Engineering topics

Schloerb, F. P.

Publications and source records attributed to Schloerb, F. P..

At least 19 records

NASA NTRS1998

HCO+ imaging of comet Hale-Bopp (C/1995 O1)

The HCO+ J = 1-0 rotational transition at 89.189 GHz has been mapped in comet Hale-Bopp (C/1995 O1) over a total of 38 individual days spanning the period 1997 March 10-June 20 with the Five College Radio Astronomy Observatory 14 m antenna. HCO+ is detectable over an extended region of the comet, with the peak emission commonly located 50,000-100,000 km in the antisolar direction. Maps made throughout the apparition show significant variability in the structure of the HCO+ coma, sometimes on timescales of several hours. The HCO+ brightness is usually depressed at the nucleus position, and on some occasions, the emission is spread into a ring around the position of the nucleus. Individual spectra within the maps display broad (approximately 4 km s-1) lines redshifted by 1-2 km s-1 or more from the nominal velocity of the nucleus, with the redshift typically increasing in the antisolar direction. The spectra and maps may be generally explained by models in which the ions are accelerated tailward at a rate on the order of 10 cm s-2, provided that HCO+ is destroyed within 50,000-100,000 km of the nucleus.

Non-NASA Center↗

NASA NTRS1998

Chemistry in cometary comae

Significant gas-phase chemistry occurs in the comae of bright comets, as is demonstrated here for the case of Comet Hale-Bopp. The abundance ratio of the two isomers, hydrogen cyanide and hydrogen isocyanide, is shown to vary with heliocentric distance in a way that is consistent with production of HNC by ion-molecule chemistry initiated by the photoionization of water. Likewise, the first maps of emission from HCO+ show an abundance and an extended distribution that are consistent with the same chemical model.

Non-NASA Center↗

NASA NTRS1997

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.

NASA Discipline Exobiology↗

NASA NTRS1997

Interstellar and cometary ices: Molecular emission from comet Hale-Bopp

Observations of rotational translations of neutral molecules, radicals and ions in the comet Hale-Bopp are reported on. Sample spectra and maps of the emission for the J = 1 to 0 transition of HCN, the J = 2 to 1 transition of CS and the J = 1 to 0 transition of HCO+ are presented. While the emission from HCN is typically centered on the position of the nucleus and is symmetric, the emission from HCO+ exhibits multiple peaks, together with evidence for acceleration away from the nucleus.

Irvine, W. M.↗

NASA NTRS1997

A survey of the chemical properties of the M17 and Cepheus A cloud cores

We present the results of a systematic survey of the chemical properties of two giant molecular cloud (GMC) cores in M17 and Cepheus A. In all, we have mapped the emission from 32 molecular transitions of 13 molecules and seven isotopic variants over a 4' x 5' region in each core. Each map includes known sites of massive star formation, as well as the more extended quiescent material. In M17 most molecules have emission peaks away from the H II region/molecular cloud interface, while two species, HC3N and CH3C2H, deviate from this structure with sharp maxima closer to this interface. In Cepheus A the core is influenced by a compact high-velocity molecular outflow and a more extended low-velocity flow. The molecular emission distributions in this source are generally quite similar, with most molecules peaking near the center of the core to the east of the compact H II region HW 2. A few molecules, SO, CH3OH, H13CN, and C18O, have more extended emission. Only two molecules, CO and HCO+, appear to trace the high- and low-velocity outflows; all other species are tracing the quiescent core. We have used the results of previous studies of the density and temperature of the dense gas in the same cloud cores to derive accurate abundances relative to CO for several positions in each core. The principal result is that the chemical composition of all the cores we have surveyed (which include OMC-1 as well as M17 and Cepheus A) show remarkable similarity, both within a given core and among the cores. This suggests that the chemical processes are similar in quiescent GMC core material. In M17 the lack of variation of molecular abundances is remarkable because the radiation field and the gas temperature are known to vary appreciably throughout the surveyed region, suggesting that the bulk of the emission arises from gas that is well shielded from radiation.

Non-NASA Center↗

NASA NTRS1997

Chemical and physical gradients along the OMC-1 ridge

We present a survey of the distribution of 20 chemical and isotopic molecular species along the central ridge of the Orion molecular cloud from 6' north to 6' south of BN-KL observed with the QUARRY focal plane array on the FCRAO 14 m telescope, which provides an angular resolution of approximately 50" in the 3 mm wavelength region. We use standard tools of multivariate analysis for a systematic investigation of the similarities and differences among the maps of integrated intensities of the 32 lines observed. The maps fall in three broad classes: first, those strongly peaked toward BN-KL; second, those having rather flat distributions along the ridge; and third, those with a clear north-south gradient or contrast. We identify six positions or regions where we calculate relative abundances. Line velocities and line widths indicate that the optically thin lines generally trace the same volume of dense gas, except in the molecular bar, where C18O, C34S, H13CO+, CN, C2H, SO, and C3H2 have velocities characteristic of the bar itself, whereas the emission from other detected species is dominated by the background cloud. The strongest abundance variations in our data are the well-known enhancements seen in HCN, CH3OH, HC3N, and SO toward BN-KL and, less strongly, toward the Orion-South outflow 1'.3S. The principal result of this study is that along the extended quiescent ridge the chemical abundances, within factors of 3-4, exhibit an impressive degree of uniformity. The northern part of the ridge has a chemistry closest to that found in quiescent dense clouds. While temperature and density are similar around the northern radical-ion peak near 3'.5N and in the southern core near 4'.2S, some abundances, in particular, those of the ions HCO+ and N2H+, are significantly lower toward 4'.2S. The areas near 4.'2S and the molecular bar itself around (1'.7E, 2'.4S) stand out with peculiar and similar properties probably caused by stronger UV fields penetrating deeper into the clumpy molecular gas. This leads to higher electron abundances and thereby reduced abundances of the ions, as well as a lack of complex molecules.

NASA Discipline Exobiology↗

NASA NTRS1997

HCO+ in the coma of comet Hale-Bopp

Maps of comet C/1995 O1 (Hale-Bopp) in the millimeter-wave emission of the ion HCO+ revealed a local minimum near the nucleus position, with a maximum about 100,000 km in the antisolar direction. These observed features of the HCO+ emission require a low abundance of HCO+ due to enhanced destruction in the inner coma of the comet, within a region of low electron temperature (Te). To set constraints on the formation of HCO+ in the coma, as well as the location and magnitude of the transition to higher Te, the data are compared with the results of ion-molecule chemistry models.

Non-NASA Center↗

NASA NTRS1997

Collisional quenching of OH radio emission from comet Hale-Bopp

Observations of comets in the 18-cm OH transitions offer a means to probe gas production, kinematics, and OH excitation in comets. We present initial results of OH observations of comet Hale-Bopp obtained with the NRAO 43 m antenna located in Greenbank, WV. Maps of the emission provide strong constraints on the amount of quenching of the inversion of the OH ground state A-doublet in the coma. Analysis of the total radio OH flux and maps of its radial brightness distribution indicate a quenched region on the order of approximately 500,000 km during March and April 1997. This large value is generally consistent with previous observations of radio OH quenching in lower production rate comets when the high production rate of comet Hale-Bopp is considered.

NASA Discipline Exobiology↗

NASA NTRS1997

The HNC/HCN ratio in comets

The abundance ratio of the isomers HCN and HNC has been investigated in comet Hale-Bopp (C/1995 O1) through observations of the J = 4-3 rotational transitions of both species for heliocentric distances 0.93 < r < 3 AU, both pre- and post-perihelion. After correcting for the optical depth of the stronger HCN line, we find that the column density ratio of HNC/HCN in our telescope beam increases significantly as the comet approaches the Sun. We compare this behavior to that predicted from an ion-molecule chemical model and conclude that the HNC is produced in significant measure by chemical processes in the coma; i.e., for comet Hale-Bopp, HNC is not a parent molecule sublimating from the nucleus.

NASA Discipline Exobiology↗

NASA NTRS1996

Spectroscopic evidence for interstellar ices in comet Hyakutake

Volatile compounds in comets are the most pristine materials surviving from the time of formation of the Solar System, and thus potentially provide information about conditions that prevailed in the primitive solar nebula. Moreover, comets may have supplied a substantial fraction of the volatiles on the terrestrial planets, perhaps including organic compounds that played a role in the origin of life on Earth. Here we report the detection of hydrogen isocyanide (HNC) in comet Hyakutake. The abundance of HNC relative to hydrogen cyanide (HCN) is very similar to that observed in quiescent interstellar molecular clouds, and quite different from the equilibrium ratio expected in the outermost solar nebula, where comets are thought to form. Such a departure from equilibrium has long been considered a hallmark of gas-phase chemical processing in the interstellar medium, suggesting that interstellar gases have been incorporated into the comet's nucleus, perhaps as ices frozen onto interstellar grains. If this interpretation is correct, our results should provide constraints on the temperature of the solar nebula, and the subsequent chemical processes that occurred in the region where comets formed.

Non-NASA Center↗

NASA NTRS1996

(abstract) Microwave Instrument for Rosetta Orbiter (MIRO)

MIRO is a scientific instrument designed for the orbiter of the Rosetta International Mission. It will address the nature of the cometary nucleus, outgassing, and the development of the coma as strongly interrelated aspects of cometary physics. Detailed parameters of the MIRO instrument and the scientific objectives to be met will be discussed. Simulated observations with the MIRO instrument will be shown.

comet microwaves MIRO Microwave Instrument for Ros↗

NASA NTRS1996

Spectroscopic Evidence for Interstellar Ice in Comet Hyakutake

Volatile compounds in comets are the most pristine materials surviving from the time of formation of the Solar System, and thus potentially provide information about conditions that prevailed in the primitive solar material. Moreover, comets may have supplied a substantial fraction of the volatiles on the terrestrial planets, perhaps including organic compounds that played a role in the origin of life on Earth. Here we report the detection of hydrogen isocyanide (HNC) in comet Hyakutake. The abundance of HNC relative to hydrogen cyanide (HCN) is very similar to that observed in quiescent interstellar molecular clouds, and quite different from the equilibrium ratio expected in the outermost solar nebula, where comets are thought to form. Such a departure from equilibrium has long been considered a hallmark of gas-phase chemical processing in the interstellar medium, suggesting that interstellar gases have been incorporated into the comet's nucleus, perhaps as ices frozen onto interstellar grains. If this interpretation is correct, our results should provide constraints on the temperature of the solar nebula, and the subsequent chemical processes that occurred in the region where comets formed.

Irvine, W. M.↗

NASA NTRS1995

The Local Variability of Interstellar Abundances: Observations of C 17O and C 18O in Three Dark Clouds

The abundance ratios of carbon monoxide with isotopes 17 and 18 were mapped in three nearby molecular clouds (B335, L134, and rho Oph), two of which are active star-forming regions. It is suggested that there may be a spatial variability of the two oxygen isotopes resulting from local enrichment and that this may have major implications for galactic nuclear evolution.

dark clouds isotopic abundances interstellar abund↗

NASA NTRS1992

A source model for the L134N molecular cloud

The dark molecular cloud L134N is observed at millimeter wavelengths in the CS (J = 3-2), OCS (J = 7-6), and HDO (1 sub 11-1 sub 10) transitions. The CS (J = 3-2) transition was observed at four positions within the cloud, while the other two transitions were observed at one position each. Fractional abundances in the LTE approximation are calculated for each emission line detected. L134N appears to have a high-density core characterized by NH3, C3H2, and H(C-13)O(+) emission maps. A lower density envelope characterized by C(0-18), CS (J = 2-1), and SO emission surrounds the core. There appears to be a gas-phase oxygen abundance gradient in L134N with atomic oxygen depleted in the high-density core. Observed molecular distributions within L134N can be explained by a model in which chemical and physical processes in icy-dust-grain mantles influence the gas-phase molecular abundances.

Swade, Daryl A.↗

NASA NTRS1990

The FCRAO extragalactic CO survey: Global properties of galaxies

Since stars form in molecular clouds, a critical element in studies of galaxy evolution is knowledge of the molecular content of a large sample of galaxies. To this end, researchers undertook a survey of CO emission from galaxies using the Fire College Radio Astronomy Observatory (FCRAO) 14-m millimeter telescope at 115 GHz. The aim was to better understand the differences found among and within galaxies with regard to the efficiency of star and cloud formation. The galaxies observed as part of the FCRAO Extragalactic CO Survey were selected on the basis of their optical or infrared properties. The galaxies observed thus far are (1) brighter than B sub T (sub o)=12.5 in the blue, or (2) brighter than 20 Jy at 100 microns. From major axis CO observations at 45 seconds resolution and spacing in over 200 galaxies, researchers determined the CO radial distributions, and derived global CO fluxes (cf. Kenney and Young 1988); H2 masses were derived using the conversion factor N(H2)/I sub CO=2.8 times 10 to the 20th power cm(-2)/K km s(-1)) (Bloemen et al. 1986). Here, researchers concentrate on the global galaxy properties within the sample. Neutral hydrogen (HI) masses for the sample galaxies were taken from Huchtmeier et al. (1983), blue luminosities and morphological types were taken from RC2. IR luminosities, colors, dust temperatures and dust masses were determined from coadded Infrared Astronomy Satellite (IRAS) data (Young et al. 1989). They have chosen to first compare absolute luminosities and masses in order to determine the slope and scatter in each correlation; next they investigate luminosity independent ratios in order to intercompare large and small galaxies.

Young, J. S.↗

NASA NTRS1987

Observations of HCN in comet P/Halley

This paper presents observations of the HCN J =1-0 rotational transition at 3.4 mm wavelength in Comet P/Halley, obtained during 56 individual observing sessions between November 1985 and May 1986 carried out at the Five College Radio Astronomy Observatory. A comparison of the HCN production with the total gas production of the comet indicates that HCN is a relatively minor constituent with 0.1 percent the abundance of H2O. A comparison of HCN and CN production suggests that HCN is a major parent molecule of CN, but probably not the sole parent. Evidence is presented of day-to-day time variability in the total HCN emission and in the hyperfine ratios. The time variation of the total emission is consistent with the known time variable behavior of the comet.

Schloerb, F. P.↗

NASA NTRS1986

CO mapping of the Orion molecular cloud: The influence of star formation on cloud structure

Regions of massive star formation have long been believed to have a profound influence on the structure of their surrounding molecular clouds. The ways in which massive star formation has altered the structure and kinematics of the Orion Molecular Cloud are discussed. The data to be discussed consists of a large scale map of the CO J=1-0 emission from approximately 3 square degrees of OMC-1. During 1985, the Five College Radio Astronomy Observatory 14M antenna was used to map a 2 deg x 1 deg region centered on alpha(1950) = 5(h)33(m)00(s) delta(1950) = -5 deg 30 min. The region mapped in 1985 covers the well known HII regions M42, M43, and NGC1977, and the CO map contains abundant evidence of the interaction between these regions and the molecular cloud. Indeed, the global structure of the cloud appears to have been strongly influenced by the continuous formation of massive stars within the cloud. Individual instances of some of these features are discussed. There appear to be two classes of features which are indicative of this interaction: CO bright rims and CO holes. During 1986, we have undertaken further mapping of OMC-1 to the south of the region covered by the 1985 map. This portion of the cloud contains significant regions of star formation, but O star formation has not occured and large HII regions have not developed to alter the appearance of the cloud. A detailed map of this region is thus an opportunity to view the structure of the molecular cloud before it has been altered by massive star formation. Preliminary analysis of data obtained in this region suggests that the structure and kinematics of the southern portion of the Orion cloud are indeed dramatically different from those of the region previously mapped. Comparison of the two regions thus supports models of the development of structure in molecular clouds through interaction with the HII regions formed within them.

Schloerb, F. P.↗

NASA NTRS1986

Radiative transfer in planetary atmospheres

The research effort was concentrated toward comet P/Halley, although observations of other comets including Giacobini-Zinner, Hartley-Good and Thiele were made as opportunities arise. In preparing for the apparition of P/Halley, detailed predictions for the emission in the 18 cm OH transitions were made in order to assist observers and to indicate what type of data would be particularly valuable. Likewise, since occultations by the comet of background radio sources can provide unique types of information, the tracks of P/Halley and P/Giacobini-Zinner were surveyed with the NRAO Very Large Array, background sources were mapped, and a catalogue of occulted sources published. Beginning in September, 1985, regular observations of the 18 cm OH lines have been made for P/Halley using the 43 m antenna of the NRAO in Greenbank, West Virginia. These data promise to provide the most accurate time sequence for the OH emission for any comet ever obtained at radio wavelengths. From the linewidth and shape, information is obtained on the kinetics of the gas in the cometary coma. An extensive series of observations have been undertaken with 14 m antenna to study emission from the HCN molecule in P/Halley. These data provide the first definitive detection of this component of the cometary ices. Interesting variations in the production of HCN were observed with time scales less than a day. Likewise, there appeared to be variations in the ratio of the hyperfine components of the J = 1-0 transition.

Irvine, W. M.↗