596 research outputs found
Dark gas in the solar neighnorhood from extinction data
When modeling infrared or gamma-ray data as a linear combination of observed
gas tracers, excess emission has been detected compared to expectations from
known neutral and atomic gas as traced by HI and CO measurements, respectively.
This excess might correspond to an additional gas component. This so-called
"dark gas" (DG) has been observed in our Galaxy, as well as the Magellanic
Clouds. For the first time, we investigate the correlation between visible
extinction (Av) data and gas tracers on large scales in the solar neighborhood.
Our work focuses on both the solar neighborhood (|b|>10\degr), and the inner
and outer Galaxy, as well as on four individual regions: Taurus, Orion,
Cepheus-Polaris and Aquila-Ophiuchus. Thanks to the recent production of an
all-sky Av map, we first perform the correlation between Av and both HI and CO
emission over the most diffuse regions, to derive the optimal (Av/NH)^(ref)
ratio. We then iterate the analysis over the entire regions to estimate the
CO-to-H2 conversion factor as well as the DG mass fraction. The average
extinction to gas column-density ratio in the solar neighborhood is found to be
(Av/NH)^(ref)=6.53 10^(-22) mag. cm^2, with significant differences between the
inner and outer Galaxy. We derive an average XCO value of 1.67 10^(20) H2
cm^(-2)/(K km s^(-1)). In the solar neighborhood, the gas mass in the dark
component is found to be 19% relative to that in the atomic component and
164 relative to the one traced by CO. These results are compatible with the
recent analysis using Planck data within the uncertainties of our measurements.
We estimate the ratio of dark gas to total molecular gas to be 0.62 in the
solar neighborhood. The HI-to-H2 and H2-to-CO transitions appear for Av
0.2 mag and Av mag, respectively, in agreement with
theoretical models of dark-H2 gas.Comment: 9 pages, 4 figures, 1 table. Accepted for publication in A&A (in
press
Extinction and dust/gas ratio in LMC molecular clouds
Aims. The goal of this paper is to measure the dust content and distribution in the Large Magellanic Cloud (LMC) by comparing extinction maps produced in the near-infrared wavelengths and the spatial distribution of the neutral and molecular gas, as traced by Hi and CO observations.
Methods. In order to derive an extinction map of the LMC, we have developed a new method to measure the color excess of dark clouds, using the 2MASS all-sky survey. Classical methods to measure the color excess (including the NICE method) tend to underestimate the true color excess if the clouds are significantly contaminated by unreddened foreground stars, as is the case in the LMC. We propose a new method that uses the color of the X percentile reddest stars and which is robust against such contamination. Using this method, it is possible to infer the positions of dark clouds with respect to the star distribution by comparing the observed color excess as a function of the percentile used and that predicted by a model.
Results. On the basis of the resulting extinction map, we perform a correlation analysis for a set of dark molecular clouds. Assuming similar infrared absorption properties for the dust in the neutral and molecular phases, we derive the absorption-to-column density ratio AV/NH and the CO-to-H2 conversion factor X_(CO). We show that AV/NH increases from the outskirts of the LMC towards the 30 Dor star-forming region. This can be explained either by a systematic increase of the dust abundance, or by the presence of an additional gas component not traced by Hi or CO, but strongly correlated to the Hi distribution. If dust abundance is allowed to vary, the derived X_(CO) factors for the selected regions are several times lower than those derived from a virial analysis of the CO data. This could indicate that molecular clouds in the LMC are not gravitationally bound, or that they are bounded by substantial external pressure. However, the X_(CO) values derived from absorption can be reconciled with the virial results assuming a constant value for the dust abundance and the existence of an additional, unseen gas component. These results are in agreement with those derived for the LMC from diffuse far-infrared emission
The nature of the dense core population in the pipe nebula: core and cloud kinematics from C18O observations
We present molecular-line observations of 94 dark cloud cores identified in
the Pipe nebula through near-IR extinction mapping. Using the Arizona Radio
Observatory 12m telescope, we obtained spectra of these cores in the J=1-0
transition of C18O. We use the measured core parameters, i.e., antenna
temperature, linewidth, radial velocity, radius and mass, to explore the
internal kinematics of these cores as well as their radial motions through the
larger molecular cloud. We find that the vast majority of the dark extinction
cores are true cloud cores rather than the superposition of unrelated
filaments. While we identify no significant correlations between the core's
internal gas motions and the cores' other physical parameters, we identify
spatially correlated radial velocity variations that outline two main kinematic
components of the cloud. The largest is a 15pc long filament that is
surprisingly narrow both in spatial dimensions and in radial velocity.
Beginning in the Stem of the Pipe, this filament displays uniformly small C18O
linewidths (dv~0.4kms-1) as well as core to core motions only slightly in
excess of the gas sound speed. The second component outlines what appears to be
part of a large (2pc; 1000 solar mass) ring-like structure. Cores associated
with this component display both larger linewidths and core to core motions
than in the main cloud. The Pipe Molecular Ring may represent a primordial
structure related to the formation of this cloud.Comment: Accepted to ApJ. 14 pages, 11 figures. Complete table at end of
documen
Existence of Dynamical Scaling in the Temporal Signal of Time Projection Chamber
The temporal signals from a large gas detector may show dynamical scaling due
to many correlated space points created by the charged particles while passing
through the tracking medium. This has been demonstrated through simulation
using realistic parameters of a Time Projection Chamber (TPC) being fabricated
to be used in ALICE collider experiment at CERN. An interesting aspect of this
dynamical behavior is the existence of an universal scaling which does not
depend on the multiplicity of the collision. This aspect can be utilised
further to study physics at the device level and also for the online monitoring
of certain physical observables including electronics noise which are a few
crucial parameters for the optimal TPC performance.Comment: 5 pages, 6 figure
A Ring Shaped Embedded Young Stellar (Proto)Cluster
We present sub-arcsec (FWHM ~ 0.5") J, H, K and L' images of a young stellar
cluster associated with a candidate massive protostar IRAS22134+5834. The
observations reveal a centrally symmetric, flattened cluster enclosing a
central dark region. The central dark region is possibly a cavity within the
flattened cluster. It is surrounded by a ring composed of 5 bright stars and
the candidate massive protostar IRAS22134+5834. We construct JHKL' color-color
and HK color-magnitude diagrams to identify the young stellar objects and
estimate their spectral types. All the bright stars in the ring are found to
have intrinsic infrared excess emission and are likely to be early to late B
type stars. We estimate an average foreground extinction to the cluster of A_v
\~ 5mag and individual extinctions to the bright stars in the range A_v ~
20-40mag indicating possible cocoons surrounding each massive star. This ring
of bright stars is devoid of any HII region. It is surrounded by an embedded
cluster making this an example of a (proto)cluster that is in one of the
dynamically least relaxed states. These observations are consistent with the
recent non-axisymmetric calculations of Li & Nakamura, who present a star
formation scenario in which a magnetically subcritical cloud fragments into
multiple magnetically supercritical cores, leading to the formation of small
stellar groups.Comment: 13 pages in preprint format, 3 figures, 1 tabl
Comparison of Magnetic Field Structures on Different Scales in and around the Filamentary Dark Cloud GF 9
New visible polarization data combined with existing IR and FIR polarization
data are used to study how the magnetic field threading the filamentary
molecular cloud GF 9 connects to larger structures in its general environment.
We find that when both visible and NIR polarization data are plotted as a
function of extinction, there is no evidence for a plateau or a saturation
effect in the polarization at Av ~ 1.3 as seen in dark clouds in Taurus. This
lack of saturation effect suggests that even in the denser parts of GF 9 we are
still probing the magnetic field. The visible polarization is smooth and has a
well-defined orientation. The IR data are also well defined but with a
different direction, and the FIR data in the core region are well defined and
with yet another direction, but are randomly distributed in the filament
region. On the scale of a few times the mean radial dimension of the molecular
cloud, it is as if the magnetic field were `blind' to the spatial distribution
of the filaments while on smaller scales within the cloud, in the core region
near the IRAS point source PSC 20503+6006, polarimetry shows a rotation of the
magnetic field lines in these denser phases. Hence, in spite of the fact that
the spatial resolution is not the same in the visible/NIR and in the FIR data,
all the data put together indicate that the field direction changes with the
spatial scale. Finally, the Chandrasekhar and Fermi method is used to evaluate
the magnetic field strength, indicating that the core region is approximately
magnetically critical. A global interpretation of the results is that in the
core region an original poloidal field could have been twisted by a rotating
elongated (core+envelope) structure. There is no evidence for turbulence and
ambipolar diffusion does not seem to be effective at the present time.Comment: 33 pages, 6 tables, 8 figures, Accepted by Ap
Non-perturbative contributions to the plane-wave string mass matrix
D-instanton contributions to the mass matrix of arbitrary excited string
states of type IIB string theory in the maximally supersymmetric plane-wave
background are calculated to leading order in the string coupling using a
supersymmetric light-cone boundary state formalism. The explicit
non-perturbative dependence of the mass matrix on the complex string coupling,
the plane-wave mass parameter and the mode numbers of the excited states is
determined.Comment: 25 pages, 1 figure. v3: corrected minor typos, added referenc
Multi-Scale Analysis of Magnetic Fields in Filamentary Molecular Clouds in Orion A
New visible and K-band polarization measurements on stars surrounding
molecular clouds in Orion A and stars in the BN vicinity are presented. Our
results confirm that magnetic fields located inside the Orion A molecular
clouds and in their close neighborhood are spatially connected. On and around
the BN object, we measured the angular offsets between the K-band polarization
data and available submm data. We find high values of the polarization degree,
P_{K}, and of the optical depth, \tau_{K}, close to an angular offset position
of 90^{\circ} whereas lower values of P_{K} and \tau_{K} are observed for
smaller angular offsets. We interpret these results as evidence for the
presence of various magnetic field components toward lines of sight in the
vicinity of BN. On a larger scale, we measured the distribution of angular
offsets between available H-band polarization data and the same submm data set.
Here we find an increase of with angular offset which we interpret as a
rotation of the magnetic field by \lesssim 60^{\circ}. This trend generalizes
previous results on small scale toward and around lines of sight to BN and is
consistent with a twist of the magnetic field on a larger scale towards OMC-1.
A comparison of our results with several other studies suggests that a
two-component magnetic field, maybe helical, could be wrapping the OMC-1
filament.Comment: 53 pages, 21 figures, 7 tables, Accepted in the Astrophysical Journa
Polarized triplet production by circularly polarized photons
A process of the pair production by a circularly polarized photon in the
field of unpolarized atomic electron has been considered in the
Weizaecker-Williams approximation. The degree of longitudinal polarization of
positron and electron has been calculated. An exclusive cross-section as well
as a spectral distribution are obtained. We estimate the accuracy of our
calculations at the level of a few percent. We show the identity of the
positron polarization for considered process and for process of pair production
in the screened Coulomb field of nucleus.Comment: 9 pages, 3 picture
A New Nearby Candidate Star Cluster in Ophiuchus at d = 170 pc
The recent discoveries of nearby star clusters and associations within a few
hundred pc of the Sun, as well as the order of magnitude difference in the
formation rates of the embedded and open cluster populations, suggests that
additional poor stellar groups are likely to be found at surprisingly close
distances to the Sun. Here I describe a new nearby stellar aggregate found by
virtue of the parallel proper motions, similar trigonometric parallaxes, and
consistent color-magnitude distribution of its early-type members. The 120
Myr-old group lies in Ophiuchus at 170 pc, with its most massive
member being the 4th-magnitude post-MS B8II-III star Oph. The group may
have escaped previous notice due to its non-negligible extinction (
0.9 mag). If the group was born with a normal initial mass function,
and the nine B- and A-type systems represent a complete system of
intermediate-mass stars, then the original population was probably of order
200 systems. The age and space motion of the new cluster are very similar
to those of the Pleiades, Per cluster, and AB Dor Moving Group,
suggesting that these aggregates may have formed in the same star-forming
complex some yr ago.Comment: 23 pages, 3 figs., to appear in Nov. 2006 A
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