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Stellar winds

Problem description:

During their lifetimes, stars loose mass via stellar winds. Winds influence the evolution of a star, objects in its vicinity, such as orbiting planets, and the general mass budget of the interstellar medium (ISM). Observations indicate that winds can be vary significantly depending in the properties and evolutionary status of a star. The most relevant parameters, which can link observations with possible driving mechanisms, are the mass-loss rate and the terminal wind velocity, which is the velocity of the wind far away from the star. More about stellar winds can be found in the review of Cranmer (2008) and Lamers & Cassinelli (1999).

The most detailed observations of stellar winds are obtained from the Sun. Observations that range from direct imaging and spectroscopy in various wavelength regimes to in-situ measurements of plasma parameters have shown that the solar wind is highly structured and its parameters vary during the activity cycle. The total mass-loss rate 2..3x10-14 M⊙/yr (Wang, 1998) and the terminal wind velocity is about 400 km/s. Coronal mass ejections (CMEs), which are often (but not always) related to flares, contribute up to 16% during solar activity maximum (Jackson & Howard, 1993).

More about the solar wind: http://solarscience.msfc.nasa.gov/SolarWind.shtml

High-density winds:

Due to their large distances, observations of winds from other stars is more difficult. If winds are massive enough (>10-10 M⊙/yr ) they produce detectable signatures seperatable from the stellar emission, such as continuum radiation excess (infrared-radio wavelengths) and spectral lines (P Cygni profiles, optical emission lines, molecular emission etc.). The spectroscopic methods can also provide the wind velocity, which remains unknown with the continuum methods.
Such strong winds are usually found on hot, massive stars (mass-loss rates up to 10-5 M⊙/yr, wind velocities up to 3000 km/s), cool giants and supergiants (similarly high mass-loss rates, but low velocities of 10-50 km/s) and young, pre-main sequence stars (mass-loss rates of 10-10..10-8 M⊙/yr).

Low-density winds:

Cool main-sequence stars like the Sun have tenous winds that cannot be observed directly. During the last years, several indirect methods have been proposed to derive mass-loss rates for solar-like stars. None of these methods is currently capable of deriving the wind velocity. Furthermore, contributions of CMEs to the steady wind cannot be determined for stars other than the Sun.

Astrospheres and Solar-like Stellar Winds

Stellar analogs for the solar wind have proven to be frustratingly difficult to detect directly. However, these stellar winds can be studied indirectly by observing the interaction regions carved out by the collisions between these winds and the interstellar medium (ISM) or an closing extrasolar planet environment.

Fig.1: Evolution of the observational based minimum and maximum stellar wind densities scaled to 1 AU (left scale: solid lines) obtained from several nearby solar-like stars. On the right scale one can see the evolution of the stellar wind velocity (dashed curve). More observations or early active stars with ages less than 700 Myr are needed to obtain a better picture of the mass-loss/activity relation shortly after the Sun arrived at the ZAMS (Lundin et al. 2007).

Methods:

a. Detecting Winds through astrospheric absorption:

The heliospheres and astropheres contain a population of hydrogen heated by charge exchange processes that can produce enough H I Lyα absorption to be detectable in UV spectra of nearby stars from the Hubble Space Telescope (HST). The amount of astrospheric absorption is a diagnostic for the strength of the stellar wind, so these observations have provided the first measurements of solar-like stellar winds. Results from these stellar wind studies and their implications for our understanding of the solar wind are reviewed in the following links to an article by Brian E. Wood at Living Reviews in solar physics. Of particular interest are results concering the past history of the solar wind and its impact on planetary atmospheres.

More Details on this Method:

The whole article can be viewed here: http://solarphysics.livingreviews.org/Articles/lrsp-2004-2/

b. Stellar wind characterization from exoplanet-ENA cloud observations:

A second way to probe stellar wind conditions is related to the observations and study of energetic neutral atom (ENA) clouds around exoplanets. For example the measured transit-associated Ly-absorption (Vidal-Madha et. al. 2003) of such a hydrogen clouds around the exoplanet HD209458b can be explained by the interaction between the exosphere and the stellar wind (Holmström et al. 2008). As the stellar wind protons are the source of the observed energetic neutral atoms, the analysis of ENA clouds contains information related to the stellar plasma flow.

 

Fig. 2/3: Left panel illustrated the stellar wind interaction with a transiting Hot Jupiter. The right planel shows a modeled planet-star velocity spectrum of hydrogen atoms in front or behind the planet. The part of the distribution that is due ti ENAs is shaded. Varying the stellar wind temperature and velocity in the model confirms that the width of this part of the distribution is proportional to the temperature of the stellar wind, with a larger width for larger temperatures, and the centre of the distribution follows the stellar wind velocity. The unshaded part of the spectrum is due to the exospheric hydrogen atoms (from Holmström et al 2008).

c. Accretion by white dwarfs:

Pre-cataclysmic binary systems consisting of an M dwarf and a white dwarf can give clues about the mass-loss rate of the M dwarf component. Some white dwarfs exhibit metal lines in their spectra that can only be maintained if there is a steady supply of material. Assuming that the wind from the M dwarf is the source of this material, the mass-loss rate can be determined by analysis of the white dwarf spectrum (Debes, 2006).

d. X-ray charge exchange emission:

By charge exchange between ions of a stellar wind ad the neutral ISM material, X-ray emission is produced. This X-ray emission can be used to determine the mass-loss rate of the star (Wargelin & Drake, 2001, 2002). No such emission has been detected yet, but with more sensitive instruments (e.g. the proposed Constellation-X and XEUS missions) this method could be a valuable diagnostic tool in the future, because it would also pe possible to determine the wind's velocity, composition and ionization state.

Until now, these methods yielded mass-loss rates for a small number of main-sequece stars in order of 10-16..10-12  M⊙/yr. Besides observational techniques, expected mass-loss rates can also be estimated using theoretical models. Holzwarth & Jardine (2007) developed a polytropic magnetized wind model applicable for winds of late-type stars. In this model the magnetic and thermal wind properties depend primarily on the stellar rotation rate. In this scenario, cool main-sequence stars can have mass-loss rates up to ten times solar.

References:

  • Cranmer, S. R.: Winds of Main-Sequence Stars: Observational Limits and a Path to Theoretical Prediction, ASP Conf. Ser., 2008, 384, 317-326
  • Holzwarth, V. & Jardine, M.: Theoretical mass loss rates of cool main-sequence stars, Astron. Astrophys., 2007, 463, 11-21
  • Jackson, B. V. & Howard, R. A.: A CME mass distribution derived from SOLWIND coronagraph observations, Solar Phys., 1993, 148, 359-370
  • Lamers, H., and Cassinelli, J. P. 1999, Introduction to Stellar Winds (Cambridge: Cambridge University Press).
  • Wang, Y.: Cyclic Magnetic Variations of the Sun, ASP Conf. Ser., 1998, 154, 131-152

a. Detecting Winds through astrospheric absorption:

  • Wood, B. E., Müller, H., Zank, G. P. & Linsky, J. L.: Measured Mass-Loss Rates of Solar-like Stars as a Function of Age and Activity, Astrophys. J., 2002, 574, 412-425
  • Wood, B. E., Müller, H., Zank, G. P., Linsky, J. L. & Redfield, S.: New Mass-Loss Measurements from Astrospheric Lyα Absorption, Astrophys. J., 2005, 628, L143-L146

b. Stellar wind characterization from exoplanet-ENA cloud observations:

  • Holmström, M.; Ekenbäck, A.; Selsis, F.; Penz, T.; Lammer, H.; Wurz, P.: Energetic neutral atoms as the explanation for the high-velocity hydrogen around HD 209458bNature, Volume 451, Issue 7181, pp. 970-972 (2008). http://adsabs.harvard.edu/abs/2008Natur.451..970H
  • Vidal-Madjar, A.; Lecavelier des Etangs, A.; Désert, J.-M.; Ballester, G. E.; Ferlet, R.; Hébrard, G.; Mayor, M.: An extended upper atmosphere around the extrasolar planet HD209458bNature, Volume 422, Issue 6928, pp. 143-146 (2003).
    http://adsabs.harvard.edu/abs/2003Natur.422..143V

c. Accretion of white dwarfs:

  • Debes, J. H.: Measuring M Dwarf Winds with DAZ White Dwarfs, Astrophys. J., 2006, 652, 636-642

d. X-ray charge exchange emission:

  • Wargelin, B. J. & Drake, J. J.: Observability of Stellar Winds from Late-Type Dwarfs via Charge Exchange X-Ray Emission, Astrophys. J., 2001, 546, L57-L60
  • Wargelin, B. J. & Drake, J. J.: Stringent X-Ray Constraints on Mass Loss from Proxima Centauri, Astrophys. J., 2002, 578, 503-514

Contacts of relevant researchers:

  • Dr. Mats Holmström:
    Swedish Institute of Space Physics, PO Box 812, SE-98128 Kiruna, Sweden.
    Email: matsh(at)irf.se