Low-mass White Dwarfs with Hydrogen Envelopes as a Missing Link in the Tidal Disruption Menu

Abstract

We construct a menu of objects that can give rise to bright flares when disrupted by massive black holes (BHs), ranging from planets to evolved stars. Through their tidal disruption, main sequence and evolved stars can effectively probe the existence of otherwise quiescent supermassive BHs and white dwarfs can probe intermediate mass BHs. Many low-mass white dwarfs possess extended hydrogen envelopes, which allow for the production of prompt flares in disruptive encounters with moderately massive BHs of 10 5 -10 7 M -masses that may constitute the majority of massive BHs by number. These objects are a missing link in two ways: (1) for probing moderately massive BHs and (2) for understanding the hydrodynamics of the disruption of objects with tenuous envelopes. A flare arising from the tidal disruption of a 0.17 M white dwarf by a 10 5 M BH reaches a maximum between 0.6 and 11 days, with a peak fallback rate that is usually super-Eddington and results in a flare that is likely brighter than a typical tidal disruption event. Encounters stripping only the envelope can provide hydrogen-only fallback, while encounters disrupting the core evolve from H-to He-rich fallback. While most tidal disruption candidates observed thus far are consistent with the disruptions of main sequence stars, the rapid timescales of nuclear transients such as Dougie and PTF10iya are naturally explained by the disruption of low-mass white dwarfs. As the number of observed flares continues to increase, the menu presented here will be essential for characterizing nuclear BHs and their environments through tidal disruptions.

Tidal Disruptions of White Dwarfs: Theoretical Models and Observational Prospects

“…Obviously, it is easier to ignite a He-white dwarf than one with a CO-composition. It should be mentioned, however, that a white dwarf below ≈ 0.46 M consists of a helium core engulfed in a hydrogen envelope that can extend to several core radii and therefore increase the effective tidal radius (Law-Smith et al 2017). Alternatively tidal compression could trigger a detonation first in a He-shell on the surface of a white dwarf followed by detonation of a CO core, 'tidal double detonation' (Tanikawa 2018b).…”

eROSITA detection rates for tidal disruptions of white dwarfs by intermediate mass black holes

“…Initial mass fallback rates are likely to be highly super Eddington (eg. Loeb & Ulmer 1997;MacLeod et al 2016;Law-Smith et al 2017), such that the event is radiatively inefficient and its luminosity output Eddington-limited (Haas et al 2012;Khabibullin et al 2014), remaining approximately constant and only starting to decay once accretion rates become sub-Eddington. We note that the process of how falling-back debris is accreted onto the central object is still not well understood, especially for super-Eddington fallback rates.…”

“…The rapid decay rates involved (the whole flaring episode occurs over hours to weeks depending on WTDE configuration, as opposed to months to years for MS TDEs -see Fig. 11 of Law-Smith et al 2017 for comparison of different TDE timescales) makes confident classification of WTDEs difficult as there is usually insufficient multi-wavelength evidence to choose one model over competing models, such as flaring stars or off-axis GRBs. eROSITA (extended Roentgen Survey with an Imaging Telescope Array) (Merloni et al 2012;Predehl 2017), which is on board the Russian-German Spektrum-Roentgen-Gamma (SRG) mission, was launched in July 2019.…”

Feedback by Massive Black Holes in Gas-rich Dwarf Galaxies

“…The main difference with TDEs of main sequence stars is that a similar amount of mass is accreted on a much more rapid timescale. This leads to luminous flares, especially via tidal disruption of the hydrogen-rich envelopes often surrounding white dwarfs and observed as nuclear transients on short time-scales (Law-Smith et al 2017). Tidal decompression can trigger SNe with possible implications for acceleration of cosmic rays to ultrahigh energies, and in particular, an intermediate/heavy composition for the observed UHECRs that is independent of specific acceleration models (Alves Batista & Silk 2017).…”