This document discusses the state of research on the internal kinematics of globular clusters. It notes that while globular clusters were once thought to contain single stellar populations, it is now known that they contain multiple stellar populations. Understanding the internal kinematics of these populations could provide clues about how they formed. However, current data is limited. Proper motions from Gaia have issues for studying globular cluster interiors. A few studies using Hubble Space Telescope data have provided some initial results but larger samples are needed. Future telescopes like JWST, WFIRST, and extremely large ground telescopes operating in the near-infrared could make major advances by providing wider-field imaging and proper motions with better precision deeper into glob

1. Andrea Bellini
05.08.2019
INTERNAL KINEMATICS OF GLOBULAR CLUSTERS:
STATE OF THE ART, ISSUES AND WHAT TO EXPECT FROM
THE NEAR FUTURE

2. Introduction
Globular Clusters: why do we care?
Multiple stellar populations
The missing piece of the puzzle: internal proper motions
What to expect from (near) future missions
Andrea Bellini

3. Globular Clusters: Why do we care?
Credits: Jay AndersonAndrea Bellini

4. Globular Clusters: Why do we care?
Credits: Jay AndersonAndrea Bellini

5. Globular Clusters: Why do we care?
Credits: Jay AndersonAndrea Bellini

6. Globular Clusters: Why do we care?
Richer et al. (2008, AJ, 135, 2141)
The ’80s & ‘90s “CMD wars”
2005: 126 HST orbits of NGC 6397 (PI: Richer)
The first complete CMD of a GC
Limited by field contamination
-> Proper Motions (PMs)!
Andrea Bellini

7. Multiple Stellar Populations
But…
ωCen anomalies!
Spectroscopically since the ‘70s:
Cannon & Stobie (1973),
Freeman & Rodgers (1975),
Norris & Bessell (1975), etc.
Photometrically since the late ‘90s:
(RGB) Lee et al. (1999)
(RGB) Pancino et al. (2000)
(MS) Anderson (1997)
Anderson, Ph.D. Thesis (1997)
Lee, Y. W., et al. (1999, Nature, 402, 55)
Andrea Bellini

8. Multiple Stellar Populations
But…
ωCen anomalies!
Piotto et al. (2005):
Low-res spectra of bMS and rMS stars
rMS more metal poor!?
Apparently only viable explanation:
Extreme Helium abundance (Y~0.4) in bMS stars
Piotto, G., et al. (2005, ApJ, 621, 777)
bMS: [Fe/H]=-1.27
rMS: [Fe/H]=-1.56
Andrea Bellini

9. UV-B filters map key molecular bands:
F275W: OH
F336W: NH
F438W: CN & CH
First-generation stars (1G) are N, Na & He poor
and C & O rich
Second-generation (2G) stars are N, Na & He rich
and C & O poor (sometimes also Fe rich)
Multiple Stellar Populations
Piotto et al. (2015, ApJ, 149, 91)
Andrea Bellini Bellini et al. (2017, ApJ, 842, 6)

10. Multiple Stellar Populations
Essentially ALL GCs host multiple stellar populations when analyzed with sufficient detail and with the right tools!
Milone et al. (2017, MNRAS, 464, 3636)Piotto et al. (2015, ApJ, 149, 91)
Andrea Bellini
(UV U) (U B)
Milone et al. (2018,
MNRAS, 481, 5098)

11. Multiple Stellar Populations
How did multiple stellar populations (MPs) in GCs form?
Andrea Bellini

12. Multiple Stellar Populations
How did multiple stellar populations (MPs) in GCs form? We don’t know!
Andrea Bellini

13. Multiple Stellar Populations
How did multiple stellar populations (MPs) in GCs form? We don’t know!
Andrea Bellini
Renzini et al. (2015, MNRAS, 454, 4197)

14. Multiple Stellar Populations
How did multiple stellar populations (MPs) in GCs form? We don’t know!
Andrea Bellini
(Corollary 1: The GCs’ cores won’t tell us much)
(Corollary 2: Kinematic fingerprints easier to spot in the outskirts of massive GCs)
IF 2G stars formed from an ICM polluted by 1G stars that sank into the GC’s
core, then:
• We would expect 2G stars to be more centrally concentrated than 1G
stars at birth. (D’Ercole et al. 2008; Decressin et al. 2008; Bekki &
Mackey 2009).
• The subsequent long-term dynamical evolution gradually erases the
differences in the spatial distribution of 1G and 2G stars.
• Stars are gradually diffuse toward the outskirts preferentially along
radial orbits.
• Stars of different generations interact dynamically and homogenize
their radial distributions and internal kinematics over time (relaxation).

15. Multiple Stellar Populations
How did multiple stellar populations (MPs) in GCs form? We don’t know!
Andrea Bellini
(Corollary 1: The GCs’ cores won’t tell us much)
(Corollary 2: Kinematic fingerprints easier to spot in the outskirts of massive GCs)
IF 2G stars formed from an ICM polluted by 1G stars that sank into the GC’s
core, then:
• We would expect 2G stars to be more centrally concentrated than 1G
stars at birth. (D’Ercole et al. 2008; Decressin et al. 2008; Bekki &
Mackey 2009).
• The subsequent long-term dynamical evolution gradually erases the
differences in the spatial distribution of 1G and 2G stars.
• Stars are expected to gradually diffuse towards the GC outskirts
preferentially along radial orbits.
• Stars of different generations interact dynamically and homogenize
their radial distributions and internal kinematics over time (relaxation).
Radial distance
σtan/σrad-1
rh rt
2G
1G
0
? ? ?
?

16. Internal Proper Motions
What about GAIA?
Andrea Bellini

17. Internal Proper Motions
What about GAIA?
DR2 issues:
• Plethora of systematic errors
hard/impossible to minimize
using DR2 alone
• No deblending (DR3)
• Not enough diagnostics in
general (DR3?)
• Not enough statistics
especially in the clusters’
cores (DR3?)
• Faint limit too bright
• PM errors too large (at the
faint limit)
Andrea Bellini

18. Internal Proper Motions
What about GAIA?
Bianchini et al. (2018, MNRAS.tmp, 2247)
DR2 issues:
• Plethora of systematic errors
hard/impossible to minimize
using DR2 alone
• No deblending (DR3)
• Not enough diagnostics in
general (DR3?)
• Not enough statistics
especially in the clusters’
cores (DR3?)
• Faint limit too bright
• PM errors too large (at the
faint limit)
Andrea Bellini

19. Internal Proper Motions
What about GAIA?
rh
1/2 rt
rt
Still, when doable:
Assuming 1 RGB star every 100 MS stars:
1000 RGB stars in a typical 105-star GC
500 RGBs within rh
<100 stars in the outer half rt
Typical σμ ~ 1–2 km/s
-> PM errors <0.5–1 km/s
@10 kpc, PM errors < 10—20 µas/yr
Andrea Bellini

20. Internal Proper Motions
What about GAIA?
rh
1/2 rt
rt
Still, when doable:
Assuming 1 RGB star every 100 MS stars:
1000 RGB stars in a typical 105-star GC
500 RGBs within rh
<100 stars in the outer half rt
Typical σμ ~ 1–2 km/s
-> PM errors <0.5–1 km/s
@10 kpc, PM errors < 10—20 µas/yr
Maybe 50 stars with appropriate PM
errors? Maybe 15-25 for each population?
-> only 1-ish point with large errorbars
Only a few close-by, massive GCs
can be studied in detail
Pancino et al. (2017, MNRAS, 467, 412)
Andrea Bellini
MS TO @ 6 kpc

21. Results
Alas, not really that much in the literature:
1. Bellini et al. (2009): ωCen (ground)
2. Anderson & van der Marel (2010): ω Cen (HST, core)
3. Richer et al. (2013): 47 Tuc (HST, outskurts)
4. Bellini et al. (2015): NGC 2808 (HST, core)
5. Bellini et al. (2018): ω Cen (HST, outskirts)
6. Libralato et al. (2018): NGC 362 (HST, core)
7. Milone et al. (2018): 47 Tuc (Gaia)
8. Libralato et al. (2019): NGC 6352 (HST, core)
Andrea Bellini

22. Results
Internal kinematics of MPs in the outskirts of ωCen
Bellini et al. (2018, ApJ, 853, 86)
Part of a large HST program to study the end of
the 2 WD CSs (Bellini et al. 2013) of the cluster
High-precision HST PMs of a field at ~17’ from the
cluster’s center (~3—4 rh)
Deep observations in Visible and IR. Shallow in UV
~10 µas/yr PM precision for well-measured stars:
>x90 better than Gaia DR2 at the same mag level
Andrea Bellini

23. Results
Internal kinematics of MPs in the outskirts of ωCen
Bellini et al. (2018, ApJ, 853, 86)
Selections based on Visible-IR
photometry
2G pops more radially
anisotropic than 1G
1G stars rotate faster!
Andrea Bellini

24. Alas, HST is not enough
• The core of most GCs too crowded even at HST resolution
• Fundamentally limited by what is available in the archive (as 1st epoch)
• Pencil-beam FoV drastically limits statistics in the clusters’ outskirts

25. The (near) Future
…is necessarily going to be near-IR dominated
JWST WFIRST E-ELT TMT (LUVOIR)
• Harder to identify multiple populations on a CMD
• New research opportunities in globular clusters
- IMBH? (need of high resolution in the core)
- Energy equipartition (need of faint stars over a wide FoV)
- Rotation (need of a wide FoV)
- Tidal-field effects (need of a wide FoV)
- HBL & beyond (necessarily limited to faint —and red— stars)

26. The (near) Future
Intermediate-Mass Black Holes (IMBHs)
NGC 6441 central 1x1 arcsec2
Greene et al. (2019) Astro2020 white paper
Very hard to detect (<1’’)
A 103 M⊙ produces a 1km/s raise in the central
velocity dispersion
Every claim (modulo G1) of a detection in the literature
(mostly from LOS velocities) followed by a counter-
claim
PMs have several advantages over LOS
measurements:
•Stars are measured individually (as opposed to
IFUs)
•PMs of the much more plentiful fainter stars
•PMs measure 2 components of the motion (no
mass/anisotropy degeneracy)
Need for high spatial resolution

27. The (near) Future
For all other cases, we need a wide field, near IR, or both
NGC 1851
R.A.
Dec.10 arcmin
NGC 104
SMC Dec.
R.A.
20 arcmin
HST
ACS/WFC
JWST
NIRCAM
HST
ACS/WFC
JWST
NIRCAM
Bellini et al. (2019) Astro2020 white paper