arXiv subjects

Tie Liu

Publications and source records attributed to Tie Liu.

At least 289 records · Page 16Linked to original sources

arXiv2013

Competitive accretion in the protocluster G10.6-0.4?

We present the results of high spatial resolution observations at 1.1 mm waveband, with the Submillimetre Array (SMA), towards the protocluster G10.6-0.4. The 1.1 mm continuum emission reveals seven dense cores, towards which infall motions are all detected with the red-shifted absorption dips in HCN (3--2) line. This is the first time that infall is seen towards multiple sources in a protocluster. We also identified four infrared point sources in this region, which are most likely Class 0/I protostars. Two jet-like structures are also identified from Spitzer/IRAC image. The dense core located in the centre has much larger mass than the off-centre cores. The clump is in overall collapse and the infall motion is supersonic. The standard deviation of core velocities and the velocity differences between the cores and the cloud/clump are all larger than the thermal velocity dispersion. The picture of G10.6-0.4 seems to favor the "competitive accretion" model but needs to be tested by further observations.

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arXiv2013

Worst-Case Expected-Capacity Loss of Slow-Fading Channels

For delay-limited communication over block-fading channels, the difference between the ergodic capacity and the maximum achievable expected rate for coding over a finite number of coherent blocks represents a fundamental measure of the penalty incurred by the delay constraint. This paper introduces a notion of worst-case expected-capacity loss. Focusing on the slow-fading scenario (one-block delay), the worst-case additive and multiplicative expected-capacity losses are precisely characterized for the point-to-point fading channel. Extension to the problem of writing on fading paper is also considered, where both the ergodic capacity and the additive expected-capacity loss over one-block delay are characterized to within one bit per channel use.

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arXiv2013

Generalized Cut-Set Bounds for Broadcast Networks

A broadcast network is a classical network with all source messages collocated at a single source node. For broadcast networks, the standard cut-set bounds, which are known to be loose in general, are closely related to union as a specific set operation to combine the basic cuts of the network. This paper provides a new set of network coding bounds for general broadcast networks. These bounds combine the basic cuts of the network via a variety of set operations (not just the union) and are established via only the submodularity of Shannon entropy. The tightness of these bounds are demonstrated via applications to combination networks.

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arXiv2012

Private Broadcasting over Independent Parallel Channels

We study private broadcasting of two messages to two groups of receivers over independent parallel channels. One group consists of an arbitrary number of receivers interested in a common message, whereas the other group has only one receiver. Each message must be kept confidential from the receiver(s) in the other group. Each of the sub-channels is degraded, but the order of receivers on each channel can be different. While corner points of the capacity region were characterized in earlier works, we establish the capacity region and show the optimality of a superposition strategy. For the case of parallel Gaussian channels, we show that a Gaussian input distribution is optimal. We also discuss an extension of our setup to broadcasting over a block-fading channel and demonstrate significant performance gains using the proposed scheme over a baseline time-sharing scheme.

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arXiv2012

Symmetrical Multilevel Diversity Coding and Subset Entropy Inequalities

Symmetrical multilevel diversity coding (SMDC) is a classical model for coding over distributed storage. In this setting, a simple separate encoding strategy known as superposition coding was shown to be optimal in terms of achieving the minimum sum rate (Roche, Yeung, and Hau, 1997) and the entire admissible rate region (Yeung and Zhang, 1999) of the problem. The proofs utilized carefully constructed induction arguments, for which the classical subset entropy inequality of Han (1978) played a key role. This paper includes two parts. In the first part the existing optimality proofs for classical SMDC are revisited, with a focus on their connections to subset entropy inequalities. First, a new sliding-window subset entropy inequality is introduced and then used to establish the optimality of superposition coding for achieving the minimum sum rate under a weaker source-reconstruction requirement. Second, a subset entropy inequality recently proved by Madiman and Tetali (2010) is used to develop a new structural understanding to the proof of Yeung and Zhang on the optimality of superposition coding for achieving the entire admissible rate region. Building on the connections between classical SMDC and the subset entropy inequalities developed in the first part, in the second part the optimality of superposition coding is further extended to the cases where there is either an additional all-access encoder (SMDC-A) or an additional secrecy constraint (S-SMDC).

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arXiv2012

Follow-up observations of Planck cold clumps in $^{12}$CO/$^{13}$CO/C$^{18}$O (1--0) transitions

The Planck Early Cold Cores Catalog (ECC) provides an unbiased list of Galactic cold clumps, which form an ideal sample for studying the early phases of star formation (\cite[Planck Collabrators et al. 2011]{Planck_etal11}). To study their properties, we have carried out a molecular line ($^{12}$CO/$^{13}$CO/C$^{18}$O) survey towards 674 Planck cold clumps in the ECC with the PMO 13.7 m telescope.

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arXiv2012

Molecular gas and triggered star formation surrounding Wolf-Rayet stars

The environments surrounding nine Wolf-Rayet stars were studied in molecular emission. Expanding shells were detected surrounding these WR stars (see left panels of Figure 1). The average masses and radii of the molecular cores surrounding these WR stars anti-correlate with the WR stellar wind velocities (middle panels of Figure 1), indicating the WR stars has great impact on their environments. The number density of Young Stellar Objects (YSOs) is enhanced in the molecular shells at $\sim$5 arcmin from the central WR star (lower-right panel of Figure 1). Through detailed studies of the molecular shells and YSOs, we find strong evidences of triggered star formation in the fragmented molecular shells (\cite[Liu et al. 2010]{liu_etal12}

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arXiv2012

Molecular environments of 51 Planck cold clumps in Orion complex

A mapping survey towards 51 Planck cold clumps projected on Orion complex was performed with J=1-0 lines of $^{12}$CO and $^{13}$CO at the 13.7 m telescope of Purple Mountain Observatory. The mean column densities of the Planck gas clumps range from 0.5 to 9.5$\times10^{21}$ cm$^{-2}$, with an average value of (2.9$\pm$1.9)$\times10^{21}$ cm$^{-2}$. While the mean excitation temperatures of these clumps range from 7.4 to 21.1 K, with an average value of 12.1$\pm$3.0 K. The averaged three-dimensional velocity dispersion $σ_{3D}$ in these molecular clumps is 0.66$\pm$0.24 km s$^{-1}$. Most of the clumps have $σ_{NT}$ larger than or comparable with $σ_{Therm}$. The H$_{2}$ column density of the molecular clumps calculated from molecular lines correlates with the aperture flux at 857 GHz of the dust emission. Through analyzing the distributions of the physical parameters, we suggest turbulent flows can shape the clump structure and dominate their density distribution in large scale, but not affect in small scale due to the local fluctuations. Eighty two dense cores are identified in the molecular clumps. The dense cores have an averaged radius and LTE mass of 0.34$\pm$0.14 pc and 38$_{-30}^{+5}$ M$_{\sun}$, respectively. And structures of low column density cores are more affected by turbulence, while those of high column density cores are more concerned by other factors, especially by gravity. The correlation of the velocity dispersion versus core size is very weak for the dense cores. The dense cores are found most likely gravitationally bounded rather than pressure confined. The relationship between $M_{vir}$ and $M_{LTE}$ can be well fitted with a power law. The core mass function here is much more flatten than the stellar initial mass function. The lognormal behavior of the core mass distribution is most likely determined by the internal turbulence.

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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.

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arXiv2012

Triggered star formation surrounding Wolf-Rayet star HD 211853

The environment surrounding Wolf-Rayet star HD 211853 is studied in molecular emission, infrared emission, as well as radio and HI emission. The molecular ring consists of well-separated cores, which have a volume density of 10$^{3}$ cm$^{-3}$ and kinematic temperature $\sim$20 K. Most of the cores are under gravitational collapse due to external pressure from the surrounding ionized gas. From SED modeling towards the young stellar objects (YSOs), sequential star formation is revealed on a large scale in space spreading from the Wolf-Rayet star to the molecular ring. A small scale sequential star formation is revealed towards core A, which harbors a very young star cluster. Triggered star formations is thus suggested. The presence of PDR, the fragmentation of the molecular ring, the collapse of the cores, the large scale sequential star formation indicate the "Collect and Collapse" process functions in this region. The star forming activities in core A seem to be affected by the "Radiation-Driven Implosion" (RDI) process.

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arXiv2012

A search for massive young stellar objects towards 98 CH$_{3}$OH maser sources

Using the 13.7 m telescope of Purple Mountain Observatory (PMO), a survey of J=1-0 lines of CO and its isotopes was carried out towards 98 methanol maser sources in January 2008. Eighty-five sources have infrared counterparts within one arcmin. In the survey, except 43 sources showing complex or multiple-peak profiles, almost all the $^{13}$CO line profiles of the other 55 sources have large line widths of 4.5 km s$^{-1}$ on average and are usually asymmetric. Fifty corresponding Infrared Astronomical Satellite (IRAS) sources of these 55 sources are with $L_{bol}$ larger than $10^{3}L_{\odot}$, which can be identified as possible high-mass young stellar sources. Statistics show that the $^{13}$CO line widths correlate with the bolometric luminosity of the associated IRAS sources. We also report the mapping results of two sources: IRAS 06117+1350 and IRAS 07299-1651 here. Two cores were found in IRAS 06117+1350 and one core was detected in IRAS 07299-1651. The northwest core in IRAS 06117+1350 and the core in IRAS 07299-1651 can be identified as precursors of UC\simH{\sc ii} regions or high-mass protostellar objects (HMPOs). The southeast core of IRAS 06117+1350 has no infrared counterpart, seeming to be on younger stages than pre-UC\simH{\sc ii} phase.

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arXiv2012

The molecular emissions and the infall motion in the high-mass young stellar object G8.68-0.37

We present a multi-wavelength observational study towards the high-mass young stellar object G8.68-0.37. A single massive gas-and-dust core is observed in the (sub)millimeter continuum and molecular line emissions. We fitted the spectral energy distribution (SED) from the dust continuum emission. The best-fit SED suggests the presence of two components with temperature of $T_{\rm d}=20$ K and 120 K, respectively. The core has a total mass of up to $1.5\times10^3$ $M_{\odot}$ and bolometric luminosity of $2.3\times10^4 L_{\odot}$. Both the mass and luminosity are dominated by the cold component ($T_{\rm d}=20$ K). The molecular lines of C$^{18}$O, C$^{34}$S, DCN, and thermally excited CH$_3$OH are detected in this core. Prominent infall signatures are observed in the $^{12}$CO $(1-0)$ and $(2-1)$. We estimated an infall velocity of 0.45 km s$^{-1}$ and mass infall rate of $7\times10^{-4} M_{\odot}$ year$^{-1}$. From the molecular lines, we have found a high DCN abundance and relative abundance ratio to HCN. The overabundant DCN may originate from a significant deuteration in the previous cold pre-protostellar phase. And the DCN should now be rapidly sublimated from the grain mantles to maintain the overabundance in the gas phase.

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arXiv2012

Secure Symmetrical Multilevel Diversity Coding

Symmetrical Multilevel Diversity Coding (SMDC) is a network compression problem introduced by Roche (1992) and Yeung (1995). In this setting, a simple separate coding strategy known as superposition coding was shown to be optimal in terms of achieving the minimum sum rate (Roche, Yeung, and Hau, 1997) and the entire admissible rate region (Yeung and Zhang, 1999) of the problem. This paper considers a natural generalization of SMDC to the secure communication setting with an additional eavesdropper. It is required that all sources need to be kept perfectly secret from the eavesdropper as long as the number of encoder outputs available at the eavesdropper is no more than a given threshold. First, the problem of encoding individual sources is studied. A precise characterization of the entire admissible rate region is established via a connection to the problem of secure coding over a three-layer wiretap network and utilizing some basic polyhedral structure of the admissible rate region. Building on this result, it is then shown that superposition coding remains optimal in terms of achieving the minimum sum rate for the general secure SMDC problem.

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arXiv2012

Secret Writing on Dirty Paper: A Deterministic View

Recently there has been a lot of success in using the deterministic approach to provide approximate characterization of Gaussian network capacity. In this paper, we take a deterministic view and revisit the problem of wiretap channel with side information. A precise characterization of the secrecy capacity is obtained for a linear deterministic model, which naturally suggests a coding scheme which we show to achieve the secrecy capacity of the degraded Gaussian model (dubbed as "secret writing on dirty paper") to within half a bit.

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arXiv2011

Outflow activities in the young high-mass stellar object G23.44-0.18

We present an observational study towards the young high-mass star forming region G23.44-0.18 using the Submillimeter Array. Two massive, radio-quiet dusty cores MM1 and MM2 are observed in 1.3 mm continuum emission and dense molecular gas tracers including thermal CH$_3$OH, CH$_3$CN, HNCO, SO, and OCS lines. The $^{12}$CO (2--1) line reveals a strong bipolar outflow originated from MM2. The outflow consists of a low-velocity component with wide-angle quasi-parabolic shape and a more compact and collimated high-velocity component. The overall geometry resembles the outflow system observed in the low-mass protostar which has a jet-driven fast flow and entrained gas shell. The outflow has a dynamical age of $6\times10^3$ years and a mass ejection rate $\sim10^{-3} M_{\odot}$ year$^{-1}$. A prominent shock emission in the outflow is observed in SO and OCS, and also detected in CH$_3$OH and HNCO. We investigated the chemistry of MM1, MM2 and the shocked region. The dense core MM2 have molecular abundances of 3 to 4 times higher than those in MM1. The abundance excess, we suggest, can be a net effect of the stellar evolution and embedded shocks in MM2 that calls for further inspection.

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arXiv2011

Evolution of the dust/gas environment around Herbig Ae/Be stars

With the KOSMA 3-m telescope, 54 Herbig Ae/Be stars were surveyed in CO and $^{13}$CO emission lines. The properties of the stars and their circumstellar environments are studied by fitting the SEDs. The mean line width of $^{13}$CO (2-1) lines of this sample is 1.87 km s$^{-1}$. The average column density of H$_{2}$ is found to be $4.9\times10^{21}$ cm$^{-2}$ for the stars younger than $10^{6}$ yr, while drops to $2.5\times10^{21}$ cm$^{-2}$ for those older than $10^{6}$ yr. No significant difference is found among the SEDs of Herbig Ae stars and Herbig Be stars at the same age. The infrared excess decreases with age. The envelope masses and the envelope accretion rates decease with age after $10^{5}$ yr. The average disk mass of the sample is $3.3\times10^{-2} M_{\sun}$. The disk accretion rate decreases more slowly than the envelope accretion rate. A strong correlation between the CO line intensity and the envelope mass is found.

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arXiv2011

Security Embedding Codes

This paper considers the problem of simultaneously communicating two messages, a high-security message and a low-security message, to a legitimate receiver, referred to as the security embedding problem. An information-theoretic formulation of the problem is presented. A coding scheme that combines rate splitting, superposition coding, nested binning and channel prefixing is considered and is shown to achieve the secrecy capacity region of the channel in several scenarios. Specifying these results to both scalar and independent parallel Gaussian channels (under an average individual per-subchannel power constraint), it is shown that the high-security message can be embedded into the low-security message at full rate (as if the low-security message does not exist) without incurring any loss on the overall rate of communication (as if both messages are low-security messages). Extensions to the wiretap channel II setting of Ozarow and Wyner are also considered, where it is shown that "perfect" security embedding can be achieved by an encoder that uses a two-level coset code.

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arXiv2011

New Results on Multiple-Input Multiple-Output Broadcast Channels with Confidential Messages

This paper presents two new results on multiple-input multiple-output (MIMO) Gaussian broadcast channels with confidential messages. First, the problem of the MIMO Gaussian wiretap channel is revisited. A matrix characterization of the capacity-equivocation region is provided, which extends the previous result on the secrecy capacity of the MIMO Gaussian wiretap channel to the general, possibly imperfect secrecy setting. Next, the problem of MIMO Gaussian broadcast channels with two receivers and three independent messages: a common message intended for both receivers, and two confidential messages each intended for one of the receivers but needing to be kept asymptotically perfectly secret from the other, is considered. A precise characterization of the capacity region is provided, generalizing the previous results which considered only two out of three possible messages.

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