arXiv subjects

Samantha Scibelli

Publications and source records attributed to Samantha Scibelli.

At least 19 recordsLinked to original sources

arXiv2026

Deuteration of Organic Molecules as a Probe of Starless Core and Filament Evolution in Barnard 10

The deuterium fractionation of molecules in starless cores is sensitive to their dynamical histories, which recent magnetohydrodynamical simulations have shown to be extremely varied. The deuterated isotopologues of formaldehyde (H$_2$CO) and methanol (CH$_3$OH) probe deuterium fractionation in simple organic molecules. This complete survey targets 11 low-mass starless cores in the small, quiescent Barnard 10 (B10) region of the Taurus Molecular Cloud. The cores were observed using the 12m Arizona Radio Observatory telescope with a 100% detection rate in transitions of o/pH$_2$CO, HDCO, pD$_2$CO, A/E-CH$_3$OH, and CH$_2$DOH and the dense gas tracer N$_2$H$^+$. The HDCO and pD$_2$CO column densities and deuterium fractions are not correlated with those of the grain-surface deuteration tracer CH$_2$DOH, indicating significant gas-phase formation of deuterated formaldehyde. The observed deuterium fractions do not correlate with evolutionary indicators (e.g. core central density) or physical conditions (e.g. core mass). We also find that cores within the northwestern filament have lower deuterium fractions than cores of similar densities in the other two filaments. These could indicate differential core evolution both between and within entire filaments. Comparing the deuterium fractions of the B10 sample to published sources across different evolutionary stages suggests the inheritance of D$_2$CO from starless cores, although more surveys of organic deuteration are needed to conclusively determine inheritance.

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arXiv2026

Inventories of Rich Carbon-Chain Chemistry in Prestellar and Starless Cores in the Perseus Molecular Cloud

Carbon-chain molecules serve as an important reservoir of reactive organic matter that will eventually be incorporated into protoplanetary disks, planets, and cometary material. Prestellar and starless cores are composed of cold (~ 10 K) and dense (~ 10$^5$ cm$^{-3}$) clumps of gas and dust within molecular clouds, and are nurseries for low-mass stars and planetary systems. Surveys of starless cores have focused on the study of complex organic molecules, COMs, whereas observations of carbon-chains in starless cores are limited. We analyze the carbon-chain inventories of 15 prestellar and starless cores in the Perseus Molecular Cloud. Using Yebes 40m single-dish observations, we detect CS, CCS, CCCS, HC$_3$N, DC$_3$N, and HC$_5$N in at least 10/15 cores and HC$_7$N in 4/15 cores. Our study also finds related isotopologues, where $^{13}$CS, C$^{34}$S, C$^{13}$CS, CC$^{34}$S, H$^{13}$CCCN, HC$^{13}$CCN, HCC$^{13}$CN, HC$^{13}$CCCCN, HCC$^{13}$CCCN, HCCC$^{13}$CCN, HCCCCC$^{15}$N, and DCCCCCN are detected. We report detection statistics, compare column density ratios with Taurus, Serpens, and protostar sources, examine DC$_3$N/HC$_3$N deuterium fractionation, and investigate the relative abundances and correlations between cyanopolyyne (HC$_n$N) and sulfur-bearing (C$_n$S) carbon-chains. The diverse suite of species detected reveals the richness of carbon-chain chemistry in Perseus and illustrates how local environmental conditions, such as density, temperature, and proximity to protostellar activity, shape each core's molecular inventory and relative evolutionary phase. Our findings provide a glimpse into the carbon-chain reservoir of starless and prestellar cores in the Perseus, which may ultimately be inherited by emerging protoplanetary disks and later integrated into planetary systems and biologically relevant material.

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arXiv2025

The Green Bank Ammonia Survey: Data Release 2

We present an overview of the final data release (DR2) from the Green Bank Ammonia Survey (GAS). GAS is a Large Program at the Green Bank Telescope to map all Gould Belt star-forming regions with $A_\mathrm{V} \gtrsim 7$~mag visible from the northern hemisphere in emission from NH$_3$ and other key molecular tracers. This final release includes the data for all the regions observed: Heiles Cloud 2 and B18 in Taurus; Barnard 1, Barnard 1-E, IC348, NGC 1333, L1448, L1451, and Per7/34 in Perseus; L1688 and L1689 in Ophiuchus; Orion A (North and South) and Orion B in Orion; Cepheus, B59 in Pipe; Corona Australis (CrA) East and West; IC5146; and Serpens Aquila and MWC297 in Serpens. Similar to what was presented in GAS DR1, we find that the NH$_3$ emission and dust continuum emission from Herschel correspond closely. We find that the NH$_3$ emission is generally extended beyond the typical 0.1 pc length scales of dense cores, and we find that the transition between coherent core and turbulent cloud is a common result. This shows that the regions of coherence are common throughout different star forming regions, with a substantial fraction of the high column density regions displaying subsonic non-thermal velocity dispersions. We produce maps of the gas kinematics, temperature, and NH$_3$ column densities through forward modeling of the hyperfine structure of the NH$_3$ (1,1) and (2,2) lines. We show that the NH$_3$ velocity dispersion, $σ_v$, and gas kinetic temperature, $T_{\rm kin}$, vary systematically between the regions included in this release, with an increase in both the mean value and spread of $σ_v$ and $T_{\rm kin}$ with increasing star formation activity. The data presented in this paper are publicly available via \dataset[DOI: 10.11570/24.0091]{https://doi.org/10.11570/24.0091}.

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arXiv2025

Nascent chemical complexity in prestellar core IRAS 16293E: complex organics and deuterated methanol

Prestellar cores represent early sites of low-mass ($M$ $\leq$ few M$_\odot$) star and planet formation and provide insight into initial chemical conditions of complex organic molecules (COMs). Deuterated COMs trace the degree of molecular inheritance and/or reprocessing, as high deuteration in protostellar systems suggests COMs forming during the prestellar stage when deuteration is enhanced. Within the L1689N molecular cloud, the prestellar core IRAS 16293E sits $90^{"}$ eastward of the chemically-rich IRAS 16293-2422 A and B protostellar system. A unique view of star formation inside a common natal cloud, IRAS 16293A, B, and E all show some of the highest levels of deuteration in the ISM, with a number of D/H ratios $10^{5}$ times higher than Solar. We investigate for the first time the deuteration levels of the simplest COM, methanol (CH$_3$OH), in IRAS 16293E. Using the Arizona Radio Observatory (ARO) 12 m telescope, we target favorable transitions of CH$_2$DOH, CHD$_2$OH, $^{13}$CH$_3$OH, and several higher complexity COMs (including acetaldehyde, CH$_3$CHO, methyl formate, HCOOCH$_3$, and dimethyl ether, CH$_3$OCH$_3$) in the 3 mm band. Follow-up observations with the Yebes 40 m telescope provided additional transitions in the 7 mm (Q-band). We report the first detections of these COMs and deuterated methanol in prestellar core IRAS 16293E and use our observations to calculate excitation temperatures, column densities, and relative abundance ratios. Striking similarities are found between relative molecular ratios and D/H values when comparing IRAS 16293E to the A and B protostars, as well as to a heterogeneous sample of other prestellar cores, protostars, and the comet 67P/Churyumov-Gerasimenko. Our results support the idea that there is a limited amount of chemical reprocessing of COMs when prestellar cores collapse and heat-up during the protostellar phase.

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arXiv2025

First Detections of PN, PO, and PO+ toward a Shocked Low-mass Starless Core

Phosphorus is a key element that plays an essential role in biological processes important for living organisms on Earth. The origin and connection of phosphorus-bearing molecules to early Solar system objects and star-forming molecular clouds is therefore of great interest, yet there are limited observations throughout different stages of low-mass ($M < $ a few M$_\odot$) star formation. Observations from the Yebes 40 m and IRAM 30 m telescopes detect for the first time in the 7mm, 3mm, and 2mm bands multiple transitions of PN and PO, as well as a single transition of PO$^{+}$, toward a low-mass starless core. The presence of PN, PO and PO$^{+}$ is kinematically correlated with bright SiO(1-0) emission. Our results reveal not only that shocks are the main driver of releasing phosphorus from dust grains and into the gas-phase, but that the emission originates from gas not affiliated with the shock itself, but quiescent gas that has been shocked in the recent past. From radiative transfer calculations, the PO/PN abundance ratio is found to be $3.1^{+0.4}_{-0.6}$, consistent with other high-mass and low-mass star-forming regions. This first detection of PO$^{+}$ toward any low-mass star-forming region reveals a PO$^{+}$/PO ratio of $0.0115^{+0.0008}_{-0.0009}$, a factor of ten lower than previously determined from observations of a Galactic Center molecular cloud, suggesting its formation can occur under more standard Galactic cosmic-ray ionization rates. These results motivate the need for additional observations that can better disentangle the physical mechanisms and chemical drivers of this precursor of prebiotic chemistry.

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arXiv2024

Molecular Distributions and Abundances in the Binary-Shaped Outflow of V Hya

Binaries are known to play a key role in the mass loss and dynamical environments of evolved stars. Stellar and sub-stellar companion interactions produce complex wind morphologies including rotating/expanding disks, bipolar outflows, and spiral wind patterns; however, the connection between these many structures and the gas phase chemistry they harbor is not well-constrained. To expand the sample of chemical inventories in interacting systems, we present a detailed spectroscopic case study of the binary C-rich Asymptotic Giant Branch (AGB) star V Hya. Using spatially resolved ALMA observations at Bands 3, 6 and 7, we characterize the rotational emission lines and distributions of molecules in its surrounding disk undergoing dynamical expansion (DUDE). We detect emission from over 15 molecules and isotopologues toward this source, and present resolved maps for the brightest tracers of carbonaceous chemistry (e.g. CCH, C4H, HC5N, HNC, CH3CN). Employing LTE and non-LTE models of emission from the DUDE, we estimate the abundance distributions for optically thin species, and compare them with prototypical carbon-rich AGB envelopes. We find that the average abundances of detected species are within a factor of ${\sim}5$ from sources with similar mass-loss rates; however, the distribution of daughter species in V Hya is much more compact, with carbon chain species (CCH, C4H, HC3N) appearing with abundances $>$10$^{-7}$ even in the innermost sampled regions (200 au) of the disk.

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arXiv2024

Survey of Complex Organic Molecules in Starless and Prestellar Cores in the Perseus Molecular Cloud

Cold ($\sim$10 K) and dense ($\sim$10$^{5}$ cm$^{-3}$) cores of gas and dust within molecular clouds, known as starless and dynamically evolved prestellar cores, are the birthplaces of low-mass ($M$ $\leq$ few M$_\odot$) stars. As detections of interstellar complex organic molecules, or COMs, in starless cores has increased, abundance comparisons suggest that some COMs might be seeded early in the star formation process and inherited to later stages (i.e., protostellar disks and eventually comets). To date observations of COMs in starless cores have been limited, with most detections reported solely in the Taurus Molecular Cloud. It is therefore still a question whether different environments affect abundances. We have surveyed 35 starless and prestellar cores in the Perseus Molecular Cloud with the Arizona Radio Observatory (ARO) 12m telescope detecting both methanol, CH$_3$OH, and acetaldehyde, CH$_3$CHO, in 100% and 49% of the sample, respectively. In the sub-sample of 15 cores where CH$_3$CHO was detected at $>3σ$ ($\sim$18 mK) with the ARO 12m, follow-up observations with the Yebes 40m telescope were carried out. Detections of formic acid, $t$-HCOOH, ketene, H$_2$CCO, methyl cyanide, CH$_3$CN, vinyl cyanide, CH$_2$CHCN, methyl formate, HCOOCH$_3$, and dimethyl ether, CH$_3$OCH$_3$, are seen in at least $20\%$ of the cores. We discuss detection statistics, calculate column densities, and compare abundances across various stages of low-mass star formation. Our findings have more than doubled COM detection statistics in cold cores and show COMs are prevalent in the gas before star and planet formation in the Perseus Molecular Cloud.

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arXiv2023

Prevalence of Complex Organic Molecules in Starless and Prestellar Cores within the Taurus Molecular Cloud

The detection of complex organic molecules (COMs) toward dense, collapsing prestellar cores has sparked interest in the fields of astrochemistry and astrobiology, yet the mechanisms for COM formation are still debated. It was originally believed that COMs initially form in ices which are then irradiated by UV radiation from the surrounding interstellar radiation field as well as forming protostars and subsequently photodesorbed into the gas-phase. However, starless and prestellar cores do not have internal protostars to heat-up and sublimate the ices. Alternative models using chemical energy have been developed to explain the desorption of COMs, yet in order to test these models robust measurements of COM abundances are needed toward representative samples of cores. We've conducted a large-sample survey of 31 starless and prestellar cores in the Taurus Molecular Cloud, detecting methanol (CH$_3$OH) in 100$\%$ of the cores targeted and acetaldehyde (CH$_3$CHO) in 70$\%$. At least two transition lines of each molecule were measured, allowing us to place tight constraints on excitation temperature, column density and abundance. Additional mapping of methanol revealed extended emission, detected down to A$_\mathrm{V}$ as low as $\sim$ 3 mag. We find complex organic molecules are detectable in the gas-phase and are being formed early, at least hundreds of thousands of years prior to star and planet formation. The precursor molecule, CH$_3$OH, may be chemically linked to the more complex CH$_3$CHO, however higher spatial resolution maps are needed to further test chemical models.

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arXiv2023

3D Radiative Transfer Modelling and Virial Analysis of Starless Cores in the B10 Region of the Taurus Molecular Cloud

Low-mass stars like our Sun begin their evolution within cold (10 K) and dense ($\sim 10^5$ cm$^{-3}$) cores of gas and dust. The physical structure of starless cores is best probed by thermal emission of dust grains. We present a high resolution dust continuum study of the starless cores in the B10 region of the Taurus Molecular Cloud. New observations at 1.2mm and 2.0mm ($12^{"}$ and $18^{"}$ resolution) with the NIKA2 instrument on the IRAM 30m have probed the inner regions of 14 low-mass starless cores. We perform sophisticated 3D radiative transfer modelling for each of these cores through the radiative transfer framework $\textit{pandora}$, which utilizes RADMC-3D. Model best-fits constrain each cores' central density, density slope, aspect ratio, opacity, and interstellar radiation field strength. These `typical' cores in B10 span central densities from $5 \times 10^4 - 1 \times 10^6$ cm$^{-3}$, with a mean value of $2.6 \times 10^5$ cm$^{-3}$. We find the dust opacity laws assumed in the 3D modelling, as well as the estimates from $\textit{Herschel}$, have dust emissivity indices, $β$'s, on the lower end of the distribution constrained directly from the NIKA2 maps, which averages to $β= 2.01\pm0.48$. From our 3D density structures and archival NH$_3$ data, we perform a self-consistent virial analysis to assess each core's stability. Ignoring magnetic field contributions, we find 9 out of the 14 cores ($64\%$) are either in virial equilibrium or are bound by gravity and external pressure. To push the bounded cores back to equilibrium, an effective magnetic field difference of only $\sim 15 μ$G is needed.

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arXiv2023

Velocity-Coherent Substructure in TMC-1: Inflow and Fragmentation

Filamentary structures have been found nearly ubiquitously in molecular clouds and yet their formation and evolution is still poorly understood. We examine a segment of Taurus Molecular Cloud 1 (TMC-1) that appears as a single, narrow filament in continuum emission from dust. We use the Regularized Optimization for Hyper-Spectral Analysis (ROHSA), a Gaussian decomposition algorithm which enforces spatial coherence when fitting multiple velocity components simultaneously over a data cube. We analyze HC$_5$N (9-8) line emission as part of the Green Bank Ammonia Survey (GAS) and identify three velocity-coherent components with ROHSA. The two brightest components extend the length of the filament, while the third component is fainter and clumpier. The brightest component has a prominent transverse velocity gradient of $2.7 \pm 0.1$ km s$^{-1}$ pc$^{-1}$ that we show to be indicative of gravitationally induced inflow. In the second component, we identify regularly spaced emission peaks along its length. We show that the local minima between pairs of adjacent HC$_5$N peaks line up closely with submillimetre continuum emission peaks, which we argue is evidence for fragmentation along the spine of TMC-1. While coherent velocity components have been described as separate physical structures in other star-forming filaments, we argue that the two bright components identified in HC$_5$N emission in TMC-1 are tracing two layers in one filament: a lower density outer layer whose material is flowing under gravity towards the higher density inner layer of the filament.

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arXiv2022

A Survey of Deuterated Ammonia in the Cepheus Star-Forming Region L1251

Understanding the chemical processes during starless core and prestellar core evolution is an important step in understanding the initial stages of star and disk formation. This project is a study of deuterated ammonia, o-NH$_2$D, in the L1251 star-forming region toward Cepheus. Twenty-two dense cores (twenty of which are starless or prestellar, and two of which have a protostar), previously identified by p-NH$_3$ (1,1) observations, were targeted with the 12m Arizona Radio Observatory telescope on Kitt Peak. o-NH$_2$D J$_{\rm{K_a} \rm{K_c}}^{\pm} =$ $1_{11}^{+} \rightarrow 1_{01}^{-}$ was detected in 13 (59\%) of the NH$_3$-detected cores with a median sensitivity of $σ_{T_{mb}} = 17$ mK. All cores detected in o-NH$_2$D at this sensitivity have p-NH$_3$ column densities $> 10^{14}$ cm$^{-2}$. The o-NH$_2$D column densities were calculated using the constant excitation temperature (CTEX) approximation while correcting for the filling fraction of the NH$_3$ source size. The median deuterium fraction was found to be 0.11 (including 3$σ$ upper limits). However, there are no strong, discernible trends in plots of deuterium fraction with any physical or evolutionary variables. If the cores in L1251 have similar initial chemical conditions, then this result is evidence of the cores physically evolving at different rates.

astro-ph.SR↗

arXiv2021

Methanol Mapping in Cold Cores: Testing Model Predictions

Chemical models predict that in cold cores gas-phase methanol is expected to be abundant at the outer edge of the CO depletion zone, where CO is actively adsorbed. CO adsorption correlates with volume density in cold cores, and, in nearby molecular clouds, the catastrophic CO freeze-out happens at volume densities above 10$^4$ cm$^{-3}$. The methanol production rate is maximized there and its freeze-out rate does not overcome its production rate, while the molecules are shielded from UV destruction by gas and dust. Thus, in cold cores, methanol abundance should generally correlate with visual extinction that depends both on volume and column density. In this work, we test the most basic model prediction that maximum methanol abundance is associated with a local $A_V\simeq$4 mag in dense cores and constrain the model parameters with the observational data. With the IRAM 30 m antenna, we mapped the CH$_3$OH (2-1) and (3-2) transitions toward seven dense cores in the L1495 filament in Taurus to measure the methanol abundance. We use the Herschel/SPIRE maps to estimate visual extinction, and the C$^{18}$O(2-1) maps from Tafalla & Hacar (2015) to estimate CO depletion. We explored the observed and modeled correlations between the methanol abundances, CO depletion, and visual extinction varying the key model parameters. The modeling results show that hydrogen surface diffusion via tunneling is crucial to reproduce the observed methanol abundances, and the needed reactive desorption efficiency matches the one deduced from laboratory experiments.

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arXiv2021

Detection of Complex Organic Molecules in Young Starless Core L1521E

Determining the level of chemical complexity within dense starless and gravitationally bound prestellar cores is crucial for constructing chemical models, which subsequently constrain the initial chemical conditions of star formation. We have searched for complex organic molecules (COMs) in the young starless core L1521E, and report the first clear detection of dimethyl ether (CH$_3$OCH$_3$), methyl formate (HCOOCH$_3$), and vinyl cyanide (CH$_2$CHCN). Eight transitions of acetaldehyde (CH$_3$CHO) were also detected, five of which (A states) were used to determine an excitation temperature to then calculate column densities for the other oxygen-bearing COMs. If source size was not taken into account (i.e., if filling fraction was assumed to be one), column density was underestimated, and thus we stress the need for higher resolution mapping data. We calculated L1521E COM abundances and compared them to other stages of low-mass star formation, also finding similarities to other starless/prestellar cores, suggesting related chemical evolution. The scenario that assumes formation of COMs in gas-phase reactions between precursors formed on grains and then ejected to the cold gas via reactive desorption was tested and was unable to reproduce observed COM abundances, with the exception of CH$_3$CHO. These results suggest that COMs observed in cold gas are formed not by gas-phase reactions alone, but also through surface reactions on interstellar grains. Our observations present a new, unique challenge for existing theoretical astrochemical models.

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arXiv2020

A Survey of CH2DOH Towards Starless and Prestellar Cores in the Taurus Molecular Cloud

Recent observations indicate that organic molecules are prevalent towards starless and prestellar cores. Deuteration of these molecules has not been well-studied during the starless phase. Published observations of singly-deuterated methanol, CH$_2$DOH, have only been observed in a couple of well-studied, dense and evolved prestellar cores (e.g. L1544, L183). Since the formation of gas-phase methanol during this cold phase is believed to occur via desorption from the icy grain surfaces, observations of CH$_2$DOH may be useful as a probe of the deuterium fraction in the ice mantles of dust grains. We present a systematic survey of CH$_2$DOH towards 12 starless and prestellar cores in the B10 region of the Taurus Molecular Cloud. Nine of the twelve cores are detected with [CH$_2$DOH]/[CH$_3$OH] ranging from $< 0.04$ to $0.23^{+0.12}_{-0.06}$ with a median value of $0.11$. Sources not detected tend to have larger virial parameters and larger methanol linewidths than detected sources. The results of this survey indicate that deuterium fractionation of organic molecules, such as methanol, during the starless phase may be more easily detectable than previously thought.

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arXiv2020

Relative Alignment between Dense Molecular Cores and Ambient Magnetic Field: The Synergy of Numerical Models and Observations

The role played by magnetic field during star formation is an important topic in astrophysics. We investigate the correlation between the orientation of star-forming cores (as defined by the core major axes) and ambient magnetic field directions in 1) a 3D MHD simulation, 2) synthetic observations generated from the simulation at different viewing angles, and 3) observations of nearby molecular clouds. We find that the results on relative alignment between cores and background magnetic field in synthetic observations slightly disagree with those measured in fully 3D simulation data, which is partly because cores identified in projected 2D maps tend to coexist within filamentary structures, while 3D cores are generally more rounded. In addition, we examine the progression of magnetic field from pc- to core-scale in the simulation, which is consistent with the anisotropic core formation model that gas preferably flow along the magnetic field toward dense cores. When comparing the observed cores identified from the GBT Ammonia Survey (GAS) and Planck polarization-inferred magnetic field orientations, we find that the relative core-field alignment has a regional dependence among different clouds. More specifically, we find that dense cores in the Taurus molecular cloud tend to align perpendicular to the background magnetic field, while those in Perseus and Ophiuchus tend to have random (Perseus) or slightly parallel (Ophiuchus) orientations with respect to the field. We argue that this feature of relative core-field orientation could be used to probe the relative significance of the magnetic field within the cloud.

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arXiv2020

Velocity-coherent Filaments in NGC 1333: Evidence for Accretion Flow?

Recent observations of global velocity gradients across and along molecular filaments have been interpreted as signs of gas accreting onto and along these filaments, potentially feeding star-forming cores and proto-clusters. The behavior of velocity gradients in filaments, however, has not been studied in detail, particularly on small scales (< 0.1 pc). In this paper, we present MUFASA, an efficient, robust, and automatic method to fit ammonia lines with multiple velocity components, generalizable to other molecular species. We also present CRISPy, a Python package to identify filament spines in 3D images (e.g., position-position-velocity cubes), along with a complementary technique to sort fitted velocity components into velocity-coherent filaments. In NGC 1333, we find a wealth of velocity gradient structures on a beam-resolved scale of ~0.05 pc. Interestingly, these local velocity gradients are not randomly oriented with respect to filament spines and their perpendicular, i.e., radial, component decreases in magnitude towards the spine for many filaments. Together with remarkably constant velocity gradients on larger scales along many filaments, these results suggest a scenario in which gas falling onto filaments is progressively damped and redirected to flow along these filaments.

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arXiv2019

Droplets II: Internal Velocity Structures and Potential Rotational Motions in Pressure-dominated Coherent Structures

We present an analysis of the internal velocity structures of the newly identified sub-0.1 pc coherent structures, droplets, in L1688 and B18. By fitting 2D linear velocity fields to the observed maps of velocity centroids, we determine the magnitudes of linear velocity gradients and examine the potential rotational motions that could lead to the observed velocity gradients. The results show that the droplets follow the same power-law relation between the velocity gradient and size found for larger-scale dense cores. Assuming that rotational motion giving rise to the observed velocity gradient in each core is a solid-body rotation of a rotating body with a uniform density, we derive the "net rotational motions" of the droplets. We find a ratio between rotational and gravitational energies, $β$, of $\sim 0.046$ for the droplets, and when including both droplets and larger-scale dense cores, we find $β\sim 0.039$. We then examine the alignment between the velocity gradient and the major axis of each droplet, using methods adapted from the histogram of relative orientations (HRO) introduced by Soler et al. (2013). We find no definitive correlation between the directions of velocity gradients and the elongations of the cores. Lastly, we discuss physical processes other than rotation that may give rise to the observed velocity field.

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arXiv2019

Droplets I: Pressure-Dominated Sub-0.1 pc Coherent Structures in L1688 and B18

We present the observation and analysis of newly discovered coherent structures in the L1688 region of Ophiuchus and the B18 region of Taurus. Using data from the Green Bank Ammonia Survey (GAS), we identify regions of high density and near-constant, almost-thermal, velocity dispersion. Eighteen coherent structures are revealed, twelve in L1688 and six in B18, each of which shows a sharp "transition to coherence" in velocity dispersion around its periphery. The identification of these structures provides a chance to study the coherent structures in molecular clouds statistically. The identified coherent structures have a typical radius of 0.04 pc and a typical mass of 0.4 Msun, generally smaller than previously known coherent cores identified by Goodman et al. (1998), Caselli et al. (2002), and Pineda et al. (2010). We call these structures "droplets." We find that unlike previously known coherent cores, these structures are not virially bound by self-gravity and are instead predominantly confined by ambient pressure. The droplets have density profiles shallower than a critical Bonnor-Ebert sphere, and they have a velocity (VLSR) distribution consistent with the dense gas motions traced by NH3 emission. These results point to a potential formation mechanism through pressure compression and turbulent processes in the dense gas. We present a comparison with a magnetohydrodynamic simulation of a star-forming region, and we speculate on the relationship of droplets with larger, gravitationally bound coherent cores, as well as on the role that droplets and other coherent structures play in the star formation process.

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