Michigan: Nearby Planet-Forming Disk Holds Water (TW Hya)

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Michigan: Nearby Planet-Forming Disk Holds Water (TW Hya)

Post by bystander » Sat Oct 22, 2011 3:09 am

Nearby planet-forming disk holds water for thousands of oceans
University of Michigan | 2011 Oct 20
For the first time, astronomers have detected around a burgeoning solar system a sprawling cloud of water vapor that’s cold enough to form comets, which could eventually deliver oceans to dry planets.

Water is an essential ingredient for life. Scientists have found thousands of Earth-oceans’ worth of it within the planet-forming disk surrounding the star TW Hydrae. TW Hydrae is 176 light years away in the constellation Hydra and is the closest solar-system-to-be.

University of Michigan astronomy professor Ted Bergin is a co-author of a paper on the findings published in the Oct. 21 edition of Science.

The researchers used the Heterodyne Instrument for the Far-Infrared (HIFI) on the orbiting Hershel Space Observatory to detect the chemical signature of water.

“This tells us that the key materials that life needs are present in a system before planets are born,” said Bergin, a HIFI co-investigator. “We expected this to be the case, but now we know it is because have directly detected it. We can see it.”

Scientists had previously found warm water vapor in planet-forming disks close to the central star. But until now, evidence for vast quantities of water extending into the cooler, far reaches of disks where comets and giant planets take shape had not emerged. The more water available in disks for icy comets to form, the greater the chances that large amounts will eventually reach new planets through impacts.

“The detection of water sticking to dust grains throughout the planet-forming disk would be similar to events in our own solar system’s evolution, where over millions of years, these dust grains would then coalesce to form comets. These would be a prime delivery mechanism for water on planetary bodies,” said principal investigator Michiel Hogerheijde of Leiden University in the Netherlands.

Other recent findings from HIFI support the theory that comets delivered a significant portion of Earth's oceans. Researchers found that the ice on a comet called Hartley 2 has the same chemical composition as our oceans. ( viewtopic.php?t=23714 )

HIFI is helping astronomers gain a better understanding of how water comes to terrestrial planets—Earth and beyond. If TW Hydrae and its icy disk are representative of many other young star systems, as researchers think they are, then the process for creating planets around numerous stars with abundant water throughout the universe appears to be in place, NASA officials say.

Detection of the Water Reservoir in a Forming Planetary System - Michiel R. Hogerheijde et al
Oceans of Water in a Planet-Forming Disk
California Institute of Technology | 2011 Oct 20

Herschel Finds Oceans of Water in Disk of Nearby Star
NASA JPL-Caltech | 2011 Oct 20

Herschel detects abundant water in planet-forming disc
ESA Space Science | 2011 Oct 20

Herschel discovers tip of cosmic iceberg around nearby young star
ESA Science & Technology | Herschel | 2011 Oct 20

Herschel Observatory Detects ‘Oceans’ of Water Around Distant Star
Universe Today | Nancy Atkinson | 2011 Oct 20

See also: viewtopic.php?t=23714
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SAO: The Water Reservoir in a Young Planetary System

Post by bystander » Sun Nov 20, 2011 9:04 pm

The Water Reservoir in a Young Planetary System
Smithsonian Astrophysical Observatory
Weekly Science Update | 2011 Nov 11
Astronomers once thought that the process of star formation was more-or-less controlled by the simple coalescence of material by gravity, leading eventually to a new star. But they have come to realize that star formation entails a very complex series of stages. In one early step, the young star assembles a circumstellar disk of gas and dust. After a few million years, this disk has matured enough to begin to develop into planets.

The star TW Hydrae, located about 150 light-years from Earth, is only about 10 million years old, and is currently in this planet-forming stage. Because TW Hydrae is relatively close and bright, and because it rotates with its pole pointed nearly directly towards the Earth, scientists can view the star's disk of material nearly face on to study what is happening. One outstanding puzzle is how rocky planets (like the Earth) can acquire their water. Most scenarios argue that the Earth's water arrived later on - via comets from the outer solar system. Thus a focus of recent astronomy has been the study of the composition of the outer parts of the young stellar disk.

CfA astronomer Gary Melnick, a leading expert on water in space, joined with a team of colleagues to use the new Herschel Space Observatory to look for traces of water around TW Hydrae. Writing in the latest issue of Science, the team reports finding convincing evidence for a reservoir of water ice in this star's disk -- with inferred quantities of water ice amounting to several thousand Earth-oceans. Moreover, they discovered from details of the ice chemistry that probably the ice comes from a mixture distributed throughout the system. The results lend convincing support to the current scenario of the origin of the Earth's oceans.

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Re: Michigan: Nearby Planet-Forming Disk Holds Water

Post by Ann » Mon Nov 21, 2011 1:47 am

According to my software, TW Hydrae has a luminosity of about 0.1 that of the Sun but a color index just a little redder than the Sun, and notably bluer than K0-type star Pollux. Should we understand TW Hydrae as a faint late G-type dwarf?

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SAO: Accretion Around a Young Star (TW Hydrae)

Post by bystander » Mon Jul 23, 2012 9:59 pm

Accretion Around a Young Star
Smithsonian Astrophysical Observatory
Weekly Science Update | 2012 July 20
The star TW Hydrae is located about 150 light-years from Earth in the direction of the constellation of Hydrae, the Water Snake. This star is relatively young, about 10 million years old, and has passed out of its infancy but is not yet mature. Astronomers are trying to understand the processes at work in stars at this stage in their lives because, for example, during this period planets might be developing from disks around the stars. The nature of the star's corona, the very hot (over a million degrees centigrade) extended gaseous outer atmosphere, is one such process. TW Hydrae provides a valuable example for two reasons: It is relatively close by and therefore bright, and it is rotating with its pole pointed nearly directly towards Earth, enabling scientists to view the star's polar region nearly face on.

Like other young stars of its size and age, TW Hydrae emits strong X-rays and lines of ionized hydrogen. These are thought to result from shocks generated as material flows onto the stellar surface, and from magnetically heated gas in the corona. SAO astronomers Andrea Dupree, Nancy Brickhouse, Steve Cranmer, Juan Luna, and Evan Schneider, along with colleagues, observed TW Hya with the Chandra X-ray Observatory, with complementary and simultaneous measurements from a suite of other telescopes. They continuously monitored the star over about seventeen days, during which time they observed both periodic and flaring events on the star.

The scientists, in a astronomical first, were able to track an accretion flare spectroscopically, providing direct information on how the excitation of the gas evolves during these events. The team successfully modeled the emission as arising in a sequence: A shock develops from accreting material and then flows down into a turbulent region, heating the star's photosphere. This ultimately leads to coronal heating and the development of stellar winds.

TW Hya: Spectral Variability, X-Rays, and Accretion Diagnostics - A. K. Dupree et al
http://asterisk.apod.com/viewtopic.php?f=31&t=18230

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MPG: In the planetary nursery (TW Hydrae)

Post by bystander » Thu Jan 31, 2013 2:21 am

In the planetary nursery
Max Planck Gesellschaft | 2013 Jan 30

Astronomers determine the mass of the disk of gas and dust surrounding the star TW Hydrae

The disk surrounding the young star TW Hydrae is regarded as a prototypical example of planetary nurseries. Due to its comparatively close proximity of 176 light-years, the object plays a key role in cosmological birth models. Using the Herschel Space Telescope, researchers including Thomas Henning from the Max Planck Institute for Astronomy in Heidelberg have, for the first time, determined the mass of the disk very precisely. The new value is larger than previous estimates and proves that planets similar to those of our solar system can form in this system. In addition, the observations are an example of how, in the world of science, not everything can be planned for.


Where Egyptologists have their Rosetta Stone and geneticists their Drosophila fruit flies, astronomers studying planet formation have TW Hydrae: A readily accessible sample object with the potential to provide foundations for an entire area of study. TW Hydrae is a young star with about the same mass as the Sun. It is surrounded by a protoplanetary disk: a disk of dense gas and dust in which small grains of ice and dust clump to form larger objects and, eventually, into planets. This is how our Solar System came into being more than 4 billion years ago.

What is special about the TW Hydrae disk is its proximity to Earth: at a distance of 176 light-years from Earth, this disk is two-and-a-half times closer to us than the next nearest specimens, giving astronomers an unparalleled view of this highly interesting specimen – if only figuratively, because the disk is too small to show up on an image; its presence and properties can only be deduced by comparing light received from the system at different wavelengths (that is, the object's spectrum) with the prediction of models.

In consequence, TW Hydrae has one of the most frequently observed protoplanetary disks of all, and its observations are a key to testing current models of planet formation. That's why it was especially vexing that one of the fundamental parameters of the disk remained fairly uncertain: The total mass of the molecular hydrogen gas contained within the disk. This mass value is crucial in determining how many and what kinds of planets can be expected to form.

Previous mass determinations were heavily dependent on model assumptions; the results had significant error bars, spanning a mass range between 0.5 and 63 Jupiter masses. The new measurements exploit the fact that not all hydrogen molecules are created equal: Some very few of them contain a deuterium atom – where the atomic nucleus of hydrogen consists of a single proton, deuterium has an additional neutron. This slight change means that these "hydrogen deuteride" molecules consisting of one deuterium and one ordinary hydrogen atom emit significant infrared radiation related to the molecule's rotation.

The Herschel Space Telescope provides the unique combination of sensitivity at the required wavelengths and spectrum-taking ability ("spectral resolution") required for detecting the unusual molecules. The observation sets a lower limit for the disk mass at 52 Jupiter masses, with an uncertainty ten times smaller than the previous result. While TW Hydrae is estimated to be relatively old for a stellar system with disk (between 3 and 10 million years), this shows that there is still ample matter in the disk to form a planetary system larger than our own (which arose from a much lighter disk).

On this basis, additional observations, notably with the millimetre/submillimetre array ALMA in Chile, promise much more detailed future disk models for TW Hydrae – and, consequently, much more rigorous tests of theories of planet formation.

The observations also throw an interesting light on how science is done – and how it shouldn't be done. Thomas Henning explains: "This project started in casual conversation between Ted Bergin, Ewine van Dishoek and me. We realized that Herschel was our only chance to observe hydrogen deuteride in this disk – way too good an opportunity to pass up. But we also realized we would be taking a risk. At least one model predicted that we shouldn't have seen anything! Instead, the results were much better than we had dared to hope."

TW Hydrae holds a clear lesson for the committees that allocate funding for scientific projects or, in the case of astronomy, observing time on major telescopes – and which sometimes take a rather conservative stance, practically requiring the applicant to guarantee their project will work. In Henning's words: "If there's no chance your project can fail, you're probably not doing very interesting science. TW Hydrae is a good example of how a calculated scientific gamble can pay off."

TW Hydrae: There's more to astronomers' favorite planetary nursery than previously thought
Max Planck Institute for Astronomy | 2013 Jan 30

How planets form: Astronomers weigh a protoplanetary disk with unprecedented accuracy
University of Michigan | Nicole Casal Moore | 2013 Jan 30

Herschel Finds Past-Prime Star May Be Making Planets
NASA | JPL-Caltech | Herschel | 2013 Jan 30

Stars can be late parents
ESA Space Science | 2013 Jan 30

Giving Birth at 10 Million Years Old
Science Shot | Lizzie Wade | 2013 Jan 30

TW Hydrae: An Infant Planetary System Analyzed
Centauri Dreams | Paul Gilster | 2013 Jan 30

An old disk still capable of forming a planetary system - Edwin A. Bergin et al
http://asterisk.apod.com/viewtopic.php?f=31&t=29149
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CfA: Young Star Suggests our Sun Was a Feisty Toddler

Post by bystander » Tue Jun 18, 2013 3:46 pm

Young Star Suggests our Sun Was a Feisty Toddler
Center for Astrophysics | Smithsonian Science | Chandra X-ray Observatory | 2013 Jun 05
If you had a time machine that could take you anywhere in the past, what time would you choose? Most people would probably pick the era of the dinosaurs in hopes of spotting a T. rex. But many astronomers would choose the period, four and a half billion years ago, that our solar system formed.

In lieu of a working time machine, we learn about the birth of our Sun and its planets by studying young stars in our galaxy. New work suggests that our Sun was both active and "feisty" in its infancy, growing in fits and starts while burping out bursts of X-rays.

"By studying TW Hydrae, we can watch what happened to our Sun when it was a toddler," said Nancy Brickhouse of the Harvard-Smithsonian Center for Astrophysics (CfA). She presented the findings today in a press conference at a meeting of the American Astronomical Society.

Brickhouse and her colleagues reached this conclusion by studying the young star TW Hydrae, located about 190 light-years from Earth in the southern constellation Hydra the Water Snake. TW Hydrae is an orange, type K star weighing about 80 percent as much as our Sun. It is about 10 million years old, and is still accreting gas from a surrounding disk of material. That same disk might contain newborn planets.

In order to grow, the star "eats" gas from the disk. However, the disk doesn't extend all the way to the star's surface, so the star can't dine from it directly. Instead, infalling gas gets funneled along magnetic field lines to the star's poles.

Fortunately, we are looking almost directly down on one of the star's poles. As a result, we can study the accretion process in detail.

"We're looking right where the action is," said team member Andrea Dupree of the CfA.

Infalling material smashes into the star, creating a shock wave and heating the accreting gas to temperatures greater than 5 million degrees Fahrenheit. The gas glows with high-energy X-rays. As it continues moving inward, the gas cools and its glow shifts to optical wavelengths of light. To study the process, Brickhouse and her team combined observations from NASA's Chandra X-ray Observatory with those from ground-based optical telescopes.

"By gathering data in multiple wavelengths we followed the gas all the way down. We traced the whole accretion process for the first time," explained Brickhouse.

They found that accretion was clumpy and episodic in building a star. At one point the amount of material landing on the star changed by a factor of five over the course of a few days.

"The accretion process changes from night to night. Things are happening all the time," stated Dupree.

Some of the infalling material is pushed away in a stellar wind much like the solar wind that fills our solar system. Some gets channeled into giant loops and stellar prominences.

Astronomers have known that young stars are much more magnetically active than our middle-aged Sun, but now they can actually probe the interplay between the star's magnetic fields and the protoplanetary disk.

"The very process of accretion is driving magnetic activity on TW Hydrae," added Brickhouse.

X-Ray Determination of the Variable Rate of Mass Accretion onto TW Hydrae - N. S. Brickhouse et al
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Hubble: Evidence for Extrasolar Planet Under Construction

Post by bystander » Tue Jun 18, 2013 4:34 pm

Hubble Uncovers Evidence for Extrasolar Planet Under Construction
NASA | STScI | HubbleSite | 2013 Jun 13
Nearly 900 extrasolar planets have been confirmed to date, but now for the first time astronomers think they are seeing compelling evidence for a planet under construction in an unlikely place, at a great distance from its diminutive red dwarf star.

The keen vision of NASA's Hubble Space Telescope has detected a mysterious gap in a vast protoplanetary disk of gas and dust swirling around the nearby star TW Hydrae, located 176 light-years away in the constellation Hydra (the Sea Serpent). The gap's presence is best explained as due to the effects of a growing, unseen planet that is gravitationally sweeping up material and carving out a lane in the disk, like a snow plow.

Researchers, led by John Debes of the Space Telescope Science Institute in Baltimore, Md., found the gap about 7.5 billion miles from the red dwarf star. If the putative planet orbited in our solar system, it would be roughly twice Pluto's distance from the Sun.

The suspected planet's wide orbit means that it is moving slowly around its host star. Finding the suspected planet in this orbit challenges current planet formation theories. The conventional planet-making recipe proposes that planets form over tens of millions of years from the slow but persistent buildup of dust, rocks, and gas as a budding planet picks up material from the surrounding disk. TW Hydrae, however, is only 8 million years old. There has not been enough time for a planet to grow through the slow accumulation of smaller debris. In fact, a planet at 7.5 billion miles from its star would take more than 200 times longer to form than Jupiter did at its distance from the Sun because of its much slower orbital speed and a deficiency of material in the disk.

An alternative planet-formation theory suggests that a piece of the disk becomes gravitationally unstable and collapses on itself. In this scenario, a planet could form more quickly, in just a few thousand years.

"If we can actually confirm that there's a planet there, we can connect its characteristics to measurements of the gap properties," Debes says. "That might add to planet formation theories as to how you can actually form a planet very far out. There's definitely a gap structure. We think it's probably a planet given the fact that the gap is sharp and circular."

What complicates the story is that the red dwarf star is only 55 percent the mass of our Sun. "It's so intriguing to see a system like this," Debes says. "This is the lowest-mass star for which we've observed a gap so far out."

The disk also lacks large dust grains in its outer regions. Observations from ALMA (the Atacama Large Millimeter Array) show that millimeter-sized (tenths-of-an-inch-sized) dust, roughly the size of a grain of sand, cuts off sharply at about 5.5 billion miles from the star, just short of the gap. The disk is 41 billion miles across.

"Typically, you need pebbles before you can have a planet. So, if there is a planet and there is no dust larger than a grain of sand farther out, that would be a huge challenge to traditional planet-formation models," Debes says.

The Hubble observations reveal that the gap, which is 1.9 billion miles wide, is not completely cleared out. The team suggests that if a planet exists, it is in the process of forming and not very massive. Based on the evidence, team member Hannah Jang-Condell at the University of Wyoming in Laramie estimates that the putative planet is 6 to 28 times more massive than Earth. Within this range lies a class of planets called super-Earths and ice giants. Such a small planet mass is also a challenge to direct-collapse planet-formation theories, which predict that clumps of material one to two times more massive than Jupiter can collapse to form a planet.

TW Hydrae has been a popular target with astronomers. The system is one of the closest examples of a face-on disk, giving astronomers an overhead view of the star's environment. Debes's team used Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) to observe the star in near-infrared light. The team then re-analyzed archival Hubble data, using more NICMOS images as well as optical and spectroscopic observations from the Space Telescope Imaging Spectrograph (STIS). Armed with these observations, they composed the most comprehensive view of the system in scattered light over many wavelengths.

When Debes accounted for the rate at which the disk dims from reflected starlight, the gap was highlighted. It was a feature that two previous Hubble studies had suspected but could not definitively confirm. These earlier observations noted an uneven brightness in the disk but did not identify it as a gap.

"When I first saw the gap structure, it just popped out like that," Debes says. "The fact that we see the gap at every wavelength tells you that it's a structural feature rather than an instrumental artifact or a feature of how the dust scatters light.

The team plans to use ALMA and NASA's upcoming James Webb Space Telescope, an infrared observatory set to launch in 2018, to study the system in more detail.

The 0.5-2.22-micron Scattered Light Spectrum of the Disk Around TW Hya:
Detection of a Partially Filled Disk Gap at 80 AU
- J. H. Debes et al
Hubble Uncovers Evidence of Farthest Planet Forming From its Star
NASA | Hubble Space Telescope | 2013 Jun 13

Exoplanet Formation Surprise
Carnegie Institution for Science | 2013 Jun 13

Should This Alien World Even Exist? This Young Disk Could Challenge Planet-Formation Theories
Universe Today | Elizabeth Howell | 2013 Jun 13
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ESO: Snow in an Infant Planetary System (TW Hydrae)

Post by bystander » Fri Jul 19, 2013 6:54 pm

Snow in an Infant Planetary System
ESO Science Release | ALMA | 2013 Jul 18

A frosty landmark for planet and comet formation
A snow line has been imaged in a far-off infant planetary system for the very first time. The snow line, located in the disc around the Sun-like star TW Hydrae, promises to tell us more about the formation of planets and comets, the factors that decide their composition, and the history of the Solar System. The results are published today in Science Express.

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have taken the first ever image of the snow line in an infant planetary system. On Earth, snow lines form at high altitudes where falling temperatures turn the moisture in the air into snow. This line is clearly visible on a mountain, where the snow-capped summit ends and the rocky face begins.

The snow lines around young stars form in a similar way, in the distant, colder reaches of the dusty discs from which planetary systems form. Starting from the star and moving outwards, water (H2O) is the first to freeze, forming the first snow line. Further out from the star, as temperatures drop, more exotic molecules can freeze and turn to snow, such as carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO). These different snows give the dust grains a sticky outer coating and play an essential role in helping the grains to overcome their usual tendency to break up in collisions, allowing them to become the crucial building blocks of planets and comets. The snow also increases how much solid matter is available and may dramatically speed up the planetary formation process.

Each of these different snow lines — for water, carbon dioxide, methane and carbon monoxide — may be linked to the formation of particular kinds of planets [1]. Around a Sun-like star in a planetary system like our own, the water snow line would correspond to a distance between the orbits of Mars and Jupiter, and the carbon monoxide snow line would correspond to the orbit of Neptune.

The snow line spotted by ALMA is the first glimpse of the carbon monoxide snow line, around TW Hydrae, a young star 175 light-years away from Earth. Astronomers believe this budding planetary system shares many of the same characteristics of the Solar System when it was just a few million years old.

“ALMA has given us the first real picture of a snow line around a young star, which is extremely exciting because of what it tells us about the very early period in the history of the Solar System,” said Chunhua “Charlie” Qi (Harvard-Smithsonian Center for Astrophysics, Cambridge, USA) one of the two lead authors of the paper. “We can now see previously hidden details about the frozen outer reaches of another planetary system similar to our own.”

But the presence of a carbon monoxide snow line could have greater consequences than just the formation of planets. Carbon monoxide ice is needed to form methanol, which is a building block of the more complex organic molecules that are essential for life. If comets ferried these molecules to newly forming Earth-like planets, these planets would then be equipped with the ingredients necessary for life.

Before now, snow lines had never been imaged directly because they always form in the relatively narrow central plane of a protoplanetary disc, so their precise location and extent could not be determined. Above and below the narrow region where snow lines exist, the star’s radiation prevents ice formation. The dust and gas concentration in the central plane is necessary to insulate the area from the radiation so that carbon monoxide and other gases can cool and freeze.

This team of astronomers succeeded in peering inside this disc to where the snow has formed with the help of a clever trick. Instead of looking for the snow — as it cannot be observed directly — they searched for a molecule known as diazenylium (N2H+), which shines brightly in the millimetre portion of the spectrum, and so is a perfect target for a telescope such as ALMA. The fragile molecule is easily destroyed in the presence of carbon monoxide gas, so would only appear in detectable amounts in regions where carbon monoxide had become snow and could no longer destroy it. In essence, the key to finding carbon monoxide snow lies in finding diazenylium.

ALMA's unique sensitivity and resolution has allowed the astronomers to trace the presence and distribution of diazenylium and find a clearly defined boundary approximately 30 astronomical units from the star (30 times the distance between the Earth and the Sun). This gives, in effect, a negative image of the carbon monoxide snow in the disc surrounding TW Hydrae, which can be used to see the carbon monoxide snow line precisely where theory predicts it should be — the inner rim of the diazenylium ring.

"For these observations we used only 26 of ALMA's eventual full complement of 66 antennas. Indications of snow lines around other stars are already showing up in other ALMA observations, and we are convinced that future observations with the full array will reveal many more of these and provide further, exciting insights into the formation and evolution of planets. Just wait and see,” concludes Michiel Hogerheijde from Leiden Observatory, the Netherlands.
  1. Notes:

    [*] For instance dry rocky planets form on the inner side of the water snow line (nearest the star), where only dust can exist. At the other extreme are the icy giant planets which form beyond the carbon monoxide snow line.

Snow Falling around Infant Solar System
Harvard Smithsonian Center for Astrophysics
National Radio Astronomy Observatory | 2013 Jul 18

Icy region gives planet and comet formation a boost

The sight of a snowfall can thrill children, but the first-ever snow line seen around a distant star gives astronomers an even greater thrill because of what it reveals about the formation of planets and our Solar System's history.

A snow line in an infant solar system
University of Michigan | 2013 Jul 18

Seeing Snow in Space
California Institute of Technology | 2013 Jul 18

Imaging of the CO Snow Line in a Solar Nebula Analog - Chunhua Qi et al
Snow's Up! Frosted Star System Discovered
Discovery News | Ian O'Neill | 2013 Jul 19

http://asterisk.apod.com/viewtopic.php?t=31736
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Re: Michigan: Nearby Planet-Forming Disk Holds Water (TW Hya

Post by Ann » Sat Jul 20, 2013 6:15 am

http://www.cfa.harvard.edu/news/2013/pr201314.html wrote:

TW Hydrae is an orange, type K star weighing about 80 percent as much as our Sun.
http://hubblesite.org/newscenter/archiv ... 3/20/text/ wrote:

The suspected planet is orbiting the diminutive red dwarf TW Hydrae
...
TW Hydrae is only 8 million years old, making it an unlikely star to host a planet, according to this theory. There has not been enough time for a planet to grow through the slow accumulation of smaller debris. Complicating the story further is that TW Hydrae is only 55 percent as massive as our sun.
It would seem that astronomers disagree very much among themselves about the nature of TW Hydrae itself. There is a huge difference between an 80% Solar mass star and a 55% Solar mass one. To illustrate what I mean, let's look at the nearest star, Alpha Centauri. This star is made up of two components, one a little more massive than the Sun, the other a little less massive.
http://stars.astro.illinois.edu/sow/rigil-kent.html wrote about Alpha Centauri A and B:

The orbit and orbital speeds yield masses of 1.10 solar for the brighter star, 0.92 for the fainter
...
Respective luminosities of Alpha Cen A and B fall around 1.52 and 0.50 times solar.
Alpha Centauri B probably contains more than 90% of the Sun's mass, yet it is only 50% as luminous. My software tells me that Alpha Cen B has a B-V index of 0.900 ± 0.020, considerably redder than the Sun.
http://stars.astro.illinois.edu/sow/61cyg.html wrote about the two components of 61 Cyg:

With temperatures of 4450 and 4120 Kelvin, they shine only at luminosities of 15 and 9 percent solar, their masses only 60 and 50 percent solar, radii just 65 and 60 percent solar.


The faintest member of the 61 Cygni pair is of spectral class K7V. Its mass is indeed about 50% solar and its luminosity 9% solar, according to Jim Kaler. That definitely resembles the values given for TW Hydrae. But the colors of TW Hya and 61 Cygni B are quite different. The B-V index of 61 Cygni B is 1.309 ± 0.012, but the B-V index for TW Hya is 0.721 ± 0.134. Clearly TW Hya is much bluer than 61 Cygni B.

However, in spite of Jim Kaler's estimate that 61 Cygni B is 9% as luminous as the Sun, the Hipparcos satellite says otherwise. The Hipparcos value for 61 Cygni B is that it is about 4% as luminous as the Sun.

In my opinion, TW Hydrae is way too blue for a late K-type star, and it is also too luminous. Admittedly TW Hya is moderately similar to 61 Cygni A. According to Jim Kaler, 61 Cygni A contains 0.6 solar masses, and according to Hipparcos its luminosity is 0.0841 ± 0.0040 that of the Sun and its B-V index is 1.069 ± 0.015. That color is red compared with TW Hya, but the masses and luminosities are similar.

Do we see TW Hya pole on? If we do, then the star might look "too blue" for its mass, due to the fact that stars are often flattened, so that we see deeper into their hot interiors from a pole-on position than from any other perspective.

It is interesting, in any case, that two prestigious astronomical sites give such very different values for TW Hya.

Ann
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SAO: Planets and the Snow Line

Post by bystander » Sat Aug 31, 2013 3:13 pm

Planets and the Snow Line
Smithsonian Astrophysical Observatory
Weekly Science Update | 2013 Jul 05
As a new star develops within a molecular cloud, a circumstellar disk forms naturally from the rotating gas and dust. These disks are called "protoplanetary disks" because astronomers expect that much of their material will gradually coagulate to form planets. The farther away from the star the gas and dust is, the cooler it is, until at some critical distance molecules in the gas will condense out onto the dust grains. This process is believed to have played a critical role in the formation of planets in the solar system.

The distance at which a molecular species freezes out is termed its "snow line." Snow lines are thought to mark regions of enhanced particle growth -- and thus planet formation – for four reasons. There is more in solid material (versus gaseous material) beyond to the snow line from which to build grains; additional freezing-out can occur as gas diffuses across the snow line; solid grains can pile-up just inside of the snow line in pressure traps; and the grains onto which gas condenses become stickier with their icy mantles, favoring yet more coagulation.

Experiments and theory on these various processes have been focused on the water snow line, but the results should be generally applicable to the snow lines of other abundant volatiles too. Determining the snow line locations for various species is key to probing grain growth and planet formation efficiencies. When our solar system was young, for example, it is thought the water snow line developed at a distance of about three astronomical units (AU; one AU is the average distance of the Earth from the Sun.) The location of the water snow line was critical to the formation of Jupiter and Saturn because at farther, colder locations methane and carbon monoxide (CO) freeze-out enhanced the sold surface density, perhaps thereby contributing to the feeding zones of Uranus and Neptune. The latest solar system models also claim that most comets formed even farther out, at about thirty-five AU.

CfA astronomers Chunhua Qi and David Wilner and their colleagues have used new the giant new radio telescopes at the Atacama Large Millimeter/Submillimeter Array (ALMA) to image the locations of a simple nitrogen-bearing molecule that traces carbon monoxide (CO) depletion. (The presence of gaseous CO chemically inhibits the formation of this nitrogen species.) The scientists report finding that, in the young star TW Hya, the CO snow line lies at about thirty AU. This result is roughly consistent with some aspects of newer solar system models, and shows how new observations and theory are making dramatic strides in explaining how our solar system formed and evolved.

Imaging of the CO Snow Line in a Solar Nebula Analog - Chunhua Qi et al
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