Hot Stars Forming Dust

Circumstellar (CS) dust is found near stars of all spectral and luminosity types. It has a protostellar origin in young objects and can be formed only around evolved stars. Dust formation is only clearly understood around cool stars (e.g., red supergiants and carbon stars, Gail & Seldmayr 1986). They have low surface temperatures (Teff  < 3000 K), and the dust (sublimation temperature ~1500-2000 K), can be formed near the stellar surface where the CS matter density is large.

In contrast, hot stars have much higher Teff, and dust formation can only occur far away from the star (~100 R_* for Teff ~ 10000 K). However, the CS matter is much less dense there, and so the molecule and dust formation process is ineffective. Also, an enhanced abundance of heavy elements, necessary for dust formation, is observed in the atmospheres and CS environments of many post-main-sequence massive stars, but only a small fraction of them show evidence for the presence of CS dust. This is the primary factor hampering studies of CS dust formation near hot stars.

At present, CS dust is known to form near only three types of hot stars: Wolf-Rayet (WR) stars, Luminous Blue Variables (LBV), and B[e] stars. The first two types contain very luminous objects with extremely dense radiatively-driven winds, and dust could form due to the presence of heavy elements, produced in their interiors, and the self-shielding of parts of their winds from the UV radiation which otherwise would destroy newly formed dust.
 

Dust formation around evolved hot stars has been extensively studied only in the case of Wolf-Rayet (WR) stars. In single carbon-rich WR stars, the dust can be formed due to high mass loss rates (> 10^-5 Mo yr^-1), hydrogen deficit, and a high carbon abundance (Cherchneff et al. 2000). In WR binaries, the large matter densities are due to colliding winds of the companions (Williams et al. 2001). Only first attempts have been made to explain dust formation around B[e] supergiants (Bjorkman 1998, Kraus & Lamers 2003). Both WR and B[e] supergiants are very luminous objects with extremely dense radiatively-driven winds, and dust could form in their environments due to the presence of heavy elements, produced in their interiors, and the shielding of parts of their winds from the UV radiation which destroys newly formed dust.

B[e] stars are a heterogeneous group including objects of mostly B spectral type (10000 K < Teff <30000 K) that show forbidden emission lines in their optical spectra and large IR excesses due to hot CS dust (Allen & Swings 1976). Although a number of B[e] stars have been identified as members of other known stellar groups (e.g., Herbig Ae/Be stars and Proto-Planetary Nebulae) or suggested to have high luminosities (B[e] supergiants, see Lamers et al. 1998), nearly half of the originally selected 65 galactic objects remained unclassified until recently.
 

The major difference of B[e]WD from other B[e] stars is their infrared (IR) SED. Analyzing the IRAS data for the originally selected B[e] stars, we noticed that 10 objects had specific colors (-0.5 < log (F₂₅/F₁₂) < 0.1, -1.1 < log (F₆₀/F₂₅) < -0.3, where F₁₂, F₂₅, and F₆₀ are the fluxes in the IRAS photometric bands centered at 12, 25, and 60 microns, respectively, see Fig. 1) which correspond to dust temperatures of  >150-200 K. Such colors are characteristic of late-type stars with CS dust (symbiotic and VV Cep binaries, and Mira stars) and may indicate either the presence of a cool companion or a compact dusty envelope (a lack of cold dust). The latter could be due to a recent dust formation.

Another characteristic of B[e]WD is their strong emission-line optical spectra. In particular, the Balmer line strength is about an order of magnitude stronger than in classical Be stars and even hot super- and hypergiants. Emission line profiles of most B[e]WD are double-peaked, suggesting that the CS gas distribution is non-spherical. They also display numerous Fe II emission lines which might result from an excessive iron abundance in the CS matter, probably due to rotationally induced mixing of the nuclear products forming in the stellar core. However, available information about the rotational velocities show that even B[e]WD, which are seen almost edge-on, are generally slow rotators with projectional rotational velocities v sin i < 100 km s^-1 (e.g., Israelian et al. 1996).
 

Properties of the main group B[e]WD

References

Allen, D.A., & Swings, J.-P. 1976, AA, 47, 29

Bjorkman, J.E. 1998, in B[e] stars, (eds.) C. Jaschek & A.-M. Hubert, Ap&SS Library, v. 233, p. 189
Cherchneff, I., Le Teuff, Y.H., Williams, P.M., & Tielens, A.G.G.M. 2000, A&A, 357, 572
Gail, H.-P., & Seldmayr, E. 1986, A&A, 166, 225

Israelian, G., et al. 1996, AA, 311, 643
Kraus, M., & Lamers, H.J.G.L.M. 2003, A&A, 405, 165

Lamers, H.J.G.L.M. et al. 1998, AA, 340, 117
Williams, P.M., et al. 2001, MNRAS, 324, 156

Last updated: 2004 October 28