61 Virginis debris disc

Herschel imaging of 61 Vir: implications for the prevalence of debris in low-mass planetary systems

The Herschel observations imply a debris disc with inclination 77 degrees with an inner edge at ~30 AU. If the planets are coplanar with the disc, this would give true masses 2.6% greater than the minimum masses.

Regarding the planetary system:

Quote :

Segransan et al. (in prep.) report 142 precise HARPS radial velocities (averaged values per night) of 61 Vir between JD = 53037 and 55948 that confirm the existence of the inner two planets, with parameters consistent with the values of Vogt et al. (2010) (i.e., a 5.1M_⊕ planet at 0.05AU and a 18.2M_⊕ planet at 0.218AU). However, the outermost planet (i.e., the 22.9M_⊕ planet at 0.478AU) has not been confirmed in the HARPS data at this point.

Also the discovery of a debris disc at ~19 AU from HD 20794 (82 Eridani).

Planet d will go to unconfirmed catalog??

Maybe. They do note there was a burst of activity on the star over part of the time range which could have affected their ability to find longer-period planets.

We shall see...

ALMA observations of the multiplanet system 61 Vir: What lies outside super-Earth systems?
https://arxiv.org/abs/1705.01944

Quote :

A decade of surveys has hinted at a possible higher occurrence rate of debris discs in systems hosting low mass planets. This could be due to common favourable forming conditions for rocky planets close in and planetesimals at large radii. In this paper we present the first resolved millimetre study of the debris disc in the 4.6 Gyr old multiplanet system 61 Vir, combining ALMA and JCMT data at 0.86 mm. We fit the data using a parametric disc model, finding that the disc of planetesimals extends from 30 AU to at least 150 AU, with a surface density distribution of millimetre sized grains with a power law slope of 0.1+1.1−0.8. We also present a numerical collisional model that can predict the evolution of the surface density of millimetre grains for a given primordial disc, finding that it does not necessarily have the same radial profile as the total mass surface density (as previous studies suggested for the optical depth), with the former being flatter. Finally, we find that if the planetesimal disc was stirred at 150 AU by an additional unseen planet, that planet should be more massive than 10 M⊕ and lie between 10-20 AU. Lower planet masses and semi-major axes down to 4 AU are possible for eccentricities ≫ 0.1.

Interesting paper

YARARA V2: Reaching sub m/s precision over a decade using PCA on line-by-line RVs

Quote :

Context. The detection of Earth-like planets with the radial-velocity (RV) method is extremely challenging today due to the presence of non-Doppler signatures such as stellar activity and instrumental signals that mimic and hide the signals of exoplanets. In a previous paper, we presented the YARARA pipeline, which implements corrections for telluric absorption, stellar activity and instrumental systematics at the spectral level, then extracts line-by-line (LBL) RVs with significantly better precision than standard pipelines. Aims. In this paper, we demonstrate that further gains in RVs precision can be achieved by performing Principal Component Analysis (PCA) decomposition on the LBL RVs. Methods. The mean-insensitive nature of PCA means that it is unaffected by true Doppler shifts, and thus can be used to isolate and correct nuisance signals other than planets. Results. We analysed the data of 20 intensively observed HARPS targets by applying our PCA approach on the LBL RVs obtained by YARARA. The first principal components show similarities across most of the stars and correspond to newly identified instrumental systematics, which we can now correct for. For several targets, this results in an unprecedented RV root-mean-square of around 90 cm/s over the full lifetime of HARPS. We use the corrected RVs to confirm a previously published 120-day signal around 61Vir, and to detect a Super-Earth candidate (K = 60 +/- 6 cm/s, m sin i = 6.6 +/- 0.7 Earth mass) around the G6V star HD20794, which spends part of its 600-day orbit within the habitable zone of the host star. Conclusions. This study highlights the potential of LBL PCA to identify and correct hitherto unknown, long-term instrumental effects and thereby extend the sensitivity of existing and future instruments towards the Earth analogue regime.