181 research outputs found
Dynamics of the radiative envelope of rapidly rotating stars: Effects of spin-down driven by mass loss
(abridged) This paper aims at deciphering the dynamics of the envelope of a
rotating star when some angular momentum loss due to mass loss is present. We
especially wish to know when the spin-down flow forced by the mass loss
supersedes the baroclinic flows that pervade the radiative envelope of rotating
stars. We consider a Boussinesq fluid enclosed in a rigid sphere whose flows
are forced both by the baroclinic torque, the spin-down of an outer layer, and
an outward mass flux. The spin-down forcing is idealized in two ways: either by
a rigid layer that imposes its spinning down velocity at some interface or by a
turbulent layer that imposes a stress at this same interface to the interior of
the star. In the case where the layer is rigid and imposes its velocity, we
find that, as the mass-loss rate increases, the flow inside the star shows two
transitions: the meridional circulation associated with baroclinic flows is
first replaced by its spin-down counterpart, while at much stronger mass-loss
rates the baroclinic differential rotation is superseded by the spin-down
differential rotation. In fact, we find three wind regimes: weak (or no wind),
moderate, and strong. In the first case, the flow in the radiative envelope is
of baroclinic origin. In the moderate case, the circulation results from the
spin-down while the differential rotation may either be of baroclinic or of
spin-down origin, depending on the coupling between mass and angular momentum
losses. For fast rotating stars, our model says that the moderate wind regime
starts when mass loss is higher than ~1e-11 Msun/yr. In the strong wind case,
the flow in the radiative envelope is mainly driven by angular momentum
advection. This latter transition depends on the mass and the rotation rate of
the star, being around 1e-8 Msun/yr for a 3 Msun ZAMS star rotating at 200 km/s
according to our model.Comment: 13 pages, 9 figures, to appear in Astronomy and Astrophysic
Theory for planetary exospheres: III. Radiation pressure effect on the Circular Restricted Three Body Problem and its implication on planetary atmospheres
The planetary exospheres are poorly known in their outer parts, since the
neutral densities are low compared with the instruments detection capabilities.
The exospheric models are thus often the main source of information at such
high altitudes. We present a new way to take into account analytically the
additional effect of the stellar radiation pressure on planetary exospheres. In
a series of papers, we present with an Hamiltonian approach the effect of the
radiation pressure on dynamical trajectories, density profiles and escaping
thermal flux. Our work is a generalization of the study by Bishop and
Chamberlain (1989). In this third paper, we investigate the effect of the
stellar radiation pressure on the Circular Restricted Three Body Problem
(CR3BP), called also the photogravitational CR3BP, and its implication on the
escape and the stability of planetary exospheres, especially for Hot Jupiters.
In particular, we describe the transformation of the equipotentials and the
location of the Lagrange points, and we provide a modified equation for the
Hill sphere radius that includes the influence of the radiation pressure.
Finally, an application to the hot Jupiter HD 209458b reveals the existence of
a blow-off escape regime induced by the stellar radiation pressure
Theory for planetary exospheres: I. Radiation pressure effect on dynamical trajectories
The planetary exospheres are poorly known in their outer parts, since the
neutral densities are low compared with the instruments detection capabilities.
The exospheric models are thus often the main source of information at such
high altitudes. We present a new way to take into account analytically the
additional effect of the radiation pressure on planetary exospheres. In a
series of papers, we present with an Hamiltonian approach the effect of the
radiation pressure on dynamical trajectories, density profiles and escaping
thermal flux. Our work is a generalization of the study by Bishop and
Chamberlain (1989). In this first paper, we present the complete exact
solutions of particles trajectories, which are not conics, under the influence
of the solar radiation pressure. This problem was recently partly solved by
Lantoine and Russell (2011) and completely by Biscani and Izzo (2014). We give
here the full set of solutions, including solutions not previously derived, as
well as simpler formulations for previously known cases and comparisons with
recent works. The solutions given may also be applied to the classical Stark
problem (Stark,1914): we thus provide here for the first time the complete set
of solutions for this well-known effect in term of Jacobi elliptic functions
Modélisation semi-analytique des exosphÚres planétaires : analyse de l'influence des collisions et de la pression de radiation stellaire
La partie externe de l'atmosphĂšre, l'exosphĂšre, est une rĂ©gion encore mal connue. Les densitĂ©s y sont trop faibles pour nombre d'instruments, et la modĂ©lisation de la dynamique des particules peut ĂȘtre complexe. Au cours de ma thĂšse, je me suis intĂ©ressĂ© Ă deux problĂ©matiques: la production de particules " satellites " Ă partir des rares collisions dans la basse exosphĂšre et l'influence de la pression de radiation sur la structure de l'exosphĂšre.
Dans la premiÚre partie de ma thÚse, nous avons modélisé l'impact des rares collisions prÚs de l'exobase sur les densités à plus haute altitude pour les cas de la Terre, Titan et Mars, au travers de la production de particules " satellites ", absentes des modÚles non-collisionnels. Dans une seconde partie, j'ai étudié l'effet de la pression de radiation sur la structure de l'exosphÚre par une approche semi-analytique. La pression de radiation affecte les profils de densité des populations " balistiques " et induit de fortes asymétries à haute altitude. Elle augmente également le flux d'échappement thermique, que nous avons déterminé analytiquement au point subsolaire. Finalement, nous avons également étudié l'influence de la pression de radiation stellaire sur le problÚme à trois corps et son impact sur la stabilité des atmosphÚres, en particulier celles des exoplanÚtes de type Jupiters chauds.The external part of the atmosphere, the exosphere, is not a well-known region. The densities are too low for many instruments compared with their detection capabilities, and the modeling of the particles dynamics can be complex. During my PhD thesis, I focused on two problems: the production of "satellite" particles from the scarce collisions in the lower exosphere and the influence of the radiation pressure on the exosphere structure. In the first part of my thesis, we modeled the influence of the scarce collisions near the exobase on the density profiles at higher altitudes for the Earth, Titan and Mars, through the production of "satellite" particles, which are neglected in the collisionless models. In a second part, I studied the effect of the radiation pressure on the structure of the exosphere with a semi-analytical approach. The radiation pressure changes the ballistic particle density profiles and implies strong asymmetries at high altitudes. It increases also the thermal escaping flux, which we determined analytically at the subsolar point. Finally, we studied its influence on the Three-Body problem and on the stability of the atmospheres, in particular for hot Jupiter exoplanets
Cometary Ionospheres: An Updated Tutorial
This chapter aims at providing the tools and knowledge to understand and
model the plasma environment surrounding comets in the innermost part near the
nucleus. In particular, our goal is to give an updated post-Rosetta view of
this ionised environment: what we knew, what we confirmed, what we overturned,
and what we still do not understand.Comment: 41 pages, 14 figures, 3 tables; To be published in Comets III (2023),
K. J. Meech and M. Combi (Eds.), University of Arizona Press, Tucso
Making waves: Mirror Mode structures around Mars observed by the MAVEN spacecraft
We present an in-depth analysis of a time interval when quasi-linear mirror
mode structures were detected by magnetic field and plasma measurements as
observed by the NASA/Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft.
We employ ion and electron spectrometers in tandem to support the magnetic
field measurements and confirm that the signatures are indeed mirror modes.
Wedged against the magnetic pile-up boundary, the low-frequency signatures last
on average 10 s with corresponding sizes of the order of 15-30 upstream
solar wind proton thermal gyroradii, or 10-20 proton gyroradii in the immediate
wake of the quasi-perpendicular bow shock. Their peak-to-peak amplitudes are of
the order of 30-35 nT with respect to the background field, and appear as a
mixture of dips and peaks, suggesting that they may have been at different
stages in their evolution. Situated in a marginally stable plasma with
1, we hypothesise that these so-called magnetic bottles,
containing a relatively higher energy and denser ion population with respect to
the background plasma, are formed upstream of the spacecraft behind the
quasi-perpendicular shock. These signatures are very reminiscent of magnetic
bottles found at other unmagnetised objects such as Venus and comets, also
interpreted as mirror modes. Our case study constitutes the first unmistakable
identification and characterisation of mirror modes at Mars from the joint
points of view of magnetic field, electron and ion measurements. Up until now,
the lack of high-temporal resolution plasma measurements has prevented such an
in-depth study.Comment: 37 pages, 11 figures, 1 tabl
Plasma-neutral gas interactions in various space environments: Assessment beyond simplified approximations as a Voyage 2050 theme
In the White Paper, submitted in response to the European Space Agency (ESA) Voyage 2050 Call, we present the importance of advancing our knowledge of plasma-neutral gas interactions, and of deepening our understanding of the partially ionized environments that are ubiquitous in the upper atmospheres of planets and moons, and elsewhere in space. In future space missions, the above task requires addressing the following fundamental questions: (A) How and by how much do plasma-neutral gas interactions influence the re-distribution of externally provided energy to the composing species? (B) How and by how much do plasma-neutral gas interactions contribute toward the growth of heavy complex molecules and biomolecules? Answering these questions is an absolute prerequisite for addressing the long-standing questions of atmospheric escape, the origin of biomolecules, and their role in the evolution of planets, moons, or comets, under the influence of energy sources in the form of electromagnetic and corpuscular radiation, because low-energy ion-neutral cross-sections in space cannot be reproduced quantitatively in laboratories for conditions of satisfying, particularly, (1) low-temperatures, (2) tenuous or strong gradients or layered media, and (3) in low-gravity plasma. Measurements with a minimum core instrument package (< 15Â kg) can be used to perform such investigations in many different conditions and should be included in all deep-space missions. These investigations, if specific ranges of background parameters are considered, can also be pursued for Earth, Mars, and Venus
IAFSS agenda 2030 for a fire safe world
The International Association of Fire Safety Science (IAFSS) is comprised of members from some 40 countries. This paper presents the Association's thinking, developed by the Management Committee, concerning pressing research needs for the coming 10 years presented as the IAFSS Agenda 2030 for a Fire Safe World. The research needs are couched in terms of two broad Societal Grand Challenges: (1) climate change, resiliency and sustainability and (2) population growth, urbanization and globalization. The two Societal Grand Challenges include significant fire safety components, that lead both individually and collectively to the need for a number of fire safety and engineering research activities and actions. The IAFSS has identified a list of areas of research and actions in response to these challenges. The list is not exhaustive, and actions within actions could be defined, but this paper does not attempt to cover all future needs
Vaccine breakthrough hypoxemic COVID-19 pneumonia in patients with auto-Abs neutralizing type I IFNs
Life-threatening `breakthrough' cases of critical COVID-19 are attributed to poor or waning antibody response to the SARS- CoV-2 vaccine in individuals already at risk. Pre-existing autoantibodies (auto-Abs) neutralizing type I IFNs underlie at least 15% of critical COVID-19 pneumonia cases in unvaccinated individuals; however, their contribution to hypoxemic breakthrough cases in vaccinated people remains unknown. Here, we studied a cohort of 48 individuals ( age 20-86 years) who received 2 doses of an mRNA vaccine and developed a breakthrough infection with hypoxemic COVID-19 pneumonia 2 weeks to 4 months later. Antibody levels to the vaccine, neutralization of the virus, and auto- Abs to type I IFNs were measured in the plasma. Forty-two individuals had no known deficiency of B cell immunity and a normal antibody response to the vaccine. Among them, ten (24%) had auto-Abs neutralizing type I IFNs (aged 43-86 years). Eight of these ten patients had auto-Abs neutralizing both IFN-a2 and IFN-., while two neutralized IFN-omega only. No patient neutralized IFN-ss. Seven neutralized 10 ng/mL of type I IFNs, and three 100 pg/mL only. Seven patients neutralized SARS-CoV-2 D614G and the Delta variant (B.1.617.2) efficiently, while one patient neutralized Delta slightly less efficiently. Two of the three patients neutralizing only 100 pg/mL of type I IFNs neutralized both D61G and Delta less efficiently. Despite two mRNA vaccine inoculations and the presence of circulating antibodies capable of neutralizing SARS-CoV-2, auto-Abs neutralizing type I IFNs may underlie a significant proportion of hypoxemic COVID-19 pneumonia cases, highlighting the importance of this particularly vulnerable population
The Comet Interceptor Mission
Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESAâs F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum ÎV capability of 600 msâ1. Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes â B1, provided by the Japanese space agency, JAXA, and B2 â that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the missionâs science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule
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