Are There Many More Small Meteors Than Hitherto Detected?

1986 ◽  
Vol 6 (4) ◽  
pp. 436-439
Author(s):  
D. I. Olsson-Steel ◽  
W. G. Elford
Keyword(s):  
Solar System ◽  
Velocity Component ◽  
High Velocity ◽  
Alternative Hypothesis ◽  
Height Distribution ◽  
Recent Past ◽  
Zodiacal Light ◽  
Low Velocity ◽  
The Past ◽  
Meteor Trails

AbstractVisual meteors, due to impinging meteoroids of radius about 1 cm, appear at a rate of a few per hour during non-shower periods. Smaller meteoroids (100 μm – 1 cm) give rise to less bright trails, but are much more abundant. These are usually detected by radars of about 10 m wavelength which, over the past 40 years, have produced a plethora of information concerning mass and height distributions, orbits, etc.Using such ‘conventional radars’, the peak of the measured height distribution is found at about 95 km, with few meteors detected above 105 km. However, the flux detected is only a few percent of the total flux (a) measured using a large (10 m) optical collector, and (b) expected from a comparison with measurements by satellite impacts and zodiacal light observations (radii < 100 μm). One possibility is that the radars detect few low-velocity (V < ~25 km s-1) meteors since these produce little ionization and thus limit their detectability: the ionizing efficiency of meteors varies as ~ V7/2. In direct opposition, our alternative hypothesis is that the undetected flux is held in a faint high-velocity component which ablates at high altitude. These are not detected by conventional radars because meteor trails have ‘initial widths’ of about 3 m at 105 km; for a radar wavelength of 10 m, components scattered from different regions of the trail therefore destructively interfere, and the probability of detecting any meteor above 105 km is small.In order to test our hypothesis we have measured the height distribution with a 150 m radar, and we are commencing ancillary observations at 50 m; compared to these wavelengths the initial width is small to at least 140 km. The results show a peak at 105 km with most meteors being above this, significant numbers occurring right up to 140 km. This suggests that the true flux is at least 10 or 20 times that previously deduced, having implications for the number of cornets in the recent past and the balance of material between the smaller bodies in the solar System.

scholarly journals Far-ultraviolet Studies. VIII. Apollo 17 Search for Zodiacal Light

1980 ◽  
Vol 90 ◽  
pp. 41-44
Author(s):  
R. C. Henry ◽  
R. C. Anderson ◽  
W. G. Fastie
Keyword(s):  
Solar System ◽  
Dust Particles ◽  
Zodiacal Light ◽  
The Past ◽  
Far Ultraviolet

Solar System dust particles reflect sunlight, producing the so-called zodiacal light (Leinert 1975). The spectrum of the zodiacal light in the far ultraviolet has been a matter of controversy in the past, and remains a subject of great interest.

scholarly journals Connection between jets, winds and accretion in T Tauri stars

2018 ◽  
Vol 609 ◽  
pp. A87 ◽  
Author(s):  
B. Nisini ◽  
S. Antoniucci ◽  
J. M. Alcalá ◽  
T. Giannini ◽  
C. F. Manara ◽  
...  
Keyword(s):  
Mass Loss ◽  
Velocity Component ◽  
High Velocity ◽  
T Tauri Stars ◽  
Young Stars ◽  
Low Velocity ◽  
Star Forming ◽  
Average Value ◽  
T Tauri ◽  
Accretion Rates

Mass loss from jets and winds is a key ingredient in the evolution of accretion discs in young stars. While slow winds have been recently extensively studied in T Tauri stars, little investigation has been devoted on the occurrence of high velocity jets and on how the two mass-loss phenomena are connected with each other, and with the disc mass accretion rates. In this framework, we have analysed the [O i]6300 Å  line in a sample of 131 young stars with discs in the Lupus, Chamaeleon and σ Orionis star forming regions. The stars were observed with the X-shooter spectrograph at the Very Large Telescope and have mass accretion rates spanning from 10-12 to 10-7M⊙ yr-1. The line profile was deconvolved into a low velocity component (LVC, | Vr | < 40 km s-1) and a high velocity component (HVC, | Vr | > 40 km s-1), originating from slow winds and high velocity jets, respectively. The LVC is by far the most frequent component, with a detection rate of 77%, while only 30% of sources have a HVC. The fraction of HVC detections slightly increases (i.e. 39%) in the sub-sample of stronger accretors (i.e. with log (Lacc/L⊙) >−3). The [O i]6300 Å  luminosity of both the LVC and HVC, when detected, correlates with stellar and accretion parameters of the central sources (i.e. L∗, M∗, Lacc, Ṁacc), with similar slopes for the two components. The line luminosity correlates better (i.e. has a lower dispersion) with the accretion luminosity than with the stellar luminosity or stellar mass. We suggest that accretion is the main drivers for the line excitation and that MHD disc-winds are at the origin of both components. In the sub-sample of Lupus sources observed with ALMA a relationship is found between the HVC peak velocity and the outer disc inclination angle, as expected if the HVC traces jets ejected perpendicularly to the disc plane. Mass ejection rates (Ṁjet) measured from the detected HVC [O i]6300 Å  line luminosity span from ~10-13 to ~10-7M⊙ yr-1. The corresponding Ṁjet/Ṁacc  ratio ranges from ~0.01 to ~0.5, with an average value of 0.07. However, considering the upper limits on the HVC, we infer a Ṁjet/Ṁacc  ratio < 0.03 in more than 40% of sources. We argue that most of these sources might lack the physical conditions needed for an efficient magneto-centrifugal acceleration in the star-disc interaction region. Systematic observations of populations of younger stars, that is, class 0/I, are needed to explore how the frequency and role of jets evolve during the pre-main sequence phase. This will be possible in the near future thanks to space facilities such as the James Webb space telescope (JWST).

scholarly journals Velocities Within 1 Arcsec of the Nucleus of NGC 4151 as Revealed by the HST Faint Object Camera

1994 ◽  
Vol 159 ◽  
pp. 441-441
Author(s):  
R.M. Catchpole ◽  
A. Boksenberg ◽  
Keyword(s):  
Velocity Component ◽  
High Velocity ◽  
Hubble Space Telescope ◽  
Position Angle ◽  
Emission Lines ◽  
Low Velocity ◽  
Strong Emission ◽  
Ngc 4151 ◽  
Zero Velocity ◽  
Faint Object

We have obtained a longslit spectrum at a position angle (PA) of 84.6° and passing within 0.38 arcsec of the nucleus of NGC 4151, using the FOC f/48 camera on the Hubble Space Telescope. The spectrum shows strong emission lines including [OII] λ 3727 and [OIII] λλ 4959, 5007. By fitting with Gaussian velocity profiles, we resolve the emission lines, within 1 arcsec of the nucleus, into a high and low velocity component. The low velocity component has a total range in radial velocity of 200 km s−1 and appears to be associated with material comprising the knots seen in the FOC, F501N [O III] image of NGC 4151, illustrated in Boksenberg (1993). The much weaker high velocity system has a range of 1000 km s−1, is more smoothly distributed in brightness and shows a peak brightness close to the nucleus. Because the slit did not intersect the nucleus it is possible to determine the PA at which the two velocity systems cross the zero velocity axis. This is at PA −26° for the low velocity system and PA +32° for the high velocity system. These PA values may be subject to a systematic error as the zero velocity is defined by the mean position of the line, in the absence of any external calibration.

The Origin of the Moon and its Relationship to the Origin of the Solar System

1962 ◽  
Vol 14 ◽  
pp. 133-148 ◽  
Author(s):  
Harold C. Urey
Keyword(s):  
Solar System ◽  
The Moon ◽  
The Past ◽  
The Earth ◽  
Short Period ◽  
Origin Of The Moon

During the last 10 years, the writer has presented evidence indicating that the Moon was captured by the Earth and that the large collisions with its surface occurred within a surprisingly short period of time. These observations have been a continuous preoccupation during the past years and some explanation that seemed physically possible and reasonably probable has been sought.

Stellar Evolution

1962 ◽  
Vol 11 (02) ◽  
pp. 137-143
Author(s):  
M. Schwarzschild
Keyword(s):  
Solar System ◽  
Stellar Evolution ◽  
Space Craft ◽  
New Techniques ◽  
Substantial Body ◽  
Individual Stars ◽  
The Past ◽  
Evolution Of Galaxies ◽  
History Of ◽  
Fast Flow

It is perhaps one of the most important characteristics of the past decade in astronomy that the evolution of some major classes of astronomical objects has become accessible to detailed research. The theory of the evolution of individual stars has developed into a substantial body of quantitative investigations. The evolution of galaxies, particularly of our own, has clearly become a subject for serious research. Even the history of the solar system, this close-by intriguing puzzle, may soon make the transition from being a subject of speculation to being a subject of detailed study in view of the fast flow of new data obtained with new techniques, including space-craft.

The Long View of Planetary Systems

2018 ◽  
Author(s):  
Karel Schrijver
Keyword(s):  
Solar System ◽  
Milky Way ◽  
Planetary Systems ◽  
Giant Planets ◽  
Milky Way Galaxy ◽  
Population Statistics ◽  
The Past ◽  
General Terms ◽  
The Galaxy ◽  
The Universe

How many planetary systems formed before our’s did, and how many will form after? How old is the average exoplanet in the Galaxy? When did the earliest planets start forming? How different are the ages of terrestrial and giant planets? And, ultimately, what will the fate be of our Solar System, of the Milky Way Galaxy, and of the Universe around us? We cannot know the fate of individual exoplanets with great certainty, but based on population statistics this chapter sketches the past, present, and future of exoworlds and of our Earth in general terms.

scholarly journals Gaia DR2 data and the evolutionary status of eight high-velocity hot post-AGB candidates

2020 ◽  
Vol 72 (6) ◽  
Author(s):  
Mudumba Parthasarathy ◽  
Tadafumi Matsuno ◽  
Wako Aoki
Keyword(s):  
Effective Temperature ◽  
High Velocity ◽  
Circumstellar Dust ◽  
Evolutionary Status ◽  
Agb Stars ◽  
Horizontal Branch ◽  
Dust Shells ◽  
The Past ◽  
Gaia Dr2 ◽  
Branch Star

Abstract From Gaia DR2 data of eight high-velocity hot post-AGB candidates, LS 3593, LSE 148, LS 5107, HD 172324, HD 214539, LS IV −12 111, LS III +52 24, and LS 3099, we found that six of them have accurate parallaxes which made it possible to derive their distances, absolute visual magnitudes (MV) and luminosity (log L/L⊙). All the stars except LS 5107 have an accurate effective temperature (Teff) in the literature. Some of these stars are metal poor, and some of them do not have circumstellar dust shells. In the past, the distances of some stars were estimated to be 6 kpc, which we find to be incorrect. The accurate Gaia DR2 parallaxes show that they are relatively nearby, post-AGB stars. When compared with post-AGB evolutionary tracks we find their initial masses to be in the range 1 M⊙ to 2 M⊙. We find the luminosity of LSE 148 to be significantly lower than that of post-AGB stars, suggesting that this is a post-horizontal-branch star or post-early-AGB star. LS 3593 and LS 5107 are new high-velocity hot post-AGB stars from Gaia DR2.

Detection of effort maximality in adults performing isokinetic wrist flexion and extension

2021 ◽  
pp. 1-7
Author(s):  
Mercè Torra ◽  
Eduard Pujol ◽  
Anna Maiques ◽  
Salvador Quintana ◽  
Roser Garreta ◽  
...  
Keyword(s):  
High Velocity ◽  
Healthy Male ◽  
Wrist Flexion ◽  
Low Velocity ◽  
Wrist Extensor ◽  
Submaximal Effort ◽  
The Difference ◽  
Muscle Groups ◽  
Maximal Effort ◽  
Flexion And Extension

BACKGROUND: The difference between isokinetic eccentric to concentric strength ratios at high and low velocities (DEC) is a powerful tool for identifying submaximal effort in other muscle groups but its efficiency in terms of the wrist extensors (WE) and flexors (WF) isokinetic effort has hitherto not been studied. OBJECTIVE: The objective of the present study is to examine the usefulness of the DEC for identifying suboptimal wrist extensor and flexor isokinetic efforts. METHODS: Twenty healthy male volunteers aged 20–40 years (28.5 ± 3.2) were recruited. Participants were instructed to exert maximal and feigned efforts, using a range of motion of 20∘ in concentric (C) and eccentric (E) WE and WF modes at two velocities: 10 and 40∘/s. E/C ratios (E/CR) where then calculated and finally DEC by subtracting low velocity E/CR from high velocity ones. RESULTS: Feigned maximal effort DEC values were significantly higher than their maximal effort counterparts, both for WF and WE. For both actions, a DEC cutoff level to detect submaximal effort could be defined. The sensitivity of the DEC was 71.43% and 62.5% for WE ad WF respectively. The specificity was 100% in both cases. CONCLUSION: The DEC may be a valuable parameter for detecting feigned maximal WF and WE isokinetic effort in healthy adults.

scholarly journals Studying the Depth Structure of the Kyrgyz Tien Shan by Using the Seismic Tomography and Magnetotelluric Sounding Methods

Geosciences ◽  
2021 ◽  
Vol 11 (3) ◽  
pp. 122
Author(s):  
Irina Medved ◽  
Elena Bataleva ◽  
Michael Buslov
Keyword(s):  
Seismic Tomography ◽  
High Velocity ◽  
Fault Zones ◽  
Magnetotelluric Sounding ◽  
Tien Shan ◽  
High Resistivity ◽  
Low Velocity ◽  
Low Resistivity ◽  
Velocity Models ◽  
Fluid Saturation

This paper presents new results of detailed seismic tomography (ST) on the deep structure beneath the Middle Tien Shan to a depth of 60 km. For a better understanding of the detected heterogeneities, the obtained velocity models were compared with the results of magnetotelluric sounding (MTS) along the Kekemeren and Naryn profiles, running parallel to the 74 and 76 meridians, respectively. We found that in the study region the velocity characteristics and geoelectric properties correlate with each other. The high-velocity high-resistivity anomalies correspond to the parts of the Tarim and Kazakhstan-Junggar plates submerged under the Tien Shan. We revealed that the structure of the Middle Tien Shan crust is conditioned by the presence of the Central Tien Shan microcontinent. It manifests itself as two anomalies lying one below the other: the lower low-velocity low-resistivity anomaly, and the upper high-velocity high-resistivity anomaly. The fault zones, limiting the Central Tien Shan microcontinent, appear as low-velocity low-resistivity anomalies. The obtained features indicate the fluid saturation of the fault zones. According to the revealed features of the Central Tien Shan geological structure, it is assumed that the lower-crustal low-velocity layer can play a significant role in the delamination of the mantle part of the submerged plates.

Sign in / Sign up

Export Citation Format

Share Document