Delta Scuti Star Newsletter

Issue 9, September 1995

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What is known and not known about Gamma Doradus stars

by: Kevin Krisciunas

• A number of early F-type stars on or just above the main sequence in the Hertzsprung-Russell Diagram have been found to be variable on time scales of 0.5 to 3 days. The V-band variability is in the range 0.05 to 0.10 mag.
• A "committee" consisting of Breger, Handler, Mantegazza, Poretti, Balona, and Krisciunas decided at the Cape Town stellar pulsation meeting in February, 1995, that these stars would be known as Gamma Dor stars. Gamma Dor itself is the brightest "bona fide" member, one of the best studied, and was the first one to be identified as variable (Cousins & Warren 1963).
• Krisciunas & Handler (1995) have recently compiled a list of 17 Gamma Dor stars, dividing them up into "bona fide" members and other candidates. Zerbi et al. are preparing a paper on 4 more candidates in NGC 2516, giving that cluster 8 known candidates.
• As shown by Krisciunas & Handler (1995), the Gamma Dor candidates are almost all to be found in very small region of the HR Diagram at the intersection of the main sequence and the cool edge of the Cepheid instability strip.
• mode pulsations. This conclusion is reached in part by the lack of evidence for other explanations, including starspots. The first big hint that non-radial g-modes might be the behavioral mechanism is the time scale of the photometric variations -- an order of magnitude slower than the fundamental radial pulsational mode. Radial velocity variations and/or line profile variations have been found in the three best studied stars, Gamma Dor itself (Balona et al. 1994; Balona et al., in preparation), 9 Aur (Krisciunas et al. 1995a), and HD 224638 (Mantegazza et al. 1995). The radial velocities range by 4 km/sec. The photometric and spectroscopic data are strong evidence that these stars are exhibiting non-radial g-modes in a low degree spherical harmonic.
• Aerts & Krisciunas (1995) used the CORAVEL data of R. F. Griffin to show that 9 Aurigae can be modelled as an l = 3, |m| = 1 non-radial pulsator. The amplitude of the radial part of the pulsation for its 1.26 day period is a factor of 4 larger than the one for its 2.89 day period. Since the photometric variability is determined mostly by temperature variations, which in turn are determined by the radial part of the pulsation, the photometric variability is dominated by the faster mode. However, the slower period is more pronounced in the spectroscopic moment variations, reflecting that the transverse displacement of the slower mode dominates the velocity behavior.
• It was 130 years after Goodricke's 1784 discovery of the variability of Delta Cephei that Shapley (1914) demonstrated the pulsational nature of Cepheids. For Gamma Dor stars it took 32 years, from 1963 to 1995.
• The evidence at present indicates that the Gamma Dor phenomenon is a characteristic of relatively young F stars (Eggen 1995; Krisciunas et al. 1995b). The Hyades (age & amp;gt; 600 Myr) apparently has no Gamma Dor stars. NGC 2516 (age ~ 140 Myr) has 8 candidates. The Pleiades (age 80 Myr) has one candidate. Gamma Dor itself is embedded in a Beta Pictoris-like shell or envelope and is a member of the IC 2391 supercluster, whose age is 50 Myr. BS 8799 is a member of the Pleiades supercluster. HD 164615 and 9 Aur have space velocities like those of young disk stars. HD 224638 and HD 224945 have very small proper motions, characteristic of young disk stars.
• We do not know how to model Gamma Dor stars from the perspective of the physics of stellar structure. Dziembowski and Pamyatnykh (see comment on p. 79 of Stobie & Whitelock 1995) have found unstable high-order g-modes in models of mid-F stars, but do not consider their results sufficiently reliable to publish. Gautschy (private communication) attempted to model these stars and found that he could excite ~ 100 pulsational modes, but not only 2 or 3. It could be that the HeII opacity bump may be responsible for the pulsations.
• We do not know the percentage of Population I stars that exhibit the Gamma Dor phenomenon. This will require the investigation of a number of clusters over a range of ages. Multi-longitude photometric campaigns will be necessary.
• We do not know how the Gamma Dor phenomenon is related to the rotation rate of the stars.
• Would a Population II star (i.e. a star with a different helium and metals content) have shown the Gamma Dor-type behavior when it was young several billion years ago?
• If the speculation that the Gamma Dor phenomenon is age-related is correct, what change in the structure of the star causes the pulsations to cease?
• If the Gamma Dor stars are an analog of the slowly pulsating B stars (Moskalik 1995), are there other regions of the HR Diagram where "slowly" pulsating stars are to be found?

References:

1. Aerts, C., Krisciunas, K. 1995, \textit{MNRAS}, in press
2. Balona, L. A., Hearnshaw, J. B., Koen, C. et al. 1994, MNRAS 267, 103
3. Cousins, A. W. J., Warren, P. R. 1963, MNASSA 22, 65
4. Eggen, O. J., 1995, IBVS No.4210
5. Krisciunas, K., Griffin, R. F., Guinan, E. F., Luedeke, K. D., McCook, G. P. 1995a, MNRAS 273, 662
6. Krisciunas, K., Crowe, R. A., Luedeke, K. D., Roberts, M. 1995b, MNRAS, in press
7. Krisciunas, K., Handler, G. 1995, IBVS No. 4195
8. Mantegazza, L., Poretti, L., Bossi, M., Zerbi, F. M. 1995, in Stobie & Whitelock, p. 339
9. Moskalik, P. 1995, in Stobie & Whitelock, p. 44
10. Shapley, H. 1914, ApJ 40, 448
11. Stobie, R. S., Whitelock, P. A. 1995, Astrophysical Applications of Stellar Pulsation, IAU Colloquium 155, ASP Conference Series 83