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RE: Orion Nebula Cluster
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 New Distance to the Orion Nebula, Part Two
Early this year, an astronomer in England reported he had used the spins of stars to determine that the Orion Nebula is much closer to Earth than had been thought. Now astronomers in California have confirmed this result by measuring the parallax of a young star in this famous stellar nursery.
Both studies find that the Orion Nebula is about 1,300 light-years from Earth--over 200 light-years closer than previously thought.
The Orion Nebula is a cloud of gas and dust visible to the unaided eye in the sword of Orion. It has spawned a cluster of more than 3,000 newborn stars, the hottest of which set the nebula's gas aglow.
Karin Sandstrom, Joshua Peek, Geoffrey Bower, Alberto Bolatto, and Richard Plambeck at the University of California at Berkeley studied a T Tauri star--a precursor of a Sunlike star--in the Orion Nebula. Named GMR A, this star is spectral type K5 and would look orange if gas and dust didn't block its light.
Sandstrom and her colleagues observed radio waves from GMR A by using the Very Long Baseline Array (VLBA), which consists of ten radio telescopes in New Hampshire, Iowa, Texas, New Mexico, Arizona, California, Washington State, Hawaii, and the Virgin Islands. By combining observations from all these telescopes, the astronomers obtained precise positions of the star and measured its parallax.

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Title: A Parallactic Distance of 389 +24/-21 parsecs to the Orion Nebula Cluster from Very Long Baseline Array Observations
Authors: Karin M. Sandstrom, J. E. G. Peek, Geoffrey C. Bower, Alberto D. Bolatto, Richard L. Plambeck

We determine the parallax and proper motion of the flaring, non-thermal radio star GMR A, a member of the Orion Nebula Cluster, using Very Long Baseline Array observations. Based on the parallax, we measure a distance of 389 +24/-21 parsecs to the source. Our measurement places the Orion Nebula Cluster considerably closer than the canonical distance of 480 +/- 80 parsecs determined by Genzel et al. (1981). A change of this magnitude in distance lowers the luminosities of the stars in the cluster by a factor of ~ 1.5. We briefly discuss two effects of this change--an increase in the age spread of the pre-main sequence stars and better agreement between the zero-age main-sequence and the temperatures and luminosities of massive stars.

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Title: The distance to the Orion Nebula Cluster
Authors: R.D. Jeffries (Keele University)

The distance to the Orion Nebula Cluster (ONC) is estimated using the rotational properties of its low-mass pre main-sequence (PMS) stars. Rotation periods, projected equatorial velocities and distance-dependent radius estimates are used to form an observational sin i distribution (where i is the axial inclination), which is modelled to obtain the distance estimate. A distance of 440 ±34 pc is found from a sample of 74 PMS stars with spectral types between G6 and M2, but this falls to 392 ±32 pc when PMS stars with accretion discs are excluded on the basis of their near-infrared excess. Since the radii of accreting stars are more uncertain and probably systematically underestimated, then this closer distance is preferred. The quoted uncertainties include statistical errors and uncertainties due to a number of systematic effects including binarity and inclination bias. This method is geometric and independent of stellar evolution models, though does rely on the assumption of random axial orientations and the Cohen & Kuhi (1979) effective temperature scale for PMS stars. The new distance is consistent with, although lower and more precise, than most previous ONC distance estimates. A closer ONC distance implies smaller luminosities and an increased age based on the positions of PMS stars in the Hertzsprung-Russell diagram.

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Highly abnormal massive star mass
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Title: A highly abnormal massive star mass function in the Orion Nebula cluster and the dynamical decay of trapezia systems
Authors: J. Pflamm-Altenburg, P. Kroupa

The ONC appears to be unusual on two grounds: The observed constellation of the OB stars of the entire Orion Nebula cluster and its Trapezium at its centre implies a time-scale problem given the age of the Trapezium, and an IMF problem for the whole OB star population in the ONC. Given the estimated crossing time of the Trapezium, it ought to have totally dynamically decayed by now. Furthermore, by combining the lower limit of the ONC mass with a standard IMF it emerges that the ONC should have formed at least about 40 stars heavier than 5 solar masses while only ten are observed. Using N-body experiments we (i) confirm the expected instability of the trapezium and (ii) show that beginning with a compact OB-star configuration of about 40 stars the number of observed OB stars after 1 Myr within 1 pc radius and a compact trapezium configuration can both be reproduced. These two empirical constraints thus support our estimate of 40 initial OB stars in the cluster. Interestingly, a more-evolved version of the ONC resembles the Upper Scorpius OB association. The N-body experiments are performed with the new C-code CATENA by integrating the equations of motion using the chain-multiple-regularisation method. In addition, we present a new numerical formulation of the initial mass function.

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Orion Magnetic Field
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Astronomers announced on Thursday, January 12 2006 what may be the first discovery of a helical magnetic field in interstellar space, coiled around a gas cloud in the constellation of Orion.

"You can think of this structure as a giant, magnetic Slinky wrapped around a long, finger-like interstellar cloud. The magnetic field lines are like stretched rubber bands; the tension squeezes the cloud into its filamentary shape'' - Timothy Robishaw, graduate student in astronomy at the University of California, Berkeley.

Astronomers have long hoped to find specific cases in which magnetic forces directly influence the shape of interstellar clouds.
The findings provide the first evidence of the magnetic field structure around a filamentary-shaped interstellar cloud known as the Orion Molecular Cloud.
The announcement by Robishaw and Carl Heiles, UC Berkeley professor of astronomy, was made during a presentation at the American Astronomical Society meeting in Washington, D.C.

Interstellar molecular clouds are the birthplaces of stars, and the Orion Molecular Cloud contains two such stellar nurseries - one in the belt and another in the sword of the Orion constellation. Interstellar clouds are dense regions embedded in a much lower-density external medium, but the "dense" interstellar clouds are, by Earth standards, a perfect vacuum. In combination with magnetic forces, it's the large size of these clouds that makes enough gravity to pull them together to make stars.

Astronomers have known for some time that many molecular clouds are filamentary structures whose shapes are suspected to be sculpted by a balance between the force of gravity and magnetic fields. In making theoretical models of these clouds, most astrophysicists have treated them as spheres rather than finger-like filaments. However, a theoretical treatment published in 2000 by Drs. Jason Fiege and Ralph Pudritz of McMaster University suggested that when treated properly, filamentary molecular clouds should exhibit a helical magnetic field around the long axis of the cloud. This is the first observational confirmation of this theory.


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"Measuring magnetic fields in space is a very difficult task; because the field in interstellar space is very weak and because there are systematic measurement effects that can produce erroneous results.'' - Timothy Robishaw.

The signature of a magnetic field pointing towards or away from the Earth is known as the Zeeman effect and is observed as the splitting of a radio frequency line.

"An analogy would be when you're scanning the radio dial and you get the same station separated by a small blank space. The size of the blank space is directly proportional to the strength of the magnetic field at the location in space where the station is being broadcast.''- Timothy Robishaw.

The signal, in this case, is being broadcast at 1420 MHz on the radio dial by interstellar hydrogen - the simplest and most abundant atom in the universe.
The transmitter is located 1750 light years away in the Orion constellation.
The antenna that received these radio transmissions is the National Science Foundation's Green Bank Telescope (GBT), operated by the National Radio Astronomy Observatory. The telescope, 148 meters tall and with a dish 100 meters in diameter, is located in West Virginia where 13,000 square miles have been set aside as the National Radio Quiet Zone. This allows radio astronomers to observe radio waves coming from space without interference from manmade signals.

Using the GBT, Robishaw and Heiles observed radio waves along slices across the Orion Molecular Cloud and found that the magnetic field reversed its direction, pointing towards the Earth on the upper side of the cloud and away from it on the bottom. They used previous observations of starlight to inspect how the magnetic field in front of the cloud is oriented. (There is no way to gain information about what's happening behind the cloud since the cloud is so dense that neither optical light nor radio waves can penetrate it.) When they combined all available measurements, the picture emerged of a corkscrew pattern wrapping around the cloud.

"These results were incredibly exciting to me for a number of reasons. There's the scientific result of a helical field structure. Then, there's the successful measurement: This type of observation is very difficult, and it took dozens of hours on the telescope just to understand how this enormous dish responds to the polarized radio waves that are the signature of a magnetic field.'' - Timothy Robishaw.

The results of these investigations suggested to Robishaw and Heiles that the GBT is not only unparalleled among large radio telescopes for measuring magnetic fields, but it is the only one that can reliably detect weak magnetic fields.
Heiles cautioned that there is one possible alternative explanation for the observed magnetic field structure: The field might be wrapped around the front of the cloud.

"It's a very dense object. It also happens to lie inside the hollowed-out shell of a very large shock wave that was formed when many stars exploded in the neighbouring constellation of Eridanus. That shock wave would have carried the magnetic field along with it, he said, "until it reached the molecular cloud! The magnetic field lines would get stretched across the face of the cloud and wrapped around the sides. The signature of such a configuration would be very similar to what we see now. What really convinces us that this is a helical field is that there seems to be a constant pitch angle to the field lines across the face of the cloud.'' - Carl Heiles.

However, the situation can be clarified by further research. Robishaw and Heiles plan to extend their measurements in this cloud and others using the GBT. They will also collaborate with Canadian colleagues to use starlight to measure the field across the face of this and other clouds.

"The hope is to provide enough evidence to understand what the true structure of this magnetic field is. A clear understanding is essential in order to truly understand the processes by which molecular clouds form stars in the Milky Way galaxy'' - Carl Heiles.

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Orion Nebula
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The image, taken by the Advanced Camera for Surveys (ACS) aboard NASA's Hubble Space Telescope, represents the sharpest view ever taken of the Orion Nebula. More than 3,000 stars of various sizes appear in this image.
Credit: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team




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A massive star is illuminating this small region, called M43, and sculpting the landscape of dust and gas.
Credit: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team


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Orion Nebula Wind
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All stars, including the Sun, give off a stream of particles as they burn. In young, hot stars like those that form the “Trapezium” at the heart of Orion this stream of particles is millions of times more dense and energetic than the solar wind.
Newborn stars, which are still shrouded in thick veils of dust and gas, often eject gas and dust from their polar regions in narrow jets, rather than broadcasting them outward in all directions. When these stellar winds impact floating clouds of dust and gas, they produce shock waves that erode and shape the clouds in a fashion similar to the way in which terrestrial winds sculpt sand dunes. When they are strong enough, such shock waves also can compress the free-floating clouds of dust and gas, triggering the formation of new stars.

Astronomer Robert O’Dell is using these shock waves as celestial “wind socks” to plot the direction of these winds in different parts of the nebula. By back-tracking older, more distant shock waves to their likely points of origin, the astronomer can also get an idea of how long major currents have been flowing.

When you look closely enough, you see that the nebula is filled with hundreds of visible shock waves” - Robert O’Dell.


Courtesy of Robert O’Dell

In his analysis, O’Dell has identified three different types of shock waves:
• Bow-shocks are stationary shock waves that are formed by the collision of two steady winds and are excellent indicators of wind direction. They are present near the hottest stars in the centre of the nebula where they show winds flowing outward at velocities of thousands of kilometres per second. They are also present in the outer nebula where they are produced by low velocity stellar winds of tens of kilometres per second.
• Jet-driven shocks are produced when narrow streams of gas and particles travelling at hundreds of kilometres per second pass through gas that is relatively stationary. There are many shockwaves of this type in the nebula that are produced by jets of material ejected by newly formed stars.
• Warped shocks are jet-driven shocks located in areas where the ambient gas is not stationary but is moving in a cross current. This bends the jets and shocks into bow-like shapes.

Using these markers, the astronomer has mapped the outflow from two of the three regions of star formation in the nebula. Both of these regions, labelled BN-KL and Orion-South, are located behind the glowing region of the nebula where the light from the central stars ionises the outer layers of the parent molecular cloud. The specific objects that are producing these winds in the two regions are not visible to optical telescopes but they stand out as hot spots in infrared images.
By tracking back the farthest shockwaves produced by these outflows, O’Dell has established that the winds blowing from BN-KL have been doing so for 900 to 1,100 years, while those from Orion-South have been going on for 200 to 1,500 years.

These observations were made during 104 orbits of the Hubble and provide the most comprehensive picture ever obtained of the Orion Nebula. The data will be combined with other Hubble and ground-based telescope observations to create a widely available archive for research scientists interested in this region, in addition to acting as a base for a detailed study that should provide new insights into the conditions required for creating stars like the sun.

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Orion Nebula
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New observations of the Orion Nebula at infrared wavelengths reveal that small dust grains located in disks around young stars are growing, taking the initial steps toward forming planets despite bathing in a flood of radiation from highly luminous stars. The properties of dust in disks around young stars plays a pivotal role in understanding star formation and determining the origins of planets in our Solar system and in extrasolar planetary systems as well. The results are presented today at the 207th meeting of the American Astronomical Society in Washington, D. C.

"One of the key questions we are trying to address is whether or not planets can form around young stars in the seemingly hostile environment of the Orion Nebula" - Dr. Marc Kassis, support astronomer at the W. M. Keck Observatory and lead author of the poster sharing the results.

The Orion Nebula, located about 1500 light years away, is an energetic stellar nursery giving birth to thousands of young, Sun-like stars with protoplanetary disks. But a few of these newborn stars are 10 to 30 times the mass of our Sun and 10,000 times as bright. These massive stars bathe the entire region in harsh ultraviolet radiation, which evaporates the protoplanetary disks of their lower mass neighbours.

"You would think that the strong ultraviolet radiation that is evaporating these disks would also inhibit planet formation, but the larger particles we see in these Orion disks seem to suggest otherwise" - team member Dr. Nathan Smith, Hubble Fellow at the University of Colorado.

To determine the relative sizes of the grains in these protoplanetary disks, the research team used the Long Wavelength Spectrometer on the Keck I 10-meter telescope and the Mid-Infrared Spectrometer and Imager at the 3-meter NASA Infrared Telescope Facility, both situated 14,000 feet atop Mauna Kea on the island of Hawaii.
In the optical part of the spectrum, these protoplanetary disks are dark and are sometimes viewed in silhouette against the bright nebula. In contrast, the dusty disks are extraordinarily bright in the infrared.
The observations revealed broad spectral signatures of silicate grains, and the overall shape of the spectra was unlike the silicate emission of relatively smaller grains typical of the interstellar medium.


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Close-up of the Trapezium region in the Orion Nebula. On the left, sources A-D are bright in the mid-infrared. On the right, the same sources are dark in optical wavelengths and sometimes are viewed in silhouette against the bright nebula.
Image credit: N. Smith, University of Colorado/Gemini/HST


"The silicate profiles from the protoplanetary disks are generally flat-topped instead of peaked, indicating the grains have increased in size since the birth of these disks. You wonder whether the grains will grow enough to start forming planets"- Dr. Marc Kassis.

"Could our own solar system have formed in such an environment?" posed Dr. Ralph Shuping, support scientist for the Stratospheric Observatory for IR Astronomy (SOFIA).

"Careful study of primitive materials in meteorites suggests that it was, and our observations show that the initial stages of grain growth that lead to planet formation can occur in protoplanetary disks born in Orion-like environments"

Most stars are born in clusters with bright, massive stars relatively nearby. The stars in clusters and their protoplanetary disks born in regions like the Orion Nebula can be exposed to the intense ultraviolet
radiation from massive stars, stellar winds, jets, gravitational pulls from their neighbours, and supernova explosions. Yet, recent theoretical work and the study of primitive meteorites indicate that our Solar
System may have been born in a region like the Orion Nebula.

"Some years ago, we thought ultraviolet radiation would be hazardous to disks. So, in disks where grains have grown and settled to the disk mid-plane, ultraviolet radiation can remove gas, leaving large particles behind to accumulate through their mutual gravitation into small, planet-like objects" - Dr. John Bally at the University of Colorado.


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Dust emission from protoplanetary disks in Orion. On the left is a mid-infrared image (11.7 microns) of the Trapezium region in the Orion Nebula. On the right are spectra from Keck Observatory that show grains in one of the protoplanetary disks have grown well beyond the sizes typical of the interstellar medium.
Image credit: N. Smith, University of Colorado/Gemini/Keck



However, recent work by Drs. Henry Throop of the Southwest Research Institute and Bally showed that ultraviolet irradiation could promote the rapid formation of planets.
The team's observations also hint at the composition of the grains. From details in the shape of the infrared spectra, the team is identifying the presence of silicate minerals such as olivine and fosterite; olivine being the same mineral found along the green sand beaches in Hawaii.

"It's amazing to think that we can study the mineralogy of these tiny grains 1500 light years away!" - Dr. Ralph Shuping.

The team responsible for the discovery of grain growth in Orion Nebula protoplanetary disks is Ralph Shuping (USRA-SOFIA), Marc Kassis (W. M. Keck Observatory), Mark Morris (UCLA), and Nathan Smith and John Bally (University of Colorado). The team acquired data at NASA's IRTF through a collaboration with the instrument team that includes Joseph Adams (Cornell University), Joseph Hora (Harvard-Smithsonian Centre for Astrophysics), James Jackson (Boston University), and Eric Tollestrup (UH-IfA, NASA IRTF).

This work was supported by the Colorado Centre for Astrobiology and the UCLA Centre for Astrobiology, both supported by the NASA Astrobiology Institute. The Infrared Telescope Facility is operated by the University of Hawaii under Cooperative Agreement no. NCC 5-538 with the National Aeronautics and Space Administration, Office of Space Science, Planetary Astronomy Program. Some of the observations for this research were provided by the W. M. Keck Observatory using Director's discretionary time, also known as "Team Keck." The W. M. Keck Observatory is operated by
the California Association for Research in Astronomy (CARA), a non-profit 501 (c) (3) corporation whose board of directors includes representatives from the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration.

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RE: Trapezium
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The fuzzy area known as the Great Nebula of Orion or M42, is found near Orion's belt.
In the nebula is a bright star cluster known as the Trapezium.
New stellar systems are forming there in gigantic globs of gas and dust known as Proplyds.
Looking closely at the picture also reveals that gas and dust surrounding some of the dimmer stars appears to form structures that point away from the brighter stars.


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The false colour image was made by combining several exposures from the orbiting Hubble Space Telescope.


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RE: Orion Nebula Cluster
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X-Ray Stars in the Orion Nebula
When our middle-aged Sun was just a few million years old it was thousands of times brighter in x-rays. In fact, it was likely similar to some of the stars found in this false-colour x-ray composite of the Orion Nebula region from the Chandra Observatory.
The image is centred on bright stars of the nebula's Trapezium star cluster, and while analyzing the Chandra data astronomers have now found examples of young, sun-like stars producing intense x-ray flares.
It sounds strange, but the situation may actually favour the formation of hospitable planetary systems like our own.
Energetic flares can produce turbulence in the planet-forming disks surrounding the stars - preventing rocky earth-like planets from spiralling uncomfortably close to and even falling into their active, young parent stars.


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About 1,500 light-years away, the Orion Nebula is the closest large stellar nursery. At that distance, this Chandra image spans about 10 light-years.



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