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NASA NTRS1979

Two new methanol transitions in Orion A

Emission from the 11(1) - 10(2) (A-) and 5(0) - 4(1) (E) transitions of CH3OH at rest frequencies of 76,247.4 and 76,509.9 MHz, respectively, has been detected toward Orion A. The velocity widths of the two transitions are estimated to be approximately 3 km/s, which is consistent with other methanol lines in Orion. It is noted that the 117.5-kayser upper-state energy of the 11(1) - 10(2) (A-) transition is the highest yet reported for methanol in any source. The observed intensity is shown to be consistent with the 90-K thermal excitation model that seems to be appropriate for all other reported CH3OH lines in Orion A. Total column densities of 3 x 10 to the 16th per sq cm for the 11(1) - 10(2) (A-) transition and of 2 x 10 to the 16th per sq cm for the 5(0) - 4(1) (E) transition are calculated.

Jennings, D. E.↗
NASA NTRS1979

Measurements of the extinction to the molecular hydrogen emission from the Orion Nebula

Observations of the reddening of two pairs of H2 emission lines in the Orion Nebula are presented. A reddening of 0.8 + or - 0.2 mag between 2.12 and 2.42 microns is derived from observations of the intensity ratio of the v = 1-0 S(1) (2.12-micron) line and the Q(3) (2.42-micron) line and a comparison of this ratio with its unreddened value. The result is checked by a measurement of the intensity ratio of the v = 1-0 S(2) (2.03-micron) line and the Q(4) (2.43-micron) line. The total IR and visual extinctions to the emission region are inferred to be 4 + or - 1 mag (2.1 microns) and 40 + or - 10 mag (visual). Previous measurements of the H2 emission lines in Orion are corrected on the basis of these results, yielding a total H2 luminosity of approximately 2000 suns, an average column density of about 3 x 10 to the 20th per sq cm, and essentially equal rotational and vibrational temperatures (2000 K) for the H2. It is concluded that the H2 emission region in Orion lies within the molecular cloud, probably at the same depth as the IR cluster.

Beckwith, S.↗
NASA NTRS1983

The kinetic temperature and CH3CCH column density profiles in SGR B2, Orion, and Dr 21

Kinetic temperatures, T(kin), are presently estimated by means of a technique employing the intensities of the adjacent rotational transitions of symmetric top molecules, under the assumptions that the metastable levels are thermalized, and that the excitation temperature connecting the upper level of the observed transition to the lowest level in the K ladder is the same for the both adjacent transitions. The method is applied to observations of the J = 5-4 and J = 6-5 transitions of CH3CCH obtained at 1-arcmin intervals along a NS-EW cross that centered on Orion KL, Sgr B2, and Dr 21. The technique's application to CH3CN lines from the central positions of Sgr B2 and Orion produces T(kin) values that are in good agreement with other determinations. The fact that these lines are broader than those of CH3CCH is interpreted in terms of a model in which CH3CN emission is confined to small and warm core region while CH3CCH emission is dominated by a more extensive, almost isothermal and cooler halo region in Orion KL and Sgr B2.

Churchwell, E.↗
NASA NTRS1986

The large system of molecular clouds in Orion and Monoceros

Emission is noted over about one-eighth of an 850-sq deg region centered on Orion and Monoceros that has been surveyed in the J = 1 to 0 line of CO; most of the emission arises from giant molecular clouds associated with Orion A and B, and Mon R2. A much smaller area was surveyed for C-13O emission. A comparison of cloud masses obtained by three independent methods indicates that CO luminosity is as accurate a measure of cloud mass as other indicators. The possible relationships among clouds in the survey are discussed, including the conjecture that the overall Orion complex of clouds is a much larger system than previously considered, incorporating most of the clouds in the present survey.

Maddalena, R. J.↗
NASA NTRS1986

H2 spectroscopy as an agent for extinction determinations The near-infrared curve for the Orion molecular cloud

A near-infrared (1.8 to 3.5) microns extinction curve for the Orion molecular cloud is presented. The curve is derived from high-resolution spectra of the Orion H2 source recorded from the Kuiper Airborne Observatory. The data reveal that the Orion extinction law is indistinguishable from a 1/lambda form in the near-infrared, except for strongly enhanced extinction near a wavelength of about 3 microns. The implications of these results, in the context of current interstellar grain models, are discussed.

Davis, D. S.↗
NASA NTRS1986

Filamentary structure in the Orion molecular cloud

A large scale 13CO map (containing 33,000 spectra) of the giant molecular cloud located in the southern part of Orion is presented which contains the Orion Nebula, NGC1977, and the LI641 dark cloud complex. The overall structure of the cloud is filamentary, with individual features having a length up to 40 times their width. This morphology may result from the effects of star formation in the region or embedded magnetic fields in the cloud. We suggest a simple picture for the evolution of the Orion-A cloud and the formation of the major filament. A rotating proto-cloud (counter rotating with respect to the galaxy) contians a b-field aligned with the galaxtic plane. The northern protion of this cloud collapsed first, perhaps triggered by the pressure of the Ori I OB association. The magnetic field combined with the anisotropic pressure produced by the OB-association breaks the symmetry of the pancake instability, a filament rather than a disc is produced. The growth of instabilities in the filament formed sub-condensations which are recent sites of star formation.

Bally, J.↗
NASA NTRS1989

Survey observations of emission-line stars in the Orion region

Survey observations were conducted for H alpha-emission stars in the Orion region using the Kiso Schmidt telescope and partly the CTIO Curtis Schmidt telescope. In the area of about 25 square degrees, a total of 236 H alpha-emission objects, mostly supposed to be T Tau type stars, have been detected among which 155 are new ones including 6 non-stellar objects. Celestial coordinates and V-magnitude are measured for the detected objects. Eye estimation of the H alpha-emission intensity is also made at three epochs in a time span of about two years, where notable variation of H alpha intensity was found in 68 out of 236 objects. Besides a remarkable concentration along the northern dark cloud complex, a loose concentration is noticed near the Orion Belt region, fairly well coinciding with the distribution of the Orion OBIb association members. A comparison with the Av-map is also made to see the relationship between the distribution of emission-line objects and that of interstellar dust.

Wiramihardja, Suhardja D.↗
NASA NTRS1990

Studies of the local interstellar medium. VIII - Morphology and kinematics of the diffuse interstellar clouds toward Orion

Interstellar clouds in the direction of the Orion association show only positive velocities for target stars within 190 pc of the sun, and both positive and negative velocities for more distant target stars, confirming an earlier prediction by Cowie, Songaila, and York (1979). The nearby positive velocity cloud, designated here as Orion-Lepus 70 (OL 70), is a standard diffuse interstellar cloud: it is subject to the ambient galactic radiation field, with properties consistent with T about equal to 100 K and n about equal to 3/cu cm. Combined with a column density log N(H) = 19.8-20.0/sq cm, these values imply a cloud thickness of about 7 pc. The kinematics of OL 70 are consistent with either an origin as part of the expanding Loop I superbubble shell, or as part of Lindbald's expanding ring, or a synthesis of the two models. The negative velocity interstellar components seen in stars at d not less than 200 pc are caused by interstellar matter accelerated by the expanding Ori-Eri superbubble. Relatively dense interstellar gas at positive LSR velocities is also found within the Orion association, so that it is difficult to pick out OL 70 components in the spectra of the distant stars.

Frisch, P. C.↗
NASA NTRS1991

Periodically spaced fragmentation in Orion A

Evidence is found for periodic density structure in the highly fragmented interior of the Orion A molecular cloud from analysis of a large scale map of C(O-18) J = 1 - 0 emission. The spatial wavelength is about 1 parsec, and extends at least over 2 degrees along the main filament of Orion A and is also observed in other filamentary structures in Orion. This periodicity in the clumps is a result of externally triggered and magnetically mediated cloud collapse and fragmentation.

Dutrey, A.↗
NASA NTRS1992

Mid-infrared imaging of Orion BN/KL - Astrometry of IRc2 and the SiO maser

Array images of Orion BN/KL at nine midinfrared (5-20 microns) wavelengths have revealed a subarcsecond structure near IRc2 which provides new constraints on the relationship between IRc2 and the Orion SiO maser. The infrared source positions have been improved, and IRc2 is found to be displaced 0.8 +/- 0.2 arcsec (at least 400 AU) from the SiO maser, an order of magnitude greater than the separation assumed in the current maser model. The SiO maser and the IRc2 midinfrared source may be physically independent objects. There are indications that IRc2 is eroded on the south side adjacent to the SiO/H2O maser cluster, suggesting that another luminous object near IRc2, most likely a stellar object associated with radio continuum point source 'I', is the host for the Orion SiO maser.

Gezari, Daniel Y.↗
NASA NTRS1993

Structure and chemistry of Orion S

We present interferometric observations of the SiO J = 2-1, H13CO(+) J = 1-0, HC3N J = 11-10, CH3OH J(K) = 2(0)-1(0), and SO2 J(KpK0) = 8(17)-8(08) transitions along with the 3.1 mm continuum toward the young stellar object Orion S. The HC3N and H13CO(+) emission trace similar spatial and velocity distributions which are extended and follow the Orion molecular ridge. The SiO emission is more spatially confined, peaking to the west of the 3.1 mm continuum source, while the CH3OH emission peaks to the southwest. Weak SO2 emission was detected southeast of the continuum source position. Column densities and fractional abundances are derived for each species at different positions in the region. In general, the molecular abundances near the continuum source are similar to those in the quiescent material near IRc 2, but the abundances decrease toward the continuum source position indicating localized depletions of at least a factor of three. The presence of strong SiO emission with much weaker SO2 emission is interpreted as resulting from high-velocity shock interactions between the outflow from Orion S and the surrounding cloud.

Mcmullin, Joseph P.↗
NASA NTRS1993

Silicon and carbon abundances in the Orion nebula

We assess the gas-phase abundances of Si and C from our recent measurements of Si(2+), C(2+), and C(+) in the Orion Nebula by expanding on our earlier 'blister' models. The C and Si abundances are derived from new IUE high-dispersion spectra of the C(2+) 1907, 1909 A and Si(2+) 1883, 1892 A lines and archival IUE data. Gas-phase Si/C = 0.016 in the Orion ionized volume and is particularly insensitive to uncertainties in extinction and temperature structure. The solar value is 0.098. Gas-phase C/H = 2.8 x 10 exp -4 and Si/H = 4.5 x 10 exp -6. Compared to solar, Si is depleted by a factor of about 8 in the ionized region, while C is much less depleted (factor of 1.3), if depleted at all. This suggests that most Si resides in dust grains even in the ionized volume. Thus, most of the observed forbidden Si II 34.8-micron emission in Orion does not arise in the H II region.

Rubin, Robert H.↗
NASA NTRS1994

Circumstellar material around young stars in Orion

The star cluster associated with the Orion nebula is one of the richest known. Lying at the nearside of the Orion Molecular cloud and at a distance of about 500 pc from us, it contains many premain-sequence stars with ages of about 300,000 yr. The nebula itself is a blister type, representing a wall of material ionized by the hottest star in the Trapezium group (member C). Although this is not the closest star formation region, it is probably the easiest place to detect circumstellar, possibly proto-planetary, material around these solar mass stars. This is because the same process of photoionization that creates the nebula also photoionizes these circumstellar clouds, thus rendering them easily visible. Moreover, their dust component is made visible by extinction of light from the background nebula. Young stars with circumstellar material were found in Orion on the second set of HST images and were called proplyds, indicating their special nature as circumstellar clouds caused to be luminous by being in or near a gaseous nebula. The brightest objects in the field had previously been seen in the optical and radio, and although their true nature had been hypothesized it was the HST images that made it clear what they are. The forms vary from cometlike when near the Trapezium to elliptical when further away, with the largest being 1000 AU and the bright portions of the smallest, which are found closest to the Trapezium, being about 100 AU in diameter. We now have a second set of HST observations made immediately after the refurbishment mission that provides even greater detail and reveals even more of these objects. About half of all the low-luminosity stars are proplyds. The poster paper describes quantitative tests about their fundamental structure and addresses the question of whether the circumstellar material is a disk or shell. One object (HST 16) is seen only in silhouette against the nebula and is easily resolved into an elliptical form of optical depth monotonically increasing toward the central star.

Odell, C. R.↗
NASA NTRS1995

A search for interstellar CH3D: Limits to the methane abundance in Orion-KL

A search has been performed for interstellar CH3D via its J(K) = 1(0) - 0(0) transition at 230 GHz and its J(K) = 2(0) - l(0) and J(K) = 2(1) - 1(1) lines at 465 GHz using the NRAO 12 m and CSO 10 m telescopes towards Orion-KL. This search was done in conjunction with laboratory measurements of all three transitions of CH3D using mm/sub-mm direct absorption spectroscopy. The molecule was not detected down to a 3 sigma level of T(A) less than 0.05 K towards Orion, which suggests an upper limit to the CH3D column density of N less than 6 x 10(exp 18)/sq cm in the hot core region and a fractional abundance (with respect to H2) of less than 6 x 10(exp -6). These measurements suggest that the methane abundance in the Orion hot core is f less than 6 x 10-4, assuming D/H approximately 0.01. Such findings are in agreement with recent hot core chemical models, which suggest CH4/H2 approximately 10(exp -4).

Womack, Maria↗
NASA NTRS1994

The complete Einstein Observatory X-ray survey of the Orion Nebula region.

We have analyzed archival Einstein Observatory images of a roughly 4.5 square degree region centered on the Orion Nebula. In all, 245 distinct X-ray sources have been detected in six High Resolution Imager (HRI) and 17 Imaging Proportional Counter (IPC) observations. An optical database of over 2700 stars has been assembled to search for candidate counterparts to the X-ray sources. Roughly half the X-ray sources are identified with a single Orion Nebula cluster member. The 10 main-sequence O6-B5 cluster stars detected in Orion have X-ray activity levels comparable to field O and B stars. X-ray emission has also been detected in the direction of four main-sequence late-B and early-A type stars. Since the mechanisms producing X-rays in late-type coronae and early-type winds cannot operate in the late-B and early-A type atmospheres, we argue that the observed X-rays, with L(sub X) approximately = 3 x 10(exp 30) ergs/s, are probably produced in the coronae of unseen late-type binary companions. Over 100 X-ray sources have been associated with late-type pre-main sequence stars. The upper envelope of X-ray activity rises sharply from mid-F to late-G, with L(sub x)/L(sub bol) in the range 10(exp -4) to 2 x 10(exp -3) for stars later than approximately G7. We have looked for variability of the late-type cluster members on timescales of a day to a year and find that 1/4 of the stars show significantly variable X-ray emission. A handful of the late-type stars have published rotational periods and spectroscopic rotational velocities; however, we see no correlation between X-ray activity and rotation. Thus, for this sample of pre-main-sequence stars, the large dispersion in X-ray activity does not appear to be caused by the dispersion in rotation, in contrast with results obtained for low-mass main-sequence stars in the Pleiades and pre-main-sequence stars in Taurus-Auriga.

Gagne, Marc↗
NASA NTRS1995

EGRET observations of gamma-ray emission from the interstellar gas in Orion

The high-energy diffuse gamma-ray emission from the interstellar gas in Orion was studied using observations from the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory (CGRO) and radio surveys of the H I and CO emission. The good correlation of the gamma-ray emission with the atomic and molecular gas permits determination of the gamma-ray emissivity per nucleon in the interstellar medium and the molecular mass calibrating ratio N(H2)/W(sub co) in Orion. The integral gamma-ray emissivity is (1.7 +/- 0.1) x 10(exp -26)/s/sr for E greater than 100 MeV, in good agreement with expectations from studies of the diffuse emission on larger scales. The N(H2)/W(sub co) ratio is (1.06 +/- 0.14) x 10(exp 20)/sq cm/(K km/s), approximately 40% less than the commonly adopted Galactic average. We find no evidence for variations of the cosmic-ray density or N(H2)/W(sub co) ratio in Orion at the sensitivity and resolution of EGRET.

Digel, S. W.↗
NASA NTRS1998

Orion Nebula and Planetary Nebulae

This report summarizes the research performed at Rice University related to NASA-Ames University consortium grant NCC2-5199 during the two year period 1996 September 1 through 1998 August 31. The research program, titled Orion Nebula and Planetary Nebulae, involved the analysis of Hubble Space Telescope (HST) imagery and spectroscopy of the Orion Nebula and of the planetary nebulae NGC 6818 and NGC 6210. In addition, we analyzed infrared spectra of the Orion Nebula taken with the Infrared Space Observatory (ISO) The primary collaborators at NASA-Ames were Drs. R. H. Rubin, A. G. C. M. Tielens, S. W. J. Colgan, and S. D. Lord (Tielens & Lord has since changed institutions). Other collaborators include Drs. P. G. Martin (CITA, Toronto), G. J. Ferland (U. KY), J. A. Baldwin (CTIO, Chile), J. J. Hester (ASU), D. K. Walter (SCSU), and P. Harrington (U. MD). In addition to the Principal Investigator, Professor Reginald J. Dufour of the Department of Space Physics & Astronomy, the research also involved two students, Mr. Matthew Browning and Mr. Brent Buckalew. Mr. Browning will be graduating from Rice in 1999 May with a B.A. degree in Physics and Mr. Buckalew continues as a graduate student in our department, having recently received a NASA GSRP research fellowship (sponsored by Ames). The collaboration was very productive, with two refereed papers already appearing in the literature, several others in preparation, numerous meeting presentations and two press releases. Some of our research accomplishments are highlighted below. Attached to the report are copies of the two major publications. Note that this research continues to date and related extensions of it recently has been awarded time with the HST for 1999-2000.

Dufour, Reginald J.↗
NASA NTRS1999

Observations of the Rotational Transitions of OH from the Orion Molecular Cold

The goal of the observations carried out under this grant was to use several transitions of OH to infer conditions within a 1-arcminute region centered on Orion-KL. OH was selected as a diagnostic because: (1) it was predicted to be sufficiently abundant to be detectable by the KAO spectrometers; (2) there exist a number of transitions in the far- infrared free from atmospheric interference at airplane altitudes; and (3) these transitions are sensitive to a range of collisional and radiative excitation conditions believed to apply toward Orion-KL These observations were carried out as a Guest Investigator in collaboration with the Townes and Houck spectrometer groups. Though not every flight series resulted in usable data -due to technical difficulties with instrumentation - sufficient data was obtained to yield a clear picture of the source of the OH emission in Orion-KL.

Melnick, Gray J.↗