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1.
The Two Micron Galactic Survey (TMGS) is the most sensitive large scale point source near infrared survey of the Galactic Plane yet attempted. The Galactic plane has been drift-scanned in several regions over the areas -5° <l < 30°, |b | 15° and 30° <l < 180°, |b | 5°. The survey is complete in the areas covered to magnitudem k = +9.8. So far, a total area of 255 square degrees has been mapped and 500000 objects have been detected, the majority of these in the Galactic plane and with no visible counterparts. In this contribution we use data from the TMGS to probe the star distribution within the Galactic disk.  相似文献   

2.
In this paper, the fourth in a series, we examine again one of the implications of the Lin‐Shu density‐wave theory, specifically, the noncircular systematic motion of the Galactic objects. Our previous investigation is extended by analyzing simultaneously both the line‐of‐sight and transversal velocities of a sample of open clusters for which velocities, distances and ages are available. The ordinary equations of the Oort‐Lindblad theory of galactic differential rotation are used. The minor effects caused by the two‐dimensional tightly‐wound density waves are also taken into account. The published data of 242 currently known optically visible clusters having distances r < 3 kpc from the Sun and ‐200 < z < 200 pc from the Galactic plane, and ages 2 × 108 < t < 2 × 109 yr are collected from Dias et al. (2014), excluding extremely far, high‐velocity, young and old objects in our fitting. The most noteworthy result is the fact that the parameters of Lin–Shu type density waves estimated from two independent line‐of‐sight and transversal along the Galactic longitude velocities are nearly equal. We argue that the resemblance of these Galactic wave structures is so remarkable that no doubt is felt as to the theory's truth with respect to these data. The results obtained allow us to conclude that several low‐m trailing density‐wave patterns with different number of spiral arms m (say, m = 1, 2, 3, and 4), pitch angles (about 5°, 8°, 11°, and 14°, respectively) and amplitudes of the perturbed gravitational potential may coexist in the Galaxy. The latter suggests the asymmetric multiarm, not well‐organized (“flocculent”) spiral structure of the system. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

3.
The combination of dispersion measures of pulsars, distances from the model of Cordes & Lazio (2002) and emission measures from the WHAM survey enabled a statistical study of electron densities and filling factors of the diffuse ionized gas (DIG) in the Milky Way. The emission measures were corrected for absorption and contributions from beyond the pulsar distance. For a sample of 157 pulsars at |b | > 5. and 60° < ℓ < 360°, located in mainly interarm regions within about 3 kpc from the Sun, we find that: (1) The average volume filling factor along the line of sight and the mean density in ionized clouds are inversely correlated: ( ) = (0.0184 ± 0.0011) –1.07 ± 0.03 for the ranges 0.03 < < 2 cm–3 and 0.8 > > 0.01. This relationship is very tight. The inverse correlation of and causes the well‐known constancy of the average electron density along the line of sight. As (z ) increases with distance from the Galactic plane |z |, the average size of the ionized clouds increases with |z |. (2) For |z| < 0.9 kpc the local density in clouds n c(z ) and local filling factor f (z ) are inversely correlated because the local electron density n e(z ) = f (z )n c(z ) is constant. We suggest that f (z ) reaches a maximum value of >0.3 near |z | = 0.9 kpc, whereas n c(z ) continues to decrease to higher |z |, thus causing the observed flattening in the distribution of dispersion measures perpendicular to the Galactic plane above this height. (3) For |z | < 0.9 kpc the local distributions n c(z ), f (z ) and (z ) have the same scale height which is in the range 250 < h ≲ 500 pc. (4) The average degree of ionization of the warm atomic gas (z ) increases towards higher |z | similarly to (z ). Towards |z | = 1 kpc, (z ) = 0.24 ± 0.05 and (z ) = 0.24 ± 0.02. Near |z | = 1 kpc most of the warm, atomic hydrogen is ionized. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

4.
We compare theW velocity dispersions of Brosche, Schwan & Schwarz (2001) with more recent results. The increase with the distance |z | from the galactic plane is confirmed, although perhaps with reduced amplitude. This could be interpreted either as one homogenous population or as a superposition of (at least) two populations with two constant dispersions and two scale heights. For each of the possibilities we propose a simple model. Combined with two observational variants for the velocity variation, we obtain surface densities up to |z | = 250 pc ranging from 5 to 44 M⊙ pc–2. Thus the case for considerable dark matter in the neighbourhood of the galactic plane is not supported  相似文献   

5.
This paper presents an analysis of the first 2MASS (The Two Micron All Sky Survey) sampler data as observed at lower Galactic latitude in our Galaxy. These new near-infrared data provide insight into the structure of the thin disk of our Galaxy, The interpretation of star counts and color distributions of stars in the near-infrared with the synthetic stellar population model, gives strong evidence that the Galactic thin disk density scale length,h R , is rather short (2.7 ± 0.1 kpc).  相似文献   

6.
A connection between climate and the Solar system's motion perpendicular to the Galactic plane during the last 200 Myr years is studied. An imprint of galactic dynamics is found in a long‐term record of the Earth's climate that is consistent with variations in the Solar system oscillation around the Galactic midplane. From small modulations in the oscillation frequency of Earth's climate the following features of the Galaxy along the Solar circle can be determined: 1) the mass distribution, 2) the timing of two spiral arm crossings (31 Myr and 142 Myr) 3) Spiral arm/interarm density ratio (ρ arm/ρ interarm ≈ 1.5–1.8), and finally, using current knowledge of spiral arm positions, a pattern speed of ΩP = 13.6 ± 1.4 km s–1 kpc–1 is determined. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

7.
Astrometric Very Long Baseline Interferometry (VLBI) observations of maser sources in the Milky Way are used to map the spiral structure of our galaxy and to determine fundamental parameters such as the rotation velocity (Θ0) and curve and the distance to the Galactic center (R0). Here, we present an update on our first results, implementing a recent change in the knowledge about the Solar motion. It seems unavoidable that the IAU recommended values for R0 and Θ0 need a substantial revision. In particular the combination of 8.5 kpc and 220 km s–1 can be ruled out with high confidence. Combining the maser data with the distance to the Galactic center from stellar orbits and the proper motion of Sgr A* gives best values of R0 = 8.3 ± 0.23 kpc and Θ0 = 239 or 246±7 km s–1, for Solar motions of V⊙ = 12.23 and 5.25 km s–1, respectively. Finally, we give an outlook to future observations in the Bar and Spiral Structure Legacy (BeSSeL) survey (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

8.
We present results from a large scale R-band survey of the nearby Leo I group to measure the luminosity function down to dwarf spheroidal luminosities and surface brightnesses. This program has utilized>7 square degrees of imaging with the Mosaic camera at the KPNO 0.9mand extensive, on-going spectroscopic follow-up. We estimate our completeness from simulations to be 90% at μR(0) ≃ 24.5, M R ≃ –11.Leo I is a nearby, low-density group for which a steep faint-end slope has been suggested (Ferguson and Sandage, 1991).We find the luminosity function to be more similar to that of the Local Group at the faint-end (flat).We also find an unusual gap in the luminosity function at intermediate luminosities, –19.5 < M R < –16. This gap cannot be explained by pure Poisson fluctuations around a typical Schechter function, nor is it likely to result from photometric incompleteness. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

9.
The Tycho-2 proper motions and Tycho-2 and 2MASS photometry are used to select 97348 red giant clump (RGC) stars. The interstellar extinction and photometric distance are calculated for each of the stars. The selected stars are shown to form a selection-unbiased sample of RGC stars within about 350 pc of the Sun with the addition of more distant stars. The distribution of the selected stars in space and their motion are consistent with the assumption that the RGC contains Galactic disk stars with various ages and metallicities, including a significant fraction of stars younger than 1 Gyr with masses of more than 2M ⊙. These young stars show differences of their statistical characteristics from those of older RGC stars, including differences in the variations of their distribution density with distance from the Galactic plane and in the dispersion of their velocities found using radial velocities and proper motions. The Sun has been found to rise above the Galactic plane by 13 ± 1 pc. The distribution density of the stars under consideration in space is probably determined by the Local Spiral Arm and the distribution of absorbing matter in the plane of the Gould Belt.  相似文献   

10.
An attempt is made to determine the scale height of the thin Galactic disk in the solar neighborhood using various stellar samples from the accurate astrometric data of the Hipparcos satellite. Using the Tycho color indices and considering the completeness of the samples, several samples were selected from the main sequence and the horizontal branch. Based on two rather complete samples, it is found that the scale height of the Galactic disk defined by the O-B type main sequence stars is 103.1 ± 3.0 pc and the Sun is located at height of 15.2 ± 7.3 pc above its mean plane; while the scale height of the Galactic disk defined by the horizontal branch stars is 144.0 the ±10.0 pc and the Sun is located at height 3.5 ± 5.4 pc above the mean plane. Owing to the incompleteness of observational samples, it is not possible to use the main sequence stars of spectral types A, F, G, K and M to make a reliable determination of the scale height.  相似文献   

11.
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12.
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13.
We present a method which separates field dwarfs and field giants by their 2MASS and V apparent magnitudes. This method is based on spectroscopically selected standards and is hence reliable.We applied it to stars in two fields, SA 54 and SA 82, and we estimated a full set of Galactic model parameters for giants including their total local space density. Our results are in agreement with the ones given in the recent literature. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

14.
Red clump giant (RCG) stars can be used as distance indicators to trace the mass distribution of the Galactic bar. We use RCG stars from 44 bulge fields from the OGLE-II microlensing collaboration data base to constrain analytic triaxial models for the Galactic bar. We find the bar major-axis is oriented at an angle of 24°–27° to the Sun–Galactic Centre line-of-sight. The ratio of semimajor and semiminor bar axis scalelengths in the Galactic plane   x 0, y 0  , and vertical bar scalelength z 0, is   x 0 :  y 0 :  z 0= 10 : 3.5 : 2.6  , suggesting a slightly more prolate bar structure than the working model of Gerhard which gives the scalelength ratios as   x 0 :  y 0 :  z 0= 10 : 4 : 3  .  相似文献   

15.
We review and discuss horizontal branch (HB) stars in a broad astrophysical context, including both variable and non-variable stars. A reassessment of the Oosterhoff dichotomy is presented, which provides unprecedented detail regarding its origin and systematics. We show that the Oosterhoff dichotomy and the distribution of globular clusters in the HB morphology-metallicity plane both exclude, with high statistical significance, the possibility that the Galactic halo may have formed from the accretion of dwarf galaxies resembling present-day Milky Way satellites such as Fornax, Sagittarius, and the LMC—an argument which, due to its strong reliance on the ancient RR Lyrae stars, is essentially independent of the chemical evolution of these systems after the very earliest epochs in the Galaxy’s history. Convenient analytical fits to isochrones in the HB type–[Fe/H] plane are also provided. In this sense, a rediscussion of the second-parameter problem is also presented, focusing on the cases of NGC 288/NGC 362, M13/M3, the extreme outer-halo globular clusters with predominantly red HBs, and the metal-rich globular clusters NGC 6388 and NGC 6441. The recently revived possibility that the helium abundance may play an important role as a second parameter is also addressed, and possible constraints on this scenario discussed. We critically discuss the possibility that the observed properties of HB stars in NGC 6388 and NGC 6441 might be accounted for if these clusters possess a relatively minor population of helium-enriched stars. A technique is proposed to estimate the HB types of extragalactic globular clusters on the basis of integrated far-UV photometry. The importance of bright type II Cepheids as tracers of faint blue HB stars in distant systems is also emphasized. The relationship between the absolute V magnitude of the HB at the RR Lyrae level and metallicity, as obtained on the basis of trigonometric parallax measurements for the star RR Lyr, is also revisited. Taking into due account the evolutionary status of RR Lyr, the derived relation implies a true distance modulus to the LMC of (m–M)0=18.44±0.11. Techniques providing discrepant slopes and zero points for the M V (RRL)–[Fe/H] relation are briefly discussed. We provide a convenient analytical fit to theoretical model predictions for the period change rates of RR Lyrae stars in globular clusters, and compare the model results with the available data. Finally, the conductive opacities used in evolutionary calculations of low-mass stars are also investigated. M. Catelan is John Simon Guggenheim Memorial Foundation Fellow.  相似文献   

16.
Using reasonable assumptions, we derive the distribution function of pulsar space velocities from the distribution of pulsar heights above the Galactic plane. We find that for 137 pulsars of the north Galactic hemisphere, the space velocity distribution is discrete, and forms two separate groups. We estimate the velocity ranges and the mean pulsar velocity of each group.Translated from Astrofizika, Vol. 37, No. 2, pp. 245–254, April–June, 1994.  相似文献   

17.
The space motions of Mira variables are derived from radial velocities, Hipparcos proper motions and a period–luminosity relation. The previously known dependence of Mira kinematics on the period of pulsation is confirmed and refined. In addition, it is found that Miras with periods in the range 145–200 d in the general Solar neighbourhood have a net radial outward motion from the Galactic Centre of 75±18 km s−1. This, together with a lag behind the circular velocity of Galactic rotation of 98±19 km s−1, is interpreted as evidence for an elongation of their orbits, with their major axes aligned at an angle of ∼17° with the Sun–Galactic Centre line, towards positive Galactic longitudes. This concentration seems to be a continuation to the Solar circle and beyond of the bar-like structure of the Galactic bulge, with the orbits of some local Miras probably penetrating into the bulge. These conclusions are not sensitive to the distance scale adopted. A further analysis is given of the short-period (SP) red group of Miras discussed in companion papers in this series. In Appendix A the mean radial velocities and other data for 842 oxygen-rich Mira-like variables are tabulated. These velocities were derived from published optical and radio observations.  相似文献   

18.
Under the two initial 1‐D one parameter velocity distribution forms (one is normal, the other is exponential), the z direction scale height evolution of normal neutron stars in the Galaxy is studied by numerical simulation. We do statistics for the cases at different time segments, also do statistics for the cumulative cases made of each time segment. The results show in the cumulative cases the evolution curves of the scale heights are smoother than in the each time segment, i.e., the cumulation improve the signal‐to‐noise ratio. Certainly the evolution cases are different at different Galactic disk locations, which also have very large difference from the average cases in the whole disk. In the initial stages of z evolution of normal neutron stars, after the beginning transient states, the cumulative scale heights increase linearly with time, and the cumulative scale height increasing rates have linear relationship with the initial velocity distribution parameters, which have larger fluctuation in the vicinity of the Sun than in the whole disk. We utilize the linear relationship of the cumulative scale height increasing rates vs. the initial velocity distribution parameters in the vicinity of the Sun to make comparison with the observation near the Sun. The results show if there is no magnetic decay, then the deserved initial velocity parameters are obvious lower than the present well known results from some authors; whereas if introducing magnetic decay, for the 1‐D normal case we can make consistence among concerning results using magnetic decay time values which are supported by some authors, while for the 1‐D exponential case the results show the lackness of young pulsar samples in the larger z in the vicinity of the Sun (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

19.
As stars close to the galactic centre have short orbital periods it has been possible to trace large fractions of their orbits in the recent years. Previously the data of the orbit of the star S2 have been fitted with Keplerian orbits corresponding to a massive black hole (MBH) with a mass of MBH = 3–4 × 106M⊙ implying an insignificant cusp mass. However, it has also been shown that the central black hole resides in a ∼1″ diameter stellar cluster of a priori unknown mass. In a spherical potential which is neither Keplerian nor harmonic, orbits will precess resulting in inclined rosetta shaped trajectories on the sky. In this case, the assumption of non‐Keplerian orbits is a more physical approach. It is also the only approach through which cusp mass information can be obtained via stellar dynamics of the cusp members. This paper presents the first exemplary modelling efforts in this direction. Using positional and radial data of star S2, we find that there could exist an unobserved extended mass component of several 105M⊙ forming a so‐called ‘cusp’ centered on the black hole position. Considering only the fraction of the cusp mass Mequation/tex2gif-inf-4.gif within the apo‐center of the S2 orbit we find as an upper limit that Mequation/tex2gif-inf-6.gif/(MBH + Mequation/tex2gif-inf-9.gif) ≤ 0.05. A large extended cusp mass, if present, is unlikely to be composed of sub‐solar mass constituents, but could be explained rather well by a cluster of high M/L stellar remnants, which we find to form a stable configuration. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

20.
The Parkes–MIT–NRAO (PMN) radio survey has been used to generate a quasi all-sky study of Galactic Supernova Remnants (SNRs) at a common frequency of 4.85 GHz (λ=6 cm). We present flux densities estimated for the sample of 110 Southern Galactic SNRs (up to δ=−65°) observed with the Parkes 64-m radio telescope and an additional sample of 54 from the Northern PMN (up to δ=+64°) survey undertaken with the Green Bank 43-m (20 SNRs) and 91-m (34 SNRs) radio telescopes. Out of this total sample of 164 selected SNRs (representing 71% of the currently 231 known SNRs in the Green catalogue) we consider 138 to provide reliable estimates of flux density and surface brightness distribution. This sub-sample represents those SNRs which fall within carefully chosen selection criteria which minimises the effects of the known problems in establishing reliable fluxes from the PMN survey data. Our selection criteria are based on a judicious restriction of source angular size and telescope beam together with careful evaluation of fluxes on a case by case basis. Direct comparison of our new fluxes with independent literature values gives excellent overall agreement. This gives confidence in the newly derived PMN fluxes when the selection criteria are respected. We find a sharp drop off in the flux densities for Galactic SNRs beyond 4 Jy and then a fairly flat distribution from 5 to 9 Jy, a slight decline and a further flat distribution from 9 to 20 Jy though the numbers of SNR in each Jy bin are low. We also re-visit the contentious Σ–D (radio surface brightness–SNRs diameter) relation to determine a new power law index for a sub-sample of shell type SNRs which yields β=−2.2±0.6. This new evaluation of the Σ–D relation, applied to the restricted sample, provides new distance estimates and their Galactic scale height distribution. We find a peak in the SNR distribution between 7–11 kpc with most restricted to ±100 pc Galactic scale height.  相似文献   

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