4,119 research outputs found
Small scale structure in diffuse molecular gas from repeated FUSE and visible spectra of HD 34078
We present preliminary results from an ongoing program devoted to a study of
small scale structure in the spatial distribution of molecular gas. Our work is
based on multi-epoch FUSE and visible observations of HD34078. A detailed
comparison of H2, CH and CH+ absorption lines is performed. No short term
variations are seen (except for highly excited H2) but long-term changes in
N(CH) are clearly detected when comparing our data to spectra taken about 10
years ago.Comment: 4 pages, 2 figures, To appear in the Proceedings of the XVII IAP
Colloquium "Gaseous Matter in Galaxies and Intergalactic Space
A far UV study of interstellar gas towards HD34078: high excitation H2 and small scale structure - Based on observations performed by the FUSE mission and at the CFHT telescope
To investigate the presence of small scale structure in the spatial
distribution of H2 molecules we have undertaken repeated FUSE UV observations
of the runaway O9.5V star, HD34078. In this paper we present five spectra
obtained between January 2000 and October 2002. These observations reveal an
unexpectedly large amount of highly excited H2. Column densities for H2 levels
from (v = 0, J = 0) up to (v = 0, J = 11) and for several v = 1 and v = 2
levels are determined. These results are interpreted in the frame of a model
involving essentially two components: i) a foreground cloud (unaffected by
HD34078) responsible for the H2 (J = 0, 1), CI, CH, CH+ and CO absorptions; ii)
a dense layer of gas (n = 10E4 cm-3) close to the O star and strongly
illuminated by its UV flux which accounts for the presence of highly excited
H2. Our model successfully reproduces the H2 excitation, the CI fine-structure
level populations as well as the CH, CH+ and CO column densities. We also
examine the time variability of H2 absorption lines tracing each of these two
components. From the stability of the J = 0, 1 and 2 damped H2 profiles we
infer a 3 sigma upper limit on column density variations Delta(N(H2))/N(H2) of
5% over scales ranging from 5 to 50 AU. This result clearly rules out any
pronounced ubiquitous small scale "density" structure of the kind apparently
seen in HI. The lines from highly excited gas are also quite stable (equivalent
to Delta(N)/N <= 30%) indicating i) that the ambient gas through which HD34078
is moving is relatively uniform and ii) that the gas flow along the shocked
layer is not subject to marked instabilitie
The BinaMIcS project: understanding the origin of magnetic fields in massive stars through close binary systems
It is now well established that a fraction of the massive (M>8 Msun) star
population hosts strong, organised magnetic fields, most likely of fossil
origin. The details of the generation and evolution of these fields are still
poorly understood. The BinaMIcS project takes an important step towards the
understanding of the interplay between binarity and magnetism during the
stellar formation and evolution, and in particular the genesis of fossil
fields, by studying the magnetic properties of close binary systems. The
components of such systems are most likely formed together, at the same time
and in the same environment, and can therefore help us to disentangle the role
of initial conditions on the magnetic properties of the massive stars from
other competing effects such as age or rotation. We present here the main
scientific objectives of the BinaMIcS project, as well as preliminary results
from the first year of observations from the associated ESPaDOnS and Narval
spectropolarimetric surveys.Comment: To appear in New Windows on Massive Stars, proceedings of the IAU
Symposium 30
Herschel observations of interstellar chloronium
Using the Herschel Space Observatory's Heterodyne Instrument for the
Far-Infrared (HIFI), we have observed para-chloronium (H2Cl+) toward six
sources in the Galaxy. We detected interstellar chloronium absorption in
foreground molecular clouds along the sight-lines to the bright submillimeter
continuum sources Sgr A (+50 km/s cloud) and W31C. Both the para-H2-35Cl+ and
para-H2-37Cl+ isotopologues were detected, through observations of their
1(11)-0(00) transitions at rest frequencies of 485.42 and 484.23 GHz,
respectively. For an assumed ortho-to-para ratio of 3, the observed optical
depths imply that chloronium accounts for ~ 4 - 12% of chlorine nuclei in the
gas phase. We detected interstellar chloronium emission from two sources in the
Orion Molecular Cloud 1: the Orion Bar photodissociation region and the Orion
South condensation. For an assumed ortho-to-para ratio of 3 for chloronium, the
observed emission line fluxes imply total beam-averaged column densities of ~
2.0E+13 cm-2 and ~ 1.2E+13 cm-2, respectively, for chloronium in these two
sources. We obtained upper limits on the para-H2-35Cl+ line strengths toward H2
Peak 1 in the Orion Molecular cloud and toward the massive young star AFGL
2591. The chloronium abundances inferred in this study are typically at least a
factor ~10 larger than the predictions of steady-state theoretical models for
the chemistry of interstellar molecules containing chlorine. Several
explanations for this discrepancy were investigated, but none has proven
satisfactory, and thus the large observed abundances of chloronium remain
puzzling.Comment: Accepted for publication in the Astrophysical Journa
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
Deuterated molecular hydrogen in the Galactic ISM. New observations along seven translucent sightlines
We present column density measurements of the HD molecule in the interstellar
gas toward 17 Galactic stars. The values for the seven most heavily reddened
sightlines, with E(B-V) = 0.38-0.72, are derived from observations with the Far
Ultraviolet Spectroscopic Explorer (FUSE). The other ten values are from a
reanalysis of spectra obtained with Copernicus. In all cases, high-resolution
ground-based observations of KI and/or the CH molecule were used to constrain
the gas velocity structure and to correct for saturation effects. Comparisons
of the column densities HD, CH, CN, and KI in these 17 sightlines indicate that
HD is most tightly correlated with CH. Stringent lower limits to the
interstellar D/H ratio, derived from the HD/2H2 ratio, range from 3.7 10^(-7)
to 4.3 10^(-6). Our results also suggest that the HD/H2 ratio increases with
the molecular fraction f(H2) and that the interstellar D/H ratio might be
obtained from HD by probing clouds with f(H2) = 1. Finally, we note an apparent
relationship between the molecular fractions of hydrogen and deuterium.Comment: Accepted in A&
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
The Dark Molecular Gas
The mass of molecular gas in an interstellar cloud is often measured using
line emission from low rotational levels of CO, which are sensitive to the CO
mass, and then scaling to the assumed molecular hydrogen H_2 mass. However, a
significant H_2 mass may lie outside the CO region, in the outer regions of the
molecular cloud where the gas phase carbon resides in C or C+. Here, H_2
self-shields or is shielded by dust from UV photodissociation, where as CO is
photodissociated. This H_2 gas is "dark" in molecular transitions because of
the absence of CO and other trace molecules, and because H_2 emits so weakly at
temperatures 10 K < T < 100 K typical of this molecular component. This
component has been indirectly observed through other tracers of mass such as
gamma rays produced in cosmic ray collisions with the gas and
far-infrared/submillimeter wavelength dust continuum radiation. In this paper
we theoretically model this dark mass and find that the fraction of the
molecular mass in this dark component is remarkably constant (~ 0.3 for average
visual extinction through the cloud with mean A_V ~ 8) and insensitive to the
incident ultraviolet radiation field strength, the internal density
distribution, and the mass of the molecular cloud as long as mean A_V, or
equivalently, the product of the average hydrogen nucleus column and the
metallicity through the cloud, is constant. We also find that the dark mass
fraction increases with decreasing mean A_V, since relatively more molecular
H_2 material lies outside the CO region in this case.Comment: 38 page, 11 figures, Accepted for Publication in ApJ, corrected
citation and typo in Appendix
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