Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004phdt.........4s&link_type=abstract
PhD Thesis, Faculty for Physics and Astronomy of the Friedrich-Schiller-University, Jena/Germany
Astronomy and Astrophysics
Astronomy
4
Rotation, Low-Mass Stars, Brown Dwarfs, Photometry
Scientific paper
This dissertation contains an investigation of the rotation of very low mass objects, i.e. Brown Dwarfs and stars with masses <0.4 MS. Today, it is well-established that there are large populations of such VLM objects in open clusters and in the field, but our knowledge about their physical properties and evolution is still very limited. Contrary to their solar-mass siblings, VLM objects are fully convective throughout their evolution. Thus, they are not able to form a large-scale magnetic field like for example the sun. The magnetic field, in turn, is crucial for the regulation of rotation: Magnetic interaction between star and circumstellar disk ("disk-locking") and angular momentum losses through stellar winds have dominant influence on the rotational evolution. Thus, we can expect major differences in the rotational behaviour of VLM objects and solar-mass stars.
The best method to investigate stellar rotation is to measure rotation periods. If a star exhibits surface features which are asymmetrically distributed, its brightness may be modulated with the rotation period. Thus, this dissertation is based on the analysis of photometric time series. Open clusters are an ideal environment for such a project, since they enable one to follow many objects at the same time. Additionally, they allow one to investigate the age and mass dependence of rotation, because distance and age of the clusters are known in good approximation. For this thesis, five open clusters were observed, which span an age range from 3 to 750 Myr. In three of them (SigmaOri, EpsilonOri, IC4665), VLM objects were identified by means of colour magnitude diagrams. The candidate lists for these three regions comprise at least 100 objects, for which photometry in at least three wavelength bands is available. About a fifth to a third of these candidates could be contaminating field stars in the fore- or background of the clusters. For the remaining two clusters (Pleiades and Praesepe), objects from the literature were selected as targets for the variability study. Masses for all these candidates were estimated by comparing the photometry with stellar evolutionary tracks.
For each of the clusters, at least one photometric monitoring campaign was carried out; three of them were observed twice. Subsequently, the magnitudes of the VLM objects were measured relative to non-variable stars in the same fields. The difference image analysis procedure was used to improve the precision for two time series. That way, a photometric precision between 5 and 20 mmag was reached for the brightest stars. A comparison of several period search techniques showed that periodogram analysis delivers by far the best results for the available time series data. Beside the Scargle and CLEAN periodogram, the period search includes several independent and robust control procedures, to assure the reliability of the results. Additionally, a test to identify even non-periodic variability was implemented.
For 87 candidates, a photometric rotation period was determined, 80 of these objects have masses <0.4 MS. Thus, this work increases the number of known VLM rotation periods in the age range between 3 and 750 Myr by a factor of 14. Altogether, about 30-50% of the candidates are variable. In the two youngest clusters, several objects show variability with very high amplitudes between 0.2 and 1.1 mag. Their lightcurves contain in the most cases a periodic component, but additionally irregular brightness variations. For two VLM stars, a flare event was detected.
The origin of the periodic variability is surface features co-rotating with the objects. In most cases, these surface features are cool magnetically induced spots. From the lightcurves, it can be concluded that the spot properties change on timescales of at most two or three weeks. The amplitudes of the lightcurves are in the VLM regime by a factor of 2.4 smaller than for solar-mass stars, indicating a change of the spot properties with mass. The best explanation for this phenomenon is a more symmetric spot distribution on VLM objects. Additionally, it is probable that the contrast between spots and photospheric environment is smaller than for more massive stars.
The lightcurves of the highly variable objects in the youngest clusters cannot be understood only with cool spots. This kind of variability resembles very much the photometric behaviour of classical T Tauri stars, i.e. stars which accrete matter from a circumstellar disk. Thus, it is likely that the highly variable VLM objects possess accretion disks as well. This interpretation is confirmed by near-infrared photometry and optical spectroscopy. For VLM objects in the SigmaOri cluster, a disk frequency of 6-14% was estimated. From this value and the age of SigmaOri it follows that VLM objects loose their disk on shorter timescales than solar-mass stars, which could be an indication for a formation through ejection from a multiple system. This result, however, needs confirmation, since the derived disk frequency should only be considered as a lower limit.
The majority of the periodic variable objects rotate with periods <2 d. Slow rotators, with periods longer than 2d, are rare, in contrast to solar-mass stars. For M<0.3 MS, a tendency of faster rotation with decreasing object mass is observed. The origin of this tendency lies very probably in the earliest phases of the rotational evolution. The lower limit of the periods is, within the statistical uncertainties, nearly independent of age and ranges from three to six hours. On the other hand, the upper period limit clearly evolves with time. Between ages of 3 and 100 Myr, it declines from at least ten days to about two days. Afterwards, it increases again up to at least four days. To investigate this behaviour in more detail, simple models were constructed which simulate the basic mechanisms of angular momentum regulation. It turns out that the basic aspects of the rotational evolution can be understood if one takes into account the contraction of the objects and exponential rotational braking through stellar winds. On the contrary, for solar-mass stars the angular momentum losses through stellar winds can be described with the Skumanich law, which predicts a period increase proportional to the squareroot of time. This Skumanich law is not applicable in the VLM regime. Moreover, in the considered age range, the influence of "disk-locking" is negligible.
Many of these results can be understood by taking into account the fact that VLM objects are fully convective and cannot possess a large-scale magnetic field. This basic physical property could be responsible for the fast rotation, the breakdown of the Skumanich law, the exponential braking of the rotation, and a more symmetric spot distribution. Thus, main results of this thesis can be ascribed to the internal structure of VLM objects.
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