Abstract
We analyze HCN and HNC emission in the nearby starburst galaxy NGC 253 to investigate its effectiveness in tracing heating processes associated with star formation. This study uses multiple ...HCN and HNC rotational transitions observed using the Atacama Large Millimeter/submillimeter Array via the ALCHEMI Large Program. To understand the conditions and associated heating mechanisms within NGC 253's dense gas, we employ Bayesian nested sampling techniques applied to chemical and radiative transfer models, which are constrained using our HCN and HNC measurements. We find that the volume density
n
H
2
and cosmic-ray ionization rate (CRIR)
ζ
are enhanced by about an order of magnitude in the galaxy’s central regions as compared to those further from the nucleus. In NGC 253's central giant molecular clouds (GMCs), where observed HCN/HNC abundance ratios are the lowest,
n
∼ 10
5.5
cm
−3
and
ζ
∼ 10
−12
s
−1
(greater than 10
4
times the average Galactic rate). We find a positive correlation in the association of both density and CRIR with the number of star formation-related heating sources (supernova remnants, H
ii
regions, and super hot cores) located in each GMC, as well as a correlation between CRIRs and supernova rates. Additionally, we see an anticorrelation between the HCN/HNC ratio and CRIR, indicating that this ratio will be lower in regions where
ζ
is higher. Though previous studies suggested HCN and HNC may reveal strong mechanical heating processes in NGC 253's CMZ, we find cosmic-ray heating dominates the heating budget, and mechanical heating does not play a significant role in the HCN and HNC chemistry.
ABSTRACT
Hydroxylamine, NH2OH, is one of the already detected interstellar molecules with the highest prebiotic potential. Yet, the abundance of this molecule found by astronomical observations is ...rather low for a relatively simple molecule, ∼10−10 relative to H2. This seemingly low abundance can be rationalized by destruction routes operating on interstellar dust grains. In this work, we tested the viability of this hypothesis under several prisms, finding that the origin of a lower abundance of NH2OH can be explained by two chemical processes, one operating at low temperature (10 K) and the other at intermediate temperature (20 K). At low temperatures, enabling the hydrogen abstraction reaction HNO + H → NO + H2, even in small amounts, partially inhibits the formation of NH2OH through successive hydrogenation of NO, and reduces its abundance on the grains. We found that enabling a 15–30 per cent of binding sites for this reaction results in reductions of NH2OH abundance of approximately one to two orders of magnitude. At warmer temperatures (20 K, in our study), the reaction NH2OH + H → HNOH + H2, which was found to be fast (k ∼ 106 s−1) in this work, followed by further abstractions by adsorbates that are immobile at 10 K (O, N) are the main route of NH2OH destruction. Our results shed light on the abundance of hydroxylamine in space and pave the way to constraining the subsequent chemistry experienced by this molecule and its derivatives in the interstellar prebiotic chemistry canvas.
Abstract
We present the first detection of (
Z
)-1,2-ethenediol, (CHOH)
2
, the enol form of glycolaldehyde, in the interstellar medium toward the G+0.693−0.027 molecular cloud located in the ...Galactic Center. We have derived a column density of (1.8 ± 0.1) × 10
13
cm
−2
, which translates into a molecular abundance with respect to molecular hydrogen of 1.3 × 10
−10
. The abundance ratio between glycolaldehyde and (
Z
)-1,2-ethenediol is ∼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.
Significance
The detection of ethanolamine (
N
H
2
C
H
2
C
H
2
OH) in a molecular cloud in the interstellar medium confirms that a precursor of phospholipids is efficiently formed by interstellar ...chemistry. Hence, ethanolamine could have been transferred from the proto-Solar nebula to planetesimals and minor bodies of the Solar System and thereafter to our planet. The prebiotic availability of ethanolamine on early Earth could have triggered the formation of efficient and permeable amphiphilic molecules such as phospholipids, thus playing a relevant role in the evolution of the first cellular membranes needed for the emergence of life.
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 presence of ethanolamine in space,
N
H
2
C
H
2
C
H
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
±
0.07)
×
10
13
c
m
−
2
, implying a molecular abundance with respect to
H
2
of
(
0.9
−
1.4
)
×
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, could have contributed to the assembling and early evolution of primitive membranes.
Abstract
The centers of starburst galaxies may be characterized by a specific gas and ice chemistry due to their gas dynamics and the presence of various ice desorption mechanisms. This may result in ...a peculiar observable composition. We analyse the abundances of CO
2
, a reliable tracer of ice chemistry, from data collected as part of the Atacama Large Millimeter/submillimeter Array large program ALCHEMI, a wide-frequency spectral scan toward the starburst galaxy NGC 253 with an angular resolution of 1.″6. We constrain the CO
2
abundances in the gas phase using its protonated form HOCO
+
. The distribution of HOCO
+
is similar to that of methanol, which suggests that HOCO
+
is indeed produced from the protonation of CO
2
sublimated from ice. The HOCO
+
fractional abundances are found to be (1–2) × 10
−9
at the outer part of the central molecular zone (CMZ), while they are lower (∼10
−10
) near the kinematic center. This peak fractional abundance at the outer CMZ is comparable to that in the Milky Way CMZ, and orders of magnitude higher than that in Galactic disk, star-forming regions. From the range of HOCO
+
/CO
2
ratios suggested from chemical models, the gas-phase CO
2
fractional abundance is estimated to be (1–20) × 10
−7
at the outer CMZ, and orders of magnitude lower near the center. We estimate the CO
2
ice fractional abundances at the outer CMZ to be (2–5) × 10
−6
from the literature. A comparison between the ice and gas CO
2
abundances suggests an efficient sublimation mechanism. This sublimation is attributed to large-scale shocks at the orbital intersections of the bar and CMZ.
Abstract
Molecular abundances are sensitive to the UV photon flux and cosmic-ray ionization rate. In starburst environments, the effects of high-energy photons and particles are expected to be ...stronger. We examine these astrochemical signatures through multiple transitions of HCO
+
and its metastable isomer HOC
+
in the center of the starburst galaxy NGC 253 using data from the Atacama Large Millimeter/submillimeter Array large program ALMA Comprehensive High-resolution Extragalactic Molecular inventory. The distribution of the HOC
+
(1−0) integrated intensity shows its association with “superbubbles,” cavities created either by supernovae or expanding H
ii
regions. The observed HCO
+
/HOC
+
abundance ratios are ∼10–150, and the fractional abundance of HOC
+
relative to H
2
is ∼1.5 × 10
−11
–6 × 10
−10
, which implies that the HOC
+
abundance in the center of NGC 253 is significantly higher than in quiescent spiral arm dark clouds in the Galaxy and the Galactic center clouds. Comparison with chemical models implies either an interstellar radiation field of
G
0
≳ 10
3
if the maximum visual extinction is ≳5, or a cosmic-ray ionization rate of
ζ
≳ 10
−14
s
−1
(3–4 orders of magnitude higher than that within clouds in the Galactic spiral arms) to reproduce the observed results. From the difference in formation routes of HOC
+
, we propose that a low-excitation line of HOC
+
traces cosmic-ray dominated regions, while high-excitation lines trace photodissociation regions. Our results suggest that the interstellar medium in the center of NGC 253 is significantly affected by energy input from UV photons and cosmic rays, sources of energy feedback.
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, NH2OH, which, despite several observational attempts, it has not been detected in space yet. Here we present the first detection of NH2OH in the interstellar medium toward 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 five transitions that are unblended or only slightly blended. The derived molecular abundance of NH2OH is (2.1 0.9) × 10−10. From the comparison of the derived abundance of NH2OH and chemically related species, with those predicted by chemical models and measured in laboratory experiments, we favor the formation of NH2OH in the interstellar medium via hydrogenation of NO on dust grain surfaces, with possibly a contribution of ice-mantle NH3 oxidation processes. Further laboratory studies and quantum chemical calculations are needed to completely rule out the formation of NH2OH in the gas phase.
The C3H3NO family of isomers is relevant in astrochemistry, even though its members are still elusive in the interstellar medium. To identify the best candidate for astronomical detection within this ...family, we developed a new computational protocol based on the minimum-energy principle. This approach aims to identify the most stable isomer of the family and consists of three steps. The first step is an extensive investigation that characterizes the vast number of compounds having the C3H3NO chemical formula, employing density functional theory for this purpose. The second step is an energy refinement, which is used to select isomers and relies on coupled cluster theory. The last step is a structural improvement with a final energy refinement that provides improved energies and a large set of accurate spectroscopic parameters for all isomers lying within 30 kJ mol−1 above the most stable one. According to this protocol, vinylisocyanate is the most stable isomer, followed by oxazole, which is about 5 kJ mol−1 higher in energy. The other stable species are pyruvonitrile, cyanoacetaldehyde, and cyanovinylalcohol. For all of these species, new computed rotational and vibrational spectroscopic data are reported, which complement those already available in the literature or fill current gaps.
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 ...compound toward the Galactic Center 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 ∼1 נ10−10. Additionally, we provide a solid confirmation of the gauche isomer of ethyl mercaptan (g-C2H5SH) with an abundance of ∼3 נ10−10, and we also detect methyl mercaptan (CH3SH) with an abundance of ∼5 נ10−9. Abundance ratios were calculated for the three SH-bearing species and their OH analogs, revealing similar trends between alcohols and thiols with increasing complexity. Possible chemical routes for the interstellar synthesis of t-HC(O)SH, CH3SH, and C2H5SH are discussed, as well as the relevance of these compounds in the synthesis of prebiotic proteins in the primitive Earth.
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