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UvA-DARE (Digital Academic Repository) Circumstellar material in young stellar objects van den Ancker, M.E. Publication date 1999 Document Version Final published version Link to publication Citation for published version (APA): van den Ancker, M. E. (1999). Circumstellar material in young stellar objects. General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl) Download date:05 Oct 2021 Circumstellar Material in Young Stellar Objects Circumstellair materiaal in jonge stellaire objecten (met een samenvatting in het Nederlands) Academisch Proefschrift ter verkrijging van de graad van doctor aan de Universiteit van Amsterdam op gezag van de rector magnificus prof. dr. J.J.M. Franse ten overstaan van een door het college voor promoties in- gestelde comissie in het openbaar te verdedigen in de Aula der Universiteit, op dinsdag 14 september 1999 te 15.00 uur door Mario Enrico van den Ancker geboren te Amsterdam Promotiecomissie: Promotor: prof. dr. A.G.G.M. Tielens Co-promotor: prof. dr. L.B.F.M. Waters Overige leden: prof. dr. E.F. van Dishoeck dr. H.F. Henrichs prof. dr. E.P.J. van den Heuvel prof. dr. P.S. Thé prof. dr. C. Waelkens dr. P.R. Wesselius Faculteit der Wiskunde, Informatica, Natuurkunde en Sterrenkunde Universiteit van Amsterdam ISBN: 9-05776-027-4 Cover illustration: Reproduction of Palomar Observatory Sky Survey print of the BD+40 4124 region (cf. Chapter 8) Contents 1 Star Formation: An Overview 1 1.1 Introduction . 1 1.2 Pre-main sequence stellar evolution . 2 1.3 Molecular clouds . 4 1.4 T Tauri Stars . 5 1.5 Herbig Ae/Be Stars . 7 1.6 Outflows and jets . 13 1.7 Protoplanetary disks . 15 2 Hipparcos Data on Herbig Ae/Be Stars: An Evolutionary Scenario 19 2.1 Introduction . 19 2.2 Data analysis . 21 2.3 Astrophysical parameters . 21 2.4 Discussion and conclusions . 23 3 Hipparcos Photometry of Herbig Ae/Be Stars 27 3.1 Introduction . 27 3.2 Data analysis . 28 3.3 Astrophysical parameters . 35 3.4 Photometric behaviour . 37 3.4.1 Inclination effects . 38 3.4.2 Evolutionary hypothesis . 40 3.4.3 Correlations with infrared excess . 40 3.5 Conclusions . 42 4 The Remarkable Photometric Behaviour of the Herbig Ae Star V351 Ori 47 4.1 Introduction . 48 4.2 Observations . 48 4.2.1 Photometry . 48 4.2.2 Spectroscopy . 51 4.3 Analysis of the photometric data . 54 4.4 The unusual blueing effect of V351 Ori . 55 4.5 The spectral type from photometric data . 56 4.6 The visual spectra . 57 4.7 The ultraviolet spectra . 58 4.8 The spectral energy distribution . 59 4.9 Position in the HR-diagram . 61 iii 4.10 Discussion and conclusions . 62 5 Circumstellar Dust in the Herbig Ae Systems AB Aur and HD 163296 65 5.1 Introduction . 65 5.2 Observations . 67 5.3 Contents of spectra . 68 5.4 Spectral energy distributions . 72 5.5 Discussion and conclusions . 72 6 Submillimeter Mapping of the R CrA Region 77 6.1 Introduction . 77 6.2 Observations . 79 6.3 Discussion and conclusions . 81 7 ISO Spectroscopy of the Young Bipolar Nebulae S106 and Cep A East 85 7.1 Introduction . 85 7.2 Observations . 88 7.3 Solid-state features . 94 7.4 Molecular hydrogen emission . 96 7.5 Carbon-monoxide emission lines . 100 7.6 Fine structure lines . 101 7.7 Discussion and conclusions . 104 8 Star Formation in the BD+40 4124 Group: A View From ISO 107 8.1 Introduction . 107 8.2 Observations . 109 8.3 Spectral energy distributions . 113 8.4 Solid-state features . 119 8.5 Gas-phase molecular absorption lines . 121 8.6 Hydrogen recombination lines . 123 8.7 Molecular hydrogen emission . 125 8.8 Carbon-monoxide emission lines . 129 8.9 Atomic fine structure lines . 131 8.10 Discussion and conclusions . 132 9 ISO Spectroscopy of Shocked Gas in the Vicinity of T Tau 139 9.1 Introduction . 139 9.2 Observations . 144 9.3 Solid-state features . 146 9.4 Gas-phase molecular absorption . 147 9.5 Hydrogen recombination lines . 148 9.6 Molecular hydrogen emission . 149 9.7 Atomic fine structure lines . 153 9.8 Discussion and conclusions . 154 iv 10 ISO Spectroscopy of PDRs and Shocks in Star Forming Regions 159 10.1 Introduction . 159 10.2 Sample selection . 161 10.3 Observations . 163 10.4 Molecular hydrogen emission . 165 10.5 Fine structure lines . 175 10.6 Notes on individual objects . 181 10.7 Discussion and conclusions . 186 11 Conclusions and Outlook 193 11.1 Introduction . 193 11.2 Herbig Ae/Be stars . 193 11.3 Emission line studies of star forming regions . 196 11.4 Concluding Remarks . 198 Nederlandse Samenvatting 199 Dankwoord 203 v vi Chapter 1 Star Formation: An Overview 1.1 Introduction One of the basic tasks of science is to understand how we and the world around us have come into existence. The thesis presented here deals with a small part of that question: how do stars form? In the eighteenth century the French astronomer Laplace asked himself the same question, and came up with the answer we still consider essentially correct today: stars form out of large clouds of gas (and dust) in interstellar space which collapse under the influence of their own gravity. However, after more than two centuries of research many important questions regarding the star formation process, and the fundamentals of planet formation, still remain unanswered. Nonetheless, the rapid progress in our knowledge of star formation in the last few decades, mainly due to the availability of a new generation of astronomical instruments, as well as a number of surprising discoveries, show that we are now at the brink of a fundamentally better understanding of the origins of our own solar system. Therefore the subject of star formation is one of the most exciting fields of astrophysics today. Today, the standard picture of the process of low-mass star formation (Shu et al. 1987) is divided in four distinct phases. First, a number of dense cores develop within a molecular cloud. At some point, the dense cores collapse to form embedded protostars which accumulate material by accretion from the surrounding cloud. As the embedded protostars accrete mass, they also lose mass due to a strong stellar wind, which eventually leads to the dispersion of the reservoir of mass from the protostellar envelope and the end of accretion. The former protostar, now a pre-main sequence star, begins to contract slowly, increasing its central temperature until hydrogen igni- tion takes place and it has become a stable star on the main sequence. Collapse of a cloud naturally leads to disk structures as material with significant angular momentum will be unable to fall directly onto the central star, and will instead accumulate in a rotating disk. The star can only accrete this material if the disk can shed angular momentum; as the mass moves inwards, the angular momentum has to move outwards and be stored in larger rotating bodies like the planets in our own solar system. Although the detailed physics behind this is by no means clear, there is strong observational evidence that in all astronomical objects where accretion disks 1 CHAPTER 1. STAR FORMATION: AN OVERVIEW Fig. 1.1. Schematic picture of the environment of a young stellar object. Infall of matter onto the circumstellar disk, accretion onto the central star and accretion-driven outflow are indicated by arrows [from Wood 1997]. are formed, energetic ejecta of material along the rotation axis of the system are also present. The ejecta can also carry away part of the angular momentum of the system, allowing accretion of mass onto the central object. These jets, commonly seen in young stellar objects (YSOs), may accelerate ambient molecular material and drive a more or less collimated bipolar molecular outflow. A schematic picture of this view of the environment of a YSO is shown in Fig. 1.1. 1.2 Pre-main sequence stellar evolution The first numerical simulations of stellar evolution were done by Henyey et al. (1955). They followed the evolution of pre-main sequence stars to the main sequence, showing that a young star remains nearly constant in brightness with increasing temperature as it contracts towards the main sequence. Later, Hayashi (1961) pointed out that these evolutionary tracks were incomplete since they assumed the star would be fully radiative until the onset of core hydrogen burning, whereas the star would be fully con- vective prior to the radiative phase described by Henyey et al. Subsequent.

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