This thesis presents new insights of protoplanetary disk evolution. It focuses on the characterisation of several elements in the earliest phases of planet formation in protoplanetary disks: the... Show moreThis thesis presents new insights of protoplanetary disk evolution. It focuses on the characterisation of several elements in the earliest phases of planet formation in protoplanetary disks: the connection between the SED and disk gaps (Chapters 2, 3 and 4), PAHs in the gas flows in disk gaps (Chapter 5) and dust processing of forsterite in evolving protoplanetary disks (Chapter 6). Show less
Maaskant, K.M.; Min, M.; Waters, L.; Tielens, A.G.G.M. 2014
Context. We have analysed far-infrared spectra of 32 circumstellar disks around Herbig Ae/Be and T Tauri stars obtained within the Herschel key programme Dust, Ice and Gas in Time (DIGIT). The... Show moreContext. We have analysed far-infrared spectra of 32 circumstellar disks around Herbig Ae/Be and T Tauri stars obtained within the Herschel key programme Dust, Ice and Gas in Time (DIGIT). The spectra were taken with the Photodetector Array Camera and Spectrometer (PACS) on board the Herschel Space Observatory. In this paper we focus on the detection and analysis of the 69 {$μ$}m emission band of the crystalline silicate forsterite. Aims: This work aims at providing an overview of the 69 {$μ$}m forsterite bands present in the DIGIT sample. We use characteristics of the emission band (peak position and FWHM) to derive the dust temperature and to constrain the iron content of the crystalline silicates. With this information, constraints can be placed on the spatial distribution of the forsterite in the disk and the formation history of the crystalline grains. Methods: The 69 {$μ$}m forsterite emission feature is analysed in terms of position and shape to derive the temperature and composition of the dust by comparison to laboratory spectra of that band. The PACS spectra are combined with existing Spitzer IRS spectra and we compare the presence and strength of the 69 {$μ$}m band to the forsterite bands at shorter wavelengths. Results: A total of 32 disk sources have been observed. Out of these 32, 8 sources show a 69 {$μ$}m emission feature that can be attributed to forsterite. With the exception of the T Tauri star AS 205, all of the detections are for disks associated with Herbig Ae/Be stars. Most of the forsterite grains that give rise to the 69 {$μ$}m bands are found to be warm (~{}100-200 K) and iron-poor (less than ~{}2% iron). AB Aur is the only source where the emission cannot be fitted with iron-free forsterite requiring approximately 3-4% of iron. Conclusions: Our findings support the hypothesis that the forsterite grains form through an equilibrium condensation process at high temperatures. The large width of the emission band in some sources may indicate the presence of forsterite reservoirs at different temperatures. The connection between the strength of the 69 and 33 {$μ$}m bands shows that at least part of the emission in these two bands originates fom the same dust grains. We further find that any model that can explain the PACS and the Spitzer IRS observations must take the effects of a wavelength dependent optical depth into account. We find weak indications of a correlation of the detection rate of the 69 {$μ$}m band with the spectral type of the host stars in our sample. However, the sample size is too small to obtain a definitive result. Appendix A is available in electronic form at http://www.aanda.org Show less
Context. We have analysed far-infrared spectra of 32 circumstellar disks around Herbig Ae/Be and T Tauri stars obtained within the Herschel key programme Dust, Ice and Gas in Time (DIGIT). The... Show moreContext. We have analysed far-infrared spectra of 32 circumstellar disks around Herbig Ae/Be and T Tauri stars obtained within the Herschel key programme Dust, Ice and Gas in Time (DIGIT). The spectra were taken with the Photodetector Array Camera and Spectrometer (PACS) on board the Herschel Space Observatory. In this paper we focus on the detection and analysis of the 69 {$μ$}m emission band of the crystalline silicate forsterite. Aims: This work aims at providing an overview of the 69 {$μ$}m forsterite bands present in the DIGIT sample. We use characteristics of the emission band (peak position and FWHM) to derive the dust temperature and to constrain the iron content of the crystalline silicates. With this information, constraints can be placed on the spatial distribution of the forsterite in the disk and the formation history of the crystalline grains. Methods: The 69 {$μ$}m forsterite emission feature is analysed in terms of position and shape to derive the temperature and composition of the dust by comparison to laboratory spectra of that band. The PACS spectra are combined with existing Spitzer IRS spectra and we compare the presence and strength of the 69 {$μ$}m band to the forsterite bands at shorter wavelengths. Results: A total of 32 disk sources have been observed. Out of these 32, 8 sources show a 69 {$μ$}m emission feature that can be attributed to forsterite. With the exception of the T Tauri star AS 205, all of the detections are for disks associated with Herbig Ae/Be stars. Most of the forsterite grains that give rise to the 69 {$μ$}m bands are found to be warm (~{}100-200 K) and iron-poor (less than ~{}2% iron). AB Aur is the only source where the emission cannot be fitted with iron-free forsterite requiring approximately 3-4% of iron. Conclusions: Our findings support the hypothesis that the forsterite grains form through an equilibrium condensation process at high temperatures. The large width of the emission band in some sources may indicate the presence of forsterite reservoirs at different temperatures. The connection between the strength of the 69 and 33 {$μ$}m bands shows that at least part of the emission in these two bands originates fom the same dust grains. We further find that any model that can explain the PACS and the Spitzer IRS observations must take the effects of a wavelength dependent optical depth into account. We find weak indications of a correlation of the detection rate of the 69 {$μ$}m band with the spectral type of the host stars in our sample. However, the sample size is too small to obtain a definitive result. Appendix A is available in electronic form at http://www.aanda.orgShow less
CO is an important component of a protoplanetary disc as it is one of the most abundant gas phase species. Furthermore, observations of CO transitions can be used as a diagnostic of the gas,... Show moreCO is an important component of a protoplanetary disc as it is one of the most abundant gas phase species. Furthermore, observations of CO transitions can be used as a diagnostic of the gas, tracing conditions in both the inner and outer disc. We present Herschel/PACS spectroscopy of a sample of 22 Herbig Ae/Be (HAEBEs) and eight T Tauri stars (TTS), covering the pure rotational CO transitions from J = 14 { arr} 13 up to J = 49 { arr} 48. CO is detected in only five HAEBEs, namely AB Aur, HD 36112, HD 97048, HD 100546, and IRS 48, and in four TTS, namely AS 205, S CrA, RU Lup, and DG Tau. The highest transition detected is J = 36 { arr} 35 with E$_{up}$ of 3669 K, seen in HD 100546 and DG Tau. We construct rotational diagrams for the discs with at least three CO detections to derive T$_{rot}$ and find average temperatures of 270 K for the HAEBEs and 485 K for the TTS. The HD 100546 star requires an extra temperature component at T$_{rot}$ ~{} 900-1000 K, suggesting a range of temperatures in its disc atmosphere, which is consistent with thermo-chemical disc models. In HAEBEs, the objects with CO detections all have flared discs in which the gas and dust are thermally decoupled. We use a small model grid to analyse our observations and find that an increased amount of flaring means higher line flux, as it increases the mass in warm gas. CO is not detected in our flat discs as the emission is below the detection limit. We find that HAEBE sources with CO detections have high L$_{UV}$ and strong PAH emission, which is again connected to the heating of the gas. In TTS, the objects with CO detections are all sources with evidence of a disc wind or outflow. For both groups of objects, sources with CO detections generally have high UV luminosity (either stellar in HAEBEs or due to accretion in TTS), but this is not a sufficient condition for the detection of the far-IR CO lines. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices are available in electronic form at http://www.aanda.orgShow less
Maaskant, K.M.; Honda, M.; Waters, L.; Tielens, A.G.G.M.; Dominik, C.; Min, M.; ... ; Ancker, M. van den 2013
Context. The evolution of young massive protoplanetary disks toward planetary systems is expected to correspond to structural changes in observational appearance, which includes the formation of... Show moreContext. The evolution of young massive protoplanetary disks toward planetary systems is expected to correspond to structural changes in observational appearance, which includes the formation of gaps and the depletion of dust and gas. Aims: A special group of disks around Herbig Ae/Be stars do not show prominent silicate emission features, although they still bear signs of flaring disks, the presence of gas, and small grains. We focus our attention on four key Herbig Ae/Be stars to understand the structural properties responsible for the absence of silicate feature emission. Methods: We investigate Q- and N-band images taken with Subaru/COMICS, Gemini South/T-ReCS, and VLT/VISIR. We perform radiative transfer modeling to examine the radial distribution of dust and polycyclic aromatic hydrocarbons (PAHs). Our solutions require a separation of inner- and outer- disks by a large gap. From this, we characterize the radial density structure of dust and PAHs in the disk. Results: The inner edge of the outer disk has a high surface brightness and a typical temperature between ~{}100-150 K and therefore, dominates the emission in the Q-band. All four disks are characterized by large gaps. We derive radii of the inner edge of the outer disk of 34$_{-4}$$^{+4}$, 23$_{-5}$$^{+3}$, 30$_{-3}$$^{+5}$ and 63$_{-4}$$^{+4}$ AU for HD 97048, HD 169142, HD 135344 B, and Oph IRS 48, respectively. For HD 97048 this is the first detection of a disk gap. The large gaps deplete the entire population of silicate particles with temperatures suitable for prominent mid-infrared feature emission, while small carbonaceous grains and PAHs can still show prominent emission at mid-infrared wavelengths. The continuum emission in the N-band is not due to emission in the wings of PAHs. This continuum emission can be due to very small grains or to thermal emission from the inner disk. We find that PAH emission is not always dominated by PAHs on the surface of the outer disk. Conclusions: The absence of silicate emission features is due to the presence of large gaps in the critical temperature regime. Many, if not all Herbig disks with spectral energy distribution classification ''group I'', are disks with large gaps and can be characterized as (pre-) transitional. An evolutionary path from the observed group I to the observed group II sources seems no longer likely. Instead, both might derive from a common ancestor. Tables A.1-A.4 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/555/A64Show less
Ellerbroek, L.; Bik, A.; Kaper, L.; Maaskant, K.M.; Paalvast, M.; Tramper, F.; ... ; Balog, Z. 2013
Context. Massive stars play a dominant role in the process of clustered star formation, with their feedback into the molecular cloud through ionizing radiation, stellar winds, and outflows. The... Show moreContext. Massive stars play a dominant role in the process of clustered star formation, with their feedback into the molecular cloud through ionizing radiation, stellar winds, and outflows. The formation process of massive stars is poorly constrained because of their scarcity, the short formation timescale, and obscuration. By obtaining a census of the newly formed stellar population, the star formation history of the young cluster and the role of the massive stars within it can be unraveled. Aims: We aim to reconstruct the formation history of the young stellar population of the massive star-forming region RCW 36. We study several dozen individual objects, both photometrically and spectroscopically, looking for signs of multiple generations of young stars and investigating the role of the massive stars in this process. Methods: We obtain a census of the physical parameters and evolutionary status of the young stellar population. Using a combination of near-infrared photometry and spectroscopy we estimate the ages and masses of individual objects. We identify the population of embedded young stellar objects (YSOs) by their infrared colors and emission line spectra. Results: RCW 36 harbors a stellar population of massive and intermediate-mass stars located around the center of the cluster. Class 0/I and II sources are found throughout the cluster. The central population has a median age of 1.1 {plusmn} 0.6 Myr. Of the stars that could be classified, the most massive ones are situated in the center of the cluster. The central cluster is surrounded by filamentary cloud structures; within these, some embedded and accreting YSOs are found. Conclusions: Our age determination is consistent with the filamentary structures having been shaped by the ionizing radiation and stellar winds of the central massive stars. The formation of a new generation of stars is ongoing, as demonstrated by the presence of embedded protostellar clumps and two exposed protostellar jets. Based on observations performed with the ESO New Technology Telescope at La Silla Observatory, as part of program 074.C-0728, and with the ESO Very Large Telescope on Cerro Paranal, Chile, as part of programs 078.C-0780, 084.C-0604, and 087.C-0442.Appendices are available in electronic form at http://www.aanda.orgShow less
