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Characterizing Exocomet Composition Thanks to Spectroscopy and Photometry (Ground and Space-Based)

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Characterizing Exocomet Composition Thanks to Spectroscopy and Photometry (Ground and Space-Based) Characterizing exocomet composition thanks to spectroscopy and photometry (ground and space-based) Flavien Kiefer Institut d'Astrophysique de Paris Exocomets worshop - 14 may 2019 Contents 1. Gas detections 1.1. Spectroscopic detections 1.2. Metals and carbon : comparison with Sun, Halley and chondrites 1.3. Difficulties of spectroscopy 2. Dust detections 2.1. Photometry of exocomet transits 2.2. Simulations and measure of dust production rate 2.3. Few systems with dust detected 3. Conclusions & Perspectives Gas detections with spectroscopy • First observations of exocomets in the Beta Pic system with ground-based visible spectroscopy (1987,90's up to now) • Detection of calcium (with Ca II) -> an ingredient of comets (silicate dust) in the Solar system with mass abundance ~1%. • Variability, saturation, redshift -> small gaseous clouds in motion towards the star • UV spectroscopy with HST and GHRS then STIS and COS (1994 and so on up until 2019) • Detection of other metals : Al, Fe, Mg mainly, all ionized and some strongly ionized (Al III, CIV). • Volatiles (ices) : C I, CII, and C IV • Spectroscopy of the disk : detection of CO, C etc. • Carbon-rich (Roberge+2006) and oxygen-rich (Brandeker+ 2016) • Clumps of CO in the disk (Wyatt+ 2014) • Presence of the metals detected in exocomets The historical observations of variability in Beta Pic Ca II K line by Ferlet+ (1987) with the Echelle Spectrograph at La Silla Ferlet+ (1987) First difficulties : - Short term variability < 1 day - Strong blends The same spectrum 27 years later with HARPS and more than 1500 spectra... Kiefer+ 2014 When we divide by the reference spectrum accounting for the circumstellar line Shallow absorptions : Often saturated High velocity Large width & Blend Deep absorptions : >50 % of the stellar disk Low velocity Blend w/ disk & saturation Always difficult to deal with the center of the CS line Almost saturated Ca II K line partial coverage Ca II H line Not saturated Saturated Kiefer+ 2014 Ca II K and Fe I simultaneous Welsh & Montgomery 2016 Ca II / Fe I ~ 0.25 with N(Ca II)~1012 cm-2 But Fe I << Fe II (x10-100) (Kiefer+ 2019) Ca / Fe ~ 0.0025-0.025 Fe II variations in Beta Pic in UV band (HST) Vidal-Madjar+ 1994, Lagrange+ 1995 Fe II variations in different lines Fe II variations at different dates - Strong blends, - Day-to-day variability, - Saturations, - Cannot relie on column densities... Al III variations in Beta Pic (UV with HST) Vidal-Madjar+ 1994 19 Dec. 1992 at 15:41 20 Dec. 1992 at 08:09 • Highly ionized species, • Different saturation regimes at different redshifts, • Small coverage of the stellar disk <50 % • UV flux is not sufficient for an A-star like Beta Pic to explain ionisation of Al II or C III. • known as the Al III problem (Beta Pic papers IV, V, VIII, XIII, XIV) • The presence of a shock surface (Beust & Tagger 1993) at the front the ejected coma in the stellar direction solves the issue. -> many collisions -> A new semi-analytical model of exocomet cloud Mg II variations in Beta Pic (UV with HST) Vidal-Madjar+ 1994 • Saturated lines • Small covering fraction of stellar disk ~50% • High redshift -> 200 km/s • star grazers at less than 5 Rstar to Beta Pic • Typical asymetrical shape => well reproduced by simulations Carbon detections (UV with HST) CI Roberge+ 2000 CI multiplets Other difficult (non) detections in UV... Vidal-Madjar+ 1994, Lagrange+ 1995, Kiefer+ (in prep.) Zn II OH Very very tiny... Nope... this is not OH... this is probably Ti II Do you see something ??? Two CO lines but only the disk... and noise ! Roberge+ 2006, Nature Roberge+ 2006, Nature Only Mg I Vidal-Madjar+ 1994 Fe II + Mg I + Zn II + Al III Lagrange+ 1995 Fe II + Ca II + Mn II Jolly+ 1998 Welsh & Montgomery 2016 - 50 km/s Ca II + C II Welsh & Montgomery 2016 Ca II + Fe I Roberge+ 2006, Nature Mg II/Mg I > 100 Roberge+ 2006 Vidal-Madjar+ 1994 Fe II + Mg I + Zn II + Al III Lagrange+ 1995 Fe II + Ca II + Mn II Jolly+ 1998 Welsh & Montgomery 2016 - 50 km/s Ca II + C II Welsh & Montgomery 2016 Ca II + Fe I UV variations in another A-star : 49 Ceti Miles+ 2016 C II, OI, Fe II dominant ions C/Fe > 10 C/O > 1.5 Volatile rich comets ? Other source of carbon ? Welsh & Montgomery 2016 - 50 km/s • Metals + carbon abundances in exocomets and in the disk of Beta Pic better compatible with Halley comet than with CI-chondrites => volatile rich objects (CO ice ?) => other carbon reservoirs ? • C & O neat overabundance in the disk ➡ A possible clue to the presence of massive amounts of CO ice in exocomet of Beta Pic. ➡ Possibly not enough to explain the excessive abundance of oxygen (Brandeker+ 2016) ➡ Means of gas braking to explain that the gas is not constantly falling onto the star. • CO absorptions detected in the inner disk + the diverse emissions in the outer disk • Yet we don't have CO or OH direct detections in exocomets... The main difficulties of spectroscopy • Absorption lines are often saturated => cannot use the curve of growth and derive column densities • Divide stellar continuum when many absorptions + blend => need a good reference spectrum, => only achievable when lots of spectra (see eg. Ca II) • Obtaining (quasi) simultaneous measurements in different bands. => Possible with eg. HST + ground-based. • Careful, one comet at less than 1 au need less than 6h to transit the stellar disk (high day-to-day, or hour-to-hour variability) • Difficult to interpret the absorptions in term of column densities (except if not saturated). Dust detection with photometry from space (Kepler, TESS...) • Dust apart from ices is one of the main ingredient of a comet in the solar system. • The various metals evaporated and observed in spectroscopy are originating from the dust. • The dust form a tail in the anti-stellar direction with a deformation due to the orbital motion of the comet. • Dust can be seen transiting with photometry. Several observations in Kepler systems (Rappaport+ 2017, Boyajian+ 2016, Kiefer+ 2017), or in Beta Pic (Zieba et al. 2019). • Composition of dust unknown: Olivine ? Pyroxène ? Halley-like ? KIC11084727 (Kepler) Rappaport+ 2018 Beta Pic (TESS) Zieba+ 2019 PC multi-coeur (simu) Lecavelier+ 1999 KIC8462852 (Kepler) Kiefer+ 2017 KIC3542116 (Kepler) Rappaport+ 2018 KIC11084727 (Rappaport+ 2018) Simu by Lecavelier Grain size ~0.1-1µm Distance ~0.5 au Dust rate ~107 kg/s This is 100 times larger than for Hale-Bopp, but for F3- star and at less than 1 au from the star. • Simulations of the 0.2%-deep Beta Pic transit event of Zieba+ (2019) would also lead to dust production rate about 107 kg/s (compatible with spectroscopy) • KIC8462852, strings of comets have production rates of the Hale-Bopp type (106 kg/s at 1 au), Kiefer+ 2017 Summary and conclusion On the composition of exocomets • Many metals (ions mostly) and carbon, • Abundances similar to Halley comet, • But saturation of spectral absorption lines (to be taken w/ care), • But different redshits, different cloud size and opacities, • Shock front ahead the coma to explain highly ionized Al and C, • Dust production rate on the order of 107 perhaps 108 kg/s. • Good compatibility of dust model with data of grain of size 0.1-1µm Issues and still unknown features: • Yet no simultaneous absorptions in spectroscopy and photometry, • Difficult measurements of gas species of a same exocomet, • Dust/gas ratio ? • Abundances of species in dust ? • OH, water ? CO ? • Isotopes ratios ? 12C/13C, D/H.... I EXOCOMETS.
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