The present study solves the transfer equations for the polarized radiation of astrophysical 22-GHz water masers in the presence of a magnetic field which causes a Zeeman splitting that is much smaller than the spectral line breadth. The emphasis is placed on the relationship between the recently detected circular polarization in this maser radiation and the strength of the magnetic field. When the observed spectral line breadth is smaller than about 0.8 km/s (FWHM), it is calculated that the uncertainty is less than a factor of about 2. The accuracy is improved significantly when the angle between the line of sight and the direction of the magnetic field does not exceed about 45 deg. Uncertainty in the strength of the magnetic field due to lack of knowledge about which hyperfine transition is the source of the 22-GHz masers is removed. The 22-GHz maser feature is found to be the result of a merger of the three strongest hyperfine components.
{"title":"The Zeeman effect in astrophysical water masers and the observation of strong magnetic fields in regions of star formation","authors":"G. Nedoluha, W. D. Watson","doi":"10.1086/170862","DOIUrl":"https://doi.org/10.1086/170862","url":null,"abstract":"The present study solves the transfer equations for the polarized radiation of astrophysical 22-GHz water masers in the presence of a magnetic field which causes a Zeeman splitting that is much smaller than the spectral line breadth. The emphasis is placed on the relationship between the recently detected circular polarization in this maser radiation and the strength of the magnetic field. When the observed spectral line breadth is smaller than about 0.8 km/s (FWHM), it is calculated that the uncertainty is less than a factor of about 2. The accuracy is improved significantly when the angle between the line of sight and the direction of the magnetic field does not exceed about 45 deg. Uncertainty in the strength of the magnetic field due to lack of knowledge about which hyperfine transition is the source of the 22-GHz masers is removed. The 22-GHz maser feature is found to be the result of a merger of the three strongest hyperfine components.","PeriodicalId":9423,"journal":{"name":"Bulletin of the American Astronomical Society","volume":"41 1","pages":"1371"},"PeriodicalIF":0.0,"publicationDate":"1992-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80043198","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A thorough reanalysis of the Einstein Observatory Deep Survey data is presented in order to determine the contribution of detected, discrete sources of X-ray emission to the cosmic X-ray background. Substantial discrepancies with previously published work on this problem are found. A detailed discussion of data editing and source algorithms buttresses a claim of having constructed a complete, flux-limited sample of the faintest sources detectable with the Einstein imaging proportional counter, the most sensitive X-ray instrument yet flown. A total of 33 sources is found in a survey region of about 3.3 sq deg down to a minimum flux threshold of 4 x 10 to the 14th ergs/sq cm/s in the 0.3-3.5 keV band. Roughly 30 percent of the objects are foreground stars, leading to an extragalactic source surface density of 70,000/sr at this threshold. The integrated contribution from discrete sources to the number of cosmic X-ray background photons measured in this same band with the same instrument is 12 percent + or - 3 percent, substantially below previous estimates. Implications of these results for the origin of the background are discussed.
{"title":"Faint X-ray source counts and the origin of the X-ray background","authors":"T. Hamilton, D. Helfand, X. Wu","doi":"10.1086/170529","DOIUrl":"https://doi.org/10.1086/170529","url":null,"abstract":"A thorough reanalysis of the Einstein Observatory Deep Survey data is presented in order to determine the contribution of detected, discrete sources of X-ray emission to the cosmic X-ray background. Substantial discrepancies with previously published work on this problem are found. A detailed discussion of data editing and source algorithms buttresses a claim of having constructed a complete, flux-limited sample of the faintest sources detectable with the Einstein imaging proportional counter, the most sensitive X-ray instrument yet flown. A total of 33 sources is found in a survey region of about 3.3 sq deg down to a minimum flux threshold of 4 x 10 to the 14th ergs/sq cm/s in the 0.3-3.5 keV band. Roughly 30 percent of the objects are foreground stars, leading to an extragalactic source surface density of 70,000/sr at this threshold. The integrated contribution from discrete sources to the number of cosmic X-ray background photons measured in this same band with the same instrument is 12 percent + or - 3 percent, substantially below previous estimates. Implications of these results for the origin of the background are discussed.","PeriodicalId":9423,"journal":{"name":"Bulletin of the American Astronomical Society","volume":"51 1","pages":"974"},"PeriodicalIF":0.0,"publicationDate":"1991-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83434752","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Computer-enhanced galaxy images reveal underlying spiral structures with three symmetric arms that extend between ∼25% and ∼85% of the outer radii of the two-arm spirals. Many of the galaxies containing multiple arms or other complex spirals are clear superpositions of simple two- and three-arm structures. The three-arm spirals appear to extend exactly from the inner to the outer 3:1 resonances in most cases, and the more prominent two-arm spirals begin outside the inner 2:1 resonance and extend to the outer 2:1 resonance
{"title":"Optical Tracers of Spiral Wave Resonances in Galaxies: II. Hidden Three-Arm Spirals in a Sample of 18 Galaxies","authors":"B. Elmegreen, D. Elmegreen, L. Montenegro","doi":"10.1086/191643","DOIUrl":"https://doi.org/10.1086/191643","url":null,"abstract":"Computer-enhanced galaxy images reveal underlying spiral structures with three symmetric arms that extend between ∼25% and ∼85% of the outer radii of the two-arm spirals. Many of the galaxies containing multiple arms or other complex spirals are clear superpositions of simple two- and three-arm structures. The three-arm spirals appear to extend exactly from the inner to the outer 3:1 resonances in most cases, and the more prominent two-arm spirals begin outside the inner 2:1 resonance and extend to the outer 2:1 resonance","PeriodicalId":9423,"journal":{"name":"Bulletin of the American Astronomical Society","volume":"13 1","pages":"1454"},"PeriodicalIF":0.0,"publicationDate":"1991-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"78826915","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1991-09-01DOI: 10.1007/978-94-011-3158-2_14
J. Beall
{"title":"The Effect of Relativistic Particle Beams on the Evolution of Supernova Envelopes: Self-Consistent Solutions","authors":"J. Beall","doi":"10.1007/978-94-011-3158-2_14","DOIUrl":"https://doi.org/10.1007/978-94-011-3158-2_14","url":null,"abstract":"","PeriodicalId":9423,"journal":{"name":"Bulletin of the American Astronomical Society","volume":"16 1","pages":"187-205"},"PeriodicalIF":0.0,"publicationDate":"1991-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76717146","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
R. Albrecht, C. Barbieri, J. Blades, A. Boksenberg, P. Crane
The first observations of a solar system target with the Faint Object Camera of the HST are reported. Observations of the Pluto-Charon system were obtained in f/96 and f/288 mode. Pluto and Charon were clearly resolved, and the observed separation and diameters are in accordance with expectations. The f/96 data were astrometrically and photometrically analyzed; preliminary results are presented.
{"title":"First Results from the Faint Object Camera: High Resolution Imaging of the Pluto-Charon System","authors":"R. Albrecht, C. Barbieri, J. Blades, A. Boksenberg, P. Crane","doi":"10.1086/186073","DOIUrl":"https://doi.org/10.1086/186073","url":null,"abstract":"The first observations of a solar system target with the Faint Object Camera of the HST are reported. Observations of the Pluto-Charon system were obtained in f/96 and f/288 mode. Pluto and Charon were clearly resolved, and the observed separation and diameters are in accordance with expectations. The f/96 data were astrometrically and photometrically analyzed; preliminary results are presented.","PeriodicalId":9423,"journal":{"name":"Bulletin of the American Astronomical Society","volume":"5 1","pages":"1279"},"PeriodicalIF":0.0,"publicationDate":"1991-06-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89003577","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Widths of slump terraces of complex craters can be used to determine the effective cohesion of the cratered region during crater collapse. We have measured terrace widths for complex craters on Mercury: these generally increase outward toward the rim for a given crater, and the width of the outermost major terrace is generally an increasing function of crater diameter. Similar observations apply to lunar complex craters, but the widths of the outermost slump terraces of Mercurian complex craters are less than those of similarly sized lunar complex craters. Using the terrace widths on Mercury and a gravity-driven slump model, we estimate the strength of the cratered region immediately after impact (specifically, during the modification stage of crater formation) to be ∼1–2 MPa. Comparison with the previous study of lunar complex craters by Pearce and Melosh indicates that the transient strength of cratered Mercurian crust is no greater than that of the Moon. The strength estimates only vary slightly with the geometric model used to restore the outermost major terrace to its precollapse configuration and are consistent with independent strength estimates from the simple-to-complex crater depth/diameter transition, in particular, the most recent depth/diameter study of Mercurian craters by Pike. Thus, contrary to previous work, the difference in “target properties” between Mercury and the Moon may be small, and systematic morphological differences between craters on the two worlds may be largely caused by the factor of 2 difference in surface gravity.
{"title":"Terrace Width Variations in Complex Mercurian Craters, and the Transient Strength of Cratered Mercurian and Lunar Crust","authors":"A. Leith, W. McKinnon","doi":"10.1029/91JE02248","DOIUrl":"https://doi.org/10.1029/91JE02248","url":null,"abstract":"Widths of slump terraces of complex craters can be used to determine the effective cohesion of the cratered region during crater collapse. We have measured terrace widths for complex craters on Mercury: these generally increase outward toward the rim for a given crater, and the width of the outermost major terrace is generally an increasing function of crater diameter. Similar observations apply to lunar complex craters, but the widths of the outermost slump terraces of Mercurian complex craters are less than those of similarly sized lunar complex craters. Using the terrace widths on Mercury and a gravity-driven slump model, we estimate the strength of the cratered region immediately after impact (specifically, during the modification stage of crater formation) to be ∼1–2 MPa. Comparison with the previous study of lunar complex craters by Pearce and Melosh indicates that the transient strength of cratered Mercurian crust is no greater than that of the Moon. The strength estimates only vary slightly with the geometric model used to restore the outermost major terrace to its precollapse configuration and are consistent with independent strength estimates from the simple-to-complex crater depth/diameter transition, in particular, the most recent depth/diameter study of Mercurian craters by Pike. Thus, contrary to previous work, the difference in “target properties” between Mercury and the Moon may be small, and systematic morphological differences between craters on the two worlds may be largely caused by the factor of 2 difference in surface gravity.","PeriodicalId":9423,"journal":{"name":"Bulletin of the American Astronomical Society","volume":"106 1","pages":"1202"},"PeriodicalIF":0.0,"publicationDate":"1991-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76737327","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Selected long-slit spectrograms of Comet P/Halley, photographed during the postperihelion period between April 16 and June 3, 1910 have been quantitatively analyzed for comparison with the 1985/86 apparition. The Fe-V-Na spark spectrum was used to determine characteristic curves for these plates, and flux calibrations based on contemporaneous broad-band photometric measurements were applied. Haser model production rates for C2 and values Afp were computed for each of the nights selected and compared with the photometric observations obtained by Schleicher et al. (1987) during the comparable postperihelion period of the 1985/86 apparition. As found for the 1985/86 apparation, the gas and dust production in 1910 varied in phase, but the rates of production were higher than observed in 1986 by a small but statistically significant amount.
{"title":"Gas and dust production by Comet P/Halley (1910 II)","authors":"E. Howell, B. Lutz, V. M. Slipher","doi":"10.1086/169942","DOIUrl":"https://doi.org/10.1086/169942","url":null,"abstract":"Selected long-slit spectrograms of Comet P/Halley, photographed during the postperihelion period between April 16 and June 3, 1910 have been quantitatively analyzed for comparison with the 1985/86 apparition. The Fe-V-Na spark spectrum was used to determine characteristic curves for these plates, and flux calibrations based on contemporaneous broad-band photometric measurements were applied. Haser model production rates for C2 and values Afp were computed for each of the nights selected and compared with the photometric observations obtained by Schleicher et al. (1987) during the comparable postperihelion period of the 1985/86 apparition. As found for the 1985/86 apparation, the gas and dust production in 1910 varied in phase, but the rates of production were higher than observed in 1986 by a small but statistically significant amount.","PeriodicalId":9423,"journal":{"name":"Bulletin of the American Astronomical Society","volume":"165 1","pages":"1097"},"PeriodicalIF":0.0,"publicationDate":"1991-04-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83320999","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Spherically symmetric model atmospheres for late-type giant stars","authors":"P. D. Bennett","doi":"10.14288/1.0085580","DOIUrl":"https://doi.org/10.14288/1.0085580","url":null,"abstract":"","PeriodicalId":9423,"journal":{"name":"Bulletin of the American Astronomical Society","volume":"60 1","pages":"919"},"PeriodicalIF":0.0,"publicationDate":"1991-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89130269","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1991-01-01DOI: 10.1007/978-94-011-3306-7_137
W. Vacca, P. Conti
{"title":"Wolf-Rayet Galaxies","authors":"W. Vacca, P. Conti","doi":"10.1007/978-94-011-3306-7_137","DOIUrl":"https://doi.org/10.1007/978-94-011-3306-7_137","url":null,"abstract":"","PeriodicalId":9423,"journal":{"name":"Bulletin of the American Astronomical Society","volume":"5 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"1991-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90868878","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1991-01-01DOI: 10.1007/978-1-4612-2988-9_6
E. Dwek
{"title":"Infrared Emission from SN 1987A: Light Echoes or Dust Formation?","authors":"E. Dwek","doi":"10.1007/978-1-4612-2988-9_6","DOIUrl":"https://doi.org/10.1007/978-1-4612-2988-9_6","url":null,"abstract":"","PeriodicalId":9423,"journal":{"name":"Bulletin of the American Astronomical Society","volume":"118 1","pages":"54-65"},"PeriodicalIF":0.0,"publicationDate":"1991-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77436310","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
京公网安备 11010802042870号
京ICP备2023020795号-1
扫码关注我们
求助内容：
应助结果提醒方式：