This is the wiki page for the 2004 CV summer student project Time slots: July 10, 21:15 to 24:00 EDT July 11, 21:15 to 24:00 EDT Of course, as the sun travels through the sky the stars visible at a given local time differ. The star time or local sidereal time corresponding to these standard times is about 1420 through 1705 or so. This is prime time for the best part of the sky--the Galactic Center and the Ophiuchus star-forming region.

The project: Formaldehyde, H2CO, is a ubiquitous molecule in space as on Earth. Its structure provides it with a moderate dipole moment which in turn makes it a useful probe of moderately dense regions. It also possesses lines at fairly low frequency, accessible with the GBT. Lines at 6cm and 2cm wavelength may be compared and used to estimate density, for example. However, owing again to the structure of the molecule and the specifics of interstellar conditions, the line is normally detected in absorption against the cosmic background radiation. In all of the sky, there are fewer than half dozen places where the line is known in emission. Wadiak et al. (1985) found one region of emission in Ophiuchus.

Wadiak et al. (1985) mapped this line in the other region in Ophiuchus where it occurs in emission, the B1 core, and found two rotating elongated structures, which may represent evolutionary phases of a cloud core as it creates and nurtures the young stars. L1689S has formed at least one star, and provides us an opportunity to glimpse the next stage in the process. If a disk is present, or a star of a solar mass or so, the gravitational velocity perturbations in pixels within 10" of the object should be as large as 1 km s$^{-1}$, easily detectable. If rotation is present, a dynamical mass estimate, which includes the central object, can be derived. The gas mass, from the line intensity, may be used to measure the gas in the rotating structure.

Another emission region is in Orion A-BN/KL. This line was mapped in continuum mode in the VLA-D configuration by Barvainis and Wootten (1986), who sought to place limits upon the polarization in the line. Weak emission at 2 cm is known to extend over a wide range in the OMC1 cloud (Bastien et al. 1985, map from Effelsberg antenna). Johnston, Wadiak, Rood and Wilson have mapped the formaldehyde distribution in the region 3' north of the dense core. The emission in the Effelsberg map is quite intense, with main beam brightness temperatures ranging from 0.2 K at the compact southern extreme of the map to 2K at BN/KL.

One was discovered in the mid-80s with the 140 foot telescope by Mangum and Wootten, near a dense portion of the Ophiuchus star-forming region. However, the large beam of that telescope and the weakness of the emission conspired to prevent those estimable authors from figuring out exactly what the relation of the emission region was to any dense material and star formation in the vicinity.

The REU Mission: Using the better sensitivity and smaller beam of the GBT, discover the true location and extent of this rare region of 2cm formaldehyde emission and how it relates to star formation in the region. Here is a summary of the old data:

Data with asterisk is from Sept 1986; all other data from Sept. 1988 (format is, from top to bottom for each map point, Ta, sigma, Vlsr, deltaV)...
L1689N ON is cold core position
16h29m27.7s -24d22'13" 1950.0
                                L1689N
                               2cm H2CO

    4E         3E        2E        1E        0E        1W        2W
--------------------------------------------------------------------------
                                              *< BAW IR3
2N   X          X       0.03        X         X         X        0.04*



1N   X        0.02   *  0.02      0.03        X         X         X
                     ^BAW IR1

              *         0.08                                    -0.14
0N  0.03      ^ X       0.02        X        0.03       X        0.02
           BAW IR2      3.60                                     3.9
                        0.40                                     0.48
                                  0.10(?)
1S   X         0.03       X       0.03        X          X        X
                                  3.80
                                  0.40
                       -0.13                 -0.14*             -0.11
2S   X          X       0.03        X         0.05*       X      0.02
                        3.81                  3.8 *              4.1
                        0.90                  1.6 *              1.01
--------------------------------------------------------------------------

Data with asterisk is from Sept 1986; all other data from Sept. 1988
Lonely asterisks denote IR source positions communicated by BAW 7/28/89.
   BAW S26  16:29:38.5 -24:21:07.  Double source, brighter 12.0m at K, JHK
                                   photometry => not particularly reddened.
   BAW S28  16:29:41.5 -24:22:00.  Second double source, neither particularly
                                   reddened. K=13.0; JHK photometry
   BAW S18  16:29:28.1 -24:20:46.  K=13.3 moderately red, JHKL photometry


  • Question 1: At what frequency does the 2cm line of H2CO lie? Hint--see the list of molecular frequencies at
http://www.cv.nrao.edu/~awootten/allmols.html

The receiver you will use is, of course, the 2cm receiver which operates in the confusingly named: Ku band Ku-Band (12.0 - 15.4 GHz)

This receiver has two beams on the sky with fixed separation, each with dual circular polarization. The feeds are corrugated horns with cooled polarizers producing circular polarizations. Internal switching modes are frequency and/or feedhorn beam switching. The user can select IF Bandwidth of 500 or 3500 MHz. Calibration is by noise injection. Receiver temperature is 14 K for typical system temperatures of 24-30K.

  • Question 2: What frequency resolution should one use on the Spectrometer? Hint--one would want several resolution elements across the line; the velocity width is given above.

  • Question 3: How long should one integrate to obtain a 5 sigma detection of the line at 2'E in the map above?

  • Question 4: For your measurments of the test source (L134N), you should have measured a strong absorption line. In this absorption line you should be able to see the hyperfine components of the H2CO 2(11)-2(12) transition. Mark the location of these hyperfine components on the L134N spectrum. Hint: See answer to the first question.

  • Question 5: What are the relative intensities of the hyperfine components?

  • Question 6: Using your answer to the last question, what is the optical depth of the absorption line in L134N?

  • Question 7: In the L134N measurement, what is this line absorbing? Hint: Remember the term "optical depth" from the lecture?

See the Answers wiki for answers to these questions.


Refs: Barvainis, R., and Wootten, A. 1986 A. J., in press.

Bastien, P., Batrla, W., Henkel, C., Pauls, T., Walmsley, C. M.,and Wilson, T. L. 1985, Astr and Ap. 146, 86.

Johnston, K., Palmer, P., Wilson, T. L., and Bieging, J. 1983, Ap. J.(Letters) 271, L89.

Wadiak, E. J., Wilson, T. L., Rood, R. T. and Johnston, K. J. 1985 Ap. J. (Letters), 295, L43.

-- JeffMangum - 22 Jul 2004
Topic revision: r1 - 2006-03-29, JeffMangum
This site is powered by FoswikiCopyright © by the contributing authors. All material on this collaboration platform is the property of the contributing authors.
Ideas, requests, problems regarding NRAO Public Wiki? Send feedback