243 research outputs found
First detection of [N II] 205 micrometer absorption in interstellar gas
We present high resolution [NII] 205 micrometer ^3P_1-^3P_0 spectra obtained
with Herschel-HIFI towards a small sample of far-infrared bright star forming
regions in the Galactic plane: W31C (G10.6-0.4), W49N (G43.2-0.1), W51
(G49.5-0.4), and G34.3+0.1. All sources display an emission line profile
associated directly with the HII regions themselves. For the first time we also
detect absorption of the [NII] 205 micrometer line by extended low-density
foreground material towards W31C and W49N over a wide range of velocities. We
attribute this absorption to the warm ionised medium (WIM) and find
N(N^+)\approx 1.5x10^17 cm^-2 towards both sources. This is in agreement with
recent Herschel-HIFI observations of [CII] 158 micrometer, also observed in
absorption in the same sight-lines, if \approx7-10 % of all C^+ ions exist in
the WIM on average. Using an abundance ratio of [N]/[H] = 6.76x10^-5 in the gas
phase we find that the mean electron and proton volume densities are ~0.1-0.3
cm^-3 assuming a WIM volume filling fraction of 0.1-0.4 with a corresponding
line-of-sight filling fraction of 0.46-0.74. A low density and a high WIM
filling fraction are also supported by RADEX modelling of the [NII] 205
micrometer absorption and emission together with visible emission lines
attributed mainly to the WIM. The detection of the 205 micrometer line in
absorption emphasises the importance of a high spectral resolution, and also
offers a new tool for investigation of the WIM.Comment: 7 pages, 4 figures, accepted for publication in Astronomy &
Astrophysics, 11 June 201
On the accretion process in a high-mass star forming region - A multitransitional THz Herschel-HIFI study of ammonia toward G34.26+0.15
[Abridged] Our aim is to explore the gas dynamics and the accretion process
in the early phase of high-mass star formation. The inward motion of molecular
gas in the massive star forming region G34.26+0.15 is investigated by using
high-resolution profiles of seven transitions of ammonia at THz frequencies
observed with Herschel-HIFI. The shapes and intensities of these lines are
interpreted in terms of radiative transfer models of a spherical, collapsing
molecular envelope. An accelerated Lambda Iteration (ALI) method is used to
compute the models. The seven ammonia lines show mixed absorption and emission
with inverse P-Cygni-type profiles that suggest infall onto the central source.
A trend toward absorption at increasingly higher velocities for higher
excitation transitions is clearly seen in the line profiles. The lines show only very weak emission, so these absorption profiles
can be used directly to analyze the inward motion of the gas. This is the first
time a multitransitional study of spectrally resolved rotational ammonia lines
has been used for this purpose. Broad emission is, in addition, mixed with the
absorption in the ortho-NH line, possibly tracing a molecular
outflow from the star forming region. The best-fitting ALI model reproduces the
continuum fluxes and line profiles, but slightly underpredicts the emission and
absorption depth in the ground-state ortho line . The derived
ortho-to-para ratio is approximately 0.5 throughout the infalling cloud core
similar to recent findings for translucent clouds in sight lines toward W31C
and W49N. We find evidence of two gas components moving inwards toward the
central region with constant velocities: 2.7 and 5.3 kms, relative
to the source systemic velocity. The inferred mass accretion rates derived are
sufficient to overcome the expected radiation pressure from G34.26+0.15.Comment: 20 pages, 18 figures, accepted by A&A 3 October 201
The killer fly hunger games : target size and speed predict decision to pursuit
© The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Brain, Behavior and Evolution 86 (2015): 28-27, doi:10.1159/000435944.Predatory animals have evolved to optimally detect their prey using exquisite sensory systems such as vision, olfaction and hearing. It may not be so surprising that vertebrates, with large central nervous systems, excel at predatory behaviors. More striking is the fact that many tiny insects, with their miniscule brains and scaled down nerve cords, are also ferocious, highly successful predators. For predation, it is important to determine whether a prey is suitable before initiating pursuit. This is paramount since pursuing a prey that is too large to capture, subdue or dispatch will generate a substantial metabolic cost (in the form of muscle output) without any chance of metabolic gain (in the form of food). In addition, during all pursuits, the predator breaks its potential camouflage and thus runs the risk of becoming prey itself. Many insects use their eyes to initially detect and subsequently pursue prey. Dragonflies, which are extremely efficient predators, therefore have huge eyes with relatively high spatial resolution that allow efficient prey size estimation before initiating pursuit. However, much smaller insects, such as killer flies, also visualize and successfully pursue prey. This is an impressive behavior since the small size of the killer fly naturally limits the neural capacity and also the spatial resolution provided by the compound eye. Despite this, we here show that killer flies efficiently pursue natural (Drosophila melanogaster) and artificial (beads) prey. The natural pursuits are initiated at a distance of 7.9 ± 2.9 cm, which we show is too far away to allow for distance estimation using binocular disparities. Moreover, we show that rather than estimating absolute prey size prior to launching the attack, as dragonflies do, killer flies attack with high probability when the ratio of the prey's subtended retinal velocity and retinal size is 0.37. We also show that killer flies will respond to a stimulus of an angular size that is smaller than that of the photoreceptor acceptance angle, and that the predatory response is strongly modulated by the metabolic state. Our data thus provide an exciting example of a loosely designed matched filter to Drosophila, but one which will still generate successful pursuits of other suitable prey.This work was funded by the Air Force Office of Scientific Research (FA9550-10-0472 to R.M. Olberg and FA9550-15-1-0188 to P.T. Gonzalez-Bellido and K. Nordström), an Isaac Newton Trust/Wellcome Trust ISSF/University of Cambridge Joint Research Grant to Paloma T. Gonzalez-Bellido, a Biotechnology and Biological Sciences Research Council David Phillips Fellowship (BBSRC, BB/L024667/1) to Trevor J. Wardill, the Swedish Research Council (2012-4740) to Karin Nordström and a Shared Equipment Grant from the School of Biological Sciences (University of Cambridge)
Circumstellar water vapour in M-type AGB stars: Constraints from H2O(1_10 - 1_01) lines obtained with Odin
Aims: Spectrally resolved circumstellar H2O(1_10 - 1_01) lines have been
obtained towards three M-type AGB stars using the Odin satellite. This provides
additional strong constrains on the properties of circumstellar H2O and the
circumstellar envelope. Methods: ISO and Odin satellite H2O line data are used
as constraints for radiative transfer models. Special consideration is taken to
the spectrally resolved Odin line profiles, and the effect of excitation to the
first excited vibrational states of the stretching modes (nu1=1 and nu3=1) on
the derived abundances is estimated. A non-local, radiative transfer code based
on the ALI formalism is used. Results: The H2O abundance estimates are in
agreement with previous estimates. The inclusion of the Odin data sets stronger
constraints on the size of the H2O envelope. The H2O(1_10 - 1_01) line profiles
require a significant reduction in expansion velocity compared to the terminal
gas expansion velocity determined in models of CO radio line emission,
indicating that the H2O emission lines probe a region where the wind is still
being accelerated. Including the nu3=1 state significantly lowers the estimated
abundances for the low-mass-loss-rate objects. This shows the importance of
detailed modelling, in particular the details of the infrared spectrum in the
range 3 to 6 micron, to estimate accurate circumstellar H2O abundances.
Conclusions: Spectrally resolved circumstellar H2O emission lines are important
probes of the physics and chemistry in the inner regions of circumstellar
envelopes around asymptotic giant branch stars. Predictions for H2O emission
lines in the spectral range of the upcoming Herschel/HIFI mission indicate that
these observations will be very important in this context.Comment: accepted in A&A, 10 pages, 8 figure
Herschel observations of the Herbig-Haro objects HH52-54
We are aiming at the observational estimation of the relative contribution to
the cooling by CO and H2O, as this provides decisive information for the
understanding of the oxygen chemistry behind interstellar shock waves. Methods.
The high sensitivity of HIFI, in combination with its high spectral resolution
capability, allows us to trace the H2O outflow wings at unprecedented
signal-to-noise. From the observation of spectrally resolved H2O and CO lines
in the HH52-54 system, both from space and from ground, we arrive at the
spatial and velocity distribution of the molecular outflow gas. Solving the
statistical equilibrium and non-LTE radiative transfer equations provides us
with estimates of the physical parameters of this gas, including the cooling
rate ratios of the species. The radiative transfer is based on an ALI code,
where we use the fact that variable shock strengths, distributed along the
front, are naturally implied by a curved surface. Based on observations of CO
and H2O spectral lines, we conclude that the emission is confined to the HH54
region. The quantitative analysis of our observations favours a ratio of the
CO-to-H2O-cooling-rate >> 1. From the best-fit model to the CO emission, we
arrive at an H2O abundance close to 1e-5. The line profiles exhibit two
components, one of which is triangular and another, which is a superposed,
additional feature. This additional feature likely originates from a region
smaller than the beam where the ortho-water abundance is smaller than in the
quiescent gas. Comparison with recent shock models indicate that a planar shock
can not easily explain the observed line strengths and triangular line
profiles.We conclude that the geometry can play an important role. Although
abundances support a scenario where J-type shocks are present, higher cooling
rate ratios than predicted by these type of shocks are derived.Comment: Accepted for publication in A&
The structure of the cometary globule CG 12: a high latitude star forming region
The structure of the high galactic latitude Cometary Globule 12 (CG 12) has
been investigated by means of radio molecular line observations. Detailed, high
signal to noise ratio maps in C18O (1-0), C18O (2-1) and molecules tracing high
density gas, CS (3-2), DCO+ (2-1) and H13CO+ (1-0), are presented. The C18O
line emission is distributed in a 10' long North-South elongated lane with two
strong maxima, CG12 N(orth) and CG12 S(outh). In CG12 S the high density
tracers delineate a compact core, DCO+ core, which is offset by 15" from the
C18O maximum. The observed strong C18O emission traces the surface of the DCO+
core or a separate, adjacent cloud component. The emission in high density
tracers is weak in CG12 N and especially the H13CO+, DCO+ and N2H+ lines are
+0.5 km/s offset in velocity with respect to the C18O lines. Evidence is
presented that the molecular gas is highly depleted. The observed strong C18O
emission towards CG12 N originates in the envelope of this depleted cloud
component or in a separate entity seen in the same line of sight. The C18O
lines in CG 12 were analyzed using Positive Matrix Factorization, PMF. The
shape and the spatial distribution of the individual PMF factors fitted
separately to the C18O (1-0) and (2-1) transitions were consistent with each
other. The results indicate a complex velocity and line excitation structure in
the cloud. Besides separate cloud velocity components the C18O line shapes and
intensities are influenced by excitation temperature variations caused by e.g,
the molecular outflow or by molecular depletion. Assuming a distance of 630 pc
the size of the CG 12 compact head, 1.1 pc by 1.8 pc, and the C18O mass larger
than 100 Msun are comparable to those of other nearby low/intermediate mass
star formation regions.Comment: 18 pages, 17 figures Accepted A&A Sep. 22 200
Gas phase production of NHD2 in L134N
We show analytically that large abundances of NH2D and NHD2 can be produced
by gas phase chemistry in the interiors of cold dense clouds. The calculated
fractionation ratios are in good agreement with the values that have been
previously determined in L134N and suggest that triply-deuterated ammonia could
be detectable in dark clouds. Grain surface reactions may lead to similar NH2D
and NHD2 enhancements but, we argue, are unlikely to contribute to the
deuteration observed in L134N.Comment: 6 pages, 2 figures, uses psfig.sty and emulateapj.sty, to appear in
Astrophysical Journal, vol 55
First detection of NH3 (1,0 - 0,0) from a low mass cloud core: On the low ammonia abundance of the rho Oph A core
Odin has successfully observed the molecular core rho Oph A in the 572.5 GHz
rotational ground state line of ammonia, NH3 (J,K = 1,0 - 0,0). The
interpretation of this result makes use of complementary molecular line data
obtained from the ground (C17O and CH3OH) as part of the Odin preparatory work.
Comparison of these observations with theoretical model calculations of line
excitation and transfer yields a quite ordinary abundance of methanol, X(CH3OH)
= 3e-9. Unless NH3 is not entirely segregated from C17O and CH3OH, ammonia is
found to be significantly underabundant with respect to typical dense core
values, viz. X(NH3) = 8e-10.Comment: 4 pages, 2 figures, 2 tables, to appear in Astron. Astrophys. Letter
Herschel/HIFI deepens the circumstellar NH3 enigma
Circumstellar envelopes (CSEs) of a variety of evolved stars have been found
to contain ammonia (NH3) in amounts that exceed predictions from conventional
chemical models by many orders of magnitude. The observations reported here
were performed in order to better constrain the NH3 abundance in the CSEs of
four, quite diverse, oxygen-rich stars using the NH3 ortho J_K = 1_0 - 0_0
ground-state line. We used the Heterodyne Instrument for the Far Infrared
aboard Herschel to observe the NH3 J_K = 1_0 - 0_0 transition near 572.5 GHz,
simultaneously with the ortho-H2O J_Ka,Kc = 1_1,0 -1_0,1 transition, toward VY
CMa, OH 26.5+0.6, IRC+10420, and IK Tau. We conducted non-LTE radiative
transfer modeling with the goal to derive the NH3 abundance in these objects'
CSEs. For the latter two stars, Very Large Array imaging of NH3
radio-wavelength inversion lines were used to provide further constraints,
particularly on the spatial extent of the NH3-emitting regions. Results. We
find remarkably strong NH3 emission in all of our objects with the NH3 line
intensities rivaling those obtained for the ground state H2O line. The NH3
abundances relative to H2 are very high and range from 2 x 10-7 to 3 x 10-6 for
the objects we have studied. Our observations confirm and even deepen the
circumstellar NH3 enigma. While our radiative transfer modeling does not yield
satisfactory fits to the observed line profiles, it leads to abundance
estimates that confirm the very high values found in earlier studies. New ways
to tackle this mystery will include further Herschel observations of more NH3
lines and imaging with the Expanded Very Large Array.Comment: 4+2 page
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