Star Formation

Although fundamental for astrophysics, the processes which produce massive stars are not well understood. Large distances, high extinction, and short timescales of critical evolutionary phases make observations of these processes challenging. Lacking good observational guidance, theoretical models have remained controversial. In an Annual Review article of Astronomy & Astrophysics by H. Zinnecker and H. W. Yorke (2007), a basic description of the collapse of a massive molecular core is offered, together with a critical discussion of the three competing concepts of massive star formation:

• monolithic collapse in isolated cores
• competitive accretion in a proto-cluster environment
• stellar collisions and mergers in very dense systems

The review also covers the observed outflows, multiplicity and clustering properties of massive stars, as well as a discussion of the upper initial mass function and the upper mass limit. It is concluded that high-mass star formation is not merely a scaled-up version of low-mass star formation with higher accretion rates, primarily due to the role of the stellar mass and radiation for the dynamics.

Galactic open clusters and associations. This is an ongoing German-Russian collaboration project supported by the DFG grant 436 RUS 113/757/0-1 and the Russian RFBR grant 03-02-04028. Based on data from the All-Sky Compiled Catalogue of 2.5 Million Stars we have identified 513 known open clusters and 7 known compact stellar associations and detected 130 new open clusters. A uniform combined spatial-kinematical-photometric cluster membership analysis was implemented for all 650 objects and new uniform measures of cluster properties (structure, photometry, evolution, and kinematics) were established. Radial velocity measurements of open cluster stars are currently under way within the RAVE survey. For more details on the open clusters project see the project homepage at the ARI/ZAH Heidelberg.

In a similar fashion to the corresponding study for low-mass stars, we derive the high-mass initial mass function in massive star-forming regions, such as 30 Doradus in the Large Magellanic Cloud (LMC). Again, our methods include optical and near-IR imaging of the related fields from which photometrical data for each source can be derived. These data are subsequently compared with theoretical isochrones to obtain the initial mass function (IMF). For a review on the theory of the IMF, see Bonnell et al. (2007).

We calibrate the extra-galactic distance scale and constrain the effect of metallicity on the Period-Luminosity relations for Cepheids and RR Lyrae stars. Cepheids are fairly young and high mass stars which can be observed to large (extra-galactic) distances and we investigate their near-IR K-band luminosity as a function of both metallicity and period using samples of Cepheids in the Milky Way as well as in the Small (SMC) and Large Magellanic Clouds (LMC). This is important because Cepheids from the metal-poor LMC are traditionally used as the reference for distance determination to much more distant and metal-rich Cepheids (Storm et al., 2004). We also tie the Baade-Wesselink type method which we employ to very recent direct parallax measurements from the Hubble Space Telescope, as well as to interferometric measurements published in recent years. Similarly, we calibrate the K-band Period-Luminosity relation for the much older RR Lyrae stars both in the Milky Way and in the LMC. In this way we attempt to bring these fundamental calibrators into agreement with each other, as well as with the distance to the LMC found e.g. from eclipsing binary stars.