369 research outputs found
Tracing early evolutionary stages of high-mass star formation with molecular lines
Despite its major role in the evolution of the interstellar medium, the
formation of high-mass stars (M > 10 Msol) is still poorly understood. Two
types of massive star cluster precursors, the so-called Massive Dense Cores
(MDCs), have been observed, which differ in their mid-infrared brightness. The
origin of this difference is not established and could be the result of
evolution, density, geometry differences, or a combination of these. We compare
several molecular tracers of physical conditions (hot cores, shocks) observed
in a sample of mid-IR weak emitting MDCs with previous results obtained in a
sample of exclusively mid-IR bright MDCs. The aim is to understand the
differences between these two types of object. We present single-dish
observations of HDO, H2O-18, SO2 and CH3OH lines at lambda = 1.3 - 3.5 mm. We
study line profiles and estimate abundances of these molecules, and use a
partial correlation method to search for trends in the results. The detection
rates of thermal emission lines are found to be very similar between mid-IR
quiet and bright objects. The abundances of H2O, HDO (1E-13 to 1E-9 in the cold
outer envelopes), SO2 and CH3OH differ from source to source but independently
of their mid-IR flux. In contrast, the methanol class I maser emission, a
tracer of outflow shocks, is found to be strongly anti-correlated with the 12
micron source brightnesses. The enhancement of the methanol maser emission in
mid-IR quiet MDCs may indicate a more embedded nature. Since total masses are
similar between the two samples, we suggest that the matter distribution is
spherical around mid-IR quiet sources but flattened around mid-IR bright ones.
In contrast, water emission is associated with objects containing a hot
molecular core, irrespective of their mid-IR brightness. These results indicate
that the mid-IR brightness of MDCs is an indicator of their evolutionary stage.Comment: 15 pages, 6 figures, 11 tables, accepted for publication in A&A the
11/06/201
The massive protostar W43-MM1 as seen by Herschel-HIFI water spectra: high turbulence and accretion luminosity
We present Herschel/HIFI observations of fourteen water lines in W43-MM1, a
massive protostellar object in the luminous star cluster-forming region W43. We
analyze the gas dynamics from the line profiles using Herschel-HIFI
observations (WISH-KP) of fourteen far-IR water lines (H2O, H217O, H218O),
CS(11-10), and C18O(9-8) lines, and using our modeling of the continuum
spectral energy distribution. As for lower mass protostellar objects, the
molecular line profiles are a mix of emission and absorption, and can be
decomposed into 'medium', and 'broad' velocity components. The broad component
is the outflow associated with protostars of all masses. Our modeling shows
that the remainder of the water profiles can be well fitted by an infalling and
passively heated envelope, with highly supersonic turbulence varying from 2.2
km/s in the inner region to 3.5 km/s in the outer envelope. Also, W43-MM1 has a
high accretion rate, between 4.0 x 10^{-4} and 4.0 x 10^{-2} \msun /yr, derived
from the fast (0.4-2.9 km/s) infall observed. We estimate a lower mass limit of
gaseous water of 0.11 \msun and total water luminosity of 1.5 \lsun (in the 14
lines presented here). The central hot core is detected with a water abundance
of 1.4 x 10^{-4} while the water abundance for the outer envelope is 8
x10^{-8}. The latter value is higher than in other sources, most likely related
to the high turbulence and the micro-shocks created by its dissipation.
Examining water lines of various energies, we find that the turbulent velocity
increases with the distance to the center. While not in clear disagreement with
the competitive accretion scenario, this behavior is predicted by the turbulent
core model. Moreover, the estimated accretion rate is high enough to overcome
the expected radiation pressure.Comment: Accepted in A&A on April 2, 2012. 12 pages 7 figure
Evolution of massive protostars: the IRAS 18151-1208 region
The study of physical and chemical properties of massive protostars is
critical to better understand the evolutionary sequence which leads to the
formation of high-mass stars. IRAS 18151-1208 is a nearby massive region (d =
3kpc, L ~ 20000 Lsun) which splits into three cores: MM1, MM2 and MM3
(separated by 1'-2'). We aim at (1) studying the physical and chemical
properties of the individual MM1, MM2 and MM3 cores; (2) deriving their
evolutionary stages; (3) using these results to improve our view of the
evolutionary sequence of massive cores. The region was observed in the CS,
C34S, H2CO, HCO+, H13CO+, and N2H+ lines at mm wavelengths with the IRAM 30m
and Mopra telescopes. We use 1D and 2D modeling of the dust continuum to derive
the density and temperature distributions, which are then used in the RATRAN
code to model the lines and constrain the abundances of the observed species.
All the lines were detected in MM1 and MM2. MM3 shows weaker emission, or even
is undetected in HCO+ and all isotopic species. MM2 is driving a newly
discovered CO outflow and hosts a mid-IR-quiet massive protostar. The abundance
of CS is significantly larger in MM1 than in MM2, but smaller than in a
reference massive protostar such as AFGL2591. In contrast the N2H+ abundance
decreases from MM2 to MM1, and is larger than in AFGL2591. Both MM1 and MM2
host an early phase massive protostar, but MM2 (and mid-IR-quiet sources in
general) is younger and more dominated by the host protostar than MM1
(mid-IR-bright). The MM3 core is probably in a pre-stellar phase. We find that
the N2H+/C34S ratio varies systematically with age in the massive protostars
for which the data are available. It can be used to identify young massive
protostars.Comment: 19 pages, 17 figures, accepted by A&A the 3 June 200
S-bearing molecules in Massive Dense Cores
Chemical composition of the massive cores forming high-mass stars can put
some constrains on the time scale of the massive star formation: sulphur
chemistry is of specific interest due to its rapid evolution in warm gas and
because the abundance of sulphur bearing species increases significantly with
the temperature. Two mid-infrared quiet and two brighter massive cores are
observed in various transitions (E_up up to 289K) of CS, OCS, H2S, SO, SO2 and
of their isotopologues at mm wavelengths with the IRAM 30m and CSO telescopes.
1D modeling of the dust continuum is used to derive the density and temperature
laws, which are then applied in the RATRAN code to model the observed line
emission, and to derive the relative abundances of the molecules. All lines,
except the highest energy SO2 transition, are detected. Infall (up to 2.9km/s)
may be detected towards the core W43MM1. The inferred mass rate is 5.8-9.4
10^{-2} M_{\odot}/yr. We propose an evolutionary sequence of our sources
(W43MM1-IRAS18264-1152-IRAS05358+3543-IRAS18162-2048), based on the SED
analysis. The analysis of the variations in abundance ratios from source to
source reveals that the SO and SO2 relative abundances increase with time,
while CS and OCS decrease. Molecular ratios, such as [OCS/H2S], [CS/H2S],
[SO/OCS], [SO2/OCS], [CS/SO] and [SO2/SO] may be good indicators of evolution
depending on layers probed by the observed molecular transitions. Observations
of molecular emission from warmer layers, hence involving higher upper energy
levels are mandatory to include.Comment: 24 pages, accepted for publicatio
Water in massive star-forming regions: HIFI observations of W3 IRS5
We present Herschel observations of the water molecule in the massive
star-forming region W3 IRS5. The o-H17O 110-101, p-H18O 111-000, p-H2O 22
202-111, p-H2O 111-000, o-H2O 221-212, and o-H2O 212-101 lines, covering a
frequency range from 552 up to 1669 GHz, have been detected at high spectral
resolution with HIFI. The water lines in W3 IRS5 show well-defined
high-velocity wings that indicate a clear contribution by outflows. Moreover,
the systematically blue-shifted absorption in the H2O lines suggests expansion,
presumably driven by the outflow. No infall signatures are detected. The p-H2O
111-000 and o-H2O 212-101 lines show absorption from the cold material (T ~ 10
K) in which the high-mass protostellar envelope is embedded. One-dimensional
radiative transfer models are used to estimate water abundances and to further
study the kinematics of the region. We show that the emission in the rare
isotopologues comes directly from the inner parts of the envelope (T > 100 K)
where water ices in the dust mantles evaporate and the gas-phase abundance
increases. The resulting jump in the water abundance (with a constant inner
abundance of 10^{-4}) is needed to reproduce the o-H17O 110-101 and p-H18O
111-000 spectra in our models. We estimate water abundances of 10^{-8} to
10^{-9} in the outer parts of the envelope (T < 100 K). The possibility of two
protostellar objects contributing to the emission is discussed.Comment: Accepted for publication in the A&A HIFI special issu
Water abundances in high-mass protostellar envelopes: Herschel observations with HIFI
We derive the dense core structure and the water abundance in four massive
star-forming regions which may help understand the earliest stages of massive
star formation. We present Herschel-HIFI observations of the para-H2O 1_11-0_00
and 2_02-1_11 and the para-H2-18O 1_11-0_00 transitions. The envelope
contribution to the line profiles is separated from contributions by outflows
and foreground clouds. The envelope contribution is modelled using Monte-Carlo
radiative transfer codes for dust and molecular lines (MC3D and RATRAN), with
the water abundance and the turbulent velocity width as free parameters. While
the outflows are mostly seen in emission in high-J lines, envelopes are seen in
absorption in ground-state lines, which are almost saturated. The derived water
abundances range from 5E-10 to 4E-8 in the outer envelopes. We detect cold
clouds surrounding the protostar envelope, thanks to the very high quality of
the Herschel-HIFI data and the unique ability of water to probe them. Several
foreground clouds are also detected along the line of sight. The low H2O
abundances in massive dense cores are in accordance with the expectation that
high densities and low temperatures lead to freeze-out of water on dust grains.
The spread in abundance values is not clearly linked to physical properties of
the sources.Comment: 8 pages, 5 figures, accepted for publication the 15/07/2010 by
Astronomy&Astrophysics as a letter in the Herschel-HIFI special issu
S-bearing molecules in massive dense cores
Context. Although few in number, high-mass stars play a major role in the interstellar energy budget and the shaping of the Galactic environment; however, the formation of high-mass stars is not well understood, because of their large distances, short time scales, and heavy extinction.
Aims. The chemical composition of the massive cores forming high-mass stars can put some constraints on the time scale of the massive star formation: sulfur chemistry is of specific interest thanks to its rapid evolution in warm gas and because the abundance of sulfur-bearing species increases significantly with the temperature.
Methods. Two mid-infrared quiet and two brighter massive cores were observed in various transitions (E_(up) to 289 K) of CS, OCS, H_2S, SO, and SO_2 and of their ^(34)S isotopologues at mm wavelengths with the IRAM 30m and CSO telescopes. The 1D modeling of the dust continuum is used to derive the density and temperature laws, which were then applied in the RATRAN code to modeling the observed line emission and to deriving the relative abundances of the molecules.
Results. All lines are detected, except the highest energy SO_2 transition. Infall (up to 2.9 km s^(-1)) may be detected towards the core W43MM1. The inferred mass rate is 5.8–9.4 10^(-2) M_⊙/yr. We propose an evolutionary sequence of our sources (W43MM1 → IRAS18264-1152 → IRAS05358+3543 → IRAS18162-2048), based on the SED analysis. The analysis of the variations in abundance ratios from source to source reveals that the SO and SO_2 relative abundances increase with time, while CS and OCS decrease.
Conclusions. Molecular ratios, such as [OCS/H_2S], [CS/H_2S], [SO/OCS], [SO_2/OCS], [CS/SO], and [SO_2/SO] may be good indicators of evolution, depending on layers probed by the observed molecular transitions. Observations of molecular emission from warmer layers, so that involving higher upper energy levels must be included
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