46 research outputs found
The emergence of interstellar molecular complexity explained by interacting networks
Recent years have witnessed the detection of an increasing number of complex organicmolecules in interstellar space, some of them being of prebiotic interest. Disentanglingthe origin of interstellar prebiotic chemistry and its connection to biochemistry andultimately, to biology is an enormously challenging scientific goal where the applicationof complexity theory and network science has not been fully exploited. Encouragedby this idea, we present a theoretical and computational framework to model theevolution of simple networked structures toward complexity. In our environment,complex networks represent simplified chemical compounds and interact optimizing thedynamical importance of their nodes. We describe the emergence of a transition fromsimple networks toward complexity when the parameter representing the environmentreaches a critical value. Notably, although our system does not attempt to model the rulesof real chemistry nor is dependent on external input data, the results describe the emer-gence of complexity in the evolution of chemical diversity in the interstellar medium.Furthermore, they reveal an as yet unknown relationship between the abundances ofmolecules in dark clouds and the potential number of chemical reactions that yieldthem as products, supporting the ability of the conceptual framework presented here toshed light on real scenarios. Our work reinforces the notion that some of the propertiesthat condition the extremely complex journey from the chemistry in space to prebioticchemistry and finally, to life could show relatively simple and universal patterns
The complex organic molecular content in the L1517B starless core
Recent observations of the pre-stellar core L1544 and the younger starless
core L1498 have revealed that complex organic molecules (COMs) are enhanced in
the gas phase toward their outer and intermediate-density shells. Our goal is
to determine the level of chemical complexity toward the starless core L1517B,
which seems younger than L1498, and compare it with the other two previously
studied cores to see if there is a chemical evolution within the cores. We have
carried out 3 mm high-sensitivity observations toward two positions in the
L1517B starless core: the core's centre and the position where the methanol
emission peaks (at a distance of 5000 au from the core's centre). Our
observations reveal that a lower number of COMs and COM precursors are detected
in L1517B with respect to L1498 and L1544, and also show lower abundances.
Besides methanol, we only detected CHO, HCCO, CHCHO, CHCN,
CHNC, HCCCN, and HCCNC. Their measured abundances are 3 times larger
toward the methanol peak than toward the core's centre, mimicking the behaviour
found toward the more evolved cores L1544 and L1498. We propose that the
differences in the chemical complexity observed between the three studied
starless cores are a consequence of their evolution, with L1517B being the less
evolved one, followed by L1498 and L1544. Chemical complexity in these cores
seems to increase over time, with N-bearing molecules forming first and
O-bearing COMs forming at a later stage as a result of the catastrophic
depletion of CO.Comment: 18 pages, 13 figure
A sequential acid-base (SAB) mechanism in the interstellar medium: The emergence of cis formic acid in dark molecular clouds
The abundance ratios between isomers of a COM observed in the ISM provides
valuable information about the chemistry and physics of the gas and eventually,
the history of molecular clouds. In this context, the origin of an abundance of
c-HCOOH acid of only 6% the isomer c-HCOOH abundance in cold cores, remains
unknown. Herein, we explain the presence of c-HCOOH in dark molecular clouds
through the destruction and back formation of c-HCOOH and t-HCOOH in a cyclic
process that involves HCOOH and highly abundant molecules such as HCO+ and NH3.
We use high-level ab initio methods to compute the potential energy profiles
for the cyclic destruction/formation routes of c-HCOOH and t-HCOOH. Global rate
constants and branching ratios were calculated based on the transition state
theory and the master equation formalism under the typical conditions of the
ISM. The destruction of HCOOH by reaction with HCO+ in the gas phase leads to
three isomers of the cation HC(OH)2+. The most abundant cation can react in a
second step with other abundant molecules of the ISM like NH3 to form back
c-HCOOH and t-HCOOH. This mechanism explains the formation of c-HCOOH in dark
molecular clouds. Considering this mechanism, the fraction of c-HCOOH with
respect t-HCOOH is 25.7%. To explain the 6% reported by the observations we
propose that further destruction mechanisms of the cations of HCOOH should be
taken into account. The sequential acid-base (SAB) mechanism proposed in this
work involves fast processes with very abundant molecules in the ISM. Thus,
HCOOH very likely suffers our proposed transformations in the conditions of
dark molecular clouds. This is a new approach in the framework of the isomerism
of organic molecules in the ISM which has the potential to try to explain the
ratio between isomers of organic molecules detected in the ISM
Mapping Large-Scale CO Depletion in a Filamentary Infrared Dark Cloud
Infrared Dark Clouds (IRDCs) are cold, high mass surface density and high
density structures, likely to be representative of the initial conditions for
massive star and star cluster formation. CO emission from IRDCs has the
potential to be useful for tracing their dynamics, but may be affected by
depleted gas phase abundances due to freeze-out onto dust grains. Here we
analyze C18O J=1-0 and J=2-1 emission line data, taken with the IRAM 30m
telescope, of the highly filamentary IRDC G035.39.-0033. We derive the
excitation temperature as a function of position and velocity, with typical
values of ~7K, and thus derive total mass surface densities, Sigma_C18O,
assuming standard gas phase abundances and accounting for optical depth in the
line, which can reach values of ~1. The mass surface densities reach values of
~0.07 g/cm^2. We compare these results to the mass surface densities derived
from mid-infrared (MIR) extinction mapping, Sigma_SMF, by Butler & Tan, which
are expected to be insensitive to the dust temperatures in the cloud. With a
significance of >10sigma, we find Sigma_C18O/Sigma_SMF decreases by about a
factor of 5 as Sigma increases from ~0.02 to ~0.2 g/cm^2, which we interpret as
evidence for CO depletion. Several hundred solar masses are being affected,
making this one of the most massive clouds in which CO depletion has been
observed directly. We present a map of the depletion factor in the filament and
discuss implications for the formation of the IRDC.Comment: 9 pages, accepted to ApJ, Mac users: Figure 1 is best viewed with
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H2CN/H2NC abundance ratio: a new potential temperature tracer for the interstellar medium
The radical is the high-energy metastable isomer of 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 / isomeric ratio, likewise the HCN/HNC ratio,
might increase with the kinetic temperature (), 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
and towards a warm galactic source, the
G+0.693-0.027 molecular cloud (with ), using IRAM
30m observations. We have detected multiple hyperfine components of the
and transitions.
We derived molecular abundances with respect to of
(6.81.3) for and of (3.10.7) for , and a / abundance ratio
of 2.20.5. These detections confirm that the /
ratio is 2 for sources with , larger than
the 1 ratios previously found in colder cores (). 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
isomerization, deserve consideration to explain the higher isomeric ratios and
abundances observed in warm sources, where the molecules can be
desorbed into the gas phase through thermal and/or shock-induced mechanisms.Comment: 12 pages, 5 figures, 3 tables, 2 appendix - Accepted for publication
in Monthly Notices of the Royal Astronomical Societ