Low luminosity observations: a test for the Galactic models

1. Low luminosity observations: a test for the Galactic models Degl’Innocenti S.1 , Cignoni M.1, Castellani V.2, Petroni S.1, Prada Moroni P.G.1 1Physics Department, University of Pisa 2 Monte Porzio Astronomical Observatory, Rome  The first Galactic models for star counts  The present situation  Galactic models and white dwarf population  Future developments

2. Galactic Models for star counts Observational data: magnitude and colour for the stars in Galactic fields Colour magnitude diagram for stars in the field l=111o, b= - 46o (Kron 1980)

3. Structure of the Galactic Models disc Two components models Bahcall & Soneira (B&S 1980,1984) spheroid disc Three components models Gilmore & Reid (G&R 1983) Robin & Crèze (1986) thick disk spheroid

4. Number of the stars at a given location of the Galaxy in a given range of apparent luminosity= X Absolute luminosity function Fi(L)  observations Spatial density law ri (r,L)from a model (where r = galactocentric distance, L = absolute luminosity, i=Galactic components) By integrating over all the distances in the chosen direction and over all the luminosities  Luminosity star counts  By adopting a colour -magnitude relation  colourstar counts  Comparison between theory and observations

5. The luminosity function adopted in the first Galactic Models Observations by McCuskey (1966), Luyten (1968), Wielen (1974) Great uncertainty for Mv> 11 but it was not a problem because at that time the observations did not reach low luminosities

6. Satisfactory fit of the observations (V<20-21) for the B&S and G&R models  confirmation of the adopted spatial distribution NGP 1 degree2 Bahcall & Soneira (1984) SA 57

7. Spatial density laws The spheroid rspher(r)/rspher(ro)(r/ro)-7/8 exp[-10.093 (r/ro)1/4] (con r=galactocentric distance, ro=galactocentric distance of the Sun, rspher(ro)=observed local density) The disc/thick disc r(r)/r(ro)  exp[-z/Hz] . exp[-(x - ro)/Hx] (z= height over the galactic disc, x= galactocentric distance on the disc, H = scale heights)

8. Present situation  Recent observational data at low luminosity  new development of the Galactic codes Few tests at low luminosity based on a relative few number of stars1 star counts do not still allow to certainly discriminate between models with and without thick disk  The analysis of the velocity distribution of local stars confirms the presence of a thick disk2   However it is not still possible to discriminate among the various thick disk parameters suggested by the different authors3 ________________________ 1 Mendez et al. 1996, Reid et al. 1996, Mendez & Guzman 1998, Castellani et al. 2001, Kerber et al. 2001 2 Casertano et al. 1990, Wyse & Gilmore 1995 ,Ojha et al. 1996. 3 vedi ad es.:Reid & Majewski 1993, Yamagata & Yoshii 1992, Ojha et al. 1996

9. Example: simulation of the field of NGC6397low latitude  relevant number of stars Observational data taken by the HST WFPC2 (King et al. 1998)

10. The spatial parameters of the three components models by Gilmore & Reid and Haywood et al. 1997

11.  Satisfactory agreement between theory and observations up to V 26.5  It is not possible to distinguish among the results of three component models with different spatial parameters ________________________ * Castellani, Degl’Innocenti, Petroni, Piotto 2001

12. The Pisa Galactic code  Three components THICK DISK: Age 9-11 Gyr, Z=0.0061 SPHEROID: Age 12 Gyr, Z=0.0002 DISK: Age 50 Myr -9 Gyr, Z=0.02 Synthetic model which adopts:  Homogeneous set of evolutionary tracks/isochrones for the different Galactic populations up to the white dwarf evolutionary phase  Updated initial mass function (IMF)3  _________________________________________ 1Gilmore, Wyse & Jones (1995) 2Cassisi, Castellani, Degl’Innocenti, Weiss 1998, Castellani, Degl’Innocenti, Marconi 1999, Salaris et al. 2000, Cassisi et al. 2000 3 Kroupa 2001

13. In this way one can obtain:  Magnitude and colour of the stars  CM diagram at the various Galactic co-ordinates  Evaluation of the evolutionary phase of the stars  “handy” code  Inclusion of the white dwarf population in an evolutionary consistent way The shape of the luminosity functions obtained for the disc and the spheroid is in agreement with the observational one within the present uncertainties

14. Fit of the globular cluster M68* (Z  0.0002) with the isochrones adopted in our code for the spheroid *Cassisi, Castellani, Degl’Innocenti, Salaris, Weiss (1998)

15. Fit of the Hyades* (Z0.024) *Castellani, Degl’Innocenti, Prada Moroni (2001)

16. Synthetic CMD for Z=0.001 Y=0.23 Age=15 Gyr (Brocato et al. 2000)

17. The white dwarf population  Updated evolutionary tracks 1  Recent relations between the progenitor mass and the white dwarf mass2  Updated colour transformations3 ________________________________________ 1 Salaris et al. (2000). Now updated white dwarf evolutionary tracks by Prada Moroni and Straniero are also available 2 Dominguez et al. 1999, Weideman 2000 3 Bergeron (2000)

19. The local white dwarfs luminosity function Good agreement with the local white dwarf LF without any normalization of the white dwarf population Castellani, Cignoni, Degl’Innocenti, Petroni, Prada Moroni (2002)

20. The WD population is barely sensitive to a change of theoretical WD models ...

21. …or to a variation of the adopted relation between the WD mass and the progenitor mass

22. Spheroid CMD for a field of 1 degree2 at the NGP

23. l=0 b=50 0.5 degree2 HST field

24. Apparent magnitude distribution  Observations up to V28 mag. include the whole disc and white dwarf disc population

25. White dwarfs only  The thick disk WD distribution is centered at V 30 mag. while the halo WD distribution at V 31 mag.

26. The Fornax field

27. V magnitude distribution

28. Color distribution V 28

29. V 28 Observations down to V~27-28 could allow to distinguish the WDs in color

30. The observational local luminosity function for the disc For Mv>11 the shape of the disc LF is uncertain

31. The halo LF is uncertain for Mv > 8

32. This affects the evaluation of the IMF: Observational LF + mass - luminosity relation IMF results  Per M>0.5 Mo dN/dM M-a(with a2.3  0.3)*  Per M<0.5 Mo the behaviour is not still well defined2 ______________________________________________________________ * see e.g. Kroupa 2001, Massey 1995 2 see e.g.. Scalo 1998, Kroupa 1998, Kroupa 2001

33.  By adopting the Kroupa (2001) IMF and our evolutionary tracks we obtain theoretical halo and disc LFs in agreement with the observational ones within their uncertaintes Note that the white dwarf population is not affected by the uncertainty on the low luminosity part of the LF (low mass IMF)

34. The effects on the results of the LF uncertainty V magnitude distribution in the field of NGC6397 for several assumptions about the disc/spheroid LFs ________________ * Castellani, Degl’Innocenti, Petroni, Piotto 2000

35. Is it possible to constrain the low luminosity part of the LF ? After the calibration of the spatial parameters thanks to the observations at high/intermediate luminosity Comparison between theory and observations for several fields at low luminosity could constrain the shape of the disc/spheroid LF

36. Models with and without thick disc At the high latitudes one expects a colour separation between the disc and the spheroid, while the thick disc should assume intermediate colours

37. Colour star counts As the luminosity decreases the differences in colour between models with and without thick disk increases ___ without thick disc ___ with thick disc ___ thick disc Star counts up to V 25 at latitudes 45o50o should discriminate between models with and without thick disk

38. …and then…age-metallicity-relations, star formation history… The work is just at the beginning!