3,057 research outputs found
Proof of two conjectures of Zuber on fully packed loop configurations
Two conjectures of Zuber [``On the counting of fully packed loops
configurations. Some new conjectures,'' preprint] on the enumeration of
configurations in the fully packed loop model on the square grid with periodic
boundary conditions, which have a prescribed linkage pattern, are proved.
Following an idea of de Gier [``Loops, matchings and alternating-sign
matrices,'' Discrete Math., to appear], the proofs are based on bijections
between such fully packed loop configurations and rhombus tilings, and the
hook-content formula for semistandard tableaux.Comment: 20 pages; AmS-LaTe
First measurements of 15N fractionation in N2H+ toward high-mass star forming cores
We report on the first measurements of the isotopic ratio 14N/15N in N2H+
toward a statistically significant sample of high-mass star forming cores. The
sources belong to the three main evolutionary categories of the high-mass star
formation process: high-mass starless cores, high-mass protostellar objects,
and ultracompact HII regions. Simultaneous measurements of 14N/15N in CN have
been made. The 14N/15N ratios derived from N2H+ show a large spread (from ~180
up to ~1300), while those derived from CN are in between the value measured in
the terrestrial atmosphere (~270) and that of the proto-Solar nebula (~440) for
the large majority of the sources within the errors. However, this different
spread might be due to the fact that the sources detected in the N2H+
isotopologues are more than those detected in the CN ones. The 14N/15N ratio
does not change significantly with the source evolutionary stage, which
indicates that time seems to be irrelevant for the fractionation of nitrogen.
We also find a possible anticorrelation between the 14N/15N (as derived from
N2H+) and the H/D isotopic ratios. This suggests that 15N enrichment could not
be linked to the parameters that cause D enrichment, in agreement with the
prediction by recent chemical models. These models, however, are not able to
reproduce the observed large spread in 14N/15N, pointing out that some
important routes of nitrogen fractionation could be still missing in the
models.Comment: 2 Figures, accepted for publication in ApJ
H_2D^+ in the High-mass Star-forming Region Cygnus X
H_2D^+ is a primary ion that dominates the gas-phase chemistry of cold dense gas. Therefore, it is hailed as a unique tool in probing the earliest, prestellar phase of star formation. Observationally, its abundance and distribution is, however, just beginning to be understood in low-mass prestellar and cluster-forming cores. In high-mass star-forming regions, H_2D^+ has been detected only in two cores, and its spatial distribution remains unknown. Here, we present the first map of the ortho-H_2D^+J_(k^+,k^-) = 1_(1,0) → 1_(1,1) and N_2H^+ 4-3 transition in the DR21 filament of Cygnus X with the James Clerk Maxwell Telescope, and N_2D^+ 3-2 and dust continuum with the Submillimeter Array. We have discovered five very extended (≤34, 000 AU diameter) weak structures in H2D+ in the vicinity of, but distinctly offset from, embedded protostars. More surprisingly, the H_2D^+ peak is not associated with either a dust continuum or N_2D^+ peak. We have therefore uncovered extended massive cold dense gas that was undetected with previous molecular line and dust continuum surveys of the region. This work also shows that our picture of the structure of cores is too simplistic for cluster-forming cores and needs to be refined: neither dust continuum with existing capabilities nor emission in tracers like N_2D^+ can provide a complete census of the total prestellar gas in such regions. Sensitive H_2D^+ mapping of the entire DR21 filament is likely to discover more of such cold quiescent gas reservoirs in an otherwise active high-mass star-forming region
Chemical evolution in the environment of intermediate mass young stellar objects: NGC7129--FIRS2 and LkH234
We have carried out a molecular survey of the Class 0 IM protostar NGC 7129
-- FIRS 2 (hereafter FIRS 2) and the Herbig Be star LkH 234 with the
aim of studying the chemical evolution of the envelopes of intermediate-mass
(IM) young stellar objects (YSOs). Both objects have similar luminosities (~500
Lsun) and are located in the same molecular cloud which minimizes the chemical
differences due to different stellar masses or initial cloud conditions.
Moreover, since they are located at the same distance, we have the same spatial
resolution in both objects. A total of 17 molecular species (including rarer
isotopes) have been observed in both objects and the structure of their
envelopes and outflows is determined with unprecedent detail.
Our results show that the protostellar envelopes are dispersed and warmed up
during the evolution to become a pre-main sequence star. In fact, the envelope
mass decreases by a factor >5 from FIRS 2 to LkH234, while the kinetic
temperature increases from ~13K to 28K. On the other hand, there is no
molecular outflow associated with LkH234. The molecular outflow seems
to stop before the star becomes visible. These physical changes strongly affect
the chemistry of their envelopes.
Based on our results in FIRS2 and LkH 234, we propose some abundance
ratios that can be used as chemical clocks for the envelopes of IM YSOs. The
SiO/CS, CN/N2H+, HCN/N2H+, DCO+/HCO+ and D2CO/DCO+ ratios are good diagnostics
of the protostellar evolutionary stage.Comment: 24 pages, 17 figure
The Deuterium Fractionation Timescale in Dense Cloud Cores: A Parameter Space Exploration
The deuterium fraction [ND]/[NH], may provide information
about the ages of dense, cold gas structures, important to compare with
dynamical models of cloud core formation and evolution. Here we introduce a
complete chemical network with species containing up to three atoms, with the
exception of the Oxygen chemistry, where reactions involving HO and its
deuterated forms have been added, significantly improving the consistency with
comprehensive chemical networks. Deuterium chemistry and spin states of H
and H isotopologues are included in this primarily gas-phase chemical
model. We investigate dependence of deuterium chemistry on model parameters:
density (), temperature, cosmic ray ionization rate, and gas-phase
depletion factor of heavy elements (). We also explore the effects
of time-dependent freeze-out of gas-phase species and dynamical evolution of
density at various rates relative to free-fall collapse. For a broad range of
model parameters, the timescales to reach large values of , observed in some low- and high-mass starless cores, are
relatively long compared to the local free-fall timescale. These conclusions
are unaffected by introducing time-dependent freeze-out and considering models
with evolving density, unless the initial 10. For fiducial
model parameters, achieving requires
collapse to be proceeding at rates at least several times slower than that of
free-fall collapse, perhaps indicating a dynamically important role for
magnetic fields in the support of starless cores and thus the regulation of
star formation.Comment: 23 pages, 18 figures, accepted by Ap
Modelling Online Gaming Metacognitions: The Role of Time Spent Gaming in Predicting Problematic Internet Use
© 2020, The Author(s). In recent years there have been growing concerns about problematic Internet use (PIU) as potential mental health problem. Among the many activities available on the Internet, the time spent gaming appears one of the most frequent risk factors in developing PIU. The aim of the current study was to model the relationship between negative affect, metacognitions about online gaming, frequency of online gaming and PIU. A total of 326 Italian gamers (mean age = 27 years, SD = 5.65 years; 93.3% males) participated in the study. The pattern of relationships specified by the theoretical model was examined through path analysis. Results showed that negative affect was directly associated with all other variables. Specifically, positive, strong and direct associations were found between negative affect and both positive and negative metacognitions about online gaming. Moreover, negative metacognitions about online gaming were strongly linked to PIU. Overall, the theoretical model was supported showing that metacognitions about online gaming may play a role in the association between time spent on online gaming to a broader pattern of PIU. Results are discussed within the context of the metacognitive model of psychopathology and clinical implications based on this model are outlined
Structure, Dynamics and Deuterium Fractionation of Massive Pre-Stellar Cores
High levels of deuterium fraction in NH are observed in some
pre-stellar cores. Single-zone chemical models find that the timescale required
to reach observed values () is longer than the free-fall
time, possibly ten times longer. Here, we explore the deuteration of turbulent,
magnetized cores with 3D magnetohydrodynamics simulations. We use an
approximate chemical model to follow the growth in abundances of NH and
ND. We then examine the dynamics of the core using each tracer for
comparison to observations. We find that the velocity dispersion of the core as
traced by ND appears slightly sub-virial compared to predictions of the
Turbulent Core Model of McKee & Tan, except at late times just before the onset
of protostar formation. By varying the initial mass surface density, the
magnetic energy, the chemical age, and the ortho-to-para ratio of H, we
also determine the physical and temporal properties required for high
deuteration. We find that low initial ortho-to-para ratios ()
and/or multiple free-fall times () of prior chemical evolution are
necessary to reach the observed values of deuterium fraction in pre-stellar
cores.Comment: 20 pages, 18 figures; accepted for publication in Ap
The dynamical properties of dense filaments in the infrared dark cloud G035.39-00.33
Infrared Dark Clouds (IRDCs) are unique laboratories to study the initial
conditions of high-mass star and star cluster formation. We present
high-sensitivity and high-angular resolution IRAM PdBI observations of N2H+
(1-0) towards IRDC G035.39-00.33. It is found that G035.39-00.33 is a highly
complex environment, consisting of several mildly supersonic filaments
(sigma_NT/c_s ~1.5), separated in velocity by <1 km s^-1 . Where multiple
spectral components are evident, moment analysis overestimates the non-thermal
contribution to the line-width by a factor ~2. Large-scale velocity gradients
evident in previous single-dish maps may be explained by the presence of
substructure now evident in the interferometric maps. Whilst global velocity
gradients are small (<0.7 km s^-1 pc^-1), there is evidence for dynamic
processes on local scales (~1.5-2.5 km s^-1 pc^-1 ). Systematic trends in
velocity gradient are observed towards several continuum peaks. This suggests
that the kinematics are influenced by dense (and in some cases, starless)
cores. These trends are interpreted as either infalling material, with
accretion rates ~(7 \pm 4)x10^-5 M_sun yr^-1 , or expanding shells with
momentum ~24 \pm 12 M_sun km s^-1 . These observations highlight the importance
of high-sensitivity and high-spectral resolution data in disentangling the
complex kinematic and physical structure of massive star forming regions.Comment: 25 pages, 23 figures, accepted for publication in MNRA
Deuteration as an evolutionary tracer in massive-star formation
Theory predicts, and observations confirm, that the column density ratio of a
molecule containing D to its counterpart containing H can be used as an
evolutionary tracer in the low-mass star formation process. Since it remains
unclear if the high-mass star formation process is a scaled-up version of the
low-mass one, we investigated whether the relation between deuteration and
evolution can be applied to the high-mass regime. With the IRAM-30m telescope,
we observed rotational transitions of N2D+ and N2H+ and derived the deuterated
fraction in 27 cores within massive star-forming regions understood to
represent different evolutionary stages of the massive-star formation process.
Results. Our results clearly indicate that the abundance of N2D+ is higher at
the pre-stellar/cluster stage, then drops during the formation of the
protostellar object(s) as in the low-mass regime, remaining relatively constant
during the ultra-compact HII region phase. The objects with the highest
fractional abundance of N2D+ are starless cores with properties very similar to
typical pre-stellar cores of lower mass. The abundance of N2D+ is lower in
objects with higher gas temperatures as in the low-mass case but does not seem
to depend on gas turbulence. Our results indicate that the N2D+-to-N2H+ column
density ratio can be used as an evolutionary indicator in both low- and
high-mass star formation, and that the physical conditions influencing the
abundance of deuterated species likely evolve similarly during the processes
that lead to the formation of both low- and high-mass stars.Comment: Accepted by A&AL, 4 pages, 2 figures, 2 appendices (one for Tables,
one for additional figures
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