Transcript 6.7 GHz Methanol Masers and the GBT

Fixing the frequency coverage
hole in C-Band
Jagadheep D. Pandian
Cornell University
GBT Future Instrumentation Workshop
Introduction
• Most telescopes in U.S. currently do not have the
capability to observe between 6 and 8 GHz of CBand.
• This band includes the 6030 and 6035 MHz OH
lines and the 6668 GHz line of methanol.
• The latter is a very strong maser line observed in
Galactic star forming regions.
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Methanol Masers at 6.7 GHz
• The 51–60 A+ transition of methanol is the
strongest of methanol masers.
– The strongest source has a peak flux density of over
5000 Jy.
• This line has not yet been detected in sources
other than that associated with massive star
formation.
– For instance, Minier et al. (2003) carried out a survey
towards 123 low-mass star forming regions in various
stages of evolution, but did not make any detections of
6.7 GHz masers.
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Methanol masers at 6.7 GHz
• This makes 6.7 GHz methanol masers unique
compared to OH and H2O masers which are also
found towards late type stars/low-mass stars.
• What makes the 6.7 GHz line even more
interesting is that it traces one of the earliest
phases of massive star formation.
• A sample of 6.7 GHz methanol masers can thus be
used for studying the poorly understood early
phases of massive star formation.
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Methanol masers at 6.7 GHz
• Since these methanol masers are often not
associated with infrared emission, they are best
discovered using blind surveys.
• To date, there have been only around four blind
surveys carried out to detect 6.7 GHz methanol
masers.
• Most of the other surveys are targeted towards
ultracompact HII (UCHII) regions and OH
masers, although blind surveys show that the peak
in the maser emission is often offset from the
position of the UCHII region itself.
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Research done in U.S.
• Currently, Arecibo is the only major facility in
U.S. which has the capability to observe at 6.7
GHz.
– However, Arecibo’s sky coverage is somewhat limited.
• Most of the research on methanol masers has been
done in Europe and Australia.
– These continents have the capability to do both single
dish work and synthesis imaging (ATCA, MERLIN,
EVN).
• The VLA is currently being fitted with new 4-8
GHz receivers as part of the EVLA project.
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The Arecibo methanol maser survey
• The Arecibo methanol maser Galactic plane
survey (AMGPS) is a blind survey for 6.7 GHz
methanol masers in the Galactic Plane done
between 35° < l < 54°, |b| < 0.4°.
• The survey, which was completed in March 2006,
produced a catalog that is complete at a flux
density of 0.27 Jy (this was achieved using just 0.5
s integration per grid point).
• The survey detected a total of 86 methanol masers,
48 of which are new detections.
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The Arecibo methanol maser survey
• 37/86 sources have possible IRAS counterparts
(within 23" of maser).
– 9 out of 37 satisfy WC89 criteria for ultracompact HII
regions, while 9 fail these criteria.
• 46/86 sources have possible MSX counterparts.
4 are clearly associated with MSX dark clouds.
• 9/86 sources have NVSS counterparts, and only
one source has a counterpart in the catalog of
Becker et al. (1991).
• Clearly, most of the HII regions associated with
methanol masers are too optically thick and
compact to be detectable at 21 cm and at 5 cm.
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Regarding the total no. of methanol
masers
• J. van der Walt (2005) estimates the number of
methanol masers in the Galaxy.
• The methodology is to combine the initial mass
function with the star formation law in the Galaxy
(as a function of position) to create a distribution
of massive stars as a function of galactic longitude.
• Then, assuming that every massive star excites a
methanol maser during its birth, one can determine
the normalized distribution of methanol masers as
a function of galactic longitude.
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Regarding the total no. of methanol
masers
• One can then use detection statistics of a blind
survey to estimate the minimum number of
methanol masers in the Galaxy.
• Using an Australian blind survey, the minimum
number of methanol maser is estimated to be 845.
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Regarding the total no. of methanol
masers
The solid line shows the observed distribution of methanol masers as a function of
longitude, and the dashed line shows the expected distribution based on the star
formation law.
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Regarding the total no. of methanol
masers
• One can then use detection statistics of a blind
survey to estimate the minimum number of
methanol masers in the Galaxy.
• Using an Australian blind survey, the minimum
number of methanol maser is estimated to be 845.
• The statistics of the Arecibo survey increases this
minimum number to 1075.
• Note that a significant number of undetected
masers are in regions not accessible to Arecibo,
but accessible to the GBT.
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Potential impact of GBT @ 6.7 GHz
• A blind survey done from GBT at l < 35° at a
comparable or slightly worse sensitivity than
Arecibo will detect a lot of methanol masers.
• The unblocked aperture of GBT will be of
immense help in detecting weak sources in the
vicinity of very bright ones.
• Catalogs from such surveys will be excellent
follow-up targets for studies in millimeter and
submillimeter wavelengths to better understand
early stages of massive star formation.
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Histogram of flux densities
all detections
new detections
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VLBI with methanol masers
• GBT and Arecibo can be combined with the VLA
to create an HSA type array to carry out high
resolution synthesis imaging on methanol masers.
• This will probe the kinematics of the massive star
forming region.
• The proper motions of maser spots can be
determined
through
multi-epoch
VLBI
observations.
– This is required to determine where the maser action
occurs in relation to the central object.
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Example of VLBI science
• Source G23.657-0.127 imaged using EVN by
Bartkiewicz et al. (2005).
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Fixing the frequency hole in C-Band
• Since NRAO has developed 4-8 GHz C-Band
receivers for the EVLA project, the amount of
time, effort and money required to fix the 6-8 GHz
hole in GBT’s coverage is expected to be much
less than otherwise.
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