arXiv subjects

S. -Y. Liu

Publications and source records attributed to S. -Y. Liu.

At least 37 records · Page 2Linked to original sources

arXiv2020

CHIMPS2: Survey description and $^{12}$CO emission in the Galactic Centre

The latest generation of Galactic-plane surveys is enhancing our ability to study the effects of galactic environment upon the process of star formation. We present the first data from CO Heterodyne Inner Milky Way Plane Survey 2 (CHIMPS2). CHIMPS2 is a survey that will observe the Inner Galaxy, the Central Molecular Zone (CMZ), and a section of the Outer Galaxy in $^{12}$CO, $^{13}$CO, and C$^{18}$O $(J = 3\rightarrow2)$ emission with the Heterodyne Array Receiver Program on the James Clerk Maxwell Telescope (JCMT). The first CHIMPS2 data presented here are a first look towards the CMZ in $^{12}$CO J = 3$\rightarrow$2 and cover $-3^{\circ}\leq\,\ell\,\leq\,5^{\circ}$ and $\mid$b$\mid \leq 0.5^{\circ}$ with angular resolution of 15 arcsec, velocity resolution of 1 km s$^{-1}$, and rms $ΔT_A ^\ast =$ 0.58 K at these resolutions. Such high-resolution observations of the CMZ will be a valuable data set for future studies, whilst complementing the existing Galactic Plane surveys, such as SEDIGISM, the Herschel infrared Galactic Plane Survey, and ATLASGAL. In this paper, we discuss the survey plan, the current observations and data, as well as presenting position-position maps of the region. The position-velocity maps detect foreground spiral arms in both absorption and emission.

astro-ph.GA↗

arXiv2020

Enhancement of the $^{81}\mathrm{Kr}$ and $^{85}\mathrm{Kr}$ count rates by optical pumping

We report an increase of up to 60% on the count rates of the rare $^{81}\mathrm{Kr}$ and $^{85}\mathrm{Kr}$ isotopes in the Atom Trap Trace Analysis method by enhancing the production of metastable atoms in the discharge source. Additional atoms in the metastable $ 1s_5 $ level (Paschen notation) are obtained via optically pumping the $1s_4-2p_6$ transition at 819 nm. By solving the master equation for the system, we identify this transition to be the most suitable one and can describe the measured increase in metastable population as a function of the 819-nm laser power. We calculate the previously unknown isotope shifts and hyperfine splittings of the $1s_4-2p_6$ transition in $^{81}\mathrm{Kr}$ and $^{85}\mathrm{Kr}$, and verify the results with count rate measurements. The demonstrated count-rate increase enables a corresponding decrease in the required sample sizes for $^{81}\mathrm{Kr}$ and $^{85}\mathrm{Kr}$ dating, a significant improvement for applications such as dating of ocean water and deep ice cores.

physics.atom-ph↗

arXiv2020

The origin of tail-like structures around protoplanetary disks

We study the origin of tail-like structures recently detected around the disk of SU Aurigae and several FU~Orionis-type stars. Dynamic protostellar disks featuring ejections of gaseous clumps and quiescent protoplanetary disks experiencing a close encounter with an intruder star were modeled using the numerical hydrodynamics code FEOSAD. Both the gas and dust dynamics were taken into account, including dust growth and mutual friction between the gas and dust components. Only plane-of-the-disk encounters were considered. Ejected clumps produce a unique type of tail that is characterized by a bow-shock shape. Such tails originate from the supersonic motion of ejected clumps through the dense envelope that often surrounds young gravitationally unstable protostellar disks. The ejected clumps either sit at the head of the tail-like structure or disperse if their mass is insufficient to withstand the head wind of the envelope. On the other hand, close encounters with quiescent protoplanetary disks produce three types of the tail-like structure; we define these as pre-collisional, post-collisional, and spiral tails. These tails can in principle be distinguished from one another by particular features of the gas and dust flow in and around them. We find that the brown-dwarf-mass intruders do not capture circumintruder disks during the encounter, while the subsolar-mass intruders can acquire appreciable circumintruder disks with elevated dust-to-gas ratios, which can ease their observational detection. However, this is true only for prograde collisions; the retrograde intruders fail to collect appreciable amounts of gas or dust from the disk of the target. The predicted mass of dust in the model tail-like structures is higher than what was inferred for similar structures in SU~Aur, FU~Ori, and Z~CMa, making their observational detection feasible. Abridged.

astro-ph.EP↗

arXiv2019

Testing the weak equivalence principle by differential measurements of fundamental constants in the Magellanic Clouds

Non-standard fields are assumed to be responsible for phenomena attributed to dark energy and dark matter. Being coupled to ordinary matter, these fields modify the masses and/or charges of the elementary particles, thereby violating the Weak Equivalence Principle. Thus, values of fundamental constants such as the proton-to-electron mass ratio, mu, and/or the fine structure constant, alpha, measured in different environment conditions can be used as probes for this coupling. Here we perform differential measurements of F = mu*alpha^2 to test a non-standard coupling in the Magellanic Clouds - dwarf galaxies where the overall mass budget is dominated by dark matter. The analysis is based on [CI] and CO lines observed with the Herschel Space Observatory. Since these lines have different sensitivities to changes in mu and alpha, the combined alpha and mu variations can be evaluated through the radial velocity offsets, Delta V, between the CO and [CI] lines. Averaging over nine positions in the Magellanic Clouds, we obtain = -0.02+/-0.07 km/s, leading to |Delta F/F| < 2*10^-7 (1sigma), where Delta F/F = (F_obs-F_lab)/F_lab}. However, for one position observed with five times higher spectral resolution we find Delta V = -0.05+/-0.02 km/s, resulting in Delta F/F = (-1.7+/-0.7)*10^-7. Whether this offset is due to changes in the fundamental constants, due to chemical segregation in the emitting gas or merely due to Doppler noise requires further investigations.

astro-ph.CO↗

arXiv2019

SCOPE: SCUBA-2 Continuum Observations of Pre-protostellar Evolution - Survey Description and Compact Source Catalogue

We present the first release of the data and compact-source catalogue for the JCMT Large Program SCUBA-2 Continuum Observations of Pre-protostellar Evolution (SCOPE). SCOPE consists of 850-um continuum observations of 1235 Planck Galactic Cold Clumps (PGCCs) made with the Submillimetre Common-User Bolometer Array 2 on the James Clerk Maxwell Telescope. These data are at an angular resolution of 14.4 arcsec, significantly improving upon the 353-GHz resolution of Planck at 5 arcmin, and allowing for a catalogue of 3528 compact sources in 558 PGCCs. We find that the detected PGCCs have significant sub-structure, with 61 per cent of detected PGCCs having 3 or more compact sources, with filamentary structure also prevalent within the sample. A detection rate of 45 per cent is found across the survey, which is 95 per cent complete to Planck column densities of $N_{H_{2}}$ $>$ 5 $\times$ 10$^{21}$ cm$^{-2}$. By positionally associating the SCOPE compact sources with YSOs, the star formation efficiency, as measured by the ratio of luminosity to mass, in nearby clouds is found to be similar to that in the more distant Galactic Plane, with the column density distributions also indistinguishable from each other.

astro-ph.GA↗

arXiv2018

The Spatial-Kinematic Structure of the Region of Massive Star Formation S255N on Various Scales

The results of a detailed analysis of SMA, VLA, and IRAM observations of the region of massive star formation S255N in CO(2---1), \nh, \nhh, \co and some other lines is presented. Combining interferometer and single-dish data has enabled a more detailed investigation of the gas kinematics in the moleclar core on various spatial scales. There are no signs of rotation or isotropic compression on the scale of the region as whole. The largest fragments of gas ($\approx$0.3 pc) are located near the boundary of the regions of ionized hydrogen S255 and S257. Some smaller-scale fragments are associated with protostellar clumps. The kinetic temperatures of these fragments lie in the range 10---80 K. A circumstellar torus with inner radius R$_{in}$ $\approx$ 8000 AU and outer radius R$_{out}$ 12 000 AU has been detected around the clump SMA1. The rotation profile indicates the existence of a central object with mass $\approx$ 8.5/ sin 2 (i) M$_\odot$ . SMA1 is resolved into two clumps, SMA1---NE and SMA1---SE, whose temperatures are $\approx$150 K and $\approx$25 K, respectively. To all appearances, the torus is involved in the accretion of surrounding gas onto the two protostellar clumps.

astro-ph.GA↗

arXiv2018

Disks and outflows in the S255IR area of high mass star formation from ALMA observations

We describe the general structure of the well known S255IR high mass star forming region, as revealed by our recent ALMA observations. The data indicate a physical relation of the major clumps SMA1 and SMA2. The driving source of the extended high velocity well collimated bipolar outflow is not the most pronounced disk-like SMA1 clump harboring a 20 M$_\odot$ young star (S255 NIRS3), as it was assumed earlier. Apparently it is the less evolved SMA2 clump, which drives the outflow and contains a compact rotating structure (probably a disk). At the same time the SMA1 clump drives another outflow, with a larger opening angle. The molecular line data do not show an outflow from the SMA3 clump (NIRS1), which was suggested by IR studies of this region.

astro-ph.GA↗

arXiv2017

Detection of a new methanol maser line with ALMA

Aims. We aimed at investigating the structure and kinematics of the gaseous disk and outflows around the massive YSO S255 NIRS3 in the S255IR-SMA1 dense clump. Methods. Observations of the S255IR region were carried out with ALMA at two epochs in the compact and extended configurations. Results. We serendipitously detected a new, never predicted, bright maser line at about 349.1 GHz, which most probably represents the CH$_3$OH $14_{1} - 14_{0}$ A$^{- +}$ transition. The emission covers most of the 6.7 GHz methanol maser emission area of almost 1$^{\prime\prime}$ in size and shows a velocity gradient in the same sense as the disk rotation. No variability was found on the time interval of several months. It is classified as Class II maser and probably originates in a ring at a distance of several hundreds AU from the central star.

astro-ph.GA↗

arXiv2016

Spectroscopic Studies of the Physical Origin of Environmental Aging Effects on Doped Graphene

The environmental aging effect of doped graphene is investigated as a function of the organic doping species, humidity, and the number of graphene layers adjacent to the dopant by studies of the Raman spectroscopy, x-ray and ultraviolet photoelectron spectroscopy, scanning electron microscopy, infrared spectroscopy, and electrical transport measurements. It is found that higher humidity and structural defects induce faster degradation in doped graphene. Detailed analysis of the spectroscopic data suggest that the physical origin of the aging effect is associated with the continuing reaction of H2O molecules with the hygroscopic organic dopants, which leads to formation of excess chemical bonds, reduction in the doped graphene carrier density, and proliferation of damages from the graphene grain boundaries. These environmental aging effects are further shown to be significantly mitigated by added graphene layers.

cond-mat.mes-hall↗

arXiv2015

The disk-outflow system in the S255IR area of high mass star formation

We report the results of our observations of the S255IR area with the SMA at 1.3 mm in the very extended configuration and at 0.8 mm in the compact configuration as well as with the IRAM-30m at 0.8 mm. The best achieved angular resolution is about 0.4 arcsec. The dust continuum emission and several tens of molecular spectral lines are observed. The majority of the lines is detected only towards the S255IR-SMA1 clump, which represents a rotating structure (probably disk) around the young massive star. The achieved angular resolution is still insufficient for conclusions about Keplerian or non-Keplerian character of the rotation. The temperature of the molecular gas reaches 130-180 K. The size of the clump is about 500 AU. The clump is strongly fragmented as follows from the low beam filling factor. The mass of the hot gas is significantly lower than the mass of the central star. A strong DCN emission near the center of the hot core most probably indicates a presence of a relatively cold ($\lesssim 80$ K) and rather massive clump there. High velocity emission is observed in the CO line as well as in lines of high density tracers HCN, HCO+, CS and other molecules. The outflow morphology obtained from combination of the SMA and IRAM-30m data is significantly different from that derived from the SMA data alone. The CO emission detected with the SMA traces only one boundary of the outflow. The outflow is most probably driven by jet bow shocks created by episodic ejections from the center. We detected a dense high velocity clump associated apparently with one of the bow shocks. The outflow strongly affects the chemical composition of the surrounding medium.

astro-ph.GA↗

arXiv2013

The Science Cases for Building a Band 1 Receiver Suite for ALMA

We present the various science cases for building Band 1 receivers as part of ALMA's ongoing Development Program. We describe the new frequency range for Band 1 of 35-52 GHz, a range chosen to maximize the receiver suite's scientific impact. We first describe two key science drivers: 1) the evolution of grains in protoplanetary disks and debris disks, and 2) molecular gas in galaxies during the era of re-ionization. Studies of these topics with Band 1 receivers will significantly expand ALMA's Level 1 Science Goals. In addition, we describe a host of other exciting continuum and line science cases that require ALMA's high sensitivity and angular resolution. For example, ALMA Band 1 continuum data will probe the Sunyaev-Zel'dovich Effect in galaxy clusters, Very Small Grains and spinning dust, ionized jets from young stars, spatial and flaring studies of Sgr A*, the acceleration sites of solar flares, pulsar wind nebulae, radio supernovae, and X-ray binaries. Furthermore, ALMA Band 1 line data will probe chemical differentiation in cloud cores, complex carbon chain molecules, extragalactic radio recombination lines, masers, magnetic fields through Zeeman effect measurements, molecular outflows from young stars, the co-evolution of star formation and active galactic nuclei, and the molecular content of galaxies at z ~ 3. ALMA provides similar to better sensitivities than the JVLA over 35-50 GHz, with differences increasing with frequency. ALMA's smaller antennas and shorter baselines, greater number of baselines, and single-dish capabilities, however, give it a significant edge for observing extended emission, making wide-field maps (mosaics), or attaining high image fidelity, as required by the described science cases.

astro-ph.IM↗

arXiv2013

The L1157 protostellar outflow imaged with the SMA

We use the Submillimeter Array to observe, at 1.4 mm, the blue-lobe of the L1157 outflow at high spatial resolution (~ 3"). We detected SiO, H_2CO, and CH_3OH lines from several molecular clumps that constitute the outflow. All three molecules were detected along the wall of the inner cavity that is supposedly related with the later ejection event. On the other hand, no emission was detected towards positions related to an old ejection episode, likely due to space filtering from the interferometer. The H_2CO and CH_3OH emission is detected only at velocities close to the systemic velocity. The spatial distributions of the H_2CO and CH_3OH are similar. These emission lines trace the U-shaped structure seen in the mid-infrared image. In contrast, the SiO emission is detected in wider velocity range with a peak at ~14 km s/s blue-shifted from the systemic velocity. The SiO emission is brightest at the B1 position, which corresponds to the apex of the U-shaped structure. There are two compact SiO clumps along the faint arc-like feature to the east of the U-shaped structure. At the B1 position, there are two velocity components; one is a compact clump with a size of ~1500 AU seen in the high-velocity and the other is an extended component with lower velocities. The kinematic structure at the B1 position is different from that expected in a single bow shock. It is likely that the high-velocity SiO clump at the B1 position is kinetically independent from the low-velocity gas. The line ratio between SiO (5--4) and SiO (2--1) suggests that the high velocity SiO clumps consist of high density gas of n ~ 10^5 - 10^6 cm^-3, which is comparable to the density of the bullets in the extremely high velocity (EHV) jets. It is likely that the high-velocity SiO clumps in L1157 have the same origin as the EHV bullets.

astro-ph.GA↗

arXiv2012

The Science Case for Building a Band 1 Receiver Suite for ALMA

We present a set of compelling science cases for the ALMA Band 1 receiver suite. For these cases, we assume in tandem the updated nominal Band 1 frequency range of 35-50 GHz with a likely extension up to 52 GHz; together these frequencies optimize the Band 1 science return. The scope of the science cases ranges from nearby stars to the re-ionization edge of the Universe. Two cases provide additional leverage on the present ALMA Level One Science Goals and are seen as particularly powerful motivations for building the Band 1 Receiver suite: (1) detailing the evolution of grains in protoplanetary disks, as a complement to the gas kinematics, requires continuum observations out to ~35 GHz (~9mm); and (2) detecting CO 3-2 line emission from galaxies like the Milky Way during the epoch of re-ionization, i.e., 6 < z < 10, also requires Band 1 receiver coverage. The range of Band 1 science is wide, however, and includes studies of very small dust grains in the ISM, pulsar wind nebulae, radio supernovae, X-ray binaries, the Galactic Center (i.e., Sgr A*), dense cloud cores, complex carbon-chain molecules, masers, magnetic fields in the dense ISM, jets and outflows from young stars, distant galaxies, and galaxy clusters (i.e., the Sunyaev-Zel'dovich Effect). A comparison of ALMA and the Jansky VLA (JVLA) at the same frequencies of Band 1 finds similar sensitivity performance at 40-50 GHz, with a slight edge for ALMA at higher frequencies (e.g., within a factor of 2 for continuum observations). With its larger number of instantaneous baselines, however, ALMA Band 1data will have greater fidelity than those from the JVLA at similar frequencies.

astro-ph.IM↗

arXiv2012

A Multi-Wavelength High Resolution Study of the S255 Star Forming Region. General structure and kinematics

We present observational data for two main components (S255IR and S255N) of the S255 high mass star forming region in continuum and molecular lines obtained at 1.3 mm and 1.1 mm with the SMA, at 1.3 cm with the VLA and at 23 and 50 cm with the GMRT. The angular resolution was from ~ 2" to ~ 5" for all instruments. With the SMA we detected a total of about 50 spectral lines of 20 different molecules (including isotopologues). About half of the lines and half of the species (in particular N2H+, SiO, C34S, DCN, DNC, DCO+, HC3N, H2CO, H2CS, SO2) have not been previously reported in S255IR and partly in S255N at high angular resolution. Our data reveal several new clumps in the S255IR and S255N areas by their millimeter wave continuum emission. Masses of these clumps are estimated at a few solar masses. The line widths greatly exceed expected thermal widths. These clumps have practically no association with NIR or radio continuum sources, implying a very early stage of evolution. At the same time, our SiO data indicate the presence of high-velocity outflows related to some of these clumps. In some cases, strong molecular emission at velocities of the quiescent gas has no detectable counterpart in the continuum. We discuss the main features of the distribution of NH3, N2H+, and deuterated molecules. We estimate properties of decimeter wave radio continuum sources and their relationship with the molecular material.

astro-ph.GA↗

arXiv2009

The Hot and Clumpy Molecular Cocoon Surrounding the Ultracompact HII Region G5.89-0.39

We present observations of CH3CN (12-11) emission at a resolution of 2" toward the shell-like ultracompact HII region G5.89-0.39 with the Submillimeter Array. The integrated CH3CN emission reveals dense and hot molecular cocoon in the periphery of the HII region G5.89-0.39, with a CH3CN deficient region roughly centered at G5.89-0.39. By analyzing the CH3CN emission using population diagram analysis, we find, for the first time, a decreasing temperature structure from 150 to 40 K with the projected distance from Feldt's star, which is thought to be responsible for powering the HII region. Our results further indicate that the majority of the heating energy in the observed dense gas is supplied by the Feldt's star. From the derived CH3CN column density profile, we conclude that the dense gas is not uniformly-distributed but centrally-concentrated, with a power-law exponent of 5.5 for r < 8000 AU, and 2.0 for 8000 AU < r < 20000 AU, where r is the distance to Feldt's star. The estimated large power index of 5.5 can be attributed to an enhancement of CH3CN abundance in the close vicinity of Feldt's star.

astro-ph.GA↗

arXiv2006

Multiple Jets from the High-Mass (Proto)stellar Cluster AFGL5142

We present studies of a massive protocluster AFGL5142 in the J=2-1 transition of the CO isotopologues, SO, CH_3OH and CH_3CN lines, as well as continuum at 225 GHz and 8.4 GHz. The 225 GHz continuum emission reveals three prominent peaks MM-1, MM-2 and MM-3. MM-1 and MM-2 are associated with strong CH_3CN emission with temperatures of 90 \pm 20 and 250 \pm 40 K, respectively, while both MM-1 and MM-3 are associated with faint continuum emission at 8.4 GHz. Additional dust continuum peaks MM-4 and MM-5 appear to be associated with \h2O masers. With many continuum sources at cm and mm wavelengths, and those already identified in the infrared, this region is forming a cluster of stars. The CO and SO emission reveals at least three molecular outflows originating from the center of the dust core. The outflows are well collimated, with terminal velocities up to 50 \kms-1 from the cloud velocity. Outflow A coincides with the SiO jet identified previously by Hunter et al. (1999). Since jet-like outflows and disk-mediated accretion process are physically connected, the well collimated outflows indicate that even in this cluster environment, accretion is responsible for the formation of individual stars in the cluster.

astro-ph↗

arXiv2005

Submillimeter Array 440$μ$m/690GHz line and continuum observations of Orion-KL

Submillimeter Array observations of Orion-KL at 1'' resolution in the 440mu/690GHz band reveal new insights about the continuum and line emission of the region. The 440mu continuum flux density measurement from source I allows us to differentiate among the various proposed physical models: Source I can be well modeled by a ``normal'' protostellar SED consisting of a proton-electron free-free emission component at low frequencies and a strong dust component in the submillimeter bands. Furthermore, we find that the protostellar object SMA1 is clearly distinct from the hot core. The non-detection of SMA1 at cm and infrared wavelengths suggests that it may be one of the youngest sources in the entire Orion-KL region. The molecular line maps show emission mainly from the sources I, SMA1 and the hot core peak position. An analysis of the CH$3CN(37_K-36_K) K-ladder (K=0...3) indicates a warm gas component of the order 600+-200K. In addition, we detect a large fraction (~58%) of unidentified lines and discuss the difficulties of line identifications at these frequencies.

astro-ph↗

arXiv2005

Submm line imaging of Orion-KL with the Submillimeter Array

We present the first submm (865mum) imaging spectral line survey at one arcsecond resolution conducted with the Submillimeter Array toward Orion-KL. Within the 2x2 GHz bandpasses (lower and upper sidebands, 337.2-339.2GHz and 347.2-349.2GHz), we find about 145 spectral lines from 13 species, 6 isotopologues, and 5 vibrational excited states. Most nitrogen-bearing molecules are strong toward the hot core, whereas the oxygen-bearing molecules peak toward the south-west in the so-called compact ridge. Imaging of spectral lines is shown to be an additional tool to improve the identifications of molecular lines. Arcsecond spatial resolution allows us to distinguish the molecular line emission of the sources I and n from that of the hot core. The only molecular species detected strongly toward source I is SiO, delineating mainly the collimated north-east south-west low-velocity outflow. The two positions close to source I, which have previously been reported to show maser emission in the v=0 28SiO(1-0) and (2-1) lines, show no detectable maser emission in the v=0 28SiO(8-7) line at our spatial resolution. SiO is weak toward source n, and thus source n may not currently be driving a molecular outflow. CH$_3$OH is the molecule with the highest number of identified lines (46) in this spectral window. This ``line forest'' allows us to estimate temperatures in the region, and we find temperatures between 50 and 350K, with the peak temperatures occurring toward the hot core. The detection of strong vibrational excited line emission from the submm continuum peak SMA1 supports the interpretation that the source SMA1 is likely of protostellar nature.

astro-ph↗