Monthly Archives: August 2020

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