From U VA via Sky and Telescope: “Metal-rich Stars Host Closer Planets”

UVA bloc

University of Virginia

Sky and Telescope

January 10, 2018
Monica Young

Artist’s impression of the view just above the surface of one of the middle planets in the TRAPPIST-1 system. Impression based on the known physical parameters for the planets and stars seen, and using a vast database of objects in the Universe. ESO/N. Bartmann/

An artist’s rendering of how the iron content of a star can impact its planets. A normal star (green label) is more likely to host a longer-period planet (green orbit), while an iron-rich star (yellow label) is more likely to host a shorter-period planet (yellow orbit). Credit: Dana Berry/SkyWorks Digital Inc.; SDSS collaboration

Iron-rich stars host planets on closer orbits than their iron-poor siblings, astronomers find. The results could help reveal how planets form.

The more iron a star contains, the closer its planet’s orbit. And astronomers aren’t quite sure why.

Robert Wilson, a graduate student at the University of Virginia, announced the puzzling result at a meeting of the American Astronomical Society in Washington, D.C.

Stars are mostly hydrogen and helium, with just a smattering of heavier elements. Since stars forge heavy elements in their core, the ones we see on the surface come from previous generations of stars. The longer a star’s lineage, the more such elements enrich it (or pollute it, depending on your point of view). The heaviest element a star can make is iron, so its abundance serves as a proxy for the presence of all the other elements in the star, or in astro-speak, the star’s metallicity.

Planets form out of the same natal gas as their parent star. So a star’s high metallicity is a sign that its planets came together within metal-enriched gas. Previous studies [Nature] have found that metallicity plays a role in planet formation — but astronomers don’t yet understand how the connection works.

Wilson studied metallicity’s effect on planet formation using data from the exoplanet-hunting Kepler mission, a space telescope that imaged a field of stars, looking for the momentary dips in brightness that signal an exoplanet’s crossing.

NASA/Kepler Telescope

Kepler has found more than 2,500 confirmed planets to date. For roughly half of these, the Sloan 2.5-meter telescope in New Mexico took additional spectroscopic data, revealing each star’s iron abundance.

SDSS Telescope at Apache Point Observatory, near Sunspot NM, USA, Altitude 2,788 meters (9,147 ft)

Apache Point Observatory, Apache Point Observatory, NM, USA. n the Sacramento Mountains in Sunspot, New Mexico, Altitude 2,788 meters (9,147 ft)

This artist’s conception shows the silhouette of a rocky planet, dubbed HD 219134b, as it passes in front of its star. NASA/JPL-Caltech.

To Wilson’s surprise, the stars richest in iron host planets on scorchingly close orbits, while stars with lower iron abundances have planets on farther-out orbits. The results point to different formation histories for the two types of planets.

A clear line divides the two groups of planets: iron-rich stars host planets with orbits of 8 days or less, while the farther-out planets circle their iron-poor stars on periods longer than 8 days. Yet the two sets of stars aren’t all that different from each other — the ones labeled iron-rich have only 25% more iron than those labeled iron-poor.

“That’s like adding five-eighths of a teaspoon of salt into a cupcake recipe that calls for half a teaspoon, among all its other ingredients,” Wilson says. When baking a planet, it turns out, even a small difference in the metallicity of a planet’s natal cloud can have surprisingly strong effects on its formation.

But how? Wilson suspects that higher-metallicity gas makes for flatter planet-forming disks. The presence of heavy elements helps gas in the planetary disk cool and collapse to the centerline — like someone forgot the baking powder when making pancakes. Thinner disks make it easier for forming planets to migrate inward, closer to the star.

The next step will be an astronomer’s version of America’s Test Kitchen: Wilson is working with theorists to cook up stars and their planet-forming disks within different metallicity environments to see if they can reproduce the same iron-rich/iron-poor divide.

See the full article here .

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UVA campus

The University of Virginia (U.Va. or UVA), frequently referred to simply as Virginia, is a public-private flagship and research university.[1][2][3] Founded in 1819 by Declaration of Independence author Thomas Jefferson, UVA is known for its historic foundations, student-run honor code, and secret societies.

UNESCO designated UVA as America’s first and only collegiate World Heritage Site in 1987, an honor shared with nearby Monticello.[7] The university was established in 1819, and its original governing Board of Visitors included Thomas Jefferson, James Madison, and James Monroe. Monroe was the sitting President of the United States at the time of its foundation. Former Presidents Jefferson and Madison were UVA’s first two rectors and the Academical Village and original courses of study were conceived and designed by Jefferson.

The university’s research endeavors are highly recognized. In 2015, Science honored UVA faculty for discovering two of its top 10 annual scientific breakthroughs; from the fields of Medicine and Psychology.[8] UVA is one of 62 institutions in the Association of American Universities (AAU), an organization of preeminent North American research universities. It is the only AAU member university located in Virginia. UVA is classified as a Research University with Very High Research by the Carnegie Foundation, and is considered Virginia’s flagship university by the College Board.[9][10][11] The university was the first non-founding member, and the first university of the American South, to attain AAU membership in 1904. UVA has been referred to as a “Public Ivy” by various sources.[12][13] Companies founded by UVA students and alumni, such as Reddit, generate more than $1.6 trillion in annual revenue – equivalent to an economy the size of Canada, 10th-largest in the world.[14][15]

UVA’s academic strength is broad, with 121 majors across the eight undergraduate and three professional schools.[16] Students compete in 26 collegiate sports and UVA leads the Atlantic Coast Conference in men’s NCAA team national championships with 17. UVA is second in women’s NCAA titles with 7. UVA was awarded the Capital One Cup in 2015 after fielding the top overall men’s athletics programs in the nation.[17]

Students come to attend the university in Charlottesville from all 50 states and 147 countries.[18][19][20] The historical campus occupies 1,682-acre (2.6 sq mi; 680.7 ha), many of which are internationally protected by UNESCO and widely recognized as some of the most beautiful collegiate grounds in the country.[21] UVA additionally maintains 2,913-acre (4.6 sq mi; 1,178.8 ha) southeast of the city, at Morven Farm.[22] The university also manages the College at Wise in Southwest Virginia, and until 1972 operated George Mason University and the University of Mary Washington in Northern Virginia.