ArXiv is a repository of electronic preprints approved for posting after moderation, but not full peer review. It consists of scientific papers in the fields of mathematics, astronomy, electrical engineering, computer science, quantitative biology, mathematical finance and economics, which can be accessed online. In many fields of mathematics and physics all scientific papers are self-archived on the arXiv repository. Begun on August 14, 1991, arXiv.org passed the half-million-article milestone on October 3, 2008, had hit a million by the end of 2014. By October 2016 the submission rate had grown to more than 10,000 per month. ArXiv was made possible by the compact TeX file format, which allowed scientific papers to be transmitted over the Internet and rendered client-side. Around 1990, Joanne Cohn began emailing physics preprints to colleagues as TeX files, but the number of papers being sent soon filled mailboxes to capacity. Paul Ginsparg recognized the need for central storage, in August 1991 he created a central repository mailbox stored at the Los Alamos National Laboratory which could be accessed from any computer.
Additional modes of access were soon added: FTP in 1991, Gopher in 1992, the World Wide Web in 1993. The term e-print was adopted to describe the articles, it began as a physics archive, called the LANL preprint archive, but soon expanded to include astronomy, computer science, quantitative biology and, most statistics. Its original domain name was xxx.lanl.gov. Due to LANL's lack of interest in the expanding technology, in 2001 Ginsparg changed institutions to Cornell University and changed the name of the repository to arXiv.org. It is now hosted principally with eight mirrors around the world, its existence was one of the precipitating factors that led to the current movement in scientific publishing known as open access. Mathematicians and scientists upload their papers to arXiv.org for worldwide access and sometimes for reviews before they are published in peer-reviewed journals. Ginsparg was awarded a MacArthur Fellowship in 2002 for his establishment of arXiv; the annual budget for arXiv is $826,000 for 2013 to 2017, funded jointly by Cornell University Library, the Simons Foundation and annual fee income from member institutions.
This model arose in 2010, when Cornell sought to broaden the financial funding of the project by asking institutions to make annual voluntary contributions based on the amount of download usage by each institution. Each member institution pledges a five-year funding commitment to support arXiv. Based on institutional usage ranking, the annual fees are set in four tiers from $1,000 to $4,400. Cornell's goal is to raise at least $504,000 per year through membership fees generated by 220 institutions. In September 2011, Cornell University Library took overall administrative and financial responsibility for arXiv's operation and development. Ginsparg was quoted in the Chronicle of Higher Education as saying it "was supposed to be a three-hour tour, not a life sentence". However, Ginsparg remains on the arXiv Scientific Advisory Board and on the arXiv Physics Advisory Committee. Although arXiv is not peer reviewed, a collection of moderators for each area review the submissions; the lists of moderators for many sections of arXiv are publicly available, but moderators for most of the physics sections remain unlisted.
Additionally, an "endorsement" system was introduced in 2004 as part of an effort to ensure content is relevant and of interest to current research in the specified disciplines. Under the system, for categories that use it, an author must be endorsed by an established arXiv author before being allowed to submit papers to those categories. Endorsers are not asked to review the paper for errors, but to check whether the paper is appropriate for the intended subject area. New authors from recognized academic institutions receive automatic endorsement, which in practice means that they do not need to deal with the endorsement system at all. However, the endorsement system has attracted criticism for restricting scientific inquiry. A majority of the e-prints are submitted to journals for publication, but some work, including some influential papers, remain purely as e-prints and are never published in a peer-reviewed journal. A well-known example of the latter is an outline of a proof of Thurston's geometrization conjecture, including the Poincaré conjecture as a particular case, uploaded by Grigori Perelman in November 2002.
Perelman appears content to forgo the traditional peer-reviewed journal process, stating: "If anybody is interested in my way of solving the problem, it's all there – let them go and read about it". Despite this non-traditional method of publication, other mathematicians recognized this work by offering the Fields Medal and Clay Mathematics Millennium Prizes to Perelman, both of which he refused. Papers can be submitted in any of several formats, including LaTeX, PDF printed from a word processor other than TeX or LaTeX; the submission is rejected by the arXiv software if generating the final PDF file fails, if any image file is too large, or if the total size of the submission is too large. ArXiv now allows one to store and modify an incomplete submission, only finalize the submission when ready; the time stamp on the article is set. The standard access route is through one of several mirrors. Sev
A protoplanetary disk is a rotating circumstellar disk of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may be considered an accretion disk for the star itself, because gases or other material may be falling from the inner edge of the disk onto the surface of the star; this process should not be confused with the accretion process thought to build up the planets themselves. Externally illuminated photo-evaporating protoplanetary disks are called proplyds. In July 2018, the first confirmed image of such a disk, containing a nascent exoplanet, named PDS 70b, was reported. Protostars form from molecular clouds consisting of molecular hydrogen; when a portion of a molecular cloud reaches a critical size, mass, or density, it begins to collapse under its own gravity. As this collapsing cloud, called a solar nebula, becomes denser, random gas motions present in the cloud average out in favor of the direction of the nebula's net angular momentum.
Conservation of angular momentum causes the rotation to increase. This rotation causes the cloud to flatten out—much like forming a flat pizza out of dough—and take the form of a disk; this occurs because centripetal acceleration from the orbital motion resists the gravitational pull of the star only in the radial direction, but the cloud remains free to collapse in the vertical direction. The outcome is the formation of a thin disc supported by gas pressure in the vertical direction; the initial collapse takes about 100,000 years. After that time the star reaches a surface temperature similar to that of a main sequence star of the same mass and becomes visible, it is now a T Tauri star. Accretion of gas onto the star continues for another 10 million years, before the disk disappears being blown away by the young star's solar wind, or simply ceasing to emit radiation after accretion has ended; the oldest protoplanetary disk yet discovered is 25 million years old. Protoplanetary disks around T Tauri stars differ from the disks surrounding the primary components of close binary systems with respect to their size and temperature.
Protoplanetary disks have radii up to 1000 AU, only their innermost parts reach temperatures above 1000 K. They are often accompanied by jets. Protoplanetary disks have been observed around several young stars in our galaxy. Recent observations by the Hubble Space Telescope have shown proplyds and planetary disks to be forming within the Orion Nebula. Protoplanetary disks are thought to be thin structures, with a typical vertical height much smaller than the radius, a typical mass much smaller than the central young star; the mass of a typical proto-planetary disk is dominated by its gas, the presence of dust grains has a major role in its evolution. Dust grains shield the mid-plane of the disk from energetic radiation from outer space that creates a dead zone in which the MRI no longer operates, it is believed that these disks consist of a turbulent envelope of plasma called the active zone, that encases an extensive region of quiescent gas called the dead zone. The dead zone located at the mid-plane can slow down the flow of matter through the disk which prohibits achieving a steady state.
The nebular hypothesis of solar system formation describes how protoplanetary disks are thought to evolve into planetary systems. Electrostatic and gravitational interactions may cause the dust and ice grains in the disk to accrete into planetesimals; this process competes against the stellar wind, which drives the gas out of the system, gravity and internal stresses, which pulls material into the central T Tauri star. Planetesimals constitute the building blocks of both giant planets; some of the moons of Jupiter and Uranus are believed to have formed from smaller, circumplanetary analogs of the protoplanetary disks. The formation of planets and moons in geometrically thin, gas- and dust-rich disks is the reason why the planets are arranged in an ecliptic plane. Tens of millions of years after the formation of the Solar System, the inner few AU of the Solar System contained dozens of moon- to Mars-sized bodies that were accreting and consolidating into the terrestrial planets that we now see.
The Earth's moon formed after a Mars-sized protoplanet obliquely impacted the proto-Earth ~30 million years after the formation of the Solar System. Gas-poor disks of circumstellar dust have been found around many nearby stars—most of which have ages in the range of ~10 million years to billions of years; these systems are referred to as "debris disks". Given the older ages of these stars, the short lifetimes of micrometer-sized dust grains around stars due to Poynting Robertson drag and radiation pressure, it is thought that this dust is from the collisions of planetesimals. Hence the debris disks around these examples are not "protoplanetary", but represent a stage of disk evolution where extrasolar analogs of the asteroid belt and Kuiper belt are home to dust-generating collisions between planetesimals. Based on recent computer model studies, the complex organic molecules necessary for life may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of the Earth.
According to the computer studies, this same process may occur around other stars that acquire planets.. Williams, J. P.. A.. "Protoplanetary Disks and Their Evolution". Annual Review of Astronomy and Astroph
Herbig Ae/Be star
A Herbig Ae/Be star is a pre-main-sequence star – a young star of spectral types A or B. These stars are still embedded in gas-dust envelopes and are sometimes accompanied by circumstellar disks. Hydrogen and calcium emission lines are observed in their spectra, they are 2-8 Solar mass objects, still existing in the star formation stage and approaching the main sequence. In the Hertzsprung–Russell diagram these stars are located to the right of the main sequence, they are named after the American astronomer George Herbig, who first distinguished them from other stars in 1960. The original Herbig criteria were: Spectral type earlier than F0, Balmer emission lines in the stellar spectrum, Projected location within the boundaries of a dark interstellar cloud, Illumination of a nearby bright reflection nebula. There are now several known isolated Herbig Ae/Be stars, thus the most reliable criteria now can be: Spectral type earlier than F0, Balmer emission lines in the stellar spectrum, Infrared radiation excess due to circumstellar dust.
Sometimes Herbig Ae/Be stars show significant brightness variability. They are believed to be due to clumps in the circumstellar disk. In the lowest brightness stage the radiation from the star becomes linearly polarized. Analogs of Herbig Ae/Be stars in the smaller mass range – F, G, K, M spectral type pre-main-sequence stars – are called T Tauri stars. More massive stars in pre-main-sequence stage are not observed, because they evolve quickly: when they become visible, the hydrogen in the center is burning and they are main-sequence objects. Planets around Herbig Ae/Be stars include: HD 95086 b around an A-type star Pérez M. R. Grady C. A. Observational Overview of Young Intermediate-Mass Objects: Herbig Ae/Be Stars, Space Science Reviews, Vol 82, p. 407-450 Waters L. B. F. M. Waelkens, C. HERBIG Ae/Be STARS, Annual Review of Astronomy and Astrophysics, Vol. 36, p. 233-266 Herbig Ae/Be stars"Molecular Hydrogen In The Circumstellar Environment Of Herbig Ae/Be Stars". Mpia-hd.mpg.de. Retrieved 2008-10-16