共查询到20条相似文献，搜索用时 31 毫秒
1.
Štefan Gajdoš 《Earth, Moon, and Planets》2008,102(1-4):117-123
A new meteroid stream—October Ursa Majorids—was announced by Japanese observers on Oct. 14–16, 2006 (Uehara et al. 2006).
Its weak manifestation was detected among coincidental major meteor showers (N/S Taurids, Orionids), as its meteors radiated
from a higher placed radiant on the northern sky. We have tried to find out previous displays of the stream throughout available
meteor orbits databases, and among ancient celestial phenomena records. Although we got no obvious identification, there are
some indications that it could be a meteor shower of cometary origin with weak/irregular activity, mostly overlayed by regular
coincidental meteor showers. With a procedure based on D-criterion (Southworth and Hawkins 1963) we found a few records in IAU MDC database of meteor photographic orbits which fulfill
common similarity limits, for October Ursae Majorids. However, their real association cannot be established, yet. With respect
to the mean orbit of this stream, we suggest for its parent body a long-period comet. 相似文献
2.
The calculation of theoretical meteor radiants is discussed for comets and asteroids whose orbits pass within, but at present do not necessarily intersect, that of the Earth, in particular from the perspective of developing a suitable method for application to Taurid Complex orbits. The main question addressed here is how to allow for dynamical evolution between epochs when an orbit isnot Earth-intersecting (as at present in most cases for macroscopic bodies) and those when itis (i.e., when meteors can actually be observed). This should be understood in terms of evolution in the past, such that meteoroids released some time ago have evolved differentially from the putative parents, allowing meteors to be detected now. Theoretical radiants for macroscopic Taurid objects are then presented and compared with observations of the nighttime and daytime Taurid meteor showers. These are found to be broadly similar in form, given the sparsity of some of the data, adding weight to the hypothesis that this sub-jovian complex contains kilometre-plus asteroids. A similar conclusion results for the group of objects in similar orbits to (2212) Hephaistos. 相似文献
3.
P.B. Babadzhanov 《Celestial Mechanics and Dynamical Astronomy》1997,69(1-2):221-234
Recent theoretical and observational work has shown that the asteroids belonging to the Taurid meteoroid complex have a cometary
nature. If so, then they might possess related meteoroid streams producing meteor showers in the Earth atmosphere. We studied
the orbital evolution of ten numbered Taurid complex asteroids by the Halphen-Goryachev method. It turned out that all of
these asteroids are quadruple crossers relative to the Earth's orbit. Therefore their proposed meteoroid streams may in theory
each produce four meteor showers. The theoretical orbital elements and geocentric radiants of these showers are determined
and compared with the available observational data. The existence of the predicted forty meteor showers of the ten Taurid
complex asteroids is confirmed by a search of the published catalogues of observed meteor shower radiants and orbits, and
of the archives of the IAU Meteor Data Center (Lund). The existence of meteor showers associated with the Taurid Complex Asteroids
confirms that, most likely, these asteroids are extinct comets.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
《Astroparticle Physics》2009,32(2):89-99
Atmospheric parameters, such as pressure (P), temperature (T) and density (ρ∝P/T), affect the development of extensive air showers initiated by energetic cosmic rays. We have studied the impact of atmospheric variations on extensive air showers by means of the surface detector of the Pierre Auger Observatory. The rate of events shows a 10% seasonal modulation and 2% diurnal one. We find that the observed behaviour is explained by a model including the effects associated with the variations of P and ρ. The former affects the longitudinal development of air showers while the latter influences the Molière radius and hence the lateral distribution of the shower particles. The model is validated with full simulations of extensive air showers using atmospheric profiles measured at the site of the Pierre Auger Observatory. 相似文献
5.
We used light curve analysis to search for evidence of the dustball meteoroid model. Leonid, Taurid, Alpha Monocerotid and
sporadic meteors from November 2003 were observed and analyzed using uniform methodology. Meteors from these four sources
were examined for evidence of fragmentation by examining light curve shape and searching for light curve irregularities. Differences
in meteoroid structure should be reflected by differences in meteor light curves. The resulting meteor light curve F-parameter values showed no statistically significant differences between the meteors from the various cometary showers or
the sporadic meteors. The F-parameter values also suggest that the meteoroids from these sources do not follow a single body ablation model, which suggests
that all four sources produce meteoroids with a dustball structure. 相似文献
6.
A technique for determining the effective temperature T
eff
and the acceleration of gravity log g of F and G supergiants is discussed using four bright stars as examples, specifically two F supergiants, α Lep(F0 Ib) and
π Sgr (F2 II), and two G supergiants, β Aqr (G0 Ib) and α Aqr (G2 Ib). In all four cases the parameter log g was derived from the high precision parallaxes recently obtained by van Leeuwen in a new reduction of data from Hipparcos.
Because of this, the accuracy of the determinations of log g is much greater than before. Estimates of the parameter T
eff
were checked using accurate values of T
eff
obtained previously by the infrared flux method (IRFM). In the case of the early F supergiants, this method confirms the
good accuracy of the T
eff
values derived from the Balmer lines and the β-index. Measurements of the Balmer lines for the G supergiants are difficult
because of strong blending, so the indices [c
1] and β serve as indicators of T
eff
. It is shown that the indices [c
1] and β yield a systematic difference in the values of T
eff
; the IRFM confirms that deriving T
eff
from the index [c
1] is more accurate. Based on the values of T
eff
and log g that have been found here, with the aid of the evolutionary tracks, we estimate the mass M and age t of each star. The Fe II lines, which are insensitive to departures from LTE, have been used to determine the microturbulence
parameter V
t
and the iron abundance. The latter is close to the solar iron abundance. Some problems concerning the chemical composition
of these stars are discussed using the supergiant α Lep as an example.
Translated from Astrofizika, Vol. 52, No. 2, pp. 237–257 (May 2009). 相似文献
7.
The phenomenon of MHD wave refraction is useful in interpreting the properties of the magnetic fluctuations in certain parcels
of solar wind. In the physics of MHD wave refraction, variations in the Alfvén speed VAlf play a dominant role. Here, we compile statistics of the 1-min averages of VAlf at the location of the ACE spacecraft during its first 5 years of operation. We find that monthly distributions of VAlf are close to log-normal, with standard deviations σV as small as 0.11 in the logarithm. Variations in the monthly mean VAlf are correlated significantly with sunspot number. We also compile monthly distributions of the plasma β parameter. The distributions
of both VAlf and β are significantly narrower than they would be if the various solar wind parameters were statistically independent.
In the Tp–VAlf plane, we find a zone of avoidance at low VAlf: for VAlf ≤10 – 15 km/s, there are no samples in the 1-min data that are cooler than Tp = 10 000 – 15 000 K. This feature can be understood in the context of MHD wave refraction, although other explanations are
also possible. 相似文献
8.
P. Jenniskens P.S. Gural L. Dynneson B.J. Grigsby K.E. Newman M. Borden M. Koop D. Holman 《Icarus》2011,216(1):40-61
First results are presented from a newly developed meteoroid orbit survey, called CAMS – Cameras for Allsky Meteor Surveillance, which combines meteor detection algorithms for low-light video observations with traditional video surveillance tools. Sixty video cameras at three stations monitor the sky above 31° elevation. Goal of CAMS is to verify meteor showers in search of their parent comets among newly discovered near-Earth objects.This paper outlines the concept of operations, the hardware, and software methods used during operation and in the data reduction pipeline, and accompanies the data release of the first batch of meteoroid orbits. During the month of November 2010, 2169 precisely reduced meteoroid trajectories from 17 nights have an error in the apparent radiant of the trajectory <2° and error in speed <10%. Median values of the error are 0.31° and 0.53 km/s, respectively, sufficient to resolve the intrinsic dispersion of annual meteor showers and resolve minor showers from the sporadic background. The limiting visual magnitude of the cameras is +5.4, recording meteors of +4 magnitude and brighter, bright enough to stand out from the mostly fainter sporadic meteors detected as under dense radar echoes.CAMS readily detected all established showers (6) active during the clear nights in November. Of the showers that needed confirmation, we confirm the theta Aurigids (THA, IAU#390), the chi Taurids (CTA, IAU#388), and the omicron Eridanids (OER, IAU#338). We conclude that the iota November Aurigids (IAR, IAU#248) are in fact the combined activity of the theta Aurigids and chi Taurids, and this shower should be dismissed from the list. Finally, there is also a clustering consistent with the zeta Cancrids (ZCN, IAU#243), but we cannot exclude that this is lower perihelion dust belonging to the Orionid shower.Data are submitted to the IAU Meteor Data Center on a semi-regular basis, and can be accessed also at http://cams.seti.org. 相似文献
9.
In 2008, we observed the spectrum of the planetary nebula He 1–5, which has had no ionization source for more than 50 years.
Comparison with the observations performed in 1972–1976 and 1988 shows that the line intensity ratio I(λ5007[O III])/I(Hβ) decreased by a factor of 2.5, the intensity ratio of the red [N II] doublet to Hα was almost constant, and the [S II]-to-Hα intensity ratio may have slightly increased. The observed changes in the spectrum agree, within the limits of the observational
errors, with the calculations of gas recombination in a medium-excitation nebula performed by Tylenda in 1980, but the nitrogen
line intensities are higher than the calculated ones by 10–15%. This is most likely related to the slightly reduced nitrogen
abundance adopted in the model. The onset of nebular recombination dates from the end of the 19th century and is consistent
with its estimate from the photometric history of the central star. 相似文献
10.
We present two new luminous blue variable (LBV) candidate stars discovered in the M33 galaxy. We identified these stars as
massive star candidates at the final stages of evolution, presumably with a notable interstellar extinction. The candidates
were selected from the Massey et al. catalog based on the following criteria: emission in H
α
, V<18./m 5 and 0.m 35 < (B - V) < 1.m 2. The spectra of both stars reveal a broad and strong H
α
emission with extended wings (770 and 1000 kms−1). Based on the spectra we estimated the main parameters of the stars. Object N45901 has a bolometric luminosity log(L/L⊙) = 6.0–6.2 with the value of interstellar extinction A
V
= 2.3 ± 0.1. The temperature of the star’s photosphere is estimated as T⋆ ∼ 13000–15000 K, its probable mass on the Zero
Age Main Sequence is M∼ 60–80 M⊙. The infrared excess in N 45901 corresponds to the emission of warm dust with the temperature Twarm ∼ 1000 K, and amounts
to 0.1%of the bolometric luminosity. A comparison of stellar magnitude estimates from different catalogs points to the probable
variability of the object N45901. Bolometric luminosity of the second object, N125093, is log(L/L⊙) = 6.3 − 6.6, the value of interstellar extinction is A
V
= 2.75 ± 0.15. We estimate its photosphere’s temperature as T⋆∼ 13000–16000K, the initial mass as M ∼ 90–120 M⊙. The infrared excess in N125093 amounts to 5–6% of the bolometric luminosity. Its spectral energy distribution reveals two
thermal components with the temperatures Twarm ∼ 1000K and Tcold ∼ 480 K. The [Ca II] λλ7291, 7323 lines, observed in LBV-like stars Var A and N93351 in M33 are also present in the spectrum of N 125093. These lines
indicate relatively recent gas eruptions and dust activity linked with them. High bolometric luminosity of these stars and
broad H
α emissions allow classifying the studied objects as LBV candidates. 相似文献
11.
This paper presents the model equations governing the nonlinear interaction between dispersive Alfvén wave (DAW) and magnetosonic
wave in the low-β plasmas (β≪m
e/m
i; known as inertial Alfvén waves (IAWs); here
\upbeta = 8pn0T /B02\upbeta = 8\pi n_{0}T /B_{0}^{2} is thermal to magnetic pressure, n
0 is unperturbed plasma number density, T(=T
e≈T
i) represents the plasma temperature, and m
e(m
i) is the mass of electron (ion)). This nonlinear dynamical system may be considered as the modified Zakharov system of equations
(MZSE). These model equations are solved numerically by using a pseudo-spectral method to study the nonlinear evolution of
density cavities driven by IAW. We observed the nonlinear evolution of IAW magnetic field structures having chaotic behavior
accompanied by density cavities associated with the magnetosonic wave. The relevance of these investigations to low-β plasmas
in solar corona and auroral ionospheric plasmas has been pointed out. For the auroral ionosphere, we observed the density
fluctuations of ∼ 0.07n
0, consistent with the FAST observation reported by Chaston et al. (Phys. Scr.
T84, 64, 2000). The heating of the solar corona observed by Yohkoh and SOHO may be produced by the coupling of IAW and magnetosonic wave via filamentation process as discussed here. 相似文献
12.
Electromagnetic instabilities in high-β plasmas, where β is the ratio of the kinetic plasma energy to the magnetic energy, have a broad range of astrophysical applications. The presence
of temperature anisotropies T
∥
/T
⊥
>1 (where ∥ and ⊥ denote directions relative to the background magnetic field) in solar flares and the solar wind is sustained by the observations
and robust acceleration mechanisms that heat plasma particles in the parallel direction. The surplus of parallel kinetic energy
can excite either the Weibel-like instability (WI) of the ordinary mode perpendicular to the magnetic field or the firehose
instability (FHI) of the circularly polarized waves at parallel propagation. The interplay of these two instabilities is examined.
The growth rates and the thresholds provided by the kinetic Vlasov – Maxwell theory are compared. The WI is the fastest growing
one with a growth rate that is several orders of magnitude larger than that of the FHI. These instabilities are however inhibited
by the ambient magnetic field by introducing a temperature anisotropy threshold. The WI admits a larger anisotropy threshold,
so that, under this threshold, the FHI remains the principal mechanism of relaxation. The criteria provided here by describing
the interplay of the WI and FHI are relevant for the existence of these two instabilities in any space plasma system characterized
by an excess of parallel kinetic energy. 相似文献
13.
Peter Brown Valerie Marchenko Danielle E. Moser Robert Weryk William Cooke 《Meteoritics & planetary science》2013,48(2):270-288
We propose that the Taurid meteor shower may contain bodies able to survive and be recovered as meteorites. We review the expected properties of meteorite‐producing fireballs, and suggest that end heights below 35 km and terminal speeds below 10 km s?1 are necessary conditions for fireballs expected to produce meteorites. Applying the meteoroid strength index (PE criteria) of Ceplecha and McCrosky (1976) to a suite of 33 photographically recorded Taurid fireballs, we find a large spread in the apparent meteoroid strengths within the stream, including some very strong meteoroids. We also examine in detail the flight behavior of a Taurid fireball (SOMN 101031) and show that it has the potential to be a (small) meteorite‐producing event. Similarly, photographic observations of a bright, potential Taurid fireball recorded in November of 1995 in Spain show that it also had meteorite‐producing characteristics, despite a very high entry velocity (33 km s?1). Finally, we note that the recent Maribo meteorite fall may have had a very high entry velocity (28 km s?1), further suggesting that survival of meteorites at Taurid‐like velocities is possible. Application of a numerical entry model also shows plausible survival of meteorites at Taurid‐like velocities, provided the initial meteoroids are fairly strong and large, both of which are characteristics found in the Taurid stream. 相似文献
14.
In this paper we have investigated the beat wave excitation of an ion-acoustic wave at the difference frequency of two kinetic
(or shear) Alfvén waves propagating in a magnetized plasma with β<1 (β=8π
n
e0
T
e/B
0
2
, where n
e0 is the unperturbed electron number density, T
e is the electron temperature, and B
0 is the external magnetic field). On account of the interaction between two kinetic Alfvén waves of frequencies ω
1 and ω
2, the ponderomotive force at the difference frequency ω
1−ω
2 leads to the generation of an ion-acoustic wave. Also because of the filamentation of the Alfvén waves, magnetic-field-aligned
density dips are observed. In this paper we propose that the ion-acoustic wave generated by this mechanism may be one of the
possible mechanisms for the heating and acceleration of solar wind particles. 相似文献
15.
For z = 0.8–2.2 redshift interval, quasar pair correlation function parameters and β redshift space distortion parameter (connected
to large-scale potential flows) values are estimated. We base them on the Main QSO Sample from SDSS Data Release 5. Standard
correlation function form ξ(r) = (r
0/r)γ is used for comoving distances r = 2–50 Mpc between quasars. We fix the parameters of the cosmological model: ΩΛ = 1 − Ω
M
= 0.726 and H
0 = 70.5 km/(s Mpc). We come to the best-fit parameter values of γ = 1.77 ± 0.20, r
0 = 5.52 ± 0.95 Mpc/h for r in the range 2–30 Mpc, γ = 1.91 ± 0.11, r
0 = 5.82 ± 0.61 Mpc for r in the range 2–50 Mpc. The mean β value is β = 0.43 ± 0.22. 相似文献
16.
Bogdan Valcu Peter L. Smith Larry D. Gardner John C. Raymond Mari-Paz Miralles John L. Kohl 《Solar physics》2007,243(1):93-104
We have tracked the spectral responsivity of the ultraviolet channels of the UVCS (Ultraviolet Coronagraph Spectrometer) instrument
on SOHO by repeated observations of a stable hot star. We demonstrate first that the ultraviolet spectral irradiance of the
Be star ζ Tau (HD 37202) for the 100- to 125-nm wavelength range has been sufficiently constant for our purposes when measured periodically
over the course of the SOHO mission. We then use ζ Tau as a radiometric transfer standard to determine an average decrease beginning in November of 1998 of 13.0% per year in
the responsivity of the UVCS O vi channel for wavelengths near H i Ly α and for a particular UVCS unvignetted aperture used for science observations. The calibration tracking method involves
separating two ζ Tau spectral regions that are overlapped on part of the detector. The change in the responsivity of UVCS/SOHO began in late
1998 as determined by comparison of simultaneous observations of the corona carried out with UVCS/SOHO and the freshly-calibrated
UVCS instrument on the Spartan 201 satellite in early November of 1998.
B. Valcu now at BrainLAB, Inc, 3 Westbrook Corporate Ctr., Tower 3, Suite 400, Westchester, IL 60154, USA, e-mail: bogdan.valcu@brainlab.com
or Bogdan_valcu@hotmail.com 相似文献
17.
Jack D. Drummond 《Icarus》1981,47(3):500-517
Sixteen comets produce recognizable meteor showers that are found in A. F. Cook's (1973, In Evolutionary and Physical Properties of Meteoroids (C. L. Hemenway, P. M. Millman, and A. F. Cook, Eds.), pp. 183–191, U.S. Govt. Printing Office, Washington, D.C.), working list of meteor streams. Of these, five are long period, including one in a parabolic and one in a hyperbolic orbit. The largest Earth-comet orbit miss distance is 0.20 AU for P/Encke and the Northern and Southern Taurids. Using this is an upper limit for meteor showers from comets, all comets which approach the Earth's orbit to within 0.20 AU were extracted from the Catalogue of Cometary Orbits (B. G. Marsden, 1979. 3rd ed., Central Bureau of Astronomical Telegrams, IAU SAO, Cambridge, Mass.). A compilation of such comets is presented by date minimum approach, along with the distance of closest approach and the theoretical geocentric radiants and velocities of possible associated meteor showers. Both pre- and postpperihelion encounters with the Earth's orbit are considered. There are 240 entries for 178 long-period comets, and 36 for 28 short-period comets. It is noted that all short-period comets that have approached the Earth's orbit to within 0.08 AU have produced meteors, except P/Lexell, P/Finlay, P/Denning-Fujikawa, and P/Grigg-Skjellerup. Attention is called to the favorable observing conditions for detecting meteors from P/Grigg-Skjellerup in April 1982, and for the possibility of another great Draconid storm from P/Giacobini-Zinner in October 1985. A comparison is made between observed sporadic meteor rates and the distribution of theoretical radiants throughout the year, from which it is concluded that the currently known comets can account for sporadic meteors. A criterion is developed to test whether or not an observed meteor shower can be associated with a given theoretical radiant. Based on known examples, a qualitative model for comet/meteor relationships is also presented. 相似文献
18.
We report the results of spectropolarimetric observations of the supergiant ζ Ori A made with 6-m BTA telescope of the SAO RAS. We found regular variations of line profiles in the spectrum of the star
with a period of 1–3 hours and point to their possible association with nonradial photospheric pulsations. We also searched
for a possible weak magnetic field in ζ Ori A, but observations did not confirm the existence of such a field in the star considered. 相似文献
19.
A. Kh. Rzaev 《Astrophysical Bulletin》2010,65(1):26-33
We use 240 CCD spectra taken in 1998–2000 with the coude echelle spectrograph of the 2-m telescope of the National Academy
of Sciences of Azerbaijan to study temporal radial velocity and line profile variations of the ion, HeI, and Hβ lines in the spectrumof the α Cyg supergiant. We demonstrate that these variations are caused by pulsation-type motions in the star’s atmosphere. Ion and
HeI lines oscillate in the main fundamental mode with a period of about 12.0 ± 0.5
d
and an amplitude of 5.0 ± 0.5 km/s. These ion-line oscillations continue for about 35 days. Then the difference between the
radial velocities of strong and weak ion lines results in a gradual decay of oscillations over a time interval of about 5.0
± 1.0
d
. Thereafter the process repeats itself. For the Hβ line we found two significant periods, two amplitudes, and three characteristic radial velocity variability behaviors for
the blue and red halves of the absorption profile: with equal variability parameters (period P and amplitude A); with equal P and A, but with a phase shift between the radial velocity variations of the blue and red halves of the absorption profile; with
different P and A for the two halves of the absorption profile. The star’s center of mass radial velocity as inferred from the γ-velocity is −4.5 ± 0.5 km/s. The average expansion velocity of the atmospheric layers, where the Hβ line forms, amounts to about −16.5 ± 0.5 km/s and varies temporally with an amplitude of about 3.0 km/s. 相似文献
20.
We present an analysis of the evolution of the thermal flare plasma during the 14 July 2000, 10 UT, Bastille Day flare event,
using spacecraft data from Yohkoh/HXT, Yohkoh/SXT, GOES, and TRACE. The spatial structure of this double-ribbon flare consists of a curved arcade with some 100 post-flare
loops which brighten up in a sequential manner from highly-sheared low-lying to less-sheared higher-lying bipolar loops. We
reconstruct an instrument-combined, average differential emission measure distribution dEM(T)/dT that ranges from T=1 MK to 40 MK and peaks at T
0=10.9 MK. We find that the time profiles of the different instrument fluxes peak sequentially over 7 minutes with decreasing
temperatures from T≈30 MK to 1 MK, indicating the systematic cooling of the flare plasma. From these temperature-dependent relative peak times
t
peak(T) we reconstruct the average plasma cooling function T(t) for loops observed near the flare peak time, and find that their temperature decrease is initially controlled by conductive
cooling during the first 188 s, T(t)∼[1+(t/τcond)]−2/7, and then by radiative cooling during the next 592 s, T(t)∼[1−(t/τrad)]3/5. From the radiative cooling phase we infer an average electron density of n
e=4.2×1011 cm−3, which implies a filling factor near 100% for the brightest observed 23 loops with diameters of ∼1.8 Mm that appear simultaneously
over the flare peak time and are fully resolved with TRACE. We reproduce the time delays and fluxes of the observed time profiles
near the flare peak self-consistently with a forward-fitting method of a fully analytical model. The total integrated thermal
energy of this flare amounts to E
thermal=2.6×1031 erg.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1014257826116 相似文献