arXiv subjects

Ci Xue

Publications and source records attributed to Ci Xue.

At least 19 recordsLinked to original sources

arXiv2026

Efficient Regression Models for Scan Statistics

We introduce a new class of regression models for scan statistics on real-valued signals. These allow for improved fitting of non-stationary signals to contrast with the interval anomalies identified by the scan statistics. Our models can represent generalized likelihood ratio statistics. While these methods naively require $O(n^4)$ for a length $n$ signal, we provide algorithmic improvements which lead to linear time algorithms (with assumptions on max interval width). Our methods, especially ones based on Nadaraya-Watson kernel regression, are demonstrated as especially effective in detecting both synthetically planted anomalies, and for identifying a real ``platforming'' issue in interferometric astronomy.

stat.ME↗

arXiv2026

Automated Assignment and Prediction of Molecules in Astronomical Line Surveys Using Machine-Learning-Based Chemical Embeddings

Modern radio telescopes generate vast amounts of observational data, offering valuable insights into the molecular composition of interstellar sources. Identifying the molecules within these datasets typically involves time-consuming and labor-intensive manual analysis. This paper presents an automated method for assigning molecules in interstellar line surveys. The algorithm operates in two main stages. First, it automatically determines key parameters of the data, including excitation temperature, line width, and source velocity. Next, it assigns the observed spectral peaks by evaluating the spectroscopic match of the molecular candidates along with analyzing their chemical relevance to the interstellar source. The chemical relevance is determined by leveraging machine-learning-based molecular embedding techniques to analyze the regions of chemical space occupied by the observed species. Following the line assignment, this information is then used to generate new molecular candidates that occupy the same regions of chemical space. These newly generated species serve as promising targets for further investigation in the observational data. The algorithm was validated on spectral line surveys of the dark molecular cloud TMC-1 and the star-forming region IRAS 16293-2422B. In both cases, it identified at least 67 molecular species, accounting for over 90 percent of the analyzed line intensity, in 17 minutes or less while maintaining a high level of accuracy.

astro-ph.GA↗

arXiv2026

Exploring the Limits of Spectral Line Stacking in Spectral Line Data and Application Toward the Detection of Bulk $^{13}$C Enrichment of Aromatics in TMC-1

The formation history of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium remains a topic of active debate, with proposed mechanisms ranging from high-temperature stellar ejecta processes to low-temperature chemistry within molecular clouds. Recently, the identification of small PAHs in the cold, dark cloud TMC-1 has provided some circumstantial evidence of the latter. If these are formed in situ, their isotopic ratios, particularly $^{12}$C/$^{13}$C, should reflect the local, bulk material of the molecular cloud; in contrast, PAHs formed in the circumstellar envelopes of evolved stars may show enhanced $^{13}$C abundances. The expected radio signals of these $^{13}$C substituted species will be faint, and thus we conducted a detailed proof-of-concept analysis examining whether spectral line stacking and matched filtering techniques can robustly retrieve signal from multiple singly substituted $^{13}$C isotopologues in aggregate. We find that while retrieved signal decreases in the limit of spectral line confusion, false-positive detections are exceedingly improbable at the adopted 5$\sigma$ matched filter response detection threshold. We then demonstrate the technique on actual observational data of isotopologues of HC$_9$N toward TMC-1. Finally, we show using synthetic data that if laboratory rotational spectra of all singly substituted $^{13}$C isotopologues of cyanonaphthalene and cyanopyrene isomers existed, current observations of TMC-1 would be sensitive enough to discriminate between a local, bulk $^{12}$C/$^{13}$C ratio and one with an enhanced $^{13}$C abundance suggestive of inheritance.

astro-ph.GA↗

arXiv2026

Detection of the Polycyclic Aromatic Hydrocarbon Phenalene (C$_{13}$H$_{10}$) in the Very Low Luminosity Object (VeLLO) MC27/L1521F

To date, 14 polycyclic aromatic hydrocarbons (PAHs) ranging in size from two to seven (including five- and six-membered) carbon rings have been detected in the starless dense core TMC-1 CP within the Taurus molecular cloud. Their detection raises questions about the distribution of PAHs in the cold interstellar medium (ISM) and their evolution during star formation. Here, we present the first interstellar detection of a three-ring PAH outside of TMC-1 CP. We detect phenalene (C$_{13}$H$_{10}$), a compact, peri-fused PAH, in the dense core MC27/L1521F, a molecular cloud in Taurus containing a very low-luminosity object (VeLLO). We compare the abundances of phenalene in the two sources with respect to the single-ring aromatic benzonitrile, and find that it is enhanced by a factor of four in MC27/L1521F. We discuss the implications for possible formation and destruction pathways in the two sources. These findings further support the widespread abundance of PAHs throughout the cold ISM and are consistent with survival, inheritance, or replenishment during the earliest stages of star formation.

astro-ph.GA↗

arXiv2026

Modeling the Effect of C/O Ratio on Complex Carbon Chemistry in Cold Molecular Clouds

Elemental abundances, which are often depleted with respect to the solar values, are important input parameters for kinetic models of interstellar chemistry. In particular, the amount of carbon relative to oxygen is known to have a strong effect on modeled abundances of many species. While previous studies have focused on comparison of modeled and observed abundances to constrain the C/O ratio, the effects of this parameter on the underlying chemistry have not been well-studied. We investigated the role of the C/O ratio on dark cloud chemistry using the NAUTILUS code and machine learning techniques for molecular representation. We find that modeled abundances are quite sensitive to the C/O ratio, especially for carbon-rich species such as carbon chains and polycyclic aromatic hydrocarbons (PAHs). CO and simple ice-phase species are found to be major carbon reservoirs under both oxygen-poor and oxygen-rich conditions. The appearance of C3H4 isomers as significant carbon reservoirs, even under oxygen-rich conditions, indicates the efficiency of gas-phase C3 formation followed by adsorption and grain-surface hydrogenation. Our model is not able to reproduce the observed, gas-phase C/H ratio of TMC-1 CP at the time of best fit with any C/O ratio between 0.1 and 3, suggesting that the modeled freeze-out of carbon-bearing molecules may be too rapid. Future investigations are needed to understand the reactivity of major carbon reservoirs and their conversion to complex organic molecules.

astro-ph.GA↗

arXiv2025

The Molecular Inventory of TMC-1 with GOTHAM Observations

Spectral line surveys of the Taurus Molecular Cloud-1 (TMC-1) have led to the detection of more than 100 new molecular species, making it the most prolific source of interstellar molecular discoveries. These wide-band, high-sensitivity line surveys have been enabled by advances in telescope and receiver technology, particularly at centimeter and millimeter wavelengths. In this work, we present a statistical analysis of the molecular inventory of TMC-1 as probed by the GOTHAM large program survey from 3.9 to 36.4 GHz. To fully unlock the potential of the $\sim$29 GHz spectral bandwidth, we developed an automated pipeline for data reduction and calibration. We applied a Bayesian approach with Markov-Chain Monte Carlo fitting to the calibrated spectra and constrained column densities for 102 molecular species detected in TMC-1, including 75 main isotopic species, 20 carbon-13 substituted species, and seven deuterium-substituted species. This list of the detected gas-phase molecules is populated by unsaturated hydrocarbons, in stark contrast to the oxygen-rich organics found in sublimated ices around protostars. Of note, ten individual aromatic molecules were identified in the GOTHAM observations, contributing 0.011% of the gas-phase carbon budget probed by detected molecules when including CO and 6% when excluding CO. This work provides a reference set of observed gas-phase molecular abundances for interstellar clouds, offering a new benchmark for astrochemical theoretical models.

astro-ph.GA↗

arXiv2025

Rotational Spectroscopy and Tentative Interstellar Detection of 3-Hydroxypropanal (HOCH$_2$CH$_2$CHO) in the G+0.693-0.027 Molecular Cloud

We synthesized the astrochemically relevant molecule 3-hydroxypropanal (HOCH$_2$CH$_2$CHO) and subsequently measured and analyzed its rotational spectrum in several frequency regions ranging from 130 to 485 GHz. We analyzed the ground vibrational state as well as the two perturbed lowest-lying vibrationally excited states. With the resulting rotational parameters, we searched for this molecule in the Sagittarius B2(N) and NGC 6334I hot cores, the IRAS 16293-2422B hot corino, and the G+0.693-0.027 and TMC-1 molecular clouds. Rotational emission of 3-hydroxypropanal was tentatively detected toward G+0.693-0.027 and a column density of (8.6$\pm$1.4)$\times$10$^{12}$ cm$^{-2}$ was determined. However, this molecule was not detected in the other sources that were investigated. The chemical implications of this tentative discovery are analyzed and several potential chemical formation pathways of this species are discussed.

astro-ph.GA↗

arXiv2025

Laboratory rotational spectra of cyanocyclohexane and its siblings (1- and 4-cyanocyclohexene) using a compact CP-FTMW spectrometer for interstellar detection

Chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy is a versatile technique to record broadband gas-phase rotational spectra, enabling detailed investigations of molecular structure, dynamics, and hyperfine interactions. Here, we present the development and application of a CP-FTMW spectrometer operating in the 6.5-18 GHz frequency range, studying cyanocyclohexane, 1-cyanocyclohexene, and 4-cyanocyclohexene using a heated pulsed supersonic expansion source. The dynamic range, experimental resolution, and high sensitivity enable observation of multiple conformers, precise measurements of hyperfine splitting arising from nuclear quadrupole coupling due to the nitrogen atom in the cyano group, as well as the observation of singly $^{13}$C- and $^{15}$N-substituted isotopic isomers in natural abundance. Using the latter, precise structures for the molecules are derived. The accurate rotational spectra enabled a search for these species toward the dark, cold molecular cloud TMC-1; no signals are found, and we discuss the implications of derived upper limits on the interstellar chemistry of the cyanocyclohexane family.

astro-ph.GA↗

arXiv2025

Discovery of the 7-ring PAH Cyanocoronene (C$_{24}$H$_{11}$CN) in GOTHAM Observations of TMC-1

We present the synthesis and laboratory rotational spectroscopy of the 7-ring polycyclic aromatic hydrocarbon (PAH) cyanocoronene (C$_{24}$H$_{11}$CN) using a laser-ablation assisted cavity-enhanced Fourier transform microwave spectrometer. A total of 71 transitions were measured and assigned between 6.8--10.6\,GHz. Using these assignments, we searched for emission from cyanocoronene in the GBT Observations of TMC-1: Hunting Aromatic Molecules (GOTHAM) project observations of the cold dark molecular cloud TMC-1 using the 100\,m Green Bank Telescope (GBT). We detect a number of individually resolved transitions in ultrasensitive X-band observations and perform a Markov Chain Monte Carlo analysis to derive best-fit parameters, including a total column density of $N(\mathrm{C}_{24}\mathrm{H}_{11}\mathrm{CN}) = 2.69^{+0.26}_{-0.23} \times 10^{12}\,\mathrm{cm}^{-2}$ at a temperature of $6.05^{+0.38}_{-0.37}\,$K. A spectral stacking and matched filtering analysis provides a robust 17.3$\,\sigma$ significance to the overall detection. The derived column density is comparable to that of cyano-substituted naphthalene, acenaphthylene, and pyrene, defying the trend of decreasing abundance with increasing molecular size and complexity found for carbon chains. We discuss the implications of the detection for our understanding of interstellar PAH chemistry and highlight major open questions and next steps.

astro-ph.GA↗

arXiv2025

The Missing Link of Sulfur Chemistry in TMC-1: The Detection of c-C3H2S from the GOTHAM Survey

We present the spectroscopic characterization of cyclopropenethione (c-C3H2S) in the laboratory and detect it in space using the Green Bank Telescope (GBT) Observations of TMC-1: Hunting Aromatic Molecules (GOTHAM) survey. The detection of this molecule - the missing link in understanding the C3H2S isomeric family in TMC-1 - completes the detection of all 3 low-energy isomers of C3H2S as both CH2CCS and HCCCHS have been previously detected in this source. The total column density of this molecule (N_T of 5.72+2.65/-1.61x10^10 cm^-2 at an excitation temperature of 4.7+1.3/-1.1 K) is smaller than both CH2CCS and HCCCHS and follows nicely the relative dipole principle (RDP), a kinetic rule-of-thumb for predicting isomer abundances which suggests that, all other chemistry among a family of isomers being the same, the member with the smallest dipole should be the most abundant. The RDP now holds for the astronomical abundance ratios of both the S-bearing and O-bearing counterparts observed in TMC-1; however, CH2CCO continues to elude detection in any astronomical source.

astro-ph.GA↗

arXiv2024

Sensitivity Analysis of Aromatic Chemistry to Gas-Phase Kinetics in a Dark Molecular Cloud Model

The increasingly large number of complex organic molecules detected in the interstellar medium necessitates robust kinetic models that can be relied upon for investigating the involved chemical processes. Such models require rate constants for each of the thousands of reactions; the values of these are often estimated or extrapolated, leading to large uncertainties that are rarely quantified. We have performed a global Monte Carlo and a more local one-at-a-time sensitivity analysis on the gas-phase rate coefficients in a 3-phase dark cloud model. Time-dependent sensitivities have been calculated using four metrics to determine key reactions for the overall network as well as for the cyanonaphthalene molecule in particular, an important interstellar species that is severely under-produced by current models. All four metrics find that reactions involving small, reactive species that initiate hydrocarbon growth have large effects on the overall network. Cyanonaphthalene is most sensitive to a number of these reactions as well as ring-formation of the phenyl cation (C6H5+) and aromatic growth from benzene to naphthalene. Future efforts should prioritize constraining rate coefficients of key reactions and expanding the network surrounding these processes. These results highlight the strength of sensitivity analysis techniques to identify critical processes in complex chemical networks, such as those often used in astrochemical modeling.

astro-ph.GA↗

arXiv2024

Discovery of interstellar 1-cyanopyrene: a four-ring polycyclic aromatic hydrocarbon in TMC-1

Polycyclic aromatic hydrocarbons (PAHs) are expected to be the most abundant class of organic molecules in space. Their interstellar lifecycle is not well understood, and progress is hampered by difficulties detecting individual PAH molecules. Here, we present the discovery of CN-functionalized pyrene, a 4-ring PAH, in the dense cloud TMC-1 using the 100-m Green Bank Telescope. We derive an abundance of 1-cyanopyrene of ~1.52 x $10^{12}$ cm$^{-2}$, and from this estimate that the un-substituted pyrene accounts for up to ~0.03-0.3% of the carbon budget in the dense interstellar medium which trace the birth sites of stars and planets. The presence of pyrene in this cold (~10 K) molecular cloud agrees with its recent measurement in asteroid Ryugu where isotopic clumping suggest a cold, interstellar origin. The direct link to the birth site of our solar system is strengthened when we consider the solid state pyrene content in the pre-stellar materials compared to comets, which represent the most pristine material in the solar system. We estimate that solid state pyrene can account for 1% of the carbon within comets carried by this one single organic molecule. The abundance indicates pyrene is an "island of stability" in interstellar PAH chemistry and suggests a potential cold molecular cloud origin of the carbon carried by PAHs that is supplied to forming planetary systems, including habitable worlds such as our own.

astro-ph.GA↗

arXiv2024

Detections of interstellar 2-cyanopyrene and 4-cyanopyrene in TMC-1

Polycyclic aromatic hydrocarbons (PAHs) are among the most ubiquitous compounds in the universe, accounting for up to ~25% of all interstellar carbon. Since most unsubstituted PAHs do not possess permanent dipole moments, they are invisible to radio astronomy. Constraining their abundances relies on the detection of polar chemical proxies, such as aromatic nitriles. We report the detection of 2- and 4-cyanopyrene, isomers of the recently detected 1-cyanopyrene. We find that these isomers are present in an abundance ratio of ~2:1:2, which mirrors the number of equivalent sites available for CN addition. We conclude that there is evidence that the cyanopyrene isomers formed by direct CN addition to pyrene under kinetic control in hydrogen-rich gas at 10 K and discuss constraints on the H/CN ratio for PAHs in TMC-1.

astro-ph.GA↗

arXiv2024

High Spectral Resolution Observations of Propynal (HCCCHO) towards TMC-1 from the GOTHAM Large Program on the Green Bank Telescope

We used new high spectral resolution observations of propynal (HCCCHO) towards TMC-1 and in the laboratory to update the spectral line catalog available for transitions of HCCCHO - specifically at frequencies lower than 30 GHz which were previously discrepant in a publicly available catalog. The observed astronomical frequencies provided high enough spectral resolution that, when combined with high-resolution (~2 kHz) measurements taken in the laboratory, a new, consistent fit to both the laboratory and astronomical data was achieved. Now with a nearly exact (<1 kHz) frequency match to the J=2-1 and 3-2 transitions in the astronomical data, using a Markov chain Monte Carlo (MCMC) analysis, a best fit to the total HCCCHO column density of 7.28+4.08/-1.94 x 10^12 cm^-2 was found with a surprisingly low excitation temperature of just over 3 K. This column density is around a factor of 5 times larger than reported in previous studies. Finally, this work highlights that care is needed when using publicly available spectral catalogs to characterize astronomical spectra. The availability of these catalogs is essential to the success of modern astronomical facilities and will only become more important as the next generation of facilities come online.

astro-ph.GA↗

arXiv2024

Maser Activity of Organic Molecules toward Sgr B2(N)

At centimeter wavelengths, single-dish observations have suggested that the Sagittarius (Sgr) B2 molecular cloud at the Galactic Center hosts weak maser emission from several organic molecules, including CH$_2$NH, HNCNH, and HCOOCH$_3$. However, the lack of spatial distribution information of these new maser species has prevented us from assessing the excitation conditions of the maser emission as well as their pumping mechanisms. Here, we present a mapping study toward Sgr B2 North (N) to locate the region where the complex maser emission originates. We report the first detection of the Class I methanol (CH$_3$OH) maser at 84 GHz and the first interferometric map of the methanimine (CH$_2$NH) maser at 5.29 GHz toward this region. In addition, we present a tool for modeling and fitting the unsaturated molecular maser signals with non-LTE radiative transfer models and Bayesian analysis using the Markov-Chain Monte Carlo approach. These enable us to quantitatively assess the observed spectral profiles. The results suggest a two-chain-clump model for explaining the intense CH$_3$OH Class I maser emission toward a region with low continuum background radiation. By comparing the spatial origin and extent of maser emission from several molecular species, we find that the 5.29 GHz CH$_2$NH maser has a close spatial relationship with the 84 GHz CH$_3$OH Class I masers. This relationship serves as observational evidence to suggest a similar collisional pumping mechanism for these maser transitions.

astro-ph.GA↗

arXiv2023

Astrochemical Modeling of Propargyl Radical Chemistry in TMC-1

Recent detections of aromatic species in dark molecular clouds suggest formation pathways may be efficient at very low temperatures and pressures, yet current astrochemical models are unable to account for their derived abundances, which can often deviate from model predictions by several orders of magnitude. The propargyl radical, a highly abundant species in the dark molecular cloud TMC- 1, is an important aromatic precursor in combustion flames and possibly interstellar environments. We performed astrochemical modeling of TMC-1 using the three-phase gas-grain code NAUTILUS and an updated chemical network, focused on refining the chemistry of the propargyl radical and related species. The abundance of the propargyl radical has been increased by half an order of magnitude compared to the previous GOTHAM network. This brings it closer in line with observations, but it remains underestimated by two orders of magnitude compared to its observed value. Predicted abundances for the chemically related C4H3N isomers within an order of magnitude of observed values corroborate the high efficiency of CN addition to closed-shell hydrocarbons under dark molecular cloud conditions. The results of our modeling provide insight into the chemical processes of the propargyl radical in dark molecular clouds and highlight the importance of resonance-stabilized radicals in PAH formation.

astro-ph.GA↗

arXiv2023

Detection of Interstellar $E$-1-cyano-1,3-butadiene in GOTHAM Observations of TMC-1

We report the detection of the lowest energy conformer of $E$-1-cyano-1,3-butadiene ($E$-1-C$_4$H$_5$CN), a linear isomer of pyridine, using the fourth data reduction of the GOTHAM deep spectral survey toward TMC-1 with the 100 m Green Bank Telescope. We performed velocity stacking and matched filter analyses using Markov chain Monte Carlo simulations and find evidence for the presence of this molecule at the 5.1$\sigma$ level. We derive a total column density of $3.8^{+1.0}_{-0.9}\times 10^{10}$ cm$^{-2}$, which is predominantly found toward two of the four velocity components we observe toward TMC-1. We use this molecule as a proxy for constraining the gas-phase abundance of the apolar hydrocarbon 1,3-butadiene. Based on the three-phase astrochemical modeling code NAUTILUS and an expanded chemical network, our model underestimates the abundance of cyano-1,3-butadiene by a factor of 19, with a peak column density of $2.34 \times 10^{10}\ \mathrm{cm}^{-2}$ for 1,3-butadiene. Compared to the modeling results obtained in previous GOTHAM analyses, the abundance of 1,3-butadiene is increased by about two orders of magnitude. Despite this increase, the modeled abundances of aromatic species do not appear to change and remain underestimated by 1--4 orders of magnitude. Meanwhile, the abundances of the five-membered ring molecules increase proportionally with 1,3-butadiene by two orders of magnitudes. We discuss implications for bottom-up formation routes to aromatic and polycyclic aromatic molecules.

astro-ph.GA↗

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 >9{\sigma} 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 ~3.2{\sigma} confidence interval at a column density of 2.02e11 cm-2. However, since the determined confidence level is currently <5{\sigma}, 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↗