7,892 research outputs found
Photodissociative Regulation of Star Formation in Metal-Free Pregalactic Clouds
We study the H2 photodissociation regions around OB stars in primordial gas
clouds whose virial temperatures are between a few hundred and a few thousand
Kelvin. In such small objects, a single O star can photodissociate a mass equal
to that of the cloud itself. As a result, the clouds deplete their molecular
coolant and cannot cool in a free-fall time, and subsequent star formation is
totally quenched. This indicates that stars do not form efficiently in small
objects and that these objects contribute little to the reionization of the
universe.Comment: 9 pages. ApJ, 518, in pres
From eco-civilization to city branding: A neo-marxist perspective of sustainable urbanization in China
While the national discourse on 'eco-civilization' drives conceptual thinking on sustainable urban development in China, in practice a systematic implementation gap appears to exist when it comes to local implementation. This paper examines how the leakage occurring in the trajectory from central government ideas to municipal and district level construction projects can be explained. More specifically, it aims to spot whether it is merely the result of mismatches resulting from ineffective interactions among players in the policy process, institutional misalignment between policy goals and policy instruments, or even if the mobilization of bias in the policy process is systematic enough to justify a neo-Marxist explanation of the abovementioned implementation gap. I found two main sources of structural bias: capital accumulation in the mechanisms for urban development and power accumulation in the mechanisms underlying the administrative process
Destruction of Molecular Hydrogen During Cosmological Reionization
We investigate the ability of primordial gas clouds to retain molecular
hydrogen (H_2) during the initial phase of the reionization epoch. We find that
before the Stromgren spheres of the individual ionizing sources overlap, the UV
background below the ionization threshold is able to penetrate large clouds and
suppress their H_2 abundance. The consequent lack of H_2 cooling could prevent
the collapse and fragmentation of clouds with virial temperatures T_vir < 10^4
K (or masses 10^8 Msun [(1+z_vir)/10]^{-3/2}). This negative feedback on
structure-formation arises from the very first ionizing sources, and precedes
the feedback due to the photoionization heating.Comment: 14 pages, uuencoded compressed Postscript, 4 figures included. To
appear in Ap
A Deep Photometric Look at Two of Andromeda's Dwarf Spheroidals: X and XVII
We use deep wide-field photometry from the Large Binocular Camera to study
the stellar and structural properties of the recently discovered Andromeda X
and Andromeda XVII (And X and And XVII) dwarf galaxies. Using the mean apparent
magnitude of the horizontal branch (HB), we derive distances of 621 +- 20 kpc
to And X and 734+- 23 kpc to And XVII, closer by >60 kpc than the previous
estimates which were based on red giant branch (RGB) observations. Thus our
results warrant against the use of the RGB tip method for determining distances
to systems with sparsely populated RGBs, and show how crucial HB observations
are in obtaining accurate distances in systems such as these. We find that And
X is a relatively faint (MV = -7.36), highly elongated (e = 0.48) system at a
distance of 174 +- 62 kpc from Andromeda. And XVII is brighter (MV = -8.61)
with an M31-centric distance of 73 kpc which makes it one of the closest
satellites to Andromeda. Both galaxies are metal-poor: we derive =-2.2
for And X, while And XVII shows = -2.0, consistent with the relation
of higher luminosity dwarfs being more metal- rich. Additionally, both galaxies
show considerable intrinsic spreads in metallicity (0.2 and 0.3 dex for And X
and And XVII respectively), consistent with multiple stellar populations.Comment: 8 pages, 6 figure
The multi-level governance of formulating regional brand identities: Evidence from three Mega City Regions in China
The emergence and expansion of clusters of large metropolitan areas also known as Mega City Regions (MCRs) is a worldwide phenomenon. In China, governmental attempts are made to develop so-called regional brand identities for them. Regional brand identities, as a subspecies of place brand identities, differ from regional identities in the sense that they are the result of conscious branding efforts on the part of public authorities rather than more historically evolved regional identities as experienced by citizens. In this study, the establishment of regional brand identities for three MCRs in China is examined, as these are constituted through and institutionally embedded in intergovernmental relations. We map the perceptions of national, provincial and municipal governments of regional brand identities based on planning documents and relevant interviews. The emergence and stabilization of the regional profiles for China's largest MCRs Pearl River Delta (PRD), Yangtze River Delta (YRD) and Jing-Jin-Ji (JJJ) are featured in their dynamic territorial boundaries, centralized institutionalization and various symbolic meanings. The national government promotes the regional brands to reinforce the strategic awareness and future direction of the region, and these profiles are diffused at the provincial and municipal level
Universality of Shot-Noise in Multiterminal Diffusive Conductors
We prove the universality of shot-noise in multiterminal diffusive conductors
of arbitrary shape and dimension for purely elastic scattering as well as for
hot electrons. Using a Boltzmann-Langevin approach we reduce the calculation of
shot-noise correlators to the solution of a diffusion equation. We show that
shot-noise in multiterminal conductors is a non-local quantity and that
exchange effects can occur without quantum phase coherence even at zero
electron temperature. Concrete numbers for shot-noise are given that can be
tested experimentally.Comment: 4 double-column pages, REVTeX, 1 eps figure embedded with eps
Nouveauté or Cliché? Assessment on island ecological vulnerability to Tourism: Application to Zhoushan, China
In comparison with coastal zones, islands are even more vulnerable to anthropogenic disturbance, especially to tourism and tourism-induced activities. Despite a great number of studies on either island tourism or island vulnerability reviewed in this paper, knowledge and practice of the impact from tourism upon island ecological vulnerability (IEV) still needs to be expanded. In this contribution, the IEV of four administrative regions in Zhoushan, China is assessed between 2012 and 2017 based on an âexposure (E)-sensitivity (S)-adaptive capacity (A)â framework and by means of coupling coordination degree modeling (CCDM) for determination of the overall development level of E-S-A subsystems in each region. The assessment results show that: (1) An index system consisting of 1 objective, 3 sub-objectives, 7 elements, and 20 indicators can be established and tested to reflect the IEV to tourism; (2) As the most attractive tourist destinations, Shengsi and Putuo inevitably have the highest IEV values; (3) Dinghai's moderate low level of IEV comes as a surprise, due to its direct connectivity to its neighboring coastal city of Ningbo; (4) The more balanced the coupling coordination degree (CCD) values among E-S-A subsystems are, the higher the IEV values in the four tourist destinations of Zhoushan. In conclusion, tourism can be a double-edged sword for islands, the overall benefits of which outweigh the negative impacts upon island ecological conditions
Toward a Commonly Shared Public Policy Perspective for Analyzing Risk Coping Strategies
The concept of risk has received scholarly attention from a variety of angles in the social, technical, and natural sciences. However, public policy scholars have not yet generated a comprehensive overview, shared understanding and conceptual framework of the main problem-solving approaches applied by governments in coping with risks. In this regard, our main aim is to examine existing perspectives on prevailing risk coping strategies, find a common denominator among them and contribute to current policy and risk science literature through providing a conceptual framework that systematically spans the spectrum of risk coping strategies and incorporates the essence of the most relevant insights. To this end, we first examine the concept of risk in-depth by exploring various definitions and types of risk. We then review different approaches proposed by different strands of research for addressing risk. Finally, we assess current knowledge and develop an amalgamated perspective for examining how risks can be addressed by classifying them into six general types of response (no response; prevention; control; precaution; toleration; and adaptation) as well as indicators to identify these responses. We argue that these strategies can function as a heuristic tool fo
Hanbury Brown Twiss effects in channel mixing normal-superconducting systems
An investigation of the role of the proximity effect in current cross
correlations in multiterminal, channel-mixing, normal-superconducting systems
is presented. The proposed experiment is an electrical analog of the optical
Hanbury Brown Twiss intensity cross correlation experiment. A chaotic quantum
dot is connected via quantum point contacts to two normal and one
superconducting reservoir. For dominating coupling of the dot to the
superconducting reservoir, a magnetic flux of the order of a flux quantum in
the dot suppresses the proximity effect and reverses the sign of the cross
correlations, from positive to negative. In the opposite limit, for a
dominating coupling to the normal reservoirs, the proximity effect is weak and
the cross correlation are positive for a nonideal contact between the dot and
the superconducting reservoir. We show that in this limit the correlations can
be explained with particle counting arguments.Comment: Invited talk at LT2
Examining the density and diversity of human activity in the built environment: The case of the pearl river delta, China
Rapid urbanization in China has been accompanied by spatial inefficiency in patterns of human activity, of which 'ghost towns' are the most visible result. In this study, we measure the density and diversity of human activity in the built environment and relate this to various explanatory factors. Using the Pearl River Delta (PRD) as an empirical case, our research demonstrates the distribution of human activity by multi-source data and then explores its dynamics within these areas. This empirical study is comprised of two parts. The first part explores location information regarding human activity in urbanized areas and shows density and diversity. Regression models are applied to explore how density and diversity are affected by urban scale, morphology and by a city's administrative level. Results indicate that: 1) cities with smaller populations are more likely to be faced with lower density and diversity, but they derive greater marginal benefits from improving land use efficiency; 2) the compactness of the layout of urban land, an index reflecting the plane shapes of the built environment, is highly correlated with density and diversity in built-up areas; and 3) the administrative importance of a city has a significant and positive impact on the density of human activity, but no obvious influence on its diversity
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