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Stellar Winds


Abstract

This page provides a brief introduction to stellar winds, their most important characteristics, implications for stellar evolution, and the forces which drive them.

Keywords: stellar wind, solar wind, hot stars

Introduction

"Stars emit not only radiation but also particles. The emission of particles is called the stellar wind" (Lamers & Cassinelli 1999, p. 8).

A key attribute of stellar winds is that they are continuous phenomena, as opposed to episodic outbursts.

The particles comprising the winds are accelerated differently, depending upon the nature of the star. In a cool star, like the sun, the wind arises from pressure-expansion in a hot corona. In hotter stars, the high radiative flux, drives the wind primarily by means of line scattering, which can be thought of as a transfer of momentum from the photons striking the atoms of gas.

 
 

Related Topics


Further Reading

  • Introduction to Stellar Winds – Henny Lamers & Joseph Cassinelli

Related Pages

Other Web Sites

 
 

Characteristics

"The two most important parameters regarding a stellar wind that can be derived from the observations are the mass loss rate, M-dot, which is the amount of mass lost by the star per unit time, and the terminal [or asymptotic] velocity v¥, which is the velocity of the stellar wind at a large distance from the star" (Lamers & Cassinelli 1999, p. 8).

All stars exhibit some kind of stellar wind. 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 20 to 60 km s-1 – but the measurements (e.g. of Ca+ line profiles) refer to layers close to the star where particle densities are high and also the radii of such stars may be 100 R¤ or more. Thus, typical mass losses are higher; in the order of 10-8 M¤ to 10-5 M¤ per year (Böhm-Vitense 1989b, pp. 216-217).
Hot luminous stars, conversely, exhibit much stronger winds, losing up to 10-5 M¤ per year via winds "blowing" at speeds up to 2,000 km s-1.
Typical values for some well-known stars are:
Star Type Mass (M*/M¤) M-dot (M¤/yr) v¥ (km/s) Reference
a Sco (Antares) M1.5 Iab-Ib 15 1 x 10-6 17 Lamers & Cassinelli 1999, Table 2.3
Sun G2V 1 1 x 10-14 200 – 700 Böhm-Vitense 1989b, p. 216
z Pup (Naos) O4I(n)f 59 2.7 x 10-6
2.4 x 10-6

2,200
Schaerer et al. 1997
Lamers & Cassinelli 1999, Table 8.1
P Cyg "B0Ia" (LBV) 30-60 1.5 x 10-5 210 Lamers & Cassinelli 1999, Table 8.1
WR1 WN5 (W-R)   6 x 10-5 2,000 Lamers & Cassinelli 1999, Table 8.1
Table 1: Typical stellar wind parameter values for some well-studied stars.

Implications for Stellar Evolution

It can be seen (Table 1) that mass loss rates for different stars vary greatly. The sun loses approximately 1 x 10-14 M¤/year and has an expected lifespan in the order of 1010 years. Thus, over its entire life, the sun is expected to lose only ~0.01% of its mass through this mechanism.

Very massive stars, to consider the opposite extreme, may be losing mass at rates in the order of 10-5 M¤/year. Although their characteristic lifespans are only in the order of a few million (106) years, this is still sufficient time for their stellar winds to carry away a few tens of solar masses; a significant proportion of the total stellar mass. Such stars may end their lives as Wolf-Rayet objects: stars which in some cases have completely shed their hydrogen envelope altogether.
"Metals" (elements heavier than helium) dredged up from deep within stars may find their way into stellar winds, contributing to the chemical enrichment of the interstellar medium.

Driving Mechanisms

Cool Stars

"The sun is a relatively low-mass, cool star with a surface temperature about 6000 K; but curiously its wind arises from pressure-expansion of the very hot, million-degree solar corona, which is somehow superheated by the mechanical energy generated from convection in the sun's subsurface layers" (Owocki 2000).

Hot Stars

"By contrast, high mass stars with much hotter surface temperatures (10,000 to 100,000 K) are thought to lack the strong convection zone needed to heat a circumstellar corona. Their stellar winds thus remain at temperatures comparable to the star's surface, and so lack the very high gas-pressure needed to drive an outward expansion against the stellar gravity. However, such hot stars have a quite high radiative flux, since by the Stefan-Boltzmann law this scales as the fourth power of the surface temperature. It is the pressure of this radiation (not of the gas itself) that drives the wind expansion" (Owocki 2000).

(See also Eddington Limit.)

"An important aspect of this radiative driving process is that it stems mostly from line scattering, wherein an electron is shuffled between two discrete, bound energy levels of an atom. In a static medium such scattering is confined to radiation with a photon energy near the energy difference between the levels, corresponding to a range of wavelengths near a distinct, line-centre value. But in an accelerating wind flow, the Doppler effect shifts the resonance to increasingly longer wavelengths, allowing the line scattering to sweep gradually through a much broader portion of the stellar spectrum" (Owocki 2000).
"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).

Compressed Disks

Massive blue stars typically rotate at high speeds. The wind compressed disk (WCD) paradigm of Jon Bjorkman and Joseph Cassinelli posits that parcels of gas driven away from a rapidly spinning star should remain in a tilted "orbital plane" that brings them over the star's equator. As wind parcels from opposite hemispheres collide over the equator, they form a disk of compressed gas. This WCD model provides a natural explanation for the strong disk emission lines seen from various classes of rapidly rotating hot stars.

However, some stars spin so fast that they flatten at the poles and bulge at the equator, leading leads to gravity darkening. The outflow of radiation is unevenly distributed over the distorted surface, with the polar regions brighter and the equatorial regions dimmer than they would be if the star didn't rotate. Computer simulations by Owocki and Cranmer, that include gravity darkening and nonradial driving forces, now indicate that such additional effects can actually inhibit formation of a WCD and even cause the bright poles to drive a denser wind than the dimmer equator.

Spiral Winds

Irregularities in a rotating star's atmosphere – sunspots, for example – can induce spiral wind structures as regions of compression between fast and slow outward streams of gas. These co-rotating interaction regions (CIRs) were first identified in our local solar wind. Dermott Mullan has suggested that similar CIRs in the wind outflows of hot stars might explain variations of certain absorption lines in their spectra. Hydrodynamical computer simulations by Cranmer and Owocki show that any region of enhanced brightness on a hot star would generate dense, low-speed wind streams. As the star rotates, these are rammed from below by the faster wind from undisturbed regions, forming the characteristic spiral pattern of a CIR.

While the simulations did produce the variable absorption line features, the extra absorption comes not from the densest region of the spiral, as expected, but from an extended layer with nearly uniform velocity that marks the initial response of the undisturbed wind to the slower CIR compression ahead. In the simulations it results in spectral features that shift slowly through the line profile over time, much as observed in real spectra.

Winds in Close Binary Systems

Hot, massive stars, some of which are Wolf-Rayet stars, often occur in close binary systems. Spectroscopic observations indicate that Wolf-Rayet stars have particularly massive winds, generally much stronger than those of the less-evolved companion star. The collision of the two stars' winds can be complex and violent.

Gayley and Owocki modeled the binary system V444 Cygni, in which the WN5 and O6III-V components orbit each other only a few diameters apart. Relatively simple hydrodynamic analyses predict that the Wolf-Rayet's wind should overpower the wind of the companion star, stopping only at a shock front near the surface of the O star.
Gayley and Owocki's simulations include the effects of radiative momentum transfer ("radiative braking") and indicate that this can significantly alter the collision's strength, geometry, and consequences, such as production of X-rays. In the simulations, the more massive wind does not overwhelm the weaker one; the geometry of the shock front depends strongly on how well the gas in the wind absorbs radiation.

Further Resources

An expert whose home page contains much additional material on this subject is Stan Owocki. Of particular note is his article Radiatively Driven Stellar Winds from Hot Stars:

"The winds from massive, hot stars – with surface temperatures above about 10,000K – form a well-defined class, distinct from the winds of cooler stars ... and characterised by the central role of the star's own radiation in driving the mass outflow."

References

Böhm-Vitense, Erika 1989b: Introduction to Stellar Astrophysics. Volume 2 – Stellar Atmospheres. Cambridge.

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.

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

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

Lamers, Henny J.G.L.M.; Cassinelli, Joseph P. 1999: Introduction to Stellar Winds. Cambridge.

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

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.

Owocki, Stan 2000: Radiatively Driven Stellar Winds from Hot Stars. In MacMillan Encyclopedia of Astronomy and Astrophysics.

Schaerer, Daniel; Schmutz, Werner; Grenon, Michel 1997: Fundamental Stellar Parameters of z Pup and g2 Vel from HIPPARCOS Data. ApJ Letters 484: L153-156.


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