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Astron. Astrophys. 325, 559-562 (1997)

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5. Oscillation pattern of BT Cnc

Breger (1980) analysed two series of observations from 1967-1968 and 1975-1976. He concluded that two modes with frequencies 9.777 and 7.88 c/d and amplitudes in B of 20.1 and 8.0 mmag, respectively, are definitely present, and that one mode with frequency 5.95 c/d and amplitude 5 mmag is probably present. For the primary oscillation our data (Table 2) are in good agreement with Bregers. However, we find no evidence for a mode with [FORMULA] = 7.88 c/d and amplitude of more than 1 mmag. The frequency [FORMULA] = 5.95 c/d is very close to the 1 c/d alias of [FORMULA]. However, if we introduce a constituent with [FORMULA] 5.95 c/d in the lsq-solution, we find a new frequency with [FORMULA] = [FORMULA], and the amplitude at 5.95 c/d drops to almost zero. Thus we find that the 1994-data cannot be described by Bregers preferred solution.

At the recent Nice-meeting Belmonte et al. (1996) discussed [FORMULA] Sct stars in Praesepe. Their Fig. 1 gives for BT Cnc two frequencies of 0.113 and 0.081 mHz, in very good agreement with [FORMULA] and [FORMULA]. The corresponding amplitudes are about 10.5 and 8.5 mmag, somewhat deviating from our data.

Thus we find that the available information on the pulsation properties of BT Cnc indicate that this star shows secular variations in the oscillation spectrum. Much more detailed observations over a long period of time are necessary to firmly establish the pulsation pattern of BT Cnc.

5.1. Modelling

We will not attempt detailed modelling of BT Cnc here (See e.g. Hernández et al. [1995] for modelling of [FORMULA] Sct stars in Praesepe). Breger (1980) estimated the value of the pulsation parameter Q for the primary oscillation with period [FORMULA] d to be 0.021 [FORMULA] 0.004 d. This can correspond to a radial overtone of order 1-4. The period ratio [FORMULA] is in agreement with [FORMULA] in the population I models of Petersen and Dalsgaard (1996). However, since [FORMULA] km/s in BT Cnc (Belmonte et al. 1996), a significant correction for rotation must be expected (Hernández et al. 1995), and precise modelling is difficult.

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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