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At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

The Gould's Belt very large array survey. III. The Orion region

We present results from a high-sensitivity (60 μJy), large-scale (2.26 deg{sup 2}) survey obtained with the Karl G. Jansky Very Large Array as part of the Gould's Belt Survey program. We detected 374 and 354 sources at 4.5 and 7.5 GHz, respectively. Of these, 148 are associated with previously known young stellar objects (YSOs). Another 86 sources previously unclassified at either optical or infrared wavelengths exhibit radio properties that are consistent with those of young stars. The overall properties of our sources at radio wavelengths such as their variability and radio to X-ray luminosity relation are consistent with previous results from the Gould's Belt Survey. Our detections provide target lists for follow-up Very Long Baseline Array radio observations to determine their distances as YSOs are located in regions of high nebulosity and extinction, making it difficult to measure optical parallaxes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Very large array and green bank telescope observations of Orion B (NGC 2024, W12): photodissociation region properties and magnetic field

We present images of C110α and H110α radio recombination line (RRL) emission at 4.8 GHz and images of H166α, C166α, and X166α RRL emission at 1.4 GHz, observed toward the star-forming region NGC 2024. The 1.4 GHz image with angular resolution ∼70'' is obtained using Very Large Array (VLA) data. The 4.8 GHz image with angular resolution ∼17'' is obtained by combining VLA and Green Bank Telescope data in order to add the short and zero spacing data in the uv plane. These images reveal that the spatial distributions of C110α line emission is confined to the southern rim of the H II region close to the ionization front whereas the C166α line emission is extended in the north-south direction across the H II region. The LSR velocity of the C110α line is 10.3 km s{sup –1} similar to that of lines observed from molecular material located at the far side of the H II region. This similarity suggests that the photodissociation region (PDR) responsible for C110α line emission is at the far side of the H II region. The LSR velocity of C166α is 8.8 km s{sup –1}. This velocity is comparable with the velocity of molecular absorption lines observed from the foreground gas, suggesting that the PDR is at the near side of the H II region. Non-LTE models for carbon line-forming regions are presented. Typical properties of the foreground PDR are T {sub PDR} ∼ 100 K, n{sub e}{sup PDR}∼5 cm{sup –3}, n {sub H} ∼ 1.7 × 10{sup 4} cm{sup –3}, and path length l ∼ 0.06 pc, and those of the far side PDR are T {sub PDR} ∼ 200 K, n{sub e}{sup PDR}∼ 50 cm{sup –3}, n {sub H} ∼ 1.7 × 10{sup 5} cm{sup –3}, and l ∼ 0.03 pc. Our modeling indicates that the far side PDR is located within the H II region. We estimate the magnetic field strength in the foreground PDR to be 60 μG and that in the far side PDR to be 220 μG. Our field estimates compare well with the values obtained from OH Zeeman observations toward NGC 2024. The H166α spectrum shows narrow (1.7 km s{sup –1}) and broad (33 km s{sup –1}) line features. The narrow line has spatial distribution and central velocity (∼9 km s{sup –1}) similar to that of the foreground carbon line emission, suggesting that they are associated. Modeling the narrow H166α emission provides physical properties T {sub PDR} ∼ 50 K, n{sub e}{sup PDR}∼4 cm{sup –3}, and l ∼ 0.01 pc and implies an ionization fraction of ∼10{sup –4}. The broad H166α line originates from the H II region. The X166α line has a different spatial distribution compared to other RRLs observed toward NGC 2024 and is probably associated with cold dust clouds. Based on the expected low depletion of sulfur in such clouds and the –8.1 km s{sup –1} velocity separation between the X166α and C166α lines, we interpret that the X166α transition arises from sulfur.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

STAR FORMATION IN ORION'S L1630 CLOUD: AN INFRARED AND MULTI-EPOCH X-RAY STUDY

X-ray emission is characteristic of young stellar objects (YSOs) and is known to be highly variable. We investigate, via an infrared and multi-epoch X-ray study of the L1630 dark cloud, whether and how X-ray variability in YSOs is related to protostellar evolutionary state. We have analyzed 11 Chandra X-Ray Observatory observations, obtained over the course of four years and totaling ∼240 ks exposure time, targeting the eruptive Class I YSO V1647 Ori in L1630. We used Two Micron All Sky Survey and Spitzer data to identify and classify IR counterparts to L1630 X-ray sources and identified a total of 52 X-ray-emitting YSOs with IR counterparts, including four Class I sources and one Class 0/I source. We have detected cool (<3 MK) plasma, possibly indicative of accretion shocks, in three classical T Tauri stars. A subsample of 27 X-ray-emitting YSOs were covered by 9 of the 11 Chandra observations targeting V1647 Ori and the vicinity. For these 27 YSOs, we have constructed X-ray light curves spanning approximately four years. These light curves highlight the variable nature of pre-main-sequence X-ray-emitting young stars; many of the L1630 YSOs vary by orders of magnitude in count rate between observations. We discuss possible scenarios to explain apparent trends between various X-ray spectral properties, X-ray variance, and YSO classification.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Massive quiescent cores in orion. VI. The internal structures and a candidate of transiting core in NGC 2024 filament

We present a multiwavelength observational study of the NGC 2024 filament using infrared to submillimeter continuum and the NH{sub 3} (1,1) and (2,2) inversion transitions centered on FIR-3, the most massive core therein. FIR-3 is found to have no significant infrared point sources in the Spitzer/IRAC bands. But the NH{sub 3} kinetic temperature map shows a peak value at the core center with T{sub k}=25 K, which is significantly higher than the surrounding level (T{sub k} = 15–19 K). Such internal heating signature without an infrared source suggests an ongoing core collapse possibly at a transition stage from first hydrostatic core (FHSC) to protostar. The eight dense cores in the filament have dust temperatures between 17.5 and 22 K. They are much cooler than the hot ridge (T{sub d}∼55 K) around the central heating star IRS-2b. Comparison with a dust heating model suggests that the filament should have a distance of 3–5 pc from IRS-2b. This value is much larger than the spatial extent of the hot ridge, suggesting that the filament is spatially separated from the hot region along the line of sight.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1972

Kinematics of the Huygenian region of the Orion nebula

The nebular (O II) lambda 3726 and (O III) lambda 5007 radial velocity measures of Wilson, et al (1959) are presented in contour map form. The space motions of theta 1 and theta 2 Orionis and the nebular measures are discussed.

Fischel, D.↗
NASA NTRS1973

Multicolor observations of stars in the vicinity of the Orion Nebula.

Photometry in nine colors from 0.3 to 3.4 microns of all stars brighter than V = 12 mag within half a degree square centered on theta super 1 Orionis is presented. Additionally about one-half of this area is searched for stars which, although visually fainter than twelfth magnitude, have 2.2-micron magnitudes brighter than about 8 mag. Several stars with infrared excesses and a few highly reddened stars are found. These two types of stars can be separated in an infrared two-color diagram. No star earlier than B9 has a significant infrared excess at 2 microns. There is a good correlation of 2-micron excess with optical variability. For those stars with spectral types, the visual absorption can be deduced. This varies greatly from star to star and does not correlate with position or luminosity and only weakly with 2-micron excess. There is, however, a probable correlation of visual absorption with optical variability. For nearly all stars more energy is absorbed in the visual region than is reemitted in the infrared at wavelengths up to 3.4 microns.

Penston, M. V.↗
NASA NTRS1973

A large carbon monoxide cloud in Orion

Carbon monoxide line emission was observed at a wavelength of 2.6 mm in an area of L1630, a diffuse dark nebula forming a bridge between NGC 2024 and the reflection nebula NGC 2068. The absence of a 21-cm counterpart to this region of high visual extinction suggests that L1630 is predominantly molecular hydrogen. The molecular hydrogen mass, estimated from the carbon monoxide observations, is found to comprise a significant fraction of the total mass of stars and gas within the boundaries of I Ori association.

Tucker, K. D.↗