Preethi Pratap

A Study of the Physics and Chemistry of TMC-1

We present a comprehensive study of the physical and chemical conditions along the TMC-1 ridge. Temperatures were estimated from observations of CH3CCH, NH3, and CO. Densities were obtained from a multitransition study of HC3N. The values of the density and temperature allow column densities for 13 molecular species to be estimated from statistical equilibrium calculations, using observations of rarer isotopomers where possible, to minimize opacity effects. The most striking abundance variations relative to HCO+ along the ridge were seen for HC3N, CH3CCH, and SO, while smaller variations were seen in CS, C2H, and HCN. On the other hand, the NH3, HNC, and N2H+ abundances relative to HCO+ were determined to be constant, indicating that the so-called NH3 peak in TMC-1 is probably a peak in the ammonia column density rather than a relative abundance peak. In contrast, the well-studied cyanopolyyne peak is most likely due to an enhancement in the abundance of long-chain carbon species. Comparisons of the derived abundances to the results of time-dependent chemical models show good overall agreement for chemical timescales around 10(5) yr. We find that the observed abundance gradients can be explained either by a small variation in the chemical timescale from 1.2 x 10(5) to 1.8 x 10(5) yr or by a factor of 2 change in the density along the ridge. Alternatively, a variation in the C/O ratio from 0.4 to 0.5 along the ridge produces an abundance gradient similar to that observed.

VLBI observations of the 6.7 GHz methanol masers toward W3(OH)

We have conducted very long baseline interferometric (VLBI) observations of the 6.668 GHz maser transition of interstellar methanol toward the ultracompact H II region W3(OH). We have determined absolute maser positions with an accuracy of 0″.05 and produced maps that show that the methanol masers have a distribution similar to the hydroxyl masers in this source. The intrinsic sizes of individual maser spots are ≃0″.0014 (FWHM), or ≃3 AU, and are not significantly affected by interstellar scattering

CLASS I METHANOL MASERS: SIGNPOSTS OF STAR FORMATION?

Class I methanol masers appear to probe very early stages of star formation. An observational survey of the 44 and 36 GHz methanol lines toward several star-forming regions was conducted using the Haystack Observatory 37 m telescope. Examining the intensities of the 36 GHz Class I maser line as compared to the 44 GHz maser line, it is seen that the 36 GHz line is enhanced toward sources where there is no apparent sign of star formation. Sources where the 36 GHz emission is absent, but the 44 GHz emission is strong, appear to be those where ultracompact H II regions and millimeter continuum sources are present. Existing models for the excitation of Class I methanol masers show strong temperature and density dependences for the presence or lack of certain methanol transitions. The 36 GHz masers appear in regimes where the temperatures are low—below 100 K. The 44 GHz masers are excited in a wider range of gas temperatures (80-200 K), supporting the hypothesis that these transitions are still masing even when the 36 GHz masers are quenched.

Multifield Mosaic of the NGC 7538 Region

We present mosaics of six Very Large Array images made in the NH3 (1, 1) and (2, 2) lines toward NGC 7538. Both the NH3 (1, 1) and (2, 2) emissions show clumpy and filamentary structures. The dense clumps are found near water maser emission or near the youngest member of groups of infrared sources. At the edges of clumps, temperature enhancements indicate external heating possibly by the nearby H II regions. The outflows in this region are conspicuously located in the voids between the clumps. Because there is more mass in the dense clumps than in the outflows, we suggest that the dense medium shapes the outflows and channels the swept-up material to the low-density regions.

Are the NGC 7538 formaldehyde masers really unusual

The molecular cloud around NGC 7358 IRS l has been studied in order to understand the excitation conditions for the unusual H 2 CO masers. Maps in the J=1→0 transition of 13 CO indicate densities with a lower bound of ∼2 × 10 3 cm -3 or more in the cloud around the H II region. The results from the study of various molecular transitions indicate that the conditions required by the Boland & de Jong (1981, A&A, 98) model for the excitation of the H 2 CO maser are not very stringent or unique. The maser can be excited by a compact H II region which is optically thin at millimeter wavelengths

Interference temperature measurements from 70 to 1500 MHz in suburban and rural environments of the Northeast

We report sensitive spectrum measurements from 70 to 1500 MHz of the radio interference (RFI) environment at locations in Westford, Massachusetts and Hancock, New Hampshire. These measurements show that below 400 MHz the interference in bands allocated to passive services is often limited by unintentional signals from power lines and part 15 devices

VLBI observations of interstellar ammonia masers

The first VLBI measurement of interstellar ammonia masers is reported. The positions, as well as size and brightness temperature limits, for the masers in the NH 3 (9, 6) inversion transition that are associated with the ultracompact H II regions W51 e1 and e2 are obtained. The masers arise from two positions, each of which is located close to but not coincident with a radio continuum peak. The NH 3 masers are closer to the H 2 O masers than to the OH masers or the ultracompact H II regions. However, at least one of the two NH 3 maser spots mapped seems offset from the region of H 2 O maser activity.

Masers and molecules near the unusual formaldehyde maser NGC 7538 IRS 1

Interferometric observations of the emission from three hyperfine lines of the J = 1-0 transition of HCN have been analyzed in order to characterize the physical conditions in the molecular cloud associated with NGC 7538 IRS 1 and its 6-cm H2CO maser. It is suggested that the lack of HCN emission around IRS 1 and IRS 2 could be the result of the expansion of the H-II region. The presence of high-density clumps around IRS 1 can account for the different excitation requirements for the H2CO and OH masers. 34 refs.

High-resolution molecular observations of NGC 7538 IRS 1

This paper presents results of high-resolution (about 3 arcsec) observations of HCN and HCO(+) toward NGC 7538 IRS 1, carried out with the Berkeley-Illinois-Maryland Array and the NRAO 12-m telescope. The J = 1-0 transition of HCO(+) shows an elongated structure extending from the northeast to the southwest. The wings of the HCO(+) line show the blue lobe of an outflow which is in the same direction as the CO outflow in the source. A detailed comparison between individual HCN and HCO(+) emission peaks shows a definite anticorrelation, which is an indication of different mechanisms which excite the two molecules. 22 refs.

Radio Astronomy: A Strong Link Between Undergraduate Education and Research

The program at the MIT Haystack Observatory has added a new opportunity for students as part of our overall national effort to encourage research by undergraduates. Radio astronomy complements other branches of astronomy, augments science education broadly, and serves to promote interdisciplinary research and education. In the past 2 years, we have conducted a successful pilot program to develop and test a program that facilitates the linking of undergraduate research and education through radio astronomy. As a result of this effort,• a small radio telescope has been developed and can be used as a hands-on introduction to radio astronomical techniques and instrument calibration. The telescope is presently being commercialized so that it can be purchased as a low-cost kit for assembly.• Remote access to the Haystack 37-m antenna has made it possible for students nationwide to access a research grade facility and we invite faculty everywhere who are interested in this possibility to use our facility. Undergraduate students can participate in the exciting world of research by performing experiments with the telescope.• Radio astronomy materials for faculty, students, and interested amateurs have been developed are posted on our web site (http://www.haystack.mit.edu). The materials will continue to be upgraded and enhanced, and the contributions from participating teachers and students nationwide will be added to the project files. We hope to make this not only a resource for someone using our facility but also for the community in general.We are now bringing the Haystack telescopes and radio astronomy to the broad undergraduate community. Based on our pilot experiences, we envision students everywhere being able to exploit the opportunity to strengthen their education through practical research using radio astronomy.