# News

## +++   Data Release 12 issued by the Sloan Digital Sky Survey +++

"Cosmic clock" - artist impression.

## Nature article: How stars reveal their ages

5 January 2015. A recently published study in the scientific journal Nature presents a method by which the age of stars can be determined very precisely: "Gyrochronology", an analytical procedure f...

By observing and surveying 30 cool solar-type stars in the 2.5 Billion-year-old cluster NGC 6819 the international research team led by Søren Meibom of the Harvard-Smithsonian Center for Astrophysics, this method has now been shown to work over a wide age range, significantly improving the accuracy of stellar age determination.

"The relationship between mass, rotation rate and age of the observed stars is now defined well enough that by measuring the first two parameters, the third, the star's age, can be determined with only 10 percent uncertainty," said Barnes. The speed of rotation of a star decreases over time, and also depends on the mass of the star; heavy stars rotate faster than smaller, lighter ones, as a rule. While some aspects of this basic behavior have been known to astronomers for a while, this work has seized an opportunity to test and clarify the precision and accuracy of the method.

The newly released study firmly provides the intimate relationship between mass,rotation rate, and age of the cool star using coeval (same age) cluster stars so that the ages of even non-cluster (so-called field'') stars can be determined. The determination of the rotation rates was carried out by observing changes in brightness caused by star spots on the surface of the observed stars rotating into and out of view. At this age, a typical star changes its brightness by much less than 1 percent. Such precise observations were only possible using NASA's Kepler Space telescope.

This most accurate determination of the age of stars is important in understanding how various astronomical phenomena evolve over time. For example, knowledge of stellar ages can be helpful in targeting the search for life outside our solar system, because it has taken billions of years for the development of the complexity of life on Earth. Planets orbiting stars with ages similar to the sun are therefore seen as particularly promising objects of study.

Caption: This artist's impression of a "cosmic clock" illustrates how astronomers have used stellar rotation to measure the ages of stars in a 2.5-billion-year-old star cluster. Their results, the latest success of gyrochronology, mark the first extension of such observations to stars with ages beyond 1 billion years, and toward the 4.6-billion-year age of the Sun. Being able to tell the ages of stars is the basis for understanding how astronomical phenomena involving stars and their companions unfold over time. (Credit: Michael Bachofner)

Scientific contacts:

Dr. Sydney Barnes, sbarnes@aip.de, +49 157-3076 1230

Dr. Søren Meibom, smeibom@cfa.harvard.edu, +1 617496-4773

Media contact: Kerstin Mork, presse@aip.de, +49 331-7499 469

The key topics of the Leibniz Institute for Astrophysics Potsdam (AIP) are cosmic magnetic fields and extragalactic astrophysics. A considerable part of the institute's efforts aim at the development of research technology in the fields of spectroscopy, robotic telescopes, and e-science. The AIP is the successor of the Berlin Observatory founded in 1700 and of the Astrophysical Observatory of Potsdam founded in 1874. The latter was the world's first observatory to emphasize explicitly the research area of astrophysics. Since 1992 the AIP is a member of the Leibniz Association.

Selected spectrum.

## A universal comb

10 December 2014. Scientists from the Leibniz Institute for Astrophysics in Potsdam (AIP) and the Centre for innovation competence innoFSPEC have tested a novel optical frequency comb using an astr...

"The special quality of the light generated by the optical frequency comb is that it consists of individual, discrete colours, with a precise frequency spacing," explains the responsible innoFSPEC scientist Jose Boggio. „The optical comb is created by the superposition of laser light with two different frequencies.“ The resulting comb spectrum is not continuous, as in a rainbow, but consists of different coloured lines with fixed spacing and dark gaps between - hence the name frequency comb.

To analyse the light from stars and galaxies, all spectrographs must be calibrated using a known light source. "The frequency comb serves as optical ruler that is more stable and regular, than the light from conventional spectral calibration lamps", explains astrophysicist Andreas Kelz. "Thanks to these methods, we will be able to determine the rotational speeds of galaxies or the chemical composition of stars more precisely."

After development in the laboratories of innoFSPEC Potsdam, the frequency comb has undergone a first practical test on sky. During a recent observing campaign at the Calar Alto Observatory in southern Spain, the AIP-built PMAS spectrograph was equipped with the frequency comb. After the successful outcome of these tests, Roger Haynes, head of the innoFSPEC research group, is sure that laser frequency combs will set new standards in  astronomical precision spectroscopy and laboratory analysis.

Back in 2005, Professor Hänsch from the Max-Planck-Institute for quantum optics, received the Nobel Prize for the development of an optical frequency comb. However, the device developed in Potsdam is based on a different principle of operation and produces comb-lines with a much larger pitch. This makes it applicable for typical astronomical night-time spectrograph operating at low and medium resolution.

Caption: Selected spectrum of the optical frequency comb (upper, blue panel) as compared to a Neon spectral lamp emission (lower red panel). The comb is a better calibrator („optical ruler“), because it features more and equally spaced emission lines than the Neon lamp.

Science contacts:

Dr. Jose Chavez-Boggio, jboggio@aip.de, +49 331-7499 665 / Dr. Andreas Kelz, akelz@aip.de, +49 331-7499 640

Media contact: Kerstin Mork, presse@aip.de, +49 331-7499 469

The key topics of the Leibniz Institute for Astrophysics Potsdam (AIP) are cosmic magnetic fields and extragalactic astrophysics. A considerable part of the institute's efforts aim at the development of research technology in the fields of spectroscopy, robotic telescopes, and e-science. The AIP is the successor of the Berlin Observatory founded in 1700 and of the Astrophysical Observatory of Potsdam founded in 1874. The latter was the world's first observatory to emphasize explicitly the research area of astrophysics. Since 1992 the AIP is a member of the Leibniz Association.

The horizontal Reynolds stress in theory (blue line) and observation (the insert).

## The Lambda effect confirmed

2 December 2014. Solar turbulence theory confirmed by solar observations: the Lambda effect exists. AIP theoreticians have long believed that turbulence on the Sun behaves opposite to what is know...

New computations of the horizontal Reynolds stress of the plasma in the (rotating) solar convection zone show it to be negative on the surface but positive in the bulk of the convection zone of the northern hemisphere (and opposite in the southern hemisphere). This is exactly what was discovered last year by the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory (SDO).

The observed acceleration on the solar surface equator, commonly known as differential rotation, is generally believed to be produced by rotating turbulent convection, albeit the details of the physics behind it are very complex and yet to be fully understood. One of the problems was that such turbulences on the Sun appear to behave opposite to the turbulences in classical experimental physics. After Boussinesq (1897), all commonly known turbulence fields transport angular momentum towards the location of the slowest rotation rate. The Sun seems to violate this law and does it exactly in the opposite way. If indeed the solar turbulence is of non-Boussinesq type then all solar-type stars should possess accelerated equators with - as the AIP model calculations show -  very similar lap-times, i.e. the time the equator finishes one rotation more than the poles. This was also recently shown to be the case by other space missions.

The formal reason for the non-Boussinesq character of solar (and likely also stellar) turbulence is the existence of the Lambda effect which dominates the angular momentum transport if the turbulence cells are huge and long-living (of the order of the solar rotation period). Numerical experiments showed that such huge cells transport angular momentum even for rigid rotation. Just recently, the Helioseismic and Magnetic Imager on board of the Solar Dynamic Observatory detected these giant solar turbulence cells with characteristic length scales of 100,000 km. And they indeed transport the angular momentum from the slow solar poles to the fast solar equator, i.e. opposite to the Boussinesq law. This finding represents the yet best evidence for the existence of the Lambda effect, which describes the angular momentum transport of rigidly rotating anisotropic turbulence, and thus improves our overall understanding of the basic phenomenon of plasma turbulence.

Publication:  G. Rüdiger, M. Küker, & I. Tereshin 2014, “The existence of the Lambda effect in the solar convection zone as indicated by SDO/HMI data”, Astronomy & Astrophysics Letters, Volume 572, December 2014.

Science Contact: Prof. Dr. G. Rüdiger, gruediger@aip.de.

The key topics of the Leibniz Institute for Astrophysics Potsdam (AIP) are cosmic magnetic fields and extragalactic astrophysics. A considerable part of the institute's efforts aim at the development of research technology in the fields of spectroscopy, robotic telescopes, and e-science. The AIP is the successor of the Berlin Observatory founded in 1700 and of the Astrophysical Observatory of Potsdam founded in 1874. The latter was the world's first observatory to emphasize explicitly the research area of astrophysics. Since 1992 the AIP is a member of the Leibniz Association.

Artist's impression of Epsilon Aurigae. Image credit: NASA/JPL-Caltech.

## Epsilon Aurigae - an extremely dynamic binary star

12 November 2014. Based on an observation campaign lasting seven years, scientists from the Leibniz Institute for Astrophysics Potsdam (AIP) published new findings about the binary star system Epsi...

Epsilon Aurigae is a bright supergiant with a diameter 300 times greater than that of the Sun; its mass is 25 times greater. Its mysterious companion star is concealed in a disk, and cannot be observed directly. Thanks to their observations, Potsdam’s astronomers were able to show that the main star pulsates non-radially, rotates very quickly and loses mass to its invisible companion, whose accretion disk also rotates.

The binary star system must be extremely dynamic for the mass to be able to flow, as proven, from the supergiant towards the disk of the invisible companion star. The astronomers determined that the giant star rotates comparatively quickly, with a period of only 540 days. In interaction with its non-radial pulsation, which has also been observed, this could be the cause of intensified mass transfer between the two stars.

“I would not like to get too close to Epsilon Aurigae with my spacecraft,” stated Professor Klaus Strassmeier, leader of the study and Research Branch Director at AIP. “What we see here is a system whose two very massive stars are simultaneously involved in all of the turbulent scenarios of stellar evolution.”

Determining the companion star in further detail remains an exciting task. In fact, the data also shows that the disk of the companion star is not extended in a circular, but in a “pear-shaped” fashion in the opposite direction to the orbital motion. Consequently, it is not possible to determine the mass of the star directly, as is the case for circular disks derived from Kepler’s laws.

Potsdam’s astronomers have been interested for a long time in Epsilon Aurigae, some 3,000 light years away in the northern sky. As early as in 1903, Hans Ludendorff and Hermann Vogel carried out the first photometric and spectroscopic observations of the star at Potsdam, and discovered that it was an eclipsing binary star with a 27-year period – the longest eclipsing period that has ever been measured.

The huge quantities of data obtained using the STELLA telescope are made available to the astronomical community for further analysis.

The study "Time-series high-resolution spectroscopy and photometry of ε Aurigae from 2006–2013: Another brick in the wall" was published in November in Astronomical Notes, Vol. 335, issue 9.

Science contact: Professor Dr. Klaus G. Strassmeier, kstrassmeier@aip.de

Media contact: Kerstin Mork, presse@aip.de, 0331-7499 469

The key topics of the Leibniz Institute for Astrophysics Potsdam (AIP) are cosmic magnetic fields and extragalactic astrophysics. A considerable part of the institute's efforts aim at the development of research technology in the fields of spectroscopy, robotic telescopes, and e-science. The AIP is the successor of the Berlin Observatory founded in 1700 and of the Astrophysical Observatory of Potsdam founded in 1874. The latter was the world's first observatory to emphasize explicitly the research area of astrophysics. Since 1992 the AIP is a member of the Leibniz Association.

SDI image of a partial solar eclipse on 23 October.

## Partial solar eclipse observed by SDI

23 October 2014. The images show a large Sunspot that appeared two days earlier. The SDI telescope uses the Sun as a guide star to keep its image to be well-projected onto the entrance fibres to th...

PEPSI is a high-resolution echelle spectrograph recently installed on the 2x8.4m Large Binocular Telescope (LBT) on Mt.Graham in Arizona and designed to obtain spectra in integral light from the Nasmyth focal station or polarized spectra with polarimeters located at the Gregorian focus of the telescope.

As an additional functionality, PEPSI uses a small 1cm binocular Solar telescope located outside the LBT in order to feed the spectrograph with the solar disk integrated (SDI) light with a resolving power of 270 000 over the whole optical spectral range. The purpose of this instrument is to take high signal-to-noise Solar spectra continuously on every day basis over the whole solar cycle to study the pressure and gravity modes of solar pulsations in high resolution, as well as, long term line profile variation over solar cycle.

The key topics of the Leibniz Institute for Astrophysics Potsdam (AIP) are cosmic magnetic fields and extragalactic astrophysics. A considerable part of the institute's efforts aim at the development of research technology in the fields of spectroscopy, robotic telescopes, and e-science. The AIP is the successor of the Berlin Observatory founded in 1700 and of the Astrophysical Observatory of Potsdam founded in 1874. The latter was the world's first observatory to emphasize explicitly the research area of astrophysics. Since 1992 the AIP is a member of the Leibniz Association.