Tag Archives: #whitedwarf

Astronomers Discovered Moon-Size, Highly Magnetized, Rapidly Rotating White Dwarf (Planetary Science)

Using ZTF and followup with CHIMERA, a team of international astronomers discovered a highly magnetized and rapidly rotating white dwarf, “ZTF J190132.9+145808.7”, which is as small as the moon. Their study recently appeared in the Journal Nature.

White dwarfs represent the last stage of evolution of stars with mass less than about eight times that of the Sun and, like other stars, are often found in binaries. If the orbital period of the binary is short enough, energy losses from gravitational-wave radiation can shrink the orbit until the two white dwarfs come into contact and merge. Depending on the component masses, the merger can lead to a supernova of type Ia or result in a massive white dwarf. In the latter case, the white dwarf remnant is expected to be highly magnetised because of the strong magnetic dynamo that should arise during the merger, and be rapidly spinning from the conservation of the orbital angular momentum.

Fig 1: Gaia colour-magnitude diagram © authors
Fig 2: ZTF J1901+1458 lightcurve: The left panels show the binned CHIMERA lightcurve phase-folded at a period of 6.94 minutes in the g0-band (a) and in the r0-band (b). The right panels show the similarly normalised ZTF discovery lightcurve in the ZTF g-band (c) and r-band (d). The error bars indicate 1σ errors © authors

Now, a team of international astronomers using the Zwicky Transient Facility (ZTF) searched for short period objects that lie below the main white dwarf cooling sequence in the Gaia colour-magnitude diagram (see Fig. 1). They found, ZTF J190132.9+145808.7 (hereafter ZTF J1901+1458) showing promising small photometric variations. Followup with CHIMERA, a high-speed imaging photometer on the 200-inch Hale telescope, confirmed that this system has a period of just 6.94 minutes (see Fig. 2).

“The period of ZTF J1901+1458 is unusually short for a white dwarf, as white dwarf rotational periods typically are upwards of hours.”

In order to identify field strength they also considered all the allowed bound-bound hydrogen transitions and found that the white dwarf’s magnetic field ranging between 600 megagauss and 900 megagauss over its surface.

Mass-radius relation © Authors

Additionally, using photometry, they found that the effective temperature, stellar radius and interstellar reddening of ZTF J1901+1458 to be Teff = 46,000 K, R∗ = 2,140 km and E(B–V) = 0.044, respectively. Moreover, it has mass between 1.327 and 1.365 solar masses. Based on this expected mass, astronomers think that it has an oxygen-neon internal composition and its central density is right at the threshold for electron capture on ²³Na.

“The radius is smaller than other white dwarfs and only slightly larger than that of the Moon”

Such a small radius implies that the star’s mass is close to the maximum white-dwarf mass and its rapid rotation, high mass and magnetism points towards the fact that it is likely a remnant of a white dwarf merger.

Finally, based on the luminosity of the white dwarf, they estimated the temperature in the core to be about 2–3 × 107 K. At such a high central temperature and density, the neutrino cooling of ZTF J1901+1458 will dominated by the ”Urca” process acting on ²³Na. This unusual neutrino cooling makes an age determination difficult.

“Over a few hundred million years, the heaviest elements, including Na, will gradually sink to the centre of ZTF J1901+1458. If the star lies at the small end of the radius constraint and if at least sixty percent of the ²³Na manages to sink and undergo beta decay before the core crystallises and sedimentation stops, electron-capture on ²⁴Mg would ensue. The star would shrink and the internal pressure would no longer be able to support the star, as the maximum allowed mass for the new composition would be lower than the mass of the white dwarf. The star would therefore collapse and heat up, leading to the onset of electron capture onto Ne and to the ignition of oxygen nuclear burning. The white dwarf would then undergo a disruptive thermonuclear supernova or implode to form a neutron star.”

— they added.

Reference: Caiazzo, I., Burdge, K.B., Fuller, J. et al. A highly magnetized and rapidly rotating white dwarf as small as the Moon. Nature 595, 39–42 (2021). https://doi.org/10.1038/s41586-021-03615-y

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Astronomers Identified A White Dwarf So Massive That It Might Collapse (Planetary Science)

Astronomers have discovered the smallest and most massive white dwarf ever seen. The smoldering cinder, which formed when two less massive white dwarfs merged, is heavy, “packing a mass greater than that of our Sun into a body about the size of our Moon,” says Ilaria Caiazzo, the Sherman Fairchild Postdoctoral Scholar Research Associate in Theoretical Astrophysics at Caltech and lead author of the new study appearing in the July 1 issue of the journal Nature. “It may seem counterintuitive, but smaller white dwarfs happen to be more massive. This is due to the fact that white dwarfs lack the nuclear burning that keep up normal stars against their own self gravity, and their size is instead regulate­­­d by quantum mechanics.”

The discovery was made by the Zwicky Transient Facility, or ZTF, which operates at Caltech’s Palomar Observatory; two Hawaiʻi telescopes – W. M. Keck Observatory on  Maunakea, Hawaiʻi Island and University of Hawaiʻi Institute for Astronomy’s Pan-STARRS (Panoramic Survey Telescope and Rapid Response System) on Haleakala, Maui – helped characterize the dead star, along with the 200-inch Hale Telescope at Palomar, the European Gaia space observatory, and NASA’s Neil Gehrels Swift Observatory.

White dwarfs are the collapsed remnants of stars that were once about eight times the mass of our Sun or lighter. Our Sun, for example, after it first puffs up into a red giant in about 5 billion years, will ultimately slough off its outer layers and shrink down into a compact white dwarf. About 97 percent of all stars become white dwarfs.

While our Sun is alone in space without a stellar partner, many stars orbit around each other in pairs. The stars grow old together, and if they are both less than eight solar-masses, they will both evolve into white dwarfs.

The new discovery provides an example of what can happen after this phase. The pair of white dwarfs, which spiral around each other, lose energy in the form of gravitational waves and ultimately merge. If the dead stars are massive enough, they explode in what is called a type Ia supernova. But if they are below a certain mass threshold, they combine together into a new white dwarf that is heavier than either progenitor star. This process of merging boosts the magnetic field of that star and speeds up its rotation compared to that of the progenitors.

Astronomers say that the newfound tiny white dwarf, named ZTF J1901+1458, took the latter route of evolution; its progenitors merged and produced a white dwarf 1.35 times the mass of our Sun. The white dwarf has an extreme magnetic field almost 1 billion times stronger than our Sun’s and whips around on its axis at a frenzied pace of one revolution every seven minutes (the zippiest white dwarf known, called EPIC 228939929, rotates every 5.3 minutes).

“We caught this very interesting object that wasn’t quite massive enough to explode,” says Caiazzo. “We are truly probing how massive a white dwarf can be.”

What’s more, Caiazzo and her collaborators think that the merged white dwarf may be massive enough to evolve into a neutron-rich dead star, or neutron star, which typically forms when a star much more massive than our Sun explodes in a supernova.

“This is highly speculative, but it’s possible that the white dwarf is massive enough to further collapse into a neutron star,” says Caiazzo. “It is so massive and dense that, in its core, electrons are being captured by protons in nuclei to form neutrons. Because the pressure from electrons pushes against the force of gravity, keeping the star intact, the core collapses when a large enough number of electrons are removed.”

If this neutron star formation hypothesis is correct, it may mean that a significant portion of other neutron stars take shape in this way. The newfound object’s close proximity (about 130 light-years away) and its young age (about 100 million years old or less) indicate that similar objects may occur more commonly in our galaxy.

Magnetic and Fast

The white dwarf was first spotted by Caiazzo’s colleague Kevin Burdge, a postdoctoral scholar at Caltech, after searching through all-sky images captured by ZTF. This particular white dwarf, when analyzed in combination with data from Gaia, stood out for being very massive and having a rapid rotation.

“No one has systematically been able to explore short-timescale astronomical phenomena on this kind of scale until now. The results of these efforts are stunning,” says Burdge, who, in 2019, led the team that discovered a pair of white dwarfs zipping around each other every seven minutes.

The team then analyzed the spectrum of the star using Keck Observatory’s Low Resolution Imaging Spectrometer (LRIS), and that is when Caiazzo was struck by the signatures of a very powerful magnetic field and realized that she and her team had found something “very special,” as she says. The strength of the magnetic field together with the seven-minute rotational speed of the object indicated that it was the result of two smaller white dwarfs coalescing into one.

Data from Swift, which observes ultraviolet light, helped nail down the size and mass of the white dwarf. With a diameter of 2,670 miles, ZTF J1901+1458 secures the title for the smallest known white dwarf, edging out previous record holders, RE J0317-853 and WD 1832+089, which each have diameters of about 3,100 miles.

The white dwarf ZTF J1901+1458 is about 2,670 miles across, while the moon is 2,174 miles across. The white dwarf is depicted above the Moon in this artistic representation; in reality, the white dwarf lies 130 light-years away in the constellation of Aquila. Credit: Giuseppe Parisi

In the future, Caiazzo hopes to use ZTF to find more white dwarfs like this one, and, in general, to study the population as a whole. “There are so many questions to address, such as what is the rate of white dwarf mergers in the galaxy, and is it enough to explain the number of type Ia supernovae? How is a magnetic field generated in these powerful events, and why is there such diversity in magnetic field strengths among white dwarfs? Finding a large population of white dwarfs born from mergers will help us answer all these questions and more.”

The study, titled “A highly magnetised and rapidly rotating white dwarf as small as the Moon,” was funded by the Rose Hills Foundation, the Alfred P. Sloan Foundation, NASA, the Heising–Simons Foundation, the A. F. Morrison Fellowship of the Lick Observatory, the NSF, and the Natural Sciences and Engineering Research Council of Canada.


Provided by W.M. Keck Observatory

Why Its Hard To Detect Highly Magnetised Super-Chandrashekhar White Dwarfs? (Planetary Science)

Mukul Bhattacharya and colleagues carried out study on the structural properties of strongly magnetized super-Chandrasekhar white dwarfs

Chandrasekhar made the startling discovery about nine decades back that the mass of compact object white dwarf has a limiting value, once nuclear fusion reactions stop therein. This is the Chandrasekhar mass-limit, which is ∼1.4M for a non-rotating non-magnetized white dwarf. On approaching this limiting mass, a white dwarf is believed to spark off with an explosion called type Ia supernova, which is considered to be a standard candle. However, observations of several over-luminous, peculiar type Ia supernovae indicate that the Chandrasekhar mass-limit to be significantly larger i.e. as high as ∼2.8 M. Such objects are called highly magnetised super-Chandrashekhar white dwarfs (B-WDs).

Now, Bhattacharya and colleagues investigated the luminosity suppression and its effect on the mass–radius relation as well as cooling evolution of highly magnetised super-Chandrashekhar white dwarfs.

In order to obtained the structural properties of these stars, they suitably modified their treatment of the radiative opacity, magnetostatic equilibrium and degenerate core equation of state, which is based on the effect of magnetic field relative to gravitational energy.

Fig 1: The effect of magnetic field on the mass radius relation of highly magnetized white dwarfs for B = (0, 0) (blue diamonds) , B = (109, 1013) (orange cross) , B = (107, 1014) G (green circles) and B = (109, 1014) G (red plus), along with the Chandrasekhar result (black squares). The luminosity is set as 10¯4 L for these results. © Bhattacharya et al.

They found that, strong central fields of about 10¹⁴ G can raise central density (ρc) significantly and yield super-Chandrasekhar WDs with masses as high as ∼ 1.87 M.

“Smaller white dwarfs tend to remain super- Chandrasekhar for sufficiently strong central fields even when their luminosity is significantly suppressed to 10¯16 L”

Over 10 billion years, when the white dwarfs field decays and they gets cold, their masses can fall upto 1.5 M. But, even though the mass of B-WD lowered significantly, majority of these systems still remain practically hidden throughout their cooling evolution due to their strong fields (which is about 109 G) and consequently low luminosities.

Fig 2: The effect of magnetic field on the mass radius relation of highly magnetized white dwarfs for B = (0, 0), B = (107, 1012), B = (107, 1013) and B = (106, 1014). The models relating to each data point were allowed to cool to L = 10¯4 L in order to remain consistent with the analytical models presented in Figure 1. © Bhattacharya et al.

They have also used stellar evolution code for numerically validating their analytical formalism and found the limiting mass of ~ 1.89 M, which is in very good agreement with the mass obtained from the analytical calculations (i.e. 1.87 M) for white dwarfs with strong fields (106¯9, 1014) G.

“Our results argue that super-Chandrasekhar white dwarfs born due to strong field effects may not remain so for long. This explains their apparent scarcity in addition to making them hard to detect because of their suppressed luminosities.”

— concluded authors of the study

Reference: Mukul Bhattacharya, Alexander J. Hackett, Abhay Gupta, Christopher A. Tout, Banibrata Mukhopadhyay, “Effect of field dissipation and cooling on the mass-radius relation of strongly magnetised white dwarfs”, Arxiv, pp. 1-15, 2021. arXiv:2106.09736

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White Dwarf Stars’ Debris Disk Formation Delayed (Planetary Science)

White dwarfs, the glowing cores of dead stars, often host disks of dusty debris. However, these debris disks only appear 10-20 millions of years following the star’s violent Red Giant phase.  A new paper by Planetary Science Institute Research Scientist Jordan Steckloff unravels the reason for this delay. 

 “When a star similar in mass to the Sun runs out of nuclear fuel, it first expands outward into a Red Giant. At the end of our own Sun’s life, it will expand into a Red Giant that envelopes and destroys the innermost planets: Mercury, Venus, and probably the Earth. During this phase, Red Giant stars also loses a large fraction of their mass, before ultimately collapsing into  a white dwarf ­— an Earth-sized ball of carbon and oxygen with half the mass of the Sun.  This destabilizes the orbits of any remaining planets, which can in turn scatter asteroids, flinging some of them toward the host white dwarf.” said Steckloff, lead author of “How Sublimation Delays the Onset of Dusty Debris Disk Formation Around White Dwarf Stars” (https://doi.org/10.3847/2041-8213/abfd39) that appears in the Astrophysical Journal Letters. PSI Senior Scientist Elisabeth R. Adams is a co-author. 

Debris disks form when planetary bodies such as these asteroids get too close to  their host star — the white dwarf — whose tidal forces can pulverize them into dust. Therefore, young, hot white dwarfs, which host destabilized planetary systems, are expected to rapidly form dusty debris disks. However, observations show that dusty debris disks form only after a long delay. 

“We found that this delay is a result of these young white dwarfs being extremely hot. So hot that any dust that forms from a tidally disrupted asteroid  rapidly vaporizes and dissipates.  We found that this dust only stops vaporizing after the white dwarf has had time to sufficiently cooled down, to a surface temperature of approximately 27,000 degrees kelvin (48,000 degrees Fahrenheit). This temperature agrees with observations of these white dwarf systems; all dusty debris disks are found around white dwarfs cooler than this critical temperature,” Steckloff said. 

“Our Solar System will follow this fate in a few billion years, when the Sun runs out of fuel, expands into a Red Giant and then ultimately collapse into a white dwarf,” Steckloff said. Most of the inner planets will be destroyed, and Jupiter will migrate outward, destabilizing the orbits of asteroids in our asteroid belt.  Some of these asteroids may end up passing very close to the Sun, where stellar tides may break them up to form dusty debris disks. In other words, we may be looking at a glimpse of our own home system in the distant future.” 

Steckloff’s work was funded by a grant to PSI from the NASA Second Exoplanet Research program. 

Featured image: This artist’s concept shows a white dwarf debris disk. Credit: NASA/JPL-Caltech.

Provided by Planetary Science Institute

Astronomers Discovered A Long Period Eclipsing White Dwarf And A Substellar Companion (Planetary Science)

Using ZTF photometry, and Gaia and Pan-STARRS data, a team of international astronomers discovered an eclipsing binary, “ZTF J0038+2030”, composed of a white dwarf and a substellar companion with an orbital period of 10 hours. Their study recently appeared in Arxiv.

A substellar object, sometimes called a substar, is an astronomical object mainly consist of hydrogen gas and are not massive enough to fuse hydrogen in their core (M ≲ MHBL ≈ 0.07 M; 73 Mjup). Substellar objects have masses in the range of ∼0.3–73 Mjup and are generally divided into two classes: brown dwarfs and giant exoplanets. There is no clear separation based on mass, but the distinction is based on the formation history. The formation of a brown dwarf is the same as that of more massive main-sequence stars: they are formed by gravitational instabilities in gas clouds and have elemental abundances similar to that of the interstellar medium. On the other hand, giant planets are formed by core accretion in a disk around a protostar, and have an enhanced metal abundance compared to the host star.

“To identify the eclipses, we used the Zwicky Transient Facility lightcurves. The system was identified in a search for deep eclipsing white dwarfs. We searched the combined PSF-photometry and alert photometry lightcurves of white dwarfs for deep eclipses and identified the period using the BLS algorithm.”

In the current study, astronomers used follow-up photometry and spectroscopy to measure the binary parameters. They found that, white dwarf has a mass of 0.50 M and a temperature of 10900 K. While, the substellar companion has a mass of 0.059 M and a small radius of 0.0783 R (as shown in table 1 below). It is one of the smallest transiting brown dwarfs known and likely old, ≳ 8 Gyr.

Table 1: Derived binary parameters: showing the radius and mass of the white dwarf (R1, M1) and brown dwarf (M2, R2) and radial velocity amplitude (K1, K2) and density (p) © Van Roestel et al.

From the mass of white dwarf, astronomers believe that, it has likely a CO core which allows the two formation scenarios. In the first scenario, the white dwarf could have formed during a common envelope phase on the Asymptotic Giant branch (AGB) after helium core exhaustion. The second scenario is that the common envelope happened at the tip of the Red giant branch (RGB), just after the helium flash which would result in a white dwarf with a mass close to 0.5 M. In that scenario the white dwarf would have after the common envelope evolved into a hot sub-dwarf (sdB) and appeared as an HW Vir system before it evolved into a white dwarf with a brown dwarf companion after helium exhaustion in the sdB.

Finally, they concluded that, the system is relatively bright, and a good prototype system where the brown dwarf suffers minimal irradiation. It is also a useful target for eclipse timing to find circumbinary objects as brown dwarf are not expected to show eclipse time variations due to Apple gate’s mechanism.

Featured image: Artist’s impression of one of this study’s superlative discoveries, the oldest known wide-separation white dwarf plus cold brown dwarf pair. The small white orb represents the white dwarf (the remnant of a long-dead Sun-like star), while the brown/orange foreground object is the newly discovered brown dwarf companion. This faint brown dwarf was previously overlooked until it was spotted by citizen scientists, because it lies right within the plane of the Milky Way. © NOIRLab/NSF/AURA/P. Marenfeld Acknowledgement: William Pendrill

For more:

Jan van Roestel, Thomas Kupfer, Keaton J. Bell, Kevin Burdge, Przemek Mróz, Thomas A. Prince, Eric C. Bellm, Andrew Drake, Richard Dekany, Ashish A. Mahabal, Michael Porter, Reed Riddle, Kyung Min Shin, David L. Shupe, “ZTFJ0038+2030: a long period eclipsing white dwarf and a substellar companion”, Arxiv, pp. 1-13, 2021. https://arxiv.org/abs/2105.08687

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Discovered: The Mechanism That Generates Huge White Dwarf Magnetic Fields (Planetary Science)

  • University of Warwick astronomer co-authors new study in Nature Astronomy that proposes solution to long-running question of how white dwarf stars generate magnetic fields
  • A dynamo mechanism, similar to how Earth generates its magnetic field, could be the answer
  • This research shows that sometimes very similar mechanisms can work in very different astronomical objects.

A dynamo mechanism could explain the incredibly strong magnetic fields in white dwarf stars according to an international team of scientists, including a University of Warwick astronomer.

One of the most striking phenomena in astrophysics is the presence of magnetic fields. Like the Earth, stars and stellar remnants such as white dwarfs have one. It is known that the magnetic fields of white dwarfs can be a million times stronger than that of the Earth. However, their origin has been a mystery since the discovery of the first magnetic white dwarf in the 1970s. Several theories have been proposed, but none of them has been able to explain the different occurrence rates of magnetic white dwarfs, both as individual stars and in different binary star environments.

This uncertainty may be resolved thanks to research by an international team of astrophysicists, including Professor Boris Gänsicke from the University of Warwick and led by Professor Dr. Matthias Schreiber from Núcleo Milenio de Formación Planetaria at Universidad Santa María in Chile. The team showed that a dynamo mechanism similar to the one that generates magnetic fields on Earth and other planets can work in white dwarfs, and produce much stronger fields. This research, part-funded by the Science and Technology Facilities Council (STFC) and the Leverhulme Trust, has been published in the prestigious scientific journal Nature Astronomy.

Professor Boris Gänsicke of the Department of Physics at the University of Warwick said: “We have known for a long time that there was something missing in our understanding of magnetic fields in white dwarfs, as the statistics derived from the observations simply did not make sense. The idea that, at least in some of these stars, the field is generated by a dynamo can solve this paradox. Some of you may remember dynamos on bicycles: turning a magnet produces electric current. Here, it works the other way around, the motion of material leads to electric currents, which in turn generate the magnetic field.”

According to the proposed dynamo mechanism, the magnetic field is generated by electric currents caused by convective motion in the core of the white dwarf. These convective currents are caused by heat escaping from the solidifying core.

“The main ingredient of the dynamo is a solid core surrounded by a convective mantle – in the case of the Earth, it is a solid iron core surrounded by convective liquid iron. A similar situation occurs in white dwarfs when they have cooled sufficiently,” explains Matthias Schreiber.

The astrophysicist explains that at the beginning, after the star has ejected its envelope, the white dwarf is very hot and composed of liquid carbon and oxygen. However, when it has sufficiently cooled, it begins to crystallize in the center and the configuration becomes similar to that of the Earth: a solid core surrounded by a convective liquid. “As the velocities in the liquid can become much higher in white dwarfs than on Earth, the generated fields are potentially much stronger. This dynamo mechanism can explain the occurrence rates of strongly magnetic white dwarfs in many different contexts, and especially those of white dwarfs in binary stars” he says.

Thus, this research could solve a decades-old problem. “The beauty of our idea is that the mechanism of magnetic field generation is the same as in planets. This research explains how magnetic fields are generated in white dwarfs and why these magnetic fields are much stronger than those on Earth. I think it is a good example of how an interdisciplinary team can solve problems that specialists in only one area would have had difficulty with,” Schreiber adds.

The next steps in this research, says the astrophysicist, are to perform a more detailed model of the dynamo mechanism and to test observationally the additional predictions of this model.

Featured image: Illustration of the origin of magnetic fields in white dwarfs in close binaries (to be read counter clockwise). Credit: Paula Zorzi

Whats The Effect Of Nova Eruption On the Donor Star In A Cataclysmic Variable? (Planetary Science)

Cataclysmic variable stars are binary stars that consist of two components; a white dwarf primary, and a mass transferring secondary. The stars are so close to each other that the gravity of the white dwarf distorts the secondary, and the white dwarf accretes matter from the companion. Therefore, the secondary is often referred to as the donor star. The infalling matter, which is usually rich in hydrogen, forms in most cases an accretion disk around the white dwarf.

Schematic of a non-magnetic cataclysmic variable. A white dwarf (on the right) accretes matter through an accretion disk, fed by a Roche-lobe-filling main-sequence donor star (on the left). Matter from the donor star travels in a stream from the Lagrange point L1 and meets the accretion disk at a hot spot. The white dwarf has a mass of 0.8 M, its companion is a main sequence star of 0.5 M and their centres are separated by 1.64 R. © Philip D. Hall

As hydrogen accumulates on the white dwarf’s surface, the density and temperature at the base of the accreted layer rise. At some point, a critical mass is reached, and the accumulated layer explodes in a thermonuclear runaway— a nova. The binary system survives the explosion, mass accumulates once again on top of the white dwarf, and the nova recurs on a time-scale that depends on the accretion rate and on the white dwarf’s mass.

During and immediately after a nova eruption, the white dwarf’s luminosity is close to the Eddington limit. The companion is irradiated and heated to a surface temperature that is an order of magnitude hotter than its pre-eruption (main sequence) effective temperature. In 1988, Kovetz and colleagues studied the response of the companion to such irradiation and calculated the heat penetration into its atmosphere. They showed that as the heated layers expand, the donor overflows its Roche lobe more than in quiescence, and the mass transfer rate increases by orders of magnitude. Recently, Hillman and colleagues demonstrated that this irradiation-driven enhanced mass transfer may dominate the long term evolution of CVs over multiple nova cycles.

Now, Ginzburg and Quataert revisited the heating of the donor star by the hot white dwarf during and after a nova eruption using a combination of analytical arguments and experiments with the mesa stellar evolution code. They calculated the enhanced mass transfer rate following a nova and improve upon Kovetz et al. and Hillman et al. by refining their power-law scaling for how this rate depends on the irradiation temperature and on the donor’s mass. More importantly, they revise upwards the normalization of the mass transfer rate.

They first showed that a nova eruption irradiates and heats the donor star in a cataclysmic variable to high temperatures Tirr, causing its outer layers to expand and overflow the Roche lobe. They then, calculated the donor’s heating and expansion both analytically and numerically and found that irradiation drives enhanced mass transfer from the donor at a rate,

which reaches m• ∼ 10¯6 M yr¯1 at the peak of the eruption — about a thousand times faster than during quiescence. As the nova subsides and the white dwarf cools down, m• drops to lower values.

Finally, they discussed the decline of the mass transfer rate back to quiescence as the white dwarf cools down after a nova. They found that under certain circumstances, the decline halts and the mass transfer persists at a self-sustaining rate of m• ∼ 10¯7 M yr¯1 for up to ∼ 10³ yr after the eruption. At this rate, irradiation by the white dwarf’s accretion luminosity is sufficient to drive the mass transfer on its own.

Figure 1. The self-sustaining (bootstrapping) irradiation temperature Tirr ∝ m•^1/4 as a function of ΔrL ≡ rL – r(0), the difference between the donor’s Roche lobe rL and its non-irradiated radius r(0) © Ginzburg and Quataert

This ‘bootstrapping’ state may explain the high quiescent accretion rates (∼ m• ∼ 10¯7 M yr¯1) recently observed for the recurrent novae T Pyxidis (T Pyx) and IM Normae (IM Nor). If the self-sustaining rate is sufficiently high, the white dwarf can burn accreted hydrogen stably, potentially explaining long-lived supersoft X-ray sources with short orbital periods like RX J0537.7–7034 and 1E 0035.4–7230 as well.

Finally, they concluded that whether or not a system reaches the self-sustaining state is sensitive to the donor’s chromosphere structure, as well as to the orbital period change during nova eruptions.

Reference: Sivan Ginzburg, Eliot Quataert, “Novae heat their food: mass transfer by irradiation”, ArXiv, pp. 1-8, 2021. https://arxiv.org/abs/2104.11250

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Dark Matter Core Collapse Leads To Type Ia Supernovae (Astronomy / Cosmology)

Riggins and colleagues studied the ignition of supernovae by the formation and self-gravitational collapse of a dark matter (DM) core containing many DM particles. For non-annihilating DM, such a core collapse may lead to a mini black hole that can ignite supernova (SN) through the emission of Hawking radiation, or possibly as a by-product of accretion. For annihilating DM, core collapse leads to an increasing annihilation rate and can ignite SN through a large number of rapid annihilations. These processes extend the previously derived constraints on DM to masses as low as 10^5 GeV.

Supernova © NASA

Dark matter (DM) accounts for over 80% of the matter density of the Universe, but its identity remains unknown. While direct detection is a promising approach to identifying the nature of DM, searches for indirect signatures of DM interactions in astrophysical systems is also fruitful, particularly if the unknown DM mass happens to be large.

Graham and colleagues recently suggested that white dwarfs (WD) act as astrophysical DM detectors: DM may heat a local region of a WD and trigger thermonuclear runaway fusion, resulting in a type Ia supernova (SN). DM ignition of sub-Chandrasekhar WDs was further studied in a companion paper, where Riggins, Graham along with their colleagues showed that generic classes of DM capable of producing high-energy standard model (SM) particles in the star can be constrained, e.g., by DM annihilations or decay to SM products. As an illustrative example, Graham’s paper placed new constraints on ultra-heavy DM with masses greater than 10^16 GeV for which a single annihilation or decay is sufficient to ignite a SN.

Now in this paper, Riggins and colleagues examined the possibility of igniting SN by the formation and self-gravitational collapse of a DM core. They studied two novel processes by which a collapsing DM core in a WD can ignite a SN. If the DM has negligible annihilation cross section, so-called asymmetric DM, collapse may result in a mini black hole (BH) that can ignite a SN via the emission of energetic Hawking radiation or possibly as it accretes. If the DM has a small but non-zero annihilation cross section, collapse can dramatically increase the number density of the DM core, resulting in SN ignition via a large number of rapid annihilations. Both of these processes extend the previously derived constraints on DM in Graham’s paper, notably to masses as low as 10^5 GeV.

“We have studied the possibility of DM core collapse triggering type Ia SN in sub-Chandrasekhar WDs, following up on previous work. Collapse of asymmetric DM can lead to the formation of a mini BH which ignites a SN by the emission of Hawking radiation, and collapse of annihilating DM can lead to large number of rapid annihilations which also ignite a SN. Such processes allow us to place novel constraints on DM parameters (as shown in Fig. 1, Fig. 2, Fig. 3, and Fig. 4). These constraints improve on the limits set by terrestrial experiments, & they are complementary to previous considerations of DM capture in compact objects.”, said Ryan Janish.

FIG. 1. Constraints on fermionic asymmetric DM which forms a DM core and collapses to a mini black hole in a WD. The black hole either ignites a supernova via Hawking emission (red) or accretes and eats the star (or possibly ignites a supernova) (blue). Also shown (purple) are the constraints on DM-nuclei scatters igniting a supernova during core collapse before formation of a black hole. © Riggins et al.
FIG. 2. Constraints on bosonic asymmetric DM which forms a DM core and collapses to a mini black hole in a WD. The black hole either ignites a supernova via Hawking emission (red) or accretes and eats the star (or possibly ignites a supernova) (blue). Also shown (purple) are the constraints on DM-nuclei scatters igniting a supernova during core collapse before formation of a black hole. © Riggins et al.
FIG. 3. Constraints on fermionic asymmetric DM which forms a DM core and ignites a supernova through annihilations (red). For sufficiently small σχχv the core first collapses to a black hole (blue), and is otherwise constrained, see Fig. 1. Also shown (purple) are the constraints on DM-nuclei scatters igniting a supernova during core collapse before annihilations could do so. © Riggins et al.
FIG. 4. Constraints on bosonic asymmetric DM which forms a DM core and ignites a supernova through annihilations (red). For sufficiently small σχχv the core first collapses to a black hole (blue), and is otherwise constrained, see Fig. 2. Also shown (purple) are the constraints on DM-nuclei scatters igniting a supernova during core collapse before annihilations could do so.

A number of potential observables of DM cores in compact objects have been considered in the literature. These include: (1) gravitational effects of DM cores on the structure of low-mass stars, white dwarfs (WDs), and neutron stars (NS), (2) BH formation & subsequent destruction of host NSs, and (3) anomalous heating from DM annihilations or scatters in white dwarfs (WDs) and (NSs). Riggins and colleagues emphasized that the signature of a DM core igniting a type Ia SN is distinct from these, and thus the constraints derived by them are complementary. For instance, while it has been known that DM cores which form evaporating mini BHs are practically unobservable in a NS, this is decidedly not the case in a WD where such BHs typically ignite a SN.

“It is interesting to contemplate that the ignition of type Ia SN through the evaporation of mini black holes represents a potential observable signature of Hawking radiation. Further, it also interesting that the extremely tiny annihilation cross sections constrained in this work, which to our knowledge have no other observable consequences, can nonetheless be capable of igniting a SN.”, said Riggins.

They claimed that DM which can cool via emission of dark radiation will be more susceptible to collapse, and is likely to be more strongly constrained than models possessing only elastic cooling. Another particularly interesting case they described is electrically charged particles or magnetic monopoles. Ultra-heavy monopoles & anti-monopoles could be captured in a white dwarf (WD) and subsequently annihilate, igniting SN—they estimated that such a process can be used to place constraints on the flux of galactic monopoles exceeding current limits.

“We emphasize that the heat deposited in the stellar matter during a DM collapse would be drastically affected by the presence of an additional cooling mechanism which drives the collapse, e.g., emitting dark radiation. In particular, if such a cooling mechanism is present and efficient in a collapsing core, ignition due to heating by nuclear scatters might not occur.”, said Riggins.

“…there are many puzzles in our understanding of the origin of type Ia SN and other WD events, such as Ca-rich transients. It is plausible that DM is responsible for a fraction of these events. To this end, it is important to identify the distinguishing features of SN that would originate from DM core collapse (e.g. the lack of a stellar companion) in order to observationally test such tantalizing possibilities.”, concluded authors of the study.

Reference: Ryan Janish, Vijay Narayan, and Paul Riggins, “Type Ia supernovae from dark matter core collapse”, Phys. Rev. D 100, 035008 – Published 9 August 2019. https://journals.aps.org/prd/abstract/10.1103/PhysRevD.100.035008

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3D Simulation Helps Revealing Accretion Process in Progenitor of Tycho’s Supernova (Astronomy)

Dr.JIAO Chengliang from Yunnan Observatories of the Chinese Academy of Sciences, collaborating with Prof. XUE Li’s group from Xiamen University, performed three-dimensional (3D) simulations of the accretion flow in the progenitor of Tycho’s supernova, which helps identifying the physical properties of the accretion process.

Images of density at the final time of the simulation run for B = 5.44×10³G. (Image by JIAO Chengliang)

The study was published in Monthly Notices of the Royal Astronomical Society on Nov. 27.

Type Ia supernovae (SNe Ia) plays an important role in astrophysics, especially in cosmology and galactic chemical evolution. SNe Ia can be triggered by a carbon-oxygen white dwarf (CO WD) accreting sufficient material from a non-degenerate companion star, i.e. the single-degenerate (SD) model.

Tycho’s supernova (SN) is a famous SN. Recent observations of its remnant suggests that the SN ejecta should have evolved in a bubble blown by a latitude-dependent wind, yet how this wind is formed is still not very clear.

The researchers studied the wind structure in different situations. They found that when the magnetic field in the accreted material was negligible, outflowing wind was concentrated near the equatorial plane. When the magnetic field had energy equipartition with internal energy, polar wind was comparable with the equatorial wind.

A carefully chosen magnetic field between the above two cases can roughly reproduce the latitude-dependent wind required to form the peculiar periphery of Tycho’s SN remnant. This magnetic field may contain the tangled magnetic field in the accreted material obtained from the surface of the companion star, as well as contributions from the WD.

The study reveals the importance of magnetic field in the progenitor of Tycho’s SN. It also provides a new source of mass-loss, other than the mass-loss caused by hydrogen and helium flashes on the WD surface, which are often considered in binary evolution researches.

The mass-loss ratio is extremely large (above 90 percent) in the simulation, yet it is consistent with researches in accretion physics, and this outflow only lasts for a limited time before the SN explosion, so it does not handicap the mass accumulation of the WD much.

Reference: Li Xue, Cheng-Liang Jiao, Yuan Li, Three-dimensional simulations of accretion flow in the progenitor of Tycho’s supernova, Monthly Notices of the Royal Astronomical Society, , staa3696, https://academic.oup.com/mnras/article-abstract/501/1/664/6007743?redirectedFrom=fulltext https://doi.org/10.1093/mnras/staa3696

Provided by Chinese Academy of Sciences