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Stellar News Feed Archive

Astronomers solve 20-year-old quasar mystery Thursday, September 11, 2014 - 09:27

Discovered in the early 1960s, quasars are highly luminous objects shining over vast intergalactic distances. Until the early 1980s, the nature of quasars was controversial, but now most astronomers agree a quasar is a supermassive black hole in the center of a distant massive galaxy. The black hole rapidly accretes (accumulates) matter toward its center to create a quasar’s powerful luminosity. Still, mysteries about quasars have remained, and now two scientists say they’ve solved a quasar mystery that astronomers have been puzzling over for 20 years. These scientists say that most observed quasar phenomena can be unified with two simple quantities: how efficiently the central black hole is being fed and the viewing orientation of the astronomer. The journal Nature published this work on September 11, 2014.

Read the full story at EarthSky


Hubble Finds Supernova Companion Star after Two Decades of Searching Tuesday, September 9, 2014 - 23:08

Using NASA’s Hubble Space Telescope, astronomers have discovered a companion star to a rare type of supernova. The discovery confirms a long-held theory that the supernova, dubbed SN 1993J, occurred inside what is called a binary system, where two interacting stars caused a cosmic explosion.

"This is like a crime scene, and we finally identified the robber," said Alex Filippenko, professor of astronomy at University of California (UC) at Berkeley. "The companion star stole a bunch of hydrogen before the primary star exploded."

SN 1993J is an example of a Type IIb supernova, unusual stellar explosions that contains much less hydrogen than found in a typical supernova.  Astronomers believe the companion star took most of the hydrogen surrounding the exploding main star and continued to burn as a super-hot helium star.

Read the full NASA press release


When Will Betelgeuse Explode? Monday, September 8, 2014 - 11:12

If there’s one star in the sky people know about, it’s Betelgeuse.

Marking the right shoulder of the hunter Orion — remember, he’s facing us, so it’s on our left — this orange-red star is one of the brightest in the night sky. It’s been studied for as long as we’ve had telescopes, yet for all our advanced technology and knowhow, details about it are maddeningly vague. We don’t even have a good determination of how far away it is!

Still, there’s a lot we do know: It’s a red supergiant, a star that started out life already a lot bigger, more massive, and far more luminous than the Sun. Stars like that go through their nuclear fuel extremely rapidly; while the Sun is only approaching middle age at 4.5 billion years old, Betelgeuse is dying now at an age of less than 10 million years old. And when it does finally give up the ghost, it’ll do so with a bang. A very, very big bang: It’ll go supernova, one of nature’s most dramatic and ridiculously violent events.

Read the full story from Phil Plait at Bad Astronomy.

APASS Landolt-Sloan BVgri photometry of RAVE stars. I. Data, effective temperatures and reddenings Wednesday, September 3, 2014 - 17:00

We provide APASS photometry in the Landolt BV and Sloan g'r'i' bands for all the 425,743 stars included in the latest 4th RAVE Data Release. The internal accuracy of the APASS photometry of RAVE stars, expressed as error of the mean of data obtained and separately calibrated over a median of 4 distinct observing epochs and distributed between 2009 and 2013, is 0.013, 0.012, 0.012, 0.014 and 0.021 mag for B, V, g', r' and i' band, respectively. The equally high external accuracy of APASS photometry has been verified on secondary Landolt and Sloan photometric standard stars not involved in the APASS calibration process, and on a large body of literature data on field and cluster stars, confirming the absence of offsets and trends. Compared with the Carlsberg Meridian Catalog (CMC-15), APASS astrometry of RAVE stars is accurate to a median value of 0.098 arcsec. Brightness distribution functions for the RAVE stars have been derived in all bands. APASS photometry of RAVE stars, augmented by 2MASS JHK infrared data, has been chi2 fitted to a densely populated synthetic photometric library designed to widely explore in temperature, surface gravity, metallicity and reddening. Resulting Teff and E(B-V), computed over a range of options, are provided and discussed, and will be kept updated in response to future APASS and RAVE data releases. In the process it is found that the reddening caused by an homogeneous slab of dust, extending for 140 pc on either side of the Galactic plane and responsible for E(B-V,poles)=0.036 +/- 0.002 at the galactic poles, is a suitable approximation of the actual reddening encountered at Galactic latitudes |b|>=25 deg.

Authors: Munari, U.; Henden, A.; Frigo, A.; Zwitter, T.; Bienayme, O.; Bland-Hawthorn, J.; Boeche, C.; Freeman, K. C.; Gilmore, G.; Gibson, B. K.; Grebel, E. K.; Helmi, A.; Kordopatis, G.; Levine, S. E.; Navarro, J. F.; Parker, Q. A.; Reid, W.; Seabroke, G. M.; Siebert, A.; Siviero, A.; Smith, T. C.; Steinmetz, M.; Templeton, M.; Terrell, D.; Welch, D. L.; Williams, M.; Wyse, R. F. G.

Read the paper on astro-ph


NY Serpentis: SU UMa-Type Nova in the Period Gap with Diversity of Normal Outbursts Friday, August 22, 2014 - 09:43

We present photometric study of NY Ser, an in-the-gap SU UMa-type nova, in 2002 and 2013. We determined the duration of the superoutburst and the mean superhump period to be 18 d and 0.10458 d, respectively. We detected in 2013 that NY Ser showed two distinct states separated by the superoutburst. A state of rather infrequent normal outbursts lasted at least 44 d before the superoutburst and a state of frequent outbursts started immediately after the superoutburst and lasted at least for 34 d. Unlike a typical SU UMa star with bimodal distribution of the outbursts duration, NY Ser displayed a diversity of normal outbursts. In the state of infrequent outbursts, we detected a wide ~12 d outburst accompanied by 0.098 d orbital modulation but without superhumps ever established in NY Ser. We classified this as the "wide normal outburst". The orbital period dominated both in quiescence and during normal outbursts in this state. In the state of the most frequent normal outbursts, the 0.10465 d positive superhumps dominated and co-existed with the orbital modulation. In 2002 we detected the normal outburst of "intermediate" 5-6 d duration that was also accompanied by orbital modulations.

Authors: Elena P. Pavlenko, Taichi Kato, Oksana I. Antonyuk, Tomohito Ohshima, Franz-Josef Hambsch, Kirill A. Antonyuk, Aleksei A. Sosnovskij, Alex V. Baklanov, Sergey Yu. Shugarov, Nikolaj V. Pit, Chikako Nakata, Gianluca Masi, Kazuhiro Nakajima,Hiroyuki Maehara, Pavol A. Dubovsky, Igor Kudzej, Maksim V. Andreev, Yuliana G. Kuznyetsova, Kirill A. Vasiliskov

Read the paper on astro-ph


Do Type Ia supernovae stem from the explosion of white dwarf mergers? Wednesday, August 20, 2014 - 11:28

Type Ia supernovae happen when a white dwarf, the “corpse” of a star similar to the Sun, absorbs material from a twin star until it reaches a critical mass--1.4 times that of the Sun—and explodes. Because of their origin, all these explosions share a very similar luminosity. This uniformity made type Ia supernovae ideal objects to measure distances in the universe, but the study of supernova 2014J suggests a scenario that would invalidate them as “standard candles".

A new model postulating the fusion of two white dwarfs is now challenging the predominant one, consisting of a white dwarf and a normal star. The new scenario does not imply the existence of a maximum mass limit and will not, therefore, necessarily produce explosions of similar luminosity. 

Radio observation makes it possible to reveal what stellar systems lie behind type Ia supernovae. If the explosion proceeds from a white dwarf being nourished by a twin star, for example, a great amount of gas should be present in the environment; after the explosion, the material ejected by the supernova will collide with this gas and produce an intense emission of X rays and radio waves. By contrast, a couple of white dwarfs will not generate this gaseous envelope and, therefore, there will be no emission of either X rays or radio waves.

Read the paper on astro-ph

Read the full press release from the Instituto de Astrofisica de Andalucia


Cool Stars and Space Weather Tuesday, August 19, 2014 - 09:50

Stellar flares, winds and coronal mass ejections form the space weather. They are signatures of the magnetic activity of cool stars and, since activity varies with age, mass and rotation, the space weather that extra-solar planets experience can be very different from the one encountered by the solar system planets. How do stellar activity and magnetism influence the space weather of exoplanets orbiting main-sequence stars? How do the environments surrounding exoplanets differ from those around the planets in our own solar system? How can the detailed knowledge acquired by the solar system community be applied in exoplanetary systems? How does space weather affect habitability? These were questions that were addressed in the splinter session "Cool stars and Space Weather", that took place on 9 Jun 2014, during the Cool Stars 18 meeting. In this paper, we present a summary of the contributions made to this session.

Read the summary paper from the Proceedings of the 18th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun.


Rare X-Ray Corona Observed Collapsing into an Extreme Supermassive Black Hole Wednesday, August 13, 2014 - 14:29

NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) has captured an extreme and rare event in the regions immediately surrounding a supermassive black hole. A compact source of X-rays that sits near the black hole, called the corona, has moved closer to the black hole over a period of just days.

"We still don't understan exactly how the corona is produced or why it changes its shape, but we see it lighting up material around the black hole, enabling us to study the regions so close in that effects described by Einstein's theory of general relativity become prominent," said NuSTAR Principal Investigator Fiona Harrison of the California Institute of Technology (Caltech).
White dwarfs crashing into Neutron Stars explain loneliest supernovae Monday, August 11, 2014 - 12:43

Astronomers and astrophysicists have found that some of the Universe’s loneliest supernovae are likely created by the collisions of white dwarf stars into neutron stars.

The study was led by the University of Warwick and involved research from the University of Leicester.  It is published by the journal Monthly Notices of the Royal Astronomical Society.

“Our paper examines so-called `calcium-rich' transients” says Dr Joseph Lyman, from Warwick. “These are luminous explosions that last on the timescales of weeks, however, they're not as bright and don't last as long as traditional supernovae, which makes them difficult to discover and study in detail”.

Previous studies had shown that calcium comprised up to half of the material thrown off in such explosions compared to only a tiny fraction in normal supernovae. This means that these curious events may actually be the dominant producers of calcium in our universe.

Read the full press relese from the University of Leicester Press Office

Image credit: © Mark A. Garlick / / University of Warwick


NOAO: Dr. Arlo Landolt: 55 years of Observing at the National Observatories Thursday, August 7, 2014 - 10:29

Dr. Landolt is known best for his photometric standard star lists. “Landolt standards” is a term familiar to the entire astronomical community. Astronomers must observe through the Earth’s atmosphere, which, as anyone who watches the weather knows, can vary greatly with the date and location. Therefore, astronomical data must be calibrated; just as your bathroom scale may not reflect the accurate weight measured at your doctor’s office and require a correction, so measurements of stellar brightness must be corrected to keep everyone’s results on the same scale. So astronomers observe the same standard stars, along with their own observations, in order to correct for differences in the sky, the telescope, and the instruments on the telescope. Almost all of Dr. Landolt’s observations have been made at the national observatories: Kitt Peak National Observatory in Arizona, and Cerro Tololo Inter-American Observatory in Chile, where Dr. Landolt was also one of the first observers in March 1965.

Read the full press release


Photometric study of the pulsating, eclipsing binary OO Dra Wednesday, August 6, 2014 - 10:30

We present a comprehensive photometric study of the pulsating, eclipsing binary OO Dra. Simultaneous B- and V-band photometry of the star was carried out on 14 nights. Revised orbital period and a new ephemeris were derived from the data. The first photometric solution of the binary system and the physical parameters of the component stars are determined. It reveals that OO Dra could be a detached system with the less-massive secondary component nearly filling in its Roche lobe. By subtracting the eclipsing light changes from the data, we obtained the intrinsic pulsating light curves of the hotter and massive primary component. Frequency analysis of the residuals light yields two confident pulsation modes in both B- and V-band data with the dominant frequency detected at 41.865 c/d. A brief discussion concerning the evolutionary status and the pulsation nature of the binary system is finally given.

Authors: X.B. Zhang, L.C. Deng, J.F. Tian, K. Wang, J.J. Sun, Q.L. Liu, H.Q. Xin, Q. Zhou, Z.Z. Yan, Z.Q. Luo, C.Q. Luo

Read the paper on astro-ph


Fermi Establishes Classical Novae as a Distinct Class of Gamma-Ray Sources Wednesday, August 6, 2014 - 09:42

A classical nova results from runaway thermonuclear explosions on the surface of a white dwarf that accretes matter from a low-mass main-sequence stellar companion. In 2012 and 2013, three novae were detected in gamma rays and stood in contrast to the first gamma-ray detected nova V407 Cygni 2010, which belongs to a rare class of symbiotic binary systems. Despite likely differences in the compositions and masses of their white dwarf progenitors, the three classical novae are similarly characterized as soft spectrum transient gamma-ray sources detected over 2-3 week durations. The gamma-ray detections point to unexpected high-energy particle acceleration processes linked to the mass ejection from thermonuclear explosions in an unanticipated class of Galactic gamma-ray sources.

Authors: C.C. Cheung, P. Jean, S.N. Shore

Read the paper on astro-ph


Revisiting CoRoT RR Lyrae stars: detection of period doubling and temporal variation of additional frequencies Wednesday, August 6, 2014 - 09:34

We search for signs of period doubling in CoRoT RR Lyrae stars. The occurrence of this dynamical effect in modulated RR Lyrae stars might help us to gain more information about the mysterious Blazhko effect. The temporal variability of the additional frequencies in representatives of all subtypes of RR Lyrae stars is also investigated. We pre-process CoRoT light curves by applying trend and jump correction and outlier removal. Standard Fourier technique is used to analyze the frequency content of our targets and follow the time dependent phenomena. The most comprehensive collection of CoRoT RR Lyrae stars, including new discoveries is presented and analyzed. We found alternating maxima and in some cases half-integer frequencies in four CoRoT Blazhko RR Lyrae stars, as clear signs of the presence of period doubling. This reinforces that period doubling is an important ingredient to understand the Blazhko effect - a premise we derived previously from the Kepler RR Lyrae sample. As expected, period doubling is detectable only for short time intervals in most modulated RRab stars. Our results show that the temporal variability of the additional frequencies in all RR Lyrae sub-types is ubiquitous. The ephemeral nature and the highly variable amplitude of these variations suggest a complex underlying dynamics of and an intricate interplay between radial and possibly nonradial modes in RR Lyrae stars. The omnipresence of additional modes in all types of RR Lyrae - except in non-modulated RRab stars - implies that asteroseismology of these objects should be feasible in the near future.

Authors: R. Szabó, J. M. Benkő, M. Paparó, E. Chapellier, E. Poretti, A. Baglin, W. W. Weiss, K. Kolenberg, E. Guggenberger, J.-F. Le Borgne

Read the paper on astro-ph


Planet-like object may have spent its youth as hot as a star Tuesday, August 5, 2014 - 09:52

Astronomers have discovered an extremely cool object that could have a particularly diverse history - although it is now as cool as a planet, it may have spent much of its youth as hot as a star. The team publish their results in the journal Monthly Notices of the Royal Astronomical Society.

The current temperature of the object is 100-150 degrees Celsius, intermediate between that of the Earth and Venus. But the object shows evidence of a possible ancient origin, implying that a large change in temperature has taken place. In the past this object would have been as hot as a star for many millions of years.

Read the full press release from the Royal Astronomical Society.


ALMA Finds Double Star with Weird and Wild Planet-forming Discs Wednesday, July 30, 2014 - 13:55

“ALMA has now given us the best view yet of a binary star system sporting protoplanetary discs  — and we find that the discs are mutually misaligned!” said Eric Jensen, an astronomer at Swarthmore College in Pennsylvania, USA.

The two stars in the HK Tauri system, which is located about 450 light-years from Earth in the constellation of Taurus (The Bull), are less than five million years old and separated by about 58 billion kilometres — this is 13 times the distance of Neptune from the Sun.

The fainter star, HK Tauri B, is surrounded by an edge-on protoplanetary disc that blocks the starlight. Because the glare of the star is suppressed, astronomers can easily get a good view of the disc by observing in visible light, or at near-infrared wavelengths.

The companion star, HK Tauri A, also has a disc, but in this case it does not block out the starlight. As a result the disc cannot be seen in visible light because its faint glow is swamped by the dazzling brightness of the star. But it does shine brightly in millimetre-wavelength light, which ALMA can readily detect.

Read the full press release from ESO


V838 Monocerotis: the central star and its environment a decade after outburst Wednesday, July 23, 2014 - 13:28

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 - 13:20

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 - 08:50

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 - 07:51

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 - 07:45

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 - 09:11

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 - 23:55

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 - 07:47

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 - 08:58

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 - 08:03

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."

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