Showing papers by "Di Li published in 2006"

The COMPLETE Survey of star-forming regions: Phase I data

Abstract: We present an overview of data available for the Ophiuchus and Perseus molecular clouds from Phase I of the COMPLETE Survey of Star-Forming Regions. This survey provides a range of data complementary to the Spitzer Legacy Program "From Molecular Cores to Planet Forming Disks." Phase I includes the following: extinction maps derived from the Two Micron All Sky Survey (2MASS) near-infrared data using the NICER algorithm; extinction and temperature maps derived from IRAS 60 and 100 µm emission; H I maps of atomic gas; 12CO and 13CO maps of molecular gas; and submillimeter continuum images of emission from dust in dense cores. Not unexpectedly, the morphology of the regions appears quite different depending on the column density tracer that is used, with IRAS tracing mainly warmer dust and CO being biased by chemical, excitation, and optical depth effects. Histograms of column density distribution are presented, showing that extinction as derived from 2MASS NICER gives the closest match to a lognormal distribution, as is predicted by numerical simulations. All the data presented in this paper, and links to more detailed publications on their implications, are publicly available at the COMPLETE Web site.

The Transition from Atomic to Molecular Hydrogen in Interstellar Clouds: 21cm Signature of the Evolution of Cold Atomic Hydrogen in Dense Clouds

Abstract: We have investigated the time scale for formation of molecular clouds by examining the conversion of HI to H2 using a time-dependent model. H2 formation on dust grains and cosmic ray and photo destruction are included in one-dimensional model slab clouds which incorporate time-independent density and temperature distributions. We calculate 21cm spectral line profiles seen in absorption against a background provided by general Galactic HI emission, and compare the model spectra with HI Narrow Self-Absorption, or HINSA, profiles absorbed in a number of nearby molecular clouds. The time evolution of the HI and H2 densities is dramatic, with the atomic hydrogen disappearing in a wave propagating from the central, denser regions which have a shorter H2 formation time scale, to the edges, where the density is lower and the time scale for H2 formation longer. The model 21cm spectra are characterized by very strong absorption at early times, when the HI column density through the model clouds is extremely large. The minimum time required for a cloud to have evolved to its observed configuration, based on the model spectra, is set by the requirement that most of the HI in the outer portions of the cloud, which otherwise overwhelms the narrow absorption, be removed. The characteristic time that has elapsed since cloud compression and initiation of the HI to H2 conversion is a few x 10^{14} s or ~ 10^7 yr. This sets a minimum time for the age of these molecular clouds and thus for the star formation that may take place within them.

The electrical and thermal conductivity and thermopower of nickel doped compounds (NixTi1−x)1+yS2 at low temperatures

Abstract: Nickel doped compounds (NixTi1−x)1+yS2 (0 ≤ x ≤ 0.06) were prepared by solid-state reaction, and their dc electrical and thermal conductivity and thermopower were investigated from 5 K to 310 K. The results indicated that Ni doping caused a metal-like to semiconductor-like behaviour transition; at low temperatures (T 0) obeys Mott's 2D variable range hopping law, ln σ ∝ T−1/3, indicating that TiS2 possesses 2D transport characteristics. The appearance of Mott's 2D law could originate from potential disorder introduced by Ni substitution for Ti in S–Ti–S slabs, while the metal-to-semiconductor transition can be ascribed to de-degeneration through reduction in electron concentration due to Ni substitution. Experiments also indicated that both lattice thermal conductivity and carrier (mainly electron) thermal conductivity of the doped compounds decreased upon doping, which can be explained as the combined effects of substitution with intercalation of Ni and reduction of carrier concentration upon doping, respectively. The absolute Seebeck coefficient |S| was found to decrease after doping, which could be attributed to generation of some holes after Ni substitution for Ti. The figure of merit, ZT, of the doped compounds (NixTi1−x)1+yS2 (x > 0) decreased as compared with TiS2 due to both a large increase in their resistivity and an obvious decrease in their Seebeck coefficient.

Massive Quiescent Cores in Orion. -- II. Core Mass Function

Abstract: We have surveyed submillimeter continuum emission from relatively quiescent regions in the Orion molecular cloud to determine how the core mass function in a high mass star forming region compares to the stellar initial mass function. Such studies are important for understanding the evolution of cores to stars, and for comparison to formation processes in high and low mass star forming regions. We used the SHARC II camera on the Caltech Submillimeter Observatory telescope to obtain 350 \micron data having angular resolution of about 9 arcsec, which corresponds to 0.02 pc at the distance of Orion. Our analysis combining dust continuum and spectral line data defines a sample of 51 Orion molecular cores with masses ranging from 0.1 \Ms to 46 \Ms and a mean mass of 9.8 \Ms, which is one order of magnitude higher than the value found in typical low mass star forming regions, such as Taurus. The majority of these cores cannot be supported by thermal pressure or turbulence, and are probably supercritical.They are thus likely precursors of protostars. The core mass function for the Orion quiescent cores can be fitted by a power law with an index equal to -0.85$\pm$0.21. This is significantly flatter than the Salpeter initial mass function and is also flatter than the core mass function found in low and intermediate star forming regions. Thus, it is likely that environmental processes play a role in shaping the stellar IMF later in the evolution of dense cores and the formation of stars in such regions.

Quiescent high mass cores in Orion region

Abstract: Our goal is to study relatively quiescent dense gas cores, isolated from disruptive stars, to understand the initial conditions of massive star formation. Determining their mass, size, dynamical status, and core mass distribution is a starting point to understand the mechanisms for formation, collapse, and the origin of their IMF. We obtained CSO 350 μm images of quiescent regions in Orion and detected 51 resolved or nearly resolved molecular cores with masses ranging from 0.1 M to 46 M (Li et al. 2006). The mean mass is 9.8 M , which is one order of magnitude higher than that of the resolved cores in low mass star forming regions, such as Taurus. Our sample includes largely thermally unstable cores, which implies that the cores are supported neither by thermal pressure nor by turbulence, and are probably supercritical. They are likely precursors of protostars. Fig. 1 shows the cores in our sample have a power law core mass function with an index α = -0.85±0.21. This mass function does not resemble the stellar IMF or turbulence cascade structure function, and it is also in contrast with core surveys done in the Ophiuchus region (Motte et al. 1998) and the Serpens region (Testi et al. 1998). We find that the differential mass function approach, while requiring more cores due to the necessity of binning, is more robust and has better defined statistical uncertainties than the cumulative mass function. Use of the cumulative mass function can erroneously suggest multiple power law indices, particularly if the core mass distribution is characterized by a power law index close to ∼−1. Our results for the quiescent cores in the Orion show that the core mass function is flatter in an environment affected by ongoing high mass star formation. Thus, environmental processes likely play a role in the evolution of dense cores and the formation of stars in such regions.