4,292 research outputs found
What is the Shell Around R Coronae Borealis?
The hydrogen-deficient, carbon-rich R Coronae Borealis (RCB) stars are known
for being prolific producers of dust which causes their large iconic declines
in brightness. Several RCB stars, including R CrB, itself, have large extended
dust shells seen in the far-infrared. The origin of these shells is uncertain
but they may give us clues to the evolution of the RCB stars. The shells could
form in three possible ways. 1) they are fossil Planetary Nebula (PN) shells,
which would exist if RCB stars are the result of a final, helium-shell flash,
2) they are material left over from a white-dwarf merger event which formed the
RCB stars, or 3) they are material lost from the star during the RCB phase.
Arecibo 21-cm observations establish an upper limit on the column density of H
I in the R CrB shell implying a maximum shell mass of 0.3
M. A low-mass fossil PN shell is still a possible source of the shell
although it may not contain enough dust. The mass of gas lost during a
white-dwarf merger event will not condense enough dust to produce the observed
shell, assuming a reasonable gas-to-dust ratio. The third scenario where the
shell around R CrB has been produced during the star's RCB phase seems most
likely to produce the observed mass of dust and the observed size of the shell.
But this means that R CrB has been in its RCB phase for 10 yr.Comment: 5 pages, 2 figures, 2 tables, Accepted for publication in A
Fluctuations and massive separation in three-dimensional shock-wave/boundary-layer interactions
Shock-wave unsteadiness was observed in rapidly compressed supersonic turbulent boundary layer flows with significant separation. A Mach 2.85 shock-wave/turbulent boundary layer flow was set up over a series of cylinder-flare bodies in the High Reynolds Number Channel 1. The transition from fully attached to fully separated flow was studied using axisymmetric flares with increasing compression angles. In the second phase, the 30 deg flare was inclined relative to the cylinder axis, so that the effect on a separated flow of increasing 3 dimensionality could be observed. Two 3-D separated cases are examined. A simple conditional sampling technique is applied to the data to group them according to an associated shock position. Mean velocities and turbulent kinetic energies, computed from the conditionally samples data, are compared to those from the unsorted data and to computed values. Three basic questions were addressed: can conditional sampling be used to provide snapshots of the flow; are averaged turbulence quantities dominated by the bimodal nature of the interaction; and is the shock unsteadiness really important to computational accuracy
The Neutral Hydrogen Bridge between M31 and M33
The Green Bank Telescope has been used to search for 21cm HI emission over a
large area between the galaxies M31 and M33 in an attempt to confirm at 9.1
arcmin angular resolution the detection by Braun and Thilker (2004) of a very
extensive neutral gas "bridge" between the two systems at the level NHI
approximately 10^{17} cm^{-2}. We detect HI emission at several locations up to
120 kpc in projected distance from M31, at least half the distance to M33, with
velocities similar to that of the galaxies, confirming the essence of the Braun
and Thilker discovery. The HI does not appear to be associated with the
extraplanar high-velocity clouds of either galaxy. In two places we measure NHI
> 3 x 10^{18} cm^{-2}, indicative of concentrations of HI with ~10^5 solar
masses on scales <2 kpc, but over most of the field we have only 5sigma upper
limits of NHI <= 1.4 x 10^{18} cm^{-2}. In very deep measurements in two
directions HI lines were detected at a few 10^{17} cm^{-2}. The absence of
emission at another location to a 5sigma limit NHI <= 1.5 x 10^{17} cm^{-2}
suggests that the HI bridge is either patchy or confined to within ~125 kpc of
M31. The measurements also cover two of M31's dwarf galaxies, And II and And
XV, but in neither case is there evidence for associated HI at the 5sigma level
of 1.4 x 10^4 solar masses of HI for And II, and 9.3 x 10^3 solar masses for
And XV.Comment: Submitted to the Astronomical Journa
HI Clouds Beyond the Galactic Disk
Recent observations in the 21cm line with the Green Bank Telescope have
changed our view of the neutral interstellar medium (ISM) in several ways. The
new data show that in the inner parts of the Milky Way the disk-halo interface
is composed of many discrete HI clouds. The clouds lie in a layer more than one
kpc thick and follow Galactic rotation. Their origin and evolution is unknown.
In the outer Galaxy, the new data show that the high-velocity cloud Complex H
is likely a satellite on a retrograde orbit interacting with some extended
component of the Milky Way's ISM. These observations place new constraints on
models of the ISM and are directly related to the work of Don Cox and Ron
Reynolds.Comment: 8 pages includes 2 figures. To appear in "How Does the Galaxy Work?",
eds. E.J. Alfaro, E. Perez, & J. Franco, Kluwer, Proceedings of a Conference
held 23-27 June 2003 in Granada, Spai
HI Detection in two Dwarf S0 Galaxies in Nearby Groups: ESO384-016 and NGC 59
An \hi survey of 10 dE/dS0 galaxies in the nearby Sculptor and Centaurus A
groups was made using the Australia Telescope Compact Array (ATCA). The
observed galaxies have accurate distances derived by Jerjen et al (1998; 2000b)
using the surface brightness fluctuation technique. Their absolute magnitudes
are in the range . Only two of the ten galaxies were
detected at our detection limit ( \msol for the Centaurus
group and \msol for the Sculptor group), the two dS0
galaxies ESO384-016 in the Centaurus A Group and NGC 59 in the Sculptor Group,
with \hi masses of \msol and \msol respectively. Those two detections were confirmed using the Green
Bank Telescope. These small \hi reservoirs could fuel future generations of low
level star formation and could explain the bluer colors seen at the center of
the detected galaxies. Similarly to what is seen with the Virgo dEs, the two
objects with \hi appear to be on the outskirt of the groups.Comment: 25 pages (11 figures), accepted by A
High Velocity Cloud Complex H: A Satellite of the Milky Way in a Retrograde Orbit?
Observations with the Green Bank Telescope of 21cm HI emission from the
high-velocity cloud Complex H suggest that it is interacting with the Milky
Way. A model in which the cloud is a satellite of the Galaxy in an inclined,
retrograde circular orbit reproduces both the cloud's average velocity and its
velocity gradient with latitude. The model places Complex H at approximately 33
kpc from the Galactic Center on a retrograde orbit inclined about 45 degrees to
the Galactic plane. At this location it has an HI mass > 6 10^6 Msun and
dimensions of at least 10 by 5 kpc. Some of the diffuse HI associated with the
cloud has apparently been decelerated by interaction with Galactic gas. Complex
H has similarities to the dwarf irregular galaxy Leo A and to some compact
high-velocity clouds, and has an internal structure nearly identical to parts
of the Magellanic Stream, with a pressure P/k about 100 cm^{-3} K.Comment: 12 pages includes 4 figures. To be published in Astrophysical Journal
Letters, 1 July 200
The interstellar oxygen-K absorption edge as observed by XMM-Newton
High resolution X-ray spectra of the Reflection Grating Spectrometer (RGS) on
board the XMM satellite are used to resolve the oxygen K absorption edge. By
combining spectra of low and high extinction sources, the observed absorption
edge can be split in the true interstellar (ISM) extinction and the
instrumental absorption. The detailed ISM edge structure closely follows the
edge structure of neutral oxygen as derived by theoretical R-matrix
calculations. However, the position of the theoretical edge requires a
wavelength shift. In addition the detailed instrumental RGS absorption edge
structure is presented. All results are verified by comparing to a subset of
Chandra LETG-HRC observations.Comment: LaTeX2e A&A style, 10 pages, 12 postscript figures, accepted for
publication in Astronomy and Astrophysic
G28.17+0.05: An unusual giant HI cloud in the inner Galaxy
New 21 cm HI observations have revealed a giant HI cloud in the Galactic
plane that has unusual properties. It is quite well defined, about 150 pc in
diameter at a distance of 5 kpc, and contains as much as 100,000 Solar Masses
of atomic hydrogen. The outer parts of the cloud appear in HI emission above
the HI background, while the central regions show HI self-absorption. Models
which reproduce the observations have a core with a temperature <40 K and an
outer envelope as much as an order of magnitude hotter. The cold core is
elongated along the Galactic plane, whereas the overall outline of the cloud is
approximately spherical. The warm and cold parts of the HI cloud have a
similar, and relatively large, line width of approximately 7 km/s. The cloud
core is a source of weak, anomalously-excited 1720 MHz OH emission, also with a
relatively large line width, which delineates the region of HI self-absorption
but is slightly blue-shifted in velocity. The intensity of the 1720 MHz OH
emission is correlated with N(H) derived from models of the cold core. There is
12CO emission associated with the cloud core. Most of the cloud mass is in
molecules, and the total mass is > 200,000 Solar Masses. In the cold core the
HI mass fraction may be 10 percent. The cloud has only a few sites of current
star formation. There may be about 100 more objects like this in the inner
Galaxy; every line of sight through the Galactic plane within 50 degrees of the
Galactic center probably intersects at least one. We suggest that G28.17+0.05
is a cloud being observed as it enters a spiral arm and that it is in the
transition from the atomic to the molecular state.Comment: 35 pages, inludes 12 figure
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