A peculiar white dwarf merger remnant with a rigidly corotating half-ring of circumstellar material
Cristea, Andrei-Alexandru; Caiazzo, Ilaria
- Publisher
- Zenodo
- Language
- en
Abstract
In the recent years, the wealth of data on potential double white dwarf merger remnants made available by time-domain surveys like the Zwicky Transient Facility (ZTF), ATLAS, Kepler and TESS, together with spectroscopic counterparts such as SDSS V and DESI, has been essential in advancing our knowledge of ompact binary evolution. In particular, the emergence of peculiar systems offers invaluable insight into the limitations of our current best theories. The periodically variable white dwarf ZTF J200832.79+444939.67, hereafter ZTF J2008+4449, is possibly one of the most exotic discovered so far: a likely merger remnant showing signs of accretion from circumstellar material without a stellar or substellar companion. The nature of ZTF J2008+4449 as a merger remnant is supported by its physical properties: hot (35, 500±300 K) and massive (1.12±0.03 M⊙), the white dwarf is rapidly rotating on a period of ≈ 6.6 minutes and likely possesses exceptionally strong magnetic fields (∼ 400 − 600 MG) at its surface. Remarkably, we detect a significant period derivative of (1.8 ± 0.1) × 10^12 s/s, indicating that the white dwarf is spinning down. This, together with the detection of soft X-rays emissions, are tell-tale signs of accretion or, more generally, interaction with circumstellar material; in the absence of a companion, we believe this likely belongs to the fallback of gravitationally bound merger ejecta. We also detect Balmer emission whose unusual variability on the spin period of the white dwarf, showing Doppler shifts as high as ≈ 2, 000 km/s, is consistent with the trapping of a toroidal half-arc of ionised gas in the white dwarf’s magnetosphere.
Full text
A peculiar white dwarf merger remnant with a rigidly corotating half-ring of circumstellar material Andrei Cristea andrei-alexandru.crist[email protected] Who is ZTF J2008+4449 (J2008)? A white dwarf: Soft, non-photospheric X-ray emission Fit: 2-temp thermal, optically thin plasma (APEC) XMM data (count rates) - EPIC PN spectra fits Fit: Power law Γ= 2.2 ±0.1 kT1= 3 ±1keV kT2= 0.23 +0.04 −0.03 keV A corotating half-ring of H -emitting gas α An increase in rotation period - angular mom. loss • - quickly rotating • - very massive • - highly magnetic Pspin ≈6.6 min M≈1.12 M⊙ B∼500 MG Found as part of a Zwicky Transient Facility (ZTF) archival search for variable white dwarfs, J2008 is young ( ) and massive ( ), located at away from the Solar system. ≈35.300 K ≈1.12 M⊙ ≈343 pc The merging of two white dwarfs may lead to the formation of a remnant white dwarf, given the right conditions. The merging process must conserve the orbital angular momentum, mass, and is believed to produce exceptionally strong magnetic dynamos. Thus, white dwarfs which are particularly massive, fast spinning, and highly magnetic are thought to likely originate from the merger of a double white dwarf binary system. J2008 is most likely a double white dwarf merger remnant. Signs of interaction with circumstellar material Γ= 2.2 ±0.1 J2008 has a surface temperature of . This is not enough to generate the X-ray luminosity observed in the 0.25-10 keV band, . ≈35.300 K LX= (2.3 ±0.4) ×1029 erg s−1 X-ray emission of J2008 is non-photospheric. Schematic representation of the fast-spinning white dwarf ZTF J2008+4449, showing a half-ring of circumstellar material in rigid body corotation with it (red), an asymmetric magnetic field structure (blue), and the emission of X-rays (green). Observed-calculated (O-C) diagram - difference (in minutes) between the observed and expected spin phases as a function of ephemeris, over the total span of the optical time-resolved photometric observations. With the help of co-authors: Ilaria Caiazzo Stephane Vennes Adela Kawka Tim Cunningham John Raymond JJ Hermes Jim Fuller Aayush Desai … The quadratic fit to the O-C diagram (on the left) reveals an increase in spin period (spin-down) of . The energy lost in the spin-down process, , is comparable to J2008’s bolometric luminosity · P= (1.8 ±0.1) ×10−12 s/s Lsd = 2 ×1032 erg s−1 The large loss of angular momentum may be linked to the ejection of material from the system. Circular movement of a half-ring of ionized circumstellar gas, in rigid-body corotation with the white dwarf. Where could this circumstellar gas be coming from? Hypothesis Caveat The fallback of gravitationally bound post-merger ejecta 1. 2. 3. Disruption of planetary body (asteroid, comet, planet) Stellar “winds” from the WD surface (e.g., Magnetically driven / reconnection driven) Why only H and no metals (C, O etc.)? Why only H and no metals (Mg, Ca, Fe etc.)? Unclear whether possible to accelerate wind. ? ? ? The H (Balmer) emission line profile oscillates over one spin period between maximally redshifted and blueshifted states, with Doppler shifts of , as seen in the two figures on the right. The morphology of the H line profile variation resembles that produced by a half-ring of ionized gas in rigid-body corotation with the white dwarf (right-hand figures). This model also provides a convincing fit of the time-averaged line (bottom left panel). The oscillating Doppler shifts of the H line profile are thus most likely associated to the circular movement of this gas distribution. α ≈1700 km/s α α Doppler velocity shift from natural H position [km/s] α H line profile variation over one spin period (bottom to top) α Data 1/2-ring Model Both Time-averaged H line profile α H line profile variation over one spin period (bottom to top) (colormap <=> flux) α H Data α 1/2-ring Model Time averaged H line profile (blue) and best-fit halfring model (red) - residuals on bottom - α Doppler velocity shift from natural H position [km/s] α
