Fixing the frequency coverage hole in C-Band

GBT Future Instrumentation Workshop

Fixing the frequency coverage hole in C-Band

Jagadheep D. Pandian

Cornell University


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.


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.


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.


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.


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.


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.


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.


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.



• 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 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.



• 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.


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.


Histogram of flux densities

all detections

new detections


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.


Example of VLBI science

• Source G23.657-0.127 imaged using EVN by Bartkiewicz et al. (2005).



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.

Documents

Fixing the frequency coverage hole in C-Band