C OMPARECENCIAS ANTE LAS C OMISIONES

Haswell et al.(2012) first suggested that a high rate of planetary mass loss can

feed diffuse circumstellar gas shrouds which absorb the stellar flux in the cores of strong resonance lines, depressing the chromospheric emission which arises in precisely these lines. In particular this naturally explains the anomalous zero flux Mg ii h & k line cores observed in the extreme HJ host WASP-12. The line

core flux is zero10_{, unique among all other stars of comparable age and spectral}

type. Extrinsic absorption across the entire stellar disc is strongly suggested by this, but cannot be explained by the interstellar medium along the line of sight, leaving the planet as a likely source of diffuse circumstellar gas.

The WASP-12 near-UV transit depth and duration observed by Haswell

et al. (2012) imply exospheric gas is overfilling the Roche lobe and escaping.

Note, however, that the hypotheses of mass loss from an exomoon (Ben-Jaffel

& Ballester 2014) and Trojan satellites (Kislyakova et al. 2016) co-orbiting with WASP-12 have also been put forward. Corroborating the hypothesis of an enshrouded exoplanet system, Fossati et al. (2013) showed that WASP-12’s Ca ii H & K lines have an extremely low value of log(R0_HK) = −5.5. If intrinsic to the star, this would be unique: WASP-12 is by far the lowest point shown in Figure1.6. Occam’s razor suggests these extreme properties of the star WASP- 12 must be related to the presence of its extreme HJ planet, WASP-12b.

UV measurements of the Mg ii h & k lines can only be made from space, and are therefore only available for a small number of stars. As discussed in Section1.3, Ca ii H & K observations can be readily carried out with ground-based facilities and are available for thousands of stars. Fossati et al.(2013) exploited this fact to show that five other HJ hosts, along with WASP-12, appear to show anoma- lously low log(R0

HK) values compared to a large sample of stars (Figure 1.6).

These objects are all well below the main sequence basal limit (Section 1.3.2). Note that Figure1.6also includes many subgiants below the basal limit.

The apparent activity suppression arising from planetary mass loss is sensitive to the planet’s surface gravity. All else being equal, the lower the gravity, the higher the mass loss rate and the degree of circumstellar absorption.

Figure 1.6: Figure taken from Fossati et al.(2013), showing the log(R0_HK) dis- tribution of stars compiled from several catalogues and of the HJ hosts from

Knutson et al.(2010). WASP-12 and five other outlying known exoplanet hosts

are highlighted.

There is a highly significant correlation between planetary surface gravity (gp) and stellar log(R

0

HK) for close-in planets11 (Hartman 2010). Figueira et al.

(2014) confirmed this correlation using a 3 times larger sample, and showed it is not due to selection biases. Lanza (2014) constructed a physical mass- loss model reproducing the observed correlation, which was refined byFossati

et al. (2015b). In the latter work, stars below and above the Vaughan-Preston

gap, with their disparate intrinsic log(R0_HK) values, are treated as two distinct subsets. This approach may enable an estimate of the average effective stellar flux powering planetary mass loss, a major uncertainty in current planetary

11_{this was based on theKnutson et al.(2010) measurements for a sample of HJ hosts.Knutson}

et al.(2010) reported a correlation between stellar activity and the presence of a temperature

evolution models. However, a larger sample of planet hosts is needed; see details inFossati et al.(2015b).

Systems enshrouded by material lost from close-in planets are highly promis- ing targets for transmission spectroscopy, as discussed in Section1.1.2. In the case of WASP-12, while the diffuse gas is present at all observed phases, it produces the most near-UV absorption close to transit (Haswell et al. 2012). Differencing the observed spectrum near transit and away from transit can reveal the additional absorption from the densest regions of the gas shroud, which can extend out to several planetary radii. Because the near-UV is such an informative wavelength region, with strong resonance lines of many abundant elements and ions, it is vital to make Hubble Space Telescope observations of the shrouded systems. There are currently no alternative means to obtain comparable information on planetary composition.

Large-scale planetary mass loss akin to that inferred from apparent activity suppression may underly the sub-Jovian desert (Kurokawa & Nakamoto 2014;

Lundkvist et al. 2016). Other explanations of this dearth in the known exo-

planet population have been suggested however (Matsakos & Königl 2016and references therein).

It should be emphasised that systems with evidence for large-scale mass loss from Ly-α observations or anomalous log(R0

HK) values are currently distinct

from systems with disintegrating rocky USPs detected via photometric dust cloud transits. The latter category is currently limited to very faint, distant stars where the former types of measurements are not feasible. Note also that planets in the former category do not necessarily have ultra-short orbital periods. Although WASP-12 and WASP-18 are in 1.1 and 0.9 day orbits, significantly longer periods are seen for other outliers highlighted in Figure1.6(P= 3.7-4.4 d); for GJ 436b (P= 2.6 d) and for 55 Cnc b (P = 14.7 d).