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Stellar News Feed Archive

On the 2015 outburst of the EXor variable star V1118 Ori Friday, February 5, 2016 - 11:44

After a long-lasting period of quiescence of about a decade, the source V1118 Ori, one of the most representative members of the EXor variables, is now outbursting. Since the initial increase of the near-infrared flux of about 1 mag (JHK bands) registered on 2015 September 22, the source brightness has remained fairly stable. We estimate DeltaV about 3 mag with respect to the quiescence phase. An optical/near-IR low-resolution spectrum has been obtained with the Large Binocular Telescope instruments MODS and LUCI2, and compared with a spectrum of similar spectral resolution and sensitivity level taken during quiescence. Together with the enhancement of the continuum, the outburst spectrum presents a definitely higher number of emission lines, in particular HI recombination lines of the Balmer, Paschen, and Brackett series, along with bright permitted lines of several species, forbidden atomic lines, and CO ro-vibrational lines. Both mass accretion and mass loss rates have significantly increased (by to about an order of magnitude, mass accretion rate = 1.2-4.8 10^-8 M_sun/yr, mass loss rate = 0.8-2 10^-9 M_sun/yr) with respect to the quiescence phase. If compared with previous outbursts, the present one appears less energetic. Alternatively, it could already be in the fading phase (with the maximum brightness level reached when the source was not visible), or, viceversa, still in the rising phase.

Authors: T.Giannini, D.Lorenzetti, S.Antoniucci, A. A.Arkharov, V.M.Larionov, A.Di Paola, S.Bisogni, A.Marchetti

Read the paper on arXiv

 

A new insight into the V1184 Tau variability Friday, February 5, 2016 - 11:36

V1184 Tau is a young variable for long time monitored at optical wavelengths. Its variability has been ascribed to a sudden and repetitive increase of the circumstellar extinction (UXor-type variable), but the physical origin of such variation, although hypothesized, has not been fully supported on observational basis. To get a new insight into the variability of V1184 Tau, we present new photometric and spectroscopic observations taken in the period 2008-2015. During these years the source has reached the same high brightness level that had before the remarkable fading of about 5 mag undergone in 2004. The optical spectrum is the first obtained when the continuum is at its maximum level. The observations are interpreted in the framework of extinction driven variability. We analyze light curves, optical and near-infrared colors, SED and optical spectrum. The emerging picture indicates that the source fading is due to an extinction increase of DeltaA_V about 5 mag, associated with a strong infrared excess, attributable to a thermal component at T=1000 K. From the flux of H(alpha) we derive a mass accretion rate between 10^-11 -5 10^-10 M_sun yr^-1 s, consistent with that of classical T Tauri stars of similar mass. The source SED was fitted for both the high and low level of brightness. A scenario consistent with the known stellar properties (such as spectral type, mass and radius) is obtained only if the distance to the source is of few hundreds of parsecs, in contrast with the commonly assumed value of 1.5 kpc. Our analysis partially supports that presented by Grinin (2009), according to which the circumstellar disk undergoes a periodical puffing, whose observational effects are both to shield the central star and to evidence a disk wind activity. However, since the mass accretion rate remains almost constant with time, the source is likely not subject to accretion bursts.

Authors: T. Giannini, D. Lorenzetti, A. Harutyunyan, G. Li Causi, S. Antoniucci, A. A. Arkharov, V. M. Larionov, F. Strafella

Read the paper on arXiv

Old Stars’ Fossil Fields Friday, January 22, 2016 - 09:03

Stars create magnetic fields through convection, the swirling, Ferris-wheel-like motion of hot, ionized gas (or boiling water, for that matter). Where convection happens in a star depends on how massive the star is: low-mass stars, including the Sun, have convective outer envelopes around a non-convective core, but stars a little bulkier — up to a couple Suns’ worth — do have convective cores.

Recently, Jim Fuller (Caltech) and colleagues found that strong core magnetic fields could explain the oddly weak, on-and-off brightening behavior of a sample of red giant stars. These stars are low- to middle-mass and have stopped fusing hydrogen in their centers, so they don’t have convective hearts. They also often have a mismatched, variable glow, with one hemisphere brightening as the other fades. What was strange about the sample the team looked at was that this group didn’t vary as much in brightness as it should have.

Read the full story at Sky & Telescope

Read the letter by Stello, Dennis et al. “A prevalence of dynamo-generated magnetic fields in the cores of intermediate-mass stars.” Nature. January 21, 2016. 

 

Record-Shattering Cosmic Blast Could Help Crack the Case of Extreme Supernova Explosions Friday, January 15, 2016 - 09:52

Records are made to be broken, as the expression goes, but rarely are records left so thoroughly in the dust. Stunned astronomers have witnessed a cosmic explosion about 200 times more powerful than a typical supernova—events which already rank amongst the mightiest outbursts in the universe—and more than twice as luminous as the previous record-holding supernova.

The record-breaking blast is thought to be an outstanding example of a "superluminous supernova," a recently discovered, supremely rare variety of explosion unleashed by certain stars when they die. Scientists are frankly at a loss, though, regarding what sorts of stars and stellar scenarios might be responsible for these extreme supernovae.

Read the full press release at Kavli Foundation News

More from OSU News and also at Carnegie Science News

 

One Thousand New Dwarf Novae from the OGLE Survey Wednesday, January 13, 2016 - 12:01

We present one of the largest collections of dwarf novae (DNe) containing 1091 objects that have been discovered in the long-term photometric data from the Optical Gravitational Lensing Experiment (OGLE) survey. They were found in the OGLE fields toward the Galactic bulge and the Magellanic Clouds. We analyze basic photometric properties of all systems and tentatively find a population of DNe from the Galactic bulge. We identify several dozen of WZ Sge-type DN candidates, including two with superhump periods longer than 0.09 d. Other interesting objects include SU UMa-type stars with "early" precursor outbursts or a Z Cam-type star showing outbursts during standstills. We also provide a list of DNe which will be observed during the K2 Campaign 9 microlensing experiment in 2016. Finally, we present the new OGLE-IV real-time data analysis system: CVOM, which has been designed to provide continuous real time photometric monitoring of selected CVs.

Authors: P. Mroz, A. Udalski, R. Poleski, P. Pietrukowicz, M.K. Szymanski, I. Soszynski, L. Wyrzykowski, K. Ulaczyk, S. Kozlowski, J. Skowron

Read the full text on arXiv

 

Multi-Mode and Non-Standard Classical Cepheids in the Magellanic System Tuesday, January 12, 2016 - 09:44

We present a sample of the most interesting classical Cepheids selected from the OGLE collection of classical Cepheids in the Magellanic System. The main selection criterion for this sample was the presence of non-standard, unique pulsational properties. 
The sample contains the first known double-mode Cepheid pulsating in the second- and third-overtone modes and a large number of objects with non-radial modes excited. We also found Cepheids revealing Blazhko-like light curve modulation, objects ceasing pulsations or showing atypical shapes of their light curves. Additionally, the status of several triple mode classical Cepheids is updated based on OGLE-IV photometry extending the time baseline to 15 years.

Authors: I. Soszynski, A. Udalski, M. K. Szymanski, R. Poleski, P. Pietrukowicz, S. Kozlowski, P. Mroz, L. Wyrzykowski, D. Skowron, J. Skowron, G. Pietrzynski, K. Ulaczyk, M. Pawlak

Read the pre-print on arXiv

 

A newly discovered stellar type: dusty post-red giant branch stars in the Magellanic Clouds Monday, January 11, 2016 - 12:13

Context: We present a newly discovered class of low-luminosity, dusty, evolved objects in the Magellanic Clouds. These objects have dust excesses, stellar parameters, and spectral energy distributions similar to those of dusty post-asymptotic giant branch (post-AGB) stars. However, they have lower luminosities and hence lower masses. We suggest that they have evolved off the red giant branch (RGB) instead of the AGB as a result of binary interaction. Aims: In this study we aim to place these objects in an evolutionary context and establish an evolutionary connection between RGB binaries (such as the sequence E variables) and our new sample of objects. Methods: We compared the theoretically predicted birthrates of the progeny of RGB binaries to the observational birthrates of the new sample of objects. Results: We find that there is order-of-magnitude agreement between the observed and predicted birthrates of post-RGB stars. The sources of uncertainty in the birthrates are discussed; the most important sources are probably the observational incompleteness factor and the post-RGB evolution rates. We also note that mergers are relatively common low on the RGB and that stars low on the RGB with mid-IR excesses may recently have undergone a merger. Conclusions: Our sample of dusty post-RGB stars most likely provides the first observational evidence for a newly discovered phase in binary evolution: post-RGB binaries with circumstellar dust.

Authors: D. Kamath, P. R. Wood, H. Van Winckel, J.D. Nie

Read the paper on astro-ph

 

Twisted Magnetic Fields Give New Insights on Star Formation Monday, December 21, 2015 - 15:40

Using new images that show unprecedented detail, scientists have found that material rotating around a very young protostar probably has dragged in and twisted magnetic fields from the larger area surrounding the star. The discovery, made with the National Science Foundation's Karl G. Jansky Very Large Array (VLA) radio telescope, has important implications for how dusty disks -- the raw material for planet formation -- grow around young stars.

"The alignment of magnetic fields in this region near young stars is very important to the development of the disks that orbit them. Depending on its alignment, the magnetic field can either hinder the growth of the disk or help funnel material onto the disk, allowing it to grow," said Leslie Looney, of the University of Illinois at Urbana-Champaign.

Read the full press release

 

CK Vul: a smorgasbord of hydrocarbons rules out a 1670 nova (and much else besides) Wednesday, December 9, 2015 - 11:13

Abstract:  We present observations of CK Vul obtained with the Spitzer Space Telescope. The infrared spectrum reveals a warm dust continuum with nebular, molecular hydrogen and HCN lines superimposed, together with the "Unidentified Infrared" (UIR) features. The nebular lines are consistent with emission by a low density gas. We conclude that the Spitzer data, combined with other information, are incompatible with CK Vul being a classical nova remnant in "hibernation" after the event of 1670, a "Very Late Thermal Pulse", a "Luminous Red Variable" such as V838 Mon, or a "Diffusion-induced nova". The true nature of CK Vul remains a mystery.

Authors: A. Evans (Astrophysics Group, Keele University), R. D. Gehrz, C. E. Woodward (University of Minnesota), P. J. Sarre (University of Nottingham), J. Th. van Loon (Keele University), L. A. Helton (SOFIA/USRA), S. Starrfield (Arizona State University), S. P. S. Eyres (University of Central Lancashire)

Read the pre-print from astro-ph

 

Student discovers stellar chamaeleon had astronomers fooled for years Friday, December 4, 2015 - 11:20

A graduate student has discovered that astronomers have for years been mistakenly interpreting the dust in the environment CW Leo, of the brightest infrared star in the Northern sky. Images released today show none of the previously identified bright spots in fact contain the famous star, which is now believed to be buried in its own dust 450 light years from earth.

CW Leo is one of the most extensively studied evolved stars, believed to be on the cusp of planetary nebula formation – the process ending the red giant’s life is potentially already under way.

Read the press release from The University of Sydney

Also see the press release from the RAS

Download the research paper from the MNRAS

 

The variable V381 Lac and its possible connection with the R CrB phenomenon Thursday, December 3, 2015 - 16:30

We have performed new medium resolution spectroscopy, optical and near infrared photometry to monitor the variability of the AGB carbon star V 381 Lac. Our observations revealed rapid and deep changes in the spectrum and extreme variability in the optical and near infrared bands. Most notably we observed the change of NaI D lines from deep absorption to emission, and the progressive growing of the [N II] doublet 6548-6584 A emission, strongly related to the simultaneous photometric fading. V381 Lac occupies regions of 2MASS and WISE colour-colour diagrams typical of stars with dust formation in the envelope. The general framework emerging from the observations of V381 Lac is that of a cool AGB carbon star undergoing episodes of high mass ejection and severe occultation of the stellar photosphere reminiscent of those characterising the RCB phenomenon.

Comparing the Spectral Energy Distribution obtained with the theoretical model for AGB evolution with dust in the circumstellar envelope, we can identify V381 Lac as the descendant of a star of initial mass ~2M_sun, in the final AGB phases, evolved into a carbon star by repeated Third Dredge Up episodes. According to our model the star is moderately obscured (tau_10 ~0.22) by dust, mainly formed by amorphous carbon (~80%) and SiC (~20%), with dust grain dimensions around ~0.2 micron and 0.08 micron respectively.

Authors: Corinne Rossi, Flavia Dell'Agli, Andrea Di Paola, Kamo S. Gigoyan, Roberto Nesci

Download the pre-print

 

Missing link between turbulence in collapsing star, hypernovae and gamma-ray bursts Monday, November 30, 2015 - 12:01

A supercomputer simulation of a mere 10 milliseconds in the collapse of a massive star into a neutron star proves that these catastrophic events, often called hypernovae, can generate the enormous magnetic fields needed to explode the star and fire off bursts of gamma rays visible halfway across the universe.

The results of the simulation, published online Nov. 30 in advance of publication in the journal Nature, demonstrate that as a rotating star collapses, the star and its attached magnetic field spin faster and faster, forming a dynamo that revs the magnetic field to a million billion times the magnetic field of Earth.

A field this strong is sufficient to focus and accelerate gas along the rotation axis of the star, creating two jets that ultimately can produce oppositely directed blasts of highly energetic gamma rays.

Read the full press release at UC Berkley News

Purchase the article from Nature

 

Aging Star’s Weight Loss Secret Revealed Wednesday, November 25, 2015 - 10:34

The star VY Canis Majoris is a red hypergiant, one of the largest known stars in the Milky Way. It is 30–40 times the mass of the Sun and 300 000 times more luminous. In its current state, the star would encompass the orbit of Jupiter, having expanded tremendously as it enters the final stages of its life.

New observations of the star using the SPHERE instrument on the VLT have clearly revealed how the brilliant light of VY Canis Majoris lights up the clouds of material surrounding it and have allowed the properties of the component dust grains to be determined better than ever before.

In this very close-up view from SPHERE the star itself is hidden behind an obscuring disc. The crosses are artefacts due to features in the instrument.

Read the full press release from ESO

 

Forged in the hearts of stars Thursday, November 19, 2015 - 09:34

Apart from hydrogen, as many have heard from the Carl Sagan and Neil deGrasse Tyson "Cosmos" series, every ingredient in the human body is made from elements forged by stars.

The calcium in our bones, the oxygen we breathe, the iron in our blood — all were forged in the element factories of stars. Even the carbon in our apple pie.

Stars are giant element furnaces. Their intense heat can cause atoms to collide, creating new elements — a process known as nuclear fusion. That process is what created chemical elements like carbon or iron, the building blocks that make up life as we know it.

It sounds pretty simple, but it is a very intricate process. And there are still many uncertainties.

Professors Sumner Starrfield and Frank Timmes, both from Arizona State University, and professor Christian Iliadis, from the University of North Carolina at Chapel Hill, hope to resolve some of those uncertainties.

Read the full press release from ASU

 

Protostar Growth Spurts Thursday, November 5, 2015 - 09:42

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have discovered an adolescent protostar that is undergoing a rapid-fire succession of growth spurts. Evidence for this fitful youth is seen in a pair of intermittent jets streaming away from the star’s poles.

Known as CARMA-7, the protostar is one of dozens of similar objects in the Serpens South star cluster, which is located approximately 1,400 light-years from Earth. This clutch of nascent stellar objects was first detected by and named for the Combined Array for Research in Millimeter-wave Astronomy (CARMA) telescope.

"This young protostar is undergoing periods of rapid growth separated by periods of relative calm,” said Adele Plunkett, previously a National Science Foundation (NSF) graduate research fellow at Yale University and now a fellow at the European Southern Observatory (ESO) in Chile. "This punctuated stellar formation provides important insights into the chaotic interplay within this tightly packed cluster of young stars.”

Read the full story at NRAO News

See also "Growing pains in a cluster of protostars" at Yale News

Mass-loss rate by the Mira in the symbiotic binary V1016 Cygni from Raman scattering Thursday, October 29, 2015 - 10:05

 

 

 

 

The mass-loss rate from Mira variables represents a key parameter in our understanding of their evolutionary tracks. We introduce a method for determining the mass-loss rate from the Mira component in D-type symbiotic binaries via the Raman scattering of atomic hydrogen in the wind from the giant. Using our method, we investigated Raman HeII 1025\AA\ --> 6545\AA\ conversion in the spectrum of the symbiotic Mira V1016 Cyg. We determined its efficiency to be 0.102 and 0.148, and the corresponding mass-loss rate 2.0 (+0.1/-0.2) x 1E-6 and 2.7 (+0.2/-0.1) x 1E-6 M(Sun)/year, using our spectra from 2006 April and 2007 July,respectively. Our values of the mass-loss rate that we derived from Raman scattering are comparable with those obtained independently by other methods. Applying the method to other Mira-white dwarf binary systems can provide a necessary constraint in the calculation of asymptotic giant branch evolution.

Authors: Matej Sekeráš, Augustin Skopal

Read the full text from astro-ph 

 

VISTA Discovers New Component of Milky Way Wednesday, October 28, 2015 - 08:50

“All of the 35 classical Cepheids discovered are less than 100 million years old. The youngest Cepheid may even be only around 25 million years old, although we cannot exclude the possible presence of even younger and brighter Cepheids,” explains the study’s second author Dante Minniti, of the Universidad Andres Bello, Santiago, Chile.

The ages of these classical Cepheids provide solid evidence that there has been a previously unconfirmed, continuous supply of newly formed stars into the central region of the Milky Way over the last 100 million years. But, this wasn’t to be the only remarkable discovery from the survey’s dataset.

Read the full press release from ESO News

 

The Inner Workings of Red Giant Stars Tuesday, October 27, 2015 - 09:19

Sunspot, filaments, plages, and flares — visible signs of the Sun’s magnetic field riddle its atmosphere. But divining magnetic activity inside the Sun is another matter entirely.

We can’t directly see the magnetic fields within stars, and that makes it difficult to connect a star’s visible tortured gas with the inner workings of its magnetic field. Everything from predicting the strength of the next solar cycle to describing stellar aging hinges on better understanding these fields.

Now, astronomers have found a way to indirectly discern magnetic fields, at least within certain types of stars.

Read the story at Sky & Telescope

Read the original press release

Full text research paper from arXiv

 

Final Kiss of Two Stars Heading for Catastrophe Wednesday, October 21, 2015 - 09:16

The double star system VFTS 352 is located about 160 000 light-years away in the Tarantula Nebula [1]. This remarkable region is the most active nursery of new stars in the nearby Universe and new observations from ESO’s VLT [2] have revealed that this pair of young stars is among the most extreme and strangest yet found.

VFTS 352 is composed of two very hot, bright and massive stars that orbit each other in little more than a day. The centres of the stars are separated by just 12 million kilometres [3]. In fact, the stars are so close that their surfaces overlap and a bridge has formed between them. VFTS 352 is not only the most massive known in this tiny class of “overcontact binaries” — it has a combined mass of about 57 times that of the Sun — but it also contains the hottest components — with surface temperatures above 40 000 degrees Celsius.

Read the full story at ESO News

 

First discovery of a magnetic field in a normal delta Scuti star Tuesday, October 20, 2015 - 09:31

Two types of pulsating stars exist among stars with a mass between 1.5 and 2.5 solar masses: the delta Scuti stars and the gamma Dor stars. Theory tells us that, when such stars have a surface temperature between 6900 and 7400 Kelvin, they can have both types of pulsations, i.e. they are called “hybrid stars”. The NASA space mission Kepler provided a wealth of new candidate “hybrid stars”, even outside the theoretically predicted physical parameter range.

Coralie Neiner (LESIA, CNRS / Observatoire de Paris / UPMC / Université Paris Diderot) and Patricia Lampens (Royal Observatory of Belgium) have therefore sought which physical phenomena could mimic the hybrid character in delta Scuti stars. One possibility could be the presence of a magnetic field which would produce spots on the rotating stellar surface and mimic the gamma Dor pulsations. However, no magnetic field had ever been observed in a delta Scuti star until now…

Read the full story at Royal Observatory of Belgium News

 

The Explosions of Stars Wrapped in Hydrogen Monday, October 19, 2015 - 09:45

The authors of today’s paper are interested in shedding light on what attributes of the star lead to the features we see in these types of supernovae, called Type II-L supernovae (where the “L” stands for “Linear”, as in linear decay).

The authors combine two popular programs to generate fake supernova light curves. First, they generate a progenitor star using the a program called MESAstar. Using this simulation, they’re able to change the amount of mass a star loses over its lifetime to beef-up that star’s outer hydrogen envelope. When their stars are ready to explode, they pass them off to another program known as STELLA. STELLA follows packets of atoms to see how they mix and heat during the supernova explosion. STELLA lets astronomers see these interactions at different frequencies, so they can map the results onto filters we can observe, like the V-band. Figure 2 shows some of the light curves the authors get from STELLA and how they compare with real Type II-L supernovae.

Read the full article in Astrobites

 

Shedding light on the growth of stars and black holes Wednesday, October 14, 2015 - 08:30

Christian Knigge, Professor in Physics and Astronomy, worked with colleagues from around the world to study one of the most important, but least understood processes in astronomy – accretion, where the mass of an object grows by gravitationally collecting material from nearby.

The article Accretion-induced variability links young stellar objects, white dwarfs, and black holes has been published in the latest edition of the journal Science Advances.

The paper reveals a close relationship between the way in which different types of accreting objects vary in brightness over time. Their results connect proto-stars resembling our Sun at the time of its birth, to accreting white dwarfs, to supermassive black holes with a billion times the mass of the Sun, located in galaxies millions of light years away.

Read the full press release here.

 

Mysterious Ripples Found Racing Through Planet-forming Disc Wednesday, October 7, 2015 - 12:38

AU Microscopii, or AU Mic for short, is a young, nearby star surrounded by a large disc of dust [1]. Studies of suchdebris discs can provide valuable clues about how planets, which form from these discs, are created.

Astronomers have been searching AU Mic’s disc for any signs of clumpy or warped features, as such signs might give away the location of possible planets. And in 2014 they used the  powerful high-contrast imaging capabilities of ESO’s newly installed SPHERE instrument, mounted on the Very Large Telescope for their search — and discovered something very unusual.

“The images from SPHERE show a set of unexplained features in the disc which have an arch-like, or wave-like, structure, unlike anything that has ever been observed before.”

Read the full story at Hubble Space Telescope News

 

Searching for Orphan Stars Amid Starbirth Fireworks Wednesday, September 30, 2015 - 09:24

Gemini Observatory has released one of the most detailed images ever obtained of emerging gas jets streaming from a region of newborn stars. The region, known as the Herbig-Haro 24 (HH 24) Complex, contains no less than six jets streaming from a small cluster of young stars embedded in a molecular cloud in the direction of the constellation of Orion.

Reipurth along with co-researcher, Colin Aspin, also at the IfA, are using the Gemini North data from the Gemini Multi-Object Spectrograph (GMOS), as well as the Gemini Near-Infrared Imager, to study the region which was discovered in 1963 by George Herbig and Len Kuhi. Located in the Orion B cloud, at a distance of about 400 parsecs, or about 1,300 light-years from our Solar System, this region is rich in young stars and has been extensively studied in all types of light, from radio waves to X-rays.

"The Gemini data are the best ever obtained from the ground of this remarkable jet complex and are showing us striking new detail," says Aspin. Reipurth and Aspin add that they are particularly interested in the fine structure and "excitation distribution" of these jets.

Read the full story at Gemini Observatory News

 

Discovery of an “Eclipse” in the WC9d-Type Wolf-Rayet Star, WR 53 Wednesday, September 30, 2015 - 09:16

AAVSO observer extraordinaire flies the flag for visual observers proudly with yet another independent discovery.

Abstract: The WC9d-type Wolf-Rayet star WR 53 was observed visually entering into an “eclipse” with a depth of 1.2 magnitude. Subsequent visual and CCD data showed a steady linear rise over 10 days to recover and return to its normal brightness level. This is the first-ever recorded “eclipse” of this star which has previously shown no photometric variability. 

Author: Rod Stubbings

Read the pre-print on astro-ph

 

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