1. Chasing disks around massive stars with Malcolm
Disks and the formation of OB stars
How HMCs studies turned into quest for disks
Before ALMA: disks versus toroids
The ALMA era: disks around B stars
The current & future challenge: disks around O stars

2. «First things first!» (Malcolm priv. comm.):
Why are disks so important?
Star formation: inside-out collapse onto protostar
Two relevant timescales:
accretion  tacc = M*/(dM/dt)
contraction  tKH = GM*/R*L*
Lowmass (< 8 MO): tacc < tKH
Highmass (> 8 MO): tacc > tKH  accretion on ZAMS  radiation pressure may halt accretion in spherical symmetry  disk may be solution! 2

3. Theory
Different models of high-mass star formation (core accretion, competitive accretion, …), but all predict circumstellar disks of ~ AU
See e.g. Bonnell 2005, Krumholz et al. 2007, Keto 2007, Kuiper et al. 2010, 2011
 stars up to 140 MO may form by disk accretion 3

4. Existence of disks
Disks are natural outcome of infall + angular momentum conservation, however:
B field  magnetic braking, pseudo disks?
Ionization by OB stars  photoevaporation?
Tidal interaction with cluster  truncation?
Merging of low-mass stars  destruction?
 Disks in OB protostars could not exist! 4

5. A little bit of history In the ’90s:
Growing evidence of disks around low mass YSOs from HST (proplyds; O’Dell et al. 1993) and mm interferometry (GG Tau, etc.; Dutrey et al. 1994, Simon et al. 2000)
Very little evidence of disks around OB stars (G ; Keto et al. 1987, 1988): too embedded, too far? But many studies of hot molecular cores! (see Friedrich’s talk)

6. My collaboration with Malcolm: From HMCs to disks
HMCs are dense, hot, chemically rich: cradles of OB stars?
Surveys of UC HIIs, H2O masers, luminous IRAS sources, etc. to identify HMCs
Tracers: NH3(4,4), CH3CN
Instruments: single-dish (IRAM, Effelsberg)  interferometry (IRAM, VLA) of selected objects

7. The special case of the G31.41+0.31 HMC
(106 LO  O type)
UCHII
HMC 7

8. strong CH3CN(6-5) emission
TB (K)
frequency (GHz)

9. Elongated distribution of velocity peaks with velocity gradient!
Rotation or outflow? Rotation  Mdyn = 900 MO ~ Mcore
2’’ resolution 9

10. IRAM PdBI observations:
Velocity gradients found also in other luminous (O- type) HMCs, but too far  limited angular resolution & sensitivity  closer HMCs needed  B-type YSOs
Selected HMC: IRAS
distance 1.6 kpc  good spatial resolution
luminosity 104 LO  massive enough
H2O masers  young and active
bipolar outflow  disk?
IRAM PdBI observations:
outflow tracer (HCO+)
High-density, high-temperature tracer (CH3CN)
3 arcsec resolution

11. PdBI in 1995: CH3CN(6-5) 3’’ resolution
Plots of peak positions in different velocity channels:
elongated perp. to outflow
velocity gradient
outflow axis 11

12. jet
disk
PdBI in 1997:
CH3CN(12-11)
0.7’’ resolution 12

13. Malcolm’s insight:
Keplerian rotation 
 FWHM  ΘS-0.5 13

14. IRAS
Cesaroni et al. (1997, 1999,
2005, 2013, 2014)
Hofner et al. (1999, 2007)
Moscadelli et al. (2005, 2010)
Kepler+infall
8 MO star
Image: H2 at 2µm
CH3CN
H2O masers 14

15. The search for disks
After year 2000, several groups engaged in search for disks around massive stars.
Selection criteria for targets:
Bolometric (IRAS) luminosity > several 103 LO  high-mass (proto)star
Association with outflow  likely disk?
Presence of massive (> 10 MO), compact (< 0.1 pc) molecular core  deeply embedded (young) high- mass object
In some cases maser and/or UCHII  OB stars 15

16. H2O, CH3OH, OH, SiO maser lines  mas resolution
Tools adopted:
Thermal lines of rare (low-abundance) molecules  trace high- density, high-temperature gas in disk
H2O, CH3OH, OH, SiO maser lines  mas resolution
(sub)mm continuum  disk mass
IR continuum/lines  disk emission and absorption
cm continuum and RRL  ionised accretion flow
Diagnostic:
Flattened (sub)mm core perpendicular to jet/outflow
Velocity gradient perpendicular to associated outflow
Peculiar (Keplerian) pattern in position-velocity plot
Dark silouhette in near-IR against bright background
Elongated emission in the mid-IR perpendicular to bipolar reflection nebula 16

17. Our choice: molecular lines  kinematical signature of rotation & outflow
core
disk
outflow
outflow 17

18. Disks Toroids Beltrán et al. (2010, 2014) Sánchez-Monge et al. (2014)
M < a few 10 MO
R ~ 1000 AU
L ~ 104 LO  B (proto)stars
large tacc/trot
 circumstellar equilibrium structures
Toroids
M > 100 MO
R ~ AU
L > 105 LO  O (proto)stars
small tacc/trot
 transient circum-cluster structures
Beltrán et al. (2010, 2014)
Sánchez-Monge et al. (2014)
Beltrán & de Wit (2017) 18

19. Chasing disks with Malcolm in the ALMA era
Is the Keplerian disk around the B-type protostar IRAS unique?
 ALMA Cycle 0: 0.4’’ observations of 2 YSOs at ~2 kpc, with 104 LO
Are there disks around O-type YSOs?
 ALMA Cycle 2: 0.2’’ observations of 6 YSOs with >105 LO

20. Typical ALMA spectra
13CH3CN
CH3CN 20

21. B-type YSOs

22. Keplerian rotation about 18 MO G35.20-0.74 N ALMA 350 GHz continuuum:
Sanchez-Monge et al. (2013, 2014)
Keplerian rotation
about 18 MO
ALMA 350 GHz continuuum:
filament (~ 40 MO) perpendicolar to bipolar nebula
~ 6 cores along filament 22

23. G
Prominent core (~4 MO) at center of bipolar nebula
Position-velocity plots along cut perpendicular to bipolar nebula
White pattern: Keplerian rotation about about 6 MO
Beltrán et al. (2014) 23

24. O-type YSOs

25. G31.41+0.31 CH3CN(1211) 97 K 778 K 10 times better resolution
Beltrán et al. (2018)
10 times better resolution
than first observation in 1994!
K=2
v8=1
97 K
778 K
high V + small offset
 small R + high T
 rotation Vrot ~ R-a 25

26. Detection rate depends more on L/M than on distance  age effect?
Name
dist.
L/M
CH3CN disk?
kpc
LO/MO
Y/N/?
G17.64
2.2
300 N
G24.78
7.7 15 N?
G29.96
6.2
< 300 Y
G31.41
7.9 17 ?
G345.49
2.4 22
G345.50
2.0 84
Y (2)
AFGL4176
time
Detection rate depends more on L/M than on distance  age effect?
Too young  disk too embedded: difficult detection
Too old  too few molecular gas

27. 41 disks/toroids around OB-type stars
Current situation:
Up do date review on disks around massive stars (Beltrán & de Wit 2017):
41 disks/toroids around OB-type stars
Red: OB stars
Blue: int. mass stars 27

28. Last paper with Malcolm (disk accretion and ejection):
Radio follow-up of IR burst (Caratti o Garatti et al. 2017) 28

29. S255 NIRS3 burst! map disk from C34S Zinchenko et al. (2015) Keplerian
PV plot
burst! 29

30. Conclusion
OK… disks, toroids, jets, bursts, etc., all these may be very exciting, but… be scheptical and always keep in mind Malcolm’s warning:

31. Io credo nulla!
(I believe nothing!) 31