arXiv subjects

Karen Willacy

Publications and source records attributed to Karen Willacy.

At least 19 recordsLinked to original sources

arXiv2026

A Comprehensive Sulfur Chemistry Network Including Excited S(1D) and SO(1Δ) for the XODIAC Photochemical Model: Accounting for Missing Sulfur Processes in Venus and Exo-Venus Analogs

Sulfur chemistry plays a central role in controlling the atmospheric structure, cloud formation, and composition of Venus and Venus-like exoplanets. However, key reactions involving ground- and excited-state sulfur species remain poorly constrained, and existing photochemical models often rely on incomplete or uncertain kinetic data under high-temperature, CO2-rich conditions. In this work, we compute kinetic parameters for reactions of ground-state S(3P) and excited-state S(1D) with CO2 under Venus-like conditions, forming SO(3Sigma), SO(1Delta), and CO. We characterize the underlying potential energy surfaces, identify intermediate complexes, and derive temperature-dependent rate coefficients using a master-equation framework based on the chemically significant eigenvalue method. We also provide NASA 7-term polynomial coefficients for S and SO in both ground and excited states to enable consistent incorporation into photochemical models. Incorporating these reactions into the one-dimensional photochemical model XODIAC for Venus produces only minor effects above 60 km due to competing pathways. While the model reproduces most observed sulfur species, discrepancies remain for S3 and S4. Introducing a 1 ppm near-surface atomic sulfur source, representing unresolved deep-atmosphere or surface processes, enhances S3 and S4 abundances by 1-2 orders of magnitude and improves agreement with observations. For exo-Venus analogs, the updated chemistry produces modest changes under isothermal conditions. In contrast, in strongly irradiated atmospheres with a high-altitude isotherm and a near-surface sulfur source, it leads to pronounced changes in most sulfur-bearing species, along with significant enhancements in S(1D) and SO(1Delta). These results highlight missing sulfur pathways, including excited states and deep sources, and potential implications for shaping Venus and exo-Venus atmospheres.

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arXiv2026

An 18 - 25 GHz spectroscopic survey of southern hemisphere dense cores

We extended the radio K-band spectroscopic survey for organics in southern hemisphere dense cores by observing seven sources using NASA's Deep Space Network 70-m antenna in Canberra, Australia, over the frequency range of 18 to 25 GHz. Molecular column densities of NH$_3$, $c$-C$_3$H$_2$, HC$_3$N, HC$_5$N, CCS, C$_3$S, and $c$-C$_3$HD were derived for each source assuming LTE. The resulting column density ratios were compared with predictions of a state-of-the art astrochemical model to constrain the C/O ratio and chemical age of each source. Most cores have similar C/O ratios of $0.5 - 0.7$, much different from the best studied TMC-1 dense core characterized by a high C/O ratio of $\sim 1.4$. The chemical ages of the cores are also similar and fall between 0.6 and 5~Myr. The less dense cores tend to have the oldest chemical ages, as might be expected given that chemical timescales scale with density. Our results showcase the synergistic approach of combining radio observations using the DSS-43 antenna with state-of-the-art astrochemical models to study the chemical composition of southern hemisphere dense cores, enabling constraints on their C/O ratios and chemical ages, which remain largely unexplored.

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arXiv2025

Unveiling the Chemical Complexity and C/O Ratio of the HD 163296 Protoplanetary Disk: Constraints from Multi-line ALMA Observations of Organics, Nitriles, Sulfur-bearing, and Deuterated Molecules

The physical and chemical conditions within a protoplanetary disk play a crucial role in determining its chemical composition, which is subsequently inherited by any forming planets. To probe these conditions, high-resolution molecular line observations, coupled with modelling, are essential. In this study, we investigate the chemistry of the nearby, massive, and relatively line-rich protoplanetary disk around HD 163296 using high-resolution observations from ALMA across Bands 3, 4, 6, and 7. We constrain the disk-averaged and radial distributions of column density and excitation temperature for the detected molecules using the new retrieval code DRive. The disk chemistry is modelled using the astrochemical code PEGASIS, with variations in the initial elemental C/O ratio. Our modelling, informed by molecular observations of HCO+, DCO+, HCN, DCN, CS, HC3N, H2CO, CH3OH, HNCO, and NH2CHO, allows us to place strong constraints on the C/O ratio, with a best-fit value of 1.1 that is broadly consistent with previous estimates. We present the highest-resolution DCO+ emission map of this disk to date, revealing triple-ringed chemical substructures that closely align with the dust continuum rings. Additionally, our results provide the first and most stringent upper limits on the column densities of NH2CHO and HNCO in this protoplanetary disk, measured at < 7e11 cm-2 and < 1e11 cm-2, respectively. Our chemical models suggest that NH2CHO and HNCO predominantly form on grain surfaces within the disk. However, physico-chemical desorption mechanisms are inefficient at releasing these species into detectable gas-phase abundances, yet they remain promising targets for future ALMA observations.

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arXiv2025

The interstellar heritage of comets

Comets have similar compositions to interstellar medium ices, suggesting at least some of their molecules maybe inherited from an earlier stage of evolution. To investigate the degree to which this might have occurred we compare the composition of individual comets to that of the well-studied protostellar region IRAS 16293-2422B. We show that the observed molecular abundance ratios in several comets correlate well with those observed in the protostellar source. However, this does not necessarily mean that the cometary abundances are identical to protostellar. We find the abundance ratios of many molecules present in comets are enhanced compared to their protostellar counterparts. For COH-molecules, the data suggest higher abundances relative to methanol of more complex species, e.g. HCOOH, CH3CHO, and HCOOCH3, are found in comets. For N-bearing molecules, the ratio of nitriles relative to CH3CN -- HC3N/CH3CN and HCN/CH3CN -- tend to be enhanced. The abundances of cometary SO and SO2 relative to H2S are enhanced, whereas OCS/H2S is reduced. Using a subset of comets with a common set of observed molecules we suggest a possible means of determining the relative degree to which they retain interstellar ices. This analysis suggests that over 84% of COH-bearing molecules can be explained by the protostellar composition. The possible fraction inherited from the protostellar region is lower for N-molecules at only 26--74%. While this is still speculative, especially since few comets have large numbers of observed molecules, it provides a possible route for determining the relative degree to which comets contain disk-processed material.

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arXiv2025

Role of Diffusive and Non-Diffusive Grain-Surface Processes in Cold Cores: Insights from the PEGASIS Three-Phase Astrochemical Model

Cold dense cores are unique among the structures found in the interstellar medium (ISM), as they harbor a rich chemical inventory, including complex organic molecules (COMs), which will be inherited by future evolutionary stages. These molecules exist both in the gas phase and as ices accreted onto grain surfaces. To model these environments, we present Pegasis, a new, fast, and extensible three-phase astrochemical code to explore the chemistry of cold cores, with an emphasis on the role of diffusive and non-diffusive chemistry in shaping their gas and grain chemical compositions. We incorporated the 2024 KIDA chemical network and compared our results with current astrochemical models. Using a traditional rate-equation-based approach, we implemented both diffusive and non-diffusive chemistry, coupled with either an inert or chemically active ice mantle. We identify crucial reactions that enhance the production of COMs through non-diffusive mechanisms on the grain surface as well as the mechanisms through which they can accumulate in the gas phase. Across all models with non-diffusive chemistry, we observe a definite enhancement in the concentration of COMs on both the grain surface as well as in the grain mantle. Finally, our model broadly reproduces the observed abundances of multiple gas-phase species in the cold dense core TMC-1 (CP) and provides insights into its chemical age. Our work demonstrates the capabilities of Pegasis in exploring a wide range of grain-surface chemical processes and modeling approaches for three-phase chemistry in the ISM, providing robust explanations for observed abundances in TMC-1 (CP). In particular, it highlights the role of non-diffusive chemistry in the production of gas-phase COMs on grain surfaces, which are subsequently chemically desorbed, especially when the precursors involved in their formation on the surfaces are heavier than atomic hydrogen.

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arXiv2025

JWST/MIRI Study of the Enigmatic Mid-Infrared Rings in the Planetary Nebula NGC 1514

While NGC 1514 is an elliptical, but complex, planetary nebula at optical wavelengths, it was discovered to have a pair of infrared-bright, axisymmetric rings contained within its faint outer shell during the course of the WISE all-sky survey. We have obtained JWST mid-infrared imaging and spectroscopy of the nebula through the use of simultaneous observations with the MIRI Imager and Medium Resolution Spectrometer, selecting the F770W, F1280W, and F2550W filters to match each of the MRS's three grating positions. These observations show that the rings are clearly resolved and relatively distinct structures, with both filamentary and clumpy detail throughout. There is also cloud-like material that has a turbulent appearance in the interior of the rings, particularly at the longest wavelengths, and faint ejecta-like structures just outside the ring boundaries. Despite their brightness, the emission from the rings within the three imager passbands is shown to be dominated by thermal emission from very small grains, not line emission from atomic hydrogen or forbidden atomic lines, shocked molecular hydrogen, or PAHs. The doppler velocities derived from the two brightest emission lines in the rings, however, suggest that the material from which the rings were formed was ejected during an early period of very heavy mass loss from the PN progenitor, then shaped by asymmetrical fast winds from the central binary pair.

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arXiv2025

An 18-25 GHz spectroscopic survey of dense cores in the Chamaeleon I molecular cloud

We extend the survey for organics in the southern hemisphere by observing two cores in the Chamaeleon complex using NASA's Deep Space Network 70-m antenna in Canberra, Australia, over the frequency range of 18 to 25 GHz. We surveyed the class 0 protostar Cha-MMS1 and the prestellar core Cha-C2, which represent two stages in the evolution of dense cores. We detect several molecules including HC$_3$N, HC$_5$N, C$_4$H, CCS, C$_3$S, NH$_3$, and c-C$_3$H$_2$. A longer cyanopolyyne, HC$_7$N, is detected with high confidence via spectral stacking analysis. While molecular column densities in the two Chamaeleon cores are typically an order of magnitude lower compared to the cynaopolyyne peak in TMC-1, the molecular abundance ratios are in general agreement with the TMC-1 values. The two exceptions are c-C$_3$H$_2$, which is enhanced by a factor of \about 25 with respect to cyanopolyynes in the Chamaeleon cores, and ammonia, which is enhanced by a factor of ~ 125. The deuterated species c-C$_3$HD is detected in both cores, with a high D/H ratio of ~0.23 in c-C$_3$H$_2$. A rare isotopologue of ammonia, $^{15}$NH$_3$, is also detected in Cha-MMS1 suggesting a high $^{14}$N/$^{15}$N ratio of ~ 690 in ammonia. However, this ratio may be artificially enhanced due to the high optical depth of the $^{14}$NH$_3$ (1,1) line, which increases the effective source size. We use the detections of ammonia, cyanopolyynes, and far-infrared dust continuum to characterize the density and temperature in the Chamaeleon cores and calculate the molecular column densities and their relative ratios. The ring molecule benzonitrile is not detected in either Chamaeleon core. The $3 σ$ upper limits for its column density are a factor of 2 higher than the value derived for TMC-1 and the upper limits for its relative abundance with respect to HC$_5$N are a factor of 3 higher than the TMC-1 value.

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arXiv2024

Water vapor as a probe of the origin of gas in debris disks

Debris disks embrace the formation and evolution histories of planetary systems. Recent detections of gas in these disks have received considerable attention, as its origin ties up ongoing disk evolution and the present composition of planet-forming materials. Observations of the CO gas alone, however, cannot reliably differentiate between two leading, competing hypotheses: (1) the observed gas is the leftover of protoplanetary disk gas, and (2) the gas is the outcome of collisions between icy bodies. We propose that such differentiation may become possible by observing cold water vapor. Order-of-magnitude analyses and comparison with existing observations are performed. We show that different hypotheses lead to different masses of water vapor. This occurs because, for both hypotheses, the presence of cold water vapor is attributed to photodesorption from dust particles by attenuated interstellar UV radiation. Cold water vapor cannot be observed by current astronomical facilities as most of its emission lines fall in the far-IR (FIR) range. This work highlights the need for a future FIR space observatory to reveal the origin of gas in debris disks and the evolution of planet-forming disks in general.

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arXiv2024

Chemistry in the GG Tau A Disk: Constraints from H2D+, N2H+, and DCO+ High Angular Resolution ALMA Observations

Resolved molecular line observations are essential for gaining insight into the physical and chemical structure of protoplanetary disks, particularly in cold, dense regions where planets form and acquire their chemical compositions. However, tracing these regions is challenging because most molecules freeze onto grain surfaces and are not observable in the gas phase. We investigated cold molecular chemistry in the triple stellar T Tauri disk GG Tau A, which harbours a massive gas and dust ring and an outer disk, using ALMA Band 7 observations. We present high angular resolution maps of N2H+ and DCO+ emission, with upper limits reported for H2D+, 13CS, and SO2. The radial intensity profile of N2H+ shows most emission near the ring outer edge, while DCO+ exhibits double peaks, one near the ring inner edge and the other in the outer disk. With complementary observations of lower-lying transitions, we constrained the molecular surface densities and rotation temperatures. We compared the derived quantities with model predictions across different cosmic ray ionization (CRI) rates, carbon-to-oxygen (C/O) ratios, and stellar UV fluxes. Cold molecular chemistry, affecting N2H+, DCO+, and H2D+ abundances, is most sensitive to CRI rates, while stellar UV flux and C/O ratios have minimal impact on these three ions. Our best model requires a low cosmic ray ionization rate of 1e-18 s-1. However, it fails to match the low temperatures derived from N2H+ and DCO+, 12 to 16 K, which are much lower than the CO freezing temperature.

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arXiv2024

Retrievals of Protoplanetary Disk Parameters using Thermochemical Models: I. Disk Gas Mass from Hydrogen Deuteride Spectroscopy

We discuss statistical relationships between the mass of protoplanetary disks and the hydrogen deuteride (HD) line emission and the dust spectral energy distribution (SED) determined using 3000 ProDiMo disk models. The models have 15 free parameters describing disk physical properties, the central star, and the local radiation field. The sampling of physical parameters is done using a Monte Carlo approach to evaluate the probability density functions of observables as a function of physical parameters. We find that the HD fractional abundance is almost constant even though the UV flux varies by several orders of magnitude. Probing the statistical relation between the physical quantities and the HD flux, we find that low-mass (optically thin) disks display a tight correlation between the average disk gas temperature and HD line flux, while massive disks show no such correlation. We demonstrate that the central star luminosity, disk size, dust size distribution, and HD flux may be used to determine the disk gas mass to within a factor of three. We also find that the far-IR and sub-mm/mm SEDs and the HD flux may serve as strong constraints for determining the disk gas mass to within a factor of two. If the HD lines are fully spectrally resolved ($R\gtrsim 1.5\times10^6, Δv=0.2~\rm km\,s^{-1}$), the 56 $μ$m and 112 $μ$m HD line profiles alone may constrain the disk gas mass to within a factor of two.

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arXiv2024

The Carbon Isotopic Ratio and Planet Formation

We present the first detection of 13CCH in a protoplanetary disk (TW Hya). Using observations of C2H we measure CCH/13CCH = 65 +/- 20 in gas with a CO isotopic ratio of 12CO/13CO = 21 +/- 5 (Yoshida et al. 2022a). The TW Hya disk exhibits a gas phase C/O that exceeds unity and C2H is the tracer of this excess carbon. We confirm that the TW Hya gaseous disk exhibits two separate carbon isotopic reservoirs as noted previously (Yoshida et al. 2022a). We explore two theoretical solutions for the development of this dichotomy. One model represents TW Hya today with a protoplanetary disk exposed to a cosmic ray ionization rate that is below interstellar as consistent with current estimates. We find that this model does not have sufficient ionization in cold (T < 40 K) layers to activate carbon isotopic fractionation. The second model investigates a younger TW Hya protostellar disk exposed to an interstellar cosmic ray ionization rate. We find that the younger model has sources of ionization deeper in a colder disk that generates two independent isotopic reservoirs. One reservoir is 12C-enriched carried by methane/hydrocarbon ices and the other is 13C-enriched carried by gaseous CO. The former potentially provides a source of methane/hydrocarbon ices to power the chemistry that generates the anomalously strong C$_2$H emission in this (and other) disk systems in later stages. The latter provides a source of gaseous 13C-rich material to generate isotopic enrichments in forming giant planets as recently detected in the super-Jupiter TYC 8998-760-1 b by Zhang et al. (2021).

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arXiv2023

Statistical Prediction of [CII] Observations by Constructing Probability Density Functions using SOFIA, Herschel, and Spitzer Observations

We present a statistical algorithm for predicting the [CII] emission from Herschel and Spitzer continuum images using probability density functions between the [CII] emission and continuum emission. The [CII] emission at 158 $μ$m is a critical tracer in studying the life cycle of interstellar medium and galaxy evolution. Unfortunately, its frequency is in the far infrared (FIR), which is opaque through the troposphere and cannot be observed from the ground except for highly red-shifted sources (z $\gtrsim$ 2). Typically [CII] observations of closer regions have been carried out using suborbital or space observatories. Given the high cost of these facilities and limited time availability, it is important to have highly efficient observations/operations in terms of maximizing science returns. This requires accurate prediction of the strength of emission lines and, therefore, the time required for their observation. However, [CII] emission has been hard to predict due to a lack of strong correlations with other observables. Here we adopt a new approach to making accurate predictions of [CII] emission by relating this emission simultaneously to several tracers of dust emission in the same region. This is done using a statistical methodology utilizing probability density functions (PDFs) among [CII] emission and Spitzer IRAC and Herschel PACS/SPIRE images. Our test result toward a star-forming region, RCW 120, demonstrates that our methodology delivers high-quality predictions with less than 30\% uncertainties over 80\% of the entire observation area, which is more than sufficient to test observation feasibility and maximize science return. The {\it pickle} dump files storing the PDFs and trained neural network module are accessible upon request and will support future far-infrared missions, for example, GUSTO and FIR Probe.

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arXiv2022

Linking atmospheric chemistry of the hot Jupiter HD 209458b to its formation location through infrared transmission and emission spectra

The elemental ratios of carbon, nitrogen, and oxygen in the atmospheres of hot Jupiters may hold clues to their formation locations in the protostellar disc. In this work, we adopt gas phase chemical abundances of C, N and O from several locations in a disc chemical kinetics model as sources for the envelope of the hot Jupiter HD 209458b and evolve the planet's atmospheric composition using a 1D chemical kinetics model, treating both vertical mixing and photochemistry. We consider two atmospheric pressure-temperature profiles, one with and one without a thermal inversion. From each of the resulting 32 atmospheric composition profiles, we find that the molecules CH4, NH3, HCN, and C2H2 are more prominent in the atmospheres computed using a realistic non-inverted P-T profile in comparison to a prior equilibrium chemistry based work which used an analytical P-T profile. We also compute the synthetic transmission and emission spectra for these atmospheres and find that many spectral features vary with the location in the disc where the planet's envelope was accreted. By comparing with the species detected using the latest high-resolution ground-based observations, our model suggests HD 209458b could have accreted most of its gas between the CO2 and CH4 icelines with a super solar C/O ratio from its protostellar disc, which in turn directly inherited its chemical abundances from the protostellar cloud. Finally, we simulate observing the planet with the James Webb Space Telescope (JWST) and show that differences in spectral signatures of key species can be recognized. Our study demonstrates the enormous importance of JWST in providing new insights into hot Jupiter's formation environments.

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arXiv2022

Vertical distribution of cyclopropenylidene and propadiene in the atmosphere of Titan

Titan's atmosphere is a natural laboratory for exploring the photochemical synthesis of organic molecules. Significant recent advances in the study of the atmosphere of Titan include: (a) detection of C$_3$ molecules: C$_3$H$_6$, CH$_2$CCH$_2$, c-C$_3$H$_2$, and (b) retrieval of C$_6$H$_6$, which is formed primarily via C$_3$ chemistry, from Cassini-UVIS data. The detection of $c$-C$_3$H$_2$ is of particular significance since ring molecules are of great astrobiological importance. Using the Caltech/JPL KINETICS code, along with the best available photochemical rate coefficients and parameterized vertical transport, we are able to account for the recent observations. It is significant that ion chemistry, reminiscent of that in the interstellar medium, plays a major role in the production of c-C$_3$H$_2$ above 1000 km.

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arXiv2022

Comets in context: Comparing comet compositions with protosolar nebula models

Comets provide a valuable window into the chemical and physical conditions at the time of their formation in the young solar system. We seek insights into where and when these objects formed by comparing the range of abundances observed for nine molecules and their average values across a sample of 29 comets to the predicted midplane ice abundances from models of the protosolar nebula. Our fiducial model, where ices are inherited from the interstellar medium, can account for the observed mixing ratio ranges of each molecule considered, but no single location or time reproduces the abundances of all molecules simultaneously. This suggests that each comet consists of material processed under a range of conditions. In contrast, a model where the initial composition of disk material is `reset', wiping out any previous chemical history, cannot account for the complete range of abundances observed in comets. Using toy models that combine material processed under different thermal conditions we find that a combination of warm (CO-poor) and cold (CO-rich) material is required to account for both the average properties of the Jupiter-family and Oort cloud comets, and the individual comets we consider. This could occur by the transport (either radial or vertical) of ice-coated dust grains in the early solar system. Comparison of the models to the average Jupiter-family and Oort cloud comet compositions suggest the two families formed in overlapping regions of the disk, in agreement with the findings of A'Hearn et al. (2012) and with the predictions of the Nice model (Gomes et al. 2005, Tsiganis et al. 2005).

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arXiv2021

Non-ideal MHD simulations of subcritical prestellar cores with non-equilibrium chemistry

Non-ideal magnetohydrodynamic (MHD) effects are thought to be gravity's closest ally in overcoming the support of magnetic fields and in forming stars. Here, we modify the publicly available version of the adaptive mesh refinement code FLASH (Fryxell et al. 2000; Dubey et al. 2008) to include a detailed treatment of non-ideal MHD and study such effects in collapsing prestellar cores. We implement two very extended non-equilibrium chemical networks, the largest of which is comprised of $\sim$ 300 species and includes a detailed description of deuterium chemistry. The ambipolar-diffusion, Ohmic and Hall resistivities are then self-consistently calculated from the abundances of charged species. We present a series of 2-dimensional axisymmetric simulations where we vary the chemical model, cosmic-ray ionization rate, and grain distribution. We benchmark our implementation against ideal MHD simulations and previously-published results. We show that, at high densities ($n_{\rm{H_2}}>~10^6~\rm{cm^{-3}}$), the ion that carries most of the perpendicular and parallel conductivities is not $\rm{H_3^+}$ as was previously thought, but is instead $\rm{D_3^+}$.

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arXiv2020

Physical and Chemical Structure of the Disk and Envelope of the Class 0/I Protostar L1527

Sub-millimeter spectral line and continuum emission from the protoplanetary disks and envelopes of protostars are powerful probes of their structure, chemistry, and dynamics. Here we present a benchmark study of our modeling code, RadChemT, that for the first time uses a chemical model to reproduce ALMA C$^{18}$O (2-1) and CARMA $^{12}$CO (1-0) and N$_{2}$H$^{+}$ (1-0) observations of L1527, that allow us to distinguish the disk, the infalling envelope and outflow of this Class 0/I protostar. RadChemT combines dynamics, radiative transfer, gas chemistry and gas-grain reactions to generate models which can be directly compared with observations for individual protostars. Rather than individually fit abundances to a large number of free parameters, we aim to best match the spectral line maps by (i) adopting a physical model based on density structure and luminosity derived primarily from previous work that fit SED and 2D imaging data, updating it to include a narrow jet detected in CARMA and ALMA data near ($\leq 75$au) the protostar, and then (ii) computing the resulting astrochemical abundances for 292 chemical species. Our model reproduces the C$^{18}$O and N$_{2}$H$^{+}$ line strengths within a factor of 3.0; this is encouraging considering the pronounced abundance variation (factor $> 10^3$) between the outflow shell and CO snowline region near the midplane. Further, our modeling confirms suggestions regarding the anti-correlation between N$_{2}$H$^{+}$ and the CO snowline between 400 au to 2,000 au from the central star. Our modeling tools represent a new and powerful capability with which to exploit the richness of spectral line imaging provided by modern submillimeter interferometers.

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arXiv2018

An Ammonia Spectral Map of the L1495-B218 Filaments in the Taurus Molecular Cloud: II CCS & HC$_7$N Chemistry and Three Modes of Star Formation in the Filaments

We present deep CCS and HC$_7$N observations of the L1495-B218 filaments in the Taurus molecular cloud obtained using the K-band focal plane array on the 100m Green Bank Telescope. We observed the L1495-B218 filaments in CCS $J_N$ = 2$_1$$-$1$_0$ and HC$_7$N $J$ = 21$-$20 with a spectral resolution of 0.038 km s$^{-1}$ and an angular resolution of 31$''$. We observed strong CCS emission in both evolved and young regions and weak emission in two evolved regions. HC$_7$N emission is observed only in L1495A-N and L1521D. We find that CCS and HC$_7$N intensity peaks do not coincide with NH$_3$ or dust continuum intensity peaks. We also find that the fractional abundance of CCS does not show a clear correlation with the dynamical evolutionary stage of dense cores. Our findings and chemical modeling indicate that the fractional abundances of CCS and HC$_7$N are sensitive to the initial gas-phase C/O ratio, and they are good tracers of young condensed gas only when the initial C/O is close to solar value. Kinematic analysis using multiple lines including NH$_3$, HC$_7$N, CCS, CO, HCN, \& HCO$^+$ suggests that there may be three different star formation modes in the L1495-B218 filaments. At the hub of the filaments, L1495A/B7N has formed a stellar cluster with large-scale inward flows (fast mode), while L1521D, a core embedded in a filament, is slowly contracting due to its self-gravity (slow mode). There is also one isolated core that appears to be marginally stable and may undergo quasi-static evolution (isolated mode).

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