arXiv subjects

Di Li

Publications and source records attributed to Di Li.

At least 415 records · Page 23Linked to original sources

arXiv2013

Quickest Change Point Detection and Identification Across a Generic Sensor Array

In this paper, we consider the problem of quickest change point detection and identification over a linear array of $N$ sensors, where the change pattern could first reach any of these sensors, and then propagate to the other sensors. Our goal is not only to detect the presence of such a change as quickly as possible, but also to identify which sensor that the change pattern first reaches. We jointly design two decision rules: a stopping rule, which determines when we should stop sampling and claim a change occurred, and a terminal decision rule, which decides which sensor that the change pattern reaches first, with the objective to strike a balance among the detection delay, the false alarm probability, and the false identification probability. We show that this problem can be converted to a Markov optimal stopping time problem, from which some technical tools could be borrowed. Furthermore, to avoid the high implementation complexity issue of the optimal rules, we develop a scheme with a much simpler structure and certain performance guarantee.

cs.IT↗

arXiv2012

FAST low frequency pulsar survey

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) is under construction and will be commissioned in September 2016. A low frequency 7-beam receiver working around 400 MHz is proposed for FAST early science. It will be optimized for a whole FAST sky drift-scan pulsar survey. Simulations show that about 1500 new normal pulsars will be discovered, as while as about 200 millisecond pulsars.

astro-ph.IM↗

arXiv2012

The Five-hundred-meter Aperture Spherical Radio Telescope Project and its Early Science Opportunities

The National Astronomical Observatories, Chinese Academy of Science (NAOC), has started building the largest antenna in the world. Known as FAST, the Five-hundred-meter Aperture Spherical radio Telescope is a Chinese mega-science project funded by the National Development and Reform Commission (NDRC). FAST also represents part of Chinese contribution to the international efforts to build the square kilometer array (SKA). Upon its finishing around September of 2016, FAST will be the most sensitive single-dish radio telescope in the low frequency radio bands between 70 MHz and 3 GHz. The design specifications of FAST, its expected capabilities, and its main scientific aspirations were described in an overview paper by Nan et al. (2011). In this paper, we briefly review the design and the key science goals of FAST, speculate the likely limitations at the initial stages of FAST operation, and discuss the opportunities for astronomical discoveries in the so-called early science phase.

astro-ph.IM↗

arXiv2012

13CO Cores in Taurus Molecular Cloud

Young stars form in molecular cores, which are dense condensations within molecular clouds. We have searched for molecular cores traced by $^{13}$CO $J=1\to 0$ emission in the Taurus molecular cloud and studied their properties. Our data set has a spatial dynamic range (the ratio of linear map size to the pixel size) of about 1000 and spectrally resolved velocity information, which together allow a systematic examination of the distribution and dynamic state of $^{13}$CO cores in a large contiguous region. We use empirical fit to the CO and CO$_2$ ice to correct for depletion of gas-phase CO. The $^{13}$CO core mass function ($^{13}$CO CMF) can be fitted better with a log-normal function than with a power law function. We also extract cores and calculate the $^{13}$CO CMF based on the integrated intensity of $^{13}$CO and the CMF from 2MASS. We demonstrate that there exists core blending, i.e.\ combined structures that are incoherent in velocity but continuous in column density. The core velocity dispersion (CVD), which is the variance of the core velocity difference $δv$, exhibits a power-law behavior as a function of the apparent separation $L$:\ CVD (km/s) $\propto L ({\rm pc})^{0.7}$. This is similar to Larson's law for the velocity dispersion of the gas. The peak velocities of $^{13}$CO cores do not deviate from the centroid velocities of the ambient $^{12}$CO gas by more than half of the line width. The low velocity dispersion among cores, the close similarity between CVD and Larson's law, and the small separation between core centroid velocities and the ambient gas all suggest that molecular cores condense out of the diffuse gas without additional energy from star formation or significant impact from converging flows.

astro-ph.GA↗

arXiv2012

Is the Taurus B213 Region a True Filament?: Observations of Multiple Cyanoacetylene Transitions

We have obtained spectra of the J=2-1 and J=10-9 transitions of cyanoacetylene (\hc3n) toward a collection of positions in the most prominent filament, B213, in the Taurus molecular cloud. The analysis of the excitation conditions of these transitions reveals an average gas H$_2$ volume density of $(1.8\pm 0.7) \times10^{4} $ \cc. Based on column density derived from 2MASS and this volume density, the line of sight dimension of the high density portion of B213 is found to be $\simeq$ 0.12 pc, which is comparable to the smaller projected dimension and much smaller than the elongated dimension of B213 ($\sim$2.4 pc). B213 is thus likely a true cylinder--like filament rather than a sheet seen edge-on. The line width and velocity gradient seen in \hc3n are also consistent with Taurus B213 being a self-gravitating filament in the early stage of either fragmentation and/or collape.

astro-ph.GA↗

arXiv2012

Gas emissions in Planck cold dust clumps---A Survey of the J=1-0 Transitions of $^{12}$CO, $^{13}$CO, and C$^{18}$O

A survey toward 674 Planck cold clumps of the Early Cold Core Catalogue (ECC) in the J=1-0 transitions of $^{12}$CO, $^{13}$CO and C$^{18}$O has been carried out using the PMO 13.7 m telescope. 673 clumps were detected with the $^{12}$CO and $^{13}$CO, and 68% of the samples have C$^{18}$O emission. Additional velocity components were also identified.A close consistency of the three line peak velocities was revealed for the first time. Kinematic distances are given out for all the velocity components and half of the clumps are located within 0.5 and 1.5 kpc. Excitation temperatures range from 4 to 27 K, slightly larger than those of $T_d$. Line width analysis shows that the majority of ECC clumps are low mass clumps. Column densities N$_{H_{2}}$ span from 10$^{20}$ to 4.5$\times10^{22}$ cm$^{-2}$ with an average value of (4.4$\pm$3.6)$\times10^{21}$ cm$^{-2}$. N$_{H_{2}}$ cumulative fraction distribution deviates from the lognormal distribution, which is attributed to optical depth. The average abundance ratio of the $^{13}$CO to C$^{18}$O in these clumps is 7.0$\pm$3.8, higher than the terrestrial value. Dust and gas are well coupled in 95% of the clumps. Blue profile, red profile and line asymmetry in total was found in less than 10% of the clumps, generally indicating star formation is not developed yet. Ten clumps were mapped. Twelve velocity components and 22 cores were obtained. Their morphologies include extended diffuse, dense isolated, cometary and filament, of which the last is the majority. 20 cores are starless.Only 7 cores seem to be in gravitationally bound state. Planck cold clumps are the most quiescent among the samples of weak-red IRAS, infrared dark clouds, UC H{\sc ii} region candidates, EGOs and methanol maser sources, suggesting that Planck cold clumps have expanded the horizon of cold Astronomy.

astro-ph.SR↗

arXiv2012

Herschel Search for O2 Toward the Orion Bar

We report the results of a search for molecular oxygen (O2) toward the Orion Bar, a prominent photodissociation region at the southern edge of the HII region created by the luminous Trapezium stars. We observed the spectral region around the frequency of the O2 N_J = 3_3 - 1_2 transition at 487 GHz and the 5_4 - 3_4 transition at 774 GHz using the Heterodyne Instrument for the Far Infrared on the Herschel Space Observatory. Neither line was detected, but the 3sigma upper limits established here translate to a total line-of-sight O2 column density < 1.5 10^16 cm^-2 for an emitting region whose temperature is between 30K and 250 K, or < 1 10^16 cm^-2 if the O2 emitting region is primarily at a temperature of ~< 100 K. Because the Orion Bar is oriented nearly edge-on relative to our line of sight, the observed column density is enhanced by a factor estimated to be between 4 and 20 relative to the face-on value. Our upper limits imply that the face-on O2 column density is less than 4 10^15 cm^-2, a value that is below, and possibly well below, model predictions for gas with a density of 10^4 - 10^5 cm^-3 exposed to a far ultraviolet flux 10^4 times the local value, conditions inferred from previous observations of the Orion Bar. The discrepancy might be resolved if: (1) the adsorption energy of O atoms to ice is greater than 800 K; (2) the total face-on Av of the Bar is less than required for O2 to reach peak abundance; (3) the O2 emission arises within dense clumps with a small beam filling factor; or, (4) the face-on depth into the Bar where O2 reaches its peak abundance, which is density dependent, corresponds to a sky position different from that sampled by our Herschel beams.

astro-ph.GA↗

arXiv2011

Herschel Measurements of Molecular Oxygen in Orion

We report observations of three rotational transitions of molecular oxygen (O2) in emission from the H2 Peak 1 position of vibrationally excited molecular hydrogen in Orion. We observed the 487 GHz, 774 GHz, and 1121 GHz lines using HIFI on the Herschel Space Observatory, having velocities of 11 km s-1 to 12 km s-1 and widths of 3 km s-1. The beam-averaged column density is N(O2) = 6.5\times1016 cm-2, and assuming that the source has an equal beam filling factor for all transitions (beam widths 44, 28, and 19"), the relative line intensities imply a kinetic temperature between 65 K and 120 K. The fractional abundance of O2 relative to H2 is 0.3 - 7.3\times10-6. The unusual velocity suggests an association with a ~ 5" diameter source, denoted Peak A, the Western Clump, or MF4. The mass of this source is ~ 10 M\odot and the dust temperature is \geq 150 K. Our preferred explanation of the enhanced O2 abundance is that dust grains in this region are sufficiently warm (T \geq 100 K) to desorb water ice and thus keep a significant fraction of elemental oxygen in the gas phase, with a significant fraction as O2. For this small source, the line ratios require a temperature \geq 180 K. The inferred O2 column density \simeq 5\times1018 cm-2 can be produced in Peak A, having N(H2) \simeq 4\times1024 cm-2. An alternative mechanism is a low-velocity (10 to 15 km s-1) C-shock, which can produce N(O2) up to 1017 cm-2.

astro-ph.GA↗

arXiv2011

The Magnetic Field in Taurus Probed by Infrared Polarization

We present maps of the plane-of-sky magnetic field within two regions of the Taurus molecular cloud: one in the dense core L1495/B213 filament, the other in a diffuse region to the west. The field is measured from the polarization of background starlight seen through the cloud. In total, we measured 287 high-quality near-infrared polarization vectors in these regions. In L1495/B213, the percent polarization increases with column density up to Av ~ 9 mag, the limits of our data. The Radiative Torques model for grain alignment can explain this behavior, but models that invoke turbulence are inconsistent with the data. We also combine our data with published optical and near-infrared polarization measurements in Taurus. Using this large sample, we estimate the strength of the plane-of-sky component of the magnetic field in nine subregions. This estimation is done with two different techniques that use the observed dispersion in polarization angles. Our values range from 5-82 microgauss and tend to be higher in denser regions. In all subregions, the critical index of the mass-to-magnetic flux ratio is sub-unity, implying that Taurus is magnetically supported on large scales (~2 pc). Within the region observed, the B213 filament makes a sharp turn to the north and the direction of the magnetic field also takes a sharp turn, switching from being perpendicular to the filament to becoming parallel. This behavior can be understood if we are observing the rim of a bubble. We argue that it has resulted from a supernova remnant associated with a recently discovered nearby gamma-ray pulsar.

astro-ph.GA↗

arXiv2011

The Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST) Project

Five-hundred-meter Aperture Spherical radio Telescope (FAST) is a Chinese mega-science project to build the largest single dish radio telescope in the world. Its innovative engineering concept and design pave a new road to realize a huge single dish in the most effective way. FAST also represents Chinese contribution in the international efforts to build the square kilometer array (SKA). Being the most sensitive single dish radio telescope, FAST will enable astronomers to jump-start many science goals, for example, surveying the neutral hydrogen in the Milky Way and other galaxies, detecting faint pulsars, looking for the first shining stars, hearing the possible signals from other civilizations, etc. The idea of sitting a large spherical dish in a karst depression is rooted in Arecibo telescope. FAST is an Arecibo-type antenna with three outstanding aspects: the karst depression used as the site, which is large to host the 500-meter telescope and deep to allow a zenith angle of 40 degrees; the active main reflector correcting for spherical aberration on the ground to achieve a full polarization and a wide band without involving complex feed systems; and the light-weight feed cabin driven by cables and servomechanism plus a parallel robot as a secondary adjustable system to move with high precision. The feasibility studies for FAST have been carried out for 14 years, supported by Chinese and world astronomical communities. The project time is 5.5 years from the commencement of work in March of 2011 and the first light is expected to be in 2016. This review intends to introduce FAST project with emphasis on the recent progress since 2006. In this paper, the subsystems of FAST are described in modest details followed by discussions of the fundamental science goals and examples of early science projects.

astro-ph.IM↗

arXiv2010

A Selection Region Based Routing Protocol for Random Mobile ad hoc Networks with Directional Antennas

In this paper, we propose a selection region based multihop routing protocol with directional antennas for wireless mobile ad hoc networks, where the selection region is defined by two parameters: a reference distance and the beamwidth of the directional antenna. At each hop, we choose the nearest node to the transmitter within the selection region as the next hop relay. By maximizing the expected density of progress, we present an upper bound for the optimum reference distance and derive the relationship between the optimum reference distance and the optimum transmission probability. Compared with the results with routing strategy using omnidirectional antennas in \cite{Di:Relay-Region}, we find interestingly that the optimum transmission probability is a constant independent of the beamwidth, the expected density of progress with the new routing strategy is increased significantly, and the computational complexity involved in the relay selection is also greatly reduced.

cs.IT↗

arXiv2010

Distribution of Water Vapor in Molecular Clouds

We report the results of a large-area study of water vapor along the Orion Molecular Cloud ridge, the purpose of which was to determine the depth-dependent distribution of gas-phase water in dense molecular clouds. We find that the water vapor measured toward 77 spatial positions along the face-on Orion ridge, excluding positions surrounding the outflow associated with BN/KL and IRc2, display integrated intensities that correlate strongly with known cloud surface tracers such as CN, C2H, 13CO J =5-4, and HCN, and less well with the volume tracer N2H+. Moreover, at total column densities corresponding to Av < 15 mag., the ratio of H2O to C18O integrated intensities shows a clear rise approaching the cloud surface. We show that this behavior cannot be accounted for by either optical depth or excitation effects, but suggests that gas-phase water abundances fall at large Av. These results are important as they affect measures of the true water-vapor abundance in molecular clouds by highlighting the limitations of comparing measured water vapor column densities with such traditional cloud tracers as 13CO or C18O. These results also support cloud models that incorporate freeze-out of molecules as a critical component in determining the depth-dependent abundance of water vapor.

astro-ph.GA↗

arXiv2010

The relation between gas and dust in the Taurus Molecular Cloud

(abridged) We report a study of the relation between dust and gas over a 100deg^2 area in the Taurus molecular cloud. We compare the H2 column density derived from dust extinction with the CO column density derived from the 12CO and 13CO J= 1-0 lines. We derive the visual extinction from reddening determined from 2MASS data. The comparison is done at an angular size of 200", corresponding to 0.14pc at a distance of 140pc. We find that the relation between visual extinction Av and N(CO) is linear between Av~3 and 10 mag in the region associated with the B213--L1495 filament. In other regions the linear relation is flattened for Av > 4 mag. We find that the presence of temperature gradients in the molecular gas affects the determination of N(CO) by ~30--70% with the largest difference occurring at large column densities. Adding a correction for this effect and accounting for the observed relation between the column density of CO and CO2 ices and Av, we find a linear relationship between the column of carbon monoxide and dust for observed visual extinctions up to the maximum value in our data 23mag. We have used these data to study a sample of dense cores in Taurus. Fitting an analytical column density profile to these cores we derive an average volume density of about 1.4e4 cm^-3 and a CO depletion age of about 4.2e5 years. We estimate the H2 mass of Taurus to be about 1.5e4 M_sun, independently derived from the Av and N(CO) maps. We derive a CO integrated intensity to H2 conversion factor of about 2.1e20 cm^-2 (K km/s)^-1, which applies even in the region where the [CO]/[H_2] ratio is reduced by up to two orders of magnitude. The distribution of column densities in our Taurus maps resembles a log--normal function but shows tails at large and low column densities.

astro-ph.GA↗

arXiv2010

A Selection Region Based Routing Protocol for Random Mobile ad hoc Networks

We propose a selection region based multi-hop routing protocol for random mobile ad hoc networks, where the selection region is defined by two parameters: a reference distance and a selection angle. At each hop, a relay is chosen as the nearest node to the transmitter that is located within the selection region. By assuming that the relay nodes are randomly placed, we derive an upper bound for the optimum reference distance to maximize the expected density of progress and investigate the relationship between the optimum selection angle and the optimum reference distance. We also note that the optimized expected density of progress scales as $Θ(\sqrtλ)$, which matches the prior results in the literature. Compared with the spatial-reuse multi-hop protocol in \cite{Baccelli:Aloha} recently proposed by Baccelli \emph{et al.}, in our new protocol the amount of nodes involved and the calculation complexity for each relay selection are reduced significantly, which is attractive for energy-limited wireless ad hoc networks (e.g., wireless sensor networks).

cs.IT↗

arXiv2010

Distance to the SNR CTB 109/AXP 1E 2259+586 by HI absorption and self-absorption

We suggest a revised distance to the supernova remnant (SNR) G109.1-1.0 (CTB 109) and its associated anomalous X-ray pulsar (AXP) 1E 2259+586 by analyzing 21cm HI-line and 12CO-line spectra of CTB 109, HII region Sh 152, and the adjacent molecular cloud complex. CTB 109 has been established to be interacting with a large molecular cloud (recession velocity at v=-55 km s^-1). The highest radial velocities of absorption features towards CTB 109 (-56 km s^-1) and Sh 152 (-65 km s^-1) are larger than the recombination line velocity (-50 km s^-1) of Sh 152 demonstrating the velocity reversal within the Perseus arm. The molecular cloud has cold HI column density large enough to produce HI self-absorption (HISA) and HI narrow self-absorption (HINSA) if it was at the near side of the velocity reversal. Absence of both HISA and HINSA indicates that the cloud is at the far side of the velocity reversal within the Perseus Arm, so we obtain a distance for CTB 109 of 4+/-0.8 kpc. The new distance still leads to a normal explosion energy for CTB 109/AXP 1E 2259+586.

astro-ph.GA↗

arXiv2008

An Improved Technique for Measurement of Cold HI in Molecular Cloud Cores

The presence of atomic gas mixed with molecular species in a "molecular" cloud may significantly affect its chemistry, the excitation of some species, and can serve as probe of the cloud's evolution. Cold neutral atomic hydrogen (HI) in molecular clouds is revealed by its self absorption of background galactic HI 21-cm emission. The properties of this gas can be investigated quantitatively through observation of HI Narrow Self-Absorption (HINSA). In this paper, we present a new technique for measuring atomic gas physical parameters from HINSA observations that utilizes molecular tracers to guide the HINSA extraction. This technique offers a significant improvement in the precision with which HI column densities can be determined over previous methods, and it opens several new avenues of study of relevance to the field of star formation.

astro-ph↗

arXiv2008

The Five College Radio Astronomy Observatory CO Mapping Survey of the Taurus Molecular Cloud

The FCRAO Survey of the Taurus Molecular Cloud observed the 12CO and 13CO J=1-0 emission from 98 square degrees of this important, nearby star forming region. This set of data with 45" resolution comprises the highest spatial dynamic range image of an individual molecular cloud constructed to date, and provides valuable insights to the molecular gas distribution, kinematics, and the star formation process. In this contribution, we describe the observations, calibration, data processing, and characteristics of the noise and line emission of the survey. The angular distribution of 12CO and 13CO emission over 1 km/s velocity intervals and the full velocity extent of the cloud are presented. These reveal a complex, dynamic medium of cold, molecular gas.

astro-ph↗

arXiv2008

Large-Scale Structure of the Molecular Gas in Taurus Revealed by High Linear Dynamic Range Spectral Line Mapping

We report the results of a 100 square degree survey of the Taurus Molecular Cloud region in the J = 1-0 transition of 12CO and 13CO. The image of the cloud in each velocity channel includes ~ 3 million Nyquist sampled pixels on a 20" grid. The high sensitivity and large linear dynamic range of the maps in both isotopologues reveal a very complex, highly structured cloud morphology. There are large scale correlated structures evident in 13CO emission having very fine dimensions, including filaments, cavities, and rings. The 12CO emission shows a quite different structure, with particularly complex interfaces between regions of greater and smaller column density defining the boundaries of the largest-scale cloud structures. The axes of the striations seen in the 12CO emission from relatively diffuse gas are aligned with the direction of the magnetic field. Using a column density-dependent model for the CO fractional abundance, we derive the mass of the region mapped to be 24,000 solar masses, a factor of three greater than would be obtained with canonical CO abundance restricted to the high column density regions. We determine that half the mass of the cloud is in regions having column density below 2.1x10^{21} per square cm. The distribution of young stars in the region covered is highly nonuniform, with the probability of finding a star in a pixel with a specified column density rising sharply for N(H2) = 6x10^{21} cm^{-2}. We determine a relatively low star formation efficiency (mass of young stars/mass of molecular gas), between 0.3 and 1.2 %, and an average star formation rate during the past 3 Myr of 8x10^{-5} stars yr^{-1}.

astro-ph↗