A statistical study of the distribution of y-ray sources

Keyword(s):

Surface Density ◽

Statistical Study ◽

Galactic Plane ◽

The Sun ◽

The Galaxy ◽

Latitude Distribution

Data from the COS-B satellite have enabled discrete sources of cosmic y-rays to be identified. We wish to estimate the contribution that such sources make to the y-ray luminosity of the Galaxy (see Protheroe et al . 1979; Rothenflug & Caraveo 1980). Since only the brightest, and hence relatively near, sources are known, only the contribution of sources to the local y-ray emissivity can be determined from them. The distances to most of the sources in the second COS-B catalogue (Hermsen 1980) are not known so that neither their mean luminosity, nor their surface density, on the galactic plane can be determined accurately. The latitude distribution of sources indicates that their distance from the Sun, r , is much greater than their distance from the galactic plane, z . We can therefore calculate the product without knowing the distances of the sources.

Wolf-Rayet Binaries: Evolutionary Causes for their Distribution in the Galaxy

10.1017/s0252921100088540 ◽

1984 ◽

Vol 80 ◽

pp. 175-190

Author(s):

Bambang Hidayat ◽

A. Gunawan Admiranto ◽

Karel A. Van Der Hucht

Keyword(s):

Galactic Plane ◽

Small Mass ◽

The Sun ◽

Relative Importance ◽

Galactocentric Distance ◽

Evolutionary Scenario ◽

The Galaxy ◽

Binary Evolution ◽

Mass Functions ◽

Galactic Distribution

AbstractOn the basis of the most recent data, the fraction of known Wolf-Rayet binaries is 0.22. In the solar neighbourhood (d < 2.5 kpc) this fraction is 0.34In order to assess the relative importance of massive binary evolution as one of the ways to produce WR stars, the galactic distribution of WR binaries is compared with that of single WR stars using improved intrinsic parameters and new data for the fainter WR stars.In the galactic plane the increase of the binary frequency with galactocentric distance is confirmed.In a direction perpendicular to the galactic plane it is demonstrated at all distances from the Sun that the single-line spectroscopic WR binaries with small mass functions have definitely larger |z|-distances than the ‘single’ WR stars and the WR binaries with massive companions. This is consistent with the evolutionary scenario for massive binaries summarized by van den Heuvel (1976). Among the ‘single’ WR stars the fraction of those with large |z|-distances is increasing with galactocentric distance, like the fraction of the known binaries. This implies that among the high-ļzļ ‘single’ WR stars as well as among the WR stars with lower |z|-values many binaries are still to be discovered.The total WR binary frequency in the Galaxy could be well above 50 %.

A New Value for the Rotation Speed of the Spiral Structure

10.1017/s0252921100067683 ◽

1977 ◽

Vol 45 ◽

pp. 293-296 ◽

Author(s):

J. Palouš

Keyword(s):

Surface Density ◽

Galactic Center ◽

Local Density ◽

Outer Part ◽

Density Variation ◽

The Sun ◽

Basic Model ◽

Rr Lyrae ◽

The Galaxy ◽

Nearby Stars

The basic model of our Galaxy, like the Schmidt (1965) model, obeys the density law ρ(R) for the Galaxy based on divers evidence, less or better known from observation. The interpretation of the interstellar hydrogen radio profiles yields the rotation curve and the run of the force component in the radial direction. The Oort constants A, B known from radial velocities and proper motions of nearby stars, the distance from the Sun to the galactic center Roestablished from the distances of RR Lyrae stars, the local density and density gradients in the vicinity of the Sun, known from the star counts, are involved in this basic model of the Galaxy. The r.m.s. velocity component in the z direction yields the approximate mass distribution in this direction. The model surface density is computed by integrating the density along the z direction in the model. The local surface density in the Schmidt model is 114 solar masses per pc2; it depends rather strongly on the assumed density variation in the outer part of the Galaxy.

The spiral potential of the Milky Way

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833755 ◽

2018 ◽

Vol 619 ◽

pp. A50 ◽

Cited By ~ 6

Author(s):

P. Grosbøl ◽

G. Carraro

Keyword(s):

Radial Velocity ◽

Galactic Center ◽

Galactic Plane ◽

Small Mass ◽

The Sun ◽

Star Forming ◽

Star Forming Regions ◽

The Galaxy ◽

Best Fit ◽

Armed Spiral

Context. The location of young sources in the Galaxy suggests a four-armed spiral structure, whereas tangential points of spiral arms observed in the integrated light at infrared and radio wavelengths indicate that only two arms are massive. Aims. Variable extinction in the Galactic plane and high light-to-mass ratios of young sources make it difficult to judge the total mass associated with the arms outlined by such tracers. The current objective is to estimate the mass associated with the Sagittarius arm by means of the kinematics of the stars across it. Methods. Spectra of 1726 candidate B- and A-type stars within 3◦ of the Galactic center (GC) were obtained with the FLAMES instrument at the VLT with a resolution of ≈6000 in the spectral range of 396–457 nm. Radial velocities were derived by least-squares fits of the spectra to synthetic ones. The final sample was limited to 1507 stars with either Gaia DR2 parallaxes or main-sequence B-type stars having reliable spectroscopic distances. Results. The solar peculiar motion in the direction of the GC relative to the local standard of rest (LSR) was estimated to U⊙ = 10.7 ± 1.3kms−1. The variation in the median radial velocity relative to the LSR as a function of distance from the sun shows a gradual increase from slightly negative values near the sun to almost 5 km s−1 at a distance of around 4 kpc. A sinusoidal function with an amplitude of 3.4 ± 1.3kms−1 and a maximum at 4.0 ± 0.6 kpc inside the sun is the best fit to the data. A positive median radial velocity relative to the LSR around 1.8 kpc, the expected distance to the Sagittarius arm, can be excluded at a 99% level of confidence. A marginal peak detected at this distance may be associated with stellar streams in the star-forming regions, but it is too narrow to be associated with a major arm feature. Conclusions. A comparison with test-particle simulations in a fixed galactic potential with an imposed spiral pattern shows the best agreement with a two-armed spiral potential having the Scutum–Crux arm as the next major inner arm. A relative radial forcing dFr ≈ 1.5% and a pattern speed in the range of 20–30 km s−1 kpc−1 yield the best fit. The lack of a positive velocity perturbation in the region around the Sagittarius arm excludes it from being a major arm. Thus, the main spiral potential of the Galaxy is two-armed, while the Sagittarius arm is an inter-arm feature with only a small mass perturbation associated with it.

High Velocity Clouds in the Galaxy

10.1017/s0074180900108496 ◽

1995 ◽

Vol 164 ◽

pp. 129-132

Author(s):

Felix J. Lockman

Keyword(s):

High Velocity ◽

Large Scale ◽

Galactic Plane ◽

Spectral Lines ◽

Galactic Rotation ◽

The Sun ◽

The Galaxy ◽

High Velocity Clouds ◽

Major Review ◽

Lower Limits

Early observers measuring 21 cm HI profiles away from the Galactic plane found not only the emission near zero velocity expected from gas in the immediate vicinity of the Sun, but also occasional emission at velocities reaching several hundred km s−1. It seemed unlikely that these spectral lines could come from gas in normal galactic rotation (they are sometimes found at |b| > 45°), and so began the puzzle of “high-velocity clouds” (HVCs). The early result that all HVCs had negative velocity implying that they were infalling was soon shown to be incorrect with the discovery of many positive velocity clouds in the southern hemisphere. Attempts to determine the distance to HVCs by searching for them in absorption against stars yielded only lower limits, typically > 1 kpc. By 1984 several large-scale surveys had established that a significant fraction of the sky was covered with high velocity HI (e.g., Oort, 1966; Giovanelli, 1980). A recent major review is by Wakker (1991a; see also van Woerden, 1993). For this brief presentation to a specialized audience, I will concentrate on issues that may be relevant to the topic of stellar populations.

The Problem of the UV Interstellar Absorption Band at 2200A

10.1017/s025292110012024x ◽

1977 ◽

Vol 43 ◽

pp. 26-26

Author(s):

D.J. Carnochan ◽

K. Nandy ◽

A.J. Willis ◽

R. Wilson

Keyword(s):

Galactic Plane ◽

Interstellar Extinction ◽

Extinction Curve ◽

Absorption Feature ◽

The Sun ◽

The Galaxy ◽

Satellite Experiment ◽

Using Data ◽

Broad Scale ◽

Pronounced Peak

The ultraviolet interstellar extinction curve from 2740Å to 1350Å has been obtained using data from the S2/68 satellite experiment. The extinction increases into the ultraviolet and shows a pronounced peak at 2200Å. This is interpreted as a general scattering continuum with a strong absorption feature superposed on it at 2200Å. The profile of the feature appears to be symmetrical and has a half-width of 360Å. There is a strong correlation between the strength of the feature and the scattering part of the curve in both the ultraviolet and the visible. On a broad scale the shape of the extinction curve is constant showing no variation with distance from the sun, direction around the galaxy, and height above the galactic plane.

Stellar encounters with giant molecular clouds

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz813 ◽

2019 ◽

Vol 489 (4) ◽

pp. 5165-5180 ◽

Cited By ~ 4

Author(s):

Giorgi Kokaia ◽

Melvyn B Davies

Keyword(s):

Molecular Clouds ◽

Galactic Plane ◽

Oort Cloud ◽

The Sun ◽

Habitable Zone ◽

Giant Molecular Clouds ◽

Thick Disc ◽

The Galaxy ◽

Pass Through ◽

Earth’S Climate

ABSTRACTGiant molecular clouds (GMCs) are believed to affect the biospheres of planets as their host star passes through them. We simulate the trajectories of stars and GMCs in the Galaxy and determine how often stars pass through GMCs. We find a strong decreasing dependence with Galactocentric radius, and with the velocity perpendicular to the Galactic plane, V$\mathrm{ z}$. The XY-component of the kinematic heating of stars was shown to not affect the GMC hit rate, unlike the Z-dependence (V$\mathrm{ z}$) implies that stars hit fewer GMCs as they age. GMCs are locations of star formation, therefore we also determine how often stars pass near supernovae. For the supernovae the decrease with V$\mathrm{ z}$ is steeper as how fast the star passes through the GMC determines the probability of a supernova encounter. We then integrate a set of Sun-like trajectories to see the implications for the Sun. We find that the Sun hits 1.6 ± 1.3 GMCs per Gyr which results in 1.5 ± 1.1 or (with correction for clustering) 0.8 ± 0.6 supernova closer than 10 pc per Gyr. The different the supernova frequencies are from whether one considers multiple supernovae per GMC crossing (few Myr) as separate events. We then discuss the effect of the GMC hits on the Oort cloud, and the Earth’s climate due to accretion, we also discuss the records of distant supernova. Finally, we determine Galactic Habitable Zone using our model. For the thin disc, we find it to lie between 5.8 and 8.7 kpc and for the thick disc to lie between 4.5 and 7.7 kpc.

6. The distribution of atomic hydrogen in the inner parts of the Galaxy

10.1017/s0074180900048816 ◽

1957 ◽

Vol 4 ◽

pp. 37-41

Author(s):

M. Schmidt

Keyword(s):

Radial Velocity ◽

Atomic Hydrogen ◽

Galactic Centre ◽

The Sun ◽

The Galaxy ◽

Latitude Distribution ◽

Inner Parts

The determination of the distribution of hydrogen from 21-cm. observations in parts of the Galaxy, which are nearer to the centre than the sun, is seriously handicapped by the fact that the observed radial velocity of the hydrogen clouds determines only the distance to the galactic centre. So two possible values of the distance to the sun correspond to one value of the frequency. We have used as a criterion to separate the contributions from the two regions the latitude distribution of the radiation.

The Local Bubble and Beyond: Summary

10.1017/s0252921100071621 ◽

1997 ◽

Vol 166 ◽

pp. 563-580 ◽

Author(s):

Christopher F. McKee

Keyword(s):

Interstellar Medium ◽

Galactic Plane ◽

Low Density ◽

The Sun ◽

Local Bubble ◽

Current State ◽

The Galaxy ◽

Theoretical Discovery ◽

Gaseous Halo

Lyman Spitzer, Jr, the founder of modern studies of the interstellar medium (ISM), passed away March 31, 1997. This conference occurred shortly thereafter and is dedicated to his memory. While many of his contributions underlie the work that was discussed at this meeting, one paper stands out in particular: his theoretical “discovery” of the hot gaseous halo of the Galaxy based on the need to confine the clouds of H I observed above the Galactic plane (Spitzer 1956). We now know that much of the ISM within about 100 pc of the Sun is largely filled by very low density gas, which is generally inferred to be hot, and as a result this region is termed the Local Bubble (Cox and Reynolds 1987). This conference was convened to establish the current state of our knowledge of the Local Bubble, both observational and theoretical, and its relation to the rest of the ISM. Because it is nearby, the Local Bubble is a laboratory for interstellar astrophysics, making the dedication to Spitzer’s memory particularly appropriate.

On the Color Excesses of Globular Clusters

10.1017/s0074180900042959 ◽

1988 ◽

Vol 126 ◽

pp. 529-530

Author(s):

V. Straižys ◽

R. Janulis

Keyword(s):

Globular Cluster ◽

Globular Clusters ◽

Galactic Center ◽

Galactic Plane ◽

The Other ◽

The Sun ◽

General Direction ◽

Dust Layer ◽

Color Indices ◽

The Galaxy

The interstellar reddening of globular clusters of the Galaxy is still an important unresolved problem, especially for metal-rich objects that are found usually at low galactic latitudes in the general direction of the galactic center. Their color excesses are needed in order to correct their color-magnitude diagrams and to determine their intrinsic integrated color indices. For this we need some method which is not related to measures of the cluster stars. One such method is to use foreground field stars in the direction of the globular cluster to measure the interstellar reddening. Because most of the globular clusters lie outside the galactic plane, we need information about the reddening in all the layer of absorbing dust in different directions. This information can be obtained by investigating stars which are at different distances from the Sun up to the edge of the absorbing dust layer. On the other hand, these stars should be as close as possible to the position of the globular cluster to avoid possible variations in the interstellar reddening in the area of the cluster.

Asymmetries in the Galactic Distribution of Pulsars

10.1017/s0074180900093219 ◽

1981 ◽

Vol 95 ◽

pp. 439-443 ◽

Author(s):

Alice K. Harding

Keyword(s):

Electron Density ◽

Galactic Plane ◽

Dispersion Measure ◽

High Electron ◽

The Sun ◽

High Electron Density ◽

Spiral Arms ◽

The Galaxy ◽

Using Data ◽

Galactic Distribution

The distribution of pulsars in galactocentric radius and z distance has been determined for opposite halves of the Galaxy, using data on 328 pulsars from three surveys. The distributions in galactocentric radius are found to be significantly different at positive and negative longitudes, although both show strong peaks between 5 and 6 kpc. There is also some indication that pulsars are located preferentially along spiral arms. Distributions in the z component of dispersion measure above and below the galactic plane also show asymmetry, with higher dispersion occurring at negative z. This may imply the existence of a narrow (~ 100 pc), high electron density layer below the plane of the Sun in the inner galaxy.