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Chuan-Peng Zhang

Publications and source records attributed to Chuan-Peng Zhang.

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arXiv2019

The JCMT BISTRO Survey: The Magnetic Field In The Starless Core $ρ$ Ophiuchus C

We report 850~$μ$m dust polarization observations of a low-mass ($\sim$12 $M_{\odot}$) starless core in the $ρ$ Ophiuchus cloud, Ophiuchus C, made with the POL-2 instrument on the James Clerk Maxwell Telescope (JCMT) as part of the JCMT B-fields In STar-forming Region Observations (BISTRO) survey. We detect an ordered magnetic field projected on the plane of sky in the starless core. The magnetic field across the $\sim$0.1~pc core shows a predominant northeast-southwest orientation centering between $\sim$40$^\circ$ to $\sim$100$^\circ$, indicating that the field in the core is well aligned with the magnetic field in lower-density regions of the cloud probed by near-infrared observations and also the cloud-scale magnetic field traced by Planck observations. The polarization percentage ($P$) decreases with an increasing total intensity ($I$) with a power-law index of $-$1.03 $\pm$ 0.05. We estimate the plane-of-sky field strength ($B_{\mathrm{pos}}$) using modified Davis-Chandrasekhar-Fermi (DCF) methods based on structure function (SF), auto-correlation (ACF), and unsharp masking (UM) analyses. We find that the estimates from the SF, ACF, and UM methods yield strengths of 103 $\pm$ 46 $μ$G, 136 $\pm$ 69 $μ$G, and 213 $\pm$ 115 $μ$G, respectively. Our calculations suggest that the Ophiuchus C core is near magnetically critical or slightly magnetically supercritical (i.e. unstable to collapse). The total magnetic energy calculated from the SF method is comparable to the turbulent energy in Ophiuchus C, while the ACF method and the UM method only set upper limits for the total magnetic energy because of large uncertainties.

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arXiv2018

Dust spectrum and polarisation at 850 um in the massive IRDC G035.39-00.33

The dust sub-millimetre polarisation of star-forming clouds carries information on dust and the role of magnetic fields in cloud evolution. With observations of a dense filamentary cloud G035.39-00.33, we aim to characterise the dust emission properties and the variations of the polarisation fraction. JCMT SCUBA-2/POL-2 data at 850um are combined with Planck 850um (353GHz) data to map polarisation fractions. With previous SCUBA-2 observations (450um and 850um) and Herschel data, the column densities are determined via modified blackbody fits and via radiative transfer modelling. Models are constructed to examine how the polarisation angles and fractions depend on potential magnetic field geometries and grain alignment. POL-2 data show clear changes in the magnetic field orientation. The filament has a peak column density of N(H2)~7 10^22 cm-2, a minimum dust temperature of T~12 K, and a mass of some 4300Msun for the area N(H2)> 5 10^21 cm-2. The estimated average value of the dust opacity spectral index is beta ~ 1.9. The ratio of sub-millimetre and J band optical depths is tau(250 um)/tau(J) ~ 2.5 10^-3, more than four times the typical values for diffuse medium. The polarisation fraction decreases as a function of column density to p ~ 1% in the central filament. Because of noise, the observed decrease of p(N) is significant only at N(H2)>2 10^22 cm-2. The observations suggest that the grain alignment is not constant. Although the data can be explained with a complete loss of alignment at densities above ~ 10^4 cm-3 or using the predictions of radiative torques alignment, the uncertainty of the field geometry and the spatial filtering of the SCUBA-2 data prevent strong conclusions. G035.39-00.33 shows strong signs of dust evolution and the low polarisation fraction is suggestive of a loss of polarised emission from its densest parts.

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arXiv2018

A holistic perspective on the dynamics of G035.39-00.33: the interplay between gas and magnetic fields

Magnetic field is one of the key agents that play a crucial role in shaping molecular clouds and regulating star formation, yet the complete information on the magnetic field is not well constrained due to the limitations in observations. We study the magnetic field in the massive infrared dark cloud G035.39-00.33 from dust continuum polarization observations at 850 $\micron$ with SCUBA-2/POL-2 at JCMT. The magnetic field tends to be perpendicular to the densest part of the main filament (F$_{M}$), whereas it has a less defined relative orientation in the rest of the structure, where it tends to be parallel to some diffuse regions. A mean plane-of-the-sky magnetic field strength of $\sim$50 $μ$G for F$_{M}$ is obtained using Davis-Chandrasekhar-Fermi method. Based on $^{13}$CO (1-0) line observations, we suggest a formation scenario of F$_{M}$ due to large-scale ($\sim$10 pc) cloud-cloud collision. Using additional NH$_3$ line data, we estimate that F$_{M}$ will be gravitationally unstable if it is only supported by thermal pressure and turbulence. The northern part of F$_{M}$, however, can be stabilized by a modest additional support from the local magnetic field. The middle and southern parts of F$_{M}$ are likely unstable even if the magnetic field support is taken into account. We claim that the clumps in F$_{M}$ may be supported by turbulence and magnetic fields against gravitational collapse. Finally, we identified for the first time a massive ($\sim$200 M$_{\sun}$), collapsing starless clump candidate, "c8", in G035.39-00.33. The magnetic field surrounding "c8" is likely pinched, hinting at an accretion flow along the filament.

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arXiv2018

The TOP-SCOPE survey of PGCCs: PMO and SCUBA-2 observations of 64 PGCCs in the 2nd Galactic Quadrant

In order to understand the initial conditions and early evolution of star formation in a wide range of Galactic environments, we carried out an investigation of 64 \textit{Planck} Galactic Cold Clumps (PGCCs) in the second quadrant of the Milky Way. Using the $^{13}$CO and C$^{18}$O $J = 1 - 0$ lines, and 850\,$μ$m continuum observations, we investigated cloud fragmentation and evolution associated with star formation. We extracted 468 clumps and 117 cores from the $^{13}$CO line and 850\,$μ$m continuum maps, respectively. We make use of the Bayesian Distance Calculator and derived the distances of all 64 PGCCs. We found that in general, the mass-size plane follows a relation of $m\sim r^{1.67}$. At a given scale, the masses of our objects are around 1/10 of that of typical Galactic massive star-forming regions. Analysis of the clump and core masses, virial parameters, densities, and mass-size relation suggests that the PGCCs in our sample have a low core formation efficiency ($\sim$3.0\%), and most PGCCs are likely low-mass star-forming candidates. Statistical study indicates that the 850\,$μ$m cores are more turbulent, more optically thick, and denser than the $^{13}$CO clumps for star formation candidates, suggesting that the 850\,$μ$m cores are likely more appropriate future star-formation candidates than the $^{13}$CO clumps.

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arXiv2018

The properties of Planck Galactic cold clumps in the L1495 dark cloud

Planck Galactic Cold Clumps (PGCCs) possibly represent the early stages of star formation. To understand better the properties of PGCCs, we studied 16 PGCCs in the L1495 cloud with molecular lines and continuum data from Herschel, JCMT/SCUBA-2 and the PMO 13.7 m telescope. Thirty dense cores were identified in 16 PGCCs from 2-D Gaussian fitting. The dense cores have dust temperatures of $T_{\rm d}$ = 11-14 K, and H$_{2}$ column densities of $N_{\rm H_{2}}$ = 0.36-2.5$\times10^{22}$ cm$^{-2}$. We found that not all PGCCs contain prestellar objects. In general, the dense cores in PGCCs are usually at their earliest evolutionary stages. All the dense cores have non-thermal velocity dispersions larger than the thermal velocity dispersions from molecular line data, suggesting that the dense cores may be turbulence-dominated. We have calculated the virial parameter $α$ and found that 14 of the dense cores have $α$ $<$ 2, while 16 of the dense cores have $α$ $>$ 2. This suggests that some of the dense cores are not bound in the absence of external pressure and magnetic fields. The column density profiles of dense cores were fitted. The sizes of the flat regions and core radii decrease with the evolution of dense cores. CO depletion was found to occur in all the dense cores, but is more significant in prestellar core candidates than in protostellar or starless cores. The protostellar cores inside the PGCCs are still at a very early evolutionary stage, sharing similar physical and chemical properties with the prestellar core candidates.

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arXiv2017

A statistical study towards the high-mass BGPS clumps with the MALT90 survey

In this work, we perform a statistical investigation towards 50 high-mass clumps using the data from the Bolocam Galactic Plane Survey (BGPS) and the Millimetre Astronomy Legacy Team 90-GHz survey (MALT90). Eleven dense molecular lines (N$_2$H$^+$(1-0), HNC(1-0), HCO$^+$(1-0), HCN(1-0), HN$^{13}$C(1-0), H$^{13}$CO$^+$(1-0), C$_2$H(1-0), HC$_3$N(10-9), SiO(2-1), $^{13}$CS(2-1) and HNCO$(4_{4,0}-3_{0,3}))$ are detected. N$_2$H$^+$ and HNC are shown to be good tracers for clumps in virous evolutionary stages since they are detected in all the fields. And the detection rates of N-bearing molecules decrease as the clumps evolve, but those of O-bearing species increase with evolution. Furthermore, the abundance ratios [N$_2$H$^+$]/[HCO$^+$] and Log([HC$_3$N]/[HCO$^+$]) decline with Log([HCO$^+$]) as two linear functions, respectively. This suggests the transformation of N$_2$H$^+$ and HC$_3$N to HCO$^+$ as the clumps evolve. We also find that C$_2$H is the most abundant molecule with an order of $10^{-8}$. Besides, three new infall candidates G010.214-00.324, G011.121-00.128, and G012.215-00.118(a) are discovered to have large-scaled infall motions and infall rates in the magnitude of $10^{-3}$ M$_\odot$ yr$^{-1}$.

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arXiv2017

Gas kinematics and star formation in the filamentary molecular cloud G47.06+0.26

We performed a multi-wavelength study toward the filamentary cloud G47.06+0.26 to investigate the gas kinematics and star formation. We present the 12CO (J=1-0), 13CO (J=1-0) and C18O (J=1-0) observations of G47.06+0.26 obtained with the Purple Mountain Observation (PMO) 13.7 m radio telescope to investigate the detailed kinematics of the filament. The 12CO (J=1-0) and 13CO (J=1-0) emission of G47.06+0.26 appear to show a filamentary structure. The filament extends about 45 arcmin (58.1 pc) along the east-west direction. The mean width is about 6.8 pc, as traced by the 13CO (J=1-0) emission. G47.06+0.26 has a linear mass density of about 361.5 Msun/pc. The external pressure (due to neighboring bubbles and H II regions) may help preventing the filament from dispersing under the effects of turbulence. From the velocity-field map, we discern a velocity gradient perpendicular to G47.06+0.26. From the Bolocam Galactic Plane Survey (BGPS) catalog, we found nine BGPS sources in G47.06+0.26, that appear to these sources have sufficient mass to form massive stars. We obtained that the clump formation efficiency (CFE) is about 18% in the filament. Four infrared bubbles were found to be located in, and adjacent to, G47.06+0.26. Particularly, infrared bubble N98 shows a cometary structure. CO molecular gas adjacent to N98 also shows a very intense emission. H II regions associated with infrared bubbles can inject the energy to surrounding gas. We calculated the kinetic energy, ionization energy, and thermal energy of two H II regions in G47.06+0.26. From the GLIMPSE I catalog, we selected some Class I sources with an age of about 100000 yr, which are clustered along the filament. The feedback from the H II regions may cause the formation of a new generation of stars in filament G47.06+0.26.

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arXiv2017

Mass-size scaling M~ r^1.67 of massive star-forming clumps -- evidences of turbulence-regulated gravitational collapse

We study the fragmentation of eight massive clumps using data from ATLASGAL 870 $μ$m, SCUBA 850 and 450 $μ$m, PdBI 1.3 and 3.5 mm, and probe the fragmentation from 1 pc to 0.01 pc scale. We find that the masses and the sizes of our objects follow $M \sim r^{1.68\pm0.05}$. The results are in agreements with the predictions of Li (2017) where $M \sim r^{5/3}$. Inside each object, the densest structures seem to be centrally condensed, with $ρ(r)\sim r^{-2}$. Our observational results support a scenario where molecular gas in the Milky Way is supported by a turbulence characterized by a constant energy dissipation rate, and gas fragments like clumps and cores are structures which are massive enough to be dynamically detached from the ambient medium.

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arXiv2017

Searching for initial stage of massive star formation around the H II region G18.2-0.3

Sometimes the early star formation can be found in cold and dense molecular clouds, such as infrared dark cloud (IRDC). Considering star formation often occurs in clustered condition, HII regions may be triggering a new generation of star formation, so we can search for initial stage of massive star formation around HII regions. Based on that above, this work is to introduce one method of how to search for initial stage of massive star formation around HII regions. Towards one sample of the HII region G18.2-0.3, multiwavelength observations are carried out to investigate its physical condition. In contrast and analysis, we find three potential initial stages of massive star formation, suggesting that it is feasible to search for initial stage of massive star formation around HII regions.

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arXiv2016

A multi-wavelength observation and investigation of six infrared dark clouds

Context. Infrared dark clouds (IRDCs) are ubiquitous in the Milky Way, yet they play a crucial role in breeding newly-formed stars. Aims. With the aim of further understanding the dynamics, chemistry, and evolution of IRDCs, we carried out multi-wavelength observations on a small sample. Methods. We performed new observations with the IRAM 30 m and CSO 10.4 m telescopes, with tracers ${\rm HCO^+}$, HCN, ${\rm N_2H^+}$, ${\rm C^{18}O}$, DCO$^+$, SiO, and DCN toward six IRDCs G031.97+00.07, G033.69-00.01, G034.43+00.24, G035.39-00.33, G038.95-00.47, and G053.11+00.05. Results. We investigated 44 cores including 37 cores reported in previous work and seven newly-identified cores. Toward the dense cores, we detected 6 DCO$^+$, and 5 DCN lines. Using pixel-by-pixel spectral energy distribution (SED) fits of the $\textit{Herschel}$ 70 to 500 $μ$m, we obtained dust temperature and column density distributions of the IRDCs. We found that ${\rm N_2H^+}$ emission has a strong correlation with the dust temperature and column density distributions, while ${\rm C^{18}O}$ showed the weakest correlation. It is suggested that ${\rm N_2H^+}$ is indeed a good tracer in very dense conditions, but ${\rm C^{18}O}$ is an unreliable one, as it has a relatively low critical density and is vulnerable to freezing-out onto the surface of cold dust grains. The dynamics within IRDCs are active, with infall, outflow, and collapse; the spectra are abundant especially in deuterium species. Conclusions. We observe many blueshifted and redshifted profiles, respectively, with ${\rm HCO^+}$ and ${\rm C^{18}O}$ toward the same core. This case can be well explained by model "envelope expansion with core collapse (EECC)".

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arXiv2016

Gas Kinematics and Star Formation in the Filamentary IRDC G34.43+0.24

We performed a multiwavelength study toward infrared dark cloud (IRDC) G34.43+0.24. New maps of 13CO $J$=1-0 and C18}O J=1-0 were obtained from the Purple Mountain Observatory (PMO) 13.7 m radio telescope. At 8 um (Spitzer - IRAC), IRDC G34.43+0.24 appears to be a dark filament extended by 18 arcmin along the north-south direction. Based on the association with the 870 um and C18O J=1-0 emission, we suggest that IRDC G34.43+0.24 should not be 18 arcmin in length, but extend by 34 arcmin. IRDC G34.43+0.24 contains some massive protostars, UC H II regions, and infrared bubbles. The spatial extend of IRDC G34.43+0.24 is about 37 pc assuming a distance of 3.7 kpc. IRDC G34.43+0.24 has a linear mass density of 1.6*10^{3} M_{sun} pc^{-1}, which is roughly consistent with its critical mass to length ratio. The turbulent motion may help stabilizing the filament against the radial collapse. Both infrared bubbles N61 and N62 show a ringlike structure at 8 um. Particularly, N61 has a double-shell structure, which has expanded into IRDC G34.43+0.24. The outer shell is traced by 8 um and 13}CO J=1-0 emission, while the inner shell is traced by 24 um and 20 cm emission. We suggest that the outer shell (9.9*10^{5} yr) is created by the expansion of H II region G34.172+0.175, while the inner shell (4.1-6.3*10^{5} yr) may be produced by the energetic stellar wind of its central massive star. From GLIMPSE I catalog, we selected some Class I sources with an age of 10^{5} yr. These Class I sources are clustered along the filamentary molecular cloud.

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arXiv2016

N131: A dust bubble born from the disruption of a gas filament

OB type stars have strong ionizing radiation, and drive energetic winds. The ultraviolet (UV) radiation from ionizing stars may heat dust and ionize gas to sweep up an expanding bubble shell. This shell may be the result of feedback leading to a new generation of stars. N131 is an infrared dust bubble residing in a molecular filament. We study the formation and fragmentation of this bubble with multi-wavelength dust and gas observations. Towards the bubble N131, we analyzed archival multi-wavelength observations including 3.6, 4.5, 5.8, 8.0, 24, 70, 160, 250, 350, 500 $μ$m, 1.1 mm, and 21 cm. In addition, we performed new observations of CO (2-1), CO (1-0), and $^{13}$CO (1-0) with the IRAM 30-m telescope. Multi-wavelength dust and gas observations reveal a ringlike shell with compact fragments, two filamentary structures, and a secondary bubble N131-A. The bubble N131 is a rare object with a large hole at 24 $μ$m and 21 cm in the direction of its center. The dust and gas clumps are compact and might have been compressed at the inner edge of the ringlike shell, while they are extended and might be pre-existing at the outer edge. The column density, excitation temperature, and velocity show a potentially hierarchical distribution from the inner to outer edge of the ringlike shell. We also detected the front and back sides of the secondary bubble N131-A in the direction of its center. The derived Lyman-continuum ionizing photon flux within N131-A is equivalent to an O9.5 star. Based on the above, we suggest that the bubble N131 might be triggered by the strong stellar winds from a group of massive stars inside the bubble. We propose a scenario in which the bubble N131 forms from the disruption of a gas filament by expansion of HII region, strong stellar winds, and fragments under self-gravity.

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arXiv2014

Submillimeter Array and Very Large Array Observations in the Hypercompact HII region G35.58-0.03

The formation of hypercompact (HC) HII regions is an important stage in massive star formation. Spectral line and continuum observations can explore its dynamic conditions. We present high angular resolution observations carried out with the Submillimeter Array (SMA) and the Very Large Array (VLA) toward the HC HII region G35.58-0.03. With the 1.3 mm SMA and 1.3 cm VLA, we detected a total of about 25 transitions of 8 different species and their isotopologues (CO, CH$_3$CN, SO$_2$, CH$_3$CCH, OCS, CS, H, and NH$_{3}$). G35.58-0.03 consists of an HC HII core with electron temperature $T_e^*\geqq5500$ K, emission measure EM $\approx 1.9\times10^{9}$ pc cm$^{-6}$, local volume electron density $n_e=3.3\times10^{5}$ cm$^{-3}$, and a same width of radio recombination line FWHM $\approx$ 43.2 km s$^{-1}$ for both H30$α$ and H38$β$ at its intrinsic core size $\sim$3714 AU. The H30$α$ line shows evidence of an ionized outflow driving a molecular outflow. Based on the derived Lyman continuum flux, there should be an early-type star equivalent to O6.5 located inside the HII region. From the continuum spectral energy distribution from 3.6 cm, 2.0 cm, 1.3 cm, 1.3 mm and 0.85 mm to 0.45 mm, we distinguished the free-free emission (25% $\sim$ 55%) from the warm dust component (75% $\sim$ 45%) at 1.3 mm. The molecular envelope shows evidence of infall and outflow with an infall rate 0.05 M$_{ \odot}$ yr$^{-1}$ and a mass loss rate $5.2\times10^{-3}$ ${\rm M_{ \odot} yr^{-1}}$. The derived momentum ($\sim0.05$ M$_{\odot}$ km s$^{-1}$) is consistent between the infalling and outflowing gas per year. It is suggested that the infall is predominant and the envelope mass of dense core is increasing rapidly, but the accretion in the inner part might already be halted.

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arXiv2013

Molecular clumps and star formation associated with the infrared dust bubble N131

Aims. The aim is to explore the interstellar medium around the dust bubble N131 and search for signatures of star formation. Methods. We perform a multiwavelength study around the N131 with data taken from large-scale surveys of infrared observation with online archive. We present new observations of three CO J = 1 - 0 isotope variants from Purple Mountain Observatory 13.7 m telescope. We analyze the distribution of the molecular gas and dust in the environment of the N131. We use color-color diagrams to search for young stellar objects and identify exciting star candidates. Results. The kinematic distance of about 8.6 kpc has been adopted as the distance of the bubble N131 from the Sun in this work. We have found a ring of clouds in CO emission coincident with the shell of N131 seen in the Spitzer telescope images, and two giant elongated molecular clouds of CO emission appearing on opposite sides of the ringlike shell of N131. There is a cavity within bubble at 1.4 GHz and 24 μm. Seven IRAS point sources are distributed along the ringlike shell of the bubble N131. 15 exciting stars and 63 YSOs candidates have been found. The clustered class I and II YSOs are distributed along the elongated clouds in the line of sight.

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arXiv2012

Star formation associated with the infrared dust bubble N68

We investigated the environment of the infrared dust bubble N68 and searched for evidence of triggered star formation in its surroundings. We performed a multiwavelength study of the nebula with data taken from several large-scale surveys: GLIMPSE, MIPSGAL, IRAS, NVSS, GRS, and JCMT. We analyzed the spectral profile and the distribution of the molecular gas (^{13}CO J = 1 - 0 and J = 3 - 2), and the dust in the environment of the N68. The position-velocity diagram clearly shows that the N68 may be expanding outward. We used two three-color images of the mid-infrared emission to explore the physical environment, and one color-color diagram to investigate the distribution of young stellar objects (YSOs). We found that the 24 μm emission is surrounded by the 8.0 μm emission. Morphologically, the 1.4 GHz continuum correlates strongly with the 24 μm emission, and the ^{13}CO J = 1 - 0 and J = 3 - 2 emissions correlate well with the 8.0 μm emission. We investigated two compact cores located at the shell of the N68. The spectral intensity ratios of ^{13}CO J = 3 - 2 to J = 1 - 0 range from 5 to 0.3. In addition, young star objects, masers, IRAS, and UC HII regions distribute at the shell of bubble. The active region may be triggered by the expanding of the bubble N68.

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