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62 YSOs in the Lupus Molecular Clouds where D is the distance to the source and F λ is the stellar flux – an integration of the NextGen model atmosphere scaled and normalized to the dereddened photometric data. Errors are derived from the uncertainty in the distance and flux, which includes an error in A V of ±0.5 mag. The disk luminosity is the integrated excess emission above the stellar photosphere. Errors are derived from the stellar luminosity errors. Those two values are shown in Table 3.3. 3.6 H-R Diagram The age and mass of a star can be derived from theoretical tracks, overlaid on physical H-R diagrams. The position of an object in the H-R diagram is determined by its temperature (derived in Section 3.4.4) and luminosity (derived in Section 3.5.1). 3.6.1 Results The pre-main sequence evolutionary tracks of Baraffe et al. (1998, 2001) and Siess et al. (2000) are overlaid in the H-R diagrams where the objects in this sample are placed (Figure 3.8). Thirteen objects are outside the range of the tracks (red and yellow circles in Figure 3.8). Three objects (marked in red) are below the tracks: ID # 65, 71 and 78. By looking at their SEDs in Figure 3.7, it can be seen that objects 65 and 78 have substantial IR excess. The stellar fluxes are also not that high. These may be objects still surrounded by envelopes, which could be responsible for their low luminosity. Another explanation is that the objects are seen edge-on, such that the dust in the disk blocks some of the light of the star. In both cases, the calculated luminosity will be too low, resulting in an incorrect placement in the HR diagram. Object 71 does not to have a lot of IR excess. The reason of its misplacement in the HR diagram could be that it is a background source and is in fact at a larger distance, which will increase the calculated luminosity. Ten objects (marked in yellow) are above the tracks: ID 3, 13, 55, 60, 61, 63, 64, 68, 74 and 75. An easy explanation would be that the assumed distance is wrong and that the objects are actually closer to us. However, even with very small distances (e.g. 60 pc), the objects are still above the tracks. Other effects have to play a role. One option is that they are part of a binary system that is unresolved, resulting in a stellar luminosity that is too high. Another option is that they are evolved AGB-stars. The working assumption is that these objects, however, do not belong to the Lupus Clouds. For those 13 objects, no ages and masses were derived. For the other objects, the effective temperature, stellar and disk luminosity, age and mass can be found Table 3.3. As can be seen from Figure 3.9, the two models agree on the range of ages and masses, albeit with a different distribution. The mean age is 3.6 Myr for the Baraffe models and 4.4 Myr for the Siess models. For the masses, both models determine a mean stellar mass of only 0.3 M ⊙ . This result is in agreement with that derived
Evolution of Dust in Protoplanetary Disks 63 Figure 3.8 – HR diagram of the objects. Overlaid are the isochrones (black) and mass tracks (gray). The top panel shows the tracks from Baraffe et al. (1998, 2001) and the bottom panel the tracks from Siess et al. (2000). The corresponding ages (in Myr) and masses (in M ⊙ ) are marked. The red and yellow circles are objects that do not follow the tracks.
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Observational <str
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Promotiecommissie Promotor: Co-Prom
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Cover Artwork: Roderik Overzier
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viii Chapter 3. YSOs in the Lupus M
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x Publications 217 Acknowledgments
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2 Introduction planets may eventual
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4 Introduction Figure 1.1 - Illustr
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6 Introduction 1.3.1.1 Disk Observa
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8 Introduction 1.3.1.3 Processes Af
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10 Introduction of them (e.g., β P
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A. Background Sources 113 Figure A.
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118 Discovery of a Dusty Planetary
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128 From Protoplanetary Disks to Pl
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199 parameter is. Na 6 tot 8 miljoe
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218 Publications 8. c2d Spitzer-IRS
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220 Thesis Acknowledgments Liesbeth
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