* Astronomy

Astronomers traditionally attribute mid-infrared emission from young newly-forming stars to circumstellar disks. However, new observations using the Thermal-Region Camera and Spectrograph (T-ReCS) on the Gemini South telescope reveal that in at least one case the mid-infrared emission from a young stellar source is associated with a spectacular outflow.

In a paper slated to appear in the May 1, 2006 Astrophysical Journal Letters, Gemini astronomer James De Buizer, offers a caution that mid-infrared emission from young stellar objects can be explained by processes other than circumstellar disks.
The Gemini observations were focused on a young massive star in the G35.20-0.74 region in the outskirts of the Serpens constellation 7500 light years away. This young star has a known collimated outflow, which was first detected from its radio emission. The new observations reveal a monopolar jet-like feature in the mid-infrared.
It is argued that this is thermal continuum emission from dust associated with the outflow.

In this case the young star is deeply embedded in a molecular clump, and therefore only the northern jet of the outflow, which is coming out of the cloud toward us, can be viewed in the mid-infrared. The southern part of the outflow can be seen in the radio image because, unlike mid-infrared emission, radio emission can penetrate through the dense molecular cloud.


a) The Gemini T-ReCS observations of the mono-polar outflow of G35.20-0.74 as seen in the mid-infrared at 12 microns. The black plus-sign marks the location of the young central star.
b) The T-ReCS mid-infrared image at 18 microns is overlaid by contours of low-resolution radio continuum emission (Heaton & Little 1988). Radio emission from the central star (black plus-sign) and the northern and southern outflow lobes can be seen.
c) A close-up look at the mid-infrared emission near the central star and outflow cavity. The central star can be seen in high-resolution radio continuum emission (black contours, from Gibb et al. 2003), with some more diffuse radio emission to the south. The hydroxyl masers are shown as asterisks (Hutawarakorn & Cohen 1999), water masers as crosses (Forster & Caswell 1989), and methanol masers as large plus-signs (A.G. Gibb, private communication). They are distributed in a V-shape along the mid-infrared walls of the outflow cavity with the apex near the central star.

Astronomers think that stars like our Sun form at the centres of disks of swirling gas and dust. These disks feed these young stars, allowing them to eventually grow to their final size. However during this circumstellar disk phase, some material is actually expelled from the system perpendicular to the disk in an outflow.
A newly forming star heats its nearby dust, and this dust radiates that heat away as mid-infrared emission. For that reason it has been traditionally thought that mid-infrared emission is produced by the hot dust in the circumstellar disks that are suspected to form around every young star as a natural consequence of their formation process.
Unexpectedly, for G35.20-0.74 it is the outflow that dominates the mid-infrared emission.
While there have been a few previous observations that have detected mid-infrared emissions from outflows, it was thought that they were dominated by shock lines of molecular hydrogen. However, these Gemini mid-infrared observations were taken through filters that did not encompass any molecular hydrogen lines. Moreover, because of the steep spectral slope from 3 to 18 microns it was concluded that the mid-infrared radiation from the outflow is dominated by continuum dust emission.

"Not only was it determined that here we are seeing an outflow dominated by mid-infrared continuum emission, but it is now suspected that the masers here are most likely associated with the outflow as well" - James De Buizer.

Masers are molecular lasers that naturally occur in space, and are often associated with star formation. In some cases, especially cases involving molecular methanol masers, they are thought to be emitted from circumstellar disks. However, with the help of the new Gemini observations it is now argued that the all of the masers in the region are associated with the outflow and emitted from the walls of the outflow cavity.

"Methanol masers require mid-infrared emission to excite them to emit. Here we see clearly for the first time that the outflow cavities close to newly forming stars are bathed in mid-infrared radiation, and may therefore be natural environments for methanol maser production. We often find that things are not what we expected." - James De Buizer.

These Gemini observations demonstrate the importance of high spatial resolution, multiple wavelength observations in deciphering the true nature of complex star forming regions.

The Hubble space telescope has found two bright debris disks of ice and dust encircling the stars HD 139664 and HD 53143 that appears to be the equivalent of our own solar system's Kuiper Belt. The disks seem to have a central area cleared of debris, perhaps by planets.

The new disks, each about 60 light-years from Earth, bring to nine the number of dusty debris disks observable at visible wavelengths. The new ones are different, however, in that they are old enough — more than 300 million years — to have settled into stable configurations akin to those in our own solar system, which is 4.6 billion years old.
The wide disk on the left, which is inclined obliquely to the line-of-sight, surrounds HD 53143, a K star slightly smaller than the Sun but about 1 billion years old in the constellation Carina. The narrow disk on the right, which is tipped nearly edge-on encircles the star HD 139664, 57 light years away in the constellation Lupus, an F star slightly larger than the Sun but only 300 million years old.
The sharp outer edges of the narrow belt may be telltale evidence for the existence of an unseen companion object that gravitationally keeps debris gravitationally corralled, in the same way that shepherding moons trim the edges of debris rings around Saturn and Uranus.



HD 53143
Position(2000): R.A. 06h 59m 59s.56 Dec. -61° 20' 09".0

A survey by the Hubble Space Telescope shows that such disks fall into two categories: those with a broad belt, wider than about 50 astronomical units; and narrow ones with a width of between 20 and 30 AU and a sharp outer boundary, seemingly like our own Kuiper Belt. Our Kuiper Belt, for example, is thought to be narrow, extending from the orbit of Neptune at 30 AU to about 50 AU.



HD 139664
Position(2000): R.A. 15 h 41 m 11 s .30 Dec. -44° 39' 41".6
Credit: NASA, ESA, and P. Kalas (University of California, Berkeley)

The false-colour images were taken with Hubble's Advanced Camera for Surveys in September 2004. The black central circle is an image artefact produced by the camera's coronagraph which blocks the glare from the central star to allow the much fainter disks to be seen. A smaller black circle at the edge of each photo is a "coronagraphic finger" also used to block light from a bright object in the field.

Source

An international group of astronomers have used the Coronagraphic Imager for Adaptive Optics (CIAO) on the Subaru telescope in Hawaii to obtain very sharp near-infrared polarized-light images of the birthplace of a massive proto-star known as the Becklin-Neugebauer (BN) object at a distance of 1500 light years from the Sun.
The group's images led to the discovery of a disk surrounding this newly forming star. This finding, described in detail in the September 1 issue of Nature, deepens our understanding of how massive stars form.



The research group, which includes astronomers from the Purple Mountain Observatory, China, National Astronomical Observatories of Japan, and University of Hertfordshire, UK, explored the region close to the Becklin-Neugebauer object and analyzed how infrared light is affected by dust. To do this, they took a polarized-light image of the object at a wavelength of 1.6 micrometers (the H band of infrared light).
Images of the brightness of the object just show a circular distribution of light. However, an image of the light's polarization shows a butterfly shape that reveals details that are undetectable by looking at the brightness distribution alone.



Polarization image of the object at a wavelength of 1.6 micrometers (the H band of infrared light). Whiter areas are regions with a larger degree of polarization. The contours superimposed on the image show the brightness distribution of the BN object at the same wavelength. The polarized image has a butterfly shape with its two wings brighter (higher polarization degrees) than in the abdomen (the dark lane). The bright wings represent the outflow cavity walls and the dark lane represents the circumstellar disk. The red lines are the polarization vectors which show the direction of polarization at different locations in the image.

To understand the environment around the star and what the butterfly shape implies, the astronomers created a computer model for comparison, along with a schematic of star formation. These models show that the butterfly shape is the signature of a disk and an outflow structure near the newborn star.


Computer simulation using Monte Carlo code


This discovery is the most concrete evidence for a disk around a massive young star and shows that massive stars like the BN object (which is about seven times the mass of the Sun) form the same way as lower-mass stars like the Sun.

There are two main theories to explain the formation of massive stars. The first states that massive stars are the results of the mergers of several low-mass stars. The second says that they are formed through gravitational collapse and mass accretion within circumstellar disks. Lower-mass stars like the Sun are most likely to have formed through the second method.
The collapse-accretion theory assumes that a system has a star associated with a bipolar outflow, a circumstellar disk and an envelope, while the merger theory does not. The presence or absence of such structures can distinguish between the two formation scenarios.

Until recently, there has been little direct observational evidence in support of either theory of massive star formation. This is because, unlike lower-mass stars, newly forming massive stars are so rare and so far away from us that they have been difficult to observe. Large telescopes and adaptive optics, which greatly improve image sharpness, now make it possible to observe these objects with unprecedented clarity. High-resolution infrared polarimetry is an especially powerful tool for probing the environment hidden behind the bright glow of a massive star.

Polarization-the direction that light waves oscillate in as they stream away from an object-is an important characteristic of radiation. Sun light doesn’t have a preferred direction of oscillation, but can become polarized when scattered by Earth’s atmosphere, or after reflecting off the surface of water.
A similar action occurs in a circumstellar cloud around a newborn star. The star lights up its surroundings-the circumstellar disk, the envelope and the cavity walls formed by the outflow streams.
The light can travel freely within the cavity and then reflect off its walls. This reflected light becomes highly polarized. By contrast, the disk and the envelope are relatively opaque to light. This reduces the polarization of light coming from those regions.

The group’s success in detecting evidence for a disk and outflow around the BN object through high-resolution infrared polarimetry suggests that the same technique can be applied to other forming stars. This would allow astronomers to obtain a comprehensive observational description of the formation of massive stars greater than ten times the mass of the Sun.

Astronomers have discovered the oldest known dust disk:
It is thought that dust disks around newborn stars disappear in a few million years. The formation of planets is the most likely reason as to why they vanish.
But now, astronomers have discovered a 25-million-year-old dust disk that shows no evidence of planet formation.

"Finding this disk is as unexpected as locating a 200-year-old person" -Lee Hartmann, Harvard-Smithsonian Center for Astrophysics astronomer, lead author on the paper announcing the find.

The discovery raises the puzzling question of why this disk has not formed planets despite its advanced age. Most protoplanetary disks last only a few million years, while the oldest previously known disks have ages of about 10 million years.

"We don't know why this disk has lasted so long, because we don't know what makes the planetary formation process start" - Nuria Calvet ,co-author.



The disk in question orbits a pair of red dwarf stars in the Stephenson 34 system, located approximately 350 light-years away in the constellation Taurus. Data from NASA's Spitzer Space Telescope shows that its inner edge is located about 65 million miles from the binary stars. The disk extends to a distance of at least 650 million miles. Additional material may orbit farther out where temperatures are too low for Spitzer to detect it.

Hartmann and Calvet hold opposite opinions about the eventual fate of the disk around Stephenson 34.

"Most stars, by the age of 10 million years, have done whatever they're going to do. If it hasn't made planets by now, it probably never will" - Lee Hartmann.
However, it maybe is that the disk is constantly being disrupted or the evolution is slower.

"This disk still has a lot of gas in it, so it may still form giant planets" -Nuria Calvet.