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.

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.

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.

Laboratory experiments and chemical models

NANOCOSMOS has developed a custom-designed vacuum chamber for the study of gas-phase molecular species through rotational spectroscopy. The innovative breakthrough is the coupling of the chamber to the new NANOCOSMOS millimeter broad band receivers. These receivers are twins of those built for the Yebes 40 meter radio telescope. We call the whole experimental setup as GACELA – Gas Cell for Laboratory Astrophysics.

Our experiments address the characterization of molecules that represent a considerable fraction of all the molecular species detected in the interstellar medium – ISM – but present rotational parameters not precise enough to allow their detection in the ISM. Therefore, we use GACELA to measure the rotational spectrum of molecular species in the frequency ranges 31.5–50 GHz (Q band) and 72–116.5 GHz (W band) in the laboratory. At the same time, we support these spectroscopic studies with high-level ab initio calculations. Finally, we use the derived experimental rotational parameters to allow the search of these molecular species in astrophysical environments.

We have complemented the GACELA experiments with other techniques like laser ablation molecular beam Fourier transform microwave (FTMW) spectroscopy at UVA, absorption spectroscopy and cryogenic trapping machine experiments.

Far below, we show a summary table on the performed experimental characterizations and techniques.

Gas cell for Laboratory Astrophysics (GACELA)

The Gas Cell for Laboratory Astrophysics (GACELA) consists of a stainless-steel chamber 1 meter long and a diameter of 60 cm. It is equipped with two teflon windows that allows the study of gases through rotational spectroscopy inside the chamber.

Hence, the team coupled the new NANOCOSMOS millimeter broad band receivers into the setup. These receivers are twins of those built for the Yebes 40 meter radio telescope. A series of vacuum chamber ports allow the injection of gas and liquids to perform plasma generation, ultraviolet photochemistry and optical spectroscopy. GACELA was built at the Segainvex Laboratories located at the Universidad Autónoma de Madrid.

Outstanding publications on our experimetal setup:

1) Broad-band high-resolution rotational spectroscopy for laboratory astrophysics  (J. Cernicharo, J. D. Gallego, J. A. López-Pérez, and 32 co-authors). Astronomy & Astrophysics, 2019 June; 626, A34. Published online 2019, June 7.

2) Using radio astronomical receivers for molecular spectroscopic characterization in astrochemical laboratory simulations: A proof of concept (I. Tanarro, B. Alemán, P. de Vicente, and 26 co-authors). Astronomy & Astrophysics, 2018 Jan; 609: A15. Published online 2017 Dec 22.

GACELA addresses an innovative potential to perform novel experiments on plasma physics, photochemistry and ices. We also address the spectroscopical characterization of a gas injected in the cell. Thus, we performed a first set of experiments in February 2018 with the detection of CH3CN in a few seconds with a very high signal-to-noise ratio (S/N). The whole system was further improved and we have made multiple runs in the full-experimental phase from May 2018.

Check our posts on the GACELA setup

HEMT receivers

The 40m radio telescope at the Yebes Observatory

Outstanding publications on our innovative development

Yebes 40 m radio telescope and the broad band NANOCOSMOS receivers at 7 mm and 3 mm for line surveys (F. Tercero, J. A. López-Pérez, J. D. Gallego and 23 co-authors, A&A, 01/2021)

Breakthroughs

  • Two new cryogenic receivers for the 31.5 − 50 GHz (Q frequency band) and the 72 − 90.5 GHz (W band).
  • A new optical circuit for the W band receiver with its mirrors, new mirrors for the Q band receiver, and a new hot-cold load calibration system.
  • Instantaneous frequency coverage to observe many molecular transitions with single tunings in single dish mode: 1) Optimization of the observing time; 2) Increase in the radio telescope output efficiency; 3) Boost in data sensitivity in comparison with previous Nobeyama (Japan) 45 m telescope surveys (less than 1 mK versus 5 mK in the Nobeyama data).

Nanocosmos has developed an experimental set-up, the Gas Cell for Laboratory Astrophysics –GACELA that operates under vacuum conditions, in order to mimick the molecular processes underlying chemical reactions of astrophysical interest. We are in particular interested in those processes occurring at the dust formation zone of AGB stars. 

We observe molecular processes in-situ by using the new NANOCOSMOS mm broad band radio astronomical receivers, which results advantageous in terms of spectral resolution and sensitivity. We have successfully applied this innovation at the 40 m radio telescope at the Yebes Observatory. Therefore, we have designed, constructed and commissioned new Q and W band receivers to foster the radio telescope capabilities and to provide wider bandwidth and better spectral resolution. The development of this instrumentation is a key aspect of the Nanocosmos project and is already providing outstanding results with the discovery of multiple molecular species.

Key Yebes internal reports to show the developments and upgrades of this instrumentation.