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The characteristic properties of type O stars including spectra, mass, and luminosity are briefly described, followed by notes about a few well-known examples.
Keywords: Spectral type O
O-type stars are massive, bright, hot stars. They are the bluest of the of the main sequence stars.
Although various individual properties, such as temperature, may be exceeded by more exotic objects such as white dwarf or Wolf-Rayet stars, type O stars are extreme by the standards of normal stars, and they are by far the rarest members of the main sequence.
In the solar vicinity, dwarf (luminosity class V) O type stars dominate the massive star population, leading to the conclusion that this is the most prolonged evolutionary phase in the lifetime of massive stars (Nota et al. 1996, p. 384; based on Garmany et al. 1982).
O is the earliest spectral type, radiating strongly at short wavelengths; thus their light appears bluish to the human eye.
Their spectra exhibit rather weak hydrogen lines, atomic helium, and also ionised helium (HeII helium atoms which have lost one electron). This is the only main sequence spectral type in which ionised helium can be seen; only O stars are sufficiently hot to produce the 24ev required to ionise helium.
|[ev = electron volts; by way of contrast, the energy requirement to ionise hydrogen is 13.6ev]|
|The hottest O-type stars display high ionisation (N III, He II) features.
These are the Of stars.
"O stars with emission may be called Oe, but those with bright lines of He II at l4686 and N III at l4634 and l4640, such as z Puppis ... are much more significant and are known as Of.... We actually see a continuous sequence between Of and pure absorption O, and intermediate types have been introduced: O(f) for stars with N III emission and no l4686 at all, and O((f)) for those with the nitrogen lines still bright, but with l4686 now dark" (Kaler 1997, p. 206).
In some Of stars, e.g. HDE 313846 and HD 152408, "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).
O-type stars are relatively rare (the following table lists all seventeen of them down to magnitude 5) owing to their fast paced lifestyle and consequently short lifetime. It will be noticed that Zeta Puppis is the earliest of those listed, and one of the brightest. Moreover, it is a single star whereas many of the others are components of multiple systems.
Mass and Luminosity
Accurate knowledge of the luminosity of these stars is important for comparing masses derived from stellar evolutionary models with those derived from stellar atmosphere models, for determining initial mass functions, and for studying stellar evolution in the high luminosity/high mass region of the Hertzsprung-Russell Diagram. The absolute magnitudes of O stars are presently poorly determined (it is only recently, with the availability of HIPPARCOS data that accurate distances to some O stars have been calculated from their trigonometric parallax). Formerly, absolute visual magnitudes were estimated primarily from O stars in clusters and OB associations whose distances are themselves uncertain, but are typically around 1-2 kpc.
One O star though scarcely a typical example which has been closely studied is Zeta Puppis. This star is thought to have an absolute magnitude of -6.06 (Schaerer et al. 1997).
Masses of observed O+O binary systems seldom show much mass difference between the components; that is the mass ratio of the components M1/M2 ~ 1 (Garmany et al. 1980).
Detailed photometric studies (e.g. as summarised in Baade 1988) reveal microvariability in "non-variable" supergiants which increases for stars with greater bolometric magnitude. The trend is true for all spectral types but most pronounced in O and early B type stars. "The amplitudes for the most luminous supergiants resemble the microvariations observed in LBVs during quiescence" (Moffat et al. 1989, p.230).
Stellar Winds and Mass Loss
Dwarf (luminosity class V) type O stars "show relatively little evidence for stellar winds in the optical line spectrum. Except for extremely wind-sensitive, such as He II l4686 or Ha, most observed spectral lines agree quite well with theoretical predictions from static, plane parallel non-LTE [local thermal equilibrium] atmospheres (Conti & Leep 1974). However, the densities in the outflow are high enough to generate characteristic wind profiles in the ultraviolet resonance line of C3+ at 1550 Å (Walborn et al. 1985). The mass loss over the main sequence lifetime inferred observationally is too small to have significant evolutionary consequences (Leitherer & Langer 1991)" (Nota et al. 1996, p. 384).
|However, supergiants "are intrinsically more luminous than [dwarf] stars of the same temperature class. Since mass loss is strongly dependent on luminosity|
|M-dot µ L1.7;||(1)|
|(Crowther & Willis 1994), stellar winds are much more significant in supergiants than in [dwarf] stars. Therefore, most observable spectral lines are formed at least partially in the wind" (Nota et al. 1996, p. 384).|
|"The [stellar wind] velocities for the hottest main sequence stars are observed to be as high as 3000 to 4000 km s-1. The mass loss rates M-dot come out to be about ~10-6 M¤ per year. [T]he lifetime of these stars is also of the order of 106 years. These stars may therefore lose a considerable fraction of their original mass during their lifetime on the main sequence, which has considerable influence on their further evolution" (Böhm-Vitense 1989b, p. 220).|
|By comparison, a star like our sun loses about 10-14 M¤ per year from winds blowing between 200 to 300 km s-1 from the quiet solar surface and 700 km s-1 from coronal holes. Cool luminous stars, typically red giants, show lower wind velocities around 50 km s-1 and typical mass losses, m-dot, in the order of 10-8 M¤ to 10-5 M¤ per year (Böhm-Vitense 1989b, pp. 216-217).|
Although only a small fraction of stars in the Galaxy are of very high mass more than a few solar masses such stars spend most of their short lives as H-burning O-type stars. They burn their hydrogen into helium after only a few millions of years, much more quickly than lower mass stars. After a high mass star burns through all of the hydrogen in its core, its internal changes place it along the supergiant branch. Eventually, the core of the star will run out of fusible material. At this point, the star comprises a central core surrounded by concentric layers of different elements.
|"In the radiation pressure driven wind scenario the stellar radiation
field supports the mass loss. Stellar photons interact with matter through one scattering
event which transfers momentum from the radiation to the wind. Nevertheless, a discrepancy
exists between the observed and predicted wind momentum for extreme O stars. The predicted
momentum becomes smaller than the observed value for stars with increasing wind density.
The observed momentum transfer is then more efficient.
"This could be due to the ionization stratification of the wind which depends on its density and allows more than one scattering event between each photon and the particles (Lamers & Leitherer 1993). In this case (not yet implemented in the calculations) every photon transfers to the wind an amount of momentum larger than hn/c up to 4hn/c (Abbott & Lucy 1985, Schulte-Ladbeck et al. 1995). The same M-dot.v¥ discrepancy is found enhanced in the case of WR stars" (Pasquali et al. 1996).
Zeta Puppis is one of the best studied O-type stars; several important parameters have been determined with good accuracy, including the distance 352 (429) 549 pc (~ 1400 light years); diameter 20 R¤; effective temperature 42,000 ± 1500 K; and period of rotation ~4.8 5.2 days.
The most luminous star known is HD93129A (O3If) which is located in part of the Carina Nebula (NGC3372). At visual wavelengths this star appears less luminous than some type B and later stars, because much of its energy is radiated at ultraviolet wavelengths. With a bolometric correction possibly as high as -4 magnitudes, HD93129A may be the most brilliant star in the Galaxy. Similar stars are also known from the Tarantula Nebula (NGC2070) in the LMC.
|Other well-known type O stars include q1 Ori C (O6), a member of the so-called 'Trapezium' cluster of type O and B stars which illuminate the famous Orion Nebula (M42, NGC1976/1982).|
|HD93250 - Kaler 1997, p. 221|
Abbott, D. C. & Lucy, L. B. 1985: Multiline Transfer and the Dynamics of Stellar Winds. Astrophysical Journal v. 288: 679-693.
Baade 1988: in O-Stars and Wolf-Rayet Stars, eds. Conti & Underhill, NASA SP-497, p.137
Böhm-Vitense, Erika 1989b: Introduction to Stellar Astrophysics. Volume 2 Stellar Atmospheres. Cambridge.
Conti, P.S.; Leep, E.M. 1974: ApJ 193: 113.
Crowther, P.A.; Willis, A.J. 1994: In Vanbeveren, D.; van Rensbergen, W.; de Loore, C. (eds.) 1994: Evolution of Massive Stars, p. 85.
Garmany, C.D.; Conti, P.S.; Chiosi, C. 1982: The Initial Mass Function for Massive Stars. Astrophysical Journal, v. 263: pp. 777-790.
Garmany, C.D.; Conti, P.S.; Massey 1980: ApJ 242: 1063
Lamers, H. J. G. L. M. & Leitherer, C. 1993, ApJ 412: 771.
Leitherer, C.; Langer, N. 1991: In Haynes, R.; Milne, D. (eds.) 1991: The Magellanic Clouds. IAU Symposium 148, p. 480.
Moffat, A.F.J.; Drissen, L.; Robert, C. 1989: Observational Connections Between LBVs and Other Stars, with Emphasis on Wolf-Rayet Stars. In K. Davidson et al. (eds.), Physics of Luminous Blue Variables, p. 229 - 240.
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.
Ochsenbien, F.; Acker, A.; Legrand, E.; Poncelet, J.M.; Thuet-Fleck, E. 1988: Le catalogue des etoiles les plus brilliantes. NASA Astronomical Data Centre, catalogue 5053.
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.
Schaerer, Daniel; Schmutz, Werner; Grenon, Michel 1997: Fundamental Stellar Parameters of z Pup and g 2 Vel from HIPPARCOS Data. ApJ Letters 484: L153-156.
Schulte-Ladbeck, R. E., Eenens, P. R. J. & Davis, K. 1995, Astroph. J. 454, 917
Smith, W.B. 1996: FK5 SAO HD Common Name Cross Index. NASA Astronomical Data Centre, catalogue 4022.
Walborn, N.R.; Nichols-Bohlin, J.; Panck, R.J. 1985: International Ultraviolet Explorer Atlas of O-Type Spectra from 1200 to 1900 Å. NASA RP-1155.
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