scholarly journals Statistical and Evolutionary Aspects of Cometary Orbits

1989 ◽  
Vol 116 (1) ◽  
pp. 487-535
Author(s):  
J.A. Fernández ◽  
W.-H. Ip
Keyword(s):  
Molecular Clouds ◽  
Oort Cloud ◽  
Tidal Force ◽  
Time Interval ◽  
Mass Relation ◽  
Galactic Latitude ◽  
Impact Cratering ◽  
Influx Rate ◽  
Short Period ◽  
The Impact

AbstractThe observed frequency of passages of Earth-crossing long-period (LP) comets (P > 200 yr) is about three per year for comets brighter than absolute magnitude H10 ∼ 10.5. About one out of six LP comets is estimated to be new, i.e., making its first passage through the inner planetary region. The sample of observed LP comets shows an excess of retrograde orbits that may be accounted for by the shorter dynamical lifetimes of comets on direct orbits due to planetary perturbations. The original semimajor axes of new comets concentrate in the range 7 × 103 ≳ aorig ≳ 4 × 104 AU, which tells us about the region of the Oort cloud where forces other than planetary perturbations act with the greatest efficiency. Yet the distribution of original semimajor axes cannot tell us anything about the existence of a dense inner core of the Oort cloud. Besides planetary perturbations, passing stars, molecular clouds and the galactic tidal force also influence the dynamical evolution of Oort cloud comets. The observed distribution of the aphelion points of near-parabolic comets shows such a dependence on the galactic latitude. Molecular clouds and stars penetrating very deeply in the Oort cloud are found to give rise to major enhancements in the influx rate of new comets, known as comet showers, at average intervals of a few 107 yr.An important issue to solve concerns how the frequency of comet passages varies with time, in particular as regards to the current level of comet appearances. Should we be passing through a highly intense phase, most aphelia of the incoming Oort comets would concentrate on the sky area where the strong perturber exerted its greatest effect. By contrast, the observed galactic latitude dependence of the aphelia suggests a dominant influence of the vertical galactic tidal force as compared with random strong perturbers. This seems to indicate that the frequency of comet passages is currently at, or near, its quiescent level. Whether intense comet showers are reflected in the impact cratering record is still a debatable issue. A periodicity of ∼ 26-30 Myr in the impact cratering rate is quite uncertain, owing to the small size of the sample of well-dated craters and the noise from background impact craters from asteroids.The family of short-period (SP) comets (orbital periods P < 20 yr) has long been regarded as the dynamical end-state of new comets on low-inclination orbits captured by Jupiter. However, if SP comets came from a spherical population of comets (e.g., incoming new comets), we should expect to find a percentage of them on retrograde orbits, which contradicts the observations. An alternative hypothesis for the origin of most SP comets is that they come from a trans-Neptunian comet belt. Extensive searches aimed at detecting faint slow-moving objects are required to assess the size of the comet population in the outer planetary region. Modeling of the transfer rate of comets from an outer belt to SP orbits gives transient populations between Saturn and Neptune on the order of 106 – 107 bodies. This is roughly comparable to the upper limit set by the most recent searches of outer solar system bodies.The impact crater production rate of comets, at the present time, can be estimated to be on the order of 10% of the value corresponding to asteroidal impacts. These estimates, however, are subject to large uncertainties in the brightness-mass relation of comets and crater scaling law. The Earth could have received about 2 × 1020 g of cometary material over the last 4 billion years — if the injection rate of new comets remained constant in the time interval. Within the context of H2O inventory, the cometary influx should have rather minor effects. On the other hand, because of the paucity of H2O content in the atmospheres of Venus and Mars, cometary impact could strongly modulate their water contents.

scholarly journals Comet Showers

1992 ◽  
Vol 152 ◽  
pp. 239-254
Author(s):  
Julio A. Fernández
Keyword(s):  
Molecular Clouds ◽  
Background Level ◽  
Oort Cloud ◽  
Giant Molecular Clouds ◽  
Impact Cratering ◽  
Influx Rate ◽  
Intermediate Size ◽  
The Impact ◽  
Trigger Mechanisms ◽  
Stellar Passages

Variations in the influx rate of incoming Oort cloud comets, leading to the occurrence of comet showers, are reviewed with special emphasis on the dynamical processes that produce them. It is found that comet showers as intense as 10-100 times the background comet flux may occur at average intervals of several 107 years, being very close stellar passages at distances ≈ 104AU and, perhaps, penetrating encounters with intermediate-size molecular clouds the trigger mechanisms. Penetrating encounters with giant molecular clouds and stellar passages at distances of a few 103AU may trigger comet showers about 102 − 103 times more intense than the background comet flux at average intervals of a few 108 years. The impact cratering record on the Earth does not show any clear evidence of past comet showers. Some orbital properties of new and dynamically young comets suggest that the influx rate of Oort cloud comets is currently near its background level.

From the Oort cloud to Halley-type comets

1999 ◽  
Vol 173 ◽  
pp. 327-338 ◽  
Author(s):  
J.A. Fernández ◽  
T. Gallardo
Keyword(s):  
Total Population ◽  
Complex Function ◽  
Dynamical Evolution ◽  
Oort Cloud ◽  
Capture Process ◽  
Influx Rate ◽  
Short Period ◽  
Dynamical Properties ◽  
Orbital Periods ◽  
Probability Of Capture

AbstractThe Oort cloud probably is the source of Halley-type (HT) comets and perhaps of some Jupiter-family (JF) comets. The process of capture of Oort cloud comets into HT comets by planetary perturbations and its efficiency are very important problems in comet ary dynamics. A small fraction of comets coming from the Oort cloud − of about 10−2− are found to become HT comets (orbital periods &lt; 200 yr). The steady-state population of HT comets is a complex function of the influx rate of new comets, the probability of capture and their physical lifetimes. From the discovery rate of active HT comets, their total population can be estimated to be of a few hundreds for perihelion distancesq &lt;2 AU. Randomly-oriented LP comets captured into short-period orbits (orbital periods &lt; 20 yr) show dynamical properties that do not match the observed properties of JF comets, in particular the distribution of their orbital inclinations, so Oort cloud comets can be ruled out as a suitable source for most JF comets. The scope of this presentation is to review the capture process of new comets into HT and short-period orbits, including the possibility that some of them may become sungrazers during their dynamical evolution.

scholarly journals Planetary Trojans – the main source of short period comets?

2010 ◽  
Vol 9 (4) ◽  
pp. 227-234 ◽  
Author(s):  
J. Horner ◽  
P. S. Lykawka
Keyword(s):  
Solar System ◽  
Short Review ◽  
Oort Cloud ◽  
Asteroid Belt ◽  
Exoplanetary Systems ◽  
Short Period ◽  
Proximate Source ◽  
Different Origins ◽  
Near Earth Asteroids ◽  
The Impact

AbstractOne of the key considerations when assessing the potential habitability of telluric worlds will be that of the impact regime experienced by the planet. In this work, we present a short review of our understanding of the impact regime experienced by the terrestrial planets within our own Solar system, describing the three populations of potentially hazardous objects which move on orbits that take them through the inner Solar system. Of these populations, the origins of two (the Near-Earth Asteroids and the Long-Period Comets) are well understood, with members originating in the Asteroid belt and Oort cloud, respectively. By contrast, the source of the third population, the Short-Period Comets, is still under debate. The proximate source of these objects is the Centaurs, a population of dynamically unstable objects that pass perihelion (closest approach to the Sun) between the orbits of Jupiter and Neptune. However, a variety of different origins have been suggested for the Centaur population. Here, we present evidence that at least a significant fraction of the Centaur population can be sourced from the planetary Trojan clouds, stable reservoirs of objects moving in 1:1 mean-motion resonance with the giant planets (primarily Jupiter and Neptune). Focussing on simulations of the Neptunian Trojan population, we show that an ongoing flux of objects should be leaving that region to move on orbits within the Centaur population. With conservative estimates of the flux from the Neptunian Trojan clouds, we show that their contribution to that population could be of order ~3%, while more realistic estimates suggest that the Neptune Trojans could even be the main source of fresh Centaurs. We suggest that further observational work is needed to constrain the contribution made by the Neptune Trojans to the ongoing flux of material to the inner Solar system, and believe that future studies of the habitability of exoplanetary systems should take care not to neglect the contribution of resonant objects (such as planetary Trojans) to the impact flux that could be experienced by potentially habitable worlds.

scholarly journals Dynamical Evolution of the Oort Cometary Cloud

1983 ◽  
Vol 6 ◽  
pp. 363-370 ◽  
Author(s):  
Paul R. Weissman
Keyword(s):  
Molecular Clouds ◽  
Dynamical Evolution ◽  
Oort Cloud ◽  
Effective Radius ◽  
Giant Molecular Clouds ◽  
Monte Carlo Techniques ◽  
Short Period ◽  
History Of ◽  
Hyperbolic Trajectories ◽  
Sphere Of Influence

The dynamical evolution of comets in the Oort cloud under the influence of stellar perturbations has been modeled using Monte Carlo techniques. It is shown that the cloud has been depleted over the history of the solar system. Comets are lost from the cloud by direct ejection due to close stellar encounters, diffusion of aphelia to distances beyond the sun’s sphere of influence, or diffusion of perihelia into the planetary region where Jupiter and Saturn perturbations either eject them on hyperbolic trajectories or capture them to short-period orbits. The population of the cloud is estimated to be 1.0 – 1.5 × 1012 comets and the total mass is on the order of 1.9 earth masses. In addition to random passing stars, less frequent encounters with giant molecular clouds may play a significant role in randomizing the orbits of comets in the cloud and reducing the effective radius of the sun’s sphere of influence.

scholarly journals Dynamical History of the Oort Cloud

1989 ◽  
Vol 116 (1) ◽  
pp. 463-486
Author(s):  
Paul R. Weissman
Keyword(s):  
Solar System ◽  
Interstellar Dust ◽  
Temporal Variations ◽  
Galactic Cosmic Rays ◽  
Oort Cloud ◽  
Interstellar Space ◽  
Satellite Systems ◽  
Short Period ◽  
History Of ◽  
The Impact

AbstractDynamical studies during the past decade have resulted in an almost explosive increase in our understanding of the Oort cloud of comets, which surrounds the solar system. Cometary orbits in the cloud evolve under the complex interaction of stellar, galactic, and giant molecular cloud perturbations, as well as planetary and nongravitational perturbations when the orbits re-enter the planetary region. Evidence has continued to build for a dense, inner Oort cloud of comets which acts as a reservoir to replenish the outer cloud as comets there are stripped away. A ring of comets beyond the orbit of Neptune, which may be the source of the short-period comets, is also likely. Both the estimated number and mass of comets in the Oort cloud have grown such that the total mass may be comparable to the mass of the planets. Temporal variations in the flux of comets from the Oort cloud into the planetary region by a factor of 50% are typical, and by factors of 20 to 200 are possible. The most intense cometary “showers” may have serious implications for biological extinction events on Earth as well as for the impact history of planets and satellite systems. Comets in the Oort cloud are processed by galactic cosmic rays, heated by nearby supernovae, eroded by interstellar dust impacts, and disrupted by mutual collisions (in the inner cloud). A detailed estimate of the Oort cloud’s dynamical history is not possible because of the inability to reconstruct the Sun’s varying galactic motion over the history of the solar system, and because of uncertainty over where comets actually formed. However, it is likely that a substantial fraction of the original Oort cloud population has been lost to interstellar space. We are approaching the time when Oort clouds around other stars may be detectable, though searches to date have so far been negative.

HBOT Application at Cases of Gingival Inflammation

2019 ◽  
Author(s):  
Ilma Robo
Keyword(s):  
Root Resorption ◽  
Periodontal Diseases ◽  
Therapeutic Effects ◽  
Clinical Effects ◽  
Gingival Inflammation ◽  
Beneficial Effects ◽  
New Therapeutic Approaches ◽  
Short Period ◽  
Mechanical Therapy ◽  
The Impact

The treatment of periodontal diseases, mainly of their origin, with the most common clinical manifestation in form of gingival inflammation, is manifold and powerful, including: mechanical therapy, antibiotic, antiseptic and various approaches to treatment, which are recommended to be used within a short period of time. New therapeutic approaches have been proven as alternative treatment to conventional therapy, or in combination with conventional therapies, to reduce the number of periodontopathic pathogens in gingival sulcus. HBOT has a detrimental effect on periodontal microorganisms, as well as beneficial effects on the healing of periodontal tissue, increasing oxygen pressure in gingival pockets. Our study is aimed at reviewing the current published literature on hyperbaric oxygen therapy and focuses on role of HBOT as a therapeutic measure for the individual with periodontal disease in general and for the impact on the recovery of gingival inflammation. HBOT and periodontal treatment together, reduce up to 99% of the gram-negative anaerobic load of subgingival flora. HBOT, significantly reduces subgingival anaerobic flora. Clinical effects in 2-year follow-up of treated patients are sensitive. Reduction of gingival hemorrhage indexes, depth of peritoneum, plaque index, occurs in cases of combination of HBOT and detraction. Reduced load persists up to 2 months after therapy. The significant increase in connective tissue removal starts at the end of 2nd week, to achieve the maximum in week 3-6 of application. HBOT used for re-implantation, stimulates the healing of periodontal membrane, pulp, prevents root resorption, healing of periodontal lining tissues. HBOT, significantly reduces the hemorrhage index with 1.2 value difference, 0.7mm probe depth, reduces gingival fluid by 2. HGH exposure is increased by gingival blood flow, with a difference of 2 in measured value. The therapeutic effects of HBOT in the value of the evaluation index can be saved up to 1-year post treatment.

scholarly journals Time-to-Treatment in Oral Cancer: Causes and Implications for Survival

Cancers ◽  
2021 ◽  
Vol 13 (6) ◽  
pp. 1321
Author(s):  
Constanza Saka-Herrán ◽  
Enric Jané-Salas ◽  
Antoni Mari-Roig ◽  
Albert Estrugo-Devesa ◽  
José López-López
Keyword(s):  
Oral Cancer ◽  
Previous Treatment ◽  
Diagnostic Procedures ◽  
Original Data ◽  
Primary Treatment ◽  
Medical Attention ◽  
Time Interval ◽  
Pre Treatment ◽  
The Impact ◽  
Lack Of Knowledge

The purpose of this review was to identify and describe the causes that influence the time-intervals in the pathway of diagnosis and treatment of oral cancer and to assess its impact on prognosis and survival. The review was structured according to the recommendations of the Aarhus statement, considering original data from individual studies and systematic reviews that reported outcomes related to the patient, diagnostic and pre-treatment intervals. The patient interval is the major contributor to the total time-interval. Unawareness of signs and/or symptoms, denial and lack of knowledge about oral cancer are the major contributors to the process of seeking medical attention. The diagnostic interval is influenced by tumor factors, delays in referral due to higher number of consultations and previous treatment with different medicines or dental procedures and by professional factors such as experience and lack of knowledge related to the disease and diagnostic procedures. Patients with advanced stage disease, primary treatment with radiotherapy, treatment at an academic facility and transitions in care are associated with prolonged pre-treatment intervals. An emerging body of evidence supports the impact of prolonged pre-treatment and treatment intervals with poorer survival from oral cancer.

scholarly journals The Impact of Time Interval between Hepatic Resection and Liver Transplantation on Clinical Outcome in Patients with Hepatocellular Carcinoma

Cancers ◽  
2021 ◽  
Vol 13 (10) ◽  
pp. 2398
Author(s):  
Matteo Serenari ◽  
Enrico Prosperi ◽  
Marc-Antoine Allard ◽  
Michele Paterno ◽  
Nicolas Golse ◽  
...  
Keyword(s):  
Hepatocellular Carcinoma ◽  
Liver Transplantation ◽  
Hepatic Resection ◽  
Primary Outcome ◽  
Polynomial Regression ◽  
Secondary Outcome ◽  
Time Interval ◽  
Comprehensive Complication Index ◽  
Fractional Polynomial ◽  
The Impact

Hepatic resection (HR) for hepatocellular carcinoma (HCC) may require secondary liver transplantation (SLT). However, a previous HR is supposed to worsen post-SLT outcomes. Data of patients treated by SLT between 2000 and 2018 at two tertiary referral centers were analyzed. The primary outcome of the study was to analyze the impact of HR on post-LT complications. A Comprehensive Complication Index ≥ 29.6 was chosen as cutoff. The secondary outcome was HCC-related death by means of competing-risk regression analysis. In the study period, 140 patients were included. Patients were transplanted in a median of 23 months after HR (IQR 14–41). Among all the features analyzed regarding the prior HR, only time interval between HR and SLT (time HR-SLT) was an independent predictor of severe complications after LT (OR = 0.98, p < 0.001). According to fractional polynomial regression, the probability of severe complications increased up to 15 months after HR (43%), then slowly decreased over time (OR = 0.88, p < 0.001). There was no significant association between HCC-related death and time HR-SLT at the multivariable competing risks regression model (SHR, 1.06; 95% CI: 0.69–1.62, p = 0.796). This study showed that time HR-SLT was key in predicting complications after LT, without affecting HCC-related death.

scholarly journals Ovarian Dysgerminoma in Pregnant Women with Viable Fetus: A Rare Case Report

2021 ◽  
pp. 141-146
Author(s):  
Reda Youssef ◽  
Gamal Sayed Ahmed ◽  
Samir Alhyassat ◽  
Sanaa Badr ◽  
Ahmed Sabry ◽  
...  
Keyword(s):  
Gestational Age ◽  
Ovarian Neoplasms ◽  
Imaging Modality ◽  
Reproductive Age ◽  
Diagnostic Challenge ◽  
Short Period ◽  
Ovarian Dysgerminoma ◽  
The Impact ◽  
In Pregnancy

Dysgerminoma is an uncommon malignant tumor arising from the germ cells of the ovary. Its association with pregnancy is extremely rare, with a reported incidence of about 0.2–1 per 100,000 pregnancies. Women in the reproductive age group are more commonly affected. It can be extremely rare to conceive naturally, without assisted reproductive interventions, in cases with ovarian dysgerminoma. If a pregnancy does occur with a concurrent dysgerminoma, it is even more unusual to carry the pregnancy to viability or childbirth without fetal or maternal compromise. We report a case of right ovarian dysgerminoma in a young female with a viable intrauterine pregnancy at 10 weeks, which is rarely diagnosed and managed at this gestational age. Numerous factors played a role in her favorable outcome, including early suspicion by ultrasound and presenting history, surgery, histopathological assessment, imaging, and involvement of the multidisciplinary oncology team. Ovarian neoplasms may rapidly increase in size within a short period with little or no symptoms. This poses a diagnostic challenge for obstetricians and oncologists. Hence, we aimed to evaluate the role of imaging in pregnancy using ultrasound as an imaging modality for both early detection of ovarian neoplasms and for follow-up. In conclusion, patients with ovarian dysgerminoma in pregnancy can have favorable outcomes. Treatment should be individualized on a case-to-case basis, depending on many factors; cancer stage, previous reproductive history, the impact of imaging in staging or follow-up of tumor on the fetus, fetal gestational age, and whether termination of the pregnancy can improve survival or morbidity for the mother.

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