Circumstellar Disks at 5-20 Myr: Observations of the Sco-Cen OB Association Marty Bitner

Star Formation Overview

Solar System Evolution CAI formation ~1Myr Moon forming impact 30+ Myr LHB 700 Myr, Gomes et al. (2005)

Observed Disk Evolution Mamajek (2009) Presence of disk inferred through H  emission or infrared excess emission Disks have largely disappeared by ~5 Myr

Dust Replenishment Timescale As large objects grow early in disk’s evolution, dust production rate declines After large objects are formed, they stir up planetesimals Collisional cascade leads to increased dust production rate Poynting-Robertson drag and radiation pressure remove dust from the system Kenyon & Bromley (2008)

The Study of Debris Disks When does disk material form into larger bodies and how does the leftover material dissipate? How do the physical conditions of the disk impact the formation and evolution of planets and small bodies? Are asteroid belts and Kuiper belts common in other planetary systems?

Planet in a Debris Disk

The Scorpius-Centaurus OB Association Sco Cen is the closest OB association to the sun ( pc) It is composed of three subgroups: Upper Scorpius, Lower Centaurus Crux, and Upper Centaurus Lupus with age estimates of 5,16, and 17 Myr Stars in our sample were identified as members of Sco Cen based on Hipparcos common proper motion with OB stars in the association

Observations Spitzer MIPS 24  m and 70  m photometry of 132 F- and G- type and 270 B- and A-type stars MIKE/Magellan high resolution optical spectroscopy of the F&G stars Science Goals: – Quantify dependence of disk evolution on age and stellar mass – Identify disk dispersal mechanisms

Interesting Source in Sco Cen F3/F5 binary aged 16 Myr T~ 440 K dust located near 2 AU Icy dust in two belts, 4-9 AU and AU Assuming stellar wind drag is dominant dust removal mechanism, lifetime of dust grains is ~12,000 years Dust grains must be replenished through collisions Lisse et al. (2008)

Disk Evolution With Age 24  m disk fractions for F- and G-type stars – 6/18 (33%) in 5 Myr Upper Sco – 19/49 (39%) in 16 Myr LCC – 8/46 (17%) in 17 Myr UCL Carpenter et al. (2006) found 0/22 F- and G-type stars possess IR excess at 16  m in 5 Myr Upper Sco Upper Sco IR excesses are smaller in magnitude than in older subgroups Primordial disk material incorporated into larger bodies at 3-5 Myr?

Corpuscular Stellar Wind Small grains can be “blown out” via transfer of momentum from corpuscular wind protons to circumstellar grains. Larger grains spiral inward because stellar wind produces a drag on circumstellar grains causing them to orbit with slower speeds. The increase in “drag” in the inward drift velocity, over that produced by Poynting- Robertson is given by the factor:

Stellar Mass Loss Rates and X-ray Activity Wood et al. (2005)

Sco-Cen Debris Disks For HD , HD , and HD , M wind c 2/ L * = 76, 27, and 77, suggesting that stellar wind drag is the dominant grain removal mechanism in these systems. At an age of 20 Myr, the Sun possessed M wind c 2/ L * = 460. Assuming stellar wind drag is larger than Poynting-Robertson drag, the lifetime of grains around a dusty star is If the grains are in a steady-state, we can estimate the mass in parent bodies assuming that the system is in steady state:

Infrared Luminosity vs. X-ray Luminosity More X-rays Less X-rays Less dust More dust

Distinct Populations? Distributions of fractional infrared luminosity separated at L x /L * = 2 x KS-test gives a 28% likelihood that the samples are drawn from the same parent population Less dustMore dust

Infrared Luminosity vs. Stellar Activity More dust Less dust Less activityMore activity

Evidence for Dust Removal by Stellar Winds Distributions of fractional infrared luminosity separated at R’ HK = -4.5 KS-test gives a 0.03% likelihood that the samples are drawn from the same parent population Suggests that stars which are more active (have stronger stellar winds), remove their disks more efficiently Less dustMore dust

An Accreting Close Binary 5 Myr old F5 primary with ~K5 secondary Hipparcos lists system as a binary with 0.15” angular separation (14 AU) No IR excess in Spitzer observations H  emission is strong and variable Accretion rate ~10 -8 M  yr -1, similar to T Tauri stars with optically thick disks

Absorption in Edge on Disks Beust et al. (1998)

Sodium Absorption in Edge on Sco-Cen Disks? 20 sources in our sample show narrow Na D absorption on top of their photospheric absorption vsini values are higher for the stars which show narrow Na absorption Compare to spectra of nearby early-type stars to test whether absorption is interstellar or circumstellar

Mass-Dependent Disk Evolution? Carpenter (2006) saw some evidence for mass-dependent disk evolution in Upper Sco With a sample of over 300 stars, we will improve the statistics Planets around more massive stars may have larger orbits because their disks disperse before they have time to migrate inward Carpenter et al. (2006) Higher mass stars Lower mass stars

Conclusions Distribution of infrared luminosity with stellar activity suggests stellar wind drag is an important dust removal mechanism Fainter IR excesses in 5 Myr Upper Sco compared with older subgroups supports the hypothesis that primordial dust may be incorporated into larger bodies at 5 Myr and replenished through collisions at Myr N arrow Na absorption may be probing gas towards edge-on debris disks