Recovering the EUV flux of HD209458 University of Warwick Supervisor: Dr. Peter Wheatley
HD209458b Evaporation UV spectroscopy with HST during transit Lyman-alpha transit significantly deeper than in broadband optical (15% vs 1.6%) Implies an expanding cloud of neutral Hydrogen (Vidal madjar Nature 422, 143 2003)
Jeans escape? At high temperatures, the tail of the MaxwellBoltzman distribution reaches escape velocity This loss mechanism should strongly prefer lighter elements, i.e Hydrogen
Hydrodynamic escape Vidal-Madjar et al find that the wind includes heavier elements, oxygen and carbon. Jeans escape should strongly preference hydrogen Suggests dramatic Hydrodynamic escape (Vidal-Madjar et al., ApJ 604 L69)
Hydrodynamic escape HST COS data taken 2009, (PiD 11534) 18 datasets with 2 grisms Mass loss observed in Si III and C II Velocity structure observed 11 Mass loss ~10 g/s (Linsky et al 2011 ApJ 717 1291)
What drives the heating? Cannot be stellar optical flux. Mass loss rates too low by a 11 factor of 10! Coronal emission is plausible. Ionising radiation (X-ray / EUV) can very efficiently heat gas to > 10,000 K (Lammer et al 2003) Miloslav Druckmuller / SWNS 2009
Measuring Ionising flux Whilst ISM extinction is very high below the Lyman edge (912 A) it is fairly transparent above. Some strong, observable UV emission features are produced in the same temperature regions as EUV lines (~ 105 K) They are also produced by multiply ionized species that are not absorbed strongly by ISM N V, O V, C IV and Si IV are the most important features.
Conveniently... COS HST data also give these high excitation lines! K. France et al. (2010, ApJ 712:1277)
X-rays X-rays also can pass through the ISM, but so far HD209458 remains un-detected. An upper limit to X-ray flux is useful however, as it constrains the High temperature coronal emission up to ~ 108 K
X-ray upper limit XMM observations taken in 2006 (P.I. Wheatley) 3 sigma upper limit - 20 counts. The Integration time was 30ks. the very low count rate and subsequent non detection was unexpected.
Differential Emission Measure The Differential Emission Measure is a measure of the quantity of emitting plasma as a function of temperature. DEM's are, to a first order, continuous functions that can be well modelled as a simple polynomial. Combining UV and X-ray data can allow the whole temperature range to be constrained, allowing recovery of EUV flux.
Plasma model CHIANTI 7.13 Well suited for coronal models UV / X-ray line lists Continuum processes Nice Python interface! (Dere et al. 1997, Landi et al. 2013) George Mason University (USA) University of Michigan (USA) University of Cambridge (UK)
Plasma model XUV Emissivity
Method Construct 40 'basis' spectra, seperated log 4 8 evenly in temperature. (Range: 10 10 K) Plasma model produces line strengths, and when passed through RMF, an XMM count rate. (produced using XMM SAS) Basis spectra weighted by a 4th order polynomial (the DEM) A simple least squares fit finds an upper limit DEM
Energy Limited mass loss Applying this spectrum to a simple energy limited model, one finds that the reported evaporation rates can only be recovered with a high efficiency (>0.5) Theory suggests that efficiency should not be greater than 0.2 (Owen et al 2012 MNRAS 425, 2931) This is just an upper limit potentially even lower flux (hence higher efficiencies) if X-ray is much lower than this upper limit Does this mean EUV heating is dead?
Mass loss structure Mass loss rates are very model dependent Simulations of mass loss require ionising flux as an input Low ionization flux favours the formation of a long lived cometary tail (Bourrier et al 2013)
Conclusions The EUV flux of HD209458 is low, if this is indeed driving the evaporation it does so efficiently May improve models of mass loss wind structure, and hence mass loss rates. EUV/X-ray evaporation might be important in sculpting the observed population of exoplanets, it's essential we understand it
Recovering the EUV flux of HD209458 University of Warwick Supervisor: Dr. Peter Wheatley