H 2 O, OH, SiO, NH 3 and CH 3 OH
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Transcript H 2 O, OH, SiO, NH 3 and CH 3 OH
Star Formation in our Galaxy
Dr Andrew Walsh (James Cook University, Australia)
Lecture 3 – High Mass Star Formation
and Masers
1.
2.
3.
4.
Is high mass star formation different?
Competitive Accretion vs Coalescence vs Monolithic Collapse
Disks around high mass stars
Masers (H2O, OH, SiO, NH3 and CH3OH)
pro⋅to⋅star
A quick note on nomenclature
Massive star
formation
High mass star
formation
Young stellar
object
High Mass
Protostellar Object
pro⋅to⋅star
[proh-toh-stahr]
an early stage in the evolution of a star, after the beginning of
the collapse of the gas cloud from which it is formed, but before
sufficient contraction has occurred to permit initiation of
nuclear reactions at its core.
What is different about High Mass Star Formation?
• Happens much faster
• Ionising radiation and HII regions
• Only happens in clusters
• Is the formation mechanism
fundamentally different?
What is different about High Mass Star Formation?
Happens much faster
• No pre-main sequence stage for high mass stars.
(tKH << tff)
• Still accreting while on the main sequence
• Rare (form quickly and low probability from the IMF)
• Very high accretion rates (10-3 or even 10-2 Mʘ /yr)
• Always embedded in natal molecular cloud
What is different about High Mass Star Formation?
Ionising
Radiation and HII Regions
Spectral Mass Luminosity Ionising flux
Type
(Mʘ)
(Lʘ)
(s-1)
O4
60
1.3 106
8.5 1049
O5
50
6.8 105
4.2 1049
O6.5
32
1.5 105
6.6 1048
O7
28
1.0 105
4.2 1048
O7.5
27
8.3 104
3.2 1048
O8
25
6.5 104
2.2 1048
O8.5
23
5.4 104
1.6 1048
O9
22
4.6 104
1.2 1048
O9.5
21
3.8 104
6.9 1047
B0
19
2.5 104
2.3 1047
B0.5
15
1.1 104
1.7 1046
B1
12
5.2 103
1.9 1045
B2
10
2.8 103
4.5 1044
B3
8
1.0 103
4.9 1043
• Stars M > 8Mʘ produce significant5 UV photons
capable of
O5.5
42
4.0 10
2.3 1049
ionising hydrogen
37
2.5 105
1.2 1049
HII regions O6
Panagia 1973
What is different about High Mass Star Formation?
Ionising Radiation and HII Regions
• Stars M > 8Mʘ produce significant UV photons capable of
ionising hydrogen
HII regions
Class of
Region
Size
(pc)
Density
(cm-3)
Emission
Measure
(pc cm-6)
Ionised Mass
(Mʘ)
Hypercompact
0.03
106
1010
~10-3
Ultracompact
0.1
104
107
~10-2
Compact
0.5
5103
107
~1
Classical
~10
~100
~102
~105
Giant
~100
~30
~5105
103-106
Supergiant
>100
~10
~105
106-108
Kurtz 2005
G305.2+0.2
Contours:
4.8GHz radio continuum
(ATCA)
What is different about High Mass Star Formation?
Ionising Radiation and HII Regions
CORNISH images of UCHII region G43.890-0.784, courtesy
Cormac (Mopra Boy) Purcell
What is different about High Mass Star Formation?
Only Happens in Clusters
Isolated low mass star formation
What is different about High Mass Star Formation?
Only Happens in Clusters
What is different about High Mass Star Formation?
Only Happens in Clusters
What is different about High Mass Star Formation?
Is the formation mechanism fundamentally different?
Problems with scaling low mass star formation up:
1. Radiation pressure is too great (spherically symmetric)
high mass star on the main sequence while still trying to accrete matter
Incredibly high accretion rates required
2. No clear cases of isolated high mass star formation
3. Fragmentation results in prestellar cores that are too small
What is different about High Mass Star Formation?
Competitive Accretion, Coalescence and Monolithic Collapse
Competitive accretion
Introduced to provide both an alternate mechanism to forming high mass stars and to
explain the Initial Mass Function (IMF)
Prestellar cores start off with similar masses.
Their location within the molecular cloud determines the final mass of the star.
High mass stars are formed in the cluster centres where the gas reservoir is denser.
What is different about High Mass Star Formation?
Competitive Accretion, Coalescence and Monolithic Collapse
Competitive accretion
One possible interpretation used ballistic motions of cores with respect to the gas to
give the final stellar mass.
Walsh et al. 2006 compared motions of high density cores (traced by N2H+) with
the low density surrounding gas (traced by 13CO and C18O), but found no clear
signs for relative motion.
What is different about High Mass Star Formation?
Competitive Accretion, Coalescence and Monolithic Collapse
Coalescence
High mass stars form in the centres of clusters
Intermediate mass prestellar cores (or the stars themselves) merge to form high mass
core/star
Will work if stellar densities are ~106 pc-3, but highest stellar densities seen
(eg. Orion) are only ~104 pc-3
Difficult to sustain a disk and an outflow during coalescence
Good evidence for outflows
Not so good evidence for disks
Matthews et al. 2007
What is different about High Mass Star Formation?
Chini et al. 2007 Finding Disks Around High Mass Stars
M17
Too big (40 000 AU)!
The Disk candidate
in
IRAS
18089-1732
VLA
Color: NH3(4,4)
Contours:
continuum
ATCA
Beuther & Walsh 2008
-
New high-excitation ammonia
NH3(4,4)/(5,5) data
Clear east-west velocity gradient.
Non-Keplerian motions.
T > 100K in rotating structure.
Dv(NH3(5,5)) ~ 4.7 km/s
Beuther & Walsh 2008
G305.21+0.21
NH3 (4,4) 0th moment NH3 (4,4) 1st moment
NH3 (5,5) 0th moment NH3 (5,5) 1st moment
G327.3-0.60
NH3 (4,4) 0th moment NH3 (4,4) 1st moment
NH3 (5,5) 0th moment
NH3 (5,5) 1st moment
G328.81+0.63
Continuum 24.142GHz
NH3 (4,4) 0th moment
Continuum 24.536GHz
NH3 (5,5) 0th moment
G351.77-0.54
NH3 (4,4) 0th moment
NH3 (4,4) 1st moment
NH3 (5,5) 0th moment
NH3 (5,5) 1st moment
Summary – disks around high mass stars
• Few good examples available
• Enough evidence to say they almost certainly exist
• Promising results from high-J NH3 observations
but so far no flattened structures.
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
Known maser lines:
Species
Frequency (GHz)
OH
1.612, 1.665, 1.667, 1.720,
6.035
H2O
22.235, 187, 233, 658
NH3
20.719, 21.071, 23.870,
24.533, 25.056
SiO
42.820, 43.122, 85.640,
86.243
CH3OH
(many)
Other maser species: H, CH, H2CO, HCCCN, HCN, SiS?
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
Known maser lines:
Species
Frequency (GHz)
OH
1.612, 1.665, 1.667, 1.720,
6.035
H2O
22.235
NH3
20.719, 21.071, 23.870,
24.533, 25.056
SiO
42.820, 43.122, 85.640,
86.243
CH3OH
6.669, 24.928, 44.069
Other maser species: H, CH, H2CO, HCCCN, HCN, SiS?
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
Water Masers
First discovered by Knowles et al. (1969)
Pumped by collisions with H2 molecules
Requires high densities ~109 cm-3
Usually associated with shocks/outflows
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
Water Masers
AFGL 5142 high mass star forming region
Goddi et al. 2007
Water masers
Methanol masers
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
W3(OH) high mass
star forming region Distance to W3(OH) is 2.04 ± 0.07 kpc
Hachisuka et al. 2006
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
OH (hydroxyl) Masers
First discovered by Wilson and Darrett (1968)
Radiatively Pumped
Found close to high mass YSOs
Forster & Caswell (1989) found OH masers tend to
be close to larger (older) HII regions, than H2O
masers.
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
OH (hydroxyl) Masers
First discovered by Wilson and Darrett (1968)
Radiatively Pumped
Found close to high mass YSOs
Forster & Caswell (1989) found OH masers tend to
be close to larger (older) HII regions, than H2O
masers.
Masers in Star
Forming Regions
Matthews
et al.
H2O, OH, SiO, NH3 and CH3OH
2007
SiO Masers
Vibrationally excited SiO transitions are masers
whilst non-vibrationally excited transitions exhibit
thermal emission.
Thermal SiO is a good outflow tracer in high mass
star forming regions.
SiO masers common in evolved stars, but only
three known in star forming regions:
Orion, SGR B2 and W51.
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
NH3 (ammonia) Masers
Some NH3 inversion transitions exhibit maser
emission: (3,3), (5,5), (6,6), (9,6), (6,3), (5,4),
(7,5), (9,8), (6,5), (10,8), (8,6) and (11,9).
Limited examples show emission closely
associated with high mass YSOs.
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
NH3 (ammonia) Masers
Walsh et al. (2007)
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
CH3OH (methanol) Masers
Two types of CH3OH masers, divided by pumping mechanism
Class I masers are pumped by collisions with H2 molecules,
are usually offset from high mass YSOs and are traditionally
associated with outflows
Class II masers are pumped by infrared photons and are usually
found coincident with high mass YSOs.
Muller et al. (2007)
H2O, OH, SiO, NH3 and CH3OH
Frequency
(GHz)
Class
6.669
II
12.178
II
19.967
II
24.928
I
24.933
I
24.934
I
24.959
I
36.169
I
44.069
I
84.521
I
95.169
I
107.013
II
108.893
II
CH3OH (methanol) Masers
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
CH3OH (methanol) Masers
Class II CH3OH masers traditionally associated with UCHII regions
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
CH3OH (methanol) Masers
Class II CH3OH masers traditionally associated with UCHII regions
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
CH3OH (methanol) Masers
Walsh et al. (1998) observed 364 CH3OH maser sites at high
spatial resolution to determine where, in relation to the UCHII
region, the masers were located.
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
CH3OH (methanol) Masers
Walsh et al. (1998) observed 364 CH3OH maser sites at high
spatial resolution to determine where, in relation to the UCHII
region, the masers were located.
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
CH3OH (methanol) Masers
Walsh et al. (1998) observed 364 CH3OH maser sites at high
spatial resolution to determine where, in relation to the UCHII
region, the masers were located.
75% of CH3OH masers are NOT associated with UCHII regions
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
CH3OH (methanol) Masers
Walsh et al. (1998) observed 364 CH3OH maser sites at high
spatial resolution to determine where, in relation to the UCHII
region, the masers were located.
75% of CH3OH masers are NOT associated with UCHII regions
Walsh et al. (1998) also found ~33% of maser sites in lines or arcs
Norris et al. (1993)
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
CH3OH (methanol) Masers
Walsh et al. (1998) observed 364 CH3OH maser sites at high
spatial resolution to determine where, in relation to the UCHII
region, the masers were located.
75% of CH3OH masers are NOT associated with UCHII regions
Walsh et al. (1998) also found ~33% of maser sites in lines or arcs
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
CH3OH (methanol) Masers
Walsh et al. (1998) observed 364 CH3OH maser sites at high
spatial resolution to determine where, in relation to the UCHII
region, the masers were located.
75% of CH3OH masers are NOT associated with UCHII regions
Walsh et al. (1998) also found ~33% of maser sites in lines or arcs
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
CH3OH (methanol) Masers
Followup observations (Walsh et al. 1997; 2001; 2003) showed
All CH3OH masers coincident with dense cores
Class II CH3OH masers are only found with sites of high
mass star formation – an easily observable and reliable
signpost
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
CH3OH (methanol) Masers
Voronkov et al. (2005)
6.7GHz Class II CH3OH masers in Orion
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
CH3OH (methanol) Masers
Voronkov et al. (2005)
Masers in Star Forming Regions
H2O, OH, SiO, NH3 and CH3OH
Maser Variability
H2O masers are highly variable (~weeks)
OH and CH3OH masers slowly variable (~years)
H2O maser monitoring in high mass star forming regions
H2O masers are highly variable (~weeks)
OH and CH3OH masers slowly variable (~years)
Felli et al. (2007)
HOPS daily monitoring of Orion
Class II CH3OH maser monitoring
Periodic flaring of the 6.7GHz CH3OH maser in G9.62
van der Walt et al. (2009)
Summary
Lecture 3 – High Mass Star Formation
and Masers
1.
2.
3.
4.
Is high mass star formation different?
Competitive Accretion vs Coalescence vs Monolithic Collapse
Disks around high mass stars
Masers (H2O, OH, SiO, NH3 and CH3OH)