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Astron. Astrophys. 342, L49-L52 (1999)

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3. Spectral evolution

Photometric data can be used to study also the spectral evolution of ON 231 during the outburst. It is well known from previous works (Lorenzetti et al. 1990, Tosti et al. 1998) that ON 231 shows a clear spectral hardening when its luminosity is increasing. Such a behaviour is fully confirmed by the 1998 observations. In Fig. 4 we plotted the spectral distributions of ON 231 in four different luminosity states derived from [FORMULA] photometric meaurements performed with a very short time separation and using the same telescope. The spectral index, computed from the zero magnitude fluxes by Mead et al. (1990), varied from -1.40[FORMULA]0.06 (February 20) to -0.52 [FORMULA] 0.05 (April 24, during the maximum phase). This change corresponds to a variation of the peak frequency in the [FORMULA] spectrum from the near infrared in the lowest state to the B band (or beyond) in the highest one. We stress that a spectrum as flat as that of April 24 is really exceptional and was never observed in the previous years: Tosti et al. (1998) report only two values of the spectral index greater than -1.0 over a set of 143 data in 1994-1997 and the greatest one was -0.87.

[FIGURE] Fig. 4. Four points spectral distributions of ON 231 in different luminosity states during the outburst. A reddening correction with [FORMULA]=0.11 was applied to the measured magnitudes. The dashed lines are the power law ([FORMULA]) best fits whose [FORMULA] values are indicated.

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999