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1.
I compute the maximum number of observable brown dwarfs for various infrared surveys by combining the maximum possible Oort limit (0.1 missing Mpc–3 with all possible brown dwarf mass and age distributions. This approach shows what limits will be placed on the contribution of brown dwarfs to any possible missing mass if no brown dwarfs are observed. I consider brown dwarfs with masses of 0.01—0.08 M and ages of 109—1010 years.The full range of predicted numbers of brown dwarfs above 6 times the noise of each of the below surveys is: IRAS Point Source Catalog 0.02—6 IRAS Faint Source Catalog |b| > 10° 0.05—16 ISO (2 week 12µm survey) 0.15—80 SIRTF (2 week 12µm survey) 2.50—1600 WIRE (4 month 12µm survey) 21.80—6000 DENIS (half sky) |b| > 10° 0.00—2000 2MASS (full sky) |b| > 10° 0.00—8000A failure to find brown dwarfs in the IRAS FSC would just barely rule out about half of the mass—age range for Oort limit total masses. A failure to find brown dwarfs in 2MASS/DENIS would rule out roughly the same mass—age range, but would set a limit of 0.1—0.01 times the Oort mass in that mass—age region. No limits would be set for the other half of the mass—age range since both IRAS and 2MASS/DENIS have insufficient sensitivity for brown dwarfs with T < 750 K.A failure to find brown dwarfs with ISO would rule out almost all of the mass—age range for Oort limit total masses, but would not set a significantly lower limit to the brown dwarf mass limit. A failure to find brown dwarfs with SIRTF or WIRE would rule out the entire mass—age range for Oort limit total masses and set an upper limit of 0.1—0.001 times the Oort mass.To date, about 18% of the IRAS FSC has been searched down to 6, and no brown dwarfs have been found. This sets a 95% upper limit of 3 in 18% of the sky, or 13 in the entire FSC for |b| > 10°. To begin to set useful limits from 2MASS or DENIS, approximately 400 square degrees needs to be analyzed. To date, only a few square degrees of results from the 2MASS prototype camera have been examined, with no brown dwarfs found so far.  相似文献   

2.
The time variation of the gravitational constantG is discussed in the light of the gravity modified form of quantum electrodynamics. From the experimental upper limit |a/| < 5 × 10–15 yr–1 on the time variation of the electromagnetic fine structure constant one finds |/G| < 5 × 10–13 yr–1.  相似文献   

3.
The 1968–2000 data on the mean magnetic field (MMF, longitudinal component) of the Sun are analysed to study long-time trends of the Sun's magnetic field and to check MMF calibration. It is found that, within the error limits, the mean intensity of photospheric magnetic field (the MMF strength, |H|), did not change over the last 33 years. It clearly shows, however, the presence of an 11-year periodicity caused by the solar activity cycle. Time variations of |H| correlate well with those of the radial component, |B r|, of the interplanetary magnetic field (IMF). This correlation (r=0.69) appears to be significantly higher than that between |B r| and the results of a potential source-surface extrapolation, to the Earth's orbit, of synoptic magnetic charts of the photosphere (using the so-called `saturation' factor –1 for magnetograph measurements performed in the line Fei 525.0 nm; Wang and Sheeley, 1995). It seems therefore that the true source surface of IMF is the `quiet' photosphere – background fields and coronal holes, like those for MMF. The average `effective' magnetic strength of the photospheric field is determined to be about 1.9 G. It is also shown that there is an approximate linear relation between |B r| and MMF intensity |H| (in gauss)|B r|(H 0)min×(1+C|H|)where =1.5×10–5 normalizes the photospheric field strength to 1 AU distance from the Sun, (H 0)min=1.2 G is some minimal `effective' intensity of photospheric background fields and C=1.3 G–1 an empirical constant. It is noted that good correlation between time variations of |H| and |B r| makes suspicious a correction of the photospheric magnetic fields with the use of saturation factor –1.  相似文献   

4.
The diffuse far UV radiation ( 1350–1480 Å) observed in the sky region ofl II180°, 0°b II40° is analyzed in connection with the distributions of stars and dust grains as well as with optical properties of grains. Its intensity (starlight+scattered light) is about 6×10–7 erg cm–2 sec–1 sr–1 Å–1 in the direction ofb II0° andl II180°. The latitude dependence of the intensity is in approximate agreement with the plane parallel slab model of the galaxy with a reasonable set of parameters. The interstellar scattering gives an albedo close to unity and forward phase function of about 0.6, which are not inconsistent with the model of interstellar grains of Wickramasinghe. The upper limit of the extragalactic UV is 2×10–8 erg cm–2 sec–1 sr–1 Å–1 in the same region of wave-length.  相似文献   

5.
The brightness distribution of diffuse soft X-rays in the pulse height range 0.15–0.3 keV (L-band) and 0.5–0.8 keV (M-band) are obtained over a quarter of the sky centered at the galactic anticenter with 1.5 m polypropylene window proportional counters on board a sounding rocket. In theL-band three enhanced regions are noticed on the map. They coincide with the northern and southern Hi holes and the inner part of the galactic radio Loop II.In the northern Hi hole theN H dependence of theL-band flux and the hardness ratioM/L can be fitted with a local hot plasma model with the absorption by a low velocity neutral hydrogen gas (|V|<25 km s–1) along the line of sight. The X-ray feature of Loop II is similar to that of Loop I. In the lowN H region (<3×1020 H atoms cm–2) theM/L value is lower than 0.3, whereas it varies in the range 0.1–0.4 at low latitudes (|b|<300). This fact seems to be interpreted in terms of a model that a number of hot plasma clouds contribute to X-ray emission.  相似文献   

6.
We deal here with the efficient starting points for Kepler's equation in the special case of nearly parabolic orbits. Our approach provides with very simple formulas that allow calculating these points on a scientific vest-pocket calculator. Moreover, srtarting with these points in the Newton's method we can calculate a root of Kepler's equation with an accuracy greater than 0.001 in 0–2 iterations. This accuracy holds for the true anomaly || 135° and |e – 1| 0.01. We explain the reason for this effect also.Dedicated to the memory of Professor G.N. Duboshin (1903–1986).  相似文献   

7.
The Monoceros ring, a circular optical nebulosity 3°.5 in diameter and centred at R.A.=6h37m, Dec.=6°30 (l ii =205°.5,b ii =0°.2) is in good structural agreement with radio observations. A neutral hydrogen shell is also accurately projected on the ring. These observations are consistent with the Monoceros ring being a supernova remnant 90–100 pc in diameter expanding at about 45 km s–1 and having an age of the order of a million years. Bright Hii regions containing early-type stars (e.g., galactic cluster NGC 2244 in the Rosette nebula) and extremely young stars of the OB association Mon OB2 lie at the edges of the ring. The positional and temporal coincidence of the Mon OB2 association with a supernova remnant suggests that probably the star formation in this region is induced or speeded up by the passage of a supernova shock wave through the clumpy interstellar medium.  相似文献   

8.
This paper summarizes and analyzes the results of radio optical, infrared, and X-ray observations of a large sector of the sky in the constellation Cygnus (19h20m-22h, =30–50°;l II=65–90°, |b II|10°). This region is associated with an extended X-ray source referred to as the Cygnus superbubble. About a quarter of the superbubble region is occupied by the extensively investigated multicomponent thermal radio source Cyg X. The region contains eight OB-associations which, when projected on the sky, duplicate the outline of the X-ray superbubble. These associations contains 110 stars of high luminosity (about 40 Wolf-Rayet and Of stars). The observations suggest that the X-ray superbubble is not a single object. Between 50 and 75% of its X-ray emission can be ascribed to discrete sources, the rest being probably due to regions of coronal gas about 100 pc in diameter, created by stellar winds and, possibly, supernova explosions in individual associations. The objects that produce the X-ray and optical radiation of the presumed superbubble are located at distances from 0.5 to 2.5 kpc from the Sun in the Carina-Cygnus spiral arm. The eastern portion of the region presumed superbubble contains the associations Cyg OB7 and Cyg OB4 and is generally less than 1 kpc distant, while the western portion contains the associations Cyg OB1, 2, 3, 8, and 9 and is 1 to 2 kpc distant.  相似文献   

9.
We study the astrophysical bounds on the change of the gravitational constant with time. We found that |/G|<10–12yr–1 is the condition that has to be satisfied in order not to cause a conflict with the observations. We find the condition to be in accord with the lower limits, the superstring theory predicts.  相似文献   

10.
A detailed investigation of the evolution of low-mass binaries is performed for the case when the secondary fills its Roche lobe at the stage of core hydrogen exhaustion. The obtained results are compared with observational data for ultra-short periodic X-ray systems MXB 1820-30 and MXB 1916-05. In the frame of the proposed evolutionary scenario it is possible to obtain for MXB 1820-30 its periodP=11.4 min twice (see Figure 2). In the first case the parameters of the system are:M 2 0.13–0.15M ,X0.05–0.13, |P/P| (3.6–6.2) } 10–7 yr–1, M2 (4.1–9.6) } 10–9 M yr–1, for the second:M 2 0.08–0.09M ,X= 0, |P/P| (1.3–1.5) } 10–7 yr–1, M2 (1.4–1.8) } 10–8 M yr–1. It is suggested that MXB 1916-05 is the progenitor of the system MXB 1820-30 (M 2 = 0.1M,X 0.221,M 2 1.8 × 10–10 M yr–1).  相似文献   

11.
Observations have been selected in Tenerife Zodiacal Light Data, which make it possible to separate the diffuse galactic light from other components. Far from the Milky Way (30°|b|60°) the diffuse galactic light shows a decreasing slope which cannot exceed –0.3S 10 vis. per degree of galactic latitude, and is probably a very weak component of the total extraterrestrial light.  相似文献   

12.
The 25-meter radio telescope of the Netherlands Foundation for Research in Astronomy in Dwingeloo has been engaged full-time during the past 5 years in a survey of galactic 21-cm emission from the entire sky accessible from the Netherlands. The new material provides coverage of the sky at -30° on a 0°.5 grid sampled with a 35 main beam, over a velocity range of 1000 km s–1 at 1 km s–1 resolution, to a limiting brightness-temperature sensitivity typically better than 0.07 K. The data have been corrected for stray radiation entering both the near and far sidelobes of the antenna.  相似文献   

13.
A theory with variable gravitational constant, based on the bimetric formulation of general relativity, is suggested. It is in agreement with the Newtonian and post-Newtonian approximations of the theory of gravitation when the theory parameter ||4×10–6. The statical spherical-symmetric distribution of gravitating masses is investigated. It is shown that the total action (including the own gravitational field of the system)S=–M, where is the time in the remote system of reference, relative to which the celestial body is in rest. In the Brans-Dicke theory ||1 andS–M. The Tolman formula for the massM in the Einstein theory is also valid in the suggestedGeneralized Bimetric Theory of Gravitation. External analytical and internal numerical solutions of the field equations are found for the case of incompressible matter. It is shown that static and supermassive configurations may exist if –0.13<0.  相似文献   

14.
A model is constructed of a spherically symmetric self-gravitating condensation of neutral hydrogen immersed in anHii region. The structure of the condensation is represented by the isothermal gas sphere at a temperature of 100°K. Typical parameters of such a condensation compatible with the estimated ultra-violet radiation field in the central regions of the Orion Nebula are, mass 1M ; radius 1016 cm; mean density 10–15 gm cm–3. The condensations are not static configurations but evolve because of mass loss by ionization from their surfaces. Perhaps 5% become gravitationally unstable and collapse. The remainder act as sources of ionized gas which flows into the surrounding nebula.  相似文献   

15.
An object located approximately atl=8°,b=–4° with a mean radial velocity of –212.3 km s–1 has been observed in the 21 cm neutral hydrogen line. The mean weighted velocity dispersion is 11.2 km s–1 and the total mass is estimated to be 190R 2 (kpc) solar masses. We discuss possible interpretations of the origin and nature of this object. The most likely interpretation is that we observe an expanding object which has been ejected from the galactic nucleus.  相似文献   

16.
Javaraiah  J. 《Solar physics》1999,189(2):289-304
We have analyzed data on sunspot groups compiled during 1874–1981 and investigated the following: (i) dependence of the `initial' meridional motion (v ini()) of sunspot groups on the life span () of the groups in the range 2–12 days, (ii) dependence of the meridional motion (v(t)) of sunspot groups of life spans 10–12 days on the age (t) of the spot groups, and (iii) variations in the mean meridional motion of spot groups of life span 2–12 days during the solar cycle. In each of the latitude intervals 0°–10°, 10°–20° and 20°–30°, the values of both v ini() and v(t) often differ significantly from zero. In the latitude interval 20°–30°, the forms of v ini() and v(t) are largely systematic and mutually similar in both the north and south hemispheres. The form of v(t) suggests existence of periodic variation in the solar meridional motion with period of 4 days and amplitude 10–20 m s–1. Using the anchoring depths of magnetic structures for spot groups of different and testimated earlier, (Javaraiah and Gokhale, 1997), we suggest that the forms of v ini() and v(t) may represent radial variation of meridional flow in the Sun's convection zone, rather than temporal variation of the flow. The meridional flows (v e(t)) determined from the data during the last few days (i.e., age t: 10–12 days) of spot groups of life spans of 10–12 days are found to have magnitudes (10–20 m s–1) and directions (poleward) similar to the those of the surface meridional plasma flows determined from the Dopplergrams and magnetograms. The mean meridional velocity of sunspot groups living 2–12 days seems to vary during the solar cycle. The velocity is not significantly different from zero during the rising phase of the cycle and there is a suggestion of equatorward motion (a few m s–1at lower latitudes and 10 m s–1at higher latitudes) during the declining phase (last few years) of the cycle. The variation during the odd numbered cycles seems to anticorrelate with the variation during the even numbered cycles, suggesting existence of 22-year periodicity in the solar meridional flow. The amplitude of the anticorrelation seems to be depending on latitude and the cycle phase. In the latitude interval 20°–30° the `surface plasma meridional motion', v e(t), is found to be poleward during maximum years (v e(t) 20 m s–1at 4th year) and equatorward during ending years of the cycle (v e(t) –17 m s–1at 10th year).  相似文献   

17.
The physical conditions under which suprathermal grains can be produced when they are accelerated by radiation pressure against the drag of ambient gas are investigated. It is found that dust grains may attain a terminal velocityU (=105 cm s–1) in most regions and move out of the midplane of the source region about a distance |z|100 pc. Once clear of the main gas/dust layer the dust grains (a3×10–6 cm) may then attain suprathermal energy (V g 3×108 cm s–1) by the Fermi process.  相似文献   

18.
We studied grain formation process and flow structure around cool luminous mass-loss stars. The nucleation and growth theory of Yamamoto and Hasegawa was extended to the case of expanding gas flow.The envelope was assumed to be steady, spherically symmetric, in thermal and radiative equilibrium, optically thin, and driven by radiation pressure on grains. For oxygen rich stars, Mg-silicate was found to be the first condensate which can drive the gas effectively. The following stellar parameters were chosen; stellar massM *=1M , effective temperatureT *=3000K, stellar luminosityL * from 7.5×103 to 2.0×104 L , and mass-loss rate |M| from 1.0×10–6 to 1.0×10–4 M yr–1.Main results of calculations are as follows; (1) grain condensation temperatureT c9801080 K; (2) total gas pressure at the condensation pointP t6×10–116×10–9 atm; (3) scale parameterA c1036×104; and (4) final grain sizer f=400Å1m. For the smaller |M| or the largerL *, these values are the smaller. We recognized two types of flow solutions (1) Dust driven flow for large |M|, which reaches the sonic point near the condensation point; and (2) Modified Parker flow for small |M| for which grain sizer f is almost independent of |M|.A comparison with observational results ofL * and gas terminal velocityV suggests that Mg-silicate grains are of submicron size, which are effective for interstellar extinction in visible and infrared. Fe-grains condense in the rarefied outflow with a size probably smaller than 100Å, which may contribute for interstellar ultraviolet extinction. The envelope has three-layer structure inner dense region with small outflow velocity, grain formation layer and outer rarefied region with fast outflow velocity.No flow solutions exist forM * greater than a critical stellar massM *cr for a given stellar luminosityL * and mass-loss rate |M|.For example, critical stellar massM *cr=1.8M forL *=104 L ,T *=3000 K, and |M|=10-5 M yr-1.  相似文献   

19.
Strong absorption satellite lines of CaI 6572 were found on spectrograms taken on three successive days just after the fourth contact of the 1971–72 eclipse of Zeta Aurigae. The radial velocities of the satellite lines are –88 km s–1, –74 km s–1, and –180 km–1, respectively, relative to the K-type primary star (K4 Ib). These absorptions should be due to a circumstellar cloud in which the column density of neutral calcium atoms is 1×1017 cm–2 and the turbulent velocities come to 20–50 km s–1. It is suggested that the cloud may be formed by the rocket-effect of the Lyman quanta of the B-type component (B6 V). We estimate the density in the cloud to be 2×1011 atoms cm–3 fors=10R K and 2×1010 atoms cm–3 fors=102 R K, wheres denotes the distance of the cloud from the K star andR K the K star's radius. The mass loss rate of the K-type component is also estimated to be about 10–7 M yr–1, assuming that the expansion of the K star occurs isotropically.  相似文献   

20.
A comparison between model atmosphere grids with and without metal blanketing has been performed in the low-temperature range (T eff=6000 K to 10000 K) and for different metal contents (logA=0, –1, –2;A=scaled solar abundance). ForA10–1 andT eff7000 K, the Johnson colour indicesU-B andB-V, together with the Strömgrenu-b, b-y and the bolometric correction (BC) are little affected by metal blanketing ( mag<0 . m 04). On the other hand, the trend of the physical quantities and them 1 andc 1 colour indices reflect the inadequateness of the models even withA=10–2 andT eff=9500 K, where hydrogen line blanketing is expected to dominate. This fact discourages once and for all the use of metal unblanketed atmospheres other than for comparison of colour indices or the calculation of bolometric corrections for Population II A-F spectral types.  相似文献   

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