Reduced-Complexity Maximum-Likelihood Detection in Downlink SDMA Systems

The literature of up-link SDMA systems is rich, but at the time of writing there is a paucity of information on the employment of SDMA techniques in the down-link. Hence, in this paper a Space Division Multiple Access (SDMA) down-link (DL) multi-user communication system invoking a novel low-complexity Maximum Likelihood (ML) space-time detection technique is proposed, which can be regarded as an advanced extension of the Complex Sphere Decoder (CSD). We demonstrate that as opposed to the previously published variants of the CSD, the proposed technique may be employed for obtaining a high effective throughput in the so-called “over-loaded” scenario, where the number of transmit antennas exceeds that of the receive antennas. The proposed method achieves the optimum performance of the ML detector even in heavily over-loaded scenarios, while the associated computational complexity is only moderately increased. As an illustrative example, the required Eb/N0 increased from 2 dB to 9 dB, when increasing the normalized system load from unity, representing the fully loaded system, to a normalized load of 1.556

Ionization in atmospheres of brown dwarfs and extrasolar planets III. Breakdown conditions for mineral clouds

Electric discharges were detected directly in the cloudy atmospheres of Earth, Jupiter, and Saturn, are debatable for Venus, and indirectly inferred for Neptune and Uranus in our solar system. Sprites (and other types of transient luminous events) have been detected only on Earth, and are theoretically predicted for Jupiter, Saturn, and Venus. Cloud formation is a common phenomenon in ultra-cool atmospheres such as in brown dwarf and extrasolar planetary atmospheres. Cloud particles can be expected to carry considerable charges which may trigger discharge events via small-scale processes between individual cloud particles (intra-cloud discharges) or large-scale processes between clouds (inter-cloud discharges). We investigate electrostatic breakdown characteristics, like critical field strengths and critical charge densities per surface, to demonstrate under which conditions mineral clouds undergo electric discharge events which may trigger or be responsible for sporadic X-ray emission. We apply results from our kinetic dust cloud formation model that is part of the Drift-Phoenix model atmosphere simulations. We present a first investigation of the dependence of the breakdown conditions in brown dwarf and giant gas exoplanets on the local gas-phase chemistry, the effective temperature, and primordial gas-phase metallicity. Our results suggest that different intra-cloud discharge processes dominate at different heights inside mineral clouds: local coronal (point discharges) and small-scale sparks at the bottom region of the cloud where the gas density is high, and flow discharges and large-scale sparks near, and maybe above, the cloud top. The comparison of the thermal degree of ionization and the number density of cloud particles allows us to suggest the efficiency with which discharges will occur in planetary atmospheres.Publisher PDFPeer reviewe

Slingshot prominences : a hidden mass loss mechanism

Funding: The authors acknowledge support from STFC consolidated grant number ST/R000824/1.Whilst "slingshot" prominences have been observed on M-dwarfs, most if not all theoretical studies have focused on solar-like stars. We present an investigation into stellar prominences around rapidly rotating young M-dwarfs. We have extrapolated the magnetic field in the corona from Zeeman-Doppler maps and determined the sites of mechanical stability where prominences may form. We analyse the prominence mass that could be supported and the latitude range over which this material is distributed. We find that for these maps, much of this prominence mass may be invisible to observation - typically <1% transits the stellar disc. On the rapidly-rotating M-dwarf V374 Peg (Prot = 0.45 days) where prominences have been observed, we find the visible prominence mass to be around only 10% of the total mass supported. The mass loss rate per unit area for prominences scales with the X-ray surface flux as Ṁ/A ∝ FX1.32 which is very close to the observationally-derived value for stellar winds. This suggests that prominence ejection may contribute significantly to the overall stellar wind loss and spin down. A planet in an equatorial orbit in the habitable zone of these stars may experience intermittent enhancements of the stellar wind due to prominence ejections. On some stars, this may occur throughout 20 per cent of the orbit.PostprintPublisher PDFPeer reviewe

Prominence formation and ejection in cool stars

We acknowledge STFC (ST/M001296/1) and H2020 (682393).The observational signatures of prominences have been detected in single and binary G and K type stars for many years now, but recently this has been extended to the M dwarf regime. Prominences carry away both mass and angular momentum when they are ejected and the impact of this mass on any orbiting planets may be important for the evolution of exoplanetary atmospheres. By means of the classification used in the massive star community, that involves knowledge of two parameters (the co-rotation and Alfvén radii, rK and rA), we have determined which cool stars could support prominences. From a model of mechanical support, we have determined that the prominence mass mp/M*=(EM/EG)(r*/rK)2F where EMB2⋆r3⋆ and EG=GM2⋆/r⋆ are magnetic and gravitational energies and F is a geometric factor. Our calculated masses and ejection frequencies (typically 1016−1017g and 0.4 d, respectively) are consistent with observations and are sufficient to ensure that an exoplanet orbiting in the habitable zone of an M dwarf could suffer frequent impacts.PostprintPeer reviewe

Could a change in magnetic field geometry cause the break in the wind-activity relation?

AAV acknowledges support from the Swiss National Science Foundation through an Ambizione Fellowship. SVJ and SBS acknowledge research funding by the Deutsche Forschungsgemeinschaft under grant SFB 963/1, project A16.Wood et al. suggested that mass-loss rate is a function of X-ray flux (dot{M}∝ F_x^{1.34}) for dwarf stars with Fx ≲ Fx,6 ≡ 106 erg cm-2 s-1. However, more active stars do not obey this relation. These authors suggested that the break at Fx,6 could be caused by significant changes in magnetic field topology that would inhibit stellar wind generation. Here, we investigate this hypothesis by analysing the stars in Wood et al. sample that had their surface magnetic fields reconstructed through Zeeman-Doppler Imaging (ZDI). Although the solar-like outliers in the dot{M} - Fx relation have higher fractional toroidal magnetic energy, we do not find evidence of a sharp transition in magnetic topology at Fx,6. To confirm this, further wind measurements and ZDI observations at both sides of the break are required. As active stars can jump between states with highly toroidal to highly poloidal fields, we expect significant scatter in magnetic field topology to exist for stars with Fx ≳ Fx,6. This strengthens the importance of multi-epoch ZDI observations. Finally, we show that there is a correlation between Fx and magnetic energy, which implies that dot{M} - magnetic energy relation has the same qualitative behaviour as the original dot{M} - Fx relation. No break is seen in any of the Fx - magnetic energy relations.Publisher PDFPeer reviewe

Magnetic activity and hot Jupiters of young Suns : the weak-line T Tauri stars V819 Tau and V830 Tau

SGG acknowledges support from the Science & Technology Facilities Council (STFC) via an Ernest Rutherford Fellowship [ST/J003255/1]. SHPA acknowledges financial support from CNPq, CAPES and Fapemig. AAV acknowledges support from the Swiss National Science Foundation (SNSF) via the allocation of an Ambizione Followship. Date of Acceptance: 06/08/2015We report results of a spectropolarimetric and photometric monitoring of the weak-line T Tauri stars (wTTSs) V819 Tau and V830 Tau within the MaTYSSE (Magnetic Topologies of Young Stars and the Survival of close-ingiant Exoplanets) programme, involving the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope. At ≃3 Myr, both stars dissipated their discs recently and are interesting objects for probing star and planet formation. Profile distortions and Zeeman signatures are detected in the unpolarized and circularly polarized lines, whose rotational modulation we modelled using tomographic imaging, yielding brightness and magnetic maps for both stars. We find that the large-scale magnetic fields of V819 Tau and V830 Tau are mostly poloidal and can be approximated at large radii by 350-400 G dipoles tilted at≃30° to the rotation axis. They are significantly weaker than the field of GQ Lup, an accreting classical T Tauri star (cTTS) with similar mass and age which can be used to compare the magnetic properties of wTTSs and cTTSs. The reconstructed brightness maps of both stars include cool spots and warm plages. Surface differential rotation is small, typically ≃4.4 times smaller than on the Sun, in agreement with previous results on wTTSs. Using our Doppler images to model the activity jitter and filter it out from the radial velocity(RV) curves, we obtain RV residuals with dispersions of 0.033 and 0.104km s-1 for V819 Tau and V830 Tau, respectively. RV residuals suggest that a hot Jupiter may be orbiting V830 Tau, though additional data are needed to confirm this preliminary result. We find no evidence for close-in giant planet around V819 Tau.Publisher PDFPeer reviewe

A spectro-polarimetric study of the planet-hosting G dwarf, HD 147513

The results from a spectro-polarimetric study of the planet-hosting Sun-like star, HD 147513 (G5V), are presented here. Robust detections of Zeeman signatures at all observed epochs indicate a surface magnetic field, with longitudinal magnetic field strengths varying between 1.0–3.2 G. Radial velocity variations from night to night modulate on a similar timescale to the longitudinal magnetic field measurements. These variations are therefore likely due to the rotational modulation of stellar active regions rather than the much longer timescale of the planetary orbit (Porb = 528 d). Both the longitudinal magnetic field measurements and radial velocity variations are consistent with a rotation period of 10 ± 2 days, which are also consistent with the measured chromospheric activity level of the star (′log R′HK = -4.64). Together, these quantities indicate a low inclination angle, i ~ 18°. We present preliminary magnetic field maps of the star based on the above period and find a simple poloidal large-scale field. Chemical analyses of the star have revealed that it is likely to have undergone a barium-enrichment phase in its evolution because of a higher mass companion. Despite this, our study reveals that the star has a fairly typical activity level for its rotation period and spectral type. Future studies will enable us to explore the long-term evolution of the field, as well as to measure the stellar rotation period, with greater accuracy.Publisher PDFPeer reviewe

Magnetic loops on rapidly rotating stars.

We present models for the thermal and mechanical equilibria of slender magnetic loops on rapidly-rotating stars. These loops are embedded within an arcade located on the stellar equator. The loop properties are governed principally by the specified base pressure and conductive flux. While rapid rotation is important in determining the pressure structure within the loop, its main effect on the loop shape is through its influence on the base values of magnetic field strength, pressure, and conductive flux. We have compared our models with observations of the temperatures and X-ray fluxes of stars with a range of rotation rates. By using the observed Variation of the differential emission measure with rotation rate Omega and allowing the base magnetic field strength to scale as B proportional to Omega(q) (q = 0.5, 0.75, 1.0) we can explain the variation of the temperature and X-ray flux of the slower rotators. For the most rapid rotators, however (approximately Omega/Omega. &gt; 10) it appears that a single value of q for all rotation rates cannot explain the observations and some other mechanism is needed to explain the saturation in the Xray flux. We have also investigated the effect of using a heating function that is proportional to the density and one that falls off exponentially with height. With the parameters that we can calculate from these models it is not possible to discriminate between these two types of heating.</p