332 research outputs found
Photoelectric effect on dust grains across the L1721 cloud in the rho Ophiuchi molecular complex
We present ISO-LWS measurements of the main gas cooling lines, C+ 158 mum and
O 63 mum towards a moderate opacity molecular cloud (Av=3), L1721, illuminated
by the B2 star nu Sco (X = 5-10). These data are combined with an extinction
map and IRAS dust emission images to test our understanding of gas heating and
cooling in photo-dissociation regions (PDRs). This nearby PDR is spatially
resolved in the IRAS images; variations in the IRAS colors across the cloud
indicate an enhanced abundance of small dust grains within the PDR. A spatial
correlation between the gas cooling lines and the infrared emission from small
dust grains illustrates the dominant role of small dust grains in the gas
heating through the photoelectric effect. The photoelectric efficiency,
determined from the observations by ratioing the power radiated by gas and
small dust grains, is in the range 2 to 3% in close agreement with recent
theoretical estimates. The brightness profiles across the PDR in the C+ 158 mum
and O 63 mum lines are compared with model calculations where the density
profile is constrained by the extinction data and where the gas chemical and
thermal balances are solved at each position. We show that abundance variations
of small dust grains across the PDR must be considered to account for the LWS
observations.Comment: 10 pages, 15 figure
Multi-wavelength observations of a nearby multi-phase interstellar cloud
High-resolution spectroscopic observations (UV HST/STIS and optical) are used
to characterize the physical state and velocity structure of the multiphase
interstellar medium seen towards the nearby (170 pc) star HD102065, located
behind the tail of a cometary-shaped, infrared cirrus-cloud, in the area of
interaction between the Sco-Cen OB association and the Local Bubble. We analyze
interstellar components present along the line of sight by fitting multiple
transitions from CO, CH, CH+, C I, S I, Fe I, Mg I, Mg II, Mn II, P II, Ni II,
C II, N I, O I, Si III, C IV, and Si IV. The absorption spectra are
complemented by H I, CO and C II emission-line spectra, H column-densities
derived from FUSE spectra, and IRAS images. Gas components of a wide range of
temperatures and ionization states are detected along the line of sight. Most
of the hydrogen column-density is in cold, diffuse, molecular gas at low LSR
velocity. This gas is mixed with traces of warmer molecular gas traced by H2 in
the J>2 levels, in which the observed CH+ must be formed. We also identify
three distinct components of warm gas at negative velocities down to -20 km/s.
The temperature and gas excitation are shown to increase with increasing
velocity shift from the bulk of the gas. Hot gas at temperatures of several
10^5 K is detected in the most negative velocity component in the
highly-ionized species. This hot gas is also detected in very strong lines of
less-ionized species (Mg II, Si II* and C II*) for which the bulk of the gas is
cooler. We relate the observational results to evidence for dynamical impact of
the Sco-Cen stellar association on the nearby ISM. We propose a scenario where
the cirrus cloud has been hit a few 10^5 yr ago by a supernova blast wave
originating from the Lower Centaurus Crux group of the Sco-Cen association.Comment: 16 pages, 9 figures, to be published in A&
Modeling of diffuse molecular gas applied to HD 102065 observations
Aims. We model a diffuse molecular cloud present along the line of sight to
the star HD 102065. We compare our modeling with observations to test our
understanding of physical conditions and chemistry in diffuse molecular clouds.
Methods. We analyze an extensive set of spectroscopic observations which
characterize the diffuse molecular cloud observed toward HD 102065. Absorption
observations provide the extinction curve, H2, C I, CO, CH, and CH+ column
densities and excitation. These data are complemented by observations of CII,
CO and dust emission. Physical conditions are determined using the Meudon PDR
model of UV illuminated gas. Results. We find that all observational results,
except column densities of CH, CH+ and H2 in its excited (J > 2) levels, are
consistent with a cloud model implying a Galactic radiation field (G~0.4 in
Draine's unit), a density of 80 cm-3 and a temperature (60-80 K) set by the
equilibrium between heating and cooling processes. To account for excited (J
>2) H2 levels column densities, an additional component of warm (~ 250K) and
dense (nH>10^4 cm-3) gas within 0.03 pc of the star would be required. This
solution reproduces the observations only if the ortho-to-para H2 ratio at
formation is 1. In view of the extreme physical conditions and the unsupported
requirement on the ortho-to-para ratio, we conclude that H2 excitation is most
likely to be accounted for by the presence of warm molecular gas within the
diffuse cloud heated by the local dissipation of turbulent kinetic energy. This
warm H2 is required to account for the CH+ column density. It could also
contribute to the CH abundance and explain the inhomogeneity of the CO
abundance indicated by the comparison of absorption and emission spectra.Comment: 10 pages, 17 figures. Accepted for publication in Astronomy and
Astrophysics. Typos correcte
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
Some empirical estimates of the H2 formation rate in photon-dominated regions
We combine recent ISO observations of the vibrational ground state lines of
H2 towards Photon-Dominated Regions (PDRs) with observations of vibrationally
excited states made with ground-based telescopes in order to constrain the
formation rate of H2 on grain surfaces under the physical conditions in the
layers responsible for H2 emission. We use steady state PDR models in order to
examine the sensitivity of different H2 line ratios to the H2 formation rate
Rf. We show that the ratio of the 0-0 S(3) to the 1-0 S(1) line increases with
Rf but that one requires independent estimates of the radiation field incident
upon the PDR and the density in order to infer Rf from the H2 line data. We
confirm the earlier result of Habart et al. (2003) that the H2 formation rate
in regions of moderate excitation such as Oph W, S140 and IC 63 is a factor of
5 times larger than the standard rate inferred from UV observations of diffuse
clouds. On the other hand, towards regions of higher radiation field such as
the Orion Bar and NGC 2023, we derive H2 formation rates consistent with the
standard value. We find also a correlation between the H2 1-0 S(1) line and PAH
emission suggesting that Rf scales with the PAH abundance. With the aim of
explaining these results, we consider some empirical models of the H2 formation
process. Here we consider both formation on big (a~0.1 microns) and small (a~10
Angstroms) grains by either direct recombination from the gas phase or
recombination of physisorbed H atoms with atoms in a chemisorbed site. We
conclude that indirect chemisorption is most promising in PDRs. Moreover small
grains which dominate the total grain surface and spend most of their time at
relatively low temperatures may be the most promising surface for forming H2 in
PDRs.Comment: A&A in press, 16 pages, 5 figure
Detection of Powerful Mid-IR H_2 Emission in the Bridge between the Taffy Galaxies
We report the detection of strong, resolved emission from warm H_2 in the Taffy galaxies and bridge. Relative to the continuum and faint polyclic aromatic hydrocarbon (PAH) emission, the H_2 emission is the strongest in the connecting bridge, approaching L(H_2)/L(PAH 8 μm) = 0.1 between the two galaxies, where the purely rotational lines of H_2 dominate the mid-infrared spectrum in a way very reminiscent of the group-wide shock in the interacting group Stephan's Quintet (SQ). The surface brightness in the 0-0 S(0) and S(1) H_2 lines in the bridge is more than twice that observed at the center of the SQ shock. We observe a warm H2 mass of 4.2 × 10^8 M_☉ in the bridge, but taking into account the unobserved bridge area, the total warm mass is likely to be twice this value. We use excitation diagrams to characterize the warm molecular gas, finding an average surface mass of ~5 × 10^6 M_☉ kpc^(–2) and typical excitation temperatures of 150-175 K. H_2 emission is also seen in the galaxy disks, although there the emission is more consistent with normal star-forming galaxies. We investigate several possible heating mechanisms for the bridge gas but favor the conversion of kinetic energy from the head-on collision via turbulence and shocks as the main heating source. Since the cooling time for the warm H_2 is short (~5000 yr), shocks must be permeating the molecular gas in the bridge region in order to continue heating the H_2
Excitation of H in photodissociation regions as seen by Spitzer
We present spectroscopic observations obtained with the infrared Spitzer
Space Telescope, which provide insight into the H physics and gas
energetics in photodissociation Regions (PDRs) of low to moderate
far-ultraviolet (FUV) fields and densities. We analyze data on six well known
Galactic PDRs (L1721, California, N7023E, Horsehead, rho Oph, N2023N), sampling
a poorly explored range of excitation conditions (), relevant
to the bulk of molecular clouds in galaxies. Spitzer observations of H
rotational lines are complemented with H data, including ro-vibrational
line measurements, obtained with ground-based telescopes and ISO, to constrain
the relative contributions of ultraviolet pumping and collisions to the H
excitation. The data analysis is supported by model calculations with the
Meudon PDR code. The observed column densities of rotationally excited H
are observed to be much higher than PDR model predictions. In the lowest
excitation PDRs, the discrepancy between the model and the data is about one
order of magnitude for rotational levels 3. We discuss whether an
enhancement in the H formation rate or a local increase in photoelectric
heating, as proposed for brighter PDRs in former ISO studies, may improve the
data-model comparison. We find that an enhancement in the H formation rates
reduces the discrepancy, but the models still fall short of the data. This
large disagreement suggests that our understanding of the formation and
excitation of H and/or of PDRs energetics is still incomplete. We discuss
several explanations, which could be further tested using the Herschel Space
TelescopeComment: A&A in pres
A Spitzer high resolution mid-infrared spectral atlas of starburst galaxies
We present an atlas of Spitzer/IRS high resolution (R~600) 10-37um spectra
for 24 well known starburst galaxies. The spectra are dominated by
fine-structure lines, molecular hydrogen lines, and emission bands of
polycyclic aromatic hydrocarbons. Six out of the eight objects with a known AGN
component show emission of the high excitation [NeV] line. This line is also
seen in one other object (NGC4194) with, a priori, no known AGN component. In
addition to strong polycyclic aromatic hydrocarbon emission features in this
wavelength range (11.3, 12.7, 16.4um), the spectra reveal other weak
hydrocarbon features at 10.6, 13.5, 14.2um, and a previously unreported
emission feature at 10.75um. An unidentified absorption feature at 13.7um is
detected in many of the starbursts. We use the fine-structure lines to derive
the abundance of neon and sulfur for 14 objects where the HI 7-6 line is
detected. We further use the molecular hydrogen lines to sample the properties
of the warm molecular gas. Several basic diagrams characterizing the properties
of the sample are also shown. We have combined the spectra of all the pure
starburst objects to create a high S/N template, which is available to the
community.Comment: 25 pages (emulate apj), 6 tables, 14 figures, Accepted for
publication in ApJ
The Science Case for PILOT I: Summary and Overview
Original article can be found at: http://www.publish.csiro.au/?nid=139&aid=108 DOI: 10.1071/AS08048 [Open access article]PILOT (the Pathfinder for an International Large Optical Telescope) is a proposed 2.5-m optical/infrared telescope to be located at Dome C on the Antarctic plateau. Conditions at Dome C are known to be exceptional for astronomy. The seeing (above ∼30 m height), coherence time, and isoplanatic angle are all twice as good as at typical mid-latitude sites, while the water-vapour column, and the atmosphere and telescope thermal emission are all an order of magnitude better. These conditions enable a unique scientific capability for PILOT, which is addressed in this series of papers. The current paper presents an overview of the optical and instrumentation suite for PILOT and its expected performance, a summary of the key science goals and observational approach for the facility, a discussion of the synergies between the science goals for PILOT and other telescopes, and a discussion of the future of Antarctic astronomy. Paper II and Paper III present details of the science projects divided, respectively, between the distant Universe (i.e. studies of first light, and the assembly and evolution of structure) and the nearby Universe (i.e. studies of Local Group galaxies, the Milky Way, and the Solar System).Peer reviewe
Herschel observations in the ultracompact HII region Mon R2: Water in dense Photon-dominated regions (PDRs)
Mon R2, at a distance of 830 pc, is the only ultracompact HII region (UC HII)
where the photon-dominated region (PDR) between the ionized gas and the
molecular cloud can be resolved with Herschel. HIFI observations of the
abundant compounds 13CO, C18O, o-H2-18O, HCO+, CS, CH, and NH have been used to
derive the physical and chemical conditions in the PDR, in particular the water
abundance. The 13CO, C18O, o-H2-18O, HCO+ and CS observations are well
described assuming that the emission is coming from a dense (n=5E6 cm-3,
N(H2)>1E22 cm-2) layer of molecular gas around the UC HII. Based on our
o-H2-18O observations, we estimate an o-H2O abundance of ~2E-8. This is the
average ortho-water abundance in the PDR. Additional H2-18O and/or water lines
are required to derive the water abundance profile. A lower density envelope
(n~1E5 cm-3, N(H2)=2-5E22 cm-2) is responsible for the absorption in the NH
1_1-0_2 line. The emission of the CH ground state triplet is coming from both
regions with a complex and self-absorbed profile in the main component. The
radiative transfer modeling shows that the 13CO and HCO+ line profiles are
consistent with an expansion of the molecular gas with a velocity law, v_e =0.5
x (r/Rout)^{-1} km/s, although the expansion velocity is poorly constrained by
the observations presented here.Comment: 4 pages, 5 figure
- …