2,038 research outputs found
Star formation in Chamaeleon I and III: a molecular line study of the starless core population
The Chamaeleon clouds are excellent targets for low-mass star formation
studies. Cha I and II are actively forming stars while Cha III shows no sign of
ongoing star formation. We aim to determine the driving factors that have led
to the very different levels of star formation activity in Cha I and III and
examine the dynamical state and possible evolution of the starless cores within
them. Observations were performed in various molecular transitions with APEX
and Mopra. Five cores are gravitationally bound in Cha I and one in Cha III.
The infall signature is seen toward 8-17 cores in Cha I and 2-5 cores in Cha
III, which leads to a range of 13-28% of the cores in Cha I and 10-25% of the
cores in Cha III that are contracting and may become prestellar. Future
dynamical interactions between the cores will not be dynamically significant in
either Cha I or III, but the subregion Cha I North may experience collisions
between cores within ~0.7 Myr. Turbulence dissipation in the cores of both
clouds is seen in the high-density tracers N2H+ 1-0 and HC3N 10-9. Evidence of
depletion in the Cha I core interiors is seen in the abundance distributions of
C17O, C18O, and C34S. Both contraction and static chemical models indicate that
the HC3N to N2H+ abundance ratio is a good evolutionary indicator in the
prestellar phase for both gravitationally bound and unbound cores. In the
framework of these models, we find that the cores in Cha III and the southern
part of Cha I are in a similar evolutionary stage and are less chemically
evolved than the central region of Cha I. The measured HC3N/N2H+ abundance
ratio and the evidence for contraction motions seen towards the Cha III
starless cores suggest that Cha III is younger than Cha I Centre and that some
of its cores may form stars in the future. The cores in Cha I South may on the
other hand be transient structures. (abridged)Comment: Accepted for publication in A&A. The resolution of Figure 2 has been
degraded and the abstract in the metadata has been shortened to fit within
the limits set by arXi
Exploring molecular complexity with ALMA (EMoCA): Detection of three new hot cores in Sagittarius B2(N)
The SgrB2 molecular cloud contains several sites forming high-mass stars.
SgrB2(N) is one of its main centers of activity. It hosts several compact and
UCHII regions, as well as two known hot molecular cores (SgrB2(N1) and
SgrB2(N2)), where complex organic molecules are detected. Our goal is to use
the high sensitivity of ALMA to characterize the hot core population in
SgrB2(N) and shed a new light on the star formation process. We use a complete
3 mm spectral line survey conducted with ALMA to search for faint hot cores in
SgrB2(N). We report the discovery of three new hot cores that we call
SgrB2(N3), SgrB2(N4), and SgrB2(N5). The three sources are associated with
class II methanol masers, well known tracers of high-mass star formation, and
SgrB2(N5) also with a UCHII region. The chemical composition of the sources and
the column densities are derived by modelling the whole spectra under the
assumption of LTE. The H2 column densities are computed from ALMA and SMA
continuum emission maps. The H2 column densities of these new hot cores are
found to be 16 up to 36 times lower than the one of the main hot core Sgr
B2(N1). Their spectra have spectral line densities of 11 up to 31 emission
lines per GHz, assigned to 22-25 molecules. We derive rotational temperatures
around 140-180 K for the three new hot cores and mean source sizes of 0.4 for
SgrB2(N3) and 1.0 for SgrB2(N4) and SgrB2(N5). SgrB2(N3) and SgrB2(N5) show
high velocity wing emission in typical outflow tracers, with a bipolar
morphology in their integrated intensity maps suggesting the presence of an
outflow, like in SgrB2(N1). The associations of the hot cores with class II
methanol masers, outflows, and/or UCHII regions tentatively suggest the
following age sequence: SgrB2(N4), SgrB2(N3), SgrB2(N5), SgrB2(N1). The status
of SgrB2(N2) is unclear. It may contain two distinct sources, a UCHII region
and a very young hot core.Comment: Accepted for publication in A&A, 24 pages, 23 figure
Silica grain catalysis of methanol formation
The specific catalytic effect of a silica grain on the formation of methanol via the sequential addition of H atoms to CO adsorbed on the surface is investigated. A negatively charged defect on a siliceous edingtonite surface is found to reduce the gas phase barriers for the H + COads and H + H2C=O-ads reactions by 770 and 399 K, respectively, when compared to the same reactions in the gas phase. The catalytic effect of negatively charged surface sites could also be applicable to the hydrogenation of other adsorbed unsaturated species. However, the activation energies on the surface defect are still too large (1150 and 2230 K) for CH3OH to form efficiently at 10-20 K in the interstellar medium via a classical mechanism. It is therefore suggested that quantum mechanical tunnelling through the activation barrier is required for these hydrogen addition reactions to proceed at such temperatures. The calculations show that because the adsorption energies of CO and H2C=O on the negatively charged defect are substantial, CH3OH may form efficiently during the warm-up period in star-forming regions
Recommended from our members
14C-Cobalamin Absorption from Endogenously Labeled Chicken Eggs Assessed in Humans Using Accelerator Mass Spectrometry.
Traditionally, the bioavailability of vitamin B-12 (B12) from in vivo labeled foods was determined by labeling the vitamin with radiocobalt (57Co, 58Co or 60Co). This required use of penetrating radioactivity and sometimes used higher doses of B12 than the physiological limit of B12 absorption. The aim of this study was to determine the bioavailability and absorbed B12 from chicken eggs endogenously labeled with 14C-B12 using accelerator mass spectrometry (AMS). 14C-B12 was injected intramuscularly into hens to produce eggs enriched in vivo with the 14C labeled vitamin. The eggs, which provided 1.4 to 2.6 μg of B12 (~1.1 kBq) per serving, were scrambled, cooked and fed to 10 human volunteers. Baseline and post-ingestion blood, urine and stool samples were collected over a one-week period and assessed for 14C-B12 content using AMS. Bioavailability ranged from 13.2 to 57.7% (mean 30.2 ± 16.4%). Difference among subjects was explained by dose of B12, with percent bioavailability from 2.6 μg only half that from 1.4 μg. The total amount of B12 absorbed was limited to 0.5-0.8 μg (mean 0.55 ± 0.19 μg B12) and was relatively unaffected by the amount consumed. The use of 14C-B12 offers the only currently available method for quantifying B12 absorption in humans, including food cobalamin absorption. An egg is confirmed as a good source of B12, supplying approximately 20% of the average adult daily requirement (RDA for adults = 2.4 μg/day)
Chemistry in Evaporating Ices: Unexplored Territory
We suggest that three-body chemistry may occur in warm high density gas
evaporating in transient co\textendash desorption events on interstellar ices.
Using a highly idealised computational model we explore the chemical conversion
from simple species of the ice to more complex species containing several heavy
atoms, as a function of density and of adopted three body rate coefficients. We
predict that there is a wide range of densities and rate coefficients in which
a significant chemical conversion may occur. We discuss the implications of
this idea for the astrochemistry of hot cores.Comment: Accepted in Ap
Beyond the pseudo-time-dependent approach: chemical models of dense core precursors
Context: Chemical models of dense cloud cores often utilize the so-called
pseudo-time-dependent approximation, in which the physical conditions are held
fixed and uniform as the chemistry occurs. In this approximation, the initial
abundances chosen, which are totally atomic in nature except for molecular
hydrogen, are artificial. A more detailed approach to the chemistry of dense
cold cores should include the physical evolution during their early stages of
formation. Aims: Our major goal is to investigate the initial synthesis of
molecular ices and gas-phase molecules as cold molecular gas begins to form
behind a shock in the diffuse interstellar medium. The abundances calculated as
the conditions evolve can then be utilized as reasonable initial conditions for
a theory of the chemistry of dense cores. Methods: Hydrodynamic shock-wave
simulations of the early stages of cold core formation are used to determine
the time-dependent physical conditions for a gas-grain chemical network. We
follow the cold post-shock molecular evolution of ices and gas-phase molecules
for a range of visual extinction up to AV ~ 3, which increases with time. At
higher extinction, self-gravity becomes important. Results: As the newly
condensed gas enters its cool post-shock phase, a large amount of CO is
produced in the gas. As the CO forms, water ice is produced on grains, while
accretion of CO produces CO ice. The production of CO2 ice from CO occurs via
several surface mechanisms, while the production of CH4 ice is slowed by
gas-phase conversion of C into CO.Comment: 9 pages, 3 figures, 2 table
- …