Constraining the IMF in Extreme Environments: Direct Detection of Young Low Mass Stars in Unresolved Starbursts Julia Greissl, Michael Meyer University of Arizona

Why Study the IMF in “Extreme Environments”? Shape of IMF can give clues to physical conditions of star formation Characteristic mass? Low-mass (high-mass) cutoff? No variation in stellar IMF in local environment -> Study more extreme environments! Starbursts clusters up to 10 6 M Sun (Orion only 10 4 M Sun ) SFR up to 1000 M Sun yr -1 Mengel et. al NGC4038/39 -> Varying M/L ratios Smith & Gallagher M82 -> Cut-Off at 2 – 3 M Sun ? Assumption that it is difficult to see low-mass stellar content BUT, PMS effects usually ignored What fraction of K-Band light is due to PMS stars at 1 Myr? Can we detect low-mass stars through integrated spectroscopy?

Methods Draw 10 6 M sun clusters from Salpeter or Chabrier IMF in Monte Carlo fashion as single burst events PMS M/L relation using Siess et al. (2000) tracks at 1/3/10 Myr all stars below 7/5/3 M Sun are PMS ~ 60 % in mass for Chabrier IMF assign each star Lum and Teff -> Using spectral library (R=1000 SNR ~ 50) of Meyer et al. (1996) compute PMS spectrum MS, post-MS stars and nebular flux using starburst99 (Leitherer et al.) Scale PMS spectrum according to MS and nebular continuum

Results BandIMFAgeF_PMSF_MSF_NEB JS551 Myr HS551 Myr KS551 Myr JCh031 Myr HCh031 Myr KCh031 Myr JCh033 Myr HCh033 Myr KCh033 Myr JCh0310 Myr HCh0310 Myr KCh0310 Myr % of light in J,H,K band from PMS stars at 1 Myr! Nebular continuum strongest at 1Myr (Mostly due to free-free emission) - decreases rapidly After 10 Myr MS stars dominate How to detect? Low-mass PMS stars (< K0) have CO features at 2.29, 2.32 and CaI at 2.26 microns. Medium-mass PMS stars (F2 - M1) have MgI feature at 2.28 microns.

Results BandIMFAgeF_PMSF_MSF_NEB JS551 Myr HS551 Myr KS551 Myr JCh031 Myr HCh031 Myr KCh031 Myr JCh033 Myr HCh033 Myr KCh033 Myr JCh0310 Myr HCh0310 Myr KCh0310 Myr % of light in J,H,K band from PMS stars at 1 Myr! Nebular continuum strongest at 1Myr (Mostly due to free-free emission) - decreases rapidly After 10 Myr MS stars dominate How to detect? Low-mass PMS stars (< K0) have CO features at 2.29, 2.32 and CaI at 2.26 microns. Medium-mass PMS stars (F2 - M1) have MgI feature at 2.28 microns.

1-2 % CO feature depth depending on age and IMF Need SNR ~ 100 spectra at R = 1000 to detect EW(CaI+ CO(2-0)) traces stars < 0.5 M Sun EW(MgI) traces stars > 0.5 M sun – 2 M sun Use EW(CaI + CO(2-0))/EW(MgI) to distinguish IMFs CaMgCO(2-0) IMFAgeEW(CaI + CO(2-0))/EW(MgI) S551 Myr8.98 +/ S553 Myr / Ch031 Myr6.27 +/ Ch033 Myr6.44 +/ > Can distinguish IMFs if we can measure age to factor of 3 S55 1 Myr S55 3 Myr Ch03 1 Myr Δ EW (1 Myr S55 – 3 Myr Ch03) =2.54 +/ Ch03 3Myr

Comments, Caveats How well do we know the age really? SED modelling of starbursts should constrain age to ~ factor of 3 Supergiants appear ~ 8 Myr dominate CO absorption Stochastic effects Neb. Flux at 10 4 M sun varies by 50% just due to random sampling Does neb. flux really dominate? Directly measure nebular flux in millimeter Spectral Resolution? Higher resolution makes it easier to distinguish between SGs and PMS stars due to differences in log(g) Metallicity? Starbursts have higher than solar z Test spectra will enable us to address these issues!