Determination of Vega’s rotational velocity based on the Fourier analysis of spectral line profiles

Abstract

ABSTRACT While it is known that the sharp-line star Vega (vesin i ∼ 20 km s−1) is actually a rapid rotator seen nearly pole-on with low $i\, ({\lt}10^{\circ })$, no consensus has yet been accomplished regarding its intrinsic rotational velocity (ve), for which rather different values have been reported so far. Methodologically, detailed analysis of spectral line profiles is useful for this purpose, since they reflect more or less the ve-dependent gravitational darkening effect. However, direct comparison of observed and theoretically simulated line profiles is not necessarily effective in practice, where the solution is sensitively affected by various conditions and the scope for combining many lines is lacking. In this study, determination of Vega’s ve was attempted based on an alternative approach making use of the first zero (q1) of the Fourier transform of each line profile, which depends on K (temperature sensitivity parameter differing from line to line) and ve. It turned out that ve and vesin i could be separately established by comparing the observed $q_{1}^{\rm obs}$ and calculated $q_{1}^{\rm cal}$ values for a number of lines of different K. Actually, independent analysis applied to two line sets (49 Fe i lines and 41 Fe ii lines) yielded results reasonably consistent with each other. The final parameters of Vega’s rotation were concluded as vesin i = 21.6(±0.3) km s−1, ve = 195(±15) km s−1, and i = 6.4(±0.5)○.