7 The stars

Among emission stars selected there are objects of different brightness V $=15\hbox{$.\!\!^{\rm m}$ }5 -19\hbox{$.\!\!^{\rm m}$ }5$. At a distance modulus 24 $.\!\!^{\rm m}$3 it is obvious that all of them are early massive stars of different luminosity classes. Because of the great dispersion of luminosities, it was a good plan to divide all the stars into three groups: bright ( $V<17\hbox{$.\!\!^{\rm m}$ }5$), medium (17 $.\!\!^{\rm m}$5 <V< 18 $.\!\!^{\rm m}$5) and faint (18 $.\!\!^{\rm m}$5 <V< 19 $.\!\!^{\rm m}$5). In Table 3 we present star numbers in these groups and mean observed colour indices with their deviations. On the two-colour diagram displayed in Fig. 4, filled circles indicate the groups. The error bars are rms of the mean values. Stellar magnitudes presented by IFM have been calibrated by photoelectric standards and have no noticeable systematic errors. The average scatter is equal to ${ \sigma(U)=0{}^{\rm m}\!\!\!.\,22}$, ${ \sigma(B)=0{}^{\rm m}\!\!\!.\,29}$ and ${ \sigma(V)=0{}^{\rm m}\!\!\!.\,20}$. The error being larger for the objects in compact groups, in isolated objects the individual error does not exceed 0 ${}^{\rm m}\!\!\!.\,$1 (IFM).

   

Table 3: Parameters of the stars

V	n	U-B	B-V	E(U-B)	E(B-V)	AV	(U-B)0	(B-V)0	MV
		rms	rms	$I\,/\,V$	$I\,/\,V$	$I\,/\,V$	$I\,/\,V$	$I\,/\,V$	$I\,/\,V$
< 17.5	20	-0.62	0.18	0.20	0.29	0.96	-0.82	-0.11	-8.3
		0.05	0.06	0.33	0.45	1.48	-0.95	-0.27
17.5-18.5	21	-0.80	0.10	0.22	0.28	0.91	-1.02	-0.18	-7.3
		0.08	0.07	0.30	0.41	1.35	-1.10	-0.31
18.5-19.5	40	-0.85	0.01	0.15	0.19	0.62	-1.00	-0.18	-5.9
		0.05	0.04	0.24	0.32	1.04	-1.09	-0.30	-6.3

In Fig. 4 two sequences are shown for stars of luminosity classes I and V as well as two reddening lines (dashed) for B0I and OV stars, respectively (Strajzhis 1977). The slopes of the reddening lines differ insignificantly. For each of these three groups and for two luminosity sequences we have found corrected colour indices ${ (U\!-\!B)_0}$ and ${(B\!-\!V)_0}$, colour excesses E(U-B) and E(B-V) and light absorption values ${ A_V=R \cdot E(B\!-\!V)}$. The values R (Strajzhis 1977) have been used for sequences I and V separately, but they are very close, R $\approx3.29$. In Table 3 we present the values obtained for both sequences. For each group of stars the top line in the table conforms to the assumption that their luminosity class is I, the bottom one -- class V.

The division of stars into three brightness groups is conventional and needed for a safer determination of their mean characteristics. The apparent difference of the groups in colours justifies the division itself. We have assumed that bright and medium stars should be referred to luminosity class I, faint ones to V. In this case we obtain throughout these three groups very close values of interstellar absorption with the mean value ${ A_V=0\hbox{$.\!\!^{\rm m}$ }97}$. However we find below that the bright and medium groups should be only used for the absorption to be determined most reliably. We conclude then from two brighter groups of stars, that the mean value of total interstellar absorption among early stars with ${\rm H}\alpha$ emission in M33 is A_V = ${\rm0\hbox{$.\!\!^{\rm m}$ }93 \pm 0\hbox{$.\!\!^{\rm m}$ }05}$. This estimate is consistent with results of other authors. Humphreys (1980) gives ${ A_V=0\hbox{$.\!\!^{\rm m}$ }8\pm0\hbox{$.\!\!^{\rm m}$ }07}$ for blue supergiants in M33. From CCD photometry of OB stars Wilson (1990) has found the mean value ${ <E(B\!-\!V)>\,=\,0\hbox{$.\!\!^{\rm m}$ }3}$ ( ${A_V=0\hbox{$.\!\!^{\rm m}$ }9\div1\hbox{$.\!\!^{\rm m}$ }0}$). The interstellar absorption in the Galaxy in this direction is $0{}^{\rm m}\!\!\!.\,2 \div 0{}^{\rm m}\!\!\!.\,3$ (Sharov 1988; van den Bergh 1991).

All the selected stars are star-like sources in ${\rm H}\alpha$. This emission is the most powerful one in HII spectra and it does not contribute to V band. One may suggest that if even HII regions do exist near these stars, it is hardly probable that their emissions contribute essentially to U, B and Vbands. Figure 11 presents the colour-luminosity diagram for three groups of stars from Table 3, where filled circles correspond to the idea that the bright and medium stars are supergiants, while faint group is main sequence stars. The open circles indicate the position of the faint stars as supergiants with ${A_V=0{}^{\rm m}\!\!\!.\,6}$ (Table 3). Bars in the figure correspond both to the colour errors (as in Fig. 4) and the intervals in stellar magnitudes. The curves of different luminosity classes are plotted from compilations of Strajzhis (1977, 1982a). The uppermost curve for hypergiant stars is constructed from data of Lang (1992).

The brightest stars do not conform to any of the classical stellar sequences (Strajzhis 1977, 1982b). The close luminosity class is Ia, in colours they fit on the average to B0Ia-B4Ia. However, the first group stars are essentially brighter ( $\bigtriangleup{ M_V=1{}^{\rm m}\!\!\!.\,1}$) than Ia stars. We conclude that they belong to hypergiants or super-supergiants. LBV stars are found among such stars, in their characteristics they fit well this kind of objects (Humphreys & Davidson 1994). It can be seen in Fig. 11 that they fully satisfy this sequence. Their mean luminosity is ${ M_V=-8{}^{\rm m}\!\!\!.\,3}$. The mass of such stars is over 60  $M_{\hbox{$\odot$ }}$. The intermediate group stars, both in colours and in luminosities, conform on average very well to B1Ia supergiants (Fig. 11). Their luminosity is ${ M_V=-7{}^{\rm m}\!\!\!.\,3}$. Masses of such stars are about 40-45  $M_{\hbox{$\odot$ }}$.

We assumed originally from the equality of absorption values for all three groups that luminosity class of the faint group is V. In that case a great luminosity excess, ${\bigtriangleup M_V=2{}^{\rm m}\!\!\!.\,3}$, appears as compared to stars of this type. If we assume these stars to be of luminosity class III, the excess ( $\bigtriangleup M_V=1{}^{\rm m}\!\!\!.\,4$) remains unexplained all the same. Since a limiting magnitude (IFM) is $V=19{}^{\rm m}\!\!\!.\,5$, it is apparent that at ${ A_V \approx 1{}^{\rm m}\!\!\!.\,0}$ only stars with ${ M_V \le -5{}^{\rm m}\!\!\!.\,8}$will be accessible to us. This practically rules out the appearance of noticeable portion of main sequence stars and giants among the faint group.

Figure 11: Colour-luminosity diagram for the bright, mean and weak groups of stars from Table 3. Filled circles show I luminosity class for stars of bright and medium groups, and V class for the faint. Open circle marks position of the faint stars, if their luminosity class is I

It is seen in Fig. 11 that these stars are located on Ib supergiant branch. In this case their absorption is ${ A_V \approx 0{}^{\rm m}\!\!\!.\,6}$and the mean luminosity ${ M_V=-5{}^{\rm m}\!\!\!.\,9}$ (Table 3). They are markedly fainter than Iab supergiants but they fit very well the Ib sequence. From the luminosity and colour indices their average spectral class satisfies completely B1Ib. Masses of such stars are about 20-25  $M_{\hbox{$\odot$ }}$. Obviously, the group of faint stars is not homogeneous, and we discuss their average properties. Both supergiants and the hottest main sequence stars may enter this group.

The mean interstellar absorption of the bright ${\rm H}\alpha$ supergiants corresponds to an optical depth of 0.86 (Strajzhis 1977). For the faint group stars this value are 1.5 times as small. This may imply the faint stars are not observable throughout the whole disk of M33 but only over the near face part of it. Stars of this group are located, on the average, in the near half accross the disk, which takes up 60$\%$ of the galaxy disk's cross-section on the line of sight. Hence, the incompleteness of our faint star group (Ib supergiants with ${\rm H}\alpha$ emission) is no less than 40$\%$. The incompleteness in general is, naturally, caused by the limiting magnitude $V=19{}^{\rm m}\!\!\!.\,5$ in the sample (IFM).

Sholukhova et al. (1997, 1999) carried out a multiobject follow up spectroscopy of a part of object from the list (Table 1). The spectra were obtained in ${\rm H}\alpha$ line region. A main goal of the observations was to isolated objects with broad ${\rm H}\alpha$, as a broad emission line suggests that it is formed (completely or partially) in stellar atmosphere. Among 170 objects studied in the follow up spectroscopy, 57 show a broad ${\rm H}\alpha$ emission. In this way we may obtain a new list of the most reliable candidates. The authors took spectra of 7 stars (s) from the bright group and found the mean ${\rm H}\alpha$ emission width is FWHM $= 310 \pm 40$ km s^-1, among 7 stars studied of the medium group FWHM $= 165 \pm 90$ km s^-1 and 13 stars of the faint group have FWHM $= 115 \pm 50$ km s^-1. 32 objects of b type and 39 objects of d type have shown correspondingly $110 \pm 30$ km s^-1 and $90 \pm 35$ km s^-1. The high dispersion of the line widths is because the morphological groups are not very homogeneous and both the stellar atmospheres and the HII regions have to contribute to the total ${\rm H}\alpha$ emission. In spite of these we see that the objects, separated only from the morphological criteria, clearly differ in ${\rm H}\alpha$ line width. The stars have broader ${\rm H}\alpha$, this confirms that their ${\rm H}\alpha$ is completely or partially intrinsic. The decrease in the FWHM in the stars as one goes from the bright group to the faint one corroborates the conclusion that the nebulae contribution to the spectrum increases with declining brightness of the star.

Thus, we conclude the blue ${\rm H}\alpha$ emission stars in our list are supergiants of luminosity classes Ia-Ib. We also conclude that the group of 20 brightest stars is well consistent with hypergiants or LBV type stars. The mean interstellar absorption estimated from the bright supergiants which are visible across the entire galaxy disk depth is ${ A_V\approx 0\hbox{$.\!\!^{\rm m}$ }93}$. Light absorption of the faintest stars is ${ A_{ V} \approx 0\hbox{$.\!\!^{\rm m}$ }6}$.