|#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.
|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.
|Discovery of molecular species in IRC+10216
|Major emission arises at 50 AU or less from the star in the dust formation zone. Constraints on chemical models
|Distribution of molecular emission in IRC+10216
|Part of the emission arises in the inner dust formation zone contrary to previous findings. Constraints on chemical models
|Detection of molecular emission in R Leo
|CO2 Infrared fluorescence
|More systematic study of the CO2 emission in O-rich stars to understand how CO2 forms
High spectral resolution infrared observations of circumstellar envelopes – CSEs – in AGB stars are essential to study important molecular species with no permanent dipole moment (e.g. H2, O2, CO2, SiH4, C2H4). This lack makes them undetectable in the millimeter range due to the absence of rotational transitions. Hence, the best possible observation of these molecules is through its vibration–rotation lines in the mid infrared range.
These observations are vital to study the amount of ejected matter in the pulsation phase and determine the chemical interactions between the ejected molecules in the CSEs. These studies help improve the underlying assumptions of currently available chemical models.
Therefore, we observed the carbon-rich star IRC+10216 with the Texas Echelon-cross-Echelle Spectrograph (TEXES) on the 3 m Infrared Telescope Facility (IRTF). We carried out observations of the oxygen rich star R Leo with the Stratospheric Observatory for Infrared Astronomy (SOFIA) with the high spectral resolution Echelon-cross-Echelle Spectrograph (EXES). Finally, we used both SOFIA/EXES and IRTF/TEXES to observe the carbon rich semi-regular star Y CVn.
Our IR observations have led to the discovery of diacetylene (C4H2) in the envelope of IRC+10216 with the major emission arising in the dust formation zone at less than 50 AU from the center of the star. Ethylene (C2H4) shows emission from the inner dust formation zone in IRC+10216 contrary to previous findings. These studies pose further constraints on current chemical models.
Summary of oustanding results with the NANOCOSMOS high-resolution infrared observations of CSEs in AGB stars
Multi-frequency high spectral resolution observations of HCN toward the circumstellar envelope of Y CVn (J. P. Fonfría et al., A&A, 07/2021)
- Analysis and Identification of 130 lines of HCN and H13CN with either P-Cygni profiles or pure absorption profiles
- Dust grains could be mostly made of silicon carbide SiC in the inner layers of the CSE (~ 3.5 stellar radii) and of amorphous carbon in the outer envelope (up to 200 stellar radii)
- The observed mid-IR lines are broader than expected due to possible high velocity matter ejections or photospheric movements related to stellar pulsation or convection.
- HCN rotational and vibrational temperatures are out of local thermodynamics equilibrium so collisions do not play any role in the gas thermalization
Detection of infrared fluorescence of carbon dioxide in R Leonis with SOFIA/EXES (J. P. Fonfría et al., A&A, 11/2020)
- CO2 (≃240 emission lines in the range 12.8−14.3 μm) – New detection in R Leo
- The observed CO2 lines can be grouped into three different populations, (warm, hot, and very hot), with approximate temperatures of 550, 1150, and 1600 K
- The CO2 emitting regions at 1600, 1150, and 550 K are located at 2.2, 3.5, and 10 stellar radii from the center of R Leo
- We need a systematic study of the CO2 emission in O-rich stars to understand how this molecule forms and the possible dependence of the column density on the mass-loss rate
Carbon Chemistry in IRC+10216: Infrared Detection of Diacetylene (J. P. Fonfría et al., ApJ, 01/2018)
- C4H2 (24 absorption features in the range 8.0 to 8.1 μm) – First detection in IRC+10216
- The major emission of C4H2 arises in the dust formation zone at radii lower than 20 stellar radii (50 Astronomical Units) from the center of IRC+10216
- Our photochemical models underestimate the observed C4H2 abundance. This finding could imply that the molecules in the envelope are photodissociated in shells closer to the star than is commonly assumed
- More info on diacetylene: Astromolecule of the Month
The Abundance of C2H4 in the Circumstellar Envelope of IRC+10216 (J. P. Fonfría et al., ApJ, 01/2017)
- C2H4 (80 ro-vibrational features in absorption) – Part of the emission arises in the inner dust formation zone contrary to previous findings
- Part of the emission of ethylene arises in the dust formation zone at radii between 14 and 28 stellar radii from the center of IRC+10216, with no evidence of C2H4 closer to the star. Previous findings supposed all C2H4 arises in the far outer envelopes.
- Our photochemical models underestimate the observed C2H4 terminal abundance by a factor of 4. We estimate that a fraction of the ethylene gas-phase could condense onto the dust grains around 20 stellar radii. this fact could affect the chemistry evolution of the envelope
For the first time, NANOCOSMOS has attempted to reproduce the complex molecular chemistry and stardust formation in circumstellar envelopes (CSEs) of asymptotic giant branch (AGB) stars and in cold molecular clouds under accurate and realistic laboratory conditions. These conditions differ from previous studies and techniques to produce stardust analogs, mostly based on laser ablation and pyrolysis, flames, and other far related conditions from those in the CSEs of AGB stars.
These achievements are based on groundbreaking innovative setups at CSIC and CNRS, e.g. Stardust, AROMA, PIRENEA 2 and cold plasma reactors, that foster the study of complex processes that lead to carbon dust formation including polycyclic aromatic hydrocarbons (PAHs) and fullerenes. We have studied the chemistry of atomic silicon and the formation of silicate dust grains. We have also investigated the aromatic content of two different meteorites, Murchison and Almahata Sitta.
In summary, our synergetic results provide significant and surprising breakthroughs in our current understanding of the chemical processes at play in CSEs, the interstellar medium (ISM) and meteoritic samples. These new and open theoretical grounds have also important implications in current chemical models. These NANOCOSMOS breakthroughs are the following:
- Aliphatic nature of carbonaceous cosmic dust analogs in CSEs. Our realistic laboratory conditions do not lead to the efficient formation of aromatic molecules (PAHs and fullerenes) in the gas phase, contrary to all previous studies (Stardust, AROMA)
- First detection of a pure PAH (indene) in the TMC-1 cold dark molecular cloud. This is totally an unexpected discovery and suggests an in-situ bottom-up formation process in these environments from smaller molecules in the gas-phase (Yebes 40m radio telescope + new mm receivers).
- Efficient mechanism for the formation of silane and disilane in the gas phase from Si, H, and H2 in the innermost regions of the CSEs around AGB stars (Stardust).
- Further evidence for the role of metal (iron) seeds to increase not only the formation of metal clusters but also catalyzed hydrocarbon growth in the CSEs of AGB stars (Cold plasma reactors, AROMA, PIRENEA 2 and ESPOIRS)
First firm detection of fullerenes in meteorites (Almahata Sitta) and co-existence of carbon clusters along with PAHs in this meteorite (AROMA).
NANOCOSMOS has performed a systematic work to identify “unknown” gas-phase species in circumstellar envelopes and interstellar molecular clouds. These species are the carriers of the so-called U-lines or unidentified features. Currently, the team has found more than 1,000 U-lines formed in the photosphere surroundings of the AGB carbon star IRC+10216. We are progressing in the identification of these carriers to complete a spectral catalogue and merge this catalogue into the MADEX radiative transfer code.
Definitely, NANOCOSMOS is characterizing different dust formation places in Space through the observations of molecular lines. So far, the team has already discovered 10 new molecular species in the inner regions of the envelope of IRC+10216 and 32 more in the interstellar medium until April 2021. These findings include the detection for the first time in Space of three new pure hydrocarbon cycles: c-C3HCCH (ethynyl cyclopropenylidene), c-C5H6 (cyclopentadiene), and the polycyclic aromatic hydrocarbon (PAH) c-C9H8 (indene). These hydrocarbons could be one of the keystones to elucidate bottom-up mechanisms in the formation of the first aromatic ring in cold molecular clouds, from which large PAHs may grow.
Our discoveries amount to approximately a 20% of known molecules in Space.
The team has made a joint effort on the following objects. IRC+10216 is the archetypal AGB carbon rich star, given its proximity ~ 130 pc, and unpaired molecular richness with more than 80 molecular species prior to NANOCOSMOS. R Leo is one of the closest AGB oxygen rich stars, at a distance of ~ 80 pc. Multi-wavelength molecular observations of this star show no detection of CO2 despite predictions from chemical models. Y CVn is a carbon rich semi-regular star at ~ 310 pc from the Earth. Its circumnuclear envelope has not been explored in detail mostly due to the lack of sensitive observations. Finally, the Taurus Molecular Cloud -1 or TMC-1, is a cold dark molecular cloud. It presents an interesting carbon-rich chemistry that leads to the formation of long neutral carbon-chain radicals and their anions, as well as cyanopolyynes, and protonated species of abundant large carbon chains.
This menu splits our stunning discoveries into the following entries:
- Yebes 40 m radio telescope new broad band receivers and complementary observations with the IRAM 30 m radio telescope. Multiple discoveries of molecular species in IRC+10216 and TMC-1. Further contraints on chemical models.
- High-resolution observations of the inner dust formation zone in IRC+10216 with the ALMA radio interferometer. Our analysis is ongoing.
- ALMA and IRAM 30m observations of the circumstellar envelopes of AGB stars, cold dark clouds and prestellar cores. These observations include two IRAM molecular surveys of 40 AGB stars. Several discoveries of molecular species in IRC+10216 and in other objects.
- High spectral resolution Infrared observations of R Leo with the airborne Stratospheric Observatory for Infrared Astronomy (SOFIA), IRC+10216 with the NASA Infrared Telescope Facility (IRTF) and Y CVn with both observatories.
The team is currently submitting new exciting results from the NANOCOSMOS legacy molecular survey of evolved stars with the Yebes 40m radio telescope. Also, we are interpreting new challenging results from ALMA high resolution observations of IRC+10216 that will boost our previous observations of this source. The best for NANOCOSMOS is yet to come!
IRC+10216: new molecules inventory
|7 or more
Interstellar Medium: new molecules inventory
|7 or more