arXiv subjects

Rachel Friesen

Publications and source records attributed to Rachel Friesen.

At least 37 records · Page 2Linked to original sources

arXiv2022

A study of 90 GHz dust emissivity on molecular cloud and filament scales

Recent observations from the MUSTANG2 instrument on the Green Bank Telescope have revealed evidence of enhanced long-wavelength emission in the dust spectral energy distribution (SED) in the Orion Molecular Cloud (OMC) 2/3 filament on 25" (0.1 pc) scales. Here we present a measurement of the SED on larger spatial scales (map size 0.5-3 degrees or 3-20 pc), at somewhat lower resolution (120", corresponding to 0.25 pc at 400 pc) using data from the Herschel satellite and Atacama Cosmology Telescope (ACT). We then extend the 120"-scale investigation to other regions covered in the Herschel Gould Belt Survey (HGBS) specifically: the dense filaments in the southerly regions of Orion A; Orion B; and Serpens-S. Our dataset in aggregate covers approximately 10 square degrees, with continuum photometry spanning from 160um to 3mm. These OMC 2/3 data display excess emission at 3mm, though less (10.9% excess) than what is seen at higher resolution. Strikingly, we find that the enhancement is present even more strongly in the other filaments we targeted, with an average excess of 42.4% and 30/46 slices showing an inconsistency with the modified blackbody to at least 4σ. Applying this analysis to the other targeted regions, we lay the groundwork for future high-resolution analyses. Additionally, we also consider a two-component dust model motivated by Planck results and an amorphous grain dust model. While both of these have been proposed to explain deviations in emission from a generic modified blackbody (MBB), we find that they have significant drawbacks, requiring many spectral points or lacking experimental data coverage.

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arXiv2022

The Central 1000 au of a Pre-stellar Core Revealed with ALMA. II. Almost Complete Freeze-out

Pre-stellar cores represent the initial conditions in the process of star and planet formation. Their low temperatures ($<$10 K) allow the formation of thick icy dust mantles, which will be partially preserved in the future protoplanetary disks, ultimately affecting the chemical composition of planetary systems. Previous observations have shown that carbon- and oxygen-bearing species, in particular CO, are heavily depleted in pre-stellar cores due to the efficient molecular freeze-out onto the surface of cold dust grains. However, N-bearing species such as NH$_3$ and, in particular, its deuterated isotopologues, appear to maintain high abundances where CO molecules are mainly in solid phase. Thanks to ALMA, we present here the first clear observational evidence of NH$_2$D freeze-out toward the L1544 pre-stellar core, suggestive of the presence of a"complete-depletion zone" within a $\simeq$1800 au radius, in agreement with astrochemical pre-stellar core model predictions. Our state-of-the-art chemical model coupled with a non-LTE radiative transfer code demonstrates that NH$_2$D becomes mainly incorporated in icy mantles in the central 2000 au and starts freezing-out already at $\simeq$7000 au. Radiative transfer effects within the pre-stellar core cause the NH$_2$D(1$_{11}$-1$_{01}$) emission to appear centrally concentrated, with a flattened distribution within the central $\simeq$3000 au, unlike the 1.3 mm dust continuum emission which shows a clear peak within the central $\simeq$1800 au. This prevented NH$_2$D freeze-out to be detected in previous observations, where the central 1000 au cannot be spatially resolved.

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arXiv2022

B-fields in Star-Forming Region Observations (BISTRO): Magnetic Fields in the Filamentary Structures of Serpens Main

We present 850 $μ$m polarimetric observations toward the Serpens Main molecular cloud obtained using the POL-2 polarimeter on the James Clerk Maxwell Telescope (JCMT) as part of the B-fields In STar-forming Region Observations (BISTRO) survey. These observations probe the magnetic field morphology of the Serpens Main molecular cloud on about 6000 au scales, which consists of cores and six filaments with different physical properties such as density and star formation activity. Using the histogram of relative orientation (HRO) technique, we find that magnetic fields are parallel to filaments in less dense filamentary structures where $N_{H_2} < 0.93\times 10^{22}$ cm$^{-2}$ (magnetic fields perpendicular to density gradients), while being perpendicular to filaments (magnetic fields parallel to density gradients) in dense filamentary structures with star formation activity. Moreover, applying the HRO technique to denser core regions, we find that magnetic field orientations change to become perpendicular to density gradients again at $N_{H_2} \approx 4.6 \times 10^{22}$ cm$^{-2}$. This can be interpreted as a signature of core formation. At $N_{H_2} \approx 16 \times 10^{22}$ cm$^{-2}$ magnetic fields change back to being parallel to density gradients once again, which can be understood to be due to magnetic fields being dragged in by infalling material. In addition, we estimate the magnetic field strengths of the filaments ($B_{POS} = 60-300~μ$G)) using the Davis-Chandrasekhar-Fermi method and discuss whether the filaments are gravitationally unstable based on magnetic field and turbulence energy densities.

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arXiv2021

The JCMT BISTRO Survey: An 850/450$μ$m Polarization Study of NGC 2071IR in OrionB

We present the results of simultaneous 450 $μ$m and 850 $μ$m polarization observations toward the massive star forming region NGC 2071IR, a target of the BISTRO (B-fields in Star-Forming Region Observations) Survey, using the POL-2 polarimeter and SCUBA-2 camera mounted on the James Clerk Maxwell Telescope. We find a pinched magnetic field morphology in the central dense core region, which could be due to a rotating toroidal disk-like structure and a bipolar outflow originating from the central young stellar object, IRS 3. Using the modified Davis-Chandrasekhar-Fermi method, we obtain a plane-of-sky magnetic field strength of 563$\pm$421 $μ$G in the central $\sim$0.12 pc region from 850 $μ$m polarization data. The corresponding magnetic energy density of 2.04$\times$10$^{-8}$ erg cm$^{-3}$ is comparable to the turbulent and gravitational energy densities in the region. We find that the magnetic field direction is very well aligned with the whole of the IRS 3 bipolar outflow structure. We find that the median value of polarization fractions, 3.0 \%, at 450 $μ$m in the central 3 arcminute region, which is larger than the median value of 1.2 \% at 850 $μ$m. The trend could be due to the better alignment of warmer dust in the strong radiation environment. We also find that polarization fractions decrease with intensity at both wavelengths, with slopes, determined by fitting a Rician noise model, of $0.59 \pm 0.03$ at 450 $μ$m and $0.36 \pm 0.04$ at 850 $μ$m, respectively. We think that the shallow slope at 850 $μ$m is due to grain alignment at the center being assisted by strong radiation from the central young stellar objects.

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arXiv2021

Revealing the diverse magnetic field morphologies in Taurus dense cores with sensitive sub-millimeter polarimetry

We have obtained sensitive dust continuum polarization observations at 850 $μ$m in the B213 region of Taurus using POL-2 on SCUBA-2 at the James Clerk Maxwell Telescope (JCMT), as part of the BISTRO (B-fields in STar-forming Region Observations) survey. These observations allow us to probe magnetic field (B-field) at high spatial resolution ($\sim$2000 au or $\sim$0.01 pc at 140 pc) in two protostellar cores (K04166 and K04169) and one prestellar core (Miz-8b) that lie within the B213 filament. Using the Davis-Chandrasekhar-Fermi method, we estimate the B-field strengths in K04166, K04169, and Miz-8b to be 38$\pm$14 $μ$G, 44$\pm$16 $μ$G, and 12$\pm$5 $μ$G, respectively. These cores show distinct mean B-field orientations. B-field in K04166 is well ordered and aligned parallel to the orientations of the core minor axis, outflows, core rotation axis, and large-scale uniform B-field, in accordance with magnetically regulated star formation via ambipolar diffusion taking place in K04166. B-field in K04169 is found to be ordered but oriented nearly perpendicular to the core minor axis and large-scale B-field, and not well-correlated with other axes. In contrast, Miz-8b exhibits disordered B-field which show no preferred alignment with the core minor axis or large-scale field. We found that only one core, K04166, retains a memory of the large-scale uniform B-field. The other two cores, K04169 and Miz-8b, are decoupled from the large-scale field. Such a complex B-field configuration could be caused by gas inflow onto the filament, even in the presence of a substantial magnetic flux.

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arXiv2020

Observations of magnetic fields surrounding LkH$α$ 101 taken by the BISTRO survey with JCMT-POL-2

We report the first high spatial resolution measurement of magnetic fields surrounding LkH$α$ 101, a part of the Auriga-California molecular cloud. The observations were taken with the POL-2 polarimeter on the James Clerk Maxwell Telescope within the framework of the B-fields In Star-forming Region Observations (BISTRO) survey. Observed polarization of thermal dust emission at 850 $μ$m is found to be mostly associated with the red-shifted gas component of the cloud. The magnetic field displays a relatively complex morphology. Two variants of the Davis-Chandrasekhar-Fermi method, unsharp masking and structure function, are used to calculate the strength of magnetic fields in the plane of the sky, yielding a similar result of $B_{\rm POS}\sim 115$ $\mathrmμ$G. The mass-to-magnetic-flux ratio in critical value units, $λ\sim0.3$, is the smallest among the values obtained for other regions surveyed by POL-2. This implies that the LkH$α$ 101 region is sub-critical and the magnetic field is strong enough to prevent gravitational collapse. The inferred $δB/B_0\sim 0.3$ implies that the large scale component of the magnetic field dominates the turbulent one. The variation of the polarization fraction with total emission intensity can be fitted by a power-law with an index of $α=0.82\pm0.03$, which lies in the range previously reported for molecular clouds. We find that the polarization fraction decreases rapidly with proximity to the only early B star (LkH$α$ 101) in the region. The magnetic field tangling and the joint effect of grain alignment and rotational disruption by radiative torques are potential of explaining such a decreasing trend.

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arXiv2020

The JCMT BISTRO Survey: Magnetic Fields Associated with a Network of Filaments in NGC 1333

We present new observations of the active star-formation region NGC 1333 in the Perseus molecular cloud complex from the James Clerk Maxwell Telescope B-Fields In Star-forming Region Observations (BISTRO) survey with the POL-2 instrument. The BISTRO data cover the entire NGC 1333 complex (~1.5 pc x 2 pc) at 0.02 pc resolution and spatially resolve the polarized emission from individual filamentary structures for the first time. The inferred magnetic field structure is complex as a whole, with each individual filament aligned at different position angles relative to the local field orientation. We combine the BISTRO data with low- and high- resolution data derived from Planck and interferometers to study the multiscale magnetic field structure in this region. The magnetic field morphology drastically changes below a scale of ~1 pc and remains continuous from the scales of filaments (~0.1 pc) to that of protostellar envelopes (~0.005 pc or ~1000 au). Finally, we construct simple models in which we assume that the magnetic field is always perpendicular to the long axis of the filaments. We demonstrate that the observed variation of the relative orientation between the filament axes and the magnetic field angles are well reproduced by this model, taking into account the projection effects of the magnetic field and filaments relative to the plane of the sky. These projection effects may explain the apparent complexity of the magnetic field structure observed at the resolution of BISTRO data toward the filament network.

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arXiv2020

Confirmation of Enhanced Long Wavelength Dust Emission in OMC 2/3

Previous continuum observations from the MUSTANG camera on the Green Bank Telescope (GBT) of the nearby star-forming filament OMC 2/3 found elevated emission at 3.3 mm relative to shorter wavelength data. As a consequence, the inferred dust emissivity index obtained from modified black body dust spectra was considerably lower than what is typically measured on $\sim 0.1 \, {\rm pc}$ scales in nearby molecular clouds. Here we present new observations of OMC 2/3 collected with the MUSTANG-2 camera on the GBT which confirm this elevated emission. We also present for the first time sensitive 1 cm observations made with the Ka-band receiver on the GBT which also show higher than expected emission. We use these observations--- along with Herschel, JCMT, Mambo, and GISMO data--- to assemble spectral energy distributions (SEDs) of a variety of structures in OMC 2/3 spanning the range $160 \, {\rm μm}$ to $1 \, {\rm cm}$. The data at 2 mm and shorter are generally consistent with a modified black body spectrum and a single value of $β\sim 1.6$. The 3 mm and 1 cm data, however, lie well above such an SED. The spectrum of the long wavelength excess is inconsistent with both free-free emission and standard "Spinning Dust" models for Anomalous Microwave Emission (AME). The 3 mm and 1 cm data could be explained by a flatter dust emissivity at wavelengths shorter than 2 mm, potentially in concert with AME in some regions.

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arXiv2019

The Formation of Stars -- From Filaments to Cores to Protostars and Protoplanetry Disks

Star formation involves the flow of gas and dust within molecular clouds into protostars and young stellar objects (YSOs) due to gravity. Along the way, these flows are shaped significantly by many other mechanisms, including pressure, turbulent motions, magnetic fields, stellar feedback, jets, and angular momentum. How all these mechanisms interact nonlinearly with each other on various length scales leads to the formation and evolution of substructures within clouds, including filaments, clumps, cores, disks, outflows, the protostars/YSOs themselves, and planets. In this white paper, prepared for the 2020 Long Range Plan panel which will recommend Canada's future directions for astronomy, we describe the observational and theoretical leadership in the star formation field that Canada's vibrant community has demonstrated over the past decade. Drawing from this extensive background, we identify five key questions that must be addressed for further progress to be made in understanding star formation in the next decade. Addressing these questions will improve our understanding of the dynamics of the dense gas and the role of the magnetic field in star formation, the optical properties of the dust used to trace mass and magnetic fields, the sources of variability in star-forming objects on short timescales, and the physical processes that specifically promote the clustering of stars. We further highlight key facilities in which Canada should become involved to continue making progress in this field. Single-dish facilities we recommend include LSST, trans-atmospheric far-infrared telescopes like BLAST-TNG and SPICA, and ground-based telescopes like JCMT, GBT, and CCAT-p. Interferometric facilities we recommend include ALMA, ngVLA, and SKA1.

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arXiv2019

The Next Generation Very Large Array

The next generation Very Large Array (ngVLA) is a transformational radio observatory being designed by the U.S. National Radio Astronomy Observatory (NRAO). It will provide order of magnitude improvements in sensitivity, resolution, and uv coverage over the current Jansky Very Large Array (VLA) at ~1.2-50 GHz and extend the frequency range up to 70-115 GHz. This document is a white paper written by members of the Canadian community for the 2020 Long Range Plan panel, which will be making recommendations on Canada's future directions in astronomy. Since Canadians have been historically major users of the VLA and have been valued partners with NRAO for ALMA, Canada's participation in ngVLA is welcome. Canadians have been actually involved in ngVLA discussions for the past five years, and have played leadership roles in the ngVLA Science and Technical Advisory Councils. Canadian technologies are also very attractive for the ngVLA, in particular our designs for radio antennas, receivers, correlates, and data archives, and our industrial capacities to realize them. Indeed, the Canadian designs for the ngVLA antennas and correlator/beamformer are presently the baseline models for the project. Given the size of Canada's radio community and earlier use of the VLA (and ALMA), we recommend Canadian participation in the ngVLA at the 7% level. Such participation would be significant enough to allow Canadian leadership in gVLA's construction and usage. Canada's participation in ngVLA should not preclude its participation in SKA; access to both facilities is necessary to meet Canada's radio astronomy needs. Indeed, ngVLA will fill the gap between those radio frequencies observable with the SKA and ALMA at high sensitivities and resolutions. Canada's partnership in ngVLA will give it access to cutting-edge facilities together covering approximately three orders of magnitude in frequency.

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arXiv2019

Investigating the Complex Velocity Structures within Dense Molecular Cloud Cores with GBT-Argus

We present the first results of high-spectral resolution (0.023 km/s) N$_2$H$^+$ observations of dense gas dynamics at core scales (~0.01 pc) using the recently commissioned Argus instrument on the Green Bank Telescope (GBT). While the fitted linear velocity gradients across the cores measured in our targets nicely agree with the well-known power-law correlation between the specific angular momentum and core size, it is unclear if the observed gradients represent core-scale rotation. In addition, our Argus data reveal detailed and intriguing gas structures in position-velocity (PV) space for all 5 targets studied in this project, which could suggest that the velocity gradients previously observed in many dense cores actually originate from large-scale turbulence or convergent flow compression instead of rigid-body rotation. We also note that there are targets in this study with their star-forming disks nearly perpendicular to the local velocity gradients, which, assuming the velocity gradient represents the direction of rotation, is opposite to what is described by the classical theory of star formation. This provides important insight on the transport of angular momentum within star-forming cores, which is a critical topic on studying protostellar disk formation.

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arXiv2019

KFPA Examinations of Young STellar Object Natal Environments (KEYSTONE): Hierarchical Ammonia Structures in Galactic Giant Molecular Clouds

We present initial results from the K-band focal plane array Examinations of Young STellar Object Natal Environments (KEYSTONE) survey, a large project on the 100-m Green Bank Telescope mapping ammonia emission across eleven giant molecular clouds at distances of $0.9-3.0$ kpc (Cygnus X North, Cygnus X South, M16, M17, MonR1, MonR2, NGC2264, NGC7538, Rosette, W3, and W48). This data release includes the NH$_3$ (1,1) and (2,2) maps for each cloud, which are modeled to produce maps of kinetic temperature, centroid velocity, velocity dispersion, and ammonia column density. Median cloud kinetic temperatures range from $11.4\pm2.2$ K in the coldest cloud (MonR1) to $23.0\pm6.5$ K in the warmest cloud (M17). Using dendrograms on the NH$_3$ (1,1) integrated intensity maps, we identify 856 dense gas clumps across the eleven clouds. Depending on the cloud observed, $40-100\%$ of the clumps are aligned spatially with filaments identified in H$_2$ column density maps derived from SED-fitting of dust continuum emission. A virial analysis reveals that 523 of the 835 clumps ($\sim63\%$) with mass estimates are bound by gravity alone. We find no significant difference between the virial parameter distributions for clumps aligned with the dust-continuum filaments and those unaligned with filaments. In some clouds, however, hubs or ridges of dense gas with unusually high mass and low virial parameters are located within a single filament or at the intersection of multiple filaments. These hubs and ridges tend to host water maser emission, multiple 70$μ$m-detected protostars, and have masses and radii above an empirical threshold for forming massive stars.

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arXiv2019

The JCMT BISTRO Survey: The Magnetic Field of the Barnard 1 Star-Forming Region

We present the POL-2 850 $μ$m linear polarization map of the Barnard 1 clump in the Perseus molecular cloud complex from the B-fields In STar-forming Region Observations (BISTRO) survey at the James Clerk Maxwell Telescope. We find a trend of decreasing polarization fraction as a function of total intensity, which we link to depolarization effects towards higher density regions of the cloud. We then use the polarization data at 850 $μ$m to infer the plane-of-sky orientation of the large-scale magnetic field in Barnard 1. This magnetic field runs North-South across most of the cloud, with the exception of B1-c where it turns more East-West. From the dispersion of polarization angles, we calculate a turbulence correlation length of $5.0 \pm 2.5$ arcsec ($1500$ au), and a turbulent-to-total magnetic energy ratio of $0.5 \pm 0.3$ inside the cloud. We combine this turbulent-to-total magnetic energy ratio with observations of NH$_3$ molecular lines from the Green Bank Ammonia Survey (GAS) to estimate the strength of the plane-of-sky component of the magnetic field through the Davis-Chandrasekhar-Fermi method. With a plane-of-sky amplitude of $120 \pm 60$ $μ$G and a criticality criterion $λ_c = 3.0 \pm 1.5$, we find that Barnard 1 is a supercritical molecular cloud with a magnetic field nearly dominated by its turbulent component.

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arXiv2019

Relative Alignment Between the Magnetic Field and Molecular Gas Structure in the Vela C Giant Molecular Cloud using Low and High Density Tracers

We compare the magnetic field orientation for the young giant molecular cloud Vela C inferred from 500-$μ$m polarization maps made with the BLASTPol balloon-borne polarimeter to the orientation of structures in the integrated line emission maps from Mopra observations. Averaging over the entire cloud we find that elongated structures in integrated line-intensity, or zeroth-moment maps, for low density tracers such as $^{12}$CO and $^{13}$CO $J$ $\rightarrow$ 1 - 0 are statistically more likely to align parallel to the magnetic field, while intermediate or high density tracers show (on average) a tendency for alignment perpendicular to the magnetic field. This observation agrees with previous studies of the change in relative orientation with column density in Vela C, and supports a model where the magnetic field is strong enough to have influenced the formation of dense gas structures within Vela C. The transition from parallel to no preferred/perpendicular orientation appears to happen between the densities traced by $^{13}$CO and by C$^{18}$O $J$ $\rightarrow$ 1 - 0. Using RADEX radiative transfer models to estimate the characteristic number density traced by each molecular line we find that the transition occurs at a molecular hydrogen number density of approximately $10^3$ cm$^{-3}$. We also see that the Centre-Ridge (the highest column density and most active star-forming region within Vela C) appears to have a transition at a lower number density, suggesting that this may depend on the evolutionary state of the cloud.

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arXiv2019

The Green Bank Ammonia Survey: A Virial Analysis of Gould Belt Clouds in Data Release 1

We perform a virial analysis of starless dense cores in three nearby star-forming regions : L1688 in Ophiuchus, NGC 1333 in Perseus, and B18 in Taurus. Our analysis takes advantage of comprehensive kinematic information for the dense gas in all of these regions made publicly available through the Green Bank Ammonia Survey Data Release 1, which used to estimate internal support against collapse. We combine this information with ancillary data used to estimate other important properties of the cores, including continuum data from the James Clerk Maxwell Telescope Gould Belt Survey for core identification, core masses, and core sizes. Additionally, we used \textit{Planck} and \textit{Herschel}-based column density maps for external cloud weight pressure, and Five College Radio Astronomy Observatory $^{13}$CO observations for external turbulent pressure. Our self-consistent analysis suggests that many dense cores in all three star-forming regions are not bound by gravity alone, but rather require additional pressure confinement to remain bound. Unlike a recent, similar study in Orion~A, we find that turbulent pressure represents a significant portion of the external pressure budget. Our broad conclusion emphasizing the importance of pressure confinement in dense core evolution, however, agrees with earlier work.

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arXiv2019

The central 1000 AU of a pre-stellar core revealed with ALMA. I. 1.3 mm continuum observations

Stars like our Sun form in self-gravitating dense and cold structures within interstellar clouds, called pre-stellar cores. Although much is known about the physical structure of dense clouds just before and soon after the switch-on of a protostar, the central few thousand astronomical units (au) of pre-stellar cores are unexplored. It is within these central regions that stellar systems assemble and fragmentation may take place, with the consequent formation of binaries and multiple systems. We present ALMA Band 6 observations (ACA and 12m array) of the dust continuum emission of the 8 Msun pre-stellar core L1544, with angular resolution of 2'' x 1.6'' (linear resolution 270 au x 216 au). Within the primary beam, a compact region of 0.1 Msun, which we call a "kernel", has been unveiled. The kernel is elongated, with a central flat zone with radius Rker ~ 10'' (~ 1400 au). The average number density within Rker is ~1 x 10^6 cm^{-3}, with possible local density enhancements. The region within Rker appears to have fragmented, but detailed analysis shows that similar substructure can be reproduced by synthetic interferometric observations of a smooth centrally concentrated dense core with a similar central flat zone. The presence of a smooth kernel within a dense core is in agreement with non-ideal magneto-hydro-dynamical simulations of a contracting cloud core with a peak number density of 1 x 10^7 cm^{-3}. Dense cores with lower central densities are completely filtered out when simulated 12m-array observations are carried out. These observations demonstrate that the kernel of dynamically evolved dense cores can be investigated at high angular resolution with ALMA.

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arXiv2018

Dense gas kinematics and a narrow filament in the Orion A OMC1 region using NH3

We present combined observations of the NH3 (J,K) = (1,1) and (2,2) inversion transitions towards OMC1 in Orion A obtained by the Karl G. Jansky Very Large Array (VLA) and the 100 m Robert C. Byrd Green Bank Telescope (GBT). With an angular resolution of 6" (0.01 pc), these observations reveal with unprecedented detail the complex filamentary structure extending north of the active Orion BN/KL region in a field covering 6' x 7'. We find a 0.012 pc wide filament within OMC1, with an aspect ratio of ~37:1, that was missed in previous studies. Its orientation is directly compared to the relative orientation of the magnetic field from the James Clerk Maxwell Telescope BISTRO survey in Orion A. We find a small deviation of ~11 deg between the mean orientation of the filament and the magnetic field, suggesting that they are almost parallel to one another. The filament's column density is estimated to be 2-3 orders of magnitude larger than the filaments studied with Herschel and is possibly self-gravitating given the low values of turbulence found. We further produce maps of the gas kinematics by forward modeling the hyperfine structure of the NH3 (J,K) = (1,1) and (2,2) lines. The resulting distribution of velocity dispersions peaks at ~0.5 km/s, close to the subsonic regime of the gas. This value is about 0.2 km/s smaller than previously measured in single-dish observations of the same region, suggesting that higher angular and spectral resolution observations will identify even lower velocity dispersions that might reach the subsonic turbulence regime in dense gas filaments.

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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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