arXiv subjects

Ci Xue

Publications and source records attributed to Ci Xue.

34 records · Page 2Linked to original sources

arXiv2023

Astronomical Detection of the Interstellar Anion C10H- towards TMC-1 from the GOTHAM Large Program on the GBT

Using data from the GOTHAM (GBT Observations of TMC-1: Hunting for Aromatic Molecules) survey, we report the first astronomical detection of the C10H- anion. The astronomical observations also provided the necessary data to refine the spectroscopic parameters of C10H-. From the velocity stacked data and the matched filter response, C10H- is detected at >9σ confidence level at a column density of 4.04e11 cm-2. A dedicated search for the C10H radical was also conducted towards TMC-1. In this case, the stacked molecular emission of C10H was detected at a ~3.2σ confidence interval at a column density of 2.02e11 cm-2. However, since the determined confidence level is currently <5σ, we consider the identification of C10H as tentative. The full GOTHAM dataset was also used to better characterize the physical parameters including column density, excitation temperature, linewidth, and source size for the C4H, C6H and C8H radicals and their respective anions, and the measured column densities were compared to the predictions from a gas/grain chemical formation model and from a machine learning analysis. Given the measured values, the C10H-/C10H column density ratio is ~2.0 - the highest value measured between an anion and neutral species to date. Such a high ratio is at odds with current theories for interstellar anion chemistry. For the radical species, both models can reproduce the measured abundances found from the survey; however, the machine learning analysis matches the detected anion abundances much better than the gas/grain chemical model, suggesting that the current understanding of the formation chemistry of molecular anions is still highly uncertain.

astro-ph.GA↗

arXiv2022

Discovery of Interstellar 2-Cyanoindene (2-C$_9$H$_7$CN) in GOTHAM Observations of TMC-1

We present laboratory rotational spectroscopy of five isomers of cyanoindene (2-, 4-, 5-, 6-, and 7-cyanoindene) using a cavity Fourier-transform microwave spectrometer operating between 6-40 GHz. Based on these measurements, we report the detection of 2-cyanoindene (1H-indene-2-carbonitrile; 2-C$_9$H$_7$CN) in GOTHAM line survey observations of the dark molecular cloud TMC-1 using the Green Bank Telescope at centimeter wavelengths. Using a combination of Markov Chain Monte Carlo (MCMC), spectral stacking, and matched filtering techniques, we find evidence for the presence of this molecule at the 6.3$σ$ level. This provides the first direct observation of the ratio of a cyano-substituted polycyclic aromatic hydrocarbon (PAH) to its pure hydrocarbon counterpart, in this case indene, in the same source. We discuss the possible formation chemistry of this species, including why we have only detected one of the isomers in TMC-1. We then examine the overall hydrocarbon:CN-substituted ratio across this and other simpler species, as well as compare to those ratios predicted by astrochemical models. We conclude that while astrochemical models are not yet sufficiently accurate to reproduce absolute abundances of these species, they do a good job at predicting the ratios of hydrocarbon:CN-substituted species, further solidifying -CN tagged species as excellent proxies for their fully-symmetric counterparts.

astro-ph.GA↗

arXiv2022

Searching for Propionamide (C2H5CONH2) Toward Sagittarius B2 at Centimeter Wavelengths

The formation of molecules in the interstellar medium (ISM) remains a complex and unresolved question in astrochemistry. A group of molecules of particular interest involves the linkage between a -carboxyl and -amine group, similar to that of a peptide bond. The detection of molecules containing these peptide-like bonds in the ISM can help elucidate possible formation mechanisms, as well as indicate the level of molecular complexity available within certain regions of the ISM. Two of the simplest molecules containing a peptide-like bond, formamide (NH2CHO) and acetamide (CH3CONH2), have previously been detected toward the star forming region Sagittarius B2 (Sgr B2). Recently, the interstellar detection of propionamide (C2H5CONH2) was reported toward Sgr B2(N) with ALMA observations at millimeter wavelengths. Yet, this detection has been questioned by others from the same set of ALMA observations as no statistically significant line emission was identified from any uncontaminated transitions. Using the PRrbiotic Interstellar MOlecule Survey (PRIMOS) observations, we report an additional search for C2H5CONH2 at centimeter wavelengths conducted with the Green Bank Telescope. No spectral signatures of C2H5CONH2 were detected. An upper limit for C2H5CONH2 at centimeter wavelengths was determined to be less than 1.8e14 cm-2 and an upper limit to the C2H5CONH2/CH3CONH2 ratio is found to be less than 2.34. This work again questions the initial detection of C2H5CONH2 and indicates that more complex peptide-like structures may have difficulty forming in the ISM or are below the detection limits of current astronomical facilities. Additional structurally related species are provided to aid in future laboratory and astronomical searches.

astro-ph.GA↗

arXiv2022

A Search for Heterocycles in GOTHAM Observations of TMC-1

We have conducted an extensive search for nitrogen-, oxygen- and sulfur-bearing heterocycles toward Taurus Molecular Cloud 1 (TMC-1) using the deep, broadband centimeter-wavelength spectral line survey of the region from the GOTHAM large project on the Green Bank Telescope. Despite their ubiquity in terrestrial chemistry, and the confirmed presence of a number of cyclic and polycyclic hydrocarbon species in the source, we find no evidence for the presence of any heterocyclic species. Here, we report the derived upper limits on the column densities of these molecules obtained by Markov Chain Monte Carlo (MCMC) analysis and compare this approach to traditional single-line upper limit measurements. We further hypothesize why these molecules are absent in our data, how they might form in interstellar space, and the nature of observations that would be needed to secure their detection.

astro-ph.GA↗

arXiv2022

FAUST III. Misaligned rotations of the envelope, outflow, and disks in the multiple protostellar system of VLA 1623$-$2417

We report a study of the low-mass Class-0 multiple system VLA 1623AB in the Ophiuchus star-forming region, using H$^{13}$CO$^+$ ($J=3-2$), CS ($J=5-4$), and CCH ($N=3-2$) lines as part of the ALMA Large Program FAUST. The analysis of the velocity fields revealed the rotation motion in the envelope and the velocity gradients in the outflows (about 2000 au down to 50 au). We further investigated the rotation of the circum-binary VLA 1623A disk as well as the VLA 1623B disk. We found that the minor axis of the circum-binary disk of VLA 1623A is misaligned by about 12 degrees with respect to the large-scale outflow and the rotation axis of the envelope. In contrast, the minor axis of the circum-binary disk is parallel to the large-scale magnetic field according to previous dust polarization observations, suggesting that the misalignment may be caused by the different directions of the envelope rotation and the magnetic field. If the velocity gradient of the outflow is caused by rotation, the outflow has a constant angular momentum and the launching radius is estimated to be $5-16$ au, although it cannot be ruled out that the velocity gradient is driven by entrainments of the two high-velocity outflows. Furthermore, we detected for the first time a velocity gradient associated with rotation toward the VLA 16293B disk. The velocity gradient is opposite to the one from the large-scale envelope, outflow, and circum-binary disk. The origin of its opposite gradient is also discussed.

astro-ph.GA↗

arXiv2021

Detection of Two Interstellar Polycyclic Aromatic Hydrocarbons via Spectral Matched Filtering

Ubiquitous unidentified infrared emission bands are seen in many astronomical sources. Although these bands are widely, if not unanimously, attributed to the collective emission from polycyclic aromatic hydrocarbons, no single species from this class has been detected in space. We present the discovery of two -CN functionalized polycyclic aromatic hydrocarbons, 1- and 2-cyanonaphthalene, in the interstellar medium aided by spectral matched filtering. Using radio observations with the Green Bank Telescope, we observe both bi-cyclic ring molecules in the molecular cloud TMC-1. We discuss potential in situ gas-phase formation pathways from smaller organic precursor molecules.

astro-ph.GA↗

arXiv2021

Interstellar Detection of 2-Cyanocyclopentadiene, C$_5$H$_5$CN, a Second Five-Membered Ring Toward TMC-1

Using radio observations with the Green Bank Telescope, evidence has now been found for a second five-membered ring in the dense cloud Taurus Molecular Cloud-1 (TMC-1). Based on additional observations of an ongoing, large-scale, high-sensitivity spectral line survey (GOTHAM) at centimeter wavelengths toward this source, we have used a combination of spectral stacking, Markov chain Monte Carlo (MCMC), and matched filtering techniques to detect 2-cyanocyclopentadiene, a low-lying isomer of 1-cyanocyclopentadiene, which was recently discovered there by the same methods. The new observational data also yields a considerably improved detection significance for the more stable isomer and evidence for several individual transitions between 23 - 32 GHz. Through our MCMC analysis, we derive total column densities of $8.3\times10^{11}$ and $1.9\times10^{11}$ cm$^{-2}$ for 1- and 2-cyanocyclopentadiene respectively, corresponding to a ratio of 4.4(6) favoring the former. The derived abundance ratios point towards a common formation pathway - most likely being cyanation of cyclopentadiene by analogy to benzonitrile.

astro-ph.GA↗

arXiv2021

Discovery of Interstellar trans-cyanovinylacetylene (HCCCH=CHCN) and vinylcyanoacetylene (H$_2$C=CHC$_3$N) in GOTHAM Observations of TMC-1

We report the discovery of two unsaturated organic species, trans-(E)-cyanovinylacetylene and vinylcyanoacetylene, using the second data release of the GOTHAM deep survey towards TMC-1 with the 100 m Green Bank Telescope. For both detections, we performed velocity stacking and matched filter analyses using Markov chain Monte Carlo simulations, and for trans-(E)-cyanovinylacetylene, three rotational lines were observed at low signal-to-noise (${\sim}$3$σ$). From this analysis, we derive column densities of $2\times10^{11}$ and $3\times10^{11}$ cm$^{-2}$ for vinylcyanoacetylene and trans-(E)-cyanovinylacetylene, respectively, and an upper limit of $<2\times10^{11}$ cm$^{-2}$ for trans-(Z)-cyanovinylacetylene. Comparisons with G3//B3LYP semi-empirical thermochemical calculations indicate abundances of the [H$_3$C$_5$N}] isomers are not consistent with their thermodynamic stability, and instead their abundances are mainly driven by dynamics. We provide discussion into how these species may be formed in TMC-1, with reference to related species like vinyl cyanide (CH$_2$=CHCN). As part of this discussion, we performed the same analysis for ethyl cyanide (CH$_3$CH$_2$CN), the hydrogenation product of CH$_2$=CHCN. This analysis provides evidence -- at 4.17$σ$ significance -- an upper limit to the column density of $<4\times10^{11}$ cm$^{-2}$; an order of magnitude lower than previous upper limits towards this source.

astro-ph.GA↗

arXiv2021

FAUST II. Discovery of a Secondary Outflow in IRAS 15398-3359: Variability in Outflow Direction during the Earliest Stage of Star Formation?

We have observed the very low-mass Class 0 protostar IRAS 15398-3359 at scales ranging from 50 au to 1800 au, as part of the ALMA Large Program FAUST. We uncover a linear feature, visible in H2CO, SO, and C18O line emission, which extends from the source along a direction almost perpendicular to the known active outflow. Molecular line emission from H2CO, SO, SiO, and CH3OH further reveals an arc-like structure connected to the outer end of the linear feature and separated from the protostar, IRAS 15398-3359, by 1200 au. The arc-like structure is blue-shifted with respect to the systemic velocity. A velocity gradient of 1.2 km/s over 1200 au along the linear feature seen in the H2CO emission connects the protostar and the arc-like structure kinematically. SO, SiO, and CH3OH are known to trace shocks, and we interpret the arc-like structure as a relic shock region produced by an outflow previously launched by IRAS 15398-3359. The velocity gradient along the linear structure can be explained as relic outflow motion. The origins of the newly observed arc-like structure and extended linear feature are discussed in relation to turbulent motions within the protostellar core and episodic accretion events during the earliest stage of protostellar evolution.

astro-ph.SR↗

arXiv2020

Interstellar Detection of the Highly Polar Five-Membered Ring Cyanocyclopentadiene

Much like six-membered rings, five-membered rings are ubiquitous in organic chemistry, frequently serving as the building blocks for larger molecules, including many of biochemical importance. From a combination of laboratory rotational spectroscopy and a sensitive spectral line survey in the radio band toward the starless cloud core TMC-1, we report the astronomical detection of 1-cyano-1,3-cyclopentadiene, $c$-C$_5$H$_5$CN}, a highly polar, cyano derivative of cyclopentadiene, $c$-C$_5$H$_6$. The derived abundance of $c$-C$_5$H$_5$CN} is far greater than predicted from astrochemical models which well reproduce the abundance of many carbon chains. This finding implies either an important production mechanism or a large reservoir of aromatic material may need to be considered. The apparent absence of its closely-related isomer, 2-cyano-1,3-cyclopentadiene, may arise from its lower stability or be indicative of a more selective pathway for formation of the 1-cyano isomer, perhaps one starting from acyclic precursors. The absence of N-heterocycles such as pyrrole and pyridine is discussed in light of the astronomical finding.

astro-ph.GA↗

arXiv2020

An Investigation of Spectral Line Stacking Techniques and Application to the Detection of HC$_{11}$N

As the inventory of interstellar molecules continues to grow, the gulf between small species, whose individual rotational lines can be observed with radio telescopes, and large ones, such as polycyclic aromatic hydrocarbons (PAHs) best studied in bulk via infrared and optical observations, is slowly being bridged. Understanding the connection between these two molecular reservoirs is critical to understanding the interstellar carbon cycle, but will require pushing the boundaries of how far we can probe molecular complexity while still retaining observational specificity. Toward this end, we present a method for detecting and characterizing new molecular species in single-dish observations toward sources with sparse line spectra. We have applied this method to data from the ongoing GOTHAM (GBT Observations of TMC-1: Hunting Aromatic Molecules) Green Bank Telescope (GBT) large program, discovering six new interstellar species. In this paper we highlight the detection of HC$_{11}$N, the largest cyanopolyyne in the interstellar medium.

astro-ph.GA↗

arXiv2020

Detection of Interstellar HC$_4$NC and an Investigation of Isocyanopolyyne Chemistry under TMC-1 Conditions

We report an astronomical detection of HC$_4$NC for the first time in the interstellar medium with the Green Bank Telescope toward the TMC-1 molecular cloud with a minimum significance of $10.5 σ$. The total column density and excitation temperature of HC$_4$NC are determined to be $3.29^{+8.60}_{-1.20}\times 10^{11}$ cm$^{-2}$ and $6.7^{+0.3}_{-0.3}$ K, respectively, using the MCMC analysis. In addition to HC$_4$NC, HCCNC is distinctly detected whereas no clear detection of HC$_6$NC is made. We propose that the dissociative recombination of the protonated cyanopolyyne, HC$_5$NH$^+$, and the protonated isocyanopolyyne, HC$_4$NCH$^+$, are the main formation mechanisms for HC$_4$NC while its destruction is dominated by reactions with simple ions and atomic carbon. With the proposed chemical networks, the observed abundances of HC$_4$NC and HCCNC are reproduced satisfactorily.

astro-ph.GA↗

arXiv2020

Early Science from GOTHAM: Project Overview, Methods, and the Detection of Interstellar Propargyl Cyanide (HCCCH$_2$CN) in TMC-1

We present an overview of the GOTHAM (GBT Observations of TMC-1: Hunting Aromatic Molecules) Large Program on the Green Bank Telescope. This and a related program were launched to explore the depth and breadth of aromatic chemistry in the interstellar medium at the earliest stages of star formation, following our earlier detection of benzonitrile ($c$-C$_6$H$_5$CN) in TMC-1. In this work, details of the observations, use of archival data, and data reduction strategies are provided. Using these observations, the interstellar detection of propargyl cyanide (HCCCH$_2$CN) is described, as well as the accompanying laboratory spectroscopy. We discuss these results, and the survey project as a whole, in the context of investigating a previously unexplored reservoir of complex, gas-phase molecules in pre-stellar sources. A series of companion papers describe other new astronomical detections and analyses.

astro-ph.GA↗

arXiv2019

Searches for Interstellar HCCSH and H$_2$CCS

A long standing problem in astrochemistry is the inability of many current models to account for missing sulfur content. Many relatively simple species that may be good candidates to sequester sulfur have not been measured experimentally at the high spectral resolution necessary to enable radioastronomical identification. On the basis of new laboratory data, we report searches for the rotational lines in the microwave, millimeter, and sub-millimeter regions of the sulfur-containing hydrocarbon HCCSH. This simple species would appear to be a promising candidate for detection in space owing to the large dipole moment along its $b$-inertial axis, and because the bimolecular reaction between two highly abundant astronomical fragments (CCH and SH radicals) may be rapid. An inspection of multiple line surveys from the centimeter to the far-infrared toward a range of sources from dark clouds to high-mass star-forming regions, however, resulted in non-detections. An analogous search for the lowest-energy isomer, H$_2$CCS, is presented for comparison, and also resulted in non-detections. Typical upper limits on the abundance of both species relative to hydrogen are $10^{-9}$-$10^{-10}$. We thus conclude that neither isomer is a major reservoir of interstellar sulfur in the range of environments studied. Both species may still be viable candidates for detection in other environments or at higher frequencies, providing laboratory frequencies are available.

astro-ph.GA↗

arXiv2019

ALMA Detection of Vibrationally Excited ($v\mathrm{_t} = 1,2$) Acetic Acid toward NGC 6334I

Vibrationally excited states of detected interstellar molecules have been shown to account for a large portion of unidentified spectral lines in observed interstellar spectra toward chemically rich sources. Here, we present the first interstellar detection of the first and second vibrationally excited torsional states of acetic acid ($v_\mathrm{t} = 1, 2$) toward the high-mass star-forming region NGC 6334I. The observations presented were taken with the Atacama Large Millimeter/submillimeter Array in bands 4, 6, and 7 covering a frequency range of 130 - 352 GHz. By comparing a single excitation temperature model to the observations, the best-fit excitation temperature and column density are obtained to be 142(25) K and $1.12(7) \times 10^{17} \mathrm{cm^{-2}}$ respectively. Based on the intensity maps of the vibrationally excited CH$_3$COOH transitions, we found that the CH$_3$COOH emissions are compact and concentrated toward the MM1 and MM2 regions with a source size smaller than 2 arcsec. After locating the emission from different CH$_3$COOH transitions, which cover a large range of excitation energies, we are able to explain the variation of the CH$_3$COOH emission peak within the MM2 core by invoking continuum absorption or outflows.

astro-ph.GA↗

arXiv2018

ALMA Observations of the Spatial Distribution of three C$_2$H$_4$O$_2$ Isomers towards Sgr B2(N)

The C$_2$H$_4$O$_2$ isomers have been previously investigated primarily via disparate sets of observations involving single dish and array measurements. The only attempt at using a uniform set of observations was performed with the IRAM 30 m observation in 2013 (Belloche et al. 2013). In this study, we present an intensive and rigorous spectral and morphological analysis of the C$_2$H$_4$O$_2$ isomers towards Sgr B2(N) with interferometers, ALMA Band 3 observations. We propose a quantitative selection method, which automates the determination of the most uncontaminated transitions and allows us to report the discovery of previously undetected transitions of the three isomers. With the least contaminated transitions, the high spatial-resolution millimeter maps of the C$_2$H$_4$O$_2$ isomers reveal that HCOOCH$_3$ and CH$_2$OHCHO each display two different velocity components, while only one velocity component of CH$_3$COOH is resolved. Moreover, the distribution of HCOOCH$_3$ is extended and offset from the continuum emission, unlike CH$_2$OHCHO and CH$_3$COOH, for which the low-velocity component is found to be compact and concentrated toward the continuum emission peak of Sgr B2(N). The distinct morphologies of these C$_2$H$_4$O$_2$ isomeric species indicate that HCOOCH$_3$ have significant differences in chemical processes than CH$_2$OHCHO and CH$_3$COOH, which display similar spatial distributions.

astro-ph.GA↗