Age and abundance structure of the central sub-kpc of the Milky Way

Transcription

1 Age and abundance structure of the central sub-kpc of the Milky Way Thomas Bensby Department of Astronomy and THeoretical Physics Sweden

2 MBD collaboration (Microlensed Bulge Dwarf) Sofia Feltzing Thomas Bensby Andy Gould Jennifer Johnson Martin Asplund Jorge Melendez Sara Lucatello Lund Univ. Lund Univ. Ohio State Univ., USA Ohio State Univ., USA Mount Stromlo, ANU, Australia Sao Paolo, Brazil INAF, Padova, Italy + MOA and OGLE people

3 Why dwarf stars? Reliable tracers of the chemical composition of the gas cloud they formed from: Long life times (>10 Gyr) and their atmospheres remain intact. Commonly used in Galactic chemical evolution studies: Edvardsson et al. (1993), Fuhrmann, Reddy, Bensby, and Adibekyan papers,...and many more... Studies of bulge giants and nearby disk dwarf stars showed very different abundance trends. Is this real? Need to compare dwarfs with dwarfs.. All detailed studies based on highresolution spectra of red giants, that can be an issue if they are metal-rich as spectral lines become very strong and numerous It is also possible to estimate stellar ages of individual stars from isochrones. Is the bulge really all old?

4 Why dwarf stars? Reliable tracers of the chemical composition of the gas cloud they formed from: Long life times (>10 Gyr) and their atmospheres remain intact. Commonly used in Galactic chemical evolution studies: Edvardsson et al. (1993), Fuhrmann, Reddy, Bensby, and Adibekyan papers,...and many more... Studies of bulge giants and nearby disk dwarf stars showed very different abundance trends. Is this real? Need to compare dwarfs with dwarfs.. All detailed studies based on highresolution spectra of red giants, that can be an issue if they are metal-rich as spectral lines become very strong and numerous It is also possible to estimate stellar ages of individual stars from isochrones. Is the bulge really all old? Fuhrmann (1998)

5 Why dwarf stars? Reliable tracers of the chemical composition of the gas cloud they formed from: Long life times (>10 Gyr) and their atmospheres remain intact. Commonly used in Galactic chemical evolution studies: Edvardsson et al. (1993), Fuhrmann, Reddy, Bensby, and Adibekyan papers,...and many more... Studies of bulge giants and nearby disk dwarf stars showed very different abundance trends. Is this real? Need to compare dwarfs with dwarfs.. All detailed studies based on highresolution spectra of red giants, that can be an issue if they are metal-rich as spectral lines become very strong and numerous It is also possible to estimate stellar ages of individual stars from isochrones. Is the bulge really all old? Fulbright et al. (2007)

6 Why dwarf stars? Reliable tracers of the chemical composition of the gas cloud they formed from: Long life times (>10 Gyr) and their atmospheres remain intact. Commonly used in Galactic chemical evolution studies: Edvardsson et al. (1993), Fuhrmann, Reddy, Bensby, and Adibekyan papers,...and many more... Studies of bulge giants and nearby disk dwarf stars showed very different abundance trends. Is this real? Need to compare dwarfs with dwarfs.. All detailed studies based on highresolution spectra of red giants, that can be an issue if they are metal-rich as spectral lines become very strong and numerous It is also possible to estimate stellar ages of individual stars from isochrones. Is the bulge really all old? [Fe/H]=0 [Fe/H]=-0.5

7 Why dwarf stars? Reliable tracers of the chemical composition of the gas cloud they formed from: Long life times (>10 Gyr) and their atmospheres remain intact. Commonly used in Galactic chemical evolution studies: Edvardsson et al. (1993), Fuhrmann, Reddy, Bensby, and Adibekyan papers,...and many more... Studies of bulge giants and nearby disk dwarf stars showed very different abundance trends. Is this real? Need to compare dwarfs with dwarfs.. All detailed studies based on highresolution spectra of red giants, that can be an issue if they are metal-rich as spectral lines become very strong and numerous It is also possible to estimate stellar ages of individual stars from isochrones. Is the bulge really all old? Zoccali et al. (2003) Old turn-off

8 Dwarf stars in the bulge are faint Dwarf stars in the bulge have V~19-20 Would require around 50 hours to get a good highresolution spectrum with UVES (S/N>50, R>40000) Clarkson et al. (2008)

9 Nature s magnifying glass A star can brighten by factors of several hundreds during a microlensing event Earth Compact object Lensed background star If the star reaches I~15 a 2 hour exposure with UVES gives S/N>50

10 OGLE and MOA OGLE-IV detected 2145 bulge microlensing events, and MOA detected 577 bulge microlensing events in 2015 Bulge fields currently monitored by OGLE-IV

11 91 microlensed bulge dwarfs Positions reflect the areas that are monitored by MOA and OGLE Distributed 2-5 degrees below the plane (or ~ pc below the plane, assuming a distance of 8 kpc)

12 The metallicity distribution Generalised histogram: each star is represented by a gaussian distribution centred at the determined [Fe/H] with a width represented by the estimated uncertainty. Independent of binning.

13 Multiple components The 5 gaussian peaks in the b=-5 ARGOS fields from Ness et al. (2013)

14 Ages and metallicities Metal-rich stars show a wide range of ages with a significant fraction (~50%) being young/ intermediate-age. Metal-poor stars mainly old, around Gyr

15 The old turn-off conspiracy Zoccali et al. (2003) Young and metal-rich stars occupy the same region in the HR diagram as old and metal-poor stars Clarkson et al. (2011) Stacking all stars in the same diagram will produce an apparent old turnoff Bensby et al. (2013, A&A, 549, A147) See also Haywood et al.(2016)

16 Chromium Background dots (nearby dwarfs from Bensby+2014): Red: older than ~9 Gyr (thick disk) Blue: younger than ~ 7 Gyr (thin disk)

17 Nickel Background dots (nearby dwarfs from Bensby+2014): Red: older than ~9 Gyr (thick disk) Blue: younger than ~ 7 Gyr (thin disk)

18 Magnesium Background dots (nearby dwarfs from Bensby+2014): Red: older than ~9 Gyr (thick disk) Blue: younger than ~ 7 Gyr (thin disk)

19 Calcium Background dots (nearby dwarfs from Bensby+2014): Red: older than ~9 Gyr (thick disk) Blue: younger than ~ 7 Gyr (thin disk)

20 Titanium Background dots (nearby dwarfs from Bensby+2014): Red: older than ~9 Gyr (thick disk) Blue: younger than ~ 7 Gyr (thin disk)

21 A shift in the knee A signature of slightly faster enrichment in the Bulge The knee in the bulge [alpha/fe] trends located at ~0.1 dex higher [Fe/H] than in the local thick disk.

22 Summary Microlensing offers a unique opportunity to study the detailed age and abundance structure of the Milky Way bulge Multiple components in the bulge MDF Wide age distribution Metal-poor mostly old Metal-rich young and old Abundance trends at [Fe/H]<0 very similar to the local thick disk, but slightly shifted to higher [Fe/H] Strong connections between the bulge components and the other Galactic populations. What is the bulge? A region (rather than a population) A conglomerate of Galactic stellar populations... and a bar!