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Trendbericht Physikalische Chemie 2023 (1/3)

Astrochemistry of star and planet formation

Von Wiley-VCH zur Verfügung gestellt

Stickstoff- und Kohlenstoffverbindungen im interstellaren Staub verraten, wie Sterne entstehen. Diese Moleküle haben Weltraumforscher:innen mit hochauflösender Rotationsspektroskopie detektiert und dazu die Verhältnisse im All im Labor nachgestellt. Wie organische Reaktionen wie nukleophile Substitution auf Molekülebene ablaufen, lässt sich mit Molekularstrahlmethoden herausfinden. Streumethoden dienen dazu, Reaktionen auf Oberflächen auf atomarer Ebene zu erfassen. Zeitaufgelöste Beugungsmethoden mit ultrakurzen Elektronen oder Röntgenphotonen werden zunehmend auf chemische Fragen angewandt, um Strukturen transienter Zustände zu beobachten.

Astrochemistry of star and planet formation

The gas composing the interstellar medium (ISM) is inherited from old stars and, at the same time, feeds into new stars and, eventually, new planets. Understanding the chemistry in the processes of star and planet formation is a crucial step into revealing not only the chemical, but also the physical processes that drive these phenomena.

Stars form in dense cloud cores that can be studied through the emission of cold dust and molecules. Dense cores have a relatively high column density of dust, which effectively blocks the ambient radiation in the optical and ultraviolet regions of the electromagnetic spectrum. Thanks to this shielding, molecules can survive, and molecular complexity can be built.

Molecular emission in such dense cores is how we can observe the process of star-formation and study the chemical evolution from clouds to stars and planets.1) Because of the interconnection between the chemistry and the physics of dense cores, identification of molecules can be used to study the dynamics of the star-formation process, as well as the chemical budget that will finally be inherited by the forming planets.

The chemical inventory of star-forming regions is a powerful diagnostic tool to study their evolution. Isotopic fractionation, and in particular deuterium fractionation, has allowed us to reveal the path of water in the evolution of the Solar System, for example, showing that a large fraction of water in the Solar System predates the Sun.2) Physical properties of the emitting gas (e.g. temperature density and kinematics) can also be traced by molecular emission. Moreover, molecules are not homogeneously distributed in star and planet forming regions. As a consequence, when observing different molecules we get a tomographic 3D view of our object of study.

The chemical and physical structure of pre-stellar cores

Pre-stellar cores are starless cores that are dynamically evolved, and on the verge of star-formation.3) Pre-stellar cores evolve towards higher central densities leading to collapse and formation of the central protostar and the orbiting protoplanetary disk providing the budget of matter that will form stars and planets. Chemical segregation has been observed in several starless and pre-stellar cores. Caselli et al. (1999) showed that CO is predominantly (>90%) frozen onto the surface of dust grains in the central 6500 au of L1544, a pre-stellar core in Taurus,4) while N2H+ and in particular N2D+ better follow the millimetre dust continuum emission showing very little depletion.5) This study was extended to other starless cores and additional molecular species.6)

Starless and pre-stellar cores present a systematic molecular differentiation, with C-bearing species such as CO, CS, C2S, CH3OH, C3H2, and HC3N, being severely affected by freeze-out and showing a sharp central hole, while N2H+ and NH3 are still present in the gas phase towards the core centres.

Carbon-chain molecules are known to preferentially trace starless and less evolved cores, where C atoms have not yet mostly been locked in as CO. While several single pointing observations have been performed to study the chemistry of pre-stellar cores, until less than a decade ago few maps were available. N2H+ and CCS are the first molecules that have been mapped towards L1554.7,8) CCS is less abundant towards the dust peak, while N2H+ suffers less depletion caused by the high density and low temperature of the core. Overall, nitrogen bearing species are very good tracers of the central parts of pre-stellar cores, but recent studies showed that they also eventually deplete in the very centre, though models have often required scaling factors to account for discrepancies in the observed data.5,9) A solution was presented in the work of Caselli et al.9) proving that ammonia also freezes-out onto the icy surface of dust grains in the very inner regions of pre-stellar cores (2000 au), finally solving the decades-long disagreement between chemical models and observations.

By observing the spatial segregation among two carbon-bearing molecules, methanol (CH3OH) and cyclopropenylidene (c-C3H2) towards a sample of starless and pre-stellar cores, we showed the effects of inhomogeneous illumination onto the chemistry observed at core scale.10,11)

In order to understand the chemical differentiation that we observe at the scale of the core, we must consider the environment where the core is embedded. Large-scale effects have a direct impact on the chemical segregation that we observe in pre-stellar cores. Methanol and cyclopropenylidene trace different regions in starless and pre-stellar cores, and give us complementary views of dense cloud cores, thus enriching our understanding of the complex interplay between physics and chemistry in the early stages of star-formation.

Towards the pre-stellar core L1544, we also performed a study on the dependencies of chemistry on physical and external conditions using the emission maps of 39 different molecular transitions belonging to 22 different molecules in the central region of L1544 identified with the single dish IRAM 30m telescope.12)

The advantage of using maps with respect to single pointing observations is that with maps it is possible to use the knowledge about the physical structure of the core and find correlation between different molecules based on their spatial distribution. The sample of emission maps was classified into four families, depending on the location of their emission peaks within the core. Figure 1 shows four molecular peaks within L1544:
Fig. 1. Left: H2 column density map pf the pre-stellar core L1544 derived from dust emission observations of Herschel/SPIRE data.10) The black dashed box shows the region where molecular emission was mapped, a sample of which is shown in the right panel of the figure. Right: Sample of maps belonging to the different molecular families observed toward L1544 using the IRAM 30m antenna.12) The black dashed lines represent the 90 %, 50 %, and 30 % of the H2 column density peak value derived from Herschel/SPIRE maps, 2.8 1022 molecules cm–2. The solid lines represent contours of the molecular integrated emission starting with 3 sigma with steps of 3 sigma (the sigma values are in Table 1 in Spezzano et al. 2017).12) The dust peak position is indicated by the black triangle. The white circles represent the HPBW of the 30 m telescope (~5000 au at 3 mm).

the dust peak, where nitrogen-bearing molecules like HCN and N2H+ are more abundant,the methanol peak, where oxygen-bearing molecules like methanol and SO are more abundant.the c-C3H2 peak, where carbon-bearing molecules like C4H and HC3N are more abundant.the HNCO peak, where HNCO and CH3CCH are more abundant.

Principal component analysis

Furthermore, to systematically study the correlations among different molecules, we performed the principal component analysis (PCA) on the integrated emission maps.

The PCA allows to reduce the amount of variables in the dataset. This method has been successfully applied in other multi-line studies of regions in our Galaxy, such as Orion.13) The results of the PCA confirmed the correlation among the molecules in the four categories depending on the spatial distribution, especially the dichotomy between the molecules peaking in the methanol and the c-C3H2 peak.10) The PCA has also shown correlation between the molecules in the HNCO and the dust peak of L1544, which could not be inferred just from a qualitative analysis.

Given the large bandwidth of new receivers of radiotelescopes and the consequent large datasets observed, machine learning algorithms will be strategic to make significant leaps in our understanding on the chemistry, physics and kinematics in the early stages of star and planet formation.

Laboratory spectroscopy as astrophysical tool

In order to use molecules as a tool for studying star and planet formation, high-resolution rotational spectroscopy is mandatory for their identification. To date almost 300 molecules have been observed in the ISM.14) Excluding few exceptions, all of these molecules have been detected through their rotational spectrum in the cm-, mm- and submm-wave range.

The rotational spectrum has the advantage of being the fingerprint of the molecule, and hence allows to make an unambiguous detection. With respect to Earth, the ISM is characterised by very low pressure. High density in the ISM means 107 cm–3, while on Earth we have a particle density of 1019 cm–3 at sea level, and high vacuum is usually on the order of 1010 cm–3. Given the very low pressure, reactive molecules such as ions and radicals are widely present in the ISM, and are of pivotal importance for the development of molecular complexity in space. A fact already recognised in the 70s, as soon as the first polyatomic molecules were detected in the ISM.15) The development of molecular complexity starts indeed from an ion, H3+, that forms the basis for an extensive network of ion-molecule reactions that are responsible for the creation of the majority of the molecules observed in interstellar space. Ion-neutral reactions are mostly barrierless, and hence proceed very fast even in cold regions of the ISM.

Figure 2shows some of the laboratory experiments built at the Center for Astrochemical Studies (Max Planck Institute for Extraterrestrial Physics) to study the spectroscopy of molecules of astrophysical interest. Figure 2 also shows the spectrum of doubly deuterated acetaldehyde, the recent spectroscopic study of which allowed its first detection in the ISM.16)
Fig. 2. Top: Spectroscopy laboratories at the Center for Astrochemical Studies (CAS) at the Max Planck Institute for Extraterrestrial Physics. Bottom: A portion of the spectrum of doubly deuterated acetaldehyde (CHD2CHO) measured in the CAS laboratories. The millimetre and sub-millimetre wave spectroscopy of CHD2CHO allowed its first detection in the ISM.16)

Molecules in star formation

Molecular spectroscopy and radioastronomical observations have a symbiotic relationship, and have pushed their development over the past 60 years. The current challenges in gas-phase spectroscopy are related to the quest to understand how far molecular complexity can go in space, and what are the reaction processes that bring us the complexity that we observe.

Complex organic molecules (COMs), defined as molecules with more than five atoms, containing carbon, oxygen and hydrogen, are ubiquitous in star-forming regions. They are very abundant around protostars, in the so-called “hot corinos” in low-mass star-forming regions, and towards “hot-cores” in high-mass star-forming regions.17,18)

Very recently, we started observing COMs also towards starless and pre-stellar cores,19) and it seems that these molecules trace the earliest phases of star-formation very well, also before the ignition of the star, suggesting that an active chemistry is at work during these early stages. Part of these species produced in pre-stellar cores and stored in ices will then be maintained during the formation of planetesimals and planets.20,21)

Among the most important open questions in the field are the ones revolving around the inheritance of molecular complexity from the pre-stellar to the protostellar phase, the level of chemical complexity that can be reached in the cold pre-stellar phase, and the chemistry happening in the icy mantles of dust grains in star-forming regions. A concerted effort of observations, modelling and, most importantly, laboratory work is the recipe to find out the answers to these important astrochemical questions, and finally understand our astrochemical origins.

Three Questions to: Silvia Spezzano

Why are you doing astrochemistry?

I serendipitously discovered astrochemistry. The idea that I could study spectroscopy not only in a laboratory but also in space, and use it to study the origins of the molecular building blocks of life, blew my mind. I enjoy the multidisciplinary aspect of astrochemistry, combining astronomical and chemical observations. Using radiotelescopes to observe molecules and astrophysical objects is one of my favourite things to do.

What do you think has been the most important outcome in astrochemistry in the last years?

The discovery of streamers in protostellar cores in 2020. Streamers are flows of material that feed the protostellar disks where planets are forming. We used to think of protostellar cores as chemically closed environments, and the detection of streamers has drastically changed this view.

What would you like to discover next?

I would like to find the initial conditions for the formation of streamers in the pre-stellar phase. And, to use isotopic fractionation to find out how much of the molecular complexity that we observe in pre-stellar cores is inherited by planets.

Silvia Spezzano is a Max Planck independent group leader at Max Planck Institute for Extraterrestrial Physics in Munich since 2020. She studied industrial chemistry in Bologna and worked on high-resolution IR spectroscopy for her bachelor and master thesis. She spent one year in Cambridge, USA, working in the Center for Astrophysics (CfA) at Harvard University. She completed her PhD in astrochemistry at the University of Cologne and subsequently moved to Munich, where she led a research group at the Max Planck Institute for Extraterrestrial Physics between 2017 and 2020.

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Wissenschaft + ForschungSchlaglichtthema: Chemie im Weltraum

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