arXiv subjects

Shaoshan Zeng

Publications and source records attributed to Shaoshan Zeng.

32 records · Page 2Linked to original sources

arXiv2023

Discovery of the elusive carbonic acid (HOCOOH) in space

After a quarter century since the detection of the last interstellar carboxylic acid, acetic acid (CH$_3$COOH), we report the discovery of a new one, the cis-trans form of carbonic acid (HOCOOH), toward the Galactic Center molecular cloud G+0.693-0.027. HOCOOH stands as the first interstellar molecule containing three oxygen atoms and also the third carboxylic acid detected so far in the interstellar medium. Albeit the limited available laboratory measurements (up to 65 GHz), we have also identified several pairs of unblended lines directly in the astronomical data (between 75-120 GHz), which allowed us to slightly improve the set of spectroscopic constants. We derive a column density for cis-trans HOCOOH of $N$ = (6.4 $\pm$ 0.4) $\times$ 10$^{12}$ cm$^{-2}$, which yields an abundance with respect to molecular H$_2$ of 4.7 $\times$ 10$^{-11}$. Meanwhile, the extremely low dipole moment (about fifteen times lower) of the lower-energy conformer, cis-cis HOCOOH, precludes its detection. We obtain an upper limit to its abundance with respect to H$_2$ of $\leq$ 1.2 $\times$10$^{-9}$, which suggests that cis-cis HOCOOH might be fairly abundant in interstellar space, although it is nearly undetectable by radio astronomical observations. We derive a cis-cis/cis-trans ratio $\leq$ 25, consistent with the smaller energy difference between both conformers compared with the relative stability of trans- and cis-formic acid (HCOOH). Finally, we compare the abundance of these acids in different astronomical environments, further suggesting a relationship between the chemical content found in the interstellar medium and the chemical composition of the minor bodies of the Solar System, which could be inherited during the star formation process.

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arXiv2023

H2CN/H2NC abundance ratio: a new potential temperature tracer for the interstellar medium

The ${\rm H_2NC}$ radical is the high-energy metastable isomer of ${\rm H_2CN}$ radical, which has been recently detected for the first time in the interstellar medium towards a handful of cold galactic sources, besides a warm galaxy in front of the PKS 1830-211 quasar. These detections have shown that the ${\rm H_2CN}$/${\rm H_2NC}$ isomeric ratio, likewise the HCN/HNC ratio, might increase with the kinetic temperature ($T_{\rm kin}$), but the shortage of them in warm sources still prevents us to confirm this hypothesis and shed light about their chemistry. In this work, we present the first detection of ${\rm H_2CN}$ and ${\rm H_2NC}$ towards a warm galactic source, the G+0.693-0.027 molecular cloud (with $T_{\rm kin} > 70 \, {\rm K}$), using IRAM 30m observations. We have detected multiple hyperfine components of the $N_{K_\text{a}K_\text{c}} = 1_{01} - 0_{00}$ and $2_{02} - 1_{01}$ transitions. We derived molecular abundances with respect to ${\rm H_2}$ of (6.8$\pm$1.3)$\times 10^{-11}$ for ${\rm H_2CN}$ and of (3.1$\pm$0.7)$\times 10^{-11}$ for ${\rm H_2NC}$, and a ${\rm H_2CN}$/${\rm H_2NC}$ abundance ratio of 2.2$\pm$0.5. These detections confirm that the ${\rm H_2CN}$/${\rm H_2NC}$ ratio is $\gtrsim$2 for sources with $T_{\rm kin} > 70 \, {\rm K}$, larger than the $\sim$1 ratios previously found in colder cores ($T_{\rm kin}\sim10 \, {\rm K}$). This isomeric ratio dependence with temperature cannot be fully explained with the currently proposed gas-phase formation and destruction pathways. Grain surface reactions, including the ${\rm H_2NC} \rightarrow {\rm H_2CN}$ isomerization, deserve consideration to explain the higher isomeric ratios and ${\rm H_2CN}$ abundances observed in warm sources, where the molecules can be desorbed into the gas phase through thermal and/or shock-induced mechanisms.

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arXiv2022

Molecular precursors of the RNA-world in space: new nitriles in the G+0.693-0.027 molecular cloud

Nitriles play a key role as molecular precursors in prebiotic experiments based on the RNA-world scenario for the origin of life. These chemical compounds could have been partially delivered to the young Earth from extraterrestrial objects, stressing the importance of establishing the reservoir of nitriles in the interstellar medium. We report here the detection towards the molecular cloud G+0.693-0.027 of several nitriles, including cyanic acid (HOCN), and three C$_4$H$_3$N isomers (cyanoallene, CH$_2$CCHCN; propargyl cyanide, HCCCH$_2$CN; and cyanopropyne (CH$_3$CCCN), and the tentative detections of cyanoformaldehyde (HCOCN), and glycolonitrile (HOCH$_2$CN). We have also performed the first interstellar search of cyanoacetaldehyde (HCOCH$_2$CN), which was not detected. Based on the derived molecular abundances of the different nitriles in G+0.693-0.027 and other interstellar sources, we have discussed their formation mechanisms in the ISM. We propose that the observed HOCN abundance in G+0.693-0.027 is mainly due to surface chemistry and subsequent shock-induced desorption, while HCOCN might be mainly formed through gas-phase chemistry. In the case of HOCH$_2$CN, several grain-surface routes from abundant precursors could produce it. The derived abundances of the three C$_4$H$_3$N isomers in G+0.693-0.027 are very similar, and also similar to those previously reported in the dark cold cloud TMC-1. This suggests that the three isomers are likely formed through gas-phase chemistry from common precursors, possibly unsaturated hydrocarbons (CH$_3$CCH and CH$_2$CCH$_2$) that react with the cyanide radical (CN). The rich nitrile feedstock found towards G+0.693-0.027 confirms that interstellar chemistry is able to synthesize in space molecular species that could drive the prebiotic chemistry of the RNA-world.

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arXiv2022

Precursors of fatty alcohols in the ISM: Discovery of n-propanol

Theories on the origins of life propose that early cell membranes were synthesized from amphiphilic molecules simpler than phospholipids such as fatty alcohols. The discovery in the interstellar medium (ISM) of ethanolamine, the simplest phospholipid head group, raises the question whether simple amphiphilic molecules are also synthesized in space. We investigate whether precursors of fatty alcohols are present in the ISM. For this, we have carried out a spectral survey at 7, 3, 2 and 1 mm toward the Giant Molecular Cloud G+0.693-0.027 located in the Galactic Center using the IRAM 30m and Yebes 40m telescopes. Here, we report the detection in the ISM of the primary alcohol n-propanol (in both conformers Ga-n-C3H7OH and Aa-n-C3H7OH), a precursor of fatty alcohols. The derived column densities of n-propanol are (5.5+-0.4)x10^13 cm^-2 for the Ga conformer and (3.4+-0.3)x10^13 cm^-2 for the Aa conformer, which imply molecular abundances of (4.1+-0.3)x10^-10 for Ga-n-C3H7OH and of (2.5+-0.2)x10^-10 for Aa-n-C3H7OH. We also searched for the AGa conformer of n-butanol (AGa-n-C4H9OH) without success yielding an upper limit to its abundance of <4.1x10^-11. The inferred CH3OH:C2H5OH:C3H7OH:C4H9OH abundance ratios go as 1:0.04:0.006:<0.0004 toward G+0.693-0.027, i.e. they decrease roughly by one order of magnitude for increasing complexity. We also report the detection of both syn and anti conformers of vinyl alcohol, with column densities of (1.11+-0.08)x10^14 cm^-2 and (1.3+-0.4)x10^13 cm^-2, and abundances of (8.2+-0.6)x10^-10 and (9.6+-3.0)x10^-11, respectively. The detection of n-propanol, together with the recent discovery of ethanolamine in the ISM, opens the possibility that precursors of lipids according to theories of the origin of life, could have been brought to Earth from outer space.

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arXiv2022

Precursors of the RNA-world in space: Detection of ($Z$)-1,2-ethenediol in the interstellar medium, a key intermediate in sugar formation

We present the first detection of ($Z$)-1,2-ethenediol, (CHOH)$_2$, the enol form of glycolaldehyde, in the interstellar medium towards the G+0.693-0.027 molecular cloud located in the Galactic Center. We have derived a column density of (1.8$\pm$0.1)$\times$10$^{13}$ cm$^{-2}$, which translates into a molecular abundance with respect to molecular hydrogen of 1.3$\times$10$^{-10}$. The abundance ratio between glycolaldehyde and ($Z$)-1,2-ethenediol is $\sim$5.2. We discuss several viable formation routes through chemical reactions from precursors such as HCO, H$_2$CO, CHOH or CH$_2$CHOH. We also propose that this species might be an important precursor in the formation of glyceraldehyde (HOCH$_2$CHOHCHO) in the interstellar medium through combination with the hydroxymethylene (CHOH) radical.

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arXiv2022

Ionise hard: interstellar PO$^{+}$ detection

We report the first detection of the phosphorus monoxide ion (PO$^{+}$) in the interstellar medium. Our unbiased and very sensitive spectral survey towards the G+0.693$-$0.027 molecular cloud covers four different rotational transitions of this molecule, two of which ($J$=1$-$0 and $J$=2$-$1) appear free of contamination from other species. The fit performed, assuming Local Thermodynamic Equilibrium conditions, yields a column density of $N$=(6.0$\pm$0.7)$\times$10$^{11}$ cm$^{-2}$. The resulting molecular abundance with respect to molecular hydrogen is 4.5$\times$10$^{-12}$. The column density of PO$^{+}$ normalised by the cosmic abundance of P is larger than those of NO$^{+}$ and SO$^{+}$, normalised by N and S, by factors of 3.6 and 2.3, respectively. The $N$(PO$^{+}$)/$N$(PO) ratio is 0.12$\pm$0.03, more than one order of magnitude higher than those of $N$(SO$^{+}$)/$N$(SO) and $N$(NO$^{+}$)/$N$(NO). These results indicate that P is more efficiently ionised in the ISM than N and S. We have performed new chemical models that confirm that the PO$^+$ abundance is strongly enhanced in shocked regions with high values of cosmic-ray ionisation rates (10$^{-15}-$10$^{-14}$ s$^{-1}$), as occurs in the G+0.693$-$0.027 molecular cloud. The shocks sputter the interstellar icy grain mantles, releasing into the gas phase most of their P content, mainly in the form of PH$_3$, which is converted into atomic P, and then ionised efficiently by cosmic rays, forming P$^+$. Further reactions with O$_2$ and OH produce PO$^{+}$. The cosmic-ray ionisation of PO might also contribute significantly, which would explain the high $N$(PO$^{+}$)/$N$(PO) observed. The relatively high gas-phase abundance of PO$^{+}$ with respect to other P-bearing species stresses the relevance of this species in the interstellar chemistry of P.

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arXiv2021

Probing the chemical complexity of amines in the ISM: detection of vinylamine (C$_2$H$_3$NH$_2$) and tentative detection of ethylamine (C$_2$H$_5$NH$_2$)

Amines, in particular primary amines (R-NH$_2$) are closely related to the primordial synthesis of amino acids since they share the same structural backbone. However, only limited number of amines has been identified in the ISM which prevents us from studying their chemistry as well as their relation to pre-biotic species that could lead to the emergence of life. In this letter, we report the first interstellar detection of vinylamine (C$_2$H$_3$NH$_2$) and tentative detection of ethylamine (C$_2$H$_5$NH$_2$) towards the Galactic Centre cloud G+0.693-0.027. The derived abundance with respect to H$_2$ is (3.3$\pm$0.4)$\times$10$^{-10}$ and (1.9$\pm$0.5)$\times$10$^{-10}$, respectively. The inferred abundance ratios of C$_2$H$_3$NH$_2$ and C$_2$H$_5$NH$_2$ with respect to methylamine (CH$_3$NH$_2$) are $\sim$0.02 and $\sim$0.008 respectively. The derived abundance of C$_2$H$_3$NH$_2$, C$_2$H$_5$NH$_2$ and several other NH$_2$-bearing species are compared to those obtained towards high-mass and low-mass star-forming regions. Based on recent chemical and laboratory studies, possible chemical routes for the interstellar synthesis of C$_2$H$_3$NH$_2$ and C$_2$H$_5$NH$_2$ are discussed.

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arXiv2021

Discovery in space of ethanolamine, the simplest phospholipid head group

Cell membranes are a key element of life because they keep the genetic material and metabolic machinery together. All present cell membranes are made of phospholipids, yet the nature of the first membranes and the origin of phospholipids are still under debate. We report here the first detection in space of ethanolamine, NH$_2$CH$_2$CH$_2$OH, which forms the hydrophilic head of the simplest and second most abundant phospholipid in membranes. The molecular column density of ethanolamine in interstellar space is $N$=(1.51$\pm$0.07)$\times$10$^{13}$ cm$^{-2}$, implying a molecular abundance with respect to H$_2$ of (0.9-1.4)$\times$10$^{-10}$. Previous studies reported its presence in meteoritic material but they suggested that it is synthesized in the meteorite itself by decomposition of amino acids. However, we find that the proportion of the molecule with respect to water in the interstellar medium is similar to the one found in the meteorite (10$^{-6}$). These results indicate that ethanolamine forms efficiently in space and, if delivered onto early Earth, it could have contributed to the assembling and early evolution of primitive membranes.

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arXiv2021

Thiols in the ISM: first detection of HC(O)SH and confirmation of C$_2$H$_5$SH

The chemical compounds carrying the thiol group (-SH) have been considered essential in recent prebiotic studies regarding the polymerization of amino acids. We have searched for this kind of compounds toward the Galactic Centre quiescent cloud G+0.693-0.027. We report the first detection in the interstellar space of the trans-isomer of monothioformic acid (t-HC(O)SH) with an abundance of $\sim\,$1$\,\times\,$10$^{-10}$. Additionally, we provide a solid confirmation of the gauche isomer of ethyl mercaptan (g-C$_2$H$_5$SH) with an abundance of $\sim\,$3$\,\times\,$10$^{-10}$, and we also detect methyl mercaptan (CH$_3$SH) with an abundance of $\sim\,$5$\,\times\,$10$^{-9}$. Abundance ratios were calculated for the three SH-bearing species and their OH-analogues, revealing similar trends between alcohols and thiols with increasing complexity. Possible chemical routes for the interstellar synthesis of t-HC(O)SH, CH$_3$SH and C$_2$H$_5$SH are discussed, as well as the relevance of these compounds in the synthesis of prebiotic proteins in the primitive Earth.

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arXiv2021

The Perseus ALMA Chemistry Survey (PEACHES). I. The Complex Organic Molecules in Perseus Embedded Protostars

To date, about two dozen low-mass embedded protostars exhibit rich spectra with lines of complex organic molecule (COM). These protostars seem to possess different enrichment in COMs. However, the statistics of COM abundance in low-mass protostars are limited by the scarcity of observations. This study introduces the Perseus ALMA Chemistry Survey (PEACHES), which aims at unbiasedly characterizing the chemistry of COMs toward the embedded (Class 0/I) protostars in the Perseus molecular cloud. Of 50 embedded protostars surveyed, 58% of them have emission from COMs. A 56%, 32%, and 40% of the protostars have CH$_3$OH, CH$_3$OCHO, and N-bearing COMs, respectively. The detectability of COMs depends neither on the averaged continuum brightness temperature, a proxy of the H$_2$ column density, nor on the bolometric luminosity and the bolometric temperature. For the protostars with detected COMs, CH$_3$OH has a tight correlation with CH$_3$CN, spanning more than two orders of magnitude in column densities normalized by the continuum brightness temperature, suggesting a chemical relation between CH$_3$OH and CH$_3$CN and a large chemical diversity in the PEACHES samples at the same time. A similar trend with more scatter is also found between all identified COMs, hinting at a common chemistry for the sources with COMs. The correlation between COMs is insensitive to the protostellar properties, such as the bolometric luminosity and the bolometric temperature. The abundance of larger COMs (CH$_3$OCHO and CH$_3$OCH$_3$) relative to that of smaller COMs (CH$_3$OH and CH$_3$CN) increases with the inferred gas column density, hinting at an efficient production of complex species in denser envelopes.

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arXiv2020

Prebiotic precursors of the primordial RNA world in space: Detection of NH$_{2}$OH

One of the proposed scenarios for the origin of life is the primordial RNA world, which considers that RNA molecules were likely responsible for the storage of genetic information and the catalysis of biochemical reactions in primitive cells, before the advent of proteins and DNA. In the last decade, experiments in the field of prebiotic chemistry have shown that RNA nucleotides can be synthesized from relatively simple molecular precursors, most of which have been found in space. An important exception is hydroxylamine, NH$_2$OH, which, despite several observational attempts, it has not been detected in space yet. Here we present the first detection of NH$_2$OH in the interstellar medium towards the quiescent molecular cloud G+0.693-0.027 located in the Galactic Center. We have targeted the three groups of transitions from the $J$=2$-$1, 3$-$2, and 4$-$3 rotational lines, detecting 5 transitions that are unblended or only slightly blended. The derived molecular abundance of NH$_2$OH is (2.1$\pm$0.9)$\times$10$^{-10}$. From the comparison of the derived abundance of NH$_2$OH and chemically related species, with those predicted by chemical models and measured in laboratory experiments, we favor the formation of NH$_2$OH in the interstellar medium via hydrogenation of NO on dust grain surfaces, with possibly a contribution of ice mantle NH$_3$ oxidation processes. Further laboratory studies and quantum chemical calculations are needed to completely rule out the formation of NH$_2$OH in the gas phase.

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arXiv2020

Propargylimine in the laboratory and in space: millimetre-wave spectroscopy and first detection in the ISM

Small imines containing up to three carbon atoms are present in the interstellar medium. As alkynyl compounds are abundant in this medium, propargylimine thus represents a promising candidate for a new interstellar detection. The goal of the present work is to perform a comprehensive laboratory investigation of the rotational spectrum of propargylimine in its ground vibrational state in order to obtain a highly precise set of rest-frequencies and to search it in space. The rotational spectra of $E$ and $Z$ geometrical isomers of propargylimine have been recorded in laboratory in the 83-500 GHz frequency interval. The measurements have been performed using a source-modulation millimetre-wave spectrometer equipped with a pyrolysis system for the production of unstable species. High-level ab initio calculations were performed to assist the analysis and to obtain reliable estimates for an extended set of spectroscopic quantities. The improved spectral data allow us to perform a successful search for this new imine in the quiescent G+0.693-0.027 molecular cloud. Eighteen lines of $Z$-propargylimine have been detected at level $>2.5 σ$, resulting in a column density estimate of $N = (0.24\pm 0.02)\times 10^{14}$ cm$^{-2}$. An upper limit was retrieved for the higher-energy $E$ isomer, which was not detected in the data. The fractional abundance (w.r.t. H$_2$) derived for $Z$-propargylimine is $1.8\times 10^{-10}$. We discuss the possible formation routes by comparing the derived abundance with those measured in the source for possible chemical precursors.

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arXiv2020

CAB: Towards the RNA-world in the interstellar medium -- detection of urea, and search of 2-amino-oxazole and simple sugars

In the past decade, Astrochemistry has witnessed an impressive increase in the number of detections of complex organic molecules. Some of these species are of prebiotic interest such as glycolaldehyde, the simplest sugar, or amino acetonitrile, a possible precursor of glycine. Recently, we have reported the detection of two new nitrogen-bearing complex organics, glycolonitrile and Z-cyanomethanimine, known to be intermediate species in the formation process of ribonucleotides within theories of a primordial ribonucleic acid (RNA)-world for the origin of life. In this paper, we present deep and high-sensitivity observations toward two of the most chemically rich sources in the Galaxy: a Giant Molecular Cloud in the center of the Milky Way (G+0.693-0.027) and a proto-Sun (IRAS16293-2422 B). Our aim is to explore whether the key precursors considered to drive the primordial RNA-world chemistry, are also found in space. Our high-sensitivity observations reveal that urea is present in G+0.693-0.027 with an abundance of about 5x10-11. This is the first detection of this prebiotic species outside a star-forming region. Urea remains undetected toward the proto-Sun IRAS16293-2422 B (upper limit to its abundance of less than 2x10-11). Other precursors of the RNA-world chemical scheme such as glycolaldehyde or cyanamide are abundant in space, but key prebiotic species such as 2- amino-oxazole, glyceraldehyde or dihydroxyacetone are not detected in either source. Future more sensitive observations targeting the brightest transitions of these species will be needed to disentangle whether these large prebiotic organics are certainly present in space.

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arXiv2019

A SiO J = 5 - 4 Survey Toward Massive Star Formation Regions

We performed a survey in the SiO $J=5\rightarrow4$ line toward a sample of 199 Galactic massive star-forming regions at different evolutionary stages with the SMT 10 m and CSO 10.4 m telescopes. The sample consists of 44 infrared dark clouds (IRDCs), 86 protostellar candidates, and 69 young \HII\ regions. We detected SiO $J=5\rightarrow4$ line emission in 102 sources, with a detection rate of 57\%, 37\%, and 65\% for IRDCs, protostellar candidates, and young \HII\ regions, respectively. We find both broad line with Full Widths at Zero Power (FWZP) $>$ 20 \kms and narrow line emissons of SiO in objects at various evolutionary stages, likely associated with high-velocity shocks and low-velocity shocks, respectively. The SiO luminosities do not show apparent differences among various evolutionary stages in our sample. We find no correlation between the SiO abundance and the luminosity-to-mass ratio, indicating that the SiO abundance does not vary significantly in regions at different evolutionary stages of star formation.

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