1,388 research outputs found
Interstellar Hydrides
Interstellar hydrides -- that is, molecules containing a single heavy element
atom with one or more hydrogen atoms -- were among the first molecules detected
outside the solar system. They lie at the root of interstellar chemistry, being
among the first species to form in initially-atomic gas, along with molecular
hydrogen and its associated ions. Because the chemical pathways leading to the
formation of interstellar hydrides are relatively simple, the analysis of the
observed abundances is relatively straightforward and provides key information
about the environments where hydrides are found. Recent years have seen rapid
progress in our understanding of interstellar hydrides, thanks largely to
far-IR and submillimeter observations performed with the Herschel Space
Observatory. In this review, we will discuss observations of interstellar
hydrides, along with the advanced modeling approaches that have been used to
interpret them, and the unique information that has thereby been obtained.Comment: Accepted for publication in Annual Review of Astronomy and
Astrophysics 2016, Vol. 5
Gravitational collapse of the OMC-1 region
We have investigated the global dynamical state of the Integral Shaped
Filament in the Orion A cloud using new NH (1-0) large-scale, IRAM30m
observations. Our analysis of its internal gas dynamics reveals the presence of
accelerated motions towards the Orion Nebula Cluster, showing a characteristic
blue-shifted profile centred at the position of the OMC-1 South region. The
properties of these observed gas motions (profile, extension, and magnitude)
are consistent with the expected accelerations for the gravitational collapse
of the OMC-1 region and explain both the physical and kinematic structure of
this cloud.Comment: 5 pages, 2 figures; Accepted by A&
Complex organic molecules in strongly UV-irradiated gas
We investigate the presence of COMs in strongly UV-irradiated interstellar
molecular gas. We have carried out a complete millimetre line survey using the
IRAM30m telescope towards the edge of the Orion Bar photodissociation region
(PDR), close to the H2 dissociation front, a position irradiated by a very
intense far-UV (FUV) radiation field. These observations have been complemented
with 8.5 arcsec resolution maps of the H2CO 5(1,5)-4(1,4) and C18O 3-2 emission
at 0.9 mm. Despite being a harsh environment, we detect more than 250 lines
from COMs and related precursors: H2CO, CH3OH, HCO, H2CCO, CH3CHO, H2CS, HCOOH,
CH3CN, CH2NH, HNCO, H13-2CO, and HC3N (in decreasing order of abundance). For
each species, the large number of detected lines allowed us to accurately
constrain their rotational temperatures (Trot) and column densities (N). Owing
to subthermal excitation and intricate spectroscopy of some COMs (symmetric-
and asymmetric-top molecules such as CH3CN and H2CO, respectively), a correct
determination of N and Trot requires building rotational population diagrams of
their rotational ladders separately. We also provide accurate upper limit
abundances for chemically related molecules that might have been expected, but
are not conclusively detected at the edge of the PDR (HDCO, CH3O, CH3NC,
CH3CCH, CH3OCH3, HCOOCH3, CH3CH2OH, CH3CH2CN, and CH2CHCN). A non-LTE LVG
excitation analysis for molecules with known collisional rate coefficients,
suggests that some COMs arise from different PDR layers but we cannot resolve
them spatially. In particular, H2CO and CH3CN survive in the extended gas
directly exposed to the strong FUV flux (Tk = 150-250 K and Td > 60 K), whereas
CH3OH only arises from denser and cooler gas clumps in the more shielded PDR
interior (Tk = 40-50 K). We find a HCO/H2CO/CH3OH = 1/5/3 abundance ratio.
These ratios are different from those inferred in hot cores and shocks.Comment: 29 pages, 22 figures, 17 tables. Accepted for publication in A&A
(abstract abridged
New two-colour light curves of Q0957+561: time delays and the origin of intrinsic variations
We extend the gr-band time coverage of the gravitationally lensed double
quasar Q0957+561. New gr light curves permit us to detect significant intrinsic
fluctuations, to determine new time delays, and thus to gain perspective on the
mechanism of intrinsic variability in Q0957+561. We use new optical frames of
Q0957+561 in the g and r passbands from January 2005 to July 2007. These frames
are part of an ongoing long-term monitoring with the Liverpool robotic
telescope. We also introduce two photometric pipelines that are applied to the
new gr frames of Q0957+561. The transformation pipeline incorporates
zero-point, colour, and inhomogeneity corrections to the instrumental
magnitudes, so final photometry to the 1-2% level is achieved for both quasar
components. The two-colour final records are then used to measure time delays.
The gr light curves of Q0957+561 show several prominent events and gradients,
and some of them (in the g band) lead to a time delay between components of 417
+/- 2 d (1 sigma). We do not find evidence of extrinsic variability in the
light curves of Q0957+561. We also explore the possibility of a delay between a
large event in the g band and the corresponding event in the r band. The gr
cross-correlation reveals a time lag of 4.0 +/- 2.0 d (1 sigma; the g-band
event is leading) that confirms a previous claim of the existence of a delay
between the g and r band in this lensed quasar. The time delays (between quasar
components and between optical bands) from the new records and previous ones in
similar bands indicate that most observed variations in Q0957+561 (amplitudes
of about 100 mmag and timescales of about 100 d) are very probably due to
reverberation within the gas disc around the supermassive black hole.Comment: 13 pages, 9 figures. Accepted for publication in A&
OH emission from warm and dense gas in the Orion Bar PDR
As part of a far-infrared (FIR) spectral scan with Herschel/PACS, we present
the first detection of the hydroxyl radical (OH) towards the Orion Bar
photodissociation region (PDR). Five OH rotational Lambda-doublets involving
energy levels out to E_u/k~511 K have been detected (at ~65, ~79, ~84, ~119 and
~163um). The total intensity of the OH lines is I(OH)~5x10^-4 erg s^-1 cm^-2
sr^-1. The observed emission of rotationally excited OH lines is extended and
correlates well with the high-J CO and CH^+ J=3-2 line emission (but apparently
not with water vapour), pointing towards a common origin. Nonlocal, non-LTE
radiative transfer models including excitation by the ambient FIR radiation
field suggest that OH arises in a small filling factor component of warm
(Tk~160-220 K) and dense (n_H~10^{6-7} cm^-3) gas with source-averaged OH
column densities of ~10^15 cm^-2. High density and temperature photochemical
models predict such enhanced OH columns at low depths (A_V<1) and small spatial
scales (~10^15 cm), where OH formation is driven by gas-phase endothermic
reactions of atomic oxygen with molecular hydrogen. We interpret the extended
OH emission as coming from unresolved structures exposed to far-ultraviolet
(FUV) radiation near the Bar edge (photoevaporating clumps or filaments) and
not from the lower density "interclump" medium. Photodissociation leads to
OH/H2O abundance ratios (>1) much higher than those expected in equally warm
regions without enhanced FUV radiation fields.Comment: Accepted for publication in A&A Letters. Figure B.2. is bitmapped to
lower resolutio
Direct estimation of electron density in the Orion Bar PDR from mm-wave carbon recombination lines
A significant fraction of the molecular gas in star-forming regions is
irradiated by stellar UV photons. In these environments, the electron density
(n_e) plays a critical role in the gas dynamics, chemistry, and collisional
excitation of certain molecules. We determine n_e in the prototypical strongly
irradiated photodissociation region (PDR), the Orion Bar, from the detection of
new millimeter-wave carbon recombination lines (mmCRLs) and existing far-IR
[13CII] hyperfine line observations. We detect 12 mmCRLs (including alpha,
beta, and gamma transitions) observed with the IRAM 30m telescope, at ~25''
angular resolution, toward the H/H2 dissociation front (DF) of the Bar. We also
present a mmCRL emission cut across the PDR. These lines trace the C+/C/CO gas
transition layer. As the much lower frequency carbon radio recombination lines,
mmCRLs arise from neutral PDR gas and not from ionized gas in the adjacent HII
region. This is readily seen from their narrow line profiles (dv=2.6+/-0.4
km/s) and line peak LSR velocities (v_LSR=+10.7+/-0.2 km/s). Optically thin
[13CII] hyperfine lines and molecular lines - emitted close to the DF by trace
species such as reactive ions CO+ and HOC+ - show the same line profiles. We
use non-LTE excitation models of [13CII] and mmCRLs and derive n_e = 60-100
cm^-3 and T_e = 500-600 K toward the DF. The inferred electron densities are
high, up to an order of magnitude higher than previously thought. They provide
a lower limit to the gas thermal pressure at the PDR edge without using
molecular tracers. We obtain P_th > (2-4)x10^8 cm^-3 K assuming that the
electron abundance is equal or lower than the gas-phase elemental abundance of
carbon. Such elevated thermal pressures leave little room for magnetic pressure
support and agree with a scenario in which the PDR photoevaporates.Comment: Accepted for publication in A&A Letters (includes language editor
corrections
The IRAM-30m line survey of the Horsehead PDR: III. High abundance of complex (iso-)nitrile molecules in UV-illuminated gas
Complex (iso-)nitrile molecules, such as CH3CN and HC3N, are relatively
easily detected in our Galaxy and in other galaxies. We constrain their
chemistry through observations of two positions in the Horsehead edge: the
photo-dissociation region (PDR) and the dense, cold, and UV-shielded core just
behind it. We systematically searched for lines of CH3CN, HC3N, C3N, and some
of their isomers in our sensitive unbiased line survey at 3, 2, and 1mm. We
derived column densities and abundances through Bayesian analysis using a large
velocity gradient radiative transfer model. We report the first clear detection
of CH3NC at millimeter wavelength. We detected 17 lines of CH3CN at the PDR and
6 at the dense core position, and we resolved its hyperfine structure for 3
lines. We detected 4 lines of HC3N, and C3N is clearly detected at the PDR
position. We computed new electron collisional rate coefficients for CH3CN, and
we found that including electron excitation reduces the derived column density
by 40% at the PDR position. While CH3CN is 30 times more abundant in the PDR
than in the dense core, HC3N has similar abundance at both positions. The
isomeric ratio CH3NC/CH3CN is 0.15+-0.02. In the case of CH3CN, pure gas phase
chemistry cannot reproduce the amount of CH3CN observed in the UV-illuminated
gas. We propose that CH3CN gas phase abundance is enhanced when ice mantles of
grains are destroyed through photo-desorption or thermal-evaporation in PDRs,
and through sputtering in shocks. (abridged)Comment: Accepted for publication in Astronomy & Astrophysic
Gravitationally lensed QSOs in the ISSIS/WSO-UV era
Gravitationally lensed QSOs (GLQs) at redshift z = 1-2 play a key role in
understanding the cosmic evolution of the innermost parts of active galaxies
(black holes, accretion disks, coronas and internal jets), as well as the
structure of galaxies at intermediate redshifts. With respect to studies of
normal QSOs, GLQ programmes have several advantages. For example, a monitoring
of GLQs may lead to unambiguous detections of intrinsic and extrinsic
variations. Both kinds of variations can be used to discuss central engines in
distant QSOs, and mass distributions and compositions of lensing galaxies. In
this context, UV data are of particular interest, since they correspond to
emissions from the immediate surroundings of the supermassive black hole. We
describe some observation strategies to analyse optically bright GLQs at z of
about 1.5, using ISSIS (CfS) on board World Space Observatory-Ultraviolet.Comment: 7 pages, 4 figures, Accepted for publication in Astrophysics & Space
Scienc
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