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Register a new userEvolutionary view through the starless cores in Taurus: deuteration in TMC 1-C and TMC 1-CP
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Added by
David Navarro-Almaida
Publication date
2021
fields
Physics
and research's language is
English
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The chemical and physical evolution of starless and pre-stellar cores are of paramount importance to understanding the process of star formation. The Taurus Molecular Cloud cores TMC 1-C and TMC 1-CP share similar initial conditions and provide an excellent opportunity to understand the evolution of the pre-stellar core phase. We investigated the evolutionary stage of starless cores based on observations towards the prototypical dark cores TMC 1-C and TMC 1-CP, mapping them in the CS $3rightarrow 2$, C$^{34}$S $3rightarrow 2$, $^{13}$CS $2rightarrow 1$, DCN $1rightarrow 0$, DCN $2rightarrow 1$, DNC $1rightarrow 0$, DNC $2rightarrow 1$, DN$^{13}$C $1rightarrow 0$, DN$^{13}$C $2rightarrow 1$, N$_2$H$^+$ $1rightarrow 0$, and N$_2$D$^+$ $1rightarrow 0$ transitions. We performed a multi-transitional study of CS and its isotopologs, DCN, and DNC lines to characterize the physical and chemical properties of these cores. We studied their chemistry using the state-of-the-art gas-grain chemical code Nautilus and pseudo time-dependent models to determine their evolutionary stage. Observational diagnostics seem to indicate that TMC 1-C is in a later evolutionary stage than TMC 1-CP, with a chemical age $sim$1 Myr. TMC 1-C shows signs of being an evolved core at the onset of star formation, while TMC 1-CP appears to be in an earlier evolutionary stage due to a more recent formation or, alternatively, a collapse slowed down by a magnetic support.
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We report the first detection in space of the cumulene carbon chain $l$-H$_2$C$_5$. A total of eleven rotational transitions, with $J_{up}$ = 7-10 and $K_a$ = 0 and 1, were detected in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40m radio telescope. We derive a column density of (1.8$pm$0.5)$times$10$^{10}$ cm$^{-2}$. In addition, we report observations of other cumulene carbenes detected previously in TMC-1, to compare their abundances with the newly detected cumulene carbene chain. We find that $l$-H$_2$C$_5$ is $sim$4.0 times less abundant than the larger cumulene carbene $l$-H$_2$C$_6$, while it is $sim$300 and $sim$500 times less abundant than the shorter chains $l$-H$_2$C$_3$ and $l$-H$_2$C$_4$. We discuss the most likely gas-phase chemical routes to these cumulenes in TMC-1 and stress that chemical kinetics studies able to distinguish between different isomers are needed to shed light on the chemistry of C$_n$H$_2$ isomers with $n$,$>$,3.
We report the detection, for the first time in space, of cyano thioformaldehyde (HCSCN) and propynethial (HCSCCH) towards the starless core TMC-1. Cyano thioformaldehyde presents a series of prominent a- and b-type lines, which are the strongest previously unassigned features in our Q-band line survey of TMC-1. Remarkably, HCSCN is four times more abundant than cyano formaldehyde (HCOCN). On the other hand, HCSCCH is five times less abundant than propynal (HCOCCH). Surprisingly, we find an abundance ratio HCSCCH/HCSCN of 0.25, in contrast with most other ethynyl-cyanide pairs of molecules for which the CCH-bearing species is more abundant than the CN-bearing one. We discuss the formation of these molecules in terms of neutral-neutral reactions of S atoms with CH2CCH and CH2CN radicals as well as of CCH and CN radicals with H2CS. The calculated abundances for the sulphur-bearing species are, however, significantly below the observed values, which points to an underestimation of the abundance of atomic sulphur in the model or to missing formation reactions, such as ion-neutral reactions.
We report on the first detection of C3N- and C5N- towards the cold dark core TMC-1 in the Taurus region, using the Yebes 40 m telescope. The observed C3N/C3N- and C5N/C5N- abundance ratios are 140 and 2, respectively; that is similar to those found in the circumstellar envelope of the carbon-rich star IRC+10216. Although the formation mechanisms for the neutrals are different in interstellar (ion-neutral reactions) and circumstellar clouds (photodissociation and radical-neutral reactions), the similarity of the C3N/C3N- and C5N/C5N- abundance ratios strongly suggests a common chemical path for the formation of these anions in interstellar and circumstellar clouds. We discuss the role of radiative electronic attachment, reactions between N atoms and carbon chain anions Cn-, and that of H- reactions with HC3N and HC5N as possible routes to form CnN-. The detection of C5N- in TMC-1 gives strong support for assigning to this anion the lines found in IRC+10216, as it excludes the possibility of a metal-bearing species, or a vibrationally excited state. New sets of rotational parameters have been derived from the observed frequencies in TMC-1 and IRC+10216 for C5N- and the neutral radical C5N.
Recent detections of complex organic molecules in dark clouds have rekindled interest in the astrochemical modeling of these environments. Because of its relative closeness and rich molecular complexity, TMC-1 has been extensively observed to study the chemical processes taking place in dark clouds. We use local thermodynamical equilibrium radiative transfer modeling coupled with a Bayesian statistical method which takes into account outliers to analyze the data from the Nobeyama spectral survey of TMC-1 between 8 and 50 GHz. We compute the abundance relative to molecular hydrogen of 57 molecules, including 19 isotopologues in TMC-1 along with their associated uncertainty. The new results are in general agreement with previous abundance determination from Ohishi & Kaifu and the values reported in the review from Agundez & Wakelam. However, in some cases, large opacity and low signal to noise effects allow only upper or lower limits to be derived, respectively.
We report the detection of the sulfur-bearing species NCS, HCCS, H2CCS, H2CCCS, and C4S for the first time in space. These molecules were found towards TMC-1 through the observation of several lines for each species. We also report the detection of C5S for the first time in a cold cloud through the observation of five lines in the 31-50 GHz range. The derived column densities are N(NCS) = (7.8 +/- 0.6)e11 cm-2, N(HCCS) = (6.8 +/- 0.6)e11 cm-2, N(H2CCS) = (7.8 +/- 0.8)e11 cm-2, N(H2CCCS) = (3.7 +/- 0.4)e11 cm-2, N(C4S) = (3.8 +/- 0.4)e10 cm-2, and N(C5S) = (5.0 +/- 1.0)e10 cm-2. The observed abundance ratio between C3S and C4S is 340, that is to say a factor of approximately one hundred larger than the corresponding value for CCS and C3S. The observational results are compared with a state-of-the-art chemical model, which is only partially successful in reproducing the observed abundances. These detections underline the need to improve chemical networks dealing with S-bearing species.
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