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Post Info TOPIC: Proxima Centauri
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Proxima Centauri b
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Title: Cosmic Rays near Proxima Centauri b
Author: Alexei Struminsky, Andrei Sadovski, Anatoly Belov

Cosmic rays are an important factor of space weather determining radiation conditions near the Earth and it seems to be essential to clarify radiation conditions near extrasolar planets too. Last year a terrestrial planet candidate was discovered in an orbit around Proxima Centauri. Here we present our estimates on parameters of stellar wind of the Parker model, possible fluxes and fluencies of galactic and stellar cosmic rays based on the available data of the Proxima Centauri activity and its magnetic field. We found that galactic cosmic rays will be practically absent near Proxima~b up to energies of 1~TeV due to the modulation by the stellar wind. Stellar cosmic rays may be accelerated in Proxima Centauri events, which are able to permanently maintain density of stellar cosmic rays in the astrosphere comparable to low energy cosmic ray density in the heliosphere. Maximal proton intensities in extreme Proxima events should be by 3--4 orders more than in solar events.

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Title: Exploring the climate of Proxima B with the Met Office Unified Model
Author: Ian A. Boutle, Nathan J. Mayne, Benjamin Drummond, James Manners, Jayesh Goyal, F. Hugo Lambert, David M. Acreman, Paul D. Earnshaw

We present results of simulations of the climate of the newly discovered planet Proxima Centauri B, performed using the Met Office Unified Model (UM). We examine the responses of both an `Earth-like' atmosphere and simplified nitrogen and trace carbon dioxide atmosphere to the radiation likely received by Proxima Centauri B. Additionally, we explore the effects of orbital eccentricity on the planetary conditions using a range of eccentricities guided by the observational constraints. Overall, our results are in agreement with previous studies in suggesting Proxima Centauri B may well have surface temperatures conducive to the presence of liquid water. Moreover, we have expanded the parameter regime over which the planet may support liquid water to higher values of eccentricity (>= 0.1) and lower incident fluxes (881.7 Wm-2) than previous work. This increased parameter space arises because of the low sensitivity of the planet to changes in stellar flux, a consequence of the stellar spectrum and orbital configuration. However, we also find interesting differences from previous simulations, such as cooler mean surface temperatures for the tidally-locked case. Finally, we have produced high resolution planetary emission and reflectance spectra, and highlight signatures of gases vital to the evolution of complex life on Earth (oxygen, ozone and carbon dioxide).

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Blobrana


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Title: On the eccentricity of Proxima b
Author: Robert A. Brown

We apply Monte Carlo projection to the radial velocity data set that Anglada Escude et al. (2016) use for the discovery of Proxima b. They find an upper limit to the orbital eccentricity of 0.35. To investigate the eccentricity issue further, we calculate a suite of monovariate and bivariate densities o eccentricity. After discarding apparent artifacts at eccentricity = 0 and = 1, we find that eccentricity has a trimodal sampling distribution: three chimeras or types of orbit compatible with the RV data set. The three modes or peaks in the density of eccentricity are located at eccentricity = 0.25, 0.75, and 0.95, with relative weights 0.79, 0.10, and 0.11. Future RV observations will clarify which of the three chimeras represents the true eccentricity of Proxima b. The most-likely estimate is eccentricity = 0.25, and our lower limit is eccentricity = 0.025. Our strategic, long-term goal is to elevate the orbital analyses of exoplanets to meet the challenges of sometimes complex probability density distributions.

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Blobrana


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RE: Proxima Centauri
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Orbit of Proxima Centauri Determined After 100 Years

Interest in our neighbouring Alpha Centauri star system has been particularly high since the recent discovery of an Earth-mass planet, known as Proxima b, orbiting the system's third star - and the closest star to the Sun - Proxima Centauri. While the system's larger stellar pair, Alpha Centauri A and B, appear to have a proper motion on the sky that is very similar to that of the smaller, fainter Proxima Centauri, it has not been possible to demonstrate that the three stars do actually form a single, gravitationally bound, triple system.
Now three astronomers, Pierre Kervella, Frédéric Thévenin and Christophe Lovis, have concluded that the three stars do indeed form a bound system.
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Blobrana


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Title: Proxima's orbit around Alpha Centauri
Author: P. Kervella, F. Thévenin

Proxima and Alpha Centauri AB have almost identical distances and proper motions with respect to the Sun. Although the probability of such similar parameters is in principle very low, the question whether they actually form a single gravitationally bound triple system has been open since the discovery of Proxima one century ago. Owing to recent high precision radial velocity measurements and the revision of the parameters of the Alpha Cen pair, we show that Proxima and Alpha Cen are gravitationally bound with a high degree of confidence. The orbital period of Proxima is approximately 600 000 years, with a moderate excentricity of 0.42 +0.07 -0.08. Proxima comes within 5.3 -0.9 +1.2 kAU of Alpha Cen at periastron, and the apastron occurs at 12.9 +0.3 -0.1 kAU. This orbital motion may have influenced the formation or evolution of the recently discovered planet orbiting Proxima as well as circumbinary planet formation around Alpha Cen.

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Proxima Centauri Might Be More Sunlike Than We Thought

Our Sun experiences an 11-year activity cycle. At the solar minimum, the Sun is nearly spot-free. At solar maximum, typically more than 100 sunspots cover less than one percent of the Sun's surface on average.
The new study finds that Proxima Centauri undergoes a similar cycle lasting seven years from peak to peak. However, its cycle is much more dramatic. At least a full one-fifth of the star's surface is covered in spots at once. Also, some of those spots are much bigger relative to the star's size than the spots on our Sun.
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Title: The Pale Green Dot: A Method to Characterize Proxima Centauri b using Exo-Aurorae
Author: Rodrigo Luger, Jacob Lustig-Yaeger, David P. Fleming, Matt A. Tilley, Eric Agol, Victoria S. Meadows, Russell Deitrick, Rory Barnes

We examine the feasibility of detecting auroral emission from the potentially habitable exoplanet Proxima Centauri b. This planet's active, late-type M dwarf host makes detection of aurorae more favourable than around a solar-type star, primarily by increasing auroral power and improving the planet-star contrast in the visible wavelength range due to strong TiO absorption in the star. Detection of aurorae would yield an independent confirmation of the planet's existence, constrain the presence and composition of its atmosphere, and determine the planet's eccentricity and inclination, thereby breaking the mass-inclination degeneracy. If Proxima b is a terrestrial world with an atmosphere and magnetic field, we estimate that its auroral power at the 5577\AA\ OI auroral line is on the order of 0.1 TW under steady-state stellar wind, or ~100 times stronger than that on Earth. This corresponds to a planet-star contrast ratio of 10^-6 - 10^-7 in a narrow band about the 5577\AA\ line, although higher contrast (10^-4 - 10^-5) may be possible during periods of strong magnetospheric disturbance (auroral power 1-10 TW). We searched the Proxima b HARPS data for the 5577\AA\ line, but find no signal, indicating that the OI auroral line power must be lower 3,000 TW, consistent with our predictions. We find that observations of 0.1 TW auroral emission lines are likely infeasible with current and planned telescopes. However, future observations with a large-aperture, space-based coronagraphic telescope or a ground-based extremely large telescope (ELT) with a coronagraph could push sensitivity down to terawatt oxygen aurorae (contrast 7 x 10^-6) with exposure times of ~1 day at high spectral resolution. If a coronagraph design contrast of 10^-7 can be achieved with negligible instrumental noise, a future concept ELT could observe steady-state auroral emission in a few nights.

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Title: The Space Weather of Proxima Centauri b
Author: Cecilia Garraffo, Jeremy J. Drake, Ofer Cohen

A planet orbiting in the "habitable zone" of our closest neighbouring star, Proxima Centauri, has recently been discovered, and the next natural question is whether or not Proxima b is "habitable". Stellar winds are likely a source of atmospheric erosion that could be particularly severe in the case of M dwarf habitable zone planets that reside close to their parent star. Here we study the stellar wind conditions that Proxima b experiences over its orbit. We construct 3-D MHD models of the wind and magnetic field around Proxima Centauri using a surface magnetic field map for a star of the same spectral type and scaled to match the observed ~ 600 G surface magnetic field strength of Proxima. We examine the wind conditions and dynamic pressure over different plausible orbits that sample the constrained parameters of the orbit of Proxima b. For all the parameter space explored, the planet is subject to stellar wind pressures of more than 2000 times those experienced by Earth from the solar wind. During an orbit, Proxima b is also subject to pressure changes of 1 to 3 orders of magnitude on timescales of a day. Its magnetopause standoff distance consequently undergoes sudden and periodic changes by a factor of 2 to 5. Proxima b will traverse the interplanetary current sheet twice each orbit, and likely crosses into regions of subsonic wind quite frequently. These effects should be taken into account in any physically realistic assessment or prediction of its atmospheric reservoir, characteristics and loss.

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Title: No Conclusive Evidence for Transits of Proxima b in MOST photometry
Author: David M. Kipping, Chris Cameron, Joel D. Hartman, James R. A. Davenport, Jaymie M. Matthews, Dimitar Sasselov, Jason Rowe, Robert J. Siverd, Jingjing Chen, Emily Sandford, Gáspár A. Bakos, Andres Jordan, Daniel Bayliss, Thomas Henning, Luigi Mancini, Kaloyan Penev, Zoltan Csubry, Waqas Bhatti, Joao Da Silva Bento, David B. Guenther, Rainer Kuschnig, Anthony F. J. Moffat, Slavek M. Rucinski, Werner W. Weiss

The analysis of Proxima Centauri's radial velocities recently led Anglada-Escud\'e et al. (2016) to claim the presence of a low mass planet orbiting the Sun's nearest star once every 11.2 days. Although the a-priori probability that Proxima b transits its parent star is just 1.5%, the potential impact of such a discovery would be considerable. Independent of recent radial velocity efforts, we observed Proxima Centauri for 12.5 days in 2014 and 31 days in 2015 with the MOST space telescope. We report here that we cannot make a compelling case that Proxima b transits in our precise photometric time series. Imposing an informative prior on the period and phase, we do detect a candidate signal with the expected depth. However, perturbing the phase prior across 100 evenly spaced intervals reveals one strong false-positive and one weaker instance. We estimate a false-positive rate of at least a few percent and a much higher false-negative rate of 20-40%, likely caused by the very high flare rate of Proxima Centauri. Comparing our candidate signal to HATSouth ground-based photometry reveals that the signal is somewhat, but not conclusively, disfavoured (1-2 sigmas) leading us to argue that the signal is most likely spurious. We expect that infrared photometric follow-up could more conclusively test the existence of this candidate signal, owing to the suppression of flare activity and the impressive infrared brightness of the parent star.

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Blobrana


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RE: Proxima Centauri
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Title: A terrestrial planet candidate in a temperate orbit around Proxima Centauri
Author: Guillem Anglada-Escudé, Pedro J. Amado, John Barnes, Zaira M. Berdiñas, R. Paul Butler, Gavin A. L. Coleman, Ignacio de la Cueva, Stefan Dreizler, Michael Endl, Benjamin Giesers, Sandra V. Jeffers, James S. Jenkins, Hugh R. A. Jones, Marcin Kiraga, Martin Kürster, María J. López-González, Christopher J. Marvin, Nicolás Morales, Julien Morin, Richard P. Nelson, José L. Ortiz, Aviv Ofir, Sijme-Jan Paardekooper, Ansgar Reiners, Eloy Rodríguez, Cristina Rodríguez-López, Luis F. Sarmiento, John P. Strachan, Yiannis Tsapras, Mikko Tuomi, Mathias Zechmeister

At a distance of 1.295 parsecs, the red-dwarf Proxima Centauri (alpha Centauri C, GL 551, HIP 70890, or simply Proxima) is the Sun's closest stellar neighbor and one of the best studied low-mass stars. It has an effective temperature of only ~ 3050 K, a luminosity of ~0.1 per cent solar, a measured radius of 0.14 solar Radii and a mass of about 12 per cent the mass of the Sun. Although Proxima is considered a moderately active star, its rotation period is ~ 83 days, and its quiescent activity levels and X-ray luminosity are comparable to the Sun's. New observations reveal the presence of a small planet orbiting Proxima with a minimum mass of 1.3~Earth masses and an orbital period of ~11.2 days. Its orbital semi-major axis is ~0.05 AU, with an equilibrium temperature in the range where water could be liquid on its surface.

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