Avaleht

Mihkel Kama

Hello, I’m Mihkel, a Senior Research Fellow at Tartu Observatory in Estonia. I study various aspects of planet formation and extrasolar planets. You can read about my scientific interests below, and see my publication list on ADS.

If you’re looking for a thesis project at any level, or a summer research project, e-mail me (mihkel.kama at ut.ee) to learn about current opportunities.

Research
We live in a remarkable age, where exopanet discoveries and solar system exploration are proceeding rapidly. We now know of over 4000 exoplanets, yet the full range of their properties remains a mystery. How did all these thousands of different planets form? What is the origin and diversity of their chemical composition? Are habitable worlds like Earth common? We can find many of the answers by studying young stars and their disks of planet-forming material.

Planets form in protoplanetary disks, containing gas, ices, and tiny rocks. I’m interested in the physical and chemical conditions and processes in planet-forming environments, and in how those relate to the properties of planets and planetary systems. Much of my work deals with elemental abundances and volatiles in protoplanetary disks, employing powerful telescopes like ALMA and APEX, and physical-chemical disk models. Optical spectroscopy of early-type stars with disks and planets is an increasingly large part of my work.

Most chemical abundance measurements in disks rely on sub-millimetre spectroscopy to derive the quantity of specific molecules in the gas, and I too use this staple technique (e.g. Kama et al. 2016b). More recently, I’ve spearheaded an effort to go beyond the state of the art. We developed a new tool for measuring the full elemental composition of the main planet-forming zone in disks. The Contaminated A-stars Method (CAM, Jermyn & Kama 2018) relies on the relatively slow mixing in the envelopes of stars with over 1.4 Solar masses, which allows freshly accreted material to dominate the surface. The photospheric composition of the star, which is relatively easily measured, becomes an almost one-to-one proxy for measuring the inner disk content of chemical elements. We are in the process of applying this technique to study new elements and stars.

We have now applied CAM to the “missing sulfur problem” in disks, and found that (89+-8)% of all sulfur atoms are, in fact, locked in refractory compounds and therefore invisible to cold gas-phase spectroscopy. This provides a novel input to disk and planet formation models incorporating chemistry, and is only a first taste of the potential of this new method.

I am also a member of the ARIEL exoplanet mission science consortium. ARIEL will launch in 2028 and is expected to measure the chemical composition of up to 1000 exoplanets.

Selected papers

Mass constraints for 15 protoplanetary disks from HD 1-0   (disk masses are hard to measure, we re-analysed Herschel Space Observatory upper limits and obtained new constraints, including a surprisingly small mass upper limit for HD 163296; Kama, Trapman, et al. 2020)

Abundant refractory sulfur in protoplanetary disks   (where we measure for the first time the fraction of sulfur, zinc, and sodium locked in rocks; Kama, Shorttle, Jermyn, et al. 2019)

Stellar photospheric abundances as a probe of discs and planets   (the theory and applications of measuring the composition of circumstellar material through accretion onto the photosphere; Jermyn & Kama 2018)

Volatile locking and release in protoplanetary disks   (detailed modelling of molecular and atomic lines from TW Hya and HD 100546, confirming a factor 100 depletion of C and O in the TW Hya disk, and determining that C/O > 1; Kama et al. 2016)

Fingerprints of giant planets in the photospheres of Herbig stars   (young A-type stars hosting transitional disks have photospheres depleted of refractory elements — matching the high gas-to-dust ratio in their dust-depleted gaps and cavities; Kama, Folsom & Pinilla 2015)

Inner rim structures of protoplanetary disks   (numerical and analytical models of the dust sublimation region; Kama et al. 2009)

Career

• Senior researcher, Tartu Observatory, University of Tartu (current)
• Marie Sklodowska-Curie Fellow, IoA, University of Cambridge, in collaboration with the groups of Oliver Shorttle, Cathie Clarke, Nikku Madhusudhan, and Mark Wyatt (2016-2019)
• Research Associate, Leiden Observatory, astrochemistry group of Ewine van Dishoeck (2013-2016)
• PhD, University of Amsterdam, thesis advisor Carsten Dominik (2008-2013)
• MSc (cum laude), University of Amsterdam, thesis advisor Carsten Dominik (2006-2008)
• BS, University of Tartu, Estonia, thesis advisor Indrek Kolka (2003-2006)