956 research outputs found
A cold complex chemistry toward the low-mass protostar B1-b: evidence for complex molecule production in ices
Gas-phase complex organic molecules have been detected toward a range of
high- and low-mass star-forming regions at abundances which cannot be explained
by any known gas-phase chemistry. Recent laboratory experiments show that UV
irradiation of CH3OH-rich ices may be an important mechanism for producing
complex molecules and releasing them into the gas-phase. To test this ice
formation scenario we mapped the B1-b dust core and nearby protostar in CH3OH
gas using the IRAM 30m telescope to identify locations of efficient non-thermal
ice desorption. We find three CH3OH abundance peaks tracing two outflows and a
quiescent region on the side of the core facing the protostar. The CH3OH gas
has a rotational temperature of ~10 K at all locations. The quiescent CH3OH
abundance peak and one outflow position were searched for complex molecules.
Narrow, 0.6-0.8 km s-1 wide, HCOOCH3 and CH3CHO lines originating in cold gas
are clearly detected, CH3OCH3 is tentatively detected and C2H5OH and HOCH2CHO
are undetected toward the quiescent core, while no complex molecular lines were
found toward the outflow. The core abundances with respect to CH3OH are ~2.3%
and 1.1% for HCOOCH3 and CH3CHO, respectively, and the upper limits are
0.7-1.1%, which is similar to most other low-mass sources. The observed complex
molecule characteristics toward B1-b and the pre-dominance of HCO-bearing
species suggest a cold ice (below 25 K, the sublimation temperature of CO)
formation pathway followed by non-thermal desorption through e.g. UV photons
traveling through outflow cavities. The observed complex gas composition
together with the lack of any evidence of warm gas-phase chemistry provide
clear evidence of efficient complex molecule formation in cold interstellar
ices.Comment: 23 pages, 7 figures, accepted for publication in Ap
Formation rates of complex organics in UV irradiated CH3OH-rich ices I: Experiments
(Abridged) Gas-phase complex organic molecules are commonly detected in the
warm inner regions of protostellar envelopes. Recent models show that
photochemistry in ices followed by desorption may explain the observed
abundances. This study aims to experimentally quantify the broad-band
UV-induced production rates of complex organics in CH3OH-rich ices at 20-70 K
under ultra-high vacuum conditions. The reaction products are mainly identified
by RAIRS and TPD experiments. Complex organics are readily formed in all
experiments, both during irradiation and during a slow warm-up of the ices to
200 K after the UV lamp is turned off. The relative abundances of photoproducts
depend on the UV fluence, the ice temperature, and whether pure CH3OH ice or
CH3OH:CH4/CO ice mixtures are used. C2H6, CH3CHO, CH3CH2OH, CH3OCH3, HCOOCH3,
HOCH2CHO and (CH2OH)2 are all detected in at least one experiment. The derived
product-formation yields and their dependences on different experimental
parameters, such as the initial ice composition, are used to estimate the CH3OH
photodissociation branching ratios in ice and the relative diffusion barriers
of the formed radicals. The experiments show that ice photochemistry in CH3OH
ices is efficient enough to explain the observed abundances of complex organics
around protostars and that ratios of complex molecules can be used to constrain
their formation pathway.Comment: Accepted for publication in A&A. 65 pages including appendice
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