Beta Pictoris : Disk, comets, planet

Transcription

1 Beta Pictoris : Disk, comets, planet Hervé Beust Institut de Planétologie et d Astrophysique de Grenoble (IPAG) 1

2 Outline of the talk 1. The star 2. The dust disk Clues for the presence of planets 3. The gas disk Falling Evaporating Bodies 4. Pictoris b Orbital fitting 5. Conclusions 2

3 Pictoris : the star An A5V type main-sequence star Magnitude m=3.86 Mass ~1.75 M, Luminosity = 8.7 L Distance = 19.4 pc (Hipparcos) (van Leeuwen et al. 2007) Effective Temperature T eff = 8050 K Radius ~ 1.7 R ; vsin i = 130 km/s Age ~ Myr ( Pic moving group) (Zuckerman et al. 2001; Ortega et al. 2004) Most recent determination : 21 4 Myr (Binks & Jeffries 2014) (Lithium depletion in Pic moving group) Used to be estimated up to 200 Myr in the past! 3

4 Pictoris : the dust disk The prototype of debris disks viewed edge-on in scattered light Under investigation since 1984 Smith & Terrile 1984 Mouillet et al

5 Characteristics of the dust disk Extending over hundreds of AUs The imaged dust is not primordial (Radiation pressure/ collisional erosion; Thebault et al. 2003) Need for an internal reservoir Disk of colliding planetesimals Many asymmetries between both branchs + warp inner part / outer part Indication of gravitational perturbations Dual power law radial profile with break at ~120 AU (Heap et al. 2000) Model : Disk of planetesimals up to 120 AU + radiation pressure (Augereau et al. 2001) Deconvolution Many rings? (Wahhaj et al. 2003) 5

6 Dust particles orbits Dust particles are produced by planetesimals on ~circular orbits (black). Due the radiation pressure, they move on another, wider orbit (green) Dust particles can be transported very far away, ejected if > 0.5. This fully explains the radial profile of Pic s disk : The 120 AU break corresponds to the edge of the planetesimal belt Asymmetries in the parent bodies distribution (due to planets ) are transported outwards this way. a ' apoastron F F a (1 1 rad grav ) 2 Q ' 1 a 2 Lecavelier et al. (1996) Augereau et al. (2001) 6

7 values depends on the grain size. For large grains, r -1 For A type stars like Pic, all grains below a threshold size are ejected. Due to erosion, smaller grains are continuously produced and ejected. Consequence : Wavy size distribution Need for a reservoir to replenish the disk. Thébault et al. (2003) 1 r 7

8 Pictoris before Pic b: Clues for the presence of planets The various asymmetries. The rings? The inner warp : could be caused by an inclined planet (Mouillet et al. 1999; Augereau et al. 2001) The «transit» = A photometric event in November 1981 Transit of the Hill sphere of a Jovian planet at ~10 AU? (Lecavelier et al. 1996) The evaporating star-grazers (FEBs) 8

9 Gas in the β Pictoris disk Circumstellar gas is detected in absorption in the spectrum of the star (thanks to edge-on orientation) : C, Na, Mg, etc (Roberge et al. 2006), H I, H 2 (Freudling et al. 1995; Lecavelier et al. 2001), O (Brandeker et al. 2016) Most ionized species in Keplerian motion, despite strong radiation pressure The abundance of Carbon can render the whole gas self-braking (Fernandez et al. 2006; Brandeker 2011) Different species at different latitudes in the outer disk (Na I in the midplane, Ca II above (Brandeker et al. 2004) Model : The gas is not primordial (as the dust). The gas disk must be continuously replenished from a inner reservoir, by evaporating planetesimals (Lagrange et al. 1998), or by grain-grain collisions or photodesorption (Kral et al. 2015) 9

10 Gas in the β Pictoris disk : variations Apart from stable components, transient additional, Doppler-shifted components are frequently observed. They vary on a very short time scale (days hours) 10

11 Characteristics of the transient events Detected in many spectral lines, but not all ( Ca II, Mg II, Al III ) : only moderately ionized species Most of the time reshifted (tens to hundreds of km/s), but some blueshifted features The higher the velocity, the shorter the variation time-scale Comparison between features in doublet lines saturated components that do not reach the zero level The absorbing clouds do not mask the whole stellar surface ~Regularly observed for ~30 years : they are frequent but their bulk frequency is erratic The components observed seem to be correlated (in Ca II) over a timescale of 2-3 weeks (Beust & Tobin 2004) 11

12 The FEBs (Falling Evaporating Bodies ) scenario (Beust et al. 1990, 1995, 1998 ) Each of these events is generated by an evaporating body (comet, planetesimal) that crosses the line of sight. These objets are star-grazing planetesimals (comets) (<0.5 AU). At this short distance the dust sublimates metallic ions in the coma. This model naturally explains : The infall velocities : projection of the velocity onto the line of sight close to the star The time variability : time to cross the line of sight The limited size of the clouds = size of the coma The chemical issue : not all species are concerned 12

13 Physics involved in the FEBs scenario The ions are subject to the radiation pressure and to a drift force by the other species If the gas composition is carbonaceous enough, il can be self-braking (Fernandez et al. 2006) The different kinds of variable features (high velocity, low velocity ) are well reproduced if we let the periastron distance vary. The longitude of periastrons are not randomly distributed (predominance of redshifted features) Several hundreds of FEBs per year Possibly several families of FEBs (Kiefer et al. 2014) Question : why so many star-grazers? What is their dynamical origin? 13

14 FEB dynamics : How do you generate star-grazers from a disk? A requirement : planetary perturbations need for planets! Direct scattering of planetesimals : possible but weakly efficient and short lived Kozai resonance on initially inclined orbits : efficient but no preferred orientation (rotational invariance) Mean-motion resonances with a Jovian planet : preferred model Bodies trapped in some inner mean-motion resonances with a Jovian planet (4:1,3:1) see their eccentricity grow up to ~1 FEBs! One requirement : The planet needs to have a moderate (>0.05) eccentricity. The orientation of the FEBs orbits is constrained explains the blue/redshift statitics. A similar phenomenon gave birth to the Kirkwood gaps in the Solar asteroid belt 14

15 Duration of the phenomenon Best planet to match the FEB statistics : ~ a few Jupiter 10 AU (Beust & Morbidelli 2000; Thébault & Beust 2001). But the resonances clear out quickly (< 1 Myr) Need for refilling to sustain the process Collisions between planetesimals are a good candidate to refill the resonances and make it last several Myrs (Thébault & Beust 2001) 15

16 A giant planet (~10 M J, 8-10 AU) detected in NACO images of 2003 (Lagrange et al. 2009) Redetected on the other side of the star in 2009, and, regularly followed since that time (Lagrange et al. 2010) Questions : What are the characteristics and the orbit of this planet? Does it match previous predictions? β Pictoris b 16

17 Physical characteristics of Pic b Mass ~10 Jupiter mass? ( for Hot start models) (Bonnefoy et al. 2014) Spectrum ~ L1 dwarf T eff = K; log g < 4.7 dex log (L/L ) = Rapid rotation : v spin 25 km/s (Snellen et al. 2014) Formation? Core accretion may be problematic (Bonnefoy et al. 2014) 17

18 Fitting Pic b s orbit Many astrometric data, from various instruments (NACO, SPHERE, GPI, MAGAO ) Our approach : fitting NACO + SPHERE data only, to avoid systematics Convention : XOY plane = sky plane; OZ direction : towards Earth Degeneracy : With just astrometric data, identical solutions but with (, ) and ( +, + ) cannot be distinguished. Use radial velocity point by Snellen et al. (2014) to lift degeneracy The rotation sense of the planet is the same as the disk (ascending node = South-West) Data set : 1 point points RV point 17/12/2013 Greatest elongation ~late 2012, planet receding quickly now. Different methods: Least squares (Levenberg-Marquardt) to derive one best fit orbit Statistical methods (MCMC, LSMC) to derive the probabilistic distribution of the orbital elements : Preferred, because the orbit is not evenly sampled! 18

19 Latest fit results (August 2016) Consistent with other independent determinations (Millar- Blanchaer al. 2015) 19

20 Semi-major axis and eccentricity Well constrained semi-major axis Moderate eccentricity but still compatible with e=0 Two branches of solutions 20

21 Inclination and Longitude of node Well constrained inclination : nealy edge-on orbit, but sligthly prograde Well constrained PA of the disk 21

22 Argument of periastron Peak at 0 20, but all values possible Solutions with e = 0 undefined 22

23 The next transit? A sky view Data points Cloud of predicted positions in

24 The next transit? A sky view : zoom (2017.7) Approximate size of Pic b s Hill sphere ~ 40 mas 24

25 The next transit : When? Peak in ~ year ahead from now 25

26 Dynamical constraint of Pic s mass? Use HARPS spectra (many) to constrain the radial velocity evolution of the star Then do a MCMC orbital fit with the astrometric data, and with the planet mass as a free parameter Result (Bonnefoy et al. 2014) : m 20 M Jup (>96% probability) Very hard to be more accurate : Mass-prior dependent (low S/B RV signal) 26

27 Is Pic b the planet predicted previously? 1. The warp? Check inclination of the orbit / Main (outer) dust disk and warped disk Closer to the warped disk than to the outer disk In agreement with predictions by Mouillet et al. (1997) and Augereau et al. (2001) Pic b could have created the warp. 27

28 Is Pic b the planet predicted previously? 2. The transit of 1981? Check past transit predictions in our posterior distribution Not perfect match, but there was a transit around 1980, and another one in the late 90 s Pic b could have generated the 1981 event... Next transit Date of the 1981 event 28

29 Is Pic b the planet predicted previously? 2. The transit of 1981? Let us try to take the Nov. 9, 1981 event as an additional astrometric point and re-fit Result : Better constraint on eccentricity, but 2 distinct families of solutions (high and low eccentricity) Explanation : In the low eccentricity regime, there was a transit in ~2000 But different predictions for the 2017 transit Free fitting cleary favours the low eccentricity scenario, but the high eccentricity solutions better fit the radial velocity point 29

30 Is Pic b the planet predicted previously? 3. The FEBs? The FEB model requires a moderate eccentricity (e>0.05) and = ( = longitude of periastron / line of sight = = 70 ) (Beust & Morbidelli 1996, 2000; Thébault & Beust 2001) Due to the predominance of redshifted events But analysis based on the 4:1 resonance. Different resonances (4:1, 3:1..) could generate the families (Kiefer et al. 2014) Pic b could trigger the FEB phenomenon (Millar-Blanchaer et al. 2015) 30

31 Conclusions and questions Pictoris : Still a nice example of debris disk! Pictoris : A fantastic laboratory to study dynamics, interactions between disk and planets, physics of a young planetary system. One of the best analogues to the young solar system known today FEBs Early asteroid belt? Outer disk Kuiper belt / Oort cloud? Pic b Jupiter? Pic b : A rare example of imaged exoplanet with detected orbital motion Pic b : Full knowledge of its orbit is of prime importance to test dynamical models Orbital plane : Probe position with respect to dust disks Eccentricity and Periastron : Probes FEB model & the 1981 transit hypothesis Be ready for 2017 «transit» and to redetect the planet on the other side! Did somebody observe the transit? Are there photometric measurements? Other radial velocity points would be appreciated! Other planets in the system? Inner to Pic b : According to FEB model, terrestrial planets only Outer to Pic b : Nothing more massive than ~Saturn according to images (Dawson 2014); but blobs (Wahhaj 2003) suggest the presence of other planets 31

32 Pictoris : Dust dynamics The dust particles are subject to gravity + radiation pressure + collisions/erosion + Poynting-Robertson drag Stellar radiation pressure : For any particle, F grav r 1 2 and F rad r 1 2 F F rad grav constant GM (1 * r 3 r ) r Under the action of radiation pressure, each particle follows a pure Keplerian orbit, but feeling an effective central mass M * (1- ) 32