Skip to main content

Stellar News Feed Archive

Doppler images of the eclipsing binary ER Vulpeculae Sunday, November 23, 2014 - 09:07

We present Doppler images of both components of the eclipsing binary system ER Vul, based on the spectra obtained in 2004 November, 2006 September and 2008 November. The least-squares deconvolution technique is used for enhancing the signal-to-noise ratios of the observed profiles. The new surface images reveal that both stars of ER Vul show strong starspot activities and the starspots appear at various latitudes. The surface maps of 2006 and 2008 both show the presence of large high-latitude starspots on each component of ER Vul. We find no obvious phase shift of the active regions during our observations. The longitude distributions of starspots are non-uniform on both stars. At low-to-mid latitudes, the active regions are almost exclusively found in the hemisphere facing the other star. However, we find no pronounced concentration of spots at the sub-stellar points.


Authors: Yue Xiang, Shenghong Gu, A. Collier Cameron, J. R. Barnes


Read the paper on astro-ph


Real-Time Stellar Evolution Saturday, November 22, 2014 - 09:38

To get an idea of how stars live and die, we can’t just pick one and watch its life unfold in real time. Most stars live for billions of years! So instead, we do a population census of sorts. Much like you can study how humans age by taking a “snapshot” of individuals ranging from newborn to elderly, so too can we study the lives of stars.

But like all good things in life (and stars), there are exceptions. Sometimes, stellar evolution happens on more human timescales—tens to hundreds of years rather than millions or billions. One such exception is the topic of today’s paper: planetary nebulae, and the rapidly dying stellar corpses responsible for all that glowing gas.

All stars similar to our Sun, or up to about eight times as massive, will end their lives embedded in planetary nebulae like these. The name is a holdover from their discovery and general appearance—we have long known that planetary nebulae have nothing to do with planets. Instead, they are the former outer layers of a star: an envelope of material hastily ejected when gravity can no longer hold a star together. In its final death throes, what’s left of the star rapidly heats up and begins to ionize gas in the nebula surrounding it.

Read the full summary on Astrobites


NASA's Swift Mission Probes an Exotic Object: ‘Kicked’ Black Hole or Mega Star? Wednesday, November 19, 2014 - 15:21

An international team of researchers analyzing decades of observations from many facilities, including NASA's Swift satellite, has discovered an unusual source of light in a galaxy some 90 million light-years away.

The object's curious properties make it a good match for a supermassive black hole ejected from its home galaxy after merging with another giant black hole. But astronomers can't yet rule out an alternative possibility. The source, called SDSS1133, may be the remnant of a massive star that erupted for a record period of time before destroying itself in a supernova explosion.

"With the data we have in hand, we can't yet distinguish between these two scenarios," said lead researcher Michael Koss, an astronomer at ETH Zurich, the Swiss Federal Institute of Technology. "One exciting discovery made with NASA's Swift is that the brightness of SDSS1133 has changed little in optical or ultraviolet light for a decade, which is not something typically seen in a young supernova remnant." 

Read the full NASA Swift press release


GSC 4560--02157: a New Long-period Eclipsing Cataclysmic Variable Star Tuesday, November 18, 2014 - 10:49

Abstract: We study the newly discovered variable star GSC 4560--02157. CCD photometry was performed in 2013--2014, and a spectrum was obtained with the 6-m telescope in June, 2014. GSC 4560--02157 is demonstrated to be a short-period (P=0.265359d) eclipsing variable star. All its flat-bottom primary minima are approximately at the same brightness level, while the star's out-of-eclipse brightness and brightness at secondary minimum varies considerably (by up to 0.6m) from cycle to cycle. Besides, there are short-term (time scale of 0.03-0.04 days) small-amplitude brightness variations out of eclipse. This behavior suggests cataclysmic nature of the star, confirmed with a spectrum taken on June 5, 2014. The spectrum shows numerous emissions of the hydrogen Balmer series, HeI, HeII.

Authors: A.V. Khruslov, A.V. Kusakin, E.A. Barsukova, V.P. Goranskij, A.F. Valeev, N.N. Samus

Read the paper on astro-ph


ASASSN-13co: A Type-Defying Supernova Tuesday, November 18, 2014 - 08:46

There are arguably a lot of things defy categorization, but it’s not everyday that we find something that suggests we do away with our categories altogether. The authors of today’s paper believe that the recently-discovered Type II supernova ASASSN-13co — read that as “assassin”, please — might just be one of the latter. Its unusual characteristics call into question the validity of the two classes (II-P and II-L, more on that later) into which we usually group Type II supernovae. As a result, they suggest that we treat Type II supernovae properties as a continuum, rather than the discrete designations we’ve become accustomed to assigning.

Death Throes of Massive Stars

Type II supernovae are identified by the hydrogen in their spectra (meaning that they still have a hydrogen envelope when they die). They are formed when a star with mass of 8-50 times that of the sun dies through core-collapse.

All stars produce energy through nuclear fusion, but massive stars can fuse much heavier nuclei than stars the size of our sun – all the way to nickel and iron, which have the highest binding energy of all elements. While the fusion of the lighter elements is an exothermic process, fusing iron uses up energy instead, so fusing elements heavier than iron isn’t energetically favorable. As a result, a core of iron and nickel (which then decays into iron) builds up in the center of a massive star. The core is supported by electron degeneracy pressure. When the mass of the iron-nickel core exceeds the Chandrasekhar limit (about 1.4 solar masses), however, electron degeneracy pressure is not enough to stop the core from collapsing. As the core collapses, the protons and electrons in the core of the star merge to form neutrons and neutrinos. The neutrinos can escape and carry away energy. At the same time, the outer layers of the star fall inward until neutrondegeneracy pressure kicks in, stopping the collapse and causing the outer layers to rebound.  The combination of the pressure from the neutrinos and the rebound of the outer layers off of the core causes the star to be torn apart in a huge explosion – a core-collapse supernovae.

Read the article on Astrobites


Astrochemical Dating of a Stellar Nursery Monday, November 17, 2014 - 21:33

An international research team led by scientists from the Coordinated Research Center (CRC) 956 “Conditions and Impact of Star Formation” at the University of Cologne has used observations made with the GREAT instrument on board the SOFIA aircraft observatory and the APEX telescope to date the core of an interstellar cloud that is forming a group of Sun-like stars.

The age of this star-forming cloud, which is located in the Ophiuchus constellation at a distance of around 400 light years, was determined by comparing the data from the telescopes with extensive computer simulations of the chemistry that is changing with time. “The simulations allow us a detailed look at the movement of our H2D+ clock,” explains Jorma Harju of the University of Helsinki. “We find that our new chemical clock is more precise than any of those used previously. Even more importantly, it keeps running when other clocks have already stopped working.” The team is confident that their new method will help to date other stellar birthplaces.

Read the press release from Uni­ver­sity of Cologne


The Party's Over for These Youthful Compact Galaxies Friday, November 14, 2014 - 10:33

Researchers using NASA's Hubble Space Telescope and Chandra X-ray Observatory have uncovered young, massive, compact galaxies whose raucous star-making parties are ending early. The firestorm of star birth has blasted out most of the remaining gaseous fuel needed to make future generations of stars. Now the party's over for these gas-starved galaxies, and they are on track to possibly becoming so-called "red and dead galaxies," composed only of aging stars.

Astronomers have debated for decades how massive galaxies rapidly evolve from active star-forming machines to star-starved graveyards. Previous observations of these galaxies reveal geysers of gas shooting into space at up to 2 million miles an hour. Astronomers have suspected that powerful monster black holes lurking at the centers of the galaxies triggered the gaseous outflows and shut down star birth by blowing out any remaining fuel.

Now an analysis of 12 merging galaxies at the end of their star-birthing frenzy is showing that the stars themselves are turning out the lights on their own star-making party. This happened when the universe was half its current age of 13.7 billion years.

Read the full HubbleSite press release


Primordial galaxy bursts with starry births Wednesday, November 12, 2014 - 16:00

An international team of researchers gazed back over 12.5 billion years to find bustling galaxies creating stars at a breakneck rate. Today, Earth’s Milky Way galaxy produces the equivalent of perhaps two to three new suns a year. The AzTEC-3 galaxy, observed to be emerging from the Big Bang’s primordial soup, creates about 1,100 suns a year, corresponding to about three suns each day.

Lead author Dominik Riechers says that galaxies with this quick rate of star production have been known to exist in the middle-aged universe, say 3 billion to 6 billion years old, but this production is surprising for galaxies in their cosmic infancy. “We expect this out of later galaxies in a more mature universe, but not from one of the earliest,” he said.

Read the press release from Cornell here

Read the paper on astro-ph


Intriguing triple-mode RR Lyrae star with period doubling Wednesday, November 12, 2014 - 09:34

We report the discovery of an intriguing triple-mode RR Lyrae star found in the OGLE Galactic bulge collection, OGLE-BLG-RRLYR-24137. In the OGLE catalog the star was identified as RRd star - double-mode pulsator, pulsating simultaneously in the fundamental and in the first overtone modes. We find that third mode is excited and firmly detect its period doubling. Period ratios are not far from that expected for triple-mode - fundamental, first and third overtone - pulsation. Unfortunately, we cannot reproduce period ratios of the three modes with a consistent set of pulsation models. Therefore the other interpretation, that additional mode is non-radial, is also likely.

Authors: R. Smolec, I. Soszyński, A. Udalski, M.K. Szymański, P. Pietrukowicz, J. Skowron, S. Kozłowski, R. Poleski, P. Moskalik, D. Skowron, G. Pietrzyński, Ł. Wyrzykowski, K. Ulaczyk, P. Mróz

Read the paper on astro-ph


Birth of Planets Revealed in Astonishing Detail in ALMA’s ‘Best Image Ever’ Thursday, November 6, 2014 - 18:24

Astronomers have captured the best image ever of planet formation around an infant star as part of the testing and verification process for the Atacama Large Millimeter/submillimeter Array’s (ALMA) new high-resolution capabilities. 

This revolutionary new image reveals in astonishing detail the planet-forming disk surrounding HL Tau, a Sun-like star located approximately 450 light-years from Earth in the constellation Taurus.  

ALMA uncovered never-before-seen features in this system, including multiple concentric rings separated by clearly defined gaps. These structures suggest that planet formation is already well underway around this remarkably young star.

"These features are almost certainly the result of young planet-like bodies that are being formed in the disk. This is surprising since HL Tau is no more than a million years old and such young stars are not expected to have large planetary bodies capable of producing the structures we see in this image," said ALMA Deputy Director Stuartt Corder. 

Read the full press release

Jets, bubbles, and bursts of light in Taurus Thursday, November 6, 2014 - 18:21


The NASA/ESA Hubble Space Telescope has snapped a striking view of a multiple star system called XZ Tauri, its neighbour HL Tauri, and several nearby young stellar objects. XZ Tauri is blowing a hot bubble of gas into the surrounding space, which is filled with bright and beautiful clumps that are emitting strong winds and jets. These objects illuminate the region, creating a truly dramatic scene.

This dark and ominous landscape is located some 450 light-years away in the constellation of Taurus (The Bull). It lies in the north-eastern part of a large, dark cloud known as LDN 1551.

Just to the left of centre in this image, embedded within a rust-coloured cloud, lies XZ Tauri. While it appears to be a single star, this bright spot actually consists of several stars. It has long been known to be a binary, but one of these two stars is thought also to be a binary, making a total of three stars within a single system.

Read the full press release


F stars: A challenge to stellar evolution Monday, November 3, 2014 - 10:50

Many main-sequence F and early G stars are too luminous for their effective temperature, surface gravity, and chemical composition. These  "overluminous stars" have two curious properties. First, their kinematics as a function of age from stellar evolution modeling (isochrone fitting) is very different from that of normal stars. Second, while X-ray luminosity of normal stars declines with age, the X-ray luminosity of overluminous F stars changes in the opposite direction, being on average higher for older stars. These properties imply that, in defiance of standard models of stellar evolution, F stars of a given mass and chemical composition can evolve very differently. Assuming that the models correctly describe normal stars, for overluminous F stars they predict too young age and the X-ray emission evolving in the direction opposite to the actually observed trend. This discrepancy between modeling results and observational data suggests that standard stellar evolution models and models of stellar activity are missing some important factors, which makes stellar age and predictions for stellar activity from these models problematic. The data and literature analysis presented in this paper point to a nonuniform rotation of the stellar interior as a plausible key factor able to reconcile the divergent trends in age-velocity relationships of normal and overluminous F stars and explain in a coherent and self-consistent way the overluminosity phenomenon.

Authors: A. A. Suchkov, S. A. Lyapustina

Read the paper on astro-ph


Detection of a Light Echo from the Otherwise Normal SN 2007af Friday, October 31, 2014 - 08:50

We present the discovery of a light echo from SN 2007af, a normal Type Ia supernova (SN Ia)
in NGC 5584. Hubble Space Telescope (HST) images taken three years post explosion reveal two
separate echoes; an outer echo and extended central region, which we propose as an unresolved
inner echo. Multiple images were obtained in the F160W, F350LP, F555W, and F814W using the
Wide Field Camera 3. If the outer echo is produced by an interstellar dust sheet perpendicular
to the line of sight, it is located ∼800 pc in front of the SN. The dust for the inner echo is 0.45 pc
< d < 90 pc away from the SN. The inner echo color is consistent with typical interstellar dust
wavelength-dependent scattering cross-sections, while the outer echo color does not match the
predictions. Both dust sheets, if in the foreground, are optically thin for scattering, with the outer
echo sheet thickness consistent with the inferred extinction from peak brightness. Whether the
inner echo is from interstellar or circumstellar dust is ambiguous. Overall, the echo characteristics
are quite similar to previously observed SN Ia echoes.

Authors: D. Drozdov, M. D. Leising, P. A. Milne, J. Pearcy, A. G. Riess, L. M. Macri, G. L. Bryngelson, P. M. Garnavich

Read the paper on astro-ph


Wind mass transfer in S-type symbiotic binaries I. Focusing by the wind compression model Wednesday, October 29, 2014 - 10:21

Context: Luminosities of hot components in symbiotic binaries require accretion rates that are higher than those that can be achieved via a standard Bondi-Hoyle accretion. This implies that the wind mass transfer in symbiotic binaries has to be more efficient.

Aims: We suggest that the accretion rate onto the white dwarfs (WDs) in S-type symbiotic binaries can be enhanced sufficiently by focusing the wind from their slowly rotating normal giants towards the binary orbital plane.

Methods: We applied the wind compression model to the stellar wind of slowly rotating red giants in S-type symbiotic binaries.

Results: Our analysis reveals that for typical terminal velocities of the giant wind, 20 to 50 km/s, and measured rotational velocities between 6 and 10 km/s, the densities of the compressed wind at a typical distance of the accretor from its donor correspond to the mass-loss rate, which can be a factor of $\sim$10 higher than for the spherically symmetric wind. This allows the WD to accrete at rates of $10^{-8} - 10^{-7}$ M(Sun)/year, and thus to power its luminosity.

Conclusions: We show that the high wind-mass-transfer efficiency in S-type symbiotic stars can be caused by compression of the wind from their slowly rotating normal giants, whereas in D-type symbiotic stars, the high mass transfer ratio can be achieved via the gravitational focusing, which has recently been suggested for very slow winds in Mira-type binaries.

Authors: Augustin Skopal, Zuzana Carikova 

Read the paper on astro-ph


Existence of a group of “quiet” quasars confirmed Wednesday, October 29, 2014 - 09:42

Quasars appear to evolve with distance: the farther away one gets, the brighter they are. This could indicate that quasars extinguish over time or it could be the result of a simple observational bias masking a different reality: that gigantic quasars evolving very quickly, most of them already extinct, coexist with a quiet population that evolves at a much slower rhythm but which our technological limitations do not yet allow us to research.  

To solve this riddle it was necessary to look for low luminosity quasars at enormous  distances and to compare their characteristics with those of nearby quasars of equal luminosity, something thus far almost impossible to do, because it requires observing objects about a hundreds of times weaker than those we are used to studying at those distances.

The tremendous  light-gathering power of the GTC telescope, has recently enabled Sulentic and his team to obtain for the first time spectroscopic data from distant, low luminosity quasars similar to typical nearby ones. Data reliable enough to establish essential parameters such as chemical composition, mass of the central black hole or rate at which it absorbs matter.

Read the full story at IAA News


Georgia State Astronomers Image the Exploding Fireball Stage of a Nova Monday, October 27, 2014 - 07:06

MOUNT WILSON, Calif.–Astronomers at Georgia State University’s Center for High Angular Resolution Astronomy (CHARA) have observed the expanding thermonuclear fireball from a nova that erupted last year in the constellation Delphinus with unprecedented clarity.

The observations produced the first images of a nova during the early fireball stage and revealed how the structure of the ejected material evolves as the gas expands and cools. It appears the expansion is more complicated than simple models previously predicted, scientists said. The results of these observations, carried out by 37 researchers from 17 institutions and led by Georgia State astronomer Gail Schaefer, are published in the current issue of Nature.

Read the full press release


Optical Dual-Band Photometry and Spectroscopy of the WZ Sge-Type Dwarf Nova EZ Lyn during the 2010 Superoutburst Thursday, October 23, 2014 - 06:11

We performed optical simultaneous dual-band (SDSS g'- and i'- band) photometry and low-resolution spectroscopy for the WZ Sge-type dwarf nova EZ Lyn during its 2010 superoutburst. Dual-band photometry revealed that the g'-i' color reddened with a decrease in brightness, during the main superoutburst and the following rebrightening phase, whereas the color became bluer with a further decrease in brightness during the slow, final decline phase. With a fit to our photometric results by a blackbody function, we estimated the disk radius ratio (ratio of the disk radius to the binary separation) and compared this with that of V455 And, a WZ Sge-type object that did not show any rebrightening in the 2007 superoutburst. 

The comparison revealed: (1) the disk radius ratio of EZ Lyn decreased more slowly than that of V455 And; and (2) the radius ratio of EZ Lyn at the end of the main superoutburst was larger than that of the V455 And. These results favor the mass reservoir model for the mechanism of rebrightening. During both the superoutburst plateau and subsequent rebrightening phase, H-alpha and H-beta lines were detected. The H-alpha line showed a double-peak profile from which we estimated the disk radius ratio. The comparison of this ratio with that derived by photometry, indicates that the H-alpha disk was larger than the photometric one, which suggests that the optically thin gas was extended to the outer region more than the optically thick gas disk and was possibly responsible for the rebrightening phenomenon. Time-series dual-band photometry during the main superoutburst revealed that color variations during the early superhump show roughly the same behavior as that of V455 And, whereas color variations during the ordinary superhump display clear anticorrelation with brightness, in contrast to that seen in the V455 And.

Authors: Mizuki Isogai, Akira Arai, Atsunori Yonehara, Hideyo Kawakita, Makoto Uemura, Daisaku Nogami

Read the paper on astro-ph


MY Camelopardalis, a very massive merger progenitor Wednesday, October 22, 2014 - 10:43

Context. The early-type binary MY Cam belongs to the young open cluster Alicante 1, embedded in Cam OB3. Aims. MY Cam consists of two early-O type main-sequence stars and shows a photometric modulation suggesting an orbital period slightly above one day. We intend to confirm this orbital period and derive orbital and stellar parameters. Methods. Timing analysis of a very exhaustive (4607 points) light curve indicates a period of 1.1754514 +- 0.0000015 d. High- resolution spectra and the cross-correlation technique implemented in the TODCOR program were used to derive radial velocities and obtain the corresponding radial velocity curves for MY Cam. Modelling with the stellar atmosphere code FASTWIND was used to obtain stellar parameters and create templates for cross-correlation. Stellar and orbital parameters were derived using the Wilson-Devinney code, such that a complete solution to the binary system could be described. Results. The determined masses of the primary and secondary stars in MY Cam are 37.7 +- 1.6 and 31.6 +- 1.4 Msol, respectively. The corresponding temperatures, derived from the model atmosphere fit, are 42 000 and 39 000 K, with the more massive component being hotter. Both stars are overfilling their Roche lobes, sharing a common envelope. Conclusions. MY Cam contains the most massive dwarf O-type stars found so far in an eclipsing binary. Both components are still on the main sequence, and probably not far from the zero-age main sequence. The system is a likely merger progenitor, owing to its very short period.

Authors: J. Lorenzo (Universidad Alicante), I. Negueruela (Universidad Alicante), A.K.F. Val Baker (University of Malaya), M. García (CSIC-INTA), S. Simón-Díaz (IAC), P. Pastor (Universidad Alicante), M. Méndez Majuelos (IES Arroyo Hondo)

Read the paper on astro-ph


NASA's Fermi Satellite Finds Hints of Starquakes in Magnetar 'Storm' Wednesday, October 22, 2014 - 08:31

Because a neutron star's solid crust is locked to its intense magnetic field, a disruption of one immediately affects the other. A fracture in the crust will lead to a reshuffling of the magnetic field, or a sudden reorganization of the magnetic field may instead crack the surface. Either way, the changes trigger a sudden release of stored energy via powerful bursts that vibrate the crust, a motion that becomes imprinted on the burst’s gamma-ray and X-ray signals.

It takes an incredible amount of energy to convulse a neutron star. The closest comparison on Earth is the 9.5-magnitude Chilean earthquake of 1960, which ranks as the most powerful ever recorded on the standard scale used by seismologists. On that scale, said Watts, a starquake associated with a magnetar giant flare would reach magnitude 23.

Read the full NASA news release


Big Black Holes Can Block New Stars Tuesday, October 21, 2014 - 09:46

Massive black holes spewing out radio-frequency-emitting particles at near-light speed can block formation of new stars in aging galaxies, a study has found.

The research provides crucial new evidence that it is these jets of “radio-frequency feedback” streaming from mature galaxies’ central black holes that prevent hot free gas from cooling and collapsing into baby stars.

“When you look into the past history of the universe, you see these galaxies building stars,” said Tobias Marriage, assistant professor of physics and astronomy at Johns Hopkins and co-lead author of the study. “At some point, they stop forming stars and the question is: Why? Basically, these active black holes give a reason for why stars stop forming in the universe.”

Read the full press release from Johns Hopkins University



A group of researchers led by Melina Bersten of Kavli IPMU recently presented a model that provides the first characterization of the progenitor for a hydrogen-deficient supernova. Their model predicts that a bright hot star, which is the binary companion to an exploding object, remains after the explosion. To verify their theory, the group secured observation time with the Hubble Space Telescope (HST) to search for such a remaining star. Their findings, which are reported in the October 2014 issue of The Astronomical Journal, have important implications for the evolution of massive stars.

Read the full press release from Kavli Institute for the Physics and Mathematics of the Universe


Observations of binaries in AGB, post-AGB stars and Planetary Nebulae Wednesday, October 15, 2014 - 11:30

During the last years, many observational studies have revealed that binaries play an active role in the shaping of non spherical planetary nebulae. We review the different works that lead to the direct or indirect evidence for the presence of binary companions during the Asymptotic Giant Branch, proto-Planetary Nebula and Planetary Nebula phases. We also discuss how these binaries can influence the stellar evolution and possible future directions in the field.

Authors: Eric Lagadec, Olivier Chesneau

Read the review paper on astro-ph


Chemical abundance analysis of symbiotic giants - II. AE Ara, BX Mon, KX Tra, and CL Sco Monday, October 13, 2014 - 09:14

Knowledge of the elemental abundances of symbiotic giants is essential to address the role of chemical composition in the evolution of symbiotic binaries, to map their parent population, and to trace their mass transfer history. However, there are few symbiotic giants for which the photospheric abundances are fairly well determined. This is the second in a series of papers on chemical composition of symbiotic giants determined from high-resolution (R ~ 50000) near-IR spectra. Results are presented for the late-type giant star in the AE Ara, BX Mon, KX TrA, and CL Sco systems. Spectrum synthesis employing standard LTE analysis and stellar atmosphere models were used to obtain photospheric abundances of CNO and elements around the iron peak (Sc, Ti, Fe, and Ni). Our analysis resulted in sub-solar metallicities in BX Mon, KX TrA, and CL Sco by [Fe/H] ~ -0.3 or -0.5 depending on the value of microturbulence. AE Ara shows metallicity closer to solar by ~0.2 dex. The enrichment in 14N isotope found in all these objects indicates that the giants have experienced the first dredge-up. In the case of BX Mon first dredge-up is also confirmed by the low 12C/13C isotopic ratio of ~8.

Authors: Cezary Galan, Joanna Mikolajewska, Kenneth H. Hinkle 

Read the paper on astro-ph


Photometric analysis of overcontact binaries AK Her, HI Dra, V1128 Tau and V2612 Oph Friday, October 10, 2014 - 11:55

We analyze new, high quality multicolor light curves of four overcontact binaries: AK Her, HI Dra, V1128 Tau and V2612 Oph, and determine their orbital and physical parameters using the modeling program of G. Djurasevic and recently published results of radial velocity studies. The achieved precision in absolute masses is between 10 and 20%, and in absolute radii between 5 and 10%. All four systems are W UMa type binaries with bright or dark spots indicative of mass and energy transfer or surface activity. We estimate the distances and the ages of the systems using the luminosities computed through our analysis, and perform an O-C study for V1128 Tau, which reveals a complex period variation that can be interpreted in terms of mass loss/exchange and either the presence of the third body, or the magnetic activity on one of the components. We conclude that further observations of these systems are needed to deepen our understanding of their nature and variability.

Authprs: S. Caliskan, O. Latkovic, G. Djurasevic, I. Ozavci, O. Basturk, A. Cseki, H. V. Senavci, T. Kilicoglu, M. Yilmaz, S. O. Selam

Read the paper on astro-ph


Luminous Blue Variables and superluminous supernovae from binary mergers Friday, October 10, 2014 - 11:49

Evidence suggests that the direct progenitor stars of some core-collapse supernovae (CCSNe) are luminous blue variables (LBVs), perhaps including some `superluminous supernovae' (SLSNe). We examine models in which massive stars gain mass soon after the end of core hydrogen burning. These are mainly intended to represent mergers following a brief contact phase during early Case B mass transfer, but may also represent stars which gain mass in the Hertzsprung Gap or extremely late during the main-sequence phase for other reasons. The post-accretion stars spend their core helium-burning phase as blue supergiants (BSGs), and many examples are consistent with being LBVs at the time of core collapse. Other examples are yellow supergiants at explosion. We also investigate whether such post-accretion stars may explode successfully after core collapse. The final core properties of post-accretion models are broadly similar to those of single stars with the same initial mass as the pre-merger primary star. More surprisingly, when early Case B accretion does affect the final core properties, the effect appears likely to favour a successful SN explosion, i.e., to make the core properties more like those of a lower-mass single star. However, the detailed structures of these cores sometimes display qualitative differences to any single-star model we have calculated. The rate of appropriate binary mergers may match the rate of SNe with immediate LBV progenitors; for moderately optimistic assumptions we estimate that the progenitor birthrate is ~1% of the CCSN rate.

Authors: Stephen Justham, Philipp Podsiadlowski, Jorick S. Vink

Read the paper on astro-ph


AAVSO 49 Bay State Rd. Cambridge, MA 02138 617-354-0484