5,455 research outputs found
First measurements of 15N fractionation in N2H+ toward high-mass star forming cores
We report on the first measurements of the isotopic ratio 14N/15N in N2H+
toward a statistically significant sample of high-mass star forming cores. The
sources belong to the three main evolutionary categories of the high-mass star
formation process: high-mass starless cores, high-mass protostellar objects,
and ultracompact HII regions. Simultaneous measurements of 14N/15N in CN have
been made. The 14N/15N ratios derived from N2H+ show a large spread (from ~180
up to ~1300), while those derived from CN are in between the value measured in
the terrestrial atmosphere (~270) and that of the proto-Solar nebula (~440) for
the large majority of the sources within the errors. However, this different
spread might be due to the fact that the sources detected in the N2H+
isotopologues are more than those detected in the CN ones. The 14N/15N ratio
does not change significantly with the source evolutionary stage, which
indicates that time seems to be irrelevant for the fractionation of nitrogen.
We also find a possible anticorrelation between the 14N/15N (as derived from
N2H+) and the H/D isotopic ratios. This suggests that 15N enrichment could not
be linked to the parameters that cause D enrichment, in agreement with the
prediction by recent chemical models. These models, however, are not able to
reproduce the observed large spread in 14N/15N, pointing out that some
important routes of nitrogen fractionation could be still missing in the
models.Comment: 2 Figures, accepted for publication in ApJ
Molecular ions in the protostellar shock L1157-B1
We perform a complete census of molecular ions with an abundance larger than
1e-10 in the protostellar shock L1157-B1 by means of an unbiased
high-sensitivity survey obtained with the IRAM-30m and Herschel/HIFI. By means
of an LVG radiative transfer code the gas physical conditions and fractional
abundances of molecular ions are derived. The latter are compared with
estimates of steady-state abundances in the cloud and their evolution in the
shock calculated with the chemical model Astrochem. We detect emission from
HCO+, H13CO+, N2H+, HCS+, and, for the first time in a shock, from HOCO+, and
SO+. The bulk of the emission peaks at blueshifted velocity, ~ 0.5-3 km/s with
respect to systemic, has a width of ~ 4-8 km/s, and is associated with the
outflow cavities (T_kin ~ 20-70 K, n(H2) ~ 1e5 cm-3). Observed HCO+ and N2H+
abundances are in agreement with steady-state abundances in the cloud and with
their evolution in the compressed and heated gas in the shock for cosmic rays
ionization rate Z = 3e-16 s-1. HOCO+, SO+, and HCS+ observed abundances,
instead, are 1-2 orders of magnitude larger than predicted in the cloud; on the
other hand they are strongly enhanced on timescales shorter than the shock age
(~2000 years) if CO2, S or H2S, and OCS are sputtered off the dust grains in
the shock. The performed analysis indicates that HCO+ and N2H+ are a fossil
record of pre-shock gas in the outflow cavity, while HOCO+, SO+, and HCS+ are
effective shock tracers and can be used to infer the amount of CO2 and
sulphur-bearing species released from dust mantles in the shock. The observed
HCS+ (and CS) abundance indicates that OCS should be one of the main sulphur
carrier on grain mantles. However, the OCS abundance required to fit the
observations is 1-2 orders of magnitude larger than observed. Further studies
are required to fully understand the chemistry of sulphur-bearing species.Comment: 12 pages, 5 figures, accepted by A&
Ionization toward the high-mass star-forming region NGC 6334 I
Context. Ionization plays a central role in the gas-phase chemistry of
molecular clouds. Since ions are coupled with magnetic fields, which can in
turn counteract gravitational collapse, it is of paramount importance to
measure their abundance in star-forming regions. Aims. We use spectral line
observations of the high-mass star-forming region NGC 6334 I to derive the
abundance of two of the most abundant molecular ions, HCO+ and N2H+, and
consequently, the cosmic ray ionization rate. In addition, the line profiles
provide information about the kinematics of this region. Methods. We present
high-resolution spectral line observations conducted with the HIFI instrument
on board the Herschel Space Observatory of the rotational transitions with Jup
> 5 of the molecular species C17O, C18O, HCO+, H13CO+, and N2H+. Results. The
HCO+ and N2H+ line profiles display a redshifted asymmetry consistent with a
region of expanding gas. We identify two emission components in the spectra,
each with a different excitation, associated with the envelope of NGC 6334 I.
The physical parameters obtained for the envelope are in agreement with
previous models of the radial structure of NGC 6334 I based on submillimeter
continuum observations. Based on our new Herschel/HIFI observations, combined
with the predictions from a chemical model, we derive a cosmic ray ionization
rate that is an order of magnitude higher than the canonical value of 10^(-17)
s-1. Conclusions. We find evidence of an expansion of the envelope surrounding
the hot core of NGC 6334 I, which is mainly driven by thermal pressure from the
hot ionized gas in the region. The ionization rate seems to be dominated by
cosmic rays originating from outside the source, although X-ray emission from
the NGC 6334 I core could contribute to the ionization in the inner part of the
envelope.Comment: This paper contains a total of 10 figures and 3 table
Sulphur-bearing species in the star forming region L1689N
We report observations of the expected main S-bearing species (SO, SO2 and
H2S) in the low-mass star forming region L1689N. We obtained large scale
(~300''x200'') maps of several transitions from these molecules with the goal
to study the sulphur chemistry, i.e. how the relative abundances change in the
different physical conditions found in L1689N. We identified eight interesting
regions, where we carried out a quantitative comparative study: the molecular
cloud (as reference position), five shocked regions caused by the interaction
of the molecular outflows with the cloud, and the two protostars IRAS16293-2422
and 16293E. In the cloud we carefully computed the gas temperature and density
by means of a non-LTE LVG code, while in other regions we used previous
results. We hence derived the column density of SO, SO2 and H2S, together with
SiO and H2CO - which were observed previously - and their relevant abundance
ratios. We find that SiO is the molecule that shows the largest abundance
variations in the shocked regions, whereas S-bearing molecules show more
moderate variations. Remarkably, the region of the brightest SiO emission in
L1689N is undetected in SO2, H2S and H2CO and only marginally detected in SO.
In the other weaker SiO shocks, SO2 is enhanced with respect to SO. We propose
a schema in which the different molecular ratios correspond to different ages
of the shocks. Finally, we find that SO, SO2 and H2S have significant abundance
jumps in the inner hot core of IRAS16293-2422 and discuss the implications of
the measured abundances.Comment: Accepted 08/10/0
Galaxy Peculiar Velocities and Infall onto Groups
We perform statistical analyses to study the infall of galaxies onto groups
and clusters in the nearby Universe. The study is based on the UZC and SSRS2
group catalogs and peculiar velocity samples. We find a clear signature of
infall of galaxies onto groups over a wide range of scales 5 h^{-1} Mpc<r<30
h^{-1} Mpc, with an infall amplitude on the order of a few hundred kilometers
per second. We obtain a significant increase in the infall amplitude with group
virial mass (M_{V}) and luminosity of group member galaxies (L_{g}). Groups
with M_{V}<10^{13} M_{\odot} show infall velocities V_{infall} \simeq 150 km
s^{-1} whereas for M_{V}>10^{13} M_{\odot} a larger infall is observed,
V_{infall} \simeq 200 km s^{-1}. Similarly, we find that galaxies surrounding
groups with L_{g}<10^{15} L_{\odot} have V_{infall} \simeq 100 km s^{-1},
whereas for L_{g}>10^{15} L_{\odot} groups, the amplitude of the galaxy infall
can be as large as V_{infall} \simeq 250 km s^{-1}. The observational results
are compared with the results obtained from mock group and galaxy samples
constructed from numerical simulations, which include galaxy formation through
semianalytical models. We obtain a general agreement between the results from
the mock catalogs and the observations. The infall of galaxies onto groups is
suitably reproduced in the simulations and, as in the observations, larger
virial mass and luminosity groups exhibit the largest galaxy infall amplitudes.
We derive estimates of the integrated mass overdensities associated with groups
by applying linear theory to the infall velocities after correcting for the
effects of distance uncertainties obtained using the mock catalogs. The
resulting overdensities are consistent with a power law with \delta \sim 1 at r
\sim 10 h^{-1}Mpc.Comment: 25 pages, 10 figure
Hot and dense water in the inner 25 AU of SVS13-A
In the context of the ASAI (Astrochemical Surveys At IRAM) project, we
carried out an unbiased spectral survey in the millimeter window towards the
well known low-mass Class I source SVS13-A. The high sensitivity reached (3-12
mK) allowed us to detect at least 6 HDO broad (FWHM ~ 4-5 km/s) emission lines
with upper level energies up to Eu = 837 K. A non-LTE LVG analysis implies the
presence of very hot (150-260 K) and dense (> 3 10^7 cm-3) gas inside a small
radius ( 25 AU) around the star, supporting, for the first time, the
occurrence of a hot corino around a Class I protostar.
The temperature is higher than expected for water molecules are sublimated
from the icy dust mantles (~ 100 K). Although we cannot exclude we are observig
the effects of shocks and/or winds at such small scales, this could imply that
the observed HDO emission is tracing the water abundance jump expected at
temperatures ~ 220-250 K, when the activation barrier of the gas phase
reactions leading to the formation of water can be overcome. We derive X(HDO) ~
3 10-6, and a H2O deuteration > 1.5 10-2, suggesting that water deuteration
does not decrease as the protostar evolves from the Class 0 to the Class I
stage.Comment: MNRAS Letter
Quasi-classical rate coefficient calculations for the rotational (de)excitation of H2O by H2
The interpretation of water line emission from existing observations and
future HIFI/Herschel data requires a detailed knowledge of collisional rate
coefficients. Among all relevant collisional mechanisms, the rotational
(de)excitation of H2O by H2 molecules is the process of most interest in
interstellar space. To determine rate coefficients for rotational de-excitation
among the lowest 45 para and 45 ortho rotational levels of H2O colliding with
both para and ortho-H2 in the temperature range 20-2000 K. Rate coefficients
are calculated on a recent high-accuracy H2O-H2 potential energy surface using
quasi-classical trajectory calculations. Trajectories are sampled by a
canonical Monte-Carlo procedure. H2 molecules are assumed to be rotationally
thermalized at the kinetic temperature. By comparison with quantum calculations
available for low lying levels, classical rates are found to be accurate within
a factor of 1-3 for the dominant transitions, that is those with rates larger
than a few 10^{-12}cm^{3}s^{-1}. Large velocity gradient modelling shows that
the new rates have a significant impact on emission line fluxes and that they
should be adopted in any detailed population model of water in warm and hot
environments.Comment: 8 pages, 2 figures, 1 table (the online material (4 tables) can be
obtained upon request to [email protected]
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