150 research outputs found
First evidence for dusty disks around Herbig Be stars
We have carried out a high-sensitivity search for circumstellar disks around
Herbig Be stars in the continuum at 1.4mm and 2.7mm using the IRAM
interferometer at the Plateau de Bure (PdBI) . In this letter, we report data
on three well studied B0 stars, MWC 1080, MWC 137 and R Mon. The two latter
have also been observed in the continuum at 0.7 cm and 1.3 cm using the NRAO
Very Large Array (VLA) . We report the detection of circumstellar disks around
MWC 1080 and R Mon with masses of Md ~ 0.003 and 0.01 Msun, respectively, while
for MWC 137 we estimate a disk mass upper limit of 0.007 Msun. Our results show
that the ratio Md/M* is at least an order of magnitude lower in Herbig Be stars
than in Herbig Ae and T Tauri stars.Comment: 5 pages (including figures
A transition from river networks to scale-free networks
A spatial network is constructed on a two dimensional space where the nodes
are geometrical points located at randomly distributed positions which are
labeled sequentially in increasing order of one of their co-ordinates. Starting
with such points the network is grown by including them one by one
according to the serial number into the growing network. The -th point is
attached to the -th node of the network using the probability: where is the degree of the -th node
and is the Euclidean distance between the points and . Here
is a continuously tunable parameter and while for one gets
the simple Barab\'asi-Albert network, the case for
corresponds to the spatially continuous version of the well known Scheidegger's
river network problem. The modulating parameter is tuned to study the
transition between the two different critical behaviors at a specific value
which we numerically estimate to be -2.Comment: 5 pages, 5 figur
Detection of reactive ions in the ultracompact HII regions Mon R2 and G29.96-0.02
We report the first detection of the reactive ions CO+ and HOC+ towards
ultracompact (UC) HII regions, particularly in Mon R2 and G29.96-0.02. We have
observed two positions in Mon R2, namely the peak of the UC HII region and a
position in the high density molecular cloud which bounds it. CO+ and HOC+ were
detected at the UC HII region but not at the molecular cloud, as expected if
the CO and HOC emissions arise in the PDR surrounding the \uch. The
measured CO and HOC column densities are of the order of 10
cm in both sources, which yields a strikingly low [HCO]/[HOC]
abundance ratio of 460 in Mon R2. These values are similar to those found in
some other well-known PDRs, like NGC 7023 and the Orion Bar. We briefly discuss
the chemical implications of these results.Comment: 10 pages, 3 figures. Accepted by Astrophysical Journal Letter
Genetic evidence of two sibling species within the Contracoecum ogmorhini Johnson & Mawson 1941 complex (Nematoda; Anisakidae) from otariid seals in boreal and austral regions
Genetic variation of Contracaecum ogmorhini (sensu lato) populations from different otariid seals of the northern
and southern hemisphere was studied on the basis of 18 enzyme loci as well as preliminary sequence analysis of
the mitochondrial cyt b gene (260 bp). Samples were collected from Zalophus californianus in the boreal region
and from Arctocephalus pusillus pusillus, A. pusillus doriferus and A. australis from the austral region. Marked
genetic heterogeneity was found between C. ogmorhini (sensu lato) samples from the boreal and austral region,
respectively. Two loci (Mdh-2 and NADHdh) showed fixed differences and a further three loci (Iddh, Mdh-1 and
6Pgdh) were highly differentiated between boreal and austral samples. Their average genetic distance was DNei =
0.36 at isozyme level. At mitochondrial DNA level, an average proportion of nucleotide substitution of 3.7% was
observed. These findings support the existence of two distinct sibling species, for which the names C. ogmorhini
(sensu stricto) and C. margolisi n. sp., respectively, for the austral and boreal taxon, are proposed. A description
for C. margolisi n. sp. is provided. No diagnostic morphological characters have so far been detected; on the other hand, two enzyme loci, Mdh-2 and NADHdh, fully diagnostic between the two species, can be used for the routine identification of males, females and larval stages. Mirounga leonina was found to host C. ogmorhini (s.s.) inmixed infections with C. osculatum (s.l.) (of which C. ogmorhini (s.l.) was in the past considered to be a synonym)
and C. miroungae; no hybrid genotypes were found,confirming the reproductive isolation of these three anisakid species. The hosts and geographical range so far recorded for C. margolisi n. sp. and C. ogmorhini (s.s.) are given
Negativity and quantum discord in Davies environments
We investigate the time evolution of negativity and quantum discord for a
pair of non-interacting qubits with one being weakly coupled to a decohering
Davies--type Markovian environment. At initial time of preparation, the qubits
are prepared in one of the maximally entangled pure Bell states. In the
limiting case of pure decoherence (i.e. pure dephasing), both, the quantum
discord and negativity decay to zero in the long time limit. In presence of a
manifest dissipative dynamics, the entanglement negativity undergoes a sudden
death at finite time while the quantum discord relaxes continuously to zero
with increasing time. We find that in dephasing environments the decay of the
negativity is more propitious with increasing time; in contrast, the evolving
decay of the quantum discord proceeds weaker for dissipative environments.
Particularly, the slowest decay of the quantum discord emerges when the energy
relaxation time matches the dephasing time.Comment: submitted for publicatio
Compact Radio Sources in Orion: New Detections, Time Variability, and Objects in OMC-1S
We present the analysis of four 3.6 cm radio continuum archival observations
of Orion obtained using the Very Large Array in its A-configuration, with
angular resolution. The observations were made during the period
1994-1997. In a region of , we detect a total of 77 compact radio
sources. Of the total of detected sources, 54 are detected in one or more of
the individual observations and 36 of these show time variability (by more than
30%) between the observed epochs. A deep image made from averaging all data
shows an additional 23 faint sources, in the range of 0.1 to 0.3 mJy. Of the
total of 77 sources, 39 are new centimeter detections. However, only 9 of the
77 sources do not have a previously reported counterpart at near-infrared,
optical, or X-ray wavelengths. In particular, we detect three faint sources in
the OMC-1S region that may be related to the sources that power the multiple
outflows that emanate from this part of the Orion nebula. %We discuss the
nature of these sources and its relation with the %near-infrared, optical, and
X-ray objects in the region.Comment: 29 pages, 9 figure
The discovery of an evolving dust scattered X-ray halo around GRB 031203
We report the first detection of a time-dependent, dust-scattered X-ray halo
around a gamma-ray burst. GRB 031203 was observed by XMM-Newton starting six
hours after the burst. The halo appeared as concentric ring-like structures
centered on the GRB location. The radii of these structures increased with time
as t^{1/2}, consistent with small-angle X-ray scattering caused by a large
column of dust along the line of sight to a cosmologically distant GRB. The
rings are due to dust concentrated in two distinct slabs in the Galaxy located
at distances of 880 and 1390 pc, consistent with known Galactic features. The
halo brightness implies an initial soft X-ray pulse consistent with the
observed GRB.Comment: 4 pages. 4 figures. Accepted for publication in ApJ Letter
Zeros of regular functions of quaternionic and octonionic variable: a division lemma and the camshaft effect
We study in detail the zero set of a regular function of a quaternionic or
octonionic variable. By means of a division lemma for convergent power series,
we find the exact relation existing between the zeros of two octonionic regular
functions and those of their product. In the case of octonionic polynomials, we
get a strong form of the fundamental theorem of algebra. We prove that the sum
of the multiplicities of zeros equals the degree of the polynomial and obtain a
factorization in linear polynomials.Comment: Proof of Lemma 7 rewritten (thanks to an anonymous reviewer
Magnetic Frustration Driven by Itinerancy in Spinel CoV2O4
Localized spins and itinerant electrons rarely coexist in geometrically-frustrated spinel lattices. They exhibit a complex interplay between localized spins and itinerant electrons. In this paper, we study the origin of the unusual spin structure of the spinel CoV2O4, which stands at the crossover from insulating to itinerant behavior using the first principle calculation and neutron diffraction measurement. In contrast to the expected paramagnetism, localized spins supported by enhanced exchange couplings are frustrated by the effects of delocalized electrons. This frustration produces a non-collinear spin state even without orbital orderings and may be responsible for macroscopic spin-glass behavior. Competing phases can be uncovered by external perturbations such as pressure or magnetic field, which enhances the frustration
Magnetic Flux Cancellation in Ellerman Bombs
Ellerman Bombs (EBs) are often found to be co-spatial with bipolar photospheric magnetic fields. We use Hα
imaging spectroscopy along with Fe I 6302.5 Å spectropolarimetry from the Swedish 1 m Solar Telescope (SST),
combined with data from the Solar Dynamic Observatory, to study EBs and the evolution of the local magnetic
fields at EB locations. EBs are found via an EB detection and tracking algorithm. Using NICOLE inversions of the
spectropolarimetric data, we find that, on average, (3.43 ± 0.49) × 1024 erg of stored magnetic energy disappears
from the bipolar region during EB burning. The inversions also show flux cancellation rates of 1014–1015 Mx s−1
and temperature enhancements of 200 K at the detection footpoints. We investigate the near-simultaneous flaring
of EBs due to co-temporal flux emergence from a sunspot, which shows a decrease in transverse velocity when
interacting with an existing, stationary area of opposite polarity magnetic flux, resulting in the formation of the
EBs. We also show that these EBs can be fueled further by additional, faster moving, negative magnetic flux
regions
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