The Öberg Astrochemistry Group

Research Goals

In the Öberg Astrochemistry Group we pursue laboratory ice experiments and astronomical observations (mainly at millimeter wavelengths) that address

  1. 1.the chemical evolution present during star and planet formation and its effects on planet compositions,

  2. 2.the fundamental physical chemical processes that underpin this evolution,

  3. 3.and the development of new molecular probes of star and planet formation.


Solar-type star formation starts with the collapse of a dense cloud core (b) into a protostar (c). Outflows and a disk form to carry away angular momentum. The circumstellar disk is the birth-place of planets (d), and may evolve into a planetary system (e).

During much of its evolution the star is deeply embedded in dust and gas and the physics is thus best traced by observing the evolving chemistry. The chemical evolution is also interesting in its own right; it determines what pre-biotic chemistry that eventually shows up in comets and planets. Much of the chemistry is proposed to take place in icy mantles on microscopic dust particles, but the fundamental processes that underpin this icy chemistry is largely unknown.

 

Specific Research Topic

Astrochemical imaging of protoplanetary disks

Protoplanetary disks are common around young stars. Their chemical composition and physical characteristics set the mass and chemical compositions of nascent planets. The figure to the left rom Henning & Semenov (2014) illustrates some important disk physics and chemistry. Studying disk chemistry is challenging because of small angular scales and intrinsically low molecular gas-phase abundances. Thanks to ALMA, a new and amazing millimeter and submillimeter interferometer in Chile, spatially resolved molecular emission on Solar System scales are now attainable.
We have used ALMA to put new constraints on isotope fractionation during planet formation, to obtain
the first ever images of a disk snow line (in this case CO ice traced by N2H+), to detect the first complex organics seen in a disk, and to assess the isotopic fractionation chemistry through a survey of H13CO+, H13CN, DCO+, DCN and HC15N in a sample of disks. An example of the DCO+ distribution in the IM Lup disk is shown to the right.


The effects of astrochemistry on planet compositions


Using astrochemical simulations and toy models we explore how the time dependent chemistry present in disks affect the bulk and organic composition of planets and planetesimals assembling at different distances away from the central star. An important example is that the gas-phase and solid C/O ratio changes with disk radius because of sequential freeze-out of the main carriers of carbon and oxygen in disks. We therefore expect that rocky planets and planetary cores and planet atmospheres will have different elemental compositions and therefore different chemical evolutions dependent on how close or far away from their star they assemble.


Laboratory simulations of interstellar ice chemistry and physics

Icy grain mantles are chemical factories. This is where water, methanol and most prebiotically interesting
molecules are proposed to form. The efficiency of this chemistry depends fundamentally on the availability of reactants and the mobility of these reactants on ice and grain surfaces and in ice interiors. At low temperatures most reactions involve radicals, atoms or ions, which generally have low or non-existent reaction barriers.

We use a specially designed ultra-high vacuum surface science set-up to investigate the the production efficiencies and branching ratios of different radicals in ices exposed to UV, X-rays and electrons (the main sources of energetic radiation in space) and the efficiencies and underlying mechanisms of radical, atom and molecule diffusion in thin ice films that simulate icy grain mantles in space. The results are used together with astrochemical theory to predict ice compositions, focused on prebiotic molecules, during star and planet formation. Key papers on photochemistry, and ice diffusion.


We also pursue experiments to constrain the interactions between grain surfaces and gas in the interstellar medium. Gas and grain chemistry are connected through freeze-out or condensation of gas-phase molecules onto grains, and through thermal and non-thermal desorption or sublimation of ices into the gas-phase. Non-thermal desorption is especially poorly understood and a focus within the group. Key papers on broadband and frequency resolved ice photodesorption.



Resolving the complex chemical evolution during the early stages of star formation

Protostars are known harborers of complex organic molecules. Abundances vary orders of magnitude between different objects and within the envelopes of individual protostars. Recently we also discovered that typical protostellar ‘hot core’ molecules are present at much earlier and colder stages of star formation.

We use a combination of ice observations, and millimeter wavelength single dish (IRAM 30m (left) and GBT) and interferometry (mostly the Submillimeter Array) to survey how the complex organic chemistry evolves between different star forming stages and within protostellar envelopes.

First results reveal an evolving chemistry within the envelope of the massive young
stellar object NGC 7538 IRS9. The extended emission of many molecules is difficult to explain with existing astrochemical models, challenging our understanding of complex molecule formation at low temperatures.