Stellar News Feed Archive
|Existence of a group of “quiet” quasars confirmed||Wednesday, October 29, 2014 - 10:42||
Quasars appear to evolve with distance: the farther away one gets, the brighter they are. This could indicate that quasars extinguish over time or it could be the result of a simple observational bias masking a different reality: that gigantic quasars evolving very quickly, most of them already extinct, coexist with a quiet population that evolves at a much slower rhythm but which our technological limitations do not yet allow us to research.
To solve this riddle it was necessary to look for low luminosity quasars at enormous distances and to compare their characteristics with those of nearby quasars of equal luminosity, something thus far almost impossible to do, because it requires observing objects about a hundreds of times weaker than those we are used to studying at those distances.
The tremendous light-gathering power of the GTC telescope, has recently enabled Sulentic and his team to obtain for the first time spectroscopic data from distant, low luminosity quasars similar to typical nearby ones. Data reliable enough to establish essential parameters such as chemical composition, mass of the central black hole or rate at which it absorbs matter.
|Georgia State Astronomers Image the Exploding Fireball Stage of a Nova||Monday, October 27, 2014 - 08:06||
MOUNT WILSON, Calif.–Astronomers at Georgia State University’s Center for High Angular Resolution Astronomy (CHARA) have observed the expanding thermonuclear fireball from a nova that erupted last year in the constellation Delphinus with unprecedented clarity.
The observations produced the first images of a nova during the early fireball stage and revealed how the structure of the ejected material evolves as the gas expands and cools. It appears the expansion is more complicated than simple models previously predicted, scientists said. The results of these observations, carried out by 37 researchers from 17 institutions and led by Georgia State astronomer Gail Schaefer, are published in the current issue of Nature.
|Optical Dual-Band Photometry and Spectroscopy of the WZ Sge-Type Dwarf Nova EZ Lyn during the 2010 Superoutburst||Thursday, October 23, 2014 - 07:11||
We performed optical simultaneous dual-band (SDSS g'- and i'- band) photometry and low-resolution spectroscopy for the WZ Sge-type dwarf nova EZ Lyn during its 2010 superoutburst. Dual-band photometry revealed that the g'-i' color reddened with a decrease in brightness, during the main superoutburst and the following rebrightening phase, whereas the color became bluer with a further decrease in brightness during the slow, final decline phase. With a fit to our photometric results by a blackbody function, we estimated the disk radius ratio (ratio of the disk radius to the binary separation) and compared this with that of V455 And, a WZ Sge-type object that did not show any rebrightening in the 2007 superoutburst.
The comparison revealed: (1) the disk radius ratio of EZ Lyn decreased more slowly than that of V455 And; and (2) the radius ratio of EZ Lyn at the end of the main superoutburst was larger than that of the V455 And. These results favor the mass reservoir model for the mechanism of rebrightening. During both the superoutburst plateau and subsequent rebrightening phase, H-alpha and H-beta lines were detected. The H-alpha line showed a double-peak profile from which we estimated the disk radius ratio. The comparison of this ratio with that derived by photometry, indicates that the H-alpha disk was larger than the photometric one, which suggests that the optically thin gas was extended to the outer region more than the optically thick gas disk and was possibly responsible for the rebrightening phenomenon. Time-series dual-band photometry during the main superoutburst revealed that color variations during the early superhump show roughly the same behavior as that of V455 And, whereas color variations during the ordinary superhump display clear anticorrelation with brightness, in contrast to that seen in the V455 And.
Authors: Mizuki Isogai, Akira Arai, Atsunori Yonehara, Hideyo Kawakita, Makoto Uemura, Daisaku Nogami
|MY Camelopardalis, a very massive merger progenitor||Wednesday, October 22, 2014 - 11:43||
Context. The early-type binary MY Cam belongs to the young open cluster Alicante 1, embedded in Cam OB3. Aims. MY Cam consists of two early-O type main-sequence stars and shows a photometric modulation suggesting an orbital period slightly above one day. We intend to confirm this orbital period and derive orbital and stellar parameters. Methods. Timing analysis of a very exhaustive (4607 points) light curve indicates a period of 1.1754514 +- 0.0000015 d. High- resolution spectra and the cross-correlation technique implemented in the TODCOR program were used to derive radial velocities and obtain the corresponding radial velocity curves for MY Cam. Modelling with the stellar atmosphere code FASTWIND was used to obtain stellar parameters and create templates for cross-correlation. Stellar and orbital parameters were derived using the Wilson-Devinney code, such that a complete solution to the binary system could be described. Results. The determined masses of the primary and secondary stars in MY Cam are 37.7 +- 1.6 and 31.6 +- 1.4 Msol, respectively. The corresponding temperatures, derived from the model atmosphere fit, are 42 000 and 39 000 K, with the more massive component being hotter. Both stars are overfilling their Roche lobes, sharing a common envelope. Conclusions. MY Cam contains the most massive dwarf O-type stars found so far in an eclipsing binary. Both components are still on the main sequence, and probably not far from the zero-age main sequence. The system is a likely merger progenitor, owing to its very short period.
Authors: J. Lorenzo (Universidad Alicante), I. Negueruela (Universidad Alicante), A.K.F. Val Baker (University of Malaya), M. García (CSIC-INTA), S. Simón-Díaz (IAC), P. Pastor (Universidad Alicante), M. Méndez Majuelos (IES Arroyo Hondo)
|NASA's Fermi Satellite Finds Hints of Starquakes in Magnetar 'Storm'||Wednesday, October 22, 2014 - 09:31||
Because a neutron star's solid crust is locked to its intense magnetic field, a disruption of one immediately affects the other. A fracture in the crust will lead to a reshuffling of the magnetic field, or a sudden reorganization of the magnetic field may instead crack the surface. Either way, the changes trigger a sudden release of stored energy via powerful bursts that vibrate the crust, a motion that becomes imprinted on the burst’s gamma-ray and X-ray signals.
It takes an incredible amount of energy to convulse a neutron star. The closest comparison on Earth is the 9.5-magnitude Chilean earthquake of 1960, which ranks as the most powerful ever recorded on the standard scale used by seismologists. On that scale, said Watts, a starquake associated with a magnetar giant flare would reach magnitude 23.
|Big Black Holes Can Block New Stars||Tuesday, October 21, 2014 - 10:46||
Massive black holes spewing out radio-frequency-emitting particles at near-light speed can block formation of new stars in aging galaxies, a study has found.
The research provides crucial new evidence that it is these jets of “radio-frequency feedback” streaming from mature galaxies’ central black holes that prevent hot free gas from cooling and collapsing into baby stars.
“When you look into the past history of the universe, you see these galaxies building stars,” said Tobias Marriage, assistant professor of physics and astronomy at Johns Hopkins and co-lead author of the study. “At some point, they stop forming stars and the question is: Why? Basically, these active black holes give a reason for why stars stop forming in the universe.”
|EXPLOSION OF A LOW-MASS HELIUM STAR IN A BINARY— FIRST EVIDENCE OF A HYDROGEN-DEFICIENT SUPERNOVA PROGENITOR||Thursday, October 16, 2014 - 13:55||
A group of researchers led by Melina Bersten of Kavli IPMU recently presented a model that provides the first characterization of the progenitor for a hydrogen-deficient supernova. Their model predicts that a bright hot star, which is the binary companion to an exploding object, remains after the explosion. To verify their theory, the group secured observation time with the Hubble Space Telescope (HST) to search for such a remaining star. Their findings, which are reported in the October 2014 issue of The Astronomical Journal, have important implications for the evolution of massive stars.
Read the full press release from Kavli Institute for the Physics and Mathematics of the Universe
|Observations of binaries in AGB, post-AGB stars and Planetary Nebulae||Wednesday, October 15, 2014 - 12:30||
During the last years, many observational studies have revealed that binaries play an active role in the shaping of non spherical planetary nebulae. We review the different works that lead to the direct or indirect evidence for the presence of binary companions during the Asymptotic Giant Branch, proto-Planetary Nebula and Planetary Nebula phases. We also discuss how these binaries can influence the stellar evolution and possible future directions in the field.
Authors: Eric Lagadec, Olivier Chesneau
|Chemical abundance analysis of symbiotic giants - II. AE Ara, BX Mon, KX Tra, and CL Sco||Monday, October 13, 2014 - 10:14||
Knowledge of the elemental abundances of symbiotic giants is essential to address the role of chemical composition in the evolution of symbiotic binaries, to map their parent population, and to trace their mass transfer history. However, there are few symbiotic giants for which the photospheric abundances are fairly well determined. This is the second in a series of papers on chemical composition of symbiotic giants determined from high-resolution (R ~ 50000) near-IR spectra. Results are presented for the late-type giant star in the AE Ara, BX Mon, KX TrA, and CL Sco systems. Spectrum synthesis employing standard LTE analysis and stellar atmosphere models were used to obtain photospheric abundances of CNO and elements around the iron peak (Sc, Ti, Fe, and Ni). Our analysis resulted in sub-solar metallicities in BX Mon, KX TrA, and CL Sco by [Fe/H] ~ -0.3 or -0.5 depending on the value of microturbulence. AE Ara shows metallicity closer to solar by ~0.2 dex. The enrichment in 14N isotope found in all these objects indicates that the giants have experienced the first dredge-up. In the case of BX Mon first dredge-up is also confirmed by the low 12C/13C isotopic ratio of ~8.
Authors: Cezary Galan, Joanna Mikolajewska, Kenneth H. Hinkle
|Photometric analysis of overcontact binaries AK Her, HI Dra, V1128 Tau and V2612 Oph||Friday, October 10, 2014 - 12:55||
We analyze new, high quality multicolor light curves of four overcontact binaries: AK Her, HI Dra, V1128 Tau and V2612 Oph, and determine their orbital and physical parameters using the modeling program of G. Djurasevic and recently published results of radial velocity studies. The achieved precision in absolute masses is between 10 and 20%, and in absolute radii between 5 and 10%. All four systems are W UMa type binaries with bright or dark spots indicative of mass and energy transfer or surface activity. We estimate the distances and the ages of the systems using the luminosities computed through our analysis, and perform an O-C study for V1128 Tau, which reveals a complex period variation that can be interpreted in terms of mass loss/exchange and either the presence of the third body, or the magnetic activity on one of the components. We conclude that further observations of these systems are needed to deepen our understanding of their nature and variability.
Authprs: S. Caliskan, O. Latkovic, G. Djurasevic, I. Ozavci, O. Basturk, A. Cseki, H. V. Senavci, T. Kilicoglu, M. Yilmaz, S. O. Selam
|Luminous Blue Variables and superluminous supernovae from binary mergers||Friday, October 10, 2014 - 12:49||
Evidence suggests that the direct progenitor stars of some core-collapse supernovae (CCSNe) are luminous blue variables (LBVs), perhaps including some `superluminous supernovae' (SLSNe). We examine models in which massive stars gain mass soon after the end of core hydrogen burning. These are mainly intended to represent mergers following a brief contact phase during early Case B mass transfer, but may also represent stars which gain mass in the Hertzsprung Gap or extremely late during the main-sequence phase for other reasons. The post-accretion stars spend their core helium-burning phase as blue supergiants (BSGs), and many examples are consistent with being LBVs at the time of core collapse. Other examples are yellow supergiants at explosion. We also investigate whether such post-accretion stars may explode successfully after core collapse. The final core properties of post-accretion models are broadly similar to those of single stars with the same initial mass as the pre-merger primary star. More surprisingly, when early Case B accretion does affect the final core properties, the effect appears likely to favour a successful SN explosion, i.e., to make the core properties more like those of a lower-mass single star. However, the detailed structures of these cores sometimes display qualitative differences to any single-star model we have calculated. The rate of appropriate binary mergers may match the rate of SNe with immediate LBV progenitors; for moderately optimistic assumptions we estimate that the progenitor birthrate is ~1% of the CCSN rate.
Authors: Stephen Justham, Philipp Podsiadlowski, Jorick S. Vink
|NASA's NuSTAR Telescope Discovers Shockingly Bright Dead Star||Thursday, October 9, 2014 - 10:28||
Astronomers have found a pulsating, dead star beaming with the energy of about 10 million suns. This is the brightest pulsar - a dense stellar remnant left over from a supernova explosion - ever recorded. The discovery was made with NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR.
"You might think of this pulsar as the 'Mighty Mouse' of stellar remnants," said Fiona Harrison, the NuSTAR principal investigator at the California Institute of Technology in Pasadena. "It has all the power of a black hole, but with much less mass."
The discovery appears in a new report in the Thursday, Oct. 9, issue of the journal Nature.
|NASA's Swift Mission Observes Mega Flares from a Mini Star||Wednesday, October 1, 2014 - 00:31||
Stars erupt with flares for the same reason the sun does. Around active regions of the star's atmosphere, magnetic fields become twisted and distorted. Much like winding up a rubber band, these allow the fields to accumulate energy. Eventually a process called magnetic reconnection destabilizes the fields, resulting in the explosive release of the stored energy we see as a flare. The outburst emits radiation across the electromagnetic spectrum, from radio waves to visible, ultraviolet and X-ray light.
At 5:07 p.m. EDT on April 23, the rising tide of X-rays from DG CVn's superflare triggered Swift's Burst Alert Telescope (BAT). Within several seconds of detecting a strong burst of radiation, the BAT calculates an initial position, decides whether the activity merits investigation by other instruments and, if so, sends the position to the spacecraft. In this case, Swift turned to observe the source in greater detail, and, at the same time, notified astronomers around the globe that a powerful outburst was in progress.
|Pulsating Components in Binary and Multiple Stellar Systems - A Catalog of Oscillating Binaries||Monday, September 29, 2014 - 09:34||
We present an up-to-date catalog of pulsating binaries, i.e. the binary and multiple stellar systems containing pulsating components, along with a statistics on them. Compared to the earlier compilation by Soydugan et al.(2006a) of 25 δ Scuti-type `oscillating Algol-type eclipsing binaries' (oEA) plus 197 candidates,the recent collection of 74 oEA by Liakos et al.(2012), and the collection of Cepheids in binaries by Szabados (2003a), both the types of pulsating variables and binaries are now extended. The total numbers of pulsating binary/multiple stellar systems have increased to be 501 as of 2014 September 16, among which 262+ are oscillating eclipsing binaries and the oEA containing δ Scuti components are updated to be 93. The catalog is intended to be a collection of various pulsating binary stars across the Hertzsprung-Russell diagram. We reviewed the open questions, advances and prospects connecting pulsation/oscillation and binarity. The observational implication of binary systems with pulsating components, to stellar evolution theories is also addressed. In addition, a catalog consisting of 434 confirmed Algol-type eclipsing binaries (EA) is provided for reference.
Author: A.-Y. Zhou
|The Immediate Environments of Two Herbig Be Stars: MWC 1080 and HD 259431||Wednesday, September 24, 2014 - 10:37||
Deep mid-infrared (10-20 $\mu$m) images with sub-arcsec resolution were obtained for two Herbig Be stars, MWC 1080 and HD 259431, to probe their immediate environments. Our goal is to understand the origin of the diffuse nebulosities observed around these two very young objects. By analyzing our new mid-IR images and comparing them to published data at other wavelengths, we demonstrate that the well extended emission around MWC 1080 traces neither a disk nor an envelope, but rather the surfaces of a cavity created by the outflow from MWC 1080A, the primary star of the MWC 1080 system. In the N-band images, the filamentary nebulosities trace the hourglass-shaped gas cavity wall out to $\sim$0.15 pc. This scenario reconciles the properties of the MWC 1080 system revealed by a wide range of observations. Our finding confirms that the environment around MWC 1080, where a small cluster is forming, is strongly affected by the outflow from the central Herbig Be star. Similarities observed between the two subjects of this study suggest that the filamentary emission around HD 259431 may arise from a similar outflow cavity structure, too.
Authors: Dan Li, Naibí Mariñas, Charles M. Telesco
|Astronomers observe mysterious winds from T Tauri star||Wednesday, September 24, 2014 - 10:21||
T Tauri stars are a class of variable stars, named for their prototype T Tauri, discovered in 1852. T Tauri stars have been known for decades as relatively normal, medium-sized, extremely young main-sequence stars. At one point, some 4.5 billion years ago, our Sun was a T Tauri star. T Tauri stars are thought to be surrounded by protoplanetary disks, containing the raw materials to build both rocky and gaseous planets. Though nearly invisible in optical light, these disks shine in both infrared and millimeter-wavelength light.
Some T Tauri stars emit infrared radiation in unexpected ways. Those stars were the focus of this study, led by astronomer Colette Salyk at the National Optical Astronomical Observatory (NOAO) in Tucson, Arizona. Many T Tauri stars have been thought to have extremely powerfulstellar winds – predicted by astronomers, but never clearly detected – and Salyk and her team proposed that, for some T Tauri stars, the winds may be emanating from within the stars’ protoplanetary disks. They say these winds could have important implications for planet formation, potentially robbing the disk of some of the gas required for the formation of giant Jupiter-like planets, or stirring up the disk and causing the building blocks of planets to change location entirely.
|The Pulsating sdB+M Eclipsing System NY Virginis and its Circumbinary Planets||Thursday, September 18, 2014 - 08:27||
We searched for circumbinary planets orbiting NY Vir in historical eclipse times including our long-term CCD data. Sixty-eight times of minimum light with accuracies better than 10 s were used for the ephemeris computations. The best fit to those timings indicated that the orbital period of NY Vir has varied due to a combination of two sinusoids with periods of P3=8.2 yr and P4=27.0 yr and semi-amplitudes of K3=6.9 s and K4=27.3 s, respectively. The periodic variations most likely arise from a pair of light-time effects due to the presence of third and fourth bodies that are gravitationally bound to the eclipsing pair. We have derived the orbital parameters and the minimum masses, M3sini3 = 2.8 MJup and M4sini4 = 4.5 MJup, of both objects. A dynamical analysis suggests that the outer companion is less likely to orbit the binary on a circular orbit. Instead we show that future timing data might push its eccentricity to moderate values for which the system exhibits long-term stability. The results demonstrate that NY Vir is probably a star-planet system, which consists of a very close binary star and two giant planets. The period ratio P3/P4 suggests that a long-term gravitational interaction between them would result in capture into a nearly 3:10 mean motion resonance. When the presence of the circumbinary planets is verified and understood more comprehensively, the formation and evolution of this planetary system should be advanced greatly.
Authors: Jae Woo Lee, Tobias Cornelius Hinse, Jae-Hyuck Youn, Wonyong Han
|Ideas for Citizen Science in Astronomy||Tuesday, September 16, 2014 - 12:02||
We review the relatively new, internet-enabled, and rapidly-evolving field of citizen science, focusing on research projects in stellar, extragalactic and solar system astronomy that have benefited from the participation of members of the public, often in large numbers. We find these volunteers making contributions to astronomy in a variety of ways: making and analyzing new observations, visually classifying features in images and light curves, exploring models constrained by astronomical datasets, and initiating new scientific enquiries. The most productive citizen astronomy projects involve close collaboration between the professionals and amateurs involved, and occupy scientific niches not easily filled by great observatories or machine learning methods: citizen astronomers are most strongly motivated by being of service to science. In the coming years we expect participation and productivity in citizen astronomy to increase, as survey datasets get larger and citizen science platforms become more efficient. Opportunities include engaging the public in ever more advanced analyses, and facilitating citizen-led enquiry by designing professional user interfaces and analysis tools with citizens in mind.
Authors: Philip J. Marshall, Chris J. Lintott, Leigh N. Fletcher
|Calling All Amateur Astronomers||Tuesday, September 16, 2014 - 11:51||
NASA’s Night Sky Network is conducting a new survey to better understand the landscape of educational outreach performed by astronomy clubs. It will then use this data to assess the needs of the amateur astronomy community for the next five years.
NASA’s Night Sky Network is a community of more than 400 astronomy clubs across the U.S. that share their time and telescopes with the public. They have held nearly 30,000 events and have inspired over 3 million members.
The survey, which will run until the end of September, is fairly straightforward. It asks questions about your local astronomy club, any astronomy activities you participate in, and any challenges you face in outreach.
|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.
|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.
|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.
|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
|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.