9 Aurigae: Strong Evidence for Non-Radial Pulsations

Abstract

We present further photometric observations of the unusual F0 V star 9 Aurigae and present evidence that this star's radial velocity, spectroscopic line widths and line depths are also variable with the same frequencies as the photometric data ($f_1 \approx 0.795$ and $f_2 \approx 0.345$ d$^{-1}$). The phases of these sinusoids are stable over time scales of longer than one year, though the amplitudes can vary, making the prediction of photometric behavior impossible. Given that a variety of other explanations have already been discounted (e.g. interactions with a close companion, the existence of a lumpy, orbiting ring of dust, or star spots) and that these variations occur on time scales an order of magnitude slower than the fundamental radial pulsation period, we have very strong evidence that 9 Aurigae exhibits non-radial {\it g}-mode pulsations. Since the power spectrum of the radial velocity data shows frequency $f_2$ but does not clearly show $f_1$, the present data suggest that $f_2$ is associated with a low degree spherical harmonic ($\ell$ = 1 or 2), while $f_1$ is associated with a higher degree harmonic. 9 Aurigae, along with such stars as $γ$ Doradus, HD 224638, HD 224945, and HD 164615, appear to constitute a new class of pulsating variables. These stars are to be found at or beyond the cool edge of the Cepheid instability strip in the HR Diagram. Prior to this, only much hotter stars have been shown to exhibit non-radial {\it g}-modes.