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SOFIA/GREAT observations offer new insights into star formation

The flying observatory was stationed at Cologne Bonn Airport until 16 March 2021

March 17, 2021

The flying observatory SOFIA (Stratospheric Observatory For Infrared Astronomy) has successfully completed its observation flights from Cologne Bonn Airport. On board, amongst others, were scientists from the University of Cologne and the Max Planck Institute for Radio Astronomy in Bonn, who gained new insights into the formation of new stars during the observations.

Left: Multi-colour image of Herschel observations of RCW36 in the constellation Vela, showing cold dust (in red) and warm dust (in green and blue). The area to be mapped with SOFIA is indicated in gray, with the lower half already observed in an earlier observing campaign in the spectroscopic line of ionized carbon (CII) at 158 µm wavelength. The resulting map is displayed at the right side. Zoom Image

Left: Multi-colour image of Herschel observations of RCW36 in the constellation Vela, showing cold dust (in red) and … [more]
© N. Schneider et al. 2020

SOFIA is a Boeing 747SP with a 2.7-m-diameter telescope on board, built for astronomical observations from the infrared to the submillimetre wavelength range. From 4 February to 16 March 2021, the aircraft was stationed at Cologne Bonn Airport. Typically, it embarks on its observation flights from its usual location in Palmdale/California. But SOFIA underwent maintenance for three months, carried out by Lufthansa in Hamburg. Since the coronavirus pandemic is currently making it impossible for German scientists to travel to California, SOFIA was used for an observation campaign in the night sky over Europe, conducted from Cologne Bonn Airport, before the aircraft’s return to California. This campaign has now been successfully completed.

The flying observatory is a joint project of NASA and the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt – DLR). SOFIA’s flight altitude is more than 13 kilometres. This allows the aircraft to fly above most of the water vapour in the Earth’s atmosphere, which would block infrared light at lower altitudes, thus enabling scientists to observe a wavelength range that is not accessible from Earth. Onboard SOFIA is the high-resolution receiver for far-infrared spectroscopy GREAT (German Receiver for Astronomy at Terahertz Frequencies), developed by the Max Planck Institute for Radio Astronomy in Bonn and the Institute for Astrophysics at the University of Cologne with the participation of the DLR’s Institute for Optical Sensor Systems (Berlin).

Scientists use the GREAT instrument, a spectrally high-resolution imaging spectrometer, to create a kind of chemical fingerprint of vast regions of the sky with high spatial and spectral resolution. The GREAT team not only performs measurements for its own research projects, but also collects data for other scientists.

Central to the Cologne campaign were observations made as part of the SOFIA legacy programme FEEDBACK, led by Dr Nicola Schneider at the University of Cologne’s Institute for Astrophysics and Professor Alexander Tielens at the University of Maryland. Several scientists from the MPIfR/Bonn participate in the FEEDBACK program. The goal of the programme is to systematically observe galactic massive star-forming regions. ‘First results from FEEDBACK and other recent SOFIA projects have yielded new discoveries, including bubbles of expanding gas caused by stellar winds, which can be observed clearly in the spectral line of ionized carbon (CII). This expansion causes further star formation,’ Dr Schneider explained. Furthermore, the rate at which new stars form in the Milky Way can also be determined by observing CII, providing insights into the evolution of our galaxy.

The GREAT far-infrared spectrometer is mounted to the telescope flange of the flying observatory SOFIA, inside the pressurized cabin. Zoom Image

The GREAT far-infrared spectrometer is mounted to the telescope flange of the flying observatory SOFIA, inside the … [more]
© Carlos Duran/MPIfR

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Background information:

SOFIA is a Boeing 747SP jetliner modified to carry a 2.7-m diameter telescope. It is a joint project of the National Aeronautics and Space Administration (NASA) in the USA and the German Aerospace Center (DLR). NASA’s Ames Research Center in California’s Silicon Valley manages the SOFIA program, science and mission operations in cooperation with the Universities Space Research Association headquartered in Columbia, Maryland, and the German SOFIA Institute (DSI) at the University of Stuttgart. The aircraft is maintained and operated from NASA’s Armstrong Flight Research Center Hangar 703, in Palmdale, California.

GREAT: The German Receiver for Astronomy at Terahertz Frequencies is a high-resolution spectrometer for astronomical observations at far-infrared wavelengths (0.06-0.60 mm), operating in a wavelength regime that generally is not accessible from ground-based observatories due to absorption in the terrestrial atmosphere. The instrument’s modular design allows integration of new technological advancements on short notice. GREAT is a development by the Max Planck Institute for Radio Astronomy and the KOSMA/Universität zu Köln, in cooperation with the DLR Institute for Optical Sensor Systems. The development of GREAT is financed by the participating institutes, by the German Aerospace Center (DLR) and within the Collaborative Research Centre 956, funded by the Deutsche Forschungsgemeinschaft (DFG).

An astonishing new three-dimensional view of the dense interstellar gas in our Milky Way

SEDIGISM, a survey with the APEX telescope, studies molecular clouds and star formation in the inner galaxy

December 03, 2020

An international research team including a number of scientists from the Max Planck Institute for Radio Astronomy in Bonn, Germany, used the APEX submillimeter telescope in 5100 m altitude in Chile to map an extended part of the Southern Galactic plane covering an area of more than 80 square degrees. Spectral lines emitted from several molecules, including the rare isotopologues 13CO and C18O of the carbon monoxide molecule, probed the moderately dense component of the interstellar medium. The resulting survey, called SEDIGISM (Structure, Excitation and Dynamics of the Inner Galactic Interstellar Medium), reveals a wide range of structures, from individual star-forming clumps to giant molecular clouds and complexes. This survey allows us to constrain the large-scale distribution of cold molecular gas in the inner Galaxy and ultimately unravel the structure of the Milky Way.

The survey data are used to study the distribution of cold molecular clouds and the amount of star formation in the inner region of the Milky Way in unprecedented detail. A catalog of over 10,000 clouds in our Milky Way has already been compiled, showing a very homogeneous distribution of physical properties. Interestingly, only a small proportion (~10%) of these clouds exhibit ongoing star formation. Altogether, the SEDIGISM survey represents a significant step forward in understanding the structure of the Galaxy and the connection between spiral arms and molecular clouds whose denser parts harbour new generations of forming stars. The first data release makes these data available to the scientific community.

The initial results from this survey are presented in three publications (with first authors from MPIfR, Cardiff University and University of Kent) within the current issue of “Monthly Notices of the Royal Astronomical Society’’.

Main image: Example of the clouds identified in a small section of the SEDIGISM survey (~5% area covered); each cloud has a different (random) colour (modified from Fig. 3 from Duarte-Cabral et al. 2020). Insert: Schematic showing the loci of the four spiral arms, the inner most 3-kpc arms, the positions of Galactic Center and central bar (these are indicated by the black cross and dark grey oval) and roman numerals in the corners indicate the Galactic quadrants. The light grey shaded regions show the survey coverage while the region shown in the main image is shaded in blue (modified from Fig. 5 from Schuller et al. 2020). Zoom Image

Main image: Example of the clouds identified in a small section of the SEDIGISM survey (~5% area covered); each cloud … [more]
© Ana Duarte-Cabral, Alex Pettitt and James Urquhart

Observations of spectral lines of the carbon monoxide molecule allow us to probe the cold and dense molecular phase of the interstellar medium, from which new stars form. In addition, the velocity of the clouds can be measured via their Doppler shift. This allows us to link the clouds to the rotation of the spiral structure of our Milky Way, thereby providing a three-dimensional view of their distribution, which shows a rich variety of structures, such as filaments and cavities, resulting from all the physical effects that shape the interstellar medium.

Using the APEX telescope in the Chilean Andes, an international team of about 50 astronomers has completed the analysis of this observational effort, which covers 84 square degrees of the southern inner Galaxy, ranging in Galactic longitude from -60 to +18 degrees with a resolution of 30 arc seconds, only about 1/60 of the projected moon diameter on the sky. With a velocity resolution of 0.25 km/s, it provides the morphology, distance information and kinematics of all Galactic molecular clouds in approximately 2/3 of the inner Milky Way disk.

The survey is called SEDIGISM (Structure, Excitation and Dynamics of the Inner Galactic Interstellar Medium) and includes data taken in the years 2013-17 that will now be released to the astronomical community together with the first three scientific publications on the full data set.

“With the publication of this unprecedentedly detailed map of cold clouds in our Milky Way a huge observational effort comes to fruition”, says Frederic Schuller from the Max Planck Institute for Radio Astronomy (MPIfR), the principal investigator of the SEDIGISM survey. “The team did a great job in delivering a new roadmap for molecular gas in the Galaxy as legacy of APEX for years to come.”

“Based on these data, a catalog of over 10,000 of these clouds in our Milky Way has been compiled, which show a highly structured Galactic distribution, albeit with relatively homogeneous physical properties, with only hints for potential environmental dependency of some cloud properties”, explains Ana Duarte-Cabral from Cardiff University, the lead author of the second paper. James Urquhart from the University of Kent, the lead author of the third publication, adds: “In conjunction with the previous survey of cold dust emission in the Galaxy (ATLASGAL), the fraction of clouds associated with dense gas could be estimated: only 10% of the clouds are sites of ongoing star formation”.

The observations targeted the rare 13CO and C18O isotopes of the carbon monoxide molecule that allow much more precise mass estimates of the clouds than the much more abundant 12CO, but require a very sensitive telescope. The 12-m APEX telescope (Fig. 2), with its precise mirror surface and a location at one of the world’s best sites for (sub)millimeter astronomy, has been key for the success of the project. Located at 5100m on the dry Chajnantor plateau in Chile, the low water vapour content of the site leads to the very high transparency of the sky that is needed for such observations.

Molecular clouds consist of the raw material from which new stars form. Imaging these clouds is therefore essential to derive important parameters such as the star formation efficiency in our Galaxy. The morphology and physical conditions of the clouds also provide the initial conditions that theories of star formation have to take into account. It is, therefore, crucial to spatially resolve the clouds, which was possible with the high-angular resolution of the survey.

The survey is not only interesting on its own, but also complements a number of other outstanding Galactic plane surveys, conducted in the last decade in the mid- to far-infrared wavelength ranges with space-based telescopes such as Spitzer and Herschel and, at longer wavelengths with APEX itself, which are all lacking the velocity information. The data of these surveys can now be re-analysed in conjunction with the new carbon monoxide line data. This will significantly enhance their role in the ongoing quest to understand the formation of stars, stellar clusters and ultimately the structure and dynamics of the Milky Way.

“Our survey represents a significant step towards understanding the structure of the Galaxy in which we live”, concludes Dario Colombo from MPIfR, co-author of all three publications, who is currently preparing another analysis of the data to establish the influence of spiral arms on molecular cloud properties.

APEX, the 12-metre Atacama Pathfinder Experiment telescope, located on Chajnantor Plateau in the Chilean Atacama Desert. Zoom Image

APEX, the 12-metre Atacama Pathfinder Experiment telescope, located on Chajnantor Plateau in the Chilean Atacama Desert.
© Carlos A. Durán/ESO

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Background Information

SEDIGISM (Structure, Excitation and Dynamics of the Inner Galactic Interstellar Medium) is a survey of the southern Galactic plane covering an area of 84 square degrees in the sky, ranging in galactic longitude from -60 to +18 degrees with a resolution of 30 arc seconds. Two spectral lines of the carbon monoxide molecule were observed in the less abundant 13CO and C18O isotopologues with the APEX telescope.

ATLASGAL, the APEX Telescope Large Area Survey of the Galaxy, is a collaboration between the Max Planck Institute for Radio Astronomy (MPIfR), the Max Planck Institute for Astronomy (MPIA), and scientists from the ESO community and the University of Chile.

APEX, the Atacama Pathfinder Experiment, is a collaboration between the Max Planck Institute for Radio Astronomy (MPIfR), the European Southern Observatory (ESO) and the Onsala Space Observatory (OSO) to construct and operate, since 2005, a single dish telescope on the Chajnantor plateau at an altitude of 5,100 metres 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.

The research team comprises a number of authors, including Dario Colombo, Timea Csengeri, Min-Young Lee, Silvia Leurini, Michael Mattern, Parichay Mazumdar, Sac Medina, Karl Menten, Alberto Sanna, Frederic Schuller, Marion Wienen, Friedrich Wyrowski, all holding a present or recent affiliation with the MPIfR.

The Distances of the Stars

Resolving Long-standing Mysteries About the First Parallaxes in Astronomy

November 19, 2020

In 1838, Friedrich Wilhelm Bessel won the race to measure the first distance to a star other than our Sun via the trigonometric parallax – setting the first scale of the Universe.

Recently, Mark Reid and Karl Menten, who are engaged in parallax measurements at radio wavelengths, revisited Bessel’s original publications on “his” star, 61 Cygni, published in the Astronomische Nachrichten (Astronomical Notes).  While they could generally reproduce the results obtained by Bessel and two contemporary 19th century astronomers, the eminent Friedrich Georg Wilhelm von Struve and Thomas Henderson, they discovered why some of these early results were statistically inconsistent with modern measurements.

Out of reverence for Bessel, Reid and Menten decided to publish their findings also in the Astronomische Nachrichten. Founded in 1821, it was one of the first astronomical journals in the world and is the oldest that is still being published.

Stamp issued by the German federal post office in 1984, on the occasion of the 200th anniversary of Friedrich Wilhelm Bessel’s birth. Zoom Image

Stamp issued by the German federal post office in 1984, on the occasion of the 200th anniversary of Friedrich Wilhelm … [more]

© Bundesministerium der Finanzen (BMF). Design by Hermann Schwahn, based on a painting by Johann Eduard Wolff.

Knowing the distance to astronomical objects is of fundamental importance for all of astronomy and for assessing our place in the Universe.  The ancient Greeks placed the unmoving “fixed” stars farther away than the celestial spheres on which they thought the planets were moving.  However, the question “how much farther?” eluded an answer for centuries after astronomers started trying to address it. Things came to a head in the late 1830s, when three astronomers zeroed in on different stars, spending many nights at their telescope, often under harsh conditions. It was Friedrich Wilhelm Bessel who won the race in 1838 by announcing that the distance to the double-star system 61 Cygni is 10.4 light years.  This proved that stars are not just a little farther away from us than planets, but more than a million times farther – a truly transformational result that totally revised the scale of the Universe as it was known in the 19th century.

Bessel’s measurement was based on the trigonometric parallax method. This technique is essentially triangulation, which is used by surveyors to determine distances on land. Astronomers measure the apparent position of a “nearby” star against much more distant stars, using the Earth’s orbit around the Sun to provide different vantage points over a year’s time.

Bessel had to make his pain-staking measurements over nearly 100 nights at his telescope. Astronomers now are far more “efficient”. The Gaia space mission is measuring accurate distances for hundreds of millions of stars, with great impact on astronomy. However, because of interstellar dust that pervades the Milky Way’s spiral arms, Gaia has difficulties observing stars within the Galactic plane that are farther from the Sun than about 10,000 light years – this is just 20% of the Milky Way’s size of more than 50,000 light years. Therefore, even a mission as powerful as Gaia will not yield the basic layout of our Galaxy, many aspects of which are still under debate – even the number of spiral arms is uncertain.

In order to better address the structure and size of the Milky Way, Mark Reid from the Center for Astrophysics | Harvard-Smithsonian and Karl Menten from the Max Planck Institute for Radio Astronomy (MPIfR) initiated a project to determine the distances to radio sources that are constrained to spiral arms of the Milky Way. Their telescope of choice is the Very Long Baseline Array, a collection of 10 radio telescopes spanning from Hawaii in the west to the eastern tips of the USA. By combining the signals of all 10 telescopes thousands of kilometers apart one can make images of what one could see were our eyes sensitive to radio waves and separated by nearly the size of the Earth.

This project is carried out by an international team, with scientists of the MPIfR making major contributions – MPIfR director Karl Menten has enjoyed a fruitful collaboration with Mark Reid for more than 30 years. When, near the start of the project, a catchy acronym was discussed, they chose to name it the Bar and Spiral Structure Legacy Survey, in short the BeSSeL Survey. Of course, they had the great astronomer and mathematician and parallax pioneer Friedrich Wilhelm Bessel on their mind.

As in all experimental or observational science, measurements only attain meaning if their uncertainties can be determined in a reliable way. This is also the bread and butter in radio astrometry and is given close attention by the BeSSeL project astronomers. In Bessel’s time, astronomers had learned to pay attention to measurement errors and to account for them when deriving results from their data. This often involved tedious calculations done entirely with pencil and paper. Naturally, a scientist of Bessel’s caliber was well aware to follow any issues that could possibly affect his observations.  He realized that temperature variations in his telescope could critically affect his delicate measurements. Bessel had a superb instrument at his observatory at Königsberg in Prussia (the present Russian Kaliningrad), which came from the genius instrument maker Joseph Fraunhofer and was the last one he built.  Nevertheless, variable temperature had a major impact on the observations required for a parallax measurement, which must be spread over an entire year; some are made in hot summer and others in cold winter nights.

Mark Reid became interested in Bessel’s original work and studied his papers on 61 Cygni. He noticed some small inconsistencies in the measurements. To address these he and Karl Menten started to dig deeper into the original literature. Bessel’s papers were first published in German, in the Astronomische Nachrichten, although some excerpts were translated into English and appeared in the Monthly Notices of the Royal Astronomical Society. Thus, the original German versions had to be examined, where Menten’s native German came in handy.

Reid and Menten also put the results of Bessel’s closest competitors under scrutiny. Thomas Henderson, who worked in Cape Town, South Africa, targeted α Centauri, the star system now known to be the closest to our Sun. Shortly after Bessel announced his result, Henderson published a distance to this star.

The eminent astronomer Friedrich Georg Wilhelm von Struve measured α Lyrae (Vega). The literature search for von Struve’s data involved some detective work. A detailed account of it was only published in Latin as a chapter of a voluminous monograph. The MPIfR librarian traced a copy to the Bavarian State library, which provided it in electronic form. It has long been a mystery as to why von Struve announced a tentative distance to Vega, one year before Bessel’s result for 61 Cygni, only to revise it to double that distance later with more measurements. It seems that von Struve first used all of his measurements, but in the end lost confidence in some and discarded those. Had he not done so, he probably would have received more credit.

Reid and Menten can generally reproduce the results obtained by all three astronomers, but found that von Struve and Henderson underestimated some of their measurement uncertainties, which made their parallaxes appear somewhat more significant than they actually were. “Looking over Bessel’s shoulder was a remarkable experience and fun,” says Mark Reid. “Viewing this work both in an astronomical and historical context has really been fascinating”, concludes Karl Menten.

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Background Information

Principle of Stellar Parallax: One wants to determine the distance, D, to a nearby (foreground) star. Over the course of a year, that star’s position apparently changes relative to the positions of faraway background stars and prescribes an ellipse that is a projection of the Earth’s orbit around the Sun. Its semi-major axis is the parallax angle π. The distance in “astronomical units” is then simply given by D = 1/ π. One astronomical unit, AU, the Earth-Sun distance is equal to approximately 150 million kilometers. The distance at which an object would have a parallax of 1 arcsecond is called one parsec (pc). It is the basic distance unit used by astronomers and corresponds to approx. 3.26 light years or 206,000 AU.

Magnetized gas flows feed a young star cluster

Polarimetric observations with SOFIA/HAWC+ show the orientation of magnetic field lines

August 18, 2020

Observations of magnetic fields in interstellar clouds made of gas and dust indicate that these clouds are strongly magnetized, and that magnetic fields influence the formation of stars within them. A key observation is that the orientation of their internal structure is closely related to that of the magnetic field.

To understand the role of magnetic fields, an international research team led by Thushara Pillai, Boston University & Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, observed the filamentary network of the dense gas surrounding a young star cluster in the solar neighboorhood, with the HAWC+ polarimeter on the airborne observatory SOFIA at infrared wavelengths. Their research shows that not all dense filaments are created equal. In some of the filaments the magnetic field succumbs to the flow of matter and is pulled into alignment with the filament. Gravitational force takes over in the denser parts of some filaments and the resulting weakly magnetized gas flow can feed the growth of young stellar clusters like a conveyor belt.

The results are published in this week’s issue of “Nature Astronomy“.

 

Composite image of the Serpens South Cluster. Magnetic fields observed by SOFIA are shown as streamlines over an image from the Spitzer Space Telescope. SOFIA indicate that gravity can overcome some of the strong magnetic fields to deliver material needed for new stars. The magnetic fields have been dragged into alignment with the most powerful flows, as seen in the lower left where the streamlines are following the direction of the narrow, dark filament. This is accelerating the flow of material from interstellar space into the cloud, and fueling the collapse needed to spark star formation.   Zoom Image

Composite image of the Serpens South Cluster. Magnetic fields observed by SOFIA are shown as streamlines over an image … [more]

© NASA/SOFIA/T. Pillai/J. Kauffmann; NASA/JPL-Caltech/L. Allen

The interstellar medium is composed of tenuous gas and dust that fills the vast amount of emptiness between stars. Stretching across the Galaxy, this rather diffuse material happens to be a significant mass reservoir in Galaxies.  An important component of this interstellar gas are the cold and dense molecular clouds which hold most of their mass in the form of molecular hydrogen. A major finding in the last decade has been that extensive network of filaments permeate every molecular cloud. A picture has emerged that stars like our own sun form preferentially in dense clusters at the intersection of filaments.

The researchers observed the filamentary network of dense gas around the Serpens South Cluster with HAWC+, a polarization-sensitive detector onboard the airborne observatory SOFIA, in order to understand the role of magnetic fields. Located about 1,400 light-years away from us, the Serpens South cluster is the youngest known cluster in the local neighborhood at the center of a network of dense filament.

The observations show that low–density gaseous filaments are parallel to the magnetic field orientation, and that their alignment becomes perpendicular at higher gas densities. The high angular resolution of HAWC+ reveals a further, previously unseen twist to the story. “In some dense filaments the magnetic field succumbs to the flow of matter and and is pulled into alignment with the filament”, says Thushara Pillai (Boston University and MPIfR Bonn), the first author of the publication.  “Gravitational force takes over in the more opaque parts of certain filaments in the Serpens Star Cluster and the resulting weakly magnetized gas flow can feed the growth of young stellar clusters like a conveyor belt”, she adds.

It is understood from theoretical simulations and observations that the filamentary nature of molecular clouds actually plays a major role in channeling mass from the larger interstellar medium into young stellar clusters whose growth is fed from the gas. The formation and evolution process of stars is expected to be driven by a complex interplay of several fundamental forces — namely turbulence, gravity, and the magnetic field. In order to get an accurate description for how dense clusters of stars form, astronomers need to pin down the relative role of these three forces. Turbulent gas motions as well as the mass content of filaments (and therefore gravitation force) can be gauged with relative ease. However, the signature of the interstellar magnetic field is weak, also because it is about 10,000–times weaker than even our own Earth’s magnetic field. This has made measurements of magnetic field strengths in filaments a formidable task.

“The magnetic field directions in this new polarization map of Serpens South align well with the direction of gas flow along the narrow southern filament. Together these observations support the idea that filamentary accretion flows can help form a young star cluster”, adds Phil Myers from the Harvard-Smithsonian Center for Astrophysics, a co-author of the paper.

A small fraction of a molecular cloud’s mass is made up by small dust grains that are mixed into the interstellar gas. These interstellar dust grains tend to align perpendicular to the direction of the magnetic field. As a result, the light emitted by the dust grains is polarized — and this polarization can be used to chart the magnetic field directions in molecular clouds.

Recently, the Planck space mission produced a highly sensitive all–sky map of the polarized dust emission at wavelengths smaller than 1 mm. This provided the first large–scale view of the magnetization in filamentary molecular clouds and their environments. Studies done with Planck data found that filaments are not only highly magnetized, but they are coupled to the magnetic field in a predictable way. The orientation of the magnetic fields is parallel to the filaments in low–density environments. The magnetic fields change their orientation to being perpendicular to filaments at high gas densities, implying that magnetic fields play an important role relative in shaping filaments, compared to the influence of turbulence and gravity.

This observation pointed towards a problem. In order to form stars in gaseous filaments, the filaments have to lose the magnetic fields. When and where does this happen? With the order of magnitude higher angular resolution of the HAWC+ instrument in comparison to Planck it was now possible to resolve the regions in filaments where the magnetic filament becomes less important.

“Planck has revealed new aspects of magnetic fields in the interstellar medium, but the finer angular resolutions of SOFIA’s HAWC+ receiver and ground-based NIR polarimetry give us powerful new tools for revealing the vital details of the processes involved”, says Dan Clemens, Professor and Chair of the Boston University Astronomy Department, another co-author.

“The fact that we were able to capture a critical transition in star formation was somewhat unexpected. This just shows how little is known about cosmic magnetic fields and how much exciting science awaits us from SOFIA with the HAWC+ receiver”, concludes Thushara Pillai.

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SOFIA, the Stratospheric Observatory for Infrared Astronomy. The HAWC+ polarimeter onboard SOFIA was used for the observations of the magnetic field in the Serpens South region. Zoom Image

SOFIA, the Stratospheric Observatory for Infrared Astronomy. The HAWC+ polarimeter onboard SOFIA was used for the … [more]

© NASA/C. Thomas

Background Information

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a Boeing 747SP jetliner modified to carry a 106-inch diameter telescope. It is a joint project of NASA and the German Aerospace Center, DLR. NASA’s Ames Research Center in California’s Silicon Valley manages the SOFIA program, science and mission operations in cooperation with the Universities Space Research Association headquartered in Columbia, Maryland, and the German SOFIA Institute (DSI) at the University of Stuttgart. The aircraft is maintained and operated from NASA’s Armstrong Flight Research Center Building 703, in Palmdale, California.

The High-resolution Airborne Wideband Camera Plus (HAWC+), SOFIA’s newest instrument, +, uses far-infrared light to observe celestial dust grains, which align perpendicular to magnetic field lines. From these results, astronomers can infer the shape and direction of the otherwise invisible magnetic field. Far-infrared light provides key information about magnetic fields because the signal is not contaminated by emission from other mechanisms, such as scattered visible light and radiation from high-energy particles. The HAWC+ instrument was developed and delivered to NASA by a multi-institution team led by the Jet Propulsion Laboratory in Pasadena, California.

The research team comprises Thushara Pillai, Dan P. Clemens, Stefan Reissl, Philip C. Myers, Jens Kauffmann, Enrique Lopez-Rodriguez, Felipe de Oliveira Alves, Gabriel A. P. Franco, Jonathan Henshaw, Karl M. Menten, Fumitaka Nakamura, Daniel Seifried, Koji Sugitani, and Helmut Wiesemeyer. Thushara Pillai, the first author, and also Karl Menten and Helmut Wiesemeyer have an affiliation with the MPIfR.

 https://www.mpifr-bonn.mpg.de/pressreleases/2020/8

First radio detection of an extrasolar planetary system around a main-sequence star

VLBA Network Finds Planet Orbiting a Small, Cool Star

August 04, 2020

An international team of astronomers including Gisela Ortiz-Leon from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn discovered a Saturn-like planet orbiting a small, cool starby detecting the “wobble” in the star’s motion caused by the gravitational pull of the planet. This is the first time that this technique is successfully employed with observations obtained at radio wavelengths. For their observations, the researchers used a network of radio antennas that are linked together to form a continent-size radio telescope. The discovery was possible thanks to the extremely high precision measurements of the star’s position that can only be achieved with such a radio telescope network.

One of the things that makes this detection exciting is that the planet, called TVLM 513b, has a similar mass to Saturn and an orbit analogous to that of Mercury in our Solar System. Only a handful of extrasolar planets with characteristics similar to TVLM 513b have been discovered so far around small, cool stars – known as ultracool dwarfs. Other planet search techniques have difficulties to study these dwarfs, mainly due to the faintness of the objects, which makes radio observations a very powerful and complementary tool to uncover many more new planets.

The results are published in the current issue of the “Astronomical Journal“.

Illustration of the planetary system TVLM 513–46546; the newly discovered Saturn-like planet is seen in front of its host star, a small and cool brown dwarf.  
 Zoom Image

Illustration of the planetary system TVLM 513–46546; the newly discovered Saturn-like planet is seen in front of its … [more]

© Luis A. Curiel Ramirez

Using the supersharp radio “vision” of the continent-wide Very Long Baseline Array (VLBA), astronomers have discovered a Saturn-sized planet closely orbiting a small, cool star 35 light-years from Earth. This is the first discovery of an extrasolar planet with a radio telescope using a technique that requires extremely precise measurements of a star’s position in the sky, and only the second planet discovery for that technique and for radio telescopes.

The technique has long been known, but has proven difficult to use. It involves tracking the star’s actual motion in space, then detecting a minuscule “wobble” in that motion caused by the gravitational effect of the planet. The star and the planet orbit a location that represents the center of mass for both combined. The planet is revealed indirectly if that location, called the barycenter, is far enough from the star’s center to cause a wobble detectable by a telescope.

This technique, called the astrometric technique, is expected to be particularly good for detecting Jupiter-like planets in orbits distant from the star. This is because when a massive planet orbits a star, the wobble produced in the star increases with a larger separation between the planet and the star, and at a given distance from the star, the more massive the planet, the larger the wobble produced.

Starting in June of 2018 and continuing for a year and a half, the astronomers tracked a star called TVLM 513–46546, a cool dwarf with less than a tenth the mass of our Sun in the constellation Boötes (The Herdsman). In addition, they used data from nine previous VLBA observations of the star between March 2010 and August 2011.

Extensive analysis of the data from those time periods revealed a telltale wobble in the star’s motion indicating the presence of a planet comparable in mass to Saturn, orbiting the star once every 221 days. This planet is closer to the star than Mercury is to the Sun.

Small, cool stars like TVLM 513–46546 are the most numerous stellar type in our Milky Way Galaxy, and many of them have been found to have smaller planets, comparable to Earth and Mars.

“Giant planets, like Jupiter and Saturn, are expected to be rare around small stars like this one, and the astrometric technique is best at finding Jupiter-like planets in wide orbits, so we were surprised to find a lower mass, Saturn-like planet in a relatively compact orbit. We expected to find a more massive planet, similar to Jupiter, in a wider orbit”, said Salvador Curiel, of the National Autonomous University of Mexico. “Detecting the orbital motions of this sub-Jupiter mass planetary companion in such a compact orbit was a great challenge”, he added.

More than 4,300 planets have been discovered orbiting stars other than the Sun, but the planet around TVLM 513–46546 is only the second to be found using the astrometric technique. Another, very successful method, called the radial velocity technique, also relies on the gravitational effect of the planet upon the star. That technique detects the slight acceleration of the star, either toward or away from Earth, caused by the star’s motion around the barycenter.

“Our method complements the radial velocity method which is more sensitive to planets orbiting in close orbits, while ours is more sensitive to massive planets in orbits further away from the star”, said Gisela Ortiz-Leon of the Max Planck Institute for Radio Astronomy in Germany. “Indeed, these other techniques have found only a few planets with characteristics such as planet mass, orbital size, and host star mass, similar to the planet we found. We believe that the VLBA, and the astrometry technique in general, could reveal many more similar planets.”

A third technique, called the transit method, also very successful, detects the slight dimming of the star’s light when a planet passes in front of it, as seen from Earth.

The astrometric method has been successful for detecting nearby binary star systems, and was recognized as early as the 19th Century as a potential means of discovering extrasolar planets. Over the years, a number of such discoveries were announced, then failed to survive further scrutiny. The difficulty has been that the stellar wobble produced by a planet is so small when seen from Earth that it requires extraordinary precision in the positional measurements.

“The VLBA, with antennas separated by as much as 5,000 miles, provided us with the great resolving power and extremely high precision needed for this discovery”, said Amy Mioduszewski, of the National Radio Astronomy Observatory. “In addition, improvements that have been made to the VLBA’s sensitivity gave us the data quality that made it possible to do this work now”, she added.

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Background Information

The research team comprises Salvador Curiel, Gisela N. Ortiz-León, Amy J. Mioduszewski and Rosa M. Torres. Gisela Ortiz-León, the second author, is affiliated with the MPIfR.

The National Radio Astronomy Observatory (NRAO) operating the “Very Long Baseline Array” (VLBA) is a facility of the National Science Foundation, under cooperative agreement by Associated Universities, Inc.

 https://www.mpifr-bonn.mpg.de/pressreleases/2020/7

SOFIA and „Christmas comet“ Wirtanen

A bright „guest star“ is showing up in the night sky: comet 46P/Wirtanen. The airborne observatory SOFIA observed the comet in the nights from December 13 to 19 with the GREAT (German REceiver for Astronomy at Terahertz frequencies) instrument, built by MPIfR and Cologne University. In parallel, 46P/Wirtanen was observed with the APEX telescope in Chile (Clemens Plank, DLR Science Blog, December 2018, in German language). [more]

CRC 956 funded for another four years

Funding of the Collaborative Research Centre (CRC) 956, entitled „Conditions and Impact of Star Formation – Astrophysics, Instrumentation and Laboratory Research“, has just been extended for another period of 4 years. CRC 956 explores basic star-formation processes and with that is contributing to worldwide exchange of extended knowledge between researchers. It is jointly run by the I. Physikalische Institut der Universität Köln, the Argelander-Institut für Astronomie der Universität Bonn and the Max-Planck-Institut für Radioastronomie in Bonn. [more]

Astronomy Walks in Effelsberg – how Sirius is coming to Chile

Scaled Extension of the Planetary Walk at the Effelsberg Radio Telescope to the Brighest Star in the Night Sky

September 14, 2018

The 100-m radio telescope of the Max Planck Institute for Radio Astronomy (MPIfR) is located in a valley near Bad Münstereifel-Effelsberg about 40 kilometers southwest of Bonn in the Eifel area. Three astronomical trails in the surroundings of the observatory, named “Planetary Walk”, “Milky Way Walk” and “Galaxy Walk” illustrate the complete cosmic distance scale from nearby planets to distant galaxies. The connection between two of them, the Planetary Walk and the Milky Way Walk, is established by the common target station “Sirius”.  In the scale of the Milky Way Walk, Sirius and our sun are neighbouring stations only 90 cm apart. In the scale of the Planetary Walk, the real distance of 8.6 light years between sun and Sirius amounts to 11,000 km, corresponding to the distance between two of MPIfR’s radio telescopes, the 100-m Effelsberg telescope in Germany and the 12-m APEX submillimeter telescope in Chile.  

The 100-m radio telescope seen from the courtyard of the visitors’ pavilion. The yellow ball marks the station “Sun” of the Effelsberg Planetary Walk. This walk includes 11 stations in total: the sun, 8 planets, dwarf planet Pluto and the nearby star Sirius as transatlantic extension.   Zoom Image

The 100-m radio telescope seen from the courtyard of the visitors’ pavilion. The yellow ball marks the station “Sun” of … [more]

© Norbert Junkes/(MPIfR

Planet trails (in German: “Planetenwege”) are a nice way to illustrate cosmic distances and sizes within our solar system. They usually consist of nine or ten stations: the sun and eight planets, sometimes also including the dwarf planet Pluto. At the widely used scale of 1 : 1 billion the trail has a total length of almost 6 kilometers (distance between sun and Pluto). The sun scales to a diameter of 1.4 meters, and the Earth to 1.3 cm in a distance of 150 meters from the sun.

The Planetary Walk at the Effelsberg Radio Observatory is a bit smaller in size. Scaled 1 : 7.7 billion it covers the walking distance of about 800 meters from the parking area to the visitors’ pavilion where talks about radio astronomy and the telescope for groups of visitors are taking place. It starts with the dwarf planet Pluto at the parking lot and continues from there to the inner solar system – the rocky planets between Mars and Mercury and the sun itself can all be found at the courtyard of the visitors’ pavilion (Fig. 1).

Pluto (0 m) Visitors‘ Parking Area

Neptune (182 m) Road to Visitors‘ Pavilion

Uranus (389 m) Road to Visitors‘ Pavilion

Saturn (584 m) Road to Visitors‘ Pavilion

Jupiter (665 m) Road to Visitors‘ Pavilion

Mars (736 m) Road to Visitors‘ Pavilion

Earth (746 m) Court of Visitors‘ Pavilion

Venus (752 m) Court of Visitors‘ Pavilion

Mercury (758 m) Court of Visitors‘ Pavilion

Sun (766 m) Court of Visitors‘ Pavilion

Sirius (11,000 km) APEX Telescope, Atacama Desert, Chile

Table: Stations of the Planetary Walk at the Effelsberg Radio Telescope

Two additional walks, the Milky Way Walk and the Galaxy Walk, are extending the cosmic distance scale far beyond the solar system. The Milky Way Walk covers a total distance of 4 kilometers from the village Burgsahr in the nearby Sahrbach valley to a viewing spot immediately in front of the giant dish of the Effelsberg telescope. At a scale of 1 : 1017 (1 : 100 quadrillion) this corresponds to 40,000 light years through our galaxy. The Milky Way Walk includes a total of 18 stations from the outer regions along the sun to the Galactic centre at a distance of 25,000 light years from the sun.

The Galaxy Walk covers the truly large distances in the Universe. It has a total length of 2.6 km, starting in the forest behind the Effelsberg radio telescope and leading to a nearby hut (Martinshütte: the “hut at the edge of the Universe”). At a scale of 1 : 5 x 1022 (1 : 50 sextillions) it includes a total of 14 stations, the most distant one with a light travel time of 12.85 billion years. In other words: we observe light from that distant galaxy (named J1148+5251) coming from a time less than 1 billion years after the formation of the Universe.

In order to connect the three astronomy trails at Effelsberg there are two target stations included in two of the trails. The station “Andromeda Galaxy M 31” is contained in both, Milky Way Walk and Galaxy Walk. It is the closest large-scale spiral galaxy, a twin of our Milky Way at a distance of 2.5 million light years. For the Galaxy Walk that scales to 50 centimeters; Milky Way and Andromeda galaxy are the first two stations only 50 cm apart. At the scale of the Milky Way Walk, however, it corresponds to a distance of 250 kilometers.

The station “Andromeda Galaxy” is mounted at the “Haus der Astronomie” in Heidelberg, 250 km away from Effelsberg. This house is indeed shaped like a spiral galaxy, contains a 1 : 100 scale model of the Effelsberg telescope in its interior and the plaque “M 31” of the Effelsberg Milky Way Walk at the main entrance.

The connecting element between Planetary Walk and Milky Way Walk will be Sirius, the brightest star in the night sky. Sirius is a nearby star at a distance of only 8.6 light years from the sun. Both stations (Sun and Sirius) can be found on the Milky Way Walk: close to the village Binzenbach in the forest, the two plaques are just 90 centimeters apart.

For the Planetary Walk it is a different story: the distance of about 9 light years to Sirius scales to 11,000 kilometers! At the same scale, the dwarf planet Pluto is less than 800 meters away from the Sun. For the space probe Voyager 1, the most distant device built by mankind, it is less than 3 km in that scale, but even for the nearest star, Proxima Centauri, the distance is already more than 5000 km!

Coincidence by chance: the required value of 11,000 km for Sirius nicely meets the distance between two radio telescopes of the Max Planck Institute for Radio Astronomy, the 100-m radio telescope at Effelsberg and the APEX telescope in the Atacama desert in Chile which is  jointly run by the MPIfR, the European Southern Observatory ESO and the Swedish Onsala Observatory.

The station “Sirius” of the Planetary Walk is mounted directly at the APEX site on the Chajnantor plateau 5100 m above sea level in the Atacama desert (Fig. 2). Sirius is the brightest star in the night sky, and it is visible from both sites, Effelsberg in Germany as well as APEX in Chile.

Since there is no general access to the APEX telescope on Chajnantor, Sirius is also presented in the nearby village San Pedro de Atacama, 2500 m above sea level. The San Pedro office of Alain Maury’s San Pedro de Atacama Celestial Explorations (SPACE) on Caracoles 400-2 presents the plaque in three different languages (Spanish, English and German).

“It is great that Sirius enables us to close the last remaining gap between our astronomical walks at the Effelsberg telescope”, concludes Norbert Junkes from MPIfR who regularly uses the cosmic distance scale in his talks for visitor groups at the Effelsberg site. “From the planets to stars and star forming regions in our Galaxy and further on to other galaxies, almost to the edge of the Universe – our three walks cover the complete range.”

The Atacama Pathfinder Experiment (APEX) at the Chajnantor plateau in Northern Chile, 5100 m above sea level. The linear distance of 11,000 km between APEX and the Effelsberg 100-m radio telescope in Germany nicely corresponds to the distance of the nearby star Sirius in the scale of the Effelsberg Planetary Walk.   Zoom Image

The Atacama Pathfinder Experiment (APEX) at the Chajnantor plateau in Northern Chile, 5100 m above sea level. The linear … [more]

© Norbert Junkes/(MPIfR

Impact of a stellar intruder on our solar system

Computer models of a fly-by show amazing resemblance to outer solar system features

August 09, 2018

The solar system was formed from a protoplanetary disk consisting of gas and dust. Since the cumulative mass of all objects beyond Neptune is much smaller than expected and the bodies there have mostly inclined, eccentric orbits it is likely that some process restructured the outer solar system after its formation. Susanne Pfalzner from the Max Planck Institute for Radio Astronomy in Bonn, Germany and her colleagues present a study showing that a close fly-by of a neighbouring star can simultaneously lead to the observed lower mass density in the outer part of the solar system and excite the bodies there onto eccentric, inclined orbits. Their numerical simulations show that many additional bodies at high inclinations still await discovery, perhaps including a sometimes postulated planet X.

The findings are published in the present issue of „The Astrophysical Journal“.

Artist’s concept of a solar system in the making with a protoplanetary disk surrounding a young star.   Zoom Image

Artist’s concept of a solar system in the making with a protoplanetary disk surrounding a young star.

 

© NASA JPL-Caltech

A near catastrophy billions of years ago might have shaped the outer parts of the solar system, while leaving the inner regions basically untouched. Researchers from the Max Planck Institute for Radio Astronomy in Bonn and their collaborators found that a close fly-by of another star can explain many of the features observed in the outer solar system. „Our group has been looking for years at what fly-bys can do to other planetary systems never considering that we actually might live right in such a system”, says Susanne Pfalzner, the leading author of the project. “The beauty of this model lies in its simplicity.”

The basic scenario of the formation of the solar system has long been known: our Sun was born from a collapsing cloud of gas and dust. In the process a flat disk was formed where not only large planets grew but also smaller objects like the asteroids, dwarf planets etc.  Due to the flatness of the disk one would expect that the planets orbit in a single plane unless something dramatic happened afterwards. Looking at the solar system right to the orbit of Neptune everything seems fine: most planets move on fairly circular orbits and their orbital inclinations vary only slightly. However, beyond Neptune things become very messy. The biggest puzzle is the dwarf planet Sedna, which moves on an inclined, highly eccentric orbit and is so far outside, that it could not have been scattered by the planets there.

Just outside Neptune’s orbit another strange thing happens. The cumulative mass of all the objects dramatically drops by almost three orders of magnitude. This happens at approximately the same distance where everything becomes messy. It might be coincidental, but such conincidences are rare in Nature.

Susanne Pfalzner and her co-workers suggest that a star was approaching the Sun at an early stage, ‘stealing’ most of the outer material from the Sun’s protoplanetary disk and throwing what was left over into inclined and eccentric orbits. Performing thousands of computer simulations they checked what would happen when a star passes very close-by and perturbs the once larger disk. It turned out that the best fit for today’s outer solar systems comes from a perturbing star which had the same mass as the Sun or somewhat lighter (0.5-1 solar masses)  and flew past at approximately 3 times the distance of Neptune.

However, the most surprising thing for the researchers was that a fly-by does not only explain the strange orbits of the objects of the outer solar system, but also gives a natural explanation for several unexplained features of our Solar System, including the mass ratio between Neptune and Uranus, and the existence of two distinct populations of Kuiper Belt objects.

“It is important to keep exploring all the possible avenues for explaining the structure of the outer solar system. The data are increasing but still too sparse, so theories have a lot of wiggle room to develop”, says Pedro Lacerda from the Queen’s University in Belfast, a co-author of the paper. “There is a certain danger that one theory crystallises as truth, not because it explains the data better but because of other pressures. Our paper shows that a lot of what we currently know can be explained by something as simple as a stellar fly-by.”

The big question is the likelihood for such an event. Nowadays, fly-bys even hundreds of times more distant are luckily rare. However, stars like our Sun are typically born in large groups of stars which are much more densely packed.  Therefore, close fly-bys where significantly more common in the distant past. Performing another type of simulations, the team found that there was a 20%-30% chance of experiencing a fly-by over the first billion years of the Sun’s life.

This is no final proof that a stellar fly-by caused the messy features of the outer Solar System, but it can reproduce many observational facts and seems relatively realistic. So far it is the simplest explanation and if simplicity is a sign for validity this model is the best candidate so far.

„In summary, our close fly-by scenario offers a realistic alternative to present models suggested to explain the unexpected features of the outer solar system“, concludes Susanne Pfalzner. „It should be considered as an option for shaping the outer solar system. The strength of the fly-by hypothesis lies in the explanation of several outer solar system features by one single mechanism.“

Simulation of the stellar intruder scenario for a mass of 0.5 solar masses and a perihelion distance of 100 astronomical units or 15 billion kilometers for the perturbing star (three times the distance between Sun and Neptune). a) average positions of the particles after the fly-by, colors showing eccentricity of their orbits increasing from blue to green. b) particle positions before the fly-by with different eccentricity populations (colors) from the top row Grey regions: particles that became unbound due to the fly-by event.   Zoom Image

Simulation of the stellar intruder scenario for a mass of 0.5 solar masses and a perihelion distance of 100 astronomical units or 15 billion kilometers for the perturbing star (three times the distance between Sun and Neptune). … [more]

© S. Pfalzner et al.: The Astrophysical Journal (2018)

IAU Prize for Doctoral Thesis of Gisela Ortiz

The IAU PhD Prize recognises the outstanding scientific achievement in astronomy by PhD students around the world. There are a series of awards, one for each of the IAU’s nine Divisions, with each division selecting a winner in its own field of astronomy.

The IAU Executive Committee awarded the IAU PhD Prize for 2017 in the Division A Fundamental Astronomy to Gisela Ortiz Leon, Instituto de Radioastronomía y Astrofísica, Mexico, for her research in Ultra-high precision astrometry with centimeter and millimeter very long baseline interferometry.

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