Download Implies dark halo for self-enrichment

The Chemical Impact of Stellar
Mass Loss
Rosemary Wyse
Johns Hopkins University
Gerry Gilmore, John Norris, Mark Wilkinson, Vasily Belokurov,
Sergei Koposov, Wyn Evans, Dan Zucker, Anna Frebel, David Yong
Elemental abundances

Field stars and dwarf spheroidal members

Massive-star mass function (core-collapse
SNe)


Mixing in interstellar medium


Invariant
Surprisingly efficient
Carbon-rich (single) stars at very low [Fe/H]
 But
also carbon-normal ultra-metal-poor stars
Elemental Abundances:
beyond metallicity

Core collapse supernovae have progenitors > 8 M and
< 107 yr, less than typical duration
explode on timescales ~
of star formation
 Main site of -elements, e.g. O, Mg, Ti, Ca, Si
 Low mass stars enriched by only Type II SNe show
enhanced ratio of -elements to iron, with value
dependent on mass distribution of SNe progenitors – if
well-mixed system, see IMF-average
 Type Ia SNe produce very significant iron, on longer
timescales, few x 108 – several 1010yr (WD in binaries)
after birth of progenitor stars
Schematic [O/Fe] vs [Fe/H]
Wyse & Gilmore 1993
IMF biased to most massive stars
Type II only
Plus Type Ia
Slow enrichment
Low SFR, winds..
Fast
Self-enriched star forming region.
Assume good mixing so IMF-average yields
IMF dependence due to different nucleosynthetic
yields of Type II progenitors of different masses
Kobayashi et al 2006
Progenitor mass
Gibson 1998
Salpeter IMF
(all progenitor
masses) gives
[/Fe] ~ 0.4;
Change of IMF
slope of ~1 gives
change in [ /Fe]
~ +0.3
(Wyse & Gilmore 92)
Elemental abundances in old stars
Ruchti et al 2011,12
Thick disk extends to -2 dex,
same enhanced [α/Fe] as halo
stars
same massive-star IMF, short
duration of star formation
little scatter – fixed IMF, good
mixing, down to [Fe/H] < -3 dex
Stars from RAVE survey, candidate
metal-poor disk, follow-up echelle data
Bulge Matches Thick Disk

same massive-star IMF
Gonzales et al 2011
Extended, low-rate star formation and slow enrichment
with gas retention, leads to expectation of ~solar (or below)
ratios of [/Fe] at low [Fe/H], such as in LMC stars
Hiatus then burst
Pompeia et al 2008
Smith et al 2003
LMC: solid
Local disk
Gilmore & Wyse 1991
dSphs vs. MWG abundances: SFH
(from A. Koch, 2009 + updates)
Boo I
Scl


Gilmore et al; Norris et al 10 BooI
Simon et al 10 Leo IV
Frebel et al 10 Scl
Leo IV
◊



Shetrone et al. (2001, 2003): 5 dSphs
Sadakane et al. (2004): Ursa Minor
Monaco et al. (2005): Sagittarius
Koch et al. (2006, 2007): Carina
Letarte (2006): Fornax
Koch et al. (2008): Hercules
Shetrone et al. (2008): Leo II
Frebel et al. (2009): Coma Ber, Ursa Major
Aoki et al. (2009): Sextans
Hill et al. (in prep): Sculptor

Same ‘plateau’ in [α/Fe] in all systems at lowest
metallicities


Type II enrichment only: massive-star IMF invariant, and
apparently well-sampled/mixed
Stellar halo could form from any system(s) in which starformation is short-lived, and inefficient so that mean
metallicity kept low


Star clusters, galaxies, transient structures…
Complementary, independent age information that bulk
of halo stars are OLD further constrains progenitors (e.g.
Unavane, Wyse & Gilmore 1996)
Star Counts: Invariant Low-Mass IMF
Main sequence luminosity functions of UMi dSph
and of globular clusters are indistinguishable.
HST star counts
Wyse et al 2002
UMi dSph stars have
narrow range of ages,
all old
M92 
M15 
0.3M
Low-Mass Stellar MF in Bulge:
Zoccali et al 2000
(M15)
Matches local disk
(Kroupa 2000)
And M15 –
which matches
the UMi dSph:
Low-mass IMF
invariant wrt
metallicity, time..
Carina dSph CMD
Stetson et al 2011
Very extended,
non-monotonic
star formation
history
Carina dSph – extended, bursty star formation history
Carina data: bursts + inhomogeneous
star formation
Massive star IMF invariant
Koch et al inc RW 2008
Age estimates: younger indeed higher [α/Fe]
Lemasle et al 2012
A much simpler system: Bootes I ‘ultra-faint’ dwarf
M* ~ 4 x 104 M, dist ~ 65 kpc
SDSS Discovery CMD
(Belokurov et al, inc RW, 2006b)
Subaru (Okamoto, PhD, 2010)
Norris, RW et al 2010
[Fe/H] distributions and radial dependence
Dwarf spheroidal galaxies
have well-defined peaks,
with low metallicity tails:
self-enriched, from
primordial gas? Then lost
most baryons – M/L high.
Segue 1, 7 stars
Very luminous globular cluster 
lacks low-metallicity tail; most
clusters do not self-enrich in Fe;
Need enough compact baryons
16 stars
Alpha Abundances:


8 stars in Boo I, VLT UVES
Double-blind analysis (Gilmore et al 2012)
 minimal scatter
Boo-119 is CEMP-no star; open dots are field CEMP
CEMP-no star Segue 1-7 has [Mg/Fe] ~ 0.94 (Norris et al 2010)
Carbon-enhanced star in Segue 1 (triangles) and BooI (circles)
Norris et al 2010a,b
No s-process plus high [Mg/Fe]
Including data for Boo I stars from Lai et al 2011
 

[Fe/H]
 

time


ISM mixing scale
Two modes of enrichment?



Unmixed, very early, enriched by individual
supernovae from zero-metal stars?
Extremely well mixed, fully sample massive-star
IMF – minimal scatter in element ratios
Boo I probably lost 90% of baryons – metals?
Conclusions

Lack of variations in elemental abundances
probably produced by core-collapse supernovae
argue for invariant massive-star IMF




Star counts imply fixed low-mass IMF
Overall patterns determined by star-formation
history
Small scatter implies well-mixed ISM
WHY? And HOW?
Large Scale Flows


Chemical evolution plus global star formation
rates argue for gas replenishment
High velocity clouds exist




Galactic Fountain
Cold Flows from Cosmic Web
Accretion from satellite galaxies (Magellanic
Stream)
Radial migration
Boötes I

MV ~ -6.3, M* ~ 4 x 104 M (Kroupa IMF), distance of ~
65kpc, half-light radius ~ 250pc (< dark matter
~
scalelength?), central velocity dispersion
~ 3-6 km/s (?),
derived (extrapolated) mass within half-light radius ~
106-7 M, M/L ~ 103, mean dark matter density ~
0.1M/pc3
 collapse at redshift z ~
> 10

Color-magnitude diagram consistent with old, metalpoor population, similar to classic halo globular cluster

More luminous dSph have very varied SFHs
Belokurov et al 06; Gilmore et al 07; Martin et al 08; Walker et al 09;
Okamoto et al 10
Koposov, et al (inc RW), 2011b
Getting the most from Flames on VLT: Bootes I field,
~1 half light radius FOV, 130 fibres, 12 x 45min integrations
Repeated observations allow detection of variability:
110 non-variable (giant) stars (< 1km/s)
Analyse spectra in pixel space; Retain full covariance:
map model spectra onto data, find ‘best’ match values of
stellar parameters (gravity, metallicity, surface temperature)
with a Bayesian classifier.
Black: data r=19; red=model
Koposov, et al (inc RW), 2011b
Identify 37 members, based on line-of-sight velocity, metallicity and stellar
gravity (should be giants, dwarfs will be foreground field halo stars)
Previous literature value
(already reduced)
Field of Streams
(and dots)
Belokurov et al (inc RW, 2006)

Boo I

Segue 1
Outer stellar halo is lumpy: but only ~15% by mass (total
mass ~ 109M) and dominated by Sgr dSph stream
SDSS data, 19< r< 22, g-r < 0.4 colour-coded by
mag (distance), blue (~10kpc), green, red (~30kpc)
Red: Segue 1
Black: Boo I
Norris, RW et al 2010
Wide-area spectroscopy
Geha et al


І
Members well beyond the nominal half-light radius in both
Stars more iron-poor than -3 dex (10-3 solar) exist in both
 Extremely rare in field halo, membership very likely


Very far out, parameters and velocity confirmed by follow-up:
 Segue 1 is very extended! (as is Boo I)
Both systems show a large spread in iron

Implies dark halo for self-enrichment (cf Simon et al 2011, 6 stars in
Segue 1, 7 in total)
Wyse & Gilmore 1992
Salpeter IMF
slope: -1.35
Scalo: -1.5
Matteucci for
Bulge: -1.1
Chemical Abundances: Boo I & Segue 1
 Spectroscopic surveys with the 2dF/AAΩ fibre-fed
MOS; stars selected from SDSS to follow discovery
CMD: wide-area mapping unique capability of 2dF


400 fibres, 2-degree FOV, dual beam, chemical abundances from
blue spectra, R ~ 5000, range 3850-4540Å. Membership based on
radial velocity (to better than 10 km/s) and the derived values of
stellar parameters
Iron from calibration of Ca II K line (3933Å, as
field halo surveys, Beers et al 99), +/- 0.2 dex (Norris
et al 08)

Carbon from synthesis of CH G-band, +/- 0.2 in
Boo I and +/- 0.4 in Segue 1 (Norris et al 10)

Follow-up UVES echelle data, [Fe/H] +/- 0.1dex; [C/Fe] for 1 star

Caveat: Segue 1 in very complex part of the
Galaxy


Very flat (bimodal?) metallicity distribution,
unlike other dwarf galaxies: contamination?
Extended structure around it

Same distance and line-of-sight as Sgr stream, but
different velocity (Niederste-Ostholt et al wrong orbit for
Sgr stream)
Distance and velocity, line-of-sight match Orphan
stream (Newberg et al 2010, Koposov et al inc RW 2011a)
 What is the `300km/s stream’?


Extremely wide-field mapping needed to be
assured of status
Segue 1

MV ~ -1.5, M* ~ 600 M, distance of ~25kpc,
half-light radius ~ 30pc (?), velocity dispersion
~ 4 km/s (?), derived mass within half-light
radius ~ 3 x 105M(?), M/L ~ 2000 (?), again
<ρ>DM ~ 0.1 M/pc3 and high collapse redshift


Superposed on Sgr tidal debris, close in distance and
velocity (?), contamination likely (Niederste-Ostholt et al
09); unlikely (Simon et al 2010)
Again old, metal-poor population
Belokurov et al. 07; Martin et al 08; Geha et al 09; Walker et al 09;
Simon et al 2010