White Dwarf Stars Found With Carbon Atmospheres

Recently, nine white dwarf stars were found with atmospheres primarily composed of carbon, with little or no trace of hydrogen or helium. The stars are considered rare. They could become a new category of star. Because they are not like the known classes of white dwarf stars, their evolution is still uncertain. An article on the study was published in the November 22 issue of the journal Nature. The report is written by Professors Patrick Dufour and James Liebert of the Department of Astronomy and Steward Observatory, University of Arizona, and their colleagues at the Université de Montréal and Paris Observatory. The discovery could offer new insight on the hearts of dying stars.

White dwarfs are the stars of relatively small mass that are in the last stage of their evolution. They start out as normal stars, but over billions of years, they expand into red giants before exhausting their energy and collapsing into objects not much bigger than Earth. Stars of larger mass usually end as either black holes or neutron stars (sometimes called pulsars).

When a star burns helium, it leaves "ashes" or "residue" of carbon and oxygen. When its nuclear fuel is exhausted, the star then dies as a white dwarf, which is an extremely dense object that packs the mass of our sun into an object about the size of Earth. The theory of stellar evolution predicts that most white dwarfs have a core made of carbon and oxygen, which is surrounded by a layer of helium and, for 80% of them, an additional layer of hydrogen. Astronomers didn't expect stars to find dwarf stars with carbon atmospheres. Some even suggest they might be seeing the bare stellar core.

The recently identified dwarf stars may have evolved in a way astronomers didn't know before. For example, they may have begun as stars that were more massive, but not quite massive enough to explode as supernovae. It seems that all but the most massive two or three percent of stars eventually die as white dwarfs rather than explode as supernovae.

The nine stars were discovered among 10,000 new white dwarf stars found in the Sloan Digital Sky Survey (SDSS). The survey found about four times the number of white dwarf stars previously known.

Liebert identified a few dozens of the newfound white dwarfs as "DQ" white dwarfs in 2003. When observed in optical light, DQ stars appear to be mostly helium and carbon. Astronomers think that convection in the helium zone pulls up carbon from the star's carbon-oxygen core.

Dufour developed a model to analyze the atmospheres of DQ stars as part of his doctoral research at the Université de Montréal. His model simulated cool DQ stars, stars at temperatures between 5,000 and 12,000 Kelvin. For reference, our sun's surface temperature is around 5,780 Kelvin.

When Dufour joined Steward Observatory in January of 2007, he updated his code to analyze hotter stars, stars as hot as 24,000 Kelvin.

When Dufour began modeling the atmospheres of the hotter DQ stars, he at first thought they were helium-rich stars with traces of carbon, just like the cooler ones, but the model did not agree with the SDSS data. Dufour gradually adjusted his model to have a larger and larger abundance of carbon, but the model still didn't agree. Then in May of 2007, in desperation, Dufour calculated a model with a pure-carbon atmosphere and it worked, exactly reproducing the observed spectra of the hotter DQ stars. Until this time, no one had ever calculated a pure carbon atmosphere model, because no one thought it existed. Astronomers are very excited by the news.

As of this writing, Dufour and his colleagues have identified eight carbon-dominated atmosphere white dwarf stars among about 200 DQ stars they've checked in the SDSS data.

For the team, the biggest question is why the carbon-atmosphere stars are found only between about 18,000 and 23,000 Kelvin. The stars are too hot to be explained by the standard convective dredge-up scenario, so there must be another explanation.

Dufour and Liebert think these stars might have evolved from a star like the unique, much hotter star called H1504+65 that was reported in 1986 by Pennsylvania State University astronomer John A. Nousek, Liebert and others. If this is so, carbon-atmosphere stars represent a previously unknown sequence of stellar evolution.

H1504+65 is a very massive and very hot star with a surface temperature of 200,000 Kelvin. Astronomers currently think the star somehow violently expelled all its hydrogen and all but a very small trace of its helium, leaving an essentially bare stellar nucleus with a surface of 50 percent carbon and 50 percent oxygen.

Dufour and Liebert think that when a star like H1504+65 cools, it eventually becomes like the pure-carbon stars. As the massive star cools, gravity separates the carbon, oxygen and trace helium. Above 25,000 Kelvin, the trace helium rises to the top, forming a thin layer above the much more massive carbon envelope, effectively disguising the star as a helium-atmosphere white dwarf, Dufour and Liebert said.

But between 18,000 and 23,000 Kelvin, convection in the carbon zone probably dilutes the thin helium layer. At these temperatures, oxygen, which is heavier than carbon, has probably sunk too deep to be pulled to the surface by convection.

Dufour and his colleagues say that models of stars of 9 to 11 solar masses might explain their peculiar carbon stars. In 1999, astronomers predicted that stars of this mass would become white dwarfs with oxygen-magnesium-neon cores and mostly carbon-oxygen atmospheres. Anything larger was thought to explode as a supernova. Now, scientists aren't sure where the dividing line is, whether stars of eight, nine, 10 or 11 solar masses are required to create supernovae.

The UA astronomers plan making new observations of the carbon atmosphere stars at the 6.5-meter MMT Observatory on Mount Hopkins, Ariz., in December to better pinpoint their masses. The observations could help define the mass limit for stars dying as white dwarfs or dying as supernovae.

To learn more about the University of Arizona Department of Astronomy and Steward Observatory, visit their home page: http://www.as.arizona.edu/

To learn more about the Sloan Digital Sky Survey (SDSS), visit their home page: http://www.sdss.org/


****************************************

MARS IS COMING, PART 5

As the December 24 opposition approaches, please enjoy this fifth installment on the Red Planet Mars.


The Polar Caps

Early each southern spring, the southern carbon dioxide cap begins to recede from its maximum extent of 50° S. As spring progresses, the cap shrinks by as much as 1° of latitude every five Earth days. The edge of the cap becomes ragged, controlled by the local topography (e.g., craters), and eventually it breaks into well-defined fragments. From year to year, the location of the fragments remains about the same, although there are small variations in detail. Over the course of one-third of the Martian year, the cap recedes to its smallest extent; it is then not usually visible from Earth, but spacecraft observations have confirmed the presence of a small remnant throughout the summer.

Re-growth of the southern cap begins, after a brief period of atmospheric clarity, with the rapid formation of obscuring clouds called the polar hood. On occasion the hood is transparent enough to red light so that spacecraft may view the formation of the cap. These limited opportunities, however, have not allowed the cap’s rate of advance toward the equator to be accurately known. The polar hood (the cloud cover) is far more extensive than the cap itself and can reach to within 35° of the equator. The hood is thought to contain particles of frozen water and carbon dioxide.

Differences in the behavior of the northern carbon dioxide cap—including its smaller maximum size in winter and complete disappearance in summer—are due to (1) differences in the lengths of seasons for the two hemispheres, (2) in the distances from the sun during their respective winters, and (3) in elevation between the two poles. The recession of the seasonal cap in the north is much more regular than in the south because of the level plains that are spread over much of the high northern latitudes. The advance of the northern polar cap has been less well observed than in the south because of thicker and more extensive polar clouds.

The composition of the seasonal polar caps was the subject of debate for nearly 200 years. Herschel was responsible for one early hypothesis that the caps were made of water ice. Herschel had imagined them to be just like those on Earth. In 1898 an Irish scientist, George J. Stoney, questioned Herschel’s theory and suggested that the caps might be made of frozen carbon dioxide, but evidence to support the idea was not available until Kuiper's 1947 discovery of carbon dioxide in the atmosphere.

In 1966 the American scientists Robert Leighton and Bruce Murray published the results of a numerical model of the thermal environment on Mars that raised considerable doubt about the water ice hypothesis. Their calculations indicated that, under Martian conditions, atmospheric carbon dioxide would freeze at the poles, and the growth and shrinkage of their model carbon dioxide caps mimicked the observed behavior of the actual caps. The model predicted that the seasonal caps were relatively thin, only a few meters deep near the poles and thinning toward the equator. Although the model was based on simplifications of the actual conditions on Mars, their results were later confirmed by thermal and spectral measurements taken by the twin Mariner 6 and 7 spacecraft when they flew by Mars in 1969.

The composition of the summer remnant caps, particularly the southern one, remains somewhat less certain despite considerable data on their ability to collect and radiate thermal energy. Measurements by the Viking orbiters showed that the ice of the northern cap remnant is definitely frozen water. Added to this evidence is the large increase in the amount of water vapor detected in the atmosphere over the summer cap. The northern remnant cap, in fact, represents the largest known reservoir of available water on the planet. At the southern pole, the carbon dioxide cap does not completely disappear in summer. A small remnant, consisting mostly of carbon dioxide but containing as much as 10 percent water, remains. Measurements by the Mars Express orbiter in 2004 revealed that water ice also is present just below the surface over wide areas around the remnant cap. Almost no water vapor is normally observed in the atmosphere above the southern remnant cap.

The topography of the polar regions is among the most distinctive on Mars. Beneath the seasonal and remnant caps at both poles are stacks of layered deposits up to 3 km (2 miles) thick that extend out to about the 80° latitude circle. The layers are exposed in escarpments and valleys that have a distinctive spiral pattern. Water ice has been detected in the upper 1 meter (about 3.3 feet) of the layered terrains at both poles, and the entire stack of layered deposits at each location is likely to be mostly water ice with variable amounts of dust. The layering is thought to result from climate-caused variations in the deposition rates of dust and water ice, which are traceable to changes in the planet's orbit and rotation. The layered terrains in the north lack impact craters, suggesting that they are very young. In contrast, the cratering of the layered terrains in the south indicate an age of roughly 100 million years.

The northern polar region also contains the largest area of sand dunes on Mars. The dunes, which occupy the northern part of the plain known as Vastitas Borealis, form a band that almost completely encircles the north polar remnant cap. Interlayering of sand and seasonal carbon dioxide snow can be seen in some locations, indicating that the dunes are active on at least a seasonal timescale.


Next time: “Surface Features, Part 1”


Bibliography

Mars. (2007). In Encyclopædia Britannica. Retrieved October 26, 2007 , from Encyclopædia Britannica Online: http://www.britannica.com/eb/article-9110149

Mars (2007). In The Columbia Encyclopedia, Sixth Edition 2007. Copyright 2007 Columbia University Press. Retrieved October 26, 2007 from Encyclopedia.com
http://www.encyclopedia.com/doc/1E1-Mars-ast.html

Planets: Mars. In NASA Solar System Exploration, Last updated October 23, 2007. Retrieved October 26, 2007, from the NASA Solar System Exploration website, maintained by NASA's Jet Propulsion Laboratory:
http://solarsystem.jpl.nasa.gov/planets/profile.cfm?Object=Mars


****************************************

THE SKY THIS WEEK


Dec 1, 7:44 A.M. ET - Last Quarter Moon

Dec 1 - Saturn 2° north of Moon

Dec 5 - Venus is 7° north of the Moon

Dec 6 - Moon at apogee, the point in the Moon's orbit when it is farthest from Earth.


****************************************

THIS WEEK IN HISTORY

Dec 7, 1972 – Apollo 17 launch, last manned mission to the Moon, 35th anniversary


****************************************

MUSICAL DIVERSION

In the late sixteenth century in the western Finland town of Turku, the rector (principal) of the Cathedral School of the local Catholic diocese took on the task of collecting, preserving and perpetuating the late medieval (thirteenth century) Latin songs that were still sung during his century in the Finnish cathedral schools. The name of the man was Jaakko Finne (or Jaakko Suomalainen), who was also a hymnist and who wrote under the Latinized penname Jacobus Finno (ca. 1540-1588). The collected songs were finally published in 1582 with the funding of a Finnish student named Theodoric Petri of Nyland, a member of an aristocratic family who was known by other names, but is remembered in print as Theodoricus Petri Nylandensi (ca. 1560- ca. 1630).

The song collection became known simply at "Piae Cantiones," but its full title was "Piae Cantiones ecclesiasticae et scholasticae veterum episcoporum" (Devout ecclesiastical and school songs of the old bishops). The collection included 74 songs that give insight to medieval Catholic culture. The origin of the songs and melodies varies and most of the songs are religious in nature, but some are secular school songs. One such school song, by author unknown, was called "Tempus Adest Floridum," Latin, meaning "Now Come the Flowers." The song celebrates the coming of spring, the warming of the earth and the blooming of the flowers.


Tempus Adest Floridum

Tempus adest floridum, surgent namque flores
Vernales in omnibus, imitantur mores
Hoc quod frigus laeserat, reparant calores
Cernimus hoc fieri, per multos labores.

Sunt prata plena floribus, iucunda aspectu
Ubi iuvat cernere, herbas cum delectu
Gramina et plantae hyeme quiescunt
Vernali in tempore virent et accrescunt.

Haec vobis pulchre monstrant Deum creatorem
Quem quoque nos credimus omnium factorem
O tempus ergo hilare, quo laetari libet
Renovato nam mundo, nos novari decet.

Terra ornatur floribus et multo decore
Nos honestis moribus et vero amore
Gaudeamus igitur tempore iucundo
Laudemusque Dominum pectoris ex fundo.


-----

In 1853 a new collection of Christmas songs was published by hymnist John Mason Neale (1818-1866) and minister and chorister Thomas Helmore (1811 – 1890). The collection was entitled "Carols for ChristmasTide" and included a carol by Neale that he may actually have written a few years earlier. The text was a tale of a young man named Václav (pronounced "VAHT-slaf"), who was the son of Duke Vratislav I of Bohemia. Václav was born around 907 in Prague, Bohemia (now part of the Czech Republic). Christianity spread through Bohemia during his reign as duke, which began about 924 or 925 when he assumed the throne at the age of eighteen. On September 28, 935, Václav was murdered on his way to church in a plot organized by his younger brother, Boleslav, who succeeded him as duke. Numerous saintly stories about Václav circulated following his death, as well as a few miracles that were attributed to his name. Václav was later venerated as a saint in both the Roman Catholic Church and the Orthodox Church, and there is a major shrine to him at St Vitus Cathedral in Prague.

The carol takes place in the Bohemian bitter cold of December 26, the feast day of Saint Stephen, whose stoning is recounted in the Acts of the Apostles (the Book of Acts). Neale set his new carol text to the tune of the old medieval Latin school song "Tempus Adest Floridum." And rather than use his Czech name of Václav, Neale called him the name by which he was better known in the west-- Wenceslas.


Good King Wenceslas

Good King Wenceslas looked out on the Feast of Stephen,
When the snow lay round about, deep and crisp and even.
Brightly shone the moon that night, though the frost was cruel,
When a poor man came in sight, gathering winter fuel.

“Hither, page, and stand by me, if you know it, telling,
Yonder peasant, who is he? Where and what his dwelling?”
“Sire, he lives a good league hence, underneath the mountain,
Right against the forest fence, by Saint Agnes’ fountain.”

“Bring me food and bring me wine, bring me pine logs hither,
You and I will see him dine, when we bear them thither.”
Page and monarch, forth they went, forth they went together,
Through the cold wind’s wild lament and the bitter weather.

“Sire, the night is darker now, and the wind blows stronger,
Fails my heart, I know not how; I can go no longer.”
“Mark my footsteps, my good page, tread now in them boldly,
You shall find the winter’s rage freeze your blood less coldly.”

In his master’s steps he trod, where the snow lay dinted;
Heat was in the very sod which the saint had printed.
Therefore, Christian men, be sure, wealth or rank possessing,
You who now will bless the poor shall yourselves find blessing.


-----

In 1919, another Christmas song was published by British-Canadian minister Joseph Simpson Cook (1859 – 1933). Cook recounted the story of the birth of Jesus and set his text to the tune of our favorite thirteenth century school song.


Gentle Mary Laid Her Child

Gentle Mary laid her Child lowly in a manger;
There He lay, the undefiled, to the world a Stranger:
Such a Babe in such a place, can He be the Savior?
Ask the saved of all the race who have found His favor.

Angels sang about His birth; wise men sought and found Him;
Heaven’s star shone brightly forth, glory all around Him:
Shepherds saw the wondrous sight, heard the angels singing;
All the plains were lit that night, all the hills were ringing.

Gentle Mary laid her Child lowly in a manger;
He is still the undefiled, but no more a stranger:
Son of God, of humble birth, beautiful the story;
Praise His Name in all the earth, hail the King of glory!


-----

To see and hear more on the hymn, "Temus Adest Floridum" visit this page of "The Cyber Hymnal" - http://www.cyberhymnal.org/non/la/tempusade.htm

To see and hear more on the hymn, "Gentle Mary Laid Her Child
" visit this page of "The Cyber Hymnal" -http://www.cyberhymnal.org/htm/g/e/gentleml.htm


To see and hear more on the hymn, "Gentle Mary Laid Her Child" visit this page of "The Cyber Hymnal" - http://www.cyberhymnal.org/htm/g/e/gentleml.htm