1. The IMF of the Orion Nebula Cluster Across the H-burning Limit Nicola Da Rio HST Orion Treasury Science Meeting STScI, Baltimore, September 12 th 13 th 2011

2. Motivation In order to derive the properties of PMS stars in Orion we need individual spectral types. We cannot derive spectral types from broad-band photometry U B V R I z J H K Most of the available spectral types are for stellar objects (Hillenbrand 1997, Da Rio+2009) In the BD mass range, spectral types are available for a small sample of sources (Slesnick+2004, Riddick+ 2007..) accretion T eff – A V degeneracy disk excess

3. Observational idea Deriving spectral types of cool stars and brown dwarfs from medium band photometry Deep photometry with WFI@MPG/ESO (35’x35’ FOV) in 4 bands. Two bands (753 and 770) are anti-correlated with the spectral type for T<4000K I-band is needed to get the extinction star by star MB 571/25 ESO#863 MB 753/18 ESO#848 MB 770/19 ESO#849 BB Ic/Iwp ESO#845 BandTotal exposure time 753 nm9360 s 770 nm14000 s I-band3360 s 571 nm11500 s

4. Color-color diagram synthetic isochrone

5. Color-color diagram synthetic isochrone  cooler hotter ר M0 = 3750K

6. Color-color diagram M0 = 3750K synthetic isochrone: inaccurate for very low temperatures Highly reddened early types (mostly BKG contaminants) VLMS and BDs (right to left) Possibly members Low A V early type members: NOT PRESENT BECAUSE OF SATURATATION Background red giants

7. Color-color diagram M0 = 3750K Additional lower limits: sources detected in 753 and I but not in 770 (which is deep into the molecular feature). If so.. must be cooler than … and therefore BD member…

8. Photometric depth HST/ACS Z vs (I-Z) CMD Black: all the ACS sources with I and Z Green: sources included in our previous work (Da Rio+ 2009, 2010), having WFI V,I and available spectral type Red: sources for which now I have photometry in all MB753, MB770 and I-band, and are placed in the 2-color diagram shown earlier.

9. Early-type background contaminants HST/ACS Z vs (I-Z) CMD Black: all the ACS sources with I and Z Blue: Teff > 3750K; Spectral type <M0 (from the position in the 2-color diagram) Red: Teff M0 (from the position in the 2-color diagram

10. Derivation of T eff e and A V Reddening independent [770 index]Extinction related [A V index] R V =3.1 from the Cardelli (1989) law

11. Derivation of T eff e and A V Calibration of the [770 index]-T eff relation based on available spectral types The index correlates with temperature for T eff < 3200K Candidate accretors are more scattered and systematically shifted. What is the effect of accretion excess on the [770 index]-T eff ?

12. Effect of accretion excess on the [770 index] Displacement of the index as a function of L accr /L star and T eff computed by synthetic photometry Bias in the determined Teff if the excess is neglected. The overall effect of accretion veiling is usually neglibile

13. Calibration of the [A V index] De-reddened sources (based on available A V from the literature and from de-reddening ISPI photometry) allow to define the A V =0 locus The A V =0 locus nicely confines the observed photometry at positive extinctions T eff and A V is derived for 1280 sources 544 of them previously assigned a spectral type 527 extra sources have T eff known and > 3200 K: 1807 sources in Orion have now a T eff estimate

14. The H-R diagram Distance: 414pc (Menten et al, 2007) Bolometric correction: from synthetic photometry with empirical calibration

15. Background contamination The number and relative fraction of background sources increases with fainter luminosities (see Robberto+ 2010). We detect a population of under-luminous sources well separated from the young population. Given the spectral type >M3 these are background red giants ONC members seen in scattered light (e.g. Guarcello+ 2010), if present, are a neglibile fraction of these sources.

16. The H-R diagram We detect BDs down to 0.02Msun Masses (and ages) are assigned by interpolation of tracks and isochrones We use Baraffe et al (1998) and D’Antona & Mazzitelli (1998) models

17. Completeness in the HRD Photometric completeness (based on artificial star tests preserving spatial distribution of the ONC Monte Carlo method to convert completeness from observed quantities to the HRD, accounting for differential extinction Completion accounting for spectral types from previous works

18. The IMF We measure a (completeness corrected) IMF which shows a strong deficiency of substellar objects. This trend was observed in previous studies (Hillenbrand & Carpenter 2000, Muench 2002, Slesnick 2004), but not as prominent as in this work. A possible explanation for this difference is our better characterization of field contamination. Chabrier log-normal Kroupa

19. Conclusions 1.Using a time-efficient pratical innovative medium-band photometric tecnique, we nearly doubled the sample of ONC sources with available spectral types and derived individual extinctions This sample is complete down to the H-burning limit and extends down the BD mass range Optical excess due to mass accretion does not affects this measurement 2.We detect a large population of background contaminants, which dominates over the ONC members in the BD mass range. In the HRD it is possible to exclude this contamination with high accuracy. 3.We derive the IMF for the ONC, which shows steep decline below the H- burning limit, suggesting a deficiency of substellar objects The paper has been sumbitted to ApJ