OMC-1 Region Research Articles

ASTE Band 10 (787–950 GHz) heterodyne receiver: System description, commissioning, and science verification

Abstract We report the system description and results of the commissioning and science verification (CSV) of the Band 10 (787–950 GHz) heterodyne receiver for ASTE (Atacama Submillimeter Telescope Experiment), a 10 m submillimeter telescope at the ALMA (Atacama Large Millimeter/submillimeter Array) site in Chile. The new ASTE Band 10 receiver cartridge was refurbished from a prototype ALMA Band 10 receiver with SIS mixers employing high critical current density junctions. We installed the new receiver on ASTE and carried out its CSV. The best double-sideband (DSB) system noise temperature achieved on ASTE toward the zenith was ∼1500 K (PWV ∼ 0.5 mm). As a part of CSV activities, we performed science verification observations to demonstrate ASTE Band 10 capabilities to the community. Wide-field images were successfully obtained toward the well-studied OMC-1 region with both [C i](3P2–3P1) and 12CO(J = 7–6) lines. In addition, we partially carried out observations toward a super star cluster RCW 38, [C i](3P1–3P0), images of which were already obtained by ASTE. We demonstrated the capability of ASTE Band 10 observations toward bright and extended objects like giant molecular clouds.

Probing the role of magnetic fields in star-forming filaments: NIKA2-Pol commissioning results toward OMC-1

Dust polarization observations are a powerful, practical tool to probe the geometry (and to some extent, the strength) of magnetic fields in starforming regions. In particular, Planck polarization data have revealed the importance of magnetic fields on large scales in molecular clouds. However, due to insufficient resolution, Planck observations are unable to constrain the B-field geometry on prestellar and protostellar scales. The high angular resolution of 11.7 arcsec provided by NIKA2-Pol 1.15 mm polarimetric imaging, corresponding to 0.02 pc at the distance of the Orion molecular cloud (OMC), makes it possible to advance our understanding of the B-field morphology in star-forming filaments and dense cores (IRAM 30m large program B-FUN). The commissioning of the NIKA2-Pol instrument has led to several challenging issues, in particular, the instrumental polarization or intensity-to-polarization “leakage” effect. In the present paper, we illustrate how this effect can be corrected for, leading to reliable exploitable data in a structured, extended source such as OMC-1. We present a statistical comparison between NIKA2-Pol and SCUBA2-Pol2 results in the OMC-1 region. We also present tentative evidence of local pinching of the B-field lines near Orion-KL, in the form of a new small-scale hourglass pattern, in addition to the larger-scale hourglass already seen by other instruments such as Pol2.

The JCMT BISTRO Survey: The Distribution of Magnetic Field Strengths toward the OMC-1 Region

Abstract Measurement of magnetic field strengths in a molecular cloud is essential for determining the criticality of magnetic support against gravitational collapse. In this paper, as part of the JCMT BISTRO survey, we suggest a new application of the Davis–Chandrasekhar–Fermi (DCF) method to estimate the distribution of magnetic field strengths in the OMC-1 region. We use observations of dust polarization emission at 450 and 850 μm, and C18O (3–2) spectral line data obtained with the JCMT. We estimate the volume density, the velocity dispersion, and the polarization angle dispersion in a box, 40″ × 40″ (5×5 pixels), which moves over the OMC-1 region. By substituting three quantities in each box with the DCF method, we get magnetic field strengths over the OMC-1 region. We note that there are very large uncertainties in the inferred field strengths, as discussed in detail in this paper. The field strengths vary from 0.8 to 26.4 mG, and their mean value is about 6 mG. Additionally, we obtain maps of the mass-to-flux ratio in units of a critical value and the Alfvén Mach number. The central parts of the BN–KL and South (S) clumps in the OMC-1 region are magnetically supercritical, so the magnetic field cannot support the clumps against gravitational collapse. However, the outer parts of the region are magnetically subcritical. The mean Alfvén Mach number is about 0.4 over the region, which implies that the magnetic pressure exceeds the turbulent pressure in the OMC-1 region.

OMC-1 dust polarization in ALMA Band 7: diagnosing grain alignment mechanisms in the vicinity of Orion Source I

ABSTRACT We present ALMA Band 7 polarization observations of the OMC-1 region of the Orion molecular cloud. We find that the polarization pattern observed in the region is likely to have been significantly altered by the radiation field of the >104 L⊙ high-mass protostar Orion Source I. In the protostar’s optically thick disc, polarization is likely to arise from dust self-scattering. In material to the south of Source I – previously identified as a region of ‘anomalous’ polarization emission – we observe a polarization geometry concentric around Source I. We demonstrate that Source I’s extreme luminosity may be sufficient to make the radiative precession time-scale shorter than the Larmor time-scale for moderately large grains ($\gt 0.005\!-\!0.1\, \mu$m), causing them to precess around the radiation anisotropy vector (k-RATs) rather than the magnetic field direction (B-RATs). This requires relatively unobscured emission from Source I, supporting the hypothesis that emission in this region arises from the cavity wall of the Source I outflow. This is one of the first times that evidence for k-RAT alignment has been found outside of a protostellar disc or AGB star envelope. Alternatively, the grains may remain aligned by B-RATs and trace gas infall on to the Main Ridge. Elsewhere, we largely find the magnetic field geometry to be radial around the BN/KL explosion centre, consistent with previous observations. However, in the Main Ridge, the magnetic field geometry appears to remain consistent with the larger-scale magnetic field, perhaps indicative of the ability of the dense Ridge to resist disruption by the BN/KL explosion.

Maps of Magnetic Field Strength in the OMC-1 Using HAWC+ FIR Polarimetric Data

Abstract Far-infrared dust polarimetry enables the study of interstellar magnetic fields via tracing of the polarized emission from dust grains that are partially aligned with the direction of the field. The advent of high-quality polarimetric data has permitted the use of statistical methods to extract both the direction and magnitude of the magnetic field. In this work, the Davis–Chandrasekhar–Fermi technique is used to make maps of the plane-of-sky (POS) component of the magnetic field in the Orion Molecular Cloud (OMC-1) by combining polarization maps at 53, 89, 154 and 214 μm from HAWC+/SOFIA with maps of density and velocity dispersion. In addition, maps of the local dispersion of polarization angles are used in conjunction with Zeeman measurements to estimate a map of the strength of the line-of-sight (LOS) component of the field. Combining these maps, information about the three-dimensional magnetic field configuration (integrated along the LOS) is inferred over the OMC-1 region. POS magnetic field strengths of up to 2 mG are observed near the BN/KL object, while the OMC-1 bar shows strengths of up to a few hundred μG. These estimates of the magnetic field components are used to produce maps of the mass-to-magnetic-flux ratio (M/Φ)—a metric for probing the conditions for star formation in molecular clouds—and determine regions of sub- and supercriticality in OMC-1. Such maps can provide invaluable input and comparison to MHD simulations of star formation processes in filamentary structures of molecular clouds.

APEX-SEPIA660 Early Science: gas at densities above 107 cm−3 towards OMC-1

Context. The star-formation rates and stellar densities found in young massive clusters suggest that these stellar systems originate from gas at densities of n(H2) > 106 cm−3. Until today, however, the physical characterization of this ultra high density material remains largely unconstrained in observations. Aims. We investigate the density properties of the star-forming gas in the OMC-1 region located in the vicinity of the Orion Nebula Cluster (ONC). Methods. We mapped the molecular emission at 652 GHz in OMC-1 as part of the APEX-SEPIA660 Early Science. Results. We detect bright and extended N2H+ (J = 7–6) line emission along the entire OMC-1 region. Comparisons with previous ALMA data of the (J = 1–0) transition and radiative transfer models indicate that the line intensities observed in this N2H+ (7–6) line are produced by large mass reservoirs of gas at densities n(H2) > 107 cm−3. Conclusions. The first detection of this N2H+ (7–6) line at parsec-scales demonstrates the extreme density conditions of the star-forming gas in young massive clusters such as the ONC. Our results highlight the unique combination of sensitivity and mapping capabilities of the new SEPIA660 receiver for the study of the ISM properties at high frequencies.

Gravitational collapse of the OMC-1 region

We have investigated the global dynamical state of the Integral Shaped Filament in the Orion A cloud using new N$_2$H$^+$ (1-0) large-scale, IRAM30m observations. Our analysis of its internal gas dynamics reveals the presence of accelerated motions towards the Orion Nebula Cluster, showing a characteristic blue-shifted profile centred at the position of the OMC-1 South region. The properties of these observed gas motions (profile, extension, and magnitude) are consistent with the expected accelerations for the gravitational collapse of the OMC-1 region and explain both the physical and kinematic structure of this cloud.

POWER SPECTRUM ANALYSIS OF THE OMC1 IMAGE AT 1.1MM WAVELENGTH

We present a 1.1mm emission map of the OMC1 region observed with AzTEC, a new large-format array composed of 144 silicon-nitride micromesh bolometers, that was in use at the James Clerk Maxwell Telescope (JCMT). These AzTEC observations reveal dozens of cloud cores and a tail of filaments in a manner that is almost identical to the submillimeter continuum emission of the entire OMC1 region at 450 and <TEX>$850{\mu}m$</TEX>. We perform Fourier analysis of the image with a modified periodogram and the density power spectrum, which provides the distribution of the length scale of the structures, is determined. The expected value of the periodogram converges to the resulting power spectrum in the mean squared sense. The present analysis reveals that the power spectrum steepens at relatively smaller scales. At larger scales, the spectrum flattens and the power law becomes shallower. The power spectra of the 1.1mm emission show clear deviations from a single power law. We find that at least three components of power law might be fitted to the calculated power spectrum of the 1.1mm emission. The slope of the best fit power law, <TEX>${\gamma}{\approx}-2.7$</TEX> is similar to those values found in numerical simulations. The effect of beam size and the noise spectrum on the shape and slope of the power spectrum are also included in the present analysis. The slope of the power law changes significantly at higher spatial frequency as the beam size increases.

SIMILARITY BETWEEN THE C18O (J= 1-0) CORE MASS FUNCTION AND THE INITIAL MASS FUNCTION (IMF) IN THE S140 REGION

We present the results of C18O(J=1-0) mapping observations of a 20'x18' area in the Lynds 1204 molecular cloud associated with the Sharpless 2-140 (S140) H II region. The C18O cube (alpha-delta-vLSR) data shows that there are three clumps with sizes of \sim 1 pc in the region. Two of them have peculiar red shifted velocity components at their edges, which can be interpreted as the results of the interaction between the cloud and the Cepheus Bubble. From the C18O cube data, the clumpfind identified 123 C18O cores, which have mean radius, velocity width in FWHM, and LTE mass of 0.36\pm0.07 pc, 0.37\pm0.09 km s-1, and 41\pm29 Msun, respectively. All the cores in S140 are most likely to be gravitationally bound by considering the uncertainty in the C18O abundance. We derived a C18O core mass function (CMF), which shows a power-law-like behavior above a turnover at 30 Msun. The best-fit power-law index of -2.1\pm0.2 is quite consistent with those of the IMF and the C18O CMF in the OMC-1 region by Ikeda & Kitamura (2009). Kramer et al. (1998) estimated the power-law index of -1.65 in S140 from the C18O(J=2-1) data, which is inconsistent with this study. However, the C18O(J=2-1) data are spatially limited to the central part of the cloud and are likely to be biased toward high-mass cores, leading to the flatter CMF. Consequently, this study and our previous one strongly support that the power-law form of the IMF has been already determined at the density of \sim 10^{3-4} cm^{-3}, traced by the C18O(J=1-0) line.

A C 18 O STUDY OF THE ORIGIN OF THE POWER-LAW NATURE IN THE INITIAL MASS FUNCTION

We have performed C$^{18}$O ($J$=1--0) mapping observations of a $20'\times20'$ area of the OMC-1 region in the Orion A cloud. We identified 65 C$^{18}$O cores, which have mean radius, velocity width in FWHM, and LTE mass of 0.18$\pm$0.03 pc, 0.40$\pm$0.15 km s$^{-1}$, and 7.2$\pm$4.5 $M_\odot$, respectively. All the cores are most likely to be gravitationally bound by considering the uncertainty in the C$^{18}$O abundance. We derived a C$^{18}$O core mass function, which shows a power-law-like behavior above 5 $M_\odot$. The best-fit power-law index of $-2.3\pm0.3$ is consistent with those of the dense core mass functions and the stellar initial mass function (IMF) previously derived in the OMC-1 region. This agreement strongly suggests that the power-law form of the IMF has been already determined at the density of $\sim10^{3}$ cm$^{-3}$, traced by the C$^{18}$O ($J$=1--0) line.

Magnetic fields in massive star forming regions: wide-field NIR polarimetry of M42 and Mon R2

AbstractMagnetic fields are believed to play an important role in star formation. We observed M42 and Mon R2 massive star forming regions using the wide-field (8′ × 8′) near-infrared imaging polarimeter SIRPOL in South Africa. Magnetic fields are mapped on the basis of dichroic polarized light from hundreds of young stars embedded in the regions. We found “hourglass shaped” magnetic field structure toward OMC-1 region, which is very consistent with magnetic fields traced by using dust emission polarimetry at sub-mm to FIR wavelengths. In the Mon R2 region, we found “S-shaped” magnetic field structure across the massive protostar IRS 1 and IRS 2. We will present the results of comparison of magnetic fields at NIR with those at other wavelengths.

NEAR-INFRARED IMAGING POLARIMETRY OF M42: APERTURE POLARIMETRY OF POINT-LIKE SOURCES

We have conducted aperture polarimetry of ~500 stars of the Orion Nebula Cluster in M42 based on our wide-field (~8'×8') JHKs-band polarimetry. Most of the near-infrared (NIR) polarizations are dichroic, with position angles of polarization agreeing, both globally and locally, with previous far-infrared (FIR) and submillimeter observations, having taken into account the 90° difference in angles between dichroic absorption and emission. This is consistent with the idea that both NIR dichroic polarizations and FIR/submillimeter thermal polarizations trace the magnetic fields in the OMC-1 region. The magnetic fields inferred from these observations show a pinch at scales less than 0.5 pc with a centroid near IRc2. The hourglass-shaped magnetic field pattern is explained by the models in which the magnetic field lines are dragged along with the contracting gas and then wound up by rotation in a disk. The highly polarized region to the northwest of IRc2 and the low-polarized region near the bright bar are also common among NIR and FIR/submillimeter data, although a few regions of discrepancy exist. We have also discerned ~50 possible highly polarized sources whose polarizations are more likely to be intrinsic rather than dichroic. Their polarization efficiencies (P(H)/A(H)) are too large to be explained by the interstellar polarization. These include ten young brown dwarfs that suggest a higher polarization efficiency, which may present geometrical evidence for (unresolved) circumstellar structures around young brown dwarfs.

Photodissociation regions. I - Basic model. II - A model for the Orion photodissociation region

view Abstract Citations (1301) References (88) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photodissociation regions. I. Basic model. Tielens, A. G. G. M. ; Hollenbach, D. Abstract A theoretical parameter study of the temperature and chemical structure of dense photodissociation regions and their resultant spectrum is presented. Models are discussed which are relevant not only to the dust and gas between molecular clouds and H II regions, but also apply to any neutral cloud illuminated by intense FUV fluxes. The models relate observed line and continuum emission from these regions to physical parameters such as the gas density and temperature, the elemental and chemical abundances, the local radiation field, and the grain properties. The results are applied to observational data from the OMC-1 region. The model shows that the observed high brightness temperature of the C I 609 microns line can be explained by emission from the C(+)/C/CO transition region. This difference with previous chemical models is due to a higher gas phase elemental abundance of carbon, to the charge exchange reactions of C(+) with S and SiO, and to carbon self-shielding. Publication: The Astrophysical Journal Pub Date: April 1985 DOI: 10.1086/163111 Bibcode: 1985ApJ...291..722T Keywords: Astronomical Models; Gas Ionization; Molecular Clouds; Nebulae; Photodissociation; Abundance; Brightness Temperature; Emission Spectra; Far Ultraviolet Radiation; Fine Structure; Gas Density; Gas Temperature; H Ii Regions; Infrared Spectra; Astrophysics full text sources ADS | data products SIMBAD (5) NED (1) Related Materials (1) Part 2: 1985ApJ...291..747T

New members of the infrared cluster in the Orion Molecular Cloud

Near-infrared high-resolution scans (3.5 arcsec) of the core of the Orion Molecular Cloud (no. 1) have revealed 26 sources. Eleven of these are identified with faint visible stars. The remainder are thought to be highly reddened stars embedded in the molecular cloud and include at least two of the previously known infrared cluster members. Comparison of the distribution of infrared and visible stars on the plane of the sky and in an infrared color-magnitude diagram shows that the obscured infrared sources form a separate cluster of stars. The newly discovered sources appear to fall into two categories: (1) optically identified stars probably on the front surface of the cloud and associated with the Trapezium cluster; and (2) unidentified infrared stars probably associated with the molecular cloud and the embedded infrared cluster. It is plausible that the newly discovered infrared sources in the OMC-1 region are stars less massive than the previously known members and younger than visible stars of similar mass in Orion Association.

Ammonia observations of the Orion Molecular Cloud

view Abstract Citations (75) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Ammonia observations of the Orion molecular cloud. Ho, P. T. P. ; Barrett, A. H. ; Myers, P. C. ; Matsakis, D. N. ; Cheung, A. L. ; Chui, M. F. ; Townes, C. H. ; Yngvesson, K. S. Abstract Emission from several metastable states of NH3 is mapped in the OMC-1 and OMC-2 regions. Rotational temperatures are deduced from a comparison of the (J, K) = (1, 1), (2, 2), and (3, 3) lines. A hot core is found at KL, embedded in the cooler OMC-1 ridge. Clumping is found to be extended over the entire OMC-1 region with only about one-tenth of the 1.4 arcmin beam filled by regions which provide most of the emission. Publication: The Astrophysical Journal Pub Date: December 1979 DOI: 10.1086/157575 Bibcode: 1979ApJ...234..912H Keywords: Ammonia; Astronomical Spectroscopy; Emission Spectra; Interstellar Gas; Metastable State; Orion Nebula; Astronomical Maps; Electron Transitions; Fine Structure; Gas Temperature; Line Spectra; Optical Thickness; Astrophysics full text sources ADS | data products SIMBAD (4)