Fixing the frequency coverage hole in C-Band

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Fixing the frequency coverage hole in C-Band

• Jagadheep D. Pandian
• Cornell University

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Introduction

• Most telescopes in U.S. currently do not have the
  capability to observe between 6 and 8 GHz of
  C-Band.
• 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 5160 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, Arecibos 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 lt l
  lt 54, b lt 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 _at_ 6.7 GHz

• A blind survey done from GBT at l lt 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
  GBTs coverage is expected to be much less than
  otherwise.