Proxima Centauri

A Multi-Year Search For Transits Of Proxima Centauri. I: Light Curves Corresponding To Published Ephemerides
https://arxiv.org/abs/1802.04284

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Proxima Centauri has become the subject of intense study since the radial-velocity discovery by Anglada-Escud\'e et al. 2016 of a planet orbiting this nearby M-dwarf every ~ 11.2 days. If Proxima Centauri b transits its host star, independent confirmation of its existence is possible, and its mass and radius can be measured in units of the stellar host mass and radius. To date, there have been three independent claims of possible transit-like event detections in light curve observations obtained by the MOST satellite (in 2014-15), the BSST telescope in Antarctica (in 2016), and the Las Campanas Observatory (in 2016). The claimed possible detections are tentative, due in part to the variability intrinsic to the host star, and in the case of the ground-based observations, also due to the limited duration of the light curve observations. Here, we present preliminary results from an extensive photometric monitoring campaign of Proxima Centauri, using telescopes around the globe and spanning from 2006 to 2017, comprising a total of 329 observations. Considering our data that coincide directly and/or phased with the previously published tentative transit detections, we are unable to independently verify those claims. We do, however, verify the previously reported ubiquitous and complex variability of the host star. We discuss possible interpretations of the data in light of the previous claims, and we discuss future analyses of these data that could more definitively verify or refute the presence of transits associated with the radial-velocity discovered planet.

Whelp, ... there go the debris disks.

Detection of a Millimeter Flare From Proxima Centauri
https://arxiv.org/abs/1802.08257

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We present new analyses of ALMA 12-m and ACA observations at 233 GHz (1.3 mm) of the Proxima Centauri system with sensitivities of 9.5 and 47 μJy beam−1, respectively, taken from 2017 January 21 through 2017 April 25. These analyses reveal that the star underwent a significant flaring event during one of the ACA observations on 2017 March 24. The complete event lasted for approximately 1 minute and reached a peak flux density of 100±4 mJy, nearly a factor of 1000× brighter than the star's quiescent emission. At the flare peak, the continuum emission is characterized by a steeply falling spectral index with frequency, Fν∝να with α=−1.77±0.45, and a lower limit on the fractional linear polarization of |Q/I|=0.19±0.02. Since the ACA observations do not show any quiescent excess emission, we conclude that there is no need to invoke the presence of a dust belt at 1−4 AU. We also posit that the slight excess flux density of 101±9 μJy observed in the 12-m observations compared to the photospheric flux density of 74±4 μJy extrapolated from infrared wavelengths may be due to coronal heating from continual smaller flares, as is seen for AU Mic, another nearby, well-studied, M dwarf flare star. If this is true, then the need for warm dust at ∼0.4 AU is also removed.

Ouch. And it's plausible the ~30 AU belt is due to background sources as well. On the upside, that might make transits a (slightly) more realistic possibility.

The First Naked-Eye Superflare Detected from Proxima Centauri
https://arxiv.org/abs/1804.02001

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Proxima b is a terrestrial-mass planet in the habitable-zone of Proxima Centauri. Proxima Centauri's high stellar activity however casts doubt on the habitability of Proxima b: sufficiently bright and frequent flares and any associated proton events may destroy the planet's ozone layer, allowing lethal levels of UV flux to reach its surface. In March 2016, the Evryscope observed the first naked-eye-visible superflare detected from Proxima Centauri. Proxima increased in brightness by a factor of ~68 during the superflare and released a bolometric energy of 10^33.5 erg, ~10X larger than any previously-detected flare from Proxima. Over the last two years the Evryscope has recorded 23 other large Proxima flares ranging in bolometric energy from 10^30.6 erg to 10^32.4 erg; coupling those rates with the single superflare detection, we predict at least five superflares occur each year. Simultaneous high-resolution HARPS spectroscopy during the Evryscope superflare constrains the superflare's UV spectrum and any associated coronal mass ejections. We use these results and the Evryscope flare rates to model the photochemical effects of NOx atmospheric species generated by particle events from this extreme stellar activity, and show that the repeated flaring is sufficient to reduce the ozone of an Earth-like atmosphere by 90% within five years. We estimate complete depletion occurs within several hundred kyr. The UV light produced by the Evryscope superflare therefore reached the surface with ~100X the intensity required to kill simple UV-hardy microorganisms, suggesting that life would struggle to survive in the areas of Proxima b exposed to these flares.

Well that's unfortunate.

Well, this kind of thing probably goes some way to explain why we find ourselves orbiting a G-type star instead of the far more common M-dwarfs.

Hopefully whatever's at Proxima has factor nine billion sunblock.

Mass of Proxima Centauri as determined by microlensing is 0.150^+0.062_-0.051 solar masses, which would increase the minimum mass of Proxima b to 1.56^+0.064_-0.053 Earth masses. The candidate transit signal does not appear to have been caused by a background eclipsing binary.

Zurlo et al. "The gravitational mass of Proxima Centauri measured with SPHERE from a microlensing event"
https://arxiv.org/abs/1807.01318

Dynamical evolution and stability maps of the Proxima Centauri system
https://arxiv.org/abs/1809.08210

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Proxima Centauri was recently discovered to host an Earth-mass planet of Proxima b, and a 215-day signal which is probably a potential planet c. In this work, we investigate the dynamical evolution of the Proxima Centauri system with the full equations of motion and semi-analytical models including relativistic and tidal effects. We adopt the modified Lagrange-Laplace secular equations to study the evolution of eccentricity of Proxima b, and find that the outcomes are consistent with those from the numerical simulations. The simulations show that relativistic effects have an influence on the evolution of eccentricities of planetary orbits, whereas tidal effects primarily affects the eccentricity of Proxima b over long timescale. Moreover, using the MEGNO (the Mean Exponential Growth factor of Nearby Orbits) technique, we place dynamical constraints on orbital parameters that result in stable or quasi-periodic motions for coplanar and non-coplanar configurations. In the coplanar case, we find that the orbit of Proxima b is stable for the semi-major axis ranging from 0.02 au to 0.1 au and the eccentricity being less than 0.4. This is where the best-fitting parameters for Proxima b exactly fall. Additional simulations show that the robust stability of this system would favor an eccentricity of Proxima b less than 0.45 and that of Proxima c below 0.65. In the non-coplanar case, we find that mutual inclinations of two planets must be lower than 50∘ in order to provide stability. Finally, we estimate the mass of Proxima c to be 3.13 M⊕≤mc≤70.7 M⊕ when 1.27 M⊕≤mb≤1.6 M⊕, if imutual≤50∘ and ΔΩ=0∘.

A Multi-Year Search For Transits Of Proxima Centauri. II: No Evidence For Transit Events With Periods Between 1-30 Days
https://arxiv.org/abs/1901.07034

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Using a global network of small telescopes, we have obtained light curves of Proxima Centauri at 329 observation epochs from 2006 to 2017. The planet Proxima b discovered by Anglada-Escude et al. (2016) with an orbital period of 11.186 d has an a priori transit probability of ~ 1.5%; if it transits, the predicted transit depth is about 5 millimagnitudes. In Blank et al. (2018), we analyzed 96 of our light curves that overlapped with predicted transit ephemerides from previously published tentative transit detections, and found no evidence in our data that would corroborate claims of transits with a period of 11.186 d. Here we broaden our analysis, using 262 high-quality light curves from our dataset to search for any periodic transit-like events over a range of periods from 1 to 30 d. We also inject a series of simulated planet transits and find that our data are sufficiently sensitive to have detected transits of 5 millimagnitude depth, with recoverability ranging from ~100% for orbital period of 1 d to ~20% for orbital period of 20 d for the parameter spaces tested. The recoverability specifically at the 11.186 d period of Proxima b is ~ 40% for the parameter spaces tested. While we are able to rule out virtually all transits of other planets at periods shorter than 5 d and depths greater than 5 millimagnitudes, we cannot rule out the possibility that transits of Proxima b are present but escaped detection. However, we are able to rule out virtually all transits of other planets at periods shorter than 5 d and depths greater than 3 millimagnitudes.

Possible second planet with six earth-masses at 1900 days (~1.5 AU). If real, it's accessible to both Gaia and direct-imaging.
https://www.scientificamerican.com/article/a-second-planet-may-orbit-earths-nearest-neighboring-star/

Breakthrough Discuss 2019

https://www.youtube.com/watch?v=EkqpBW4h44c

Mario Damasso & Fabio Del Sordo, “Things Behind the Sun: Proxima Strikes Again”

Prudence : remember Alpha Cen Bb

Proxima Centauri b is not a transiting exoplanet
https://arxiv.org/abs/1905.01336

TESS Light curve. Lots of flares, no evidence of transits.

Flaring activity of Proxima Centauri from TESS observations: quasi-periodic oscillations during flare decay and inferences on the habitability of Proxima b
https://arxiv.org/abs/1907.12580

From ExSS4 :

Goodbye HD 114762 b, welcome Proxima Centauri c:

http://exoplanet.eu/catalog/proxima_centauri_c/

Reference

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A low-mass planet candidate orbiting Proxima Centauri at a distance of 1.5 au
2019 DAMASSO M., DEL SORDO F., ANGLADA-ESCUDE G., GIACOBBE P., SOZZETTI A. et al.
Science Advance, accepted

Et voilà:

Damasso et al. (2020) "A low-mass planet candidate orbiting Proxima Centauri at a distance of 1.5 AU"
https://advances.sciencemag.org/content/6/3/eaax7467

Now we can read the Proxima c paper. The possibility that the RV variations are due to activity cannot be entirely ruled out, but Gaia results should be able to decide one way or the other. ALMA follow-up would also be useful to determine whether the candidate source at ~1.6 AU represents the planet.

https://arxiv.org/abs/2003.00984

The UV surface habitability of Proxima b: first experiments revealing probable life survival to stellar flares

We use a new interdisciplinary approach to study the UV surface habitability of Proxima b under quiescent and flaring stellar conditions. We assumed planetary atmospheric compositions based on CO2 and N2 and surface pressures from 100 to 5000 mbar. Our results show that the combination of these atmospheric compositions and pressures provide enough shielding from the most damaging UV wavelengths, expanding the "UV-protective" planetary atmospheric compositions beyond ozone. Additionally, we show that the UV radiation reaching the surface of Proxima b during quiescent conditions would be negligible from the biological point of view, even without an atmosphere. Given that high UV fluxes could challenge the existence of life, then, we experimentally tested the effect that flares would have on microorganisms in a "worst-case scenario" (no UV-shielding). Our results show the impact that a typical flare and a superflare would have on life: when microorganisms receive very high fluences of UVC, such as those expected to reach the surface of Proxima b after a typical flare or a superflare, a fraction of the population is able to survive. Our study suggests that life could cope with highly UV irradiated environments in exoplanets under conditions that cannot be found on Earth.

From the exoplanet.eu bibliography:

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Orbital inclination and mass of the exoplanet candidate Proxima c
KERVELLA P., ARENOU F. & SCHNEIDER J.
2020 | Astron. & Astrophys., accepted,

That sounds like it might be an astrometric detection so I'm guessing it'll turn out to be low inclination and higher planetary mass, but we shall see.

On the subject of confirming Proxima c:

Demasso & Del Sordo "Expectations for the confirmation of Proxima c from a long-term radial velocity follow-up"
https://arxiv.org/abs/2003.09305

It's finally out!

Orbital inclination and mass of the exoplanet candidate Proxima c
https://arxiv.org/abs/2003.13106

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We analyse the orbital parameters of the exoplanet candidate Proxima c recently discovered by Damasso et al. (2020) using a combination of its spectroscopic orbital parameters and Gaia DR2 proper motion anomaly. We obtain an orbital inclination of i=152±14° for the prograde solution, corresponding to a planet mass of mc=12+12−5 M⊕, comparable to Uranus or Neptune. While the derived orbital parameters are too uncertain to predict accurately the position of the planet for a given epoch, we present a map of its probability of presence relatively to its parent star in the coming years.

If the two planets are coplanar, we can expect a mass of ~2.1 M_earth for the b planet.

I thought the disk was disproven, but:

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The inclination of the dust rings identified by Anglada et al. (2017) (≈ 45°) from ALMA observations of Proxima is compatible with our derived inclination. The position angle of the major axis of the ring (≈ 140°) is also in agreement with the position angle of the line of nodes of the orbit of Proxima c

An independent detection of Proxima Centauri c with HST FGS astromtry!
https://iopscience.iop.org/article/10.3847/2515-5172/ab84f3

Possible direct-imaging detection of Proxima Centauri c, but the implied orbit is not consistent with the Gaia astrometry results.

Searching for the near infrared counterpart of Proxima c using multi-epoch high contrast SPHERE data at VLT
https://arxiv.org/abs/2004.06685

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While we are not able to provide a firm detection of Proxima c, we found a possible candidate that has a rather low probability of being a false alarm. If our direct NIR/optical detection of Proxima c is confirmed (and the comparison with early Gaia results indicates that we should take it with extreme caution), it would be the first optical counterpart of a planet discovered from radial velocities. A dedicated survey to look for RV planets with SPHERE lead to non-detections (Zurlo et al. 2018b). If real, the detected object (contrast of about 16-17 mag in the H-band) is clearly too bright to be the RV planet seen due to its intrinsic emission; it should then be circumplanetary material shining through reflected star-light. In this case we envision either a conspicuous ring system (Arnold & Schneider 2004), or dust production by collisions within a swarm of satellites (Kennedy & Wyatt 2011; Tamayo 2014), or evaporation of dust boosting the planet luminosity...

Precision RV's with ESPRESSO!! Let's go!

Revisiting Proxima with ESPRESSO
https://arxiv.org/abs/2005.12114

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We aim to confirm the presence of Proxima b using independent measurements obtained with the new ESPRESSO spectrograph, and refine the planetary parameters taking advantage of its improved precision. We analysed 63 spectroscopic ESPRESSO observations of Proxima taken during 2019. We obtained radial velocity measurements with a typical radial velocity photon noise of 26 cm/s. We ran a joint MCMC analysis on the time series of the radial velocity and full-width half maximum of the cross-correlation function to model the planetary and stellar signals present in the data, applying Gaussian process regression to deal with stellar activity. We confirm the presence of Proxima b independently in the ESPRESSO data. The ESPRESSO data on its own shows Proxima b at a period of 11.218 ± 0.029 days, with a minimum mass of 1.29 ± 0.13 Me. In the combined dataset we measure a period of 11.18427 ± 0.00070 days with a minimum mass of 1.173 ± 0.086 Me. We find no evidence of stellar activity as a potential cause for the 11.2 days signal. We find some evidence for the presence of a second short-period signal, at 5.15 days with a semi-amplitude of merely 40 cm/s. If caused by a planetary companion, it would correspond to a minimum mass of 0.29 ± 0.08 Me. We find that the FWHM of the CCF can be used as a proxy for the brightness changes and that its gradient with time can be used to successfully detrend the radial velocity data from part of the influence of stellar activity. The activity-induced radial velocity signal in the ESPRESSO data shows a trend in amplitude towards redder wavelengths. Velocities measured using the red end of the spectrograph are less affected by activity, suggesting that the stellar activity is spot-dominated. The data collected excludes the presence of extra companions with masses above 0.6 Me at periods shorter than 50 days.

(emphases mine)

From AAS 236 :

https://aas.org/sites/default/files/2020-06/benedict_aas236.pdf

Masses for Proxima Centauri and the Exoplanet Companion Proxima Centauri c

and

https://phys.org/news/2020-06-astronomer-year-old-hubble-planet-proxima.html

New tricks from old data: Astronomer uses 25-year-old Hubble data to confirm planet Proxima Centauri c