Stellar News Feed Archive
|The Magnetospheric Boundary in Cataclysmic Variables||Tuesday, December 17, 2013 - 21:56||
The magnetic cataclysmic variables (MCVs) present a wealth of observational diagnostics for studying accretion flows interacting with a magnetosphere. Spin-period pulsations from the rotation of the white dwarf are seen in optical light, in the UV and X-ray bands, and in polarimetry, and modelling these can constrain the size and location of the accretion footprints on the white-dwarf surface. Tracing these back along field lines can tell us about the transition region between the stream or disk and the magnetosphere. Further, optical emission lines give us velocity information, while analysis of eclipses gives spatial information.
Author: Coel Hellier
|J075141 and J174140: Doubling Down With Rare White Dwarf Systems||Tuesday, December 17, 2013 - 15:59||
Despite being known for almost 50 years, the question has remained: where do AM CVn systems come from? New X-ray and optical observations have begun to answer that with the discovery of the first known systems of double stars that astronomers think will evolve into AM CVn systems.
AM CVn systems are of interest to scientists because they are predicted to be sources of gravitational waves. This is important because even though such waves have yet to be detected, many scientists and engineers are working on instruments that should be able to detect them in the near future. This will open a significant new observational window to the universe.
The paper reporting these results is available online and is published in the Monthly Notices of the Royal Astronomical Society Letters. The authors are Mukremin Kilic, from the University of Oklahoma in Norman, OK; J.J. Hermes from the University of Texas at Austin in TX; Alexandros Gianninas from the University of Oklahoma; Warren Brown from Smithsonian Astrophysical Observatory in Cambridge, MA; Craig Heinke from University of Alberta, in Edmonton, Canada; Marcel Agüeros from Columbia University in New York, NY; Paul Chote and Denis Sullivan from Victoria University of Wellington, New Zealand; and Keaton Bell and Samuel Harrold from University of Texas at Austin.
|RS Puppis puts on a spectacular light show||Tuesday, December 17, 2013 - 10:09||
The NASA/ESA Hubble Space Telescope has observed the variable star RS Puppis over a period of five weeks, showing the star growing brighter and dimmer as it pulsates. These pulsations have created a stunning example of a phenomenon known as a light echo, where light appears to reverberate through the murky environment around the star.
RS Puppis is unusual as it is shrouded by a nebula — thick, dark clouds of gas and dust. Hubble observed this star and its murky environment over a period of five weeks in 2010, capturing snapshots at different stages in its cycle and enabling scientists to create a time-lapse video of this ethereal object (heic1323a).
|Fast Radio Bursts Might Come From Nearby Stars||Thursday, December 12, 2013 - 11:22||
First discovered in 2007, "fast radio bursts" continue to defy explanation. These cosmic chirps last for only a thousandth of a second. The characteristics of the radio pulses suggested that they came from galaxies billions of light-years away. However, new work points to a much closer origin - flaring stars within our own galaxy.
"We propose that fast radio bursts aren't as exotic as astronomers first thought," says lead author Avi Loeb of the Harvard-Smithsonian Center for Astrophysics (CfA).
Fast radio bursts are both brief and bright, packing a lot of energy into a short time. Only six have been discovered to date, all of them in archival data. Each was detected only once, making follow-up studies difficult.
|NOAO/SOAR: Where do stars end and brown dwarfs begin?||Monday, December 9, 2013 - 14:15||
Stars come in a tremendous size range, from many tens of times bigger than the Sun to a tiny fraction of its size. But the answer to just how small an astronomical body can be, and still be a star, has never been known. What is known is that objects below this limit are unable to ignite and sustain hydrogen fusion in their cores: these objects are referred to as brown dwarfs.
“We can now point to a temperature (2100K), radius (8.7% that of our Sun), and luminosity (1/8000 of the Sun) and say ‘the main sequence ends there’ and we can identify a particular star (with the designation 2MASS J0513-1403) as a representative of the smallest stars.”
|Observing the Next Galactic Supernova||Saturday, December 7, 2013 - 15:46||
The last time a supernova was observed within the Milky Way was in 1604 by Johannes Kepler, and was only appreciated by the human eye, since optical telescopes and other measurement devices had not yet been invented. Despite a lack of hard observational data, astronomers have a theoretical framework to describe the processes that occur during a supernova, and numerical simulations are always growing more detailed and sophisticated. Still, without observation, neither theory nor numerical result can be put to the test.
While supernovae in our galaxy are relatively rare, extragalactic supernovae are not. That is because there are countless galaxies that have supernova rates similar to that of the Milky Way. But, due to their distance from Earth are not resolvable and offer little insight into the mechanisms at work during the explosion. Although astronomers haven’t observed supernovae in the Milky Way for several hundred years (read on to find out why this may be), the good news here is that astronomers are developing methods to be ready when the next one happens...
|Explosive growth of young star||Wednesday, December 4, 2013 - 09:03||
Authors: Jes K. Jorgensen, Ruud Visser, Nami Sakai, Edwin A. Bergin, Christian Brinch, Daniel Harsono, Johan E. Lindberg, Ewine F. van Dishoeck, Satoshi Yamamoto, Suzanne E. Bisschop, Magnus V. Persson
|The Star of Bethlehem is Not the Nova DO Aquilae (Nor Any Other Nova, Supernova, or Comet)||Monday, December 2, 2013 - 08:56||
The Star of Bethlehem is only known from a few verses in the Gospel of Matthew, with the Star inspiring and leading the Magi (i.e., Persian astrologers) to Jerusalem and ultimately worshipping the young Jesus Christ in Bethlehem. In the last four centuries, astronomers have put forth over a dozen greatly different naturalistic explanations, all involving astronomical events, often a bright nova, supernova, or comet. This paper will evaluate one prominent recent proposal, that the Star was a 'recurrent nova' now catalogued as DO Aquilae, and provide three refutations. In particular, (1) DO Aql is certainly not a recurrent nova, but rather an ordinary nova with a recurrence time scale of over a million years, (2) in its 1925 eruption, DO Aql certainly never got brighter than 8.5 mag, and the physics of the system proves that it could never get to the required luminosity of a supernova, and (3) the Magi were astrologers who had no recognition or interpretation for novae (or supernovae or comets) so any such event is completely irrelevant and meaningless to them.
Author: Bradley Schaefer
|Orbital Decay of X-ray Binaries||Sunday, December 1, 2013 - 11:07||
The authors investigate the orbital decay of two black hole X-ray binaries. Combined with previous data, they were able to measure how fast the orbit is decaying and make predictions about the exact cause of this orbital decay. These systems are some of the best to look for this orbital decay because of their extreme masses compared to other systems. By obtaining spectra to create a radial velocity curve, the authors were able to add an additional O-C point with a larger baseline than before. This larger baseline is important because it allows time for small changes to build up.
|A Fiery Drama of Star Birth and Death||Wednesday, November 27, 2013 - 09:49||
The Large Magellanic Cloud is one of the closest galaxies to our own. Astronomers have now used the power of ESO’s Very Large Telescope to explore one of its lesser known regions. This new image shows clouds of gas and dust where hot new stars are being born and are sculpting their surroundings into odd shapes. But the image also shows the effects of stellar death — filaments created by a supernova explosion.
|Something cataclysmic in the Kepler field||Tuesday, November 26, 2013 - 14:15||
BOKS 45906 was identified as having colors consistent with other CVs in a pre-launch survey of the Kepler spacecraft’s field of view, and was also seen to go through an outburst period, increasing in brightness by a factor of ~25 for five days. Ramsay et al. were looking to study CVs in the Kepler field, so they included it as one of their targets for further monitoring during the Kepler mission. In the end, they were able to use three years of Kepler data, as well as observations from the Isaac Newton Telescope, Swift, and the Hale 200″ Telescope, to learn more.
|Imaging the circumstellar environment of the young T Tauri star SU Aurigae||Wednesday, November 20, 2013 - 08:54||
The circumstellar environments of classical T Tauri stars are challenging to directly image because of their high star-to-disk contrast ratio. One method to overcome this is by using imaging polarimetry where scattered and consequently polarised starlight from the star's circumstellar disk can be separated from the unpolarised light of the central star. We present images of the circumstellar environment of SU Aur, a classical T Tauri star at the transition of T Tauri to Herbig stars. The images directly show that the disk extends out to ~500 au with an inclination angle of $\sim$ 50$^\circ$. Using interpretive models, we derived very small grains in the surface layers of its disk, with a very steep size- and surface-density distribution. Additionally, we resolved a large and extended nebulosity in our images that is most likely a remnant of the prenatal molecular cloud. The position angle of the disk, determined directly from our images, rules out a polar outflow or jet as the cause of this large-scale nebulosity.
Authors: S. V. Jeﬀers, M. Min, H. Canovas, M. Rodenhuis, and C. U. Keller
|BOKS 45906: a CV with an orbital period of 56.6 min in the Kepler field?||Tuesday, November 19, 2013 - 07:55||
BOKS 45906 was found to be a blue source in the Burrell-Optical-Kepler-Survey which showed a 3 mag outburst lasting ~5 d. We present the Kepler light curve of this source which covers nearly 3 years. We find that it is in a faint optical state for approximately half the time and shows a series of outbursts separated by distinct dips in flux. Using data with 1 min sampling, we find clear evidence that in its low state BOKS 45906 shows a flux variability on a period of 56.5574+/-0.0014 min and a semi-amplitude of ~3 percent. Since we can phase all the 1 min cadence data on a common ephemeris using this period, it is probable that 56.56 min is the binary orbital period. Optical spectra of BOKS 45906 show the presence of Balmer lines in emission indicating it is not an AM CVn (pure Helium) binary. Swift data show that it is a weak X-ray source and is weakly detected in the bluest of the UVOT filters. We conclude that BOKS 45906 is a cataclysmic variable with a period shorter than the `period-bounce' systems and therefore BOKS 45906 could be the first helium-rich cataclysmic variable detected in the Kepler field.
Authors: Gavin Ramsay (Armagh Observatory), Steve B. Howell, Matt A. Wood, Alan Smale, Thomas Barclay, Sally A. Seebode, Dawn Gelino, Martin Still, John K. Cannizzo
|Radial velocity variations in the young eruptive star EX Lup||Monday, November 18, 2013 - 22:37||
EX Lup-type objects (EXors) are low-mass pre-main sequence objects characterized by outbursts attributed to highly enhanced disk accretion. The trigger mechanism of EXor outbursts is still debated. One theory requires a close (sub)stellar companion that perturbs the inner disk and triggers the onset of the outburst. Here, we study the radial velocity (RV) variations of EX Lup, the prototype of EXors. We conducted a 5-year RV survey with HARPS and FEROS.
We discuss two possibilities to explain the RV data: a geometry with two accretion columns rotating with the star, and a single accretion flow synchronized with the orbital motion of the hypothetical companion. In the companion scenario, the companion's mass would fall into the brown dwarf desert, which, together with the unusually small separation would make EX Lup a unique binary system, with interesting implications on the physical mechanisms responsible for triggering the outburst.
Authors: Á. Kóspál, M. Mohler-Fischer, A. Sicilia-Aguilar, P. Ábrahám, M. Curé, Th. Henning, Cs. Kiss, R. Launhardt, A. Moór, A. Müller
|A review of pulsating stars from the ASAS data||Monday, November 18, 2013 - 09:28||
The All-Sky Automated Survey (ASAS) appeared to be extremely useful in establishing the census of bright variable stars in the sky. A short review of the characteristics of the ASAS data and discoveries based on these data and related to pulsating stars is presented here by an enthusiastic user of the ASAS data.
Author: Andrzej Pigulski
|Hubble views an old and mysterious cluster||Thursday, November 14, 2013 - 10:44||
The NASA/ESA Hubble Space Telescope has captured the best ever image of the globular cluster Messier 15, a gathering of very old stars that orbits the centre of the Milky Way. Astronomers studying the cluster with Hubble in 2002 found there to be something dark and mysterious lurking at its heart. It could either be a collection of dark neutron stars, or an intermediate-mass black hole. Of the two possibilities it is more likely that Messier 15 harbours a black hole at its centre.
Read the press release
|Photometric evolution of Nova Del 2013 (V339 Del) during the optically thick phase||Wednesday, November 13, 2013 - 08:33||
We present and discuss the BVRI photometric evolution of Nova Del 2013 from the time of discovery, which occurred a few days before maximum brightness, to day +77, when the optically thick phase was over.
Authors: U. Munari, A. Henden, S. Dallaporta, G. Cherini
|Towards a better understanding of the distance scale from RR Lyrae variable stars: A case study for the inner halo globular cluster NGC 6723||Monday, November 11, 2013 - 09:59||
Understanding the formation and evolution of our Galaxy has always been one of the key quests in modern astrophysics for decades (e.g. Freeman & Bland-Hawthorn 2002). Since RRLs are easily identiﬁable and they can provide a powerful means to probe the chemical compositions and dynamical properties of the old stellar populations, RRLs in the Galactic globular cluster (GC) systems or in the ﬁeld are of particular importance to address the question of the early history of our Galaxy (see, for example, Smith 1995). Also, being a primary distance indicator, the distance to RRLs can be accurately measured and RRLs can help to yield important insights into the structure of our Galaxy. Recent studies by Drake et al. (2013) or Pietrukowicz et al. (2012) are excellent examples of making use of RRLs to understand the substructures in the Galactic halo expected from the theory of the hierarchical structure formation, and to delineate the bar structure in the central part of our Galaxy.
Authors: Jae-Woo Lee, Mercedes Lopez-Morales, Kyeong-Soo Hong, Young-Woon Kang, Brian L. Pohl, Alistair Walker
|A Study of the Unusual Z Cam Systems IW Andromedae and V513 Cassiopeia||Thursday, November 7, 2013 - 21:29||
The Z Cam stars IW And and V513 Cas are unusual in having outbursts following their standstills in contrast to the usual Z Cam behavior of quiescence following standstills. In order to gain further understanding of these little-studied systems, we obtained spectra correlated with photometry from the AAVSO throughout a 3-4 month interval in 2011. In addition, time-resolved spectra were obtained in 2012 that provided orbital periods of 3.7 hrs for IW And and 5.2 hrs for V513 Cas. The photometry of V513 Cas revealed a regular pattern of standstills and outbursts with little time at quiescence, while IW And underwent many excursions from quiescence to outburst to short standstills. The spectra of IW And are similar to normal dwarf novae, with strong Balmer emission at quiescence and absorption at outburst. In contrast, V513 Cas shows a much flatter/redder spectrum near outburst with strong HeII emission and prominent emission cores in the Balmer lines. Part of this continuum difference may be due to reddening effects. While our attempts to model the outburst and standstill states of IW And indicate a mass accretion rate near 3×10−9 solar masses per year, we could find no obvious reason why these systems behave differently following standstill compared to normal Z Cam stars.
Authors: Paula Szkody, Meagan Albright, Albert P. Linnell, Mark E. Everett, Russet McMillan, Gabrelle Saurage, Joseph Huehnerhoff, Steve B. Howell, Mike Simonsen, Nick Hunt-Walker
|Multi-Wavelength Photometry of the T Tauri Binary V582 Mon (KH 15D): A New Epoch of Occultations||Thursday, November 7, 2013 - 08:52||
We present multi-wavelength (VRIJHK) observations of KH 15D obtained in 2012/13, as well as a master table of standard photometry spanning the years 1967 to 2013. The system is a close, eccentric T Tauri binary embedded in an inclined precessing circumbinary (CB) ring. The most recent data show the continued rise of star B with respect to the trailing edge of the occulting horizon as the system's maximum brightness steadily increases. The wealth of data in time and wavelength domains allows us to track the long-term CCD color evolution of KH 15D. We find that the V-I behavior is consistent with direct and scattered light from the composite color of two stars with slightly different temperatures. There is no evidence for any reddening or bluing associated with extinction or scattering by ISM-sized dust grains. Furthermore, we probe the system's faint phase behavior at near-infrared wavelengths in order to investigate extinction properties of the ring and signatures of a possible shepherding planet sometimes invoked to confine the CB ring at ~ 5 AU. The wavelength independence of eclipse depth at second contact is consistent with the ring material being fully opaque to 2.2 microns. The color-magnitude diagrams demonstrate excess flux in J and H at low light levels, which may be due to the presence of a hot, young Jupiter-mass planet.
Authors: Diana Windemuth, William Herbst
|Mystery of 'Hot, Young Stars' in Ancient Red Galaxies --Solved!||Thursday, November 7, 2013 - 08:13||
New evidence from NASA's Wide-field Infrared Survey Explorer (WISE) and Galaxy Evolution Explorer (GALEX) missions provide support for the "inside-out" theory of galaxy evolution, which holds that star formation starts at the core of the galaxy and spreads outward. Like tree rings, inner and outer portions of a galaxy's disk are a historical record. Two NASA missions find evidence that star formation bursts started in galaxy centers and spread outward. Unexplainedultraviolet light might come from a late phase in the lives of older stars .
|Symbiotic stars in X-rays||Tuesday, November 5, 2013 - 07:10||
Until recently, symbiotic binary systems in which a white dwarf accretes from a red giant were thought to be mainly a soft X-ray population. Here we describe the detection with the X-ray Telescope (XRT) on the Swift satellite of nine white dwarf symbiotics that were not previously known to be X-ray sources and one that had previously been detected as a supersoft X-ray source. The nine new X-ray detections were the result of a survey of 41 symbiotic stars, and they increase the number of symbiotic stars known to be X-ray sources by approximately 30%. The Swift/XRT telescope detected all of the new X-ray sources at energies greater than 2 keV. Their X-ray spectra are consistent with thermal emission and fall naturally into three distinct groups. The first group contains those sources with a single, highly absorbed hard component that we identify as probably coming from an accretion-disk boundary layer. The second group is composed of those sources with a single, soft X-ray spectral component that probably originates in a region where low-velocity shocks produce X-ray emission, i.e., a colliding-wind region.
Authors: G. J. M. Luna, J. L. Sokoloski, K. Mukai, T. Nelson
|Face to phase with RU Lupi||Monday, November 4, 2013 - 08:42||
We present new results on the classical T Tauri star RU Lupi based on three observing runs collecting high-resolution spectra, complementary NIR spectra, multicolour photometric data, and X-ray observations. The photospheric absorption lines are weakened. This veiling becomes extremely strong on occasion, and we show that this effect is due to narrow emission lines that fill in the photospheric lines. The blue-shifted wings in the optical emission lines of He I, attributed to a stellar wind, are remarkably stable in equivalent width. In contrast, the red-shifted wings change dramatically in strength depending on rotational phase. From the pattern of variability we infer that these wings originate in accreting gas close to the star, and that the accretion funnels are bent and trail the hot spot. Forbidden emission lines are very stable over the entire observing period and originate in the disk wind. A system of narrow blue-shifted absorption features seen in lines of Ca II and Na I can be traced to a disk wind as well. Slightly blue-shifted emission components are present in the forbidden lines and might be related to a wide angle molecular disk wind.
Authors: Gösta F. Gahm, Henricus C. Stempels, Frederick M. Walter, Peter P. Petrov, Gregory J. Herczeg
|Could a Milky Way Supernova Be Visible from Earth in Next 50 Years?||Friday, November 1, 2013 - 08:02||
The good news: they’ve calculated the odds to be nearly 100 percent that such a supernova would be visible to telescopes in the form of infrared radiation.
The bad news: the odds are much lower—dipping to 20 percent or less—that the shining stellar spectacle would be visible to the naked eye in the nighttime sky.
Yet, all this is great news to astronomers, who, unlike the rest of us, have high-powered infrared cameras to point at the sky at a moment’s notice. For them, this study suggests that they have a solid chance of doing something that’s never been done before: detect a supernova fast enough to witness what happens at the very beginning of a star’s demise. A massive star “goes supernova” at the moment when it’s used up all its nuclear fuel and its core collapses, just before it explodes violently and throws off most of its mass into space.
“We see all these stars go supernova in other galaxies, and we don’t fully understand how it happens. We think we know, we say we know, but that’s not actually 100 percent true,” said Christopher Kochanek, professor of astronomy at Ohio State and the Ohio Eminent Scholar in Observational Cosmology. "Today, technologies have advanced to the point that we can learn enormously more about supernovae if we can catch the next one in our galaxy and study it with all our available tools.
|Nova Aquilae 1918 (V603 Aql) Faded by 0.44 mag/century from 1938-2013||Monday, October 28, 2013 - 08:14||
We present the light curve of the old nova V603 Aql (Nova Aql 1918) from 1898-1918 and 1934-2013 using 22,722 archival magnitudes. All of our magnitudes are either in, or accurately transformed into, the Johnson
Authors: Christopher B. Johnson (LSU), Bradley E. Schaefer (LSU), Peter Kroll (Sonneberg Observatory), Arne Henden (AAVSO)