1. MALT 90 Millimetre Astronomy Legacy Team 90 GHz survey
James Jackson (Boston University)
Kate Brooks, Jill Rathborne (ATNF), Jonathan Foster (Boston U), Gary Fuller (Manchester), Friedrich Wyrowski (MPIfR), and 38 others….
Great Barriers Townsville 2010

2. High-mass star formation: dense cores
Cold
Dark in the mid- and far-IR
Starless core
Giant Molecular
Cloud
OH Masers
CH3OH + H2O Masers
High-mass
Star
HII Region
Pre-UCHII
+ IR
UCHII
HCHII
+ cm
Hot Molecular Core
mm only
Starless Core
Hot Core
Hot
H II region embedded
Strong, extended mid-IR emission
Strong 8 mm PAH emission
Warm
Compact mid-IR sources
4.5 mm “green fuzzies”
Protostellar core
As I mentioned, Malt 90 will focus primarily on the early phases of high-mass star formation. While the exact mechanisms that give rise to a high-mass star are unknown, all theories of high-mass star formation begin with a cold, dense molecular clump inside a giant molecular cloud. From an observational point of view, the early stages of high-mass star formation are often referred to as the starless core phase, the pre-UCHII region phase and the Hot molecular core phase. Each one of these describes the massive dense core as it evolves from a cold, dense core to when the central star has begun to turn on to the phase when the UV radiation from the central star is effecting the surroundings.
Slide courtesy of Kate Brooks

3. The stages of high-mass star formation
Pre-stellar
Cold
IR-dark
Proto-stellar
Warm
Compact mid-IR emission (and sometimes “green fuzzy”)
H II region (stellar)
Hot
Extended mid-IR emission, PAH emission.
To get a better understanding of high-mass star formation we need to first identify a large sample of cores in the very earliest stages. These cores ought to be massive and dense. Those that are cold with no obvious signs of star formation will be the starless, pre-stellar cores. Only a handful of these cores have been identified to date. Cores that are associated with infrared emission (which could arise from heated dust of be tracing outflows) are likely in the pre-UCHII region or protostellar phase. Those cores that have very rich molecular spectra are likely in the later phases, after the central star has turned on and is heating the gas.
Fortunately, recent galactic plane surveys such as GLIMPSE and MIPSGAL have revealed clouds that contain massive dense cores that have exactly these characteristics. These clouds are referred to as Infrared Dark Clouds.

4. Cores in the Nessie Nebula
1.2 degrees
0.2 degrees
One remarkable aspect of these clouds is that they tend to be very filamentary. An extreme example of this is shown here. This IRDC has an extremely large aspect ratio, extending more than a degree along the Galactic plane.
What is particularly interesting about this IRDC, is that, when we mapped it in molecular line emission, we discovered that the velocity of the molecular line emission is uniform across the whole filament, which implies that it is a single molecular cloud.
What is also interesting are the number of cores that are located along this filament.
Image credit: NASA/JPL-Caltech/Univ. of Wisconsin
Blue - 3.6m, Green - 8m, Red - 24m

5. Cores in the Nessie Nebula
One remarkable aspect of these clouds is that they tend to be very filamentary. An extreme example of this is shown here. This IRDC has an extremely large aspect ratio, extending more than a degree along the Galactic plane.
What is particularly interesting about this IRDC, is that, when we mapped it in molecular line emission, we discovered that the velocity of the molecular line emission is uniform across the whole filament, which implies that it is a single molecular cloud.
What is also interesting are the number of cores that are located along this filament.
Cold core
Image credit: NASA/JPL-Caltech/Univ. of Wisconsin
Blue - 3.6m, Green - 8m, Red - 24m

6. Cores in the Nessie Nebula
One remarkable aspect of these clouds is that they tend to be very filamentary. An extreme example of this is shown here. This IRDC has an extremely large aspect ratio, extending more than a degree along the Galactic plane.
What is particularly interesting about this IRDC, is that, when we mapped it in molecular line emission, we discovered that the velocity of the molecular line emission is uniform across the whole filament, which implies that it is a single molecular cloud.
What is also interesting are the number of cores that are located along this filament.
Protostellar core
Image credit: NASA/JPL-Caltech/Univ. of Wisconsin
Blue - 3.6m, Green - 8m, Red - 24m

7. Cores in the Nessie Nebula
One remarkable aspect of these clouds is that they tend to be very filamentary. An extreme example of this is shown here. This IRDC has an extremely large aspect ratio, extending more than a degree along the Galactic plane.
What is particularly interesting about this IRDC, is that, when we mapped it in molecular line emission, we discovered that the velocity of the molecular line emission is uniform across the whole filament, which implies that it is a single molecular cloud.
What is also interesting are the number of cores that are located along this filament.
Stellar “H II region” core
Image credit: NASA/JPL-Caltech/Univ. of Wisconsin
Blue - 3.6m, Green - 8m, Red - 24m

8. MALT 90: The Millimetre Astronomy Legacy Team 90 GHz Survey
MALT 90 will image 3,000 high-mass cores with Mopra in key 90 GHz molecular lines, e.g. N2H+, HCO+, HCN, HNC…
The survey will be complete: all high-mass star forming cores (M > 200 M) to 10 kpc.
It will provide key information for Herschel and ATLASGAL studies
MALT 90 sources will be key targets for ALMA
ALMA will be able to image any core detected in MALT 90 at 1” angular resolution with excellent signal-to-noise

9. MALT 90 science goal: How do high-mass star-forming cores evolve?
MALT 90 will provide
Kinematic distances
Column densities
Molecular chemical abundances
Virial masses
Core kinematics
A large sample of the elusive youngest cores
The MDCS will be valuable not only in its own right, but also as an important finding chart to identify key ALMA targets. With its unprecedented large sample of dense cores, the MDCS will identify the rare, extremely interesting objects that demand ALMA’s capabilities. Because the MDCS will provide physical conditions, chemical conditions, and kinematic distances, it will be an essential resource for ALMA scientists who study star-formation. For instance, the MDCS will identify huge numbers of the heretofore elusive high-mass, cold pre-stellar cores. These objects will be key laboratories to test the ideas of monolithic collapse and competitive accretion at high angular resolution.

10. Why 90 GHz ?
Dense core
13CO
C18O CS
Molecular lines at ~90 GHz require high densities for their excitation (n > 105 cm-3)
These lines are therefore sensitive ONLY to dense star-forming cores.
We have selected to conduct this survey at 90 GHz because here the spectrum is rich in diagnostic lines. Because these lines have high dipole moments, they require high densities for their excitation. These lines have the advantage of pinpointing only the dense gas that is associated with star-forming cores and not from the surrounding, diffuse molecular cloud.
Moreover, because the lines near 90 GHz span a large range of excitation energies and critical densities, they indicate distinct physical conditions and chemical evolution.
Also, with the measured VLSR, we can derive a kinematic distance to the cores which will allow us to more reliably determine their masses and the luminosities of any protostars. We can then also answer the question about whether or not adjacent cores are physically related.

11. For more on Nessie see Jackson et al. 2010 ApJL
All of these cores are strong molecular line emitters easily detected by Mopra
1.2 degrees
0.2 degrees
One remarkable aspect of these clouds is that they tend to be very filamentary. An extreme example of this is shown here. This IRDC has an extremely large aspect ratio, extending more than a degree along the Galactic plane.
What is particularly interesting about this IRDC, is that, when we mapped it in molecular line emission, we discovered that the velocity of the molecular line emission is uniform across the whole filament, which implies that it is a single molecular cloud.
What is also interesting are the number of cores that are located along this filament.
For more on Nessie see Jackson et al ApJL
Mopra HNC (1-0) integrated emission
Size ~100 pc x 0.5 pc
Image credit: NASA/JPL-Caltech/Univ. of Wisconsin
Blue - 3.6m, Green - 8m, Red - 24m

12. The MALT 90 strategy
A blind fully-sampled 90 GHz Galactic plane survey is impractical
90 GHz emission is relatively weak
Dense cores have a small solid angle
BUT submm thermal dust emission indicates cores
The ATLASGAL 870 mm survey of the Galactic plane has now identified thousands of cores.
ATLASGAL sources will be imaged with Mopra in molecular lines.
Our aim is to capitalize on the recent progress that has been made in potentially identifying the earliset phases of high-mass star formation. To do this we are conducting a large, molecular line survey of massive, dense cores using the 22m Mopra telescope.
Because emission from dense cores will be fairly compact and the emission relatively weak (compared to CO for example), a blind, fully sampled Galactic plane survey is impractical. Moreover, we don’t need a blind survey, because we already know where to search for the dense cores. ATLASGAL will give us the locations of the cores.

13. Mopra HNC (1-0) integrated emission
Spitzer/MIPS 24 m
ATLASGAL 0.87 mm
When we look toward these cores in the IR at 24um or the sub-millimeter continuum emission from ATLASGAL, we find that the dense gas traced by the HNC emission is associated with continuum emission detected in ATLASGAL and sometimes, but not always, with 24um emission.
Our aim is to identify and characterize a large sample of these types of cores.
Mopra HNC (1-0) integrated emission

14. Core classification : Spitzer GLIMPSE/MIPSGAL
Pre-stellar
Protostellar
H II region
These are examples of cores in each of these groups. The first is dark in GLIMPSE and most likely represents a pre-stellar core. The second contains two cores, both of which are bright at 24um indicating warm dust, while the third example contains a core with very bright and extended 8um emission - indiciating it contains an already formed high-mass star.
The primary goal of MALT 90 is to characterize each of these stages in the star formination process.
The primary goal of the MALT 90 GHz Survey is to characterize star- forming cores and to study their physical and chemical evolution
Blue 3.6 mm Green 8 mm, Red 24 mm Spitzer: GlIMPLSE?MIPSGAL

15. MALT 90 Survey Observing Parameters
16 lines
3’ x 3’ maps
Two orthogonally scanned “on-the-fly” maps for each source
38” angular resolution
0.05 K sensitivity
0.1 km s-1 spectral resolution
To test the fesability of this project, we conducted a set of pilot observations last July. In total, 199 cores were mapped - these cores were chosen to be representative of the three groups we are considering.
Not surprisingly, we found signifiant differences between the morphologies of the various lines, their profiles and also between the different types of cores.
ATNF Mopra 22 m

16. 720 hours scheduled for July-Sept 2010
Survey overview
This is a summary of the MALT 90 survey - in total, we’d like to obtain maps toward about 3000 dense cores, covering the groups defined as pre-stellar, protostellar and HII regions.
We’ll have 38 arcsec angular resolution, with 0.1 k/s spectral resolution. We will image 16 molecular lines simultaneously. Each map will be 3 arcmin by 3 arcmins. The sensitivity will be 0.2 K per channel.
When complete the survey will comprise 3 tera bytes. To map all 3000 sources requires about 240 nights, which we hope to observe over the next three years. We have 72 hours already scheduled for this coming winter season.
720 hours scheduled for July-Sept 2010

17. Selected lines IF Line Frequency (MHz) Tracer 1 N2H+ 93,173.772
Density, chemically robust 2
13CS
92,
Optical depth, Column density, VLSR 3
H41
92,
Ionized gas 4
CH3CN
91,
Hot core 5
HC3N
91, 6
13C34S
90, 7
HNC
90,
Density; cold chemistry 8
HC13CCN
90, 9
HCO+
89,
Density 10
HCN
88, 11
HNCO 413
88, 12
HNCO 404
87, 13
C2H
87,
Photodissociation region 14
SiO
86,
Shock/outflow 15
H13CO+
86, 16
H13CN
86,
Because we can image 16 lines simultaneously, we can obtain a great deal of information about the chemistry and physical conditions within the cores. We have selected these lines as they offer the best combination of optically thin/thick tracers, early time and late time molecules, shocks and outflows, and density probes.
The ground state lines of N2H+, HNC, HCO+ and HCN will be the brightest molecular lines in the band. These lines are used to reveal the dense gas.
In addition to these high-density tracers, we have also selected a number of rarer, optically thin isotopomers. Thses lines will be used to estimate the optical depths and column densities of the cores. Moreover, they will be invaluable to help understand any complex line profiles as they reveal the true systemic velocity of the core.
The third group of lines we have selected require very high densities and temperatures for their excitation. These lines are only found toward very warm, dense ‘hot cores’ which are associated with the protostellar and HII region phases. These will be used as an independent check on the evolutionary state.
The final group of lines require specific conditions for their excitation. The H41 alpha recombination line only arises in HII regions. SiO is only formed in shock, hence, it is a tracer of outflow activity. The C2H line is a distinct tracer of ionized and molecular gas, which will reveal any PDRs.
The combination of these lines will trace both the optically thin and thick gas, the cold and warm gas.

18. Hot vs. Cold Core 90 GHz spectra
HCO+
N2H+
HNC
HCN
H13CO+
C2H
H13CN
HC3N
CH3CN
HNCO
HCO+
N2H+
HNC
HCN
H13CO+
C2H
H13CN
HC3N
CH3CN
HNCO
Hot
Cold

19. Differing chemical morphologies: need to map
N2H+
HCO+
For instances, in many cases the emission from various molecules were coincident. However, toward a number of the cores, the different molecules showed very different morphologies. In some cases the N2H+ is enhanced, while toward others, the HCN, HCO+ is enhanced. The differences in the morphologies of the emission reinforces the need to map rather than obtain a single spectrum.
GLIMPSE 3-colour image

20. Complex line profiles Infall Outflow Self-absorption HCO+ 1-0
Many of the maps also contain valuable spectral information. Toward many of the cores we see assymetric line profiles, indicating outflows, infall or optical depth effects. Because we have a combination of optically thick and thin lines, this will help with the interpretation of the line profiles.
HCO+ 1-0

21. Complex Chemistry: An N2H+ “only” source; associated with starless or protostellar cores
HNC
HCN
HCO+
Blue - 3.6m, Green m, Red – 8 m from GLIMPSE survey

22. Complex Chemistry: An N2H+ “drop out”; typically associated with H II regions
HNC
HCN
HCO+
Blue - 3.6m, Green m, Red – 8 m from GLIMPSE survey

23. A combination of N2H+ “only” and “drop out” morphologies
HNC
HCN
HCO+
Blue - 3.6m, Green m, Red – 8 m from GLIMPSE survey

24. Source Velocities
Source: CfA-Columbia CO survey

25. CO l-v (position-velocity) diagram
Source Velocities
CO l-v (position-velocity) diagram
VLSR
Source: CfA-Columbia CO survey

26. Galactic Distribution of Cores: High-mass stars forming in spiral arms

27. Deep integration on “standard” source

28. Current status Observations began in July 2010 Over 600 sources mapped
Data pipeline in place
Analysis underway
All data will be made public after verification and calibration

29. MALT 90 Team Meeting Tomorrow (Tuesday) from 12:00—13:30
New team members are welcome!

30. Summary
MALT 90 will map ~3000 dense, star-forming cores with the Mopra telescope in 16 different molecular lines near 90 GHz
ATLASGAL cores are the targets
Spitzer GLIMPSE/MIPSGAL images will allow us to classify cores:
Pre-protostellar cores
Protostellar cores
HII regions
MALT 90 will be an enormous, systematic molecular line survey of dense cores and an excellent resource for Herschel and ALMA.