Year 3 Observatory lab projects

Wolf-Rayet stars

Wolf-Rayet (W-R) stars are hot, very luminous stars whose spectra are dominated by very broad emission lines, which are formed in a dense and supersonic stellar wind. W-R stars represent the late stages of stellar evolution of the most massive stars (those with initial masses of ≥20M₀

Medium dispersion spectra of nearby W-R stars will show several broad emission lines and will allow the classification into the two main W-R spectral subtypes (WN and WC) which primarily depend on the elements present in the wind (and which in turn depends on the evolutionary state of the star). These observations will also enable a simple investigation into the ionization structure of the stellar wind.
 

A sample of W-R stars in the northern hemisphere brighter than V=9, visible in Semester 1 from Wast Hills. The WR names are included as they are often used to identify the stars

Observations

Select a small sample of W-R stars to observe (2 or 3), making sure you include both a WN and a WC star. From Underhill (1968), list the strongest spectral lines that you are likely to observe for each star you have decided to observe. Some examples of spectra of W-R stars can be found on the web at http://astrosurf.com/buil/us/peculiar2/wolf.htm. Some of the W-R stars have more and stronger lines than others so you will need to do some research before selecting your targets.

Obtain medium resolution spectra of the target stars over as wide a spectral range as is possible with the Wast Hills set up, extending from the blue into the red (wavelength range ~4500-8500 Angstroms), encompassing as many of the expected spectral lines as possible. To do this will require two exposures for each star, and care will have to be taken over the spectrograph settings to get the necessary wavelength coverage in the two exposures. You should also obtain as long exposures as possible, without saturating the CCD. Because of the strong emission lines calculating reasonable exposure times is quite difficult, so care must be taken here.

Analysis

From an analysis of the spectral lines in your spectra and their widths attempt to determine (as far as you are able) the spectral types of the stars you observe (see references below, particularly Underhill 1968 for a discussion of classification). Note, there is more than one scheme to classify W-R stars, and some references are given below

Fit the spectral lines with Gaussian profiles. Note, some lines will be blended. For each star plot the derived values of the line-widths (in velocity units) versus the excitation potential of the line. What trends are apparent in these plots, and discuss the possible physical reasons behind these trends? What do they tell us about the ionization structure of the stellar winds of W-R stars?

See also:

Perry Berlind's page on W-R stars at CfA, Harvard

Christian Buil's page on Astrosurf

General articles can be found in the following books:

Lamers, J. G. L. M. & Cassinelli, J. P., Introduction to Stellar Winds, Cambridge University Press, 1999.
This is and excellent an relevant book: look for a detailed discussion on Wolf-Rayet stars in Section 2.7.2, then turn to the index

Percy, John R., Understanding Variable stars, Cambridge University Press, 2007.
Very useful advanced textbook

Underhill, A. B., 1996, The Early Stars, Reidel (Book in Y3 Laboratory Library and University Library)
Old classic- the discussion is largely qualitative.

Other Useful (more advanced) References

Abbott D.C. & Conti P.S., 1987, Annual reviews of astronomy and astrophysics, 25, 113..
Physical Properties of Wolf-Rayet Stars Crowther, P., 2007, Annual reviews of astronomy and astrophysics, 45, 177-219
Lang K.R., Astrophysical Data: Planets and Stars, Chapter 11.
Schulte-Ladbeck R.E. et al. 1995, ApJ, 454, 917.
Smith L.F., 1968, Monthly Notices of the Royal Astronomical Society, 138, 109.
Underhill A.B., 1966, The Early-Type Stars, Reidel.
Underhill A.B., 1968, Annual Reviews of Astronomy and Astrophysics, vol 6, pp 39-78.

(Some of these books and articles are available in Y3 Obs Lab collection, upstairs in Astrolab- ask Barry Weston)