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.

Yebes 40m broad band receivers

HighlightsFeaturesDescription
IRC+10216: Discovery of molecular speciesHC9N (first time in Space), HC7N, MgC3N and MgC4HFormation in the external layers of the circumstellar envelope
TMC-1: Discovery of molecular species25 molecules, including 1 PAH (indene), 2 hydrocarbon cycles, 3 protonated forms and 2 nitrile anions (15 of them, first time in Space)See dedicated descriptions below
Complementary laboratory experimentsHDCCN, CH3CO+, HC3S+ and HC3O+Laboratory production and full spectroscopical characterization
Table: Major NANOCOSMOS highlights with the “Yebes 40m broad band receivers” (see dedicated descriptions below)

NANOCOSMOS successfully designed and constructed two new millimeter broad band receivers for the Q frequency band (31.5 − 50 GHz) and the W band (72 − 90.5 GHz) for the Yebes 40m radio telescope. One of our main achievements is the instantaneous frequency coverage in order to observe many molecular transitions with single tunings in single-dish mode.

Our Yebes observations have led to the discovery of multiple molecular species. These were made in 2019 (Q2, Q4), and 2020 (Q1) and were complemented with the IRAM 30m radio telescope in Granada (W band).

  • Objects: IRC+10216 as the archetypal AGB carbon rich star and the cold dark Taurus Molecular Cloud -1 or TMC-1.
  • Methodology: observations, laboratory characterization, ab-initio quantum calculations, rotational diagrams, gas-phase chemical models and radiative transfer calculations.

Summary of results with the NANOCOSMOS mm broad band receivers at Yebes 40m radio telescope:

Detection of vibrationally excited HC7N and HC9N in IRC+10216 (J. R. Pardo et al., A&A, 08/2020)

  • HC9N (26 doublets, vib. exc.) – First time detection and characterization in Space
  • HC7N (17 doublets, vib. exc.) – New detection
  • Emission arising from the external layers of the circumstellar envelope
  • Possibe effects on intensity line variations with the stellar phase
  • These vibrationally excited states must be taken into account for precise abundance determinations of long carbon chains

MgCCCN (top) and MgCCCCH (bottom) optimized structures

Discovery of two new magnesium-bearing species in IRC+10216: MgC3N and MgC4H (J. Cernicharo et al., A&A, 10/2019)

  • MgC3N (16 doublets) – New detection
  • MgC4H (6 doublets) – New detection
  • MgCCH (2 doublets) – Confirmation

TMC-1, the starless core sulfur factory: Discovery of NCS, HCCS, H2CCS, H2CCCS, and C4S and detection of C5S (J. Cernicharo et al., A&A, 04/2021)

  • Sulfur-bearing species NCS, HCCS, H2CCS, H2CCCS, and C4S – First time discovery in Space (TMC-1)
  • C5S – First time detection in a cold dark cloud (TMC-1)
  • State-of-the-art gas-phase chemical networks fail to reproduce the observed column densities. Thus, important reactions involving S and S+(neutral-neutral, neutral-ion) and those on dust grain surfaces are missing and much laboratory and theoretical work need be performed in order to understand the chemistry of sulfur
  • The analysis of C4S and C5S shows that S-bearing carbon chains do not follow the smooth decrease in abundance observed in cold dark molecular clouds and circumstellar envelopes for other carbon chains such as cyanopolyynes (HC2n + 1N; a factor 3–5 between members of this molecular species)

Discovery of the propargyl radical (CH2CCH) in TMC-1: One of the most abundant radicals ever found and a key species for cyclization to benzene in cold dark clouds (M. Agúndez et al., A&A, 03/2021)

  • Propargyl radical (CH2CCH) 6 strongest hyperfine components of the 20, 2–10, 1 rotational transition – First time discovery in interstellarr Space (TMC-1)
  • Similar abundance as methyl acetylene. Thus, it is one of the most abundant radicals detected in TMC-1. Moreover, it is probably the most abundant organic radical with a certain chemical complexity ever found in a cold dark cloud
  • The observed high abundance, points out that CH2CCH probably plays a key role in the synthesis of large organic molecules and, in particular, the cyclization towards the first aromatic ring (benzene). This also happens in combustion processes where the CH2CCH radical has a key role in the synthesis of benzene and polycyclic aromatic hydrocarbons (PAHs).

Discovery of allenyl acetylene, H2CCCHCCH, in TMC-1. A study of the isomers of C5H4 (J. Cernicharo et al., A&A, 03/2021)

  • Allenyl Acetylene (H2CCCHCCH) 19 rotational transitions – First time detection in Space (TMC-1)
  • We find that allenyl acetylene and methyl diacetylene are the two most stable C5H4 isomers and both have similar observed abundances in TMC-1
  • State-of-the-art chemical models understimate the observed abundances by an order of magnitude. We need to assess the main formation routes in the chemical models, mainly the reactions of the CCH radical with methyl acetylene (CH3CCH) and allene (H2CCCH2)

Discovery of CH2CHCCH and detection of HCCN, HC4N, CH3CH2CN, and, tentatively, CH3CH2CCH in TMC-1 (J. Cernicharo et al., A&A, 03/2021)

  • Vynil Acetylene (CH2CHCCH) – First time detection in Space (TMC-1)
  • HCCN, HC4N, and CH3CH2CN – First time detection in a cold dark cloud (TMC-1)
  • Ethylene could be a likely precursor of CH2CHCCH and CH2CHCN through reactions with CCH and CN, respectively
  • The reaction between CN and vinyl acetylene is a viable route to the C5H3N isomers recently found in TMC-1 at very low temperatures
  • Our observations show that the cyano methylene radical HCCN and the linear cyano ethynyl-methylene radical HC4N have similar abundances unlike predictions from current chemical models

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