Context. The TESS satellite was launched in 2018 to perform high-precision photometry from space over almost the whole sky in a search for exoplanets orbiting bright stars. This instrument has opened new opportunities to study variable hot subdwarfs, white dwarfs, and related compact objects. Targets of interest include white dwarf and hot subdwarf pulsators, both carrying high potential for asteroseismology. Aims. We present the discovery and detailed asteroseismic analysis of a new g -mode hot B subdwarf (sdB) pulsator, EC 21494−7018 (TIC 278659026), monitored in TESS first sector using 120-s cadence. Methods. The TESS light curve was analyzed with standard prewhitening techniques, followed by forward modeling using our latest generation of sdB models developed for asteroseismic investigations. By simultaneously best-matching all the observed frequencies with those computed from models, we identified the pulsation modes detected and, more importantly, we determined the global parameters and structural configuration of the star. Results. The light curve analysis reveals that EC 21494−7018 is a sdB pulsator counting up to 20 frequencies associated with independent g -modes. The seismic analysis singles out an optimal model solution in full agreement with independent measurements provided by spectroscopy (atmospheric parameters derived from model atmospheres) and astrometry (distance evaluated from Gaia DR2 trigonometric parallax). Several key parameters of the star are derived. Its mass (0.391 ± 0.009 M ⊙ ) is significantly lower than the typical mass of sdB stars and suggests that its progenitor has not undergone the He-core flash; therefore this progenitor could originate from a massive (≳2 M ⊙ ) red giant, which is an alternative channel for the formation of sdBs. Other derived parameters include the H-rich envelope mass (0.0037 ± 0.0010 M ⊙ ), radius (0.1694 ± 0.0081 R ⊙ ), and luminosity (8.2 ± 1.1 L ⊙ ). The optimal model fit has a double-layered He+H composition profile, which we interpret as an incomplete but ongoing process of gravitational settling of helium at the bottom of a thick H-rich envelope. Moreover, the derived properties of the core indicate that EC 21494−7018 has burnt ∼43% (in mass) of its central helium and possesses a relatively large mixed core ( M core = 0.198 ± 0.010 M ⊙ ), in line with trends already uncovered from other g-mode sdB pulsators analyzed with asteroseismology. Finally, we obtain for the first time an estimate of the amount of oxygen (in mass; X (O) core = 0.16 +0.13 −0.05 ) produced at this stage of evolution by an helium-burning core. This result, along with the core-size estimate, is an interesting constraint that may help to narrow down the still uncertain 12 C( α , γ ) 16 O nuclear reaction rate.
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Gearing up for the 21st century space race
A new space race is imminent, with several industry players
working towards satellite-based Internet connectivity. While
satellite networks are not themselves new, these recent proposals are aimed at orders of magnitude higher bandwidth and
much lower latency, with constellations planned to comprise
thousands of satellites. These are not merely far future plans
— the first satellite launches have already commenced, and
substantial planned capacity has already been sold. It is thus
critical that networking researchers engage actively with this
research space, instead of missing what may be one of the
most significant modern developments in networking.
In our first steps in this direction, we find that this new
breed of satellite networks could potentially compete with
today’s ISPs in many settings, and in fact offer lower latencies than present fiber infrastructure over long distances. We
thus elucidate some of the unique challenges these networks
present at virtually all layers, from topology design and ISP
economics, to routing and congestion control.
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- NSF-PAR ID:
- 10140456
- Date Published:
- Journal Name:
- ACM Workshop on Hot Topics in Networks
- Page Range / eLocation ID:
- 113 to 119
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Summary This paper studies a problem for seamless migration of legacy networks of Internet service providers to a software‐defined networking (SDN)‐based architecture along with the transition to the full adoption of the Internet protocol version 6 (IPv6) connectivity. Migration of currently running legacy IPv4 networks into such new approaches requires either upgrades or replacement of existing networking devices and technologies that are actively operating. The joint migration to SDN and IPv6 network is considered to be vital in terms of migration cost optimization, skilled human resource management, and other critical factors. In this work, we first present the approaches of SDN and IPv6 migration in service providers' networks. Then, we present the common concerns of IPv6 and SDN migration with joint transition strategies so that the cost associated with joint migration is minimized to lower than that of the individual migration. For the incremental adoption of software‐defined IPv6 (SoDIP6) network with optimum migration cost, a greedy algorithm is proposed based on optimal path and the customer priority. Simulation and empirical analysis show that a unified transition planning to SoDIP6 network results in lower migration cost.
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null (Ed.)The primary objectives of International Ocean Discovery Program (IODP) Expedition 367/368 to the northern South China Sea (SCS) margin were to (1) examine its history of continental breakup and (2) compare it with other nonvolcanic or magma-poor rifted margins with the broader goal of testing models for continental breakup. A secondary objective was to further our understanding of the paleoceanographic and environmental development of the SCS and southeast Asia during the Cenozoic. Four primary sites were selected for the overall program: one in the outer margin high (OMH) and three seaward of the OMH on distinct, margin-parallel basement ridges. These three ridges are informally labeled A, B, and C and are located in the continent–ocean transition (COT) zone ranging from the OMH to the interpreted steady-state oceanic crust (Ridge C) of the SCS. The main scientific objectives include the following: Determining the nature of the basement in crustal units across the COT of the SCS that are critical to constrain style of rifting, Constraining the time interval from initial crustal extension and plate rupture to the initial generation of igneous ocean crust, Constraining vertical crustal movements during breakup, and Examining the nature of igneous activity from rifting to seafloor spreading. In addition, the sediment cores from the drill sites targeting primarily tectonic and basement objectives will provide information on the Cenozoic regional environmental development of the Southeast Asia margin. Site U1499 on Ridge A and Site U1500 on Ridge B were drilled during Expedition 367. Expedition 368 was planned to drill at two primary sites (U1501 and U1503) at the OMH and Ridge C, respectively, but based on drilling results from Expedition 367, Expedition 368 chose to insert an alternate site on Ridge A (Site U1502). In addition, Expedition 368 added two more sites on the OMH (Sites U1504 and U1505). Expedition 367/368 completed operations at six of the seven sites (U1499–U1502, U1504, and U1505). Site U1503, however, was not completed beyond casing without coring to 990 m because of mechanical problems with the drilling equipment that prevented the expedition, after 25 May 2017, from operating with a drill string longer than 3400 m. New alternate Site U1504, proposed during Expedition 367, met this condition. Original Site U1505 also met the operational constraints of the 3400 m drill string (total) and was an alternate site for the already-drilled Site U1501. At Site U1499, we cored to 1081.8 m in 22.1 days with 52% recovery and then logged downhole data from 655 to 1020 m. In 31 days at Site U1500, we penetrated to 1529 m, cored a total of 1012.8 m with 37% recovery, and collected log data from 842 to 1133 m. At Site U1501, we cored to 697.1 m in 9.4 days with 78.5% recovery. We also drilled ahead for 433.5 m in Hole U1501D and then logged downhole data from 78.3 to 399.3 m. In 19.3 days at Site U1502, we penetrated 1679.0 m in Holes U1502A (758 m) and U1502B (921 m), set 723.7 m of casing and cored a total of 576.3 m with 53.5% recovery, and collected downhole log data from 785.3 to 875.3 m and seismic data through the 10¾ inch casing. At Site U1503, we penetrated 995.1 m and set 991.5 m of 10¾ inch casing, but no cores were taken because of a mechanical problem with the drawworks. At Site U1504, we took 40 rotary core barrel (RCB) cores over two holes. The cored interval between both holes was 277.3 m with 26.8% recovery. An 88.2 m interval was drilled in Hole U1504B. At Site U1505, we cored 668.0 m with 101.1% recovery. Logging data was collected from 80.1 to 341.2 m. Operations at this site covered 6.1 days. Except for Sites U1503 and U1505, all sites were drilled to acoustic basement. A total of 6.65 days were lost due to mechanical breakdown or waiting on spare supplies for repair of drilling equipment, but drilling options were severely limited from 25 May to the end of the expedition by the defective drawworks limiting deployment of drill string longer than 3400 m. At Site U1499, coring ~200 m into the interpreted acoustic basement sampled sedimentary rocks, possibly including early Miocene chalks underlain by Oligocene polymict breccias and poorly cemented gravels of unknown age comprising sandstone pebbles and cobbles. Preliminary structural and lithologic analysis suggests that the gravels might be early to late synrift sediment. At Site U1500, the main seismic reflector corresponds to the top of a basalt sequence at ~1379.1 m. We cored 149.90 m into this volcanic package and recovered 114.92 m (77%) of sparsely to moderately plagioclase-phyric basalt comprising numerous lava flows, including pillow lavas with glass, chilled margins, altered veins, hyaloclastites, and minor sediment. Preliminary geochemical analyses indicate that the basalt is tholeiitic. Sampling of the Pleistocene to lower Miocene sedimentary section at Sites U1499 and U1500 was not continuous for two reasons. First, there was extremely poor recovery in substantial intervals interpreted to be poorly lithified sands, possibly turbidites. Second, we chose to drill down without coring in some sections at Site U1500 to ensure sufficient time to achieve this site’s high-priority deep drilling objectives. The upper Miocene basin sequence, which consists of interbedded claystone, siltstone, and sandstone can be correlated between the two sites by seismic stratigraphic mapping and biostratigraphy. At Site U1501 on the OMH, coring ~45 m into the acoustic basement sampled prerift(?) deposits comprising sandstone to conglomerate of presumed Mesozoic age. These deposits are overlain by siliciclastic synrift sediments of Eocene to Oligocene age followed by primarily carbonaceous postrift sediments of early Miocene to Pleistocene age. Site U1502 on Ridge A was cased to 723.7 m. No coring was attempted shallower than 380 m to save operational time and because of low expectations for core recovery in the upper Plio–Pleistocene sequence. At this site, we recovered 180 m of hydrothermally altered brecciated basalts comprising sheet and pillow lavas below deep-marine sediments of Oligocene to late Miocene age. At Site U1503 on Ridge C, 991.5 m of casing was installed in preparation for the planned deep drilling to ~1800 m. No coring was performed due to mechanical failures, and the site was abandoned without further activity except for installation of a reentry cone. Coring at Site U1504 on the OMH, located ~45 km east of Site U1501, recovered mostly foliated, greenschist facies metamorphic rocks below late Eocene(?) carbonate rocks (partly reef debris) and early Miocene to Pleistocene sediments. At Site U1505, we cored to 480.15 m through Pleistocene to late Oligocene mainly carbonaceous ooze followed at depth by early Oligocene siliciclastic sediments. Efforts were made at every drill site to correlate the core with the seismic data and seismic stratigraphic unconformities interpreted in the Eocene to Plio–Pleistocene sedimentary sequence prior to drilling. The predrilling interpretation of ages of these unconformities was in general confirmed by drilling results, although some nontrivial corrections can be expected from detailed postexpedition work on integrating seismic stratigraphic interpretations with detailed bio- and lithostratigraphy. As a result of the limited length of drill string that could be deployed during the later part of Expedition 368, the secondary expedition objectives addressing the environmental history of the SCS and Southeast Asia received more focus than originally planned, allowing Site U1505 (alternate to Site U1501) to be included. Despite this change in focus, Expedition 367/368 provided solid evidence for a process of breakup that included vigorous synrift magmatism as opposed to the often-favored interpretation of the SCS margin as a magma-starved margin or a margin possibly overprinted at a much later stage by plume-related magmatism. In this broader perspective, Expedition 367/368 accomplished a fundamental objective of the two-expedition science program.more » « less
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We present the first Bayesian method for tomographic decomposition of the plane-of-sky orientation of the magnetic field with the use of stellar polarimetry and distance. This standalone tomographic inversion method presents an important step forward in reconstructing the magnetized interstellar medium (ISM) in three dimensions within dusty regions. We develop a model in which the polarization signal from the magnetized and dusty ISM is described by thin layers at various distances, a working assumption which should be satisfied in small-angular circular apertures. Our modeling makes it possible to infer the mean polarization (amplitude and orientation) induced by individual dusty clouds and to account for the turbulence-induced scatter in a generic way. We present a likelihood function that explicitly accounts for uncertainties in polarization and parallax. We develop a framework for reconstructing the magnetized ISM through the maximization of the log-likelihood using a nested sampling method. We test our Bayesian inversion method on mock data, representative of the high Galactic latitude sky, taking into account realistic uncertainties from Gaia and as expected for the optical polarization survey P ASIPHAE according to the currently planned observing strategy. We demonstrate that our method is effective at recovering the cloud properties as soon as the polarization induced by a cloud to its background stars is higher than ~0.1% for the adopted survey exposure time and level of systematic uncertainty. The larger the induced polarization is, the better the method’s performance, and the lower the number of required stars. Our method makes it possible to recover not only the mean polarization properties but also to characterize the intrinsic scatter, thus creating new ways to characterize ISM turbulence and the magnetic field strength. Finally, we apply our method to an existing data set of starlight polarization with known line-of-sight decomposition, demonstrating agreement with previous results and an improved quantification of uncertainties in cloud properties.more » « less
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Monero has emerged as one of the leading cryptocurrencies with privacy by design. However, this comes at the price of reduced expressiveness and interoperability as well as severe scalability issues. First, Monero is restricted to coin exchanges among individual addresses and no further functionality is supported. Second, transactions are authorized by linkable ring signatures, a digital signature scheme used in Monero, hindering thereby the interoperability with virtually all the rest of cryptocurrencies that support different digital signature schemes. Third, Monero transactions require an on-chain footprint larger than other cryptocurrencies, leading to rapid ledger growth and thus scalability issues. This work extends Monero expressiveness and interoperability while mitigating its scalability issues. We present Dual Linkable Spontaneous Anonymous Group Signature for Ad Hoc Groups (DLSAG), a linkable ring signature scheme that enables for the first time non-interactive refund transactions natively in Monero: DLSAG can seamlessly be implemented along with other cryptographic tools already available in Monero such as commitments and range proofs. We formally prove that DLSAG provides the same security and privacy notions introduced in the original linkable ring signature [31] namely, unforgeability, signer ambiguity, and linkability. We have evaluated DLSAG and showed that it imposes even slightly lower computation and similar communication overhead than the current digital signature scheme in Monero, demonstrating its practicality. We further show how to leverage DLSAG to enable off-chain scalability solutions in Monero such as payment channels and payment-channel networks as well as atomic swaps and interoperable payments with virtually all cryptocurrencies available today. DLSAG is currently being discussed within the Monero community as an option for adoption as a key building block for expressiveness, interoperability, and scalability.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1145/3286062.3286079
