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Astrophysical artefact in the astrometric detection of ?

Jean Schneider LUTh – Paris Observatory

Work in progress

● Dynamical and brightness astrometry

● Astrophysical sources of excess brightness – Simulations – Observations

● Conclusion

12 Oct 2011 1 Context

Ultimate goal: the precise physical characterization of -mass in the Habitable Zone (~ 1 AU) by direct spectro- polarimetric imaging It will also require a good knowledge of their mass. Two approaches (also used to find Earth-mass planets):

– measurements – Astrometry

12 Oct 2011 2 Context

Radial Velocity and Astrometric mass measurements have both their limitations .

Here we investigate a possible artefact of the astrometric approach for the Earth-mass regime at 1 AU. ==> not applicable to or PRIMA/ESPRI Very simple idea: can a blob in a disc mimic the astrometric signal of an Earth-mass at 1 AU?

12 Oct 2011 3 Dynamical and brightness astrometry

Baryc. M M * C  B   I Ph I I << I 1 2 2 1 Photoc.

a

M a   = C ● Dynamical astrometry B M* D

I I M ● 2 a 2 C a Brightness (photometric) astrometry  Ph= − B= − I1 D  I1 M*  D Question: can   ph be >   B ?

12 Oct 2011 4 Dynamical and brightness astrometry

Baryc. M M * C  B   I Ph I I << I 1 2 2 1 Photoc.

MC a −6 a ●   B = ~ 3 x 10 for a 1 Earth-mass planet M* D D

I a I a   = 2 −  2 -6 ● Ph B ~ Can I /I be > 3x10 ? I1 D I1 D 2 1

12 Oct 2011 5 Dynamical and brightness astrometry

Baryc. M M * C  B   I Ph I I << I 1 2 2 1 Photoc.

For 1 Jup. MC a −6 a  Ph ●   B = ~ 3 x 10 M* D D  B

2 2 ● I2 a A RC a A RC   ~ = = Ph I D 4  a  D 4 aD 1

~0.1 AU a

12 Oct 2011 6 Dynamical and brightness astrometry

MC a −6 a  Ph 1 Jup at 1 AU ●   B = ~ 3 x 10 M* D D

2 2 I2 a A RC a A RC  B ●   ~ = = Ph I D 4  a  D 4 aD 1

~0.1 AU a

Can   Ph be larger than   B at 1 AU ? 2 −6 2 2 2 Condition: A/4RC /1AU> 3.10 ~100 R Jup ==> ARC > 50R Jup Explore sources of low mass objects brighter than 50 Jupiter at 1 AU Past experience: Anything can happen in 12 Oct 2011 7

Astrophysical sources of brighness excess

● Simulations ''Irregular Satellite Swarms: Detectable Dust around and Extrasolar Planets'' Kennedy & Wyatt, MNRAS, 412, 2137, 2011

Satell. swarm

103

1 Jupiter

12 Oct 2011 8 Astrophysical sources of brighness excess

● Observations – Earth's dust ring (Reach, Icarus, 209, 848, 2011) About N = 10 blobs 0.1 AU x 0.1 AU = 200 x 200 R 2 jup Assuming a dust albedo 0.01,

2 3 2 ARC >~10 R Jup

for each blob and at 1 AU:

2 2 1 A R a A R R   ~ C = C =0.015 Jup Ph  N 4  a  D 12 aD D

~ B for 1MEarth at 1 AU

12 Oct 2011 9 Astrophysical sources of brighness excess

● Observations – b Companion 400 brighter than a Jupiter at 100 AU

Interpretation: dust cloud around a planet

Open question: Can such dust clouds exist at 1 AU?

12 Oct 2011 10 Conclusion

The photocenter variation can possibly be larger than the 1 Earth mass astrometric signal at 1 AU.

To measure masses at 1 AU, astrometric measurements must be combined with high contrast high angular resolution imaging

12 Oct 2011 11