M16 (NGC 6611): The Eagle Nebula (II)
M16 is associated with a diffuse emission nebula, or H II region, which is catalogued as IC 4703.
The star cluster NGC6611 is associated with the Eagle Nebula, and contains the largest
population of intermediate-mass pre-main-sequence stars known in any
young cluster in the Galaxy (Hillenbrand et al 1993). It is made up
of several hundred optically visible stars whose masses lie in the range
3 < (M/M) < 8,
and ages range from ~ 0.25 to 3 million years.
While most of the stars in this cluster seem to have formed in a mini-starburst
~ 2 million years ago, high-mass star-formation has continued over a
period of at least 6 million years (Hillenbrand et al 1993). A
study with the Hubble Space Telescope (HST - Hester et al 1996) revealed
a population of optical globules, at least some of which may be associated
with circumstellar discs around stars which are in the final stages
of pre-main sequence development (McCaughrean 1997).
The November 1995 Hubble shots of M16
Gas Pillars in the Eagle Nebula M16
Undersea corral? Enchanted castles? Space serpents?
Pillars of Creation in a star-forming region
Image credit: HST
FIG. 1.— M16, HST full field
The color image is constructed from three separate images taken in the light of emission from different
types of atoms.
Red shows emission from singly-ionized sulfur atoms.
Green shows emission from hydrogen.
Blue shows light emitted by doubly- ionized oxygen atoms.
Image credit: HST
FIG. 2.— M16, HST, details
An animation was obtained from the HST images in this page, simulating the approach to the star
forming EGGs in the Eagle Nebula.
These eerie, dark pillar-like structures are actually columns of cool interstellar hydrogen gas
and dust that are also incubators for new stars. The pillars protrude from the
interior wall of a dark molecular cloud like stalagmites from the floor of a
cavern. They are part of the "Eagle Nebula" M16, a nearby star-forming region
1.8 kpc away in the constellation Serpens.
The pillars are in some ways akin to buttes in the desert, where basalt and
other dense rock have protected a region from erosion, while the surrounding
landscape has been worn away over millennia.
In this celestial case, it is especially
• dense clouds of molecular hydrogen gas and
• dust that have survived longer than their surroundings in the face
of a flood of ultraviolet light from hot, massive newborn stars (off the top edge of the picture).
This process is called "photoevaporation". This
ultraviolet light is also responsible for illuminating the convoluted surfaces
of the columns and the ghostly streamers of gas boiling away from their
surfaces, producing the dramatic visual effects that highlight the
three-dimensional nature of the clouds. The tallest pillar (left) is about a light-year long from base to tip.
As the pillars themselves are slowly eroded away by the ultraviolet light,
small globules of even denser gas buried within the pillars are uncovered. These
globules have been dubbed "EGGs."
EGGs is an acronym for "Evaporating Gaseous Globules," but it is also a word that describes what
these objects are.
Forming inside at least some of the EGGs are embryonic stars -- stars that abruptly stop
growing when the EGGs are uncovered and they are separated from the larger
reservoir of gas from which they were drawing mass. Eventually, the stars
themselves emerge from the EGGs as the EGGs themselves succumb to photoevaporation.
The picture was taken on April 1, 1995 with the Hubble Space Telescope Wide Field and Planetary Camera 2.
Star-Birth Clouds in M16
Stellar "EGGs" emerge from Molecular Clouds
This eerie, dark structure, resembling an imaginary sea serpent's head, is a
column of cool molecular hydrogen gas (two atoms of hydrogen in each molecule)
and dust that is an incubator for new stars.
The stars are embedded inside
finger-like protrusions extending from the top of the nebula. Each "fingertip"
is somewhat larger than our own solar system.
The pillar is slowly eroding away by the ultraviolet light from nearby hot
stars, a process called "photoevaporation". As it does, small globules of
especially dense gas buried within the cloud is uncovered.
The shadows of the EGGs protect gas behind them, resulting in the finger-like structures at
the top of the cloud.
Forming inside at least some of the EGGs are embryonic stars -- stars that
abruptly stop growing when the EGGs are uncovered and they are separated from
the larger reservoir of gas from which they were drawing mass. Eventually the
stars emerge, as the EGGs themselves succumb to photoevaporation.
Evaporating globules in M16
The stellar EGGS are found, appropriately enough, in the "Eagle Nebula".
These pictures were taken on April 1, 1995 with the Hubble Space Telescope
Wide Field and Planetary Camera 2. The color image is constructed from three
separate images taken in the light of emission from different types of atoms.
Red shows emission from singly-ionized sulfur atoms. Green shows emission from
hydrogen. Blue shows light emitted by doubly- ionized oxygen atoms.
FIG. 3.— Zoom
The following mosaic allows you to identify which regions of the Eagle nebula
the Hubble telescope has exposed:
Image credit: HST
K2 The Eagle Has Risen: Stellar Spire in the Eagle Nebula
Appearing like a winged fairy-tale creature poised on a pedestal, this
object is actually a billowing tower of cold gas and dust rising from a
stellar nursery called the Eagle Nebula. The soaring tower is 9.5
light-years high, about twice the distance from our Sun to the next nearest star.
Stars in the Eagle Nebula are born in clouds of cold hydrogen gas that
reside in chaotic neighborhoods, where energy from young stars sculpts
fantasy-like landscapes in the gas. The tower may be a giant incubator
for those newborn stars. A torrent of ultraviolet light from a band of
massive, hot, young stars [off the top of the image] is eroding the
Image credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
This Hubble Space Telescope view shows a portion of the Eagle Nebula (Messier 16).
The image has been vastly reduced in resolution and only hints at the incredible detail
in the full-resolution image from the Advanced Camera for Surveys.
The starlight also is responsible for illuminating the tower's rough
surface. Ghostly streamers of gas can be seen boiling off this surface,
creating the haze around the structure and highlighting its
three-dimensional shape. The column is silhouetted against the background glow of more distant gas.
The edge of the dark hydrogen cloud at the top of the tower is resisting
erosion, in a manner similar to that of brush among a field of prairie
grass that is being swept up by fire. The fire quickly burns the grass
but slows down when it encounters the dense brush. In this celestial
case, thick clouds of hydrogen gas and dust have survived longer than
their surroundings in the face of a blast of ultraviolet light from the hot, young stars.
Inside the gaseous tower, stars may be forming. Some of those stars may
have been created by dense gas collapsing under gravity. Other stars may
be forming due to pressure from gas that has been heated by the neighboring hot stars.
The first wave of stars may have started forming before the massive star
cluster began venting its scorching light. The star birth may have begun
when denser regions of cold gas within the tower started collapsing under their own weight to make stars.
The bumps and fingers of material in the center of the tower are
examples of these stellar birthing areas. These regions may look small
but they are roughly the size of our solar system. The fledgling stars
continued to grow as they fed off the surrounding gas cloud. They
abruptly stopped growing when light from the star cluster uncovered
their gaseous cradles, separating them from their gas supply.
Ironically, the young cluster's intense starlight may be inducing star
formation in some regions of the tower. Examples can be seen in the
large, glowing clumps and finger-shaped protrusions at the top of the
structure. The stars may be heating the gas at the top of the tower and
creating a shock front, as seen by the bright rim of material tracing the
edge of the nebula at top, left. As the heated gas expands, it acts like a
battering ram, pushing against the darker cold gas. The intense pressure
compresses the gas, making it easier for stars to form. This scenario
may continue as the shock front moves slowly down the tower.
The dominant colors in the image were produced by gas energized by the
star cluster's powerful ultraviolet light. The blue color at the top is
from glowing oxygen. The red color in the lower region is from glowing
hydrogen. The Eagle Nebula image was taken in November 2004 with the
Advanced Camera for Surveys aboard NASA's Hubble Space Telescope.
Chandra — YSOs near the Pillars of Creation
M16 — X-ray sources at the heart of the Eagle Nebula|
||Authors: Jeffrey L. Linsky, Marc Gagne, Anna Mytyk, Mark McCaughrean, Morten Andersen|
||Journal-ref: ApJ 654 (2007) 347 [astro-ph/0610279 ]|
||Title: Chandra Observations of the Eagle Nebula. I.
Embedded Young Stellar Objects near the Pillars of Creation|
We present and analyze the first high-resolution X-ray images ever obtained of the Eagle Nebula star-forming
region. On 2001 July 30 the Chandra X-ray Observatory obtained a 78 ks image of the Eagle Nebula (M 16) that
includes the core of the young galactic cluster NGC 6611 and the dark columns of dust and
cold molecular gas in M 16 known as the Pillars of Creation.
We find a total of 1101 X-ray sources in the 17'x17' ACIS-I field of view. Most of the X-ray
sources are low mass pre-main-sequence or high-mass main-sequence stars in this
young cluster. A handful of hard X-ray sources in the pillars are spatially
coincident with deeply embedded young stellar objects seen in high-resolution
near-infrared images recently obtained with the VLT (McCaughrean & Andersen 2002).
In this paper, we focus on the 40 X-ray sources in and around Pillars
1-4 at the heart of the Eagle Nebula.
None of the X-ray sources are associated
with the evaporating gaseous globules (EGGs) first observed by Hester et al.
(1996) in HST WFPC2 images of M 16, implying that either the EGGs do not
contain protostars or that the protostars have not yet become X-ray active.
Eight X-ray counts are coincident with the Herbig-Haro object HH216, implying log(L_X) ~ 30.0.
Hester et al. (2004) presented evidence suggesting that the Sun was born in a massive
star-forming region. For example, the discovery of radioactive 60Ni, a decay product of
radioactive 60Fe, in two chondritic meteorites (Tachibana & Huss 2003) implies that 60Fe
was present when the Ca-Al-rich inclusions (CAIs) were formed 4.5 Gyr ago. Because 60Fe
cannot be formed efficiently by particle irradiation, its presence requires input from a nearby
supernova, which prior to the explosion was a massive O star. Because the half-life of 60Fe is
only 1.49 Myr, the CAIs must have formed within a few million years of a nearby supernova
explosion. While the star-forming regions within 200 pc of the Sun are generally classified
as low-mass star-forming regions (Taurus-Auriga, Ophichus, Upper Scorpius), it is not clear
whether most stars in the Galaxy are formed in such small clouds and loose associations
rather than in high-mass star-forming regions like the Orion Nebula and the Eagle Nebula (M16).
Clusters of young stellar objects (YSOs) emerging from their molecular clouds are the
laboratories of choice for addressing fundamental questions of star formation. Such clusters
include YSOs with different masses, ages, and distances from hot O stars. These clusters
often contain YSOs in diverse environments, including
(a) completely embedded very young Class 0 objects visible only at millimeter and sub-millimeter wavelengths,
(b) partially embedded Class I sources visible at infrared and X-ray wavelengths,
(c) Class II T Tauri stars with circumstellar disks and their more massive cousins, the Herbig Ae/Be stars, and
(d) Class III naked or weak-lined T Tauri stars that have completely emerged from their nascent cloud environments.
Both low-mass and high-mass star-forming regions can be studied at a variety of wavelengths from radio waves
to X-rays. Observations at radio wavelengths, in Ha and other
emission lines are useful for studying the gas in H II regions ionized by nearby O stars.
Observations in the near- to far-infrared are useful for identifying deeply embedded YSOs
and their cold disks. X-ray imaging of clusters is valuable for identifying cluster membership
since YSOs have very high X-ray luminosities, as large as log LX/Lbol ~ 10-3,
and they can be detected through column densities as large as
NH I ~ 1023 cm-2.
Hester, J. J., et al. 1996, AJ 111, 2349
Hester, J. J., Desch, S. J., Healy, K. R., & Leshin, L. A. 2004, Science 304, 1116
McCaughrean, M. J., & Andersen, M. 2002, A&A 389, 513
The Eagle's EGGs: Fertile or sterile?
K4 SST: Embedded Star Formation
| — |
||Authors: R. Indebetouw, T.R. Robitaille, B.A. Whitney, E. Churchwell, B. Babler, M. Meade,
C. Watson, M. Wolfire|
||Journal-ref: ApJ (2007) [0707.1895 ]|
||Title: Embedded Star Formation in the Eagle Nebula with Spitzer/GLIMPSE|
We present new Spitzer photometry of the Eagle Nebula (M16, containing the optical
cluster NGC 6611) combined with near-infrared photometry from 2MASS.
We use dust radiative transfer models, mid-infrared and near-infrared color-color
analysis, and mid-infrared spectral indices to analyze point source spectral
energy distributions, select candidate young stellar objects (YSOs), and
constrain their mass and evolutionary state.
Comparison of the different protostellar selection methods shows that mid-infrared methods are consistent,
but as has been known for some time, near-infrared-only analysis misses some young objects.
We reveal more than 400 protostellar candidates, including one
massive young stellar object (YSO) that has not been previously highlighted.
The YSO distribution supports a picture of distributed low-level star
formation, with no strong evidence of triggered star formation in the ``pillars''.
We confirm the youth of NGC 6611 by a large fraction of
infrared-excess sources, and reveal a younger cluster of YSOs in the nearby molecular cloud.
Analysis of the YSO clustering properties shows a possible
imprint of the molecular cloud's Jeans length. Multiwavelength mid-IR imaging
thus allows us to analyze the protostellar population, to measure the dust
temperature and column density, and to relate these in a consistent picture of star formation in M16.
The fusion energy generated in stars dominates the evolution of galaxies, heating and processing their
interstellar media. However, our understanding of how stars form is incomplete.
This issue must be clarified by making detailed assessment of whether triggered star formation is happening in
particular molecular clouds.
FIG. 4.— The Eagle Nebula as seen by IRAC (RGB = [8.0],[4.5],[3.6]).
IRAC [8.0] has been supplemented with MSX A (8 µm) in the NW corner.
Some fairly reliable empirical
scaling laws exist, and a believable scenario exists for how an isolated low-mass star can form via disk
accretion, but we lack a detailed understanding of how an entire molecular cloud turns into a cluster of stars.
Most stars form in cluster environments, and stellar densities imply
some interaction between YSOs during their formation.
We must therefore analyze entire star forming regions, at
long enough wavelengths to not be limited by extinction,
and determine the physical properties of whole populations of young stellar objects (YSOs).
One particularly vexing aspect of the complex and important topic of star formation is that of triggering.
The formation of massive stars feeds energy back into the nearby interstellar medium, compressing,
irradiating, and heating the natal molecular cloud. This feedback can have destructive or constructive effects,
but it is not clear which dominates in a given cloud or overall in a galaxy.
Although many examples exist of regions in our Galaxy and other galaxies with suggestive spatial
distributions of young and somewhat younger stars, determining the relevant physical conditions and timescales
to make a strong argument for triggering is more difficult.
For some examples of these arguments, see the discussion of the Galactic region W3/4 by Oey et al. (2005),
30 Doradus by Walborn et al. (2002), various regions and theory by Deharveng et al. (2005), the particular
case of cloud-crushing described by Lefloch et al. (1997), and further theoretical considerations in
Elmegreen et al. (2002).
The Eagle Nebula (M16, containing the optical cluster NGC 6611) contains a well-studied region of
possible triggered star formation, in the widely publicized
“pillars of creation” or “elephant trunks” imaged with
HST by Hester et al. (1996, hereafter H96).
NGC 6611 is a young open cluster, likely still in its formation
stages. Hillenbrand et al. (1993) and de Winter et al.
(1997) found a spread in ages of the optically visible cluster members:
The massive (M* >
10M) stellar population is ~2±1 Myr old,
with the most massive members apparently having begun to evolve off of the ZAMS, and
evidence of at least one evolved (6 Myr old)
(Hillenbrand et al. 1993). There are also optically visible (no longer embedded) intermediate mass stars (3–
8M) that are still evolving onto the ZAMS.
The famous pillars are dense knots of molecular gas (Pound 1998) and dust which shield the region behind them
from the destructive radiation of the O stars in the central
cluster. H96 identified “Evaporating Gaseous Globules”
(EGGs) in and around the pillars, and a central question has been whether the EGGs are being compressed
and forming stars fast enough before they are photodestroyed. Detailed near-infrared (NIR) searches find
that about 15% of the EGGs have evidence of YSOs.
|The Eagle Nebula with Spitzer/GLIMPSE|
FIG. 5.— A GLIMPSE of M16
IRAC 8µ image of M16 with stars of various inferred evolutionary states marked in different colors:
Sources that can be constrained to have relatively massive accretion disks are yellow,
and those that in addition likely have massive circumstellar envelopes are red.
YSO candidates for which the data do not strongly constrain the mass of circumstellar dust (merely that they
have some, and are inconsistent with stellar atmospsheres) are green.
• blue circles
NGC 6611 and a new cluster of YSO candidates to the north are marked with large blue circles.
The edge of the region for which coverage exists in all IRAC bands and with MIPS at 24 µ is marked
as a thin black line.
Image credit: SST / Indebetouw et al. (2007)
Deharveng, L., Zavagno, A., Caplan, J. 2005, A&A 433, 565
Elmegreen, B. G., Palous, J., Ehlerova, S. 2002, MNRAS 334, 693
Hester, J. J., et al. 1996, AJ 111, 2349
Hillenbrand, L. A., Massey, P., Strom, S. E., & Merrill, K. M. 1993, AJ 106, 1906
Lefloch, B., Lazareff, B., & Castets, A. 1997, A&A 324, 249
Oey, M. S., Watson, A. M., Kern, K., Walth, G. L. 2005, AJ 129, 393
Pound, M. W. 1998, ApJ 493, L113
Walborn, N. R., Maiz-Apellaniz, J., Barba, R.H. 2002, AJ 124, 1601
de Winter, D., Koulis, C., The, P.S., van den Ancker, M.E., Perez, M.R., Bibo, E.A. 1997, A&AS 121, 223
|Literatur zu "M16 (II)"
|J.L. Linsky, M. Gagne, A. Mytyk, M. McCaughrean, M. Andersen||2007||ApJ 654, 347||
"Chandra Observations I. Embedded YSOs near the Pillars of Creation"
|H. Heintzmann||( Eintrag vom 20.9.2006) ||
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