Hot Luminous Stars

Abstract

Hot luminous stars occupying the top left corner of the Hertzsprung-Russell Diagram are some of the most spectacular inhabitants of the galaxy. They evolve quickly and encompass a range of stellar types during their lifetimes.

Keywords: stars, hot, luminous, Type O, Type Of, Ofpe/WN9, Wolf-Rayet, WNL, LBV, B[e]

Introduction

Hot luminous stars occupy the top left corner of the Hertzsprung-Russell Diagram, home to some of the most spectacular inhabitants of the galaxy.  Rare and exotic, these giant objects live life in the fast lane, courting disaster at the farthest extremes of mass and luminosity known to exist.  This is where supernova 1987A, and perhaps many others, came from.

At the place where the zero-age main sequence for hydrogen burning stars (H-ZAMS) and the supergiant branch come together, we find the earliest (Þ bluest, hottest) O-type stars. Although relatively ‘normal’ by comparison with some of their neighbours in the top left corner, they are violently unstable by the standards of a star like our Sun.

Sometimes plotting next to the O stars, sometimes far to the right where the red giants are found, the Luminous Blue Variables "careen across the H-R diagram in repeated blue-red-blue excursions, [losing] mass through constant winds and occasional outbursts" (Parker et al. 1993, p. 770). The premier example, Eta Carinae, is a peculiar, erratically fluctuating variable, one of the most luminous stellar objects of our Galaxy, and, if single, one of the most massive stars known.

Wolf-Rayet Stars are supernovas waiting to happen. They are believed to be essentially the naked cores of massive stars from which extreme stellar winds have stripped off the atmosphere. As they age, they move towards the lower left of the H-R diagram, becoming smaller and hotter before ending in a final apocalyptic fling as a type IIb supernova.

The finest examples of each of these classes are to be found in the deep southern skies, wheeling right above our heads throughout Autumn. Sadly, the bulk of the lay astronomical literature is written by American or European authors with a northern hemisphere audience in mind, who are rather sparing of their attention to the objects in our southern skies. Thus, whole classes of object – including those described here – are passed over because there is no good example to be seen from New York or London.

Characteristics

Luminosity

"The most luminous stars known in our local universe have luminosities around L = 10⁶ L_¤ and temperatures of 30,000 K < T_eff < 50,000 K. When plotted on a Hertzsprung-Russell diagram (HRD), they occupy a fairly well defined region whose high temperature boundary is the zero-age main-sequence (ZAMS) corresponding to stars with masses around M = 80 ± 50/20 M_¤. ... The low temperature boundary is defined by the observed lack of cool high-luminosity stars, and important detection made by Humphreys & Davidson (1979)" (Nota et al. 1996, p. 383; also see de Jager 1994).

Spectra

The pioneering work in our understanding of the spectral appearance of hot massive stars was published by Conti 1971 and Walborn 1971.

Stellar Winds and Mass Loss

"Most hot, massive stars initiate and maintain powerful stellar winds whose kinetic momentum flux is about the same order of magnitude as the radiative momentum flux (Lamers & Leitherer 1993). Terminal wind velocities v_¥ exceed the surface escape velocities by large factors. Typical observed values in the hottest stars are v_¥ » 2000 km s^-1 (Cassinelli & Lamers 1987), but terminal velocities which are lower by an order of magnitude have been determined in luminous blue variables (LBVs; see Lamers 1989) and related objects. As a consequence of such strong stellar winds, these stars experience mass loss at rates of up to M-dot » 10^-4 M_¤ yr^-1 (Howarth & Prinja 1989)" (Nota et al. 1996, p. 383).

Evolution

Mass loss and strong stellar winds dictate the course of massive star evolution. At high stellar masses the core H exhaustion and loss of the H-rich envelope become comparable in timescale (Maeder 1990).

References

Cassinelli, J.P.; Lamers, H.J.G.L.M. 1987: In Kondo, Y. (ed.) 1987: Exploration of the Universe with the IUE Satellite. Reidel, 139.

Conti, P.S. 1971: Mem. Soc. R. Sci. Liège 5: 261.

de Jager, C. 1994: In Vanbeveren, D.; van Rensbergen, W.; de Loore, C. (eds.) 1994: Evolution of Massive Stars.

Howarth, I.D.; Prinja, R.K. 1989: ApJ Supp. 69: 527.

Humphreys, Roberta M.; Davidson K. (1979): Astrophys. J. 232, 409.

Lamers, H.J.G.L.M. 1989: In K. Davidson et al. (eds.) 1989: Physics of Luminous Blue Variables, p. 135.

Lamers, H.J.G.L.M.; Leitherer, Claus 1993: ApJ 412: 771.

Maeder, A. 1990: A&AS 84: 139.

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.

Parker, Joel W.; Clayton, Geoffrey C.; Winge, Cláudia; Conti, Peter S. 1993: A New Luminous Blue Variable: R143 in 30 Doradus. The Astrophysical Journal, 409: 770-775.

Walborn, Nolan R. 1971: Some Spectroscopic Characteristics of the OB Stars: an Investigation of the Space Distribution of Certain OB Stars and the Reference Frame of the Classification. Astrophysical Journal Supplement v. 23: 257.