In this article, we simulated the collisions of an OH
•
projectile impacting on a methanol cluster formed by ten units of methanol to mimic an ice mantle (CH
3
OH)
10
. The chemical processes ...occurring after the impact were studied through Born-Oppenheimer (ab-initio) molecular dynamics. We focus on collisions with initial kinetic impact energy of 10–22 eV, where the richest chemistry happens. We report the formation mechanisms of stable complex organic molecules (COMs) such as methoxymethanol CH
3
OCH
2
OH, formic acid HCOOH, formyl radical HCO, formaldehyde H
2
CO and its elusive HCOH isomer. We show that CH
2
(OH)
2
,
•
CH
2
OH or
+
CH
2
OH are key intermediates to generate H
2
CO and other COMs. We compare the outcomes using OH
•
with those using OH
−
projectiles. These processes are likely relevant to the production of COMs in astrophysical environments. We discuss its formation mechanism and the astrophysical implications of these chemical pathways in star-forming regions.
Abstract
Although episodic star formation (SF) has been suggested for nearby SF regions, a panoramic view of the recent episodic SF history in the solar neighborhood is still missing. By uniformly ...constraining the slope
α
of infrared spectral energy distributions (SEDs) of young stellar objects (YSOs) in the 13 largest Gould’s Belt (GB) protoclusters surveyed by the Spitzer Space Telescope, we have constructed a cluster-averaged histogram of
α
representing the YSO evolution lifetime as a function of the
α
value. Complementary to the traditional SED classification scheme (
0
,
i
,
f
,
ii
, and
iii
) that is based on different
α
values, a staging scheme (A, B, C, D, and E) of SED evolution is proposed on the basis of the
α
statistical features that can be better matched to the physical stages of disk dissipation and giant planet formation. This has also allowed us to unravel the fluctuations of SF rate (SFR) in the 3 Myr history of these GB protoclusters. Diverse evolutionary patterns such as single peaks, double peaks, and ongoing acceleration of SFR are revealed. The SFR fluctuations are between 20% and 60% (∼40% on average) and no dependence on the average SFR or the number of SFR episodes is found. However, spatially close protoclusters tend to have similar SFR fluctuation trends, indicating that the driving force of the fluctuations should be at size scales beyond the typical cluster sizes of several parsecs.
ABSTRACT
Hot cores characterized by rich lines of complex organic molecules are considered as ideal sites for investigating the physical and chemical environments of massive star formation. We ...present a search for hot cores by using typical nitrogen- and oxygen-bearing complex organic molecules (C2H5CN, CH3OCHO, and CH3OH), based on ALMA Three-millimeter Observations of Massive Star-forming regions (ATOMS). The angular resolutions and line sensitivities of the ALMA observations are better than 2 arcsec and 10 mJy beam−1, respectively. A total of 60 hot cores are identified with 45 being newly detected, in which the complex organic molecules have high gas temperatures (> 100 K) and hot cores have small source sizes (< 0.1 pc). So far, this is the largest sample of hot cores observed with similar angular resolution and spectral coverage. The observations have also shown nitrogen and oxygen differentiation in both line emission and gas distribution in 29 hot cores. Column densities of CH3OH and CH3OCHO increase as rotation temperatures rise. The column density of CH3OCHO correlates tightly with that of CH3OH. The pathways for production of different species are discussed. Based on the spatial position difference between hot cores and ultracompact H ii (UC H ii) regions, we conclude that 24 hot cores are externally heated, while the other hot cores are internally heated. The observations presented here will potentially help establish a hot core template for studying massive star formation and astrochemistry.
ABSTRACT
Investigating the physical and chemical structure of massive star-forming regions is critical for understanding the formation and early evolution of massive stars. We performed a detailed ...line survey toward six dense cores, named MM1, MM4, MM6, MM7, MM8, and MM11, in the G9.62+0.19 star-forming region resolved in Atacama Large Millimeter/submillimeter Array (ALMA) band 3 observations. Toward these cores, about 172 transitions have been identified and attributed to 16 species, including organic oxygen-, nitrogen-, and sulphur-bearing molecules and their isotopologues. Four dense cores, MM7, MM8, MM4, and MM11, are line-rich sources. Modelling of these spectral lines reveals that the rotational temperature lies in the range 72–115, 100–163, 102–204, and 84–123 K for MM7, MM8, MM4, and MM11, respectively. The molecular column densities are 1.6 × 1015–9.2 × 1017 cm−2 toward the four cores. The cores MM8 and MM4 show a chemical difference between oxygen- and nitrogen-bearing species, i.e. MM4 is rich in oxygen-bearing molecules, while nitrogen-bearing molecules, especially vibrationally excited HC3N lines, are mainly observed in MM8. The distinct initial temperatures at the accretion phase may lead to this N/O differentiation. Through analysing column densities and spatial distributions of O-bearing complex organic molecules (COMs), we found that C2H5OH and CH3OCH3 might have a common precursor, CH3OH. CH3OCHO and CH3OCH3 are likely chemically linked. In addition, the observed variation in HC3N and HC5N emission may indicate their different formation mechanisms in hot and cold regions.
Abstract
In this paper, we present a new gas-grain chemical code for interstellar clouds written in pure Python (GGCHEMPY (GGCHEMPY is available on
https://github.com/JixingGE/GGCHEMPY
)). By ...combining with the high-performance Python compiler Numba, GGCHEMPY is as efficient as the Fortran-based version. With the Python features, flexible computational workflows and extensions become possible. As a showcase, GGCHEMPY is applied to study the general effects of three-dimensional projection on molecular distributions using a two-core system which can be easily extended for more complex cases. By comparing the molecular distribution differences between two overlapping cores and two merging cores, we summarized the typical chemical differences such as N
2
H
+
, HC
3
N, C
2
S, H
2
CO,
HCN
and C
2
H, which can be used to interpret 3D structures in molecular clouds.
In this paper,we present a new gas-grain chemical code for interstellar clouds written in pure Python(GGCHEMPY(GGCHEMPY is available on https://github.com/JixingGE/GGCHEMPY)).By combining with the ...high-performance Python compiler Numba,GGCHEMPY is as efficient as the Fortran-based version.With the Python features,flexible computational workflows and extensions become possible.As a showcase,GGCHEMPY is applied to study the general effects of three-dimensional projection on molecular distributions using a two-core system which can be easily extended for more complex cases.By comparing the molecular distribution differences between two overlapping cores and two merging cores,we summarized the typical chemical differences such as N2H+,HC3N,C2S,H2CO,HCN and C2H,which can be used to interpret 3D structures in molecular clouds.
Abstract Infrared (IR) spectral energy distribution (SED) is the major tracer of protoplanetary disks. It was recently proposed to use the near-to-mid IR (or K-24) SED slope α defined between 2 and ...24 μ m as a potential quantitative tracer of disk age. We critically examine the viability of this idea and confront it with additional statistics of IR luminosities and SED shapes. We point out that, because the statistical properties of most of the complicated physical factors involved in disk evolution are still poorly understood in a quantitative sense, the only viable way is to assume them to be random so that an idealized “average disk” can be defined, which allows the α histogram to trace its age. We confirm that the statistics of the zeroth order (luminosity), first order (slope α ), and second order characteristics (concavity) of the observed K-24 SEDs indeed carry useful information upon the evolutionary processes of the “average disk”. We also stress that intrinsic diversities in K-24 SED shapes and luminosities are always large at the level of individual stars so that the application of the evolutionary path of the “average disk” to individual stars must be done with care. The data of most curves in plots are provided on GitHub (Disk-age package https://github.com/starage/disk-age/ ).
We modeled the collisions between OH^+ projectiles with kinetic energies ranging from 10 to 22 eV and an amorphous cold (CH3OH)_10 substrate using Born-Oppenheimer molecular dynamics (BOMD) ...simulations. We conducted the simulations for a collision time of 400 femtoseconds (fs), during which we followed multiple bond-forming and breaking reactions. Here, we report four new pathways for the formation of formic acid HCOOH . We find new precursors such as CH3(OH)2+ HC(OH)2^+ CH2OH^+ and CH2(OH)2 which are essential in these pathways for the formation of formic acid. The methanodiol CH2(OH)2 and hydroxymethyl CH2OH^+ cations have previously been identified as key precursors of formaldehyde. These pathways suggest new ways to form formic acid in methanol ice mantles on dust grains, offering alternative mechanisms leading to the formation of complex organic molecules (COMs) in space.
ABSTRACT
The dust continuum and molecular distributions observed on the sky plane always show irregular shapes of molecular clouds. However, it is hard to directly reproduce the observed ...distributions using symmetrical models. In this work, for the first time, we present a three-dimensional (3D) gas-grain chemical simulation using an irregular 3D density structure derived by the Abel inversion taking the starless core L1544 as an example. We found that most (∼70 per cent) of the observed features (molecular distributions, peak positions, and column density values of 16 species) can be reproduced directly. The previously reached conclusion of non-uniform illumination at C3H2 peak is confirmed as the contribution from the gas component with density of a few 104 cm−3 at distance of ∼3000–8000 au from the core centre along the line of sight.
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