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  • richardmitnick 1:13 pm on March 1, 2018 Permalink | Reply
    Tags: , , , , , ESO FORS, , , Rho Ophiuchi A, Spectropolarimetry   

    From ESA via Manu: “Rho Ophiuchi A confirmed as a cosmic lighthouse” 


    Manu Garcia, a friend from IAC.

    The universe around us.
    Astronomy, everything you wanted to know about our local universe and never dared to ask.

    27 February 2018.

    Ignazio Pillitteri
    INAF-Osservatorio Astronomico di Palermo
    Palermo, Italy
    Phone: +39 091 233 420
    pilliastropa.inaf.it

    Lida Oskinova
    Institute of Physics and Astronomy, University of Potsdam
    Potsdam, Germany
    Phone: +49 331-9775910
    lidaastro.physik.uni-potsdam.de

    Norbert Schartel
    XMM-Newton Project Scientist
    European Space Agency
    norbert.Schartelesa.int

    ESAC Communication Office
    comunicacionesac@esa.int

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    European Space Agency

    XMM-Newton detects the first X-ray flares of a massive stellar beacon.

    ESA/XMM Newton

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    Credit : ESA / XMM-Newton; I. Pillitteri (INAF-Astronomical Observatory of Palermo).
    This image of space observatory XMM-Newton of the ESA shows a massive star called Rho Ophiuchi A . The star, visible in the center of the frame, is in the heart of Rho Ophiuchi dark cloud, a nearby region known for actively forming new stars, located about 350 light years away.

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    A rich collection of colourful astronomical objects is revealed in this picturesque image of the Rho Ophiuchi cloud complex from NASA’s Wide-field Infrared Explorer, or WISE.

    NASA/WISE Telescope

    The Rho Ophiuchi cloud (pronounced ‘oh-fee-yoo-ki’ and named after a bright star in the region) is found rising above the plane of the Milky Way in the night sky, bordering the constellations Ophiuchus and Scorpius. It’s one of the nearest star-forming regions to Earth, allowing us to resolve much more detail than in more distant similar regions, like the Orion nebula.

    In 2014, a team of scientists used X – rays with the X-ray observatory XMM-Newton ESA emanating from the massive star Rho Ophiuchi A. Numerous and subsequent telescope observations showed that these periodically fluctuated as intense flames , ranging over a period of about 1.2 days as the star he turned. The team used the ESO Very Large Telescope to discover that the star has a strong magnetic field, confirming its status as cosmic X – ray lighthouse.

    ESO VLT Platform at Cerro Paranal elevation 2,635 m (8,645 ft)

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    The FORS instruments (Focal Reducer and low dispersion Spectrograph 2) located at ESO’s VLT. The FORS2 is in the foreground while the FORS1 is at the bottom. The two seem similar instruments but perform completely different tasks. Credit: ESO.

    This finding was unexpected given what is known about the massive stars and their behavior: it is known that sun – like stars produce flares of X rays strong, but massive stars seem to be very different. In stars more than eight solar masses, the X – ray emission is constant, and has not been observed with certainty that said star SHINES repeatedly in this part of the spectrum before observing Rho Ophiuchi A.

    It is known that sun-like stars produce powerful X-ray flares, but massive stars seem to be very different. In stars from eight solar masses, the X-ray emission is continuous and had not reached any star to issue such flares repeatedly in that part of the spectrum … Until recently observed.

    “We spent almost 40 hours observing the star with XMM-Newton and discovered something even more unexpected, recognizes Ignazio Pillitteri, INAF-Osservatorio Astronomico the di Palermo (Italy) and head of the research team. Instead of a homogeneous and continuous emission periodically pulsed X-rays toward the outside of Rho Ophiuchi A, with a variation of about 1.2 days as rotating star-just like a lighthouse X-ray! This is a new phenomenon in greater than the Sun “stars.

    Rho Ophiuchi A is much hotter and more massive than our parent star. It is not yet known how X – rays are generated in this type of stars; One possibility is a strong intrinsic magnetism, which would be observable by signs of surface magnetism. However, it remains unclear how the magnetic field would originate and how to be associated with X-ray emissions

    “We suspect that there may be a giant active magnetic point on the surface of Rho Ophiuchi A, something like a sunspot, only much larger and more stable, added Pillitteri. As the star rotates, this stain would be hidden or visible repeatedly, causing the observed X-ray pulses. However, this hypothesis is not very likely: stains stars form when a magnetic field inside comes to the surface, and we know that only one in ten massive stars have a measurable magnetic field. ”

    Furthermore, the ‘lighthouse effect’ pulse also may be due to low mass companion orbital, which would add their own and abundant X rays to light attributed to Rho Ophiuchi A . This X – ray emission power would vary due to the passage of this hypothetical smaller star ahead or behind Rho Ophiuchi A during orbit of 1.2 days. The team also considered the possibility that Rho Ophiuchi A could have an inconspicuous, small, low – mass companion in a close orbit.

    “To confirm what was the case, we hasten to obtain measurements of Rho Ophiuchi A using one of the largest ground-based observatories: the ESO VLT says Lida Oskinova, University of Potsdam (Germany) and member of the international team that carried the study. Fortunately, one of our measurements confirmed predictions by showing that X-rays were probably due to magnetic structures on the surface of the star. ”

    The measurements were made in visible light with a technique known as spectropolarimetry, which involves studying various wavelengths of polarized light emitted by a star. The data showed that Rho Ophiuchi A has a strong magnetic field, about 500 times stronger than the Sun ‘s .

    “Such a strong field may easily produce the type of detected flares, Pillitteri points. This confirms that what we found with XMM-Newton were really X-ray emissions from Rho Ophiuchi A massive stars can be magnetically active (as shown by optical observations) and that this activity can be seen in X-rays “.

    The combined data indicate that Rho Ophiuchi A is the only star of this type in which confirmed an active magnetic region on the surface that emits X – ray search for similar behavior in stars like Rho Ophiuchi A will help scientists understand the prevalence of this phenomenon and learn more about the magnetic properties of these stars.

    “This study is important for the exploration of massive stars, as there is much we do not know about these objects emphasizes Norbert Schartel, XMM-Newton scientist ESA project. By combining the extraordinary capabilities of XMM-Newton and the VLT we have managed to fit another piece of the puzzle. ”

    “In addition, it illustrates perfectly the scientific process: find something interesting or unusual, investigate and launched several hypotheses, then keep watching to find out which one is correct. It is a fantastic example of international collaboration between telescopes, both orbiting and ground, coming together to explore and explain the phenomena we see in the cosmos. ”

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    X-ray flares Rho Ophiuchi A. The flickering light of the massive star Rho Ophiuchi A is observed by the XMM-Newton space observatory of the ESA in 2016.

    These and earlier observations XMM-Newton showed that this star periodically throws flares X-ray of its surface as it rotates, a behavior something like a cosmic lighthouse. Follow-up observations made by the research team using the Very Large Telescope of ESO confirmed that this star has a strong magnetic field and the X-ray flares are connected to an active magnetic region on the surface of the star turns in and out of sight.

    This sequence consists of 40 frames obtained between 22 and 23 February 2016 each taken approximately one hour apart. It shows the emission of the star on the X – ray part of the spectrum; The clearer it is blue tone, stronger is the issue, and the white represents the maximum intensity. It can be seen that the intensity of X – ray emission of Rho Ophiuchi A rises sharply at the beginning and end of this sequence; This is because the data cover more than one cycle period X – ray burning star, which lasts 1.2 days.

    These findings are described in three papers published in Astronomy & Astrophysics:
    Smooth X-ray variability from ρ Ophiuchi A + B: A strongly magnetized primary B2 star? By Pillitteri et al. (2014), doi: 10.1051 / 0004-6361 / 201424243;
    The early B-type star Rho Ophiuchi A is an X-ray lighthouse of Pillitteri et al. (2017), doi: 10.1051 / 0004-6361 / 201630070; Y
    Detection of magnetic field in the B2 star ρ Oph A with ESO FORS2 of Pillitteri et al. (2018), doi: 10.1051 / 0004-6361 / 201732078.

    See the full article here .

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  • richardmitnick 3:40 pm on June 9, 2016 Permalink | Reply
    Tags: An ancient tale in astronomical time, , , ESO FORS   

    From ESO: “FORS1 at the VLT UT1: First Spectra Obtained” 1998, but too good to pass up 

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    This post is dedicated to the well traveled ESO ambassador O.S. I hope she sees it.

    European Southern Observatory

    7 October 1998 (retrieved from current post http://www.eso.org/public/images/eso9846a/)
    No writer credit in those ancient days

    First commissioning phase successfully completed

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    The FORS Team at Paranal has concluded the first phase of the extensive FORS1 commissioning tests at the first 8.2-m VLT Unit Telescope (UT1), successfully and according to the plan. Although this work was primarily aimed at testing the technical performance of this new instrument, it has also been possible to obtain some spectacular images already at this early stage. And now, for the first time, spectra [1] have also been observed with the VLT.

    ESO/FORS1
    ESO/FORS1

    After three weeks of intense work, the FORS team reports that FORS1 has now been trimmed to a very high level of performance. A large amount of test data was obtained in all FORS1 observing modes. They include direct images through various optical filtres of star fields, galactic nebulae, galaxies, galaxy clusters, gravitational arcs as well as spectroscopic and also spectro-polarimetric observations of single and multiple objects. Towards the end, the work concentrated on streamlining certain functions of the control software in order to make observations safe and easy to perform, thereby further optimizing the use of the telescope time.

    The purpose of this Commission Phase 1 was first of all to get the instrument on-line and to prove that all observing modes work correctly. This goal was fully achieved and mostly involved observations of comparatively bright objects although, already now, a spectrum of a 25-magnitude galaxy proved to be no problem. Later, during Commission Phase 2 and especially the subsequent FORS Science Verification programme, observations will also be made of extremely faint objects at the limit of what is possible with FORS1.

    The FORS team is now on its way back to Europe, elated but also quite exhausted after one month of continuous hard work, from the initial installation of the instrument to the final checks during this commissioning phase.

    The following pictures are based on test observations done during the commissioning period now terminated. Full details about the exposures are given below as “Technical Information”.

    The Dumbbell Nebula

    The Dumbbell Nebula – also known as Messier 27 or NGC 6853 – is a typical planetary nebula and is located in the constellation Vulpecula (The Fox). The distance is rather uncertain, but is believed to be around 1200 light-years. It was first described by the French astronomer and comet hunter Charles Messier who found it in 1764 and included it as no. 27 in his famous list of extended sky objects [2].

    Despite its class, the Dumbbell Nebula has nothing to do with planets. It consists of very rarified gas that has been ejected from the hot central star (well visible on this photo), now in one of the last evolutionary stages. The gas atoms in the nebula are excited (heated) by the intense ultraviolet radiation from this star and emit strongly at specific wavelengths.

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    eso9846a

    eso9846a is the beautiful by-product of a technical test of some FORS1 narrow-band optical interference filtres. They only allow light in a small wavelength range to pass and are used to isolate emissions from particular atoms and ions.

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    eso9846b

    eso9846b is an enlargment that shows well the intricate structure in the central part of the nebula.
    In this three-colour composite, a short exposure was first made through a wide-band filtre registering blue light from the nebula. It was then combined with exposures through two interference filtres in the light of double-ionized oxygen atoms and atomic hydrogen. They were colour-coded as “blue”, “green” and “red”, respectively, and then combined to produce this picture that shows the structure of the nebula in “approximately true” colours.

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    eso9846c

    eso9846c shows a direct image of the entire sky field (square and outlined by a blue line) with 19 horizontal strips that define the allowed areas for each of the 19 vertical slits. The positions of the slits that were chosen for this exposure are also indicated by vertical double lines.

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    eso9846d

    eso9846d shows the recorded spectra of the stars in this cluster that were selected for this observation. They appear as bright lines spanning the full field in horizontal direction. Spectrum no. 4 from the top (of star “Be41”) is indicated. The shorter, bright vertical lines are spectral emission lines originating in the terrestrial atmosphere (air glow); they show the extent of the individual slits. Note that in some slits, more than one star spectrum has been registered, thus further increasing the observing efficiency.

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    eso9846e
    FORS1 at the VLT UT1: First Spectra

    Multi-object spectra of extragalactic stars

    One of the main features of the FORS instruments is their ability to do multi-object spectroscopy (MOS), i.e., to obtain spectra of several objects at the same time. Many conventional spectrographs in use at telescopes around the world are only capable of observing one spectrum at a time. This necessitates a large amount of precious observing time when spectra of several stars or galaxies shall be observed, e.g., for comparison, or when searching for objets with unusual physical properties.

    FORS1 and FORS2 are designed in such a way that they can register spectra of up to 19 astronomical objects simultaneously. Moreover, they can change from one set of objects to the next within seconds. This greatly increases the observing efficiency and ensures that valuable data can be obtained much faster. That is particularly useful during especially excellent, but relatively rare observing conditions.

    The MOS mode of FORS1 is here illustrated by example of spectra of stars in the open cluster NGC 330 in the Small Magellanic Cloud (SMC)[No illustration present]. The SMC is a companion galaxy of our Milky Way galaxy at a distance of about 150,000 light-years. It is seen deep down in the southern sky and will be a main object of future studies with the VLT.

    Small Magellanic Cloud. NASA/ESA Hubble and ESO/Digitized Sky Survey 2
    Small Magellanic Cloud. NASA/ESA Hubble and ESO/Digitized Sky Survey 2

    Sophisticated software was written by the FORS consortium that allows interactive allocation of target objects in the sky field to the individual slits of the multi-object spectroscopy unit (MOS) of FORS1. The dispersing element that separates the incoming light into different wavelengths (colours) is a grism (a glass prism with a ruled grating replicated onto a thin resin layer on one of the prism surfaces). FORS1 has different grisms that produce spectra with different spectral resolutions. This allows a wide range of projects to be carried out, from quite detailed spectra of brighter objects, to low-resolution spectra of very faint objects, e.g., extremely distant galaxies.

    The FORS1 MOS spectrum was taken for technical reasons, in order to verify the accuracy with which the positions of the individual MOS slits can be set. Therefore, fairly bright stellar objects (down to about 19th magnitude) and a comparatively short exposure time were used. However, already on such “technical” spectra, it will be possible to perform very useful science, as explained below.

    NGC 330 is an extraordinary, young open star cluster. It is famous because it is extremely metal poor, even more than its surroundings in the Small Magellanic Cloud. It furthermore contains unusually many Be stars . In fact, no less than about 70% of its B stars belong to this peculiar variety, compared to about 10% in star clusters of our own Milky Way Galaxy. Be stars are fairly young and hot (~30,000 K) and rotate comparatively fast. Their spectra show broad emission lines of hydrogen from a rotating circumstellar disk. The reason for the overabundance of Be stars in NGC 330 is not known with certainty; the reason may be the very low content of heavy elements.

    Until now, the fainter Be stars in NGC 330 have only been identified by means of photometric observations of their colours. Now, however, the FORS Team was able to obtain the first spectra of some of these stars and confirm the presence of emission lines. eso9846e displays the tracing (brightness vrs. wavelength) of the spectrum of star “Be41” in NGC 330. It is of magnitude 17 and the spectrum isa the fourth from the top in eso9846d. In addition to broad absorption lines of hydrogen and helium (a doppler effect of the rapid rotation), there is a sharp H-beta emission peak from hydrogen near the center of the spectrum, thus confirming it as a Be star . Note that this emission line can also be seen as a bright spot in spectrum no. 4 in eso9846d.
    Notes

    [1] A spectrum is the dispersion of light from an object into the colours of the rainbow. Spectroscopy is a key technique in astronomy: From a spectrum, it is possible to deduce important information about the object emitting the light, e.g. its chemical composition, surface temperature and the direction and speed of its motion (relative to us). This is especially important in the investigation of very distant objects as it allows the determination of their distance due to the expansion of the universe. This will be one of the main domains of the work with FORS.

    [2] More information about this impressive object is available on the web at various locations, e.g., http://messier.seds.org/m/m027.html.

    See the full article here .

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