Baade-Wesselink technology

The Baade-Wesselink technique or Baade-Wesselink method (based on the astronomers Walter Baade and Adriaan Wesselink, who significantly developed the method in the first half of the 20th century) is a method in astronomy for monoperiodic radially pulsating stars determine the radius and thus indirectly the distance from spectroscopic and photometric data. The Baade-Wesselink technique is a method to calibrate the period-luminosity relationship for Cepheids and RR Lyrae stars .

requirements

• The variability of the star in terms of radius and brightness must be strictly periodic
• The photosphere must be small compared to the star's radius. This criterion excludes all types of red giants such as B. Mira-Stern from the analysis by the Baade-Wesselink method.

approach

If the same temperature is measured in two phases of the pulsation light change, the different brightness is a direct result of the change in the radius. More precisely, the photometrically measured difference in brightness is proportional to the squares of the radii at the two points in time. The radii can be described as the mean radius of the variable star plus the respective change in radius derived from the spectrographically measured radial velocities . The mean radius can be derived from this formula if the velocity gradients in the atmosphere and the edge darkening are taken into account accordingly.

The near infrared surface brightness method

A further increase in accuracy could be achieved using the near infrared surface brightness method. The measurement in the near infrared allows a calibration of the radii on the surface brightness of invariable giant stars, which are known with high accuracy from interferometric measurements of nearby stars. The error of this method in determining the distance of individual Cepheids is less than 6 percent.

Expanding photosphere method

A variant of the Baade-Wesselink method can be applied to the expanding shell of supernovae . In this method of expanding photosphere it is assumed that in the early phase of the eruption the outer atmosphere is still dense enough to resemble a black body as a first approximation . The expansion speed of the stellar atmosphere can be derived from the radial speed and the distance to the eruptive variable can be calculated from the change in brightness and colors .

literature

• W. Strohmeier: Variable stars . Treugesell-Verlag, Düsseldorf 1974.
• Wolfgang Gieren et al .: Cepheid distances from the Baade – Wesselink method . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.7150 .
• Richard de Grijs: An Introduction to Distance Measurement in Astronomy . Wiley, Chicester 2011, ISBN 978-0-470-51180-0 .