7,087 research outputs found
Confirmation of circumstellar phosphine
Phosphine (PH3) was tentatively identified a few years ago in the carbon star
envelopes IRC+10216 and CRL2688 from observations of an emission line at 266.9
GHz attributable to the J=1-0 rotational transition. We report the detection of
the J=2-1 rotational transition of PH3 in IRC+10216 using the HIFI instrument
on board Herschel, which definitively confirms the identification of PH3.
Radiative transfer calculations indicate that infrared pumping to excited
vibrational states plays an important role in the excitation of PH3 in the
envelope of IRC+10216, and that the observed lines are consistent with
phosphine being formed anywhere between the star and 100 R* from the star, with
an abundance of 1e-8 relative to H2. The detection of PH3 challenges chemical
models, none of which offers a satisfactory formation scenario. Although PH3
locks just 2 % of the total available phosphorus in IRC+10216, it is together
with HCP, one of the major gas phase carriers of phosphorus in the inner
circumstellar layers, suggesting that it could be also an important phosphorus
species in other astronomical environments. This is the first unambiguous
detection of PH3 outside the solar system, and a further step towards a better
understanding of the chemistry of phosphorus in space.Comment: Accepted for publication in ApJ Letter
Initial Conditions for Large Cosmological Simulations
This technical paper describes a software package that was designed to
produce initial conditions for large cosmological simulations in the context of
the Horizon collaboration. These tools generalize E. Bertschinger's Grafic1
software to distributed parallel architectures and offer a flexible alternative
to the Grafic2 software for ``zoom'' initial conditions, at the price of large
cumulated cpu and memory usage. The codes have been validated up to resolutions
of 4096^3 and were used to generate the initial conditions of large
hydrodynamical and dark matter simulations. They also provide means to generate
constrained realisations for the purpose of generating initial conditions
compatible with, e.g. the local group, or the SDSS catalog.Comment: 12 pages, 11 figures, submitted to ApJ
Infall near clusters of galaxies: comparing gas and dark matter velocity profiles
We consider the dynamics in and near galaxy clusters. Gas, dark matter and
galaxies are presently falling into the clusters between approximately 1 and 5
virial radii. At very large distances, beyond 10 virial radii, all matter is
following the Hubble flow, and inside the virial radius the matter particles
have on average zero radial velocity. The cosmological parameters are imprinted
on the infall profile of the gas, however, no method exists, which allows a
measurement of it. We consider the results of two cosmological simulations
(using the numerical codes RAMSES and Gadget) and find that the gas and dark
matter radial velocities are very similar. We derive the relevant dynamical
equations, in particular the generalized hydrostatic equilibrium equation,
including both the expansion of the Universe and the cosmological background.
This generalized gas equation is the main new contribution of this paper. We
combine these generalized equations with the results of the numerical
simulations to estimate the contribution to the measured cluster masses from
the radial velocity: inside the virial radius it is negligible, and inside two
virial radii the effect is below 40%, in agreement the earlier analyses for DM.
We point out how the infall velocity in principle may be observable, by
measuring the gas properties to distance of about two virial radii, however,
this is practically not possible today.Comment: 7 pages, 3 figures, to appear in MNRA
On the filamentary environment of galaxies
The correlation between the large-scale distribution of galaxies and their
spectroscopic properties at z=1.5 is investigated using the Horizon MareNostrum
cosmological run.
We have extracted a large sample of 10^5 galaxies from this large
hydrodynamical simulation featuring standard galaxy formation physics. Spectral
synthesis is applied to these single stellar populations to generate spectra
and colours for all galaxies. We use the skeleton as a tracer of the cosmic web
and study how our galaxy catalogue depends on the distance to the skeleton. We
show that galaxies closer to the skeleton tend to be redder, but that the
effect is mostly due to the proximity of large haloes at the nodes of the
skeleton, rather than the filaments themselves.
This effects translate into a bimodality in the colour distribution of our
sample. The origin of this bimodality is investigated and seems to follow from
the ram pressure stripping of satellite galaxies within the more massive
clusters of the simulation.
The virtual catalogues (spectroscopical properties of the MareNostrum
galaxies at various redshifts) are available online at
http://www.iap.fr/users/pichon/MareNostrum/cataloguesComment: 18 pages, 27 figures, accepted for publication in MNRA
Nitrogen isotopic ratios in Barnard 1: a consistent study of the N2H+, NH3, CN, HCN and HNC isotopologues
The 15N isotopologue abundance ratio measured today in different bodies of
the solar system is thought to be connected to 15N-fractionation effects that
would have occured in the protosolar nebula. The present study aims at putting
constraints on the degree of 15N-fractionation that occurs during the
prestellar phase, through observations of D, 13C and 15N-substituted
isotopologues towards B1b. Both molecules from the nitrogen hydride family,
i.e. N2H+ and NH3, and from the nitrile family, i.e. HCN, HNC and CN, are
considered in the analysis. As a first step, we model the continuum emission in
order to derive the physical structure of the cloud, i.e. gas temperature and
H2 density. These parameters are subsequently used as an input in a non-local
radiative transfer model to infer the radial abundances profiles of the various
molecules. Our modeling shows that all the molecules are affected by depletion
onto dust grains, in the region that encompasses the B1-bS and B1-bN cores.
While high levels of deuterium fractionation are derived, we conclude that no
fractionation occurs in the case of the nitrogen chemistry. Independently of
the chemical family, the molecular abundances are consistent with 14N/15N~300,
a value representative of the elemental atomic abundances of the parental gas.
The inefficiency of the 15N-fractionation effects in the B1b region can be
linked to the relatively high gas temperature ~17K which is representative of
the innermost part of the cloud. Since this region shows signs of depletion
onto dust grains, we can not exclude the possibility that the molecules were
previously enriched in 15N, earlier in the B1b history, and that such an
enrichment could have been incorporated into the ice mantles. It is thus
necessary to repeat this kind of study in colder sources to test such a
possibility.Comment: accepted in A&
High-velocity hot CO emission close to Sgr A*: Herschel/HIFI submillimeter spectral survey toward Sgr A*
The properties of molecular gas, the fuel that forms stars, inside the cavity
of the circumnuclear disk (CND) are not well constrained. We present results of
a velocity-resolved submillimeter scan (~480 to 1250 GHz}) and [CII]158um line
observations carried out with Herschel/HIFI toward Sgr A*; these results are
complemented by a ~2'x2' CO (J=3-2) map taken with the IRAM 30 m telescope at
~7'' resolution. We report the presence of high positive-velocity emission (up
to about +300 km/s) detected in the wings of CO J=5-4 to 10-9 lines. This wing
component is also seen in H2O (1_{1,0}-1_{0,1}) a tracer of hot molecular gas;
in [CII]158um, an unambiguous tracer of UV radiation; but not in [CI]492,806
GHz. This first measurement of the high-velocity CO rotational ladder toward
Sgr A* adds more evidence that hot molecular gas exists inside the cavity of
the CND, relatively close to the supermassive black hole (< 1 pc). Observed by
ALMA, this velocity range appears as a collection of CO (J=3-2) cloudlets lying
in a very harsh environment that is pervaded by intense UV radiation fields,
shocks, and affected by strong gravitational shears. We constrain the physical
conditions of the high positive-velocity CO gas component by comparing with
non-LTE excitation and radiative transfer models. We infer T_k~400 K to 2000 K
for n_H~(0.2-1.0)x10^5 cm^-3. These results point toward the important role of
stellar UV radiation, but we show that radiative heating alone cannot explain
the excitation of this ~10-60 M_Sun component of hot molecular gas inside the
central cavity. Instead, strongly irradiated shocks are promising candidates.Comment: Accepted for publication in A&A Letters ( this v2 includes
corrections by language editor
The hyperfine structure in the rotational spectrum of CF+
Context. CF+ has recently been detected in the Horsehead and Orion Bar
photo-dissociation regions. The J=1-0 line in the Horsehead is double-peaked in
contrast to other millimeter lines. The origin of this double-peak profile may
be kinematic or spectroscopic. Aims. We investigate the effect of hyperfine
interactions due to the fluorine nucleus in CF+ on the rotational transitions.
Methods. We compute the fluorine spin rotation constant of CF+ using high-level
quantum chemical methods and determine the relative positions and intensities
of each hyperfine component. This information is used to fit the theoretical
hyperfine components to the observed CF+ line profiles, thereby employing the
hyperfine fitting method in GILDAS. Results. The fluorine spin rotation
constant of CF+ is 229.2 kHz. This way, the double-peaked CF+ line profiles are
well fitted by the hyperfine components predicted by the calculations. The
unusually large hyperfine splitting of the CF+ line therefore explains the
shape of the lines detected in the Horsehead nebula, without invoking intricate
kinematics in the UV-illuminated gas.Comment: 2 pages, 1 figure, Accepted for publication in A&
Systematic uncertainties in the determination of the local dark matter density
A precise determination of the local dark matter density and an accurate
control over the corresponding uncertainties are of paramount importance for
Dark Matter (DM) searches. Using very recent high-resolution numerical
simulations of a Milky Way like object, we study the systematic uncertainties
that affect the determination of the local dark matter density based on
dynamical measurements in the Galaxy. In particular, extracting from the
simulation with baryons the orientation of the Galactic stellar disk with
respect to the DM distribution, we study the DM density for an observer located
at 8 kpc from the Galactic center {\it on the stellar disk}, .
This quantity is found to be always larger than the average density in a
spherical shell of same radius , which is the quantity inferred
from dynamical measurements in the Galaxy, and to vary in the range
. This suggests that the actual dark matter
density in the solar neighbourhood is on average 21\% larger than the value
inferred from most dynamical measurements, and that the associated systematic
errors are larger than the statistical errors recently discussed in the
literature.Comment: 6 pages, 3 figures, matches published versio
Optical properties of BiTeBr and BiTeCl
We present a comparative study of the optical properties - reflectance,
transmission and optical conductivity - and Raman spectra of two layered
bismuth-tellurohalides BiTeBr and BiTeCl at 300 K and 5 K, for light polarized
in the a-b planes. Despite different space groups, the optical properties of
the two compounds are very similar. Both materials are doped semiconductors,
with the absorption edge above the optical gap which is lower in BiTeBr (0.62
eV) than in BiTeCl (0.77 eV). The same Rashba splitting is observed in the two
materials. A non-Drude free carrier contribution in the optical conductivity,
as well as three Raman and two infrared phonon modes, are observed in each
compound. There is a dramatic difference in the highest infrared phonon
intensity for the two compounds, and a difference in the doping levels. Aspects
of the strong electron-phonon interaction are identified. Several interband
transitions are assigned, among them the low-lying absorption which has
the same value 0.25 eV in both compounds, and is caused by the Rashba spin
splitting of the conduction band. An additional weak transition is found in
BiTeCl, caused by the lower crystal symmetry.Comment: Accepted in PR
- …