Masers Surveys with Mopra: Which is best 7 or 3 mm? Simon Ellingsen, Maxim Voronkov & Shari Breen 3 November 2008

Masers Surveys with Mopra: Which is best 7 or 3 mm?

Simon Ellingsen, Maxim Voronkov & Shari Breen

3 November 2008

Maser Surveys with Mopra: Which is best 7 or 3 mm?

Overview

• What maser transitions lie in the 7 and

3 mm frequency bands?

• What science could be done with large-

scale surveys in these transitions?

• The optimal maser configuration.

• What sensitivity is required?


Masers at millimetre wavelengths

• Most of the strong, well studied maser transitions are at centimetre wavelengths - why?

• At higher frequencies the number of maser transitions increases, but they are typically less intense and less common than those at centimetre wavelengths.

• In both the 7 and 3 mm bands the dominant maser transitions are SiO from late-type stars and class I methanol masers.

• The class I methanol masers trace outflows and are quite distinct from the better studied 6.7 GHz (class II) masers.


Masers in the 7 mm bandFrequency

(GHz)Molecule/Type Comments

36.17Methanol (class I)

Common, strong

37.70, 38.29, 38.45

Methanol (class II)

Rare, weak

42.37 - 43.12SiO (late-type

stars)Common,

strong

44.07Methanol (class I)

Common, strong


Masers in the 3 mm bandFrequency

(GHz)Molecule/Type Comments

84.52, 95.17Methanol (class I)

Common, strong

86.14 - 86.85SiO (late-type

stars)Common,

strong

89.07HCN (late-type

stars)Weak, rare

86.2, 86.6, 86.9, 107.01,

108.0

Methanol (class II)

Weak, rare Intermediate


Class I methanol masers - poor cousins

• Methanol maser transitions are empirically divided into two classes on the basis of where they arise.

• Class II methanol masers are associated with high-mass star formation regions. They are typically found close to IR sources, OH and water masers (e.g. 6.7, 12.2 GHz).

• Class I methanol masers are associated with interactions between outflows and molecular gas, they are offset from IR sources and other types of masers (e.g. 36, 44, 95 GHz).

• Theoretically the empirical difference arises from different pumping schemes - collisional for class I, radiative for class II.



Class I methanol masers - poor cousins

• Methanol maser transitions are empirically divided into two classes on the basis of where they arise.

• Class II methanol masers are associated with high-mass star formation regions. They are typically found close to IR sources, OH and water masers (e.g. 6.7, 12.2 GHz).

• Class I methanol masers are associated with interactions between outflows and molecular gas, they are offset from IR sources and other types of masers (e.g. 36, 44, 95 GHz).

• Theoretically the empirical difference arises from different pumping schemes - collisional for class I, radiative for class II.


Science with Class I masers

• The association of class I methanol masers with outflows has led to the suggestion that they may trace a very early stage of high-mass star formation.

• There are no substantial, sensitive unbiased searches for class I masers. The best targeting strategy is to search towards known class II methanol masers (Ellingsen 2005).

• It appears that some class I masers are associated with lower-mass star formation (Kalenskii et al. 2006).

• They are potentially powerful probes of the physical conditions at multiple locations in the star forming region.


Class I in IRAS16547-4247

Red contours = 25 GHz continuum

Yellow contours = 95 GHz class I

Colour image = Shock-excited H2

(Voronkov et al. 2006)


SiO - Probing Galactic dynamics

• Searches for OH/IR stars at 1612 MHz have previously been used to study Galactic structure (Sevenster, Chapman, Habing et al.)

• More than 2000 SiO masers are now known, primarily in the Northern hemisphere and from targeted searches (Deguchi et al. 2004)

• In addition to Galactic structure SiO masers can be used to investigate stellar evolution, stellar mergers, open star and globular clusters.


Galactic Distribution of SiO Masers(Deguchi et al. 2007)


SiO - Probing Galactic dynamics

• Searches for OH/IR stars at 1612 MHz have previously been used to study Galactic structure (Sevenster, Chapman, Habing et al.)

• More than 2000 SiO masers are now known, primarily in the Northern hemisphere and from targeted searches (Deguchi et al. 2004)

• In addition to Galactic structure SiO masers can be used to investigate stellar evolution, stellar mergers, open star and globular clusters.


95 GHz Class I Methanol in the DQS

• The DQS CS observations included the 95 GHz class I methanol maser transition in the band.

• The DQS observations (zoom mode) had TRMS ~ 0.2-0.3 K at the maser frequency.

• There are two previously known masers in the region covered, one of which was detected.

• This suggests that there would be around 3 detections per sq. degree at this sensitivity.


95 GHz class I methanol masers in the DQS

(cube courtesy of Nadia Lo)

Known source Possible New Detection


95 GHz Class I Methanol in the DQS

• The DQS CS observations included the 95 GHz class I methanol maser transition in the band.

• The DQS observations (zoom mode) had TRMS ~ 0.2-0.3 K at the maser frequency.

• There are two previously known masers in the region covered, one of which was detected.

• This suggests that there would be around 3 detections per sq. degree at this sensitivity.


The Best Option for Masers• The best maser science would be achieved from a sensitive

search for class I methanol and SiO masers.• The best wavelength range in which to do this is the 7mm

band (both system performance and maser intensity are better there).

• With 42-50 GHz frequency setup you get multiple SiO transitions, class I methanol, thermal methanol at 48 GHz, CS(1-0) + isotopes, HCCCN, CCS.

• Zoom mode gives approximately 900 km/s velocity coverage at 0.21 km/s resolution for up to 16 sub-bands.

• Using the ‘strawman’ 7mm survey specifications and CMZ results as a guide we would expect TRMS = 0.12 K (1 pass)


Does the Strawman stand up?• Deguchi et al. (2004) surveyed 400 colour-selected IRAS

sources and detected 254 SiO masers in the 43 GHz lines (RMS ~0.07 K).

• A 1-pass Mopra survey would detect 37% of the Nobeyama sources and a 2-pass survey would detect 51%.

• Targeted 44 GHz class I methanol maser (Slysh et al. 1994 & Kurtz et al. 2004) suggest a similar scenario for those.

• Assuming the SiO and class I masers show similar flux density distributions, I estimate approximately 20 class I maser detections and a similar number of SiO per sq deg.

• A 2-pass strawman survey is 3.1 years of telescope time to cover 90 sq degrees (HOPS)



Does the Strawman stand up?• Deguchi et al. (2004) surveyed 400 colour-selected IRAS

sources and detected 254 SiO masers in the 43 GHz lines (RMS ~0.07 K).

• A 1-pass Mopra survey would detect 37% of the Nobeyama sources and a 2-pass survey would detect 51%.

• Targeted 44 GHz class I methanol maser (Slysh et al. 1994 & Kurtz et al. 2004) suggest a similar scenario for those.

• Assuming the SiO and class I masers show similar flux density distrubutions, I estimate approximately 20 class I maser detections and a similar number of SiO per sq deg.

• A 2-pass strawman survey is 3.1 years of telescope time to cover 90 sq degrees (HOPS)


Thermal + Masers = Complementary

• Masers are good at signposting interesting regions (e.g. young star formation regions, outflows etc) and can be used to measure kinematics.

• The masers trace “special” conditions, but generally it is easier and more reliable to use thermal lines to estimate important physical parameters (temperature, density etc).

• Thermal observations at the maser locations are critical to better understanding the star formation and/or outflow physics (as well as how the masers are produced).

• Thermal methanol can measure the temperature, CS traces the dense gas, thermal SiO the outflows etc.


Kalenskii et al. (2007):

Rotation diagrams from 96.7 GHz series of methanol (similar to 48 GHz series).

Multiple series gives better constraints.


ATCA + CABB better for masers?

• To determine associations and make the most of any science from maser surveys requires accurate positions for the masers => ATCA observations of the detections.

• The ATCA achieves the same sensitivity as the Mopra Strawman 7mm survey in 10 seconds onsource.

• The ATCA could potentially observe 0.1 sq deg per hour (onsource), but a practical implementation with current limitations probably requires a factor of several slower.

• With CABB it would be possible to simultaneously observe the SiO, Class I methanol and continuum (hypercompact HII region search).


Mopra observations (Maxim Voronkov) of 36 GHz class I methanol masers in the low-mass star forming region BHR71.


Conclusions• The requirements of maser and thermal

surveys are not particularly compatible:– At 7mm the masers are stronger, but the

thermal lines are weaker and less prevalent.– At 3mm the masers are weaker, the system is

worse, achieving worthwhile sensitivity very difficult.

• A smaller-scale maser survey (with the ATCA) of part of the Mopra survey region may yield the best science.

Documents

Masers Surveys with Mopra: Which is best 7 or 3 mm? Simon Ellingsen, Maxim Voronkov & Shari Breen 3 November 2008