共查询到20条相似文献,搜索用时 46 毫秒
1.
The structure of the AGN object 1803+784 has been investigated at a wavelength of 7 mm with a limiting angular resolution
reaching 20 μas. The ejector nozzle surrounded by a ring structure, an accretion disk, has been identified. The nozzle size
is ∼0.1 pc, the diameter of the ring structure is ∼1.4 pc, and its width is ∼0.25 pc. The reaction of the plasma flow produces
a multimode precession responsible for the conical helical structure of the jet with a variable step and a curved axis. The
viewing angle of the flow ejection is ∼40°. The central part of the ejected flow moving along the axis accelerates to a relativistic
velocity. The apparent velocity reaches 12 s at a distance of ∼1 mas or ∼6 pc from the ejector. The outer part of the flow
moves along a helix around a high-velocity component whose step is a factor of 4 smaller, because the longitudinal velocity
is relatively low. The plasma is ejected almost toward the observer, as confirmed by its high brightness temperature T
b ≈ 8 × 1013 K and highly beamed emission. The polarized emission from the nozzle is axisymmetric. The orientation of the polarization
of the flow along the whole length is aligned with the direction of its motion, suggesting the excitation of a ring magnetic
field around it and self-focusing. 相似文献
2.
K. S. Dwarakanath W. M. Goss J. H. Zhao C. C. Lang 《Journal of Astrophysics and Astronomy》2004,25(3-4):129-141
We have imaged a region of ∼ 5′ extent surrounding Sgr A* in the HI 21 cm-line absorption using the Very Large Array. A Gaussian
decomposition of the optical depth spectra at positions within ∼ 2′ (∼ 5 pc at 8.5 kpc) of Sgr A* detects a wide line underlying
the many narrow absorption lines. The wide line has a mean peak optical depth of 0.32 ± 0.12 centered at a mean velocity of
V1sr = −4 ± 15 km s{−1}. The mean full width at half maximum is 119 ± 42 km s−1. Such a wide line is absent in the spectra at positions beyond ∼ 2′ from Sgr A*. The position-velocity diagrams in optical
depth reveal that the wide line originates in various components of the circumnuclear disk (radius ∼ 1.3′ ) surrounding Sgr
A*. These components contribute to the optical depth of the wide line in different velocity ranges. The position-velocity
diagrams do not reveal any diffuse feature which could be attributed to a large number of HI clouds along the line of sight
to Sgr A*. Consequently, the wide line has no implications either to a global population of shocked HI clouds in the Galaxy
or to the energetics of the interstellar medium as was earlier thought. 相似文献
3.
We present the results of a search for carbon recombination lines in the Galaxy at 34.5 MHz (C575α) made using the dipole
array at Gauribidanur near Bangalore. Observations made towards 32 directions resulted in detections of lines, in absorption
at nine positions. Followup observations at 328 MHz (C272α) using the Ooty Radio Telescope detected these lines in emission.
A VLA D-array observation of one of the positions at 330 MHz yielded no detection implying a lower limit of 10′ for the angular
size of the line forming region.
The longitude-velocity distribution of the observed carbon lines indicate that the line forming regions are located mainly
between 4 kpc and 7 kpc from the Galactic centre. Combining our results with published carbon recombination line data near
76 MHz (Erickson, McConnell & Anantharamaiah 1995), we obtain constraintson the physical parameters of the line forming regions.
We find thatif the angular size of the line forming regions is ≥ 4°, then the range of parameters that fit the data are:T
e
=20–40 K,n
e
∼ 0.1–0.3 cm−3 and pathlengths ∼ 0.07–0.9 pc which may correspond to thin photodissociated regions around molecular clouds. On the other
hand, if the line forming regions are ∼ 2° in extent, then warmer gas (T
e
∼ 60–300 K) with lower electron densities (n
e
∼ 0.03–0.05 cm−3) extending over several tens of parsecs along the line of sight and possibly associated with atomic HI gas can fit the data.
Based on the range of derived parameters, we suggest that the carbon line regions are most likely associated with photo-dissociation
regions. 相似文献
4.
N. G. Kantharia W. M. Goss D. Anish Roshi Niruj R. Mohan Francois Viallefond 《Journal of Astrophysics and Astronomy》2007,28(1):41-53
We report multi-frequency radio continuum and hydrogen radio recombination line observations of HII regions near l = 24.8°, b = 0.1° using the Giant Metrewave Radio Telescope (GMRT) at 1.28 GHz (n = 172), 0.61 GHz (n = 220) and the Very Large Array (VLA) at 1.42 GHz (n = 166). The region consists of a large number of resolved HII regions and a few compact HII regions as seen in our continuum
maps, many of which have associated infrared (IR) point sources. The largest HII region at l = 24.83° and b = 0.1° is a few arcmins in size and has a shell-type morphology. It is a massive HII region enclosing ∼550 M⊙ with a linear size of 7 pc and an rms electron density of ∼110 cm−3 at a kinematic distance of 6 kpc. The required ionization can be provided by a single star of spectral type O5.5.
We also report detection of hydrogen recombination lines from the HII region at l = 24.83° and b = 0.1° at all observed frequencies near V
lsr
= 100 km s−1. We model the observed integrated line flux density as arising in the diffuse HII region and find that the best fitting model
has an electron density comparable to that derived from the continuum. We also report detection of hydrogen recombination
lines from two other HII regions in the field. 相似文献
5.
Mathieu de Naurois 《Astrophysics and Space Science》2007,309(1-4):277-284
Recent observations of the binary system LS 5039 with the High Energy Stereoscopic System (H.E.S.S.) revealed that its Very
High Energy (VHE) γ-ray emission is modulated at the 3.9 days orbital period of the system. The bulk of the emission is largely confined to half
of the orbit, peaking around the inferior conjunction epoch of the compact object. The flux modulation provides the first
indication of γ-ray absorption by pair production on the intense stellar photon field. This implies that the production region size must
be not significantly greater than the gamma-gamma photosphere size (∼1 AU), thus excluding the large scale collimated outflows
or jets (extending out to ∼1000 AU). A hardening of the spectrum is also observed at the same epoch between 0.2 and a few
TeV which is unexpected under a pure absorption scenario and could rather arise from variation with phase in the maximum electron
energy and/or the dominant VHE γ-ray production mechanism. This first-time observation of modulated γ-ray emission allows precise tests of the acceleration and emission models in binary systems.
Mathieu de Naurois for the H.E.S.S. Collaboration. 相似文献
6.
E. Sánchez-Ayaso J. A. Combi J. F. Albacete Colombo J. López-Santiago J. Martí A. J. Mu?oz-Arjonilla 《Astrophysics and Space Science》2012,337(2):573-579
We report XMM-Newton observations of the Galactic supernova remnant G296.8-0.3, together with complementary radio and infrared data. The spatial
and spectral properties of the X-ray emission, detected towards G296.8-0.3, was investigated in order to explore the possible
evolutionary scenarios and the physical connexion with its unusual morphology detected at radio frequencies. G296.8-0.3 displays
diffuse X-ray emission correlated with the peculiar radio morphology detected in the interior of the remnant and with the
shell-like radio structure observed to the northwest side of the object. The X-ray emission peaks in the soft/medium energy
range (0.5–3.0 keV). The X-ray spectral analysis confirms that the column density is high (N
H∼0.64×1022 cm−2) which supports a distant location (d>9 kpc) for the SNR. Its X-ray spectrum can be well represented by a thermal (PSHOCK) model, with kT∼0.86 keV, an ionization timescale of 6.1×1010 cm−3 s, and low abundance (∼0.12 Z
⊙). The 24 μm observations show shell-like emission correlated with part of the northwest and southeast boundaries of the SNR.
In addition a point-like X-ray source is also detected close to the geometrical center of the radio SNR. The object presents
some characteristics of the so-called compact central objects (CCO). Its X-ray spectrum is consistent with those found at
other CCOs and the value of N
H is consistent with that of G296.8-0.3, which suggests a physical connexion with the SNR. 相似文献
7.
We present here a review of the radio observations of the remarkable comet Hale-Bopp C/1995 O1 in which most major radio astronomical
facilities have been involved. These observations started in August 1995, soon after the discovery of the comet (it was then
at ∼7 AU from the sun), and well before its perihelion on April 1st, 1997; they are still going on, hopefully up to end of
1998. Extended cartographies have been obtained using multibeam receivers and on-the-fly techniques. High spatial resolution
(a few ″) has been achieved with interferometers. Submillimetric observations are playing an increasing role, and high resolution
(R ∼ 106−107) spectroscopy of cometary lines is now performed from decimetric to submillimetric wavelengths.
The number of species observed at radio wavelengths now reaches ∼28,when it was ∼14 for comet C/1996 B2 Hyakutake. Most of
these species are parent molecules. However, ions have been observed for the first time at radio wavelengths, and their velocities
measured. Several isotopic species (involving D,13C,34S,15N) have been sought, allowing isotopic enrichment determinations.
The abundances of cometary molecules present many similarities and some differences with the abundances of interstellar molecules
in regions where grain mantles are believed to be evaporated to the gas phase (hot cores, bipolar flows). They will be discussed
for their implications on the origin of cometary ices and of comets themselves.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
M. Stupar M. D. Filipović Q. A. Parker G. L. White T. G. Pannuti P. A. Jones 《Astrophysics and Space Science》2007,307(4):423-435
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
9.
G. Swarup 《Journal of Astrophysics and Astronomy》1984,5(2):139-148
A 4-km synthesis radio telescope has recently been commissioned at Ootacamund, India for operation at 327 MHz. It consists
of the Ooty Radio Telescope (530 m × 30 m) and 7 small antennas which are distributed over an area of about 4 km × 2 km. It
has a coverage of about ± 40‡ in declination δ. The beam-width is about 40 arcsec × 90 arcsec at δ = 0‡ and about 40 arcsec
× 50 arcsec at δ = 40‡. The sensitivity attained for a 5:1 signal-to-noise ratio is about 15 m Jy after a 10-hour integration.
The observational programmes undertaken and some of the results obtained recently are summarized. The radio halo around the
edge-on spiral NGC 4631 is found to have a larger scale-height at 327 MHz than is known at higher frequencies. Mapping of
interesting radio galaxies at 327 MHz is being carried out; preliminary results for 0511-305 (∼2 Mpc) and 1333-337 (∼750 kpc)
are summarized. The very-steep-spectrum radio source in the Abell cluster A85 is found to be resolved; since it has no obvious
optical counterpart, it is conceivable that it is a remnant of past activity of a galaxy that has drifted away in about 109 years. 相似文献
10.
Preliminary data are presented on seven “special” objects found in low-dispersion spectral plates of the FBS survey. The first
object, FBS 0250+167, is found to be a class M7 dwarf with a high proper motion (∼5.130 arcsec/year) lying about 3 pc from
the sun. Three of the objects, with spectra of classes M8–M9, are suspected of being long-period Mirids with large mass losses
and surrounded by dense clouds. The three remaining objects have very short low-dispersion spectra on the FBS survey plates
(clearly they are non stellar objects) and are of interest for later detailed study.
__________
Translated from Astrofizika, Vol. 50, No. 1, pp. 73–85 (February 2007). 相似文献
11.
Jokūbas Sūdžius Saulius Raudeliūnas Arūnas Kučinskas Audrius Bridžius Vladas Vansevičius 《Astrophysics and Space Science》2002,280(1-2):109-114
We investigate the possibilities for tracing interstellar extinction with the ESA's astrometric space mission GAIA. The analysis
is based on detailed simulations of the GAIA photometry, which are used to derive the distribution of interstellar matter
in a modelled Galaxy. We find that `small' diffuse clouds (diameter D = 4 pc, E
B-V = 0.06) will be easily traced with GAIA up to the distances of ∼ 800 pc. `Large' diffuse interstellar clouds (D = 10 pc, E
B-V = 0.13) will be located up to the distances of ∼ 2.5 kpc. This holds for the reddening tracers of spectral types O – K2 brighter
than V = 17. Inmost cases, due to their low spatial density, the early type stars (O– A2) cannot provide reliable information about
the distribution of interstellar matter. None of the reddening tracers measured by GAIA will provide reliable identification
of the individual interstellar clouds beyond the distances of ∼ 3 kpc. Therefore, we conclude that the information available
from photometric observations will be not sufficient for the detailed reconstruction of the 3-D distribution of Galactic interstellar
matter. It is therefore extremely important to define the new strategies which would allow to combine all the available information,
including the earlier space- and/or ground-based investigations, together with the information which will be provided by GAIA
itself (parallaxes, E
B-V etc.).
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
12.
By systematically searching regions around planetary nebulae (PNe) for signs of interactions of their precursors’ wind with
ambient matter we found a number of huge IRAS dust structures. Some of them may be chance projections, but a few appear to
be real, like those around NGC 6826 and NGC 2899. In the case of NGC 6826 we noticed a giant (∼2°) bipolar dust emission,
whose axis is along the proper motion of the central star. The PN itself is offset in the direction of motion both as to the
center of this ∼30 pc large dust structure and to the center of a similarly large new Hα nebula. NGC 2899 was found in the center of a 14×11 pc quadrupolar cavity, whose directions of axes coincide with the directions
of the main axes of the optical PN. In both cases, the formation of these structures appears to have commenced in the asymptotic
giant branch (AGB) phase. 相似文献
13.
A. De Luca P. A. Caraveo S. Mereghetti A. Tiengo G. F. Bignami 《Astrophysics and Space Science》2007,308(1-4):231-238
1E 161348-5055 (1E) is a compact object lying at the center of the 2000 year old Supernova Remnant (SNR) RCW103. Its original
identification as an isolated, radio-quiet neutron star has been questioned in recent years by the observation of a significant
long-term variability, as well as by reports of a possible periodicity at ∼6 hours. Here we report conclusive evidence for
a strong (nearly 50%) periodic modulation of 1E at 6.67±0.03 hours, discovered during a long (90 ks) XMM-Newton observation
performed in August 2005, when the source was in a “low state”. The source spectrum varies along the 6.67 hr cycle. No fast
pulsations are seen. 1E could be a very young binary system, possibly composed of a compact object and a low-mass star in
an eccentric orbit. This would be the first example of a low-mass X-ray binary (LMXB) associated with a SNR, and thus the
first LMXB for which we know the precise birth date, just 2000 years ago. Alternatively, if it is an isolated neutron star,
the unprecedented combination of age, period and variability may only fit in a very unusual scenario, featuring a peculiar
magnetar, dramatically slowed-down over 2000 years, possibly by a supernova debris disc.
相似文献
14.
In this paper the results of multiwavelength investigation of an unusual nebular object SNO 85 are presented. In 2MASS images
this object looks like a star with a jet. In DSS2 R image the end of the jet is connected with an interesting symmetric structure, consisting of arcs and loops. Such a structure
is seen also in the opposite direction from the central star; it favors the existence of two opposite jets, which repeat the
rotation and precession movements of the central star. The results of 12CO observations of the dark nebula LDN 288, connected with SNO 85, are also given. From these observations the following results
were obtained: SNO 85 is situated in a dense condensation and the neighbor B type star GSC 0625400181 is surrounded by a hollow
cavity. The velocity of the dark cloud is ∼2.5 km/s and its distance is estimated as (380–990) pc. The object SNO 85 itself
is associated with an IRAS point source IRAS 17547-1832, the infrared colors of this source are typical for a non-evolved
source embedded in the dense dark cloud. This region is perhaps a star formation one because there is also another star with
a straight jet in the vicinity of B type star GSC 0625400181.
Published in Astrofizika, Vol. 49, No. 4, pp. 621–629 (August 2006). 相似文献
15.
The gravitational instability of expanding shells evolving in a homogeneous and static medium is discussed. In the low density
environment (n = 1 cm-3), the fragmentation starts in shells with diameters of a few 100 pc and fragment masses are in the range of 5 × 103 - 106
M
⊙. In the high density environment (n = 105 - 107 cm-3), shells fragment at diameters of
pc producing clumps of stellar masses. The mass spectrum in both environments is approximated by a power law dN/dm ∼ m
-2.3. This is close to the slope of the stellar IMF. To reproduce the observed mass spectrum of clouds (the spectral index close
to ∼ -2.0) we have to assume, that the cloud formation time is independent of the cloud size, similarly to the Jeans unstable
medium.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
16.
Observations of the H272α recombination line towards the galactic centre show features near VLSR= 0, −50 and + 36 kms−1. We have combined the parameters of these features with the available H166α measurements to obtain the properties of the
ionized gas present along the line of sight and also in the ‘3 kpc arm’. For the line-of-sight ionized gas we get an electron
density around 7 cm−3 and a pathlength through it ∼ 10–60 pc. The emission measure and the electron temperature are in the range 500–2900 pc cm−6 and 2000–6000 K. respectively. The ionized gas in the 3 kpc arm has an electron density of 30 cm−3 and extends over 9 pc along the line of sight if we assume an electron temperature of 104 K. Using the available upper limit to the intensity of the H351α recombination line, we show that the distributed ionized
gas responsible for the dispersion of pulsar signals should have a temperature >4500 K. and a minimum filling factor of 20
per cent. We also show that recombination lines from the ‘warm ionized’ gas proposed by McKee & Ostriker (1977) should be
detectable in the frequency range 100–150 MHz towards the galactic centre with the sensitivity available at present. 相似文献
17.
The galactic distribution of pulsars and shell remnants of supernovae (SN) as investigated on the basis of newly-estimated parameters. Special attention was paid to taking into account all possible selection effects and an attempt was made to reveal a statistically-pure ensemble of objects. On the basis of this ensemble we studied the radial andz-distribution of pulsars and supernova remnants (SNR).It is shown that the radial distribution of both objects is considered to have an annular structure with the maximum surface density at a distance of 4.5–5.3 kpc (if the distance of the Sun from the galactic centre is assumed to be equal to 8.5 kpc). The scle-height of the progenitors of SNRs is not more than 110 pc and only 15% of the SNRs, whose progenitors may also be massive runaway stars, are situated at 300 pc. The mean application of the pulsars is not more than 300 pc which agrees with the hypothesis about the genetic connection with type-II SN outbursts at the kinematic aget
k5×106 yr and thez-component of spatial velocity beingV
z=120 km s–1.The possible precursors of type-I SNRs by the shape of their radial distribution in late spirals and the various model calculations given in the literature, are also discussed. 相似文献
18.
K. R. Anantharamaiah 《Journal of Astrophysics and Astronomy》1985,6(4):177-201
A survey of the H 272 α recombination line at 325 MHz has been made towards 53 directions in the galactic plane using the
Ooty Radio Telescope (ORT). 34 of these directions correspond to well-known Hn regions, 12 to SNRs and 6 to ‘blank’ areas
selected so that the 5 GHz continuum is a minimum over the telescope beam of 2ℴ x 6 arcmin. Observing procedure and spectra
of 47 sources towards which lines are detected are presented. Hydrogen recombination lines have been detected towards all
the observed directions havingl <40ℴ. Carbon recombination lines are identified in 12 of the directions. The hydrogen line intensities are found to correlate
well with the total continuum intensity (which includes the nonthermal galactic background) indicating that most of the lines
arise due to stimulated emission by the background radiation. A preliminary discussion on the nature of the line-emitting
regions is also presented. 相似文献
19.
We present the results of a year-long monitoring campaign on J1819+3845.We interpret the results of this WSRT campaign to
infer critical source parameters such as source lifetime and structure on tens of microarcseconds. The long lifetime of the
source at such high brightness temperatures requires continuous energy injection or exotic emission processes. We have previously
interpreted the extreme scintillation ofJ1819+3845 as due to a relatively nearby (∼ 20 pc) scattering screen. We show this
screen has a velocity w.r.t. the LSR of ∼ 25 kms-1, as measured by the changing scintillation properties throughout the year: the `velocity parallax'.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
The radio recombination line intensities of heavy elements of helium, carbon and oxygen are calculated with accounting for
dielectronic recombination. Dielectronic recombination rates are determined accurate to the second order of a perturbation
theory and the rates are described as function of principal quantum number for helium-like atom or ion. Balance equations
are solved for the departure coefficients from LTE bn. The collision and spontaneous transition rates are accounted for the balance equations, in which non-equilibrium distribution
source is dielectronic recombination. Non-equilibrium amplification coefficients are found as functions of a medium temperature,
density and ion charge z = 1–3 for radio recombination lines.
Optical depths are calculated for the heavy element low-frequency lines with the numbers 300 > n > 1200. For the chosen electronic temperatures and densities Te = 0.8× 104–10× 104 K, Ne = 0.05–0.1 cm−3 the line optical depth is determined by the values τL∼ 0.1× 10−4–100× 10−4. Calculated for free-free transition rates, the optical depth is given by using the value τff∼ 10−2τL. 相似文献