Luminous blue variable

CNO equilibrium it can be reached by massive stars in the Luminous Blue Variable (LBV) stage and by WN stars. The changes in H and He contents are rather smooth and cases of CNO equilibrium with H (LBV and WNL stars) and without H (WNE stars) may be distinguished (see also 6). 

Stars in the location of LBV (luminous blue variable), i.e. blue supergiants with M q -IO, are predicted to exhibit CNO equilibrium abundances (cf. Fig. 1 and 2), whether or not overshooting is present. The observations (22) for n Carinae and the models agree, which confirms the evolutionary status of this intriguing object as a post-MS supergiant. 

A reduced width of the main sequence band and the avoidance of the domain of the Luminous Blue Variables (LBVs) in the HR diagram. 

The uppermost part of the Hertzsprung-Russell diagram is of particular interest since the stars in that area are apparently close to their limit of existence, which is shown by their stochastic variability, pulsations, large rate of mass loss and occasional episodic mass loss. The curve above which no stars appear to exist is called the Humphreys-Davidson limit (Humphreys and Davidson 1979 De Jager, 1980) cf. Figure 2. Stars close to that limit exhibit many of the properties listed above. In that area one also finds the Luminous Blue Variables, which are stars that erratically expell a large amount of mass. At some distance from the star the gas condenses into dust particles and thus the star becomes reddened. Sometimes the expelled gas is optically... 

But such behaviour is not really restricted to the Luminous Blue Variables. Humphreys (1987) described a cool star ("variable A") that shows the same behaviour, and so does the cool hypergiant HR 8752 (Piters et al., 1987) here an episodical mass ejection started around 1968 the star obtained a later spectral type the expelled gas remained detectable till 1980-1982. It would make sense to include such variables in the sample and to speak just of Very Luminous Variables, hence adding the word "Very" and deleting "Bright". 

Figure 2.3 Dust production and gas mass return rate by different stellar types in solar masses per year and kpc-2 in the galaxy at the solar cycle. Stars produce mainly silicate or carbon dust only in some cases is a different kind of dust material formed, probably iron or some iron alloy (peculiar dust). Many additional dust components with much smaller abundance are formed in most cases (Data from Tielens 1999 Zhukovska el al. 2008). Abbreviations of stellar types AGB = asymptotic giant branch stars of spectral types M, S, or C OB = massive stars of spectral types O and B on or close to the main sequence RGB = massive stars on the red giant branch LBV = luminous blue variables WCL = Wolf-Rayet stars from the lower temperature range Novae = mass ejecta from novae SN = mass ejecta from supemovae.

Laboratory studies of presolar dust grains also show that dust is formed in the mass ejected after an SN explosion, as will also be discussed in Section 2.2. Observations show that the ejected mass shells occasionally do form dust some time after the SN explosions (e.g. Bianchi Schneider 2007), but generally the efficiency of dust production seems to be rather low (Bianchi Schneider 2007 Zhukovska et al. 2008). Other important sources of stardust are red supergiants (mostly silicate dust). Most of the dust from red supergiants, however, is not expected to survive the shock wave from the subsequent SN explosion of the star (Zhukovska et al. 2008). Some dust is also formed by novae (Amari et al. 2001b), Wolf-Rayet stars (WRs, Crowther 2007), and luminous blue variables (LBVs, Voors et al. 2000), but the dust quantities formed by these are very small. Stardust - i.e. dust that is formed in stellar outflows or ejecta - in the interstellar medium is dominated by dust from AGB stars. 

Luminous Blue Variable stars are regarded as precursors of WR stars with the most massive progenitors. They are usually found to be associated with small ejecta type nebulae like r] Car, AG Car (Nota et al. 1995). 

After the imtial discovery of about 11 Hel emission line stars (Krabbe et al. 91) a new data-cube with mudi improved S/N and about twice the spatial resolution ( 1" FWHM at 0.5"/pixel) has been obt2uned at the WHT, La Palma, revealing now 15 to 18 Hel emitting stars (Fig. 2). These stars seem to be related to the class of Luminous Blue Variables (LBV) indicated by their brightness, strong, broad Hel and H emission lines and the absence of HeH, C, or N NIR emission lines which would be indicators of WN stars. Despite their common nature, the members show differences in Hel/Bry, Hel (2.06/ian)/HeI(2.11 ), Line/continuum, brightness, and the depth of the P-Cygni profile. 

The spectrum of the luminous blue variable P Cygni is dominated by lines of HI and Hel with weak lines of Mgll and [FeH]. The Hel to Bry line ratios indicate a probable overabundance of Helium. Line widths imply stellar winds no greater than about 200 km/s. 

The K band of IRS 13 bears a striking resemblance to P Cygni and the AF source. The linewidths are, at best, only marginaUy resolved (FWHM 390 km/s). We conclude that a luminous blue variable may be one of the exciting sources for IRS 13. 

The existence of one WN9/0 e star (the AF source), and possibly a luminous blue variable (IRS 13) at the Galactic center makes more plausible the existence of a duster of early-type, mass-loss stars there. A duster of 10-100 such stars in the central parsec would rule out the need for a black hole to contribute significantly to the luminosity of the Galactic center. 

Flame radiation is a function of many variables C/H ratio of the fuel, air/fuel ratio, air and fuel temperatures, mixing and atomization of the fuel, and thickness of the flame—some of which may change with distance from the burner. Fuels with higher C/H ratio, such as oils, tend to make more soot, so they usually create luminous flames, although blue flames are possible with light oils. Many gases have a low C/H ratio, and tend to burn clear or blue. It is difficult to burn tar without luminosity. It is equally difficult to produce a visible flame with blast furnace gas or with hydrogen. 

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