Nanocosmos confirms there are PAHs in the interstellar medium

Image of the Heiles Cloud 2 as part of the massive Taurus Molecular Cloud (TMC). The magnifying glass shows the region, called TMC-1, where our line survey observations were made. Image captured at Grand Mesa Observatory in Colorado (USA). Image credit and copyright Terry Hancock and Tom Masterson.

First detection of a pure PAH (indene c-C9H8) in an unexpected place!

Polycyclic Aromatic Hydrocarbons (PAHs) are organic compounds formed by rings. Their bad reputation on Earth is due to their toxicity, as they are mostly the result of oil and coal combustion. However, in space they have another role that, despite waiting for confirmation, may be related even to the origin of life.

In the interstellar medium observations, there are infrared bands that, until now, were unidentified. The hypothesis (for more than 40 years) was that these bands were probably PAHs, but final confirmation was lacking.

The first milestone of this work is the confirmation, for the first time, of the presence of a pure PAH (indene) in the interstellar medium. The second milestone is that we have confirmed the discovery in an unexpected place: a cold dark cloud called TMC-I.

The TMC-1 cold dark cloud

It was originally thought that PAHs could form in circumstellar envelopes around evolved stars. These stars are in the final stages of their lives and expel much of their matter into the interstellar medium. In fact, twenty years ago benzene, (an aromatic ring present in many PAHs) was first detected in the hot and ultraviolet light illuminated regions around an evolved star. This made astronomers think that the formation of PAHs requires high temperatures and ultraviolet radiation. Therefore, the presence of PAHs in the interstellar medium would have an exogenous origin. That is, PAHs would form in circumstellar envelopes and would later be dragged into the interstellar medium by stellar winds.

However, the first detection has been carried out in an unexpected environment: the cold pre-stellar core TMC-1 in the Taurus Molecular Cloud complex, which is well protected from ultraviolet radiation. In this environment, in addition to the indene (c-C9H8), the presence of ethynyl cyclopropenylidene (c-C3HCCH) and cyclopentadiene (c-C5H6) has been detected. It should be noted that cyclopentadiene and indene, molecules formed by rings of five and six carbon atoms, are exceptionally abundant despite their large size.

With these observations, it is demonstrated not only the unambiguous presence of PAHs in the interstellar medium, but also that they are formed in situ and from less complex molecules. They are not dragged from other environments (e.g. on the surface of dust grains), but are formed according to what is called a bottom-up formation mechanism, that is, from smaller molecules that join in the gas phase.

Although some theories relate PAHs to the origin of life, more studies are still needed to confirm the role they could have played in the formation of nucleobases, which are part of the RNA. While astronomers gather more data that may or may not confirm this hypothesis, this NANOCOSMOS-ERC discovery is a major breakthrough in our current understanding to explain the formation mechanisms of complex molecules, which remain, for the most part, a mystery.

The Yebes 40m radio telescope

The TMC-1 observations have been carried out with the 40m radio telescope at Yebes Observatory (IGN, the Spanish National Geographic Institute). This was possible thanks to the Nanocosmos new receivers, built within the Nanocosmos-ERC project, funded by the European Research Council. Since they were installed, these high-sensitivity new receivers are providing valuable new information on the interstellar medium.

More information

Pure hydrocarbon cycles in TMC-1: Discovery of ethynyl cyclopropenylidene, cyclopentadiene and indene (Astronomy & Astrophysics, May 2021, DOI: 10.1051/0004-6361/202141156). Authors: J. Cernicharo, M. Agúndez, C. Cabezas, B. Tercero, N. Marcelino, J. R. Pardo, & P. de Vicente.

AstroPAH: A Newsletter on Astronomical PAHs (Leiden University, the Netherlands), issue 78, May 21, 2021. A new golden age era for Astrochemistry: Discovering PAHs with millikelvin sensitive radio astronomical molecular line surveys (by Prof. José Cernicharo, on behalf of the NANOCOSMOS ERC team).

CSIC press release: Hallados hidrocarburos policíclicos aromáticos en el medio interestelar

IGN press release: Hidrocarburos policíclicos aromáticos en el medio interestelar

El Mundo newspaper (May 22, 2021): ¿Por qué es importante el indeno hallado en el espacio por astrónomos españoles? (by Dr. Rafael Bachiller, director of the Observatorio Astronómico Nacional, IGN, Madrid).

Breaking: First Time Discovery of the PAH Indene in Space

The NANOCOSMOS team reports the first time detection of the simplest polycyclic aromatic hydrocarbon (PAH) carrying a five-membered ring—indene (c-C9H8) in Space (TMC-1 cold dark molecular cloud) with rotational spectroscopy. This major challenging breakthrough is the first step to understand the potential formation mechanisms of these species in the interstellar medium. Moreover, the team derives a high abundance of indene that needs to be explained through alternative and efficient chemical routes.

The team also reports the first time discovery in space of two other organic compounds, the c-C3HCCH (ethynyl cyclopropenylidene), and c-C5H6 (cyclopentadiene).

These discoveries are the result from the groundbreaking Yebes 40m Observatory sensitive survey with the new NANOCOSMOS Q-band receiver of the TMC-1 cold molecular cloud. This survey has led to the discovery of multiple molecular species since 2020 with more than 25 molecules, 15 of them for the first time in Space.

The best for NANOCOSMOS is yet to come. Stay tuned!

Pure hydrocarbon cycles in TMC-1: Discovery of ethynyl cyclopropenylidene, cyclopentadiene and indene (Accepted for publication in A&A Letters, 2021). Authors: J. Cernicharo, M. Agúndez, C. Cabezas, B. Tercero, N. Marcelino, J. R. Pardo, & P. de Vicente.

New outreach video

Today we have 2 great audiovisual news: we present a new short video summarizing some of the scientific and technological achievements of our project. The premiere is today at 10:00h in our youtube channel:

And we also release the documentary “Nanocosmos: un viaje a lo pequeño” (subtitles available in Spanish, English and French): 40 minutes to talk about the journey of cosmic dust and the seek of our researchers to go deep into the dust grains formation.

We want to thank all the professionals that made possible those two audiovisuals, Filmociencia and LUZLUX, the institutions that support us, the European Research Council, the CSIC (Consejo Superior de Investigaciones Científicas), the CNRS (Centre National de la Recherche Scientifique) and the funding from Fecyt (Fundación española para la Ciencia y la Tecnología). And, of course, thanks to all the researchers of our participating institutions for their work and their enthusiasm.

We hope those videos will help to understand how astrochemistry and fundamental science improves a better knowledge of the universe and helps Humanity to go beyond the frontiers of the unknown. Because, as Carl Sagan said: “The cosmos is also within us, we’re made of star stuff. We are a way for the cosmos to know itself.”

Why do we study chemical equilibrium in red giants?

You may have often read that “we are stardust.” It is a rather accurate expression, especially if we think that most of the elements that make us up (this scarce 5% of the baryonic matter of the universe) emerged from the core of a star and from a whole process of death and destruction. But what do we call stardust?

Nanocosmos has participated in this study, explained in the outreach article “Funambulist stars”, that you can continue reading by clicking here.

Carbon grains around evolved stars

The Nanocosmos team published in October 21, 2019, at Nature Astronomy (available free at Europe PubMed Central), the results of a set of laboratory experiments showing that gas-phase chemistry, under conditions similar to those of a red giant star environment, can produce very efficiently small amorphous carbon grains and carbon chains similar to those found in oil.

Stardust, an ultra-high vacuum machine built in the ERC Nanocosmos project (a Synergy project funded by the European Research Council), was specifically conceived to simulate, with a high level of control, the complex conditions of stardust formation and processing in the environment of evolved stars. In addition, the AROMA setup was built to analyse the molecular content of the samples synthesized by Stardust.

In the words of José Ángel Martín-Gago (Institute of Materials Science of Madrid, ICMM-CSIC, Spain), responsible for the Stardust instrument, “Mimicking the conditions of the envelope of an evolved star, laboratory experiments allow scientists to follow, step by step, the formation process of dust grains, from atoms to simple molecules and their growth to more complex clusters of molecules.”

For José Cernicharo (Institute of Fundamental Physics, IFF-CSIC, Spain), lead co-investigator of the project together with Martín-Gago and Christine Joblin (Institut de Recherche en Astrophysique et Planétologie, IRAP-CNRS, France), “That process is important because those grains of dust, which emerge from the final stages of the evolution of medium-sized stars like our Sun will provide the fundamental pieces needed for the birth of the planets and the main ingredients for the onset of life once injected into the interstellar medium.”

This is why it is essential to develop experiments combining laboratory astrophysics, surface science and astronomical observations to unveil the chemical routes that operate in the inner layers of the envelope of evolved stars.

The results obtained show the formation of amorphous carbon nanograins and aliphatic carbon clusters with traces of aromatic species and no fullerenes. This shows that the latter species cannot form effectively by gas-phase condensation at these temperatures in the zone of the evolved star where the dust is formed, a region that extends up to a few stellar radii.

Chemical complexity

Carbon dust analogues were produced in Stardust and analysed with several characterization techniques including Scanning Tunneling Microscopy and mass spectrometry with the AROMA setup. To produce them only gas carbon atoms and molecular hydrogen were used in a ratio close to that in the atmospheres of AGB stars.

The results showed two types of products: amorphous carbonaceous nanograins – the most abundant, considered to be the main component of carbonaceous star dust – and aliphatic carbon groups. But almost no aromatic molecules were found in the analysis.

According to Joblin, “Polycyclic aromatic hydrocarbons (PAHs) are widespread in massive star-forming regions and in carbon-rich protoplanetary and planetary nebulae. Large carbonaceous molecules like buckminsterfullerene C60 have also been detected in some of these environments. But it seems that they need different conditions to be formed”.

One possible pathway could be through thermal processing of aliphatic material on the surface of dust, which could take place as a result of the significant rise in the temperature of nanograins that occurs in highly UV-irradiated environments. Those results give us new insights into the chemistry of carbonaceous stardust seed formation and foster new observations in order to constrain the physical and chemical conditions in the inner shells of the envelops of evolved stars.

About the ERC

The European Research Council, set up by the European Union in 2007, is the premier European funding organisation for excellent frontier research. Every year it selects and funds the very best, creative researchers of any nationality and age to run projects based in Europe. The ERC has three grant schemes for individual principal investigators – Starting Grants, Consolidator Grants, and Advanced Grants – and Synergy Grants for small groups of excellent researchers.

To date, the ERC has funded more than 9,000 top researchers at various stages of their careers, and over 50,000 postdoctoral fellows, PhD students and other staff working in their research teams. The ERC strives to attract top researchers from anywhere in the world to come to Europe.

The ERC is led by an independent governing body, the Scientific Council. The ERC current President is Professor Jean-Pierre Bourguignon. The ERC has an annual budget of €2 billion for the year 2019. The overall ERC budget from 2014 to 2020 is more than €13 billion, as part of the Horizon 2020 programme, for which European Commissioner for Research, Innovation and Science Carlos Moedas is currently responsible.