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Stellar News Feed Archive

HD 54272, a classical lambda Bootis star and gamma Doradus pulsator Thursday, March 20, 2014 - 15:40

Another remarkable star, formerly misclassified as a lambda Bootis and RR Lyrae star,  featured in a pre-print on arXiv

We detect the second known lambda Bootis star (HD 54272) which exhibits gamma Doradus-type pulsations. The star was formerly misidentified as a RR Lyrae variable. The lambda Bootis stars are a small group (only 2 per cent) of late B to early F-type, Population I stars which show moderate to extreme (up to a factor 100) surface underabundances of most Fe-peak elements and solar abundances of lighter elements (C, N, O, and S). The photometric data from the Wide Angle Search for Planets (WASP) and All Sky Automated Survey (ASAS) projects were analysed. They have an overlapping time base of 1566 d and 2545 d, respectively. Six statistically significant peaks were identified (f1 = 1.410 116 c/d, f2 = 1.283 986 c/d, f3 = 1.293 210 c/d, f4 = 1.536 662 c/d, f5 = 1.157 22 c/d and f6 = 0.226 57 c/d). The spacing between f1 and f2, f1 and f4, f5 and f2 is almost identical. Since the daily aliasing is very strong, the interpretation of frequency spectra is somewhat ambiguous. From spectroscopic data, we deduce a high rotational velocity (250+-25 km/s) and a metal deficiency of about -0.8 to -1.1 dex compared to the Sun. A comparison with the similar star, HR 8799, results in analogous pulsational characteristics but widely different astrophysical parameters. Since both are lambda Bootis-type stars, the main mechanism of this phenomenon, selective accretion, may severely influence gamma Doradus-type pulsations.

Authors: E. Paunzen, M. Skarka, D.L. Holdsworth, B. Smalley, R. G. West

Read the abstract and paper on arXiv

An exotic eclipsing binary system composed of two classical overtone Cepheids in a 413-day orbit. Tuesday, March 18, 2014 - 13:10

We have obtained extensive high-quality spectroscopic observations of the OGLE-LMC-CEP-1718 eclipsing binary system in the Large Magellanic Cloud which Soszynski et al. (2008) had identified as a candidate system for containing two classical Cepheids in orbit. Our spectroscopic data clearly demonstrate binary motion of the Cepheids in a 413-day eccentric orbit, rendering this eclipsing binary system the first ever known to consist of two classical Cepheid variables. After disentangling the four different radial velocity variations in the system we present the orbital solution and the individual pulsational radial velocity curves of the Cepheids. We show that both Cepheids are extremely likely to be first overtone pulsators and determine their respective dynamical masses, which turn out to be equal to within 1.5 %. Since the secondary eclipse is not observed in the orbital light curve we cannot derive the individual radii of the Cepheids, but the sum of their radii derived from the photometry is consistent with overtone pulsation for both variables.

The existence of two equal-mass Cepheids in a binary system having different pulsation periods (1.96 and 2.48 days, respectively) may pose an interesting challenge to stellar evolution and pulsation theories, and a more detailed study of this system using additional datasets should yield deeper insight about the physics of stellar evolution of Cepheid variables. Future analysis of the system using additional near-infrared photometry might also lead to a better understanding of the systematic uncertainties in current Baade-Wesselink techniques of distance determinations to Cepheid variables.

Authors: W. Gieren, B. Pilecki, G. Pietrzynski, D. Graczyk, I.B. Thompson, I. Soszynski, P. Konorski, R. Smolec, A. Udalski, N. Nardetto, G. Bono, P.G. Prada Moroni, J. Storm, A. Gallenne

Read the pre-print paper

Discovery of an SU UMa-type eclipsing cataclysmic variable star inside the CV "period gap'' Monday, March 17, 2014 - 09:48

IBVS 6097 on a recently discovered eclipsing cataclysmic variable

We report the discovery of CzeV404 - eclipsing SU UMa-type cataclysmic variable star with an orbital period inside the period gap. Using photometric observations taken during 15 nights from June to September 2012 and 8 nights from July to August 2013 we determined an orbital period of 2.35 hours and superhump period of 2.50 hours. From the observed period excess, theestimated mass ratio of the system is q = 0.30.

Authors:  Cagas, Pavel; Cagas, Petr

Read IBVS 6097

Stellar luminosity: The true brightness of stars Thursday, March 13, 2014 - 06:24

Nearly every star that you see with the unaided eye is larger and more luminous than our sun. Astronomers speak of a star’s true brightness as its “luminosity.”

Some stars look bright because they’re near Earth. Others are truly extremely bright members of our Milky Way galaxy. Astronomers call the true, intrinsic brightness of a star its luminosity. The luminosity of any star depends on size and surface temperature. Some extremely large and hot stars blaze away with the luminosity of a million suns!

Read the full explanation at Earthsky.org

 

VLT Spots Largest Yellow Hypergiant Star Wednesday, March 12, 2014 - 07:38

ESO’s Very Large Telescope Interferometer has revealed the largest yellow star — and one of the ten largest stars found so far. This hypergiant has been found to measure more than 1300 times the diameter of the Sun, and to be part of a double star system, with the second component so close that it is in contact with the main star. Observations spanning over sixty years, some from amateur observers, also indicate that this rare and remarkable object is changing very rapidly and has been caught during a very brief phase of its life.

Read the full press release from ESO
 

Towards a Determination of Definitive Parameters for the Long Period Cepheid S Vulpeculae Tuesday, March 11, 2014 - 15:05

AAVSO Councilor, David G. Turner, utilizes APASS data to expand on an oral paper given at the spring meeting of the AAVSO held in Boone, North Carolina, May 17–18, 2013.

A new compilation of UBV data for stars near the Cepheid S Vul incorporates BV observations from APASS and NOMAD to augment UBV observations published previously. A reddening analysis yields mean colour excesses and distance moduli for two main groups of stars in the field: the sparse cluster Turner 1 and an anonymous background group of BA stars. The former appears to be 1.07+-0.12 kpc distant and reddened by E(B-V)=0.45+-0.05, with an age of 10^9 yrs. The previously overlooked latter group is 3.48+-0.19 kpc distant and reddened by E(B-V)=0.78+-0.02, with an age of 1.3x10^7 yrs. Parameters inferred for S Vul under the assumption that it belongs to the distant group, as also argued by 2MASS data, are all consistent with similar results for other cluster Cepheids and Cepheid-like supergiants.

Read the paper on arXiv

The 2011 Outburst of Recurrent Nova T Pyx: Radio Observations Reveal the Ejecta Mass and Hint at Complex Mass Loss Friday, March 7, 2014 - 10:34

An updated version of this paper appeared on arXiv March 5, 2014.

Despite being the prototype of its class, T Pyx is arguably the most unusual and poorly understood recurrent nova. Here, we use radio observations from the Karl G. Jansky Very Large Array to trace the evolution of the ejecta over the course of the 2011 outburst of T Pyx. The radio emission is broadly consistent with thermal emission from the nova ejecta. However, the radio flux began rising surprisingly late in the outburst, indicating that the bulk of the radio-emitting material was either very cold, or expanding very slowly, for the first ~50 days of the outburst. Considering a plausible range of volume filling factors and geometries for the ejecta, we find that the high peak flux densities of the radio emission require a massive ejection of 1-30 x 10^{-5} solar masses. This ejecta mass is much higher than the values normally associated with recurrent novae, and is more consistent with a nova on a white dwarf well below the Chandrasekhar limit.

Authors: Thomas Nelson, Laura Chomiuk, Nirupam Roy, J. L. Sokoloski, Koji Mukai, Miriam I. Krauss, Amy J. Mioduszewski, Michael P. Rupen, Jennifer Weston

Read the pre-print on astro-ph

OQ Carinae: A New Southern Z Cam Type Dwarf Nova Tuesday, March 4, 2014 - 22:49

Rod Stubbings has discovered a new southern Z Cam hiding in the weeds, disguised as an ordinary dwarf nova no one has paid attention to in years.

Congratulations to Rod. His story is a testament to the value of patience, persistence and visual observations.

Abstract: Long term optical monitoring of the dwarf nova OQ Car has been conducted to study the previously unknown behaviour of this star system. The observations have shown OQ Car to have frequent dwarf nova outbursts and revealed the first recorded standstill of this star system. Based on this, we conclude that OQ Car is a new member of the Z Cam type dwarf novae.

Authors: Rod Stubbings and Mike Simonsen

Read the pre-print paper on arXiv

GALEX J194419.33+491257.0: An Unusually Active SU UMa-Type Dwarf Nova with a Very Short Orbital Period in the Kepler Data Tuesday, March 4, 2014 - 09:57

We studied the background dwarf nova of KIC 11412044 in the Kepler public data and identified it with GALEX J194419.33+491257.0. This object turned out to be a very active SU UMa-type dwarf nova having a mean supercycle of about 150 d and frequent normal outbursts having intervals of 4-10 d. The object showed strong persistent signal of the orbital variation with a period of 0.0528164(4) d (76.06 min) and superhumps with a typical period of 0.0548 d during superoutbursts. Most of the superoutbursts were accompanied by a precursor outburst. All these features are unusual for this very short orbital period. We succeeded in detecting the evolving stage of superhumps (stage A superhumps) and obtained a mass ratio of 0.141(2), which is unusually high for this orbital period. We suggest that the unusual outburst properties are a result of this high mass ratio. We suspect that this object is a member of the recently recognized class of cataclysmic variables (CVs) with a stripped core evolved secondary which are evolving toward AM CVn-type CVs. The present determination of the mass ratio using stage A superhumps makes the first case in such systems.

Authors:  Taichi Kato (Kyoto U.), Yoji Osaki (U. of Tokyo)

Dowload the pre-print from arXiv

Standard-Candle Supernovae are Still Standard, but Why? Monday, March 3, 2014 - 16:40

The Nearby Supernova Factory based at Berkeley Lab shows that Type Ia supernovae have a surprisingly large range of masses

Until recently, scientists thought they knew why Type Ia supernovae are all so much alike. But their favorite scenario was wrong.

The assumption was that carbon-oxygen white dwarf stars, the progenitors of the supernovae, capture additional mass by stripping it from a companion star or by merging with another white dwarf; when they approach the Chandrasekhar limit (40 percent more massive than our sun) they experience thermonuclear runaway. Type Ia brightnesses were so similar, scientists thought, because the amounts of fuel and the explosion mechanisms were always the same.

Greg Aldering summarizes the most basic result of the new analysis: “The white dwarfs exploding as Type Ia supernovae have a range of masses, and the resulting light-curve width is directly proportional to the total mass involved in the explosion.”

Read the full press release at Berkley Lab

Read the pre-print paper on astro-ph 

Closest, brightest supernova in decades is also a little weird Thursday, February 27, 2014 - 13:55

When University of California, Berkeley, astronomer Alex Filippenko’s research team looked for the supernova in data collected by the Katzman Automatic Imaging Telescope (KAIT) at Lick Observatory near San Jose, Calif., they discovered that the robotic telescope had actually taken a photo of it 37 hours after it appeared, unnoticed, on Jan. 14.

Combining this observation with another chance observation by a Japanese amateur astronomer, Filippenko’s team was able to calculate that SN 2014J had unusual characteristics – it brightened faster than expected for a Type Ia supernova and, even more intriguing, it exhibited the same unexpected, rapid brightening as another supernova that KAIT discovered and imaged last year – SN 2013dy.

“Now, two of the three most recent and best-observed Type Ia supernovae are weird, giving us new clues to how stars explode,” said Filippenko, referring to a third, though apparently ‘normal,’ Type Ia supernova, SN 2011fe, discovered three years ago. “This may be teaching us something general about Type Ia supernovae that theorists need to understand. Maybe what we think of as ‘normal’ behavior for these supernovae is actually unusual, and this weird behavior is the new normal.”

Read the full press release from UC Berkley

Pre-outburst observations of Nova Del 2013 from Pan-STARRS 1 Wednesday, February 26, 2014 - 09:42

Nova Delphini 2013 was identified on the 14th of August 2013 and eventually rose to be a naked eye object. We sought to study the behaviour of the object in the run-up to outburst and to compare it to the pre-outburst photometric characteristics of other novae. We searched the Pan-STARRS 1 datastore to identify pre-outburst photometry of Nova Del 2013 and identified twenty-four observations in the 1.2 years before outburst. The progenitor of Nova Delphini showed variability of a few tenths of a magnitude but did not brighten significantly in comparison with archival plate photometry. We also found that the object did not vary significantly on the approximately half hour timescale between pairs of Pan-STARRS 1 observations.

Authors: N.R. Deacon, D.W. Hoard, E.A. Magnier, et al.

Read the paper on astro-ph

Study of Negative and Positive Superhumps in ER Ursae Majoris Monday, February 24, 2014 - 22:18

We carried out the photometric observations of the SU UMa-type dwarf nova ER UMa during 2011 and 2012, which showed the existence of persistent negative superhumps even during the superoutburst. We performed two-dimensional period analysis of its light curves by using a method called "least absolute shrinkage and selection operator" (Lasso) and "phase dispersion minimization" (PDM) analysis, and we found that the period of negative superhumps systematically changed between a superoutburst and the next superoutburst. The trend of the period change can beinterpreted as reflecting the change of the disk radius. This change of the disk radius is in good agreement with the predicted change of the disk radius by the thermal-tidal instability (TTI) model. The normal outbursts within a supercycle showed a general trend that the rising rate to maximum becomes slower as the next superoutburst approaches. The change can be interpreted as the consequence of the increased gas-stream flow onto the inner region of the disk as the result of the tilted disk. Some of the superoutbursts were found to be triggered by a precursor normal outburst when the positive superhumps appeared to develop. The positive and negative superhumps co-existed during the superoutburst. The positive superhumps were prominent only during four or five days after the supermaximum, while the signal of the negative superhumps became strong after the middle phase of the superoutburst plateau. A simple combination of the positive and negative superhumps was found to be insufficient in reproducing the complex profile variation. We were able to detect the developing phase of positive superhumps (stage A superhumps) for the first time in ER UMa-type dwarf novae. Using the period of stage A superhumps, we obtained a mass ratio of 0.100(15), which indicates that ER UMa is on the ordinary evolutional track of CVs.

Authors: Tomohito Ohshima, Taichi Kato, Elena Pavlenko, Hidehiko Akazawa, Kazuyoshi Imamura, Kenji Tanabe, Enrique de Miguel, William Stein, Hiroshi Itoh, Franz-Josef Hambsch, Pavol A. Dubovsky, Igor Kudzej, Thomas Krajci, Alex Baklanov, Denis Samsonov, Oksana Antonyuk, Viktor Malanushenko, Maksim Andreev, Ryo Noguchi, Kazuyuki Ogura, Takashi Nomoto, Rikako Ono, Shin'ichi Nakagawa, Keisuke Taniuchi, Tomoya Aoki, Miho Kawabata, Hitoshi Kimura, Kazunari Masumoto, Hiroshi Kobayashi, Katsura Matsumoto, Kazuhiko Shiokawa, Sergey Yu. Shugarov, Natalia Katysheva, Irina Voloshina, Polina Zemko, Kiyoshi Kasai, Javier Ruiz, Hiroyuki Maehara, Natalia Virnina, Jani Virtanen, Ian Miller, Boyd Boitnott, Colin Littlefield, Nick James, Tamas Tordai, Fidrich Robaert, Stefono Padovan, Atsushi
Miyashita

Chemical abundance analysis of symbiotic giants. RW Hya, SY Mus, BX Mon, and AE Ara Monday, February 24, 2014 - 15:56

Symbiotic stars are the long period, binary systems of strongly interacting stars at the final stages of evolution which can be useful tool to understand the chemical evolution of the Galaxy and the formation of stellar populations. Knowledge of the chemical composition of the symbiotic giants is essential to advancing our understanding of these issues but unfortunately reliably determinations exist only in a few cases. We perform a program for detailed chemical composition analysis in over 30 symbiotic giants, based on the high resolution, near-IR spectra, obtained with Phoenix/Gemini South spectrometer. The methods of the standard LTE analysis is used to obtain photospheric abundances of CNO and elements around iron peak. Here we present results obtained for four objects: RW Hya, SY Mus, BX Mon, and AE Ara. Our analysis revealed a significantly sub-solar metallicity (Me/H ~ -0.75) for RW Hya, a slightly sub-solar metallicities (Me/H ~ 0.2-0.3) in BX Mon and AE Ara, and a near-solar metallicity in SY Mus. 12C/13C isotopic ratios are low in all cases, ranging from ~6 to ~10, and indicate that the giants have experienced the first dredge-up.

Authors: Cezary Galan, Joanna Mikolajewska, Kenneth H. Hinkle, Miroslaw R. Schmidt, Mariusz Gromadzki

Read the paper on astro-ph

The Purest Star Tells an Ancient Tale Sunday, February 23, 2014 - 15:30

Astronomers have discovered the purest star to date. Composed almost exclusively of hydrogen and helium — with 15 million times less iron than our Sun — it illuminates what happened among the first supernovae in the early universe. 

The young universe was virtually pure. Only hydrogen, helium, and a tiny trace of lithium emerged from the Big Bang nearly 13.8 billion years ago. And for hundreds of millions of years the universe was too hot to handle anything else.

But over time the universe cooled and giant clouds of the primordial elements collapsed to form the first stars. Without traces of heavier elements available to cool the gas clouds, the first “Population III” stars were extremely massive and bright, erupting as supernovae after relatively short lifetimes of just a few million years. These explosions, in turn, began seeding the young universe with heavier elements.

Read the full story at Sky & Telescope.com

How Supernova 2014J Will Help Determine the Extragalactic Distance Scale and Impact Cosmology Thursday, February 13, 2014 - 10:08

In only three weeks since its discovery on January 21, 2014, much has been learned about the new supernova SN 2014J in Messier 82, the “Cigar” galaxy. In addition to early confirmation based on its spectrum that it is indeed a type Ia supernova, it is now understood to be the nearest type Ia explosion to our Milky Way galaxy since 1986.

Its unique proximity alone makes SN 2014J one the most important supernova ever observed. It will impact our understanding both of the type Ia-class of supernovae and of the Universe as a whole, because our Universe’s size, age, and ultimate fate are linked intimately to observations of type Ia supernovae, and because the precision with which they can be applied to estimate Universal-scale distances depends crucially on the nearest examples. SN 2014J is very likely to remain the nearest anchor-point in the type Ia supernovae-based distance scale for decades to come.

“Being the nearest supernova of this kind, SN 2014J will help us to better calibrate the expansion of the Universe,” said Adam Riess, co-leader of the Supernova H0 for Equation of State (SHOES) project, and co-winner of the 2011 Nobel Prize in Physics.

Read the full story at Universe Today

How stellar death can lead to twin celestial jets Wednesday, February 12, 2014 - 12:57

Astronomers know that while large stars can end their lives as violently cataclysmic supernovae, smaller stars end up as planetary nebulae – colourful, glowing clouds of dust and gas. In recent decades these nebulae, once thought to be mostly spherical, have been observed to often emit powerful, bipolar jets of gas and dust. But how do spherical stars evolve to produce highly aspherical planetary nebulae?

In a theoretical paper published this week in the Monthly Notices of the Royal Astronomical Society, a University of Rochester professor and his undergraduate student conclude that only “strongly interacting” binary stars – or a star and a massive planet – can feasibly give rise to these powerful jets.

Read the full press release at the Royal Astronomical Socierty news archive.
 

Shedding Twice the Light on Circumbinary Systems Monday, February 10, 2014 - 11:55

Astronomers are beginning to understand the unlikely formation and dangerous survival of exoplanets circling binary stars.

Reality is catching up with science fiction. In 2011 astronomers detected a planet orbiting two stars and nicknamed it Tatooine after the fictional Star Wars planet. To date, six similar planets have joined the list of wacky circumbinary planets. 

But even with a half dozen of these systems to study, astronomers are baffled. Binary star systems are downright dangerous. Powerful tidal forces from the two stars can easily grind a planet to dust, let alone prevent it from forming in the first place. 

This week, however, two binary star systems are shedding twice the light on their circling exoplanets, providing promising clues to these exotic systems.

Read the full story at Sky & Telescope.com

Astronomers discover oldest star: Formed shortly after the Big Bang 13.7 billion years ago Monday, February 10, 2014 - 11:44

A team led by astronomers at The Australian National University has discovered the oldest known star in the Universe, which formed shortly after the Big Bang 13.7 billion years ago.

The discovery has allowed astronomers for the first time to study the chemistry of the first stars, giving scientists a clearer idea of what the Universe was like in its infancy.

"This is the first time that we've been able to unambiguously say that we've found the chemical fingerprint of a first star," said lead researcher, Dr Stefan Keller of the ANU Research School of Astronomy and Astrophysics.

"This is one of the first steps in understanding what those first stars were like. What this star has enabled us to do is record the fingerprint of those first stars."

Read the full press release from the Australian National University.
 

Z Cam Stars in the Twenty-First Century Monday, February 3, 2014 - 22:08

AAVSO observers help unravel Z Cam mysteries

Z Cam stars are a small subset of dwarf novae that exhibit standstills in their light curves. Most modern literature and catalogs of cataclysmic variables quote the number of known Z Cams to be on the order of 30 or so systems. After a four-year observing campaign and an exhaustive examination of the data in the AAVSO International Database we have trimmed that number by a third. One of the reasons for the misclassification of some systems is the fact that the definition of what a Z Cam is has changed over the last 85 years to what it is today. This has caused many stars formerly assumed to be Z Cams or rumored to be Z Cams to be eliminated from the final list. In this paper we present the results of our investigation into 65 stars listed at one time or another in the literature as Z Cams or possible Z Cams.

Authors: Mike Simonsen, David Boyd, Bill Goff, Tom Krajci, Kenneth Menzies, Sebastian Otero, Stefano Padovan, Gary Poyner, James Roe, Richard Sabo, George Sjoberg, Bart Staels, Rod Stubbings, John Toone, Patrick Wils

Read the pre-print on astro-ph

Astronomers Reconstruct a Short-Lived Nova Explosion Saturday, February 1, 2014 - 12:54

On 11 November 2011, astronomers witnessed a distant star erupt in an incredibly powerful explosion. An international research team including Mikio Morii and colleagues from the MAXI Team at the RIKEN Global Research Cluster has now reconstructed the event from a handful of telescopic snapshots, revealing for the first time the runaway fusion reaction that triggered the blast.

 The whole process happened more quickly than is typical of novae. Yet it was also much fainter, implying that relatively little mass was ejected in the explosion. The team’s findings suggest that the white dwarf was unusually massive, giving it a higher surface gravity that put the accumulated material under even greater pressure. This meant that less fuel was needed to trigger the explosion and a briefer nova with less ejecta was produced.

Read the full story at SciTechDaily

Read the paper on astro-ph

Stormy with a Chance of Molten Iron Rain: First Ever Map of Exotic Weather on Brown Dwarfs Thursday, January 30, 2014 - 11:46

Think the weather is nasty this winter here on Earth? Try vacationing on the brown dwarf Luhman 16B sometime.

Two studies out this week from the Max Planck Institute for Astronomy based at Heidelberg, Germany offer the first look at the atmospheric features of a brown dwarf.

A brown dwarf is a substellar object which bridges the gap between at high mass planet at over 13 Jupiter masses, and a low mass red dwarf star at above 75 Jupiter masses. To date, few brown dwarfs have been directly imaged. For the study, researchers used the recently discovered brown dwarf pair Luhman 16A & B. At about 45(A) and 40(B) Jupiter masses, the pair is 6.5 light years distant and located in the constellation Vela. Only Alpha Centauri and Barnard’s Star are closer to Earth. Luhman A is an L-type brown dwarf, while the B component is a T-type substellar object.


Read more: http://www.universetoday.com/108693/stormy-with-a-chance-of-molten-iron-rain-first-ever-map-of-exotic-weather-on-brown-dwarfs/#ixzz2rtfsBLYL

 

Eta Carinae is heating up Thursday, January 30, 2014 - 09:56

Eta Carinae is one of the biggest and brightest stars in our galaxy, with a mass over 100 times and a luminosity more than a million times that of our sun. Scientists have been observing it one way or another for almost 400 years, especially because it has large variations in brightness, and there was even a period in the 19th century called the Great Eruption when it was the second brightest star in the sky. The causes of that are still under discussion. Eta Carinae is furthermore a binary system (the secondary star is about 30 solar masses), and in 1998 it reached periastron, the point of closest approach for the two stars.

Eta Carinae has been observed with the South African Astronomical Observatory (SAAO) in the JHKL infrared bands for over forty years. From the start of observations in 1976 until periastron in 1998, astronomers saw a linear increase in the star’s brightness (although semi-periodic variations are also seen). But after 1998, the linear trend changed significantly, and the star began to brighten much faster in the J and H bands. 

Read the full story on Astrobites

The UBV Color Evolution of Classical Novae. I. Nova-Giant Sequence in the Color-Color Diagram Wednesday, January 29, 2014 - 09:34

We identified a general course of classical nova outbursts in the B-V versus U-B color-color diagram. It is reported that novae show spectra similar to those of A--F supergiants near optical light maximum. However, they do not follow the supergiant sequence in the color-color diagram, neither the blackbody nor the main-sequence sequence. Instead, we found that novae evolve along a new sequence in the pre-maximum and near-maximum phases, which we call ``the nova-giant sequence.'' This sequence is parallel to but \Delta (U-B) \approx -0.2 mag bluer than the supergiant sequence. This is because the mass of a nova envelope is much (\sim10^{-4} times) less than that of a normal supergiant. After optical maximum, its color quickly evolves back blueward along the same nova-giant sequence and reaches the point of free-free emission (B-V=-0.03, U-B=-0.97), which coincides with the intersection of the blackbody sequence and the nova-giant sequence, and remains there for a while. Then the color evolves leftward (blueward in B-V but almost constant in U-B), owing mainly to the development of strong emission lines. This is the general course of nova outbursts in the color-color diagram, which was deduced from eight well-observed novae in various speed classes. For a nova with unknown extinction, we can determine a reliable value of the color excess by matching the observed track of the target nova with this general course. This is a new and convenient method for obtaining the color excesses of classical novae. Using this method, we redetermined the color excesses of twenty well-observed novae. The obtained color excesses are in reasonable agreement with the previous results, which in turn supports the idea of our general track of nova outbursts.

Authors:  Izumi Hachisu (Univ. of Tokyo), Mariko Kato (Keio Univ.)

Read the pre-print paper on arXiv

The North Star Polaris Is Getting Brighter Tuesday, January 28, 2014 - 16:35

The North Star has remained an eternal reassurance for northern travelers over the centuries. But recent and historical research reveals that the ever-constant star is actually changing.

After dimming for the last few decades, the North Star is beginning to shine brightly again. And over the last two centuries, the brightening has become rather dramatic.

"It was unexpected to find," Scott Engle of Villanova University in Pennsylvania told SPACE.com. Engle investigated the fluctuations of the star over the course of several years, combing through historical records and even turning the gaze of the famed Hubble Space Telescope onto the star.

Read the rest of the story on Space.com

 

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