arXiv subjects

Di Li

Publications and source records attributed to Di Li.

At least 397 records · Page 22Linked to original sources

arXiv2015

Planck cold clumps in the $λ$ Orionis complex: I. Discovery of an extremely young Class 0 protostellar object and a proto-brown dwarf candidate in a bright rimmed clump PGCC G192.32-11.88

We are performing a series of observations with ground-based telescopes toward Planck Galactic cold clumps (PGCCs) in the $λ$ Orionis complex in order to systematically investigate the effects of stellar feedback. In the particular case of PGCC G192.32-11.88, we discovered an extremely young Class 0 protostellar object (G192N) and a proto-brown dwarf candidate (G192S). G192N and G192S are located in a gravitationally bound bright-rimmed clump. The velocity and temperature gradients seen in line emission of CO isotopologues indicate that PGCC G192.32-11.88 is externally heated and compressed. G192N probably has the lowest bolometric luminosity ($\sim0.8$ L$_{\sun}$) and accretion rate (6.3$\times10^{-7}$ M$_{\sun}$~yr$^{-1}$) when compared with other young Class 0 sources (e.g. PACS Bright Red sources (PBRs)) in the Orion complex. It has slightly larger internal luminosity ($0.21\pm0.01$ L$_{\sun}$) and outflow velocity ($\sim$14 km~s$^{-1}$) than the predictions of first hydrostatic cores (FHSCs). G192N might be among the youngest Class 0 sources, which are slightly more evolved than a FHSC. Considering its low internal luminosity ($0.08\pm0.01$ L$_{\odot}$) and accretion rate (2.8$\times10^{-8}$ M$_{\sun}$~yr$^{-1}$), G192S is an ideal proto-brown dwarf candidate. The star formation efficiency ($\sim$0.3\%-0.4\%) and core formation efficiency ($\sim$1\%) in PGCC G192.32-11.88 are significantly smaller than in other giant molecular clouds or filaments, indicating that the star formation therein is greatly suppressed due to stellar feedback.

astro-ph.GA↗

arXiv2015

Distributed Bayesian Quickest Change Detection in Sensor Networks via Two-layer Large Deviation Analysis

We propose a distributed Bayesian quickest change detection algorithm for sensor networks, based on a random gossip inter-sensor communication structure. Without a control or fusion center, each sensor executes its local change detection procedure in a parallel and distributed fashion, interacting with its neighbor sensors via random inter-sensor communications to propagate information. By modeling the information propagation dynamics in the network as a Markov process, two-layer large deviation analysis is presented to analyze the performance of the proposed algorithm. The first-layer analysis shows that the relation between the probability of false alarm and the conditional averaged detection delay satisfies the large deviation principle, implying that the probability of false alarm according to a rare event decays to zero at an exponentially fast rate when the conditional averaged detection decay increases, where the Kullback-Leibler information number is established as a crucial factor. The second-layer analysis shows that the probability of the rare event that not all observations are available at a sensor decays to zero at an exponentially fast rate when the averaged number of communications increases, where the large deviation upper and lower bounds for this rate are also derived, based on which we show that the performance of the distributed algorithm converges exponentially fast to that of the centralized one, by proving that the defined distributed Kullback-Leibler information number converges to the centralized Kullback-Leibler information number.

cs.IT↗

arXiv2015

Learning-Based Distributed Detection-Estimation in Sensor Networks with Unknown Sensor Defects

We consider the problem of distributed estimation of an unknown deterministic scalar parameter (the target signal) in a wireless sensor network (WSN), where each sensor receives a single snapshot of the field. We assume that the observation at each node randomly falls into one of two modes: a valid or an invalid observation mode. Specifically, mode one corresponds to the desired signal plus noise observation mode (\emph{valid}), and mode two corresponds to the pure noise mode (\emph{invalid}) due to node defect or damage. With no prior information on such local sensing modes, we introduce a learning-based distributed procedure, called the mixed detection-estimation (MDE) algorithm, based on iterative closed-loop interactions between mode learning (detection) and target estimation. The online learning step re-assesses the validity of the local observations at each iteration, thus refining the ongoing estimation update process. The convergence of the MDE algorithm is established analytically. Asymptotic analysis shows that, in the high signal-to-noise ratio (SNR) regime, the MDE estimation error converges to that of an ideal (centralized) estimator with perfect information about the node sensing modes. This is in contrast to the estimation performance of a naive average consensus based distributed estimator (without mode learning), whose estimation error blows up with an increasing SNR.

cs.IT↗

arXiv2015

Outflows and Bubbles in Taurus: Star-formation Feedback Sufficient to Maintain Turbulence

We have identified outflows and bubbles in the Taurus molecular cloud based on the $\sim 100$ deg$^2$ Five College Radio Astronomy Observatory $^{12}$CO(1-0) and $^{13}$CO(1-0) maps and the Spitzer young stellar object catalogs. In the main 44 deg$^2$ area of Taurus we found 55 outflows, of which 31 were previously unknown. We also found 37 bubbles in the entire 100 deg$^2$ area of Taurus, all of which had not been found before. The total kinetic energy of the identified outflows is estimated to be $\bf \sim 3.9 \times 10^{45}$ erg, which is \textbf{1\%} of the cloud turbulent energy. The total kinetic energy of the detected bubbles is estimated to be $\sim 9.2 \times 10^{46}$ erg, which is 29\% of the turbulent energy of Taurus. The energy injection rate from outflows is $\bf \sim 1.3 \times 10^{33}~\rm erg\ s^{-1}$, \textbf{0.4 - 2 times} the dissipation rate of the cloud turbulence. The energy injection rate from bubbles is $\sim 6.4 \times 10^{33}$ erg s$^{-1}$, \textbf{2 - 10 times} the turbulent dissipation rate of the cloud. The gravitational binding energy of the cloud is $\bf \sim 1.5 \times 10^{48}$ {\bf erg}, \textbf{385} and 16 times the energy of outflows and bubbles, respectively. We conclude that neither outflows nor bubbles can \textbf{provide enough energy to balance the overall gravitational binding energy and the turbulent energy of Taurus. However,} in the current epoch, stellar feedback is sufficient to maintain the observed turbulence in Taurus.

astro-ph.GA↗

arXiv2015

A New Method for Constraining Molecular Cloud Thickness: A study of Taurus, Perseus and Ophiuchus

The core velocity dispersion (CVD) is a potentially useful tool for studying the turbulent velocity field of molecular clouds. CVD is based on centroid velocities of dense gas clumps, thus is less prone to density fluctuation and reflects more directly the cloud velocity field. Prior work demonstrated that the Taurus molecular cloud CVD resembles the well-known Larson's linewidth-size relation of molecular clouds. In this work, we studied the dependence of the CVD on the line-of-sight thickness of molecular clouds, a quantity which cannot be measured by direct means. We produced a simple statistical model of cores within clouds and analyzed the CVD of a variety of hydrodynamical simulations. We show that the relation between the CVD and the 2D projected separation of cores ($L_{2D}$) is sensitive to the cloud thickness. When the cloud is thin, the index of CVD-$L_{2D}$ relation ($γ$ in the relation CVD$\sim L_{2D}^γ$) reflects the underlying energy spectrum ($E(k)\sim k^{-β}$) in that $γ\sim(β-1)/2$. The CVD-$L_{2D}$ relation becomes flatter ($γ\to 0$) for thicker clouds. We used this result to constrain the thicknesses of Taurus, Perseus, and Ophiuchus. We conclude that Taurus has a ratio of cloud depth to cloud length smaller than about 1/10-1/8, i.e. it is a sheet. A simple geometric model fit to the linewidth-size relation indicates that the Taurus cloud has a $\sim 0.7$ pc line-of-sight dimension. In contrast, Perseus and Ophiuchus are thicker and have ratios of cloud depth to cloud length larger than about 1/10-1/8.

astro-ph.GA↗

arXiv2015

Extremely Energetic Outflow and Decelerated Expansion in W49N

W49N is a mini-starburst in the Milky Way and thus an ideal laboratory for high-mass star formation studies. Due to its large distance (11.1$_{-0.7}^{+0.9}$ kpc), the kinematics inside and between the dense molecular clumps in W49N are far from well understood. The SMA observations resolved the continuum emission into two clumps. The molecular line observation of SO$_{2}$ (28$_{4,24}$-28$_{3,25}$) suggests that the two clumps have a velocity difference of $\sim$7 km~s$^{-1}$. The eastern clump is very close to two radio sources "G1" and "G2", and the western clump coincides with a radio source "B". The HCN (3-2) line reveals an extremely energetic outflow, which is among the most energetic molecular outflows in the Milky Way. This is the first report of high-velocity molecular outflow detection in W49N. The outflow jet might be in precession, which could account for the distribution, velocity and rotation of water maser spots. Three absorption systems are identified in HCO$^{+}$ (3-2) spectra. The absorption features are blueshifted with respect to the emission of SO$_{2}$ (28$_{4,24}$-28$_{3,25}$) lines, indicating that a cold layer is expanding in front of the warm gas. Further analysis indicates that the expansion is decelerated from the geometric expansion centers.

astro-ph.SR↗

arXiv2015

Radio Afterglows and Host Galaxies of Gamma-Ray Bursts

Considering the contribution of the emission from the host galaxies of gamma-ray bursts (GRBs) to the radio afterglows, we investigate the effect of host galaxies on observations statistically. For the three types of events, e.g. low-luminosity, standard and high-luminosity GRBs, it is found that a tight correlation exists between the ratio of the radio flux (RRF) of host galaxy to the total radio peak emission and the observational frequency. Especially, toward lower frequencies, the contribution from the host increases significantly. The correlation can be used to get a useful estimate for the radio brightness of those host galaxies which only have very limited radio afterglow data. Using this prediction, we re-considered the theoretical radio afterglow light curves for four kinds of events, i.e. high-luminosity, low-luminosity, standard and failed GRBs, taking into account the contribution from the host galaxies and aiming at exploring the detectability of these events by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Lying at a typical redshift of $z=1$, most of the events can be detected by FAST easily. For the less fierce low-luminosity GRBs, their radio afterglows are not strong enough to exceed the sensitivity limit of FAST at such distances. However, since a large number of low luminosity bursts actually happen very near to us, it is expected that FAST will still be able to detect many of them.

astro-ph.HE↗

arXiv2015

Quantifying Dark Gas

A growing body of evidence has been supporting the existence of so-called "dark molecular gas" (DMG), which is invisible in the most common tracer of molecular gas, i.e., CO rotational emission. DMG is believed to be the main gas component of the intermediate extinction region between A$\rm_v$$\sim$0.05-2, roughly corresponding to the self-shielding threshold of H$_2$ and $^{13}$CO. To quantify DMG relative to HI and CO, we are pursuing three observational techniques, namely, HI self-absorption, OH absorption, and TeraHz C$^+$ emission. In this paper, we focus on preliminary results from a CO and OH absorption survey of DMG candidates. Our analysis show that the OH excitation temperature is close to that of the Galactic continuum background and that OH is a good DMG tracer co-existing with molecular hydrogen in regions without CO. Through systematic "absorption mapping" by Square Kilometer Array (SKA) and ALMA, we will have unprecedented, comprehensive knowledge of the ISM components including DMG in terms of their temperature and density, which will impact our understanding of galaxy evolution and star formation profoundly.

astro-ph.GA↗

arXiv2015

Power vs. Spectrum 2-D Sensing in Energy Harvesting Cognitive Radio Networks

Energy harvester based cognitive radio is a promising solution to address the shortage of both spectrum and energy. Since the spectrum access and power consumption patterns are interdependent, and the power value harvested from certain environmental sources are spatially correlated, the new power dimension could provide additional information to enhance the spectrum sensing accuracy. In this paper, the Markovian behavior of the primary users is considered, based on which we adopt a hidden input Markov model to specify the primary vs. secondary dynamics in the system. Accordingly, we propose a 2-D spectrum and power (harvested) sensing scheme to improve the primary user detection performance, which is also capable of estimating the primary transmit power level. Theoretical and simulated results demonstrate the effectiveness of the proposed scheme, in term of the performance gain achieved by considering the new power dimension. To the best of our knowledge, this is the first work to jointly consider the spectrum and power dimensions for the cognitive primary user detection problem.

cs.IT↗

arXiv2015

Sulfur-Bearing Molecules In Massive Star-Forming Regions: Observations Of OCS, CS, H2S and SO

We studied sulfur chemistry of massive star-forming regions through single-dish submillimeter spectroscopy. OCS, O13CS, 13CS, H2S and SO transitions were observed toward a sample of massive star-forming regions with embedded UCH II or CH II regions. These sources could be divided into H II-hot core and H II-only sources based on their CH3CN emission. Our results show that the OCS line of thirteen sources is optically thick, with optical depth ranging from 5 to 16. Column densities of these molecules were computed under LTE conditions. CS column densities were also derived using its optically thin isotopologue 13CS. H2S is likely to be the most abundant gas-phase sulfuretted molecules in hot assive cores. Both the column density and abundance of sulfur-bearing molecules decrease significantly from H II-hot core to H II-only sources. Ages derived from hot core models appear to be consistent with star-formation theories, suggesting that abundance ratios of [CS]/[SO], [SO]/[OCS] and [OCS]/[CS] could be used as chemical clocks in massive star-forming regions.

astro-ph.SR↗

arXiv2015

Superconductivity in efficient thermoelectric Cu$_3$Sb$_{0.98}$Al$_{0.02}$Se$_4$

Both superconductivity and thermoelectricity offer promising prospects for daily energy efficiency applications. The advancements of thermoelectric materials have led to the huge improvement of the thermoelectric figure of merit in the past decade. By applying pressure on a highly efficient thermoelectric material Cu$_{3}$Sb$_{0.98}$Al$_{0.02}$Se$_4$, we achieve dome-shape superconductivity developing at around 8.5 GPa but having a maximum critical temperature of 3.2 K at pressure of 12.7 GPa. The novel superconductor is realized through the first-order structural transformation from its initial phase to an orthorhombic one. The superconducting phase is determined in the ultimate formation of the Cu-Al-Sb-Se alloy.

cond-mat.supr-con↗

arXiv2015

Distributed Kalman Filtering over Massive Data Sets: Analysis Through Large Deviations of Random Riccati Equations

This paper studies the convergence of the estimation error process and the characterization of the corresponding invariant measure in distributed Kalman filtering for potentially unstable and large linear dynamic systems. A gossip network protocol termed Modified Gossip Interactive Kalman Filtering (M-GIKF) is proposed, where sensors exchange their filtered states (estimates and error covariances) and propagate their observations via inter-sensor communications of rate $\overlineγ$; $\overlineγ$ is defined as the averaged number of inter-sensor message passages per signal evolution epoch. The filtered states are interpreted as stochastic particles swapped through local interaction. The paper shows that the conditional estimation error covariance sequence at each sensor under M-GIKF evolves as a random Riccati equation (RRE) with Markov modulated switching. By formulating the RRE as a random dynamical system, it is shown that the network achieves weak consensus, i.e., the conditional estimation error covariance at a randomly selected sensor converges weakly (in distribution) to a unique invariant measure. Further, it is proved that as $\overlineγ \rightarrow \infty$ this invariant measure satisfies the Large Deviation (LD) upper and lower bounds, implying that this measure converges exponentially fast (in probability) to the Dirac measure $δ_{P^*}$, where $P^*$ is the stable error covariance of the centralized (Kalman) filtering setup. The LD results answer a fundamental question on how to quantify the rate at which the distributed scheme approaches the centralized performance as the inter-sensor communication rate increases.

cs.IT↗

arXiv2015

Galactic and Magellanic Evolution with the SKA

As we strive to understand how galaxies evolve it is crucial that we resolve physical processes and test emerging theories in nearby systems that we can observe in great detail. Our own Galaxy, the Milky Way, and the nearby Magellanic Clouds provide unique windows into the evolution of galaxies, each with its own metallicity and star formation rate. These laboratories allow us to study with more detail than anywhere else in the Universe how galaxies acquire fresh gas to fuel their continuing star formation, how they exchange gas with the surrounding intergalactic medium, and turn warm, diffuse gas into molecular clouds and ultimately stars. The $λ$21-cm line of atomic hydrogen (HI) is an excellent tracer of these physical processes. With the SKA we will finally have the combination of surface brightness sensitivity, point source sensitivity and angular resolution to transform our understanding of the evolution of gas in the Milky Way, all the way from the halo down to the formation of individual molecular clouds.

astro-ph.GA↗

arXiv2014

Follow-up observations toward Planck cold clumps with ground-based radio telescopes

The physical and chemical properties of prestellar cores, especially massive ones, are still far from being well understood due to the lack of a large sample. The low dust temperature ($<$14 K) of Planck cold clumps makes them promising candidates for prestellar objects or for sources at the very initial stages of protostellar collapse. We have been conducting a series of observations toward Planck cold clumps (PCCs) with ground-based radio telescopes. In general, when compared with other star forming samples (e.g. infrared dark clouds), PCCs are more quiescent, suggesting that most of them may be in the earliest phase of star formation. However, some PCCs are associated with protostars and molecular outflows, indicating that not all PCCs are in a prestellar phase. We have identified hundreds of starless dense clumps from the mapping survey with the Purple Mountain Observatory (PMO) 13.7-m telescope. Follow-up observations suggest that these dense clumps are ideal targets to search for prestellar objects.

astro-ph.SR↗

arXiv2014

Evidence for Large Grains in the Star-forming Filament OMC-2/3

We present a new 3.3 mm continuum map of the OMC-2/3 region. When paired with previously published maps of 1.2mm continuum and NH3-derived temperature, we derive the emissivity spectral index of dust emission in this region, tracking its changes across the filament and cores. We find that the median value of the emissivity spectral index is 0.9, much shallower than previous estimates in other nearby molecular clouds. We find no significant difference between the emissivity spectral index of dust in the OMC-2/3 filament and the starless or protostellar cores. Furthermore, the temperature and emissivity spectral index, beta, are anti-correlated at the 4 sigma level. The low values of the emissivity spectral index found in OMC-2/3 can be explained by the presence of millimeter-sized dust grains in the dense regions of the filaments to which these maps are most sensitive. Alternatively, a shallow dust emissivity spectral index may indicate non-powerlaw spectral energy distributions, significant free-free emission, or anomalous microwave emission. We discuss the possible implications of millimeter-sized dust grains compared to the alternatives.

astro-ph.GA↗

arXiv2014

Characterization of Molecular Outflows in The Substellar Domain

We report here our latest search for molecular outflows from young brown dwarfs and very low-mass stars in nearby star-forming regions. We have observed three sources in Taurus with the Submillimeter Array and the Combined Array for Research in Millimeter-wave Astronomy at 230 GHz frequency to search for CO J=2-1 outflows. We obtain a tentative detection of a redshifted and extended gas lobe at about 10 arcsec from the source GM Tau, a young brown dwarf in Taurus with an estimated mass of 73 M_J, which is right below the hydrogen-burning limit. No blueshifted emission around the brown dwarf position is detected. The redshifted gas lobe that is elongated in the northeast direction suggests a possible bipolar outflow from the source with a position angle of about 36 degrees. Assuming that the redshifted emission is outflow emission from GM Tau, we then estimate a molecular outflow mass in the range from 1.9x10^-6 M_Sun to 2.9x10^-5 M_Sun and an outflow mass-loss rate from 2.7x10^-9 M_Sun yr^-1 to 4.1x10^-8 M_Sun yr^-1. These values are comparable to those we have observed in the young brown dwarf ISO-Oph 102 of 60 M_J in rho Ophiuchi and the very low-mass star MHO 5 of 90 M_J in Taurus. Our results suggest that the outflow process in very low-mass objects is episodic with duration of a few thousand years and the outflow rate of active episodes does not significantly change for different stages of the formation process of very low-mass objects. This may provide us with important implications that clarify the formation process of brown dwarfs.

astro-ph.SR↗

arXiv2014

IC348-SMM2E: a Class 0 proto-brown dwarf candidate forming as a scaled-down version of low-mass stars

We report on Submillimeter Array observations of the 870 micron continuum and CO(3-2), 13CO(2-1) and C18O(2-1) line emission of a faint object, SMM2E, near the driving source of the HH797 outflow in the IC348 cluster. The continuum emission shows an unresolved source for which we estimate a mass of gas and dust of 30 Mjup, and the CO(3-2) line reveals a compact bipolar outflow centred on SMM2E, and barely seen also in 13CO(2-1). In addition, C18O(2-1) emission reveals hints of a possible rotating envelope/disk perpendicular to the outflow, for which we infer a dynamical mass of ~16 Mjup. In order to further constrain the accreted mass of the object, we gathered data from Spitzer, Herschel, and new and archive submillimetre observations, and built the Spectral Energy Distribution (SED). The SED can be fitted with one single modified black-body from 70 micron down to 2.1 cm, using a dust temperature of ~24 K, a dust emissivity index of 0.8, and an envelope mass of ~35 Mjup. The bolometric luminosity is 0.10 Lsun, and the bolometric temperature is 35 K. Thus, SMM2E is comparable to the known Class 0 objects in the stellar domain. An estimate of the final mass indicates that SMM2E will most likely remain substellar, and the SMM2E outflow force matches the trend with luminosity known for young stellar objects. Thus, SMM2E constitutes an excellent example of a Class 0 proto-brown dwarf candidate which forms as a scaled-down version of low-mass stars. Finally, SMM2E seems to be part of a wide (~2400 AU) multiple system of Class 0 sources.

astro-ph.SR↗

arXiv2014

Massive Quiescent Cores in Orion: V. The Physical Structures on Sub-Jeans Scale and the Chemical Properties in Two Extremely Dense Cores

We present a high-resolution $1.5"$ observational study towards two massive dust-and-gas cores, ORI8nw\_2 and ORI2\_6 in Orion Molecular Cloud using the Combined Array for Research in Millimeter-wave Astronomy (CARMA). In both regions the 3.2 mm continuum emission reveals a dense and compact dust core of 1 to 3 solar masses. The cores are estimated to have number densities more than $10^9$ cm$^{-3}$, which are among the highest volume density ever published for the star-forming cores. In both regions the ${\rm N_2H^+}$ shows multiple gas clumps which are spatially displaced from the dense gas traced by the 3.2 mm continuum as well as the ${\rm HCO^+}$. The ${\rm N_2H^+}$ clumps in ORI8nw\_2 are gravitationally unbound. The clumps have masses comparable to the thermal Jeans mass, and are separated with intervals comparable with the Jeans length, thus likely result from thermal Jeans fragmentation. The fragmentation may have prohibited the high-mass star formation in the two cores which originally have larger total masses. In both regions the abundance ratio of ${\rm [N_2H^+]/[HCO^+]}$ appears to be higher than those in infrared dark clouds as a result of CO depletion. The extreme high central density in these Orion cores implies a very small Jeans scale ($<500$ AU). To further reveal the evolution of such dense cores requires higher resolution and sensitivity.

astro-ph.SR↗