A Dead Star Brightens

by Paul Gilster on July 21, 2006

RS Ophiuchi, a binary system some 5000 light years from Earth, has given astronomers plenty to talk about since February, when it suddenly brightened. The phenomenon wasn’t unusual — RS Ophiuchi undergoes periodic outbursts — but this was the first since 1985, allowing powerful radio telescope arrays to study the results. By coordination between radio telescopes from South Africa to China, Hawaii to the UK, astronomers have pieced together the sequence of events that led to the explosion and have studied its aftermath.

The results are reported in the July 20 issue of Nature, revealing that a mere two weeks after first reports of the stellar eruption, an expanding blast wave extended to a distance of 10 AU. It was triggered by a nuclear explosion on the surface of a white dwarf that had been capturing gas from the nearby gas giant it orbits. Once enough gas collects on the white dwarf, a thermonuclear reaction begins, with the white dwarf’s energy output increasing to over 100,000 times that of the Sun.

Explosion in a binary system

Image: Artist’s impression of the binary system RS Ophiuchi. Hydrogen-rich gas is transferred from a red giant onto the surface of a white dwarf and has just exploded there. Credit: David A. Hardy & PPARC (www.astroart.org).

The gas, meanwhile, is ejected at several thousand kilometers per second, striking the atmosphere of the red giant and setting up blast waves that accelerate electrons close to the speed of light. The radio emissions of these electrons as they move through magnetic fields in the vicinity of the binaries are what the astronomers are examining.

The beauty of modern radio telescope work is that it can quickly become global in scope. With initial observations on the Very Long Baseline Array (VLBA), which extends from Hawaii to the Caribbean, the team continued to track the outburst using facilities in the USA (the Very Large Array) and the UK (MERLIN), as well as Europe’s VLBI Network, which includes telescopes in South Africa and China.

The work has already yielded its share of surprises, says Richard Porcas (Max Planck Institute for Radio Astronomy):

“A week after our first observations, we combined telescopes across Europe with two in China and another in South Africa and were surprised to find that the blast wave had become distorted. Over the next few months our observations have shown it turning from a ring into a cigar-like shape. It’s going to need a lot more work to understand exactly what causes this but either the explosion shoots jets of matter in opposite directions or somehow the atmosphere of the red giant is shaping the ejected material.”

Further explosions on RS Ophiuchi are inevitable as gas builds up again on the white dwarf. Are we seeing a supernova in the making as the white dwarf increases in mass, or is the tiny star shedding everything it captures from the red giant with each dramatic outburst? That’s one major question that future work on this distant binary should answer. The paper is O’Brien, Bode, Porcas et al., “An asymmetric shock wave in the 2006 outburst of the recurrent nova RS Ophiuchi,” Nature 442 (20 July 2006), pp. 279-281 (abstract here).

{ 6 comments }

Adam Crowl July 22, 2006 at 20:24

Novae are one of those cosmic hazards which would drive ETIs from their home systems – if, for example, their biosphere became hospitable as the second star began evolving off the Main Sequence. Imagine a moon of a Jovian, or a stripped back Neptune that had suffered mass loss during the first star’s evolution.

Hmmm… that’s quite a good scenario, in a fictional sense.

Adam

ljk January 17, 2007 at 22:53

Some Rare Abnormal Stars may have White Dwarf Parents to Blame

Monday, 08 January 2007

Page 1 of 3

For embargoed release at 9:30 AM (Pacific Time) on Tuesday, January 9, 2007

Astronomers have announced the discovery of huge quantities of an unusual variety of oxygen in two very rare types of stars. The finding suggests that the origin of these oddball stars may lie in the physics behind the mergers of white dwarf star pairs.

The unusual stars are known as hydrogen-deficient (HdC) and R Coronae Borealis (RCB) stars. Both types have almost no hydrogen – an element that makes up about 90% of most stars. Surprisingly, they contain up to a thousand times more of the isotope oxygen-18 than normal stars like our Sun. The discovery of abnormal quantities of oxygen-18 is based on near-infrared spectroscopic observations from the Gemini Near-Infrared Spectrograph (GNIRS) on the 8-meter Gemini-South telescope in Chile.

Full story here:

http://www.gemini.edu/index.php?option=content&task=view&id=220

ljk June 23, 2007 at 8:38

The Future is Now: the Formation of Single Low Mass White Dwarfs in the Solar Neighborhood

Authors: Mukremin Kilic, K. Z. Stanek, M. H. Pinsonneault (Ohio State)

(Submitted on 21 Jun 2007)

Abstract: Low mass white dwarfs (M

ljk June 23, 2007 at 8:39

The Future is Now: the Formation of Single Low Mass White Dwarfs in the Solar Neighborhood

Authors: Mukremin Kilic, K. Z. Stanek, M. H. Pinsonneault (Ohio State)

(Submitted on 21 Jun 2007)

Abstract: Low mass white dwarfs (M less than 0.45 Msun) can be produced from interacting binary systems, and traditionally all of them have been attributed to this channel. However, a helium white dwarf could also result from a single star that experiences severe mass loss on the first ascent giant branch. A large population of low mass He white dwarfs has been discovered in the old metal-rich cluster NGC 6791. There is therefore a mechanism in clusters to produce low mass white dwarfs without requiring binary star interactions, and we search for evidence of a similar population in field white dwarfs. We argue that there is a significant field population (of order half of the detected systems) that arises from old metal rich stars which truncate their evolution prior to the helium flash from severe mass loss. There is a consistent absence of evidence for nearby companions in a large fraction of low mass white dwarfs. The number of old metal-rich field dwarfs is also comparable with the apparently single low mass white dwarf population, and our revised estimate for the space density of low mass white dwarfs produced from binary interactions is also compatible with theoretical expectations.

One strong implication of our model is that single low mass white dwarfs should be good targets for planet searches because they are likely to arise from metal-rich progenitors. We also discuss other observational tests and implications, including the potential impact on SN Ia rates and the frequency of planetary nebulae.

Comments: submitted to ApJ

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0706.3045v1 [astro-ph]

Submission history

From: Mukremin Kilic [view email]

[v1] Thu, 21 Jun 2007 16:35:59 GMT (19kb)

http://arxiv.org/abs/0706.3045

ljk June 27, 2007 at 10:30

Planet around a white dwarf

Category: astro

Posted on: June 27, 2007 4:17 AM, by Steinn Sigurðsson

The long rumoured planet candiate around a nearby white dwarf has finally been made public

Fergal Mullally (U Texas) announced that they have strong evidence for a 2+ Jupiter mass planet around the white dwarf GD66 – a pulsating moderate temperature hydrogen white dwarf that is about 50 pc away.

Detection was through timing of pulsations, over a period of few years they have significant non-monotonic timing variation consistent with a planet in a circular 4 year orbit (or somewhat period longer if eccentric) – probably have to wait for JWST for imaging confirmation, but timing will verify, or fail, the presence of a planet on a 2-3 year time scale.

Planet is bordeline Spitzer candidate, and Spitzer should look for the associated mid-infrared excess this autumn – if the orbit inclination is high enough (ie mass is higher than minimum timing mass) and if atmospheric models for mid-IR emission are good then there is a chance Spitzer will see the object, blended with the white dwarf emission.

Looks very interesting, almost there.

http://scienceblogs.com/catdynamics/2007/06/planet_around_a_white_dwarf.php

ljk January 29, 2008 at 17:08

Milliarcsecond N-Band Observations of the Nova RS Ophiuchi: First Science with the Keck Interferometer Nuller

Authors: R. K. Barry, W. C. Danchi, W. A. Traub, J. L. Sokoloski, J. P. Wisniewski, E. Serabyn, M. J. Kuchner, R. Akeson, E. Appleby, J. Bell, A. Booth, H. Brandenburg, M. Colavita, S. Crawford, M. Creech-Eakman, W. Dahl, C. Felizardo, J. Garcia, J. Gathright, M. A. Greenhouse, J.Herstein, E. Hovland, M. Hrynevych, C. Koresko, R. Ligon, B. Mennesson, R. Millan-Gabet, D. Morrison, D. Palmer, T. Panteleeva, S. Ragland, M. Shao, R. Smythe, K. Summers, M. Swain, K. Tsubota, C. Tyau, G. Vasisht, E. Wetherell, P. Wizinowich, J. Woillez

(Submitted on 27 Jan 2008)

Abstract: We report observations of the nova RS Ophiuchi (RS Oph) using the Keck Interferometer Nuller (KIN), approximately 3.8 days following the most recent outburst that occurred on 2006 February 12. These observations represent the first scientific results from the KIN, which operates in N-band from 8 to 12.5 microns in a nulling mode. By fitting the unique KIN data, we have obtained an angular size of the mid-infrared continuum of 6.2, 4.0, or 5.4 mas for a disk profile, gaussian profile (FWHM), and shell profile respectively. The data show evidence of enhanced neutral atomic hydrogen emission and atomic metals including silicon located in the inner spatial regime near the white dwarf (WD) relative to the outer regime. There are also nebular emission lines and evidence of hot silicate dust in the outer spatial region, centered at ! 17 AU from the WD, that are not found in the inner regime.

Our evidence suggests that these features have been excited by the nova flash in the outer spatial regime before the blast wave reached these regions.

These identifications support a model in which the dust appears to be present between outbursts and is not created during the outburst event. We further discuss the present results in terms of a unifying model of the system that includes an increase in density in the plane of the orbit of the two stars created by a spiral shock wave caused by the motion of the stars through the cool wind of the red giant star.

These data show the power and potential of the nulling technique which has been developed for the detection of Earth-like planets around nearby stars for the Terrestrial Planet Finder Mission and Darwin missions.

Comments: 41 pages, 10 figures

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.4165v1 [astro-ph]

Submission history

From: William Danchi [view email]

[v1] Sun, 27 Jan 2008 22:50:53 GMT (1324kb)

http://arxiv.org/abs/0801.4165

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