arXiv subjects

Keping Qiu

Publications and source records attributed to Keping Qiu.

At least 109 records · Page 6Linked to original sources

arXiv2018

Magnetic fields towards Ophiuchus-B derived from SCUBA-2 polarization measurements

We present the results of dust emission polarization measurements of Ophiuchus-B (Oph-B) carried out using the Submillimetre Common-User Bolometer Array 2 (SCUBA-2) camera with its associated polarimeter (POL-2) on the James Clerk Maxwell Telescope (JCMT) in Hawaii. This work is part of the B-fields In Star-forming Region Observations (BISTRO) survey initiated to understand the role of magnetic fields in star formation for nearby star-forming molecular clouds. We present a first look at the geometry and strength of magnetic fields in Oph-B. The field geometry is traced over $\sim$0.2 pc, with clear detection of both of the sub-clumps of Oph-B. The field pattern appears significantly disordered in sub-clump Oph-B1. The field geometry in Oph-B2 is more ordered, with a tendency to be along the major axis of the clump, parallel to the filamentary structure within which it lies. The degree of polarization decreases systematically towards the dense core material in the two sub-clumps. The field lines in the lower density material along the periphery are smoothly joined to the large scale magnetic fields probed by NIR polarization observations. We estimated a magnetic field strength of 630$\pm$410 $μ$G in the Oph-B2 sub-clump using a Davis-Chandeasekhar-Fermi analysis. With this magnetic field strength, we find a mass-to-flux ratio $λ$= 1.6$\pm$1.1, which suggests that the Oph-B2 clump is slightly magnetically supercritical.

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arXiv2018

First observations of the magnetic field inside the Pillars of Creation: Results from the BISTRO survey

We present the first high-resolution, submillimeter-wavelength polarimetric observations of -- and thus direct observations of the magnetic field morphology within -- the dense gas of the Pillars of Creation in M16. These 850$\,μ$m observations, taken as part of the BISTRO (B-Fields in Star-forming Region Observations) Survey using the POL-2 polarimeter on the SCUBA-2 camera on the James Clerk Maxwell Telescope (JCMT), show that the magnetic field runs along the length of the pillars, perpendicular to, and decoupled from, the field in the surrounding photoionized cloud. Using the Chandrasekhar-Fermi method we estimate a plane-of-sky magnetic field strength of $170-320\,μ$G in the Pillars, consistent with their having been formed through compression of gas with initially weak magnetization. The observed magnetic field strength and morphology suggests that the magnetic field may be slowing the pillars' evolution into cometary globules. We thus hypothesize that the evolution and lifetime of the Pillars may be strongly influenced by the strength of the coupling of their magnetic field to that of their parent photoionized cloud -- i.e. that the Pillars' longevity results from magnetic support

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arXiv2018

An Isothermal Outflow in High-mass Star-forming Region G240.31+0.07

We present Atacama Pathfinder EXperiment (APEX) observations toward the massive star-forming region G240.31+0.07 in the CO J = 3--2, 6--5, and 7--6 lines. We detect a parsec-sized, bipolar, and high velocity outflow in all the lines, which allow us, in combination with the existing CO J = 2--1 data, to perform a multi-line analysis of physical conditions of the outflowing gas. The CO 7--6/6--5, 6--5/3--2, and 6--5/2--1 ratios are found to be nearly constant over a velocity range of $\sim$5--25 km s$^{-1}$ for both blueshifted and redshifted lobes. We carry out rotation diagram and large velocity gradient (LVG) calculations of the four lines, and find that the outflow is approximately isothermal with a gas temperature of $\sim$50 K, and that the the CO column density clearly decreases with the outflow velocity. If the CO abundance and the velocity gradient do not vary much, the decreasing CO column density indicates a decline in the outflow gas density with velocity. By comparing with theoretical models of outflow driving mechanisms, our observations and calculations suggest that the massive outflow in G240.31+0.07 is being driven by a wide-angle wind and further support a disk mediated accretion at play for the formation of the central high-mass star.

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arXiv2018

A First Look at BISTRO Observations of The $ρ$ Oph-A core

We present 850 $μ$m imaging polarimetry data of the $ρ$ Oph-A core taken with the Submillimeter Common-User Bolometer Array-2 (SCUBA-2) and its polarimeter (POL-2), as part of our ongoing survey project, BISTRO (B-fields In STar forming RegiOns). The polarization vectors are used to identify the orientation of the magnetic field projected on the plane of the sky at a resolution of 0.01 pc. We identify 10 subregions with distinct polarization fractions and angles in the 0.2 pc $ρ$ Oph A core; some of them can be part of a coherent magnetic field structure in the $ρ$ Oph region. The results are consistent with previous observations of the brightest regions of $ρ$ Oph-A, where the degrees of polarization are at a level of a few percents, but our data reveal for the first time the magnetic field structures in the fainter regions surrounding the core where the degree of polarization is much higher ($> 5 \%$). A comparison with previous near-infrared polarimetric data shows that there are several magnetic field components which are consistent at near-infrared and submillimeter wavelengths. Using the Davis-Chandrasekhar-Fermi method, we also derive magnetic field strengths in several sub-core regions, which range from approximately 0.2 to 5 mG. We also find a correlation between the magnetic field orientations projected on the sky with the core centroid velocity components.

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arXiv2018

CSO CO(2-1) and Spitzer IRAC Observations of a Bipolar Outflow in High-mass Star-forming Region IRAS 22506+5944

We present Caltech Submillimeter Observatory CO (2-1) and Spitzer IRAC observations toward IRAS 22506+5944, which is a 10$^4$ $L_{\odot}$ massive star-forming region. The CO (2-1) maps show an east-west bipolar molecular outflow originating from the 3 mm dust continuum peak. The Spitzer IRAC color-composite image reveals a pair of bow-shaped tips which are prominent in excess 4.5 $μ$m emission and are located to the leading fronts of the bipolar outflow, providing compelling evidence for the existence of bow-shocks as the driving agents of the molecular outflow. By comparing our CO (2-1) observations with previously published CO (1-0) data, we find that the CO (2-1)/(1-0) line ratio increases from low (~5 km s$^{-1}$) to moderate (~8 - 12 km s$^{-1}$) velocities, and then decreases at higher velocities. This is qualitatively consistent with the scenario that the molecular outflow is driven by multiple bow-shocks. We also revisit the position-velocity diagram of the CO (1-0) data, and find two spur structures along the outflow axis, which are further evidence for the presence of multiple jet bow-shocks. Finally, power-law fittings to the mass spectrum of the outflow gives power law indexes more consistent with the jet bow-shock model than the wide-angle wind model.

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arXiv2018

Detection of Dust Condensations in the Orion Bar Photon-dominated Region

We report Submillimeter Array dust continuum and molecular spectral line observations toward the Orion Bar photon-dominated region (PDR). The 1.2 mm continuum map reveals, for the first time, a total of 9 compact (r < 0.01pc) dust condensations located within a distance of ~0.03 pc from the dissociation front of the PDR. Part of the dust condensations are seen in spectral line emissions of CS (5-4) and H$_2$CS ($7_{1,7}$-$6_{1,6}$), though the CS map also reveals dense gas further away from the dissociation front. We detect compact emissions in H$_2$CS ($6_{0,6}$-$5_{0,5}$), ($6_{2,4}$-$5_{2,3}$) and C$^{34}$S, C$^{33}$S (4-3) toward bright dust condensations. The line ratio of H$_2$CS ($6_{0,6}$-$5_{0,5}$)/($6_{2,4}$-$5_{2,3}$) suggests a temperature of $73\pm58$ K. A non-thermal velocity dispersion of ~0.25 - 0.50 km s$^{-1}$ is derived from the high spectral resolution C$^{34}$S data, and indicates a subsonic to transonic turbulence in the condensations. The masses of the condensations are estimated from the dust emission, and range from 0.03 to 0.3 $M_{\odot}$, all significantly lower than any critical mass that is required for self-gravity to play a crucial role. Thus the condensations are not gravitationally bound, and could not collapse to form stars. In cooperating with recent high resolution observations of the surface layers of the molecular cloud in the Bar, we speculate that the condensations are produced as a high-pressure wave induced by the expansion of the HII region compresses and enters the cloud. A velocity gradient along a direction perpendicular to the major axis of the Bar is seen in H$_2$CS ($7_{1,7}$-$6_{1,6}$), and is consistent with the scenario that the molecular gas behind the dissociation front is being compressed.

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arXiv2018

Re-visiting the extended Schmidt law: the important role of existing stars in regulating star formation

We revisit the proposed extended Schmidt law (Shi et al. 2011) which points that the star formation efficiency in galaxies depends on the stellar mass surface density, by investigating spatially-resolved star formation rates (SFRs), gas masses and stellar masses of star formation regions in a vast range of galactic environments, from the outer disks of dwarf galaxies to spiral disks and to merging galaxies as well as individual molecular clouds in M33. We find that these regions are distributed in a tight power-law as Sigma_SFR ~(Sigma_star^0.5 Sigma_gas )^1.09, which is also valid for the integrated measurements of disk and merging galaxies at high-z. Interestingly, we show that star formation regions in the outer disks of dwarf galaxies with Sigma_SFR down to 10^(-5) Msun/yr/kpc^2, which are outliers of both Kennicutt-Schmidt and Silk-Elmegreen law, also follow the extended Schmidt law. Other outliers in the Kennicutt-Schmidt law, such as extremely-metal poor star-formation regions, also show significantly reduced deviations from the extended Schmidt law. These results suggest an important role for existing stars in helping to regulate star formation through the effect of their gravity on the mid-plane pressure in a wide range of galactic environments.

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arXiv2017

The JCMT BISTRO Survey: The magnetic field strength in the Orion A filament

We determine the magnetic field strength in the OMC 1 region of the Orion A filament via a new implementation of the Chandrasekhar-Fermi method using observations performed as part of the James Clerk Maxwell Telescope (JCMT) B-Fields In Star-Forming Region Observations (BISTRO) survey with the POL-2 instrument. We combine BISTRO data with archival SCUBA-2 and HARP observations to find a plane-of-sky magnetic field strength in OMC 1 of $B_{\rm pos}=6.6\pm4.7$ mG, where $δB_{\rm pos}=4.7$ mG represents a predominantly systematic uncertainty. We develop a new method for measuring angular dispersion, analogous to unsharp masking. We find a magnetic energy density of $\sim1.7\times 10^{-7}$ Jm$^{-3}$ in OMC 1, comparable both to the gravitational potential energy density of OMC 1 ($\sim 10^{-7}$ Jm$^{-3}$), and to the energy density in the Orion BN/KL outflow ($\sim 10^{-7}$ Jm$^{-3}$). We find that neither the Alfvén velocity in OMC 1 nor the velocity of the super-Alfvénic outflow ejecta is sufficiently large for the BN/KL outflow to have caused large-scale distortion of the local magnetic field in the $\sim$500-year lifetime of the outflow. Hence, we propose that the hour-glass field morphology in OMC 1 is caused by the distortion of a primordial cylindrically-symmetric magnetic field by the gravitational fragmentation of the filament and/or the gravitational interaction of the BN/KL and S clumps. We find that OMC 1 is currently in or near magnetically-supported equilibrium, and that the current large-scale morphology of the BN/KL outflow is regulated by the geometry of the magnetic field in OMC 1, and not vice versa.

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arXiv2017

Magnetized converging flows towards the hot core in the intermediate/high-mass star-forming region NGC 6334 V

We present Submillimeter Array (SMA) observations at 345 GHz towards the intermediate/high-mass cluster-forming region NGC 6334 V. From the dust emission we spatially resolve three dense condensations, the brightest one presenting the typical chemistry of a hot core. The magnetic field (derived from the dust polarized emission) shows a bimodal converging pattern towards the hot core. The molecular emission traces two filamentary structures at two different velocities, separated by 2 km/s, converging to the hot core and following the magnetic field distribution. We compare the velocity field and the magnetic field derived from the SMA observations with MHD simulations of star-forming regions dominated by gravity. This comparison allows us to show how the gas falls in from the larger-scale extended dense core (~0.1 pc) of NGC 6334 V towards the higher-density hot core region (~0.02 pc) through two distinctive converging flows dragging the magnetic field, whose strength seems to have been overcome by gravity.

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arXiv2017

First results from BISTRO -- a SCUBA-2 polarimeter survey of the Gould Belt

We present the first results from the B-fields In STar-forming Region Observations (BISTRO) survey, using the Sub-millimetre Common-User Bolometer Array 2 (SCUBA-2) camera, with its associated polarimeter (POL-2), on the James Clerk Maxwell Telescope (JCMT) in Hawaii. We discuss the survey's aims and objectives. We describe the rationale behind the survey, and the questions which the survey will aim to answer. The most important of these is the role of magnetic fields in the star formation process on the scale of individual filaments and cores in dense regions. We describe the data acquisition and reduction processes for POL-2, demonstrating both repeatability and consistency with previous data. We present a first-look analysis of the first results from the BISTRO survey in the OMC 1 region. We see that the magnetic field lies approximately perpendicular to the famous 'integral filament' in the densest regions of that filament. Furthermore, we see an 'hour-glass' magnetic field morphology extending beyond the densest region of the integral filament into the less-dense surrounding material, and discuss possible causes for this. We also discuss the more complex morphology seen along the Orion Bar region. We examine the morphology of the field along the lower-density north-eastern filament. We find consistency with previous theoretical models that predict magnetic fields lying parallel to low-density, non-self-gravitating filaments, and perpendicular to higher-density, self-gravitating filaments.

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arXiv2017

Magnetic fields in the massive dense cores of DR21 filament: weakly magnetized cores in a strongly magnetized filament

We present Submillimeter Array 880 $μ$m dust polarization observations of six massive dense cores in the DR21 filament. The dust polarization shows complex magnetic field structures in the massive dense cores with sizes of 0.1 pc, in contrast to the ordered magnetic fields of the parsec-scale filament. The major axes of the massive dense cores appear to be aligned either parallel or perpendicular to the magnetic fields of the filament, indicating that the parsec-scale magnetic fields play an important role in the formation of the massive dense cores. However, the correlation between the major axes of the cores and the magnetic fields of the cores is less significant, suggesting that during the core formation, the magnetic fields below 0.1 pc scales become less important than the magnetic fields above 0.1 pc scales in supporting a core against gravity. Our analysis of the angular dispersion functions of the observed polarization segments yields the plane-of-sky magnetic field strengths of 0.4--1.7 mG of the massive dense cores. We estimate the kinematic, magnetic, and gravitational virial parameters of the filament and the cores. The virial parameters show that in the filament, the gravitational energy is dominant over magnetic and kinematic energies, while in the cores, the kinematic energy is dominant. Our work suggests that although magnetic fields may play an important role in a collapsing filament, the kinematics arising from gravitational collapse must become more important than magnetic fields during the evolution from filaments to massive dense cores.

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arXiv2017

Mass growth in MYSO fed by companion

We present a Submillimeter Array (SMA) observation towards the young massive double-core system G350.69-0.49. This system consists of a northeast (NE) diffuse gas Bubble and a southwest (SW) massive young stellar object (MYSO), both clearly seen in the Spitzer images. The SMA observations reveal a gas flow between the NE Bubble and the SW MYSO in a broad velocity range from 5 to 30 km/s with respect to the system velocity. The gas flow is well confined within the interval between the two objects, and traces a significant mass transfer from the NE gas Bubble to the SW massive core. The transfer flow can supply the material accreted onto the SW MYSO at a rate of $4.2\times10^{-4} year^{-1}$. The whole system therefore suggests a mode for the mass growth in MYSO from a gas transfer flow launched from its companion gas clump, despite that the driving mechanism of the transfer flow is not yet fully determined from the current data.

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arXiv2015

Local Magnetic Field Role in Star Formation

We highlight distinct and systematic observational features of magnetic field morphologies in polarized submm dust continuum. We illustrate this with specific examples and show statistical trends from a sample of 50 star-forming regions.

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arXiv2015

Self-similar Fragmentation Regulated by Magnetic Fields in a Massive Star Forming Filament

Most molecular clouds are filamentary or elongated. Among those forming low-mass stars, their long axes tend to be either parallel or perpendicular to the large-scale (10-100 pc) magnetic field (B-field) in the surrounding inter cloud medium. This arises because, along the dynamically dominant B-fields, the competition between self-gravity and turbulent pressure will shape the cloud to be elongated either perpendicular or parallel to the fields. Recent study also suggested that, on the scales of 0.1-0.01 pc, fields are dynamically important within cloud cores forming massive stars. But whether the core field morphologies are inherited from the inter cloud medium or governed by cloud turbulence is under vigorous debate, so is the role played by B-fields in cloud fragmentation at 10 - 0.1 pc scales. Here we report B-field maps covering 100-0.01 pc scales inferred from polarimetric observations of a massive-star forming region, NGC 6334. First, the main filament also lies perpendicular to the ambient field. NGC 6334 hosts young star-forming sites where fields are not severely affected by stellar feedback, and their directions do not change significantly over the entire scale range. This means that the fields are dynamically important. At various scales, we find that the hourglass-shaped field lines are pinched where the gas column density peaks and the field strength is proportional to the 0.4-power of the density. We conclude that B-fields play a crucial role in the fragmentation of NGC 6334.

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arXiv2014

The Importance of the Magnetic Field from an SMA-CSO-Combined Sample of Star-Forming Regions

Submillimeter dust polarization measurements of a sample of 50 star-forming regions, observed with the SMA and the CSO covering pc-scale clouds to mpc-scale cores, are analyzed in order to quantify the magnetic field importance. The magnetic field misalignment $δ$ -- the local angle between magnetic field and dust emission gradient -- is found to be a prime observable, revealing distinct distributions for sources where the magnetic field is preferentially aligned with or perpendicular to the source minor axis. Source-averaged misalignment angles $\langle|δ|\rangle$ fall into systematically different ranges, reflecting the different source-magnetic field configurations. Possible bimodal $\langle|δ|\rangle$-distributions are found for the separate SMA and CSO samples. Combining both samples broadens the distribution with a wide maximum peak at small $\langle|δ|\rangle$-values. Assuming the 50 sources to be representative, the prevailing source-magnetic field configuration is one that statistically prefers small magnetic field misalignments $|δ|$. When interpreting $|δ|$ together with an MHD force equation, as developed in the framework of the polarization-intensity gradient method, a sample-based scaling fits the magnetic field tension-to-gravity force ratio $\langleΣ_B\rangle$ versus $\langle|δ|\rangle$ with $\langleΣ_B\rangle = 0.116 \cdot \exp(0.047\cdot \langle|δ|\rangle)\pm 0.20$ (mean error), providing a way to estimate the relative importance of the magnetic field, only based on measurable field misalignments $|δ|$. The force ratio $Σ_B$ discriminates systems that are collapsible on average ($\langle Σ_B\rangle <1$) from other molecular clouds where the magnetic field still provides enough resistance against gravitational collapse ($\langle Σ_B\rangle >1$) (abridged).

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arXiv2014

Submillimeter Array Observations of Magnetic Fields in G240.31+0.07: an Hourglass in a Massive Cluster-forming Core

We report the first detection of an hourglass magnetic field aligned with a well-defined outflow-rotation system in a high-mass star-forming region. The observations were performed with Submillimeter Array toward G240.31+0.07, which harbors a massive, flattened, and fragmenting molecular cloud core and a wide-angle bipolar outflow. The polarized dust emission at 0.88 mm reveals a clear hourglass-shaped magnetic field aligned within 20 degree of the outflow axis. Maps of high-density tracing spectral lines, e.g., H13CO+ (4-3), show that the core is rotating about its minor axis, which is also aligned with the magnetic field axis. Therefore, both the magnetic field and kinematic properties observed in this region are surprisingly consistent with the theoretical predictions of the classic paradigm of isolated low-mass star formation. The strength of the magnetic field in the plane of sky is estimated to be about 1.1 mG, resulting in a mass-to-magnetic flux ratio of 1.4 times the critical value and a turbulent to ordered magnetic energy ratio of 0.4. We also find that the specific angular momentum almost linearly decreases from r~0.6 pc to 0.03 pc scales, which is most likely attributed to magnetic braking.

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arXiv2014

Magnetic Fields and Massive Star Formation

Massive stars ($M > 8$ \msun) typically form in parsec-scale molecular clumps that collapse and fragment, leading to the birth of a cluster of stellar objects. We investigate the role of magnetic fields in this process through dust polarization at 870 $μ$m obtained with the Submillimeter Array (SMA). The SMA observations reveal polarization at scales of $\lsim$ 0.1 pc. The polarization pattern in these objects ranges from ordered hour-glass configurations to more chaotic distributions. By comparing the SMA data with the single dish data at parsec scales, we found that magnetic fields at dense core scales are either aligned within $40^\circ$ of or perpendicular to the parsec-scale magnetic fields. This finding indicates that magnetic fields play an important role during the collapse and fragmentation of massive molecular clumps and the formation of dense cores. We further compare magnetic fields in dense cores with the major axis of molecular outflows. Despite a limited number of outflows, we found that the outflow axis appears to be randomly oriented with respect to the magnetic field in the core. This result suggests that at the scale of accretion disks ($\lsim 10^3$ AU), angular momentum and dynamic interactions possibly due to close binary or multiple systems dominate over magnetic fields. With this unprecedentedly large sample massive clumps, we argue on a statistical basis that magnetic fields play an important role during the formation of dense cores at spatial scale of 0.01 - 0.1 pc in the context of massive star and cluster star formation.

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arXiv2013

From Poloidal to Toroidal: Detection of Well-ordered Magnetic Field in High-mass Proto-cluster G35.2-0.74N

We report on detection of an ordered magnetic field (B field) threading a massive star-forming clump in the molecular cloud G35.2-0.74, using Submillimeter Array observations of polarized dust emission. Thanks to the sensitive and high-angular-resolution observations, we are able to resolve the morphology of the B field in the plane of sky and detect a great turn of 90 degree in the B field direction: Over the northern part of the clump, where a velocity gradient is evident, the B field is largely aligned with the long axis of the clump, whereas in the southern part, where the velocity field appears relatively uniform, the B field is slightly pinched with its mean direction perpendicular to the clump elongation. We suggest that the clump forms as its parent cloud collapses more along the large scale B field. In this process, the northern part carries over most of the angular momentum, forming a fast rotating system, and pulls the B field into a toroidal configuration. In contrast, the southern part is not significantly rotating and the B field remains in a poloidal configuration. A statistical analysis of the observed polarization dispersion yields a B field strength of ~ 1 mG, a turbulent-to-magnetic energy ratio of order unity, and a mass-to-magnetic flux ratio of ~ 2--3 in units of the critical value. Detailed calculations support our hypothesis that the B field in the northern part is being rotationally distorted. Our observations, in conjunction with early single-dish data, suggest that the B field may play a critical role in the formation of the dense clump, whereas rotation and turbulence could also be important in further dynamical evolution of the clump. The observations also provide evidence for a wide-angle outflow driven from a strongly rotating region whose B field is largely toroidal.

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