Research Topics

In the interstellar medium group at Leiden Observatory, we study the origin and evolution of the organic inventory of space and combine that with studies on the structure and evolution of the interstellar medium of galaxies. In addition, we study the coagulation of dust particles, to aggregates, pebbles, and larger bodies in the general ISM and in protoplanetary disks.

Molecular Universe

Over the last 20 years, we have discovered that we live in a molecular universe: a universe where molecules are abundant and widespread; a universe with a rich organic inventory particularly in regions of star and planet formation; a universe where the formation of stars and the evolution of galaxies is driven in many ways by the presence of molecules; a universe where prebiotic interstellar molecules may represent the first steps toward life; a universe where molecules can be used as "dye" to trace important processes in the interstellar medium; a universe where molecules provide unique information on the physical conditions of a wide variety of regions; and a universe where molecules can work together to form such complex species as you and me. In Leiden, we focus on the characteristics of large Polycyclic Aromatic Hydrocarbon molecules, their relationship to molecular clusters and dust grains, their evolution under the influence of radiation and on planetary bodies, their contribution to the organic inventory of regions of planet formation, and their role in the Universe.

Interstellar Medium

The interstellar medium (ISM) plays a central role in the evolution of galaxies. The formation of new stars slowly consumes the ISM, locking it up for millions to billions of year while stars, as they age, return much of their mass increased in metallicity, back to the ISM. Stars also inject radiative and kinetic energy into the ISM and this controls the physical characteristics (density, temperature and pressure) as well as the dynamics of the gas as revealed in observed spectra. On a macroscopic scale, we study the dust mass budget of the Milky Way and other galaxies and the dynamical interaction of stellar radiation with interstellar dust. We also perform a carbon radio recombination line survey of the Northern galactic plane, probing the physical characteristics of diffuse clouds. To address issues such as the interrelationship of atomic and molecular clouds and the cosmic ray ionization rate in the galaxy. On a microscopic level, heating and cooling processes involving large molecules and molecular clusters regulate the interplay of stars and surroundings. In addition, we use PAHs and dust - and their spectroscopic characteristics - to trace this interaction.

Dust Aggregation

In recent years, we have discovered that almost every star in the Milky Way has a planetary system and one in every five solar-type stars has a terrestrial planet in the habitable zone. With this realization, understanding the formation of planets and the processes that control the architecture of planetary systems have become the key questions in astrophysics. In Leiden, we study the first steps in this, the aggregation of small dust grains into larger structures in protoplanetary disks and their subsequent growth to cometesimals and planetesmals.

PDRs and PAHs

Two main topics which our group are specialized are Photodissociation Regions (PDRs) and Polycyclic Aromatic Hydrocarbons (PAHs). PDRs sre formed by the interaction of far-ultraviolet photons with dense neutral atomic gas separating the highly ionized hydrogen plasma from the surrounding molecular cloud in which stars are born. Essentially the entire neutral interstellar medium of galaxies is a PDR governed by the same physics and chemistry. Indeed, PDR studies cover surfaces of protoplanetary disks, photo-evaporation of globules and pillars, planetary nebula, characteristics of diffuse interstellar clouds, and the nuclei of galaxies, including starburst and Ultra-Luminous InfraRed Galaxies (ULIRGS) and range from the here and now all the way back through the era of ubiquitous star formation when galaxies were assembled.
Broad emission features dominate the mid IR spectra of almost all objects, including HII regions, reflection nebulae, young stellar objects, planetary nebulae, post-asymptotic giant branch (AGB) objects, nuclei of galaxies, and ultraluminous infrared galaxies (ULIRGs). These features are generally attributed to IR fluorescence of far-ultraviolet-pumped PAH molecules. PAHs are an important component of the ISM, and their presence has major ramifications.



The ISM being the repository of stellar ejecta as well as the birth site of new stars is a key factor in driving the evolution of galaxies. Cold, diffuse, atomic clouds are an important component of the ISM, bridging the warm ionized (HII) and cold molecular (H2) phases. However, the cold, atomic phase has been difficult to study, because its main tracer, the HI 21 cm line, does not constrain the basic physical information of the gas (e.g., temperature, density) well. A promising complementary tracer exists in the form of so-called Radio Recombination Lines from Carbon (CRRL) and Hydrogen (HRRL). Driven by a new generation of sensitive, high-resolution radio telescopes and recent advances in our understanding of atomic physics these lines do now provide us with a sensitive probe of the physical conditions in cold, diffuse clouds. Over the next years with LOFAR we will provide a new view on the role of the cold, diffuse atomic gas in our own Milky Way and beyond.

CII emission from the Orion Molecular Cloud

We will use the upGREAT instrument on SOFIA to map the Orion Molecular cloud in the [CII] 1.9 THz (158 μm) fine-structure line. The CII line is the brightest far-IR line in the spectrum of the Milky Way with an integrated intensity of 0.5% of the dust continuum, but its origin is not well understood with potential major contributions from dense PDRs, low density ionized gas (the Warm Ionized Medium), surfaces of molecular clouds, and the diffuse clouds in the ISM (the Cold Neutral Medium). Notwithstanding, this line is widely used as a star formation indicator in ALMA studies at high redshifts. In low metallicity regions, this line may also be the best tracer of CO-dark molecular gas. Our SOFIA proposal – together with David Tessier (ESAC, Madrid), Olivier Berne (IRAP, Toulouse), Javier Goicoechea & Pepe Cernicharo (ICMM, Madrid) and Mark Wolfire (University of Maryland)– was approved to map a region of about 0.5 square degrees at high spectral (sub km/s) and spatial (15") resolution. For comparison, HIFI/Herschel mapped the central portion (about 100 square arc minutes) in about 20 hours. The improved sensitivity, multi-element array and nimble telescope of upGREAT on SOFIA will allow us to map the complete cloud – a region about 20 times larger in size – in some 55 hours. This will allow us to study the energetics of the interaction of massive stars with their environment (radiative & kinematic), the relationship of atomic and molecular gas, molecular cloud formation and evaporation, and the physics of atomic gas. This will be part of Cornelia Pabst PhD thesis.


Velocity-resolved observations of the C+ line obtained by the upGREAT instrument onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) reveal distinct kinematic structures in the Orion Nebula complex. With increasing LSR velocity the Veil shell, that is enveloping the Orion Nebula, is seen to move outward, attesting to its large-scale expansion. At higher velocity (v_LSR~8-10 km/s) the dense interaction zone of the massive stars with the nearby molecular cloud becomes visible. To the north, the expanding shell surrounding the reflection nebulae NGC 1973, 1975 and 1977 lights up at v_LSR~12 km/s.

Credit to Universitat zu Koln/NASA/SOFIA.

Observations of the C+ line obtained by the upGREAT instrument onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) provide a 3D view of the Orion Nebula, that is our nearest massive star-forming region. The 3D space-velocity cube reveals a rich dynamic structure of the entire nebula, showing multiple filaments, outflows, a complementary view of the molecular gas, and, not least, the bubble created by the wind of the most massive star, theta1 Ori C. The cropped rotated structure has been nicknamed Orion's Dragon.

Credit to NASA/SOFIA.