Transitional YSOs: Candidates from Flat-Spectrum IRAS Sources

We are searching for Young Stellar Objects (YSOs) near the boundary between protostars and pre-main sequence objects, what we have termed transitional YSOs. We have identified a sample of 125 objects as candidate transitional YSOs on the basis of IRAS colors and optical appearance on DSS images. We find that the majority of our objects are associated with star-forming regions, confirming our expectation that the bulk of these are YSOs. We present optical, near-IR and high-resolution IRAS images of 92 objects accessible from the northern and 62 from the southern hemisphere. The objects have been classified on the basis of their morphology and spectral index. Of the 125 objects, 28 have a variety of characteristics very similar to other transitional YSOs, while another 22 show some of these characteristics, suggesting that these transitional YSOs are not as rare as predicted by theory.Comment: 4 pages, 3 figures, to appear in proc. 33rd ESLAB Symposium ``Star Formation from the Small to the Large Scale'', eds. F. Favata et al., ESA SP-44

The composition of the circumstellar dust around the Herbig Ae stars AB Aur and HD 163296

Abstract. We have analysed the 2–200 µm ISO spectra of two bright Herbig Ae stars, AB Aur and HD 163296, in order to derive the composition and mass over temperature distribution of the circumstellar dust. We find that the two stars have a similar composition of the dust, however also with some important differences. We address the onset of the strong near-infrared emission between 1–4 µm. We show that this emission can be explained with thermal emission from metallic iron or dust species containing iron. The circumstellar dust surrounding both stars shows two distinct regimes in the mass over temperature distributions with temperatures of ∼ 10 3 and ∼ 10 2 K. Modelling of the ISO spectra further reveal a significant deviation of the derived grain sizes compared to the interstellar grain size distribution. A population of large ( ∼ 1 mm) grains is required to explain the observed (sub-) millimetre fluxes of HD 163296. The presence of crystalline silicates and the larger grain sizes in HD 163296 compared to AB Aur suggests that the former star is a more evolved system. Amorphous silicates make out the bulk of the dust mass in both stars. It is likely that a significant fraction of the dust is carbon-rich. We find strong evidence for a 20–30 µm band which we tentatively attribute to FeO