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Stellar News Feed Archive

AAVSO Acronym of the Day: AID Tuesday, May 6, 2014 - 09:27

Today's featured AAVSO Acronym is AID – The AAVSO International Database

The AAVSO International Database has over 25 million variable star observations going back over one hundred years. It is the largest and most comprehensive digital variable star database in the world. Over 1,000,000 new variable star brightness measurements are added to the database every year by observers from all over the world. Professional astronomers, educators, students and amateur researchers around the globe use these data.

This is just one example of the tools and programs the AAVSO provides to its members, observers and the astronomical community. Please help support these services by contributing to this year's Annual Campaign.

You can mail a check to AAVSO headquarters, or you can make a donation online. Just click the Donate Now button on our home page and select Annual Campaign in the drop down menu.

Maybe today or this week isn't the best time for you to make a donation. That's okay, make a pledge. We'll take you at your word if you promise to make a donation to the Annual Campaign before June 30, 2014. To make a pledge click here.
 

AAVSO Acronym of the Day: APASS Monday, May 5, 2014 - 07:19

Today we begin a series of short informational pieces, highlighting some of the tools, projects and programs we know by their acronym names.

Our first featured acronym is APASS- The AAVSO Photometric All Sky Survey.

Through a grant from the Robert Martin Ayers Sciences Fund, the AAVSO is performing an all-sky photometric survey. This survey is conducted in five filters: Johnson B and V, plus Sloan g’, r’, i’. It is valid from approximately 10th magnitude to 17th magnitude. Precise, reliable standardized photometry in this magnitude range is in high demand, both from our observers and from the professional community. This survey will have an impact on the astronomical community for decades to come.

This is just one example of the tools and programs the AAVSO provides to its members, observers and the astronomical community. Please help support these services by contributiong to this year's Annual Campaign.

You can mail a check to AAVSO headquarters, or you can make a donation online. Just click the Donate Now button on our home page and select Annual Campaign in the drop down menu.
 

Identifying and Quantifying Recurrent Novae Masquerading as Classical Novae Thursday, May 1, 2014 - 21:45

Recurrent novae (RNe) are cataclysmic variables with two or more nova eruptions within a century. Classical novae (CNe) are similar systems with only one such eruption. Many of the so-called 'CNe' are actually RNe for which only one eruption has been discovered. Since RNe are candidate Type Ia supernova progenitors, it is important to know whether there are enough in our galaxy to provide the supernova rate, and therefore to know how many RNe are masquerading as CNe. To quantify this, we collected all available information on the light curves and spectra of a Galactic, time-limited sample of 237 CNe and the 10 known RNe, as well as exhaustive discovery efficiency records. We recognize RNe as having (a) outburst amplitude smaller than 14.5 - 4.5 * log(t_3), (b) orbital period >0.6 days, (c) infrared colors of J-H > 0.7 mag and H-K > 0.1 mag, (d) FWHM of H-alpha > 2000 km/s, (e) high excitation lines, such as Fe X or He II near peak, (f) eruption light curves with a plateau, and (g) white dwarf mass greater than 1.2 M_solar. Using these criteria, we identify V1721 Aql, DE Cir, CP Cru, KT Eri, V838 Her, V2672 Oph, V4160 Sgr, V4643 Sgr, V4739 Sgr, and V477 Sct as strong RN candidates. We evaluate the RN fraction amongst the known CNe using three methods to get 24% +/- 4%, 12% +/- 3%, and 35% +/- 3%. With roughly a quarter of the 394 known Galactic novae actually being RNe, there should be approximately a hundred such systems masquerading as CNe.

Authors:  Ashley Pagnotta, Bradley E. Schaefer

Read this paper on astro-ph

Diversity Of Short Gamma-Ray Burst Afterglows From Compact Binary Mergers Hosting Pulsars Thursday, May 1, 2014 - 21:33

Short gamma-ray bursts (sGRBs) are widely believed to result from the mergers of compact binaries. This model predicts an afterglow that bears the characteristic signatures of a constant, low density medium, including a smooth prompt-afterglow transition, and a simple temporal evolution. However, these expectations are in conflict with observations for a non-negligible fraction of sGRB afterglows. In particular, the onset of the afterglow phase for some of these events appears to be delayed and, in addition, a few of them exhibit late-time rapid fading in their lightcurves. These facts have prompted speculation that the central engine activity continues to operate effectively for tens of seconds following the prompt emission. We show that these peculiar observations can be explained independently of ongoing central engine activity if some sGRB progenitors are compact binaries hosting at least one pulsar. The Poynting flux emanating from the pulsar companion can excavate a bow-shock cavity surrounding the binary. If this cavity is larger than the shock deceleration length scale in the undisturbed interstellar medium, then the onset of the afterglow will be delayed. Should the deceleration occur entirely within the swept-up thin shell, a rapid fade in the lightcurve will ensue. We identify two types of pulsar that can achieve the conditions necessary for altering the afterglow: low field, long lived pulsars, and high field pulsars. We find that a sizable fraction (2050%) of low field pulsars are likely to reside in neutron star binaries based on observations, while their high field counterparts are not.

Authors:  Cole Holcomb, Enrico Ramirez-Ruiz, Fabio De Colle, Gabriela Montes

Read the preprint on astro-ph

Massive Stars Don’t Make Good Neighbors Wednesday, April 30, 2014 - 06:38

Big stars are bullies. When one dies, it does so spectacularly, exploding in a supernova with energy output equal to that of the Sun over its entire lifetime. However, a massive star’s mischief starts much sooner. As it is born, it can steal gas from its neighbors. During its life, its strong radiation and winds plow through the neighborhood, wreaking havoc and disrupting the births of other stars. Is this feedback enough to explain why we see such a small fraction of the gas in stellar nurseries ending up in stars? New analysis using simulations investigates the details.

Read the full story on Astrobites

 

NASA's Spitzer and WISE Telescopes Find Close, Cold Neighbor of Sun Saturday, April 26, 2014 - 11:13

NASA's Wide-field Infrared Survey Explorer (WISE) and Spitzer Space Telescope have discovered what appears to be the coldest "brown dwarf" known -- a dim, star-like body that, surprisingly, is as frosty as Earth's North Pole.

Images from the space telescopes also pinpointed the object's distance to 7.2 light-years away, earning it the title for fourth closest system to our sun. The closest system, a trio of stars, is Alpha Centauri, at about 4 light-years away.

"It's very exciting to discover a new neighbor of our solar system that is so close," said Kevin Luhman, an astronomer at Pennsylvania State University's Center for Exoplanets and Habitable Worlds, University Park. "And given its extreme temperature, it should tell us a lot about the atmospheres of planets, which often have similarly cold temperatures."

Read the full press release

A long-term UBVRI photometric study of the pre-main sequence star V350 Cep Thursday, April 24, 2014 - 13:01

Results from UBVRI optical photometric observations of the pre-main sequence star V350 Cep during the period 2004-2014 are presented in the paper. The star is discovered in 1977 due to the remarkable increase in brightness with more than 5 mag (R). In the previous studies V350 Cep is considered a potential member of the groups of FUors or EXors eruptive variables. Our data suggest that during the period of observations the star keeps its maximum brightness with low amplitude photometric variations. Our conclusion is that V350 Cep is probably an intermediate object between FUors and EXors, similar to V1647 Ori.

Authors:  Sunay Ibryamov, Evgeni Semkov, Stoyanka Peneva

Read the pre-print on arXiv

Red giant pulsations from the suspected symbiotic star StHA 169 detected in Kepler data Thursday, April 24, 2014 - 09:01

We present Kepler and Swift observations of StHa 169 which is currently classified as a symbiotic binary. The Kepler light curve shows quasi periodic behaviour with a mean period of 34 d and an amplitude of a few percent. Using Swift data we find a relatively strong UV source at the position of StHa 169 but no X-ray counterpart. Using a simple two component blackbody fit to model the combined Swift and 2MASS spectral energy distribution and an assessment of the previously published optical spectrum, we find that the source has a hot (~10,000K) component and a cooler (~3700K) component. The Kepler light is dominated by the cool component and we attribute the variability to pulsations in a red giant star. If we remove this approximate month long modulation from the light curve, we find no evidence for additional variability in the light curve. The hotter source is assigned to a late B or early A main sequence star. We briefly discuss the implications of these findings and conclude that StHA 169 is a red giant plus main sequence binary.

Authors: Gavin Ramsay (Armagh Observatory), Pasi Hakala (FINCA), Steve Howell (NASA Ames)

Read the pre-print om arXiv

Ancient story tells of star's variable nature Monday, April 21, 2014 - 15:16

Aboriginal people of central Australia appear to have known about the variable star Betelgeuse, long before modern European astronomers, according to a new study.

The discovery reported in the Journal of Astronomical History and Heritage, was uncovered while examining the records of famous amateur anthropologist Daisy Bates.

Early last century, Bates spent 16 years living among the people of South Australia's Great Victoria Desert, recording their language, customs, and oral traditions, according to one of the study's authors, Dr Duane Hamacher of the University of New South Wales.

Hamacher and co-author Trevor Leaman collected Bates' published accounts and journal entries as part of an ongoing project to develop a complete picture of Aboriginal sky knowledge and star lore.

Read thew full story at ABC Science

 

The Light Curve Shapes as a Key to Resolving the Origin of Long Secondary Periods in Red Giant Stars Friday, April 18, 2014 - 14:17

We present a study of OGLE light curves of red giant stars exhibiting long secondary periods (LSPs) - an enigmatic phenomenon commonly observed in stars on the upper red giant branch and asymptotic giant branch. We show that the light curves of LSP stars are essentially identical to those of the spotted variables with one dark spot on their photospheres. Such a behavior can be explained by a presence of a dusty cloud orbiting the red giant together with a low-mass companion in a close, circular orbit. We argue that the binary scenario is in agreement with most of the observational properties of LSP variables, including non-sinusoidal shapes of their radial velocity curves.

Authors:  I. Soszynski, A. Udalski

Read the paper on arXiv

Cataclysmic Variables from the Catalina Real-time Transient Survey Wednesday, April 16, 2014 - 14:47

We present 855 cataclysmic variable candidates detected by the Catalina Real-time Transient Survey (CRTS) of which at least 137 have been spectroscopically confirmed and 705 are new discoveries. The sources were identified from the analysis of five years of data, and come from an area covering three quarters of the sky. We study the amplitude distribution of the dwarf novae CVs discovered by CRTS during outburst, and find that in quiescence they are typically two magnitudes fainter compared to the spectroscopic CV sample identified by SDSS. However, almost all CRTS CVs in the SDSS footprint have ugriz photometry. We analyse the spatial distribution of the CVs and find evidence that many of the systems lie at scale heights beyond those expected for a Galactic thin disc population. We compare the outburst rates of newly discovered CRTS CVs with the previously known CV population, and find no evidence for a difference between them. However, we find that significant evidence for a systematic difference in orbital period distribution. We discuss the CVs found below the orbital period minimum and argue that many more are yet to be identified among the full CRTS CV sample. We cross-match the CVs with archival X-ray catalogs and find that most of the systems are dwarf novae rather than magnetic CVs.

Authors:  A.J. Drake, B.T. Gaensicke, S.G. Djorgovski, P. Wils, A.A. Mahabal, M.J. Graham, T-C. Yang, R. Williams, M. Catelan, J.L. Prieto,C. Donalek, S. Larson, E. Christensen

Read the paper on astro-ph

Long-term photometry of the eclipsing dwarf nova V893 Scorpii: Orbital period, oscillations, and a possible giant planet Tuesday, April 15, 2014 - 15:38

The cataclysmic variable V893 Sco is an eclipsing dwarf nova which, apart from outbursts with comparatively low amplitudes, exhibits a particularly strong variability during quiescence on timescales of days to seconds.The present study aims to update the outdated orbital ephemerides published previously, to investigate deviations from linear ephemerides, and to characterize non-random brightness variations in a range of timescales. Light curves of V893 Sco were observed on 39 nights, spanning a total time base of about 14 years. They contain 114 eclipses which were used to significantly improve the precision of the orbital period and to study long-term variations of the time of revolution. Oscillations and similar brightness variations were studied with Fourier techniques in the individual light curves. The orbital period exhibits long-term variations with a cycle time of 10.2 years. They can be interpreted as a light travel time effect caused by the presence of a giant planet with approximately 9.5 Jupiter masses in a 4.5 AU orbit around V893 Sco. On some nights transient semi-periodic variations on timescales of several minutes can be seen which may be identified as quasi-periodic oscillations. However, it is difficult to distinguish whether they are caused by real physical mechanisms or if they are the effect of an accidental superposition of unrelated flickering flares. Simulations to investigate this question are presented.

Author: Albert Bruch

Read the paper on astro-ph

Chandra Resolves the T Tauri Binary System RW Aur Tuesday, April 15, 2014 - 15:32

RW Aur is a multiple T Tauri system consisting of an early-K type primary (A) and a K5 companion (B) at a separation of 1.4 arcsec. RW Aur A drives a bipolar optical jet that is well-characterized optically. We present results of a sensitive Chandra observation whose primary objective was to search for evidence of soft extended X-ray emission along the jet, as has been seen for a few other nearby T Tauri stars. The binary is clearly resolved by Chandra and both stars are detected as X-ray sources. The X-ray spectra of both stars reveal evidence for cool and hot plasma. Suprisingly, the X-ray luminosity of the less-massive secondary is at least twice that of the primary and is variable. The disparity is attributed to the primary whose X-ray luminosity is at the low end of the range for classical T Tauri stars of similar mass based on established correlations. Deconvolved soft-band images show evidence for slight outward elongation of the source structure of RW Aur A along the blueshifted jet axis inside the central arcsecond. In addition, a faint X-ray emission peak is present on the redshifted axis at an offset of 1.2 +- 0.2 arcsec from the star. Deprojected jet speeds determined from previous optical studies are too low to explain this faint emission peak as shock-heated jet plasma. Thus, unless flow speeds in the redshifted jet have been underestimated, other mechanisms such as magnetic jet heating may be involved.

 Authors: Stephen L. Skinner, Manuel Guedel

Read the pre-print on astro-ph

GG Tau: the fifth element Thursday, April 10, 2014 - 09:48

We aim at unveiling the observational imprint of physical mechanisms that govern planetary formation in young, multiple systems. In particular, we investigate the impact of tidal truncation on the inner circumstellar disks. We observed the emblematic system GG Tau at high-angular resolution: a hierarchical quadruple system composed of low-mass T Tauri binary stars surrounded by a well-studied, massive circumbinary disk in Keplerian rotation. We used the near-IR 4-telescope combiner PIONIER on the VLTI and sparse-aperture-masking techniques on VLT/NaCo to probe this proto-planetary system at sub-AU scales. We report the discovery of a significant closure-phase signal in H and Ks bands that can be reproduced with an additional low-mass companion orbiting GG Tau Ab, at a (projected) separation rho = 31.7 +/- 0.2mas (4.4 au) and PA = 219.6 +/- 0.3deg. This finding offers a simple explanation for several key questions in this system, including the missing-stellar-mass problem and the asymmetry of continuum emission from the inner dust disks observed at millimeter wavelengths. Composed of now five co-eval stars with 0.02 <= Mstar <= 0.7 Msun, the quintuple system GG Tau has become an ideal test case to constrain stellar evolution models at young ages (few 10^6yr).

Authors:  E. Di Folco, A. Dutrey, J.-B. Le Bouquin, S. Lacour, J.-P. Berger, R. Köhler, S. Guilloteau, V. Piétu, J. Bary, T. Beck, H. Beust, E. Pantin

Read the pre-print on arXiv

The oEA stars QY Aql, BW Del, TZ Dra, BO Her and RR Lep: Photometric analysis, frequency search and evolutionary status Thursday, April 10, 2014 - 08:38

New and complete multi-band light curves of the oEA stars QY Aql, BW Del, TZ Dra, BO Her and RR Lep were obtained and analysed with the Wilson-Devinney code. The light curves residuals were further analysed with the Fourier method in order to derive the pulsation characteristics of the oscillating components. All the reliable observed times of minimum light were used to examine orbital period irregularities. The orbital period analyses revealed secular changes for QY Aql and BW Del, while the Light-Time Effect seems to be the best explanation for the cyclic period changes in TZ Dra and BO Her. RR Lep has a rather steady orbital period. Light curve solutions provided the means to calculate the absolute parameters of the components of the systems, which subsequently were used to make an estimate of their present evolutionary status.

Authors: Alexios Liakos and Panagiotis Niarchos

Read the pre-print on arXiv

Light Echoes of Historic Transients Thursday, April 3, 2014 - 14:50

AAVSO councilor, Doug Welch, co-authors an interesting paper on light echoes of historic transients.

Light echoes, light from a variable source scattered off dust, have been observed for over a century. The recent discovery of light echoes around centuries-old supernovae in the Milky Way and the Large Magellanic Cloud have allowed the spectroscopic characterization of these events, even without contemporaneous photometry and spectroscopy using modern instrumentation. Here we review the recent scientific advances using light echoes of ancient and historic transients, and focus on our latest work on SN 1987A's and Eta Carinae's light echoes.

Authors: A. Rest, B. Sinnott, D. L. Welch, J. L. Prieto, F. Bianco

Read the pre-print from astro-ph

Also see the article in Astrobites discussing the paper
 

NOAO/Gemini: Sakurai's Object - Stellar Evolution in Real Time Thursday, April 3, 2014 - 07:39

Stellar lifetimes are measured in billions of years, so changes in their appearance rarely take place on a human timescale. Thus an opportunity to observe a star passing from one stage of life to another on a timescale of months to years is very exciting, as there are only a very few examples known. One such star is Sakurai’s Object (V4334 Sgr). First reported by a Japanese amateur astronomer in 1996 as a “nova-like object,” Sakurai’s Object had been only a few years before the faint central star of a planetary nebula. In the 1990’s Sakurai’s Object brightened by a factor of 10000. This brightening has been attributed to a final helium shell flash. In this process the burned out core of the star at the center of the planetary nebula re-ignites.

The final helium shell flash is violent, ejecting a cloud of dust and gas that forms a thick cocoon around the star blocking all visible light. By 2000 the dust cloud was so thick that Sakurai’s Object was not visible even with the Hubble Space Telescope (HST). Scientists at the National Optical Astronomy Observatory (NOAO) have been observing the sky in the area of Sakurai’s Object waiting for infrared radiation to break through the dust cloud. Infrared radiation penetrates dust much more efficiently than optical light. A detection of the infrared light would mean that the dust cloud is breaking apart, ultimately permitting light from the star to escape.

Read the full story at either Gemini or NOAO

http://www.gemini.edu/node/12195

http://www.noao.edu/news/2014/pr1402.php

A Mystery Star Wrapped in a Stingray Tuesday, April 1, 2014 - 13:19

SAO 244567 is nestled in the heart of the Stingray Nebula, a tiny planetary nebula. Planetary nebulae are formed when an intermediate mass (roughly 0.6–10 solar masses) star enters the last stages of stellar evolution.

Observations of the Stingray Nebula in 1989 led astronomers to conclude that SAO 244567 was a post-AGB star, since it had already produced a planetary nebula.

But spectra taken in 1971 and analyzed in 1995 indicated the star was still evolving at the time, meaning it would have made the transition from giant to post-AGB in twenty years, which is far too fast for any known evolutionary models.  Further, the 1971 observations yielded an effective temperature of 21,000 K for the star, but in 2002 different observations found the star to be at 60,000 K. Spectral observations in between those times support the star growing steadily hotter, though it appears lately (since 2006) to be cooling again slightly. That is a massive change in temperature in roughly thirty years.

Read the rest of the story on Astrobites

Novalike Cataclysmic Variables in the Infrared Tuesday, April 1, 2014 - 13:07

Novalike cataclysmic variables have persistently high mass transfer rates and prominent steady state accretion disks. We present an analysis of infrared observations of twelve novalikes obtained from the Two Micron All Sky Survey, the Spitzer Space Telescope, and the Wide-field Infrared Survey Explorer All Sky Survey. The presence of an infrared excess at >3-5 microns over the expectation of a theoretical steady state accretion disk is ubiquitous in our sample. The strength of the infrared excess is not correlated with orbital period, but shows a statistically significant correlation (but shallow trend) with system inclination that might be partially (but not completely) linked to the increasing view of the cooler outer accretion disk and disk rim at higher inclinations. We discuss the possible origin of the infrared excess in terms of emission from bremsstrahlung or circumbinary dust, with either mechanism facilitated by the mass outflows (e.g., disk wind/corona, accretion stream overflow, and so on) present in novalikes. Our comparison of the relative advantages and disadvantages of either mechanism for explaining the observations suggests that the situation is rather ambiguous, largely circumstantial, and in need of stricter observational constraints.

Authors:  D. W. Hoard, Knox S. Long, Steve B. Howell, Stefanie Wachter, Carolyn S. Brinkworth, Christian Knigge, J. E. Drew, Paula Szkody, S. Kafka, Kunegunda Belle, David R. Ciardi, Cynthia S. Froning, Gerard T. van Belle, M. L. Pretorius

Read the pre-print on astro-ph

New SDO Image of X-Class Solar Flare Tuesday, April 1, 2014 - 12:57

From SciTech Daily

This new image of an X-class solar flare was captured by NASA’s Solar Dynamics Observatory on March 29, 2014.

The sun emitted a significant solar flare, peaking at 1:48 p.m. EDT March 29, 2014, and NASA’s Solar Dynamics Observatory captured images of the event. Solar flares are powerful bursts of radiation. Harmful radiation from a flare cannot pass through Earth’s atmosphere to physically affect humans on the ground, however — when intense enough — they can disturb the atmosphere in the layer where GPS and communications signals travel.

To see how this event impacted Earth, please visit NOAA’s Space Weather Prediction Center at http://spaceweather.gov, the U.S. government’s official source for space weather forecasts, alerts, watches and warnings.

This flare is classified as an X.1-class flare. X-class denotes the most intense flares, while the number provides more information about its strength. An X2 is twice as intense as an X1, an X3 is three times as intense, etc.

Source: Karen C. Fox, NASA’s Goddard Space Flight Center

Image: NASA/SDO
 

Theory suggests source of white dwarf pollution … and ominous fate for Earth Thursday, March 27, 2014 - 09:38

Hot, young, white dwarfs – the super-dense remains of sunlike stars – have atmospheres made essentially of pure hydrogen or pure helium. But these stars’ atmospheres are contaminated by other elements, too – for example, carbon, silicon and iron – known to astronomers as metals. A decades-old space mystery is how the metals came to be in white dwarf atmospheres. And now an international team of astronomers led by Professor Martin Barstow of the University of Leicester in the U.K. say they have solved the mystery. Their solution points to an ominous fate for planet Earth. Professor Barstow said in a press release:

The mystery of the composition of these stars is a problem we have been trying to solve for more than 20 years. It is exciting to realize that they are swallowing up the leftovers from planetary systems, perhaps like our own.

Read the full story on Earthsky.org

OGLE-BLG-RRLYR-12245: An RR Lyrae Star that Switched from a Double to Single-mode Pulsation Thursday, March 27, 2014 - 08:40

We report the discovery of an RR Lyrae star that experienced a switching of its pulsation mode. OGLE-BLG-RRLYR-12245 was discovered as a double-mode RRd star from the observations conducted in years 2001-2006 during the third phase of the Optical Gravitational Lensing Experiment (OGLE-III). The OGLE-IV observations carried out since 2010 reveal that this object is now a fundamental-mode RRab star, with no sign of the first-overtone pulsation. The analysis of the OGLE photometry shows that the final stage of the mode switching occurred on a relatively short timescale of a few months in 2005. We study the behavior of the star during this process, showing changes of the pulsational amplitudes and periods. We also discuss possible causes for the mode switching in RR Lyr stars.

Authors:  I. Soszynski, W. A. Dziembowski, A. Udalski, M. K. Szymanski, M. Kubiak, G. Pietrzynski, L. Wyrzykowski, K. Ulaczyk, R. Poleski, S. Kozlowski, P. Pietrukowicz, J. Skowron, P. Mroz

Read the paper on arXiv

A CLOSE LOOK AT THE NEAREST “STANDARD CANDLE” SUPERNOVA IN SEVERAL DECADES Monday, March 24, 2014 - 12:40

Type Ia supernovae are among the best tools to measure cosmological distances. Thanks to their consistent peak brightness, these ”standard candles” are used to map the expansion history of the Universe. In 1998 distance measurements using supernovae lead to the a paradigm shift in cosmology and fundamental physics: the expansion of the Universe is speeding up, contrary to the expectations from the attractive nature of gravitational forces: a mysterious new cosmic component, ”dark energy”, has been invoked to explain this unexpected phenomenon. This discovery was awarded the 2011 Nobel Prize in physics.

“Since Type Ia supernovae are very rare, occurring only once every several hundred years in a galaxy like ours, there have been very few opportunities to study these explosions in great detail. SN2014J in the nearby galaxy M82 is a very welcome exception”, says Rahman Amanullah a researcher at OKC.

Read the full press release at The Oscar Klein Centre blog

Download the paper (paywall) from IOP Science

HD 54272, a classical lambda Bootis star and gamma Doradus pulsator Thursday, March 20, 2014 - 15:40

Another remarkable star, formerly misclassified as a lambda Bootis and RR Lyrae star,  featured in a pre-print on arXiv

We detect the second known lambda Bootis star (HD 54272) which exhibits gamma Doradus-type pulsations. The star was formerly misidentified as a RR Lyrae variable. The lambda Bootis stars are a small group (only 2 per cent) of late B to early F-type, Population I stars which show moderate to extreme (up to a factor 100) surface underabundances of most Fe-peak elements and solar abundances of lighter elements (C, N, O, and S). The photometric data from the Wide Angle Search for Planets (WASP) and All Sky Automated Survey (ASAS) projects were analysed. They have an overlapping time base of 1566 d and 2545 d, respectively. Six statistically significant peaks were identified (f1 = 1.410 116 c/d, f2 = 1.283 986 c/d, f3 = 1.293 210 c/d, f4 = 1.536 662 c/d, f5 = 1.157 22 c/d and f6 = 0.226 57 c/d). The spacing between f1 and f2, f1 and f4, f5 and f2 is almost identical. Since the daily aliasing is very strong, the interpretation of frequency spectra is somewhat ambiguous. From spectroscopic data, we deduce a high rotational velocity (250+-25 km/s) and a metal deficiency of about -0.8 to -1.1 dex compared to the Sun. A comparison with the similar star, HR 8799, results in analogous pulsational characteristics but widely different astrophysical parameters. Since both are lambda Bootis-type stars, the main mechanism of this phenomenon, selective accretion, may severely influence gamma Doradus-type pulsations.

Authors: E. Paunzen, M. Skarka, D.L. Holdsworth, B. Smalley, R. G. West

Read the abstract and paper on arXiv

An exotic eclipsing binary system composed of two classical overtone Cepheids in a 413-day orbit. Tuesday, March 18, 2014 - 13:10

We have obtained extensive high-quality spectroscopic observations of the OGLE-LMC-CEP-1718 eclipsing binary system in the Large Magellanic Cloud which Soszynski et al. (2008) had identified as a candidate system for containing two classical Cepheids in orbit. Our spectroscopic data clearly demonstrate binary motion of the Cepheids in a 413-day eccentric orbit, rendering this eclipsing binary system the first ever known to consist of two classical Cepheid variables. After disentangling the four different radial velocity variations in the system we present the orbital solution and the individual pulsational radial velocity curves of the Cepheids. We show that both Cepheids are extremely likely to be first overtone pulsators and determine their respective dynamical masses, which turn out to be equal to within 1.5 %. Since the secondary eclipse is not observed in the orbital light curve we cannot derive the individual radii of the Cepheids, but the sum of their radii derived from the photometry is consistent with overtone pulsation for both variables.

The existence of two equal-mass Cepheids in a binary system having different pulsation periods (1.96 and 2.48 days, respectively) may pose an interesting challenge to stellar evolution and pulsation theories, and a more detailed study of this system using additional datasets should yield deeper insight about the physics of stellar evolution of Cepheid variables. Future analysis of the system using additional near-infrared photometry might also lead to a better understanding of the systematic uncertainties in current Baade-Wesselink techniques of distance determinations to Cepheid variables.

Authors: W. Gieren, B. Pilecki, G. Pietrzynski, D. Graczyk, I.B. Thompson, I. Soszynski, P. Konorski, R. Smolec, A. Udalski, N. Nardetto, G. Bono, P.G. Prada Moroni, J. Storm, A. Gallenne

Read the pre-print paper

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