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.

ALMA/IRAM observations

Discovery of molecular species in IRC+10216Methyl 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 AGBsSiC2, CS, SiO and SiS gas-phase precursors of dustDecline 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 AGBsSiO and SO gas-phase precursors of dustDecline 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 Medium7 molecules, including one protonated form and isotopologs (2 of them, first time in Space)See dedicated descriptions below (under construction)
Major NANOCOSMOS highlights in “ALMA/IRAM observations” (see dedicated descriptions below)

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.

High-resolution Infrared Observations

Discovery of molecular species in IRC+10216Diacetylene (C4H2)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+10216Ethylene (C2H4)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 LeoCO2 Infrared fluorescence More systematic study of the CO2 emission in O-rich stars to understand how CO2 forms
Major NANOCOSMOS highlights in “High spectral resolution IR observations” (see dedicated descriptions below)

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, submitted)

  • 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

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