Authors: Habart, Emilie; Le Gal, Romane; Alvarez, Carlos; Peeters, Els; Berne, Olivier; Wolfire, Mark G.; Goicoechea, Javier R.; Schirmer, Thiebaut; Bron, Emeric; Roellig, Markus

Journal: ASTRONOMY & ASTROPHYSICS

Publication date: 2023/05/24

DOI: 10.1051/0004-6361/202244034

Abstract: Context. Nearby photo-dissociation regions (PDRs), where the gas and dust are heated by the far-ultraviolet (FUV) irradiation emitted from stars, are ideal templates with which to study the main stellar feedback processes. Aims. With this study, we aim to probe the detailed structures at the interfaces between ionized, atomic, and molecular gas in the Orion Bar. This nearby prototypical strongly irradiated PDR are among the first targets of the James Webb Space Telescope (JWST) within the framework of the PDRs4All Early Release Science program. Methods. We employed the subarcsecond resolution accessible with Keck-II NIRC2 and its adaptive optics system to obtain images of the vibrationally excited line H-2 1-0 S(1) at 2.12 mu m that are more detailed and complete than ever before. H-2 1-0 S(1) traces the dissociation front (DF), and the [FeII] and Br gamma lines, at 1.64 and 2.16 mu m, respectively, trace the ionization front (IF). The former is a powerful tracer of the FUV radiation field strength and gas density distribution at the PDR edge, while the last two trace the temperature and density distribution from the ionized gas to the PDR. We obtained narrow-band filter images in these key gas line diagnostics over similar to 40 at spatial scales of similar to 0.1 (similar to 0.0002 pc or similar to 40 AU at 414 pc). Results. The Keck/Near Infrared Camera 2 (NIRC2) observations spatially resolve a plethora of irradiated substructures such as ridges, filaments, globules, and proplyds. This portends what JWST should accomplish and how it will complement the highest resolution Atacama Large Millimeter/submillimeter Array (ALMA) maps of the molecular cloud. We observe a remarkable spatial coincidence between the H-2 1-0 S(1) vibrational and HCO+ J = 4-3 rotational emission previously obtained with ALMA. This likely indicates the intimate link between these two molecular species and highlights that in high-pressure PDRs, the H/H-2 and C+/C/CO transitions zones come closer than in a typical layered structure of a constant density PDR. The H/H-2 dissociation front appears as a highly structured region containing substructures with a typical thickness of a few similar to 10(-3) pc.

Authors: Agundez, M.; Loison, J. -c.; Hickson, K. M.; Wakelam, V.; Fuentetaja, R.; Cabezas, C.; Marcelino, N.; Tercero, B.; de Vicente, P.; Cernicharo, J.

Journal: ASTRONOMY & ASTROPHYSICS

Publication date: 2023/05/03

DOI: 10.1051/0004-6361/202346076

Abstract: We present the detection of ethanol (C2H5OH), acetone (CH3COCH3), and propanal (C2H5CHO) toward the cyanopolyyne peak of TMC-1. These three O-bearing complex organic molecules are known to be present in warm interstellar clouds but had never been observed in a starless core. The addition of these three new pieces to the puzzle of complex organic molecules in cold interstellar clouds highlights the rich chemical diversity of cold dense cores in stages prior to the onset of star formation. The detections of ethanol, acetone, and propanal were made in the framework of QUIJOTE, a deep line survey of TMC-1 in the Q band that is being carried out with the Yebes 40m telescope. We derive column densities of (1.1 +/- 0.3) x 10(12) cm(-2) for C2H5OH, (1.4 +/- 0.6) x 10(11) cm(-2) for CH3COCH3, and (1.9 +/- 0.7) x 10(11) cm(-2) for C2H5CHO. We investigated the formation of these three O-bearing complex organic molecules with the aid of a detailed chemical model that includes gas and ice chemistry. The calculated abundances at a time around 2 x 10(5) yr are in reasonable agreement with the values derived from the observations. The formation mechanisms of these molecules in our chemical model are as follows. Ethanol is formed on grains via the addition of atomic carbon on methanol followed by hydrogenation and nonthermal desorption. Acetone and propanal are produced by the gas-phase reaction between atomic oxygen and two different isomers of the C3H7 radical, which itself forms from the hydrogenation of C-3 on grains followed by nonthermal desorption. A gas-phase route involving the formation of (CH3)(2)COH+ through several ion-neutral reactions followed by its dissociative recombination with electrons also contributes to the formation of acetone.

Authors: Quintana-Lacaci, G.; Velilla-Prieto, L.; Agundez, M.; Fonfria, J. P.; Cernicharo, J.; Decin, L.; Castro-Carrizo, A.

Journal: ASTRONOMY & ASTROPHYSICS

Publication date: 2023/05/01

DOI: 10.1051/0004-6361/202244396

Abstract: Context. Red supergiant stars (RSGs, M-init=10-40 M-circle dot) are known to eject large amounts of material, as much as half of their initial mass during this evolutionary phase. However, the processes powering the mass ejection in low- and intermediate-mass stars do not work for RSGs and the mechanism that drives the ejection remains unknown. Different mechanisms have been proposed as responsible for this mass ejection including Alfven waves, large convective cells, and magnetohydrodynamical (MHD) disturbances at the photosphere, but so far little is known about the actual processes taking place in these objects. Aims. Here we present high angular resolution interferometric ALMA maps of VY CMa continuum and molecular emission, which resolve the structure of the ejecta with unprecedented detail. The study of the molecular emission from the ejecta around evolved stars has been shown to be an essential tool in determining the characteristics of the mass loss ejections. Our aim is thus to use the information provided by these observations to understand the ejections undergone by VY CMa and to determine their possible origins. Methods. We inspected the kinematics of molecular emission observed. We obtained position-velocity diagrams and reconstructed the 3D structure of the gas traced by the different species. It allowed us to study the morphology and kinematics of the gas traced by the different species surrounding VY CMa. Results. Two types of ejecta are clearly observed: extended, irregular, and vast ejecta surrounding the star that are carved by localized fast outflows. The structure of the outflows is found to be particularly flat. We present a 3D reconstruction of these outflows and proof of the carving. This indicates that two different mass loss processes take place in this massive star. We tentatively propose the physical cause for the formation of both types of structures. These results provide essential information on the mass loss processes of RSGs and thus of their further evolution.

Authors: Agundez, M.; Roncero, O.; Marcelino, N.; Cabezas, C.; Tercero, B.; Cernicharo, J.

Journal: ASTRONOMY & ASTROPHYSICS

Publication date: 2023/04/28

DOI: 10.1051/0004-6361/202346279

Abstract: We carried out an observational search for the recently discovered molecule H2NC, and its more stable isomer H2CN, toward eight cold dense clouds (L1544, L134N, TMC-2, Lupus-1A, L1489, TMC-1 NH3, L1498, and L1641N) and two diffuse clouds (B0415+379 and B0355+508) in an attempt to constrain its abundance in different types of interstellar regions and shed light on its formation mechanism. We detected H2NC in all but one of the cold dense clouds targeted, while H2CN was only detected in five out of the eight clouds. The column densities derived for both H2NC and H2CN are in the range 10(11)-10(12) cm(-2), and the abundance ratio H2NC/H2CN varies between 0.51 and >2.7. The metastable isomer H2NC is therefore widespread in cold dense clouds, where it is present with an abundance similar to that of H2CN. We did not detect H2NC or H2CN in any of the two diffuse clouds targeted, meaning we can make no conclusions regarding how the chemistry of H2NC and H2CN varies between dense and diffuse clouds. We find that the column density of H2NC is correlated with that of NH3, which strongly suggests that these two molecules are chemically linked, ammonia most likely being a precursor of H2NC through the C + NH3 reaction. We performed electronic structure and statistical calculations that show that both H2CN and H2NC can be formed in the C + NH3 reaction through two different channels involving two different transition states that are very close in energy. The predicted product branching ratio H2NC/H2CN is very method dependent, but values between 0.5 and 0.8 are the most likely. Therefore, both the astronomical observations and the theoretical calculations support the reaction C + NH3 being the main source of H2NC in interstellar clouds.

Authors: Cernicharo, J.; Cabezas, C.; Pardo, J. R.; Agundez, M.; Roncero, O.; Tercero, B.; Marcelino, N.; Guelin, M.; Endo, Y.; de Vicente, P.

Journal: ASTRONOMY & ASTROPHYSICS

Publication date: 2023/04/21

DOI: 10.1051/0004-6361/202346467

Abstract: We found four series of harmonically related lines in IRC +10216 with the Yebes 40 m and IRAM 30 m telescopes. The first series corresponds to a molecule with a rotational constant, B, of 1448.5994 +/- 0.0013 MHz and a distortion constant, D, of 63.45 +/- 1.15 Hz and covers upper quantum numbers from J(u) = 11 up to 33 (B1449). The second series is fitted with B = 1446.9380 +/- 0.0098 MHz and D = 91 +/- 23 Hz and covers upper quantum numbers from J(u) = 11 up to 17 (B1447). The third series is fitted with B = 598.7495 +/- 0.0011 MHz and D = 6.13 +/- 0.43 Hz and covers quantum numbers from J(u) = 26 up to 41 (B599). Finally, the frequencies of the last series of lines can be reproduced with B = 594.3176 +/- 0.0026 MHz and D = 4.92 +/- 1.16 Hz (B594). The large values of D point toward four metal-bearing carriers. After exploring all plausible candidates containing Na, Al, Mg, and other metals, our ab initio calculations indicate that the cations MgC4H+, MgC3N+, MgC6H+, and MgC5N+ must be the carriers of B1449, B1447, B599, and B594, respectively. These cations could be formed by the radiative association of Mg+ with C4H, C3N, C6H, and C5N, respectively. We calculated the radiative association rate coefficient of Mg+ with C4H, C3N, C6H, and C5N and incorporated them in our chemical model. The results confirm that the Mg-bearing cations can be formed through these radiative association reactions in the outer layers of IRC +10216. This is the first time that cationic metal-bearing species have been found in space. These results provide a new paradigm on the reactivity of ionized metals with abundant radicals and open the door for further characterization of similar species in metal-rich astrophysical environments.