Tag Archives: Star formation

Fortifying the Brick and Charming the Snake

Fortifying the Brick and Charming the Snake

Magnetic Fields are crucial in shaping the Cradles of Massive Stars

January 16, 2015

Magnetic fields in massive dark clouds are strong enough to support the regions against collapse due to their own gravity. A study lead by researchers at the Max–Planck–Institut für Radioastronomie in Bonn, Germany, shows for the first time that high magnetization sets the stage for the formation of stars much more massive than the sun. This is demonstrated in observations of polarized dust emission from two of the most massive clouds in our Milky Way, the “Brick” and “Snake”. The results are published in this week’s issue of the “Astrophysical Journal”.

<p><em>The left panel shows the &ldquo;Brick&rdquo; as a shadow against the mid&ndash;infrared emission from warm gas and dust in the vicinity of the Galactic Center. The background false&ndash;color image and white contours in the right panel give the emission of cold dust in the Brick itself. Markers indicate the orientation of the magnetic field deduced from polarization observations. The area shown on the right is indicated by a white box in the left&ndash;hand panel.</em></p> Zoom Image

The left panel shows the “Brick” as a shadow against the mid–infrared emission from warm gas and dust in the vicinity of the Galactic Center. The background false–color image and white contours in the right panel give the emission of cold dust in the Brick itself. Markers indicate the orientation of the magnetic field deduced from polarization observations. The area shown on the right is indicated by a white box in the left–hand panel.
© T. Pillai & J. Kauffmann, based on Spitzer GLIMPSE & MIPSGAL images (NASA / JPL–Caltech / Univ. of Wisconsin) and Hertz data from the CSO (J. Dotson)

Stars much more massive than the Sun (with 8 solar masses or more) live wild and die young. They spew out powerful stellar winds and sometimes explode violently to end up as supernovae. Even their birth is spectacular: massive stars form out of very dense and massive gaseous cores that are deeply embedded within dark clouds of gas and dust. In fact, the high mass of these cores has puzzled researchers for many years: the cores should quickly collapse due to their own gravity and destroy themselves before telescopes on Earth can detect them.

“For the first time we witness how magnetic fields thread a massive cloud and help stabilize the region while it gets ready to form high–mass stars” says Thushara Pillai from the Max–Planck–Institut für Radioastronomie (MPIfR) in Bonn (Germany), the lead author of the study. “The cloud would already be collapsing if there were no magnetic support”, she adds. “In that case the young forming cores would never become massive enough to form stars much larger than the Sun.”

It has long been suspected that magnetic fields help to support clouds against collapse. But magnetic fields are elusive: it is difficult to tease the weak signal from magnetic fields from the noise. Every region has to be observed over several nights to finally achieve a significant detection. The current study therefore only targets two regions. The “Brick” is an unusually dense cloud that is as opaque as its namesake. It resides just a few dozen light years away from the Galactic Center Black Hole in a distance of about 26,000 light years. The nickname of the “Snake” is inspired by its serpent–like shape. This cloud is about 12,000 light years away from Earth. The team used archival data from two telescopes on top of Mauna Kea (Hawaii, USA) to conduct this research, the James Clerk Maxwell Telescope and the Caltech Submillimeter Observatory.

The magnetic field geometry can be studied by observing the dust particles aligned with the magnetic field. These grains emit polarized radiation that can be detected with telescopes. The magnetic field lines are constantly disturbed by random gas motions in the clouds. “You can think of a guitar string being plucked”, suggests Paul Goldsmith, a team member from the Jet Propulsion Laboratory at the California Institute of Technology in Pasadena (California, USA). “On a stringed instrument such as a guitar, the tension in the string tries to hold it straight. In our clouds, the magnetic field tries to do this, and the degree of straightness of the field lines is a measure of the magnetic field strength.” Researchers Chandrasekhar and Fermi already suggested this technique in 1953. But only recently have telescopes become sensitive enough to conduct this experiment throughout the Milky Way.

<p><em>In the left panel the &ldquo;Snake&rdquo; is seen as a dark silhouette against the diffuse mid&ndash;infrared glow of the Milky Way. The right panel zooms in on a dense section of the cloud that is outlined by a box in the overview panel. The background false&ndash;color image and contours indicate emission from cold dust. Markers give the magnetic field orientation derived from polarization observations.</em></p><br /><br /><br />
<p><em>&nbsp;</em></p> Zoom Image

In the left panel the “Snake” is seen as a dark silhouette against the diffuse mid–infrared glow of the Milky Way. The right panel zooms in on a dense section of the cloud that is outlined by a box in the overview panel. The background false–color image and contours indicate emission from cold dust. Markers give the magnetic field orientation derived from polarization observations.
© T. Pillai & J. Kauffmann, based on Spitzer GLIMPSE & MIPSGAL images (NASA / JPL-Caltech / S. Carey [SSC/Caltech]) and SCUPOL data from the JCMT (P. Redman / B. Matthews)

This study opens a new chapter in research that started in the early 1980’s at the Effelsberg 100m–telescope of the MPIfR. First surveys of dense gas near the center of the Milky Way revealed unusually massive clouds, including the “Brick”. This discovery inspired several follow–up studies, as co–author Jens Kauffmann from the MPIfR explains. “Two years ago we successfully revealed for the first time the internal structure of the Brick. We were surprised to find very little substructure in this cloud: something seemed to stop the gas from clumping up. Now we know that the strong magnetic field might do this.”

The team has now started a project that will observe many more such clouds. This time the researchers will use MPIfR’s APEX telescope. “APEX is currently the only telescope worldwide that is equipped to make these observations”, concludes Thushara Pillai. “It is an exciting possibility to use this observatory to explore more of our Galactic backyard”.


The research team is comprised of Thushara Pillai, Jens Kauffmann and Karl M. Menten (all MPIfR), moreover Jonathan C. Tan (University of Florida), Paul F. Goldsmith (Jet Propulsion Laboratory, California Institute of Technology), and Sean J. Carey (IPAC, California Institute of Technology).

The Age of Stellar Nurseries

The Age of Stellar Nurseries

Astrochemical Dating with Molecular Line Observations from SOFIA und APEX

November 17, 2014

An international research team led by scientists from the Coordinated Research Center (CRC) 956 “Conditions and Impact of Star Formation” at the University of Cologne has used observations made with the GREAT instrument on board the SOFIA aircraft observatory and the APEX telescope to date the core of an interstellar cloud that is forming a group of Sun-like stars. This work, to which scientists from the University of Helsinki as well as from the Max-Planck-Institutes for Radio Astronomy (MPIfR) and Extraterrestrial Physics (MPE) contributed, is published in this week’s issue of “Nature”.
Our own solar system formed many billion years ago when a dark interstellar cloud started to contract to form our protostar &ndash; which later became the Sun. The duration of this first step in stellar evolution has now been determined to last at least 1 million years in the similar system IRAS 16293-2422, a collection of protostars ~400 light years away in the constellation Ophiuchus (background image). This has been achieved by using molecular hydrogen, H<sub>2</sub>, as a chemical clock. As hydrogen is not directly detectable, the chemically closely related species H<sub>2</sub>D<sup>+</sup> has been observed instead in the submm- and far-infrared wavelength range, using the ground-based telescope APEX in the Chilean Andes and the airborne observatory SOFIA. Zoom Image

Our own solar system formed many billion years ago when a dark interstellar cloud started to contract to form our … [more]
© Martina Markus & Oskar Asvany, based upon: NASA/Carla Thomas, C. Durán/ESO/APEX (MPIfR/ESO/OSO), ESO/Digitized Sky Survey 2/Davide De Martin, ESO/ L. Calçada, Bill Saxton, NRAO/AUI/NSF)

 

Stars like our Sun and their planetary systems are born inside clouds consisting of dust and molecular gas. Stellar evolution begins with the contraction of dense material in these stellar nurseries until an embryonic star, the protostar, is formed. How this collapse happens exactly, and on what timescales, is not very well understood. Is the gas “free-falling” towards the center due to gravity or is the collapse slowed down by other factors? “Since this process takes much longer than human history, it cannot just be followed as a function of time. Instead, one needs to find an internal clock that allows to read off the age of a particular star forming cloud,” says the leading author Sandra Brünken from the University of Cologne.

The hydrogen molecule (H2), by far the most abundant molecule in space, could act as such an internal, “chemical” clock. Molecular hydrogen exists in two different forms, called ortho and para, which correspond to different orientations of the spins of the two hydrogen nuclei. In the cold and dense molecular clouds out of which stars are formed, the relative abundance of these two forms changes continuously with time by chemical exchange reactions. Therefore the current abundance ratio observed is a measure of the time elapsed since the formation of H2, and thereby the molecular cloud itself. Unfortunately, H2 cannot be directly detected in the very cold interstellar “breeding grounds” of stars. However, H2D+, an ionized variant in which a deuteron particle is attached to the H2 molecule, can be observed. Indeed, the ortho and para forms of H2D+ emit and absorb at characteristic wavelengths, forming “spectral lines” that are observable with different telescopes. “We knew from our own laboratory experiments and from theory that H2D+ has a very close chemical relation to H2,” says Stephan Schlemmer from the University of Cologne who proposed the observations. “For the first time we could now observe both variants of H2D+, which allowed us to indirectly determine the ratio of ortho H2 to para H2. Reading this chemical clock we find an age of at least one million years for the parental cloud that is right now giving birth to Sun-like stars.” This result is questioning theories of rapid star formation.

The astronomical observations were very challenging. The relevant spectral line of para-H2D+ lies in the far infrared wavelength range (at 219 µm), where the Earth’s atmosphere absorbs most of the radiation. “Its first unambiguous detection was only possible due to the unique capabilities of the GREAT (German REceiver for Astronomy at Terahertz Frequencies) instrument on board the SOFIA (Stratospheric Observatory for Infrared Astronomy) aircraft,” says Jürgen Stutzki, whose group at the University of Cologne is involved in the development of GREAT. SOFIA is a joint project between NASA and the DLR (Deutsches Zentrum für Luft- und Raumfahrt) carrying a 2.7 meter diameter telescope as high as 13.7 km, above the atmosphere’s absorbing layers. The team also observed the corresponding rotational line of ortho-H2D+ at mm-wavelengths with the ground-based APEX (Atacama Pathfinder Experiment) telescope located in the Chilean Andes at an altitude of 5100 m. APEX Principal Investigator, the MPIfR’s Karl Menten points out that “It’s great to see the synergy between both telescopes!”

The age of this star-forming cloud, which is located in the Ophiuchus constellation at a distance of around 400 light years, was determined by comparing the data from the telescopes with extensive computer simulations of the chemistry that is changing with time. “The simulations allow us a detailed look at the movement of our H2D+ clock,” explains Jorma Harju of the University of Helsinki. “We find that our new chemical clock is more precise than any of those used previously. Even more importantly, it keeps running when other clocks have already stopped working.” The team is confident that their new method will help to date other stellar birthplaces.


<p>The GREAT far-infrared spectrometer is mounted to the telescope flange, inside the pressurized cabin. During observations GREAT rotates &plusmn;20 degrees from the vertical.</p> Zoom Image

The GREAT far-infrared spectrometer is mounted to the telescope flange, inside the pressurized cabin. During … [more]
© R. Güsten

Background Information:

SOFIA, the Stratospheric Observatory for Infrared Astronomy, is a joint project of the National Aeronautics and Space Administration (NASA) and the Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR; German Aerospace Centre, grant: 50OK0901). The German component of the SOFIA project is being carried out under the auspices of DLR, with funds provided by the Federal Ministry of Economics and Technology (Bundesministerium für Wirtschaft und Technologie; BMWi) under a resolution passed by the German Federal Parliament, and with funding from the State of Baden-Württemberg and the University of Stuttgart. Scientific operations are coordinated by the German SOFIA Institute (DSI) at the University of Stuttgart and the Universities Space Research Association (USRA) headquartered in Columbia, Maryland, U.S.A.

GREAT, the German Receiver for Astronomy at Terahertz Frequencies, is a receiver for spectroscopic observations in the far infrared spectral regime at frequencies between 1.25 and 5 terahertz (wavelengths of 60 to 220 microns), which are not accessible from the ground due to absorption by water vapor in the atmosphere. GREAT is a first-generation German SOFIA instrument, developed and maintained by the Max-Planck Institute for Radio Astronomy (MPIfR) and KOSMA at the University of Cologne, in collaboration with the Max Planck Institute for Solar System Research and the DLR Institute of Planetary Research. Rolf Guesten (MPIfR) is the principal investigator for GREAT. The development of the instrument was financed by the participating institutes, the Max Planck Society and the German Research Foundation (Deutsche Forschungsgemeinschaft; DFG).

APEX, the Atacama Pathfinder Experiment, is a collaboration between the Max Planck Institute for Radio Astronomy (MPIfR), Onsala Space Observatory (OSO), and the European Southern Observatory (ESO) to construct and operate a modified prototype antenna of ALMA (Atacama Large Millimetre Array) as a single dish on the Chajnantor plateau at an altitude of 5,100 meters above sea level (Atacama Desert, Chile). The telescope was manufactured by VERTEX Antennentechnik in Duisburg, Germany. The operation of the telescope is entrusted to ESO.

Hidden Nurseries in the Milky Way

Hidden Nurseries in the Milky Way

APEX reveals cradles of massive star-formation throughout our Galaxy

May 10, 2014

APEX, the Atacama Pathfinder Experiment, is a telescope of 12 m diameter at an exceptional site on Earth: the Chajnantor plateau is located 5100 m above sea level in the Atacama desert in Chile. It was used to map the whole inner  part of the plane of our Milky Way, ranging from the Southern constellations of Vela and Carina all the way to the Northern constellations of Aquila and the great Cygnus rift. The APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) mapped the Galactic Plane at a wavelength of 0.87 mm. Cold interstellar dust emits strongly in this part of the electromagnetic spectrum, called the sub-millimeter range, while it is blocking visible and infrared wavelengths. The survey has revealed an unprecedented number of cold dense clumps of gas and dust as the cradles of massive stars, thus providing a complete view of their birthplaces in the Milky Way. Based on this census, an international team of scientists led by Timea Csengeri from the Max Planck Institute for Radio Astronomy in Bonn has estimated the time scale for these nurseries to grow stars. This has been found to be a very fast process: with only 75,000 years on average it is much shorter than the corresponding time scales typically found for nurseries of lower mass stars.
The ATLASGAL survey coverstwo thirds of the surface area of the Galaxy within 50,000 light years of the Galactic center. Thus it includes practically all (97%) of the star-formation within the Solar Circle, i.e. the inner Galaxy. The image displays a part of ATLASGAL, a region located between the giant molecular complexes called W33 and M17 in the Sagittarius constellation. Zooms in color scale show the 3-color emission from the mid-infrared GLIMPSE survey, and sub-millimeter dust emission from ATLASGAL is shown in red and traced with contours. One region corresponds to a cold, pristine massive clump (upper left inset), and another one to a young massive star (upper right inset). Both objects have sizes of only a few light-years across. In the lower right inset we present a schematic of the Milky Way and show the position of the Solar Circle (green) and region of the Galaxy covered by ATLASGAL (shaded region). Zoom Image

The ATLASGAL survey coverstwo thirds of the surface area of the Galaxy within 50,000 light years of the Galactic center … [more]
© ATLASGAL Team

 

Stars significantly more massive than the Sun end their fast and furious lives in violent supernova explosions producing the heavy elements in the Universe. Throughout their lives, their powerful stellar winds and high-energy radiation shape their local environments and have a significant impact on the appearance and future evolution of their host galaxies. These stars form at the densest and coldest places in the Milky Way deeply embedded in dust cocoons, which are so dense that they absorb most of the radiation from the young stars within. It is in these dense cocoons   of gas and dust, hidden from visible and infrared wavelengths, where the next generation of stars are being born.

An international team of astronomers used the APEX telescope with its sub-millimeter camera, LABOCA, built at the Max Planck Institute for Radio Astronomy (MPIfR), to survey the inner Galaxy to search for the birthplaces of the most massive stars currently forming in the Milky Way. The APEX telescope is located on the Chajnantor Plateau in Chile at 5100 m altitude, which is one of the few places on Earth where observations at sub-millimeter wavelengths are possible. The ATLASGAL survey covers more than 420 square degrees of the Galactic plane, which corresponds to 97% of the inner Galaxy within the Solar circle. Thus it includes large sections of all four spiral arms, and approximately two thirds of the entire molecular disc of the Milky Way (see lower right inset of Fig. 1). This data set therefore includes the majority of all massive star forming nurseries in the Galaxy and is being used to construct a 3D map of the Milky Way.

The APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) provides an unprecedented census of the cold and dense environments where the most massive stars in our Galaxy begin their lives. The material in these stellar nurseries is so dense that optical and infrared light emitted from the embedded young high-mass stars cannot escape. Therefore, the earliest stages of star formation are effectively hidden at these short wavelengths and longer wavelengths are required to probe these regions. The ATLASGAL survey detects emission at sub-millimeter wavelengths, which is dominated by emission from cold dust. It provides a detailed view of the birthplaces where the next generation of massive stars is being formed.

As these dusty corners of our Galaxy are very difficult to access, such surveys provide the large scale coverage to search for the stellar nurseries forming the most massive stars in our Galaxy. “Our team has now analyzed this survey revealing the largest sample of the so-far hidden places of massive star-formation”, states Timea Csengeri from MPIfR, the lead author of the study. “We have identified many new potential sites where the most massive stars currently form in our Galaxy.”

Providing an unprecedented statistics, scientists reveal that the processes to build up the cold, dense sites where the most massive stars in our Galaxy form, occur rapidly, taking place within only 75,000 years, which is  much shorter than the corresponding timescales in nurseries of lower mass –stars like our Sun. This is the first global indication that star-formation is a fast process in our Galaxy.

“We characterized these places to search for signatures revealing how massive stars form within them,” continues James Urquhart, also from MPIfR. “The fast and furious life of the most massive stars was already known. And now we could also show that it is initiated by a pretty short infancy within their stellar cocoons.”  The lifetime of massive stars is about 1000 times shorter than the lifetime of stars like the Sun, and the new results reveal that they also form on short timescales and in a much more dynamic star formation process.

APEX and the night sky. The image shows the southern part of the Milky Way including the pointer stars, the Southern Cross and the Eta Carina region (bright reddish nebula above the cross). The ATLASGAL survey covers the southern Milky Way to the Carina region. Zoom Image

APEX and the night sky. The image shows the southern part of the Milky Way including the pointer stars, the Southern … [more]
© ESO/Y. Beletsky (Sky Image); ESO (APEX telescope); Composite image created by C. Urquhart.

 

“Only telescopes at exceptional locations, such as the high and dry Chajnantor Plateau in Chile at 5100 m are capable to observe in this frequency range”, adds Frederic Schuller from ESO, co-author of the study. “This is the largest area in the sky surveyed from a ground-based telescope in the sub-millimeter wavelength regime”.

“ATLASGAL also provides a “finding chart” for the most extreme dust cocoons, where the innermost processes of stellar birth can be studied at much higher angular-resolution with the new ALMA interferometer, located just next to the APEX telescope” concludes Friedrich Wyrowski, the APEX project scientist at MPIfR.

Orion’s Hidden Fiery Ribbon

Orion’s Hidden Fiery Ribbon

May 15, 2013

Picture Release No. 1321 of European Southern Observatory (ESO). A dramatic new image of cosmic clouds in the constellation of Orion reveals what seems to be a fiery ribbon in the sky. The orange glow represents faint light coming from grains of cold interstellar dust, at wavelengths too long for human eyes to see. It was observed by the MPIfR-built & ESO-operated Atacama Pathfinder Experiment (APEX) in Chile.

Molecule Hunter in the Universe

Molecule Hunter in the Universe

HIFI has opened up new window to explore the cosmos

April 30, 2013

After 3.5 years the liquid helium coolant on board of ESA’s Herschel Space Observatory has finally run out. This means that the molecule hunter HIFI – one of its three instruments – has ceased to collect spectra. But the scientific heritage is impressive already. Thanks to HIFI, scientists now much better understand the cosmic cycle of gas which leads to for instance the birth of stars and planets, and the role (water) molecules play.
<em> Cygnus X star-forming complex, observed with Herschel </em> Zoom Image

Cygnus X star-forming complex, observed with Herschel
© ESA/PACS/SPIRE/M. Hennemann & F. Motte, Laboratoire AIM Paris-Saclay, CEA/Irfu – CNRS/INSU – Univ. Paris Diderot, France

 

HIFI has extracted a wealth of unique information from very different environments – from shells of dying stars to galaxy cores and comets. The Molecule Hunter was able to do this because of an unbeatable combination of uninterrupted spectral coverage, high spectral resolution and calibration accuracy. This combination may not be available in space for another 40 years.

Water trail
Frank Helmich (SRON), HIFI’s Principal Investigator: “Water has been one of the most important targets for HIFI because of its rich spectrum and high abundance, and because it plays such an important role in star formation. Because of the high water vapor content of our own atmosphere, the data from space obtained with HIFI are a true legacy for decades to come.”

Water has been used to trace new types of shock waves present in the outflows that are part of the formation processes of stars. It has also been used to detect material flowing onto a star in the very first stages of its formation and to probe the cold water reservoir in the outer regions of planet-forming disks, with the water released from ice by a weak UV radiation field triggered by cosmic rays. Together, these data will allow to put together the water trail from collapsing clouds to planetary systems.

Investigation of the solar system
An unexpected discovery was a huge water torus surrounding Saturn, apparently fed by cryovolcanic activity of Enceladus. This torus is the long sought-after source of the stratospheric water in Saturn and Titan.

HIFI found for the first time water in comets showing the same isotopic composition than water on Earth. “This pioneering result contributes substantially to the on-going paradigm change about the formation and evolution of the solar system” says Paul Hartogh from the MPI for Solar System research.

Molecules in CO dark gas
HIFI was looking for hydride molecules, which are very important building blocks of chemistry, but unobservable from the ground. The amount and relative abundances of the detected molecules came as a large surprise. Molecules such as ionized water (H2O+) and OH+ were, contrary to expectation, much more abundant than protonated water (H3O+). HIFI Co-I Volker Ossenkopf (University of Cologne) says “HIFI observations have revolutionized our view of the interstellar medium in ways we hadn’t even dared to dream about when Herschel was launched, leading to completely new insight into the chemical pathways.”

Cosmic rays were not only detected through the ionizing influence they have on interstellar gas. Several times observations were interrupted by a hit of such a high energy particle in the electronics of HIFI (local cosmic rays). Every time HIFI was successfully restarted.

<em>HIFI - the "Heterodyne Instrument for the Far-Infrared" </em> Zoom Image

HIFI – the “Heterodyne Instrument for the Far-Infrared”
© HIFI Instrument Control Center, SRON, Groningen

 

Spectral scans
Besides observations of single molecules, HIFI also excels in making complete spectral scans. Such scans contain tens of thousands of lines from, in some cases, over 50 molecular species arising from within water and organic rich interstellar gas clouds. These observations directly characterize the chemical composition of star-forming gas with unprecedented accuracy while offering the unprecedented ability to probe gas physics with hundreds to thousands of lines of a single molecule. University of Cologne researcher Peter Schilke, who was involved in two Key Programs of spectral scans: “We have done ground-based line surveys before, but the HIFI results have blown us out of the water. The high sensitivity, high quality and the access to the high-frequency range, unavailable before HIFI, have lifted this research to a new level, which will take many years to fully digest.”

“With HIFI we have been able to decypher many details of the physical and chemical state of the interstellar medium that were previously not accessable to astronomical observations”, says HIFI Co-PI Jürgen Stutzki from the Submm-Astronomy group at the University of Cologne.

“We had the rare occasion to scientifically exploit a completely new part of the electromagnetic spectrum”, concludes Co-I Karl Menten from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn. “HIFI delivered a whole number of stunning surprises.” The Submillimeter Astronomy groups at the University of Cologne and at the MPIfR provided a significant contribution to the HIFI instrument and hence for the success of the mission.

Sun Block for the “Big Dog”

Sun block for the “Big Dog”

Astronomers detect titanium oxide and titanium dioxide around the giant star VY Canis Majoris

March 27, 2013

An international team of astronomers, including researchers from the Max Planck Institute for Radio Astronomy and from the University of Cologne, successfully identified two titanium oxides in the extended atmosphere around a giant star. The object VY Canis Major is one of the largest stars in the known universe and close to the end of its life. The detection was made using telescope arrays in the USA and in France.

Gone with the stellar wind: an extended dusty nebula surrounds VY CMa in the constellation Big Dog, one of the largest known stars in the universe. In the atmosphere of this huge sun, astronomers discovered the molecules TiO and TiO<sub>2</sub>. Zoom Image

Gone with the stellar wind: an extended dusty nebula surrounds VY CMa in the constellation Big Dog, one of the largest … [more]
© Molecule symbols: CDMS/T. Kamiński. Background image: NASA/ESA and R. Humphreys (University of Minnesota)

 

The discovery was made in the course of a study of a spectacular star, VY Canis Majoris or VY CMa for short, which is a variable star located in the constellation Canis Major (Greater Dog). “VY CMa is not an ordinary star, it is one of the largest stars known, and it is close the end of its life,” says Tomasz Kamiński from the Max Planck Institute for Radio Astronomy (MPIfR). In fact, with a size of about one to two thousand times that of the Sun, it could extend out to the orbit of Saturn if it were placed in the centre of our Solar System.

The star ejects large quantities of material which forms a dusty nebula. It becomes visible because of the small dust particles that form around it which reflect light from the central star. The complexity of this nebula has been puzzling astronomers for decades. It has been formed as a result of stellar wind, but it is not understood well why it is so far from having a spherical shape.

Neither is known what physical process blows the wind, i.e. what lifts the material up from the stellar surface and makes it expand. “The fate of VY CMa is to explode as a supernova, but it is not known exactly when it will happen”, adds Karl Menten, head of the “Millimetre and Submillimetre Astronomy” Department at MPIfR.

Observations at different wavelengths provide different pieces of information which is characteristic for atomic and molecular gas and from which physical properties of an astronomical object can be derived. Each molecule has a characteristic set of lines, something like a ’bar code’, that allows to identify what molecules exist in the nebula.

“Emission at short radio wavelengths, in so-called submillimetre waves, is particularly useful for such studies of molecules”, says Sandra Brünken from the University of Cologne. “The identification of molecules is easier and usually a larger abundance of molecules can be observed than at other parts of the electromagnetic spectrum.”

Observatory at the volcano: the SMA interferometer where the discovery of the new molecules in VY CMa was made. Zoom Image

Observatory at the volcano: the SMA interferometer where the discovery of the new molecules in VY CMa was made.
© N. Patel/SMA

 

The research team observed TiO and TiO2 for the first time at radio wavelengths. In fact, titanium dioxide has been seen in space unambiguously for the first time. It is known from every-day life as the main component of the commercially most important white pigment (known by painters as “titanium white”) or as an ingredient in sunscreens. It is also quite possible that the reader consumed some amounts of it as it is used to colour food (coded as E171 in the labels).

However, stars, especially the coolest of them, are expected to eject large quantities of titanium oxides, which, according to theory, form at relatively high temperatures close to the star. “They tend to cluster together to form dust particles visible in the optical or in the infrared,” says Nimesh Patel from the Harvard-Smithsonian Center for Astrophysics. “And the catalytic properties of TiO2 may influence the chemical processes taking place on these dust particles, which are very important for forming larger molecules in space”, adds Holger Müller from the University of Cologne.

Absorption features of TiO have been known from spectra in the visible region for more than a hundred years. In fact, these features are used in part to classify some types of stars with low surface temperatures (M- and S-type stars). The pulsation of Mira stars, one specific class of variable stars, is thought to be caused by titanium oxide. Mira stars, supergiant variable stars in a late stage of their evolution, are named after their prototype star “Mira” (the wonderful) in the constellation of Cetus (the ‘sea monster’ or the ‘whale’).

The observations of TiO and TiO2 show that the two molecules are easily formed around VY CMa at a location that is more or less as predicted by theory. It seems, however, that some portion of those molecules avoid forming dust and are observable as gas phase species. Another possibility is that the dust is destroyed in the nebula and releases fresh TiO molecules back to the gas. The latter scenario is quite likely as parts of the wind in VY CMa seem to collide with each other.

The new detections at submillimetre wavelengths are particularly important because they allow studying the process of dust formation. Also, at optical wavelengths, the radiation emitted by the molecules is scattered by dust present in the extended nebula which blurs the picture, while this effect is negligible at radio wavelengths allowing for more precise measurements.

The discoveries of TiO and TiO2 in the spectrum of VY CMa have been made with the Submillimetre Array (SMA), a radio interferometer located at Hawaii, USA. Because the instrument combines eight antennas which worked together as one big telescope 226-meters in size, astronomers were able to make observations at unprecedented sensitivity and angular resolution. A confirmation of the new detections was successively made later with the IRAM Plateau de Bure Interferometer (PdBI) located in the French Alps.