table 2.

References

Bohannan, B., & Walborn, N. R. 1989: PASP 101: 520.

Hutsemékers, D.; Kohoutek, L. 1988: Astr. Ap. Suppl. 73: 217.

Nota, Antonella; Pasquali, Anna; Drissen, Laurent; Leitherer, Claus; Robert, Carmelle; Moffat, Anthony F. J.; Schmutz, Werner 1996: O Stars in Transition. I. Optical Spectroscopy of Ofpe/WN9 and Related Stars. Astrophysical Journal Supplement v.102, pp. 383-410.

Pasquali, A.; Nota, A.; Clampin, M. 1999: Spatially Resolved Nebulae Around the Ofpe/WN9 Stars S61 and BE381. Astronomy and Astrophysics, 343: 536-544.

Pasquali, Anna; Schmutz, Werner; Leitherer, Claus; Nota, Antonella; Hubeny, Ivan; Langer, Norbert; Drissen, Laurent; Robert, Carmelle 1996: Fundamental Properties of Ofpe/WN9 Stars from Ultraviolet HST Spectra. Science with the HST 2: 386-392.

Pasquali et al. 1997: O Stars in Transition II

Schild, H. 1987: Astr. Ap. 173: 405.

Smith, L.J.; Crowther, P.A.; Prinja, R.K. 1994: A&A 281: 833.

Stahl, O. 1986: A&A 164: 321 (errata 170: 197).

Stahl, O.; Wolf, B. 1986a: Astr. Ap. 154: 243.

Stahl, O.; Wolf, B. 1986b: Astr. Ap. 158: 371.

Stahl, O.; Wolf, B.; Klare, G.; Cassatella, A.; Krautter, J.; Persi, P.; Ferrari-Toniolo, M. 1983: A&A 127: 49.

Walborn, N.R. 1977: ApJ. 215: 53.

Walborn, N.R. 1982: ApJ. 256: 45/452 ???.

Walborn, N.R. 1984: In van den Bergh, S.; de Boer, K.S. (eds.) IAU Symposium 108 - Structure and Evolution of the Magellanic Clouds, p. 239.

Walborn, Nolan R. 1989: Evolutionary Diagnostics of LBV Spectra and Systems in the LMC. In K. Davidson et al. (eds.) Physics of Luminous Blue Variables, pp. 27-34.

Wolf, B.; Stahl, O. 1982: Astr. Ap. 112: 111.

Wolf, B.; Stahl, O.; Smolinski, J.; Cassatella, A. 1988: Astr. Ap. Suppl. 74: 239.
 Peripatus Home Page  pix1Black.gif (807 bytes)  Astronomy Page >> Ofpe/WN9 Stars Contact me.