150 research outputs found
Evaporating Very Small Grains as tracers of the UV radiation field in Photo-dissociation Regions
Context. In photo-dissociation regions (PDRs), Polycyclic Aromatic
Hydrocarbons (PAHs) could be produced by evaporation of Very Small Grains
(VSGs) by the impinging UV radiation field from a nearby star. Aims. We
investigate quantitatively the transition zone between evaporating Very Small
Grains (eVSGs) and PAHs in several PDRs. Methods. We study the relative
contribution of PAHs and eVSGs to the mid-IR emission in a wide range of
excitation conditions. We fit the observed mid-IR emission of PDRs by using a
set of template band emission spectra of PAHs, eVSGs and gas lines. The fitting
tool PAHTAT (PAH Toulouse Astronomical Templates) is made available to the
community as an IDL routine. From the results of the fit, we derive the
fraction of carbon f_eVSG locked in eVSGs and compare it to the intensity of
the local UV radiation field. Results. We show a clear decrease of f_eVSG with
increasing intensity of the local UV radiation field, which supports the
scenario of photo-destruction of eVSGs. Conversely, this dependence can be used
to quantify the intensity of the UV radiation field for different PDRs,
including non resolved ones. Conclusions. PAHTAT can be used to trace the
intensity of the local UV radiation field in regions where eVSGs evaporate,
which correspond to relatively dense (nH = [100, 10^5 ] cm-3) and UV irradiated
PDRs (G0 = [100, 5x10^4]) where H2 emits in rotational lines.Comment: 13 pages, 11 figures. Accepted for publication in A&A. Typos
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Kinematics of the ionized-to-neutral interfaces in Monoceros R2
Context. Monoceros R2 (Mon R2), at a distance of 830 pc, is the only
ultra-compact H ii region (UC H ii) where its associated photon-dominated
region (PDR) can be resolved with the Herschel Space Observatory. Aims. Our aim
is to investigate observationally the kinematical patterns in the interface
regions (i.e., the transition from atomic to molecular gas) associated with Mon
R2. Methods. We used the HIFI instrument onboard Herschel to observe the line
profiles of the reactive ions CH+, OH+ and H2O+ toward different positions in
Mon R2. We derive the column density of these molecules and compare them with
gas-phase chemistry models. Results. The reactive ion CH+ is detected both in
emission (at central and red-shifted velocities) and in absorption (at
blue-shifted velocities). OH+ is detected in absorption at both blue- and
red-shifted velocities, with similar column densities. H2O+ is not detected at
any of the positions, down to a rms of 40 mK toward the molecular peak. At this
position, we find that the OH+ absorption originates in a mainly atomic medium,
and therefore is associated with the most exposed layers of the PDR. These
results are consistent with the predictions from photo-chemical models. The
line profiles are consistent with the atomic gas being entrained in the ionized
gas flow along the walls of the cavity of the H ii region. Based on this
evidence, we are able to propose a new geometrical model for this region.
Conclusions. The kinematical patterns of the OH+ and CH+ absorption indicate
the existence of a layer of mainly atomic gas for which we have derived, for
the first time, some physical parameters and its dynamics.Comment: 6 pages, 5 figures. Accepted for publication in A&
Star Formation Near Photodissociation Regions: Detection of a Peculiar Protostar Near Ced 201
We present the detection and characterization of a peculiar low-mass
protostar (IRAS 22129+7000) located ~0.4 pc from Ced 201 Photodissociation
Region (PDR) and ~0.2 pc from the HH450 jet. The cold circumstellar envelope
surrounding the object has been mapped through its 1.2 mm dust continuum
emission with IRAM-30m/MAMBO. The deeply embedded protostar is clearly detected
with Spitzer/MIPS (70 um), IRS (20-35 um) and IRAC (4.5, 5.8, and 8 um) but
also in the K_s band (2.15 um). Given the large "near- and mid-IR excess" in
its spectral energy distribution, but large submillimeter-to-bolometric
luminosity ratio (~2%), IRAS 22129+7000 must be a transition Class 0/I source
and/or a multiple stellar system. Targeted observations of several molecular
lines from CO, 13CO, C18O, HCO+ and DCO+ have been obtained. The presence of a
collimated molecular outflow mapped with the CSO telescope in the CO J=3-2 line
suggests that the protostar/disk system is still accreting material from its
natal envelope. Indeed, optically thick line profiles from high density tracers
such as HCO+ J=1-0 show a red-shifted-absorption asymmetry reminiscent of
inward motions. We construct a preliminary physical model of the circumstellar
envelope (including radial density and temperature gradients, velocity field
and turbulence) that reproduces the observed line profiles and estimates the
ionization fraction. The presence of both mechanical and (non-ionizing)
FUV-radiative input makes the region an interesting case to study triggered
star formation
Mid-infrared PAH and H2 emission as a probe of physical conditions in extreme PDRs
Mid-infrared (IR) observations of polycyclic aromatic hydrocarbons (PAHs) and
molecular hydrogen emission are a potentially powerful tool to derive physical
properties of dense environments irradiated by intense UV fields. We present
new, spatially resolved, \emph{Spitzer} mid-IR spectroscopy of the high
UV-field and dense photodissocation region (PDR) around Monoceros R2, the
closest ultracompact \hII region, revealing the spatial structure of ionized
gas, PAHs and H emissions. Using a PDR model and PAH emission feature
fitting algorithm, we build a comprehensive picture of the physical conditions
prevailing in the region. We show that the combination of the measurement of
PAH ionization fraction and of the ratio between the H 0-0 S(3) and S(2)
line intensities, respectively at 9.7 and 12.3 m, allows to derive the
fundamental parameters driving the PDR: temperature, density and UV radiation
field when they fall in the ranges K, cm,
respectively. These mid-IR spectral tracers thus provide a tool
to probe the similar but unresolved UV-illuminated surface of protoplanetary
disks or the nuclei of starburst galaxies.Comment: Accepted for publication in ApJ Letter
Deuteration around the ultracompact HII region Mon R2
The massive star-forming region Mon R2 hosts the closest ultra-compact HII
region that can be spatially resolved with current single-dish telescopes. We
used the IRAM-30m telescope to carry out an unbiased spectral survey toward two
important positions (namely IF and MP2), in order to studying the chemistry of
deuterated molecules toward Mon R2. We found a rich chemistry of deuterated
species at both positions, with detections of C2D, DCN, DNC, DCO+, D2CO, HDCO,
NH2D, and N2D+ and their corresponding hydrogenated species and isotopologs.
Our high spectral resolution observations allowed us to resolve three velocity
components: the component at 10 km/s is detected at both positions and seems
associated with the layer most exposed to the UV radiation from IRS 1; the
component at 12 km/s is found toward the IF position and seems related to the
molecular gas; finally, a component at 8.5 km/s is only detected toward the MP2
position, most likely related to a low-UV irradiated PDR. We derived the column
density of all the species, and determined the deuterium fractions (Dfrac). The
values of Dfrac are around 0.01 for all the observed species, except for HCO+
and N2H+ which have values 10 times lower. The values found in Mon R2 are well
explained with pseudo-time-dependent gas-phase model in which deuteration
occurs mainly via ion-molecule reactions with H2D+, CH2D+ and C2HD+. Finally,
the [H13CN]/[HN13C] ratio is very high (~11) for the 10 km/s component, which
also agree with our model predictions for an age of ~0.01-0.1 Myr. The
deuterium chemistry is a good tool for studying star-forming regions. The
low-mass star-forming regions seem well characterized with Dfrac(N2H+) or
Dfrac(HCO+), but it is required a complete chemical modeling to date massive
star-forming regions, because the higher gas temperature together with the
rapid evolution of massive protostars.Comment: 14 pages of manuscript, 17 pages of apendix, 7 figures in the main
text, accepted for publication in A&
Spatial distribution of small hydrocarbons in the neighborhood of the Ultra Compact HII region Monoceros R2
We study the chemistry of small hydrocarbons in the photon-dominated regions
(PDRs) associated with the ultra-compact HII region Mon R2. Our goal is to
determine the variations of the abundance of small hydrocarbons in a high-UV
irradiated PDR and investigate their chemistry. We present an observational
study of CH, CCH and c-CH in Mon R2 combining data obtained with the
IRAM 30m telescope and Herschel. We determine the column densities of these
species, and compare their spatial distributions with that of polycyclic
aromatic hydrocarbon (PAH). We compare the observational results with different
chemical models to explore the relative importance of gas-phase, grain-surface
and time-dependent chemistry in these environments. The emission of the small
hydrocarbons show different patterns. The CCH emission is extended while CH and
c-CH are concentrated towards the more illuminated layers of the PDR.
The ratio of the column densities of c-CH and CCH shows spatial
variations up to a factor of a few, increasing from
_3_2 in the envelope to a maximum of
towards the 8m emission peak. Comparing these results
with other galactic PDRs, we find that the abundance of CCH is quite constant
over a wide range of G, whereas the abundance of c-CH is higher in
low-UV PDRs. In Mon R2, the gas-phase steady-state chemistry can account
relatively well for the abundances of CH and CCH in the most exposed layers of
the PDR, but falls short by a factor of 10 to reproduce c-CH.
In the molecular envelope, time-dependent effects and grain surface chemistry
play a dominant role in determining the hydrocarbons abundances. Our study
shows that CCH and c-CH present a complex chemistry in which UV
photons, grain-surface chemistry and time dependent effects contribute to
determine their abundances.Comment: 18 pages, 11 figures, 7 tables. Proposed for acceptance in A&A.
Abstract abridge
The Role of Canyons in Strata Formation
This paper provides a spatial and temporal multi-scale approach of European submarine canyons. We fi rst present the long-term geologic view of European margins as related to controls on submarine canyon development. Then we discuss the extent to which submarine canyon systems resemble river systems because both essentially form drainage networks. Finally, we deal with the hortest-term, highestresolution scale to get a fl avor of the current functioning and health of modern submarine canyons in the northwestern Mediterranean Sea. Submarine canyons are unique features of the seafl oor whose existence was known by European fi shermen centuries ago, especially for those canyons that have their heads at short distance from shoreline. Popular names given to specifi c canyons in the different languages spoken in European coastal communities refer to the concepts of a"deep" or"trench." In the old times it was also common thinking that submarine canyons where so deep that nobody could measure their depth or even that they had no bottom. Submarine canyons are just one of the seven different types of seafl oor valleys identifi ed by Shepard (1973) in his pioneering morphogenetic classifi cation. Shepard (1973) defined submarine canyons as"steep-walled, sinuous valleys, with V-shaped cross sections, and relief comparable even to the largest of land canyons; tributaries are found in most of the canyons and rock outcrops abound on their walls." Canyons are features typical of continental slopes with their upper reaches and heads cut into the continental shelf
Herschel / HIFI observations of CO, H2O and NH3 in Mon R2
Context. Mon R2 is the only ultracompact HII region (UCHII) where the
associated photon-dominated region (PDR) can be resolved with Herschel. Due to
its brightness and proximity, it is the best source to investigate the
chemistry and physics of highly UV-irradiated PDRs. Aims. Our goal is to
estimate the abundance of H2O and NH3 in this region and investigate their
origin. Methods. We present new observations obtained with HIFI and the
IRAM-30m telescope. Using a large velocity gradient approach, we model the line
intensities and derive an average abundance of H2O and NH3 across the region.
Finally, we model the line profiles with a non-local radiative transfer model
and compare these results with the abundance predicted by the Meudon PDR code.
Results. The variations of the line profiles and intensities indicate complex
geometrical and kinematical patterns. The H2O lines present a strong absorption
at the ambient velocity and emission in high velocity wings towards the HII
region. The spatial distribution of the o-H2^18O line shows that the its
emission arises in the PDR surrounding the HII region. By modeling the o-H2^18O
emission we derive a mean abundance of o-H2O of ~10^-8 relative to H2. The
ortho-H2O abundance is however larger, ~1x10^-7, in the high velocity wings.
Possible explanations for this larger abundance include an expanding hot PDR
and/or an outflow. Ammonia seems to be present only in the envelope with an
average abundance of ~2x10^-9 relative to H2. Conclusions. The Meudon PDR code
can account for the measured water abundance in the high velocity gas as long
as we assume that it originates from a <1 mag hot expanding layer of the PDR,
i.e. that the outflow has only a minor contribution to this emission. To
explain the abundances in the rest of the cloud the molecular freeze out and
grain surface chemistry would need to be included.Comment: 12 pages, 7 figures, 3 tables. Accepted for publication in A&A.
Abstract shortened. Updated references, language editing applied in v
Gas morphology and energetics at the surface of PDRs: new insights with Herschel observations of NGC 7023
We investigate the physics and chemistry of the gas and dust in dense
photon-dominated regions (PDRs), along with their dependence on the
illuminating UV field. Using Herschel-HIFI observations, we study the gas
energetics in NGC 7023 in relation to the morphology of this nebula. NGC 7023
is the prototype of a PDR illuminated by a B2V star and is one of the key
targets of Herschel. Our approach consists in determining the energetics of the
region by combining the information carried by the mid-IR spectrum (extinction
by classical grains, emission from very small dust particles) with that of the
main gas coolant lines. In this letter, we discuss more specifically the
intensity and line profile of the 158 micron (1901 GHz) [CII] line measured by
HIFI and provide information on the emitting gas. We show that both the [CII]
emission and the mid-IR emission from polycyclic aromatic hydrocarbons (PAHs)
arise from the regions located in the transition zone between atomic and
molecular gas. Using the Meudon PDR code and a simple transfer model, we find
good agreement between the calculated and observed [CII] intensities. HIFI
observations of NGC 7023 provide the opportunity to constrain the energetics at
the surface of PDRs. Future work will include analysis of the main coolant line
[OI] and use of a new PDR model that includes PAH-related species.Comment: Accepted for publication in Astronomy and Astrophysics Letters
(Herschel HIFI special issue), 5 pages, 5 figure
Spectral line survey of the ultracompact HII region Mon R2
Ultracompact (UC) HII regions constitute one of the earliest phases in the
formation of a massive star and are characterized by extreme physical
conditions (Go>10^5 Habing field and n>10^6 cm^-3). The UC HII Mon R2 is the
closest one and therefore an excellent target to study the chemistry in these
complex regions.
We carried out a 3mm and 1mm spectral survey using the IRAM 30-m telescope
towards three positions that represent different physical environments in Mon
R2: (i) the ionization front (IF) at (0",0"); two peaks in the molecular cloud
(ii) MP1 at the offset (+15",-15") and (iii) MP2 at the farther offset
(0",40"). In addition, we carried out extensive modeling to explain the
chemical differences between the three observed regions.
We detected more than thirty different species. We detected SO+ and C4H
suggesting that UV radiation plays an important role in the molecular chemistry
of this region. We detected the typical PDR molecules CN, HCN, HCO, C2H, and
c-C3H2. While the IF and the MP1 have a chemistry similar to that found in high
UV field and dense PDRs like the Orion Bar, the MP2 is more similar to lower
UV/density PDRs like the Horsehead nebula.
We also detected complex molecules that are not usually found in PDRs (CH3CN,
H2CO, HC3N, CH3OH and CH3C2H). Sulfur compounds CS, HCS+, C2S, H2CS, SO and SO2
and the deuterated species DCN and C2D were also identified. [DCN]/[HCN]=0.03
and [C2D]/[C2H]=0.05, are among the highest in warm regions.
Our results show that the high UV/dense PDRs present a different chemistry
from that of the low UV case. Abundance ratios like [CO+]/[HCO+] or
[HCO]/[HCO+] are good diagnostics to differentiate between them. In Mon R2 we
have the two classes of PDRs, a high UV PDR towards the IF and the adjacent
molecular bar and a low-UV PDR which extends towards the north-west following
the border of the cloud.Comment: 31 page
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