Tides and the evolution of planetary habitability

Tides raised on a planet by the gravity of its host star can reduce the planet's orbital semi-major axis and eccentricity. This effect is only relevant for planets orbiting very close to their host stars. The habitable zones of low-mass stars are also close in, and tides can alter the orbits of planets in these locations. We calculate the tidal evolution of hypothetical terrestrial planets around low-mass stars and show that tides can evolve planets past the inner edge of the habitable zone, sometimes in less than 1 billion years. This migration requires large eccentricities (>0.5) and low-mass stars ( less or similar to 0.35 M(circle)). Such migration may have important implications for the evolution of the atmosphere, internal heating, and the Gaia hypothesis. Similarly, a planet that is detected interior to the habitable zone could have been habitable in the past. We consider the past habitability of the recently discovered, approximately 5 M(circle) planet, Gliese 581 c. We find that it could have been habitable for reasonable choices of orbital and physical properties as recently as 2 Gyr ago. However, when constraints derived from the additional companions are included, most parameter choices that indicate past habitability require the two inner planets of the system to have crossed their mutual 3:1 mean motion resonance. As this crossing would likely have resulted in resonance capture, which is not observed, we conclude that Gl 581 c was probably never habitable.     

X-ray broad line emission in Seyfert galaxies: Mrk 766 and NGC 4051

Long (>100 ks) observations of the bright Seyfert galaxies Mrk 766 and NGC 4051 have been obtained using XMM-Newton. The RGS 5–38 Å spectra reveal evidence of broad features. These can be modelled with relativistic emission lines coming from the immediate vicinity of a massive rotating black hole. Lines of OVIII, NVII and CVI are required to reproduce the spectrum of Mrk 766, whereas the spectrum of NGC 4051 can be modelled using a single, even broader OVIII line. Both Seyferts also exhibit broad iron line emission in the 2–8 keV range, and the data available thus far suggest that the strength of the low-energy emission lines and the strength of the iron line may be correlated.     

STEREO IMPACT Investigation Goals, Measurements, and Data Products Overview

The IMPACT (In situ Measurements of Particles And CME Transients) investigation on the STEREO mission was designed and developed to provide multipoint solar wind and suprathermal electron, interplanetary magnetic field, and solar energetic particle information required to unravel the nature of coronal mass ejections and their heliospheric consequences. IMPACT consists of seven individual sensors which are packaged into a boom suite, and a SEP suite. This review summarizes the science objectives of IMPACT, the instruments that comprise the IMPACT investigation, the accommodation of IMPACT on the STEREO twin spacecraft, and the overall data products that will flow from the IMPACT measurements. Accompanying papers in this volume of Space Science Reviews highlight the individual sensor technical details and capabilities, STEREO project plans for the use of IMPACT data, and modeling activities for IMPACT (and other STEREO) data interpretation.     

Long-Term Fluences of Solar Energetic Particles from H to Fe

Data from ACE and GOES have been used to measure Solar Energetic Particle (SEP) fluence spectra for H, He, O, and Fe, over the period from October 1997 to December 2005. The measurements were made by four instruments on ACE and the EPS sensor on three GOES satellites and extend in energy from ∼0.1 MeV/nuc to ∼100 MeV/nuc. Fluence spectra for each species were fit by conventional forms and used to investigate how the intensities, composition, and spectral shapes vary from year to year.     

The discovery of a 25 min regular modulation in the X-ray flux from 2S0142+61

A 13 hr observation of 2S0142+61 on 1984 August 27 by EXOSAT shows the X-ray flux of 2S0142+61 to be modulated with a period of 1456+/-6 s. The 1–10 keV spectrum is two component with a 0.7 keV thermal and 0.0 energy index power law, with 30% of the total luminosity in the thermal component. The spectrum is absorbed by 1 × 10²² H cm^-2. Only the hard component is pulsed with a 3 to 10 keV peak to mean amplitude of 35%. Below 2 keV the modulation is less than a few percent. The total 1–10 keV luminosity is 3.5 × 1032 erg s^-1 for a distance of 100 pc. Possible optical counterparts are discussed.     

Impact of ITRS 2014 realizations on altimeter satellite precise orbit determination

This paper evaluates orbit accuracy and systematic error for altimeter satellite precise orbit determination on TOPEX, Jason-1, Jason-2 and Jason-3 by comparing the use of four SLR/DORIS station complements from the International Terrestrial Reference System (ITRS) 2014 realizations with those based on ITRF2008. The new Terrestrial Reference Frame 2014 (TRF2014) station complements include ITRS realizations from the Institut National de l’Information Géographique et Forestière (IGN) ITRF2014, the Jet Propulsion Laboratory (JPL) JTRF2014, the Deutsche Geodätisches Forschungsinstitut (DGFI) DTRF2014, and the DORIS extension to ITRF2014 for Precise Orbit Determination, DPOD2014. The largest source of error stems from ITRF2008 station position extrapolation past the 2009 solution end time. The TRF2014 SLR/DORIS complement impact on the ITRF2008 orbit is only 1–2 mm RMS radial difference between 1992–2009, and increases after 2009, up to 5 mm RMS radial difference in 2016. Residual analysis shows that station position extrapolation error past the solution span becomes evident even after two years, and will contribute to about 3–4 mm radial orbit error after seven years. Crossover data show the DTRF2014 orbits are the most accurate for the TOPEX and Jason-2 test periods, and the JTRF2014 orbits for the Jason-1 period. However for the 2016 Jason-3 test period only the DPOD2014-based orbits show a strong and statistically significant margin of improvement. The positive results with DTRF2014 suggest the new approach to correct station positions or normal equations for non-tidal loading before combination is beneficial. We did not find any compelling POD advantage in using non-linear over linear station velocity models in our SLR & DORIS orbit tests on the Jason satellites. The JTRF2014 proof-of-concept ITRS realization demonstrates the need for improved SLR+DORIS orbit centering when compared to the Ries (2013) CM annual model. Orbit centering error is seen as an annual radial signal of 0.4 mm amplitude with the CM model. The unmodeled CM signals show roughly a 1.8 mm peak-to-peak annual variation in the orbit radial component. We find the TRF network stability pertinent to POD can be defined only by examination of the orbit-specific tracking network time series. Drift stability between the ITRF2008 and the other TRF2014-based orbits is very high, the relative mean radial drift error over water is no larger than 0.04 mm/year over 1993–2015. Analyses also show TRF induced orbit error meets current altimeter rate accuracy goals for global and regional sea level estimation.     

X-ray emission from cataclysmic variables

EXOSAT results on cataclysmic variables are reviewed. The long continuous X-ray observations afforded by this observatory, coupled with the sensitivity of its instruments to medium energy and very low energy X-rays, have enabled the rotational and orbital X-ray light curves of these stars to be measured in unprecedented detail. Examples are given of data on synchronously and asynchronously rotating magnetic systems, and on disc accreting stars. The impact of the new observations on our understanding of cataclysmic variables is discussed.     

An EXOSAT observation of the peculiar X-ray binary 2S 0921-630 during optical eclipse

An x-ray observation of 2S 0921-630 has been made coincident with the time of optical eclipse of this 9-day binary. No significant reduction in X-ray flux is measured. This can be explained if the binary orbit is viewed almost exactly edge-on, so that the central X-ray emitting star is obscured by the accretion disk from direct view. The X-rays that are seen are scattered into the line of sight by material above and below the disk and the apparent size of the X-ray emission region is thus large compared to the size of the occulting star.     

Evidence for a Two-Stage Acceleration Process in Large Solar Energetic Particle Events

Using high-resolution mass spectrometers on board the Advanced Composition Explorer (ACE), we surveyed the event-averaged ∼0.1–60 MeV/nuc heavy ion elemental composition in 64 large solar energetic particle (LSEP) events of cycle 23. Our results show the following: (1) The Fe/O ratio decreases with increasing energy up to ∼10 MeV/nuc in ∼92% of the events and up to ∼60 MeV/nuc in ∼64% of the events. (2) The rare isotope ³He is greatly enhanced over the corona or the solar wind values in 46% of the events. (3) The heavy ion abundances are not systematically organized by the ion’s M/Q ratio when compared with the solar wind values. (4) Heavy ion abundances from C–Fe exhibit systematic M/Q-dependent enhancements that are remarkably similar to those seen in ³He-rich SEP events and CME-driven interplanetary (IP) shock events. Taken together, these results confirm the role of shocks in energizing particles up to ∼60 MeV/nuc in the majority of large SEP events of cycle 23, but also show that the seed population is not dominated by ions originating from the ambient corona or the thermal solar wind, as previously believed. Rather, it appears that the source material for CME-associated large SEP events originates predominantly from a suprathermal population with a heavy ion enrichment pattern that is organized according to the ion’s mass-per-charge ratio. These new results indicate that current LSEP models must include the routine production of this dynamic suprathermal seed population as a critical pre-cursor to the CME shock acceleration process.     

Origin, Injection, and Acceleration of CIR Particles: Observations Report of Working Group 6

This report emphasizes new observational aspects of CIR ions revealed by advanced instruments launched on the Ulysses, WIND, SOHO, and ACE spacecraft, and by the unique vantage point of Ulysses which carried out the first survey of Corotating Interaction Region (CIR) properties over a very wide range of heliolatitudes. With this more complete observational picture established, this review is the basis to consider the status of theoretical models on origin, injection, and acceleration of CIR particles reported by Scholer, Mann et al. (1999) in this volume. This revised version was published online in August 2006 with corrections to the Cover Date.