Stellar News Feed Archive
|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
|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)
|"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."
|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.
|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
|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.”
|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
|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.
|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.
|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.
|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
|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.
|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.
|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
|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.
|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.
|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.