Untitled

Koter

et al. 2001

A&A

290

349

Abstract. We have analysed the 10 µm spectral region of a sample of Herbig Ae/Be (HAEBE) stars. The spectra are dominated by a broad emission feature caused by warm amorphous silicates, and by polycyclic aromatic hydrocarbons. In HD 163296 we find aliphatic carbonaceous dust, the first detection of this material in a HAEBE star. The silicate band shows a large variation in shape, due to variable contributions of three components: (i) a broad shoulder at 8.6 µm; (ii) a broad maximum at 9.8 µm; and (iii) a narrow feature with a broad underlying continuum at 11.3 µm. From detailed modeling these features can be identified with silica (SiO2), sub-micrometer sized amorphous olivine grains and micrometer sized amorphous olivine grains in combination with forsterite (Mg2SiO4), respectively. Typical mass fractions are 5 to 10 per cent of crystalline over amorphous olivine, and a few per cent of silica compared to the olivines. The detection of silica in emission implies that this material is heated by thermal contact with other solids that have a high absorptivity at optical to near-IR wavelengths. The observed change in peak position of the silicate band in HAEBE stars from 9.7 µm to 11.3 µm is dominated by an increase in average grain size, while changes in composition play only a minor rôle. The HAEBE stars, β Pic and the solar system comet Halley form a sequence of increasing crystallinity. We find that the abundance of SiO2 tends to increase with increasing crystallinity. This is consistent with the compositional changes expected from thermal annealing of amorphous grains in the inner regions of the disk. We confirm earlier studies that the timescale for crystallisation of silicates in disks is longer than that of coagulation. Our results indicate that the processes that governed grain processing in the proto-solar nebula, are also at work in HAEBE stars.

Section: The Chemical Composition Of the Silicate Dustmentioning

confidence: 99%

Section: Adopted Dust Components and Grain Shapesmentioning

confidence: 99%

Processing of silicate dust grains in Herbig Ae/Be systems

Bouwman

Koter

et al. 2001

A&A

290

349

“…Changes in the composition, structure, and size are expected to occur during the subsequent evolution of the star+disk system, possibly leading to a planetary system. Laboratory experiments have suggested that, because of thermal annealing, amorphous silicates gradually turn into crystalline forsterite and silica (e.g., Rietmeijer 1989;Hallenbeck & Nuth 1997). Bouwman et al (2001) found a correlation between the amount of forsterite and silica in the disks of Herbig Ae/Be (HAeBe) stars, indicating that thermal annealing indeed takes place in these objects.…”

Section: Silicatesmentioning

confidence: 99%

Mid-IR observations of circumstellar disks

Schuetz¹,

Sterzik³

et al. 2009

A&A

We present new mid-infrared spectra of 15 targets (1 FU Orionis object, 4 Herbig Ae stars, 5 T Tauri stars, and 5 Vega-type stars), obtained with the TIMMI2 camera at La Silla Observatory (ESO). Three targets are members of the β Pic moving group (HD 155 555, HD 181 296, and HD 319 139). PAH bands are observed towards the T Tauri star HD 34 700 and the Herbig Ae star PDS 144 N. For HD 34 700, the band profiles indicate processed PAHs. The spectrum of the Vega-type object η Corvi (HD 109 085), for which a resolved disk at sub-mm wavelengths is known, appears stellar between 8-13 μm, but a small excess emission was reported by Spitzer observations. Similarly, no indication of circumstellar matter at mid-infrared wavelengths is found towards the Vega-like stars HD 3003, HD 80 951, HD 181 296, and, surprisingly, the T Tauri system HD 155 555. The silicate emission features of the remaining eight sources are modelled with a mixture of silicates of different grain sizes and composition. Unprocessed dust dominates FU Ori, HD 143 006, and CD-43 344. Large amorphous grains are the main dust component around HD 190 073, HD 319 139, KK Oph, and PDS 144 S. Both small grains and crystalline dust is found for the Vega-type HD 123 356, with a dominance of small amorphous grains. We show that the infrared emission of the binary HD 123 356 is dominated by its late-type secondary, but optical spectroscopy is still required to confirm the age of the system and the spectral class of the companion. For most targets, this is their first mid-infrared spectroscopic observation. We investigate trends between stellar, disk, and silicate properties and confirm correlations identified in previous studies. Several objects present an exciting potential for follow-up high-resolution disk studies.

“…Laboratory experiments have shown that, due to thermal annealing, amorphous silicates gradually turn into crystalline forsterite and silica (e.g. Rietmeijer 1989;Hallenbeck & Nuth 1997). Bouwman et al (2001) found a correlation between the amount of forsterite and silica in the disks of Herbig Ae/Be (HAeBe) stars, showing that thermal annealing indeed takes place in these objects.…”

Section: Mid-ir Featuresmentioning

confidence: 99%

Mid-IR observations of circumstellar disks

Schütz¹,

Sterzik

2005

A&A

Abstract. We present spectral energy distributions and new N-band photometry and spectroscopy for a sample of six main sequence stars and one post-MS object using the ESO TIMMI2 camera at La Silla observatory (Chile). All objects are thought to possess circumstellar material and for the majority of the targets this is their first N-band spectroscopic observation. The emission spectra (observed in three cases), modelled with a mixture of silicates consisting of different grain sizes and composition, confirm the suspected presence of disks around these targets. The most important discovery is that HD 113766, a young Vega-type star, is host to highly processed dust which is probably second generation. It is the first time a Vega-type star with such highly evolved dust has been observed. Silicate emission of basically unevolved dust is seen in case of the post-MS object HD 41511 and the Vega-type star HD 172555. In addition, to study the cold dust, we observed a subsample at 1200 µm with the bolometer array SIMBA at the SEST in La Silla but we only got upper limits for those five objects. This shows that these Vega-type stars have a smaller amount of dust than their precursors, the T Tauri and Herbig Ae/Be stars.