Astron. Astrophys. 329, 131-136 (1998)

3. Photometry

3.1. Spectral energy distribution

The reddening of the program stars was estimated as follows. First, the total extinction was computed by comparing the observed -color with the intrinsic color which corresponds to the spectral type of the star (Meylan et al. 1980) and adopting . The interstellar extinction

was derived by computing the extinction as a function of distance modulus for B-stars near the line of sight ( or slightly more, if less than 20 B-stars with Geneva-photometry were found) of each program star. This method was developed by Cramer & Maeder (1979) who computed for B-stars and by Cramer (1982) who computed the intrinsic color indices and for B-stars. Combining these two methods, the above mentioned vs. diagram could be made, and since Herbig Ae/Be stars are population I, could be estimated from (Mihalas & Binney, 1981) . The - and the -values for the stars are given in Table 3. For HD 139614 and HD 144432 the estimated interstellar and total reddenings agree fairly well, indicating that the amount of circumstellar reddening is small or negligible; for HD 142666 a substantial amount of circumstellar reddening occurs.

Table 3. Interstellar and total extinction for the program stars

To construct the spectral energy distributions, the UV and optical data were corrected for the total extinction. An average extinction curve (Savage & Mathis, 1979) was used with , to calculate the reddening (in magnitudes) at each wavelength . Then

where is the intrinsic flux at wavelength , and the observed flux at wavelength .

The optical observations and near-UV observations were fitted by a LTE atmosphere model (Kurucz R.L., 1994) with parameters initially estimated from the spectral type and the luminosity class of the stars; in all cases a solar composition was adopted. After some iterations, a Kurucz model with = 8250 K and log g = 4.5 resulted in the best fit for HD 139614 and HD 142666, while HD 144432 needed a somewhat cooler ( = 7750 K, log g = 4.5) model. In Fig. 1, the spectral energy distributions for the program stars are presented. In all three stars excess radiation is observed in the UV and the IR, caused by circumstellar gas and dust, respectively.

Fig. 1a-c. The energy distributions of HD 139614, HD 142666 and HD 144432. is plotted normalised to the V-filter as a function of wavelength in nm. The observed points are fitted by a Kurucz atmosphere model. The three stars show an excess both in the UV and in the IR.

3.2. Optical photometric variability

The Geneva photometric data obtained for HD 139614 and HD 144432 do not show any significant variability; on the other hand, HD 142666 displays large ( ) brightness variations. The visual magnitude ( ) of this star as a function of the Julian Date (JD) is shown in Fig. 2. It should be noted that the star is most frequently in its bright state. A period analysis for this star was performed, but no period was found. Therefore it can be concluded that the variations are irregular.

Fig. 2a and b. Upper: The optical brightness of HD 142666 as a function of JD (244+), showing substantial variability ( ). Lower: The color-magnitude diagram for HD 142666. Here a linear relation can be seen between and the color index . The star reddens when it becomes fainter.

In Fig. 2, the color-magnitude diagram for HD 142666 is shown. It can be seen that, for this star, there is a linear relation between and the color-index : the star becomes redder as the apparent magnitude decreases. This shows that HD 142666 is a member of class R (Red behaviour), as defined by Bibo & Thé (1991), in their study of the photometric variability of Herbig Ae/Be stars in a color-magnitude-diagram. The slope of the ( ) curve is similar to that of the IS extinction law by Savage & Mathis (1979). This suggests that, like the Herbig Ae star HR 5999 (Bibo & Thé 1991), the brightness variations of HD 142666 can be ascribed to variable obscuration by CS dust. It can be concluded that the star HD 142666 shows nonperiodic Algol-like minima, and can be considered as a member of the UXor class.

© European Southern Observatory (ESO) 1998

Online publication: November 24, 1997
helpdesk.link@springer.de