From the observed random velocities of 269 stars of spectral type F the relation for the increase of the velocity dispersion upon age σ∼tα is obtained. Both the value α=0.2–0.3 for star’s velocities in the plane of our Galaxy and the value α=0.4–0.6 in vertical direction are determined.Oscillations and stability of stellar‐giant molecular clouds disk of the Galaxy with account taken on binary collisions are considered. Kinetic equation with a model Bhatnagar‐Gross‐Krook collisional integral and the Poisson equation are the basis for our investigation. A dispersion relation is derived to describe the dynamics of small perturbations in the plane of a differentially rotating disk. It is shown: In parts of a colliding two‐component system that are at the boundary of gravitational stability (Jeans’ stability) a specific dissipative instability may grow. As a result, this dissipative instability can increase the dispersion of the random velocities of stars, as the observations have shown.
{"title":"Relaxation and stability of the disk of our Galaxy","authors":"E. Griv","doi":"10.1063/1.43957","DOIUrl":"https://doi.org/10.1063/1.43957","url":null,"abstract":"From the observed random velocities of 269 stars of spectral type F the relation for the increase of the velocity dispersion upon age σ∼tα is obtained. Both the value α=0.2–0.3 for star’s velocities in the plane of our Galaxy and the value α=0.4–0.6 in vertical direction are determined.Oscillations and stability of stellar‐giant molecular clouds disk of the Galaxy with account taken on binary collisions are considered. Kinetic equation with a model Bhatnagar‐Gross‐Krook collisional integral and the Poisson equation are the basis for our investigation. A dispersion relation is derived to describe the dynamics of small perturbations in the plane of a differentially rotating disk. It is shown: In parts of a colliding two‐component system that are at the boundary of gravitational stability (Jeans’ stability) a specific dissipative instability may grow. As a result, this dissipative instability can increase the dispersion of the random velocities of stars, as the observations have shown.","PeriodicalId":310353,"journal":{"name":"Back to the Galaxy","volume":"98 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2008-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"122650956","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}
G. Amici, G. Smoot, M. Bensadoun, M. Limon, W. Vinje, C. Witebsky, S. Torres, A. Umaña, M. Becerra
Uncertainties in the gain and zero‐level of the existing radio and microwave frequency sky surveys often dominate the error budget in studies of anisotropies and spectral distortions of the Cosmic Background Radiation.This paper discusses our existing prototype and the planned instruments and the observation techniques which are necessary for conducting an extended observational campaign to map the total sky brightness. Some experimental difficulties are outlined, and possible solutions are given.
{"title":"Mapping the absolute brightness of the sky at low frequencies","authors":"G. Amici, G. Smoot, M. Bensadoun, M. Limon, W. Vinje, C. Witebsky, S. Torres, A. Umaña, M. Becerra","doi":"10.1063/1.44014","DOIUrl":"https://doi.org/10.1063/1.44014","url":null,"abstract":"Uncertainties in the gain and zero‐level of the existing radio and microwave frequency sky surveys often dominate the error budget in studies of anisotropies and spectral distortions of the Cosmic Background Radiation.This paper discusses our existing prototype and the planned instruments and the observation techniques which are necessary for conducting an extended observational campaign to map the total sky brightness. Some experimental difficulties are outlined, and possible solutions are given.","PeriodicalId":310353,"journal":{"name":"Back to the Galaxy","volume":"30 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2008-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"124565443","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}
We present results of numerical simulations on the dynamical evolution of a two‐component plasma system consisting of charged dust grains and oppositely charged low‐mass ions. We consider the case that the electrostatic and the gravitational forces are similar in magnitude (charge‐to‐mass ratio is of order of square root of the gravitational constant). Wollman (1988, Phys. Rev. A37, 3052) showed that such case is possible in some situations and that the gravitational condensation of the system can explain many observed properties of our galaxy. Morphological changes of the plasma system are studied by two‐dimensional particle‐in‐cell (PIC) simulation.
{"title":"Simulations on the gravito‐electrodynamical evolution of charged dust grain plasma: A model of galaxy formations","authors":"G. Lee","doi":"10.1063/1.44032","DOIUrl":"https://doi.org/10.1063/1.44032","url":null,"abstract":"We present results of numerical simulations on the dynamical evolution of a two‐component plasma system consisting of charged dust grains and oppositely charged low‐mass ions. We consider the case that the electrostatic and the gravitational forces are similar in magnitude (charge‐to‐mass ratio is of order of square root of the gravitational constant). Wollman (1988, Phys. Rev. A37, 3052) showed that such case is possible in some situations and that the gravitational condensation of the system can explain many observed properties of our galaxy. Morphological changes of the plasma system are studied by two‐dimensional particle‐in‐cell (PIC) simulation.","PeriodicalId":310353,"journal":{"name":"Back to the Galaxy","volume":"788 ","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2008-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"120876323","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}
During the Astro‐1 space shuttle mission of December 1990, the Hopkins Ultraviolet Telescope was used to carry out an absorption study of the local interstellar medium (LISM). Through EUV observations of the hot DA white dwarf stars G191‐B2B and HZ43, neutral hydrogen and neutral helium column densities have been determined along two lines of sight through the local interstellar cloud surrounding the Sun. The neutral hydrogen to helium ratios observed (in comparison to the cosmic abundance ratio of the two elements) provide an assessment of the relative ionization of the two species in the LISM. We find, in contrast to some previous indirect determinations, that hydrogen is not preferentially ionized compared with helium in the LISM and thus, that exotic sources of ionization are not required to explain the ionization state of the local cloud.
{"title":"Observations of the local interstellar medium with the Hopkins Ultraviolet Telescope","authors":"R. Kimble, A. Davidsen, K. Long, P. Feldman","doi":"10.1063/1.44009","DOIUrl":"https://doi.org/10.1063/1.44009","url":null,"abstract":"During the Astro‐1 space shuttle mission of December 1990, the Hopkins Ultraviolet Telescope was used to carry out an absorption study of the local interstellar medium (LISM). Through EUV observations of the hot DA white dwarf stars G191‐B2B and HZ43, neutral hydrogen and neutral helium column densities have been determined along two lines of sight through the local interstellar cloud surrounding the Sun. The neutral hydrogen to helium ratios observed (in comparison to the cosmic abundance ratio of the two elements) provide an assessment of the relative ionization of the two species in the LISM. We find, in contrast to some previous indirect determinations, that hydrogen is not preferentially ionized compared with helium in the LISM and thus, that exotic sources of ionization are not required to explain the ionization state of the local cloud.","PeriodicalId":310353,"journal":{"name":"Back to the Galaxy","volume":"16 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2008-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"130907335","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}
We present high‐resolution (30‘;0.3 pc) 12CO(2−1), 13CO(2−1), and C18O(2−1) observations of the 7’×5’ region comprising the W3‐Main molecular cloud core. An analysis of velocity channel‐maps of 13CO optical depth and column density reveals seven major clumps, ranging in mass from 180 M⊙ to 965 M⊙. The total H2 mass of the mapped region is ∼6500 M⊙; 58% of the total mass of the mapped region is contained within these seven major clumps, and 26% within just two clumps, one centered on IRS‐4 and the other 30‘ south of that position. The IRS‐4South clump is coincident with a recently discovered peak in submillimeter (350–800 μm) continuum emission, but appears not to be associated with previously identified IRS or radio continuum sources. IRS‐4South is likely the newest center of massive star formation activity in W3‐Main.
{"title":"Massive clumps in the W3-Main cloud core","authors":"J. Deane, E. Ladd, D. Sanders","doi":"10.1063/1.43997","DOIUrl":"https://doi.org/10.1063/1.43997","url":null,"abstract":"We present high‐resolution (30‘;0.3 pc) 12CO(2−1), 13CO(2−1), and C18O(2−1) observations of the 7’×5’ region comprising the W3‐Main molecular cloud core. An analysis of velocity channel‐maps of 13CO optical depth and column density reveals seven major clumps, ranging in mass from 180 M⊙ to 965 M⊙. The total H2 mass of the mapped region is ∼6500 M⊙; 58% of the total mass of the mapped region is contained within these seven major clumps, and 26% within just two clumps, one centered on IRS‐4 and the other 30‘ south of that position. The IRS‐4South clump is coincident with a recently discovered peak in submillimeter (350–800 μm) continuum emission, but appears not to be associated with previously identified IRS or radio continuum sources. IRS‐4South is likely the newest center of massive star formation activity in W3‐Main.","PeriodicalId":310353,"journal":{"name":"Back to the Galaxy","volume":"276 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2008-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132884497","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}
G. Berriman, M. Hauser, N. Boggess, T. Kelsall, S. Moseley, R. Silverberg, T. Murdock
This paper describes polarimetry of the Galactic plane and the zodiacal light, surveyed by the COBE Diffuse Infrared Background Experiment. Methods of analysis and the overall polarization characteristics of the diffuse sky are emphasized over interpretation. Results are illustrated by polarimetry at 2.2 μm obtained from 1990 August 5–12, when most of the Galactic plane was in view. Similar results are found at 1.2 μm and 3.5 μm.
{"title":"Polarimetry with the diffuse infrared background experiment aboard COBE","authors":"G. Berriman, M. Hauser, N. Boggess, T. Kelsall, S. Moseley, R. Silverberg, T. Murdock","doi":"10.1063/1.44026","DOIUrl":"https://doi.org/10.1063/1.44026","url":null,"abstract":"This paper describes polarimetry of the Galactic plane and the zodiacal light, surveyed by the COBE Diffuse Infrared Background Experiment. Methods of analysis and the overall polarization characteristics of the diffuse sky are emphasized over interpretation. Results are illustrated by polarimetry at 2.2 μm obtained from 1990 August 5–12, when most of the Galactic plane was in view. Similar results are found at 1.2 μm and 3.5 μm.","PeriodicalId":310353,"journal":{"name":"Back to the Galaxy","volume":"40 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2008-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115675998","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 thick disk of ionized gas similar to the Reynolds‐layer of the Galaxy has recently been detected in several edge‐on galaxies. We have measured line ratios of diagnostic emission lines such as [NII]/Hα, [SII]/Hα originating in this extraplanar diffuse ionized gas (DIG). In addition, we have compiled from the literature emission line ratios measured for diffuse ionized gas in nearby late‐type spiral, early‐type disk (S0/Sa), and Magellanic‐type irregular galaxies.From this compilation one trend emerges. For most objects [SII] and [NII] lines are both stronger in DIG spectra than in the emission from HII regions. It is only for DIG in Milky Way and irregular galaxies that [SII] lines are strong while [NII] line strengths are comparable to HII regions. The DIG in the Milky Way is therefore not typical for physical conditions in the diffuse ionized gas in external galaxies.The observed spread in excitation conditions allows us to confine possible ionization models. It is concluded that photoionization by a d...
{"title":"The ionization of diffuse ionized gas in galaxies: A comparison with the Reynolds‐layer in the Milky Way","authors":"R. Dettmar","doi":"10.1063/1.43989","DOIUrl":"https://doi.org/10.1063/1.43989","url":null,"abstract":"A thick disk of ionized gas similar to the Reynolds‐layer of the Galaxy has recently been detected in several edge‐on galaxies. We have measured line ratios of diagnostic emission lines such as [NII]/Hα, [SII]/Hα originating in this extraplanar diffuse ionized gas (DIG). In addition, we have compiled from the literature emission line ratios measured for diffuse ionized gas in nearby late‐type spiral, early‐type disk (S0/Sa), and Magellanic‐type irregular galaxies.From this compilation one trend emerges. For most objects [SII] and [NII] lines are both stronger in DIG spectra than in the emission from HII regions. It is only for DIG in Milky Way and irregular galaxies that [SII] lines are strong while [NII] line strengths are comparable to HII regions. The DIG in the Milky Way is therefore not typical for physical conditions in the diffuse ionized gas in external galaxies.The observed spread in excitation conditions allows us to confine possible ionization models. It is concluded that photoionization by a d...","PeriodicalId":310353,"journal":{"name":"Back to the Galaxy","volume":"432 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2008-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115934115","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}
Bars in galaxies acquire a pronounced peanut shape when seen from the side. It is shown that the peanut‐shaped bar in a 3‐D N‐body model has an appearance closely resembling the COBE image of the Milky Way bulge, when viewed from within the disk. Moreover, the projected velocity distribution is not far from isotropic, in agreement with modern kinematic data. Although the model is not a perfect match to the Milky Way, it exhibits a straightforward mechanism for the formation of the observed shape of the Milky Way ‘‘bulge’’ and provides a further argument in favor of a bar in the Galaxy.
{"title":"A bar model for the Galactic bulge","authors":"J. Sellwood","doi":"10.1063/1.44022","DOIUrl":"https://doi.org/10.1063/1.44022","url":null,"abstract":"Bars in galaxies acquire a pronounced peanut shape when seen from the side. It is shown that the peanut‐shaped bar in a 3‐D N‐body model has an appearance closely resembling the COBE image of the Milky Way bulge, when viewed from within the disk. Moreover, the projected velocity distribution is not far from isotropic, in agreement with modern kinematic data. Although the model is not a perfect match to the Milky Way, it exhibits a straightforward mechanism for the formation of the observed shape of the Milky Way ‘‘bulge’’ and provides a further argument in favor of a bar in the Galaxy.","PeriodicalId":310353,"journal":{"name":"Back to the Galaxy","volume":"90 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2008-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"114564509","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}
Near‐infrared imaging and spectroscopy have revealed a rich collection of stars whose character are now being understood as the result of several epochs of star formation. The collection of objects known as IRS16 may be intermediate temperature supergiants which need not contribute significantly to the ionizing radiation present in the central few parsecs.
{"title":"Stellar populations at the galactic center","authors":"M. Rieke","doi":"10.1063/1.43954","DOIUrl":"https://doi.org/10.1063/1.43954","url":null,"abstract":"Near‐infrared imaging and spectroscopy have revealed a rich collection of stars whose character are now being understood as the result of several epochs of star formation. The collection of objects known as IRS16 may be intermediate temperature supergiants which need not contribute significantly to the ionizing radiation present in the central few parsecs.","PeriodicalId":310353,"journal":{"name":"Back to the Galaxy","volume":"10 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2008-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"128334327","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}
We discuss a numerical algorithm based on maximum likelihood for estimating parameters for models of the Galaxy. We use simultaneously all the information available in a catalog of stellar data in a global optimization, to derive unbiased estimates of intrinsic stellar properties, such as luminosity and velocity dispersion. The likelihood function is defined in the observed domain using quantities such as photoelectric photometry, line‐of‐sight velocity, proper motion, trigonometric parallax, and metallicity. Individual stars included in the statistical analysis can have different amounts of information available. This method includes an explicit treatment of observational errors, can identify outliers objectively and allows use of stellar data with relatively large errors. It can self‐consistently detect and correct for systematic deviations in the observations, such as zero point residuals or underestimated errors.
{"title":"Galaxy model parameters using numerical maximum likelihood estimation","authors":"K. Ratnatunga, S. Casertano","doi":"10.1063/1.44002","DOIUrl":"https://doi.org/10.1063/1.44002","url":null,"abstract":"We discuss a numerical algorithm based on maximum likelihood for estimating parameters for models of the Galaxy. We use simultaneously all the information available in a catalog of stellar data in a global optimization, to derive unbiased estimates of intrinsic stellar properties, such as luminosity and velocity dispersion. The likelihood function is defined in the observed domain using quantities such as photoelectric photometry, line‐of‐sight velocity, proper motion, trigonometric parallax, and metallicity. Individual stars included in the statistical analysis can have different amounts of information available. This method includes an explicit treatment of observational errors, can identify outliers objectively and allows use of stellar data with relatively large errors. It can self‐consistently detect and correct for systematic deviations in the observations, such as zero point residuals or underestimated errors.","PeriodicalId":310353,"journal":{"name":"Back to the Galaxy","volume":"278 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2008-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"129240254","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}
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