The chemical evolution of the Galactic Bulge seen through micro-lensing events

Transcription

1 The chemical evolution of the Galactic Bulge seen through micro-lensing events Sofia Feltzing Lund Observatory Main collaborators: Martin Asplund (MPA),Thomas Bensby (Lund), Andy Gold (Ohio), Jennifer Johnson (Ohio), Sara Lucatello (Padoa), Jorge Melendez (Sao Paolo), Jennifer Yee (Ohio)

2 But bulge dwarfs are faint Proper motion cleaned bulge CMD VTO ~19-20 Clarkson et al. (2008 ApJ )

3 Why dwarf stars? Very little, if any, changes in the composition of the atmosphere over time Spectra are easy to analyse (i.e., warm) For stars close to the turn-off, ages can be determined Large samples of dwarf stars in the solar neighbourhood to compare with differential analysis

4 Micro-lensing to the rescue MOA 2009 BLG 489S Transient phenomena Target-of-Opportunity program on UT2/VLT An event (P82 P92; PI: SF) I s = Amax 1000 before HJD >65 micro-lensed dwarfs observed, (>50 with VLT) SNR: μ-lens

5 OGLE-IV fields l=+10 b=+5 b=+10 l=+5 l=±0 l= 5 b=+5 red: per night yellow: 3-10 per night green: 1-3 per night blue: per night cyan: less than 0.5 per night transparent: observed occasionally b=±0 Micro-lensed dwarfs mainly in this area. Bensby et al. (2014, in prep.) Bensby et al. (2013 A&A 549 A147) Bensby et al. (2011 A&A 533 A134) Bensby et al. (2010 A&A 521 L57) Bensby et al. (2010 A&A 512 A41) Epstein et al. (2010 ApJ ) b= 5 Bensby et al. (2009 ApJ 699 L174) Bensby et al. (2009 A&A ) Cohen et al. (2008 ApJ ) Johnson et al. (2008 ApJ ) Johnson et al. (2007 ApJ 655 L33)

6 Abundance trends Magnesium Oxygen Nickel Chromium Size = Age Bensby et al. (2013 A&A 549 A147)

7 Abundance trends Dwarf stars in the bulge 700 disk dwarf stars near the Sun Kinematic thick disk Kinematic thin disk Bensby et al. (2013 A&A 549 A147) Large span in [Fe/H]; follows the thick disk trend Bensby et al. (2013 A&A 549 A147)

8 Abundances from giants Bulge giants Thick disk giants Thin disk giants Giant stars Alves-Brito et al. (2010 A&A 513 A35) Inner disk: 4<R sun <7 kpc Solar neighbourhood: R sun =8 kpc Outer disk: 9<R sun <13 kpc [Mg/Fe] [Fe/H] [Fe/H] [Fe/H] Bensby et al. (2011 ApJ 735 L46) See also Melendez et al. (2008 A&A 484 L21) and Gonzalez et al. (2011 A&A 530 A54) and preliminary results from the Gaia-ESO Survey.

9 Metallicity distribution(s) MDFs are important constraints on models Selection biases (all studies) Bensby et al. (2013 A&A 549 A147)

10 Complex interpretation Fig. 3. Metallicity histograms (MDFs) for model particles in 4 fields alongtheminoraxisoftheboxybulge(left panels). The mean value for each MDF is given by the red arrow pointing up from the bottom. ThecorrespondingmeanvalueforthedataofZoccalietal. (2008) is indicated by the blue arrow from the top. The histograms in the right panels are based data from Zoccali et al. (2008) for b = 4, 6, 12,andfromJohnsonetal.(2011)forb = 8. has several noteworthy properties: (i) The approximate outline of the boxy bulge can be seen together with low-metallicity indentations in the Galactic plane. The latter are due to foregroud/background stars in the planar bar, which in these positions dominate the bulge stars. (ii) A metallicity gradient is present in all directions on the sky, both vertically and radially. This is expected from the binding energy argument of Sect However, in the central few degrees the gradients become shallower. (iii) Iso-metallicity contours are more elongated vertically than horizontally, whereas the surface density contours are flattened to the plane (MVG11). (iv) The asymmetry between l>0andl<0 at intermediate latitudes is due to perspective effects and signifies the bar origin. In Fig. 4 high metallicities extend to larger l on the l>0 side at b 5,butfor b 10 they extend to larger l on the l<0 side. This map has some remarkable similarities with that being constructed from the near-infrared photometric VISTA VVV survey (Gonzalez et al. 2013). For now we can compare quantitatively with a few more measured values in off-axis fields. Details will of course depend on the parametrisation of our simple model. In the longitudinal direction the gradient along b =2 is α l = 0.05 dex/deg (see Fig. 2). Minniti et al. (1995) found mean metallicities [Fe/H]= 0.3 in two fields at (l, b) =(8, 7 )and(10, 7 ) for bulge stars selected with [Fe/H]> 1. The model values in these fields are and Model Model Martinez-Valpuesta & Gerhard ( ApJL 3) Fig. 4. Metallicity map of the model bulge and bar in galactic coordinates. Foreground and background disk particles with distances < 4kpcand> 12 kpc from the solar position are excluded. The colour in each square corresponds to the average metallicity in a cone with radius 0.5 centered at this position. APOGEE, Gaia-ESO Survey 3.3. Origin of the vertical gradient be quantified by considering the change in Jacobi energy Clearly, while the bar and buckling instabilities scram

11 Summary Elemental abundance trends are very tight with no broad [α/fe] spread as in the S.N. No high α-abundances at large [Fe/H], consistent with most recent RGB/RC studies There are not only old stars in the bulge but potentiall also younger stars. Compare van Loon et al. 2003, Cole & Weinberg 2002, Uttenthaler et al van Loon et al MNRAS Cole & Weinberg 2002 ApJ 574 L43 Uttenthaler et al A&A

12 Analysis Wλ LTE All parameters derived from the spectral analysis Compare with colours from micro-lensing Check with Hα line Bensby et al. (2013 A&A 549 A147)

13 Ages [Fe/H] 14 [Fe/H] > < [Fe/H] < [Fe/H] 0 15 Bensby et al. (2013 A&A 549 A147) [Fe/H] 0.3?

14 Three things to solve How important are secular/internal (e.g., migration) vs external (e.g., mergers) processes for shaping the stellar disk(s) and bulge? Is the bulge a separate entity or just an extension (mainly) of the disk(s)? Requires interplay between models and observations and models with refined details

15 A bit more precise 65 micro-lensed dwarf and subgiant stars Baade s window Innermost three fields of AAOmega survey (Ness et al.)

16 Li in the Bulge Spite plateau First clear detection of Li in the bulge Metal-poor part of the bulge follows the Spite plateau Bensby et al. (2010 A&A 521 L57)