2,051 research outputs found
Proper Motions of H2O Masers in the Water Fountain Source IRAS 19190+1102
We report on the results of two epochs of Very Long Baseline Array (VLBA)
observations of the 22 GHz water masers toward IRAS 19190+1102. The water maser
emission from this object shows two main arc-shaped formations perpendicular to
their NE-SW separation axis. The arcs are separated by ~280 mas in position,
and are expanding outwards at an angular rate of 2.35 mas/yr. We detect maser
emission at velocities between -53.3 km/s to +78.0 km/s and there is a distinct
velocity pattern where the NE masers are blueshifted and the SW masers are
redshifted. The outflow has a three-dimensional outflow velocity of 99.8 km/s
and a dynamical age of about 59 yr. A group of blueshifted masers not located
along the arcs shows a change in velocity of more than 35 km/s between epochs,
and may be indicative of the formation of a new lobe. These observations show
that IRAS 19190+1102 is a member of the class of "water fountain"'
pre-planetary nebulae displaying bipolar structureComment: Accepted for publication in ApJ, corrected typo
Young Planetary Nebulae: Hubble Space Telescope Imaging and a New Morphological Classification System
Using Hubble Space Telescope images of 119 young planetary nebulae, most of
which have not previously been published, we have devised a comprehensive
morphological classification system for these objects. This system generalizes
a recently devised system for pre-planetary nebulae, which are the immediate
progenitors of planetary nebulae (PNs). Unlike previous classification studies,
we have focussed primarily on young PNs rather than all PNs, because the former
best show the influences or symmetries imposed on them by the dominant physical
processes operating at the first and primary stage of the shaping process.
Older PNs develop instabilities, interact with the ambient interstellar medium,
and are subject to the passage of photoionization fronts, all of which obscure
the underlying symmetries and geometries imposed early on. Our classification
system is designed to suffer minimal prejudice regarding the underlying
physical causes of the different shapes and structures seen in our PN sample,
however, in many cases, physical causes are readily suggested by the geometry,
along with the kinematics that have been measured in some systems. Secondary
characteristics in our system such as ansae indicate the impact of a jet upon a
slower-moving, prior wind; a waist is the signature of a strong equatorial
concentration of matter, whether it be outflowing or in a bound Keplerian disk,
and point symmetry indicates a secular trend, presumably precession, in the
orientation of the central driver of a rapid, collimated outflow.Comment: (to appear in The Astronomical Journal, March 2011.) The quality of
the figures as it appears in the arXiv pdf output is not up-to-par; the full
ms with high-quality figures is available by anonymous FTP at
ftp://ftp.astro.ucla.edu/pub/morris/sahai_AJ_360163.pd
Heterojunction solar cell calculations
Solar cell efficiencies computed for semiconductor heterojunction cell
Development of an (AlGaAs-Ga As) graded band gap solar cell
The results of an experimental program to develop the epitaxial growth techniques and analytical characterization techniques to fabricate graded bandgap solar cells are reported
The Coldest Place in the Universe: Probing the Ultra-Cold Outflow and Dusty Disk in the Boomerang Nebula
Our Cycle 0 ALMA observations confirmed that the Boomerang Nebula is the
coldest known object in the Universe, with a massive high-speed outflow that
has cooled significantly below the cosmic background temperature. Our new CO
1-0 data reveal heretofore unseen distant regions of this ultra-cold outflow,
out to AU. We find that in the ultra-cold outflow, the
mass-loss rate (dM/dt) increases with radius, similar to its expansion velocity
() - taking , we find . The mass in
the ultra-cold outflow is Msun, and the Boomerang's main-sequence
progenitor mass is Msun. Our high angular resolution (0".3) CO
J=3-2 map shows the inner bipolar nebula's precise, highly-collimated shape,
and a dense central waist of size (FWHM) 1740 AU AU. The
molecular gas and the dust as seen in scattered light via optical HST imaging
show a detailed correspondence. The waist shows a compact core in thermal dust
emission at 0.87-3.3 mm, which harbors Msun~of very large
(mm-to-cm sized), cold ( K) grains. The central waist
(assuming its outer regions to be expanding) and fast bipolar outflow have
expansion ages of yr and yr: the "jet-lag" (i.e.,
torus age minus the fast-outflow age) in the Boomerang supports models in which
the primary star interacts directly with a binary companion. We argue that this
interaction resulted in a common-envelope configuration while the Boomerang's
primary was an RGB or early-AGB star, with the companion finally merging into
the primary's core, and ejecting the primary's envelope that now forms the
ultra-cold outflow.Comment: accepted ApJ, 12 Apr, 201
Shocked and Scorched: The Tail of a Tadpole in an Interstellar Pond
We report multi-wavelength observations of the far-infrared source IRAS
20324+4057, including high-resolution optical imaging with HST, and
ground-based near-infrared, millimeter-wave and radio observations. These data
show an extended, limb-brightened, tadpole-shaped nebula with a bright,
compact, cometary nebula located inside the tadpole head. Our molecular line
observations indicate that the Tadpole is predominantly molecular, with a total
gas mass exceeding 3.7 Msun. Our radio continuum imaging, and archival Spitzer
IRAC images, show the presence of additional tadpole-shaped objects in the
vicinity of IRAS 20324+4057 that share a common E-W head-tail orientation: we
propose that these structures are small, dense molecular cores that originated
in the Cygnus cloud and are now being (i) photoevaporated by the ultraviolet
radiation field of the Cyg OB2 No. 8 cluster located to the North-West, and
(ii) shaped by ram pressure of a distant wind source or sources located to the
West, blowing ablated and photoevaporated material from their heads eastwards.
The ripples in the tail of the Tadpole are interpreted in terms of
instabilities at the interface between the ambient wind and the dense medium of
the former.Comment: (accepted by the Astrophysical Journal
Binarity in Cool Asymptotic Giant Branch Stars: A Galex Search for Ultraviolet Excesse
The search for binarity in AGB stars is of critical importance for our
understanding of how planetary nebulae acquire the dazzling variety of
aspherical shapes which characterises this class. However, detecting binary
companions in such stars has been severely hampered due to their extreme
luminosities and pulsations. We have carried out a small imaging survey of AGB
stars in ultraviolet light (using GALEX) where these cool objects are very
faint, in order to search for hotter companions. We report the discovery of
significant far-ultraviolet excesses towards nine of these stars. The
far-ultraviolet excess most likely results either directly from the presence of
a hot binary companion, or indirectly from a hot accretion disk around the
companion.Comment: revised for Astrophysical Journa
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