共查询到20条相似文献，搜索用时 93 毫秒
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J. N. McCallum S. P. Ellingsen J. E. J. Lovell 《Monthly notices of the Royal Astronomical Society》2007,376(2):549-556
We have used the Australia Telescope Compact Array (ATCA) to search for polarized emission from the Circinus galaxy H2 O megamasers. No linear or circular polarized emission was detected with 3σ upper limits of 24 and 39 mJy, respectively, corresponding to fractional polarizations of 0.6 and 0.9 per cent for the strongest line. These results allow us to place upper limits on the strength of the nuclear magnetic field of ≤150 mG near the outer edge of the masing disc and ≤360 mG at the inner edge.
Rapid variability, thought to be due to interstellar scintillation, is a well-known feature of the Circinus H2 O megamasers. Our Stokes-I data show evidence for changes in the shape of the maser line spectral profiles, as well as their intensity. In addition to the rapid variations, the maser light curves also exhibit variations with a time-scale of the order of days (although our observations are too short to characterize this well). These behaviours support the hypothesis that the Circinus H2 O megamasers undergo diffractive interstellar scintillation, which has previously only been observed in pulsars and in one active galactic nucleus. 相似文献
Rapid variability, thought to be due to interstellar scintillation, is a well-known feature of the Circinus H
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H. Sugai R. I. Davies M. Ishii M. J. Ward 《Monthly notices of the Royal Astronomical Society》2000,317(2):447-452
We have obtained an H2 v =1–0 S(1) image of a merging galaxy system, NGC 6090, by using a Fabry–Perot imager. The H2 emission originates between the double nuclei of pre-merger galaxies, and exhibits an arc-like or ring-like structure almost connecting the double nuclei. This structure is similar to that suggested for Arp 220 from the velocity field measured by CO radio emission. The separation of the double nuclei in NGC 6090 is 5–6 arcsec, corresponding to a projected distance of 3–4 kpc. This is much larger than that of Arp 220 and suggests that the molecular gas distribution can form an organized shape between the nuclei, such as a ring, in a rather early phase of merging. 相似文献
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R. J. Ivison J. A. Yates & P. J. Hall 《Monthly notices of the Royal Astronomical Society》1998,295(4):813-818
We report high spatial and spectral resolution measurements of masers towards R Aqr and H1−36, both of which are examples of the sub-class of symbiotic stars that contain a long-period Mira-type variable. Our observations have resulted in the first detection of 321-GHz H2 O maser action towards a symbiotic Mira — R Aqr. Comparison with simultaneous 22-GHz H2 O maser data suggests that the masers do not have the same properties as those in the circumstellar envelopes of field Miras. R Aqr's 22-/321-GHz peak flux density and luminosity ratios are low, as is the line width ratio. Continuum and spectral-line maps indicate that the 22-GHz maser and free–free emission are aligned. Three mechanisms can reproduce the data with varying degrees of success. All three lead naturally to normal levels of maser emission in SiO and 321-GHz H2 O and anomalously weak OH and 22-GHz H2 O masers. In the most convincing model, UV radiation and a fast wind from the companion remove the Mira's envelope of dusty, molecular gas, leaving a relatively small cavity of dense, neutral material within a large, ionized nebula. Excitation temperatures suggest that 321-GHz masers are normally excited close to the Mira whilst 22-GHz masers are more remote; in R Aqr, therefore, the 22-GHz masers do not form under optimum conditions. Instead, we see weak and narrow lines that form closer to the Mira, consistent with our high-resolution maps. 相似文献
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Andrew Harrison Phil Puxley Adrian Russell & Peter Brand 《Monthly notices of the Royal Astronomical Society》1998,297(2):624-632
We present measurements of several near-infrared emission lines from the nearby galaxy NGC 253. We have been able to measure four H2 lines across the circumnuclear starburst, from which we estimate the ortho- to para- ratio of excited H2 to be ∼2. This indicates that the bulk of the H2 emission arises from photodissociation regions (PDRs), rather than from shocks. This is the case across the entire region of active star formation.
As the H2 emission arises from PDRs, it is likely that the ratio of H2 to Brγ (the bright hydrogen recombination line) is a measure of the relative geometry of O and B stars and PDRs. Towards the nucleus of NGC 253 the geometry is deduced to be tightly clustered O and B stars in a few giant H II regions that are encompassed by PDRs. Away from the nuclear region, the geometry becomes that of PDRs bathed in a relatively diffuse ultraviolet radiation field.
The rotation curves of 1–0 S(1) and Brγ suggest that the ionized gas is tracing a kinetic system different from that of the molecular gas in NGC 253, particularly away from the nucleus. 相似文献
As the H
The rotation curves of 1–0 S(1) and Brγ suggest that the ionized gas is tracing a kinetic system different from that of the molecular gas in NGC 253, particularly away from the nucleus. 相似文献
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S. C. O. Glover P. W. J. L. Brand 《Monthly notices of the Royal Astronomical Society》2001,321(3):385-397
The first star formation in the Universe is expected to take place within small protogalaxies, in which the gas is cooled by molecular hydrogen. However, if massive stars form within these protogalaxies, they may suppress further star formation by photodissociating the H2 . We examine the importance of this effect by estimating the time-scale on which significant H2 is destroyed. We show that photodissociation is significant in the least massive protogalaxies, but becomes less so as the protogalactic mass increases. We also examine the effects of photodissociation on dense clumps of gas within the protogalaxy. We find that while collapse will be inhibited in low-density clumps, denser ones may survive to form stars. 相似文献
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We study the prospects for observing H2 emission during the assembly of primordial molecular cloud kernels. The primordial molecular cloud cores, which resemble those at the present epoch, can emerge around 1+ z ∼20 according to recent numerical simulations. The kernels form inside the cores, and the first stars will appear inside the kernels. A kernel typically contracts to form one of the first generation stars with an accretion rate that is as large as ∼0.01 M⊙ yr−1 . This occurs owing to the primordial abundances, which result in a kernel temperature of order 1000 K, and the collapsing kernel emits H2 line radiation at a rate ∼1035 erg s−1 . Predominantly J =5-3 ( v =0) rotational emission of H2 is expected. At redshift 1+ z ∼20 , the expected flux is ∼0.01 μJy for a single kernel. While an individual object is not observable by any facilities available in the near future, the expected assembly of primordial star clusters on subgalactic scales can result in fluxes at the sub-mJy level. This is marginally observable with ASTRO-F and ALMA. We also examine the rotational J =2-0 ( v =0) and vibrational δv =1 emission lines. The former may possibly be detectable with ALMA. 相似文献
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J. S.Lazendic J. B.Whiteoak I.Klamer P. D.Harbison T. B. H.Kuiper 《Monthly notices of the Royal Astronomical Society》2002,331(4):969-974
Positions with subarcsecond accuracy have been measured for seven 22-GHz H2 O masers associated with H ii regions in the Large Magellanic Cloud (LMC); two of the masers are new detections. Initial position measurements were obtained with the 70-m antenna of the Canberra NASA Deep Space Network during a period of more than two years in which the antenna was used to monitor the maser emission. The positions were further improved using 22-GHz observations involving three antennas of the Australia Telescope Compact Array.
The positions have been compared with those of 1.6-GHz continuum emission and other LMC masers (of OH and CH3 OH). The H2 O maser positions range from within 1 arcsec (270 mpc) of the centre of a compact H ii component to beyond the boundary of significant continuum emission. Three of the four masers located near continuum peaks are close to OH masers. In two cases the positional agreement is better than 0.2 arcsec (53 mpc); in the third case the agreement is worse (0.9 arcsec) but the positions of the individual H2 O features appear to be spread over more than 1 arcsec. The velocities of the OH masers are within the spread of the H2 O velocities. The three H2 O masers offset from continuum centres are located 3–7 arcsec from optical or infrared phenomena probably associated with very early stages of star formation; no other molecular masers are known in these directions. 相似文献
The positions have been compared with those of 1.6-GHz continuum emission and other LMC masers (of OH and CH
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C. Codella R. Bachiller M. Benedettini P. Caselli 《Monthly notices of the Royal Astronomical Society》2003,341(2):707-716
The Cepheus A star-forming region has been investigated through a multiline H2 S and SO2 survey at millimetre wavelengths. Large-scale maps and high-resolution line profiles reveal the occurrence of several outflows. Cep A East is associated with multiple mass-loss processes: in particular, we detect a 0.6-pc jet-like structure which shows for the first time that the Cep A East young stellar objects are driving a collimated outflow moving towards the south.
The observed outflows show different clumps associated with definitely different H2 S/SO2 integrated emission ratios, indicating that the gas chemistry in Cepheus A has been altered by the passage of shocks. H2 S appears to be more abundant than SO2 in high-velocity clumps, in agreement with chemical models. However, we also find quite small H2 S linewidths, suggestive of regions where the evaporated H2 S molecules had enough time to slow down but not to freeze out on to dust grains. Finally, comparison between the line profiles indicates that the excitation conditions increase with the velocity, as expected for a propagation of collimated bow shocks. 相似文献
The observed outflows show different clumps associated with definitely different H
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Richard Saunders Rüdiger Kneissl Keith Grainge William F. Grainger Michael E. Jones Alessia Maggi Rhiju Das † Alastair C. Edge Anthony N. Lasenby G. G. Pooley Shigeru J. Miyoshi Taisuke Tsuruta Koujun Yamashita Yuzuru Tawara Akihiro Furuzawa Akihiro Harada Izamu Hatsukade 《Monthly notices of the Royal Astronomical Society》2003,341(3):937-940
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Stephen P. Todd Suzanne K. Ramsay Howat 《Monthly notices of the Royal Astronomical Society》2006,367(1):238-246
The results of B -band CCD imaging linear polarimetry obtained for stars from the Hipparcos catalogue are used to re-examine the distribution of the local interstellar medium towards the IRAS 100-μm emission void in the Lupus dark clouds. The analysis of the obtained parallax–polarization diagram assigns to the dark cloud Lupus 1 a distance between 130 and 150 pc and assures the existence of a low column density region coincident with the observed infrared void. Moreover, there are clear indications of the existence of absorbing material at distances closer than 60–100 pc, which may be associated with the interface boundary between the Local Bubble and its neighbourhood Loop I superbubble. 相似文献
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We calculate analytically and numerically the distance–redshift equation in perfect fluid quintessence models and give an accurate fit to the numerical solutions for all the values of the density parameter and the quintessence equation of state. Then we apply our solutions to the estimation of H 0 from multiple image time delays and find that the inclusion of quintessence modifies significantly the likelihood distribution of H 0 , generally reducing the best estimate with respect to a pure cosmological constant. Marginalizing over the other parameters (Ω m and the quintessence equation of state) we obtain H 0 =71±6 km s−1 Mpc−1 for an empty beam and H 0 =64±4 km s−1 Mpc−1 for a filled beam. These errors, however, do not take into account the uncertainty on the modelling of the lens. We also discuss the future prospects for distinguishing quintessence from a cosmological constant with time delays. 相似文献
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