47 research outputs found
Modelling study of interstellar ethanimine isomers
Ethanimine (CH3CHNH) , including both the E- and Z- isomers, were detected towards the star-forming region Sgr B2(N) using the GBT PRIMOS data (Loomis et al 2013), and were recently imaged by the ACTA (Corby et al. 2015). These aldimines can serve as precursors of biological molecules such as amino acids thus are considered prebiotic molecules in interstellar medium. In this study, we present chemical simulations of ethanimine with various physical conditions. From models for Sgr B2(N) and environs, calculated ethanimine abundances show reasonable agreement with observed values, while the translucent cloud models yield much lower abundances. These results agree with locations suggested by observations that ethanimine isomers were detected in the foreground of the shells of the hot core
Gas-Grain Modeling of Isocyanic Acid (HNCO), Cyanic Acid (HOCN), Fulminic Acid (HCNO), and Isofulminic Acid (HONC) in Assorted Interstellar Environments
Isocyanic acid (HNCO) is a well-known interstellar molecule. Evidence also exists for the presence of two of its metastable isomers in the interstellar medium: HCNO (fulminic acid) and HOCN (cyanic acid). Fulminic acid has been detected toward cold and lukewarm sources, while cyanic acid has been detected both in these sources and in warm sources in the Galactic Center. Gas-phase models can reproduce the abundances of the isomers in cold sources, but overproduce HCNO in the Galactic Center. Here we present a detailed study of a gas-grain model that contains these three isomers, plus a fourth isomer, isofulminic acid (HONC), for four types of sources: hot cores, the warm envelopes of hot cores, lukewarm corinos, and cold cores. The current model is partially able to rationalize the abundances of HNCO, HOCN, and HCNO in cold and warm sources. Predictions for HONC in all environments are also made
Practical Distributed Control for VTOL UAVs to Pass a Tunnel
Unmanned Aerial Vehicles (UAVs) are now becoming increasingly accessible to
amateur and commercial users alike. An air traffic management (ATM) system is
needed to help ensure that this newest entrant into the skies does not collide
with others. In an ATM, airspace can be composed of airways, intersections and
nodes. In this paper, for simplicity, distributed coordinating the motions of
Vertical TakeOff and Landing (VTOL) UAVs to pass an airway is focused. This is
formulated as a tunnel passing problem, which includes passing a tunnel,
inter-agent collision avoidance and keeping within the tunnel. Lyapunov-like
functions are designed elaborately, and formal analysis based on invariant set
theorem is made to show that all UAVs can pass the tunnel without getting
trapped, avoid collision and keep within the tunnel. What is more, by the
proposed distributed control, a VTOL UAV can keep away from another VTOL UAV or
return back to the tunnel as soon as possible, once it enters into the safety
area of another or has a collision with the tunnel during it is passing the
tunnel. Simulations and experiments are carried out to show the effectiveness
of the proposed method and the comparison with other methods
Coverage Dependent H Desorption Energy: a Quantitative Explanation Based on Encounter Desorption Mechanism
Recent experiments show that the desorption energy of H on a diamond-like
carbon (DLC) surface depends on the H coverage of the surface. We aim to
quantitatively explain the coverage dependent H desorption energy measured
by the experiments. We derive a math formula to calculate an effective H
desorption energy based on the encounter desorption mechanism. The effective
H desorption energy depends on two key parameters, the desorption energy of
H on H substrate and the ratio of H diffusion barrier to its
desorption energy. The calculated effective H desorption energy
qualitatively agrees with the coverage dependent H desorption energy
measured by the experiments if the values of these two parameters in literature
are used in the calculations. We argue that the difference between the
effective H desorption energy and the experimental results is due to the
lacking of knowledge about these two parameters. So, we recalculate these two
parameters based on experimental data. Good agreement between theoretical and
experimental results can be achieved if these two updated parameters are used
in the calculations.Comment: 6 pages,6 figures,2 tables, accepted for publication in MNRA
New Theoretical Results Concerning the Interstellar Abundance of Molecular Oxygen
The low abundance of molecular oxygen in cold cores of interstellar clouds poses a continuing problem to modelers of the chemistry of these regions. In chemical models O_2 is formed principally by the reaction between O and OH, which has been studied experimentally down to 39 K. It remains possible that the rate coefficient of this reaction at 10 K is considerably less than its measured value at 39 K, which might inhibit the production of O_2 and possibly bring theory and observation closer together over a wider range of times. Two theoretical determinations of the rate coefficient for the O + OH reaction at temperatures down to 10 K have been undertaken recently; both results show that the rate coefficient is indeed lower at 10 K than at 39 K, although they differ in the magnitude of the decrease. Here we show, using gas-phase models, how the calculated interstellar O_2 abundance in cold cores is affected by a decrease in the rate coefficient. We also consider its effect on other species. Our major finding is that for standard O-rich abundances, the calculated abundance of O_2 in cold cores is sufficiently low to explain observations only at early times regardless of the value of k_1 in the range investigated here. For C-rich abundances, on the other hand, late-time solutions can also be possible
Mapping Observations of Peptide-like molecules around Sagittarius B2
Peptide-like molecule, which has a close connection with the origin of life,
has been detected in universe. Mapping observations of HCONH and
CHCONH, two simplest peptide-like molecules, are performed towards
Sagittarius B2 (Sgr B2) complex with the IRAM 30m telescope. Seven transitions
of HCONH and five transitions of CHCONH are used in analysis. The
spatial distribution of excitation temperature and column density of HCONH
in the molecular envelope of Sgr B2 are obtained by the rotation diagrams.
Assuming the same excitation temperature as HCONH, the column densities of
CHCONH are also calculated. The results show that excitation
temperature ranges from 6 K to 46 K in the molecular envelope of Sgr B2. The
abundance ratio between HCONH and CHCONH are calculated to explore
the relationship among them, as well as HNCO mentioned in our pervious
research. The abundance ratio of CHCONH/HCONH varies from 10% to
20%, while that of HCONH/HNCO ranges from 1.5% to 10%. CHCONH is
enhanced with respect to HCONH in the northwest region of Sgr B2. One
transition of HCONH is detected toward 12 positions of Sgr B2, from
which a C/C ratio of 28.7 is obtained. A time-dependent chemical
model with a short duration of X-ray burst is used to explain the observed
abundances of HCONH and CHCONH, with the best fitting result at
T = 53-56 K. More chemical reactions are required to be included
into the model since the modeled abundance is lower than the observed one at
the observed T