606 research outputs found
C18O (3-2) observations of the Cometary Globule CG 12: a cold core and a C18O hot spot
The feasibility of observing the C18O (3-2) spectral line in cold clouds with
the APEX telescope has been tested. As the line at 329.330 GHz lies in the wing
of a strong atmospheric H2O absorption it can be observed only at high altitude
observatories. Using the three lowest rotational levels instead of only two
helps to narrow down the physical properties of dark clouds and globules. The
centres of two C18O maxima in the high latitude low mass star forming region CG
12 were mapped in C18O (3-2) and the data were analyzed together with spectral
line data from the SEST. The T_MB(3-2)/T_MB(2-1) ratio in the northern C18O
maximum, CG 12 N, is 0.8, and in the southern maximum, CG 12 S, ~2. CG 12 N is
modelled as a 120'' diameter (0.4pc) cold core with a mass of 27 Msun. A small
size maximum with a narrow, 0.8 kms-1, C18O (3-2) spectral line with a peak
temperature of T_MB ~11 K was detected in CG 12 S. This maximum is modelled as
a 60'' to 80'' diameter (~0.2pc) hot (80 K < Tex < 200 K) ~1.6 Msun clump. The
source lies on the axis of a highly collimated bipolar molecular outflow near
its driving source. This is the first detection of such a compact, warm object
in a low mass star forming region.Comment: APEX A&A special issue, accepte
A new proof for the decidability of D0L ultimate periodicity
We give a new proof for the decidability of the D0L ultimate periodicity
problem based on the decidability of p-periodicity of morphic words adapted to
the approach of Harju and Linna.Comment: In Proceedings WORDS 2011, arXiv:1108.341
High Spatial Resolution Observations of Two Young Protostars in the R Corona Australis Region
We present multi-wavelength, high spatial resolution imaging of the IRS 7
region in the R Corona Australis molecular cloud. Our observations include 1.1
mm continuum and HCO^+ J = images from the SMA, ^{12}CO J =
outflow maps from the DesertStar heterodyne array receiver on the HHT, 450
m and 850 m continuum images from SCUBA, and archival Spitzer IRAC
and MIPS 24 \micron images. The accurate astrometry of the IRAC images allow us
to identify IRS 7 with the cm source VLA 10W (IRS 7A) and the X-ray source X_W.
The SMA 1.1 mm image reveals two compact continuum sources which are also
distinguishable at 450 m. SMA 1 coincides with X-ray source CXOU
J190156.4-365728 and VLA cm source 10E (IRS 7B) and is seen in the IRAC and
MIPS images. SMA 2 has no infrared counterpart but coincides with cm source VLA
9. Spectral energy distributions constructed from SMA, SCUBA and Spitzer data
yield bolometric temperatures of 83 K for SMA 1 and 70 K for SMA 2. These
temperatures along with the submillimeter to total luminosity ratios indicate
that SMA 2 is a Class 0 protostar, while SMA 1 is a Class 0/Class I
transitional object (L= \Lsun). The ^{12}CO J = outflow map
shows one major and possibly several smaller outflows centered on the IRS 7
region, with masses and energetics consistent with previous work. We identify
the Class 0 source SMA 2/VLA 9 as the main driver of this outflow. The complex
and clumpy spatial and velocity distribution of the HCO^+ J =
emission is not consistent with either bulk rotation, or any known molecular
outflow activity.Comment: 31 pages, 8 figures, Accepted to Ap
Generalized tight-binding transport model for graphene nanoribbon-based systems
An extended tight-binding model that includes up to third-nearest-neighbor hopping and a Hubbard mean-field interaction term is tested against ab initio local spin-density approximation results of band structures for armchair- and zigzag-edged graphene nanoribbons. A single tight-binding parameter set is found to accurately reproduce the ab initio results for both the armchair and zigzag cases. Transport calculations based on the extended tight-binding model faithfully reproduce the results of ab initio transport calculations of graphene nanoribbon-based systems.Peer reviewe
Physical properties of dense cores in Orion B9
We aim to determine the physical and chemical properties of dense cores in
Orion B9. We observed the NH3(1,1) and (2,2), and the N2H+(3-2) lines towards
the submm peak positions. These data are used in conjunction with our LABOCA
870 micron dust continuum data. The gas kinetic temperature in the cores is
between ~9.4-13.9 K. The non-thermal velocity dispersion is subsonic in most of
the cores. The non-thermal linewidth in protostellar cores appears to increase
with increasing bolometric luminosity. The core masses are very likely drawn
from the same parent distribution as the core masses in Orion B North. Starless
cores in the region are likely to be gravitationally bound, and thus
prestellar. Some of the cores have a lower radial velocity than the systemic
velocity of the region, suggesting that they are members of the "low-velocity
part" of Orion B. The observed core-separation distances deviate from the
corresponding random-like model distributions. The distances between the
nearest-neighbours are comparable to the thermal Jeans length. The fractional
abundances of NH3 and N2H+ in the cores are ~1.5-9.8x10^{-8} and
~0.2-5.9x10^{-10}, respectively. The NH3 abundance appears to decrease with
increasing H2 column and number densities. The NH3/N2H+ column density ratio is
larger in starless cores than in cores with embedded protostars. The core
population in Orion B9 is comparable in physical properties to those in nearby
low-mass star-forming regions. It is unclear if the origin of cores could be
explained by turbulent fragmentation. On the other hand, many of the core
properties conform with the picture of dynamic core evolution. The Orion B9
region has probably been influenced by the feedback from the nearby Ori OB 1b
group, and the fragmentation of the parental cloud into cores could be caused
by gravitational instability.Comment: 17 pages, 11 figures, 7 tables. Accepted for publication in Astronomy
and Astrophysics. Version 2: minor language corrections adde
Broken Symmetry in Density-Functional Theory: Analysis and Cure
We present a detailed analysis of the broken-symmetry mean-field solutions
using a four-electron rectangular quantum dot as a model system. Comparisons of
the density-functional theory predictions with the exact ones show that the
symmetry breaking results from the single-configuration wave function used in
the mean-field approach. As a general cure we present a scheme that
systematically incorporates several configurations into the density-functional
theory and restores the symmetry. This cure is easily applicable to any
density-functional approach.Comment: 4 pages, 4 figures, submitted to PR
Millimeter and Submillimeter Survey of the R Corona Australis Region
Using a combination of data from the Antarctic Submillimeter Telescope and
Remote Observatory (AST/RO), the Arizona Radio Observatory Kitt Peak 12m
telescope and the Arizona Radio Observatory 10m Heinrich Hertz Telescope, we
have studied the most active part of the R CrA molecular cloud in multiple
transitions of Carbon Monoxide, HCO and 870\micron continuum emission.
Since R CrA is nearby (130 pc), we are able to obtain physical spatial
resolution as high as 0.01pc over an area of 0.16 pc, with velocity
resolution finer than 1 km/s. Mass estimates of the protostar driving the
mm-wave emission derived from HCO, dust continuum emission and kinematic
techniques point to a young, deeply embedded protostar of 0.5-0.75
M, with a gaseous envelope of similar mass. A molecular outflow is
driven by this source that also contains at least 0.8 M of molecular
gas with 0.5 L of mechanical luminosity. HCO lines show the
kinematic signature of infall motions as well as bulk rotation. The source is
most likely a Class 0 protostellar object not yet visible at near-IR
wavelengths. With the combination of spatial and spectral resolution in our
data set, we are able to disentangle the effects of infall, rotation and
outflow towards this young object.Comment: 29 pages, 9 figures. Accepted for publication in the Astrophysical
Journa
Detection of H2D+ in a massive prestellar core in Orion B
Aims. The purpose of this study is to examine the prediction that the deuterated H3+ ion, H2D+, can be found exclusively in the coldest regions of molecular cloud cores. This is also a feasibility study for the detection of the ground-state line of ortho-H2D+ at 372 GHz with APEX. Methods. The 1(10)-1(11) transition of H2D+ at 372 GHz was searched towards selected positions in the massive star forming cloud OriB9, in the dark cloud L183, and in the low- to intermediate mass star-forming cloud R CrA. Results. The line was detected in cold, prestellar cores in the regions of OriB9 and L183, but only upper limits were obtained towards other locations which either have elevated temperatures or contain a newly born star. The H2D+ detection towards OriB9 is the first one in a massive star-forming region. The fractional ortho-H2D+ abundances (relative to H2) are estimated to be about 1.0E-10 in two cold cores in OriB9, and 3.0E-10 in the cold core of L183. Conclusions. The H2D+ detection in OriB9 shows that also massive star forming regions contain very cold prestellar cores which probably have reached matured chemical composition characterized, e.g., by a high degree of deuterium fractionation. Besides as a tracer of the interior parts of prestellar cores, H2D+ may therefore be used to put contraints on the timescales related to massive star formation
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