jueves, junio 28, 2007
ESO 25/07 - Chronicle of a Death Foretold
ESO 25/07 - Science Release
31 May 2007
For Immediate Release
Chronicle of a Death Foretold
Two of the World's Largest Interferometric Facilities Team-up to Study a Red Giant Star
Using ESO's VLTI on Cerro Paranal and the VLBA facility operated by NRAO, an international team of astronomers has made what is arguably the most detailed study of the environment of a pulsating red giant star. They performed, for the first time, a series of coordinated observations of three separate layers within the star's tenuous outer envelope: the molecular shell, the dust shell, and the maser shell, leading to significant progress in our understanding of the mechanism of how, before dying, evolved stars lose mass and return it to the interstellar medium.
S Orionis (S Ori) belongs to the class of Mira-type variable stars. It is a solar-mass star that, as will be the fate of our Sun in 5 billion years, is nearing its gloomy end as a white dwarf. Mira stars are very large and lose huge amounts of matter. Every year, S Ori ejects as much as the equivalent of Earth's mass into the cosmos.
" Because we are all stardust, studying the phases in the life of a star when processed matter is sent back to the interstellar medium to be used for the next generation of stars, planets... and humans, is very important, " said Markus Wittkowski, lead author of the paper reporting the results. A star such as the Sun will lose between a third and half of its mass during the Mira phase.
S Ori pulsates with a period of 420 days. In the course of its cycle, it changes its brightness by a factor of the order of 500, while its diameter varies by about 20%.
Although such stars are enormous - they are typically larger than the current Sun by a factor of a few hundred, i.e. they encompass the orbit of the Earth around the Sun - they are also distant and to peer into their deep envelopes requires very high resolution. This can only be achieved with interferometric techniques.
" Astronomers are like medical doctors, who use various instruments to examine different parts of the human body," said co-author David Boboltz. " While the mouth can be checked with a simple light, a stethoscope is required to listen to the heart beat. Similarly the heart of the star can be observed in the optical, the molecular and dust layers can be studied in the infrared and the maser emission can be probed with radio instruments. Only the combination of the three gives us a more complete picture of the star and its envelope. "
The maser emission comes from silicon monoxide (SiO) molecules and can be used to image and track the motion of gas clouds in the stellar envelope roughly 10 times the size of the Sun.
The astronomers observed S Ori with two of the largest interferometric facilities available: the ESO Very Large Telescope Interferometer (VLTI) at Paranal, observing in the near- and mid-infrared, and the NRAO-operated Very Long Baseline Array (VLBA), that takes measurements in the radio wave domain.
Because the star's luminosity changes periodically, the astronomers observed it simultaneously with both instruments, at several different epochs. The first epoch occurred close to the stellar minimum luminosity and the last just after the maximum on the next cycle.
The astronomers found the star's diameter to vary between 7.9 milliarcseconds and 9.7 milliarcseconds. At the distance of S Ori, this corresponds to a change of the radius from about 1.9 to 2.3 times the distance between the Earth and the Sun, or between 400 and 500 solar radii!
As if such sizes were not enough, the inner dust shell is found to be about twice as big. The maser spots, which also form at about twice the radius of the star, show the typical structure of partial to full rings with a clumpy distribution. Their velocities indicate that the gas is expanding radially, moving away at a speed of about 10 km/s.
The multi-wavelength analysis indicates that near the minimum there is more dust production and mass ejection: in these phases indeed the amount of dust is significantly higher than in the others. After this intense matter production and ejection the star continues its pulsation and when it reaches the maximum luminosity, it displays a much more expanded dust shell. This clearly supports a strong connection between the Mira pulsation and the dust production and expulsion.
Furthermore, the astronomers found that grains of aluminum oxide - also called corundum - constitute most of S Ori's dust shell: the grain size is estimated to be of the order of 10 millionths of a centimetre, that is one thousand times smaller than the diameter of a human hair.
" We know one chapter of the secret life of a Mira star, but much more can be learned in the near future, when we add near-infrared interferometry with the AMBER instrument on the VLTI to our (already broad) observational approach, " said Wittkowski.
More Information
The research presented here is reported in a paper in press in the journal Astronomy and Astrophysics ("The Mira variable S Ori: Relationships between the photosphere, molecular layer, dust shell, and SiO maser shell at 4 epochs", by M. Wittkowski et al.). It is available in PDF format from the publisher's web site.
The team consists of Markus Wittkowski (ESO), David A. Boboltz (U.S. Naval Observatory, USA), Keiichi Ohnaka and Thomas Driebe (MPIfR Bonn, Germany), and Michael Scholz (University of Heidelberg, Germany and University of Sydney, Australia).
Notes
A maser is the microwave equivalent to a laser, which emits visible light. A maser emits powerful microwave radiation instead and its study requires radio telescopes. An astrophysical maser is a naturally occurring source of stimulated emission that may arise in molecular clouds, comets, planetary atmospheres, stellar atmospheres, or from various conditions in interstellar space.
ESO operates the Very Large Telescope Interferometer at Paranal Observatory, Chile, with four fixed 8.2-m telescopes and four relocatable 1.8-m telescopes, working at optical/infrared wavelengths. NRAO operates the Very Long Baseline Array with 10 stations across the U.S. working at radio wavelengths between 3 mm and 90 cm (0.3-90 GHz). ESO, NRAO and other partners will operate the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, working at millimetre wavelengths between 0.3 and 10 mm (30-950 GHz).
Contacts
Markus Wittkowski
ESO
Phone: +49 89 3200 6769
Email: mwittkow (at) eso.org
David A. Boboltz
U.S. Naval Observatory, USA
Phone: +1 202 762 1488
Email: dboboltz (at) usno.navy.mil
National contacts for the media:
Belgium - Dr. Rodrigo Alvarez +32-2-474 70 50 rodrigo.alvarez@oma.be
Czech Republic - Pavel Suchan +420 267 103 040 suchan@astro.cz
Finland - Ms. Tiina Raivo +358 9 7748 8369 tiina.raivo@aka.fi
Denmark - Dr. Michael Linden-Vørnle +45-33-18 19 97 mykal@tycho.dk
France - Dr. Daniel Kunth +33-1-44 32 80 85 kunth@iap.fr
Germany - Dr. Jakob Staude +49-6221-528229 staude@mpia.de
Italy - Dr. Leopoldo Benacchio +39-347-230 26 51 benacchio@inaf.it
The Netherlands - Ms. Marieke Baan +31-20-525 74 80 mbaan@science.uva.nl
Portugal - Prof. Teresa Lago +351-22-089 833 mtlago@astro.up.pt
Spain - Dr. Miguel Mas-Hesse +34918131196 mm@laeff.inta.es
Sweden - Dr. Jesper Sollerman +46-8-55 37 85 54 jesper@astro.su.se
Switzerland - Dr. Martin Steinacher +41-31-324 23 82 martin.steinacher@sbf.admin.ch
United Kingdom - Mr. Peter Barratt +44-1793-44 20 25 peter.barratt@stfc.ac.uk
ESO 26/07 - Matter Flashed at Ultra Speed
ESO PR Photo 26a/07
The REM Telescope 
ESO PR Photo 26b/07
Light Curve of a Gamma-ray Burst
ESO 26/07 - Science Release
12 June 2007
For Immediate Release
Matter Flashed at Ultra Speed
Robotic Telescope Measures Speed of Material Ejected in Cosmic Death
Using a robotic telescope at the ESO La Silla Observatory, astronomers have for the first time measured the velocity of the explosions known as gamma-ray bursts. The material is travelling at the extraordinary speed of more than 99.999% of the velocity of light, the maximum speed limit in the Universe.
ESO PR Photo 26a/07
The REM Telescope
" With the development of fast-slewing ground-based telescopes such as the 0.6-m REM telescope at ESO La Silla, we can now study in great detail the very first moments following these cosmic catastrophes," says Emilio Molinari, leader of the team that made the discovery.
Gamma-ray bursts (GRBs) are powerful explosions occurring in distant galaxies, that often signal the death of stars. They are so bright that, for a brief moment, they almost rival the whole Universe in luminosity. They last, however, for only a very short time, from less than a second to a few minutes. Astronomers have long known that, in order to emit such incredible power in so little time, the exploding material must be moving at a speed comparable with that of light, namely 300 000 km per second. By studying the temporal evolution of the burst luminosity, it has now been possible for the first time to precisely measure this velocity.
Gamma-ray bursts, which are unseen by our eyes, are discovered by artificial satellites. The collision of the gamma-ray burst jets into the surrounding gas generates an afterglow visible in the optical and near-infrared that can radiate for several weeks. An array of robotic telescopes were built on the ground, ready to catch this vanishing emission (see e.g. ESO 17/07). On 18 April and 7 June 2006, the NASA/PPARC/ASI Swift satellite detected two bright gamma-ray bursts. In a matter of a few seconds, their position was transmitted to the ground, and the REM telescope began automatically to observe the two GRB fields, detecting the near-infrared afterglows, and monitored the evolution of their luminosity as a function of time (the light curve). The small size of the telescope is compensated by its rapidity of slewing, which allowed astronomers to begin observations very soon after each GRB's detection (39 and 41 seconds after the alert, respectively), and to monitor the very early stages of their light curve.
The two gamma-ray bursts were located 9.3 and 11.5 billion light-years away, respectively.
ESO PR Photo 26b/07
Light Curve of a Gamma-ray Burst
For both events, the afterglow light curve initially rose, then reached a peak, and eventually started to decline, as is typical of GRB afterglows. The peak is, however, only rarely detected. Its determination is very important, since it allows a direct measurement of the expansion velocity of the explosion of the material. For both bursts, the velocity turns out to be very close to the speed of light, precisely 99.9997% of this value. Scientists use a special number, called the Lorentz factor, to express these high velocities. Objects moving much slower than light have a Lorentz factor of about 1, while for the two GRBs it is about 400.
" Matter is thus moving with a speed that is only different from that of light by three parts in a million," says Stefano Covino, co-author of the study. " While single particles in the Universe can be accelerated to still larger velocities - i.e. much larger Lorentz factors - one has to realise that in the present cases, it is the equivalent of about 200 times the mass of the Earth that acquired this incredible speed. "
" You certainly wouldn't like to be in the way," adds team member Susanna Vergani.
The measurement of the Lorentz factor is an important step in understanding gamma-ray burst explosions. This is in fact one of the fundamental parameters of the theory which tries to explain these gigantic explosions, and up to now it was only poorly determined.
" The next question is which kind of 'engine' can accelerate matter to such enormous speeds," says Covino.
More Information
"REM observations of GRB060418 and GRB060607A: the onset of the afterglow and the initial fireball Lorentz factor determination", by E. Molinari, S. D. Vergani, D. Malesani, S. Covino, et al. The paper is available at http://dx.doi.org/10.1051/0004-6361:20077388 (A&A, 469, L13-L16, 2007).
The REM team is formed by G. Chincarini, E. Molinari, F.M. Zerbi, L.A. Antonelli, S. Covino, P. Conconi, L. Nicastro, E. Palazzi, M. Stefanon, V. Testa, G. Tosti, F. Vitali, A. Monfardini, F. D'Alessio, P. D'Avanzo, D. Fugazza, G. Malaspina, S. Piranomonte, S.D. Vergani, P.A. Ward, S. Campana, P. Goldoni, D. Guetta, D. Malesani, N. Masetti, E.J.A. Meurs, L. Norci, E. Pian, A. Fernandez-Soto, L. Stella, G. Tagliaferri, G. Ihle, L. Gonzalez, A. Pizarro, P. Sinclair, and J. Valenzuela.
Notes
Gamma-ray bursts (GRBs) are short flashes of energetic gamma-rays lasting from less than a second to several minutes. They release a tremendous quantity of energy in this short time making them the most powerful events since the Big Bang. They come in two different flavours, long and short ones. Over the past few years, international efforts have convincingly shown that long gamma-ray bursts are linked with the ultimate explosion of massive stars (hypernovae; see e.g. ESO PR 16/03) while the short ones most likely originate from the violent collision of neutron stars and/or black holes (see e.g. ESO PR 26/05 and 32/05). Irrespective of the original source of the GRB energy, the injection of so much energy into a confined volume will cause a fireball to form.
Gamma-ray photons have nearly a million times more energy than the 'visual' photons the eye can see.
Strictly speaking, the Lorentz factor is the ratio between the total and rest-mass energy of the fireball.
REM (Rapid Eye Mount) is a small (60 cm mirror diameter) rapid reaction automatic telescope dedicated to monitor the prompt afterglow of Gamma Ray Burst events. It is located at the ESO La Silla Observatory in Chile. For more information, see http://www.rem.inaf.it and Chincarini et al. ( ESO Messenger, 113, 40, 2003).
Contacts
Emilio Molinari
INAF / Osservatorio Astronomico di Brera
Merate, Italy
Phone: +39 039 9991158
Email: emilio.molinari (at) brera.inaf.it
National contacts for the media:
Belgium - Dr. Rodrigo Alvarez +32-2-474 70 50 rodrigo.alvarez@oma.be
Czech Republic - Pavel Suchan +420 267 103 040 suchan@astro.cz
Finland - Ms. Tiina Raivo +358 9 7748 8369 tiina.raivo@aka.fi
Denmark - Dr. Michael Linden-Vørnle +45-33-18 19 97 mykal@tycho.dk
France - Dr. Daniel Kunth +33-1-44 32 80 85 kunth@iap.fr
Germany - Dr. Jakob Staude +49-6221-528229 staude@mpia.de
Italy - Dr. Leopoldo Benacchio +39-347-230 26 51 benacchio@inaf.it
The Netherlands - Ms. Marieke Baan +31-20-525 74 80 mbaan@science.uva.nl
Portugal - Prof. Teresa Lago +351-22-089 833 mtlago@astro.up.pt
Spain - Dr. Miguel Mas-Hesse +34918131196 mm@laeff.inta.es
Sweden - Dr. Jesper Sollerman +46-8-55 37 85 54 jesper@astro.su.se
Switzerland - Dr. Martin Steinacher +41-31-324 23 82 martin.steinacher@sbf.admin.ch
United Kingdom - Mr. Peter Barratt +44-1793-44 20 25 peter.barratt@stfc.ac.uk
ESO 28/07 - Back on Track
ESO 24/07 - A Brown Dwarf Joins the Jet-Set
ESO 24/07 - Science Release
23 May 2007
For Immediate Release
A Brown Dwarf Joins the Jet-Set
VLT Finds Smallest Galactic Object with Jets
Jets of matter have been discovered around a very low mass 'failed star', mimicking a process seen in young stars. This suggests that these 'brown dwarfs' form in a similar manner to normal stars but also that outflows are driven out by objects as massive as hundreds of millions of solar masses down to Jupiter-sized objects.
The brown dwarf with the name 2MASS1207-3932 is full of surprises [1]. Its companion, a 5 Jupiter-mass giant, was the first confirmed exoplanet for which astronomers could obtain an image (see ESO 23/04 and 12/05), thereby opening a new field of research - the direct detection of alien worlds. It was then later found (see ESO 19/06) that the brown dwarf has a disc surrounding it, not unlike very young stars.
Now, astronomers using ESO's Very Large Telescope (VLT) have found that the young brown dwarf is also spewing jets, a behaviour again quite similar to young stars.
The mass of the brown dwarf is only 24 Jupiter-masses. Hence, it is by far the smallest object known to drive an outflow. "This leads us to the tantalizing prospect that young giant planets could also be associated with outflows," says Emma Whelan, the lead-author of the paper reporting the results.
The outflows were discovered using an amazing technique known as spectro-astrometry, based on high resolution spectra taken with UVES on the VLT. Such a technique was required due to the difficulty of the task. While in normal young stars - known as T-Tauri stars for the prototype of their class - the jets are large and bright enough to be seen directly, this is not the case around brown dwarfs: the length scale of the jets, recovered with spectro-astrometry is only about 0.1 arcsecond long, that is, the size of a two Euro coin seen from 40 km away.
The jets stretch about 1 billion kilometres and the material is rushing away from the brown dwarf with a speed of a few kilometres per second.
The astronomers had to rely on the power of the VLT because the observed emission is extremely faint and only UVES on the VLT could provide both the sensitivity and the spectral resolution they required.
"Discoveries like these are purely reliant on excellent telescopes and instruments, such as the VLT," says Whelan. "Our result also highlights the incredible level of quality which is available today to astronomers: the first telescopes built by Galileo were used to observe the moons of Jupiter. Today, the largest ground-based telescopes can be used to observe a Jupiter size object at a distance of 200 light-years and find it has outflows!"
Using the same technique and the same telescope, the team had previously discovered outflows in another young brown dwarf. The new discovery sets a record for the lowest mass object in which jets are seen [2].
Outflows are ubiquitous in the Universe, as they are observed rushing away from the active nuclei of galaxies - AGNs - but also emerging from young stars. The present observations show they even arise in still lower mass objects. The outflow mechanism is thus very robust over an enormous range of masses, from several tens of millions of solar mass (for AGNs) down to a few tens of Jupiter masses (for brown dwarfs).
More Information
These results were reported in a Letter to the Editor in the Astrophysical Journal (vol. 659, p. L45): "Discovery of a Bipolar Outflow from 2MASSW J1207334-393254 a 24 MJup Brown Dwarf", by E.T. Whelan et al.
The team is composed of Emma Whelan and Tom Ray (Dublin Institute for Advanced Studies, Ireland), Ray Jayawardhana (University of Toronto, Canada), Francesca Bacciotti, Antonella Natta and Sofia Randich (Osservatorio Astrofisico di Arcetri, Italy), Leonardo Testi (ESO), and Subu Mohanty (Harvard-Smithsonian CfA, USA).
Notes
[1]: Brown dwarfs are objects whose masses are below those of normal stars - the borderline is believed to be about 8% of the mass of our Sun - but larger than those of planets. Unlike normal stars, brown dwarfs are unable to sustain stable nuclear fusion of hydrogen.
[2]: The brown dwarf 2MASS1207-3932 belongs to the TW Hydrae Association and is therefore about 8 million years old. Albeit this is relatively young, this also implies that this brown dwarf is one of the oldest Galactic objects with a resolved jet, highlighting the fact that outflows can persist for a relatively long time.
Technical information: Spectro-astrometry is simply Gaussian fitting of the spatial profile of the continuum and emission line regions of a spectrum in order to very accurately measure positions. In this way spatial information is recovered beyond the limitations of the seeing of an observation. For example spectro-astrometry has been primarily used to investigate binarity in sources where the binary separation is far less than the seeing and to confirm outflow activity where the line emission tracing the outflow originates at a distance again much smaller than the seeing and therefore appears confined to the source. The first step is to measure the continuum centroid i.e. the source position. The spatial profile of the continuum is extracted at many positions along the dispersion axis. Each extracted profile is fitted with a Gaussian to measure the centroid position of the continuum emission and the result is a position spectrum of the continuum. This map of the continuum position is easily corrected for curvature or tilting in the spectrum. Next the continuum is removed and the position of a pure emission line region is measured (again with Gaussian fitting) with respect to the continuum position. The presence of the continuum will tend to drag the position of an emission line region back towards the source so it must be removed. The accuracy with which one can measure positions with spectro-astrometry is strongly dependent on the signal to noise of the observation and is given by sigma=seeing/[ 2.3548(sqrt{Np})] where Np is the number of detected photons. For example, for a seeing of 1 arcsecond and a value of Np of 10,000, positions can be recovered to an accuracy of less than 5 milliarcseconds. The forbidden emission lines found in the spectra of some young brown dwarfs were a strong indication of outflow activity. However the regions were not extended and therefore that they originated in an outflow could not be directly confirmed. Using spectro-astrometry the astronomers were able to show that the line regions were shifted by small amounts with respect to the brown dwarf continuum (shifts were small relative to the seeing) and therefore were indeed tracing an outflow. Please see http://www.nature.com/nature/journal/v435/n7042/suppinfo/nature03598.html for a more in depth discussion of the spectro-astrometric technique.
Contact
Emma Whelan
School of Cosmic Physics
Dublin Institute for Advanced Studies, Ireland
Phone: +353-1-6621333
Email: ewhelan (at) cp.dias.ie
National contacts for the media:
Belgium - Dr. Rodrigo Alvarez +32-2-474 70 50 rodrigo.alvarez@oma.be
Czech Republic - Pavel Suchan +420 267 103 040 suchan@astro.cz
Finland - Ms. Tiina Raivo +358 9 7748 8369 tiina.raivo@aka.fi
Denmark - Dr. Michael Linden-Vørnle +45-33-18 19 97 mykal@tycho.dk
France - Dr. Daniel Kunth +33-1-44 32 80 85 kunth@iap.fr
Germany - Dr. Jakob Staude +49-6221-528229 staude@mpia.de
Italy - Dr. Leopoldo Benacchio +39-347-230 26 51 benacchio@inaf.it
The Netherlands - Ms. Marieke Baan +31-20-525 74 80 mbaan@science.uva.nl
Portugal - Prof. Teresa Lago +351-22-089 833 mtlago@astro.up.pt
Spain - Dr. Miguel Mas-Hesse +34918131196 mm@laeff.inta.es
Sweden - Dr. Jesper Sollerman +46-8-55 37 85 54 jesper@astro.su.se
Switzerland - Dr. Martin Steinacher +41-31-324 23 82 martin.steinacher@sbf.admin.ch
United Kingdom - Mr. Peter Barratt +44-1793-44 20 25 peter.barratt@stfc.ac.uk
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