304 research outputs found
Analytical Formulas of Molecular Ion Abundances and N2H+ Ring in Protoplanetary Disks
We investigate the chemistry of ion molecules in protoplanetary disks,
motivated by the detection of NH ring around TW Hya. While the ring
inner radius coincides with the CO snow line, it is not apparent why NH
is abundant outside the CO snow line in spite of the similar sublimation
temperatures of CO and N. Using the full gas-grain network model, we
reproduced the NH ring in a disk model with millimeter grains. The
chemical conversion of CO and N to less volatile species (sink effect
hereinafter) is found to affect the NH distribution. Since the
efficiency of the sink depends on various parameters such as activation
barriers of grain surface reactions, which are not well constrained, we also
constructed the no-sink model; the total (gas and ice) CO and N abundances
are set constant, and their gaseous abundances are given by the balance between
adsorption and desorption. Abundances of molecular ions in the no-sink model
are calculated by analytical formulas, which are derived by analyzing the
full-network model. The NH ring is reproduced by the no-sink model, as
well. The 2D (R-Z) distribution of NH, however, is different among the
full-network model and no-sink model. The column density of NH in the
no-sink model depends sensitively on the desorption rate of CO and N, and
the flux of cosmic ray. We also found that NH abundance can peak at the
temperature slightly below the CO sublimation, even if the desorption energies
of CO and N are the same.Comment: accepted to Ap
ALMA Observations of the IRDC Clump G34.43+00.24 MM3: DNC/HNC Ratio
We have observed the clump G34.43+00.24 MM3 associated with an infrared dark
cloud in DNC =3--2, HNC =3--2, and NH =3--2 with the
Atacama Large Millimeter/submillimeter Array (ALMA). The NH emission is
found to be relatively weak near the hot core and the outflows, and its
distribution is clearly anti-correlated with the CS emission. This result
indicates that a young outflow is interacting with cold ambient gas. The
HNC emission is compact and mostly emanates from the hot core, whereas
the DNC emission is extended around the hot core. Thus, the DNC and HNC
emission traces warm regions near the protostar differently. The DNC emission
is stronger than the HNC emission toward most parts of this clump. The
DNC/HNC abundance ratio averaged within a area around the phase center is higher than 0.06. This ratio
is much higher than the value obtained by the previous single-dish observations
of DNC and HNC =1--0 (0.003). It seems likely that the DNC and
HNC emission observed with the single-dish telescope traces lower density
envelopes, while that observed with ALMA traces higher density and highly
deuterated regions. We have compared the observational results with
chemical-model results in order to investigate the behavior of DNC and HNC in
the dense cores. Taking these results into account, we suggest that the low
DNC/HNC ratio in the high-mass sources obtained by the single-dish observations
are at least partly due to the low filling factor of the high density regions.Comment: accepted to Ap
Depletion of 15N in the center of L1544: Early transition from atomic to molecular nitrogen?
We performed sensitive observations of the N15ND+(1-0) and 15NND+(1-0) lines
toward the prestellar core L1544 using the IRAM 30m telescope. The lines are
not detected down to 3 sigma levels in 0.2 km/s channels of around 6 mK. The
non-detection provides the lower limit of the 14N/15N ratio for N2D+ of
~700-800, which is much higher than the elemental abundance ratio in the local
ISM of ~200-300. The result indicates that N2 is depleted in 15N in the central
part of L1544, because N2D+ preferentially traces the cold dense gas, and
because it is a daughter molecule of N2. In-situ chemistry is unlikely
responsible for the 15N depletion in N2; neither low-temperature gas phase
chemistry nor isotope selective photodissociation of N2 explains the 15N
depletion; the former prefers transferring 15N to N2, while the latter requires
the penetration of interstellar FUV photons into the core center. The most
likely explanation is that 15N is preferentially partitioned into ices compared
to 14N via the combination of isotope selective photodissociation of N2 and
grain surface chemistry in the parent cloud of L1544 or in the outer regions of
L1544 which are not fully shielded from the interstellar FUV radiation. The
mechanism is the most efficient at the chemical transition from atomic to
molecular nitrogen. In other words, our result suggests that the gas in the
central part of L1544 already went trough the transition from atomic to
molecular nitrogen in the earlier evolutionary stage, and that N2 is currently
the primary form of gas-phase nitrogen.Comment: 5 pages, 2 figures, 2 tables, Accepted for publication in A&A Letter
Diffusion activation energy and desorption activation energy for astrochemically relevant species on water ice show no clear relation
The activation energy for desorption (Edes) and that for surface diffusion
(Esd) of adsorbed molecules on dust grains are two of the most important
parameters for the chemistry in the interstellar medium. Although Edes is often
measured by laboratory experiments, the measurement of Esd is sparse. Due to
the lack of data, astrochemical models usually assume a simple scaling
relation, Esd = fEdes, where f is a constant, irrespective of adsorbed species.
Here, we experimentally measure Esd for CH4, H2S, OCS, CH3OH, and CH3CN on
water-ice surfaces using an ultra-high-vacuum transmission electron microscope
(UHV-TEM). Compiling the measured Esd values and Edes values from the
literature, we find that the value of f ranges from ~0.2 to ~0.7, depending on
the species. Unless f (or Esd) for the majority of species is available, a
natural alternative approach for astrochemical models is running multiple
simulations, varying f for each species randomly. In this approach, ranges of
molecular abundances predicted by multiple simulations, rather than abundances
predicted by each simulation, are important. We here run 10,000 simulations of
astrochemical models of molecular clouds and protostellar envelopes, randomly
assigning a value of f for each species. In the former case, we identify
several key species whose Esd most strongly affects the uncertainties of the
model predictions; Esd for those species should be investigated in future
laboratory and quantum chemical studies. In the latter case, uncertainties in
the Esd of many species contribute to the uncertainties in the model
predictions.Comment: Accepted for publication in ApJ
Security Impact Analysis of Degree of Field Extension in Lattice Attacks on Ring-LWE Problem
Modern information communications use cryptography to keep the contents of
communications confidential. RSA (Rivest-Shamir-Adleman) cryptography and
elliptic curve cryptography, which are public-key cryptosystems, are widely
used cryptographic schemes. However, it is known that these cryptographic
schemes can be deciphered in a very short time by Shor's algorithm when a
quantum computer is put into practical use. Therefore, several methods have
been proposed for quantum computer-resistant cryptosystems that cannot be
cracked even by a quantum computer. A simple implementation of LWE-based
lattice cryptography based on the LWE (Learning With Errors) problem requires a
key length of to ensure the same level of security as existing
public-key cryptography schemes such as RSA and elliptic curve cryptography. In
this paper, we attacked the Ring-LWE (RLWE) scheme, which can be implemented
with a short key length, with a modified LLL (Lenstra-Lenstra-Lov\'asz) basis
reduction algorithm and investigated the trend in the degree of field extension
required to generate a secure and small key. Results showed that the
lattice-based cryptography may be strengthened by employing Cullen or Mersenne
prime numbers as the degree of field extension.Comment: accepted in COMPSAC 2023 Workshop DSML: The 1st IEEE International
Workshop on Data Science & Machine Learning for Cybersecurity, IoT & Digital
Forensic
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