2,742 research outputs found
Astrochemistry: overview and challenges
This paper provides a brief overview of the journey of molecules through the
Cosmos, from local diffuse interstellar clouds and PDRs to distant galaxies,
and from cold dark clouds to hot star-forming cores, protoplanetary disks,
planetesimals and exoplanets. Recent developments in each area are sketched and
the importance of connecting astronomy with chemistry and other disciplines is
emphasized. Fourteen challenges for the field of Astrochemistry in the coming
decades are formulated.Comment: 20 pages, 5 figures, to appear in IAU Symposium 332 "Astrochemistry
VII: Through the Cosmos from Galaxies to Planets", eds. M. Cunningham, T.J.
Millar, Y. Aikaw
Astrochemistry of dust, ice and gas: introduction and overview
A brief introduction and overview of the astrochemistry of dust, ice and gas
and their interplay is presented, aimed at non-specialists. The importance of
basic chemical physics studies of critical reactions is illustrated through a
number of recent examples. Such studies have also triggered new insight into
chemistry, illustrating how astronomy and chemistry can enhance each other.
Much of the chemistry in star- and planet-forming regions is now thought to be
driven by gas-grain chemistry rather than pure gas-phase chemistry, and a
critical discussion of the state of such models is given. Recent developments
in studies of diffuse clouds and PDRs, cold dense clouds, hot cores,
protoplanetary disks and exoplanetary atmospheres are summarized, both for
simple and more complex molecules, with links to papers presented in this
volume. In spite of many lingering uncertainties, the future of astrochemistry
is bright: new observational facilities promise major advances in our
understanding of the journey of gas, ice and dust from clouds to planets.Comment: Introductory paper for Faraday Discussions 168 conference, April 201
Chemistry of massive young stellar objects with a disk-like structure
Our goal is to take an inventory of complex molecules in three well-known
high-mass protostars for which disks or toroids have been claimed and to study
the similarities and differences with a sample of massive YSOs without evidence
of such flattened disk-like structures. With a disk-like geometry, UV radiation
can escape more readily and potentially affect the ice and gas chemistry on
hot-core scales. A partial submillimeter line survey, targeting CH3OH, H2CO,
C2H5OH, HCOOCH3, CH3OCH3, CH3CN, HNCO, NH2CHO, C2H5CN, CH2CO, HCOOH, CH3CHO,
and CH3CCH, was made toward three massive YSOs with disk-like structures,
IRAS20126+4104, IRAS18089-1732, and G31.41+0.31. Rotation temperatures and
column densities were determined by the rotation diagram method, as well as by
independent spectral modeling. The molecular abundances were compared with
previous observations of massive YSOs without evidence of any disk structure,
targeting the same molecules with the same settings and using the same analysis
method. Consistent with previous studies, different complex organic species
have different characteristic rotation temperatures and can be classified
either as warm (>100 K) or cold (<100 K). The excitation temperatures and
abundance ratios are similar from source to source and no significant
difference can be established between the two source types. Acetone, CH3COCH3,
is detected for the first time in G31.41+0.31 and IRAS18089-1732. Temperatures
and abundances derived from the two analysis methods generally agree within
factors of a few. The lack of chemical differentiation between massive YSOs
with and without observed disks suggest either that the chemical complexity is
already fully established in the ices in the cold prestellar phase or that the
material experiences similar physi- cal conditions and UV exposure through
outflow cavities during the short embedded lifetime
Water Vapor in the Inner 25 AU of a Young Disk around a Low-Mass Protostar
Water is one of the key molecules in the physical and chemical evolution of
star- and planet-forming regions. We here report the first spatially resolved
observation of thermal emission of (an isotopologue of) water with the Plateau
de Bure Interferometer toward the deeply embedded Class 0 protostar NGC
1333-IRAS4B. The observations of the H2-18-O 3_13-2_20 transition at 203.4 GHz
resolve the emission of water toward this source with an extent of about 0.2"
corresponding to the inner 25 AU (radius). The H2-18-O emission reveals a
tentative velocity gradient perpendicular to the extent of the protostellar
outflow/jet probed by observations of CO rotational transitions and water
masers. The line is narrow, about 1 km/s (FWHM), significantly less than what
would be expected for emission from an infalling envelope or accretion shock,
but consistent with emission from a disk seen at a low inclination angle. The
water column density inferred from these data suggests that the water emitting
gas is a thin warm layer containing about 25 M_Earth of material, 0.03% of the
total disk mass traced by continuum observations.Comment: accepted for publication in ApJ Letters; 12 pages, 3 figure
Volatile snowlines in embedded disks around low-mass protostars
(Abridged*) Models of the young solar nebula assume a hot initial disk with
most volatiles are in the gas phase. The question remains whether an actively
accreting disk is warm enough to have gas-phase water up to 50 AU radius. No
detailed studies have yet been performed on the extent of snowlines in an
embedded accreting disk (Stage 0). Quantify the location of gas-phase volatiles
in embedded actively accreting disk system. Two-dimensional physical and
radiative transfer models have been used to calculate the temperature structure
of embedded protostellar systems. Gas and ice abundances of HO, CO, and
CO are calculated using the density-dependent thermal desorption formulation.
The midplane water snowline increases from 3 to 55 AU for accretion rates
through the disk onto the star between -. CO can remain in the solid phase within the disk for down to AU. Most of the CO
is in the gas phase within an actively accreting disk independent of disk
properties and accretion rate. The predicted optically thin water isotopolog
emission is consistent with the detected HO emission toward the Stage
0 embedded young stellar objects, originating from both the disk and the warm
inner envelope (hot core). An accreting embedded disk can only account for
water emission arising from AU, however, and the extent rapidly
decreases for low accretion rates. Thus, the radial extent of the emission can
be measured with ALMA observations and compared to this limit. Volatiles
sublimate out to 50 AU in young disks and can reset the chemical content
inherited from the envelope in periods of high accretion rates. A hot young
solar nebula out to 30 AU can only have occurred during the deeply embedded
Stage 0, not during the T-Tauri phase of our early solar system.Comment: 15 pages, 10 figures, accepted for publication in A&
Limits on the cosmic-ray ionization rate toward massive young stars
Recent models of the envelopes of seven massive protostars are used to
analyze observations of H3+ infrared absorption and H13CO+ submillimeter
emission lines toward these stars, and to constrain the cosmic-ray ionization
rate zeta. The H13CO+ gives best-fit values of zeta=(2.6+/- 1.8) x 10^-17 s^-1,
in good agreement with diffuse cloud models and with recent Voyager/Pioneer
data but factors of up to 7 lower than found from the H3+ data. No relation of
zeta with luminosity or total column density is found, so that local (X-ray)
ionization and shielding against cosmic rays appear unimportant for these
sources. The difference between the H3+ and H13CO+ results and the correlation
of N(H3+) with heliocentric distance suggest that intervening clouds contribute
significantly to the H3+ absorptions in the more distant regions. The most
likely absorbers are low-density (<~10^4 cm^-3) clouds with most carbon in
neutral form or in CO.Comment: To be published in A&A 358 (Letters); 4 pages including 3 figure
Disk masses in the Orion Molecular Cloud-2: distinguishing time and environment
The mass evolution of protoplanetary disks is driven by both internal
processes and external factors, such as photoevaporation. Disentangling these
two effects, however, has remained difficult. We measure the dust masses of a
sample of 132 disks in the Orion Molecular Cloud (OMC)-2 region, and compare
them to (i) externally photoevaporated disks in the Trapezium cluster, and (ii)
disks in nearby low-mass star forming regions (SFRs). This allows us to test if
initial disk properties are the same in high- and low-mass SFRs, and enables a
direct measurement of the effect of external photoevaporation on disks. A ~
mosaic of 3 mm continuum observations from the Atacama Large
Millimeter/submillimeter Array (ALMA) was used to measure the fluxes of 132
disks and 35 protostars >0.5 pc away from the Trapezium. We identify and
characterize a sample of 34 point sources not included in the Spitzer catalog
on which the sample is based. Of the disks, 37 (28%) are detected, with masses
ranging from 7-270 M_e. The detection rate for protostars is higher at 69%.
Disks near the Trapezium are found to be less massive by a factor
, implying a mass loss rate of M_sun/yr.
Our observations allow us to distinguish the impact of time and environment on
disk evolution in a single SFR. The disk mass distribution in OMC-2 is
statistically indistinguishable from that in nearby low-mass SFRs, like Lupus
and Taurus. We conclude that age is the main factor determining the evolution
of these disks. This result is robust with respect to assumptions of dust
temperature, sample incompleteness and biases. The difference between the OMC-2
and Trapezium cluster samples is consistent with mass loss driven by
far-ultraviolet radiation near the Trapezium. Together, this implies that in
isolation, disk formation and evolution proceed similarly, regardless of cloud
mass.Comment: Accepted for publication in A&A. 16 pages, 6 figure
Interstellar water chemistry: from laboratory to observations
Water is observed throughout the universe, from diffuse interstellar clouds
to protoplanetary disks around young stars, and from comets in our own solar
system and exoplanetary atmospheres to galaxies at high redshifts. This review
summarizes the spectroscopy and excitation of water in interstellar space as
well as the basic chemical processes that form and destroy water under
interstellar conditions. Three major routes to water formation are identified:
low temperature ion-molecule chemistry, high-temperature neutral-neutral
chemistry and gas-ice chemistry. The rate coefficients of several important
processes entering the networks are discussed in detail; several of them have
been determined only in the last decade through laboratory experiments and
theoretical calculations. Astronomical examples of each of the different
chemical routes are presented using data from powerful new telescopes, in
particular the Herschel Space Observatory. Basic chemical physics studies
remain critically important to analyze astronomical data.Comment: Authors' manuscript 138 pages, 34 figures, 4 tables, published in a
Thematic Issue "Astrochemistry" in Chemical Reviews (December 2013), volume
113, 9043-9085 following peer review by the American Chemical Society. The
published paper is available as open access at
http://pubs.acs.org/doi/abs/10.1021/cr400317
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