Chemical constraints on Theories of Planet Formation

Vincent GeersInstitute for Astronomy, ETH Zurich

Star & Planet Formation group

A. Banzatti, S. Bruderer, F. Lahuis, I. Pascucci, Th. Henning, P. Ábrahám, A. Juhász

M. Meyer, U. Gorti, E. Mamajek, D. Hollenbach, A. Benz

Outline

• EX Lup: molecular emission lines toward variable YSO

• Limits on timescale ice-giant formation with Herschel

Different Flavors of Planet Formation

The carbon problem

Lee, Bergin & Nomura 2010

• C under abundant in Earth and meteorites compared to what is available at formation => primordial carbon grains are destroyed, while silicon grains remain intact

Different Flavors of Planet Formation

Discontinuities in disks provide observational tests

From M. Meyer, Physics World, November, 2009 Based on Dullemond et al. (2001) with artwork from R. Hurt (NASA)

Chemistry in planet-forming zone

• Wide range of molecules now detected in planet-forming zone (0.1-10AU) around few dozen YSOs(H2O, HCN, C2H2, OH, CO)

• Concurrent C, N, O in inner disks imply complex chemistry!

Pontoppidan et al. 2010

Gas disk chemistry may vary with stellar mass

Pascucci et al. (2009)

EX Lup: temporal domain experiment

• EX Lup– young M star with disk– Eruptive variable star, on timescale of decades– Recent outburst in January 2008, accretion rate

up ~40, luminosity ~4 (Aspin et al. 2010)– Spitzer observed it before and during outburst

• Unique experiment:What happens to the gas and dust content when only 1 parameter, luminosity, is changed?

Episodic formation of cometary material

• Witnessed formation of silicate crystals during outburst• Previous big outburst in 1955 => no trace in 2005 =>

efficient removal of crystalline silicates from surface

Ábrahám et al. 2009 Nature

ISM Pre-outburst Outburst Halley/Tempelcomets

Water line variations in disk around EX Lup

H2O features (Banzatti et al. in prep.)

Quiescent phaseMarch 2005

Outburst phaseApril 2008

Results of water modeling

• Line ratios suggest larger surface area with constant water abundance in outburst

• Grid of simple LTE models (NH2O, T, area)– best fit outburst : cooler

water and 4-5 x larger emitting area than when in quiescence

• Ice line moving outward during outburst?

Modified from Pontoppidan et al. 2010

C2H2, HCN, OH also change!

• OH: undetected in quiescence, detected in outburst– photo-dissociation from H2O ? (Tappe et al. 2008)

– compare with predictions for self-shielding by Bethell & Bergin 2010

• HCN & C2H2: detected in quiescence, not in outburst– Line flux ratio HCN/C2H2 in quiescence consistent with solar-type

star, cf. Pascucci et al. (2009)

Banzatti et al. (in prep.)

C2H2HCNOH OH

Quiescent phaseMarch 2005

Outburst phaseApril 2008

What is the timescale for forming ice-giants?

• Do young 10-100 Myr stars with debris disk systems have enough gas (> 10 Mearth) to form planets like Uranus and Neptune?

• Debris disks assumed gas-poor, but counter-examples exist: 49 Cet, Beta Pic (10-20 Myr)

• Modest amounts of gas may still significantly influence grain dynamics, thus planetesimals growth

Herschel is best for limits of ice-giant formation!

• [OI] one of the strongest gas emission lines originating from 10-50 AU region (ice-giant planet forming zone)=> sensitive probe of remnant gas available to form ice-giants

Based on Gorti & Hollenbach 2008

Limits on ice-giant formation with Herschel

• Search for remnant gas in 10-100 Myr debris disks, with detected planets and/or signs of planet formation:– GT program (2.5 hr) : HR 8799, HD 15115

to be scheduled hopefully next window Nov-Jan– OT program (4.9 hr) : 4 young stars with well-studied dust

distributions: HD 61005, HD377, MML17, RXJ1852.3-3700

• Observations will probe down to 0.01 – 4 Mearth of gas:– Non-detections => strong upper limits for formation of ice-

giants (M ~ 10 Mearth)

– Detections => will need follow-up (e.g. CO, [CII]) to determine relative abundances of C, N and O in photo-evaporating disks

Take home message

• EX Lup: molecular emission lines changing dramatically with luminosity during outburst :– H2O, OH lines stronger, C2H2 and HCN weaker

– Simple LTE model of water consistent with cooler water and larger emitting area

• Herschel will provide important constraints on timescale for formation of ice-giants

Gordon Research Conferences

Origins of Solar SystemsComposition of Forming Planets: A Tool to Understand Processes

July 17-22, 2011Mount Holyoke College

South Hadley, MA

Program available later this fall. Registration open athttp://www.grc.org/

or Google “GRC 2011 Origins”