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At least 289 records · Page 16
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DOE OSTI2017

Radio measurements of the stellar proper motions in the core of the orion nebula cluster

Using multi-epoch Very Large Array observations, covering a time baseline of 29.1 years, we have measured the proper motions of 88 young stars with compact radio emission in the core of the Orion Nebula Cluster (ONC) and the neighboring BN/KL region. Our work increases the number of young stars with measured proper motion at radio frequencies by a factor of 2.5 and enables us to perform a better statistical analysis of the kinematics of the region than was previously possible. Most stars (79 out of 88) have proper motions consistent with a Gaussian distribution centered on (μ{sub α}cos δ)-bar =1.07±0.09 mas yr{sup −1}, and (μ{sub δ})-bar =−0.84±0.16 mas yr{sup −1}, with velocity dispersions of σ{sub α}=1.08±0.07 mas yr{sup −1}, σ{sub δ}=1.27±0.15 mas yr{sup −1}. We looked for organized movements of these stars but found no clear indication of radial expansion/contraction or rotation. The remaining nine stars in our sample show peculiar proper motions that differ from the mean proper motions of the ONC by more than 3σ. One of these stars, V 1326 Ori, could have been expelled from the Orion Trapezium 7000 years ago. Two could be related to the multi-stellar disintegration in the BN/KL region, in addition to the previously known sources BN, I and n. The others either have high uncertainties (so their anomalous proper motions are not firmly established) or could be foreground objects.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

THE GOULD’S BELT DISTANCES SURVEY (GOBELINS). II. DISTANCES AND STRUCTURE TOWARD THE ORION MOLECULAR CLOUDS

We present the results of the Gould’s Belt Distances Survey of young star-forming regions toward the Orion Molecular Cloud Complex. We detected 36 young stellar objects (YSOs) with the Very Large Baseline Array, 27 of which have been observed in at least three epochs over the course of two years. At least half of these YSOs belong to multiple systems. We obtained parallax and proper motions toward these stars to study the structure and kinematics of the Complex. We measured a distance of 388 ± 5 pc toward the Orion Nebula Cluster, 428 ± 10 pc toward the southern portion L1641, 388 ± 10 pc toward NGC 2068, and roughly ∼420 pc toward NGC 2024. Finally, we observed a strong degree of plasma radio scattering toward λ Ori.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

ALMA Observations of Vibrationally Excited HC{sub 3}N Lines Toward Orion KL

We present high spatial resolution ALMA observations of vibrational transitions of HC{sub 3}N toward Orion KL in the 214–247 GHz frequency band. 41 transitions of HC{sub 3}N in 7 vibrationally excited states, and 23 transitions of {sup 13}C isotopologues of HC{sub 3}N in 2 vibrational states are detected. The line images show that vibrationally excited HC{sub 3}N lines originate mainly from the hot core of Orion and IRc7. The images of HC{sub 3}N vibrationally excited lines show that the line emission peaks associated with the hot core move from south to northeast as E{sub u} increases. Based on multiple transitions of each vibrationally excited state, we performed local thermodynamic equilibrium calculations in the XCLASS suite toward the hot core and IRc7 positions. Generally, transitions in highly excited states have higher rotational temperatures and lower column densities. The rotational temperatures and column densities of the hot core range from 93 to 321 K, and from 1.0×10{sup 14} to 4.9×10{sup 16} cm{sup −2}, respectively. Lower rotational temperatures ranging from 88 to 186 K and column densities from 1.0×10{sup 14} to 3.2×10{sup 16} cm{sup −2} are obtained toward IRc7. The facts that the hot core emission peaks of vibrationally excited HC{sub 3}N lines move from south to northeast with increasing E{sub u}, and that higher-energy HC{sub 3}N lines have higher rotational temperatures and lower column densities, appear to support that the hot core is externally heated. The emission peaks are moving along the major axis of the SiO outflow, which may indicate that higher-energy HC{sub 3}N transitions are excited by interaction between pre-existing dense medium and shocks generated by SiO outflows.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

ALMA Images of the Orion Hot Core at 349 GHz

We present ALMA images of the dust and molecular line emission in the Orion Hot Core at 349 GHz. At 0.″2 angular resolution the images reveal multiple clumps in an arc ∼1″ east of Orion Source I, the protostar at the center of the Kleinmann–Low Nebula, and another chain of peaks from IRc7 toward the southwest. The molecular line images show narrow filamentary structures at velocities >10 km s{sup −1} away from the heavily resolved ambient cloud velocity ∼5 km s{sup −1}. Many of these filaments trace the SiO outflow from Source I, and lie along the edges of the dust emission. Molecular line emission at excitation temperatures 300–2000 K, and velocities >10 km s{sup −1} from the ambient cloud, suggest that the Hot Core may be heated in shocks by the outflow from Source I or from the Becklin–Neugebauer (BN)/SrcI explosion. The spectral line observations also reveal a remarkable molecular ring, ∼2″ south of SrcI, with a diameter ∼600 au. The ring is seen in high-excitation transitions of HC{sub 3}N, HCN v 2 = 1, and SO{sub 2}. An impact of ejecta from the BN/SrcI explosion with a dense dust clump could result in the observed ring of shocked material.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The JCMT BISTRO Survey: The Magnetic Field Strength in the Orion A Filament

We determine the magnetic field strength in the OMC 1 region of the Orion A filament via a new implementation of the Chandrasekhar–Fermi method using observations performed as part of the James Clerk Maxwell Telescope (JCMT) B-Fields In Star-forming Region Observations (BISTRO) survey with the POL-2 instrument. We combine BISTRO data with archival SCUBA-2 and HARP observations to find a plane-of-sky magnetic field strength in OMC 1 of B{sub pos}=6.6±4.7 mG, where δB{sub pos}=4.7 mG represents a predominantly systematic uncertainty. We develop a new method for measuring angular dispersion, analogous to unsharp masking. We find a magnetic energy density of ∼1.7×10{sup −7} J m{sup −3} in OMC 1, comparable both to the gravitational potential energy density of OMC 1 (∼10{sup −7} J m{sup −3}) and to the energy density in the Orion BN/KL outflow (∼10{sup −7} J m{sup −3}). We find that neither the Alfvén velocity in OMC 1 nor the velocity of the super-Alfvénic outflow ejecta is sufficiently large for the BN/KL outflow to have caused large-scale distortion of the local magnetic field in the ∼500 yr lifetime of the outflow. Hence, we propose that the hourglass field morphology in OMC 1 is caused by the distortion of a primordial cylindrically symmetric magnetic field by the gravitational fragmentation of the filament and/or the gravitational interaction of the BN/KL and S clumps. We find that OMC 1 is currently in or near magnetically supported equilibrium, and that the current large-scale morphology of the BN/KL outflow is regulated by the geometry of the magnetic field in OMC 1, and not vice versa.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

New Evidence for the Dynamical Decay of a Multiple System in the Orion Kleinmann–Low Nebula

We have measured astrometry for members of the Orion Nebula Cluster with images obtained in 2015 with the Wide Field Camera 3 on board the Hubble Space Telescope . By comparing those data to previous measurements with the Near-Infrared Camera and Multi-Object Spectrometer on Hubble in 1998, we have discovered that a star in the Kleinmann–Low Nebula, source x from Lonsdale et al., is moving with an unusually high proper motion of 29 mas yr{sup −1}, which corresponds to 55 km s{sup −1} at the distance of Orion. Previous radio observations have found that three other stars in the Kleinmann–Low Nebula (the Becklin–Neugebauer object and sources I and n) have high proper motions (5–14 mas yr{sup −1}) and were near a single location ∼540 years ago, and thus may have been members of a multiple system that dynamically decayed. The proper motion of source x is consistent with ejection from that same location 540 years ago, which provides strong evidence that the dynamical decay did occur and that the runaway star BN originated in the Kleinmann–Low Nebula rather than the nearby Trapezium cluster. However, our constraint on the motion of source n is significantly smaller than the most recent radio measurement, which indicates that it did not participate in the event that ejected the other three stars.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1972

Energy Distributions and spectra of Orion B stars

New MK spectral types and energy distributions are presented for B stars in Orion for which far ultraviolet flux excesses have recently been discovered. Significant differences between HD spectral energy distributions show the Orion late B stars to have smaller Balmer discontinuities than do field stars of the same spectral types. For the late B stars, these effects cause the 1500 A fluxes to be under-estimated by approximately 0.5 mag. No comparable systematic effects were found for the early B stars.

Schild, R. E.↗
NASA NTRS1974

Search for deuterium in Orion and detection of high-velocity features

Results of a search for D alpha emission near the bright H alpha line in Orion. The observations revealed a high-velocity H alpha feature near -60 km/sec, whose intensity varied across the nebula, ranging from 0.005 to 0.005 relative to H alpha; another much weaker feature, possibly also H alpha, was observed at about -100 km/sec. Because these high-velocity H alpha features partially overlap the expected position of D alpha, only an upper limit D/H less than .00013 for the number ratio in Orion can be derived.

Traub, W. A.↗
NASA NTRS1975

Photometry of a bright filament in the Orion nebula

The spectrum of a bright filament in the outer portion of the Orion nebula, 14 min from the Trapezium, was studied. The temperature of the filament, derived from /N II/ lambda 5755/6583 ratio, is 9560 K. The electron density (from /S II/ lambda 6717, 6731) is low: 360 plus or minus 150 per cu cm. The ionic abundances of He(+), N(+), O(++), and S(-) are about the same as Peimbert and Costero (1969) found in regions in Orion closer to the Trapezium. It is concluded that the filament is photoionized, presumably by the Trapezium, rather than a shock front, as had originally been suspected.

Dufour, R. J.↗
NASA NTRS1976

A photometric study of the Orion OB 1 association. 2: Photometric analysis

The procedures adopted for analysis of photometric data in terms of color excesses, intrinsic color indexes, absolute visual magnitudes, and rotational-velocity effects are discussed in detail for Orion association B-, intermediate (I)-, and AF-type stars. The effects of the nebular environment and a comparison of various calibrations of Balmer-line and four-color indexes are considered for the determination of individual absolute magnitudes for B-type stars. When absolute magnitudes of stars in the region of the Orion Nebula are determined from the beta index, emission mechanisms appear to spuriously brighten them. A detailed comparison of absolute magnitudes derived from Balmer-line indexes and MK spectral-type calibrations is presented. The data are also examined with regard to the effects of polarization and infrared excesses. The results suggest a complex combination of intracluster and circumstellar origins for these processes.

Warren, W. H., Jr.↗
NASA NTRS1976

Millimeter emission lines in Orion A

During the course of a search of Orion A for signals from three large organic molecules, several millimeter-wave lines from known interstellar molecules were observed. Results are reported for observations of methanol (CH3OH), methyl cyanide (CH3CN), methyl acetylene (CH3CCH), acetaldehyde (CH3CHO) and (Si-29)O. Emission signals from two hydrogen recombination lines (H41-alpha and H42-alpha) detected from the H II region of Orion A are also reported. Negative results were obtained for several millimeter-wave transitions of ethylene oxide, acetone, and cyclopropenone.

Lovas, F. J.↗
NASA NTRS1976

Emission object in Orion

The note draws attention to a structural feature in the Orion nebula, consisting of a parachute-shaped object converging on 64 Orionis, noted on H alpha photographs of the Orion nebula taken with the 76-cm Allegheny refractor. The same feature shows up clearly on a photograph taken in 1909 with the Mount Wilson 152-cm reflector, and on a photograph taken with the Palomar 5-m reflector. Measurements made on these three plates indicate a possible expansion of the object of about 3.6 seconds of arc per century, corresponding to a tangential velocity of about 90 to 100 km/sec. If the expansion continued at a linear rate, an explosive event may have occurred approximately 900 years ago.

Feibelman, W. A.↗
NASA NTRS1977

Scanner K-line photometry of Orion stars

Results are presented for two-channel scanner measurements of calcium K-line strengths in 39 Orion sword and belt stars. Values of the calcium k index and its associated standard error are given for each observed star, and the K-line strengths are compared with those of K-line standard stars and Hyades stars. Plots of k index against reddening-corrected color and of k-index deviation against metal-strength index deviation are provided which show that the Orion sword and belt stars do not differ significantly in their calcium and metal abundances from general field stars.

Hesser, J. E.↗
NASA NTRS1977

Vibrationally excited molecular hydrogen in Orion

Physical mechanisms for producing vibrationally excited molecular hydrogen, such as has recently been detected toward the Orion Nebula, are discussed. The most likely mechanisms are collisional excitation behind a shock moving into a molecular cloud and near-ultraviolet pumping in the H2 Lyman and Werner bands and subsequent cascade. The absolute intensities of the Orion lines require either a 10-km/s shock moving into a cloud with a density of 300,000 per cu cm or an incident near-UV flux 1 million times the mean interstellar value. The shock model is favored because it matches the observed relative line intensities and because the near-UV source, Theta-1 Ori C, may be too weak to provide the required flux. Intensities of other H2 lines in the shock model are predicted as a further observational discriminant.

Hollenbach, D. J.↗
NASA NTRS1977

Contraction of the Orion Nebula cluster-molecular cloud A complex

Observational evidence is presented in support of the view that the width of the molecular lines in Orion is produced by a contraction of the cloud with a velocity directly proportional to the distance from the cloud center. The main support is provided by close agreement between the motions of the cloud and that of the stars embedded in it. A further implication of this agreement is that star formation occurred throughout the Orion cloud, rather than only in the dense center as predicted by the conventional picture of star formation.

Fallon, F. W.↗
NASA NTRS1978

Methane detected in Orion A

This letter reports the detection of line emission from the Orion Molecular Cloud corresponding to six distinct pure-rotational delta J = 0 transitions in CH4. The transitions observed and the transition frequencies are: J = 11 E(2) - E(1), 4600.359 MHz; J = 18 F1(4) - F2(1), 76,231.45 MHz; J = 18 A1(2) - A2(1), 76,700.02 MHz; J = 19 F1(4) - F2(1), 75,944.99 MHz; J = 19 F2(4) - F1(1), 78,233.59 MHz; and J = 20 F2(5) - F1(2), 82,873.59 MHz. The observed spectra at the six transition frequencies are plotted, and all but two of the methane lines are found to be very narrow (about 2 km/s). It is noted that the small velocity widths of the four weaker lines and an observed variability of the J = 18 A line are classic characteristics of a maser and that the excitation appears to be nonthermal. An effective excitation temperature of 1100 to 2100 K is estimated for the methane in Orion A.

Fox, K.↗
NASA NTRS1979

Observations of the 63 micron forbidden OI emission line in the Orion and Omega Nebulae

Observations of 63-micron neutral oxygen emission from the Orion and Omega Nebulae are reported which were carried out from the NASA Lear Jet flying at an altitude of approximately 13.7 km. The best estimate for the 3 P 1 - 3 P 2 transition wavelength is shown to be 63.2 microns, and the detected fluxes are found to be extraordinarily high (amounting to approximately 600 suns in M42 at 0.5 kpc and to about 2900 suns in the line in M17 at 2 kpc). Attempts are made to estimate the minimum temperature and other parameters of the emitting region in Orion. It is concluded that conditions not too different from those permitted by some current models appear to provide fluxes that agree in order of magnitude with those observed.

Melnick, G.↗
NASA NTRS1979

Fine-structure lines and elemental abundances in the Orion Nebula

Infrared observations of the forbidden fine-structure lines of Ne II at 12.81 microns, S IV at 10.51 microns, and Ar III at 8.99 microns in the Orion Nebula are presented. These measurements are combined with optical observations of forbidden Ne III, S II, S III, O II, and O III to derive total elemental abundances of neon, sulfur, and oxygen, Ne/H = 7.91, S/H = 7.34, and O/H = 8.60, which are very similar to solar abundances. While the commonly employed ionization correction formula scheme is found to work satisfactorily for neon, the method overcompensates for the abundance of S(3+) and leads to a higher sulfur abundance than is actually present, though the net discrepancy in the elemental abundance of sulfur in Orion is small.

Lester, D. F.↗