arXiv subjects
Publications and source records attributed to Na'ama Hallakoun.
We present a summary of our understanding of Type Ia Supernova progenitors, mostly discussing the observational approach. The main goal of this review is to provide the non-specialist with a sufficiently comprehensive view of where we stand.
We obtain new and precise information on the double white dwarf (DWD) population and on its gravitational-wave-driven merger rate, by combining the constraints on the DWD population from two previous radial-velocity-variation studies: One based on a sample of white dwarfs (WDs) from the Sloan Digital Sky Survey (SDSS, which with its low spectral resolution probes systems at separations a<0.05 au), and one based on the ESO-VLT Supernova-Ia Progenitor surveY (SPY, which, with high spectral resolution, is sensitive to a<4 au). From a joint likelihood analysis, the DWD fraction among WDs is fbin=0.095+/-0.020 (1-sigma, random) +0.010 (systematic) in the separation range ~<4 au. The index of a power-law distribution of initial WD separations (at the start of solely gravitational-wave-driven binary evolution), N(a)da ~ a^alpha da, is alpha=-1.30+/-0.15 (1-sigma) +0.05 (systematic). The Galactic WD merger rate per WD is R_merge=(9.7+/-1.1)e-12 /yr. Integrated over the Galaxy lifetime, this implies that 8.5-11 per cent of all WDs ever formed have merged with another WD. If most DWD mergers end as more-massive WDs, then some 10 per cent of WDs are DWD-merger products, consistent with the observed fraction of WDs in a "high-mass bump" in the WD mass function. The DWD merger rate is 4.5-7 times the Milky Way's specific Type-Ia supernova (SN Ia) rate. If most SN Ia explosions stem from the mergers of some DWDs (say, those with massive-enough binary components) then ~15 per cent of all WD mergers must lead to a SN Ia.
Recent Kepler photometry has revealed that about half of white dwarfs (WDs) have periodic, low-level (~ 1e-4 - 1e-3), optical variations. Hubble Space Telescope (HST) ultraviolet spectroscopy has shown that up to about one half of WDs are actively accreting rocky planetary debris, as evidenced by the presence of photospheric metal absorption lines. We have obtained HST ultraviolet spectra of seven WDs that have been monitored for periodic variations, to test the hypothesis that these two phenomena are causally connected, i.e. that the optical periodic modulation is caused by WD rotation coupled with an inhomogeneous surface distribution of accreted metals. We detect photospheric metals in four out of the seven WDs. However, we find no significant correspondence between the existence of optical periodic variability and the detection of photospheric ultraviolet absorption lines. Thus the null hypothesis stands, that the two phenomena are not directly related. Some other source of WD surface inhomogeneity, perhaps related to magnetic field strength, combined with the WD rotation, or alternatively effects due to close binary companions, may be behind the observed optical modulation. We report the marginal detection of molecular hydrogen in WD J1949+4734, only the fourth known WD with detected H_2 lines. We also re-classify J1926+4219 as a carbon-rich He-sdO subdwarf.
We present a photometric detection of the first brightness dips of the unique variable star KIC 8462852 since the end of the Kepler space mission in 2013 May. Our regular photometric surveillance started in October 2015, and a sequence of dipping began in 2017 May continuing on through the end of 2017, when the star was no longer visible from Earth. We distinguish four main 1-2.5% dips, named "Elsie," "Celeste," "Skara Brae," and "Angkor", which persist on timescales from several days to weeks. Our main results so far are: (i) there are no apparent changes of the stellar spectrum or polarization during the dips; (ii) the multiband photometry of the dips shows differential reddening favoring non-grey extinction. Therefore, our data are inconsistent with dip models that invoke optically thick material, but rather they are in-line with predictions for an occulter consisting primarily of ordinary dust, where much of the material must be optically thin with a size scale <<1um, and may also be consistent with models invoking variations intrinsic to the stellar photosphere. Notably, our data do not place constraints on the color of the longer-term "secular" dimming, which may be caused by independent processes, or probe different regimes of a single process.
From a sample of spectra of 439 white dwarfs (WDs) from the ESO-VLT Supernova-Ia Progenitor surveY (SPY), we measure the maximal changes in radial-velocity (DRVmax) between epochs (generally two epochs, separated by up to 470d), and model the observed DRVmax statistics via Monte-Carlo simulations, to constrain the population characteristics of double WDs (DWDs). The DWD fraction among WDs is fbin=0.100+/-0.020 (1-sigma, random) +0.02 (systematic), in the separation range ~<4AU within which the data are sensitive to binarity. Assuming the distribution of binary separation, a, is a power-law, dN/da ~ a^alpha, at the end of the last common-envelope phase and the start of solely gravitational-wave-driven binary evolution, the constraint by the data is alpha=-1.3+/-0.2 (1-sigma) +/-0.2 (systematic). If these parameters extend to small separations, the implied Galactic WD merger rate per unit stellar mass is R_merge=(1-80)e-13 /yr/Msun (2-sigma), with a likelihood-weighted mean of R_merge=(7+/-2)e-13 /yr/Msun (1-sigma). The Milky Way's specific Type-Ia supernova (SN Ia) rate is likely R_Ia~1.1e-13 /yr/Msun and therefore, in terms of rates, a possibly small fraction of all merging DWDs (e.g. those with massive-enough primary WDs) could suffice to produce most or all SNe Ia.