1,411 research outputs found
Supply of renewable energy sources and the cost of EU climate policy
What are the excess costs of a separate 20% target for renewable energy as a part of the EU climate policy for 2020? We answer this question using a computable general equilibrium model, WorldScan, which has been extended with a bottom-up module of the electricity sector. The model set-up makes it possible to directly use available estimates of costs and capacity potentials for renewable energy sources for calibration. In our base case simulation, the costs of EU climate policy with the renewables target are 6% higher than those of a policy without this target. As information on the supply of renewable energy is scarce and uncertain, we perform an extensive sensitivity analysis with respect to the level and steepness of the supply curves for wind energy and biomass. In the range we explore, the excess costs vary from zero (when the target is not binding) to 23% (when the cost progression and the initial cost disadvantage for renewables are doubled).
Spitzer/MIPS Imaging of NGC 650: Probing the History of Mass Loss on the Asymptotic Giant Branch
We present the far-infrared (IR) maps of a bipolar planetary nebula (PN), NGC
650, at 24, 70, and 160 micron taken with the Multiband Imaging Photometer for
Spitzer (MIPS) on-board the Spitzer Space Telescope. While the two-peak
emission structure seen in all MIPS bands suggests the presence of a near
edge-on dusty torus, the distinct emission structure between the 24 micron map
and the 70/160 micron maps indicates the presence of two distinct emission
components in the central torus. Based on the spatial correlation of these two
far-IR emission components with respect to various optical line emission, we
conclude that the 24 micron emission is largely due to the [O IV] line at 25.9
micron arising from highly ionized regions behind the ionization front, whereas
the 70 and 160 micron emission is due to dust continuum arising from
low-temperature dust in the remnant asymptotic giant branch (AGB) wind shell.
The far-IR nebula structure also suggests that the enhancement of mass loss at
the end of the AGB phase has occurred isotropically, but has ensued only in the
equatorial directions while ceasing in the polar directions. The present data
also show evidence for the prolate spheroidal distribution of matter in this
bipolar PN. The AGB mass loss history reconstructed in this PN is thus
consistent with what has been previously proposed based on the past optical and
mid-IR imaging surveys of the post-AGB shells.Comment: 9 pages in the emulated ApJ format with 6 figures, to appear in Ap
ORFEUS II echelle spectra: Absorption by H_2 in the LMC
We present the first detection of molecular hydrogen (H_2) UV absorption
profiles on the line of sight to the LMC. The star LH 10:3120 in the LMC was
measured with the ORFEUS telescope and the Tuebingen echelle spectrograph
during the space shuttle mission of Nov./Dec. 1996. 16 absorption lines from
the Lyman band are used to derive the column densities of H_2 for the lowest 5
rotational states in the LMC gas. For these states we find a total column
density of N(H_2)=6.6 x 10^18$ cm^-2 on this individual line of sight. We
obtain equivalent excitation temperatures of T < 50 K for the rotational ground
state and T = 470 K for 0 < J < 6 by fitting the population densities of the
rotational states to theoretical Boltzmann distributions. We conclude that UV
pumping dominates the population of the higher rotational levels, as known from
the H_2 gas in the Milky Way. (Research supported in part by the DARA)Comment: Astronomy & Astrophysics, Letter, in pres
Inferring physical conditions in interstellar clouds of H_2
We have developed a code that models the formation, destruction, radiative
transfer, and vibrational/rotational excitation of H_2 in a detailed fashion.
We discuss how such codes, together with FUSE observations of H_2 in diffuse
and translucent lines of sight, may be used to infer various physical
parameters. We illustrate the effects of changes in the major physical
parameters (UV radiation field, gas density, metallicity), and we point out the
extent to which changes in one parameter may be mirrored by changes in another.
We provide an analytic formula for the molecular fraction, f_H2, as a function
of cloud column density, radiation field, and grain formation rate of H_2. Some
diffuse and translucent lines of sight may be concatenations of multiple
distinct clouds viewed together. Such situations can give rise to observables
that agree with the data, complicating the problem of uniquely identifying one
set of physical parameters with a line of sight. Finally, we illustrate the
application of our code to an ensemble of data, such as the FUSE survey of H_2
in the Large and Small Magellanic Clouds (LMC/SMC), in order to constrain the
elevated UV radiation field intensity and reduced grain formation rate of H_2
in those low- metallicity environments.Comment: 33 pages (aastex, manuscript), 9 figures (3 color). accepted to Ap
On the use of verb-based implicit causality in sentence comprehension: Evidence from self-paced reading and eye tracking
In two experiments, we examined the recent claim (Stewart, Pickering, & Sanford, 2000) that verb-based implicit causality information is used during sentence–final clausal integration only. We did so by looking for mid-sentence reading delays caused by pronouns that are inconsistent with the bias of a preceding implicit causality verb (e.g., “David praised Linda because he…”). In a self-paced reading task, such pronouns immediately slowed down reading, at the two words immediately following the pronoun. In eye tracking, bias-inconsistent pronouns also immediately perturbed the reading process, as indexed by significant delays in various first pass measures at and shortly after the critical pronoun. Hence, readers can recruit verb-based implicit causality information in the service of comprehension rapidly enough to impact on the interpretation of a pronoun early in the subordinate clause. We take our results to suggest that implicit causality is used proactively, allowing readers to focus on, and perhaps even predict, who or what will be talked about next
A Near-Infrared L Band Survey of the Young Embedded Cluster NGC 2024
We present the results of the first sensitive L band (3.4 micron) imaging
study of the nearby young embedded cluster NGC 2024. Two separate surveys of
the cluster were acquired in order to obtain a census of the circumstellar disk
fraction in the cluster. From an analysis of the JHKL colors of all sources in
our largest area, we find an infrared excess fraction of > 86%. The JHKL colors
suggest that the infrared excesses arise in circumstellar disks, indicating
that the majority of the sources which formed in the NGC 2024 cluster are
currently surrounded by, and likely formed with circumstellar disks. The excess
fractions remain very high, within the errors, even at the faintest L
magnitudes from our deeper surveys suggesting that disks form around the
majority of the stars in very young clusters such as NGC 2024 independent of
mass. From comparison with published JHKL observations of Taurus, we find the K
- L excess fraction in NGC 2024 to be consistent with a high initial incidence
of circumstellar disks in both NGC 2024 and Taurus. Because NGC 2024 represents
a region of much higher stellar density than Taurus, this suggests that disks
may form around most of the YSOs in star forming regions independent of
environment. We find a relatively constant JHKL excess fraction with increasing
cluster radius, indicating that the disk fraction is independent of location in
the cluster. In contrast, the JHK excess fraction increases rapidly toward the
central region of the cluster, and is most likely due to contamination of the K
band measurements by bright nebulosity in the central regions of the cluster.
We identify 45 candidate protostellar sources in the central regions of the NGC
2024 cluster, and find a lower limit on the protostellar phase of early stellar
evolution of 0.4 - 1.4 X 10^5 yr, similar to that in Taurus.Comment: 37 pages, 8 figures, 3 tables, To appear in the Astronomical Journa
Infrared Photometry of Starless Dense Cores
Deep JHKs photometry was obtained towards eight dense molecular cores and J-H
vs. H-Ks color-color plots are presented. Our photometry, sensitive to the
detection of a 1 solar mass, 1 X 10^6 year old star through approx. 35 - 50
magnitudes of visual extinction, shows no indication of the presence of
star/disk systems based on J-H vs. H-Ks colors of detected objects. The stars
detected towards the cores are generally spatially anti-correlated with core
centers suggesting a background origin, although we cannot preclude the
possibility that some stars detected at H and Ks alone, or Ks alone, are not
low mass stars or brown dwarfs (< 0.3 Solar Masses) behind substantial amounts
of visual extinction (e.g. 53 magnitudes for L183B). Lower limits to optical
extinctions are estimated for the detected background stars, with high
extinctions being encountered, in the extreme case ranging up to at least Av =
46, and probably higher. The extinction data are used to estimate cloud masses
and densities which are comparable to those determined from molecular line
studies. Variations in cloud extinctions are consistent with a systematic
nature to cloud density distributions and column density variations and
extinctions are found to be consistent with submillimeter wave continuum
studies of similar regions. The results suggest that some cores have achieved
significant column density contrasts (approx. 30) on sub-core scales (approx.
0.05 pc) without having formed known stars.Comment: 44 pages including tables and figures, accepted ApJ, March 24, 200
Looking for Distributed Star Formation in L1630: A Near-infrared (J, H, K) Survey
We have carried out a simultaneous, multi-band (J, H, K) survey over an area
of 1320 arcmin^2 in the L1630 region, concentrating on the region away from the
dense molecular cores and with modest visual extinctions (\leq 10 mag).
Previous studies found that star formation in L1630 occurs mainly in four
localized clusters, which in turn are associated with the four most massive
molecular cores (Lada et al. 1991; Lada 1992). The goal of this study is to
look for a distributed population of pre-main-sequence stars in the outlying
areas outside the known star-forming cores. More than 60% of the
pre-main-sequence stars in the active star forming regions of NGC 2024 and NGC
2023 show a near-infrared excess in the color-color diagram. In the outlying
areas of L1630, excluding the known star forming regions, we found that among
510 infrared sources with the near-infrared colors ((J-H) and (H-K)) determined
and photometric uncertainty at K better than 0.10 mag, the fraction of the
sources with a near-infrared excess is 3%--8%; the surface density of the
sources with a near-infrared excess is less than half of that found in the
distributed population in L1641, and 1/20 of that in the young cluster NGC
2023. This extremely low fraction and low surface density of sources with a
near-infrared excess strongly indicates that recent star formation activity has
been very low in the outlying region of L1630. The sources without a
near-infrared excess could be either background/foreground field stars, or
associated with the cloud, but formed a long time ago (more than 2 Myrs). Our
results are consistent with McKee's model of photoionization-regulated star
formation.Comment: 30 pages, 10 figures To appear in ApJ Oct 1997, Vol 48
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