Observations of the 18.3-GHz Transition of C3H2 towards Sgr B2

1992 ◽  
Vol 10 (2) ◽  
pp. 113-117
Author(s):  
R.S. Peng ◽  
J.B. Whiteoak ◽  
J.E. Reynolds ◽  
T.B.H. Kuiper ◽  
W.L. Peters
Keyword(s):  
Optical Depth ◽  
Molecular Clouds ◽  
Column Density ◽  
Radial Velocities ◽  
Continuum Emission ◽  
Fractional Abundance ◽  
Right Ascension ◽  
The Continuum

AbstractWe have mapped the molecular clouds of Sgr B2 in the 110 → 101 ortho-transition of C3H2 at 18.3 GHz, using the 70-m NASA telescope at Tidbinbilla (beamwidth 55 arcsec). Three clouds show absorption against the Sgr B2 continuum emission at radial velocities of 50, 65 and 80 km s−1. The 65-km s−1 cloud covers most of the observed area (4 × 6 arcmin in right ascension and declination), has a peak optical depth of 2.7 and a corresponding C3H2 column density of 7.6 × 1015 cm−2. The C3H2 fractional abundance relative to H2 is 1.5 × 10−9. The 80-km s−1 cloud, located north of the Sgr B2 continuum peak, has a peak optical depth of 0.9 and a C3H2 column density of 1.9 × 1015 cm−2. The 50-km s−1 cloud is centred 2 arcmin south of the continuum peak; here the minimum optical depth of 0.5 yields a column density of 5.3 × 1014cm−2.

scholarly journals A 45 AU radius source around L1551-IRS 5: a possible accretion disk

1991 ◽  
Vol 147 ◽  
pp. 361-372
Author(s):  
J. Keene ◽  
C. R. Masson
Keyword(s):  
Accretion Disk ◽  
Optical Depth ◽  
Column Density ◽  
Angular Resolution ◽  
Continuum Emission ◽  
Millimeter Wavelengths ◽  
High Column ◽  
Rule Out ◽  
The Continuum

Our interferometric and single-dish observations of the continuum emission from L1551-IRS 5 show that, at millimeter wavelengths, there are two distinct components to the source, an envelope with a radius ≥ 2000 AU, and a compact core with a radius ≤ 64 AU. The compact core has a large optical depth, indicating a high column density (∼ 1000 g cm—2). By modeling the temperature in the region of the compact core, we show that its size must lie in the range 45 ± 20 AU. The compact core is most plausibly identified with an accretion, or preplanetary, disk around the star, although the present observations do not have sufficient angular resolution to rule out other structures.

scholarly journals A 45 AU radius source around L1551-IRS 5: a possible accretion disk

1991 ◽  
Vol 147 ◽  
pp. 361-372
Author(s):  
J. Keene ◽  
C. R. Masson
Keyword(s):  
Accretion Disk ◽  
Optical Depth ◽  
Column Density ◽  
Angular Resolution ◽  
Continuum Emission ◽  
Millimeter Wavelengths ◽  
High Column ◽  
Rule Out ◽  
The Continuum

Our interferometric and single-dish observations of the continuum emission from L1551-IRS 5 show that, at millimeter wavelengths, there are two distinct components to the source, an envelope with a radius ≥ 2000 AU, and a compact core with a radius ≤ 64 AU. The compact core has a large optical depth, indicating a high column density (∼ 1000 g cm—2). By modeling the temperature in the region of the compact core, we show that its size must lie in the range 45 ± 20 AU. The compact core is most plausibly identified with an accretion, or preplanetary, disk around the star, although the present observations do not have sufficient angular resolution to rule out other structures.

scholarly journals Studies of the 3C 345 Quasar Structure at dm-Wavelengths

1987 ◽  
Vol 121 ◽  
pp. 301-304
Author(s):  
L.I. Matveyenko ◽  
I.I.K. Pauliny-Toth ◽  
B. Sherwood ◽  
L. Bååth ◽  
A. Kus
Keyword(s):  
Optical Depth ◽  
Continuum Emission ◽  
The Continuum

The structure of the quasar 3C 345 has been studied at 18 and 49 cm wavelengths by VLBI method. The structure has a nucleus and a jet, which changes direction with distance from the nucleus. The continuum emission of the nucleus and the jet regions correspond to a source with a large optical depth, the cut frequence goes down with increasing distance from nucleus.

scholarly journals Cosmic-ray ionisation in circumstellar discs

2018 ◽  
Vol 614 ◽  
pp. A111 ◽  
Author(s):  
Marco Padovani ◽  
Alexei V. Ivlev ◽  
Daniele Galli ◽  
Paola Caselli
Keyword(s):  
Molecular Clouds ◽  
Column Density ◽  
Cosmic Ray ◽  
Galactic Cosmic Rays ◽  
Charged Dust ◽  
Interstellar Gas ◽  
Complex Behaviour ◽  
Electron Positron ◽  
Fractional Abundance ◽  
Ionisation Rate

Context. Galactic cosmic rays (CRs) are a ubiquitous source of ionisation of the interstellar gas, competing with UV and X-ray photons as well as natural radioactivity in determining the fractional abundance of electrons, ions, and charged dust grains in molecular clouds and circumstellar discs. Aims. We model the propagation of various components of Galactic CRs versus the column density of the gas. Our study is focussed on the propagation at high densities, above a few g cm−2, especially relevant for the inner regions of collapsing clouds and circumstellar discs. Methods. The propagation of primary and secondary CR particles (protons and heavier nuclei, electrons, positrons, and photons) is computed in the continuous slowing down approximation, diffusion approximation, or catastrophic approximation by adopting a matching procedure for the various transport regimes. A choice of the proper regime depends on the nature of the dominant loss process modelled as continuous or catastrophic. Results. The CR ionisation rate is determined by CR protons and their secondary electrons below ≈130 g cm−2 and by electron-positron pairs created by photon decay above ≈600 g cm−2. We show that a proper description of the particle transport is essential to compute the ionisation rate in the latter case, since the electron and positron differential fluxes depend sensitively on the fluxes of both protons and photons. Conclusions. Our results show that the CR ionisation rate in high-density environments, such as the inner parts of collapsing molecular clouds or the mid-plane of circumstellar discs, is higher than previously assumed. It does not decline exponentially with increasing column density, but follows a more complex behaviour because of the interplay of the different processes governing the generation and propagation of secondary particles.

12.2 GHz CH3OH Absorption in Southern Molecular Clouds

1991 ◽  
Vol 9 (2) ◽  
pp. 287-288
Author(s):  
R. Peng ◽  
J. B. Whiteoak
Keyword(s):  
Optical Depth ◽  
Molecular Clouds ◽  
Column Density ◽  
H Ii Regions ◽  
Maser Emission ◽  
Dark Clouds

AbstractWe have used the Parkes 64m telescope to observe the 20 → 3−1E absorption of CH3 OH at 12.2 GHz towards 58 Galactic H II regions and dark clouds, yielding 38 detections. The results show that CH3 OH absorbing clouds have a typical optical depth of 0.25 and a column density of 8.4 × 1015 cm−2. CH3 OH absorption is often accompanied by unsaturated maser emission and is closely associated with background H II regions. CH3 OH absorption against the 2.7 K background is also observed in several dark clouds.

scholarly journals A Survey of OH in the Galactic Centre Region

1996 ◽  
Vol 169 ◽  
pp. 311-316
Author(s):  
P.J. Boyce ◽  
R. J. Cohen
Keyword(s):  
Central Region ◽  
Molecular Clouds ◽  
Radio Continuum ◽  
Continuum Emission ◽  
Galactic Centre ◽  
Molecular Gas ◽  
The Galaxy ◽  
Centre Region ◽  
Line Survey ◽  
The Continuum

The galactic centre contains the largest concentration of molecular clouds in the Galaxy. The clouds in the central region are unusual in having large linewidths and masses, and large non-circular motions. Previous surveys of their distribution in the central region have been carried out in OH (Robinson & McGee 1970; Cohen & Few 1976), H2CO (Whiteoak & Gardner 1979; Cohen & Few 1981), CO (Bania 1977; Dame et al. 1987; Bally et al. 1987, 1988) and CS (Bally et al. 1987, 1988). The OH groundstate lines at 18cm wavelength have certain advantages for such a survey. The OH lines appear in absorption against the galactic centre continuum sources, and against the continuum emission from the disk of the Galaxy. The absorption spectra are sensitive to relatively small molecular column densities. In addition they can give information on the relative positions of the molecular gas and the radio continuum sources. This paper describes results from an absorption line survey of the galactic centre region in the OH main lines at 1667.359 MHz and 1665.402 MHz (Boyce & Cohen 1994).

scholarly journals High-resolution observations of the galactic centre at 1420 Mc/s in the continuum and in the neutral hydrogen line

1964 ◽  
Vol 20 ◽  
pp. 191-195
Author(s):  
N. F. Ryzhkov ◽  
T. M. Egorova ◽  
I. V. Gossachinsky ◽  
N. V. Bystrova
Keyword(s):  
Neutral Hydrogen ◽  
Continuum Emission ◽  
Galactic Centre ◽  
Hydrogen Line ◽  
Sagittarius A ◽  
The Mean ◽  
The Right ◽  
Right Ascension ◽  
Large Pulkovo Radio Telescope ◽  
The Continuum

The large Pulkovo radio telescope has a beamwidth of 7 min of arc in R.A. at a frequency of 1420 Mc/s. The continuum emission from the source Sagittarius A was measured with a bandwidth of 5 Mc/s. The mean drift curve is shown in Figure 1. It can be separated into components according to Drake (1959) as indicated in the figure. The galactic ridge is also shown. The right ascension, the antenna temperature, the observed angular diameter, and the flux density of each component are given in Table 1. The errors given in the table can be somewhat higher in the case of components 2 and 3 because of the difficulties of separating them. The calibration of the antenna and receiver was made using the flux densities of the sources IAU 19N4A, 05N2A, 05S0A, and 18S1A according to Westerhout (1958) with the corrections given by Altenhoff et al. (1960). The atmospheric extinction at λ = 21 cm was taken from these works as well.

scholarly journals Effects of grain alignment efficiency on synthetic dust polarization observations of molecular clouds

2019 ◽  
Vol 490 (2) ◽  
pp. 2760-2778 ◽  
Author(s):  
Patrick K King ◽  
Che-Yu Chen ◽  
L M Fissel ◽  
Zhi-Yun Li
Keyword(s):  
Magnetic Field ◽  
Power Law ◽  
Molecular Clouds ◽  
Column Density ◽  
Local Magnetic Field ◽  
Continuum Emission ◽  
Field Orientation ◽  
Local Conditions ◽  
Grain Alignment ◽  
Polarization Fraction

ABSTRACT It is well known that the polarized continuum emission from magnetically aligned dust grains is determined to a large extent by local magnetic field structure. However, the observed significant anticorrelation between polarization fraction and column density may be strongly affected, perhaps even dominated by variations in grain alignment efficiency with local conditions, in contrast to standard assumptions of a spatially homogeneous grain alignment efficiency. Here we introduce a generic way to incorporate heterogeneous grain alignment into synthetic polarization observations of molecular clouds (MCs), through a simple model where the grain alignment efficiency depends on the local gas density as a power law. We justify the model using results derived from radiative torque alignment theory. The effects of power-law heterogeneous alignment models on synthetic observations of simulated MCs are presented. We find that the polarization fraction-column density correlation can be brought into agreement with observationally determined values through heterogeneous alignment, though there remains degeneracy with the relative strength of cloud-scale magnetized turbulence and the mean magnetic field orientation relative to the observer. We also find that the dispersion in polarization angles-polarization fraction correlation remains robustly correlated despite the simultaneous changes to both observables in the presence of heterogeneous alignment.

scholarly journals Recurring obscuration in NGC 3783

2018 ◽  
Vol 619 ◽  
pp. A112 ◽  
Author(s):  
J. S. Kaastra ◽  
M. Mehdipour ◽  
E. Behar ◽  
S. Bianchi ◽  
G. Branduardi-Raymont ◽  
...  
Keyword(s):  
Column Density ◽  
Extreme Ultraviolet ◽  
Galactic Nuclei ◽  
Seyfert Galaxies ◽  
Continuum Emission ◽  
X Ray ◽  
Transient Nature ◽  
Hardness Ratio ◽  
Ultraviolet Photons ◽  
The Continuum

Context. Obscuration of the continuum emission from active galactic nuclei by streams of gas with relatively high velocity (>1000 km s−1) and column density (>3 × 1025 m−2) has been seen in a few Seyfert galaxies. This obscuration has a transient nature. In December 2016 we witnessed such an event in NGC 3783. Aims. The frequency and duration of these obscuration events is poorly known. Here we study archival data of NGC 3783 in order to constrain this duty cycle. Methods. We use archival Chandra/NuSTAR spectra taken in August 2016. We also study the hardness ratio of all Swift XRT spectra taken between 2008 and 2017. Results. In August 2016, NGC 3783 also showed evidence of obscuration. While the column density of the obscuring material is ten times lower than in December 2016, the opacity is still sufficient to block a significant fraction of the ionising X-ray and extreme ultraviolet photons. From the Swift hardness ratio behaviour we find several other epochs with obscuration. Obscuration with columns >1026 m−2 may take place about half of the time. Also, in archival X-ray data taken by the Advanced Satellite for Cosmology and Astrophysics (ASCA) in 1993 and 1996 we find evidence of obscuration. Conclusions. Obscuration of the ionising photons in NGC 3783 occurs more frequently than previously thought. This may not always have been recognised due to low-spectral-resolution observations, overly limited spectral bandwidth or confusion with underlying continuum variations.

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