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Stellar News Feed Archive

V838 Monocerotis: the central star and its environment a decade after outburst Wednesday, July 23, 2014 - 12:28

Aims. V838 Monocerotis erupted in 2002, brightened in a series of outbursts, and eventually developed a spectacular light echo. A very red star emerged a few months after the outburst. The whole event has been interpreted as the result of a merger. Methods. We obtained near-IR and mid-IR interferometric observations of V838 Mon with the AMBER and MIDI recombiners located at the Very Large Telescope Interferometer (VLTI) array. The MIDI two-beam observations were obtained with the 8m Unit Telescopes between October 2011 and February 2012. The AMBER three-beam observations were obtained with the compact array (B≤35m) in April 2013 and the long array (B≤140m) in May 2014, using the 1.8m Auxiliary Telescopes. Results. A significant new result is the detection of a compact structure around V838 Mon, as seen from MIDI data. The extension of the structure increases from a FWHM of 25 mas at 8 {\mu}m to 70 mas at 13 {\mu}m. At the adopted distance of D = 6.1 ± 0.6 kpc, the dust is distributed from about 150 to 400 AU around V838 Mon. The MIDI visibilities reveal a flattened structure whose aspect ratio increases with wavelength. The major axis is roughly oriented around a position angle of −10◦, which aligns with previous polarimetric studies reported in the literature. This flattening can be interpreted as a relic of the 2002 eruption or by the influence of the currently embedded B3V companion. The AMBER data provide a new diameter for the pseudo-photosphere, which shows that its diameter has decreased by about 40% in 10yrs, reaching a radius R∗ = 750 ± 200 R⊙ (3.5 ±1.0 AU). Conclusions. After the 2002 eruption, interpreted as the merging of two stars, it seems that the resulting source is relaxing to a normal state. The nearby environment exhibits an equatorial over-density of dust up to several hundreds of AU.

Authors: Olivier Chesneau (LAGRANGE), Florentin Millour (LAGRANGE), Orsola De Marco, S.N. Bright (LAGRANGE), Alain Spang (LAGRANGE), D. P. K. Banerjee (PRL), N. M. Ashok (PRL), T. Kaminski (MPIFR), John P. Wisniewski, Anthony Meilland (LAGRANGE), Eric Lagadec (LAGRANGE)

Read the paper on astro-ph

 

The RCB star V854 Cen is surrounded by a hot dusty shell Wednesday, July 23, 2014 - 12:20

Aims. The hydrogen-deficient supergiants known as R Coronae Borealis Stars might be the result of a double degenerate merger of two white dwarfs (WDs), or a final helium shell flash in a PN central star. In this context, any information on the geometry of their circumstellar environment and, in particular, the potential detection of elongated structures is of great importance. Methods. We obtained near-IR observations of \object{V854\,Cen} with the {{\sc AMBER}} recombiner located at the Very Large Telescope Interferometer ({{\sc VLTI}}) array with the compact array (B≤35m) in 2013 and the long array (B≤140m) in 2014. At each time, \object{V854\,Cen} was at maximum light. The H- and K-band continua were investigated by means of spectrally-dependant geometric models. These data are supplemented with mid-IR {{\sc VISIR}}/VLT images. Results. A dusty slightly elongated over-density is discovered both in the H and K-band images. With the compact array, the central star is unresolved (Θ≤2.5\,mas), but a flattened dusty environment of 8×11 mas is discovered whose flux raises from about ∼20\% in the H band to reach about ∼50\% at 2.3\micron, indicative of the presence of hot (T∼1500\,K) dust in the close vicinity of the star. The major axis is oriented at a position angle (P.A.) of 126±29\deg. Adding the long array configuration dataset provides tighter constraints on the star diameter (Θ≤1.0\,mas), a slight increase of the over-density representing 12×15 mas and a consistent P.A. of 133±49\deg. The closure phases, sensitive to asymmetries, are null and compatible with a centro-symmetric, unperturbed environment excluding point sources at the level of 3\% of the total flux in 2013 and 2014. The VISIR images exhibit at larger distances (∼1\arcsec) a flattened aspect ratio at the 15-20\% level with a position angle of 92±19\deg, marginally consistent with the interferometric observations. Conclusions. This is the first time that a moderately elongated structure has been observed around an RCB star. These observations confirm the numerous suggestions for this star for a bipolar structure proposed in the literature, mainly based on polarimetric and spectroscopic observations.

Authors: Olivier Chesneau (LAGRANGE), Florentin Millour (LAGRANGE), Orsola De Marco, S.N. Bright (LAGRANGE), Alain Spang (LAGRANGE), D. P. K. Banerjee (PRL), N. M. Ashok (PRL), T. Kaminski (MPIFR), John P. Wisniewski, Anthony Meilland (LAGRANGE), Eric Lagadec (LAGRANGE)

Read the paper on astro-ph

 

Recurrent Novae - A Review Friday, July 18, 2014 - 07:50

In recent years, recurrent nova eruptions are often observed very intensely in wide range of wavelengths from radio to optical to X-rays. Here I present selected highlights from recent multi-wavelength observations. The enigma of T Pyx is at the heart of this paper. While our current understanding of CV and symbiotic star evolution can explain why certain subset of recurrent novae have high accretion rate, that of T Pyx must be greatly elevated compared to the evolutionary mean. At the same time, we have extensive data to be able to estimate how the nova envelope was ejected in T Pyx, and it turns to be a rather complex tale. One suspects that envelope ejection in recurrent and classical novae in general is more complicated than the textbook descriptions. At the end of the review, I will speculate that these two may be connected.

Author: Koji Mukai

Read the review paper on astro-ph

 

AL Pictoris and FR Piscis: two regular Blazhko RR Lyrae stars Thursday, July 17, 2014 - 06:51

Another paper by AAVSO observers-

The results presented are a continuation of observing campaigns conducted by a small group of amateur astronomers interested in the Blazhko effect of RR Lyrae stars. The goal of these observations is to confirm the RR Lyrae Blazhko effect and to detect any additional Blazhko modulation which cannot be identified from all sky survey data-mining. The Blazhko effect of the two observed stars is confirmed, but no additional Blazhko modulations have been detected. 
The observation of the RR Lyrae star AL Pic during 169 nights was conducted from San Pedro de Atacama (Chile). From the observed light curve, 49 pulsation maxima have been measured. Fourier analyses of (O-C), magnitude at maximum light (Mmax) and the complete light curve have provided a confirmation of published pulsation and Blazhko periods, 0.548622 and 34.07 days, respectively. The second multi-longitude observation campaign focused on the RR Lyrae star FR Psc was performed from Europe, United States and Chile. Fourier analyses of the light curve and of 59 measured brightness maxima have improved the accuracy of pulsation and Blazhko periods which are 0.45568 and 51.31 days, respectively. For both stars, no additional Blazhko modulations have been detected.

Authors: Pierre de Ponthiere, Franz-Josef (Josch) Hambsch, Kenneth Menzies, Richard Sabo

Read the paper on astro-ph

Variable Accretion Processes in the Young Binary-Star System UY Aur Thursday, July 17, 2014 - 06:45

We present new K-band spectroscopy of the UY Aur binary star system. Our data are the first to show H2 emission in the spectrum of UY Aur A and the first to spectrally resolve the Br{\gamma} line in the spectrum of UY Aur B. We see an increase in the strength of the Br{\gamma} line in UY Aur A and a decrease in Br{\gamma} and H2 line luminosity for UY Aur B compared to previous studies. Converting Br{\gamma} line luminosity to accretion rate, we infer that the accretion rate onto UY Aur A has increased by 2×10−9 M⊙ yr−1 per year since a rate of zero was observed in 1994. The Br{\gamma} line strength for UY Aur B has decreased by a factor of 0.54 since 1994, but the K-band flux has increased by 0.9 mags since 1998. The veiling of UY Aur B has also increased significantly. These data evince a much more luminous disk around UY Aur B. If the lower Br{\gamma} luminosity observed in the spectrum of UY Aur B indicates an intrinsically smaller accretion rate onto the star, then UY Aur A now accretes at a higher rate than UY Aur B. However, extinction at small radii or mass pile-up in the circumstellar disk could explain decreased Br{\gamma} emission around UY Aur B even when the disk luminosity implies an increased accretion rate. In addition to our scientific results for the UY Aur system, we discuss a dedicated pipeline we have developed for the reduction of echelle-mode data from the ARIES spectrograph.

Authors: Jordan M. Stone, J. A. Eisner, Colette Salyk, Craig Kulesa, Don McCarthy

Read the paper on astro-ph

 

Followup Observations of SDSS and CRTS Candidate Cataclysmic Variables Tuesday, July 15, 2014 - 08:11

We present photometry of 11 and spectroscopy of 35 potential cataclysmic variables from the Sloan Digital Sky Survey, the Catalina Real-Time Transient Survey and vsnet-alerts. The photometry results include quasi-periodic oscillations during the decline of V1363 Cyg, nightly accretion changes in the likely Polar (AM Herculis binary) SDSS J1344+20, eclipses in SDSS J2141+05 with an orbital period of 76+/-2 min, and possible eclipses in SDSS J2158+09 at an orbital period near 100 min. Time-resolved spectra reveal short orbital periods near 80 min for SDSS J0206+20, 85 min for SDSS J1502+33, and near 100 min for CSS J0015+26, RXS J0150+37, SDSS J1132+62, SDSS J2154+15 and SDSS J2158+09. The prominent HeII line and velocity amplitude of SDSS J2154+15 are consistent with a Polar nature for this object, while the lack of this line and a low velocity amplitude argue against this classification for RXS J0150+37. Single spectra of 10 objects were obtained near outburst and the rest near quiescence, confirming the dwarf novae nature of these objects.

Authors: Paula Szkody, Mark E. Everett, Steve B. Howell, Arlo U. Landolt, Howard E. Bond, David R. Silva, Stephanie Vasquez-Soltero

Read the pre-print on astro-ph

 

Anomalous Z Cam stars: a response to mass-transfer outbursts Sunday, July 13, 2014 - 22:55

Recent observations of two unusual Z Cam systems, V513 Cas and IW And, by Szkody et al. (2013) have shown light-curves that seem to contradict the disc instability model for dwarf novae: outbursts are appearing during standstills of the system when, according to the model, the disc is supposed to be in a hot quasi-equilibrium state. We investigate what additional physical processes should be included in the model to reconcile it with observations of such anomalous Z Cam systems. We use our code for modeling thermal-viscous outbursts of the accretion discs, and determine what kind of mass-transfer variations reproduce the observed light curves. We find that outbursts of mass transfer (duration a few days, with a short rise time and an exponential decay) from the stellar companion will account for the observed properties of V513 Cas and IW And, provided they are followed by a short but significant mass-transfer dip. The total mass involved in outbursts is of the order of 1023g. Conclusions: We study the possible origins of such mass transfer outbursts, and show that they most probably result from a giant flare near the secondary star surface, possibly due to absence of star-spots in the L1 region.

Authors: Jean-Marie Hameury, Jean-Pierre Lasota

Read the paper on astro-ph

 

The Most Distant Stars in the Milky Way Saturday, July 12, 2014 - 06:47

We report on the discovery of the most distant Milky Way (MW) stars known to date: ULAS J001535.72$+$015549.6 and ULAS J074417.48$+$253233.0. These stars were selected as M giant candidates based on their infrared and optical colors and lack of proper motions. We spectroscopically confirmed them as outer halo giants using the MMT/Red Channel spectrograph. Both stars have large estimated distances, with ULAS J001535.72$+$015549.6 at $274 \pm 74$ kpc and ULAS J074417.48$+$253233.0 at 238 $\pm$ 64 kpc, making them the first MW stars discovered beyond 200 kpc. ULAS J001535.72$+$015549.6 and ULAS J074417.48$+$253233.0 are both moving away from the Galactic center at $52 \pm 10$ km s$^{-1}$ and $24 \pm 10$ km s$^{-1}$, respectively. Using their distances and kinematics, we considered possible origins such as: tidal stripping from a dwarf galaxy, ejection from the MW's disk, or membership in an undetected dwarf galaxy. These M giants, along with two inner halo giants that were also confirmed during this campaign, are the first to map largely unexplored regions of our Galaxy's outer halo.

Authors:  John J. Bochanski, Beth Willman, Nelson Caldwell, Robyn Sanderson, Andrew A. West, Jay Strader, Warren Brown

Read the paper on astro-ph

 

One thousand cataclysmic variables from the Catalina Real-time Transient Survey Wednesday, July 9, 2014 - 07:58

Over six years of operation, the Catalina Real-time Transient Survey (CRTS) has identified 1043 cataclysmic variable (CV) candidates --- the largest sample of CVs from a single survey to date. Here we provide spectroscopic identification of 85 systems fainter than g<19, including three AMCVn binaries, one helium-enriched CV, one polar and one new eclipsing CV. We analyse the outburst properties of the full sample and show that it contains a large fraction of low accretion rate CVs with long outburst recurrence times. We argue that most of the high accretion rate dwarf novae in the survey footprint have already been found and that future CRTS discoveries will be mostly low accretion rate systems. We find that CVs with white dwarf dominated spectra have significantly fewer outbursts in their CRTS light curves compared to disc-dominated CVs, reflecting the difference in their accretion rates. Comparing the CRTS sample to other samples of CVs, we estimate the overall external completeness to be 23.6 per cent, but show that as much as 56 per cent of CVs have variability amplitudes that are too small to be selected using the transient selection criteria employed by current ground-based surveys. The full table of CRTS CVs, including their outburst and spectroscopic properties examined in this paper, is provided in the online materials.

Authors: E. Breedt (1), B.T. Gaensicke (1), A.J. Drake (2), P. Rodriguez-Gil (3 and 4), S.G. Parsons (5), T.R. Marsh (1), P. Szkody (6), M.R. Schreiber (5), S.G. Djorgovski (2) ((1) University of Warwick, UK, (2) California Institute of Technology, USA, (3) Instituto de Astrofisica de Canarias, Spain, (4) Universidad de La Laguna, Spain, (5) Universidad de Valparaiso, Chile, (6) University of Washington, USA)

Read the paper on arXiv

 

Astronomers Bring The Third Dimension To A Doomed Star's Outburst Wednesday, July 9, 2014 - 07:03

In the middle of the 19th century, the massive binary system Eta Carinae underwent an eruption that ejected at least 10 times the sun's mass and made it the second-brightest star in the sky. Now, a team of astronomers has used extensive new observations to create the first high-resolution 3-D model of the expanding cloud produced by this outburst.

"Our model indicates that this vast shell of gas and dust has a more complex origin than is generally assumed," said Thomas Madura, a NASA Postdoctoral Program fellow at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and a member of the study team. "For the first time, we see evidence suggesting that intense interactions between the stars in the central binary played a significant role in sculpting the nebula we see today."

Read the full press release at nasa.gov

 

The 2014 Eclipse of EE Cep- Announcement for a Third International Observational Campaign Tuesday, July 8, 2014 - 07:33

EE Cep is a unique system in which a Be star is eclipsed by a dark dusty disk, making this star similar to the famous epsilon Aur in many respects. The depth and the duration of the EE Cep eclipses change to a large extent. The last two eclipses were observed in the framework of extensive international campaigns. The joint analysis of these campaigns data and historical photometry, enabled us to propose a model of this system, which implies a disk precession with a period approximately 11-12 times larger than the orbital period. This model predicts that the forthcoming eclipse should be among the deepest observed, reaching about 2 mag. The next eclipse approaches - the photometric minimum should occur around August 23, 2014. Here we would like to announce a new, third international campaign with purpose to verify the disk precession model and to put more constraints on the physical parameters of this system.

Authors: C. Galan, P. Wychudzki, M. Mikolajewski, T. Tomov, D. Dimitrov

Read the paper on astro-ph

 

New Technique Provides a Clear and Rapid Means of Classifying Supernova Remnants Friday, July 4, 2014 - 08:10

An international team of astronomers using data from the Japan-led Suzaku X-ray observatory has developed a powerful technique for analyzing supernova remnants, the expanding clouds of debris left behind when stars explode. The method provides scientists with a way to quickly identify the type of explosion and offers insights into the environment surrounding the star before its destruction.

“Supernovae imprint their remnants with X-ray evidence that reveals the nature of the explosion and its surroundings,” said lead researcher Hiroya Yamaguchi, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Thanks to Suzaku, we are now learning how to interpret these signals.”

The technique involves observing specific X-ray emissions from iron atoms in the core of supernova remnants. Even after thousands of years, these atoms remain extremely hot, stripped of most of the 26 electrons that accompany iron atoms under normal conditions on Earth. The metal is formed in the centers of shattered stars toward the end of their energy-producing lives and in their explosive demise, which makes it a key witness to stellar death.

Read the full press release with links to the paper here.

 

Luminous Blue Variables are Antisocial: Their Isolation Implies that they are Kicked Mass Gainers in Binary Evolution Tuesday, July 1, 2014 - 06:17

Based on their relatively isolated environments, we argue that LBVs must be primarily the product of binary evolution, challenging the traditional single-star view wherein LBVs mark a brief transition between massive O stars and Wolf-Rayet (WR) stars. If the latter were true, then LBVs should be concentrated in young clusters and found alongside main-sequence stars with similarly high inferred initial mass. This is decidedly not the case. Examining locations of LBVs compared to O stars in our Galaxy and the Magellanic Clouds reveals that LBVs systematically avoid clusters of O stars, and many reside over 100 pc from any O star. In the LMC, LBVs are statistically much more isolated than O-type stars, and (perhaps most surprisingly) even more isolated than most WR stars. This makes it impossible for LBVs to be massive stars in transition to WR stars. Instead, we propose that massive stars and supernova (SN) subtypes are dominated by bifurcated evolutionary paths in interacting binaries, wherein most WR stars and SNeIbc correspond to the mass donors, while LBVs (and their lower-mass analogs like B[e] supergiants, which we show to be even more isolated) are the mass gainers. LBVs are essentially the late evolutionary stage of massive blue stragglers. Through binary mass transfer, rejuvinated mass gainers get enriched, spun up, and sometimes kicked far from their clustered birthsites by their companion's SN. This scenario agrees better with LBVs exploding as SNeIIn and the observed isolation of SNe~IIn and SN impostors. We argue that environmental trends of various SN subtypes are influenced more by binarity and SN kicks, rather than tracing initial mass as is generally assumed. Mergers or Thorne-Zykow objects might also give rise to LBVs, but these scenarios may have a harder time explaining why LBVs avoid clusters.

Authors: Nathan Smith, Ryan Tombleson

Read the paper on astro-ph

 

Survey of Period Variations of Superhumps in SU UMa-Type Dwarf Novae. VI: The Sixth Year (2013-2014) Thursday, June 26, 2014 - 11:56

Continuing the project described by Kato et al. (2009, PASJ, 61, S395, arXiv:0905.1757), we collected times of superhump maxima for 56 SU UMa-type dwarf novae mainly observed during the 2013-2014 season and characterized these objects. We detected negative superhumps in VW Hyi and indicated that the low number of normal outbursts in some supercycle can be interpreted as a result of the disk tilt. This finding, combined with the Kepler observation of V1504 Cyg and V344 Lyr, suggests that the disk tilt is responsible for modulating the outburst pattern in SU UMa-type dwarf novae. We also studied the deeply eclipsing WZ Sge-type dwarf nova MASTER OT J005740.99+443101.5 and found evidence of a sharp eclipse during the phase of early superhumps. The profile can be reproduced by a combination of the eclipse of the axisymmetric disk and the uneclipsed light source of early superhumps. This finding confirms the lack of evince of a greatly enhanced hot spot during the early stage of WZ Sge-type outburst. We detected growing (stage A) superhumps in MN Dra and give a suggestion that some of SU UMa-type dwarf novae situated near the critical condition of tidal instability may show long-lasting stage A superhumps. The large negative period derivatives reported in such systems can be understood a result of the combination of stage A and B superhumps. The WZ Sge-type dwarf novae AL Com and ASASSN-13ck showed a long-lasting (plateau-type) rebrightening. In the early phase of the rebrightening, both objects showed a precursor-like outburst, suggesting that the long-lasting rebrightening is triggered by a precursor outburst.

Authors: Taichi Kato (Kyoto U), Pavol A. Dubovsky, Igor Kudzej, Franz-Josef Hambsch, Ian Miller, Tomohito Ohshima, Chikako Nakata, Miho Kawabata, Hirochika Nishino, Kazunari Masumoto, Sahori Mizoguchi, Masayuki Yamanaka, Katsura Matsumoto, Daisuke Sakai, Daiki Fukushima, Minami Matsuura, Genki Bouno, Megumi Takenaka, Shinichi Nakagawa, Ryo Noguchi, Eriko Iino, Roger D. Pickard, Yutaka Maeda, Arne Henden, Kiyoshi Kasai, Seiichiro Kiyota, Hidehiko Akazawa, Kazuyoshi Imamura, Enrique de Miguel, Hiroyuki Maehara, Berto Monard, Elena P. Pavlenko, Kirill Antonyuk, Nikolaj Pit, Oksana I. Antonyuk, Aleksei V. Baklanov, Javier Ruiz, Michael Richmond, Arto Oksanen, Caisey Harlingten, Sergey Yu. Shugarov, Drahomir Chochol, Gianluca Masi, Francesca Nocentini, Patrick Schmeer, Greg Bolt, Peter Nelson, Joseph Ulowetz, Richard Sabo, William N. Goff, William Stein, Raul Michel, Shawn Dvorak, Irina B. Voloshina, Vladimir Metlov, Natalia Katysheva, Vitaly V. Neustroev, George Sjoberg, Colin Littlefield, Bartlomiej Debski, Paulina Sowicka, Marcin Klimaszewski, Malgorzata Curylo, Etienne Morelle, Ivan A. Curtis, Hidetoshi Iwamatsu, Neil D. Butterworth, Maksim V. Andreev, Nikolai Parakhin, Aleksandr Sklyanov, Kazuhiko Shiokawa, Rudolf Novak, Tat'yana R. Irsmambetova, Hiroshi Itoh, Yoshiharu Ito, Kenji Hirosawa, Denis Denisenko, Christopher S. Kochanek, Benjamin Shappee, Krzysztof Z. Stanek, Jose L. Prieto, Koh-ichi Itagaki, Rod Stubbings, Jose Ripero, Eddy Muyllaert, Gary Poyner

Read the paper on arXiv

 

Outburst activity of symbiotic system AG Dra Wednesday, June 25, 2014 - 10:23

AG Dra is a well known bright symbiotic binary with a white dwarf and a pulsating red giant. The long-term photometry monitoring and a new behaviour of the system are presented. The detailed period analysis of photometry as well as spectroscopy was carried out. In the system of AG Dra, two periods of variability are detected. The longer one around 550 days is related to the orbital motion, and the shorter one around 355 days is interpreted as pulsations of the red giant in our older paper. In addition the active stages change distinctively, but the outbursts are repeated with the periods from 359 to 375 days.

Authors: Ladislav Hric, Rudolf Galis, Laurits Leedjärv, Mari Burmeister, Emil Kundra

Read the pre-print on arXiv

 

Remarkable White Dwarf Star Possibly Coldest, Dimmest Ever Detected Monday, June 23, 2014 - 13:30

A team of astronomers has identified possibly the coldest, faintest white dwarf star ever detected. This ancient stellar remnant is so cool that its carbon has crystallized, forming -- in effect -- an Earth-size diamond in space.

“It’s a really remarkable object,” said David Kaplan, a professor at the University of Wisconsin-Milwaukee. “These things should be out there, but because they are so dim they are very hard to find.” 

Read the full press release from the National Radio Astronomy Observatory.
 

On the Effect of Explosive Thermonuclear Burning on the Accreted Envelopes of White Dwarfs in Cataclysmic Variables Monday, June 23, 2014 - 00:21

The detection of heavy elements at suprasolar abundances in the atmospheres of some accreting white dwarfs in cataclysmic variables, coupled with the high temperatures needed to produce these elements requires explosive thermonuclear burning. The central temperatures of any formerly more massive secondary stars in CVs undergoing hydrostatic CNO burning are far too low to produce these elements. Evidence is presented that at least some cataclysmic variables contain donor secondaries that have been contaminated by repeated novae ejecta and are transferring this material back to the white dwarf. This scenario does not exclude the channel in which formerly more massive donor stars underwent CNO processing in ystems that underwent thermal timescale mass transfer. Implications for the progenitors of CVs are discussed.

Author: Edward M. Sion

Read the paper on astro-ph
 

Ultra-short Period Binaries from the Catalina Surveys Thursday, June 19, 2014 - 07:44

We investigate the properties of 367 ultra-short period binary candidates selected from 31,000 sources recently identified from Catalina Surveys data. Based on light curve morphology, along with WISE, SDSS and GALEX multi-colour photometry, we identify two distinct groups of binaries with periods below the 0.22 day contact binary minimum. In contrast to most recent work, we spectroscopically confirm the existence of M-dwarf+M-dwarf contact binary systems. By measuring the radial velocity variations for five of the shortest-period systems, we find examples of rare cool-white dwarf+M-dwarf binaries. Only a few such systems are currently known. Unlike warmer white dwarf systems, their UV flux and their optical colours and spectra are dominated by the M-dwarf companion. We contrast our discoveries with previous photometrically-selected ultra-short period contact binary candidates, and highlight the ongoing need for confirmation using spectra and associated radial velocity measurements. Overall, our analysis increases the number of ultra-short period contact binary candidates by more than an order of magnitude.

Authors: A.J. Drake, S.G. Djorgovski, D. Garcia-Alvarez, M.J. Graham, M. Catelan, A.A. Mahabal, C. Donalek, J.L. Prieto, G. Torrealba, S. Abraham, R. Williams, S. Larson, E. Christensen

Read the paper on astro-ph
 

The puzzling new class of variable stars in NGC 3766 : old friend pulsators? Thursday, June 19, 2014 - 07:37

The recent variability survey of the NGC 3766 cluster revealed a considerable number of periodic variable stars in a region of the H-R diagram where no pulsation is expected. This region lies between the instability strips of the delta Scuti and SPB stars. Moreover the periods of the new phenomenon, P~0.1-0.7 d, do not allow to associate it a priori to either of these two types of pulsations. Stars in the NGC 3766 cluster are known as fast rotators with rotational velocities typically larger than half of their critical velocity. Rotation can affect both the geometrical properties and period domain of pulsations. It also alters the apparent stellar luminosity through gravity darkening, effect seldom taken considered in theoretical studies of the rotation-pulsation interaction. We explore if both of these effects are able to deliver a consistent interpretation for the observed properties of the "new variables" in NGC 3766: explaining their presence outside the known instability strips and their variability periods. We carry out an instability analysis of SPB models within the framework of the Traditional Approximation of Rotation and study the visibility of modes according to the angle of view and rotation. We also check how gravity darkening affects the effective temperature and luminosity of stellar models for different angles of view and rotation velocities. At the red (cold) border of the instability strip, prograde sectoral modes are preferentially excited and their visibilities are maximum when seen equator-on. Furthermore low-mass SPB models seen equator-on can appear in the gap between non-rotating SPB and delta Scuti stars due to gravity darkening. In that case, periods of these most visible modes are shifted to the 0.2-0.5 d range due to the effects of the Coriolis force. We hence suggest that the new variable stars observed in NGC 3766 are actually fast rotating SPB pulsators.

Authors: S. J. A. J. Salmon, J. Montalbàn, D. R. Reese, M.-A. Dupret, P. Eggenberger

Read the paper on astro-ph
 

The asynchronous polar V1432 Aquilae and its path back to synchronism Tuesday, June 17, 2014 - 13:36

A paper from the Society for Astronomical Sciences 33rd Annual Symposium on Telescope Science held last week in Ontario, California.

V1432 Aquilae is the only known eclipsing asynchronous polar. In this respect it is unique and therefore merits our attention. We report the results of a 15-year campaign by the globally distributed Center for Backyard Astrophysics to observe V1432 Aql and investigate its return to synchronism. Originally knocked out of synchrony by a nova explosion before observing records began, the magnetic white dwarf in V1432 Aql is currently rotating slower than the orbital period but is gradually catching up. The fortuitously high inclination of the binary orbit affords us the bonus of eclipses providing a regular clock against which these temporal changes can be assessed. At the present rate, synchronism should be achieved around 2100. The continually changing trajectory of the accretion stream as it follows the magnetic field lines of the rotating white dwarf produces a complex pattern of light emission which we have measured and documented, providing comprehensive observational evidence against which physical models of the system can be tested.

Authors: David Boyd, Joseph Patterson, William Allen, Greg Bolt, Michel Bonnardeau, Tut, Jeannie Campbell, David Cejudo, Michael Cook, Enrique de Miguel, Claire Ding, Shawn Dvorak, Jerrold Foote, Robert Fried, Franz-Josef Hambsch, Jonathan Kemp,Thomas Krajci, Berto Monard, Yenal Ogmen, Robert Rea, George Roberts, David Skillman, Donn Starkey, Joseph Ulowetz, Helena Uthas, Stan Walker

Read the paper on astro-ph

The Smallest Star Friday, June 13, 2014 - 09:21

Astronomers may have identified what may be the smallest known star. And not just the smallest known star but quite possibly the smallest possible star. If it were any smaller, it might not even be a star anymore.

The star in question is called 2MASS J05233822-1403022, but I’ll call it J0523 for short (the name comes from its discovery in the 2-Micron All Sky Survey (2MASS), together with its coordinates on the sky). As weaklings go, it’s just about the weakest: It shines only 1/8,000th as brightly as the Sun, has a temperature of 1,800° C (compared with the Sun’s 5,600°), and a diameter a mere 0.09 times the Sun’s—smaller than Jupiter!

Read the full article on Bad Astronomy
 

Accretion in Young Stars Friday, June 13, 2014 - 09:17

Previous observations of T Tauri stars have found that the accretion rate declines with age. Accretion rates have been observed as high as 10-4 solar masses per year for young T Tauri stars. Older T Tauri stars have been observed with accretion rates as low as 10-10 solar masses per year. There are a couple reasons why this happens. First, as material from the inner part of the disk accretes onto the star, the accretion disk is depleted in that region. If the disk does not evolve and move in to fill that region with more material, the accretion rate will drop. Alternatively, radiation from the newly-formed star could blow away the inner parts of the accretion disk, reducing the overall accretion rate. Once the accretion rate drops, astronomers refer to the systems as weak T Tauri stars. Classical T Tauri stars are those with higher accretion rates.

A few T Tauri stars have been found with high accretion rates up to 10 Myr, well after the accretion rate typically drops. Since not many of these systems have been observed, it is unclear whether they are anomalous or represent a different evolutionary path. In this paper, the authors look at eight T Tauri stars to measure their ages and mass accretion rates. They find seven systems which have accretion rates higher than would be expected.

Read the rest of this interesting story on Astrobites.
 

Astronomers discover first Thorne-Zytkow object, a bizarre type of hybrid star Friday, June 6, 2014 - 07:16

In a discovery decades in the making, scientists have detected the first of a “theoretical” class of stars first proposed in 1975 by physicist Kip Thorne and astronomer Anna Żytkow. Thorne-Żytkow objects (TŻOs) are hybrids of red supergiant and neutron stars that superficially resemble normal red supergiants, such as Betelgeuse in the constellation Orion. They differ, however, in their distinct chemical signatures that result from unique activity in their stellar interiors.

TŻOs are thought to be formed by the interaction of two massive stars―a red supergiant and a neutron star formed during a supernova explosion―in a close binary system. While the exact mechanism is uncertain, the most commonly held theory suggests that, during the evolutionary interaction of the two stars, the much more massive red supergiant essentially swallows the neutron star, which spirals into the core of the red supergiant.

Read the joint press release from CU-Boulder and Lowell Observatory

 

V473 Lyrae, a unique second-overtone Cepheid with two modulation cycles Wednesday, June 4, 2014 - 09:05

V473 Lyrae is the only Galactic Cepheid with confirmed periodic amplitude and phase variations similar to the Blazhko effect observed in RR Lyrae stars. We collected all available photometric data and some radial velocity measurements to investigate the nature of the modulation. The comparison of the photometric and radial velocity amplitudes confirmed that the star pulsates in the second overtone. The extensive data set, spanning more than 40 years, allowed us to detect a secondary modulation cycle with a period of approximately 5300 days or 14.5 years. The secondary variations can be detected in the period of the primary modulation, as well. 
Phenomenologically, the light variations are analogous to the Blazhko effect. To find a physical link, we calculated linear hydrodynamic models to search for potential mode resonances that could drive the modulation and found two viable half-integer (n:2) and three n:4 resonances between the second overtone and other modes. If any of these resonances will be confirmed by non-linear models, it may confirm the mode resonance model, a common mechanism that can drive modulations both in RR Lyrae and Cepheid stars.

Authors: László MolnárLászló Szabados

Read the paper on astro-ph
 

Astronomers View Pulsar Encased in Supernova Bubble Tuesday, June 3, 2014 - 09:14

Massive stars end their lives with a bang: exploding as spectacular supernovas, they release huge amounts of mass and energy into space. These explosions sweep up any surrounding material, creating bubble remnants that expand into interstellar space. At the heart of bubbles like these are small, dense neutron stars or black holes, the remains of what once shone brightly as a star.

Since supernova-carved bubbles shine for only a few tens of thousands of years before dissolving, it is rare to come across neutron stars or black holes that are still enclosed within their expanding shell. This image captures such an unusual scene, featuring both a strongly magnetized, rotating neutron star – known as a pulsar – and its cosmic cloak, the remains of the explosion that generated it.

Read the full story at SciTechDaily.com
 

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