Fr.: V2052 Oph
A hot star of → apparent visual magnitude V = 5.82 lying in the constellation → Ophiuchus, also known as HR 6684, HD 163472, and HIP 87812. V2052 Oph is of → subgiant → spectral type B2 IV-V and belongs to the family of magnetic → Beta Cephei → pulsators. It has a luminosity of 620 Lsol, a radius of 6.83 Rsol and lies about 830 → light-years away. Three stellar pulsation modes have been detected and studied from ground-based spectroscopy and multi-color photometry. Its magnetic field was deduced to be → dipolar dipolar with a strength of ~ 400 G, inclined 35° to the rotation axis. The star's → convective overshooting region is small, with no extra mixing in spite of the relatively large rotational velocity. This clearly illustrated the effect of the magnetic field: the inhibition of chemical mixing processes by the magnetic field. This result is in agreement with theoretical criteria, which predict that a surface magnetic field of ~ 70 G is sufficient to limit the overshooting in this star. V2052 Oph is now considered as a prime example of what magneto-asteroseismology can achieve (Neiner et al., 2012, A&A, 537, A148).
1) A space completely empty of matter but not achievable in practice on Earth.
L. vacuum "an empty space, void," noun use of neuter of vacuus "empty," related to vacare "to be empty."
Xala' loan from Ar.
Fr.: biréfringence du vide
A highly → magnetized vacuum behaving as a prism for the propagation of light, as predicted by → quantum electrodynamics (QED). Attempts to detect this phenomenon in the laboratory have not yet succeeded in the 80 years since it was predicted (Heisenberg & Euler, 1936, Z. Physik, 98, 714). This effect can be detected only in the presence of enormously strong → magnetic fields, such as those around → neutron stars. Owing to the large inferred magnetic fields (B ~ 1013 G, → gauss), radiation from these sources is expected to be substantially polarized, independently of the mechanism actually responsible for the → thermal emission. The strongest magnetic field so far created in a laboratory is less than 106 G lasting only for several tens of milliseconds. A large observed → polarization degree is, however, expected only if QED polarization effects are present in the magnetized vacuum around the star. The detection of a strongly → linearly polarized signal would therefore provide the observational evidence of QED effects in the strong-field regime. Recently a team of astrophysicists (Mignani et al. 2016, arXiv/1610.08323) have detected → linear polarization toward the neutron star RXJ1856.5-3754 (at a significant degree of around 16%). This finding is likely due to the boosting effect of vacuum birefringence occurring in the area of empty space surrounding the neutron star.
Fr.: chambre à vide
An enclosure from which air is removed.
Fr.: énergie du vide
In particle physics the lowest energy allowed by field quantization when all fields are in their → ground states. Vacuum energy is predicted to arise from → virtual particles that fluctuate in and out of existence, as manifested by the → Casimir effect. The cosmological → dark energy is postulated to be related to vacuum fluctuations. There is however an enormous discrepancy with the predictions of quantum field theory. In this theory the value of vacuum energy density is expected to be roughly of the order ρv≅ Emax4, where Emax is the maximum energy at which the field theory is valid. At energies of the order of the → Planck energy, EPl≅ 1019 GeV, vacuum energy might be roughly: ρv≅ EPl4≅ 1076 GeV4. On the other hand, the vacuum energy density in standard cosmological model is given by: ρΛ = ΩΛ.ρcrit, where ΩΛ is the → density parameter for the → cosmological constant and ρcrit is the → critical density. More explicitly, ρΛ = ΩΛ . 3 H2/(8πG). Using present-day values of ΩΛ (0.7) and H (70) leads to ρΛ = 10-46 GeV4. Therefore, the discrepancy between the prediction and the observed value is 122 orders of magnitude.
Fr.: polarisation du vide
A quantum field theory a process in which an electromagnetic field gives rise to virtual electron-positron pairs that in turn exert electromagnetic fields of their own, in a manner similar to classical dielectric polarization.
1) Not clearly or explicitly stated or expressed.
M.E., from M.Fr. vague "empty, vacant; wild, uncultivated; wandering," from L. vagus "wandering, rambling," of unknown origin.
Gereng, from Kalidari gereng "wandering," Kurd. (Kurmanji) garin, (Sorani) garân "to wander," Dehxodâ gereng "shattered;" ultimately from Proto-Ir. *gar-an-ka-, from *gar- "to turn, to wind" (Cheung 2007).
sâl-e gardân, ~ gereng
Fr.: année vague
A year of 365 days that overlooks the fraction of less than 0.25 days corresponding to the whole length of the → tropical year. The vague year was used in the → calendars of ancient Egypt, Iran, Mayas, and some other civilizations. Typically the vague year was divided into 12 months of 30 days each plus 5 → epagomenal days.
A measure of the number of chemical bonds formed by the atoms of a given element. It represents the relative ability of an atom of an element to combine with other atoms. For example, the valence of O in water, H2O, is 2. Also called valency.
M.E., from O.Fr., from L valentia "strength, worth," from valentem (nominative valens), pr.p. of valere "to be strong."
Arzâyi, noun from arzâ, agent noun/adj. from arzidan "to be worth," arzân "worthy; of small value, cheap," arj "esteem, honour, price, worth" (Mid.Pers. arz "value, worth," arzidan "be worth," arzân "valuable;" Av. arəjaiti "is worth," arəja- "valuable," arəg- "to be worth;" cf. Skt. arh- "to be worth, to earn," árhant- "worthy person;" Gk. alphanein "to bring in as profit," alphein "to ear, obtain;" Lith. algà "salary, pay;" PIE base *algwh- "to earn; price, value").
Fr.: bande de valence
The range of energy states in the spectrum of a solid crystal which includes the energies of all the electrons binding the crystal together.
Fr.: électron de valence
An electron of an atom lying farthest from the nucleus. Valence electrons are shared when atoms combine to form molecules.
Same as → valence.
Logic: Of an argument, if the premises are true, then the conclusion cannot be denied without contradiction. The truth of the premises logically guarantees the truth of the conclusion.
M.E., from M.Fr. valide, from L. validus "strong, effective," from valere "to be strong."
Pâymand, from pâ, pây "foot; base, foundation, firmness" (Mid.Pers. pâd, pây; Av. pad- "foot;" cf. Skt. pat; Gk. pos, genitive podos; L. pes, genitive pedis; P.Gmc. *fot; E. foot; Ger. Fuss; Fr. pied; PIE *pod-/*ped-) + -mand possession suffix.
Logic: Of an argument, whether or not the conclusion follows logically from the premises and the allowable syllogisms of the logical system being used.
Quality noun from → valid.
Fr.: Valles Marineris
A system of canyons located just south of the Martian equator. The system is about 4000 km long. The central individual troughs, generally 50 to 100 km wide, merge into a depression as much as 600 km wide. In places the canyon floor reaches a depth of 10 km, 6 to 7 times deeper than the Grand Canyon on Earth.
L. Valles Marineris "Mariner's Valleys," named after the Mars orbiter Mariner 9, which discovered the Martian canyon in 1971-72. → valley.
A long, narrow region of low land between ranges of mountains, hills, or other high areas, often having a river or stream running along the bottom.
M.E. valeie, valey, from O.Fr. valee "valley," from V.L. *vallata, from L. vallis "valley," of unknown origin.
Darré "valley," from Mid.Pers. dar, darrak "ravine, cleft;" cf. Skt. pradará- "cleft."
Math.: Magnitude; quantity; a numerical quantity assigned to a mathematical symbol.
M.E., from O.Fr. value "worth, value," noun use of feminine p.p. of valoir "be worth," from L. valere "be strong, be well, be of value."
Arzeš, verbal noun of arzidan "to be worth," arzân "worthy; of small value, cheap," arj "esteem, honour, price, worth;" Mid.Pers. arz "value, worth," arzidan "be worth," arzân "valuable;" Av. arəjaiti "is worth," arəja- "valuable," arəg- "to be worth;" cf. Skt. arh- "to be worth, to earn," árhant- "worthy person;" Gk. alphanein "to bring in as profit," alphein "to ear, obtain;" Lith. algà "salary, pay;" PIE base *algwh- "to earn; price, value."
Fr.: valve, soupape
Any device that halts or regulates the flow of a fluid through a passage, pipe, etc.
From L. valvae "leaves of a door."
Daricé, leterally "small door, window," → stop. Supâp, loan from Fr. soupape, from O.Fr. sourpape, probably from souspape "a blow under chin," from sous "under," from L. subtus, → sub-, + *pape "jaw," from paper, from L. pap(p)are "to eat."
Fr.: mécanisme de valve
A mechanism proposed by Eddington to explain → stellar pulsations. Same as the → kappa mechanism. In this analogy the stellar layer acts like a heat engine with radiation taking the role of stream. The expanding and contracting layer acts as the piston, and the opacity of the layer behaves as the valve mechanism (Eddington, 1917, The pulsation theory of → Cepheid variables, The Observatory 40, 290).
Van Allen belts
kamarbandhâ-ye Van Allen
Fr.: ceintures de Van Allen
The ring-shaped regions of charged particles surrounding the Earth from 1 to 6 Earth radii into space. The charged particles are trapped in by the Earth's magnetic field. The inner belt is between 1.2 and 4.5 Earth radii and contains high-energy electrons and protons which originate mainly from interactions between cosmic rays and the upper atmosphere. The outer belt, located between 4.5 and 6.0 Earth radii, contains lower-energy charged particles mainly coming from the solar wind.
Named after James Van Allen (1914-2006), who discovered the belts in 1958 based on measurements made by Explorer 1, the USA's first successful artificial satellite; → belt.