Our understanding of Be stars is still incomplete, see Chapter 1. Theoretical models can only be constrained, or indeed ruled out, if their predictions can be confronted with observations of well understood Be star samples. Hitherto, a homogeneous data set involving extensive wavelength coverage across all Be spectral types has been lacking.
This demand has been addressed by defining and observing in a homogeneous fashion a
representative sample of Be stars. The sample contains objects from O9 to B8.5 and of
luminosity classes III (giants) to V (dwarfs), as well as three shell stars. It was selected in an
attempt to contain several objects that were typical of each spectral and luminosity class in
the above range; it therefore does not reflect the spectral and luminosity class space
distribution of Be stars, but only the average properties of each subclass in temperature and
luminosity. A spectral type and measure of v sin(i) were derived for each object in the sample
and presented in Steele et al. (1999). In Clark and Steele (2000), K band spectroscopy of the
sample is presented, and in Steele and Clark (2001) H band spectroscopy. In a
forthcoming paper, Steele and Negueruela (2002), present spectra in the regions of the
H
and Paschen series. In an attempt to ascertain whether Strömgren photometry
can be successfully separated into the constituent parts of circumstellar excess,
interstellar reddening and intrinsic colour, the results of the separation attempted in
this chapter will be confronted with the physics of circumstellar excess line and
continuum production. The results of the separation will also be used to derive the
fundamental stellar properties that the Strömgren filter set was originally designed
to.
One of the main problems which prevents the detailed study and modelling of Be stars is the
difficulty in determining the precise astrophysical parameters (MV , Teff, log g) of the
underlying B star (Fabregat et al., 1996). This is because the equivalent widths of Balmer lines
are distorted by emission lines of the circumstellar shell and photometric measurements are
hampered by the circumstellar continuum radiation. Due to these effects the usual techniques
of uvby
calibration are not suitable (Fabregat and Torrejon, 1998). Those spectral lines
referred to as the Balmer lines are generated by electrons in hydrogen atoms which fall
(emission line) from higher energy states to the n = 2 shell, or are energised from (absorption
line) the n = 2 shell.
The uvby and H photometric systems, defined by Strömgren (1966) and Crawford and
Mander (1966), were designed to measure fundamental spectral signatures in early and
intermediate type stars (Fabregat and Torrejon, 1998). The intermediate width filters and
narrow tails allows for high transmission efficiency (see Table 2.2 and Figure 6.1), this design
was forged such that the system would be almost independent of the detector and thus easily
transformed to a standard set of references. The passband of each filter was picked to
correspond to a particular astrophysical effect. The u filter relates to the Balmer
discontinuity (see Section 6.1.1) while v was chosen to coincide with that section of the
spectrum that shows metal excess. The wavelength of the b and y filters were chosen to
correspond to that section of the spectrum almost purely determined by stellar
temperature.
Fabregat and Torrejon (1998) have developed a method to allow Strömgren filters to be used to observe Be stars, (see Section 6.5 later in this chapter), this chapter attempts to use this method to find out detailed information about the stars in the representative sample. The Strömgren filters are one of the possible filter options on the LT, it is therefore necessary for the LT PL to be able to reduce data obtained from these filters, accordingly the data in this chapter will be processed using the LT PL.
|
The usual colour indices employed are given by
| (b - y) | (6.1) | |
| c1 = | (u - b) - (v - b) | (6.2) |
| m1 = | (u - b) - (b - y) | (6.3) |
| = | Hnarrow - Hwide, | (6.4) |
is a luminosity indicator for hot stars and a
temperature indicator for cooler stars. For cooler stars m1 measures the amount of
line-blanketing, the dimming of the blue part of the spectrum caused by millions of
heavy-element absorption lines
3646Å, therefore the measured radiative flux decreases. This sudden break in
the continuum spectrum of a star is known as the Balmer discontinuity. It occurs at the
point where hydrogen becomes ionised, this increases density and therefore opacity, its size
depends on the fraction of hydrogen atoms that are in the first excited state, which in turn
depends on the temperature.
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