Spectral Type O Stars

Abstract

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: type O stars

Introduction

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).

Only a small fraction of stars in the Galaxy are more massive than 20 solar masses. However, such stars, which spend most of their short lives as H-burning O-type stars, play an important role in galactic structure and evolution.

Characteristics

Spectra

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 l4634and l4640, such as ζ 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 1989, 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).

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).

Variability/Mass Loss

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 pronouncedin 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).

Interpretation

Progenitors

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).

Evolution

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 asHe 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 havesignificant 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 ∝ L^1.7 (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^-14M_¤ per year from winds blowing between 200 to 300km 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 10-8M_¤ to 10-5 M_¤ per year (Böhm-Vitense 1989b, pp. 216-217).

Occurrence and Examples

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.

HD#	Bayer	R.A.	Dec	Type	Lum	Mag(V)	#Components
66811	Zeta Pup	08:03:35.052	-40:00:11.64	O5Iaf	1	2.25	1
24912	Xi (46) Per	03:58:57.902	+35:47:27.71	O7e	0	4.04
57060	29 CMa	07:18:40.3	-24:33:32	O7e+O7	0	4.95
47839	15 Mon	06:40:58.6	+09:53:44	O7Ve	5	4.66	?
68273	Gamma 2 Vel	08:09:31.965	-47:20:11.91	WC8+O7.5e	0	1.83	5
36861	Lambda (39) Ori	05:35:08.2	+09:56:03	O8e	0	3.66	4
203064	60 Cyg	21:18:27	+43:56:46	O8e	0	5.00
57061	Tau (30) CMa	07:18:42.4	-24:57:15	O9Ib	1	4.40	5
37043	Iota (44) Ori	05:35:25.974	-05:54:35.61	O9III	3	2.76	3
214680	10 Lac	22:39:15.685	+39:03:01.01	O9V	5	4.89	2
36486	Delta (34) Ori	05:32:00.398	+00:17:56.88	B0III+O9V	3	2.24	3
147165	Sigma (20) Sco	16:21:11.317	-25:35:34.17	B2III+O9V	3	2.88	4
42933	Delta Pic	06:10:17.898	-54:58:07.23	B3III+O9V	3	4.81
30614	Alpha (9) Cam	04:54:03.016	+66:20:33.64	O9.5Iae	1	4.29
37742	Zeta (50) Ori	05:40:45.5	-01:56:34	O9.5Ibe	1	2.05	3
37468	Sigma (48) Ori	05:38:44.7	-02:36:0	O9.5V	5	3.80	5
149757	Zeta (13) Oph	16:37:09.542	-10:34:01.56	O9.5Vn	5	2.56
Table 2: O-type stars, magnitude 5 and greater. HD# = Henry Draper catalogue number, Bayer = Bayer (Flamsteed) reference, R.A. = right ascension, Dec = declination, Type = spectral type, Lum = luminosity class, Mag(V) = visual magnitude, #Components = number of visual components in multiple systems (after Ochsenbien et al. 1988, Smith 1996).

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).

Zeta Puppis

Zeta Puppis is one of the best studied O-type stars; several important parameters havebeen determined with good accuracy, including the distance 352 (429) 549 pc (~1400 lightyears); diameter 20 R_¤; effective temperature 42,000 ± 1500 K; and period of rotation ~4.8 – 5.2 days.

(Read more.)

HD93129A

The most luminous star known is HD93129A (O3If) which is located in part of the h 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, however, HD93129A may be the most brilliant star in the Galaxy. Similar stars are also known from the Tarantula Nebula (NGC2070) in the LMC.

HD93250

This is another O3 star from the h Carina nebula.

Conclusion

The bright object near the centre of the Tarantula Nebula (30 Doradus) in the LMC, known as R136a (also as HD38268) was for a brief time “thought to be a single superstar of as much as 1000 solar masses, but we have recently learned that it is a remarkable, tightly compacted cluster of early O and WR stars. … It is doubtful that we will find any stars much more massive than those of class O3” (Kaler 1989, p. 224).

References

Abbott, D. C.; Lucy, L. B. 1985: Multiline transfer and the dynamics of stellar winds. Astrophysical Journal 288: 679-693.

Baade, D. 1988: Ground-based Observations of Intrinsic Variations in O, Of, and Wolf-Rayet Stars. In Conti, P.S.; Underhill, A.B. (ed.) 1988: O-Stars and Wolf-Rayet Stars. NASA Special Publications 497 : 137.

Böhm-Vitense, E. 1989b: Introduction to stellar astrophysics. Volume 2. Stellar atmospheres. Cambridge: 1-264.

Conti, P.S.; Leep, E.M. 1974: Spectroscopic observations of O-type stars. V. The hydrogen lines and lambda 4686 He II. Astrophysical Journal 193: 113-124.

Crowther, P.A.; Willis, A.J. 1994: Observations of the atmospheres and winds of O-stars, LBVs and Wolf-Rayet Stars. In Vanbeveren, D.; van Rensbergen, W.; de Loore, C. (ed.) 1994: Evolution of massive stars. A confrontation between theory and observation. Springer : 85-103.

Garmany, C.D.; Conti, P.S.; Chiosi, C. 1982: The initial mass function for massive stars. Astrophysical Journal 263: 777-790.

Garmany, C.D.; Conti, P.S.; Massey, P. 1980: Spectroscopic studies of O type stars. IX - Binary frequency. Astrophysical Journal 242: 1063-1076.

Kaler, J.B. 1989: Stars and their spectra: an introduction to the spectral sequence. Cambridge: 1-300.

Lamers, H.J.G.L.M.; Leitherer, C. 1993: What are the mass-loss rates of O stars? Astrophysical Journal 412 (2): 771-791.

Leitherer, C.; Langer, N. 1991: Mass loss and evolution of massive stars in the Magellanic Clouds. In Haynes, R.; Milne, D. (ed.) 1991: The Magellanic Clouds. Proceedings of the 148th Symposium of the International Astronomical Union, held in Sydney, Australia, July 9-13, 1990. International Astronomical Union Symposia 148 : 480-482.

Moffat, A.F.J.; Drissen, L.; Robert, C. 1989: Observational connections between LBVs and other stars, with emphasis on Wolf-Rayet stars. In Davidson, K.; Moffat, A.F.J.; Lamers, H.J.G.L.M. (ed.) 1989: Physics of Luminous Blue Variables. Kluwer Academic Publishers : 229-240.

Nota, A.; Pasquali, A.; Drissen, L.; Leitherer, C.; Robert, C.; Moffat, Anthony F. J.; Schmutz, W. 1996: O stars in transition. I. Optical spectroscopy of Ofpe/WN9 and related stars. Astrophysical Journal Supplement 102: 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, A.; Schmutz, W.; Leitherer, C.; Nota, A.; Hubeny, I.; Langer, N.; Drissen, L.; Robert, C. 1996: Fundamental properties of Ofpe/WN9 stars from ultraviolet HST spectra. Science with the HST 2: 386-392.

Schaerer, D.; Schmutz, W.; Grenon, M. 1997: Fundamental Stellar Parameters of ζ Pup and γ² Vel from HIPPARCOS Data. Astrophysical Journal Letters 484: L153-L156.

Schulte-Ladbeck, R.E.; Eenens, P.R.J.; Davis, K. 1995: The Structure of Wolf-Rayet Winds. I. Observation of Ionization Stratification in WR 6 and WR 111. Astrophysical Journal 454: 917-926.

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.