409,565 research outputs found
A Cold Nearby Cloud Inside the Local Bubble
The high-latitude Galactic H I cloud toward the extragalactic radio source 3C
225 is characterized by very narrow 21 cm emission and absorption indicative of
a very low H I spin temperature of about 20 K. Through high-resolution optical
spectroscopy, we report the detection of strong, very narrow Na I absorption
corresponding to this cloud toward a number of nearby stars. Assuming that the
turbulent H I and Na I motions are similar, we derive a cloud temperature of 20
(+6, -8) K (in complete agreement with the 21 cm results) and a line-of-sight
turbulent velocity of 0.37+/-0.08 km/s from a comparison of the H I and Na I
absorption linewidths. We also place a firm upper limit of 45 pc on the
distance of the cloud, which situates it well inside the Local Bubble in this
direction and makes it the nearest-known cold diffuse cloud discovered to date.Comment: 11 pages, 3 figures, accepted for publication in ApJ Letter
Large-Scale Structure of the Molecular Gas in Taurus Revealed by High Linear Dynamic Range Spectral Line Mapping
We report the results of a 100 square degree survey of the Taurus Molecular
Cloud region in the J = 1-0 transition of 12CO and 13CO. The image of the cloud
in each velocity channel includes ~ 3 million Nyquist sampled pixels on a 20"
grid. The high sensitivity and large linear dynamic range of the maps in both
isotopologues reveal a very complex, highly structured cloud morphology. There
are large scale correlated structures evident in 13CO emission having very fine
dimensions, including filaments, cavities, and rings. The 12CO emission shows a
quite different structure, with particularly complex interfaces between regions
of greater and smaller column density defining the boundaries of the
largest-scale cloud structures. The axes of the striations seen in the 12CO
emission from relatively diffuse gas are aligned with the direction of the
magnetic field. Using a column density-dependent model for the CO fractional
abundance, we derive the mass of the region mapped to be 24,000 solar masses, a
factor of three greater than would be obtained with canonical CO abundance
restricted to the high column density regions. We determine that half the mass
of the cloud is in regions having column density below 2.1x10^{21} per square
cm. The distribution of young stars in the region covered is highly nonuniform,
with the probability of finding a star in a pixel with a specified column
density rising sharply for N(H2) = 6x10^{21} cm^{-2}. We determine a relatively
low star formation efficiency (mass of young stars/mass of molecular gas),
between 0.3 and 1.2 %, and an average star formation rate during the past 3 Myr
of 8x10^{-5} stars yr^{-1}.Comment: 53 pages, 21 figure
Modelling the local and global cloud formation on HD 189733b
Context. Observations suggest that exoplanets such as HD 189733b form clouds
in their atmospheres which have a strong feedback onto their thermodynamical
and chemical structure, and overall appearance. Aims. Inspired by mineral cloud
modelling efforts for Brown Dwarf atmospheres, we present the first spatially
varying kinetic cloud model structures for HD 189733b. Methods. We apply a
2-model approach using results from a 3D global radiation-hydrodynamic
simulation of the atmosphere as input for a detailed, kinetic cloud formation
model. Sampling the 3D global atmosphere structure with 1D trajectories allows
us to model the spatially varying cloud structure on HD 189733b. The resulting
cloud properties enable the calculation of the scattering and absorption
properties of the clouds. Results. We present local and global cloud structure
and property maps for HD 189733b. The calculated cloud properties show
variations in composition, size and number density of cloud particles which are
strongest between the dayside and nightside. Cloud particles are mainly
composed of a mix of materials with silicates being the main component. Cloud
properties, and hence the local gas composition, change dramatically where
temperature inversions occur locally. The cloud opacity is dominated by
absorption in the upper atmosphere and scattering at higher pressures in the
model. The calculated 8{\mu}m single scattering Albedo of the cloud particles
are consistent with Spitzer bright regions. The cloud particles scattering
properties suggest that they would sparkle/reflect a midnight blue colour at
optical wavelengths.Comment: Accepted for publication (A&A) - 21/05/2015 (Low Resolution Maps
A peculiar HI cloud near the distant globular cluster Pal 4
We present 21-cm observations of four Galactic globular clusters, as part of
the on-going GALFA-HI Survey at Arecibo. We discovered a peculiar HI cloud in
the vicinity of the distant (109 kpc) cluster Pal 4, and discuss its properties
and likelihood of association with the cluster. We conclude that an association
of the HI cloud and Pal 4 is possible, but that a chance coincidence between
Pal 4 and a nearby compact high-velocity cloud cannot be ruled out altogether.
New, more stringent upper limits were derived for the other three clusters: M
3, NGC 5466, and Pal 13. We briefly discuss the fate of globular cluster gas
and the interaction of compact clouds with the Galactic Halo gas.Comment: Accepted for publication in MNRA
The interstellar C18O/C17O ratio in the solar neighbourhood: The rho Oph cloud
Observations of up to ten carbon monoxide (CO and isotopomers) transitions
are presented to study the interstellar C18O/C17O ratio towards 21 positions in
the nearby (d~140pc) low-mass star forming cloud rho Oph. A map of the C18O
J=1-0 distribution of parts of the cloud is also shown. An average
12C18O/12C17O isotopomeric ratio of 4.11 +/- 0.14, reflecting the 18O/17O
isotope ratio, is derived from Large Velocity Gradient (LVG) calculations. From
LTE column densities we derive a ratio of 4.17 +/-0.26. These calculations also
show that the kinetic temperature decreases from about 30 K in the cloud
envelope to about 10 K in the cloud cores. This decrease is accompanied by an
increase of the average molecular hydrogen density from 10^4 cm-3 to >10^5
cm-3. Towards some lines of sight C18O optical depths reach values of order
unity.Comment: 13 pages, 9 figures; accepted for publication in A&
Severe storm initiation and development from satellite infrared imagery and Rawinsonde data
The geographical distribution of potential temperatures, mixing ratio, and streamlines of flow patterns at 850, 700, and 500 mb heights are used to understand the prestorm convection and the horizontal convergence of moisture. From the analysis of 21 tornadoes the following conclusions are reached: (1) Strong horizontal convergence of moisture appeared at the 850, 700, and 500 mb levels in the area 12 hours before the storm formation; (2) An abundantly moist atmosphere below 3 km (700 mb) becomes convectively unstable during the time period between 12 and 24 hours before the initiation of the severe storms; (3) Strong winds veering with height with direction parallel to the movement of a dryline, surface fronts, etc; (4) During a 36-hour period, a tropopause height in the areas of interest is lowest at the time of tornadic cloud formation; (5) A train of gravity waves is detected before and during the cloud formation period. Rapid-scan infrared imagery provides near real-time information on the life cycle of the storm which can be summarized as follows: (1) Enhanced convection produced an overshooting cloud top penetrating above the tropopause, making the mass density of the overshooting cloud much greater than the mass density of the surrounding air; (2) The overshooting cloud top collapsed at the end of the mature stage of the cloud development; (3) The tornado touchdown followed the collapse of the overshooting cloud top
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