RV Vetting & CCFs

Often, before performing intensive follow-up of an exoplanet candidate found from a transit survey, it is necessary to check that our candidate planet is not in fact another star orbiting the host or potentially another nearby star. This can be done by measuring two or three initial RV points and vetting their cross-correlation functions.

RV measurements are obtained by comparing the spectra obtained from the star (see Section1.6.1) with a binary mask. A mask is merely an array where spaces within the mask match the location of the absorption lines in the spectrum. As the location of the absorption lines move due to the Doppler effect, and the size of this shift needs to be accurately measured, the mask is offset by small steps and cross-correlated with the spectrum. Once done over a range, this generates a function known as a cross-correlation function (CCF). The peak of the CCF gives the radial velocity of the star at the time of that observation.

To vet the initial CCFs, there are several things to be checked before continuing with more observations (see Figure2.3for an example):

1) CCF Shape: One simple check is the general shape of the CCF(s). They must be Gaussian in appearance and symmetric. Any large asymmetries can be an indicator of a stellar binary system.

2) Check for Spectral Binaries: A spectral binary is a binary system of stars where it is possible to detect both bodies via their individual spectra, which can change a CCF in two ways.

SB1 Over two or more CCFs, the values of the RVs will dramatically change (e.g. many 100s of ms⁻¹). This indicates that the object orbiting our perceived host has a very high mass and is therefore not planetary (Figure2.4).

SB2 Within each CCF, there will be more than one peak. Most commonly it appears as one, large peak with an additional, smaller peak nearby. This shows that there is another stellar object in this system which has a different radial velocity compared to the perceived host, which would suggest the detected candidate may not be planetary (Figure2.5).

3) CCF Contrast: The contrast (‘depth’) of the CCF should ideally be around 30-40%.

Significantly less may suggest the wrong mask has been used for this star, and a different spectral type mask should be applied.

4) CCF FWHM: TheFullWidth at Half Maximum can be used to determine the stellar rotational velocity (vsini). Whilst having a high vsini does not ‘kill’ the exoplanet

Figure 2.3: K2-140: an example of a good CCF, where the criteria are met. It has a contrast of 40%, a FWHM of 8.4kms⁻¹ and a BIS close to zero (the BIS in the diagram has been exaggerated to ease of view).

candidate, if thevsiniis much greater than 7-8kms⁻¹, then it becomes more difficult to accurately measure the planetary mass.

5) BIS: Check the bisectors (BIS) of the CCF, which is an additional check of the CCF shape. The bisector describes the curvature of the line which tracks the midpoint of the CCF at different depths. For a perfect Gaussian, the value would be 0 (as it is symmetrical). However, given the CCF suffers from noise, the value is rarely exactly 0.

But the BIS value, with its errors taken into account, should always be near 0. If a BIS value is significantly far from 0, this can suggest a non-planetary cause.

6) Radial Velocity Value: The actual value of the RV measurement should not be an extreme (either very small or large). Additionally, the error should be of the order of 10-20ms⁻¹. If the error is much larger than this, it would suggest a longer exposure is needed for the observation to improve the accuracy. Additionally there should be no correlation between the BIS values and RV values, as this may indicate an SB2 or stellar activity.

Many of the above criteria are there for vetting that the object is planetary in nature, rather

Figure 2.4: EPIC245946030: an example of an SB1, where the CCFs vary signific-antly between each exposure. This would indicate a non-planetary-like mass of the companion to the star.

Figure 2.5: 1SWASPJ212938.68-032434.9: an example of an SB2 where there is a secondary CCF peak which also evolves over time. This would indicate that there is a stellar companion to the star of interest.

Figure 2.6: The very red Swiss Euler telescope (a) is housed within the dome of image (b), based at ESO’s La Silla Observatory in Chile.

than a stellar companion. But of course, if candidates fail the vetting, this does not mean that there is not a planetary object which is in a more complicated system with two stars. For example a ‘Tatooine’-like planet was found by the Kepler mission, Kepler-16b, which is a Saturn-like planet orbiting a pair of low-mass stars (Doyle et al. 2011). Therefore it is always worth comparing what has been seen in the initial CCFs and in the light curve.

2.2.2.1 Euler & CORALIE

For conducting RV follow-up, two things are needed: a telescope to gather the stellar light; and a spectrograph to split the light into a spectrum which is projected onto a CCD.

Run by the Observatoire de Genève, the Swiss Euler telescope is based at ESO’s La Silla Observatory in Chile (Figure 2.6). Named after the famous Swiss mathematician, Leonhard Euler, the 1.2m reflecting telescope was first pointed at the sky in April 1998 and comes equipped with two instruments: EulerCam, a CCD for performing photometry; and CORALIE, an echelle spectrograph. CORALIE is fed by an optical fibre and has a resolution R=60,000 which can deliver an accuracy of 6ms⁻¹or less for the brightest targets (Queloz et al. 2000).

Euler and CORALIE have been heavily involved in exoplanet follow-up efforts since first-light, providing follow-up data for transiting exoplanet discoveries from missions like the ground-based WASP (Pollacco et al. 2006) and NGTS (Wheatley et al. 2013) projects; and space-based missions such as K2 (Koch et al. 2010;Borucki et al. 2010), K2(Howell et al.

2014), and TESS (Ricker et al. 2015). Euler and CORALIE have also performed their own

long-term RV surveys to detect non-transiting planets (seeQueloz et al. 2000;Udry et al. 2000;

Santos et al. 2000, and others in the series).