Mathematics
Scientific paper
Nov 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003phdt.........2l&link_type=abstract
PhD Thesis, Combined Faculties for the Natural Sciences and for Mathematics of the Ruperto-Carola University of Heidelberg, Germ
Mathematics
7
Pre-Main Sequence Stars, Angular Momentum, Evolution, Open Clusters
Scientific paper
This thesis presents a detailed investigation of the rotational behaviour of young pre-main sequence (PMS) stars in order to achieve a better understanding of their angular momentum evolution during this evolutionary stage. For that purpose I have carried out an extensive photometric monitoring program which enabled me to identify a total of 405 periodic and 184 irregular variable PMS stars in the young open cluster NGC 2264 (age: 2-4 Myr). Hence, I could increase the number of known PMS stars in NGC 2264 by more than a factor of three and the number of published periods by more than a factor of ten. Apart from the Orion Nebular Cluster (ONC, age: 1 Myr), with about 370 known rotation periods, NGC 2264 is therefore the only cluster for which a statistically significant number of rotation periods of PMS stars is known. For the first time it was possible to compare the rotational behaviour of stars of two young clusters with each other. I have shown that the period distribution of stars in NGC 2264 strongly depends on mass and quantitatively agrees with that of the ONC. However, the stars in NGC 2264 rotate with shorter rotation periods on average. A quantitative analysis which took into account the age ratio and the different stellar radii showed that a large fraction of stars spin up with conserved angular momentum while increasing in age. However, I also found that some stars clearly rotate with longer rotation periods even though they are aging. Apparently, these stars lose angular momentum, which is interpreted as a result of magnetic coupling to their circumstellar disk ("disk-locking"). The resulting locking period is about 8 days. My analysis further showed that "disk-locking" in NGC 2264 or the ONC is important only for higher mass stars (M >~ 0.3 Msolar). There is evidence that also the lower mass stars (M <~ 0.3 Msolar) interact magnetically with their disks, but this interaction is in most cases not strong enough to remove angular momentum with sufficiently high rates to lock the star with a constant rotation period. These findings are closely connected with a surprising result. The typical brightness variations of the stars in NGC 2264 (caused by star spots) differ for the two mass regimes considered here. While the peak-to-peak variation of the higher mass stars is typically up to 0.2 mag, the lower mass stars show brightness modulations only up to 0.06 mag. I argue that this is evidence for a different magnetic field structure, caused by different dynamo processes, and could be the decisive factor for the different rotational behaviour of the lower mass stars.
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