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|New λ6 cm observations of the Cygnus Loop|
Radio continuum and polarization observations of the entire Cygnus Loopat λ6 cm wavelength were made with the Urumqi 25 m telescope. Theλ6 cm map is analysed together with recently published maps fromthe Effelsberg 100 m telescope at λ21 cm and λ11 cm. Theintegrated flux density of the Cygnus Loop at λ6 cm is 90± 9 Jy, which implies a spectral index of α=-0.40 ±0.06. This rules out any global spectral steepening up to λ6 cm.However, small spectral index variations in some regions of the sourceare possible, but there are no indications for any spectral curvature.The linear polarization data at λ6 cm show percentagepolarizations up to 35% in some areas of the Cygnus Loop, exceedingthose observed at λ11 cm. The Rotation Measure is around -21 radm-2 in the southern area, which agrees with previousobservations. However, the distribution of Rotation Measures is rathercomplex in the northern part of the Cygnus Loop, where the λ21 cmemission is totally depolarized. Rotation Measures based on λ11cm and λ6 cm data are significantly larger than in the southernpart. The difference in the polarization characteristics between thenorthern and southern part supports previous ideas that the Cygnus Loopconsists of two supernova remnants.
|New galactic open cluster candidates from DSS and 2MASS imagery|
An inspection of the DSS and 2MASS images of selected Milky Way regionshas led to the discovery of 66 stellar groupings whose morphologies,color-magnitude diagrams, and stellar density distributions suggest thatthese objects are possible open clusters that do not yet appear to belisted in any catalogue. For 24 of these groupings, which we consider tobe the most likely to be candidates, we provide extensive descriptionson the basis of 2MASS photometry and their visual impression on DSS and2MASS. Of these cluster candidates, 9 have fundamental parametersdetermined by fitting the color-magnitude diagrams with solarmetallicity Padova isochrones. An additional 10 cluster candidates havedistance moduli and reddenings derived from K magnitudes and (J-K) colorindices of helium-burning red clump stars. As an addendum, we alsoprovide a list of a number of apparently unknown galactic andextragalactic objects that were also discovered during the survey.
|Hubble Space Telescope Imaging of the Primary Shock Front in the Cygnus Loop Supernova Remnant|
We present Hubble Space Telescope WFPC2 narrowband Hα (F656N) and[O III] λ5007 (F502N) imaging of two fields on the northeasternlimb of the Cygnus Loop supernova remnant. This region provides anoutstanding example of the initial encounter between the primary blastwave and the surrounding interstellar medium. The Hα images showthe primary nonradiative shock front, which, when viewed edge-on, isunresolved at WFPC2 resolution. The [O III] images show portions offilaments that are beginning to become radiative, and in these imagesthe filaments are resolved, appearing fuzzy at WFPC2 resolution. The [OIII] filament regions are not bounded directly by Hα filaments,indicating that the shock emission from the nascent radiative region issufficient to fully ionize the local preshock gas. One field, imaged 4yr earlier in Hα with WFPC2, is used to study the proper motion ofthe filament and constrain any brightness variations over this timeperiod. In conjunction with improved models of nonradiative shocks,these data are used to place limits on the possible deceleration of thefilament and refine the distance to the Cygnus Loop, arriving at arevised value of d=540+100-80 pc (assuming nodeceleration). The second field imaged contains examples of coherentHα filaments with much more dramatic curvatures than identifiedpreviously. We discuss the possible reasons for the different morphologyof these filaments and conclude that they can be accommodated withrelatively modest variations in local density and shock velocity.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute (STScI), which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS5-26555.
|Radio emission from the Cygnus Loop and its spectral characteristics|
We present a new sensitive 2675 MHz radio continuum map of the CygnusLoop, which is used in conjunction with 408 MHz, 863 MHz and 1420 MHzmaps from both the Effelsberg 100-m telescope and the DRAO SynthesisTelescope for a spectral analysis. Between 408 MHz and 2675 MHz we findan overall integrated spectral index of α = -0.42 ± 0.06 (S˜ να), close to previous results. There is noindication of a spectral break in the integrated spectrum. Spatiallyhighly varying and rather strong spectral curvature was previouslyreported, but is not confirmed on the basis of new, higher sensitivityobservations. We found spectral variations across the Cygnus Loopreaching up to Δα = 0.2 from a TT-plot analysis. Theflattest spectra are seen towards enhanced emission areas. Spectralindex maps produced between different frequency pairs, as well as allfour maps, revealed that there are at least three flat spectrum regions.In regions interior to the high emission filaments, we have detected atleast two spectral components across the whole object with α=-0.4and α=-0.6 towards northern and southern parts of the object,respectively.Based on observations with the Effelsberg 100-m radio telescope operatedby the Max-Planck-Institut für Radioastronomie (MPIfR), near Bonn,Germany.
|The European Large-Area ISO Survey (ELAIS): the final band-merged catalogue|
We present the final band-merged European Large-Area ISO Survey (ELAIS)Catalogue at 6.7, 15, 90 and 175 μm, and the associated data at U,g', r', i', Z, J, H, K and 20 cm. The origin of the survey, infrared andradio observations, data-reduction and optical identifications arebriefly reviewed, and a summary of the area covered and the completenesslimit for each infrared band is given. A detailed discussion of theband-merging and optical association strategy is given. The totalCatalogue consists of 3762 sources. 23 per cent of the 15-μm sourcesand 75 per cent of the 6.7-μm sources are stars. For extragalacticsources observed in three or more infrared bands, colour-colour diagramsare presented and discussed in terms of the contributing infraredpopulations. Spectral energy distributions (SEDs) are shown for selectedsources and compared with cirrus, M82 and Arp220 starburst, and activegalactic nuclei (AGN) dust torus models.Spectroscopic redshifts are tabulated, where available. For the N1 andN2 areas, the Isaac Newton Telescope ugriz Wide Field Survey permitsphotometric redshifts to be estimated for galaxies and quasars. Theseagree well with the spectroscopic redshifts, within the uncertainty ofthe photometric method [~10 per cent in (1 +z) for galaxies]. Theredshift distribution is given for selected ELAIS bands andcolour-redshift diagrams are discussed.There is a high proportion of ultraluminous infrared galaxies(log10 of 1-1000 μm luminosity Lir > 12.22)in the ELAIS Catalogue (14 per cent of 15-μm galaxies with known z),many with Arp220-like SEDs. 10 per cent of the 15-μm sources aregenuine optically blank fields to r'= 24: these must have very highinfrared-to-optical ratios and probably have z > 0.6, so arehigh-luminosity dusty starbursts or Type 2 AGN. Nine hyperluminousinfrared galaxies (Lir > 13.22) and nine extremely redobjects (EROs) (r-K > 6) are found in the survey. The latter areinterpreted as ultraluminous dusty infrared galaxies at z~ 1. The largenumbers of ultraluminous galaxies imply very strong evolution in thestar formation rate between z= 0 and 1. There is also a surprisinglylarge population of luminous (Lir > 11.5), cool(cirrus-type SEDs) galaxies, with Lir-Lopt > 0,implying AV > 1.
|Classification of Spectra from the Infrared Space Observatory PHT-S Database|
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
|Is the Cygnus Loop two supernova remnants?|
The Cygnus Loop is classified as a middle-aged supernova remnant (SNR)located below the Galactic equator (l=74o,b=-8.6o) and 770 pc away from us. Its large size and littleconfusion with Galactic emission makes it an ideal test ground forevolutionary and structural theories of SNRs. New radio continuummapping of the Cygnus Loop at 2695 MHz with the Effelsberg 100-mtelescope provides indications that the Cygnus Loop consists of twoseparate SNRs. Combining this result with data from the literature weargue that a secondary SNR exists in the south with a recently detectedneutron star close to its center. Two interacting SNRs seem to be thebest explanation to account for the Cygnus Loop observations at allwavelengths.
|Far Ultraviolet Spectroscopic Explorer and Hopkins Ultraviolet Telescope Observations of Radiative Shocks in the Cygnus Loop|
We report a combination of high and low spectral resolutionfar-ultraviolet spectral data for several groupings of bright radiativeshock filaments in the Cygnus Loop supernova remnant. Thehigh-resolution spectra were obtained with the Far UltravioletSpectroscopic Explorer (FUSE) satellite and cover the 905-1187 Åspectral region at 0.1-0.3 Å resolution, depending on observingaperture. The low-resolution data were obtained with the HopkinsUltraviolet Telescope (HUT) and cover the 850-1850 Å spectralregion at ~3 Å resolution. These data sets complement each other,with the HUT data providing the broader context of a wider range ofemission lines and the FUSE data resolving line profiles from nearbyairglow emissions and allowing detailed line profiles to be studied.Relative line intensities and line profiles change on a wide variety ofspatial scales. O VI λλ1032, 1038 emission is much moregenerally distributed than the optical filaments. The FUSE data providedirect insight into the effects and variability of self-absorption inthe strong resonance lines of O VI and C III λ977. Variability ofthe central reversals in O VI with position indicate a patchy O VIdistribution, with self-absorption occurring locally within the remnant.Self-absorption in C III could be due to the interstellar medium, thelocal regions of the remnant, or a combination. We also find evidencefor overlying molecular hydrogen absorption effects on some lines andsuggest an interstellar (foreground) origin. It is likely that theseeffects have had significant impacts on many previous investigationsthat based their results on the interpretation of low-resolutionspectral data where these effects could not be seen. Based onobservations made with the NASA-CNES-CSA Far Ultraviolet SpectroscopicExplorer. FUSE is operated for NASA by the Johns Hopkins Universityunder NASA contract NAS5-32985.
|The rise and rise of the deep sky image|
Presidential Address to the British Astronomical Association, 2000October 25
|The first seven UK supernova discoveries|
This paper outlines the early history of amateur patrolling forsupernovae from the United Kingdom, describes the methods employed bythe patrollers, and provides a detailed description in the words of theobservers of the first seven successful discoveries, made between 1996October and 1998 April.
|The Life of a Neutron Star|
|Emission-Line Properties of the Large Magellanic Cloud Bubble N70|
We present a spectrophotometric imaging study of the emission bubble N70(DEM 301) in the Large Magellanic Cloud. N70 is approximately 100 pc insize with a nearly circular shell-like morphology. The nebular emissionis powered by an uncertain combination of EUV photons, intense winds,and supernova shock waves from the central population of high-mass stars(the OB association LH 114). We have obtained narrowband images (FWHM~6Å) of N70 in the light of Hα lambda6563, [N II] lambda6584, [SII] lambdalambda6717, 6731, and [O III] lambda5007, along with thecorresponding red and green continua. The resulting line fluxes and fluxratios are used to derive ionization rates, nebular densities, volumefilling fractions, and excitation indices. The photoionizing luminosityinferred from the embedded stellar population is more than adequate toaccount for the observed hydrogen ionization rate. We compare theemission-line photometry with that derived from similar imaging of theOrion Nebula and with data collected from the literature on otheremission-line regions in the LMC. Compared with the Orion Nebula, N70shows much higher [S II]/Hα intensity ratios that increase smoothlywith radius-from less than 0.3 near the center to greater than 1.0toward the outer filamentary shell. The measured intensity ratios in N70more closely match the range of excitation spanned by giant andsupergiant H II shells and by some of the supernova remnants observed inthe LMC. The contending ionization and excitation processes in theinterior and outer shell of N70 are evaluated in terms of the availabledata. EUV photons probably contribute most of the inner nebula'sionization, whereas a combination of photoionization plus collisionalionization and excitation of sulfur atoms by low-velocity shocks seemsto best fit the emission-line luminosities and intensity ratios observedin the outer shell. Considerations of the radiative and mechanicalenergetics that are involved may indicate the need for one or twosupernova explosions having occurred during the last ~Myr.
|Panoramic Views of the Cygnus Loop|
We present a complete atlas of the Cygnus Loop supernova remnant in thelight of [O III] (lambda5007), Hα, and [S II] (lambdalambda6717,6731). We include low-resolution (25") global maps and smaller fields at6" resolution from observations using the Prime Focus Corrector on the0.8 m telescope at McDonald Observatory. Despite its shell-likeappearance, the Cygnus Loop is not a current example of a Sedov-Taylorblast wave. Rather, the optical emission traces interactions of thesupernova blast wave with clumps of gas. The surrounding interstellarmedium forms the walls of a cavity through which the blast wave nowpropagates, including a nearly complete shell in which nonradiativefilaments are detected. We identify nonradiative shocks around half theperimeter of the Cygnus Loop, and they trace a circle of radius R = 1.4d(19 pc) in the spherical cavity walls. The Cygnus Loop blast wave is notbreaking out of a dense cloud but is instead running into confiningwalls. Modification of the shock velocity and gas temperature due tointeraction of the blast wave with the surrounding medium introduceserrors in estimates of the age of this supernova remnant. The opticalemission of radiative shocks arises only where the blast wave encountersinhomogeneities in the ambient medium; it is not a consequence ofgradual evolution to a global radiative phase. Distance measurementsthat rely on this uniform blast wave evolution are uncertain, but theradiative shocks can be used as distance indicators because of thespherical symmetry of the surrounding medium. The interstellar mediumdominates not only the appearance of the Cygnus Loop but also thecontinued evolution of the blast wave. If this is a typical example of asupernova remnant, then global models of the interstellar medium mustaccount for such significant blast wave deceleration.
|Standard Stars for CCD Photometry in the Vilnius System|
The results of seven-color photometry of 73 stars of magnitudes 9--13are given in 11 areas. The areas are of 10times 10 arcmin size at lowgalactic latitudes in the transparent parts of the Milky Way in theconstellations of Serpens, Aquila, Vulpecula, Lyra and Cygnus. Thesestars can be used as zero-point standards for future CCD photometry.
|From Swords to Supernovae|
|The ROSAT HRI X-Ray Survey of the Cygnus Loop|
We describe and report progress on the joint U.S. and German campaign tomap the X-ray emission from the entire Cygnus Loop with the ROSAT HighResolution Imager. The Cygnus Loop is the prototype for a supernovaremnant that is dominated by interactions with the interstellar mediumand supplies fundamental physical information on this basic mechanismfor shaping the interstellar medium. The global view that thesehigh-resolution (FWHM ~ 10") observations provide emphasizes theinhomogeneity of the interstellar medium and the pivotal nature ofcloud--blast-wave interactions in determining the X-ray morphology ofthe supernova remnant. While investigating the details of the evolutionof the blast wave, we also describe the interstellar medium in thevicinity of the Cygnus Loop, which the progenitor star has processed.Although we do not expect the X-ray observations to be complete until1997 September, the incomplete data combined with deep H alpha imagesprovide definitive evidence that the Cygnus Loop was formed by anexplosion within a preexisting cavity.
|Comet Hale-Bopp: Your Viewing Guide|
|Exploring southern nebulae.|
|Briefly noted: Irish astronomy (poem)|
|Nebular filters in deep sky astronomy|
Visual deep sky astronomy has been revolutionised in the last 15 yearsby two major advances. The first was the availability of large apertureportable telescopes based on the design created by John Dobson, and thesecond was the development of the deep sky filter originally popularisedby the filter sets made by Lumicon. There have been a number of reviewsof deep sky filters in the popular astronomical literature1-5 and thispaper updates some of this information. Some challenging targets arealso included for users of the more specialist filters.
|The Veil nebula.|
|A project for investigation of the stellar population of the galactic disk.|
|Bring on the photo CD revolution.|
|Glittering realms of the summer Milky Way.|
|H-alpha images of the Cygnus Loop - A new look at shock-wave dynamics in an old supernova remnant|
Attention is given to deep H-alpha images of portions of the east, west,and southwest limbs of the Cygnus Loop which illustrate several aspectsof shock dynamics in a multiphase interstellar medium. An H-alpha imageof the isolated eastern shocked cloud reveals cloud deformation and gasstripping along the cloud's edges, shock front diffraction andreflection around the rear of the cloud, and interior remnant emissiondue to upstream shock reflection. A faint Balmer-dominated filament isidentified 30 arcmin further west of the remnant's bright line ofwestern radiative filaments. This detection indicates a far morewesterly intercloud shock front position than previously realized, andresolves the nature of the weak X-ray, optical, and nonthermal radioemission observed west of NGC 6960. Strongly curved Balmer-dominatedfilaments along the remnant's west and southwest edge may indicate shockdiffraction caused by shock wave passage in between clouds.
|Der Blick in die Milchstrasse.|
|The Cygnus Loop at 408 MHz - Spectral variations, and a better overall view|
Observations of the Cygnus Loop made at 408 MHz are presented. Radiospectral variations within the remnant are revealed by comparing the newobservations with existing data taken at 2.695 GHz. The brightnortheastern arc of the Cygnus Loop has a radio spectral index alpha ofroughly 0.35, whereas the southern portion of the remnant generallyshows steeper spectra, with alpha up to roughly 0.55. There is probablya portion of the faint radio emission from the northeastern arc that hasa flatter spectrum than the brighter radio emission nearby. Thesedifferences are discussed in relation to mechanism for producing radioemission in SNRs and to the overall structure of the remnant. Theobservation confirm that there is faint radio emission outside thebright limb-brightened 'shell' of the remnant, but this is limited tothe west only. The faint halo of X-rays around the remnant is discussedand interpreted as originating from dust scattering.
|Find a supernova remnant.|
|The chaotic material between the stars.|
|The Cygnus Loop - an Older Supernova Remnant|
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