2,644 research outputs found
A Self-Occulting Accretion Disk in the SW Sex Star DW UMa
We present the ultraviolet spectrum of the SW Sex star and nova-like variable
DW UMa in an optical low state, as observed with the Space Telescope Imaging
Spectrograph on board the Hubble Space Telescope (HST). The data are well
described by a synthetic white dwarf (WD) spectrum with T_eff = 46,000 +/- 1000
K, log g = 7.60 +/- 0.15, v*sin(i) = 370 +/- 100 km/s and Z/Z_solar = 0.47 +/-
0.15. For this combination of T_eff and log g, WD models predict M_WD = 0.48
+/- 0.06 M_solar and R_WD = (1.27 +/- 0.18) * 10^9 cm. Combining the radius
estimate with the normalization of the spectral fit, we obtain a distance
estimate of d = 830 +/-150 pc.
During our observations, DW UMa was approximately 3 magnitudes fainter in V
than in the high state. A comparison of our low-state HST spectrum to a
high-state spectrum obtained with the International Ultraviolet Explorer shows
that the former is much bluer and has a higher continuum level shortward of
1450 A. Since DW UMa is an eclipsing system, this suggests that an optically
thick accretion disk rim blocks our view of the WD primary in the high state.
If self-occulting accretion disks are common among the SW Sex stars, we can
account for (i) the preference for high-inclination systems within the class
and (ii) their V-shaped continuum eclipses. Moreover, even though the emission
lines produced by a self-obscured disk are generally still double-peaked, they
are weaker and narrower than those produced by an unobscured disk. This may
allow a secondary line emission mechanism to dominate and produce the
single-peaked, optical lines that are a distinguishing characteristic of the SW
Sex stars.Comment: 9 pages, including 2 figures; accepted for publication in
Astrophysical Journal Letters; New version matches version in press (footnote
added to discussion section; figures now use color
Semi-infinite parabolic IC-sheaf
Let G be a connected reductive group, P its parabolic subgroup. We consider
the parabolic semi-infinite category of sheaves on the affine Grassmanian of G
and construct the parabolic version of the semi-infinite IC-sheaf of each
orbit. We establish some of its properties and relate it to sheaves on the
Drinfeld compactification of the moduli stack Bun_P of P-torsors on a curve. We
also relate the parabolic semi-infinite IC-sheaf with the dual baby Verma
object on the spectral side.Comment: 85
Load-Aware Modeling and Analysis of Heterogeneous Cellular Networks
Random spatial models are attractive for modeling heterogeneous cellular
networks (HCNs) due to their realism, tractability, and scalability. A major
limitation of such models to date in the context of HCNs is the neglect of
network traffic and load: all base stations (BSs) have typically been assumed
to always be transmitting. Small cells in particular will have a lighter load
than macrocells, and so their contribution to the network interference may be
significantly overstated in a fully loaded model. This paper incorporates a
flexible notion of BS load by introducing a new idea of conditionally thinning
the interference field. For a K-tier HCN where BSs across tiers differ in terms
of transmit power, supported data rate, deployment density, and now load, we
derive the coverage probability for a typical mobile, which connects to the
strongest BS signal. Conditioned on this connection, the interfering BSs of the
tier are assumed to transmit independently with probability ,
which models the load. Assuming - reasonably - that smaller cells are more
lightly loaded than macrocells, the analysis shows that adding such access
points to the network always increases the coverage probability. We also
observe that fully loaded models are quite pessimistic in terms of coverage.Comment: to appear, IEEE Transactions on Wireless Communication
Seed dormancy and endogenous growth substances in Anab-e-Shahi grapes
Auxin, gibberellin and inhibitor contents were estimated in the freshly extracted seeds as well as in those moist stratified for 30, 45, 60, 75 and 90 days. The maximum seed germination was obtained when the seeds were stratified for 60 days. Auxin activity increased with the increase in the period of stratification and was found to be at its maximum in the germinating seeds. Fully ripe freshly extracted seeds did not show any gibberellin-like activity. It was maximum in the extracts of the seeds moist-stratified for 60 days. Abscisic acid-like inhibitor was present in the extracts of the fully ripe fresh seeds. Its quantity decreased with the period of stratification and was minimum when the seeds were stratified for 45 and 90 days. From these studies, it may be postulated that the seed germination in grapes is regulated by the appearance of growth promoting substances like auxins and gibberellins during stratification and disappearance of inhibitors like abscisic acid. In all the cultivars, stratification for 60 days was sufficient to create these conditions for the best seed germination.Samenruhe und endogene Wachstumsregulatoren bei der Rebensorte Anab-e-ShahiDer Gehalt an Auxin, Gibberellin und Hemmstoffen in frisch isolierten Samen von Anab-e-Shahi sowie in Samen, die 30, 45, 60, 75 und 90 Tage lang feucht stratifiziert worden waren, wurde bestimmt. Durch 60tägige Stratifikation wurde die Keimung am stärksten gefördert. Mit der Dauer der Stratifikation nahm auch die Auxinaktivität zu, um ihr Maximum in den keimenden Samen zu erreichen. Voll ausgereifte, frisch isolierte Samen zeigten keinerlei Gibberellin-artige Aktivität. Eine solche war in den Extrakten von Samen, die 60 Tage lang feucht stratifiziert worden waren, im Maximum vorhanden. In den Extrakten voll ausgereifter, frischer Samen trat ein Abscisinsäure-artiger Hemmstoff auf. Seine Konzentration nahm mit fortschreitender Stratifikation ab und erreichte ihr Minimum nach 45 und 90 Tagen. Aufgrund der vorliegenden Untersuchungen kann angenommen werden, daß die Samenkeimung bei Reben durch das verstärkte Auftreten wachstumsfördernder Substanzen wie Auxine und Gibberelline im Laufe der Stratifikation und den Rückgang Abscisinsäure-artiger Hemmstoffe reguliert wird. Auch bei anderen Rebensorten genügte durchweg eine 60tägige Stratifikation, um die bestmöglichen Bedingungen für die Samenkeimung zu schaffen
Fundamentals of Heterogeneous Cellular Networks with Energy Harvesting
We develop a new tractable model for K-tier heterogeneous cellular networks
(HetNets), where each base station (BS) is powered solely by a self-contained
energy harvesting module. The BSs across tiers differ in terms of the energy
harvesting rate, energy storage capacity, transmit power and deployment
density. Since a BS may not always have enough energy, it may need to be kept
OFF and allowed to recharge while nearby users are served by neighboring BSs
that are ON. We show that the fraction of time a k^{th} tier BS can be kept ON,
termed availability \rho_k, is a fundamental metric of interest. Using tools
from random walk theory, fixed point analysis and stochastic geometry, we
characterize the set of K-tuples (\rho_1, \rho_2, ... \rho_K), termed the
availability region, that is achievable by general uncoordinated operational
strategies, where the decision to toggle the current ON/OFF state of a BS is
taken independently of the other BSs. If the availability vector corresponding
to the optimal system performance, e.g., in terms of rate, lies in this
availability region, there is no performance loss due to the presence of
unreliable energy sources. As a part of our analysis, we model the temporal
dynamics of the energy level at each BS as a birth-death process, derive the
energy utilization rate, and use hitting/stopping time analysis to prove that
there exists a fundamental limit on \rho_k that cannot be surpassed by any
uncoordinated strategy.Comment: submitted to IEEE Transactions on Wireless Communications, July 201
Modeling and Analysis of K-Tier Downlink Heterogeneous Cellular Networks
Cellular networks are in a major transition from a carefully planned set of
large tower-mounted base-stations (BSs) to an irregular deployment of
heterogeneous infrastructure elements that often additionally includes micro,
pico, and femtocells, as well as distributed antennas. In this paper, we
develop a tractable, flexible, and accurate model for a downlink heterogeneous
cellular network (HCN) consisting of K tiers of randomly located BSs, where
each tier may differ in terms of average transmit power, supported data rate
and BS density. Assuming a mobile user connects to the strongest candidate BS,
the resulting Signal-to-Interference-plus-Noise-Ratio (SINR) is greater than 1
when in coverage, Rayleigh fading, we derive an expression for the probability
of coverage (equivalently outage) over the entire network under both open and
closed access, which assumes a strikingly simple closed-form in the high SINR
regime and is accurate down to -4 dB even under weaker assumptions. For
external validation, we compare against an actual LTE network (for tier 1) with
the other K-1 tiers being modeled as independent Poisson Point Processes. In
this case as well, our model is accurate to within 1-2 dB. We also derive the
average rate achieved by a randomly located mobile and the average load on each
tier of BSs. One interesting observation for interference-limited open access
networks is that at a given SINR, adding more tiers and/or BSs neither
increases nor decreases the probability of coverage or outage when all the
tiers have the same target-SINR.Comment: IEEE Journal on Selected Areas in Communications, vol. 30, no. 3, pp.
550 - 560, Apr. 201
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