Data taken over several seasons reveal two principal frequencies of f1 = 0.7948 and f2 = 0.3429 cycles/day. 1994/5 data indicate a third frequency f3 = 0.7679 cycles/day, which is present in the 1993/4 data as well.

Radial velocities obtained by Roger Griffin at Haute Provence show that the radial velocities vary by 4 km/sec and follow frequency f2 only. It is significant (see below) that the light maximum of f2 coincides in time with the most negative radial velocity.

Griffin's data also show that 9 Aur exhibits line profile variations. The line profile rock back and forth, something akin to cow's udders. Given that we are likely dealing with pulsations, this would indicate that we are dealing with a low spherical harmonic order. (For one to observe the photometric and radial velocity variations, the star cannot be sliced up to a large number of sections.)

By good fortune the referee of a previous paper on this star was Luis Balona (SAAO), who pointed out that the star gamma Doradus has nearly the same spectral type and similar behavior to 9 Aurigae. Since then it has been shown that gamma Dor exhibits line profile and radial velocity variations similar to 9 Aur.

Our Italian colleagues Luciano Mantegazza, Ennio Poretti, and Filippo Zerbi have found some similar stars, namely HD 224638 and HD 224945. Two years ago Gerald Handler and I compiled a list of 17 candidates that appear to exhibit the "gamma Doradus phenomenon". We show the positions (as of 17 June 1997) of the bona fide members of this class (circles) and candidates (dots) in the color-magnitude diagram below. Eight stars in NGC 2516 are indicated by triangles.

An updated list of gamma Dor-type stars, with references to the literature on the observations, can be obtained by clicking here .

One of the ideas to explain the behavior of these stars was rotational modulation of starspots. For a star with a single period (like HD 164615) or two closely spaced periods one can construct such a model. But now that three frequencies are known for 9 Aur and gamma Doradus, and much data directly supports the pulsational idea, starspots are no longer a very viable alternative.

Another problem with starspots is that a starspot model would predict that the radial velocity maximum should be 90 degrees out of phase with the light maximum. For 9 Aur and gamma Dor the maximum blue shifted radial velocity coincides with the light maximum of a given period, rather than being 90 degrees out of phase.

The evidence is that these stars are pulsating, but not in the radial mode like Cepheids. Rather, they exhibit low-order non-radial gravity modes. This is to say that the time scale for the variations is much slower than the fundamental radial pulsation mode (1 to 3 hours) for stars of this density. These stars are found on or just above the main sequence in the Hertzsprung- Russell Diagram, and at or just beyond the cool edge of the Cepheid instability strip. This behavior was completely unexpected.

On the theoretical front, Aerts and Krisciunas (1996) have recently modelled 9 Aur (also now known as V398 Aur) as an l = 3, |m| = 1 non-radial pulsator. At an inclination angle of 55 degrees, it would look like this:

Balona et al. (1996) find from a similar analysis of gamma Doradus that its behavior can be represented by sectoral retrograde modes with spherical harmonic degrees, (l,m), as follows: f_1 = (3,3), f_2 = (1,1), and f_4 = (1,1). The frequencies, obtained from photometry, are f_1 = 1.32098, f_2 = 1.36354, and f_4 = 1.47447 cycles/day. To view abstract of this paper from Los Alamos eprint archvies, click here . You may then choose whether or not to download a 417 kb file to view the whole paper plus graphics.

Analogous images of gamma Doradus are given below. On the left shows the velocity distribution, while on the right we see the temperature (or flux) distribution):

It is likely that the gamma Dor phenomenon is a characteristic of relatively young F stars. We observed the early F stars in the Hyades star cluster (age > 600 Myr) -- stars not known to be delta Scuti stars -- and found no evidence for gamma Dor-like behavior. The cluster NGC 2516 (age 137 Myr) now has 8 gamma Dor candidates. The Pleiades (age 78 Myr) has one gamma Dor candidate. gamma Dor itself is imbedded in a beta Pictoris-like shell, so is presumably young. 9 Aur has space motions like other young disk stars. The open cluster M 34, which has an estimated age of 250 Myr, has several gamma Doradus candidates (Krisciunas and Crowe 1997).

Regarding multi-longitude campaigns of 1995, analysis is proceeding on HD 164615, BS 8799, HD 224638, and HD 224945. Photometry of HD 164615 over 16 years shows one period of constant phase, with varying amplitude. BS 8799 seems to be a similar example.

The interested reader may want to consult these papers:

Aerts, C., Krisciunas, K., 1996, "Mode identification of the slowly pulsating F0 V star V398 Aur (9 Aur)," Monthly Notices of the Royal Astr. Soc. , 278 , 877-882.

Balona, L. A., Krisciunas, K., and Cousins, A. W. J., 1994, "gamma Doradus: evidence for a new class of pulsating star," Monthly Notices of the Royal Astr. Soc., 270 , 905-913.

Balona, L. A., Bohm, T., Foing, B. H., et al. 1996, "Line profile variations in gamma Doradus," Monthly Notices of the Royal Astr. Soc. 281 , 1315.

Krisciunas, K., Griffin, R. F., Guinan, E. F., Luedeke, K. D., and McCook, G. P., 1995, "9 Aurigae: strong evidence for non-radial pulsations," Monthly Notices of the Royal Astr. Soc., 273 , 662-674.

Krisciunas, K., and Handler, G., 1995, "A list of variable stars similar to gamma Dor," Information Bulletin on Variable Stars , No. 4195.

Krisciunas, K., and Crowe, R. A., 1997, "A search for gamma Doradus-type variables in the open cluster M 34," Information Bulletin on Variable Stars , No. 4430.

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This document last revised on 11 July 1997.