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Anthony J. Remijan NRAO-CV GBT K-Band Focal Plane Array Science and Data Pipeline Workshop Green Bank, WV Nov 26-28, 2007 Chemistry with the K-band Focal Plane Array Presentation Outline (2)Sources of large interstellar molecules Chemistry of molecules discovered with the GBT

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Presentation Transcript
slide1
Anthony J. Remijan

NRAO-CV

GBT K-Band Focal Plane Array Science and Data Pipeline Workshop

Green Bank, WV

Nov 26-28, 2007

Chemistry with the K-bandFocal Plane Array

GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WV

presentation outline
GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WVPresentation Outline
  • (2)Sources of large interstellar molecules
  • Chemistry of molecules discovered with the GBT
  • VLA/BIMA Mapping Projects
  • Interstellar molecule “myths”
  • Why a GBT spectral line mapping array.
sgrb2 and tmc 1
GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WVSgrB2 and TMC-1

Sagittarius B2(N) is the largest repository of complex molecules in the interstellar medium. Of the 140 interstellar molecules detected, more than half have been detected first in the Sagittarius B2 star-forming region.

TMC-1 is a prototypical dark cloud that contains a number of different carbon chain sequences and thus, TMC-1 is the optimal source for investigating the formation of carbon chain species.

chemistry of molecules discovered with the gbt
GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WVChemistry of molecules discovered with the GBT

The molecules discovered with the GBT over the last 2 years, have opened new questions regarding their formation.

From the previous slides, you see that the GBT is “sensitive” to detecting “large” (>5 atoms) molecules.

By understanding the distribution and excitation of a molecular species, we obtain a better understanding of its formation in astronomical environments and the physical and chemical conditions of the surrounding environment.

Ultimately, this is essential in testing and confirming formation models especially of large organic molecules as are found in Sgr.

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GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WV

Species Detected Towards Sgr B2N with the GBT

HCONH2 + CH2 ==> CH3CONH2

species detected towards tmc 1 with the gbt
GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WVSpecies Detected Towards TMC-1 with the GBT

CH2CCH2 + CN ==> CH2CCHCN + H

and/or

CH3CCH + CN ==> CH2CCHCN + H

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GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WV

hcooh with the bima array
HCOOH with the BIMA Array
  • Formic acid is the simplest carboxylic acid. It is structurally similar to larger acids
  • In nature, it is found in the stings and bites of many, including bees and ants.
  • In 1975, it was the first organic acid detected in space.
  • A large survey of formic acid sources was conducted by Liu et al. in 2001 (Sgr B2N map shown on right).

GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WV

hcooh with the bima array11
HCOOH with the BIMA Array
  • Follow-up observations of HCOOH with higher resolution and better sensitivity toward Orion showed it may be tracing shocked regions of molecular gas (Liu et al. 2002):

GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WV

recent array observations hcooh toward orion with the vla
GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WVRecent Array Observations – HCOOH toward Orion with the VLA
  • Furthermore, observations of HCOOH and HCOOCH3 at 43 GHz have shown much the same morphology as is seen at 1 mm.
  • Hollis et al. 2003 showed the distribution of HCOOH (greyscale) at 7 cm was very similar to the distribution to the HCOOH distribution at 1 mm
  • Thus more supporting evidence of HCOOH tracing a shock region.
extended distribution of ch 3 cho toward sgr b2n
Extended distribution of CH3CHOtoward Sgr B2N
  • Integrated CH3CHO emission towards Sgr B2 (contours) overlaid on the 1065 MHz continuum emission (greyscale).
  • The spatial resolution is 34'‘x19''. The greyscale ranges from 0 to 3.0 Jy/Beam. The contours are at 90, 150, 250, 350, 415, 710, 1000 and 1275 Jy/Beam m/s.
  • Chengalur & Kanekar 2003, A&A, 403, L43
  • They were motivated to search for large scale emission of aldehydes due to detection of CH2OHCHO.

GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WV

extended distribution of ch 3 cho toward sgr b2n15
Extended distribution of CH3CHOtoward Sgr B2N
  • Integrated CH3CHO emission towards Sgr B2. The spatial resolution is 6.6‘’x3.8''.
  • The contour levels are at 30, 48.8, 79.4, 129.1 and 210 Jy/Beam m/s. The position of Sgr B2 (LMH) is marked with a cross in the figure.

GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WV

gbt sgrb2 n spectra
GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WVGBT SgrB2(N) Spectra

The VLA detects this

15NH3 transition at 21.7 GHz with a scale size of ~5”

The VLA cannot detect

either of these CH2OHCHO transitions at 15.2 GHz and 22.1 GHz. The scale size

must be >19”

vla sgrb2 n spectra
VLA SgrB2(N) Spectra

With no compact source of emission or absorption, that couples to the beam of the array, there is no way to map the distribution of the large, extended organic molecules in interstellar environments.

GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WV

interstellar molecule myths
GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WVInterstellar Molecule Myths
  • The most interesting interstellar chemistry occurs in compact hot cores.
  • Large complex molecules are rendered into the gas phase primarily by grain mantle evaporation as the hot core heats up.
  • Large saturated molecules are confined to compact hot cores.
  • Isomers of complex molecules are expected to be cospatial and have the same spatial morphology.
  • Large interstellar molecules are best studied at frequencies in the millimeter range or even at higher frequencies.
  • The really interesting complex interstellar molecules will be found and studied with high spatial resolution interferometric arrays able to couple well to compact molecular sources.
the distribution of ch 3 cn vs ch 3 nc
The distribution of CH3CN vs. CH3NC

GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WV

why a gbt spectral line mapping array
GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WVWhy a GBT Spectral Line Mapping Array
  • Weak Line Intensities of complex molecules found with the GBT take hours of integration to gain acceptable signal to noise in a single pointing. Thus, mapping by multiple pointings is precluded.
  • Low State Temperatures of complex molecules found with the GBT indicate these species are not characteristic of hot cores but reside in the cold extended halo regions of dense and dark clouds.
  • Low-Energy Level Transitions of complex molecules occur in the GBT frequency range (I.e., 300 MHz to 48 GHz). Such transitions sample the coldest regions of interstellar clouds.
  • Information about Cloud Density, Temperature, Kinematics, & Structure based on observations of complex molecules with widespread spatial distribution is practically non-existent. This information is essential to constrain molecule formation mechanisms.
  • No Interferometric Arrays are planned or now operating in the GBT frequency range that can tune to low-energy transitions of complex interstellar molecules and couple to sources with scale sizes on the order of an arcminute.
gbt spectral line mapping array characteristics
GBT Kband FPA Science and Data Pipeline Workshop – Green Bank, WVGBT Spectral Line Mapping Array Characteristics
  • Nominally operate at 22 GHz (1.36 cm) -- a GBT mid-range frequency that gets a number of important low-energy transitions of glycolaldehyde, acetamide, ketenimine, etc.
  • An n by n Array of Feedhorns at 22 GHz would afford a manageable (28 cm x 28 cm) Gregorian focal plane footprint

for n = 5 .

  • Arcminute Mapping Scale Size. For individual beams overlapping by half a beamwidth at 22 GHz, the arrays sample the sky as follows:

3 x 3 array ==> 68” x 68”

4 x 4 array ==> 85” x 85”

5 x 5 array ==> 102” x 102”