Anatoly Miroshnichenko

Hot star candidates with recently formed dust

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, whose sublimation temperature is a little lower (~1500-2000 K), can be formed near the stellar surface where the CS matter density is large.

Dust formation around evolved hot stars requires special conditions and 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.

We have identified a distinct group of hot stars with IRAS fluxes suggestive of a recent CS dust formation process (Miroshnichenko et al. 2002). These objects (22 B[e] stars to date; hot stars with dust or HSWD hereafter) have a wide range of luminosities (log L/Lo=2.5-5). Their existence suggests that new dust can be formed near intermediate- and even low-luminosity stars. However, no systematic study of this process has been carried out yet.  HSWD occupy a well-defined region of the IRAS color-color diagram (Fig. 1); however, cool stars with CS dust are also found in this region. The latter can be identified from other color-color diagrams. We found ~4500 IRAS sources with colors within the HSWD box, and so far have identified 10% of them in the 2MASS near-IR catalog and the astrometric optical USNO-B1.0 catalog (Monet et al. 2003). From these data, we separated ~40 objects whose color-indices imply the presence of a hot star (Fig. 2). They are definitely not young stars, because of the lack of far-IR flux. Near-IR low-resolution spectroscopy of 2 of them (IRAS00470+6429 and IRAS02155+6410) shows that are indeed early-type emission-line stars. The large fraction of HSWD candidates suggests that many more of them may be found among the remaining 90% of the initial sample. Thus the group may become the largest among the family of dust making early-type stars.

Evolved hot stars are significant contributors of gas to the IS medium (comparable to supernovae). Similarly, they could supply a largeamount of dust. However, the scale of this process is not clear, as only a few dozen HSWD are currently known. Our findings of intermediate-luminosity HSWD suggest that existing IR catalogs may contain many more of them. We will continue to search for new HSWD candidates using the results of this project to sharpen the search criteria. Extending the HSWD group may lead to a completely new view of CS dust formation, stimulate studies of mass loss from intermediate-mass and luminosity single stars and various binary systems, and suggest new research directions for the forthcoming space missions (e.g., SIRTF, SIM, Herschel).

References

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
Kraus, M., & Lamers, H.J.G.L.M. 2003, A&A, 405, 165
Miroshnichenko, A.S., Bjorkman, K.S., Chentsov, E.L., & Klochkova, V.G. 2002, ASP Conf. Ser., 279, 303
Monet, D.G., et al. 2003, AJ, 125, 984
Williams, P.M., et al. 2001, MNRAS, 324, 156

Links

List of newly found HSWD candidates
Letters of support.
Poster presentation at the 203 AAS Meeting (Atlanta, January 2004)

Last updated: 2004 August 12