7,603 research outputs found
The abundance of 28Si32S, 29Si32S, 28Si34S, and 30Si32S in the inner layers of the envelope of IRC+10216
We present high spectral resolution mid-IR observations of SiS towards the
C-rich AGB star IRC+10216 carried out with the Texas Echelon-cross-Echelle
Spectrograph mounted on the NASA Infrared Telescope Facility. We have
identified 204 ro-vibrational lines of 28Si32S, 26 of 29Si32S, 20 of 28Si34S,
and 15 of 30Si32S in the frequency range 720-790 cm-1. These lines belong to
bands v=1-0, 2-1, 3-2, 4-3, and 5-4, and involve rotational levels with
Jlow<90. About 30 per cent of these lines are unblended or weakly blended and
can be partially or entirely fitted with a code developed to model the mid-IR
emission of a spherically symmetric circumstellar envelope composed of
expanding gas and dust. The observed lines trace the envelope at distances to
the star <35R* (~0.7 arcsec). The fits are compatible with an expansion
velocity of 1+2.5(r/R*-1) km/s between 1 and 5R*, 11 km/s between 5 and 20R*,
and 14.5 km/s outwards. The derived abundance profile of 28Si32S with respect
to H2 is 4.9e-6 between the stellar photosphere and 5R*, decreasing linearly to
1.6e-6 at 20R* and to 1.3e-6 at 50R*. 28Si32S seems to be rotationally under
LTE in the region of the envelope probed with our observations and
vibrationally out of LTE in most of it. There is a red-shifted emission excess
in the 28Si32S lines of band v=1-0 that cannot be found in the lines of bands
v=2-1, 3-2, 4-3, and 5-4. This excess could be explained by an enhancement of
the vibrational temperature around 20R* behind the star. The derived isotopic
ratios 28Si/29Si, and 32S/34S are 17 and 14, compatible with previous
estimates.Comment: 11 pages, 5 figures, and 4 tables. Accepted for publication in MNRA
Variable stars in the globular cluster M28 (NGC 6626)
We present a new search for variable stars in the Galactic globular cluster
M28 (NGC 6626). The search is based on a series of BVI images obtained with the
SMARTS Consortium's 1.3m telescope at Cerro Tololo Inter-American Observatory,
Chile. The search was carried out using the ISIS v2.2 image subtraction
package. We find a total of 25 variable stars in the field of the cluster, 9
being new discoveries. Of the newly found variables, 1 is an ab-type RR Lyrae
star, 6 are c-type RR Lyrae, and 2 are long-period/semi-regular variables. V22,
previously classified as a type II Cepheid, appears as a bona-fide RRc in our
data. In turn, V20, previously classified as an ab-type RR Lyrae, could not be
properly phased with any reasonable period. The properties of the ab-type RR
Lyrae stars in M28 appear most consistent with an Oosterhoff-intermediate
classification, which is unusual for bona-fide Galactic globulars clusters.
However, the cluster's c-type variables do not clearly support such an
Oosterhoff type, and a hybrid Oosterhoff I/II system is accordingly another
possibility, thus raising the intriguing possibility of multiple populations
being present in M28. Coordinates, periods, and light curves in differential
fluxes are provided for all the detected variables.Comment: A&A, in pres
Nesting Induced Peierls-type Instability for Compressed Li-CI16
Alkalies are considered to be simple metals at ambient conditions. However,
recently reported theoretical and experimental results have shown an unexpected
and intriguing correlation between complex structures and an enhanced
superconducting transition temperature in lithium under pressure. In this
article we analyze the pressure induced Fermi surface deformation in bcc
lithium, and its relation to the observed cI16 structure. According to our
calculations, the Fermi surface becomes increasingly anisotropic with pressure
and develops an extended nesting along the bcc [121] direction. This nesting
induces a phonon instability of both transverse modes at N, so that a
Peierls-type mechanism is proposed to explain the stability of Li-cI16.Comment: Proceedings of Fukuoka 2006 Conference on Novel Pressure-induced
Phenomena in Condensed Matter Systems. To be published in J. Phys. Soc. Jpn.
2 pages and 3 figure
A -Vertex Kernel for Maximum Internal Spanning Tree
We consider the parameterized version of the maximum internal spanning tree
problem, which, given an -vertex graph and a parameter , asks for a
spanning tree with at least internal vertices. Fomin et al. [J. Comput.
System Sci., 79:1-6] crafted a very ingenious reduction rule, and showed that a
simple application of this rule is sufficient to yield a -vertex kernel.
Here we propose a novel way to use the same reduction rule, resulting in an
improved -vertex kernel. Our algorithm applies first a greedy procedure
consisting of a sequence of local exchange operations, which ends with a
local-optimal spanning tree, and then uses this special tree to find a
reducible structure. As a corollary of our kernel, we obtain a deterministic
algorithm for the problem running in time
A Survey About Nothing: Monitoring a Million Supergiants for Failed Supernovae
Extragalactic transient searches have historically been limited to looking
for the appearance of new sources such as supernovae. It is now possible to
carry out a new kind of survey that will do the opposite, that is, search for
the disappearance of massive stars. This will entail the systematic observation
of galaxies within a distance of 10 Mpc in order to watch ~10^6 supergiants.
Reaching this critical number ensures that something will occur yearly, since
these massive stars must end their lives with a core collapse within ~10^6
years. Using deep imaging and image subtraction it is possible to determine the
fates of these stars whether they end with a bang (supernova) or a whimper
(fall out of sight). Such a survey would place completely new limits on the
total rate of all core collapses, which is critical for determining the
validity of supernova models. It would also determine the properties of
supernova progenitors, better characterize poorly understood optical
transients, such as eta Carina-like mass ejections, find and characterize large
numbers of Cepheids, luminous blue variables and eclipsing binaries, and allow
the discovery of any new phenomena that inhabit this relatively unexplored
parameter space.Comment: final version, 7 pages, 5 figures, ApJ in pres
The puzzling interpretation of NIR indices: The case of NaI2.21
We present a detailed study of the Na I line strength index centered in the
-band at , {\AA} (NaI2.21 hereafter) relying on different samples of
early-type galaxies. Consistent with previous studies, we find that the
observed line strength indices cannot be fit by state-of-art scaled-solar
stellar population models, even using our newly developed models in the NIR.
The models clearly underestimate the large NaI2.21 values measured for most
early-type galaxies. However, we develop a Na-enhanced version of our newly
developed models in the NIR, which - together with the effect of a bottom-heavy
initial mass function - yield NaI2.21 indices in the range of the observations.
Therefore, we suggest a scenario in which the combined effect of [Na/Fe]
enhancement and a bottom-heavy initial mass function are mainly responsible for
the large NaI2.21 indices observed for most early-type galaxies. To a smaller
extent, also [C/Fe] enhancement might contribute to the large observed NaI2.21
values.Comment: 13 pages, 4 figures, accepted for publication in MNRA
HS 1857+5144 : a hot and young pre-cataclysmic variable
Aims. We report the discovery of a new white dwarf/M dwarf binary, HS 1857+5144, identified in the Hamburg Quasar Survey (HQS).
Methods. Time-resolved optical spectroscopy and photometry were carried out to determine the properties of this new cataclysmic variable progenitor (pre-CV).
Results. The light curves of HS 1857+5144 display a sinusoidal variation with a period of Porb = 383.52 min and peak-to-peak amplitudes of 0.7 mag and 1.1mag in the B-band and R-band, respectively. The large amplitude of the brightness variation results from a reflection effect on the heated inner hemisphere of the companion star, suggesting a very high temperature of the white
dwarf. Our radial velocity study confirms the photometric period as the orbital period of the system. A model atmosphere fit to the spectrum of the white dwarf obtained at minimum light provides limits to its mass and temperature of Mwd 0.6−1.0 M and Twd 70 000−100 000 K, respectively. The detection of He II λ4686 absorption classifies the primary star of HS 1857+5144 as a
DAO white dwarf. Combining the results from our spectroscopy and photometry, we estimate the mass of the companion star and the binary inclination to be Msec 0.15−0.30 M and i 45◦−55◦, respectively.
Conclusions. We classify HS 1857+5144 as one of the youngest pre-CV known to date. The cooling age of the white dwarf suggests that the present system has just emerged from a common envelope phase ∼105 yr ago. HS 1857+5144 will start mass transfer within or below the 2–3 h period gap
The Discovery of a Companion to the Lowest Mass White Dwarf
We report the detection of a radial velocity companion to SDSS
J091709.55+463821.8, the lowest mass white dwarf currently known with
M~0.17Msun. The radial velocity of the white dwarf shows variations with a
semi-amplitude of 148.8 km/s and a period of 7.5936 hours, which implies a
companion mass of M > 0.28Msun. The lack of evidence of a companion in the
optical photometry forces any main-sequence companion to be smaller than
0.1Msun, hence a low mass main sequence star companion is ruled out for this
system. The companion is most likely another white dwarf, and we present
tentative evidence for an evolutionary scenario which could have produced it.
However, a neutron star companion cannot be ruled out and follow-up radio
observations are required to search for a pulsar companion.Comment: ApJ, in press. See the Press Release at
http://www.cfa.harvard.edu/press/2007/pr200708.htm
Transient interference of transmission and incidence
Due to a transient quantum interference during a wavepacket collision with a
potential barrier, a particular momentum, that depends on the potential
parameters but is close to the initial average momentum, becomes suppressed.
The hole left pushes the momentum distribution outwards leading to a
significant constructive enhancement of lower and higher momenta. This is
explained in the momentum complex-plane language in terms of a saddle point and
two contiguous ``structural'' poles, which are not associated with resonances
but with incident and transmitted components of the wavefunction.Comment: 4 pages of text, 6 postscript figures, revte
Cohomological tautness for Riemannian foliations
In this paper we present some new results on the tautness of Riemannian
foliations in their historical context. The first part of the paper gives a
short history of the problem. For a closed manifold, the tautness of a
Riemannian foliation can be characterized cohomologically. We extend this
cohomological characterization to a class of foliations which includes the
foliated strata of any singular Riemannian foliation of a closed manifold
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