Anja C. Andersen's research

I hold a permanent position at the Niels Bohr Institute which is part of the University of Copenhagen. Since 2005 I have been part of the Dark Cosmology Centre (PI Jens Hjorth), where I am part of the management team, whom are in charge of 7 postdocs and 10 PhD students. I am currently the main supervisor of two of the PhD-students and co-supervisor for another two.

During the first years of my research career I performed laboratory measurements on presolar grains from meteorites - direct specimens of surviving physical material formed in past stellar environments that can be quantitatively analysed in the laboratory. These micro-analytical studies provided important clues regarding the nature of interstellar dust (Andersen et al. 1998, A&A 330, 1080; 1999, A&A 343, 933). The first paper was described in News and Views, Nature 392, page 133; Frankfurter Allgemeine Zeitung 4/2-1998, page 4 and New Scientist 10/1-1998, page 23, as well as in several Danish media. Despite the success of these laboratory measurements, it became clear that the grains found in the interstellar medium and - even more so - within the Solar System have undergone some degree of processing which may have modified their original nature. Thus, the cosmic dust connection was unclear and ab initio theory of dust formation had also to be considered. I therefore added theoretical modeling of dust nucleation and grain growth in cool evolved (AGB) stars to my research area in order to properly disentangle the laboratory measurements (Andersen et al. 1999, A&A 349, 243; Andersen et al. 2003, A&A 400, 981).

The figure shows a piece of the Allende meteorite from which meteoritic nano-diamonds have been extracted by boiling the meteorite in acid solutions. The individual nano-diamond are hardly visible in the Transmission Electron Microscope picture, where each cluster consists of about 1000 diamond grains. The sample is mounted in the spectrometer to obtain a near-IR spectrum. The spectrum on the right shows the extinction features of the meteoritic nano-diamonds from the UV to the far-IR. Such a measurement needs 4 different instruments, as no single spectrometer can measure the full wavelength band. The extraction procedure was mainly done at the Max-Planck-Institute for Cosmochemie in Mainz in collaboration with Dr. Uli Ott, while most of the spectroscopy was done at the Astrophysics Laboratory in Jena in collaboration with Prof. Thomas Henning’s group, while the numerical modeling was done at Vienna Observatory with Docent Susanne Höfner.

From the theoretical investigations I performed with Docent Susanne Höfner it became clear that a correct micro-physical description of the dust is crucial for predicting the mass loss rates of AGB stars (Andersen et al. 2003, A&A 400, 981). To obtain a better understanding of the interplay between the micro-physical structure of the dust grains and the predicted dust driven mass loss I divided my time between laboratory work and numerical modeling. In the laboratory we focused on cosmic dust analogues, where the dust grains were produced in the laboratory in such a way that we could control their size, morphology, mineralogy, crystallinity and purity and thereby disentangle the different effects (Mutschke, Andersen et al. 1999, A&A 345, 187). With these measurements we showed that the spectral variation among crystal types of i.e. SiC grains are smaller than the variations due to different grain size, shape and impurities. It is therefore not possible to distinguish, by IR spectroscopy, between crystal types of SiC dust in carbon stars as inferred by many researchers at that time (e.g. Groenewegen 1995, A&A 293, 463; Speck et al. 1997, MNRAS 288, 431; Blanco et al. 1998, A&A 330, 505).

The collaboration with Docent Susanne Höfner have continued over the years and we showed last year that although current knowledge suggests that the dust-driven wind scenario provides a realistic framework for understanding mass loss from carbon-rich AGB stars (C-type), then for oxygen-rich AGB stars (M-type) radiation pressure on silicate grains is not sufficient to drive the observed winds, contrary to previous expectations (Höfner & Andersen, 2007, A&A 465, L39). This paper was highlighted by A&A as “paper of the week” and it was also described on, le Scienze - edizione italiana di Scientific American as well as in the Russian, Swedish and Danish news. The novelty of the paper is that it provide a natural explanation for the observed similarities in wind properties of M-type and C-type AGB stars and implies a smooth transition for stars with increasing carbon abundance, from solar- composition to C-rich AGB stars, possibly solving the longstanding problem of the driving mechanism for stars with a C/O close to one. Several observational attempts are currently underway to prove or disprove this prediction of ours (Sofia Ramstad, Hans Olofsson and Iain McDonald, Jacco van Loon).

Along side with the numerical dust driven mass loss models we did, I also took up studies of how the optical properties of dust grains change once they leave the circumstellar envelope of the star and become part of the interstellar medium. The changes are mainly due to the formation of ice mantles, sputtering, growth and clustering. For this project I collaborated with the solid state physics group at Uppsala University, where I performed numerical modeling of how the extinction properties of dust grains will change as a result of clustering (Andersen et al., 2002, A&A 386, 296). We found that it unfortunately will not be possible to distinguish observationally between small fractal or small compact clusters as previously assumed.

My research has been more and more directed towards collaborating with observers on comparing the obtained laboratory measurements and the numerical models with observations of dusty environments. This has resulted in explanations for the 21-micron feature observed in some planetary nebulae (Posch et al., 2004, ApJ 616, 1167), the far-UV quasar break (Binette et al., 2005, ApJ 631, 661), the 3.5 micron emission in quasars (de Diego et al., 2007, A&A 467, L7), the environment of the gamma-ray burst GRB050401 (Watson et al., 2006, ApJ 652, 1011) as well as a characterisation of the highest redshift (z=2.45) detected 2175Å dust bump (Elíasdóttier et al, 2009, ApJ submitted). I have also been in collaborations where we have investigated the very early stages of planetformation in accretion disks, both theoretically (Johansen 2004, A&A 417, 361) and from the observational side (Goto et al., 2008, ApJ submitted). As well as participated in developing a mathematical model to describe homochiral growth of polymers (Brandenburg et al., 2005, OLEB 35; 225; OLEB 35, 507; Nilsson et al., 2005, IJAsB 4, 233). Last but not least I have proposed an autonomous space mission (named Bering) to explore the asteroide belt as a way to determine the link between meteorites and asteroids (e.g. Andersen et al., AAJ 59, 966).

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May 2009.