I used a large compilation of archival activity values to identify objects show- ing both transient and consistently low outliers. Possible causes of short- lived activity outliers (“dropouts”) were investigated, including planetary and cometary transit signals and spurious systematic effects. I found the latter explanation is likely for the majority of outliers, in particular those within the
Isaacson & Fischer (2010) dataset. I confirmed that the only Ca ii planetary
transit detection reported to date in the literature has a clear effect on the cor- responding activity timeseries. The ∼1% activity depression in-transit is well below the 30 - 90% amplitude of the activity dropouts I identified among my large stellar sample. I then selected main sequence stars consistently below either the basal limit or the relevant cluster distribution. These are the targets for testing the hypothesis that anomalously low activity levels can provide a short-cut to identifying mass-losing, short-period exoplanets.
Details on collaborative contributions
The idea of searching for stars with anomalously low activity levels as targets for a short-period planet search originated from my supervisor C.A. Haswell. J.R. Barnes and G. Anglada-Escude contributed important analysis of the public HARPS, low SNR observations (covering known HJ transits, see Section 2.2). This led to the conclusion that the data was unsuitable for the detection of Ca
iitransit signals. I identified the apparent, high SNR Ca ii transit signal in the HD189733 dataset. All detailed analysis of this signal and its origin (extending to other spectral lines) was then carried out by J. R. Barnes, leading to the publication ofBarnes et al.(2016). I provided feedback for this manuscript. J.R. Barnes reduced and analysed the archival HIRES spectra used as a comparison for cluster stars in Section2.5and provided Figure2.12.
SALT observations of the
Chromospheric Activity of
Transiting Planet Hosts: Mass Loss
and Star Planet Interactions
A significantly shortened version of this chapter was published as
Staab et al.(2017).
3.1
Introduction
As detailed in Section 1.4, there has been significant interest in the chromo- spheric Ca ii H & K line flux in HJ host stars, with the main hypotheses that:
1. A HJ planet can stimulate stellar activity through magnetic and/or tidal star planet interactions (SPI;Cuntz et al. 2000).
2. A HJ planet can suppress stellar activity through tidal interactions (Miller et al. 2012;Pillitteri et al. 2014).
3. Mass loss from a HJ planet can form a diffuse circumstellar gas cloud which absorbs in the cores of strong resonance lines (e.g. Ca ii H & K and Mg ii h & k) suppressing the measured stellar activity below its true value (Haswell et al. 2012;Fossati et al. 2013).
As discussed in Chapter1, it is important to identify individual systems host- ing known short period planets where mass loss appears to be masking the intrinsic activity, and cases where the stellar activity appears boosted by SPI. Ca ii H & K emission enhancements from SPI should be a good predictor of a system’s radio brightness (e.g. See et al. 2015). With enhancements in radio astronomy capabilities (ALMA, ALMA Partnership et al. 2015; SKA, Carilli
& Rawlings 2004), detections of exoplanetary radio emissions are imminent
(Vidotto et al. 2015). These are expected to yield planet rotation periods and
magnetic moments, with important implications for exoplanetary magneto- spheric physics and the transfer of energy and angular momentum between the host star and planet, driving orbital evolution. SPI processes themselves are active areas of research (see Section1.1.1). Identifying additional systems with evidence for SPI can inform detailed follow-up observations for character- ising SPI. Systems where the Ca ii H & K emission is absorbed by gas lost from the planet offer the potential to determine the planet’s chemical composition through transmission spectroscopy.
3.1.1
Importance of stellar activity data from SALT
There are few southern hemisphere spectrographs calibrated to produce log(R0HK),
and large telescopes or significant exposure times are required to achieve suf- ficient signal to noise in the Ca ii H & K cores of typical HJ host stars (V= 11 or fainter).
At the time of writing, approximately 200 out of 2900 confirmed planet hosts (as listed on exoplanet.org) have published S-values, i.e. less than 10%. While this database is not exhaustive for activity data, it is certainly the case that the majority of exoplanet hosts lack log(R0HK) data. Currently ∼40% of the
∼ 170 known transiting HJ systems (defined as Mp > 0.5 MJ, P < 10d) have
known activity values, as listed byFigueira et al. (2014). The vast majority of HJs are discovered through ground-based transit surveys. WASP-S and HAT-S are responsible for almost all such discoveries in the southern hemisphere, and both routinely publish without measuring log(R0HK) values and lack follow-up to achieve this goal. Note that only ∼17 out of 75 known southern transiting HJs currently have a log(R0HK) value published inFigueira et al.(2014).
In this chapter I report observations using the Robert Stobie Spectrograph (RSS; Kobulnicky et al. 2003) at the Southern African Large Telescope (SALT;
Buckley et al. 2006), calibrating it to measure log(R0HK), and “pilot project”
results for four HJ host stars. With the calibration, the RSS can now be used as an efficient facility for stellar activity measurements: it is attached to a 10 m class telescope, enabling high SNR observations of HJ hosts within modest time allocations. Following the increased instrumental throughput in 2015, we found that ∼ 400s exposures ensured SNR> 15 in the Ca ii H & K cores of stars with V . 13, even in poor observing conditions. Using our RSS calibration, homogeneous log(R0HK) measurements for the majority of southern HJ hosts will be published after additional semesters of ongoing SALT observations.
neous, measured with a variety of spectrographs (see Chapter 2) and often lacks rigorous uncertainty estimates. For high SNR observations, the different instruments’ calibrations to the Mount Wilson system can dominate the uncer- tainty on log(R0HK) (seeJenkins et al. 2011and Section3.3.3). Analysis of a large,
consistent RSS dataset, using the methodology described in this chapter, will not suffer these disadvantages.
In this chapter, Section3.2 describes observations and data reduction; Sec- tion 3.3 describes the RSS calibration; Section 3.4 discusses the planet host measurements in the context of large stellar samples and the three hypotheses listed above; Section3.5gives conclusions and implications for future work.