Using Molecular Bands to Measure Starspot Properties

We are using spectra obtained at Penn State, Kitt Peak, and McDonald Observatory to directly measure the total area and the temperature of magnetic "spots" in the photospheres of active stars. The observed spectrum of an active star (top) is composed mostly of emission from the non-spotted photosphere, which is much warmer than the starspots. The starspots are cool enough that molecular TiO bands form in them, producing a radically different spectrum. The spectrum of an active star (II Peg) is reproduced here by a suitably weighted sum of a K dwarf star (Gl 688) to represent the non-spotted photosphere plus an M dwarf star (FS Com) to represent the spots.

We have observed a ``grid'' of comparison stars spanning all possible temperatures along with our sample of active stars. We have observed bandheads with different temperature sensitivity. The absolute depth of the bands is a measure of starspot area, while the ratio of band depths independently constrains the starspot temperature. Given the total area and temperature of spots, Doppler imaging can provide a unique map of the surface distrution of those spots.

We are studying the variation of these parameters on rotational and on stellar cycle timescales. This involves repeated observations of our target stars on approximately uniform intervals from several days to several weeks (for rotational variation; over many years for the cycle variation).

PUBLICATIONS:

1. Spot Temperatures and Area Coverages on Active Dwarf Stars, S.H. Saar and J.E. Neff, in Cool Stars, Stellar Systems, and the Sun, ed. G. Wallerstein, (ASP Conf. Series), p. 171, 1990.
2. Absolute Measurements of Starspot Area and Temperature, J.E. Neff, D. O'Neal, S.H. Saar, in Inside The Stars, ed. W.W. Weiss and A. Baglin, (ASP Conf. Series), p. 193, 1993.
3. Measurements of Starspot Area and Temperature on II Pegasi, D. O'Neal, J.E. Neff, S.H. Saar, in Cool Stars, Stellar Systems, and the Sun, ed. J.-P. Caillault, (ASP Conf. Series), p. 726, 1994.
4. Measurements of Starspot Area and Temperature on II Pegasi In 1989 October, J.E. Neff, D. O'Neal, S.H. Saar, Astrophys. J., 452, 879, 1995.
5. Using TiO Spectroscopy To Further Constrain Doppler Imaging, D. O'Neal, J.E. Neff, S.H. Saar, N.E. Piskunov, in Stellar Surface Structure: Poster Proceedings, ed. K.G. Strassmeier, (Vienna Obs.), p. 32, 1995.
6. A Possible Relationship Between Surface Gravity and Temperature Deficit in Stellar Spots, S.H. Saar, D. O'Neal, J.E. Neff, in Stellar Surface Structure: Poster Proceedings, ed. K.G. Strassmeier, (Vienna Obs.), p. 105, 1995.
7. Multiwavelength Observations of Two Moderate Rotation RS CVn Systems: V815 Hercules and IM Pegasi, R.C. Dempsey, J.E. Neff, D. O'Neal, K. Olah, Astron. J., 111, 1356, 1996.
8. Measurements of Starspot Area and Temperature on Four Active, Evolved Stars, D. O'Neal, S.H. Saar, J.E. Neff, Astrophys. J., 463, 766, 1996.
9. TiO Band Measurement of Starspot Temperature and Area on Active Stars, D. O'Neal, J.E. Neff, S.H. Saar, in Cool Stars, Stellar Systems, and the Sun, ed. R. Pallavicini and A.K. Dupree, (ASP Conf. Series 109), p. 621, 1996.
10. OH 1.563 micron Absorption From Starspots on Active Stars, D. O'Neal, J.E. Neff, Astron. J., 113, 1129, 1997.
11. II Peg: Spectroscopic Evidence for Multiple Spot Temperatures, D. O'Neal, S.H. Saar, J.E. Neff, in Cool Stars, Stellar Systems, and the Sun, ed. R.A. Donahue and J.A. Bookbinder, (ASP Conf. Series 154), p. 1439, 1998.
12. Spectroscopic Evidence for Non-Uniform Starspot Properties on II Peg, D. O'Neal, S.H. Saar, J.E. Neff, Astrophys. J. (Letters), 501, L73, 1998.
13. Measurements of Starspot Parameters on Active Stars using Molecular Bands in Echelle Spectra, D. O'Neal, J.E. Neff, S.H. Saar, Astrophys. J., 507, 919, 1998.
14. Starspot Temperature and Filling Factor Measurements for Active Dwarfs, S.H. Saar, A. Peterchev, D. O'Neal, J.E. Neff, in Cool Stars, Stellar Systems, and the Sun, eds. R.J. Garcia Lopes, R. Rebolo, M.R. Zapatero Osorio, (ASP Conf. Series 223), p. CD-1057, 2001.
15. Hydroxl 1.53 micron Absorption From Starspots on Active Stars, D. O'Neal, J.E. Neff, S.H. Saar, J.K. Mines, Astron. J., 122, 1954, 2001.