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(1999) Walsh, Andrew; Burton, Michael; Hyland, A; Robinson, GarryJournal Article
(1998) Walsh, Andrew; Burton, Michael; Robinson, Garry; Nyland, AJournal Article
(1999) Walsh, Andrew; Burton, Michael; Hyland, A; Robinson, GarryJournal ArticleA survey towards a selection of 35 methanol maser and/or ultracompact (UC) H ii regions, reported in Papers I and II and by Norris et al., has been conducted in the near-infrared (NIR). Out of 25 methanol maser sites surveyed, 12 are associated with a NIR counterpart. Out of 18 UC H ii regions (8 of which overlap with maser emission), 12 are associated with a NIR counterpart. Counterparts can be confidently identified not only by the positional agreements, but also by their unusually red colours. Spectral types for the embedded stars can be unambiguously determined for six sources, all of which imply massive, ionizing stars. One of these infrared sources has methanol maser emission, but no UC H ii region. It is possible that the maser emission associated with this source arises from a pre-UC H ii phase of massive stellar evolution or it could be that nearly all the ultraviolet photons are absorbed by dust within the UC H ii region. We have modelled the spectral energy distributions (SEDs) for some sources and find that a single blackbody can be used to estimate the stellar luminosity, but cannot represent the whole infrared SED. A two-component blackbody model and a radiative transfer model were also used to derive essential parameters of the infrared sources. The radiative transfer model also indicates which infrared sources are relatively young and which are older. Both models show that silicate absorption at 9.7 μm must be a dominant feature of these SEDs.
Studies of ultracompact H II regions - II. High-resolution radio continuum and methanol maser survey(1998) Walsh, Andrew; Burton, Michael; Hyland, A; Robinson, GarryJournal ArticleHigh spatial resolution radio continuum and 6.67-GHz methanol spectral line data are presented for methanol masers previously detected by Walsh et al. (1997). Methanol maser and/or radio continuum emission is found in 364 cases towards IRAS-selected regions. For those sources with methanol maser emission, relative positions have been obtained to an accuracy of typically 0.05 arcsec, with absolute positions accurate to around 1 arcsec. Maps of selected sources are provided. The intensity of the maser emission does not seem to depend on the presence of a continuum source. The coincidence of water and methanol maser positions in some regions suggests there is overlap in the requirements for methanol and water maser emission to be observable. However, there is a striking difference between the general proximity of methanol and water masers to both cometary and irregularly shaped ultracompact (UC) H ii regions, indicating that, in other cases, there must be differing environments conducive to stimulating their emission. We show that the methanol maser is most likely present before an observable UC H ii region is formed around a massive star and is quickly destroyed as the UC H ii region evolves. There are 36 out of 97 maser sites that are linearly extended. The hypothesis that the maser emission is found in a circumstellar disc is not inconsistent with these 36 maser sites, but is unlikely. It cannot, however, account for all other maser sites. An alternative model which uses shocks to create the masing spots can more readily reproduce the maser spot distributions.
(1998) Stolovy, S; Burton, Michael; Erickson, E; Kaufman, Michael; Chrysostomou, A; Young, E; Colgan, Sean; Axon, D; Thompson, Roger; Rieke, M; Schneider, GJournal ArticleThe core of OMC-1 has been imaged with the Near-Infrared Camera and Multiobject Spectrometer (NICMOS) on board the Hubble Space Telescope at 0 2 resolution in 1% filters at 2.12 and 2.15 μm, revealing intricate structures in both continuum and shocked molecular hydrogen line emission. Numerous H -emitting clumps surrounding the BN/KL region have been resolved for the first time, several of which exhibit prototypical bow-shock morphologies with -shaped tips. We interpret these to be lower excitation analogs of similar structures 2 northwest of the core observed with ground-based telescopes. Many of the elongated H structures and bow-shock features appear to radiate outward from a region within a few arcseconds of radio source I, which suggests that the H energetics are dominated by one or more outflow sources in this region. However, the orientations of some features are unrelated to this apparent outflow pattern. The deeply embedded, suspected outflow source I remains undetected at 2 μm, although two faint new continuum sources have been detected within 1 of it. The newly resolved H features with bow-shock morphologies are located in regions previously identified as bow shocks by highly blueshifted components in their line profiles. In contrast, regions of H emission that are diffuse in the NICMOS image have broad, smooth line profiles. Several continuum features have an arclike appearance, which suggests interactions of winds with the ambient medium. At least 40 stellar or protostellar continuum sources have been detected, including at least one proplyd and four pairs of binary stars.
(1998) Stolovy, S; Young, E; Thompson, Roger; Rieke, M; Schneider, G; Burton, Michael; Erickson, E; Kaufman, Michael; Colgan, Sean; Chrysostomou, A; Axon, DConference PaperThe core of OMC--1 has been imaged with NICMOS on the Hubble Space Telescope at 0.2'' resolution in 1% filters at 2.12 and 2.15mu m, revealing intricate structures in both continuum and shocked molecular hydrogen line emission. Numerous H_2-emitting clumps surrounding the BN/KL region have been resolved for the first time, several of which exhibit prototypical bow-shock morphologies with V-shaped tips. We interpret these to be lower-excitation analogs of similar structures ~ 2' NW of the core observed with ground based telescopes. Many of the elongated H_2 structures and bow-shock features appear to radiate outward from a region within a few arcseconds of radio source 'I', suggesting that the H_2 energetics are dominated by one or more outflow sources in this region. However, the orientations of some features are unrelated to this apparent outflow pattern. The deeply embedded, suspected outflow source `I' remain s undetected at 2 mu m, although two faint new continuum sources have be en detected within ~ 1'' of it. The newly resolved H_2 features with bow-shock morphologies are located in regions previously identified as bow-shocks by highly blueshifted components in their line profiles. Several continuum features have an arc-like appearance, suggesting interactions of winds with the ambient medium. At least 40 stellar or protostellar continuum sources have been detected, including at least one proplyd and four pairs of binary stars.
(1997) Walsh, Andrew; Hyland, A; Robinson, Garry; Burton, MichaelJournal Article
(1995) Zhao, Yong; Cadogan, John; Campbell, StewartConference Paper
(1997) Picard, Dominique; Hall, Peter; Penev, Spiridon; Kerkyacharian, GerardJournal ArticleUsually, methods for thresholding wavelet estimators are implemented term by term, with empirical coefficients included or excluded depending on whether their absolute values exceed a level that reflects plausible moderate deviations of the noise. We argue that performance may be improved by pooling coefficients into groups and thresholding them together. This procedure exploits the information that coefficients convey about the sizes of their neighbours. In the present paper we show that in the context of moderate to low signal-to-noise ratios, this lsquoblock thresholdingrsquo approach does indeed improve performance, by allowing greater adaptivity and reducing mean squared error. Block thresholded estimators are less biased than term-by-term thresholded ones, and so react more rapidly to sudden changes in the frequency of the underlying signal. They also suffer less from spurious aberrations of Gibbs type, produced by excessive bias. On the other hand, they are more susceptible to spurious features produced by noise, and are more sensitive to selection of the truncation parameter.