Download STScI 2005

An upper limit
to the masses
of stars
Donald F. Figer
STScI
Collaborators:
Sungsoo Kim (KHU)
Paco Najarro (CSIC)
Rolf Kudritzki (UH)
Mark Morris (UCLA)
Mike Rich (UCLA)
Arches Cluster Illustration
Outline
1.
2.
3.
4.
5.
Introduction to the problem
Observations
Analysis
Violators?
Conclusions
1. Introduction
An upper mass limit has been elusive
• There is no accepted upper mass limit for stars.
• Theory: incomplete understanding of star
formation/destruction.
– accretion may be inhibited by opacity to radiation
pressure/winds
– formation may be aided by collisions of protostellar
clumps
– destruction may be due to pulsational instability
• Observation: incompleteness in surveying
massive stars in the Galaxy.
– the most massive stars known have M~150 M
– most known clusters are not massive enough
Radial pulsations and an upper limit
1941, ApJ, 94, 537
Also see Eddington (1927, MNRAS, 87, 539)
Upper mass limit: theoretical predictions
Stothers & Simon (1970)
Upper mass limit: theoretical predictions
Ledoux (1941)
radial pulsation, e- opacity,
H
100 M
Schwarzchild & Härm (1959)
radial pulsation, e- opacity,
H and He, evolution
65-95 M
Stothers & Simon (1970)
radial pulsation, e- and atomic
Larson & Starrfield (1971)
pressure in HII region
50-60 M
Cox & Tabor (1976)
e- and atomic opacity
Los Alamos
80-100 M
Klapp et al. (1987)
e- and atomic opacity
Los Alamos
440 M
Stothers (1992)
e- and atomic opacity
Rogers-Iglesias
120-150 M
80-120 M
Upper mass limit: observation
R136
Feitzinger et al. (1980)
250-1000 M
Eta Car
various
120-150 M
R136a1
Massey & Hunter (1998)
136-155 M
Figer et al. (1998)
140-180 M
Damineli et al. (2000)
~70+? M
LBV 1806-20
Eikenberry et al. (2004)
150-1000 M
LBV 1806-20
Figer et al. (2004)
130 (binary?) M
HDE 269810
Walborn et al. (2004)
150 M
WR20a
Bonanos et al. (2004)
Rauw et al. (2004)
82+83 M
Pistol Star
Eta Car
The initial mass function: a tutorial
• Stars generally form with a frequency that
decreases with increasing mass for masses
greater than ~1 M:
d( log N)/d( log m)  
• Stars with M>150 M can only be observed in
clusters with total stellar mass >104 M.
• This requirement limits the potential sample of
stellar clusters that can constrain the upper mass
limit to only a few in the Galaxy.
The initial mass function: observations
=-1.35
=-1.35
1-120 M
Salpeter 1955
Kroupa 2002
2. Observations
Upper mass limit: an observational test
• Target sample must satisfy many criteria.
–
–
–
–
–
–
–
massive enough to populate massive bins
young enough to be pre-supernova phase
old enough to be free of natal molecular material
close enough to discern individual stars
at known distance
coeval enough to constitute a single event
of a known age
• Number of "expected" massive stars given by
extrapolating observed initial mass function.
Lick 3-m (1995)
Keck 10-m (1998)
HST (1999)
VLT (2003)
Galactic Center Clusters
too old (~4 Myr)
3. Analysis
Arches Cluster CMD
Figer et al. 1999, ApJ, 525, 750
Luminosity function
Stellar evolution models
O
WNL
WNE
WCL
WCE
WO
SN
Meynet, Maeder et al. 1994, A&AS, 103, 97
NICMOS 1.87 mm image of Arches Cluster
No WNE
or WC!
Figer et al. 2002, ApJ, 581, 258
NIII
NIII
HeII
HeI/HI
NIII
HeI
HeI
Arches stars: WN9 stars
enhanced Nitrogen
Figer et al. 2002, ApJ, 581, 258
Arches stars: O stars
HI
HeI
68
27
Figer et al. 2002, ApJ, 581, 258
NIII
NIII
NIII
Arches stars: quantitative spectroscopy
Najarro et al. 2004
Age through nitrogen abundances
Najarro, Figer, Hillier, & Kudritzki 2004, ApJ, 611, L105
Mass vs. magnitude for t=2 Myr
Initial mass function
Arches Cluster mass function: confirmation
HST•NICMOS
Flat Mass Function in the Arches Cluster
VLT•NAOS•CONICA
Stolte et al. 2003
Monte Carlo simulation
• Simulate 100,000 model clusters, each with 39 stars in four
highest mass bins.
• Repeat for two IMF slopes: =-1.35 and -0.90.
• Repeat for IMF cutoffs: 130, 150, 175, 200 M.
• Assign ages: = tCL± s = (2.0-2.5) ± 0.3 Myr.
• Apply evolution models to determine apparent magnitudes.
• Assign extinction: = AK,CL± s = 3.1 ± 0.3.
• Assign photometric error: s=0.2.
• Transform "observed" magnitudes into initial masses
assuming random cluster age (2.0-2.5 Myr) and AK=3.1.
• Estimate N(NM>130 M=0).
Simulated effects of errors
true initial mass function
inferred initial mass function
Results of Monte Carlo simulation
Does R136 have a cutoff?
• Massey & Hunter (1998) claim no upper mass cutoff.
• Weidner & Kroupa (2004) claim a cutoff of 150 M.
– deficit of 10 stars with M>150 M for Mc~50,000 M.
– deficit of 4 stars with M>150 M for Mc~20,000 M.
• Oey & Clark (2005) claim a cutoff of 120-200 M.
• Metallicity in LMC is less than in Arches: ZLMC~Z/3.
• Upper mass cutoff to IMF is roughly the same over a factor
of three in metallicity.
4. Violators?
Figer et al. 1999, ApJ, 525, 759
Is the Pistol Star "too" massive?
tracks by Langer
Figer et al. 1998, ApJ, 506, 384
Two Violators in the Quintuplet Cluster?
Pistol Star and #362 have ~ same mass.
Pistol Star
Star #362
Figer et al. 1999, ApJ, 525, 759
Geballe et al. 2000, ApJ, 530, 97
LBV 1806-20
• Claim
• 1-7 LPistol*
• 150-1000 M⊙
• Primary uncertainties
• distance
• temperature
• singularity
LBV
SGR
LBV 1806-20 is a binary?
double lines
Figer, Najarro, Kudritzki 2004, ApJ, 610, L109
Conclusions
• The Arches Cluster has an upper mass cutoff to the stellar
initial mass function.
• The upper mass cutoff is ~150 M.
• The upper mass cutoff may be invariant over a range of a
factor of three in metallicity.
The next step: search the Galaxy!
• Find massive stellar cluster candidates
– 2MASS
– Spitzer (GLIMPSE)
• Target for intensive observation
– NICMOS/HST (128 orbits proposed)
– Chandra (50 ks approved, 50 ks proposed)
– NIRSPEC/Keck (2 half nights appoved)
– Phoenix/Gemini (30 hours approved)
– IRMOS/KPNO 4-m (10 nights contingent on HST)
– EMIR/GTC (10 nights approved)
– VLA (~100 hours approved)
128 New Galactic Clusters from 2MASS
Candidate 2MASS Clusters
Massive Young Clusters in X-rays
Arches and Quintuplet Clusters in X-rays
Chandra
Law & Yusef-Zadeh 2003
Massive Young Clusters in Radio
Arches and Quintuplet Clusters in Radio
VLA
Lang et al. 2001