Stellar News Feed Archive
|Subaru Telescope Observers Superflare Stars with Large Starspots||Wednesday, May 13, 2015 - 10:07||
A team of astronomers has used the High Dispersion Spectrograph on the Subaru Telescope to conduct spectroscopic observations of Sun-like "superflare" stars first observed and cataloged by the Kepler Space Telescope. The investigations focused on the detailed properties of these stars and confirmed that Sun-like stars with large starspots can experience superflares.
The team targeted a set of solar-type stars emitting very large flares that release total energies 10-10000 times greater than the biggest solar flares.
This work follows up on observations made in 2012 (Maehara et al. Nature on 2012 May 24), where the team reported finding several hundred superflares on solar-type stars by analyzing stellar observation data from Kepler Space Telescope. This discovery was very important since it enabled the astronomers to conduct statistical analysis of superflares for the first time. However, more detailed observations were needed to investigate detailed properties of superflare stars and whether such massive flares can occur on ordinary single stars similar to our Sun.
|Delta Cephei's hidden companion||Tuesday, May 12, 2015 - 13:07||
Delta Cephei, prototype of the cepheids, which has given its name to all similar variable stars, was discovered 230 years ago by the English astronomer John Goodricke. Since the early 20th century, scientists have been interested in measuring cosmic distances using a relationship between these stars' periods of pulsation and their luminosities (intrinsic brightness), discovered by the American Henrietta Leavitt. Today, researchers from the Astronomical Observatory of UNIGE, Johns Hopkins University and the ESA show that Delta Cephei is, in fact, a double star, made up of a cepheid-type variable star and a companion that had thus far escaped detection, probably because of its low luminosity. Yet, pairs of stars, called binaries, complicate the calibration of the period-luminosity relationship, and can bias the measurement of distances. This is a surprising discovery, since Delta Cephei is one of the most studied stars, of which scientists thought they knew almost everything.
|ALMA Discovers Proto Super Star Cluster -- a Cosmic 'Dinosaur Egg' About to Hatch||Thursday, May 7, 2015 - 12:59||
Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have discovered what may be the first known example of a globular cluster about to be born: an incredibly massive, extremely dense, yet star-free cloud of molecular gas.
“We may be witnessing one of the most ancient and extreme modes of star formation in the universe,” said Kelsey Johnson, an astronomer at the University of Virginia in Charlottesville and lead author on a paper accepted for publication in the Astrophysical Journal. “This remarkable object looks like it was plucked straight out of the very early universe. To discover something that has all the characteristics of a globular cluster, yet has not begun making stars, is like finding a dinosaur egg that’s about to hatch.”
|PHL 1445: An eclipsing cataclysmic variable with a substellar donor near the period minimum||Friday, May 1, 2015 - 14:06||
We present high-speed, three-colour photometry of the eclipsing dwarf nova PHL 1445, which, with an orbital period of 76.3 min, lies just below the period minimum of ~82 min for cataclysmic variable stars. Averaging four eclipses reveals resolved eclipses of the white dwarf and bright spot. We determined the system parameters by fitting a parameterised eclipse model to the averaged lightcurve. We obtain a mass ratio of q = 0.087 +- 0.006 and inclination i = 85.2 +- 0.9 degrees. The primary and donor masses were found to be Mw = 0.73 +- 0.03 Msun and Md = 0.064 +- 0.005 Msun, respectively. Through multicolour photometry a temperature of the white dwarf of Tw = 13200 +- 700 K and a distance of 220 +- 50 pc were determined. The evolutionary state of PHL 1445 is uncertain. We are able to rule out a significantly evolved donor, but not one that is slightly evolved. Formation with a brown dwarf donor is plausible; though the brown dwarf would need to be no older than 600 Myrs at the start of mass transfer, requiring an extremely low mass ratio (q = 0.025) progenitor system. PHL 1445 joins SDSS 1433 as a sub-period minimum CV with a substellar donor. These existence of two such systems raises an alternative possibility; that current estimates for the intrinsic scatter and/or position of the period minimum may be in error.
Authors: M. J. McAllister, S. P. Littlefair, I. Baraffe et al
|Strange Supernova is "Missing Link" in Gamma-Ray Burst Connection||Monday, April 27, 2015 - 14:19||
"This is a striking result that provides a key insight about the mechanism underlying these explosions," said Sayan Chakraborti, of the Harvard-Smithsonian Center for Astrophysics (CfA). "This object fills in a gap between GRBs and other supernovae of this type, showing us that a wide range of activity is possible in such blasts," he added.
|The Lives of the Longest Lived Stars||Tuesday, April 21, 2015 - 13:22||
Heavy stars live like rock stars: they live fast, become big, and die young. Low mass stars, on the other hand, are more persistent, and live longer. The ages of the former stars are measured in millions to billions of years; the expected lifetimes of the latter are measured in trillions. Low mass stars are the turtle that beats the hare.
But why do we want to study the evolution of low mass stars, and their less than imminent demise? There are various good reasons. First, galaxies are composed of stars —and other things, but here we focus on the stars. Second, low-mass stars are by far the most numerous stars in the galaxy, about 70% of stars in the Milky Way are less than 0.3 solar masses (also denoted as 0.3M☉). Third, low-mass stars provide useful insights into stellar evolution: if you want to understand why heavier mass stars evolve in a certain way —e.g. develop into red giants— it is helpful to take a careful look at why the lowest mass stars do not.
|Discovering the mysterious companions of Cepheids||Wednesday, April 15, 2015 - 09:34||
Though the physics behind Cepheid variability is well-understood, we still have significant difficulties to overcome in order to improve the zero-point of Leavitt’s law. Cepheids are supergiants. They are stars several times the mass of our Sun that have evolved off of the main sequence of the stellar color-magnitude diagram (in other words they’re in the stellar ‘afterlife’). Because bigger stars burn their fuel faster than smaller stars, Cepheids are also young stars. Thus they are often found in the dusty regions of galaxies so we have to deal with absorption, reddening, and dust scattering when we observe them. The period-luminosity relationship may also have a dependence on metallicity (the fraction of atoms in the star that are heavier than helium) that we still don’t fully understand.
Another common problem that we face when using Cepheids—and the focus of today’s paper!—is the presence of a binary companion. In fact, more than 50% of Galactic Cepheids are expected to have at least one companion. The number of Cepheids with binary companions is so high that we can’t deal with them by simply throwing out the ones that have companions. Separating the luminosity of the Cepheid from its companion is important if we want to use the period luminosity relationship.
|Accelerating Universe? Not So Fast||Monday, April 13, 2015 - 15:16||
A University of Arizona-led team of astronomers found that the type of supernovae commonly used to measure distances in the universe fall into distinct populations not recognized before. The findings have implications for our understanding of how fast the universe has been expanding since the Big Bang.
"To be clear, this research does not suggest that there is no acceleration," Milne said, "just that there might be less of it."
"We're proposing that our data suggest there might be less dark energy than textbook knowledge, but we can't put a number on it," he added. "Until our paper, the two populations of supernovae were treated as the same population. To get that final answer, you need to do all that work again, separately for the red and for the blue population."
|Super-bright Supernovae are Single-Degenerate?||Wednesday, April 8, 2015 - 09:42||
Type Ia supernovae (SNe) are often the archetype of an astronomical standardizable candle — something that has a known luminosity which we can use to measure its distance. Scientists famously used type Ia SNe to discover that our universe is accelerating and won the Nobel prize in 2011. However, one of astronomy’s dirtiest secrets is that we don’t know exactly how type Ia SNe materialize or why they might even be standard candles.
Supernovae are the explosive deaths of stars. They have a range of spectral types and energies that depend on the nature of the explosion and the progenitor stars. Type Ia SNe detonate in one of two ways: via the single degenerate or double degenerate model. In the single degenerate model, a white dwarf orbits a massive main-sequence star and eats aways at its partner’s outer layers. The white dwarf gains mass and eventually tips over the Chandrasekhar limit and collapses on itself and explodes. In the double-degenerate model, a binary system of two white dwarfs loses energy due to gravitational waves and the white dwarfs eventually collide.
|Suzaku Studies Supernova 'Crime Scene,' Shows a Single White Dwarf to Blame||Friday, April 3, 2015 - 09:06||
Until recently, astronomers thought the most likely way for a white dwarf to gain mass would be as a member of a close binary system with a normal sun-like star. By accumulating matter from its companion, the white dwarf can, over millions of years, nudge itself closer to the limit and explode. The companion stars are expected to survive, but astronomers find scant evidence for them, suggesting the need for an alternative model. In the merger scenario, the blast is triggered by a pair of lower-mass white dwarfs, whose orbits tighten over time until they eventually merge and explode.
"We can distinguish which of these scenarios is responsible for a given supernova remnant by tallying the nickel and manganese in the expanding cloud," said Goddard astrophysicist Brian Williams. "An explosion from a single white dwarf near its mass limit will produce significantly different amounts of these elements than a merger."
|Astronomers solve decades-long mystery of the "lonely old stars"||Thursday, April 2, 2015 - 08:41||
An overwhelming majority of the known members of a very important family of stars, known to astronomers as RR Lyrae variables, have for long appeared to live their lives all alone. These stars, being among the oldest known in the cosmos, contain precious information about the origin and evolution of the stellar systems that harbour them, such as the Milky Way itself. However, the lack of RR Lyrae stars in binary systems has made a direct assessment of some of their key properties difficult. Most often, theory had to be invoked to fill the gap.
Authors: G. Hajdu, M. Catelan1, J. Jurcsik, I. D´ek´any, A. J. Drake and J.-B. Marquette
|HST Images Flash Ionization of Old Ejecta by the 2011 Eruption of Recurrent Nova T Pyxidis||Wednesday, April 1, 2015 - 10:42||
T Pyxidis is the only recurrent nova surrounded by knots of material ejected in previous outbursts. Following the eruption that began on 2011 April 14.29, we obtained seven epochs (from 4 to 383 days after eruption) of Hubble Space Telescope narrowband Ha images of T Pyx . The flash of radiation from the nova event had no effect on the ejecta until at least 55 days after the eruption began. Photoionization of hydrogen located north and south of the central star was seen 132 days after the beginning of the eruption. That hydrogen recombined in the following 51 days, allowing us to determine a hydrogen atom density of at least 7e5 cm^-3 - at least an order of magnitude denser than the previously detected, unresolved [NII] knots surrounding T Pyx. Material to the northwest and southeast was photoionized between 132 and 183 days after the eruption began. 99 days later that hydrogen had recombined. Both then (282 days after outburst) and 101 days later, we detected almost no trace of hydrogen emission around T Pyx. There is a large reservoir of previously unseen, cold diffuse hydrogen overlapping the previously detected, [NII] - emitting knots of T Pyx ejecta. The mass of this newly detected hydrogen is probably an order of magnitude larger than that of the [NII] knots. We also determine that there is no significant reservoir of undetected ejecta from the outer boundaries of the previously detected ejecta out to about twice that distance, near the plane of the sky. The lack of distant ejecta is consistent with the Schaefer et al (2010) scenario for T Pyx, in which the star underwent its first eruption within five years of 1866 after many millennia of quiescence, followed by the six observed recurrent nova eruptions since 1890. This lack of distant ejecta is not consistent with scenarios in which T Pyx has been erupting continuously as a recurrent nova for many centuries or millennia.
Authors: Michael M. Shara, David Zurek, Bradley E. Schaefer, Howard E. Bond, Patrick Godon, Mordecai-Mark Mac Low, Ashley Pagnotta, Dina Prialnik, Edward M. Sion, Jayashree Toraskar, Robert E. Williams
|Resolving the stellar activity of the Mira AB binary with ALMA||Saturday, March 28, 2015 - 09:44||
We present the size, shape and flux densities at millimeter continuum wavelengths, based on ALMA science verification observations in Band 3 (~94.6 GHz) and Band 6 (~228.7 GHz), from the binary Mira A (o Ceti) and Mira B. The Mira AB system has been observed with ALMA at a spatial resolution of down to ~25 mas. The extended atmosphere of Mira A and the wind around Mira B sources are resolved and we derive the size of Mira A and of the ionized region around Mira B. The spectral indices within Band 3 (between 89-100 GHz) and between Band 3 and Band 6 are also derived. The spectral index of Mira A is found to change from 1.71+-0.05 within Band 3 to 1.54+-0.04 between Band 3 and 6. The spectral index of Mira B is 1.3+-0.2 in Band 3, in good agreement with measurements at longer wavelengths. However it rises to 1.72+-0.11 between the bands. For the first time the extended atmosphere of a star is resolved at these frequencies and for Mira A the diameter is ~3.7x2.9 AU in Band 3 (with brightness temperature Tb~6000 K) and ~4.0x3.6 AU in Band 6 (Tb~2500 K). Additionally, a bright hotspot of ~0.4 AU and with Tb~10000 K is found on the stellar disc of Mira A. The size of the ionized region around the accretion disk of Mira B is found to be ~2.4 AU. The emission around Mira B is consistent with that from a partially ionized wind of gravitationally bound material from Mira A close to the accretion disk of Mira B. The Mira A atmosphere does not fully match predictions, with brightness temperatures in Band 3 significantly higher than expected, potentially due to shock heating. The hotspot is likely due to magnetic activity and could be related to the previously observed X-ray flare of Mira A.
Authors: W.H.T. Vlemmings, S. Ramstedt, E. O'Gorman, E.M.L. Humphreys, M. Wittkowski, A. Baudry, M. Karovska
|This International Year of Light, Switch Off the Lights for the Planet||Friday, March 27, 2015 - 09:03||
This International Year of Light, as we celebrate the role of light and light-based technology in sustainable development, let us also pledge to take action for a sustainable future with the flick of a light switch. Celebrate your commitment to our planet by switching off the lights on Saturday 28 March at 8:30 PM local time.
|Colliding Stars Explain Enigmatic Seventeenth Century Explosion||Monday, March 23, 2015 - 12:11||
New observations made with APEX and other telescopes reveal that the star that European astronomers saw appear in the sky in 1670 was not a nova, but a much rarer, violent breed of stellar collision. It was spectacular enough to be easily seen with the naked eye during its first outburst, but the traces it left were so faint that very careful analysis using submillimetre telescopes was needed before the mystery could finally be unravelled more than 340 years later.
The lead author of the new study, Tomasz Kamiński (ESO and the Max Planck Institute for Radio Astronomy, Bonn, Germany) explains: “For many years this object was thought to be a nova, but the more it was studied the less it looked like an ordinary nova — or indeed any other kind of exploding star.”
The results appear online in the journal Nature on 23 March 2015.
|Spectacular aurora from severe solar storm light up northern skies||Saturday, March 21, 2015 - 08:41||
The St. Patrick’s Day geomagnetic storm, the most intense since fall of 2013, spurred dazzling aurora before and after sunset, and then finally faded.
On Tuesday evening, forecasts indicated a slight chance aurora would be viewable as far south as the Mid-Atlantic, but the farthest south we’ve seen reports were in Illinois, Ohio and northern New Jersey – which is nonetheless quite unusual.
Pictures from the Washington Post article.
Cloud camera YouTube video from Hankasalmi Observatory, compliments of Arto Okasanen.
Image above taken by Marketa Murray on March 17, 2015 @ Dalton HWY , Alaska
|Another deep dimming of the classical T Tauri star RW Aur A||Monday, March 16, 2015 - 09:07||
Context. RW Aur A is a classical T Tauri star (CTTS) with an unusually rich emission line spectrum. In 2014 the star faded by ~ 3 magnitudes in the V band and went into a long-lasting minimum. In 2010 the star suffered from a similar fading, although less deep. These events in RW Aur A are very unusual among the CTTS, and have been attributed to occultations by passing dust clouds. Aims. We want to find out if any spectral changes took place after the last fading of RW Aur A with the intention to gather more information on the occulting body and the cause of the phenomenon. Methods. We collected spectra of the two components of RW Aur. Photometry was made before and during the minimum. Results. The overall spectral signatures reflecting emission from accretion flows from disk to star did not change after the fading. However, blue-shifted absorption components related to the stellar wind had increased in strength in certain resonance lines, and the profiles and strengths, but not fluxes, of forbidden lines had become drastically different. Conclusions. The extinction through the obscuring cloud is grey indicating the presence of large dust grains. At the same time, there are no traces of related absorbing gas. The cloud occults the star and the interior part of the stellar wind, but not the wind/jet further out. The dimming in 2014 was not accompanied by changes in the accretion flows at the stellar surface. There is evidence that the structure and velocity pattern of the stellar wind did change significantly. The dimmings could be related to passing condensations in a tidally disrupted disk, as proposed earlier, but we also speculate that large dust grains have been stirred up from the inclined disk into the line-of-sight through the interaction with an enhanced wind.
Authors: P. P. Petrov, G. F. Gahm, A. A. Djupvik, E. V. Babina, S. A. Artemenko, K. N. Grankin
|Revealing δ Cephei's Secret Companion and Intriguing Past||Monday, March 16, 2015 - 08:27||
Classical Cepheid variable stars are crucial calibrators of the cosmic distance scale thanks to a relation between their pulsation periods and luminosities. Their archetype, δ Cephei, is an important calibrator for this relation. In this paper, we show that δ Cephei is a spectroscopic binary based on newly-obtained highprecision radial velocities. We combine these new data with literature data to determine the orbit, which has period 2201 days, semi-amplitude 1.5 km s−1 , and high eccentricity (e = 0.647). We re-analyze Hipparcos intermediate astrometric data to measure δ Cephei’s parallax ($ = 4.09 ± 0.16 mas) and find tentative evidence for an orbital signature, although we cannot claim detection. We estimate that Gaia will fully determine the astrometric orbit. Using the available information from spectroscopy, velocimetry, astrometry, and Geneva stellar evolution models (MδCep ∼ 5.0 − 5.25 M ), we constrain the companion mass to within 0.2 < M2 < 1.2 M . We discuss the potential of ongoing and previous interactions between the companion and δ Cephei near pericenter passage, informing reported observations of circumstellar material and bow-shock. The orbit may have undergone significant changes due to a Kozai-Lidov mechanism driven by the outer (visual and astrometric) companion HD 213307. Our discovery of δ Cephei’s nature as a spectroscopic binary exposes a hidden companion and reveals a rich and dynamical history of the archetype of classical Cepheid variables.
Authors: Richard I. Anderson, Johannes Sahlmann, Berry Holl, Laurent Eyer, Lovro Palaversa, Nami Mowlavi, Maria Süveges, Maroussia Roelens
|How old is the Hyades?||Monday, March 9, 2015 - 10:40||
The Hyades cluster forms the head of Taurus the bull in the zodiac constellation. It is one of the most famous open clusters—a group of stars that all formed at the same time from the same cloud of gas. This cluster was thought to be 625 million years old, however new research suggests that the Hyades is much older. This makes for a slightly awkward situation; the Hyades underpins our understanding of stellar ages. If its age is wrong then a lot of other ages are wrong too.
This conflict may be resolved soon—the Kepler spacecraft (now reincarnated as K2) is currently observing the Hyades. It will be able to detect asteroseismic oscillations in some of its stars, revealing their true ages. Hundreds of inferences rely on the age of this cluster—unveiling the mystery will be an exciting moment for stellar astronomy!
Read the story on Astrobites
|A young star takes centre stage||Wednesday, March 4, 2015 - 09:44||
What makes V1331Cyg special is the fact that we look almost exactly at one of its poles. Usually, the view of a young star is obscured by the dust from the circumstellar disc and the envelope that surround it. However, with V1331Cyg we are actually looking in the exact direction of a jet driven by the star that is clearing the dust and giving us this magnificent view.
Image credit: ESA/Hubble, NASA, Karl Stapelfeldt (GSFC), B. Stecklum and A. Choudhary (Thüringer Landessternwarte Tautenburg, Germany)
Read the full press release from ESA Hubble Images
|Fourteen new eclipsing white dwarf plus main-sequence binaries from the SDSS and Catalina surveys||Monday, February 23, 2015 - 07:51||
We report on the search for new eclipsing white dwarf plus main-sequence (WDMS) binaries in the light curves of the Catalina surveys. We use a colour selected list of almost 2000 candidate WDMS systems from the Sloan Digital Sky Survey, specifically designed to identify WDMS systems with cool white dwarfs and/or early M type main-sequence stars. We identify a total of 17 eclipsing systems, 14 of which are new discoveries. We also find 3 candidate eclipsing systems, 2 main-sequence eclipsing binaries and 22 non-eclipsing close binaries. Our newly discovered systems generally have optical fluxes dominated by the main-sequence components, which have earlier spectral types than the majority of previously discovered eclipsing systems. We find a large number of ellipsoidally variable binaries with similar periods, near 4 hours, and spectral types M2--3, which are very close to Roche-lobe filling. We also find that the fraction of eclipsing systems is lower than found in previous studies and likely reflects a lower close binary fraction among WDMS binaries with early M-type main-sequence stars due to their enhanced angular momentum loss compared to fully convective late M type stars, hence causing them to become cataclysmic variables quicker and disappear from the WDMS sample. Our systems bring the total number of known detached, eclipsing WDMS binaries to 71.
Authors: S. G. Parsons, C. Agurto-Gangas, B. T. Gaensicke, A. Rebassa-Mansergas, M. R. Schreiber, T. R. Marsh, V. S. Dhillon, S. P. Littlefair, A. J. Drake, M. C. P. Bours, E. Breedt, C. M. Copperwheat, L. K. Hardy, C. Buisset, P. Prasit, J. J. Ren
|Every interacting double white dwarf binary may merge||Friday, February 20, 2015 - 10:45||
Interacting double white dwarf binaries can give rise to a wide variety of astrophysical outcomes ranging from faint thermonuclear and Type Ia supernovae to the formation of neutron stars and stably accreting AM Canum Venaticorum systems. One key factor affecting the final outcome is whether mass transfer remains dynamically stable or instead diverges, leading to the tidal disruption of the donor and the merger of the binary. It is typically thought that for low ratios of the donor mass to the accretor mass, mass transfer remains stable, especially if accretion occurs via a disk. In this Letter, we examine disk-accreting binaries with extremely low mass ratios and find that the initial phase of hydrogen-rich mass transfer leads to a classical nova-like outburst on the accretor. Dynamical friction within the expanding nova shell shrinks the orbit and causes the mass transfer rate to increase dramatically above the accretor's Eddington limit, resulting in a binary merger. While further calculations are necessary to confirm this outcome for the entire range of binaries previously thought to be dynamically stable, it appears likely that most, if not all, interacting double white dwarf binaries will merge during the course of their evolution.
Author: Ken J. Shen
|'Golden stars' pulsate in a strange, non-chaotic way||Tuesday, February 17, 2015 - 14:22||
This star – called KIC 5520878 – is a type of periodic variable star known as an "RRc Lyrae" variable. It pulsates at a large number of frequencies that are all related to two frequencies – f1 and f2 – that have a golden ratio. The golden ratio or "golden mean" is an irrational number that has significance in geometry, biology and art. Its presence in a dynamical system can mean that the system behaves as a "strange non-chaotic attractor". In this case, "strange" means that the system can be characterized as fractal, and "non-chaotic" means that the dynamics falls in the middle ground between order and chaos.
Read the article in physicsworld.com News
|Stars akin to the sun also explode when they die||Tuesday, February 17, 2015 - 12:10||
IRAS 15103-5754 stands out because it has been observed that the velocity of the material inside the jet increases in proportion to the distance from the central star. "Water molecules are generally destroyed soon after the planetary nebula is formed, and in the rare cases where a maser emission has been detected, the velocity has always been very low”, says Luis F. Miranda (IAA-CSIC, University of Vigo). “In IRAS 15103-5754 we are seeing for the first time a water maser emission at velocities of hundreds of kilometers per second. We are witnessing the transition of a star into a planetary nebula in real time".
“The high velocity can only be explained by the occurrence of an explosion”.
Read the full press release from the Institute of Astrophysics of Andalusia (IAA-CSIC)
|Astronomers Catch Multiple-Star System in First Stages of Formation||Friday, February 13, 2015 - 22:48||
For the first time, astronomers have caught a multiple-star system in the beginning stages of its formation, and their direct observations of this process give strong support to one of several suggested pathways to producing such systems.
When the research team used the VLA to map radio emission from methane molecules, they discovered that filaments of gas in B5 are fragmenting, and the fragments are beginning to form into additional stars that will become a multiple-star system.
Read the full story at NRAO News