2,821 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
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
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
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&
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
Abundances and rotational temperatures of the C2 interstellar molecule towards six reddened early-type stars
Using high-resolution (~85000) and high signal-to-noise ratio (~200) optical
spectra acquired with the European Southern Observatory Ultraviolet and Visual
Echelle Spectrograph, we have determined the interstellar column densities of
C2 for six Galactic lines of sight with E(B- V) ranging from 0.33 to 1.03. For
our purposes, we identified and measured absorption lines belonging to the (1,
0), (2, 0) and (3, 0) Phillips bands A1{\Pi}u-X1{\Sigma}+g. We report on the
identification of a few lines of the C2 (4, 0) Phillips system towards HD
147889. The curve-of-growth method is applied to the equivalent widths to
determine the column densities of the individual rotational levels of C2. The
excitation temperature is extracted from the rotational diagrams. The physical
parameters of the intervening molecular clouds (e.g. gas kinetic temperatures
and densities of collision partners) were estimated by comparison with the
theoretical model of excitation of C2.Comment: 11 pages, 3 figures, MNRAS 201
Warm water deuterium fractionation in IRAS 16293-2422 - The high-resolution ALMA and SMA view
Measuring the water deuterium fractionation in the inner warm regions of
low-mass protostars has so far been hampered by poor angular resolution
obtainable with single-dish ground- and space-based telescopes. Observations of
water isotopologues using (sub)millimeter wavelength interferometers have the
potential to shed light on this matter. Observations toward IRAS 16293-2422 of
the 5(3,2)-4(4,1) transition of H2-18O at 692.07914 GHz from Atacama Large
Millimeter/submillimeter Array (ALMA) as well as the 3(1,3)-2(2,0) of H2-18O at
203.40752 GHz and the 3(1,2)-2(2,1) transition of HDO at 225.89672 GHz from the
Submillimeter Array (SMA) are presented. The 692 GHz H2-18O line is seen toward
both components of the binary protostar. Toward one of the components, "source
B", the line is seen in absorption toward the continuum, slightly red-shifted
from the systemic velocity, whereas emission is seen off-source at the systemic
velocity. Toward the other component, "source A", the two HDO and H2-18O lines
are detected as well with the SMA. From the H2-18O transitions the excitation
temperature is estimated at 124 +/- 12 K. The calculated HDO/H2O ratio is (9.2
+/- 2.6)*10^(-4) - significantly lower than previous estimates in the warm gas
close to the source. It is also lower by a factor of ~5 than the ratio deduced
in the outer envelope. Our observations reveal the physical and chemical
structure of water vapor close to the protostars on solar-system scales. The
red-shifted absorption detected toward source B is indicative of infall. The
excitation temperature is consistent with the picture of water ice evaporation
close to the protostar. The low HDO/H2O ratio deduced here suggests that the
differences between the inner regions of the protostars and the Earth's oceans
and comets are smaller than previously thought.Comment: Accepted for publication in Astronomy & Astrophysic
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