7,572 research outputs found
High angular resolution imaging of the circumstellar material around intermediate mass (IM) stars
In this Paper we present high angular resolution imaging of 3
intermediate-mass (IM) stars using the Plateau de Bure Interferometer (PdBI).
In particular we present the chemical study we have carried out towards the IM
hot core NGC 7129--FIRS 2. This is the first chemical study in an IM hot core
and provides important hints to understand the dependence of the hot core
chemistry on the stellar luminosity. We also present our high angular
resolution (0.3") images of the borderline Class 0-Class I object IC1396 N.
These images trace the warm region of this IM protostar with unprecedent detail
(0.3"\sim200 AU at the distance of IC1396 N) and provide the first detection of
a cluster of IM hot cores. Finally, we present our interferometric continuum
and spectroscopic images of the disk around the Herbig Be star R Mon. We have
determined the kinematics and physical structure of the disk associated with
this B0 star. The low spectral index derived from the dust emission as well as
the flat geometry of the disk suggest a more rapid evolution of the disks
associated with massive stars. In the Discussion, we dare to propose a possible
evolutionary sequence for the warm circumstellar material around IM stars.Comment: 4 pages, 2 figures. Proceedings of the conference "Science with ALMA:
a new era for Astrophysics" hold in Madrid in November, 13-17, 200
Detection of CO+ in the nucleus of M82
We present the detection of the reactive ion CO+ towards the prototypical
starburst galaxy M82. This is the first secure detection of this short-lived
ion in an external galaxy. Values of [CO+]/[HCO+]>0.04 are measured across the
inner 650pc of the nuclear disk of M82. Such high values of the [CO+]/[HCO+]
ratio had only been previously measured towards the atomic peak in the
reflection nebula NGC7023. This detection corroborates that the molecular gas
reservoir in the M82 disk is heavily affected by the UV radiation from the
recently formed stars. Comparing the column densities measured in M82 with
those found in prototypical Galactic photon-dominated regions (PDRs), we need
\~20 clouds along the line of sight to explain our observations. We have
completed our model of the molecular gas chemistry in the M82 nucleus. Our PDR
chemical model successfully explains the [CO+]/[HCO+] ratios measured in the
M~82 nucleus but fails by one order of magnitude to explain the large measured
CO+ column densities (~1--4x10^{13} cm^{-2}). We explore possible routes to
reconcile the chemical model and the observations.Comment: 12 pages, 2 figure
Molecular line probes of activity in galaxies
The use of specific tracers of the dense molecular gas phase can help to
explore the feedback of activity on the interstellar medium (ISM) in galaxies.
This information is a key to any quantitative assessment of the efficiency of
the star formation process in galaxies. We present the results of a survey
devoted to probe the feedback of activity through the study of the excitation
and chemistry of the dense molecular gas in a sample of local universe
starbursts and active galactic nuclei (AGNs). Our sample includes also 17
luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). From the
analysis of the LIRGs/ULIRGs subsample, published in Gracia-Carpio et al.(2007)
we find the first clear observational evidence that the star formation
efficiency of the dense gas, measured by the L_FIR/L_HCN ratio, is
significantly higher in LIRGs and ULIRGs than in normal galaxies. Mounting
evidence of overabundant HCN in active environments would even reinforce the
reported trend, pointing to a significant turn upward in the Kennicutt-Schmidt
law around L_FIR=10^11 L_sun. This result has major implications for the use of
HCN as a tracer of the dense gas in local and high-redshift luminous infrared
galaxies.Comment: 4 pages, 2 figures, contributed paper to Far-Infrared Workshop 07
(FIR 2007
Detection of CO+ toward the reflection nebula NGC 7023
We have detected CO+ toward the photon-dominated region (PDR) associated with
the reflection nebula NGC 7023. This is the first detection of CO+ in the
vicinity of a Be star. A CO+ column density of ~ 3E11 cm-2 has been derived
toward the PDR peak. We have, however, not detected CO+ in a well shielded
clump of the adjacent molecular cloud, where the CO+/HCO+ abundance ratio is at
least 100 times lower than in the PDR. Our results show, for the first time,
that CO^+ column densities as large as ~ 3E11 cm-2 can be produced in regions
with incident UV fields of just a few 1E3 (in units of Habing field) and
densities of ~ 1E5 cm-3. Furthermore, since the ionization potential of CO is
larger than 13.6 eV, our data rule out the direct photoionization of CO as a
significant CO+ formation mechanism.Comment: 14 pages, 1 Postscript figure, uses aasms4.sty. to appear in
Astrophysical Journal Letter
Herschel observations in the ultracompact HII region Mon R2: Water in dense photon-dominated regions (PDRs)
Context. Monoceros R2, at a distance of 830 pc, is the only ultracompact Hii region (UC H_(II)) where the photon-dominated region (PDR) between
the ionized gas and the molecular cloud can be resolved with Herschel. Therefore, it is an excellent laboratory to study the chemistry in extreme
PDRs (G_0 > 10^5 in units of Habing field, n > 10^6 cm^9−3)).
Aims. Our ultimate goal is to probe the physical and chemical conditions in the PDR around the UC H_(II) Mon R2.
Methods. HIFI observations of the abundant compounds ^(13)CO, C^(18)O, o-H_2^(18)O, HCO^+, CS, CH, and NH have been used to derive the physical
and chemical conditions in the PDR, in particular the water abundance. The modeling of the lines has been done with the Meudon PDR code and
the non-local radiative transfer model described by Cernicharo et al.
Results. The ^(13)CO, C^(18)O, o-H^(18)_2O, HCO^+ and CS observations are well described assuming that the emission is coming from a dense (n =
5 × 10^6 cm^(−3), N(H_2) > 10^(22) cm^(−2)) layer of molecular gas around the H_(II) region. Based on our o-H^(18)_2O observations, we estimate an o-H_2O
abundance of ≈2 × 10^(−8). This is the average ortho-water abundance in the PDR. Additional H^(18)_2O and/or water lines are required to derive the
water abundance profile. A lower density envelope (n ~ 10^5 cm^(−3), N(H_2) = 2−5 × 10^(22) 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 ^(13)CO and HCO^+ line profiles are consistent with an expansion of the molecular gas with a velocity
law, v_e = 0.5 × (r/R_(out))^(−1) km s^(−1), although the expansion velocity is poorly constrained by the observations presented here.
Conclusions. We determine an ortho-water abundance of ≈2 × 10^(−8) in Mon R2. Because shocks are unimportant in this region and our estimate is
based on H^(18)_2O observations that avoids opacity problems, this is probably the most accurate estimate of the water abundance in PDRs thus far
First evidence for dusty disks around Herbig Be stars
We have carried out a high-sensitivity search for circumstellar disks around
Herbig Be stars in the continuum at 1.4mm and 2.7mm using the IRAM
interferometer at the Plateau de Bure (PdBI) . In this letter, we report data
on three well studied B0 stars, MWC 1080, MWC 137 and R Mon. The two latter
have also been observed in the continuum at 0.7 cm and 1.3 cm using the NRAO
Very Large Array (VLA) . We report the detection of circumstellar disks around
MWC 1080 and R Mon with masses of Md ~ 0.003 and 0.01 Msun, respectively, while
for MWC 137 we estimate a disk mass upper limit of 0.007 Msun. Our results show
that the ratio Md/M* is at least an order of magnitude lower in Herbig Be stars
than in Herbig Ae and T Tauri stars.Comment: 5 pages (including figures
Diffusive Transport Enhanced by Thermal Velocity Fluctuations
We study the contribution of advection by thermal velocity fluctuations to
the effective diffusion coefficient in a mixture of two indistinguishable
fluids. The enhancement of the diffusive transport depends on the system size L
and grows as \ln(L/L_0) in quasi two-dimensional systems, while in three
dimensions it scales as L_0^{-1}-L^{-1}, where L_0 is a reference length. The
predictions of a simple fluctuating hydrodynamics theory are compared to
results from particle simulations and a finite-volume solver and excellent
agreement is observed. Our results conclusively demonstrate that the nonlinear
advective terms need to be retained in the equations of fluctuating
hydrodynamics when modeling transport in small-scale finite systems.Comment: To appear in Phys. Rev. Lett., 201
Structure and electronic properties of molybdenum monoatomic wires encapsulated in carbon nanotubes
Monoatomic chains of molybdenum encapsulated in single walled carbon
nanotubes of different chiralities are investigated using density functional
theory. We determine the optimal size of the carbon nanotube for encapsulating
a single atomic wire, as well as the most stable atomic arrangement adopted by
the wire. We also study the transport properties in the ballistic regime by
computing the transmission coefficients and tracing them back to electronic
conduction channels of the wire and the host. We predict that carbon nanotubes
of appropriate radii encapsulating a Mo wire have metallic behavior, even if
both the nanotube and the wire are insulators. Therefore, encapsulating Mo
wires in CNT is a way to create conductive quasi one-dimensional hybrid
nanostructures.Comment: 8 pages, 10 figure
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