HARPS data release #3, independent analyzes & more

12th September 2017 Guillem Anglada-Escude Science Logs 1
This unusual Picture of the Week showcases the latest data on Proxima Cenaturi gathered by ESO's exoplanet hunter, the High Accuracy Radial velocity Planet Searcher (HARPS), during the ongoing Red Dots campaign. The top left graph displays the 2016 data that confirmed the existence of Proxima b, showing how the planet is causing its parent star, Proxima Centauri, to move towards and away from Earth over time. The curved line represents the wobbling signal of the star, with the regular pattern of changing radial velocities (RV) repeating every 11.2 days. The top right graph shows new measurements made with HARPS during the Red Dots campaign in 2017. The new data once again supports the presence of Proxima b's signal (in yellow), but also includes additional patterns visible here as a downward slope in both the 2016 and 2017 campaigns, hinting that there may be more to be discovered. To make a firmer statement on what is causing these patterns, astronomers need to use quantitative mathematical tools. One such mathematical tool is called a periodogram, which searches for repeating signals in the data displayed here as prominent peaks. Several periods seem promising but it is hard to make a quantitative argument favouring one or another. This typically happens on poorly sampled signals and when the variability is caused by stellar activity.

by Guillem Anglada-Escude, Mario Damasso & Fabio del Sordo

As featured in the current ESO picture of the Week, Proxima Centauri keeps showing extra variability beyond the wobble caused by Proxima b. The nature of the variability remains unclear so conclusive evidence will require the combination of all available data (HARPS, UVES spectrometers; but also photometric time-series being collected in both professional and Pro-Am observatories). We will be posting the photometric measurements in short.

The new radial velocity measurements are now available for download on our website. Share your thoughts or analyses in the comments or via social media Twitter @RedDotsSpace or Facebook.

For example, our colleagues Mario Damasso and Fabio Del Sordo also have been looking at the new Proxima data (based on HARPS data release #2). This is what they have found so far.

Proxima re-reloaded! Vol. 1

by Mario Damasso and Fabio Del Sordo

We have analysed the radial velocities of Proxima including the new dataset, for a total of 248 measurements. We have modeled the stellar “noise” component through a Gaussian process regression, as we did in our previous work (Damasso & Del Sordo, A&A, 599A 126D, 2017), using a quasi-periodic covariance function, which is particularly suitable when a signal with a frequency related to the stellar rotation period is present in the data.

Here we briefly summarize the results of the first model we have tested, that takes into account the existence of only one planet. Our discussion is based on the results corresponding to the Maximum a Posteriori likelihood (MAP), i.e. we are presenting single values for all the free parameters of our model corresponding to the maximum of the likelihood function we have used as figure of merit.

First of all, we recover a planetary signal with semi-amplitude K=1.46 m/s, with orbital period P=11.186 days, moving on a nearly circular orbit (see fig 1.): the signal, already evident in the previous dataset, is therefore once again clearly confirmed.

Fig 1: Folded phase curve for all the radial velocities collected for Proxima. The red curve represents the best-fit planetary signal we have derived for Proxima b.

These new data also confirm our previous results. The rotation period of Proxima is present in the radial velocity dataset, because we find a signal with rotational period of about 87 days modeled as correlated noise. One parameter of our model can be seen as  the average lifetime of the active regions, for which we find a value of 312 days. The correlated stellar noise induced by stellar activity has an estimated amplitude of 1.82 m/s. All these results are consistent with our previous findings.

We have analysed the residuals of this model, and we do not find evidence for additional, significant frequencies left in the data. We are now running a model which includes two planets. Stay tuned!

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