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Keene, Jocelyn

Publications and source records attributed to Keene, Jocelyn.

14 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

A New Look at Stellar Outflows: Spitzer Observations of the HH 46/47 System

We present the Early Release Observations of the HH 46/47 system and HH 46 IRS 1 source, taken with the three instruments aboard the Spitzer Space Telescope. The optically invisible southwest lobe, driven by the HH 47C bow shock, is revealed in full detail by the Infrared Array Camera (IRAC) images and displays a 'loop'-like morphology. Both of the mid-infrared outflow lobes are narrower than those of CO flow. We believe that the combination of emission by H2 rotational lines [S(11)-S(4)] and some atomic lines, which fall within the IRAC passbands, are responsible for the bulk of the observed emission, although contributions from the 3.3, 6.2, and 7.7 micron polycyclic aromatic hydrocarbon emission bands cannot be ruled out. Weak spectral features corresponding to these emitters are present in the Infrared Spectrograph spectrum of the HH 47A bow shock. The spectrum of HH 46 IRS 1 shows remarkable similarities to those of high-mass protostars, which include the presence of H2O, CO2, CH4, and possibly NH3, CH3OH, and ices. The high ice abundances and the lack of signs of thermal processing indicate that these ices in the envelope are well shielded from the powerful outflow and its cavity. Emission from the Bok globule at 24 micron is detected and displays a similar structure to that observed at 8 micron.

Herbig-Haro objects↗
NASA NTRS2001

Atomic Oxygen Abundance in Molecular Clouds: Absorption Toward Sagittarius B2

We have obtained high-resolution (approximately 35 km/s) spectra toward the molecular cloud Sgr B2 at 63 micrometers, the wavelength of the ground-state fine-structure line of atomic oxygen (O(I)), using the ISO-LWS instrument. Four separate velocity components are seen in the deconvolved spectrum, in absorption against the dust continuum emission of Sgr B2. Three of these components, corresponding to foreground clouds, are used to study the O(I) content of the cool molecular gas along the line of sight. In principle, the atomic oxygen that produces a particular velocity component could exist in any, or all, of three physically distinct regions: inside a dense molecular cloud, in the UV illuminated surface layer (PDR) of a cloud, and in an atomic (H(I)) gas halo. For each of the three foreground clouds, we estimate, and subtract from the observed O(I) column density, the oxygen content of the H(I) halo gas, by scaling from a published high-resolution 21 cm spectrum. We find that the remaining O(I) column density is correlated with the observed (13)CO column density. From the slope of this correlation, an average [O(I)]/[(13)CO] ratio of 270 +/- 120 (3-sigma) is derived, which corresponds to [O(I)]/[(13)CO] = 9 for a CO to (13)CO abundance ratio of 30. Assuming a (13)CO abundance of 1x10(exp -6) with respect to H nuclei, we derive an atomic oxygen abundance of 2.7x10(exp -4) in the dense gas phase, corresponding to a 15% oxygen depletion compared to the diffuse ISM in our Galactic neighborhood. The presence of multiple, spectrally resolved velocity components in the Sgr B2 absorption spectrum allows, for the first time, a direct determination of the PDR contribution to the O(I) column density. The PDR regions should contain O(I) but not (13)CO, and would thus be expected to produce an offset in the O(I)-(13)CO correlation. Our data do not show such an offset, suggesting that within our beam O(I) is spatially coexistent with the molecular gas, as traced by (13)CO. This may be a result of the inhomogeneous nature of the clouds.

Lis, D. C.↗
NASA NTRS1998

Statistical Properties of Line Centroid Velocity Increments in the rho Ophiuchi Cloud

We present a comparison of histograms of CO (2-1) line centroid velocity increments in the rho Ophiuchi molecular cloud with those computed for spectra synthesized from a three-dimensional, compressible, but non-starforming and non-gravitating hydrodynamic simulation. Histograms of centroid velocity increments in the rho Ophiuchi cloud show clearly non-Gaussian wings, similar to those found in histograms of velocity increments and derivatives in experimental studies of laboratory and atmospheric flows, as well as numerical simulations of turbulence. The magnitude of these wings increases monotonically with decreasing separation, down to the angular resolution of the data. This behavior is consistent with that found in the phase of the simulation which has most of the properties of incompressible turbulence. The time evolution of the magnitude of the non-Gaussian wings in the histograms of centroid velocity increments in the simulation is consistent with the evolution of the vorticity in the flow. However, we cannot exclude the possibility that the wings are associated with the shock interaction regions. Moreover, in an active starforming region like the rho Ophiuchi cloud, the effects of shocks may be more important than in the simulation. However, being able to identify shock interaction regions in the interstellar medium is also important, since numerical simulations show that vorticity is generated in shock interactions.

Lis, D. C.↗
NASA NTRS1995

Submillimeter spectroscopy of interstellar hydrides

We discuss airborne observations of rotational transitions of various hydride molecules in the interstellar medium, including H2O-18 and HCl. The detection of these transitions is now feasible with a new, sensitive submillimeter receiver which has been developed for the NASA Kuiper Airborne Observatory (KAO) over the past several years.

Zmuidzinas, Jonas↗
NASA NTRS1994

Circumprotostellar environments. 2: Envelopes, activity, and evolution

We have obtained 800 and 1100 micron photometric observations of a complete, flux-limited, IRAS-selected sample of cold sources in Taurus, whose infrared and molecular properties indicate them to be low-mass protostars which are younger than T Tauri stars. The goal of this study is to understand the role of the circumstellar envelope and disk during low-mass star formation and to search for signs of evolution. We find a good correlation between the submillimeter flux density (as well as of envelope mass) with both the mechanical luminosity of the molecular outflow and with the far-infrared luminosity, presumably produced by accretion onto the protostar. This suggests that the mass of the disk/envelope may be linked to the rate of accretion onto the protostar and to the generation of outflows. However, in contrast to the results of Cabrit & Andre (1991) we see no evidence for a lower limit to the disk/envelope mass which can produce outflow. We also find a significant difference between the spectral energy distributions at submillimeter wavelengths of embedded protostars compared to optically visible T Tauri stars. The more evolved (T Tuari) objects tend to have shallower spectral indices than embedded sources, suggesting (1) an evolution of dust grains, and/or (2) a more compact envelope and optically thick disk in the older objects.

Moriarty-Schieven, G. H.↗
NASA NTRS1993

IRAS 21391 + 5802 - A study in intermediate mass star formation

We present infrared and millimeter wavelength observations of the cold IRAS source 21391 + 5802 and its associated molecular core. Infrared observations at lambda = 3.5 microns reveal a heavily obscured, central point source which is coincident with a compact lambda = 2.7 mm continuum and C18O emission region. The source radiates about 310 solar luminosities, primarily at FIR wavelengths, suggesting that it is a young stellar object of intermediate mass. The steeply rising spectral energy distribution and the large fraction of the system mass residing in circumstellar material imply that IRAS 21391 + 5802 is in an early stage of evolution. The inferred dust temperature indicates a temperature gradient in the core. A comprehensive model for the surrounding core of dust and gas is devised to match the observed dust continuum emission and multitransition CS emission from this and previous studies. We find a r exp -1.5 +/- 0.2 density gradient consistent with that of a gravitationally evolved core and a total core mass of 380 solar masses. The observed dust emission is most consistent with a lambda exp -1.5 - lambda exp -2 dust emissivity law; for a lambda exp -2 law, the data are best fit by a mass opacity coefficient of 3.6 x 10 exp -3 sq cm/g at lambda = 1.25 mm.

Wilking, Bruce↗
NASA NTRS1992

Circumprotostellar environments. I - Molecular outflows from protostar candidates in Taurus

We report on the search for molecular outflows from a complete, flux-limited, IRAS-selected sample of cold, low-mass protostar candidates located in the Taurus cloud complex. We find that 18 of the 24 sources have outflows associated with them, only four of which have been previously reported, implying that these sources are predominantly protostars in an early stage of evolution. We argue that at least a few of the outflows are very small in extent, indicating that their dynamical ages are less than a few thousand years.

Moriarty-Schieven, G. H.↗
NASA NTRS1992

1.25-mm observations of luminous infrared galaxies

Measurements at a wavelength of 1.25 mm have been obtained for 17 IRAS galaxies selected on the basis of high far-infrared luminosity. These measurements are used to estimate the lower and upper limits to the mass of cold dust in infrared galaxies. As a lower limit on dust mass, all of the galaxies can be successfully modeled without invoking any dust colder than the dust responsible for the 60 and 100 micron emission that was detected by IRAS. As an upper limit, it is possible that the dust mass in a number of the galaxies may actually be dominated by cold dust. This large difference between the lower and upper limits is due primarily to uncertainty in the long-wavelength absorption efficiency of the astrophysical dust grains.

Carico, David P.↗
NASA NTRS1992

Herbig Ae/Be stars - Intermediate-mass stars surrounded by massive circumstellar accretion disks

The proposition that Herbig Ae/Be stars are young intermediate mass stars surrounded by optically thick accretion disks is explored. From a study of 47 such objects, a subset of 30 stars is identified whose spectral energy distributions can be interpreted convincingly in terms of pre-main sequence stars surrounded by massive optically thick circumstellar accretion disks. Constraints on the physical properties of the disks, such as size, mass, accretion rate, lifetime, and radial structure are derived from the photometric data.

Hillenbrand, Lynne A.↗
NASA NTRS1991

Millimeter continuum observations of Galactic center giant molecular cloud cores

Results are presented of observations of 1.3- and 0.8-mm continuum emission toward the cores of three Galactic center molecular clouds with ongoing massive star formation, Sagittarius B2, C, and D, which were made in order to study possible variations in the high-mass star formation rate per unit mass between the Galactic center and the disk. The luminosity-to-mass ratio, based on the mass estimates derived from the millimeter continuum emission, is used as a tracer of the high-mass star formation rate in GMC cores. The magnitude of errors involved in using millimeter continuum emission for determining the core mass is estimated through radiative transfer modeling. It is inferred from the present millimeter data, along with previously published far-infrared data, that the Sgr C and D cores are very similar in terms of mean dust optical depth and temperature. The luminosity-to-mass ratios derived for the Sgr C and D cores are found to be consistent with those of typical disk GMC cores with comparable far-infrared luminosities.

Lis, D. C.↗
NASA NTRS1990

CI, CII, and CO as tracers of gas phase carbon

In the dense interstellar medium, we find that about 20 percent of the total carbon abundance is in the form of CO, about 3 percent in C(sub I), and 100 percent in C(sub II) with uncertainties of factors of order 2. The abundance of other forms of gaseous carbon is negligible. CO is widespread throughout molecular clouds as is C(sub I). C(sub II) has only been observed near bright star-formation regions so far because of its high excitation energy. Further from ultraviolet sources it may be less abundant. Altogether we have accounted for about 1/3 of the total carbon abundance associated with dense molecular clouds. Since the other gaseous forms are thought to have negligible abundances, the rest of the carbon is probably in solid form.

Keene, Jocelyn↗
NASA NTRS1989

The abundances of atomic carbon and carbon monoxide compared with visual extinction in the Ophiuchus molecular cloud complex

Emission from the 492 GHz lines of C I have been observed toward six positions in the Ophiuchus molecular cloud complex for which accurate visual extinctions are available. The column density of C I increases with A(V) to greater than 2 x 10 to the 17th/sq cm at 100 mag, the column-averaged fractional abundance reaches a peak of about 2.2 x 10 to the -5th for A(V) in the range 4-11 mag and the column-averaged abundance ratio of C I to CO decreases with A(V) from about 1 at 2 mag to greater that about 0.03 at 100 mag. These results imply that, while C I is not the primary reservoir of gaseous carbon even at cloud edges, its fractional abundance remains high for at least 10 mag into the cloud and may be significant at even greater depths.

Frerking, Margaret A.↗
NASA NTRS1987

Comparison of submillimeter and ultraviolet observations of neutral carbon toward Zeta Ophiuchi

The 3P1 yields 3P0 line of neutral C I toward Z Oph was observed at 492 GHz to demonstrate that C I column densities and line velocity measurements can be made with both submillimeter and UV techniques. The data were collected with an InSb cyclotron-resonance heterodyne receiver on the NASA Kuiper Airborne Observatory at an altitude of 12.5 km. The radio data densities were compared with densities calculated from Copernicus satellite UV data. It was found that the radio data densities improved that accuracy of column densities previously determined from UV measurements. It is concluded that follow-up high resolution submillimeter data are a useful aid in the interpretation of UV data on column densities.

Keene, Jocelyn↗