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IMF Constraints in Extreme Environments: Direct Detection of Young Low-Mass Stars in Unresolved Starbursts

This study aims to investigate the initial mass function (IMF) in extreme environments by detecting young low-mass stars in unresolved starburst clusters. The shape and cutoff of the IMF can provide insights into the physical conditions of star formation. The study focuses on starbursts with masses up to 10^6 MSun and star formation rates up to 1000 MSun/yr. The presence and contribution of pre-main sequence (PMS) stars are investigated using integrated spectroscopy and spectral modeling. Results show that 7-10% of the light in J, H, and K bands at 1 Myr is due to PMS stars. The CO and CaI features at 2.29, 2.32, and 2.26 microns, respectively, can be used to identify low-mass PMS stars (<K0), while the MgI feature at 2.28 microns can identify medium-mass PMS stars (F2-M1). The depth of CO and MgI features can be used to distinguish between different IMFs. Spectral resolution and metallicity are important factors in this analysis. Additional measurements and tests are needed to further understand and constrain the IMF in extreme environments.

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IMF Constraints in Extreme Environments: Direct Detection of Young Low-Mass Stars in Unresolved Starbursts

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

  2. 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 106 MSun (Orion only 104 MSun) SFR up to 1000 MSun yr-1 Mengel et. al NGC4038/39 -> Varying M/L ratios Smith & Gallagher M82 -> Cut-Off at 2 – 3 MSun? 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?

  3. Methods Draw 106 Msun 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 MSun 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

  4. Results 7- 10 % 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.

  5. Results 7- 10 % 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.

  6. 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 MSun EW(MgI) traces stars > 0.5 Msun – 2 Msun Use EW(CaI + CO(2-0))/EW(MgI) to distinguish IMFs S55 1 Myr S55 3 Myr Ch03 1 Myr Ch03 3Myr Ca Mg CO(2-0) Δ EW (1 Myr S55 – 3 Myr Ch03) =2.54 +/- 0.87 -> Can distinguish IMFs if we can measure age to factor of 3

  7. 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 104 Msun 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!

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