11,418 research outputs found
Trade-Offs Between Size and Degree in Polynomial Calculus
Building on [Clegg et al. \u2796], [Impagliazzo et al. \u2799] established that if an unsatisfiable k-CNF formula over n variables has a refutation of size S in the polynomial calculus resolution proof system, then this formula also has a refutation of degree k + O(?(n log S)). The proof of this works by converting a small-size refutation into a small-degree one, but at the expense of increasing the proof size exponentially. This raises the question of whether it is possible to achieve both small size and small degree in the same refutation, or whether the exponential blow-up is inherent. Using and extending ideas from [Thapen \u2716], who studied the analogous question for the resolution proof system, we prove that a strong size-degree trade-off is necessary
Automating Resolution is NP-Hard
We show that the problem of finding a Resolution refutation that is at most
polynomially longer than a shortest one is NP-hard. In the parlance of proof
complexity, Resolution is not automatizable unless P = NP. Indeed, we show it
is NP-hard to distinguish between formulas that have Resolution refutations of
polynomial length and those that do not have subexponential length refutations.
This also implies that Resolution is not automatizable in subexponential time
or quasi-polynomial time unless NP is included in SUBEXP or QP, respectively
Transmission spectroscopy of the inflated exo-Saturn HAT-P-19b
We observed the Saturn-mass and Jupiter-sized exoplanet HAT-P-19b to refine
its transit parameters and ephemeris as well as to shed first light on its
transmission spectrum. We monitored the host star over one year to quantify its
flux variability and to correct the transmission spectrum for a slope caused by
starspots. A transit of HAT-P-19b was observed spectroscopically with OSIRIS at
the Gran Telescopio Canarias in January 2012. The spectra of the target and the
comparison star covered the wavelength range from 5600 to 7600 AA. One
high-precision differential light curve was created by integrating the entire
spectral flux. This white-light curve was used to derive absolute transit
parameters. Furthermore, a set of light curves over wavelength was formed by a
flux integration in 41 wavelength channels of 50 AA width. We analyzed these
spectral light curves for chromatic variations of transit depth. The transit
fit of the combined white-light curve yields a refined value of the
planet-to-star radius ratio of 0.1390 pm 0.0012 and an inclination of 88.89 pm
0.32 degrees. After a re-analysis of published data, we refine the orbital
period to 4.0087844 pm 0.0000015 days. We obtain a flat transmission spectrum
without significant additional absorption at any wavelength or any slope.
However, our accuracy is not sufficient to significantly rule out the presence
of a pressure-broadened sodium feature. Our photometric monitoring campaign
allowed for an estimate of the stellar rotation period of 35.5 pm 2.5 days and
an improved age estimate of 5.5^+1.8_-1.3 Gyr by gyrochronology.Comment: 14 pages, 9 figures, Accepted for publication in A&
CO ro-vibrational lines in HD100546: A search for disc asymmetries and the role of fluorescence
We have studied the emission of CO ro-vibrational lines in the disc around
the Herbig Be star HD100546 with the final goal of using these lines as a
diagnostic to understand inner disc structure in the context of planet
formation. High-resolution IR spectra of CO ro-vibrational emission at eight
different position angles were taken with CRIRES at the VLT. From these spectra
flux tables, CO ro-vibrational line profiles, and population diagrams were
produced. We have investigated variations in the line profile shapes and line
strengths as a function of slit position angle. We used the thermochemical disc
modelling code ProDiMo based on the chemistry, radiation field, and temperature
structure of a previously published model for HD100546. Comparing observations
and the model, we investigated the possibility of disc asymmetries, the
excitation mechanism (UV fluorescence), the geometry, and physical conditions
of the inner disc. The observed CO ro-vibrational lines are largely emitted
from the inner rim of the outer disc at 10-13 AU. The line shapes are similar
for all v levels and line fluxes from all vibrational levels vary only within
one order of magnitude. All line profile asymmetries and variations can be
explained with a symmetric disc model to which a slit correction and pointing
offset is applied. Because the angular size of the CO emitting region (10-13
AU) and the slit width are comparable the line profiles are very sensitive to
the placing of the slit. The model reproduces the line shapes and the fluxes of
the v=1-0 lines as well as the spatial extent of the CO ro-vibrational
emission. It does not reproduce the observed band ratios of 0.5-0.2 with higher
vibrational bands. We find that lower gas volume densities at the surface of
the inner rim of the outer disc can make the fluorescence pumping more effcient
and reproduce the observed band ratios.Comment: 20 pages, 21 figure
Do galaxies that leak ionizing photons have extreme outflows?
To reionize the early universe, high-energy photons must escape the galaxies
that produce them. It has been suggested that stellar feedback drives galactic
outflows out of star-forming regions, creating low density channels through
which ionizing photons escape into the inter-galactic medium. We compare the
galactic outflow properties of confirmed Lyman continuum (LyC) leaking galaxies
to a control sample of nearby star-forming galaxies to explore whether the
outflows from leakers are extreme as compared to the control sample. We use
data from the Cosmic Origins Spectrograph on the Hubble Space Telescope to
measure the equivalent widths and velocities of Si II and Si III absorption
lines, tracing neutral and ionized galactic outflows. We find that the Si II
and Si III equivalent widths of the LyC leakers reside on the low-end of the
trend established by the control sample. The leakers' velocities are not
statistically different than the control sample, but their absorption line
profiles have a different asymmetry: their central velocities are closer to
their maximum velocities. The outflow kinematics and equivalent widths are
consistent with the scaling relations between outflow properties and host
galaxy properties -- most notably metallicity -- defined by the control sample.
Additionally, we use the Ly\alpha\ profiles to show that the Si II equivalent
width scales with the Ly\alpha\ peak velocity separation. We determine that the
low equivalent widths of the leakers are likely driven by low metallicities and
low H I column densities, consistent with a density-bounded ionization region,
although we cannot rule out significant variations in covering fraction. While
we do not find that the LyC leakers have extreme outflow velocities, the low
maximum-to-central velocity ratios demonstrate the importance of the
acceleration and density profiles for LyC and Ly\alpha\ escape. [abridged]Comment: 17 pages, 8 Figures. Accepted for publication in Astronomy &
Astrophysic
Probing the initial conditions of high-mass star formation -- IV. Gas dynamics and NHD chemistry in high-mass precluster and protocluster clumps
The initial stage of star formation is a complex area study because of its
high density and low temperature. Under such conditions, many molecules become
depleted from the gas phase by freezing out onto dust grains. However, the
deuterated species could remain gaseous and are thus ideal tracers. We
investigate the gas dynamics and NHD chemistry in eight massive
pre/protocluster clumps. We present NHD 1-1 (at 85.926 GHz),
NH (1, 1) and (2, 2) observations in the eight clumps using the PdBI and
the VLA, respectively. We find that the distribution between deuterium
fractionation and kinetic temperature shows a number density peak at around
K, and the NHD cores are mainly located at a temperature
range of 13.0 to 22.0 K. We detect seven instances of extremely high deuterium
fractionation of . We find that the
NHD emission does not appear to coincide exactly with either dust continuum
or NH peak positions, but often surrounds the star-formation active
regions. This suggests that the NHD has been destroyed by the central
young stellar object (YSO) due to its heating. The detected NHD lines are
very narrow with a median width of . The extracted
NHD cores are gravitationally bound (), are likely
prestellar or starless, and can potentially form intermediate-mass or high-mass
stars. Using NH (1, 1) as a dynamical tracer, we find very complicated
dynamical movement, which can be explained by a combined process with outflow,
rotation, convergent flow, collision, large velocity gradient, and rotating
toroids. High deuterium fractionation strongly depends on the temperature
condition. NHD is a poor evolutionary indicator of high-mass star formation
in evolved stages, but a useful tracer in the starless and prestellar cores.Comment: 27 pages, 25 figures, 6 tables, accepted for publication in A&
A new astrophysical solution to the Too Big To Fail problem - Insights from the MoRIA simulations
We test whether advanced galaxy models and analysis techniques of simulations
can alleviate the Too Big To Fail problem (TBTF) for late-type galaxies, which
states that isolated dwarf galaxy kinematics imply that dwarfs live in
lower-mass halos than is expected in a {\Lambda}CDM universe. Furthermore, we
want to explain this apparent tension between theory and observations. To do
this, we use the MoRIA suite of dwarf galaxy simulations to investigate whether
observational effects are involved in TBTF for late-type field dwarf galaxies.
To this end, we create synthetic radio data cubes of the simulated MoRIA
galaxies and analyse their HI kinematics as if they were real, observed
galaxies. We find that for low-mass galaxies, the circular velocity profile
inferred from the HI kinematics often underestimates the true circular velocity
profile, as derived directly from the enclosed mass. Fitting the HI kinematics
of MoRIA dwarfs with a theoretical halo profile results in a systematic
underestimate of the mass of their host halos. We attribute this effect to the
fact that the interstellar medium of a low-mass late-type dwarf is continuously
stirred by supernova explosions into a vertically puffed-up, turbulent state to
the extent that the rotation velocity of the gas is simply no longer a good
tracer of the underlying gravitational force field. If this holds true for real
dwarf galaxies as well, it implies that they inhabit more massive dark matter
halos than would be inferred from their kinematics, solving TBTF for late-type
field dwarf galaxies.Comment: 21 pages, 21 figures. Accepted for publication in A&A. Corrected
certain values in Table
A necklace of dense cores in the high-mass star forming region G35.20-0.74N: ALMA observations
The present study aims at characterizing the massive star forming region
G35.20N, which is found associated with at least one massive outflow and
contains multiple dense cores, one of them recently found associated with a
Keplerian rotating disk. We used ALMA to observe the G35.20N region in the
continuum and line emission at 350 GHz. The observed frequency range covers
tracers of dense gas (e.g. H13CO+, C17O), molecular outflows (e.g. SiO), and
hot cores (e.g. CH3CN, CH3OH). The ALMA 870 um continuum emission map reveals
an elongated dust structure (0.15 pc long and 0.013 pc wide) perpendicular to
the large-scale molecular outflow detected in the region, and fragmented into a
number of cores with masses 1-10 Msun and sizes 1600 AU. The cores appear
regularly spaced with a separation of 0.023 pc. The emission of dense gas
tracers such as H13CO+ or C17O is extended and coincident with the dust
elongated structure. The three strongest dust cores show emission of complex
organic molecules characteristic of hot cores, with temperatures around 200 K,
and relative abundances 0.2-2x10^(-8) for CH3CN and 0.6-5x10^(-6) for CH3OH.
The two cores with highest mass (cores A and B) show coherent velocity fields,
with gradients almost aligned with the dust elongated structure. Those velocity
gradients are consistent with Keplerian disks rotating about central masses of
4-18 Msun. Perpendicular to the velocity gradients we have identified a
large-scale precessing jet/outflow associated with core B, and hints of an
east-west jet/outflow associated with core A. The elongated dust structure in
G35.20N is fragmented into a number of dense cores that may form massive stars.
Based on the velocity field of the dense gas, the orientation of the magnetic
field, and the regularly spaced fragmentation, we interpret this elongated
structure as the densest part of a 1D filament fragmenting and forming massive
stars.Comment: 24 pages, 26 figures, accepted for publication in Astronomy and
Astrophysics (abstract modified to fit arXiv restrictions
A multi-wavelength observation and investigation of six infrared dark clouds
Context. Infrared dark clouds (IRDCs) are ubiquitous in the Milky Way, yet
they play a crucial role in breeding newly-formed stars.
Aims. With the aim of further understanding the dynamics, chemistry, and
evolution of IRDCs, we carried out multi-wavelength observations on a small
sample.
Methods. We performed new observations with the IRAM 30 m and CSO 10.4 m
telescopes, with tracers , HCN, , ,
DCO, SiO, and DCN toward six IRDCs G031.97+00.07, G033.69-00.01,
G034.43+00.24, G035.39-00.33, G038.95-00.47, and G053.11+00.05.
Results. We investigated 44 cores including 37 cores reported in previous
work and seven newly-identified cores. Toward the dense cores, we detected 6
DCO, and 5 DCN lines. Using pixel-by-pixel spectral energy distribution
(SED) fits of the 70 to 500 m, we obtained dust
temperature and column density distributions of the IRDCs. We found that emission has a strong correlation with the dust temperature and column
density distributions, while showed the weakest correlation. It
is suggested that is indeed a good tracer in very dense
conditions, but is an unreliable one, as it has a relatively
low critical density and is vulnerable to freezing-out onto the surface of cold
dust grains. The dynamics within IRDCs are active, with infall, outflow, and
collapse; the spectra are abundant especially in deuterium species.
Conclusions. We observe many blueshifted and redshifted profiles,
respectively, with and toward the same core. This
case can be well explained by model "envelope expansion with core collapse
(EECC)".Comment: 24 pages, 11 figures, 4 tables. To be published in A&A. The
resolutions of the pictures are cut dow
Structure and Fragmentation of a high line-mass filament: Nessie
An increasing number of hundred-parsec scale, high line-mass filaments have
been detected in the Galaxy. Their evolutionary path, including fragmentation
towards star formation, is virtually unknown. We characterize the fragmentation
within the Nessie filament, covering size-scales between 0.1-100 pc. We
also connect the small-scale fragments to the star-forming potential of the
cloud. We combine near-infrared data from the VVV survey with mid-infrared
GLIMPSE data to derive a high-resolution dust extinction map and apply a
wavelet decomposition technique on it to analyze the fragmentation
characteristics of the cloud, which are compared with predictions from
fragmentation models. We compare the detected objects to those identified in
10 times coarser resolution from ATLASGAL data. We present a
high-resolution extinction map of Nessie. We estimate the mean line-mass of
Nessie to be 627 M/pc and the distance to be 3.5 kpc. We
find that Nessie shows fragmentation at multiple size scales. The
nearest-neighbour separations of the fragments at all scales are within a
factor of 2 of the Jeans' length at that scale. However, the relationship
between the mean densities of the fragments and their separations is
significantly shallower than expected for Jeans' fragmentation. The
relationship is similar to the one predicted for a filament that exhibits a
Larson-like scaling between size-scale and velocity dispersion; such a scaling
may result from turbulent support. Based on the number of YSOs in Nessie, we
estimate that the star formation rate is 371 M/Myr; similar
values result if using the number of dense cores, or the amount of dense gas,
as the proxy of star formation. The star formation efficiency is 0.017. These
numbers indicate that Nessie's star-forming content is comparable to the Solar
neighborhood giant molecular clouds like Orion A
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