Radio Frequency Models of Novae in Eruption. I. The Free-Free Process in Bipolar Morphologies

Abstract

Observations of novae at radio frequencies provide us with a measure of the total ejected mass, density profile, and kinetic energy of a nova eruption. The radio emission is typically well characterized by the free-free emission process. Most models to date have assumed spherical symmetry for the eruption, although for as long as there have been radio observations of these systems, it has been known that spherical eruptions are too simplistic a geometry. In this paper, we build bipolar models of the nova eruption, assuming the free-free process, and show the effects of varying different parameters on the radio light curves. The parameters considered include the ratio of the minorto major-axis, the inclination angle, and shell thickness. We also show the uncertainty introduced when fitting spherical-model synthetic light curves to bipolar-model synthetic light curves. We find that the optically thick phase rises with the same power law (S ν ∝ t 2 ) for both the spherical and bipolar models. In the bipolar case, there is a "plateau" phase-depending on the thickness of the shell as well as the ratio of the minor-to major-axis-before the final decline, which follows the same power law (S ν ∝ t −3 ) as in the spherical case. Finally, fitting spherical models to the bipolar-model synthetic light curves requires, in the worst-case scenario, doubling the ejected mass, more than halving the electron temperature, and reducing the shell thickness by nearly a factor of 10. This implies that in some systems we have been over-predicting the ejected masses and under-predicting the electron temperature of the ejecta.

Internal shocks from variable outflows in classical novae

“…Also note that the light curve shape from the true bipolar ejecta morphology, including both the fast polar and slower equatorial components, will differ in detail from the predictions of a 1D model presented here (e.g. Ribeiro et al 2014b). Figure 12 shows the 10 GHz radio light curves (top panel) and synchrotron brightness temperatures (bottom panel) for the multiple-transition model.…”

“…Further evidence regarding the nature of the shocks in novae comes from late-time ( several month to year timescale) radio observations, both of thermal emission from the expanding photo-ionized ejecta (e.g. Ribeiro et al 2014a;Cunningham et al 2015) as well non-thermal synchrotron emission from shocks (e.g. Krauss et al 2011;Weston et al 2016).…”

Non-thermal radio emission from colliding flows in classical nova V1723 Aql

“…Therefore, that for V1723, Aql α trans appears to flatten slightly with time may indicate that the inner remnant is more confined towards the equatorial plane than the outer remnant -as in the case of the biconical flow collimated by a dense, slow torus in V959 Mon . If this is the case, a bi-polar outflow could result in an over-estimate of mass by up to a factor of two (Ribeiro et al 2014). Nevertheless, since the density profile of 1/r 2 in the outer regions of the ejecta suggest that any bipolar flow must have a wide opening angle, we can nevertheless use a spherical model to obtain reasonable estimates of the ejecta parameters.…”

Binary orbits as the driver of γ-ray emission and mass ejection in classical novae

“…With these assumption, the light curve is best fit with an electron temperature 2 × 10 4 K, v min = 200 km/s, and M ej = 4 × 10 −5 M . The geometry of V959 Mon was clearly aspherical, and departures from spherical symmetry could decrease the ejected mass by as much as a factor of three 42 . This ejecta mass estimate is well within the range of theoretical expectations and observed values for normal classical novae 1,13,43 .…”