119 research outputs found
Aromatic emission from the ionised mane of the Horsehead nebula
We study the evolution of the Aromatic Infrared Bands (AIBs) emitters across
the illuminated edge of the Horsehead nebula and especially their survival and
properties in the HII region. We present spectral mapping observations taken
with the Infrared Spectrograph (IRS) at wavelengths 5.2-38 microns. A strong
AIB at 11.3 microns is detected in the HII region, relative to the other AIBs
at 6.2, 7.7 and 8.6 microns. The intensity of this band appears to be
correlated with the intensity of the [NeII] at 12.8 microns and of Halpha,
which shows that the emitters of the 11.3 microns band are located in the
ionised gas. The survival of PAHs in the HII region could be due to the
moderate intensity of the radiation field (G0 about 100) and the lack of
photons with energy above about 25eV. The enhancement of the intensity of the
11.3 microns band in the HII region, relative to the other AIBs can be
explained by the presence of neutral PAHs. Our observations highlight a
transition region between ionised and neutral PAHs observed with ideal
conditions in our Galaxy. A scenario where PAHs can survive in HII regions and
be significantly neutral could explain the detection of a prominent 11.3
microns band in other Spitzer observations.Comment: 9 pages, 9 figures, accepted for publication in A&
Density structure of the Horsehead nebula photo-dissociation region
We present high angular resolution images of the H 1-0 S(1) line emission
obtained with the Son of ISAAC (SOFI) at the New Technology Telescope (NTT) of
the Horsehead nebula. These observations are analysed in combination with
H line emission, aromatic dust, CO and dust continuum emissions. The
Horsehead nebula illuminated by the O9.5V star Ori ( 60)
presents a typical photodissociation region (PDR) viewed nearly edge-on and
offers an ideal opportunity to study the gas density structure of a PDR. The
H fluorescent emission observations reveal extremely sharp and bright
filaments associated with the illuminated edge of the nebula which spatially
coincides with the aromatic dust emission. Analysis of the H fluorescent
emission, sensitive to both the far-UV radiation field and the gas density, in
conjunction with the aromatic dust and H line emission, brings new
constraints on the illumination conditions and the gas density in the outer PDR
region. Furthermore, combination of this data with millimeter observations of
CO and dust continuum emission allows us to trace the penetration of the far-UV
radiation field into the cloud and probe the gas density structure throughout
the PDR. From comparison with PDR model calculations, we find that i) the gas
density follows a steep gradient at the cloud edge, with a scale length of 0.02
pc (or 10'') and and cm in the H emitting and
inner cold molecular layers respectively, and ii) this density gradient model
is essentially a constant pressure model, with 4 K cm. The
constraints derived here on the gas density profile are important for the study
of physical and chemical processes in PDRs and provide new insight into the
evolution of interstellar clouds.Comment: To be published in A&
A Spitzer Infrared Spectrograph Survey of Warm Molecular Hydrogen in Ultra-luminous Infrared Galaxies
We have conducted a survey of Ultra-luminous Infrared Galaxies (ULIRGs) with
the Infrared Spectrograph on the Spitzer Space Telescope, obtaining spectra
from 5.0-38.5um for 77 sources with 0.02<z <0.93. Observations of the pure
rotational H2 lines S(3) 9.67um, S(2) 12.28um, and S(1) 17.04um are used to
derive the temperature and mass of the warm molecular gas. We detect H2 in 77%
of the sample, and all ULIRGs with F(60um)>2Jy. The average warm molecular gas
mass is ~2x10^8solar-masses. High extinction, inferred from the 9.7um silicate
absorption depth, is not observed along the line of site to the molecular gas.
The derived H2 mass does not depend on F(25um)/F(60um), which has been used to
infer either starburst or AGN dominance. Similarly, the molecular mass does not
scale with the 25 or 60um luminosities. In general, the H2 emission is
consistent with an origin in photo-dissociation regions associated with star
formation. We detect the S(0) 28.22um emission line in a few ULIRGs. Including
this line in the model fits tends to lower the temperature by ~50-100K,
resulting in a significant increase in the gas mass. The presence of a cooler
component cannot be ruled out in the remainder of our sample, for which we do
not detect the S(0) line. The measured S(7) 5.51um line fluxes in six ULIRGs
implies ~3x10^6 solar-masses of hot (~1400K) H2. The warm gas mass is typically
less than 1% of the cold gas mass derived from CO observations.Comment: Accepted ApJ 01 September 2006, v648n1 issue. 14 pages 12 figures
IRAS 06361-6217 the f25/f60 ratio is 0.10 not 1.0
The structure of the protoplanetary disk surrounding three young intermediate mass stars. II. Spatially resolved dust and gas distribution
[Abridged] We present the first direct comparison of the distribution of the
gas, as traced by the [OI] 6300 AA emission, and the dust, as traced by the 10
micron emission, in the protoplanetary disk around three intermediate-mass
stars: HD 101412, HD 135344 B and HD 179218. N-band visibilities were obtained
with VLTI/MIDI. Simple geometrical models are used to compare the dust emission
to high-resolution optical spectra in the 6300 AA [OI] line of the same
targets. The disks around HD 101412 and HD 135344 B appear strongly flared in
the gas, but self-shadowed in the dust beyond ~ 2 AU. In both systems, the 10
micron emission is rather compact (< 2 AU) while the [OI] brightness profile
shows a double peaked structure. The inner peak is strongest and is consistent
with the location of the dust, the outer peak is fainter and is located at 5-10
AU. Spatially extended PAH emission is found in both disks. The disk around HD
179218 is flared in the dust. The 10 micron emission emerges from a double
ring-like structure with the first ring peaking at ~ 1 AU and the second at ~
20 AU. No dust emission is detected between ~ 3 -- 15 AU. The oxygen emission
seems also to come from a flared structure, however, the bulk of this emission
is produced between ~ 1 -- 10 AU. This could indicate a lack of gas in the
outer disk or could be due to chemical effects which reduce the abundance of OH
-- the parent molecule of the observed [OI] emission -- further away from the
star. The three systems, HD 179218, HD 135344 B and HD 101412, may form an
evolutionary sequence: the disk initially flared becomes flat under the
combined action of gas-dust decoupling, grain growth and dust settling.Comment: Accepted for publication in A&
The IRAM-30m line survey of the Horsehead PDR: I. CF+ as a tracer of C+ and a measure of the Fluorine abundance
C+ is a key species in the interstellar medium but its 158 {\mu}m fine
structure line cannot be observed from ground-based telescopes. Current models
of fluorine chemistry predict that CF+ is the second most important fluorine
reservoir, in regions where C+ is abundant. We detected the J = 1-0 and J = 2-1
rotational lines of CF+ with high signal-to-noise ratio towards the PDR and
dense core positions in the Horsehead. Using a rotational diagram analysis, we
derive a column density of N(CF+) = (1.5 - 2.0) \times 10^12 cm^-2. Because of
the simple fluorine chemistry, the CF+ column density is proportional to the
fluorine abundance. We thus infer the fluorine gas-phase abundance to be F/H =
(0.6 - 1.5) \times 10^-8. Photochemical models indicate that CF+ is found in
the layers where C+ is abundant. The emission arises in the UV illuminated skin
of the nebula, tracing the outermost cloud layers. Indeed, CF+ and C+ are the
only species observed to date in the Horsehead with a double peaked line
profile caused by kinematics. We therefore propose that CF+, which is
detectable from the ground, can be used as a proxy of the C+ layers.Comment: Accepted to A&A, 4 pages, 4 figures, 2 table
Physical structure of the photodissociation regions in NGC 7023: Observations of gas and dust emission with <i>Herschel</i>
The determination of the physical conditions in molecular clouds is a key step towards our understanding of their formation and evolution of associated star formation. We investigate the density, temperature, and column density of both dust and gas in the photodissociation regions (PDRs) located at the interface between the atomic and cold molecular gas of the NGC 7023 reflection nebula. We study how young stars affect the gas and dust in their environment. Our approach combining both dust and gas delivers strong constraints on the physical conditions of the PDRs. We find dense and warm molecular gas of high column density in the PDRs
Deuterium fractionation in the Horsehead edge
Deuterium fractionation is known to enhance the [DCO+]/[HCO+] abundance ratio
over the D/H elemental ratio of about 1e-5 in the cold and dense gas typically
found in pre-stellar cores. We report the first detection and mapping of very
bright DCO+ J=3-2 and J=2-1 lines (3 and 4 K respectively) towards the
Horsehead photodissociation region (PDR) observed with the IRAM-30m telescope.
The DCO+ emission peaks close to the illuminated warm edge of the nebula (< 50"
or about 0.1 pc away). Detailed nonlocal, non-LTE excitation and radiative
transfer analyses have been used to determine the prevailing physical
conditions and to estimate the DCO+ and H13CO+ abundances from their line
intensities. A large [DCO+]/[HCO+] abundance ratio (>= 0.02) is inferred at the
DCO+ emission peak, a condensation shielded from the illuminating far-UV
radiation field where the gas must be cold (10-20 K) and dense (>= 2x10^5
cm-3). DCO+ is not detected in the warmer photodissociation front, implying a
lower [DCO+]/[HCO+] ratio (< 1e-3). According to our gas phase chemical
predictions, such a high deuterium fractionation of HCO+ can only be explained
if the gas temperature is below 20 K, in good agreement with DCO+ excitation
calculations.Comment: 4 pages, 3 PostScript figures. Accepted for publication in Astronomy
& Astrophysics in the letter section. Uses aa LaTeX macro
Evidence for CO depletion in the inner regions of gas-rich protoplanetary disks
We investigate the physical properties and spatial distribution of Carbon
Monoxide (CO) gas in the disks around the Herbig Ae/Be stars HD 97048 and HD
100546.
Using high-spectral-resolution 4.588-4.715 m spectra containing
fundamental CO emission taken with CRIRES on the VLT, we probe the
circumstellar gas and model the kinematics of the emission lines. By using
spectro-astrometry on the spatially resolved targets, we constrain the physical
size of the emitting regions in the disks. We resolve, spectrally and
spatially, the emission of the CO v(1-0) vibrational band and the
CO and vibrational bands in both targets,
as well as the CO band in HD 100546. Modeling of the CO emission
with a homogeneous disk in Keplerian motion, yields a best fit with an inner
and outer radius of the CO emitting region of 11 and 100 AU for HD
97048. HD 100546 is not fit well with our model, but we derive a lower limit on
the inner radius of 8 AU. The fact that gaseous [OI] emission was previously
detected in both targets at significantly smaller radii suggests that CO may be
effectively destroyed at small radii in the surface layers of these disksComment: v2: Letter format has been changed to Paper format; Change in the
focus of the paper towards CO depletion; Major changes in text; Change of
title. Submitted to A&A, 14/10/2008. Accepted by A&A, 17/04/200
H2 formation on interstellar dust grains: the viewpoints of theory, experiments, models and observations
Molecular hydrogen is the most abundant molecule in the universe. It is the first one to form and survive photo-dissociation in tenuous environments. Its formation involves catalytic reactions on the surface of interstellar grains. The micro-physics of the formation process has been investigated intensively in the last 20 years, in parallel of new astrophysical observational and modeling progresses. In the perspectives of the probable revolution brought by the future satellite JWST, this article has been written to present what we think we know about the H formation in a variety of interstellar environments.VW’s research is funded by an ERC Starting Grant (3DICE, grant agreement 336474). GV acknowledges financial support from the National Science Foundation’s Astronomy & Astrophysics Division (Grants No. 1311958 and 1615897). LH acknowledges support from ERC Consolidator Grant GRANN (grant agreement no. 648551). GN acknowledges support from the Swedish Research Council. VW, FD and SM acknowledge the CNRS program ”Physique et Chimie du Milieu Interstellaire” (PCMI) co-funded bythe Centre National d’Etudes Spatiales (CNES). SDP acknowledges funding from STFC, UK. V.V acknowledges funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (MagneticYSOS project, grant agreement No 679937)
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