1. Alain KLOTZ : C.E.S.R. Toulouse - France Nicolas LODIEU : A.I.P. Postdam - Germany Emmanuel CAUX : C.E.S.R. Toulouse - France Jean-Louis MONIN : L.A.O.G. Grenoble - France

2. Characteristics of the Serpens cloud Star forming region Galactic location : l=31.5° b=5.4° Apparent dimensions : ~ 20 arcmin for Av>5 Central core : SVS 20 and its nebula Estimated distance : 259 +/- 37 pc (Straizys et al. 1996) Extinction : >30 for radius < 1.5 arcmin from SVS 20 Identified young stellar objects : > 100 (Kaas 1999, …) Estimated age : < 10 7 years (Giovannetti et al 1998, …) High Background contamination 1 low mass star 0.05 M sun identified (Lodieu et al. 2002)

3. Goal of this study Plot the mass function specially for stars lighter than 0.5 M sun (the “low mass” slope of the mass function is almost unknown). Find new brown dwarfs candidates. Establish a high resolution Av chart of the cloud in order to constraint spectral types (and mass) of individual stars. Determination of Av, distance modulus and mass for all stars from JHK magnitudes using (J-H)/(H-K) and K/(H-K) diagrams.

5. Observations JHK 2x5 arcmin images from Pic du Midi (TBL-MOICAM) JHK 6x11 arcmin images from ESO (NTT-SOFI) JHK spectra from ESO (VLT-ISAAC) JHK images from Pic-du-Midi-TBL, allowed us to identified an interesting field to observe with ESO-NTT. JHK images from ESO-NTT allowed us to identify very low mass star candidates. JHK spectra from ESO-VLT allowed us to find a very low mass star (Ser-BD 1) embedded in the Serpens cloud. A three step strategy

6. Extinction and distance modulus determination (1) From NTT-SOFI images: A total of ~9000 stars measured ~ 2300 stars measured in JHK ~ 3000 stars measured in HK For each star measured in JHK Av is measured from colour/colour diagram. MD is measured from colour/magnitude diag. Three types of class are considered: dwarfs (blue on diagrams) giants (red on diagrams) young 3. 10 6 yr (magenta on diags.) Right : an example for a star J=18.5, H=17.5, K=17. There are four couples of solutions (Av, MD) for a cool young (4.7, 13), a cool dwarf (4.9, 11), a hot giant (6.5,18) and a hot dwarf (6.7, 12). Uncertainties are 1.5 for Av and 1.5 for MD.

7. For each star measured in HK J magnitude is estimated supposing the star is in the middle of the reddening band of the colour/colour diagram: slope comes from Rieke & Lebosky (1985) Aj/Av=0.282, Ah/Av=0.175, Ak/Av=0.112 For each star outside the reddening band A star having a red colour excess is considered in the Serpens cloud J = H + slope*(H-K) + 0.15 Extinction and distance modulus determination (2)

8. Spatial distribution of extinction Building the Av chart of the field The observed field is divided in cells of 15x15 arcsec. In each cell, an average value of Av is computed from Av previously computed from stars, supposed to be cool dwarfs, measured in JHK or HK bands (method adapted from Lada et al. 1994). Cells which are empty of JHK or HK stars, will be computed from an empirical relation giving Av from the density of stars in the K band. This relation is established from cells treated rigorously. Av chart. White is for Av>60. JHK image from NTT-SOFI Dark is for Av~5.

9. Spectral types and mass determination (1) (Av,MD) determination for stars lead also to the estimation of possible spectral types and masses. A maximum of 6 solutions are possible. One must select the “most” probable solution: The simplified algorithm of the choice of the spectral class is: A young star if its MD is compatible with Serpens cloud and if Av is not higher than background stars. A giant star if its MD is less than 14 (maximum distance allowed for giants at this galactic latitude). A cool dwarf if its Av is not too lower compared to the mean Av of the corresponding cell (expected if its distance modulus is lower or equal to that of Serpens cloud). Else the star is assigned as a hot dwarf. A method for a direct assignment N.B. The majority of stars are assigned as cool dwarfs

10. Spectral types and mass determination (2) Advantages of that method Disadvantages of that method Good accuracy for low and very low mass stars (M<0.5 M sun ) Possibility to extract a mass function for low masses. Individual assignments of (Av,MD, spectral type, mass) Finding peculiar stars is easy. A brown dwarf candidate was found by this way. High precision photometry is needed 0.05 mag error typically change 1.5 mag for Av and 1.5 mag for MD. Three JHK magnitudes are needed for a non ambiguous determination. Poor accuracy for masses between 0.8 and 1.5 M sun Comes from the “elbow” around M0 spectral types in colour/colour diagr. The choice of the most probable solution is ambiguous.

11. Luminosity function Derived from stars observed in JHK bands without IR excess & age=3 10 6 yr & mass < 0.8 M sun 202 stars were found at the distance modulus (+/- 1) of the Serpens cloud: 28 stars (only !) fit the conditions (see upper histograms) 17 other stars have an infrared colour excess 11 other stars have only HK magnitude 55 other stars have a blue colour excess 91 stars were assigned as classical dwarfs

12. Mass function & conclusion Derived from stars observed in JHK bands without IR excess & age=3 10 6 yr & mass < 0.8 M sun Not enough stars to conclude about the slope of low mass part of the mass function Location of brown dwarfs candidates (Ser-BD 1 was discovered by this method). There is not a dramatic fall of the mass function for masses lower than 0.5 solar mass. This work is still in progress ! A refinement of the method will allow to growth the number of stars to include in the mass function...