|#1 – BREAKTHROUGHS in CARBON CHEMISTRY in CIRCUMSTELLAR ENVELOPES (CSEs)|
Breakthroughs in our current understanding on the formation of carbonaceous dust and complex molecules in the circumstellar envelopes (CSEs) of AGB stars and the interstellar medium (ISM).
Production and analysis of carbon dust seeds in conditions resembling those in the dust formation zones of CSEs contrasting to all previous experiments in the field.
|Innovative experimental methodology|
1 – Production of carbon dust seeds at the Stardust Machine using exclusively gas-phase carbon atoms and molecular hydrogen in a ratio close to that in the atmospheres of AGB stars under ultra-high vacuum (UHV) conditions.
2 – Expansion of the previous study to investigate the interaction of atomic carbon and diatomic carbon with acetylene.
3 – First use of the sputtering gas aggregation source (SGAS) in Laboratory Astrophysics to generate small clusters of nanometre-sized particles by gas-phase aggregation of individual atoms in a weakly ionized environment, thus resembling what happens in the dust formation zones of CSEs.
4 – Full experimental analysis: atomic force microscopy (AFM), scanning tunnel microscopy (STM), transmission electron microscopy (TEM), optical emission spectroscopy (OES), infrared spectroscopy in transmission geometry and quadrupole mass spectrometry (QMS) at the Stardust Machine. Ex-situ laser desorption ionization/mass spectrometry (LDI-MS) in the AROMA machine that resembles the reactions on the surfaces of dust grains.
1) From experiment 1. Efficient production of carbonaceous nanometre-sized grains, nanometer-sized small amorphous carbon clusters, acetylene (C2H2), along with fragments of ethylene (C2H4), ethane (C2H6) and larger aliphatic molecules, saturated aliphatic species and marginal detection of aromatic species (benzene, small PAHs like naphthalene) and no fullerenes. We reproduce the abundances of the acetylene and ethylene found in CSEs around AGB stars.
2) From experiment 2. Production of a non-negligible amount of pure and hydrogenated carbon clusters as well as aromatics with aliphatic substitutions, both being a direct consequence of the addition of atomic carbon
Our experiments, that closely resemble the chemistry involved in the CSEs, do not favour the formation of aromatic species (PAHs and fullerenes), which can account for up to 18% of the total carbon species in the interstellar medium. We also show that aromatics with aliphatic substitutions as well as pure and hydrogenated carbon clusters can be produced as a direct consequence of the addition of atomic carbon.
1) SGAS, a technique not previously used in laboratory astrophysics, can be a very valuable tool to gain information on the chemistry operating in CSEs and the interstellar medium.
2) PAHs might not be efficiently formed during gas-phase growth in CSEs.
3) New theoretical plausible scenario: Thermal processing of aliphatic species deposited on dust grains in CSEs could lead to the formation of larger molecules or aromatic species. Such a temperature rise happens in later stages of stellar evolution (protoplanetary nebula PPNe) when the star emits UV radiation that leads to photo-processing of the carbon dust. Indeed, aromatic infrared bands, the signature for PAHs, are not convincingly detected in AGBs, but are observed at these later stages.
4) Unveiling of chemical routes: these results could unveil chemical routes leading to the formation of acetylene-based molecular species in the external layers of AGB stars and in PPNe, and to foster the search for alkyl-substituted aromatics in these environments.
– Prevalence of non-aromatic carbonaceous molecules in the inner regions of circumstellar envelopes (L. Martínez et al., Nature Astronomy, volume 4, pages 97–105, 2020), DOI link. EPMC link.
– A new take on circumstellar carbon chemistry (M. Gatchell, News and Views, Nature Astronomy, volume 4, pages 21 – 22, 2020), share link).
– The Chemistry of Cosmic Dust Analogs from C, C2, and C2H2 in C-rich Circumstellar Envelopes (G. Santoro et al., The Astrophysical Journal, volume 895, number 2, 2020). DOI link. EPMC link.
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.
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).
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.
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 have used GACELA to measure the rotational spectrum of 4 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 Fourier transform microwave (FTMW) spectroscopy at UVA (11 molecular compounds), absorption spectroscopy (2 molecular compounds) and cryogenic trapping machine experiments (7 protonated molecular species).
Below, we show a summary table on the performed experimental characterizations and techniques. We also include the Nanocosmos publication when clicking on the compound. For Polycyclic aromatic hydrocarbons (PAHs), proceed far below after the table (new page).
|Discovery of molecular species in IRC+10216||Methyl silane (CH3SiH3) and silyl cyanide (SiH3CN, first time in Space)||We propose a formation mechanism through catalytic reactions on the surface of dust grains by hydrogenation of silicon-carbon species in the inner dust formation zone|
|Formation of SiC dust in C-rich AGBs||SiC2, CS, SiO and SiS gas-phase precursors of dust||Decline in the abundances of these molecular species with increasing density in the envelopes of C-rich AGB stars. Important constraints for Stardust experiments on SiC dust formation.|
|Formation of dust in O-rich AGBs||SiO and SO gas-phase precursors of dust||Decline in the abundances of these molecular species with increasing density in the envelopes of O-rich AGB stars|
|Discovery of molecular species in the Interstellar Medium||7 molecules, including one protonated form and isotopologs (2 of them, first time in Space)||See dedicated descriptions below (under construction)|
NANOCOSMOS has performed several key observations of the circumnuclear envelopes -CSEs- of AGB stars with the IRAM 30m radio telescope and the ALMA interferometer. These observations are mandatory to foster the study of the gas-phase precursors of dust in these envelopes. We have made fruitful efforts in the study of the Si-C chemistry in these objects.
NANOCOSMOS has discovered methyl silane, CH3SiH3 and silyl cyanide (SiH3CN) in the envelope of the C-rich AGB star IRC +10216. We suggest that both are formed in the inner zones of the circumstellar envelope through catalytic reactions on the surface of dust grains by hydrogenation of silicon-carbon species.
We have also performed two molecular surveys with the IRAM facility, one to study the envelopes of 25 C-rich AGB stars to search for emission lines of SiC2, SiC, Si2C, CS, SiO and SiS and another one with a sample of 30 O-rich AGB stars to investigate the potential role of SiO, CS, SiS, SO, and SO2 in the formation of dust in these environments.
Our results show strong evidences that the observed decline in the molecular abundances of these species with increasing density in the envelopes are due to their incorporation to the solid phase. Furthermore, we establish that SiC2, CS, SiO and SiS (tentatively) are very likely gas-precursors of SiC dust in C-rich envelopes of AGB stars and SiO and SO (tentatively) in O-rich AGB stars.
Finally, the team has detected 7 molecules in the Interstellar Medium, some of them of key importance to constrain chemical models. These are the c-C3D isotopologs, the metastable and polar isomer isocyanogen (CNCN), the isocyanate radical NCO, the thioformyl radical (HCS) and its metastable isomer HSC, all of them in the dark cold cloud core L483, which contains a low-mass protostar. We have also detected ethyl formate (CH3CH2OCOH) and NS+ in the young protostellar system Barnard 1b with ALMA and IRAM respectively.