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Stellar News Feed Archive

Progenitors of supernova Ibc: a single Wolf-Rayet star as the possible progenitor of the SN Ib iPTF13bvn Friday, August 2, 2013 - 10:03

Core-collapse supernova (SN) explosions mark the end of the tumultuous life of massive stars. Determining the nature of their progenitors is a crucial step towards understanding the properties of SNe. Until recently, no progenitor has been directly detected for SN of type Ibc, which are believed to come from massive stars that lose their Hydrogen envelope through stellar winds and from binary systems where the companion has stripped the H envelope from the primary. Here we analyze recently-reported observations of iPTF13bvn, which could possibly be the first detection of a SN Ib progenitor based on pre-explosion images.

Authors: Jose H. Groh (Geneva Observatory, Switzerland), Cyril Georgy (Keele University, UK), Sylvia Ekstrom (Geneva Observatory, Switzerland)

Read the full abstract on arXiv

Under leaden skies - where heavy metal clouds the stars Thursday, August 1, 2013 - 09:01

The team believes that these heavy-metal stars are a crucial link between bright red giants, stars thirty or forty times the size of the Sun, and faint blue subdwarfs, stars one fifth the size, but seven times hotter and seventy times brighter than the Sun. A few red giants lose their thick hydrogen skin and shrink to become hot subdwarfs, or nearly-naked helium stars. As they shrink, conditions become favourable for the pressure of light from the helium stars to act on individual atoms to sort the elements into separate layers, where they are concentrated by a factor of ten thousand or more.

Read the full story from the RAS website

Spitzer Discovers Young Stars with a 'Hula Hoop' Wednesday, July 31, 2013 - 22:55

As the two inner stars whirl around each other, they periodically peek out from the disk that girds them like a hula hoop. The hoop itself appears to be misaligned from the central star pair, probably due to the disrupting gravitational presence of the third star orbiting at the periphery of the system. The whole system cycles through bright and faint phases, with the central stars playing a sort of cosmic peek-a-boo as the tilted disk twirls around them. It is believed that this disk should go on to spawn planets and the other celestial bodies that make up a solar system. 

Spitzer observed infrared light from YLW 16A, emitted by the warmed gas and dust in the disk that still swathes the young stars. Other observations came from the ground-based 2MASS survey, as well as from the NACO instrument at the European Southern Observatory's Very Large Telescope in Chile. 

Read the story from JPL

Powering the Second 2012 Outburst of SN 2009ip by Repeating Binary Interaction Tuesday, July 30, 2013 - 09:02

We propose that the major 2012 outburst of the supernova impostor SN 2009ip was powered by an extended and repeated interaction between the Luminous Blue Variable (LBV) and a more compact companion. Motivated by the recent analysis of Margutti et al. (2013) of ejected clumps and shells we consider two scenarios. In both scenarios the major 2012b outburst of ~5 * 10^{49} erg was powered by accretion of ~ 2-5 solar masses onto the companion during a periastron passage (the first passage) of the binary system approximately 20 days before the observed maximum of the light curve. In the first, the surviving companion scenario, the companion was not destructed and still exists in the system after the outburst. It ejected partial shells (or collimated outflows or clumps) for two consecutive periastron passages after the major one. The orbital period was reduced from ~38 days to ~25 days as a result of the mass transfer process that took place during the first periastron passage. In the second, the merger scenario, some partial shells/clumps were ejected also in a second periastron passage that took place ~20 days after the first one. After this second periastron passage the companion dived too deep into the LBV envelope to launch more outflows, and merged with the LBV.

Authors: Amit Kashi (UNLV), Noam Soker (Technion), Nitsan Moskovitz (Technion)

Download the paper from arXiv

Dynamical Fragmentation of the T Pyxidis Nova Shell During Recurrent Eruptions Tuesday, July 30, 2013 - 07:12

Hubble Space Telescope images of the ejecta surrounding the nova T Pyxidis resolve the emission into more than two thousand bright knots. We simulate the dynamical evolution of the ejecta from T Pyxidis during its multiple eruptions over the last 150 years using the adaptive mesh refinement capability of the gas dynamics code Ramses. We demonstrate that the observed knots are the result of Richtmeyer-Meshkov gas dynamical instabilities (the equivalent of Rayleigh-Taylor instabilities in an accelerated medium). These instabilities are caused by the overrunning of the ejecta from the classical nova of 1866 by fast moving ejecta from the subsequent six recurrent nova outbursts. The model correctly predicts the observed expansion and dimming of the T Pyx ejecta as well as the knotty morphology. The model also predicts that deeper, high resolution imagery will show filamentary structure connecting the knots. We show reprocessed Hubble Space Telescope imagery that shows the first hints of such structure.

Authors: Jayashree Toraskr, Mordecai-Mark Mac Low, Michael M. Shara, David R. Zurek

Download the paper from arXiv

Binary Cepheids: Separations and Mass Ratios in 5 Solar Mass Binaries Tuesday, July 30, 2013 - 07:00

Deriving the distribution of binary parameters for a particular class of stars over the full range of orbital separations usually requires the combination of results from many different observing techniques (radial velocities, interferometry, astrometry, photometry, direct imaging), each with selection biases. However, Cepheids---cool, evolved stars of 5 Mdot---are a special case because ultraviolet spectra will immediately reveal any companion star hotter than early type A, regardless of the orbital separation. We have used  International Ultraviolet Explorer (IUE) UV spectra of a complete sample of all 76 Cepheids brighter than V=8 to create a list of all 18 Cepheids with companions more massive than 2.0 Mdot. Orbital periods of many of these binaries are available from radial-velocity studies, or can be estimated for longer-period systems from detected velocity variability. In an imaging survey with the Hubble Space Telescope Wide Field Camera 3, we resolved three of the companions (those of eta Aql, S Nor, and V659 Cen), allowing us to make estimates of the periods out to the long-period end of the distribution. Combining these separations with orbital data in the literature, we derive an unbiased distribution of binary separations, orbital periods, and mass ratios. The distribution of orbital periods shows that the 5 Mdot binaries have systematically shorter periods than do 1 Mdot stars. Our data also suggest that the distribution of mass ratios depends both on binary separation and system multiplicity. The distribution of mass ratios as a function of orbital separation, however, does not depend on whether a system is a binary or a triple.

Authors: Nancy Remage Evans (SAO), Howard E. Bond (PSU, STScI), Gail H. Schaefer (The CHARA Array, GSU), Brian D. Mason (USNO), Margarita Karovska (SAO), Evan Tingle (SAO)

Download the paper from arXiv

"The Dark Cloud"--Emergence of a Monster Star: Largest Ever Observed in the Milky Way Monday, July 29, 2013 - 14:01

“The remarkable observations from ALMA allowed us to get the first really in-depth look at what was going on within this cloud,” says Nicolas Peretto, of Cardiff University and CEA/AIM Parsis-Saclay. “We wanted to see how monster stars form and grow, and we certainly achieved our aim! One of the sources we have found is an absolute giant — the largest protostellar core ever spotted in the Milky Way."

Read the entire article at The Daily Galaxy

Third Bright Supernova Discovered In Spiral Galaxy M74 Sunday, July 28, 2013 - 09:31

I love this galaxy. Not only does M74 display a near perfect spiral form but if this latest supernova is the third to “go boom” in the galaxy in just 11 years. The new object, designated PSN J01364816+1545310, was discovered blazing near 12.4 magnitude by the Lick Observatory Supernova Search at Lick Observatory near San Jose, Calif. “PSN” stands for “possible supernova” and the long string of numbers give the object’s position in the sky using the celestial equivalents of latitude and longitude.

Read the full article by AAVSO member Bob King at Universe Today

Kepler observations of the eclipsing cataclysmic variable KIS J192748.53+444724.5 Saturday, July 27, 2013 - 10:06

We present results from long cadence Kepler observations covering 97.6 days of the newly discovered eclipsing cataclysmic variable KIS J192748.53+444724.5/KIC 8625249. We detect deep eclipses of the accretion disk by the donor star every 3.97 hours. Additionally, the Kepler observations also cover a full outburst for this cataclysmic variable, making KIS J192748.53+444724.5 the second known eclipsing cataclysmic variable system in the Kepler field of view. We show how in quiescence a significant component associated to the hot-spot is visible preceding the eclipse, and that this component is swamped by the brightness increase during the outburst, potentially associated with the accretion disk. Furthermore we present evidence for accretion disk radius changes during the outburst by analysing the out-of-eclipse light levels and eclipse depth through each orbital cycle. We show how these parameters are linearly correlated in quiescence, and discuss how their evolution during the outburst is suggesting disk radius changes and/or radial temperature gradient variations in the disk.

Authors: S. Scaringi, P.J. Groot, M. Still

Download the paper from arXiv

New discoveries about quasars Friday, July 26, 2013 - 12:48

Dartmouth astrophysicists Ryan Hickox and Kevin Hainline and colleagues have a paper scheduled for publication in The Astrophysical Journal, detailing discoveries based upon observations of 10 quasars. They documented the immense power of quasar radiation, which reaches out for many thousands of light years to the limits of the quasar’s galaxy.

“For the first time, we are able to see the actual extent to which these quasars and their black holes can affect their galaxies, and we see that it is limited only by the amount of gas in the galaxy,” says Hainline, a Dartmouth postdoctoral research associate. “The radiation excites gas all the way to the margins of the galaxy and stops only when it runs out of gas.”


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WZ Sge-type dwarf novae with multiple rebrightenings: MASTER OT J211258.65+242145.4 and MASTER OT J203749.39+552210.3 Friday, July 26, 2013 - 08:49

From the abstract: We have made a survey of WZ Sge-type dwarf novae with multiple rebrightenings, and confirmed that the superhump periods of WZ Sge-type dwarf novae with multiple rebrightenings were longer than those of WZ Sge-type dwarf novae without a rebrightening. Although WZ Sge-type dwarf novae with multiple rebrightenings have been thought to be the good candidates for period bouncers based on their low mass ratio (q) from inferred from the period of fully grown (stage B) superhumps, our new method using the period of growing superhumps (stage A superhumps), however, implies higher q than those expected from stage B superhumps. These q values appear to be consistent with the duration of the stage A superoutbursts, which likely reflects the growth time of the 3:1 resonance. We present a working hypothesis that the small fractional superhump excesses for stage B superhumps in these systems may be explained as a result that a higher gas pressure effect works in these systems than in ordinary SU UMa-type dwarf novae. This result leads to a new picture that WZ Sge-type dwarf novae with multiple rebrightenings and SU UMa-type dwarf novae without a rebrightening (they are not period bouncers) are located in the same place on the evolutionary track.

Authors: Chikako Nakata, Tomohito Ohshima, Taichi Kato, Daisaku Nogami, Gianluca Masi, Enrique de Miguel, Joseph Ulowetz, Colin Littlefield, William N. Goff, Thomas Krajci, Hiroyuki Maehara, William Stein, Richard Sabo, Ryo Noguchi, Rikako Ono, Miho Kawabata, Hisami Furukawa, Katsura Matsumoto, Takehiro Ishibashi, Pavol A. Dubovsky, Igor Kudzej, Shawn Dvorak, Franz-Josef Hambsch, Roger D. Pickard, Etienne Morelle, Eddy Muyllaert, Stefano Padovan, Arne Henden

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Orbital, Superhump, and Superorbital Periods in the Cataclysmic Variables AQ Mensae and IM Eridani Thursday, July 25, 2013 - 09:45

This paper features AAVSO observers as co-authors.

Abstract: We report photometric detections of orbital and superorbital signals, and negative orbital sidebands, in the light curves of the nova-like cataclysmic variables AQ Mensae and IM Eridani. The frequencies of the orbital, superorbital, and sideband signals are 7.0686 (3), 0.263 (3), and 7.332 (3) cycles per day (c/d) in AQ Mensae, and 6.870 (1), 0.354 (7), and 7.226 (1) c/d in IM Eridani. We also find a spectroscopic orbital frequency in IM Eridani of 6.86649 (2) c/d. These observations can be reproduced by invoking an accretion disc that is tilted with respect to the orbital plane. This model works well for X-ray binaries, in which irradiation by a primary neutron star can account for the disc's tilt. A likely tilt mechanism has yet to be identified in CVs, yet the growing collection of observational evidence indicates that the phenomenon of tilt is indeed at work in this class of object. The results presented in this paper bring the number of CVs known to display signals associated with retrograde disc precession to twelve. 

We also find AQ Mensae to be an eclipsing system. The eclipse depths are highly variable, which suggests that the eclipses are grazing. This finding raises the possibility of probing variations in disc tilt by studying systematic variations in the eclipse profile.

Download the paper from arXiv

The Weakest Solar Cycle in 100 Years Wednesday, July 24, 2013 - 16:54

The Sun is acting weird. It typically puts on a pageant of magnetic activity every 11 years for aurora watchers and sungazers alike, but this time it overslept. When it finally woke up (a year late), it gave the weakest performance in 100 years.

Read the full story at Sky & Telescope

Starburst to Star Bust: ALMA Sheds Light on Mystery of Missing Massive Galaxies Wednesday, July 24, 2013 - 13:33

New observations from the ALMA telescope in Chile have given astronomers the best view yet of how vigorous star formation can blast gas out of a galaxy and starve future generations of stars of the fuel they need to form and grow. The dramatic images show enormous outflows of molecular gas ejected by star-forming regions in the nearby Sculptor Galaxy. These new results help to explain the strange paucity of very massive galaxies in the Universe. The study is published in the journal Nature on 25 July 2013.

Read the press release

Variability of the Spin Period of the White Dwarf in the Intermediate Polar V405 Aur Wednesday, July 24, 2013 - 10:35

We present the results of photometric CCD observations of the magnetic cataclysmic variable V405 Aurigae obtained using different instruments. We analysed variability of the spin period of the white dwarf in the V405 Aur system using our observations and previously published maxima timings. The spin period of the system in 2010-2012 is P=545.4558163(94)s. As we have gaps in observational data, we present 2 hypotheses of the spin period variability of this system - a cubic ephemeris which may be interpreted by a precession of the magnetic white dwarf or a periodic change with a period of 6.2 years and semi-amplitude of 17.2\pm1.8 sec. The periodic variations may be interpreted by a light-time effect caused by a low-mass star. In this case, the system belongs to a rare class of cataclysmic variables with a third body.

Image credit Mark Garlick

Download the paper from arXiv

Metal Abundances, Radial Velocities and other Physical Characteristics for the RR Lyrae Stars in the Kepler Field Wednesday, July 24, 2013 - 10:08

Spectroscopic iron-to-hydrogen ratios, radial velocities, atmospheric parameters, and new photometric analyses are presented for 41 RR Lyrae stars (and one probable high-amplitude delta Scuti star) located in the field-of-view of the Kepler space telescope. Thirty-seven of the RR Lyrae stars are fundamental-mode pulsators (i.e., RRab stars) of which 16 exhibit the Blazhko effect. Four of the stars are multiperiodic RRc pulsators oscillating primarily in the first-overtone mode.

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Download the paper from arXiv

The discovery of gamma-ray emission from Nova Sco 2012 Wednesday, July 24, 2013 - 10:02

In March 2010 the Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope discovered for the first time >100 MeV gamma-ray emission from a nova within our galaxy, V407 Cyg. The high-energy spectrum and light curve was explained as a consequence of shock acceleration in the nova shell as it interacts with the local ambient medium. It was suspected that the necessary conditions for high-energy emission from novae would be rare. In June 2012 the LAT detected a new flaring source, Fermi J1750-3243, that is spatially coincident and contemporaneous with a new nova, Nova Sco 2012. We report on the exciting discovery of this new 'gamma-ray' nova and present a detailed analysis of its high-energy properties.

Download the paper from arXiv

Supernova Explosions of Super-Asymptotic Giant Branch Stars Wednesday, July 24, 2013 - 09:56

An electron-capture supernova (ECSN) is a core-collapse supernova (CCSN) explosion of a super-asymptotic giant branch (SAGB) star with a main-sequence mass 7-9.5 that of the Sun. The explosion takes place in accordance with core bounce and subsequent neutrino heating and is a unique example successfully produced by first-principle simulations. This allows us to derive a first self-consistent multicolor light curves of a CCSN.  

Image credit: Argonne National Laboratory via Flickr (Creative Commons license); Image courtesy Hongfeng Yu

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Download the paper from arXiv

Hubble Shows Link Between Stars' Ages and Their Orbits in Dense Cluster Saturday, July 20, 2013 - 08:32

Astronomers using NASA's Hubble Space Telescope have for the first time linked two distinct populations of stars in an ancient globular star cluster to their unique orbital dynamics, offering proof that the stars do not share the same birth date.

The analysis of the globular cluster 47 Tucanae shows that the two populations differ in age by less than 100 million years. The cluster resides roughly 16,700 light-years away in the southern constellation Tucana.

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Pulsating stars in NGC 6231 Saturday, July 20, 2013 - 08:09

We identified 32 variable stars in the field of the cluster out of which 21 are confirmed members and twelve are newly detected variable stars. Ten stars were classified as Slowly Pulsating B (SPB) stars in NGC 6231 out of which seven are new discoveries. We also analyzed six previously reported beta Cephei variables in more detail. One of them may be a hybrid beta Cephei/SPB pulsator.

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Download the paper from arXiv

Astronomers Detect "Snow Line" Around TW Hydrae Saturday, July 20, 2013 - 07:15

ALMA spotted a never-before-seen CO snow line around TW Hydrae, a young star 175 light-years away from Earth. Astronomers believe this nascent solar system has many of the same characteristics that our own Solar System had when it was just a few million years old.

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